Abstracts

Rationale: Traumatic brain injury (TBI) is known to trigger the development of epilepsy [1]. Chronic changes in neuronal activity that promote or accompany TBI-induced epileptogenesis remain poorly understood to date. Here, we implement the Tetanus Toxin (TeT) model of post-traumatic epilepsy in mouse visual cortex and use chronic 2-photon imaging via GCamP6 or GCaMP7 to study the evolution of cortical activity in the same population of cells for up to 5 months, with simultaneous EEG recording. TeT acutely disinhibits neuronal activity by blocking GABA and glycine release from inhibitory interneurons [2,3]. However, we do not know how this translates to the development of network hypersynchrony and epileptogenesis over time.  Methods: Briefly, we inject either 0.15 ng or 5 ng TeT in 200 nL cortex buffer with 1.5% BSA at a depth of 600 µm below dura in area V1 of C57/BL6 mice of either sex. Precise spatial and temporal control over the TeT injection allows us to obtain standardized results around the cortical focus, as previously described [4]. Reference activity is recorded prior to the TeT injection, while in a control cohort we also record activity before and after vehicle (BSA) injection. Pupil size and animal movements are monitored throughout via an infrared camera and a treadmill sensor.  Results: Following unilateral injection of TeT, mice develop several different types of abnormal EEG discharge patterns, including bi-hemispheric seizure events. To reduce experimenter bias, we identify abnormal EEG discharges via an automated spectral analysis, and subsequently manually classify events into four categories: 1) seizures, 2) single spikes, 3) spike-wave complexes, and 4) irregular oscillations. These patterns evolve dynamically over several weeks, peak around 10-30 days post TeT injection, and gradually subside thereafter. Figure 1 shows summary data from n=4 mice which received initially BSA (controls; orange bars) and subsequently high-dose TeT (blue bars) injections. We do not observe any seizures, spikes and spike-wave discharges following BSA injections. A low rate of 'irregular events' does occur but is similar to the 'irregular event' rate observed in intact cortex, and much lower than the rate seen post TeT injection (1-2 events/10 min vs 7-8 events/10 min on average at D1-10 and D30-40). All 4 types of abnormal events were more likely to be observed in the high-dose versus the low-dose injections.  Conclusions: Preliminary analysis shows that calcium activity elicited during seizures and single spike events differs from calcium activity elicited during spike and wave discharge periods recorded on EEG. Interestingly, we also identified epochs of abnormal calcium activity localized in relatively small groups of neurons near the TeT injection site that do not appear to correlate with electrographic anomalies (figure 2 illustrates such an event of hypersynchronization in a local group of neurons). Even though they lack an obvious correlate in EEG amplitude, these episodes of local hypersynchrony are likely to be important for understanding the progression of epileptogenesis. We are in the process of analyzing how synchronous firing patterns evolve over time and how neurons of different types contribute to the generation of seizure events.References: - Xinyao DeGrauw, David Thurman, Likang Xu, Vijaya Kancherla, Ton DeGrauw, Epidemiology of traumatic brain injury-associated epilepsy and early use of anti-epilepsy drugs: An analysis of insurance claims data, 2004-2014, Epilepsy Research, Volume 146, 2018, Pages 41-49, ISSN 0920-1211, https://doi.org/10.1016/j.eplepsyres.2018.07.012 - Timothy Benke, John Swann, The Tetanus Toxin Model of Chronic Epilepsy, in: Binder DK, Scharfman HE. Recent Advances in Epilepsy Research. New York: CRC Press [CAM]; 2004 - Giampietro Schiavo, Michela Matteoli, Cesare Montecucco, Neurotoxins Affecting Neuroexocytosis, Physiological Reviews, Volume 80, Issue 2, 2000, pages 717-766 - Marco Mainardi, Marta Pietrasanta, Eleonora Vannini, Ornella Rossetto, Matteo Caleo, Tetanus neurotoxin-induced epilepsy in mouse visual cortex, Epilepsia, Volume 53, Issue 7, 2012, pages e132-e136. Funding: No funding