title: A small pool of vesicles maintains synaptic activity in vivo.
abstract: Chemical synapses contain substantial numbers of neurotransmitter-filled synaptic vesicles, ranging from approximately 100 to many thousands. The vesicles fuse with the plasma membrane to release neurotransmitter and are subsequently reformed and recycled. Stimulation of synapses in vitro generally causes the majority of the synaptic vesicles to release neurotransmitter, leading to the assumption that synapses contain numerous vesicles to sustain transmission during high activity. We tested this assumption by an approach we termed cellular ethology, monitoring vesicle function in behaving animals (10 animal models, nematodes to mammals). Using FM dye photooxidation, pHluorin imaging, and HRP uptake we found that only approximately 1-5% of the vesicles recycled over several hours, in both CNS synapses and neuromuscular junctions. These vesicles recycle repeatedly, intermixing slowly (over hours) with the reserve vesicles. The latter can eventually release when recycling is inhibited in vivo but do not seem to participate under normal activity. Vesicle recycling increased only to ≈ 5% in animals subjected to an extreme stress situation (frog predation on locusts). Synapsin, a molecule binding both vesicles and the cytoskeleton, may be a marker for the reserve vesicles: the proportion of vesicles recycling in vivo increased to 30% in synapsin-null Drosophila. We conclude that synapses do not require numerous reserve vesicles to sustain neurotransmitter release and thus may use them for other purposes, examined in the accompanying paper.
authors: European Neuroscience Institute,  Deutsche Forschungsgemeinschaft Center for Molecular Physiology of the Brain/Excellence Cluster 171, 37077 Göttingen,  Germany.
authors: Annette
authors: A
authors: Denker
authors: Ioanna
authors: I
authors: Bethani
authors: Katharina
authors: K
authors: Kröhnert
authors: Christoph
authors: C
authors: Körber
authors: Heinz
authors: H
authors: Horstmann
authors: Benjamin G
authors: BG
authors: Wilhelm
authors: Sina V
authors: SV
authors: Barysch
authors: Thomas
authors: T
authors: Kuner
authors: Erwin
authors: E
authors: Neher
authors: Silvio O
authors: SO
authors: Rizzoli
mesh_terms: D000818:Animals; D030801:Animals, Genetically Modified; D017173:Caenorhabditis elegans / Q000502:physiology; D002642:Chick Embryo; D004331:Drosophila melanogaster / Q000235:genetics / Q000502:physiology / Q000648:ultrastructure; D005260:Female; D055786:Gene Knockout Techniques; D017344:Genes, Insect; D006110:Grasshoppers / Q000502:physiology; D006863:Hydrogen-Ion Concentration; D008297:Male; D051379:Mice; D046529:Microscopy, Electron, Transmission; D008959:Models, Neurological; D009154:Mutation; D018377:Neurotransmitter Agents / Q000378:metabolism; D011893:Rana esculenta / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D013312:Stress, Physiological; D016704:Synapsins / Q000502:physiology; D013572:Synaptic Vesicles / Q000502:physiology* / Q000648:ultrastructure; D015027:Zebrafish / Q000502:physiology
chemical_list: D018377:Neurotransmitter Agents; D016704:Synapsins