Slice preparation and recording conditions

Mice were anesthetized with isoflurane and rapidly decapitated to excise brain tissue. Brains were quickly transferred to an ice-cold, oxygenated (95% CO₂/5% O₂ bubbled) cutting solution containing (mm): 30 NaCl, 4.5 KCl, 1 MgCl₂, 26 NaHCO₃, 1.2 NaH₂PO₄, 10 glucose, and 194 sucrose. Coronal brain slices containing the striatum were taken at 280 μm using a VT1200S vibratome (Leica). These sections were placed into a holding chamber held at 32°C filled with oxygenated artificial CSF (aCSF) containing (mm): 124 NaCl, 4.5 KCl, 2 CaCl2, 1 MgCl2, 26 NaHCO3, 1.2 NaH₂PO₄, and 10 glucose. Slices were incubated at 32°C for 1 h before being transferred to room temperature until the time of recording.

Whole-cell voltage and current-clamp recordings were acquired using a Multiclamp 700B amplifier and Digidata 1550B (Molecular Devices). Brain slices, which were held at 32°C and continuously perfused with oxygenated aCSF at a rate of ∼1.5 ml/min, were moved to a recording chamber for recordings. Slices were visualized on a BX51WI microscope (Olympus). MSNs in the DMS and DLS were confirmed by their membrane resistance (80–400 MΩ) and capacitance (100–200 pF), as previously reported (Gertler et al., 2008). Borosilicate glass recording pipettes of 2–4 MΩ were filled with the appropriate internal solutions (see below; adjusted to 295–310 mOsm). All recordings were filtered at 2.2 kHz and digitized at 10 kHz. MSNs were in voltage-clamp mode and held at −60 mV. Data were acquired using Clampex 10 software (Molecular Devices). Series resistance was continuously monitored, and only cells with a stable access resistance (<25 MΩ, and that did not change ≥15%) were included for data analysis.

For recordings of glutamate transmission, excitatory currents were isolated by adding 50 μm picrotoxin to the aCSF.