Biological and Radioimmunological Evidence for Melanocyte Stimulating Hormones (MSH) of Extrapituitary Origin in the Rat Brain

Abstract

The possible existence of extrapituitary melanocyte stimulating hormone (MSH) in various regions of the rat brain was studied in intact and hypophysectomized rats. Using a sensitive and specific radioimmunoassay (RIA), .alpha.MSH has been found in a number of brain regions in intact rats. The standard curves of synthetic .alpha.MSH and the dilution curves for pars intermedia nervosa (PIN), pars distalis (PD), hypothalamus and thalamus extracts were strictly parallel. The .alpha.MSH concentrations were measured in PIN (6225 .+-. 962 ng/mg wet tissue); PD (12.5 .+-. 1.41 ng/mg); pineal (380 .+-. 29 ng/g wet tissue); hypothalamus (645 .+-. 161 ng/g) and thalamus (33.3 .+-. 5.26 ng/g). In rats hypophysectomized for 1 or 2 mo., the highest concentrations of immunoreactive .alpha.MSH were found in pineal (353 .+-. 140 ng/g wet tissue), hypothalamus (85.8 .+-. 14.1 ng/g) and thalamus (39.8 .+-. 13.9 ng/g). Hypophysectomy significantly reduced hypothalamic MSH content and concentration but did not alter MSH concentration in pineal and thalamus. Apparently hypothalamic .alpha.MSH is, in part, of hypophyseal origin while pineal and thalamus .alpha.MSH does not originate from the pituitary. After Sephadex G-25 gel filtration, synthetic .alpha.MSH and PIN extracts showed a single peak of both bioactive and immunoreactive .alpha.MSH. In the same conditions, extracts from the 5 brain regions studied in hypophysectomized rats chromatographed as a single peak of immunoreactive MSH but as 2 peaks of apparent bioactive MSH, 1 concident with synthetic .alpha.MSH and the other far after the salt volume. .alpha.MSH is found in a number of brain areas and its presence after hypophysectomy would indicate synthesis within the CNS.