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Background: Vagus Nerve Stimulation (VNS) has seizure-suppressing effects but the underlying mechanism is not fully understood. To further elucidate the mechanisms underlying VNS-induced seizure suppression at a neurophysiological level, the present study examined effects of VNS on hippocampal excitability using dentate gyrus evoked potentials (EPs) and hippocampal electroencephalography (EEG)., Methods: Male Sprague-Dawley rats were implanted with a VNS electrode around the left vagus nerve. A bipolar stimulation electrode was implanted in the left perforant path and a bipolar recording electrode was implanted in the left dentate gyrus for EEG and dentate field EP recording. Following recovery, VNS was applied in freely moving animals, using a duty cycle of 7 s on/18 s off, 30 Hz frequency, 250 µs pulse width, and an intensity of either 0 (SHAM), 25 µA or 1000 µA, while continuously monitoring EEG and dentate field EPs., Results: VNS at 1000 µA modulated dentate field EPs by decreasing the field excitatory post-synaptic potential (fEPSP) slope and increasing the latency and amplitude of the population spike. It additionally influenced hippocampal EEG by slowing theta rhythm from 7 Hz to 5 Hz and reducing theta peak and gamma band power. No effects were observed in the SHAM or 25 µA VNS conditions., Conclusion: VNS modulated hippocampal excitability of freely moving rats in a complex way. It decreased synaptic efficacy, reflected by decreased fEPSP slope and EEG power, but it simultaneously facilitated dentate granule cell discharge indicating depolarization of dentate granule cells., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Background: In many temporal lobe epilepsy (TLE) patients both hippocampi are seizure onset zones. These patients are unsuitable candidates for epilepsy surgery but may be amenable to hippocampal deep brain stimulation (DBS). The optimal DBS parameters for these patients are unknown. Recent observations suggest that even in patients with a unilateral focus switching from unilateral hippocampal DBS to bilateral hippocampal DBS could improve seizure control., Objective: Compare the effect of unilateral with bilateral hippocampal DBS on seizures in a rat model for TLE., Methods: In the post status epilepticus (SE) kainic acid rat model for TLE continuous EEG monitoring was performed for 50 days during which rats were subjected to 10 days of unilateral and 10 days of bilateral Poisson-distributed high frequency hippocampal DBS in a cross-over trial. During bilateral DBS, each hippocampus was stimulated with a separate stimulator and its own generated Poisson distribution with a mean and variance of 1/130 s., Results: Electrographic seizure rate was 23% lower during bilateral compared to unilateral hippocampal DBS (P < 0.05). No effect of unilateral nor bilateral hippocampal DBS was observed on seizure duration. When bilateral hippocampal DBS was applied, lower stimulation intensities were required to evoke after discharges (P < 0.05), reflecting a higher potency of bilateral hippocampal DBS compared to unilateral hippocampal DBS to affect hippocampal networks., Conclusions: Superior outcome in seizure control with bilateral compared to unilateral hippocampal DBS indicates that targeting larger regions of the hippocampal formation with more than one stimulation electrode may be more successful in suppressing seizures in TLE., (Copyright © 2015 Elsevier Inc. All rights reserved.)