Late pregnancy progesterone treatment as a modulatory signal for postpartum behavior.

Changes in plasma progesterone levels during late pregnancy are a determining factor in the expression of maternal behavior during lactation. Previous studies showed that mild opioidergic stimulation during late pregnancy makes lactating females more sensitive to opioidergic-induced inhibition of maternal behavior and more willing to display hunting behavior. Such previous behaviorally meaningful opioidergic stimulation also selectively increased serum progesterone levels. The present study tested whether progesterone treatment during late pregnancy interferes with the display of maternal behavior and behavioral selection during lactation. In Experiment 1, rats were treated with progesterone (400 and 500 μg per day) from the 17th day to the 22nd day of pregnancy. The lowest progesterone dose did not interfere with pregnancy or parturition, and this dose was used in Experiments 2 and 3, in which the rats were treated with subcutaneous progesterone or peanut oil for 5 days beginning on pregnancy day 17. On day 5 of lactation, dams were challenged with subcutaneous morphine (1.5 mg/kg), or saline. The rats were then tested for maternal care (Experiment 2) or behavioral selection with pups and cockroaches (Experiment 3). Animals treated with progesterone during late pregnancy and challenged with morphine during lactation exhibited a significant decrease in maternal behavior in both Experiments 2 and 3. Predatory hunting was not modified by progesterone treatment. These results indicate that sensitivity to opioidergic-mediated inhibition of maternal behavior is enhanced by prepartum progesterone administration. Thus progesterone might be part of the opioid-triggered prepartum signaling leading to behavioral changes during lactation.