ATTENUATION OF THE GROWTH-HORMONE SECRETAGOGUE INDUCTION OF FOS PROTEIN IN THE RAT ARCUATE NUCLEUS BY CENTRAL SOMATOSTATIN ACTION

We set out to determine whether the central action of growth hormone ( GH) secretagogues to induce Fos protein expression in the arcuate nucl eus is influenced by central somatostatin action. Conscious male rats were injected i.v. with 100 mu g sandostatin (octreotide? a long-actin g somatostatin analogue) or saline, 10 min before an i.v. injection of either 50 mu g GH-releasing peptide (GHRP-6), 50 mu g MK-0677 (a non- peptide GH secretagogue) or saline. In separate study, conscious male rats were injected i.c.v. with either 2 mu g sandostatin or artificial cerebrospinal fluid (aCSF) vehicle 20 min before an i.v. injection of 50 mu g GHRP-6. In all studies, rats were anaesthetized 90 min follow ing GH secretagogue injection, perfused with fixative and the brains p rocessed for the immunocytochemical detection of Fos protein. The numb er of Fos-positive nuclei detected in the arcuate nucleus of the i.v, sandostatin/i.v. GHRP-6 treated rats(28 +/- 5 nuclei/section) and the i.v. sandostatin/i.v. MK-0677-injected rats (8 +/- 2 nuclei/section) w as significantly less than the i.v. saline/i.v. GHRP-6-treated group ( 56 +/- 5 nuclei/section) and the i.v. saline/ i.v. MK-0677-treated gro up (20 +/- 2 nuclei/section) respectively, Intracerebroventricular san dostatin injection attenuated the GHRP-6-induced Fos response, from 53 +/- 6 nuclei/section in the i.c.v. aCSF/i.v. GHRP-6 group, to 39 +/- 5 nuclei/section in the i.c.v. sandostatin/i.v. GHRP-6 group, Thus, th e central action of GH secretagogues to induce Fos protein expression in the arcuate nucleus appears to be subject to central inhibitory con trol by somatostatin.