Antinociceptive effects of heterotopic electroacupuncture in formalin-induced pain

This study examined the antinociceptive effect of electroacupuncture (EA) to heterotopic acupoints on formalin-induced pain in rats. EA (2 ms, 10 Hz, and 3 mA) was delivered to heterotopic acupoints HE(7) and PE(7), or non-acupoints at the right fore limb, for 30 min and was immediately followed by subcutaneous formalin injection into the left hind paw, respectively. The quantified pain score, electromyogram (EMG) response of the C-fiber reflex, and cFos immunoreactivity were assessed, respectively. EA to heterotopic acupoints significantly reduced both early- and late-phase pain-like behaviors and significantly decreased the EMG responses of the C-fiber reflex after formalin injection. By contrast, EA to non-acupoints had no significant effects on pain-like behavior or the EMG response. In addition, EA to heterotopic acupoints decreased cFos immunoreactivity in the lumbar spinal dorsal horn. Therefore, EA induced pre-emptive antinociception via the extra-segmental inhibition of the formalin-induced pain, suggesting that EA to heterotopic acupoints is a useful treatment for inflammatory pain.