Gonadotrophin-releasing hormone and oxytocin secretion from the hypothalamus in vitro during prooestrus: The effects of time of day and melatonin

An accurately timed surge of luteinizing hormone (LH), during the second ha lf of the day of pro-oestrus in rats, is a crucial part of the endocrine si gnal that leads to expulsion of an ovum from an ovarian follicle. LH releas e is partly controlled by a number of peptides, including gonadotrophin-rel easing hormone (GnRH) and oxytocin, which travel from the hypothalamus to t he pituitary. The profile of secretion of these peptides is poorly understo od. Therefore, the amounts of GnRH and oxytocin that were secreted from hyp othalamic explants were determined at several time points during the day of pro-oestrus. Basal secretion of oxytocin from hypothalami taken rater in p ro-oestrus was greater than from hypothalami taken earlier in the day (p < 0.02). On the other hand, basal secretion of GnRH decreased during the day of pro-oestrus (p < 0.03). The different trends of GnRH and oxytocin secret ion reveal that their secretion is regulated by distinct mechanisms. GnRH s ecretion was higher at midpro-oestrus than late in the day (p < 0.05) consi stent with a peak of GnRH having been observed by others in portal blood in the second half of the day of pro-oestrus. Responsiveness of oxytocin to s timulation by K+ of the hypothalami declined from the early light hours to the evening dark hours (p < 0.02). Thus, oxytocin modulation might be achie ved partly by modification of intracellular processes. Melatonin, secreted during hours of darkness, is frequently involved in modulating time-depende nt events in mammals, but its contribution to peptide regulation during the ovulatory cycle is unclear. Melatonin was observed to inhibit basal oxytoc in secretion from hypothalami collected during light hours (p < 0.05). The investigation has, therefore, revealed the potential for melatonin to modul ate peptide secretion from the hypothalamus during the day of pro-oestrus. We also observed that secretion from the hypothalamus of the two LH-regulat ing peptides, GnRH and oxytocin, are differently regulated during the day o f pro-oestrus. (C) 1999 Elsevier Science Inc.