Schmidt Hartmut
Carl-Ludwig Institute for Physiology, Medical Faculty, University of Leipzig, Leipzig, Germany.
Front Syst Neurosci. 2019 Jul 17;13:30. doi: 10.3389/fnsys.2019.00030. eCollection 2019.
Parallel fiber (PF) synapses show pronounced and lasting facilitation during bursts of high-frequency activity. They typically connect to their target neurons a single active zone (AZ), harboring few release sites (2-8) with moderate initial vesicular release probability (0.2-0.4). In light of these biophysical characteristics, it seems surprising that PF synapses can sustain facilitation during high-frequency periods of tens of action potentials (APs). Recent findings suggest an increase in the number of occupied release sites due to ultra-rapid (~180 s), Ca dependent recruitment of synaptic vesicles (SVs) from replenishment sites as major presynaptic mechanism of this lasting facilitation. On the molecular level, Synaptotagmin 7 or Munc13s have been suggested to be involved in mediating facilitation at PF synapses. The recruitment of SVs from replenishment sites appears to be reversible on a slower time-scale, thereby, explaining that PF synapses rapidly depress and ultimately become silent during low-frequency activity. Hence, PF synapses show high-frequency facilitation (HFF) but low-frequency depression (LFD). This behavior is explained by regulation of the number of occupied release sites at the AZ by AP frequency.
平行纤维（PF）突触在高频活动爆发期间表现出明显且持久的易化作用。它们通常通过单个活性区（AZ）与靶神经元相连，该活性区含有较少的释放位点（约2 - 8个），初始囊泡释放概率适中（约0.2 - 0.4）。鉴于这些生物物理特性，PF突触在数十个动作电位（AP）的高频期能够维持易化作用，这似乎令人惊讶。最近的研究结果表明，由于超快速（约180秒）、钙依赖的突触囊泡（SV）从补充位点募集，导致被占据的释放位点数量增加，这是这种持久易化作用的主要突触前机制。在分子水平上，已表明突触结合蛋白7或Munc13s参与介导PF突触的易化作用。从补充位点募集SV似乎在较慢的时间尺度上是可逆的，从而解释了PF突触在低频活动期间迅速抑制并最终沉默的现象。因此，PF突触表现出高频易化（HFF）但低频抑制（LFD）。这种行为可以通过动作电位频率对活性区被占据的释放位点数量的调节来解释。
