Glial transporters for glutamate, glycine and GABA I. Glutamate transporters.

The termination of chemical neurotransmission in the CNS involves the rapid removal of neurotransmitter from synapses by specific transport systems. Such mechanism operates for the three major amino acid neurotransmitters glutamate, gamma-aminobutyric acid (GABA) and glycine. To date, five different high-affinity Na(+)-dependent glutamate (Glu) transporters have been cloned: GLT1, GLAST, EAAC1, EAAT4 and EAAT5. The first two are expressed mainly by glial cells, and seem to be the predominant Glu transporters in the brain. A major function of Glu uptake in the nervous system is to prevent extracellular Glu concentrations from raising to neurotoxic levels in which glial transporters seem to play a critical role in protecting neurons from glutamate-induced excitotoxicity. Under particular conditions, glial GluTs have been shown to release Glu by reversal of activity, in a Ca(2+)--and energy-independent fashion. Furthermore, an activity of these transporters as ion channels or transducing units coupled to G-proteins has recently been reported. The localization, stoichiometry, and regulation of glial GluTs are outlined, as well as their possible contributions to nervous system diseases as ALS, AD and ischemic damage.