Toll-like receptor 4 is involved in ischemic tolerance of postconditioning in hippocampus of tree shrews to thrombotic cerebral ischemia

Background: Toll-like receptor 4 (TLR4) is an important mediator of the innate immune response. It significantly contributes to neuroinflammation and may be involved in ischemic tolerance. It is unknown how cerebral ischemia in the cortex and postconditioning might affect inflammatory reactions in the hippocampus or whether TLR4 expression plays a role. Objective: This study explored the mechanistic hypothesis that postconditioning modulates TLR4 expression and thus improves inflammatory reactions in the hippocampus. Methods: Thrombotic focal cerebral ischemia was induced by a photochemical reaction in tree shrews. Four hours after the photochemical reaction onset, ischemic postconditioning was established with three repeated five minute cycles of temporary right carotid artery clipping and a five minute reperfusion. Histological changes were assessed over 72 h in hippocampal morphology (hematoxylin-and-eosin), myeloperoxidase (MPO) expression (immunohistochemistry), TLR4 expression (Western blot analysis and immunohistochemistry), and TLR4 mRNA expression (semiquantitative RT-PCR). Results: We found extensive neuronal degeneration in the hippocampus that peaked at 24 h after cerebral ischemia. This was significantly attenuated after postconditioning. Cerebral ischemia caused a predominant increase in TLR4 protein expression from 4 to 24 h (P < 0.05). In contrast, postconditioning caused a decrease in TLR4 protein expression from 4 to 24 h (P < 0.05), which increased at 72 h (P < 0.05). Hippocampal TLR4 mRNA levels showed the same trends as those observed in protein expression. Conclusion: These findings indicated that TLR4 signaling and innate immunity may be involved in the protective mechanisms of postconditioning and ischemic tolerance. (C) 2011 Elsevier B.V. All rights reserved.