Electroacupuncture Treatment Alleviated Intracolonic Capsaicin-Induced Rectal Visceral Pain by Inhibiting the Expressions of TRPV1 and P2X4R and Blocking MAPK Pathway

Background: Electroacupuncture (EA) treatment can alleviate chronic neuropathic pain, like visceral pain, but the underlying mechanism remains unclear.Methods: Herein, rats were intracolonically injected with capsaicin to create a visceral (rectal) pain model. Then rats were treated with EA. The number of visceral pain-related behaviors, along with the mechanical pain thresholds of the rats' tail, metapedes and abdomen, were recorded. Meanwhile, the pathology of rectal tissue was observed by hematoxylin-eosin staining. The expressions of inflammatory factors (interleukin (IL)-1 beta, IL-6 and tumor necrosis factor (TNF-alpha)) and glial fibrillary acidic protein (GFAP) in the rat spinal cord or serum were assessed using enzyme-linked immunosorbent assay or Western blot. Likewise, the expression level of oxycocin-42 (OX42), GFAP, transient receptor potential vanilloid 1 (TRPV1), P2X4 receptor (P2X4R) and mitogen-activated protein kinase (MAPK) pathway-related proteins were determined by immunofluorescence or Western blot in the rats' spinal cord.Results: The increased visceral behavioral pain, mechanical pain threshold, secretion of inflammatory factors, edema of rectal mucosa and activation of spinal glial cells induced by capsaicin were all reversed by EA treatment. Additionally, the upregulation of TRPV1 and P2X4R as well as the activation of the MAPK pathway caused by capsaicin was offset by EA treatment.Conclusions: EA treatment ameliorates rats' rectal visceral pain by repressing TRPV1 and P2X4R expression levels as well as blocking MAPK pathway.