Different behavioral functions of dopamine in the nucleus accumbens and ventrolateral striatum: a microdialysis and behavioral investigation.

Three experiments were conducted to investigate the behavioral functions of dopamine in the nucleus accumbens and ventrolateral striatum. In the first experiment, dialysis probes were implanted in the nucleus accumbens or ventrolateral striatum of rats previously trained to respond on fixed interval lever pressing schedules for food reinforcement. During the dialysis test session, both schedule- and site-dependent effects on dopamine release were observed. Overall, lever pressing on a fixed interval 30-s schedule produced a greater increase in extracellular dopamine than did responding on a fixed interval 120-s schedule. The fixed interval 30-s schedule was also accompanied by a higher rate of lever pressing. Rats with nucleus accumbens probe placements showed significantly higher increases in dopamine release than rats with ventrolateral striatal placements. An additional dialysis experiment showed that baseline levels of dopamine were suppressed by 1.0 microM tetrodotoxin to a similar extent in the nucleus accumbens and ventrolateral striatum. In the third experiment, 6-hydroxydopamine was injected locally into either the nucleus accumbens or the ventrolateral striatum in order to deplete dopamine. Nucleus accumbens dopamine depletions produced only a minor decrease in operant responding, whereas rats with ventrolateral striatal dopamine depletions showed low levels of responding that differed from both the control group and from the group that had accumbens dopamine depletions. Thus, these results are somewhat paradoxical, in that the structure that showed the greatest increase in dopamine release (i.e. the nucleus accumbens) was also the terminal region at which dopamine depletions had very little effect on operant responding. Ventrolateral striatal dopamine appears to be largely permissive over lever pressing, in that normal levels of dopamine in the ventrolateral striatum are critical for responding, although dopamine levels do not fluctuate much during behavioral sessions.