Ghrelin
A 28 amino acid peptide produced primarily by the stomach that stimulates appetite and growth hormone release. Often called the hunger hormone, ghrelin acts through the GHS-R receptor — the same receptor targeted by research peptides GHRP-2, GHRP-6, and ipamorelin.
Technical Context
Ghrelin is unique as the only known orexigenic (appetite-stimulating) circulating hormone. It requires octanoylation (attachment of an 8-carbon fatty acid to Ser3 by the enzyme GOAT — ghrelin O-acyltransferase) for GHS-R1a activation. Plasma ghrelin rises before meals and falls after eating, creating a hormonal hunger signal. GHS-R1a is expressed on pituitary somatotrophs (mediating GH release), hypothalamic neurons (mediating appetite stimulation), and vagal afferent neurons (gastrointestinal signalling). Research GHRPs (GHRP-2, GHRP-6, ipamorelin, hexarelin) are synthetic peptides that activate GHS-R1a but have distinct selectivity and side effect profiles. Ipamorelin is noted for more selective GH release with less effect on cortisol and prolactin than GHRP-2 or GHRP-6.