Reward, memory and substance abuse: functional neuronal circuits in the nucleus accumbens.

The firing patterns of neurons in the nucleus accumbens (NA) are examined and discussed with respect to different types of rewards and reward conditions. Comparisons and contrasts between individually identified NA neuron responses to cocaine self-administration and water reinforcement are presented with an emphasis on the fact that the same neurons do not respond in a phasic manner to both types of rewards. However, the phasic firing patterns, even though segregated for each reinforcer, are quite similar, suggesting that the method of differentiation between rewarding stimuli in the NA is by sorting cell populations into distinct ensembles or networks for each type of reinforcer. These neural networks appear to be 'tuned' to respond to particular associative behavioral contexts that couple response execution to reward delivery, and in the process acquire a reciprocity to firing within reward contexts. This maintains the specificity of each reinforcer for the response and associated stimuli that produce it and, makes it possible to attach different NA networks to different reinforcing circumstances. Comparisons of cocaine and water reinforced NA cell firing patterns during rapid switching between these two reinforcers suggests that the networks are negatively coupled and mutually inhibit each other to maintain accurate encoding of immediately experienced, as well as expected (i.e. future) reward contingencies.