Local suppression of epileptiform activity by electrical stimulation in rat hippocampus in vitro.

High frequency electrical stimulation of deep brain structures (DBS) has been effective at controlling abnormal neuronal activity in Parkinson's patients and is now being applied for the treatment of pharmacologically intractable epilepsy. The mechanisms underlying the therapeutic effects of DBS are unknown. In particular, the effect of the electrical stimulation on neuronal firing remains poorly understood. Previous reports have showed that uniform electric fields with both AC (continuous sinusoidal) or DC waveforms could suppress epileptiform activity in vitro. In the present study, we tested the effects of monopolar electrode stimulation and low-duty cycle AC stimulation protocols, which more closely approximate those used clinically, on three in vitro epilepsy models. Continuous sinusoidal stimulation, 50 % duty-cycle sinusoidal stimulation, and low (1.68 %) duty-cycle pulsed stimulation (120 micros, 140 Hz) could completely suppress spontaneous low-Ca2+ epileptiform activity with average thresholds of 71.11 +/- 26.16 microA, 93.33 +/- 12.58 microA and 300 +/- 100 microA, respectively. Continuous sinusoidal stimulation could also completely suppress picrotoxin- and high-K+-induced epileptiform activity with either uniform or localized fields. The suppression generated by the monopolar electrode was localized to a region surrounding the stimulation electrode. Potassium concentration and transmembrane potential recordings showed that AC stimulation was associated with an increase in extracellular potassium concentration and neuronal depolarization block; AC stimulation efficacy was not orientation-selective. In contrast, DC stimulation blocked activity by membrane hyperpolarization and was orientation-selective, but had a lower threshold for suppression.