Developmental presence and disappearance of postsynaptically silent synapses on dendritic spines of rat layer 2/3 pyramidal neurons

H-Ras induces exuberant de novo dendritic protrusion growth in mature neurons regardless of cell type

Dendritic protrusions, mainly spines and filopodia, correlate with excitatory synapses in pyramidal neurons (PyNs), but this relationship may not apply universally. We found that ectopic H-Ras expression increased protrusions across various cortical cell types, including layer 2/3 PyNs, parvalbumin (PV)-, and vasoactive intestinal peptide (VIP)-positive interneurons (INs) in the primary motor cortex. The probability of detecting protrusions correlated with local H-Ras activity, indicating its role in protrusion formation. H-Ras overexpression led to high turnover rates by adding protrusions. Two-photon photolysis of glutamate induced de novo spine formation in mature H-Ras expressing neurons, suggesting H-Ras's effect is not limited to early development. In PyNs and PV-INs, but not VIP-INs, spine neck lengths shifted to filopodia-like phenotypes. H-Ras primarily induced filopodia in PyNs and spines in PV- and VIP-INs. Increased protrusions in H-Ras-transfected PyNs lacked key excitatory synaptic proteins and did not affect miniature excitatory postsynaptic currents (mEPSCs), suggesting multifaceted roles beyond excitatory synapses.

Half a century legacy of long-term potentiation

In 1973, two papers from Bliss and Lømo and from Bliss and Gardner-Medwin reported that high-frequency synaptic stimulation in the dentate gyrus of rabbits resulted in a long-lasting increase in synaptic strength. This form of synaptic plasticity, commonly referred to as long-term potentiation (LTP), was immediately considered as an attractive mechanism accounting for the ability of the brain to store information. In this historical piece looking back over the past 50 years, we discuss how these two landmark contributions directly motivated a colossal research effort and detail some of the resulting milestones that have shaped our evolving understanding of the molecular and cellular underpinnings of LTP. We highlight the main features of LTP, cover key experiments that defined its induction and expression mechanisms, and outline the evidence supporting a potential role of LTP in learning and memory. We also briefly explore some ramifications of LTP on network stability, consider current limitations of LTP as a model of associative memory, and entertain future research orientations.

Synaptic memory and CaMKII

Ca2+/calmodulin-dependent protein kinase II (CaMKII) and long-term potentiation (LTP) were discovered within a decade of each other and have been inextricably intertwined ever since. However, like many marriages, it has had its up and downs. Based on the unique biochemical properties of CaMKII, it was proposed as a memory molecule before any physiological linkage was made to LTP. However, as reviewed here, the convincing linkage of CaMKII to synaptic physiology and behavior took many decades. New technologies were critical in this journey, including in vitro brain slices, mouse genetics, single-cell molecular genetics, pharmacological reagents, protein structure, and two-photon microscopy, as were new investigators attracted by the exciting challenge. This review tracks this journey and assesses the state of this marriage 40 years on. The collective literature impels us to propose a relatively simple model for synaptic memory involving the following steps that drive the process: 1) Ca2+ entry through N-methyl-d-aspartate (NMDA) receptors activates CaMKII. 2) CaMKII undergoes autophosphorylation resulting in constitutive, Ca2+-independent activity and exposure of a binding site for the NMDA receptor subunit GluN2B. 3) Active CaMKII translocates to the postsynaptic density (PSD) and binds to the cytoplasmic C-tail of GluN2B. 4) The CaMKII-GluN2B complex initiates a structural rearrangement of the PSD that may involve liquid-liquid phase separation. 5) This rearrangement involves the PSD-95 scaffolding protein, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), and their transmembrane AMPAR-regulatory protein (TARP) auxiliary subunits, resulting in an accumulation of AMPARs in the PSD that underlies synaptic potentiation. 6) The stability of the modified PSD is maintained by the stability of the CaMKII-GluN2B complex. 7) By a process of subunit exchange or interholoenzyme phosphorylation CaMKII maintains synaptic potentiation in the face of CaMKII protein turnover. There are many other important proteins that participate in enlargement of the synaptic spine or modulation of the steps that drive and maintain the potentiation. In this review we critically discuss the data underlying each of the steps. As will become clear, some of these steps are more firmly grounded than others, and we provide suggestions as to how the evidence supporting these steps can be strengthened or, based on the new data, be replaced. Although the journey has been a long one, the prospect of having a detailed cellular and molecular understanding of learning and memory is at hand.

Exuberant de novo dendritic spine growth in mature neurons

Dendritic spines are structural correlates of excitatory synapses maintaining stable synaptic communications. However, this strong spine-synapse relationship was mainly characterized in excitatory pyramidal neurons (PyNs), raising a possibility that inferring synaptic density from dendritic spine number may not be universally applied to all neuronal types. Here we found that the ectopic expression of H-Ras increased dendritic spine numbers regardless of cortical cell types such as layer 2/3 pyramidal neurons (PyNs), parvalbumin (PV)- and vasoactive intestinal peptide (VIP)-positive interneurons (INs) in the primary motor cortex (M1). The probability of detecting dendritic spines was positively correlated with the magnitude of H-Ras activity, suggesting elevated local H-Ras activity is involved in the process of dendritic spine formation. H-Ras overexpression caused high spine turnover rate via adding more spines rather than eliminating them. Two-photon photolysis of glutamate triggered de novo dendritic spine formation in mature neurons, suggesting H-Ras induced spine formation is not restricted to the early development. In PyNs and PV-INs, but not VIP-INs, we observed a shift in average spine neck length towards longer filopodia-like phenotypes. The portion of dendritic spines lacking key excitatory synaptic proteins were significantly increased in H-Ras transfected neurons, suggesting that these increased spines have other distinct functions. High spine density caused by H-Ras did not result in change in the frequency or the amplitude of miniature excitatory postsynaptic currents (mEPSCs). Thus, our results propose that dendritic spines possess more multifaceted functions beyond the morphological proxy of excitatory synapse.

miR-34a regulates silent synapse and synaptic plasticity in mature hippocampus

AMPAR-lacking silent synapses are prevailed and essential for synaptic refinement and synaptic plasticity in developing brains. In mature brain, they are sparse but could be induced under several pathological conditions. How they are regulated molecularly is far from clear. miR-34a is a highly conserved and brain-enriched microRNA with age-dependent upregulated expression profile. Its neuronal function in mature brain remains to be revealed. Here by analyzing synaptic properties of the heterozygous miR-34a knock out mice (34a_ht), we have discovered that mature but not juvenile 34a_ht mice have more silent synapses in the hippocampus accompanied with enhanced synaptic NMDAR but not AMPAR function and increased spine density. As a result, 34a_ht mice display enhanced long-term potentiation (LTP) in the Schaffer collateral synapses and better spatial learning and memory. We further found that Creb1 is a direct target of miR-34a, whose upregulation and activation may mediate the silent synapse increment in 34a_ht mice. Hence, we reveal a novel physiological role of miR-34a in mature brains and provide a molecular mechanism underlying silent synapse regulation.

How Are Synapses Born? A Functional and Molecular View of the Role of the Wnt Signaling Pathway

Synaptic transmission is a dynamic process that requires precise regulation. Early in life, we must be able to forge appropriate connections (add and remove) to control our behavior. Neurons must recognize appropriate targets, and external soluble factors that activate specific signaling cascades provide the regulation needed to achieve this goal. Wnt signaling has been implicated in several forms of synaptic plasticity, including functional and structural changes associated with brain development. The analysis of synapses from an electrophysiological perspective allows us to characterize the functional role of cellular signaling pathways involved in brain development. The application of quantal theory to principles of developmental plasticity offers the possibility of dissecting the function of structural changes associated with the birth of new synapses as well as the maturation of immature silent synapses. Here, we focus on electrophysiological and molecular evidence that the Wnt signaling pathway regulates glutamatergic synaptic transmission, specifically N-methyl-d-aspartate receptors (NMDARs), to control the birth of new synapses. We also focus on the role of Wnts in the conversion of silent synapses into functional synapses.

Filopodia are a structural substrate for silent synapses in adult neocortex

Newly generated excitatory synapses in the mammalian cortex lack sufficient AMPA-type glutamate receptors to mediate neurotransmission, resulting in functionally silent synapses that require activity-dependent plasticity to mature. Silent synapses are abundant in early development, during which they mediate circuit formation and refinement, but they are thought to be scarce in adulthood¹. However, adults retain a capacity for neural plasticity and flexible learning that suggests that the formation of new connections is still prevalent. Here we used super-resolution protein imaging to visualize synaptic proteins at 2,234 synapses from layer 5 pyramidal neurons in the primary visual cortex of adult mice. Unexpectedly, about 25% of these synapses lack AMPA receptors. These putative silent synapses were located at the tips of thin dendritic protrusions, known as filopodia, which were more abundant by an order of magnitude than previously believed (comprising about 30% of all dendritic protrusions). Physiological experiments revealed that filopodia do indeed lack AMPA-receptor-mediated transmission, but they exhibit NMDA-receptor-mediated synaptic transmission. We further showed that functionally silent synapses on filopodia can be unsilenced through Hebbian plasticity, recruiting new active connections into a neuron's input matrix. These results challenge the model that functional connectivity is largely fixed in the adult cortex and demonstrate a new mechanism for flexible control of synaptic wiring that expands the learning capabilities of the mature brain.

Wnt-5a induces the conversion of silent to functional synapses in the hippocampus

Synapse unsilencing is an essential mechanism for experience-dependent plasticity. Here, we showed that the application of the ligand Wnt-5a converts glutamatergic silent synapses into functional ones by increasing both α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) currents (IAMPA and INMDA, respectively). These effects were mimicked by the hexapeptide Foxy-5 and inhibited by secreted frizzled-related protein sFRP-2. INMDA potentiation was produced by increased synaptic potency, followed by an increase in the probability of release (Pr), even in the presence of 7-nitro-2,3-dioxo-1,4-dihydroquinoxaline-6-carbonitrile (CNQX). At a longer time of Wnt-5a exposure, the Pr increments were higher in INMDA than in IAMPA. In the presence of NMDAR inhibitors, Wnt-5a-induced conversion was fully inhibited in 69.0% of silent synapses, whereas in the remaining synapses were converted into functional one. Our study findings showed that the Wnt-5a-activated pathway triggers AMPAR insertion into mammalian glutamatergic synapses, unsilencing non-functional synapses and promoting the formation of nascent synapses during the early postnatal development of the brain circuits.

A Synthetic Likelihood Solution to the Silent Synapse Estimation Problem

Functional features of synaptic populations are typically inferred from random electrophysiological sampling of small subsets of synapses. Are these samples unbiased? Here, we develop a biophysically constrained statistical framework to address this question and apply it to assess the performance of a widely used method based on a failure-rate analysis to quantify the occurrence of silent (AMPAR-lacking) synapses. We simulate this method in silico and find that it is characterized by strong and systematic biases, poor reliability, and weak statistical power. Key conclusions are validated by whole-cell recordings from hippocampal neurons. To address these shortcomings, we develop a simulator of the experimental protocol and use it to compute a synthetic likelihood. By maximizing the likelihood, we infer silent synapse fraction with no bias, low variance, and superior statistical power over alternatives. Together, this generalizable approach highlights how a simulator of experimental methodologies can substantially improve the estimation of physiological properties.

Single Synapse LTP: A Matter of Context?

The most commonly studied form of synaptic plasticity is long-term potentiation (LTP). Over the last 15 years, it has been possible to induce structural and functional LTP in dendritic spines using two-photon glutamate uncaging, allowing for studying the signaling mechanisms of LTP with single synapse resolution. In this review, we compare different stimulation methods to induce single synapse LTP and discuss how LTP is expressed. We summarize the underlying signaling mechanisms that have been studied with high spatiotemporal resolution. Finally, we discuss how LTP in a single synapse can be affected by excitatory and inhibitory synapses nearby. We argue that single synapse LTP is highly dependent on context: the choice of induction method, the history of the dendritic spine and the dendritic vicinity crucially affect signaling pathways and expression of single synapse LTP.

Layer-specific cholinergic modulation of synaptic transmission in layer 2/3 pyramidal neurons of rat visual cortex

It is known that top-down associative inputs terminate on distal apical dendrites in layer 1 while bottom-up sensory inputs terminate on perisomatic dendrites of layer 2/3 pyramidal neurons (L2/3 PyNs) in primary sensory cortex. Since studies on synaptic transmission in layer 1 are sparse, we investigated the basic properties and cholinergic modulation of synaptic transmission in layer 1 and compared them to those in perisomatic dendrites of L2/3 PyNs of rat primary visual cortex. Using extracellular stimulations of layer 1 and layer 4, we evoked excitatory postsynaptic current/potential in synapses in distal apical dendrites (L1-EPSC/L1-EPSP) and those in perisomatic dendrites (L4-EPSC/L4-EPSP), respectively. Kinetics of L1-EPSC was slower than that of L4-EPSC. L1-EPSC showed presynaptic depression while L4-EPSC was facilitating. In contrast, inhibitory postsynaptic currents showed similar paired-pulse ratio between layer 1 and layer 4 stimulations with depression only at 100 Hz. Cholinergic stimulation induced presynaptic depression by activating muscarinic receptors in excitatory and inhibitory synapses to similar extents in both inputs. However, nicotinic stimulation enhanced excitatory synaptic transmission by ~20% in L4-EPSC. Rectification index of AMPA receptors and AMPA/NMDA ratio were similar between synapses in distal apical and perisomatic dendrites. These results provide basic properties and cholinergic modulation of synaptic transmission between distal apical and perisomatic dendrites in L2/3 PyNs of the visual cortex, which might be important for controlling information processing balance depending on attentional state.

The Effects of NMDA Receptor Blockade on Sensory-Evoked Responses in Superficial Layers of the Rat Barrel Cortex

Transmission of excitation from L4 to L2/3 is a part of a canonical circuit of cortical sensory signal processing. While synapses from L4 to L2/3 are mediated by both AMPA and NMDA glutamate receptors, previous studies suggested that sensory-evoked excitation of neurons in supragranular layers is almost entirely mediated by NMDA receptors. Here, we readdressed this question using extracellular recordings of sensory-evoked potentials (SEPs) and multiple unit activity (MUA) in the rat barrel cortex. We found that blockade of NMDA receptors using the selective antagonist dAPV profoundly inhibited the late part of L2/3 SEP, the associated sink, and MUA response but did not affect its initial part. Our results indicate that both non-NMDA and NMDA receptors are involved in sensory signal transmission from L4 to L2/3. While non-NMDA receptors mediate fast transmission of sensory signals, NMDA-Rs are importantly involved in the generation of the late phase of the sensory-evoked response in supragranular layers.

Precision in the development of neocortical architecture: From progenitors to cortical networks

Of all brain regions, the 6-layered neocortex has undergone the most dramatic changes in size and complexity during mammalian brain evolution. These changes, occurring in the context of a conserved set of organizational features that emerge through stereotypical developmental processes, are considered responsible for the cognitive capacities and sensory specializations represented within the mammalian clade. The modern experimental era of developmental neurobiology, spanning 6 decades, has deciphered a number of mechanisms responsible for producing the diversity of cortical neuron types, their precise connectivity and the role of gene by environment interactions. Here, experiments providing insight into the development of cortical projection neuron differentiation and connectivity are reviewed. This current perspective integrates discussion of classic studies and new findings, based on recent technical advances, to highlight an improved understanding of the neuronal complexity and precise connectivity of cortical circuitry. These descriptive advances bring new opportunities for studies related to the developmental origins of cortical circuits that will, in turn, improve the prospects of identifying pathogenic targets of neurodevelopmental disorders.

The Eloquent Silent Synapse

The ability of central synapses to undergo long-term potentiation (LTP) still captures the imagination of scientists and has become one of the most fascinating and deeply studied questions in modern neuroscience. By the mid-1990s, however, the field was deeply ensnarled in trying to answer a passionately dichotomous question: is LTP expressed by a pre- or a postsynaptic mechanism? Experimental results that could only be seen by many as being incontrovertibly contradictory presented a perplexing conundrum. However, two papers published in 1995 fundamentally redefined critical assumptions and provided a cunningly simple and elegant solution to an otherwise inextricable impasse.

(Micro)Glia as Effectors of Cortical Volume Loss in Schizophrenia

Contrary to the notion that neurology but not psychiatry is the domain of disorders evincing structural brain alterations, it is now clear that there are subtle but consistent neuropathological changes in schizophrenia. These range from increases in ventricular size to dystrophic changes in dendritic spines. A decrease in dendritic spine density in the prefrontal cortex (PFC) is among the most replicated of postmortem structural findings in schizophrenia. Examination of the mechanisms that account for the loss of dendritic spines has in large part focused on genes and molecules that regulate neuronal structure. But the simple question of what is the effector of spine loss, ie, where do the lost spines go, is unanswered. Recent data on glial cells suggest that microglia (MG), and perhaps astrocytes, play an important physiological role in synaptic remodeling of neurons during development. Synapses are added to the dendrites of pyramidal cells during the maturation of these neurons; excess synapses are subsequently phagocytosed by MG. In the PFC, this occurs during adolescence, when certain symptoms of schizophrenia emerge. This brief review discusses recent advances in our understanding of MG function and how these non-neuronal cells lead to structural changes in neurons in schizophrenia.

Metaplasticity at CA1 Synapses by Homeostatic Control of Presynaptic Release Dynamics

Hebbian and homeostatic forms of plasticity operate on different timescales to regulate synaptic strength. The degree of mechanistic overlap between these processes and their mutual influence are still incompletely understood. Here, we report that homeostatic synaptic strengthening induced by prolonged network inactivity compromised the ability of CA1 synapses to exhibit LTP. This effect could not be accounted for by an obvious deficit in the postsynaptic capacity for LTP expression, since neither the fraction of silent synapses nor the ability to induce LTP by two-photon glutamate uncaging were reduced by the homeostatic process. Rather, optical quantal analysis reveals that homeostatically strengthened synapses display a reduced capacity to maintain glutamate release fidelity during repetitive stimulation, ultimately impeding the induction, and thus expression, of LTP. By regulating the short-term dynamics of glutamate release, the homeostatic process thus influences key aspects of dynamic network function and exhibits features of metaplasticity.

Conserved regulators of cognitive aging: From worms to humans

Cognitive decline is a major deficit that arises with age in humans. While some research on the underlying causes of these problems can be done in humans, harnessing the strengths of small model systems, particularly those with well-studied longevity mutants, such as the nematode C. elegans, will accelerate progress. Here we review the approaches being used to study cognitive decline in model organisms and show how simple model systems allow the rapid discovery of conserved molecular mechanisms, which will eventually enable the development of therapeutics to slow cognitive aging.

Early hyperactivity and precocious maturation of corticostriatal circuits in Shank3B(-/-) mice

Some autistic individuals exhibit abnormal development of the caudate nucleus and associative cortical areas, suggesting potential dysfunction of cortico-basal ganglia (BG) circuits. Using optogenetic and electrophysiological approaches in mice, we identified a narrow postnatal period that is characterized by extensive glutamatergic synaptogenesis in striatal spiny projection neurons (SPNs) and a concomitant increase in corticostriatal circuit activity. SPNs during early development have high intrinsic excitability and respond strongly to cortical afferents despite sparse excitatory inputs. As a result, striatum and corticostriatal connectivity are highly sensitive to acute and chronic changes in cortical activity, suggesting that early imbalances in cortical function alter BG development. Indeed, a mouse model of autism with deletions in Shank3 (Shank3B(-/-)) shows early cortical hyperactivity, which triggers increased SPN excitatory synapse and corticostriatal hyperconnectivity. These results indicate that there is a tight functional coupling between cortex and striatum during early postnatal development and suggest a potential common circuit dysfunction that is caused by cortical hyperactivity.

Correlated Synaptic Inputs Drive Dendritic Calcium Amplification and Cooperative Plasticity during Clustered Synapse Development

The mechanisms that instruct the assembly of fine-scale features of synaptic connectivity in neural circuits are only beginning to be understood. Using whole-cell electrophysiology, two-photon calcium imaging, and glutamate uncaging in hippocampal slices, we discovered a functional coupling between NMDA receptor activation and ryanodine-sensitive intracellular calcium release that dominates the spatiotemporal dynamics of activity-dependent calcium signals during synaptogenesis. This developmentally regulated calcium amplification mechanism was tuned to detect and bind spatially clustered and temporally correlated synaptic inputs and enacted a local cooperative plasticity rule between coactive neighboring synapses. Consistent with the hypothesis that synapse maturation is spatially regulated, we observed clustering of synaptic weights in developing dendritic arbors. These results reveal developmental features of NMDA receptor-dependent calcium dynamics and local plasticity rules that are suited to spatially guide synaptic connectivity patterns in emerging neural networks.

Optical control of NMDA receptors with a diffusible photoswitch

N-methyl-D-aspartate receptors (NMDARs) play a central role in synaptic plasticity, learning and memory, and are implicated in various neuronal disorders. We synthesized a diffusible photochromic glutamate analogue, azobenzene-triazole-glutamate (ATG), which is specific for NMDARs and functions as a photoswitchable agonist. ATG is inactive in its dark-adapted trans-isoform, but can be converted into its active cis-isoform using one-photon (near UV) or two-photon (740 nm) excitation. Irradiation with violet light photo-inactivates ATG within milliseconds, allowing agonist removal on the timescale of NMDAR deactivation. ATG is compatible with Ca²⁺ imaging and can be used to optically mimic synaptic coincidence detection protocols. Thus, ATG can be used like traditional caged glutamate compounds, but with the added advantages of NMDAR specificity, low antagonism of GABAR-mediated currents, and precise temporal control of agonist delivery.

N-methyl-D-aspartate receptors (NMDARs) play a central role in synaptic plasticity, learning and memory. Here the authors describe azobenzene-triazole-glutamate (ATG), a new diffusible photoswitchable agonist that allows precise temporal control over NMDAR activity.

Effects of homeostatic constraints on associative memory storage and synaptic connectivity of cortical circuits

The impact of learning and long-term memory storage on synaptic connectivity is not completely understood. In this study, we examine the effects of associative learning on synaptic connectivity in adult cortical circuits by hypothesizing that these circuits function in a steady-state, in which the memory capacity of a circuit is maximal and learning must be accompanied by forgetting. Steady-state circuits should be characterized by unique connectivity features. To uncover such features we developed a biologically constrained, exactly solvable model of associative memory storage. The model is applicable to networks of multiple excitatory and inhibitory neuron classes and can account for homeostatic constraints on the number and the overall weight of functional connections received by each neuron. The results show that in spite of a large number of neuron classes, functional connections between potentially connected cells are realized with less than 50% probability if the presynaptic cell is excitatory and generally a much greater probability if it is inhibitory. We also find that constraining the overall weight of presynaptic connections leads to Gaussian connection weight distributions that are truncated at zero. In contrast, constraining the total number of functional presynaptic connections leads to non-Gaussian distributions, in which weak connections are absent. These theoretical predictions are compared with a large dataset of published experimental studies reporting amplitudes of unitary postsynaptic potentials and probabilities of connections between various classes of excitatory and inhibitory neurons in the cerebellum, neocortex, and hippocampus.

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.

The expression of long-term potentiation: reconciling the preists and the postivists

Long-term potentiation (LTP) of excitatory synaptic transmission in the hippocampus has been investigated in great detail over the past 40 years. Where and how LTP is actually expressed, however, remain controversial issues. Considerable evidence has been offered to support both pre- and postsynaptic contributions to LTP expression. Though it is widely held that postsynaptic expression mechanisms are the primary contributors to LTP expression, evidence for that conclusion is amenable to alternative explanations. Here, we briefly review some key contributions to the 'locus' debate and describe data that support a dominant role for presynaptic mechanisms. Recognition of the state-dependency of expression mechanisms, and consideration of the consequences of the spatial relationship between postsynaptic glutamate receptors and presynaptic vesicular release sites, lead to a model that may reconcile views from both sides of the synapse.

Expression mechanisms underlying long-term potentiation: a postsynaptic view, 10 years on

This review focuses on the research that has occurred over the past decade which has solidified a postsynaptic expression mechanism for long-term potentiation (LTP). However, experiments that have suggested a presynaptic component are also summarized. It is argued that the pairing of glutamate uncaging onto single spines with postsynaptic depolarization provides the final and most elegant demonstration of a postsynaptic expression mechanism for NMDA receptor-dependent LTP. The fact that the magnitude of this LTP is similar to that evoked by pairing synaptic stimulation and depolarization leaves little room for a substantial presynaptic component. Finally, recent data also require a revision in our thinking about the way AMPA receptors (AMPARs) are recruited to the postsynaptic density during LTP. This recruitment is independent of subunit type, but does require an adequate reserve pool of extrasynaptic receptors.

Contribution of intracolumnar layer 2/3-to-layer 2/3 excitatory connections in shaping the response to whisker deflection in rat barrel cortex

This computational study integrates anatomical and physiological data to assess the functional role of the lateral excitatory connections between layer 2/3 (L2/3) pyramidal cells (PCs) in shaping their response during early stages of intracortical processing of a whisker deflection (WD). Based on in vivo and in vitro recordings, and 3D reconstructions of connected pairs of L2/3 PCs, our model predicts that: 1) AMPAR and NMDAR conductances/synapse are 0.52 ± 0.24 and 0.40 ± 0.34 nS, respectively; 2) following WD, connection between L2/3 PCs induces a composite EPSPs of 7.6 ± 1.7 mV, well below the threshold for action potential (AP) initiation; 3) together with the excitatory feedforward L4-to-L2/3 connection, WD evoked a composite EPSP of 16.3 ± 3.5 mV and a probability of 0.01 to generate an AP. When considering the variability in L4 spiny neurons responsiveness, it increased to 17.8 ± 11.2 mV; this 3-fold increase in the SD yielded AP probability of 0.35; 4) the interaction between L4-to-L2/3 and L2/3-to-L2/3 inputs is highly nonlinear; 5) L2/3 dendritic morphology significantly affects L2/3 PCs responsiveness. We conclude that early stages of intracortical signaling of WD are dominated by a combination of feedforward L4-L2/3 and L2/3-L2/3 lateral connections.

LY404187, a potentiator of AMPARs, enhances both the amplitude and 1/CV2 of AMPA EPSCs but not NMDA EPSCs at CA3-CA1 synapses in the hippocampus of neonatal rats

Cyclothiazide is a well-known AMPAR potentiator, but it has also been shown to enhance the probability of presynaptic release in some cases. Interestingly, cyclothiazide has been shown to reveal AMPA EPSCs at silent CA3-CA1 synapses (which exhibit NMDA EPSCs but not AMPA EPSCs) in the hippocampus of neonatal or developing rats, but this particular result has not been reproduced at other types of synapses. Although this discrepancy may be due to the different mechanisms underlying silent synapses in distinct brain subregions, it is also possible that cyclothiazide has pre- and postsynaptic molecular targets that are differentially expressed at the different types (or different developing stages) of synapses. In this study, we reexamined, using a new AMPAR potentiator, LY404187, whether AMPAR potentiation leads to the conversion of silent CA3-CA1 synapses into functional synapses (exhibiting both AMPA and NMDA EPSCs) in the hippocampus of neonatal rats. LY404187 did not appear to alter the probability of presynaptic release, as evidenced by the lack of significant changes in both the amplitude and the paired-pulse facilitation ratio (an index of release probability) of NMDA EPSCs. LY404187 enhanced both the amplitude and 1/CV(2) (CV: coefficient of variation) of AMPA EPSCs but not NMDA EPSCs. Because an increase in 1/CV(2) reflects an increased number of functional synapses and/or an enhanced release probability, the LY404187-induced increase in the 1/CV(2) value of AMPA EPSCs, but not NMDA EPSCs, likely indicates an increased number of synapses exhibiting AMPA EPSCs but not an increased number of synapses exhibiting NMDA EPSCs. Because AMPARs and NMDARs are co-localized at the same synapses, our findings are consistent with a scenario in which LY404187 enables silent synapses to acquire AMPA EPSCs.

Spike-timing dependence of structural plasticity explains cooperative synapse formation in the neocortex

Structural plasticity governs the long-term development of synaptic connections in the neocortex. While the underlying processes at the synapses are not fully understood, there is strong evidence that a process of random, independent formation and pruning of excitatory synapses can be ruled out. Instead, there must be some cooperation between the synaptic contacts connecting a single pre- and postsynaptic neuron pair. So far, the mechanism of cooperation is not known. Here we demonstrate that local correlation detection at the postsynaptic dendritic spine suffices to explain the synaptic cooperation effect, without assuming any hypothetical direct interaction pathway between the synaptic contacts. Candidate biomolecular mechanisms for dendritic correlation detection have been identified previously, as well as for structural plasticity based thereon. By analyzing and fitting of a simple model, we show that spike-timing correlation dependent structural plasticity, without additional mechanisms of cross-synapse interaction, can reproduce the experimentally observed distributions of numbers of synaptic contacts between pairs of neurons in the neocortex. Furthermore, the model yields a first explanation for the existence of both transient and persistent dendritic spines and allows to make predictions for future experiments.

Structural plasticity has been observed even in the adult mammalian neocortex – in seemingly static neuronal circuits structural remodeling is continuously at work. Still, it has been shown that the connection patterns between pairs of neurons are not random. In contrast, there is evidence that the synaptic contacts between a pair of neurons cooperate: several experimental studies report either zero or about 3–6 synapses between neuron pairs. The mechanism by which the synapses cooperate, however, has not yet been identified. Here we propose a model for structural plasticity that relies on local processes at the dendritic spine. We combine and extend the previous models and determine the equilibrium probability distribution of synaptic contact numbers of the model. By optimizing the parameters numerically for each of three reference datasets, we obtain equilibrium contact number distributions that fit the references very well. We conclude that the local dendritic mechanisms that we assume suffice to explain the cooperative synapse formation in the neocortex.

Deep molecular diversity of mammalian synapses: why it matters and how to measure it

Pioneering studies in the middle of the twentieth century revealed substantial diversity among mammalian chemical synapses and led to a widely accepted classification of synapse type on the basis of neurotransmitter molecule identity. Subsequently, powerful new physiological, genetic and structural methods have enabled the discovery of much deeper functional and molecular diversity within each traditional neurotransmitter type. Today, this deep diversity continues to pose both daunting challenges and exciting new opportunities for neuroscience. Our growing understanding of deep synapse diversity may transform how we think about and study neural circuit development, structure and function.

Examining form and function of dendritic spines

The majority of fast excitatory synaptic transmission in the central nervous system takes place at protrusions along dendrites called spines. Dendritic spines are highly heterogeneous, both morphologically and functionally. Not surprisingly, there has been much speculation and debate on the relationship between spine structure and function. The advent of multi-photon laser-scanning microscopy has greatly improved our ability to investigate the dynamic interplay between spine form and function. Regulated structural changes occur at spines undergoing plasticity, offering a mechanism to account for the well-described correlation between spine size and synapse strength. In turn, spine structure can influence the degree of biochemical and perhaps electrical compartmentalization at individual synapses. Here, we review the relationship between dendritic spine morphology, features of spine compartmentalization and synaptic plasticity. We highlight emerging molecular mechanisms that link structural and functional changes in spines during plasticity, and also consider circumstances that underscore some divergence from a tight structure-function coupling. Because of the intricate influence of spine structure on biochemical and electrical signalling, activity-dependent changes in spine morphology alone may thus contribute to the metaplastic potential of synapses. This possibility asserts a role for structural dynamics in neuronal information storage and aligns well with current computational models.

The ageing cortical synapse: hallmarks and implications for cognitive decline

Normal ageing is associated with impairments in cognitive function, including memory. These impairments are linked, not to a loss of neurons in the forebrain, but to specific and relatively subtle synaptic alterations in the hippocampus and prefrontal cortex. Here, we review studies that have shed light on the cellular and synaptic changes observed in these brain structures during ageing that can be directly related to cognitive decline in young and aged animals. We also discuss the influence of the hormonal status on these age-related alterations and recent progress in the development of therapeutic strategies to limit the impact of ageing on memory and cognition in humans.

Hypoxia-induced neonatal seizures diminish silent synapses and long-term potentiation in hippocampal CA1 neurons

Neonatal seizures can lead to epilepsy and long-term cognitive deficits into adulthood. Using a rodent model of the most common form of human neonatal seizures, hypoxia-induced seizures (HS), we aimed to determine whether these seizures modify long-term potentiation (LTP) and silent NMDAR-only synapses in hippocampal CA1. At 48-72 h after HS, electrophysiology and immunofluorescent confocal microscopy revealed a significant decrease in the incidence of silent synapses, and an increase in AMPARs at the synapses. Coincident with this decrease in silent synapses, there was an attenuation of LTP elicited by either tetanic stimulation of Schaffer collaterals or a pairing protocol, and persistent attenuation of LTP in slices removed in later adulthood after P10 HS. Furthermore, postseizure treatment in vivo with the AMPAR antagonist 2,3-dihydroxy-6-nitro-7-sulfonyl-benzo[f]quinoxaline (NBQX) protected against the HS-induced depletion of silent synapses and preserved LTP. Thus, this study demonstrates a novel mechanism by which early life seizures could impair synaptic plasticity, suggesting a potential target for therapeutic strategies to prevent long-term cognitive deficits.

Glutamate spillover promotes the generation of NMDA spikes

NMDA spikes are prominent in the basal dendrites of cortical pyramidal neurons and greatly expand their ability to integrate synaptic inputs. Calcium (Ca) signals during these spikes are important for synaptic plasticity and fundamentally depend on activation of NMDA receptors. However, the factors that shape the activation of these receptors and the initiation of NMDA spikes remain unclear. Here we examine the properties of NMDA spikes in the basal dendrites of layer 5 pyramidal neurons in the mouse prefrontal cortex. Using two-photon imaging, we demonstrate that NMDA spikes evoke large Ca signals in both postsynaptic spines and nearby dendrites. We find that the dendrite Ca signals depend on NMDA and AMPA receptors but not sodium (Na) or Ca channels. Using voltage-clamp recordings, we show that activation of dendrite NMDA receptors is enhanced by concerted synaptic activity. Blocking glutamate reuptake further increases activation of these receptors and promotes the initiation of NMDA spikes. We conclude that glutamate spillover and recruitment of extrasynaptic receptors contribute to the initiation of NMDA spikes. These results have important implications for how synaptic activity generates both electrical and biochemical signals in dendrites and spines.

Maturation of a recurrent excitatory neocortical circuit by experience-dependent unsilencing of newly formed dendritic spines

Local recurrent excitatory circuits are ubiquitous in neocortex, yet little is known about their development or architecture. Here we introduce a quantitative technique for efficient single-cell resolution circuit mapping using 2-photon (2P) glutamate uncaging and analyze experience-dependent neonatal development of the layer 4 barrel cortex local excitatory circuit. We show that sensory experience specifically drives a 3-fold increase in connectivity at postnatal day (P) 9, producing a highly recurrent network. A profound dendritic spinogenesis occurs concurrent with the connectivity increase, but this is not experience dependent. However, in experience-deprived cortex, a much greater proportion of spines lack postsynaptic AMPA receptors (AMPARs) and synaptic connectivity via NMDA receptors (NMDARs) is the same as in normally developing cortex. Thus we describe a approach for quantitative circuit mapping and show that sensory experience sculpts an intrinsically developing template network, which is based on NMDAR-only synapses, by driving AMPARs into newly formed silent spines.

Boosting of synaptic potentials and spine Ca transients by the peptide toxin SNX-482 requires alpha-1E-encoded voltage-gated Ca channels

The majority of glutamatergic synapses formed onto principal neurons of the mammalian central nervous system are associated with dendritic spines. Spines are tiny protuberances that house the proteins that mediate the response of the postsynaptic cell to the presynaptic release of glutamate. Postsynaptic signals are regulated by an ion channel signaling cascade that is active in individual dendritic spines and involves voltage-gated calcium (Ca) channels, small conductance (SK)-type Ca-activated potassium channels, and NMDA-type glutamate receptors. Pharmacological studies using the toxin SNX-482 indicated that the voltage-gated Ca channels that signal within spines to open SK channels belong to the class Ca(V)2.3, which is encoded by the Alpha-1E pore-forming subunit. In order to specifically test this conclusion, we examined the effects of SNX-482 on synaptic signals in acute hippocampal slices from knock-out mice lacking the Alpha-1E gene. We find that in these mice, application of SNX-482 has no effect on glutamate-uncaging evoked synaptic potentials and Ca influx, indicating that that SNX-482 indeed acts via the Alpha-1E-encoded Ca(V)2.3 channel.

Glutamate induces de novo growth of functional spines in developing cortex

Mature cortical pyramidal neurons receive excitatory inputs onto small protrusions emanating from their dendrites called spines. Spines undergo activity-dependent remodeling, stabilization, and pruning during development and similar structural changes can be triggered by learning and changes in sensory experiences1–4. However, the biochemical triggers and mechanisms of de novo spine formation in the developing brain and the functional significance of new spines to neuronal connectivity are largely unknown. We developed an approach to induce and monitor de novo spine formation in real-time using combined two-photon laser-scanning microscopy and two-photon laser uncaging of glutamate. Our data demonstrate that, in mouse cortical layer 2/3 pyramidal neurons, glutamate is sufficient to trigger de novo spine growth from the dendrite shaft in a location-specific manner. We find that glutamate-induced spinogenesis requires opening of NMDA-type glutamate receptors and activation of PKA but is independent of CaMKII and TrkB receptors. Furthermore, newly formed spines express glutamate receptors and are rapidly functional such that they transduce presynaptic activity into postsynaptic signals. Together, our data demonstrate that early neural connectivity is shaped by activity in a spatially precise manner and that nascent dendrite spines are rapidly functionally incorporated into cortical circuits.

AMPA receptor subunit GluR1 (GluA1) serine-845 site is involved in synaptic depression but not in spine shrinkage associated with chemical long-term depression

The structure of dendritic spines is highly plastic and can be modified by neuronal activity. In addition, there is evidence that spine head size correlates with the synaptic α-amino-3-hydroxy-5-methylisoxazole propionic acid (AMPA) receptor (AMPAR) content, which suggests that they may be coregulated. Although there is evidence that there are overlapping mechanisms for structural and functional plasticity, the extent of the overlap needs further investigation. Specifically, it is unknown whether AMPAR levels determine spine size or whether both are regulated via parallel pathways. We studied the correlation between spine structural plasticity and long-term synaptic plasticity following chemical-induced long-term depression (chemLTD). In particular, we examined whether the regulation of AMPARs, which is implicated in LTD, is critical for spine morphological plasticity. We used mutant mice specifically lacking the serine-845 site on the type 1 glutamate receptor (GluR1, or GluA1) subunit of AMPARs (mutants). These mice specifically lack N-methyl-D-aspartate (NMDA) receptor (NMDAR)-dependent LTD and NMDAR activation-induced AMPAR endocytosis. We found that chemLTD causes a rapid and persistent shrinkage in spine head volume of hippocampal CA1 pyramidal neurons in wild types similar to that reported in other studies using low-frequency stimulation (LFS)-induced LTD. Surprisingly, we found that although S845A mutant mice display impaired chemLTD, the shrinkage of spine head volume occurred to a similar magnitude to that observed in wild types. Our results suggest that there is dissociation in the molecular mechanisms underlying functional LTD and spine shrinkage and that GluR1-S845 regulation is not necessary for spine morphological plasticity.

Cocaine-induced metaplasticity in the nucleus accumbens: silent synapse and beyond

The neuroadaptation theory of addiction suggests that, similar to the development of most memories, exposure to drugs of abuse induces adaptive molecular and cellular changes in the brain which likely mediate addiction-related memories or the addictive state. Compared to other types of memories, addiction-related memories develop fast and last extremely long, suggesting that the cellular and molecular processes that mediate addiction-related memories are exceptionally adept and efficient. We recently demonstrated that repeated exposure to cocaine generated a large portion of "silent" glutamatergic synapses within the nucleus accumbens (NAc). Silent glutamatergic synapses are synaptic connections in which only N-methyl-D-aspartic acid receptor (NMDAR)-mediated responses are readily detected whereas alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are absent or highly labile. Extensive experimental evidence suggests that silent synapses are conspicuously efficient plasticity sites at which long-lasting plastic changes can be more easily induced and maintained. Thus, generation of silent synapses can be regarded as a process of metaplasticity, which primes the NAc for subsequent durable and robust plasticity for addiction-related memories. Focusing on silent synapse-based metaplasticity, this review discusses how key brain regions, such as the NAc, utilize the metaplasticity mechanism to optimize the plasticity machineries to achieve fast and durable plastic changes following exposure to cocaine. A summary of recent related results suggests that upon cocaine exposure, newly generated silent synapses may prime excitatory synapses within the NAc for long-term potentiation (LTP), thus setting the direction of future plasticity. Furthermore, because cocaine-generated silent synapses are enriched in NMDARs containing the NR2B subunit, the enhanced NR2B-signaling may set up a selective recruitment of certain types of AMPARs. Thus, silent synapse-based metaplasticity may lead to not only quantitative but also qualitative alterations in excitatory synapses within the NAc. This review is one of the first systematic analyses regarding the hypothesis that drugs of abuse induce metaplasticity, which regulates the susceptibility, the direction, and the molecular details of subsequent plastic changes. Taken together, metaplasticity ultimately serves as a key step in mediating cascades of addiction-related plastic alterations.

Arc-dependent synapse-specific homeostatic plasticity

Both theoretical and experimental research has indicated that the synaptic strength between neurons in a network needs to be properly fine-tuned and controlled by homeostatic mechanisms to ensure proper network function. One such mechanism that has been extensively characterized is synaptic homeostatic plasticity or global synaptic scaling. This mechanism refers to the bidirectional ability of all synapses impinging on a neuron to actively compensate for changes in the neuron's overall excitability. Here, using a combination of electrophysiological, two-photon glutamate uncaging and imaging methods, we show that mature individual synapses, independent of neighboring synapses, have the ability to autonomously sense their level of activity and actively compensate for it in a homeostatic-like fashion. This synapse-specific homeostatic plasticity, similar to global synaptic plasticity, requires the immediate early gene Arc. Together, our results document an extra level of regulation of synaptic function that bears important computational consequences on information storage in the brain.

M1 muscarinic receptors boost synaptic potentials and calcium influx in dendritic spines by inhibiting postsynaptic SK channels

Acetylcholine release and activation of muscarinic cholinergic receptors (mAChRs) enhance synaptic plasticity in vitro and cognition and memory in vivo. Within the hippocampus, mAChRs promote NMDA-type glutamate receptor-dependent forms of long-term potentiation. Here, we use calcium (Ca) imaging combined with two-photon laser glutamate uncaging at apical spines of CA1 pyramidal neurons to examine postsynaptic mechanisms of muscarinic modulation of glutamatergic transmission. Uncaging-evoked excitatory postsynaptic potentials and Ca transients are increased by muscarinic stimulation; however, this is not due to direct modulation of glutamate receptors. Instead, mAChRs modulate a negative feedback loop in spines that normally suppresses synaptic signals. mAChR activation reduces the Ca sensitivity of small conductance Ca-activated potassium (SK) channels that are found in the spine, resulting in increased synaptic potentials and Ca transients. These effects are mediated by M1-type muscarinic receptors and occur in a casein kinase-2-dependent manner. Thus, muscarinic modulation regulates synaptic transmission by tuning the activity of nonglutamatergic postsynaptic ion channels.

Competitive regulation of synaptic Ca2+ influx by D2 dopamine and A2A adenosine receptors

Striatal D2-type dopamine receptors (D2Rs) have been implicated in the pathophysiology of neuropsychiatric disorders, including Parkinson's disease and schizophrenia. Although these receptors regulate striatal synaptic plasticity, the mechanisms underlying dopaminergic modulation of glutamatergic synapses are unclear. We combined optogenetics, two-photon microscopy and glutamate uncaging to examine D2R-dependent modulation of glutamatergic synaptic transmission in mouse striatopallidal neurons. We found that D2R activation reduces corticostriatal glutamate release and attenuates both synaptic- and action potential-evoked Ca2+ influx into dendritic spines by approximately 50%. Modulation of Ca2+ signaling was mediated by a protein kinase A (PKA)-dependent regulation of Ca2+ entry through NMDA-type glutamate receptors that was inhibited by D2Rs and enhanced by activation of 2A-type adenosine receptors (A2ARs). D2Rs also produced a PKA- and A2AR-independent reduction in Ca2+ influx through R-type voltage-gated Ca2+ channels. These findings reveal that dopamine regulates spine Ca2+ by multiple pathways and that competitive modulation of PKA controls NMDAR-mediated Ca2+ signaling in the striatum.

Fragile X mental retardation protein is required for synapse elimination by the activity-dependent transcription factor MEF2

Fragile X syndrome (FXS), the most common genetic form of mental retardation and autism, is caused by loss-of-function mutations in an RNA-binding protein, Fragile X Mental Retardation Protein (FMRP). Neurons from patients and the mouse Fmr1 knockout (KO) model are characterized by an excess of dendritic spines, suggesting a deficit in excitatory synapse elimination. In response to neuronal activity, myocyte enhancer factor 2 (MEF2) transcription factors induce robust synapse elimination. Here, we demonstrate that MEF2 activation fails to eliminate functional or structural excitatory synapses in hippocampal neurons from Fmr1 KO mice. Similarly, inhibition of endogenous MEF2 increases synapse number in wild-type but not Fmr1 KO neurons. MEF2-dependent synapse elimination is rescued in Fmr1 KO neurons by acute postsynaptic expression of wild-type but not RNA-binding mutants of FMRP. Our results reveal that active MEF2 and FMRP function together in an acute, cell-autonomous mechanism to eliminate excitatory synapses.

Enhanced infragranular and supragranular synaptic input onto layer 5 pyramidal neurons in a rat model of cortical dysplasia

Cortical dysplasias frequently underlie neurodevelopmental disorders and epilepsy. Rats with a neonatally induced cortical microgyrus [freeze-lesion (FL)], a model of human polymicrogyria, display epileptiform discharges in vitro. We probed excitatory and inhibitory connectivity onto neocortical pyramidal neurons in layers 2/3 and 5 of postnatal day 16-22 rats, approximately 1-2 mm lateral of the lesion, using laser scanning photostimulation (LSPS)/glutamate uncaging. Excitatory input from deep and supragranular layers to layer 5 pyramidal cells was greater in FL cortex, while no significant differences were seen in layer 2/3 cells. The increased input was due to a greater number of LSPS-evoked excitatory postsynaptic currents (EPSCs), without differences in amplitude or kinetics. Inhibitory input was increased in a region-specific manner in pyramidal cells in FL cortex, due to an increased inhibitory postsynaptic current (IPSC) amplitude. Connectivity within layer 5, parts of which are destroyed during lesioning, was more severely affected than connectivity in layer 2/3. Thus, we observed 2 distinct mechanisms of altered synaptic input: 1) increased EPSC frequency suggesting an increased number of excitatory synapses and 2) higher IPSC amplitude, suggesting an increased strength of inhibitory synapses. These increases in both excitatory and inhibitory connectivity may limit the extent of circuit hyperexcitability.

Properties of glutamatergic synapses in immature layer Vb pyramidal neurons: coupling of pre- and postsynaptic maturational states

Following initial contact formation, glutamatergic synapses in cortical neurons undergo pronounced functional maturation. These maturational events, occurring both pre- and postsynaptically, have been well described in the developing hippocampus. In this paper, we characterized glutamatergic synapses in immature layer Vb pyramidal neurons of the mouse somatosensory cortex during early postnatal development. At postnatal day 7, a significant subpopulation of glutamatergic synapses exhibited a low release probability that was accompanied by strong paired-pulse facilitation of AMPA EPSCs (paired-pulse ratio C > or = 2). Increasing extracellular Ca(2+) concentration increased release probability and led to paired-pulse depression. During further postnatal development, these functionally immature synapses disappeared. As shown pharmacologically,these synapses expressed postsynaptic NMDA receptors containing NR2B subunits, while NMDA receptors with NR2A subunits were lacking. Taken together, a low release probability presynaptically was coupled to postsynaptic NR2B signaling. This subpopulation of neocortical synapses thus differed from the majority of synapses in the developing hippocampus, where high release probability is coupled to NR2B signaling. The novel type of functionally immature glutamatergic synapse described here might play an important role in early developmental synapse elimination and in the activity-dependent refinement of the neocortical synaptic microcircuitry.

BDNF signaling in the formation, maturation and plasticity of glutamatergic and GABAergic synapses

In the past 15 years numerous reports provided strong evidence that brain-derived neurotrophic factor (BDNF) is one of the most important modulators of glutamatergic and GABAergic synapses. Remarkable progress regarding localization, kinetics, and molecular mechanisms of BDNF secretion has been achieved, and a large number of studies provided evidence that continuous extracellular supply of BDNF is important for the proper formation and functional maturation of glutamatergic and GABAergic synapses. BDNF can play a permissive role in shaping synaptic networks, making them more susceptible for the occurrence of plastic changes. In addition, BDNF appears to be also an instructive factor for activity-dependent long-term synaptic plasticity. BDNF release just in response to synaptic stimulation might be a molecular trigger to convert high-frequency synaptic activity into long-term synaptic memories. This review attempts to summarize the current knowledge in synaptic secretion and synaptic action of BDNF, including both permissive and instructive effects of BDNF in synaptic plasticity.

Cholinergic modulation of multivesicular release regulates striatal synaptic potency and integration

The pleiotropic actions of neuromodulators on pre- and postsynaptic targets make disentangling the mechanisms underlying regulation of synaptic transmission challenging. In the striatum, acetylcholine modulates glutamate release via activation of muscarinic receptors (mAchRs), although the consequences for postsynaptic signaling are unclear. Using two-photon microscopy and glutamate uncaging to examine individual synapses in the rat striatum, we found that glutamatergic afferents have a high degree of multivesicular release (MVR) in the absence of postsynaptic receptor saturation. We found that mAchR activation decreased both the probability of release and the concentration of glutamate in the synaptic cleft. The corresponding decrease in synaptic potency reduced the duration of synaptic potentials and limited temporal summation of afferent inputs. These findings reveal a mechanism by which a combination of basal MVR and low receptor saturation allow the presynaptic actions of a neuromodulator to control the engagement of postsynaptic nonlinearities and regulate synaptic integration.

The dynamics of excitatory synapse formation on dendritic spines

Dendritic spines, the postsynaptic compartments of most functional excitatory synapses in the Central Nervous System (CNS), are highly dynamic structures, having the ability to grow, change shape, or retract in response to varying levels of neuronal activity. This dynamic nature of spines allows modifications in brain circuitry and connectivity, thus participating in fundamental processes such as learning, recall, and emotional behaviors. Although many studies have characterized the precise molecular identities and signaling pathways by which spines initially form, little is known about the actual time course over which they mature into functional postsynaptic compartments of excitatory synapses. A recent publication in Neuron addresses this issue by studying dendritic spine growth in response to multiphoton glutamate uncaging, simultaneously monitoring the amplitudes of the resultant postsynaptic currents and intracellular Ca(2+) transients within individual spines in CA1 pyramidal neurons in organotypic cultures of postnatal hippocampal slices. The authors describe that dendritic spines are able to respond to glutamate shortly after their formation, leading to the conclusion that spine growth and glutamate receptor recruitment are closely coupled temporally. AMPA receptor-mediated currents exhibited similar amplitudes in newly formed spines compared with older, more mature spines when their volume was taken into account. In addition, NMDA receptor-mediated currents also appeared early after spine formation, although the amount of Ca(2+) entry through these receptors was significantly lower in newly formed spines compared to older, mature spines. Within just a couple of hours, these newly formed spines were contacted by presynaptic terminals, thus acquiring a morphological appearance indistinguishable from already existing mature excitatory synapses.