Diversity in neuronal intrinsic excitability during the early phase of spinal cord development: an example in a pioneer interneuron, the Renshaw cells.
Invited by : Igor MEDINA
Abstract : Spontaneous network activity (SNA) emerges in the spinal cord (SC) before the formation of peripheral sensory inputs and central descending inputs (E12.5 in the mouse embryo). 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 (V1R) activity at the onset of SNA (E12.5) in the embryonic mouse SC. V1R are one of the interneurons known to contact MNs, which are generated early in the embryonic SC. We found that V1R can already produce GABA and can make synaptic-like contacts with MNs at early developmental stages. We also found the presence of putative extrasynaptic release sites for GABA at E12.5, which may explain why we observe paracrine release of GABA at this developmental stage.
We discovered that V1R are spontaneously active at the onset of 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 (INap). Although motoneurons progressively acquire the ability to sustain repetitive firing during the early phase of embryonic spinal cord network development, the ability of V1R to sustain repetitive firing is no longer observed between E12.5 and E14.5 but recovers from E16.5. This was related to changes in INap density. These results indicate that the development of the neuronal intrinsic activity does not necessarily follow a linear process in the embryonic spinal cord. This raises the question of the role of this unconventional developmental pattern during the development of the spinal cord network in the embryo.