Voltage-gated K⁺ channels in layer 5 neocortical pyramidal neurones from young rats: Subtypes and gradients

1. We investigated the types and distribution of voltage-gated K⁺ channels in the soma and apical dendrite of layer 5 (L5) neocortical pyramidal neurones, of young rats (postnatal days 13-15), in acute brain slices. 2. A slow inactivating outward K⁺ current and a fast inactivating outward K⁺ current were detected in nucleated patches. The slow K⁺ current was completely blocked by tetraethylammonium (TEA) with an IC₅₀ of 5 ± 1 mM (mean ± S.E.M.) and was partially blocked by 4-aminopyridine (4-AP). The fast K⁺ current was blocked by 4-AP with an IC₅₀ of 4.2 ± 0.5 mM, but was not blocked by TEA. 3. The activation kinetics of the slow K⁺ current were described by a second order Hodgkin-Huxley model. The slow K⁺ current displayed bi-exponential inactivation. A fourth order Hodgkin-Huxley model for activation and first order for inactivation described the kinetics of the fast K⁺ current. 4. In somatic cell-attached recordings, three classes of single K⁺ channels could be differentiated based on their unitary conductance and inactivation kinetics, a fast inactivating channel having a conductance of 13 ± 1 pS, a slow inactivating channel having a conductance of 9.5 ± 0.5 pS, and a very slowly inactivating channel having a conductance of 16 ± 1 pS. 5. The inactivation time constants of the slow and of the very slow K⁺ channel corresponded to the two inactivation time constants of the slow K⁺ current observed in nucleated patches. This suggested that two distinct K⁺ channels mediated the slow K⁺ current in nucleated patches. 6. The three subtypes of K⁺ channels that were observed in somatic recordings were present along the apical dendrite. The amplitude of ensemble K⁺ currents in cell-attached patches decreased along the apical dendrite as the distance from the soma increased, with a slope of -0.9 ± 0.3 pA per 100 μm. 7. The results suggest that the decrease of the voltage-gated K⁺ channel density from the soma along the apical dendrite of L5 pyramidal neurones helps to define a distal, low threshold region for the initiation of dendritic regenerative potentials.