Growth Hormone Releasing Peptide-6 (GHRP-6) is a synthetic hexapeptide (His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂) and one of the first characterised growth hormone secretagogues (GHS). Originally developed from enkephalin analogues in the 1980s, GHRP-6 played a foundational role in identifying the ghrelin receptor (GHSR-1a) and understanding GH pulse regulation.
GHRP-6 was synthesised by Cyril Bowers and colleagues at Tulane University as part of a programme investigating enkephalin opioid peptide analogues. Bowers observed unexpectedly potent GH-releasing activity from certain analogues that did not bind opioid receptors, indicating a novel receptor pathway. This work, spanning the late 1970s through the 1980s, eventually led to the discovery and characterisation of the ghrelin receptor (GHSR-1a) — a G-protein coupled receptor that mediates GH secretion from pituitary somatotrophs.
GHRP-6 became the reference compound for the GHS drug class and was central to understanding the endogenous ligand ghrelin, which was not discovered until 1999 (Kojima et al., Nature).
GHRP-6 binds GHSR-1a on pituitary somatotroph cells, stimulating GH release through multiple pathways: activation of phospholipase C, increased intracellular calcium, and inhibition of somatostatin release at hypothalamic level. Unlike GHRH, which acts exclusively through pituitary GHRH receptors, GHRP-6 also acts centrally in the hypothalamus and peripheral tissues where GHSR-1a is expressed.
GHRP-6 is less selective than newer GHRPs: at doses required for substantial GH release, it also stimulates cortisol and prolactin secretion to a measurable degree, and produces marked appetite stimulation via its ghrelin-like orexigenic effect in the arcuate nucleus.
In published human studies, GHRP-6 at 1 µg/kg IV produces peak GH at approximately 15–30 minutes with return to baseline by 60–90 minutes. Higher doses (10 µg/kg) produce correspondingly larger but not proportionally greater GH peaks, suggesting receptor saturation. Synergy with exogenous GHRH (and by extension CJC-1295 no DAC) follows the same dual-pathway mechanism seen with ipamorelin.
Bowers (1998) published comprehensive characterisation of GHRP-6 GH release kinetics in humans, including comparisons with GHRP-2 and ipamorelin. GHRP-6 produces larger GH peaks than ipamorelin at equivalent doses but with significantly more cortisol co-secretion — a property exploited in some research contexts where cortisol responses are the endpoint of interest.
GHRP-6's potent orexigenic effect, mediated via GHSR-1a activation in the hypothalamic arcuate nucleus, makes it a valuable tool in appetite regulation research. It mimics endogenous ghrelin signalling and has been used in preclinical models to study feeding behaviour, energy homeostasis, and the GH/appetite axis. Its use as a research tool is distinct from newer selective GHRPs precisely because its pleotropic activity makes it useful for studying the full GHSR-1a receptor pharmacology rather than isolated GH secretion.
A separate and growing body of research has investigated GHRP-6 in cardioprotection and hepatoprotection independent of its GH-releasing activity. Rodent models of ischaemia-reperfusion injury have demonstrated GHRP-6 pre-treatment reduces infarct size and myocardial apoptosis. This cardioprotective effect appears to involve activation of pro-survival PI3K/Akt and ERK1/2 signalling pathways at cardiac GHSR-1a sites and may be GH-independent (Gomes et al., 2016).
GHRP-2 and ipamorelin were developed subsequently with improved selectivity profiles. Ipamorelin in particular shows minimal cortisol and prolactin co-secretion. GHRP-6 remains valuable in research specifically because its broader pharmacological profile provides a more complete model of GHSR-1a physiology, including appetite, cortisol, and GH axis interactions simultaneously.
