Vesicle Priming and Depriming A SNAP Decision

AbstractSynapses have a limited pool of vesicles that are docked and primed for rapid release. In neuroendocrine cells, splice variants of the SNARE protein SNAP-25 and phosphorylation of SNAP-25 independently influence the size of the releasable vesicle pool, possibly by altering the rate of vesicle depriming. Pre- and posttranslational modifications of SNAP-25 may therefore affect synaptic strength

Synaptic vesicle dynamics in mouse rod bipolar cells.

To better understand synaptic signaling at the mammalian rod bipolar cell terminal and pave the way for applying genetic approaches to the study of visual information processing in the mammalian retina, synaptic vesicle dynamics and intraterminal calcium were monitored in terminals of acutely isolated mouse rod bipolar cells and the number of ribbon-style active zones quantified. We identified a releasable pool, corresponding to a maximum of 7 s. The presence of a smaller, rapidly releasing pool and a small, fast component of refilling was also suggested. Following calcium channel closure, membrane surface area was restored to baseline with a time constant that ranged from 2 to 21 s depending on the magnitude of the preceding Ca2+ transient. In addition, a brief, calcium-dependent delay often preceded the start of onset of membrane recovery. Thus, several aspects of synaptic vesicle dynamics appear to be conserved between rod-dominant bipolar cells of fish and mammalian rod bipolar cells. A major difference is that the number of vesicles available for release is significantly smaller in the mouse rod bipolar cell, both as a function of the total number per neuron and on a per active zone basis

Quantitative Analysis of Synaptic Release at the Photoreceptor Synapse

AbstractExocytosis from the rod photoreceptor is stimulated by submicromolar Ca2+ and exhibits an unusually shallow dependence on presynaptic Ca2+. To provide a quantitative description of the photoreceptor Ca2+ sensor for exocytosis, we tested a family of conventional and allosteric computational models describing the final Ca2+-binding steps leading to exocytosis. Simulations were fit to two measures of release, evoked by flash-photolysis of caged Ca2+: exocytotic capacitance changes from individual rods and postsynaptic currents of second-order neurons. The best simulations supported the occupancy of only two Ca2+ binding sites on the rod Ca2+ sensor rather than the typical four or five. For most models, the on-rates for Ca2+ binding and maximal fusion rate were comparable to those of other neurons. However, the off-rates for Ca2+ unbinding were unexpectedly slow. In addition to contributing to the high-affinity of the photoreceptor Ca2+ sensor, slow Ca2+ unbinding may support the fusion of vesicles located at a distance from Ca2+ channels. In addition, partial sensor occupancy due to slow unbinding may contribute to the linearization of the first synapse in vision

Case report 107

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

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

Roles of ATP in Depletion and Replenishment of the Releasable

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