Cyclophosphamide Depletes Ovarian Follicles in Mice During Both the Light and Dark Phases of the Circadian Cycle

Abstract

The alkylating agent cyclophosphamide (CY) is a potent ovarian toxicant. It damages growing follicles and causes premature activation and depletion of the resting follicles that constitute the ovarian reserve. While there is abundant information on the impact of CY on the ovary and its toxicity mechanisms, the influence of the circadian rhythm on ovarian toxicity has not been evaluated. To test the hypothesis that time of exposure affects ovarian toxicity of CY, C57BL/6 mice were treated with a single injection of CY (75 mg/kg) at either two hours after lights on (Zeitgeber time (ZT) 02) or two hours after lights off (ZT14). Toxicity was evaluated one week after treatment by counting ovarian follicles in histological sections. Fewer primordial follicles were counted in the ovaries of CY-treated animals at both treatment times, and fewer antral follicles were counted in the ovaries of animals treated at ZT02. There was no difference in the number of primordial follicles in the ovaries of CY-treated animals between the two treatment times. These results demonstrate that CY-induced depletion of the ovarian reserve occurs when mice are exposed early in the light phase and early in the circadian cycle’s dark phase. There is no impact of the circadian rhythm on follicle depletion by CY at these time points. KEYWORDS: Cyclophosphamide; ovary; circadian; ovarian follicles; toxicity; mouse; chronotherapy; alkylating agent