Effect of electroacupuncture on spontaneous pain during the synovial inflammatory response stage and allodynia at the later stage in rats with knee osteoarthritis

Abstract

To observe the effect of electroacupuncture (EA) on pain behavior, synovial inflammatory response and demyelination of saphenous nerve in the rats modeled with knee osteoarthritis (KOA) and explore the effect mechanism of EA for reliving allodynia. Eighty-four male SD rats were randomly divided into a control group, a model group and an EA group, 28 rats in each one. Intra-articular injection of sodium monoiodoacetate (MIA) was administered in right knee joint of each rat in the model group and the EA group to establish the KOA model. In the EA group, separately, on day 5, 7 and 9 after modeling, EA was applied at "Zusanli" (ST 36) and "Yanglingquan" (GB 34) on the right side, with disperse-dense wave (2 Hz/15 Hz), 1 mA in current intensity, for 30 min in one intervention, once a day, and 3 interventions were required. On the 9th day after modeling, the weight-bearing rate was calculated for the affected limbs of the rats in each group, the synovial morphological changes were observed using HE and Masson staining, flow cytometry was adopted to detect the synovial immunocyte counts, and MSD multi-spot assay was used to detected the synovial inflammatory cytokine content. On the 14th day after modeling, the hind-paw mechanical withdrawal threshold was observed in each group and the ultrastructure of the saphenous nerve was observed under transmission electron microscopy. On the 9th day after modeling, compared with the control group, the weight-bearing rate of the affected limb was reduced (P<0.01), the synovial hyperplasia, inflammatory cell infiltration and synovial fibrosis occurred in the affected limb; the counts of synovial CD11b+ cells and M1 macrophages (CD11b+CD86+) were increased (P<0.01), the contents of tumor necrosis factor-α (TNF-α), interleukin (IL)-6, IL-1β, IL-10 and IL-13 in the synovial tissue were elevated (P<0.01, P<0.05) for the rats of the model group. Compared with the model group, the weight-bearing rate of the affected limb was increased (P<0.05), the synovial hyperplasia, inflammatory cell infiltration and synovial fibrosis were mitigated, the counts of CD11b+ cells and M1 macrophages (CD11b+CD86+) in the synovial tissue, and the contents of TNF-α and IL-6 were reduced (P<0.01, P<0.05) in the EA group. On the 14th day after modeling, the hind-paw mechanical withdrawal threshold was reduced in the model group when compared with the control group (P<0.01), and it was increased in the EA group when compared with the model group (P<0.05). Besides, in the model group, obviously, the myelin sheath structure was destroyed, the myelin layer was disintegrated and loosened, the axon was extruded or the layer thicken and cracked. Compared with the model group, the injury of saphenous nerve was alleviated remarkably in the EA group. The intervention with EA may attenuate the synovial inflammatory response and the injury of saphenous nerve in the affected limb of the rat with KOA, so that the spontaneous pain during the synovial inflammatory response stage and allodynia at the later stage are relieved.