INDUCTION OF C-FOS MESSENGER-RIBONUCLEIC-ACID IN NEUROPEPTIDE-Y AND GROWTH-HORMONE (GH)-RELEASING FACTOR NEURONS IN THE RAT ARCUATE NUCLEUSFOLLOWING SYSTEMIC INJECTION OF THE GH SECRETAGOGUE, GH-RELEASING PEPTIDE-6

In this study we investigated the neurochemical identity of the arcuat e cells activated following GH-releasing peptide-6 (GHRP-6) injection by comparing, on consecutive sections, the distribution c-ibs messenge r RNA (mRNA) with that of mRNAs for peptides synthesized in arcuate ce lls, including neuropeptide Y (NPY), GH-releasing factor (GRF), tyrosi ne hydroxylase, POMC, and somatostatin. Rats bearing chronically impla nted jugular catheters were injected with either 50 mu g GHRP-6 or veh icle. Thirty minutes later they were terminally anesthetized and perfu sed with fixative. Paraffin-embedded sections of 7 mu m thickness were processed using in situ hybridization for either c-ibs mRNA or mRNAs for the neurochemical markers. In GHRP-6-treated rats the mean (+/- SE M) number of cells expressing c-fos mRNA in the arcuate nucleus (23 +/ - 2 cells/section per rat; n = 5) was significantly higher than for ve hicle-treated controls (2 +/- 1 cells/section per rat; n = 5; P < 0.00 1, Mann-Whitney U test). Superimposed camera lucida maps indicated tha t, in GHRP-6-injected rats, neurochemically identifiable cells express ing c-fos mRNA also express NPY mRNA (51 +/- 4%), GRF mRNA (23 +/- 1%) tyrosine hydroxylase mRNA (11 +/- 3%), POMC mRNA (11 +/- 2%), or soma tostatin mRNA (4 +/- 1%). Thus, the majority of cells expressing c-ibs mRNA following GHRP-6 injection are NPY and GRF-containing cells.