Vesicle pool partitioning influences presynaptic diversity and weighting in rat hippocampal synapses

Parallel processing of quickly and slowly mobilized reserve vesicles in hippocampal synapses

Vesicles within presynaptic terminals are thought to be segregated into a variety of readily releasable and reserve pools. The nature of the pools and trafficking between them is not well understood, but pools that are slow to mobilize when synapses are active are often assumed to feed pools that are mobilized more quickly, in a series. However, electrophysiological studies of synaptic transmission have suggested instead a parallel organization where vesicles within slowly and quickly mobilized reserve pools would separately feed independent reluctant- and fast-releasing subdivisions of the readily releasable pool. Here, we use FM-dyes to confirm the existence of multiple reserve pools at hippocampal synapses and a parallel organization that prevents intermixing between the pools, even when stimulation is intense enough to drive exocytosis at the maximum rate. The experiments additionally demonstrate extensive heterogeneity among synapses in the relative sizes of the slowly and quickly mobilized reserve pools, which suggests equivalent heterogeneity in the numbers of reluctant and fast-releasing readily releasable vesicles that may be relevant for understanding information processing and storage.

Pericytes and the Control of Blood Flow in Brain and Heart

Pericytes, attached to the surface of capillaries, play an important role in regulating local blood flow. Using optogenetic tools and genetically encoded reporters in conjunction with confocal and multiphoton imaging techniques, the 3D structure, anatomical organization, and physiology of pericytes have recently been the subject of detailed examination. This work has revealed novel functions of pericytes and morphological features such as tunneling nanotubes in brain and tunneling microtubes in heart. Here, we discuss the state of our current understanding of the roles of pericytes in blood flow control in brain and heart, where functions may differ due to the distinct spatiotemporal metabolic requirements of these tissues. We also outline the novel concept of electro-metabolic signaling, a universal mechanistic framework that links tissue metabolic state with blood flow regulation by pericytes and vascular smooth muscle cells, with capillary K_ATP and Kir2.1 channels as primary sensors. Finally, we present major unresolved questions and outline how they can be addressed.

Heterogeneity of glutamatergic synapses: cellular mechanisms and network consequences

Chemical synapses are commonly known as a structurally and functionally highly diverse class of cell-cell contacts specialized to mediate communication between neurons. They represent the smallest "computational" unit of the brain and are typically divided into excitatory and inhibitory as well as modulatory categories. These categories are subdivided into diverse types, each representing a different structure-function repertoire that in turn are thought to endow neuronal networks with distinct computational properties. The diversity of structure and function found among a given category of synapses is referred to as heterogeneity. The main building blocks for this heterogeneity are synaptic vesicles, the active zone, the synaptic cleft, the postsynaptic density, and glial processes associated with the synapse. Each of these five structural modules entails a distinct repertoire of functions, and their combination specifies the range of functional heterogeneity at mammalian excitatory synapses, which are the focus of this review. We describe synapse heterogeneity that is manifested on different levels of complexity ranging from the cellular morphology of the pre- and postsynaptic cells toward the expression of different protein isoforms at individual release sites. We attempt to define the range of structural building blocks that are used to vary the basic functional repertoire of excitatory synaptic contacts and discuss sources and general mechanisms of synapse heterogeneity. Finally, we explore the possible impact of synapse heterogeneity on neuronal network function.

Single synapse glutamate imaging reveals multiple levels of release mode regulation in mammalian synapses

Mammalian central synapses exhibit vast heterogeneity in signaling strength. To understand the extent of this diversity, how it is achieved, and its functional implications, characterization of a large number of individual synapses is required. Using glutamate imaging, we characterized the evoked release probability and spontaneous release frequency of over 24,000 individual synapses. We found striking variability and no correlation between action potential-evoked and spontaneous synaptic release strength, suggesting distinct regulatory mechanisms. Subpixel localization of individual evoked and spontaneous release events reveals tight spatial regulation of evoked release and enhanced spontaneous release outside of evoked release region. Using on-stage post hoc immune-labeling of vesicle-associated proteins, Ca²⁺-sensing proteins, and soluble presynaptic proteins we were able to show that distinct molecular ensembles are associated with evoked and spontaneous modes of synaptic release.

Mechanism of Stiff Substrates up-Regulate Cultured Neuronal Network Activity

Our previous work provided compelling evidence showing that substrate stiffness is crucial for regulating synaptic connectivity and excitatory synaptic transmission among neurons in the neuronal network. However, the underlying mechanisms remain elusive. In our study, polydimethylsiloxane (PDMS) substrates with different stiffness have been fabricated to investigate the mechanisms by which the substrate stiffness upregulates the formation and activity of the cultured neuronal network. Here we report that stiff substrate increased both the number of synapses and the efficacy of excitatory synaptic transmission. More colocalization of synaptotagmin and PSD-95 was observed in the neuronal network on stiff substrate, which indicated the synapse number has increased. We also found that the increased synapse number was mediated by Hevin and SPARC that are secreted from astrocyte. The increased efficacy of excitatory synaptic transmission induced by stiff substrate was explored in three aspects. First, stiff substrate enhanced the presynaptic activity through increasing the vesicular release probability (Pr) of neurotransmitters as well as the calcium influx. Second, stiff substrate reduced voltage-dependent Mg²⁺ blockade to N-methyl-d-aspartate receptor (NMDAR) channels, which led to higher postsynaptic activity. Third, our work suggested that the increased excitatory synaptic transmission in the neural network on stiff substrate involved the upregulated synaptic glutamate concentration. Taken together, these findings may provide a molecular mechanism underlying substrate stiffness regulation of excitatory synaptic transmission in the cultured neural network.

Stationary-State Statistics of a Binary Neural Network Model with Quenched Disorder

In this paper, we study the statistical properties of the stationary firing-rate states of a neural network model with quenched disorder. The model has arbitrary size, discrete-time evolution equations and binary firing rates, while the topology and the strength of the synaptic connections are randomly generated from known, generally arbitrary, probability distributions. We derived semi-analytical expressions of the occurrence probability of the stationary states and the mean multistability diagram of the model, in terms of the distribution of the synaptic connections and of the external stimuli to the network. Our calculations rely on the probability distribution of the bifurcation points of the stationary states with respect to the external stimuli, calculated in terms of the permanent of special matrices using extreme value theory. While our semi-analytical expressions are exact for any size of the network and for any distribution of the synaptic connections, we focus our study on networks made of several populations, that we term "statistically homogeneous" to indicate that the probability distribution of their connections depends only on the pre- and post-synaptic population indexes, and not on the individual synaptic pair indexes. In this specific case, we calculated analytically the permanent, obtaining a compact formula that outperforms of several orders of magnitude the Balasubramanian-Bax-Franklin-Glynn algorithm. To conclude, by applying the Fisher-Tippett-Gnedenko theorem, we derived asymptotic expressions of the stationary-state statistics of multi-population networks in the large-network-size limit, in terms of the Gumbel (double exponential) distribution. We also provide a Python implementation of our formulas and some examples of the results generated by the code.

Photoconversion of FM1-43 Reveals Differences in Synaptic Vesicle Recycling and Sensitivity to Pharmacological Disruption of Actin Dynamics in Individual Synapses

The cycling of synaptic vesicles ensures that neurons can communicate adequately through their synapses on repeated occasions when activity is sustained, and several steps in this cycle are modulated by actin. The effects of pharmacological stabilization of actin with jasplakinolide or its depolymerization with latrunculin A was assessed on the synaptic vesicle cycle at individual boutons of cerebellar granule cells, using FM1-43 imaging to track vesicle recycling and its photoconversion to specifically label recycled organelles. Remarkable differences in the recycling capacity of individual boutons are evident, and their dependence on the actin cytoskeleton for recycling is clear. Disrupting actin dynamics causes a loss of functional boutons, and while this indicates that exo/endocytotic cycling in boutons is fully dependent on such events, this dependence is only partial in other boutons. Indeed, exocytosis and vesicle trafficking are impaired significantly by stabilizing or depolymerizing actin, whereas repositioning recycled vesicles at the active zone seems to be dependent on actin polymerization alone. These findings support the hypothesis that different steps of synaptic vesicle cycling depend on actin dynamics and that such dependence varies among individual boutons.

Functional Viability: Measurement of Synaptic Vesicle Pool Sizes

Neurons and their function of conveying information across a chemical synapse are highly regulated systems. Impacts on their functional viability can occur independently from changes in morphology. Here we describe a method to assess the size of synaptic vesicle pools using live cell fluorescence imaging and a genetically encoded probe (pHluorin). Assessing functional parameters such as the size of synaptic vesicle pools can be a valuable addition to common assays of neuronal cell viability as they demonstrate that key cellular functions are intact.

Dynamic Partitioning of Synaptic Vesicle Pools by the SNARE-Binding Protein Tomosyn

Neural networks engaged in high-frequency activity rely on sustained synaptic vesicle recycling and coordinated recruitment from functionally distinct synaptic vesicle (SV) pools. However, the molecular pathways matching neural activity to SV dynamics and release requirements remain unclear. Here we identify unique roles of SNARE-binding Tomosyn1 (Tomo1) proteins as activity-dependent substrates that regulate dynamics of SV pool partitioning at rat hippocampal synapses. Our analysis is based on monitoring changes in distinct functionally defined SV pools via V-Glut1-pHluorin fluorescence in cultured hippocampal neurons in response to alterations in presynaptic protein expression. Specifically, we find knockdown of Tomo1 facilitates release efficacy from the Readily Releasable Pool (RRP), and regulates SV distribution to the Total Recycling Pool (TRP), which is matched by a decrease in the SV Resting Pool. Notably, these effects were reversed by Tomo1 rescue and overexpression. Further, we identify that these actions of Tomo1 are regulated via activity-dependent phosphorylation by cyclin-dependent kinase 5 (Cdk5). Assessment of molecular interactions that may contribute to these actions identified Tomo1 interaction with the GTP-bound state of Rab3A, an SV GTPase involved in SV targeting and presynaptic membrane tethering. In addition, Tomo1 via Rab3A-GTP was also observed to interact with Synapsin 1a/b cytoskeletal interacting proteins. Finally, our data indicate that Tomo1 regulation of SV pool sizes serves to adapt presynaptic neurotransmitter release to chronic silencing of network activity. Overall, the results establish Tomo1 proteins as central mediators in neural activity-dependent changes in SV distribution among SV pools.

SIGNIFICANCE STATEMENT

Although information transfer at central synapses via sustained high-frequency neural activity requires coordinated synaptic vesicle (SV) recycling, the mechanism(s) by which synapses sense and dynamically modify SV pools to match network demands remains poorly defined. To advance understanding, we quantified SV pool sizes and their sensitivity to neural activity while altering Tomo1 expression, a putative regulator of the presynaptic Readily Releasable Pool. Remarkably, we find Tomo1 actions to extend beyond the Readily Releasable Pool to mediate the Total Recycling Pool and SV Resting Pool distribution, and this action is sensitive to neural activity through Cdk5 phosphorylation of Tomo1. Moreover, Tomo1 appears to exert these actions through interaction with Rab3A-GTP and synapsin proteins. Together, our results argue that Tomo1 is a central mediator of SV availability for neurotransmission.

Superpriming of synaptic vesicles as a common basis for intersynapse variability and modulation of synaptic strength

Glutamatergic synapses show large variations in strength and short-term plasticity (STP). We show here that synapses displaying an increased strength either after posttetanic potentiation (PTP) or through activation of the phospholipase-C-diacylglycerol pathway share characteristic properties with intrinsically strong synapses, such as (i) pronounced short-term depression (STD) during high-frequency stimulation; (ii) a conversion of that STD into a sequence of facilitation followed by STD after a few conditioning stimuli at low frequency; (iii) an equalizing effect of such conditioning stimulation, which reduces differences among synapses and abolishes potentiation; and (iv) a requirement of long periods of rest for reconstitution of the original STP pattern. These phenomena are quantitatively described by assuming that a small fraction of "superprimed" synaptic vesicles are in a state of elevated release probability (p ∼ 0.5). This fraction is variable in size among synapses (typically about 30%), but increases after application of phorbol ester or during PTP. The majority of vesicles, released during repetitive stimulation, have low release probability (p ∼ 0.1), are relatively uniform in number across synapses, and are rapidly recruited. In contrast, superprimed vesicles need several seconds to be regenerated. They mediate enhanced synaptic strength at the onset of burst-like activity, the impact of which is subject to modulation by slow modulatory transmitter systems.

Different pools of postsynaptic GABAA receptors mediate inhibition evoked by low- and high-frequency presynaptic stimulation at hippocampal synapses

Patterns of short-term synaptic plasticity could considerably differ between synapses of the same axon. This may lead to separation of synaptic receptors transmitting either low- or high-frequency signals and, therefore, may have functional consequences for the information transfer in the brain. Here, we estimated a degree of such separation at hippocampal GABAergic synapses using a use-dependent GABAA receptor antagonist, picrotoxin, to selectively suppress a pool of GABAA receptors monosynaptically activated during the low-frequency stimulation. The relative changes in postsynaptic responses evoked by the high-frequency stimulation before and after such block were used to estimate the contribution of this GABAA receptor pool to synaptic transmission at high frequencies. Using this approach, we have shown that IPSCs evoked by low-frequency (0.2 Hz) stimulation and asynchronous currents evoked by high-frequency (20-40 Hz) stimulation are mediated by different pools of postsynaptic GABAA receptors. Thus, our findings suggest that inhibition produced by a single hippocampal interneuron may be selectively routed to different postsynaptic targets depending on the presynaptic discharge frequency.

A preferentially segregated recycling vesicle pool of limited size supports neurotransmission in native central synapses

At small central synapses, efficient turnover of vesicles is crucial for stimulus-driven transmission, but how the structure of this recycling pool relates to its functional role remains unclear. Here we characterize the organizational principles of functional vesicles at native hippocampal synapses with nanoscale resolution using fluorescent dye labeling and electron microscopy. We show that the recycling pool broadly scales with the magnitude of the total vesicle pool, but its average size is small (∼45 vesicles), highly variable, and regulated by CDK5/calcineurin activity. Spatial analysis demonstrates that recycling vesicles are preferentially arranged near the active zone and this segregation is abolished by actin stabilization, slowing the rate of activity-driven exocytosis. Our approach reveals a similarly biased recycling pool distribution at synapses in visual cortex activated by sensory stimulation in vivo. We suggest that in small native central synapses, efficient release of a limited pool of vesicles relies on their favored spatial positioning within the terminal.

Independent regulation of basal neurotransmitter release efficacy by variable Ca²+ influx and bouton size at small central synapses

Concurrent imaging of vesicular release and calcium dynamics in small presynaptic boutons shows that the fusion probability of readily releasable vesicles is a major determinant of the overall variability in release probability.

The efficacy of action potential evoked neurotransmitter release varies widely even among synapses supplied by the same axon, and the number of release-ready vesicles at each synapse is a major determinant of this heterogeneity. Here we identify a second, equally important, mechanism for release heterogeneity at small hippocampal synapses, the inter-synaptic variation of the exocytosis probability of release-ready vesicles. Using concurrent measurements of vesicular pool sizes, vesicular exocytosis rates, and presynaptic Ca²⁺ dynamics, in the same small hippocampal boutons, we show that the average fusion probability of release-ready vesicles varies among synapses supplied by the same axon with the size of the spike-evoked Ca²⁺ concentration transient. We further show that synapses with a high vesicular release probability exhibit a lower Ca²⁺ cooperativity, arguing that this is a direct consequence of increased Ca²⁺ influx at the active zone. We conclude that variability of neurotransmitter release under basal conditions at small central synapses is accounted for not only by the number of release-ready vesicles, but also by their fusion probabilities, which are set independently of bouton size by variable spike-evoked presynaptic Ca²⁺ influx.

Synaptic vesicle pools and dynamics

Synaptic vesicles release neurotransmitter at chemical synapses, thus initiating the flow of information in neural networks. To achieve this, vesicles undergo a dynamic cycle of fusion and retrieval to maintain the structural and functional integrity of the presynaptic terminals in which they reside. Moreover, compelling evidence indicates these vesicles differ in their availability for release and mobilization in response to stimuli, prompting classification into at least three different functional pools. Ongoing studies of the molecular and cellular bases for this heterogeneity attempt to link structure to physiology and clarify how regulation of vesicle pools influences synaptic strength and presynaptic plasticity. We discuss prevailing perspectives on vesicle pools, the role they play in shaping synaptic transmission, and the open questions that challenge current understanding.

Determinants of synaptic strength vary across an axon arbor

We used synaptophysin-pHluorin expressed in hippocampal neurons to address how functional properties of terminals, namely, evoked release, total vesicle pool size, and release fraction, vary spatially across individual axon arbors. Consistent with previous reports, over short arbor distances (≈ 100 μm), evoked release was spatially heterogeneous when terminals contacted different postsynaptic dendrites or neurons. Regardless of the postsynaptic configuration, the evoked release and total vesicle pool size spatially covaried, suggesting that the fraction of synaptic vesicles available for release (release fraction) was similar over short distances. Evoked release and total vesicle pool size were highly correlated with the amount of NMDA receptors and PSD-95 in postsynaptic specialization. However, when individual axons were followed over longer distances (several hundred micrometers), a significant increase in evoked release was observed distally that was associated with an increased release fraction in distal terminals. The increase in distal release fraction can be accounted for by changes in individual vesicle release probability as well as readily releasable pool size. Our results suggest that for a single axon arbor, presynaptic strength indicated by evoked release over short distances is correlated with heterogeneity in total vesicle pool size, whereas over longer distances presynaptic strength is correlated with the spatial modulation of release fraction. Thus the mechanisms that determine synaptic strength differ depending on spatial scale.

Use dependence of presynaptic tenacity

Recent studies indicate that synaptic vesicles (SVs) are continuously interchanged among nearby synapses at very significant rates. These dynamics and the lack of obvious barriers confining synaptic vesicles to specific synapses would seem to challenge the ability of synapses to maintain a constant amount of synaptic vesicles over prolonged time scales. Moreover, the extensive mobilization of synaptic vesicles associated with presynaptic activity might be expected to intensify this challenge. Here we examined the ability of individual presynaptic boutons of rat hippocampal neurons to maintain their synaptic vesicle content, and the degree to which this ability is affected by continuous activity. We found that the synaptic vesicle content of individual boutons belonging to the same axons gradually changed over several hours, and that these changes occurred independently of activity. Intermittent stimulation for 1 h accelerated rates of vesicle pool size change. Interestingly, however, following stimulation cessation, vesicle pool size change rates gradually converged with basal change rates. Over similar time scales, active zones (AZs) exhibited substantial remodeling; yet, unlike synaptic vesicles, AZ remodeling was not affected by the stimulation paradigms used here. These findings indicate that enhanced activity levels can increase synaptic vesicle redistribution among nearby synapses, but also highlight the presence of forces that act to restore particular set points in terms of SV contents, and support a role for active zones in preserving such set points. These findings also indicate, however, that neither AZ size nor SV content set points are particularly stable, questioning the long-term tenacity of presynaptic specializations.

Examining size-strength relationships at hippocampal synapses using an ultrastructural measurement of synaptic release probability

Release probability (p(r)) is a fundamental presynaptic parameter which is critical in defining synaptic strength. Knowledge of how synapses set and regulate their p(r) is a fundamental step in understanding synaptic transmission and communication between neurons. Despite its importance, p(r) is difficult to measure directly at single synapses. One important strategy to achieve this has relied on the application of fluorescence-based imaging methods, but this is always limited by the lack of detailed information on the morphological and structural properties of the individual synapses under study, and thus precludes an investigation of the relationship between p(r) and synaptic anatomy. Here we outline a powerful methodology based on using FM-styryl dyes, photoconversion and correlative ultrastructural analysis in dissociated hippocampal cultured neurons, which provides both a direct readout of p(r) as well as nanoscale detail on synaptic organization and structure. We illustrate the value of this approach by investigating, at the level of individual reconstructed terminals, the relationship between release probability and defined vesicle pools. We show that in our population of synapses, p(r) is highly variable, and while it is positively correlated with the number of vesicles docked at the active zone it shows no relationship with the total number of synaptic vesicles. The lack of a direct correlation between total synaptic size and performance in these terminals suggests that factors other than the absolute magnitude of the synapse are the most important determinants of synaptic efficacy.

A study of clustered data and approaches to its analysis

Statistical analysis is critical in the interpretation of experimental data across the life sciences, including neuroscience. The nature of the data collected has a critical role in determining the best statistical approach to take. One particularly prevalent type of data is referred to as "clustered data." Clustered data are characterized as data that can be classified into a number of distinct groups or "clusters" within a particular study. Clustered data arise most commonly in neuroscience when data are compiled across multiple experiments, for example in electrophysiological or optical recordings taken from synaptic terminals, with each experiment providing a distinct cluster of data. However, there are many other types of experimental design that can yield clustered data. Here, we provide a statistical model for intracluster correlation and systematically investigate a range of methods for analyzing clustered data. Our analysis reveals that it is critical to take data clustering into account and suggests appropriate statistical approaches that can be used to account for data clustering.

Functional analysis of calcium channel-mediated exocytosis in synaptic terminals by FM imaging technique

OBJECTIVE

Presynaptic voltage-gated Ca(2+) channels mediate rapid Ca(2+) influx into the synaptic terminal which triggers synaptic vesicle exocytosis and neurotransmitter release. The FM 1-43 dye was firstly introduced as a fluorescence probe by Betz and his colleagues in 1992, and has been used to monitor exocytosis, endocytosis and endosomal traffic in a variety of cell types. The present study aims to investigate the feasibility of applying the FM 1-43 dye in the functional analysis of calcium channel-mediated exocytosis in synaptic terminals.

METHODS

The hippocampi were isolated from embryos of pregnant rats, and hippocampal neurons were then transfected with Ds-Red conjugated plasmid. The neurons were then loaded with 8 micromol/L FM 1-43 and 47 mmol/L KCl for 90 s after transfection. After that, 90 mmol/L KCl was employed to induce FM dye destaining, which was recorded by FM imaging system.

RESULTS

The neuron synaptic terminals of rat hippocampus could be effectively stained by the FM 1-43 dye. Besides, the destaining of the labeled neuron terminals was in accordance with the transmitter release, which could be blocked by the application of nifedipine (inhibitor for L-type calcium channel).

CONCLUSION

The FM imaging technique is an advanced and effective method for analyzing synaptic vesicle exocytosis and neurotransmitter release, and can be applied in various synaptic functional studies.

Functional heterogeneity at dopamine release sites

Although drugs used to treat several neurological diseases are presumed to target synapses that secrete dopamine (DA), relatively little is known about synaptic vesicle (SV) release mechanisms at single DA synapses. We found that the relative probability of release (Pr) varied between individual DA synapses. Furthermore, DA terminals generally exhibited lower Pr than glutamatergic hippocampal (Hpc) terminals, suggesting that DA release is less reliable than the release of glutamate. Our mathematical model of fluorescence loss shows that Pr is regulated by two independent and heterogeneous elements. First, the size of the recycling SV pool regulates Pr. Second, Pr is also independently regulated by additional factors, which are reflected in the time constant of FM 1-43 destaining, tau. We found that the observed difference in Pr between Hpc and DA neurons results because the recycling SV pool is smaller in DA neurons than in Hpc neurons. However, tau does not vary between these two neuron populations. We also identified a population of functional nonsynaptic boutons in DA axons, which are not associated with a postsynaptic element and which are not functionally different from boutons that formed conventional synapses. Our work provides a new approach to the study of SV exocytosis in DA neurons and shows that synaptic terminals of DA neurons are functionally heterogeneous and differ from excitatory terminals in terms of Pr.

Optical tracking of phenotypically diverse individual synapses on solitary tract nucleus neurons

The solitary tract nucleus (NTS) is the termination site for cranial visceral afferents-peripheral primary afferent neurons which differ by phenotype (e.g. myelinated and unmyelinated). These afferents have very uniform glutamate release properties calculated by variance mean analysis. In the present study, we optical measured the inter-terminal release properties across individual boutons by assessing vesicle membrane turnover with the dye FM1-43. Single neurons were mechanically micro-harvested from medial NTS without enzyme treatment. The TRPV1 agonist capsaicin (CAP, 100 nM) was used to identify afferent, CAP-sensitive terminals arising from unmyelinated afferents. Isolated NTS neurons retained both glutamatergic and inhibitory terminals that generated EPSCs and IPSCs, respectively. Visible puncta on the neurons were stained positively with monoclonal antibody for synaptophysin, a presynaptic marker. Elevating extracellular K(+) concentration to 10 mM increased synaptic release measured at individual terminals by FM1-43. Within single neurons, CAP destained some but not other individual terminals. FM1-43 positive terminals that were resistant to CAP could be destained with K(+) solution. Individual terminals responded to depolarization with similar vesicle turnover kinetics. Thus, vesicular release was relatively homogenous across individual release sites. Surprisingly, conventionally high K(+) concentrations (>50 mM) produced erratic synaptic responses and at 90 mM K(+) overt neuron swelling--results that suggest precautions about assuming consistent K(+) responses in all neurons. The present work demonstrates remarkably uniform glutamate release between individual unmyelinated terminals and suggests that the homogeneous EPSC release properties of solitary tract afferents result from highly uniform release properties across multiple contacts on NTS neurons.

Dynamic modulation of phasic and asynchronous glutamate release in hippocampal synapses

Although frequency-dependent short-term presynaptic plasticity has been of long-standing interest, most studies have emphasized modulation of the synchronous, phasic component of transmitter release, most evident with a single or a few presynaptic stimuli. Asynchronous transmitter release, vesicle fusion not closely time locked to presynaptic action potentials, can also be prominent under certain conditions, including repetitive stimulation. Asynchrony has often been attributed to residual Ca(2+) buildup in the presynaptic terminal. We verified that a number of manipulations of Ca(2+) handling and influx selectively alter asynchronous release relative to phasic transmitter release during action potential trains in cultured excitatory autaptic hippocampal neurons. To determine whether other manipulations of vesicle release probability also selectively modulate asynchrony, we probed the actions of one thoroughly studied modulator class whose actions on phasic versus asynchronous release have not been investigated. We examined the effects of the phorbol ester PDBu, which has protein kinase C (PKC) dependent and independent actions on presynaptic transmitter release. PDBu increased phasic and asynchronous release in parallel. However, while PKC inhibition had relatively minor inhibitory effects on PDBu potentiation of phasic and total release during action potential trains, PKC inhibition strongly reduced phorbol-potentiated asynchrony, through actions most evident late during stimulus trains. These results lend new insight into PKC-dependent and -independent effects on transmitter release and suggest the possibility of differential control of synchronous versus asynchronous vesicle release.

The probability of neurotransmitter release: variability and feedback control at single synapses

Information transfer at chemical synapses occurs when vesicles fuse with the plasma membrane and release neurotransmitter. This process is stochastic and its likelihood of occurrence is a crucial factor in the regulation of signal propagation in neuronal networks. The reliability of neurotransmitter release can be highly variable: experimental data from electrophysiological, molecular and imaging studies have demonstrated that synaptic terminals can individually set their neurotransmitter release probability dynamically through local feedback regulation. This local tuning of transmission has important implications for current models of single-neuron computation.

Counting the number of releasable synaptic vesicles in a presynaptic terminal

Synaptic transmission depends on the continued availability of neurotransmitter-filled synaptic vesicles (SVs) for triggered release from presynaptic boutons. Surprisingly, small boutons in the brain, that already contain comparatively few SVs, are thought to retain the majority of these SVs in a "reserve" pool that is not mobilized under physiological conditions. Why such a scarce synaptic resource is normally inaccessible has been a matter of debate. Here, we readdress this issue by developing an electrophysiological approach for counting SVs released from boutons formed by a single, isolated neuron on itself ("autapses"). We show that, after treatment with Bafilomycin A1 to prevent reloading of discharged SVs with glutamate, each SV is counted only once on first-time release. Hence, by integrating all autaptic currents as they run down over time, we can estimate the total number of SVs released by a single neuron. This total can be normalized to the number of boutons on the neuron, giving the mean number of SVs released per bouton. We estimate that up to approximately 130 vesicles can be released per bouton over approximately 10 min of stimulation at 0.2 Hz. This number of vesicles represents a substantial proportion of the total number of SVs (100-200) that have been counted in these boutons by using electron microscopy. Thus, mild electrical stimulation, when maintained for sufficient time, causes the eventual release of many of the SVs in a bouton, including those in the putative reserve pool. This result suggests that SVs are functionally homogeneous in that the majority can contribute to basal synaptic transmission.

Spontaneous and evoked glutamate release activates two populations of NMDA receptors with limited overlap

In a synapse, spontaneous and action-potential-driven neurotransmitter release is assumed to activate the same set of postsynaptic receptors. Here, we tested this assumption using (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate (MK-801), a well characterized use-dependent blocker of NMDA receptors. NMDA-receptor-mediated spontaneous miniature EPSCs (NMDA-mEPSCs) were substantially decreased by MK-801 within 2 min in a use-dependent manner. In contrast, MK-801 application at rest for 10 min did not significantly impair the subsequent NMDA-receptor-mediated evoked EPSCs (NMDA-eEPSCs). Brief stimulation in the presence of MK-801 significantly depressed evoked NMDA-eEPSCs but only mildly affected the spontaneous NMDA-mEPSCs detected on the same cell. Optical imaging of synaptic vesicle fusion showed that spontaneous and evoked release could occur at the same synapse albeit without correlation between their kinetics. In addition, modeling glutamate diffusion and NMDA receptor activation revealed that postsynaptic densities larger than approximately 0.2 microm(2) can accommodate two populations of NMDA receptors with nonoverlapping responsiveness. Collectively, these results support the premise that spontaneous and evoked neurotransmissions activate distinct sets of NMDA receptors and signal independently to the postsynaptic side.

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.

PKC modulation of transmitter release by SNAP-25 at sensory-to-motor synapses in aplysia

Activation of phosphokinase C (PKC) can increase transmitter release at sensory-motor neuron synapses in Aplysia, but the target of PKC phosphorylation has not been determined. One putative target of PKC at synapses is the synaptosomal-associated protein of 25 kDa (SNAP-25), a member of the SNARE protein complex implicated in synaptic vesicle docking and fusion. To determine whether PKC regulated transmitter release through phosphorylation of SNAP-25, we cloned Aplysia SNAP-25 and expressed enhanced green fluorescent protein (EGFP)-coupled SNAP-25 constructs mutated at the PKC phosphorylation site Ser198 in Aplysia sensory neurons. We found several distinct effects of expression of EGFP-SNAP-25 constructs. First, the rates of synaptic depression were slowed when cells contained SNAP-25 with phosphomimetic residues Glu or Asp. Second, PDBu-mediated increases in transmitter release at naïve synapses were blocked in cells expressing nonphosphorylated-state SNAP-25. Finally, expression of EGFP-coupled SNAP-25 but not uncoupled SNAP-25 inhibited 5-HT-mediated reversal of depression and the ability of EGFP-coupled SNAP-25 to inhibit the reversal of depression was affected by changes at Ser198. These results suggest SNAP-25 and phosphorylation of SNAP-25 by PKC can regulate transmitter release at Aplysia sensory-motor neuron synapses by a number of distinct processes.

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.

SNAP-29-mediated modulation of synaptic transmission in cultured hippocampal neurons

Identifying the molecules that regulate both the recycling of synaptic vesicles and the SNARE components required for fusion is critical for elucidating the molecular mechanisms underlying synaptic plasticity. SNAP-29 was initially isolated as a syntaxin-binding and ubiquitously expressed protein. Previous studies have suggested that SNAP-29 inhibits SNARE complex disassembly, thereby reducing synaptic transmission in cultured superior cervical ganglion neurons in an activity-dependent manner. However, the role of SNAP-29 in regulating synaptic vesicle recycling and short-term plasticity in the central nervous system remains unclear. In the present study, we examined the effect of SNAP-29 on synaptic transmission in cultured hippocampal neurons by dual patch clamp whole-cell recording, FM dye imaging, and immunocytochemistry. Our results demonstrated that exogenous expression of SNAP-29 in presynaptic neurons significantly decreased the efficiency of synaptic transmission after repetitive firing within a few minutes under low and moderate frequency stimulations (0.1 and 1 Hz). In contrast, SNAP-29 did not affect the density of synapses and basal synaptic transmission. Whereas neurotransmitter release was unaffected during intensive stimulation, recovery after synaptic depression was impaired by SNAP-29. Furthermore, knockdown of SNAP-29 expression in neurons by small interfering RNA increased the efficiency of synaptic transmission during repetitive firing. These findings suggest that SNAP-29 acts as a negative modulator for neurotransmitter release, probably by slowing recycling of the SNARE-based fusion machinery and synaptic vesicle turnover.

Trafficking of the plasma membrane gamma-aminobutyric acid transporter GAT1. Size and rates of an acutely recycling pool

Plasma membrane neurotransmitter transporters rapidly traffic to and from the cell surface in neurons. This trafficking may be important in regulating neuronal signaling. Such regulation will be subject to the number of trafficking transporters and their trafficking rates. In the present study, we define an acutely recycling pool of endogenous gamma-aminobutyric acid transporters (GAT1) in cortical neurons that comprises approximately one-third of total cellular GAT1. Kinetic analysis of this pool estimates exocytosis and endocytosis time constants of 1.6 and 0.9 min, respectively, and thus approximately one-third of the recycling pool is plasma membrane resident in the basal state. Recent evidence shows that GAT1 substrates, second messengers, and interacting proteins regulate GAT1 trafficking. These triggers could act by altering trafficking rates or by changing the recycling pool size. In the present study we examine three GAT1 modulators. Calcium depletion decreases GAT1 surface expression by diminishing the recycling pool size. Sucrose increases GAT1 surface expression by blocking clathrin- and dynamin-dependent endocytosis, but it does not change the recycling pool size. Protein kinase C decreases surface GAT1 expression by increasing the endocytosis rate, but it does not change the exocytosis rate or the recycling pool size. Based upon estimates of GAT1 molecules in cortical boutons, the present data suggest that approximately 1000 transporters comprise the acutely recycling pool, of which 300 are on the surface in the basal state, and five transporters insert into the plasma membrane every second. This insertion could represent the fusion of one transporter-containing vesicle.