A MICRODIALYSIS STUDY OF NUCLEUS-ACCUMBENS CORE AND SHELL DOPAMINE DURING OPERANT RESPONDING IN THE RAT

This investigation examined dopamine release and metabolism in nucleus accumbens core and shell during three operant tasks in the rat. Rats were trained to lever press on a fixed-ratio 5, variable-interval 30 s , or a tandem variable interval 30/fixed-ratio 5 schedules, these thre e schedules were chosen because they generate a wide range of response and reinforcement rates. After several weeks of training, dialysis pr obes were implanted into nucleus accumbens core or shell subregions. A single 30 min behavioural session was conducted during the dialysis t est session. Rats lever pressing on each of the three operant schedule s showed a significant increase in extracellular dopamine relative to the food-deprived control group during the behavioural session. In add ition, increases in dopamine in nucleus accumbens shell were found to be significantly greater than in the core during the lever pressing pe riod. Across all three schedules, extracellular dopamine in the nucleu s accumbens was significantly correlated with the number of lever pres ses performed, but was not correlated with the number of food pellets delivered. Analysis of covariance, which used amount of food consumed as the covariate, showed an overall group difference, indicating that dopamine levels increased in lever pressing animals even if one correc ted for the amount of food consumed. These results indicate that dopam ine release was more responsive in the nucleus accumbens shell than in the core during operant responding, and that increases in extracellul ar dopamine in nucleus accumbens are related to response rate rather t han reinforcement magnitude. (C) 1998 IBRO. Published by Elsevier Scie nce Ltd.