Circuit Map of Frontal Lobe Seizures

The central hypothesis of this dissertation is that anatomical connectivity of the seizure focus and properties of neurons drive seizure circuits. We combined novel techniques such as activity reporter TRAP2 mice, local field potential recordings, tissue clearing, viral tracing, chemogenetics, optogenetics, lesioning, and super resolution microscopy to interrogate the circuits active during frontal lobe focal motor to bilateral tonic-clonic seizures.
We found neuronal activation in the striatum, globus pallidus externus, subthalamic nucleus, substantial nigra pars reticulata, and neurons of the indirect pathway. Simultaneous LFP recordings from these structures showed that seizures activate structures via short and long latency loops, and activation of the basal ganglia modulates seizures. These studies led to the hypothesis that connectivity of excitatory neurons primarily drives long-distance seizure spread.
Seizures also preferentially activated dopamine D2 receptor-expressing neurons over D1 in the striatum, which have different projections. D2 neurons are more excitable than D1; thus, the properties of neurons also determine seizure circuits.
The D2 receptor agonist infused directly into the striatum through a bilateral cannula exerted an anticonvulsant effect. Systemic injection of the D2 agonist was also anticonvulsant in frontal lobe seizures. We found that injection of D2R agonist led to extensive activation of parvalbumin interneurons in the cortex and striatum ipsilateral to the seizure focus. D2R agonists activate PV interneurons, which in turn inhibit principal neurons, potentially explaining the anticonvulsant effect of D2R agonists.
Previous studies indicate that the thalamus is essential for seizure maintenance and generalization to the contralateral hemisphere. Surprisingly, we found that seizures spread faster to the contralateral cortex than to the contralateral thalamus. The seizure focus in the ipsilateral cortex was strongly connected to the contralateral cortex via the corpus callosum as indicated by viral track tracing, whereas the ipsilateral thalamus lacked direct monosynaptic connections to the contralateral thalamus. We propose that seizures spread from the seizure focus to the contralateral cortex by engaging the cortico-cortical commissure, corpus callosum, rather than via commissural projections between the two thalami or through bilateral spread from the thalamus via the brainstem.
After chemogenetic inhibition of the ipsilateral thalamus, we still found contralateral seizure spread, although seizure duration decreased significantly. Anterior callosotomy, on the other hand, prevented contralateral seizure spread during initial seizures. Thus, we propose that the thalamus amplifies seizures, whereas the corpus callosum allows transmission to the contralateral hemisphere.
Recent studies also indicate that superficial layers 2/3 are recruited ahead of the deep layers 5/6 locally, but it is not known how seizures spread intracortically over a long distance. We showed that superficial layers were more activated after a seizure compared to deep layers throughout the cortex. Seizures spread faster posteriorly through the superficial layers and arrived to the posterior deep layers with a delay. AAV9 GFP injected at the seizure focus labeled posterior superficial layers stronger than posterior deep layers, suggesting more direct monosynaptic projections exist across layers 2/3 than layers 5/6. We further showed that activated cells in the posterior cortex receive direct synaptic connections from the seizure focus. However, not all neurons that received direct projections from the seizure focus became active, which might suggest that those neurons were electrophysiologically different to begin with.