Abstracts

Rationale: Neural stem cells in the adult mammalian brain, found in the hippocampus, continually generate new neurons throughout adulthood. Pathological changes to the hippocampus during the formation of temporal lobe epilepsy (TLE) accelerates the production of aberrant new neurons that contribute to epileptogenesis (Cho et al., Nat Commun. 2015 Mar 26;6:660). These aberrant new neurons take on an abnormal pathology of ectopic granule cell (EGC) migration, basal dendrite formation, mossy fiber sprouting, and are hyperexcitable. While these young granule cells use activity dependent mechanisms to properly mature and integrate into extant hippocampus circuitry, it is unclear if increased intrinsic activity itself regulates aberrant neurogenesis and spontaneous seizure development. Methods: To determine the role of intrinsic activity in the development of aberrant new neurons, we use a dual in vivo Designer Receptor Exclusively Activated by Designer Drugs (DREADDs) with simultaneous video-EEG recordings to specifically silence seizure-induced granule cells. A retrovirus packaging the inhibitory DREADD (CAG-hM4di-IRES GFP) or control construct (CAG-IRES GFP) was stereotaxically injected bilaterally into the mouse hippocampus. Status epilepticus (SE) was induced using a pilocarpine model of TLE and clozapine-N-oxide (CNO) was administered daily for 2 weeks following SE to silence aberrant new neurons. Seizures were monitored beginning 1 month after SE using video-EEG. Following EEG recordings mice were perfused for immunohistochemistry analysis. Results: Transiently silencing seizure-induced new neurons following SE leads to a reduction in TLE pathology. Most notably, there is a trend in the reduction in the number of labeled ectopic granule cells (27.43 8.162, GFP; 12.0 4, hM4Di; p=0.0822) and a significant reduction in the migration distance of the EGCs (18.83.7 m, GFP; 8.81.5 m; p=0.0200. Silencing aberrant new neurons also led to ~43% persistent reduction in SRS (p= .0274; n=9 hM4Di, n=7 GFP) with no change in seizure duration (33.141.64, hM4Di; 33.750.39, GFP). Conclusions: Silencing aberrant new neurons reduces ectopic migration of new neurons and SRS development suggesting that intrinsic activity is necessary for aberrant neurogenesis. This further implicates adult born granule cells playing a causative role in epileptogenesis. Further, targeting aberrant new neurons offers a promising therapeutic intervention to prevent the development of spontaneous seizures. Future studies are needed to further understand the mechanisms underlying activity dependent aberrant neurogenesis. Funding: Supported by NIH: R21 NS090926 and 1 RO1NS093992-01A1, NIH training grant 5T32Hl007360-34 (For ZRL)