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Prolactin, gonadotropin, and hair shedding responses to daily sulpiride administration in geldings in winter.

This experiment was designed to determine 1) the efficacy of daily s.c. injections of a dopamine antagonist, sulpiride, for increasing prolactin secretion in geldings in winter and 2) whether increasing prolactin concentrations would hasten the onset of hair shedding or enhance gonadotropin secretion. Five geldings each received vehicle (vegetable oil) or sulpiride (100 mg in vehicle) daily from February 8 through March 29. On February 8 and every 7 d thereafter through March 29, blood samples were drawn around treatment injections and hair samples were collected. On March 30, all geldings received an injection of GnRH and thyrotropin-releasing hormone. Over the 8-wk sampling period, prolactin response to sulpiride varied in a quadratic manner (P < .002). Average area under the 2-h response curve for sulpiride-treated geldings was 24.9 h.ng.mL-1 on February 8, declined to 4.3 after 4 wk, and then increased to 14.8 by the 8th wk of treatment (pooled SE = 4.1 h.ng.mL-1). Prolactin concentrations in control geldings did not vary (P > .1) after injection or over the 8-wk period. Weight of hair pulled from sulpiride-treated geldings did not peak as sharply or as high as that from control gelding (P < .05) and continued to be high through the 11th wk. Sulpiride treatment reduced (P = .071) the LH response to GnRH on March 30; the FSH and prolactin responses to secretagogue were not altered (P > .25). In conclusion, even though prolactin concentrations were increased by sulpiride, the effects on gonadotropin secretion and hair shedding were minor and opposite of those expected.