Previous studies have demonstrated the effects of different afferent fibers on electroacupuncture (EA)-induced analgesia. However, contributions of functional receptors expressed on afferent fibers to the EA analgesia remain unclear. This study investigates the roles of acid-sensing ion channel 3 (ASIC3) and transient receptor potential vanilloid 1 (TRPV1) receptors in EA-induced segmental and systemic analgesia. Effects of EA at acupoint ST36 with different intensities on the C-fiber reflex and mechanical and thermal pain thresholds were measured among the ASIC3-/-, TRPV1-/-, and C57BL/6 mice. Compared with C57BL/6 mice, the ipsilateral inhibition of EA with 0.8 C-fiber threshold (0.8Tc) intensity on C-fiber reflex was markedly reduced in ASIC3-/- mice, whereas the bilateral inhibition of 1.0 and 2.0Tc EA was significantly decreased in TRPV1-/- mice. The segmental increase in pain thresholds induced by 0.3 mA EA was significantly reduced in ASIC3-/- mice, whereas the systemic enhancement of 1.0 mA EA was markedly decreased in TRPV1-/- mice. Thus, segmental analgesia of EA with lower intensity is partially mediated by ASIC3 receptor on Aβ-fiber, whereas systemic analgesia induced by EA with higher intensity is more likely induced by TRPV1 receptor on Aδ- and C-fibers.
Keywords: ASIC3; C-fiber reflex; TRPV1; analgesia; electroacupuncture.