ObjectiveTo explore the mechanism of neural protection after hypoxia preconditioning. MethodsMice were randomly divided into control group (H0 group) and hypoxia group (H4 group). H4 group were built up models of hypoxia preconditioning by keeping them in hypoxia repeatedly, H0 group were kept intact. The expression of neuropeptide Y (NPY) and synaptophysin in hippocampus was assessed with immunohistochemistry, and morphology of asymmetric synapses and perforated asymmetric synapses in the CA1 of hippocampus was observed by electron microscopy and their numbers were counted. ResultsComparing with H0 group, there was a moderate increase in the number of NPY positive cells,and a significant increase in the number of synaptophysin positive cells respectively in hippocampus(EM>n/EM>=30), but the number of asymmetric synapses and perforated asymmetric synapses presenting in CA1 region of hippocampus decreased in H4 group(EM>n/EM>=6). ConclusionThe changes took place in hippocampus after hypoxia preconditioning might decrease the neuronal exciting so that increase the brain’s tolerance to hypoxia/ischemia and thus protected neurons.