EFFECTS OF GONADAL-STEROIDS ON THE OPIOID REGULATION OF LH AND PROLACTIN-RELEASE IN OVARIECTOMIZED PONY MARES

The aim of the present study was to investigate the role of ovarian st eroids in the opioid regulation of LH and prolactin release in mares. Effects of the opioid antagonist naloxone on LH and prolactin secretio n were determined in ovariectomized pony mares. The animals were pretr eated with either progesterone (500 mu g kg(-1)) or oestradiol benzoat e (10 mu g kg(-1)) for 8 days and subsequently with a combination of p rogesterone and oestradiol for an additional 8 days. Naloxone administ ration (0.5 mg kg(-1) i.v.) resulted in a significant release of LH as well as prolactin in mares after pretreatment with either oestradiol benzoate or progesterone plus oestradiol benzoate (P < 0.05). No signi ficant changes in LH and prolactin secretion were detected in progeste rone-treated and non-steroid-treated ovariectomized mares. These resul ts indicate that a prolonged oestrogen influence activates the opioid inhibition of LH and prolactin release in mares. In contrast to other species, progesterone alone does not activate a tonic opioid inhibitio n of LH and prolactin secretion, but modulates the effect of oestrogen s. The opioid systems therefore seem to be regulated by a sequence of different steroid environments, as found during the oestrous cycle. Th e parallel increases in prolactin and LH secretion in mares may indica te a common regulatory pathway for these two hormones.