Abstracts

Rationale: Motor seizures are dangerous because they increase the risk of injury and sudden unexpected death in epilepsy (SUDEP). In order to understand the pathophysiology of motor seizures, circuits generating and propagating them must be delineated first. Methods: To label neurons activated by the seizure, we used TRAP mice (Targeted Recombination in Active Populations), which utilize the promoter region of immediate early genes (IEGs) to drive the expression of tamoxifen-dependent CreERT2. To initiate frontal motor seizures, we implanted 1.7 mg of cobalt wire in the secondary motor cortex of TRAP mice. To visualize anatomical projections from the seizure focus, we injected AAV9-GFP virus two weeks prior to Co insertion at the same site. Sections were processed for staining or CLARITY. We also recorded local field potentials (LFPs) at specific locations in the circuit to measure seizure spread. Results: We hypothesized that seizures follow the efferent connections of the premotor cortex focus. Surprisingly, our results indicate consistent activation of the striatum, globus pallidus, and substantia nigra reticulata during premotor onset seizures. Seizures engage these structures as indicated by tdTomato expression. We also confirmed striatal activation by implanting microelectrodes to measure local field potentials. Viral track tracing from the premotor cortex indicates that there are direct connections from the premotor cortex to the striatum, globus pallidus, and substantia nigra reticulata, and seizures followed these connections as indicated by tdTomato colocalization with GFP. Electrophysiological recordings also indicate that seizure spread to the contralateral cortex was faster than to the contralateral thalamus, showing that more complex circuits are engaged during secondarily generalized motor seizures in addition to the canonical thalamocortical circuit. During secondarily generalized seizures, layer 2/3 pyramidal neurons in the contralateral cortex expressed tdTomato and were in close proximity to the GFP positive axons. Corpus callosum showed colocalization of tdTomato and GFP, indicating that seizures travel through the callosal fibers to the contralateral hemisphere. To characterize spread from motor to somatosensory cortex, we quantified tdTomato expression using cortical layer markers (n=7) and LFPs. Layer 2/3 was more active than layer 5/6 throughout all cortical regions as indicated by tdTomato expression. Seizures spread to the contralateral premotor cortex consistently within 30 mS, whereas they consistently took longer than 200 mS to reach ipsilateral somatosensory cortex. Focal seizures consistently engaged layer 2/3 of the somatosensory cortex (100%, 50-100%, p=0.002) but not layer 5/6 (71.4%, 0-85.7%, p=0.002). Layer 5/6 was engaged later than 2/3 during generalized seizures. Conclusions: Our results indicate that the spread of motor seizures is more complex than previously proposed thalamocortical circuit. Funding: NIH 2R01NS040337-13