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 sensit ization to the locomotor effect of morphine and the link between this behav iour and plasma corticosterone concentrations. Four weeks isolation induced an increase in the locomotor effect of morphine. In social and isolated mi ce, repeated administrations (6) of morphine (one injection every 3 or 4 da ys) followed by 3 h in an actimeter induced behavioural sensitization to th e locomotor effect of morphine. No interaction was observed between social isolation and behavioural sensitization to morphine. Resocializing: previou sly isolated mice for 3 weeks reduced the morphine-induced locomotor effect without altering the behavioural sensitization. Corticosterone plasma leve ls were more increased (416%) in mice isolated 5 weeks than in mice isolate d for 2 weeks (243%) and they return to the control levels following 3 week s of resocialization. Since there was no interaction between the increase i n morphine locomotor effect induced by social isolation and the morphine-in duced behavioural sensitization, it is suggested that each of these two eve nts acts independently. Whether or not a common mechanism (plasma corticost erone levels?) partly underlies both effects. the result resembles a simple additive effect. (C) 2000 Elsevier Science B.V. All rights reserved.