# Initial Distance‐Dependent Mean Force Drives Synaptic Vesicle Motion Toward Fusion Sites in Stimulated Hippocampal Neurons

## TL;DR

This study reveals how synaptic vesicles move toward fusion sites in stimulated neurons, showing that those farther away move straighter and faster, impacting how neurons communicate.

## Contribution

The study introduces a quantitative model explaining how initial distance influences synaptic vesicle motion and fusion dynamics during neuronal stimulation.

## Key findings

- Type II synaptic vesicles show unconfined motion before tethering and exocytosis.
- Electrical stimulation increases the straightness of Type II vesicle trajectories toward fusion sites.
- Fusion time depends non-monotonically on the initial distance of Type II vesicles from fusion sites.

## Abstract

Neuronal communication occurs through transport and exocytosis of synaptic vesicles (SVs). However, their dynamics during neuronal stimulation remains poorly understood. Here, real‐time, 3D motion of individual SVs undergoing exocytosis in presynaptic terminals is quantitatively investigated. SVs are categorized into two types: Type I showing confined motion near fusion sites until exocytosis and Type II SVs exhibiting unconfined motion before tethering and exocytosis. Type II SVs have a broader fusion time distribution with a higher mean value than Type I SVs. Electrical stimulation increases the straightness of the Type II trajectories toward their fusion sites approximately tenfold. To quantify the straightness of the SV trajectories, a straightness parameter is introduced and its relationship to the mean force exerted on SVs is established. Interestingly, the straightness parameter, and hence mean velocity, increase in a sigmoidal manner with the initial distances of Type II SVs from their fusion sites upon stimulation, which results in a counterintuitive non‐monotonic dependence of their fusion time on the initial distances. A quantitative model is presented that simultaneously explains various experimental results regarding SV transport and fusion dynamics. This work offers new insights into mysterious SV motion at presynaptic terminals and its consequences on synaptic transmission of stimulated neurons.

Synaptic vesicle (SV) motion and exocytosis dynamics in the presynaptic terminals of live neurons are intimately linked to synaptic transmission during neuronal activity. In this work, it is revealed that, during electrical stimulation, SVs located farther from their fusion sites exhibit greater straightness and experience larger mean forces directed toward the fusion sites compared to those positioned closer.