INHIBITION OF GONADOTROPIN-RELEASE IN MARES DURING THE LUTEAL-PHASE OF THE ESTROUS-CYCLE BY ENDOGENOUS OPIOIDS

Effects of the opioid antagonist naloxone on concentrations of LH and FSH in plasma were measured in mares during different stages of the oestrous cycle. During the follicular phase of the cycle, naloxone (300 mg i.v.) had no discernible effects on basal concentrations of LH and FSH. A significant increase in plasma LH (P < 0.01) and FSH (P < 0.05) concentrations was observed after naloxone in mares during the luteal phase. This response was not different between suckled and non-suckled mares. The gonadotrophin-releasing hormone analogue buserelin (0.02 mg i.v.) caused a significant (P < 0.05) LH and FSH release irrespective of the stage of the oestrous cycle and a previous naloxone treatment. The results of this study indicate that endogenous opioid peptides are involved in the inhibition of LH and FSH release during the luteal phase of the oestrous cycle in mares and may partially mediate the suppressive influence of progesterone on gonadotrophin secretion. The opioid-mediated suppression of LH and FSH release does not seem to be affected by suckling.