The rAAV-SYP1-miniSOG-Citrine titer was measured by quantitative PCR to be 6.6 × 1013 genome copy (GC)/ml (Salk Vector Core). The rAAV-SYP1-miniSOG-T2A-mCherry titer was estimated to be 2.3 × 1013 GC/ml with Quant-IT picogreen dsDNA dye (Life Technologies). Sindbis BGB324 nmr virus containing the tdTomato transgene is produced as described previously ( Malinow et al., 2010). In brief, BHK cells were electroporated with RNAs transcribed from pSinRep5-tdTomato and DH(26S) plasmids. The media was collected

40 hr later and centrifuged to obtain the concentrated virus. Hippocampal microisland cultures were made by a protocol modified from Bekkers (2005). In brief, a collagen (0.5mg/ml, Affymetrix)/poly-D-lysine (0.1mg/ml) mixture was sprayed onto the glass surface of glass bottom dishes (MatTek) with an atomizer. Hippocampal and cortical neurons were extracted from P2 Sprague-Dawley rat pups with papain digestion and mechanical trituration. Hippocampal neurons were transfected by electroporation (Lonza) and plated

at 1.5–3 × 104 cells per dish. Cortical Epigenetic inhibitor in vivo neurons were plated on poly-D-lysine coated dish and infected with rAAV three days after plating. The procedures of extracting cultured neurons and organotypic slices (below) from rat pups were approved by the UCSD Institutional Animal Care and Use Committee. Cultured hippocampal neurons were placed on an Olympus IX71 microscope with 20× air phase contrast objective for the recording (Olympus). Illumination (9.8 mW/mm2) from a xenon arc lamp (Opti-quip) was filtered through a 480/40 nm filter and reflected to the specimen with a full-reflective no mirror (Chroma). Illumination was controlled with a mechanical shutter (Sutter Instrument). Recordings were performed with an Axopatch 200B patch amplifier, Digidata 1332A digitizer, and pCLAMP 9.2 software (Molecular Devices). EPSCs were evoked with a 2 ms voltage step from −60 mV to 0 mV at 0.2 Hz. Illumination

was initiated after 1.5 min of stable baseline (changes <10%) of EPSC amplitude. One hundred percent response for each cell was the mean EPSC amplitude of the 1 min prior to light illumination and the amplitudes of each EPSC were normalized to this 100% response. Reduction of EPSC amplitudes was measured as the mean amplitudes of 6 EPSCs (25 s) after light illumination. Only cells with series resistance <10 MΩ and changes of series resistance <20% after light illumination were analyzed. The external solution contained 118 mM NaCl, 3 mM KCl, 2 mM CaCl2, 1 mM MgCl2, 10 mM HEPES, and 20 mM glucose (pH 7.35, 315 mOsm). The intracellular pipette solution contained 110 mM K-gluconate, 30 mM KCl, 5 mM NaCl, 2 mM MgCl2, 0.1 mM CaCl2, 2 mM MgATP, 0.3 mM TrisGTP, and 10 mM HEPES (pH 7.25, 285 mOsm). Cortical neurons were recorded with intracellular solution containing 110 mM Cs methanesulfonate, 30 mM tetraethylammonium chloride, 10 mM EGTA, 10 mM HEPES, 1 mM CaCl2, 1 mM MgCl2, 2 mM Mg-ATP, 0.