Opiate regulation of behavioral selection during lactation.

Treatment of postpartum female rats with morphine inhibits maternal behavior. The same type of treatment stimulates foraging in adult animals. The aim of the present study was to investigate, in lactating rats, the functional role of opioid systems in the choice between caring for pups versus hunting insects. Experiment 1 was designed to test how acute morphine treatment with 3.0 mg/kg interferes with choosing between caring for pups versus predatory behavior. Morphine-treated dams decreased maternal behavior while increasing efficiency in hunting insects. The next step was to test the opioid antagonist naloxone in the same context of maternal versus predatory behavior. Naloxone restored maternal care and reduced hunting in morphine-treated rats. Finally, in order to test the role of endogenous opioidergic stimulation in this scenario, lactating rats were treated with the opioid antagonist naloxone alone. Consistently, naloxone treatment induced a decrease in number of insects captured and an increase in the percentage of animals displaying nursing behavior. These results provide important insight into the role of opioidergic transmission in the regulation of behavioral selection during lactation. The present results suggest that endogenous opioids may stimulate hunting by replacing maternal behavior during lactation.