Effect of morphine during pregnancy and lactation in mice.

The influence of pregnancy and lactation on morphine-induced hypothermia and hyperactivity was investigated in Rockland-Swiss mice. Pregnant mice were slower to recover from the hypothermic effect of morphine than nonpregnant controls. The greatest hypothermic response was seen in mice at day 18, the day before parturition. On the day after parturition, mice recovered faster from morphine-induced hypothermia than controls. During lactation, mice were again slower to recover from the morphine-induced hypothermia. Morphine-induced locomotor activity, however, was attenuated by both pregnancy and lactation. A further experiment compared the hypothermic effect of morphine in postpartum mice that were allowed to remain with pups and mice that were separated from pups. The enhanced hypothermia in the postpartum period was abolished by removing pups. This indicates that the altered response to morphine in the postpartum period was an effect of lactation rather than an aftereffect of pregnancy or parturition.