SYNAPTIC ACTIVATION OF GLUTAMATE TRANSPORTERS IN HIPPOCAMPAL ASTROCYTES

Glutamate transporters in the CNS are expressed in neurons and glia an d mediate high affinity, electrogenic uptake of extracellular glutamat e. Although glia have the highest capacity for glutamate uptake, the a mount of glutamate that reaches glial membranes following release and the rate that glial transporters bind and sequester transmitter is not known. We find that stimulation of Schaffer collateral/commissural fi bers in hippocampal slices evokes glutamate transporter currents in CA 1 astrocytes that activate rapidly, indicating that a significant amou nt of transmitter escapes the synaptic cleft shortly after release. Tr ansporter currents in outside-out patches from astrocytes have faster kinetics than synaptically elicited currents, suggesting that the glut amate concentration attained at astrocytic membranes is lower but rema ins elevated for longer than in the synaptic cleft.