Melanin-stimulating hormone

Elevation of temperature increases tyrosinase activity, shortens the induction period and stimulates melanin formation within physiological limits. The enzyme activity is optimal in the pH range 6.1-1.2 during in vitro reactions 183). The temperature responses of melanin-stimulating hormones have also been studied (226). 

The lateral hypothalamic area has been identified as a feeding centre by studies involving electric stimulation and discrete lesions. Neurons in the lateral hypothalamic area and the neighbouring perifornical area express neuropeptides that stimulate feeding when injected into cerebral ventricles (orexins 1 and 2, melanin-concentrating hormone (MCH)). 

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. 

Melanin concentrating hormone (MCH) acts as a functional antagonist to melanocyte stimulating hormone (see below) (Truant et al. 1972). Direct injection of MCH into the lateral ventricle of laboratory animals leads to an immediate increase in food intake. 

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. 

B. Excess phenylalanine inhibits tyrosinase the first step toward melanin production, thus resulting in hypopigmentation. Excess melanin leads to hyperpigmentation. Melatonin is a hormone involved in the sleep cycle. Excessive stimulation of tyrosinase would lead to more melanin and therefore hyperpigmentation. Para-hydroxyphenylpyruvate means less transamination and perhaps more tyrosine converted to melanin and hyperpigmentation. 

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. 

Among sex hormones, the female hormones are strong stimulants of melanogenesis (251). The reports reveal that administration of small doses of estrogen to ovariectomized guinea pigs increases the melanin content of melanocytes in all skin regions examined, while the effect of... 

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