Properties of synchronous and asynchronous release during pulse train depression in cultured hippocampal neurons

Kinetic efficiency of endocytosis at mammalian CNS synapses requires synaptotagmin I

At nerve terminals, synaptic vesicle components are retrieved from the cell surface and recycled for local reuse soon after exocytosis. The kinetics of this coupling is critical for the proper functioning of synapses during repetitive action potential firing, because deficiencies in this process lead to abnormal depletion of the releasable vesicle pool. Although the molecular basis of this coupling is poorly understood, numerous biochemical data point to a role for synaptotagmin I (SytI), an essential synaptic vesicle protein required for fast calcium-dependent exocytosis. Here, using synapto-pHluorin in an approach that allows the dissection of endocytosis and exocytosis into separate components during periods of stimulation, we examined exocytic-endocytic coupling in synapses from SytI knockout mice and their WT littermates. We show that endocytosis is significantly impaired in the absence of SytI with the relative rates of endocytosis compared with exocytosis reduced approximately 3-fold with respect to WT. Thus, in addition to regulating exocytosis, SytI also controls the kinetic efficiency of endocytosis at nerve terminals.

Presynaptic plasticity at two giant auditory synapses in normal and deaf mice

Large calyceal synapses are often regarded as simple relay points, built for high-fidelity and high-frequency synaptic transmission and a minimal requirement for synaptic plasticity, but this view is oversimplified. Calyceal synapses can exhibit surprising activity-dependent developmental plasticity. Here we compare basal synaptic transmission and activity-dependent plasticity at two stereotypical calyceal synapses in the auditory pathway, the endbulb and the calyx of Held. Basal synaptic transmission was more powerful at the calyx than the endbulb synapse: the amplitude of evoked AMPA receptor-mediated excitatory postsynaptic currents (eEPSCs) was significantly greater at the calyx, as were the release probability, and the number of release sites. The quantal amplitude was smaller at the calyx, consistent with the smaller amplitude of spontaneous miniature EPSCs at this synapse. High-frequency trains of stimuli revealed that the calyx had a larger readily releasable pool of vesicles (RRP), less tetanic depression and less asynchronous transmitter release. Activity-dependent synaptic plasticity was assessed in congenitally deaf mutant mice (dn/dn). Previously we showed that a lack of synaptic activity in deaf mice increases synaptic strength at the endbulb of Held via presynaptic mechanisms. In contrast, we have now found that deafness does not affect synaptic transmission at the calyx synapse, as eEPSC and mEPSC amplitude, release probability, number of release sites, size of RRP, tetanic depression and asynchronous release were unchanged compared to normal mice. Synaptic transmission at the calyx synapse is more powerful and has less capacity for developmental plasticity compared to the endbulb synapse.

The synaptic vesicle cycle

Neurotransmitter release is mediated by exocytosis of synaptic vesicles at the presynaptic active zone of nerve terminals. To support rapid and repeated rounds of release, synaptic vesicles undergo a trafficking cycle. The focal point of the vesicle cycle is Ca2+-triggered exocytosis that is followed by different routes of endocytosis and recycling. Recycling then leads to the docking and priming of the vesicles for another round of exo- and endocytosis. Recent studies have led to a better definition than previously available of how Ca2+ triggers exocytosis and how vesicles recycle. In particular, insight into how Munc18-1 collaborates with SNARE proteins in fusion, how the vesicular Ca2+ sensor synaptotagmin 1 triggers fast release, and how the vesicular Rab3 protein regulates release by binding to the active zone proteins RIM1 alpha and RIM2 alpha has advanced our understanding of neurotransmitter release. The present review attempts to relate these molecular data with physiological results in an emerging view of nerve terminals as macromolecular machines.

Light-directed electrical stimulation of neurons cultured on silicon wafers

Dissociated neurons cultured in vitro can serve as a model system for studying the dynamics of neural networks. Such studies depend on techniques for stimulating patterns of neural activity. We show a technique for extracellular stimulation of dissociated neurons cultured on silicon wafers. When the silicon surface is reverse biased, electrical current can be generated near any neuron by pulsing a laser. Complex spatiotemporal stimulation patterns can be produced by directing a single beam with an acousto-optic deflector. The technique can generate a stimulating current at any location in the culture. This contrasts with multielectrode arrays (MEAs), which can stimulate only at fixed electrode locations. To characterize reliability and spatial selectivity of stimulation, we used intracellular (patch-clamp) recordings to monitor the effect of targeted laser pulses on cultured hippocampal neurons. Action potentials could be stimulated with submillisecond precision and 100-micron spatial resolution at rates exceeding 100 Hz. Optimal control parameters for stimulation are discussed.

Multiple Roles for the Active Zone Protein RIM1α in Late Stages of Neurotransmitter Release

The active zone protein RIM1alpha interacts with multiple active zone and synaptic vesicle proteins and is implicated in short- and long-term synaptic plasticity, but it is unclear how RIM1alpha's biochemical interactions translate into physiological functions. To address this question, we analyzed synaptic transmission in autaptic neurons cultured from RIM1alpha-/- mice. Deletion of RIM1alpha causes a large reduction in the readily releasable pool of vesicles, alters short-term plasticity, and changes the properties of evoked asynchronous release. Lack of RIM1alpha, however, had no effect on synapse formation, spontaneous release, overall Ca2+ sensitivity of release, or synaptic vesicle recycling. These results suggest that RIM1alpha modulates sequential steps in synaptic vesicle exocytosis through serial protein-protein interactions and that this modulation is the basis for RIM1alpha's role in synaptic plasticity.

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.

Excitatory inputs to CA1 interneurons show selective synaptic dynamics

The dynamic properties of synapses between neurons in the hippocampal CA1 area are important for the frequency-dependent signal transfer of the network. We have examined the synaptic dynamics of excitatory inputs to CA1 interneurons and pyramidal cells using whole cell voltage-clamp recordings. The CA1 network was activated using extracellular stimulation electrodes at the Schaffer collaterals (feedforward activation) or at the Alveus (activation of the feedback loop). The dynamic properties of input from the Schaffer collaterals to CA1 interneurons (basket and bistratified cells) were different from the synaptic dynamics of input from the Alveus. Synaptic input from the Schaffer collaterals to CA1 interneurons showed facilitation for most frequencies. After 10 stimuli the synaptic response reached a plateau level that was approximately 150% of the first response in the train. In contrast, the plateau levels of Alveus inputs to interneurons were not different from the first responses for frequencies <or=40 Hz. Paired-pulse facilitation of Schaffer input was stronger than for Alveus input. Cells in stratum oriens with horizontal dendritic trees appeared to be a special group of interneurons because Alveus input to these cells showed strong facilitation with plateau levels of 200% of the first responses. Schaffer input to CA1 basket and bistratified cells showed similar synaptic dynamics compared with Schaffer input to pyramidal cells for frequencies <or=80 Hz. The synaptic dynamics of Schaffer and Alveus input depended only weakly on the stimulus intensity. The difference between the dynamics of Alveus and Schaffer input to CA1 interneurons implies that the relative contribution of feedforward and -back inhibition to network activity depends on the frequency of the input signal at the afferent fibers, adding a level of complexity to transient responses.

Inhibition of mitochondrial Ca2+ uptake affects phasic release from motor terminals differently depending on external [Ca2+]

We investigated how inhibition of mitochondrial Ca2+ uptake affects stimulation-induced increases in cytosolic [Ca2+] and phasic and asynchronous transmitter release in lizard motor terminals in 2 and 0.5 mM bath [Ca2+]. Lowering bath [Ca2+] reduced the rate of rise, but not the final amplitude, of the increase in mitochondrial [Ca2+] during 50-Hz stimulation. The amplitude of the stimulation-induced increase in cytosolic [Ca2+] was reduced in low-bath [Ca2+] and increased when mitochondrial Ca2+ uptake was inhibited by depolarizing mitochondria. In 2 mM Ca2+, end-plate potentials (epps) depressed by 53% after 10 s of 50-Hz stimulation, and this depression increased to 80% after mitochondrial depolarization. In contrast, in 0.5 mM Ca2+ the same stimulation pattern increased epps by approximately 3.4-fold, and this increase was even greater (transiently) after mitochondrial depolarization. In both 2 and 0.5 mM [Ca2+], mitochondrial depolarization increased asynchronous release during the 50-Hz train and increased the total vesicular release (phasic and asynchronous) measured by destaining of the styryl dye FM2-10. These results suggest that by limiting the stimulation-induced increase in cytosolic [Ca2+], mitochondrial Ca2+ uptake maintains a high ratio of phasic to asynchronous release, thus helping to sustain neuromuscular transmission during repetitive stimulation. Interestingly, the quantal content of the epp reached during 50-Hz stimulation stabilized at a similar level ( approximately 20 quanta) in both 2 and 0.5 mM Ca2+. A similar convergence was measured in oligomycin, which inhibits mitochondrial ATP synthesis without depolarizing mitochondria, but quantal contents fell to <20 when mitochondria were depolarized in 2 mM Ca2+.

Intraterminal Ca2+ concentration and asynchronous transmitter release at single GABAergic boutons in rat collicular cultures

Neurotransmitter release in response to a single action potential has a precise time course. A significant fraction of the releasable vesicles is exocytosed synchronously, within a few milliseconds after the arrival of an action potential. If repeatedly activated, stimulus-locked phasic synchronous release declines, but synaptic transmission can be maintained through tonic asynchronous transmitter release. The desynchronisation of release during repetitive activation is generally attributed to a build-up of intraterminal Ca2+ concentration. However, the precise relationship between presynaptic Ca2+ level and asynchronous release rate at small central synapses has remained unclear. Here we characterise this relationship for single GABAergic terminals in rat collicular cultures. In the presence of tetrodotoxin, inhibitory postsynaptic currents (IPSCs) and presynaptic Ca2+ transients were recorded in response to direct presynaptic depolarisation of individual boutons. Repetitive stimulation indeed resulted in a shift from phasic to asynchronous neurotransmitter release. A clear dominance of the asynchronous release mode was observed after 10 pulses. The steady-state asynchronous release rate showed a third-power dependency on the presynaptic Ca2+ concentration, which is similar to that of evoked release. The Ca2+ sensor for asynchronous release exhibited a high affinity for Ca2+ and was far from saturation. These properties of the Ca2+ sensor should make the asynchronous release very sensitive to any modification of presynaptic Ca2+ concentration, including those resulting from changes in presynaptic activity patterns. Thus, asynchronous release represents a powerful but delicately regulated mechanism that ensures the maintenance of appropriate inhibition when the readily releasable pool of vesicles is depleted.

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.

Mechanism of the distance-dependent scaling of Schaffer collateral synapses in rat CA1 pyramidal neurons

Schaffer collateral axons form excitatory synapses that are distributed across much of the dendritic arborization of hippocampal CA1 pyramidal neurons. Remarkably, AMPA-receptor-mediated miniature EPSP amplitudes at the soma are relatively independent of synapse location, despite widely different degrees of dendritic filtering. A progressive increase with distance in synaptic conductance is thought to produce this amplitude normalization. In this study we examined the mechanism(s) responsible for spatial scaling by making whole-cell recordings from the apical dendrites of CA1 pyramidal neurons. We found no evidence to suggest that there is any location dependence to the range of cleft glutamate concentrations found at Schaffer collateral synapses. Furthermore, we observed that release probability (Pr), paired-pulse facilitation and the size of the readily releasable vesicular pool are not dependent on synapse location. Thus, there do not appear to be any changes in the fundamental presynaptic properties of Schaffer collateral synapses that could account for distance-dependent scaling. On the other hand, two-photon uncaging of 4-methoxy-7-nitroindolinyl-caged L-glutamate onto isolated dendritic spines shows that the number of postsynaptic AMPA receptors per spine increases with distance from the soma. We conclude, therefore, that the main synaptic mechanism involved in the production of distance-dependent scaling of Schaffer collateral synapses is an elevated postsynaptic AMPA receptor density.

Quantitative estimate of mitochondrial [Ca2+] in stimulated motor nerve terminals

Peak values reported for mitochondrial matrix [Ca(2+)] following stimulation have ranged from micromolar to near-millimolar in various cells. Measurements using fluorescent indicators have traditionally used high-affinity dyes such as rhod-2, whose fluorescence would be expected to saturate if matrix [Ca(2+)] approaches millimolar levels. To avoid this potential problem, we loaded lizard motor terminal mitochondria with the low-affinity indicator rhod-5N (K(d) approximately 320 microM). During trains of action potentials at 50Hz, matrix fluorescence transients (measured as F/F(rest)) increased to a plateau level that was maintained throughout the stimulus train. This plateau of matrix [Ca(2+)] occurred in spite of evidence that Ca(2+) continued to enter the terminal and continued to be sequestered by mitochondria. When the stimulation frequency was increased, or when Ca(2+) entry per action potential was increased with the K(+) channel blocker 3,4-diaminopyridine (3,4-DAP), or reduced by lowering bath [Ca(2+)], the rate of rise of matrix [Ca(2+)] changed, but the plateau amplitude remained constant. Calculations demonstrated that the F/F(rest) measured at this plateau corresponded to a matrix [Ca(2+)] of approximately 1 microM. The high K(d) of rhod-5N ensures that this value is not a result of dye saturation, but rather reflects a powerful Ca(2+) buffering mechanism within the matrix of these mitochondria.

Membrane fusion

Membrane fusion, one of the most fundamental processes in life, occurs when two separate lipid membranes merge into a single continuous bilayer. Fusion reactions share common features, but are catalyzed by diverse proteins. These proteins mediate the initial recognition of the membranes that are destined for fusion and pull the membranes close together to destabilize the lipid/water interface and to initiate mixing of the lipids. A single fusion protein may do everything or assemblies of protein complexes may be required for intracellular fusion reactions to guarantee rigorous regulation in space and time. Cellular fusion machines are adapted to fit the needs of different reactions but operate by similar principles in order to achieve merging of the bilayers.

Sr2+ binding to the Ca2+ binding site of the synaptotagmin 1 C2B domain triggers fast exocytosis without stimulating SNARE interactions

Sr(2+) triggers neurotransmitter release similar to Ca(2+), but less efficiently. We now show that in synaptotagmin 1 knockout mice, the fast component of both Ca(2+)- and Sr(2+)-induced release is selectively impaired, suggesting that both cations partly act by binding to synaptotagmin 1. Both the C(2)A and the C(2)B domain of synaptotagmin 1 bind Ca(2+) in phospholipid complexes, but only the C(2)B domain forms Sr(2+)/phospholipid complexes; therefore, Sr(2+) binding to the C(2)B domain is sufficient to trigger fast release, although with decreased efficacy. Ca(2+) induces binding of the synaptotagmin C(2) domains to SNARE proteins, whereas Sr(2+) even at high concentrations does not. Thus, triggering of the fast component of release by Sr(2+) as a Ca(2+) agonist involves the formation of synaptotagmin/phospholipid complexes, but does not require stimulated SNARE binding.

Short-term synaptic plasticity, simulation of nerve terminal dynamics, and the effects of protein kinase C activation in rat hippocampus

Phorbol esters are hypothesised to produce a protein kinase C (PKC)-dependent increase in the probability of transmitter release via two mechanisms: facilitation of vesicle fusion or increases in synaptic vesicle number and replenishment. We used a combination of electrophysiology and computer simulation to distinguish these possibilities. We constructed a stochastic model of the presynaptic contacts between a pair of hippocampal pyramidal cells that used biologically realistic processes and was constrained by electrophysiological data. The model reproduced faithfully several forms of short-term synaptic plasticity, including short-term synaptic depression (STD), and allowed us to manipulate several experimentally inaccessible processes. Simulation of an increase in the size of the readily releasable vesicle pool and the time of vesicle replenishment decreased STD, whereas simulation of a facilitation of vesicle fusion downstream of Ca(2+) influx enhanced STD. Because activation of protein kinase C with phorbol ester enhanced STD of EPSCs in rat hippocampal slice cultures, we conclude that an increase in the sensitivity of the release process for Ca(2+) underlies the potentiation of neurotransmitter release by PKC.

Synaptic transmission in the auditory brainstem of normal and congenitally deaf mice

The deafness (dn/dn) mutant mouse provides a valuable model of human congenital deafness. We investigated the properties of synaptic transmission in the anteroventral cochlear nucleus (AVCN) of normal and congenitally deaf dn/dn mice. Excitatory postsynaptic currents (EPSCs) were evoked by focal stimulation of single auditory nerve fibres, and measured by whole-cell recordings from neurones in AVCN slices (mean postnatal age = P13). Absolute amplitudes of both AMPA- and NMDA-mediated components of evoked EPSCs were greater (170 %) in deaf versus control animals. Enhanced transmission in deaf mice was due to a presynaptic mechanism. Variance-mean analysis revealed that the probability of transmitter release was significantly greater in deaf (P(r) = 0.8) versus control animals (P(r) = 0.5). Following high frequency stimulation, deaf mice showed a greater depression of evoked EPSCs, and a significant increase in the frequency of delayed-release (asynchronous) miniature EPSCs (aEPSCs) (deaf 100 Hz vs. control 7 Hz). The acetoxymethyl ester of EGTA (EGTA-AM) blocked the increase in miniature aEPSCs and returned tetanic depression to control values. In deaf mice, reduction of mean P(r) using cadmium caused an expected increase in paired-pulse ratio (PPR). However, in the same cells, a similar reduction in release by EGTA-AM did not result in a change in PPR, demonstrating that a change may occur in P(r) without a concomitant change in PPR. In many respects, transmission in deaf mice was found to be remarkably similar to control mice, implying that many parameters of synaptic transmission develop normally in these animals. The two significant differences (higher P(r) and asynchronous release in deaf mice) could both be reversed by the addition of EGTA-AM, suggesting that endogenous calcium buffering may be impaired or undeveloped in the presynaptic terminals of the auditory nerve in deaf mice.

Elevated postsynaptic [Ca2+]i and L-type calcium channel activity in aged hippocampal neurons: relationship to impaired synaptic plasticity

Considerable evidence supports a Ca(2+) dysregulation hypothesis of brain aging and Alzheimer's disease. However, it is still not known whether (1) intracellular [Ca(2+)](i) is altered in aged brain neurons during synaptically activated neuronal activity; (2) altered [Ca(2+)](i) is directly correlated with impaired neuronal plasticity; or (3) the previously observed age-related increase in L-type voltage-sensitive Ca(2+) channel (L-VSCC) density in hippocampal neurons is sufficient to impair synaptic plasticity. Here, we used confocal microscopy to image [Ca(2+)](i) in single CA1 neurons in hippocampal slices of young-adult and aged rats during repetitive synaptic activation. Simultaneously, we recorded intracellular EPSP frequency facilitation (FF), a form of short-term synaptic plasticity that is impaired with aging and inversely correlated with cognitive function. Resting [Ca(2+)](i) did not differ clearly with age. Greater elevation of somatic [Ca(2+)](i) and greater depression of FF developed in aged neurons during 20 sec trains of 7 Hz synaptic activation, but only if the activation triggered repetitive action potentials for several seconds. Elevated [Ca(2+)](i) and FF also were negatively correlated in individual aged neurons. In addition, the selective L-VSCC agonist Bay K8644 increased the afterhyperpolarization and mimicked the depressive effects of aging on FF in young-adult neurons. Thus, during physiologically relevant firing patterns in aging neurons, postsynaptic Ca(2+) elevation is closely associated with altered neuronal plasticity. Moreover, selectively increasing postsynaptic L-VSCC activity, as occurs in aging, negatively regulated a form of short-term plasticity that enhances synaptic throughput. Together, the results elucidate novel processes that may contribute to impaired cognitive function in aging.

Remodeling of synaptic actin induced by photoconductive stimulation

Use-dependent synapse remodeling is thought to provide a cellular mechanism for encoding durable memories, yet whether activity triggers an actual structural change has remained controversial. We use photoconductive stimulation to demonstrate activity-dependent morphological synaptic plasticity by video imaging of GFP-actin at individual synapses. A single tetanus transiently moves presynaptic actin toward and postsynaptic actin away from the synaptic junction. Repetitive spaced tetani induce glutamate receptor-dependent stable restructuring of synapses. Presynaptic actin redistributes and forms new puncta that label for an active synapse marker FM5-95 within 2 hr. Postsynaptic actin sprouts projections toward the new presynaptic actin puncta, resembling the axon-dendrite interaction during synaptogenesis. Our results indicate that activity-dependent presynaptic structural plasticity facilitates the formation of new active presynaptic terminals.