br Ghrelin Ghrelin was purified from rat stomach about

Ghrelin Ghrelin was purified from rat stomach about twenty years ago as a 28-amino 5-BrdU mg octanoylated peptide and shown to be the endogenous ligand of the growth hormone (GH) secretagogue receptor (now termed GHSR1a, Howard et al., 1996, Kojima et al., 1999). GHSR1a is a 7-transmembrane receptor coupled to Gαq/11, which activates phospholipase Cγ and Ca2+ release from internal stores [see (Camina, 2006) for a review]. Beyond the initial results in relation with GH releasing effects (Kojima et al., 1999, Seoane et al., 2000, Takaya et al., 2000), most of the studies focused on ghrelin's ability to regulate food-related behaviors and metabolism [reviewed in (Al Massadi et al., 2017, Perello and Dickson, 2015)]. Ghrelin is produced by gut endocrine cells mostly located in gastric oxyntic glands, in the fundus of the stomach (Date et al., 2000). Ghrelin production in the brain is controversial [see (Cabral et al., 2017) for a review]. Although ghrelin immunoreactivity was reported in hypothalamic neurons (Cowley et al., 2003, Kojima et al., 1999), other authors suggested this likely resulted from unspecific staining (Furness et al., 2011). In addition, several ghrelin reporter mouse lines failed to reveal positive cells in the brain (Sakata et al., 2009a). Although sensitive techniques such as RT-PCR revealed some ghrelin transcripts in the brain, the significance of these results remains unclear (Cabral et al., 2017). Interestingly, ghrelin-O-acyltransferase (GOAT), the enzyme that catalyzes the octanoylation of ghrelin, is expressed (Gahete et al., 2010, Sakata et al., 2009b, Wellman and Abizaid, 2015) and active in specific brain regions (Murtuza and Isokawa, 2018), raising the possibility that local processing of ghrelin might exist, although the source of the putative unmodified ghrelin remains to be identified. Peripheral ghrelin can act in the CNS by penetrating into the ventromedial part of the hypothalamus through the fenestrated capillaries of the median eminence close to the arcuate nucleus (ARC, Schaeffer et al., 2013, Fig. 1). Ghrelin can also cross the blood-brain barrier in other regions such as the area postrema [see (Cabral et al., 2017)] or reach the cerebrospinal fluid through the choroid plexus and tanycytes (Uriarte et al., 2018). Another route by which peripheral ghrelin could impact on brain function is the activation of GHSR1a in the vagus nerve terminals and transmission of information to the nucleus of the tractus solitarius (NTS), which is indirectly connected to the hypothalamus, providing a potential pathway to regulate food intake or other behaviors (Date et al., 2006, Fig. 1). It should also be pointed out that ghrelin-independent activity of GHSR1a may be relevant in some cells, either through its constitutive activity or as a result of heterodimerization and potential sensitivity to other ligands (Schellekens et al., 2013). The contribution of these mechanisms to the actions attributed to ghrelin remains however to be determined. With respect to ghrelin regulation, the levels of this hormone are high when nutrient availability is low, as during fasting, and decrease when energy supply is sufficient, as happens after consumption of a meal [for review see (Al Massadi et al., 2014)]. Ghrelin levels in the circulation are inversely correlated with the body mass index, and are up-regulated in under-nourished states, such as anorexia nervosa, and down-regulated in states of positive energy balance, like obesity (Mequinion et al., 2013, Otto et al., 2001, Tschop et al., 2001). Ghrelin is considered to act as a meal initiation cue and its levels rise just before eating in rodents and humans (Cummings et al., 2001, Drazen et al., 2006). It is also involved in anticipatory locomotor activity (Blum et al., 2009). Indeed, reports using fixed meal patterns suggest that ghrelin is increased preprandially, as part of an anticipatory and learned response corresponding to the animal expectation to eat, and this increase is independent of the state of fasting or feeding (Drazen et al., 2006, Merkestein et al., 2012). Ghrelin secretion in the stomach can also be regulated through exposition to food-associated stimuli and the consequent activation of the vagal efferent fibers of the NTS (Arosio et al., 2004, Seoane et al., 2007, Sugino et al., 2002). Therefore, factors such as anticipatory response, cephalic or oropharyngeal stimulation, and food-related sensory stimuli appear to contribute to the regulation of ghrelin [see (Al Massadi et al., 2014)].