Contribution of associative and nonassociative processes to the development of morphine tolerance

The contribution of associative and nonassociative processes to the development of tolerance to the analgesic effects of morphine in rats was investigated in two experiments. Associative contingencies were manipulated by administering a series of moderately high morphine doses (20 mg/kg) either explicitly paired or explicitly unpaired with a distinctive context. During distinctive context exposures, animals were placed for 60 min in plastic boxes located in a room adjacent to the colony room. The distinctiveness of this environment was enhanced by the presence of white noise and a pine scent. Nonassociative processes were manipulated by administering the morphine at either a very short (6 h) or relatively long (96 h) interdose-interval (IDI). Analgesia was measured on a tail-flick test. At the 96 h IDI, tolerance, as indexed by shifts in dose-response curves, was controlled primarily by associative processes. Associative control over tolerance at the long IDI was evident at an immediate test (experiment 1) and was retained for a 30 day interval (experiment 2). In contrast, tolerance that developed at the 6 h IDI was not influenced by associative contingencies at the immediate test (experiment 1) and showed no retention over a 30 day interval (experiment 2). These data suggest that tolerance that developed at the short IDI was nonassociative. Overall, the results indicate that conditions conductive to the development of non-associative tolerance disrupt the acquisition of associative tolerance. Hypotheses regarding the absence of associative effects at the short IDI are reviewed. Methodological implications of these results for evaluations of associative and nonassociative morphine tolerance are also discussed.