Timing of Post-partum Ovulation in the Rat1

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Abstract
The ovulatory surge of LH is released between 2 and 4 PM of proestrus in the cyclic rat, as shown by the ability of drugs or hypophysectomy at 2 PM on the afternoon of proestrus to block ovulation. In the present study, the time of occurrence of the ovulation following parturition in the rat was examined with respect to the time of delivery, and the time of LH release responsible for the ovulation was investigated. Post-partum animals which delivered between 4 PM (Day 1) and 8 AM (Day 2) had not ovulated by 4 PM of Day 2 but had ovulated by 10 AM on Day 3; of animals delivering between 8 AM and 4 PM (Day 1), about ⅔ ovulated between 4 PM of Day 1 and 10 AM of Day 2, and the rest ovulated between 4 PM of Day 2 and 10 AM of Day 3. The LH release responsible for this ovulation appeared to be occurring no earlier than 10 AM and no later than 6 PM (assuming a 12-hr latent period to LH), since animals observed during the night ovulated between 10 PM and 6 AM. Atropine, pentobarbital, morphine and barbital were not as effective in blocking the post-partum ovulation as in blocking cyclic ovulation. Hypophysectomy was not effective unless performed at 1 PM or earlier. These results indicate that the post-partum LH release can begin earlier and last until later than the ovulatory release of LH in the 4-day cyclic rat. (Endocrinology76: 620, 1965)