Abstract

The only molecularly identified ghrelin receptor is the growth hormone secretagogue receptor GHSR1a. Its natural ligand, ghrelin, is an acylated peptide whose unacylated counterpart (UAG) is almost inactive at GHSR1a. A truncated, nonfunctional receptor, GHSR1b, derives from the same gene. We have critically evaluated evidence for effects of ghrelin receptor ligands that are not consistent with actions at GHSR1a. Effects of ghrelin are observed in cells or tissues where the expression of GHSR1a is not detectable or after the Ghsr gene has been inactivated. In several, effects of ghrelin are mimicked by UAG, and ghrelin binding is competitively reduced by UAG. Effects in the absence of GHSR1a and sites at which ghrelin and UAG have similar potency suggest the presence of novel nonspecific ghrelin receptors (ghrelin receptor–like receptors [GRLRs]). A third class of receptor, the UAG receptors, at which UAG, but not ghrelin, is an agonist has been proposed. None of the novel receptors, with the exception of the glycoprotein CD36, which accounts for ghrelin action at a limited number of sites, have been identified. GHSR1a and GHSR1b combine with other G protein–coupled receptors to form heterodimers, whose pharmacologies differ from their components. Thus, it is feasible some GRLRs and some UAG receptors are heterodimers. Effects mediated through GRLRs or UAG receptors include adipocyte lipid accumulation, myoblast differentiation, osteoblast proliferation, insulin release, cardioprotection, coronary artery constriction, vascular endothelial cell proliferation, and tumor cell proliferation. The molecular identification and pharmacologic characterization of novel ghrelin receptors are thus important objectives.