Interneuron Desynchronization Precedes Seizures in a Mouse Model of Dravet Syndrome

Abstract

Recurrent seizures, which define epilepsy, are transient abnormalities in the electrical activity of the brain. The mechanistic basis of seizure initiation, and the contribution of defined neuronal subtypes to seizure pathophysiology, remains poorly understood. We performedin vivotwo-photon calcium imaging in neocortex during temperature-induced seizures in male and female Dravet syndrome (Scn1a+/−) mice, a neurodevelopmental disorder with prominent temperature-sensitive epilepsy. Mean activity of both putative principal cells and parvalbumin-positive interneurons (PV-INs) was higher inScn1a+/− relative to wild-type controls during quiet wakefulness at baseline and at elevated core body temperature. However, wild-type PV-INs showed a progressive synchronization in response to temperature elevation that was absent in PV-INs fromScn1a+/− mice. Hence, PV-IN activity remains intact interictally inScn1a+/− mice, yet exhibits decreased synchrony immediately before seizure onset. We suggest that impaired PV-IN synchronization may contribute to the transition to the ictal state during temperature-induced seizures in Dravet syndrome.SIGNIFICANCE STATEMENTEpilepsy is a common neurological disorder defined by recurrent, unprovoked seizures. However, basic mechanisms of seizure initiation and propagation remain poorly understood. We performedin vivotwo-photon calcium imaging in an experimental model of Dravet syndrome (Scn1a+/− mice)—a severe neurodevelopmental disorder defined by temperature-sensitive, treatment-resistant epilepsy—and record activity of putative excitatory neurons and parvalbumin-positive GABAergic neocortical interneurons (PV-INs) during naturalistic seizures induced by increased core body temperature. PV-IN activity was higher inScn1a+/− relative to wild-type controls during quiet wakefulness. However, wild-type PV-INs showed progressive synchronization in response to temperature elevation that was absent in PV-INs fromScn1a+/− mice before seizure onset. Hence, impaired PV-IN synchronization may contribute to transition to seizure in Dravet syndrome.

Gene Therapy in Models of Severe Epilepsy due to Sodium Channelopathy

“…Acute brain slices were prepared at postnatal day (P) 21 to 28, and electrophysiological recordings demonstrated that deficits in action potential generation in parvalbumin-expressing fast-spiking GABAergic interneurons (PV-INs)-known to be dysfunctional in Scn1a þ/À mice-were rescued by the Scn1a.dCas9 system. Recent data suggest that PV-INs in Scn1a þ/À mice that survive to P35 or beyond recover normal intrinsic excitability 7 and in vivo 2photon calcium imaging of PV-INs in awake, behaving Scn1a þ/À mice shows that activity during quiet wakefulness and running behavior is similar to wild-type, 8 supporting the conclusion that PV-IN excitability normalizes over time. These data raise the question as to whether increasing Nav1.1 expression exclusively in PV-INs in a chronic phase of Dravet syndrome would actually be efficacious at all, as these neurons seem to have at least partially "fixed themselves."…”

2P or not 2P: The Question of Seizure Initiation

“…However, naturalistic seizures have been studied using 2P imaging in mouse models of genetic epilepsies including absence seizures 3 and temperature-induced generalized tonic-clonic seizures in Dravet syndrome (Scn1aþ/-mice). 4 Despite these limitations, the highlighted study takes an important step forward both conceptually and methodologically toward greater understanding of seizure mechanisms. Two-photon calcium imaging and other large-scale imaging modalities, particularly when validated by or in combination with electrophysiology and combined with manipulation of activity, promise to be powerful tools for the studying of mechanisms of seizure initiation and propagation.…”

“…And of course electrophysiological methods also continue to advance, including optogenetic tagging of neuronal populations to identify specific cell types 1 and novel probes that facilitate recording of on the order of a thousand single units. 2 Recent studies 3 - 6 have used an optical imaging method, 2-photon laser scanning microscopy (2P imaging) combined with use of genetically encoded calcium indicators (2P calcium imaging) to record seizures. This technique uses a high-powered, pulsed infrared laser that facilitates imaging at depth in light scattering environments such as the intact brain.…”

“…However, naturalistic seizures have been studied using 2P imaging in mouse models of genetic epilepsies including absence seizures 3 and temperature-induced generalized tonic–clonic seizures in Dravet syndrome (Scn1a+/-mice). 4 …”

The Heated Relationship Between Neural Activity and Seizures

“…A recent paper by Tran et al used 2P MCI to monitor neural activity in Ex neurons and parvalbumin-positive (PV + ) In neurons leading up to seizures in Scn1a +/− mice, which model the human disease Dravet Syndrome, 6 involving loss of function of the voltage-gated sodium channel Na(v) 1.1. This design is innovative because it places further constraints on the issue by (1) using a mouse genetic model of epilepsy in which the cellular effects of the causative gene variant are relatively well understood, (2) imaging a genetically defined cell population that is known to be vulnerable to Scn1a loss, and (3) taking advantage of the fact that these mice have temperature-dependent seizures.…”