LAYER-I NEURONS OF THE RAT NEOCORTEX .2. VOLTAGE-DEPENDENT OUTWARD CURRENTS

1. Whole cell patch-clamp techniques, combined with direct visualizati on of neurons, were used to study voltage-dependent potassium currents in layer I neurons and layer II/III pyramidal cells. 2. In the presen ce of tetrodotoxin, step depolarizations evoked an outward current. Th is current had a complex waveform and appeared to be a composite of ea rly and late components. The early peak of the composite K+ outward cu rrent was larger in layer I neurons. 3. In both layer I and pyramidal cells, the composite outward K+ current could be separated into two co mponents based on kinetic and pharmacological properties. The early co mponent was termed I-(A) because it was a transient outward current ac tivating rapidly and then decaying. I-(A) was more sensitive to blocki ng by 4-aminopyridine (4-AP) than tetraethylammonium (TEA). The second component, termed the delayed rectifier or I-(DR), activated relative ly slowly and did not decay significantly during a 200-ms test pulse. I-(DR) was insensitive to blocking by 4-AP at concentrations up to 4mM and blocked by >60% by 40-60mM TEA. 4. I-(A) kinetics were examined i n the presence of 40-60mM TEA. Under these conditions, I-(A) began to activate between -40 and -30mV. Half-maximal activation occurred aroun d 0mV. In both layer I and pyramidal cells, the half-inactivation pote ntial (V-h-inact) was around or more positive than -50mV. At -60mV, >7 0% of I-(A) conductance was available. I-(A) decayed along a single ex ponential time course with a time constant of similar to 15ms. This de cay showed little voltage dependence. 5. In both layer I and pyramidal cells, I-(DR) was studied in the presence of 4mM 4-AP to block I-(A) and in saline containing 0.2mM Ca2+ and 3.6mM Mg2+ to reduce contribut ions from Ca2+-dependent K+ currents. Under these conditions, I-(DR) b egan to activate at -35 to -25mV with V-h-act of 3.6 plus or minus 4.5 mV (mean plus or minus SD). The 10-90% rise time of I-(DR) was 15ms at 30mV. At 2.2ms after the onset of the command potential to +30mV, I-( DR) could reach a significant amplitude (similar to 1.5nA in layer I n eurons and 2.2nA in pyramidal cells depending on the cell size). When long test pulses (greater than or equal to 1000ms) were used, a decay time constant similar to 800ms at +40mV was observed. In both layer I and pyramidal cells, steady state inactivation of I-(DR) was minimal. 6. These results indicate that I-(A) and I-(DR) are the two major hype rpolarizing currents in layer I and pyramidal cells. The kinetics and pharmacological properties of I-(A) and I-(DR) were not significantly different in fast-spiking layer I neurons and regular-spiking layer II /III pyramidal cells. The relatively positive activation threshold (mo re than or equal to -40mV) of both I-(A) and I-(DR) suggest that they do not play a role in neuronal behavior below action potential (AP) th reshold and that their properties are more suitable to repolarize AP. The greater density of I-A in layer I neurons appears responsible for fast spike generation.