Dependence in the absence of tolerance to morphine.

Intravenous morphine depressed the firing of cat dorsal horn neurones to impulses in unmyelinated primary afferents. Electrophoretic administration of naloxone in the substantia gelatinosa between 10 min and 6 h after morphine revealed hyper-responsiveness which was not associated with apparent tolerance to morphine. The observations suggest the changes underlying dependence occur very rapidly in the dorsal horn probably at a site different from that at which morphine acts.