Local intramuscular injection of a plasmid encoding human proenkepahlin attenuates incision pain in rats

We investigated the antinociceptive effect of local intramuscular injection of a plasmid encoding human proenkephalin (pVAX1-hPPE) on postoperative pain in rats. Male Sprague-Dawley rats with incision-induced pain were intramuscularly injected into injured plantaris muscle with empty vector (pVAX1) or pVAX1-hPPE, respectively. Paw mechanical threshold and thermal latency in the 200 μg pVAX1-hPPE treated rats were significantly higher at 6 h and on 1 day, and lasted until day 7 after intramuscular administration, respectively. The analgesic effects were reversed by methylnaltrexone, suggesting that the antinociceptive effect of pVAX1-hPPE was mediated through peripheral opioid receptor pathway. In contrast, incisional or pVAX1-treated rats did not significantly affect pain thresholds. These results demonstrated that single intramuscular injection of pVAX1-hPPE attenuated incision-induced pain in rats, and it is worthy of further study as a potential gene therapy for postoperative pain.