Orexins, Sleep, and Blood Pressure
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Current Hypertension Reports (2018) 20: 79 https://doi.org/10.1007/s11906-018-0879-6 SLEEP AND HYPERTENSION (SJ THOMAS, SECTION EDITOR) Orexins, Sleep, and Blood Pressure Mariusz Sieminski1 & Jacek Szypenbejl1 & Eemil Partinen2,3 Published online: 10 July 2018 # The Author(s) 2018 Abstract Purpose of Review The aim of this review was to summarize collected data on the role of orexin and orexin neurons in the control of sleep and blood pressure. Recent Findings Although orexins (hypocretins) have been known for only 20 years, an impressive amount of data is now available regarding their physiological role. Hypothalamic orexin neurons are responsible for the control of food intake and energy expenditure, motivation, circadian rhythm of sleep and wake, memory, cognitive functions, and the cardiovascular system. Multiple studies show that orexinergic stimulation results in increased blood pressure and heart rate and that this effect may be efficiently attenuated by orexinergic antagonism. Increased activity of orexinergic neurons is also observed in animal models of hypertension. Summary Pharmacological intervention in the orexinergic system is now one of the therapeutic possibilities in insomnia. Although the role of orexin in the control of blood pressure is well described, we are still lacking clinical evidence that this is a possibility for a new approach in the treatment of cardiovascular diseases. Keywords Orexin . Hypocretin . Blood pressure . Sleep . Narcolepsy . Autonomic nervous system Introduction cardiovascular system. With such a variety of functions, orexins appear to be a promising target for therapeutic in- We are celebrating the twentieth anniversary of the discov- terventions aimed at solving the most pivotal health prob- ery of hypocretins/orexins. Those two decades appeared to lems of our civilization: psychiatric disorders, cognitive be extremely fruitful: We identified a new neuronal system dysfunctions, addictions, insomnia, and diseases of the car- within the central nervous system with projections to mul- diovascular system. So far, we were able to introduce new tiple areas of the brain and discovered its involvement in treatment options for insomnia based on drugs that block almost all crucial life processes. It was almost immediately the orexin system. This review presents the most recent, discovered that orexins are responsible for the regulation significant discoveries related to the role of orexins in the of appetite; next, it became clear that they steer also the control of sleep and blood pressure. circadian rhythm of the sleep-wake cycle. Later, new facts emerged, showing their role in energy expenditure, loco- motor activity, motivation and addiction, cognition, emo- Orexins Uncovered: the Peptides, tional control, and, last but not least, control of the the Receptors, the Anatomy It was in 1998, when simultaneously, independently, and with This article is part of the Topical Collection on Sleep and Hypertension different purposes, the work of two groups of scientists led to the discovery of two novel neurotransmitters produced in the * Mariusz Sieminski lateral part of the hypothalamus. De Lecea et al. decided to [email protected] name the peptides hypocretin-1 and hypocretin-2 to indicate the hypothalamus as the locus of their origin and to signify that 1 Department of Emergency Medicine, Medical University of Gdansk, they belong to the family of incretins [1]. Sakurai and col- Smoluchowskiego 17, 80-235 Gdansk, Poland leagues while seeking for a natural ligand for G-protein- 2 Department of Neurology, University of Helsinki, Helsinki, Finland coupled receptors described earlier also discovered two pep- 3 Vitalmed Helsinki Sleep Clinic, Helsinki, Finland tides produced in the lateral hypothalamus. As it was quickly 79 Page 2 of 8 Curr Hypertens Rep (2018) 20: 79 observed that those peptides injected directly into the brains of orexinergic stimulation, e.g., as a result from the injection of rats led to an increase in the animals’ appetite, they were called orexin into the ventral tegmental area, leads to increased food orexin-A and orexin-B (from the Greek word orexis,meaning intake in laboratory animals. This observation is further sup- appetite) [2]. ported by the finding of the same group that the specific Orexin-A is composed of 33 amino acids, while orexin-B is OX1R antagonist SB334867 blocks the food intake [20]. a peptide that contains 28 amino acids. Both peptides are González et al. observed that activity of orexinergic neurons products of a proteolytic process using the common precursor is suppressed by the act of eating, i.e., the activity of these peptide prepro-orexin. A gene localized on the chromosome neurons is reduced during feeding regardless of the kind of 17 encodes this precursor peptide [3]. Expression of the food consumed by the laboratory animals, and complete lack prepro-orexin gene is dependent on numerous physiological of orexin signaling, e.g., in orexin knockout mice, leads to factors like fasting status or sleep duration [4, 5]. massive overeating and obesity [21]. These findings suggest Orexins interact with two G-protein-coupled receptors that orexins play a dual role in the process of food intake: they (GPCRs): OX1R and OX2R. Orexin-A has a similar affinity induce eating behavior and simultaneously regulate the to both receptors while orexin-B preferentially binds to OX2R amount of food intake. This role of orexins in the metabolic receptors. Binding of orexin to any of those receptors evokes balance of organisms is additionally reinforced by their influ- an excitatory action, most frequently leading to a depolariza- ence on the level of physical activity. Kosse et al. recently tion of the postsynaptic neuron. The electrical effect of the discovered that activation of orexinergic neurons stimulates orexin receptor activation is a consequence of the inhibition a population of GAD65-positive nerve cells (neurons contain- of potassium channels, the opening of non-selective cation ing glutamic acid decarboxylase 65) in hypothalamus, which channels, and the influx of calcium ions [6, 7, 8••]. in turn are responsible for increasing the mobility of laborato- Orexins are exclusively produced by a population of no ry animals [22]. This may suggest that orexins stimulate en- more than 80,000 hypothalamic neurons located in the lateral ergy expenditure through increased mobility. Coborn and col- and dorsomedial parts of the hypothalamus as well as in the leagues who performed microinjections of dual orexin recep- perifornical area [9]. Apart from orexins, those neurons also tor antagonist into the ventrolateral preoptic area of rats and secrete dynorphin [10], glutamate [11], and neuronal activity- observed a significant decrease in orexin-induced spontaneous regulated pentraxin (NARP) [12]. Orexin neurons receive nu- physical activity as well as lower energy expenditure during merous efferents from cortical and limbic areas (e.g., prefron- sleep recently confirmed this hypothesis. In the same series of tal cortex, bed nucleus of stria terminals, or amygdala) as well experiments, injected orexin-A led to a shortened sleep and an as from brainstem regions (e.g., dorsal raphe nucleus, rostral increased energy expenditure [23]. It seems that the metabolic ventrolateral medulla) [13–15]. The afferent projections of role of orexin may be simply described by the sentence “eat as orexinergic neurons reach the ventral tegmental area, the nu- much as possible and burn it as quickly as you can”. cleus accumbens, the basal forebrain, the locus coeruleus, the The most significant clinical role of orexin observed so far dorsal raphe nuclei, the amygdala, and the reticular formation is the regulation of the diurnal sleep-wake rhythm. Deficiency [16•, 17, 18]. As it may be concluded from this diversity of of orexins leads to narcolepsy type 1, a disease manifesting in brain areas interconnected with orexinergic neurons, they increased daytime somnolence, involuntary naps, disorga- must play a role in many various processes. nized sleep architecture, increased presence and shortened la- tency of REM sleep, and cataplexy, a sudden loss of muscle tone. In humans, narcolepsy is caused by an extremely re- Mighty Transmitter—the Roles of Orexins duced number of hypothalamic orexin neurons and deficiency of circulating orexin. These observations imply that orexins Immediately after its discovery, it became apparent that cen- have stimulating, wake-promoting actions. This was recently trally injected orexin provokes food-seeking behavior in lab- confirmed by findings of Vassalli and Franken who showed oratory animals [2]. This observation was confirmed in further electrophysiological evidence that orexins are necessary to studies. Van Holst et al. demonstrated recently that orexin- sustain active, spontaneous wakefulness (during which an an- deficient humans (patients with narcolepsy type I) more fre- imal undertakes explorative or goal-oriented behaviors) [24]. quently reach for salty or sweet snacks compared to controls The role of orexin in narcolepsy is well described and re- and to patients with idiopathic hypersomnia. It was especially cently published papers show that they are also important interesting that these patients consumed snacks even after sa- factors in insomnia. Tang et al. found that orexin-A plasma tiety and after being educated about which snacks should be levels were higher in patients with insomnia and correlated avoided. These findings suggest that orexin regulates the