Nonetheless, regardless of the extent to which the drug (even when its DA-enhancing effects are attenuated) predicts reward, the act of its administration (injection, smoke, drink) becomes a conditioned cue so that the mere expectation of the reward during administration of the drug may increase DA in the NAc. On the basis of these findings and given the role of DA in predicting reward, we postulate a shift in the source of DA increases: Although drugs may initially lead to DA release in the NAc (i.e., signaling reward), with repeated administration and as habits develop, the DA increases become transferred from the pharmacological effects of the drug to the conditioned responses (i.e., to the behaviors and objects linked with administering the drugs, such as the act of injecting or seeing a syringe). Such an effect is reported to occur also with repeated exposure to natural reinforcers (23).
