Calcium accelerates endocytosis of vSNAREs at hippocampal synapses

Pharmacology of neurotransmitter release: measuring exocytosis

Neurotransmission in the nervous system is initiated at presynaptic terminals by fusion of synaptic vesicles with the plasma membrane and subsequent exocytic release of chemical transmitters. Currently, there are multiple methods to detect neurotransmitter release from nerve terminals, each with their own particular advantages and disadvantages. For instance, most commonly employed methods monitor actions of released chemical substances on postsynaptic receptors or artificial substrates such as carbon fibers. These methods are closest to the physiological setting because they have a rapid time resolution and they measure the action of the endogenous neurotransmitters rather than the signals emitted by exogenous probes. However, postsynaptic receptors only indirectly report neurotransmitter release in a form modified by the properties of receptors themselves, which are often nonlinear detectors of released substances. Alternatively, released chemical substances can be detected biochemically, albeit on a time scale slower than electrophysiological methods. In addition, in certain preparations, where presynaptic terminals are accessible to whole cell recording electrodes, fusion of vesicles with the plasma membrane can be monitored using capacitance measurements. In the last decade, in addition to electrophysiological and biochemical methods, several fluorescence imaging modalities have been introduced which report synaptic vesicle fusion, endocytosis, and recycling. These methods either take advantage of styryl dyes that can be loaded into recycling vesicles or exogenous expression of synaptic vesicle proteins tagged with a pH-sensitive GFP variant at regions facing the vesicle lumen. In this chapter, we will provide an overview of these methods with particular emphasis on their relative strengths and weaknesses and discuss the types of information one can obtain from them.

Single-vesicle imaging reveals that synaptic vesicle exocytosis and endocytosis are coupled by a single stochastic mode

The nature of synaptic vesicle recycling at nerve terminals has been a subject of considerable debate for >35 years. Here, we report the use of an optical strategy that allows the exocytosis and retrieval of synaptic components to be tracked in real time at single-molecule sensitivity in living nerve terminals. This approach has allowed us to examine the recycling of synaptic vesicles in response to single action potentials. Our results show that, after exocytosis, individual synaptic vesicles are retrieved by a stochastic process with an exponential distribution of delay times, with a mean time of approximately 14 s. We propose that evidence for fast endocytosis, such as that proposed to support the presence of kiss-and-run, is likely explained by the stochastic nature of a slower process.

Studying vesicle cycling in presynaptic terminals using the genetically encoded probe synaptopHluorin

Genetically encoded fluorescent probes have become indispensable tools in the biological sciences. Studies of synaptic vesicle recycling have been facilitated by a group of GFP-derived probes called pHluorins. These probes exploit changes in pH that accompany exocytosis and recapture of synaptic vesicles. Here we describe how these synaptic tracers can be used in rodent hippocampal neurons to monitor the synaptic vesicle cycle in real time and to obtain mechanistic insights about it. Synapses can be observed in living samples using a wide-field fluorescence microscope and a cooled charge-coupled device camera. A simple specimen chamber allows electrical stimulation of synapses to evoke exocytosis in a precisely controlled manner. We present protocols to measure various parameters of the synaptic vesicle cycle. This technique can be easily adapted to study different classes of synapses from wild-type and mutant mice. Once cultured neurons expressing synaptopHluorin are available, the whole procedure should take about 2 h.

Vesicle pool heterogeneity at hippocampal glutamate and GABA synapses

Glutamate and GABA are the major fast excitatory and inhibitory neurotransmitters, respectively, in the CNS. Although glutamate and GABA have clearly distinct postsynaptic actions, we are just beginning to appreciate that presynaptic differences between glutamatergic and GABAergic neurons may contribute to distinct functions of these transmitter systems. We therefore probed possible differences between the functional synaptic vesicle populations of glutamatergic and GABAergic neurons. We examined superecliptic synaptopHluorin (SpH) fluorescence during 20 Hz electrical stimulation in transfected hippocampal neurons and identified the phenotype of SpH-fluorescent synapses with post hoc immunostaining. With 200 stimuli (10 s), individual glutamate synapses displayed considerably more variability in peak SpH fluorescence than GABA synapses, without a strong difference in the mean SpH fluorescence increase. This spatial heterogeneity could not be accounted for by differences in endocytosis, which was nearly constant over these short time periods across glutamate and GABA synapses. Instead, variability in vesicle exocytosis correlated with variability in total vesicle staining and in measures of the total recycling pool size. Differences were also evident using FM1-43 [N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl) pyridinium dibromide] uptake. These data support the idea that the population of glutamate synapses exhibits more heterogeneity in release properties than the population of GABA synapses, possibly correlated with glutamatergic synaptic malleability.

Synaptic vesicle dynamics in the mossy fiber-CA3 presynaptic terminals of mouse hippocampus

The mossy fiber (MF)-CA3 synapse in the hippocampus is unique in the CNS because of its wide dynamic range of transmitter release during short- and long-term plasticity. The presynaptic mechanisms underlying the fidelity of transmission were investigated for the MF-CA3 synapses. The relative size of readily releasable pool (RRP) of vesicles was estimated by counting the number of docked vesicles at an active zone (AZ) on the transmission electron microscopy (TEM) image. The size of the releasable pool and the exo-endocytosis kinetics were directly measured from individual large MF boutons in hippocampal slices of transgenic mice that selectively express synaptopHluorin (SpH), a pH-sensitive GFP fused to the lumenal aspect of one of the vesicular membrane proteins, VAMP-2, in these boutons. Here we found (1) there are distinct two vesicle pools, the resting pool which is resistant to exocytosis, and the releasable pool, (2) the initially docked vesicles are easily depleted and the RRP is maintained by refilling from the reserve subpopulation of releasable pool ("reserve" releasable pool), and (3) the contribution of rapid reuse of recycled vesicles is relatively small. Therefore, the fidelity of transmission is suggested to be ensured by the rapid refilling rate of RRP.

Multiple vesicle recycling pathways in central synapses and their impact on neurotransmission

Short-term synaptic depression during repetitive activity is a common property of most synapses. Multiple mechanisms contribute to this rapid depression in neurotransmission including a decrease in vesicle fusion probability, inactivation of voltage-gated Ca(2+) channels or use-dependent inhibition of release machinery by presynaptic receptors. In addition, synaptic depression can arise from a rapid reduction in the number of vesicles available for release. This reduction can be countered by two sources. One source is replenishment from a set of reserve vesicles. The second source is the reuse of vesicles that have undergone exocytosis and endocytosis. If the synaptic vesicle reuse is fast enough then it can replenish vesicles during a brief burst of action potentials and play a substantial role in regulating the rate of synaptic depression. In the last 5 years, we have examined the impact of synaptic vesicle reuse on neurotransmission using fluorescence imaging of synaptic vesicle trafficking in combination with electrophysiological detection of short-term synaptic plasticity. These studies have revealed that synaptic vesicle reuse shapes the kinetics of short-term synaptic depression in a frequency-dependent manner. In addition, synaptic vesicle recycling helps maintain the level of neurotransmission at steady state. Moreover, our studies showed that synaptic vesicle reuse is a highly plastic process as it varies widely among synapses and can adapt to changes in chronic activity levels.

Optical glutamate sensor for spatiotemporal analysis of synaptic transmission

Imaging neurotransmission is expected to greatly improve our understanding of the mechanisms and regulations of synaptic transmission. Aiming at imaging glutamate, a major excitatory neurotransmitter in the CNS, we developed a novel optical glutamate probe, which consists of a ligand-binding domain of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor glutamate receptor GluR2 subunit and a small molecule fluorescent dye. We expected that such fluorescent conjugates might report the microenvironmental changes upon protein conformational changes elicited by glutamate binding. After more than 100 conjugates were tested, we finally obtained a conjugate named E (glutamate) optical sensor (EOS), which showed maximally 37% change in fluorescence intensity upon binding of glutamate with a dissociation constant of 148 nm. By immobilizing EOS on the cell surface of hippocampal neuronal culture preparations, we pursued in situ spatial mapping of synaptically released glutamate following presynaptic firing. Results showed that a single firing was sufficient to obtain high-resolution images of glutamate release, indicating the remarkable sensitivity of this technique. Furthermore, we monitored the time course of changes in presynaptic activity induced by phorbol ester and found heterogeneity in presynaptic modulation. These results indicate that EOS can be generally applicable to evaluation of presynaptic modulation and plasticity. This EOS-based glutamate imaging method is useful to address numerous fundamental issues about glutamatergic neurotransmission in the CNS.

A selective activity-dependent requirement for dynamin 1 in synaptic vesicle endocytosis

Dynamin 1 is a neuron-specific guanosine triphosphatase thought to be critically required for the fission reaction of synaptic vesicle endocytosis. Unexpectedly, mice lacking dynamin 1 were able to form functional synapses, even though their postnatal viability was limited. However, during spontaneous network activity, branched, tubular plasma membrane invaginations accumulated, capped by clathrin-coated pits, in synapses of dynamin 1-knockout mice. Synaptic vesicle endocytosis was severely impaired during strong exogenous stimulation but resumed efficiently when the stimulus was terminated. Thus, dynamin 1-independent mechanisms can support limited synaptic vesicle endocytosis, but dynamin 1 is needed during high levels of neuronal activity.

Fast synaptic vesicle reuse slows the rate of synaptic depression in the CA1 region of hippocampus

During short-term synaptic depression, neurotransmission rapidly decreases in response to repetitive action potential firing. Here, by blocking the vacuolar ATPase, alkalinizing the extracellular pH, or exposing hippocampal slices to pH buffers, we impaired neurotransmitter refilling, and electrophysiologically tested the role of vesicle reuse in synaptic depression. Under all conditions, synapses onto hippocampal CA1 pyramidal cells showed faster depression with increasing stimulation frequencies. At 20 Hz, compromising neurotransmitter refilling increased depression within 300 ms reaching completion within 2 s, suggesting a minimal contribution of reserve vesicles to neurotransmission. In contrast, at 1 Hz, depression emerged gradually and became significant within 100 s. Moreover, the depression induced by pH buffers was reversible with a similar frequency dependence, suggesting that the frequency-dependent increase in depression was caused by impairment of rapid synaptic vesicle reuse. These results indicate that synaptic vesicle trafficking impacts the kinetics of short-term synaptic plasticity at an extremely rapid time scale.

Neurotransmitter depletion by bafilomycin is promoted by vesicle turnover

Accumulation of neurotransmitter into synaptic vesicles is powered by the vacuolar proton ATPase. We show here that, in brain slices, application of the H(+)-ATPase inhibitors bafilomycin or concanamycin does not efficiently deplete glutamatergic vesicles of transmitter unless vesicle turnover is increased. Simulations of vesicle energetics suggest either that bafilomycin and concanamycin act on the H(+)-ATPase from inside the vesicle, or that the vesicle membrane potential is maintained after the H(+)-ATPase is inhibited.

Formation of presynaptic terminals at predefined sites along axons

What determines where synapses will form along an axon or how proteins are deposited at nascent synapses remains unknown. Here, we show that the initial formation of presynaptic terminals occurs preferentially at predefined sites within the axons of cortical neurons. Time-lapse imaging of synaptic vesicle protein transport vesicles (STVs) indicates that STVs pause repeatedly at these sites, even in the absence of neuronal or glial contact. Contact with a neuroligin-expressing non-neuronal cell induces formation of presynaptic terminals specifically at these STV pause sites. Remarkably, formation of stable contacts with dendritic filopodia also occurs selectively at STV pause sites. Although it is not yet known which molecules comprise the predefined sites, STV pausing is regulated by cues that affect synaptogenesis. Overall, these data are consistent with the hypothesis that regulation of STV pausing might be an important mechanism for accumulation of presynaptic proteins at nascent synapses and support a new model in which many en passant synapses form specifically at predefined sites in young axons.

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.

Clathrin-mediated endocytosis is the dominant mechanism of vesicle retrieval at hippocampal synapses

The maintenance of synaptic transmission requires that vesicles be recycled after releasing neurotransmitter. Several modes of retrieval have been proposed to operate at small synaptic terminals of central neurons, including a fast "kiss-and-run" mechanism that releases neurotransmitter through a fusion pore. Using an improved fluorescent reporter comprising pHluorin fused to synaptophysin, we find that only a slow mode of endocytosis (tau = 15 s) operates at hippocampal synapses when vesicle fusion is triggered by a single nerve impulse or short burst. This retrieval mechanism is blocked by overexpression of the C-terminal fragment of AP180 or by knockdown of clathrin using RNAi, and it is associated with the movement of clathrin and vesicle proteins out of the synapse. These results indicate that clathrin-mediated endocytosis is the major, if not exclusive, mechanism of vesicle retrieval after physiological stimuli.

Protein sorting in the synaptic vesicle life cycle

At early stages of differentiation neurons already contain many of the components necessary for synaptic transmission. However, in order to establish fully functional synapses, both the pre- and postsynaptic partners must undergo a process of maturation. At the presynaptic level, synaptic vesicles (SVs) must acquire the highly specialized complement of proteins, which make them competent for efficient neurotransmitter release. Although several of these proteins have been characterized and linked to precise functions in the regulation of the SV life cycle, a systematic and unifying view of the mechanisms underlying selective protein sorting during SV biogenesis remains elusive. Since SV components do not share common sorting motifs, their targeting to SVs likely relies on a complex network of protein-protein and protein-lipid interactions, as well as on post-translational modifications. Pleiomorphic carriers containing SV proteins travel and recycle along the axon in developing neurons. Nevertheless, SV components appear to eventually undertake separate trafficking routes including recycling through the neuronal endomembrane system and the plasmalemma. Importantly, SV biogenesis does not appear to be limited to a precise stage during neuronal differentiation, but it rather continues throughout the entire neuronal lifespan and within synapses. At nerve terminals, remodeling of the SV membrane results from the use of alternative exocytotic pathways and possible passage through as yet poorly characterized vacuolar/endosomal compartments. As a result of both processes, SVs with heterogeneous molecular make-up, and hence displaying variable competence for exocytosis, may be generated and coexist within the same nerve terminal.

C2B polylysine motif of synaptotagmin facilitates a Ca2+-independent stage of synaptic vesicle priming in vivo

Synaptotagmin I, a synaptic vesicle protein required for efficient synaptic transmission, contains a highly conserved polylysine motif necessary for function. Using Drosophila, we examined in which step of the synaptic vesicle cycle this motif functions. Polylysine motif mutants exhibited an apparent decreased Ca2+ affinity of release, and, at low Ca2+, an increased failure rate, increased facilitation, and increased augmentation, indicative of a decreased release probability. Disruption of Ca2+ binding, however, cannot account for all of the deficits in the mutants; rather, the decreased release probability is probably due to a disruption in the coupling of synaptotagmin to the release machinery. Mutants exhibited a major slowing of recovery from synaptic depression, which suggests that membrane trafficking before fusion is disrupted. The disrupted process is not endocytosis because the rate of FM 1-43 uptake was unchanged in the mutants, and the polylysine motif mutant synaptotagmin was able to rescue the synaptic vesicle depletion normally found in syt(null) mutants. Thus, the polylysine motif functions after endocytosis and before fusion. Finally, mutation of the polylysine motif inhibits the Ca2+-independent ability of synaptotagmin to accelerate SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor)-mediated fusion. Together, our results demonstrate that the polylysine motif is required for efficient Ca2+-independent docking and/or priming of synaptic vesicles in vivo.

Distinct endocytic pathways control the rate and extent of synaptic vesicle protein recycling

Synaptic vesicles have been proposed to form through two mechanisms: one directly from the plasma membrane involving clathrin-dependent endocytosis and the adaptor protein AP2, and the other from an endosomal intermediate mediated by the adaptor AP3. However, the relative role of these two mechanisms in synaptic vesicle recycling has remained unclear. We now find that vesicular glutamate transporter VGLUT1 interacts directly with endophilin, a component of the clathrin-dependent endocytic machinery. In the absence of its interaction with endophilin, VGLUT1 recycles more slowly during prolonged, high-frequency stimulation. Inhibition of the AP3 pathway with brefeldin A rescues the rate of recycling, suggesting a competition between AP2 and -3 pathways, with endophilin recruiting VGLUT1 toward the faster AP2 pathway. After stimulation, however, inhibition of the AP3 pathway prevents the full recovery of VGLUT1 by endocytosis, implicating the AP3 pathway specifically in compensatory endocytosis.

Synaptic vesicles interchange their membrane proteins with a large surface reservoir during recycling

During recycling of synaptic vesicles (SVs), the retrieval machinery faces the challenge of recapturing SV proteins in a timely and precise manner. The significant dilution factor that would result from equilibration of vesicle proteins with the much larger cell surface would make recapture by diffusional encounter with the endocytic retrieval machinery unlikely. If SV proteins exchanged with counterparts residing at steady state on the cell surface, the dilution problem would be largely avoided. In this scenario, during electrical activity, endocytosis would be driven by the concentration of a pre-existing pool of SVs residing on the axonal or synaptic surface rather than the heavily diluted postfusion vesicular pool. Using both live cell imaging of endogenous synaptotagmin Ia (sytIa) as well as pHluorin-tagged sytIa and VAMP-2, we show here that synaptic vesicle proteins interchange with a large pool on the cell axonal surface whose concentration is approximately 10-fold lower than that in SVs.

Kiss-and-run and full-collapse fusion as modes of exo-endocytosis in neurosecretion

Neurotransmitters and hormones are released from neurosecretory cells by exocytosis (fusion) of synaptic vesicles, large dense-core vesicles and other types of vesicles or granules. The exocytosis is terminated and followed by endocytosis (retrieval). More than fifty years of research have established full-collapse fusion and clathrin-mediated endocytosis as essential modes of exo-endocytosis. Kiss-and-run and vesicle reuse represent alternative modes, but their prevalence and importance have yet to be elucidated, especially in neurons of the mammalian CNS. Here we examine various modes of exo-endocytosis across a wide range of neurosecretory systems. Full-collapse fusion and kiss-and-run coexist in many systems and play active roles in exocytotic events. In small nerve terminals of CNS, kiss-and-run has an additional role of enabling nerve terminals to conserve scarce vesicular resources and respond to high-frequency inputs. Full-collapse fusion and kiss-and-run will each contribute to maintaining cellular communication over a wide range of frequencies.

The synaptic vesicle: cycle of exocytosis and endocytosis

Synaptic vesicles are clustered at the presynaptic terminal where they fuse and recycle in response to stimulation. Vesicles appear to be sorted into pools, but we do not yet understand how physiologically defined pools relate to morphological pools. The advent of dynamic imaging approaches has led to an appreciation of the regulation of vesicle mobility. Newly endocytosed vesicles are highly mobile but appear to become transiently trapped as they re-enter the recycling pool. Recent experiments indicate that endocytosis might have a constant rate, but limited capacity. How endocytosis is linked to exocytosis remains unclear, although calcium emerges as an important player.

Activity-dependent acceleration of endocytosis at a central synapse

Accumulated evidence indicates the existence of rapid and slow endocytosis at many synapses. It has been proposed that rapid endocytosis is activated by intense stimulation when vesicle recycling needs to be speeded up to supply vesicles at hippocampal synapses. However, the evidence, as obtained with imaging techniques, which are somewhat indirect in indicating rapid endocytosis, is controversial. Furthermore, a slower time course of endocytosis is often found after more intense nerve activity, casting doubt on the role of rapid endocytosis at synapses. Here, we addressed this issue at a mammalian central synapse, the calyx of Held, using a capacitance measurement technique that provides a higher time resolution than imaging techniques. We found that rapid endocytosis with a time constant of approximately 1-2 s was activated during intense nerve activity. Reducing the presynaptic calcium current or buffering the intracellular calcium with EGTA significantly inhibited rapid endocytosis, suggesting that calcium triggers rapid endocytosis. During intense stimulation, rapid endocytosis retrieved up to approximately eight vesicles per second per active zone, approximately eightfold larger than reported in the hippocampus, and thus played a dominant role during and within 3 s after intense stimulation. Slow endocytosis became dominant 3 s after intense stimulation likely because of the fall of the intracellular calcium level that deactivated rapid endocytosis. These results underscore the importance of calcium-triggered rapid endocytosis, which offers the nerve terminal the plasticity to speed up vesicle cycling during intense nerve activity.

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.

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.

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.

Stonin 2 Is an AP-2-Dependent Endocytic Sorting Adaptor for Synaptotagmin Internalization and Recycling

Clathrin-mediated endocytosis is involved in the internalization, recycling, and degradation of cycling membrane receptors as well as in the biogenesis of synaptic vesicle proteins. While many constitutively internalized cargo proteins are recognized directly by the clathrin adaptor complex AP-2, stimulation-dependent endocytosis of membrane proteins is often facilitated by specialized sorting adaptors. Although clathrin-mediated endocytosis appears to be a major pathway for presynaptic vesicle cycling, no sorting adaptor dedicated to synaptic vesicle membrane protein endocytosis has been indentified in mammals. Here, we show that stonin 2, a mammalian ortholog of Drosophila stoned B, facilitates clathrin/AP-2-dependent internalization of synaptotagmin and targets it to a recycling vesicle pool in living neurons. The ability of stonin 2 to facilitate endocytosis of synaptotagmin is dependent on its association with AP-2, an intact mu-homology domain, and functional AP-2 heterotetramers. Our data identify stonin 2 as an AP-2-dependent endocytic sorting adaptor for synaptotagmin internalization and recycling.

Optical imaging and control of genetically designated neurons in functioning circuits

Proteins with engineered sensitivities to light are infiltrating the biological mechanisms by which neurons generate and detect electrochemical signals. Encoded in DNA and active only in genetically specified target cells, these proteins provide selective optical interfaces for observing and controlling signaling by defined groups of neurons in functioning circuits, in vitro and in vivo. Light-emitting sensors of neuronal activity (reporting calcium increase, neurotransmitter release, or membrane depolarization) have begun to reveal how information is represented by neuronal assemblies, and how these representations are transformed during the computations that inform behavior. Light-driven actuators control the electrical activities of central neurons in freely moving animals and establish causal connections between the activation of specific neurons and the expression of particular behaviors. Anchored within mathematical systems and control theory, the combination of finely resolved optical field sensing and finely resolved optical field actuation will open new dimensions for the analysis of the connectivity, dynamics, and plasticity of neuronal circuits, and perhaps even for replacing lost--or designing novel--functionalities.

Developmental change in the calcium sensor for synaptic vesicle endocytosis in central nerve terminals

Synaptic vesicle endocytosis is stimulated by calcium influx in mature central nerve terminals via activation of the calcium-dependent protein phosphatase, calcineurin. However, in different neuronal preparations calcineurin activity is either inhibitory, stimulatory or irrelevant to the process. We addressed this inconsistency by investigating the requirement for calcineurin activity in synaptic vesicle endocytosis during development, using vesicle recycling assays in isolated nerve terminals. We show that endocytosis occurs independently of calcineurin activity in immature nerve terminals, and that a calcineurin requirement develops 2-4 weeks after birth. Calcineurin-independent endocytosis is not due to the absence of calcineurin activity, since calcineurin is present in immature nerve terminals and its substrate, dynamin I, is dephosphorylated on depolarization. Calcineurin-independent endocytosis is calcium-dependent, since substitution of the divalent cation, barium, inhibits the process. Finally, we demonstrated that in primary neuronal cultures derived from neonatal rats, endocytosis that was initially calcineurin-independent developed a calcineurin requirement on maturation in culture. Our data account for the apparent inconsistencies regarding the role of calcineurin in synaptic vesicle endocytosis, and we propose that an unidentified calcium sensor exists to couple calcium influx to endocytosis in immature nerve terminals.

Release kinetics, quantal parameters and their modulation during short-term depression at a developing synapse in the rat CNS

We have characterized developmental changes in the kinetics and quantal parameters of action potential (AP)-evoked neurotransmitter release during maturation of the calyx of Held synapse. Quantal size (q) and peak amplitudes of evoked EPSCs increased moderately, whereas the fraction of vesicles released by single APs decreased. During synaptic depression induced in postnatal day (P) 5-7 synapses by 10-100 Hz stimulation, q declined rapidly to 40-12% of its initial value. The decrease in q was generally smaller in more mature synapses (P12-14), but quite severe for frequencies > or = 300 Hz. The stronger decline of q in immature synapses resulted from a slower recovery from desensitization, presumably due to delayed glutamate clearance. Recovery from this desensitization followed an exponential time course with a time constant of approximately 480 ms in P5-7 synapses, and sped up > 20-fold during maturation. Deconvolution analysis of EPSCs revealed a significant acceleration of the release time course during development, which was accompanied by a 2-fold increase of the peak release rate. During long 100 Hz trains, more mature synapses were able to sustain average rates of 8-10 quanta s(-1) per active zone for phasic release. The rates of asynchronous vesicle release increased transiently > 35-fold immediately after such stimuli and decayed rapidly with an exponential time constant of approximately 50 ms to low resting levels of spontaneous release. However, even following extended periods of 100 Hz stimulation, the amount of asynchronous release was relatively minor with peak rates of less than 5% of the average rate of synchronous release measured at steady state during the tetani. Therefore, a multitude of mechanisms seems to converge on the generation of fast, temporally precise and reliable high-frequency transmission at the mature calyx of Held synapse.

Non-stoichiometric relationship between clathrin heavy and light chains revealed by quantitative comparative proteomics of clathrin-coated vesicles from brain and liver

We used tandem mass spectrometry with peptide counts to identify and to determine the relative levels of expression of abundant protein components of highly enriched clathrin-coated vesicles (CCVs) from rat liver. The stoichiometry of stable protein complexes including clathrin heavy chain and clathrin light chain dimers and adaptor protein (AP) heterotetramers was assessed. We detected a deficit of clathrin light chain compared with clathrin heavy chain in non-brain tissues, suggesting a level of regulation of clathrin cage formation specific to brain. The high ratio of AP-1 to AP-2 in liver CCVs is reversed compared with brain where there is more AP-2 than AP-1. Despite this, general endocytic cargo proteins were readily detected in liver but not in brain CCVs, consistent with the previous demonstration that a major function for brain CCVs is recycling synaptic vesicles. Finally we identified 21 CCV-associated proteins in liver not yet characterized in mammals. Our results further validate the peptide accounting approach, reveal new information on the properties of CCVs, and allow for the use of quantitative proteomics to compare abundant components of organelles under different experimental and pathological conditions.

Clathrin-mediated endocytosis in snake motor terminals is directly facilitated by intracellular Ca2+

At the snake neuromuscular junction, low temperature (LT, 5-7 degrees C) blocks clathrin-mediated endocytosis (CME) while exocytosis is largely unaffected. Thus compensatory endocytosis that normally follows transmitter release is inhibited, or 'delayed' until the preparation is warmed to room temperature (RT). This delay was exploited to observe how changes in bulk [Ca(2+)](i) directly affect CME. Motor terminals were loaded with fura-2 to monitor [Ca(2+)](i). With brief stimulation at LT, [Ca(2+)](i) transiently increased but returned to baseline ( approximately 63 nm) in < 8 min. After 15 min at LT, [Ca(2+)](i) was altered by incubating preparations in the Ca(2+) ionophore ionomyocin. Preparations were then warmed to RT to initiate delayed endocytosis, which was quantified as uptake of the fluorescent optical probe sulforhodamine 101. Endocytosis was more rapid when [Ca(2+)](i) increased; the rate at 300 nm Ca(2+) was approximately double that under basal conditions. Thus the rate of CME - isolated from stimulation, transmitter release, and other forms of endocytosis - is directly influenced by intraterminal Ca(2+).

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.

The General Anesthetic Isoflurane Depresses Synaptic Vesicle Exocytosis

General anesthetics have marked effects on synaptic transmission, but the mechanisms of their presynaptic actions are unclear. We used quantitative laser-scanning fluorescence microscopy to analyze the effects of the volatile anesthetic isoflurane on synaptic vesicle cycling in cultured neonatal rat hippocampal neurons monitored using either transfection of a pH-sensitive form of green fluorescent protein fused to the luminal domain of VAMP (vesicle-associated membrane protein), (synapto-pHluorin) or vesicle loading with the fluorescent dye FM 1-43. Isoflurane reversibly inhibited action potential-evoked exocytosis over a range of concentrations, with little effect on vesicle pool size. In contrast, exocytosis evoked by depolarization in response to an elevated extracellular concentration of KCl, which is insensitive to the selective Na+ channel blocker tetrodotoxin, was relatively insensitive to isoflurane. Inhibition of exocytosis by isoflurane was resistant to bicuculline, indicating that this presynaptic effect is not caused by the well known GABA(A) receptor modulation by volatile anesthetics. Depression of exocytosis was mimicked by a reduction in stimulus frequency, suggesting a reduction in action potential initiation, conduction, or coupling to Ca2+ channel activation. There was no evidence for a direct effect on endocytosis. The effects of isoflurane on synaptic transmission are thus caused primarily by inhibition of action potential-evoked synaptic vesicle exocytosis at a site upstream of Ca2+ entry and exocytosis, possibly as a result of Na+ channel blockade and/or K+ channel activation, with the possibility of lesser contributions from Ca2+ channel blockade and/or soluble N-ethylmaleimide-sensitive factor attachment protein receptor-mediated vesicle fusion.

An Isolated Pool of Vesicles Recycles at Rest and Drives Spontaneous Neurotransmission

Spontaneous synaptic vesicle fusion is a common property of all synapses. To trace the origin of spontaneously fused vesicles in hippocampal synapses, we tagged vesicles with fluorescent styryl dyes, antibodies against synaptotagmin-1, or horseradish peroxidase. We could show that synaptic vesicles recycle at rest, and after spontaneous exo-endocytosis, they populate a reluctantly releasable pool of limited size. Interestingly, vesicles in this spontaneously labeled pool were more likely to re-fuse spontaneously compared to vesicles labeled with activity. We found that blocking vesicle refilling at rest selectively depleted neurotransmitter from spontaneously fusing vesicles without significantly altering evoked transmission. Furthermore, in the absence of the vesicle SNARE protein synaptobrevin (VAMP), activity-dependent and spontaneously recycling vesicles could mix, suggesting a role for synaptobrevin in the separation of the two pools. Taken together these results suggest that spontaneously recycling vesicles and activity-dependent recycling vesicles originate from distinct pools with limited cross-talk with each other.

Fast endocytosis is inhibited by GABA-mediated chloride influx at a presynaptic terminal

Although multiple kinetic components of synaptic vesicle endocytosis have been identified, it has remained unclear whether neurons can differentially modulate these components. Using membrane capacitance measurements from isolated goldfish bipolar cell terminals, we found that the kinetics of endocytosis in retinal slices (single exponential decay; tau > 10 s) were significantly slower than those in acutely dissociated terminals (double exponential decay; tau(fast) approximately 1-2 s; tau(slow) > 10 s). Surprisingly, GABA(A) and/or GABA(C) receptor antagonists restored the fast component of endocytosis to terminals in retinal slices. Blocking GABAergic feedback from reciprocal synapses or removing external Cl(-) ions also allowed for fast endocytosis. Elevating internal Cl(-) via the patch pipette invariably slowed endocytosis, even in terminals dialyzed with increased Ca(2+) buffer. These results suggest a new role for GABA and Cl(-) ions in blocking the trigger for fast endocytosis at this ribbon-type synapse.

Mobilization and fusion of a non-recycling pool of synaptic vesicles under conditions of endocytic blockade

At vertebrate central synapses, it has been demonstrated that a resting pool of synaptic vesicles (SVs) exists that normally does not participate in SV release and recycling. It remains unclear whether SVs within the resting pool are capable of mobilization and fusion. Here, we combine live imaging of SV exo- and endocytosis using pH-sensitive GFP (synapto-pHluorins) with pharmacological and genetic manipulations of the SV cycle at the Drosophila NMJ. We demonstrate that a resting pool of SVs exists at this synapse that encompasses 30-41% of the total SV pool. Under conditions of endocytic blockade, using a temperature-sensitive dynamin mutation, the resting pool of SVs can be mobilized and released. We present a model for the presence of a resting pool of SVs that does not require molecular specification of a subpopulation of SVs.

Transgenic mouse lines expressing synaptopHluorin in hippocampus and cerebellar cortex

We generated six transgenic mouse lines in which synaptopHluorin (SpH), one of green fluorescent protein-based sensors of vesicular exocytosis, was expressed under the control of neuron-specific Thy-1.2 promoter. In situ hybridization study revealed that SpH mRNA was expressed in a broad spectrum of brain regions in four of them, whereas in others it was expressed in the specific regions of the hippocampus. In one particular line, SpH immunoreactivity was specifically observed in the mossy fiber presynaptic terminals of both hippocampus and cerebellar cortex. The fluorescence intensity of these presynaptic terminals was somewhat decreased by acidic buffer superfusion and greatly increased by vesicular neutralization of pH, indicating that the SpH molecules are mainly distributed in the synaptic vesicles. The exocytosis-dependent fluorescence increment was measured upon activation of a single presynaptic terminal. These transgenic lines are expected to facilitate morphological and physiological studies of presynaptic terminals in a variety of regions of the brain.

Distribution of proteins associated with synaptic vesicle endocytosis in the mouse and goldfish retina

Current models of synaptic transmission require retrieval of membrane from the presynaptic terminal following neurotransmitter exocytosis. Dynamin, a GTPase, is thought to be critical for this retrieval process. At ribbon synapses of retinal bipolar neurons, however, compensatory endocytosis does not require GTP hydrolysis, suggesting that endocytosis mechanisms may differ among synapses. To understand better the synaptic vesicle recycling at conventional and ribbon synapses, the distributions of dynamin and two associated proteins, amphiphysin and clathrin, were examined in the retinas of goldfish and mouse by using immunocytochemical methods. Labeling for dynamin, clathrin, and amphiphysin was distributed differentially among conventional and ribbon synapses in retinas of both species. Ribbon synapses of photoreceptors and most bipolar cells labeled only weakly for dynamin relative to conventional synapses. Amphyiphysin labeling was strong at many ribbon synapses, and labeling in rod terminals was stronger than in cone terminals in the mouse retina. Clathrin labeling was heterogeneous among ribbon synapses. Similarly to the case with amphiphysin, mouse rod terminals showed stronger clathrin labeling than cone terminals. Among conventional synapses, there was heterogeneous labeling for all three endocytic proteins. Some labeling for each protein might have been associated with postsynaptic terminals. The differential distribution of labeling for these proteins among identified synapses in the retina suggests considerable heterogeneity in the molecular mechanisms underlying synaptic membrane retrieval, even among synapses with similar active zone ultrastructure. Thus, as with exocytosis, mechanisms of synaptic membrane retrieval may be tuned by the precise complement of proteins expressed within the synaptic terminal.

Kinetic efficiency of endocytosis at mammalian CNS synapses requires synaptotagmin I

At nerve terminals, synaptic vesicle components are retrieved from the cell surface and recycled for local reuse soon after exocytosis. The kinetics of this coupling is critical for the proper functioning of synapses during repetitive action potential firing, because deficiencies in this process lead to abnormal depletion of the releasable vesicle pool. Although the molecular basis of this coupling is poorly understood, numerous biochemical data point to a role for synaptotagmin I (SytI), an essential synaptic vesicle protein required for fast calcium-dependent exocytosis. Here, using synapto-pHluorin in an approach that allows the dissection of endocytosis and exocytosis into separate components during periods of stimulation, we examined exocytic-endocytic coupling in synapses from SytI knockout mice and their WT littermates. We show that endocytosis is significantly impaired in the absence of SytI with the relative rates of endocytosis compared with exocytosis reduced approximately 3-fold with respect to WT. Thus, in addition to regulating exocytosis, SytI also controls the kinetic efficiency of endocytosis at nerve terminals.

Synaptobrevin is essential for fast synaptic-vesicle endocytosis

Synaptobrevin-2 (VAMP-2), the major SNARE protein of synaptic vesicles, is required for fast calcium-triggered synaptic-vesicle exocytosis. Here we show that synaptobrevin-2 is also essential for fast synaptic-vesicle endocytosis. We demonstrate that after depletion of the readily releasable vesicle pool, replenishment of the pool is delayed by knockout of synaptobrevin. This delay was not from a loss of vesicles, because the total number of pre-synaptic vesicles, docked vesicles and actively recycling vesicles was unaffected. However, altered shape and size of the vesicles in synaptobrevin-deficient synapses suggests a defect in endocytosis. Consistent with such a defect, the stimulus-dependent endocytosis of horseradish peroxidase and fluorescent FM1-43 were delayed, indicating that fast vesicle endocytosis may normally be nucleated by a SNARE-dependent coat. Thus, synaptobrevin is essential for two fast synapse-specific membrane trafficking reactions: fast exocytosis for neurotransmitter release and fast endocytosis that mediates rapid reuse of synaptic vesicles.

Enlargeosome, an exocytic vesicle resistant to nonionic detergents, undergoes endocytosis via a nonacidic route

Enlargeosomes, a new type of widely expressed cytoplasmic vesicles, undergo tetanus toxin-insensitive exocytosis in response to cytosolic Ca(2+) concentration ([Ca(2+)](i)) rises. Cell biology of enlargeosomes is still largely unknown. By combining immunocytochemistry (marker desmoyokin-Ahnak, d/A) to capacitance electrophysiology in the enlargeosome-rich, neurosecretion-defective clone PC12-27, we show that 1) the two responses, cell surface enlargement and d/A surface appearance, occur with similar kinetics and in the same low micromolar [Ca(2+)](i) range, no matter whether induced by photolysis of the caged Ca(2+) compound o-nitrophenyl EGTA or by the Ca(2+) ionophore ionomycin. Thus, enlargeosomes seem to account, at least in large part, for the exocytic processes triggered by the two stimulations. 2. The enlargeosome membranes are resistant to nonionic detergents but distinct from other resistant membranes, rich in caveolin, Thy1, and/or flotillin1. 3. Cell cholesterol depletion, which affects many membrane fusions, neither disrupts enlargeosomes nor affects their regulated exocytosis. 4. The postexocytic cell surface decline is [Ca(2+)](i) dependent. 5. Exocytized d/A-rich membranes are endocytized and trafficked along an intracellular pathway by nonacidic organelles, distinct from classical endosomes and lysosomes. Our data define specific aspects of enlargeosomes and suggest their participation, in addition to cell differentiation and repair, for which evidence already exists, to other physiological and pathological processes.

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.

Impaired PtdIns(4,5)P2 synthesis in nerve terminals produces defects in synaptic vesicle trafficking

Phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) has an important function in cell regulation both as a precursor of second messenger molecules and by means of its direct interactions with cytosolic and membrane proteins. Biochemical studies have suggested a role for PtdIns(4,5)P2 in clathrin coat dynamics, and defects in its dephosphorylation at the synapse produce an accumulation of coated endocytic intermediates. However, the involvement of PtdIns(4,5)P2 in synaptic vesicle exocytosis remains unclear. Here, we show that decreased levels of PtdIns(4,5)P2 in the brain and an impairment of its depolarization-dependent synthesis in nerve terminals lead to early postnatal lethality and synaptic defects in mice. These include decreased frequency of miniature currents, enhanced synaptic depression, a smaller readily releasable pool of vesicles, delayed endocytosis and slower recycling kinetics. Our results demonstrate a critical role for PtdIns(4,5)P2 synthesis in the regulation of multiple steps of the synaptic vesicle cycle.

It's green outside: tracking cell surface proteins with pH-sensitive GFP

Green fluorescent protein (GFP) and mutated GFP variants have proved to be immensely powerful tools that have had a profound impact on research in biological sciences. This review considers the development, use and future implications of pH-dependent GFP variants (e.g. pHluorins). These proteins hold considerable promise for the relatively non-invasive monitoring of events such as exocytosis, endocytosis and protein surface expression in living neurons with high spatial and temporal resolution.

Calcium- and dynamin-independent endocytosis in dorsal root ganglion neurons

Synaptic vesicle endocytosis is believed to require calcium and the GTPase dynamin. We now report a form of rapid endocytosis (RE) in dorsal root ganglion (DRG) neurons that, unlike previously described forms of endocytosis, is independent of calcium and dynamin. The RE is tightly coupled to calcium-independent but voltage-dependent secretion (CIVDS). Using FM dye and capacitance measurements, we show that membrane depolarization induces RE in the absence of calcium. Inhibition of dynamin function does not affect RE. The magnitude of RE is proportional to that of preceding CIVDS and stimulation frequency. Inhibitors of protein kinase A (PKA) suppress RE induced by high-frequency depolarization, while PKA activators enhance RE induced by low-frequency depolarization. Biochemical experiments demonstrate that depolarization directly upregulates PKA activity in calcium-free medium. These results reveal a calcium- and dynamin-independent form of endocytosis, which is controlled by neuronal activity and PKA-dependent phosphorylation, in DRG neurons.

Vesicular glutamate transporter-dependent glutamate release from astrocytes

Astrocytes exhibit excitability based on variations of their intracellular Ca2+ concentrations, which leads to glutamate release, that in turn can signal to adjacent neurons. This glutamate-mediated astrocyte-neuron signaling occurs at physiological intracellular Ca2+ levels in astrocytes and includes modulation of synaptic transmission. The mechanism underlying Ca2+-dependent glutamate release from astrocytes is most likely exocytosis, because astrocytes express the protein components of the soluble N-ethyl maleimide-sensitive fusion protein attachment protein receptors complex, including synaptobrevin 2, syntaxin, and synaptosome-associated protein of 23 kDa. Although these proteins mediate Ca2+-dependent glutamate release from astrocytes, it is not well understood whether astrocytes express functional vesicular glutamate transporters (VGLUTs) that are critical for vesicle refilling. Here, we find in cultured and freshly isolated astrocytes the presence of brain-specific Na+-dependent inorganic phosphate cotransporter and differentiation-associated Na+-dependent inorganic phosphate cotransporter that have recently been identified as VGLUTs 1 and 2. Indirect immunocytochemistry showed a punctate pattern of VGLUT immunoreactivity throughout the entire cell body and processes, whereas pharmacological inhibition of VGLUTs abolished mechanically and agonist-evoked Ca2+-dependent glutamate release from astrocytes. Taken together, these data indicate that VGLUTs play a functional role in exocytotic glutamate release from astrocytes.

The kinetics of synaptic vesicle pool depletion at CNS synaptic terminals

During sustained action potential (AP) firing at nerve terminals, the rates of endocytosis compared to exocytosis determine how quickly the available synaptic vesicle pool is depleted, in turn influencing presynaptic efficacy. Mechanisms, including rapid kiss-and-run endocytosis as well as local, preferential recycling of docked vesicles, have been proposed as a means to allow endocytosis and recycling to keep up with stimulation. We show here that, for CNS nerve terminals at physiological temperatures, endocytosis is sufficiently fast to avoid vesicle pool depletion during continuous AP firing at 10 Hz. This endocytosis-exocytosis balance persists for turnover of the entire releasable pool of vesicles and allows for efficient escape of FM 4-64, indicating that it is a non-kiss-and-run endocytic event. Thus, under physiological conditions, the sustained speed of vesicle membrane retrieval for the entire releasable pool appears to be sufficiently fast to compensate for exocytosis, avoiding significant vesicle pool depletion during robust synaptic activity.