Contribution of a glial glutamate transporter to GABA synthesis in the retina

Neuronal glutamate transporters have been shown to play a role in GABA synthesis by enhancing glutamate uptake. In the present study, we have examined whether a glial glutamate transporter, GLAST, has a role in GABA synthesis in the mammalian retina. We found that the retinal GABA level was about two-fold higher in the GLAST^-/- mouse retina compared to that in the wild type. Endogenous glutamate level was also increased about 2-fold in the mutant. Therefore, loss of GLAST results in a higher retinal GABA level, probably due to increased availability of its precursor, glutamate. An increase in GABAergic activity can be expected to affect trigger features such as directional selective response of neurons in the GLAST^-/- mouse retina.