Seventy-three Sprague–Dawley rats of either gender, aged PD15–115, were used in this study. The rats were divided into juvenile (PD15−28), adolescent (PD31−63) and adult (PD86−115) groups as previously reported (Spear, 2000; Tseng & O’Donnell, 2007; Wang & Gao, 2009). The rats were cared for according to National Institutes of Health guidelines, and the experimental protocol was approved by the Institutional Animal Care and Use Committee at Drexel University College of Medicine. The experiments also complied with the policies and regulations of ethical matters in The Journal of Physiology (Drummond, 2009). The detailed procedure can be found in our previous studies (Gao & Goldman-Rakic, 2003; Gao et al. 2003; Gao, 2007; Wang & Gao, 2009). The rats were deeply anaesthetized with Euthasol (0.2 ml kg−1, i.p.), rapidly perfused with ice-cold (< 4°C) sucrose solution containing (in mm): KCl 2.5, NaH2PO4 1.25, NaHCO3 26, CaCl2 0.5, MgSO4 7.0, and sucrose 213, and aerated with 95% O2 and 5% CO2. The rats were decapitated with a guillotine; the brains were quickly removed and placed in the same sucrose solution. Horizontal brain slices at 300 μm were made with a Vibratome (Vibratome Co., St Louis, MO, USA), and the slices were incubated in an oxygenated sucrose solution at 35°C for 1 h. The slices were incubated at room temperature until being transferred into a submerged recording chamber. The recordings were conducted with cortical slices perfused with Ringer solution containing the following ingredients (in mm): NaCl 128, KCl 2.5, NaH2PO4 1.25, CaCl2 2, MgSO4 1, NaHCO3 26, and dextrose 10, pH 7.4. Whole-cell patch clamp recordings were conducted in the PFC slices through an upright microscope (Olympus BX61, Olympus Optics, Japan) equipped with infrared differential interference contrast optics (IR-DIC). The recordings were conducted at ∼36°C. Resistance of the recording pipette (1.2 mm borosilicate glass) was ∼9 MΩ. Tips of the recording pipettes were first filled with a potassium gluconate-based intracellular solution (∼1 mm from the tip) and then backfilled with a Cs+-containing solution. The potassium gluconate solution contained (in mm): potassium gluconate 120, KCl 6, ATP-Mg 4, Na2GTP 0.3, EGTA 0.1, Hepes 10, and 0.3% biocytin, pH 7.3, 310 mosmol L−1; the Cs+ solution contained (in mm): caesium gluconate 120, QX-314 chloride 5, CsCl2 6, ATP-Mg 1, Na2GTP 0.2, Hepes 10, spermine 0.05, and 0.3% biocytin at pH 7.3 (adjusted with CsOH). With this strategy, we were able to record the action potentials immediately (usually within 1 min) after forming a giga-seal due to the presence of a K+ internal solution at the tip of the patch pipette. Then we could record AMPAR- and NMDAR-mediated currents with minimal K+ current contamination due to the delayed diffusion (∼5 min) of Cs+ ions into the recorded cell (Wang & Gao, 2009). The excitatory postsynaptic currents (EPSCs) were evoked by a bipolar electrode placed about 300 μm away from the recorded neurons (0.1 ms, 40–400 μA, 0.1 Hz) in the presence of the GABAA antagonist picrotoxin (50 μm, Sigma-Aldrich, St Louis, MO, USA). The AMPA receptor-mediated AMPA EPSCs were recorded at −60 mV for the paired-pulse stimulation, whereas the inwardly rectifying AMPA EPSCs were recorded at −60, 0 and +60 mV to calculate the RI (see ‘Data analysis’) in the presence of picrotoxin and NMDA receptor antagonist d-(−)-2-amino-5-phosphonopentanoic acid (d-APV, 50 μm, Sigma-Aldrich). The I–V relationships of AMPA EPSCs were recorded 10 min after break-in to allow sufficient time for diffusion of spermine, and measurements were made from the averages of 15 responses evoked by intracortical stimulation with membrane potentials held at 20 mV steps from −80 mV to +80 mV. The NMDA receptor-mediated currents (NMDA EPSCs) were recorded at +60 mV under conditions of bath-applied picrotoxin and the AMPA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (NBQX) (20 μm, Sigma-Aldrich). NMDA EPSCs were confirmed by bath application of d-APV (50 μm) in some cases. To record NMDA receptor-mediated miniature EPSCs (mEPSCs), membrane potentials of FS interneurons were held either at −60 mV in Mg2+-free external solution with bath perfusion of the sodium channel blocker tetrodotoxin (TTX, 0.5 μm) and picrotoxin (100 μm), as described previously (Myme et al. 2003) or at +60 mV in the presence of picrotoxin, NBQX and TTX. The access resistance ranged from 18 to 30 MΩ, and the series resistances were constantly monitored through a test hyperpolarizing pulse (5 mV, 200 ms) applied in each sweep and were compensated at regular intervals throughout the recordings. The electric signals were recorded using MultiClamp 700B (Molecular Devices) and acquired at sampling intervals of 20–50 μs through pCLAMP 9.2 software (Molecular Devices).