Short day inhibition of gonadal function in male hamsters is associated with an increase in sensitivity to negative feedback effects of gonadal steroids on gonadotropin secretion. Conversely, in many mammalian species, pubertal activation of gonadal function is associated with a decrease in sensitivity to steroid negative feedback effects. We examined the developmental time course of these opposing pubertal and photoperiodic neuroendocrine changes in golden hamsters. Male hamsters maintained from birth on a light-dark cycle of either 14-h light, 10-h dark, or 6-h light, 18 dark were either left intact or at 2 weeks of age were castrated and implanted with either a 4-mm testosterone-filled Silastic capsule or an empty capsule. Groups of hamsters from each treatment group were decapitated at various ages through 14 weeks of age. Testis weights were obtained from intact animals and trunk blood was collected for RIA of LH, FSH, and testosterone from all animals. Pubertal growth of the testes occurred through 9 weeks of age in intact hamsters on both long and short days, and was followed by testicular regression only in hamsters on short days. In testosterone-treated castrated hamsters on both photoperiods, serumgonadotropin levels were very low at 3 weeks of age, but gradually rose to high levels by 7 weeks. Thereafter, serum LH and FSH levels remained elevated in the hamsters on long days, whereas in the hamsters on short days, gonadotropins returned to low levels by 9 weeks. These fluctuations in LH and FSH concentrations reflect changing responsiveness to testosterone negative feedback on gonadotropin release since 1) testosterone levels were equal across time and photoperiod in testosterone-treated castrated hamsters, and 2) parallel fluctuations in LH and FSH concentrations were not observed in castrated hamsters in the absence of steroid. These results demonstrate that a pubertal decrease in the negative feedback effects of testosterone on gonadotropin release precedes the development of photic-induced changes in steroid negative feedback sensitivity in hamsters exposed to short days from birth. This study also corroborates the unresponsiveness of the prepubertal golden hamster to the inhibitory effects of short days.
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