Glutamate Transporters Contribute to the Time Course of Synaptic Transmission in Cerebellar Granule Cells

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Abstract
Transporters are thought to assist in the termination of synaptic transmission at some synapses by removing neurotransmitter from the synapse. To investigate the role of glutamate transport in shaping the time course of excitatory transmission at the mossy fiber–granule cell synapse, the effects of transport impairment were studied using whole-cell voltage- and current-clamp recordings in slices of rat cerebellum. Impairment of transport byl-trans-pyrrolidine-2,4-dicarboxylate (PDC) produced a prolongation of the decay of the AMPA receptor-mediated current after a repetitive stimulus, as well as prolongation of single stimulus-evoked EPSCs when AMPA receptor desensitization was blocked. PDC also produced a prolongation of both single and repetitive-evoked NMDA receptor-mediated EPSCs. Enzymatic degradation of extracellular glutamate did not reverse the PDC-induced prolongation of AMPA receptor-mediated current after a repetitive stimulus, suggesting that transporter binding sites participate in limiting glutamate spillover. In current-clamp recordings, PDC dramatically increased the total area of the EPSP and the burst duration evoked by single and repetitive stimuli. These data indicate that glutamate transporters play a significant role in sculpting the time course of synaptic transmission at granule cell synapses, most likely by limiting the extent of glutamate spillover. The contribution of transporters is particularly striking during repetitive stimulus trains at physiologically relevant frequencies. Hence, the structural arrangement of the glomerulus may enhance the contribution of transporters to information processing by limiting the extent of glutamate spillover between adjacent synapses.