Slice preparation
All housing of animals and procedures were approved by the University of Michigan Institutional Animal Care and Use Committee. Multiple mouse lines were used in this study, including PV-IRES-Cre (Jackson Laboratories, 008069), CaMKII-Cre (Jackson Laboratories, 005359), Ai32 (Jackson Laboratories, 024109), Ai14 (Jackson Laboratories, 007914), PV-IRES-Cre x Ai14 (crossed in house), PV-IRES-Cre x Ai32 (crossed in house), CaMKII-Cre x Ai32 (crossed in house), and NTSR1-Cre (MMRRC, 030648-UCD). A total of 167 recordings are included in this study from the following mouse lines: PV-IRES-Cre (55), CaMKII-Cre (15), Ai32 (3), PV-IRES-Cre x Ai14 (3), PV-IRES-Cre x Ai32 (64), CaMKII-Cre x Ai32 (20), and NTSR1-Cre (7). Mice of both sexes between the ages of P21-31 and P62-63 were included in the experiments. No differences in cell type properties were observed across mouse lines, while both age and sex were explicitly analyzed in terms of their relationship to cell type properties (see results).
Mice underwent deep isoflurane anesthesia before decapitation. Brains were removed within one minute of decapitation and placed in an ice-cold high-sucrose slicing solution that had been saturated with carbogen gas for at least 30 minutes prior to use. Coronal slices (300um) were cut using a Leica 1200 VT vibratome. Slices were allowed to rest in the slicing solution for about 2 minutes before being placed in a carbogen-saturated high-magnesium artificial CSF (ACSF) solution to incubate at body temperature (32°C) for 20 minutes. The entire bubbling bath was then removed from the heater, allowing the slices to gradually cool to room temperature. Slices rested an additional 30 minutes at room temperature before use.
Slices were submerged in a recording chamber with a constant flow of ACSF containing 126 mM NaCl, 1.25 mM NaH2PO4, 26 mM NaHCO3, 3 mM KCl, 10 mM dextrose, 1.20 mM CaCL2, and 1 mM MgSO4. Recordings were done between 29-31°C with an ACSF flow rate of 2 mL per minute. All recordings were done within 8 hours of slicing to ensure reputable health of the cells. Patch pipettes with 2-3 um diameter and resistances of 3-6 MΩ were filled with a potassium gluconate internal solution containing 130 mM K-gluconate, 2 mM NaCl, 4 mM KCl, 10 mM HEPES, 0.2 mM EGTA, 0.3 mM GTP-Tris, 14 mM phosphocreatine-Tris, and 4 mM ATP-Mg (pH 7.25, ~290 mOsm).