While it is clear that the length of the day can alter the negative feedback effect of steroid hormones on pituitary gonadotropin release, the neural components involved in this response are not known. In the present study we investigated the possibility that the suprachiasmatic nuclei (SCN) of the hypothalamus are involved in photically induced changes in steroid feedback sensitivity. Castrated male hamsters which had been exposed to a 6-h light, 18-h dark schedule (LD 6:18) for 75 days were either shamoperated or received electrolytic lesions aimed at the region of the SCN. The animals were then implanted with either empty or 4-mm long testosterone-filled capsules that maintained circulating levels of testosterone at about 0.9 ng/ml. Half of the animals were returned to LD 6:18, while the remaining half were transferred to LD 14:10. Blood was collected for analysis of immunoreactive LH, FSH, and testosterone on days 7, 25, and 42 after capsule implantation. Regardless of photoperiod, shamlesioned animals implanted with empty capsules had high postcastration levels of serum LH (∼10-25 ng/ml) and FSH (∼3000-4, 000 ng/ml). As expected, sham-lesioned animals maintained on short days were extremely sensitive to the negative feedback effects of testosterone, as evidenced by a 20-fold reduction in serum LH (<0.67 ng/ml) and FSH (<180 ng/ml) levels after 42 days of testosterone treatment. In contrast, SCN-lesioned animals maintained on LD 6:18 did not remain hypersensitive to the negative feedback effects of testosterone, nor did the sham-lesioned or the SCN-lesioned animals that were transferred to LD 14:10. In addition, on day 42, serum FSH levels were found to be significantly lower in animals implanted with empty capsules that had sustained SCN lesions than in shamlesioned animals. This effect was not influenced by the photoperiod. These results suggest that the SCN may influence pituitary FSH release in the castrated hamster and, in addition, appear to be involved in photically induced changes in the feedback relationships within the hypothalamic-pituitary-gonadal axis.
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