147 Meiotic competence of oocytes obtained from seasonally anovulatory mares treated with estradiol and sulpiride

Stimulation of prolactin by administration of a dopamine antagonist has been shown to stimulate ovarian follicular growth and hasten the date to first ovulation in seasonally anovulatory mares. The objective of this study was to compare recovery and maturation rates of oocytes collected from seasonally anovulatory mares with oocytes collected from naturally transitional mares (controls). Starting on February 23, 14 light horse and pony-cross anovulatory mares were blocked by breed and age before random assignment to treatment or control. Treatment consisted of 50 mg oestradiol cypionate (ECP) followed by 3 g sulpiride 1 day later (n = 7), while controls received vehicle only (n = 7). Jugular blood samples were collected before treatment and continued daily for 12 days. Ovarian activity was assessed by daily rectal ultrasonography. Prolactin and LH concentrations were determined via radioimmunoassay. All mares were subjected to ovum pick-up (OPU) when the first 30-mm follicle of the season was detected. All follicles ≥8 mm were targeted for OPU, and aspirates from three follicle diameter categories (8–19 mm, 20–29 mm, and >30 mm) were pooled within treatment groups. Recovery rate was noted as the number of oocytes collected from the total number of follicles successfully aspirated and flushed. Oocytes were held overnight at room temperature in holding media (Emcare, ICPbio Reproduction) before being placed in maturation media (EQ-IVM, IVF Bioscience) and incubated at 38.2°C at 6% CO₂ for 30 h. Oocytes were then placed in 45% acetic acid in ethanol mixture for 3 days, mounted onto microscope slides, and stained with orcein to visualise a metaphase II plate. All 7 ECP-sulpiride treated mares responded within 14 days of treatment. Prolactin and LH concentrations were compared using ANOVA with repeated sampling. Prolactin and LH were stimulated (P < 0.0001 and P < 0.01, respectively) in mares treated with ECP-sulpiride. Date to first 30-mm follicle was advanced (P = 0.017) in treated mares compared to controls (March 6 ± 1 day vs. March 26 ± 7 days). Recovery and maturation rates were compared using chi-squared analysis. Recovery rates of oocytes from treated mares and control mares were 46.4% (13/28) and 33.3% (7/21), respectively, and did not differ. Meiotic competence, as defined by visualisation of a metaphase II plate, did not differ between treated and control mares (66.7% vs. 71.4%). This experiment was the first to assess IVM potential of oocytes obtained from mares induced to cycle early following treatment with ECP and sulpiride. Oocytes obtained from these mares appeared to mature equally to oocytes derived from naturally transitional mares. The developmental competence of these oocytes compared to oocytes derived from cycling mares has yet to be determined.