The effect of aggregation on the lethality of morphine in mice.

Morphine was more lethal to mice housed in groups (aggregated) than to mice housed individually (isolated). The aggregation effect on morphine lethality was increased when ambient temperature was increased to 29 degrees C and abolished when ambient temperature was lowered to 19 degrees C. At 29 degrees C morphine produced hypothermia in isolated mice and hyperthermia in aggregated mice. Both isolated and aggregated mice were hypothermic after morphine administration at 19 degrees C. At 29 degrees C the locomotor activity of morphine-treated aggregated mice was significantly greater than that of morphine-treated isolated mice, whereas at 19 degrees C no significant difference was found. The locomotor activity time course of morphine-treated aggregated mice at 29 degrees C closely resembled the rectal temperature time course, which suggests that the hyperthermia in those animals involves an increase in heat production via an increase in locomotor activity. However, the hyperthermia observed was not sufficient to explain the increased lethality in aggregated mice. Differences between the aggregation effect on morphine lethality and that previously reported for amphetamine were discussed.