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Ghrelin is a hormone produced by specialized cells that line the stomach and the pancreas.
In the stomach, cells that secrete ghrelin include the P/D1 cells in the fundus or upper part of the stomach and in the pancreas, ghrelin secreting cells are called epsilon cells.
Ghrelin is one of the main hormones to stimulate hunger. Ghrelin levels increase before meals and decrease after meals, a mechanism that has its roots in the hypothalamus. If the lateral hypothalamus is removed (as seen in animal studies), feeding becomes less frequent leading to severe weight loss and death. If the ventromedial hypothalamus is removed, feeding increases, leading to weight gain and severe obesity.
A hormone that counteracts the effects of ghrelin is leptin, which is produced by the fat or adipose tissue in the body. Leptin induces satiation or a feeling of fullness after a meal. When the leptin level is high, hunger is decreased. Since ghrelin increases hunger, several weight loss procedures aim to reduce the ghrelin level in order to increase satiation, even with a small meal.
Ghrelin plays an important role in hunger and therefore weight gain. Also called the growth hormone secretagogue (GHS) receptor, the ghrelin receptor was discovered in 1996. The term ghrelin is based on the hormone’s role as a growth hormone-releasing peptide.
Ghrelin is also important for a process called neurotrophy, which refers to the brain’s ability to adapt to new environments and learn new processes.
Studies suggest that ghrelin enters the hippocampus of the brain from the blood and alters the connections between nerves and cells to enhance learning and memory. Learning is most effective throughout the day and when the stomach is empty, which is when ghrelin levels are higher.