Acupuncture analgesia in a new rat model of ankle sprain pain.

The lack of suitable experimental animal models for persistent pain showing clear acupuncture analgesia, has been the major stumbling block in the investigation of the physiological mechanisms of acupuncture analgesia. The present study developed a new rat model of ankle sprain pain and the effect of electroacupuncture (EA) on this model was examined. A common source of persistent pain in humans is the lateral ankle sprain. To model this condition, the rat's right ankle was bent repeatedly, overextending lateral ligaments, for 4 min under halothane anesthesia. The rat subsequently showed swelling of the ankle and a reduced stepping force of the affected limb for the next several days. The reduced stepping force of the limb was presumably due to a painful ankle since systemic injection of morphine produced temporary improvement of weight bearing. EA was applied to the SI-6 acupuncture point on the contralateral forelimb for 30 min under halothane anesthesia. After the termination of EA, behavioral tests measuring stepping force were periodically conducted during the next 4h. EA produced a 40% recovery in the stepping force of the sprained foot lasting for at least 2h. The magnitude of this improvement was equivalent to that obtained after a systemic injection of 2mg/kg of morphine and this improvement of stepping pressure was interpreted as an analgesic effect. The analgesic effect was specific to the acupuncture point since (1). the analgesic effect on the ankle sprain pain model could not be mimicked by EA applied to a nearby point, LI-4 and (2). EA applied to the SI-6 point was not effective in the knee arthritis pain model. The analgesic effect could not be blocked by systemic injection of opioid antagonists naloxone or naltrexone. These data suggest that EA produces a potent analgesic effect on the ankle sprain pain model in the rat. This analgesic effect is produced by applying EA to a site remote from the painful area in a stimulus point-specific way. The present study provides a powerful experimental animal model that can be used for investigating the unique physiological mechanisms involved in acupuncture analgesia.