Pulsatile administration of gonadotropin-releasing hormone advances ovulation in cycling mares.

The research article investigates the effect of gonadotropin-releasing hormone (GnRH) on ovulation in cycling mares and finds that pulsatile administration of this hormone can speed up the ovulation cycle, but the frequency of its administration is significant.

Overview of the Research

• This study involved cycling standardbred mares, and commenced on the 16th day of their estrous cycle — the female horse’s reproductive cycle.
• The mares were infused with either saline solution (the control group) or 20 micrograms of GnRH in a pulsatile manner — meaning it was given in regular pulses instead of a continuous dose. This pulsing occurred either once every hour for 5 seconds, or over 2 or 4 hours.

GnRH and Luteinizing Hormone Levels

• The researchers noticed that luteinizing hormone (LH) levels elevated significantly quicker in all three groups given GnRH — this occurred within a day of starting the infusion.
• However, there was no difference in periovulatory LH concentrations between the different groups; the overall average was 8.98 +/- 0.55 ng/ml.

Time to Ovulation

• They found that mares infused with 20 micrograms of GnRH every hour ovulated earlier: 2.9 +/- 0.6 days from the start of the treatment, or 18.9 days post their previous ovulation.
• This is in comparison to mares given saline instead, who took 5.9 +/- 0.3 days, or 21.9 days from their last ovulation.
• Mares given GnRH every 4 hours took around 5.4+/- 0.9 days to ovulate after the initiation of the treatment, translating to 21.4 days from their previous ovulation.
• Meanwhile, mares given the hormone every 2 hours took around 4.3 +/- 0.7 days from the start of the treatment (20.3 days post their previous ovulation) to ovulate. This, however, was not significantly different from the control or the other GnRH treatments.

Post-Ovulation Findings

• There was no notable difference in the length of the resulting luteal phase or the overall estrous cycle between any of the treatment groups. On average, these durations were 14.7 +/- 0.2 days and 21.3 +/- 0.4 days respectively (the luteal phase is a period of the estrous cycle following ovulation).

Conclusions

• From this study, the researchers concluded that the pulsatile administration of GnRH is useful in advancing the time of ovulation in cycling mares. However, the frequency of the pulse infusion proved to be an important variable, making a difference in how effective the treatments were.