GHRP-6

GHRP-6, a polypeptide hormone, is a growth hormone secretagogue and ghrelin mimetic and analog. GHRP-6 is from the first generation of GHRPs and causes significant release of growth hormone by itself, due both to its suppression of somatostatin (an antagonist to GHRH) and stimulation of release of GH from the anterior pituitary.[1] The cells that produce and release GH are known as somatotropes.[2] Like GHRP-2, GHRP-6 does not have ghrelin’s lipogenic properties. Unlike GHRP-2, GHRP-6 induces hunger consistently in mammals. GHRP-6 acts synergistically when applied during a native GHRH (growth-hormone releasing hormone) pulse or when coadministered with GHRH or a GHRH analog such as Sermorelin or GRF 1-29 (growth releasing factor, aminos 1-29).[1] The synergy comes both due to the suppression of somatostatin and the fact that GHRP-6 increases GH release per-somatotrope, while GHRH increases the number of somatotropes releasing GH.[1,2] There is also a secondary effect of neuronal excitation in the hypothalamus caused by GHRP-6, which lasts for approximately 3 hours after application, similar to GHRP-2.

Because they increase GH release to a degree dependent on their application, any GH secretagogue used at an effective dose offers the benefits of GH. Increased collagen production, better cellular repair of internal organs, increased healing capability, increased energy, improved sleep, increased lean body mass, and reduced body fat are all documented effects of GH. However, unlike the exogenous synthetic e. coli-derived 22kDa growth hormone in application, the resultant GH pulse from GHRP-6 lasts about three hours instead of eight. Overexposure to GH – that is, GH release that does not adhere to normal pulsatile rhythms of the body (a steeper curve, with a sharp peak and rapid decline) but instead lasts longer and does not reach the same amplitude (a shallow curve, with a slow rise and descent over eight hours) – results in cellular desensitization to the effects of GH. GH secretagogues do not result in cellular desensitization to GH in any quantity; the pituitary may stop responding to the signal the compounds indicate if they are used in extreme quantities, but when administered up to every three hours GHRP-6 can result in supraphysiological levels of GH that are nonetheless reasonably safe, unlike exogenous GH.

Each GH secretagogue – GHRP-2, GHRP-6, ipamorelin, hexarelin, etc – has unique properties beyond release of GH. As demonstrated in the rat, GHRP-6 does have some lipogenic properties that are additive but are dependent on insulin/glucose states. Granado et al find that “GHRP-6 and insulin exert an additive effect on weight gain and visceral fat mass accrual in diabetic rats, indicating that some of GHRP-6's metabolic effects depend on the insulin/glucose status.” [3] The rats who were administered GHRP-6 when insulin was quiet did not experience the same effects on adiposity and weight gain when compared to either group (insulin or insulin with GHRP-6).

Delagado et al document that the most important unique effect of GHRP-6 could be a neuroprotective effect that is independent of the established IGF-1 pathway:

Treatment of the fetal hypothalamic neuronal cell line RCA-6 with growth hormone-releasing peptide 6, an agonist of the ghrelin receptor, or insulin-like growth factor I activates intracellular signalling cascades associated with anti-apoptotic actions. Abnormally high concentrations of glutamate provoke over-excitation of neurons leading to cell damage and apoptosis. Thus, the aim of this study was to investigate whether the administration of growth hormone-releasing peptide 6 and insulin-like growth factor I attenuates monosodium glutamate-induced apoptosis in RCA-6 neurons and the mechanisms involved. Two different mechanisms are involved in glutamate-induced cell death, one by means of caspase activation and the second through activation of a caspase-independent pathway of apoptosis mediated by the translocation of apoptosis-inducing factor. Growth hormone-releasing peptide 6 partially reversed glutamate-induced cell death but not the activation of caspases, suggesting blockage of the caspase-independent cell death pathway, which included interference with the translocation of apoptosis-inducing factor to the nucleus associated with the induction of Bcl-2. In contrast, the addition of insulin-like growth factor I to RCA-6 neurons abolished glutamate-induced caspase activation and cell death. These data demonstrate for the first time a neuroprotective role for growth hormone secretagogues in the caspase-independent cell death pathway and indicate that these peptides have neuroprotective effects independent of its induction of insulin-like growth factor I.[4]

Referances:

[1] Bowers CY, Momany F, Reynolds GA. In vitro and in vivo activity of a small synthetic peptide with potent GH releasing activity. 64th Annual Meeting of the Endocrine Society, San Francisco, 1982, p. 205.
[2]Bowers CY, Momany F, Reynolds GA, Sartor O. Multiple receptors mediate GH release. 7th International Congress of Endocrinology, Quebec, Canada, 1984, p. 464.

[3]Granado M, García-Cáceres C, Frago LM, Argente J, Chowen JA. The positive effects of growth hormone-releasing peptide-6 on weight gain and fat mass accrual depend on the insulin/glucose status. Endocrinology. 2010 May;151(5):2008-18.

[4] Delgado-Rubín A, Chowen JA, Argente J, Frago LM. Growth hormone-releasing peptide 6 protection of hypothalamic neurons from glutamate excitotoxicity is caspase independent and not mediated by insulin-like growth factor I. Eur J Neurosci. 2009 Jun;29(11):2115-24.

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