Morphine Taste Conditioning and Analgesia: Assessing Conditioned and Novelty-Induced Analgesia

In previous work showing a taste-elicited decrease in pain sensitivity (J. S. Miller, K. S. Kelly, J. L. Neisewander, D. F. McCoy, & M. T. Bardo, 1990), the rats (Rattus norvegicus) were always habituated to an inactive hot plate after each drug injection. The present report examined whether the analgesic response was a conditioned response to the taste or a response to the novelty of the hot plate resulting from morphine disrupting the habituation process. In 3 experiments, it was found that hot plate novelty was mainly responsible for the analgesic response. For example, increasing the number of conditioning trials did not enhance analgesia in morphine-treated rats. Rather, it attenuated analgesia in saline-treated controls (habituation). Also, rats given habituation in a drug-free state failed to show an analgesic response compared with controls.