Glutamate transporters and retinal excitotoxicity

Glutamate appears to play a major role in several degenerative retinal disorders. However, exogenous glutamate is only weakly toxic to the retina when glutamate transporters on Müller glial cells are operational. In an ex vivo rat retinal preparation, we previously found that exogenous glutamate causes Müller cell swelling but does not trigger excitotoxic neurodegeneration unless very high concentrations that overwhelm the capacity of glutamate transporters are administered. To determine the role of glutamate transporters in Müller cell swelling and glutamate-mediated retinal degeneration, we examined the effects of DL-threo-beta-benzyloxyaspartate (TBOA), an agent that blocks glutamate transport but that unlike most available transport inhibitors is neither a substrate for transport nor a glutamate receptor agonist. We found that TBOA triggered severe retinal neurodegeneration attenuated by ionotropic glutamate receptor antagonists. TBOA-induced neuronal damage was also diminished by riluzole, an agent that inhibits endogenous glutamate release. In the presence of riluzole, to inhibit glutamate release plus TBOA to block glutamate uptake, the addition of low concentrations of exogenous glutamate triggered severe excitotoxic neuronal damage without inducing Müller cell swelling. We conclude that TBOA-sensitive glutamate transporters play an important role in regulating the neurodegenerative effects of glutamate in the rat retina.