Role of spinal Toll-like receptor 4 signaling pathway in development of inflammatory pain in rats

Objective To evaluate the role of spinal Toll-like receptor 4(TLR4)signaling pathway in the development of inflammatory pain in rats. Methods Thirty-six adult male Sprague-Dawley rats were divided into 3 groups(n=12 each)using a random number table: control group, inflammatory pain group and TLR4 signaling pathway inhibitor epigallocatechin gallate(EGCG)group(EGCG group). Inflammatory pain was induced by injecting 50 μl of complete Freund′s adjuvant(CFA)into the ankle joint cavity of the left hindpaw of rats anesthetized with isoflurane.At 1-3 days after injection of CFA, EGCG 30 μg was intrathecally injected once a day in group EGCG.At 1, 3(30 min after intrathecal injection), 5 and 7 days after injection of CFA, the mechanical paw withdrawal threshold(MWT)and thermal paw withdrawal latency(TWL)were measured.The ipsilateral L4, 5 segments of the spinal cord were removed at 3 days after CFA injection for determination of TLR4 expression(by Western blot)and contents of tumor necrosis factor-alpha(TNF-α), interleukin-6(IL-6)and IL-1β in the spinal dorsal horn(by enzyme-linked immunosorbent assay). Results Compared with control group, the MWT was significantly decreased and the TWL was shortened at each time point after injection of CFA, the expression of TLR4 in the spinal dorsal horn was up-regulated, and contents of TNF-α, IL-6 and IL-1β in the spinal dorsal horn were increased in inflammatory pain group(P 0.05). Compared with inflammatory pain group, the MWT was significantly increased and the TWL was prolonged at each time point after injection of CFA, the expression of TLR4 in the spinal dorsal horn was down-regulated, and contents of TNF-α, IL-6 and IL-1β in the spinal dorsal horn were decreased in EGCG group(P<0.05). Conclusion Spinal TLR4 signaling pathway is involved in the development of inflammatory pain in rats. Key words: Toll-like receptor 4; Spinal cord; Inflammation; Pain