834 Inhibition of the Epileptic Engram Prevents Seizures in a Mouse Model of Temporal Lobe Epilepsy

Abstract

INTRODUCTION: The cellular basis of temporal lobe epilepsy (TLE) remains elusive and consequently has hindered efforts to develop novel therapies. Similar challenges have plagued the field of learning and memory until recent work identified plasticity of a subset of neurons in a multi-focal network as the cellular source for memory (the memory engram). Building on these results, we ask if an engram exists for TLE and if it can be targeted to modulate the epileptic phenotype? METHODS: We used the intra-amygdala kainic acid status-epilepticus (SE) model in which a brief episode of SE results in spontaneous seizures days later. To identify cells active during epileptogenesis and thus possibly comprising the engram, we performed immunohistochemistry for the immediate early gene product c-Fos (a marker of cellular activity) at various time points after SE (1 to 96 hours). To modulate the persistently active cells, we used a chemical-genetic approach to first label these cells with an inhibitory DREADD (designer receptors exclusively activated by designer drugs) and then inhibit their activity with clozapine-N-oxide (a small molecular activator of DREADDs). RESULTS: We found a pattern of c-Fos expression that first appears after SE in the dentate gyrus, begins to localize in subsets of CA3 and CA1 pyramidal neurons by 12 hours, and then persists in these cells for approximately 72 hours. With our chemical-genetic approach, we successfully labeled the cells active after SE with an inhibitory DREADD and then silenced their activity once the animals were epileptic. Such silencing proved effective in significantly reducing the occurrence of subsequent seizure activity. CONCLUSIONS: Following an epilepsy-inducing insult, a subset of hippocampal neurons remains persistently active for several days. Subsequent inhibition of these neurons in epileptic animals prevents seizures, consistent with them comprising the epileptic engram.