Role of spinal Toll-like receptor 4 in glial cell activation mediated persistent postoperative pain in a rat model of persistent postoperative pain evoked by skin/muscle incision and retraction

Objective To investigate the role of Toll-like receptor 4 (TLR4) in glial cell activation mediated persistent postoperative pain in a rat model of persistent postoperative pain evoked by skin/muscle incision and retraction (SMIR).Methods One hundred twenty male SD rats weighting 200 g-250 g,were randomly divided into 5 groups (n=24):sham operation group(group S),persistent postoperative pain group (group SMIR),group SMIR+intrathecal fiuorocitrate (group F),group SMIR+intrathecal minocycline (group M) and group SMIR+intrathecal minocycline plus fluorocitrate (group F+M).The rat model of persistent postoperative pain evoked by SMIR was made according to the method described by Flatters.Pain behavior was assessed by paw mechanical withdrawal threshold (MWT) to yon Frey filament stimulation at 1 d before and 3,7,14,21,32 d after operation.The expression of TLR4 protein in the spinal cord was assessed by western blot analysis.Results Compared with their preoperative values and corresponding values in group S,the MWT values in SMIR were significantly decreased[the MWT values were (7.3±0.7),(6.2±1.0),(4.5±1.2),(11.4±1.1) g] (P＜0.01),and the expression of TLR4 protein in the spinal cord were significantly increased in group SMIR at 3,7,14 d and 21 d after operation [the expression of TLR4 protein were (1.647±0.042),(1.804±0.058),(2.023±0.161),(1.365±0.130)](P＜0.05 or P＜0.01).Compared with corresponding values in group SMIR,the MWT values in group F,group M and group F+M were significantly increased at 3,7,14 d and 21 d after operation [the MWT values of postoperative 14 d were (7.9±0.7),(8.4±0.7),(11.9±1.0) g](P＜0.05 or P＜0.01),the expression of TLR4 protein in spinal cord in group F at 7,14 d after operation,and group M and group F+M at 3,7,14 d after operation significantly decreased [the expression of TLR4 protein of postoperative 14 d in group F,group M,group F+M were (1.437±0.092),(1.432±0.129),(1.047±0.026),respectively](P＜0.05 or P＜0.01).Compared with corresponding values in group F and group M,the MWT values in group F+M were significantly increased at 3,7 d and 14 d [the MWT values were (11.3±0.8),(10.9±0.6),(11.9±1.0) g](P＜0.05) (P＜0.05),and the expression of TLR4 protein in spinal cord decreased significantly at 14 d after operation (the expression of TLR4 protein was 1.047±0.026)(P＜0.05),the MWT values and the expression of TLR4 protein in spinal cord had no significant differences between group F and group M.Conclusions The activation of TLR4 in spinal cord may be related to the glial cell activation mediated persistent postoperative pain in a rat model of persistent postoperative pain evoked by SMIR. Key words: Toll-like receptor 4;  Postoperative pain;  Astrocyte;  Microglia;  Spinal cord