Endocrine and reproductive responses to implants of deslorein acetate in horses

Abstract

Four experiments were performed to study the effects of the gonadotropin releasing hormone (GnRH) analog, deslorelin acetate (Ovuplant™), on endocrine and reproductive characteristics in mares. The first experiment tested whether anecdotal field reports of Ovuplant causing extended interovulatory intervals would be detectable under controlled, experimental conditions. The use of Ovuplant to hasten ovulation in 13 mares, compared to 12 controls, increased (P < 0.05) the interovulatory interval by 6.2 d and suppressed (P < 0.05) plasma concentrations of both luteinizing hormone (LH) and follicle stimulating hormone (FSH) for approximately 11 d. Two mares receiving Ovuplant did not return to estrus within 30 d. In the second experiment, 10 control mares and 10 mares induced to ovulate with Ovuplant were administered GnRH (50 μg) on d 1, 4, 7, and 10 after ovulation. Again, treated mares had a longer (4.4 d, P < 0.05) interovulatory interval and suppressed LH and FSH concentrations in daily plasma samples. The gonadotropin response to GnRH was lower (P < 0.05) in the deslorelin mares on d 1, 4, and 7, indicating a lack of pituitary responsiveness. In the third experiment, 9 stallions and 12 steroid-treated geldings were used to determine if males were potential models for studying the deslorelin-induced gonadotropin suppression. In both cases, treatment with Ovuplant caused an initial rise in both gonadotropins followed by suppression for about 14 d. In the last experiment, 21 mares were used to determine if multiple doses of deslorelin would cause complete ovarian shutdown. Mares received either sham injections, three Ovuplant implants on the first day, or one implant per day for 3 d (n = 7 per group). Treatment with multiple deslorelin implants increased (P < 0.05) the interovulatory interval by 14.8 d and suppressed LH and FSH concentrations for approximately 25 d, however no mares exhibited complete ovarian shutdown. In conclusion, deslorelin acetate implants in horses in the form of Ovuplant induce short-term increases in LH and FSH secretion followed by long-term suppression of these concentrations and an insensitivity of the pituitary to GnRH. In a small percentage of mares, long-term ovarian shutdown is a possibility.