Properties of two voltage-activated potassium currents in acutely isolated juvenile rat dentate gyrus granule cells

1. The properties of outward currents were investigated in acutely isolated dentate gyrus granule cells at postnatal ages of day 5-7, 10-14, 18-24 (P5-7, P10-14, P18-24) and at adulthood (2-3 mo), with the use of the whole-cell patch-clamp technique. 2. Kinetic analysis and pharmacological properties showed that an A-type K+ current (IA) and a delayed rectifier current (IK) were present in these cells. 3. IA in P10-14 cells activated and inactivated rapidly with a decay time constant of 7.5 +/- 2.1 (SD) ms with command pulses to +30 mV. The removal of inactivation was monoexponential with a time constant of 23.1 ms (holding potential, -50 mV; conditioning voltage steps of varying duration to -110 mV). V 1/2 of the Boltzmann function describing steady-state inactivation was -65.1 +/- 1.8 mV with a slope factor of -6.0. IA was sensitive to 5 mM 4-aminopyridine (4-AP) but not to 10 mM tetraethylammonium (TEA). 4. IK in P10-14 cells displayed a voltage-dependent activation time constant (4.3 +/- 0.8 ms for command pulses to +30 mV and 16.2 +/- 2.4 for command pulses to -10 mV) and a double-exponential decay (time constants 194 +/- 21 and 1,625 +/- 254 ms). The rate constant of removal of inactivation was 332.1 ms. IK showed a reduction by 61.4 +/- 5.3% with 10 mM TEA and was partially blocked by 5 mM 4-AP in a subpopulation of cells. 5. Whereas IA remained stable over time, IK showed a substantial reduction of current amplitude by 67% after 30 min of cell perfusion through the patch pipette. The time course of this reduction was monoexponential with a time constant of 6.9 min and was partly due to a shift in V1/2 of the steady-state inactivation from -79.2 to -99.6 mV. 6. IA and IK remained stable with respect to kinetic properties during ontogenesis. However, the relative contribution and pharmacological properties of the investigated K+ currents varied with age. Although IA dominated in P5-7 cells, IK was prominent in most older cells. Five millimolars 4-AP reduced IA by 40.7 +/- 26.7% in P5-7 cells and blocked IA completely in 80% of investigated P10-14 cells. Similar changes were observed for the effects of 4-AP on IK (18.7% depression in the age group P5-8, 46.1% in the age group P10-14, and 45.7% in adult animals).