NEURONAL EXPRESSION OF THE GLUTAMATE TRANSPORTER GLT-1 IN HIPPOCAMPALMICROCULTURES

To address the question of the relative contributions of glial and neu ronal glutamate transport in the vertebrate CNS, we studied the distri bution of forebrain glutamate transporters in rat hippocampal microcul tures, a preparation in which physiological functions of glutamate tra nsporters have been well characterized. Two of the three transporters, GLAST (EAAT1) and EAAC1 (EAAT3), are localized to microculture glia a nd neurons, respectively, as expected. However, we find strong immunor eactivity for the third glutamate transporter GLT-1 (EAAT2), a putativ ely glial transporter, in microculture neurons and in a small subset o f microculture glia. Indistinguishable immunohistochemical staining pa tterns for GLT-1 were obtained with antibodies directed against both t he N terminal and C terminal of the GLT-1 protein. Double-labeling exp eriments suggest that neuronal GLT-1 protein is primarily localized to the dendrites of excitatory neurons. Neuronal electrogenic transport currents in response to D-aspartate applications were occluded by the selective GLT-1 inhibitor dihydrokainate. In contrast, glia exhibited a larger transporter current density than did neurons, and the glial t ransport current was less sensitive to dihydrokainate, Neuronal transp ort currents were potentiated less than were glial currents when the c haotropic anion thiocyanate was substituted for gluconate in the whole -cell recording pipette, consistent with the previously reported lower anion permeability of EAAC1 and GLT-1 compared with that of GLAST. Af ter microculture glia were rendered nonviable, excitatory autaptic cur rents (EACs) were prolonged in the presence of dihydrokainate, suggest ing that neuronal GLT-1 is capable of participating in the clearance o f synaptically released glutamate. Our results suggest that the initia lly proposed characterization of GLT-1 as a purely glial transporter i s too simplistic and that under certain conditions functional GLT-1 pr otein can be expressed in brain neurons. The study suggests that chang es in GLT-1 levels that occur with pathology or experimental manipulat ions cannot be assumed to be glial.