High-affinity glutamate transporters in the rat retina: A major role of the glial glutamate transporter GLAST-1 in transmitter clearance

Glutamate is the major excitatory neurotransmitter of the mammalian retina and glutamate uptake is essential for normal transmission at glutamatergic synapses. The reverse transcriptase-polymerase chain reaction (RT-PCR) has revealed the presence of three different high-affinity glutamate transporters in the rat retina, viz. GLAST-1, GLT-1 and EAAC-1. No message has been found in the retina for EAAT-4, a transporter recently cloned from human brain. By using membrane vesicle preparations of total rat retina, we show that glutamate uptake in the retina is a high-affinity electrogenic sodium-dependent transport process driven by the transmembrane sodium ion gradient. Autoradiography of intact and dissociated rat retinae indicates that glutamate uptake by Muller glial cells dominates total retinal glutamate transport and that this uptake is strongly influenced by the activity of glutamine synthetase. RT-PCR, immunoblotting and immunohistochemistry have revealed that Muller cells express only GLAST-1. The K(m) for glutamate of GLAST-1 is 2.1 ± 0.4 μM. This study suggests a major role for the Muller cell glutamate transporter GLAST-1 in retinal transmitter clearance. By regulating the extracellular glutamate concentration, the action of GLAST-1 in Muller cells may extend beyond the protection of neurons from excitotoxicity; we suggest a mechanism by which Muller cell glutamate transport might play an active role in shaping the time course of excitatory transmission in the retina.