Inhibition of electroacupuncture on nociceptive responses of dorsal horn neurons evoked by noxious colorectal distention in an intensity-dependent manner

Background: The transmission of visceral nociception can be inhibited by electroacupuncture (EA) at the spinal level. However, relationships between current intensity and EA-induced analgesia are still lacking. This study compares the effects of different intensities of EA at local acupoints and heterotopic acupoints on nociceptive responses of spinal wide dynamic range (WDR) neurons induced by noxious colorectal distension (CRD). Materials and methods: Experiments were conducted on 40 Sprague Dawley rats anesthetized with 10% urethane. Discharges of WDR neurons in the L1-L3 segments of the dorsal horn of the spinal cord were recorded extracellularly by glass micropipettes. Different intensities of EA (0.5, 1, 2, 4, 6, and 8 mA, 0.5 ms, 2 Hz) were applied to contralateral "Zusanli" (ST 36) or "Neiguan" (PC 6), with either the same or different segmental innervation of the colon. Results: In local acupoints, the increased discharges of WDR neurons evoked by CRD were significantly inhibited by EA at 0.5-8 mA. A positive relationship between current intensity and the inhibiting rate was observed within 0.5-4 mA, but the inhibiting rate reached a plateau when EA exceeded 4 mA. In heterotopic acupoints, the increased discharges of WDR neurons evoked by CRD were significantly inhibited by EA at 2-8 mA. A positive relationship between current intensity and the inhibiting rate was observed within 2-6 mA. Further increase in the current beyond 6 mA also resulted in a plateau effect. Conclusion: Within a certain range, the nociceptive responses of dorsal horn neurons induced by CRD could be inhibited by EA in an intensity-dependent manner.