Therapeutic effects of electroacupuncture at different sites on rats with neuropathic pain via the spinal Nrf2/HO-1 pathway

Objective

This study aimed to assess the effects of electroacupuncture (EA) at the contralateral, ipsilateral, or bilateral “Zusanli (ST36)” and “Yanglingquan(GB34)” on neuropathic pain caused by chronic contractile injury (CCI) and to explore the role of the nuclear factor erythroid 2-related factor 2 (Nrf2) pathway in the effects of EA.

Methods

Male Sprague-Dawley rats were subjected to the CCI model to induce neuropathic pain. A total of 45 rats were randomly divided into five groups (n = 9): sham, CCI, EA-Co (CCI + EA at contralateral acupoints), EA-Ip (CCI + EA at ipsilateral acupoints), and EA-Bi (CCI + EA at bilateral acupoints). The rats received EA treatment on day 8 after CCI, once every alternate day, for a total of eight times. The time courses of mechanical pain threshold (MWT), hind paw withdrawal latency (HWL), and sciatic functional index (SFI) were determined. The expression levels of 8-hydroxydeoxyguanosine (8-OHdG), glutathione (GSH), superoxide dismutase (SOD) activity, interleukin-6 (IL-6), interleukin-1 beta (IL-1β), and tumor necrosis factor factor-alpha (TNF-α) in the spinal cord were measured. The distribution of Nrf2, its expression of Nrf2 in both the cytosol and nucleus, and the protein levels of its downstream target genes, NQO1 and HO-1, were detected via double immunofluorescence staining and western blotting, respectively.

Results

Following CCI, both MWT and HWL in the CCI group significantly decreased from day 14 after surgery (P < 0.001). EA treatment exhibited significant antinociceptive effects induced by CCI by increasing the MWT and HWL values, especially bilateral EA (P < 0.05). The SFI of the CCI group was significantly lower than that of the sham group (P < 0.001). Only bilateral EA improved the SFI scores compared to the CCI group (P < 0.05). 8-OHdG levels in the spinal cord of the CCI group were significantly higher than those in the sham group (P < 0.05), whereas GSH levels and SOD activity in the spinal cord of the CCI group were significantly lower than those in the sham group (P < 0.001 and P < 0.01, respectively). Bilateral EA administration significantly downregulated 8-OHdG levels (P < 0.01) and upregulated GSH levels and SOD activity in the spinal cord (P < 0.01). CCI significantly enhanced the production of IL-1β, IL-6, and TNF-α in the spinal cord compared with that in the sham group (all P < 0.001). Meanwhile, the effects of EA were also accompanied by markedly decreased expression of IL-1β and IL-6 in the spinal cord (P < 0.05). TNF-α levels were only decreased in the EA-Ip and EA-Bi groups compared with those in the CCI group (P < 0.001). Confocal microscopy revealed that Nrf2 was mainly localized in the neurons of the spinal cord. Notably, EA treatment enhanced nuclear translocation of Nrf2 in neurons. CCI significantly decreased the production of Nrf2, HO-1, and NQO1 in the spinal cord compared to the sham group (P < 0.001), and bilateral EA up-regulated the protein levels of Nrf2 and its target genes HO-1 and NQO1(all P < 0.001).

Conclusion

Our results suggest that bilateral EA is an optimal therapeutic strategy for neuropathic pain. The effects of EA on neuropathic pain may be mediated by the restoration of the Nrf2 pathway in the spinal cord.