Administration of a gonadotropin-releasing hormone antagonist to mares at different times during the luteal phase of the estrous cycle.

The research article investigates the impact of administering the GnRH antagonist cetrorelix on mares during different periods of their luteal phase. Specifically, it examines the subsequent effects on the secretion of FSH and LH hormones, alongside impacts on corpus luteum function and inter-ovulatory intervals.

Research Methodology and Approach

• The antagonist cetrorelix was administered to mares at different times during their luteal phase. These periods included the early phase (Days 1-5), mid-luteal phase (Days 6-10 or 5-12), and the entire luteal phase (Days 1-16), with Day 0 representing day of ovulation.
• The researchers frequently sampled the blood of the mares, specifically from Day 6 to Day 14, to track the exact time-course of the suppression effects of cetrorelix, when it was given during Days 6-10.

Findings: Impact on FSH and LH concentrations

• Cetrorelix administration resulted in decreased FSH and LH concentrations, observed within 8 and 16 hours after treatment, respectively.
• This treatment entirely removed the response to exogenous GnRH when provided 24 hours after treatment onset.
• However, the treatment never suppressed FSH and LH concentrations to undetectable levels. The lowest (nadir) concentrations reached were 46.2±6% and 33.1±11% of the original pre-treatment concentrations for FSH and LH respectively.

Findings: Impact on Progesterone concentrations, Inter-ovulatory interval and Corpus Luteum function

• All treatment groups experienced a decrease in daily FSH concentrations, and daily LH concentrations were found to be lower only when treatment commenced at the beginning of the luteal phase.
• Progesterone concentrations varied according to the time of cetrorelix administration. The alternations in these concentrations implicated a function of LH in the corpus luteum.
• The inter-ovulatory interval was found to be longer than controls when cetrorelix was administered during the mid or entire luteal phase – but not affected when given during the early phase.

Conclusions

• The researchers concluded that FSH concentrations were consistently higher around 6-10 days before the subsequent ovulation, across all treatment groups. This consistent relationship suggested a strict requirement for GnRH-induced elevation of FSH above a certain threshold during this specific time frame (approximately 6-10 days before ovulation), but not at any other times.