The voltage‐dependent conductances of rat neocortical layer I neurons

Whole cell patch-clamp techniques were used to study voltage-dependent sodium (Na⁺), calcium (Ca²⁺), and potassium (K⁺) conductances in acutely isolated neurons from cortical layer I of adult rats. Layer I cells were identified by means of γ-aminobutyric acid (GABA) immunocytochemistry. Positive stainings for the Ca²⁺-binding protein calretinin in a subset of cells, indicated the presence of Cajal–Retzius (C-R) cells. All investigated cells displayed a rather homogeneous profile of voltage-dependent membrane currents. A fast Na⁺ current activated at about −45 mV, was half-maximal steady-state inactivated at −66.6 mV, and recovery from inactivation followed a two-exponential process (τ₁ = 8.4 ms and τ₂ = 858.8 ms). Na⁺ currents declined rapidly with two voltage-dependent time constants, reaching baseline current after some tens of milliseconds. In a subset of cells (< 50%) a constant current level of < 65 pA remained at the end of a 90 ms step. A transient outward current (I_fast) activated ≈–40 mV, declined rapidly with a voltage-insensitive time constant (τ≈ 350 ms) and was relatively insensitive to tetraethylammonium (TEA, 20 mm ). I_fast was separated into two components based on their sensitivity to 4-aminopyridine (4-AP): one was blocked by low concentrations (40 μm ) and a second by high concentrations (6 mm ). After elimination of I_fast by a conditioning prepulse (50 ms to −50 mV), a slow K⁺ current (I_KV) could be studied in isolation. I_KV was only moderately affected by 4-AP (6 mm ), while TEA (20 mm ) blocked most (> 80%) of the current. I_KV activated at about −40 mV, declined monoexponentially in a voltage-dependent manner (τ≈ 850 ms at −30 mV), and revealed an incomplete steady-state inactivation. In addition to I_fast and I_KV, indications of a Ca²⁺-dependent outward current component were found. When Na⁺ currents, I_fast, and I_KV were blocked by tetrodotoxin (TTX, 1 μm ), 4-AP (6 mm ) and TEA (20 mm ) an inward current carried by Ca²⁺ was found. Ca²⁺ currents activated at depolarized potentials at about −30 mV, were completely blocked by 50 μm cadmium (Cd²⁺), were sensitive to verapamil (≈ 40% block by 10 μm ), and were not affected by nickel (50 μm ). During current clamp recordings, isolated layer I neurons displayed fast spiking behaviour with short action potentials (≈ 2 ms, measured at half maximal amplitude) of relative small amplitude (≈ 83 mV, measured from the action potential threshold).