Potassium-induced enhancement of persistent inward current in hippocampal neurons in isolation and in tissue slices.

Previous work suggested a role for the voltage-dependent persistent sodium current, I(Na,P), in the generation of seizures and spreading depression (SD). Ordinarily, I(Na,P) is small in hippocampal neurons. We investigated the effect of raising external K(+) concentration, [K(+)](o), on whole-cell persistent inward current in freshly isolated hippocampal CA1 pyramidal neurons. I(Na,P) was identified by TTX-sensitivity and dependence on external Na(+) concentration. When none of the ion channels were blocked, I(Na,P) was not usually detectable, probably because competing K(+) current masked it, but after raising [K(+)](o) I(Na,P) appeared, while K(+) currents diminished. With K(+) channels blocked, I(Na,P) could usually be evoked in control solution and raising [K(+)](o) caused its reversible increase in most cells. The increase did not depend on external calcium [Ca(2+)](o). In CA1 pyramidal neurons in hippocampal slices a TTX-sensitive persistent inward current was always recorded and when [K(+)](o) was raised, it was reversibly enhanced. Strong depolarization evoked irregular current fluctuations, which were also augmented in high [K(+)](o). The findings support a role of potassium-mediated positive feedback in the generation of seizures and spreading depression.