MK-801 effectively blocks synaptic NMDA receptors during bicuculline induced bursting. (A) The MK-801 protocol used to selectively block synaptic NMDA receptors is shown schematically (see Methods). NMDA receptor-mediated EPSCs were evoked by single stimuli (arrows) and measured in voltage clamp mode (VC) at +40 mV in the presence of 10 μM CNQX, 50 μM bicuculline (bic) and 10 μM glycine (gly). Following CNQX washout, bursting returned as was recorded in current clamp mode (IC) before 10 μM MK801 was applied for 4 to 8 bursts (42 to 188 spikes, 2.2 to 6.2 min) to selectively block synaptic NMDA receptors (synaptic blockade). Bursting was then silenced in CNQX in the absence of glycine to prevent unblocking of NMDA receptors. EPSCs were recorded under the same stimulation parameters. (B) Representative recording from the IC component of the protocol. (C) NMDA receptor-mediated EPSCs recorded before (black trace) and after (grey trace) the MK801 protocol recording shown in B. (D) Cumulative data from 5 neurons showing the EPSC amplitude before (control) and after the MK-801 protocol (MK-801). Bars represent the mean and whiskers the SEM.