DistributionDistribution, Subcellular Localization and Identity of Immunoreactive α-Melanotropin in the Pituitary Gland and Brain

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