INDUCTION OF REPRODUCTIVE FUNCTION IN ANESTROUS MARES USING A DOPAMINE ANTAGONIST

We investigated the role of dopamine in the regulation of seasonal rep roductive activity in mates. Nine seasonal anestrous mares, maintained under a natural photoperiod, were treated daily with a dopamine D2 an tagonist, [-]-sulpiride (200 mg/mare, im), beginning February 5 (day o f year = 36) until the first ovulation of the year or for a maximum of 58. Nine untreated anestrous mates were maintained under the same con ditions. The ovaries were examined by ultrasonography twice a week, an d blood was collected three times a week for progesterone, LH FSH and prolactin determinations. Mean day of first ovulation was significantl y advanced for [-]-sulpiride-treated mares than control mares (mean da y of year +/- SEM = 77.3 +/- 7.9 and 110.0 +/- 6.8, respectively; P<0. 01). Eight mares ovulated during [-]-sulpiride treatment while one mar e failed to ovulate. Ovulation occurred 91 d after the start of treatm ent or on Day 127. All mates continued to have normal estrous cycles a fter the first ovulation. First cycle length and luteal progesterone c oncentrations did not differ between [-]-sulpiride-treated and control mares. Plasma prolactin concentrations were significantly increased a t 2 and 9 h after [-]-sulpiride administration (P<0.05), and had retur ned to basal Levels by 24 h. At the time of the LH surge associated wi th the first ovulation, mean LH and FSH secretion was significantly hi gher in [-]-sulpiride-treated mares than in control mates (P<0.05). Th ese results suggest that dopamine plays a role in the control of repro ductive seasonality in mares and exerts a tonic inhibition on reproduc tive activity during the anovulatory season. (C) 1997 by Elsevier Scie nce Inc.