Cyclical changes in concentrations of alpha-melanotrophin in the plasma of male and female rats.

Radioimmunoassay measurements of alpha-melanotrophin in plasma have identified a diurnal rhythm in male rats. Animals maintained on a 12 h light : 12 h darkness photoperiod had raised levels of plasma alpha-melanotrophin during the dark phase. Time-series analysis gave a fitted mean level of alpha-melanotrophin of 52.4 pmol/l, an amplitude of 12.1 pmol/l and peak levels 2.2 h before dawn. Measurements throughout the oestrous cycle in female rats showed that similar variations between the dark and light phases occurred on the 2 days of dioestrus. The raised levels during the dark period were, however, absent on the nights of pro-oestrus and oestrus. During this pro-oestrous/oestrous period, plasma alpha-melanotrophin levels were below average but higher than the normal minimum levels found during the light period.