GHRP-6 (growth-hormone releasing peptide 6) is a GH secretagogue and and partial analog and mimetic of ghrelin. GHRP-6 is a member of the first generation of synthetic GHRPs, a class of partial ghrelin analogs that induce release of endogenous growth hormone by suppressing somatostatin (an antagonist to the separate hypothalamic hormone GHRH or growth-hormone releasing hormone) and by stimulating the release of GH directly from somatotropes (anterior pituitary cells that specialize in the production and storage of GH). GHRP-6 is sometimes used in a provocation test to rule out or diagnose pituitary GH deficiency.
GHRP-6, like other GHRPs, is synergistic with GHRH and its analogs such as Sermorelin (1-44) and Modified GRF (1-29). In addition to provoking direct release of GH, by suppressing somatostatin GHRP-6 allows GHRH to exert an effect on a greater number of somatotropes and increase the absolute GH release relative to the sum total of each peptide administered alone.
Endogenous ghrelin and GHRH both induce GH release; however, ghrelin plays a key role in fasting-induced GH release:
GH-releasing hormone and somatostatin release [were] considered as the main mechanisms that induce this elevated GH secretion during fasting. Ghrelin is an endogenous ligand of the GH secretagogue receptor and is synthesized in the stomach. As ghrelin administration in man stimulates GH release, while serum ghrelin concentrations are elevated during fasting in man, this increase in ghrelin levels might be another mechanism whereby fasting results in stimulation of GH release. In ten healthy non-obese males we performed a double-blind placebo-controlled crossover study comparing fasting with and fasting without GH receptor blockade. GH, ghrelin, insulin, glucose and free fatty acids were assessed. While ghrelin levels do not vary considerably in the fed state, fasting rapidly induced a diurnal rhythm in ghrelin concentrations. These changes in serum ghrelin concentrations during fasting were followed by similar, profound changes in serum GH
The close association between fasting, ghrelin, and changes in GH levels caused Muller et al (2002) to conclude that in fact “ghrelin is the driving force of increased GH secretion during fasting”. While GHRP-6 behaves somewhat similarly to ghrelin in inducing GH release, it can also be administered in the fed state with minimal effect on the release of GH.
GH secretagogues like GHRP-6 have unique properties that are distinct from the release of GH, and sometimes distinct from the traits exhibited by ghrelin. GHRP-6 is neuroprotective:
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….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. 
Although GHRP-6 is relatively selective for GH release and (similar to GHRP-2) does not reliably induce lipogenesis like ghrelin, GHRP-6 does provoke hunger in mammals under most circumstances, resulting in increased food intake. When insulin and glucose are present in sufficient levels upon administration, GHRP-6 increases bodyweight, some of which weight gain may be from fat mass accrual.
 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.
Bowers CY, Momany F, Reynolds GA, Sartor O. Multiple receptors mediate GH release. 7th International Congress of Endocrinology, Quebec, Canada, 1984, p. 464.
Muller AF, Lamberts SW, Janssen JA, Hofland LJ, Koetsveld PV, Bidlingmaier M, Strasburger CJ, Ghigo E, Van der Lely AJ. Ghrelin drives GH secretion during fasting in man. Eur J Endocrinol. 2002 Feb;146(2):203-7.
 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.
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.