A significant diurnal variation in α-melanocyte stimulating hormone (α-MSH) concentrations was observed in discrete regions of the rat brain. During the 12 h light: 12 h dark cycle, α-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 α-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 α-MSH concentrations at 13.00 h. In the medial preoptic nucleus, the peak α-MSH concentrations appeared at 17.00 h. These changes in α-MSH content may reflect an α-MSH role in circadian variations in behavioral and neuroendocrine processes.

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