Effect of photoperiod length on feedback sensitivity to estradiol in the female rat.

Abstract

The effect of photoperiod on the negative feedback effect of estradiol on LH release was examined in this study. Female ARS/Sprague-Dawley rats were exposed to lighting schedules of 2, 8, 16 or 22 h of light per 24 h from 60 days of age until autopsy. After 5 to 6 weeks of acclimatization to the photoperiod, all rats were ovariectomized, given various treatments during the next 14 days and sacrificed on the 15th day. Treatments consisted of either daily injections of various doses of estradiol benzoate (EB) in sesame oil or of implantation of a Silastic capsule containing crystalline estradiol-17<i>β </i>(E₂) at the time of ovariectomy; in addition to untreated controls in each group, some animals received either oil injections or implantation of an empty Silastic capsule, as appropriate. At the time of autopsy, blood was taken for radioimmunoassay of serum LH and body and uterine weights were recorded. Untreated ovariectomized rats, and those receiving oil injections or an empty capsule implant, showed wide variations in serum LH levels; no significant effect of photoperiod length could be demonstrated. At low dose ranges of EB, very little effect on serum LH was seen. A dose-related effect was seen on relative uterine weight, and the response to EB was significantly greater (p < 0.05) in the 22 h photoperiod than in the others, but this effect was not seen at moderate or at presumptively high doses of estradiol. At these higher doses, however, there was a significant effect of photoperiod length on serum LH levels (p < 0.05). In both of these dose ranges, the longer the photoperiod, the higher the LH level at any given dose of estrogen. These results suggest that photoperiod length can alter the sensitivity of the LH control system to the negative feedback effects of estrogen: in the case of the rat, the shorter the photoperiod, the more sensitive the negative feedback system.