Acid-ethanol extracts of various regions of the rat brain were analyzed by radioimmunoassay for α-MSH. The highest concentrations of α-MSH in the brain, exluding the pituitary, were found in the pineal gland (163 ± 44 pg/mg, mean ± SE) and hypothalamus (143 ± 22 pg/mg). 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. However, these values were much lower than those in the posterior pituitary (1.1 ± 0.12 μg/mg) and anterior pituitary (4.5 ± 0.83 ng/mg). Immunoreactive α-MSH extracted from the rat brain and synthetic α-MSH 1), react similarly with a specific antiserum to synthetic α-MSH; 2), are equally susceptible to degradation by plasma enzymes; 3), have similar chromatographic properties; and 4), possess 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. However, when the extracts were fractionated by gel filtration on Sephadex G-25, MSH bioactivity and α-MSH immunoreactivity were recovered in the same fractions of the column eluate. Even so, the quantity of immunoreactive α-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 α-MSH was still present in various regions of the rat brain 3 weeks after hypophysectomy. These findings are consistent with the conclusion 1), that α-MSH is present in appreciable quantity in the rat brain and 2), that the pituitary gland is not the sole source of α-MSH extracted from the rat brain.
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