02 November 2005
Ghrelin, the natural ligand for the growth hormone secretagogue-1a (GHS-1a) receptor, has received a great deal of attention due to its ability to stimulate weight gain and the hope that an antagonist of the GHS-1a receptor could be a treatment for obesity. We have discovered an analog of full-length human ghrelin, BIM-28163, which fully antagonizes GHS-1a by binding to but not activating the receptor. We further demonstrate that BIM-28163 blocks ghrelin activation of the GHS-1a receptor, and inhibits ghrelin-induced GH secretion in vivo. Unexpectedly, however, BIM-28163 acts as an agonist with regard to stimulating weight gain. These results may suggest the presence of an unknown ghrelin receptor that modulates ghrelin actions on weight gain. In keeping with our results on growth hormone (GH) secretion, BIM-28163 acts as an antagonist of ghrelin-induced Fos protein immunoreactivity (Fos-IR) in the medial arcuate nucleus, an area involved in the ghrelin modulation of GH secretion. However, in the dorsal medial hypothalamus (DMH), a region associated with regulation of food intake, both ghrelin and BIM-28163 act as agonists to upregulate Fos-IR. The observation that ghrelin and BIM-28163 have different efficacies in inducing Fos-IR in the DMH, and that concomitant administration of ghrelin and an excess of BIM-28163 results in the same level of Fos-IR as BIM-28163 administered alone may demonstrate that in the DMH both ghrelin and BIM-28163 act via the same receptor. If so, it is unlikely that this receptor is GHS-1a. Collectively, our findings suggest that the action of ghrelin to stimulate increased weight gain may be mediated by a novel receptor other than GHS-1a, and further imply that GHS-1a may not be the appropriate target for anti-obesity strategies.