Excitatory Amino Acid‐Induced Toxicity in Chick Retina: Amino Acid Release, Histology, and Effects of Chloride Channel Blockers

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Abstract
Acute excitotoxicity in embryonic chick retina and the ability of Cl- channel blockers to prevent toxicity were evaluated by measurement of endogenous amino acid release and histology. Treatment of retinal with kainate, quisqualate, or N-methyl-D-aspartate resulted in a large dose-dependent release of .gamma.-aminobutyric acid and taurine, moderate release of glutamine and alanine, and no measureable release of glutamate or aspartate. Concentrations inducing maximal .gamma.-aminobutyric acid release were 50 .mu.M quisqualate, 100 .mu.M kainate, and 100 .mu.M N-methyl-D-aspartate. Treatment with 1 mM glutamate resulted in significant .gamma.-aminobutyric acid release, as well as an elevation in medium aspartate levels. Typical excitotoxic retinal lesions were produced by the agonists and, at the lower concentrations tested, revealed a regional sensitivity. There was a positive correlation between the amount of .gamma.-aminobutyric acid release and the extent of tissue swelling, suggesting that release may be secondary to toxic cellular events. Omission of Cl- completely blocked cytotoxic effects due to kainate or glutamate. Likewise, addition of the Cl-/bicarbonate anion channel blocker 4,4''-diisothiocyanatostilbene-2,2''-disulfonate at 600 .mu.M protected retina from cytotoxic damage from all excitotoxic analogs and restored amino acid levels to baseline values. Furosemide, which blocks Na+/K+/2Cl- cotransport, was only minimally effective in reducing amino acid release induced by the agonists. Consistent with the latter, histological examination showed the continued presence of the lesion but with general reduction of cellular edema. These results indicate that although influx of Cl- is a central component of the acute excitotoxic phenomenon, mechanisms other than passive Cl- flux may be involved.