Administration of sulpiride to anovulatory mares in winter: effects on prolactin and gonadotropin concentrations, ovarian activity, ovulation and hair shedding

Abstract

Sixteen seasonally anovulatory mares were randomly allotted to two groups and injected daily with either sulpiride (1 mg/kg body weight) or vehicle from 14 January to 14 February. Sulpiride administration increased daily plasma prolactin concentrations ( P<0.05), although the prolactin response during the 6 h following sulpiride injections decreased markedly from the 1st to the 6th day of treatment (treatment by day, P<0.0001). Plasma concentrations of LH and FSH were not affected by treatment ( P>0.1). Injection of GnRH and TRH on 15 February showed that the response of plasma prolactin to secretagogue was increased in sulpiride-treated mares ( P<0.005), while there was no effect ( P>0.1) of sulpiride treatment on the response of LH or FSH. Both treatment groups had similar changes in numbers of follicles 10–19 and ≥20 mm during the experiment ( P>0.1). Similarly, the mean change in maximal follicular size was not affected by treatment ( P>0.9). No mare ovulated during the study, and plasma progesterone concentrations were similar in both groups ( P>0.1), always at levels <1 ng/ml. Hair shedding increased with time in all mares ( P<0.001) and was increased by sulpiride injections ( P=0.09). It was concluded that sulpiride administration to seasonally anovulatory mares under the conditions of our experiment increased daily plasma prolactin levels but did not stimulate gonadotropin secretion or ovarian activity.