Distribution and Characterization of a-Melanocyte-Stimulating Hormone in the Rat Brain*

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Abstract
Acid-ethanol extracts of various regions of the rat brain were analyzed by radioimmunoassay for .alpha.-MSH [melanocyte stimulating hormone]. The highest concentrations of .alpha.-MSH in the brain, excluding the pituitary, were found in the pineal gland (163 .+-. 44 pg/mg, mean .+-. SE) and hypothalamus (143 .+-. 22 pg/ml). The levels in the thalamus, brain stem, cerebrum and cerebellum were 25 .+-. 2.7, 11 .+-. 0.9, 5.7 .+-. 0.7 and 2.3 .+-. 0.6 pg/mg, respectively. These values were much lower than those in the posterior pituitary (1.1 .+-. 0.12 .mu.g/mg) and anterior pituitary (4.5 .+-. 0.83 ng/mg). Immunoreactive .alpha.-MSH extracted from the rat brain and synthetic .alpha.-MSH: reacted similarly with a specific antiserum to synthetic .alpha.-MSH; were equally susceptible to degradation by plasma enzymes; had similar chromatographic properties; and possessed comparable electrophoretic mobilities. When unpurified extracts from various regions of the brain were assayed for MSH-like activity by using the skin of the lizard, Anolis carolinensis, no activity was detected. When the extracts were fractionated by gel filtration on Sephadex G-25, MSH bioactivity and .alpha.-MSH immunoreactivity were recovered in the same fractions of the column eluate. The quantity of immunoreactive .alpha.-MSH in these fractions was generally higher than that of bioactive MSH. This difference was attributed to the presence of substance inhibiting skin darkening in the purified fractions. Immunoreactive .alpha.-MSH was still present in various regions of the rat brain 3 wk after hypophysectomy. .alpha.-MSH is present in appreciable quantity in the rat brain and the pituitary gland is not the sole source of .alpha.-MSH extracted from the rat brain.