Synaptic activation of glutamate transporters in hippocampal astrocytes.

Glutamate transporters in the CNS are expressed in neurons and glia and mediate high affinity, electrogenic uptake of extracellular glutamate. Although glia have the highest capacity for glutamate uptake, the amount 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 fibers in hippocampal slices evokes glutamate transporter currents in CA1 astrocytes that activate rapidly, indicating that a significant amount of transmitter escapes the synaptic cleft shortly after release. Transporter currents in outside-out patches from astrocytes have faster kinetics than synaptically elicited currents, suggesting that the glutamate concentration attained at astrocytic membranes is lower but remains elevated for longer than in the synaptic cleft.