Abstracts

Rationale: Temporal lobe epilepsy (TLE) is a common type of adult epilepsy and is characterized by the occurrence of spontaneous, recurrent seizures that originate from limbic structures of the temporal lobe. Seizures associated with TLE can be difficult to treat with up to 30% of patients being therapy resistant. The subiculum remains largely understudied in TLE despite the fact it contains a large population of endogenously bursting neurons and is often considered the output of the hippocampus.Alterations in voltage gated sodium (Na) channel physiology have been implicated in facilitating and maintaining increases in neuronal excitability in TLE. The Na channel isoform Nav1.6 has received much attention in the development of neuronal hyperexcitability since it is expressed along the axon initial segment and nodes of Ranvier, where it plays a significant role in the initiation and propagation of action potentials (APs). In this study, we sought to determine if the subiculum becomes hyperexcitable in TLE, and what role Nav1.6 plays in facilitating this increase in neuronal excitability. Methods: All recordings were performed in adult male Sprague-Dawley rats. TLE was induced using the continuous hippocampal stimulation (CHS) protocol. Seizures in TLE were confirmed by video/EEG recording. Brain slices were prepared and electrophysiology recordings were taken using either the whole-cell or outside-out patch clamp electrophysiology techniques. Recordings of membrane properties, persistent and resurgent sodium channel currents, and Na channel physiology were obtained from both WT and TLE rats.  Results: In this study, we show that bursting subiculum TLE neurons are hyperexcitable, firing a higher number of action potential spikes than WT neurons in response to both somatic current injections (WT=36.6±1.3 Hz, n=16; TLE= 44.4±1.0 Hz, n=12; pv1.6 in subiculum hyperexcitability, we used the tetrodotoxin metabolite, 4,9-anhydro-TTX which has a higher affinity for Nav1.6 over other Na channel isoforms. We show that application of 4,9-anhydro-TTX  significantly decreases persistent (by 64.5%; n=6, p < 0.01) and resurgent (by 48.5%; n=9, p < 0.05) sodium currents in TLE subiculum neurons, as well as reducing spike frequency in response to somatic current injections (by 49.1% at a current injection of 470 pA, n=6, p < 0.05) and synaptic stimulation (59.4% reduction, n=8, p < 0.01). Conclusions: These studies delineate a role for Nav1.6 in subiculum hyperexcitability in TLE.  The subiculum has remained vastly understudied in TLE despite its large population of endogenously bursting neurons and its role as the output of the hippocampus.  These proexcitatory alterations in Na­v­1.6 activity would contribute to subiculum neuronal hyperexcitability and likely plays an important role in the initiation and propagation of spontaneous seizures in TLE. Funding: NIH R01 NS075157 (MKP) , AES Predoctoral Fellowship (BSB)