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

Abstract

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) activity at the onset of SNA (E12.5) in the embryonic mouse SC (both sexes). V1 are one of the interneurons known to contact MNs, which are generated early in the embryonic SC. Here, we show that V1 already produce GABA in E12.5 embryo, and that V1 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 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 (). This is the first demonstration that 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, 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 in embryonic SC neurons may play a role in the early development of mammalian locomotor networks. 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) are GABAergic at E12.5 and spontaneously active during SNA. We uncover a new role for persistent sodium currents () in driving plateau potential in V1 and in SNA patterning in the embryonic SC. Our study thus sheds light on a role for NaP in the excitability of V1 and the developing SC.

Two opposite voltage-dependent currents control the unusual early development pattern of embryonic Renshaw cell electrical activity

“…We also uncovered a relationship between ! "# and the ability of embryonic V1 R to sustain repetitive firing (Boeri et al 2018). However, the heterogeneous firing patterns of V1 R observed at E12.5 could not be fully explained by variations in !…”

“…We previously highlighted that V1 R are spontaneously active at E12.5. Their response to a 2 s suprathreshold depolarizing current steps revealed four main patterns, depending of the recorded interneuron (Boeri et al 2018): V1 R firing only 1-3 APs at the onset depolarizing pulse, which were named single spiking (SS) V1 R , repetitive spiking (RS) V1 R ( Figure 1A), long-lasting sodium-dependent plateau potentials (PP) V1 R and mixed events (ME) V1 R that shows an alternation of action potentials (APs) and plateau potentials (Boeri et al 2018).…”

“…However, little is known about the maturation of the intrinsic properties and the firing pattern of embryonic V1 R . We recently found that V1 R exhibit heterogeneous excitability properties when SNA emerges in the SC (Boeri et al 2018) in contrast to adult Renshaw cells that constitute a functionally homogeneous population (Bikoff et al 2016, Perry et al 2015. activity during SNA episodes is also unknown.…”

“…During SNA episodes, long lasting giant depolarization potentials (GDPs) are evoked in the SC, mainly by the massive release of GABA onto MNs (Czarnecki et al 2014). Renshaw cells (V1 R ) are likely the first GABAergic partners of MNs in the mouse embryo (Benito-Gonzalez & Alvarez 2012, Boeri et al 2018, and the massive release of GABA during SNA probably requires that many of them display repetitive action potential firing or plateau potential activity (Boeri et al 2018).…”

Contributions of h- and Na+/K+ pump currents to the generation of episodic and continuous rhythmic activities

“…We provide an analysis of cellular dynamics when each element in our model is individually manipulated, and we find parallel changes in biological network behaviour when these elements are individually blocked pharmacologically. Critically, some of the currents we studied here, such as INaP, are present in spinal neurons as early as embryonic day (E) 12 (Boeri et al, 2018;Kuo et al, 2006), suggesting that modulating conductances within a simple circuit can produce diverse output patterns. Similarly, Ih may contribute to circuit development in rodents (Yang et al, 2018), and is present in rat cultured motoneurons from E14 (Elliott et al, 1999).…”