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 gamma-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 (tau(1) = 8.4 ms and tau(2) = 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 approximate to -40 mV, declined rapidly with a voltage-insensitive time constant (tau approximate to 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 mu M) 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 (tau approximate to 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 mu M), 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 mu M cadmium (Cd2+), were sensitive to verapamil (approximate to 40% block by 10 mu M), and were not affected by nickel (50 mu M). During current clamp recordings, isolated layer I neurons displayed fast spiking behaviour with short action potentials (approximate to 2 ms, measured at half maximal amplitude) of relative small amplitude (approximate to 83 mV, measured from the action potential threshold).