Effect of prolonged exposure to nonstimulatory photoperiods on the activity of the neuroendocrine-testicular axis of golden hamsters

Male golden hamsters were exposed to nonstimulatory short days for 25 weeks, and the effects on gonadal weight and serum gonadotropin levels were assayed at biweekly intervals.Within 11 weeks after animals were transferred from a photostimulatory (LD 14:10) to a nonstimulatory (LD 6:18) lightcycle, serum LH and FSH titers,as well as testicularweight had decreased markedly (4-, 5- and 13-fold, respectively). Continued exposure to LD 6:18 resulted in gradual increases in serum gonadotropin levels and subsequently in the complete recrudescence (regrowth) of the testes by 25 weeks. The testis itself does not appear to be involved in the “spontaneous ” increase in serum gonadotropin levels since these levels also increased gradually in hamsters that were castrated after 10 weeks of LD 6:18 and then were exposed to LD 6:18 for an additional 17 weeks. The observation of a gradual increase in the activity of the neuroendocrine-testicular axis during prolonged exposure to LD 6:18 suggested to us that the hamster’s reproductive system might become more responsive to photostimulation with increasing length of prior exposure to nonstimulatory photoperiods. To examine this possibility hamsters maintained on LD 6:18 for either 10 or 13.5 weeks were transferred to LD 16:8 for 3.5 weeks, and then sacrificed. In those hamsters exposed to nonstimulatory photoperiods for 13.5 weeks there was a greater increase in testicular size after photostimulation (5.5-fold increase) than in those animals exposed for only 10 weeks (2.2-fold increase). A similar pattern was observed in serum gonadotropin levels, indicating that the neuroendocrine-gonadal axis of the male hamster increases in sensitivityto photostimula-lion with a longer exposure to nonstimulatory photoperiods