Daily Patterns of Pituitary Prolactin Secretion and their Role in Regulating Maternal Serum Progesterone Concentrations Across Pregnancy in the Djungarian Hamster (Phodopus Campbelli)1

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.