Control of follicular development and luteal function in the mare: effects of a GnRH antagonist.

The research focuses on the control of the equine estrous cycle and how it can be influenced by suppressing gonadotropin secretion with a GnRH antagonist in mares.

Research Methodology

• The researchers conducted their study on four cyclic pony mares, which were given a vehicle (control cycle) or a GnRH antagonist, known as Antarelix.
• This substance was administered on Day 8 of diestrus at a dosage of 100 micrograms per kilogram.
• Blood samples were collected at various intervals, ranging from 15 minutes to 36 hours, continuing daily until the next ovulation,
• Ovarian activity was kept under surveillance using transrectal ultrasonography and checking plasma concentrations of progesterone and estradiol.

Findings

• Following the administration of the antagonist, the researchers observed that large diestrous pulses of luteinizing hormone (LH) were eliminated.
• Progesterone concentrations in all mares had significantly dropped by the day following treatment, with three out of the four mares maintaining low levels until luteolysis (the degradation of the corpus luteum).
• However, the timing of luteolysis was not influenced by the antagonist treatment, that is, progesterone concentrations remained below 1 ng/mL.
• This treatment also significantly delayed the preovulatory surges of estradiol and LH, along with the appearance of a preovulatory follicle larger than 30 mm.
• As a result, the cycle length during the treatment was notably longer than the control cycle.

Conclusion

• The research findings illustrate that treatment of diestrous mares with a GnRH antagonist reduced progesterone secretion, suggesting that LH plays a crucial role in controlling CL (corpus luteum) function in mares.
• It also demonstrates that ovulation was delayed, likely due to the lack of gonadotropic support.