Nonhomogeneous distribution of subtypes of NMDARs among individual synapses, as revealed by voltage dependence of mEPSCs. A, Left, Cumulative distribution of time intervals between NMDAR-mediated mEPSCs recorded at -70 mV (dashed line) and +40 mV (continuous line) in the presence of 2 mm Mg²⁺. Note that at -70 mV, the intervals between detectable mEPSCs are significantly longer. Inset, Sample current traces at -70 and +40 mV. Right, After Mg²⁺ wash out, mEPSC frequency at -70 mV increased, such that cumulative distribution of intervals between NMDAR-mediated mEPSCs at the two potentials did not differ. B, The voltage dependence of frequency of NMDAR-mediated mEPSCs. The bar graphs show mean frequency at -70 and +40 mV recorded in the presence of 2 mm Mg²⁺ (filled bars) and in Mg²⁺-free solution (open bars). C, In a different set of experiments, after 3 h of incubation in the presence of the open-channel NMDAR blocker MK-801 (40 μm), mEPSCs were completely blocked at -70 mV (data not shown), and mEPSC frequency at +40 mV was significantly decreased.