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(Abst. 217), 2020

Altered excitability, synaptic properties, and plasticity in the Scn1b knockout mouse model of Dravet syndrome
Authors: Jessica Hotard Chancey, University of Texas at Austin; Alisha Ahmed - University of Texas at Austin; Alexandra McConnell - University of Texas at Austin; MacKenzie Howard - University of Texas at Austin;
Mutations in the SCN1B gene have been linked to Generalized Epilepsy with Febrile Seizures Plus (GEFS+) and Dravet syndrome (DS). DS is a severe epileptic encephalopathy characterized by frequent, difficult to control seizures, and developmental and cognitive deficits. DS has limited treatment options in part due to a lack of understanding of the cellular and circuit level dysfunction underlying the phenotype. The SCN1B gene product, the β1 protein, regulates the trafficking and biophysical properties of several ion channels that are critical regulators of action potential initiation and dendritic excitability. β1 has also been shown to play roles in neurite outgrowth and neuron development. The Scn1b knockout (KO) mouse phenocopies human DS and has previously been shown to have hyperexcitable pyramidal neurons. The goal of this project is to define how the loss of β1 perturbs the interactions between synaptic and intrinsic properties in forebrain principal neurons, leading to deficits in synaptic integration and plasticity, to discover cellular mechanisms underlying the seizures and cognitive deficits in DS.
We performed whole-cell patch clamp and field recordings from CA1 pyramidal cells (PCs) in acute hippocampal slices from Scn1b KO and wild-type (WT) littermates, age p15-20. We measured intrinsic and synaptic properties and summation of patterned of synaptic stimulation.  We also examined theta-burst induced long-term potentiation (LTP) in slices from WT and KO mice.  We then reconstructed filled neurons to examine neuronal morphology.
We found that CA1 PCs from Scn1b KO mice fire more in response to current injection, yet do not exhibit other physiological changes normally associated with cellular hyperexcitability, such as resting membrane potential or threshold. KO neurons display modest increases in input resistance and reduced capacitance compared to WT neurons, which is indicative of reduced cell size. However, we found no difference in total dendrite length between KO and WT neurons, and an unexpected increase in branching in basal dendrites of KO neurons. In response to synaptic stimulation, PCs from KO mice have smaller and more facilitating excitatory and inhibitory postsynaptic currents compared to WT. Despite this, KO neurons have larger, prolonged depolarizations and increased occurrence of simple and complex spikes in response to patterned synaptic stimulation. We hypothesized that this combination of hyperexcitability and enhanced synaptic integration would lead to enhanced LTP in KO slices, but instead found deficits in LTP in slices from Scn1b KO mice.
Our data show that modest changes in intrinsic excitability and complex alterations in synaptic properties lead to a great enhancement of synaptic integration and input/output functions in Scn1b KO neurons. This is accompanied by deficits in synaptic plasticity, a cellular correlate of learning and memory. Our data indicate that loss of β1 fundamentally changes network excitability and information processing in the hippocampus, providing insights into cellular mechanisms underlying the complex phenotypes of DS.
:Current: AES Postdoctoral Fellowship to J.H.C; R01 NS112500; Dell Medical School at the University of Texas at Austin startup funds Previous: AES Young Investigator Award to M.A.H.; Dravet Syndrome Foundation Postdoctoral Fellowship to J.H.C