Linking kindling to increased glutamate release in the dentate gyrus of the hippocampus through the STXBP5/tomosyn-1 gene.

Seth R Batten,Greg A Gerhardt,John T Slevin,Sidney W Whiteheart,Elena A Matveeva,Thomas C Vanaman

doi:10.1002/brb3.795

Abstract

IntroductionIn kindling, repeated electrical stimulation of certain brain areas causes progressive and permanent intensification of epileptiform activity resulting in generalized seizures. We focused on the role(s) of glutamate and a negative regulator of glutamate release, STXBP5/tomosyn‐1, in kindling.MethodsStimulating electrodes were implanted in the amygdala and progression to two successive Racine stage 5 seizures was measured in wild‐type and STXBP5/tomosyn‐1−/− (Tom−/−) animals. Glutamate release measurements were performed in distinct brain regions using a glutamate‐selective microelectrode array (MEA).ResultsNaïve Tom−/− mice had significant increases in KCl‐evoked glutamate release compared to naïve wild type as measured by MEA of presynaptic release in the hippocampal dentate gyrus (DG). Kindling progression was considerably accelerated in Tom−/− mice, requiring fewer stimuli to reach a fully kindled state. Following full kindling, MEA measurements of both kindled Tom+/+ and Tom−/− mice showed significant increases in KCl‐evoked and spontaneous glutamate release in the DG, indicating a correlation with the fully kindled state independent of genotype. Resting glutamate levels in all hippocampal subregions were significantly lower in the kindled Tom−/− mice, suggesting possible changes in basal control of glutamate circuitry in the kindled Tom−/− mice.ConclusionsOur studies demonstrate that increased glutamate release in the hippocampal DG correlates with acceleration of the kindling process. Although STXBP5/tomosyn‐1 loss increased evoked glutamate release in naïve animals contributing to their prokindling phenotype, the kindling process can override any attenuating effect of STXBP5/tomosyn‐1. Loss of this “braking” effect of STXBP5/tomosyn‐1 on kindling progression may set in motion an alternative but ultimately equally ineffective compensatory response, detected here as reduced basal glutamate release.

Highlights

In kindling, repeated electrical stimulation of certain brain areas causes progressive and permanent intensification of epileptiform activity resulting in generalized seizures
The minimal protein requirements needed for neurotransmitter (NT) release are the plasma membrane-bound t-SNAREs, syntaxin and synaptosomal- associated protein 25 (SNAP-25), and the vesicle-bound v-S NARE, synaptobrevin/vesicle-associated membrane protein 2 (VAMP-2) (Jahn & Scheller, 2006)
Basal glutamate levels were lower in kindled Tom−/− mice suggesting a compensatory mechanism that may be triggered by kindling progression. These results suggest that a critical factor, possibly a driver, in the kindling phenomenon is the evolution of increased glutamate release in the hippocampal dentate gyrus (DG); once a fully kindled state is reached, STXBP5/tomosyn-1 may not be important in modulating glutamate release and maintaining the kindled state

Summary

| INTRODUCTION

Kindling is a process whereby repeated electrical stimulation of certain brain areas, the limbic system, with a fixed current strength and duration causes progressive and permanent intensification of epileptiform activity resulting in generalized seizures (Goddard, McIntyre, & Leech, 1969). STXBP5/ tomosyn-1 is thought to function as a negative regulator of SNARE- mediated membrane fusion, primarily by sequestering the syntaxin-1a and SNAP-25 heterodimer and inhibiting 7SC formation (Ashery, Bielopolski, Barak, & Yizhar, 2009; Sakisaka et al, 2008; Willams et al, 2011). Basal glutamate levels were lower in kindled Tom−/− mice suggesting a compensatory mechanism that may be triggered by kindling progression These results suggest that a critical factor, possibly a driver, in the kindling phenomenon (and potentially in epileptogenesis) is the evolution of increased glutamate release in the hippocampal DG; once a fully kindled state is reached, STXBP5/tomosyn-1 may not be important in modulating glutamate release and maintaining the kindled state

| MATERIALS AND METHODS

SCa NSFb SV2b

| DISCUSSION

Findings

CONFLICT OF INTEREST