The study shows that when you give GHRP‑6 to rats, it turns on a large share (about 70‑80%) of hypothalamic neurons that send signals out of the brain to release hormones. These activated cells are not the dopamine‑producing (TH) or beta‑endorphin cells, meaning GHRP‑6 mainly stimulates other hormone‑releasing neurons, likely the ones that trigger growth hormone release.

Previously, we demonstrated that the synthetic hexapeptide GH-releasing peptide (GHRP-6) activates a subpopulation of arcuate neurones, as reflected by increased electrical activation and by the detection of Fos protein in cell nuclei. Here we set out to determine (1) what proportion 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 retrograde tracer, Fluorogold, to detect cells which project outside the blood-brain barrier (and are therefore likely to be neurosecretory neurones). Three days later the conscious rats were injected i.v. with 50 micrograms GHRP-6 and the brains processed for the immunocytochemical detection of Fos protein. Between 68% and 82% of the arcuate neurones expressing Fos protein following GHRP-6 injection were retrogradely labelled with Fluorogold. In the second study, conscious male rats, bearing a chronically implanted jugular catheter, were killed 90 min following an i.v. injection of 50 micrograms GHRP-6 and the brains were processed for the double immunocytochemical detection of Fos protein and either TH or beta-endorphin. Less 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 cells detected only four cells were identified as containing Fos protein. Thus, the majority of arcuate neurones activated by GHRP-6 (1) project outside the blood-brain barrier (and are therefore likely to be neuro-secretory neurones) and (2) were not identified as TH- or beta-endorphin-containing cells.