GHRP-6 is studied in endocrine and receptor-pharmacology models involving GHSR activation, pituitary growth-hormone release and appetite-associated signalling. It is historically important in growth-hormone secretagogue research because it helped demonstrate that ghrelin-receptor pathways can regulate pituitary signalling independently of the GHRH receptor (Learn more), alongside related secretagogue peptides such as Ipamorelin and CJC-1295 with DAC.
From an enkephalin lead to a growth-hormone hexapeptide
GHRP-6 was designed long before anyone knew what it was imitating. In the late 1970s, Cyril Bowers’ laboratory at Tulane University in New Orleans noticed that two altered versions of methionine enkephalin amide, one of the brain’s own opioid peptides, with D-tryptophan or D-phenylalanine at position two, weakly released growth hormone from rat pituitary tissue. Working with Frank Momany, who used conformational energy calculations to predict which shapes should be active, the team cycled through design, synthesis and testing. By 1981 a pentapeptide, Tyr-D-Trp-Ala-Trp-D-Phe-NH2, was about a thousand times more active than the enkephalin starting points.
The 1984 Endocrinology paper that introduced His-D-Trp-Ala-Trp-D-Phe-Lys-NH2, then labelled [His1,Lys6]GHRP, described a peptide that released growth hormone without shifting LH, FSH, TSH or prolactin. It was active in rats, monkeys, lambs and calves, and in chicks under special conditions, so it showed little species dependence. Despite its opioid ancestry, later work established that it acted through non-opiate receptors, an early hint that an undiscovered signalling system lay behind it.
A receptor found first, its hormone three years later
For more than a decade, GHRP-6 and its successors released growth hormone through a target nobody could name. Bowers’ group detected specific binding sites in pig pituitary and hypothalamic membranes, but the receptor itself stayed elusive until a Merck team led by Andrew Howard isolated it. Using the company’s non-peptide secretagogue MK-0677 to pick it out, they cloned a G-protein-coupled receptor from the pituitary and the arcuate and infundibular hypothalamus of pigs and humans, and wrote in Science that the synthetic secretagogues seemed to be mimicking a hormone yet to be identified.
That hormone turned up in an unexpected organ. Masayasu Kojima, Kenji Kangawa and colleagues purified it from rat stomach and called it ghrelin, after ghre, the Proto-Indo-European root of “grow”. It is a 28-residue peptide carrying an octanoyl fatty-acid chain on serine 3, and that modification is essential for activity. GHRP-6 therefore belongs to a rare group of compounds designed against a pathway almost two decades before its natural messenger came to light.
Feeding circuits in rodent models: arcuate neurons and eating
GHRP-6 was among the first tools to show that the secretagogue receptor reaches into brain circuits for feeding. Suzanne Dickson, Gareth Leng and Iain Robinson reported that systemic GHRP-6 switched on Fos, a marker of recent neuronal activity, in a subpopulation of rat hypothalamic neurons, and increased the firing of arcuate nucleus cells thought to make GHRH. Two years later, a study in sated adult rats found that GHRP-6 delivered into the brain’s ventricles triggered eating in a dose-related way while plasma growth hormone showed no matching change, suggesting the feeding effect ran on a separate track from GH release.
Ghrelin’s discovery later made sense of those results, and GHRP-6 is still in use as a probe. In a recent mouse study from Dickson’s group, intranasal GHRP-6 increased both meal frequency and meal size and activated arcuate neurons, about four in five of which expressed AgRP, a peptide made by the neurons that drive feeding. All of this is rodent work, and that is exactly where GHRP-6 earns its place as a feeding-circuit tool.
Cytoprotection research in heart, liver and gut models
A separate strand of GHRP-6 research has little to do with growth hormone. A group at the Center for Genetic Engineering and Biotechnology in Havana has spent years asking whether the peptide shields tissue from lethal stress. Their best-known experiment used Cuban Creole pigs with a coronary artery closed for an hour and then reopened. Compared with saline, GHRP-6 reduced infarct mass by 78% and infarct thickness by half, more than half of the treated pigs showed no pathological Q waves on ECG, and oxidative stress markers pointed to fewer reactive oxygen species as part of the mechanism.
The same themes recur in other models. In rats subjected to hepatic ischaemia and reperfusion, pretreatment curbed liver and intestinal damage, neutrophil infiltration and lipid peroxidation, and in cultured intestinal epithelial cells GHRP-6 tripled the migration rate without changing proliferation. Newer rat studies have examined doxorubicin-induced heart damage and permanent coronary ligation. A review from the Havana team proposes that part of this protection runs through CD36, a second binding site, which switches on PI-3K/AKT1 survival signalling.
