Postpartum estrus in Norway rats. II. Physiology

This investigation continues our descriptive analysis of postpartum estrus in Norway rats. Specifically, it is concerned with the occurrence of ovulation, the vaginal cytology, and the plasma levels of estradiol-17β and progesterone. Females were sacrificed by decapitation at 3 h intervals following delivery of the first pup. Plasma levels of estradiol-17β and progesterone were analyzed throughout the 24 h postpartum estrus period. Estradiol levels ranged from 130.35 pg/ml at 3 h to a low of 5.35 pg/ml at 24 h. Progesterone exhibited a peak level of 37.04 ng/ml at 12 h, and the intermediate levels ranged from 6.51 ng/ml to 23.01 ng/ml. Attempts were made to correlate the plasma steroid levels with vaginal cytology. Ovulation was noted as early as 3 h postpartum with 100% ovulation occurring by 18 h. Females exhibited a 'diestrous' smear throughout the 24 h period. In contrast to changes in vaginal cytology in normal estrous cycles, onset and duration of postpartum estrus could not be determined by change in the vaginal cytology. This investigation demonstrates that ovulation can occur as early as 3 h following parturition.