Induction of follicular development and ovulation in seasonally acyclic mares using gonadotrophin-releasing hormones and progesterone.

This research explores the use of gonadotrophin-releasing hormones (GnRH) and progesterone to induce ovulation in seasonally acyclic (non-ovulating) mares. Despite partial success with follicular development, final maturation and corpus luteum (CL) development were not effectively achieved, potentially due to insufficient luteinizing hormone (LH) and oestradiol-17 beta surges.

Methodology and Treatment Procedure

• The research involved treating seasonally acyclic mares with GnRH or a GnRH analogue to incite follicular growth and ovulation.
• The treatment consisted of the administration of 3 to 4 courses of GnRH at approximately 10-day intervals. This was to mimic the gonadotrophin surges that precede ovulation in a normal cycle.
• Exogenous progesterone was also introduced in an attempt to replicate the pattern of the luteal phase of a normal cycle.

Treatment Outcomes and Observations

• The induced follicle-stimulating hormone (FSH) concentrations were in the range that causes follicular development in a regular cycle.
• However, the induced levels of LH were found to be lower and of shorter duration compared to those of the periovulatory surge during a normal ovulation cycle.
• Three out of four mares treated with GnRH appeared to ovulate, but did not successfully establish the corpus luteum (CL), an important aspect of normal ovulation.
• Nine out of ten mares treated with the GnRH analogue developed follicles during the final phase of treatment.
• Mares treated only with progesterone also showed follicular development, whilst only one out of five untreated control mares showed any ovarian development.

Interpretation and Further Conclusions

• The research suggests that the failure to induce the final stages of follicular maturation and CL development might be due to inadequate LH surge at the expected time of ovulation.
• This could also be associated with a low preovulatory oestradiol-17 beta surge, which may have been caused by the preceding FSH stimulation being insufficient or inappropriate.
• Additionally, the treatment with progesterone increased the baseline FSH concentrations in the GnRH-treated mares and stimulated follicular development in mares not treated with GnRH.
• This suggests a possible role for progesterone in the process of folliculogenesis (the development of ovarian follicles) and indirectly, in ovulation. The specific mechanisms of this potential function, however, require further investigation.