Rate and extent of recovery from dark rearing in the visual cortex of the mouse

Neuroplasticity-driven timing modulations revealed by ultrafast functional magnetic resonance imaging

Detecting neuroplasticity in global brain circuits in vivo is key for understanding myriad processes such as memory, learning, and recovery from injury. Functional Magnetic Resonance Imaging (fMRI) is instrumental for such in vivo mappings, yet it typically relies on mapping changes in spatial extent of activation or via signal amplitude modulations, whose interpretation can be highly ambiguous. Importantly, a central aspect of neuroplasticity involves modulation of neural activity timing properties. We thus hypothesized that this temporal dimension could serve as a new marker for neuroplasticity. To detect fMRI signals more associated with the underlying neural dynamics, we developed an ultrafast fMRI (ufMRI) approach facilitating high spatiotemporal sensitivity and resolution in distributed neural pathways. When neuroplasticity was induced in the mouse visual pathway via dark rearing, ufMRI indeed mapped temporal modulations in the entire visual pathway. Our findings therefore suggest a new dimension for exploring neuroplasticity in vivo.

Experience-Dependent Development of Dendritic Arbors in Mouse Visual Cortex

The dendritic arbor of neurons constrains the pool of available synaptic partners and influences the electrical integration of synaptic currents. Despite these critical functions, our knowledge of the dendritic structure of cortical neurons during early postnatal development and how these dendritic structures are modified by visual experience is incomplete. Here, we present a large-scale dataset of 849 3D reconstructions of the basal arbor of pyramidal neurons collected across early postnatal development in visual cortex of mice of either sex. We found that the basal arbor grew substantially between postnatal day 7 (P7) and P30, undergoing a 45% increase in total length. However, the gross number of primary neurites and dendritic segments was largely determined by P7. Growth from P7 to P30 occurred primarily through extension of dendritic segments. Surprisingly, comparisons of dark-reared and typically reared mice revealed that a net gain of only 15% arbor length could be attributed to visual experience; most growth was independent of experience. To examine molecular contributions, we characterized the role of the activity-regulated small GTPase Rem2 in both arbor development and the maintenance of established basal arbors. We showed that Rem2 is an experience-dependent negative regulator of dendritic segment number during the visual critical period. Acute deletion of Rem2 reduced directionality of dendritic arbors. The data presented here establish a highly detailed, quantitative analysis of basal arbor development that we believe has high utility both in understanding circuit development as well as providing a framework for computationalists wishing to generate anatomically accurate neuronal models.SIGNIFICANCE STATEMENT Dendrites are the sites of the synaptic connections among neurons. Despite their importance for neural circuit function, only a little is known about the postnatal development of dendritic arbors of cortical pyramidal neurons and the influence of experience. Here we show that the number of primary basal dendritic arbors is already established before eye opening, and that these arbors primarily grow through lengthening of dendritic segments and not through addition of dendritic segments. Surprisingly, visual experience has a modest net impact on overall arbor length (15%). Experiments in KO animals revealed that the gene Rem2 is positive regulator of dendritic length and a negative regulator of dendritic segments.

Congenital olfactory impairment is linked to cortical changes in prefrontal and limbic brain regions

The human sense of smell is closely associated with morphological differences of the fronto-limbic system, specifically the piriform cortex and medial orbitofrontal cortex (mOFC). Still it is unclear whether cortical volume in the core olfactory areas and connected brain regions are shaped differently in individuals who suffer from lifelong olfactory deprivation relative to healthy normosmic individuals. To address this question, we examined if regional variations in gray matter volume were associated with smell ability in seventeen individuals with isolated congenital olfactory impairment (COI) matched with sixteen normosmic controls. All subjects underwent whole-brain magnetic resonance imaging, and voxel-based morphometry was used to estimate regional variations in grey matter volume. The analyses showed that relative to controls, COI subjects had significantly larger grey matter volumes in left middle frontal gyrus and right superior frontal sulcus (SFS). COI subjects with severe olfactory impairment (anosmia) had reduced grey matter volume in the left mOFC and increased volume in right piriform cortex and SFS. Within the COI group olfactory ability, measured with the "Sniffin' Sticks" test, was positively associated with larger grey matter volume in right posterior cingulate and parahippocampal cortices whereas the opposite relationship was observed in controls. Across COI subjects and controls, better olfactory detection threshold was associated with smaller volume in right piriform cortex, while olfactory identification was negatively associated with right SFS volume. Our findings suggest that lifelong olfactory deprivation trigger changes in the cortical volume of prefrontal and limbic brain regions previously linked to olfactory memory.

Changes in neuroplasticity following early-life social adversities: the possible role of brain-derived neurotrophic factor

Social adversities experienced in childhood can have a profound impact on the developing brain, leading to the emergence of psychopathologies in adulthood. Despite the burden this places on both the individual and society, the neurobiological aspects mediating this transition remain unclear. Recent advances in preclinical and clinical research have begun examining neuroplasticity-the nervous system's ability to form adaptive changes in response to new experience-in the context of early-life vulnerability to social adversities and plasticity-related alterations following such traumatic events. A key mediator of plasticity-related molecular processes is the brain-derived neurotrophic factor (BDNF), which has also been implicated in various psychiatric disorders related to childhood social adversities. Preclinical and clinical data suggest early-life social adversities (ELSA) might be associated with accelerated maturation of social network circuitry, a possible ontogenic adaptation to the adverse environment. Neural plasticity decreases by adulthood, lessening the efficacy of treatment in ELSA-related psychiatric disorders. However, literature data suggest that by increasing BDNF/TrkB signalling through antidepressant treatment a juvenile-like plasticity state can be induced, which allows for reorganization of the social circuitry when guided by psychotherapy and surrounded by a safe and positive environment.

Rem2 stabilizes intrinsic excitability and spontaneous firing in visual circuits

Sensory experience plays an important role in shaping neural circuitry by affecting the synaptic connectivity and intrinsic properties of individual neurons. Identifying the molecular players responsible for converting external stimuli into altered neuronal output remains a crucial step in understanding experience-dependent plasticity and circuit function. Here, we investigate the role of the activity-regulated, non-canonical Ras-like GTPase Rem2 in visual circuit plasticity. We demonstrate that Rem2-/- mice fail to exhibit normal ocular dominance plasticity during the critical period. At the cellular level, our data establish a cell-autonomous role for Rem2 in regulating intrinsic excitability of layer 2/3 pyramidal neurons, prior to changes in synaptic function. Consistent with these findings, both in vitro and in vivo recordings reveal increased spontaneous firing rates in the absence of Rem2. Taken together, our data demonstrate that Rem2 is a key molecule that regulates neuronal excitability and circuit function in the context of changing sensory experience.

Neural architecture: from cells to circuits

Circuit operations are determined jointly by the properties of the circuit elements and the properties of the connections among these elements. In the nervous system, neurons exhibit diverse morphologies and branching patterns, allowing rich compartmentalization within individual cells and complex synaptic interactions among groups of cells. In this review, we summarize work detailing how neuronal morphology impacts neural circuit function. In particular, we consider example neurons in the retina, cerebral cortex, and the stomatogastric ganglion of crustaceans. We also explore molecular coregulators of morphology and circuit function to begin bridging the gap between molecular and systems approaches. By identifying motifs in different systems, we move closer to understanding the structure-function relationships that are present in neural circuits.

Neglect as a Violation of Species-Expectant Experience: Neurodevelopmental Consequences

The human brain requires a wide variety of experiences and environmental inputs in order to develop normally. Children who are neglected by caregivers or raised in institutional environments are deprived of numerous types of species-expectant environmental experiences. In this review, we articulate a model of how the absence of cognitive stimulation and sensory, motor, linguistic, and social experiences common among children raised in deprived early environments constrains early forms of learning, producing long-term deficits in complex cognitive function and associative learning. Building on evidence from animal models, we propose that deprivation accelerates the neurodevelopmental process of synaptic pruning and limits myelination, resulting in age-specific reductions in cortical thickness and white matter integrity among children raised in deprived early environments. We review evidence linking early experiences of psychosocial deprivation to reductions in cognitive ability, associative and implicit learning, language skills, and executive functions as well as atypical patterns of cortical and white matter development-domains that should be profoundly influenced by deprivation through the learning and neural mechanisms we propose. These patterns of atypical development are difficult to explain with existing models that emphasize stress pathways and accelerated limbic system development. A learning account of how deprived early environments influence cognitive and neural development provides a complementary perspective to stress models and highlights novel pathways through which deprivation might confer risk for internalizing and externalizing psychopathology. We end by reviewing evidence for plasticity in cognitive and neural development among children raised in deprived environments following interventions that improve caregiving quality.

Synapse-specific control of experience-dependent plasticity by presynaptic NMDA receptors

Sensory experience orchestrates the development of cortical circuitry by adaptively modifying neurotransmission and synaptic connectivity. However, the mechanisms underlying these experience-dependent modifications remain elusive. Here we demonstrate that visual experience suppresses a presynaptic NMDA receptor (preNMDAR)-mediated form of timing-dependent long-term depression (tLTD) at visual cortex layer (L) 4-2/3 synapses. This tLTD can be maintained during development, or reinstated in adulthood, by sensory deprivation. The changes in tLTD are mirrored by changes in glutamate release; visual deprivation enhances both tLTD and glutamate release. These effects require the GluN3A NMDAR subunit, the levels of which are increased by visual deprivation. Further, by coupling the pathway-specific optogenetic induction of tLTD with cell-type-specific NMDAR deletion, we find that visual experience modifies preNMDAR-mediated plasticity specifically at L4-L2/3 synapses.

Visual acuity development and plasticity in the absence of sensory experience

Visual circuits mature and are refined by sensory experience. However, significant gaps remain in our understanding how deprivation influences the development of visual acuity in mice. Here, we perform a longitudinal study assessing the effects of chronic deprivation on the development of the mouse subcortical and cortical visual circuits using a combination of behavioral optomotor testing, in vivo visual evoked responses (VEP) and single-unit cortical recordings. As previously reported, orientation tuning was degraded and onset of ocular dominance plasticity was delayed and remained open in chronically deprived mice. Surprisingly, we found that the development of optomotor threshold and VEP acuity can occur in an experience-independent manner, although at a significantly slower rate. Moreover, monocular deprivation elicited amblyopia only during a discrete period of development in the dark. The rate of recovery of optomotor threshold upon exposure of deprived mice to light confirmed a maturational transition regardless of visual input. Together our results revealed a dissociable developmental trajectory for visual receptive-field properties in dark-reared mice suggesting a differential role for spontaneous activity within thalamocortical and intracortical circuits.

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

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

Dendritic spines and development: towards a unifying model of spinogenesis--a present day review of Cajal's histological slides and drawings

Dendritic spines receive the majority of excitatory connections in the central nervous system, and, thus, they are key structures in the regulation of neural activity. Hence, the cellular and molecular mechanisms underlying their generation and plasticity, both during development and in adulthood, are a matter of fundamental and practical interest. Indeed, a better understanding of these mechanisms should provide clues to the development of novel clinical therapies. Here, we present original results obtained from high-quality images of Cajal's histological preparations, stored at the Cajal Museum (Instituto Cajal, CSIC), obtained using extended focus imaging, three-dimensional reconstruction, and rendering. Based on the data available in the literature regarding the formation of dendritic spines during development and our results, we propose a unifying model for dendritic spine development.

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

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

Bilateral whisker trimming during early postnatal life impairs dendritic spine development in the mouse somatosensory barrel cortex

The rodent somatosensory barrel cortex is an ideal model for studying the impact of sensory experience on developing brain circuitry. To examine whether and how interference with sensory perception in the early postnatal period can affect the development of synaptic networks in this system, we took advantage of a transgenic mouse strain expressing the yellow fluorescent protein in layer 5B pyramidal neurons of the somatosensory cortex. By using ex vivo confocal imaging, we first demonstrate a cortical-layer-specific increase in the number of dendritic spines during postnatal development on apical dendritic shafts of these cells extending up to cortical layer 1. Next, by performing bilateral whisker trimming at distinct developmental stages, we show that disruption of sensory perception before postnatal day 20 impairs dendritic spine development in apical dendritic segments within layers 1 and 2/3 but not in layer 4. The whisker trimming-induced decrease in dendritic spine density during this period is accompanied by a highly significant decrease in dendritic spine head diameter. Finally, we also show that these whisker trimming-induced morphological alterations of dendritic spines during the early postnatal period are no longer detectable in adult animals. Altogether, these findings further emphasize the important role of sensory activity in synaptic network assembly in the developing barrel cortex. They also support an as yet unidentified structural mechanism that might contribute to the layer- and cell-type-specific physiological effects of whisker trimming during the early postnatal period.

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.

Deprivation-induced dendritic shrinkage might be oppositely affected by the expression of wild-type and mutated human amyloid precursor protein

The physiological role of the amyloid precursor protein (APP) and its proteolytic fragments in the brain is associated with neuronal survival, neurite outgrowth, synaptic formation, and neuronal plasticity. However, malregulation of APP processing leads to disordered balance of fragments, which may results in opposite, degenerative neuronal effects. In the present study, we analyzed in vivo effects of the expression of wild-type or mutated human APP on afferent deprivation-induced changes of dendritic morphology. After vibrissectomy, expression of wild-type human APP prevented diameter shrinkage of dendritic segments as well as dendritic rarefaction of apical arbors. In contrast, mutant human APP expression exacerbated degenerative changes of deprived barrel neurons. Degradation of apical arbors was especially pronounced. Results demonstrate for the first time opposite effects of the expression of wild-type and mutated human APP on deprivation-induced dendritic restructuring in vivo.

Septins and the lateral compartmentalization of eukaryotic membranes

Eukaryotic cells from neurons and epithelial cells to unicellular fungi frequently rely on cellular appendages such as axons, dendritic spines, cilia, and buds for their biology. The emergence and differentiation of these appendages depend on the formation of lateral diffusion barriers at their bases to insulate their membranes from the rest of the cell. Here, we review recent progress regarding the molecular mechanisms and functions of such barriers. This overview underlines the importance and conservation of septin-dependent diffusion barriers, which coordinately compartmentalize both plasmatic and internal membranes. We discuss their role in memory establishment and the control of cellular aging.

The discovery of dendritic spines by Cajal in 1888 and its relevance in the present neuroscience

The year 2006 marks the centenary of the Nobel Prize for Physiology or Medicine awarded to Santiago Ramón y Cajal and Camilo Golgi, "in recognition of their work on the structure of the nervous system". Their discoveries are keys to understanding the present neuroscience, for instance, the discovery of dendritic spines. Cajal discovered dendritic spines in 1888 with the Golgi method, although other contemporary scientists thought that they were silver precipitates. Dendritic spines were demonstrated definitively as real structures by Cajal with the Methylene Blue in 1896. Many of the observations of Cajal and other contemporary scientists about dendritic spines are active fields of research of present neuroscience, for instance, their morphology, distribution, density, development and function. This article will deal with the main contributions of Cajal and other contemporary scientists about dendritic spines. We will analyse their contributions from the historical and present point of view. In addition, we will show high quality images of Cajal's original preparations and drawings related with this discovery.

Synaptic modifications at the CA3-CA1 synapse after chronic AMPA receptor blockade in rat hippocampal slices

Maintenance of dendritic spines, the postsynaptic elements of most glutamatergic synapses in the central nervous system, requires continued activation of AMPA receptors. In organotypic hippocampal slice cultures, chronic blockade of AMPA receptors for 14 days induces a substantial loss of dendritic spines on CA1 pyramidal neurons. Here, using serial section electron microscopy, we show that loss of dendritic spines is paralleled by a significant reduction in synapse density. In contrast, we observed an increased number of asymmetric synapses onto the dendritic shaft, suggesting that spine retraction does not inevitably lead to synapse elimination. Functional analysis of the remaining synapses revealed that hippocampal circuitry compensates for the anatomical loss of synapses by increasing synaptic efficacy. Moreover, we found that the observed morphological and functional changes were associated with altered bidirectional synaptic plasticity. We conclude that continued activation of AMPA receptors is necessary for maintaining structure and function of central glutamatergic synapses.

Brain plasticity and mental processes: Cajal again

The year 2006 marks the 100th anniversary of the first Nobel Prize for Physiology or Medicine for studies in the field of the Neurosciences jointly awarded to Camillo Golgi and Santiago Ramón y Cajal for their key contributions to the study of the nervous system. This award represented the beginning of the modern era of neuroscience. Using the Golgi method, Cajal made fundamental, but often unappreciated, contributions to the study of the relationship between brain plasticity and mental processes. Here, I focus on some of these early experiments and how they continue to influence studies of brain plasticity.

A Comparison of Experience-Dependent Plasticity in the Visual and Somatosensory Systems

In the visual and somatosensory systems, maturation of neuronal circuits continues for days to weeks after sensory stimulation occurs. Deprivation of sensory input at various stages of development can induce physiological, and often structural, changes that modify the circuitry of these sensory systems. Recent studies also reveal a surprising degree of plasticity in the mature visual and somatosensory pathways. Here, we compare and contrast the effects of sensory experience on the connectivity and function of these pathways and discuss what is known to date concerning the structural, physiological, and molecular mechanisms underlying their plasticity.

Long-term sensory deprivation prevents dendritic spine loss in primary somatosensory cortex

A substantial decrease in the number of synapses occurs in the mammalian brain from the late postnatal period until the end of life. Although experience plays an important role in modifying synaptic connectivity, its effect on this nearly lifelong synapse loss remains unknown. Here we used transcranial two-photon microscopy to visualize postsynaptic dendritic spines in layer I of the barrel cortex in transgenic mice expressing yellow fluorescent protein. We show that in young adolescent mice, long-term sensory deprivation through whisker trimming prevents net spine loss by preferentially reducing the rate of ongoing spine elimination, not by increasing the rate of spine formation. This effect of deprivation diminishes as animals mature but still persists in adulthood. Restoring sensory experience after adolescent deprivation accelerates spine elimination. Similar to sensory manipulation, the rate of spine elimination decreases after chronic blockade of NMDA (N-methyl-D-aspartate) receptors with the antagonist MK801, and accelerates after drug withdrawal. These studies of spine dynamics in the primary somatosensory cortex suggest that experience plays an important role in the net loss of synapses over most of an animal's lifespan, particularly during adolescence.

Experience-dependent pruning of dendritic spines in visual cortex by tissue plasminogen activator

Sensory experience physically rewires the brain in early postnatal life through unknown processes. Here, we identify a robust anatomical consequence of monocular deprivation (MD) in layer II/III of visual cortex that corresponds to the rapid, functional loss of responsiveness preceding any changes in axonal input. Protrusions on pyramidal cell apical dendrites increased steadily after eye opening, but were transiently lost through competitive mechanisms after brief MD only during the physiological critical period. Proteolysis by tissue-type plasminogen activator (tPA) conversely declined with age and increased with MD only in young mice. Targeted disruption of tPA release or its upstream regulation by glutamic acid decarboxylase (GAD65) prevented MD-induced spine loss that was pharmacologically rescued concomitant with critical period plasticity. An extracellular mechanism for structural remodeling that is limited to the binocular zone upon proper detection of competing inputs thus links early sensory experience to visual function.

A morphological correlate of synaptic scaling in visual cortex

We studied the response of dendritic spines in the thalamic-recipient zone of rat visual cortex to simple manipulations of the visual environment. We measured the morphologies of a total of 3824 spines located on the basal dendrites of 60 layer 3 pyramidal cells. As expected from previous studies, we found a significantly lower spine density in dark-reared animals at postnatal day 30 (P30) compared with light-reared controls. Additional analysis revealed that the spines in dark-reared animals were significantly shorter and more bulbous than in light-reared animals. When these two results were combined, we found that the total synaptic area per unit length of dendrite was conserved, compatible with the phenomenon of "synaptic scaling." We also found that the increase in average spine head diameter is reversed by 10 d of light exposure (starting at P20), but surprisingly, the decrease in spine density is not. Thus, not all effects of dark rearing can be reversed by subsequent visual experience, even when the experience occurs during the third postnatal week.

Structural plasticity in the developing visual system

The visual system has been used extensively to study cortical plasticity during development. Seminal experiments by Hubel and Wiesel (Wiesel, T.N. and Hubel, D.H. (1963) Single cell responses in striate cortex of kittens deprived of vision in one eye. J. Neurophysiol., 26: 1003-1017.) identified the visual cortex as a very attractive model for studying structural and functional plasticity regulated by experience. It was discovered that the thalamic projections to the visual cortex, and neuronal connectivity in the visual cortex itself, were organized in alternating columns dominated by input from the left or the right eye. This organization was shown to be strongly influenced by manipulating binocular input during a specific time point of postnatal development known as the critical period. Two chapters in this volume review the molecular and functional aspects of this form of plasticity. This chapter reviews the structural changes that occur during ocular dominance (OD) plasticity and their possible functional relevance, and discusses developments in the methods that allow the analysis of the molecular and cellular mechanisms that regulate them.

Ubiquitin-proteasome-mediated local protein degradation and synaptic plasticity

A proteolytic pathway in which attachment of a small protein, ubiquitin, marks the substrates for degradation by a multi-subunit complex called the proteasome has been shown to function in synaptic plasticity and in several other physiological processes of the nervous system. Attachment of ubiquitin to protein substrates occurs through a series of highly specific and regulated steps. Degradation by the proteasome is subject to multiple levels of regulation as well. How does the ubiquitin-proteasome pathway contribute to synaptic plasticity? Long-lasting, protein synthesis-dependent, changes in the synaptic strength occur through activation of molecular cascades in the nucleus in coordination with signaling events in specific synapses. Available evidence indicates that ubiquitin-proteasome-mediated degradation has a role in the molecular mechanisms underlying synaptic plasticity that operate in the nucleus as well as at the synapse. Since the ubiquitin-proteasome pathway has been shown to be versatile in having roles in addition to proteolysis in several other cellular processes relevant to synaptic plasticity, such as endocytosis and transcription, this pathway is highly suited for a localized role in the neuron. Because of its numerous roles, malfunctioning of this pathway leads to several diseases and disorders of the nervous system. In this review, I examine the ubiquitin-proteasome pathway in detail and describe the role of regulated proteolysis in long-term synaptic plasticity. Also, using synaptic tagging theory of synapse-specific plasticity, I provide a model on the possible roles and regulation of local protein degradation by the ubiquitin-proteasome pathway.

Developmental regulation of spine and filopodial motility in primary visual cortex: reduced effects of activity and sensory deprivation

Dendritic protrusions are highly motile during postnatal development. Although spine morphological plasticity could be associated with synaptic plasticity, the function of rapid spine/filopodial motility is still unknown. To investigate the role of spine motility in the development of the visual cortex and its relation with critical periods, we used two-photon imaging of neurons from layers receiving visual input in developing mouse primary visual cortex and compared motility between control and visually deprived animals. Spine and filopodia motility was prominent during early synaptogenesis (P11-P13) but greatly decreased after P15. This "switch" was coincident with a 2.5-fold increase in protrusion density and spine formation. Spine motility was not regulated during the critical period for monocular deprivation (P19-P34). Moreover, delaying the critical period by dark rearing did not delay the normal developmental decrease of spine motility, but caused a modest further reduction in motility at P28-P35. Dark rearing and enucleation also mildly reduced spine motility before eye opening and dark rearing reduced the proportion of filopodia. We conclude that (1) rapid spine motility is not related to critical period plasticity, but is likely to play a role in early synaptogenesis, and (2) neuronal activity stimulates spine motility during synaptogenesis and promotes the appearance of dendritic filopodia.

Neuronal changes during development and evolution (an overview)

This overview highlights the presentations made by several authors, with sidelights commentaries on the topics covered in this session. The Neurotropic hypothesis, formulated by Cajal more than one century ago, has been one of the fundamental tenets of modern Neuroscience. Research work is unveiling highly complex molecular mechanisms by means of which neuronal processes grow in the right direction leading to the formation of neural networks. Another interesting topic covered in this session pertains to the remarkably conserved similarity of most parts of the brain between reptiles, birds and mammals. The presence of the neocortex, which occurs in mammals, is discussed on the basis of different migratory behavior during development. The question of neuronal stability throughout the life cycle raises interesting aspects on the generation of new neurons. This is a particularly timely topic because recent evidence, denied by others, has shown the generation of new neocortical neurons in the adult. In another session, it was discussed the evidence showing that pyramidal cells and intrinsic cells of the neocortex have different developmental origins. Finally, several modern analyses show the role of different transcription factors in the formation of brain cortical maps before the arrival of their proper afferent connections.

BMP-7 and excess glutamate: opposing effects on dendrite growth from cerebral cortical neurons in vitro

Glutamate is an important regulator of dendrite development. During cerebral ischemia, however, there is massive release of glutamate reaching millimolar concentrations in the extracellular space. An early consequence of this excess glutamate is reduced dendrite growth. Bone morphogenetic protein-7 (BMP-7) a member of the transforming growth factor-beta (TGF-beta) superfamily has been demonstrated to enhance dendrite output from cerebral cortical and hippocampal neurons in vitro. However, it is not known whether BMP-7can prevent the reduced dendrite growth associated with excess glutamate or enhance dendrite growth after glutamate exposure. Therefore we quantified axon and primary, secondary, and total dendrite growth from embryonic mouse cortical neurons (E18) grown at low density in vitro in a chemically defined medium and exposed to glutamate (1 or 2 mM) for 48 h. Morphology and double immunolabeling (MAP2, NF-H) were used to identify cortical dendrites and axons after 3 DIV. In these short-term cultures, glutamate did not influence neuron survival. The addition of glutamate to cortical neurons, however, significantly attenuated dendrite output. This effect was mimicked by the addition of NMDA but not AMPA agonists and inhibited by the specific NMDA receptor antagonist MK-801. The reduction in dendrite growth mediated by excess glutamate was ameliorated by the administration of 30 or 100 ng/ml of BMP-7. In addition, when administered in a delayed fashion between 1 and 24 h after the initial glutamate exposure, BMP-7 was able to enhance dendrite growth, including primary dendrite number, primary dendrite length, and secondary dendritic branching. These findings demonstrate that BMP-7 can ameliorate reduced dendrite growth from cerebral cortical neurons associated with excess glutamate in vitro and are important because they may help explain why BMP-7 administration is associated with enhanced functional recovery in models of cerebral ischemia.

Structure and function of dendritic spines

Spines are neuronal protrusions, each of which receives input typically from one excitatory synapse. They contain neurotransmitter receptors, organelles, and signaling systems essential for synaptic function and plasticity. Numerous brain disorders are associated with abnormal dendritic spines. Spine formation, plasticity, and maintenance depend on synaptic activity and can be modulated by sensory experience. Studies of compartmentalization have shown that spines serve primarily as biochemical, rather than electrical, compartments. In particular, recent work has highlighted that spines are highly specialized compartments for rapid large-amplitude Ca(2+) signals underlying the induction of synaptic plasticity.

BDNF/trkB signaling in the developmental sculpting of visual connections

Neurotrophins are a family of secreted molecules that have multiple, profound actions on the structure and function of both developing and mature neurons. Neurotrophins exert their influences by signaling through the trk family of receptor tyrosine kinases and the p75 low affinity neurotrophin receptor. Here we review the contributions of neurotrophins to the development of neural circuitry in the mammalian visual system. We emphasize: (1) the role of neurotrophins as components of the cellular mechanisms by which neuroelectric activity sculpts pattern of brain connectivity; and (2) the results of recent experiments suggesting that the trafficking of neurotrophin proteins may be activity dependent.

Morphological changes in dendritic spines associated with long-term synaptic plasticity

Dendritic spines are morphological specializations that receive synaptic inputs and compartmentalize calcium. In spite of a long history of research, the specific function of spines is still not well understood. Here we review the current status of the relation between morphological changes in spines and synaptic plasticity. Since Cajal and Tanzi proposed that changes in the structure of the brain might occur as a consequence of experience, the search for the morphological correlates of learning has constituted one of the central questions in neuroscience. Although there are scores of studies that encompass this wide field in many species, in this review we focus on experimental work that has analyzed the morphological consequences of hippocampal long-term potentiation (LTP) in rodents. Over the past two decades many studies have demonstrated changes in the morphology of spines after LTP, such as enlargements of the spine head and shortenings of the spine neck. Biophysically, these changes translate into an increase in the synaptic current injected at the spine, as well as shortening of the time constant for calcium compartmentalization. In addition, recent online studies using time-lapse imaging have reported increased spinogenesis. The currently available data show a strong correlation between synaptic plasticity and morphological changes in spines, although at the same time, there is no evidence that these morphological changes are necessary or sufficient for the induction or maintenance of LTP. Still, they highlight once more how form and function go hand in hand in the central nervous system.

Comparison of hippocampal dendritic spines in culture and in brain

We have quantified hippocampal spine structure at the light and ultrastructural levels in cell cultures approximately 1- 3 weeks old and in the brains of rodents 5 and 21 d old. The number of spines bearing synapses increases with age in cultures and in brain, but the structures are similar in both. In culture, about half of the synapses are formed on spines and the remainder are formed on dendritic shafts. In the 5-d-old brain, about half of the synapses occur on dendritic shafts, by 3 weeks of age only approximately 20% of synapses are found on dendritic shafts, and in the adult shaft synapses are very rare.

A method for vibratome sectioning of Golgi-Cox stained whole rat brain

A method for impregnating the whole rat brain with Golgi-Cox stain and sectioning with the vibratome is described. The method is simple, inexpensive and provides good resolution of dendrites and spines.

Expression of DMAP-45R in the rat visual cortex is modulated by visual experience

Effects of visual experience upon expression of a developmentally regulated microtubule-associated protein (MAP) were studied in the visual cortex of monocularly deprived rats. The antibody Drosophila MAP-45 (DMAP-45) recognizes proteins in the developing ventral nerve cord of Drosophila and in rat brain. Monocular deprivation from day 12, before eye opening, to day 80 reduced the number of DMAP-45 immunoreactive layer V pyramidal cell apical dendrites in the monocular segment (Oc1M) of the visual cortex contralateral to the deprived eye. No significant visual deprivation effects were seen in the binocular segment (Oc1B). Immunoreactivity was restored to control levels in Oc1M of rats in which the monocular sutures were removed at day 75, subsequently allowing 5 days of exposure to light. These results indicate potential involvement of this MAP in experience-dependent structural plasticity.

Monocular deprivation alters the morphology of glial fibrillary acidic protein-immunoreactive astrocytes in the rat visual cortex

Monocular deprivation was used to examine the experience-dependent structural plasticity of astrocytes in Oc1M and Oc1B visual cortex of young and adult rats. Stereological techniques were employed to assess the numerical density (Nv) of cells and surface density (Sv) of processes immunoreactive for glial fibrillary acidic protein in laminae II/III, IV, V and VI in the hemisphere opposite the deprived eye. In one group of pups eyelids were sutured on postnatal day 12 (P12) and maintained until P80 (MD), while a second group had the sutures removed at P75 followed by 5 days of light exposure (MD + L). An unoperated light experienced group was used for comparisons (L). The Sv of astrocytic processes in lamina IV but not laminae II/III, V and VI was significantly decreased in the MD group. The ratio of Sv to the Nv of neurons, an estimate of the amount of astrocytic membrane per neuron, was also significantly decreased in layer IV. The Nv of astrocytes was not significantly different among the three groups. In adults that were monocularly deprived for 5, 10 and 30 days the Nv of astrocytes and Sv of their processes were not significantly altered in layer IV. There was however an increase in the Nv of all types of glial cells combined in layer IV following 10 and 30 days. These results indicate that the structure of astrocytes is influenced by visual experience during development whereas merely altering the level of visually-driven activity in the adult was not sufficient to induce astrocytic structural change.

Activity-dependent regulation of dendritic spine density on cortical pyramidal neurons in organotypic slice cultures

In order to examine the effects of activity on spine production and/or maintenance in the cerebral cortex, we have compared the number of dendritic spines on pyramidal neurons in slices of P0 mouse somatosensory cortex maintained in organotypic slice cultures under conditions that altered basal levels of spontaneous electrical activity. Cultures chronically exposed to 100 microM picrotoxin (PTX) for 14 days exhibited significantly elevated levels of electrical activity when compared to neurons in control cultures. Pyramidal neurons raised in the presence of PTX showed significantly higher densities of dendritic spines on primary apical, secondary apical, and secondary basal dendrites when compared to control cultures. The PTX-induced increase in spine density was dose dependent and appeared to saturate at 100 microM. Cultures exhibiting little or no spontaneous activity, as a result of growth in a combination of PTX and tetrodotoxin (TTX), showed significantly fewer dendritic spines compared to cultures maintained in PTX alone. These results demonstrate that the density of spines on layers V and VI pyramidal neurons can be modulated by growth conditions that alter the levels of spontaneous electrical activity.

Topographic differences of slow event-related brain potentials in blind and sighted adult human subjects during haptic mental rotation

Twelve blindfolded sighted, nine congenitally blind, and seven adventitiously blind subjects were tested in a haptic mental rotation task while slow event-related brain potentials in the EEG were recorded from 17 scalp locations. The overall topography of the slow wave pattern which prevailed during the task differed for sighted and for blind, but not for congenitally and adventitiously blind subjects. While the tactile stimuli were encoded, the blind showed a pronounced occipital and the sighted a pronounced frontal activation. The task-specific amplitude increment of a negative slow wave which can be understood as a manifestation of the process of mental rotation proper, showed a different topography for sighted and for blind subjects too. It had its maximum over central to parietal cortical areas in both groups, but it extended more towards occipital regions in the blind. In both groups, the effects were very similar to those observed in former studies with visual versions of the mental rotation task, i.e. the slow wave amplitude over central to parietal areas increased monotonously with an increasing angular disparity of the two stimuli to be compared. These results are discussed with respect to the question of whether visual deprivation in the blind can cause a reorganization of cortical representational maps.

Dendritic development in the dorsal lateral geniculate nucleus of ferrets in the postnatal absence of retinal input: a Golgi study

In order to determine the ongoing role of retinal fibers in the development of dorsal lateral geniculate nucleus (dLGN) neurons during postnatal development, the development of dLGN neurons in the postnatal absence of retinal input was studied in pigmented ferrets using the Golgi-Hortega technique. The development of four dLGN cell classes, defined on the basis of somatic and dendritic morphology, was described previously in normal ferrets (Sutton and Brunso-Bechtold, 1991, J. Comp. Neurol. 309:71-85). The present results indicate that the morphological development of dLGN neurons is strikingly similar in normal and experimental ferrets. The exuberant dendritic appendages that appear after eye opening in normal ferrets are overproduced and eliminated in the postnatal absence of retinal input; however, the final reduction of these transient appendages is delayed. Because exuberant appendages develop in the absence of retinal input, their production cannot depend upon visual experience. Differences in cell body size between normal and experimental ferrets are apparent only after neurons can be classified at the end of the first postnatal month. Cell body size is markedly reduced for class 1 neurons; class 2 cells also are reduced in size but to a far lesser extent. As there is a general trend for class 1 neurons to have the functional properties of Y-cells, it is likely that the dLGN neurons most affected by the absence of retinal input also are Y-cells.

Reversible loss of dendritic spines and altered excitability after chronic epilepsy in hippocampal slice cultures

The morphological and functional consequences of epileptic activity were investigated by applying the convulsants bicuculline and/or picrotoxin to mature rat hippocampal slice cultures. After 3 days, some cells in all hippocampal subfields showed signs of degeneration, including swollen somata, vacuolation, and dendritic deformities, whereas others displayed only a massive reduction in the number of their dendritic spines. Intracellular recordings from CA3 pyramidal cells revealed a decrease in the amplitude of evoked excitatory synaptic potentials. gamma-Aminobutyric acid-releasing interneurons and inhibitory synaptic potentials were unaffected. Seven days after withdrawal of convulsants, remaining cells possessed a normal number of dendritic spines, thus demonstrating a considerable capacity for recovery. The pathological changes induced by convulsants are similar to those found in the hippocampi of human epileptics, suggesting that they are a consequence, rather than a cause, of epilepsy.

Neuronal organization and plasticity in adult monkey visual cortex: immunoreactivity for microtubule-associated protein 2

Immunocytochemical staining for microtubule-associated protein 2 (MAP 2) was used to examine the morphology of neurons, the organization of neuronal groups, and the neurochemical plasticity of cells in adult monkey area 17. MAP 2-immunostained neurons are present through the depth of area 17 but are most intensely immunoreactive in layers IVB and VI. From layer IVB, separate groups of MAP 2-positive cells invade layers IVA and IVC alpha. Clusters of cells protrude upward from superficial layer IVB and occupy the central core regions of the cytochrome oxidase (CO)-stained honeycomb in layer IVA, while large neurons typical of layer IVB are distributed in irregular clusters in the subjacent layer IVC alpha. The somata in the layer IVA honeycomb cores give off immunostained dendrites which remain largely within the core regions. Patches of MAP 2-positive neurons are also present in layers II and III, where they coincide with the CO-stained puffs. Intravitreal injections of tetrodotoxin (TTX) into one eye of adult monkeys produce stripes of alternating light and dark MAP 2 immunostaining in layer IVC. Stripes of light immunostaining coincide with stripes of light CO staining, and correspond to reduced MAP 2 immunoreactivity within cortical neurons dominated by the TTX-injected eye. In layers II and III, the MAP 2 immunostaining of patches overlying the injected-eye columns is similarly reduced. No change occurs in the MAP 2 immunostaining of layer IVA. These data suggest that the anatomical and physiological heterogeneity of layers IVA and IVC alpha arises from the periodic invasion of neurons characteristic of layer IVB, that the neurons in layer IVA have dendrites confined to thalamocortical-recipient or nonrecipient zones and that the expression of MAP 2 changes in adult cortical neurons following the loss of retinal input.

Ultrastructural development of the medial superior olive (MSO) in the ferret

When ferrets are born, four weeks before the onset of hearing, few synapses are evident in the medial superior olive (MSO). The synapses present are immature and almost exclusively found in the neuropil. The MSO somata are virtually devoid of synaptic contacts but are contacted by fine glial processes that increasingly ensheathe the somata during the first postnatal week. By P12, somatic synaptogenesis in the MSO is evident. Initially the terminals contain vesicles of irregular shape, size, and distribution. The glial lamellae appear to withdraw as the synaptic contacts form but continue to cover the asynaptic portions of the cell surface. The lamellae frequently extend from ensheathing the soma to encapsulate the immature terminals. During the next two weeks, synaptic density and terminal encapsulation proceed until the somata is surrounded by encapsulated synaptic terminals as in the adult ferret MSO. While most immature terminals contain round vesicles, during the first postnatal week some terminals with nonround vesicles can be distinguished. The first distinction between types of nonround vesicle-containing terminals, i.e., pleiomorphic and ovoid, is in the second postnatal week. This distinction becomes increasingly clear and by the end of the first postnatal month, terminal types can be reliably categorized. These observations indicate that: (1) synapses are present in the MSO neuropil one month prior to the onset of hearing, (2) the major period of synaptogenesis begins approximately two weeks prior to the onset of hearing, and (3) glial lamellae ensheathe MSO somata prior to the onset of somatic synaptogenesis, withdraw as synapses form, and subsequently re-extend to encapsulate newly formed synapses.

Repeated changes of dendritic morphology in the hippocampus of ground squirrels in the course of hibernation

Quantitative Golgi study of hippocampal pyramidal neurons of ground squirrels showed rapid and profound transformation of their apical dendrites in the course of hibernation. The dendrites were significantly shorter, less branched and had fewer dendritic spines in the middle of hibernation bout than in the active euthermic ground squirrels between bouts. After arousal from torpor, within 2 h dendrites completely restored their structure. During hibernation, season remodelling of the hippocampal dendrites occurs repeatedly during each torpor-activity cycle.

Cortical local circuit axons do not mature after early deafferentation

The processes underlying development, refinement, and retention of the intracortical connections critical for the function of the mammalian brain are unknown. Horseradish peroxidase-labeled fibers in mouse somatosensory barrel cortex, which is patterned like the whiskers on the contralateral face from which it receives inputs, were evaluated by automated image analysis. The sensory nerve to the whiskers was sectioned on postnatal day 7, after the whisker map is set. The deprived barrel cortices, examined in adults, showed drastically diminished intracortical projections relative to normal controls, although the map of the whiskers in the cortex was unchanged. This demonstrates anatomically that the normal pattern of intracortical connections, like the normal sensory map, is dependent upon the sensory periphery four synapses away.