Studies on Maintaining the Lactational Diestrum after Early Litter Weaning

Abstract

The litters of lactating rats were adjusted to 8 pups on the day of parturition (day 1 of lactation) and various methods of cervical stimuli were performed. The litters were weaned at regular intervals and vaginal smears were examined for the return of estrous cycles. Thirty percent of mothers who either were not suckled or were suckled for 48 hr showed diestrus smears for 10 days or more. Mothers whose litters were removed on days 6, 9, and 12 of lactation did not exhibit an extended diestrum. Mechanical stimulation of the cervix of mothers weaned on day 3 of lactation did not increase the incidence of extended diestrus from the controls; electrical stimulation, however, almost doubled the number responding (50 vs. 30%). If the initiation of vaginal smearing was delayed from the first day of lactation until the third, none of the animals had pseudopregnancy-like vaginal smears. Estradiol administration on day 1 of lactation resulted in 60% of the animals exhibiting an extended diestrum. Combining both estradiol injection and mechanical cervical stimulation resulted in a slightly lower percentage (40%). The suckling stimulus is believed to induce an acute pattern of prolactin secretion while cervical stimulation induces a chronic pattern. The summation of these two secretory patterns may be observed early in lactation.