Abstracts

Rationale: Dravet Syndrome (DS) is characterized by prominent seizures, with non-seizure aspects of the phenotype progressively becoming more significant over time. DS primarily arises from pathogenic variants in SCN1A, which lead to impaired expression of Nav 1.1, a sodium channel crucial for neuronal signal transmission. Nav 1.1 is expressed throughout the brain, including established ictogenic regions such as the neocortex and hippocampus, as well as subcortical regions including the substantia nigra pars reticulata (SNr) and pedunculopontine nucleus (PPN). Despite the widespread expression of Nav 1.1, the specific contributions of subcortical networks, particularly the nigrotegmental circuit, to the DS phenotype remain poorly understood.

Methods: We investigated the nigrotegmental circuit in an Scn1a+/- mouse model of DS using whole-cell patch clamp recordings and fiber photometry. We compared the intrinsic cell properties of parvalbumin (PV+) inhibitory neurons in the SNr and cholinergic (ChAT+) neurons in the PPN between Scn1a+/- and wild-type littermate mice. We also employed simultaneous EEG and fiber photometry recordings to monitor SNr PV+ and PPN ChAT+ population activity during seizures induced by hyperthermia.

Results: SNr PV+ neuronal firing in response to depolarizing current steps was significantly decreased in Scn1a+/- mice, in addition to increased individual spike width. In contrast, Scn1a+/- PPN ChAT+ neurons were indistinguishable from wild-type. Imaging population level fluorescence activity via fiber photometry revealed that SNr PV+ neurons had a profound decrease in activity during hyperthermia-evoked seizures. In contrast, PPN ChAT+ neurons had a significant increase in calcium transients prior to seizure onset and significant increase in baseline fluorescence during the seizure, followed by post-ictal attenuation.

Conclusions: Dysfunction observed here in Scn1a+/- SNr PV+ neuronal spike generation has been seen in PV+ neurons in neocortex and hippocampus as well, underscoring the conceptualization of DS as a whole-brain disorder. Surprisingly, our data revealed no change in PPN ChAT+ neurons despite prior reports that a substantial fraction express Nav1.1, suggesting that Nav1.1 may not be crucial for somatic spike initiation in slower spiking cholinergic neurons. Multiple previous studies have shown that exogenous SNr inhibition suppresses seizures. If our finding of endogenous SNr inhibition during hyperthermia-evoked seizures is replicated in spontaneous seizures, it may be the case that the SNr contributes to intrinsic seizure termination mechanisms in this mouse model. We hypothesize that PPN ChAT+ excitation observed during pre-ictal and ictal periods may result from a combination of excitatory cortical input and SNr disinhibition, and that PPN activation may serve as an additional endogenous seizure termination mechanism. Future experiments manipulating these neuronal populations during seizures will test the causal role of this circuit in seizure initiation and termination, thereby assessing its potential as a focal therapeutic target.

Funding: NIH NINDS T32 NS115724 (C.C.R.); Taubman Institute Emerging Scholar Award (J.M.); NIH NINDS K08 NS121464 (J.M.).