Repeated intermittent exposure to psychostimulants can lead to long-lasting sensitization of the drugs' behavioral and biochemical effects. Such findings have figured importantly in recent theories of drug addiction proposing that sensitized nucleus accumbens (NAcc) dopamine (DA) overflow in particular acts in concert with other alterations in the neurochemistry of this nucleus to promote drug seeking and self-administration. Yet, experiments in rodents, nonhuman primates and humans have not always detected behavioral or biochemical sensitization following drug exposure, bringing into doubt the utility of this model. In an effort to reconcile apparent discrepancies in the literature, this review assesses conditions that might affect the expression of sensitization during testing. Specifically, the role played by conditioned cues is reviewed. A number of reports strongly support a potent and critical role for conditioned stimuli in the expression of sensitization. Findings suggest that stimuli associated either with the presence or absence of drug can respectively facilitate or inhibit sensitized responding. It is concluded that the presence or absence of such stimuli during testing for sensitization in animal and human studies could significantly affect the results obtained. It is necessary to consider this possibility especially when interpreting the results of studies that fail to observe sensitized responding.Keywords conditioned inhibition; conditioning; dopamine; drug addiction; drug self-administration; excitatory Pavlovian conditioning; facilitation; occasion setters; sensitization
Sensitization in animals and humansThere is general agreement that rats repeatedly exposed to psychostimulants like amphetamine will exhibit enhanced -sensitized -locomotor responding when subsequently challenged with the drug some time later. In these animals, the reactivity of mesoaccumbens dopamine (DA) neurons to the drug challenge is also enhanced (for critical reviews of the preclinical literature, see Kalivas and Stewart, 1991;Vanderschuren and Kalivas, 2000;Vezina, 2004). These are long-lasting effects in the rat. Sensitized locomotor responding has been reported up to one year (Paulson et al., 1991) and enhanced nucleus accumbens (NAcc) DA overflow up to three *Correspondence: Department of Psychiatry The University Of Chicago 5841 S. Maryland Avenue, MC3077 Chicago, IL 60637 TEL: 773/702-2890 FAX: 773/702-0857 E-MAIL: pvezina@yoda.bsd.uchicago.edu. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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