Central actions of peptide and non-peptide growth hormone secretagogues in the rat

Evidence for a central site of action of growth-hormone-releasing peptide (GHRP-6) was sought by (1) counting the number of Fos-positive nuclei within the brain following intracerebroventricular or intravenous injection of peptide and non-peptide GH secretagogues and (2) characterizing the electrophysiological responses of neuroendocrine arcuate neurones (recorded in vivo) following intravenous injection of GHRP-6. Conscious male rates were chronically implanted with intracerebroventricular or intravenous catheters. Dense nuclear Fos staining was induced throughout the ventral arcuate nucleus of rats injected intracerebroventricularly with low doses of GHRP-6 but not in rats injected with the endogenous GH-releasing hormone GHRH or in vehicle-treated controls. The non-peptidyl GH secretagogues L-692,585 and L-692,429 also induced Fos expression in the arcuate nucleus, and the pattern of distribution was similar to that described for GHRP-6. No increase in Fos expression was observed in rats given a systemic injection of a high dose of GHRH. In pentobarbitone-anaesthetized male rats, the effects of intravenous injection of GHRP-6 on the electrical activity of arcuate neurones was predominantly excitatory for putative neuroendocrine cells and inhibitory for the remaining unidentified cells. These results suggest that (1) GHRP-6 and non-peptidyl GH secretagogues have a central site of action involving the activation of a subpopulation of arcuate neurones and (2) this action is not mimicked by the central or peripheral effects of GHRH.