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

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Abstract
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 (IA) and a delayed rectifier current (IK), which were blocked by 4‐aminopyridine (5 mm) and tetraethylammonium (30 mm), respectively. 3 Recovery from inactivation was mono‐exponential (for IA) or bi‐exponential (for IK) and strongly voltage dependent. Both IA and IK 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 (V½) and slope factor (k) values of −81.6 mV and −6.7 mV for IA, and −66.6 mV and −9.2 mV for IK. Peak activation curves were described by the Boltzmann equation with V½ and k values of −18.8 mV and 16.6 mV for IA, and −9.6 mV and 13.2 mV for IK. 4 I A inactivated mono‐exponentially during a depolarising test pulse, with a time constant (∼7 ms) that was weakly dependent on membrane potential. IK inactivated bi‐exponentially with time constants (∼460 ms, ∼4.2 s) that were also weakly voltage dependent. The time to peak of both IA and IK depended strongly on membrane potential. The kinetics of IA and IK were described by a Hodgkin‐Huxley‐style equation of the form mNh, where N was 3 for IA and 1 for IK. 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.