Abstracts

Rationale: Cortical trauma is one of the most common etiologies for acquired epilepsy world-wide. Currently, there is no effective way to prevent epileptogenesis following traumatic brain injury (TBI). For successful prophylaxis, it would be important to intervene with therapy timed to interrupt the critical processes that initiate epileptogenesis. Axonal sprouting and formation of excitatory synapses are important events that may contribute early-on to the epileptogenesis following TBI. However, little is known about when axonal sprouting is initiated following epileptogenic injury. Our previous work (Graber and Prince, 2004; Li et al., 2012) suggested a critical period for maladaptive excitatory circuit formation and chronic neocortical hyperexcitability within the first 3 days of injury in the partial neocortical isolation (undercut or “UC”) model of injury-induced epileptogenesis. In the present experiments, we tested the hypothesis that aberrant excitatory connections form during this critical period in the UC injured neocortex.Methods: Partial cortical isolations were produced in Sprague-Dawley rats at P21. Immunocytochemical experiments were performed 3 days after UC to identify potential aberrant synapses on layer V biocytin-filled Pyr neurons. Immunofluorescence was assessed with laser scanning confocal microscopy. Confocal image stacks were analyzed in Volocity 2.6.1 for volumetric analysis and ImageJ for 2-dimensional analysis.Results: Our results show that VGLUT1/PSD95 colocalization (putative excitatory synapses) can be seen only on dendrites of biocytin-filled cells in control slices. However, in slices from UC cortex, colocalization was identified not only on dendrites, but also on somata. This result was confirmed by analyzing rotating images of 3D reconstructed neurons from confocal stacks and volumetric analysis of a perisomatic ROI containing VGLUT1/PSD95 co-labeled voxels. Volumetric analysis of the VGLUT1 /PSD95 co-labeled voxels within the ROI showed that ~2.73 ± 0.11 % of the perisomatic ROI in 5 UC cells was occupied by colocalized VGLUT1/PSD95 in close apposition to somata. No colocalization of VGLUT1/PSD95 was found in the perisomatic ROI of the 5 control cells.Conclusions: Our data strongly support the presence of aberrant excitatory synapses on layer V Pyr somata soon after neocortical injury that would contribute to hyperexcitability. Early intervention that targets such new excitatory connections could serve as a potential prophylactic therapy for posttraumatic epilepsy. Supported by grants NS039579 and NS090076 from the NINDS and the Pimley Research Fund.