ROLE OF ENDOGENOUS OPIOIDS FOR REGULATION OF THE ESTROUS-CYCLE IN THEHORSE

In the horse mare, endogenous opioids are involved in the regulation o f LH and prolactin release from the pituitary. Opioids inhibit LH rele ase in anovulatory mares during the non-breeding season and in cyclic mares during the luteal but not the follicular phase as indicated by a n increase in plasma LH concentrations after injection of the opioid a ntagonist naloxone. Naloxone did not change plasma prolactin concentra tions in ovary-intact mares, irrespective of the time of the year. In ovariectomizied mares, pretreatment with oestradiol or with progestero ne combined with oestradiol activated an opioid inhibition of LH and p rolactin release, whereas progesterone alone did not induce an opioid effect on pituitary hormone release. However, progesterone seems to mo dulate the activity of primarily oestrogen-dependent mechanisms. It ca n be concluded that in cyclic mares, the opioid inhibition of anterior pituitary hormone release is regulated not by one specific steroid ho rmone but by a sequence of steroid hormone environments characteristic of the oestrous cycle.