Attenuation of mechanical but not thermal hyperalgesia by electroacupuncture with the involvement of opioids in rat model of chronic inflammatory pain

Opioid peptides have been proven effective in reducing the sign of hyperalgesia associated with inflammation. Electroacupuncture (EA) produces antinociception via release of endogenous opioid peptides in normal rats. Moreover, intrathecal injection of dynorphin has antinociceptive effect in rats. The present study was designed to examine whether EA has effect on the thermal and mechanical hyperalgesia in rat model of complete Freund's adjuvant (CFA)-induced inflammatory pain. The results are the following: (1) single session of 100Hz EA (0.5-1.0-1.5 mA, 10 min for each intensity) at both Zusanli (ST 36) and Sanyinjiao acupoints (SP 6) significantly increased mechanical withdrawal threshold determined by von Frey filaments but not with thermal withdrawal latency that is determined by hot plate (52 +/- 0.2 degrees C); (2) 100 Hz EA applied twice a week for 4 weeks and showed a significant decrease in the mechanical hyperalgesia at the third and fourth week, with no effect on thermal hyperalgesia; (3) naloxone (20 mg kg(-1)) had the ability to reverse the inhibition of the mechanical hyperalgesia produced by a single session of EA. In conclusion, the present results indicate that a single or repetitive EA could reduce mechanical hyperalgesia, but not thermal hyperalgesia, in CFA-inflammatory pain rats, and the opioid system might be involved in these effects.