Differential Expression of Motivational Stimulus Properties by Dopamine in Nucleus Accumbens Shell versus Core and Prefrontal Cortex

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Abstract
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.