RETROGRADELY LABELED NEUROSECRETORY NEURONS OF THE RAT HYPOTHALAMIC ARCUATE NUCLEUS EXPRESS FOS PROTEIN FOLLOWING SYSTEMIC INJECTION OF GH-RELEASING PEPTIDE-6

Previously, we demonstrated that the synthetic hexapeptide GH-releasin g peptide (GHRP-6) activates a subpopulation of arcuate neurones, as r eflected by increased electrical activation and by the detection of Fo s protein in cell nuclei. Here we set out to determine (1) what propor tion of the cells activated by GHRP-6 are neurosecretory neurones and (2) whether the cells activated by GHRP-6 contain tyrosine hydroxylase (TH; a marker of dopaminergic cells in this region) or beta-endorphin . In the first study, adult male rats were injected i.v. with the retr ograde tracer, Fluorogold, to detect cells which project outside the b lood-brain barrier (and are therefore likely to be neurosecretory neur ones). Three days later the conscious rats were injected i.v. with 50 mu g GHRP-6 and the brains processed for the immunocytochemical detect ion of Fos protein. Between arcuate neurones expressing Fos protein fo llowing GHRP-6 injection were retrogradely labelled with Fluorogold. I n the second study, conscious male rats, bearing a chronically implant ed jugular catheter, were killed 90 min following an i.v. injection of 50 mu g GHRP-6 and the brains were processed for the double immunocyt ochemical detection of Fos protein and either TH or beta-endorphin. Le ss than 7% (mean +/- S.E.M. = 6.7 +/- 2.6% nuclei/section per rat) of the arcuate neurones expressing Fos protein following GHRP-6 injection were TH-containing cells. Of 143 beta-endorphin-containing arcuate ce lls detected only four cells were identified as containing Fos protein . Thus, the majority of arcuate neurones activated by GHRP-6 (1) proje ct outside the blood-brain barrier (and an therefore likely to be neur osecretory neurones) and (2) were not identified as TH- or beta-endorp hin-containing cells.