Fig. 7.

Synaptically activated single NMDA receptor channels of NR2C −/− granule cells are made up exclusively of high-conductance events. A, In the presence of nonsaturating concentrations of the competitive NMDA receptor antagonistd-AP5, the pharmacologically isolated NMDA receptor-mediated mossy fiber-evoked EPSC (i) (macroscopic EPSC recorded before the application of d-AP5) is made up of a flurry of directly resolvable single-channel openings (ii–iv). These channels closely resemble the spontaneously active NMDA receptor channels (B,C, open circles), as well as the channels activated by NMDA in an excised patch from the same cell (B,C, filled circles) and do not appear to express any small-conductance openings. The complete lack of small-conductance openings at any cellular locus argues against a nonspecific effect of localization. The similarity of synaptic and somatic channels lends credence to inferences about synaptic currents based on somatic receptors in preparations in which synaptic receptors are not as accessible.