POTASSIUM CONDUCTANCES UNDERLYING REPOLARIZATION AND AFTERHYPERPOLARIZATION IN RAT CA1 HIPPOCAMPAL INTERNEURONS

1. The roles of multiple potassium conductances underlying action potential repolarization and after-hyperpolarization (AHP) in visually identified st. oriens-alveus (st.O-A) inhibitory interneurones of neonatal rat CA1 hippocampal slices were determined using whole-cell patch clamp techniques. 2. 4-Aminopyridine dose-dependently prolonged the action potential repolarization. The effects of 4-AP persisted in Ca2+-free conditions. Action potentials evoked from hyper polarized potentials possessed an increased rate of repolarization. These data suggest an involvement of the rapidly activating transient current, I-A, in spike repolarization. 3. Action potential duration was increased in the presence of Ca2+-free, Cd2+-containing solution, iberiotoxin or 1 mM TEA. The fast component of the AHP was attenuated by these agents suggesting that the Ca2+-activated K+ conductance, I-C, underlies both the spike repolarization and fast AHP. 4. In Ca2+-free conditions, TEA (>1 mM) dose-dependently prolonged the action potential duration by blocking a late conductance in action potential repolarization, suggesting a role for the sustained current, I-K. 5. The slow AHP was attenuated by Ca2+ free medium, apamin or the Ca2+ chelator EGTA, suggesting a role for the Ca2+-activated K+ conductance, I-AHP. 6. We conclude that action potential repolarization and AHP of st. O-A interneurones result from the activation of pharmacologically distinct, temporally overlapping potassium conductances. These findings are discussed with reference to the voltage clamp data presented in the preceding manuscript.