N. Accumbens by conditioned stimuli

The issue of the ability of conditioned stimuli to release DA in the NAc is debated. Existing discrepancies might be related to a number of variables including the positive or negative valence of the stimulus, the specific terminal area where DA is monitored, the contingent versus noncontingent modality of presentation of the CS and its discrete (cue) versus contextual nature. 

Recently, we have completed a series of studies comparing the effect of Pavlovian stimuli conditioned to morphine and food on DA relase in the NAc shell, NAc core and PFCX (Bassareo et al., in preparation). A more detailed account of these studies will be given in the Section Drug-induced stimulation of DA transmission and abnormal Pavlovian incentive learning. Here it will suffice to say that while drug-conditioned stimuli elicit a sustained release of DA in the NAc shell but not in the NAc core, food-conditioned stimuli release DA in the NAc core but not in the shell (Figs. 6-8). 

Presentation of a response contingent CS does not result in release of DA in the NAc as estimated by microdialysis (Bradberry et al., 2000 Neisewander et al., 1996). Similarly, Ito et al. (2000) did not observe any change of dialysate DA in the NAc shell or in the NAc core in relation to bar-pressing for a cocaine-conditioned stimulus. 

These studies show that phasic DA transmission is activated by Pavlovian stimuli in a manner consistent with its postulated role in incentive motivation. 

These studies indicate the existence within the striatum of a gradient of increasing responsiveness to psychostimulants directed ventromedially that corresponds to the notion of a preferential action in the ventral striatum. 

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