Lighting the Fuse: Deconstructing Complex Network Interactions Using On-Demand Seizures

Commentary The unpredictability of seizures severely affects the quality of life of patients with epilepsy and is a major impediment to understanding the microcircuit dynamics in the seconds to minutes preceding a seizure (1). Although pharmacological approaches have been used to elicit and examine seizures, the onset is still unpredictable, and the pharmacology confounds mechanistic analyses. Thus, despite the explosion of techniques ranging from optogenetic manipulation of cellular activity, physiology, and neuronal imaging, the mechanisms underlying transition to seizure have remained elusive due to the inability to repeatedly and reliably predict seizure onset. In particular, the role of GABAergic interneurons has been difficult to decipher due to their diversity, distinct connections, and activity patterns. GABAergic neurons shape patterned activation of neuronal ensembles during essential brain functions (2). Traditionally, a reduction or transient collapse in GABAergic inhibition is thought to promote hyperexcitability and seizures; however, recent studies have shown an aberrant increase in GABAergic signaling at seizure onset suggesting an ictogenic role for GABA, possibly through pathological synchronization of networks (3–5). Nevertheless, the identity and contribution of interneuron classes to seizure initiation is not fully known, largely due to the unpredictable nature of seizures. Khoshkhoo and colleagues introduced a novel “optical-kindling” technique to evoke seizures on-demand through focal optogenetic activation of neurons in mouse primary motor cortex. They showed that 20–40 Hz optogenetic stimulation evokes seizures with a predictable 1–2-minute delay, enabling analysis of processes underlying seizure onset. Interestingly, the majority of optically evoked seizures were primarily generalized. However a few seizures had a focal origin often with a focus contralateral to the optically kindled hemisphere. The authors exploited this on-demand seizure model and mouse lines with Cre driven by cell specific promoters to examine the contribution of parvalbumin (PV) and somatostatin (SOM) interneurons, which are derived from the medial ganglionic eminence (MGE) and vasoactive intestinal peptide (VIP) expressing “interneuron-targeting interneurons” to seizure initiation and termination. By combining optical kindling with bilateral EEG and contralateral fiber photometric recordings of bulk calcium signals from neurons expressing a fluorescent reporter GCaMP6f in the primary motor cortex, they showed that all three interneuron classes exhibit rapid increases in calcium signals at seizure onset, approximately 10 sec ahead of excitatory neurons. While PV and SOM neurons showed sustained calcium signals until seizure termination, calcium signals in VIP and excitatory neurons decreased prior to seizure termination, indicating celltype specific differences in neuronal contribution to seizure maintenance and termination. The authors used cre-driven expression of Archaerhodopsin (eArch 3.0)—a microbial proton pump that hyperpolarizes the membrane on light activation—to silence specific interneurons and determine their role Dynamic, Cell Type-Specific Roles for GABAergic Interneurons in a Mouse Model of Optogenetically Inducible Seizures.