The acquisition, retention, and extinction of context-specific morphine withdrawal in rats were investigated in two experiments. In Experiment 1, context-specific withdrawal was observed after a placebo injection following 11 sessions in which the context was paired with either a 10-mg/kg or a 75-mg/kg dose of morphine. Contextual withdrawal was retained during 10 days of drug abstinence in both dose conditions. In Experiment 2, context-specific withdrawal (rearing) was retained after 21 days of morphine abstinence. In addition, conditioned rearing was extinguished by exposure to the drug context with or without a placebo injection. II\iection cues did not contribute to conditioned withdrawal.Chronic administration of morphine and other opiates produces both tolerance and dependence. Tolerance is defined as a decrease in drug effectiveness that occurs after repeated administration, and dependence is identified by the appearance of withdrawal symptoms after the drug is abruptly discontinued. A large body of research has shown that tolerance to many of the effects of morphine (e.g., analgesia, hyperthermia, sedation) is at least partly mediated by environmental or contextual cues paired with the drug (for a review, see Goudie & Demellweek, 1986).Siegel (1975, 1989) proposed a classical conditioning model of drug tolerance in which contextual cues present during drug administration serve as a conditioned stimulus (CS) and the drug acts as the unconditioned stimulus (US). After repeated pairings of these cues and the drug, the cues elicit compensatory, drug-opposite responses. These compensatory conditioned responses (CRs) serve a homeostatic function and reduce the drug's effects, which results in tolerance.Siegel 's model (l975, 1989) also incorporates drug dependence. In the absence of drug administration, the environmental cues still elicit the compensatory CRs, which are withdrawal signs. Tolerance and dependence, according to this model, result from the same underlying process. Although contextual control of tolerance to opiates is well documented (see Goudie & Demellweek, 1986; Siegel, 1989), its mediation by compensatory CRs is con-troversiaL Siegel and others have found cue-elicited compensatory hyperalgesia (
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