Melanocyte-stimulating hormone
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Melanocyte-stimulating hormone (MSH) is a peptide hormone produced by cells in the intermediate lobe of the pituitary gland. It stimulates the production and release of melanin (melanogenesis) by melanocytes in skin and hair.
In some animals (such as the clawed toad Xenopus laevis) production of MSH is increased when the animal is in a dark location. This causes pigment to be released from cells in the toad's skin, making it become darker, and harder for predators to spot.
An increase in MSH will cause a darkening in humans too. Melanocyte-stimulating hormone increases in humans during pregnancy. This, along with increased estrogens, causes increased pigmentation in pregnant women. In Addison's disease high levels of adrenocorticotropic hormone (ACTH), a compound similar to MSH causes an abnormal darkening.
Different levels of MSH are not the major cause of racial variation in skin colour. In many red headed people, and other people who do not tan well, there are variations in their hormone receptors, causing them to not respond to MSH in the blood.
Melanocyte-stimulating hormone belongs to a group called the melanocortins. This group includes ACTH, alpha-MSH, beta-MSH and gamma-MSH. Alpha-MSH is the most important in pigmentation.
The different melanocyte-stimulating hormones have the following amino acid sequences:
| alpha-MSH: | Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2 |
| beta-MSH (human): | Ala-Glu-Lys-Lys-Asp-Glu-Gly-Pro-Tyr-Arg-Met-Glu-His-Phe-Arg-Trp-Gly-Ser-Pro-Pro-Lys-Asp |
| beta-MSH (porcine): | Asp-Glu-Gly-Pro-Tyr-Lys-Met-Glu-His-Phe-Arg-Trp-Gly-Ser-Pro-Pro-Lys-Asp |
| gamma-MSH: | Tyr-Val-Met-Gly-His-Phe-Arg-Trp-Asp-Arg-Phe-Gly |
Two synthetic analogs of alpha-MSH, called Melanotans I and II are being developed by an Australian company. Melanotan II causes enhanced libido and erections in some male test subjects. PT-141 is a close analog of melanotan II that shares its aphrodisiac effects and is currently in clinical trials to treat erectile dysfunction and sexual arousal disorder.