Development of long-term dendritic spine stability in diverse regions of cerebral cortex

Chemical and morphological alterations of spines within the hippocampus and entorhinal cortex precede the onset of Alzheimer's disease pathology in double knock-in mice

Mice with knock-in of two mutations that affect beta amyloid processing and levels (2xKI) exhibit impaired spatial memory by 9-12 months of age, together with synaptic plasticity dysfunction in the hippocampus. The goal of this study was to identify changes in the molecular and structural characteristics of synapses that precede and thus could exert constraints upon cellular mechanisms underlying synaptic plasticity. Drebrin A is one protein reported to modulate spine sizes and trafficking of proteins to and from excitatory synapses. Thus, we examined levels of drebrin A within postsynaptic spines in the hippocampus and entorhinal cortex. Our electron microscopic immunocytochemical analyses reveal that, by 6 months, the proportion of hippocampal spines containing drebrin A is reduced and this change is accompanied by an increase in the mean size of spines and decreased density of spines. In the entorhinal cortex of 2xKI brains, we detected no decrement in the proportion of spines labeled for drebrin A and no significant change in spine density at 6 months, but rather a highly significant reduction in the level of drebrin A immunoreactivity within each spine. These changes are unlike those observed for the somatosensory cortex of 2xKI mice, in which synapse density and drebrin A immunoreactivity levels remain unchanged at 6 months and older. These results indicate that brains of 2xKI mice, like those of humans, exhibit regional differences of vulnerability, with the hippocampus exhibiting the first signatures of structural changes that, in turn, may underlie the emergent inability to update spatial memory in later months.

Dendritic spine plasticity: Looking beyond development

Most excitatory synapses in the CNS form on dendritic spines, tiny protrusions from the dendrites of excitatory neurons. As such, spines are likely loci of synaptic plasticity. Spines are dynamic structures, but the functional consequences of dynamic changes in these structures in the mature brain are unclear. Changes in spine density, morphology, and motility have been shown to occur with paradigms that induce synaptic plasticity, as well as altered sensory experience and neuronal activity. These changes potentially lead to an alteration in synaptic connectivity and strength between neuronal partners, affecting the efficacy of synaptic communication. Here, we review the formation and modification of excitatory synapses on dendritic spines as it relates to plasticity in the central nervous system after the initial phase of synaptogenesis. We will also discuss some of the molecular links that have been implicated in both synaptic plasticity and the regulation of spine morphology.

Postnatal development of synaptic transmission in local networks of L5A pyramidal neurons in rat somatosensory cortex

The probability of synaptic transmitter release determines the spread of excitation and the possible range of computations at unitary connections. To investigate whether synaptic properties between neocortical pyramidal neurons change during the assembly period of cortical circuits, whole-cell voltage recordings were made simultaneously from two layer 5A (L5A) pyramidal neurons within the cortical columns of rat barrel cortex. We found that synaptic transmission between L5A pyramidal neurons is very reliable between 2 and 3 weeks of postnatal development with a mean unitary EPSP amplitude of approximately 1.2 mV, but becomes less efficient and fails more frequently in the more mature cortex of approximately 4 weeks of age with a mean unitary EPSP amplitude of 0.65 mV. Coefficient of variation and failure rate increase as the unitary EPSP amplitude decreases during development. The paired-pulse ratio (PPR) of synaptic efficacy at 10 Hz changes from 0.7 to 1.04. Despite the overall increase in PPR, short-term plasticity displays a large variability at 4 weeks, ranging from strong depression to strong facilitation (PPR, range 0.6-2.1), suggesting the potential for use-dependent modifications at this intracortical synapse. In conclusion, the transmitter release probability at the L5A-L5A connection is developmentally regulated in such a way that in juvenile animals excitation by single action potentials is efficiently transmitted, whereas in the more mature cortex synapses might be endowed with a diversity of filtering characteristics.

Impaired spine stability underlies plaque-related spine loss in an Alzheimer's disease mouse model

Dendritic spines, the site of most excitatory synapses in the brain, are lost in Alzheimer's disease and in related mouse models, undoubtedly contributing to cognitive dysfunction. We hypothesized that spine loss results from plaque-associated alterations of spine stability, causing an imbalance in spine formation and elimination. To investigate effects of plaques on spine stability in vivo, we observed cortical neurons using multiphoton microscopy in a mouse model of amyloid pathology before and after extensive plaque deposition. We also observed age-matched nontransgenic mice to study normal effects of aging on spine plasticity. We found that spine density and structural plasticity are maintained during normal aging. Tg2576 mice had normal spine density and plasticity before plaques appeared, but after amyloid pathology is established, severe disruptions were observed. In control animals, spine formation and elimination were equivalent over 1 hour of observation ( approximately 5% of observed spines), resulting in stable spine density. However, in aged Tg2576 mice spine elimination increased, specifically in the immediate vicinity of plaques. Spine formation was unchanged, resulting in spine loss. These data show a small population of rapidly changing spines in adult and even elderly mouse cortex; further, in the vicinity of amyloid plaques, spine stability is markedly impaired leading to loss of synaptic structural integrity.

Distinct structural and ionotropic roles of NMDA receptors in controlling spine and synapse stability

NMDA-type glutamate receptors (NMDARs) play a central role in the rapid regulation of synaptic transmission, but their contribution to the long-term stabilization of glutamatergic synapses is unknown. We find that, in hippocampal pyramidal neurons in rat organotypic slices, pharmacological blockade of NMDARs does not affect synapse formation and dendritic spine growth but does increase the motility of spines. Physical loss of synaptic NMDARs induced by RNA interference against the NR1 subunit of the receptor also increases the motility of spines. Furthermore, knock-down of NMDARs, but not their pharmacological block, destabilizes spine structure and over time leads to loss of spines and excitatory synapses. Maintenance of normal spine density requires the coexpression of two specific splice isoforms of the NR1 subunit that contain the C-terminal C2 cassette. Thus, although ionotropic properties of NMDARs induce synaptic plasticity, it is the physical interactions of the C-tail of the receptor that mediate the long-term stabilization of synapses and spines.

Dendritic pathology in prion disease starts at the synaptic spine

Spine loss represents a common hallmark of neurodegenerative diseases. However, little is known about the underlying mechanisms, especially the relationship between spine elimination and neuritic destruction. We imaged cortical dendrites throughout a neurodegenerative disease using scrapie in mice as a model. Two-photon in vivo imaging over 2 months revealed a linear decrease of spine density. Interestingly, only persistent spines (lifetime > or = 8 d) disappeared, whereas the density of transient spines (lifetime < or = 4 d) was unaffected. Before spine loss, dendritic varicosities emerged preferentially at sites where spines protrude from the dendrite. These results implicate that the location where the spine protrudes from the dendrite may be particularly vulnerable and that dendritic varicosities may actually cause spine loss.

Interactive effects of age and estrogen on cognition and pyramidal neurons in monkey prefrontal cortex

We previously reported that long-term cyclic estrogen (E) treatment reverses age-related impairment of cognitive function mediated by the dorsolateral prefrontal cortex (dlPFC) in ovariectomized (OVX) female rhesus monkeys, and that E induces a corresponding increase in spine density in layer III dlPFC pyramidal neurons. We have now investigated the effects of the same E treatment in young adult females. In contrast to the results for aged monkeys, E treatment failed to enhance dlPFC-dependent task performance relative to vehicle control values (group young OVX+Veh) but nonetheless led to a robust increase in spine density. This response was accompanied by a decline in dendritic length, however, such that the total number of spines per neuron was equivalent in young OVX+Veh and OVX+E groups. Robust effects of chronological age, independent of ovarian hormone status, were also observed, comprising significant age-related declines in dendritic length and spine density, with a preferential decrease in small spines in the aged groups. Notably, the spine effects were partially reversed by cyclic E administration, although young OVX+Veh monkeys still had a higher complement of small spines than did aged E treated monkeys. In summary, layer III pyramidal neurons in the dlPFC are sensitive to ovarian hormone status in both young and aged monkeys, but these effects are not entirely equivalent across age groups. The results also suggest that the cognitive benefit of E treatment in aged monkeys is mediated by enabling synaptic plasticity through a cyclical increase in small, highly plastic dendritic spines in the primate dlPFC.

Functional maturation of excitatory synapses in layer 3 pyramidal neurons during postnatal development of the primate prefrontal cortex

In the primate dorsolateral prefrontal cortex (DLPFC), the density of excitatory synapses decreases by 40-50% during adolescence. Although such substantial circuit refinement might underlie the adolescence-related maturation of working memory performance, its functional significance remains poorly understood. The consequences of synaptic pruning may depend on the properties of the eliminated synapses. Are the synapses eliminated during adolescence functionally immature, as is the case during early brain development? Or do maturation-independent features tag synapses for pruning? We examined excitatory synaptic function in monkey DLPFC during postnatal development by studying properties that reflect synapse maturation in rat cortex. In 3-month-old (early postnatal) monkeys, excitatory inputs to layer 3 pyramidal neurons had immature properties, including higher release probability, lower alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/N-methyl-D-aspartate (NMDA) ratio, and longer duration of NMDA-mediated synaptic currents, associated with greater sensitivity to the NMDA receptor subunit B (NR2B) subunit-selective antagonist ifenprodil. In contrast, excitatory synaptic inputs in neurons from preadolescent (15 months old) and adult (42 or 84 months old) monkeys had similar functional properties. We therefore conclude that the contribution of functionally immature synapses decreases significantly before adolescence begins. Thus, remodeling of excitatory connectivity in the DLPFC during adolescence may occur in the absence of widespread maturational changes in synaptic strength.

Interplay between laminar specificity and activity-dependent mechanisms of thalamocortical axon branching

Target and activity-dependent mechanisms of axonal branching were studied in the thalamocortical (TC) projection using organotypic cocultures of the thalamus and cortex. TC axons were labeled with enhanced yellow fluorescent protein (EYFP) by a single-cell electroporation method and observed over time by confocal microscopy. Changes in the firing activity of cocultures grown on multielectrode dishes were also monitored over time. EYFP-labeled TC axons exhibited more branch formation in and around layer 4 of the cortical explant during the second week in vitro, when spontaneous firing activity increased in both thalamic and cortical cells. Time-lapse imaging further demonstrated that branching patterns were generated dynamically by addition and elimination with a bias toward branch accumulation in the target layer. To examine the relationship between neural activity and TC branch formation, the dynamics of axonal branching was analyzed under various pharmacological treatments. Chronic blockade of firing or synaptic activity reduced the remodeling process, in particular, branch addition in the target layer. However, extension of branches was not affected by this treatment. Together, these findings suggest that neural activity can modify the molecular mechanisms that regulate lamina-specific TC axon branching.

Activity-dependent PSD formation and stabilization of newly formed spines in hippocampal slice cultures

Development and remodeling of synaptic networks occurs through a continuous turnover of dendritic spines. However, the mechanisms that regulate the formation and stabilization of newly formed spines remain poorly understood. Here, we applied repetitive confocal imaging to hippocampal slice cultures to address these issues. We find that, although the turnover rate of protrusions progressively decreased during development, the process of stabilization of new spines remained comparable both in terms of time course and low level of efficacy. Irrespective of the developmental stage, most new protrusions were quickly eliminated, in particular filopodia, which only occasionally lead to the formation of stable dendritic spines. We also found that the stabilization of new protrusions was determined within a critical period of 24 h and that this coincided with an enlargement of the spine head and the expression of tagged PSD-95. Blockade of postsynaptic AMPA and NMDA receptors significantly reduced the capacity of new spines to express tagged PSD-95 and decreased their probability to be stabilized. These results suggest a model in which synaptic development is associated with an extensive, nonspecific growth of protrusions followed by stabilization of a few of them through a mechanism that involves activity-driven formation of a postsynaptic density.

Do thin spines learn to be mushroom spines that remember?

Dendritic spines are the primary site of excitatory input on most principal neurons. Long-lasting changes in synaptic activity are accompanied by alterations in spine shape, size and number. The responsiveness of thin spines to increases and decreases in synaptic activity has led to the suggestion that they are 'learning spines', whereas the stability of mushroom spines suggests that they are 'memory spines'. Synaptic enhancement leads to an enlargement of thin spines into mushroom spines and the mobilization of subcellular resources to potentiated synapses. Thin spines also concentrate biochemical signals such as Ca(2+), providing the synaptic specificity required for learning. Determining the mechanisms that regulate spine morphology is essential for understanding the cellular changes that underlie learning and memory.

Extensive turnover of dendritic spines and vascular remodeling in cortical tissues recovering from stroke

Recovery of function after stroke is thought to be dependent on the reorganization of adjacent, surviving areas of the brain. Macroscopic imaging studies (functional magnetic resonance imaging, optical imaging) have shown that peri-infarct regions adopt new functional roles to compensate for damage caused by stroke. To better understand the process by which these regions reorganize, we used in vivo two-photon imaging to examine changes in dendritic and vascular structure in cortical regions recovering from stroke. In adult control mice, dendritic arbors were relatively stable with very low levels of spine turnover (<0.5% turnover over 6 h). After stroke, however, the organization of dendritic arbors in peri-infarct cortex was fundamentally altered with both apical dendrites and blood vessels radiating in parallel from the lesion. On a finer scale, peri-infarct dendrites were exceptionally plastic, manifested by a dramatic increase in the rate of spine formation that was maximal at 1-2 weeks (5-8-fold increase), and still evident 6 weeks after stroke. These changes were selective given that turnover rates were not significantly altered in ipsilateral cortical regions more distant to the lesion (>1.5 mm). These data provide a structural framework for understanding functional and behavioral changes that accompany brain injury and suggest new targets that could be exploited by future therapies to rebuild and rewire neuronal circuits lost to stroke.

Sensory experience alters cortical connectivity and synaptic function site specifically

Neocortical circuitry can alter throughout life with experience. However, the contributions of changes in synaptic strength and modifications in neuronal wiring to experience-dependent plasticity in mature animals remain unclear. We trimmed whiskers of rats and made electrophysiological recordings after whisker cortical maps have developed. Measurements of miniature EPSPs suggested that synaptic inputs to layer 2/3 pyramidal neurons were altered at the junction of deprived and spared cortex in primary somatosensory cortex. Whole-cell recordings were made from pairs of synaptically connected pyramidal neurons to investigate possible changes in local excitatory connections between layer 2/3 pyramidal neurons. The neurons were filled with fluorescent dyes during recording and reconstructed in three dimensions using confocal microscopy and image deconvolution to identify putative synapses. We show that sensory deprivation induces a striking reduction in connectivity between layer 2/3 pyramidal neurons in deprived cortex without large-scale, compensatory increases in the strength of remaining local excitatory connections. A markedly different situation occurs in spared cortex. Connection strength is potentiated, but local excitatory connectivity and synapse number per connection are unchanged. Our data suggest that alterations in local excitatory circuitry enhance the expansion of spared representations into deprived cortex. Moreover, our findings offer one explanation for how the responses of spared and deprived cortex to sensory deprivation can be dissociated in developed animals.

Choice of cranial window type for in vivo imaging affects dendritic spine turnover in the cortex

Determining the degree of synapse formation and elimination is essential for understanding the structural basis of brain plasticity and pathology. We show that in vivo imaging of dendritic spine dynamics through an open-skull glass window, but not a thinned-skull window, is associated with high spine turnover and substantial glial activation during the first month after surgery. These findings help to explain existing discrepancies in the degree of dendritic spine plasticity observed in the mature cortex.

Axon pruning: an essential step underlying the developmental plasticity of neuronal connections

Regressive events play a key role in modifying neural connectivity in early development. An important regressive event is the pruning of neuronal processes. Pruning is a strategy often used to selectively remove exuberant neuronal branches and connections in the immature nervous system to ensure the proper formation of functional circuitry. In the following review, we discuss our present understanding of the cellular and molecular mechanisms that regulate the pruning of axons during neuronal development as well as in neurological diseases. The evidence suggests that there are several similarities between the mechanisms that are involved in developmental axon pruning and axon elimination in disease. In summary, these findings provide researchers with a unique perspective on how developmental plasticity is achieved and how to develop strategies to treat complex neurological diseases.

Immediate-early gene expression in the barrel cortex

Since their detection in the early 1980s immediate-early genes (most of them being inducible transcription factors) have been regarded as molecular keys to the orchestration of late-effector genes that ultimately would enable functional and structural adaptation of the brain to changing external and internal demands. This is called neuronal plasticity and it has been intensively studied in the somatosensory (barrel) cortex of rodents. This brain region is intimately involved in the processing and probably also the storage of tactile information, stemming from the large facial whiskers, necessary for object detection or spatial navigation in the environment. On the other hand, several of the inducible transcription factors have been found to function as neuronal activity markers providing a cellular resolution, thus, enabling the cell-type specific mapping of activated neuronal circuits. Some recent data on both topics in the rodent barrel cortex will be presented in this topical review.

Dendritic development and plasticity of adult-born neurons in the mouse olfactory bulb

The mammalian brain maintains few developmental niches where neurogenesis persists into adulthood. One niche is located in the olfactory system where the olfactory bulb continuously receives functional interneurons. In vivo two-photon microscopy of lentivirus-labeled newborn neurons was used to directly image their development and maintenance in the olfactory bulb. Time-lapse imaging of newborn neurons over several days showed that dendritic formation is highly dynamic with distinct differences between spiny neurons and non-spiny neurons. Once incorporated into the network, adult-born neurons maintain significant levels of structural dynamics. This structural plasticity is local, cumulative and sustained in neurons several months after their integration. Thus, I provide a new experimental system for directly studying the pool of regenerating neurons in the intact mammalian brain and suggest that regenerating neurons form a cellular substrate for continuous wiring plasticity in the olfactory bulb.

Rapid redistribution of synaptic PSD-95 in the neocortex in vivo

Most excitatory synapses terminate on dendritic spines. Spines vary in size, and their volumes are proportional to the area of the postsynaptic density (PSD) and synaptic strength. PSD-95 is an abundant multi-domain postsynaptic scaffolding protein that clusters glutamate receptors and organizes the associated signaling complexes. PSD-95 is thought to determine the size and strength of synapses. Although spines and their synapses can persist for months in vivo, PSD-95 and other PSD proteins have shorter half-lives in vitro, on the order of hours. To probe the mechanisms underlying synapse stability, we measured the dynamics of synaptic PSD-95 clusters in vivo. Using two-photon microscopy, we imaged PSD-95 tagged with GFP in layer 2/3 dendrites in the developing (postnatal day 10–21) barrel cortex. A subset of PSD-95 clusters was stable for days. Using two-photon photoactivation of PSD-95 tagged with photoactivatable GFP (paGFP), we measured the time over which PSD-95 molecules were retained in individual spines. Synaptic PSD-95 turned over rapidly (median retention times τ_r ~ 22–63 min from P10–P21) and exchanged with PSD-95 in neighboring spines by diffusion. PSDs therefore share a dynamic pool of PSD-95. Large PSDs in large spines captured more diffusing PSD-95 and also retained PSD-95 longer than small PSDs. Changes in the sizes of individual PSDs over days were associated with concomitant changes in PSD-95 retention times. Furthermore, retention times increased with developmental age (τ_r ~ 100 min at postnatal day 70) and decreased dramatically following sensory deprivation. Our data suggest that individual PSDs compete for PSD-95 and that the kinetic interactions between PSD molecules and PSDs are tuned to regulate PSD size.

Using two-photon microscopy and photoactivation of a fluorescently tagged synaptic protein (PSD-95), the authors demonstrated rapid turnover of these molecules in dendritic spines of the mouse sensory cortex in vivo.

Anatomical and physiological plasticity of dendritic spines

In excitatory neurons, most glutamatergic synapses are made on the heads of dendritic spines, each of which houses the postsynaptic terminal of a single glutamatergic synapse. We review recent studies demonstrating in vivo that spines are motile and plastic structures whose morphology and lifespan are influenced, even in adult animals, by changes in sensory input. However, most spines that appear in adult animals are transient, and the addition of stable spines and synapses is rare. In vitro studies have shown that patterns of neuronal activity known to induce synaptic plasticity can also trigger changes in spine morphology. Therefore, it is tempting to speculate that the plastic changes of spine morphology reflect the dynamic state of its associated synapse and are responsible to some extent for neuronal circuitry remodeling. Nevertheless, morphological changes are not required for all forms of synaptic plasticity, and whether changes in the spine shape and size significantly impact synaptic signals is unclear.

Local sharing as a predominant determinant of synaptic matrix molecular dynamics

Recent studies suggest that central nervous system synapses can persist for weeks, months, perhaps lifetimes, yet little is known as to how synapses maintain their structural and functional characteristics for so long. As a step toward a better understanding of synaptic maintenance we examined the loss, redistribution, reincorporation, and replenishment dynamics of Synapsin I and ProSAP2/Shank3, prominent presynaptic and postsynaptic matrix molecules, respectively. Fluorescence recovery after photobleaching and photoactivation experiments revealed that both molecules are continuously lost from, redistributed among, and reincorporated into synaptic structures at time-scales of minutes to hours. Exchange rates were not affected by inhibiting protein synthesis or proteasome-mediated protein degradation, were accelerated by stimulation, and greatly exceeded rates of replenishment from somatic sources. These findings indicate that the dynamics of key synaptic matrix molecules may be dominated by local protein exchange and redistribution, whereas protein synthesis and degradation serve to maintain and regulate the sizes of local, shared pools of these proteins.

To understand processes involved in synaptic maintenance, the authors examine the loss, redistribution, reincorporation and replenishment dynamics of two key synaptic proteins, Synapsin I and ProSAP2/Shank3.

AMPAR removal underlies Abeta-induced synaptic depression and dendritic spine loss

Beta amyloid (Abeta), a peptide generated from the amyloid precursor protein (APP) by neurons, is widely believed to underlie the pathophysiology of Alzheimer's disease. Recent studies indicate that this peptide can drive loss of surface AMPA and NMDA type glutamate receptors. We now show that Abeta employs signaling pathways of long-term depression (LTD) to drive endocytosis of synaptic AMPA receptors. Synaptic removal of AMPA receptors is necessary and sufficient to produce loss of dendritic spines and synaptic NMDA responses. Our studies indicate the central role played by AMPA receptor trafficking in Abeta-induced modification of synaptic structure and function.

Automatic dendritic spine analysis in two-photon laser scanning microscopy images

Dendritic spine expression plays an important role in the central nervous system. Modern fluorescence microscopy and green fluorescent protein technology have facilitated the research on spines. To quantitatively analyze the spines in fluorescence microscopy images, an automatic dendritic spine analysis method is proposed. Because of the limit of axial resolution, our method is designed to process the projection image along the z-axis and analyze the lateral spines. The method can automatically extract the dendrite centerlines and segment the spines along the dendrites according to width-based criteria. The criteria utilize a common morphological feature of the spines. It can detect some shapes of spines missed by previous methods. In addition, the proposed method is automatic once a few parameters are set. Spine numbers, lengths, and densities, which biologists are interested in, are analyzed both manually and automatically. The results of the two methods match well. The proposed method provides automatic and accurate dendritic spine analysis. It can serve as a useful tool for spine image analysis to avoid tedious manual labor.

Binding and complementary expression patterns of semaphorin 3E and plexin D1 in the mature neocortices of mice and monkeys

Although axon guidance molecules play critical roles in neural circuit formation during development, their roles in the adult circuit are not well understood. In this study we examined the expression patterns of Semaphorin 3E (Sema3E), a member of the semaphorin family, in the mature neocortices of monkeys and mice by in situ hybridization (ISH). We found that Sema3E mRNA is highly specific to layer VI throughout the macaque monkey neocortex. We further examined the ratio of Sema3E+ cells among the layer VI excitatory neurons in areas M1, S1, TE, and V1 by fluorescence double ISH, using the vesicular glutamate transporter 1 (VGluT1) gene as a specific marker for excitatory neurons. Among these areas, 34-63% of the VGluT1+ neurons expressed Sema3E mRNA. In the mouse cortex, two significant differences were observed in the pattern of Sema3E mRNA distribution. 1) Sema3E mRNA was expressed in layer Vb, in addition to layer VI in mice. 2) A subset of GABAergic interneurons expressed Sema3E mRNA in mice. By an in vitro binding experiment, we provide evidence that Plexin D1 is the specific receptor for Sema3E. Plexin D1 mRNA was preferentially expressed in layers II-V in both monkey and mouse cortices. The detailed lamina analysis by double ISH, however, revealed that Plexin D1 mRNA is expressed in layers II-Va, but not in layer Vb in the mouse cortex. Thus, the Plexin D1 and Sema3E mRNAs exhibit conserved complementary lamina patterns in mice and monkeys, despite the species differences in the pattern of each gene.

Two-photon imaging of synaptic plasticity and pathology in the living mouse brain

Two-photon microscopy (TPM) has become an increasingly important tool for imaging the structure and function of brain cells in living animals. TPM imaging studies of neuronal structures over intervals ranging from seconds to years have begun to provide important insights into the structural plasticity of synapses and the modulating effects of experience in the intact brain. TPM has also started to reveal how neuronal connections are altered in animal models of neurodegeneration, acute brain injury, and cerebrovascular disease. Here, we review some of these studies with special emphasis on the degree of structural dynamism of postsynaptic dendritic spines in the adult mouse brain as well as synaptic pathology in mouse models of Alzheimer's disease and cerebral ischemia. We also discuss technical considerations that are critical for the acquisition and interpretation of data from TPM in vivo.

“The seven sins” of the Hebbian synapse: Can the hypothesis of synaptic plasticity explain long-term memory consolidation?

Memorizing new facts and events means that entering information produces specific physical changes within the brain. According to the commonly accepted view, traces of memory are stored through the structural modifications of synaptic connections, which result in changes of synaptic efficiency and, therefore, in formations of new patterns of neural activity (the hypothesis of synaptic plasticity). Most of the current knowledge on learning and initial stages of memory consolidation ("synaptic consolidation") is based on this hypothesis. However, the hypothesis of synaptic plasticity faces a number of conceptual and experimental difficulties when it deals with potentially permanent consolidation of declarative memory ("system consolidation"). These difficulties are rooted in the major intrinsic self-contradiction of the hypothesis: stable declarative memory is unlikely to be based on such a non-stable foundation as synaptic plasticity. Memory that can last throughout an entire lifespan should be "etched in stone." The only "stone-like" molecules within living cells are DNA molecules. Therefore, I advocate an alternative, genomic hypothesis of memory, which suggests that acquired information is persistently stored within individual neurons through modifications of DNA, and that these modifications serve as the carriers of elementary memory traces.

Cooperative astrocyte and dendritic spine dynamics at hippocampal excitatory synapses

Accumulating evidence is redefining the importance of neuron-glial interactions at synapses in the CNS. Astrocytes form "tripartite" complexes with presynaptic and postsynaptic structures and regulate synaptic transmission and plasticity. Despite our understanding of the importance of neuron-glial relationships in physiological contexts, little is known about the structural interplay between astrocytes and synapses. In the past, this has been difficult to explore because studies have been hampered by the lack of a system that preserves complex neuron-glial relationships observed in the brain. Here we present a system that can be used to characterize the intricate relationship between astrocytic processes and synaptic structures in situ using organotypic hippocampal slices, a preparation that retains the three-dimensional architecture of astrocyte-synapse interactions. Using time-lapse confocal imaging, we demonstrate that astrocytes can rapidly extend and retract fine processes to engage and disengage from motile postsynaptic dendritic spines. Surprisingly, astrocytic motility is, on average, higher than its dendritic spine counterparts and likely relies on actin-based cytoskeletal reorganization. Changes in astrocytic processes are typically coordinated with changes in spines, and astrocyte-spine interactions are stabilized at larger spines. Our results suggest that dynamic structural changes in astrocytes help control the degree of neuron-glial communication at hippocampal synapses.

Organelles and trafficking machinery for postsynaptic plasticity

Neurons are among the largest and most complex cells in the body. Their immense size and intricate geometry pose many unique cell-biological problems. How is dendritic architecture established and maintained? How do neurons traffic newly synthesized integral membrane proteins over such long distances to synapses? Functionally, protein trafficking to and from the postsynaptic membrane has emerged as a key mechanism underlying various forms of synaptic plasticity. Which organelles are involved in postsynaptic trafficking, and how do they integrate and respond to activity at individual synapses? Here we review what is currently known about long-range trafficking of newly synthesized postsynaptic proteins as well as the local rules that govern postsynaptic trafficking at individual synapses.

Changes of brain activity in the aged SAMP mouse

This study investigates characteristics of aging in the central nervous system of the senescence accelerated prone mice (SAMP8). We examined 3 and 10-months old senescence-accelerated-prone mice (SAMP8) for functional and molecular changes in their brains, specifically in the hippocampus and somatosensory cortex. There was no statistically significant increase in the apoptosis indicators as revealed by Western Blotting for BAD and TUNEL experiments. However, the functional magnetic resonance imaging showed an increase in the area of BOLD images from the 3-month old to the 10-months old SAMP mice upon the application of tail stimulus. These results demonstrated a lack of neuronal deaths but an increase in the activated brain area with age.

Postsynaptic excitability is necessary for strengthening of cortical sensory responses during experience-dependent development

Sensory experience is necessary for normal cortical development. This has been shown by sensory deprivation and pharmacological perturbation of the cortex. Because these manipulations affect the cortical network as a whole, the role of postsynaptic cellular properties during experience-dependent development is unclear. Here we addressed the developmental role of somatodendritic excitability, which enables postsynaptic spike timing-dependent forms of plasticity, in rat somatosensory cortex. We used short interfering RNA (siRNA)-based knockdown of Na+ channels to suppress the somatodendritic excitability of small numbers of layer 2/3 pyramidal neurons in the barrel cortex, without altering the ascending sensory pathway. In vivo recordings from siRNA-expressing cells revealed that this manipulation interfered with the normal developmental strengthening of sensory responses. The sensory responsiveness of neighboring cortical neurons was unchanged, indicating that the cortical network was unchanged. We conclude that somatodendritic excitability of the postsynaptic neuron is needed for the regulation of synaptic strength in the developing sensory cortex.

Synapse development: still looking for the forest, still lost in the trees

Synapse development in the vertebrate central nervous system is a highly orchestrated process occurring not only during early stages of brain development, but also (to a lesser extent) in the mature nervous system. During development, the formation of synapses is intimately linked to the differentiation of neuronal cells, the extension of their axons and dendrites, and the course wiring of the nervous system. Subsequently, the stabilization, elimination, and strengthening of synaptic contacts is coupled to the refinement of axonal and dendritic arbors, to the establishment of functionally meaningful connections, and probably also to the day-to-day acquisition, storage, and retrieval of memories, higher order thought processes, and behavioral patterns.

Long-term rearrangements of hippocampal mossy fiber terminal connectivity in the adult regulated by experience

We investigated rearrangements of connectivity between hippocampal mossy fibers and CA3 pyramidal neurons. We found that mossy fibers establish 10-15 local terminal arborization complexes (LMT-Cs) in CA3, which exhibit major differences in size and divergence in adult mice. LMT-Cs exhibited two types of long-term rearrangements in connectivity in the adult: progressive expansion of LMT-C subsets along individual dendrites throughout life, and pronounced increases in LMT-C complexities in response to an enriched environment. In organotypic slice cultures, subsets of LMT-Cs also rearranged extensively and grew over weeks and months, altering the strength of preexisting connectivity, and establishing or dismantling connections with pyramidal neurons. Differences in LMT-C plasticity reflected properties of individual LMT-Cs, not mossy fibers. LMT-C maintenance and growth were regulated by spiking activity, mGluR2-sensitive transmitter release from LMTs, and PKC. Thus, subsets of terminal arborization complexes by mossy fibers rearrange their local connectivities in response to experience and age throughout life.

Neuroanatomical, molecular genetic, and behavioral correlates of fragile X syndrome

Fragile X syndrome (FXS) is a leading cause of inherited mental retardation. In the vast majority of cases, this X-linked disorder is due to a CGG expansion in the 5' untranslated region of the fmr-1 gene and the resulting decreased expression of its associated protein, FMRP. FXS is characterized by a number of cognitive, behavioral, anatomical, and biological abnormalities. FXS provides a unique opportunity to study the consequence of mutation in a single gene on the development and proper functioning of the CNS. The current focus on the role of FMRP in neuronal maturation makes it timely to assemble the extant information on how reduced expression of the fmr-1 gene leads to neuronal dysmorphology. The purpose of this review is to summarize recent genetic, neuroanatomical, and behavioral studies of fragile X syndrome and to offer potential mechanisms to account for the pleiotropic phenotype of this disorder.

Common molecular mechanisms in explicit and implicit memory

Cellular and molecular studies of both implicit and explicit memory suggest that experience-dependent modulation of synaptic strength and structure is a fundamental mechanism by which these memories are encoded and stored within the brain. In this review, we focus on recent advances in our understanding of two types of memory storage: (i) sensitization in Aplysia, a simple form of implicit memory, and (ii) formation of explicit spatial memories in the mouse hippocampus. These two processes share common molecular mechanisms that have been highly conserved through evolution.

Plasticity and specificity of cortical processing networks

The cerebral cortex is subdivided into discrete functional areas that are defined by specific properties, including the presence of different cell types, molecular expression patterns, microcircuitry and long-range connectivity. These properties enable different areas of cortex to carry out distinct functions. Emerging data argue that the particular structure and identity of cortical areas derives not only from specific inputs but also from unique processing networks. The aim of this review is to summarize current information on the interplay of intrinsic molecular cues with activity patterns that are driven by sensory experience and shape cortical networks as they develop, emphasizing synaptic connections in networks that process vision. This review is part of the TINS special issue on The Neural Substrates of Cognition.

Principles of Two-Photon Excitation Microscopy and Its Applications to Neuroscience

The brain is complex and dynamic. The spatial scales of interest to the neurobiologist range from individual synapses (approximately 1 microm) to neural circuits (centimeters); the timescales range from the flickering of channels (less than a millisecond) to long-term memory (years). Remarkably, fluorescence microscopy has the potential to revolutionize research on all of these spatial and temporal scales. Two-photon excitation (2PE) laser scanning microscopy allows high-resolution and high-sensitivity fluorescence microscopy in intact neural tissue, which is hostile to traditional forms of microscopy. Over the last 10 years, applications of 2PE, including microscopy and photostimulation, have contributed to our understanding of a broad array of neurobiological phenomena, including the dynamics of single channels in individual synapses and the functional organization of cortical maps. Here we review the principles of 2PE microscopy, highlight recent applications, discuss its limitations, and point to areas for future research and development.

Cell type-specific structural plasticity of axonal branches and boutons in the adult neocortex

We imaged axons in layer (L) 1 of the mouse barrel cortex in vivo. Axons from thalamus and L2/3/5, or L6 pyramidal cells were identified based on their distinct morphologies. Their branching patterns and sizes were stable over times of months. However, axonal branches and boutons displayed cell type-specific rearrangements. Structural plasticity in thalamocortical afferents was mostly due to elongation and retraction of branches (range, 1-150 microm over 4 days; approximately 5% of total axonal length), while the majority of boutons persisted for up to 9 months (persistence over 1 month approximately 85%). In contrast, L6 axon terminaux boutons were highly plastic (persistence over 1 month approximately 40 %), and other intracortical axon boutons showed intermediate levels of plasticity. Retrospective electron microscopy revealed that new boutons make synapses. Our data suggest that structural plasticity of axonal branches and boutons contributes to the remodeling of specific functional circuits.

Should I stay or should I go? Presynaptic boutons in the adult cortex still haven't made up their minds

Previous studies demonstrating turnover of the dendritic spines of cortical neurons have suggested a modest rate of turnover of synaptic connections. Now, two papers in this issue of Neuron address this question from the other side of the synapse, the presynaptic boutons. Both studies use in vivo multiphoton imaging of cortical axons to show that synaptic boutons come and go, just like spines. One of the studies shows remarkable diversity in the lability of boutons depending on the cell type from which they originate, with some boutons displaying nearly complete turnover in just a few months. The other study shows that bouton turnover occurs in primates as well as rodents.

Axons and synaptic boutons are highly dynamic in adult visual cortex

While recent studies of synaptic stability in adult cerebral cortex have focused on dendrites, how much axons change is unknown. We have used advances in axon labeling by viruses and in vivo two-photon microscopy to investigate axon branching and bouton dynamics in primary visual cortex (V1) of adult Macaque monkeys. A nonreplicative adeno-associated virus bearing the gene for enhanced green fluorescent protein (AAV.EGFP) provided persistent labeling of axons, and a custom-designed two-photon microscope enabled repeated imaging of the intact brain over several weeks. We found that large-scale branching patterns were stable but that a subset of small branches associated with terminaux boutons, as well as a subset of en passant boutons, appeared and disappeared every week. Bouton losses and gains were both approximately 7% of the total population per week, with no net change in the overall density. These results suggest ongoing processes of synaptogenesis and elimination in adult V1.

Telencephalin slows spine maturation

Dendritic filopodia are highly dynamic structures, and morphological maturation from dendritic filopodia to spines is intimately associated with the stabilization and strengthening of synapses during development. Here, we report that telencephalin (TLCN), a cell adhesion molecule belonging to the Ig superfamily, is a negative regulator of spine maturation. Using cultured hippocampal neurons, we examined detailed localization and functions of TLCN in spine development and synaptogenesis. At early stages of synaptogenesis, TLCN immunoreactivity gradually increased and was present in dendritic shafts and filopodia. At later stages, TLCN tended to be excluded from mature spine synapses in which PSD-95 (postsynaptic density-95) clusters were apposed to presynaptic synaptophysin clusters. To elucidate the function of TLCN in spine maturation, we analyzed the dendrite morphology of TLCN-overexpressing and TLCN-deficient neurons. Overexpression of TLCN caused a dramatic increase in the density of dendritic filopodia and a concomitant decrease in the density of spines. Conversely, TLCN-deficient mice showed a decreased density of filopodia and an acceleration of spine maturation in vitro as well as in vivo. These results demonstrate that TLCN normally slows spine maturation by promoting the filopodia formation and negatively regulating the filopodia-to-spine transition. In addition, we found that spine heads of mature neurons were wider in TLCN-deficient mice compared with wild-type mice. Thus, the preservation of immature synapses by TLCN may be an essential step for refinement of functional neural circuits in the telencephalon, that take charge of higher brain functions such as learning, memory, and emotion.

Remodeling of synaptic structure in sensory cortical areas in vivo

Although plastic changes are known to occur in developing and adult cortex, it remains unclear whether these changes require remodeling of cortical circuitry whereby synapses are formed and eliminated or whether they rely on changes in the strength of existing synapses. To determine the structural stability of dendritic spines and axon terminals in vivo, we chose two approaches. First, we performed time-lapse two-photon imaging of dendritic spine motility of layer 5 pyramidal neurons in juvenile [postnatal day 28 (P28)] mice in visual, auditory, and somatosensory cortices. We found that there were differences in basal rates of dendritic spine motility of the same neuron type in different cortices, with visual cortex exhibiting the least structural dynamics. Rewiring visual input into the auditory cortex at birth, however, failed to alter dendritic spine motility, suggesting that structural plasticity rates might be intrinsic to the cortical region. Second, we investigated the persistence of both the presynaptic (axon terminals) and postsynaptic (dendritic spine) structures in young adult mice (P40-P61), using chronic in vivo two-photon imaging in different sensory areas. Both terminals and spines were relatively stable, with >80% persisting over a 3 week period in all sensory regions. Axon terminals were more stable than dendritic spines. These data suggest that changes in network function during adult learning and memory might occur through changes in the strength and efficacy of existing synapses as well as some remodeling of connectivity through the loss and gain of synapses.

Development and regulation of dendritic spine synapses

Dendritic spines are small protrusions from neuronal dendrites that form the postsynaptic component of most excitatory synapses in the brain. They play critical roles in synaptic transmission and plasticity. Recent advances in imaging and molecular technologies reveal that spines are complex, dynamic structures that contain a dense array of cytoskeletal, transmembrane, and scaffolding molecules. Several neurological and psychiatric disorders exhibit dendritic spine abnormalities.

Molecular mechanisms of dendritic spine morphogenesis

Excitatory synapses are formed on dendritic spines, postsynaptic structures that change during development and in response to synaptic activity. Once mature, however, spines can remain stable for many months. The molecular mechanisms that control the formation and elimination, motility and stability, and size and shape of dendritic spines are being revealed. Multiple signaling pathways, particularly those involving Rho and Ras family small GTPases, converge on the actin cytoskeleton to regulate spine morphology and dynamics bidirectionally. Numerous cell surface receptors, scaffold proteins and actin binding proteins are concentrated in spines and engaged in spine morphogenesis.

Postsynaptic TrkB signaling has distinct roles in spine maintenance in adult visual cortex and hippocampus

In adult primary visual cortex (V1), dendritic spines are more persistent than during development. Brain-derived neurotrophic factor (BDNF) increases synaptic strength, and its levels rise during cortical development. We therefore asked whether postsynaptic BDNF signaling through its receptor TrkB regulates spine persistence in adult V1. This question has been difficult to address because most methods used to alter TrkB signaling in vivo affect cortical development or cannot distinguish between pre- and postsynaptic mechanisms. We circumvented these problems by employing transgenic mice expressing a dominant negative TrkB-EGFP fusion protein in sparse pyramidal neurons of the adult neocortex and hippocampus, producing a Golgi-staining-like pattern. In adult V1, expression of dominant negative TrkB-EGFP resulted in reduced mushroom spine maintenance and synaptic efficacy, accompanied by an increase in long and thin spines and filopodia. In contrast, mushroom spine maintenance was unaffected in CA1, indicating that TrkB plays fundamentally different roles in structural plasticity in these brain areas.

Deep tissue two-photon microscopy

With few exceptions biological tissues strongly scatter light, making high-resolution deep imaging impossible for traditional-including confocal-fluorescence microscopy. Nonlinear optical microscopy, in particular two photon-excited fluorescence microscopy, has overcome this limitation, providing large depth penetration mainly because even multiply scattered signal photons can be assigned to their origin as the result of localized nonlinear signal generation. Two-photon microscopy thus allows cellular imaging several hundred microns deep in various organs of living animals. Here we review fundamental concepts of nonlinear microscopy and discuss conditions relevant for achieving large imaging depths in intact tissue.

Spatial organization of cofilin in dendritic spines

Synaptic plasticity is associated with morphological changes in dendritic spines. The actin-based cytoskeleton plays a key role in regulating spine structure, and actin reorganization in spines is critical for the maintenance of long term potentiation. To test the hypothesis that a stable pool of F-actin rests in the spine "core," while a dynamic pool lies peripherally in its "shell," we performed immunoelectron microscopy in the stratum radiatum of rat hippocampus to elucidate the subcellular distribution of cofilin, an actin-depolymerizing protein that mediates reorganization of the actin cytoskeleton. We provide direct evidence that cofilin in spines avoids the core, and instead concentrates in the shell and within the postsynaptic density. These data suggest that cofilin may link synaptic plasticity to the actin remodeling that underlies changes in spine morphology.

Multimodal imaging of mouse development: Tools for the postgenomic era

With the sequence of the mouse genome known, it is now possible to create or identify mutations in every gene to determine the molecules necessary for normal development. Consequently, there is a growing need for advanced phenotyping tools to best understand defects produced by altering gene function. Perhaps nothing is more satisfying than to directly observe a process in action; to disturb it and see for ourselves how the process changes before our very eyes. No doubt, this desire is what drove the invention of the very first microscopes and continues to this day to fuel progress in the field of biological imaging. Because mouse embryos are small and develop embedded within many tissue layers within the nurturing environment of the mother, directly observing the dynamic, micro- and nanoscopic events of early mammalian development has proven to be one of the greater challenges for imaging scientists. Here, I will review some of the imaging methods being used to study mouse development, highlighting the results obtained from imaging.

Dynamic remodeling of dendritic arbors in GABAergic interneurons of adult visual cortex

Despite decades of evidence for functional plasticity in the adult brain, the role of structural plasticity in its manifestation remains unclear. To examine the extent of neuronal remodeling that occurs in the brain on a day-to-day basis, we used a multiphoton-based microscopy system for chronic in vivo imaging and reconstruction of entire neurons in the superficial layers of the rodent cerebral cortex. Here we show the first unambiguous evidence (to our knowledge) of dendrite growth and remodeling in adult neurons. Over a period of months, neurons could be seen extending and retracting existing branches, and in rare cases adding new branch tips. Neurons exhibiting dynamic arbor rearrangements were GABA-positive non-pyramidal interneurons, while pyramidal cells remained stable. These results are consistent with the idea that dendritic structural remodeling is a substrate for adult plasticity and they suggest that circuit rearrangement in the adult cortex is restricted by cell type–specific rules.

Chronic in vivo imaging of fluorescent-labeled neurons in adult mice reveals extension and retraction of dendrites in GABAergic non-pyramidal interneurons of the cerebral cortex.

How to build a central synapse: clues from cell culture

Central neurons develop and maintain molecularly distinct synaptic specializations for excitatory and inhibitory transmitters, often only microns apart on their dendritic arbor. Progress towards understanding the molecular basis of synaptogenesis has come from several recent studies using a coculture system of non-neuronal cells expressing molecules that generate presynaptic or postsynaptic "hemi-synapses" on contacting neurons. Together with molecular properties of these protein families, such studies have yielded interesting clues to how glutamatergic and GABAergic synapses are assembled. Other clues come from heterochronic cultures, manipulations of activity in subsets of neurons in a network, and of course many in vivo studies. Taking into account these data, we consider here how basic parameters of synapses--competence, placement, composition, size and longevity--might be determined.

Map Plasticity in Somatosensory Cortex

Sensory maps in neocortex are adaptively altered to reflect recent experience and learning. In somatosensory cortex, distinct patterns of sensory use or disuse elicit multiple, functionally distinct forms of map plasticity. Diverse approaches—genetics, synaptic and in vivo physiology, optical imaging, and ultrastructural analysis—suggest a distributed model in which plasticity occurs at multiple sites in the cortical circuit with multiple cellular/synaptic mechanisms and multiple likely learning rules for plasticity. This view contrasts with the classical model in which the map plasticity reflects a single Hebbian process acting at a small set of cortical synapses.