INFLUENCE OF MELATONIN AND ESTRADIOL ON THE OPIOIDERGIC REGULATION OFLH AND PROLACTIN-RELEASE IN PONY MARES

The aim of this study was to investigate the influence of oestradiol, melatonin and season on the opioid regulation of LH and prolactin rele ase. Effects of the opioid antagonist naloxone (0.5 mg/kg) on LH and p rolactin secretion were determined in ovariectomized pony mares. In ex periment 1, mares in January (n=6) were pretreated with oestradiol ben zoate (5 mu g/kg) for 20 days. In experiment 2, beginning in May, mare s (n=7) received melatonin (15 mg) for 15 days and subsequently a comb ination of melatonin plus oestradiol for 20 days. In experiment 3, beg inning in May, mares (n=6) were pretreated with oestradiol for 30 days , left untreated for 12 days and then given melatonin for 35 days. In all experiments the animals were injected with the opioid antagonist n aloxone and saline on 2 consecutive days prior to treatment. In experi ment 1, animals received naloxone and saline on days 10 and 11 and 20 and 21 following oestradiol treatment. In experiment 2, naloxone and s aline were administered on days 15 and 16 following melatonin treatmen t and on days 10 and 11 and 20 and 21 of melatonin plus oestradiol tre atment. In experiment 3, the animals received naloxone and saline on d ays 10 and 11, 20 and 21 and 30 and 31 of oestradiol treatment, prior to melatonin treatment and on days 15 and 16, 25 and 26 and 35 and 36 following melatonin. In January (experiment 1), naloxone evoked a sign ificant (P<0.05) LH release at all times, however the LH increment in response to naloxone increased during oestradiol pretreatment (P<0.05) . During the breeding season (experiments 2 and 3), naloxone induced a significant (P<0.05) increase in plasma LH concentrations when mares had not been pretreated with oestradiol or melatonin and after oestrad iol pretreatment. Basal LH concentrations and the LH increment in resp onse to naloxone increased significantly (P<0.05) during the 30-day oe stradiol pretreatment. Melatonin decreased the naloxone-induced LH rel ease and the LH release in response to naloxone and saline no longer d iffered after 25 and 35 days of melatonin pretreatment. When melatonin was given together with oestradiol for 20 days, again a significant ( P<0.05) LH release in response to naloxone occurred. Prolactin release was significantly (P<0.05) increased by naloxone when mares had been pretreated with only melatonin. The opioid antagonist did not affect p rolactin release in mares that had not been pretreated or received oes tradiol either alone or in combination with melatonin. In conclusion, in longterm ovariectomized mares, opioids inhibit LH secretion indepen dent from ovarian factors. This opioid inhibition of LH secretion is e nhanced by oestradiol and reduced by melatonin. Although short-term me latonin treatment inactivates the opioid regulation of LH release, a p rolonged influence of melatonin as occurs in winter does not prevent a ctivation of the opioid system. This indicates that effects of melaton in on the opioid regulation of LH release change with time. An opioid inhibition of prolactin secretion is activated by melatonin given for 15-35 days but is lost under the prolonged influence of a short-day me latonin signal in winter.