Biology of Reproduction, Vol 10, 111-115, Copyright © 1974 by Society for the Study of Reproduction

Association Between Pregnant Mare Serum-Induced Ovulation and Luteolysis in Rats and its Dissociation by Progesterone Treatment

¹ Department of Obstetrics and Gynecology, University of Western Ontario, London, Ontario, Canada and Department of Physiology, University of Western Ontario, London, Ontario, Canada

The ability of prolactin and progesterone to inhibit ovulation was utilized to investigate the relationship between ovulation and luteolysis. The first estrus and ovulation were synchronized in 27-day-old, prepubertal rats by the subcutaneous (sc) administration of 4 IU of pregnant mane serum gonadotropin (PMS). Pseudopregnancy was induced by the sc administration of prolactin (100 µg, NIH-P-B₂ administered twice daily) beginning on the day of ovulation (Day 0) and maintained through Day 2. If prolactin therapy was discontinued on Day 2, a second injection of PMS on Day 3 resulted in ovulation accompanied by involution of the original corpora lutea. Continuous prolactin treatment from Day 0 through Day 6 not only prevented ovulation as a result of PMS administration on Day 3, but prevented luteolysis as well. Progesterone (8 mg/day), if administered on Days 3 through 6, inhibited ovulation in response to PMS administered on Day 3. It did not, however, prevent the luteolysis as a result of the ovulatory stimulus provided by PMS. Progesterone, in the absence of PMS, did not affect luteal weight. The dissociation of luteolysis from ovulation demonstrated that actual ovulation was not a requirement for luteolysis to occur, but leaves open the possibility that an ovulatory stimulus may be an effective luteolytic stimulus.