The Postpartum Gonadotropin Surge in the Rat Depends upon the Presence of Nonluteal Ovarian Tissues

We have determined which ovarian tissues and their associated hormones must be present in periparturient rats to evoke normal patterns of serum LH and prolactin in the immediate postpartum period. Ectopic corpora lutea were established in rats by autotransplantation of follicles beneath the kidney capsule on the evening of proestrus (day 0). Rats with ectopic corpora lutea (group 1), ectopic corpora lutea and a single ovary from which corpora lutea had been removed (group 2), or a single intact ovary and no ectopic corpora lutes (group 3) were allowed to litter and blood samples obtained by heart punctures at intervals after the onset of delivery. Pups were removed to prevent suckling. All rats delivered on days 22 to 24. Surges of LH (>1.0 µg/ml serum) were detected at 4.5 or 9 h after delivery in rats with ectopic corpora lutes and a follicular ovary (group 2) and in rats with a single intact ovary (group 3). However, no surges of LH were found in rats of group 1, as late as 18 h after delivery; mean LH concentration was 100 ± 17 ng/ml. With one exception, elevated levels of prolactin (>85 ng/ml) were found only in those animals which showed surges of LH. The data can be interpreted to suggest that parturition and associated withdrawal of luteal and placental hormones permit the postpartum gonadotropin surge but do not comprise the necessary stimulus; the stimulus appears to be provided by hormones produced by nonluteal tissues of the ovary.