Evaluation of a Role for Prolactin in the Recrudescense of Ovarian Activity in Seasonally Anovulatory Mares

Abstract

A series of experiments studied the possible role for prolactin in the spring-time recrudescence of ovarian activity in mares. The first experiment was based on cabergoline administration to seasonally anovulatory mares to prevent the seasonal rise in endogenous plasma prolactin to assess whether reduced prolactin altered onset of ovarian activity or first ovulation. Although prolactin concentrations were reduced, basal prolactin concentrations recovered from suppression by the time of the next injection (10 days later), even though low-dose sulpiride stimulation of prolactin was still suppressed by 85%. The subsequent experiment tested whether season affected the duration and degree of suppression produced by a standard dose of cabergoline. Mares were administered cabergoline at 1.5 mg/500 kg body weight in March, June, September, and December. Duration of suppression was affected little by month of assessment. Follow-up assessments with either low dose sulpiride (July) or thyrotropin releasing hormone (October) challenges after administration of 5 mg cabergoline indicated that basal prolactin concentrations always rebounded earlier than secretagogue-induced secretion. The third experiment studied possible physical and hormonal characteristics of seasonally anovulatory mares that might affect their response to a combined estradiol and sulpiride treatment for inducing ovulation in winter. All mares received 50 mg of estradiol cypionate (ECP) followed by 3 g of sulpiride. Factors that were commonly associated with early-induced ovulation were: adequate body condition, elevated plasma concentrations of leptin and insulin, and a greater LH response to secretagogue prior to the start of the experiment. The final experiment studied the hormonal production aspects of the first pre-ovulatory follicle of the breeding season in seasonally anovulatory mares treated with ECP followed by injections of sulpiride 5 and 12 days later, or ECP plus vehicle (controls), beginning in January. Date to first 35-mm follicle was advanced in sulpiride-treated mares. It was concluded that early-induced follicles are equally as competent in terms of steroid production and ability to respond to circulating LH as pre-ovulatory follicles occurring naturally in spring. These experiments support the model in which an increase in circulating prolactin during winter can hasten follicular growth and advance the date to first ovulation in most mares.