Induction of reproductive function in anestrous mares using a dopamine antagonist

We investigated the role of dopamine in the regulation of seasonal reproductive activity in mares. Nine seasonal anestrous mares, maintained under a natural photoperiod, were treated daily with a dopamine D2 antagonist, [-]-sulpiride (200 mg/mare, im), beginning February 5 (day of year = 36) until the first ovulation of the year or for a maximum of 58. Nine untreated anestrous mares were maintained under the same conditions. The ovaries were examined by ultrasonography twice a week, and blood was collected three times a week for progesterone, LH, FSH and prolactin determinations. Mean day of first ovulation was significantly advanced for [-]-sulpiride-treated mares than control mares (mean day of year +/- SEM = 77.3 +/- 7.9 and 110.0 +/- 6.8, respectively; P < 0.01). Eight mares ovulated during [-]-sulpiride treatment while one mare failed to ovulate. Ovulation occurred 91 d after the start of treatment or on Day 127. All mares continued to have normal estrous cycles after the first ovulation. First cycle length and luteal progesterone concentrations did not differ between [-]-sulpiride-treated and control mares. Plasma prolactin concentrations were significantly increased at 2 and 9 h after [-]-sulpiride administration (P < 0.05), and had returned to basal levels by 24 h. At the time of the LH surge associated with the first ovulation, mean LH and FSH secretion was significantly higher in [-]-sulpiride-treated mares than in control mares (P < 0.05). These results suggest that dopamine plays a role in the control of reproductive seasonality in mares and exerts a tonic inhibition on reproductive activity during the anovulatory season.