Long‐lasting hyperpolarization underlies seizure reduction by low frequency deep brain electrical stimulation
Authors: Toprani Sheela , Durand Dominique M. 
Published: August 2013

Key points
Deep brain electrical stimulation (DBS) is a promising treatment for mesial temporal lobe epilepsy (MTLE). However, treatment optimization and clinical application are limited by the fact that the mechanisms of seizure reduction by electrical stimulation remain unknown.
We have shown that low frequency electrical stimulation (LFS) of a white matter target connecting the hippocampi effectively reduces chemically induced epileptic activity in bilateral hippocampi.
LFS induces long‐lasting hyperpolarization (1–2 s) in the inter‐stimulus interval that protects cells from seizure activity.
This long‐lasting hyperpolarization is mediated by (1) GABAB IPSPs and (2) the slow afterhyperpolarization (sAHP). Its magnitude, as measured by amplitude and area, is correlated with LFS efficacy of seizure reduction.
Understanding the mechanisms of LFS could have therapeutic applications for seizure reduction in patients with MTLE.
Abstract  Mesial temporal lobe epilepsy (MTLE) is a common medically refractory neurological disease. Deep brain electrical stimulation (DBS) of grey matter has been used for MTLE with limited success. However, stimulation of a white matter tract connecting the hippocampi, the ventral hippocampal commissure (VHC), with low frequencies that simulate interictal discharges has shown promising results, with seizure reduction greater than 98% in bilateral hippocampi during stimulation and greater than 50% seizure reduction in bilateral hippocampi after treatment. A major hurdle to the implementation and optimization of this treatment is that the mechanisms of seizure reduction by low frequency electrical stimulation (LFS) are not known. The goal of this study is to understand how commissural fibre tract stimulation reduces bilateral hippocampal epileptic activity in an in vitro slice preparation containing bilateral hippocampi connected by the VHC. It is our hypothesis that electrical stimuli induce hyperpolarization lasting hundreds of milliseconds following each pulse which reduces spontaneous epileptic activity during each inter‐stimulus interval (ISI). Stimulus‐induced long‐lasting‐hyperpolarization (LLH) can be mediated by GABAB inhibitory post‐synaptic potentials (IPSPs) or slow after‐hyperpolarization (sAHP). To test the role of LLH in effective bilateral seizure reduction by fibre tract stimulation, we measured stimulus‐induced hyperpolarization during LFS of the VHC using electrophysiology techniques. Antagonism of the GABAB IPSP and/or sAHP diminished stimulus‐induced hyperpolarization concurrently with LFS efficacy (greater than 50% reduction). Blocking both the GABAB IPSP and sAHP simultaneously eliminated the effect of electrical stimulation on seizure reduction entirely. These data show that LFS of the VHC is an effective protocol for bilateral hippocampal seizure reduction and that its efficacy relies on the induction of long‐lasting hyperpolarization mediated through GABAB IPSPs and sAHP. Based on this study, optimization of the timing of LFS and LFS‐induced‐LLH may lead to improved outcomes from DBS treatments for human epilepsy.