Electroacupuncture effects in a rat model of complete Freund's adjuvant-induced inflammatory pain: antinociceptive effects enhanced and tolerance development accelerated.

We have previously shown that electroacupuncture (EA) produced antinociception through the release of endogenous opioid peptides to activate opioid receptors during acute nociception. EA produced tolerance after its prolonged application. It has reported that 100 Hz EA could reduce mechanical hyperalgesia in complete Freund's adjuvant (CFA)-induced inflammatory nociception rats. The present study aims to investigate the antinociceptive effect of EA and the development of EA tolerance in chronic inflammatory nociception rats with CFA injection into the hind paw plantar. The results showed that the antinociceptive effect of 100 Hz EA was significantly enhanced in CFA-induced inflammatory nociception rats. Naloxone at 20 mg/kg could significantly block this antinociceptive effect. Chronic tolerance to EA was developed faster in CFA-induced inflammatory nociception rats than in normal rats. Therefore, 100 Hz EA could enhance antinociceptive effects and accelerate tolerance development in CFA-induced inflammatory nociception rats. The enhancement of EA antinociceptive effect in CFA-induced inflammatory nociception rats might involve the endogenous opioid peptides such as dynorphin.