Functional and molecular analysis of transient voltage-dependent K+ currents in rat hippocampal granule cells

1. We have investigated voltage-dependent outward K+ currents of dentate gr anule cells (DGCs) in acute brain slices from young and adult rats using nu cleated and outside-out patch recordings. 2. In adult DGCs, the outward current pattern was dominated by a transient K+ current component. One portion of this current (similar to 60%) was bloc ked by micromolar concentrations of tetraethylammonium (TEA; IC50 42 muM) a nd BDS-I, a specific blocker of Kv3.4 subunits (2.5 muM), A second componen t was insensitive to tetraethylammonium (10 mm) and BDS-I. The transient ou tward current could be completely blocked by 4-aminopyridine (IC50 296 muM) . 3. The TEA- and BDS-I-sensitive and the TEA-resistant current components we re isolated pharmacologically. The current component, that was blocked by B DS-I and TEA showed a depolarized threshold of activation (similar to -30 m V) reminiscent of Kv3.4 subunits, while the current component resistant to TEA activated at more hyperpolarized potentials (similar to -60 mV). 4. In nucleated patches obtained by placing the patch pipette adjacent to t he apical dendrite, only small Na+ currents and small BDS-I-sensitive trans ient currents were detected. Nucleated patches obtained from either the cel l soma (see above) or the axon hillock showed significantly larger amplitud e Na+ currents as well as larger BDS-I-sensitive currents, indicating that this current was predominantly localized within the axosomatic compartment. This result, was in good agreement with the distribution of Kv3.4 protein as determined by immunohistochemistry. 5. Current-clamp as well as mock action potential-clamp experiments reveale d that the BDS-sensitive current component contributes to action potential repolarization. 6. A comparison of the two age groups (4-10 days and 60-100 days) revealed a marked developmental up-regulation of the BDS-I-sensitive component. Thes e functional changes are paralleled by a developmental increase in Kv3.4 mR NA expression determined by quantitative real-time RT-PCR, as well as a pro nounced up-regulation of Kv3.4 on the protein level determined by immunohis tochemistry. 7. These functional and molecular results argue that Kv3.4 channels located predominantly in the axosoinatic compartment underlie a transient K+ curre nt in adult DGCs, and that these channels are functionally important for re gulating spike repolarization. The marked developmental regulation suggests an important role of Kv3.4 in neuronal maturation.