The neurotoxicity of excitatory amino acids is produced by passive chloride influx

Abstract

In the 15 years since the neurotoxic properties of glutamate and related amino acids were first described, there has been no thoroughly convincing explanation of the pathophysiology of excitatory amino acid-induced neuronal death. These substances depolarize central neurons, increases the frequency of neuronal discharge, and augment synaptic activity, leading to the suggestion that one or more of these properties may in some way be responsible for toxicity. More recently, an excessive calcium influx triggered by amino acids has been implicated in this process. As isolation of the different factors potentially involved in amino acid neurotoxicity is virtually impossible in vivo, dispersed hippocampal cultures were used to define the pathophysiology of this process in vitro. The toxicity of glutamate, N-methyl-D-aspartate, and kainate was unaffected when calcium was deleted and tetrodotoxin added to the balanced salt solution bathing the cultures. In parallel experiments, the calcium ionophore A23187 was not toxic in the presence of calcium. These experiments failed to confirm a role for neuronal activity or calcium influx in this process. However, when depolarization was blocked by deleting sodium from the control salt solution, neither glutamate, N-methyl-D-aspartate, nor kainate produced obvious changes. Alternately, when passive chloride influx was prevented by largely deleting chloride from the bath, the cells were also unchanged by the amino acids. Further experiments showed that depolarization produced by high external potassium concentrations or veratridine was also toxic, but only in the presence of external chloride. These experiments suggest that the pathophysiology of amino acid neurotoxicity may be rather straightforward. These substances produce a steady depolarization which leads to an influx of chloride. Cations are then drawn into the neuron, which results in water entry and cell lysis.