Electroacupuncture (EA) Modulates the Expression of NMDA Receptors in Primary Sensory Neurons in Relation to Hyperalgesia in Rats

N-methyl-D-aspartate (NMDA) receptor on the central terminals of the dorsal root ganglion (DRG) appears to be playing an important role in the development of central sensitization related to persistent inflammatory pain. Acupuncture analgesia has been confirmed by numerous clinical observations and experimental studies to be a useful treatment to release different kinds of pains, including inflammatory pain and hyperalgesia. However, the underlying mechanisms of the analgesic effect of acupuncture are not fully understood. In the present study, using a rat model of inflammatory pain induced by complete Freund's adjuvant (CFA), we observed the effect of electroacupuncture (EA) on animal behavior with regard to pain and the expression of a subunit of NMDA receptor (NR1) and isolectin B4 (IB4) in the neurons of the lumbar DRG. Intraplantar injection of 50 microl CFA resulted in considerable changes in thermal hyperalgesia, edema of the hind paw and "foot-bend" score, beginning 5 h post-injection and persisting for a few days, after which a gradual recovery occurred. The changes were attenuated by EA treatment received on the ipsilateral "Huan Tiao" and "Yang Ling Quan" once a day from the first day post-injection of CFA. Using an immunofluorescence double staining, we found that the number of double-labeled cells to the total number of the IB4 and NR1-labeled neurons increased significantly on days 3 and 7 after CFA injection. The change was attenuated by EA treatment. These results suggest that EA affects the progress of experimental inflammatory pain by modulating the expression of NMDA receptors in primary sensory neurons, in particular, IB4-positive small neurons.