Evidence that the opiate receptors of the substantia gelatinosa contribute to the depression, by intravenous morphine, of the spinal transmission of impulses in unmyelinated primary afferents.

In barbiturate anaesthetized cats, dorsal horn neurones were excited by electrical stimulation of the common tibial nerve. The numbers of action potentials evoked by excitation of C fibres were depressed by analgesic doses of morphine (1-4 mg/kg i.v.). Naloxone, administered electrophoretically in the substantia gelatinosa, fully reversed the inhibitory action of morphine. The results suggest that opiate receptors in the substantia gelatinosa play a role in morphine analgesia.