Biology of Reproduction, Vol 25, 1-5, Copyright © 1981 by Society for the Study of Reproduction

Prolactin Response to Metoclopramide in the Castrated Male Rat

¹ Department of Endocrinology and Metabolism, Shaare Zedek Medical Center, Jerusalem 91000, Israel

The effect of castration on the prolactin (PRL) response to the dopaminergic antagonist metoclopramide (MET) has been 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, however, 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. These experiments demonstrate that castration can depress pituitary PRL concentration as well as the PRL response to the dopaminergic antagonist metoclopramide without altering basal PRL levels.

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ACKNOWLEDGMENTS The generous gift of ketamine-HCl (Ketalar) from Warner-Lambert International, NJ, is gratefully acknowledged. We thank the National Pituitary Agency, NIAMDD, for providing the rat PRL kit. We thank H. Lapidus for typing the manuscript. The study was supported by a grant from the Wolinsky Bessin Research Fund, Toronto, Canada.