Contralateral electroacupuncture pretreatment suppresses carrageenan-induced inflammatory pain via the opioid-mu receptor

Acupuncture has been used to treat various clinical diseases in Eastern medicine. To investigate the analgesic effect of electroacupuncture (EA) pretreatment on carrageenan-induced inflammatory pain, we studied on the effect of EA parameters on an animal model of acute arthritic pain. Pretreatment with 1 mA, 10 Hz EA prior to carrageenan injection under halothane anesthesia suppressed carrageenan-induced pain. Interestingly, EA stimulation of the 'Zu-San-Li' (ST36) acupuncture point (1 mA, 10 Hz) contralateral to the site of the carrageenan injection in the rat synovial cavity produced significantly greater improvement of the weight-bearing force compared with EA stimulation of the 'San-Yin-Jiao' acupuncture point. To determine how ST36 EA treatment suppresses carrageenan-induced inflammatory pain, we examined the effect of a mu opioid receptor antagonist on ST36 EA-induced analgesia. The selective antagonist of the mu opioid receptor (OR) significantly suppressed contralateral ST36 EA-induced analgesia against carrageenan-induced inflammation. These results suggested that the analgesic effect mediated by the mu OR during low-frequency contralateral EA pretreatment has an anti-nociceptive action against inflammatory pain and that it may provide a potential strategy to treat inflammatory arthritic pain.