Role of extracellular dopamine in the initiation and long-term expression of behavioral sensitization to cocaine

Repeated intermittent administration of cocaine has been shown to sensitize animals to the locomotor-activating effects of this agent. The neurobiochemical basis of this phenomenon, however, remains only partially understood. The present study sought to characterize basal dialysate dopamine (DA) concentrations within the nucleus accumbens (NAc), 2, 12 or 22 days after the cessation of either repeated cocaine (20 mg/kg/day x 5 days) or saline (1.0 ml/kg/day x 5 days) treatment. Locomotor activity and dialysate DA levels in response to a subsequent cocaine administration (20 mg/kg i.p.) were assessed at the same time intervals. Cocaine-pretreated animals exhibited an enhanced motor response to a cocaine injection 2 days after cessation of cocaine treatment. The magnitude of this effect increased progressively over time. Basal DA overflow was elevated 2 days after termination of cocaine treatment; at this time, however, a blunted response of DA neurons to the cocaine administration was observed. As the duration of withdrawal increased, basal dialysate DA concentrations gradually declined, whereas the response of DA neurons to cocaine progressively increased. By day 22 of withdrawal, a significant enhancement of cocaine-induced DA overflow was seen. These findings demonstrate that increased DA overflow in response to cocaine cannot account for the short-term expression of behavioral sensitization to cocaine. Rather, an enhanced DA response develops during later stages of the sensitization process and, therefore, may be one of the mechanisms responsible for the long-term expression of cocaine sensitization.