Suppression of epileptiform activity by high frequency sinusoidal fields in rat hippocampal slices

1. Sinusoidal high frequency (20-50 Hz) electric fields induced across rat hippocampal slices were found to suppress zero-Ca2+, low-Ca2+, picrotoxin, and high-K+ epileptiform activity for the duration of the stimulus and for up to sever;El minutes following the stimulus. 2. Suppression of spontaneous activity by high frequency stimulation was found to be frequency (<500 Hz) but not orientation or waveform dependent. 3. Potassium-sensitive microelectrodes showed that block: of epileptiform activity was always coincident with a stimulus-induced rise in extracellular potassium concentration during stimulation. Post-stimulus inhibition was always associated with a decrease in extracellular potassium activity below baseline levels. 4. Intracellular recordings and optical imaging with voltage-sensitive dyes showed that during suppression neurons were depolarized yet did not fire action potentials. 5. Direct injection of sinusoidal current into individual pyramidal cells did not result in a tonic depolarization. Injection of lar ge direct current (DC) depolarized neurons and suppressed action potential generation. 6. These findings: suggest that high frequency stimulation suppresses epileptiform activity by inducing potassium efflux and depolarization block.