Abstract: Objective To investigate the effect of esketamine (S-KET) on pain in knee osteoarthritis (KOA) rats by modulating the TLR4/TNF-α/MMP-9 signaling pathway. Methods A KOA rat model was established by intra-articular injection of papain. Successfully modeled rats were assigned into KOA group, L-S-KET, M-S-KET, H-S- KET groups (intraperitoneal injection of 10, 20, and 40 mg/kg S-KET), and H-S-KET+LPS group (intraperitoneal injection of 40 mg/kg S-KET+intra-articular injection of 0.25 mg/kg TLR4 activator LPS) according to the random number table method, with 10 rats in each group. Another 10 normal rats served as control group. The control group and KOA group were given equal amounts of physiological saline once a day for 28 consecutive days. The thermal radiation method and pain threshold measurement instrument were used to detect the pain threshold of rats. HE staining was used to observe pathological changes in rat cartilage tissue, and the pathological scoring was performed. TUNEL staining was used to observe the apoptosis of chondrocytes in cartilage tissue. ELISA was performed to detect IL-6, TNF-α, and IL-1β in serum. Western blot was performed to detect changes in the expression of TLR4/TNF-α/MMP-9 signaling pathway proteins in joint tissues. Results Compared with the control group, the cartilage tissue of rats in KOA group lost normal structure and the Mankin score increased (P＜0.05). Compared with KOA group, the L-S-KET group, M-S-KET group, and H-S-KET group showed reduced cartilage damage, relatively uniform cell arrangement, and decreased Mankin score(P＜0.05). Compared with the H-S-KET group, the H-S-KET+LPS group showed cracks in the cartilage tissue of rats, uneven cell arrangement, and an increase in Mankin score (P＜0.05). Compared with the control group, the KOA group had lower thermal pain threshold and mechanical pain threshold, higher chondrocyte apoptosis rate, IL-6, TNF-α, IL-1β levels, and TLR4, TNF-α, MMP-9 protein expression (P＜0.05). Compared with the KOA group, the L-S-KET group, M-S-KET group, and H-S-KET group had higher thermal pain threshold and mechanical pain threshold, lower chondrocyte apoptosis rate, IL-6, TNF-α, IL-1β levels, and TLR4, TNF-α, MMP-9 protein expression(P＜0.05). Compared with the H-S-KET group, the H-S-KET+LPS group had lower thermal pain threshold and mechanical pain threshold, higher chondrocyte apoptosis rate, IL-6, TNF-α, IL-1β levels, and TLR4, TNF-α, MMP-9 protein expression(P＜0.05). Conclusions S-KET can reduce inflammatory factors and alleviate KOA pain in rats. This may be achieved by modulating the TLR4/TNF-α/MMP-9 signaling pathway.

Key words: esketamine(S-KET), toll-like receptor 4(TLR4), tumor necrosis factor-α(TNF-α), matrix metalloproteinase-9(MMP-9), knee osteoarthritis(KOA)