Organoids were embedded in a 4% agarose block. Slices (250 μm) were cut in chilled oxygenated (95% O2/5% CO2) sucrose solution containing (in mM: 85 NaCl, 75 sucrose, 2.5 KCl, 25 glucose, 1.25 NaH2PO4, 4 MgCl2, 0.5 CaCl2, and 24 NaHCO3) using Vibratome slicer (Leica 1200S). After 1 h incubation in oxygenated sucrose solution at 33°C, electrophysiological recordings were obtained using artificial cerebrospinal fluid (ACSF) containing (in mM: 126 NaCl, 2.5 KCl, 26 NaHCO3, 2 CaCl2, 2 MgCl2, 1.25 NaH2PO4, and 10 glucose). Individual cells were visualized with an upright microscope (Nikon Eclipse FN1) with infrared differential interference contrast optics with 60× objective. Whole-cell patch-clamp recordings were obtained from individual cells with borosilicate glass pipettes (5–7 MΩ) when filled with internal solution containing 126 mM K-gluconate, 4 mM KCl, 10 mM HEPES, 4 mM ATP-Mg, 0.3 mM GTP-Na, 10 mM phosphocreatine, and 0.5% biocytin with a pH 7.2, and osmolality of 290 mOsm. During recordings, slices were continuously perfused with oxygenated (95% O2/5% CO2) ACSF at the rate of 2.5 ml/min. Recordings were obtained using MultiClamp700B amplifiers (Molecular Devices), filtered
