Behavioral effects of low and high acute doses of morphine in solitary mice.

The effects of morphine (5, 10, 100, and 150 mg/kg SC) on locomotor activity, object manipulation, grooming, rearing, and responsiveness to social stimulation were observed in naive, nontolerant mice. Morphine induced significant changes in the behavior elements recorded. Five and 10 mg/kg morphine caused an initial phase of about 1 h with inhibition of all activities. After 1 h the mice gradually increased activity and exceeded the corresponding placebo level at the end of the sessions. 100 and 150 mg/kg morphine caused an increase in locomotor activity. This hyperactive continuous running was stereotyped, restricted as it was to only a certain part of the experimental cage. Concurrently all other behavior elements were abolished. The animals did not normalize within the observation period. Neither the sedated mice with low doses nor the mice with high doses of morphine responded socially to the presence of another untreated mouse which was placed in the cage as a social response test.