The interpretation of the measurement of nucleus accumbens dopamine by in vivo dialysis: the kick, the craving or the cognition?

Psychopharmacological studies have implicated the dopaminergic innervation of the nucleus accumbens (NAC) in reward and reinforcement, in the actions of addictive drugs, and in the control of the symptoms of schizophrenia. Recent developments in in vivo dialysis, and other in vivo neurochemical techniques have permitted a more direct analysis of the behavioural correlates of increased dopamine release in rats, and have largely confirmed these findings in relation to reward, and drugs of abuse potential. However, dopamine release has also been found to be increased by many other stimuli/situations including aversive stimuli, stimuli conditioned to aversive stimuli, complex novel stimuli, and in the process of conditioning itself. These results contrast with electrophysiological data obtained in the behaving monkey, where rewarding stimuli, or stimuli predictive of reward are associated with increased firing of presumptive dopamine neurones projecting to the NAC (and indeed to the striatum), but mild aversive stimuli are not, leading to the suggestion that this system subserves a more purely reward function, or indeed that it provides a reward error signal. Further exploration of these issues will depend upon a comparison of increased dopamine cell firing and increased dopamine release, and an analysis of the behavioural effects of blocking these increases in dopamine transmission. One suggestion, deriving from work on latent inhibition, is that the significance of dopamine release by salient stimuli is to allow learning about stimuli which would otherwise be excluded on the basis of familiarity. This suggests that in addition to a role in some types of learning about salient stimuli, dopamine release in NAC may have a role in controlling the attention paid to familiar stimuli. Since it is difficult to see a connection between simple learning about rewards, and the symptoms of schizophrenia, this provides a more convincing link between the dopamine theory of schizophrenia, and the attentional difficulties held by many theorists to underlie schizophrenic symptoms.