Protein kinase Cε is required for spinal analgesic synergy between delta opioid and alpha-2A adrenergic receptor agonist pairs

We recently showed that spinal synergistic interactions between δ opioid receptors (δORs) and α_2A adrenergic receptors (α_2AARs) require protein kinase C (PKC). To identify which PKC isoforms contribute to analgesic synergy, we evaluated the effects of various PKC-isoform-specific peptide inhibitors on synergy between δORs and α_2AARs using the tail flick assay of thermal nociception in mice. Only a PKCε inhibitor abolished synergy between a δOR agonist and an α_2AAR agonist. We tested a panel of combinations of opioid and adrenergic agonists in PKCε knock-out mice and found that all four combinations of a SOR agonist and an α_2AAR agonist required PKCε for antinociceptive synergy. None of the combinations of a μOR agonist with an α₂AR agonist required PKCε. Immunohistochemistry confirmed that PKCε could be found in the population of peptidergic primary afferent nociceptors where δORs and α_2AARs have been found to extensively colocalize. Immunoreactivity for PKCε was found in the majority of dorsal root ganglion neurons and intensely labeled laminae I and II of the spinal cord dorsal horn. PKCε is widespread in the spinal nociceptive system and in peptidergic primary afferents it appears to be specifically involved in mediating the synergistic interaction between δORs and α_2AARs.