Timing of Ovulation Following Progesterone Withdrawal in the Rat1

Abstract

Ovulation in cycling rats was blocked by administration of progesterone, 10 mg daily for 7, 8 or 9 days, and the animals were autopsied on Days 1, 4 or 5 after the last injection. Regardless of treatment length, the majority of animals had not ovulated by Day 4, but had ovulated by the morning of Day 5 after withdrawal of progesterone. Observation of the actual time of occurrence of ovulation in some animals and results of administration of ovulation-blocking drugs to others suggest that the facilitation for LH release responsible for the first ovulation following progesterone withdrawal in the rat begins about 2 PM but lasts somewhat longer than in the 4-day proestrous cyclic animal. Pituitary LH content on Day 1 after cessation of progesterone treatment was higher than maximal cyclic proestrous values, but dropped to normal low estrous levels once ovulation had occurred. Ovarian and uterine weights, which were decreased following treatment, rose to normal values by the time of the first ovulation. (Endocrinology76: 626, 1965)