Progesterone inhibits, on a circadian basis, the release of melatonin by rat pineal perifusion

The effects of 10(-6) and 10(-9) M of progesterone were documented on isopr oterenol-stimulated melatonin release by perifused pineal glands removed fr om female rats in diestrous at two different times of a 12: 12 h light/dark cycle, 7 and 19 h after light onset (which corresponds to daytime and nigh ttime, respectively, to look for the existence of a circadian stage-depende nce of the hormone effects. Three weeks before the experiment, the rats wer e synchronized with a 12: 12 lighting regimen. Progesterone decreased by si milar to 50% the release of melatonin during the light span, but not during the dark span. These results show the direct effects of this ovarian hormo ne on pineal melatonin release and strongly suggest a time-related effect o f progesterone on pineal function. (C) 2000 Elsevier Science Inc. All right s reserved.