RAPID CA2-PERMEABLE AMPA( ENTRY THROUGH CA2+)KAINATE CHANNELS TRIGGERS MARKED INTRACELLULAR CA2+ RISES AND CONSEQUENT OXYGEN RADICAL PRODUCTION/

The widespread neuronal injury that results after brief activation of highly Ca2+-permeable NMDA channels may, in large part, reflect mitoch ondrial Ca2+ overload and the consequent production of injurious oxyge n radicals. In contrast, AMPA/kainate receptor activation generally ca uses slower toxicity, and most studies have not found evidence of comp arable oxygen radical production. Subsets of central neurons, composed mainly of GABAergic inhibitory interneurons, express AMPA/kainate cha nnels that are directly permeable to Ca2+ ions. Microfluorometric tech niques were performed by using the oxidationsensitive dye hydroethidin e (HEt) to determine whether the relatively rapid Ca2+ flux through AM PA/kainate channels expressed on GABAergic neurons results in oxygen r adical production comparable to that triggered by NMDA. Consistent wit h previous studies, NMDA exposures triggered increases in fluorescence in most cultured cortical neurons, whereas high K+ (50 mM) exposures (causing depolarization-induced Ca2+ influx through voltage-sensitive Ca2+ channels) caused little fluorescence change. In contrast, kainate exposure caused fluorescence increases in a distinct subpopulation of neurons; immunostaining for glutamate decarboxylase revealed the resp onding neurons to constitute mainly the GABAergic population. The effe ct of NMDA, kainate, and high K+ exposures on oxygen radical productio n paralleled the effect of these exposures on intracellular Ca2+ level s when they were monitored with the low-affinity Ca2+-sensitive dye fu ra-2FF, but not with the high-affinity dye fura-2. Inhibition of mitoc hondrial electron transport with CN- or rotenone almost completely blo cked kainate-triggered oxygen radical production. Furthermore, antioxi dants attenuated neuronal injury resulting from brief exposures of NMD A or kainate. Thus, as with NMDA receptor activation, rapid Ca2+ influ x through Ca2+-permeable AMPA/kainate channels also may result in mito chondrial Ca2+ overload and consequent injurious oxygen radical produc tion.