Excitatory amino acid release from astrocytes during energy failure by reversal of sodium-dependent uptake

Non-synaptic release may be the major route of excitatory amino acid (EAA) efflux during cerebral ischemia. Possible routes of non-synaptic release include non-specific anion channels, reversal of Na(+)-, Cl(-)-, or Ca(2+)-dependent uptake, and cell lysis. In the present study we employ a novel approach to show reversal of Na(+)-dependent uptake as a major route of EAA efflux from astrocyte cultures under conditions of energy failure. Primary rat astrocyte cultures were subjected to combined blockade of glycolytic and oxidative metabolism after incubation with [3H]-D-aspartate (D-ASP). Energy failure produced an efflux of D-ASP that was maximal by 90 minutes. The efflux over this period was reduced by more than 50% in cells that had been pre-loaded with PDC (L-transpyrrolidine-2,4-dicarboxylic acid) or TBHA (threo-beta-hydroxyaspartic acid), compounds that are competitive inhibitors of Na(+)-dependent glutamate uptake. The effect of pre-loading with the inhibitors was concentration dependent. No effect was seen if the inhibitors were added after induction of energy failure, suggesting that the attenuation of D-ASP efflux resulted from binding of the inhibitors to an intracellular site. These results provide strong evidence that EAA efflux from astrocytes under conditions of energy failure occurs largely through reversal of Na(+)-dependent uptake.