Roles of ATP in Depletion and Replenishment of the Releasable Pool of Synaptic Vesicles

Synaptic terminals of retinal bipolar neurons contain a pool of readily releasable synaptic vesicles that undergo rapid calcium-dependent release. ATP hydrolysis is required for the functional refilling of this vesicle pool. However, it was unclear which steps required ATP hydrolysis: delivery of vesicles to their anatomical release sites or preparation of synaptic vesicles and/or the secretory apparatus for fusion. To address this, we dialyzed single synaptic terminals with ATP or the poorly hydrolyzable analogue ATP-γS and examined the size of the releasable pool, refilling of the releasable pool, and the number of vesicles at anatomical active zones. After minutes of dialysis with ATP-γS, vesicles already in the releasable pool could still be discharged. This pool was not functionally refilled despite the fact that its anatomical correlate, the number of synaptic vesicles tethered to active zone synaptic ribbons, was completely normal. We conclude 1) because the existing releasable pool is stable during prolonged inhibition of ATP hydrolysis, whereas entry into the functional pool is blocked, a vesicle on entering the pool will tend to remain there until it fuses; 2) because the anatomical pool is unaffected by inhibition of ATP hydrolysis, failure to refill the functional pool is not caused by failure of vesicle movement; 3) local vesicle movements important for pool refilling and fusion are independent of conventional ATP-dependent motor proteins; and 4) ATP hydrolysis is required for the biochemical transition of vesicles and/or release sites to fusion-competent status.

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Diurnal Changes in Exocytosis and the Number of Synaptic Ribbons at Active Zones of an on-Type Bipolar Cell Terminal

Journal of Neurophysiology ◽

10.1152/jn.00364.2006 ◽

2006 ◽

Vol 96 (4) ◽

pp. 2025-2033 ◽

Cited By ~ 50

Author(s):

Court Hull ◽

Keith Studholme ◽

Stephen Yazulla ◽

Henrique von Gersdorff

Keyword(s):

Synaptic Vesicle ◽

Bipolar Cell ◽

Bipolar Cells ◽

Synaptic Ribbons ◽

Diurnal Changes ◽

Synaptic Vesicle Exocytosis ◽

Pulse Ratio ◽

Vesicle Exocytosis ◽

Active Zones ◽

Vesicle Pool

The number and morphology of synaptic ribbons at photoreceptor and bipolar cell terminals has been reported to change on a circadian cycle. Here we sought to determine whether this phenomenon exists at goldfish Mb-type bipolar cell terminals with the aim of exploring the role of ribbons in transmitter release. We examined the physiology and ultrastructure of this terminal around two time points: midday and midnight. Nystatin perforated-patch recordings of membrane capacitance ( Cm) revealed that synaptic vesicle exocytosis evoked by short depolarizations was reduced at night, even though Ca2+ currents were larger. The efficiency of exocytosis (measured as the Δ Cm jump per total Ca2+ charge influx) was thus significantly lower at night. The paired-pulse ratio remained unchanged, however, suggesting that release probability was not altered. Hence the decreased exocytosis likely reflects a smaller readily releasable vesicle pool at night. Electron microscopy of single sections from intact retinas averaged 65% fewer ribbons at night. Interestingly, the number of active zones did not change from day to night, only the probability of finding a ribbon at an active zone. Additionally, synaptic vesicle halos surrounding the ribbons were more completely filled at night when these on-type bipolar cells are more hyperpolarized. There was no change, however, in the physical dimensions of synaptic ribbons from day to night. These results suggest that the size of the readily releasable vesicle pool and the efficiency of exocytosis are reduced at night when fewer ribbons are present at bipolar cell terminal active zones.

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Nicotine Enhancement of Dopamine Release by a Calcium-Dependent Increase in the Size of the Readily Releasable Pool of Synaptic Vesicles

Journal of Neuroscience ◽

10.1523/jneurosci.1559-04.2004 ◽

2004 ◽

Vol 24 (50) ◽

pp. 11328-11336 ◽

Cited By ~ 40

Author(s):

T. J. Turner

Keyword(s):

Synaptic Vesicles ◽

Dopamine Release ◽

Calcium Dependent ◽

Releasable Pool ◽

Readily Releasable Pool

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RNA editing of CAPS1 regulates synaptic vesicle organization, release and retrieval

10.1101/178202 ◽

2017 ◽

Cited By ~ 1

Author(s):

Randi J. Ulbricht ◽

Sarah J. Sun ◽

Claire E. DelBove ◽

Kristina E. Kitko ◽

Saad C. Rehman ◽

...

Keyword(s):

Rna Editing ◽

Neurotransmitter Release ◽

Synaptic Vesicles ◽

Hippocampal Neurons ◽

Protein Isoforms ◽

Editing Event ◽

Calcium Dependent ◽

Presynaptic Vesicle ◽

Vesicle Pool ◽

Fine Tune

ABSTRACTCalcium-dependent activator protein for secretion 1 (CAPS1) facilitates the docking and priming of synaptic and dense core vesicles. A conserved hairpin structure in the CAPS1 pre-mRNA allows an post-transcriptional adenosine-to-inosine RNA editing event to alter a genomically-encoded glutamate to a glycine codon. Functional comparisons of CAPS1 protein isoforms in primary hippocampal neurons show that elevation of edited CAPS1 isoforms facilitates presynaptic vesicle clustering and turnover. Conversely, non-edited CAPS1 isoforms slow evoked release, increase spontaneous fusion, and loosen the clustering of synaptic vesicles. Therefore, CAPS1 editing promotes organization of the vesicle pool in a way that is beneficial for evoked release, while non-edited isoforms promote more lax vesicle organization that widens distribution, attenuates evoked release and eases the control of spontaneous fusion. Overall, RNA editing of CAPS1 is a mechanism to fine tune neurotransmitter release.IMPACT STATEMENTPost-transcriptional RNA editing of CAPS1 is a mechanism to regulate neurotransmitter release from synaptic vesicles.

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Active Zones and the Readily Releasable Pool of Synaptic Vesicles at the Neuromuscular Junction of the Mouse

Journal of Neuroscience ◽

10.1523/jneurosci.4663-10.2011 ◽

2011 ◽

Vol 31 (6) ◽

pp. 2000-2008 ◽

Cited By ~ 54

Author(s):

R. Ruiz ◽

R. Cano ◽

J. J. Casanas ◽

M. A. Gaffield ◽

W. J. Betz ◽

...

Keyword(s):

Neuromuscular Junction ◽

Synaptic Vesicles ◽

Releasable Pool ◽

Readily Releasable Pool ◽

Active Zones

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Adenosine Triphosphate and the Late Steps in Calcium-dependent Exocytosis at a Ribbon Synapse

The Journal of General Physiology ◽

10.1085/jgp.111.2.225 ◽

1998 ◽

Vol 111 (2) ◽

pp. 225-241 ◽

Cited By ~ 54

Author(s):

Ruth Heidelberger

Keyword(s):

Synaptic Vesicles ◽

Flash Photolysis ◽

Average Rate ◽

Small Sample Size ◽

Small Sample ◽

Calcium Dependent ◽

Synaptic Terminals ◽

First Response ◽

Synaptic Vesicle Fusion ◽

Kinetics Of

The ATP dependence of the kinetics of Ca2+-dependent exocytosis after flash photolysis of caged Ca2+ was studied by capacitance measurements with submillisecond resolution in single synaptic terminals of retinal bipolar neurons. After control experiments verified that this combination of techniques is valid for the study of exocytosis in synaptic terminals, a comparison was made between the Ca2+ dependence of the rate of exocytosis in synaptic terminals internally dialyzed with MgATP, MgATP-γ-S, or no added Mg2+ or nucleotide. The Ca2+ threshold for release, the maximum rate of release, and the overall relationship between the rate of synaptic vesicle fusion and [Ca2+]i were found to be independent of MgATP. A decrease in the average rate at near-threshold [Ca2+]i was observed in terminals with MgATP-γ-S, but due to the small sample size is of unclear significance. The Ca2+ dependence of the delay between the elevation of [Ca2+]i and the beginning of the capacitance rise was also found to be independent of MgATP. In contrast, MgATP had a marked effect on the ability of terminals to respond to multiple stimuli. Terminals with MgATP typically exhibited a capacitance increase to a second stimulus that was >70% of the amplitude of the first response and to a third stimulus with a response amplitude that was >50% of the first, whereas terminals without MgATP responded to a second stimulus with a response <35% of the first and rarely responded to a third flash. These results suggest a major role for MgATP in preparing synaptic vesicles for fusion, but indicate that cytosolic MgATP may have little role in events downstream of calcium entry, provided that [Ca2+]i near release sites is elevated above ≈30 μM.

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Direct Observation Of Vesicle Transport On The Synaptic Ribbon Provides Evidence That Vesicles Are Mobilized And Prepared Rapidly For Release

10.1101/2020.02.05.936153 ◽

2020 ◽

Author(s):

Christina Joselevitch ◽

David Zenisek

Keyword(s):

Release Kinetics ◽

Bipolar Cells ◽

Synaptic Ribbons ◽

Synaptic Ribbon ◽

Paired Pulse ◽

Maturation Rate ◽

Interstimulus Intervals ◽

Vesicle Movement ◽

Vesicle Pool ◽

Paired Pulse Depression

SUMMARYSynaptic ribbons are thought to provide vesicles for continuous synaptic transmission in some retinal non-spiking neurons, yet recent studies indicate that genetic removal of the ribbon has little effect on vesicle release kinetics. To investigate vesicle replenishment at synaptic ribbons, we imaged synaptic vesicles and ribbons in retinal bipolar cells with TIRF microscopy during stimulation with trains of 30-ms depolarizations. Analysis of vesicles released by the stimuli revealed that the vast majority of releasable vesicles reside within 300 nm of the ribbon center. A single 30-ms step to 0 mV was sufficient to deplete the most membrane-proximal vesicle pool, while triggering rapid stepwise movements of distal vesicles along the ribbon and toward the plasma membrane.Replenishment only becomes rate-limiting for recovery from paired-pulse depression for interstimulus intervals shorter than 250 ms. For longer interstimulus intervals, vesicle movement down the ribbon is fast enough to replenish released vesicles, but newly arrived vesicles are not release-ready. Notably, vesicle re-supply is 40-to 50-fold faster than previously measured in non-ribbon conventional synapses, whereas vesicle maturation rate is comparable. Moreover, in contrast to conventional synapses, vesicles docked at the base of the ribbon release with high fidelity. Lastly, our data show that with multiple stimuli, the delay in vesicle departure increases. Our results support a role for ribbons in the rapid supply and efficient preparation of vesicles for release, provide direct measurements of vesicle movement down the synaptic ribbon and suggest that multiple factors contribute to paired-pulse depression.

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