Kojima, M. et al. (Nature, 1999) reported an endogenous ligand specific for GHSR,and they designate the GH-releasing peptide 'ghrelin' (ghre is the Proto-Indo-European root of the word 'grow').

Cortistatin, but not somatostatin, binds to growth hormone secretagogue (GHS) receptors of human pituitary gland Deghenghi R. et al. (J Endocrinol Invest. 2001).
Cortistatin (CST)-14, a neuropeptide with high homology with somatostatin (SS)-14, binds all sst subtypes but, unlike SS, also ghrelin's receptor. Endocrine activities of cortistatin-14 and its interaction with GHRH and ghrelin in humans. Broglio F. et al. (J Clin Endocrinol Metab. 2002)