A new GLT1 splice variant: cloning and immunolocalization of GLT1c in the mammalian retina and brain

We have identified a novel carboxyl-terminal splice-variant of the glutamate transporter GLT1, which we denote as GLT1c. Within the rat brain only low levels of protein and message were detected, protein expression being restricted to end feet of astrocytes apposed to blood vessels or some astrocytes adjacent to the ventricles. Conversely, within the retina, this variant was selectively and heavily expressed in the synaptic terminals of both rod- and cone-photoreceptors in both humans and rats. Double-immunolabelling with antibodies to the carboxyl region of GLT1b/GLT1v, which is strongly expressed in apical dendrites of bipolar cells and in cone photoreceptors revealed that in the rat GLT1c was co-localised with GLT1b/GLT1v in cone photoreceptors but not with GLT1b/GLT1v in bipolar cells. GLT1c expression was developmentally regulated, only appearing at around postnatal day 7 in the rat retina, when photoreceptors first exhibit a dark current. Since the glutamate transporter EAAT5 is also expressed in terminals of rod photoreceptor terminals these data indicate that rod photoreceptors express two glutamate transporters with distinct properties. Similarly, cone photoreceptors express two glutamate transporters. We suggest that differential usage of these transporters by rod and cone photoreceptors may influence the kinetics of glutamate transmission by these neurons.