MK-801 blocks NMDA receptor-mediated EPSCs in PCC/RSC interneurons. A, Current-clamp recording of a PCC/RSC layer IV pyramidal cell in response to depolarizing current injection. The firing pattern of this pyramidal cell shows slow action potentials and frequency adaptation, characteristics typical for pyramidal cells. B, Current-clamp recording of a PCC/RSC layer III interneuron in response to depolarizing current injection. This interneuron fires rapidly and lacks frequency adaptation, characteristics typical for interneurons. Calibration:A, B, 200 msec, 20 mV, 80 pA.C, At the holding potential of −70 mV and in the presence of BMI (20 μm), DNQX-sensitive fast inward EPSCs are dominant in the interneuron. Calibration: 500 msec, 50 pA.D, When this interneuron was voltage-clamped at +50 mV and in the presence of BMI (20 μm), the long slow EPSCs were dominant. Calibration: 1000 msec, 100 pA. E, The slow EPSCs were blocked after administration of MK-801 (40 μm). Calibration: 1000 msec, 50 pA.