Day-night differences in the effects of gonadal hormones on melatonin release from perifused rat pineals. Evidence of a circadian control.

The effects of testosterone (10(-3) M, 10(-6) M, and 10(-9) M) and 17 beta-estradiol (10(-3) M and 10(-9) M) were looked for upon isoproterenol-stimulated melatonin release by perifused pineal glands removed from male rats and female rats in diestrus. Two different times of a 12/12 hour light/dark cycle, i.e., 7 and 19 hours after light onset, were documented in order to look for the existence of a circadian stage-dependence of the hormone effects. Both testosterone (45-60%) and estradiol (60-80%) markedly increased melatonin release by glands removed during the dark span and not during the light span. These results show a direct effect of gonadal hormones on pineal melatonin release and strongly suggest a time-related effect of these hormones on pineal function.