Repeated Cocaine Modifies the Mechanism by which Amphetamine Releases Dopamine

This study determined whether daily cocaine administration initiates a calcium requirement for the increase in extracellular dopamine produced by psychostimulants. The increase in extracellular dopamine induced by perfusion of amphetamine through a microdialysis probe in the nucleus accumbens shell was enhanced in cocaine- relative to saline-pretreated rats. The augmented portion of the amphetamine-induced increase in nucleus accumbens dopamine was abolished by the coperfusion of L- or N-type calcium channel blockers. Inhibition of calcium/calmodulin-dependent protein kinase II (CaM-KII) also prevented the augmented increase in dopamine by amphetamine, whereas inhibition of vesicular exocytosis by botulinum toxin B was ineffective. When the concentration of extracellular dopamine in the nucleus accumbens was elevated by blocking the plasmallemal dopamine transporter with GBR-12909, the augmented increase in extracellular dopamine in rats sensitized to repeated cocaine was blocked by a CaM-KII inhibitor. Pretreatment with botulinum toxin B prevented the increase in extracellular dopamine by GBR-12909 in both cocaine-pretreated and control rats. Taken together, these results demonstrate that the psychostimulant-induced enhanced increase in extracellular dopamine in the nucleus accumbens shell of cocaine-pretreated rats arises from the induction of calcium- and CaM-KII-dependent mechanisms.