Objective: To observe the effect of electroacupuncture (EA) and moxibustion-like thermal stimulation with different temperatures at "Zusanli" (ST 36) on pain reactions in transient receptor potential vanilloid type 1 knock-out (TRPV 1 -/-) mice so as to study its mechanisms underlying analgesia.
Methods: Ten C 57 BL/6 mice and 10 TRPV 1 -/- mice were respectively divided into control and TRPV 1 -/- groups. EA (0.3 mA, 1.0 mA, 3.0 mA) or thermal water-bath (38 degrees C, 43 degrees C, 46 degrees C) stimulation was applied to the right ST 36 for 20 min and 10 min, respectively. Mechanical and thermal pain thresholds of the bilateral paws were detected before and immediately after stimulation, and 5 min after EA stimulation at 0.3 mA, 3.0 mA and thermal water-bath stimulation at 38 degrees C, 46 degrees C, separately.
Results: (1) The thermal threshold was significantly higher in TRPV 1 -/- mice than in C 57 BL/6 mice (P < 0.001). (2) In C 57 BL/6 mice, both homolateral and contralateral mechanical and thermal pain thresholds were significantly increased following 1.0 mA, 3.0 mA EA, and 43 degrees C and 46 degrees C thermal stimulation of the right Zusanli (ST 36) (P < 0.05, P < 0.01, P < 0.001); while only homolateral mechanical and thermal pain thresholds were increased after 0.3 mA EA and 38 degrees C thermal stimulation of ST36 (P < 0.05, P < 0.01). In TRPV 1 -/- mice group, both homolateral and contralateral mechanical and thermal pain thresholds were significantly increased by 3.0 mA EA stimulation of ST 36 (P < 0.05, P < 0.01). Bilateral mechanical thresholds and homolateral thermal threshold were significantly increased by 1.0 mA EA stimulation of ST 36 (P < 0.05, P < 0.01), while only homolateral mechanical and thermal pain thresholds were increased by 0.3 mA EA, and 43 degrees C and 46 degrees C thermal stimulation (P < 0.05, P < 0.01). Additionally, only the homolateral mechanical threshold was increased by 38 degrees C thermal stimulation (P < 0.05). (3) The mechanical and thermal pain thresholds of the bilateral limbs were increased to some extent in TRPV 1 -/- mice compared with those of C 57 BL/6 mice following EA and thermal stimulation of the right ST 36. (4) Five minutes after 0.3 mA, 3.0 mA EA and 38 degrees C, 46 degrees C thermal water-bath stimulation, both mechanical and thermal pain thresholds recovered to their baseline levels in both TRPV 1 -/- and C 57 BL/6 mice.
Conclusion: Both EA and thermal stimulation of the right ST 36 can raise mechanical and thermal pain thresholds in TRPV 1 -/- and C 57 BL/6 mice, but needing a stronger stimulation in TRPV 1 -/- mice, suggesting an involvement of TRPV 1 receptors in analgesia.