Low concentrations of glutamate induce apoptosis in cultured neurons: implications for amyotrophic lateral sclerosis

Evidence is accumulating that excessive glutamate concentration in the extracellular space is neurotoxic and plays a role in amyotrophic lateral sclerosis (ALS). However, the published results on glutamate levels in cerebrospinal fluid (CSF) and on glutamate-mediated toxicity of CSF in ALS disease remain controversial. In this report, we studied CSF from patients with sporadic ALS and controls to determine glutamate concentrations, and then analyzed the neurotoxic effect of glutamate at the concentrations present in CSF from ALS patients on cultured cortical neuronal cells. Our study shows that glutamate, at the concentrations found in CSF from ALS patients (5.8 microM), diminished cell viability and increased apoptosis determined by the fluorescent DNA-binding dye Hoechst 33342 as well as by Terminal deoxynucleotidyl transferase (TdT)-mediated dUTP Nick End-Labeling (TUNEL) reaction in cultured neuronal cells. However, glutamate concentrations as those found in CSF from controls (2.8 microM or below) did not induce any effect. Both significant glutamate-induced effects were inhibited in the presence of NBQX (2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline-2,3-dione), an alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate-sensitive glutamate receptor antagonist. These results demonstrate that AMPA/kainate receptors are involved in the glutamate-mediated neurotoxic effects on cultured neurons, according to reports that implicate these receptors in ALS disease. We conclude that the glutamate-mediated neuronal apoptosis through AMPA/kainate receptors could occur in ALS patients who have elevated CSF glutamate concentration.