Role of Toll-like receptor 4 in mechanical ventilation-induced activation of microglias in hippocampi of mice

Objective To evaluate the role of Toll-like receptor 4(TLR4)in mechanical ventilation-induced activation of microglias in the hippocampi of mice. Methods Forty-two healthy male TLR4 gene knocked out C57BL/10ScNJNju mice and 42 wild type C57BL/6 mice, aged 10-12 weeks, weighing 20-25 g, were used in the study.The mice of either group were divided into 2 subgroups(n=21 each)using a random number table: spontaneous breathing subgroup(subgroup SB)and mechanical ventilation subgroup(subgroup MV). In subgroup SB, the mice were exposed to isoflurane for 6 h in an anesthesia chamber.After tracheal intubation, the mice were mechanically ventilated for 6 h when anesthesia was maintained with isoflurane in subgroup MV.Twelve mice were selected from each subgroup at 1 and 3 days after ventilation, fear conditioning test was performed to assess the cognitive function, and the rate of freezing time was recorded.Three mice in each subgroup were sacrificed at 1 day after ventilation, and the brains were removed to observe the morphological changes of microglias(by double immunofluorescent staining)and to count CD11b positive cells and co-expression of TLR4 with CD11b(TLR4/CD11b)positive cells in hippocampal microglias.Six mice in each subgroup were sacrificed at 1 day after ventilation, and the hippocampi were isolated for determination of the content of tumor necrosis factor-alpha(TNF-α)in hippocampal tissues by enzyme-linked immunosorbent assay. Results Compared with subgroup SB of wild type group, the rate of freezing time was significantly decreased at 1 and 3 days after ventilation, and the number of CD11b positive cells and TLR4/CD11b positive cells were increased, and the content of TNF-α was increased in subgroup MV of wild type group(P<0.05), and the number of CD11b positive cells and content of TNF-α were significantly increased in subgroup MV of gene knockout group(P<0.05). Compared with subgroup SB of gene knockout group, the rate of freezing time was significantly decreased at 1 and 3 days after ventilation, and the number of CD11b positive cells and content of TNF-α were increased in subgroup MV of gene knockout group(P<0.05). Compared with subgroup MV of wild type group, the rate of freezing time was significantly increased at 1 and 3 days after ventilation, and the number of CD11b positive cells and content of TNF-α were decreased in subgroup MV of gene knockout group(P<0.05). Conclusion The mechanism by which mechanical ventilation induces activation of microglias in hippocampi is partially related to TLR4 in mice. Key words: Toll like receptor 4; Respiration, artificial; Hippocampus; Microglia