The role of predrug signals in morphine analgesic tolerance: Support for a Pavlovian conditioning model of tolerance

According to a model of morphine tolerance, which emphasizes Pavlovian conditioning principles, tolerance results from an association between predrug environmental cues and the systemic effects of the drug. To assess this model, groups of rats were administered morphine on either three or nine occasions, with a complex environmental stimulus either paired or not paired with each injection. Control groups had equivalent experience with the …