Excitatory amino acid neurotoxicity at the N-methyl- d-aspartate receptor in cultured neurons: role of the voltage-dependent magnesium block

Results of the present report show that cerebellar neurons in primary culture are resistant to glutamate concentrations as high as 5 mM in the presence of glucose and Mg 2+, but sensitive to glutamate concentrations lower than 35 μM when the neurons are deprived of glucose. Glutamate toxicity is also potentiated when Mg 2+ is removed but glucose and EDTA are present; in this case, higher concentrations of glutamate (1 mM) are required for full toxicity. Glucose concentrations as low as 50 μM are fully protective against the toxicity of 100 μM glutamate; pyruvate and, to a lesser extent, lactate are also protective. Significantly, increasing concentrations of extracellular Mg 2+ are fully protective against the toxicity of 100 μM glutamate in the absence of glucose and against the toxicity of 1 mM glutamate in the presence of glucose and EDTA. We interpret these results as support for our hypothesis that the pivotal event in glutamate's transition to neurotoxin is relief of the Mg 2+ block of the N-methyl- d-aspartate (NMDA) receptor channel, which is known to be voltage-dependent. Partial depolarization in response to depletion of high-energy phosphates relieves the voltage-dependent block enabling glutamate to stimulate an excessive ion influx which results in the death of the neuron by a mechanism which is not yet understood. We propose that this mechanism may be operative in the neuronal damage associated with a variety of neurodegenerative disorders.