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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.
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