Kinetics of synaptic depression and vesicle recycling after tetanic stimulation of frog motor nerve terminals

Mechanism of Purinergic Regulation of Neurotransmission in Mouse Neuromuscular Junction: The Role of Redox Signaling and Lipid Rafts

Acetylcholine is the main neurotransmitter at the vertebrate neuromuscular junctions (NMJs). ACh exocytosis is precisely modulated by co-transmitter ATP and its metabolites. It is assumed that ATP/ADP effects on ACh release rely on activation of presynaptic Gi protein-coupled P2Y13 receptors. However, downstream signaling mechanism of ATP/ADP-mediated modulation of neuromuscular transmission remains elusive. Using microelectrode recording and fluorescent indicators, the mechanism underlying purinergic regulation was studied in the mouse diaphragm NMJs. Pharmacological stimulation of purinoceptors with ADP decreased synaptic vesicle exocytosis evoked by both low and higher frequency stimulation. This inhibitory action was suppressed by antagonists of P2Y13 receptors (MRS 2211), Ca2+ mobilization (TMB8), protein kinase C (chelerythrine) and NADPH oxidase (VAS2870) as well as antioxidants. This suggests the participation of Ca2+ and reactive oxygen species (ROS) in the ADP-triggered signaling. Indeed, ADP caused an increase in cytosolic Ca2+ with subsequent elevation of ROS levels. The elevation of [Ca2+]in was blocked by MRS 2211 and TMB8, whereas upregulation of ROS was prevented by pertussis toxin (inhibitor of Gi protein) and VAS2870. Targeting the main components of lipid rafts, cholesterol and sphingomyelin, suppressed P2Y13 receptor-dependent attenuation of exocytosis and ADP-induced enhancement of ROS production. Inhibition of P2Y13 receptors decreased ROS production and increased the rate of exocytosis during intense activity. Thus, suppression of neuromuscular transmission by exogenous ADP or endogenous ATP can rely on P2Y13 receptor/Gi protein/Ca2+/protein kinase C/NADPH oxidase/ROS signaling, which is coordinated in a lipid raft-dependent manner.

Acute adaptation of central and peripheral motor unit features to exercise-induced fatigue differs with concentric and eccentric loading

NEW FINDINGS

What is the central question of this study? Conflicting evidence exists on motor unit (MU) firing rate in response to exercise-induced fatigue, possibly due to the contraction modality used: Do MU properties adapt similarly following concentric and eccentric loading? What is the main finding and its importance? MU firing rate increased following eccentric loading only despite a decline in absolute force. Force steadiness deteriorated following both loading methods. Central and peripheral MU features are altered in a contraction type-dependant manner, which is an important consideration for training interventions.

ABSTRACT

Force output of muscle is partly mediated by the adjustment of motor unit (MU) firing rate (FR). Disparities in MU features in response to fatigue may be influenced by contraction type, as concentric (CON) and eccentric (ECC) contractions demand variable amounts of neural input, which alters the response to fatigue. This study aimed to determine the effects of fatigue following CON and ECC loading on MU features of the vastus lateralis (VL). High-density surface (HD-sEMG) and intramuscular (iEMG) electromyography were used to record MU potentials (MUPs) from bilateral VLs of 12 young volunteers (six females) during sustained isometric contractions at 25% and 40% of the maximum voluntary contraction (MVC), before and after completing CON and ECC weighted stepping exercise. Multi-level mixed effects linear regression models were performed with significance assumed as P < 0.05. MVC decreased in both CON and ECC legs post-exercise (P < 0.0001), as did force steadiness at both 25% and 40% MVC (P < 0.004). MU FR increased in ECC at both contraction levels (P < 0.001) but did not change in CON. FR variability increased in both legs at 25% and 40% MVC following fatigue (P < 0.01). From iEMG measures at 25% MVC, MUP shape did not change (P > 0.1) but neuromuscular junction transmission instability increased in both legs (P < 0.04), and markers of fibre membrane excitability increased following CON only (P = 0.018). These data demonstrate that central and peripheral MU features are altered following exercise-induced fatigue and differ according to exercise modality. This is important when considering interventional strategies targeting MU function.

A three-pool model dissecting readily releasable pool replenishment at the calyx of held

Although vesicle replenishment is critical in maintaining exo-endocytosis recycling, the underlying mechanisms are not well understood. Previous studies have shown that both rapid and slow endocytosis recycle into a very large recycling pool instead of within the readily releasable pool (RRP), and the time course of RRP replenishment is slowed down by more intense stimulation. This finding contradicts the calcium/calmodulin-dependence of RRP replenishment. Here we address this issue and report a three-pool model for RRP replenishment at a central synapse. Both rapid and slow endocytosis provide vesicles to a large reserve pool (RP) ~42.3 times the RRP size. When moving from the RP to the RRP, vesicles entered an intermediate pool (IP) ~2.7 times the RRP size with slow RP-IP kinetics and fast IP-RRP kinetics, which was responsible for the well-established slow and rapid components of RRP replenishment. Depletion of the IP caused the slower RRP replenishment observed after intense stimulation. These results establish, for the first time, a realistic cycling model with all parameters measured, revealing the contribution of each cycling step in synaptic transmission. The results call for modification of the current view of the vesicle recycling steps and their roles.

A high-intensity, intermittent exercise protocol and dynamic postural control in men and women

CONTEXT

Deficits in dynamic postural control predict lower limb injury. Differing fatiguing protocols negatively affect dynamic postural control. The effect of high-intensity, intermittent exercise on dynamic postural control has not been investigated.

OBJECTIVE

To investigate the effect of a high-intensity, intermittent exercise protocol (HIIP) on the dynamic postural control of men and women as measured by the Star Excursion Balance Test (SEBT).

DESIGN

Descriptive laboratory study.

SETTING

University gymnasium.

PATIENTS OR OTHER PARTICIPANTS

Twenty male (age = 20.83 ± 1.50 years, height = 179.24 ± 7.94 cm, mass = 77.67 ± 10.82 kg) and 20 female (age = 20.45 ± 1.34 years, height = 166.08 ± 5.83 cm, mass = 63.02 ± 6.67 kg) athletes.

INTERVENTION(S)

We recorded SEBT measurements at baseline, pre-HIIP, and post-HIIP. The HIIP consisted of 4 repetitions of 10-m forward sprinting with a 90° change of direction and then backward sprinting for 5 m, 2 repetitions of 2-legged jumping over 5 hurdles, 2 repetitions of high-knee side stepping over 5 hurdles, and 4 repetitions of lateral 5-m shuffles. Participants rested for 30 seconds before repeating the circuit until they reported a score of 18 on the Borg rating of perceived exertion scale.

MAIN OUTCOME MEASURE(S)

A mixed between- and within-subjects analysis of variance was conducted to assess time (pre-HIIP, post-HIIP) × sex interaction effects. Subsequent investigations assessed the main effect of time and sex on normalized maximal SEBT scores. We used intraclass correlation coefficients to determine the test-retest reliability of the SEBT and paired-samples t tests to assess the HIIP effect on circuit times.

RESULTS

We found a time × sex effect (F(8,69) = 3.5; P range, <.001-.04; η(2) range, 0.057-0.219), with women less negatively affected. We also noted a main effect for time, with worse normalized maximal SEBT scores postfatigue (F(8,69) = 22.39; P < .001; η(2) range, 0.324-0.695), and for sex, as women scored better in 7 SEBT directions (F(8,69) = 0.84; P range, <.001-008; η(2) range, 0.088-0.381). The intraclass correlation coefficients demonstrated high (0.77-0.99) test-retest repeatability. Paired-samples t tests demonstrated increases in circuit time post-HIIP (P < .001).

CONCLUSIONS

The HIIP-induced fatigue negatively affected normalized maximal SEBT scores. Women had better scores than men and were affected less negatively by HIIP-induced fatigue.

Presynaptic clathrin levels are a limiting factor for synaptic transmission

To maintain communication, neurons must recycle their synaptic vesicles with high efficiency. This process places a huge burden on the clathrin-mediated endocytic machinery, but the consequences of this are poorly understood. We found that the amount of clathrin in a presynaptic terminal is not fixed. During stimulation, clathrin moves out of synapses as a function of stimulus strength and neurotransmitter release probability, which, together with membrane coat formation, transiently reduces the available pool of free clathrin triskelia. Correlative functional and morphological experiments in cholinergic autapses established by superior cervical ganglion neurons in culture show that presynaptic terminal function is compromised if clathrin levels fall by 20% after clathrin heavy chain knock down using RNAi. Synaptic transmission is depressed due to a reduction of cytoplasmic and readily releasable pools of vesicles. However, synaptic depression reverts after dialysis of exogenous clathrin, thus compensating RNAi-induced depletion. Lowering clathrin levels also reduces quantal size, which occurs concomitantly with a decrease in the size of synaptic vesicles. Large dense-core vesicles are unaffected by clathrin knock down. Together, our results show that clathrin levels are a dynamic property of presynaptic terminals that can influence short-term plasticity in a stimulus-dependent manner.

Presynaptic membrane receptors in acetylcholine release modulation in the neuromuscular synapse

Over the past few years, we have studied, in the mammalian neuromuscular junction (NMJ), the local involvement in transmitter release of the presynaptic muscarinic ACh autoreceptors (mAChRs), purinergic adenosine autoreceptors (P1Rs), and trophic factor receptors (TFRs; for neurotrophins and trophic cytokines) during development and in the adult. At any given moment, the way in which a synapse works is largely the logical outcome of the confluence of these (and other) metabotropic signalling pathways on intracellular kinases, which phosphorylate protein targets and materialize adaptive changes. We propose an integrated interpretation of the complementary function of these receptors in the adult NMJ. The activity of a given receptor group can modulate a given combination of spontaneous, evoked, and activity-dependent release characteristics. For instance, P1Rs can conserve resources by limiting spontaneous quantal leak of ACh (an A1 R action) and protect synapse function, because stimulation with adenosine reduces the magnitude of depression during repetitive activity. The overall outcome of the mAChRs seems to contribute to upkeep of spontaneous quantal output of ACh, save synapse function by decreasing the extent of evoked release (mainly an M2 action), and reduce depression. We have also identified several links among P1Rs, mAChRs, and TFRs. We found a close dependence between mAChR and some TFRs and observed that the muscarinic group has to operate correctly if the tropomyosin-related kinase B receptor (trkB) is also to operate correctly, and vice versa. Likewise, the functional integrity of mAChRs depends on P1Rs operating normally.

Synaptic activity slows vesicular replenishment at excitatory synapses of rat hippocampus

Short-term synaptic depression mainly reflects the depletion of the readily releasable pool (RRP) of quanta. Its dynamics, and especially the replenishment rate of the RRP, are still not well characterized in spite of decades of investigation. Main reason is that the vesicular storage and release system is treated as time-independent. If it is time-dependent all parameters thus estimated become problematic. Indeed the reports about how prolonged stimulation affects the dynamics are contradictory. To study this, we used patterned stimulation on the Schaeffer collateral fiber pathway and model-fitting of the excitatory post-synaptic currents (EPSC) recorded from CA1 neurons in rat hippocampal slices. The parameters of a vesicular storage and release model with two pools were estimated by minimizing the squared difference between the ESPC amplitudes and simulated model output. This yields the 'basic' parameters (release coupling, replenishment coupling and RRP size) that underlie the 'derived' and commonly used parameters (fractional release and replenishment rate). The fractional release increases when [Ca(++)]o is raised, whereas the replenishment rate is [Ca(++)]o independent. Fractional release rises because release coupling increases, and the RRP becomes less able to contain quanta. During prolonged stimulation, the fractional release remains generally unaltered, whereas the replenishment rate decreases down to ~10 % of its initial value with a decay time of ~15 s, and this decrease in the replenishment rate significantly contributes to synaptic depression. In conclusion, the fractional release is [Ca(++)]o-dependent and stimulation-independent, whereas the replenishment rate is [Ca(++)]o-independent and stimulation-dependent.

A realistic neural mass model of the cortex with laminar-specific connections and synaptic plasticity - evaluation with auditory habituation

In this work we propose a biologically realistic local cortical circuit model (LCCM), based on neural masses, that incorporates important aspects of the functional organization of the brain that have not been covered by previous models: (1) activity dependent plasticity of excitatory synaptic couplings via depleting and recycling of neurotransmitters and (2) realistic inter-laminar dynamics via laminar-specific distribution of and connections between neural populations. The potential of the LCCM was demonstrated by accounting for the process of auditory habituation. The model parameters were specified using Bayesian inference. It was found that: (1) besides the major serial excitatory information pathway (layer 4 to layer 2/3 to layer 5/6), there exists a parallel "short-cut" pathway (layer 4 to layer 5/6), (2) the excitatory signal flow from the pyramidal cells to the inhibitory interneurons seems to be more intra-laminar while, in contrast, the inhibitory signal flow from inhibitory interneurons to the pyramidal cells seems to be both intra- and inter-laminar, and (3) the habituation rates of the connections are unsymmetrical: forward connections (from layer 4 to layer 2/3) are more strongly habituated than backward connections (from Layer 5/6 to layer 4). Our evaluation demonstrates that the novel features of the LCCM are of crucial importance for mechanistic explanations of brain function. The incorporation of these features into a mass model makes them applicable to modeling based on macroscopic data (like EEG or MEG), which are usually available in human experiments. Our LCCM is therefore a valuable building block for future realistic models of human cognitive function.

Adenosine A₁ and A₂A receptor-mediated modulation of acetylcholine release in the mice neuromuscular junction

Immunocytochemistry shows that purinergic receptors (P1Rs) type A1 and A2A (A1 R and A2 A R, respectively) are present in the nerve endings at the P6 and P30 Levator auris longus (LAL) mouse neuromuscular junctions (NMJs). As described elsewhere, 25 μm adenosine reduces (50%) acetylcholine release in high Mg(2+) or d-tubocurarine paralysed muscle. We hypothesize that in more preserved neurotransmission machinery conditions (blocking the voltage-dependent sodium channel of the muscle cells with μ-conotoxin GIIIB) the physiological role of the P1Rs in the NMJ must be better observed. We found that the presence of a non-selective P1R agonist (adenosine) or antagonist (8-SPT) or selective modulators of A1 R or A2 A R subtypes (CCPA and DPCPX, or CGS-21680 and SCH-58261, respectively) does not result in any changes in the evoked release. However, P1Rs seem to be involved in spontaneous release (miniature endplate potentials MEPPs) because MEPP frequency is increased by non-selective block but decreased by non-selective stimulation, with A1 Rs playing the main role. We assayed the role of P1Rs in presynaptic short-term plasticity during imposed synaptic activity (40 Hz for 2 min of supramaximal stimuli). Depression is reduced by micromolar adenosine but increased by blocking P1Rs with 8-SPT. Synaptic depression is not affected by the presence of selective A1 R and A2 A R modulators, which suggests that both receptors need to collaborate. Thus, A1 R and A2 A R might have no real effect on neuromuscular transmission in resting conditions. However, these receptors can conserve resources by limiting spontaneous quantal leak of acetylcholine and may protect synaptic function by reducing the magnitude of depression during repetitive activity.

Neuromodulatory changes in short-term synaptic dynamics may be mediated by two distinct mechanisms of presynaptic calcium entry

Although synaptic output is known to be modulated by changes in presynaptic calcium channels, additional pathways for calcium entry into the presynaptic terminal, such as non-selective channels, could contribute to modulation of short term synaptic dynamics. We address this issue using computational modeling. The neuropeptide proctolin modulates the inhibitory synapse from the lateral pyloric (LP) to the pyloric dilator (PD) neuron, two slow-wave bursting neurons in the pyloric network of the crab Cancer borealis. Proctolin enhances the strength of this synapse and also changes its dynamics. Whereas in control saline the synapse shows depression independent of the amplitude of the presynaptic LP signal, in proctolin, with high-amplitude presynaptic LP stimulation the synapse remains depressing while low-amplitude stimulation causes facilitation. We use simple calcium-dependent release models to explore two alternative mechanisms underlying these modulatory effects. In the first model, proctolin directly targets calcium channels by changing their activation kinetics which results in gradual accumulation of calcium with low-amplitude presynaptic stimulation, leading to facilitation. The second model uses the fact that proctolin is known to activate a non-specific cation current I ( MI ). In this model, we assume that the MI channels have some permeability to calcium, modeled to be a result of slow conformation change after binding calcium. This generates a gradual increase in calcium influx into the presynaptic terminals through the modulatory channel similar to that described in the first model. Each of these models can explain the modulation of the synapse by proctolin but with different consequences for network activity.

Rapid reversal of impaired inhibitory and excitatory transmission but not spine dysgenesis in a mouse model of mental retardation

Intellectual disability affects 2-3% of the population: those due to mutations of the X-chromosome are a major cause of moderate to severe cases (1.8/1000 males). Established theories ascribe the cellular aetiology of intellectual disability to malformations of dendritic spines. Recent work has identified changes in synaptic physiology in some experimental models. Here, we investigated the pathophysiology of a mouse model of intellectual disability using electrophysiological recordings combined with confocal imaging of dentate gyrus granule neurons. Lack of oligophrenin-1 resulted in reductions in dendritic tree complexity and mature dendritic spine density and in evoked and spontaneous EPSCs and IPSCs. In the case of inhibitory transmission, the physiological change was associated with a reduction in the readily releasable pool and vesicle recycling which impaired the efficiency of inhibitory synaptic transmission. Acute inhibition of the downstream signalling pathway of oligophrenin-1 fully reversed the functional changes in synaptic transmission but not the dendritic abnormalities. The impaired inhibitory (as well as excitatory) synaptic transmission at frequencies associated with cognitive function suggests a cellular mechanism for the intellectual disability, because cortical oscillations associated with cognition normally depend on inhibitory neurons firing on every cycle.

Altered neurotransmitter release machinery in mice deficient for the deubiquitinating enzyme Usp14

Homozygous ataxic mice (ax(J)) express reduced levels of the deubiquitinating enzyme Usp14. They develop severe tremors by 2-3 wk of age, followed by hindlimb paralysis, and death by 6-8 wk. While changes in the ubiquitin proteasome system often result in the accumulation of ubiquitin protein aggregates and neuronal loss, these pathological markers are not observed in the ax(J) mice. Instead, defects in neurotransmission were observed in both the central and peripheral nervous systems of ax(J) mice. We have now identified several new alterations in peripheral neurotransmission in the ax(J) mice. Using the two-microelectrode voltage clamp technique on diaphragm muscles of ax(J) mice, we observed that under normal neurotransmitter release conditions ax(J) mice lacked paired-pulse facilitation and exhibited a frequency-dependent increase in rundown of the end plate current at high-frequency stimulation (HFS). Combined electrophysiology and styryl dye staining revealed a significant reduction in quantal content during the initial and plateau portions of the HFS train. In addition, uptake of styryl dyes (FM dye) during HFS demonstrated that the size of the readily releasable vesicle pool was significantly reduced. Destaining rates for styryl dyes suggested that ax(J) neuromuscular junctions are unable to mobilize a sufficient number of vesicles during times of intense activity. These results imply that ax(J) nerve terminals are unable to recruit a sufficient number of vesicles to keep pace with physiological rates of transmitter release. Therefore, ubiquitination of synaptic proteins appears to play an important role in the normal operation of the neurotransmitter release machinery and in regulating the size of pools of synaptic vesicles.

A new kinetic framework for synaptic vesicle trafficking tested in synapsin knock-outs

At least two rate-limiting mechanisms in vesicle trafficking operate at mouse Schaffer collateral synapses, but their molecular/physical identities are unknown. The first mechanism determines the baseline rate at which reserve vesicles are supplied to a readily releasable pool. The second causes the supply rate to depress threefold when synaptic transmission is driven hard for extended periods. Previous models invoked depletion of a reserve vesicle pool to explain the reductions in the supply rate, but the mass-action assumption at their core is not compatible with kinetic measurements of neurotransmission under maximal-use conditions. Here we develop a new theoretical model of rate-limiting steps in vesicle trafficking that is compatible with previous and new measurements. A physical interpretation is proposed where local reserve pools consisting of four vesicles are tethered to individual release sites and are replenished stochastically in an all-or-none fashion. We then show that the supply rate depresses more rapidly in synapsin knock-outs and that the phenotype can be fully explained by changing the value of the single parameter in the model that would specify the size of the local reserve pools. Vesicle-trafficking rates between pools were not affected. Finally, optical imaging experiments argue against alternative interpretations of the theoretical model where vesicle trafficking is inhibited without reserve pool depletion. This new conceptual framework will be useful for distinguishing which of the multiple molecular and cell biological mechanisms involved in vesicle trafficking are rate limiting at different levels of synaptic throughput and are thus candidates for physiological and pharmacological modulation.

The number of components of enhancement contributing to short-term synaptic plasticity at the neuromuscular synapse during patterned nerve Stimulation progressively decreases as basal release probability is increased from low to normal levels by changing extracellular Ca2+

Presynaptic short-term plasticity (STP) dynamically modulates synaptic strength in a reversible manner on a timescale of milliseconds to minutes. For low basal vesicular release probability (prob0), four components of enhancement, F1 and F2 facilitation, augmentation (A), and potentiation (P), increase synaptic strength during repetitive nerve activity. For release rates that exceed the rate of replenishment of the readily releasable pool (RRP) of synaptic vesicles, depression of synaptic strength, observed as a rundown of postsynaptic potential amplitudes, can also develop. To understand the relationship between enhancement and depression at the frog (Rana pipiens) neuromuscular synapse, data obtained over a wide range of prob0 using patterned stimulation are analyzed with a hybrid model to reveal the components of STP. We find that F1, F2, A, P, and depletion of the RRP all contribute to STP during repetitive nerve activity at low prob0. As prob0 is increased by raising Ca(o)(2+) (extracellular Ca2+), specific components of enhancement no longer contribute, with first P, then A, and then F2 becoming undetectable, even though F1 continues to enhance release. For levels of prob0 that lead to appreciable depression, only F1 and depletion of the RRP contribute to STP during rundown, and for low stimulation rates, F2 can also contribute. These observations place prob0-dependent limitations on which components of enhancement contribute to STP and suggest some fundamental mechanistic differences among the components. The presented model can serve as a tool to readily characterize the components of STP over wide ranges of prob0.

Neuromuscular fatigue in healthy muscle: underlying factors and adaptation mechanisms

OBJECTIVES

This review aims to define the concept of neuromuscular fatigue and to present the current knowledge of the central and peripheral factors at the origin of this phenomenon. This review also addresses the literature that focuses on the mechanisms responsible for the adaption to neuromuscular fatigue.

METHOD

One hundred and eighty-two articles indexed in PubMed (1954-2010) have been considered.

RESULTS

Neuromuscular fatigue has central and peripheral origins. Central fatigue, preponderant during long-duration, low-intensity exercises, may involve a drop in the central command (motor, cortex, motoneurons) elicited by the activity of cerebral neurotransmitters and muscular afferent fibers. Peripheral fatigue, associated with an impairment of the mechanisms from excitation to muscle contraction, may be induced by a perturbation of the calcium ion movements, an accumulation of phosphate, and/or a decrease of the adenosine triphosphate stores. To compensate for the consequent drop in force production, the organism develops several adaptation mechanisms notably implicating motor units.

CONCLUSION

Fatigue onset is associated with an alteration of the mechanisms involved in force production. Then, the interaction between central and peripheral mechanisms leads to a series of events that ultimately contribute to the observed decrease in force production.

Intersectin regulates dendritic spine development and somatodendritic endocytosis but not synaptic vesicle recycling in hippocampal neurons

Intersectin-short (intersectin-s) is a multimodule scaffolding protein functioning in constitutive and regulated forms of endocytosis in non-neuronal cells and in synaptic vesicle (SV) recycling at the neuromuscular junction of Drosophila and Caenorhabditis elegans. In vertebrates, alternative splicing generates a second isoform, intersectin-long (intersectin-l), that contains additional modular domains providing a guanine nucleotide exchange factor activity for Cdc42. In mammals, intersectin-s is expressed in multiple tissues and cells, including glia, but excluded from neurons, whereas intersectin-l is a neuron-specific isoform. Thus, intersectin-I may regulate multiple forms of endocytosis in mammalian neurons, including SV endocytosis. We now report, however, that intersectin-l is localized to somatodendritic regions of cultured hippocampal neurons, with some juxtanuclear accumulation, but is excluded from synaptophysin-labeled axon terminals. Consistently, intersectin-l knockdown (KD) does not affect SV recycling. Instead intersectin-l co-localizes with clathrin heavy chain and adaptor protein 2 in the somatodendritic region of neurons, and its KD reduces the rate of transferrin endocytosis. The protein also co-localizes with F-actin at dendritic spines, and intersectin-l KD disrupts spine maturation during development. Our data indicate that intersectin-l is indeed an important regulator of constitutive endocytosis and neuronal development but that it is not a prominent player in the regulated endocytosis of SVs.

The vesicle cycle in motor nerve endings of the mouse diaphragm

Experiments on the mouse diaphragm muscle using intracellular microelectrode recordings and fluorescence microscopy were performed to study the dynamics of transmitter secretion and synaptic vesicle recycling processes (the exocytosis-endocytosis cycle) in motor nerve endings (NE) during prolonged rhythmic stimulation (20 impulses/sec). During stimulation, there were triphasic changes in the amplitude of endplate potentials (EPP): an initial rapid reduction, followed by prolonged (1-2 min) stabilization of amplitude, i.e., a plateau, and then a further slow decrease. Restoration of EPP amplitude after stimulation for 3 min occurred over a period of several seconds. Loading of synaptic vesicles with the fluorescent endocytic stain FM1-43 showed that rhythmic stimulation led to a gradual (over 5-6 min) decrease in NE fluorescence, demonstrating exocytosis of synaptic vesicles. Quantum analysis of the electrophysiological data and comparison of these data with results from fluorescence studies suggested that mouse NE have a high rate of endocytosis and reutilization of synaptic vesicles (the mean recycling time was about 50 sec), which may support the maintenance of reliable synaptic transmission during prolonged high-frequency activity. The sizes of the release-ready and recycling pools of synaptic vesicles were determined quantitatively. It is suggested that vesicle recycling in mouse NE occurs via a short, rapid pathway with incorporation into the recycling pool. Vesicles of the reserve pool are not used for transmitter secretion in the stimulation conditions used here.

A kinetic model unifying presynaptic short-term facilitation and depression

Short-term facilitation and depression refer to the increase and decrease of synaptic strength under repetitive stimuli within a timescale of milliseconds to seconds. This phenomenon has been attributed to primarily presynaptic mechanisms such as calcium-dependent transmitter release and presynaptic vesicle depletion. Previous modeling studies that aimed to integrate the complex short-term facilitation and short-term depression data derived from varying synapses have relied on computer simulation or abstract mathematical approaches. Here, we propose a unified theory of synaptic short-term plasticity based on realistic yet tractable and testable model descriptions of the underlying intracellular biochemical processes. Analysis of the model equations leads to a closed-form solution of the resonance frequency, a function of several critical biophysical parameters, as the single key indicator of the propensity for synaptic facilitation or depression under repetitive stimuli. This integrative model is supported by a broad range of transient and frequency response experimental data including those from facilitating, depressing or mixed-mode synapses. Specifically, the theory predicts that high calcium initial concentration and large gain of calcium action result in low resonance frequency and hence depressing behavior. In contrast, for synapses that are less sensitive to calcium or have higher recovery rate, resonance frequency becomes higher and thus facilitation prevails. The notion of resonance frequency therefore allows valuable quantitative parametric assessment of the contributions of various presynaptic mechanisms to the directionality of synaptic short-term plasticity. Thus, the model provides the reasons behind the switching behavior between facilitation and depression observed in experiments. New experiments are also suggested to control the short-term synaptic signal processing through adjusting the resonance frequency and bandwidth.

A heterogeneous "resting" pool of synaptic vesicles that is dynamically interchanged across boutons in mammalian CNS synapses

Using pHluorin-tagged synaptic vesicle proteins we have examined the partitioning of these probes into recycling and nonrecycling pools at hippocampal nerve terminals in cell culture. Our studies show that for three of the major synaptic vesicle components, vGlut-1, VAMP-2, and Synaptotagmin I, approximately 50-60% of the tagged protein appears in a recycling pool that responds readily to sustained action potential stimulation by mobilizing and fusing with the plasma membrane, while the remainder is targeted to a nonrecycling, acidic compartment. The fraction of recycling and nonrecycling (or resting) pools varied significantly across boutons within an individual axon, from 100% resting (silent) to 100% recycling. Single-bouton bleaching studies show that recycling and resting pools are dynamic and exchange between synaptic boutons. The quantitative parameters that can be extracted with the approaches outlined here should help elucidate the potential functional role of the resting vesicle pool.

Kinetic isolation of a slowly recovering component of short-term depression during exhaustive use at excitatory hippocampal synapses

This study examines the kinetics of the longest lasting form of short-term depression at excitatory hippocampal synapses. After initial depletion of the readily releasable pool (RRP), continued 20-Hz stimulation was found to be fast enough to maximally drive presynaptic neurotransmitter exocytosis; maximal is defined here as the rate needed to maintain the RRP in a nearly empty steady state. Induction of depression proceeded in two distinct phases. The first was caused by RRP depletion, whereas the second is shown to reflect the progressive reduction of the overall rate at which new vesicles are supplied to the RRP and is termed "supply-rate depression." Supply-rate depression is identified further with the emergence, during heavy use, of a rate-limiting vesicle trafficking step that slows the timing of RRP replenishment by switching from a fast (tau congruent with 7 s) to a slow (tau congruent with 1 min) vesicle supply mechanism. Both mechanisms apparently follow first-order kinetics. After the induction of the maximum amount of depression, individual synapses were able to output only <1 quantum of neurotransmitter per synapse per second, matching previous predictions based on cell biological measurements of synaptic vesicle cycling. Surprisingly, the onset of supply-rate depression occurred with a marked delay, not having a detectable impact on synaptic function until after several seconds of continuous use. The delayed onset is not consistent with traditional vesicle trafficking models, but may be important for limiting the impact of supply-rate depression to pathological episodes and might function as a native antiepilepsy device.

Interactions between multiple sources of short-term plasticity during evoked and spontaneous activity at the rat calyx of Held

Sustained activity at most central synapses is accompanied by a number of short-term changes in synaptic strength which act over a range of time scales. Here we examine experimental data and develop a model of synaptic depression at the calyx of Held synaptic terminal that combines many of these mechanisms (acting at differing sites and across a range of time scales). This new model incorporates vesicle recycling, facilitation, activity-dependent vesicle retrieval and multiple mechanisms affecting calcium channel activity and release probability. It can accurately reproduce the time course of experimentally measured short-term depression across different stimulus frequencies and exhibits a slow decay in EPSC amplitude during sustained stimulation. We show that the slow decay is a consequence of vesicle release inhibition by multiple mechanisms and is accompanied by a partial recovery of the releasable vesicle pool. This prediction is supported by patch-clamp data, using long duration repetitive EPSC stimulation at up to 400 Hz. The model also explains the recovery from depression in terms of interaction between these multiple processes, which together generate a stimulus-history-dependent recovery after repetitive stimulation. Given the high rates of spontaneous activity in the auditory pathway, the model also demonstrates how these multiple interactions cause chronic synaptic depression under in vivo conditions. While the magnitude of the depression converges to the same steady state for a given frequency, the time courses of onset and recovery are faster in the presence of spontaneous activity. We conclude that interactions between multiple sources of short-term plasticity can account for the complex kinetics during high frequency stimulation and cause stimulus-history-dependent recovery at this relay synapse.

Truncations of amphiphysin I by calpain inhibit vesicle endocytosis during neural hyperexcitation

Under normal physiological conditions, synaptic vesicle endocytosis is regulated by phosphorylation and Ca(2+)-dependent dephosphorylation of endocytic proteins such as amphiphysin and dynamin. To investigate the regulatory mechanisms that may occur under the conditions of excessive presynaptic Ca(2+) influx observed preceding neural hyperexcitation, we examined hippocampal slices following high-potassium or high-frequency electrical stimulation (HFS). In both cases, three truncated forms of amphiphysin I resulted from cleavage by the protease calpain. In vitro, the binding of truncated amphiphysin I to dynamin I and copolymerization into rings with dynamin I were inhibited, but its interaction with liposomes was not affected. Moreover, overexpression of the truncated form of amphiphysin I inhibited endocytosis of transferrin and synaptic vesicles. Inhibiting calpain prevented HFS-induced depression of presynaptic transmission. Finally, calpain-dependent amphiphysin I cleavage attenuated kainate-induced seizures. These results suggest that calpain-dependent cleavage of amphiphysin I inhibits synaptic vesicle endocytosis during neural hyperexcitation and demonstrate a novel post-translational regulation of endocytosis.

Connecdenn, a novel DENN domain-containing protein of neuronal clathrin-coated vesicles functioning in synaptic vesicle endocytosis

Clathrin-coated vesicles (CCVs) are responsible for the endocytosis of multiple cargo, including synaptic vesicle membranes. We now describe a new CCV protein, termed connecdenn, that contains an N-terminal DENN (differentially expressed in neoplastic versus normal cells) domain, a poorly characterized protein module found in multiple proteins of unrelated function and a C-terminal peptide motif domain harboring three distinct motifs for binding the alpha-ear of the clathrin adaptor protein 2 (AP-2). Connecdenn coimmunoprecipitates and partially colocalizes with AP-2, and nuclear magnetic resonance and peptide competition studies reveal that all three alpha-ear-binding motifs contribute to AP-2 interactions. In addition, connecdenn contains multiple Src homology 3 (SH3) domain-binding motifs and coimmunoprecipitates with the synaptic SH3 domain proteins intersectin and endophilin A1. Interestingly, connecdenn is enriched on neuronal CCVs and is present in the presynaptic compartment of neurons. Moreover, connecdenn has a uniquely stable association with CCV membranes because it resists extraction with Tris and high-salt buffers, unlike most other CCV proteins, but it is not detected on purified synaptic vesicles. Together, these observations suggest that connecdenn functions on the endocytic limb of the synaptic vesicle cycle. Accordingly, disruption of connecdenn interactions with its binding partners through overexpression of the C-terminal peptide motif domain or knock down of connecdenn through lentiviral delivery of small hairpin RNA both lead to defects in synaptic vesicle endocytosis in cultured hippocampal neurons. Thus, we identified connecdenn as a component of the endocytic machinery functioning in synaptic vesicle endocytosis, providing the first evidence of a role for a DENN domain-containing protein in endocytosis.

Augmentation increases vesicular release probability in the presence of masking depression at the frog neuromuscular junction

Synaptic augmentation is a short-term component of synaptic plasticity that increases transmitter release during repetitive stimulation and decays thereafter with a time constant of approximately 7 sec. Augmentation has typically been observed under conditions where there is little or no depression because of depletion of synaptic vesicles from the readily releasable pool (RRP) of transmitter. We now study augmentation under conditions of pronounced depression at the frog neuromuscular junction to gain additional insight into mechanism. If augmentation reflects an increase in the size of the RRP of transmitter, then augmentation should not be present with depression. Our findings using four different experimental approaches suggested that augmentation was still present in the presence of pronounced depression: mathematical extraction of augmentation from the changes in transmitter release after repetitive stimulation, identification of augmentation with Ba2+, correction of the data for the measured depletion of the RRP, and identification of an augmentation component of residual Ca2+. We conclude that the augmentation machinery still acts to increase transmitter release when depression reduces the RRP sufficiently to mask obvious augmentation. The masked augmentation was found to increase transmitter release by increasing the probability of releasing individual vesicles from the depressed RRP, countering the effects of depression. Because augmentation and depression have similar time courses, either process can mask the other, depending on their relative magnitudes. Consequently, the apparent absence of one of the processes does not exclude that it is still contributing to short-term synaptic plasticity.

The role of GABAergic inhibition in shaping the response size and duration selectivity of bat inferior collicular neurons to sound pulses in rapid sequences

Natural sounds, such as vocal communication sounds of many animal species typically occur as sequential sound pulses. Therefore, the response size of auditory neurons to a sound pulse would be inevitably affected when the sound pulse is preceded and succeeded by another sound pulse (i.e., forward and backward masking). The present study presents data to show that increasing strength of GABAergic inhibition relative to excitation contributes to decreasing response size and sharpening of duration selectivity of bat inferior collicular (IC) neurons to sound pulses in rapid sequences. The response size in number of impulses and duration selectivity of IC neurons were studied with a pulse train containing 9 sound pulses. A family of duration tuning curves was plotted for IC neurons using the number of impulses discharged to each presented sound pulse against pulse duration. Our data show that the response size of IC neurons progressively decreased and duration selectivity increased when determined with sequentially presented sound pulses. This variation in the response size and duration selectivity of IC neurons with sequentially presented sound pulses was abolished or reduced during bicuculline and GABA application. Bicuculline application increased the response size and broadened the duration tuning curve of IC neurons while GABA application produced opposite results. Possible mechanisms underlying increasing strength of GABAergic inhibition with sequentially presented sound pulses are presented. Biological significance of these findings in relation to acoustic signal processing is also discussed.

Modulation of biphasic rate of end-plate potential recovery in rat diaphragm

Previous diaphragm studies found that during intermittent stimulation, intratrain end-plate potential (EPP) amplitude rundown is accelerated by increasing stimulation frequency, whereas intertrain EPP rundown is independent of frequency. We hypothesized that increasing stimulation frequency accelerates rundown recovery, and with a biphasic time course. Intracellular recordings were made in vitro from rat phrenic nerve-hemidiaphragm preparations. EPP amplitude recovery after a 100-ms stimulation train and 100 ms of quiescence was significantly greater following stimulation at 200 HZ than at 20-100 HZ, despite larger antecedent EPP decline. EPP amplitudes recovered with a biphasic pattern: an early component with a fast time-constant (0.03-0.06 s) and a late component with a slow time-constant (0.5-5 s). Increased antecedent stimulation frequency accelerated the early component, but stimulation duration or pulse number modulated the late component. When interpreted in the context of vesicle recycling and replenishment models involving multiple pools and pathways, these data suggest that antecedent stimulation frequency regulates predominantly the fast pathways. This may have important implications for the development of respiratory failure in diseases of the neuromuscular junction, such as myasthenia gravis, when the firing duration and frequency are altered in association with changes in breathing pattern.

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.

Competition between phasic and asynchronous release for recovered synaptic vesicles at developing hippocampal autaptic synapses

Developing hippocampal neurons in microisland culture undergo rapid and extensive transmitter release-dependent depression of evoked (phasic) excitatory synaptic activity in response to 1 sec trains of 20 Hz stimulation. Although evoked phasic release was attenuated by repeated stimuli, asynchronous (miniature like) release continued at a high rate equivalent to approximately 2.8 readily releasable pools (RRPs) of quanta/sec. Asynchronous release reflected the recovery and immediate release of quanta because it was resistant to sucrose-induced depletion of the RRP. Asynchronous and phasic release appeared to compete for a common limited supply of release-ready quanta because agents that block asynchronous release, such as EGTA-AM, led to enhanced steady-state phasic release, whereas prolongation of the asynchronous release time course by LiCl delayed recovery of phasic release from depression. Modeling suggested that the resistance of asynchronous release to depression was associated with its ability to out-compete phasic release for recovered quanta attributable to its relatively low release rate (up to 0.04/msec per vesicle) stimulated by bulk intracellular Ca2+ concentration ([Ca2+]i) that could function over prolonged intervals between successive stimuli. Although phasic release was associated with a considerably higher peak rate of release (0.4/msec per vesicle), the [Ca2+]i microdomains that trigger it are brief (1 msec), and with asynchronous release present, relatively few quanta can accumulate within the RRP to be available for phasic release. We conclude that despite depression of phasic release during train stimulation, transmission can be maintained at a near-maximal rate by switching to an asynchronous mode that takes advantage of a bulk presynaptic [Ca2+]i.

Enhanced neuromuscular transmission efficacy in overloaded rat plantaris muscle

To better understand the effect of muscle hypertrophy on the physiological properties of transmitter release, we investigated neuromuscular transmission (NMT) efficacy in overloaded rat plantaris muscle in situ. In the overload group, following bilateral tenotomy of plantaris synergists, rats were confined to wheel-cages. Age-matched rats in the control group were confined to plastic cages. During the terminal experiment, muscle action potentials were blocked using micro-conotoxin, and full-sized endplate potentials (EPPs) were recorded at 25, 50, and 75 HZ to determine their amplitude rundown. Quantal contents for the control and overload groups were 37.0 and 74.3, respectively (P <0.01). There was a significant group difference in EPP amplitude rundown at all frequencies examined, with increased rundown occurring in the overload group (P < 0.01). Cumulative quantal release was 139% and 153% higher in the overload group at 25 and 50 HZ, respectively (P < 0.05). Together, these data suggest the safety factor for NMT is increased by neuromuscular overload. Furthermore, these findings support and supplement previously reported activity-dependent improvements in NMT efficacy that are probably mediated via presynaptic adaptations.

Loading and recycling of synaptic vesicles in the Torpedo electric organ and the vertebrate neuromuscular junction

In vertebrate motor nerve terminals and in the electromotor nerve terminals of Torpedo there are two major pools of synaptic vesicles: readily releasable and reserve. The electromotor terminals differ in that the reserve vesicles are twice the diameter of the readily releasable vesicles. The vesicles contain high concentrations of ACh and ATP. Part of the ACh is brought into the vesicle by the vesicular ACh transporter, VAChT, which exchanges two protons for each ACh, but a fraction of the ACh seems to be accumulated by different, unexplored mechanisms. Most of the vesicles in the terminals do not exchange ACh or ATP with the axoplasm, although ACh and ATP are free in the vesicle interior. The VAChT is controlled by a multifaceted regulatory complex, which includes the proteoglycans that characterize the cholinergic vesicles. The drug (-)-vesamicol binds to a site on the complex and blocks ACh exchange. Only 10-20% of the vesicles are in the readily releasable pool, which therefore is turned over fairly rapidly by spontaneous quantal release. The turnover can be followed by the incorporation of false transmitters into the recycling vesicles, and by the rate of uptake of FM dyes, which have some selectivity for the two recycling pathways. The amount of ACh loaded into recycling vesicles in the readily releasable pool decreases during stimulation. The ACh content of the vesicles can be varied over eight-fold range without changing vesicle size.

Synaptic vesicle pools at the frog neuromuscular junction

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

Mitochondrial Ca2+ uptake prevents desynchronization of quantal release and minimizes depletion during repetitive stimulation of mouse motor nerve terminals

We investigated how inhibition of mitochondrial Ca2+ uptake affects transmitter release from mouse motor terminals during brief trains of action potentials (500 at 50 Hz) in physiological bath [Ca2+]. When mitochondrial Ca2+ uptake was inhibited by depolarizing mitochondria with antimycin A1 or carbonyl cyanide m-chlorophenyl-hydrazone, the stimulation-induced increase in cytosolic [Ca2+] was greater (> 10 microM, compared to < or = 1 microM in control solution), the quantal content of the endplate potential (EPP) depressed more rapidly (approximately 84 % depression compared to approximately 8 % in controls), and asynchronous release during the stimulus train reached higher frequencies (peak rates of approximately 6000 s-1 compared to approximately 75 s-1 in controls). These effects of mitochondrial depolarization were not accompanied by a significant change in EPP quantal content or the rate of asynchronous release during 1 Hz stimulation, and were not seen in oligomycin, which blocks mitochondrial ATP synthesis without depolarizing mitochondria. Inhibition of endoplasmic reticular Ca2+ uptake with cyclopiazonic acid also had little effect on stimulation-induced changes in cytosolic [Ca2+] or EPP amplitude. We hypothesize that the high rate of asynchronous release evoked by stimulation during mitochondrial depolarization was produced by the elevation of cytosolic [Ca2+], and contributed to the accelerated depression of phasic release by reducing the availability of releasable vesicles. During mitochondrial depolarization, the post-tetanic potentiation of the EPP observed under control conditions was replaced by a post-tetanic depression with a slow time course of recovery. Thus, mitochondrial Ca2+ uptake is essential for sustaining phasic release, and thus neuromuscular transmission, during and following tetanic stimulation.

Secretory granule exocytosis

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

Incomplete recovery of endplate potential amplitude while intermittently activating rat soleus neuromuscular junctions in situ

Studies dealing with neuromuscular transmission efficacy typically employ continuous patterns of activation to demonstrate decrements in endplate potential (epp) amplitude. Recent evidence from rat diaphragm muscle has shown that including periods of quiescence to the stimulation protocol allows epp amplitude to recover between series of contractions. Whether similar recovery occurs in rat hindlimb muscle is unknown. In this study, we have measured declines in epp amplitude in rat soleus muscle during trains of stimulation evoked either continuously (10 s) or intermittently (400 ms repeated every second), using an in situ approach. As in diaphragm, we found that rest periods within the intermittent trains significantly improved neuromuscular transmission efficacy. However, unlike the diaphragm, epp amplitude recovery was incomplete even by the second train in the intermittent protocols, recovery being frequency-dependent and ranging from 40 to 50%. The results suggest that the kinetics of epp amplitude rundown and recovery may be muscle-specific, and should be considered when evaluating situations in which neuromuscular transmission efficacy may be altered.

GABAergic inhibition contributes to pulse repetition rate-dependent frequency selectivity in the inferior colliculus of the big brown bat, Eptesicus fuscus

This study examined the effect of bicuculline application on sharpness of frequency tuning curves (FTCs) of bat inferior collicular neurons plotted under three different pulse repetition rates (PRRs) of 10, 30 and 90 pulses per second. The sharpness of FTCs of collicular neurons, which was expressed in Q(n) (Q(10), Q(20), Q(30)) and bandwidths (90, 75 and 50% of the maximal response at the best frequency), improved with increasing PRR. However, this PRR-dependent frequency selectivity of collicular neurons was abolished during bicuculline application. This observation suggests that GABAergic inhibition contributes more effectively to sharpening of FTCs at higher than at lower PRRs.

Short-term synaptic plasticity

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

Fast vesicle recycling supports neurotransmission during sustained stimulation at hippocampal synapses

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

Fast vesicle recycling supports neurotransmission during sustained stimulation at hippocampal synapses

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

Development of vesicle pools during maturation of hippocampal synapses

We studied the emergence of vesicle pool organization at developing hippocampal synapses by monitoring vesicle recycling and neurotransmitter release as well as examining electron micrographs. Our analysis suggests that presynaptic boutons go through three distinct functional states to mature. At the onset the synapses lack readily releasable vesicles although they possess a pool of recycling vesicles that can release neurotransmitters under strong stimulation. In the next stage the majority of these recycling vesicles switches to a functionally docked state and forms the readily releasable pool (RRP). After assembly of the RRP, new vesicles build the reserve pool. At the mature state the size of the RRP increases linearly with increasing recycling pool size. Furthermore, this preferential filling of the RRP during early synapse maturation is reduced strikingly in synapses deficient in synapsin I and II. Taken together, these results expose a mechanism that ensures functionally effective allocation of a limited number of vesicles in a CNS synapse.

Development of vesicle pools during maturation of hippocampal synapses

We studied the emergence of vesicle pool organization at developing hippocampal synapses by monitoring vesicle recycling and neurotransmitter release as well as examining electron micrographs. Our analysis suggests that presynaptic boutons go through three distinct functional states to mature. At the onset the synapses lack readily releasable vesicles although they possess a pool of recycling vesicles that can release neurotransmitters under strong stimulation. In the next stage the majority of these recycling vesicles switches to a functionally docked state and forms the readily releasable pool (RRP). After assembly of the RRP, new vesicles build the reserve pool. At the mature state the size of the RRP increases linearly with increasing recycling pool size. Furthermore, this preferential filling of the RRP during early synapse maturation is reduced strikingly in synapses deficient in synapsin I and II. Taken together, these results expose a mechanism that ensures functionally effective allocation of a limited number of vesicles in a CNS synapse.

Synaptojanin 1 contributes to maintaining the stability of GABAergic transmission in primary cultures of cortical neurons

Inhibitory synapses in the CNS can exhibit a considerable stability of neurotransmission over prolonged periods of high-frequency stimulation. Previously, we showed that synaptojanin 1 (SJ1), a presynaptic polyphosphoinositide phosphatase, is required for normal synaptic vesicle recycling (Cremona et al., 1999). We asked whether the stability of inhibitory synaptic responses was dependent on SJ1. Whole-cell patch-clamp recordings of unitary IPSCs were obtained in primary cortical cultures between cell pairs containing a presynaptic, fast-spiking inhibitory neuron (33.5-35 degrees C). Prolonged presynaptic stimulation (1000 stimuli, 2-20 Hz) evoked postsynaptic responses that decreased in size with a bi-exponential time course. A fast component developed within a few stimuli and was quantified with paired-pulse protocols. Paired-pulse depression (PPD) appeared to be independent of previous GABA release at intervals of >/=100 msec. The characteristics of PPD, and synaptic depression induced within the first approximately 80 stimuli in the trains, were unaltered in SJ1-deficient inhibitory synapses. A slow component of depression developed within hundreds of stimuli, and steady-state depression showed a sigmoidal dependence on stimulation frequency, with half-maximal depression at 6.0 +/- 0.5 Hz. Slow depression was increased when release probability was augmented, and there was a small negative correlation between consecutive synaptic amplitudes during steady-state depression, consistent with a presynaptic depletion process. Slow depression was increased in SJ1-deficient synapses, with half-maximal depression at 3.3 +/- 0.9 Hz, and the recovery was retarded approximately 3.6-fold. Our studies establish a link between a distinct kinetic component of physiologically monitored synaptic depression and a molecular modification known to affect synaptic vesicle reformation.

Synaptojanin 1 contributes to maintaining the stability of GABAergic transmission in primary cultures of cortical neurons

Inhibitory synapses in the CNS can exhibit a considerable stability of neurotransmission over prolonged periods of high-frequency stimulation. Previously, we showed that synaptojanin 1 (SJ1), a presynaptic polyphosphoinositide phosphatase, is required for normal synaptic vesicle recycling (Cremona et al., 1999). We asked whether the stability of inhibitory synaptic responses was dependent on SJ1. Whole-cell patch-clamp recordings of unitary IPSCs were obtained in primary cortical cultures between cell pairs containing a presynaptic, fast-spiking inhibitory neuron (33.5-35 degrees C). Prolonged presynaptic stimulation (1000 stimuli, 2-20 Hz) evoked postsynaptic responses that decreased in size with a bi-exponential time course. A fast component developed within a few stimuli and was quantified with paired-pulse protocols. Paired-pulse depression (PPD) appeared to be independent of previous GABA release at intervals of >/=100 msec. The characteristics of PPD, and synaptic depression induced within the first approximately 80 stimuli in the trains, were unaltered in SJ1-deficient inhibitory synapses. A slow component of depression developed within hundreds of stimuli, and steady-state depression showed a sigmoidal dependence on stimulation frequency, with half-maximal depression at 6.0 +/- 0.5 Hz. Slow depression was increased when release probability was augmented, and there was a small negative correlation between consecutive synaptic amplitudes during steady-state depression, consistent with a presynaptic depletion process. Slow depression was increased in SJ1-deficient synapses, with half-maximal depression at 3.3 +/- 0.9 Hz, and the recovery was retarded approximately 3.6-fold. Our studies establish a link between a distinct kinetic component of physiologically monitored synaptic depression and a molecular modification known to affect synaptic vesicle reformation.

Drosophila stoned proteins regulate the rate and fidelity of synaptic vesicle internalization

At an initial step during synaptic vesicle recycling, dynamin and adaptor proteins mediate the endocytosis of synaptic vesicle components from the plasma membrane. StonedA and stonedB, novel synaptic proteins encoded by a single Drosophila gene, have predicted structural similarities to adaptors and other proteins implicated in endocytosis. Here, we test possible roles of the stoned proteins in synaptic vesicle internalization via analyses of third instar larval neuromuscular synapses in two Drosophila stoned (stn) mutants, stn(ts) and stn(8P1). Both mutations reduce presynaptic levels of stonedA and stonedB, although stn(ts) has relatively weak effects. The mutations cause retention of synaptic vesicle proteins on the presynaptic plasma membrane but do not alter the levels or distribution of endocytosis proteins, dynamin, alpha-adaptin, and clathrin. In addition, stn(8P1) mutants exhibit depletion and enlargement of synaptic vesicles. To determine whether these defects arise from altered synaptic vesicle endocytosis or from defects in synaptic vesicle biogenesis, we implemented new methods to assess directly the efficiency of synaptic vesicle recycling and membrane internalization at Drosophila nerve terminals. Behavioral and electrophysiological analyses indicate that stn(ts), an allele with normal evoked release and synaptic vesicle number, enhances defects in synaptic vesicle recycling shown by Drosophila shi(ts) mutants. A dye uptake assay demonstrates that slow synaptic vesicle recycling in stn(ts) is accompanied by a reduced rate of synaptic vesicle internalization after exocytosis. These observations are consistent with a model in which stonedA and stonedB act to facilitate the internalization of synaptic vesicle components from the plasma membrane.

Drosophila stoned proteins regulate the rate and fidelity of synaptic vesicle internalization

At an initial step during synaptic vesicle recycling, dynamin and adaptor proteins mediate the endocytosis of synaptic vesicle components from the plasma membrane. StonedA and stonedB, novel synaptic proteins encoded by a single Drosophila gene, have predicted structural similarities to adaptors and other proteins implicated in endocytosis. Here, we test possible roles of the stoned proteins in synaptic vesicle internalization via analyses of third instar larval neuromuscular synapses in two Drosophila stoned (stn) mutants, stn(ts) and stn(8P1). Both mutations reduce presynaptic levels of stonedA and stonedB, although stn(ts) has relatively weak effects. The mutations cause retention of synaptic vesicle proteins on the presynaptic plasma membrane but do not alter the levels or distribution of endocytosis proteins, dynamin, alpha-adaptin, and clathrin. In addition, stn(8P1) mutants exhibit depletion and enlargement of synaptic vesicles. To determine whether these defects arise from altered synaptic vesicle endocytosis or from defects in synaptic vesicle biogenesis, we implemented new methods to assess directly the efficiency of synaptic vesicle recycling and membrane internalization at Drosophila nerve terminals. Behavioral and electrophysiological analyses indicate that stn(ts), an allele with normal evoked release and synaptic vesicle number, enhances defects in synaptic vesicle recycling shown by Drosophila shi(ts) mutants. A dye uptake assay demonstrates that slow synaptic vesicle recycling in stn(ts) is accompanied by a reduced rate of synaptic vesicle internalization after exocytosis. These observations are consistent with a model in which stonedA and stonedB act to facilitate the internalization of synaptic vesicle components from the plasma membrane.

Effect of Temperature on Endplate Potential Rundown and Recovery in Rat Diaphragm

The amplitude of neuromuscular junction end-plate potentials (EPPs) decreases quickly within a train but recovers nearly completely from train to train during intermittent stimulation. Rundown has been shown to be dependent not only on the rate of transmitter release but also on the rate of replenishment of the depleted neurotransmitter at the site of release. Two groups of processes have been proposed for synaptic vesicle recycling, both of which involve multiple energy-requiring steps and enzymatic reactions and which therefore would be expected to be very temperature-sensitive. The present study tested the hypothesis that low temperature therefore increases the rate of EPP amplitude rundown. Studies were performed in vitro on rat diaphragm and used μ-conotoxin to allow normal-sized EPPs to be recorded from intact fibers. EPP amplitude rundown during intermittent stimulation at 20 and 50 Hz (duty cycle 333 ms) was greater at 20°C than it was at 37°C. Initially, temperature affected only intra-train rundown but, over longer periods of stimulation, both intra- and inter-train rundown were significantly accelerated by cold temperature. Cumulative EPP amplitudes were calculated by successively adding the amplitudes of each EPP during the stimulation period to provide an estimate of total neurotransmitter release in the neuromuscular junction. The cumulative EPP amplitude was significantly lower at 20°C than it was at 37°C during both 20 and 50 Hz stimulation. These data indicate that the mechanism involved in EPP amplitude rundown and recovery is temperature-sensitive, with a greater decrement in EPP amplitude at cold than at warm temperatures.