DISTRIBUTION OF ALPHA-MELANOCYTE-STIMULATING HORMONE IN THE RAT-BRAIN - EVIDENCE THAT ALPHA-MSH-CONTAINING CELLS IN THE ARCUATE REGION SEND PROJECTIONS TO EXTRAHYPOTHALAMIC AREAS

The distribution and concentration of α-MSH in the rodent brain has been determined by radioimmunoassay. The limbic system contained substantial quantities of α-MSH. Forty per cent of the α-MSH present in the brain was localized in the hypothalamus, with the highest concentration of α-MSH in the arcuate nucleus. More than 40% of the extrahypothalamic α-MSH in the brain was found in the following areas: midbrain (16%), preoptic area (13%), septum (7%), and thalamus (7%). To determine the source of the hypothalamic and extrahypothalamic α-MSH, the anterior hypothalamic preoptic area of the brain was surgically separated from more caudal diencephalic structures, and the arcuate region of the hypothalamus was surgically isolated from the remainder of the brain. Following these deafferentations, no significant reduction in hypothalamic α-MSH levels was observed; however, a significant reduction in extrahypothalamic α-MSH levels was demonstrated. This dramatic decrease of α-MSH in extrahypothalamic areas of the rodent brain strongly suggests that the bulk of the extrahypothalamic α-MSH arises from neuronal perikarya in the arcuate region. These findings are consistent with the hypothesis that a population of neuronal cell bodies producing α-MSH originate in the arcuate region of the hypothalamus and that they send axonal projections to many areas of the limbic system and brain stem. © 1979.