Naloxone-induced hypoalgesia: effects of heat, cold and novelty.

Experiment 1 confirmed that pairings of the opioid antagonist naloxone and a heated floor served to induce and then to maintain a conditioned hypoalgesia in rats. Experiments 2a and b demonstrated that this was not specific to some property of the heated floor: pairings of the drug and a cold floor (a) resulted in conditioned hypoalgesia and (b) maintained the hypoalgesia provoked by pairings of the drug with a heated floor. Experiments 3a and 4 showed that pairings of naloxone with the non-heated floor of the apparatus provoked hypoalgesia and provided evidence that this was due to the drug's disruption of a familiarization process. Experiment 3b revealed that pairings of naloxone with the non-heated floor maintained the hypoalgesia induced by pairings of the drug with a heated one. The results were interpreted to mean that naloxone interacts selectively with a stressor so as to maintain or enhance its aversive properties.