Figure 1. Increasing cytosolic glutamate levels in cone terminals enhanced the amplitude of excitatory postsynaptic currents (EPSCs) recorded from postsynaptic horizontal cells. A: Examples of horizontal cell EPSCs recorded the first minute (thin black trace) after obtaining whole-cell configuration, and 8 min later (dashed gray trace) when using a pipette solution containing 40 mM glutamate. EPSCs were evoked by depolarizing the cone from −70 mV to −10 mV for 100 ms. B: EPSCs recorded every minute with 40 mM glutamate (open squares, n=8) in the patch pipette are much larger than EPSCs recorded without glutamate (filled squares, n=8) in the patch pipette. Use of 10 mM glutamate produced less enhancement (open circles, n=7). For three of the cell pairs with 10 mM glutamate, measurements were made for only 10 min. C: EPSCs recorded with 40 mM glutamate (open squares) were larger than EPSCs with 40 mM glutamate plus 0.5 mM 1S,3R-1-aminocyclopentane-1,3-dicarboxylate (open triangles, n=4), a potent vesicular glutamate transport inhibitor. D: Substituting 40 mM alpha ketoglutarate (open gray circles, n=4) for glutamate did not enhance the EPSC amplitude above that of EPSCs recorded without glutamate (filled squares). Substituting 40 mM glutamine (n=8, filled diamonds) for glutamate slightly enhanced the EPSCs. Error bars show SEM.