Variability of neurotransmitter concentration and nonsaturation of postsynaptic AMPA receptors at synapses in hippocampal cultures and slices

Estimation of nonuniform quantal parameters with multiple-probability fluctuation analysis: theory, application and limitations

Synapses are a key determinant of information processing in the central nervous system. Investigation of the mechanisms underlying synaptic transmission at central synapses is complicated by the inaccessibility of synaptic contacts and the fact that their temporal dynamics are governed by multiple parameters. Multiple-probability fluctuation analysis (MPFA) is a recently developed method for estimating quantal parameters from the variance and mean amplitude of evoked steady-state synaptic responses recorded under a range of release probability conditions. This article describes the theoretical basis and the underlying assumptions of MPFA, illustrating how a simplified multinomial model can be used to estimate mean quantal parameters at synapses where quantal size and release probability are nonuniform. Interpretations of the quantal parameter estimates are discussed in relation to uniquantal and multiquantal models of transmission. Practical aspects of this method are illustrated including a new method for estimating quantal size and variability, approaches for optimising data collection, error analysis and a method for identifying multivesicular release. The advantages and limitations of investigating synaptic function with MPFA are explored and contrasted with those for traditional quantal analysis and more recent optical quantal analysis methods.

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.

Heterogeneity of postsynaptic receptor occupancy fluctuations among glycinergic inhibitory synapses in the zebrafish hindbrain

The amplitude of glycinergic miniature inhibitory postsynaptic currents (mIPSCs) varies considerably in neurons recorded in the isolated hindbrain of 50-h-old zebrafish larvae. At this age, glycinergic synapses are functionally mature. In order to measure the occupancy level of postsynaptic glycine receptors (GlyRs) and to determine the pre- and/or postsynaptic origin of its variability, we analysed mIPSCs within bursts evoked by alpha-latrotoxin (0.1-1 nM). Two types of burst were observed according to their mIPSC frequencies: 'slow' bursts with clearly spaced mIPSCs and 'fast' bursts characterised by superimposed events. Non-stationary noise analysis of mIPSCs in some 'slow' bursts recorded in the presence or in the absence of Ca2+ denoted that mIPSC amplitude variance did not depend on the quantity of neurotransmitters released (presynaptic origin), but rather on intrinsic stochastic behaviour of the same group of GlyRs (postsynaptic origin). In these bursts, the open probability measured at the peak of the mIPSCs was close to 0.5 while the maximum open probability is close to 0.9 for the synaptic isoform of GlyRs (heteromeric alpha1/beta GlyRs). In 'fast' bursts with superimposed events, a correlation was found between the amplitude of mIPSCs and the basal current level measured at their onset, which could suggest that the same group of GlyRs is activated during such bursts. Altogether, our results indicate that glycine synapses can display different release modes in the presence of alpha-latrotoxin. They also indicate that, in our model, postsynaptic GlyRs cannot be saturated by the release of a single vesicle.

Secretory vesicles membrane area is regulated in tandem with quantal size in chromaffin cells

The number of transmitter molecules released in a quantal event can be regulated, and recent studies suggest that the modulation of quantal size is associated with corresponding changes in vesicle volume (Colliver et al., 2000; Pothos et al., 2002). If so, this could occur either by distension of the vesicle membrane or by incorporation and removal of vesicle membrane. We performed simultaneous measurements of vesicle membrane area and catecholamine release in individual quantal events from chromaffin cells using cell-attached patch amperometry. Cells were treated with reserpine, a vesicular monoamine transport blocker that decreases quantal size, or l-dopa, a catecholamine precursor that increases quantal size. We show that decrease and increase in quantal size are associated with a respective decrease and increase in vesicle membrane area. These results point to a novel mechanism of vesicle membrane dynamics by which vesicles physically change their membrane area in response to changes in transmitter content such that the intravesicular concentration of transmitter is maintained.

Stochastic fluctuations of the quantal EPSC amplitude in computer simulated excitatory synapses of hippocampus

The postsynaptic response in glutamatergic synapses of hippocampus, produced by the release of a single presynaptic vesicle, shows a large variability in amplitude not only among the synapses, but also for a single synapse. A mathematical modelling based on a Brownian motion for the diffusion of glutamate molecules and receptor binding was applied to study the possible sources of the quantal variability. Detailed, geometric and functional, descriptions of the vesicle, of the fusion pore and of the synaptic cleft were used and quantal (or miniature) EPSCs were computed. Our results show non-saturation of AMPA receptors, attributable to the small number of molecules contained in the glutamate vesicles of hippocampus. NMDA receptor saturation was obtained rarely, only in very specific instances. We concluded that the lack of AMPA saturation and intrinsic random variations in basic presynaptic elements, such as the vesicle volume and the vesicle docking position, are the main causes of the observed stochastic variability of the quantal EPSC amplitude. Only minor effects can be ascribed to postsynaptic sources.

Developmental increase in vesicular glutamate content does not cause saturation of AMPA receptors at the calyx of Held synapse

Whether a quantal packet of transmitter saturates postsynaptic receptors is a fundamental question in central synaptic transmission. However, this question remains open with regard to saturation at mature synapses. The calyx of Held, a giant glutamatergic synapse in the auditory brainstem, becomes functionally mature during the fourth postnatal week in rats. During postnatal development, the mean amplitude of miniature (i.e., quantal) EPSCs (mEPSCs) becomes significantly larger. Experiments using the rapidly dissociating glutamate receptor antagonist kynurenate suggested that vesicular glutamate content increases with development. To test whether AMPA receptors are saturated by a packet of transmitter, we infused a high concentration of l-glutamate into mature calyceal terminals. This caused a marked increase in the mean amplitude of mEPSCs. We conclude that a single packet of transmitter glutamate does not saturate postsynaptic AMPA receptors even at the mature calyx of Held synapse with increased vesicular transmitter content.

Presynaptic control of quantal size: kinetic mechanisms and implications for synaptic transmission and plasticity

Although the strength of quantal synaptic transmission is jointly controlled by pre- and post-synaptic mechanisms, the presynaptic mechanisms remain substantially less well characterized. Recent studies reveal that a single package of neurotransmitter is generally insufficient to activate all available postsynaptic receptors, whereas the sum of transmitter from multiple vesicles can result in receptor saturation. Thus, depending upon the number of vesicles released, a given synaptic pathway might be either 'reliable' or 'unreliable'. A lack of receptor saturation in turn makes it possible to modify quantal size by altering the flux of transmitter through the synaptic cleft. Studies are now illuminating several new mechanisms behind the regulation of this transmitter flux--characteristics that control how transmitter is loaded into vesicles, how it is released and the manner by which it interacts with postsynaptic receptors.

Independent sources of quantal variability at single glutamatergic synapses

Variability in the size of single postsynaptic responses is a feature of most central neurons, although the source of this variability is not completely understood. The dominant source of variability could be either intersynaptic or intrasynaptic. To quantitatively examine this question, a biophysically realistic model of an idealized central axospinous synapse was used to assess mechanisms underlying synaptic variability measurements. Three independent sources of variability were considered: stochasticity of postsynaptic receptors ("channel noise"), variations of glutamate concentration in the synaptic cleft (Deltaq), and differences in the potency of vesicles released from different locations on the active zone [release-location dependence (RLD)]. As expected, channel noise was small (8% of the total variance) and Deltaq was the dominant source of variability (58% of total variance). Surprisingly, RLD accounted for a significant amount of variability (36%). Our simulations show that potency of release sites decreased with a length constant of approximately 100 nm, and that receptors were not activated by release events >300 nm away, which is consistent with the observation that single active zones are rarely >300 nm. RLD also predicts that the manner in which receptors are added or removed from synapses can dramatically affect the nature of the synaptic response, with increasing receptor density being more efficient than merely increasing synaptic area. Saturation levels and synaptic geometry were also important in determining the size and shape of the distribution of amplitudes recorded at different synapses.

Long-term potentiation: outstanding questions and attempted synthesis

This article attempts an overview of the mechanism of NMDAR-dependent long-term potentiation (LTP) and its role in hippocampal networks. Efforts are made to integrate information, often in speculative ways, and to identify unresolved issues about the induction, expression and molecular storage processes. The pre/post debate about LTP expression has been particularly difficult to resolve. The following hypothesis attempts to reconcile the available physiological evidence as well as anatomical evidence that LTP increases synapse size. It is proposed that synapses are composed of a variable number of trans-synaptic modules, each having presynaptic release sites and a postsynaptic structure that can be AMPAfied by the addition of a hyperslot assembly that anchors 10-20 AMPA channels. According to a newly developed view of transmission, the quantal response is generated by AMPA channels near the site of vesicle release and so will depend on whether the module where release occurs has been AMPAfied. LTP expression may involve two structurally mediated processes: (i) the AMPAfication of existing modules by addition of hyperslot assemblies: this is a purely postsynaptic process and produces an increase in the probability of an AMPA response, with no change in the NMDA component; and (ii) the addition of new modules: this is a structurally coordinated pre/post process that leads to LTP-induced synapse enlargement and potentiation of the NMDA component owing to an increase in the number of release sites (the number of NMDA channels is assumed to be fixed). The protocol used for LTP induction appears to affect the proportion of these two processes; pairing protocols that involve low-frequency presynaptic stimulation induce only AMPAfication, making LTP purely postsynaptic, whereas high-frequency stimulation evokes both processes, giving rise to a presynaptic component. This model is capable of reconciling much of the seemingly contradictory evidence in the pre/post debate. The structural nature of the postulated changes is relevant to a second debate: whether a CaMKII switch or protein-dependent structural change is the molecular memory mechanism. A possible reconciliation is that a reversible CaMKII switch controls the construction of modules and hyperslot assemblies from newly synthesized proteins.

Fusion pore modulation as a presynaptic mechanism contributing to expression of long-term potentiation

Working on the idea that postsynaptic and presynaptic mechanisms of long-term potentiation (LTP) expression are not inherently mutually exclusive, we have looked for the existence and functionality of presynaptic mechanisms for augmenting transmitter release in hippocampal slices. Specifically, we asked if changes in glutamate release might contribute to the conversion of 'silent synapses' that show N-methyl-D-aspartate (NMDA) responses but no detectable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) responses, to ones that exhibit both. Here, we review experiments where NMDA receptor responses provided a bioassay of cleft glutamate concentration, using opposition between peak [glu](cleft )and a rapidly reversible antagonist, L-AP5. We discuss findings of a dramatic increase in peak [glu](cleft) upon expression of pairing-induced LTP (Choi). We present simulations with a quantitative model of glutamatergic synaptic transmission that includes modulation of the presynaptic fusion pore, realistic cleft geometry and a distributed array of postsynaptic receptors and glutamate transporters. The modelling supports the idea that changes in the dynamics of glutamate release can contribute to synaptic unsilencing. We review direct evidence from Renger et al., in accord with the modelling, that trading off the strength and duration of the glutamate transient can markedly alter AMPA receptor responses with little effect on NMDA receptor responses. An array of additional findings relevant to fusion pore modulation and its proposed contribution to LTP expression are considered.

The high variance of AMPA receptor- and NMDA receptor-mediated responses at single hippocampal synapses: evidence for multiquantal release

Most of our knowledge about transmission at central synapses has been obtained by studying populations of synapses, but some important properties of synapses can be determined only by studying them individually. An important issue is whether a presynaptic action potential causes, at most, a single vesicle to be released, or whether multiquantal transmission is possible. Previous work in the CA1 region has shown that the response to stimulation of a single axon can be highly variable, apparently because it is composed of a variable number of quantal elements ( approximately 5 pA in amplitude). These quantal events have a low coefficient of variation (CV). Because the number of synaptic contacts involved is not known, the response could be because of uniquantal transmission at a varying number of synapses, or to multliquantal transmission at a single synapse. The former predicts that the CV at individual synapses should be small. We have used optical methods to measure the N-methyl-D-aspartate receptor-mediated Ca(2+) elevation at single active synapses. Our main finding is that the amplitude of nonfailure responses could be highly variable, having a CV as large as 0.63. In one fortuitous experiment, the optically studied synapse was the only active synapse, and we could therefore measure both its N-methyl-D-aspartate (NMDA) receptor- and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated signals. At this synapse, both signals varied over a 10-fold range and were highly correlated. These results strongly suggest that transmission at single CA1 synapses can be multiquantal. Furthermore, the individual quantal response is very far from saturation, allowing the effective summation of many quanta. The existence of multiquantal release has important implications for defining synaptic strength and understanding the mechanisms of synaptic plasticity.

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.

Silence analysis of AMPA receptor mutated at the CaM-kinase II phosphorylation site

Direct phosphorylation of the GluR1 subunit of postsynaptic AMPA receptors by Ca(2+)/calmodulin-dependent protein kinase II (CaM-KII) is believed to be one of the major contributors to the enhanced strength of glutamatergic synapses in CA1 area of hippocampus during long-term potentiation. The molecular mechanism of AMPA receptor regulation by CaM-KII is examined here by a novel approach, silence analysis, which is independent of previously used variance analysis. I show that three fundamental channel properties-single-channel conductance, channel open probability, and the number of functional channels-can be measured in an alternative way, by analyzing the probability of channels to be simultaneously closed (silent). Validity of the approach was confirmed by modeling, and silence analysis was applied then to the GluR1 AMPA receptor mutated at S831, the site phosphorylated by CaM-KII during long-term potentiation. Silence analysis indicates that a negative charge at S831 is a critical determinant for the enhanced channel function as a charge carrier. Silence and variance analyses, when applied to the same sets of data, were in agreement on the receptor regulation upon mutations. These results provide independent evidences for the mechanism of AMPA receptor regulation by CaM-KII and further strengthens the idea how calcium-dependent phosphorylation of AMPA receptors can contribute to the plasticity at central glutamatergic synapses.

Quantal size and variation determined by vesicle size in normal and mutant Drosophila glutamatergic synapses

Quantal size and variation at chemical synapses could be determined presynaptically by the amount of neurotransmitter released from synaptic vesicles or postsynaptically by the number of receptors available for activation. We investigated these possibilities at Drosophila glutamatergic neuromuscular synapses formed by two separate motor neurons innervating the same muscle cell. At wild-type synapses of the two neurons we found a difference in quantal size corresponding to a difference in mean synaptic vesicle volume. The same finding applied to two mutants (dlg and lap) in which synaptic vesicle size was altered. Quantal variances at wild-type and mutant synapses were similar and could be accounted for by variation in vesicular volume. The linear relationship between quantal size and vesicular volume for several different genotypes indicates that glutamate is regulated homeostatically to the same intravesicular concentration in all cases. Thus functional differences in synaptic strength among glutamatergic neurons of Drosophila result in part from intrinsic differences in vesicle size.

Interaction of postsynaptic receptor saturation with presynaptic mechanisms produces a reliable synapse

Synapses that reliably activate their postsynaptic targets typically release neurotransmitter with high probability, are not very sensitive to changes in calcium entry, and depress. We have determined the mechanisms that give rise to these characteristic features at the climbing fiber to Purkinje cell synapse. We find that saturation of presynaptic calcium entry, of presynaptic release, and of postsynaptic receptors combine to produce a postsynaptic response that is near maximal. Postsynaptic receptor saturation also accelerates recovery from depression, in part by accentuating a rapid calcium-dependent recovery phase. Thus, postsynaptic receptor saturation interacts with presynaptic mechanisms to produce highly reliable synapses that can effectively drive their targets even during sustained activation.

Quantal analysis suggests strong involvement of presynaptic mechanisms during the initial 3 h maintenance of long-term potentiation in rat hippocampal CA1 area in vitro

Long-term potentiation (LTP) is the most prominent model to study neuronal plasticity. Previous studies using quantal analysis of an early stage of LTP in the CA1 hippocampal region (<1 h after induction) suggested increases in both the mean number of transmitter quanta released by each presynaptic pulse (m, quantal content) and postsynaptic effect of a single quantum (v, quantal size). When LTP was large, it was m that increased predominantly suggesting prevailing presynaptic contribution. However, LTP consists of several temporary phases with presumably different mechanisms. Here we recorded excitatory postsynaptic potentials from CA1 hippocampal slices before and up to 3.5 h after LTP induction. A new version of the noise deconvolution revealed significant increases in m with smaller and often not statistically significant changes in v. The changes in m were similar for both early (<1 h) and later (1-3 h) post-tetanic periods and correlated with LTP magnitude. The coefficient of variation of the response amplitude and the number of failures decreased during both early and late post-tetanic periods. The results suggest that both early (<0.5 h) and later LTP components (0.5-3 h) are maintained by presynaptic changes, which include increases in release probabilities and the number of effective release sites. In addition initially silent synapses can be converted into effective ones due to either pre- or postsynaptic rearrangements. If this occurs, our data indicate that the number and the efficacy of the receptors in the new transmission sites are approximately similar to those in the previously effective sites.

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.

A Monte Carlo model reveals independent signaling at central glutamatergic synapses

We have developed a biophysically realistic model of receptor activation at an idealized central glutamatergic synapse that uses Monte Carlo techniques to simulate the stochastic nature of transmission following release of a single synaptic vesicle. For the a synapse with 80 AMPA and 20 NMDA receptors, a single quantum, with 3000 glutamate molecules, opened approximately 3 NMDARs and 20 AMPARs. The number of open receptors varied directly with the total number of receptors, and the fraction of open receptors did not depend on the ratio of co-localized AMPARs and NMDARs. Variability decreased with increases in either total receptor number or quantal size, and differences between the variability of AMPAR and NMDAR responses were due solely to unequal numbers of receptors at the synapse. Despite NMDARs having a much higher affinity for glutamate than AMPARs, quantal release resulted in similar occupancy levels in both receptor types. Receptor activation increased with number of transmitter molecules released or total receptor number, whereas occupancy levels were only dependent on quantal size. Tortuous diffusion spaces reduced the extent of spillover and the activation of extrasynaptic receptors. These results support the conclusion that signaling is spatially independent within and between central glutamatergic synapses.

Stochastic fluctuations of the synaptic function

The peak amplitudes of the quantal Excitatory Post Synaptic Currents in single hippocampal synapses show a large variability. Here, we present the results of a mathematical, computational investigation on the main sources of this variability. A detailed description of the synaptic cleft, rigorously based on empirically-derived parameters, was used. By using a Brownian motion model of neurotransmitter molecule diffusion, quantal EPSCs were computed by a simple kinetic schema of AMPA receptor dynamics. Our results show that the lack of saturation of AMPA receptors obtained in these conditions, combined with stochastic variations in basic presynaptic elements, such as the vesicle volume, the vesicle docking position, and the vesicle neurotransmitter concentration can explain almost the entire range of EPSC variability experimentally observed.

Spine motility. Phenomenology, mechanisms, and function

Throughout the history of neuroscience, dendritic spines have been considered stable structures, but in recent years, imaging techniques have revealed that spines are constantly changing shape. Spine motility is difficult to categorize, has different forms, and possibly even represents multiple phenomena. It is influenced by synaptic transmission, intracellular calcium, and a multitude of ions and other molecules. An actin-based cascade mediates this phenomenon, and while the precise signaling pathways are still unclear, the Rho family of GTPases could well be a "common denominator" controlling spine morphology. One role of spine motility might be to enable a searching function during synaptogenesis, allowing for more efficacious neuronal connectivity in the neuronal thicket. This idea revisits concepts originally formulated by Cajal, who proposed over a hundred years ago that spines might help to increase and modify synaptic connections.

Multimodal quantal release at individual hippocampal synapses: evidence for no lateral inhibition

Most CNS synapses investigated thus far contain a large number of vesicles docked at the active zone, possibly forming individual release sites. At the present time, it is unclear whether these vesicles can be discharged independently of one another. To investigate this problem, we recorded miniature excitatory currents by whole-cell and single-synapse recordings from CA3-CA1 hippocampal neurons and analyzed their stochastic properties. In addition, spontaneous release was investigated by ultrastructural analysis of quickly frozen synapses, revealing vesicle intermediates in docking and spontaneous fusion states. In these experiments, no signs of inhibitory interactions between quanta could be detected up to 1 msec from the previous discharge. This suggests that exocytosis at one site does not per se inhibit vesicular fusion at neighboring sites. At longer intervals, the output of quanta diverged from a random memoryless Poisson process because of the presence of a bursting component. The latter, which could not be accounted for by random coincidences, was independent of Ca2+ elevations in the cytosol, whether from Ca2+ flux through the plasma membrane or release from internal stores. Results of these experiments, together with the observation of spontaneous pairs of omega profiles at the active zone, suggest that multimodal release is produced by an enduring activation of an integrated cluster of release sites.

Dendritic spine plasticity in hippocampus

Most excitatory input in the hippocampus and cerebral cortex impinges on dendritic spines. Alterations in dendritic spine density or shape are suspected to be morphological manifestations of changes in physiology or behavior. The links between spine plasticity and physiological responses have probably been best studied in the hippocampus in the context of changes in the circulating levels of steroid hormones or long-term potentiation. Here we review and present data which indicate that both the age of the preparation and the timing of the analysis can dramatically effect the results obtained. Collectively the data suggest that different cellular and morphological strategies may be utilized at different ages and under different circumstances to effect similar physiological responses or behaviors.

Presynaptic and postsynaptic mechanisms underlie paired pulse depression at single GABAergic boutons in rat collicular cultures

Paired pulse depression (PPD) is a common form of short-term synaptic plasticity. The aim of this study was to characterise PPD at the level of a single inhibitory bouton. Low-density collicular cultures were loaded with the Ca2+ indicator Oregon Green-1, active boutons were stained with RH414, and action potentials were blocked with TTX. Evoked IPSCs (eIPSCs) and presynaptic Ca2+ transients were recorded in response to direct presynaptic depolarisation of an individual bouton. The single bouton eIPSCs had a low failure rate (< 0.1), large average quantal content (3-6) and slow decay (tau(1) = 15 ms, tau(2) = 81 ms). The PPD of eIPSCs had two distinct components: PPD(fast) and PPD(slow) (tau = 86 ms and 2 s). PPD(slow) showed no dependence on extracellular Ca2+ concentration, or on the first eIPSC's failure rate or amplitude. Most probably, it reflects a release-independent inhibition of exocytosis. PPD(fast) was only observed in normal or elevated Ca2+. It decreased with the failure rate and increased with the amplitude of the first eIPSC. It coincided with paired pulse depression of the presynaptic Ca2+ transients (tau = 120 ms). The decay of the latter was accelerated by EGTA, which also reduced PPD(fast). Therefore, a suppressive effect of residual presynaptic Ca2+ on subsequent Ca2+ influx is considered the most likely cause of PPD(fast). PPD(fast) may also have a postsynaptic component, because exposure to a low-affinity GABA(A) receptor antagonist (TPMPA; 300 microM) counteracted PPD(fast), and asynchronous IPSC amplitudes were depressed for a short interval following an eIPSC. Thus, at these synapses, PPD is produced by at least two release-independent presynaptic mechanisms and one release-dependent postsynaptic mechanism.

Glutamate release during LTD at cerebellar climbing fiber-Purkinje cell synapses

It is widely thought that persistent, use-dependent alterations in synaptic strength such as long-term synaptic potentiation (LTP) and depression (LTD) underlie at least a portion of memory traces in the brain, but the exact cellular locus of expression for these alterations remains to be determined. They could be expressed presynaptically as a decrease in transmitter release, postsynaptically as a decrease in the synaptic current evoked by a fixed delivery of transmitter, as an increase in the number of functional synaptic contacts, or by a combination of these mechanisms. Here we report that LTD at the climbing fiber-Purkinje cell synapse in rat cerebellum was not associated with changes in a synaptic cleft glutamate transient, indicating that this type of LTD is most likely expressed postsynaptically.

Release dependence to a paired stimulus at a synaptic release site with a small variable pool of immediately releasable vesicles

Monte Carlo simulations were performed on a release model based on experimental data from single glutamatergic synapses containing a single release site in the hippocampal CA1 region of the neonatal rat. These simulations explored what can be learned about the release process by examining how the release probability in response to the second stimulus (P(2)) of a paired stimulus to a synapse depends on the release in response to the first stimulus. Comparisons between experimental data from a number of individual synapses and the simulated data support the notion that the immediately releasable vesicle pool is small (approximately one) and shows substantial intertrial variation. The simulations also show that the release dependence of P(2) is not necessarily an indicator of either intertrial variation in Ca(2+) influx, feedback effects of released transmitter, or activation failure of the axon.

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.

AMPA receptor-mediated miniature synaptic calcium transients in GluR2 null mice

AMPA-type glutamate receptors are normally Ca(2+) impermeable due to the expression of the GluR2 receptor subunit. By using GluR2 null mice we were able to detect miniature synaptic Ca(2+) transients (MSCTs) associated with AMPA-type receptor-mediated miniature synaptic currents at single synapses in primary cortical cultures. MSCTs and associated Ca(2+) transients were monitored under conditions that isolated responses mediated by AMPA or N-methyl-D-aspartate (NMDA) receptors. As expected, addition of the antagonist 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX, 3 microM) blocked the AMPA receptor-mediated MSCTs. Voltage-gated Ca(2+) channels did not contribute to AMPA MSCTs because CdCl(2) (0.1-0.2 mM) did not significantly alter the frequency or the amplitude of the MSCTs. The amplitude of AMPA MSCTs appeared to be regulated independently from event frequency since the two measures were not correlated (R = 0.023). Synapses were identified that only expressed MSCTs attributed to either NMDA or AMPA receptors. At synapses with only NMDA responses, MSCT amplitude was significantly lower (by 40%) than synapses expressing both NMDA and AMPA responses. At synapses that showed MSCTs mediated by both AMPA and NMDA receptors, the amplitude of the transients in each condition was positively correlated (R = 0.94). Our results suggest that when AMPA and NMDA receptors are co-expressed at synapses, mechanisms exist to ensure proportional scaling of each receptor type that are distinct from the presynaptic factors controlling the frequency of miniature release.

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.

A single packet of transmitter does not saturate postsynaptic glutamate receptors

Neurotransmitter is stored in synaptic vesicles and released by exocytosis into the synaptic cleft. One of the fundamental questions in central synaptic transmission is whether a quantal packet of transmitter saturates postsynaptic receptors. To address this question, we loaded the excitatory transmitter L-glutamate via whole-cell recording pipettes into the giant nerve terminal, the calyx of Held, in rat brainstem slices. This caused marked potentiations of both quantal and action potential-evoked EPSCs mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or N-methyl-D-aspartate (NMDA) receptors. These results directly demonstrate that neither AMPA nor NMDA receptors are saturated by a single packet of transmitter, and indicate that vesicular transmitter content is an important determinant of synaptic efficacy.

High-resolution iontophoresis for single-synapse stimulation

We present a technique for stimulating post-synaptic receptors with neurotransmitter locally at a single synapse and with a concentration profile that is comparable to endogenous stimulation. We modify the technique of iontophoresis to use a 0.1 microm electrode tip for local stimulation, and we combine it with fast capacitance compensation to achieve high-speed application from a high-resistance tip. Ejection of fluorescent dye from the electrode shows that transmitter can be limited to the width of a single synapse and to a time scale similar to an endogenous event. The speed and localization of transmitter is confirmed by iontophoretically stimulating single labeled synapses in cultured hippocampal neurons held under voltage clamp. The amount of transmitter ejected is linear and reproducible over a physiologically relevant range, making this technique useful for examining receptor kinetics and receptor insertion/removal. The system should be capable of delivering any charged neurotransmitter, and we show examples using glutamate and GABA. The technique is also combined with computer-controlled manipulation to study the strength and plasticity of multiple synapses in real-time.

Modulation of synaptic transmission by astrocytes in the rat supraoptic nucleus

One of the functions of astroglial cells in the central nervous system is to clear synaptically-released glutamate from the extracellular space. This is performed thanks to specific transporters of the excitatory amino acid expressed on their surface. The way by which astrocytic glutamate uptake contributes to synaptic transmission has been investigated via numerous experimental approaches but has never been addressed under conditions where neuroglial interactions are physiologically modified. Recently, we took advantage of the neuroglial plastic properties of the hypothalamo-neurohypophysial system to examine the consequences of a physiological reduction in the astrocytic coverage of neurons on glutamatergic synaptic transmission. This experimental model has brought some insights on the physiological interactions between glial cells and neurons at the level of the synapse. In particular, it has revealed that the degree of glial coverage of neurons influences glutamate concentration at the vicinity of excitatory synapses and, as a consequence, affects the level of activation of presynaptic glutamate receptors. Astrocytes, therefore, appear to contribute to the regulation of neuronal excitability by modulating synaptic efficacy at glutamatergic nerve terminals.

Synaptically released glutamate activates extrasynaptic NMDA receptors on cells in the ganglion cell layer of rat retina

NMDA and AMPA receptors (NMDARs and AMPARs) are colocalized at most excitatory synapses in the CNS. Consequently, both receptor types are activated by a single quantum of transmitter and contribute to miniature and evoked EPSCs. However, in amphibian retina, miniature EPSCs in ganglion cell layer neurons are mediated solely by AMPARs, although both NMDARs and AMPARs are activated during evoked EPSCs. One explanation for this discrepancy is that NMDARs are located outside of the synaptic cleft and are activated only when extrasynaptic glutamate levels increase during coincident release from multiple synapses. Alternatively, NMDARs may be segregated at synapses that either are not spontaneously active or yield miniature EPSCs that are too small to detect. In this study, we examined excitatory, glutamatergic synaptic inputs to neurons in the ganglion cell layer of acute slices of rat retina. EPSCs, elicited by electrically stimulating presynaptic bipolar cells, exhibited both NMDAR- and AMPAR-mediated components. However, spontaneous EPSCs exhibited only an AMPAR-mediated component. The effects of low-affinity, competitive receptor antagonists indicated that NMDARs encounter less glutamate than AMPARs during an evoked synaptic response. Reducing glutamate uptake or changing the probability of release preferentially affected the NMDAR component in evoked EPSCs; reducing uptake revealed an NMDAR component in spontaneous EPSCs. These results indicate that NMDARs are located extrasynaptically and that glutamate transporters prevent NMDAR activation by a transmitter released from a single vesicle and limit their activation during evoked responses.

Persistent, exocytosis-independent silencing of release sites underlies homosynaptic depression at sensory synapses in Aplysia

The synaptic connections of Aplysia sensory neurons (SNs) undergo dramatic homosynaptic depression (HSD) with only a few low-frequency stimuli. Strong and weak SN synapses, although differing in their probabilities of release, undergo HSD at the same rate; this suggests that the major mechanism underlying HSD in these SNs may not be depletion of the releasable pool of vesicles. In computational models, we evaluated alternative mechanisms of HSD, including vesicle depletion, to determine which mechanisms enable strong and weak synapses to depress with identical time courses. Of five mechanisms tested, only release-independent, stimulus-dependent switching off of release sites resulted in HSD that was independent of initial synaptic strength. This conclusion that HSD is a release-independent phenomenon was supported by empirical results: an increase in Ca2+ influx caused by spike broadening with a K+ channel blocker did not alter HSD. Once induced, HSD persisted during 40 min of rest with no detectable recovery; thus, release does not recover automatically with rest, contrary to what would be expected if HSD represented an exhaustion of the exocytosis mechanism. The hypothesis that short-term HSD involves primarily a stepwise silencing of release sites, rather than vesicle depletion, is consistent with our earlier observation that HSD is accompanied by only a modest decrease in release probability, as indicated by little change in the paired-pulse ratio. In contrast, we found that there was a dramatic decrease in the paired-pulse ratio during serotonin-induced facilitation; this suggests that heterosynaptic facilitation primarily involves an increase in release probability, rather than a change in the number of functional release sites.

Functional expression of distinct NMDA channel subunits tagged with green fluorescent protein in hippocampal neurons in culture

We generated expression vectors for N-terminally green fluorescent protein -tagged NR2A and NR2B subunits (GFP-NR2A and GFP-NR2B). Both constructs expressed GFP and formed functional NMDA channels with similar properties to untagged controls when co-transfected with NR1 subunit partner in HEK293 cells. Primary cultured hippocampal neurons were transfected at five days in vitro with these vectors. Fifteen days after transfection, well-defined GFP clusters were observed for both GFP-NR2A and GFP-NR2B subunits being co-localized with endogenous NR1 subunit. Whole-cell recordings showed that the GFP-NR2A subunit determined the decay of NMDA-mediated miniature spontaneous excitatory postsynaptic currents (NMDA-mEPSCs) in transfected neurons. Live staining with anti-GFP antibody demonstrated the surface expression of GFP-NR2A and GFP-NR2B subunits that was partly co-localized a presynaptic marker. Localization of NMDA receptor clusters in dendrites was studied by co-transfection of CFP-actin and GFP-NR2 subunits followed by anti-GFP surface staining. Within one week after plating most surface NMDAR clusters were distributed on dendritic shafts. Later in development, a large portion of surface clusters for both GFP-NR2A and GFP-NR2B subunits were clearly localized at dendritic spines. Our report provides the basis for studies of NMDA receptor location together with dendritic dynamics in living neurons during synaptogenesis in vitro.

The effects of temperature on vesicular supply and release in autaptic cultures of rat and mouse hippocampal neurons

Membrane fusion plays a central role in the synaptic vesicle cycle. While many of the pre- and postfusion events have been investigated at room temperature, few researchers have investigated these processes at more physiologically relevant temperatures. We have used autaptic cultures of hippocampal neurons to investigate changes in the size and refilling rate of the readily releasable pool (RRP) of synaptic vesicles brought about by an increase in temperature from 25 to 35 degrees C. We have also examined temperature-dependent changes in spontaneous and action potential (AP)-evoked release as well as the fraction of the RRP that is released during an AP. Although we found a threefold increase in the refilling rate of the RRP at the higher temperature, there was no apparent change in the size of the RRP with increased temperature. Moreover, we observed a slight but significant decrease in the quanta released during an AP. This increased refilling rate and decreased release probability resulted in a reduction of both the degree and time course of synaptic depression during high frequency stimulation at the higher temperature. This reduction in synaptic depression was accompanied by an increased maintenance of the synchronous component of release during high frequency stimulation. These findings indicate that the dynamics of vesicular supply and release in hippocampal neurons at room temperature are significantly different at near physiological temperatures and could affect our present understanding of the way in which individual neurons and networks of neurons process information.

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.

Mechanisms shaping fast excitatory postsynaptic currents in the central nervous system

How different factors contribute to determine the time course of the basic element of fast glutamate-mediated excitatory postsynaptic currents (mEPSCs) in the central nervous system has been a focus of interest of neurobiologists for some years. In spite of intensive investigations, these mechanisms are not well understood. In this review, basic hypotheses are summarized, and a new hypothesis is proposed, which holds that desensitization of AMPA receptors plays a major role in shaping the time course of fast mEPSCs. According to the new hypothesis, desensitization shortens the time course of mEPSCs largely by reducing the buffering of glutamate molecules by AMPA receptors. The hypothesis accounts for numerous findings on fast mEPSCs and is expected to be equally fruitful as a framework for further experimental and theoretical investigations.

Giant miniature EPSCs at the hippocampal mossy fiber to CA3 pyramidal cell synapse are monoquantal

The mechanisms generating giant miniature excitatory postsynaptic currents (mEPSCs) were investigated at the hippocampal mossy fiber (MF) to CA3 pyramidal cell synapse in vitro. These giant mEPSCs have peak amplitudes as large as 1,700 pA (13.6 nS) with a mean maximal mEPSC amplitude of 366 +/- 20 pA (mean +/- SD; 5 nS; n = 25 cells). This is compared with maximal mEPSC amplitudes of <100 pA typically observed at other cortical synapses. We tested the hypothesis that giant mEPSCs are due to synchronized release of multiple vesicles across the release sites of single MF boutons by directly inducing vesicular release using secretagogues. If giant mEPSCs result from simultaneous multivesicular release, then secretagogues should increase the frequency of small mEPSCs selectively. We found that hypertonic sucrose and spermine increased the frequency of both small and giant mEPSCs. The peptide toxin secretagogues alpha-latrotoxin and pardaxin failed to increase the frequency of giant mEPSCs, but the possible lack of tissue penetration of the toxins make these results equivocal. Because a multiquantal release mechanism is likely to be mediated by a spontaneous increase in presynaptic calcium concentration, a monoquantal mechanism is further supported by results that giant mEPSCs were not affected by manipulations of extracellular or intracellular calcium concentrations. In addition, reducing the temperature of the bath to 15 degrees C failed to desynchronize the rising phases of giant mEPSCs. Together these data suggest that the giant mEPSCs are generated via a monovesicular mechanism. Three-dimensional analysis through serial electron microscopy of the MF boutons revealed large clear vesicles (50 to 160 nm diam) docked presynaptically at the MF synapse in sufficient numbers to account for the amplitude and frequency of giant mEPSCs recorded electrophysiologically. It is concluded that release of the contents of a single large clear vesicle generates giant mEPSCs at the MF to CA3 pyramidal cell synapse.

Paired-pulse plasticity at the single release site level: an experimental and computational study

CA3-CA1 glutamatergic synapses in the hippocampus exhibit a large heterogeneity in release probability (p) and paired-pulse (PP) plasticity, established already in the early neonatal period when the CA3-CA1 connections consist of only a single release site. At such a site two factors decide initial release probability: the number of immediately releasable vesicles (preprimed pool) and the vesicle release probability (P(ves1)). Depletion and replenishment of this pool, an alteration in P(ves), and desensitization of postsynaptic receptors may contribute to PP plasticity. A model based on data from single neonatal CA3-CA1 synapses has been used to address the relative importance of these factors for the heterogeneity in PP plasticity. At a 20 msec PP interval, the PP ratio (P(2)/P(1)) varied from 0.1 to 4.5 among the synapses. At this interval desensitization and replenishment were of little importance. The heterogeneity was explained mostly by the variation in P(ves1), whereas the preprimed pool size was of minor importance. P(ves) altered from the first to the second stimulus such that P(ves2) was rather uniform among the synapses. Its variation thus contributed little to the heterogeneity in PP ratio. The model also shows that the relationship between alterations in release probability and PP ratio is complex. Thus, an increase in release probability can be associated with an increase, a decrease, or no change at all in PP ratio, depending on the original values of P(ves1) and the preprimed pool and on which one of these factors is altered to produce the increase in release probability.

Factors explaining heterogeneity in short-term synaptic dynamics of hippocampal glutamatergic synapses in the neonatal rat

1. Quantal release from single hippocampal glutamatergic (CA3-CA1) synapses was examined in the neonatal rat during a 10 impulse, 50 Hz stimulus train. These synapses contain a single release site only, thus allowing for an analysis of frequency facilitation/depression at the single release site level. 2. These synapses displayed a considerable heterogeneity with respect to short-term synaptic dynamics, from a pronounced facilitation to a pronounced depression. Facilitation/depression was the same whether evaluated using the magnitude or the probability of occurrence of the postsynaptic response. This result suggests that postsynaptic factors, such as desensitisation, play little role. 3. Release probabilities initially and late during the train were uncorrelated. Initially, release is determined by the number of immediately release-ready vesicles and by the probability of releasing such vesicles (P(ves)). Within the first five stimuli this vesicle pool is depleted. The deciding factor for release is thereafter the rate at which new vesicles can be recruited for release, rather than P(ves). 4. Heterogeneity in facilitation/depression among the synapses was strongly correlated with heterogeneity in initial P(ves) but not with that of the immediately release-ready vesicle pool. Thus, the main factors deciding short-term synaptic dynamics are heterogeneity in initial P(ves) and in vesicle recruitment rate among the synapses.

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.

An evaluation of synapse independence

If, as is widely believed, information is stored in the brain as distributed modifications of synaptic efficacy, it can be argued that the storage capacity of the brain will be maximized if the number of synapses that operate independently is as large as possible. The majority of synapses in the brain are glutamatergic; their independence will be compromised if glutamate released at one synapse can significantly activate receptors at neighboring synapses. There is currently no agreement on whether "spillover" after the liberation of a vesicle will significantly activate receptors at neighboring synapses. To evaluate the independence of central synapses, it is necessary to compare synaptic responses with those generated at neighboring synapses by glutamate spillover. Here, synaptic activation and spillover responses are simulated in a model, based on data for hippocampal synapses, that includes an approximate representation of the extrasynaptic space. Recently-published data on glutamate transporter distribution and properties are incorporated. Factors likely to influence synaptic or spillover responses are investigated. For release of one vesicle, it is estimated that the mean response at the nearest neighboring synapse will be <5% of the synaptic response. It is concluded that synapses can operate independently.

Dendritic spine geometry is critical for AMPA receptor expression in hippocampal CA1 pyramidal neurons

Dendritic spines serve as preferential sites of excitatory synaptic connections and are pleomorphic. To address the structure-function relationship of the dendritic spines, we used two-photon uncaging of glutamate to allow mapping of functional glutamate receptors at the level of the single synapse. Our analyses of the spines of CA1 pyramidal neurons reveal that AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid)-type glutamate receptors are abundant (up to 150/spine) in mushroom spines but sparsely distributed in thin spines and filopodia. The latter may be serving as the structural substrates of the silent synapses that have been proposed to play roles in development and plasticity of synaptic transmission. Our data indicate that distribution of functional AMPA receptors is tightly correlated with spine geometry and that receptor activity is independently regulated at the level of single spines.

Multivesicular release at climbing fiber-Purkinje cell synapses

Synapses driven by action potentials are thought to release transmitter in an all-or-none fashion; either one synaptic vesicle undergoes exocytosis, or there is no release. We have estimated the glutamate concentration transient at climbing fiber synapses on Purkinje cells by measuring the inhibition of excitatory postsynaptic currents (EPSCs) produced by a low-affinity competitive antagonist of AMPA receptors, gamma-DGG. The results, together with simulations using a kinetic model of the AMPA receptor, suggest that the peak glutamate concentration at this synapse is dependent on release probability but is not affected by pooling of transmitter released from neighboring synapses. We propose that the mechanism responsible for the elevated glutamate concentration at this synapse is the simultaneous release of multiple vesicles per site.

Retrograde signaling at central synapses

Transcellular retrograde signaling from the postsynaptic target cell to the presynaptic neuron plays critical roles in the formation, maturation, and plasticity of synaptic connections. We here review recent progress in our understanding of the retrograde signaling at developing central synapses. Three forms of potential retrograde signals-membrane-permeant factors, membrane-bound factors, and secreted factors-have been implicated at both developing and mature synapses. Although many of these signals may be active constitutively, retrograde factors produced in association with activity-dependent synaptic plasticity, e.g., long-term potentiation and long-term depression, are of particular interest, because they may induce modification of neuronal excitability and synaptic transmission, functions directly related to the processing and storage of information in the nervous system.

Regulation of dendritic spine morphology and synaptic function by Shank and Homer

The Shank family of proteins interacts with NMDA receptor and metabotropic glutamate receptor complexes in the postsynaptic density (PSD). Targeted to the PSD by a PDZ-dependent mechanism, Shank promotes the maturation of dendritic spines and the enlargement of spine heads via its ability to recruit Homer to postsynaptic sites. Shank and Homer cooperate to induce accumulation of IP3 receptors in dendritic spines and formation of putative multisynapse spines. In addition, postsynaptic expression of Shank enhances presynaptic function, as measured by increased minifrequency and FM4-64 uptake. These data suggest a central role for the Shank scaffold in the structural and functional organization of the dendritic spine and synaptic junction.

Dynamic modulation of excitation and inhibition during stimulation at gamma and beta frequencies in the CA1 hippocampal region

Fast oscillations at gamma and beta frequency are relevant to cognition. During this activity, excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) are generated rhythmically and synchronously and are thought to play an essential role in pacing the oscillations. The dynamic changes occurring to excitatory and inhibitory synaptic events during repetitive activation of synapses are therefore relevant to fast oscillations. To cast light on this issue in the CA1 region of the hippocampal slice, we used a train of stimuli, to the pyramidal layer, comprising 1 s at 40 Hz followed by 2--3 s at 10 Hz, to mimic the frequency pattern observed during fast oscillations. Whole cell current-clamp recordings from CA1 pyramidal neurons revealed that individual stimuli at 40 Hz produced EPSPs riding on a slow biphasic hyperpolarizing-depolarizing waveform. EPSP amplitude initially increased; it then decreased concomitantly with the slow depolarization and with a large reduction in membrane resistance. During the subsequent 10-Hz train: the cells repolarized, EPSP amplitude and duration increased to above control, and no IPSPs were detected. In the presence of GABA(A) receptor antagonists, the slow depolarization was blocked, and EPSPs of constant amplitude were generated by 10-Hz stimuli. Altering pyramidal cell membrane potential affected the time course of the slow depolarization, with the peak being reached earlier at more negative potentials. Glial recordings revealed that the trains were associated with extracellular potassium accumulation, but the time course of this event was slower than the neuronal depolarization. Numerical simulations showed that intracellular chloride accumulation (due to massive GABAergic activation) can account for these observations. We conclude that synchronous activation of inhibitory synapses at gamma frequency causes a rapid chloride accumulation in pyramidal neurons, decreasing the efficacy of inhibitory potentials. The resulting transient disinhibition of the local network leads to a short-lasting facilitation of polysynaptic EPSPs. These results set constraints on the role that synchronous, rhythmic IPSPs may play in pacing oscillations at gamma frequency in the CA1 hippocampal region.

Single cocaine exposure in vivo induces long-term potentiation in dopamine neurons

How do drugs of abuse modify neural circuitry and thereby lead to addictive behaviour? As for many forms of experience-dependent plasticity, modifications in glutamatergic synaptic transmission have been suggested to be particularly important. Evidence of such changes in response to in vivo administration of drugs of abuse is lacking, however. Here we show that a single in vivo exposure to cocaine induces long-term potentiation of AMPA (alpha-amino-3-hydroxy-5-methyl-isoxazole propionic acid)-receptor-mediated currents at excitatory synapses onto dopamine cells in the ventral tegmental area. Potentiation is still observed 5 but not 10 days after cocaine exposure and is blocked when an NMDA (N-methyl-d-aspartate) receptor antagonist is administered with cocaine. Furthermore, long-term potentiation at these synapses is occluded and long-term depression is enhanced by in vivo cocaine exposure. These results show that a prominent form of synaptic plasticity can be elicited by a single in vivo exposure to cocaine and therefore may be involved in the early stages of the development of drug addiction.

Control of glutamate clearance and synaptic efficacy by glial coverage of neurons

Analysis of excitatory synaptic transmission in the rat hypothalamic supraoptic nucleus revealed that glutamate clearance and, as a consequence, glutamate concentration and diffusion in the extracellular space, is associated with the degree of astrocytic coverage of its neurons. Reduction in glutamate clearance, whether induced pharmacologically or associated with a relative decrease of glial coverage in the vicinity of synapses, affected transmitter release through modulation of presynaptic metabotropic glutamate receptors. Astrocytic wrapping of neurons, therefore, contributes to the regulation of synaptic efficacy in the central nervous system.