Mechanism of focal cerebral ischemic tolerance in rats with ischemic preconditioning involves MyD88- and TRIF-dependent pathways

The aim of this study was to explore the involvement of Toll-like receptor 4 (TLR4) and the downstream myeloid differentiation factor 88 (MyD88)-dependent and -independent pathways in the mechanisms of cerebral ischemic tolerance. Using an improved middle cerebral artery occlusion method, we constructed a preconditioned ischemic brain model in rats. Sham and ischemia-reperfusion groups were also established. The expression levels of proteins in the MyD88/nuclear factor-κB (NF-κB) pathway (MyD88-dependent) were compared with those in the Toll/interleukin-1 receptor-domain-containing adaptor-inducing interferon-β (TRIF)/interferon regulatory factor-3 (IRF-3) pathway (MyD88-independent) by western blot analysis. NF-κB and IRF-3 protein expression levels within cells were determined by immunofluorescence staining of frozen tissue sections. Western blot analysis showed a downregulation of MyD88 protein expression in the brain tissue of ischemic preconditioned rats; however, NF-κB, TRIF and IRF-3 protein expression levels were upregulated. Immunofluorescence staining showed that NF-κB protein was mainly located in the cytoplasm in ischemic preconditioned rats and IRF-3 was predominantly located in the nucleus. The results indicate that changes in the two TLR4 downstream pathways are the main mechanisms involved in the development of brain ischemic tolerance with ischemic pretreatment.