Estrogen accelerates gonadal recrudescence in photo-regressed male Siberian hamsters

Seasonal gonadal recrudescence in the male Siberian hamster is accompanied by the initiation of spermatogenesis, weight gain, and darkening of coat color. The downstream endocrine regulators responsible for these changes have not been definitively identified. We have previously shown that the administration of exogenous 17beta-estradiol (E-2) to adult male Siberian hamsters kept under long-day photoperiod increased testicular mass without altering spermatogenesis. In this study, we examine if E-2 can initiate testicular growth in photo-regressed adult Siberian hamsters and if this testicular growth is accompanied by weight gain and pelage color change. Photo-regressed adult male Siberian hamsters were subcutaneously implanted with a 1 mm silastic capsule containing E-2 or cholesterol control. After 15 days, robust initiation of spermatogenesis was observed in E-implanted animals in the absence of body weight and pelage color change. While circulating follicle-stimulating hormone (FSH) remained undetectable in both control and E-2-treated animals, E-2 significantly reduced pituitary gonadotropin stores. Overall, we showed E-2 stimulated gonadal recrudescence via a pathway that has diverged from body,weight and pelage color change. Further, we demonstrated a novel role of E-2 in the initiation of spermatogenesis, possibly via a mechanism independent of FSH.