Glutamate-mediated influx of extracellular Ca2+ is coupled with reactive oxygen species generation in cultured hippocampal neurons but not in astrocytes.

Generation of reactive oxygen species (ROS) in brain tissue leads to neurodegeneration. The major source of ROS is the mitochondrial respiratory chain. We studied regulation of Ca2+ level, mitochondrial potential, and ROS generation in defined mixed hippocampal cell cultures exposed to glutamate (100 microM). Recordings were made from individually identified astrocytes and neurons to compare the physiologic responses in both cell types. Neurons identified by synaptotagmin immunoreactivity were characterized functionally by the fast Ca2+ increase with K+ (50 mM) stimulation, and the astrocytes identified by glial fibrillary acidic protein (GFAP) staining had the functional characteristic of a transient Ca2+ peak in response to ATP (10 microM) stimulation. We found that the glutamate-mediated Ca2+ response in neurons is due largely to influx of extracellular Ca2+. This is consistent with our finding that in cultured hippocampal neurons, stores depending on the activity of the sarcoendoplasmic reticulum Ca2+ ATPase (SERCA) pump had a low Ca2+ content, regardless of whether the neurons were challenged or not with K+ before applying the SERCA inhibitor cyclopiazonic acid (CPA). Astrocytes displayed a large CPA-mediated Ca2 response, indicating a high level of Ca2+ load in the stores in astrocytes. Importantly, the rise in ROS generation due to glutamate application was cell-type specific. In neurons, glutamate induced a marked rise in generation of ROS, but not in astrocytes. In both astrocytes and neurons, the mitochondrial potential was increased in response to glutamate challenge. We conclude that in neurons, Ca2+ influx accounts for the increased ROS generation in response to glutamate. This might explain the high vulnerability of neurons to glutamate challenge compared to the vulnerability of astrocytes. The high resistance of astrocytes is accompanied by an efficient downregulation of cytosolic Ca2+, which is not found in neurons.