Post-tetanic potentiation is caused by two signalling mechanisms affecting quantal size and quantal content

Membrane transformations of fusion and budding

Membrane fusion and budding mediate fundamental processes like intracellular trafficking, exocytosis, and endocytosis. Fusion is thought to open a nanometer-range pore that may subsequently close or dilate irreversibly, whereas budding transforms flat membranes into vesicles. Reviewing recent breakthroughs in real-time visualization of membrane transformations well exceeding this classical view, we synthesize a new model and describe its underlying mechanistic principles and functions. Fusion involves hemi-to-full fusion, pore expansion, constriction and/or closure while fusing vesicles may shrink, enlarge, or receive another vesicle fusion; endocytosis follows exocytosis primarily by closing Ω-shaped profiles pre-formed through the flat-to-Λ-to-Ω-shape transition or formed via fusion. Calcium/SNARE-dependent fusion machinery, cytoskeleton-dependent membrane tension, osmotic pressure, calcium/dynamin-dependent fission machinery, and actin/dynamin-dependent force machinery work together to generate fusion and budding modes differing in pore status, vesicle size, speed and quantity, controls release probability, synchronization and content release rates/amounts, and underlies exo-endocytosis coupling to maintain membrane homeostasis. These transformations, underlying mechanisms, and functions may be conserved for fusion and budding in general.

Synapsin Is Required for Dense Core Vesicle Capture and cAMP-Dependent Neuropeptide Release

Release of neuropeptides from dense core vesicles (DCVs) is essential for neuromodulation. Compared with the release of small neurotransmitters, much less is known about the mechanisms and proteins contributing to neuropeptide release. By optogenetics, behavioral analysis, electrophysiology, electron microscopy, and live imaging, we show that synapsin SNN-1 is required for cAMP-dependent neuropeptide release in Caenorhabditis elegans hermaphrodite cholinergic motor neurons. In synapsin mutants, behaviors induced by the photoactivated adenylyl cyclase bPAC, which we previously showed to depend on ACh and neuropeptides (Steuer Costa et al., 2017), are altered as in animals with reduced cAMP. Synapsin mutants have slight alterations in synaptic vesicle (SV) distribution; however, a defect in SV mobilization was apparent after channelrhodopsin-based photostimulation. DCVs were largely affected in snn-1 mutants: DCVs were ∼30% reduced in synaptic terminals, and their contents not released following bPAC stimulation. Imaging axonal DCV trafficking, also in genome-engineered mutants in the serine-9 protein kinase A phosphorylation site, showed that synapsin captures DCVs at synapses, making them available for release. SNN-1 colocalized with immobile, captured DCVs. In synapsin deletion mutants, DCVs were more mobile and less likely to be caught at release sites, and in nonphosphorylatable SNN-1B(S9A) mutants, DCVs traffic less and accumulate, likely by enhanced SNN-1 dependent tethering. Our work establishes synapsin as a key mediator of neuropeptide release.SIGNIFICANCE STATEMENT Little is known about mechanisms that regulate how neuropeptide-containing dense core vesicles (DCVs) traffic along the axon, how neuropeptide release is orchestrated, and where it occurs. We found that one of the longest known synaptic proteins, required for the regulation of synaptic vesicles and their storage in nerve terminals, synapsin, is also essential for neuropeptide release. By electrophysiology, imaging, and electron microscopy in Caenorhabditis elegans, we show that synapsin regulates this process by tethering the DCVs to the cytoskeleton in axonal regions where neuropeptides are to be released. Without synapsin, DCVs cannot be captured at the release sites and, consequently, cannot fuse with the membrane, and neuropeptides are not released. We suggest that synapsin fulfills this role also in vertebrates, including humans.

Disruption of Cholinergic Circuits as an Area for Targeted Drug Treatment of Alzheimer's Disease: In Vivo Assessment of Short-Term Plasticity in Rat Brain

The search for new therapeutics for the treatment of Alzheimer’s disease (AD) is still in progress. Aberrant pathways of synaptic transmission in basal forebrain cholinergic neural circuits are thought to be associated with the progression of AD. However, the effect of amyloid-beta (Aβ) on short-term plasticity (STP) of cholinergic circuits in the nucleus basalis magnocellularis (NBM) is largely unknown. STP assessment in rat brain cholinergic circuitry may indicate a new target for AD cholinergic therapeutics. Thus, we aimed to study in vivo electrophysiological patterns of synaptic activity in NBM-hippocampus and NBM-basolateral amygdala circuits associated with AD-like neurodegeneration. The extracellular single-unit recordings of responses from the hippocampal and basolateral amygdala neurons to high-frequency stimulation (HFS) of the NBM were performed after intracerebroventricular injection of Aβ 25–35. We found that after Aβ 25–35 exposure the number of hippocampal neurons exhibiting inhibitory responses to HFS of NBM is decreased. The reverse tendency was seen in the basolateral amygdala inhibitory neural populations, whereas the number of amygdala neurons with excitatory responses decreased. The low intensity of inhibitory and excitatory responses during HFS and post-stimulus period is probably due to the anomalous basal synaptic transmission and excitability of hippocampal and amygdala neurons. These functional changes were accompanied by structural alteration of hippocampal, amygdala, and NBM neurons. We have thus demonstrated that Aβ 25–35 induces STP disruption in NBM-hippocampus and NBM-basolateral amygdala circuits as manifested by unbalanced excitatory/inhibitory responses and their frequency. The results of this study may contribute to a better understanding of synaptic integrity. We believe that advancing our understanding of in vivo mechanisms of synaptic plasticity disruption in specific neural circuits could lead to effective drug searches for AD treatment.

Reduced presynaptic vesicle stores mediate cellular and network plasticity defects in an early-stage mouse model of Alzheimer's disease

Background

Identifying effective strategies to prevent memory loss in AD has eluded researchers to date, and likely reflects insufficient understanding of early pathogenic mechanisms directly affecting memory encoding. As synaptic loss best correlates with memory loss in AD, refocusing efforts to identify factors driving synaptic impairments may provide the critical insight needed to advance the field. In this study, we reveal a previously undescribed cascade of events underlying pre and postsynaptic hippocampal signaling deficits linked to cognitive decline in AD. These profound alterations in synaptic plasticity, intracellular Ca²⁺ signaling, and network propagation are observed in 3–4 month old 3xTg-AD mice, an age which does not yet show overt histopathology or major behavioral deficits.

Methods

In this study, we examined hippocampal synaptic structure and function from the ultrastructural level to the network level using a range of techniques including electron microscopy (EM), patch clamp and field potential electrophysiology, synaptic immunolabeling, spine morphology analyses, 2-photon Ca²⁺ imaging, and voltage-sensitive dye-based imaging of hippocampal network function in 3–4 month old 3xTg-AD and age/background strain control mice.

Results

In 3xTg-AD mice, short-term plasticity at the CA1-CA3 Schaffer collateral synapse is profoundly impaired; this has broader implications for setting long-term plasticity thresholds. Alterations in spontaneous vesicle release and paired-pulse facilitation implicated presynaptic signaling abnormalities, and EM analysis revealed a reduction in the ready-releasable and reserve pools of presynaptic vesicles in CA3 terminals; this is an entirely new finding in the field. Concurrently, increased synaptically-evoked Ca²⁺ in CA1 spines triggered by LTP-inducing tetani is further enhanced during PTP and E-LTP epochs, and is accompanied by impaired synaptic structure and spine morphology. Notably, vesicle stores, synaptic structure and short-term plasticity are restored by normalizing intracellular Ca²⁺ signaling in the AD mice.

Conclusions

These findings suggest the Ca²⁺ dyshomeostasis within synaptic compartments has an early and fundamental role in driving synaptic pathophysiology in early stages of AD, and may thus reflect a foundational disease feature driving later cognitive impairment. The overall significance is the identification of previously unidentified defects in pre and postsynaptic compartments affecting synaptic vesicle stores, synaptic plasticity, and network propagation, which directly impact memory encoding.

Electronic supplementary material

The online version of this article (10.1186/s13024-019-0307-7) contains supplementary material, which is available to authorized users.

A single episode of high intensity sound inhibits long-term potentiation in the hippocampus of rats

Exposure to loud sounds has become increasingly common. The most common consequences of loud sound exposure are deafness and tinnitus, but emotional and cognitive problems are also associated with loud sound exposure. Loud sounds can activate the hipothalamic-pituitary-adrenal axis resulting in the secretion of corticosterone, which affects hippocampal synaptic plasticity. Previously we have shown that long-term exposure to short episodes of high intensity sound inhibited hippocampal long-term potentiation (LTP) without affecting spatial learning and memory. Here we aimed to study the impact of short term loud sound exposure on hippocampal synaptic plasticity and function. We found that a single minute of 110 dB sound inhibits hippocampal Schaffer-CA1 LTP for 24 hours. This effect did not occur with an 80-dB sound exposure, was not correlated with corticosterone secretion and was also observed in the perforant-dentate gyrus synapse. We found that despite the deficit in the LTP these animals presented normal spatial learning and memory and fear conditioning. We conclude that a single episode of high-intensity sound impairs hippocampal LTP, without impairing memory and learning. Our results show that the hippocampus is very responsive to loud sounds which can have a potential, but not yet identified, impact on its function.

Calcium-Dependent Protein Kinase C Is Not Required for Post-Tetanic Potentiation at the Hippocampal CA3 to CA1 Synapse

Post-tetanic potentiation (PTP) is a widespread form of short-term synaptic plasticity in which a period of elevated presynaptic activation leads to synaptic enhancement that lasts tens of seconds to minutes. A leading hypothesis for the mechanism of PTP is that tetanic stimulation elevates presynaptic calcium that in turn activates calcium-dependent protein kinase C (PKC) isoforms to phosphorylate targets and enhance neurotransmitter release. Previous pharmacological studies have implicated this mechanism in PTP at hippocampal synapses, but the results are controversial. Here we combine genetic and pharmacological approaches to determine the role of classic PKC isoforms in PTP. We find that PTP is unchanged in PKC triple knock-out (TKO) mice in which all calcium-dependent PKC isoforms have been eliminated (PKCα, PKCβ, and PKCγ). We confirm previous studies and find that in wild-type mice 10 μm of the PKC inhibitor GF109203 eliminates PTP and the PKC activator PDBu enhances neurotransmitter release and occludes PTP. However, we find that the same concentrations of GF109203 and PDBu have similar effects in TKO animals. We also show that 2 μm GF109203 does not abolish PTP even though it inhibits the PDBu-dependent phosphorylation of PKC substrates. We conclude that at the CA3 to CA1 synapse Ca(2+)-dependent PKC isoforms do not serve as calcium sensors to mediate PTP.

SIGNIFICANCE STATEMENT

Neurons dynamically regulate neurotransmitter release through many processes known collectively as synaptic plasticity. Post-tetanic potentiation (PTP) is a widespread form of synaptic plasticity that lasts for tens of seconds that may have important computational roles and contribute to short-term memory. According to a leading mechanism, presynaptic calcium activates protein kinase C (PKC) to increase neurotransmitter release. Pharmacological studies have also implicated this mechanism at hippocampal CA3 to CA1 synapses, but there are concerns about the specificity of PKC activators and inhibitors. We therefore used a molecular genetic approach and found that PTP was unaffected when all calcium-dependent PKC isozymes were eliminated. We conclude that PKC isozymes are not the calcium sensors that mediate PTP at the CA3 to CA1 synapse.

Fast cAMP Modulation of Neurotransmission via Neuropeptide Signals and Vesicle Loading

Cyclic AMP (cAMP) signaling augments synaptic transmission, but because many targets of cAMP and protein kinase A (PKA) may be involved, mechanisms underlying this pathway remain unclear. To probe this mechanism, we used optogenetic stimulation of cAMP signaling by Beggiatoa-photoactivated adenylyl cyclase (bPAC) in Caenorhabditis elegans motor neurons. Behavioral, electron microscopy (EM), and electrophysiology analyses revealed cAMP effects on both the rate and on quantal size of transmitter release and led to the identification of a neuropeptidergic pathway affecting quantal size. cAMP enhanced synaptic vesicle (SV) fusion by increasing mobilization and docking/priming. cAMP further evoked dense core vesicle (DCV) release of neuropeptides, in contrast to channelrhodopsin (ChR2) stimulation. cAMP-evoked DCV release required UNC-31/Ca²⁺-dependent activator protein for secretion (CAPS). Thus, DCVs accumulated in unc-31 mutant synapses. bPAC-induced neuropeptide signaling acts presynaptically to enhance vAChT-dependent SV loading with acetylcholine, thus causing increased miniature postsynaptic current amplitudes (mPSCs) and significantly enlarged SVs.

Dynamin-1 deletion enhances post-tetanic potentiation and quantal size after tetanic stimulation at the calyx of Held

KEY POINTS

Post-tetanic potentiation (PTP) is attributed mainly to an increase in release probability (P_r ) and/or readily-releasable pool (RRP) in many synapses, but the role of endocytosis in PTP is unknown. Using the calyx of Held synapse from tissue-specific dynamin-1 knockout (cKO) mice (P16-20), we report that cKO synapses show enhanced PTP compared to control. We found significant increases in both spontaneous excitatory postsynaptic current (spEPSC) amplitude and RRP size (estimated by a train of 30 APs at 100 Hz) in cKO over control during PTP. Actin depolymerization blocks the increase in spEPSC amplitude in both control and cKO, and it abolishes the enhancement of PTP in cKO. PTP is sensitive to the PKC inhibitor GF109203X in both control and cKO. We conclude that an activity-dependent quantal size increase contributes to the enhancement of PTP in cKO over control and an altered endocytosis affects short-term plasticity through quantal size changes.

ABSTRACT

High-frequency stimulation leads to post-tetanic potentiation (PTP) at many types of synapses. Previous studies suggest that PTP results primarily from a protein kinase C (PKC)-dependent increase in release probability (P_r ) and/or readily-releasable pool (RRP) of synaptic vesicles (SVs), but the role of SV endocytosis in PTP is unknown. Using the mature calyx of Held (P16-20), we report that tissue-specific ablation of dynamin-1 (cKO), an endocytic protein crucial for SV regeneration, enhances PTP in cKO over control. To explore the mechanism of this enhancement, we estimated the changes in paired-pulse ratios (PPRs) and RRP size during PTP. RRP was estimated by the back-extrapolation of cumulative EPSC amplitudes during a train of 30 action potentials at 100 Hz (termed RRP_train ). We found an increase in RRP_train during PTP in both control and cKO, but no significant changes in the PPR. Moreover, the amplitude and frequency of spontaneous excitatory postsynaptic currents (spEPSCs) increased during PTP in both control and cKO; however, the spEPSC amplitude in cKO during PTP was significantly larger than in control. Actin depolymerization reagent latrunculin-B (Lat-B) abolished the activity-dependent increase in spEPSC amplitude in both control and cKO, but selectively blocked the enhancement of PTP in cKO, without affecting PTP in control. PKC inhibitor GF109203X nearly abolished PTP in both control and cKO. These data suggest that the quantal size increase contributes to the enhancement of PTP in dynamin-1 cKO, and this change depends on strong synaptic activity and actin polymerization.

The calyx of Held in the auditory system: Structure, function, and development

The calyx of Held synapse plays an important role in the auditory system, relaying information about sound localization via fast and precise synaptic transmission, which is achieved by its specialized structure and giant size. During development, the calyx of Held undergoes anatomical, morphological, and physiological changes necessary for performing its functions. The large dimensions of the calyx of Held nerve terminal are well suited for direct electrophysiological recording of many presynaptic events that are difficult, if not impossible to record at small conventional synapses. This unique accessibility has been used to investigate presynaptic ion channels, transmitter release, and short-term plasticity, providing invaluable information about basic presynaptic mechanisms of transmission at a central synapse. Here, we review anatomical and physiological specializations of the calyx of Held, summarize recent studies that provide new mechanisms important for calyx development and reliable synaptic transmission, and examine fundamental presynaptic mechanisms learned from studies using calyx as a model nerve terminal. This article is part of a Special Issue entitled <Annual Reviews 2016>.

Molecular Machines Regulating the Release Probability of Synaptic Vesicles at the Active Zone

The fusion of synaptic vesicles (SVs) with the plasma membrane of the active zone (AZ) upon arrival of an action potential (AP) at the presynaptic compartment is a tightly regulated probabilistic process crucial for information transfer. The probability of a SV to release its transmitter content in response to an AP, termed release probability (Pr), is highly diverse both at the level of entire synapses and individual SVs at a given synapse. Differences in Pr exist between different types of synapses, between synapses of the same type, synapses originating from the same axon and even between different SV subpopulations within the same presynaptic terminal. The Pr of SVs at the AZ is set by a complex interplay of different presynaptic properties including the availability of release-ready SVs, the location of the SVs relative to the voltage-gated calcium channels (VGCCs) at the AZ, the magnitude of calcium influx upon arrival of the AP, the buffering of calcium ions as well as the identity and sensitivity of the calcium sensor. These properties are not only interconnected, but can also be regulated dynamically to match the requirements of activity patterns mediated by the synapse. Here, we review recent advances in identifying molecules and molecular machines taking part in the determination of vesicular Pr at the AZ.

Extremely Low Frequency Electromagnetic Fields Facilitate Vesicle Endocytosis by Increasing Presynaptic Calcium Channel Expression at a Central Synapse

Accumulating evidence suggests significant biological effects caused by extremely low frequency electromagnetic fields (ELF-EMF). Although exo-endocytosis plays crucial physical and biological roles in neuronal communication, studies on how ELF-EMF regulates this process are scarce. By directly measuring calcium currents and membrane capacitance at a large mammalian central nervous synapse, the calyx of Held, we report for the first time that ELF-EMF critically affects synaptic transmission and plasticity. Exposure to ELF-EMF for 8 to 10 days dramatically increases the calcium influx upon stimulation and facilitates all forms of vesicle endocytosis, including slow and rapid endocytosis, endocytosis overshoot and bulk endocytosis, but does not affect the RRP size and exocytosis. Exposure to ELF-EMF also potentiates PTP, a form of short-term plasticity, increasing its peak amplitude without impacting its time course. We further investigated the underlying mechanisms and found that calcium channel expression, including the P/Q, N, and R subtypes, at the presynaptic nerve terminal was enhanced, accounting for the increased calcium influx upon stimulation. Thus, we conclude that exposure to ELF-EMF facilitates vesicle endocytosis and synaptic plasticity in a calcium-dependent manner by increasing calcium channel expression at the nerve terminal.

Translating neuronal activity at the synapse: presynaptic calcium sensors in short-term plasticity

The complex manner in which patterns of presynaptic neural activity are translated into short-term plasticity (STP) suggests the existence of multiple presynaptic calcium (Ca(2+)) sensors, which regulate the amplitude and time-course of STP and are the focus of this review. We describe two canonical Ca(2+)-binding protein domains (C2 domains and EF-hands) and define criteria that need to be met for a protein to qualify as a Ca(2+) sensor mediating STP. With these criteria in mind, we discuss various forms of STP and identify established and putative Ca(2+) sensors. We find that despite the multitude of proposed sensors, only three are well established in STP: Munc13, protein kinase C (PKC) and synaptotagmin-7. For putative sensors, we pinpoint open questions and potential pitfalls. Finally, we discuss how the molecular properties and modes of action of Ca(2+) sensors can explain their differential involvement in STP and shape net synaptic output.

Protein kinase C is a calcium sensor for presynaptic short-term plasticity

In presynaptic boutons, calcium (Ca(2+)) triggers both neurotransmitter release and short-term synaptic plasticity. Whereas synaptotagmins are known to mediate vesicle fusion through binding of high local Ca(2+) to their C2 domains, the proteins that sense smaller global Ca(2+) increases to produce short-term plasticity have remained elusive. Here, we identify a Ca(2+) sensor for post-tetanic potentiation (PTP), a form of plasticity thought to underlie short-term memory. We find that at the functionally mature calyx of Held synapse the Ca(2+)-dependent protein kinase C isoforms α and β are necessary for PTP, and the expression of PKCβ in PKCαβ double knockout mice rescues PTP. Disruption of Ca(2+) binding to the PKCβ C2 domain specifically prevents PTP without impairing other PKCβ-dependent forms of synaptic enhancement. We conclude that different C2-domain-containing presynaptic proteins are engaged by different Ca(2+) signals, and that Ca(2+) increases evoked by tetanic stimulation are sensed by PKCβ to produce PTP.DOI: http://dx.doi.org/10.7554/eLife.03011.001.

Myosin light chain kinase accelerates vesicle endocytosis at the calyx of Held synapse

Neuronal activity triggers endocytosis at synaptic terminals to retrieve efficiently the exocytosed vesicle membrane, ensuring the membrane homeostasis of active zones and the continuous supply of releasable vesicles. The kinetics of endocytosis depends on Ca(2+) and calmodulin which, as a versatile signal pathway, can activate a broad spectrum of downstream targets, including myosin light chain kinase (MLCK). MLCK is known to regulate vesicle trafficking and synaptic transmission, but whether this kinase regulates vesicle endocytosis at synapses remains elusive. We investigated this issue at the rat calyx of Held synapse, where previous studies using whole-cell membrane capacitance measurement have characterized two common forms of Ca(2+)/calmodulin-dependent endocytosis, i.e., slow clathrin-dependent endocytosis and rapid endocytosis. Acute inhibition of MLCK with pharmacological agents was found to slow down the kinetics of both slow and rapid forms of endocytosis at calyces. Similar impairment of endocytosis occurred when blocking myosin II, a motor protein that can be phosphorylated upon MLCK activation. The inhibition of endocytosis was not accompanied by a change in Ca(2+) channel current. Combined inhibition of MLCK and calmodulin did not induce synergistic inhibition of endocytosis. Together, our results suggest that activation of MLCK accelerates both slow and rapid forms of vesicle endocytosis at nerve terminals, likely by functioning downstream of Ca(2+)/calmodulin.

Munc18-1 is a dynamically regulated PKC target during short-term enhancement of transmitter release

Transmitter release at synapses is regulated by preceding neuronal activity, which can give rise to short-term enhancement of release like post-tetanic potentiation (PTP). Diacylglycerol (DAG) and Protein-kinase C (PKC) signaling in the nerve terminal have been widely implicated in the short-term modulation of transmitter release, but the target protein of PKC phosphorylation during short-term enhancement has remained unknown. Here, we use a gene-replacement strategy at the calyx of Held, a large CNS model synapse that expresses robust PTP, to study the molecular mechanisms of PTP. We find that two PKC phosphorylation sites of Munc18-1 are critically important for PTP, which identifies the presynaptic target protein for the action of PKC during PTP. Pharmacological experiments show that a phosphatase normally limits the duration of PTP, and that PTP is initiated by the action of a ‘conventional’ PKC isoform. Thus, a dynamic PKC phosphorylation/de-phosphorylation cycle of Munc18-1 drives short-term enhancement of transmitter release during PTP.

DOI:http://dx.doi.org/10.7554/eLife.01715.001

Brain function depends on the rapid transfer of information from one brain cell to the next at junctions known as synapses. Small packages called vesicles play an important role in this process. The arrival of an electrical action potential at the nerve terminal of the first cell causes some vesicles in the cell to fuse with the cell membrane, and this leads to the neurotransmitters inside the vesicles being released into the synapse. The neurotransmitters then bind to receptors on the second cell, which leads to an electrical signal in the second cell. A protein called Munc18-1 has a central role in the fusion of the vesicle at the cell membrane.

The strength of a synapse—that is, how easily the first brain cell can impact the electrical behaviour of the second—can change, and this ‘synaptic plasticity’ is thought to underlie learning and memory. Long-term changes in synaptic strength require additional receptors to be inserted into the membrane of the second cell. However, synapses can also be temporarily strengthened: the arrival of a burst of action potentials—a tetanus—causes some synapses to increase the amount of neurotransmitter they release in response to any subsequent, single, action potential.

This temporary increase in synaptic strength, which is known as post-tetanic potentiation, requires an enzyme called protein kinase C; the role of this enzyme is to phosphorylate specific target proteins (i.e., to add phosphate groups to them). Now, Genç et al. have genetically modified a mouse synapse in vivo and shown that protein kinase C brings about post-tetanic potentiation by phosphorylating Munc18-1. Furthermore, pharmacological experiments show that proteins called phosphatases, which de-phosphorylate proteins, normally terminate the post-tetanic potentiation after about one minute. Taken together, the study identifies a target protein which is phosphorylated by protein kinase C during post-tetanic potentiation. The study also suggests that in addition to its fundamental role in vesicle fusion, the phosphorylation state of Munc18-1 can change the probability of vesicle fusion in a more subtle way, thereby contributing to synaptic plasticity.

DOI:http://dx.doi.org/10.7554/eLife.01715.002

Exocytosis and endocytosis: modes, functions, and coupling mechanisms

Vesicle exocytosis releases content to mediate many biological events, including synaptic transmission essential for brain functions. Following exocytosis, endocytosis is initiated to retrieve exocytosed vesicles within seconds to minutes. Decades of studies in secretory cells reveal three exocytosis modes coupled to three endocytosis modes: (a) full-collapse fusion, in which vesicles collapse into the plasma membrane, followed by classical endocytosis involving membrane invagination and vesicle reformation; (b) kiss-and-run, in which the fusion pore opens and closes; and (c) compound exocytosis, which involves exocytosis of giant vesicles formed via vesicle-vesicle fusion, followed by bulk endocytosis that retrieves giant vesicles. Here we review these exo- and endocytosis modes and their roles in regulating quantal size and synaptic strength, generating synaptic plasticity, maintaining exocytosis, and clearing release sites for vesicle replenishment. Furthermore, we highlight recent progress in understanding how vesicle endocytosis is initiated and is thus coupled to exocytosis. The emerging model is that calcium influx via voltage-dependent calcium channels at the calcium microdomain triggers endocytosis and controls endocytosis rate; calmodulin and synaptotagmin are the calcium sensors; and the exocytosis machinery, including SNARE proteins (synaptobrevin, SNAP25, and syntaxin), is needed to coinitiate endocytosis, likely to control the amount of endocytosis.

Developmental upregulation of presynaptic NCKX underlies the decrease of mitochondria-dependent posttetanic potentiation at the rat calyx of Held synapse

The sensitivity of posttetanic potentiation (PTP) to high-frequency stimulation (HFS) steeply decays during the first 2 postnatal weeks. We investigated the underlying mechanisms for the developmental change of PTP induced by HFS (100 Hz, 2 s) at postnatal days 4-6 and 9-11 at the rat calyx of Held synapse. Low-concentration tetraphenylphosphonium (2 μM), an inhibitor of mitochondrial Na(+)/Ca(2+) exchanger, suppressed the amount of posttetanic residual Ca(2+) and PTP to a larger extent at the immature calyx synapse, indicating a developmental reduction of mitochondrial contribution to PTP. The higher amount of mitochondrial Ca(2+) uptake during HFS and consequent posttetanic residual Ca(2+) at the immature calyx of Held was associated with higher peak of HFS-induced Ca(2+) transients, most likely because the mitochondrial Ca(2+) uptake during HFS was supralinearly dependent on the presynaptic resting Ca(2+) level. Probing into the contribution of Na(+)/Ca(2+) exchangers to Ca(2+) clearance, we found a specific upregulation of the K(+)-dependent Na(+)/Ca(2+) exchanger (NCKX) activity in the mature calyx of Held. We conclude that the upregulation of NCKX limits the Ca(2+) buildup and inhibits mitochondrial Ca(2+) uptake during HFS, which in turn results in the reduction of posttetanic residual Ca(2+) and PTP at the mature calyx of Held synapse.

Short-term presynaptic plasticity

Different types of synapses are specialized to interpret spike trains in their own way by virtue of the complement of short-term synaptic plasticity mechanisms they possess. Numerous types of short-term, use-dependent synaptic plasticity regulate neurotransmitter release. Short-term depression is prominent after a single conditioning stimulus and recovers in seconds. Sustained presynaptic activation can result in more profound depression that recovers more slowly. An enhancement of release known as facilitation is prominent after single conditioning stimuli and lasts for hundreds of milliseconds. Finally, tetanic activation can enhance synaptic strength for tens of seconds to minutes through processes known as augmentation and posttetantic potentiation. Progress in clarifying the properties, mechanisms, and functional roles of these forms of short-term plasticity is reviewed here.

Synaptic vesicle recycling at the calyx of Held

Efficient endocytosis is crucial for maintaining synaptic transmission because of its role in retrieving constituent membrane and associated proteins. In the past three decades three modes of endocytosis have been proposed involving the central nervous system: clathrin-mediated endocytosis, kiss-and-run endocytosis and bulk endocytosis. These forms of endocytosis can be induced under different conditions, but their detailed molecular mechanisms and functions are largely unknown. Here, we review the existence and initiation of all three modes of endocytosis at a giant glutamatergic synapse, the calyx of Held. The possibility of direct electrophysiology recording in this synapse allows for accurate tracking of exocytosis and endocytosis via capacitance measurements. Future aims will be focused on identifying the molecules that undergo the different mechanisms of endocytosis and the conditions under which different forms of endocytosis predominate.