The Excitotoxin Hypothesis in Relation to Cerebral Ischemia

The distribution of brain cell injury following transient ischemia is remarkable because only certain neurons in distinct brain regions are destroyed (selective neuronal death). Because excitatory neurotransmitters (glutamate and aspartate) cause a similar pattern of selective neuronal death, it seemed only natural to associate these effects with the trauma of ischemia. This led to the formulation of the excitotoxin hypothesis, which explains selective neuronal death as a result of excessive interstitial concentration increases of excitatory amino acids during ischemia, resulting in the opening of receptor-coupled ionophores, of which calcium channels are of particular interest. A large influx of calcium associated with impaired intracellular calcium sequestration mechanisms due to energy failure activates a host of catabolic enzymes that ultimately will cause neuronal death. The purpose of this work was (a) to measure extracellular glutamate concentration increases during ischemia in a selective vulnerable brain region (rat CA1 hippocampus), (b) to evaluate the toxicity of such a concentration increase, and (c) to investigate the relationship between ischemia-induced glutamate accumulation and changes of calcium homeostasis. The execution of these experiments required a method that was able to sample excitatory amino acids in the brain extracellular space for subsequent analysis by high performance liquid chromatography (HPLC). The choice of the microdialysis technique proved most satisfactory and further mathematical analysis made it possible to transform dialysate glutamate concentrations to extracellular concentrations. The study demonstrated that extracellular glutamate in CA1 reached toxic concentrations during ischemia. There appeared to be a clear correlation between ischemia-induced glutamate accumulation and the decrease in extracellular calcium since both changes were prevented in the denervated CA1 (the destruction of glutamatergic innervation from CA3 protects CA1 pyramidal neurons from ischemic damage). By contrast, blockade of N-methyl-D-aspartate (NMDA) receptors with the glutamate antagonist APV was only partially effective in preventing the ischemia-induced calcium changes in CA1. Taken together, these results support the excitotoxin hypothesis but question the rational of treating neuronal injury caused by transient global ischemia exclusively with NMDA antagonists.