ANIMALS PREDISPOSED TO DEVELOP AMPHETAMINE SELF-ADMINISTRATION SHOW HIGHER SUSCEPTIBILITY TO DEVELOP CONTEXTUAL CONDITIONING OF BOTH AMPHETAMINE-INDUCED HYPERLOCOMOTION AND SENSITIZATION

It has been shown that rats, like humans, display individual differenc es in the propensity to develop psychostimulant self-administration. A nimals showing the highest locomotor reactivity to novelty (HRs: High Responders) are more prone to develop amphetamine self-administration than rats having a low locomotor response to novelty (LRs: Low Respond ers). The present study was designed to ascertain whether individual d ifferences are also present in the conditioning of drug effects, a pro cess involved in the maintenance of addiction. After pairing the drug effect with a particular set of environmental cues, only HRs showed co nditioned hyperlocomotion and environment-specific sensitization to th e effect of amphetamine. Unconditioned sensitization was, however, obs erved in LRs but not in HRs. The environment-specific sensitization di sappeared on extinction of the conditioned hyperlocomotion in HRs, ind icating that conditioning facilitates the expression of sensitization. In contrast, an inhibitory influence of conditioning on sensitization emerged from the analysis of the same results over all the experiment al groups, without taking individual differences into account. In conc lusion, our results shaw that: (i) locomotor reactivity to novelty pre dicts both vulnerability to develop self-administration and contextual conditioning of drug effects, which suggests that the two phenomena a re two related features and that conditioning plays an important role not only in the maintenance of drug intake but also in its development ; (ii) conditioned and unconditioned sensitization can be developed se parately in different individuals which suggests that they are indepen dent phenomena; (iii) analysis of individual differences is relevant t o pharmacological studies, especially with respect to drugs of abuse.