Ca_V3.2 drives sustained burst-firing, which is critical for absence seizure propagation in reticular thalamic neurons

Objective: Genetic alterations have been identified in the CACNA1H gene, encoding the Ca_V3.2 T-type calcium channel in patients with absence epilepsy, yet the precise mechanisms relating to seizure propagation and spike-wave-discharge (SWD) pacemaking remain unknown. Neurons of the thalamic reticular nucleus (TRN) express high levels of Ca_V3.2 calcium channels, and we investigated whether a gain-of-function mutation in the Cacna1h gene in Genetic Absence Epilepsy Rats from Strasbourg (GAERS) contributes to seizure propagation and pacemaking in the TRN. Methods: Pathophysiological contributions of Ca_V3.2 calcium channels to burst firing and absence seizures were assessed in vitro using acute brain slice electrophysiology and quantitative real-time polymerase chain reaction (PCR) and in vivo using free-moving electrocorticography recordings. Results: TRN neurons from GAERS display sustained oscillatory burst-firing that is both age- and frequency-dependent, occurring only in the frequencies overlapping with GAERS SWDs and correlating with the expression of a Ca_V3.2 mutation-sensitive splice variant. In vivo knock-down of Ca_V3.2 using direct thalamic injection of lipid nanoparticles containing Ca_V3.2 dicer small interfering (Dsi) RNA normalized TRN burst-firing, and in free-moving GAERS significantly shortened seizures. Significance: This supports a role for TRN Ca_V3.2 T-type channels in propagating thalamocortical network seizures and setting the pacemaking frequency of SWDs.