OPIOIDERGIC PATHWAYS IN ANIMALS REPRODUCTION - THEIR ROLE AND EFFECTSOF THEIR PHARMACOLOGICAL CONTROL

The endogenous opioid peptides (EOP) comprise a family of neurotransmi tters which profoundly influence reproduction. A major component of th eir effects is modulation of gonadotrophin secretion via interactions with other neural pathways or systems impinging upon luteinising hormo ne releasing hormone (LHRH) neurones. Limited evidence also exists for direct effects of EOP at the pituitary level. Generally, the EOP inhi bit luteinising hormone (LH) secretion, although the scale of inhibiti on depends upon the peptide and receptor subtype (mu K, delta, sigma a nd E) involved. This paper briefly reviews more recent evidence for EO P modulation of gonadotrophin secretion and reproductive function in c attle, sheep and pigs from prepuberty to lactation, including modulati on of ovine reproduction by photoperiod. Additionally, data are presen ted from one of our own studies of EOP modulation of LH secretion (and consequent ovarian responses) in 11 sows during early lactation. EOP antagonism by naloxone during the initial 72 h post-partum failed to o vercome suppression of LH secretion. At Day 10 of lactation, however, naloxone administration elevated plasma LH levels. There was no effect of naloxone administration on ovarian follicular diameter. These resu lts and those of others indicate potential applications of opioid agon ists and antagonists to reproductive management of several domestic sp ecies. Such applications are briefly discussed in the context of our o wn results and the literature reviewed.