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Profiles of serum prolactin (PRL) and progesterone (P4) were determined in repeatedly (every 2 h) sampled female Djungarian hamsters (Phodopus campbelli) over a 24-h period on Days 3, 6, 9, 12, and 15 of the 18-day pregnancy. The first half of pregnancy was characterized by significant surges of PRL within a 2-h period around dawn (0500 h) and dusk (1900 h), with some females also showing a weak midday surge at 1300 h. By Day 9, dusk and midday surges were absent, but the dawn surge remained at its initial amplitude. On Day 12, no PRL surges were seen. Resumption of both the dusk and dawn PRL surges occurred on Day 15 of gestation. Considerable interindividual variability in the amplitude of PRL surges, the timing of PRL surges, and the number of surges per day was detected and would complicate any assessment of PRL levels based on single samples per female. Serum P4 concentrations were 8-10 ng.ml-1 before doubling on Day 15. A 3-day treatment of 50 and 300 micrograms bromocryptine (CB 154; on Days 13-15) effectively suppressed PRL during late pregnancy (Day 15) but did not alter serum P4 concentrations or interfere with parturition. Therefore, surges of PRL are not an essential luteotropic stimulus during late gestation. Compared to oil-injected controls, CB 154-treated females had a higher incidence of infanticide postpartum. Growth rates of the pups, mammary gland development, and successful delivery of milk to pups, however, did not differ between groups. Further studies will be required to determine the function of late-gestation PRL surges.

In contrast to results from previous research, a surge of progesterone (P4) on the afternoon of proestrus was consistently detected in Djungarian hamster females. However, the timing of maximal P4 levels varied from 1200 to 1900 h across females; this impeded detection of the surge in earlier studies. No role for P4 in the induction of behavioral receptivity was found. Behavioral receptivity was induced in ovariectomized females with physiological levels of estradiol typical of an estrous cycle (60-180 pg/ml). P4 did, however, terminate receptive behavior within 48 h. Prolactin (PRL) was present as surge levels on each day of the cycle except diestrus 1, during which PRL remained at basal levels in all females. This pattern distinguishes the Djungarian hamster from the rat and mouse, which have PRL surges only on the afternoon of proestrus, and the golden hamster, which has a PRL surge on each of the 4 days of the estrous cycle. Diestrus 1, with P4 high and PRL low, was clearly distinct from Day 2 of Djungarian hamster pregnancy, during which P4 is low and PRL is surging. Therefore, postcoital P4 levels change within 34 h and before rescue of the corpus luteum. As Day 2 of pregnancy is also the most sensitive time for a mate-removal pregnancy-blocking response in P. campbelli, this hormonal profile may be associated with sensitivity to those stimuli.