Abstracts

Rationale: Aberrant dentate granule cell (DGC) mossy fiber sprouting (MFS) into the dentate inner molecular layer is among the many types of structural plasticity observed in hippocampal tissue from patients with temporal lobe epilepsy (TLE) and animal models of the disease. MFS is known to result in the formation of de novo excitatory connections, primarily onto DGCs [1]. These connections are hypothesized to play an important role in the formation of aberrant synchronicity of DGCs that may contribute to seizure activity in the epileptic brain. Our recent work suggests that immature or newborn DGCs, rather than pre-existing DGCs, give rise to MFS in the pilocarpine model of TLE [2]. Whether DGCs also show an age-dependent susceptibility for innervation by sprouted mossy fibers is unknown. We hypothesized that immature DGCs are more likely to be innervated by sprouted mossy fibers. 1. Buckmaster, P.S., G.F. Zhang, and R. Yamawaki, J Neurosci, 2002. 22:6650-8 2. Kron, M.M., H. Zhang, and J.M. Parent, J Neurosci, 2010 30:2051-9 Methods: To investigate this idea, we birthdated DGCs by stereotaxic injection of GFP-expressing retrovirus into the dentate gyrus of rats at specific ages between postnatal day (P) 7 and 60, induced status epilepticus (SE) with systemic pilocarpine at P56 and killed animals 4-10 weeks after SE. Mossy fiber synapse formation was assessed by double-label immunofluorescence for zinc-transporter member 3 (Znt3), which labels mossy fiber boutons, and GFP. Confocal microscopic images were analyzed for the percentage of GFP dendritic spines with apposed Znt3 terminals. Ongoing studies involve labeling DGC progenitors with two different RV reporters at different times before or after epileptogenic injury to assess the ages of DGCs that are most likely to be synaptically coupled by mossy fiber inputs onto apical dendrites in the molecular layer, as well as onto hilar basal dendrites. Results: We found that cells born after pilocarpine-induced SE form about twice as many synapses with mossy fiber terminals compared to cells born 2 or 7 weeks before SE. Conclusions: These preliminary findings suggest that DGCs born after SE are more likely to form recurrent excitatory circuits than preexisting DGCs.