Vesicular Release Statistics and Unitary Postsynaptic Current at Single GABAergic Synapses

The existence of vesicular docking sites in central synapses is supported by morphological and biochemical evidence, but their functional role remains elusive. To investigate this role we have studied single depressing GABAergic synapses where multivesicular release and postsynaptic receptor saturation have been documented. We used failure/success patterns to estimate the number of vesicular docking sites, which varied from one to six among synapses. Variations of docking site numbers account for differences in release probability, as well as in the amplitude and decay kinetics of unitary postsynaptic currents. Upon repetitive stimulation, decreasing docking site occupancy likewise accounts for changes both in presynaptic and postsynaptic parameters. Finally steady-state docking site occupancy during train stimulations can be modulated by applying subthreshold presynaptic conditioning potential steps. The results suggest that differences in docking site numbers determine intersynaptic variability and that docking site occupancy is a key parameter controlling single synapse signaling.