The floor-plate of His is a non-neuronal electrical conduction pathway
In the developing central nervous system, electrical signaling is thought to rely exclusively on differentiating neurons as they acquire the ability to generate action potentials. Accordingly, the neuroepithelial progenitors (NEPs) giving rise to all neurons and glial cells during development have been reported to remain electrically passive. Here, we investigated the physiological properties of NEPs in the mouse spinal cord at the onset of spontaneous neural activity (SNA) initiating motor behavior in embryos. Using patch-clamp recordings, we discovered that spinal NEPs exhibit spontaneous membrane depolarizations during episodes of SNA. These recurrent depolarizations exhibited a ventral-to-dorsal gradient with the highest amplitude located in the floor-plate - the ventral-most part of the neuroepithelium. Paired-recordings revealed that NEPs are extensively coupled via gap-junctions and form a single electrical syncytium. Although other NEPs were electrically passive, we discovered that floor-plate NEPs have the unique ability to generate large Na+/Ca++ action potentials. Unlike neurons, floor-plate action potentials relied primarily on the activation of voltage-gated T-type calcium channels (TTCCs). In situ hybridization showed that all 3 known subtypes of TTCCs are highly and predominantly expressed in the floor-plate. During SNA, we found that acetylcholine released by motoneurons recurrently trigger floor-plate action potentials by acting through nicotinic acetylcholine receptors. Finally, by expressing the genetically encoded calcium indicator GCaMP6f in the floor plate, we demonstrated that neuroepithelial action potentials are associated with calcium waves and propagate along the entire length of the spinal cord. By unraveling a novel physiological mechanism generating electrical signals which can propagate independently from neurons across a neural structure, our work significantly changes our understanding of the development, origin and extent of electrical signaling in the central nervous system. HIGHLIGHTSO_LISpinal neuroepithelial progenitors (NEP) are depolarized during spontaneous neural activity C_LIO_LINEPs form a single electrical syncytium connected by gap junctions C_LIO_LIFloor-plate NEPs generate large Na+/Ca++ action potentials in response to acetylcholine C_LIO_LINeuroepithelial action potentials propagate across the entire spinal cord C_LI