TIME COURSE OF THE PHOTOPERIOD-INDUCED CHANGE IN SENSITIVITY OF THE HYPOTHALAMIC-PITUITARY AXIS TO TESTOSTERONE FEEDBACK IN CASTRATED MALE HAMSTERS

Exposure to short days [6 h of light/24 h (LD 6:18)] for 10 wk 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 them implanted s.c. with either empty or testosterone-filled Silastic capsules that were 4, 8, or 20 mm long. Half of the animals 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 [lutropin] and FSH [follitropin], 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 10-66 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.