Environmental enrichment induces synaptic structural modification after transient focal cerebral ischemia in rats

Environmental enrichment (EE), where animals are exposed to a complex novel environment, has been shown to induce synaptic plasticity in both intact and injured animals. The purpose of this study was to investigate the effects of EE on spatial memory and structural modifications of synaptic junctions in rats following transient focal cerebral ischemia. Adult male Sprague-Dawley rats underwent right middle cerebral artery occlusion (MCAO) for 40 min and reperfusion. On day 3 after MCAO or sham surgery, rats were randomly assigned for 14 days to enriched or standard environmental housing. Spatial memory was then tested by the Morris water maze. Parietal cortex and the CA1 region of hippocampus were processed for electron microscopy and stereological techniques were used to evaluate plasticity of synaptic junctions. EE after MCAO improved spatial memory, with shortened escape length, increased frequency of crossings at the location of the platform, and increased percentage of time spent in the quadrant where the platform was previously located. Synaptic ultrastructural analysis showed that EE after MCAO increased numeric synaptic density in parietal cortex, and induced structural changes in synaptic junctions, with a decreased width of synaptic clefts and increased thickness of postsynaptic densities (PSD) in parietal cortex and hippocampus, accompanying improved performance on the spatial memory task. Using Western blot analysis, we determined the expression of glutamate receptor NMDAR1, and PSD-95, the best characterized protein member of the PSD-95 family, that was abundantly expressed in the PSD of excitatory synapses. The results showed that the content of NMDAR1 was not altered in MCAO rats of EE; however, the phosphorylated NMDAR1 increased significantly when compared with the standard environment housing MCAO rats. In addition, EE inhibited the impaired expression of PSD-95 induced by MCAO in parietal cortex and hippocampus. These data suggest that improved spatial memory of cerebral ischemic rats by EE is associated with structural modifications of synaptic junctions in several brain regions.