A neurochemical and behavioral investigation of the involvement of nucleus accumbens dopamine in instrumental avoidance.

Two experiments were conducted to investigate the role of nucleus accumbens dopamine in the performance of a lever press avoidance task in the rat. In this task, 0.5 mA shock was presented for 5 s every 30 s, but the rat could escape shock presentation, or avoid the shock for 30 s, by pressing a lever. In the first experiment, dialysis probes were implanted into the nucleus accumbens of rats previously trained on the avoidance procedure, and also two groups of untrained rats. On the day after dialysis probe implantation, rats trained to press a lever to avoid shock (n = 10) were exposed to a 45 min avoidance test session. Untrained rats were either exposed to periodic shock (n = 9) or the control procedure in which lights were dimmed but no shock was presented (n = 8). Performance of the avoidance task led to significant increases in extracellular dopamine and dopamine metabolites. There was a significant positive correlation between increases in extracellular dopamine and the number of avoidance responses emitted. In the second experiment, groups of rats were trained on the lever press avoidance procedure. After training, rats received intra-accumbens injections of the neurotoxic agent 6-hydroxydopamine or ascorbic acid vehicle. Dopamine depletion produced by 6-hydroxydopamine injection led to a substantial decrease in lever pressing to avoid or escape shock. These results indicate that dopamine in nucleus accumbens is important for operant avoidance responding, and that the involvement of accumbens dopamine in instrumental behavior is not unique to positively reinforced responses.