Distribution, subcellular localization and identity of immunoreactive alpha-melanotropin in the pituitary gland and brain.

In homogenates of adult human or rat hypothalamic tissue, immunoreactive alpha-MSH (alpha-MSHi) is concentrated in synaptosomes, suggesting that it is localized in neurons. This conclusion is supported by immunohistochemical results on the localization of alpha-MSHi in the brain. The function of alpha-MSHi in the hypothalamus is undefined but the amount of alpha-MSH in this tissue increases in young rats and decreases in old rats. There is little alpha-MSHi in pituitary glands from human abortuses before the 15th gestational week. Thereafter the amount of alpha-MSHi in the anterior and neurointermediate lobes increases appreciably. When acetic acid extracts of each lobe were analysed by high-pressure liquid chromatography the alpha-MSHi in extracts of the anterior lobe had a retention time equal to that of desacetyl alpha-MSH; little if any alpha-MSHi had a retention time identical to that of desacetyl alpha-MSH; alpha-MSH, if present in these extracts, appeared to be a minor component. A similar analysis of extracts of adult human hypothalamic tissue yielded results suggesting that the alpha-MSHi in this tissue is also attributable largely to desacetyl alpha-MSH.