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C. Joselevitch, D. Zenisek; Vesicle Transport on the Synaptic Ribbon and Its Role in the Recovery From Synaptic Depression. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1628.
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Virtually all synapses are subject to use-dependent plastic changes. Short-term synaptic depression is one of these changes. It is characterized by a decrease in transmission efficiency triggered by previous activity and can last seconds. The aim of this study was to determine whether short-term depression is caused by the slow replenishment of released vesicles.
Voltage-clamp recordings of goldfish mixed-input bipolar cell somata were combined with single-vesicle imaging in TIRF microscopy of the axon terminals loaded with FM1-43. The timing of release events and vesicle arrivals at the membrane were correlated with the onset of voltage stimuli, in order to evaluate how trains of voltage steps change vesicle dynamics. Active zones were labeled with a RIBEYE-binding fluorescent peptide.
Three types of vesicle movement were identified: mobile vesicles, newly immobilized vesicles and released vesicles. Only the last two cluster at active zones and are correlated with membrane potential changes. When trains of voltage steps were applied to the bipolar cells, the number of released vesicles decreased after the first voltage pulse. The number of newly immobilized vesicles at the terminals, however, followed closely that of released vesicles for all stimuli in the trains, even for very short (<250 ms) interstimulus intervals.
The replenishment of released vesicles at active zones is a very fast process and does not seem to be a rate-limiting step in synaptic transmission. Since synaptic depression has a much slower recovery than vesicle replenishment, it is unlikely that the recruitment of new vesicles to release sites is the underlying mechanism. Additional steps may need to take place before docked vesicles can fuse with the cell membrane.
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