Ghrelin receptor (GHS-R1a)

The ghrelin receptor, formally the growth hormone secretagogue receptor type 1a, sits on pituitary cells and in the hypothalamus. Its natural ligand is ghrelin, the stomach hormone that rises before meals and signals hunger. Activating the receptor in the pituitary releases growth hormone through a pathway independent of GHRH, which is why the two families of secretagogues are additive when studied together.

The synthetic peptides that act here were discovered before ghrelin itself. GHRP-6 was designed in the early 1980s as a growth-hormone-releasing peptide; its receptor was cloned in 1996 and ghrelin, the natural ligand, was found in 1999. GHRP-2 is a more potent successor, and ipamorelin is the selective member of the family: it releases growth hormone without the rise in cortisol, prolactin and appetite that GHRP-6 produces through the same receptor's other actions.

Because the receptor also mediates hunger, the older GHRPs are known in the literature for strong appetite stimulation, a property that is either a nuisance or the point depending on the model being studied.