The Mechanisms Of Ischemic Postconditioning On TLR4 Dependent Ischemic Tolerance In Tree Shrews To Thrombotic Cerebral Ischemia

Objective:To observe the effects of thrombotic cerebral ischemia on toll-like receptor 4 (TLR4) expressions in cortex and hippocampus of tree shrews, and to investigate the effect of ischemia postconditioning on the expresssion of MPO (myeloperoxidase), TLR4 and inflammation in cortex and hippocampus. The aim of this study is to explore the possible mechanism of ischemia postconditioning in modulating TLR4 expression and improving inflammatory reaction in cortex and hippocampus.Methods:The focal thrombotic cerebral ischemia was induced by photochemical reaction in tree shrews, and ischemic postconditioning was established by 3 repeated cycles of 5 min of temporary right carotid arteries clipped at 4h after the onset of photochemistry and 5 min reperfusion. The changes of histology of cortex and hippocampus (by Hematoxylin-and-eosin). MPO expression (by immunohistochemistry), TLR4 expression (by Western Blot Analysis and immunohistochemistry) and TLR4 mRNA (by RT-PCR) in tree shrews were observed.Results:Our study found extensive neuronal injury and inflammation in cortex at 4,24 and 72h, the peak at 24h after the cerebral ischemia, while significantly-attenuated after the postconditionng with less infiltration of inflammatory cells. Immunohistochemistry analysis showed that cerebral ischemia caused an increase of MPO expression in cortex, and ischemic postcondionting decreased the expression at 24h and 72h(P<0.05). Cerebral ischemia caused an increase of the TLR4 expression in cortex from 4h to 72h (P<0.05), peak at 24h. Compared with the ischemic group, postcondionting caused a decrease in the TLR4 expression in cortex at 4 and 24h (P <0.05), but an increase at 72h (P<0.05). The expression of the TLR4 in tree shrews' cortex after postconditioning had the trend of increase gradually. The levels of TLR4 mRNA in cortex were also lower with postconditioning at 4h and 24h and increased at 72h (P<0.05), showing the similar variation trend in accordance with the protein expression tendency in cortex. Our study found extensive neuronal degeneration in hippocampus from 4h to 24h,72h, and peak at 24h after the cerebral ischemia, while significantly attenuated after the postconditionng at 24 and 72h (P< 0.05). Cerebral ischemia caused an increase in the TLR4 protein expression at 4h and 24h (P<0.05), and 72h decrease (P<0.05) in hippocampus. In contrast to ischemic group postcondionting caused a decrease in expression of the TLR4 protein at 4h and 24h (P<0.05), but an increase in expression of the TLR4 at 72h (P<0.05) in hippocampus.The levels of TLR4 mRNA in hippocampus showed the similar variation trend with the protein expression.Conclusions:The TLR4 expression increased in cortex and hippocampus after cerebral ischemia. The protection mechanisms of ischemia postconditioning may partially be associated with modulating TLR4 expression and restraining inflammation.