Differential excitatory and inhibitory effects of opiates on non-nociceptive and nociceptive neurones in the spinal cord of the cat.

Morphine, levorphanol, dextrorphan and naloxone were applied microelectrophoretically to cells identified as either having nociceptive inputs or non-nociceptive inputs in the dorsal horn of the cat. Morphine excited non-nociceptive cells and depressed nociceptive cells. Naloxone reversed morphine excitations on non-nociceptive cells, but only reversed about one-third of morphine depressions on nociceptive cells. Levorphanol depressed nociceptive cells, whilst dextrophan ejected with similar currents caused less depression or had no effect. It is concluded that excitation of non-nociceptive cells may constitute a spinal action relevant to the analgesic action of opiates, acting synergistically with a depressant effect on nociceptive neurones.