Abstracts

Rationale: Focal neocortical seizures develop at one brain site and propagate through contiguous ictal wave expansion as well as through long axonal connections that recruit distal regions. Recent studies have found differences in excitatory (E) and inhibitory (I) neural activity across brain regions during seizures, and our recent work studying E:I balance in other seizure models and interictal spikes has led us to test here how excitatory and inhibitory activity differ across a well-defined bilateral seizure network in awake seizing animals. Understanding the role of E:I in seizure propagation through interconnected brain sites is an important next-step for translational experiments aimed at manipulating this activity to control epilepsy.

Methods: Transgenic mice expressing GCaMP6f in either Thy1 excitatory or PV inhibitory neurons were injected with 4-Aminopyridine (4AP, 2.5 mM) after being implanted with a large (8.2 mm) transparent cranial window. Widefield calcium imaging and electrophysiology were simultaneously performed to measure seizure propagation across a network of connected brain regions in awake animals. The ictal focus was placed in right somatosensory cortex (iS1), which projects heavily to ipsilateral M2 (iM2), and to contralateral S1 (cS1) by callosal projections; a secondary, di-synaptically connected network site, contralateral M2 (cM2) receives callosal input from iM2 and non-callosal projections from cS1. Allen Atlas brain maps were used to initially identify this network, and experiments with electrical stimulation and bicuculline were used to confirm the connectivity of this seizure network.

Results: Prior to electrographic ictal onset, PV and Thy1 neural activity increases slightly at the seizure focus. Following onset, on average, both PV and Thy1 neural activity show that propagation time in contralateral nodes connected by callosal projections (cM2 and cS1) is protracted relative to ipsilateral non-callosal node propagation (iS1 to iM2). Contralateral S1, despite receiving direct callosal projections from the focus, is recruited significantly later than the other contralateral site cM2 (a di-synaptic node), as measured in both PV and Thy1 activity.

Conclusions: The difference in seizure propagation speeds between the focus and ipsilaterally vs. contralaterally connected nodes supports the claim that callosal projections may vary in their function depending on their source (e.g., S1 vs M2), providing either predominantly excitatory or inhibitory inputs, though the specific feedforward inhibition mechanism (cell type) is not clear. These differences may underlie seizure propagation patterns and suggest the local E:I ratio could potentially be manipulated to control seizure networks by selectively targeting inhibitory or excitatory activity within connected nodes.

Funding: Mitchell Alan Ross Award, Weill Cornell Medicine