Properties of voltage-gated potassium currents in nucleated patches from large layer 5 cortical pyramidal neurons of the rat

1. Voltage-gated potassium currents were studied in nucleated outside-out patches obtained from large layer 5 pyramidal neurons in acute slices of sensorimotor cortex from 13- to 15-day-old Wistar rats (22-25°C). 2. Two main types of current were found, an A-current (I(A)) and a delayed rectifier current (I(K)), which were blocked by 4-aminopyridine (5 mM) and tetraethylammonium (30 mM), respectively. 3. Recovery from inactivation was mono-exponential (for I(A)) or bi-exponential (for I(K)) and strongly voltage dependent. Both I(A) and I(K) could be almost fully inactivated by depolarising prepulses of sufficient duration. Steady-state inactivation curves were well fitted by the Boltzmann equation with half-maximal voltage (V( 1/2 )) and slope factor (k) values of -81.6 mV and -6.7 mV for I(A), and -66.6 mV and -9.2 mV for I(K). Peak activation curves were described by the Boltzmann equation with V( 1/2 ) and k values of -18.8 mV and 16.6 mV for I(A), and -9.6 mV and 13.2 mV for I(K). 4. I(A) inactivated mono-exponentially during a depolarising test pulse, with a time constant (~ 7 ms) that was weakly dependent on membrane potential. I(K) inactivated bi-exponentially with time constants (~ 460 ms, ~ 4.2 s) that were also weakly voltage dependent. The time to peak of both I(A) and I(K) depended strongly on membrane potential. The kinetics of I(A) and I(K) were described by a Hodgkin-Huxley-style equation of the form m(N)h, where N was 3 for I(A) and 1 for I(K). 5. These results provide a basis for understanding the role of voltage-gated potassium currents in the firing properties of large layer 5 pyramidal neurons of the rat neocortex.