Figure 4.

The source of glutamate that accumulates during transporter blockade is nonvesicular. (A1) Left and Right traces: same cell. Cd²⁺ (200 μM) does not significantly modify extracellular glutamate accumulation during inhibition of uptake with TBOA (200 μM). (A1, B1, C, and D) Representative current traces. (A2) Cd²⁺ abolishes voltage-gated Ca²⁺ currents. (B) BoNT A (100 ng/ml), which blocks vesicular release of glutamate (B2, AMPA receptor-mediated miniature excitatory postsynaptic currents at −60 mV, same two cells as B1), does not alter responses to TBOA (C) Left and Right traces: same cell. The anion channel inhibitor SITS (2 mM) does not prevent extracellular glutamate accumulation after transporter blockade. In B1 and C, upward deflections represent truncated responses to brief focal application of NMDA (500 μM in puffer pipette, 40 ms). (D) Inhibition of glutamine synthase with MSO increases the extent of extracellular glutamate accumulation on inhibition of uptake. Note the scale for the MSO recording. (E) Pooled results for the responses to Cd²⁺, BoNT A or TeNT, SITS or NPPB, and MSO.