Delayed clearance of transmitter and the role of glutamate transporters at synapses with multiple release sites

The roles of glutamate diffusion, uptake, and channel kinetics in shaping the AMPA receptor EPSC were examined at a calyceal synapse. The EPSC decay was described by three exponential components: two matching desensitizing channel kinetics, and a third component at least 10 times slower. The slowest component had identical voltage dependence to the steady-state AMPA current and was selectively increased and prolonged by blockade of glutamate uptake, indicating that the slow EPSC represented rebinding of glutamate at partially desensitized AMPA receptors. The data were in strong agreement with the predictions of a model of transmitter diffusion from multiple release sites into a large synaptic cleft. Within the first millisecond after release, transmitter concentrations in the cleft fell below millimolar levels, causing the fastest parts of the EPSC to be shaped by channel kinetics. The slowest component was determined by the removal over tens of milliseconds of the final 10-100 μM glutamate by diffusion and uptake. The data and modeling indicate that transmitter uptake and cooperation between release sites are significant determinants of a slow 'tail' of glutamate in the synaptic cleft. This slow clearance of glutamate is likely to limit postsynaptic receptor availability through desensitization.