Ghrelin is a GH-releasing peptide that also has an important role as an orexigenic hormone-stimulating food intake. By measuring inositol phosphate turnover or by using a reporter assay for transcriptional activity controlled by cAMP-responsive elements, the ghrelin receptor showed strong, ligand-independent signaling in transfected COS-7 or human embryonic kidney 293 cells. Ghrelin and a number of the known nonpeptide GH secretagogues acted as agonists stimulating inositol phosphate turnover further. In contrast, the low potency ghrelin antagonist, [D-Arg1,D-Phe5,D-Trp7,9,Leu11]-substance P was surprisingly found to be a high potency (EC50 = 5.2 nm) full inverse agonist as it decreased the constitutive signaling of the ghrelin receptor down to that observed in untransfected cells. The homologous motilin receptor functioned as a negative control as it did not display any sign of constitutive activity; however, upon agonist stimulation the motilin receptor signaled as strongly as the unstimulated ghrelin receptor. It is concluded that the ghrelin receptor is highly constitutively active and that this activity could be of physiological importance in its role as a regulator of both GH secretion and appetite control. It is suggested that inverse agonists for the ghrelin receptor could be particularly interesting for the treatment of obesity.

The human cytomegalovirus (CMV) proteins US28 and UL33 are homologous to chemokine receptors (CKRs). Knockout of the mouse CMV M33 protein (UL33 homologue) results in substantial attenuation of salivary gland infection/replication and reduced efficiency of reactivation from tissue explants. M33-mediated G protein-coupled signaling is critical for the salivary gland phenotype. In this report, we demonstrate that US28 and (to a lesser degree) UL33 restore reactivation from tissue explants and partially restore replication in salivary glands (compared to a signaling-deficient M33 mutant). These studies provide a novel small animal model for evaluation of therapies targeting the human CMV CKRs.Chemokine receptor (CKR) homologues and other seventransmembrane receptor (7TMR) homologues are characteristic of beta-and gammaherpesviruses and are potential therapeutic targets (7,14,16). Human cytomegalovirus (HCMV) encodes four 7TMRs: UL33 and UL78 are conserved in all betaherpesviruses, whereas US28/US27 homologues are restricted to primate betaherpesviruses.Clues to the roles of HCMV 7TMRs have come from in vivo studies of mouse and rat CMVs (MCMV and RCMV, respectively). Acute-phase replication in primary organs is followed by dissemination to secondary sites, such as salivary glands, where virus may replicate for several days, attaining high titers. Following immune clearance of productive infection, latently infected cells remain distributed in a number of organs. In the absence of M33 (UL33 homologue), MCMV failed to attain detectable levels of infectious virus in salivary glands, with similar observations for R33 of RCMV (1, 5), suggesting evolutionarily conserved functions. The mechanisms underlying salivary gland attenuation may include defects in dissemination to, initial infection of, or maintenance of productive infection within that organ. Definitive studies are lacking, but there is some evidence for attenuation at a postdissemination step; the M33-null mutant was defective for replication following direct intrasalivary gland inoculation, and the R33-null mutant was detectable in salivary glands (by PCR), similar to the wild type at early times postinfection (1, 5). We and others have demonstrated the importance of constitutive M33-induced signaling for the salivary gland phenotype (4, 15). We further demonstrated that an M33-null mutant was deficient for reactivation in an ex vivo tissue explant model (2), although whether M33 plays a specific role during the establishment of, maintenance of, or reactivation from latency is not yet known.