Unique dynamics and exocytosis of single inhibitory synaptic vesicles

The balance between excitation and inhibition is essential for maintaining proper brain function in the central nervous system. Inhibitory synaptic transmission plays an important role in maintaining this balance. Inhibitory synaptic transmission faces greater kinetic demands than excitatory synaptic transmission, yet remains less well understood. In particular, the dynamics and exocytosis of single inhibitory vesicles have not been investigated due to both technical and practical limitations. Using quantum dots (QDs)-conjugated antibodies against the luminal domain of the vesicular GABA transporter (VGAT) to selectively label single GABAergic inhibitory vesicles and dual-focus imaging optics, we tracked single inhibitory vesicles up to the moment of exocytosis (i.e., fusion) in three dimensions in real time. Using three-dimensional trajectories, we found that the total travel length before fusion of inhibitory synaptic vesicles was smaller than that of synaptotagmin-1 (Syt1)-labeled vesicles. Fusion times of inhibitory vesicles were shorter compared with those of Syt1-labeled vesicles. We also found a close relationship between release probability to the proximity to fusion sites and total travel length of inhibitory synaptic vesicles. Furthermore, inhibitory synaptic vesicles exhibited a higher prevalence of kiss-and-run fusion than Syt1-labeled vesicles. Thus, our results showed that inhibitory synaptic vesicles have the unique dynamics and fusion properties that facilitate their ability to support fast synaptic inhibition.