A diurnal rhythm of immunoreactive alpha-melanocyte-stimulating hormone in discrete regions of the rat brain.

A significant diurnal variation in alpha-melanocyte stimulating hormone (alpha-MSH) concentrations was observed in discrete regions of the rat brain. During the 12 h light: 12 h dark cycle, alpha-MSH concentrations in each case were highest during the light period and lowest during the dark period. At 09.00 h, 3 h after lights were turned on, the peak alpha-MSH concentration occurred in the median eminence, the arcuate and dorsomedial hypothalamic nuclei and the periventricular thalamic nucleus. The paraventricular and anterior hypothalamic nuclei had highest alpha-MSH concentrations at 13.00 h. In the medial preoptic nucleus, the peak alpha-MSH concentrations appeared at 17.00 h. These changes in alpha-MSH content may reflect an alpha-MSH role in circadian variations in behavioral and neuroendocrine processes.