Ghrelin is a hormone created by P/D1 cells, which are found in the stomach lining. Ghrelin is also found in smaller amounts in the pituitary gland, hypothalamus, kidney, and placenta. In addition, it is created by a small number of neurons located in the arcuate nucleas.

Ghrelin is known to encourage the secretion of growth hormones from the anterior pituitary gland. This is because the anterior pituitary gland contains special ghrelin receptors, which were discovered before ghrelin itself ever was. For this reason, the ghrelin receptor is also referred to as the growth hormone secretagoue receptor (GHS-R). In addition to the anterior pituitary gland, ghrelin receptors are also found in the heart, hypothalamus, and adipose tissue.

Ghrelin is also responsible for stimulating the appetite and has been found to increase the appetite before eating and to decrease it afterward. In laboratory tests, humans who were injected with ghrelin reported an increase in hunger. In addition, research seems to demonstrate that ghrelin suppresses the utilization of fat in the adipose tissue. In essence, ghrelin appears to be at least partially responsible for letting the body know when it is hungry and for keeping the body informed about the energy balance of the brain and the body.

Ghrelin also has an effect on the gastrointestinal system in that it stimulates emptying of the gastric system. It also appears to effect the function of the cardiovascular system positively in a variety of ways, such as by increasing cardiac output. Research is still unclear, however, as to whether ghrelin provides these positive effects itself or whether they are due to the growth hormone secretion that is stimulated by ghrelin.

Ghrelin also has an effect on brain function. It appears to play a large part in neurotrophy, especially as it applies to the hippocampus. In addition, ghrelin is important in helping the brain make cognitive adaptations and other changes in response to the environment. Therefore, it is critical to the learning process.