Synaptic vesicle pools and vesicle recycling in nerve terminals

In a temperature-sensitive Drosophila mutant，shibire，synaptic vesicles are completely depleted in nerve terminals after stimulation at 34°C，but upon returning to 22°C endocytosis resumes.In this study synaptic vesicles in the boutons of nerve terminals at the mutant neuromuscular junction were loaded with a fluorescent dye，FM1-43，during vesicle reformation at 22°C after complete depletion at 34°C.We found two distinct pools of synaptic vesicles，namely an exo/endo cycling pool，located in the periphery of the bouton and a reserve pool，located in the periphery of the bouton and a reserve pool，located in its center.Cytochalasin D treatment eliminated the reserve pool and reduced synaptic transmission evoked by high frequency stimulation.Thus the reserve pool may play a crucial role for sustaining high frequency synaptic transmission.To determine the role of exo/endo cycling vesicle pool in synaptic transmission，we estimated the quantal content electrophysiologically while the pool size was determined optically using fluorescent dye…More FM1-43.We then manipulated the size of the pool with following treatments.First，to change the state of boutons of terminals motoneuronal axons were severed.With this treatment the size of exo/endo cycling vesicle pool decreased together with the quantal content。Secondly，we promoted the FM1-43uptake using cyclosporin A which inhibits calcineurin activities and enhances endocytosis。Cyclosporin A increased the total uptake of FM1-43，but neither the size of exo/endo cycling vesicle pool nor the quantal content changed.Thirdly，we increased the size of exo/endo cycling vesicle pool by forskolin which enhances synaptic transmission。The forskolin-treatment increased both the size of exo/endo cycling vesicle pool and the quantal content。Thus，we found that the quantal content was closely correlated with the size of exo/endo cycling vesicle pool but not necessarily with the total uptake of FM1-43fluorescence by boutons.The results suggest that vesicles in the exo/endo cycling pool primarily participate in evoked exocytosis of vesicles。Less：Less

In a temperature-sensitive Drosophila mutant, shibire, synaptic vesicles are completely depleted in nerve terminals after stimulation at 34゜C, but upon returning to 22゜C endocytosis resumes. In this study synaptic vesicles in the boutons of nerve terminals at the mutant neuromuscular junction were loaded with a fluorescent dye, FM1-43, during vesicle reformation at 22゜C after complete depletion at 34゜C. We found two distinct pools of synaptic vesicles, namely an exo/endo cycling pool, located in the periphery of the bouton and a reserve pool, located in its center. Cytochalasin D treatment eliminated the reserve pool and reduced synaptic transmission evoked by high frequency stimulation. Thus the reserve pool may play a crucial role for sustaining high frequency synaptic transmission. To determine the role of exo/endo cycling vesicle pool in synaptic transmission, we estimated the quantal content electrophysiologically while the pool size was determined optically using fluorescent dye … More FM1-43. We then manipulated the size of the pool with following treatments. First, to change the state of boutons of terminals motoneuronal axons were severed. With this treatment the size of exo/endo cycling vesicle pool decreased together with the quantal content. Secondly, we promoted the FM1-43 uptake using cyclosporin A which inhibits calcineurin activities and enhances endocytosis. Cyclosporin A increased the total uptake of FM1-43, but neither the size of exo/endo cycling vesicle pool nor the quantal content changed . Thirdly, we increased the size of exo/endo cycling vesicle pool by forskolin which enhances synaptic transmission. The forskolin-treatment increased both the size of exo/endo cycling vesicle pool and the quantal content. Thus, we found that the quantal content was closely correlated with the size of exo/endo cycling vesicle pool but not necessarily with the total uptake of FM1-43 fluorescence by boutons. The results suggest that vesicles in the exo/endo cycling pool primarily participate in evoked exocytosis of vesicles. Less