Abstracts

Rationale: Dravet syndrome (DS) is a severe epileptic encephalopathy that does not respond well to current treatments and has a high incidence of mortality. Approximately 70% of patients with DS have a loss-of-function mutation in one allele of the sodium-channel gene SCN1A. Although most evidence so far supports hippocampal and neocortical abnormalities in DS seizures, the thalamus, a deep brain structure with extensive and vital interconnections with cortical seizure circuits, also expresses high levels of Scn1a in inhibitory neurons within the thalamic reticular nucleus (TRN), which strategically positions the thalamus to regulate seizure circuits in DS. Methods: For our studies, we used a well-established mouse model of DS in which Scn1a is haploinsufficient. We first assessed thalamic circuit function by performing multiunit linear array recordings of thalamic neurons. To determine whether the intrinsic properties of TRN neurons and thalamocortical (TC) neurons in ventral basal thalamus (VB) and the anterior nucleus of the thalamus (AN) are altered in DS, we performed whole cell electrophysiology recordings in TRN and TC neurons in slices in vitro. We also expressed opsins in these neuron populations to determine how opsin activation modifies TRN and TC neuron firing. To interrogate the role of the thalamus in seizure expression in DS, we combined optogenetics with in vivo thalamic multiunit recordings, cortical EEG, and video behavior monitoring. Results: Results: Analysis of thalamic circuit function revealed that the somatosensory thalamic circuit is hyperexcitable and has widespread hyperactivity in DS (mean oscillation duration S.D. for WT slices (n = 9) is 2.4 s 1.6 s vs. DS slices (n = 12) is 10.5 s 7.5 s, Analysis by Mann-Whitney U, p < 0.01). This hyperexcitability can be explained in part by the increased rebound burst firing that we observe in TRN neurons in DS mice. We also found that TRN and thalamocortical (TC) cells fire high-frequency bursts of action potentials in phase with electrocorticographic spikes during spontaneous convulsive and non-convulsive seizures in DS mice. Thus, we hypothesized that rhythmic bursts of thalamic firing facilitate seizure expression in DS. To this end, we found that selectively inducing rhythmic bursts in TC neurons in VB is sufficient to trigger complex, generalized tonic-clonic seizure (GTCS) activity in freely behaving DS mice. Further, unilateral disruption of thalamic neuron bursting in VB with stable step function opsins is sufficient to interrupt non-convulsive seizures in DS mice. As deep brain stimulation in AN has been shown to reduce seizure frequency in a human patient with DS, we are exploring how manipulation of TC neuron firing in AN can alter seizures in DS. Future studies aim to determine whether opsin manipulation in VB and/or AN can interrupt convulsive seizures in DS. Conclusions: This work may provide new insight into the therapeutic potential of targeting the thalamus to treat seizures in DS and direct our future efforts in determining whether targeting the thalamus can also prevent behavioral deficits and/or Sudden Unexpected Death in Epilepsy in DS. Funding: Supported by a grant from the American Epilepsy Society and the Dravet Syndrome Foundation.