[Analgesic effects of electroacupuncture at different intensities on visceral pain in rats].

Objectives: To observe the analgesic effect of electroacupuncture (EA) at different intensities on visceral pain in rats and explore the supraspinal central mechanism and dose-response relationship.

Methods: Male SD rats were randomly divided into a normal group, a model group and the groups of EA at different intensities (0.5, 1, 2, 3 and 6 mA, 6 rats/group). The visceral pain rat model was established using mustard oil enema. Abdominal rectus electromyography (EMG) was used to observe visceral motor response in the rats. A colorectal distension (CRD) of 60 mm Hg was adopted to induce abdominal rectus muscle discharge for 20 s in each group. Afterwards, EA (0.5, 1, 2, 3, and 6 mA of intensity, 0.5 ms of pulse width, 10 Hz of frequency) was employed at "Zusanli" (ST36) for 1 min. After the end of EA, CRD was performed again for 20 s. Before and after EA, the areas under the curve (AUC) of CRD-induced abdominal rectus EMG were compared. Another set of rats were randomly divided into a model group and the groups of EA at different intensities (0.5, 1, 2, 3 and 6 mA, 8 rats/group). Multi-channel electrophysiological technique was used to record the changes in electric activity of the subnucleus reticularis dorsalis (SRD) neurons in the medulla oblongate of rats, and SRD neural nociceptive electrical activity was observed 1 h after rectal mustard oil infusion. EA at different intensities was administered for 1 min at the 10-minute mark after rectal mustard oil infusion, and the changes in discharge frequency of nociceptive electrical activity of SRD neurons were compared before and after EA at different intensities. The proportion of SRD neurons whose discharge frequency decreased by more than 20% after EA was calculated.

Results: Compared to the normal group, the AUC of abdominal rectus EMG induced by 60 mm Hg CRD was increased in the model group and EA of different intensity groups before intervention (<0.001). After intervention, compared with the model group, AUC was decreased in the high-intensity (2, 3 and 6 mA) EA groups (<0.05, <0.001). When compared with the low-intensity (0.5 and 1 mA) EA groups, AUC was reduced in the high-intensity EA groups (<0.01, <0.001, <0.05). In comparison with the indicators before modeling, the electrical activity of SRD neurons after rectal mustard oil infusion increased and remained excited and relatively stable in the duration from the 5-minute mark to the 30-minute mark in the rats (<0.05);and after high-intensity EA at 11-minute mark, the discharge frequency of SRD neurons was reduced (<0.01, <0.001). Compared with the low-intensity EA, after the high-intensity EA, the reduced difference of discharge frequency increased (<0.05, <0.01, <0.001). The frequency difference, and the proportion of discharge frequency increasing over 20% after EA were elevated with the increase of EA intensity.

Conclusions: High-intensity EA can alleviate visceral pain and inhibit nociceptive electrical activity of SRD neurons in rats and the inhibitory effects are strengthened with the increase of EA intensity within a certain range.