4). Slices were transferred to normal artificial cerebrospinal fluid (ACSF) for at least 1 hr prior to recording. Normal ACSF was similar to the dissection buffer except that sucrose was replaced by 124 mM NaCl, MgCl2 was lowered to 1 mM, and CaCl2 was raised to 2 mM. Visualized dual whole-cell voltage-clamp recordings were made from pairs of FS (PV) INs Caspase-independent apoptosis and pyramidal neurons with glass pipettes filled with 130 mM K-gluconate, 0.2 mM CaCl2, 8 mM NaCl, 2 mM EGTA, 0.5 mM NaGTP, 4 mM MgATP, and 10 mM HEPES (pH 7.2). Only cells with membrane potentials

<−65 mV, series resistance <20 MW, input resistance >100 MW (with <15% variation over the experiment) were studied. Data were filtered at 5 kHz and digitized at 10 kHz using Igor Pro (Wave Metrics). uEPSCs were recorded in voltage clamp in the FS (PV) INs at −70 mV and evoked by suprathreshold somatic current injection (2 ms) in presynaptic pyramidal neurons. uIPSCs were recorded in voltage clamp in pyramidal neurons at 0 mV and evoked by suprathreshold somatic current injection (2 ms)

in presynaptic FS (PV) INs (Jiang et al., 2010). At least 20 responses evoked at 0.1 Hz with paired pulse stimulation (interstimulus interval: 50 ms for Pyr→FS [PV] IN pairs; 100 ms for FS [PV] IN→Pyr pairs) were used to confirm a synaptic connection and to compute the amplitudes of the unitary responses. Mean variance analysis was performed on responses evoked by 15 stimulus trains (5 or 10 stimuli at 50 Hz) ABT888 delivered at 20 s intervals. The uEPSC amplitude was measured for each stimulus, and the mean (I) and variance (Var) were plotted against each other. Synaptic parameters including number of release sites (N) and quantal size (q) were obtained by fitting the data to the parabola: Var = qI − I2/N as previously described (Scheuss and Neher, 2001). We considered only those cases in which the R2 value of the fit was >0.5. In vivo electrophysiology was performed

under isoflurane anesthesia (∼1.5% in 100% O2 via modified nose cone). The dura covering binocular visual cortex was exposed through a hole (∼3 mm diameter) in the skull. The exposed brain was kept moist with artificial cerebral spinal fluid (ACSF), and the room humidity below was supplemented (ZD300Y0, Zenith). Subjects were retained in a stereotaxic device in a darkened room (without visual stimulation) between measurements. Body temperature was maintained at 37°C via circulating water heating pad (T/PUMP; Gaymar Industries), monitored with a rectal probe (BAT-12; Sensortek). A broad-band signal was collected from the lateral aspect (binocular region) of the primary visual cortex (site of largest ipsilateral eye VEP, typically 3.3 mm lateral to the intersection of lambda and the midline), with a tungsten microelectrode (0.5 MΩ) relative to a ground screw in the frontal bone (Figure S3).