Interaction of oestradiol and gonadotrophin-releasing hormone on LH release in the mare.

This research investigates how the hormones oestradiol and gonadotrophin-releasing hormone (GnRH) affect the release of Luteinizing Hormone (LH) in horses. The study finds that the timing of the oestradiol surge, especially in relation to other hormonal events, is critical in controlling the LH surge typically observed during ovulation.

Interaction between Oestradiol and GnRH in Regulating LH Release

• The research began with acyclic mares being treated with oestradiol for three days to imitate the preovulatory plasma oestradiol surge, which resulted in a non-significant 37% decrease in plasma LH during the treatment period.
• When Gonadotropin-Releasing Hormone (GnRH) analogues were introduced with oestradiol between the first and third days, it was found that oestradiol had no effect on each of the GnRH-induced LH increases.

Influence of Oestradiol on the Prolongation of LH Release

• However, there was an observation that increases in LH were more extended after subsequent injections of GnRH between the fourth and seventh days, an interval when oestradiol was no longer administered.
• Interestingly, a considerably longer period of LH secretion occurred when subsequent doses of GnRH commenced after the termination of the oestradiol treatment. The responsiveness was almost equivalent to the prolonged periovulatory LH surge, typically seen in the mare’s normal cycle.

Conclusion: The Role of Oestradiol Timing in LH Surge

• These findings highlight the significant role of timing during the oestradiol surge with respect to other hormonal activities in triggering the horse’s uniquely extended periovulatory LH surge.
• This research strengthens the understanding of how hormonal interactions and timing underpin the physiological processes of ovulation in horses and potentially other related mammalian species.