Kinetic properties of a transient outward current in rat neocortical neurons.

1. Whole-cell patch-clamp techniques were used to record outward currents in embryonic rat neocortical neurons maintained in culture. In the presence of tetrodotoxin and cadmium, depolarization evoked an outward current with a complex waveform. This outward current consisted of an initial fast transient component and a late, slowly inactivating component. 2. The two outward current components could be separated pharmacologically with the use of tetraethylammonium (TEA) and 4-aminopyridine (4-AP). TEA (20 mM) applied extracellularly completely blocked the late component, unmasking a fast transient outward current (TOC). 4-AP (5 mM) applied extracellularly blocked the early component while reducing the late component by 27.8 +/- 9.7% (mean +/- SE). 3. The TOC activated after a short delay and rose rapidly to a peak. The time to peak was voltage dependent and decreased with depolarization. In the presence of 200 microM extracellular cadmium, activation threshold was around -25 mV, and current amplitude increased with depolarization. The voltage-conductance relationship was well fitted by the use of the Boltzmann equation with a Vm of +19 mV for half activation and a slope factor of +6 mV. 4. On sustained depolarization the TOC rapidly inactivated and decayed to baseline within 500-600 ms. The decay phase followed a single exponential time course with a time constant of 55-65 ms. The decay time was most rapid at potentials from +5 to +20 mV and increased slightly with further depolarization. 5. Steady-state inactivation of the TOC, in the presence of cadmium, was complete near -10 mV and was totally relieved at potentials more negative than -75 mV. With the use of the Boltzmann equation, a Vm of -34 mV for half inactivation and a slope factor of -8.6 mV were found. 6. Recovery of the TOC from steady-state inactivation followed a single exponential time course and was voltage dependent. When the membrane potential was held at -84 mV during the conditioning pulse, the time constant of recovery was 17 ms, increasing to 45.2 and 58.1 ms at holding potentials of -64 and -44 mV, respectively. Holding at potentials more negative than -84 mV produced no further change in the recovery time course. 7. The presence of 200 microM external cadmium altered the TOC activation and inactivation curves. Removal of cadmium produced a -16-mV shift in the Vm for half activation and a -25-mV shift in the inactivation curve. This sensitivity to cadmium is higher than that reported in other systems.(ABSTRACT TRUNCATED AT 400 WORDS)