157 Effect of a slow-release gonadotrophin-releasing hormone analogue on ovarian activity and oestrous behaviour in mares

Abstract

Oestrus behaviour in mares can contribute to problems in their handleability and reduced performance in equestrian sports. Therefore, methods of transient suppression of oestrous cyclicity in mares are of interest. The aim of our study was to determine whether treatment of mares with slow-release implants containing the gonadotrophin-releasing hormone (GnRH) analogue deslorelin downregulates pituitary GnRH receptors and reduces ovarian function and oestrous behaviour. Shetland mares (age = 11.0 ± 1.4 years; bodyweight = 185.5 ± 7 kg) were oestrous synchronised with two injections of the prostaglandin F_2α analogue luprostiol (3.725 mg) at an interval of 12 days. One day after the second injection (Day 0), mares were randomly assigned to three groups: slow-release implant with 9.4 mg of deslorelin (Suprelorin, Virbac; group D1; n = 6), implant with 4.7 mg of deslorelin (group D2; n = 5), and intramuscular injection of 1.25 mg of short-acting deslorelin (control, group C; n = 5). Collection of blood samples for analysis of progesterone, LH, and anti-Müllerian hormone (AMH) using established and validated enzyme immunoassays (Scarlet et al. 2018 Theriogenology 117, 72-77), testing for oestrus-like behaviour with a Shetland stallion, and ultrasonography of the genital tract were performed at 2-day intervals until Day 10 after treatment and at 5-day intervals from there. On Days 10 and 45 after treatment, LH stimulation tests with the GnRH agonist buserelin (4 µg IV) were performed. Data were normally distributed; differences among groups were analysed using analysis of variance and subsequent Tukey test. Values are means ± s.e.m. In all mares without a corpus luteum on Day 0 (progesterone <1 ng mL⁻¹; one mare in group D1 and two in group C), ovulation was detected within 9 days after deslorelin treatment. These ovulations were classified as deslorelin induced, whereas ovulations after Day 10 were classified as spontaneous ovulations. The mean interval from deslorelin until the first spontaneous ovulation was 62.0 ± 8.6, 44.2 ± 14.1, and 22.2 ± 3.1 days in groups D1, D2, and C (P < 0.05), respectively. Subsequent oestrous cycles were regular. Oestrus-like behaviour until day 50 was reduced in groups D1 (2.0 ± 0.9 days) and D2 (2.4 ± 1.3 days) compared with group C (6.4 ± 1.2 days; P < 0.05). Concentration of plasma LH and AMH decreased in group D1 (P < 0.05) but not in groups D2 and C. The GnRH stimulation test on Day 10 resulted in an increase (P < 0.001) in plasma LH concentration in group C but not in groups D1 and D2 (treatment × time P < 0.05). On Day 45, LH concentration increased in all mares in response to buserelin (NS among groups). Within 100 days of treatment, LH concentrations but not AMH concentrations in mares of group D1 returned to baseline. In conclusion, deslorelin slow-release implants transiently suppress ovarian function and oestrus behaviour in mares. Spontaneous ovulation is delayed in a dose-dependent manner. A decrease in AMH concentration suggests inhibitory effects of deslorelin on small antral follicles. Long-term effects on follicular dynamics and fertility in larger horses also need to be assessed.