GLIAL CONTRIBUTION TO GLUTAMATE UPTAKE AT SCHAFFER COLLATERAL-COMMISSURAL SYNAPSES IN THE HIPPOCAMPUS

Astrocytes in the hippocampus express high-affinity glutamate transpor ters that are important for lowering the concentration of extracellula r glutamate after release at excitatory synapses. These transporters e xhibit a permeability to chaotropic anions that is associated with tra nsport, allowing their activity to be monitored in cell-fee patches wh en highly permeant anions are present. Astrocyte glutamate transporter s are highly temperature sensitive, because L-glutamate-activated, ani on-potentiated transporter currents in outside-out patches from these cells exhibited larger amplitudes and faster kinetics at 36 degrees C than at 24 degrees C. The cycling rate of these transporters was estim ated by using paired applications of either L-glutamate or D-aspartate to measure the time necessary for the peak of the transporter current to recover from the steady-state level. Transporter currents in patch es recovered with a time constant of 11.6 msec at 36 degrees C, sugges ting that either the turnover rate of native transporters is much fast er than previously reported for expressed EAAT2 transporters or the ef ficiency of these transporters is very low. Synaptically activated tra nsporter currents persisted in astrocytes at physiological temperature s, although no evidence of these currents was found in CAI pyramidal n eurons in response to afferent stimulation. L-glutamate-gated transpor ter currents were also not detected in outside-out patches from pyrami dal neurons. These results are consistent with the hypothesis that ast rocyte transporters are responsible for taking up the majority of glut amate released at Schaffer collateral-commissural synapses in the hipp ocampus.