Review Current Understanding of the Hypothalamic Ghrelin Pathways Inducing Appetite and Adiposity Omar Al Massadi,1,2,* Miguel López,1,2 Matthias Tschöp,3,4 Carlos Diéguez,1,2 and Ruben Nogueiras1,2,* Ghrelin is a multifaceted regulator of metabolism. Ghrelin regulates energy Trends balance in the short term via induction of appetite and in the long term via The lack of ghrelin in adulthood has no increased body weight and adiposity. Recently, several central pathways effect on feeding or body weight. modulating the metabolic actions of ghrelin were unmasked, and it was shown Ghrelin- or ghrelin[32_TD$IF] receptor-deficient to act through different hypothalamic nuclei to induce feeding. Ghrelin also mice fed a high-fat diet after weaning and neuronal deletion of ghrelin recep- modulates glucose homeostasis, but the central mechanisms responsible for tor are diet-induced resistant. Ghrelin this action have not been studied in detail. Although ghrelin also acts through inhibition before weaning caused increased adiposity and feeding. extrahypothalamic areas to promote feeding, this review specifically dissects hypothalamic control of ghrelin’s orexigenic and adipogenic actions and Energy sensors controlling neuronal presents current understanding of the intracellular ghrelin orexigenic pathways, function and plasticity are located in the hypothalamus and ghrelin acts including their dependence on other relevant systems implicated in energy through these energy sensors to mod- balance. ulate feeding. Ghrelin: Precursor, Products, and Metabolic Actions The orexigenic but not the adipogenic action of ghrelin is impaired in obese The hypothalamus, a central structure composed of anatomically distinct nuclei interconnected animals. via axonal projections, integrates central and peripheral information to regulate energy balance [1]. Among these nuclei, the arcuate (ARC), ventromedial (VMH), dorsomedial (DMH), para- Mutations in the ghrelin receptor that ventricular (PVH), and the lateral hypothalamic area (LHA) regions play major roles in the prevented its binding to beta-arrestin did not influence ghrelin orexigenic modulation of energy metabolism. action but increased its effects on adiposity and insulin resistance. Ghrelin was discovered as the endogenous ligand of the growth hormone (GH) secretagogue ’ receptor 1a (thereafter called ghrelin receptor) [2]. Ghrelin is primarily produced in the Ghrelin s actions on energy and glu- cose homeostasis are of clinical rele- fi – stomach, and rst described as a potent inducer of GH secretion [2 4].However,soon vance: ghrelin agonists show beneficial after its discovery, its ability to induce food intake and adiposity was reported [5,6].The effects in patients with cancer ghrelin gene generates a peptide of 117 amino acids called preproghrelin, which encodes cachexia; and an agonist of des-acyl different subproducts, the most abundant being the acylated ghrelin (AG) and des-acyl ghrelin ghrelin improves insulin sensitivity in humans. (DAG) forms of the peptide, both composed of 28 amino acids [2]. The AG form represents 10% of the total amount of ghrelin and is acylated with an octanoic acid in the Ser3, a process that is mediated by the ghrelin O-acyl transferase [7,8]. Ghrelin modulates feeding behavior, adiposity, and glucose homeostasis through the ghrelin receptor, which is abundantly 1Department of Physiology, School of expressed in the hypothalamus and in extrahypothalamic areas. This review focuses on Medicine-CiMUS, Instituto de the actions of ghrelin in the hypothalamus and the resultant effects on appetite, adiposity, Investigación Sanitaria (IDIS), and glucose metabolism. We will discuss the relevance of hypothalamic ghrelin receptor, University of Santiago de Compostela, Av de Barcelona s/n Santiago de the physiological relevance of the ghrelin system at the central level, and the factors Compostela (A Coruña), 15782, constituting the different hypothalamic ghrelin signaling pathways that control feeding, such Spain Trends in Neurosciences, March 2017, Vol. 40, No. 3 http://dx.doi.org/10.1016/j.tins.2016.12.003 167 © 2016 Elsevier Ltd. All rights reserved. 2 as neuropeptides in the ARC, energy and[3_TD$IF] nutrient sensors, fatty acid metabolism, neuro- CIBER Fisiopatología de la Obesidad y Nutrición (CIBERobn), Spain[30_TD$IF] peptides in the LHA, and its interaction with the dopamine, opioid, cannabinoid, and 3Helmholtz Diabetes Center, serotonin systems.[34_TD$IF] Finally, the main feeding-independent actions of central ghrelin system Helmholtz Zentrum München and on adiposity and glucose metabolism will be discussed. German Center for Diabetes Research (DZD), Neuherberg, Germany 4Division of Metabolic Diseases, Ghrelin Receptor Forms Dimers and Heterodimers with Other Technische Universität München, G-Protein-Coupled Receptors to Regulate Feeding Munich, Germany The ghrelin receptor gene generates two isoforms, the functional ghrelin receptor (ghrelin receptor subtype 1a) and another truncated isoform termed ghrelin receptor subtype 1b. The *Correspondence: ghrelin receptor is a G-protein-coupled receptor (GPCR) that can form heterodimers with other [email protected] key components of body weight regulation, for example, with melanocortin receptor 3 (MC3 (O. Al Massadi) and [email protected] receptor). Both receptors are co-localized in ARC neurons [9] and the interaction stimulates (R. Nogueiras). MC3 receptor signaling while simultaneously inhibiting ghrelin receptor signaling [9]. Ghrelin receptor also interacts with GPR83, a rhodopsin-like or family A orphan GPCR [10]. GPR83 co-localizes with ghrelin receptor in the ARC, and its hetero-dimerization represses the activity of ghrelin receptor [10]. Thus, ghrelin-induced food intake and adiposity are potentiated in GPR83-null animals [10]. Ghrelin receptor also heterodimerizes with dopamine receptor 2 (D2 receptor), another GPCR. These two receptors are co-expressed in subsets of neurons in the hypothalamus as well as in the striatum and in the hippocampus [11]. In opposition to wild-type (WT) mice, the treatment of ghrelin receptor-null mice with a D2 receptor agonist does not induce anorexia, and a ghrelin antagonist inhibits D2 receptor agonist signaling in vivo and in vitro, indicating that these two receptors interact by allosteric mechanisms [11]. Interestingly, a similar interaction was also proposed between ghrelin receptor and dopamine receptor 1 (D1 receptor) in extrahypotha- lamic areas [12]. Finally, ghrelin receptor can form heterodimers with serotonin receptor 2c (5-HT2c receptor) [13,14]. These two receptors are co-localized in primary hypothalamic and hippocampal rat 2+ neurons [14] and 5-HT2c receptor attenuates ghrelin signaling by reducing Ca [329_TD$IF]release (see the text in the following section describing ghrelin and the serotonin system)[35_TD$IF] [14]. Lessons from Animal Models: The Physiological Relevance of Central Ghrelin on Energy Balance and Glucose Metabolism The endogenous role of the ghrelin system has been studied comprehensively (reviewed in [15,16]). The adult-onset ablation of ghrelin-producing cells has no effect on feeding or body weight [17]. However, ghrelin- or ghrelin receptor-deficient mice given early exposure to a high-fat diet showed reduced weight and adiposity, features explained by a decrease in fuel efficiency and an increase in fat oxidation [18,19]. This high[36_TD$IF] fat diet-resistance is mediated by the central nervous system (CNS), as neuronal deletion of ghrelin receptor almost completely prevented diet-induced obesity by enhancing thermogenesis in brown adipose tissue and the induction of browning in subcutaneous adipose tissue, thereby stimulating[37_TD$IF] energy expenditure but not affecting[38_TD$IF] energy intake [20]. Ghrelin-deficient mice fed with a high-fat diet also showed improved glucose disposal and insulin sensitivity compared to controls [18] and the double lack of ghrelin and leptin enhanced glucose tolerance and insulin sensitivity [21]. These effects are likely[39_TD$IF] regulated by the CNS, since deletion of ghrelin receptor exclusively in the neurons yielded the same improvement in insulin sensitivity as observed in whole-body-deficient mice [20]. More specifically, neuropep- tide Y/agouti-related peptide (NPY/AgRP) neurons are essential, since in mice deficient for ghrelin receptor, the re-expression of physiological levels of ghrelin receptor in NPY/AgRP neurons in adulthood recovered the lowered blood glucose levels observed upon caloric 168 Trends in Neurosciences, March 2017, Vol. 40, No. 3 restriction without affecting body weight [22]. Thus, it seems that ghrelin signaling in the brain plays a key physiological role in the control of energy expenditure, fatty acid metabolism, and insulin sensitivity. Inhibiting ghrelin with NOX-B11-2, an antighrelin compound that specifically binds and inacti- vates acyl ghrelin during the preweaning period (from postnatal Day 4 to Day 18), gives different results than those observed when ghrelin was inhibited in adult animals. Specifically, inhibiting ghrelin before weaning promoted higher body weight, adiposity, and food intake; increased glucose and leptin levels; and altered leptin sensitivity when the mice became adults (Figure 1) [23]. In addition, the inhibition of ghrelin during the neonatal period induced an increase in the subsequent density of ARC NPY/AgRP and a-melanocyte stimulating hormone neuronal fiber projections to the PVH,[340_TD$IF] DMH, and LHA in adulthood (conversely, no changes