RESUMO
ObjectiveTo explore the possible peripheral analgesic mechanism of electroacupuncture （EA） at promimal and distal acupoints in treatment of myofascial pain syndrome （MPS）. MethodsTwenty-four SD rats were randomly divided into blank group， model group， proximal group， and distal group， with six rats in each group. MPS model was prepared by “strike combined with centrifugal exercise” in all groups except for the blank group. After modeling， the rats in the proximal group received EA at the local myofascial trigger points （MTrPs）， namely the Ashi points， with dilatational waves of frequency of 2/100 HZ and voltage of 2-4 V， current intensity depending on a slight trembling of the left lower limbs， once a day， 15min each time，for 14 days. The rats in the distal group received EA at “Yanglingquan” （GB 34） and “Yinlingquan” （SP 9）， with the same operations as the proximal group. The rats in the blank group and the model group were only grasped and hedged， without other interventions. After intervention， the paw withdrawl mechanical threshold （PWMT） was measured， and variability between the left and right hind paws was calculated. Musculoskeletal ultrasound imaging and electromyography monitoring were performed on the left lower extremity vastus medialis. The morphological changes of vastus medialis muscle of the left lower extremity were observed by HE staining. The positive expression of substance P （SP）， calcitonin gene-related peptide （CGRP）， CD68 and CD206 in muscle tissue was detected by immunohistochemistry. Abdominal aortic serum interleukin-1β （IL-1β）， interleukin-6 （IL-6）， tumor necrosis factor-α （TNF-α）， interleukin-10 （IL-10）， and interleukin-8 （interleukin-8） were detected by ELISA. ResultsCompared to those in the blank group， the fibers of the vastus medial muscle of the rats in the model group were broken and distorted with thickness in variation， and the myofascia was broken， with fibrillation potential， enlarged muscle cells， inward moved nucleus， and widened muscle space； the variability of PWMT between the left and right hind paws significantly increased， as well as the levels of SP， CGRP， CD68， and CD206 in the vastus medialis muscle （P＜0.01）， and the serum IL-8 and TNF-αlevels were significantly elevated （P＜0.05 or P＜0.01）. Compared to those in the model group， the muscle fibers in the proximal and distal group were complete in shape and arranged in an orderly manner， with continued non-broken myofascia， regular shape of muscle cells， and significantly reduced level of IL-8 （P＜0.01）； the amplitude and frequency of spontaneous discharge in the proximal group significantly decreased， as well as the variability of PWMT between the left and right hind paws， and the levels of SP， CGRP， and CD68 in the vastus medialis muscle， while the CD206 level increased significantly （P＜0.05 or P＜0.01 ）； there was complex discharges in the distal group， with significantly decreased level of CD68 in the vastus medialis muscle and increased level of CD206 （P＜0.01）. Compared to the proximal group， the level of IL-8 in the distal group was significantly higher （P＜0.05）. ConclusionsEA at proximal acupoints can significantly improve the pain threshold and local muscle tissue morpho-logy in rats， and its mechanism may be related to reducing the levels of pain-causing substances and related inflammatory factors and promoting the polarization of macrophages. The analgesic effect of EA at distal acupoints is not obvious， and the mechanism is still unclear.