Dopamine D1 receptors in the nucleus accumbens: involvement in the reinforcing effect of tegmental stimulation.

Subtypes of dopamine receptors involved in the mechanism of reinforcement were examined. Twenty-five rats were implanted with bipolar electrodes into the ventral tegmental area (VTA) and cannulae into the nucleus accumbens (NAc) or the caudate-putamen complex (CPu). The rats were trained to press a bar to receive VTA stimulation, and dopamine antagonists were injected into the brain while the animals were responding. SCH 23390 (D1-specific antagonist) injected into NAc ipsilateral to the electrode site suppressed responding, but injections into contralateral NAc or ipsilateral CPu had no effect. Haloperidol was less effective. Sulpiride (D2-specific antagonist) produced no effect when injected into NAc, though slightly suppressed responding when injected into CPu. The animals that have reduced their response rate after accumbens injection of SCH 23390 responded at a normal rate when the frequency of pulses in the tegmental stimulation was increased, indicating that they were not incapacitated by the D1 antagonist. These findings suggest that dopamine D1 receptors in the nucleus accumbens are critically involved in the reinforcement produced by ventral tegmental stimulation.