Synaptic vesicles have two distinct recycling pathways

Kinetic regulation of vesicle endocytosis at synapses

Studies from a variety of synapses indicate that the time course of endocytosis ranges from less than a second to hundreds of seconds. This raises questions about how the time course of endocytosis is regulated and why different rates of endocytosis are needed. Recent progress sheds light on these issues. Neuronal firing frequency and duration determine the time course of endocytosis. The dynamic nature of this time course could be a result of multiple endocytic pathways and/or of regulation by a variety of modulators. Because endocytosis is crucial for maintaining transmitter release during repetitive stimulation, regulation of endocytosis could thus provide a mechanism by which synaptic plasticity is achieved.

A Drosophila temperature-sensitive seizure mutant in phosphoglycerate kinase disrupts ATP generation and alters synaptic function

A novel paralytic mutant, nubian, was identified in a behavioral screen for conditional temperature-sensitive seizure mutants in Drosophila melanogaster. nubian mutants display reduced lifespan, abnormal motor behavior, altered synaptic structure, and defective neurotransmitter release. The nubian mutant disrupts phosphoglycerate kinase (PGK), an enzyme required for ATP generation in the terminal stage of the glycolytic pathway. Consistent with altered ATP generation in nubian animals, brain extracts show a threefold reduction in resting ATP levels compared with controls. Microarray analysis of nubian mutants reveals altered transcription of genes implicated in glucose and lipid metabolism. Disruption of ATP generation in nubian animals is accompanied by temperature-dependent defects in neuronal activity, with initial seizure activity, followed by an activity-dependent loss of synaptic transmission. nubian mutants also display structural defects at the synapse, with larger varicosity size but normal varicosity number, indicating that these synaptic parameters are regulated independently. Both exocytotic (NSF) and endocytotic (dynamin) ATPase/GTPase activity are required for normal synaptic transmission. Biochemical and physiological analyses indicate that synaptic defects in nubian animals are secondary to defective endocytosis, suggesting that endocytotic pathways may be generally more sensitive to altered ATP levels than those used for exocytosis. Alterations in ATP metabolism likely disrupt similar pathways in humans, because PGK deficiency is associated with mental retardation, seizures, and exercise intolerance. Given the behavioral similarities between disruptions of PGK function in Drosophila and humans, the analysis of nubian animals may reveal conserved neuronal responses associated with altered ATP generation within the brain.

Biochemical characterization of the coating mechanism of the endosomal donor compartment of synaptic vesicles

The heterotetrameric adaptor protein complex, AP-3, sorts proteins to both the endosome/lysosome and the synaptic vesicles. We have characterized the recruitment of pure AP-3 complex and ADP-ribosylation factor (ARF) onto the endosomal donor compartments that give rise to synaptic vesicles. We demonstrated that endosomes become heavier in a sucrose gradient after incubation with rat brain cytosol and a nonhydrolyzable GTP analog, GTPgammaS. This process requires a small GTPase, ARF-1. Furthermore, the endosomal coating is specific for AP-3 but not the AP-2 complex. This process requires only two soluble proteins AP-3 and ARF, with the recruitment of AP-3 being saturable at about 30 nM. These results establish that the synaptic vesicle's donor membrane is coated with AP-3 before vesiculation, in a coat-protein-specific and dose-dependent fashion.

Dap160/Intersectin Acts as a Stabilizing Scaffold Required for Synaptic Development and Vesicle Endocytosis

We describe the isolation of mutations in dynamin-associated protein 160 kDa (dap160), the Drosophila homolog of intersectin, a putative adaptor for proteins involved in endocytosis, cytoskeletal regulation, and signaling. We show that partial loss-of-function mutants display temperature-sensitive (ts) paralysis, whereas null mutants show ts defects in endocytosis. Loss-of-function mutants exhibit bouton overgrowth at larval neuromuscular junctions (NMJs), but evoked neurotransmission is normal. Mutant NMJs show a mild endocytic defect at 22 degrees C, which is strongly enhanced at 34 degrees C. The levels of dynamin, synaptojanin and endophilin are severely reduced in dap160 mutant NMJs, suggesting that Dap160 serves to stabilize an endocytic macromolecular complex. Electron microscopy reveals fewer vesicles, aberrant large vesicles, and an accumulation of endocytic intermediates at active and periactive zones in mutant terminals. Our data suggest that Dap160, like dynamin, is involved in synaptic vesicle retrieval at active and periactive zones.

Endocytosis and synaptic plasticity: might the tail wag the dog?

In nerve terminals, endocytosis in compensation for transmitter exocytosis was once thought 'housekeeping'. Now, several lines of research utilizing optical endocytic probes have coalesced into a new functional model. The model comprises two distinct endocytosis and vesicle-processing paradigms that are differentially regulated in response to demand for transmitter. Under some circumstances, such as recovery from short-term depression, it appears that endocytosis, and not exocytosis, is rate-limiting for transmitter release and is therefore a principal determinant of synaptic strength.

Effects of wortmannin and latrunculin A on slow endocytosis at the frog neuromuscular junction

Phosphoinositides are key regulators of synaptic vesicle cycling and endocytic traffic; the actin cytoskeleton also seems to be involved in modulating these processes. We investigated the effects of perturbing phosphoinositide signalling and actin dynamics on vesicle cycling in frog motor nerve terminals, using fluorescence and electron microscopy, and electrophysiology. Antibody staining for beta-actin revealed that actin surrounds but does not overlap with synaptic vesicle clusters. Latrunculin A, which disrupts actin filaments by binding actin monomers, and wortmannin, an inhibitor of phosphatidyl inositol-3-kinase (PI3-kinase), each disrupted the pattern of presynaptic actin staining, but not vesicle clusters in resting terminals. Latrunculin A, but not wortmannin, also reduced vesicle mobilization and exocytosis. Both drugs inhibited the stimulation-induced uptake of the styryl dye FM1-43 and blocked vesicle reformation from internalized membrane objects after tetanic stimulation. These results are consistent with a role of PI3-kinase and the actin cytoskeleton in the slow pathway of vesicle endocytosis, used primarily by reserve pool vesicles.

Synaptotagmin 7 splice variants differentially regulate synaptic vesicle recycling

The speed of synaptic vesicle recycling determines the efficacy of neurotransmission during repetitive stimulation. Synaptotagmins are synaptic C(2)-domain proteins that are involved in exocytosis, but have also been linked to endocytosis. We now demonstrate that upon expression in transfected neurons, a short splice variant of synaptotagmin 7 that lacks C(2)-domains accelerates endocytic recycling of synaptic vesicles, whereas a longer splice variant that contains C(2)-domains decelerates recycling. These results suggest that alternative splicing of synaptotagmin 7 acts as a molecular switch, which targets vesicles to fast and slow recycling pathways.

Endophilin promotes a late step in endocytosis at glial invaginations in Drosophila photoreceptor terminals

Retrieval of synaptic vesicles from the membrane of neurons is crucial to maintain normal rates of neurotransmitter release. Photoreceptor terminals of the fly's eye release neurotransmitter in a tonic manner. They therefore rely heavily on vesicle regeneration. Null mutations in endophilin (endo) block clathrin-mediated endocytosis at the Drosophila neuromuscular junction, where previous analysis of hypomorphic mutations has suggested a function for Endophilin (Endo) before vesicle fission, during membrane bending. Here, at fly photoreceptor synapses, we show that Endo is localized to synaptic vesicles at sites of endocytosis that are glial invaginations called capitate projections, and that when the photoreceptor synapses lack Endo they are impaired in their ability to release neurotransmitter. Detailed ultrastructural analysis of endo null mutant photoreceptor synapses fails to reveal a defect at early stages of vesicle reformation but, instead, reveals an accumulation of clusters of electron-dense, apparently nonfunctional, late endocytotic vesicles. Using dynamin;endo double-mutant photoreceptors, we provide further evidence that ultimately the function of Endophilin is required late in endocytosis, allowing vesicles to progress through the synaptic vesicle cycle.

Two modes of vesicle recycling in the rat calyx of Held

Vesicle recycling was studied in the rat calyx of Held, a giant brainstem terminal involved in sound localization. Stimulation of brain slices containing the calyx-type synapse with a high extracellular potassium ion concentration in the presence of horseradish peroxidase resulted within several minutes in a reduction of the number of neurotransmitter vesicles and in the appearance of labeled endosome-like structures. After returning to normal solution, the endosome-like structures disappeared over a period of several minutes, whereas simultaneously the number of labeled vesicles increased. A comparison with afferent stimulation suggested that the endosome-like structures normally do not participate in the vesicle cycle. Afferent stimulation at 5 Hz resulted in sustained synaptic transmission, without vesicle depletion but with an estimated endocytotic activity of <0.2 synaptic vesicles per active zone per second. At 20 Hz, the presynaptic action potentials generally failed during prolonged stimulation. In identified synapses, the number of vesicles labeled by photoconversion after stimulation at 5 Hz in the presence of the styryl dye RH414 was much lower than the number of vesicles that were released, as determined by measuring EPSCs. No more than approximately 5% of the vesicles were labeled after 20 min stimulation at 5 Hz, whereas this stimulation protocol was sufficient to largely destain a terminal after previous loading. The results support a scheme for recycling in which two different modes coexist. At physiological demands, a pool of approximately 5% of all vesicles provides sufficient vesicles for release. During intense stimulation, such as occurs in the presence of high extracellular K+, the synapse resorts to bulk endocytosis, a very slow mode of recycling.

Endocytosis at the synaptic terminal

Exocytosis of neurotransmitter from a synaptic vesicle is followed by efficient retrieval of its constituent membrane and proteins. Real-time measurements indicate that fast and slow modes of retrieval operate in parallel at a number of presynaptic terminals. Two mechanisms can be distinguished by electron microscopy: clathrin-mediated retrieval of small vesicles and bulk retrieval of large cisternae. Methods that investigate the behaviour of individual vesicles have recently demonstrated a third route of retrieval: the rapid reversal of a pore-like connection between the vesicle and surface ('kiss-and-run'). Key aims for the future are to identify the molecules underlying different mechanisms of endocytosis at the synapse and the signals that select between them.

Synaptotagmin I is necessary for compensatory synaptic vesicle endocytosis in vivo

Neurotransmission requires a balance of synaptic vesicle exocytosis and endocytosis. Synaptotagmin I (Syt I) is widely regarded as the primary calcium sensor for synaptic vesicle exocytosis. Previous biochemical data suggest that Syt I may also function during synaptic vesicle endocytosis; however, ultrastructural analyses at synapses with impaired Syt I function have provided an indirect and conflicting view of the role of Syt I during synaptic vesicle endocytosis. Until now it has not been possible experimentally to separate the exocytic and endocytic functions of Syt I in vivo. Here, we test directly the role of Syt I during endocytosis in vivo. We use quantitative live imaging of a pH-sensitive green fluorescent protein fused to a synaptic vesicle protein (synapto-pHluorin) to measure the kinetics of endocytosis in sytI-null Drosophila. We then combine live imaging of the synapto-pHluorins with photoinactivation of Syt I, through fluorescein-assisted light inactivation, after normal Syt I-mediated vesicle exocytosis. By inactivating Syt I only during endocytosis, we demonstrate that Syt I is necessary for the endocytosis of synaptic vesicles that have undergone exocytosis using a functional Syt I protein.

Endophilin promotes a late step in endocytosis at glial invaginations in Drosophila photoreceptor terminals

Retrieval of synaptic vesicles from the membrane of neurons is crucial to maintain normal rates of neurotransmitter release. Photoreceptor terminals of the fly's eye release neurotransmitter in a tonic manner. They therefore rely heavily on vesicle regeneration. Null mutations in endophilin (endo) block clathrin-mediated endocytosis at the Drosophila neuromuscular junction, where previous analysis of hypomorphic mutations has suggested a function for Endophilin (Endo) before vesicle fission, during membrane bending. Here, at fly photoreceptor synapses, we show that Endo is localized to synaptic vesicles at sites of endocytosis that are glial invaginations called capitate projections, and that when the photoreceptor synapses lack Endo they are impaired in their ability to release neurotransmitter. Detailed ultrastructural analysis of endo null mutant photoreceptor synapses fails to reveal a defect at early stages of vesicle reformation but, instead, reveals an accumulation of clusters of electron-dense, apparently nonfunctional, late endocytotic vesicles. Using dynamin;endo double-mutant photoreceptors, we provide further evidence that ultimately the function of Endophilin is required late in endocytosis, allowing vesicles to progress through the synaptic vesicle cycle.

Two modes of vesicle recycling in the rat calyx of Held

Vesicle recycling was studied in the rat calyx of Held, a giant brainstem terminal involved in sound localization. Stimulation of brain slices containing the calyx-type synapse with a high extracellular potassium ion concentration in the presence of horseradish peroxidase resulted within several minutes in a reduction of the number of neurotransmitter vesicles and in the appearance of labeled endosome-like structures. After returning to normal solution, the endosome-like structures disappeared over a period of several minutes, whereas simultaneously the number of labeled vesicles increased. A comparison with afferent stimulation suggested that the endosome-like structures normally do not participate in the vesicle cycle. Afferent stimulation at 5 Hz resulted in sustained synaptic transmission, without vesicle depletion but with an estimated endocytotic activity of <0.2 synaptic vesicles per active zone per second. At 20 Hz, the presynaptic action potentials generally failed during prolonged stimulation. In identified synapses, the number of vesicles labeled by photoconversion after stimulation at 5 Hz in the presence of the styryl dye RH414 was much lower than the number of vesicles that were released, as determined by measuring EPSCs. No more than approximately 5% of the vesicles were labeled after 20 min stimulation at 5 Hz, whereas this stimulation protocol was sufficient to largely destain a terminal after previous loading. The results support a scheme for recycling in which two different modes coexist. At physiological demands, a pool of approximately 5% of all vesicles provides sufficient vesicles for release. During intense stimulation, such as occurs in the presence of high extracellular K+, the synapse resorts to bulk endocytosis, a very slow mode of recycling.

Low frequency stimulation of mouse adrenal slices reveals a clathrin-independent, protein kinase C-mediated endocytic mechanism

Evidence suggests that chromaffin cells employ separate mechanisms for evoked endocytosis and granule recycling when stimulated at basal (approximately 0.5 Hz) and stress-activated (approximately 15 Hz) rates. Previous studies have focused mainly on elucidating the cellular mechanisms responsible for membrane recycling under conditions similar to the stress-activated state and indicate a clathrin/dephosphin-mediated retrieval via coated pits. However, the mechanism for membrane internalisation at basal stimulus intensity remains largely unexplored. We electrically stimulated chromaffin cells in adrenal tissue slices at the sympathetic basal firing rate and measured cell capacitance in the perforated voltage clamp configuration. A new method for the separation of non-secretory from secretory cell capacitance signals is presented. Simultaneous catecholamine release was measured electrochemically to isolate the exocytic from endocytic components of the capacitance responses. Using this approach we demonstrate that firing patterns that mimic basal sympathetic input results in rapid and graded membrane retrieval. We show that block of the calcium-mediated protein phosphatase 2B, a common step in clathrin-mediated processes, did not alter endocytosis elicited at basal firing levels. We further blocked clathrin-mediated retrieval with a clathrin/dephosphin-disrupting peptide (PP-19) and found endocytosis to be blocked at 15 Hz stimulation but complete and indistinguishable from control cells at 0.5 Hz stimulation. Lastly, pharmacological treatments show that conventional isoforms of protein kinase C (cPKC) are required for the 0.5 Hz-evoked retrieval mechanism. From these data we conclude that unlike endocytosis evoked under stress conditions, basal firing activity results in a clathrin-independent rapid membrane retrieval mediated through conventional isoforms of PKC.

An essential role of Rab5 in uniformity of synaptic vesicle size

Rab5 small GTPase is a famous regulator of endocytic vesicular transport from plasma membrane to early endosomes. In neurons, Rab5 is found not only on endocytic vesicles in cell bodies but also on synaptic vesicles in nerve terminals. However, the function of Rab5 on synaptic vesicles remains unclear. Here, we elucidate the function of Rab5 on synaptic vesicles with in vivo and in vitro experiments using Drosophila photoreceptor cells. Functional inhibition of Rab5 with Rab5N142I, a dominant negative version of Drosophila Rab5, induced enlargement of synaptic vesicles. This enlargement was, however, suppressed by enhancing synaptic vesicle recycling under light illumination. In addition, synaptic vesicles prepared from Rab5N142I-expressing flies exhibited homotypic fusion in vitro. These results indicate that Rab5 functions to keep the size of synaptic vesicles uniform by preventing their homotypic fusion. By contrast, Rab5 was not involved in the endocytic reformation of synaptic vesicles, contrary to expectation from its conventional function. Furthermore, we electrophysiologically and behaviourally showed that the function of Rab5 is essential for efficient signal transmission across synapses.

"Delayed" endocytosis is regulated by extracellular Ca2+ in snake motor boutons

When cooled below approximately 7 degrees C, recently endocytosed vesicles in the motor terminals of the garter snake fail to shed their clathrin coats. Perhaps as a result, the terminals complete only about one-half of the compensatory endocytosis expected after a given period of stimulation. Upon return to room temperature (RT), endocytosis resumes immediately and is complete within minutes. This "delayed" endocytosis following release from cold block provides an opportunity to study clathrin-dependent endocytotic mechanisms in temporal isolation from those events, such as Ca2+ entry and consequent exocytosis, that are normally associated with the activation of nerve terminals. We have taken advantage of clathrin decoating blockade to examine the rate, temperature dependence and extracellular Ca2+ dependence of endocytosis at the snake nerve-muscle synapse. Endocytosis was fast at RT (complete in < 1 min) and markedly faster still at 35 degrees C. Moreover, the rate of endocytosis varied significantly with change in [Ca2+]o; the rate at 7.2 mM (single exponential time constant, approximately 3 s) was approximately double that at 0 mM (single exponential time constant, approximately 7 s). Thus, membrane retrieval via clathrin is rapid and, due to its dependence on [Ca2+]o, potentially regulated by changes in the milieu of the synaptic cleft during neural activity.

Targeted expression of shibire ts and semaphorin 1a reveals critical periods for synapse formation in the giant fiber of Drosophila

In order to determine the timing of events during the assembly of a neural circuit in Drosophila we targeted expression of the temperature-sensitive shibire gene to the giant fiber system and then disrupted endocytosis at various times during development. The giant fiber retracted its axon or incipient synapses when endocytosis was blocked at critical times, and we perceived four phases to giant fiber development: an early pathfinding phase, an intermediate phase of synaptogenesis, a late stabilization process and, finally, a mature synapse. By co-expressing shibire(ts) and semaphorin 1a we provided evidence that Semaphorin 1a was one of the proteins being regulated by endocytosis and its removal was a necessary part of the program for synaptogenesis. Temporal control of targeted expression of the semaphorin 1a gene showed that acute excess Semaphorin 1a had a permanent disruptive effect on synapse formation.

Synaptic vesicle pools at the frog neuromuscular junction

We have characterized the morphological and functional properties of the readily releasable pool (RRP) and the reserve pool of synaptic vesicles in frog motor nerve terminals using fluorescence microscopy, electron microscopy, and electrophysiology. At rest, about 20% of vesicles reside in the RRP, which is depleted in about 10 s by high-frequency nerve stimulation (30 Hz); the RRP refills in about 1 min, and surprisingly, refilling occurs almost entirely by recycling, not mobilization from the reserve pool. The reserve pool is depleted during 30 Hz stimulation with a time constant of about 40 s, and it refills slowly (half-time about 8 min) as nascent vesicles bud from randomly distributed cisternae and surface membrane infoldings and enter vesicle clusters spaced at regular intervals along the terminal. Transmitter output during low-frequency stimulation (2-5 Hz) is maintained entirely by RRP recycling; few if any vesicles are mobilized from the reserve pool.

Secretion: kiss and run caught on film

Recent results have provided graphic support for the hypothesis that vesicle secretion involves a 'kiss-and-run' mechanism. Evanescent field microscopy has shown that, during exocytosis, intravesicular markers escape without collapse of the vesicular membrane into the surface membrane and that the empty vesicle is immediately retrieved back into the cell.

Role of Drosophila Rab5 during endosomal trafficking at the synapse and evoked neurotransmitter release

During constitutive endocytosis, internalized membrane traffics through endosomal compartments. At synapses, endocytosis of vesicular membrane is temporally coupled to action potential-induced exocytosis of synaptic vesicles. Endocytosed membrane may immediately be reused for a new round of neurotransmitter release without trafficking through an endosomal compartment. Using GFP-tagged endosomal markers, we monitored an endosomal compartment in Drosophila neuromuscular synapses. We showed that in conditions in which the synaptic vesicles pool is depleted, the endosome is also drastically reduced and only recovers from membrane derived by dynamin-mediated endocytosis. This suggests that membrane exchange takes place between the vesicle pool and the synaptic endosome. We demonstrate that the small GTPase Rab5 is required for endosome integrity in the presynaptic terminal. Impaired Rab5 function affects endo- and exocytosis rates and decreases the evoked neurotransmitter release probability. Conversely, Rab5 overexpression increases the release efficacy. Therefore, the Rab5-dependent trafficking pathway plays an important role for synaptic performance.

Bulk membrane retrieval in the synaptic terminal of retinal bipolar cells

The mechanism of bulk membrane uptake at the synapse remains poorly defined, although exocytosis of synaptic vesicles is followed by compensatory membrane retrieval into both small vesicles and large cisternas or vacuoles. We investigated bulk retrieval in the presynaptic terminal of retinal bipolar cells. Fluorescence imaging of the membrane dye FM1-43 indicated that Ca2+-triggered exocytosis was followed by endocytosis into small vesicles and larger vacuoles that could be selectively labeled using large fluorescent dextrans. Disruption of actin filaments with cytochalasin D or latrunculin B inhibited the formation and transport of vacuoles, but exocytosis and endocytosis continued at normal rates. Bulk retrieval was linked to remodeling of the actin network, and both processes were inhibited by 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one, an inhibitor of phosphatidylinositol 3-kinase (PI 3-kinase). The regulation of F-actin dynamics by Ca2+ and PI 3-kinase therefore played an important role in compensatory endocytosis at this synapse, but this role was confined to bulk membrane uptake. Capacitance measurements demonstrated that fast endocytosis and refilling of the rapidly releasable pool of vesicles were not dependent on F-actin or PI 3-kinase activity. The basic properties of bulk membrane retrieval at this synapse were very similar to macropinocytosis described in non-neural cells. Bulk retrieval did not play an essential role in maintaining the vesicle cycle during maintained stimulation, but we suggest that it may play a role in the structural plasticity of this synaptic terminal.

Bulk membrane retrieval in the synaptic terminal of retinal bipolar cells

The mechanism of bulk membrane uptake at the synapse remains poorly defined, although exocytosis of synaptic vesicles is followed by compensatory membrane retrieval into both small vesicles and large cisternas or vacuoles. We investigated bulk retrieval in the presynaptic terminal of retinal bipolar cells. Fluorescence imaging of the membrane dye FM1-43 indicated that Ca2+-triggered exocytosis was followed by endocytosis into small vesicles and larger vacuoles that could be selectively labeled using large fluorescent dextrans. Disruption of actin filaments with cytochalasin D or latrunculin B inhibited the formation and transport of vacuoles, but exocytosis and endocytosis continued at normal rates. Bulk retrieval was linked to remodeling of the actin network, and both processes were inhibited by 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one, an inhibitor of phosphatidylinositol 3-kinase (PI 3-kinase). The regulation of F-actin dynamics by Ca2+ and PI 3-kinase therefore played an important role in compensatory endocytosis at this synapse, but this role was confined to bulk membrane uptake. Capacitance measurements demonstrated that fast endocytosis and refilling of the rapidly releasable pool of vesicles were not dependent on F-actin or PI 3-kinase activity. The basic properties of bulk membrane retrieval at this synapse were very similar to macropinocytosis described in non-neural cells. Bulk retrieval did not play an essential role in maintaining the vesicle cycle during maintained stimulation, but we suggest that it may play a role in the structural plasticity of this synaptic terminal.

Temporal and spatial coordination of exocytosis and endocytosis

In secretory cells, exocytosis and compensatory endocytosis are tightly coupled membrane trafficking processes that control the surface area and composition of the plasma membrane. While exocytic and endocytic processes have been studied independently in great detail, at present there is much interest in understanding the mode of their coupling. This review discusses emerging insights into the coupling of these processes, both in the chemical synapses of neurons and in non-neuronal cells.

Synaptic vesicle endocytosis: the races, places, and molecular faces

The classical experiments on synaptic vesicle recycling in the 1970s by Heuser and Reese, Ceccarelli, and their colleagues raised opposing theories regarding the speed, mechanisms, and locations of membrane retrieval at the synapse. The Heuser and Reese experiments supported a model in which synaptic vesicle recycling is mediated by the formation of coated vesicles, is relatively slow, and occurs distally from active zones, the sites of neurotransmitter release. Because heavy levels of stimulation were needed to visualize the coated vesicles, Ceccarelli's experiments argued that synaptic vesicle recycling does not require the formation of coated vesicles, is relatively fast, and occurs directly at the active zone in a "kiss-and-run" reversal of exocytosis under more physiological conditions. For the next thirty years, these models have provided the foundation for studies of the rates, locations, and molecular elements involved in synaptic vesicle endocytosis. Here, we describe the evidence supporting each model and argue that the coated vesicle pathway is the most predominant physiological mechanism for recycling synaptic vesicles.

Optical monitoring of synaptic vesicle trafficking in ribbon synapses

Synaptic transmission constitutes the major basis of communication among nerve cells. Upon nerve terminal depolarisation, calcium influx triggers the exocytosis of synaptic vesicles at active zones. Vesicles are then retrieved by endocytosis, recycled and refilled with neurotransmitter. Fluorescent styryl dyes have proven very useful as tools for studying several aspects of the synaptic vesicle cycle. Here, we review recent imaging studies using styryl FM dyes and bipolar cells of goldfish retina, which have a giant synaptic terminal containing ribbon-type active zones. Optical techniques applied to this unique synaptic terminal have provided novel insights into the trafficking of synaptic vesicles during and following strong stimulation.

Okadaic acid disrupts synaptic vesicle trafficking in a ribbon-type synapse

Protein phosphorylation plays an essential role in regulating synaptic transmission and plasticity. However, regulation of vesicle trafficking towards and away from the plasma membrane is poorly understood. Furthermore, the extent to which phosphorylation modulates ribbon-type synapses is unknown. Using the phosphatase inhibitor okadaic acid (OA), we investigated the influence of persistent phosphorylation on vesicle cycling in goldfish bipolar cells. We followed uptake of FM1-43 during vesicle recycling in control and OA-treated cells. FM1-43 fluorescence spread to the center of control synaptic terminals after depolarization elicited Ca2+ influx. However, OA (1-50 nm) impaired this spatial spread of FM1-43 in a dose-dependent manner. Capacitance measurements revealed that OA (50 nm) did not modify either the amount or kinetics of exocytosis and endocytosis evoked by depolarizing pulses. The extremely low concentrations of OA (1-5 nm) sufficient to observe the inhibition of vesicle mobility implicate phosphatase 2A (PP2A) as a major regulator of vesicle trafficking after endocytosis. These results contrast with those at the neuromuscular junction where OA enhances lateral movement of vesicles between distinct vesicle clusters. Thus, our results suggest that phosphatases regulate vesicle translocation at ribbon synapses in a different manner than conventional active zones.

Perturbation of synaptic vesicle delivery during neurotransmitter release triggered independently of calcium influx

Although much evidence suggests that calcium (Ca(2+)) usually triggers synaptic vesicle exocytosis and neurotransmitter release, the role of Ca(2+) in vesicle endocytosis and in the delivery of fusion-competent vesicles (i.e. mobilisation and/or priming) in nerve terminals remains unclear. To address this issue, we have studied synaptic vesicle dynamics in cultured rat neurones under conditions where neurotransmitter release is triggered independently of Ca(2+) using the secretagogue Ruthenium Red (RR). Using a prolonged stimulation protocol, we find that RR causes a rapid increase in neurotransmitter release followed by a gradually decrementing response. In contrast, when release is triggered by moderate membrane depolarisation caused by saline containing 18 mM K(+), release is sustained. These observations suggest that when release is triggered independently of a rise in Ca(2+), endocytosis or vesicle mobilisation/priming are perturbed. Using FM2-10, a fluorescent indicator of synaptic vesicle cycling, we find that neurotransmitter release triggered by RR is accompanied by both uptake and release of this dye, thereby suggesting that vesicle endocytosis is not blocked. To evaluate whether synaptic vesicle mobilisation/priming is perturbed in the absence of a rise in Ca(2+), we compared the kinetics of FM2-10 loss during prolonged stimulation. While 18 mM K(+) induced gradual and continuous dye loss, RR only induced substantial dye loss during the first minute of stimulation. In the presence of low concentrations of the Ca(2+) ionophore ionomycin, release caused by RR was prolonged. Taken together, these results provide evidence suggesting that, although a rise in intraterminal Ca(2+) is not required for endocytosis, it is essential for the continuous delivery of fusion-competent vesicles and to maintain neurotransmitter release during prolonged stimulation.

Selective replenishment of two vesicle pools depends on the source of Ca2+ at the Drosophila synapse

After synaptic vesicles (SVs) undergo exocytosis, SV pools are replenished by recycling SVs at nerve terminals. At Drosophila neuromuscular synapses, there are two distinct SV pools (i.e., the exo/endo cycling pool (ECP), which primarily maintains synaptic transmission, and the reserve pool (RP), which participates in synaptic transmission only during tetanic stimulation). Labeling endocytosed vesicular structures with a fluorescent styryl dye, FM1-43, and measuring intracellular Ca2+ concentrations with a Ca2+ indicator, rhod-2, we show here that the ECP is replenished by SVs endocytosed during stimulation, and this process depends on external Ca2+. In contrast, the RP is refilled after cessation of tetanus by a process mediated by Ca2+ released from internal stores.

Do morphogen gradients arise by diffusion?

Many patterns of cell and tissue organization are specified during development by gradients of morphogens, substances that assign different cell fates at different concentrations. Gradients form by morphogen transport from a localized site, but whether this occurs by simple diffusion or by more elaborate mechanisms is unclear. We attempt to resolve this controversy by analyzing recent data in ways that appropriately capture the complexity of systems in which transport, receptor interaction, endo- and exocytosis, and degradation occur together. We find that diffusive mechanisms of morphogen transport are much more plausible-and nondiffusive mechanisms much less plausible-than has generally been argued. Moreover, we show that a class of experiments, endocytic blockade, thought to effectively distinguish between diffusive and nondiffusive transport models actually fails to draw useful distinctions.

Short-term synaptic plasticity

Synaptic transmission is a dynamic process. Postsynaptic responses wax and wane as presynaptic activity evolves. This prominent characteristic of chemical synaptic transmission is a crucial determinant of the response properties of synapses and, in turn, of the stimulus properties selected by neural networks and of the patterns of activity generated by those networks. This review focuses on synaptic changes that result from prior activity in the synapse under study, and is restricted to short-term effects that last for at most a few minutes. Forms of synaptic enhancement, such as facilitation, augmentation, and post-tetanic potentiation, are usually attributed to effects of a residual elevation in presynaptic [Ca(2+)]i, acting on one or more molecular targets that appear to be distinct from the secretory trigger responsible for fast exocytosis and phasic release of transmitter to single action potentials. We discuss the evidence for this hypothesis, and the origins of the different kinetic phases of synaptic enhancement, as well as the interpretation of statistical changes in transmitter release and roles played by other factors such as alterations in presynaptic Ca(2+) influx or postsynaptic levels of [Ca(2+)]i. Synaptic depression dominates enhancement at many synapses. Depression is usually attributed to depletion of some pool of readily releasable vesicles, and various forms of the depletion model are discussed. Depression can also arise from feedback activation of presynaptic receptors and from postsynaptic processes such as receptor desensitization. In addition, glial-neuronal interactions can contribute to short-term synaptic plasticity. Finally, we summarize the recent literature on putative molecular players in synaptic plasticity and the effects of genetic manipulations and other modulatory influences.

Sustained stimulation shifts the mechanism of endocytosis from dynamin-1-dependent rapid endocytosis to clathrin- and dynamin-2-mediated slow endocytosis in chromaffin cells

Transient stimulation of secretion in calf chromaffin cells is invariably followed by rapid endocytosis (RE), a clathrin- and K(+)-independent process with a half time of several seconds. Here we show that when exocytosis is triggered in a more sustained manner, a much slower form of endocytosis (SE) replaces RE. SE is complete within 10 min and is abolished when anticlathrin antibodies are introduced into the cell or when intracellular K(+) is removed. RE, but not SE, is blocked by intracellular administration of antidynamin-1 antibodies; the inverse specificity was found for antidynamin-2 antibodies. Replacement of extracellular Ca(2+) by Ba(2+) or Sr(2+) completely blocked RE but had little effect on SE. Thus chromaffin cells exhibit two kinetically and mechanistically distinct forms of endocytosis that are coupled to different extents of exocytosis and are mediated by different isoforms of dynamin. We surmise that RE is associated with the transient fusion ("kiss-and-run") mechanism of transmitter release and is the prevalent means of vesicle recapture and recycling under normal physiological conditions, whereas the clathrin-based SE mechanism comes into play only at higher levels of stimulation and may be associated with complete fusion of vesicles with the plasma membrane.

Sequential changes in synaptic vesicle pools and endosome-like organelles during depolarization near the active zone of central nerve terminals

During periods of high-frequency stimulation the maintenance of synaptic transmission depends on a continued supply of synaptic vesicles. Local recycling in the terminals ensures synaptic vesicle replenishment, but the intermediate steps are still a matter of debate. We analyzed changes in synaptic vesicle pools and endosome-like organelles near the active zone in central nerve terminals during depolarization at the ultrastructural level by electron microscopy. A short, 100 ms, depolarization-induced recruitment of synaptic vesicles was observed from a reserve pool to a recruited pool, within 150 nm of the active zone, and the docked pool at the active zone was increased as well. Prolonged, 15 s or 3 min, depolarization decreased the total amount of synaptic vesicles, which was accompanied by a parallel increase in size and amount of endosome-like organelles. After a period of rest, the number of endosome-like organelles decreased and the amount of synaptic vesicles was restored to control level. The endocytotic nature of part of the endosome-like organelles after 15 s and 3 min depolarization was indicated by their labeling with extracellularly added horseradish peroxidase (HRP). In addition, a small number of synaptic vesicles entrapped HRP under these conditions. After repolarization, the number of HRP-loaded endosome-like structures decreased. Simultaneously, a strong increase in amount of HRP-loaded small vesicles did occur. These results indicate that during sub-second depolarization, synaptic vesicles were rapidly recruited from the reserve pool to replenish the releasable pool, whereas prolonged depolarization (s-min) induced local endocytosis in at least two ways, i.e. either directly as vesicles or via endosome-like organelles from which synaptic vesicles were reformed.

Endophilin mutations block clathrin-mediated endocytosis but not neurotransmitter release

We have identified mutations in Drosophila endophilin to study its function in vivo. Endophilin is required presynaptically at the neuromuscular junction, and absence of Endophilin dramatically impairs endocytosis in vivo. Mutant larvae that lack Endophilin fail to take up FM1-43 dye in synaptic boutons, indicating an inability to retrieve synaptic membrane. This defect is accompanied by an expansion of the presynaptic membrane, and a depletion of vesicles from the bouton lumen. Interestingly, mutant larvae are still able to sustain release at 15%-20% of the normal rate during high-frequency stimulation. We propose that kiss-and-run maintains neurotransmission at active zones of the larval NMJ in endophilin animals.

Regulating morphogen gradients in the Drosophila wing

During development, diffusible ligands, known as morphogens, are thought to move across fields of cells, regulating gene expression in a concentration dependent manner. The case for morphogens has been convincingly made for the Decapentapleigic (Dpp), Wingless (Wg) and Hedgehog (Hh) proteins in the Drosophila wing. In each case, the concentration of the morphogen's receptor plays an important role in shaping the morphogen gradient, through influencing ligand transport and/or stability. However, the relationships between each ligand/receptor pair are different. The role of heparan sulfated proteoglycans, endocytosis and novel exovesicles called argosomes in regulating morphogen distribution will also be discussed.

Fast vesicle recycling supports neurotransmission during sustained stimulation at hippocampal synapses

High-frequency induced short-term synaptic depression is a common feature of central synapses in which synaptic responses rapidly decrease to a sustained level. A limitation in the availability of release-ready vesicles is thought to be a major factor underlying this phenomenon. Here, we studied the kinetics of vesicle reavailability and reuse during synaptic depression at hippocampal synapses. High-intensity stimulation of neurotransmitter release was induced by hyperosmolarity, high potassium, or action potential firing at 30 Hz to produce synaptic depression. Under these conditions, synaptic transmission rapidly depressed to a plateau level that was typically 10-40% of the initial response and persisted at this level for at least 5 min regardless of the developmental stage of synapses. This nondeclining phase of transmission was partly sustained by fast recycling and reuse of synaptic vesicles even after minutes of stimulation. Simultaneous electrical recording of postsynaptic responses and styryl dye destaining showed that after an initial round of exocytosis, vesicles were available for reuse with a delay between 1 and 3 sec during 30 Hz action potential or hypertonicity-induced stimulation. During these stimulation paradigms, there was a limited mobilization of vesicles from the reserve pool. During 10 Hz stimulation, however, the extent of vesicle reuse was minimal during the first 20 sec. These results suggest a role for fast vesicle recycling as a functional homeostatic mechanism that prevents vesicle depletion and maintains synaptic responses in the face of intense stimulation.

Fast vesicle recycling supports neurotransmission during sustained stimulation at hippocampal synapses

High-frequency induced short-term synaptic depression is a common feature of central synapses in which synaptic responses rapidly decrease to a sustained level. A limitation in the availability of release-ready vesicles is thought to be a major factor underlying this phenomenon. Here, we studied the kinetics of vesicle reavailability and reuse during synaptic depression at hippocampal synapses. High-intensity stimulation of neurotransmitter release was induced by hyperosmolarity, high potassium, or action potential firing at 30 Hz to produce synaptic depression. Under these conditions, synaptic transmission rapidly depressed to a plateau level that was typically 10-40% of the initial response and persisted at this level for at least 5 min regardless of the developmental stage of synapses. This nondeclining phase of transmission was partly sustained by fast recycling and reuse of synaptic vesicles even after minutes of stimulation. Simultaneous electrical recording of postsynaptic responses and styryl dye destaining showed that after an initial round of exocytosis, vesicles were available for reuse with a delay between 1 and 3 sec during 30 Hz action potential or hypertonicity-induced stimulation. During these stimulation paradigms, there was a limited mobilization of vesicles from the reserve pool. During 10 Hz stimulation, however, the extent of vesicle reuse was minimal during the first 20 sec. These results suggest a role for fast vesicle recycling as a functional homeostatic mechanism that prevents vesicle depletion and maintains synaptic responses in the face of intense stimulation.

Synaptic vesicle alterations in rod photoreceptors of synaptophysin-deficient mice

The abundance of the integral membrane protein synaptophysin in synaptic vesicles and its multiple possible functional contributions to transmitter exocytosis and synaptic vesicle formation stand in sharp contrast to the observed lack of defects in synaptophysin knockout mice. Assuming that deficiencies are compensated by the often coexpressed synaptophysin isoform synaptoporin, we now show that retinal rod photoreceptors, which do not synthesize synaptoporin either in wild-type or in knockout mice, are affected by the loss of synaptophysin. Multiple pale-appearing photoreceptors, as seen by electron microscopy, possess reduced cytoplasmic electron density, swollen mitochondria, an enlarged cell surface area, and, most importantly, a significantly reduced number of synaptic vesicles with an unusually bright interior. Quantification of the number of synaptic vesicles per unit area, not only in these, but also in all other rod terminals of knockout animals, reveals a considerable reduction in vesicles that is even more pronounced during the dark period, i.e., at times of highest synaptic activity. Moreover, activity-dependent reduction in synaptic vesicle diameter, typically occurring in wild-type mice, is not detected in knockout animals. The large number of clathrin-coated pits and vesicles in dark-adapted synaptophysin knockout mice is taken as an indication of compensatory usage of synaptophysin-independent pathway(s), and, conversely, in view of the overall reduction in the number of synaptic vesicles, as an indication for the presence of another synaptophysin-dependent synaptic vesicle recycling pathway. Our results provide in vivo evidence for the importance of the integral membrane protein synaptophysin for synaptic vesicle recycling and formation.

Neurotransmission and the synaptic vesicle cycle

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Physiological stimuli evoke two forms of endocytosis in bovine chromaffin cells

1. Exocytosis and endocytosis were measured following single, or trains of, simulated action potentials (sAP) in bovine adrenal chromaffin cells. Catecholamine secretion was measured by oxidative amperometry and cell membrane turnover was measured by voltage clamp cell capacitance measurements. 2. The sAPs evoked inward Na(+) and Ca(2+) currents that were statistically identical to those evoked by native action potential waveforms. On average, a single secretory granule underwent fusion following sAP stimulation. An equivalent amount of membrane was then quickly internalised (tau = 560 ms). 3. Stimulation with sAP trains revealed a biphasic relationship between cell firing rate and endocytic activity. At basal stimulus frequencies (single to 0.5 Hz) cells exhibited a robust membrane internalisation that then diminished as firing increased to intermediate levels (1.9 and 6 Hz). However at the higher stimulation rates (10 and 16 Hz) endocytic activity rebounded and was again able to effectively maintain cell surface near pre-stimulus levels. 4. Treatment with cyclosporin A and FK506, inhibitors of the phosphatase calcineurin, left endocytosis characteristics unaltered at the lower basal stimulus levels, but blocked the resurgence in endocytosis seen in control cells at higher sAP frequencies. 5. Based on these findings we propose that, under physiological electrical stimulation, chromaffin cells internalise membrane via two distinct pathways that are separable. One is prevalent at basal stimulus frequencies, is lessened with increased firing, and is insensitive to cyclosporin A and FK506. A second endocytic form is activated by increased firing frequencies, and is selectively blocked by cyclosporin A and FK506.

Exocytosis of catecholamine (CA)-containing and CA-free granules in chromaffin cells

Recent evidence suggests that endocytosis in neuroendocrine cells and neurons can be tightly coupled to exocytosis, allowing rapid retrieval from the plasma membrane of fused vesicles for future use. This can be a much faster mechanism for membrane recycling than classical clathrin-mediated endocytosis. During a fast exo-endocytotic cycle, the vesicle membrane does not fully collapse into the plasma membrane; nevertheless, it releases the vesicular contents through the fusion pore. Once the vesicle is depleted of transmitter, its membrane is recovered without renouncing its identity. In this report, we show that chromaffin cells contain catecholamine-free granules that retain their ability to fuse with the plasma membrane. These catecholamine-free granules represent 7% of the total population of fused vesicles, but they contributed to 47% of the fusion events when the cells were treated with reserpine for several hours. We propose that rat chromaffin granules that transiently fuse with the plasma membrane preserve their exocytotic machinery, allowing another round of exocytosis.

ATP is required at an early step in compensatory endocytosis in synaptic terminals

Whole-terminal capacitance measurements were used to examine membrane retrieval that follows Ca(2+)-triggered exocytosis in single synaptic terminals. Exocytosis was followed by endocytosis only when the internal solution contained a hydrolyzable analog of ATP. ATP-gamma-S, a poorly hydrolyzable ATP analog, did not support endocytosis but instead produced a rapid and profound inhibition of membrane retrieval. Under similar conditions, the GTP analogs GTP-gamma-S and GDP-beta-S failed to block endocytosis, suggesting that ATP is the preferred substrate. Furthermore, the requirement for ATP was independent of the role of ATP in regulating intraterminal Ca(2+), and the role of Ca(2+) in endocytosis was different from that of ATP. The results suggest a direct, acute requirement for ATP hydrolysis in compensatory fast endocytosis in synaptic terminals. Given that the capacitance technique detects changes in membrane surface area, ATP must be required for the membrane fission step or at a step that is a prerequisite for membrane fission.

ATP is required at an early step in compensatory endocytosis in synaptic terminals

Whole-terminal capacitance measurements were used to examine membrane retrieval that follows Ca(2+)-triggered exocytosis in single synaptic terminals. Exocytosis was followed by endocytosis only when the internal solution contained a hydrolyzable analog of ATP. ATP-gamma-S, a poorly hydrolyzable ATP analog, did not support endocytosis but instead produced a rapid and profound inhibition of membrane retrieval. Under similar conditions, the GTP analogs GTP-gamma-S and GDP-beta-S failed to block endocytosis, suggesting that ATP is the preferred substrate. Furthermore, the requirement for ATP was independent of the role of ATP in regulating intraterminal Ca(2+), and the role of Ca(2+) in endocytosis was different from that of ATP. The results suggest a direct, acute requirement for ATP hydrolysis in compensatory fast endocytosis in synaptic terminals. Given that the capacitance technique detects changes in membrane surface area, ATP must be required for the membrane fission step or at a step that is a prerequisite for membrane fission.

Regulation of presynaptic phosphatidylinositol 4,5-biphosphate by neuronal activity

Phosphatidylinositol 4,5-biphosphate (PIP2) has been implicated in a variety of cellular processes, including synaptic vesicle recycling. However, little is known about the spatial distribution of this phospholipid in neurons and its dynamics. In this study, we have focused on these questions by transiently expressing the phospholipase C (PLC)-delta1 pleckstrin homology (PH) domain fused to green fluorescent protein (GFP) in cultured hippocampal neurons. This PH domain binds specifically and with high affinity to PIP2. Live confocal imaging revealed that in resting cells, PH-GFP is localized predominantly on the plasma membrane. Interestingly, no association of PH-GFP with synaptic vesicles in quiescent neurons was observed, indicating the absence of detectable PIP2 on mature synaptic vesicles. Electrical stimulation of hippocampal neurons resulted in a decrease of the PH-GFP signal at the plasma membrane, most probably due to a PLC-mediated hydrolysis of PIP2. This was accompanied in the majority of presynaptic terminals by a marked increase in the cytoplasmic PH-GFP signal, localized most probably on freshly endocytosed membranes. Further investigation revealed that the increase in PH-GFP signal was dependent on the activation of N-methyl-D-aspartate receptors and the consequent production of nitric oxide (NO). Thus, PIP2 in the presynaptic terminal appears to be regulated by postsynaptic activity via a retrograde action of NO.

Interaction of stoned and synaptotagmin in synaptic vesicle endocytosis

The Drosophila dicistronic stoned locus encodes two distinctive presynaptic proteins, Stoned A (STNA) and Stoned B (STNB); STNA is a novel protein without homology to known synaptic proteins, and STNB contains a domain with homology to the endocytotic protein AP50. Both Stoned proteins colocalize precisely with endocytotic proteins including the AP2 complex and Dynamin in the "lattice network" characteristic of endocytotic domains in Drosophila presynaptic terminals. FM1-43 dye uptake studies in stoned mutants demonstrate a striking decrease in the size of the endo-exo-cycling synaptic vesicle pool and loss of spatial regulation of the vesicular recycling intermediates. Mutant synapses display a significant delay in vesicular membrane retrieval after depolarization and neurotransmitter release. These studies suggest that the Stoned proteins play a role in mediating synaptic vesicle endocytosis. We have documented previously a highly specific synaptic mislocalization and degradation of Synaptotagmin I in stoned mutants. Here we show that transgenic overexpression of Synaptotagmin I rescues stoned embryonic lethality and restores endocytotic recycling to normal levels. Furthermore, overexpression of Synaptotagmin I in otherwise wild-type animals results in increased synaptic dye uptake, indicating that Synaptotagmin I directly regulates the endo-exo-cycling synaptic vesicle pool size. In parallel with recent biochemical studies, this genetic analysis strongly suggests that Stoned proteins regulate the AP2-Synaptotagmin I interaction during synaptic vesicle endocytosis. We conclude that Stoned proteins control synaptic transmission strength by mediating the retrieval of Synaptotagmin I from the plasma membrane.

Nucleoside diphosphate kinase, a source of GTP, is required for dynamin-dependent synaptic vesicle recycling

Nucleoside diphosphate kinase (NDK), an enzyme encoded by the Drosophila abnormal wing discs (awd) or human nm23 tumor suppressor genes, generates nucleoside triphosphates from respective diphosphates. We demonstrate that NDK regulates synaptic vesicle internalization at the stage where function of the dynamin GTPase is required. awd mutations lower the temperature at which behavioral paralysis, synaptic failure, and blocked membrane internalization occur at dynamin-deficient, shi(ts), mutant nerve terminals. Hypomorphic awd alleles display shi(ts)-like defects. NDK is present at synapses and its enzymatic activity is essential for normal presynaptic function. We suggest a model in which dynamin activity in nerve terminals is highly dependent on NDK-mediated supply of GTP. This connection between NDK and membrane internalization further strengthens an emerging hypothesis that endocytosis, probably of activated growth factor receptors, is an important tumor suppressor activity in vivo.

Clathrin-mediated endocytosis near active zones in snake motor boutons

We have used the activity-dependent probe FM1-43 with electron microscopy (EM) to examine endocytosis at the vertebrate nerve-muscle synapse. Preparations were fixed after very brief neural stimulation at reduced temperature, and internalized FM1-43 was photoconverted into an electron-dense reaction product. To locate the reaction product, we reconstructed computer renderings of individual terminal boutons from serial EM sections. Most of the reaction product was seen in 40-60 nm vesicles. All of the labeled vesicles were clathrin-coated, and 92% of them were located within 300 nm of the plasma membrane, suggesting that they had undergone little processing after retrieval from their endocytic sites. The vesicles (and by inference the sites) were not dispersed randomly near the plane of the membrane but instead were clustered significantly near active zones. Additional reaction product was found within putative macropinosomes; these appeared to form from deep membrane invaginations near active zones. Thus two mechanisms of endocytosis were evident after brief stimulation. Endocytosis near active zones is consistent with the existence of local exo/endocytic cycling pools. This mechanism also might serve to maintain alignment of active zones with postsynaptic folds during periods of activity when vesicular and plasma membranes are interchanged.