Central mammalian synapses release synaptic vesicles in dedicated structures called docking/release sites. It has been assumed that when voltage-dependent calcium entry is sufficiently large, synaptic output attains a maximum value of one synaptic vesicle per action potential and per site. Here we use deconvolution to count synaptic vesicle output at single sites (mean site number per synapse: 3.6). When increasing calcium entry with tetraethylammonium in 1.5 mM external calcium concentration, we find that synaptic output saturates at 0.22 vesicle per site, not at 1 vesicle per site. Fitting the results with current models of calcium-dependent exocytosis indicates that the 0.22 vesicle limit reflects the probability of docking sites to be occupied by synaptic vesicles at rest, as only docked vesicles can be released. With 3 mM external calcium, the maximum output per site increases to 0.47, indicating an increase in docking site occupancy as a function of external calcium concentration.

中文翻译：

---

不完全的水泡对接限制了高释放概率条件下的突触强度

中央哺乳动物的突触在称为对接/释放位点的专用结构中释放突触小泡。已经假定，当电压依赖性钙进入足够大时，突触输出在每个动作电位和每个部位达到一个突触小泡的最大值。在这里，我们使用反卷积来计数单个位点的突触小泡输出（每个突触的平均位点数：3.6）。当在1.5 mM外部钙浓度下用四乙铵增加钙的进入时，我们发现突触输出在每个位点的0.22囊泡处饱和，而不在每个位点的1囊泡处饱和。用目前的钙依赖性胞吐模型对结果进行拟合表明，0.22的囊泡极限反映了停靠位点在静止时被突触囊泡占据的可能性，因为只能释放停靠的囊泡。加上3 mM的外部钙，