Demonstration of an endogenous circadian rhythm of α-melanocyte stimulating hormone in the rat pineal gland

Abstract

α-Melanocyte stimulating hormone (α-MSH) has been identified and characterized in the rat pineal gland by a combination of immunochemical and high pressure liquid chromatographic techniques. The immunoreactivity in pineal extracts was separated into two chromatographic components. The major component had a retention time identical to that of α-MSH while the minor component eluted just slightly before standard α-MSH. Male rats maintained in a 12 h photoperiod demonstrated a marked circadian rhythm in pineal α-MSH concentration. Concentrations that peaked at 07.00 h, 1 h after the lights were turned on, were greater than 5 times the nadir which occurred at 01.00 h. Animals housed in chronic dark for 7 days maintained the diurnal variation of α-MSH concentrations. However, in chronically dark housed rats, the peak shifted to 05.00 h and was greater than 10 times the nadir of this rhythm and approximately 4 times the peak at 07.00 in alternating light/dark conditions. Rats exposed to chronic light for 7 days maintained a pineal α-MSH rhythm although the amplitude of the peak was significantly decreased compared to the rhythm in animals housed in alternating light/dark conditions. Neither hypophysectomy nor superior cervical ganglionectomy had any effect on the α-MSH rhythm. Lesion of the arcuate nucleus, the major source of α-MSH-containing nerves in the brain, did not significantly affect pineal α-MSH concentrations. These data demonstrate a circadian α-MSH rhythm in the rat pineal and suggest an α-MSH involvement in the rhythmic processes of the pineal gland.