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Feeding a novel food (Fonzies) to rats fed ad libitum with standard food increased extracellular dopamine (DA) in the medial prefrontal cortex (PFCX) and in the medial nucleus accumbens (NAc). Previous Fonzies feeding, although it did not affect the increase of extracellular DA in the PFCX in response to Fonzies feeding, blunted that increase in the NAc (habituation); recovery from habituation in the NAc was complete 5 d after previous Fonzies feeding. Predictive association of an otherwise neutral stimulus extrinsic to Fonzies (empty plastic box) with Fonzies feeding resulted in the acquisition by the stimulus of the property to elicit incentive responses directed toward the stimulus and to increase extracellular DA in the PFCX. However, the same stimulus, or a more complex stimulus including intrinsic stimuli (Fonzies-filled plastic box), failed to acquire the ability to modify extracellular DA in the NAc.Pseudoconditioning, i.e., nonpredictive association of the extrinsic stimulus (empty box) with Fonzies feeding, did not result in acquisition by the stimulus of the property to elicit incentive responses and to increase extracellular DA in the PFCX. Repeated nonreinforced presentation of previously conditioned extrinsic stimuli (empty box) resulted in extinction of the property to elicit incentive responses and to increase extracellular DA in the PFCX. These results indicate that in rats fed ad libitum, phasic activation of mesocortical and mesolimbic DA systems by motivational stimuli is differentially influenced by associative (conditioning) and nonassociative (habituation) learning mechanisms and is differentially related to acquisition and expression of incentive motivation.

The response of extracellular dopamine (DA) and its relationship to motivational valence (positive or negative) and novelty of motivational stimuli was investigated by brain microdialysis in the nucleus accumbens (NAc) shell and core and prefrontal cortex (PFCX) of rats. Stimuli were elicited by intraoral infusion of 20% sucrose, sucrose plus chocolate, quinine, and NaCl solutions, feeding of a palatable food (Fonzies), or smelling of a predator (red fox) urine. Sucrose elicited appetitive reactions and increased DA in the PFCX but not in the NAc shell. An unfamiliar appetitive taste such as that of sweet chocolate and Fonzies, increased DA in all three areas. Habituation of the stimulatory DA response to intraoral chocolate or to Fonzies feeding was observed in the NAc shell after a single pre-exposure to the same taste or food; no habituation was observed in the NAc core nor in the PFCX. Aversive taste stimuli (quinine, saturated NaCl solutions) rapidly increased DA in the PFCX and in the NAc core, and this response did not undergo one-trial habituation. In the NAc shell, instead, no effect (10 min exposure) or a delayed, transitory increase of DA (5 min exposure) sensitive to one-trial habituation was obtained in response to the aversive taste (quinine and saturated NaCl) or olfactory (red fox urine) stimuli. These observations indicate that DA responsiveness is an integrated function of the motivational valence and novelty of stimuli in the NAc shell and an expression of generic motivational value in the NAc core and PFCX.

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