Influence of opioids on LH secretion in gilts during the estrous cycle.

The effect of endogenous opioid peptides (EOP) on LH secretion is variable during different physiological states. A series of experiments concerning the role of EOP on LH secretion in cyclic gilts was performed. They were comprised of (1) an administration of an opioid antagonist or agonist in gilts during the estrous cycle and in ovariectomized (OVX) gilts in which the LH surge was induced with estradiol benzoate (EB) and (2) in vitro studies on GnRH release from the stalk median eminence (SME) of cyclic gilts and OVX estrogen and progesterone primed gilts in response to naloxone (NAL). Naloxone and met-enkephalin analogue (FK 33-824) administration as a single independent injections did not affect LH secretion during the early (Day 16) or late (Day 19 or 20) follicular phase. However, continuous infusion of FK 33-824 for 4 h decreased LH secretion during the infusion period on Day 19 of the estrous cycle. Morphine also exerts an inhibitory effect on the EB-induced LH surge during the positive feedback phase (60-64 h after EB administration) in OVX gilts. On the contrary, NAL infusion in OVX gilts during the negative feedback phase (30-34 h after EB administration) did not alter LH secretion. A single injection of FK 33-824 in luteal phase gilts decreased the number of LH pulses for a 3 h period. This allows to hypothesize that EOP participates in the regulation of pulsatile LH secretion in pigs during the luteal phase. In vitro studies indicate that influence of EOP on LH secretion also takes place at the SME level. GnRH efflux from the SME of gilts during the luteal and late follicular phases was augmented in the presence of NAL. Unexpectedly, the priming of OVX gilts with estrogens caused the highest increase in GnRH release from the SME in vitro in response to NAL. These results confirm the variety of functional links between the opioid system and LH secretion in gilts during different stages of the estrous cycle.