Kinetics of both synchronous and asynchronous quantal release during trains of action potential-evoked EPSCs at the rat calyx of Held

Two forms of asynchronous release with distinctive spatiotemporal dynamics in central synapses

Asynchronous release is a ubiquitous form of neurotransmitter release that persists for tens to hundreds of milliseconds after an action potential. How asynchronous release is organized and regulated at the synaptic active zone (AZ) remains debatable. Using nanoscale-precision imaging of individual release events in rat hippocampal synapses, we observed two spatially distinct subpopulations of asynchronous events, ~75% of which occurred inside the AZ and with a bias towards the AZ center, while ~25% occurred outside of the functionally defined AZ, that is, ectopically. The two asynchronous event subpopulations also differed from each other in temporal properties, with ectopic events occurring at significantly longer time intervals from synchronous events than the asynchronous events inside the AZ. Both forms of asynchronous release did not, to a large extent, utilize the same release sites as synchronous events. The two asynchronous event subpopulations also differ from synchronous events in some aspects of exo-endocytosis coupling, particularly in the contribution from the fast calcium-dependent endocytosis. These results identify two subpopulations of asynchronous release events with distinctive organization and spatiotemporal dynamics.

Complexins: Ubiquitously Expressed Presynaptic Regulators of SNARE-Mediated Synaptic Vesicle Fusion

Neurotransmitter release is a spatially and temporally tightly regulated process, which requires assembly and disassembly of SNARE complexes to enable the exocytosis of transmitter-loaded synaptic vesicles (SVs) at presynaptic active zones (AZs). While the requirement for the core SNARE machinery is shared by most membrane fusion processes, SNARE-mediated fusion at AZs is uniquely regulated to allow very rapid Ca2+-triggered SV exocytosis following action potential (AP) arrival. To enable a sub-millisecond time course of AP-triggered SV fusion, synapse-specific accessory SNARE-binding proteins are required in addition to the core fusion machinery. Among the known SNARE regulators specific for Ca2+-triggered SV fusion are complexins, which are almost ubiquitously expressed in neurons. This chapter summarizes the structural features of complexins, models for their molecular interactions with SNAREs, and their roles in SV fusion.

A sequential two-step priming scheme reproduces diversity in synaptic strength and short-term plasticity

Central nervous system synapses are diverse in strength and plasticity. Short-term plasticity has traditionally been evaluated with models postulating a single pool of functionally homogeneous fusion-competent synaptic vesicles. Many observations are not easily explainable by such simple models. We established and experimentally validated a scheme of synaptic vesicle priming consisting of two sequential and reversible steps of release–machinery assembly. This sequential two-step priming scheme faithfully reproduced plasticity at a glutamatergic model synapse. The proposed priming and fusion scheme was consistent with the measured mean responses and with the experimentally observed heterogeneity between synapses. Vesicle fusion probability was found to be relatively uniform among synapses, while the priming equilibrium at rest of mature versus immature vesicle priming states differed greatly.

Glutamatergic synapses display variable strength and diverse short-term plasticity (STP), even for a given type of connection. Using nonnegative tensor factorization and conventional state modeling, we demonstrate that a kinetic scheme consisting of two sequential and reversible steps of release–machinery assembly and a final step of synaptic vesicle (SV) fusion reproduces STP and its diversity among synapses. Analyzing transmission at the calyx of Held synapses reveals that differences in synaptic strength and STP are not primarily caused by variable fusion probability (p_fusion) but are determined by the fraction of docked synaptic vesicles equipped with a mature release machinery. Our simulations show that traditional quantal analysis methods do not necessarily report p_fusion of SVs with a mature release machinery but reflect both p_fusion and the distribution between mature and immature priming states at rest. Thus, the approach holds promise for a better mechanistic dissection of the roles of presynaptic proteins in the sequence of SV docking, two-step priming, and fusion. It suggests a mechanism for activity-induced redistribution of synaptic efficacy.

Postnatal maturation of glutamate clearance and release kinetics at the rat and mouse calyx of Held synapses

Although calyx of Held synapses undergo dramatic changes around the hearing onset, previous in vivo studies suggest that the calyx synapses undergo further post-hearing maturation process. While developmental changes over the hearing onset have been extensively studied, this post-hearing maturation process remained relatively little investigated. Because of post-hearing maturation, previous results from studies around hearing onset and studies of post-hearing calyx synapses are somewhat inconsistent. Here, we characterized the post-hearing maturation of calyx synapses with regard to in vitro electrophysiological properties in rats and mice. We found that parameters for residual glutamate in the cleft during a train, EPSC kinetics, and vesicle pool size became close to a full mature level by P14, but they further matured until P16 in the rats. Consistently, the phasic and slow EPSCs evoked by action potential trains at P16 calyx synapses were not different from those at P18 or P25 under physiological extracellular [Ca²⁺ ]_o (1.2 mM). In contrast, the parameters for residual current and EPSC kinetics displayed drastic changes until P16 in mice, and slow EPSCs during the train further decreased between P16 and P18, suggesting that maturation of calyx synapses progresses at least up to P16 in rats and P18 in mice.

Short-term synaptic depression can increase the rate of information transfer at a release site

The release of neurotransmitters from synapses obeys complex and stochastic dynamics. Depending on the recent history of synaptic activation, many synapses depress the probability of releasing more neurotransmitter, which is known as synaptic depression. Our understanding of how synaptic depression affects the information efficacy, however, is limited. Here we propose a mathematically tractable model of both synchronous spike-evoked release and asynchronous release that permits us to quantify the information conveyed by a synapse. The model transits between discrete states of a communication channel, with the present state depending on many past time steps, emulating the gradual depression and exponential recovery of the synapse. Asynchronous and spontaneous releases play a critical role in shaping the information efficacy of the synapse. We prove that depression can enhance both the information rate and the information rate per unit energy expended, provided that synchronous spike-evoked release depresses less (or recovers faster) than asynchronous release. Furthermore, we explore the theoretical implications of short-term synaptic depression adapting on longer time scales, as part of the phenomenon of metaplasticity. In particular, we show that a synapse can adjust its energy expenditure by changing the dynamics of short-term synaptic depression without affecting the net information conveyed by each successful release. Moreover, the optimal input spike rate is independent of the amplitude or time constant of synaptic depression. We analyze the information efficacy of three types of synapses for which the short-term dynamics of both synchronous and asynchronous release have been experimentally measured. In hippocampal autaptic synapses, the persistence of asynchronous release during depression cannot compensate for the reduction of synchronous release, so that the rate of information transmission declines with synaptic depression. In the calyx of Held, the information rate per release remains constant despite large variations in the measured asynchronous release rate. Lastly, we show that dopamine, by controlling asynchronous release in corticostriatal synapses, increases the synaptic information efficacy in nucleus accumbens.

Fatigue is an intrinsic property of living systems and synapses are no exception. Synaptic depression reduces the ability of synapses to release vesicles in response to an incoming action potential. Whether synaptic depression simply reflects the exhaustion of neuronal resources or whether it serves some additional function is still an open question. We ask how synaptic depression modulates the information transfer between neurons by keeping the synapse in an appropriate operating range. Using a tractable mathematical model for synaptic depression of both synchronous spike-evoked and asynchronous release of neurotransmitter, we derive a closed-form expression for the mutual information rate. Depression, it turns out, can both enhance or impair information transfer, depending on the relative level of depression for synchronous spike-evoked and asynchronous releases. We also study the compromise a synapse makes between its energy consumption and the rate of information transmission. Interestingly, we show that synaptic depression can regulate energy use without affecting the information (measured in bits) per synaptic release. By applying our mathematical framework to experimentally measured synapses, we show that some synapses can compensate for intrinsic variability in asynchronous release rates; moreover, we show how neuromodulators such as dopamine act to improve the information transmission rate.

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

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

Complexin stabilizes newly primed synaptic vesicles and prevents their premature fusion at the mouse calyx of held synapse

Complexins (Cplxs) are small synaptic proteins that cooperate with SNARE-complexes in the control of synaptic vesicle (SV) fusion. Studies involving genetic mutation, knock-down, or knock-out indicated two key functions of Cplx that are not mutually exclusive but cannot easily be reconciled, one in facilitating SV fusion, and one in "clamping" SVs to prevent premature fusion. Most studies on the role of Cplxs in mammalian synapse function have relied on cultured neurons, heterologous expression systems, or membrane fusion assays in vitro, whereas little is known about the function of Cplxs in native synapses. We therefore studied consequences of genetic ablation of Cplx1 in the mouse calyx of Held synapse, and discovered a developmentally exacerbating phenotype of reduced spontaneous and evoked transmission but excessive asynchronous release after stimulation, compatible with combined facilitating and clamping functions of Cplx1. Because action potential waveforms, Ca(2+) influx, readily releasable SV pool size, and quantal size were unaltered, the reduced synaptic strength in the absence of Cplx1 is most likely a consequence of a decreased release probability, which is caused, in part, by less tight coupling between Ca(2+) channels and docked SV. We found further that the excessive asynchronous release in Cplx1-deficient calyces triggered aberrant action potentials in their target neurons, and slowed-down the recovery of EPSCs after depleting stimuli. The augmented asynchronous release had a delayed onset and lasted hundreds of milliseconds, indicating that it predominantly represents fusion of newly recruited SVs, which remain unstable and prone to premature fusion in the absence of Cplx1.

Ca2+ channel to synaptic vesicle distance accounts for the readily releasable pool kinetics at a functionally mature auditory synapse

Precise regulation of synaptic vesicle (SV) release at the calyx of Held is critical for auditory processing. At the prehearing calyx of Held, synchronous and asynchronous release is mediated by fast and slow releasing SVs within the readily releasable pool (RRP). However, the posthearing calyx has dramatically different release properties. Whether developmental alterations in RRP properties contribute to the accelerated release time course found in posthearing calyces is not known. To study these questions, we performed paired patch-clamp recordings, deconvolution analysis, and numerical simulations of buffered Ca(2+) diffusion and SV release in postnatal day (P) 16-19 mouse calyces, as their release properties resemble mature calyces of Held. We found the P16-P19 calyx RRP consists of two pools: a fast pool (τ ≤ 0.9 ms) and slow pool (τ ∼4 ms), in which release kinetics and relative composition of the two pools were unaffected by 5 mm EGTA. Simulations of SV release from the RRP revealed that two populations of SVs were necessary to reproduce the experimental release rates: (1) SVs located close (∼5-25 nm) and (2) more distal (25-100 nm) to VGCC clusters. This positional coupling was confirmed by experiments showing 20 mm EGTA preferentially blocked distally coupled SVs. Lowering external [Ca(2+)] to in vivo levels reduced only the fraction SVs released from the fast pool. Therefore, we conclude that a dominant parameter regulating the mature calyx RRP release kinetics is the distance between SVs and VGCC clusters.

Different types of retinal inhibition have distinct neurotransmitter release properties

Neurotransmitter release varies between neurons due to differences in presynaptic mechanisms such as Ca(2+) sensitivity and timing. Retinal rod bipolar cells respond to brief dim illumination with prolonged glutamate release that is tuned by the differential release of GABA and glycine from amacrine cells in the inner retina. To test if differences among types of GABA and glycine release are due to inherent amacrine cell release properties, we directly activated amacrine cell neurotransmitter release by electrical stimulation. We found that the timing of electrically evoked inhibitory currents was inherently slow and that the timecourse of inhibition from slowest to fastest was GABAC receptors > glycine receptors > GABAA receptors. Deconvolution analysis showed that the distinct timing was due to differences in prolonged GABA and glycine release from amacrine cells. The timecourses of slow glycine release and GABA release onto GABAC receptors were reduced by Ca(2+) buffering with EGTA-AM and BAPTA-AM, but faster GABA release on GABAA receptors was not, suggesting that release onto GABAA receptors is tightly coupled to Ca(2+). The differential timing of GABA release was detected from spiking amacrine cells and not nonspiking A17 amacrine cells that form a reciprocal synapse with rod bipolar cells. Our results indicate that release from amacrine cells is inherently asynchronous and that the source of nonreciprocal rod bipolar cell inhibition differs between GABA receptors. The slow, differential timecourse of inhibition may be a mechanism to match the prolonged rod bipolar cell glutamate release and provide a way to temporally tune information across retinal pathways.

Vesicles derived via AP-3-dependent recycling contribute to asynchronous release and influence information transfer

Action potentials trigger synchronous and asynchronous neurotransmitter release. Temporal properties of both types of release could be altered in an activity-dependent manner. While the effects of activity-dependent changes in synchronous release on postsynaptic signal integration have been studied, the contribution of asynchronous release to information transfer during natural stimulus patterns is unknown. Here we find that during trains of stimulations, asynchronous release contributes to the precision of action potential firing. Our data show that this form of release is selectively diminished in AP-3b2 KO animals, which lack functional neuronal AP-3, an adaptor protein regulating vesicle formation from endosomes generated during bulk endocytosis. We find that in the absence of neuronal AP-3, asynchronous release is attenuated and the activity-dependent increase in the precision of action potential timing is compromised. Lack of asynchronous release decreases the capacity of synaptic information transfer and renders synaptic communication less reliable in response to natural stimulus patterns.

Biophysical properties of presynaptic short-term plasticity in hippocampal neurons: insights from electrophysiology, imaging and mechanistic models

Hippocampal neurons show different types of short-term plasticity (STP). Some exhibit facilitation of their synaptic responses and others depression. In this review we discuss presynaptic biophysical properties behind heterogeneity in STP in hippocampal neurons such as alterations in vesicle priming and docking, fusion, neurotransmitter filling and vesicle replenishment. We look into what types of information electrophysiology, imaging and mechanistic models have given about the time scales and relative impact of the different properties on STP with an emphasis on the use of mechanistic models as complementary tools to experimental procedures. Taken together this tells us that it is possible for a multitude of different mechanisms to underlie the same STP pattern, even though some are more important in specific cases, and that mechanistic models can be used to integrate the biophysical properties to see which mechanisms are more important in specific cases of STP.

Kinetics of neurotransmitter release in neuromuscular synapses of newborn and adult rats

The kinetics of the phasic synchronous and delayed asynchronous release of acetylcholine quanta was studied at the neuromuscular junctions of aging rats from infant to mature animals at various frequencies of rhythmic stimulation of the motor nerve. We found that in infants 6 (P6) and 10 (P10) days after birth a strongly asynchronous phase of quantal release was observed, along with a reduced number of quanta compared to the synapses of adults. The rise time and decay of uni-quantal end-plate currents were significantly longer in infant synapses. The presynaptic immunostaining revealed that the area of the synapses in infants was significantly (up to six times) smaller than in mature junctions. The intensity of delayed asynchronous release in infants increased with the frequency of stimulation more than in adults. A blockade of the ryanodine receptors, which can contribute to the formation of delayed asynchronous release, had no effect on the kinetics of delayed secretion in the infants unlike synapses of adults. Therefore, high degree of asynchrony of quantal release in infants is not associated with the activity of ryanodine receptors and with the liberation of calcium ions from intracellular calcium stores.

Estimation of the time course of neurotransmitter release at central synapses from the first latency of postsynaptic currents

Measurement of the release time course (RTC) and of the quantal content is important for quantifying synaptic precision and understanding the molecular basis of the release process at central synapses. In theory, the RTC can be determined directly from the histogram of first latencies of quantal events only if a maximum of one vesicle is released per trial, but at most synapses multiple vesicles are released. Traditionally, first latency histograms have been corrected for multiple releases using a simple correction, derived by Barrett and Stevens (BS; 1972b) for quantifying release at the neuromuscular junction. This correction has also been used to quantify release at central synapses. We show, by combining an analytical approach and numerical simulations of stochastic quantal release, that the BS correction gives a biased estimate for RTC and quantal content. The bias increases with release probability, and is therefore particularly problematic for central synapses. We show that this is due to assuming infinite availability of releasable vesicles and we derive a formula for estimating the RTC from first latencies without this assumption. The resulting ‘binomial correction’ requires knowledge of the maximum number of quanta that can be released following an action potential (N), which can be estimated with variance-mean analysis. We show with simulations that estimating RTC and quantal content from first latencies using the binomial correction is robust in the presence of noise and when release probability is non-uniform. We also provide an alternative method for estimating RTC from the first latencies when N cannot be determined.

Desynchronization of multivesicular release enhances Purkinje cell output

The release of neurotransmitter-filled vesicles after action potentials occurs with discrete time courses: submillisecond phasic release that can be desynchronized by activity followed by "delayed release" that persists for tens of milliseconds. Delayed release has a well-established role in synaptic integration, but it is not clear whether desynchronization of phasic release has physiological consequences. At the climbing fiber to Purkinje cell synapse, the synchronous fusion of multiple vesicles is critical for generating complex spikes. Here we show that stimulation at physiological frequencies drives the temporal dispersion of vesicles undergoing multivesicular release, resulting in a slowing of the EPSC on the millisecond timescale. Remarkably, these changes in EPSC kinetics robustly alter the Purkinje cell complex spike in a manner that promotes axonal propagation of individual spikelets. Thus, desynchronization of multivesicular release enhances the precise and efficient information transfer by complex spikes.

Synaptotagmin increases the dynamic range of synapses by driving Ca²+-evoked release and by clamping a near-linear remaining Ca²+ sensor

Ca²+-evoked transmitter release shows a high dynamic range over spontaneous release. We investigated the role of the Ca²+ sensor protein, Synaptotagmin2 (Syt2), in both spontaneous and Ca²+-evoked release under direct control of presynaptic [Ca²+](i), using an in vivo rescue approach at the calyx of Held. Re-expression of Syt2 rescued the highly Ca²+ cooperative release and suppressed the elevated spontaneous release seen in Syt2 KO synapses. This latter release clamping function was partially mediated by the poly-lysine motif of the C₂B domain. Using an aspartate mutation in the C₂B domain (D364N) in which Ca²+ triggering was abolished but release clamping remained intact, we show that Syt2 strongly suppresses the action of another, near-linear Ca²+ sensor that mediates release over a wide range of [Ca²+](i). Thus, Syt2 increases the dynamic range of synapses by driving release with a high Ca²+ cooperativity, as well as by suppressing a remaining, near-linear Ca²+ sensor.

Regulation of transmitter release by Ca(2+) and synaptotagmin: insights from a large CNS synapse

Transmitter release at synapses is driven by elevated intracellular Ca(2+) concentration ([Ca(2+)](i)) near the sites of vesicle fusion. [Ca(2+)](i) signals of profoundly different amplitude and kinetics drive the phasic release component during a presynaptic action potential, and asynchronous release at later times. Studies using direct control of [Ca(2+)](i) at a large glutamatergic terminal, the calyx of Held, have provided significant insight into how intracellular Ca(2+) regulates transmitter release over a wide concentration range. Synaptotagmin-2 (Syt2), the major isoform of the Syt1/2 Ca(2+) sensors at these synapses, triggers highly Ca(2+)-cooperative release above 1μM [Ca(2+)](i), but suppresses release at low [Ca(2+)](i). Thus, neurons utilize a highly sophisticated release apparatus to maximize the dynamic range of Ca(2+)-evoked versus spontaneous release.

Modelling vesicular release at hippocampal synapses

We study local calcium dynamics leading to a vesicle fusion in a stochastic, and spatially explicit, biophysical model of the CA3-CA1 presynaptic bouton. The kinetic model for vesicle release has two calcium sensors, a sensor for fast synchronous release that lasts a few tens of milliseconds and a separate sensor for slow asynchronous release that lasts a few hundred milliseconds. A wide range of data can be accounted for consistently only when a refractory period lasting a few milliseconds between releases is included. The inclusion of a second sensor for asynchronous release with a slow unbinding site, and thereby a long memory, affects short-term plasticity by facilitating release. Our simulations also reveal a third time scale of vesicle release that is correlated with the stimulus and is distinct from the fast and the slow releases. In these detailed Monte Carlo simulations all three time scales of vesicle release are insensitive to the spatial details of the synaptic ultrastructure. Furthermore, our simulations allow us to identify features of synaptic transmission that are universal and those that are modulated by structure.

Chemical synaptic transmission in neurons takes place when a neurotransmitter released from a nerve terminal of the presynaptic neuron signals to the postsynaptic neuron that an event has occurred. The goal of our research was to model the release at a type of synapse found in the hippocampus, a part of the brain that is involved with learning and memory. The synapse model was simulated in a computer that kept track of all of the important molecules in the nerve terminal. The model led to a better understanding of the extant experimental data including exact conditions that lead to the release of a single packet of neurotransmitter. According to our model, the release of more than one packet can be triggered by a single presynaptic event but the packets are released one at a time. Furthermore, we uncovered the mechanisms underlying an extremely fast form of release that had not been previously studied. The model made predictions for other properties of the synapse that can be tested experimentally. A better understanding of how the normal synapses in the hippocampus work will help us to better understand what goes wrong with synapses in mental disorders such as depression and schizophrenia.

Desynchronization of neocortical networks by asynchronous release of GABA at autaptic and synaptic contacts from fast-spiking interneurons

Networks of specific inhibitory interneurons regulate principal cell firing in several forms of neocortical activity. Fast-spiking (FS) interneurons are potently self-inhibited by GABAergic autaptic transmission, allowing them to precisely control their own firing dynamics and timing. Here we show that in FS interneurons, high-frequency trains of action potentials can generate a delayed and prolonged GABAergic self-inhibition due to sustained asynchronous release at FS-cell autapses. Asynchronous release of GABA is simultaneously recorded in connected pyramidal (P) neurons. Asynchronous and synchronous autaptic release show differential presynaptic Ca(2+) sensitivity, suggesting that they rely on different Ca(2+) sensors and/or involve distinct pools of vesicles. In addition, asynchronous release is modulated by the endogenous Ca(2+) buffer parvalbumin. Functionally, asynchronous release decreases FS-cell spike reliability and reduces the ability of P neurons to integrate incoming stimuli into precise firing. Since each FS cell contacts many P neurons, asynchronous release from a single interneuron may desynchronize a large portion of the local network and disrupt cortical information processing.

Bidirectional neuron-glia interactions triggered by deficiency of glutamate uptake at spinal sensory synapses

Bidirectional interactions between neurons and glial cells are crucial to the genesis of pathological pain. The mechanisms regulating these interactions and the role of this process in relaying synaptic input in the spinal dorsal horn remain to be established. We studied the role of glutamate transporters in the regulation of such interactions. On pharmacological blockade of glutamate transporters, slow inward currents (SICs) appeared spontaneously and/or were evoked by peripheral synaptic input in the spinal superficial dorsal horn neurons, including the spinothalamic tract neurons. We showed that the SICs were induced by the release of glutamate from glial cells. On inhibition of glutamate uptake, the stimulation-induced, synaptically released glutamate activated glial cells and caused glial cells to release glutamate. Glial-derived glutamate acted on extrasynaptic N-methyl-d-aspartate (NMDA) receptors mainly composed of NR2B receptors and generated SICs, which led to depolarization and action potential generation in superficial spinal dorsal horn neurons. Thus glutamate transporters regulate glutamatergic neuron-glia interactions at spinal sensory synapses. When glutamate uptake is impaired, glial cells function like excitatory interneurons-they are activated by peripheral synaptic input and release glutamate to activate postsynaptic neurons in spinal pain pathways.

A comparison of deterministic and stochastic simulations of neuronal vesicle release models

We study the calcium-induced vesicle release into the synaptic cleft using a deterministic algorithm and MCell, a Monte Carlo algorithm that tracks individual molecules. We compare the average vesicle release probability obtained using both algorithms and investigate the effect of the three main sources of noise: diffusion, sensor kinetics and fluctuations from the voltage-dependent calcium channels (VDCCs). We find that the stochastic opening kinetics of the VDCCs are the main contributors to differences in the release probability. Our results show that the deterministic calculations lead to reliable results, with an error of less than 20%, when the sensor is located at least 50 nm from the VDCCs, corresponding to microdomain signaling. For smaller distances, i.e. nanodomain signaling, the error becomes larger and a stochastic algorithm is necessary.

Mechanisms of short-term plasticity at neuromuscular active zones of Drosophila

DURING SHORT BURSTS OF NEURONAL ACTIVITY, CHANGES IN THE EFFICACY OF NEUROTRANSMITTER RELEASE ARE GOVERNED PRIMARILY BY TWO COUNTERACTING PROCESSES: (1) Ca(2+)-dependent elevations of vesicle release probability and (2) depletion of synaptic vesicles. The dynamic interplay of both processes contributes to the expression of activity-dependent synaptic plasticity. Here, we exploited various facets of short-term plasticity at the Drosophila neuromuscular junction to dissect these two processes. This enabled us to rigorously analyze different models of synaptic vesicle pools in terms of their size and mobilization properties. Independent of the specific model, we estimate approximately 300 readily releasable vesicles with an average release probability of approximately 50% in 1 mM extracellular calcium ( approximately 5% in 0.4 mM extracellular calcium) under resting conditions. The models also helped interpreting the altered short-term plasticity of the previously reported mutant of the active zone component Bruchpilot (BRP). Finally, our results were independently confirmed through fluctuation analysis. Our data reveal that the altered short-term plasticity observed in BRP mutants cannot be accounted for by delocalized Ca(2+) channels alone and thus suggest an additional role of BRP in short-term plasticity.

Synaptotagmin has an essential function in synaptic vesicle positioning for synchronous release in addition to its role as a calcium sensor

A multitude of synaptic proteins interact at the active zones of nerve terminals to achieve the high temporal precision of neurotransmitter release in synchrony with action potentials. Though synaptotagmin has been recognized as the Ca2+ sensor for synchronous release, it may have additional roles of action. We address this question at the calyx of Held, a giant presynaptic terminal, that allows biophysical dissection of multiple roles of molecules in synaptic transmission. Using high-level expression recombinant adenoviruses, in conjunction with a stereotactic surgery in postnatal day 1 rats, we overcame the previous inability to molecular perturb the calyx by overexpression of a mutated synaptotagmin. We report that this mutation leaves intrinsic Ca2+ sensitivity of vesicles intact while it destabilizes the readily releasable pool of vesicles and loosens the tight coupling between Ca2+ influx and release, most likely by interfering with the correct positioning of vesicles with respect to Ca2+ channels.

Inhibitory regulation of electrically coupled neurons in the inferior olive is mediated by asynchronous release of GABA

Inhibitory projection neurons in the deep cerebellar nuclei (DCN) provide GABAergic input to neurons of the inferior olive (IO) that in turn produce climbing fiber synapses onto Purkinje cells. Anatomical evidence suggests that DCN to IO synapses control electrical coupling between IO neurons. In vivo studies suggest that they also control the synchrony of IO neurons and play an important role in cerebellar learning. Here we describe the DCN to IO synapse. Remarkably, GABA release was almost exclusively asynchronous, with little conventional synchronous release. Synaptic transmission was extremely frequency dependent, with low-frequency stimulation being largely ineffective. However, due to the prominence of asynchronous release, stimulation at frequencies above 10 Hz evoked steady-state inhibitory currents. These properties seem ideally suited to transform the firing rate of DCN neurons into sustained inhibition that both suppresses the firing of IO cells and regulates the effective coupling between IO neurons.

Fluctuation analysis of tetanic rundown (short-term depression) at a corticothalamic synapse

Hypothetical scenarios for "tetanic rundown" ("short-term depression") of synaptic signals evoked by stimulus trains differ in evolution of quantal amplitude (Q) and covariances between signals. With corticothalamic excitatory postsynaptic currents (EPSCs) evoked by 2.5- to 20-Hz trains, we found Q (estimated using various corrections of variance/mean ratios) to be unchanged during rundown and close to the size of stimulus-evoked "miniatures". Except for covariances, results were compatible with a depletion model, according to which incomplete "refill" after probabilistic quantal release entails release-site "emptying". For five neurons with 20 train repetitions at each frequency, there was little between-neuron variation of rundown; pool-refill rate increased with stimulus frequency and evolved during rundown. Covariances did not fit the depletion model or theoretical alternatives, being excessively negative for adjacent EPSCs early in trains, absent at equilibrium, and anomalously positive for some nonadjacent EPSCs. The anomalous covariances were unaltered during pharmacological blockade of receptor desensitization and saturation. These findings suggest that pool-refill rate and release probability at each release site are continually modulated by antecedent outputs in its neighborhood, possibly via feedback mechanisms. In all data sets, sampling errors for between-train variances were much less than theoretical, warranting reconsideration of the probabilistic nature of quantal transmitter release.

Synaptic vesicles in mature calyx of Held synapses sense higher nanodomain calcium concentrations during action potential-evoked glutamate release

During development of the calyx of Held synapse, presynaptic action potentials (APs) become substantially faster and briefer. Nevertheless, this synapse is able to upregulate quantal output triggered by arriving APs. Briefer APs lead to less effective gating of voltage-gated Ca(2+) channels (VGCCs). Therefore, mechanisms downstream of Ca(2+) entry must effectively compensate for the attenuated Ca(2+) influx associated with shorter APs in more mature calyces. This compensation could be achieved by tighter spatial coupling between VGCCs and synaptic vesicles, so that the latter are exposed to higher intracellular Ca(2+) concentration ([Ca(2+)](i)). Alternatively or additionally, the Ca(2+) sensitivity of the release apparatus may increase during synapse development. To differentiate between these possibilities, we combined paired patch-clamp recordings with Ca(2+) imaging and flash photolysis of caged Ca(2+) and estimated the [Ca(2+)](i) requirements for vesicle release in the developing mouse calyx of Held synapse. Surprisingly, the dose-response relationship between [Ca(2+)](i) and release rate was shifted slightly to the right in more mature calyces, rendering their vesicles slightly less sensitive to incoming Ca(2+). Taking into account the time course and peak rates of AP-evoked release transients for the corresponding developmental stages, we estimate the local [Ca(2+)](i)"seen" by the Ca(2+) sensors on synaptic vesicles to increase from 35 to 56 mum [from postnatal day 9 (P9)-P11 to P16-P19]. Our results reinforce the idea that developmental tightening of the spatial coupling between VGCCs and synaptic vesicles plays a predominant role in enhancing quantal output at this synapse and possibly other central synapses.