Growth hormone secretagogues (GHSs) are small synthetic molecules. They stimulate the growth hormone (GH) release from the pituitary gland through the G-protein-coupled receptor (GHS-R).
Growth hormone secretagogues (GHSs) are small synthetic molecules. They stimulate the growth hormone (GH) release from the pituitary gland through the G-protein-coupled receptor (GHS-R). 
In the past, GHS-R was an orphan receptor, because it had no known natural ligand. Recently, researchers have identified the natural ligand for the GHS-R and they called it ghrelin. 
Ghrelin is a 28-amino acid acylated peptide.
The release of GH from the pituitary grand might be regulated not only by the GHRH (GH-releasing hormone) but also by ghrelin produced by the stomach, intestine, placenta, pituitary and possibly in the hypothalamus.[1,2, 4] Ghrelin and its receptor are widely distributed in the body, however, the greatest expression of glrelin is in stomach endocrine cells.
Admiinstration of exogenous ghrelin has been shown to stimulate pituitary GH secretion, appetitie, body growth and fat deposition. Thus, it is an anabolic hormone. 
Ghrelin stimulates appetite by acting on the hypothalamic arcuate nucleus. Ghrelin is secreted from the stomach and circulates in the bloodstream under fasting conditions. This activity may transmit a hunger singal to central nervous system or induces appetite-stimulating effect and adiposity in rat.  Ghrelin is a natural leptin antagonist.  In general, plasma ghrelin levels are low in obese human subjects and after food intake. And, it increases during starvation and in patients with mental anorexia. In addition, ghrelin plasma levels are negatively correlated with body mass index, amount of body fat, adipocyte size, and leptin, insulin and glucose levels. [6,7]
REFERENCES  Kojima M et al, Ghrelin: structure and function, Physiol Rev 2005 Apr,85(2):495-522.  Kojima M et al, Ghrelin: discovery of the natural endogenous ligand for the growth hormone secretagogue receptor. Trends Endocrinol Metab. 2001 Apr;12(3):118-22.  Kojima M Ghrelin: a novel growth-hormone releasing peptide. Nippon Rinsho. 2001 Jul;59(7):1400-7.  Horath TL et al, Minireview: ghrelin and the regulation of energy balance-a hypothalamic perspective. Endocrinology. 2001 oct;142(10):4163-9.  Wang G et al Ghrelin-not just another stomach hormone. Regul Pept. 2002 May 15;105(2):75-81.  Ukkola O et al, Ghrelin, growth and obesity. Ann Med. 2002;34(2):102-8.  Bronsky J et al, Ghrelin-structure, function and clinical applications. Cesk Fysiol 2004;53(2):80-5.