Mechanisms of glutamate release elicited in rat cerebrocortical nerve endings by ‘pathologically’ elevated extraterminal K + concentrations

Extracellular [K + ] can increase during some pathological conditions, resulting into excessive glutamate release through multiple mechanisms. We here investigate the overflow of [ 3 H] d ‐aspartate ([ 3 H] d ‐ASP) and of endogenous glutamate elicited by increasing [K + ] from purified rat cerebrocortical synaptosomes. Depolarization with [K + ] ≤ 15 mmol/L provoked [ 3 H] d ‐ASP and glutamate overflows almost totally dependent on external Ca 2+ . Consistent with release by exocytosis, the overflow of [ 3 H] d ‐ASP evoked by 12 mmol/L K + was sensitive to clostridial toxins. The overflows evoked by 35/50 mmol/L K + remained external Ca 2+ ‐dependent by more than 50%. The Ca 2+ ‐independent components of the [ 3 H] d ‐ASP overflows evoked by [K + ] > 15 mmol/L were prevented by the glutamate transporter inhibitors dl ‐ threo ‐beta‐benzyloxyaspartate ( dl ‐TBOA) and dihydrokainate. Differently, the overflows of endogenous glutamate provoked by [K + ] > 15 mmol/L were insensitive to both inhibitors; the external Ca 2+ ‐independent glutamate overflow caused by 50 mmol/L KCl was prevented by bafilomycin, by chelating intraterminal Ca 2+ , by blocking the mitochondrial Na + /Ca 2+ exchanger and, for a small portion, by blocking anion channels. In contrast to purified synaptosomes, the 50 mmol/L K + ‐evoked release of endogenous glutamate or [ 3 H]D‐ASP was inhibited by dl ‐TBOA in crude synaptosomes; moreover, it was external Ca 2+ ‐insensitive and blocked by dl ‐TBOA in purified gliosomes, suggesting that carrier‐mediated release of endogenous glutamate provoked by excessive [K + ] in CNS tissues largely originates from glia.