NMDA receptor currents were recorded using an internal pipette solution containing (in mM): 102 Cs-gluconate, 5 TEA-chloride, 3.7 NaCl, 20 HEPES, 0.3 (Na)GTP, 4 (Mg)ATP, 0.2 EGTA, 10 BAPTA, 5 QX-314 (pH 7.2, ~300 mOsm) under voltage clamp (Vh=+40 mV). To isolate NMDA receptor currents and minimize multisynaptic responses, ACSF in the recording chamber contained 2.5 μM gabazine, 25μM CNQX, 1μM glycine, 4mM CaCl2, and 4mM MgCl2. A stimulating electrode was placed in the middle of the cortical thickness and used to evoke responses of at least 100 pA. Slices were stimulated every 15 seconds until a stable baseline of 30 consecutive responses was obtained. Ifenprodil (3μM; Tocris) was then delivered to the recording chamber. After a new stable amplitude was reached, 30 consecutive responses were recorded. The average peak amplitude was compared before and after Ifenprodil. Baseline sweeps free of noise were averaged and NMDA receptor deactivation kinetics were measured by fitting a double exponential function to the current decay (Igor Pro). For quantification, we calculated the weighted decay constant, τw, as: τw=τf(If/(If+Is))+τs(Is/(If+Is)) 9 (link),52 (link).