Smell and Stress Response in the Brain: Review of the Connection Between Chemistry and Neuropharmacology

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Review Smell and Stress Response in the Brain: Review of the Connection between Chemistry and Neuropharmacology

Yoshinori Masuo 1,*, Tadaaki Satou 2 , Hiroaki Takemoto 3 and Kazuo Koike 3

1 Laboratory of Neuroscience, Department of Biology, Faculty of Science, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274-8510, Japan 2 Department of Pharmacognosy, Faculty of Pharmaceutical Sciences, International University of Health and Welfare, 2600-1 Kitakanemaru, Ohtawara, Tochigi 324-8501, Japan; [email protected] 3 Department of Pharmacognosy, Faculty of Pharmaceutical Sciences, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274-8510, Japan; [email protected] (H.T.); [email protected] (K.K.) * Correspondence: [email protected]; Tel.: +81-47-472-5257

Abstract: The stress response in the brain is not fully understood, although stress is one of the risk factors for developing mental disorders. On the other hand, the stimulation of the olfactory system can influence stress levels, and a certain smell has been empirically known to have a stress- suppressing effect, indeed. In this review, we first outline what stress is and previous studies on stress-responsive biomarkers (stress markers) in the brain. Subsequently, we confirm the olfactory system and review previous studies on the relationship between smell and stress response by species, such as humans, rats, and mice. Numerous studies demonstrated the stress-suppressing effects of aroma. There are also investigations showing the effects of odor that induce stress in experimental animals. In addition, we introduce recent studies on the effects of aroma of coffee beans and essential oils, such as lavender, cypress, α-pinene, and thyme linalool on the behavior and the expression of stress marker candidates in the brain. The transfer of volatile components into the brain is also Citation: Masuo, Y.; Satou, T.; Takemoto, H.; Koike, K. Smell and discussed while using the results of thyme linalool as an example. These studies may provide a Stress Response in the Brain: Review good opportunity to connect chemical research at the molecular level with neuropharmacological of the Connection between Chemistry approaches in the future. and Neuropharmacology. Molecules 2021, 26, 2571. https://doi.org/ Keywords: stress; smell; aroma; olfaction; brain; anxiety; fatigue; human; rat; mouse 10.3390/molecules26092571

Academic Editor: Farid Chemat 1. Introduction Received: 15 March 2021 Stress is a risk factor in the development of mental disorders. Anxiety regarding the Accepted: 17 April 2021 future and life that dominates society as a whole has become a major issue for mental health. Published: 28 April 2021 A large number of researchers have studied the effects of stress on animals, including humans, and they have demonstrated changes in the endocrine system, especially via the Publisher’s Note: MDPI stays neutral hypothalamic-pituitary-adrenal (HPA) axis [1,2]. Although the stress response of the brain with regard to jurisdictional claims in and the onset mechanism of mental disorders have not yet been elucidated, the suppression published maps and institutional afﬁl- iations. of stress is important in preventing the onset of mental disorders. It has been empirically known from ancient times that the aroma and fragrance have the effect of suppressing stress. Human beings have become more dependent on vision than other species in the process of evolution, and the olfactory system is thought to be degenerated [3]. However, humans are also a member of mammals, and the olfaction can affect emotions, higher Copyright: © 2021 by the authors. functions, and the autonomic nervous system [4,5]. Indeed, the relationship between stress Licensee MDPI, Basel, Switzerland. and olfactory memory has been suggested in humans [6,7]. The subjective discomfort of This article is an open access article odors per se provokes the stress response in emotion and the sympathetic nervous system distributed under the terms and conditions of the Creative Commons in humans [8]. On the contrary, certain smells suppress stress level [9], which is supported Attribution (CC BY) license (https:// by the evidence showing the effects of exogenous volatile components on the brain [10]. creativecommons.org/licenses/by/ There are a wide range of words regarding smell, which are not academic terms 4.0/). and the difference in meaning is not clear. For this review, the authors have endeavored

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to be based on the Cambridge Advanced Learner’s Dictionary, 4th Edition, Cambridge University Press, as much as possible. Specifically, “smell” is the quality something has that you notice by using your nose. “Fragrance” and “scent” represent a good smell. “Aroma” expresses a nice smell that usually comes from food or drink. However, the “aroma” used in aromatherapy has the meaning of smell and fragrance. “Odor” means a smell, often one that is unpleasant. Although these different expressions are intricate, the target chemicals can sometimes be the same. Even if certain smell is pleasant at low concentrations, it may turn into unpleasant at high concentrations. It is also possible that a high dose of volatile compound may have toxicity. However, the dose-response of volatile components has often been unclear in the literature. In addition, the effects of volatile compounds may appear not only via the olfactory pathway but also via the transport of molecules to the brain (brain transferability), which makes it difficult to study the effects of smell on the brain. This review first outlines what stress is and previous studies on stress-responsive biomarkers (stress markers). Subsequently, we would like to focus on the relationship between the smell and the stress response in the brain. Research on stress-suppressing aromas is conspicuous, but some odors induce stress, which, in turn, enables the survival of the species. Moreover, we introduce recent animal studies showing the effects of aroma on the behavior and expression levels of stress marker candidates in the brain as examples of coffee bean aroma and fragrance of essential oils, such as lavender, cypress, α-pinene, and thyme linalool. In addition, here we discuss the permeability of volatile components to the brain by taking thyme linalool as an example.

2. Stress Evaluation in Mammals 2.1. Stress A short article that was published by Hans Selye in the Nature [11] is well known as “Stress Theory”. He calls the state in which the living body is distorted by external stimuli, such as “stress”, and the stimulus that causes stress as “stressor”. There are four kinds of stressors, such as physical, chemical, biological, and mental stressors. When exposed to a stressor, the organism not only specifically responds to the type of stimulus, but also to a series of non-specific reactions that are independent of the type of stimulus. This non-specific reaction is called stress reaction. The word “stress” that is commonly used today means “stressor”. It may be of interest that stress is divided into “eustress” and “distress”, depending on whether it has a positive or negative effect on the organism. The balance between these two affects the state of health. Stress causes alterations in the HPA axis and it affects the function of whole body [2,3]. These changes lead to modifications in the brain with negative feedback of the endocrine system [12]. The effects of stress include increased levels of cortisol in human blood (corticosterone in rodents [13]) and decreased function of the prefrontal cortex. Figure1 summarizes what is currently known about the stress response in the brain. The functions of monoaminergic neurons, such as dopamine (DA), noradrenaline (NA), and serotonin (5-HT), which are projected to the prefrontal cortex, are enhanced early after stress exposure. This reaction is the so-called “fight or flight response”, in which the function of the prefrontal cortex is relatively low. Afterwards, monoamine neurotransmitters are depleted, and the neuronal functions becomes lowered. Stress-induced decline in limbic DAergic neurons (projection from the ventral tegmental area to the nucleus accumbens, frontal cortex, and amygdala, the so-called reward system leads to a loss of pleasure. Stress levels are strongly suggested to be related to reduced activities of 5-HTergic neurons in the amygdala and hippocampus [14]. Moreover, an increase in cortisol concentration due to stress causes a decrease in the expression of brain-derived neurotrophic factor (BDNF) in the brain, which results in neuronal cell death, especially in the fragile hippocampus. Thus, the brain function deteriorates, which leads to an increase in the incidence of mental disorders, such as depression [15]. Molecules 2021, 26, x FOR PEER REVIEW 3 of 28

(BDNF) in the brain, which results in neuronal cell death, especially in the fragile hippo- Molecules 2021, 26, 2571 3 of 29 campus. Thus, the brain function deteriorates, which leads to an increase in the incidence of mental disorders, such as depression [15].

Figure 1. Sagittal sections cut in the middle of the human brain. ( (AA):): The The main main pathways pathways of of monoaminergic neurons. neurons. Dopamine (DA) neurons are highly locate locatedd in the midbrain (substantia nigra pars compacta and and ventral ventral tegmental tegmental area). area). The nigro-striatal pathway is mainly related to the motor activity. The mesolimbic pathway to the nucleus accumbens, frontal cortexcortex andand amygdalaamygdala isis involved involved in in emotions, emotions, which which is is called called the the reward reward system. system. Noradrenaline Noradrenalin (NA)e (NA) and and serotonin seroto- (5-HT)nin (5-HT) neurons neurons in thein the brain brain stem stem (locus (locus coeruleus coeruleus and and raphe raphe nucleus, nucleus, respectively) respectively) project project extensively. extensively. ((BB):): TheThe brain with the limbic system. The hippocampus plays important roles in recent memory. The amygdala is involved in emotion with the limbic system. The hippocampus plays important roles in recent memory. The amygdala is involved in emotion and remote memory, which is related to fear memory. (C): Effects of acute stress. Stress activates the amygdala and the and remote memory, which is related to fear memory. (C): Effects of acute stress. Stress activates the amygdala and the release of neurotransmitters from monoaminergic neurons. The sympathetic nervous system becomes dominant over the releaseparasympathetic of neurotransmitters nervous system. from monoaminergic(D): Effects of chronic neurons. stress. The sympatheticNeurons are nervousthen depleted system of becomes neurotransmitters dominant overand thebe- parasympatheticcome less active. nervousCortisol system.level enhanced (D): Effects by stress of chronic in the stress. blood Neurons reduces are brain-derived then depleted neurotrophic of neurotransmitters factor (BDNF) and become levels lessin the active. brain, Cortisol which causes level enhanced neurodegeneration. by stress in the blood reduces brain-derived neurotrophic factor (BDNF) levels in the brain, which causes neurodegeneration. Regarding changes in the brain that are related to stress and olfaction, corticotropin releasingRegarding factor (CRF)-containing changes in the brain neurons that are from related the central to stress amygdala and olfaction, [16,17] corticotropinprojecting to thereleasing bed nucleus factor (CRF)-containingof the stria terminalis neurons [16,18] from are the suggested central amygdala to be involved [16,17] projectingin the stress to response.the bed nucleus This projection of the stria is highly terminalis related [16 to,18 γ]-aminobutyric are suggested acid to be (G involvedABA) containing in the stress neu- ronsresponse. [18]. ThisChronic projection stress in is highlyadulthood related causes to γ -aminobutyricalterations in the acid expression (GABA) containing of glucocorti- neu- coidrons receptors [18]. Chronic and stressCRF in in the adulthood central amygdala, causes alterations which results in the expressionin visceral ofhypersensitivity glucocorticoid [19].receptors The NA and neurons CRF in thein the central locus amygdala, coeruleus which have been results implicated in visceral in hypersensitivity cognitive aspects [19 of]. theThe stress NA neurons response, in in the part locus through coeruleus its regulation have been by implicated opioid and in the cognitive stress-related aspects neuro- of the stress response, in part through its regulation by opioid and the stress-related neuropep- peptide, CRF [20]. Moreover, somatostatin- and neuropeptide Y-containing neurons in the tide, CRF [20]. Moreover, somatostatin- and neuropeptide Y-containing neurons in the amygdala [21] and hypothalamus [22] may respond to stress. amygdala [21] and hypothalamus [22] may respond to stress.

2.2. Stress-Responsive Biomarkers (Stress Markers) Stress markers will enable an objective understanding of stress levels in pre-disorder and contribute to early detection and the prevention of psychiatric and neurodegenerative disorders, including depression and dementia. In a relatively early paper [1], the use of Molecules 2021, 26, 2571 4 of 29

blood plasma was a major material for measuring glucocorticoid hormones cortisol or corticosterone). Alternative methods were developed, such as the measurement of corticosteroids in saliva, urine, or feces to overcome the stress that is induced by blood sampling itself. Concerning the effects of chronic stress, hormonal secretions are usually unchanged, but functional changes are observed at several levels of the system, including the sensitiza- tion of the adrenal cortex to ACTH and the resistance of the HPA axis to feedback inhibition by corticosteroids (dexamethasone suppression test). BDNF, which is expressed in the brain and blood, has been demonstrated as a stress marker [22]. Cortisol or corticosterone levels in the blood elevated by stress provide feedback to the brain. In the hippocampus, which is a region vulnerable to stress [23], a decrement in the expression of BDNF causes neuronal cell death, resulting in cognitive decline. Because BDNF concentration in the blood of patients with major depressive disorder is lower than that of healthy subjects under certain conditions [15], BDNF is expected to be a candidate for a biomarker for a diagnosis of depression. Salivary biomarkers are readily accessible and easily obtained. α-Amylase in saliva has been suggested as a sensitive biomarker for stress-related changes in the body reflecting the activity of the sympathetic nervous system [8,24]. Salivary stress markers are suggested to include α-amylase, cortisol, chromogranin A (CgA), and immunoglobulin A (IgA) [25]. Farah et al. [26] reviewed the literature on salivary biomarkers for diagnosing and monitoring psychiatric and neurodegenerative disorders, such as autism and Alzheimer’s, Parkinson’s, and Huntington’s disease. These biomarker candidates are amyloid β peptide, tau protein, lactoferrin, α-synuclein, DJ-1 protein, chromogranin A, huntingtin protein, DNA methylation disruptions, and micro-RNA profiles. Giacomello et al. [27] described that cortisol may be the currently best evaluated stress marker, and NA or its metabolite, 3- methoxy-4-hydroxyphenylglycol, is also considered to be a good stress marker. They cited, as other stress marker candidates, testosterone, dehydroepiandrosterone (DHEA) and its sulfonated analogue DHEA-S, α-amylase, secretory immunoglobulin A, and chromogranin A. Quite recently, Chojnowska et al. [28] suggested, in their review article, that promising salivary biomarkers of stress, anxiety, or depression are cortisol, immunoglobulin A (IgA), lysozyme, melatonin, α-amylase, chromogranin A, and fibroblast growth factor 2 (FGF-2). Among these, they mentioned cortisol, lysozyme, α-amylase, and chromogranin A as the most valuable potential markers of stress. Cortisol and melatonin are also suggested to be useful for the differentiation of depression from stress.

2.3. Recent Exploratory Research on Novel Stress Marker Candidates Cortisol, α-amylase, chromogranin A, etc. have been established as stress markers, as mentioned above. Although these factors are effective for an evaluation of acute stress in healthy subjects, it is not suitable for the purpose of grasping changes in brain function due to chronic stress. In addition, many studies conducted around the world aim to find novel biomarkers of disorders, contributing to post-onset diagnostic methods. The authors have loaded various types of stress on experimental animals and they have found a group of new stress marker candidates in the brain. We have mainly studied developmental disorders due to chemical stress [29–31], schizophrenia [32], and chronic alcoholism [33]. As for chemical stress, we have been studying the effects of environmental chemical substances [34–36] and the effects of radiation as physical stress [37,38]. Furthermore, the effects of restraint, disturbance of the light-dark cycle, and water immersion (insomnia due to an unpleasant environment) were investigated in order to clarify the process from stress to the onset of depression. OMICS analysis was applied to screen for novel stress markers. Marker candidate genes were identified by DNA array analysis, and the proteins by two-dimensional electrophoresis and mass spectrometry. We have found several new stress marker candidate genes that show changes in expression in both the brain and blood. Alterations in the expression of biochemical parameters in the brain are unlikely to be immediately reflected in peripheral blood, and the mechanisms that connect changes in the brain and blood Molecules 2021, 26, 2571 5 of 29

should be further clariﬁed. At least, the, stress response in the brain can be extrapolated by the results of blood tests if the correlation is established between changes in the brain and blood. By establishing an extrapolation method for changes in the brain by measuring blood stress markers, it will be possible to objectively grasp the stress level in the future. This will contribute to the prevention of the onset of mental disorders and the evaluation of therapeutic effects.

3. Olfaction and Stress Response in the Brain 3.1. Olfactory System The olfactory pathway has been clarified relatively recently [39] (Figure2). The existence of a unique subsystem is suggested in the main mammalian olfactory system. This system may be comprised of sensory neurons expressing odorant receptors (ORs) of the OR37 subfamily. OR37 receptor subtypes A, B, and C were activated by the long- chain aliphatic aldehydes pentadecanal, hexadecanal, and heptadecanal, respectively, and the OR37 system was suggested to mediate social buffering [40]. Scientific knowledge with the keywords, “olfaction”, “stress”, and “brain” has been accumulated. Olfaction, which is the oldest phylogenetic sensation, is characterized by a unique intimacy with the emotion system, and certain smell causes stress and others suppress stress. The olfactory system has been suggested to be dynamic, with neuroplasticity in olfactory perception and memory [41]. Smell perception may be strongly related to functions of amygdala and hippocampus [41]. Indeed, human and animal studies suggest that olfaction may be modulated by experience and/or physical state, and some smells induce emotion and lead to the recall of emotional memories [42,43]. Moreover, it was demonstrated that stress affects smell recognition, including the development of olfactory system [44–46]. Numerous papers reported the effects of odors of mammalian endogenous substances as predator odor stress [47,48]. Chronic stress leads to long-term deficits in odor-driven behaviors that are critical for species survival and reproduction, and to neuroplastic changes in the nucleus of the lateral olfactory tract [49]. Social isolation has been suggested to modulate olfaction and social memory persistence, probably by independent mechanisms [50]. Odor may provoke a physiological response in the autonomic nervous system. An unpleasant aversive odor causes non-invasive stress, while a pleasant smell promotes healing and relaxation in mammals [51]. Moreover, it was demonstrated that the sweat odor of patients with panic disorder was positively correlated with the severity of psychopathology [52].

3.2. Stress-Inducing Smell In humans, it was demonstrated that the secretion of salivary α-amylase was increased upon exposure to malodors, which suggested that subjective unpleasantness of odors can induce the stress response [8]. However, any other human studies on the induction of stress by speciﬁc odors were not found in the literature. In fact, experiments investigating the effects of certain odors that induce stress on humans must be ethically difﬁcult to perform. In animal studies, prenatal stress can interfere with the development of the olfactory system during the prenatal period, leading to neurogenesis and altered olfactory capacity in the adulthood [53]. Maternal odor affects child development on the cognitive function [54] and in the social buffering, a phenomenon that social supports attenuate stress response [55]. A large number of studies were carried out on the effects of endogenous odor of animals, which suggested that predator-emitted odor molecules are stressors for prey animals. It is really important for species conservation to identify the dangers to life among the volatile components in the environment. These studies examined the stress response that odors cause in rodents as stressors. It has been suggested that the predator odors are a strong stressor [5], which leads to post-traumatic stress disorder (PTSD) [4]. Indeed, stress due to predator odor activates the HPA axis [56]. Gu et al. [57] demonstrated, in the Brandt’s vole, that maternal stress induced by predator odors during gestation restrained the growth of female offspring and increased the duration of exploring and foraging behaviors of male offspring by regulating the HPA axis. Therefore, the offspring became Molecules 2021, 26, 2571 6 of 29

less sensitive to unfamiliar environments and more likely to be prey. As a predator’s stress on rodents, many researchers used cats [58–60], hamsters [61], guinea pigs [61], and ferrets [16,21,62]. In the case of cat odors, the effects of its urine [17,56,63,64] and feces [65] were examined. The urine of fox [66,67] and coyote [66] was also tested and it suggested a Molecules 2021, 26, x FOR PEER REVIEW 6 of 28 role for adolescent stress as a risk factor for psychosis, particularly in those with a heritable predisposition [67].

(A)

Olfactory glomerulus

External (B) Olfactory bulb olfactory tract Olfactory cortex (5 regions) Mitral cell Tufted cell Anterior Olfactory Piriform Amygdala Entorhinal olfactory nucleus tubercle cortex cortex Olfactory epithelium Hypo- Contralateral thalamus olfactory bulb Hippocampus

Thalamus

Orbitofrontal Frontal cortex cortex

FigureFigure 2. 2.The The human human olfactory olfactory system. system. ( A(A):): Structure Structure of of the the olfactory olfactor epithelium.y epithelium. There There are are 50 50 million mil- olfactorylion olfactory sensory sensory neurons neurons in the in the nasal nasal olfactory olfactor epithelium.y epithelium. Each Each neuron neuron expressesexpresses one type type of of olfactoryolfactory receptorsreceptors on the the cilia cilia protruding protruding toward toward the the nasal nasal cavity. cavity. From From the other the other side of side the of neu- the neuron,ron, an axon an axon projects projects into the into ol thefactory olfactory glomerulus glomerulus in the olfactory in the olfactory bulb, where bulb, it wheresynapses it synapseswith the dendrites of the mitral and tufted cells. (B): Diagram of the olfactory pathway. Mitral cells project with the dendrites of the mitral and tufted cells. (B): Diagram of the olfactory pathway. Mitral to the five regions, and tufted cells project to two regions in the olfactory cortex. Four regions con- cells project to the ﬁve regions, and tufted cells project to two regions in the olfactory cortex. Four nect to the thalamus and frontal cortex. Conscious discrimination of smell depends on the orbito- regionsfrontal connectand frontal to the cortices. thalamus Emotive and frontalaspects cortex.of olfaction Conscious derive discriminationfrom limbic projections of smell depends(amygdala on theand orbitofrontal hypothalamus). and Thus, frontal olfactory cortices. information Emotive aspects may be of closely olfaction related derive to frommemory limbic and projections emotion, (amygdalawhich affects and stress hypothalamus). response. This Thus, figure olfactory is derived information from Kandel, may beE.R.; closely Schwartz, related J.H.; to memoryJessell, T.M. and emotion,(Eds) Principles which affectsof Neural stress Science, response. 4th ed. This Mc ﬁgureGraw-Hill, is derived New fromYork, Kandel,N.Y., USA, E.R.; 2000 Schwartz, [39] with J.H.; a Jessell,copyright T.M. permission. (Eds) Principles of Neural Science, 4th ed. McGraw-Hill, New York, NY, USA, 2000 [39] with a copyright permission. 3.2. Stress-Inducing Smell InIn view humans, of molecules it was demonstrated that stimulate thethat olfaction, the secretion 2,5-dihydro-2,4,5-trimethylthiazoline of salivary α-amylase was in- (TMT)creased is upon the main exposure component to malodors, of fox feces. which Therefore, suggested TMT that issubjective a predator unpleasantness odor for rodents, of andodors it causescan induce PTSD the as stress a serious response stressor [8]. However, [13,18,68– any74]. other Miyazono human et studies al. [75 ]on found the induc- that a mixturetion of stress of three by pyrazinespecific od analoguesors were not (P-mix) found present in the litera at highture. levels In fact, in wolfexperiments urine induced inves- fear-relatedtigating the effects responses of certain in mice, odors which that induce was attenuated stress on humans by etizolam, must be an ethically anxiolytic. difficult On theto perform. other hand, In animal stressed studies, rats release prenatal odor stress cues, can which interfere allows with other the ratsdevelopment to avoid danger.of the olfactory system during the prenatal period, leading to neurogenesis and altered olfactory capacity in the adulthood [53]. Maternal odor affects child development on the cognitive function [54] and in the social buffering, a phenomenon that social supports attenuate stress response [55].

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Inflammatory cytokine responses in the brain can mediate such stress-induced chemical signals [76]. Moreover, Inagaki et al. [77] reported the identification of a pheromone that induces anxiety in rats. Stressed rats release a specific odor into the air, and this odor enhances the anxiety levels in other rats. Therefore, anxiety-causing molecules may be present in the stress-related odorants, which may be useful in understanding mammalian chemical communication. They suggested that this odor may be involved in 4- methylpentanal and hexanal. On the response to odor stress, gender differences are pointed out [13,18,70,78,79]. Harris et al. [80] demonstrated the social defeat paradigm that works in female mice. If male odorants are applied to female mice, females repeatedly undergo social defeat stress by aggressive behavior of resident males. Subsequently, females develop social avoidance, reduce the function of reward system, and enhance anxiety level. Such differences are also reported in social play behavior [62]. Concerning brain regions that are responsible for odor stress, Kondoh et al. [48] suggest that the amygdalo-piriform transition area, a small region of the olfactory cortex, may play a key part in the hormonal component of the instinctive fear response to volatile predator scents. The relationship between the olfaction and the amygdala and hippocampus has been suggested. Fujimoto and Aou [68] observed that prenatal treatment with low-level bisphenol A, one of environmental chemicals, impaired gender-specific behavior, enhanced depression-like behavior, and augmented behavioral responses to predator odor. Interestingly, mild exposure to a predator odor during hippocampus-dependent learning leads to increasing performance through the co-activation of the amygdala, which is probably by a stress mediated mechanism [69]. Moreover, it was shown that trpa1-mediated nociception plays an important role in the innate fear/defense behavior that is caused by predator odors [74].

3.3. Stress-Suppressing Smell Many studies mainly demonstrated the stress-suppressing effect of essential oils. Table1 summarizes the literature. The articles are introduced separately for humans, rats, and mice.

3.3.1. Effect on Humans Even at non-life-threatening levels, chronic stress is a risk factor for the development of various mental disorders, and it is desirable to control the stress levels. The stress-suppressing effects of certain smell have been recognized, and the effects of fragrances on human psychophysiological activity have been studied for a long time [10]. Granqvist et al. [9] pointed out that, in stressful situations, access to adult attachment ﬁgures (e.g., romantic partners) is an important means by which adults regulate stress responses. They observed in humans that partner body odor reduced subjective discomfort during a stressful event, as compared with the odor of oneself, which suggests that partner odor is a scent of security, especially for attachment-secure adults. Yoto et al. [81] investigated the alleviating effect of black tea aroma on physical and psychological stress in humans that is induced by the Uchida–Kraepelin test, based on salivary chromogranin-A (CgA) levels as a stress marker and subjective evaluations. It was observed that inhaling black tea aroma can reduce stress levels, and Darjeeling tea aroma tends to improve mood before exposure to mental stress.

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Table 1. Summary of smell andTable its main1. Summary component(s) of smell in and previous its main studies. componen The scientiﬁct(s) in previous name of studies. plants andThe thescienti namefic name of main of component(s)plants and the are name asdescribed of in the reference. A general TableTable 1.1. SummarySummary ofof smellsmell andand itsits mainmain componencomponent(s)t(s) inin previousprevious studies.studies. TheThe scientiscientificfic namename ofof plantsplants andand thethe namename ofof name appears in the case ofTable nomain indication component(s)1.Table SummarySummary 1. in Summary the ofof article.are smellsmell as of described andand smell itsits and mainmain in itsthe componencomponen main reference. component(s)t(s) A inin general previousprevioust(s) in name previous studies.studies. appears studies. TheThe in scientiscienti the The case ficficscienti namenameof nofic indicationof ofname plantsplants of andplantsand in the thethe and article. namename the of ofname of mainmainTable component(s)component(s) 1. Summary areareof smell asas describeddescribed and its main inin thethe componen reference.reference. t(s) AA general generalin previous namename studies. appearsappears The inin thescientithe casecasefic ofof name nono indicationindication of plants ininand thethe the article.article. name of main component(s)main component(s) are as describedare as described in the reference. reference.in the reference. AA generalgeneral A general namename appears appearsname appears inin thethe case casein the ofof case nono indicationindication of no indication inin thethe article. article.in the article. Smellmain component(s)Scientific are asName described in theMain reference. Component(s) A general name appears in the case of no indicationReferences in the article. SmellSmellSmell Scientiﬁc NameScientificScientific Name Name MainMainMain Component(s) Component(s) ReferencesReferences References Smell Smell ScientificScientific Name Name Main Component(s)Main Component(s) ReferencesReferences Smell Scientific Name Main Component(s) Human ReferencesRat Mouse HumanHumanHuman RatRat MouseMouse Rat Mouse HumanHuman Rat RatMouse Mouse Many kinds of aroma, such asMany lavender, kinds of rosemary, aroma, such bitter as lavender, orange, rosema peppermint,ry, bitter linalool, orange, peppermint, limonene linalool, limonene [4 (Review)]Human [4] (Review) Rat Mouse ManyMany kinds kinds of of aroma, aroma, such such as as lavender, lavender, rosema rosemary,ry, bitter bitter orange, orange, peppermint, peppermint, linalool, linalool, limonene limonene [4[4 (Review)] (Review)] Many kindsMany of kinds aroma, of sucharoma, as suchlavender, as lavender, rosema ry,ry,rosema bitterbitterry, orange,orange, bitter orange, peppermint,peppermint, peppermint, linalool,linalool, linalool, limonenelimonene limonene [4[4 (Review)](Review)][4 (Review)] Partner smell PartnerMany kindssmell of aroma, such as lavender, rosema ry, bitter orange, peppermint, linalool, limonene [4 (Review)][9] [9 ] PartnerPartner smell smell [9][9] Partner Partnersmell smell [9][9] [9] BlackPartner tea smell Geraniol Benzyl alcohol 2-Phenylethanol [81][9] BlackBlack tea tea GeraniolGeraniol BenzylBenzyl alcohol alcohol 2-Phenylethanol2-Phenylethanol [81][81] Black tea Black teaBlack tea GeraniolGeraniol Benzyl alcoholBenzyl alcohol 2-Phenylethanol 2-Phenylethanol [81][81] [81][81] Black tea Geraniol Benzyl alcohol 2-Phenylethanol [81]

Lavender Lavandula [82–87] [88] [89,90] LavenderLavender LavandulaLavandula [82–87][82–87] [88][88] [89,90][89,90] Lavender LavenderLavenderLavandula LavandulaLavandulaLavandula [82–87][82–87] [[82–87]82–87[88]][ [88] [88][89,90][89,90] [89,90]88][89,90] Lavender Lavandula Linalool Linalyl acetate [82–87] [88] [89,90] LinaloolLinalool LinalLinalyyll acetate acetate LinaloolLinalool Linal yyl l acetateLinalacetateyl acetate Linalool Linalyl acetate Chamomile Matricaria recutita [83] Chamomile ChamomileChamomileMatricaria recutitaMatricariaMatricaria recutita recutita Chamizulene [83][83] [83] ChamomileChamomile MatricariaMatricaria recutita recutita ChamizuleneChamizulene [83][83] [83] Chamomile Matricaria recutita ChamizuleneChamizulene [83] Chamizulene Rosemary Rosmarinus officinalis α-Pinene Cineole (Eucalyptol) Camphor [86] RosemaryRosemary RosmarinusRosmarinus officinalis officinalis αα-Pinene-Pinene CineoleCineole (Eucalyptol) (Eucalyptol) CamphorCamphor [86][86] RosemaryRosemary RosmarinusRosmarinus officinalis officinalis α -Pinene-Pineneα -Pinene Cineole Cineole(Eucalyptol) (Eucalyptol) Camphor Camphor [86][86] [86] Rosemary Rosmarinus officinalis α-Pinene Cineole (Eucalyptol) Camphor [86] Rosemary Rosmarinus ofﬁcinalis [86]

Rose Rosa 2-Phenylethanol Citronellol Geraniol [91] [91] [72] RoseRose RosaRosa 2-Phenylethanol2-Phenylethanol CitronellolCitronellol GeraniolGeraniol [91][91] [91][91] [72][72] Rose Rose Rosa Rosa 2-Phenylethanol2-Phenylethanol2-Phenylethanol CitronellolCitronellol Citronellol GeraniolGeraniol Geraniol [91][91] [91] [91][91] [91][72][72] [72] Rose Rosa 2-Phenylethanol Citronellol Geraniol [91] [91] [72]

Rose Rosa [91][ 91][72]

2-Phenylethanol 2-Phenylethanol2-Phenylethanol 2-Phenylethanol2-Phenylethanol2-Phenylethanol 2-Phenylethanol [92] 2-Phenylethanol 2-Phenylethanol2-Phenylethanol [92][92] 2-Phenylethanol 2-Phenylethanol2-Phenylethanol2-Phenylethanol [92][92] [92] [92] 2-Phenylethanol2-Phenylethanol [92] Bergamot Citrus bergamia Limonen Linalyl acetate Linalool [93–95] [96] BergamotBergamot CitrusCitrus bergamia bergamia LimonenLimonen LinalylLinalyl acetate acetate LinaloolLinalool [93–95][93–95] [96][96] BergamotBergamot Citrus bergamiaCitrus bergamia LimonenLimonen Linalyl acetateLinalyl acetate Linalool Linalool [93–95][93–95] [93–95][96] [96] [96] Bergamot Citrus bergamia Limonen Linalyl acetate Linalool [93–95] [96] Bergamot Citrus bergamia [93–95][96]

Grapefruit Citrus paradisi Limonene Nootkatone β-Myrcene [97] GrapefruitGrapefruit CitrusCitrus paradisi paradisi LimoneneLimonene NootkatoneNootkatone ββ-Myrcene-Myrcene [97][97] GrapefruitGrapefruit Citrus paradisiCitrus paradisi LimoneneLimonene Nootkatone Nootkatone ββ -Myrcene-Myrcene β-Myrcene [97][97] [97] Grapefruit Citrus paradisi Limonene Nootkatone β-Myrcene [97]

Neroli Citrus aurantium L. Linalool [98] NeroliNeroli CitrusCitrus aurantium aurantium L. L. LinaloolLinalool [98][98] Neroli Neroli Citrus aurantiumCitrus aurantium L. L. LinaloolLina lool [98][98] [98] Neroli var.Citrus amara aurantium L. Linalool [98] var.var. amara amara var.var. amaraamaravar. amara var. amara Cypress Chamaecyparis obtusa [99] [100] [101] CypressCypress ChamaecyparisChamaecyparis obtusa obtusa α-Pinene [99][99] [100][100] [101][101] CypressCypress ChamaecyparisChamaecyparis obtusa obtusa αα-Pinene-Pinene [99][99] [99] [100][100] [100][101][101] [101] Cypress Chamaecyparis obtusa α-Pinene-Pineneα -Pinen e [99] [100] [101] α-Pinene

Cedar Cryptomeria japonica [102] CedarCedar CryptomeriaCryptomeria japonica japonica α-Pinene [102][102] Cedar Cedar CryptomeriaCryptomeria japonica japonica αα-Pinene-Pinene [102][102] [102] Cedar Cryptomeria japonica α-Pinene-Pineneα -Pinen e [102] α-Pinene

α-Pinene [103] αα-Pinene-Pinene α-Pinene [103][103] α-Pinene-Pineneα -Pinene αα-Pinene-Pinene [103][103] [103] α-Pinene α-Pinene-Pineneα -Pinen e [103] α-Pinene Mixture of essential oils MixtureMixture of of essential essential oils oils [104] MixtureMixture of essential of essential oils oils [104][104] Mixture of essential oils [104][104] [104] [104]

Lavender Lavandula officinalis LavenderLavender LavandulaLavandula officinalis officinalis LavenderLavender LavandulaLavandulaLavandula officinalisofficinalis officinalis Lavender Lavandula officinalis Linalool Linalyl acetate LinaloolLinalool LinalylLinalyl acetate acetate LinaloolLinalool Linalyl acetateLinalyl acetate Linalool Linalyl acetate

Molecules 2021, 26, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/molecules MoleculesMolecules 2021 2021, ,26 26, ,x. x. https://doi.org/10.3390/xxxxx https://doi.org/10.3390/xxxxx www.mdpi.com/journwww.mdpi.com/journal/moleculesal/molecules Molecules 20212021,, 2626,, x.x. https://doi.org/10.3390/xxxxxhttps://doi.org/10.3390/xxxxx www.mdpi.com/journal/moleculesal/molecules Molecules 2021, 26, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/molecules

TableTable 1. 1. Summary Summary of of smell smell and and its its main main componen component(s)t(s) in in previous previous studies. studies. The The scienti scientificfic name name of of plants plants and and the the name name of of mainmain component(s) component(s) are are as as described described in in the the reference. reference. A A general general name name appears appears in in the the case case of of no no indication indication in in the the article. article. Table 1. Summary of smell and its main component(s) in previous studies. The scientific name of plants and the name of SmellSmell ScientificScientific Name Name MainMain Component(s) Component(s) ReferencesReferences TablemainTable component(s)1. 1. Summary Summary of ofare smell smell as described and and its its main main in the componen componen reference.t(s)t(s) A in generalin previous previous name studies. studies. appears The The in scienti scientithe caseficfic nameof name no indicationof of plants plants and andin the the the article. name name of of mainmain component(s) component(s) are are as as described described in in the the reference. reference. A A general general name name appears appears in in the the case case of of no no indication indicationHumanHuman in in the theRat Ratarticle. article. Mouse Mouse Smell Scientific Name Main Component(s) References SmellManySmellMany kinds kinds of of aroma, aroma,Scientific Scientificsuch such as as lavender, lavender,Name Name rosema rosemary,ry,Main Mainbitter bitter Component(s) Component(s)orange, orange, peppermint, peppermint, linalool, linalool, limonene limonene [4[4 (Review)] (Review)]ReferencesReferences Human Rat Mouse Partner Partner smell smell HumanHuman[9][9] RatRat MouseMouse Many kinds of aroma, such as lavender, rosemary, bitter orange, peppermint, linalool, limonene [4 (Review)] ManyBlackManyBlack kindstea kindstea of of aroma, aroma, such such as as lavender, lavender, rosema rosemary, ry, Geraniolbitter Geraniolbitter orange, orange, peppermint, peppermint, Benzyl Benzyl alcohol linalool,alcohol linalool, limonene limonene 2-Phenylethanol 2-Phenylethanol [4[4 (Review)] (Review)][81][81] Partner smell [9] Partner Partner smell smell [9][9] Black tea Geraniol Benzyl alcohol 2-Phenylethanol [81] BlackBlack tea tea GeraniolGeraniol BenzylBenzyl alcohol alcohol 2-Phenylethanol2-Phenylethanol [81][81]

LavenderLavender LavandulaLavandula [82–87][82–87] [88][88] [89,90][89,90]

LinaloolLinalool Linal Linalyly acetatel acetate Lavender Lavandula [82–87] [88] [89,90] LavenderLavender LavandulaLavandula [82–87][82–87] [88][88] [89,90][89,90] ChamomileChamomile MatricariaMatricaria recutita recutita Linalool Linalyl acetate [83][83] ChamizuleneChamizulene LinaloolLinalool LinalLinalyyl lacetate acetate

ChamomileRosemaryRosemary MatricariaRosmarinusRosmarinus recutita officinalis officinalis αα-Pinene-Pinene Cineole Cineole (Eucalyptol) (Eucalyptol) Camphor Camphor [83][86][86] ChamomileChamomile MatricariaMatricaria recutita recutita Chamizulene [83][83] ChamizuleneChamizulene Rosemary Rosmarinus officinalis α-Pinene Cineole (Eucalyptol) Camphor [86] RosemaryRosemary RosmarinusRosmarinus officinalis officinalis αα-Pinene-Pinene CineoleCineole (Eucalyptol) (Eucalyptol) CamphorCamphor [86][86] RoseRose RosaRosa 2-Phenylethanol2-Phenylethanol Citronellol Citronellol Geraniol Geraniol [91][91] [91][91] [72][72]

Rose Rosa 2-Phenylethanol Citronellol Geraniol [91] [91] [72] RoseRose RosaRosa 2-Phenylethanol2-Phenylethanol CitronellolCitronellol GeraniolGeraniol [91][91] [91][91] [72][72]

2-Phenylethanol2-Phenylethanol Molecules 2021, 26, 2571 9 of 29 2-Phenylethanol2-Phenylethanol [92] [92] 2-Phenylethanol 2-Phenylethanol Bergamot2-PhenylethanolBergamot CitrusCitrus bergamia bergamia 2-PhenylethanolLimonenLimonen Linalyl Linalyl acetate acetate Linalool Linalool [93–95][93–95] [96][96] [92] [92] 2-Phenylethanol2-Phenylethanol Table 1. Cont . [92]

Bergamot Citrus bergamia Limonen Linalyl acetate Linalool [93–95] [96] SmellBergamotBergamot Scientiﬁc NameCitrusCitrus bergamia bergamia MainLimonenLimonen Component(s) LinalylLinalyl acetate acetate LinaloolLinalool [93–95][93–95] [96][96] References GrapefruitGrapefruit CitrusCitrus paradisi paradisi LimoneneLimonene Nootkatone Nootkatone β -Myrceneβ-Myrcene [97][97] Human Rat Mouse

Grapefruit Citrus paradisi Limonene Nootkatone β-Myrcene [97] GrapefruitGrapefruit CitrusCitrus paradisi paradisi LimoneneLimonene NootkatoneNootkatone ββ-Myrcene-Myrcene [97] Grapefruit NeroliNeroli Citrus paradisiCitrusCitrus aurantium aurantium L. L. LinaloolLinalool [98][97][98] [97]

var.var. amara amara Neroli Citrus aurantium L. Linalool [98] NeroliNeroli Citrus aurantiumCitrusCitrus aurantiumL. aurantium L. L. LinaloolLinalool [98][98] Neroli CypressCypress var.ChamaecyparisChamaecyparis amara obtusa obtusa [99][99] [98] [100][100] [101][101] var. amara var. amara αα-Pinene-Pinene var. amara Linalool Cypress Chamaecyparis obtusa [99] [100] [101] α-Pinene Cypress CypressCypress ChamaecyparisChamaecyparisChamaecyparis obtusa obtusa obtusa [99][99] [99][[100][100] [101][101] 100][101] CedarCedar CryptomeriaCryptomeria japonica japonica αα-Pinene-Pinene [102][102] αα-Pineneα-Pinene-Pinene

Cedar Cedar CryptomeriaCryptomeria japonica japonica [102] [102] α-Pinene CedarCedar CryptomeriaCryptomeria japonica japonica α-Pinene [102][102] αα-Pinene-Pinene αα-Pinene-Pinene [103][103] αα-Pinene-Pinene α-Pinene [103] α-Pinene α-Pinene [103] MixtureMixture of of essential essential oils oils α -Pinene αα-Pinene-Pinene [104][104] [103][103] Mixture of essential oils αα-Pinene-Pinene [104] Mixture of essential oils Mixture of essential oils [104] MoleculesMoleculesMixtureMolecules 2021of2021 2021 essential,, 2626, 26,, xx, xFORFOR oilsFOR PEERPEER PEER REVIEWREVIEW REVIEW [104] 22 2ofof of 33 3 Lavender LavenderLavenderLavandula ofﬁcinalisLavandulaLavandula officinalis officinalis [104] LinaloolLinaloolLinalool LinalylLinalyl Linalyl acetate acetate acetate Lavender Lavandula officinalis LavenderLavender LavandulaLavandula officinalis officinalis Linalool Linalyl acetate Ylang-ylang Ylang-ylangYlang-ylangYlang-ylangCananga odorataCanangaCanangaCananga odorataodorata odorata LinaloolLinalool LinalylLinalyl acetate acetate MoleculesMolecules 2021 2021, 26, 26, x., x. https://doi.org/10.3390/xxxxx https://doi.org/10.3390/xxxxx Germacrene GermacreneGermacreneGermacrene D D D www.mdpi.com/journwww.mdpi.com/journal/moleculesal/molecules

Majoram Molecules MajoramMajoramMajoram 2021Origanum , 26, x. https://doi.org/10.3390/xxxxx majoranaOriganumOriganumOriganum majoranamajorana majorana www.mdpi.com/journ al/molecules MoleculesMolecules 2021 2021, ,26 26, ,x. x. https://doi.org/10.3390/xxxxx https://doi.org/10.3390/xxxxx Terpinen-4-olTerpinen-4-olTerpinen-4-olTerpinen-4-ol γγγ -Terpineneγ-Terpinene-Terpinene www.mdpi.com/journwww.mdpi.com/journal/moleculesal/molecules

NeroliNeroliNeroli CitrusCitrusCitrus aurantiumaurantium aurantium LinaloolLinaloolLinalool SmellSmellSmell ScientificScientificScientific namename name MainMainMain component(s)component(s) component(s) ReferencesReferencesReferences

HumanHumanHuman RatRatRat MouseMouseMouse

LimoneneLimoneneLimonene [105][105][105] LimoneneLimoneneLimonene

GreenGreenGreen cis-3-Hexenolcis-3-Hexenolcis-3-Hexenol trans-2-Hexenaltrans-2-Hexenal trans-2-Hexenal [106][106][106] [73,107][73,107][73,107]

YomogiYomogiYomogi ArtemisiaArtemisiaArtemisia montanamontana montana [108][108][108] CineolCineolCineol (Eucalyptol)(Eucalyptol) (Eucalyptol)

CannabisCannabisCannabis CannabisCannabisCannabis sativasativa sativa ββ-Myrceneβ-Myrcene-Myrcene ββ -Caryophylleneβ-Caryophyllene-Caryophyllene [109][109][109]

ManyManyMany kindskinds kinds ofof of volitilevolitile volitile componentscomponents components [110][110][110]

CoffeeCoffeeCoffee beanbean bean CoffeaCoffeaCoffea [111][111][111] FurfurylFurfurylFurfuryl alcoholalcohol alcohol

ThymeThymeThyme TymusTymusTymus vulgarisvulgaris vulgaris [112][112][112] CarvacrolCarvacrolCarvacrol ThymolThymol Thymol

LemongrassLemongrassLemongrass CymbopogonCymbopogonCymbopogon flexuosusflexuosus flexuosus [113][113][113] CitralCitralCitral

TetraclinisTetraclinisTetraclinis TetraclinisTetraclinisTetraclinis articulataarticulata articulata [114][114][114] HinokitiolHinokitiolHinokitiol αα -Pineneα-Pinene-Pinene

MixtureMixtureMixture ofof of essentialessential essential oiloil oil componentscomponents components LimoneneLimoneneLimonene γγ -Terpineneγ-Terpinene-Terpinene ββ -Myrceneβ-Myrcene-Myrcene αα α-Pinene-Pinene-Pinene ββ -Pineneβ-Pinene-Pinene [92][92][92]

SabineneSabineneSabinene LinaloolLinalool Linalool CymeneCymene Cymene ValenceneValencene Valencene

RomanRomanRoman chamomilechamomile chamomile ChamaemelumChamaemelumChamaemelum nobilenobile nobile [115][115][115] AngericAngericAngeric acidacid acid

Molecules 2021, 26, x FOR PEER REVIEW 2 of 3

MoleculesMolecules 2021 20212021, ,26, 2626, ,x, xx FOR FORFOR PEER PEERPEER REVIEW REVIEWREVIEW 222 of ofof 3 33 Molecules Molecules 2021 2021, 26, 26, x, xFOR FOR PEER PEER REVIEW REVIEW 2 2of of 3 3 Molecules 2021, 26, x FOR PEER REVIEW 2 of 3 Molecules 2021, 26, x FOR PEER REVIEW 2 of 3

Molecules 2021, 26, 2571 10 of 29 Ylang-ylang Cananga odorata Germacrene D Ylang-ylangYlang-ylang CanangaCananga odorata odorata Ylang-ylangYlang-ylang CanangaCananga odorata odorata Ylang-ylang Cananga odorata GermacreneGermacrene D D Ylang-ylang Cananga odorata GermacreneGermacrene D D Majoram Origanum majorana Germacrene D Table 1. Cont. Terpinen-4-ol γ-Terpinene MajoramMajoram OriganumOriganum majorana majorana MajoramMajoram OriganumOriganum majorana majorana SmellMajoram Scientiﬁc NameOriganum majorana MainTerpinen-4-olTerpinen-4-ol Component(s) γγ-Terpinene-Terpinene-Terpinene References Majoram Origanum majorana Terpinen-4-olTerpinen-4-ol γ γ-Terpinene-Terpinene-Terpinene Neroli Citrus aurantium Terpinen-4-ol γ-Terpinene Human Rat Mouse NeroliNeroli CitrusCitrus aurantium aurantium Linalool NeroliNeroli CitrusCitrus aurantium aurantium Neroli Citrus aurantium Linalool SmellNeroli CitrusScientific aurantium name LinaloolMain component(s) References Neroli Citrus aurantium LinaloolLinalool Smell Scientific name MainLinalool component(s) SmellSmell ScientificScientific name name LinaloolMainMain component(s) component(s) References Smell Scientific name LinaloolMain component(s) Human ReferencesRat Mouse SmellSmellSmell ScientificScientificScientific namename name MainMain component(s) component(s) ReferencesReferences SmellSmell Scientiﬁc nameScientific name MainMain component(s) Human ReferencesRat Mouse References Human Rat Mouse HumanHuman RatRat MouseMouse Human HumanRat Mouse Rat Mouse Limonene [105] Limonene LimoneneLimonene [105][105][105] LimoneneLimonene [105][105] Limonene Limonene LimoneneLimonene [105][105] [105] Limonene LimoneneLimonene [105] Green Limonenecis-3-HexenolLimonene trans-2-Hexenal [106] [73,107] Limonene Green cis-3-Hexenolcis-3-Hexenol trans-2-Hexenaltrans-2-Hexenal [106][106] [73,107][73,107] Green cis-3-Hexenol trans-2-Hexenal [106] [73,107] GreenGreen cis-3-Hexenolcis-3-Hexenol trans-2-Hexenal trans-2-Hexenal [106][106] [73,107][73,107] Green Green cis-3-Hexenol trans-2-Hexenal [106] [106[73,107]][ 73,107] Green cis-3-Hexenol trans-2-Hexenal [106] [73,107]

Yomogi Artemisia montana [108] Cineol (Eucalyptol) YomogiYomogi ArtemisiaArtemisia montana montana [108][108][108] Yomogi YomogiYomogi Artemisia montanaArtemisiaArtemisia montana montana [108][108] [108] Yomogi Artemisia montana CineolCineol (Eucalyptol) (Eucalyptol) [108] Cannabis Cannabis sativa β-Myrcene β-Caryophyllene [109] Yomogi Artemisia montana CineolCineolCineol (Eucalyptol) (Eucalyptol) (Eucalyptol) [108] CannabisCannabis CannabisCannabis sativa sativa Cineolββ-Myrcene-Myrcene-Myrcene (Eucalyptol) β β-Caryophyllene-Caryophyllene-Caryophyllene [109][109][109] Cannabis Cannabis sativa β-Myrcene β-Caryophyllene CannabisCannabis CannabisCannabis sativa sativa ββ-Myrcene-Myrcene-Myrcene β β-Caryophyllene-Caryophyllene-Caryophyllene [109][109][109]

Cannabis Cannabis sativa β-Myrcene β-Caryophyllene [109] Cannabis Cannabis sativa [109]

Many kinds of volitile components [110]

ManyMany kinds kinds of of volitile volitile components components [110][110][110] Many kinds of volitile components ManyMany kinds kinds of of volitile volitile components components [110][110][110] Many kinds of volitile componentsManyCoffee kinds bean of volitile Coffeacomponents [110][111] [110] Furfuryl alcohol CoffeeCoffee bean bean CoffeaCoffea [111][111][111] CoffeeCoffee bean bean CoffeaCoffea FurfurylFurfurylFurfuryl alcohol alcoholalcohol [111][111] Coffee bean Coffee beanCoffea Coffea Furfuryl alcohol [111] [111] Coffee bean Coffea FurfurylFurfurylFurfurylFurfuryl alcohol alcoholalcohol alcohol [111] Furfuryl alcohol Thyme Tymus vulgaris [112] Thyme ThymeThyme Tymus vulgarisTymusTymus vulgaris vulgaris Carvacrol Thymol [112][112][112] [112] ThymeThyme TymusTymus vulgaris vulgaris [112][112] Thyme Tymus vulgaris CarvacrolCarvacrol ThymolThymol [112][112] Thyme Tymus vulgaris Carvacrol Thymol [112] CarvacrolCarvacrol Thymol Thymol Carvacrol Thymol Lemongrass LemongrassCymbopogon Cymbopogon ﬂexuosus flexuosus [113] [113] CitralCitral LemongrassLemongrass CymbopogonCymbopogon flexuosus flexuosus [113][113][113] LemongrassLemongrass CymbopogonCymbopogon flexuosus flexuosus CitralCitral [113][113] Lemongrass Cymbopogon flexuosus Citral [113] Lemongrass Cymbopogon flexuosus CitralCitral [113] Citral Tetraclinis Tetraclinis articulata [114] Hinokitiol α-Pinene Tetraclinis Tetraclinis articulata [114][114] Tetraclinis Tetraclinis articulata Hinokitiol α-Pinene-Pinene [114] TetraclinisTetraclinis TetraclinisTetraclinis articulata articulata Hinokitiol α-Pinene [114][114][114] Tetraclinis Tetraclinis articulata HinokitiolHinokitiol α α-Pinene-Pinene-Pinene [114] Mixture of essential oil components HinokitiolLimonene γ-Terpinene α-Pinene β-Myrcene α-Pinene β-Pinene [92] Mixture of essential oil components Limonene γ-Terpinene-Terpinene β-Myrcene-Myrcene α-Pinene-Pinene β-Pinene-Pinene [92] Mixture of essential oil components Limonene γ-Terpinene β-Myrcene α-Pinene β-Pinene [92][92] Mixture of essential oil components LimoneneLimonene γ γ-Terpinene-Terpinene β β-Myrcene-Myrcene αα-Pinene-Pinene β β-Pinene-Pinene [92] MixtureMixture of of essential essential oil oil components components Limonene γ-Terpinene-Terpinene β-Myrcene-Myrcene α-Pinene-Pinene β-Pinene-Pinene [92][92] [92] Limonene γ-Terpinene β-Myrcene α-Pinene β-Pinene Mixture of essential oil components [92]

Sabinene Linalool Cymene Valencene

SabineneSabineneSabinene LinaloolLinaloolLinalool CymeneCymeneCymene Valencene ValenceneValencene Sabinene Linalool Cymene Valencene SabineneSabineneSabinene Linalool LinaloolLinalool Cymene CymeneCymene Valencene ValenceneValencene

Sabinene Linalool Cymene Valencene

Roman chamomile Chamaemelum nobile [115] Angeric acid Roman chamomile Chamaemelum nobile [115][115] Roman chamomile Chamaemelum nobile Angeric acid [115] RomanRoman chamomile chamomile ChamaemelumChamaemelum nobile nobile Angeric acid [115][115] Roman chamomile Chamaemelum nobile AngericAngeric acid acid [115] Roman chamomile Chamaemelum nobile Angeric acid [115] Angeric acid

Molecules 2021, 26, x FOR PEER REVIEW 2 of 3 Molecules MoleculesMolecules 2021 2021 2021, ,26 ,26 26, ,x ,x x FOR FOR FOR PEER PEER PEER REVIEW REVIEW REVIEW 22 2 of ofof 3 3 3

Ylang-ylang Cananga odorata Ylang-ylang Cananga odorata Ylang-ylangYlang-ylang CanangaCananga odorata odorata Germacrene D GermacreneGermacreneGermacrene D D D

Majoram Origanum majorana MajoramMajoram OriganumOriganum majorana majorana Majoram Origanum majorana Terpinen-4-ol γ-Terpinene Terpinen-4-olTerpinen-4-olTerpinen-4-ol γ γγ-Terpinene-Terpinene-Terpinene

Neroli Citrus aurantium NeroliNeroliNeroli CitrusCitrusCitrus aurantium aurantium aurantium Linalool LinaloolLinaloolLinalool Smell Scientific name Main component(s) References SmellSmellSmell ScientificScientificScientific name namename MainMainMain component(s) component(s) component(s) ReferencesReferencesReferences Human Rat Mouse HumanHumanHuman RatRatRat MouseMouseMouse

Limonene [105] Limonene [105] LimoneneLimonene Limonene [105][105] LimoneneLimoneneLimonene

Green cis-3-Hexenol trans-2-Hexenal [106] [73,107] GreenGreenGreen cis-3-Hexenolcis-3-Hexenolcis-3-Hexenol trans-2-Hexenal trans-2-Hexenaltrans-2-Hexenal [106][106][106] [73,107][73,107][73,107]

Yomogi Artemisia montana [108] YomogiYomogi ArtemisiaArtemisia montana montana [108][108] Yomogi Artemisia montana Cineol (Eucalyptol) [108] CineolCineolCineol (Eucalyptol) (Eucalyptol) (Eucalyptol) Cannabis Cannabis sativa β-Myrcene β-Caryophyllene [109] CannabisCannabisCannabis CannabisCannabisCannabis sativa sativa sativa βββ-Myrcene-Myrcene-Myrcene β ββ-Caryophyllene-Caryophyllene-Caryophyllene [109][109][109]

Many kinds of volitile components [110] ManyManyMany kinds kinds kinds of of of volitile volitilevolitile components components components [110][110][110]

Coffee bean Coffea [111] CoffeeCoffeeCoffee bean bean bean CoffeaCoffeaCoffea Furfuryl alcohol [111][111][111] FurfurylFurfurylFurfuryl alcohol alcohol alcohol

Molecules 2021 26 , , 2571 Thyme Tymus vulgaris [112] 11 of 29 ThymeThymeThyme TymusTymusTymus vulgaris vulgaris vulgaris [112][112][112] Carvacrol Thymol CarvacrolCarvacrolCarvacrol Thymol ThymolThymol

Table 1. Cont. Lemongrass Cymbopogon flexuosus [113] LemongrassLemongrassLemongrass CymbopogonCymbopogonCymbopogon flexuosus flexuosus flexuosus Citral [113][113][113] Smell Scientiﬁc Name MainCitralCitralCitral Component(s) References Human Rat Mouse

Tetraclinis Tetraclinis articulata [114] Tetraclinis TetraclinisTetraclinisTetraclinisTetraclinis articulataTetraclinisTetraclinisTetraclinis articulata articulata articulata Hinokitiol α-Pinene [114][114][114] [114] HinokitiolHinokitiolHinokitiolHinokitiol α αα-Pinene-Pinene-Pinene

Mixture of essential oil components Limonene γ-Terpinene β-Myrcene α-Pinene β-Pinene [92] MixtureMixtureMixture of of of essential essential essential oil oil oil components components components LimoneneLimoneneLimonene γ γγ-Terpinene-Terpinene-Terpinene β ββ-Myrcene-Myrcene-Myrcene α αα-Pinene-Pinene-Pinene β β β-Pinene-Pinene-Pinene [92][92][92]

Mixture of essential oil components Sabinene Linalool Cymene Valencene [92] SabineneSabineneSabinene Linalool LinaloolLinalool Cymene CymeneCymene Valencene ValenceneValencene

Molecules 20212021,, 2626,, xx FORFOR PEERPEER REVIEWREVIEW 103 ofof 28 3 Molecules Molecules 20212021,, 2626,, xx FORFOR PEERPEER REVIEWREVIEW 33 ofof 33 Molecules 2021, 26, x FOR PEER REVIEW 3 of 3

Roman chamomile Chamaemelum nobile [115] Roman chamomile RomanRomanRoman chamomile chamomile Chamaemelumchamomile ChamaemelumChamaemelumChamaemelum nobile nobile nobilenobile Angeric acid [115][115][115] [115] AngericAngericAngericAngeric acidacid acid

Lemon Citrus limon [116][116] Lemon Lemon Citrus limon Citrus limon [116] [116] LemonLemon CitrusCitrus limon limon Limonene [116][116] Limonene LimoneneLimoneneLimonene

Hinokitiol Hinokitiol [117][117] [117] HinokitiolHinokitiol HinokitiHinokitiolol [117][117] Hinokitiol HinokitiolHinokitiol [117] HinokitiolHinokitiol Navel orange Citrus sinensis (L.)(L.) Limonene α-Pinene-Pinene [118][118] Navel orangeCitrus sinensisCitrus(L.) sinensis (L.) Limonene α-Pinene [118] Navel orange NavelNavel orange orange CitrusCitrus sinensis sinensis (L.) (L.) LimoneneLimonene αα-Pinene-Pinene [118][118] [118] Osbeck Osbeck Osbeck OsbeckOsbeck Thyme linalool Thymus vulgaris ct.ct. Linalool Terpinen-4-ol [119,120][119,120] [121,122][121,122] ThymeThyme linaloollinalool ThymusThymus vulgarisvulgaris ct.ct. LinaloolLinalool Terpinen-4-olTerpinen-4-ol [119,120] [121,122] Thyme linaloolThymus vulgarisThymusct. vulgaris ct. Linalool Terpinen-4-ol [119,120][119,120] [121,122][121,122] Thyme linalool linaloollinalool [119,120][ 121,122] linalool linalool linaloollinalool

SesameSesame oiloil [123][123] Sesame oil 2-Meth2-Methylpyrazineylpyrazine [123] Sesame oil SesameSesame oil oil 2-Methylpyrazine [123][123] [123] 2-Methylpyrazine2-Methylpyrazine2-Methylpyrazine

3.3.1. Effect on Humans Even at non-life-threatening levels, chronic stress is a risk factor for the development of various mental disorders, and it is desirable to control the stress levels. The stress-suppressing effects of certain smell have been recognized, and the effects of fragrances on human psychophysiological activity have been studied for a long time [10]. Granqvist et al. [9] pointed out that, in stressful situations, access to adult attachment figures (e.g., romantic partners) is an important means by which adults regulate stress responses. They observed in humans that partner body odor reduced subjective discomfort during a stressful event, as compared with the odor of oneself, which suggests that partner odor is a scent of security, especially for attachment-secure adults. Yoto et al. [81] investigated the alleviating effect of black tea aroma on physical and psychological stress in humans that is induced by the Uchida–Kraepelin test, based on salivary chromogranin-A (CgA) levels as a stress marker and subjective evaluations. It was observed that inhaling black tea aroma can reduce stress levels, and Darjeeling tea aroma tends to improve mood before exposure to mental stress. Several lines of evidence in human studies suggest the stress suppression effects of essential oils, such as lavender [82–86], rose [91], rosemary [86], bergamot [93–95], chamomile [83], grapefruit [97], and neroli (Citrus aurantium L. var. amara) obtained from the flowers of bitter orange [98]. Stress attenuating effects are also observed in essential oils that are derived from trees, such as cypress [99] and cedar [102]. The majority of human studies relate to assessing the effects of essential oils in clinical applications. Essential oils can reduce the stress levels at various levels. In human studies, psychiatric interviews are possible, and conditions, such as mood, can be evaluated. It is also possible to measure vital signs such as body temperature, blood pressure, and heart rate to evaluate the sympathetic nerve activity, and the concentration of stress hormones, such as cortisol in blood and stress markers in saliva. Therefore, the data showing that the aroma of essential oils suppressed the changes in these parameters caused by stress must be the evidence that the smell suppressed stress. However, it should be noted that there is a difficulty to control the dosage of essential oil, since the main component of essential oils are volatile. Atsumi and Tonosaki [86] observed that lavender and rosemary increase the free radical scavenging activity and decrease the cortisol level in saliva in humans. The effects of lavender aroma have been the most studied in the essential oils, but lavender-containing mixtures have also been investigated. For example, it was demonstrated that a mixture of lavender, ylang-ylang, marjoram, and neroli (20:15:10:2) suppresses the stress level and may have relaxation effects for controlling hypertension [104]. Aroma of limonene, a typical monoterpene contained in citrus fruits, has been also studied, since it is suggested that the aroma may be useful for human health [105]. Li et al. [99] examined the effects of essential oils from trees on male subjects who stayed at an urban hotel for three nights.

Molecules 2021, 26, 2571 12 of 29

Several lines of evidence in human studies suggest the stress suppression effects of essential oils, such as lavender [82–86], rose [91], rosemary [86], bergamot [93–95], chamomile [83], grapefruit [97], and neroli (Citrus aurantium L. var. amara) obtained from the flowers of bitter orange [98]. Stress attenuating effects are also observed in essential oils that are derived from trees, such as cypress [99] and cedar [102]. The majority of human studies relate to assessing the effects of essential oils in clinical applications. Essential oils can reduce the stress levels at various levels. In human studies, psychiatric interviews are possible, and conditions, such as mood, can be evaluated. It is also possible to measure vital signs such as body temperature, blood pressure, and heart rate to evaluate the sympathetic nerve activity, and the concentration of stress hormones, such as cortisol in blood and stress markers in saliva. Therefore, the data showing that the aroma of essential oils suppressed the changes in these parameters caused by stress must be the evidence that the smell suppressed stress. However, it should be noted that there is a difficulty to control the dosage of essential oil, since the main component of essential oils are volatile. Atsumi and Tonosaki [86] observed that lavender and rosemary increase the free radical scavenging activity and decrease the cortisol level in saliva in humans. The effects of lavender aroma have been the most studied in the essential oils, but lavender-containing mixtures have also been investigated. For example, it was demonstrated that a mixture of lavender, ylang-ylang, marjoram, and neroli (20:15:10:2) suppresses the stress level and may have relaxation effects for controlling hypertension [104]. Aroma of limonene, a typical monoterpene contained in citrus fruits, has been also studied, since it is suggested that the aroma may be useful for human health [105]. Li et al. [99] examined the effects of essential oils from trees on male subjects who stayed at an urban hotel for three nights. The aromatic volatiles (phytoncides) were produced by vaporizing cypress (Chamaecyparis obtusa) stem oil with a humidifier in the hotel room during the night stay. The inhalation of the essential oil caused significant increases in the activity and proportion of natural killer (NK) cells in the blood. Decrements were found in the percentage of T cells and the concentrations of adrenaline and NA in the urine. Phytoncides, such as α-pinene and β-pinene, were detected in the air of hotel rooms. Therefore, phytoncide exposure may contribute to lowering stress hormone levels and increasing NK activity, which results in the enhancement of immune function. Matsubara and Ohira [102] reported, in female subjects, that olfactory stimulation with the essential oil that is extracted from Japanese cedar (Cryptomeria japonica) wood may modulate mood states, and transiently suppress sympathetic nervous activity. Thus, the volatile compounds of the essential oil could contribute to mental health. One of the aliphatic aldehydes, cis-3-hexenol, has another name of green leaf aldehyde, together with the isomer trans-2-hexenal, and it is a major component of the smell of grass and leaf. It was demonstrated that a mixture of cis-3-hexenol and trans-2-hexenal has anti-fatigue effects, which suggests that the olfactory receptors in the olfactory system may be involved in the attenuation of fatigue [106]. Unique aromas have also been studied. Kunihiro et al. [108] demonstrated that volatile components in the essential oil of “Yomogi (Artemisia montana)” shows stress-reducing effects following nasal exposure. The sedative effects of yomogi oil is thought to be due to 1,8-cineol according to the near-infrared spectroscopy (NIRS) analysis. The inhalation of essential oil from legal hemp variety, in which the main components were myrcene and β-caryophyllene, affected the brain wave activity and autonomic nervous system, which suggested that the essential oil of the Cannabis has a neuro modular activity in cases of stress, depression, and anxiety [109]. As described above, stress-suppressing smells are well recognized, and essential oils are actually used for aromatherapy for stress reduction in humans [87]. Thus, the development of effective essential oil inhalers is underway [124]. However, the mechanism underlying the effect of aroma on the stress response in the brain has not yet been elucidated. Molecules 2021, 26, 2571 13 of 29

3.3.2. Effect on Rats Numerous investigators have revealed the stress-suppressing effect of essential oils, especially on the behavior of rodents [110]. First, let us look at previous studies in rats. It was demonstrated that the aroma of coffee beans reduced stress [111]. Hritcu et al. [88] reported the effect of lavender on spatial memory impairment by scopolamine. Thyme (Tymus vulgaris)[112] has the property of suppressing aflatoxin-induced oxidative stress. The essential oil from a lemongrass variety of Cymbopogon flexuosus (lemongrass oil) is used in various food and aroma industry products. Li et al. [113] demonstrated that lemongrass oil and its major component, citral, may affect the activities of drug-metabolizing enzymes and reduce the oxidative stress in the liver. Fukada et al. [91] reported that the destruction of the skin barrier due to chronic stress can be restricted or prevented by rose essential oil inhalation on the HPA axis in rats and human. Sadiki et al. [114] demonstrated that the essential oil of Tetraclinis (Tetraclinis articulata) could be a potent pharmacological agent against dementia by modulating the cholinergic activity and promoting antioxidant action in the rat hippocampus. Bergamot essential oil may act like an anxiolytic, similarly to di- azepam, and reduce the corticosterone response to stress [96]. Behavioral studies combined with the measurement of c-fos expression in the rat amygdala suggested that vetiver essential oil may have the anxiolytic properties that might be associated with altering neuronal activation in the central amygdaloid nucleus [125]. It has been suggested that olfactory signals may mediate a phenomenon, called social buffering of conditioned fear responses in the rat [126]. The amygdala may play important roles in social buffering [55,126,127]. The smell of green, a mixture of cis-3-hexenol and trans-2-hexenal, can affect neuronal activity in the prelimbic and intralimbic of medial prefrontal cortex [73], which may have a preventive as well as a therapeutic effect on the depressive-like state [107]. These effects, at least in part, may be due to an up-regulation induced by the smell of green in the hippocampus. Park et al. [100] studied the effects of an essential oil from cypress on early life stress by means of maternal separation (MS) rats. Cypress inhalation decreased anxiety-related behavior in the elevated plus-maze test. The aroma also downregulated the expressions of cytokine genes, such as Cxcl10, Ccl19, and Il1rl, in addition to Ccl2 and Il6 in the hippocampus of MS rats. Thus, the aroma of cypress may attenuate MS- induced anxiety-related behaviors, and modulate cytokines, particularly Ccl2 and Il6, in the hippocampus. Boiangiu et al. [92] examined the effects of an essential oil mix consisting of limonene (91.11%), γ-terpinene (2.02%), β-myrcene (1.92%), β-pinene (1.76%), α-pinene (1.01%), sabinene (0.67%), linalool (0.55%), cymene (0.53%), and valencene (0.43%) on the rat. This oil reversed scopolamine-induced memory deficits and oxidative stress in the brain, along with cholinesterase inhibitory effects, an important mechanism of anti-amnestic effects. These findings suggest that the inhalation of the essential oil mix can improve memory deficits through the activity of the cholinergic system and the restoration of antioxidant status in the brain.

3.3.3. Effect on Mice A growing body of evidence suggests the effects of aroma in the mouse, as follows. Matsukawa et al. [72] demonstrated, in mice, that the aroma of rose can counteract the odor of predators. Ueno et al. [128] suggested that 2-phenylethanol, the main aroma component of rose oil, elicits neuropsychological effects that alter the behavior of mice and may also elicit anti-depressive effects. Thus, the inhalation of rose oil containing 2-phenylethanol may be effective against depression and stress-related diseases. It was also demonstrated that by lavender [89,90], cypress [101], α-pinene [103], and thyme linalool suppressed stress [121,122]. It was Hashikawa-Hobara et al. [115] suggested that inhalation of roman chamomile (Chamaemelum nobile) may enhance the antidepressant effect of clomipramine by increasing hippocampal neurogenesis and regulating corticosterone levels in patients with treatment-resistant depression. Komiya et al. [116] showed a strong anti-stress effect of lemon essential oil by the behavior of mice. Their phenomenological Molecules 2021, 26, 2571 14 of 29

results suggest that the antidepressant-like effects of lemon oil are closely associated with the 5-HTergic pathway, especially via the 5-HT (1A) receptor. They also found that lemon oil significantly accelerated the metabolic turnover of hippocampal DA and prefrontal cortex and striatal 5-HT. The effects of woody (hinokitiol) aroma were examined in mice by plasma corticotropin levels, c-Fos immunoreactivity in the piriform cortex, and bed nucleus of stria terminalis, and it was suggested that hinokitiol ameliorates TMT-induced stress [117]. In phytotherapy, essential oils tend to be used daily for a period of days or weeks rather than a single application. Satou et al. [129] investigated the effects of the inhalation of α-pinene, an essential oil component, on the mouse behavior and accumulation in the brain and liver. Significant anxiolytic-like activity was consistently observed during the five-day inhalation of α-pinene. On the other hand, the accumulation of α-pinene in the brain and liver peaked on the third day of inhalation. Stress appears to affect the accumulation of α-pinene in the internal organs, keeping the anxiolytic-like action constant. Zhang et al. [118] investigated the effects of navel orange (Citrus sinensis (L.) Osbeck) essential oil and its main volatile component, limonene, on the mouse receiving chronic unpredictable mild stress. The essential oil inhalation significantly restored the depression-like behaviors and symptoms in the stressed mice. Limonene significantly improved depressive behavior, hyperactivity of the HPA axis, and decreased monoamine neurotransmitter levels, as well as suppressing the down-regulation of BDNF and its receptor expression in the hippocampus. These data suggest that limonene plays important roles in antidepressant effects.

3.3.4. Biochemical Parameters in the Brain in Response to Olfaction Regarding alterations in the expression of biochemical parameters in the brain to clarify the relationship between olfaction and stress, immediate-early genes have been frequently measured. Many of the immediate-early genes in neurons are “activity-dependent genes”, whose expression is induced by synaptic activity and the inﬂux of calcium ions that are associated with action potentials [130]. Typical immediate-early genes in the brain include genes encoding transcriptional regulators, such as c-fos and Egr-1, and gene encoding synapse-related proteins, such as Arc and Homer1a/Vesl-1s. The mRNAs and protein products of these genes are widely used as markers of neural activity [131,132]. Indeed, numerous researchers reported an alteration in the expression of c-fos gene [16,21,59,68] and Arc gene [132]. c-Fos immunoreactivity was also analyzed in the rat amygdala [96], bed nucleus of the stria terminalis [57], and hypothalamus [74,126]. Kigar et al. [59] suggested that the methylation of adenosine in mammalian DNA can be used as an epigenetic biomarker for investigating the development of stress-induced neuropathology. Exceptionally, studies on PTSD model rats showed changes in the expression of GABAA receptor subunit α1 in the amygdala and hippocampus [133]. In other PTSD model rats by the odor of dirty cat litter, Aykac et al. [134] found alterations in the expression of muscarine receptors, M1, M2, M4, and M5 in the frontal cortex, hippocampus, and amygdala. Fluoxetine and propranolol ameliorated these changes in the frontal cortex. Ozbeyli et al. [135] suggested that, when vortioxetine is administered immediately after trauma, it may reduce anxiety, cognitive, and neuronal impairment, and it may prevent the development of PTSD by predator odor. Based on the action of vortioxetine, a possible decrease in 5-HTergic neurons is considered to be very important in the brain of PTSD. However, analysis of biochemical parameters in the brain, such as stress marker candidates, including neurotransmitters, proteins, peptides, etc., has not yet been sufﬁciently accumulated.

4. Recent Studies on the Effects of Aroma on Biochemical Parameters in the Brain As described above, the effect of aroma on the stress response in the brain has not been well documented yet. The authors focused on the stress-suppressing effect of certain aroma in the process of analyzing the effect of stress on the brain. Conventionally, it is empirically recognized that a certain aroma has a stress-suppressing effect, although its Molecules 2021, 26, 2571 15 of 29

effectiveness has not been fully proven [10]. To attempt a proof of the effect of aroma, we examined, in experimental animals, some kinds of smell by means of behavioral analysis and measuring biochemical parameters that have recently been identiﬁed as potential stress marker candidates in the brain and blood. Here, we would like to introduce our recent studies on the effects of aroma of coffee beans, lavender, cypress, α-pinene, and thyme linalool.

4.1. Coffee Beans The debate about the health effects of coffee has not yet been settled, but the beneficial effects of drinking coffee are known to reduce the risk of developing depression and suicide [136] and relieve stress [137]. It should be noted that previous studies have primarily focused on the non-volatile component caffeine, whose awakening effect can be stressful. On the other hand, changes in the brain due to volatile components have not well been clarified. Interestingly, Tabuchi et al. [119] demonstrated that experimental animal rats prefer the aroma of coffee. The authors [111] divided rats into four groups (control group, stress group, coffee group, and stress + coffee group) and the rats were bred in each environment for 24 h. For the stress and stress + coffee groups, a very mild water immersion stress (stress of putting water instead of wood chips for control and coffee groups) was applied. After that, the whole brain was removed, and the expression of genes and proteins was analyzed. As a result, it was assumed that alterations in the expression of some genes and proteins that are caused by stress was suppressed by the coffee bean aroma. Such stress-suppressing effects of the aroma was notably observed in the expression of nerve growth factor receptor (NGFR), high affinity neurotrophic tyrosine kinase, receptor type 3 (trkC), glucocorticoid-induced receptor (GIR), and thiol-specific antioxidant protein and heat shock 70 kDa protein 5. Based on the annotation regarding the function of each factor, it was suggested that the coffee bean aroma might have antioxidant and stress-relieving functions. For example, the nerve growth factor (NGF) is known to be resistant to oxidative stress and it promotes cell survival by activating free radical scavenging enzymes and glutathione peroxidase. The expression levels of NGFR gene were decreased in the stress group, and the stress + coffee group showed higher levels than that of the stress group, which suggests that the aroma of coffee beans suppressed oxidative stress induced by water immersion stress. GIR is a G protein-coupled receptor for which no ligand has been identified (orphan receptor). The GIR gene expression levels were also decreased due to stress, and the expression in the stress + coffee group was increased comparing to the stress group. Because GIR is involved in anxiety and neuroendocrine regulation, the aroma of coffee beans may have anxiolytic effects. Thiol-specific antioxidant proteins are known to suppress oxidative stress. The expression level in the stress + coffee group was higher than that in the stress group. These results suggest that the aroma of coffee beans may have an antioxidant effect. These factors in the brain were considered to be potential stress marker candidates. The roasted coffee bean aroma could change the mRNA and protein expression levels of the rat brain, providing clues as to the potential antioxidant or stress relaxation activities of volatile molecules of coffee beans. These data on the effects of volatile components of coffee beans are completely different from those investigating the effects of caffeine intake.

4.2. Lavender Previous studies suggest that lavender has a stress-suppressing effect and it may be effective in preventing depression [89]. As the next step, the authors analyzed the effect of lavender (Lavandula ofﬁcinalis) essential oil (LvEO) [90]. The mice were divided into four groups (control group, stress group, LvEO group, stress + LvEO group) and then bred in each environment for 24 h. As the stress, water immersion stress was applied as in the coffee bean aroma experiment [111]. Subsequently, distilled water or LvEO was inhaled by the mice for 90 min. An elevated plus-maze test, which is one of the evaluation methods Molecules 2021, 26, x FOR PEER REVIEW 15 of 28 Molecules 2021, 26, x FOR PEER REVIEW 15 of 28 Molecules 2021, 26, 2571 16 of 29

but no significant change was observed in any of the groups. It might be possible that the but no significant change was observed in any of the groups. It might be possible that the stress level in this study was not the intensity that significantly reduced BDNF expression. stressfor anxiety-related level in this study behavior, was not suggested the intensity that LvEO that significantly had anxiolytic reduced effects. BDNF When expression. LvEO was Because BDNF expression is significantly reduced after the onset of depression, BDNF is Becauseinhaled byBDNF the mice,expression the rate is ofsignificantly entry into openreduced arms after and the the onset time of that depression, was spent BDNF in open is considered to be an effective biomarker when stress causes serious brain damage. The consideredarms were increasedto be an significantlyeffective biomarker regardless when of the stress presence causes or absenceserious brain of stress. damage. Moreover, The results of behavioral analysis suggested that LvEO had anxiolytic activity and was accom- resultsthe rate of of behavioral entering andanalysis the timesuggested spent that in open LvEO arms had anxiolytic in the stress activity + LvEO and groupwas accom- were panied by an increase in locomotor activity, as described above. From the results of ex- paniedsignificantly by an higher increase than in thoselocomotor in the activity, control as group. described Furthermore, above. Fr theom total the distanceresults of that expression analysis of genes and proteins in the brain, it was suggested that LvEO had an waspression traveled analysis in the of stressgenes + and LvEO proteins group in was the significantly brain, it was longer suggested than that inLvEO the controlhad an anti-stress effect, and it exerted a sedative effect. The relationship between sedation and anti-stressgroup, indicating effect, and an increase it exerted in a locomotor sedative effect. activity. The These relationship results strongly between suggested sedation thatand increased locomotor activity should be further investigated. As a volatile component of increasedLvEO had locomotor anxiolytic activity.activity should be further investigated. As a volatile component of lavender, the molecules of linalool and linalyl acetate may considered to affect the olfac- lavender,In the the whole molecules brain, the of linalool expression and level linalyl of Arc acetate mRNA may of considered the stress group to affect and the LvEOolfac- tory system. torygroup system. was significantly lower than that of the control group. On the other hand, the stress Recently, Yoshida et al. [120] investigated gene expression profiles by means of DNA + LvEORecently, group Yoshida was equivalent et al. [120] to the investigated control group gene level expression (Figure profiles3A). The by gene means expression of DNA microarray analysis in the hypothalamus, and the found that inhalation of racemic (R,S) - microarrayof NGFR showed analysis similar in the changes hypothalamus, as Arc, but and the the stress found + LvEO that inhalation group showed of racemic a significant (R,S) - linalool restored the expression of numerous stress-induced genes to normal. These genes linaloolincrease restored when compared the expression to the of control numerous group stress-induced (Figure3B).These genes resultsto normal. suggested These genes that were associated with synaptic transmission via neurotransmitters, including anxiolytic wereLvEO associated had an anti-stress with synaptic effect. transmission In immunohistochemistry, via neurotransmitters, the ratio including of the number anxiolytic of neuropeptides, such as oxytocin and neuropeptide Y, and it also contained several major positiveneuropeptides, cells in dentatesuch as gyrusoxytocin of theand hippocampus neuropeptide to Y, the and number it also of contained Nissl-stained several cells major was histocompatibility complex (MHC) class I molecules that were required for neurodevel- histocompatibilitycalculated. The expression complex level (MHC) of galactokinase class I molecules (GLK) that 1 was were significantly required forincreased neurodevel- in the opment and plasticity. In addition, it was suggested that inhalation of (R,S) -linalool under opmentstress group and plasticity. and decreased In addition, to the sameit was level suggested in the that stress inhalation + LvEO groupof (R,S as) -linalool in the control under stress might activate prolactin. These results suggested some of the molecular mecha- stressgroup might (Figure activate4). Therefore, prolactin. LvEO These might resu havelts asuggested sedative effect some by of suppressing the molecular the energymecha- nisms associated with aroma inhalation in the hypothalamus of stressed organisms. nismsexpenditure associated (hypermetabolism) with aroma inhalation due to arousal/insomnia. in the hypothalamus of stressed organisms.

Figure 3. The effects of lavender on gene expression levels in the mouse brain. RT-PCR analysis was performed using Figure 3. The effects of lavender on gene expression levels in the mouse brain. RT-PCR analysis was performed using the theFigure whole 3. The brain effects of mice of lavender inhaled on with gene distilled expression water levels or lavender in the mouse essential brain. oil RT-PCR (LvEO) afteranalysis water was immersion performed stress. using (theA): whole brain of mice inhaled with distilled water or lavender essential oil (LvEO) after water immersion stress. (A): Ex- Expressionwhole brain of of Arc mice gene. inhaled (B): Expression with distilled of nerve water growth or lavender factor receptoressential (NGFR) oil (LvEO) gene. after The water results immersion are expressed stress. as the(A): ratio Ex- pression of Arc gene. (B): Expression of nerve growth factor receptor (NGFR) gene. The results are expressed as the ratio ofpression the mRNA of Arc expression gene. (B): level Expression to that ofof glyceraldehyde-3-phosphatenerve growth factor receptor dehydrogenase (NGFR) gene. (GAPDH)The results (%), are andexpressed are means as the± SEMratio of the mRNA expression level to that of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (%), and are means ± SEM of the mRNA expression level to that of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (%), and are means ± SEM (n = 4). * * p < 0.05 0.05 (Tukey–Kramer’s (Tukey–Kramer’s HSD HSD test). test). This This figure figure is modified modified based on the original figure figure published [[90].90]. (n = 4). * p < 0.05 (Tukey–Kramer’s HSD test). This figure is modified based on the original figure published [90].

Figure 4. The effects of lavender on protein expressionexpression levelslevels inin thethe mousemouse brain.brain. After waterwater immersion stress, brain Figure 4. The effects of lavender on protein expression levels in the mouse brain. After water immersion stress, brain sections of mice inhaled with distilleddistilled waterwater oror LvEOLvEO werewere stainedstained immunohistochemicallyimmunohistochemically with anti-galactokinase 1 sections of mice inhaled with distilled water or LvEO were stained immunohistochemically with anti-galactokinase 1 (GLK1) antibody. (A): Nissl-stained mouse brain frontal section at the level of the hippocampus. (B): Expression of GLK1 (GLK1)(GLK1) antibody.antibody. ((AA):): Nissl-stainedNissl-stained mousemouse brainbrain frontalfrontal sectionsection atat thethe levellevel ofof thethe hippocampus.hippocampus. (B): Expression of GLK1 positive cells in the dentate gyrus, inside the photo square indicated in (A). The results are expressed as the ratio of the positive cells in the dentate gyrus, insideinside the photo square indicated in (A). The resultsresults areare expressedexpressed as thethe ratioratio ofof thethe number of GLK1 positive cells to that of Nissl-stained cells (%), and are means ± SEM (n = 4). * p < 0.05 (Tukey–Kramer’s number ofof GLK1GLK1 positivepositive cellscells toto that that ofof Nissl-stainedNissl-stained cellscells(%), (%),and and are are means means± ± SEMSEM (n(n == 4).4). * p < 0.05 (Tukey–Kramer’s HSD test). This figure is modified based on the original figure published [90]. HSD test). This figurefigure is modifiedmodified basedbased onon thethe originaloriginal figurefigure publishedpublished [[90].90].

Molecules 2021, 26, 2571 17 of 29

The expression level of BDNF, which was regarded as a promising stress marker, tended to decrease in the order of LvEO group, stress group, and stress + LvEO group, but no significant change was observed in any of the groups. It might be possible that the stress level in this study was not the intensity that significantly reduced BDNF expression. Because BDNF expression is significantly reduced after the onset of depression, BDNF is considered to be an effective biomarker when stress causes serious brain damage. The results of behavioral analysis suggested that LvEO had anxiolytic activity and was accompanied by an increase in locomotor activity, as described above. From the results of expression analysis of genes and proteins in the brain, it was suggested that LvEO had an anti-stress effect, and it exerted a sedative effect. The relationship between sedation and increased locomotor activity should be further investigated. As a volatile component of lavender, the molecules of linalool and linalyl acetate may considered to affect the olfactory system. Recently, Yoshida et al. [120] investigated gene expression profiles by means of DNA microarray analysis in the hypothalamus, and the found that inhalation of racemic (R,S) -linalool restored the expression of numerous stress-induced genes to normal. These genes were associated with synaptic transmission via neurotransmitters, including anxiolytic neuropeptides, such as oxytocin and neuropeptide Y, and it also contained several major histocompatibility complex (MHC) class I molecules that were required for neurodevelop- ment and plasticity. In addition, it was suggested that inhalation of (R,S) -linalool under stress might activate prolactin. These results suggested some of the molecular mechanisms associated with aroma inhalation in the hypothalamus of stressed organisms.

4.3. Cypress Cypress is a tree that is especially familiar to Japanese people and it has been used as a fragrance and building material for a long time. The authors investigated the effect of cypress aroma on the emotional behavior of mice and the expression level of stress marker candidates in the brain [101]. Rodents, such as mice and rats, are comfortable in an environment where multiple individuals are bred in the same cage, and single breeding is stressful. In this study, five-week-old male ICR mice were bred alone for one week (one animal/cage), and then each mouse was placed in a glass container and inhaled with cypress essential oil or distilled water for 90 min. Immediately after inhalation, an elevated plus-maze test was performed. After the behavioral test, the whole brain was removed, and the expression level of the stress marker candidates was measured. The gene expression of NGFR and Arc was analyzed by RT-PCR, and the protein expression of BDNF and GLK1 by immunohistochemistry. As a result of the elevated plus-maze test, the rate of visiting open arms and time spent in the open arms of the group inhaled cypress essential oil (7.0 mg/L air) were significantly increased when compared with the control group (p < 0.05). Mice with low anxiety levels entered and stayed in open arms more often, which suggested that the aroma of cypress might reduce the anxiety levels. The expression levels of Arc and NGFR in the cypress group (7.0 mg/L air inhalation) were significantly higher than those in the control group (p < 0.05). Arc plays important roles in long-term potentiation and long-term memory, and NGFR is a receptor for NGF that is resistant to oxidative stress. Because the expression levels of these genes were decreased by stress, the present results of the gene expression in the brain also suggested that the cypress aroma might suppress stress level. In immunohistochemistry, the number of BDNF and GLK1-positive cells in the cypress group was not significantly different from that in the control group. Alterations in protein expression could take longer time than those in gene expression, so changes in expression due to the aroma could not be observed under the present experimental conditions. These results suggested that the aroma of cypress had anxiolytic and stress-suppressing effects. Molecules 2021, 26, 2571 18 of 29

4.4. α-Pinene α-Pinene, which accounts for 50–60% of the cypress essential oil, is a pine-derived monoterpene, and it is the source of the unique aroma that is found in many conifers. The essential oils listed in descending order of α-pinene content are cypress, pine, rosemary Molecules 2021, 26, x FOR PEER REVIEWverbenone, eucalyptus globulus, rosemary cineole, chamomile roman, and thyme linalool.17 of 28 The authors studied the effect of α-pinene on stressed mice [103]. After the mice were bred alone for one week, these animals were divided into four groups: 60 min. control group (distilled water inhalation for 60 min.), 60 min. α-pinene group (α-pinene inhalation for 60For min.), genes 90 in min. the brain control regions, group the (distilled BDNF ge waterne expression inhalation level for 90was min.), significantly 90 min. in-α- pinenecreased groupin the (olfactoryα-pinene bulb inhalation of the for90-min. 90 min.). α-pinene After group inhalation, and the an hippocampus elevated plus-maze of the test60-minute was performed, α-pinene group and the (Figure total distance6A, B). Therefore, traveled wasBDNF significantly genes in the increased olfactory in bulb the 60and min. hippocampusα-pinene groupmight asbe comparedinvolved in to the the anxiolytic 60 min. control effect of group α-pinene. (Figure Nevertheless,5A). These resultsthe inhalation suggested time that of αα-pinene,-pinene which might caused have an a significant excitatory increase effect. The in BDNF 90 min. geneα-pinene expres- groupsion, was showed different a significant between increasethe olfactory in the bu visitslb and to hippocampus. open arms and The the effects time spent of inhaled in open α- arms,pinene and might the differ 60 min. in time,α-pinene depending group on caused the brain a significant region, and increase such ina time the visitslag should to open be armsclarified (Figure in the5B,C future.), suggesting As mentioned that α ab-pineneove, behavioral might have analyses anxiolytic revealed effects. that However,α-pinene thehad inhalationan excitatory-like time of αeffect.-pinene, Therefore, which causedwe measured significant tyrosine changes hydroxylase in the excitatory (TH), a rate- and anxiolyticlimiting enzyme effects, for did the not synthesis match between of catecholamines, 60 min. and gene 90 min. expression Therefore, in the we midbrain. investigated α- thePinene transfer caused of α a-pinene significant into theaugment brain. in As the a result TH expression of measuring level the inα -pinenethe 60-min. concentration α-pinene ingroup the mouse(Figure brain 6C). afterThese inhalation, results suggested the brain that transferability α-pinene might by 60-min. activate inhalation DA neurons, was significantlywhich results higher in hyperactivities. than that by 90-min.On the other inhalation. hand, Thus,α-pinene the braindid not transferability cause significant of α- pinenechanges was in the not expression proportional of to TH inhalation gene in time,the medulla which suggested oblongata, that suggesting the effect that of α -pinenethe NA wasneurons due in to this both region olfactory were stimulation not significantly and transfer affected. to the brain.

Figure 5. Effects of αα-pinene-pinene on on mouse mouse behavior. behavior. Stressed Stressed mice mice were were inhaled inhaled with with distilled distilled water water or orα-pineneα-pinene for for 60 60or or90 90min., min., and and then then an anelevated elevated plus-maze plus-maze test test was was performed. performed. (A (A): ):Total Total distance distance traveled traveled (m). (m). (B (B):): Visits Visits to to open open arms arms (%). (%). ((CC):): TimeTime spentspent inin openopen armsarms (sec).(sec). TheThe results show means ± SEMSEM (n (n = = 5). 5). * *pp << 0.05, 0.05, ** ** p p< <0.01, 0.01, *** *** p p< <0.001 0.001 (Student’s (Student’s t- ttest).-test). This This figure figure is is mo modifieddified based based on on the the orig originalinal figure figure [103] [103 ]with with the the copyright copyright permission. permission.

For genes in the brain regions, the BDNF gene expression level was significantly increased in the olfactory bulb of the 90-min. α-pinene group and the hippocampus of the 60-min α-pinene group (Figure6A, B). Therefore, BDNF genes in the olfactory bulb and hippocampus might be involved in the anxiolytic effect of α-pinene. Nevertheless, the inhalation time of α-pinene, which caused a significant increase in BDNF gene expression, was different between the olfactory bulb and hippocampus. The effects of inhaled α-pinene might differ in time, depending on the brain region, and such a time lag should be clarified in the future. As mentioned above, behavioral analyses revealed that α-pinene had an excitatory-like effect. Therefore, we measured tyrosine hydroxylase (TH), a rate-limiting enzyme for the synthesis of catecholamines, gene expression in the midbrain. α-Pinene caused a significant augment in the TH expression level in the 60-min. α-pinene group (Figure6C). These results suggested that α-pinene might activate DA neurons, which Figure 6. Effects of α-pineneresults on gene in expression hyperactivities. levels in On the the brain. other After hand, inhalingα-pinene distilled did water not causeor α-pinene significant into stressed changes in mice, the olfactory bulb, hippocampus, and midbrain were used for RT-PCR analysis. (A): Expression of BDNF gene in the olfactory bulb. (B): Expression of BDNF gene in the hippocampus. (C): Expression of tyrosine hydroxylase (TH) gene in the midbrain. The results are expressed as the ratio of the mRNA expression level of BDNF or TH to that of GAPDH (%) and are means ± SEM (n = 5). * p < 0.05, ** p < 0.01 (Student’s t-test). This figure is modified based on the original figure [103] with the copyright permission.

Although the amount of α-pinene transferred to the brain after inhalation for 90 min. was lower than that inhaled for 60 min., anxiolytic activity was observed. These results suggest that α-pinene and its metabolites might stimulate the olfactory system. An excitatory effect was observed after 60-min. inhalation, and an anxiolytic effect was observed

Molecules 2021, 26, x FOR PEER REVIEW 17 of 28

For genes in the brain regions, the BDNF gene expression level was significantly increased in the olfactory bulb of the 90-min. α-pinene group and the hippocampus of the 60-minute α-pinene group (Figure 6A, B). Therefore, BDNF genes in the olfactory bulb and hippocampus might be involved in the anxiolytic effect of α-pinene. Nevertheless, the inhalation time of α-pinene, which caused a significant increase in BDNF gene expression, was different between the olfactory bulb and hippocampus. The effects of inhaled α- pinene might differ in time, depending on the brain region, and such a time lag should be clarified in the future. As mentioned above, behavioral analyses revealed that α-pinene had an excitatory-like effect. Therefore, we measured tyrosine hydroxylase (TH), a rate- limiting enzyme for the synthesis of catecholamines, gene expression in the midbrain. α- Pinene caused a significant augment in the TH expression level in the 60-min. α-pinene group (Figure 6C). These results suggested that α-pinene might activate DA neurons, which results in hyperactivities. On the other hand, α-pinene did not cause significant changes in the expression of TH gene in the medulla oblongata, suggesting that the NA neurons in this region were not significantly affected.

Molecules 2021, 26, 2571 19 of 29

Figure 5. Effects of α-pinene on mouse behavior. Stressed mice were inhaled with distilled water or α-pinene for 60 or 90 min., and then an elevated plus-maze test was performed. (A): Total distance traveled (m). (B): Visits to open arms (%). (C): Time spent in open armsthe (sec). expression The results of show TH gene means in ± the SEM medulla (n = 5). oblongata,* p < 0.05, ** suggestingp < 0.01, *** pthat < 0.001 the (Student’s NA neurons t- in test). This figure is modifiedthis based region on the were original not figure signiﬁcantly [103] with affected. the copyright permission.

Figure 6. 6. EffectsEffects of of αα-pinene-pinene on on gene gene expression expression levels levels in in th thee brain. brain. After After inhaling inhaling distilled distilled water water or or αα-pinene-pinene into into stressed stressed mice, the olfactory bulb,bulb, hippocampus,hippocampus, andand midbrain midbrain were were used used for for RT-PCR RT-PCR analysis. analysis. (A ():A Expression): Expression of BDNFof BDNF gene gene in the in theolfactory olfactory bulb. bulb. (B): ( ExpressionB): Expression of BDNF of BDNF gene gene in the in hippocampus. the hippocampus. (C): Expression(C): Expression of tyrosine of tyrosine hydroxylase hydroxylase (TH) gene(TH) ingene the inmidbrain. the midbrain. The results The results are expressed are expressed as the ratioas the of ratio the mRNAof the mRNA expression expression level of level BDNF of orBDNF TH to or that TH ofto GAPDHthat of GAPDH (%) and (%)are meansand are± meansSEM (n± SEM = 5). (n * p = <5). 0.05, * p < ** 0.05,p < 0.01** p < (Student’s 0.01 (Student’st-test). t-test). This figureThis figure is modified is modified based based on the on original the original figure figure [103] [103]with thewith copyright the copyright permission. permission.

AlthoughAlthough the the amount amount of of αα-pinene-pinene transferred transferred to to the the brain brain after after inhalation inhalation for for 90 90 min. min. waswas lower lower than that inhaled for 60 min., anxiolytic activity was observed. These results suggestsuggest that that αα-pinene-pinene and and its metabolites might stimulatestimulate thethe olfactoryolfactory system.system. AnAn excita-excit- atorytory effecteffect waswas observedobserved afterafter 60-min.60-min. inhalation,inhalation, and an anxiolytic effect was observed after 90-min. inhalation. Therefore, the biological effects of volatile components could be mediated by both olfactory stimulation and the transfer of aroma molecules to the brain.

4.5. Thyme Linalool Stress is thought to lead to anxiety and fatigue. Various mechanisms may be involved in fatigue, but brain fatigue has recently been of particular interest. Brain fatigue is caused by inflammation that occurs in the brain and affects the whole body. In the process of studying the effects of aroma on the brain, we have come to aim to scientifically prove the anti-fatigue effect of aromatherapy. We investigated the effect of thyme linalool on brain fatigue model animals [121]. There are a variety of types of thyme, with the most general one being common thyme (Thymus vulgaris), and one of the chemotypes is thyme linalool (Thymus vulgaris ct. linalool). Thyme linalool is an essential oil containing 51.2% linalool and 14.0% terpinen-4-ol. We studied the effect of thyme linalool (thyme). Mice received intraperitoneal (i.p.) administration of polyinosinic:polycytidylic acid (poly I:C) (20 mg/kg) as a brain fatigue model. Locomotor activities were measured using a rotating cage after the inhalation (i.h.) of thyme or distilled water with 2 µL/L air. The effect of thyme i.h on inflammatory cytokines was also analyzed. The locomotor activity was significantly increased in the thyme i.h. group as compared with the control (Poly (I:C), i.p./distilled water, i.h.) group (Figure7). These data suggested the anti-fatigue effect of thyme. The inhalation of thyme significantly reduced the mRNA expression level of interleukin-6, one of the inflammatory cytokines, in the hippocampus (Figure8A). The expression level of BDNF could have been reduced in the hippocampus of stressed animals and the blood of patients with depression [15]. However, BDNF mRNA expression levels in the hippocampus were significantly increased in the thyme i.h group than the control group in the present study (Figure8B). Molecules 2021, 26, x FOR PEER REVIEW 18 of 28

after 90-min. inhalation. Therefore, the biological effects of volatile components could be mediated by both olfactory stimulation and the transfer of aroma molecules to the brain.

4.5. Thyme Linalool Stress is thought to lead to anxiety and fatigue. Various mechanisms may be involved in fatigue, but brain fatigue has recently been of particular interest. Brain fatigue is caused by inflammation that occurs in the brain and affects the whole body. In the process of studying the effects of aroma on the brain, we have come to aim to scientifically prove the anti-fatigue effect of aromatherapy. We investigated the effect of thyme linalool on brain fatigue model animals [121]. There are a variety of types of thyme, with the most general one being common thyme (Thymus vulgaris), and one of the chemotypes is thyme linalool (Thymus vulgaris ct. linalool). Thyme linalool is an essential oil containing 51.2% linalool and 14.0% terpinen-4-ol. We studied the effect of thyme linalool (thyme). Mice received intraperitoneal (i.p.) administration of polyinosinic:polycytidylic acid (poly I:C) (20 mg/kg) as a brain fatigue model. Locomotor activities were measured using a rotating cage after the inhalation (i.h.) of thyme or distilled water with 2 μL/L air. The effect of thyme i.h on inflammatory cytokines was also analyzed. The locomotor activity was significantly increased in the thyme i.h. group as compared with the control (Poly (I:C), i.p./distilled water, i.h.) group (Figure 7). These data suggested the anti-fatigue effect of thyme. The inhalation of thyme significantly reduced the mRNA expression level of interleukin-6, one of the inflammatory cytokines, in the hippocampus (Figure 8A). The expression level of BDNF could have been reduced in the hippocampus of stressed animals and the blood of patients with depression [15]. How- Molecules 2021, 26, 2571 20 of 29 ever, BDNF mRNA expression levels in the hippocampus were significantly increased in the thyme i.h group than the control group in the present study (Figure 8B).

Figure 7.7. The effects of thyme on mouse activity. A brainbrain fatigue model was prepared by intraperi- tonealtoneal (i.p.)(i.p.) administrationadministration ofof polypoly I:CI:C (10(10 mg/kg).mg/kg). Saline was similarly administeredadministered toto thethe controlcontrol group. After inhalation (i.h.) (i.h.) of of thyme thyme (EOT) (EOT) (2 (2 mL mL/L/L air) air) oror distilled distilled water, water, mice mice were were examined examined by bythe the wheel wheel rotation rotation test test for for 15 15 h.h. The The results results are are expressed expressed as as the the rotation rotation speed speed rate (%) andand areare means ± SEM (n = 8). *** p < 0.001 (Tukey–Kramer test). This figure is modified based on the original Molecules 2021, 26, x FOR PEER REVIEWmeans ± SEM (n = 8). *** p < 0.001 (Tukey–Kramer test). This figure is modified based on the original19 of 28 figure [121] with the copyright permission. figure [121] with the copyright permission.

Figure 8. Effects ofof thymethyme on on gene gene expression expression levels levels in thein the brain. brain. Mice Mice received received i.p. injectioni.p. injection of Poly of I:CPoly (20 I:C mg/kg) (20 mg/kg) or saline. or saline.After i.h. After of thymei.h. of (EOT)thyme (2(EOT) mL/L (2 air)mL/L or air) distilled or distilled water, hippocampuswater, hippocampus was used was for used RT-PCR for RT-PCR analysis. analysis. (A): Interleukin-6 (A): Inter- leukin-6(IL-6) gene (IL-6) expression gene expression in the hippocampus. in the hippocampus. (B): BDNF ( geneB): BDNF expression gene expression in the hippocampus. in the hippocampus. The results areThe expressed results are as ex- the pressedratio of mRNAas the ratio expression of mRNA level expression of IL-6 or BDNFlevel of to IL-6 that or of GAPDHBDNF to (%),that andof GAPDH are means (%),± andSEM are (n =means 5–7). *±p SEM< 0.05, (n **= 5–7).p < 0.01, * p ***< 0.05,p < 0.001** p < (Tukey–Kramer 0.01, *** p < 0.001 test). (Tukey–Kramer This figure is modifiedtest). This based figure on is the modi originalfied based figure on [121 the] with original the copyright figure [121] permission. with the copyright permission. The anti-fatigue effects of linalool and (+)-terpinen-4-ol, which are the main com- ponentsThe ofanti-fatigue the essential effects oil of used linalool in this and study, (+)-terpinen-4-ol, were also examined. which are Thethe main inhalation compo- of nents(+)-terpinen-4-ol of the essential (2 µ L/Loil used air) causedin this study, a significant were also increase examined. in locomotor The inhalation activity of of (+)- the terpinen-4-olbrain fatigue (2 model μL/L miceair) caused in comparison a significant with increase the control in locomotor group (Figure activity9). The of the linalool- brain fatigueadministered model groupmice in showed comparison a tendency with the of increasecontrol group in locomotor (Figure activity,9). The linalool-admin- which was not isteredstatistically group significant. showed a Thetendency concentration of increase of linaloolin locomotor in the activity, brain was which more was than not 10 statis- times ticallythe (+)-terpinen-4-ol significant. The concentration. concentration These of linalool results in suggested the brain was that themore inhalation than 10 times of thyme the (+)-terpinen-4-olmight lead to anti-fatigue concentration. effects throughThese result anti-inflammatorys suggested that effects the and inhalation activating of neurons. thyme mightIn this lead study, to itanti-fatigue was unclear effects whether through the anti-fatigue anti-inflammatory effect of effects thyme and was activating due to linalool neurons.or (+)-terpinen-4-ol, In this study, it although was unclear it could whether not be the denied anti-fatigue that both effect of the of moleculesthyme was might due beto linaloolinvolved or in (+)-terpinen-4-ol, the anti-fatigue effect.although it could not be denied that both of the molecules might be involved in the anti-fatigue effect.

Figure 9. Effects of thyme components, linalool and (+)-terpinen-4-ol, on mouse behavior. A brain fatigue model mouse was prepared by i.p. injection of Poly I:C (20 mg/kg) or saline. After i.h. administration of (−)-linalool or (+)-terpinen-4-ol, mice were examined by the wheel rotation test for 15 h. The results are expressed as the rotation speed rate (%) and are means ± SEM (n = 4–8). Lin: (−)-Linalool (2 mL/L air), Ter: (+)-Terpinen-4-ol (2 mL/L air). ** p < 0.01, *** p < 0.001 (Tukey–Kramer test). This figure is modified based on the original figure [121] with the copyright permission.

Molecules 2021, 26, x FOR PEER REVIEW 19 of 28

Figure 8. Effects of thyme on gene expression levels in the brain. Mice received i.p. injection of Poly I:C (20 mg/kg) or saline. After i.h. of thyme (EOT) (2 mL/L air) or distilled water, hippocampus was used for RT-PCR analysis. (A): Inter- leukin-6 (IL-6) gene expression in the hippocampus. (B): BDNF gene expression in the hippocampus. The results are expressed as the ratio of mRNA expression level of IL-6 or BDNF to that of GAPDH (%), and are means ± SEM (n = 5–7). * p < 0.05, ** p < 0.01, *** p < 0.001 (Tukey–Kramer test). This figure is modified based on the original figure [121] with the copyright permission.

The anti-fatigue effects of linalool and (+)-terpinen-4-ol, which are the main components of the essential oil used in this study, were also examined. The inhalation of (+)- terpinen-4-ol (2 μL/L air) caused a significant increase in locomotor activity of the brain fatigue model mice in comparison with the control group (Figure 9). The linalool-administered group showed a tendency of increase in locomotor activity, which was not statistically significant. The concentration of linalool in the brain was more than 10 times the (+)-terpinen-4-ol concentration. These results suggested that the inhalation of thyme might lead to anti-fatigue effects through anti-inflammatory effects and activating neurons. In this study, it was unclear whether the anti-fatigue effect of thyme was due to Molecules 2021, 26, 2571 21 of 29 linalool or (+)-terpinen-4-ol, although it could not be denied that both of the molecules might be involved in the anti-fatigue effect.

FigureFigure 9. 9.EffectsEffects of of thyme thyme components, components, linalool linalool and and (+)- (+)-terpinen-4-ol,terpinen-4-ol, on on mouse mouse behavior. behavior. A Abrain brain fatiguefatigue model model mouse mouse was was prepared prepared by byi.p. i.p. injection injection of Poly of Poly I:C (20 I:C mg/kg) (20 mg/kg) or saline. or saline. After Afteri.h. ad- i.h. ministrationadministration of (− of)-linalool (−)-linalool or (+)-terpinen-4-ol, or (+)-terpinen-4-ol, mice mice were were examined examined by bythe the wheel wheel rotation rotation test test for for 15 h. The results are expressed as the rotation speed rate (%) and are means ± SEM (n = 4–8). Lin: 15 h. The results are expressed as the rotation speed rate (%) and are means ± SEM (n = 4–8). Lin: (−)-Linalool (2 mL/L air), Ter: (+)-Terpinen-4-ol (2 mL/L air). ** p < 0.01, *** p < 0.001 (Tukey–Kramer (−)-Linalool (2 mL/L air), Ter: (+)-Terpinen-4-ol (2 mL/L air). ** p < 0.01, *** p < 0.001 (Tukey–Kramer test). This figure is modified based on the original figure [121] with the copyright permission. test). This figure is modified based on the original figure [121] with the copyright permission.

Subsequently, we investigated the anxiolytic effect of thyme with the aim of clarifying the relationship between anxiety, fatigue, and stress [122]. Male ICR mice were bred alone and divided into three groups: no treatment and i.p. administration of saline or poly I:C. After inhaling thyme for 90 min., the anxiety-related behavior was assessed by an elevated plus-maze test. The inhalation of thyme did not cause anxiolytic effects in mice with no treatment (Figure 10A). On the other hand, thyme showed significant anxiolytic effects in brain fatigue model mice with poly I:C. Indeed, the rate of visits to open arms and time spent in open arms were significantly increased by i.h. of thyme in the brain fatigue model mice (Figure 10C). Thyme also caused a significant increase in the time that was spent in open arms in mice with i.p. administration of saline (Figure 10B).

4.6. Transfer of Fragrant Molecules to the Brain The biological effects of fragrant molecules can include not only the olfactory pathway, but also the blood flow due to nasal and transdermal absorption of volatile molecules. Monoterpenes are the main volatile component of plant essential oils. Olfactory receptors may recognize monoterpenes as smell and affect emotions. It is possible that the components of the essential oil act directly on the central nervous system as well as on the olfactory pathway. Terpenes have been known to cross the blood-brain barrier and have proven to interact with potential receptors, such as cannabinoid receptor type 1 [138]. However, the transferability of monoterpenes after inhalation, the main method of administration of essential oils in aromatherapy, has not been well clarified. The components of thyme that were transferred into the brain after thyme inhalation were analyzed by gas chromatography-mass spectrometry (GC/MS) [122]. Concentrations of linalool in the brain of the poly I:C group were significantly higher than those of mice inhaled with thyme alone (Figure 11A). An increment in the linalool concentration was also observed in mice that were treated with saline (Figure 11A). Terpinene-4-ol levels in the brain showed changes similar to those of linalool. A significant increase of terpinene- 4-ol level was found in the saline group inhaled with thyme 2 mL/L air. An increasing tendency was observed in the saline group inhaled with thyme 4 mL/L air, which was not statistically significant (Figure 11B). These results suggested that stress and inflammation might disrupt the blood-brain barrier and, as a result, the anxiolytic and anti-fatigue effects of thyme could be enhanced. Further analysis was required on the mechanism by which the amount of volatile component transported to the brain by mechanical stressors, such as i.p. administration. Molecules 2021, 26, x FOR PEER REVIEW 20 of 28

Subsequently, we investigated the anxiolytic effect of thyme with the aim of clarifying the relationship between anxiety, fatigue, and stress [122]. Male ICR mice were bred alone and divided into three groups: no treatment and i.p. administration of saline or poly I:C. After inhaling thyme for 90 min., the anxiety-related behavior was assessed by an elevated plus-maze test. The inhalation of thyme did not cause anxiolytic effects in mice with no treatment (Figure 10A). On the other hand, thyme showed significant anxiolytic effects in brain fatigue model mice with poly I:C. Indeed, the rate of visits to open arms Molecules 2021, 26, 2571 and time spent in open arms were significantly increased by i.h. of thyme in the 22brain of 29 fatigue model mice (Figure 10C). Thyme also caused a significant increase in the time that was spent in open arms in mice with i.p. administration of saline (Figure 10B).

FigureFigure 10. 10. TheThe effects effects of of thyme thyme on on mouse mouse behavior. behavior. ( (AA):): Elevated Elevated plus-maze plus-maze test test immediately immediately after after i.h.i.h. of of thyme thyme (EOT) (EOT) (2 (2 mL/L mL/L air) air) to to mice mice bred bred alone alone for for seven seven days. ( (BB):): Elevated Elevated plus-maze test immediatelyimmediately after after i.h. of of thyme thyme (EOT) (EOT) (2 mL/L air) air) 6 6 h h after after i.p. i.p. administration administration of of saline saline to to mice mice bred bred alone for seven days. (C): Elevated plus-maze test immediately after i.h. of thyme (EOT) (2 mL/L alone for seven days. (C): Elevated plus-maze test immediately after i.h. of thyme (EOT) (2 mL/L air) air) 6 h after i.p. administration of Poly I: C to mice bred alone for seven days. The results show 6 h after i.p. administration of Poly I: C to mice bred alone for seven days. The results show means ± means ± SEM (n = 5). * p < 0.05, ** p < 0.01, *** p < 0.001 (Student’s t-test). This figure is modified basedSEM (non = the 5). *originalp < 0.05, figure ** p < [122] 0.01, with *** p the< 0.001 copyright (Student’s permission.t-test). This figure is modified based on the original figure [122] with the copyright permission.

4.6. TransferThe authors of Fragrant also investigated Molecules to thethe Brain brain migration of (+)-α-pinene, (+)-limonene, (−)- linalool,The biological and 1,8-cineole, effects which of fragrant are the molecules major monoterpenes can include not of plantonly the essential olfactory oils path- [139]. way,After but inhalation also the of blood each compoundflow due to into nasal mice, and the transdermal brain was removed,absorption and of eachvolatile molecule molecules.was analyzed Monoterpenes by GC/MS. are the Asmain a result,volatile it co wasmponent clarified of plant that α essential-pinene oils. maximized Olfactory brain re- ceptorstransferability may recognize when inhaled monoterpenes for 30 min. as smell This an mightd affect be emotions. due to the It high is possible volatility that of theα- pinene. Limonene and linalool were maximally translocated to the brain after 90 min. of inhalation. The amount of linalool that was transferred to the brain was less than that of α-pinene and limonene. Brain transfer of 1,8-cineole was at its lowest level after 30 min. of inhalation. Furthermore, 1,8-cineole was easily transferred to the brain after i.p. administration. These results might be indicators for maximizing the effects of essential oils in aromatherapy. The transfer of volatile components of aroma to the brain needs to be clarified in the future. Under such circumstances, Aponso et al. [110] recently studied the identification of physicochemical properties that affect the volatility of essential oils and pathways of brain uptake by inhalation. The pharmacokinetics of volatile components are a major challenge in clarifying the effects of aroma on the brain. Molecules 2021, 26, x FOR PEER REVIEW 21 of 28

components of the essential oil act directly on the central nervous system as well as on the olfactory pathway. Terpenes have been known to cross the blood-brain barrier and have proven to interact with potential receptors, such as cannabinoid receptor type 1 [138]. However, the transferability of monoterpenes after inhalation, the main method of administration of essential oils in aromatherapy, has not been well clarified. The components of thyme that were transferred into the brain after thyme inhalation were analyzed by gas chromatography-mass spectrometry (GC/MS) [122]. Concentrations of linalool in the brain of the poly I:C group were significantly higher than those of mice inhaled with thyme alone (Figure 11A). An increment in the linalool concentration was also observed in mice that were treated with saline (Figure 11A). Terpinene-4-ol levels in the brain showed changes similar to those of linalool. A significant increase of terpinene- 4-ol level was found in the saline group inhaled with thyme 2 mL/L air. An increasing tendency was observed in the saline group inhaled with thyme 4 mL/L air, which was not statistically significant (Figure 11B). These results suggested that stress and inflammation might disrupt the blood-brain barrier and, as a result, the anxiolytic and anti-fatigue effects of thyme could be enhanced. Further analysis was required on the mechanism by Molecules 2021, 26, 2571 23 of 29 which the amount of volatile component transported to the brain by mechanical stressors, such as i.p. administration.

(A) Concentration of linalool in the brain ＊＊ 1.5 1.5 ＊＊

）＊＊）

1 1 μg/g tissueμg/g tissueμg/g （（ 0.5 0.5 Linalool Linalool 0 0 EOT2 （i.h.） Saline (i.p.) Poly I:C (i.p.) EOT4 （i.h.） Saline (i.p.) Poly I:C (i.p.) EOT2 （i.h.） EOT2 (i.h.) EOT4 （i.h.） EOT4 (i.h.)

(B) Concentration of terpinene-4-ol in the brain ）） 1.5 ＊ 1.5

0.4 0.4 μg/g tissue μg/g μg/g tissueμg/g 0.3 0.3 （（ 0.2 0.2

0.1 0.1

0 0 Terpinen-4-ol Terpinen-4-ol Terpinen-4-ol Terpinen-4-ol EOT2 （i.h.） Saline (i.p.) Poly I:C (i.p.) EOT4 （i.h.） Saline (i.p.) Poly I:C (i.p.) EOT2 （i.h.） EOT2 (i.h.) EOT4 （i.h.） EOT4 (i.h.)

Figure 11. TheThe concentrations concentrations of of linalool linalool and and terpinen-4 terpinen-4-ol-ol in in the the mouse mouse brain brain after after i.h. i.h. of ofthyme. thyme. Three groups of of mice mice loaded loaded with with weak weak stress stress by by breeding breeding alone: alone: thyme thyme (EOT) (EOT) (i.h.) (i.h.) group, group, EOT EOT (i.h.) after moderate stressstress byby i.p.i.p. injectioninjection ofof salinesaline or or Poly Poly I:C. I:C. ( A(A):): Linalool Linalool concentration concentration after after EOT EOT (2 or 4 mL/L air, i.h.), (B): Terpinen-4-ol concentration after EOT (2 or 4 mL/L air, i.h.). The (2 or 4 mL/L air, i.h.), (B): Terpinen-4-ol concentration after EOT (2 or 4 mL/L air, i.h.). The results results show means ± SEM (n = 4). * p < 0.05, ** p < 0.01 (Tukey–Kramer test). This figure is modi- show means ± SEM (n = 4). * p < 0.05, ** p < 0.01 (Tukey–Kramer test). This figure is modified based fied based on the original figure [122] with the copyright permission. on the original figure [122] with the copyright permission. The authors also investigated the brain migration of (+)-α-pinene, (+)-limonene, (−)- For investigations on the effects of smell, it cannot be avoided that volatile components linalool,may affect and organisms 1,8-cineole, not which only by are the the olfactory major monoterpenes pathway, but also of plant by other essential pathways, oils [139]. such Afteras nasal inhalation and transdermal of each compound absorption. into It should mice, alsothe brain be considered was removed, that even andstress-reducing each molecule wasfragrant analyzed molecules by GC can/MS. be As toxic a result, [140 ].it was For example,clarified that terpenes, α-pinene important maximized components brain trans- of ferabilityessential oils,when have inhaled been for suggested 30 min. This to have might hepatotoxicity. be due to the Because high volatility terpenes of haveα-pinene. been Limoneneshown to penetrateand linalool the were blood-brain maximally barrier, translocated it may be to possiblethe brain that after the 90 molecules min. of inhala- have tion.toxic The effects amount in the of brain. linalool Intensive that was research transferred on metabolism to the brain and was the less toxicity than of that terpenes of α-pi- is nenerequired and to limonene. reduce the Brain risk oftransfer injury of from 1,8-cineol terpene-containinge was at its lowest products, level including after 30 essential min. of oils [141].

5. Conclusions This review briefly described the stress response of the brain and provided information on the biochemical parameters of the brain. From such a point of view, we summarized the literature on the relationship between the smell and stress response in the brain. The effects of stress on the brain and the pathogenic mechanism of mental disorders are being elucidated. Research on stress markers have continued to progress and the understanding of brain function is further evolving. It is interesting that the effect of smell on stress response in the brain is becoming clarified. Most of the previous studies have shown that a certain aroma can suppresses stress levels, and neuropharmacological and neurochemical studies are providing scientific evidence of the stress-suppressing effects of the smell. On the other hand, the smell also has stress-enhancing effects. The investigation of the relationship between the smell and stress response in the brain requires multifaceted analysis in the future. From this point of view, it is necessary to devise ways to accurately control the dose of volatile components. Elucidating the mechanisms of stress suppression by the smell will deepen the comprehension of living organisms and enable the effective use of volatile components. Multifaceted research on volatile molecules may contribute not only to aromatherapy, but also to food development. Indeed, we recently demonstrated the effect of sesame oil on mouse behavior and the expression of stress marker candidates [123]. Molecules 2021, 26, 2571 24 of 29

In the future, it will be necessary to link chemical research at the level of molecules with neuropharmacology and neurochemistry.

Author Contributions: Original draft preparation, Y.M.; Review and Editing, Y.M., T.S., H.T., and K.K. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable to this article. Acknowledgments: Our research on the effects of aroma was started in 2007 as part of stress research at the National Institute of Advanced Industrial Science and Technology (AIST). The authors thank Etsuo Niki (University of Tokyo), Hitoshi Iwahashi (Gifu University), and Rakwal Randeep (University of Tsukuba) for their guidance and encouragement during the tenure of Y.M. at AIST. The authors would also like to express sincere gratitude to Masami Ishido (National Institute for Environmental Studies, a classmate of Y.M. in graduate school) for his great help in the joint research. The authors thank all the undergraduate and graduate students who studied in the Neuroscience Laboratory, Department of Biology, Faculty of Science, and the Department of Pharmacognosy, Faculty of Pharmaceutical Sciences, Toho University. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Mormède, P.; Andanson, S.; Aupérin, B.; Beerda, B.; Guémené, D.; Malmkvist, J.; Manteca, X.; Manteuffel, G.; Prunet, P.; van Reenen, C.G.; et al. Exploration of the hypothalamic–pituitary–adrenal function as a tool to evaluate animal welfare. Physiol. Behav. 2007, 92, 317–339. [CrossRef][PubMed] 2. Misiak, B.; Łoniewski, I.; Marlicz, W.; Frydecka, D.; Szulc, A.; Rudzki, L.; Samochowiec, J. The HPA axis dysregulation in severe mental illness: Can we shift the blame to gut microbiota? Prog. Neuro-Psychopharmacol. Biol. Psychiatry 2020, 102, 109951. [CrossRef][PubMed] 3. Niimura, Y.; Matsui, A.; Touhara, K. Acceleration of olfactory receptor gene loss in primate evolution: Possible link to anatomical change in sensory systems and dietary transition. Mol. Biol. Evol. 2018, 35, 1437–1450. [CrossRef] 4. Angelucci, F.L.; Silva, V.V.; Dal Pizzol, C.; Spir, L.G.; Praes, C.E.O.; Maibach, H. Physiological effect of olfactory stimuli inhalation in humans: An overview. Int. J. Cosmet. Sci. 2014, 36, 117–123. [CrossRef][PubMed] 5. Cortese, B.M.; Schumann, A.Y.; Howell, A.N.; McConnell, P.A.; Yang, Q.X.; Uhde, T.W. Preliminary evidence for differential olfactory and trigeminal processing in combat veterans with and without PTSD. NeuroImage Clin. 2018, 17, 378–387. [CrossRef] 6. Daniels, J.K.; Vermetten, E. Odor-induced recall of emotional memories in PTSD–Review and new paradigm for research. Exp. Neurol. 2016, 284, 168–180. [CrossRef][PubMed] 7. Herten, N.; Pomrehn, D.; Wolf, O.T. Memory for objects and startle responsivity in the immediate aftermath of exposure to the Trier Social Stress Test. Behav. Brain Res. 2017, 326, 272–280. [CrossRef][PubMed] 8. Hirasawa, Y.; Shirasu, M.; Okamoto, M.; Touhara, K. Subjective unpleasantness of malodors induces a stress response. Psychoneu- roendocrinology 2019, 106, 206–215. [CrossRef][PubMed] 9. Granqvist, P.; Vestbrant, K.; Döllinger, L.; Liuzza, M.T.; Olsson, M.J.; Blomkvist, A.; Lundström, J.N. The scent of security: Odor of romantic partner alters subjective discomfort and autonomic stress responses in an adult attachment-dependent manner. Physiol. Behav. 2019, 198, 144–150. [CrossRef][PubMed] 10. Sowndhararajan, K.; Kim, S. Influence of fragrances on human psychophysiological activity: With special reference to human electroencephalographic response. Sci. Pharm. 2016, 84, 724–751. [CrossRef][PubMed] 11. Selye, H. A Syndrome produced by Diverse Nocuous Agents. Nature 1936, 138, 32. [CrossRef] 12. Frankiensztajn, L.M.; Elliott, E.; Koren, O. The microbiota and the hypothalamus-pituitary-adrenocortical (HPA) axis, implications for anxiety and stress disorders. Curr. Opin. Neurobiol. 2020, 62, 76–82. [CrossRef][PubMed] 13. Homiack, D.; O’Cinneide, E.; Hajmurad, S.; Barrileaux, B.; Stanley, M.; Kreutz, M.R.; Schrader, L.A. Predator odor evokes sex- independent stress responses in male and female Wistar rats and reduces phosphorylation of cyclic-adenosine monophosphate response element binding protein in the male, but not the female hippocampus. Hippocampus 2017, 27, 1016–1029. [CrossRef] [PubMed] 14. Jovanovic, H.; Perski, A.; Berglund, H.; Savić,I. Chronic stress is linked to 5-HT1A receptor changes and functional disintegration of the limbic networks. NeuroImage 2011, 55, 1178–1188. [CrossRef] 15. Kreinin, A.; Lisson, S.; Nesher, E.; Schneider, J.; Bergman, J.; Farhat, K.; Farah, J.; Lejbkowicz, F.; Yadid, G.; Raskin, L.; et al. Blood BDNF level is gender specific in severe depression. PLoS ONE 2015, 10, e0127643. [CrossRef] Molecules 2021, 26, 2571 25 of 29

16. Butler, R.K.; Oliver, E.M.; Sharko, A.C.; Parilla-Carrero, J.; Kaigler, K.F.; Fadel, J.R.; Wilson, M.A. Activation of corticotropin releasing factor-containing neurons in the rat central amygdala and bed nucleus of the stria terminalis following exposure to two different anxiogenic stressors. Behav. Brain Res. 2016, 304, 92–101. [CrossRef] 17. Itoga, C.A.; Hellard, E.A.R.; Whitaker, A.M.; Lu, Y.-L.; Schreiber, A.L.; Baynes, B.B.; Baiamonte, B.A.; Richardson, H.N.; Gilpin, N.W. Traumatic stress promotes hyperalgesia via Corticotropin-Releasing Factor-1 Receptor (CRFR1) signaling in central amygdala. Neuropsychopharmacology 2016, 41, 2463–2472. [CrossRef][PubMed] 18. Janitzky, K.; Peine, A.; Kröber, A.; Yanagawa, Y.; Schwegler, H.; Roskoden, T. Increased CRF mRNA expression in the sexually dimorphic BNST of male but not female GAD67 mice and TMT predator odor stress effects upon spatial memory retrieval. Behav. Brain Res. 2014, 272, 141–149. [CrossRef][PubMed] 19. Prusator, D.K.; Meerveld, B.G.-V. Amygdala-mediated mechanisms regulate visceral hypersensitivity in adult females following early life stress: Importance of the glucocorticoid receptor and corticotropin-releasing factor. Pain 2017, 158, 296–305. [CrossRef] 20. Curtis, A.L.; Leiser, S.C.; Snyder, K.; Valentino, R.J. Predator stress engages corticotropin-releasing factor and opioid systems to alter the operating mode of locus coeruleus norepinephrine neurons. Neuropharmacology 2012, 62, 1737–1745. [CrossRef] 21. Butler, R.K.; White, L.C.; Frederick-Duus, D.; Kaigler, K.F.; Fadel, J.R.; Wilson, M.A. Comparison of the activation of somatostatin- and neuropeptide Y-containing neuronal populations of the rat amygdala following two different anxiogenic stressors. Exp. Neurol. 2012, 238, 52–63. [CrossRef] 22. Cohen, H.; Zohar, J.; Kaplan, Z.; Arnt, J. Adjunctive treatment with brexpiprazole and escitalopram reduces behavioral stress responses and increase hypothalamic NPY immunoreactivity in a rat model of PTSD-like symptoms. Eur. Neuropsychopharmacol. 2018, 28, 63–74. [CrossRef][PubMed] 23. Masuo, Y.; Ogura, A.; Kobayashi, M.; Masaki, T.; Furuta, Y.; Ono, T.; Takamatsu, K. Hippocalcin protects hippocampal neurons against excitotoxin damage by enhancing calcium extrusion. Neuroscience 2007, 145, 495–504. [CrossRef][PubMed] 24. Nater, U.; Rohleder, N. Salivary alpha-amylase as a non-invasive biomarker for the sympathetic nervous system: Current state of research. Psychoneuroendocrinology 2009, 34, 486–496. [CrossRef][PubMed] 25. Obayashi, K. Salivary mental stress proteins. Clin. Chim. Acta 2013, 425, 196–201. [CrossRef][PubMed] 26. Farah, R.; Haraty, H.; Salame, Z.; Fares, Y.; Ojcius, D.M.; Sadier, N.S. Salivary biomarkers for the diagnosis and monitoring of neurological diseases. Biomed. J. 2018, 41, 63–87. [CrossRef] 27. Giacomello, G.; Scholten, A.; Parr, M.K. Current methods for stress marker detection in saliva. J. Pharm. Biomed. Anal. 2020, 191, 113604. [CrossRef][PubMed] 28. Chojnowska, S.; Ptaszyńska-Sarosiek, I.; K˛epka,A.; Kna´s,M.; Waszkiewicz, N. Salivary biomarkers of stress, anxiety and depression. J. Clin. Med. 2021, 10, 517. [CrossRef][PubMed] 29. Hirano, M.; Rakwal, R.; Shibato, J.; Sawa, H.; Nagashima, K.; Ogawa, Y.; Yoshida, Y.; Iwahashi, H.; Niki, E.; Masuo, Y. Proteomics- and transcriptomics-based screening of differentially expressed proteins and genes in brain of Wig rat: A model for attention deficit hyperactivity disorder (ADHD) research. J. Proteome Res. 2008, 7, 2471–2489. [CrossRef] 30. Masuo, Y.; Ishido, M.; Morita, M.; Oka, S.; Niki, E. Motor activity and gene expression in rats with neonatal 6-hydroxydopamine lesions. J. Neurochem. 2004, 91, 9–19. [CrossRef] 31. Masuo, Y.; Ishido, M.; Morita, M.; Sawa, H.; Nagashima, K.; Niki, E. Behavioural characteristics and gene expression in the hyperactive wiggling (Wig) rat. Eur. J. Neurosci. 2007, 25, 3659–3666. [CrossRef] 32. Kobayashi, Y.; Kulikova, S.P.; Shibato, J.; Rakwal, R.; Satoh, H.; Pinault, D.; Masuo, Y. DNA microarray unravels rapid changes in transcriptome of MK-801 treated rat brain. World J. Biol. Chem. 2015, 6, 389–408. [CrossRef] 33. Masuo, Y.; Imai, T.; Shibato, J.; Hirano, M.; Jones, O.A.H.; Maguire, M.L.; Satoh, K.; Kikuchi, S.; Rakwal, R. Omic analyses unravels global molecular changes in the brain and liver of a rat model for chronic Sake (Japanese alcoholic beverage) intake. Electrophoresis 2009, 30, 1259–1275. [CrossRef] 34. Ishido, M.; Masuo, Y.; Sayato-Suzuki, J.; Oka, S.; Niki, E.; Morita, M. Dicyclohexylphthalate causes hyperactivity in the rat concomitantly with impairment of tyrosine hydroxylase immunoreactivity. J. Neurochem. 2004, 91, 69–76. [CrossRef][PubMed] 35. Masuo, Y.; Ishido, M.; Morita, M.; Oka, S. Effects of neonatal treatment with 6-hydroxydopamine and endocrine disruptors on motor activity and gene expression in rats. Neural Plast. 2004, 11, 59–76. [CrossRef][PubMed] 36. Masuo, Y.; Morita, M.; Oka, S.; Ishido, M. Motor hyperactivity caused by a deficit in dopaminergic neurons and the effects of endocrine disruptors: A study inspired by the physiological roles of PACAP in the brain. Regul. Pept. 2004, 123, 225–234. [CrossRef][PubMed] 37. Hirano, M.; Rakwal, R.; Kouyama, N.; Katayama, Y.; Hayashi, M.; Shibato, J.; Ogawa, Y.; Yoshida, Y.; Iwahashi, H.; Masuo, Y. Gel-based proteomics of unilateral irradiated striatum after gamma knife surgery. J. Proteome Res. 2007, 6, 2656–2668. [CrossRef] [PubMed] 38. Hirano, M.; Shibato, J.; Rakwal, R.; Kouyama, N.; Katayama, Y.; Hayashi, M.; Masuo, Y. Transcriptomic analysis of rat brain tissue following gamma knife surgery: Early and distinct bilateral effects in the un-irradiated striatum. Mol. Cells 2009, 27, 263–268. [CrossRef][PubMed] 39. Kandel, E.R.; Schwartz, J.H.; Jessell, T.M. (Eds.) Principles of Neural Science, 4th ed.; McGraw-Hill: New York, NY, USA, 2000. 40. Klein, B.; Bautze, V.; Maier, A.-M.; Deussing, J.; Breer, H.; Strotmann, J. Activation of the mouse odorant receptor 37 subsystem coincides with a reduction of novel environment-induced activity within the paraventricular nucleus of the hypothalamus. Eur. J. Neurosci. 2015, 41, 793–801. [CrossRef] Molecules 2021, 26, 2571 26 of 29

41. Gheusi, G.; Lledo, P.-M. Adult neurogenesis in the olfactory system shapes odor memory and perception. Prog. Brain Res. 2014, 208, 157–175. [CrossRef][PubMed] 42. Kadohisa, M. Effects of odor on emotion, with implications. Front. Syst. Neurosci. 2013, 7, 66. [CrossRef] 43. Krusemark, E.A.; Novak, L.R.; Gitelman, D.R.; Li, W. When the sense of smell meets emotion: Anxiety-state-dependent olfactory processing and neural circuitry adaptation. J. Neurosci. 2013, 33, 15324–15332. [CrossRef] 44. Drobyshevsky, A.; Yu, L.; Yang, Y.; Khalid, S.; Luo, K.; Jiang, R.; Ji, H.; Derrick, M.; Kay, L.; Silverman, R.B.; et al. Antenatal insults modify newborn olfactory function by nitric oxide produced from neuronal nitric oxide synthase. Exp. Neurol. 2012, 237, 427–434. [CrossRef][PubMed] 45. Landers, M.S.; Sullivan, R.M. The development and neurobiology of infant attachment and fear. Dev. Neurosci. 2012, 34, 101–114. [CrossRef] 46. Davies, D.A.; Molder, J.J.; Greba, Q.; Howland, J.G. Inactivation of medial prefrontal cortex or acute stress impairs odor span in rats. Learn. Mem. 2013, 20, 665–669. [CrossRef] 47. Albrechet-Souza, L.; Gilpin, N.W. The predator odor avoidance model of post-traumatic stress disorder in rats. Behav. Pharmacol. 2019, 30, 105–114. [CrossRef] 48. Kondoh, K.; Lu, Z.; Ye, X.; Olson, D.P.; Lowell, B.B.; Buck, L.B. A specific area of olfactory cortex involved in stress hormone responses to predator odours. Nature 2016, 532, 103–106. [CrossRef] 49. Vaz, R.P.; Cardoso, A.; Serrão, P.; Pereira, P.A.; Madeira, M.D. Chronic stress leads to long-lasting deficits in olfactory-guided behaviors, and to neuroplastic changes in the nucleus of the lateral olfactory tract. Horm. Behav. 2018, 98, 130–144. [CrossRef] [PubMed] 50. Gusmão, I.D.; Monteiro, B.M.; Cornélio, G.O.; Fonseca, C.S.; Moraes, M.F.; Pereira, G.S. Odor-enriched environment rescues long-term social memory, but does not improve olfaction in social isolated adult mice. Behav. Brain Res. 2012, 228, 440–446. [CrossRef][PubMed] 51. Horii, Y.; Nagai, K.; Nakashima, T. Order of exposure to pleasant and unpleasant odors affects autonomic nervous system response. Behav. Brain Res. 2013, 243, 109–117. [CrossRef] 52. Wintermann, G.-B.; Donix, M.; Joraschky, P.; Gerber, J.; Petrowski, K. Altered olfactory processing of stress-related body odors and artificial odors in patients with panic disorder. PLoS ONE 2013, 8, e74655. [CrossRef] 53. Belnoue, L.; Grosjean, N.; Ladevèze, E.; Abrous, D.N.; Koehl, M. Prenatal stress inhibits hippocampal neurogenesis but spares olfactory bulb neurogenesis. PLoS ONE 2013, 8, e72972. [CrossRef] 54. Debiec, J.; Sullivan, R.M. The neurobiology of safety and threat learning in infancy. Neurobiol. Learn. Mem. 2017, 143, 49–58. [CrossRef][PubMed] 55. Al Aïn, S.; Perry, R.E.; Nuñez, B.; Kayser, K.; Hochman, C.; Brehman, E.; LaComb, M.; Wilson, N.A.; Sullivan, R.M. Neurobe- havioral assessment of maternal odor in developing rat pups: Implications for social buffering. Soc. Neurosci. 2016, 12, 32–49. [CrossRef] 56. Whitaker, A.M.; Gilpin, N.W. Blunted hypothalamo-pituitary adrenal axis response to predator odor predicts high stress reactivity. Physiol. Behav. 2015, 147, 16–22. [CrossRef][PubMed] 57. Gu, C.; Wang, W.; Ding, X.; Yang, S.; Wang, A.; Yin, B.; Wei, W. Effects of maternal stress induced by predator odors during gestation on behavioral and physiological responses of offspring in Brandt’s vole (Lasiopodomys brandtii). Integr. Zool. 2018, 13, 723–734. [CrossRef] 58. Kenny, S.L.; Wright, L.D.; Green, A.D.; Mashoodh, R.; Perrot, T.S. Expression of maternal behavior and activation of the bed nucleus of the stria terminalis during predatory threat exposure: Modulatory effects of transport stress. Physiol. Behav. 2014, 123, 148–155. [CrossRef][PubMed] 59. Kigar, S.L.; Chang, L.; Guerrero, C.R.; Sehring, J.R.; Cuarenta, A.; Parker, L.L.; Bakshi, V.P.; Auger, A.P. N6-methyladenine is an epigenetic marker of mammalian early life stress. Sci. Rep. 2017, 7, 18078. [CrossRef] 60. Úbeda-Contreras, J.; Marín-Blasco, I.; Nadal, R.; Armario, A. Brain c-fos expression patterns induced by emotional stressors differing in nature and intensity. Brain Struct. Funct. 2018, 223, 2213–2227. [CrossRef] 61. Cherng, C.-F.G.; Chang, C.P.; Su, C.-C.; Tzeng, W.-Y.; Chuang, J.-Y.; Chen, L.-H.; Lin, K.-Y.; Yu, L. Odors from proximal species reverse the stress-decreased neurogenesis via main olfactory processing. Behav. Brain Res. 2012, 229, 106–112. [CrossRef] [PubMed] 62. Stockman, S.L.; McCarthy, M.M. Predator odor exposure of rat pups has opposite effects on play by juvenile males and females. Pharmacol. Biochem. Behav. 2017, 152, 20–29. [CrossRef] 63. Dremencov, E.; Lapshin, M.; Komelkova, M.; Alliluev, A.; Tseilikman, O.; Karpenko, M.; Pestereva, N.; Manukhina, E.; Downey, H.F.; Tseilikman, V. Chronic predator scent stress alters serotonin and dopamine levels in the rat thalamus and hypothalamus, respectively. Gen. Physiol. Biophys. 2019, 38, 187–190. [CrossRef] 64. Manukhina, E.B.; Tseilikman, V.E.; Karpenko, M.N.; Pestereva, N.S.; Tseilikman, O.B.; Komelkova, M.V.; Kondashevskaya, M.V.; Goryacheva, A.V.; Lapshin, M.S.; Platkovskii, P.O.; et al. Intermittent hypoxic conditioning alleviates post-traumatic stress disorder-induced damage and dysfunction of rat visceral organs and brain. Int. J. Mol. Sci. 2020, 21, 345. [CrossRef] 65. Hegab, I.M.; Shang, G.; Ye, M.; Jin, Y.; Wang, A.; Yin, B.; Yang, S.; Wei, W. Defensive responses of Brandt’s voles (Lasiopodomys brandtii) to chronic predatory stress. Physiol. Behav. 2014, 126, 1–7. [CrossRef][PubMed] Molecules 2021, 26, 2571 27 of 29

66. Dopfel, D.; Perez, P.D.; Verbitsky, A.; Bravo-Rivera, H.; Ma, Y.; Quirk, G.J.; Zhang, N. Individual variability in behavior and functional networks predicts vulnerability using an animal model of PTSD. Nat. Commun. 2019, 10, 2372. [CrossRef][PubMed] 67. Perez, S.M.; Lodge, D.J. Adolescent stress contributes to aberrant dopamine signaling in a heritable rodent model of susceptibility. Prog. Neuro-Psychopharmacol. Biol. Psychiatry 2019, 95, 109701. [CrossRef][PubMed] 68. Fujimoto, T.; Aou, S. Prenatal bisphenol A exposure is associated with medial amygdala neuron hyperresponsiveness to predator odor in rats. J. Toxicol. Sci. 2018, 43, 531–536. [CrossRef][PubMed] 69. Galliot, E.; Levaillant, M.; Beard, E.; Millot, J.-L.; Pourié, G. Enhancement of spatial learning by predator odor in mice: Involvement of amygdala and hippocampus. Neurobiol. Learn. Mem. 2010, 93, 196–202. [CrossRef] 70. Homiack, D.; O’Cinneide, E.; Hajmurad, S.; Dohanich, G.P.; Schrader, L.A. Effect of acute alarm odor exposure and biological sex on generalized avoidance and glutamatergic signaling in the hippocampus of Wistar rats. Stress 2018, 21, 292–303. [CrossRef] 71. Janitzky, K.; D’hanis, W.; Kröber, A.; Schwegler, H. TMT predator odor activated neural circuit in C57BL/6J mice indicates TMT-stress as a suitable model for uncontrollable intense stress. Brain Res. 2015, 1599, 1–8. [CrossRef] 72. Matsukawa, M.; Imada, M.; Murakami, T.; Aizawa, S.; Sato, T. Rose odor can innately counteract predator odor. Brain Res. 2011, 1381, 117–123. [CrossRef][PubMed] 73. Nikaido, Y.; Nakashima, T. Different patterns of neuronal activities in the infralimbic and prelimbic cortices and behavioral expression in response to two affective odors, 2,5-dihydro-2,4,5-trimethylthiazoline and a mixture of cis-3-hexenol and trans-2- hexenal, in the freely moving rat. Behav. Brain Res. 2011, 218, 218–227. [CrossRef][PubMed] 74. Wang, Y.; Cao, L.; Lee, C.-Y.; Matsuo, T.; Wu, K.; Asher, G.; Tang, L.; Saitoh, T.; Russell, J.; Klewe-Nebenius, D.; et al. Large-scale forward genetics screening identifies Trpa1 as a chemosensor for predator odor-evoked innate fear behaviors. Nat. Commun. 2018, 9, 2041. [CrossRef] 75. Miyazono, S.; Hasegawa, K.; Miyazaki, S.; Sakakima, H.; Konno, S.; Meguro, S.; Sasajima, H.; Noguchi, T.; Osada, K.; Kashi- wayanagi, M. Etizolam attenuates the reduction in cutaneous temperature induced in mice by exposure to synthetic predator odor. Eur. J. Pharmacol. 2018, 824, 157–162. [CrossRef] 76. Arakawa, H.; Arakawa, K.; Blandino, P., Jr.; Deak, T. The role of neuroinflammation in the release of aversive odor cues from footshock-stressed rats: Implications for the neural mechanism of alarm pheromone. Psychoneuroendocrinology 2011, 36, 557–568. [CrossRef][PubMed] 77. Inagaki, H.; Kiyokawa, Y.; Tamogami, S.; Watanabe, H.; Takeuchi, Y.; Mori, Y. Identification of a pheromone that increases anxiety in rats. Proc. Natl. Acad. Sci. USA 2014, 111, 18751–18756. [CrossRef][PubMed] 78. Konar, A.; Rastogi, M.; Bhambri, A. Brain region specific methylation and Sirt1 binding changes in MAOA promoter is associated with sexual dimorphism in early life stress induced aggressive behavior. Neurochem. Int. 2019, 129, 104510. [CrossRef] 79. Tzeng, W.-Y.; Chen, L.-H.; Cherng, C.G.; Tsai, Y.-N.; Yu, L. Sex differences and the modulating effects of gonadal hormones on basal and the stressor-decreased newly proliferative cells and neuroblasts in dentate gyrus. Psychoneuroendocrinology 2014, 42, 24–37. [CrossRef][PubMed] 80. Harris, A.Z.; Atsak, P.; Bretton, Z.H.; Holt, E.S.; Alam, R.; Morton, M.P.; Abbas, A.I.; Leonardo, E.D.; Bolkan, S.S.; Hen, R.; et al. A novel method for chronic social defeat stress in female mice. Neuropsychopharmacology 2018, 43, 1276–1283. [CrossRef][PubMed] 81. Yoto, A.; Fukui, N.; Kaneda, C.; Torita, S.; Goto, K.; Nanjo, F.; Yokogoshi, H. Black tea aroma inhibited increase of salivary chromogranin-A after arithmetic tasks. J. Physiol. Anthr. 2018, 37, 3. [CrossRef] 82. Cho, E.H.; Lee, M.-Y.; Hur, M.-H. The effects of aromatherapy on intensive care unit patients’ stress and sleep quality: A nonrandomised controlled trial. Evid.-Based Complement. Altern. Med. 2017, 2017, 2856592. [CrossRef][PubMed] 83. Ebrahimi, H.; Mardani, A.; Basirinezhad, M.H.; Hamidzadeh, A.; Eskandari, F. The effects of Lavender and Chamomile essential oil inhalation aromatherapy on depression, anxiety and stress in older community-dwelling people: A randomized controlled trial. Explore 2021.[CrossRef][PubMed] 84. Shimada, K.; Fukuda, S.; Maeda, K.; Kawasaki, T.; Kono, Y.; Jissho, S.; Taguchi, H.; Yoshiyama, M.; Yoshikawa, J. Aromatherapy alleviates endothelial dysfunction of medical staff after night-shift work: Preliminary observations. Hypertens. Res. 2010, 34, 264–267. [CrossRef][PubMed] 85. Toda, M.; Morimoto, K. Effect of lavender aroma on salivary endocrinological stress markers. Arch. Oral Biol. 2008, 53, 964–968. [CrossRef][PubMed] 86. Atsumi, T.; Tonosaki, K. Smelling lavender and rosemary increases free radical scavenging activity and decreases cortisol level in saliva. Psychiatry Res. 2007, 150, 89–96. [CrossRef] 87. Hur, M.-H.; Song, J.-A.; Lee, J.; Lee, M.S. Aromatherapy for stress reduction in healthy adults: A systematic review and meta-analysis of randomized clinical trials. Maturitas 2014, 79, 362–369. [CrossRef][PubMed] 88. Hritcu, L.; Cioanca, O.; Hancianu, M. Effects of lavender oil inhalation on improving scopolamine-induced spatial memory impairment in laboratory rats. Phytomedicine 2012, 19, 529–534. [CrossRef][PubMed] 89. Takahashi, M.; Satou, T.; Ohashi, M.; Hayashi, S.; Sadamoto, K.; Koike, K. Interspecies comparison of chemical composition and anxiolytic-like effects of lavender oils upon inhalation. Nat. Prod. Commun. 2011, 6, 1769–1774. [CrossRef][PubMed] 90. Takahashi, M.; Yoshino, A.; Yamanaka, A.; Asanuma, C.; Satou, T.; Hayashi, S.; Masuo, Y.; Sadamoto, K.; Koike, K. Effects of inhaled lavender essential oil on stress-loaded animals: Changes in anxiety-related behavior and expression levels of selected mRNAs and proteins. Nat. Prod. Commun. 2012, 7, 1539–1544. [CrossRef][PubMed] Molecules 2021, 26, 2571 28 of 29

91. Fukada, M.; Kano, E.; Miyoshi, M.; Komaki, R.; Watanabe, T. Effect of "rose essential oil" inhalation on stress-induced skin-barrier disruption in rats and humans. Chem. Senses 2011, 37, 347–356. [CrossRef][PubMed] 92. Boiangiu, R.S.; Brinza, I.; Hancianu, M.; Orhan, I.E.; Eren, G.; Gündüz, E.; Ertas, H.; Hritcu, L.; Cioanca, O. Cognitive facilitation and antioxidant effects of an essential oil mix on scopolamine-induced amnesia in rats: Molecular modeling of in vitro and in vivo approaches. Molecules 2020, 25, 1519. [CrossRef][PubMed] 93. Chang, K.-M.; Shen, C.-W. Aromatherapy benefits autonomic nervous system regulation for elementary school faculty in Taiwan. Evid.-Based Complement. Altern. Med. 2011, 2011, 946537. [CrossRef][PubMed] 94. Peng, S.-M.; Koo, M.; Yu, Z.-R. Effects of music and essential oil inhalation on cardiac autonomic balance in healthy individuals. J. Altern. Complement. Med. 2009, 15, 53–57. [CrossRef][PubMed] 95. Watanabe, E.; Kuchta, K.; Kimura, M.; Rauwald, H.W.; Kamei, T.; Imanishi, J. Effects of bergamot (Citrus bergamia (Risso) Wright & Arn.) essential oil aromatherapy on mood states, parasympathetic nervous system activity, and salivary cortisol levels in 41 healthy females. Forsch. Komplementmedizin 2015, 22, 43–49. [CrossRef] 96. Saiyudthong, S.; Marsden, C.A. Acute effects of bergamot oil on anxiety-related behaviour and corticosterone level in rats. Phytother. Res. 2010, 25, 858–862. [CrossRef][PubMed] 97. Kawai, E.; Takeda, R.; Ota, A.; Morita, E.; Imai, D.; Suzuki, Y.; Yokoyama, H.; Ueda, S.-Y.; Nakahara, H.; Miyamoto, T.; et al. Increase in diastolic blood pressure induced by fragrance inhalation of grapefruit essential oil is positively correlated with muscle sympathetic nerve activity. J. Physiol. Sci. 2020, 70, 2. [CrossRef] 98. Choi, S.Y.; Kang, P.; Lee, H.S.; Seol, G.H. Effects of inhalation of essential oil of Citrus aurantium L. var. amara on menopausal symptoms, stress, and estrogen in postmenopausal women: A randomized controlled trial. Evid.-Based Complement. Altern. Med. 2014, 2014, 796518. [CrossRef] 99. Li, Q.; Kobayashi, M.; Wakayama, Y.; Inagaki, H.; Katsumata, M.; Hirata, Y.; Hirata, K.; Shimizu, T.; Kawada, T.; Park, B.; et al. Effect of phytoncide from trees on human natural killer cell function. Int. J. Immunopathol. Pharmacol. 2009, 22, 951–959. [CrossRef] [PubMed] 100. Park, H.J.; Kim, S.K.; Kang, W.S.; Woo, J.-M.; Kim, J.W. Effects of essential oil from Chamaecyparis obtusa on cytokine genes in the hippocampus of maternal separation rats. Can. J. Physiol. Pharmacol. 2014, 92, 95–101. [CrossRef] 101. Kasuya, H.; Hata, E.; Satou, T.; Yoshikawa, M.; Hayashi, S.; Masuo, Y.; Koike, K. Effect on emotional behavior and stress by inhalation of the essential oil from Chamaecyparis obtusa. Nat. Prod. Commun. 2013, 8, 515–518. [CrossRef][PubMed] 102. Matsubara, E.; Ohira, T. Inhalation of Japanese cedar (Cryptomeria japonica) wood odor causes psychological relaxation after monotonous work among female participants. Biomed. Res. 2018, 39, 241–249. [CrossRef][PubMed] 103. Kasuya, H.; Okada, N.; Kubohara, M.; Satou, T.; Masuo, Y.; Koike, K. Expression of BDNF and TH mRNA in the brain following inhaled administration of α-pinene. Phytother. Res. 2015, 29, 43–47. [CrossRef][PubMed] 104. Kim, I.-H.; Kim, C.; Seong, K.; Hur, M.-H.; Lim, H.M.; Lee, M.S. Essential oil inhalation on blood pressure and salivary cortisol levels in prehypertensive and hypertensive subjects. Evid.-Based Complement. Altern. Med. 2012, 2012, 984203. [CrossRef] [PubMed] 105. Vieira, A.; Beserra, F.; Souza, M.; Totti, B.; Rozza, A. Limonene: Aroma of innovation in health and disease. Chem.-Biol. Interact. 2018, 283, 97–106. [CrossRef][PubMed] 106. Saito, N.; Yamano, E.; Ishii, A.; Tanaka, M.; Nakamura, J.; Watanabe, Y. Involvement of the olfactory system in the induction of anti-fatigue effects by odorants. PLoS ONE 2018, 13, e0195263. [CrossRef][PubMed] 107. Watanabe, T.; Fujihara, M.; Murakami, E.; Miyoshi, M.; Tanaka, Y.; Koba, S.; Tachibana, H. Green odor and depressive-like state in rats: Toward an evidence-based alternative medicine? Behav. Brain Res. 2011, 224, 290–296. [CrossRef] 108. Kunihiro, K.; Myoda, T.; Tajima, N.; Gotoh, K.; Kaneshima, T.; Someya, T.; Toeda, K.; Fujimori, T.; Nishizawa, M. Volatile components of the essential oil of Artemisia montana and their sedative effects. J. Oleo Sci. 2017, 66, 843–849. [CrossRef][PubMed] 109. Gulluni, N.; Re, T.; Loiacono, I.; Lanzo, G.; Gori, L.; Macchi, C.; Epifani, F.; Bragazzi, N.; Firenzuoli, F. Cannabis essential oil: A preliminary study for the evaluation of the brain effects. Evid.-Based Complement. Altern. Med. 2018, 2018, 1709182. [CrossRef] 110. Aponso, M.; Patti, A.; Bennett, L.E. Dose-related effects of inhaled essential oils on behavioural measures of anxiety and depression and biomarkers of oxidative stress. J. Ethnopharmacol. 2020, 250, 112469. [CrossRef][PubMed] 111. Seo, H.-S.; Hirano, M.; Shibato, J.; Rakwal, R.; Hwang, I.K.; Masuo, Y. Effects of coffee bean aroma on the rat brain stressed by sleep deprivation: A selected transcript- and 2D gel-based proteome analysis. J. Agric. Food Chem. 2008, 56, 4665–4673. [CrossRef] 112. El-Nekeety, A.A.; Mohamed, S.R.; Hathout, A.S.; Hassan, N.S.; Aly, S.E.; Abdel-Wahhab, M.A. Antioxidant properties of Thymus vulgaris oil against aflatoxin-induce oxidative stress in male rats. Toxicon 2011, 57, 984–991. [CrossRef] 113. Li, C.-C.; Yu, H.-F.; Chang, C.-H.; Liu, Y.-T.; Yao, H.-T. Effects of lemongrass oil and citral on hepatic drug-metabolizing enzymes, oxidative stress, and acetaminophen toxicity in rats. J. Food Drug Anal. 2018, 26, 432–438. [CrossRef] 114. Sadiki, F.Z.; El Idrissi, M.; Cioanca, O.; Trifan, A.; Hancianu, M.; Hritcu, L.; Postu, P.A. Tetraclinis articulata essential oil mitigates cognitive deficits and brain oxidative stress in an Alzheimer’s disease amyloidosis model. Phytomedicine 2019, 56, 57–63. [CrossRef] 115. Hashikawa-Hobara, N.; Otsuka, A.; Ishikawa, R.; Hashikawa, N. Roman chamomile inhalation combined with clomipramine treatment improves treatment-resistant depression-like behavior in mice. Biomed. Pharmacother. 2019, 118, 109263. [CrossRef] 116. Komiya, M.; Takeuchi, T.; Harada, E. Lemon oil vapor causes an anti-stress effect via modulating the 5-HT and DA activities in mice. Behav. Brain Res. 2006, 172, 240–249. [CrossRef] Molecules 2021, 26, 2571 29 of 29

117. Murakami, T.; Matsukawa, M.; Katsuyama, N.; Imada, M.; Aizawa, S.; Sato, T. Stress-related activities induced by predator odor may become indistinguishable by hinokitiol odor. NeuroReport 2012, 23, 1071–1076. [CrossRef][PubMed] 118. Zhang, L.-L.; Yang, Z.-Y.; Fan, G.; Ren, J.-N.; Yin, K.-J.; Pan, S.-Y. Antidepressant-like effect of Citrus sinensis (L.) Osbeck essential oil and its main component limonene on mice. J. Agric. Food Chem. 2019, 67, 13817–13828. [CrossRef][PubMed] 119. Tabuchi, E.; Ono, T.; Uwano, T.; Takashima, Y.; Kawasaki, M. Rat preference for food-related odors. Brain Res. Bull. 1991, 27, 387–391. [CrossRef] 120. Yoshida, K.; Yamamoto, N.; Fujiwara, S.; Kamei, A.; Abe, K.; Nakamura, A. Inhalation of a racemic mixture (R,S)-linalool by rats experiencing restraint stress alters neuropeptide and MHC class I gene expression in the hypothalamus. Neurosci. Lett. 2017, 653, 314–319. [CrossRef] 121. Hayakawa, M.; Satou, T.; Koike, K.; Masuo, Y. Anti-fatigue activity of essential oil from thyme (linalool chemotype) in the polyriboinosinic:polyribocytidylic acid-induced brain fatigue mouse. Flavour Fragr. J. 2016, 31, 395–399. [CrossRef] 122. Satou, T.; Hayakawa, M.; Goto, Y.; Masuo, Y.; Koike, K. Anxiolytic-like effects of essential oil from Thymus vulgaris was increased during stress. Flavour Fragr. J. 2018, 33, 191–195. [CrossRef] 123. Takemoto, H.; Take, C.; Kojima, K.; Kuga, Y.; Hamada, T.; Yasugi, T.; Kato, N.; Koike, K.; Masuo, Y. Effects of sesame oil aroma on mice after exposure to water immersion stress: Analysis of behavior and gene expression in the brain. Molecules 2020, 25, 5915. [CrossRef][PubMed] 124. Schneider, R. Essential oil inhaler (AromaStick®) improves heat tolerance in the Hot Immersion Test (HIT). Results from two randomized, controlled experiments. J. Therm. Biol. 2020, 87, 102478. [CrossRef][PubMed] 125. Saiyudthong, S.; Pongmayteegul, S.; Marsden, C.A.; Phansuwan-Pujito, P. Anxiety-like behaviour andc-fosexpression in rats that inhaled vetiver essential oil. Nat. Prod. Res. 2015, 29, 2141–2144. [CrossRef][PubMed] 126. Takahashi, Y.; Kiyokawa, Y.; Kodama, Y.; Arata, S.; Takeuchi, Y.; Mori, Y. Olfactory signals mediate social buffering of conditioned fear responses in male rats. Behav. Brain Res. 2013, 240, 46–51. [CrossRef][PubMed] 127. Kiyokawa, Y.; Honda, A.; Takeuchi, Y.; Mori, Y. A familiar conspecific is more effective than an unfamiliar conspecific for social buffering of conditioned fear responses in male rats. Behav. Brain Res. 2014, 267, 189–193. [CrossRef][PubMed] 128. Ueno, H.; Shimada, A.; Suemitsu, S.; Murakami, S.; Kitamura, N.; Wani, K.; Matsumoto, Y.; Okamoto, M.; Ishihara, T. Anti- depressive-like effect of 2-phenylethanol inhalation in mice. Biomed. Pharmacother. 2019, 111, 1499–1506. [CrossRef][PubMed] 129. Satou, T.; Kasuya, H.; Maeda, K.; Koike, K. Daily inhalation of α-pinene in mice: Effects on behavior and organ accumulation. Phytother. Res. 2014, 28, 1284–1287. [CrossRef][PubMed] 130. Okuno, H. Regulation and function of immediate-early genes in the brain: Beyond neuronal activity markers. Neurosci. Res. 2011, 69, 175–186. [CrossRef] 131. Morgan, J.; Cohen, D.; Hempstead, J.; Curran, T. Mapping patterns of c-fos expression in the central nervous system after seizure. Science 1987, 237, 192–197. [CrossRef][PubMed] 132. Guzowski, J.F.; McNaughton, B.L.; Barnes, C.A.; Worley, P.F. Environment-specific expression of the immediate-early gene Arc in hippocampal neuronal ensembles. Nat. Neurosci. 1999, 2, 1120–1124. [CrossRef] 133. Ardi, Z.; Albrecht, A.; Richter-Levin, A.; Saha, R.; Richter-Levin, G. Behavioral profiling as a translational approach in an animal model of posttraumatic stress disorder. Neurobiol. Dis. 2016, 88, 139–147. [CrossRef][PubMed] 134. Aykaç, A.; Aydın, B.; Cabadak, H.; Gören, M.Z. The change in muscarinic receptor subtypes in different brain regions of rats treated with fluoxetine or propranolol in a model of post-traumatic stress disorder. Behav. Brain Res. 2012, 232, 124–129. [CrossRef] [PubMed] 135. Ozbeyli, D.; Aykac, A.; Alaca, N.; Hazar-Yavuz, A.N.; Ozkan, N.; Sener, G. Protective effects of vortioxetine in predator scent stress model of post-traumatic stress disorder in rats: Role on neuroplasticity and apoptosis. J. Physiol. Pharmacol. 2019, 70. [CrossRef] 136. Kawachi, I.; Willett, W.C.; Colditz, G.A.; Stampfer, M.J.; Speizer, F.E. A prospective study of coffee drinking and suicide in women. Arch. Intern. Med. 1996, 156, 521–525. [CrossRef][PubMed] 137. Yamato, T.; Yamasaki, S.; Misumi, Y.; Kino, M.; Obata, T.; Aomine, M. Modulation of the stress response by coffee: An in vivo microdialysis study of hippocampal serotonin and dopamine levels in rat. Neurosci. Lett. 2002, 332, 87–90. [CrossRef] 138. Stasiłowicz, A.; Tomala, A.; Podolak, I.; Cielecka-Piontek, J. Cannabis sativa L. as a natural drug meeting the criteria of a multitarget approach to treatment. Int. J. Mol. Sci. 2021, 22, 778. [CrossRef][PubMed] 139. Satou, T.; Hayakawa, M.; Kasuya, H.; Masuo, Y.; Koike, K. Mouse brain concentrations of α-pinene, limonene, linalool, and 1,8-cineole following inhalation. Flavour Fragr. J. 2017, 32, 36–39. [CrossRef] 140. Ude, C.; Schubert-Zsilavecz, M.; Wurglics, M. Ginkgo biloba extracts: A review of the pharmacokinetics of the active ingredients. Clin. Pharmacokinet. 2013, 52, 727–749. [CrossRef] 141. Zárybnický, T.; Boušová, I.; Ambrož, M.; Skálová, L. Hepatotoxicity of monoterpenes and sesquiterpenes. Arch. Toxicol. 2018, 92, 1–13. [CrossRef][PubMed]