"Kiss and run" exocytosis at hippocampal synapses

What is Rate-Limiting during Sustained Synaptic Activity: Vesicle Supply or the Availability of Release Sites

For some types of synapses the availability of release-ready vesicles is a limiting factor during ongoing activity. Synaptic strength in this case is determined both by the recruitment of such vesicles and the probability of their release during an action potential. Here it is argued that not the availability of vesicles is the limiting factor for recruitment, but rather the availability of specific sites to which vesicles can dock.

Analytical approaches to investigate transmitter content and release from single secretory vesicles

The vesicle serves as the primary intracellular unit for the highly efficient storage and release of chemical messengers triggered during signaling processes in the nervous system. This review highlights conventional and emerging analytical methods that have used microscopy, electrochemistry, and spectroscopy to resolve the location, time course, and quantal content characteristics of neurotransmitter release. Particular focus is on the investigation of the synaptic vesicle and its involvement in the fundamental molecular mechanisms of cell communication.

Biophysical characterization of styryl dye-membrane interactions

Styryl dyes (also referred to as FM dyes) become highly fluorescent upon binding to membranes and are often used to study synaptic vesicle recycling in neurons. To date, however, no direct comparisons of the fluorescent properties, or time-resolved (millisecond) measurements of dye-membrane binding and unbinding reactions, for all members of this family of probes have been reported. Here, we compare the fluorescence intensities of each member of the FM dye family when bound to membranes. This analysis included SGC5, a new lipophilic fluorescent dye with a unique structure. Fluorescence intensities depended on the length of the lipophilic tail of each dye, with a rank order as follows: SGC5 > FM1-84 > FM1-43 > SynaptoGreen C3 > FM2-10/FM4-64/FM5-95. Stopped-flow measurements revealed that dye hydrophobicity determined the affinity and departitioning rates for dye-membrane interactions. All of the dyes dissociated from membranes on the millisecond timescale, which is orders of magnitude faster than the overall destaining rate (timescale of seconds) of these dyes from presynaptic boutons. Departitioning kinetics were faster at higher temperatures, but were unaffected by pH or cholesterol. The data reported here aid interpretation of dye-release kinetics from single synaptic vesicles, and indicate that these probes dissociate from membranes on more rapid timescales than previously appreciated.

Proteomics of regulated secretory organelles

Regulated secretory organelles are important subcellular structures of living cells that allow the release in the extracellular space of crucial compounds, such as hormones and neurotransmitters. Therefore, the regulation of biogenesis, trafficking, and exocytosis of regulated secretory organelles has been intensively studied during the last 30 years. However, due to the large number of different regulated secretory organelles, only a few of them have been specifically characterized. New insights into regulated secretory organelles open crucial perspectives for a better comprehension of the mechanisms that govern cell secretion. The combination of subcellular fractionation, protein separation, and mass spectrometry is also possible to study regulated secretory organelles at the proteome level. In this review, we present different strategies used to isolate regulated secretory organelles, separate their protein content, and identify the proteins by mass spectrometry. The biological significance of regulated secretory organelles-proteomic analysis is discussed as well.

Exocytosis of post-Golgi vesicles is regulated by components of the endocytic machinery

Post-Golgi vesicles target and deliver most biosynthetic cargoes to the cell surface. However, the molecules and mechanisms involved in fusion of these vesicles are not well understood. We have employed a system to simultaneously monitor release of luminal and membrane biosynthetic cargoes from individual post-Golgi vesicles. Exocytosis of these vesicles is not calcium triggered. Release of luminal cargo can be accompanied by complete, partial, or no release of membrane cargo. Partial and no release of membrane cargo of a fusing vesicle are fates associated with kiss-and-run exocytosis, and we find that these are the predominant mode of post-Golgi vesicle exocytosis. Partial cargo release by post-Golgi vesicles occurs because of premature closure of the fusion pore and is modulated by the activity of clathrin, actin, and dynamin. Our results demonstrate that these components of the endocytic machinery modulate the nature and extent of biosynthetic cargo delivery by post-Golgi vesicles at the cell membrane.

The dynamic control of kiss-and-run and vesicular reuse probed with single nanoparticles

Vesicular secretion of neurotransmitter is essential for neuronal communication. Kiss-and-run is a mode of membrane fusion and retrieval without the full collapse of the vesicle into the plasma membrane and de novo regeneration. The importance of kiss-and-run during efficient neurotransmission has remained in doubt. We developed an approach for loading individual synaptic vesicles with single quantum dots. Their size and pH-dependent photoluminescence change allowed us to distinguish kiss-and-run from full-collapse fusion and to track single vesicles through multiple rounds of kiss-and-run and reuse, without perturbing vesicle cycling. Kiss-and-run dominated at the beginning of stimulus trains, reflecting the preference of vesicles with high release probability. Its incidence was increased by rapid firing, a response appropriate to shape the kinetics of neurotransmission during a wide range of firing patterns.

Imaging of evoked dense-core-vesicle exocytosis in hippocampal neurons reveals long latencies and kiss-and-run fusion events

Evoked neuropeptide secretion in the central nervous system occurs slowly, but the basis for slow release is not fully understood. Whereas exocytosis of single synaptic vesicles in neurons and of dense-core vesicles (DCVs) in endocrine cells have been directly visualized, single DCV exocytic events in neurons of the central nervous system have not been previously studied. We imaged DCV exocytosis in primary cultured hippocampal neurons using fluorescent propeptide cargo and total internal reflectance fluorescence microscopy. The majority of Ca(2+)-triggered exocytic events occurred from immobile plasma-membrane-proximal DCVs in the cell soma, whereas there were few events in the neurites. Strikingly, DCVs in the cell soma exhibited 50-fold greater release probabilities than those in neurites. Latencies to depolarization-evoked fusion for DCVs were surprisingly long, occurring with an average time constant (tau) of 16 seconds for DCVs in the soma and even longer for DCVs in neurites. All of the single DCV release events exhibited rapid fusion-pore openings and closures, the kinetics of which were highly dependent upon Ca(2+) levels. These ;kiss-and-run' events were associated with limited cargo secretion. Thus, the slow evoked release of neuropeptides could be attributed to very prolonged latencies from stimulation to fusion and transient fusion-pore openings that might limit cargo secretion.

Probing synaptic vesicle fusion by altering mechanical properties of the neuronal surface membrane

Because synaptic vesicle exocytosis is a nano-mechanical process, it should be influenced by the mechanical properties of the cell membrane to which the vesicle fuses. By dissolving surfactants at various concentrations in the neuronal membrane, we have perturbed mechanical properties of the membrane and have found that dissolved surfactants lower the probability that a synaptic vesicle will open its fusion pore when the fusion machinery of the vesicle is activated by binding calcium. By using standard theories from the physics and chemistry of surfaces, we can account for this decrease in fusion probability and can infer that a vesicle, when activated, opens its fusion pore approximately 3 times out of 4 and that the area of the fusion pore is approximately 4 nm(2).

Synaptic vesicle endocytosis: fast and slow modes of membrane retrieval

Several modes of synaptic vesicle release, retrieval and recycling have been identified. In a well-established mode of exocytosis, termed 'full-collapse fusion', vesicles empty their neurotransmitter content fully into the synaptic cleft by flattening out and becoming part of the presynaptic membrane. The fused vesicle membrane is then reinternalized via a slow and clathrin-dependent mode of compensatory endocytosis that takes several seconds. A more fleeting mode of vesicle fusion, termed 'kiss-and-run' exocytosis or 'flicker-fusion', indicates that during synaptic transmission some vesicles are only briefly connected to the presynaptic membrane by a transient fusion pore. Finally, a mode that retrieves a large amount of membrane, equivalent to that of several fused vesicles, termed 'bulk endocytosis', has been found after prolonged exocytosis. We are of the opinion that both fast and slow modes of endocytosis co-exist at central nervous system nerve terminals and that one mode can predominate depending on stimulus strength, temperature and synaptic maturation.

Magnitude and direction of vesicle dynamics in growing pollen tubes using spatiotemporal image correlation spectroscopy and fluorescence recovery after photobleaching

The delivery of cell wall material and membrane to growing plant cell surfaces requires the spatial and temporal coordination of secretory vesicle trafficking. Given the small size of vesicles, their dynamics is difficult to quantify. To quantitatively analyze vesicle dynamics in growing pollen tubes labeled with the styryl dye FM1-43, we applied spatiotemporal correlation spectroscopy on time-lapse series obtained with high-speed confocal laser scanning microscopy recordings. The resulting vector maps revealed that vesicles migrate toward the apex in the cell cortex and that they accumulate in an annulus-shaped region adjacent to the extreme tip and then turn back to flow rearward in the center of the tube. Fluorescence recovery after photobleaching confirmed vesicle accumulation in the shoulder of the apex, and it revealed that the extreme apex never recovers full fluorescence intensity. This is consistent with endocytotic activity occurring in this region. Fluorescence recovery after photobleaching analysis also allowed us to measure the turnover rate of the apical vesicle population, which was significantly more rapid than the theoretical rate computed based on requirements for new cell wall material. This may indicate that a significant portion of the vesicles delivered to the apex does not succeed in contacting the plasma membrane for delivery of their contents. Therefore, we propose that more than one passage into the apex may be needed for many vesicles before they fuse to the plasma membrane and deliver their contents.

Model for calcium dependent oscillatory growth in pollen tubes

Experiments have shown that pollen tubes grow in an oscillatory mode, the mechanism of which is poorly understood. We propose a theoretical growth model of pollen tubes exhibiting such oscillatory behaviour. The pollen tube and the surrounding medium are represented by two immiscible fluids separated by an interface. The physical variables are pressure, surface tension, density and viscosity, which depend on relevant biological quantities, namely calcium concentration and thickness of the cell wall. The essential features generally believed to control oscillating growth are included in the model, namely a turgor pressure, a viscous cell wall which yields under pressure, stretch-activated calcium channels which transport calcium ions into the cytoplasm and an exocytosis rate dependent on the cytosolic calcium concentration in the apex of the cell. We find that a calcium dependent vesicle recycling mechanism is necessary to obtain an oscillating growth rate in our model. We study the variation in the frequency of the growth rate by changing the extracellular calcium concentration and the density of ion channels in the membrane. We compare the predictions of our model with experimental data on the frequency of oscillation versus growth speed, calcium concentration and density of calcium channels.

Discharge of the readily releasable pool with action potentials at hippocampal synapses

A readily releasable pool (RRP) of synaptic vesicles has been identified at hippocampal synapses with application of hypertonic solution. RRP size correlates with important properties of synaptic function such as release probability. However, a discrepancy in RRP size has been reported depending on the method used to evoke synaptic release. This study was undertaken to determine quantitative relationships between the RRP defined with hypertonic solution and that released with trains of action potentials. We find that asynchronous release at cell culture synapses contributes significantly to the discharge of the RRP with trains of action potentials and that RRP size is the same when elicited by either nerve stimuli or hypertonic challenge.

Kiss-and-run, Collapse and ‘Readily Retrievable’ Vesicles

Two models of synaptic vesicle recycling have been intensely debated for decades: kiss-and-run, in which the vesicle opens and closes transiently, presumably through a small fusion pore, and full fusion, in which the vesicle collapses into the plasma membrane and is retrieved by clathrin-coat-dependent processes. Conceptually, it seems that kiss-and-run would be faster and would retrieve vesicles with greater fidelity. Is this the case? This review discusses recent evidence for both models. We conclude that both mechanisms allow for high fidelity of vesicle recycling. Also, the presence in the plasma membrane of a depot of previously fused vesicles that are already interacting with the endocytotic machinery (the 'readily retrievable' vesicles) allows full fusion to trigger quite fast endocytosis, further blurring the efficiency differences between the two models.

Presynaptic G-protein-coupled receptors regulate synaptic cleft glutamate via transient vesicle fusion

When synaptic vesicles fuse with the plasma membrane, they may completely collapse or fuse transiently. Transiently fusing vesicles remain structurally intact and therefore have been proposed to represent a form of rapid vesicle recycling. However, the impact of a transient synaptic vesicle fusion event on neurotransmitter release, and therefore on synaptic transmission, has yet to be determined. Recently, the molecular mechanism by which a serotonergic presynaptic G-protein-coupled receptor (GPCR) regulates synaptic vesicle fusion and inhibits synaptic transmission was identified. By making paired electrophysiological recordings in the presence and absence of low-affinity antagonists, we now demonstrate that activation of this presynaptic GPCR lowers the peak synaptic cleft glutamate concentration independently of the probability of vesicle fusion. Furthermore, this change in cleft glutamate concentration differentially inhibits synaptic NMDA and AMPA receptor-mediated currents. We conclude that a presynaptic GPCR regulates the profile of glutamate in the synaptic cleft through altering the mechanism of vesicle fusion leading to qualitative as well as quantitative changes in neural signaling.

Subnanometer fusion pores in spontaneous exocytosis of peptidergic vesicles

Kiss-and-run exocytosis, consisting of reversible fusion between the vesicle membrane and the plasma membrane, is considered to lead to full fusion after stimulation of vesicles containing classical transmitters. However, whether this is also the case in the fusion of peptidergic vesicles is unknown. Previously, we have observed that spontaneous neuropeptide discharge from a single vesicle is slower than stimulated release, because of the kinetic constraints of fusion pore opening. To explore whether slow spontaneous release also reflects a relatively narrow fusion pore, we analyzed the permeation of FM 4-64 dye and HEPES molecules through spontaneously forming fusion pores in lactotroph vesicles expressing synaptopHluorin, a pH-dependent fluorescent fusion marker. Confocal imaging showed that half of the spontaneous exocytotic events exhibited fusion pore openings associated with a change in synaptopHluorin fluorescence but were impermeable to FM 4-64 and HEPES. Together with membrane capacitance measurements, these findings indicate an open fusion pore diameter <0.5 nm, much smaller than the neuropeptides. In stimulated cells, >70% of exocytotic events exhibited a larger, FM 4-64-permeable pore (>1 nm). Interestingly, capacitance measurements showed that the majority of exocytotic events in spontaneous and stimulated conditions were transient. Stimulation increased the frequency of transient events and the fusion pore dwell time but decreased the fraction of events with lowest measurable fusion pore. Kiss-and-run is the predominant mode of exocytosis in resting and in stimulated peptidergic vesicles. Stimulation prolongs the effective opening of the fusion pore and expands its primary subnanometer diameter to enable hormone secretion without full fusion.

In search of the fusion pore of exocytosis

Research on calcium-triggered exocytosis has converged on the fusion pore as a critical kinetic intermediate. Using sensitive biophysical methods to record signals from living cells in the act of releasing neurotransmitter or hormone has provided clues about the structure and composition of fusion pores. The dynamics of fusion pore opening, closing, and dilating has revealed how specific proteins transduce a calcium binding signal to catalyze membrane fusion. The fusion pore determines how rapidly neurotransmitter is expelled from a vesicle into the synaptic cleft. This rate places constraints on the form of a synaptic response during different modes of release.

Activity-dependent control of slow synaptic vesicle endocytosis by cyclin-dependent kinase 5

The stimulated dephosphorylation of the dephosphin group of endocytic proteins by calcineurin and their subsequent rephosphorylation by cyclin-dependent kinase 5 (cdk5) is required for synaptic vesicle (SV) retrieval in central nerve terminals. However, the specific endocytic pathway(s) controlled by these enzymes is unknown. To address this issue, we combined functional and morphological assays of endocytosis in primary neuronal cultures with pharmacological and molecular ablation of calcineurin and cdk5 activity. During strong stimulation, inhibition of calcineurin or cdk5 blocked uptake of the activity-dependent membrane marker FM1-43, but not the more hydrophilic FM2-10. However, FM2-10 uptake-measured poststimulation was sensitive to cdk5 and calcineurin inhibition, indicating that a slow form of endocytosis persists after termination of stimulation. In parallel EM studies, inhibition of cdk5 during strong stimulation greatly reduced horseradish peroxidase labeling of plasma membrane-derived nerve terminal endosomes, but not SVs. Furthermore, during mild stimulation, FM1-43 uptake was unaffected by cdk5 inhibition and the SV membrane was exclusively retrieved via a single SV route, suggesting that recruitment of the endosomal route of membrane retrieval is activity dependent. Thus, we propose that the calcineurin/cdk5-dependent phosphorylation cycle of the dephosphins specifically controls a slow endocytic pathway that proceeds via endosomal intermediates and is activated by strong physiological stimulation in central nerve terminals.

Amyloid precursor protein overexpression depresses excitatory transmission through both presynaptic and postsynaptic mechanisms

Overexpression of the amyloid precursor protein (APP) in hippocampal neurons leads to elevated beta-amyloid peptide (Abeta) production and consequent depression of excitatory transmission. The precise mechanisms underlying APP-induced synaptic depression are poorly understood. Uncovering these mechanisms could provide insight into how neuronal function is compromised before cell death during the early stages of Alzheimer's disease. Here we verify that APP up-regulation leads to depression of transmission in cultured hippocampal autapses; and we perform whole-cell recording, FM imaging, and immunocytochemistry to identify the specific mechanisms accounting for this depression. We find that APP overexpression leads to postsynaptic silencing through a selective reduction of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated currents. This effect is likely mediated by Abeta because expression of mutant APP incapable of producing Abeta did not depress transmission. In addition, although we eliminate presynaptic silencing as a mechanism underlying APP-mediated inhibition of transmission, we did observe an Abeta-induced presynaptic deficit in vesicle recycling with sustained stimulation. These findings demonstrate that APP elevation disrupts both presynaptic and postsynaptic compartments.

Inhibition of dynamin completely blocks compensatory synaptic vesicle endocytosis

The ability of synapses to sustain signal propagation relies on rapid recycling of transmitter-containing presynaptic vesicles. Clathrin- and dynamin-mediated retrieval of vesicular membrane has an undisputed role in synaptic vesicle recycling. There is also evidence for other modes of vesicle retrieval, including bulk retrieval and the so-called kiss-and-run recycling. Whether dynamin in required for these other modes of synaptic vesicle endocytosis remains unclear. Here, we have tested the role of dynamin in synaptic vesicle endocytosis by using a small molecule called dynasore, which rapidly inhibits the GTPase activity of dynamin with high specificity. Endocytosis after sustained or brief stimuli was completely and reversibly blocked by dynasore in cultured hippocampal neurons expressing the fluorescent tracer synaptopHluorin. By contrast, dynasore had no effect on exocytosis. In the presence of dynasore, low-frequency stimulation led to sustained accumulation of synaptopHluorin and other vesicular proteins on the surface membrane at a rate predicted from net exocytosis. These vesicular components remained on surface membranes even after the stimulus was terminated, suggesting that all endocytic events rely on dynamin during low-frequency activity as well as in the period after it. Ultrastructural analysis revealed a reduction in the density of synaptic vesicles and the presence of endocytic structures only at synapses that were stimulated in the presence of dynasore. In sum, our data indicate that dynamin is essential for all forms of compensatory synaptic vesicle endocytosis including any kiss-and-run events.

A unified model of the presynaptic and postsynaptic changes during LTP at CA1 synapses

Long-term potentiation (LTP) has been studied extensively at CA1 synapses of the hippocampus, and there is evidence implicating both postsynaptic and presynaptic changes in this process. These changes include (i) addition of AMPA channels to the extrasynaptic membrane and diffusional equilibrium of extrasynaptic receptors with synaptic receptors, (ii) sudden addition of AMPA channels to the synapse in large groups, (iii) a change in the mode of glutamate release (presumably from kiss-and-run to full fusion), and (iv) a delayed increase in the number of vesicles released. However, it remains unclear whether (or how) these changes work together. We have incorporated all of these processes into a structural model of the synapse. We propose that the synapse is composed of transsynaptic modules that function quasi-independently in AMPA-mediated transmission. Under basal conditions, synapses are partially silent; some modules are AMPA-silent (but contribute to NMDA-mediated transmission), whereas others are functional (and contribute to both AMPA- and NMDA-mediated transmission). During LTP, there is both a rapid change in the mode of vesicle fusion and a rapid insertion of a postsynaptic complex (a hyperslot) containing many proteins (slots) capable of binding AMPA channels. The combined effect of these pre- and postsynaptic changes is to convert AMPA-silent modules into functional modules. Slot filling is transiently enhanced by a rapid increase in extrasynaptic GluR1, a form of the AMPA-type receptor. A slower transsynaptic growth process adds AMPA-silent modules to the synapse, enhancing the number of vesicles released and thereby enhancing the NMDA response. This model accounts for a broad range of data, including the LTP-induced changes in quantal parameters. The model also provides a coherent explanation for the diverse effects of GluR1 knockout on basal transmission, LTP, and distance-dependent scaling.

Influence of cholesterol depletion in plasma membrane of rat brain synaptosomes on calcium-dependent and calcium-independent exocytosis

It is well established that calcium-dependent neurotransmitter release and exocytosis can be regulated by altering the cholesterol content of the plasma membrane. We have compared the influence of cholesterol depletion of synaptosomal plasma membrane by 15 mM methyl-beta-cyclodextrin (MCD) treatment on calcium-dependent release of D-[(3)H]aspartate induced by the calcium ionophore A23187 and on calcium-independent release induced by hypertonic shrinking or polyvalent cations. We found that decrease of cholesterol concentration by 9.3% inhibited calcium-dependent release of d-[(3)H]aspartate induced by calcium ionophore A23187 by four times while release induced by 300 microM Gd(3+), 150 mM and 500 mM sucrose remained unchanged. Further we have investigated the influence of MCD on exocytosis monitored by the fluorescent dye, acridine orange. Cholesterol depletion inhibited calcium-dependent exocytosis induced by calcium ionophore A23187 but had virtually no influence on calcium-independent exocytosis induced by hypertonic shrinking or Gd(3+). In summary, we found that the cholesterol content in synaptosomal plasma membrane is important for calcium-dependent exocytosis.

Fusion pores and fusion machines in Ca2+-triggered exocytosis

Exocytosis is initiated within a highly localized region of contact between two biological membranes. Small areas of these membranes draw close, molecules on the two surfaces interact, and structural transformations take place. Membrane fusion requires the action of proteins specialized for this task, and these proteins act as a fusion machine. At a critical point in this process, a fusion pore forms within the membrane contact site and then expands as the spherical vesicle merges with the flat target membrane. Hence, the operation of a fusion machine must be realized through the formation and expansion of a fusion pore. Delineating the relation between the fusion machine and the fusion pore thus emerges as a central goal in elucidating the mechanisms of membrane fusion. We summarize present knowledge of fusion machines and fusion pores studied in vitro, in neurons, and in neuroendocrine cells, and synthesize this knowledge into some specific and detailed hypotheses for exocytosis.

BDNF increases release probability and the size of a rapidly recycling vesicle pool within rat hippocampal excitatory synapses

Exerting its actions pre-, post- and peri-synaptically, brain-derived neurotrophic factor (BDNF) is one of the most potent modulators of hippocampal synaptic function. Here, we examined the effects of BDNF on a rapidly recycling pool (RRP) of vesicles within excitatory synapses. First, we estimated vesicular release in hippocampal cultures by performing FM4-64 imaging in terminals impinging on enhanced green fluorescent protein (eGFP)-labelled dendritic spines - a hallmark of excitatory synapses. Consistent with a modulation of the RRP, BDNF increased the evoked destaining rate of FM4-64 only during the initial phase of field stimulation. Multiphoton microscopy in acute hippocampal slices confirmed these observations by selectively imaging the RRP, which was loaded with FM1-43 by hyperosmotic shock. Slices exposed to BDNF showed an increase in the evoked and spontaneous rates of FM1-43 destaining from terminals in CA1 stratum radiatum, mostly representing excitatory terminals of Schaffer collaterals. Variance-mean analysis of evoked EPSCs in CA1 pyramidal neurons further confirmed that release probability is increased in BDNF-treated slices, without changes in the number of independent release sites or average postsynaptic quantal amplitude. Because BDNF was absent during dye loading, imaging, destaining and whole-cell recordings, these results demonstrate that BDNF induces a long-lasting enhancement in the probability of transmitter release at hippocampal excitatory synapses by modulating the RRP. Since the endogenous BDNF scavenger TrkB-IgG prevented the enhancement of FM1-43 destaining rate caused by induction of long-term potentiation in acute hippocampal slices, the modulation of a rapidly recycling vesicle pool may underlie the role of BDNF in hippocampal long-term synaptic plasticity.

High throughput analysis of endogenous glutamate release using a fluorescence plate reader

Recent discoveries of different modes of exocytosis and a plethora of molecules involved in neurotransmitter release has resulted in demand for more rapid and efficient methods for monitoring endogenous glutamate release from various tissue sources. In this article, we describe a high throughput microplate version of the enzyme-linked fluorescence detection method for the measurement of released glutamate, which utilises glutamate dehydrogenase, and the reduction of NADP to NADPH. Previous versions of this method rely upon cuvette-based fluorimeters for detection that are limited by large sample volumes and small numbers of samples that can be measured simultaneously. Comparison between the two methods shows that the microplate assay has comparable performance to the cuvette-based assay but has the capacity to analyse many times more samples in a given run. This increased capacity provides improved experimental design opportunities, higher experimental throughput and better comparison between experimental conditions.

Double patch clamp reveals that transient fusion (kiss-and-run) is a major mechanism of secretion in calf adrenal chromaffin cells: high calcium shifts the mechanism from kiss-and-run to complete fusion

Transient fusion ("kiss-and-run") is accepted as a mode of transmitter release both in central neurons and neuroendocrine cells, but the prevalence of this mechanism compared with full fusion is still in doubt. Using a novel double patch-clamp method (whole cell/cell attached), permitting the recording of unitary capacitance events while stimulating under a variety of conditions including action potentials, we show that transient fusion is the predominant (>90%) mode of secretion in calf adrenal chromaffin cells. Raising intracellular Ca2+ concentration ([Ca]i) from 10 to 200 microM increases the incidence of full fusion events at the expense of transient fusion. Blocking rapid endocytosis that normally terminates transient fusion events also promotes full fusion events. Thus, [Ca]i controls the transition between transient and full fusion, each of which is coupled to different modes of endocytosis.

Light-induced exocytosis in cell development and differentiation

Calcium-dependent exocytosis of fluorescently labeled single secretory vesicles in PC12 cells and primary embryonic telencephalon cells can be triggered by illumination with visible light and imaged by TIRF or epifluorescence microscopy. Opsin 3 was identified by quantitative PCR expression analysis as the putative light receptor molecule for light-induced exocytosis. In primary chicken telencephalon cells, light-induced exocytosis is restricted to a specific period during embryonic development, and involves fusion of rather large vesicles. Strictly calcium-dependent exocytosis starts after a delay of a few seconds of illumination and lasts for up to 2 min. We analyzed the frequency, time course and spatial distribution of exocytotic events. Exocytosis in PC12 cells and telencephalon cells occurs at the periphery or the interface between dividing cells, and the duration of single secretion events varies considerably. Our observation strongly supports the idea that light induced exocytosis is most likely a mechanism for building plasma membrane during differentiation, development and proliferation rather than for calcium-dependent neurotransmitter release.

Synaptic vesicle recycling adapts to chronic changes in activity

Synaptic vesicle recycling is essential for maintaining neurotransmission during rhythmic activity. To test whether the demands imposed by ambient activity influences synaptic vesicle trafficking, we compared the kinetics of synaptic depression in hippocampal versus neocortical cultures, which have high and low levels of intrinsic activity, respectively. In response to moderate 10 Hz stimulation, hippocampal synapses depressed less compared with neocortical synapses, although they reused vesicles more slowly. Therefore, during stimulation, hippocampal synapses used more vesicles from the reserve pool, whereas neocortical synapses relied on vesicle reuse. In hippocampal cultures, chronic block of network activity increased synaptic depression by decreasing the rate of vesicle mobilization, with little effect on the rate of vesicle reuse. In contrast, in neocortical cultures, an increase in the normally low network activity reduced synaptic depression by robustly increasing vesicle reuse with no effect on vesicle mobilization. These results suggest that synaptic vesicle trafficking and the resulting synaptic dynamics adapt to meet the changing demands on neurotransmitter release. Furthermore, during these functional modifications, synapses use alternate strategies to adjust to changes in activity.

The kinetics of synaptic vesicle reacidification at hippocampal nerve terminals

After exocytosis, synaptic vesicles are recycled locally in the synaptic terminal and are refilled with neurotransmitter via vesicular transporters. The biophysical mechanisms of refilling are poorly understood, but it is clear that the generation of a proton gradient across the vesicle membrane is crucial. To better understand the determinants of vesicle refilling, we developed a novel method to measure unambiguously the kinetics of synaptic vesicle reacidification at individual synaptic terminals. Hippocampal neurons transfected with synapto-pHluorin (SpH), a synaptic vesicle-targeted lumenal GFP (green fluorescent protein), whose fluorescence is quenched when protonated (pKa approximately 7.1), were rapidly surface-quenched immediately after trains of repetitive electrical stimulation. The recently endocytosed alkaline pool of SpH is protected from such surface quenching, and its fluorescence decay reflects reacidification kinetics. These measurements indicate that, after compensatory endocytosis, synaptic vesicles reacidify with first-order kinetics (tau approximately 4-5 s) and that their rate of reacidification is subject to slowing by increased external buffer.

SH3P7/mAbp1 deficiency leads to tissue and behavioral abnormalities and impaired vesicle transport

The intracellular adaptor protein SH3P7 is the mammalian ortholog of yeast actin-binding protein 1 and thus alternatively named as mAbp1 (or HIP55). Structural properties, biochemical analysis of its interaction partners and siRNA studies implicated mAbp1 as an accessory protein in clathrin-mediated endocytosis (CME). Here, we describe the generation and characterization of mice deficient for SH3P7/mAbp1 owing to targeted gene disruption in embryonic stem cells. Mutant animals are viable and fertile without obvious deficits during the first weeks of life. Abnormal structure and function of organs including the spleen, heart, and lung is observed at about 3 months of age in both heterozygous and homozygous mouse mutants. A moderate reduction of both receptor-mediated and synaptic endocytosis is observed in embryonic fibroblasts and in synapses of hippocampal neurons, respectively. Recycling of synaptic vesicles in hippocampal boutons is severely impaired and delayed four-fold. The presynaptic defect of SH3P7/mAbp1 mouse mutants is associated with their constricted physical capabilities and disturbed neuromotoric behaviour. Our data reveal a nonredundant role of SH3P7/mAbp1 in CME and places its function downstream of vesicle fission.

Synaptic vesicle reuse and its implications

Presynaptic nerve terminals are exquisite vesicle trafficking machines. Neurotransmission is sustained by constant recycling of a handful of vesicles. Therefore, the rate and the pathway of vesicle trafficking can critically determine synaptic efficacy during activity. However, it is yet unclear whether synaptic vesicle recycling becomes rate limiting on a rapid time scale during physiologically relevant forms of activity in the brain. Several forms of synaptic plasticity arise from persistent alterations in the dynamics of vesicle trafficking in presynaptic terminals. What makes presynaptic forms of plasticity particularly interesting is that they not only increase or decrease the amplitude of synaptic responses but also cause frequency-dependent changes in neurotransmission. In this manner, plasticity can alter the information coding in neural circuits beyond simple scaling of synaptic responses. However, studying the synaptic vesicle cycle beyond exocytosis and endocytosis has been difficult. In the past decade, several methods have been developed to infer vesicles' trajectory during their cycle in the synapse. Nevertheless, several questions remain. A better understanding of the role of synaptic vesicle trafficking in neurotransmission will require novel approaches that either combine existing methods or the development of new methods to trace vesicles during their cycle. Recent evidence suggests that various presynaptic proteins involved in the synaptic function and homeostasis are either mutated or altered in their expression in several neurological and psychiatric disorders. Therefore, elucidation of the mechanisms that underlie the synaptic vesicle cycle may reveal novel therapeutic targets for brain disorders.

Clathrin and synaptic vesicle endocytosis: studies at the squid giant synapse

The role of clathrin-mediated endocytosis in SV (synaptic vesicle) recycling has been studied by combining molecular biology, physiology and electron microscopy at the squid giant synapse. Procedures that prevent clathrin from assembling into membrane coats, such as impairment of binding of the AP180 and AP-2 adaptor proteins, completely prevent membrane budding during endocytosis. These procedures also reduce exocytosis, presumably an indirect effect of a reduction in the number of SVs following block of endocytosis. Disrupting the binding of auxilin to Hsc70 (heat-shock cognate 70) prevents clathrin-coated vesicles from uncoating and also disrupts SV recycling. Taken together, these results indicate that a clathrin-dependent pathway is the primary means of SV recycling at this synapse under physiological conditions.

G protein betagamma-subunits activated by serotonin mediate presynaptic inhibition by regulating vesicle fusion properties

Neurotransmitters are thought to be released as quanta, where synaptic vesicles deliver packets of neurotransmitter to the synaptic cleft by fusion with the plasma membrane. However, synaptic vesicles may undergo incomplete fusion. We provide evidence that G protein-coupled receptors inhibit release by causing such incomplete fusion. 5-hydroxytryptamine (5-HT) receptor signaling potently inhibits excitatory postsynaptic currents (EPSCs) between lamprey reticulospinal axons and their postsynaptic targets by a direct action on the vesicle fusion machinery. We show that 5-HT receptor-mediated presynaptic inhibition, at this synapse, involves a reduction in EPSC quantal size. Quantal size was measured directly by comparing unitary quantal amplitudes of paired EPSCs before and during 5-HT application and indirectly by determining the effect of 5-HT on the relationship between mean-evoked EPSC amplitude and variance. Results from FM dye-labeling experiments indicate that 5-HT prevents full fusion of vesicles. 5-HT reduces FM1-43 staining of vesicles with a similar efficacy to its effect on the EPSC. However, destaining of FM1-43-labeled vesicles is abolished by lower concentrations of 5-HT that leave a substantial EPSC. The use of a water-soluble membrane impermeant quenching agent in the extracellular space reduced FM1-43 fluorescence during stimulation in 5-HT. Thus vesicles contact the extracellular space during inhibition of synaptic transmission by 5-HT. We conclude that 5-HT, via free Gbetagamma, prevents the collapse of synaptic vesicles into the presynaptic membrane.

Synaptic vesicles caught kissing again

In this issue of Neuron, Harata et al. use a novel quenching technique to provide compelling evidence that kiss-and-run is the dominant mode of vesicle fusion at hippocampal synapses and that the prevalence of kiss-and-run can be modulated by stimulus frequency. The increased incidence of kiss-and-run at lower frequencies may ensure that vesicles are available for use during periods of high demand.

Frequency-dependent kinetics and prevalence of kiss-and-run and reuse at hippocampal synapses studied with novel quenching methods

The kinetics of exo-endocytotic recycling could restrict information transfer at central synapses if neurotransmission were entirely reliant on classical full-collapse fusion. Nonclassical fusion retrieval by kiss-and-run would be kinetically advantageous but remains controversial. We used a hydrophilic quencher, bromophenol blue (BPB), to help detect nonclassical events. Upon stimulation, extracellular BPB entered synaptic vesicles and quenched FM1-43 fluorescence, indicating retention of FM dye beyond first fusion. BPB also quenched fluorescence of VAMP (synaptobrevin-2)-EGFP, thus indicating the timing of first fusion of vesicles in the total recycling pool. Comparison with FM dye destaining revealed that kiss-and-run strongly prevailed over full-collapse fusion at low frequency, giving way to a near-even balance at high frequency. Quickening of kiss-and-run vesicle reuse was also observed at higher frequency in the average single vesicle fluorescence response. Kiss-and-run and reuse could enable hippocampal nerve terminals to conserve scarce vesicular resources when responding to widely varying input patterns.

Presynaptically silent GABA synapses in hippocampus

Mammalian central synapses commonly specialize in one fast neurotransmitter, matching the content of their presynaptic vesicles with the appropriate receptors in their postsynaptic membrane. Here, I show that hippocampal cultures contain autaptic glutamatergic synapses that contravene this rule: in addition to postsynaptic glutamate receptors, they also express clusters of functional postsynaptic GABA(A) receptors yet lack presynaptic GABA. Hence, these synapses are presynaptically silent with respect to GABA. They can be unsilenced by loading GABA into presynaptic vesicles by endocytosis, after which a postload IPSC appears. This IPSC is similar to native IPSCs recorded from GABAergic interneurons in the same cultures. Thus, these "mistargeted" GABA(A) receptors, which apparently lack a signal that confers synaptic specificity, function almost normally. After GABA loading, glutamatergic miniature postsynaptic currents acquire a slow tail that is mediated by GABA(A) receptors, showing that synaptic vesicles can accommodate both the usual concentration of native glutamate and a saturating concentration of loaded GABA. After brief Ca(2+)-dependent exocytosis, endocytosis of GABA can proceed in low-Ca(2+) external solution. The amplitude of the postload IPSC declines exponentially with repetitive stimulation as the endocytosed GABA passes through the presynaptic vesicle cycle and is depleted. Hence, by using GABA as an exogenous but physiological tracer, the properties of these presynaptically silent synapses can provide novel insights into the content and cycling of vesicles in presynaptic terminals.

Distinct transmitter release properties determine differences in short-term plasticity at functional and silent synapses

Recent evidence suggests that functional and silent synapses are not only postsynaptically different but also presynaptically distinct. The presynaptic differences may be of functional importance in memory formation because a proposed mechanism for long-term potentiation is the conversion of silent synapses into functional ones. However, there is little direct experimentally evidence of these differences. We have investigated the transmitter release properties of functional and silent Schaffer collateral synapses and show that on the average functional synapses displayed a lower percentage of failures and higher excitatory postsynaptic current (EPSC) amplitudes than silent synapses at +60 mV. Moreover, functional but not silent synapses show paired-pulse facilitation (PPF) at +60 mV and thus presynaptic short-term plasticity will be distinct in the two types of synapse. We examined whether intraterminal endoplasmic reticulum Ca2+ stores influenced the release properties of these synapses. Ryanodine (100 microM) and thapsigargin (1 microM) increased the percentage of failures and decreased both the EPSC amplitude and PPF in functional synapses. Caffeine (10 mM) had the opposite effects. In contrast, silent synapses were insensitive to both ryanodine and caffeine. Hence we have identified differences in the release properties of functional and silent synapses, suggesting that synaptic terminals of functional synapses express regulatory molecular mechanisms that are absent in silent synapses.

Encoding light intensity by the cone photoreceptor synapse

How cone synapses encode light intensity determines the precision of information transmission at the first synapse on the visual pathway. Although it is known that cone photoreceptors hyperpolarize to light over 4-5 log units of intensity, the relationship between light intensity and transmitter release at the cone synapse has not been determined. Here, we use two-photon microscopy to visualize release of the synaptic vesicle dye FM1-43 from cone terminals in the intact lizard retina, in response to different stimulus light intensities. We then employ electron microscopy to translate these measurements into vesicle release rates. We find that from darkness to bright light, release decreases from 49 to approximately 2 vesicles per 200 ms; therefore, cones compress their 10,000-fold operating range for phototransduction into a 25-fold range for synaptic vesicle release. Tonic release encodes ten distinguishable intensity levels, skewed to most finely represent bright light, assuming release obeys Poisson statistics.

Synaptotagmin IV does not alter excitatory fast synaptic transmission or fusion pore kinetics in mammalian CNS neurons

Synaptotagmin IV (Syt IV) is a brain-specific isoform of the synaptotagmin family, the levels of which are strongly elevated after seizure activity. The dominant hypothesis of Syt IV function states that Syt IV upregulation is a neuroprotective mechanism for reducing neurotransmitter release. To test this hypothesis in mammalian CNS synapses, Syt IV was overexpressed in cultured mouse hippocampal neurons, and acute effects on fast excitatory neurotransmission were assessed. We found neurotransmission unaltered with respect to basal release probability, Ca2+ dependence of release, short-term plasticity, and fusion pore kinetics. In contrast, expression of a mutant Syt I with diminished Ca2+ affinity (R233Q) reduced release probability and altered the Ca2+ dependence of release, thus demonstrating the sensitivity of the system to changes in neurotransmission resulting from changes to the Ca2+ sensor. Together, these data refute the dominant model that Syt IV functions as an inhibitor of neurotransmitter release in mammalian neurons.

A slowed classical pathway rather than kiss-and-run mediates endocytosis at synapses lacking synaptojanin and endophilin

The extent to which a "kiss-and-run" mode of endocytosis contributes to synaptic-vesicle recycling remains controversial. The only genetic evidence for kiss-and-run at the synapse comes from mutations in the genes encoding synaptojanin and endophilin, proteins that together function to uncoat vesicles in classical clathrin-mediated endocytosis. Here we have characterized the endocytosis that persists in null alleles of Drosophila synaptojanin and endophilin. In response to high-frequency stimulation, the synaptic-vesicle pool can be reversibly depleted in these mutants. Recovery from this depletion is slow and indicates the persistence of an impaired form of classical endocytosis. Steady-state exocytosis rates reveal that endocytosis saturates in mutant neuromuscular terminals at approximately 80 vesicles/s, 10%-20% of the wild-type rate. Analyses of quantal size, FM1-43 loading, and dynamin function further demonstrate that, even in the absence of synaptojanin or endophilin, vesicles undergo full fusion and re-formation. Therefore, no genetic evidence remains to indicate that synaptic vesicles undergo kiss-and-run.

Osmolarity-induced renin secretion from kidneys: evidence for readily releasable renin pools

Our study aimed to characterize the influence of changes in extracellular osmolarity on renin secretion from the whole kidney. For this purpose, the osmolarity of the perfusion medium of isolated rat or mouse kidneys was either decreased by lowering the NaCl concentration by 20% or was increased up to 133% by the addition of various salts or sugars. It turned out that changes in osmolarity led to instantaneous transient changes followed by a plateau of renin secretion, in that increases in osmolarity stimulated renin secretion, whereas decreases attenuated renin secretion. The peak amplitude of changes in renin secretion was related to steady-state renin secretion rates before the osmotic challenge but was independent of the maneuver used to modulate steady-state renin secretion. Osmolarity-induced changes in renin secretion were more related to relative rather than to absolute changes in osmolarity and were not dependent on the formation of nitric oxide or of prostanoids and did not require Na-K-2Cl cotransport function or swelling-activated chloride channels. Moreover, we obtained evidence that the pool of renin secretion excitable by hyperosmolarity is exhaustible and that its complete refilling takes at least 2 min. The observed behavior of renin secretion fits the concept about exocytosis proposing the existence of different pools of committed secretory vesicles, which have not yet undergone the final modification for initiation of exocytosis. Probably, a pool of readily releasable vesicles determines steady-state secretion rates from kidneys.

Regulation of synaptic vesicles pools within motor nerve terminals during short-term facilitation and neuromodulation

The reserve pool (RP) and readily releasable pool (RRP) of synaptic vesicles within presynaptic nerve terminals were physiologically differentiated into distinctly separate functional groups. This was accomplished in glutamatergic nerve terminals by blocking the glutamate transporter with dl-threo-beta-benzyloxyaspartate (TBOA; 10 microM) during electrical stimulation with either 40 Hz of 10 pulses within a train or 20- or 50-Hz continuous stimulation. The 50-Hz continuous stimulation decreased the excitatory postsynaptic potential amplitude 60 min faster than for the 20-Hz continuous stimulation in the presence of TBOA (P < 0.05). There was no significant difference between the train stimulation and 20-Hz continuous stimulation in the run-down time in the presence of TBOA. After TBOA-induced synaptic depression, the excitatory postsynaptic potentials were rapidly (<1 min) revitalized by exposure to serotonin (5-HT, 1 microM) in every preparation tested (P < 0.05). At this glutamatergic nerve terminal, 5-HT promotes an increase probability of vesicular docking and fusion. Quantal recordings made directly at nerve terminals revealed smaller quantal sizes with TBOA exposure with a marked increase in quantal size as well as a continual appearance of smaller quanta upon 5-HT treatment after TBOA-induced depression. Thus 5-HT was able to recruit vesicles from the RP that were not rapidly depleted by acute TBOA treatment and electrical stimulation. The results support the notion that the RRP is selectively activated during rapid electrical stimulation sparing the RP; however, the RP can be recruited by the neuromodulator 5-HT. This suggests at least two separate kinetic and distinct regulatory paths for vesicle recycling within the presynaptic nerve terminal.

Fusion pore regulation of transmitter release

During the last decade a wealth of new information about the properties of the exocytotic fusion pore is changing our current view of exocytosis. The exocytotic fusion pore, a necessary stage before the full merging of the vesicle membrane with the plasma membrane, is becoming a key cellular structure that might critically control the amount of neurotransmitter released into the synaptic cleft and that can be subjected to control by second messengers and phosphorylated proteins. Fusion pores form, expand to fully merge membranes, or can close leaving an intact and identical synaptic vesicle in place for a new round of exocytosis. Transient formation of fusion pores is the mechanistic representation of the "kiss-and-run" hypothesis of transmitter release and offers new alternatives for synaptic vesicle recycling besides to the classical mechanism mediated by clathrin coat endocytosis. For vesicle recycling transient fusion pores ensures a fast mechanism for maintaining an active pool of synaptic vesicles. The size reached by transient fusion pores and the time spent on the open state can determine the release of subquantal synaptic transmission, which could be a mechanism of synaptic potentiation. In this review we will described the electrophysiological and fluorescence methods that contribute to further explore the biophysical properties of the exocytotic fusion pore and the relevant experiments obtained by these methods.

Synaptotagmin mutants Y311N and K326/327A alter the calcium dependence of neurotransmission

Synaptotagmin I, a calcium-binding synaptic vesicle protein, is thought to act as the calcium sensor for fast neurotransmission, but what synaptotagmin I does, upon binding calcium, to trigger exocytosis is still unknown. To begin to examine the role of synaptotagmin I's interactions with calcium-dependent binding partners, three mutant versions of synaptotagmin I reported to affect calcium-dependent self-oligomerization (Y311N, K327A, and K326/327A) were expressed in cultured mouse hippocampal neurons lacking endogenous synaptotagmin I, and effects on neurotransmission were evaluated by comparison with transmission rescued by wild-type synaptotagmin I. All three mutants reduced transmitter release. To separate effects on calcium binding from effects on calcium-dependent oligomerization, we measured the calcium dependence of exocytosis for two of the mutants. Both showed apparent calcium affinity much lower than wild-type, a reduction sufficient to account for the neurotransmission defects. We conclude that self-oligomerization is unlikely to play any significant role in triggering synaptic vesicle exocytosis.

Cooling blocks rat hippocampal neurotransmission by a presynaptic mechanism: observations using 2-photon microscopy

Over the past decade there has been great interest in the therapeutic potential of brain cooling for epilepsy, stroke, asphyxia and other neurological diseases. However, there is still no consensus regarding the neurophysiological effect(s) of brain cooling. We employed standard physiological techniques and 2-photon microscopy to directly examine the effect of temperature on evoked neurotransmitter release in rat hippocampal slices. We observed a monotonic decline in extracellular synaptic potentials and their initial slope over the temperature range 33-20 degrees C, when the slices were cooled to a new set point in less than 5 s. Imaging the fluorescent synaptic marker FM1-43 with 2-photon microscopy showed that the same cooling protocol dramatically reduced transmitter release between 33 and 20 degrees C. Cooling also reduced the terminal FM1-43 destaining that was induced by direct depolarization with elevated K+, indicating that axonal conduction block cannot account for our observations. The temperature dependence of FM1-43 destaining correlated well with the effect of temperature on field potential slope, compatible with a presynaptic explanation for our electrophysiological observations. Optical measurement of FM1-43 dissociation from cell membranes was not affected by temperature, and rapid cooling of slices loaded with FM1-43 did not increase their fluorescence. Our experiments provide visible evidence that a major neurophysiological effect of cooling in the mammalian brain is a reduction in the efficacy of neurotransmitter release. This presynaptic effect may account for some of the therapeutic benefits of cooling in epilepsy and possibly stroke.

The sperm, a neuron with a tail: 'neuronal' receptors in mammalian sperm

A number of plasma membrane receptor types originally thought to be specific to neurons have been found in other somatic cells. More surprisingly, the mammalian sperm and neuron appear to share many of these 'neuronal' receptors. The morphology, chromosome number, genomic activity, and functions of those two cell types are as unlike as any two cells in the body, but they both achieve their highly disparate goals with the aid of a number of the same receptors. Exocytosis in neurons and sperm is essential to the functions of these cells and is strongly influenced by similar receptors. 'Neuronal' receptor types in sperm may also play a role in the control of sperm motility (a function of course not shared by neurons). This review will consider the evidence for the presence of sperm plasma membrane 'neuronal' receptors and for their significance to mammalian sperm function. The persuasiveness of the evidence varies depending on the receptor being considered, but there is strong experimental support for the presence and importance of a number of 'neuronal' receptors in sperm.

Two modes of exocytosis at hippocampal synapses revealed by rate of FM1-43 efflux from individual vesicles

We have examined the kinetics by which FM1-43 escapes from individual synaptic vesicles during exocytosis at hippocampal boutons. Two populations of exocytic events were observed; small amplitude events that lose dye slowly, which made up more than half of all events, and faster, larger amplitude events with a fluorescence intensity equivalent to single stained synaptic vesicles. These populations of destaining events are distinct in both brightness and kinetics, suggesting that they result from two distinct modes of exocytosis. Small amplitude events show tightly clustered rate constants of dye release, whereas larger events have a more scattered distribution. Kinetic analysis of the association and dissociation of FM1-43 with membranes, in combination with a simple pore permeation model, indicates that the small, slowly destaining events may be mediated by a narrow ∼1-nm fusion pore.

Synaptic vesicle recycling studied in transgenic mice expressing synaptopHluorin

Synaptic vesicles are recycled locally within presynaptic specializations. We examined how vesicles are reused after endocytosis, using transgenic mice expressing the genetically encoded fluorescent indicator synaptopHluorin in subsets of neurons. At both excitatory and inhibitory synapses in cultured hippocampal neurons, newly endocytosed vesicles did not preferentially enter the releasable pool of vesicles. Rather, they entered the reserve pool first and subsequently the readily releasable pool over a period of several minutes. All vesicles in the recycling pool could be accessed by spaced stimuli, arguing against preferential local reuse of the readily releasable vesicles. Interestingly, nearly half the vesicles at excitatory synapses, and a third at inhibitory synapses, could not be recruited for release even by sustained stimuli. We conclude that, at presynaptic terminals in the hippocampus, most vesicles vacate release sites after exocytosis and are replaced by existing vesicles from the reserve pool, placing constraints on kiss-and-run recycling.