Kelatorphan potentiates the effect of [Met5]enkephalin in the substantia gelatinosa of the cat spinal cord.

In anaesthetized spinal cats, kelatorphan, an inhibitor of enkephalin degradation, was administered microelectrophoretically while recording the excitation of lumbar dorsal horn neurones by noxious and innocuous peripheral stimuli. When administered near the cell bodies of laminae IV and V neurons, kelatorphan neither altered evoked responses nor potentiated the inhibition by [Met5]enkephalin of these cells. When ejected in the substantia gelatinosa, however, kelatorphan reduced the nociceptive responses of some laminae IV and V neurones, an effect blocked by electrophoretic naloxone. The selective inhibition of nociceptive responses by [Met5]enkephalin administered in the substantia gelatinosa was markedly potentiated by co-administration of kelatorphan, and this effect was also blocked by electrophoretic naloxone. Neurones inhibited by administration of kelatorphan alone in the substantia gelatinosa were excited by administration of naloxone alone at the same site. The results suggest that some dorsal horn neurones are tonically inhibited by an action of opioid peptides in the substantia gelatinosa, and indicate that enzymic degradation limits the action of both exogenous and endogenous enkephalin in this spinal region.