The postpartum gonadotrophin surge in the rat depends upon the presence of nonluteal ovarian tissues

The role of nonluteal ovarian hormones in the surge of luteinizing hormone (LH) which occurs in the hours following parturition was studied in 3 groups of rats. Ectopic corpora lutea were established in rats by autotransplantation of follicles beneath the kidney capsule on the evening of proestrus (Day 0). Group 1 rats had ectopic corpora lutea Group 2 had ectopic corpora lutea and a single ovary from which corpora lutea had been removed and Group 3 had a single intact ovary and were w ithout corpora lutea. All rats carried pregnancies for 22-24 days and the pups were removed after delivery to prevent suckling. Blood samples were obtained by heart puncture at intervals after the onset of delivery. Surges of LH (greater than 1.0 mcg/ml serum) were detected at 4.5-9 hours after delivery in rats in Groups 2 and 3. LH surges were absent in rats in Group 1 as late as 18 hours after delivery. Mean LH concentration was 100 + or -17 ng/ml. Prolactin levels were elevated only in animals that showed an LH surge (except 1). The data suggest th at parturition and associated withdrawal of luteal and placental hormones permit postpartum gonadotropin surge. However the necessary stimulus for the LH surge must be provided by the nonluteal ovarian hormones.