Project Summary Many neurological diseases present with comorbid gastrointestinal (GI) abnormalities, suggesting either parallel altered functionality in the neurons of the central nervous system (CNS) and enteric nervous system (ENS) or aberrations in axonal pathfinding mechanisms that dictate neuronal connections in both brain and the GI tract. A potential mechanistic link between neurological and GI disorders is aberrant function of voltage- gated sodium channel (VGSC) and β subunits, which are expressed in the CNS and ENS. VGSCs control the generation and propagation of action potentials in neurons and therefore play a critical role in determining neuronal signaling. VGSC β1/β1B subunits, encoded by SCN1B, modulate channel plasma membrane trafficking, gating, and voltage-dependence. In addition, VGSC β1/β1B subunits mediate cell-cell adhesion, neuronal pathfinding, and axon fasciculation in the CNS. VGSC gene variants are linked not only to multiple forms of epileptic encephalopathies (EEs) but also to GI motility disorders, such as inflammatory bowel syndrome and Hirschsprung's disease. Additionally, EE patients with VGSC gene variants often exhibit chronic constipation – suggesting a connection between VGSCs and colonic motility. A subset of these EE patients has homozygous loss-of-function variants in SCN1B, which encodes the VGSC β1/β1B subunits. In addition to aberrations in neuronal firing within the CNS, multiple physicians report SCN1B-linked EE patients presenting with GI symptomology, such as constipation. Our Scn1b-null mice also exhibit spontaneous generalized seizures and colonic dysmotility. The overall objective of this work is to understand how Scn1b loss-of-function contributes to the GI abnormalities observed in the Scn1b-null mouse model. We hypothesize that β1/β1B subunits regulate (1) excitability in colonic myenteric neurons and (2) axonal fasciculation and pathfinding of the colonic myenteric plexus. Successful completion of this work will provide novel insights that may lead to new therapeutics for the treatment of EE patients with co-morbid neurological and GI symptomology.