In vivo electroporation of proopiomelanocortin induces analgesia in a formalin-injection pain model in rats.

Opioids remain the most efficacious pharmacological agents for various clinical pain syndromes. Recently, various engineered cells capable of secreting opioidergic peptides have been applied to relieve pain in animal models. In vivo gene delivery by viruses encoding endogenous opioids has also been used with success. In this study, we attempted non-viral intrathecal in vivo gene delivery by electroporation to induce analgesia. Thirty Sprague-Dawley rats were used in this study, six in each of five groups. Rats were treated as follows: vehicle without electroporation (group A), vehicle with electroporation (group B), 100 microg of pCMV-hPOMC plasmid without electroporation (group C), or 100 microg of pCMV-hPOMC plasmid with electroporation (group D). Group E was treated with both pCMV-hPOMC plasmid and electroporation, and given naloxone (1mg/kg) 1h before the formalin test. The tail flick, paw withdrawal latency from radiant heat, and formalin test results for each groups were compared. Radioimmunoassay (RIA) and reverse transcription-polymerase chain reaction (RT-PCR) were used to determine the levels of expression of beta-endorphin in the spinal cord. beta-Endorphin expression was localized by immunohistochemistry. A significant decrease in the number of flinches in phase 2 of the formalin test was observed in the group treated with both plasmid and electroporation (group D), whereas the other measures of pain did not differ between groups. RIA and RT-PCR both showed increased expression of beta-endorphin in group D. The expression of beta-endorphin was highest in laminae I and II of the dorsal horn of the spinal cord. We conclude that electroporation successfully delivered intrathecally administered pCMV-hPOMC into the dorsal horn cells of the spinal cord, and induced analgesia in phase 2 of the formalin test in rats.