Synaptic and intrinsic mechanisms impair reticular thalamus and thalamocortical neuron function in a Dravet syndrome mouse model

Abstract

Thalamocortical network dysfunction contributes to seizures and sleep deficits in Dravet syndrome (DS), an infantile epileptic encephalopathy, but the underlying molecular and cellular mechanisms remain elusive. DS is primarily caused by mutations in the SCN1A gene encoding the voltage-gated sodium channel NaV1.1, which is highly expressed in GABAergic reticular thalamus (nRT) neurons as well as glutamatergic thalamocortical neurons. We hypothesized that NaV1.1 haploinsufficiency alters somatosensory corticothalamic circuit function through both intrinsic and synaptic mechanisms in nRT and thalamocortical neurons. Using Scn1a heterozygous mice of both sexes aged P25-P30, we discovered reduced intrinsic excitability in nRT neurons and thalamocortical neurons in the ventral posterolateral (VPL) thalamus, while thalamocortical ventral posteromedial (VPM) neurons exhibited enhanced excitability. NaV1.1 haploinsufficiency enhanced GABAergic synaptic input and reduced ascending glutamatergic sensory input to VPL neurons, but not VPM neurons. In addition, glutamatergic cortical input to nRT neurons was reduced in Scn1a heterozygous mice, whereas cortical input to VPL and VPM neurons remained unchanged. These findings introduce input-specific alterations in glutamatergic synapse function and aberrant glutamatergic neuron excitability in the thalamus as disease mechanisms in Dravet syndrome, which has been widely considered a disease of GABAergic neurons. This work reveals additional complexity that expands current models of thalamic dysfunction in Dravet syndrome and identifies new components of corticothalamic circuitry as potential therapeutic targets. HIGHLIGHTS GABAergic reticular thalamus neurons have impaired tonic and burst firing properties in a NaV1.1 haploinsufficiency mouse model of Dravet syndrome. NaV1.1 haploinsufficiency has opposing effects on spike firing in two distinct glutamatergic thalamocortical neuron populations. NaV1.1 haploinsufficiency alters glutamatergic synaptic connectivity in an input-specific manner in the thalamus. Dysregulation of both intrinsic and synaptic mechanisms contribute to imbalanced thalamic excitation and inhibition in this Dravet syndrome mouse model.