GABAergic signalling in a neurogenic niche of the turtle spinal cord

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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 GABAA 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.

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 Ca2+ 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 GABAA receptors. In CC-contacting neurones, GABAA 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 Ca2+ that required extracellular Ca2+ and was blocked by the selective GABAA 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.

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