Abstracts

Rationale: Absence seizures are associated with impaired quality of life, including cognitive, psychosocial and learning deficits that may persist into adulthood, even after seizure suppression. Animal models represent an important tool in both our understanding of the pathophysiology of absence seizures as well as the development of new therapies. Previous work used sedated or anesthetized animals for functional magnetic resonance imaging (fMRI) studies and found cortical increases, unlike the predominant cortical decreases that are seen in human studies. Moreover, behavioral impairments in GAERS during absence seizures have not been clearly demonstrated in the literature. These limitations may partly explain why the mechanisms of how absence seizures impair cognition and behavior remain unclear. Here we investigate whether seizure hemodynamics and behavioral profile in awake, drug-free animals are consistent with findings in human seizures. We further define patterns of individual thalamocortical neuronal activity in relation to severity of behavior and physiologic changes in absence seizures. Methods: Genetic Absence Epilepsy Rats from Strasbourg (GAERS), an established animal model of absence seizures, were utilized. Rats aged 4-8 months were trained to allow for awake body and head restraint. A 9.4 tesla fMRI was performed in awake, drug-free animals with simultaneous skull carbon-electrode electroencephalography (EEG) for detection of spike-wave discharges. To test behavior, GAERs implanted with frontoparietal skull electrodes were trained on one of two behavioral tasks with concurrent video EEG. A goal-oriented sensory detection task involved an 8kHz tone to signify reward availability of 20% sucrose water. During a repetitive spontaneous licking task, rats were encouraged to lick a spout intermittently by the presentation of a 20% sucrose water reward at varying intervals. Single unit thalamocortical recordings were obtained with a custom-built Microdrive with bilateral 4-shank, 32-channel A4x2-tet silicon probes (NeuroNexus, USA) with RHD2000 Intan headstage amplifier chips (Intan Technologies, USA) and the open-electrophysiology platform, Open Ephys, sampled at 30 kHz. NDManager, Neuroscope and Klusters suite software was used for analysis. Results: fMRI was obtained from 18 animals during 117 sessions, for which a total of 1,906 seizures were recorded. Time-course analysis showed a decrease in blood oxygen level dependent (BOLD) signal in multiple cortical regions and an increase in BOLD signal in the thalamus during seizure activity compared to baseline. 17 rats underwent the sensory detection task, with 5,836 recorded seizures. Responses to the auditory stimulus were severely impaired throughout the seizure period and rapidly recovered after seizure offset. 24 rats performed the spontaneous licking task, with 18,797 recorded seizures. While there was an average behavioral impairment during seizure activity, the degree of behavioral impairment on the spontaneously licking task varied significantly between seizures. Severity of behavioral impairment on the spontaneous licking task correlated with electrophysiologic parameters. Spectral analysis for both behavioral tasks suggests pre-ictal state-related electrophysiologic changes with gradual decrease in high gamma (65-500 Hz) power and increases in delta and alpha power closer to seizure onset. Single unit neuronal recordings in 8 animals showed heterogenicity in corticothalamic firing patterns during seizure activity. Conclusions: Hemodynamic and behavioral changes in our awake GAERS model of absence seizures were similar to humans. BOLD signal during seizures in awake GAERS have significant differences between the cortex and thalamus. Behavior was impaired during seizure activity in GAERS. The degree of impairment varied significantly between seizures, with electrographic correlates. State-related preictal changes may relate to physiologic and behavioral severity. The neuronal and hemodynamic determinants of seizure severity may lead to a better understanding of the mechanisms underlying absence seizure and guide improved therapy options. Funding: NIH R37 NS100901 – Neuroimaging, neuronal firing and behavior in spike-wave seizures. PI: Hal BlumenfeldPostdoctoral Fellowship 337053, Epilepsy Foundation – fMRI and neural activity underlying impaired behavior in absence seizures. PI: Cian McCafferty