Through its circadian pattern of melatonin (MT) secretion, the pineal gland helps to keep the internal physiology of animals in synchrony with the day-night cycle and with the annually changing seasons.
Currently, however, the influence of MT on the human reproductive axis appears to be marginal and the available evidence supporting its physiological role is largely circumstantial; there is no clear consensus whether MT levels change during the menstrual cycle.
Objective: The present study evaluated the diurnal rhythm both of melatonin and gonadotropins in healthy women, in order to clarify the implications of circadian neuroendocrin axis.
Methods: Hormone measurements were performed in early follicular phase, ovulation and midluteal phase. aMT6s, a reliable index of MT secretion and gonadotropins, LH and FSH were assayed in urine samples sequentially collected for 3036 h. The reproductive hormones were measured in blood samples prelevated at 8 a.m.
Results: As expected, a marked 24-h variation in MT secretion was found during menstrual cycle. Cosinor analysis of circadian profiles of aMT6s showed discrete changes among phases of menstrual cycle. Mean of amplitudes at ovulation showed a rise at limit signification (F: 11.62±2.49; O: 18.08±5.63; L: 15.5±3.58). Melatonin and gonadotropin secretion showed a positive correlation in early follicular phase and negative one in ovulation. Gonadotropins, LH and FSH, exhibited an evident diurnal rhythm only in ovulation.
Conclusions: The data suggest the hypothesis that melatonin induces chemical changes in secretion of reproductive hormones that respond to light/dark cycles for keeping the balance of reproductive axis. Melatonin secretion positively correlated with gonadotropin secretion in early follicular phase could be explained by a synergic action with gonadotropins, necessary for normal follicular development. At ovulation, melatonin secretion negatively correlated with rising levels of gonadotropin secretion may exert a tonic negative feedback effect for the ovulatory stimulus.