Induction of ovulation in anestrous mares with pulsatile administration of gonadotropin-releasing hormone.

This research explores how the use of hormone treatment can stimulate ovulation in mares that are in anestrous phase, even in absence of stimulatory light conditions. Researchers found that treatment with gonadotropin-releasing hormone (GnRH), regardless of dosage or timing, can notably increase luteinizing hormone and result in ovulation in these horses.

Experiment Overview

• The experiment was carried out on four seasonally anestrous mares (Standardbred), i.e., mares not presently in their breeding cycle.
• These mares were kept in a photoperiod condition of 8 light hours and 16 dark hours per day, which is not meant to stimulate breeding.
• The hormone GnRH was administered to these mares in a pulsatile pattern, i.e. a continuous stream with interruptions, of either 50 or 250 micrograms per hour.
• The administration went on for approximately eight to eighteen days in the months of February and March of 1985.

Results and Findings

• It was found that the GnRH treatment, irrespective of the dosage or the month, resulted in an increase in serum luteinizing hormone in the mares.
• The pre-treatment value average was characteristic of anestrus and was around 0.58 ng/ml. However, on the 8th day of the GnRH treatment, the value rose to around 10.84 ng/ml.
• Ovulation in the mares occurred approximately 8.8 days after the pulsatile administration of the GnRH hormone initiated.
• Each ovulation was followed by a functional corpus luteum – a temporary endocrine structure in female ovaries essential for maintenance of a pregnancy – as indicated by a luteal phase. This phase was defined as the number of days on which serum levels of progesterone were above 1.0 ng/ml and lasted around 14.5 days on average.

Conclusion

• The results indicate that the administration of the hormone GnRH in a pulsatile pattern is effective in inducing follicular development and ovulation in anestrous mares, even in the absence of a stimulatory photoperiod.