The abnormal influx of Ca2+ through glutamate receptor channels is thought to contribute to the loss of neurons associated with a number of brain disorders. Until recently, the NMDA receptor was the only glutamate receptor known to be Ca(2+)-permeable. It is now well established that AMPA receptors exist not only in Ca(2+)-impermeable but also in Ca(2+)-permeable forms. AMPA receptors are encoded by four genes designated gluR1 (gluR-A) through gluR4 (gluR-D). The presence of the gluR2 subunit renders heteromeric AMPA receptor assemblies Ca(2+)-impermeable. Recent studies involving animal models of transient forebrain ischemia and epilepsy show that gluR2 mRNA is downregulated in vulnerable neurons. These observations suggest that downregulation of gluR2 gene expression may serve as a 'molecular switch' leading to the formation of Ca(2+)-permeable AMPA receptors and enhanced toxicity of endogenous glutamate following a neurological insult.