Spine-neck geometry determines NMDA receptor-dependent Ca2+ signaling in dendrites

Selective estrogen receptor modulators regulate dendritic spine plasticity in the hippocampus of male rats

Some selective estrogen receptor modulators, such as raloxifene and tamoxifen, are neuroprotective and reduce brain inflammation in several experimental models of neurodegeneration. In addition, raloxifene and tamoxifen counteract cognitive deficits caused by gonadal hormone deprivation in male rats. In this study, we have explored whether raloxifene and tamoxifen may regulate the number and geometry of dendritic spines in CA1 pyramidal neurons of the rat hippocampus. Young adult male rats were injected with raloxifene (1 mg/kg), tamoxifen (1 mg/kg), or vehicle and killed 24 h after the injection. Animals treated with raloxifene or tamoxifen showed an increased numerical density of dendritic spines in CA1 pyramidal neurons compared to animals treated with vehicle. Raloxifene and tamoxifen had also specific effects in the morphology of spines. These findings suggest that raloxifene and tamoxifen may influence the processing of information by hippocampal pyramidal neurons by affecting the number and shape of dendritic spines.

Depolarization gates spine calcium transients and spike-timing-dependent potentiation

Timing-dependent long-term potentiation (t-LTP) is induced when synaptic activity is immediately followed by one or more back-propagating action potentials (bAPs) in the postsynaptic cell. As a mechanistic explanation, it has been proposed that the bAP removes the Mg2+ block of synaptic NMDA receptors, allowing for rapid Ca2+ entry at the active synapse. Recent experimental studies suggest that this model is incomplete: NMDA receptor-based coincidence detection requires strong postsynaptic depolarization, usually provided by AMPA receptor currents. Apparently, the brief AMPA-EPSP does not only enable t-LTP, it is also responsible for the very narrow time window for t-LTP induction. The emerging consensus puts the spine in the center of coincidence detection, as active conductances on the spine together with the electrical resistance of the spine neck regulate the depolarization of the spine head and thus Ca2+ influx during pairing. A focus on postsynaptic voltage during synaptic activation not only encompasses spike-timing-dependent plasticity (STDP), but explains also the cooperativity and frequency-dependence of plasticity.

Calcium signaling in synapse-to-nucleus communication

Changes in the intracellular concentration of calcium ions in neurons are involved in neurite growth, development, and remodeling, regulation of neuronal excitability, increases and decreases in the strength of synaptic connections, and the activation of survival and programmed cell death pathways. An important aspect of the signals that trigger these processes is that they are frequently initiated in the form of glutamatergic neurotransmission within dendritic trees, while their completion involves specific changes in the patterns of genes expressed within neuronal nuclei. Accordingly, two prominent aims of research concerned with calcium signaling in neurons are determination of the mechanisms governing information conveyance between synapse and nucleus, and discovery of the rules dictating translation of specific patterns of inputs into appropriate and specific transcriptional responses. In this article, we present an overview of the avenues by which glutamatergic excitation of dendrites may be communicated to the neuronal nucleus and the primary calcium-dependent signaling pathways by which synaptic activity can invoke changes in neuronal gene expression programs.

Distribution of extracellular glutamate in the neuropil of hippocampus

Reported values of extracellular glutamate concentrations in the resting state depend on the method of measurement and vary ∼1000-fold. As glutamate levels in the micromolar range can cause receptor desensitization and excitotoxicity, and thus affect neuronal excitability, an accurate determination of ambient glutamate is important. Part of the variability of previous measurements may have resulted from the sampling of glutamate in different extracellular compartments, e.g., synaptic versus extrasynaptic volumes. A steep concentration gradient of glutamate between these two compartments could be maintained, for example, by high densities of glutamate transporters arrayed at the edges of synapses. We have used two photon laser scanning microscopy and electrophysiology to investigate whether extracellular glutamate is compartmentalized in acute hippocampal slices. Pharmacological blockade of NMDARs had no effect on Ca(2+) transients generated in dendritic shafts or spines of CA1 pyramidal neurons by depolarization, suggesting that ambient glutamate is too low to activate a significant number of NMDARs. Furthermore, blockade of transporters did not flood the synapse with glutamate, indicating that synaptic NMDARs are not protected from high concentrations of extrasynaptic glutamate. We suggest that, in the CA1 region of hippocampus, glutamate transporters do not create a privileged space within the synapse but rather keep ambient glutamate at very low levels throughout the neuropil.

A Ca(2+) threshold for induction of spike-timing-dependent depression in the mouse striatum

The striatum is the principal input nucleus of the basal ganglia, receiving glutamatergic afferents from the cerebral cortex. There is much interest in mechanisms of synaptic plasticity in the corticostriatal synapses. We used two-photon microscopy and whole-cell recording to measure changes in intracellular calcium concentration ([Ca(2+)](i)) associated with spike-time-dependent plasticity in mouse striatum. Uncaging glutamate adjacent to a dendritic spine caused a postsynaptic potential at the soma and a rise in spine [Ca(2+)](i). Action potentials elicited at the soma raised both dendrite and spine [Ca(2+)](i). Pairing protocols in which glutamate uncaging preceded action potentials by 10 ms (pre-post protocol) produced supralinear increases in spine [Ca(2+)](i) compared with the sum of increases seen with uncaging and action potentials alone, or timing protocols in which the uncaging followed the action potentials (post-pre protocols). The supralinear component of the increases in [Ca(2+)](i) were eliminated by the voltage-sensitive calcium channel blocker nimodipine. In the adjacent parent dendrites, the increases in [Ca(2+)](i) were neither supralinear nor sensitive to the relative pre-post timing. In parallel experiments, we investigated the effects of these pairing protocols on spike-timing-dependent synaptic plasticity. Long-term depression (t-LTD) of corticostriatal inputs was induced by pre-post but not post-pre protocols. Intracellular calcium chelators and calcium antagonists blocked pre-post t-LTD, confirming that elevated calcium entering via voltage-sensitive calcium channels is necessary for t-LTD. These findings confirm a spine [Ca(2+)](i) threshold for induction of t-LTD in the corticostriatal pathway, mediated by the supralinear increase in [Ca(2+)](i) associated with pre-post induction protocols.

Automated 4D analysis of dendritic spine morphology: applications to stimulus-induced spine remodeling and pharmacological rescue in a disease model

Uncovering the mechanisms that regulate dendritic spine morphology has been limited, in part, by the lack of efficient and unbiased methods for analyzing spines. Here, we describe an automated 3D spine morphometry method and its application to spine remodeling in live neurons and spine abnormalities in a disease model. We anticipate that this approach will advance studies of synapse structure and function in brain development, plasticity, and disease.

Lipocalin-2 controls neuronal excitability and anxiety by regulating dendritic spine formation and maturation

Psychological stress causes adaptive changes in the nervous system directed toward maintaining homoeostasis. These biochemical and structural mechanisms regulate animal behavior, and their malfunction may result in various forms of affective disorders. Here we found that the lipocalin-2 (Lcn2) gene, encoding a secreted protein of unknown neuronal function, was up-regulated in mouse hippocampus following psychological stress. Addition of lipocalin-2 to cultured hippocampal neurons reduced dendritic spine actin's mobility, caused retraction of mushroom spines, and inhibited spine maturation. These effects were further enhanced by inactivating iron-binding residues of Lcn-2, suggesting that they were facilitated by the iron-free form of Lcn-2. Concurrently, disruption of the Lcn2 gene in mice promoted stress-induced increase in spine density and caused an increase in the proportion of mushroom spines. The above changes correlated with higher excitability of CA1 principal neurons and with elevated stress-induced anxiety in Lcn-2(-/-) mice. Our study demonstrates that lipocalin-2 promotes stress-induced changes in spine morphology and function to regulate neuronal excitability and anxiety.

Imaging synaptic plasticity

Over the past decade, the use and development of optical imaging techniques has advanced our understanding of synaptic plasticity by offering the spatial and temporal resolution necessary to examine long-term changes at individual synapses. Here, we review the use of these techniques in recent studies of synaptic plasticity and, in particular, long-term potentiation in the hippocampus.

Structural plasticity of dendritic spines

Dendritic spines are small mushroom-like protrusions arising from neurons where most excitatory synapses reside. Their peculiar shape suggests that spines can serve as an autonomous postsynaptic compartment that isolates chemical and electrical signaling. How neuronal activity modifies the morphology of the spine and how these modifications affect synaptic transmission and plasticity are intriguing issues. Indeed, the induction of long-term potentiation (LTP) or depression (LTD) is associated with the enlargement or shrinkage of the spine, respectively. This structural plasticity is mainly controlled by actin filaments, the principal cytoskeletal component of the spine. Here we review the pioneering microscopic studies examining the structural plasticity of spines and propose how changes in actin treadmilling might regulate spine morphology.

Spatial distributions of GABA receptors and local inhibition of Ca2+ transients studied with GABA uncaging in the dendrites of CA1 pyramidal neurons

GABA(γ-amino-butylic acid)-mediated inhibition in the dendrites of CA1 pyramidal neurons was characterized by two-photon uncaging of a caged-GABA compound, BCMACM-GABA, and one-photon uncaging of RuBi-GABA in rat hippocampal slice preparations. Although we found that GABA_A-mediated currents were diffusely distributed along the dendrites, currents elicited at the branch points of the apical dendritic trunk were approximately two times larger than those elsewhere in the dendrite. We examined the inhibitory action of the GABA-induced currents on Ca²⁺ transients evoked with a single back-propagating action potential (bAP) in oblique dendrites. We found that GABA uncaging selectively inhibited the Ca²⁺ transients in the region adjacent (<20 µm) to the uncaging site, and that GABA uncaging was effective only within a short period after bAP (<20 ms). The strength of inhibition was linearly related to the amplitudes of the GABA currents, suggesting that the currents inhibited a sustained, subthreshold after-depolarization without preventing propagation of bAP. GABA uncaging at the dendritic branch points inhibited Ca²⁺ transients farther into dendritic branches (>20 µm). Our data indicate that GABA inhibition results in spatially confined inhibition of Ca²⁺ transients shortly after bAP, and suggest that this effect is particularly potent at the dendritic branch points where GABA receptors cluster.

Modulation of dendritic spine development and plasticity by BDNF and vesicular trafficking: fundamental roles in neurodevelopmental disorders associated with mental retardation and autism

The process of axonal and dendritic development establishes the synaptic circuitry of the central nervous system (CNS) and is the result of interactions between intrinsic molecular factors and the external environment. One growth factor that has a compelling function in neuronal development is the neurotrophin brain-derived neurotrophic factor (BDNF). BDNF participates in axonal and dendritic differentiation during embryonic stages of neuronal development, as well as in the formation and maturation of dendritic spines during postnatal development. Recent studies have also implicated vesicular trafficking of BDNF via secretory vesicles, and both secretory and endosomal trafficking of vesicles containing synaptic proteins, such as neurotransmitter and neurotrophin receptors, in the regulation of axonal and dendritic differentiation, and in dendritic spine morphogenesis. Several genes that are either mutated or deregulated in neurodevelopmental disorders associated with mental retardation have now been identified, and several mouse models of these disorders have been generated and characterized. Interestingly, abnormalities in dendritic and synaptic structure are consistently observed in human neurodevelopmental disorders associated with mental retardation, and in mouse models of these disorders as well. Abnormalities in dendritic and synaptic differentiation are thought to underlie altered synaptic function and network connectivity, thus contributing to the clinical outcome. Here, we review the roles of BDNF and vesicular trafficking in axonal and dendritic differentiation in the context of dendritic and axonal morphological impairments commonly observed in neurodevelopmental disorders associated with mental retardation.

Colocalization of protein kinase A with adenylyl cyclase enhances protein kinase A activity during induction of long-lasting long-term-potentiation

The ability of neurons to differentially respond to specific temporal and spatial input patterns underlies information storage in neural circuits. One means of achieving spatial specificity is to restrict signaling molecules to particular subcellular compartments using anchoring molecules such as A-Kinase Anchoring Proteins (AKAPs). Disruption of protein kinase A (PKA) anchoring to AKAPs impairs a PKA-dependent form of long term potentiation (LTP) in the hippocampus. To investigate the role of localized PKA signaling in LTP, we developed a stochastic reaction-diffusion model of the signaling pathways leading to PKA activation in CA1 pyramidal neurons. Simulations investigated whether the role of anchoring is to locate kinases near molecules that activate them, or near their target molecules. The results show that anchoring PKA with adenylyl cyclase (which produces cAMP that activates PKA) produces significantly greater PKA activity, and phosphorylation of both inhibitor-1 and AMPA receptor GluR1 subunit on S845, than when PKA is anchored apart from adenylyl cyclase. The spatial microdomain of cAMP was smaller than that of PKA suggesting that anchoring PKA near its source of cAMP is critical because inactivation by phosphodiesterase limits diffusion of cAMP. The prediction that the role of anchoring is to colocalize PKA near adenylyl cyclase was confirmed by experimentally rescuing the deficit in LTP produced by disruption of PKA anchoring using phosphodiesterase inhibitors. Additional experiments confirm the model prediction that disruption of anchoring impairs S845 phosphorylation produced by forskolin-induced synaptic potentiation. Collectively, these results show that locating PKA near adenylyl cyclase is a critical function of anchoring.

The hippocampus is a part of the cerebral cortex involved in formation of certain types of long term memories. Activity-dependent change in the strength of neuronal connections in the hippocampus, known as synaptic plasticity, is one mechanism used to store memories. The ability to form crisp and distinguishable memories of different events implies that learning produces plasticity of specific and distinct subsets of synapses within each neuron. Synaptic activity leads to production of intracellular signaling molecules, which ultimately cause changes in the properties of the synapses. The requirement for synaptic specificity seems incompatible with the diffusibility of intracellular signaling molecules. Anchoring proteins restrict signaling molecules to particular subcellular compartments thereby combating the indiscriminate spread of intracellular signaling molecules. To investigate whether the critical function of anchoring proteins is to localize proteins near their activators or their targets, we developed a stochastic reaction-diffusion model of signaling pathways leading to synaptic plasticity in hippocampal neurons. Simulations demonstrate that colocalizing proteins with their activator molecules is more important due to inactivation mechanisms that limit the spatial extent of the activator molecules.

The decade of the dendritic NMDA spike

In the field of cortical cellular physiology, much effort has been invested in understanding thick apical dendrites of pyramidal neurons and the regenerative sodium and calcium spikes that take place in the apical trunk. Here we focus on thin dendrites of pyramidal cells (basal, oblique, and tuft dendrites), and we discuss one relatively novel form of an electrical signal ("NMDA spike") that is specific for these branches. Basal, oblique, and apical tuft dendrites receive a high density of glutamatergic synaptic contacts. Synchronous activation of 10-50 neighboring glutamatergic synapses triggers a local dendritic regenerative potential, NMDA spike/plateau, which is characterized by significant local amplitude (40-50 mV) and an extraordinary duration (up to several hundred milliseconds). The NMDA plateau potential, when it is initiated in an apical tuft dendrite, is able to maintain a good portion of that tuft in a sustained depolarized state. However, if NMDA-dominated plateau potentials originate in proximal segments of basal dendrites, they regularly bring the neuronal cell body into a sustained depolarized state, which resembles a cortical Up state. At each dendritic initiation site (basal, oblique, and tuft) an NMDA spike creates favorable conditions for causal interactions of active synaptic inputs, including the spatial or temporal binding of information, as well as processes of short-term and long-term synaptic modifications (e.g., long-term potentiation or long-term depression). Because of their strong amplitudes and durations, local dendritic NMDA spikes make up the cellular substrate for multisite independent subunit computations that enrich the computational power and repertoire of cortical pyramidal cells. We propose that NMDA spikes are likely to play significant roles in cortical information processing in awake animals (spatiotemporal binding, working memory) and during slow-wave sleep (neuronal Up states, consolidation of memories).

Interactive effects of age and estrogen on cortical neurons: implications for cognitive aging

In the past few decades it has become clear that estrogen signaling plays a much larger role in modulating the cognitive centers of the brain than previously thought possible. We have developed a nonhuman primate (NHP) model to investigate the relationships between estradiol (E) and cognitive aging. Our studies of cyclical E treatment in ovariectomized (OVX) young and aged rhesus monkeys have revealed compelling cognitive and synaptic effects of E in the context of aging. Delayed response (DR), a task that is particularly dependent on integrity of dorsolateral prefrontal cortex (dlPFC) area 46 revealed the following: (1) that young OVX rhesus monkeys perform equally well whether treated with E or vehicle (V), and (2) that aged OVX animals given E perform as well as young adults with or without E, whereas OVX V-treated aged animals display significant DR impairment. We have analyzed the structure of layer III pyramidal cells in area 46 in these same monkeys. We found both age and treatment effects on these neurons that are consistent with behavioral data. Briefly, reconstructions of pyramidal neurons in area 46 from these monkeys showed that cyclical E increased the density of small, thin spines in both young and aged monkeys. However, this effect of E was against a background of age-related loss of small, thin spines, leaving aged V-treated monkeys with a particularly low density of these highly plastic spines, and vulnerable to cognitive decline. Our current interpretation is that E not only plays a critically important role in maintaining spine number, but also enables synaptic plasticity through a cyclical increase in small highly plastic spines that may be stabilized in the context of learning. Interestingly, recent studies demonstrate that chronic E is less effective at inducing spinogenesis than cyclical E. We have begun to link certain molecular attributes of excitatory synapses in area 46 to E effects and cognitive performance in these monkeys. Given the importance of synaptic estrogen receptor α (ER-α) in rat hippocampus, we focused our initial studies on synaptic ER-α in area 46. Three key findings have emerged from these studies: (1) synaptic ER-α is present in axospinous synapses in area 46; (2) it is stable across treatment and age groups (which is not the case in rat hippocampus); and (3) the abundance and distribution of synaptic ER-α is a key correlate of individual variation in cognitive performance in certain age and treatment groups. These findings have important implications for the design of hormone treatment strategies for both surgically and naturally menopausal women. This article is part of a Special Issue entitled: Neuroactive Steroids: Focus on Human Brain.

Quantification of postsynaptic density proteins: glutamate receptor subunits and scaffolding proteins

The postsynaptic density (PSD) protein complex has long been a major target of proteomics in neuroscience. As the number of glutamate receptors on a synapse is one of the main determinants of synaptic efficacy, determining the absolute numbers of receptors in the PSD is necessary for estimating the amplitude of the excitatory postsynaptic current (EPSC) in individual synapses. Moreover, as the receptor molecules are embedded in a macromolecular complex within the PSD, stoichiometry between the receptors and other PSD proteins could help explain the functional and regional specialization of the synapses and their possible roles in synaptic plasticity. Here, I review various studies concerned with the quantification of PSD proteins.

Loss of prestin does not alter the development of auditory cortical dendritic spines

Disturbance of sensory input during development can have disastrous effects on the development of sensory cortical areas. To examine how moderate perturbations of hearing can impact the development of primary auditory cortex, we examined markers of excitatory synapses in mice who lacked prestin, a protein responsible for somatic electromotility of cochlear outer hair cells. While auditory brain stem responses of these mice show an approximately 40 dB increase in threshold, we found that loss of prestin produced no changes in spine density or morphological characteristics on apical dendrites of cortical layer 5 pyramidal neurons. PSD-95 immunostaining also showed no changes in overall excitatory synapse density. Surprisingly, behavioral assessments of auditory function using the acoustic startle response showed only modest changes in prestin KO animals. These results suggest that moderate developmental hearing deficits produce minor changes in the excitatory connectivity of layer 5 neurons of primary auditory cortex and surprisingly mild auditory behavioral deficits in the startle response.

Two-photon uncaging microscopy

Two-photon uncaging takes advantage of the inherent optical sectioning power of two-photon excitation to generate highly localized concentration increases of neurotransmitters such as glutamate. This can be used to activate isolated clusters of receptors and, thus, produce maps of receptor densities, or activate intracellular signal transduction under these receptors, in three dimensions and in complex structures such as hippocampal brain slices. Used in combination with two-photon imaging, two-photon uncaging provides a means to study the long-term structural and functional consequences of stimulation of structures such as dendritic spines. This protocol gives an overview of the procedures used for two-photon uncaging microscopy. It includes a detailed description of the development of a microscope that enables effective two-photon release of caged neurotransmitters and provides several examples of its use in cultured and acutely isolated hippocampal neurons.

Two-photon microscopy for chemical neuroscience

Microscopes using non-linear excitation of chromophores with pulsed near-IR light can generate highly localized foci of molecules in the electronic singlet state that are concentrated in volumes of less than one femtoliter. The three-dimensional confinement of excitation arises from the simultaneous absorption of two IR photons of approximately half the energy required for linear excitation. Two-photon microscopy is especially useful for two types of interrogation of neural processes. First, uncaging of signaling molecules such as glutamate, as stimulation is so refined it can be used to mimic normal unitary synaptic levels. In addition, uncaging allows complete control of the timing and position of stimulation, so the two-photon light beam provides the chemical neuroscientist with an "optical conductor's baton" which can command synaptic activity at will. A second powerful feature of two-photon microscopy is that when used for fluorescence imaging it enables the visualization of cellular structure and function in living animals at depths far beyond that possible with normal confocal microscopes. In this review I provide a survey of the many important applications of two-photon microscopy in these two fields of neuroscience, and suggest some areas for future technical development.

Sequestration of CaMKII in dendritic spines in silico

Calcium calmodulin dependent kinase II (CaMKII) is sequestered in dendritic spines within seconds upon synaptic stimulation. The program Smoldyn was used to develop scenarios of single molecule CaMKII diffusion and binding in virtual dendritic spines. We first validated simulation of diffusion as a function of spine morphology. Additional cellular structures were then incorporated to simulate binding of CaMKII to the post-synaptic density (PSD); binding to cytoskeleton; or their self-aggregation. The distributions of GFP tagged native and mutant constructs in dissociated hippocampal neurons were measured to guide quantitative analysis. Intra-spine viscosity was estimated from fluorescence recovery after photo-bleach (FRAP) of red fluorescent protein. Intra-spine mobility of the GFP-CaMKIIα constructs was measured, with hundred-millisecond or better time resolution, from FRAP of distal spine tips in conjunction with fluorescence loss (FLIP) from proximal regions. Different FRAP \ FLIP profiles were predicted from our Scenarios and provided a means to differentiate binding to the PSDs from self-aggregation. The predictions were validated by experiments. Simulated fits of the Scenarios provided estimates of binding and rate constants. We utilized these values to assess the role of self-aggregation during the initial response of native CaMKII holoenzymes to stimulation. The computations revealed that self-aggregation could provide a concentration-dependent switch to amplify CaMKII sequestration and regulate its activity depending on its occupancy of the actin cytoskeleton.

In vivo two-photon uncaging of glutamate revealing the structure-function relationships of dendritic spines in the neocortex of adult mice

Two-photon (2P) uncaging of caged neurotransmitters can efficiently stimulate individual synapses and is widely used to characterize synaptic functions in brain slice preparations. Here we extended 2P uncaging to neocortical pyramidal neurons in adult mice in vivo where caged glutamate was applied from the pial surface. To validate the methodology, we applied a small fluorescent probe using the same method, and confirmed that its concentrations were approximately homogenous up to 200 μm below the cortical surface, and that the extracellular space of the neocortex was as large as 22%. In fact, in vivo whole-cell recording revealed that 2P glutamate uncaging could elicit transient currents (2pEPSCs) very similar to excitatory postsynaptic currents (EPSCs). A spatial resolution of glutamate uncaging was 0.6-0.8 μm up to the depth of 200 μm, and in vivo 2P uncaging was able to stimulate single identified spines. Automated three-dimensional (3-D) mapping of such 2pEPSCs which covered the surfaces of dendritic branches revealed that functional AMPA receptor expression was stable and proportional to spine volume.Moreover, in vivo 2P Ca2+ imaging and uncaging suggested that the amplitudes of glutamate-induced Ca2+ transients were inversely proportional to spine volume. Thus, the key structure-function relationships hold in dendritic spines in adult neocortex in vivo, as in young hippocampal slice preparations. In vivo 2P uncaging will be a powerful tool to investigate properties of synapses in the neocortex.

The mechanisms underlying the spatial spreading of signaling activity

During the induction of plasticity of dendritic spines, many intracellular signaling pathways are spatially and temporally regulated to co-ordinate downstream cellular processes in different dendritic micron-domains. Recent advent of imaging technology based on fluorescence resonance energy transfer (FRET) has allowed the direct monitoring of the spatiotemporal regulation of signaling activity in spines and dendrites during synaptic plasticity. In particular, the activity of three small GTPase proteins HRas, Cdc42, and RhoA, which share similar structure and mobility on the plasma membrane, displayed different spatial spreading patterns: Cdc42 is compartmentalized in the stimulated spines while RhoA and HRas spread into dendrites over 5-10 μm. These measurements thus provide the basis for understanding the mechanisms underlying the spatiotemporal regulation of signaling activity. Further, using spatiotemporally controlled spine stimulations, some of the roles of signal spreading have been revealed.

Simultaneous two-photon activation of presynaptic cells and calcium imaging in postsynaptic dendritic spines

Background

Dendritic spines of pyramidal neurons are distributed along the complicated structure of the dendritic branches and possess a variety of morphologies associated with synaptic strength. The location and structure of dendritic spines determine the extent of synaptic input integration in the postsynaptic neuron. However, how spine location or size relates to the position of innervating presynaptic cells is not yet known. This report describes a new method that represents a first step toward addressing this issue.

Results

The technique combines two-photon uncaging of glutamate over a broad area (~500 × 250 × 100 μm) with two-photon calcium imaging in a narrow region (~50 × 10 × 1 μm). The former was used for systematic activation of layer 2/3 pyramidal cells in the rat motor cortex, while the latter was used to detect the dendritic spines of layer 5 pyramidal cells that were innervated by some of the photoactivated cells. This technique allowed identification of various sizes of innervated spine located <140 μm laterally from the postsynaptic soma. Spines distal to their parent soma were preferentially innervated by cells on the ipsilateral side. No cluster of neurons innervating the same dendritic branch was detected.

Conclusions

This new method will be a powerful tool for clarifying the microarchitecture of synaptic connections, including the positional and structural characteristics of dendritic spines along the dendrites.

Activity-dependent calcium signaling and ERK-MAP kinases in neurons: a link to structural plasticity of the nucleus and gene transcription regulation

Activity-dependent gene expression is important for the formation and maturation of neuronal networks, neuronal survival and for plastic modifications within mature networks. At the level of individual neurons, expression of new protein is required for dendritic branching, synapse formation and elimination. Experience-driven synaptic activity induces membrane depolarization, which in turn evokes intracellular calcium transients that are decoded according to their source and strength by intracellular calcium sensing proteins. In order to activate the gene transcription machinery of the cell, calcium signals have to be conveyed from the site of their generation in the cytoplasm to the cell nucleus. This can occur via a variety of mechanisms and with different kinetics depending on the source and amplitude of calcium influx. One mechanism involves the propagation of calcium itself, leading to nuclear calcium transients that subsequently activate transcription. The mitogen-activated protein kinase (MAPK) cascade represents a second central signaling module that transduces information from the site of calcium signal generation at the plasma membrane to the nucleus. Nuclear signaling of the MAPK cascades catalyzes the phosphorylation of transcription factors but also regulates gene transcription more globally at the level of chromatin remodeling as well as through its recently identified role in the modulation of nuclear shape. Here we discuss the possible mechanisms by which the MAPKs ERK1 and ERK2, activated by synaptically evoked calcium influx, can signal to the nucleus and regulate gene transcription. Moreover, we describe how MAPK-dependent structural plasticity of the nuclear envelope enhances nuclear calcium signaling and suggest possible implications for the regulation of gene transcription in the context of nuclear geometry.

Intrinsic programs regulating dendrites and synapses in the upper layer neurons of the cortex

Dendrites and spines are key regulators of neuronal function often affected in cognitive disorders. Neuronal subclasses are characterized by a wide range of dendritic morphologies that aid their specific functions. However, how subclass-specific dendritic trees arise during vertebrate development remains largely unknown. We have recently reported that the restricted expression of Cux1 and Cux2 genes in the upper layers of the cerebral cortex determines the specific morphology of dendrites and spines and the function of these neurons. Since Cux genes are the vertebrate homologs of Drosophila Cut, which specifies the dendritic morphologies of certain sensory neuron populations, our findings suggest that mechanisms of dendrite differentiation are conserved between Drosophila and mammals, which had yet to be demonstrated. Importantly, we found that Cux genes not only modulate dendritic branching, but also dendritic spine morphogenesis, the functional synapse and cognition. Dendritic spine stabilization was partly mediated by direct repression of genes of the Xlr family, previously implicated in cognitive defects in a model of Turner syndrome. Hence, our work indicates that neuronal subclass specific determinants may intrinsically affect synaptic activity beyond expected. The functions of Cux1 and Cux2 were additive and complement each other to establish the final pattern of the dendritic tree and the number and strength of the synapses. This work unravels novel mechanisms of dendritogenesis and synaptogenesis and illustrates how regulating dendritic structures contributes to the specialization of upper layer neurons. It will be interesting to dissect how these mechanisms regulate cortical activity, area specialization and cognitive functions.

Methods of measuring activity at individual synapses: a review of techniques and the findings they have made possible

Neurons in the brain are often linked by single synaptic contacts (Gulyás et al., 1993) and the probabilistic character of synaptic activity makes it desirable to increase the resolution of physiological experiments by observing the function of the smallest possible number of synaptic terminals, ideally, one. Because they are critically important and technically difficult to resolve, several of the core questions investigated in singe-site experiments have been under study for decades (Auger and Marty, 2000). Many approaches have been taken toward the goal of measuring activity at few synapses, and consideration of the capabilities and limitations of each of these methods permits a review of the contributions each has made possible to present understanding of synaptic function. A number of methodological advances in recent years have increased resolving power. New techniques often build on previous developments and many effective approaches combine components of existing specialized methods with new technology. One theme that emerges is that synaptic properties vary among regions, reducing the utility of general questions such as whether synaptic glutamate saturates receptors or how rapidly synaptic vesicle pools are depleted. For several core questions, multiple studies using different methods have reached similar conclusions, suggesting that consensus may be emerging for some anatomic synapses.

Structural dynamics of synapses in vivo correlate with functional changes during experience-dependent plasticity in visual cortex

The impact of activity on neuronal circuitry is complex, involving both functional and structural changes whose interaction is largely unknown. We have used optical imaging of mouse visual cortex responses and two-photon imaging of superficial layer spines on layer 5 neurons to monitor network function and synaptic structural dynamics in the mouse visual cortex in vivo. Total lack of vision due to dark-rearing from birth dampens visual responses and shifts spine dynamics and morphologies toward an immature state. The effects of vision after dark rearing are strongly dependent on the timing of exposure: over a period of days, functional and structural changes are temporally related such that light stabilizes spines while increasing visually driven activity. The effects of long-term light exposure can be partially mimicked by experimentally enhancing inhibitory signaling in the darkness. Brief light exposure, however, results in a rapid, transient, NMDA-dependent increase of cortical responses, accompanied by increased dynamics of dendritic spines. These findings indicate that visual experience induces rapid reorganization of cortical circuitry followed by a period of stabilization, and demonstrate a close relationship between dynamic changes at single synapses and cortical network function.

CHMP2B mutants linked to frontotemporal dementia impair maturation of dendritic spines

The highly conserved ESCRT-III complex is responsible for deformation and cleavage of membranes during endosomal trafficking and other cellular activities. In humans, dominant mutations in the ESCRT-III subunit CHMP2B cause frontotemporal dementia (FTD). The decade-long process leading to this cortical degeneration is not well understood. One possibility is that, akin to other neurodegenerative diseases, the pathogenic protein affects the integrity of dendritic spines and synapses before any neuronal death. Using confocal microscopy and 3D reconstruction, we examined whether expressing the FTD-linked mutants CHMP2B(intron5) and CHMP2B(Delta10) in cultured hippocampal neurons modified the number or structure of spines. Both mutants induced a significant decrease in the proportion of large spines with mushroom morphology, without overt degeneration. Furthermore, CHMP2B(Delta10) induced a drop in frequency and amplitude of spontaneous excitatory postsynaptic currents, suggesting that the more potent synapses were lost. These effects seemed unrelated to changes in autophagy. Depletion of endogenous CHMP2B by RNAi resulted in morphological changes similar to those induced by mutant CHMP2B, consistent with dominant-negative activity of pathogenic mutants. Thus, CHMP2B is required for spine growth. Taken together, these results demonstrate that a mutant ESCRT-III subunit linked to a human neurodegenerative disease can disrupt the normal pattern of spine development.

CaMKII control of spine size and synaptic strength: role of phosphorylation states and nonenzymatic action

CaMKII is an abundant synaptic protein strongly implicated in plasticity. Overexpression of autonomous (T286D) CaMKII in CA1 hippocampal cells enhances synaptic strength if T305/T306 sites are not phosphorylated, but decreases synaptic strength if they are phosphorylated. It has generally been thought that spine size and synaptic strength covary; however, the ability of CaMKII and its various phosphorylation states to control spine size has not been previously examined. Using a unique method that allows the effects of overexpressed protein to be monitored over time, we found that all autonomous forms of CaMKII increase spine size. Thus, for instance, the T286D/T305D/T306D form increases spine size but decreases synaptic strength. Further evidence for such dissociation is provided by experiments with the T286D form that has been made catalytically dead. This form fails to enhance synaptic strength but increases spine size, presumably by a structural process. Thus very different mechanisms govern how CaMKII affects spine structure and synaptic function.

Mitochondrial trafficking and the provision of energy and calcium buffering at excitatory synapses

Neuronal postsynaptic currents consume most of the brain's energy supply. Delineating how neurons control the distribution, morphology and function of the energy-producing mitochondria that fuel synaptic communication is therefore important for our understanding of nervous system function and pathology. Here we review recent insights into the molecular mechanisms that control activity-dependent regulation of mitochondrial trafficking, morphology and activity at excitatory synapses. We also consider some implications of this regulation for synaptic function and plasticity and discuss how this may contribute to synaptic dysfunction and signalling in neurological disease, with a focus on Alzheimer's disease.

Dendritic organization of sensory input to cortical neurons in vivo

In sensory cortex regions, neurons are tuned to specific stimulus features. For example, in the visual cortex, many neurons fire predominantly in response to moving objects of a preferred orientation. However, the characteristics of the synaptic input that cortical neurons receive to generate their output firing pattern remain unclear. Here we report a novel approach for the visualization and functional mapping of sensory inputs to the dendrites of cortical neurons in vivo. By combining high-speed two-photon imaging with electrophysiological recordings, we identify local subthreshold calcium signals that correspond to orientation-specific synaptic inputs. We find that even inputs that share the same orientation preference are widely distributed throughout the dendritic tree. At the same time, inputs of different orientation preference are interspersed, so that adjacent dendritic segments are tuned to distinct orientations. Thus, orientation-tuned neurons can compute their characteristic firing pattern by integrating spatially distributed synaptic inputs coding for multiple stimulus orientations.

Structural dynamics of dendritic spines in memory and cognition

Recent studies show that dendritic spines are dynamic structures. Their rapid creation, destruction and shape-changing are essential for short- and long-term plasticity at excitatory synapses on pyramidal neurons in the cerebral cortex. The onset of long-term potentiation, spine-volume growth and an increase in receptor trafficking are coincident, enabling a 'functional readout' of spine structure that links the age, size, strength and lifetime of a synapse. Spine dynamics are also implicated in long-term memory and cognition: intrinsic fluctuations in volume can explain synapse maintenance over long periods, and rapid, activity-triggered plasticity can relate directly to cognitive processes. Thus, spine dynamics are cellular phenomena with important implications for cognition and memory. Furthermore, impaired spine dynamics can cause psychiatric and neurodevelopmental disorders.

Activity-dependent localization in spines of the F-actin capping protein CapZ screened in a rat model of dementia

Actin reorganization in dendritic spines is hypothesized to underlie neuronal plasticity. Actin-related proteins, therefore, might serve as useful markers of plastic changes in dendritic spines. Here, we utilized memory deficits induced by fimbria-fornix transection (FFT) in rats as a dementia model to screen candidate memory-associated molecules by using a two-dimensional gel method. Comparison of protein profiles between the transected and control sides of hippocampi after unilateral FFT revealed a reduction in the F-actin capping protein (CapZ) signal on the FFT side. Subsequent immunostaining of brain sections and cultured hippocampal neurons revealed that CapZ localized in dendritic spines and the signal intensity in each spine varied widely. The CapZ content decreased after suppression of neuronal firing by tetrodotoxin treatment in cultured neurons, indicating rapid and activity-dependent regulation of CapZ accumulation in spines. To test input specificity of CapZ accumulation in vivo, we delivered high-frequency stimuli to the medial perforant path unilaterally in awake rats. This path selectively inputs to the middle molecular layer of the dentate gyrus, where CapZ immunoreactivity increased. We conclude that activity-dependent, synapse-specific regulation of CapZ redistribution might be important in both maintenance and remodeling of synaptic connections in neurons receiving specific spatial and temporal patterns of inputs.

Overexpression of the cell adhesion protein neuroligin-1 induces learning deficits and impairs synaptic plasticity by altering the ratio of excitation to inhibition in the hippocampus

Trans-synaptic cell-adhesion molecules have been implicated in regulating CNS synaptogenesis. Among these, the Neuroligin (NL) family (NLs 1-4) of postsynaptic adhesion proteins has been shown to promote the development and specification of excitatory versus inhibitory synapses. NLs form a heterophilic complex with the presynaptic transmembrane protein Neurexin (NRX). A differential association of NLs with postsynaptic scaffolding proteins and NRX isoforms has been suggested to regulate the ratio of excitatory to inhibitory synapses (E/I ratio). Using transgenic mice, we have tested this hypothesis by overexpressing NL1 in vivo to determine whether the relative levels of these cell adhesion molecules may influence synapse maturation, long-term potentiation (LTP), and/or learning. We found that NL1-overexpressing mice show significant deficits in memory acquisition, but not in memory retrieval. Golgi and electron microscopy analysis revealed changes in synapse morphology indicative of increased maturation of excitatory synapses. In parallel, electrophysiological examination indicated a shift in the synaptic activity toward increased excitation as well as impairment in LTP induction. Our results demonstrate that altered balance in the expression of molecules necessary for synapse specification and development (such as NL1) can lead to defects in memory formation and synaptic plasticity and outline the importance of rigidly controlled synaptic maturation processes.

Regulation of NMDA receptor Ca2+ signalling and synaptic plasticity

NMDARs (N-methyl-D-aspartate receptors) are critical for synaptic function throughout the CNS (central nervous system). NMDAR-mediated Ca(2+) influx is implicated in neuronal differentiation, neuronal migration, synaptogenesis, structural remodelling, long-lasting forms of synaptic plasticity and higher cognitive functions. NMDAR-mediated Ca(2+) signalling in dendritic spines is not static, but can be remodelled in a cell- and synapse-specific manner by NMDAR subunit composition, protein kinases and neuronal activity during development and in response to sensory experience. Recent evidence indicates that Ca(2+) permeability of neuronal NMDARs, NMDAR-mediated Ca(2+) signalling in spines and induction of NMDAR-dependent LTP (long-term potentiation) at hippocampal Schaffer collateral-CA1 synapses are under control of the cAMP/PKA (protein kinase A) signalling cascade. Thus, by enhancing Ca(2+) influx through NMDARs in spines, PKA can regulate the induction of LTP. An emerging concept is that activity-dependent regulation of NMDAR-mediated Ca(2+) signalling by PKA and by extracellular signals that modulate cAMP or protein phosphatases at synaptic sites provides a dynamic and potentially powerful mechanism for bi-directional regulation of synaptic efficacy and remodelling.

Structural plasticity can produce metaplasticity

BACKGROUND

Synaptic plasticity underlies many aspect of learning memory and development. The properties of synaptic plasticity can change as a function of previous plasticity and previous activation of synapses, a phenomenon called metaplasticity. Synaptic plasticity not only changes the functional connectivity between neurons but in some cases produces a structural change in synaptic spines; a change thought to form a basis for this observed plasticity. Here we examine to what extent structural plasticity of spines can be a cause for metaplasticity. This study is motivated by the observation that structural changes in spines are likely to affect the calcium dynamics in spines. Since calcium dynamics determine the sign and magnitude of synaptic plasticity, it is likely that structural plasticity will alter the properties of synaptic plasticity.

METHODOLOGY/PRINCIPAL FINDINGS

In this study we address the question how spine geometry and alterations of N-methyl-D-aspartic acid (NMDA) receptors conductance may affect plasticity. Based on a simplified model of the spine in combination with a calcium-dependent plasticity rule, we demonstrated that after the induction phase of plasticity a shift of the long term potentiation (LTP) or long term depression (LTD) threshold takes place. This induces a refractory period for further LTP induction and promotes depotentiation as observed experimentally. That resembles the BCM metaplasticity rule but specific for the individual synapse. In the second phase, alteration of the NMDA response may bring the synapse to a state such that further synaptic weight alterations are feasible. We show that if the enhancement of the NMDA response is proportional to the area of the post synaptic density (PSD) the plasticity curves most likely return to the initial state.

CONCLUSIONS/SIGNIFICANCE

Using simulations of calcium dynamics in synaptic spines, coupled with a biophysically motivated calcium-dependent plasticity rule, we find under what conditions structural plasticity can form the basis of synapse specific metaplasticity.

Hebbian crosstalk prevents nonlinear unsupervised learning

Learning is thought to occur by localized, activity-induced changes in the strength of synaptic connections between neurons. Recent work has shown that induction of change at one connection can affect changes at others (“crosstalk”). We studied the role of such crosstalk in nonlinear Hebbian learning using a neural network implementation of independent components analysis. We find that there is a sudden qualitative change in the performance of the network at a threshold crosstalk level, and discuss the implications of this for nonlinear learning from higher-order correlations in the neocortex.

Biphasic synaptic Ca influx arising from compartmentalized electrical signals in dendritic spines

Dendritic spines compartmentalize synaptically-evoked biochemical signals. The authors show that electrical compartmentalization provided by a spine endows the associated synapse with additional modes of calcium signaling by shaping the kinetics of synaptic calcium currents.

Excitatory synapses on mammalian principal neurons are typically formed onto dendritic spines, which consist of a bulbous head separated from the parent dendrite by a thin neck. Although activation of voltage-gated channels in the spine and stimulus-evoked constriction of the spine neck can influence synaptic signals, the contribution of electrical filtering by the spine neck to basal synaptic transmission is largely unknown. Here we use spine and dendrite calcium (Ca) imaging combined with 2-photon laser photolysis of caged glutamate to assess the impact of electrical filtering imposed by the spine morphology on synaptic Ca transients. We find that in apical spines of CA1 hippocampal neurons, the spine neck creates a barrier to the propagation of current, which causes a voltage drop and results in spatially inhomogeneous activation of voltage-gated Ca channels (VGCCs) on a micron length scale. Furthermore, AMPA and NMDA-type glutamate receptors (AMPARs and NMDARs, respectively) that are colocalized on individual spine heads interact to produce two kinetically and mechanistically distinct phases of synaptically evoked Ca influx. Rapid depolarization of the spine triggers a brief and large Ca current whose amplitude is regulated in a graded manner by the number of open AMPARs and whose duration is terminated by the opening of small conductance Ca-activated potassium (SK) channels. A slower phase of Ca influx is independent of AMPAR opening and is determined by the number of open NMDARs and the post-stimulus potential in the spine. Biphasic synaptic Ca influx only occurs when AMPARs and NMDARs are coactive within an individual spine. These results demonstrate that the morphology of dendritic spines endows associated synapses with specialized modes of signaling and permits the graded and independent control of multiple phases of synaptic Ca influx.

The vast majority of excitatory synapses in the mammalian central nervous system are made onto dendritic spines, small (< 1 fL) membranous structures stippled along the dendrite. The head of each spine is separated from its parent dendrite by a thin neck – a morphological feature that intuitively suggests it might function to limit the transmission of electrical and biochemical signals. Unfortunately, the extremely small size of spines has made direct measurements of their electrical properties difficult and, therefore, the functional implications of electrical compartmentalization have remained elusive. In this study, we use spatiotemporally controlled stimulation to generate calcium signals within the spine head and/or neighboring dendrite. By comparing these measurements we demonstrate that spines create specialized electrical signaling compartments, which has at least two functional consequences. First, synaptic stimulation, but not similar dendritic depolarization, can trigger the activation of voltage-gated calcium channels within the spine. Second, voltage changes in the spine head arising from compartmentalization shape the time course of synaptically evoked calcium influx such that it is biphasic. Thus, the electrical compartmentalization provided by spines allows for multiple modes of calcium signaling at excitatory synapses.

Glial ephrin-A3 regulates hippocampal dendritic spine morphology and glutamate transport

Increasing evidence indicates the importance of neuron-glia communication for synaptic function, but the mechanisms involved are not fully understood. We reported that the EphA4 receptor tyrosine kinase is in dendritic spines of pyramidal neurons of the adult hippocampus and regulates spine morphology. We now show that the ephrin-A3 ligand, which is located in the perisynaptic processes of astrocytes, is essential for maintaining EphA4 activation and normal spine morphology in vivo. Ephrin-A3-knockout mice have spine irregularities similar to those observed in EphA4-knockout mice. Remarkably, loss of ephrin-A3 or EphA4 increases the expression of glial glutamate transporters. Consistent with this, glutamate transport is elevated in ephrin-A3-null hippocampal slices whereas Eph-dependent stimulation of ephrin-A3 signaling inhibits glutamate transport. Furthermore, some forms of hippocampus-dependent learning are impaired in the ephrin-A3-knockout mice. Our results suggest that the interaction between neuronal EphA4 and glial ephrin-A3 bidirectionally controls synapse morphology and glial glutamate transport, ultimately regulating hippocampal function.

Synaptic Mechanisms of Activity-Dependent Remodeling in Visual Cortex during Monocular Deprivation

It has long been appreciated that in the visual cortex, particularly within a postnatal critical period for experience-dependent plasticity, the closure of one eye results in a shift in the responsiveness of cortical cells toward the experienced eye. While the functional aspects of this ocular dominance shift have been studied for many decades, their cortical substrates and synaptic mechanisms remain elusive. Nonetheless, it is becoming increasingly clear that ocular dominance plasticity is a complex phenomenon that appears to have an early and a late component. Early during monocular deprivation, deprived eye cortical synapses depress, while later during the deprivation open eye synapses potentiate. Here we review current literature on the cortical mechanisms of activity-dependent plasticity in the visual system during the critical period. These studies shed light on the role of activity in shaping neuronal structure and function in general and can lead to insights regarding how learning is acquired and maintained at the neuronal level during normal and pathological brain development.

Long-term potentiation in isolated dendritic spines

Background

In brain, N-methyl-D-aspartate (NMDA) receptor (NMDAR) activation can induce long-lasting changes in synaptic α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor (AMPAR) levels. These changes are believed to underlie the expression of several forms of synaptic plasticity, including long-term potentiation (LTP). Such plasticity is generally believed to reflect the regulated trafficking of AMPARs within dendritic spines. However, recent work suggests that the movement of molecules and organelles between the spine and the adjacent dendritic shaft can critically influence synaptic plasticity. To determine whether such movement is strictly required for plasticity, we have developed a novel system to examine AMPAR trafficking in brain synaptosomes, consisting of isolated and apposed pre- and postsynaptic elements.

Methodology/Principal Findings

We report here that synaptosomes can undergo LTP-like plasticity in response to stimuli that mimic synaptic NMDAR activation. Indeed, KCl-evoked release of endogenous glutamate from presynaptic terminals, in the presence of the NMDAR co-agonist glycine, leads to a long-lasting increase in surface AMPAR levels, as measured by [³H]-AMPA binding; the increase is prevented by an NMDAR antagonist 2-amino-5-phosphonopentanoic acid (AP5). Importantly, we observe an increase in the levels of GluR1 and GluR2 AMPAR subunits in the postsynaptic density (PSD) fraction, without changes in total AMPAR levels, consistent with the trafficking of AMPARs from internal synaptosomal compartments into synaptic sites. This plasticity is reversible, as the application of AMPA after LTP depotentiates synaptosomes. Moreover, depotentiation requires proteasome-dependent protein degradation.

Conclusions/Significance

Together, the results indicate that the minimal machinery required for LTP is present and functions locally within isolated dendritic spines.

Characterization of dendritic spines in the Drosophila central nervous system

Dendritic spines are a characteristic feature of a number of neurons in the vertebrate nervous system and have been implicated in processes that include learning and memory. In spite of this, there has been no comprehensive analysis of the presence of spines in a classical genetic system, such as Drosophila, so far. Here, we demonstrate that a subset of processes along the dendrites of visual system interneurons in the adult fly central nervous system, called LPTCs, closely resemble vertebrate spines, based on a number of criteria. First, the morphology, size, and density of these processes are very similar to those of vertebrate spines. Second, they are enriched in actin and devoid of tubulin. Third, they are sites of synaptic connections based on confocal and electron microscopy. Importantly, they represent a preferential site of localization of an acetylcholine receptor subunit, suggesting that they are sites of excitatory synaptic input. Finally, their number is modulated by the level of the small GTPase dRac1. Our results provide a basis to dissect the genetics of dendritic spine formation and maintenance and the functional role of spines.

Connectivity patterns revealed by mapping of active inputs on dendrites of thalamorecipient neurons in the auditory cortex

Despite being substantially outnumbered by intracortical inputs on thalamorecipient neurons, thalamocortical projections efficiently deliver acoustic information to the auditory cortex. We hypothesized that thalamic projections may achieve effectiveness by forming synapses at optimal locations on dendritic trees of cortical neurons. Using two-photon calcium imaging in dendritic spines, we constructed maps of active thalamic and intracortical inputs on dendritic trees of thalamorecipient cortical neurons in mouse thalamocortical slices. These maps revealed that thalamic projections synapse preferentially on stubby dendritic spines within 100 microm of the soma, whereas the locations and morphology of spines that receive intracortical projections have a less-defined pattern. Using two-photon photolysis of caged glutamate, we found that activation of stubby dendritic spines located perisomatically generated larger postsynaptic potentials in the soma of thalamorecipient neurons than did activation of remote dendritic spines or spines of other morphological types. These results suggest a novel mechanism of reliability of thalamic projections: the positioning of crucial afferent inputs at optimal synaptic locations.

Activation of CaMKII in single dendritic spines during long-term potentiation

Ca²⁺ / Calmodulin-dependent kinase II (CaMKII) plays a central role in long-term potentiation (LTP), which underlies some forms of learning and memory. Here we monitored the spatiotemporal dynamics of CaMKII activation in individual dendritic spines during LTP using 2-photon fluorescence lifetime imaging microscopy in combination with 2-photon glutamate uncaging. Induction of LTP and associated spine enlargement in single spines triggered transient (∼ 1 min) CaMKII activation restricted to the stimulated spines. CaMKII in spines was specifically activated by NMDA receptors and L-type voltage sensitive calcium channels, presumably via nanodomain Ca²⁺ near the channels, in response to glutamate uncaging and depolarization, respectively. The high degree of compartmentalization and channel specificity of CaMKII signalling allow stimuli-specific spatiotemporal patterns of CaMKII signalling and may be important for synapse-specificity of synaptic plasticity.

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.

Activity of the AMPA receptor regulates drebrin stabilization in dendritic spine morphogenesis

Spine morphogenesis mainly occurs during development as a morphological shift from filopodia-like thin protrusions to bulbous ones. We have previously reported that synaptic clustering of the actin-binding protein drebrin in dendritic filopodia governs spine morphogenesis and synaptic PSD-95 clustering. Here, we report the activity-dependent cellular mechanisms for spine morphogenesis, in which the activity of AMPA receptors (AMPARs) regulates drebrin clustering in spines by promoting drebrin stabilization. In cultured developing hippocampal neurons, pharmacological blockade of AMPARs, but not of other glutamate receptors, suppressed postsynaptic drebrin clustering without affecting presynaptic clustering of synapsin I (synapsin-1). Conversely, the enhancement of the action of AMPARs promoted drebrin clustering in spines. When we explored drebrin dynamics by photobleaching individual spines, we found that AMPAR activity increased the fraction of stable drebrin without affecting the time constant of drebrin turnover. An increase in the fraction of stable drebrin corresponded with increased drebrin clustering. AMPAR blockade also suppressed normal morphological maturation of spines and synaptic PSD-95 clustering in spines. Together, these data suggest that AMPAR-mediated stabilization of drebrin in spines is an activity-dependent cellular mechanism for spine morphogenesis.

Miro1 is a calcium sensor for glutamate receptor-dependent localization of mitochondria at synapses

Energy use, mainly to reverse ion movements in neurons, is a fundamental constraint on brain information processing. Trafficking of mitochondria to locations in neurons where there are large ion fluxes is essential for powering neural function. Mitochondrial trafficking is regulated by Ca²⁺ entry through ionotropic glutamate receptors, but the underlying mechanism is unknown. We show that the protein Miro1 links mitochondria to KIF5 motor proteins, allowing mitochondria to move along microtubules. This linkage is inhibited by micromolar levels of Ca²⁺ binding to Miro1. With the EF hand domains of Miro1 mutated to prevent Ca²⁺ binding, Miro1 could still facilitate mitochondrial motility, but mitochondrial stopping induced by glutamate or neuronal activity was blocked. Activating neuronal NMDA receptors with exogenous or synaptically released glutamate led to Miro1 positioning mitochondria at the postsynaptic side of synapses. Thus, Miro1 is a key determinant of how energy supply is matched to energy usage in neurons.