GABAergic signalling in a neurogenic niche of the turtle spinal cord

Abstract

Non-technical summary Neurogenesis is tightly regulated by epigenetic factors that assure the correct assembly of neural circuits. Neurotransmitters play a fundamental role in this type of control. We show that GABA signals on progenitors and immature neurones within a neurogenic niche around the central canal (CC) of the turtle spinal cord. GABA depolarized progenitors whereas the effect on immature neurones varied from excitation to inhibition. In both cell types GABA A receptor activation induced an increase in intracellular calcium. Our findings imply that GABAergic signalling around the CC shares fundamental properties with those in the embryo and adult neurogenic niches in the brain, suggesting that GABA is part of the mechanisms regulating the production and integration of neurones to already operational spinal circuits. Understanding the GABAergic modulation of progenitors and neuroblasts may provide useful clues about key mechanisms needed for functional neurogenesis in the spinal cord.Abstract The region that surrounds the central canal (CC) in the turtle spinal cord is a neurogenic niche immersed within already functional circuits, where radial glia expressing brain lipid binding protein (BLBP) behave as progenitors. The behaviour of both progenitors and neuroblasts within adult neurogenic niches must be regulated to maintain the functional stability of the host circuit. In the brain, GABA plays a major role in this kind of regulation but little is known about GABAergic signalling in neurogenic niches of the postnatal spinal cord. Here we explored the action of GABA around the CC of the turtle spinal cord by combining patch-clamp recordings of CC-contacting cells, immunohistochemistry for key components of GABAergic signalling and Ca 2+ imaging. Two potential sources of GABA appeared around the CC: GABAergic terminals and CC-contacting neurones. GABA depolarized BLBP + progenitors via GABA transporter-3 (GAT3) and/or GABA A receptors. In CC-contacting neurones, GABA A receptor activation generated responses ranging from excitation to inhibition. This functional heterogeneity appeared to originate from different ratios of activity of the Na + -K + -2Cl − co-transporter (NKCC1) and the K + -Cl − co-transporter (KCC2). In both progenitors and immature neurones, GABA induced an increase in intracellular Ca 2+ that required extracellular Ca 2+ and was blocked by the selective GABA A receptor antagonist gabazine. Our study shows that GABAergic signalling around the CC shares fundamental properties with those in the embryo and adult neurogenic niches, suggesting that GABA may be part of the mechanisms regulating the production and integration of neurones within operational spinal circuits in the turtle.

GABAergic responses of mammalian ependymal cells in the central canal neurogenic niche of the postnatal spinal cord

“…The strong correlation between the change in input resistance and the change in membrane potential in response to 18β-GA indicates that the depolarisation is a direct effect of gap junction blockade rather than a non-gap junction specific effect of 18β-GA. This effect is similar to that observed in progenitor cells surrounding the turtle CC [20]. A possible reason for ependymal cells to form gap junctions is to allow the control of cellular proliferation, as seen in the embryonic neocortex and in the adult SVZ [3,11]…”

“…It was investigated whether the remainder of the GABAergic response could be mediated by GABA transporters, as observed in ependymal cells of turtle spinal cord [20]. The bath application of GABA in the presence of nipecotic acid (900 μM) or guvacine (50 μM), non-selective GABA transporter blockers, resulted in a significant depolarisation of ependymal cells (4.11 ± 0.67 mV; n  = 7; Fig.…”

“…A recent study by Reali et al [20], explored the effects of GABA on brain lipid binding protein (BLBP) immunoreactive progenitor cells that are found surrounding the CC of the turtle spinal cord; these cells appear to be similar to the radial ependymal cells or potential radial glia that surround the mammalian CC [16,17]. GABA depolarised these progenitor cells inducing increases in intracellular Ca 2+ , suggesting that GABA is capable of stimulating long lasting intracellular effects within these cells [20]. The response to GABA was mediated by either GABA A receptors, the GABA uptake transporter, GAT3, or a combination of both [20], highlighting variation within the population of progenitor cells.…”

“…GABA depolarised these progenitor cells inducing increases in intracellular Ca 2+ , suggesting that GABA is capable of stimulating long lasting intracellular effects within these cells [20]. The response to GABA was mediated by either GABA A receptors, the GABA uptake transporter, GAT3, or a combination of both [20], highlighting variation within the population of progenitor cells. Neural stem and progenitor cells within other mammalian postnatal neurogenic niches such as the subventricular zone (SVZ) or the dentate gyrus (DG) also respond to GABA and as a result GABA is capable of influencing the proliferation and differentiation of these cells [12,23].…”

The late and dual origin of cerebrospinal fluid-contacting neurons in the mouse spinal cord

“…On the physiological properties and the role of CSF-cNs CSF-cNs have been found in every vertebrate species examined so far, from lamprey to primates (Vigh and Vigh-Teichmann, 1971;Vigh et al, 1977Vigh et al, , 1983Barber et al, 1982;Dale et al, 1987b;Stoeckel et al, 2003;Jalalvand et al, 2014;Djenoune et al, 2014;Reali et al, 2011). However, despite this evolutionary conservation, their function has not yet been fully elucidated.…”