Morphine tolerance in rats: Congruence with a Pavlovian paradigm

Evidence for associational and nonassociational mechanisms of tolerance development was found in 4 experiments using an automated flinch/jump assessment of morphine's analgesic effects on a total of 175 male Holtzman rats. Ss receiving morphine in a distinctive environment displayed greater tolerance when tested in that environment than Ss receiving equal but unpaired exposure to the drug and environment. This context effect occurred at low (5 mg/kg) and moderate (20 mg/kg) doses and was attenuated by predrug exposure to the tolerance-test environment. This attenuation was found to be due to latent inhibition rather than to stress induction or novelty reduction. In general, results support a Pavlovian model of contextually mediated tolerance and are consistent with other explanations. (39 ref) (PsycINFO Database Record (c) 2006 APA, all rights reserved).