The study shows that GHRP‑6 triggers a big jump in growth hormone (GH) when it reaches the brain, and this effect can be shut down by somatostatin, a hormone that normally blocks GH. Giving GHRP‑6 directly into the bloodstream needs a lot more of the peptide to get the same GH boost, meaning its main action is in the brain, not the pituitary.

Growth hormone (GH) release is stimulated by a variety of synthetic secretagogues, of which growth hormone-releasing hexapeptide (GHRP-6) has been most thoroughly studied; it is thought to have actions at both pituitary and hypothalamic sites. To evaluate the central actions of this peptide, we have studied GH release in response to direct i.c.v. injections in anaesthetized guinea pigs. GHRP-6 (0.04-1 microgram) stimulated GH release > 10-fold 30-40 min after i.c.v. injection. The same GH response required > 20-fold more GHRP-6 when given by i.v. injection. GH release could also be elicited by a non-peptide GHRP analogue (L-692,585, 1 microgram i.c.v.), whereas a growth hormone-releasing factor (GRF) analogue (human GRF27Nle(1-29)NH2, 2 micrograms, i.c.v.) was ineffective. A long acting somatostatin analogue (Sandostatin, SMS 201-995, 10 micrograms i.c.v.) (SMS) given 20 min before 200 ng GHRP-6 blocked GH release. This was unlikely to be due to a direct effect of SMS leaking out to the pituitary, since central SMS injections did not affect basal GH release, nor did they block GH release in response to i.v. GRF injections. We conclude that the hypothalamus is a major target for GHRP-6 in vivo. Since the GH release induced by central GHRP-6 injections can be inhibited by a central action of somatostatin, and other data indicate that GHRP-6 activates GRF neurones, we suggest that somatostatin may block this activation via receptors known to be located on or near the GRF cells themselves. Somatostatin may therefore be a functional antagonist of GHRP-6 acting centrally, as well as at the pituitary gland.