Comparison of the discriminative stimulus properties of nefopam and morphine

Separate groups of rats were trained to discriminate nefopam or morphine from saline in a two-lever operant task. Nefopam produced saline-appropriate responding in morphine-trained rats. The highest dose of morphine tested in nefopam-trained rats produced responding appropriate for neither training treatment. This intermediate morphine responding was antagonized by a naloxone treatment that had no effect on the nefopam discrimination. These results suggest that the stimulus properties of nefopam and morphine are qualitatively different but also that morphine produces effects related to those that result in stimulus control by nefopam.