The extracellular concentration of glutamate, an excitatory neurotransmitter throughout the CNS, including the retina, must be kept low to ensure proper glutamate-mediated neurotransmission during synaptic activation and to prevent neuronal excitotoxic damage from prolonged exposure to glutamate and subsequent activation of glutamate receptors. Maintenance of extracellular glutamate is under the control of high-affinity, Na+-dependent glutamate transporters in the plasma membranes of neurons and glial cells, including glial cells in the retina. Reversed transport of glutamate, from inside the cell to outside the cell may be an important mechanism contributing to neuronal damage in a variety diseases, including stroke and glaucoma, where ischemic conditions are a factor. Release of glutamate from glial cells has been demonstrated to occur through the reversal of a glutamate transporter, but recent studies of neuronal glutamate transporters cloned from neurons and expressed in oocytes suggest that reversed transport of glutamate does not occur in the neuronal glutamate transporter.
A further example of the functional differences between neuronal and glial cell glutamate transporters is that dysfunctional glial cell transporters, rendered dysfunctional by transgenic methods, mediate glutamate neurotoxcity in the brain, while dysfunctional neuronal transporters contribute little to glutamate neurotoxicity. In this study we have shown that a retinal glial cell exhibits reversed transport of glutamate, releasing a large, dense cloud of extracellular glutamate surrounding retinal ganglion cells that is sufficient to mediate long-term glutamate neurotoxicity. Calcium-dependent release and reversed transport of glutamate from retinal neurons plays a relatively minor role in this release pattern.
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