Postoperative cognitive dysfunction (POCD) is a common complication in elderly patients after surgical operations, mainly characterized by loss of attention, damaged memory, personality changes, and the mental disorder for several weeks or more.1 POCD is associated with poorer recovery, increased utilization of social financial assistance, and higher morbidity with the hazard ratio of 1.63.2,3 Thus, it is urgently needed to explore the mechanisms and interventions for preventing or reducing the occurrence of POCD.
It has been suggested that apoptotic neurodegeneration in brain might be a potential pathway mediating the development of POCD since anesthesia could induce neuronal cell death in the developing rat brain via the intrinsic and extrinsic apoptotic pathways.4 The changes caused by surgical procedures and anesthesia induces the increased neuroinflammation (such as interleukin‐1β [IL‐1β], IL‐6, and microgliosis), decreased brain‐derived neurotrophic factor levels, and reduced neurogenesis, which might be the underlying mechanisms for POCD.5 Meanwhile, it has been found that surgery could increase pro‐inflammatory cytokines (IL‐1β and IL‐6) and then inhibit GluR1 trafficking to cause learning and memory impairment, also indicating that neuroinflammation plays a pivotal role in the pathogenesis of POCD.6 Thus, inhibiting the release of inflammatory cytokines in brain may be a promising prevention and therapeutic strategy for POCD.
Dexmedetomidine (DEX) is a highly selective α2 adrenergic receptor agonist with effects of sedation, analgesic, anti‐anxiety, inhibition of the sympathetic activity, and stabilizing the hemodynamics.7 DEX is widely applied in clinical practice to keep hemodynamic stability and reduce intraoperative anesthetic requirements.8 Xiong et al. have suggested that DEX may improve cognitive functioning in aged rats by inhibiting neural overexcitability through restraining relaxin‐3 and c‐fos expression.9 According to some retrospective studies, DEX can ameliorate the postoperative cognitive functions and effectively decrease the incidence rate of POCD of aged patients who received the surgery under general anesthesia.10,11 However, it also reported that intraoperative infusion of DEX did not reduce the incidence of delirium or affect postoperative cognition in patients older than 70 years undergoing major elective noncardiac surgery.12 Therefore, more studies are needed to examine the effects of DEX on POCD and explore the involved molecular mechanisms, so as to offer theoretical basis for clinical practice.
In our study, we evaluate the effects pretreatment of DEX on lipopolysaccharide (LPS)‐induced microglia activation and the postoperative cognition in aged mice. Meanwhile, the pro‐inflammatory cytokines including tumor necrosis factor‐alpha (TNF‐α), interleukin (IL)‐1β, and IL‐6 were detected to monitor the neuroinflammation. Moreover, Toll‐like receptor 4 (TLR4) and nuclear factor kappa B (NF‐κB) were measured to elucidate its possible signaling pathway involved in anti‐inflammatory effects.
BV2 microglial cells were obtained from Cell Resource Center Chinese Academy of Medical Sciences (Beijing, China). BV2 Cells were cultured in Dulbecco's Eagle's Medium (Gibco, Grand Island, New York) supplemented with 10% fetal bovine serum (Gibco, Grand Island, New York), 100 IU/mL penicillin (Gibco, Grand Island, New York), and 100 μg/mL streptomycin (Gibco, Grand Island, New York) at 37°C in a humidified incubator (Thermo Scientific) containing 5% CO2.
BV2 microglial cells were seeded in 6‐well plates at a density of 1 × 105 cells/well and cultured overnight. Control cells were treated with fresh medium. For DEX or LPS treatment, cells were incubated in fresh medium containing 0.1 μg/mL DEX (Jiangsu Hengrui Medicine Co, Ltd, Jiangsu, China) or LPS (Sigma, St. Louis, Missouri) for 24 hours. For the combination treatment of DEX and LPS, the cells were incubated with LPS (0.1 μg/mL) for 24 hours after culturing for 1 hour in the presence of 0.1 μg/mL DEX. Culture medium supernatant was collected and total protein was extracted from BV2 microglial cells using radioimmunoprecipitation assay (RIPA) buffer.
Male elderly C57BL/6 mice (weight: 30‐45 g; mean age: 15 months) were purchased from the Center of Experimental Animal (China Agriculture University). The mice were housed in standard cages in groups of three to four in a 12 hours light/dark cycle with controlled temperature and humidity, with free access to standard rodent chow and water. All experiments were approved by the Animal Care and Use Committee of China Agriculture University and all the procedures were performed in accordance with the guidelines recommended by the National Institutes of Health (NIH) Guide for the Care and Use of Laboratory Animals.
Sixty mice were randomly divided into four groups: control group (n = 15; group C), injected intraperitoneally with equivalent volumes of saline; DEX group (n = 15; group D), injected intraperitoneally with DEX (25 mg/kg; Jiangsu Hengrui Medicine Co, Ltd, Jiangsu, China) dissolved in 0.9% saline; surgery group (n = 15; group S), underwent partial hepatectomy; and surgery plus DEX group (n = 15; group DS), injected intraperitoneally with DEX (25 mg/kg) 10 minutes before surgery and underwent partial hepatectomy.
The surgical animals underwent partial hepatectomy. Briefly, mice were anesthetized by intraperitoneal injection of sodium pentobarbital (50 mg/kg). The liver was exposed through a 1 to 2 cm midline abdominal incision under aseptic surgical conditions. The left lateral lobes of the liver (approximately 30% of the organ) were excised. The wound was closed by sterile suture and the surgery‐associated pain was treated by eutectic mixture of local anesthetics cream (2.5% lidocaine and 2.5% prilocaine) every 8 hours for 2 days.
The place navigation test and spatial probe test for mice were analyzed by Morris water maze. The maze consisted of a circular pool (diameter: 120 cm; depth: 50 cm) and a clear round platform (diameter: 9 cm; depth: 30 cm). The platform was placed 2 cm beneath the water surface. Swimming was recorded by a video tracking system, and the data were analyzed using the motion‐detection software for the Morris water maze (Institute of Materia Medica, Chinese Academy of Medical Sciences, Beijing, China). Mice were released at the edge of the pool, facing toward the wall of the pool from one of four randomly assigned release points. If the mice failed to find the escape platform within 90 seconds, a guide stick was used to help the mice to find the platform and leave there for 20 seconds. All mice were trained for four trials per day for five consecutive days. After 1 day rest, mice underwent surgery on day 7. On postoperative days 1, 3, and 7, the hidden platform was removed from the pool, and the mice (n = 10 for each group) were tested in the water maze for 90 seconds. Time spent in the target quadrant and the number of the crossings over the former platform was recorded to evaluate spatial memory.
One day after surgery, the mice (n = 5 for each group) were sacrificed by decapitation under deep anesthesia. The hippocampal tissues were isolated and homogenized in RIPA buffer (Sigma, St. Louis, Missouri) supplemented with protease inhibitors on ice for 30 minutes and then centrifuged at 12 000 rpm at 4°C for 15 minutes.
The expression levels of TNF‐α, IL‐1β, and IL‐6 in cell culture medium supernatant and hippocampal tissue samples were measured by enzyme‐linked immunosorbent assay (ELISA) kit following the manufacturer's protocol (Dakewe Biotech Co., Ltd. Beijing, China). The absorbance in each well was measured at 450 nm using a microplate reader (Model 550, Bio‐Rad, Laboratories, Inc). The expression values were then calculated according to the standard curves.
Protein concentration of each samples was determined using a bicinchoninic acid protein assay kit (Thermo Scientific). The extracted proteins were denatured by adding a buffer solution and boiled for 10 minutes at 100°C. An equal amount of protein from each sample (20 μg) was resolved in 10% sodium dodecyl sulfate polyacrylamide gel electrophoresis. The electrophoresis voltages were 80 V in concentrated gels for 45 minutes, and then changed to separation gels for 1 hour at 120 V. After electrophoresis, the separated protein bands were transferred to the polyvinylidene fluoride membrane (Millipore, Bedford, Massachusetts) with a transferring voltage of 400 mA. Nonspecific bindings to membrane were blocked with 5% milk in Tris‐buffered saline with 0.1% Tween20 for 1 hour at room temperature. Then, the membranes were incubated at 4°C with the primary antibodies against NF‐κB‐P65 (1:1000; Abcam, UK), Histone 3 (1:1000; Abcam, UK), TLR4 (1:1000; Abcam, UK), and β‐actin (1:1000; Abcam, UK) for 24 hours. Afterward, the membranes were rinsed and then incubated for 1 hour at room temperature with secondary horse radish peroxidase‐conjugated secondary antibodies (1:2000, Abcam, UK). Immunoreactivity was detected using SuperSignals West Pico (Thermo Scientific, Rockford, Illinois) and the blots were photographed using the Image Lab Software Systems (BIO‐RAD, Hercules, California). Software Image Lab 3.0 (BIO‐RAD) was used to analyze the relative intensity of the bands normalized to Histone 3 or β‐actin.
The hippocampal tissues were fixed with formalin, embedded in paraffin, and sliced into sections (thickness: 4‐7 μm). The sections were then processed for immunofluorescence staining after de‐paraffinizin. Briefly, after 10 to 15 minutes of microwave heating in citrate buffer (0.01 mol/L, pH 6.0) for antigen retrieval, the sections were incubated with primary antibodies against IL‐1β (12 703, CTS), IL‐6 (21865‐1‐AP, Proteintech), or TNF‐α (11 948, CTS) together with primary antibodies against CD11b (66519‐1‐Ig, Proteintech) at 4°C overnight after treated with blocking buffer for 30 minutes. The sections were washed in phosphate buffered saline (PBS) and incubated with fluorescence‐conjugated secondary antibodies for 30 minutes at room temperature. After washing with PBS, the sections were incubated with 4′,6‐diamidino‐2‐phenylindole (DAPI) staining solution in the dark for 2 minutes. The sections were examined by fluorescent microscope (Olympus).
Statistical analyses were performed by the Statistical Package for the Social Sciences (SPSS; version 19.0, Illinois). Data were expressed as mean ± SD. Repeated‐measures analysis of variance (ANOVA) followed by Bonferroni post‐hoc test was used to analyze the Morris water maze data. Other differences among multiple groups were assessed by one‐way ANOVA. The P < .05 was regarded as statistically significant.
Compared with the control cells, significantly increased expressions of IL‐1β (206.11 ± 26.02 pg/mL vs 31.62 ± 7.26 pg/mL, P < .001), IL‐6 (265.65 ± 50.78 pg/mL vs 12.35 ± 0.48 pg/mL, P < .001), and TNF‐α (1738.69 ± 187.76 pg/mL vs 14.67 ± 3.28 pg/mL, P < .001) were observed in BV2 microglia cells when incubated in fresh medium containing 0.1 μg/mL LPS for 24 hours (Figure 1). Compared with the cells only treated with LPS, cells pretreated with 0.1 μg/mL DEX for 1 hour and then incubated with 0.1 μg/mL LPS for 24 hours, DEX had significantly inhibited expressions of IL‐1β (90.34 ± 15.40 pg/mL vs 206.11 ± 26.02 pg/mL, P < .01), IL‐6 (153.54 ± 41.03 pg/mL vs 265.65 ± 50.78 pg/mL, P < .001), and TNF‐α (585.05 ± 227.16 pg/mL vs 1738.69 ± 187.76 pg/mL, P < .001) (Figure 1).
Enlarge this image.1 FIGURE. Effects of dexmedetomidine (DEX) on the expression levels of interleukin (IL)‐1β, IL‐6, and tumor necrosis factor‐alpha (TNF‐α) in lipopolysaccharide (LPS)‐induced BV2 microglia cells. The levels of IL‐1β, A; IL‐6, B; and TNF‐α, C in the culture media were measured by enzyme‐linked immunosorbent assay for the four groups (group C: Control cells; group D: Cells incubated in fresh medium containing 0.1 μg/mL DEX for 24 hours; group L: Cells incubated in fresh medium containing 0.1 μg/mL LPS for 24 hours; and group DL: Cells incubated with 0.1 μg/mL LPS for 24 hours after culturing for 1 hour in the presence of 0.1 μg/mL DEX). Results are presented as the mean ± SD. ###P < .001, compared with group C; **P < .01, ***P < .001, compared with the group L
According to the western blotting analysis, LPS stimulation significantly increased the expressions of TLR4 (2.74 ± 0.53 vs 1, P < .01) and NF‐κB (6.45 ± 1.31 vs 1, P < .001) of BV2 microglia cells comparing with the control cells (Figure 2). Pretreatment with 0.1 μg/mL DEX for 1 hour significantly inhibited the LPS‐induced overexpressions of TLR4 (1.99 ± 0.57 vs 2.74 ± 0.53, P < .05) and NF‐κB (3.34 ± 1.44 vs 6.45 ± 1.31, P < .01) (Figure 2).
Enlarge this image.2 FIGURE. Effects of dexmedetomidine (DEX) on lipopolysaccharide (LPS)‐induced Toll‐like receptor 4 (TLR4) and nuclear factor kappa B (NF‐κB) activity in BV2 microglia cells. Western blot analysis of TLR4, A and NF‐κB, B in BV2 microglial cells of four groups (group C: Control cells; group D: Cells incubated in fresh medium containing 0.1 μg/mL DEX for 24 hours; group L: Cells incubated in fresh medium containing 0.1 μg/mL LPS for 24 hours; and group DL: Cells incubated with 0.1 μg/mL LPS for 24 hours after culturing for 1 hour in the presence of 0.1 μg/mL DEX). The protein expression level was normalized to β‐actin or histone H3. The results of densitometric analysis were expressed as the mean ± SD of three independent experiments. ###P < .001 compared with group C; *P < .05, **P < .01, compared with group L
All aged mice were trained to find the platform in the Morris water maze for five consecutive days before surgery. The parameters of escape latency, swimming distance, and swimming speed were used to evaluate the spatial memory function of the mice. During the 5 days training, the swimming speed remained unchanged, the escape latency and swimming distance showed obviously decreased tendencies. On the fifth day of training, all the aged mice were able to find the platform within 30 seconds, indicating that their spatial memory had been gradually consolidated (Figure 3).
Enlarge this image.3 FIGURE. Swimming date of aged mice for five consecutive training days in the Morris water maze prior to surgery. A, The escape latency; B, swimming distance; and C, swimming speed for the four groups (group C: Injected intraperitoneally with equivalent volumes of saline; group D: Injected intraperitoneally with 25 mg/kg dexmedetomidine (DEX); group S: Underwent partial hepatectomy; and group DS: Injected intraperitoneally with 25 mg/kg DEX 10 minutes before surgery and underwent partial hepatectomy). Results are presented as the mean ± SD
Subsequently, Morris water maze assay was performed on postoperative days 1, 3, and 7. The percentages of time spent in the target quadrant decreased to 24.32 ± 7.17% (P < .01), 27.98 ± 4.85% (P < .01), and 34.20 ± 11.63% (P < .05) of mice in the group S at the first, third, and seventh days after surgery comparing with those of mice in the group C, respectively. Meanwhile, a reduction was revealed in the number of crossings over the former platform location on postoperative days 1 (2.8 ± 2.39, P < .01), 3 (3.1 ± 1.85, P < .01), and 7 (4.8 ± 3.05, P < .05), suggesting that their spatial memory function was impaired shortly following surgery. Compared with group S, the percentages of time spent in the target quadrant increased to 33.25 ± 9.81% (P < .05), 35.13 ± 9.21% (P < .05), and 46.31 ± 9.07% (P < .05) of mice in the group DS on the first, third, and seventh days after surgery, respectively. At the same time, the number of crossings over the former platform location, respectively, increased to 4.9 ± 1.45 (P < .05), 5.0 ± 1.69 (P < .05), and 6.7 ± 1.42 (P < .05), suggesting that their cognitive function had recovered to some extent. On postoperative days 1, 3, 7, there was no significant difference in the percentage of time spent in the target quadrant and the number of crossings over the former platform location between group D and group C (P > .05), indicating that DEX did not lead to spatial memory deficit in aged mice (Figure 4).
Enlarge this image.4 FIGURE. Assessment of the effects of dexmedetomidine (DEX) on the cognitive performance of mice in the Morris water maze test. A, Effects of DEX on the number of crossings over the former platform location; B, time spent in target quadrant; and C, speed of aged mice in the four groups (group C: Injected intraperitoneally with equivalent volumes of saline; group D: Injected intraperitoneally with 25 mg/kg DEX; group S: Underwent partial hepatectomy; and group DS: Injected intraperitoneally with 25 mg/kg DEX 10 minutes before surgery and underwent partial hepatectomy. Results are presented as the mean ± SD. #P < .05, ##P < .01, compared with group C; *P < .05, compared with group DS
The cytokine productions in hippocampal tissues of different groups were measured using ELISA. The results showed that surgery significantly increased the expressions of IL‐1β (131.30 ± 34.12 vs 29.0 ± 3.63 pg/mL, P < .01), IL‐6 (54.79 ± 9.65 vs 18.58 ± 4.35 pg/mL, P < .01), and TNF‐α (39.11 ± 6.07 vs 11.73 ± 2.07 pg/mL, P < .01) in comparison with the control group (Figure 5). What is more, intraperitoneal injection of DEX before partial hepatectomy could markedly decrease the expression levels of TNF‐α (26.46 ± 5.31 vs 39.11 ± 6.07 pg/mL, P < .05), IL‐1β (89.61 ± 24.28 vs 131.30 ± 34.12 pg/mL, P < .05), and IL‐6 (38.53 ± 6.05 vs 54.79 ± 9.65 pg/mL, P < .05) in the hippocampus comparing with the surgery group (Figure 5).
Enlarge this image.5 FIGURE. Expression levels of interleukin (IL)‐1β, IL‐6, and tumor necrosis factor‐alpha (TNF‐α) in the hippocampus after partial hepatectomy in aged mice. The levels of, A, IL‐1β; B, IL‐6; and C, TNF‐α in the hippocampus were measured by enzyme‐linked immunosorbent assay for the four groups (group C: Injected intraperitoneally with equivalent volumes of saline; group D: Injected intraperitoneally with 25 mg/kg dexmedetomidine (DEX); group S: Underwent partial hepatectomy; and group DS: Injected intraperitoneally with 25 mg/kg DEX 10 minutes before surgery and underwent partial hepatectomy). Results are presented as the mean ± SD. ##P < .01, ###P < .001, compared with group C; *P < .05, **P < .01, compared with the group S
The western blotting analysis showed that the protein expressions of TLR4 (3.56 ± 0.27 vs 1, P < .01) and NF‐κB (4.19 ± 1.44 vs 1, P < .01) after partial hepatectomy were significantly higher than those in the control group (Figure 6). However, the aged mice received intraperitoneal injection of DEX had significantly decreased expressions of TLR4 (2.71 ± 0.19 vs 3.56 ± 0.27, P < .05) and NF‐κB (2.73 ± 0.41 vs 4.19 ± 1.44, P < .05) in the hippocampus samples comparing with the mice in the surgery group (Figure 6).
Enlarge this image.6 FIGURE. Representative protein expressions of Toll like receptor 4 (TLR4), and nuclear factor kappa B (NF‐κB) in the hippocampus 1 day after partial hepatectomy in aged mice. A, TLR4 and B, NF‐κB were measured by western blot analysis for the four groups (group C: Injected intraperitoneally with equivalent volumes of saline; group D: Injected intraperitoneally with 25 mg/kg dexmedetomidine (DEX); group S: Underwent partial hepatectomy; and group DS: Injected intraperitoneally with 25 mg/kg DEX 10 minutes before surgery and underwent partial hepatectomy). Results are presented as the mean ± SD. ##P < .01, compared with group C; *P < .05, compared with group S
The immunofluorescence staining results indicated that the pro‐inflammatory cytokines IL‐1β (Figure 7), IL‐6 (Figure 8), and TNF‐α (Figure 9) were obviously increased in group S, and reduced tendency was observed in group DS. No significant differences were found in the immunofluorescence staining results between group C and group D. Moreover, the pro‐inflammatory cytokines IL‐1β, IL‐6, and TNF‐α were occurred in the same position with the microglia activation marker CD11b.
Enlarge this image.7 FIGURE. Colocation of pro‐inflammatory cytokine interleukin (IL)‐1β with microglia activation in hippocampal tissues of rats in four groups (group C: Injected intraperitoneally with equivalent volumes of saline; group D: Injected intraperitoneally with 25 mg/kg dexmedetomidine (DEX); group S: Underwent partial hepatectomy; and group DS: Injected intraperitoneally with 25 mg/kg DEX 10 minutes before surgery and underwent partial hepatectomy). Representative images of double immunofluorescent staining for IL‐1β (green) and microglia activation marker CD11b (red). 4′,6‐diamidino‐2‐phenylindole (blue) counter staining for cell nucleus. Scale: 100 μm
Enlarge this image.8 FIGURE. Colocation of pro‐inflammatory cytokine interleukin (IL)‐6 with microglia activation in hippocampal tissues of rats in four groups (group C: Injected intraperitoneally with equivalent volumes of saline; group D: Injected intraperitoneally with 25 mg/kg dexmedetomidine (DEX); group S: Underwent partial hepatectomy; and group DS: Injected intraperitoneally with 25 mg/kg DEX 10 minutes before surgery and underwent partial hepatectomy). Representative images of double immunofluorescent staining for IL‐6 (green) and microglia activation marker CD11b (red). 4′,6‐diamidino‐2‐phenylindole (blue) counter staining for cell nucleus. Scale: 100 μm
Enlarge this image.9 FIGURE. Colocation of pro‐inflammatory cytokine tumor necrosis factor alpha (TNF‐α) with microglia activation in hippocampal tissues of rats in four groups (group C: Injected intraperitoneally with equivalent volumes of saline; group D: Injected intraperitoneally with 25 mg/kg dexmedetomidine (DEX); group S: Underwent partial hepatectomy; and group DS: Injected intraperitoneally with 25 mg/kg DEX 10 minutes before surgery and underwent partial hepatectomy). Representative images of double immunofluorescent staining for TNF‐α (green) and microglia activation marker CD11b (red). 4′,6‐diamidino‐2‐phenylindole (blue) counter staining for cell nucleus. Scale: 100 μm
Neurodegeneration is the major cause of cognitive and motor dysfunction, characterized with progressive dysfunction and loss of neurons in the central nervous system.13 Microglia are the primary mediators of the immune defense system of the central nervous system and microglial activation is integral to the response of central nervous system tissue to injury.14 Microglia can become chronically activated by a single stimulus, (such as lipopolysaccharide or neuron damage), and activated microglia can produce a myriad of inflammatory mediators (such as TNF‐α, nitric oxide, IL‐1β, and reactive oxygen species) that involved in important defense functions against invading neurotropic pathogens, brain damage, and neuroinflammatory disease.15,16 A line of studies indicated that downregulations of pro‐inflammatory mediators derived from microglia can alleviate the severity of neuroinflammation.17,18 DEX has been widely applied in clinical practice, which provides excellent sedation and analgesia without causing respiratory depression.7 In our study, BV2 microglial cells stimulated by LPS significantly increased the release of IL‐1β, IL‐6, and TNF‐α. Pretreatment with DEX markedly inhibited LPS‐induced expressions of IL‐1β, IL‐6, and TNF‐α in BV2 microglial cells. Therefore, it could be speculated that DEX significantly inhibited microglial activation by downregulating expressions of IL‐1β, IL‐6, and TNF‐α.
Although the detailed molecular mechanisms for POCD still remain unclear, it has demonstrated that neuroinflammation plays a key role in the pathogenesis of POCD.5 In this present study, the aged C57BL/6 mice received partial hepatectomy to mimic human liver surgery. Morris water maze test showed that, the percentage of time staying in the target quadrant and the number of crossings over the former platform location was decreased on the first, third, and seventh day after the surgery. Therefore, spatial memory functions of the aged mice were impaired after the partial hepatectomy, indicating cognitive dysfunction models have been successfully established. Meanwhile, administration of DEX before surgery could increase the percentage of time staying in the target quadrant and the number of crossings over the former platform location. Therefore, these results suggested that DEX could improve spatial memory function after the partial hepatectomy in aged mice.
The memory and learning impairments have been reported to be paralleled by elevations of pro‐inflammatory cytokines (IL‐1β, IL‐4, IL‐6, and TNF‐α) in the hippocampus of aged rats after laparotomy, and intracisternal administration of IL‐1 receptor antagonist, IL‐6 receptor antagonist, or TNF‐α receptor antagonist at the time of surgery could notably attenuated the surgery‐induced cognitive deficit and the neuroinflammatory response.19–21 Impaired cognitive functions and increased expression of IL‐1β, TNF‐α, Bax, and caspase‐3 in hippocampus were observed after splenectomy in aged mice, while these changes were significantly inversed by DEX.22 In our study, the expression levels of IL‐1β, IL‐6, and TNF‐α were also significantly upregulated in hippocampus following surgery. Administration of DEX before surgery could cause obvious downregulation of the surgery‐induced IL‐1β, IL‐6, and TNF‐α expressions in hippocampus. These results showed that the hippocampal inflammatory response might be involved in the occurrence and development of POCD.
TLR4 participates in the inflammatory response by producing numerous pro‐inflammatory factors, and NF‐κB is an important transcription factor downstream in the TLR4‐mediated signaling pathway.23,24 Activation of TLR4 can promote the activity of NF‐κB through a MyD88‐dependent pathway, and then regulate the mRNA transcriptions of multiple pro‐inflammatory cytokines, including TNF‐α, IL‐1β, IL‐6, and IL‐8.25,26 Thus, the TLR4/NF‐κB signaling pathway is pivotal in inflammation. Zhu et al. have found that pretreatment with DEX in a rat tibial fracture model could significantly suppress the inflammatory responses by reducing TNF‐α and IL‐1β levels, significantly inhibit NF‐κB activity, and alleviate overexpressions of microglia and astrocytes in the hippocampus.27 It has been reported that DEX may decrease mortality and inhibit inflammatory reaction in lung tissues of septic rats by suppressing TLR4/MyD88/NF‐κB pathway.28 Zhang et al. revealed that DEX preconditioning may attenuate myocardial ischemia/reperfusion injury by reducing the levels of pro‐inflammatory cytokines (IL‐6 and TNF‐α) through the HMGB1‐TLR4‐MyD88‐NF‐кB signaling pathway.29 In our study, the protein expressions of TLR4 and NF‐κB after partial hepatectomy were significantly higher than those in the control group. The aged mice that received intraperitoneal injection of DEX had significantly decreased expressions of TLR4 in the hippocampus samples comparing with the mice in the surgery group. Therefore, it could be speculated that DEX inhibited the release of pro‐inflammatory factors possibly by suppressing the TLR4/NF‐κB signaling and to alleviate POCD.
In summary, the present study demonstrated that DEX is an effective inhibitor of inflammation response in LPS‐stimulated BV2 microglial cells and surgery‐induced mice. Additionally, pretreatment of DEX may improve POCD by inhibiting the TLR4/NF‐κB signaling pathway.
All authors declare no conflict of interest.
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Details
1 School of Clinical Medicine, Tsinghua University, Beijing, China; Department of Anesthesiology, Beijing Tsinghua Changgung Hospital, Beijing, China
2 Anesthesia and Operation Center, Chinese PLA General Hospital, Beijing, China
3 Department of Anesthesiology, The 305 Hospital of Chinese PLA, Beijing, China