Ie190

Borosilicate glass capillaries (World Precision Instruments TW150F-4) were washed for 6 h with 1 M HCl and rinsed with 70% Ethanol, followed by incubation @ 120 °C overnight. Capillaries were then stored in an airtight container with anhydrous calcium sulfate desiccant until ready for pulling. Tips were created with a Narishige PC-10 pipette puller set to 84.8 and 61.2. Tips were then placed in a glass coplin jar and silanized by applying 50 µL of dicholorodimethylsilane (Tokyo Chemical Industry B2150) below the tips and sealed. The coplin jars were then placed in an oven @ 200 °C for 30 min. Following silanization, tips were visualized under a light microscope and tips manually broken to a size of ~5−10 µM. Tips were then loaded with 2 µL of potassium sensitive ionophore (World Precision Instruments IE190) and backfilled with KCl (100 mM). Microelectrodes were stored with tips down in a solution of KCl (100 mM) for up to ~1 h prior to use.

Borosilicate glass capillary tubes (Warner Instrument Corp.; GC200F-10; OD: 2.0 mm, ID: 1.16 mm) were washed in concentrated nitric acid, water, then 100% EtOH and dried overnight at 200°C. Pipettes were pulled on Sutter Instrument Model P-97 and then placed upright and incubated above a dish containing silane [bis(dimethylamino)dimethylsilane] for 5 min at 200°C. Using a fine-tipped syringe (Microfil 28 AWG, World Precision Instruments), a K⁺ ion exchanger (IE190, World Precision Instruments) was filled at the pipette tip, with care taken to ensure no bubbles. Pipettes were then backfilled with 100 mm KCl. K⁺-sensitive microelectrodes were calibrated with solutions containing in mm: 133 NaCl, 3 KCl, 1.5 CaCl₂, 1.2 NaH₂PO₄, 1 MgCl₂, 10 glucose, and 8.55 Hepes or 106 NaCl, 30 KCl, 1.5 CaCl₂, 1.2 NaH₂PO₄, 1 MgCl₂, 10 glucose, and 8.55 Hepes after every experiment. All experiments were performed using a World Precision Instruments electrometer (model FD 223), digitized with a Digidata 1440A (Molecular Devices), and stored and analyzed using Clampex 10.2. K⁺ responses were recorded, at depths 40–75 μm into the slice, in layer II/III of the cortex resulting from 3 consecutive 1-s stimulations of 100 μA at 20 Hz (0.1-μs pulse width) in layer IV/V of the cortex. Signals were low-pass filtered at 10 Hz.