Progesterone, estradiol, and prolactin involvement in lactation, including lactation following a postpartum mating, in the Djungarian hamster (Phodopus campbelli)

Abstract

The endocrinology of lactation in the Djungarian hamster was investigated through once-daily sampling of females on Days 0, 3, 6, 9, 12, 15, and 18 postpartum. Serum prolactin (PRL) was elevated during early lactation but declined with time. Serum progesterone (P4) levels were uniformly low, and serum estradiol (E2) levels increased after Day 9. New CL were formed after the postpartum ovulation and persisted throughout lactation. During the first 9 days, the CL had the high P4 content typical of pregnancy. After Day 9, each individual CL decreased in P4 content and increased in E2 content. As evidenced by the reappearance of large antral follicles, lactating females probably returned to ovulatory cycles at about that time. The Djungarian hamster has a postpartum estrus and is capable of delivering a second litter without a diapause (18 days), although the majority of second litters are delayed. Mating during the postpartum estrus had no detected effect on growth or survival of the first litter. Second pregnancies were developmentally delayed by 3 to 7 days. P4 was effectively absent from serum for the first 12 days after mating. Although CL contained high levels of P4, the CL remained small and poorly vascularized. Mated females did not have higher serum P4 levels than nonpregnant, lactating females until Day 18. In response to the competing demands of lactation and concurrent gestation, the endocrine conditions associated with pregnancy were the most affected.