Abstract
Classic experiments have shown that ovulation and estrous cyclicity are under circadian control and that surgical ablation of the suprachiasmatic nuclei (SCN) results in estrous acyclicity in rats. Here, we characterized reproductive function in the circadian Clock mutant mouse and found that the circadian Clock mutation both disrupts estrous cyclicity and interferes with the maintenance of pregnancy. Clock mutant females have extended, irregular estrous cycles, lack a coordinated luteinizing hormone (LH) surge on the day of proestrus, exhibit increased fetal reabsorption during pregnancy, and have a high rate of full-term pregnancy failure. Clock mutants also show an unexpected decline in progesterone levels at midpregnancy and a shortened duration of pseudopregnancy, suggesting that maternal prolactin release may be abnormal. In a second set of experiments, we interrogated the function of each level of the hypothalamic-pituitary-gonadal (HPG) axis in order to determine how the Clock mutation disrupts estrous cyclicity. We report that Clock mutants fail to show an LH surge following estradiol priming in spite of the fact that hypothalamic levels of gonadotropin-releasing hormone (GnRH), pituitary release of LH, and serum levels of estradiol and progesterone are all normal in Clock/Clock females. These data suggest that Clock mutants lack an appropriate circadian daily-timing signal required to coordinate hypothalamic hormone secretion. Defining the mechanisms by which the Clock mutation disrupts reproductive function offers a model for understanding how circadian genes affect complex physiological systems.