Dravet Syndrome is a severe neurodevelopmental disorder caused by heterozygous loss-of-function mutations in the SCN1A gene. Clinically, it is characterized by febrile and afebrile seizures, developmental delay, cognitive impairments, and a significantly increased risk of sudden unexpected death in epilepsy (SUDEP). The SCN1A gene encodes Nav1.1, a voltage-gated sodium channel primarily expressed in inhibitory GABAergic interneurons, including parvalbumin-positive (PV+) basket cells.

In Dravet models, PV+ basket cells exhibit reduced sodium currents and decreased action potential (AP) firing frequency. This dysfunction contributes to impaired inhibition and neuronal hyperexcitability. However, while basket cells have been implicated in Dravet, another class of PV+ interneurons called chandelier cells remains largely understudied. Chandelier cells are a distinct subtype of GABAergic interneurons that form axo-axonic synapses exclusively onto the axon initial segment (AIS) of pyramidal neurons. The AIS is the specialized domain at the beginning of the axon where action potentials are initiated and is enriched with voltage-gated sodium channels that determine whether a neuron fires. Pyramidal neurons, the major excitatory projection cells of the cortex, are responsible for integrating synaptic input and transmitting signals across cortical and subcortical networks. By directly targeting the AIS, chandelier cells exert precise inhibitory control over the most critical site for action potential generation, tightly regulating the timing and output of pyramidal cells. This unique positioning makes chandelier cells powerful modulators of cortical activity and particularly relevant in the context of Dravet Syndrome. To investigate chandelier cell contributions to Dravet pathology, we conducted both structural and functional analyses in a mouse model of Dravet Syndrome. Biocytin-filled chandelier cells from postnatal day P18–21 and P35-56 wild-type and Dravet mice were processed using tissue clearing and immunostained for ankyrin-G to label the AIS. Confocal microscopy and Neurolucida software enabled 3D reconstruction and quantification of morphological features, including mean bouton count per cartridge, cartridge number, number of axon terminals, etc. Preliminary structural data (n = 1), suggests subtle abnormalities in chandelier cell axonal morphology in Dravet mice. Structural reconstructions showed that at P18–21, Dravet chandelier cells showed slightly higher cartridge counts compared to wild-type. However by P35–56 this trend reversed as Dravet cells exhibited fewer cartridges, reduced axon length, and decreased axonal complexity and branch order. The mean amount of boutons per cartridge remained largely unchanged across groups, suggesting bouton density remained the same despite cartridge differences. Electrophysiological recordings further demonstrated functional impairments, with Dravet chandelier cells displaying higher rheobase and reduced input resistance at P18–21, and a significant reduction in firing frequency at higher current injections by P35–56.

Together, these findings provide preliminary evidence that chandelier cell dysfunction may contribute to impaired cortical inhibition and hyperexcitability in Dravet Syndrome. Future aims with this study include completing data sets to strengthen conclusions and exploring the developmental progression of chandelier cell abnormalities in Dravet mice by looking at various developmental time points and comparing them to current-clamp patch recordings.