A developmental switch in neurotransmitter flux enhances synaptic efficacy by affecting AMPA receptor activation

Formation of glutamatergic synapses entails development of "silent" immature contacts into mature functional synapses. To determine how this transformation occurs, we investigated the development of neurotransmission at single synapses in vitro. Maturation of presynaptic function, assayed with endocytotic markers, followed accumulation of synapsin I. During this period, synaptic transmission was primarily mediated by activation of NMDA receptors, suggesting that most synapses were functionally silent. However, local glutamate application to silent synapses indicated that these synapses contained functional AMPA receptors, suggesting a possible presynaptic locus for silent transmission. Interference with presynaptic vesicle fusion by exposure to tetanus toxin reverted functional to silent transmission, implicating SNARE-mediated fusion as a determinant of the ratio of NMDA:AMPA receptor activation. This work reveals that functional maturation of synaptic transmission involves transformation of presynaptic silent secretion into mature synaptic transmitter release.     

State-dependent heterogeneity in synaptic depression between pyramidal cell pairs

Paired recordings between CA3 pyramidal neurons were used to study the properties of synaptic plasticity in active and silent synapses. Synaptic depression is accompanied by decreases in both AMPAR and NMDAR function. The mechanisms of synaptic depression, and the potential to undergo activity-dependent plastic changes in efficacy, differ depending on whether a synapse is active, recently silent, or potentiated. These results suggest that silent and active synapses represent distinct synaptic "states," and that once unsilenced, synapses express plasticity in a graded manner. The state in which a synapse resides, and the states recently visited, determine its potential and mechanism for undergoing subsequent plastic changes.     

Pair recordings reveal all-silent synaptic connections and the postsynaptic expression of long-term potentiation

The activation of silent synapses is a proposed mechanism to account for rapid increases in synaptic efficacy such as long-term potentiation (LTP). Using simultaneous recordings from individual pre- and postsynaptic neurons in organotypic hippocampal slices, we show that two CA3 neurons can be connected entirely by silent synapses. Increasing release probability or application of cyclothiazide does not produce responses from these silent synapses. Direct measurement of NMDAR-mediated postsynaptic responses in all-silent synaptic connections before and after LTP induction show no change in failure rate, amplitude, or area. These data do not support hypotheses that synapse silent results from presynaptic factors or that LTP results from increases in presynaptic glutamate release. LTP is also associated with an increase in postsynaptic responsiveness to exogenous AMPA. We conclude that synapse silence, activation, and expression of LTP are postsynaptic.     

Optimal information storage and the distribution of synaptic weights: perceptron versus Purkinje cell

It is widely believed that synaptic modifications underlie learning and memory. However, few studies have examined what can be deduced about the learning process from the distribution of synaptic weights. We analyze the perceptron, a prototypical feedforward neural network, and obtain the optimal synaptic weight distribution for a perceptron with excitatory synapses. It contains more than 50% silent synapses, and this fraction increases with storage reliability: silent synapses are therefore a necessary byproduct of optimizing learning and reliability. Exploiting the classical analogy between the perceptron and the cerebellar Purkinje cell, we fitted the optimal weight distribution to that measured for granule cell-Purkinje cell synapses. The two distributions agreed well, suggesting that the Purkinje cell can learn up to 5 kilobytes of information, in the form of 40,000 input-output associations.     

AMPA receptor activation causes silencing of AMPA receptor-mediated synaptic transmission in the developing hippocampus

Agonist-induced internalization of transmembrane receptors is a widespread biological phenomenon that also may serve as a mechanism for synaptic plasticity. Here we show that the agonist AMPA causes a depression of AMPA receptor (AMPAR) signaling at glutamate synapses in the CA1 region of the hippocampus in slices from developing, but not from mature, rats. This developmentally restricted agonist-induced synaptic depression is expressed as a total loss of AMPAR signaling, without affecting NMDA receptor (NMDAR) signaling, in a large proportion of the developing synapses, thus creating AMPAR silent synapses. The AMPA-induced AMPAR silencing is induced independently of activation of mGluRs and NMDARs, and it mimics and occludes stimulus-induced depression, suggesting that this latter form of synaptic plasticity is expressed as agonist-induced removal of AMPARs. Induction of long-term potentiation (LTP) rendered the developing synapses resistant to the AMPA-induced depression, indicating that LTP contributes to the maturation-related increased stability of these synapses. Our study shows that agonist binding to AMPARs is a sufficient triggering stimulus for the creation of AMPAR silent synapses at developing glutamate synapses.     

Incorporation of inwardly rectifying AMPA receptors at silent synapses during hippocampal long-term potentiation

Despite decades of study, the mechanisms by which synapses express the increase in strength during long-term potentiation (LTP) remain an area of intense interest. Here, we have studied how AMPA receptor subunit composition changes during the early phases of hippocampal LTP in CA1 pyramidal neurons. We studied LTP at silent synapses that initially lack AMPA receptors, but contain NMDA receptors. We show that strongly inwardly rectifying AMPA receptors are initially incorporated at silent synapses during LTP and are then subsequently replaced by non-rectifying AMPA receptors. These findings suggest that silent synapses initially incorporate GluA2-lacking, calcium-permeable AMPA receptors during LTP that are then replaced by GluA2-containing calcium-impermeable receptors. We also show that LTP consolidation at CA1 synapses requires a rise in intracellular calcium concentration during the early phase of expression, indicating that calcium influx through the GluA2-lacking AMPA receptors drives their replacement by GluA2-containing receptors during LTP consolidation. Taken together with previous studies in hippocampus and in other brain regions, these findings suggest that a common mechanism for the expression of activity-dependent glutamatergic synaptic plasticity involves the regulation of GluA2-subunit composition and highlights a critical role for silent synapses in this process.     

Energy Efficient Sparse Connectivity from Imbalanced Synaptic Plasticity Rules

It is believed that energy efficiency is an important constraint in brain evolution. As synaptic transmission dominates energy consumption, energy can be saved by ensuring that only a few synapses are active. It is therefore likely that the formation of sparse codes and sparse connectivity are fundamental objectives of synaptic plasticity. In this work we study how sparse connectivity can result from a synaptic learning rule of excitatory synapses. Information is maximised when potentiation and depression are balanced according to the mean presynaptic activity level and the resulting fraction of zero-weight synapses is around 50%. However, an imbalance towards depression increases the fraction of zero-weight synapses without significantly affecting performance. We show that imbalanced plasticity corresponds to imposing a regularising constraint on the L ₁-norm of the synaptic weight vector, a procedure that is well-known to induce sparseness. Imbalanced plasticity is biophysically plausible and leads to more efficient synaptic configurations than a previously suggested approach that prunes synapses after learning. Our framework gives a novel interpretation to the high fraction of silent synapses found in brain regions like the cerebellum.     

Activity-dependent wiring of the developing hippocampal neuronal circuit

In the developing hippocampus, functional excitatory synaptic connections seem to be recruited from a preformed, initially silent synaptic network. This functional synapse induction requires presynaptic action potentials paired with postsynaptic depolarization, thus obeying Hebb's rule of association. During early postnatal development the hippocampus exhibits an endogenous form of patterned neuronal activity that is driven by GABAergic depolarization. We propose that this recurrent activity promotes the input-specific induction of functional synapses in the CA1 region. Thus, activity-dependent synaptic reorganization in the developing hippocampus appears to be dominated by an active recruitment of new synapses rather than an active elimination of redundant connections.     

Comparing fluctuations of synaptic responses mediated via AMPA and NMDA receptor channels--implications for synaptic plasticity

Glutamate-releasing synapses are essential in fast neuronal signalling. Plasticity at these synapses is important for learning and memory as well as for the activity-dependent control of neuronal development. We have evaluated the trial-to-trial fluctuations of excitatory postsynaptic currents mediated by glutamate receptors of the AMPA and NMDA types in CA1 pyramidal cells. By using the whole cell patch clamp technique in brain slices from young rats, we have demonstrated that the relative variability of AMPA and NMDA receptor mediated responses, expressed as the coefficient of variation, is similar for these two types of responses [Brain Res. 800 (1998) 253-259]. The present paper summarizes and discusses these results in relation to current theories on hippocampal synaptic plasticity, especially with regard to the ideas of glutamate spillover and silent synapses. Our finding of a correspondence between AMPA and NMDA responses with respect to fluctuations is compatible with our previous finding of equal relative changes of the two during activity induced synaptic plasticity. However, the results argue against the glutamate spillover model according to which the effect of glutamate--and hence the induction of plasticity--may spread unspecifically between synapses. But how can silent synapses become functional if no spread of glutamate occurs and no initial signal is present to trigger the functionalization? Is it necessary that NMDA responses are present at these synapses, which are then silent merely with respect to AMPA receptors, or do other alternatives exist? Our discussion aims to elucidate these questions.     

Fueling synapses

The transmission of information across neuronal synapses is an energetically taxing business. Sheng and colleagues monitored the localization of mitochondria following different levels of synaptic activation and discovered that these organelles change their distribution in interesting ways, stalling near synapses when neurons are activated and increasing their movement when neurons are silent (Li et al., 2004 [this issue of Cell]).     

Dopaminergic stimulation of local protein synthesis enhances surface expression of GluR1 and synaptic transmission in hippocampal neurons

The use-dependent modification of synapses is strongly influenced by dopamine, a transmitter that participates in both the physiology and pathophysiology of animal behavior. In the hippocampus, dopaminergic signaling is thought to play a key role in protein synthesis-dependent forms of synaptic plasticity. The molecular mechanisms by which dopamine influences synaptic function, however, are not well understood. Using a GFP-based reporter, as well as a small-molecule reporter of endogenous protein synthesis, we show that dopamine D1/D5 receptor activation stimulates local protein synthesis in the dendrites of hippocampal neurons. We also identify the GluR1 subunit of AMPA receptors as one protein upregulated by dopamine receptor activation, with increased incorporation of surface GluR1 at synaptic sites. The insertion of new GluRs is accompanied by an increase in the frequency of miniature synaptic events. Together, these data suggest a local protein synthesis-dependent activation of previously silent synapses as a result of dopamine receptor stimulation.     

Silent synapses and the emergence of a postsynaptic mechanism for LTP

Silent synapses abound in the young brain, representing an early step in the pathway of experience-dependent synaptic development. Discovered amidst the debate over whether long-term potentiation reflects a presynaptic or a postsynaptic modification, silent synapses--which in the hippocampal CA1 subfield are characterized by the presence of NMDA receptors but not AMPA receptors--have stirred some mechanistic controversy of their own. Out of this literature has emerged a model for synapse unsilencing that highlights the central role for postsynaptic AMPA-receptor trafficking in the expression of excitatory synaptic plasticity.     

突触长时程增强形成机制的研究进展

高等动物脑内突触传递的可塑性是近30年来神经科学研究的热点,突触传递长时程增强（long-term potentiation,LTP)是神经元可塑性的反映,其形成主要与突触后机制有关。过去关于LTP机制的研究主要集中于N－甲基－D门冬氨酸（NMDA）受体的特征及该受体被激活后的细胞内级联反应,现认为脑内存在只具有NMDA受体而不具有α－氨基羟甲基恶唑丙酸（AMPA）受体的“静寂突触（silent synapse)”,这一概念的提出,使人们认识到AMPA受体在LTP表达的突触后机制中的重要作用。     

Differential modulation of chemical and electrical components of mixed synapses in the lobster stomatogastric ganglion

1. Two pairs of neurons in the pyloric network of the spiny lobster, Panulirus interruptus, communicate through mixed graded chemical and rectifying electrical synapses. The anterior burster (AB) chemically inhibits and is electrically coupled to the ventricular dilator (VD); the lateral pyloric (LP) and pyloric (PY) neurons show reciprocal chemical inhibition and electrical coupling. We examined the effects of dopamine (DA), serotonin (5HT) and octopamine (Oct) on these mixed synapses to determine the plasticity possible with opposing modes of synaptic interaction.
2. Dopamine increased net inhibition at all three pyloric mixed synapses by both reducing electrical coupling and increasing chemical inhibition. This reversed the sign of the net synaptic interaction when electrotonic coupling dominated some mixed synapses, and activated silent chemical components of other mixed synapses.
3. Serofonin weakly enhanced LP → PY net inhibition, by reducing electrical coupling without altering chemical inhibition. Serotonin reduced AB→ VD electrical coupling, but variability in its effect on the chemical component made the net effect non-significant.
4. Octopamine enhanced LP→ PY and PY→ LP net inhibition by enhancing the chemical inhibitory component without altering electrical coupling.
5. Differential modulation of chemical and electrical components of mixed synapses markedly changes the net synaptic interactions. This contributes to the flexible outputs that modulators evoke from anatomically defined neural networks.

     

Reduced expression of neuropeptide genes in a genome-wide screen of a secretion-deficient mouse

Activity-dependent changes in synapses rely on functional changes in resident proteins and on gene expression. We addressed the relationship between synapse activity and the expression of synaptic genes by comparing RNA levels in the neocortex of normal mice versus secretion-deficient and therefore synaptically silent munc18-1 (mammalian homologue of Caenorhabditis elegans uncoordinated locomotion-18) null mutants, using microarray expression analysis, real-time quantitative PCR and northern blotting. We hypothesized that genes under the control of synaptic activity would be differentially expressed between mutants and controls. We found that few synaptic genes were differentially expressed. However, most neuropeptide genes with detectable expression on the microarray were differentially expressed, being expressed 3-20-fold higher in control cortex. Several other secreted proteins were also differentially expressed, but genes encoding their receptors and many other synaptic components were not. Differential expression was confirmed by real-time quantitative PCR analysis. In situ hybridization indicated that the difference in neuropeptide expression was uniform and not due to the loss of specific cells in the mutant. In primary sensory neurons, which do not depend on synaptic activity for their input, the differential expression of neuropeptides was not observed. These data argue against a general relationship between the activity of synapses and the expression of their resident proteins, but suggest a link between secretion and the expression of genes encoding the secreted products.     

Eye opening rapidly induces synaptic potentiation and refinement

NMDA receptor (NMDAR)-mediated increases in AMPA receptor (AMPAR) currents are associated with long-term synaptic potentiation (LTP). Here, we provide evidence that similar changes occur in response to normal increases in sensory stimulation during development. Experiments discriminated between eye opening-induced and age-dependent changes in synaptic currents. At 6 hr after eye opening (AEO), a transient population of currents mediated by NR2B-rich NMDARs increase significantly, and silent synapses peak. Sustained increases in evoked and miniature AMPAR currents occur at 12 hr AEO. Significant changes in AMPAR:NMDAR evoked current ratios, contacts per axon, and inputs per cell are present at 24 hr AEO. The AMPAR current changes are those seen in vitro during NMDAR-dependent LTP. Here, they are a consequence of eye opening and are associated with a new wave of synaptic refinement. These data also suggest that new NR2B-rich NMDAR currents precede and may initiate this developmental synaptic potentiation and functional tuning.     

Silent synapses: what are they telling us about long-term potentiation?

At several cortical synapses glutamate release events can be mediated exclusively by NMDA receptors, with no detectable contribution from AMPA receptors. This observation was originally made by comparing the trial-to-trial variability of the two components of synaptic signals evoked in hippocampal neurons, and was subsequently confirmed by recording apparently pure NMDA receptor-mediated EPSCs with stimulation of small numbers of axons. It has come to be known as the 'silent synapse' phenomenon, and is widely assumed to be caused by the absence of functional AMPA receptors, which can, however, be recruited into the postsynaptic density by long-term potentiation (LTP) induction. Thus, it provides an important impetus for relating AMPA receptor trafficking mechanisms to the expression of LTP, a theme that is taken up elsewhere in this issue. This article draws attention to several findings that call for caution in identifying silent synapses exclusively with synapses without AMPA receptors. In addition, it attempts to identify several missing pieces of evidence that are required to show that unsilencing of such synapses is entirely accounted for by insertion of AMPA receptors into the postsynaptic density. Some aspects of the early stages of LTP expression remain open to alternative explanations.