MELATONIN DIRECTLY SUPPRESSES STEROID-PRODUCTION BY PREOVULATORY FOLLICLES IN THE CYCLIC HAMSTER

The purpose of these studies was to investigate the effects of melaton in on the production of steroids (progesterone, testosterone, and estr adiol) and cAMP by preovulatory follicles and to examine changes in me latonin concentrations in the ovary during the estrous cycle. Adult cy clic hamsters were used in this study. Melatonin concentrations in the ovary, pineal gland, and serum were measured at mid-light and mid-dar k during the estrous cycle. Effects of melatonin on steroidogenesis by preovulatory follicles, thecae, and granulosa cells were examined, an d its effect on cAMP production by preovulatory follicles was also inv estigated. Melatonin (0.1-10 ng/ml) had no effect on steroid productio n in the absence of hCG, but melatonin decreased progesterone and estr adiol production by preovulatory follicles in a dose-dependent and tim e-dependent manner in the presence of hCG (100 mIU/ml). The target of melatonin was thecae but not granulosa cells, and melatonin significan tly reduced cAMP production by preovulatory follicles. Melatonin conce ntrations in the ovary showed a similar phasic variation with high lev els during mid-dark and low during mid-light, as in the pineal gland a nd serum. These results show that the ovarian melatonin levels also ex hibit a circadian rhythm and suggest that the high melatonin milieu in the ovary may induce gonadal regression in the cyclic hamster.