Glycine Release from Radial Cells Modulates the Spontaneous Activity and Its Propagation during Early Spinal Cord Development

Abstract

Rhythmic electrical activity is a hallmark of the developing embryonic CNS and is required for proper development in addition to genetic programs. Neurotransmitter release contributes to the genesis of this activity. In the mouse spinal cord, this rhythmic activity occurs after embryonic day 11.5 (E11.5) as waves spreading along the entire cord. At E12.5, blocking glycine receptors alters the propagation of the rhythmic activity, but the cellular source of the glycine receptor agonist, the release mechanisms, and its function remain obscure. At this early stage, the presence of synaptic activity even remains unexplored. Using isolated embryonic spinal cord preparations and whole-cell patch-clamp recordings of identified motoneurons, we find that the first synaptic activity develops at E12.5 and is mainly GABAergic. Using a multiple approach including direct measurement of neurotransmitter release (i.e., outside-out sniffer technique), we also show that, between E12.5 and E14.5, the main source of glycine in the embryonic spinal cord is radial cell progenitors, also known to be involved in neuronal migration. We then demonstrate that radial cells can release glycine during synaptogenesis. This spontaneous non-neuronal glycine release can also be evoked by mechanical stimuli and occurs through volume-sensitive chloride channels. Finally, we find that basal glycine release upregulates the propagating spontaneous rhythmic activity by depolarizing immature neurons and by increasing membrane potential fluctuations. Our data raise the question of a new role of radial cells as secretory cells involved in the modulation of the spontaneous electrical activity of embryonic neuronal networks.

Microglia Proliferation Is Controlled by P2X7 Receptors in a Pannexin-1-Independent Manner during Early Embryonic Spinal Cord Invasion

“…Microglia in the SC of the mouse embryo already express functional P2X7R at the onset of MN developmental cell death and synaptogenesis (Scain et al, 2010;Rigato et al, 2011). P2X7R activation by high concentration of ATP evokes a biphasic current in embryonic microglia, but our results indicate that although these cells express Panx1, there is no functional coupling of P2X7R to Panx1 in the embryo.…”

“…Drug application and pharmacological agents. All drugs were applied using a 0.5 mm diameter quartz tubing positioned 1 mm away from the recording area (Scain et al, 2010). The quartz tubing was connected using a manifold to reservoirs filled with a control solution or with P2X receptor agonists and/or channel blockers or P2X receptor antagonists.…”

Pattern of invasion of the embryonic mouse spinal cord by microglial cells at the time of the onset of functional neuronal networks

“…This is especially the case for the spinal cord. Embryonic development of the spinal cord has been extensively studied, from both a cellular and functional perspective, revealing critical developmental stages for the emergence of functional neuronal networks (Branchereau et al, 2000;Goulding, 2009;Myers et al, 2005;Scain et al, 2010). The pattern of microglial cell distribution in the embryonic spinal cord was first investigated in the human fetus (Rezaie et al, 1999).…”

Electrical maturation of spinal neurons in the human fetus: comparison of ventral and dorsal horn

“…Electrical activity is thought to exist in the embryonic spinal cords of mice at E12, based on recordings of spontaneous rhythmic activity in the sciatic nerve (Hanson and Landmesser 2003). Similarly, in the rat ventral horn (VH), presumptive motoneurons exhibit glycinergic postsynaptic currents from as early as E12.5 (Scain et al 2010) and discharge antidromic action potentials (APs) by E18 (Di Pasquale et al 1996). This occurs later in the dorsal horn (DH), where small and very broad APs have been recorded in neurons from E15 mice (Walsh et al 2009).…”