Effects of ovariectomy and constant light on responsiveness to estrogen in the rat

The hypothesis that the responsiveness of sexual behavior and LH secretion to exogenous gonadal steroid treatment is dependent on the endogenous steroid environment existing prior to treatment was tested in female rats. The major finding was that estrogen was more effective in stimulating lordosis behavior when treatment was commenced immediately after ovariectomy than when it was delayed for 6 weeks. This indicates that the sensitivity of behavior regulating mechanisms in the female rat declines after removal of the “activating” hormones, as previously reported for testosterone in the male. Similar results were obtained in groups of animals whose pattern of steroid secretion prior to ovariectomy had been changed by 2 months' exposure to constant light. The constant illumination itself showed no significant effect on behavioral responsiveness in spayed estrogen-treated rats. Results are also reported for plasma LH determinations and uterine weights in each of the experiments. Plasma LH levels were found to be lower under conditions of constant as compared to cycling light, both in spayed untreated and spayed estrogen-treated animals.