ESTROGENIC EFFECTS ON URINARY 6-SULPHATOXYMELATONIN EXCRETION IN THE FEMALE RAT

The pattern of melatonin production during the estrous cycle of the ra t was measured by monitoring urinary 6-sulphatoxymelatonin (aMT.6S) ex cretion. Adult rats were maintained under a 14L:10D photoperiod and ur ine was collected at hourly intervals over a 5-day period using an aut omated collection system; the concentration of aMT.6S was assayed by R IA and hourly outputs were calculated. Each nightly collection of urin e was assigned to an estrous cycle stage as determined by the vaginal smear of the preceding morning. Total aMT.6S excretions (mean +/- SEM) during estrous, metestrous, diestrous, and proestrous stages were wer e 493 +/- 49, 539 +/- 44, 562 +/- 40, and 646 +/- 51 pmol/night. respe ctively (n = 7). The excretion of aMT.6S was significantly higher on t he night of proestrus compared to each of the other stages (P < 0.05). To determine whether estrogen was responsible for the increased aMT.6 S excretion during proestrus, rats were studied before and after ovari ectomy and following implantation with estradiol implants. Total overn ight aMT.6S excretion was reduced by 31% in ovariectomized animals rel ative to the intact state (P < 0.05) and restored to the intact levels by administration of estradiol (P < 0.05). It was concluded that estr adiol can modulate melatonin production in adult rats, and that the ch anging pattern of aMT.6S excretion throughout the estrous cycle may pr ovide a basis for a functional relationship between pineal activity an d reproduction in this species.