MELATONIN ENHANCES THE LUTEINIZING-HORMONE AND FOLLICLE-STIMULATING-HORMONE RESPONSES TO GONADOTROPIN-RELEASING-HORMONE IN THE FOLLICULAR, BUT NOT IN THE LUTEAL, MENSTRUAL PHASE

Exogenous melatonin enhances LH pulse amplitude and mean LH levels in women during the follicular, but not the luteal, menstrual phase. In this study we investigated whether an increased pituitary response to GnRH is involved in the stimulatory effect of melatonin. Eight normal cycling women were studied on 2 consecutive days during the follicular stage (days 4-6), and eight were studied during the luteal phase (days 19-21) of the menstrual cycle. On 2 consecutive days, each woman received, randomly and in a double blind fashion, placebo or 3 mg melatonin (1 mg at 0800 1000, and 1200 h), whereas the pituitary LH and FSH responses to GnRH were tested by the iv administration of three submaximal doses of GnRH (1 mu g at 0900 h, 5 mu g at 1100 h, and 10 mu g at 1300 h). In the follicular phase, melatonin administration enhanced the LH and FSH responses to all three GnRH stimuli, whereas in the luteal phase, melatonin administration was ineffective. The present data indicate that an enhancing effect of melatonin on the LH and FSH responses to submaximal GnRH stimuli is evident in the follicular, but not the luteal, phase of the menstrual cycle and infer an endocrine window for the effect of melatonin on gonadotropin secretion.