Testosterone-filled Silastic capsules, of various sizes, were placed in the peritoneal cavity of adult House Sparrows to determine the response of the testis to exogenous testosterone in (a) photostimulated birds with developed testes and (b) nonphotostimulated birds with regressed testes. In photostimulated sparrows, "low doses" of testosterone, administered via 5- or 10-mm Silastic capsules, induced complete gonadal atrophy, whereas "high-doses" of testosterone, administered via 40-, 80-, or 120-mm Silastic capsules, maintained paired testis weight and spermatogenic activity at values that were similar to those of control birds. Plasma LH levels declined twofold below control values in birds receiving testosterone capsules that inhibited and/or maintained spermatogenesis. Similar concentrations of total androgens were found in the plasma of control birds and those receiving "low doses" of testosterone that induced gonadal atrophy. However, the concentration of total androgens in plasma rose about tenfold above control values in birds exposed to "high doses" of testosterone that maintained spermatogenesis. In another study, testicular growth was initiated in birds exposed to nonstimulatory light cycles (LD 8:16) by implanting testosterone-filled Silastic capsules. The results support the hypothesis that "low doses" of testosterone induce testicular atrophy by suppressing pituitary gonadotropin secretion, whereas "high doses" of testosterone initiate and/or maintain spermatogenic activity via a direct action on the testis.
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