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 prola ctin secretion in geldings in winter and 2) whether increasing prolact in concentrations would hasten the onset of hair shedding or enhance g onadotropin secretion. Five geldings each received vehicle (vegetable oil) or sulpiride (100 mg in vehicle) daily from February 8 through Ma rch 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, prolact in response to sulpiride varied in a quadratic manner (P < .002). Aver age area under the 2-h response curve for sulpiride-treated geldings w as 24.9 h.ng.ml(-1) on February 8, declined to 4.3 after 4 wk, and the n increased to 14.8 by the 8th wk of treatment (pooled SE = 4.1 h.ng.m l(-1)). Prolactin concentrations in control geldings did not vary (P > .1) after injection or over the 8-wk period. Weight of hair pulled fr om sulpiride-treated geldings did not peak as sharply or as high as th at from control geldings (P < .05) and continued to be high through th e 11th wk. Sulpiride treatment reduced (P = .071) the LH response to G nRH on March 30; the FSH and prolactin responses to secretagogue were not altered (P > .25). In conclusion, even though prolactin concentrat ions were increased by sulpiride, the effects on gonadotropin secretio n and hair shedding were minor and opposite of those expected.