Ghrelin is a hormone produced by P/D1 cells lining the acer of the human stomach that stimulate appetite. In rodents, X/A-like cells produce ghrelin. Ghrelin levels increase before meals and decrease after meals. It is considered the counterpart of the hormone leptin, produced by adipose tissue, which induces satiation when present at higher levels. Ghrelin also stimulates the secretion of growth hormone from the anterior pituitary gland. In some bariatric procedures, the level of ghrelin is reduced in patients, thus causing satiation before it would normally occur.

Receptors for ghrelin are expressed by neurons in the arcuate nucleus and the ventromedial hypothalamus. The ghrelin receptor is a G protein-coupled receptor, formerly known as the GHS receptor (growth hormone secretagogue receptor). Ghrelin is also made by a small population of neurons in the arcuate nucleus. Ghrelin plays a significant role in neurotrophy, particularly in the hippocampus, and is essential for cognitive adaptation to changing environments and the process of learning. Recently, ghrelin has been shown to activate the endothelial isoform of nitric oxide synthase in a pathway that depends on various kinases including Akt.