Gonadotropin-Releasing Hormone Treatment Induces Follicular Growth and Ovulation in Seasonally Anestrous Mares1

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Abstract
A study was conducted to evaluate the effectiveness of gonadotropin-releasing hormone (GnRH) pulse infusion to stimulate follicular development and induce ovulation in seasonally anestrous standardbred mares. Seventeen mares were selected for use in this experiment, on the basis of a previous normal reproductive history, and were housed under a photoperiod of 8L:16D beginning one week prior to the start of the experiment (second week in January). Mares were infused with 20 µg (n=7) or 2 µg (n=6) GnRH/h, or were subjected to photoperiod treatment only (controls, n=4). Serum concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and progesterone did not vary, and neither significant follicular development nor ovulation was observed in any control mare throughout the experimental period (>60 days). By contrast, both groups of GnRH-treated mares showed elevated serum concentrations of LH and FSH within one day after the start of infusion. Mares infused with 20 µg GnRH/h had at least one follicle greater than or equal to 25 mm in 7.4 ± 1.3 (mean ± SEM) days following the start of infusion, and ovulated in 12.0 ± 0.7 days. In the 2-µg-GnRII/h treatment group, a 25-mm follicle was detected in 5.7 ± 0.7 days, and ovulation occurred after 10.0 ± 0.3 days of infusion. Ovulation in every instance was followed by a functional luteal phase, as indicated by the profiles of progesterone secretion. Finally, the number of follicles ovulated per mare was related to the dose of GnRH infused, in that mares infused with 20 µg GnRH/h had 3.0 ± 0.6 ovulations per mare, whereas there were 1.2 ± 0.2 ovulations per mare in the 2-µg-GnRH/h-infused group (p < 0.05). Results indicate that the infusion of GnRH in a pulsatile pattern can induce follicular development and ovulation in seasonally anestrous mares in the absence of photoperiodic stimulation. These data provide additional evidence that the physiological state of seasonal anestrus in the mare is the result of a decrease in the quantity and/or pulse frequency of hypothalamic GnRH secretion.