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Evidence against High Extracellular Glutamate Promoting the Elicitation of Spreading Depression by Potassium

This study ascertains whether high extracellular glutamate contributes to the initiation of spreading depression (SD) by K+. Two microdialysis probes, each incorporating an electrode to record the extracellular direct current (DC) potential at the elicitation site, were implanted symmetrically in the cortex of anesthetized rats. Recurrent SD was triggered by perfusion of 130 mM K+ through the microdialysis probe for 20 min. On one side, this medium was supplemented with increasing concentrations of glutamate (0.1–1 mM) or of the selective glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylate (L-trans-PDC; 1–10 mM). The effects of L-trans-PDC on extracellular glutamate and basal DC potential were studied in separate experiments. Application of K+ for 20 min consistently elicited five to seven waves of SD. Increasing the concentration of glutamate in the perfusion medium did not alter SD elicitation. Application of L-trans-PDC concentration dependently increased the dialysate levels of glutamate (by ∼ 19-fold with 10 mM L-trans-PDC) but, unexpectedly, reduced SD elicitation. These data do not support the hypothesis that SD is elicited because high extracellular glutamate resulting from exocytosis and/or reversal of glutamate uptake depolarizes adjacent neurons. As SD elicitation requires activation of N-methyl-D-aspartate (NMDA) receptors, these results also illustrate that sensitivity of a pathological or experimental event to NMDA receptor antagonists does not necessarily imply involvement of increased extracellular glutamate. This does not rule out a selective action of glutamate, transiently released from presynaptic vesicles, on immediately juxtaposed postsynaptic receptors.