Abstracts

Rationale: Traumatic brain injury (TBI) is among the most common causes of acquired temporal lobe epilepsy (TLE). Multiple forms of synaptic reorganization occur in the dentate gyrus after brain injury, which may contribute to TLE development. Inhibition of the mammalian target of rapamycin (mTOR) signaling with rapamycin has demonstrated therapeutic promise in animal models of epilepsy, but the underlying mechanisms are unclear. This study used a murine model of posttraumatic epilepsy to test the hypothesis that mTOR inhibition suppresses post-injury reorganization of synaptic input to hilar inhibitory interneurons. Methods: Severe unilateral controlled cortical impact (CCI; impact depth= 1.0 mm) was administered to 6-8 week old male CD-1 or FVB-Tg(GadGFP)4570Swn/J (i.e., GIN) mice that express green fluorescent protein (GFP) in a subset of hilar inhibitory interneurons. CCI injured mice received either vehicle or rapamycin (3 mg/kg) treatment daily. In vitro electrophysiological recordings were performed on GFP+ hilar inhibitory interneurons 8-12 weeks post-injury to assess action potential (AP) firing and spontaneous or glutamate photolysis-evoked excitatory postsynaptic currents (sEPSCs) arising from dentate granule cells (DGCs) or CA3 pyramidal neurons. DCX immunoreactivity and mossy fiber sprouting were also assessed. Results: DCX immunolabeling (2 weeks post-CCI) and mossy fiber sprouting (8-13 wks post-CCI) were significantly increased in the ipsilateral DGC layer after CCI injury (p < 0.05). Daily rapamycin treatment significantly reduced both these measures (p < 0.05 vs CCI alone). Increased sEPSC frequency and AP firing was seen in GFP+ hilar inhibitory interneurons ipsilateral to CCI (p < 0.05 vs control). Relative to CCI injury alone, daily rapamycin treatment for 8-12 weeks after CCI injury reduced, but did not normalize this increased excitability of GFP+ hilar interneurons (p < 0.05 vs CCI ipsi; p>0.05 vs control). After CCI injury, GFP+ hilar interneurons exhibited increased responses to glutamate photostimulation of both DGCs and CA3 pyramidal cells, relative controls (p < 0.05). Rapamycin treatment reduced interneuron responses to photostimulation of DGCs (p < 0.05), but did not reduce the increased response to photostimulation of CA3 pyramidal cells (p>0.05). Increased CA3 back projections to DGCs were not detected after CCI (p=0.15). Conclusions: Increased synaptic input from both DGCs and CA3 pyramids contributes to the increased excitability of hilar inhibitory interneurons after CCI. Rapamycin treatment reduces synaptic reorganization originating from DGCs, but not from CA3 pyramidal cells. DGCs ipsilateral to CCI-injury exhibited numerically, but not significantly increased responsiveness to CA3 back projections, suggesting that axon sprouting of CA3 pyramidal cells after CCI injury is directed toward hilar inhibitory neurons. Whereas rapamycin treatment reduces axon sprouting of DGCs, it does not prevent injury-induced synaptic reorganization of CA3 pyramidal neurons. Funding: NIH Grant NS088608