Opioidergic pathways in animal reproduction: Their role and effects of their pharmacological control

The endogenous opioid peptides (EOP) comprise a family of neurotransmitters which profoundly influence reproduction. A major component of their effects is modulation of gonadotrophin secretion via interactions with other neural pathways or systems impinging upon luteinising hormone releasing hormone (LHRH) neurones. Limited evidence also exists for direct effects of EOP at the pituitary level. Generally, the EOP inhibit luteinising hormone (LH) secretion, although the scale of inhibition depends upon the peptide and receptor subtype (μ, K, δ, σ and E) involved. This paper briefly reviews more recent evidence for EOP modulation of gonadotrophin secretion and reproductive function in cattle, sheep and pigs from prepuberty to lactation, including modulation of ovine reproduction by photoperiod. Additionally, data are presented 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 overcome 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 results and those of others indicate potential applications of opioid agonists and antagonists to reproductive management of several domestic species. Such applications are briefly discussed in the context of our own results and the literature reviewed. © 1993.