Inhibition of gonadotrophin release in mares during the luteal phase of the oestrous cycle by endogenous opioids.

This article focuses on the research into the effects of opioid antagonist naloxone on hormone release during the reproductive cycle of mares. The research suggests endogenous opioids play a significant role in inhibiting the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) during the luteal phase of the mare’s cycle and are not impacted by suckling.

Objective and Methodology

• The researchers set out to study how opioid antagonist naloxone impacted the levels of LH and FSH in the oestrous, or reproductive cycle, of mares.
• Naloxone was administered during different stages of the oestrous cycle, and the effects on hormone concentration were measured.
• The study also used buserelin, a gonadotrophin-releasing hormone analogue, to investigate if any effects on hormone release were cycle-dependent or influenced by naloxone treatment.

Findings during the Follicular and Luteal Phases

• During the follicular phase of the cycle, naloxone did not appear to have an effect on the baseline concentrations of LH and FSH.
• In contrast, during the luteal phase, there was a notable increase in plasma LH and FSH concentrations following naloxone administration.
• This finding indicated that endogenous opioid peptides may be functioning to inhibit the release of these hormones during the luteal phase of the oestrous cycle.

Naloxone, Buserelin, and Hormone Release

• In a further test, whether or not there was previous naloxone treatment, buserelin caused a significant release of LH and FSH at any stage of the oestrous cycle.
• This suggests that the inhibitory action of the opioids is specific to the endogenous hormones and is not influenced by external factors related to naloxone treatment.

Influence of Suckling

• The researchers noted that there was no discernible difference in the response to naloxone between mares that had been suckled and those that had not.
• This indicates that the opioid-mediated suppression of LH and FSH release is not affected by suckling, reinforcing the importance of endogenous opioids in these hormonal processes.

Conclusion

• In conclusion, the research suggests that endogenous opioids play an essential role in inhibiting the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) during the luteal phase of the oestrous cycle in mares.
• These findings provide further insight about the mechanisms underpinning reproductive hormone regulation and may have implications for understanding fertility in mares and potentially other mammals also.