Antagonism by naloxone of morphine-induced single-dose dependence and antinociception in mice

Single-dose physical dependence on morphine, as indicated by the mouse withdrawal jumping test, was reduced in a dose-related fashion by co-administration of naloxone. In the same dose range, naloxone also antagonized the antinociceptive effect of morphine in the mouse writhing test. Dose-response data revealed essentially the same ED50's for naloxone in both tests. These results indicate that naloxone does indeed block the development of morphine-induced single-dose physical dependence in mice and that it does so as effectively as it blocks morphine-induced inhibition of writhing.