Narcotic cuing and analgesic activity of narcotic analgesics: associative and dissociative characteristics.

By using a discrete-trial, two-lever, food-reinforced discrimination learning paradigm, rats were trained to discriminate the narcotic analgesic fentanyl (0.05 mg/kg) from saline. Stimulus generalization experiments with lower fentanyl doses (0.0025 to 0.02 mg/kg) were carried out to generate individual threshold doses. The latter were compared with the sensitivity of the same rats to the analgesic effect of fentanyl, and it was found that there is no correlation between these two sets of data. In a time-effect experiment, the duration of fentanyl's cuing effect was compared with that of its analgesic effect, and it was found that the time-effect characteristics of the narcotic cue are similar to those of analgesia. Again, however, there was no correlation between the duration of both effects within the same group of animals. The results further deliniate the associative and dissociative characteristics of the narcotic cue and narcotic analgesia.