Separation of Recycling and Reserve Synaptic Vesicles from Cholinergic Nerve Terminals of the Myenteric Plexus of Guinea-Pig Ileum
Our understanding of the dynamics of vesicle recycling and transmitter synthesis, storage and release at a cholinergic synapse has been greatly increased by the discovery that synaptic vesicles isolated from stimulated cholinergic electromotor nerve terminals of Torpedo marmorata can be separated into two subpopulations (designated VP1 and VP2) by methods utilizing differences in size (6) or density (18, 19). Studies utilizing dextran particles which are recognizable in the electron microscope but are unable to diffuse through lipoprotein membranes and whose presence within the lumen of synaptic vesicles could only have occurred during a cycle of exo- and endocytosis, have enabled the VP2 vesicles to be identified as the recycling subpopulation and as the smaller vesicles seen in whole tissue sections near the presynaptic plasma membrane which accumulate in the terminal cytoplasm as stimulation continues (17, 18). Work with radioactive precursors of acetylcholine have shown that newly synthesized acetylcholine is preferentially taken up into these smaller, denser vesicles (6, 17) and that on the restimulation of already labelled tissue blocks, the acetylcholine released has a specific radioactivity not significantly different from that in the VP2 fraction (13). Finally, false transmitters that distribute themselves differently from newly synthesized acetylcholine and from each other between cytoplasm and VP2 vesicles are released on restimulation in a ratio equal to that in which they are stored in the VP2 subpopulation and not in that in which they occur in the cytoplasm (9). Thus VP2 vesicles have been identified, in this terminal, as the immediate source of released transmitter.
KeywordsSynaptic Vesicle Specific Radioactivity Vesicle Recycling Vesicle Population Cholinergic Nerve Terminal
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