Quantitative relationship between transmitter release and calcium current at the calyx of held synapse

Augmentation of corticogeniculate EPSCs in principal cells of the dorsal lateral geniculate nucleus of the rat investigated in vitro

Augmentation is a component of short-term synaptic plasticity with a gradual onset and duration in seconds. To investigate this component at the corticogeniculate synapse, whole cell patch-clamp recordings were obtained from principal cells in a slice preparation of the rat dorsal lateral geniculate nucleus. Trains with 10 stimuli at 25 Hz evoked excitatory postsynaptic currents (EPSCs) that grew in amplitude, primarily from facilitation. Such trains also induced augmentation that decayed exponentially with a time constant tau= 4.6 +/- 2.6 s (mean +/- standard deviation). When the trains were repeated at 1-10 s intervals, augmentation markedly increased the size of the first EPSCs, leaving late EPSCs unaffected. The magnitude of augmentation was dependent on the number of pulses, pulse rate and intervals between trains. Augmented EPSCs changed proportionally to basal EPSC amplitudes following alterations in extracellular calcium ion concentration. The results indicate that augmentation is determined by residual calcium remaining in the presynaptic terminal after repetitive spikes, competing with fast facilitation. We propose that augmentation serves to maintain a high synaptic strength in the corticogeniculate positive feedback system during attentive visual exploration.

Dynamic properties of corticogeniculate excitatory transmission in the rat dorsal lateral geniculate nucleus in vitro

The feedback excitation from the primary visual cortex to principal cells in the dorsal lateral geniculate nucleus (dLGN) is markedly enhanced with firing frequency. This property presumably reflects the ample short-term plasticity at the corticogeniculate synapse. The present study aims to explore corticogeniculate excitatory postsynaptic currents (EPSCs) evoked by brief trains of stimulation with whole-cell patch-clamp recordings in dLGN slices from DA-HAN rats. The EPSCs rapidly increased in amplitude with the first two or three impulses followed by a more gradual growth. A double exponential function with time constants 39 and 450 ms empirically described the growth for 5-25Hz trains. For lower train frequencies (down to 1Hz) a third component with time constant 4.8 s had to be included. The different time constants are suggested to represent fast and slow components of facilitation and augmentation. The time constant of the fast component changed with the extracellular calcium ion concentration as expected for a facilitation mechanism involving an endogenous calcium buffer that is more efficiently saturated with larger calcium influx. Concerning the function of the corticogeniculate feedback pathway, the different components of short-term plasticity interacted to increase EPSC amplitudes on a linear scale to firing frequency in the physiological range. This property makes the corticogeniculate synapse well suited to function as a neuronal amplifier that enhances the thalamic transfer of visual information to the cortex.

Combining deconvolution and fluctuation analysis to determine quantal parameters and release rates

Analysis methods are described which integrate information from fluctuation analysis with that from deconvolution. Together the two approaches allow to derive a consistent quantitative description of quantal release (both evoked, spontaneous and asynchronous) under conditions in which quantal parameters may change during a repetitively applied stimulation protocol. Specifically, our methods take into account the effects of accumulating transmitter in the synaptic cleft and postsynaptic receptor desensitization, which may develop during strong stimulation. Several ways to handle non-stationarities are described. Examples are provided for the Calyx of Held, a glutamatergic synapse, in which both the pre- and the postsynaptic compartments can be voltage-clamped.

Two modes of vesicle recycling in the rat calyx of Held

Vesicle recycling was studied in the rat calyx of Held, a giant brainstem terminal involved in sound localization. Stimulation of brain slices containing the calyx-type synapse with a high extracellular potassium ion concentration in the presence of horseradish peroxidase resulted within several minutes in a reduction of the number of neurotransmitter vesicles and in the appearance of labeled endosome-like structures. After returning to normal solution, the endosome-like structures disappeared over a period of several minutes, whereas simultaneously the number of labeled vesicles increased. A comparison with afferent stimulation suggested that the endosome-like structures normally do not participate in the vesicle cycle. Afferent stimulation at 5 Hz resulted in sustained synaptic transmission, without vesicle depletion but with an estimated endocytotic activity of <0.2 synaptic vesicles per active zone per second. At 20 Hz, the presynaptic action potentials generally failed during prolonged stimulation. In identified synapses, the number of vesicles labeled by photoconversion after stimulation at 5 Hz in the presence of the styryl dye RH414 was much lower than the number of vesicles that were released, as determined by measuring EPSCs. No more than approximately 5% of the vesicles were labeled after 20 min stimulation at 5 Hz, whereas this stimulation protocol was sufficient to largely destain a terminal after previous loading. The results support a scheme for recycling in which two different modes coexist. At physiological demands, a pool of approximately 5% of all vesicles provides sufficient vesicles for release. During intense stimulation, such as occurs in the presence of high extracellular K+, the synapse resorts to bulk endocytosis, a very slow mode of recycling.

Neurotransmitter Release at Central Synapses

Our understanding of synaptic transmission has grown dramatically during the 15 years since the first issue of Neuron was published, a growth rate expected from the rapid progress in modern biology. As in all of biology, new techniques have led to major advances in the cell and molecular biology of synapses, and the subject has evolved in ways (like the production of genetically engineered mice) that could not even be imagined 15 years ago. My plan for this review is to summarize what we knew about neurotransmitter release when Neuron first appeared and what we recognized we did not know, and then to describe how our views have changed in the intervening decade and a half. Some things we knew about synapses--"knew" in the sense that the field had reached a consensus--are no longer accepted, but for the most part, impressive advances have led to a new consensus on many issues. What I find fascinating is that in certain ways nothing has changed--many of the old arguments persist or recur in a different guise--but in other ways the field would be unrecognizable to a neurobiologist time-transported from 1988 to 2003.

Examining synaptotagmin 1 function in dense core vesicle exocytosis under direct control of Ca2+

We tested the long-standing hypothesis that synaptotagmin 1 is the Ca2+ sensor for fast neurosecretion by analyzing the intracellular Ca2+ dependence of large dense-core vesicle exocytosis in a mouse strain carrying a mutated synaptotagmin C2A domain. The mutation (R233Q) causes a twofold increase in the KD of Ca2+-dependent phospholipid binding to the double C2A-C2B domain of synaptotagmin. Using photolysis of caged calcium and capacitance measurements we found that secretion from mutant cells had lower secretory rates, longer secretory delays, and a higher intracellular Ca2+-threshold for secretion due to a twofold increase in the apparent KD of the Ca2+ sensor for fast exocytosis. Single amperometric fusion events were unchanged. We conclude that Ca2+-dependent phospholipid binding to synaptotagmin 1 mirrors the intracellular Ca2+ dependence of exocytosis.

The mechanism of action of aniracetam at synaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors: indirect and direct effects on desensitization

The mechanism of action of aniracetam on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors was examined in outside-out patches and at glutamatergic synapses in neurons of the chick cochlear nucleus. A combination of rapid-flow analysis, using glutamate as an agonist, and kinetic modeling indicated that aniracetam slows both the rate of channel closing, and the microscopic rates of desensitization, even for partially liganded receptors. Little effect was observed on the rate of recovery from desensitization or on the response to the weakly desensitizing agonist kainate. Aniracetam's effects on receptor deactivation saturated at lower concentrations than its effects on desensitization, suggesting that cooperativity between homologous binding sites was required to regulate desensitization. Analysis of responses to paired pulses of agonist also indicated that AMPA receptors must desensitize partially even after agonist exposures too brief to permit rebinding. In the presence of aniracetam, evoked excitatory synaptic currents (EPSCs) and miniature EPSCs in low quantal-content conditions had decay times similar to the time course of receptor deactivation. Under these conditions, the time course of both transmitter release and clearance must be <1 to 2 ms. However, in high quantal-content conditions, the evoked EPSC in aniracetam decayed with a time course intermediate between deactivation and desensitization, suggesting that the time course of transmitter clearance is prolonged because of pooling of transmitter in the synaptic cleft. Moreover, by comparing the amounts of paired-pulse synaptic depression and patch desensitization prevented by aniracetam, we conclude that significant desensitization occurs in response to rebinding of transmitter to the AMPA receptors.

Presynaptic modulation of the retinogeniculate synapse

Modulatory projections from brainstem nuclei and intrinsic thalamic interneurons play a significant role in modifying sensory information as it is relayed from the thalamus to the cortex. In the lateral geniculate nucleus (LGN), neurotransmitters released from these modulatory inputs can affect the intrinsic conductances of thalamocortical relay neurons, thus altering their firing properties. Here, we show that in addition to postsynaptic effects, neuromodulators such as serotonin (5-HT) and GABA can act presynaptically to regulate neurotransmitter release at the synapse between retinal ganglion cells (RGCs) and relay neurons in the LGN, the retinogeniculate synapse. Activation of 5HT1 and GABA(B) receptors significantly decreased EPSC amplitude. This inhibition was accompanied by a decrease in the extent of paired-pulse depression, suggesting that it is presynaptic in origin. In addition, fluorometric calcium measurements from retinal axon terminals labeled with Calcium Green-1 dextran revealed that 5HT1 and GABA(B) receptor agonists decreased presynaptic calcium influx. Taken together, our data indicate that serotonin and GABA can act presynaptically to decrease calcium influx at the retinogeniculate synapse and modify transmission of visual information in the LGN.

Exocytosis at the ribbon synapse of retinal bipolar cells studied in patches of presynaptic membrane

The distribution of exocytic sites and ion channels in the synaptic terminal of retinal bipolar cells was investigated by measuring capacitance and conductance changes in cell-attached patches of presynaptic membrane. Patch depolarization evoked capacitance and conductance increases that were inhibited by blocking Ca(2+) influx or loading the terminal with EGTA. The increase in capacitance declined as the depolarization approached the reversal potential for Ca(2+), indicating that it was a result of Ca(2+)-dependent exocytosis. The conductance increase was caused by K(Ca) channels that were also activated by Ca(2+) influx. Two observations indicated that sites of exocytosis and endocytosis colocalized with clusters of Ca(2+) channels and K(Ca) channels; the initial rate of exocytosis was correlated with the activation of K(Ca) channels, and exocytosis did not occur in the 41% of patches lacking this conductance. Electron microscopy demonstrated that there were approximately 16 vesicles docked to the plasma membrane at each active zone marked by a ribbon, but vesicles were also attached to the rest of the membrane at a density of 1.5/microm(2). The density of ribbons was 0.10 +/- 0.02/microm(2), predicting that approximately 43% of cell-attached patches would lack an active zone. The density of Ca(2+) channel clusters assayed by capacitance and conductance responses was therefore similar to the density of ribbons. These results are consistent with the idea that Ca(2+) channel clusters were colocalized with ribbons but do not exclude the possibility that calcium channels also occurred at other sites. The wide distribution of vesicles docked to the plasma membrane suggests that exocytosis might also be triggered by the spread of Ca(2+) from Ca(2+) channel clusters.

Heterogeneous presynaptic release probabilities: functional relevance for short-term plasticity

We discuss a model of presynaptic vesicle dynamics, which allows for heterogeneity in release probability among vesicles. Specifically, we explore the possibility that synaptic activity is carried by two types of vesicles; first, a readily releasable pool and, second, a reluctantly releasable pool. The pools differ regarding their probability of release and time scales on which released vesicles are replaced by new ones. Vesicles of both pools increase their release probability during repetitive stimulation according to the buildup of Ca(2+) concentration in the terminal. These properties are modeled to fit data from the calyx of Held, a giant synapse in the auditory pathway. We demonstrate that this arrangement of two pools of releasable vesicles can account for a variety of experimentally observed patterns of synaptic depression and facilitation at this synapse. We conclude that synaptic transmission cannot be accurately described unless heterogeneity of synaptic release probability is taken into account.

Involvement of actin polymerization in vesicle recruitment at the calyx of Held synapse

Depletion and replenishment of pools of synaptic vesicles are important determinants of short-term synaptic plasticity, but the underlying molecular mechanisms are not yet clear. As a first step toward understanding the process of vesicle recruitment, we have applied various specific agents directly to the presynaptic terminal of the calyx of Held synapse. Here we show that the nonhydrolyzable ATP analog ATP-gammaS retards the recovery from vesicle pool depletion, as does latrunculin A. Phalloidin has no effects on recovery, suggesting that dynamic actin reorganization is not necessary. Unexpectedly, neither N-ethylmaleimide nor staurosporine affected the recovery, calling into question the role of N-ethylmaleimide-sensitive factor and protein kinases. The results suggest that intact actin polymerization is involved in vesicle recruitment.

Optimizing synaptic architecture and efficiency for high-frequency transmission

Bursts of neuronal activity are transmitted more effectively as synapses mature. However, the mechanisms that control synaptic efficiency during development are poorly understood. Here, we study postnatal changes in synaptic ultrastructure and exocytosis in a calyx-type nerve terminal. Vesicle pool size, exocytotic efficiency (amount of exocytosis per Ca influx), Ca current facilitation, and the number of active zones (AZs) increased with age, whereas AZ area, number of docked vesicles per AZ, and release probability decreased with age. These changes led to AZs that are less prone to multivesicular release, resulting in reduced AMPA receptor saturation and desensitization. A greater multiplicity of small AZs with few docked vesicles, a larger pool of releasable vesicles, and a higher efficiency of release thus promote prolonged high-frequency firing in mature synapses.

Three-dimensional reconstruction of a calyx of Held and its postsynaptic principal neuron in the medial nucleus of the trapezoid body

The three-dimensional morphology of the axosomatic synaptic structures between a calyx of Held and a principal neuron in the medial nucleus of the trapezoid body (MNTB) in the brainstem of young postnatal day 9 rats was reconstructed from serial ultrathin sections. In the apposition zone between the calyx and the principal neuron two types of membrane specializations were identified: synaptic contacts (SCs) with active zones (AZs) and their associated postsynaptic densities (PSDs) constituted approximately 35% (n = 554) of the specializations; the remaining 65% (n = 1010) were puncta adherentia (PA). Synaptic contacts comprised approximately 5% of the apposition area of presynaptic and postsynaptic membranes. A SC had an average area of 0.100 microm(2), and the nearest neighbors were separated, on average, by 0.59 microm. Approximately one-half of the synaptic vesicles in the calyx were clustered within a distance of 200 nm of the AZ membrane area, a cluster consisting of approximately 60 synaptic vesicles (n = 52 SCs). Approximately two synaptic vesicles per SC were "anatomically docked." Comparing the geometry of the synaptic structure with its previously studied functional properties, we find that during a single presynaptic action potential (AP) (1) approximately 35% of the AZs release a transmitter quantum, (2) the number of SCs and anatomically docked vesicles is comparable with the low estimates of the readily releasable pool (RRP) of quanta, and (3) the broad distribution of PSD areas [coefficient of variation (CV) = 0.9] is likely to contribute to the large variability of miniature EPSC peaks. The geometry of the reconstructed synapse suggests that each of the hundreds of SCs is likely to contribute independently to the size and rising phase of the EPSC during a single AP.

Estimation of quantal parameters at the calyx of Held synapse

The calyx of Held has recently emerged as a convenient model system to study CNS synapses. In order to understand the mechanisms of synaptic transmission and short-term synaptic plasticity, quantal parameters and their changes should be estimated precisely. For this purpose, various methods have been applied to the calyx of Held synapse. The results confirm many aspects of the early findings on transmission at the neuromuscular junction. On the other hand, the simplest quantal hypothesis does not work at the calyx of Held, because of additional factors such as heterogeneous release probability of synaptic vesicles, intra- and intersite quantal variability, an overlap of facilitation and depression of transmitter release, changes in quantal sizes due to desensitization and saturation of postsynaptic receptors, and delayed clearance of transmitter from the synaptic cleft. These factors should always be taken into account for fully understanding the mechanisms of synaptic transmission and plasticity.

Limit on the role of activity in controlling the release-ready supply of synaptic vesicles

Typical fast chemical synapses in the brain weaken transiently during normal high-frequency use after expending their presynaptic supply of release-ready vesicles. Although it takes several seconds for the readily releasable pool (RRP) to refill during periods of rest, it has been suggested that the replenishment process may be orders of magnitude faster when synapses are active. Here, we measure this replenishment rate at active Schaffer collateral terminals by determining the maximum rate of release that can still be elicited when the RRP is almost completely exhausted. On average, we find that spent vesicles are replaced at a maximum unitary rate of 0.24/sec during periods of intense activity. Because the replenishment rate is similar during subsequent periods of rest, we conclude that no special mechanism accelerates the mobilization of neurotransmitter in active terminals beyond the previously reported, several-fold, residual calcium-driven modulation that persists for several seconds after bouts of intense synaptic activity. In the course of this analysis, we provide new evidence supporting the hypothesis that a simple enzymatic step limits the rate at which reserve synaptic vesicles become ready to undergo exocytosis.

Emerging roles of presynaptic proteins in Ca++-triggered exocytosis

The twinning of techniques from biophysics and molecular biology has led to remarkable progress in understanding the molecular mechanisms of synaptic transmission. Here we review the current picture of Ca++-triggered exocytosis, which has emerged from studies of a simple cellular model, the adrenal chromaffin cell. We discuss the molecular players that have been assigned a specific role in a particular step of this process and give a brief outlook on what these insights might tell us about mechanisms of short-term plasticity at classical synapses.

Diversification of synaptic strength: presynaptic elements

Synapses are not static; their performance is modified adaptively in response to activity. Presynaptic mechanisms that affect the probability of transmitter release or the amount of transmitter that is released are important in synaptic diversification. Here, we address the diversity of presynaptic performance and its underlying mechanisms: how much of the variation can be accounted for by variation in synaptic morphology and how much by molecular differences? Significant progress has been made in defining presynaptic structural contributions to synaptic strength; by contrast, we know little about how presynaptic proteins produce normally observed functional differentiation, despite abundant information on presynaptic proteins and on the effects of their individual manipulation. Closing the gap between molecular and physiological synaptic diversification still represents a considerable challenge.

Release probability is regulated by the size of the readily releasable vesicle pool at excitatory synapses in hippocampus

Synapses in the central nervous system can be very unreliable: stimulation of an individual synapse by an action potential often does not lead to release of neurotransmitter. The probability of transmitter release is not always the same, however, which enables the average strength of synaptic transmission to be regulated by modulation of release probability. Release probability is believed to be determined by the number of fusion competent vesicles (the readily releasable vesicle pool) and the release probability per vesicle. Studies from single synapses have shown that release probability correlates with the size of the readily releasable pool of vesicles across the population of excitatory CA3-CA1 synapses, both in hippocampal slices and in cultured cells. Here I present evidence that the same relationship exists between release probability and the size of the readily releasable vesicle pool within individual synapses, further suggesting that the size of the readily releasable pool helps determine release probability. In addition, using a simple model, I examine how both the number of readily releasable vesicles and the average release probability per vesicle change during trains of high frequency stimulation, and present evidence for non-uniformity of the release probability among vesicles.

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.

Noradrenaline increases high-frequency firing at the calyx of Held synapse during development by inhibiting glutamate release

The mammalian auditory brain stem receives profuse adrenergic innervation, whose function is poorly understood. Here we investigate, during postnatal development, the effect of noradrenaline (NA) at the calyx of Held synapse in the rat medial nucleus of the trapezoid body (MNTB). We observed that NA inhibits the large glutamatergic EPSC, evoked by afferent fiber stimulation, in a dose-dependent manner. The inhibition was maximal (approximately 48%) at the concentration of 2 microM. It was antagonized by yohimbine and mimicked by the alpha2-adrenergic specific agonist UK14304. Both AMPA and NMDA receptor-mediated EPSCs were inhibited in parallel by NA, suggesting a presynaptic effect. Presynaptic recordings showed that NA inhibits the action potential (AP) generated Ca current by about 20%; however, NA did not significantly affect the presynaptic AP waveform. We thus conclude that the calyx of Held presynaptic terminal expresses alpha2-adrenergic receptors that inhibit its Ca current and thus glutamate release. Noradrenaline was effective in all cells tested from postnatal days 6 to 7 (P6-P7), and thereafter the number of responsive cells diminished, although half of the P14 cells tested still had EPSCs that were inhibited by NA. By contrast, activation by L-2-amino-5-phosphonovaleric acid-sensitive metabotropic glutamate receptors strongly inhibited the EPSCs of all cells tested from P6 to P14. The effect of NA on postsynaptic action potential firing was dependent on the stimulus frequency. At 10 Hz, NA had no effect on firing probability; however, NA helped MNTB cells fire more action potentials during a 100-Hz train of stimuli, even though it did not increase the steady-state depressed EPSC, because it produced a smaller N-methyl-D-aspartate (NMDA) receptor-activated depolarizing plateau. We therefore suggest that the reduction by NA of the first few EPSCs in a train leads to a smaller NMDA depolarizing plateau and thus to increased firing probability at 100 Hz in young synapses. Surprisingly, the inhibition of glutamate release by NA can thus actually increase the excitability of MNTB neurons during early postnatal development.

Vesicle pools and short-term synaptic depression: lessons from a large synapse

Depletion of a pool of readily releasable vesicles during repetitive presynaptic activity is a candidate mechanism for the induction of short-term synaptic depression. The large, calyx-type synaptic terminals in the brainstem auditory pathway, and especially the calyx of Held, offer unique possibilities for studying the cellular mechanisms leading to synaptic depression. Recent work at these synapses using presynaptic whole-cell patch-clamp recordings has revealed a large pool of readily releasable vesicles. During prolonged presynaptic depolarization, vesicles are released in kinetically distinct phases, indicating heterogeneity of release probability between vesicles. Heterogeneity might endow synapses with a rapid phase of depression at the onset of activity, followed by sustained and surprisingly large synaptic strength during the steady-state phase of depression. By influencing the synaptic output during repetitive activity, vesicle pool dynamics are expected to modulate information processing in neuronal networks of the CNS.

Calcium secretion coupling at calyx of Held governed by nonuniform channel-vesicle topography

Phasic transmitter release at synapses in the mammalian CNS is regulated by local [Ca2+] transients, which control the fusion of readily releasable vesicles docked at active zones (AZs) in the presynaptic membrane. The time course and amplitude of these [Ca2+] transients critically determine the time course and amplitude of the release and thus the frequency and amplitude tuning of the synaptic connection. As yet, the spatiotemporal nature of the [Ca2+] transients and the number and location of release-controlling Ca2+ channels relative to the vesicles, the "topography" of the release sites, have remained elusive. We used a time-dependent model to simulate Ca2+ influx, three-dimensional buffered Ca2+ diffusion, and the binding of Ca2+ to the release sensor. The parameters of the model were constrained by recent anatomical and biophysical data of the calyx of Held. Comparing the predictions of the model with previously measured release probabilities under a variety of experimental conditions, we inferred which release site topography is likely to operate at the calyx: At each AZ one or a few clusters of Ca2+ channels control the release of the vesicles. The distance of a vesicle to the cluster(s) varies across the multiple release sites of a single calyx (ranging from 30 to 300 nm; average approximately 100 nm). Assuming this topography, vesicles in different locations are exposed to different [Ca2+] transients, with peak amplitudes ranging from 0.5 to 40 microm (half-width approximately 400 microsec) during an action potential. Consequently the vesicles have different release probabilities ranging from <0.01 to 1. We demonstrate how this spatially heterogeneous release probability creates functional advantages for synaptic transmission.

Separation of presynaptic and postsynaptic contributions to depression by covariance analysis of successive EPSCs at the calyx of Held synapse

Synaptic short-term plasticity is considered to result from multiple cellular mechanisms, which may include presynaptic and postsynaptic contributions. We have recently developed a nonstationary EPSC fluctuation analysis (Scheuss and Neher, 2001) to estimate synaptic parameters and their transient changes during short-term synaptic plasticity. Extending the classical variance-mean approach, a short train of stimuli is applied repetitively, and the resulting EPSCs are analyzed for means, variances, and covariances. This provides estimates of the quantal size and quantal content for each EPSC in the train, and furthermore, an estimate of the number of release sites. The latter is less sensitive to heterogeneity in the release probability than that of the variance-mean approach. Here, we applied this analysis to the calyx of Held synapse in brainstem slices of young rats (postnatal day 8-10). We found significant negative covariance in the amplitude of successive EPSCs in a train. The analysis showed that the 10-fold depression in the EPSC amplitude during 100 Hz trains at elevated extracellular Ca(2+) concentration resulted from a 2.5-fold reduction in quantal size caused by postsynaptic AMPA receptor desensitization and saturation, and a fourfold reduction in quantal content, which was partially relieved by application of cyclothiazide. The number of release sites estimated by covariance analysis was approximately 2000 and significantly larger than estimates from variance-mean parabolas.

Beta phorbol ester- and diacylglycerol-induced augmentation of transmitter release is mediated by Munc13s and not by PKCs

Munc13-1 is a presynaptic protein with an essential role in synaptic vesicle priming. It contains a diacylglycerol (DAG)/beta phorbol ester binding C(1) domain and is a potential target of the DAG second messenger pathway that may act in parallel with PKCs. Using genetically modified mice that express a DAG/beta phorbol ester binding-deficient Munc13-1(H567K) variant instead of the wild-type protein, we determined the relative contribution of PKCs and Munc13-1 to DAG/beta phorbol ester-dependent regulation of neurotransmitter release. We show that Munc13s are the main presynaptic DAG/beta phorbol ester receptors in hippocampal neurons. Modulation of Munc13-1 activity by second messengers via the DAG/beta phorbol ester binding C(1) domain is essential for use-dependent alterations of synaptic efficacy and survival.

Endogenous calcium buffers regulate fast exocytosis in the synaptic terminal of retinal bipolar cells

Calcium-triggered exocytosis at the synapse is suppressed by addition of calcium chelators, but the effects of endogenous Ca(2+) buffers have not been tested. We find that 80% of Ca(2+) binding sites in the synaptic terminal of retinal bipolar cells were associated with mobile molecules that suppressed activation of Ca(2+)-sensitive K(+) channels with an efficiency equivalent to approximately 1.2 mM BAPTA. Removing these buffers caused a 30-fold increase in the number of vesicles released by Ca(2+) tail currents lasting approximately 0.5 ms and a 2-fold increase in the rapidly releasable pool of vesicles (RRP). The effects of BAPTA and EGTA indicate that vesicles comprising the RRP were docked at variable distances from Ca(2+) channels. We propose that endogenous Ca(2+) buffers regulate the size of the RRP by suppressing the release of vesicles toward the periphery of the active zone.

Short-term plasticity at the calyx of Held

Synapses show widely varying degrees of short-term facilitation and depression. Several mechanisms have been proposed to underlie short-term plasticity, but the contributions of presynaptic mechanisms have been particularly difficult to study because of the small size of synaptic boutons in the mammalian brain. Here we review the functional properties of the calyx of Held, a giant nerve terminal that has shed new light on the general mechanisms that control short-term plasticity. The calyx of Held has also provided fresh insights into the strategies used by synapses to extend their dynamic range of operation and preserve the timing of sensory stimuli.

Calmodulin mediates rapid recruitment of fast-releasing synaptic vesicles at a calyx-type synapse

In many synapses, depletion and recruitment of releasable synaptic vesicles contribute to use-dependent synaptic depression and recovery. Recently it has been shown that high-frequency presynaptic stimulation enhances recovery from depression, which may be mediated by Ca2+. We addressed this issue by measuring quantal release rates at the calyx of Held synapse and found that transmission is mediated by a heterogeneous population of vesicles, with one subset releasing rapidly and recovering slowly and another one releasing reluctantly and recovering rapidly. Ca2+ promotes refilling of the rapidly releasing synaptic vesicle pool and calmodulin inhibitors block this effect. We propose that calmodulin-dependent refilling supports recovery from synaptic depression during high-frequency trains in concert with rapid recovery of the slowly releasing vesicles.

Estimating transmitter release rates from postsynaptic current fluctuations

A method is presented that allows one to estimate transmitter release rates from fluctuations of postsynaptic current records under conditions of stationary or slowly varying release. For experimental applications, we used the calyx of Held, a glutamatergic synapse, in which "residual current," i.e., current attributable to residual glutamate in the synaptic cleft, is present. For a characterization of synaptic transmission, several postsynaptic parameters, such as the mean amplitude of the miniature postsynaptic current and an apparent single channel conductance, have to be known. These were obtained by evaluating variance and two more higher moments of the current fluctuations. In agreement with Fesce et al. (1986), we found both by simulations and by analyzing experimental records that high-pass filtering of postsynaptic currents renders the estimates remarkably tolerant against nonstationarities. We also found that release rates and postsynaptic parameters can be reliably obtained when release rates are low ( approximately 10 events/msec). Furthermore, during a long-lasting stimulus, the transmitter release at the calyx of Held was found to decay to a low, stationary rate of 10 events/msec after depletion of the "releasable pool" of synaptic vesicles. This stationary release rate is compatible with the expected rate of recruitment of new vesicles to the release-ready pool of vesicles. MiniatureEPSC (mEPSC) size is estimated to be similar to the value of spontaneously occurring mEPSC under this condition.

Protein kinase c increases the apparent affinity of the release machinery to Ca2+ by enhancing the release machinery downstream of the Ca2+ sensor

Modulation of the release probability of releasable vesicles in response to Ca(2+) influx (Prob(Ca)) is involved in mediating several forms of synaptic plasticity, including short-term depression, short-term augmentation, and potentiation induced by protein kinases. Given such an important role, however, the mechanism underlying modulation of the Prob(Ca) is unclear. We addressed this question by investigating how the activation of protein kinase C modulates the Prob(Ca) at a calyx-type nerve terminal in rat brainstem. Various lengths of step depolarization were applied to the nerve terminal to evoke different amounts of Ca(2+) currents and capacitance jumps, the latter of which reflect vesicle release. The relationship between the capacitance jump and the Ca(2+) current integral was sigmoidal and was fit well with a Hill function. The sigmoidal relationship was shifted significantly to the left during the application of the PKC activator 12-myristate 13-acetate (PMA), suggesting that PMA increases the apparent affinity of the release machinery to Ca(2+). This effect was blocked in large part by the application of the PKC inhibitor bisindolylmaleimide, suggesting that the effect is mediated mainly by the activation of PKC. We also found that PMA increased the rate of miniature EPSCs evoked by the application of hypertonic sucrose solution, which triggers release downstream of the Ca(2+) influx. Taken together, our results suggest that PKC enhances the apparent affinity of the release machinery to Ca(2+) by a mechanism downstream of the binding between Ca(2+) and its sensor. These results have provided the first example of the mechanisms underlying modulation of the Prob(Ca).

Estimation of quantal size and number of functional active zones at the calyx of Held synapse by nonstationary EPSC variance analysis

At the large excitatory calyx of Held synapse, the quantal size during an evoked EPSC and the number of active zones contributing to transmission are not known. We developed a nonstationary variant of EPSC fluctuation analysis to determine these quantal parameters. AMPA receptor-mediated EPSCs were recorded in slices of young (postnatal 8-10 d) rats after afferent fiber stimulation, delivered in trains to induce synaptic depression. The means and the variances of EPSC amplitudes were calculated across trains for each stimulus number. During 10 Hz trains at 2 mm Ca(2+) concentration ([Ca(2+)]), we found linear EPSC variance-mean relationships, with a slope that was in good agreement with the quantal size obtained from amplitude distributions of spontaneous miniature EPSCs. At high release probability with 10 or 15 mm [Ca(2+)], competitive antagonists were used to partially block EPSCs. Under these conditions, the EPSC variance-mean plots could be fitted with parabolas, giving estimates of quantal size and of the binomial parameter N. With the rapidly dissociating antagonist kynurenic acid, quantal sizes were larger than with a slowly dissociating antagonist, suggesting that the effective glutamate concentration was increased at high release probability. Considering the possibility of multivesicular release and moderate saturation of postsynaptic AMPA receptors, we conclude that the binomial parameter N (637 +/- 117; mean +/- SEM) represents an upper limit estimate of the number of functional active zones. We estimate that during normal synaptic transmission, the probability of vesicle fusion at single active zones is in the range of 0.25-0.4.

Estimating synaptic parameters from mean, variance, and covariance in trains of synaptic responses

Fluctuation analysis of synaptic transmission using the variance-mean approach has been restricted in the past to steady-state responses. Here we extend this method to short repetitive trains of synaptic responses, during which the response amplitudes are not stationary. We consider intervals between trains, long enough so that the system is in the same average state at the beginning of each train. This allows analysis of ensemble means and variances for each response in a train separately. Thus, modifications in synaptic efficacy during short-term plasticity can be attributed to changes in synaptic parameters. In addition, we provide practical guidelines for the analysis of the covariance between successive responses in trains. Explicit algorithms to estimate synaptic parameters are derived and tested by Monte Carlo simulations on the basis of a binomial model of synaptic transmission, allowing for quantal variability, heterogeneity in the release probability, and postsynaptic receptor saturation and desensitization. We find that the combined analysis of variance and covariance is advantageous in yielding an estimate for the number of release sites, which is independent of heterogeneity in the release probability under certain conditions. Furthermore, it allows one to calculate the apparent quantal size for each response in a sequence of stimuli.

Intracellular calcium dependence of large dense-core vesicle exocytosis in the absence of synaptotagmin I

Synaptotagmin I is a synaptic vesicle-associated protein essential for synchronous neurotransmission. We investigated its impact on the intracellular Ca(2+)-dependence of large dense-core vesicle (LDCV) exocytosis by combining Ca(2+)-uncaging and membrane capacitance measurements in adrenal slices from mouse synaptotagmin I null mutants. Synaptotagmin I-deficient chromaffin cells displayed prolonged exocytic delays and slow, yet Ca(2+)-dependent fusion rates, resulting in strongly reduced LDCV release in response to short depolarizations. Vesicle recruitment, the shape of individual amperometric events, and endocytosis appeared unaffected. These findings demonstrate that synaptotagmin I is required for rapid, highly Ca(2+)-sensitive LDCV exocytosis and indicate that it regulates the equilibrium between a slowly releasable and a readily releasable state of the fusion machinery. Alternatively, synaptotagmin I could function as calcium sensor for the readily releasable pool, leading to the destabilization of the pool in its absence.

The actions of barium and strontium on exocytosis and endocytosis in the synaptic terminal of goldfish bipolar cells

1. We investigated the properties of Ca2+-sensitive steps in the cycling of synaptic vesicles by comparing the actions of Ca2+, Ba2+ and Sr2+ in the synaptic terminal of depolarizing bipolar cells isolated from the retina of goldfish. FM1-43 fluorescence and capacitance measurements demonstrated that exocytosis, endocytosis and vesicle mobilization were maintained when external Ca2+ was replaced by either Ba2+ or Sr2+. 2. The rapidly releasable pool of vesicles (RRP) was equivalent to 1.5 % of the membrane surface area when measured in the presence of 2.5 mM Ca2+, but only 0.4 % in 2.5 mM Sr2+. The relative sizes of the RRP in Ca2+, Sr2+ and Ba2+ were 1.0, 0.28 and 0.1, respectively. We conclude that a smaller proportion of docked vesicles are available for fast exocytosis triggered by the influx of Sr2+ or Ba2+ compared to Ca2+. 3. The slow phase of exocytosis was not altered when Ca2+ was replaced by Ba2+, but it was accelerated 1.6-fold in Sr2+. The peak concentrations of Ca2+, Sr2+ and Ba2+ (measured using Mag-fura-5) were approximately 4, approximately 14 and approximately 60 microM, respectively. The order of efficiency for the stimulation of slow exocytosis was Ca2+ approximately Sr2+ > Ba2+. 4. Exocytosis was prolonged after the influx of Sr2+ and Ba2+. Sr2+ was cleared from the synaptic terminal with the same time constant as Ca2+ (1.3 s), but Ba2+ was cleared 10-100 times more slowly. Although Ba(2+) stimulates the slow release of a large number of vesicles, it did so less efficiently than Ca2+ or Sr2+. 5. The recovery of the membrane capacitance was equally rapid in Sr2+ and Ca2+, demonstrating that the fast mode of endocytosis could be triggered by either cation.

Combining deconvolution and noise analysis for the estimation of transmitter release rates at the calyx of held

The deconvolution method has been used in the past to estimate release rates of synaptic vesicles, but it cannot be applied to synapses where nonlinear interactions of quanta occur. We have extended this method to take into account a nonlinear current component resulting from the delayed clearance of glutamate from the synaptic cleft. We applied it to the calyx of Held and verified the important assumption of constant miniature EPSC (mEPSC) size by combining deconvolution with a variant of nonstationary fluctuation analysis. We found that amplitudes of mEPSCs decreased strongly after extended synaptic activity. Cyclothiazide (CTZ), an inhibitor of glutamate receptor desensitization, eliminated this reduction, suggesting that postsynaptic receptor desensitization occurs during strong synaptic activity at the calyx of Held. Constant mEPSC sizes could be obtained in the presence of CTZ and kynurenic acid (Kyn), a low-affinity blocker of AMPA-receptor channels. CTZ and Kyn prevented postsynaptic receptor desensitization and saturation and also minimized voltage-clamp errors. Therefore, we conclude that in the presence of these drugs, release rates at the calyx of Held can be reliably estimated over a wide range of conditions. Moreover, the method presented should provide a convenient way to study the kinetics of transmitter release at other synapses.