The distinctions between drugs in the induction and expression of sensitization indicate that behavioral sensitization can arise from multiple neuroadaptations in multiple brain nuclei. This is not only the result of distinct molecular targets for the drugs, but may also include a differential involvement of learned associations. It is postulated that the relatively more robust pharmacological capacity of amphetamine to release dopamine may induce a form of sensitization that is more dependent on adaptations in mesoaccumbens dopamine transmission compared with cocaine and morphine sensitization.

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Compulsive drug use in the face of adverse consequences is a hallmark feature of addiction, yet there is little preclinical evidence demonstrating the actual progression from casual to compulsive drug use. Presentation of an aversive conditioned stimulus suppressed drug seeking in rats with limited cocaine self-administration experience, but no longer did so after an extended cocaine-taking history. In contrast, after equivalent extended sucrose experience, sucrose seeking was still suppressed by an aversive conditioned stimulus. Persistent cocaine seeking in the presence of signals of environmental adversity after a prolonged cocaine-taking history was not due to impaired fear conditioning, nor to an increase in the incentive value of cocaine, and may reflect the establishment of compulsive behavior.

No abstract