Differential influence of morphine sensitization on accumbens shell and core dopamine responses to morphine- and food-conditioned stimuli

Rationale Sensitization of the incentive and dopamine (DA) stimulant properties of drug-conditioned stimuli (CSs) by repeated exposure to drugs of abuse has been assigned an important role in the genesis of drug addiction. Objective To test in rats if morphine-induced sensitization potentiates incentive and DA-releasing properties in the nucleusaccumbens(NAc)shellandcoreelicitedbypresentation of a morphine-conditioned stimulus(CS) and if this property generalizes to a non-drug-(palatable food, Fonzies)-CS. Methods Controls and rats previously sensitized by morphine were trained via three daily sessions consisting of a 10-min presentation of CS (Fonzies filled box, FB) followed by s.c. saline and morphine (1 mg/kg) or by standard food and Fonzies. Rats were implanted with microdialysis probes and the next-day incentive reactions and NAc shell and core DAwere monitored during CS presentation and subsequent morphine (1 mg/kg) administration or Fonzies feeding. Results Morphine sensitization increased incentive and NAc shell and core DA responses to morphine-CS. Morphine conditioning per se increased incentive reactions and NAc shell but not core DA responses to FB presentation. Morphine sensitization potentiated incentive responses but did not affect NAc shell and core DA responses to FonziesCS. Fonzies conditioning increased incentive reactions and NAc core but not shell DA responses to FB presentation. Conclusions These observations confirm the prediction of the incentive sensitization theory in the case of drug-CS but not of non-drug-CS. NAc DA might be differentially involved in the expression of incentive sensitization of drugand non-drug-CSs, thus providing a clue for the abnormal incentive properties of drug CSs.