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Introduction: The membrane localization of the N-Methyl-D-aspartic acid receptor (NMDAR) influences the consequences of receptor activation: synaptic activation is associated with signaling pathways favoring cell survival.Extrasynaptic receptors are associated with cell death and are preferably activated by glutamate released by astrocytes or spilled over from the synaptic cleft.Hence, enhanced extrasynaptic receptor expression could contribute to neuron loss and seizure generation in epilepsy.Therefore we evaluated the membrane localization of the NMDAR subunit NR2B during epileptogenesis.Methods: Spontaneous recurrent seizures were evoked in adult male Sprague-Dawley rats following status epilepticus induced by electrical stimulation of the CA3 region of the hippocampus.Membrane localization during the epileptogenic phase was analyzed through: 1) Western Blot analysis of CREB-phosphorylation, which is associated with synaptic NMDAR activation;2) co-immunoprecipitation of NR2B and the synaptic NMDA-receptor associated scaffolding protein PSD-95;3) immuno-electron microscopy of NR2B-labeled particles.Involvement of the NR2B-containing receptors in neuronal cell loss was evaluated by pharmacological blockade of these receptors.Results: All analyses indicated increased extrasynaptic localization ofNR2B subunit during the epileptogenic phase: 1) CREB-phosphorylation decreased with 53± 10% compared to sham controls 2) Co-immunoprecipitation showed a drastic decrease in the association of NR2B and PSD-95 3) Electron microscopy confirmed decreased synaptie and increased extrasynpatie NR2B expression.Neuronal cell loss decreased significantly after pharmacological blockade of NR2B receptors.Conclusions: During epileptogenesis NR2B is increasingly expressed at extrasynaptic sites.This extrasynaptic expression is involved in increased neuronal cell loss during epilepsy development.