Glutamate induces the rapid formation of spine head protrusions in hippocampal slice cultures

Developmental lead exposure alters synaptogenesis through inhibiting canonical Wnt pathway in vivo and in vitro

Lead (Pb) exposure has been implicated in the impairment of synaptic plasticity in the developing hippocampus, but the mechanism remains unclear. Here, we investigated whether developmental lead exposure affects the dendritic spine formation through Wnt signaling pathway in vivo and in vitro. Sprague–Dawley rats were exposed to lead throughout the lactation period and Golgi-Cox staining method was used to examine the spine density of pyramidal neurons in the hippocampal CA1 area of rats. We found that lead exposure significantly decreased the spine density in both 14 and 21 days-old pups, accompanied by a significant age-dependent decline of the Wnt7a expression and stability of its downstream protein (β-catenin). Furthermore, in cultured hippocampal neurons, lead (0.1 and 1 µM lead acetate) significantly decreased the spine density in a dose-dependent manner. Exogenous Wnt7a application attenuated the decrease of spine density and increased the stability of the downstream molecules in Wnt signaling pathway. Together, our results suggest that lead has a negative impact on spine outgrowth in the developing hippocampus through altering the canonical Wnt pathway.

Estradiol regulates markers of synaptic plasticity in the hypothalamic ventromedial nucleus and amygdala of female rats

Ovarian hormones act in multiple brain regions to modulate specific behaviors and emotional states. For example, ovarian hormones promote female sexual receptivity in the hypothalamic ventromedial nucleus (VMH) and modulate anxiety in the amygdala. Hormone-induced changes within the VMH include structural modifications, such as changes in dendritic spines, dendrite length and the number of synapses. In some situations, dendrite remodeling requires actin polymerization, which depends on phospho-deactivation of the enzyme cofilin, or the ionotropic AMPA-type glutamate receptors, especially the GluA1 and GluA2 subunits. The present experiments used immunohistochemistry to test the hypothesis that ovarian hormone-induced neural plasticity in the VMH and amygdala involves the regulation of phospho-cofilin, GluA1 and GluA2. These proteins were assessed acutely after estradiol administration (0.5, 1.0 and 4.0h), as well as three days after hormone treatment. Both brain regions displayed rapid (4.0h or less) and transient estradiol-induced increases in the level of phospho-cofilin. At the behaviorally relevant time point of three days, differential changes in AMPA receptor subunits were observed. Using Golgi impregnation, the effect of estradiol on amygdala dendrites was examined. Three days after estradiol treatment, an increase in the length of dendrites in the central nucleus of the amygdala was observed. Thus, estradiol initiates structural changes in dendrites in both the VMH and amygdala associated with an early phospho-deactivation of cofilin, followed by dynamic, brain region-specific changes in AMPA receptor composition.

Prolonged ampakine exposure prunes dendritic spines and increases presynaptic release probability for enhanced long-term potentiation in the hippocampus

CX 546, an allosteric positive modulator of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type ionotropic glutamate receptors (AMPARs), belongs to a drug class called ampakines. These compounds have been shown to enhance long-term potentiation (LTP), a cellular model of learning and memory, and improve animal learning task performance, and have augmented cognition in neurodegenerative patients. However, the chronic effect of CX546 on synaptic structures has not been examined. The structure and integrity of dendritic spines are thought to play a role in learning and memory, and their abnormalities have been implicated in cognitive disorders. In addition, their structural plasticity has been shown to be important for cognitive function, such that dendritic spine remodeling has been proposed as the morphological correlate for LTP. Here, we tested the effect of CX546 on dendritic spine remodeling following long-term treatment. We found that, with prolonged CX546 treatment, organotypic hippocampal slice cultures showed a significant reduction in CA3-CA1 excitatory synapse and spine density. Electrophysiological approaches revealed that the CA3-CA1 circuitry compensates for this synapse loss by increasing synaptic efficacy through enhancement of presynaptic release probability. CX546-treated slices showed prolonged and enhanced potentiation upon LTP induction. Furthermore, structural plasticity, namely spine head enlargement, was also more pronounced after CX546 treatment. Our results suggest a concordance of functional and structural changes that is enhanced with prolonged CX546 exposure. Thus, the improved cognitive ability of patients receiving ampakine treatment may result from the priming of synapses through increases in the structural plasticity and functional reliability of hippocampal synapses.

The role of glutamate in the morphological and physiological development of dendritic spines

Dendritic spines form the postsynaptic half of the synapse but how they form during CNS development remains uncertain, as are the factors that promote their morphological and physiological maturation. One hypothesis posits that filopodia, long motile dendritic processes that are present prior to spine formation, are the precursors to spines. Another hypothesis posits that they form directly from the dendritic shaft. We used microphotolysis of caged glutamate to stimulate individual dendritic processes in young hippocampal slice cultures while recording their morphological and physiological responses. We observed that brief trains of stimuli delivered to immature processes triggered morphological changes within minutes that resulted, in about half of experiments, in a more mature, spine-like appearance such as decreased spine neck length and increased spine head width. We also observed that glutamate-induced inward currents elicited from immature processes were mostly or entirely mediated by NMDARs, whereas responses in those processes with a more mature morphology, regardless of actual developmental age, were mediated by both AMPARs and NMDARs. Consistent with this observation, glutamate-induced morphological changes were largely, but not entirely, prevented by blocking NMDARs. Our observations thus favor a model in which filopodia in the developing nervous system sense and respond to release of glutamate from developing axons, resulting in physiological and morphological maturation.

The relationship between PSD-95 clustering and spine stability in vivo

The appearance and disappearance of dendritic spines, accompanied by synapse formation and elimination may underlie the experience-dependent reorganization of cortical circuits. The exact temporal relationship between spine and synapse formation in vivo remains unclear, as does the extent to which synapse formation enhances the stability of newly formed spines and whether transient spines produce synapses. We used in utero electroporation of DsRedExpress- and eGFP-tagged postsynaptic density protein 95 (PSD-95) to investigate the relationship between spine and PSD stability in mouse neocortical L2/3 pyramidal cells in vivo. Similar to previous studies, spines and synapses appeared and disappeared, even in naive animals. Cytosolic spine volumes and PSD-95-eGFP levels in spines covaried over time, suggesting that the strength of many individual synapses continuously changes in the adult neocortex. The minority of newly formed spines acquired PSD-95-eGFP puncta. Spines that failed to acquire a PSD rarely survived for more than a day. Although PSD-95-eGFP accumulation was associated with increased spine lifetimes, most new spines with a PSD did not convert into persistent spines. This indicates that transient spines may serve to produce short-lived synaptic contacts. Persistent spines that were destined to disappear showed, on average, reduced PSD-95-eGFP levels well before the actual pruning event. Altogether, our data indicate that the PSD size relates to spine stability in vivo.

Synaptic vesicle recycling: steps and principles

Synaptic vesicle recycling is one of the best-studied cellular pathways. Many of the proteins involved are known, and their interactions are becoming increasingly clear. However, as for many other pathways, it is still difficult to understand synaptic vesicle recycling as a whole. While it is generally possible to point out how synaptic reactions take place, it is not always easy to understand what triggers or controls them. Also, it is often difficult to understand how the availability of the reaction partners is controlled: how the reaction partners manage to find each other in the right place, at the right time. I present here an overview of synaptic vesicle recycling, discussing the mechanisms that trigger different reactions, and those that ensure the availability of reaction partners. A central argument is that synaptic vesicles bind soluble cofactor proteins, with low affinity, and thus control their availability in the synapse, forming a buffer for cofactor proteins. The availability of cofactor proteins, in turn, regulates the different synaptic reactions. Similar mechanisms, in which one of the reaction partners buffers another, may apply to many other processes, from the biogenesis to the degradation of the synaptic vesicle.

Dendritic spines: the locus of structural and functional plasticity

The introduction of high-resolution time lapse imaging and molecular biological tools has changed dramatically the rate of progress towards the understanding of the complex structure-function relations in synapses of central spiny neurons. Standing issues, including the sequence of molecular and structural processes leading to formation, morphological change, and longevity of dendritic spines, as well as the functions of dendritic spines in neurological/psychiatric diseases are being addressed in a growing number of recent studies. There are still unsettled issues with respect to spine formation and plasticity: Are spines formed first, followed by synapse formation, or are synapses formed first, followed by emergence of a spine? What are the immediate and long-lasting changes in spine properties following exposure to plasticity-producing stimulation? Is spine volume/shape indicative of its function? These and other issues are addressed in this review, which highlights the complexity of molecular pathways involved in regulation of spine structure and function, and which contributes to the understanding of central synaptic interactions in health and disease.

Astrocyte-synapse structural plasticity

The function and efficacy of synaptic transmission are determined not only by the composition and activity of pre- and postsynaptic components but also by the environment in which a synapse is embedded. Glial cells constitute an important part of this environment and participate in several aspects of synaptic functions. Among the glial cell family, the roles played by astrocytes at the synaptic level are particularly important, ranging from the trophic support to the fine-tuning of transmission. Astrocytic structures are frequently observed in close association with glutamatergic synapses, providing a morphological entity for bidirectional interactions with synapses. Experimental evidence indicates that astrocytes sense neuronal activity by elevating their intracellular calcium in response to neurotransmitters and may communicate with neurons. The precise role of astrocytes in regulating synaptic properties, function, and plasticity remains however a subject of intense debate and many aspects of their interactions with neurons remain to be investigated. A particularly intriguing aspect is their ability to rapidly restructure their processes and modify their coverage of the synaptic elements. The present review summarizes some of these findings with a particular focus on the mechanisms driving this form of structural plasticity and its possible impact on synaptic structure and function.

Nitric oxide mediates local activity-dependent excitatory synapse development

Learning related paradigms play an important role in shaping the development and specificity of synaptic networks, notably by regulating mechanisms of spine growth and pruning. The molecular events underlying these synaptic rearrangements remain poorly understood. Here we identify NO signaling as a key mediator of activity-dependent excitatory synapse development. We find that chronic blockade of NO production in vitro and in vivo interferes with the development of hippocampal and cortical excitatory spine synapses. The effect results from a selective loss of activity-mediated spine growth mechanisms and is associated with morphological and functional alterations of remaining synapses. These effects of NO are mediated by a cGMP cascade and can be reproduced or prevented by postsynaptic expression of vasodilator-stimulated phosphoprotein phospho-mimetic or phospho-resistant mutants. In vivo analyses show that absence of NO prevents the increase in excitatory synapse density induced by environmental enrichment and interferes with the formation of local clusters of excitatory synapses. We conclude that NO plays an important role in regulating the development of excitatory synapses by promoting local activity-dependent spine-growth mechanisms.

A simple rule for dendritic spine and axonal bouton formation can account for cortical reorganization after focal retinal lesions

Lasting alterations in sensory input trigger massive structural and functional adaptations in cortical networks. The principles governing these experience-dependent changes are, however, poorly understood. Here, we examine whether a simple rule based on the neurons' need for homeostasis in electrical activity may serve as driving force for cortical reorganization. According to this rule, a neuron creates new spines and boutons when its level of electrical activity is below a homeostatic set-point and decreases the number of spines and boutons when its activity exceeds this set-point. In addition, neurons need a minimum level of activity to form spines and boutons. Spine and bouton formation depends solely on the neuron's own activity level, and synapses are formed by merging spines and boutons independently of activity. Using a novel computational model, we show that this simple growth rule produces neuron and network changes as observed in the visual cortex after focal retinal lesions. In the model, as in the cortex, the turnover of dendritic spines was increased strongest in the center of the lesion projection zone, while axonal boutons displayed a marked overshoot followed by pruning. Moreover, the decrease in external input was compensated for by the formation of new horizontal connections, which caused a retinotopic remapping. Homeostatic regulation may provide a unifying framework for understanding cortical reorganization, including network repair in degenerative diseases or following focal stroke.

The adult brain is less hard-wired than traditionally thought. About ten percent of synapses in the mature visual cortex is continually replaced by new ones (structural plasticity). This percentage greatly increases after lasting changes in visual input. Due to the topographically organized nerve connections from the retina in the eye to the primary visual cortex in the brain, a small circumscribed lesion in the retina leads to a defined area in the cortex that is deprived of input. Recent experimental studies have revealed that axonal sprouting and dendritic spine turnover are massively increased in and around the cortical area that is deprived of input. However, the driving forces for this structural plasticity remain unclear. Using a novel computational model, we examine whether the need for activity homeostasis of individual neurons may drive cortical reorganization after lasting changes in input activity. We show that homeostatic growth rules indeed give rise to structural and functional reorganization of neuronal networks similar to the cortical reorganization observed experimentally. Understanding the principles of structural plasticity may eventually lead to novel treatment strategies for stimulating functional reorganization after brain damage and neurodegeneration.

PIP₃ regulates spinule formation in dendritic spines during structural long-term potentiation

Dendritic spines are small, highly motile structures on dendritic shafts that provide flexibility to neuronal networks. Spinules are small protrusions that project from spines. The number and the length of spinules increase in response to activity including theta burst stimulation and glutamate application. However, what function spinules exert and how their formation is regulated still remains unclear. Phosphatidylinositol-3,4,5-trisphosphate (PIP₃) plays important roles in cell motility such as filopodia and lamellipodia by recruiting downstream proteins such as Akt and WAVE to the membrane, respectively. Here we reveal that PIP₃ regulates spinule formation during structural long-term potentiation (sLTP) of single spines in CA1 pyramidal neurons of hippocampal slices from rats. Since the local distribution of PIP₃ is important to exert its functions, the subcellular distribution of PIP₃ was investigated using a fluorescence lifetime-based PIP₃ probe. PIP₃ accumulates to a greater extent in spines than in dendritic shafts, which is regulated by the subcellular activity pattern of proteins that produce and degrade PIP₃. Subspine imaging revealed that when sLTP was induced in a single spine, PIP₃ accumulates in the spinule whereas PIP₃ concentration in the spine decreased.

Maturation and integration of adult born hippocampal neurons: signal convergence onto small Rho GTPases

Adult neurogenesis, restricted to specific regions in the mammalian brain, represents one of the most interesting forms of plasticity in the mature nervous system. Adult-born hippocampal neurons play important roles in certain forms of learning and memory, and altered hippocampal neurogenesis has been associated with a number of neuropsychiatric diseases such as major depression and epilepsy. Newborn neurons go through distinct developmental steps, from a dividing neurogenic precursor to a synaptically integrated mature neuron. Previous studies have uncovered several molecular signaling pathways involved in distinct steps of this maturational process. In this context, the small Rho GTPases, Cdc42, Rac1, and RhoA have recently been shown to regulate the morphological and synaptic maturation of adult-born dentate granule cells in vivo. Distinct upstream regulators, including growth factors that modulate maturation and integration of newborn neurons have been shown to also recruit the small Rho GTPases. Here we review recent findings and highlight the possibility that small Rho GTPases may act as central assimilators, downstream of critical input onto adult-born hippocampal neurons contributing to their maturation and integration into the existing dentate gyrus (DG) circuitry.

Matrix metalloproteinases regulate the formation of dendritic spine head protrusions during chemically induced long-term potentiation

Dendritic spines are are small membranous protrusions that extend from neuronal dendrites and harbor the majority of excitatory synapses. Increasing evidence has shown that matrix metalloproteinases (MMPs), a family of extracellularly acting and Zn²⁺-dependent endopeptidases, are able to rapidly modulate dendritic spine morphology. Spine head protrusions (SHPs) are filopodia-like processes that extend from the dendritic spine head, representing a form of postsynaptic structural remodeling in response to altered neuronal activity. Herein, we show that chemically induced long-term potentiation (cLTP) in dissociated hippocampal cultures upregulates MMP-9 activity that controls the formation of SHPs. Blocking of MMPs activity or microtubule dynamics abolishes the emergence of SHPs. In addition, autoactive recombinant MMP-9, promotes the formation of SHPs in organotypic hippocampal slices. Furthermore, spines with SHPs gained postsynaptic α-amino-3-hydroxyl-5-methyl-4-isoxazole propionic acid (AMPA) receptors upon cLTP and the synaptic delivery of AMPA receptors was controlled by MMPs. The present results strongly imply that MMP-9 is functionally involved in the formation of SHPs and the control of postsynaptic receptor distribution upon cLTP.

Varenicline and neuronal nicotinic acetylcholine receptors: a new approach to the treatment of co-occurring alcohol and nicotine addiction?

BACKGROUND AND OBJECTIVES

Alcohol and nicotine dependence are serious public health problems worldwide. They are associated with substantial morbidity and mortality, as well as adverse social effects and increased healthcare costs. Although efficacious treatments are available for these disorders, additional therapeutic options are required to ensure greater treatment utilization. In this paper, we describe the empirical basis on which varenicline, a nicotinic partial agonist approved for smoking cessation, may also have utility in the treatment of alcohol addiction.

METHODS

We sought to identify papers examining nicotine dependence, alcohol dependence, smoking, alcohol, and varenicline for possible inclusion in the present review. We identified over 600 papers through Pubmed/Medline, PsychINFO, and Google Scholar. We found 12 papers taking into consideration the following criteria: original language English, varenicline effect on alcohol consumption.

RESULTS

Animal studies have shown that varenicline reduces alcohol consumption. Two recent studies showed that varenicline also reduces alcohol consumption in humans. Both nicotine and alcohol interact with α4β2 and α3β4 nicotinic acetylcholine (ACh) receptors located in the ventral tegmental area of the brain, inducing dopamine (DA) release at the nucleus accumbens. Varenicline binds to nicotinic ACh receptors, where it has partial agonist effects, producing a moderate and constant level of DA release both in the mesolimbic pathway and in the prefrontal cortex.

CONCLUSIONS AND SCIENTIFIC SIGNIFICANCE

Through these effects, varenicline may reduce alcohol craving, seeking, and consumption, in addition to promoting smoking cessation. Additional studies are needed to confirm the efficacy of varenicline in the treatment of alcohol dependence.

Selective regulation of GluA subunit synthesis and AMPA receptor-mediated synaptic function and plasticity by the translation repressor 4E-BP2 in hippocampal pyramidal cells

The eukaryotic initiation factor 4E-binding protein-2 (4E-BP2) is a repressor of cap-dependent mRNA translation and a major downstream effector of the mammalian target of rapamycin (mTOR) implicated in hippocampal long-term synaptic plasticity and memory. Yet, synaptic mechanisms regulated by 4E-BP2 translational repression remain unknown. Combining knock-out mice, whole-cell recordings, spine analysis, and translation profiling, we found that 4E-BP2 deletion selectively upregulated synthesis of glutamate receptor subunits GluA1 and GluA2, facilitating AMPA receptor (AMPAR)-mediated synaptic transmission and affecting translation-dependent chemically induced late long-term potentiation (cL-LTP). In 4E-BP2 knock-out (4E-BP2(-/-)) mice, evoked and miniature EPSCs were increased, an effect mimicked by short-hairpin RNA knockdown of 4E-BP2 in wild-type mice, indicating that 4E-BP2 level regulates basal transmission at mature hippocampal AMPAR-containing synapses. Remarkably, in 4E-BP2(-/-) mice, the AMPA to NMDA receptor (NMDAR) EPSC ratio was increased, without affecting NMDAR-mediated EPSCs. The enhanced AMPAR function concurred with increased spine density and decreased length resulting from greater proportion of regular spines and less filopodia in 4E-BP2(-/-) mice. Polysome profiling revealed that translation of GluA1 and GluA2 subunits, but not GluN1 or GluN2A/B, was selectively increased in 4E-BP2(-/-) hippocampi, consistent with unaltered I-V relation of EPSCs mediated by GluA1/GluA2 heteromers. Finally, translation-dependent cL-LTP of unitary EPSCs was also affected in 4E-BP2(-/-) mice, lowering induction threshold and removing mTOR signaling requirement while impairing induction by normal stimulation. Thus, translational control through 4E-BP2 represents a unique mechanism for selective regulation of AMPAR synthesis, synaptic function, and long-term plasticity, important for hippocampal-dependent memory processes.

Enhanced recruitment of endosomal Na+/H+ exchanger NHE6 into Dendritic spines of hippocampal pyramidal neurons during NMDA receptor-dependent long-term potentiation

Postsynaptic endosomal trafficking has emerged as a principal regulatory mechanism of structural and functional plasticity of glutamatergic synapses. Recycling endosomes perform activity-dependent transport of AMPA receptors (AMPARs) and lipids to the postsynaptic membrane, activities that are known to contribute to long-term synaptic potentiation and hypothesized to subserve learning and memory processes in the brain. Recently, genetic defects in a widely expressed vesicular pH-regulating transporter, the Na(+)/H(+) exchanger NHE6 isoform, have been implicated in neurodevelopmental disorders including severe X-linked mental retardation and autism. However, little information is available regarding the cellular properties of this transporter in the CNS. Here, we show by quantitative light microscopy that the protein abundance of NHE6 is developmentally regulated in area CA1 of the mouse hippocampus. Within pyramidal neurons, NHE6 was found to localize to discrete puncta throughout the soma and neurites, with noticeable accumulation at dendritic spines and presynaptic terminals. Dual immunolabeling of dendritic spines revealed that NHE6 partially colocalizes with typical markers of early and recycling endosomes as well as with the AMPAR subunit GluA1. Significantly, NHE6-containing vesicles exhibited enhanced translocation to dendritic spine heads during NMDA receptor (NMDAR)-dependent long-term potentiation. These data suggest that NHE6 may play a unique, previously unrecognized, role at glutamatergic synapses that are important for learning and memory.

Perisynaptic chondroitin sulfate proteoglycans restrict structural plasticity in an integrin-dependent manner

During early postnatal development of the CNS, neuronal networks are configured through the formation, elimination, and remodeling of dendritic spines, the sites of most excitatory synaptic connections. The closure of this critical period for plasticity correlates with the maturation of the extracellular matrix (ECM) and results in reduced dendritic spine dynamics. Chondroitin sulfate proteoglycans (CSPGs) are thought to be the active components of the mature ECM that inhibit functional plasticity in the adult CNS. These molecules are diffusely expressed in the extracellular space or aggregated as perineuronal nets around specific classes of neurons. We used organotypic hippocampal slices prepared from 6-d-old Thy1-YFP mice and maintained in culture for 4 weeks to allow ECM maturation. We performed live imaging of CA1 pyramidal cells to assess the effect of chondroitinase ABC (ChABC)-mediated digestion of CSPGs on dendritic spine dynamics. We found that CSPG digestion enhanced the motility of dendritic spines and induced the appearance of spine head protrusions in a glutamate receptor-independent manner. These changes were paralleled by the activation of β1-integrins and phosphorylation of focal adhesion kinase at synaptic sites, and were prevented by preincubation with a β1-integrin blocking antibody. Interestingly, microinjection of ChABC close to dendritic segments was sufficient to induce spine remodeling, demonstrating that CSPGs located around dendritic spines modulate their dynamics independently of perineuronal nets. This restrictive action of perisynaptic CSPGs in mature neural tissue may account for the therapeutic effects of ChABC in promoting functional recovery in impaired neural circuits.

Functional circuits of new neurons in the dentate gyrus

The hippocampus is crucial for memory formation. New neurons are added throughout life to the hippocampal dentate gyrus (DG), a brain area considered important for differential storage of similar experiences and contexts. To better understand the functional contribution of adult neurogenesis to pattern separation processes, we recently used a novel synapse specific trans-neuronal tracing approach to identify the (sub) cortical inputs to new dentate granule cells (GCs). It was observed that newly born neurons receive sequential innervation from structures important for memory function. Initially, septal-hippocampal cells provide input to new neurons, including transient innervation from mature GCs as well as direct feedback from area CA3 pyramidal neurons. After about 1 month perirhinal (PRH) and lateral entorhinal cortex (LEC), brain areas deemed relevant to integration of novel sensory and environmental information, become substantial input to new GCs. Here, we review the developmental time-course and proposed functional relevance of new neurons, within the context of their unique neural circuitry.

Deficits in morphofunctional maturation of hippocampal mossy fiber synapses in a mouse model of intellectual disability

The grik2 gene, coding for the kainate receptor subunit GluK2 (formerly GluR6), is associated with autism spectrum disorders and intellectual disability. Here, we tested the hypothesis that GluK2 could play a role in the appropriate maturation of synaptic circuits involved in learning and memory. We show that both the functional and morphological maturation of hippocampal mossy fiber to CA3 pyramidal cell (mf-CA3) synapses is delayed in mice deficient for the GluK2 subunit (GluK2⁻/⁻). In GluK2⁻/⁻ mice this deficit is manifested by a transient reduction in the amplitude of AMPA-EPSCs at a critical time point of postnatal development, whereas the NMDA component is spared. By combining multiple probability peak fluctuation analysis and immunohistochemistry, we have provided evidence that the decreased amplitude reflects a decrease in the quantal size per mf-CA3 synapse and in the number of active synaptic sites. Furthermore, we analyzed the time course of structural maturation of CA3 synapses by confocal imaging of YFP-expressing cells followed by tridimensional (3D) anatomical reconstruction of thorny excrescences and presynaptic boutons. We show that major changes in synaptic structures occur subsequently to the sharp increase in synaptic transmission, and more importantly that the course of structural maturation of synaptic elements is impaired in GluK2⁻/⁻ mice. This study highlights how a mutation in a gene linked to intellectual disability in the human may lead to a transient reduction of synaptic strength during postnatal development, impacting on the proper formation of neural circuits linked to memory.

Spontaneous transmitter release is critical for the induction of long-term and intermediate-term facilitation in Aplysia

Long-term plasticity can differ from short-term in recruiting the growth of new synaptic connections, a process that requires the participation of both the presynaptic and postsynaptic components of the synapse. How does information about synaptic plasticity spread from its site of origin to recruit the other component? The answer to this question is not known in most systems. We have investigated the possible role of spontaneous transmitter release as such a transsynaptic signal. Until recently, relatively little has been known about the functions of spontaneous release. In this paper, we report that spontaneous release is critical for the induction of a learning-related form of synaptic plasticity, long-term facilitation in Aplysia. In addition, we have found that this signaling is engaged quite early, during an intermediate-term stage that is the first stage to involve postsynaptic as well as presynaptic molecular mechanisms. In a companion paper, we show that spontaneous release from the presynaptic neuron acts as an orthograde signal to recruit the postsynaptic mechanisms of intermediate-term facilitation and initiates a cascade that can culminate in synaptic growth with additional stimulation during long-term facilitation. Spontaneous release could make a similar contribution to learning-related synaptic plasticity in mammals.

Differential modulation of drug-induced structural and functional plasticity of dendritic spines

Drug-induced plasticity of excitatory synapses has been proposed to be the cellular mechanism underlying the aberrant learning associated with addiction. Exposure to various drugs of abuse causes both morphological plasticity of dendritic spines and functional plasticity of excitatory synaptic transmission. Chronic activation of μ-opioid receptors (MOR) in cultured hippocampal neurons causes two forms of synaptic plasticity: loss of dendritic spines and loss of synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. With use of live imaging, patch-clamp electrophysiology, and immunocytochemistry, the present study reveals that these two forms of synaptic plasticity are mediated by separate, but interactive, intracellular signaling cascades. The inhibition of Ca(2+)/calmodulin-dependent protein kinase II with 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-l-tyrosyl]-4-phenylpiperazine (KN-62) blocks MOR-mediated structural plasticity of dendritic spines, but not MOR-mediated cellular redistribution of GluR1 and GluR2 AMPA receptor subunits. In contrast, the inhibition of calcineurin with tacrolimus (FK506) blocks both cellular processes. These findings support the idea that drug-induced structural and functional plasticity of dendritic spines is mediated by divergent, but interactive, signaling pathways.

Ultrastructure of synapses in the mammalian brain

The morphology and molecular composition of synapses provide the structural basis for synaptic function. This article reviews the electron microscopy of excitatory synapses on dendritic spines, using data from rodent hippocampus, cerebral cortex, and cerebellar cortex. Excitatory synapses have a prominent postsynaptic density, in contrast with inhibitory synapses, which have less dense presynaptic or postsynaptic specializations and are usually found on the cell body or proximal dendritic shaft. Immunogold labeling shows that the presynaptic active zone provides a scaffold for key molecules involved in the release of neurotransmitter, whereas the postsynaptic density contains ligand-gated ionic channels, other receptors, and a complex network of signaling molecules. Delineating the structure and molecular organization of these axospinous synapses represents a crucial step toward understanding the mechanisms that underlie synaptic transmission and the dynamic modulation of neurotransmission associated with short- and long-term synaptic plasticity.

Glial glutamate transport modulates dendritic spine head protrusions in the hippocampus

Accumulating evidence supports the idea that synapses are tripartite, whereby perisynaptic astrocytes modulate both pre- and postsynaptic function. Although some of these features have been uncovered by using electrophysiological methods, less is known about the structural interplay between synapses and glial processes. Here, we investigated how astrocytes govern the plasticity of individual hippocampal dendritic spines. Recently, we uncovered that a subgroup of innervated dendritic spines is able to undergo remodeling by extending spine head protrusions (SHPs) toward neighboring functional presynaptic boutons, resulting in new synapses. Although glutamate serves as a trigger, how this behavior is regulated is unknown. As astrocytes control extracellular glutamate levels through their high-affinity uptake transporters, together with their privileged access to synapses, we investigated a role for astrocytes in SHP formation. Using time-lapse confocal microscopy, we found that the volume overlap between spines and astrocytic processes decreased during the formation of SHPs. Focal application of glutamate also reduced spine-astrocyte overlap and induced SHPs. Importantly, SHP formation was prevented by blocking glial glutamate transporters, suggesting that glial control of extracellular glutamate is important for SHP-mediated plasticity of spines. Hence, the dynamic changes of both spines and astrocytes can rapidly modify synaptic connectivity.

Targeting of the Arpc3 actin nucleation factor by miR-29a/b regulates dendritic spine morphology

Previous studies have demonstrated that microribonucleic acids (miRs) are key regulators of protein expression in the brain and modulate dendritic spine morphology and synaptic activity. To identify novel miRs involved in neuronal plasticity, we exposed adult mice to chronic treatments with nicotine, cocaine, or amphetamine, which are psychoactive drugs that induce well-documented neuroadaptations. We observed brain region- and drug-specific changes in miR expression levels and identified miR-29a/b as regulators of synaptic morphology. In vitro imaging experiments indicated that miR-29a/b reduce mushroom-shaped dendritic spines on hippocampal neurons with a concomitant increase in filopodial-like outgrowths, suggesting an effect on synapse formation via actin cytoskeleton remodeling. We identified Arpc3, a component of the ARP2/3 actin nucleation complex, as a bona fide target for down-regulation by miR-29a/b. This work provides evidence that targeting of Arpc3 by miR-29a/b fine tunes structural plasticity by regulating actin network branching in mature and developing spines.

Heroin relapse requires long-term potentiation-like plasticity mediated by NMDA2b-containing receptors

Persistent relapse to addictive drugs constitutes the most challenging problem in addiction therapy, and is linked to impaired prefrontal cortex regulation of motivated behaviors involving the nucleus accumbens. Using a rat model of heroin addiction, we show that relapse requires long-term potentiation (LTP)-like increases in synaptic strength in the prefrontal cortex projection to the nucleus accumbens. The increased synaptic strength was paralleled by dendritic spine enlargement in accumbens spiny neurons and required up-regulated surface expression of NMDA2b-containing receptors (NR2B). Accordingly, blocking NR2B before reinstating heroin-seeking prevented the induction of LTP-like changes in spine remodeling and synaptic strength, and inhibited heroin relapse. These data show that LTP-like neuroplasticity in prefrontal-accumbens synapses is initiated by NR2B stimulation and strongly contributes to heroin relapse. Moreover, the data reveal NR2B-containing NMDA receptors as a previously unexplored therapeutic target for treating heroin addiction.

Coordination of size and number of excitatory and inhibitory synapses results in a balanced structural plasticity along mature hippocampal CA1 dendrites during LTP

Enlargement of dendritic spines and synapses correlates with enhanced synaptic strength during long-term potentiation (LTP), especially in immature hippocampal neurons. Less clear is the nature of this structural synaptic plasticity on mature hippocampal neurons, and nothing is known about the structural plasticity of inhibitory synapses during LTP. Here the timing and extent of structural synaptic plasticity and changes in local protein synthesis evidenced by polyribosomes were systematically evaluated at both excitatory and inhibitory synapses on CA1 dendrites from mature rats following induction of LTP with theta-burst stimulation (TBS). Recent work suggests dendritic segments can act as functional units of plasticity. To test whether structural synaptic plasticity is similarly coordinated, we reconstructed from serial section transmission electron microscopy all of the spines and synapses along representative dendritic segments receiving control stimulation or TBS-LTP. At 5 min after TBS, polyribosomes were elevated in large spines suggesting an initial burst of local protein synthesis, and by 2 h only those spines with further enlarged synapses contained polyribosomes. Rapid induction of synaptogenesis was evidenced by an elevation in asymmetric shaft synapses and stubby spines at 5 min and more nonsynaptic filopodia at 30 min. By 2 h, the smallest synaptic spines were markedly reduced in number. This synapse loss was perfectly counterbalanced by enlargement of the remaining excitatory synapses such that the summed synaptic surface area per length of dendritic segment was constant across time and conditions. Remarkably, the inhibitory synapses showed a parallel synaptic plasticity, also demonstrating a decrease in number perfectly counterbalanced by an increase in synaptic surface area. Thus, TBS-LTP triggered spinogenesis followed by loss of small excitatory and inhibitory synapses and a subsequent enlargement of the remaining synapses by 2 h. These data suggest that dendritic segments coordinate structural plasticity across multiple synapses and maintain a homeostatic balance of excitatory and inhibitory inputs through local protein-synthesis and selective capture or redistribution of dendritic resources.

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.

Rapid and reversible formation of spine head filopodia in response to muscarinic receptor activation in CA1 pyramidal cells

A key feature at excitatory synapses is the remodelling of dendritic spines, which in conjunction with receptor trafficking modifies the efficacy of neurotransmission. Here we investigated whether activation of cholinergic receptors, which can modulate synaptic plasticity, also mediates changes in dendritic spine structure. Using confocal time-lapse microscopy in mouse slice cultures we found that brief activation of muscarinic receptors induced the emergence of fine filopodia from spine heads in all CA1 pyramidal cells examined. This response was widespread occurring in 48% of imaged spines, appeared within minutes, was reversible, and was blocked by atropine. Electron microscopic analyses showed that the spine head filopodia (SHFs) extend along the presynaptic bouton. In addition, the decay time of miniature EPSCs was longer after application of the muscarinic acetylcholine receptor agonist methacholine (MCh). Both morphological and electrophysiological changes were reduced by preventing microtubule polymerization with nocodazole. This extension of SHFs during cholinergic receptor activation represents a novel structural form of subspine plasticity that may regulate synaptic properties by fine-tuning interactions between presynaptic boutons and dendritic spines.

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.

Proteins that promote filopodia stability, but not number, lead to more axonal-dendritic contacts

Dendritic filopodia are dynamic protrusions that are thought to play an active role in synaptogenesis and serve as precursors to spine synapses. However, this hypothesis is largely based on a temporal correlation between filopodia formation and synaptogenesis. We investigated the role of filopodia in synapse formation by contrasting the roles of molecules that affect filopodia elaboration and motility, versus those that impact synapse induction and maturation. We used a filopodia inducing motif that is found in GAP-43, as a molecular tool, and found this palmitoylated motif enhanced filopodia number and motility, but reduced the probability of forming a stable axon-dendrite contact. Conversely, expression of neuroligin-1 (NLG-1), a synapse inducing cell adhesion molecule, resulted in a decrease in filopodia motility, but an increase in the number of stable axonal contacts. Moreover, RNAi knockdown of NLG-1 reduced the number of presynaptic contacts formed. Postsynaptic scaffolding proteins such as Shank1b, a protein that induces the maturation of spine synapses, increased the rate at which filopodia transformed into spines by stabilization of the initial contact with axons. Taken together, these results suggest that increased filopodia stability and not density, may be the rate-limiting step for synapse formation.

Tau mislocalization to dendritic spines mediates synaptic dysfunction independently of neurodegeneration

The microtubule-associated protein tau accumulates in Alzheimer's and other fatal dementias, which manifest when forebrain neurons die. Recent advances in understanding these disorders indicate that brain dysfunction precedes neurodegeneration, but the role of tau is unclear. Here, we show that early tau-related deficits develop not from the loss of synapses or neurons, but rather as a result of synaptic abnormalities caused by the accumulation of hyperphosphorylated tau within intact dendritic spines, where it disrupts synaptic function by impairing glutamate receptor trafficking or synaptic anchoring. Mutagenesis of 14 disease-associated serine and threonine amino acid residues to create pseudohyperphosphorylated tau caused tau mislocalization while creation of phosphorylation-deficient tau blocked the mistargeting of tau to dendritic spines. Thus, tau phosphorylation plays a critical role in mediating tau mislocalization and subsequent synaptic impairment. These data establish that the locus of early synaptic malfunction caused by tau resides in dendritic spines.

Extracerebellar role for Cerebellin1: modulation of dendritic spine density and synapses in striatal medium spiny neurons

Cerebellin1 (Cbln1) is a secreted glycoprotein that was originally isolated from the cerebellum and subsequently found to regulate synaptic development and stability. Cbln1 has a heterogeneous distribution in brain, but the only site in which it has been shown to have central effects is the cerebellar cortex, where loss of Cbln1 causes a reduction in granule cell-Purkinje cell synapses. Neurons of the thalamic parafascicular nucleus (PF), which provide glutamatergic projections to the striatum, also express high levels of Cbln1. We first examined Cbln1 in thalamostriatal neurons and then determined if cbln1 knockout mice exhibit structural deficits in striatal neurons. Virtually all PF neurons express Cbln1-immunoreactivity (-ir). In contrast, only rare Cbln1-ir neurons are present in the central medial complex, the other thalamic region that projects heavily to the dorsal striatum. In the striatum Cbln1-ir processes are apposed to medium spiny neuron (MSN) dendrites; ultrastructural studies revealed that Cbln1-ir axon terminals form axodendritic synapses with MSNs. Tract-tracing studies found that all PF cells retrogradely labeled from the striatum express Cbln1-ir. We then examined the dendritic structure of Golgi-impregnated MSNs in adult cbln1 knockout mice. MSN dendritic spine density was markedly increased in cbln1(-/-) mice relative to wildtype littermates, but total dendritic length was unchanged. Ultrastructural examination revealed an increase in the density of MSN axospinous synapses in cbln1(-/-) mice, with no change in postsynaptic density length. Thus, Cbln1 determines the dendritic structure of striatal MSNs, with effects distinct from those seen in the cerebellum.

CXCR4-mediated glutamate exocytosis from astrocytes

The role of astrocytes as structural and metabolic support for neurons is known since the beginning of the last century. Because of their strategic localization between neurons and capillaries they can monitor and control the level of synaptic activity by providing energetic metabolites to neurons and remove excess of neurotransmitters. During the last two decades number of papers further established that the astrocytic plasma-membrane G-protein coupled receptors (GPCR) can sense external inputs (such as the spillover of neurotransmitters) and transduce them as intracellular calcium elevations and release of chemical transmitters such as glutamate. The chemokine CXCR4 receptor is a GPCR widely expressed on glial cells (especially astrocytes and microglia). Activation of the astrocytic CXCR4 by its natural ligand CXCL12 (or SDF1 alpha) results in a long chain of intracellular and extracellular events (including the release of the pro-inflammatory cytokine TNFalpha and prostanglandins) leading to glutamate release. The emerging role of CXCR4-CXCL12 signalling axis in brain physiology came from the recent observation that glutamate in astrocytes is released via a regulated exocytosis process and occurs with a relatively fast time-scale, in the order of few hundred milliseconds. Taking into account that astrocytes are electrically non-excitable and thus exocytosis rely only on a signalling pathway that involves the release Ca(2+) from the internal stores, these results suggested a close relationship between sites of Ca(2+) release and those of fusion events. Indeed, a recent observation describes structural sub-membrane microdomains where fast ER-dependent calcium elevations occur in spatial and temporal correlation with fusion events.

Dendritic spines, synaptic plasticity and neuronal survival: activity shapes dendritic spines to enhance neuronal viability

An emerging view of structure-function relations of synapses in central spiny neurons asserts that larger spines produce large synaptic currents and that these large spines are persistent ('memory') compared to small spines which are transient. Furthermore, 'learning' involves enlargement of small spine heads and their conversion to being large and stable. It is also assumed that the number of spines, hence the number of synapses, is reflected in the frequency of miniature excitatory postsynaptic currents (mEPSCs). Consequently, there is an assumption that the size and number of mEPSCs are closely correlated with, respectively, the physical size of synapses and number of spines. However, several recent observations do not conform to these generalizations, necessitating a reassessment of the model: spine dimension and synaptic responses are not always correlated. It is proposed that spines are formed and shaped by ongoing network activity, not necessarily by a 'learning' event, to the extent that, in the absence of such activity, new spines are not formed and existing ones disappear or convert into thin filopodia. In the absence of spines, neurons can still maintain synapses with afferent fibers, which can now terminate on its dendritic shaft. Shaft synapses are likely to produce larger synaptic currents than spine synapses. Following loss of their spines, neurons are less able to cope with the large synaptic inputs impinging on their dendritic shafts, and these inputs may lead to their eventual death. Thus, dendritic spines protect neurons from synaptic activity-induced rises in intracellular calcium concentrations.

Glutamate receptor-mediated phosphorylation of ezrin/radixin/moesin proteins is implicated in filopodial protrusion of primary cultured hippocampal neuronal cells

Previously, we reported the phosphorylation of moesin induced by electroconvulsive shock in rat brain and by glutamate in immortalized rat hippocampal cells. However, the function of phosphorylated moesin in differentiated neurons is not well understood. In this study, we observed that glutamate induces phosphorylation of ezrin/radixin/moesin proteins (ERM) in cultured hippocampal cells and that phosphorylated ERM localizes at the newly formed filopodia of neurites. The glutamate-induced phosphorylation of ERM is calcium-dependent, and inhibition of protein kinase C abolishes ERM phosphorylation as well as RhoA activation. The inhibitions of RhoA and RhoA kinase also diminishes the glutamate-induced ERM phosphorylation in cultured hippocampal cells. The knock-down of moesin or the inhibition of ERM phosphorylation results in the reduction of glutamate-induced filopodia protrusion and diminishes the increase in active synaptic boutons induced by glutamate treatment. These results indicate that glutamate-induced phosphorylation of ERM proteins in primary cultured differentiated hippocampal neurons is mediated by calcium-dependent protein kinase C, RhoA and RhoA kinase, and the phosphorylated ERM protein is necessary for the formation of filopodial protrusion and may be involved in pre-synaptic trafficking.

Excitatory amino acid involvement in dendritic spine formation, maintenance and remodelling

In the central nervous system, most excitatory synapses occur on dendritic spines, which are small protrusions from the dendritic tree. In the mature cortex and hippocampus, dendritic spines are heterogeneous in shape. It has been shown that the shapes of the spine can affect synapse stability and synaptic function. Dendritic spines are highly motile structures that can undergo actin-dependent shape changes, which occur over a time scale ranging from seconds to tens of minutes or even days. The formation, remodelling and elimination of excitatory synapses on dendritic spines represent ways of refining the microcircuitry in the brain. Here I review the current knowledge on the effects of modulation of AMPA and NMDA ionotropic glutamate receptors on dendritic spine formation, motility and remodelling.

Dendritic spine dynamics

Dendritic spines are the postsynaptic components of most excitatory synapses in the mammalian brain. Spines accumulate rapidly during early postnatal development and undergo a substantial loss as animals mature into adulthood. In past decades, studies have revealed that the number and size of dendritic spines are regulated by a variety of gene products and environmental factors, underscoring the dynamic nature of spines and their importance to brain plasticity. Recently, in vivo time-lapse imaging of dendritic spines in the cerebral cortex suggests that, although spines are highly plastic during development, they are remarkably stable in adulthood, and most of them last throughout life. Therefore, dendritic spines may provide a structural basis for lifelong information storage, in addition to their well-established role in brain plasticity. Because dendritic spines are the key elements for information acquisition and retention, understanding how spines are formed and maintained, particularly in the intact brain, will likely provide fundamental insights into how the brain possesses the extraordinary capacity to learn and to remember.

Experience-dependent structural synaptic plasticity in the mammalian brain

Synaptic plasticity in adult neural circuits may involve the strengthening or weakening of existing synapses as well as structural plasticity, including synapse formation and elimination. Indeed, long-term in vivo imaging studies are beginning to reveal the structural dynamics of neocortical neurons in the normal and injured adult brain. Although the overall cell-specific morphology of axons and dendrites, as well as of a subpopulation of small synaptic structures, are remarkably stable, there is increasing evidence that experience-dependent plasticity of specific circuits in the somatosensory and visual cortex involves cell type-specific structural plasticity: some boutons and dendritic spines appear and disappear, accompanied by synapse formation and elimination, respectively. This Review focuses on recent evidence for such structural forms of synaptic plasticity in the mammalian cortex and outlines open questions.

Differential susceptibility to excitotoxic stress in YAC128 mouse models of Huntington disease between initiation and progression of disease

Huntington disease (HD) is a neurodegenerative disorder caused by an expanded CAG tract in the HD gene. Polyglutamine expansion of huntingtin (htt) results in early, progressive loss of medium spiny striatal neurons, as well as cortical neurons that project to the striatum. Excitotoxicity has been postulated to play a key role in the selective vulnerability of striatal neurons in HD. Early excitotoxic neuropathological changes observed in human HD brain include increased quinolinate (QUIN) concurrent with proliferative changes such as increased spine density and dendritic length. In later stages of the disease, degenerative-type changes are apparent, such as loss of dendritic arborization, a reduction in spine density and reduced levels of 3-hydroxykynurenine and QUIN. It is currently unknown whether sensitivity to excitotoxic stress varies between initiation and progression of disease. Here, we have assessed the excitotoxic phenotype in the YAC128 mouse model of HD by examining the response to excitotoxic stress at different stages of disease. Our results demonstrate that YAC128 mice display enhanced sensitivity to NMDA ex vivo and QUIN in vivo before obvious phenotypic changes. In contrast, 10-month-old symptomatic YAC128 mice are resistant to QUIN-induced neurotoxicity. These findings are paralleled by a significant increase in NMDAR-mediated membrane currents in presymptomatic YAC128 dissociated medium spiny neurons progressing to reduced NMDAR-mediated membrane currents with disease progression. These data highlight the dynamic nature of the mutant htt-mediated excitotoxic phenotype and suggests that therapeutic approaches to HD may need to be altered, depending on the stage and development of the disease.

GABAergic synapses are formed without the involvement of dendritic protrusions

Synaptogenesis and the role of dendritic protrusions in this process are well studied in glutamatergic synapses. Much less is known about the formation of GABAergic synapses, which are located predominantly on the dendritic shaft. We used genetically labeled interneurons in mature hippocampal slice cultures and two-photon laser-scanning microscopy to examine contact formation between GABAergic axons and the dendrites of CA1 pyramidal cells. Dendritic protrusions distinguished and selected between glutamatergic and GABAergic boutons. In contrast with contacts with glutamatergic boutons, which can be long lasting, the contacts of dendritic protrusions with GABAergic boutons were always short lived. Similarly, the contacts made by GABAergic axonal protrusions were always transient. New putative GABAergic synapses were formed exclusively by new boutons appearing at pre-existing axon-dendrite crossings without the involvement of any dendritic or axonal protrusions. These findings imply that fundamentally different mechanisms underlie the generation of GABAergic and glutamatergic synapses.

Experience-dependent, rapid structural changes in hippocampal pyramidal cell spines

Morphological changes in dendritic spines may contribute to the fine tuning of neural network connectivity. The relationship between spine morphology and experience-dependent neuronal activity, however, is largely unknown. In the present study, we combined 2 histological analyses to examine this relationship: 1) Measurement of spines of neurons whose morphology was visualized in brain sections of mice expressing membrane-targeted green florescent protein (Thy1-mGFP mice) and 2) Categorization of CA1 neurons by immunohistochemical monitoring of Arc expression as a putative marker of recent neuronal activity. After mice were exposed to a novel, enriched environment for 60 min, neurons that expressed Arc had fewer small spines and more large spines than Arc-negative cells. These differences were not observed when the exploration time was shortened to 15 min. This net-balanced structural change is consistent with both synapse-specific enhancement and suppression. These results provide the first evidence of rapid morphological changes in spines that were preferential to a subset of neurons in association with an animal's experiences.

Activity-dependent dynamic microtubule invasion of dendritic spines

Dendritic spines are the primary sites of contact with presynaptic axons on excitatory hippocampal and cortical neurons. During development and plasticity spines undergo marked changes in structure that directly affect the functional communication between neurons. Elucidating the cytoskeletal events that induce these structural changes is fundamental to understanding synaptic biology. Actin plays a central role in the spine cytoskeleton, however the role of microtubules in spine function has been studied little. Although microtubules have a prominent role in transporting material throughout the dendrite that is destined for spines, they are not thought to directly influence spine structure or function. Using total internal reflectance fluorescent microscopy we discovered that microtubules rapidly invade dendritic protrusions of mature CNS neurons (up to 63 d in vitro), occasionally being associated with marked changes in spine morphology in the form of transient spine head protrusions (tSHPs). Two microtubules can occupy a spine simultaneously and microtubule targeting can occur from both the proximal and distal dendrite. A small percentage of spines are targeted at a time and all targeting events are transient, averaging only a few minutes. Nevertheless, over time many spines on a dendrite are targeted by microtubules. Importantly, we show that increasing neuronal activity enhances both the number of spines invaded by microtubules and the duration of these invasions. This study provides new insight into the dynamics of the neuronal cytoskeleton in mature CNS neurons and suggests that microtubules play an important, direct role in spine morphology and function.

AMPA glutamate receptor subunits 1 and 2 regulate dendrite complexity and spine motility in neurons of the developing neocortex

Within neurons of several regions of the CNS, mature dendrite architecture is attained via extensive reorganization of arbor during the developmental period. Since dendrite morphology determines the firing patterns of the neuron, morphological refinement of dendritic arbor may have important implications for mature network activity. In the neocortex, a region of brain that is sensitive to activity-dependent structural rearrangement of dendritic arbor, the proportion of AMPA receptors increases over the developmental period. However, it is unclear whether changes in AMPA receptor expression contribute to maturation of dendritic architecture. To determine the effects of increasing AMPA receptor expression on dendrite morphology and connectivity within the neocortex, and to determine whether these effects are dependent on specific AMPA receptor subunits, we overexpressed the AMPA glutamate receptor subunit 1 (GluR1) and glutamate receptor subunit 2 (GluR2) in cultured rat neocortical neurons at the time that AMPA receptors would normally be incorporated into synapses. Following expression of GluR1 or GluR2 we observed increases in the length and complexity of dendritic arbor of cortical neurons, and a concurrent reduction in motility of spines. In addition, expression of either subunit was associated with an increased density of excitatory postsynaptic puncta. These results suggest that AMPA receptor expression is an important determinant of dendrite morphology and connectivity in neocortical neurons, and further, that contrary to other regions of the CNS, the effects of AMPA receptors on dendrite morphology are not subunit-specific.

Dichotomous anatomical properties of adult striatal medium spiny neurons

Principal medium spiny projection neurons (MSNs) of the striatum have long been thought to be homogeneous in their somatodendritic morphology and physiology. Recent work using transgenic mice, in which the two major classes of MSN are labeled, has challenged this assumption. To explore the basis for this difference, D(1) and D(2) receptor-expressing MSNs (D(1) and D(2) MSNs) in brain slices from adult transgenic mice were characterized electrophysiologically and anatomically. These studies revealed that D(1) MSNs were less excitable than D(2) MSNs over a broad range of developmental time points. Although M(1) muscarinic receptor signaling was a factor, it was not sufficient to explain the dichotomy between D(1) and D(2) MSNs. Reconstructions of biocytin-filled MSNs revealed that the physiological divergence was paralleled by a divergence in total dendritic area. Experimentally grounded simulations suggested that the dichotomy in MSN dendritic area was a major contributor to the dichotomy in electrophysiological properties. Thus, rather than being an intrinsically homogenous population, striatal MSNs have dichotomous somatodendritic properties that mirror differences in their network connections and biochemistry.