EFFECTS OF THE OPIOID ANTAGONIST NALOXONE ON RELEASE OF LUTEINIZING-HORMONE IN MARES DURING THE ANOVULATORY SEASON

To investigate an involvement of endogenous opioids in the regulation of circannual changes in reproductive activity, effects of the opioid antagonist naloxone on the concentration of immunoreactive and bioacti ve luteinizing hormone (LH) in plasma were measured in mares during th e anovulatory season. Naloxone (0.5 mg/kg i.v.) caused a significant i ncrease (P<0.05) in immunoreactive as well as bioactive LH concentrati on in plasma. The amplitude of the increase in LH concentrations measu red with an in vitro bioassay was more pronounced than the amplitude o f the increase in LH secretion determined by radioimmunoassay. This in dicates that although in seasonal anovulatory mares the bioactivity of LH in plasma is low, highly bioactive LH is present in the anterior p ituitary and can be released by naloxone. The LH response to naloxone did not depend on the degree of ovarian follicular activity. it can be concluded that a tonic opioid inhibition of LH release is present in mares during at least part of the anovulatory season and that endogeno us opioids seem to be involved in the regulation of seasonal reproduct ive activity in the horse. In contrast to the situation during the bre eding season, the opioid systems regulating LH release are activated i ndependently of luteal progesterone.