Expression of glutamate transporters in rat optic nerve oligodendrocytes

To investigate the role of glutamate transport in non-synaptic glia, we characterized the expression of three major glutamate transporters (EAAC1, GLAST and GLT-1) in rat optic nerve in situ using reverse transcription-polymerase chain reaction in combination with Western blot and immunochemistry with specific antibodies. GLAST was localized to interfascicular oligodendrocytes, whereas a subpopulation of cells, probably immature oligodendrocyte cells, expressed EAAC1. In contrast, astrocytes, expressed only GLT-1, consistent with the idea that this is the major glutamate transporter in this cell type. In addition, we observed that glutamine synthetase, a key enzyme in glutamate metabolism, was localized in oligodendrocytes in situ. To examine the properties of these glutamate transporters, we conducted uptake experiments in glial cultures. The kinetics of sodium-dependent glutamate uptake in cultured oligodendrocytes from the perinatal rat optic nerve were markedly different from those observed in type-1 astrocytes from the newborn rat cerebral cortex, with higher affinity and lower Vmax. In both cell types, glutamate transport was inhibited by L-trans-pyrrolidine-2,4-dicarboxylate (t-PDC). In contrast, dihydrokainate exhibited significantly more uptake inhibition in oligodendrocytes than in type-1 astrocytes. These results provide evidence for the expression of functional sodium-dependent glutamate transporters in optic nerve oligodendrocytes, and suggest that this cell type may play a role in the glutamate-glutamine cycle.