Université de Paris, SPPIN - Saints-Pères Paris Institute for the Neurosciences, Centre National de la Recherche Scientifique, UMR 8003, Paris, France.
J Gen Physiol. 2020 May 4;152(5). doi: 10.1085/jgp.201912523.
In several types of central mammalian synapses, sustained presynaptic stimulation leads to a sequence of two components of synaptic vesicle release, reflecting the consecutive contributions of a fast-releasing pool (FRP) and of a slow-releasing pool (SRP). Previous work has shown that following common depletion by a strong stimulation, FRP and SRP recover with different kinetics. However, it has remained unclear whether any manipulation could lead to a selective enhancement of either FRP or SRP. To address this question, we have performed local presynaptic calcium uncaging in single presynaptic varicosities of cerebellar interneurons. These varicosities typically form "simple synapses" onto postsynaptic interneurons, involving several (one to six) docking/release sites within a single active zone. We find that strong uncaging laser pulses elicit two phases of release with time constants of ∼1 ms (FRP release) and ∼20 ms (SRP release). When uncaging was preceded by action potential-evoked vesicular release, the extent of SRP release was specifically enhanced. We interpret this effect as reflecting an increased likelihood of two-step release (docking then release) following the elimination of docked synaptic vesicles by action potential-evoked release. In contrast, a subthreshold laser-evoked calcium elevation in the presynaptic varicosity resulted in an enhancement of the FRP release. We interpret this latter effect as reflecting an increased probability of occupancy of docking sites following subthreshold calcium increase. In conclusion, both fast and slow components of release can be specifically enhanced by certain presynaptic manipulations. Our results have implications for the mechanism of docking site replenishment and the regulation of synaptic responses, in particular following activation of ionotropic presynaptic receptors.
在几种类型的哺乳动物中枢突触中，持续的突触前刺激会导致突触囊泡释放的两个成分的序列，反映出快速释放池（FRP）和缓慢释放池（SRP）的连续贡献。以前的工作表明，在强烈刺激下共同耗尽后，FRP 和 SRP 以不同的动力学恢复。然而，目前仍不清楚任何操作是否可以选择性地增强 FRP 或 SRP。为了解决这个问题，我们在小脑中间神经元的单个突触前小泡中进行了局部突触前钙释放。这些小泡通常形成“简单突触”到突触后中间神经元，涉及单个活性区中的几个（一到六个）停靠/释放位点。我们发现，强烈的 uncaging 激光脉冲引发了两个释放阶段，时间常数分别为约 1ms（FRP 释放）和约 20ms（SRP 释放）。当 uncaging 之前是动作电位诱发的囊泡释放时，SRP 释放的程度会特异性增强。我们将这种效应解释为反映了两步释放（停靠然后释放）的可能性增加，这是在动作电位诱发释放消除停靠的突触囊泡之后。相比之下，在突触前小泡中阈下激光诱发的钙升高导致 FRP 释放增强。我们将这种后一种效应解释为反映了在阈下钙增加后，停靠位点占据的可能性增加。总之，快速和缓慢的释放成分都可以通过某些突触前操作来特异性增强。我们的结果对停靠位点补充的机制和突触反应的调节具有影响，特别是在离子型突触前受体激活后。
