Annual changes in and photoperiodic influence oh the weight of gonads, a parameter of gonadal activity, are much smaller in female birds than in males. Effect of season and photoperiod on the follicle-stimulating hormone receptors in the testis or ovary was studied using a subtropical weaver finch. The number of follicle-stimulating hormone binding sites per unit testicular weight showed a peak in the non-breeding phase; while the total number of binding sites per two testes was maximal in the breeding phase and minimal in the regressive phase. In contrast, seasonal changes in follicle-stimulating hormone binding sites in the ovary were less marked. Exposure to short-day during the breeding phase induced marked decreases in the numbers of binding sites per unit testicular weight and per two testes. These numbers markedly increased after transfer to long-day during the non-breeding phase. However, there was no significant effect of short-day or long-day exposure on follicle-stimulating hormone binding sites in the ovary. These results suggest that photoperiod is an effective environmental factor in the regulation of follicle-stimulating hormone receptors in the testis and the effect is manifested by pronounced changes in the testicular weight during annual breeding cycle.
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