Whole-cell voltage clamp recordings were performed on cultured hippocampal neurons as described above using an Axopatch-200b amplifier (Molecular Devices, Sunnyvale, CA). Cells were continuously perfused (1 ml·min−1) with normal ACSF (nACSF) that contained (in mM): 125 NaCl, 2.5 KCl, 2 CaCl2, 1 MgCl2, 5 Hepes and 33 glucose; pH was adjusted to 7.3 using NaOH. Osmolarity was adjusted to 290 mosmol·l−1. For isolating miniature excitatory postsynaptic currents (mEPSCs), gabazine (10 μM), strychnine (3 μM), and tetrodotoxin (0.5 μM) were added to the external buffer to block GABAA receptor, glycine receptor, and Na channel activity, respectively. Patch electrodes were pulled from thin-walled borosilicate glass capillaries (tip resistance ranged from 4–6 MΩ) and filled with internal buffer solution that contained (in mM): 100 cesium methanesulfonate, 25 CsCl, 2 MgCl2, 4 Mg2+-ATP, 0.4 Na-GTP, 10 phosphocreatine, 0.4 EGTA, and 10 Hepes (pH 7.4; 284 mosmol·l−1). All experiments were carried out at room temperature (22 °C). Whole-cell recordings were only established after a high-resistance seal (> 2 GΩ) was achieved. Only cells that had an input resistance of > 150 MΩ and resting membrane potentials < −50 mV were considered for experiments. Resting membrane potentials were measured immediately upon breaking into whole-cell mode by setting the current to 0 pA. Cells were then voltage clamped at a holding potential of -70 mV unless otherwise noted. LTP was induced by switching perfusate to ACSF containing (in mM): 125 NaCl, 2.5 KCl, 2 CaCl2, 5 Hepes, 33 glucose, 0.2 glycine, 0.02 bicuculline, and 0.003 strychnine for 10 min at room temp before returning back to nACSF. Access resistance (Ra) was monitored at the beginning and end of each experiment with small voltage pulses and typically ranged between 10 and 15 MΩ and was not compensated. Cells were rejected from analysis if Ra increased by more than 15% during the course of the experiment or if the input resistance fell below 150 MΩ.