Evidence that endogenous enkephalins produce delta-opiate receptor mediated neuronal inhibitions in rat dorsal horn.

Kelatorphan, a full inhibitor of aminopeptidases, enkephalinase and dipeptidylaminopeptidase, enzymes which degrade the enkephalins, produced inhibitions (around 50%) of dorsal horn C fibre evoked responses in the rat, following local application. The inhibitions were reversed by the selective delta-opiate receptor antagonists ICI 174,864. Furthermore the inhibitions produced by two doses of Tyr-D-Ala-Gly-MePhe-Gly-ol (DAGO), a potent selective mu-opiate receptor agonist, were not altered in the presence of kelatorphan, so demonstrating an additive effect of the mu-agonist and the enzyme inhibitor. The inhibitions produced by the enzyme inhibitor would seem due to an increased availability of endogenous opioids, presumably the enkephalins which act via the delta-opiate receptor.