Prolonged exposure of castrated male hamsters to a nonstimulatory photoperiod: Spontaneous change in sensitivity of the hypothalamic-pituitary axis to testosterone feedback

Maintenance of male golden hamsters on nonstimulatory short days for 8-10 weeks induces testicular atrophy, while prolonged exposure (18-30 weeks) results in a refractoriness to the inhibitory effects of short days, and a spontaneous recrudescence of the testes. In view of recent reports that photicinduced changes in the reproductive axis involve an alteration in sensitivity of the gonadotropin control center to steroid feedback, the present investigation was designed to determine if spontaneous changes in the reproductive axis may also involve a change in the responsiveness of the hypothalamic-pituitary system to steroid feedback. Hamsters were castrated and exposed to short days [6 h of light/(LD 6:18)] for 8 weeks. Castrates were then implanted with either empty or testosterone-filled Silastic capsules which were 2, 4, 8, or 20 mm long, and maintained on LD 6:18 for another 18 weeks. Blood samples were obtained by cardiac puncture before capsule implantation and after 11, 14, 18, 22, and 26 weeks of exposure to LD 6:18. Serum levels of LH and FSH were suppressed to or near baseline by all doses of testosterone through 18 weeks of LD 6:18 exposure. However, after 22 weeks of LD 6:18, animals implanted with 2-mm testosterone capsules exhibited mean serum LH and FSH levels that were not significantly different from those found in animals implanted with empty capsules, and serum LH and FSH levels were elevated above baseline in some of the animals treated with 4- and 8-mm testosterone implants. By 26 weeks of short days, serum LH and FSH levels in animals implanted with 2-, 4-, or 8-mm testosterone capsules were similar to those found in animals implanted with empty capsules. The 20-mm capsules suppressed serum gonadotropins to baseline levels at all time points. Importantly, serum testosterone levels yielded by any given capsule size did not differ between weeks 14-22 of LD 6:18 exposure. axis of the castrated male hamster maintained on a nonstimulatory photoperiod beyond 18 weeks becomes less sensitive to the negative feedback effect of exogenous testosterone as exposure to short days proceeds. Thus, the condition in which hamsters become refractory to the inhibitory effect of a nonstimulatory photoperiod involves a spontaneous decrease in steroid feedback sensitivity of the neuroendocrine axis. Both the refractoriness found in other photoperiodic species and circannual changes in reproductive function may also involve similar alterations in feedback relationships within the neuroendocrine-gonadal axis.