Objectives were to determine if low-dose, continuous infusion of GnRH from Fall to Spring, would prevent seasonal anovulation in mares. Twenty Quarter Horse mares, ages 18 mo to 24 yrs, were stratified by age and body condition score and assigned randomly to either a saline control (n = 9) or GnRH (n = 11) treatment group. Treatments were instituted between September 23 and October 9, 2002. Gonadotropinreleasing hormone was delivered in 0.9% physiological saline via Alzet osmotic minipumps (Model 2004) placed sc at the base of the neck, with Silastic sham pumps placed in control mares. Pumps were inserted on day 3 following ovulation or during the follicular phase if ovulation had not occurred. Delivery rate of GnRH was 2.5 ug/h (60 ug/d) for the first 60 d, followed by 5.0 ug/h (120 ug/d) thereafter, with all pumps replaced every 30 d. By December 1, all mares had become anovulatory and remained anovulatory until February. Mean serum concentrations of LH were not affected by treatment in anovulatory mares. In contrast, control mares that exhibited ovulatory cycles after treatment onset had higher (P < 0.05) mean concentrations of LH during all phases of the estrous cycle except diestrus. Mean serum concentrations of FSH were not affected by treatment, but were lower (P < 0.05) from November though January relative to all other months in anovulatory mares. Interovulatory intervals in mares that cycled temporarily did not differ between groups. Ovulatory control mares had slightly larger (P < 0.10) follicles overall than GnRH-treated mares; however, ovulatory follicle diameters for control and GnRH-treated mares did not differ. Ovulatory control mares had higher (P < 0.10) mean concentrations of progesterone during metestrus and late diestrus. In a subgroup of control (n =5) and GnRH-treated (n = 5) mares, total releasable pools of LH in response to 1 mg GnRH did not differ between groups. Ovulation resumed in 3 control and 3 GnRH-treated mares by March 30. Results indicate that continuous infusion of native GnRH at the doses employed herein is not sufficient to maintain ovulatory cycles during the anovulatory season.

The seasonal termination of ovarian cycles in mares initiated near the time of the autumnal equinox is a significant managerial issue for horse breeders world-wide. Studies presented herein had two over-arching aims. In Aim I, objectives were to develop the principals needed to apply gonadotropin-releasing hormone (GnRH) therapeutics for routinely establishing pregnancies in the winter anovulatory mare. We first tested the hypothesis that continuous administration of native GnRH, beginning in either early February or March, would induce ovulation without reversion to an anovulatory state following treatment withdrawal. Continuous 28-d treatments elevated circulating luteinizing hormone (LH) and stimulated spontaneous ovulation much earlier than controls. However, mares treated only in February ceased ovarian cycles at termination of treatment. In contrast, mares administered GnRH in March continued to exhibit estrous cycles. Thus, we concluded that GnRH treatment must continue through March to ensure continued escape from winter anovulation. We then tested the hypothesis that the Julian day of conception could be accelerated in winter anovulatory mares treated continuously with native GnRH for 56 d beginning on February 1. Indeed, GnRH treatment caused a marked increase in the frequency of pregnancy compared to controls. Data illustrated that continuous administration of native GnRH is a practical and highly efficient option for managing seasonal anovulation. In Aim II, we examined hypothalamic distribution, adenohyphyseal receptor gene expression, and ligand functionality of RFRP3 in the mare during the breeding and non-breeding seasons. Hypothalamic RFRP3 mRNA was detected in the mare; however, neither hypothalamic expression of RFRP3 nor its anterior pituitary receptor differed between reproductive states. We then used equine adenohypophyseal cell culture to test the hypothesis that RFRP3 reduces the responsiveness of the equine gonadotrope to GnRH. Addition of RFRP3 to cell culture failed to counter the effects of GnRH. Finally, the effects of a RFRP3 receptor-signaling antagonist (RF9) were examined in winter anovulatory mares. A robust increase in circulating concentrations of LH relative to controls was observed in response to RF9 treatments, but treatments had no effect on adenohypophyseal responsiveness to GnRH. Data provide indirect evidence that antagonism of the RFRP3 system by RF9 may be at the GnRH neuronal level.