VOLTAGE-GATED POTASSIUM CURRENTS IN STRATUM ORIENS-ALVEUS INHIBITORY NEURONS OF THE RAT CA1 HIPPOCAMPUS

1. Voltage-activated K+ currents mere recorded from visually identifie d inhibitory interneurones of the CA1 stratum oriens-alveus region in neonatal rat hippocampal slices using outside-out patch and whole-cell voltage clamp techniques. 2. Outward currents comprised both a transi ent and a sustained component when elicited from a holding potential o f -90 mV. Tail current analysis of current reversal potentials showed that outward currents were carried by potassium ions. 3. The transient current, I-A, was activated with a time to peak within 5 ms, inactiva ted with a time constant of similar to 15 ms at 0 mV and possessed hal f-activation at -14 mV. Half-inactivation of the transient current occ urred at -71 mV. At -90 mV, the transient current recovered from inact ivation with a time constant of 142 ms. 4. Activation of currents from a holding potential of -50 mV permitted isolation of the sustained cu rrent, I-K. In Ca2+-free conditions the sustained current showed rapid activation, reaching about 80% of its maximum within 1.5 ms, and show ed little inactivation during Is depolarizing steps. The majority of s ustained outward currents showed no voltage-dependent inactivation. In similar to 20% of cells, a slow time-dependent inactivation of the su stained current was observed, suggesting the presence of a second type of sustained current in these cells. 5. A Ca2+-dependent K+ current c omprised a significant portion of the total sustained current; this cu rrent was activated at voltages positive to -30 mV and showed no time- dependent inactivation over a 1 s depolarizing step. This current comp onent was removed in Ca2+-free conditions or by iberiotoxin. 6. Low co ncentrations of 4-AP (50 mu m) attenuated both the transient and susta ined current components recorded in a Ca2+-free solution. Higher conce ntrations of 4-AP (<10 mM) were without further effect on the sustaine d current but completely blocked the transient current with an IC50 of 1.8 mM. TEA blocked the sustained current with an IC50 of 7.9 mM with out significantly reducing the transient current. Both current compone nts were resistant to dendrotoxin (500 nM).