ENDOGENOUS OPIOIDS AND REPRODUCTIVE FUNCTIONS IN THE HORSE

In the horse, endogenous opioids participate in the regulation of repr oductive functions. Opioids inhibit LH release in mares during the lut eal phase as well as in seasonally anovulatory mares and in stallions. The opioid inhibition of LH release in cyclic mares requires exposure to oestradiol and progesterone, and thus is regulated by a sequence o f altering steroid environments. In seasonally anovulatory mares, an o pioid inhibition of LH secretion might either be activated by low oest rogen concentrations or be independent from ovarian factors. An opioid regulation of prolactin secretion could not be detected in ovary-inta ct mares, irrespective of the time of the year. In ovariectomized mare s, however, pretreatment with oestradiol and with oestradiol plus prog esterone activated a naloxone-reversible inhibition of prolactin relea se. Opioids affect LH and prolactin release in stallions also. The opi oid mechanisms are affected by gonadal hormones, undergo seasonal chan ges and, for LH, are most active during the non-breeding season. This could explain an increase in plasma LH concentrations that is seen at the beginning of the breeding season. An opioid regulation of prolacti n secretion is evident in stallions, but seasonal changes do not paral lel variations in the regulation of LH release.