Melanin-stimulating hormone synthesis

The anterior lobe secretes various trophic hormones, the posterior lobe is responsible for the secretion of oxytocin and antidiuretic hormone (vasopressin) and middle lobe secretes melanocyte-stimulating hormone (MSH) which may affect the synthesis of melanin. 

MSHs consist of three peptide hormones a-MSH, 3-MSH, and y-MSH, which are secreted by intermediate lobe of the pituitary gland. They are cleaved from the same precursor peptide as ACTH. Their basic function is stimulation of melanocytes to darken skin and stimulation of melanin synthesis to darken the skin and hair. They also have been found to be released in the brain affecting appetite, sexual arousal, and many other functions. 

In addition, both ACTH and MSH stimulate lipolysis in rabbit adipose tissue. That the melanophore stimulating activity associated with purified ACTH is intrinsic to the hormone and not due to contamination with the melanotropins, was conclusively demonstrated by the synthesis of a nonadecapeptide corresponding to the first 19 residues of ACTH ( ). This synthetic peptide stimulated steroidogenesis in the adrenal gland and also elicited melanin dispersion. 

Tyrosinase activity decreases after hypophysectomy, but can be restored by chronic administration of pituitary hormones like prolactin, MSH, and ACTH to a greater or lesser extent (131). Prolactin can promote in vivo melanin synthesis not only by stimulating tjn osinase activity, but also by increasing the supply of available substrate (131). However, it is still unknown which of the pituitary hormones is responsible for hyperpigmentation in Addison s disease in man. 

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