A comparative study of the analgesic and respiratory effects of N-allylnorcodeine (nalodeine), nalorphine, codeine and morphine.

In this comparative study, the abdominal constriction test was used to determine analgesia in mice, and the body plethysmograph was used to study respiratory effects of nalodeine, nalorphine, naloxone, codeine, morphine and various agonist-antagonist combinations in rats. The analgesia dose-response curves for the surrogate pairs, nalodeine-nalorphine and codeine-morphine, were parallel but had significantly different slopes. Naloxone was a more potent antagonist of morphine and codeine than of nalorphine and nalodeine. In antagonizing morphine and codeine analgesia, naloxone was the most potent antagonist, nalorphine had a biphasic effect with decreasing activity at higher doses and nalodeine was not an antagonist. Moderate doses of nalorphrine depressed minute volume largely by their effect on tidal volume, but high doses stimulated respiratory rate and therefore had less effect on minute volume. Nalodeine depressed minute volume by depressing tidal volume, since all doses initially stimulated and then variably affected respiratory rate. Metabolic rate was not increased by either drug short of convulsant doses. Nalodeine depresses the ventilatory response to CO2 and weakly antagonizes the respiratory depressant actions of morphine.