Chloride Homeostasis in Developing Motoneurons

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Abstract
Maturation of GABA/Glycine chloride-mediated synaptic inhibitions is crucial for the establishment of a balance between excitation and inhibition. GABA and glycine are excitatory neurotransmitters on immature neurons that exhibit elevated [Cl−]i. Later in development [Cl−]i drops leading to the occurrence of inhibitory synaptic activity. This ontogenic change is closely correlated to a differential expression of two cation-chloride cotransporters that are the Cl− channel K+/Cl− co-transporter type 2 (KCC2) that extrudes Cl− ions and the Na+-K+-2Cl− cotransporter NKCC1 that accumulates Cl− ions. The classical scheme built from studies performed on cortical and hippocampal networks proposes that immature neurons display high [Cl−]i because NKCC1 is overexpressed compared to KCC2 and that the co-transporters ratio reverses in mature neurons, lowering [Cl−]i. In this chapter, we will see that this classical scheme is not true in motoneurons (MNs) and that an early alteration of the chloride homeostasis may be involved in pathological conditions. Keywords Chloride co-transporters (CCCs) KCC2 NKCC1 ALS disease SOD1G93A mouse; time course Inhibitory synaptic events GABA/glycine Patch-clamp Modelling Spinal cord motoneuron

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