Changes in Postsynaptic Densities After Brain Ischemia

Summary

Synaptic dysfunction and neurological deficits resulting from transient cerebral ischemia have been observed in numerous studies. However, the molecular mechanisms for the alteration of synaptic function are not completely understood. We conducted a series of studies to investigate the molecular and morphological mechanisms underlying these synaptic alterations in the hippocampus. Rat brains were subjected to 15 min ischemia followed by 0 min, 30 min, 4, 24 and 48 h, and 7 days of reperfusion. Postsynaptic densities (PSDs) were analyzed by biochemical and electron microscopic methods. Fifteen minutes of ischemia induced a marked accumulation of ubiquitinated proteins (ubi-proteins) in PSDs at 4 h of reperfusion, indicating that proteins in PSDs are denatured during reperfusion. Several groups of ATP-binding proteins are highly accumulated in PSDs after ischemia. These include both inactive forms of protein kinases, such as P38 kinase, c-Jun kinases (JNKs), calcium/calmodulin-dependent protein kinase II (CaM- kinase II), protein kinase C and TrkB, and ATPases for membranous protein assembly such as N-ethylmaleimide-sensitive fusion protein (NSF) and heat-shock cognate protein-70 (HSC70). Two- and three-dimensional ultra-structural analyses of PSDs showed a persistent and progressive destructive change, after ischemia, in synapses from the hippocampal area CA1, an area particularly vulnerable to transient ischemia. These changes were not as pronounced in the dentate gyrus, an area relatively resistant to ischemic cell death. We conclude that transient cerebral ischemia of 15 min duration causes degenerative changes in synapses. The degenerative changes are persistent and progressive in CA1 dying neurons, starting at 2-4 h of reperfusion until cell death, but only transient in DG neurons destined to survive. The degenerative changes in synapses may contribute to synaptic dysfunction and neurological deficits after ischemia.