Follicular and endocrine responses of anoestrous mares to administration of native GnRH or a GnRH agonist.

The research revolves around investigating the efficiency of Gonadotropin-Releasing Hormone (GnRH) and a GnRH agonist in triggering ovulation among mares in anestrous condition. The study found that the success of a GnRH agonist hinged on the season, whereas pulsatile GnRH administration showed a consistent response throughout various seasons.

Research Context

• The study focuses on mares in a particular physiological state known as anoestrus, during which they do not undergo the usual estrous (or ‘heat’) cycles and, hence, ovulation. This is common in certain periods, often in response to seasonal changes.
• The main aim of the researchers was to evaluate the effectiveness of native Gonadotropin-Releasing Hormone (GnRH) and a type of GnRH agonist (GnRHa) on these mares. Both these hormones play a significant role in the reproductive system, stimulating the production and release of essential reproductive hormones like luteinizing hormone (LH).

Research Methodology

• Thirty-seven seasonally anoestrous mares were divided into two groups, a treatment group and a control group.
• The mares were administered either native GnRH or a GnRHa, starting on specific dates, either January 13, February 13 or March 14. The treatment varied in frequency and method. Native GnRH was given per hour using a peristaltic pump, while GnRHa was administered twice daily.

Research Findings

• Administration of GnRH was found to effectively induce ovulation in consistently high proportions among the mares across January, February, and March.
• On the other hand, the GnRHa treatment was most successful in February and March, while its effectiveness was notably lower in January.
• A comparison of LH concentrations revealed higher values in mares induced to ovulate with GnRH.
• Urinary estrogen levels were also monitored, revealing a parallel increase with follicular diameter during the treatment process.
• Moreover, normal luteal phases followed the ovulations induced by either GnRH or GnRHa. This phase in the estrous cycle is characterized by the presence of a corpus luteum, a hormone-secreting structure that forms after ovulation.
• All mares that were triggered to ovulate in January and February returned to an anoestrus state after withdrawal of GnRH.

Conclusion

• It was concluded that the efficacy of GnRHa in inducing ovulation in anoestrous mares is influenced by the time of year. This season-dependent influence was not observed in cases where pulsatile native GnRH was administered.