Diversity of glutamate transporter expression and function in the mammalian retina.

Glutamate is the major excitatory neurotransmitter of the mammalian retina and glutamate uptake is essential for normal transmission at glutamatergic synapses. Between photoreceptors and second order neurons, increases in light intensity are signaled by decreases in the concentration of glutamate within the synaptic cleft. In such a system the precise control of glutamate in the synaptic cleft is thus essential and glutamate transporters are thought to contribute to this process. As demonstrated here, all neuronal and macroglial cells of the retina appear to express high-affinity glutamate transporters. GLAST1, GLT1, EAAC1 and EAAT5 are expressed in the retina and exhibit unique localisation and functional properties. In the present study we summarize retinal glutamate transporter expression, identify the major glutamate uptake site in the mammalian retina and discuss the possible functional roles of different glutamate transporter subtypes in glutamatergic neurotranmission in the retina.