Effects of melatonin and time of day on in vitro pituitary gonadotropin basal secretion and GnRH responsiveness in the male golden hamster.

Melatonin, a pineal hormone, is known to play an important role in mediating changes in the reproductive system which occur in response to seasonal changes in the length of the day. An in vitro pituitary perifusion system has been used to examine both direct and indirect effects of melatonin on pituitary gonadotropin basal secretion and responsiveness to GnRH stimulation. Anterior pituitaries from adult male golden hamsters were perifused with an APS 10 perifusion system for 6 h. Both basal secretion and secretion in response to hourly GnRH stimulation (1 min, 35 ng/ml pulses) were evaluated for LH and FSH. In order to expose pituitaries to shorter and longer durations of melatonin, tissue was perifused with melatonin-containing medium following dissection in the daytime (when endogenous melatonin levels are low) and following dissection in the nighttime (when endogenous melatonin levels are high). Exposure of pituitary tissue from golden hamsters to melatonin during perifusion caused a decrease in basal secretion of LH but did not affect GnRH stimulated LH or FSH release regardless of the time of dissection. There was, however, an effect of some diurnal factor to lower FSH responsiveness to GnRH stimulation from tissue dissected in the nighttime. It is suggested that melatonin may be responsible for this diurnal difference. Therefore, melatonin can act directly at the anterior pituitary to lower basal LH secretion, and melatonin or some other diurnal factor can act in vivo to lower pituitary FSH secretion in response to GnRH stimulation during the night.