Ovulation in Postpartum Rats1

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Abstract
In a long—term study, the vast majority of Sprague—Dawley rats from the Holtzman Co. have been found to deliver between 6 AM and 4 PM on Day 23 of pregnancy. Temporal examination for the presence of ova in the oviducts following parturition demonstrated that the time of follicle rupture occurred between 9 PM to 12 midnight postpartum whereas that in cyclic rats is from 12 midnight to 4 AM. Determination of the ovulatory surge of gonadotropin in the postpartum rat was carried out by phenobarbital blockade experiments, neutralization of circulating LH with ovine LH antiserum and direct measurement of serum LH by radioimmunoassay. It was observed that as in the cyclic rats the period occupied by the initial triggering process for ovulation in the individual postpartum rat is relatively brief. However, for the whole population, the period stretched from early morning to late evening. Ovulation was observed the day after a single iv injection of either LH or FSH during pregnancy. A single dose of estradiol benzoate given sc on Days 11–20 of pregnancy induced ovulation within 48 hr in over 60% of animals. The measurement of serum LH in pregnant animals which received a single injection of EB on Day 17 demonstrated a dramatic increase in the afternoon of Day 18, suggesting an EB—induced ovulatory surge of LH. However, progesterone given with EB on Day 19 or Days 19 and 20 inhibited the EB—induced ovulation. (Endocrinology92: 108, 1973)

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