Expression of glutamate transporters in rat optic nerve oligodendrocytes

To investigate the role of glutamate transport in non-synaptic glia, we cha racterized the expression of three major glutamate transporters (EAAC1, GLA ST and GLT-1) in rat optic nerve in situ using reverse transcription-polyme rase chain reaction in combination with Western blot and immunochemistry wi th specific antibodies. GLAST was localized to interfascicular oligodendroc ytes, whereas a subpopulation of cells, probably immature oligodendrocyte c ells, expressed EAAC1. In contrast, astrocytes, expressed only GLT-1, consi stent with the idea that this is the major glutamate transporter in this ce ll type. In addition, we observed that glutamine synthetase, a key enzyme i n glutamate metabolism, was localized in oligodendrocytes in situ. To exami ne the properties of these glutamate transporters, we conducted uptake expe riments in glial cultures. The kinetics of sodium-dependent glutamate uptak e in cultured oligodendrocytes from the perinatal rat optic nerve were mark edly different from those observed in type-1 astrocytes from the newborn ra t 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 i nhibition in oligodendrocytes than in type-1 astrocytes. These results prov ide evidence for the expression of functional sodium-dependent glutamate tr ansporters in optic nerve oligodendrocytes, and suggest that this cell type may play a role in the glutamate-glutamine cycle.