Persistent Sodium Current Drives Excitability of Immature Renshaw Cells in Early Embryonic Spinal Networks

Spontaneous network activity (SNA) emerges in the spinal cord (SC) before the formation of peripheral sensory inputs and central descending inputs. SNA is characterized by recurrent giant depolarizing potentials (GDPs). Because GDPs in motoneurons (MNs) are mainly evoked by prolonged release of GABA, they likely necessitate sustained firing of interneurons. To address this issue we analyzed, as a model, embryonic Renshaw cell (V1^R) activity at the onset of SNA (E12.5) in the embryonic mouse SC (both sexes). V1^R are one of the interneurons known to contact MNs, which are generated early in the embryonic SC. Here, we show that V1^R already produce GABA in E12.5 embryo, and that V1^R make synaptic-like contacts with MNs and have putative extrasynaptic release sites, while paracrine release of GABA occurs at this developmental stage. In addition, we discovered that V1^R are spontaneously active during SNA and can already generate several intrinsic activity patterns including repetitive-spiking and sodium-dependent plateau potential that rely on the presence of persistent sodium currents (I_Nap). This is the first demonstration that I_Nap is present in the embryonic SC and that this current can control intrinsic activation properties of newborn interneurons in the SC of mammalian embryos. Finally, we found that 5 μm riluzole, which is known to block I_NaP, altered SNA by reducing episode duration and increasing inter-episode interval. Because SNA is essential for neuronal maturation, axon pathfinding, and synaptogenesis, the presence of I_NaP in embryonic SC neurons may play a role in the early development of mammalian locomotor networks.

SIGNIFICANCE STATEMENT The developing spinal cord (SC) exhibits spontaneous network activity (SNA) involved in the building of nascent locomotor circuits in the embryo. Many studies suggest that SNA depends on the rhythmic release of GABA, yet intracellular recordings of GABAergic neurons have never been performed at the onset of SNA in the SC. We first discovered that embryonic Renshaw cells (V1^R) are GABAergic at E12.5 and spontaneously active during SNA. We uncover a new role for persistent sodium currents (I_NaP) in driving plateau potential in V1^R and in SNA patterning in the embryonic SC. Our study thus sheds light on a role for INaP in the excitability of V1^R and the developing SC.

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