[Signal transduction of cloned opioid receptors]

Complementary DNAs encoding delta, mu and kappa-opioid receptors have now been cloned and characterized. These receptors, which are members of the superfamily of seven transmembrane spanning receptors, share a high degree of amino acid sequence similarity among these receptors. From the similarity of the sequence, it is speculated that both the 1st and 2nd extracellular loop and the 4th membrane spanning domain are supposed to be involved in the opioid binding and subtype specificity. Because of the high similarity of the cytoplasmic regions' amino acid sequence, however, it seems that the signal transductions of delta, mu and kappa are very similar. In Xenopus oocytes expressing delta-opioid receptors and various kinds of GTP-binding protein alpha-subunits, the delta-agonist DSLET caused currents through Gi1 alpha (or Gi2 alpha)-phospholipase C mechanisms Neither G(o) alpha, Gq alpha, G11 alpha nor G14 alpha was involved in such delta-receptor-mediated responses. The higher concentration of DSLET (3-10 microM) showed a rapid desensitization upon repeated challenges. Such a rapid desensitization was purely homologous, and this was rescued by the pretreatment with protein kinase C inhibitor. Similar findings were also observed with mu and kappa-opioid receptors. These results suggest that the phosphorylation by protein kinase C is involved in the acute tolerance.