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_½) 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_½ 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^Nh, 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.