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 granule cells (DGCs) in acute brain slices from young and adult rats using nucleated 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 (approximately 60%) was blocked by micromolar concentrations of tetraethylammonium (TEA; IC50 42 microM) and BDS-I, a specific blocker of Kv3.4 subunits (2.5 microM). A second component was insensitive to tetraethylammonium (10 mM) and BDS-I. The transient outward current could be completely blocked by 4-aminopyridine (IC50 296 microM). 3. The TEA- and BDS-I-sensitive and the TEA-resistant current components were isolated pharmacologically. The current component that was blocked by BDS-I and TEA showed a depolarized threshold of activation (approximately -30 mV) reminiscent of Kv3.4 subunits, while the current component resistant to TEA activated at more hyperpolarized potentials (approximately -60 mV). 4. In nucleated patches obtained by placing the patch pipette adjacent to the apical dendrite, only small Na+ currents and small BDS-I-sensitive transient currents were detected. Nucleated patches obtained from either the cell soma (see above) or the axon hillock showed significantly larger amplitude 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 revealed 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. These functional changes are paralleled by a developmental increase in Kv3.4 mRNA expression determined by quantitative real-time RT-PCR, as well as a pronounced up-regulation of Kv3.4 on the protein level determined by immunohistochemistry. 7. These functional and molecular results argue that Kv3.4 channels located predominantly in the axosomatic compartment underlie a transient K+ current in adult DGCs, and that these channels are functionally important for regulating spike repolarization. The marked developmental regulation suggests an important role of Kv3.4 in neuronal maturation.