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Context-specific morphine tolerance on the paw-pressure and tail-shock vocalization tests: evidence of associative
A Cepeda-Benito1, S T Tiffany, L S Cox
1Texas A&M University, Texas, USA.
Rationale:
Demonstrations of associative tolerance to the analgesic effects of morphine, not confounded by practice or novelty effects, have been restricted to the tail-flick and flinch-jump tests.
Objectives:
Experiment 1 investigated whether associative tolerance would be found on two other nociceptive assessment methods: the paw-pressure withdrawal and tail-shock vocalization thresholds. Experiment 2 tested the hypothesis that conditioned compensatory behavioral responses are the substrate of associative morphine tolerance in the paw-pressure, tail-shock, and tail-flick tests.
Methods:
Rats were given eight morphine injections (20 mg/kg, i.p.) explicitly paired or unpaired with a distinctive context. Control animals were given saline injections over the course of conditioning. Animals were then tested after morphine (experiment 1) or placebo injections (experiment 2) in the context.
Results:
There was evidence of context-specific tolerance across both testing methods, with a rightward shift of dose-response curves of paired relative to unpaired animals. No evidence of conditioned compensatory responding was found on any of the three testing methods.
Conclusions:
The data indicated that, although Pavlovian processes can play a major role in tolerance acquisition, there was little support for the thesis that the conditioned tolerance response is a behavioral effect that is opposite in direction to the direct effects of the drug.