Morphine-induced sensitization of locomotor activity in mice: effect of social isolation on plasma corticosterone levels.

This study examined the influence of social isolation on behavioural sensitization to the locomotor effect of morphine and the link between this behaviour and plasma corticosterone concentrations. Four weeks isolation induced an increase in the locomotor effect of morphine. In social and isolated mice, repeated administrations (6) of morphine (one injection every 3 or 4 days) followed by 3 h in an actimeter induced behavioural sensitization to the locomotor effect of morphine. No interaction was observed between social isolation and behavioural sensitization to morphine. Resocializing previously isolated mice for 3 weeks reduced the morphine-induced locomotor effect without altering the behavioural sensitization. Corticosterone plasma levels were more increased (416%) in mice isolated 5 weeks than in mice isolated for 2 weeks (243%) and they return to the control levels following 3 weeks of resocialization. Since there was no interaction between the increase in morphine locomotor effect induced by social isolation and the morphine-induced behavioural sensitization, it is suggested that each of these two events acts independently. Whether or not a common mechanism (plasma corticosterone levels?) partly underlies both effects, the result resembles a simple additive effect.