Slice cultures were prepared as described73, with small modifications. Briefly, neonatal mice (7–9 d) were anesthetized with halothane in an induction chamber and decapitated. Brains were quickly removed and hippocampi dissected into ice-cold and oxygenated (95% O2/5% CO2) cutting medium: Minimal Essential Media (MEM) supplemented with 25 mM HEPES, 10 mM Tris-base, 10 mM glucose, and 3 mM MgCl2 (pH 7.2). Transverse hippocampal slices (400 μm) were cut using a tissue slicer (Stoelting Co.; Wood Dale, IL). Slices were placed onto transwell membrane inserts (Corning, Inc.; Corning, NY) in 6-well plates; each well was pre-filled with 1.6 mL of a 2:1 mixture of Basal Medium Eagle (Sigma) and Earle’s Balanced Salt Solution (Sigma) supplemented with (in mM): 20 NaCl, 5 NaHCO3, 0.2 CaCl2, 1.7 MgSO4 48 glucose, 26.7 HEPES, penicillin (40,000 U/L) (Thermo Fisher Scientific), streptomycin (40 mg/L) (Thermo Fisher Scientific), insulin (1 mg/L) (Sigma-Aldrich), 0.5 ascorbic acid and 5% horse serum (Thermo Fisher Scientific) (pH 7.2). Slices were maintained in an incubator at 34 °C/5% CO2, and were fed 3 times per week by replacing half of the growth
