Endogenous opioids and reproductive functions in the horse

Abstract

In the horse, endogenous opioids participate in the regulation of reproductive functions. Opioids inhibit LH release in mares during the luteal 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 of altering steroid environments. In seasonally anovulatory mares, an opioid inhibition of LH secretion might either be activated by low oestrogen concentrations or be independent from ovarian factors. An opioid regulation of prolactin secretion could not be detected in ovary-intact mares, irrespective of the time of the year. In ovariectomized mares, however, pretreatment with oestradiol and with oestradiol plus progesterone activated a naloxone-reversible inhibition of prolactin release. Opioids affect LH and prolactin release in stallions also. The opioid mechanisms are affected by gonadal hormones, undergo seasonal changes 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 prolactin secretion is evident in stallions, but seasonal changes do not parallel variations in the regulation of LH release.