Plasma levels of testosterone (T) and estradiol (E) were determined in intact adult male rhesus monkeys before bilateral castration. Beginning on the day of orchidectomy, the precastration diurnal patterns in plasma T concentrations were replicated quantitatively by the sc implantation of T-containing Silastic capsules and by sc injections of T in oil. Preoperative plasma E levels were replicated by the sc implantation of E-containing Silastic capsules. Replacement with T and E maintained serum LH and FSH concentrations within or below the preoperative control ranges. Replacement with T alone also maintained circulating gonadotropins at concentrations similar to those observed before castration. Withdrawal of T replacement, while maintaining plasma E at precastration levels, resulted in a rise in circulating LH and FSH to concentrations approximating those observed when all steroid treatment was discontinued. These findings fail to provide evidence in support of the “inhibin” hypothesis and suggest that testicular T and its metabolites can account for the gonadal component of the negative feedback loop regulating the secretion of both LH and FSH in the male rhesus monkey. Physiological T replacement given to chronically castrated males did not suppress LH and FSH secretion. This observation suggests that the long term orchidectomized monkey is not suitable for the study of neuroendocrine mechanisms involved in the negative feedback action of testicular steroids on gonadotropin secretion.
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