Conditioned changes in nucleus accumbens dopamine signal established by intravenous cocaine in rats.

To explore the role of cocaine-induced conditioning in the regulation of mesolimbic dopamine (DA) activity, high-speed chronoamperometry with Nafion-coated carbon fiber electrodes was used to measure changes in nucleus accumbens (NAc) DA-dependent signal induced by a light stimulus previously paired and not paired with intravenous cocaine (0.85 mg/kg per injection), and cocaine injections paired and not paired with light. A small phasic DA signal increase was found after light presentation in drug-naive rats, whereas greater and longer signal increases were induced by the same light after it had been repeatedly paired with cocaine. The DA signal changes associated with repeated cocaine injections paired with light differed from those induced by cocaine alone and occurring during drug self-administrations. These data suggest that cocaine-related sensory cues, acting alone, may induce a conditioned mesolimbic DA activation, and, acting in the presence of the drug, may significantly modify the DA response to the drug.