Discriminative stimulus properties of cocaine: modulation by dopamine D₁ receptors in the nucleus accumbens

Dopamine (DA) D₁ and D₂ receptors are involved in mediating the behavioral effects of cocaine, including its discriminative stimulus properties. The purpose of the present study was to investigate the role of the nucleus accumbens and, in particular, accum bens DA D₁ receptors in modulating the stimulus effects of cocaine. Thus, rats were trained to discriminate cocaine (10 mg/kg, IP) from saline using a two-lever, water-reinforced FR 20 drug discrimination task. In substitution tests, systemic (IP) administration of cocaine (0.625-20 mg/kg) produced a dose-related increase in cocaine-appropriate responding. Microinjections of cocaine (2.5-40 μg) into the nucleus accumbens also engendered dose-dependent and complete substitutions (> 80% drug-lever responding) for the systemic training dose of cocaine, whereas intra-accumbens artificial cerebrospinal fluid (1 μl/side) produced primarily saline-appropriate responding. In antagonism tests, pretreatment with the DA D₁ antagonist SCH 23390 (3-12 μg/kg) completely antagonized (<20% drug-lever responding) a dose of cocaine (5 mg/kg) that produced greater than 90% cocaine-lever responding when given alone. Additionally, intra-accumbens injections of SCH 23390 (0.025-0.4 μg) prior to systemic cocaine (5 mg/kg) also significantly blocked the cocaine stimulus. The present results confirm the importance of the nucleus accumbens in mediating the discriminative stimulus properties of cocaine and suggest a primary role of accumbens DA D₁ receptors in modulating this behavior.