Roles of calcium- and voltage-sensitive potassium currents in the generation of neuromagnetic signals and field potentials in a CA3 longitudinal slice of the guinea-pig

Objectives: Roles of calcium- and voltage-sensitive potassium currents in generation of neuromagnetic signals and field potentials were evaluated using the longitudinal CA3 slice preparation of the guinea-pig. Methods: Their roles were evaluated by using selective channel blockers (tetraethyl-ammonium (TEA) and 4-aminopyridine (4AP)) and measuring their effects on the two types of signals and intracellular potentials. Fast γ-aminobutyric acid type A inhibition was blocked with picrotoxin. Results: Stimulation of the apical dendrites with an array of extracellular bipolar electrodes produced triphasic evoked magnetic fields with a spike and a slow wave typical of the slices. The evoked potentials in the apical and basal areas of the pyramidal cells closely resembled the magnetic field waveforms. Blockade of the potassium currents with TEA and 4AP had only subtle effects on the initial spike, but dramatically altered the slow wave. They also induced long-lasting spontaneous burst discharges synchronized across the slice. The results could be interpreted in terms of their known pre- and postsynaptic effects. Their post-synaptic effects were confirmed with intracellular recordings. Conclusion: Our results are consistent with a hypothesis that the calcium- and voltage-sensitive potassium currents, especially the A and C currents, play important roles in shaping the slow wave of the neuromagnetic and field potential signals produced by the mammalian hippocampus.