HIGH-AFFINITY GLUTAMATE TRANSPORTERS IN THE RAT RETINA - A MAJOR ROLEOF THE GLIAL GLUTAMATE TRANSPORTER GLAST-1 IN TRANSMITTER CLEARANCE

Glutamate is the major excitatory neurotransmitter of the mammalian re tina and glutamate uptake is essential for normal transmission at glut amatergic synapses. The reverse transcriptase-polymerase chain reactio n (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 preparatio ns of total rat retina, we show that glutamate uptake in the retina is a high-affinity electrogenic sodium-dependent transport process drive n 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 Mu ller cells express only GLAST-1. The K-m for glutamate of GLAST-1 is 2 .1+/-0.4 mu M. This study suggests a major role for the Muller cell gl utamate transporter GLAST-1 in retinal transmitter clearance. By regul ating the extracellular glutamate concentration, the action of GLAST-1 in Muller cells may extend beyond the protection of neurons from exci totoxicity; we suggest a mechanism by which Muller cell glutamate tran sport might play an active role in shaping the time course of excitato ry transmission in the retina.