Morphine effects on maternal aggression, pup care and analgesia in mice.

In order to assess the respective contribution of opioid receptors to the behavioral and physiological characteristics of lactating animals, we challenged mice with morphine at different phases of the lactation period. Sensitivity to morphine's effects on aggressive behavior, pup care, pain response and body temperature were measured. Lactating mice were assigned to 1 of the 3 weeks of lactation and to 1 of 5 doses of morphine sulfate (0, 1, 3, 6, 10 mg/kg IP). After morphine administration, rectal temperature and tail flick were assessed. Behavior towards three pups was observed for 5 min, followed by an aggression test with a female intruder. Morphine significantly increased the latency to retrieve pups and decreased aggressive behavior at doses that do not decrease motoric activity. Compared to virgin mice, lactating females are less sensitive to the analgesic actions of morphine but similarly sensitive to its hypothermic properties. The fact that virgin and lactating females can be distinguished on the basis of their sensitivity to morphine-induced analgesia suggests that lactating animals undergo functionally relevant changes in opioid regulation of pain sensitivity. Furthermore, morphine's specific and potent inhibition of pup retrieval supports the hypothesis that decreased opioid peptide activity is important for the expression of certain postpartum behaviors.