Time Course of the Photoperiod-Induced Change in Sensitivity of the Hypothalamic-Pituitary Axis to Testosterone Feedback in Castrated Male Hamsters*

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Abstract
Exposure to short days [6 h of light/24 h (LD 6: 18)] for 10 weeks renders the hypothalamic-pituitary axis of the castrated male golden hamster extremely sensitive to the negative feedback effect of testosterone. The present study investigated the time course of this change in sensitivity to steroid feedback, in both animals shifted from a stimulatory to a nonstimulatory photoperiod and animals shifted from a nonstimulatory to a stimulatory photoperiod. Hamsters were castrated and maintained on LD 14:10 for 60 days and then implanted sc with either empty or testosteronefilled Silastic capsules that were 4, 8, or 20 mm long. Half of theanimals remained on LD 14:10 and half were transferred to LD 6:18. In animals maintained on LD 14:10, the 4-mm testosterone implants had no effect on serum levels of LH and FSH, while the larger doses of testosterone induced a reduction in serum gonadotropin levels. The effect of testosterone on serum gonadotropin levels in LD 14:10 animals did not vary from 10 days after capsule implantation. In animals transferred to LD 6:18, serum gonadotropin levels were reduced at day 10 by 8- or 20- mm implants and at day 20 by 4-mm implants. Serum LH and FSH levels continued to fall as exposure to LD 6:18 proceeded in animals implanted with either 4- or 8-mm testosterone capsules In a second study, hamsters were castrated and exposed to LD 6:18 for 60 days and then implanted with 0, 4-, 8-, or 20-mm testosterone capsules. Serum LH and FSH levels were reduced to baseline in all animals at 10 days after implantation. In animals maintained on LD 6:18, serum gonadotropin levels remained suppressed throughout the study. The first significant rise in serum LH and FSH in animals shifted to LD 14:10 and implanted with a 4-, 8-, or 20-mm testosterone capsule was observed after 20, 42, or 66 days, respectively. Furthermore, in animals implanted with 4-, or 8-mm testosterone capsules, serum LH and FSH levels continued to rise with increased exposure to long days. Importantly, serum testosterone levels in implanted animals did not change from 5-42 days after capsule placement. A shift from stimulatory long days to nonstimulatory short days induces a gradual increase in the responsiveness of the hypothalamic-pituitary axis to steroid feedback, while photostimulation induces a gradual decrease in the steroid-feedback sensitivity of the neuroendocrine axis. The present results suggest that the photoperiodic control of reproduction in the hamster represents an extended period of accommodation of the feedback controls within the neuroendocrine-gonadal complex to a change in environmental daylength.