Astroglial cells are heavily involved in transportation of glycine, which acts as a neurotransmitter (inhibitory as well as excitatory) in several regions of the nervous system, particularly in the spinal cord. In addition, glycine is an important neuromodulator acting on NMDA receptors throughout the brain. Glycine transporters present in neurones and astrocytes are functionally and structurally different. Astrocytes predominantly express Glycine transporter 1 (GlyTl), while neurones express GlyT2; both cotransport glycine with Na+ and Cl-, yet their stoi-chiometry is different. Glycine translocation by GlyTl is coupled with cotransport of 2 Na+ and 1 Cl-, whereas GlyT2 requires cotransporting 3 Na+ and 1 Cl-. As a consequence, the reversal potential of glial GlyT1 is very close to the resting membrane potential, and therefore even a slight depolarization may promote the reversal of transporter and efflux of glycine into the extracellular space; this in turn can be important for modulating neuronal excitability. Neuronal GlyT2, in contrast is primarily involved with glycine clearance.
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