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

Early presynaptic changes during plasticity in cultured hippocampal neurons

Long-lasting increase in synaptic strength is thought to underlie learning. An explosion of data has characterized changes in postsynaptic (pstS) AMPA receptor cycling during potentiation. However, changes occurring within the presynaptic (prS) terminal remain largely unknown. We show that appearance of new release sites during potentiation between cultured hippocampal neurons is due to (a) conversion of nonrecycling sites to recycling sites, (b) formation of new releasing sites from areas containing diffuse staining for the prS marker Vesicle-Associated Membrane Protein-2 and (c) budding of new recycling sites from previously existing recycling sites. In addition, potentiation is accompanied by a release probability increase in pre-existing boutons depending upon their individual probability. These prS changes precede and regulate fluorescence increase for pstS GFP-tagged-AMPA-receptor subunit GluR1. These results suggest that potentiation involves early changes in the prS terminal including remodeling and release probability increase of pre-existing synapses.

Parallel processing in two transmitter microenvironments at the cone photoreceptor synapse

A cone photoreceptor releases glutamate at ribbons located atop narrow membrane invaginations that empty onto a terminal base. The unique shape of the cone terminal suggests that there are two transmitter microenvironments: within invaginations, where concentrations are high and exposures are brief; and at the base, where concentrations are low and exposure is smoothed by diffusion. Using multicell voltage-clamp recording, we show that different subtypes of Off bipolar cells sample transmitter in two microenvironments. The dendrites of an AMPA receptor-containing cell insert into invaginations and sense rapid fluctuations in glutamate concentration that can lead to transient responses. The dendrites of kainate receptor-containing cells make basal contacts and respond to a smoothed flow of glutamate that produces sustained responses. Signaling at the cone to Off bipolar cell synapse illustrates how transmitter spillover and synapse architecture can combine to produce distinct signals in postsynaptic neurons.

Temperature-dependent shift of balance among the components of short-term plasticity in hippocampal synapses

Studies of short-term plasticity (STP) in the hippocampus, performed mostly at room temperature, have shown that small central synapses rapidly depress in response to high-frequency stimulation. This decrease in synaptic strength with synapse use places constraints on the use of STP as a dynamic filter for processing of natural high-frequency input. Here we report that, because of a strong but differential temperature dependence of STP components, the properties of STP in excitatory hippocampal synapses change dramatically with temperature. By separating the contributions of various STP processes during spike trains at different temperatures, we found a shift from dominating depression at 23 degrees C to prevailing facilitation and augmentation at 33-38 degrees C. This shift of balance among STP components resulted from a large increase in amplitudes of facilitation and augmentation (Q10 approximately 2.6 and approximately 5.1, respectively) and little change in the amplitude of depression (Q10 approximately 1.1) with temperature. These changes were accompanied by the accelerated decay of all three processes (Q10 = 3.2, 6.6, and 2.1, respectively). The balance of STP components achieved at higher temperatures greatly improved the maintenance of synaptic strength during prolonged synaptic use and had a strong effect on the processing of natural spike trains: a variable mixture of facilitated and depressed responses at 23 degrees C changed into a significantly more reproducible and depression-free filtering pattern at 33-38 degrees C. This filtering pattern was highly conserved among cells, slices, and animals, and under various physiological conditions, arguing for its physiological significance. Therefore, the fine balance among STP components, achieved only at near body temperatures, is required for the robust function of STP as a dynamic filter during natural stimulation.

Glial glutamate transporters maintain one-to-one relationship at the climbing fiber-Purkinje cell synapse by preventing glutamate spillover

A glial glutamate transporter, GLAST, is expressed abundantly in Bergmann glia and plays a major role in glutamate uptake at the excitatory synapses in cerebellar Purkinje cells (PCs). It has been reported that a higher percentage of PCs in GLAST-deficient mice are multiply innervated by climbing fibers (CFs) than in the wild-type (WT) mice, and that CF-mediated EPSCs with small amplitude and slow rise time, designated as atypical slow CF-EPSCs, are observed in these mice. To clarify the mechanism(s) underlying the generation of these atypical CF-EPSCs, we used (2S,3S)-3-[3-(4-methoxybenzoylamino)benzyloxy]aspartate (PMB-TBOA), an inhibitor of glial glutamate transporters. After the application of PMB-TBOA, slow-rising CF-EPSCs were newly detected in WT mice, and their rise and decay kinetics were different from those of conventional fast-rising CF-EPSCs but similar to those of atypical CF-EPSCs in GLAST-deficient mice. Furthermore, both slow-rising CF-EPSCs in the presence of PMB-TBOA in WT mice and atypical CF-EPSCs in GLAST-deficient mice showed much greater paired-pulse depression compared with fast-rising CF-EPSCs. In addition, both of them were more markedly inhibited by gamma-d-glutamyl-glycine, a low-affinity competitive antagonist of AMPA receptors. These results indicated that both of these types of EPSCs were mediated by a low concentration of glutamate released from neighboring CFs. Based on all of these findings, we suggest that glial transporters prevent glutamate released from a single CF from spilling over to neighboring PCs other than the synaptically connected PC, and play an essential role in the maintenance of the functional one-to-one relationship between CFs and PCs.

Distance-dependent differences in synapse number and AMPA receptor expression in hippocampal CA1 pyramidal neurons

The ability of synapses throughout the dendritic tree to influence neuronal output is crucial for information processing in the brain. Synaptic potentials attenuate dramatically, however, as they propagate along dendrites toward the soma. To examine whether excitatory axospinous synapses on CA1 pyramidal neurons compensate for their distance from the soma to counteract such dendritic filtering, we evaluated axospinous synapse number and receptor expression in three progressively distal regions: proximal and distal stratum radiatum (SR), and stratum lacunosum-moleculare (SLM). We found that the proportion of perforated synapses increases as a function of distance from the soma and that their AMPAR, but not NMDAR, expression is highest in distal SR and lowest in SLM. Computational models of pyramidal neurons derived from these results suggest that they arise from the compartment-specific use of conductance scaling in SR and dendritic spikes in SLM to minimize the influence of distance on synaptic efficacy.

The influence of multivesicular release and postsynaptic receptor saturation on transmission at granule cell to Purkinje cell synapses

The properties of a synapse are crucially dependent on whether an action potential can trigger the release of multiple vesicles at an individual release site [multivesicular release (MVR)] and whether fusion of a single vesicle leads to receptor saturation. MVR and receptor saturation both occur at some high p synapses, but it is not known whether they also occur at low p synapses. Here we examine this issue at the low p synapse between parallel fibers and Purkinje cells using the low-affinity antagonist DGG (gamma-D-glutamylglycine) to relieve AMPA receptor saturation. We find that the presence of MVR and receptor saturation at this synapse alters the calcium dependence of synaptic transmission and reduces the extent of facilitation. These findings establish that MVR and postsynaptic receptor saturation can influence transmission even at synapses with a low initial probability of release and suggest that these properties may be common at synapses in the mammalian brain.

Multisynaptic activity in a pyramidal neuron model and neural code

The highly irregular firing of mammalian cortical pyramidal neurons is one of the most striking observation of the brain activity. This result affects greatly the discussion on the neural code, i.e. how the brain codes information transmitted along the different cortical stages. In fact it seems to be in favor of one of the two main hypotheses about this issue, named the rate code. But the supporters of the contrasting hypothesis, the temporal code, consider this evidence inconclusive. We discuss here a leaky integrate-and-fire model of a hippocampal pyramidal neuron intended to be biologically sound to investigate the genesis of the irregular pyramidal firing and to give useful information about the coding problem. To this aim, the complete set of excitatory and inhibitory synapses impinging on such a neuron has been taken into account. The firing activity of the neuron model has been studied by computer simulation both in basic conditions and allowing brief periods of over-stimulation in specific regions of its synaptic constellation. Our results show neuronal firing conditions similar to those observed in experimental investigations on pyramidal cortical neurons. In particular, the variation coefficient (CV) computed from the inter-spike intervals (ISIs) in our simulations for basic conditions is close to the unity as that computed from experimental data. Our simulation shows also different behaviors in firing sequences for different frequencies of stimulation.

Multivesicular release at developing Schaffer collateral-CA1 synapses: an analytic approach to describe experimental data

We developed and analytically solved a simple and general stochastic model to distinguish the univesicular from the multivesicular mode of glutamate release. The model solution gives analytical mathematical expressions for average values of quantities that can be measured experimentally. Comparison of these quantities with the experimental measures allows one to discriminate the release mode and to determine the most probable values of model parameters. The model has been validated at glutamatergic CA3-CA1 synapses in the hippocampus from newborn (P1-P5 old) rats. Our results strongly support a multivesicular type of release process requiring a variable pool of immediately releasable vesicles. Moreover, computing quantities that are functions of the model parameters, the mean amplitude of the synaptic response to the release of a single vesicle (q) was estimated to be 5-10 pA, in very good agreement with experimental findings. In addition a multivesicular type of release was supported by the following experimental evidences: 1) a high variability of the amplitude of successes, with a coefficient of variation ranging from 0.12 to 0.73; 2) an average potency ratio a2/a1 between the second and first response to a pair of stimuli >1; and 3) changes in the potency of the synaptic response to the first stimulus when the release probability was modified by increasing or decreasing the extracellular calcium concentration. Our results indicate that at Schaffer collateral-CA1 synapses of the neonatal rat hippocampus a single action potential may induce the release of more than one vesicle from the same release site.

Noradrenaline triggers multivesicular release at glutamatergic synapses in the hypothalamus

The origin of large-amplitude miniature EPSCs (mEPSCs) at central synapses remains to be firmly established. Here, we show that at excitatory synapses onto magnocellular neurosecretory cells in the hypothalamus, noradrenaline induces a rapid and robust increase in mEPSC amplitude that requires alpha1-adrenoceptor activation but is impervious to postsynaptic manipulations that block the putative insertion of AMPA receptors. In response to noradrenaline, mEPSCs exhibit a putative multimodal amplitude histogram distribution that is not attributable to random temporal summation, the unveiling of a quiescent synapse, or the release of large vesicles. Large-amplitude mEPSCs are sensitive to a high dose of ryanodine and are associated with an enhanced glutamate cleft concentration. Together, these data are consistent with the hypothesis that large-amplitude mEPSCs result from the synchronous release of multiple vesicles via rapid presynaptic calcium expulsion from intracellular stores.

Activity-dependent regulation of vesicular glutamate and GABA transporters: a means to scale quantal size

The functional balance of glutamatergic and GABAergic signaling in neuronal cortical circuits is under homeostatic control. That is, prolonged alterations of global network activity leads to opposite changes in quantal amplitude at glutamatergic and GABAergic synapses. Such scaling of excitatory and inhibitory transmission within cortical circuits serves to restore and maintain a constant spontaneous firing rate of pyramidal neurons. Our recent work shows that this includes alterations in the levels of expression of vesicular glutamate (VGLUT1 and VGLUT2) and GABA (VIAAT) transporters. Other vesicle markers, such as synaptophysin or synapsin, are not regulated in this way. Endogenous regulation at the level of mRNA and synaptic protein controls the number of transporters per vesicle and hence, the level of vesicle filling with transmitter. Bidirectional and opposite activity-dependent regulation of VGLUT1 and VIAAT expression would serve to adjust the balance of glutamate and GABA release and therefore the level of postsynaptic receptor saturation. In some excitatory neurons and synapses, co-expression of VGLUT1 and VGLUT2 occurs. Bidirectional and opposite changes in the levels of two excitatory vesicular transporters would enable individual neocortical neurons to scale up or scale down the level of vesicular glutamate storage, and thus, the amount available for release at individual synapses. Regulated vesicular transmitter storage and release via selective changes in the level of expression of vesicular glutamate and GABA transporters indicates that homeostatic plasticity of synaptic strength at cortical synapses includes presynaptic elements.

G protein betagamma-subunits activated by serotonin mediate presynaptic inhibition by regulating vesicle fusion properties

Neurotransmitters are thought to be released as quanta, where synaptic vesicles deliver packets of neurotransmitter to the synaptic cleft by fusion with the plasma membrane. However, synaptic vesicles may undergo incomplete fusion. We provide evidence that G protein-coupled receptors inhibit release by causing such incomplete fusion. 5-hydroxytryptamine (5-HT) receptor signaling potently inhibits excitatory postsynaptic currents (EPSCs) between lamprey reticulospinal axons and their postsynaptic targets by a direct action on the vesicle fusion machinery. We show that 5-HT receptor-mediated presynaptic inhibition, at this synapse, involves a reduction in EPSC quantal size. Quantal size was measured directly by comparing unitary quantal amplitudes of paired EPSCs before and during 5-HT application and indirectly by determining the effect of 5-HT on the relationship between mean-evoked EPSC amplitude and variance. Results from FM dye-labeling experiments indicate that 5-HT prevents full fusion of vesicles. 5-HT reduces FM1-43 staining of vesicles with a similar efficacy to its effect on the EPSC. However, destaining of FM1-43-labeled vesicles is abolished by lower concentrations of 5-HT that leave a substantial EPSC. The use of a water-soluble membrane impermeant quenching agent in the extracellular space reduced FM1-43 fluorescence during stimulation in 5-HT. Thus vesicles contact the extracellular space during inhibition of synaptic transmission by 5-HT. We conclude that 5-HT, via free Gbetagamma, prevents the collapse of synaptic vesicles into the presynaptic membrane.

Rapid vesicular release, quantal variability, and spillover contribute to the precision and reliability of transmission at a glomerular synapse

The amplitude and shape of EPSC waveforms are thought to be important determinants of information processing and storage in the brain, yet relatively little is known about the origins of EPSC variability or how it affects synaptic signaling. We investigated the stochastic determinants of AMPA receptor-mediated EPSC variability at cerebellar mossy fiber-granule cell (MF-GC) connections by combining multiple-probability fluctuation analysis (MPFA) and deconvolution methods. The properties of MF connections with a single release site and the effects of the rapidly equilibrating competitive antagonist kynurenic acid on EPSCs suggest that receptors are not saturated by glutamate during a quantal event and that quanta sum linearly over a wide range of release probabilities. MPFA revealed an average of five vesicular release sites per MF-GC connection. Our results show that the time course of vesicular release is rapid (decay, tau = 75 micros) and independent of release probability, introducing little jitter in the shape or timing of the quantal component of the EPSC at physiological temperature. Moreover, the peak vesicular release rate per release site after an action potential (AP) (approximately 3 ms(-1)) is substantially higher than previously reported for central synapses. Interaction of amplitude fluctuations arising from quantal release and quantal size with the slower, low variability spillover-mediated current produce substantial variability in EPSC shape. Our simulations of MF-GC transmission suggest that quantal variability and transmitter spillover extend the voltage from which AP threshold can be crossed, improving reliability, and that fast vesicular release allows precise signaling across MF connections with heterogeneous weights.

Presynaptic regulation of quantal size by the vesicular glutamate transporter VGLUT1

A fundamental question in synaptic physiology is whether the unitary strength of a synapse can be regulated by presynaptic characteristics and, if so, what those characteristics might be. Here, we characterize a newly proposed mechanism for altering the strength of glutamatergic synapses based on the recently identified vesicular glutamate transporter VGLUT1. We provide direct evidence that filling in isolated synaptic vesicles is subject to a dynamic equilibrium that is determined by both the concentration of available glutamate and the number of vesicular transporters participating in loading. We observe that changing the number of vesicular transporters expressed at hippocampal excitatory synapses results in enhanced evoked and miniature responses and verify biophysically that these changes correspond to an increase in the amount of glutamate released per vesicle into the synaptic cleft. In addition, we find that this modulation of synaptic strength by vesicular transporter expression is endogenously regulated, both across development to coincide with a maturational increase in vesicle cycling and quantal amplitude and by excitatory and inhibitory receptor activation in mature neurons to provide an activity-dependent scaling of quantal size via a presynaptic mechanism. Together, these findings underscore that vesicular transporter expression is used endogenously to directly regulate the extent of glutamate release, providing a concise presynaptic mechanism for controlling the quantal efficacy of excitatory transmission during synaptic refinement and plasticity.

NMDA receptor subunit-dependent [Ca2+] signaling in individual hippocampal dendritic spines

Ca2+ influx through synaptic NMDA receptors (NMDA-Rs) triggers a variety of adaptive cellular processes. To probe NMDA-R-mediated [Ca2+] signaling, we used two-photon glutamate uncaging to stimulate NMDA-Rs on individual dendritic spines of CA1 pyramidal neurons in rat brain slices. We measured NMDA-R currents at the soma and NMDA-R-mediated [Ca2+] transients in stimulated spines (Delta[Ca2+]). Uncaging-evoked NMDA-R current amplitudes were independent of the size of the stimulated spine, implying that smaller spines contain higher densities of functional NMDA-Rs. The ratio of Delta[Ca2+] over NMDA-R current was highly variable (factor of 10) across spines, especially for small spines. These differences were not explained by heterogeneity in spine sizes or diffusional coupling between spines and their parent dendrites. In addition, we find that small spines have NMDA-R currents that are sensitive to NMDA-R NR2B subunit-specific antagonists. With block of NR2B-containing receptors, the range of Delta[Ca2+]/NMDA-R current ratios and their average value were much reduced. Our data suggest that individual spines can regulate the subunit composition of their NMDA-Rs and the effective fractional Ca2+ current through these receptors.

Homeostatic scaling of vesicular glutamate and GABA transporter expression in rat neocortical circuits

Homeostatic control of pyramidal neuron firing rate involves a functional balance of feedforward excitation and feedback inhibition in neocortical circuits. Here, we reveal a dynamic scaling in vesicular excitatory (vesicular glutamate transporters VGLUT1 and VGLUT2) and inhibitory (vesicular inhibitory amino acid transporter VIAAT) transporter mRNA and synaptic protein expression in rat neocortical neuronal cultures, using a well established in vitro protocol to induce homeostatic plasticity. During the second and third week of synaptic differentiation, the predominant vesicular transporters expressed in neocortical neurons, VGLUT1 and VIAAT, are both dramatically upregulated. In mature cultures, VGLUT1 and VIAAT exhibit bidirectional and opposite regulation by prolonged activity changes. Endogenous coregulation during development and homeostatic scaling of the expression of the transporters in functionally differentiated cultures may serve to control vesicular glutamate and GABA filling and adjust functional presynaptic excitatory/inhibitory balance. Unexpectedly, hyperexcitation in differentiated cultures triggers a striking increase in VGLUT2 mRNA and synaptic protein, whereas decreased excitation reduces levels. VGLUT2 mRNA and protein are expressed in subsets of VGLUT1-encoded neocortical neurons that we identify in primary cultures and in neocortex in situ and in vivo. After prolonged hyperexcitation, downregulation of VGLUT1/synaptophysin intensity ratios at most synapses is observed, whereas a subset of VGLUT1-containing boutons selectively increase the expression of VGLUT2. Bidirectional and opposite regulation of VGLUT1 and VGLUT2 by activity may serve as positive or negative feedback regulators for cortical synaptic transmission. Intracortical VGLUT1/VGLUT2 coexpressing neurons have the capacity to independently modulate the level of expression of either transporter at discrete synapses and therefore may serve as a plastic interface between subcortical thalamic input (VGLUT2) and cortical output (VGLUT1) neurons.

Short-term plasticity of Bergmann glial cell extrasynaptic currents during parallel fiber stimulation in rat cerebellum

Bergmann glial cells (BGC) enclose the synapses of Purkinje neurons (PN) and interneurons in the molecular layer of the cerebellar cortex. During synaptic transmission, glutamate evokes inward currents in the glia by activation of Ca2+-permeable aminohydroxymethylisoxazole propionic acid receptors (AMPAR) and electrogenic transporters. We describe the plasticity of BGC currents during paired-pulse and repetitive stimulation of parallel fibers in cerebellar slices. Paired-pulse facilitation (PPF) of BGC AMPAR currents was 4-fold, twice that of PN PPF. Experiments with a low-affinity AMPAR antagonist showed an increase in extrasynaptic glutamate concentration during the second pulse of the pair. PPF of glial transporter currents was 1.8-fold, similar to synaptic PPF. Tetanic stimulation revealed that facilitation of BGC AMPAR currents is not sustained during high-frequency stimulation, and substantial depression is observed after a few pulses. Consequently, Ca2+ influx through glial AMPARs would initially be facilitated but subsequently depressed, generating a transient Ca2+ influx in response to a sustained tetanus. This pattern of plasticity may be important in enabling Bergmann glial cell processes to detect and support synapses with high-frequency input. Finally, a new current was observed in BGC during repetitive stimulation. It was blocked by NBQX and intracellular GDP-beta-S, increased by glutamate uptake inhibition, had PPF similar to synaptic PPF, and was unaffected by an inhibitor of fast glial AMPAR currents. The evidence suggests that activation of neuronal AMPARs causes the release of a paracrine messenger to activate a G-protein coupled receptor in the BGC.

Multivesicular release at Schaffer collateral-CA1 hippocampal synapses

Whether an individual synapse releases single or multiple vesicles of transmitter per action potential is contentious and probably depends on the type of synapse. One possibility is that multivesicular release (MVR) is determined by the instantaneous release probability (Pr) and therefore can be controlled by activity-dependent changes in Pr. We investigated transmitter release across a range of Pr at synapses between Schaffer collaterals (SCs) and CA1 pyramidal cells in acute hippocampal slices using patch-clamp recordings. The size of the synaptic glutamate transient was estimated by the degree of inhibition of AMPA receptor EPSCs with the rapidly equilibrating antagonist gamma-D-glutamylglycine. The glutamate transient sensed by AMPA receptors depended on Pr but not spillover, indicating that multiple vesicles are essentially simultaneously released from the same presynaptic active zone. Consistent with an enhanced glutamate transient, increasing Pr prolonged NMDA receptor EPSCs when glutamate transporters were inhibited. We suggest that MVR occurs at SC-CA1 synapses when Pr is elevated by facilitation and that MVR may be a phenomenon common to many synapses throughout the CNS.

Adaptation to synaptic inactivity in hippocampal neurons

In response to activity deprivation, CNS neurons undergo slow adaptive modification of unitary synaptic transmission. The changes are comparable in degree to those induced by brief intense stimulation, but their molecular basis is largely unknown. Our data indicate that prolonged AMPAR blockade acts through loss of Ca2+ entry through L-type Ca2+ channels to bring about an increase in both vesicle pool size and turnover rate, as well as a postsynaptic enhancement of the contribution of GluR1 homomers, concentrated at the largest synapses. The changes were consistent with a morphological scaling of overall synapse size, but also featured a dramatic shift toward synaptic drive contributed by the Ca2+-permeable homomeric GluR1 receptors. These results extend beyond "synaptic homeostasis" to involve more profound changes that can be better described as "metaplasticity".

Target-specific regulation of synaptic amplitudes in the neocortex

In layers 2/3 in the rat visual cortex, glutamatergic synapses, between pyramidal neurons and GABAergic interneurons, show target-specific depression or facilitation. To study the mechanisms regulating these short-term synaptic modifications, we recorded from synaptically connected pyramidal neurons (presynaptic) and multipolar or bitufted interneurons (postsynaptic). Evoked AMPA receptor (AMPAR)- or NMDA receptor (NMDAR)-mediated EPSCs were pharmacologically isolated at these pyramidal-to-interneuron synapses while altering release probability (P(r)) by changing the extracellular Ca2+ concentration ([Ca2+]o). At the pyramidal-to-multipolar synapse, which shows paired-pulse depression, elevation of [Ca2+]o from physiological concentrations (2 mm) to 3 mm increased the amplitude of the initial AMPAR-mediated EPSC and enhanced paired-pulse depression. In contrast, the initial NMDAR-mediated EPSC did not change in amplitude with raised P(r) nor was paired-pulse depression altered. This lack of an increase of NMDAR-mediated currents is not a result of Ca2+-dependent effects on the NMDAR. Rather, at the pyramidal-to-multipolar synapse, raised P(r) increases the transient glutamate concentration at individual release sites, possibly reflecting multivesicular release. In contrast, at the pyramidal-to-bitufted synapse, which shows facilitation, AMPAR- and NMDAR-meditated EPSCs showed parallel increases in response to raised P(r). Thus, our results reveal differential recruitment of AMPA and NMDARs at depressing and facilitating synapses in layers 2/3 of the cortex and suggest that the mechanisms regulating dynamic aspects of synaptic transmission are target specific.

Number and density of AMPA receptors in single synapses in immature cerebellum

The number of ionotropic receptors in synapses is an essential factor for determining the efficacy of fast transmission. We estimated the number of functional AMPA receptors at single postsynaptic sites by a combination of two-photon uncaging of glutamate and the nonstationary fluctuation analysis in immature rat Purkinje cells (PCs), which receive a single type of excitatory input from climbing fibers. Areas of postsynaptic membrane specialization at the recorded synapses were measured by reconstruction of serial ultrathin sections. The number of functional AMPA receptors was proportional to the synaptic area with a density of approximately 1280 receptors/microm2. Moreover, highly sensitive freeze-fracture replica labeling revealed a homogeneous density of immunogold particles for AMPA receptors in synaptic sites (910 +/- 36 particles/microm2) and much lower density in extrasynaptic sites (19 +/- 2 particles/microm2) in the immature PCs. Our results indicate that in this developing synapse, the efficacy of transmission is determined by the synaptic area.

Release kinetics, quantal parameters and their modulation during short-term depression at a developing synapse in the rat CNS

We have characterized developmental changes in the kinetics and quantal parameters of action potential (AP)-evoked neurotransmitter release during maturation of the calyx of Held synapse. Quantal size (q) and peak amplitudes of evoked EPSCs increased moderately, whereas the fraction of vesicles released by single APs decreased. During synaptic depression induced in postnatal day (P) 5-7 synapses by 10-100 Hz stimulation, q declined rapidly to 40-12% of its initial value. The decrease in q was generally smaller in more mature synapses (P12-14), but quite severe for frequencies > or = 300 Hz. The stronger decline of q in immature synapses resulted from a slower recovery from desensitization, presumably due to delayed glutamate clearance. Recovery from this desensitization followed an exponential time course with a time constant of approximately 480 ms in P5-7 synapses, and sped up > 20-fold during maturation. Deconvolution analysis of EPSCs revealed a significant acceleration of the release time course during development, which was accompanied by a 2-fold increase of the peak release rate. During long 100 Hz trains, more mature synapses were able to sustain average rates of 8-10 quanta s(-1) per active zone for phasic release. The rates of asynchronous vesicle release increased transiently > 35-fold immediately after such stimuli and decayed rapidly with an exponential time constant of approximately 50 ms to low resting levels of spontaneous release. However, even following extended periods of 100 Hz stimulation, the amount of asynchronous release was relatively minor with peak rates of less than 5% of the average rate of synchronous release measured at steady state during the tetani. Therefore, a multitude of mechanisms seems to converge on the generation of fast, temporally precise and reliable high-frequency transmission at the mature calyx of Held synapse.

Amplitude/frequency of spontaneous mEPSC correlates to the degree of long-term depression in the CA1 region of the hippocampal slice

Prior synaptic or cellular activity influences degree or threshold for subsequent induction of synaptic plasticity, a process known as metaplasticity. Thus, the continual synaptic activity, spontaneous miniature excitatory synaptic current (mEPSC) may correlate to the induction of long-term depression (LTD). Here, we recorded whole-cell EPSC and mEPSC alternately in the Schaffer-CA1 synapses in brain slice of young rats, and found that this recording configuration affected neither EPSC nor mEPSC. Low frequency stimulation (LFS) induced variable magnitudes of LTD. Remarkably, larger magnitudes of LTD were significantly correlated to smaller amplitude/lower frequency of the basal mEPSC. Furthermore, under the conditions reduced amplitude/frequency of the basal mEPSC by exposure to behavioral stress immediately before slice preparation or low concentration of calcium in bath solution, the magnitudes of LTD were still inversely correlated to mEPSC amplitude/frequency. These new findings suggest that spontaneous mEPSC may reflect functional and/or structural aspects of the synapses, the synaptic history ongoing metaplasticity.

Developmental changes of GABA synaptic transient in cerebellar granule cells

The time course of synaptic currents is largely determined by the microscopic gating of the postsynaptic receptors and the temporal profile of the synaptic neurotransmitter concentration. Although several lines of evidence indicate that developmental changes of GABAergic synaptic current time course are clearly correlated with a switch in postsynaptic receptors, much less is known about the modification of GABA release during development. To address this issue, we studied the sensitivity of miniature inhibitory postsynaptic currents (mIPSCs) to a quickly dissociating competitive antagonist, 1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid (TPMPA), in neurons cultured for 6 to 8 days in vitro (DIV) ("young") and for 12 to 14 DIV ("old"). mIPSCs recorded in young neurons were significantly more resistant to the block by TPMPA. This observation was interpreted as a consequence of a more efficient displacement of TPMPA from GABA(A) receptors caused by a stronger GABA release in young neurons. The change in mIPSC sensitivity to TPMPA during development was not affected by the deletion of alpha(1) subunit, supporting its presynaptic origin. The effects of a second quickly dissociating antagonist, SR-95103 [2-(carboxy-3'-propyl)-3-amino-4-methyl-6-phenylpyridazinium chloride], on young, old, and alpha(1) -/- neurons were qualitatively the same as those obtained with TPMPA. Moreover, the analysis of current responses to ultrafast GABA applications showed that the unbinding rates of TPMPA in DIV 6 to 8 and in DIV 12 to 14 neurons are not significantly different, ruling out the postsynaptic mechanism of differential TPMPA action. Thus, we provide evidence that presynaptic GABA uniquantal release is developmentally regulated.

Short-term synaptic plasticity during development of rat mossy fibre to granule cell synapses

Changes occur during the postnatal development of the rat glutamatergic mossy fibre to granule cell synapse: to the morphology of synapses, glutamate transporter expression, AMPA receptor expression and the kinetics of AMPA receptor-mediated synaptic transmission. For example, both the rise and decay times of AMPA receptor-mediated excitatory postsynaptic currents significantly shorten. To further define the development of mossy fibre to granule cell synaptic transmission, the properties and mechanisms of short-term plasticity have been described. The characterization of short-term plasticity will aid our understanding of the mechanisms that define the parameters of synaptic transmission during development and furthermore short-term plasticity may play an important role in determining information transfer between mossy fibres and granule cells. In response to pairs of stimuli (2-100-ms interval), depression (second excitatory postsynaptic current amplitude smaller than the first) was observed at both mature (older than 40 postnatal days) and immature (between 8 and 12 postnatal days) synapses. The degree of depression was similar at both stages of development, although recovery from depression was slower at mature synapses (tau 22 vs 12.5 ms). Several experimental approaches (coefficient of variation, low-affinity antagonists and cyclothiazide) suggest that depression at immature synapses results from multiple mechanisms. At mature synapses, postsynaptic receptor desensitization appears to be the major cause of depression.

The effect of rare human sequence variants on the function of vesicular monoamine transporter 2

The extent to which genetic variation in a population contributes to phenotypic variation depends on the frequency of sequence polymorphisms and the effect of these polymorphisms on function. The frequency of polymorphisms might also reflect the severity of their effects on function. We therefore examined the effect of very rare single nucleotide polymorphisms (SNPs) on the activity of the vesicular monoamine transporter 2 (VMAT2, SLC18A2), a gene implicated in neuropsychiatric disease. Of the two rare SNPs identified in an ethnically diverse population, neither eliminates transport, but one that involves replacement of a highly conserved residue with a very similar amino acid impairs substrate recognition. This variant, and another affecting an unconserved residue, also affect inhibition by the clinically used drug reserpine. Because VMAT2 influences a form of toxicity similar to Parkinson's disease, we extended the analysis to two SNPs identified in a population with Parkinson's disease. These two SNPs have no detectable effect on most aspects of VMAT2 function, but one that affects a highly conserved residue may increase sensitivity to the inhibitor tetrabenazine. The results illustrate the relationship between conservation of the affected residue, the nature of the substitution and effects on substrate versus inhibitor interaction.

Attenuated plasticity of postsynaptic kainate receptors in hippocampal CA3 pyramidal neurons

Kainate receptor-mediated components of postsynaptic currents at hippocampal mossy fiber synapses have markedly slower kinetics than currents arising from AMPA receptors. Here, we demonstrate that other aspects of kainate and AMPA receptor function at this synapse are distinct; in particular, kainate receptor currents are less sensitive to short- and long-term increases in presynaptic strength. EPSCs arising predominantly from AMPA receptors exhibited well characterized paired-pulse facilitation, frequency facilitation, and NMDA receptor-independent long-term potentiation, whereas isolated kainate receptor synaptic currents (KA-EPSCs) exhibited attenuated facilitation and long-term potentiation. In addition, KA-EPSCs varied in their sensitivity to a low-affinity competitive antagonist, suggestive of a synaptic heterogeneity greater than that of EPSCs comprised predominantly of AMPA receptors. These data suggest that the proportional contribution of AMPA and kainate receptors to ensemble synaptic currents will vary depending on the firing frequency of mossy fiber afferents. These synaptic features may be a mechanism for limiting activation of kainate receptors at mossy fiber synapses, which has been shown to be involved in seizurogenic firing of the CA3 network.

Enhancement of synaptic plasticity through chronically reduced Ca2+ flux during uncorrelated activity

The plasticity of synapses within neural circuits is regulated by activity, but the underlying mechanisms remain elusive. Using the dye FM1-43 to directly image presynaptic function, we found that large numbers of presynaptic terminals in hippocampal cultures have a low release probability. While these terminals were not readily modifiable, a transient but not permanent long-term reduction of network activity or Ca2+ influx could increase their modifiability. This modulation of plasticity was mediated by Ca2+ flux through NMDA and voltage-gated calcium channels and was lost within 48 hr. A more permanent enhancement of synaptic plasticity was achieved by selectively reducing the Ca2+ flux associated with uncorrelated activity via adjustment of the voltage-dependent Mg2+ block of the NMDAR. Upregulation of NR2B-containing NMDARs induced by this treatment is an important but not sole contributor to the enhancement of plasticity. Thus, quantity and quality of activity have differential effects on the intrinsic plasticity of neurons.

Presynaptic source of quantal size variability at GABAergic synapses in rat hippocampal neurons in culture

The variability of quantal size depends on both presynaptic (profile of the neurotransmitter concentration in the cleft) and postsynaptic (number and gating properties of postsynaptic receptors) factors. Here we have examined the possibility that at nonsaturated synapses in cultured hippocampal neurons, changes in both the transmitter concentration peak and its clearance from the synaptic cleft may influence the variability of spontaneous miniature synaptic GABAergic currents (mIPSCs). We found that, in contrast to the slow-off GABAA receptor antagonist bicuculline, fast-off competitive antagonists such as SR-95103 and TPMPA differentially blocked small and large mIPSCs. In the presence of flurazepam, a drug believed to increase the affinity of GABA for GABAAR, small mIPSCs were enhanced more efficiently than large events. Moreover, the addition of dextran, which increases the viscosity of the extracellular fluid, preferentially increased small mIPSCs with respect to large ones. These observations suggest that changes in the concentration peak and the speed of GABA clearance in the cleft may be an important source of synaptic variability. The study of the correlation between peak amplitude and kinetics of mIPSCs allowed determination of the relative contribution of transmitter peak concentration vs. time of GABA clearance. Small synaptic responses were associated with fast onset and decay kinetics while large amplitude currents were associated with slow kinetics, indicating a crucial role for GABA synaptic clearance in variability of mIPSCs. By using model simulations we were able to estimate the range of variability of both the concentration and the speed of clearance of the GABA transient in the synaptic cleft.

Dynamism of GABAA receptor activation shapes the “personality” of inhibitory synapses

The kinetics of synaptic currents is largely determined by the postsynaptic receptor gating and the concentration time course of synaptic neurotransmitter. While the analysis of current responses to rapid agonist application provides the means to study the ligand-gated receptor gating, no direct tools are available to measure the neurotransmitter transient at GABAergic and glutamatergic synapses. Several lines of evidence indicate that the synaptic agonist transient is very brief suggesting that the activation of postsynaptic receptors occurs in conditions of extreme non-equilibrium. Such a dynamic pattern of activation has a crucial impact not only on the kinetics of synaptic currents but also on their susceptibility to pharmacological modulation. Thus, changes in the synaptic agonist waveform due to, for example modulation of the release machinery or uptake system may considerably alter both kinetics and pharmacology of synaptic currents. The use of modifiers of GABA(A) receptor gating and low-affinity antagonists provides a tool to estimate the time course of the agonist transient revealing that synaptic neurotransmitter is not saturating and that the agonist clearance occurs at a sub-millisecond time scale. It is proposed that dynamic conditions of synaptic receptor activation assure a broad spectrum of performance rendering the synapse extremely susceptible to a variety of modulatory processes.

Dissociation of experience-dependent and -independent changes in excitatory synaptic transmission during development of barrel cortex

A fundamental problem in the study of cortical development is the extent to which the formation and refinement of synaptic circuitry depends upon sensory experience. The barrel cortex is a useful model system to study experience-dependent cortical development because there is a simple mapping of individual whiskers to the corresponding barrel columns in the cortex. We investigated experience-dependent and -independent changes in glutamatergic synaptic transmission in the barrel cortex during the second postnatal week by comparing synaptic responses from whisker-intact mice at postnatal day (P) 7 and P14 with those from whisker-deprived mice at P14. alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA)-receptor-mediated excitatory synaptic responses were recorded from layer 2/3 pyramidal cells in vitro during voltage-clamp in response to stimulation in layer 4. We observed that the ratio of synaptic AMPA- to NMDA-receptor-mediated current (A/N ratio) increased with developmental age. The development of the A/N ratio was unchanged by deprivation of the whisker input throughout the second postnatal week. In contrast, the NMDA-receptor current decay and sensitivity to the NMDA receptor 2B subunit-selective antagonist ifenprodil was affected strongly by such deprivation. These results demonstrate a concurrent dissociation between sensory experience-dependent and -independent changes of glutamatergic transmission in the barrel cortex during the second postnatal week. Furthermore, they suggest that the development of subunit composition of synaptic receptors is dependent on sensory experience, whereas maturation of the synaptic A/N ratio is independent of such experience. Thus, different components of synaptic development may be governed by different developmental rules.

Properties of Quantal Transmission at CA1 Synapses

We have used Monte Carlo simulations to understand the generation of quantal responses at the single active zones of CA1 synapses. We constructed a model of AMPA channel activation that accounts for the responses to controlled glutamate application and a model of glutamate diffusion in the synaptic cleft. With no further adjustments to these models, we simulated the response to the release of glutamate from a single vesicle. The predicted response closely matches the rise time of observed responses, which recent measurements show is much faster (<100 μs) than previously thought. The simulations show that initial channel opening is driven by a brief (<100 μs) glutamate spike near the site of vesicle fusion, producing a hotspot of channel activation (diameter: ∼250 nm) smaller than many synapses. Quantal size therefore depends more strongly on the density of channels than their number, a finding that has important implications for measuring synaptic strength. Recent measurements allow estimation of AMPA receptor density at CA1 synapses. Using this value, our simulations correctly predicts a quantal amplitude of ∼10 pA. We have also analyzed the properties of excitatory postsynaptic currents (EPSCs) generated by the multivesicular release that can occur during evoked responses. We find that summation is nearly linear and that the existence of multiple narrow peaks in amplitude histograms can be accounted for. It has been unclear how to reconcile the existence of these narrow peaks, which indicate that the variation of quantal amplitude is small (CV < 0.2) with the highly variable amplitude of miniature EPSCs (mEPSCs; CV ∼ 0.6). According to one theory, mEPSC variability arises from variation in vesicle glutamate content. However, both our modeling results and recent experimental results indicate that this view cannot account for the observed rise time/amplitude correlation of mEPSCs. In contrast, this correlation and the high mEPSC variability can be accounted for if some mEPSCs are generated by two or more vesicles released with small temporal jitter. We conclude that a broad range of results can be accounted for by simple principles: quantal amplitude (∼10 pA) is stereotyped, some mEPSCs are multivesicular at moderate and large synapses, and evoked responses are generated by quasi-linear summation of multiple quanta.

Differential modulation of beta2 and beta4 subunits of human neuronal nicotinic acetylcholine receptors by acidification

We have shown previously that acidification increases the affinity of agonists to rat alpha3beta4 nicotinic acetylcholine receptors (nAChR) and accelerates both the activation and decay kinetics of agonist-induced currents recorded from human embryonic kidney 293 cells stably expressing the receptor (Abdrakhmanova et al., 2002b). Here, we report on experiments examining the effect of rapid acidification on four different subtypes (alpha3beta4alpha5, alpha4beta2, alpha3beta2, and alpha3beta2alpha5) of human neuronal nAChRs stably expressed in tsA201 cells using a piezoelectric device for rapid (<5 ms) solution application. Application of ACh, at its EC(50) concentration for each nAChR subtype, at pH values 7.4 and 6.0, showed that acidification, similarly to that reported for rat alpha3beta4 acetylcholine receptors (AChRs), increased the amplitude and accelerated the activation and decay kinetics of the currents in human alpha3beta4alpha5 AChRs by increasing their affinity to the agonist. In sharp contrast, acidification reduced the amplitude but accelerated the decay kinetics of the current in all human beta2-containing nAChR subtypes (alpha3beta2, alpha3alpha5beta2, alpha4beta2) examined in this study. Brief application of ACh at saturating concentration (1 mM) on alpha3beta4alpha5 AChRs induced a "rebound current" upon rapid washout of the agonist at pH 7.4, but no "rebound current" was observed in alpha3beta2 AChRs. Surprisingly, acidification, pH 6.0, applied only during the agonist pulse also accelerated the decay kinetics of the "rebound current". Our data provide evidence for the specificity of proton-induced modulation of neuronal nAChRs based on their beta subunit composition. Furthermore, in alpha3beta4alpha5 AChR, we find that protonation effects may persist, after washout of acidic solutions, consistent with proton-induced conformational changes of the receptor.

Contribution of astrocytes to synaptic transmission in the rat supraoptic nucleus

Astrocytes, besides supporting metabolic and scaffolding functions, play a prominent role in the modulation of neuronal communication. In particular, they are responsible for clearing synaptically-released glutamate via highly specific transporters located on their plasma membrane. Since glutamate is the main excitatory neurotransmitter in the central nervous system (CNS), astrocytes are likely to play a central role in the regulation of synaptic processing and overall cellular excitability. We recently investigated the influence of astrocytes on glutamatergic and GABAergic transmission in the rat supraoptic nucleus (SON) of the hypothalamus. This nucleus is part of the hypothalamus-neurohypophysial system (HNS), which constitutes a conspicuous example of activity-dependent neuroglial plasticity, in which certains physiological conditions, such as parturition, lactation, and dehydration are accompanied by a structural remodeling of the neurones, their synaptic inputs and their surrounding glia. The use of pharmacological inhibitors of glutamate transporters on this model, in which a physiological change in the astrocyte environment occurs, has brought new insights on the contribution of astrocytes to both excitatory and inhibitory neurotransmissions. The astrocytic environment of neurons appears to control glutamate uptake and diffusion in the extracellular space. This has direct repercussions on the tonic level of activation of presynaptic glutamate receptors and, as a consequence, on the release of neurotransmitter. This short review summarizes data obtained so far, which clearly support the view that astrocytes are indeed a third partner in synaptic transmission, and which show that the supraoptic nucleus represents a remarkable model to study dynamic physiological interactions between astrocytes and neurons.

Distinct quantal features of AMPA and NMDA synaptic currents in hippocampal neurons: implication of glutamate spillover and receptor saturation

Excitatory postsynaptic currents (EPSCs) were studied in the CA1 pyramidal cells of rat hippocampal slices. Components mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) and by N-methyl-D-aspartate (NMDA) receptors were separated pharmacologically. Quantal parameters of AMPA and NMDA receptor-mediated EPSCs were obtained using both maximal likelihood and autocorrelation techniques. Enhancement of transmitter release with 4-aminopyridine caused a significant increase in quantal size of NMDA EPSC. This was accompanied by a slowing of the EPSC decay. The maximal number of quanta in the NMDA current was unchanged, while the probability of quantal event dramatically enhanced. In contrast, neither the quantal size nor the kinetics of AMPA EPSC was altered by 4-aminopyridine, while the maximal number of quanta increased. These changes in the quantal parameters are consistent with a transition to multivesicular release of the neurotransmitter. Spillover of excessive glutamate on the nonsynaptic areas of dendritic spines causes an increase in the quantal size of NMDA synaptic current. The difference in quantal behavior of AMPA and NMDA EPSCs implies that different mechanisms underlie their quantization: the additive response of nonsaturated AMPA receptors contrasts with the variable involvement of saturated intrasynaptic and nonsaturated extrasynaptic NMDA receptors.

Variance-Mean Analysis in the Presence of a Rapid Antagonist Indicates Vesicle Depletion Underlies Depression at the Climbing Fiber Synapse

Many types of synapses throughout the nervous system are transiently depressed during high-frequency stimulation. Several mechanisms have been proposed to account for this depression, including depletion of release-ready vesicles. However, numerous studies have raised doubts about the importance of depletion in depression of central synapses and have implicated alternative mechanisms, such as decreased release probability. We use variance-mean analysis to determine the mechanism of depression at the climbing fiber to Purkinje cell synapse. We find that postsynaptic receptor saturation makes it difficult to distinguish between a decrease in available vesicles and a reduction in release probability. When AMPA receptor saturation is relieved with a low-affinity antagonist, variance-mean analysis reveals that depression arises from a decrease in the number of release-ready vesicles. Vesicle depletion is prominent, despite numerous docked vesicles at each release site, due to multivesicular release. We conclude that vesicle depletion can contribute significantly to depression of central synapses.

Does the fusion pore contribute to synaptic plasticity?

How synapses change their strength in response to impinging neural activity is a fundamental issue for understanding how the brain molds its circuitry in response to behavioral experience. Although a growing number of studies reveal the involvement of postsynaptic changes contributing to synaptic plasticity in brain circuits, the involvement of presynaptic factors has been implied by several studies. Most recently, several works point to the mechanism of vesicle fusion as a new possible locus for the modification of presynaptic synaptic strength. However, it is not yet clear to what extent such changes affect the simplest form of information transfer in the brain--the transmission of a single action potential.

Wavelet analysis of nonstationary fluctuations of Monte Carlo-simulated excitatory postsynaptic currents

Tracking spectral changes of rapidly varying signals is a demanding task. In this study, we explore on Monte Carlo-simulated glutamate-activated AMPA patch and synaptic currents whether a wavelet analysis offers such a possibility. Unlike Fourier methods that determine only the frequency content of a signal, the wavelet analysis determines both the frequency and the time. This is owing to the nature of the basis functions, which are infinite for Fourier transforms (sines and cosines are infinite), but are finite for wavelet analysis (wavelets are localized waves). In agreement with previous reports, the frequency of the stationary patch current fluctuations is higher for larger currents, whereas the mean-variance plots are parabolic. The spectra of the current fluctuations and mean-variance plots are close to the theoretically predicted values. The median frequency of the synaptic and nonstationary patch currents is, however, time dependent, though at the peak of synaptic currents, the median frequency is insensitive to the number of glutamate molecules released. Such time dependence demonstrates that the "composite spectra" of the current fluctuations gathered over the whole duration of synaptic currents cannot be used to assess the mean open time or effective mean open time of AMPA channels. The current (patch or synaptic) versus median frequency plots show hysteresis. The median frequency is thus not a simple reflection of the overall receptor saturation levels and is greater during the rise phase for the same saturation level. The hysteresis is due to the higher occupancy of the doubly bound state during the rise phase and not due to the spatial spread of the saturation disk, which remains remarkably constant. Albeit time dependent, the variance of the synaptic and nonstationary patch currents can be accurately determined. Nevertheless the evaluation of the number of AMPA channels and their single current from the mean-variance plots of patch or synaptic currents is not highly accurate owing to the varying number of the activatable AMPA channels caused by desensitization. The spatial nonuniformity of open, bound, and desensitized AMPA channels, and the time dependence and spatial nonuniformity of the glutamate concentration in the synaptic cleft, further reduce the accuracy of estimates of the number of AMPA channels from synaptic currents. In conclusion, wavelet analysis of nonstationary fluctuations of patch and synaptic currents expands our ability to determine accurately the variance and frequency of current fluctuations, demonstrates the limits of applicability of techniques currently used to evaluate the single channel current and number of AMPA channels, and offers new insights into the mechanisms involved in the generation of unitary quantal events at excitatory central synapses.

Two Loci of expression for long-term depression at hippocampal mossy fiber-interneuron synapses

Two distinct forms of long-term depression (LTD) exist at mossy fiber synapses between dentate gyrus granule cells and hippocampal CA3 stratum lucidum interneurons. Although induction of each form of LTD requires an elevation of postsynaptic intracellular Ca2+, at Ca2+-impermeable AMPA receptor (CI-AMPAR) synapses, induction is NMDA receptor (NMDAR) dependent, whereas LTD at Ca2+-permeable AMPA receptor (CP-AMPAR) synapses is NMDAR independent. However, the expression locus of either form of LTD is not known. Using a number of criteria, including the coefficient of variation, paired-pulse ratio, AMPA-NMDA receptor activity, and the low-affinity AMPAR antagonist gamma-D-glutamyl-glycine, we demonstrate that LTD expression at CP-AMPAR synapses is presynaptic and results from reduced transmitter release, whereas LTD expression at CI-AMPAR synapses is postsynaptic. The N-ethylmaleimide-sensitive fusion protein-AP2-clathrin adaptor protein 2 inhibitory peptide pep2m occluded LTD expression at CI-AMPAR synapses but not at CP-AMPAR synapses, confirming that CI-AMPAR LTD involves postsynaptic AMPAR trafficking. Thus, mossy fiber innervation of CA3 stratum lucidum interneurons occurs via two parallel systems targeted to either Ca2+-permeable or Ca2+-impermeable AMPA receptors, each with a distinct expression locus for long-term synaptic plasticity.

Intracellular Chloride Channels: Determinants of Function in the Endosomal Pathway

Endosomes, and related subcellular compartments, contain various Cl- channels in the ClC family. In this review, we describe the known roles of intracellular Cl- channels and also explore some of the functional implications of transmembrane Cl- flux in these organelles. Cl- influx acts to control intralumenal pH, both by shunting the effects of the proton pump on membrane potential and, possibly, through direct effects of Cl- on the proton pump. Changes in intralumenal pH likely help regulate membrane trafficking. We propose that changes in intralumenal Cl- concentration ([Cl-]) could theoretically play a direct role in regulating membrane trafficking and organellar function through effects on chloride-sensitive proteins in the vesicular membrane, which could transduce information about intralumenal [Cl-] to the outside of the vesicle and thereby recruit various signaling molecules. We present a model in which regulation of cytosolic [Cl-] and vesicular Cl- conductance could help control the amount or type of neurotransmitter stored in a particular population of synaptic vesicles.

Postfusional Control of Quantal Current Shape

Whether glutamate is released rapidly, in an all-or-none manner, or more slowly, in a regulated manner, is a matter of debate. We analyzed the time course of excitatory postsynaptic currents (EPSCs) at glutamatergic neuromuscular junctions of Drosophila and found that the decay phase of EPSCs was protracted to a variable extent. The protraction was more pronounced in evoked and spontaneous quantal EPSCs than in action potential-evoked multiquantal EPSCs; reduced in quantal EPSCs from endophilin null mutants, which maintain release via kiss-and-run; and dependent on synaptotagmin isoform, calcium, and protein phosphorylation. Our data indicate that glutamate is released from individual synaptic vesicles for milliseconds through a fusion pore. Quantal glutamate discharge time course depends on presynaptic calcium inflow and the molecular composition of the release machinery.

Saturation in excitatory synapses of hippocampus investigated by computer simulations

The standard view of the synaptic function in excitatory synapses has been deeply questioned by recent experimental data on hippocampal glutamate synapses both for possible receptor nonsaturation and for larger and non-Gaussian peak amplitude fluctuations. Our previous investigations of the mechanisms involved in the variability of the response of hippocampal glutamatergic synapses, carried out by computer simulation of simple Brownian models of glutamate diffusion, furnished initial evidence about their presynaptic character. A new, refined model, reported here, assumes a collision volume for the glutamate molecule and a more realistic description of receptors and their binding dynamics. Based on this model, conditions for AMPA and NMDA receptor saturation have been investigated and new miniature (or quantal) EPSC parameters have been computed. The results corroborate the hypothesis that the lack of AMPA and NMDA receptor saturation and the EPSC stochastic variability are attributable to the small volume of glutamatergic synaptic vesicles and hence to the small number of glutamate molecules diffusing in the cleft after a vesicle release. The investigations better characterize some not well-known elements of the synaptic structure, such as the fusion pore, and provide useful information on AMPA receptor dynamics. Indeed, a nice fit between computed EPSCs and some miniature EPSCs in recent experimental literature allowed for the computation of new transition time values among the different AMPA receptor states through a trial-and-error optimization procedure. Moreover, the model has been used to evaluate two hypotheses on the genesis of the long-term potentiation phenomenon.

The number of glutamate receptors opened by synaptic stimulation in single hippocampal spines

The number of receptors opening after glutamate release is critical for understanding the sources of noise and the dynamic range of synaptic transmission. We imaged [Ca2+] transients mediated by synaptically activated NMDA receptors (NMDA-Rs) in individual spines in rat brain slices. We show that Ca2+ influx through single NMDA-Rs can be reliably detected, allowing us to estimate the number of receptors opening after synaptic transmission. This number is small: at the peak of the synaptic response, less than one NMDA-R is open, on average. Therefore, stochastic interactions between transmitter and receptor contribute substantially to synaptic noise, and glutamate occupies a small fraction of receptors. The number of receptors opening did not scale with spine volume, and smaller spines experience larger [Ca2+] transients during synaptic transmission. Our measurements further demonstrate that optical recordings can be used to study single receptors in intact systems.

An essential role for vesicular glutamate transporter 1 (VGLUT1) in postnatal development and control of quantal size

Quantal neurotransmitter release at excitatory synapses depends on glutamate import into synaptic vesicles by vesicular glutamate transporters (VGLUTs). Of the three known transporters, VGLUT1 and VGLUT2 are expressed prominently in the adult brain, but during the first two weeks of postnatal development, VGLUT2 expression predominates. Targeted deletion of VGLUT1 in mice causes lethality in the third postnatal week. Glutamatergic neurotransmission is drastically reduced in neurons from VGLUT1-deficient mice, with a specific reduction in quantal size. The remaining activity correlates with the expression of VGLUT2. This reduction in glutamatergic neurotransmission can be rescued and enhanced with overexpression of VGLUT1. These results show that the expression level of VGLUTs determines the amount of glutamate that is loaded into vesicles and released and thereby regulates the efficacy of neurotransmission.

Structural contributions to short-term synaptic plasticity

Synaptic ultrastructure is critical to many basic hypotheses about synaptic transmission. Various aspects of synaptic ultrastructure have also been implicated in the mechanisms of short-term plasticity. These forms of plasticity can greatly affect synaptic strength during ongoing activity. We review the evidence for how synaptic ultrastructure may contribute to facilitation, depletion, saturation, and desensitization.

Local structural balance and functional interaction of excitatory and inhibitory synapses in hippocampal dendrites

Theoretical and experimental studies on the computation of neural networks suggest that neural computation results from a dynamic interplay of excitatory and inhibitory (E/I) synaptic inputs. Precisely how E/I synapses are organized structurally and functionally to facilitate meaningful interaction remains elusive. Here we show that E/I synapses are regulated across dendritic trees to maintain a constant ratio of inputs in cultured rat hippocampal neurons. This structural arrangement is accompanied by an E/I functional balance maintained by a 'push-pull' feedback regulatory mechanism that is capable of adjusting E/I efficacies in a coordinated fashion. We also found that during activity, inhibitory synapses can determine the impact of adjacent excitatory synapses only if they are colocalized on the same dendritic branch and are activated simultaneously. These fundamental relationships among E/I synapses provide organizational principles relevant to deciphering the structural and functional basis for neural computation within dendritic branches.

Roles of glutamate transporters in shaping excitatory synaptic currents in cerebellar Purkinje cells

Several subtypes of glutamate transporters are abundantly expressed near the excitatory synapses on cerebellar Purkinje cells. We investigated the roles of the glutamate transporters in shaping the excitatory postsynaptic currents (EPSCs) and regulating the levels of extracellular glutamate in the mouse cerebellum using a potent blocker of glutamate transporters, dl-threo-beta-benzyloxyaspartate (dl-TBOA). This drug markedly prolonged AMPA receptor-mediated EPSCs in Purkinje cells evoked by stimulating both parallel fibres and climbing fibres. The decay phase of the prolonged EPSCs was fitted by double exponentials, of which the slower component was preferentially inhibited by a low-affinity competitive antagonist of AMPA receptors, gamma-d-glutamyl-glycine, indicating that the slow component induced by dl-TBOA was the AMPA receptor-mediated current activated by lower concentrations of glutamate than those contributing to the peak of the EPSC. This result suggests that dl-TBOA prolongs the stay of synaptically released glutamate in the synaptic cleft and also induces glutamate spillover to extrasynaptic targets as well as neighbouring synapses. Furthermore, high concentrations of dl-TBOA in the presence of cyclothiazide generated a continuous inward current in Purkinje cells, of which the amplitude reached the peak level of the climbing-fibre EPSC. This continuous inward current was abolished by the blocker of AMPA receptors, indicating that the strong inhibition of glutamate uptake causes the rapid accumulation of glutamate in the extracellular space. These results highlight the importance of glutamate transporters in maintaining the proper glutamatergic transmission in Purkinje cell synapses.

Paired-pulse depression of unitary quantal amplitude at single hippocampal synapses

At central synapses, quantal size is generally regarded as fluctuating around a fixed mean with little change during short-term synaptic plasticity. We evoked quantal release by brief electric stimulation at single synapses visualized with FM 1-43 dye in hippocampal cultures. The majority of quantal events evoked at single synapses were monovesicular, based on examination of amplitude distribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-receptor-mediated responses. Consistent with previous findings, the quantal size did not change during paired-pulse facilitation (PPF), supporting the notion that the evoked events were monoquantal. However, during paired-pulse depression (PPD), there was a significant decrease in unitary quantal size, which was not due to postsynaptic receptor desensitization. This asymmetry of quantal modulation during PPF and PPD was demonstrated at the same single synapse at different extracellular calcium concentrations. Our results indicate that PPF can be fully accounted for by an increase of release probability, whereas PPD may be caused by decreases in both release probability and quantal size. One possible explanation is that the release of a quantum of neurotransmitter from synaptic vesicles is not invariant but subject to rapid calcium-dependent modulation during short-term synaptic plasticity.

Simulation modeling of ligand receptor interactions at non-equilibrium conditions: processing of noisy inputs by ionotropic receptors

The first event in signal transduction at a synapse is the binding of transmitters to receptors. Because of rapidly changing transmitter levels this binding is unlikely to occur at equilibrium. We describe a mathematical approach that models complex receptor interactions in which the timing and amplitude of transmitter release are noisy. We show that exact solutions for simple bimolecular interactions and receptor transitions can be used to model complex reaction schemes by expressing them in sets of difference equations. Results from the difference equation method to describe binding and channel opening at extended time points compare well with standard solutions using ordinary differential equations. Because it is applicable to noisy systems we used the difference method to investigate the information processing capabilities of GABA receptors and predict how pharmacological agents may modify these properties. As previously demonstrated, the response to a single pulse of GABA is prolonged through entry into a desensitized state. During trains of stimuli the signal to noise ratio can change, and even increase progressively, but the overall transmitted fidelity of the signal decreases with increased driving frequency. The GABA modulator chlorpromazine (primarily affects agonist on and off rates) is predicated to increase receptor signal to noise ratio at all frequencies whereas pregnenolone sulfate (affects receptor desensitization) completely inhibits information transfer.

Glial modulation of synaptic transmission: Insights from the supraoptic nucleus of the hypothalamus

Astrocytes clear synaptically released glutamate from the extracellular space through high-affinity transporters present on their plasma membrane. By controlling the extracellular level of the main excitatory transmitter in the central nervous system, astrocytes thus contribute prominently to the regulation of overall cellular excitability and synaptic information processing. We recently investigated the influence of the glial environment on glutamatergic and GABAergic neurotransmission in the supraoptic nucleus of the rat hypothalamus under physiological conditions such as lactation that significantly reduce astrocytic coverage of its neurons. By performing electrophysiological analyses on this unique model of dynamic neuronal-glial interactions, we have been able to show that the fine astrocytic processes normally enwrapping synapses serve two important functions. First, they govern the level of activation of presynaptic metabotropic glutamate receptors on glutamatergic terminals, thereby regulating synaptic efficacy at excitatory synapses. Second, they act as a physical and functional barrier to diffusion in the extracellular space, limiting spillover of glutamate and other neuroactive substances and therefore contributing to the regulation of heterosynaptic transmission and intercellular communication.