noun, plural: melanocyte-stimulating hormones
Melanocyte-stimulating hormone is a peptide hormone secreted by the cells of the anterior pituitary, particularly the corticotrophic cell. Corticotrophic cells are one of the different types of cells in the anterior pituitary. Apart from the melanocyte-stimulating hormone (MSH), they also produce and release adrenocorticotropic hormone (ACTH) and lipotropin. Apart from the anterior pituitary, MSH is also secreted by the intermediate part (pars intermedia) of the pituitary gland. MSH secreted by the pars intermedia may be called intermedins. The pars intermedia or the intermediate lobe of the pituitary gland secretes melanocyte-stimulating hormone (MSH). MSH produced by the pars intermedia of the pituitary gland may be in the form of α-MSH, β-MSH, and γ-MSH. Some neurons in the arcuate nucleus of the hypothalamus also produce and secrete MSH (particularly, α-MSH) in response to leptin.
The target cells of MSH are melanocytes (melanin-producing cells) and melanophores (melanin-containing cells). MSH stimulates melanocytes to produce melanin and the melanophores to expand, thereby, resulting in the darkening of the skin. Ultraviolet light is one of the factors stimulating melanocytes (in the skin) to produce MSH. MSH is produced by cleaving certain proopiomelanocortin protein (which is also a precursor to other neuropeptides like adrenocorticotropin).
α-MSH acts through the melanocortin 1 receptor on the surface of the target cell, e.g. of melanocyte, stimulating the latter to produce and release melanin via melanogenesis. In the hypothalamus, α-MSH plays a role in suppressing appetite and in sexual arousal.
MSH is also available as synthetic analogues. Examples are afamelanotide, melatonin II, and bremelanotide.