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.