Analgesic effect of electroacupuncture on inflammatory pain in collagen-induced arthritis rats: mediation by alpha2- and beta-adrenoceptors.

This study was designed to investigate whether alpha- and beta-adrenergic systems mediate the analgesic effect of electroacupucture (EA) on inflammatory pain in a rat model of collagen-induced arthritis (CIA). To induce CIA, male Sprague-Dawley rats were immunized with bovine type II collagen emulsified in Freund's incomplete adjuvant, followed by a booster injection 14 days later. After induction of arthritis, the inflammatory pain threshold by the tail flick latency decreased as time passed and reached the minimum value at 4th week. Four weeks after the first immunization, low-frequency EA stimulation (2 Hz, 0.07 mA, 0.3 ms) delivered to Zusanli (ST(36)) for 30 min showed the analgesic effect. And also, the analgesic effect of EA was blocked by pretreatment with yohimbine (alpha2-adrenoceptor antagonist, 2 mg/kg, i.p.), but not by pretreatment with prazosin (alpha1-adrenoceptor antagonist, 1 mg/kg, i.p.) and propranolol (non-selective beta-adrenoceptor antagonist, 10 mg/kg, i.p.). These results suggest that the low-frequency EA can relieve the inflammatory pain in CIA, and the analgesic effect of EA can be mediated by alpha2- and beta-adrenoceptor, but not by alpha1-adrenoceptor.