In conclusion we have identified ten novel

In conclusion, we have identified ten novel GPCR genes and a pseudogene. Transcripts for GPR78, GPR81, GPR94, GPR95, GPR101, GPR103 and P2Y were detected in various CNS and peripheral tissues. Given the high levels of identity observed within paralogous oGPCR gene clusters, future efforts will likely discover common endogenous ligands for each of these novel GPCR subfamilies. The increasing number of oGPCRs that continue to be isolated with unique distribution profiles in brain and periphery is indicative that the search for novel transmitter ligands should be intensified. These efforts have the tremendous potential to uncover novel physiological roles for these as yet unknown receptor-transmitter signaling systems.
Acknowledgements
Introduction Ghrelin is an octanoylated peptide produced mainly by gastric mucosal endocrine cells. Since its initial description as a potent stimulator of growth hormone secretion via engagement of the growth hormone secretagogue receptor [1], the known functions of ghrelin have broadened considerably to include a large number of physiologic processes and behaviors that influence eating, storage of energy, body weight, and blood glucose via mainly central but also peripheral mechanisms [2], [3], [4]. Plasma levels of ghrelin rise before expected meals and drop rapidly upon food intake [5], [6] through molecular mechanisms that are poorly characterized. In contrast to most other enteroendocrine cells, gastric ghrelin cells are round, closed-type endocrine cells which do not contact the gastric lumen and are therefore not regulated directly by dietary components present in the stomach [7]. Gastric distension does not change plasma ghrelin levels; nor does a solution of glucose if it is retained in the stomach [5], [8]. However, after reaching the small intestine, dietary glucose, lipids and amino acids all reduce plasma ghrelin [5], [8], [9], [10], an effect most likely mediated through hormonal, neuronal and metabolite signals arising from nutrient sensing and protease inhibitor in the intestine. As examples, inhibition of ghrelin secretion by glucose and insulin has been demonstrated using primary cultures of rodent stomach cells [11], [12], perfused rat stomach [13], and infusion studies in humans [14], [15], [16], [17], [18] – although this was not observed in all studies [19], [20]. Several gastrointestinal peptides can suppress ghrelin secretion, including cholecystokinin (CCK) [21], [22], glucose-dependent insulinotropic polypeptide (GIP) [23], glucagon-like peptide 1 (GLP-1) [13], [24], peptide YY (PYY) [25] and somatostatin [20], [26], [27], [28]. However, many of these effects may be indirect and/or have been difficult to reproduce and their physiological relevance remains unclear [13], [22], [28], [29], [30]. Also, metabolites efficiently decrease ghrelin secretion, as demonstrated by intravenous triglycerides [31] and total parenteral nutrition [32]. While known signals inhibiting ghrelin secretion are mainly endocrine and paracrine, known stimulatory signals are mainly neuronal. Both sympathetic nerve [33], [34] and vagal stimulation [35], [36] increase ghrelin secretion. Sympathetic stimulation most likely occurs directly at the ghrelin cell via β1 adrenergic receptors [33], [37]. Vagal stimulation depends on cholinergic muscarinic mechanisms, but whether this effect is direct or indirect is unclear [35], [37]. Inhibition of these pathways prevents fasting-induced elevation of plasma ghrelin levels [33], [38]. Although we know much about how food, stress, sleep and circadian rhythmicity affects circulating ghrelin levels, the molecular machinery within the ghrelin cell which receives the stimulatory and inhibitory secretion signals remains mostly unknown. The large family of seven transmembrane segment (7TM), G protein-coupled receptors function as receivers for neurotransmitters, hormones and paracrine lipid messengers and as sensors for nutrients and metabolites [39], [40]. In the present study, we identify the full repertoire of gastric ghrelin cell 7TM receptors and heterotrimeric GTP-binding protein Gα subunits and functionally characterize those that are both highly expressed and highly enriched in gastric ghrelin cells, in respect to their effects on ghrelin secretion.