Novel patterns of progesterone and prolactin in plasma during the estrous cycle in the Djungarian hamster (Phodopus campbelli) as determined by repeated sampling of individual females

In contrast to results from previous research, a surge of progesterone (P-4) on the afternoon of proestrus was consistently detected in Djungarian hamster females. However, the timing of maximal P-4 levels varied from 1200 to 1900 h across females; this impeded detection of the surge in earlier studies. No role for P-4 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). P-4 did, however, terminate receptive behavior within 48 h. Prolactin (PRL) was present at 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 P-4 high and PRL low, was clearly distinct from Day 2 of Djungarian hamster pregnancy, during which P-4 is low and PRL is surging. Therefore, postcoital P-4 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.