Nitrous Oxide Activates the Supraspinal Pain Inhibition System

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Abstract
The neurophysiological mechanism of nitrous oxide‐induced analgesia was studied in decerebrate non‐anesthetized cats. Intra‐arterial injection of bradykinin was used as the test noxious stimulus and the neural response in the lateral funiculus of the spinal cord was measured by the multi‐unit recording technique. The effects of nitrous oxide on the neural activity of spinal cord were compared before and after the spinal cord transection. Nitrous oxide activated the spontaneous firing level, but depressed the bradykinin‐induced response. These effects were more significant before the cord transection than after it. These findings indicate that nitrous oxide exerts its analgesic action through both the direct intraspinal anti‐nociceptive action and an indirect action by activating the supraspinal descending inhibition system.