In layer 4 of the mouse barrel cortex, NMDAR-mediated EPSCs at voltages near the resting potential appear after the first postnatal week. A, Top, Unstained 400-μm-thick tangential slice from a P20 mouse, containing layer 4 of the posteromedial barrel subfield, visualized at low magnification. Bottom, Whole-cell recording from identified layer 4 SpS neuron at higher magnification. B, At P6, whole-cell recording from a layer 4 SpS cell does not show any NMDAR-mediated activity at -70 mV, whereas activity is prominent when the membrane is depolarized to +40 mV. C, At P12, under the same experimental conditions, sEPSCs were prominent at both membrane potentials. D, Representative traces from a P22 SpS neuron demonstrate that, whereas inhibitory transmission was intact (no BMI was added into extracellular solution), APV-sensitive NMDAR-mediated sEPSCs were prominent at -70 mV. The recordings were performed with a CsGlu-containing electrode at -70 mV (EIPSCs). E, Brief pressure application (1-5 ms; 30 psi) of glutamate (0.2 mm) and glycine (0.1 mm) onto an outside-out patch (from the slice in A) elicited a long-lasting barrage of channel activity, which was completely and reversibly blocked by 50 μm APV. In this and all subsequent figures, aCSF contained 10 μm BMI and 20 μm DNQX.
