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

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

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

Long-term sensory deprivation selectively rearranges functional inhibitory circuits in mouse barrel cortex

Long-term whisker removal alters the balance of excitation and inhibition in rodent barrel cortex, yet little is known about the contributions of individual cells and synapses in this process. We studied synaptic inhibition in four major types of neurons in live tangential slices that isolate layer 4 in the posteromedial barrel subfield. Voltage-clamp recordings of layer 4 neurons reveal that fast decay of synaptic inhibition requires alpha1-containing GABA(A) receptors. After 7 weeks of deprivation, we found that GABA(A)-receptor-mediated inhibitory postsynaptic currents (IPSCs) in the inhibitory low-threshold-spiking (LTS) cell recorded in deprived barrels exhibited faster decay kinetics and larger amplitudes in whisker-deprived barrels than those in nondeprived barrels in age-matched controls. This was not observed in other cell types. Additionally, IPSCs recorded in LTS cells from deprived barrels show a marked increase in zolpidem sensitivity. To determine if the faster IPSC decay in LTS cells from deprived barrels indicates an increase in alpha1 subunit functionality, we deprived alpha1(H101R) mutant mice with zolpidem-insensitive alpha1-containing GABA(A) receptors. In these mice and matched wild-type controls, IPSC decay kinetics in LTS cells were faster after whisker removal; however, the deprivation-induced sensitivity to zolpidem was reduced in alpha1(H101R) mice. These data illustrate a change of synaptic inhibition in LTS cells via an increase in alpha1-subunit-mediated function. Because alpha1 subunits are commonly associated with circuit-specific plasticity in sensory cortex, this switch in LTS cell synaptic inhibition may signal necessary circuit changes required for plastic adjustments in sensory-deprived cortex.

A nonfibrin macromolecular cofactor for tPA-mediated plasmin generation following cellular injury

Tissue-type plasminogen activator (tPA) is an extracellular protease that converts plasminogen into plasmin. For tPA to generate plasmin under biologic conditions, a cofactor must first bring tPA and plasminogen into physical proximity. Fibrin provides this cofactor for tPA-mediated plasmin generation in blood. Despite being naturally devoid of fibrin(ogen), tPA-mediated plasmin formation also occurs in the brain. The fibrin-like cofactor(s) that facilitates plasmin formation in the injured brain has remained unknown. Here we show that protein aggregates formed during neuronal injury provide a macromolecular, nonfibrin cofactor that promotes tPA-mediated plasmin formation and subsequent cell breakdown. The binding of plasminogen and tPA to these protein aggregates occurs via distinct mechanisms. Importantly, nonneuronal cell types also exhibit this cofactor effect upon injury, indicating a general phenomenon. This novel cofactor identified in nonviable cells has ramifications for ischemic stroke where tPA is used clinically and where plasmin activity within the injured brain is unwanted. A means of selectively inhibiting the binding of tPA to nonviable cells while preserving its association with fibrin may be of benefit for the treatment of ischemic stroke.

A synaptic trek to autism

Autism spectrum disorders (ASD) are diagnosed on the basis of three behavioral features namely deficits in social communication, absence or delay in language, and stereotypy. The susceptibility genes to ASD remain largely unknown, but two major pathways are emerging. Mutations in TSC1/TSC2, NF1, or PTEN activate the mTOR/PI3K pathway and lead to syndromic ASD with tuberous sclerosis, neurofibromatosis, or macrocephaly. Mutations in NLGN3/4, SHANK3, or NRXN1 alter synaptic function and lead to mental retardation, typical autism, or Asperger syndrome. The mTOR/PI3K pathway is associated with abnormal cellular/synaptic growth rate, whereas the NRXN-NLGN-SHANK pathway is associated with synaptogenesis and imbalance between excitatory and inhibitory currents. Taken together, these data strongly suggest that abnormal synaptic homeostasis represent a risk factor to ASD.

Dendritic spine formation and stabilization

Formation, elimination and remodeling of excitatory synapses on dendritic spines represent a continuous process that shapes the organization of synaptic networks during development. The molecular mechanisms controlling dendritic spine formation and stabilization therefore critically determine the rules of network selectivity. Recent studies have identified new molecules, such as Ephrins and Telencephalin that regulate filopodia motility and their transformation into dendritic spines. Trans-synaptic signaling involving nitric oxide, protease, adhesion molecules and Rho GTPases further controls contact formation or the structural remodeling of spines and their stability. Evidence also suggests that activity and induction of plasticity participate to the selection of persistent spines. Together these new data provide a better understanding of the mechanisms, speed and steps leading to the establishment of a stable excitatory synapse.

Intrinsic patterning and experience-dependent mechanisms that generate eye-specific projections and binocular circuits in the visual pathway

A defining feature of the mammalian nervous system is its complex yet precise circuitry. The mechanisms which underlie the generation of neural connectivity are the topic of intense study in developmental neuroscience. The mammalian visual pathway demonstrates precise retinotopic organization in subcortical and cortical pathways, together with the alignment and matching of eye-specific projections, and sophisticated cortical circuitry that enables the extraction of features underlying vision. New approaches employing molecular-genetic analyses, transgenic mice, novel recombinant probes, and high-resolution imaging are contributing to rapid progress and a new synthesis in the field. These approaches are revealing the ways in which intrinsic patterning mechanisms act in concert with experience-dependent mechanisms to shape visual projections and circuits.

Extracellular matrix in plasticity and epileptogenesis

Extracellular matrix (ECM) in the brain is composed of molecules synthesized and secreted by neurons and glial cells in a cell-type-specific and activity-dependent manner. During development, ECM plays crucial roles in proliferation, migration and differentiation of neural cells. In the mature brain, ECM undergoes a slow turnover and supports multiple physiological processes, while restraining structural plasticity. In the first part of this review, we discuss the contribution of ECM molecules to different forms of plasticity, including developmental plasticity in the cortex, long-term potentiation and depression in the hippocampus, homeostatic scaling of synaptic transmission and metaplasticity. In the second part, we focus on pathological changes associated with epileptogenic mutations in ECM-related molecules or caused by seizure-induced remodeling of ECM. The available data suggest that ECM components regulating physiological plasticity are also engaged in different aspects of epileptogenesis, such as dysregulation of excitatory and inhibitory neurotransmission, sprouting of mossy fibers, granule cell dispersion and gliosis. At the end, we discuss combinatorial approaches that might be used to counteract seizure-induced dysregulation of both ECM molecules and extracellular proteases. By restraining ECM modification and preserving the status quo in the brain, these treatments might prove to be valid therapeutic interventions to antagonize the progression of epileptogenesis.

Watching moving images specifically promotes development of medial area of secondary visual cortex in rat

It is generally accepted that the cortex can be divided into numerous regions depending on the type of information each processes, and that specific input is effective in improving the development of related regions. In visual cortex, many subareas are distinguished on the basis of their adequate information. However, whether the development of a subarea can be specifically improved by its particular input is still largely unknown. Here, we show the specific effects of motion information on the development of the medial area of secondary visual cortex (V2M), a subarea associated with processing the movement component of visual information. Although watching a moving or a still image had similar effects in primary visual cortex, the moving image induced multistage development of V2M in dark-reared rats: both mRNA and protein levels of GluR2 were upregulated, the density and protein content of GluR2-positive synapses increased, and the spine density and the frequency of spontaneous excitatory postsynaptic currents (EPSCs) of pyramidal neurons in Layer 5 were elevated. Our results suggest that rats are able to identify motion information, distribute it to V2M, and then use this input to specifically improve the development of V2M.

Plasminogen activator activity is inhibited while neuroserpin is up-regulated in the Alzheimer disease brain

Amyloid-beta plaques are a pathological hallmark of Alzheimer's disease. Several proteases are known to cleave/remove amyloid-beta, including plasmin, the product of tissue plasminogen activator cleavage of the pro-enzyme plasminogen. Although plasmin levels are lower in Alzheimer brain, there has been little analysis of the plasminogen activator/plasmin system in the brains of Alzheimer patients. In this study, zymography, immunocapture, and ELISAs were utilized to show that tissue plasminogen activator activity in frontal cortex tissue of Alzheimer patients is dramatically reduced compared with age-matched controls, while tissue plasminogen activator and plasminogen protein levels are unchanged; suggesting that plasminogen activator activity is inhibited in the Alzheimer brain. Analysis of endogenous plasminogen activator inhibitors shows that while plasminogen activator inhibitor-1 and protease nexin-1 levels are unchanged, the neuroserpin levels are significantly elevated in brains of Alzheimer patients. Furthermore, elevated amounts of tissue plasminogen activator-neuroserpin complexes are seen in the Alzheimer brain, and immunohistochemical studies demonstrate that both tissue plasminogen activator and neuroserpin are associated with amyloid-beta plaques in Alzheimer brain tissue. Thus, neuroserpin inhibition of tissue plasminogen activator activity leads to reduced plasmin and may be responsible for reduced clearance of amyloid-beta in the Alzheimer disease brain. Furthermore, decreased tissue plasminogen activator activity in the Alzheimer brain may directly influence synaptic activity and impair cognitive function.

Bidirectional synaptic mechanisms of ocular dominance plasticity in visual cortex

As in other mammals with binocular vision, monocular lid suture in mice induces bidirectional plasticity: rapid weakening of responses evoked through the deprived eye followed by delayed strengthening of responses through the open eye. It has been proposed that these bidirectional changes occur through three distinct processes: first, deprived-eye responses rapidly weaken through homosynaptic long-term depression (LTD); second, as the period of deprivation progresses, the modification threshold determining the boundary between synaptic depression and synaptic potentiation becomes lower, favouring potentiation; and third, facilitated by the decreased modification threshold, open-eye responses are strengthened via homosynaptic long-term potentiation (LTP). Of these processes, deprived-eye depression has received the greatest attention, and although several alternative hypotheses are also supported by current research, evidence suggests that α-amino-3- hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor endocytosis through LTD is a key mechanism. The change in modification threshold appears to occur partly through changes in N-methyl-d-aspartate (NMDA) receptor subunit composition, with decreases in the ratio of NR2A to NR2B facilitating potentiation. Although limited research has directly addressed the question of open-eye potentiation, several studies suggest that LTP could account for observed changes in vivo. This review will discuss evidence supporting this three-stage model, along with outstanding issues in the field.

Molecular mechanisms of experience-dependent plasticity in visual cortex

A remarkable amount of our current knowledge of mechanisms underlying experience-dependent plasticity during cortical development comes from study of the mammalian visual cortex. Recent advances in high-resolution cellular imaging, combined with genetic manipulations in mice, novel fluorescent recombinant probes, and large-scale screens of gene expression, have revealed multiple molecular mechanisms that underlie structural and functional plasticity in visual cortex. We situate these mechanisms in the context of a new conceptual framework of feed-forward and feedback regulation for understanding how neurons of the visual cortex reorganize their connections in response to changes in sensory inputs. Such conceptual advances have important implications for understanding not only normal development but also pathological conditions that afflict the central nervous system.

From brain formation to plasticity: insights on Otx2 homeoprotein

The shaping of neuronal circuits is essential during postnatal brain development. A window of neuronal remodeling by sensory experience typically occurs during a unique time in early life. The many types of behavior and perception, like human language, birdsong, hearing and vision are refined by experience during these distinct 'critical periods'. The onset of critical periods for vision is delayed in animals that remain in complete darkness from birth. It is then predicted that a 'messenger' within the visual pathway signals the amount of sensory experience that has occurred. Our recent results indicate that Otx2 homeoprotein, an essential morphogen for embryonic head formation, is reused later in life as this 'messenger' for critical period plasticity. The homeoprotein is stimulated by visual experience to propagate into the visual cortex, where it is internalized by GABAergic interneurons, especially Parvalbumin-positive cells (PV-cells). Otx2 promotes the maturation of PV-cells, consequently activating critical period onset in the visual cortex. Here, we discuss recent data that are beginning to illuminate the physiological function of non-cell autonomous homeoproteins, as well as the restriction of their transfer to PV-cells in vivo.

Tissue plasminogen activator promotes axonal outgrowth on CNS myelin after conditioned injury

Following CNS injury, myelin-associated inhibitors represent major obstacles to axonal regeneration and functional recovery. The following study suggests that the proteolytic enzyme tissue plasminogen activator (tPA) plays a major function in 'conditioning-injury induced' axon regeneration. In this paradigm, prior peripheral nerve injury leads to an enhanced ability of sensory neurons to regenerate their central axons in the presence of the CNS inhibitory microenvironment. tPA is widely expressed by CNS and PNS neurons and plays major roles in synaptic reorganization and plasticity. This study shows that cultured neurons from mice deficient in tPA, in contrast to wild-type mice, fail to undergo conditioning-injury induced axonal regeneration in the presence of purified myelin membranes. Interestingly, neurons from mice deficient in plasminogen, the best known substrate for tPA, showed active axon regeneration. These results suggest a novel plasminogen-independent role for tPA in promoting axonal regeneration on CNS myelin.

Experience leaves a lasting structural trace in cortical circuits

Sensory experiences exert a powerful influence on the function and future performance of neuronal circuits in the mammalian neocortex. Restructuring of synaptic connections is believed to be one mechanism by which cortical circuits store information about the sensory world. Excitatory synaptic structures, such as dendritic spines, are dynamic entities that remain sensitive to alteration of sensory input throughout life. It remains unclear, however, whether structural changes at the level of dendritic spines can outlast the original experience and thereby provide a morphological basis for long-term information storage. Here we follow spine dynamics on apical dendrites of pyramidal neurons in functionally defined regions of adult mouse visual cortex during plasticity of eye-specific responses induced by repeated closure of one eye (monocular deprivation). The first monocular deprivation episode doubled the rate of spine formation, thereby increasing spine density. This effect was specific to layer-5 cells located in binocular cortex, where most neurons increase their responsiveness to the non-deprived eye. Restoring binocular vision returned spine dynamics to baseline levels, but absolute spine density remained elevated and many monocular deprivation-induced spines persisted during this period of functional recovery. However, spine addition did not increase again when the same eye was closed for a second time. This absence of structural plasticity stands out against the robust changes of eye-specific responses that occur even faster after repeated deprivation. Thus, spines added during the first monocular deprivation experience may provide a structural basis for subsequent functional shifts. These results provide a strong link between functional plasticity and specific synaptic rearrangements, revealing a mechanism of how prior experiences could be stored in cortical circuits.

Notch1 signaling in pyramidal neurons regulates synaptic connectivity and experience-dependent modifications of acuity in the visual cortex

How the visual cortex responds to specific stimuli is strongly influenced by visual experience during development. Monocular deprivation, for example, changes the likelihood of neurons in the visual cortex to respond to input from the deprived eye and reduces its visual acuity. Because these functional changes are accompanied by extensive reorganization of neurite morphology and dendritic spine turnover, genes regulating neuronal morphology are likely to be involved in visual plasticity. In recent years, Notch1 has been shown to mediate contact inhibition of neurite outgrowth in postmitotic neurons and implicated in the pathogenesis of various degenerative diseases of the CNS. Here, we provide the first evidence for the involvement of neuronal Notch1 signaling in synaptic morphology and plasticity in the visual cortex. By making use of the Cre/Lox system, we expressed an active form of Notch1 in cortical pyramidal neurons several weeks after birth. We show that neuronal Notch1 signals reduce dendritic spine and filopodia densities in a cell-autonomous manner and limit long-term potentiation in the visual cortex. After monocular deprivation, these effects of Notch1 activity predominantly affect responses to visual stimuli with higher spatial frequencies. This results in an enhanced effect of monocular deprivation on visual acuity.

Endogenous modulators for drug dependence

Drug addiction/dependence is defined as a chronically relapsing disorder that is characterized by compulsive drug taking, inability to limit intake, and intense drug cravings. The positive reinforcing/rewarding effects of drugs primarily depend on the mesocorticolimbic dopamine system innervating the nucleus accumbens while the craving for drugs is associated with activation of the prefrontal cortex. The chronic intake of drugs causes homeostatic molecular and functional changes in synapses, which may be critically associated with the development of drug dependence. Recent studies have demonstrated that various cytokines and proteinases are produced in the brain on treatment with drugs of abuse, and play a role in drug dependence. These endogenous modulators of drug dependence are classified into two groups, pro-addictive and anti-addictive factors. The former including basic fibroblast growth factor, brain-derived neurotrophic factor, tissue plasminogen activator, matrix metalloproteinase (MMP)-2 and MMP-9 act to potentiate the rewarding effects of drugs, while the latter such as tumor necrosis factor-alpha and glial cell line-derived neurotrophic factor reduce the reward. These findings suggest that an imbalance between pro-addictive and anti-addictive factors contributes to the development and relapse of drug dependence. Furthermore, targeting these endogenous modulators would provide new therapeutic approaches to the treatment of drug dependence.

Roles of endocannabinoids in heterosynaptic long-term depression of excitatory synaptic transmission in visual cortex of young mice

Tetanic stimulation of one of two afferent pathways converging to neurons in the visual cortex induces long-term depression (LTD) of synaptic transmission in the other, nonactivated pathway under a certain condition. This form of synaptic plasticity called heterosynaptic LTD (hetero-LTD) was not systematically investigated in previous studies, whereas homosynaptic LTD has been extensively studied. To determine whether hetero-LTD is induced in visual cortical slices of mice and, if so, through what mechanisms, we recorded EPSPs evoked in layer II/III neurons by alternating test stimulation of two sites in layer IV at 0.05 Hz. After theta-burst stimulation of one site, EPSPs evoked by test stimulation of the other site were depressed for a long time in most of the neurons, whereas homosynaptic long-term potentiation was induced at activated synapses. Such a hetero-LTD was induced in most mice at postnatal day 7-20 (P7-P20), but not induced in mice at P35-P41. Tests using the paired-pulse stimulation protocol and coefficient of variation analysis suggested that hetero-LTD was expressed at presynaptic sites. Pharmacological analysis indicated that this form of LTD was induced through activation of the type 5 of metabotropic glutamate receptors, not through the NMDA type of glutamate receptors. Additional analysis using a cannabinoid type 1 receptor agonist and an antagonist suggested that endocannabinoids (eCBs) are involved in this type of LTD. Moreover, results suggest that brain-derived neurotrophic factor, which may be released from strongly activated presynaptic sites, prevents eCBs from suppressing the release of transmitters from these sites.

Plasticity in the adult brain: lessons from the visual system

While cortical circuits display maximal sensitivity to sensory experience during critical periods of early postnatal development, far less plasticity is present in the mature brain. Ocular dominance shift of visual cortical neurons in response to eye occlusion and recovery of visual functions from a period of sensory deprivation are two classical models in the study of critical period determinants in the visual cortex. Recent papers employing various pharmacological and environmental strategies have shown that it is possible to reinstate much greater levels of plasticity in the adult visual cortex than previously suspected. These studies point toward intracortical inhibition as a crucial determinant for critical period regulation in the visual system and have a great potential for therapeutic rehabilitation and recovery from injury in the adult brain.

Coordination of synaptic adhesion with dendritic spine remodeling by AF-6 and kalirin-7

Remodeling of central excitatory synapses is crucial for synapse maturation and plasticity, and contributes to neurodevelopmental and psychiatric disorders. Remodeling of dendritic spines and the associated synapses has been postulated to require the coordination of adhesion with spine morphology and stability; however, the molecular mechanisms that functionally link adhesion molecules with regulators of dendritic spine morphology are mostly unknown. Here, we report that spine size and N-cadherin content are tightly coordinated. In rat mature cortical pyramidal neurons, N-cadherin-dependent adhesion modulates the morphology of existing spines by recruiting the Rac1 guanine-nucleotide exchange factor kalirin-7 to synapses through the scaffolding protein AF-6/afadin. In pyramidal neurons, N-cadherin, AF-6, and kalirin-7 colocalize at synapses and participate in the same multiprotein complexes. N-cadherin clustering promotes the reciprocal interaction and recruitment of N-cadherin, AF-6, and kalirin-7, increasing the content of Rac1 and in spines and PAK (p21-activated kinase) phosphorylation. N-cadherin-dependent spine enlargement requires AF-6 and kalirin-7 function. Conversely, disruption of N-cadherin leads to thin, long spines, with reduced Rac1 contact, caused by uncoupling of N-cadherin, AF-6, and kalirin-7 from each other. By dynamically linking N-cadherin with a regulator of spine plasticity, this pathway allows synaptic adhesion molecules to rapidly coordinate spine remodeling associated with synapse maturation and plasticity. This study hence identifies a novel mechanism whereby cadherins, a major class of synaptic adhesion molecules, signal to the actin cytoskeleton to control the morphology of dendritic spines, and outlines a mechanism that underlies the coordination of synaptic adhesion with spine morphology.

Automated quantification of dendritic spine density and spine head diameter in medium spiny neurons of the nucleus accumbens

Dendritic spines are postsynaptic specializations thought to regulate the strength of synaptic transmission and play a critical role in neuronal plasticity. While changes in dendritic spine density can be pharmacologically- or environmentally-induced, the widespread utility of this important measure of synaptic plasticity in vivo has been hampered by the labor-intensive nature, and potential for bias and inconsistency inherent in manual spine counting. Here we report a method for obtaining high-resolution, three-dimensional confocal images of accumbens spiny neurons labeled with a diolistically delivered lipophilic fluorescence dye (DiI) that permits automated analysis of spine density and spine head diameter. The automated quantification was verified by manual counts of spine density and electron microscopic measures of spine head diameter. The density of spines was relatively constant over 2nd to 4th order dendrites within a neuron, and spine density was normally distributed. The mean spine density (2.68 spines/microm; N = 45 neurons) was higher than previous reports, due in part to analysis in three rather than two dimensions and the capacity of lipophilic dyes to fill very thin spines. The distribution of spine head diameters was continuous and skewed to the right (mean = 0.43 microm; N = 8,891), and approximately 25% of all spines were thin and filopodia-like (< or = 0.20 microm diameter). The density of spines was not correlated with average spine head diameter or with the number of filopodia-like spines. The capacity to rapidly assess spine density and spine head diameter will facilitate quantifying spine plasticity induced by pharmacological and environmental manipulations.

Neurodevelopmental trajectories of the human cerebral cortex

Understanding the organization of the cerebral cortex remains a central focus of neuroscience. Cortical maps have relied almost exclusively on the examination of postmortem tissue to construct structural, architectonic maps. These maps have invariably distinguished between areas with fewer discernable layers, which have a less complex overall pattern of lamination and lack an internal granular layer, and those with more complex laminar architecture. The former includes several agranular limbic areas, and the latter includes the homotypical and granular areas of association and sensory cortex. Here, we relate these traditional maps to developmental data from noninvasive neuroimaging. Changes in cortical thickness were determined in vivo from 764 neuroanatomic magnetic resonance images acquired longitudinally from 375 typically developing children and young adults. We find differing levels of complexity of cortical growth across the cerebrum, which align closely with established architectonic maps. Cortical regions with simple laminar architecture, including most limbic areas, predominantly show simpler growth trajectories. These areas have clearly identified homologues in all mammalian brains and thus likely evolved in early mammals. In contrast, polysensory and high-order association areas of cortex, the most complex areas in terms of their laminar architecture, also have the most complex developmental trajectories. Some of these areas are unique to, or dramatically expanded in primates, lending an evolutionary significance to the findings. Furthermore, by mapping a key characteristic of these development trajectories (the age of attaining peak cortical thickness) we document the dynamic, heterochronous maturation of the cerebral cortex through time lapse sequences ("movies").

Social isolation rearing-induced impairment of the hippocampal neurogenesis is associated with deficits in spatial memory and emotion-related behaviors in juvenile mice

Experiences during brain development may influence the pathogenesis of developmental disorders. Thus, social isolation (SI) rearing after weaning is a useful animal model for studying the pathological mechanisms of such psychiatric diseases. In this study, we examined the effect of SI on neurogenesis in the hippocampal dentate gyrus (DG) relating to memory and emotion-related behaviors. When newly divided cells were labeled with 5-bromo-2'-deoxyuridine (BrdU) before SI, the number of BrdU-positive cells and the rate of differentiation into neurons were significantly decreased after 4-week SI compared with those in group-housed mice. Repeated treatment of fluoxetine prevented the SI-induced impairment of survival of newly divided cells and ameliorated spatial memory impairment and part of aggression in SI mice. Furthermore, we investigated the changes in gene expression in the DG of SI mice by using DNA microarray and real-time PCR. We finally found that SI reduced the expression of development-related genes Nurr1 and Npas4. These findings suggest that communication in juvenile is important in the survival and differentiation of newly divided cells, which may be associated with memory and aggression, and raise the possibility that the reduced expression of Nurr1 and/or Npas4 may contribute to the impairment of neurogenesis and memory and aggression induced by SI.

The EphA4 receptor regulates dendritic spine remodeling by affecting beta1-integrin signaling pathways

Remodeling of dendritic spines is believed to modulate the function of excitatory synapses. We previously reported that the EphA4 receptor tyrosine kinase regulates spine morphology in hippocampal pyramidal neurons, but the signaling pathways involved were not characterized (Murai, K.K., L.N. Nguyen, F. Irie, Y. Yamaguchi, and E.B. Pasquale. 2003. Nat. Neurosci. 6:153–160). In this study, we show that EphA4 activation by ephrin-A3 in hippocampal slices inhibits integrin downstream signaling pathways. EphA4 activation decreases tyrosine phosphorylation of the scaffolding protein Crk-associated substrate (Cas) and the tyrosine kinases focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2 (Pyk2) and also reduces the association of Cas with the Src family kinase Fyn and the adaptor Crk. Consistent with this, EphA4 inhibits β1-integrin activity in neuronal cells. Supporting a functional role for β1 integrin and Cas inactivation downstream of EphA4, the inhibition of integrin or Cas function induces spine morphological changes similar to those associated with EphA4 activation. Furthermore, preventing β1-integrin inactivation blocks the effects of EphA4 on spines. Our results support a model in which EphA4 interferes with integrin signaling pathways that stabilize dendritic spines, thus modulating synaptic interactions with the extracellular environment.

Plasminogen activator induction facilitates recovery of respiratory function following spinal cord injury

The possibility that plasminogen activator (PA) plays a role in synaptic plasticity was explored in the spinal cord during the crossed phrenic phenomenon (CPP), where respiratory functional plasticity develops following spinal cord injury. Synaptic remodeling on phrenic motorneurons occurs during the characteristic delay period following spinal cord injury before CPP recovery of respiratory function. The molecular mechanisms underlying this plasticity are not well-defined. During the critical 1-2 h delay period required for this synaptic plasticity following a C2 hemisection in mice, uPA and tPA mRNAs are rapidly induced in C4-5 ventral spinal cord neurons in the ipsilateral phrenic motor nucleus (PMN), as are uPA and tPA protein levels. A role for uPA in CPP spinal cord plasticity is confirmed by the impaired ability of uPA knockout mice to acquire a good CPP response by 6 h post-hemisection and their lack of structural remodeling of PMN synapses that underlies development of the CPP response.

Homeostatic regulation of eye-specific responses in visual cortex during ocular dominance plasticity

Experience-dependent plasticity is crucial for the precise formation of neuronal connections during development. It is generally thought to depend on Hebbian forms of synaptic plasticity. In addition, neurons possess other, homeostatic means of compensating for changes in sensory input, but their role in cortical plasticity is unclear. We used two-photon calcium imaging to investigate whether homeostatic response regulation contributes to changes of eye-specific responsiveness after monocular deprivation (MD) in mouse visual cortex. Short MD durations decreased deprived-eye responses in neurons with binocular input. Longer MD periods strengthened open-eye responses, and surprisingly, also increased deprived-eye responses in neurons devoid of open-eye input. These bidirectional response adjustments effectively preserved the net visual drive for each neuron. Our finding that deprived-eye responses were either weaker or stronger after MD, depending on the amount of open-eye input a cell received, argues for both Hebbian and homeostatic mechanisms regulating neuronal responsiveness during experience-dependent plasticity.

Visual stimulation activates ERK in synaptic and somatic compartments of rat cortical neurons with parallel kinetics

BACKGROUND

Extracellular signal-regulated kinase (ERK) signalling pathway plays a crucial role in regulating diverse neuronal processes, such as cell proliferation and differentiation, and long-term synaptic plasticity. However, a detailed understanding of the action of ERK in neurons is made difficult by the lack of knowledge about its subcellular localization in response to physiological stimuli. To address this issue, we have studied the effect of visual stimulation in vivo of dark-reared rats on the spatial-temporal dynamics of ERK activation in pyramidal neurons of the visual cortex.

METHODOLOGY/PRINCIPAL FINDINGS

Using immunogold electron microscopy, we show that phosphorylated ERK (pERK) is present in dendritic spines, both at synaptic and non-synaptic plasma membrane domains. Moreover, pERK is also detected in presynaptic axonal boutons forming connections with dendritic spines. Visual stimulation after dark rearing during the critical period causes a rapid increase in the number of pERK-labelled synapses in cortical layers I-II/III. This visually-induced activation of ERK at synaptic sites occurs in pre- and post-synaptic compartments and its temporal profile is identical to that of ERK activation in neuronal cell bodies.

CONCLUSIONS/SIGNIFICANCE

Visual stimulation in vivo increases pERK expression at pre- and post-synaptic sites of axo-spinous junctions, suggesting that ERK plays an important role in the local modulation of synaptic function. The data presented here support a model in which pERK can have early and late actions both centrally in the cell nucleus and peripherally at synaptic contacts.

Possible involvement of protease-activated receptor-1 in the regulation of morphine-induced dopamine release and hyperlocomotion by the tissue plasminogen activator-plasmin system

We have previously demonstrated that tissue plasminogen activator (tPA)-plasmin system participates in the rewarding effect of morphine, by regulating dopamine release in the nucleus accumbens (NAc). However, it is unclear how plasmin increases the morphine-induced release of dopamine and hyperlocomotion. In the present study we investigated whether protease activated receptor-1 (PAR-1) is involved in the regulation of acute morphine-induced dopamine release by the tPA-plasmin system. Morphine significantly but transiently increased extracellular tPA activity in the NAc, which was completely blocked by naloxone. Microinjection of a PAR-1 antagonist, (tyr(-1))-thrombin receptor activating peptide 7, into the NAc significantly reduced morphine-induced dopamine release in the NAc and hyperlocomotion although the treatment had no effect on basal dopamine release and spontaneous locomotor activity. Furthermore, the PAR-1 antagonist blocked the ameliorating effect of plasmin on the defect of morphine-induced dopamine release in the NAc of tPA-deficient mice. In contrast, intracerebroventricular injection of the PAR-1 antagonist had no effect on the antinociceptive effects of morphine in mice. These results suggest that PAR-1 is a target for the tPA-plasmin system in the regulation of acute morphine-induced dopamine release in the NAc.

In vivo roles for matrix metalloproteinase-9 in mature hippocampal synaptic physiology and plasticity

Extracellular proteolysis is an important regulatory nexus for coordinating synaptic functional and structural plasticity, but the identity of such proteases is incompletely understood. Matrix metalloproteinases (MMPs) have well-known, mostly deleterious roles in remodeling after injury or stroke, but their role in nonpathological synaptic plasticity and function in intact adult brains has not been extensively investigated. Here we address the role of MMP-9 in hippocampal synaptic plasticity using both gain- and loss-of-function approaches in urethane-anesthetized adult rats. Acute blockade of MMP-9 proteolytic activity with inhibitors or neutralizing antibodies impairs maintenance, but not induction, of long-term potentiation (LTP) at synapses formed between Schaffer-collaterals and area CA1 dendrites. LTP is associated with significant increases in levels of MMP-9 and proteolytic activity within the potentiated neuropil. By introducing a novel application of gelatin-substrate zymography in vivo, we find that LTP is associated with significantly elevated numbers of gelatinolytic puncta in the potentiated neuropil that codistribute with immunolabeling for MMP-9 and for markers of synapses and dendrites. Such increases in proteolytic activity require NMDA receptor activation. Exposing intact area CA1 neurons to recombinant-active MMP-9 induces a slow synaptic potentiation that mutually occludes, and is occluded by, tetanically evoked potentiation. Taken together, our data reveal novel roles for MMP-mediated proteolysis in regulating nonpathological synaptic function and plasticity in mature hippocampus.

Neurodevelopmental sequelae of postnatal maternal care in rodents: clinical and research implications of molecular insights

Parental care plays an important role in the emotional and cognitive development of the offspring. Children who have been exposed to abuse or neglect are more likely to develop numerous psychopathologies, while good parent-infant bonding is associated with improved resiliency to stress. Similar observations have also been reported in non-human primates and rodents, suggesting that at least some neurodevelopmental aspects of parent-offspring interactions are conserved among mammals and could therefore be studied in animals. We present data to suggest that frequency of licking and grooming provided by the dam during a critical period in development plays an important role in modifying neurodevelopment. These findings are examined in the broader context in which exposure to other sensory modalities such as vision or hearing during a specific period in development shapes brain development with functional consequences that persist into adulthood. We also discuss recent rodent work showing that increased frequency of licking and grooming provided by the dam during the first week of life is associated with changes in DNA methylation of promoter elements that control expression of these genes and behavior. The stability of DNA methylation in postmitotic cells provides a possible molecular scaffold by which changes in gene expression and behavioral traits induced by postnatal maternal care are maintained throughout life. Finally, the relevance of findings reported in rodents to those noted in non-human primates and humans are assessed and the research and clinical implications of these observations for future work are explored.

Early pharmacological treatment of autism: a rationale for developmental treatment

Autism is a dynamic neurodevelopmental syndrome in which disabilities emerge during the first three postnatal years and continue to evolve with ongoing development. We briefly review research in autism describing subtle changes in molecules important in brain development and neurotransmission, in morphology of specific neurons, brain connections, and in brain size. We then provide a general schema of how these processes may interact with particular emphasis on neurotransmission. In this context, we present a rationale for utilizing pharmacologic treatments aimed at modifying key neurodevelopmental processes in young children with autism. Early treatment with selective serotonin reuptake inhibitors (SSRIs) is presented as a model for pharmacologic interventions because there is evidence in autistic children for reduced brain serotonin synthesis during periods of peak synaptogenesis; serotonin is known to enhance synapse refinement; and exploratory studies with these agents in autistic children exist. Additional hypothetical developmental interventions and relevant published clinical data are described. Finally, we discuss the importance of exploring early pharmacologic interventions within multiple experimental settings in order to develop effective treatments as quickly as possible while minimizing risks.

A Critical Role for System A Amino Acid Transport in the Regulation of Dendritic Development by Brain-derived Neurotrophic Factor (BDNF)

Dendritic development is essential for the establishment of a functional nervous system. Among factors that control dendritic development, brain-derived neurotrophic factor (BDNF) has been shown to regulate dendritic length and complexity of cortical neurons. However, the cellular and molecular mechanisms that underlie these effects remain poorly understood. In this study, we examined the role of amino acid transport in mediating the effects of BDNF on dendritic development. We show that BDNF increases System A amino acid transport in cortical neurons by selective up-regulation of the sodium-coupled neutral amino acid transporter (SNAT)1. Up-regulation of SNAT1 expression and System A activity is required for the effects of BDNF on dendritic growth and branching of cortical neurons. Further analysis revealed that induction of SNAT1 expression and System A activity by BDNF is necessary in particular to enhance synthesis of tissue-type plasminogen activator, a protein that we demonstrate to be essential for the effects of BDNF on cortical dendritic morphology. Together, these data reveal that stimulation of neuronal differentiation by BDNF requires the up-regulation of SNAT1 expression and System A amino acid transport to meet the increased metabolic demand associated with the enhancement of dendritic growth and branching.

Experience-dependent orientation plasticity in the visual cortex of rats chronically exposed to a single orientation

We used the intrinsic signal optical imaging technique to assess the effect of orientation-restricted visual experience on response properties of the rat visual cortex. We placed young animals wearing goggles fitted with plano-convex cylindrical lenses in a stimulus-enriched environment for 3 weeks. Experienced orientation was over-represented in the visual cortex, which was associated with the under-representation of orthogonal orientation. These findings suggest that chronic exposure to a single orientation can modify orientation preferences even in rats lacking in orderly arrangement of preferred orientations.

Local cortical circuit model inferred from power-law distributed neuronal avalanches

How cortical neurons process information crucially depends on how their local circuits are organized. Spontaneous synchronous neuronal activity propagating through neocortical slices displays highly diverse, yet repeatable, activity patterns called "neuronal avalanches". They obey power-law distributions of the event sizes and lifetimes, presumably reflecting the structure of local circuits developed in slice cultures. However, the explicit network structure underlying the power-law statistics remains unclear. Here, we present a neuronal network model of pyramidal and inhibitory neurons that enables stable propagation of avalanche-like spiking activity. We demonstrate a neuronal wiring rule that governs the formation of mutually overlapping cell assemblies during the development of this network. The resultant network comprises a mixture of feedforward chains and recurrent circuits, in which neuronal avalanches are stable if the former structure is predominant. Interestingly, the recurrent synaptic connections formed by this wiring rule limit the number of cell assemblies embeddable in a neuron pool of given size. We investigate how the resultant power laws depend on the details of the cell-assembly formation as well as on the inhibitory feedback. Our model suggests that local cortical circuits may have a more complex topological design than has previously been thought.

Factors critical for the plasticity of dendritic spines and memory storage

The structure of dendritic spines is highly plastic and responds to synaptic activity, including activity patterns that induce long-term potentiation (LTP) and depression (LTD). Induction of LTP causes enlargement of spine heads, while LTD causes spine head shrinkage. In addition, spine structure is well associated with synaptic weight and the extent of synaptic plasticity, such that structural changes of the spine may represent forms of memory storage. While the correlation between structural and functional plasticity appears to be simple, the underlying mechanisms of spine plasticity are intricate. Spine plasticity requires multiple molecular interactions, and is affected by the surrounding environment and by cellular metabolic state. Here, I synthesize the latest progress in this field by defining six determinants of spine plasticity, and discuss the role of each factor in memory storage.

The rewards of nicotine: regulation by tissue plasminogen activator-plasmin system through protease activated receptor-1

Nicotine, a primary component of tobacco, is one of the most abused drugs worldwide. Approximately four million people die each year because of diseases associated with tobacco smoking. Mesolimbic dopaminergic neurons mediate the rewarding effects of abused drugs, including nicotine. Here we show that the tissue plasminogen activator (tPA)-plasmin system regulates nicotine-induced reward and dopamine release by activating protease activated receptor-1 (PAR1). In vivo microdialysis revealed that microinjection of either tPA or plasmin into the nucleus accumbens (NAc) significantly potentiated whereas plasminogen activator inhibitor-1 reduced the nicotine-induced dopamine release in the NAc in a dose-dependent manner. Nicotine-induced dopamine release was markedly diminished in tPA-deficient (tPA-/-) mice, and the defect of dopamine release in tPA-/- mice was restored by microinjection of either exogenous tPA or plasmin into the NAc. Nicotine increased tPA protein levels and promoted the release of tPA into the extracellular space in the NAc. Immunohistochemistry revealed that PAR1 immunoreactivity was localized to the nerve terminals positive for tyrosine hydroxylase in the NAc. Furthermore, we demonstrated that plasmin activated PAR1 and that nicotine-induced place preference and dopamine release were diminished in PAR1-deficient (PAR1-/-) mice. Targeting the tPA-plasmin-PAR1 system would provide new therapeutic approaches to the treatment of nicotine dependence.

Enduring critical period plasticity visualized by transcranial flavoprotein imaging in mouse primary visual cortex

Experience-dependent plasticity in the visual cortex was investigated using transcranial flavoprotein fluorescence imaging in mice anesthetized with urethane. On- and off-responses in the primary visual cortex were elicited by visual stimuli. Fluorescence responses and field potentials elicited by grating patterns decreased similarly as contrasts of visual stimuli were reduced. Fluorescence responses also decreased as spatial frequency of grating stimuli increased. Compared with intrinsic signal imaging in the same mice, fluorescence imaging showed faster responses with approximately 10 times larger signal changes. Retinotopic maps in the primary visual cortex and area LM were constructed using fluorescence imaging. After monocular deprivation (MD) of 4 d starting from postnatal day 28 (P28), deprived eye responses were suppressed compared with nondeprived eye responses in the binocular zone but not in the monocular zone. Imaging faithfully recapitulated a critical period for plasticity with maximal effects of MD observed around P28 and not in adulthood even under urethane anesthesia. Visual responses were compared before and after MD in the same mice, in which the skull was covered with clear acrylic dental resin. Deprived eye responses decreased after MD, whereas nondeprived eye responses increased. Effects of MD during a critical period were tested 2 weeks after reopening of the deprived eye. Significant ocular dominance plasticity was observed in responses elicited by moving grating patterns, but no long-lasting effect was found in visual responses elicited by light-emitting diode light stimuli. The present results indicate that transcranial flavoprotein fluorescence imaging is a powerful tool for investigating experience-dependent plasticity in the mouse visual cortex.

Experience-Dependent Plasticity in Adult Visual Cortex

Experience-dependent plasticity is a prominent feature of the mammalian visual cortex. Although such neural changes are most evident during development, adult cortical circuits can be modified by a variety of manipulations, such as perceptual learning and visual deprivation. Elucidating the underlying mechanisms at the cellular and synaptic levels is an essential step in understanding neural plasticity in the mature animal. Although developmental and adult plasticity share many common features, notable differences may be attributed to developmental cortical changes at multiple levels. These range from shifts in the molecular profiles of cortical neurons to changes in the spatiotemporal dynamics of network activity. In this review, we will discuss recent progress and remaining challenges in understanding adult visual plasticity, focusing on the primary visual cortex.

Visual cortex: overcoming a no-go for plasticity

Normally, the brain can be shaped by sensory experience only during a so-called critical period early in life. Recent research has shed light on the factors determining the end of the critical period, and on how cortical plasticity might be re-established in adulthood.

Stress-induced spine loss in the medial amygdala is mediated by tissue-plasminogen activator

The amygdala, which exerts a regulatory influence on the stress response, is itself affected by stress. It has been reported that the serine protease tissue-plasminogen activator (tPA), a key mediator of spine plasticity, is required for stress-induced facilitation of anxiety-like behavior. Importantly, tPA is also involved in stress-induced activation of molecular signals that have the potential to contribute to neuronal remodeling in the medial amygdala (MeA). However, little is known about the precise nature of, and specific role played by tPA in, stress-induced structural plasticity in the MeA. Hence, we compared the impact of chronic restraint stress on spine density of medium spiny stellate neurons in MeA in wild-type mice with mice in which the tPA gene is disrupted (tPA-/-). In wild-type mice, chronic stress caused significant reduction in MeA spine density, which was in contrast to enhanced spine density in the neighboring basolateral amygdala (BLA). Strikingly, tPA-/- mice exhibited significant attenuation of stress-induced spine retraction in the MeA, but BLA spinogenesis was not affected. Therefore, tPA-dependence of stress-induced modulation in spine density was restricted to the MeA. Further, MeA neurons in tPA-/- mice, even when challenged with repeated stress, were able to maintain levels of spine density that were comparable to that of wild-type mice without stress. Our findings provide novel evidence for a permissive role for tPA in amygdalar spine plasticity elicited by behavioral stress.

Lifelong learning: ocular dominance plasticity in mouse visual cortex

Ocular dominance plasticity has long served as a successful model for examining how cortical circuits are shaped by experience. In this paradigm, altered retinal activity caused by unilateral eye-lid closure leads to dramatic shifts in the binocular response properties of neurons in the visual cortex. Much of the recent progress in identifying the cellular and molecular mechanisms underlying ocular dominance plasticity has been achieved by using the mouse as a model system. In this species, monocular deprivation initiated in adulthood also causes robust ocular dominance shifts. Research on ocular dominance plasticity in the mouse is starting to provide insight into which factors mediate and influence cortical plasticity in juvenile and adult animals.

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.

Structural and functional recovery from early monocular deprivation in adult rats

Visual deficits caused by abnormal visual experience during development are hard to recover in adult animals. Removal of chondroitin sulfate proteoglycans from the mature extracellular matrix with chondroitinase ABC promotes plasticity in the adult visual cortex. We tested whether chondroitinase ABC treatment of adult rats facilitates anatomical, functional, and behavioral recovery from the effects of a period of monocular deprivation initiated during the critical period for monocular deprivation. We found that chondroitinase ABC treatment coupled with reverse lid-suturing causes a complete recovery of ocular dominance, visual acuity, and dendritic spine density in adult rats. Thus, manipulations of the extracellular matrix can be used to promote functional recovery in the adult cortex.

Matrix metalloproteinase-9 is required for hippocampal late-phase long-term potentiation and memory

Matrix metalloproteinases (MMPs) are extracellular proteases that have well recognized roles in cell signaling and remodeling in many tissues. In the brain, their activation and function are customarily associated with injury or pathology. Here, we demonstrate a novel role for MMP-9 in hippocampal synaptic physiology, plasticity, and memory. MMP-9 protein levels and proteolytic activity are rapidly increased by stimuli that induce late-phase long-term potentiation (L-LTP) in area CA1. Such regulation requires NMDA receptors and protein synthesis. Blockade of MMP-9 pharmacologically prevents induction of L-LTP selectively; MMP-9 plays no role in, nor is regulated during, other forms of short-term synaptic potentiation or long-lasting synaptic depression. Similarly, in slices from MMP-9 null-mutant mice, hippocampal LTP, but not long-term depression, is impaired in magnitude and duration; adding recombinant active MMP-9 to null-mutant slices restores the magnitude and duration of LTP to wild-type levels. Activated MMP-9 localizes in part to synapses and modulates hippocampal synaptic physiology through integrin receptors, because integrin function-blocking reagents prevent an MMP-9-mediated potentiation of synaptic signal strength. The fundamental importance of MMP-9 function in modulating hippocampal synaptic physiology and plasticity is underscored by behavioral impairments in hippocampal-dependent memory displayed by MMP-9 null-mutant mice. Together, these data reveal new functions for MMPs in synaptic and behavioral plasticity.