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 neuro nes, 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 compl etely 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 b locked by TEA. 3. The activation kinetics of the slow K+ current were described by a secon d 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 inactivatio n 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+ chan nel corresponded to the two inactivation time constants of the slow K+ curr ent observed in nucleated patches. This suggested that two distinct K+ chan nels mediated the slow K+ current in nucleated patches. 6. The three subtypes of K+ channels that were observed in somatic recordin gs were present along the apical dendrite. The amplitude of ensemble K+ cur rents in cell-attached patches decreased along the apical dendrite as the d istance from the soma increased, with a slope of -0.9 +/- 0.3 pA per 100 mu m. 7. The results suggest that the decrease of the voltage-gated K+ channel de nsity from the soma along the apical dendrite of L5 pyramidal neurones help s to define a distal, low threshold region for the initiation of dendritic regenerative potentials.