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Ghrelin receptors are structures which bind to the hormone ghrelin, and which are found on the surfaces of cells in the pituitary gland. The pituitary gland is situated at the base of the brain where it produces a number of important hormones which control the body's vital functions. While much remains unknown about ghrelin, the hormone is thought to be involved in appetite regulation and fat metabolism. It is known that, when ghrelin receptors are stimulated by ghrelin binding to them, the pituitary gland secretes growth hormone. Ghrelin receptors are found in other parts of the brain, including the hypothalamus, which relays information between the brain and the pituitary gland.
Long before the hormone ghrelin was first recognized, ghrelin receptors were discovered. Initially, they were known as growth hormone secretagogue receptors, because their most notable feature when stimulated was to cause growth hormone secretion. It was only later, when ghrelin was found and named, that the receptors acquired their more specific name. Ghrelin is mainly produced by the stomach, and levels are known to rise when people are fasting and to fall after a meal. Although it is not fully understood what controls the secretion of ghrelin by the stomach, it is known that the hormones leptin and estrogen affect its release.
As well as stimulating growth hormone release, the activation of ghrelin receptors causes other effects in the body. When ghrelin binds to receptors in the hypothalamus, in areas which are known to be concerned with appetite control, the sensation of hunger is increased. Ghrelin receptor activation is also known to cause a reduction in the body's use of fat stored in fatty tissues. Both of these effects associated with ghrelin action tend to lead to obesity. It is thought that the development of ghrelin receptor antagonists, drugs which would bind to ghrelin receptors and block them, could provide useful treatments for obesity.
In research studies involving humans and rats, ghrelin has been found to play an important part in those brain systems which make certain behaviors rewarding and predispose people to developing addictions. Addictions to sweet foods and alcohol were both found to be associated with increased levels of ghrelin. Consumption of alcohol and sugary food is associated with the activation of ghrelin receptors in parts of the brain such as the hippocampus. This is thought to trigger chemical processes which are experienced as rewarding by the individual. When researchers used antagonists to block ghrelin receptors, they found that levels of consumption of alcohol and sweet foods decreased.