Abstracts

Rationale: Status epilepticus (SE) in adult rodents induces hippocampal remodeling and stimulates neurogenesis. Some newborn dentate granule cells (DGCs) appear to integrate normally and restore inhibition, while others integrate aberrantly and are hyperexcitable. The net influence of this neurogenesis on epileptogenesis, however, remains unclear. To gain insight into the impact of SE on DGC neurogenesis and its role in seizure-induced plasticity, we used a transgenic neural progenitor-specific conditional reporter mouse line to visualize adult-born DGCs and examine their integration, both normal and aberrant, after SE. We also injected a retroviral (RV) reporter into adult rat dentate gyrus (DG) before or after SE to label dividing cells. Last, fractioned X-irradiation was used to suppress neurogenesis before or after SE. These combined strategies tested the hypothesis that different forms of SE-induced hippocampal plasticity arise selectively from DGCs of specific developmental stages, ranging from precursors to fully mature neurons. Methods: SE was induced in 2-3 mon, Nestin-CreERT2 X R26R-YFP transgenic mice with kainic acid (KA), or in 2-3 mon Sprague-Dawley rats with pilocarpine. In mice, exons 1-3 and the nestin 2nd intronic enhancer drives a cre-recombinase-mutated estrogen receptor (ERT2) fusion gene. The mice were crossed with a floxed-stop-R26R-YFP line so that tamoxifen (TMX) treatment induced YFP labeling in nestin-expressing neural progenitors and their progeny. Mice received daily i.p. TMX on days 5-9 after SE and were perfused on day 30. Controls received saline instead of KA or vehicle in place of TMX. We also injected high-titer, GFP-expressing RV to label DGC progenitors. To suppress neurogenesis, rats received 6 Gray (Gy) whole-brain irradiation either 5 wk before or 1 wk after SE. Controls received sham irradiation or saline instead of pilocarpine. Animals were killed 4 wk after SE and immunostained for Prox1, DCx, GFP and NeuN. Timm stain was used to detect mossy fiber sprouting (MFS). Results: Transgenic mice not given TMX expressed no YFP. TMX-treated controls showed YFP+ cells exclusively in the subgranular zone and DGC layer that had only apical dendrites. KA-treated mice, in contrast, showed YFP+ cells in the dentate hilus and in more external aspects of the DGC layer, and many had hilar basal dendrites (HBDs). Rats receiving 6 Gy irradiation early after SE had less Prox1+ hilar ectopic DGCs vs. controls. When RV was injected 7 wk before SE, supraganular mossy fibers were GFP+. Morever, irradiating rats 5 wk before SE failed to alter MFS. Rats injected with RV-GFP 2 wk before SE had YFP+ cells with HBDs but only rare ectopic GFP+ DGCs. Conclusions: These findings suggest that neuronal progenitors proliferating after KA or pilocarpine-induced SE in adult rodents migrate aberrantly and form ectopic DGCs with persistent HBDs. The abnormal integration of these cells may contribute to chronic seizures or cognitive dysfunction. Progenitors proliferating before SE only rarely give rise to ectopic DGCs, but fully mature DGCs at the time of SE contribute to MFS.