Prolactin Response to Metoclopramide in the Castrated Male Rat

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Abstract
The effect of castration on the prolactin (PRL) response to the dopaminergic antagonist metoclopramide (MET) was examined in the male rat. The mean .+-. SD PRL content in pituitaries of intact rats (1802 .+-. 293 ng/mg) was significantly higher (P < 0.001) than that of rats 10 days after castration (1170 .+-. 157 ng/mg). Basal plasma PRL levels were unaffected by castration. In intact animals the maximum PRL response to metoclopramide (0.25 mg/kg) was 144.2 .+-. 49.6 ng/ml, and increasing doses of MET (0.5, 1.0 and 2.5 mg/kg) did not augment the response. In the castrated rat, the mean maximum PRL response to MET (0.25 mg/kg) was only 59.8 .+-. 21.8 ng/ml (P < 0.01 compared with intact animals.) Higher doses of MET (0.5, 1.0 and 2.5 mg/kg) increased the maximum PRL response, but this was still less (P < 0.05) than the peak response in intact animals. With a larger dose of MET (10 mg/kg), a further increase in the peak PRL response was not observed in the castrated rats. Castration apparently can depress pituitary PRL concentration and the PRL response to the dopaminergic antagonist metoclopramide without altering basal PRL levels.