Sequential changes in synaptic vesicle pools and endosome-like organelles during depolarization near the active zone of central nerve terminals.
During periods of high-frequency stimulation the maintenance of synaptic transmission depends on a continued supply of synaptic vesicles. Local recycling in the terminals ensures synaptic vesicle replenishment, but the intermediate steps are still a matter of debate. We analyzed changes in synaptic vesicle pools and endosome-like organelles near the active zone in central nerve terminals during depolarization at the ultrastructural level by electron microscopy. A short, 100 ms, depolarization-induced recruitment of synaptic vesicles was observed from a reserve pool to a recruited pool, within 150 nm of the active zone, and the docked pool at the active zone was increased as well. Prolonged, 15 s or 3 min, depolarization decreased the total amount of synaptic vesicles, which was accompanied by a parallel increase in size and amount of endosome-like organelles. After a period of rest, the number of endosome-like organelles decreased and the amount of synaptic vesicles was restored to control level. The endocytotic nature of part of the endosome-like organelles after 15 s and 3 min depolarization was indicated by their labeling with extracellularly added horseradish peroxidase (HRP). In addition, a small number of synaptic vesicles entrapped HRP under these conditions. After repolarization, the number of HRP-loaded endosome-like structures decreased. Simultaneously, a strong increase in amount of HRP-loaded small vesicles did occur. These results indicate that during sub-second depolarization, synaptic vesicles were rapidly recruited from the reserve pool to replenish the releasable pool, whereas prolonged depolarization (s-min) induced local endocytosis in at least two ways, i.e. either directly as vesicles or via endosome-like organelles from which synaptic vesicles were reformed.
Leenders
A G M
AG
Swammerdam Institute for Life Sciences, Section Neurobiology, Faculty of Science, University of Amsterdam, Kruislaan 320, 1090 GB Amsterdam, The Netherlands.
Scholten
G
G
de Lange
R P J
RP
Lopes da Silva
F H
FH
Ghijsen
W E J M
WE
D000200:Action Potentials / Q000187:drug effects / Q000502:physiology; D000818:Animals; D002490:Central Nervous System / Q000378:metabolism* / Q000648:ultrastructure; D011992:Endosomes / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D006735:Horseradish Peroxidase; D051379:Mice; D008810:Mice, Inbred C57BL; D018377:Neurotransmitter Agents / Q000378:metabolism; D011188:Potassium / Q000494:pharmacology; D017729:Presynaptic Terminals / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D013570:Synaptic Membranes / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D013572:Synaptic Vesicles / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D013574:Synaptosomes / Q000378:metabolism / Q000648:ultrastructure
D018377:Neurotransmitter Agents; D006735:Horseradish Peroxidase; D011188:Potassium