A well-defined readily releasable pool with fixed capacity for storing vesicles at calyx of held

The readily releasable pool (RRP) of vesicles is a core concept in studies of presynaptic function. However, operating principles lack consensus definition and the utility for quantitative analysis has been questioned. Here we confirm that RRPs at calyces of Held from 14 to 21 day old mice have a fixed capacity for storing vesicles that is not modulated by Ca2+. Discrepancies with previous studies are explained by a dynamic flow-through pool, established during heavy use, containing vesicles that are released with low probability despite being immediately releasable. Quantitative analysis ruled out a posteriori explanations for the vesicles with low release probability, such as Ca2+-channel inactivation, and established unexpected boundary conditions for remaining alternatives. Vesicles in the flow-through pool could be incompletely primed, in which case the full sequence of priming steps downstream of recruitment to the RRP would have an average unitary rate of at least 9/s during heav...囊泡易释放池（readily releasable pool, RRP）是突触前功能研究中的核心概念。然而，其运作机制尚未形成统一定义，且其在定量分析中的应用价值也曾受到学界质疑。本研究证实，14至21日龄小鼠赫尔德杯状突触（calyces of Held）处的RRP具有固定的囊泡储存容量，且不受钙离子（Ca²+）调控。本研究与既往研究的分歧可通过动态流通池得到解释：该池体在高强度激活过程中形成，其内的囊泡即便处于即刻可释放状态，其释放概率仍较低。定量分析排除了这类低释放概率囊泡的后验解释，例如钙离子通道失活，并为剩余的潜在机制确立了意料之外的边界条件。若流通池内的囊泡未完全启动，则招募至RRP后的启动步骤全序列的平均单位速率在高强度激活期间至少可达9次/秒……