GHRP-6

GHRP-6 (Growth Hormone Releasing Peptide-6) is a synthetic hexapeptide that acts as a potent agonist of the growth hormone secretagogue receptor (GHS-R1a), also known as the ghrelin receptor. It was one of the first synthetic GH secretagogues developed and remains one of the most potent in terms of raw GH release amplitude.

Upon binding to GHS-R1a on anterior pituitary somatotrophs, GHRP-6 triggers GH release through an IP3/DAG/PKC signalling cascade that is distinct from the cAMP pathway used by GHRH. This means GHRP-6 and GHRH stimulate GH release through complementary, non-overlapping mechanisms — which is why their combination produces synergistic rather than merely additive GH pulses.

GHRP-6 also stimulates the release of ghrelin from the stomach and hypothalamus, which accounts for its pronounced appetite-stimulating effect — a characteristic that distinguishes it from more selective GH secretagogues like Ipamorelin. Additionally, GHRP-6 causes modest, transient increases in cortisol and prolactin levels, reflecting its broader activation profile across pituitary cell types. It also demonstrates gastric cytoprotective properties, similar to ghrelin itself.

GHRP-6 was developed in the late 1970s and early 1980s by Cyril Bowers and colleagues at Tulane University, as part of pioneering work to create synthetic peptides capable of releasing growth hormone independently of GHRH. The discovery that short, modified peptides could activate a then-unknown receptor on pituitary cells was groundbreaking — this receptor was later identified as GHS-R1a (the ghrelin receptor), years before ghrelin itself was discovered in 1999.

GHRP-6 played a central role in the identification and characterisation of the ghrelin signalling system, making it historically significant beyond its pharmacological applications. Extensive research throughout the 1990s and 2000s documented its effects on GH secretion, appetite, cardiac protection, and gastric cytoprotection. It has been superseded in some research contexts by more selective analogues (Ipamorelin, Hexarelin), but remains widely used due to its well-characterised pharmacology and potent GH-releasing efficacy.