Evidence for only one reserve pool of vesicles subserving all release ready pools at active zones in terminals of the amphibian neuromuscular junction.
Etherington, S.J. and Everett, A.W. (2003) Evidence for only one reserve pool of vesicles subserving all release ready pools at active zones in terminals of the amphibian neuromuscular junction. In: Australian Neuroscience Society 23rd Annual Meeting, 28 - 31 January, Adelaide.
We compared the distribution of vesicles in terminals determined by EM and functional labelling with FM2-10 to investigate the structural basis for the spotted appearance of terminals stained for synaptic vesicles. Non-serial cross sections of 12 terminal branches from an iliofibularis muscle of a double-pithed toad (Bufo marinus) were analysed by stereological procedures to obtain indices of the terminal and vesicle areas. Terminal x-sectional area varied 3-fold on average along branches and was largest when active zone was present. The vesicle area index (a measure of vesicle abundance) was highly correlated with the terminal area index, suggesting that the average density of vesicles is constant throughout the branches. We found only a small (26%) increase in the average density of vesicles in sections from active zone compared with non-active zone regions in the terminal. The spotty staining in terminals loaded with FM2-1O suggested that vesicles were distributed in concentrated clusters. However, variation between spot and inter-spot staining intensities was similar in magnitude to that for terminal cross sectional area determined by the EM. We conclude that the spotty FM2- 10 staining pattern results primarily from the varicose nature of the terminal. The slightly higher density of vesicles at active zone regions is most likely due to a minority of vesicles concentrated around the presynaptic density; these may be the only vesicles (making up the release ready pools) that are genuinely clustered in the terminal.
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