Afferent innervation influences the development of dendritic branches and spines via both activity-dependent and non-activity-dependent mechanisms

Development of artificial synapse organizers liganded with a peptide tag for molecularly inducible neuron-microelectrode interface

It has been proposed that cell-type-specific bioelectronic interfaces for neuronal circuits could be established by utilizing the function of synapse organizers. For this purpose, using neurexin-1β and a peptide tag, we engineered compact synapse organizers that do not interact with the naturally occurring receptors but induce presynaptic differentiation upon contact with nanobody-decorated objects in cultured mammalian and chick forebrain neurons. In chick neurons, the engineered organizer exerted synaptogenesis typically in ∼4 h after the contact, even under an air atmosphere at room temperature, thereby providing a useful cellular model for establishing the molecularly inducible neuron-microelectrode interface.

Chronic and Acute Manipulation of Cortical Glutamate Transmission Induces Structural and Synaptic Changes in Co-cultured Striatal Neurons

In contrast to the prenatal topographic development of sensory cortices, striatal circuit organization is slow and requires the functional maturation of cortical and thalamic excitatory inputs throughout the first postnatal month. While mechanisms regulating synapse development and plasticity are quite well described at excitatory synapses of glutamatergic neurons in the neocortex, comparatively little is known of how this translates to glutamate synapses onto GABAergic neurons in the striatum. Here we investigate excitatory striatal synapse plasticity in an in vitro system, where glutamate can be studied in isolation from dopamine and other neuromodulators. We examined pre-and post-synaptic structural and functional plasticity in GABAergic striatal spiny projection neurons (SPNs), co-cultured with glutamatergic cortical neurons. After synapse formation, medium-term (24 h) TTX silencing increased the density of filopodia, and modestly decreased dendritic spine density, when assayed at 21 days in vitro (DIV). Spine reductions appeared to require residual spontaneous activation of ionotropic glutamate receptors. Conversely, chronic (14 days) TTX silencing markedly reduced spine density without any observed increase in filopodia density. Time-dependent, biphasic changes to the presynaptic marker Synapsin-1 were also observed, independent of residual spontaneous activity. Acute silencing (3 h) did not affect presynaptic markers or postsynaptic structures. To induce rapid, activity-dependent plasticity in striatal neurons, a chemical NMDA receptor-dependent “long-term potentiation (LTP)” paradigm was employed. Within 30 min, this increased spine and GluA1 cluster densities, and the percentage of spines containing GluA1 clusters, without altering the presynaptic signal. The results demonstrate that the growth and pruning of dendritic protrusions is an active process, requiring glutamate receptor activity in striatal projection neurons. Furthermore, NMDA receptor activation is sufficient to drive glutamatergic structural plasticity in SPNs, in the absence of dopamine or other neuromodulators.

Mitotically heritable effects of BMAA on striatal neural stem cell proliferation and differentiation

The widespread environmental contaminant β-methylamino-L-alanine (BMAA) is a developmental neurotoxicant that can induce long-term learning and memory deficits. Studies have shown high transplacental transfer of 3H-BMAA and a significant uptake in fetal brain. Therefore, more information on how BMAA may influence growth and differentiation of neural stem cells is required for assessment of the risk to the developing brain. The aim of this study was to investigate direct and mitotically inherited effects of BMAA exposure using primary striatal neurons and embryonic neural stem cells. The neural stem cells were shown to be clearly more susceptible to BMAA exposure than primary neurons. Exposure to 250 µM BMAA reduced neural stem cell proliferation through apoptosis and G2/M arrest. At lower concentrations (50–100 µM), not affecting cell proliferation, BMAA reduced the differentiation of neural stem cells into astrocytes, oligodendrocytes, and neurons through glutamatergic mechanisms. Neurons that were derived from the BMAA-treated neuronal stem cells demonstrated morphological alterations including reduced neurite length, and decreased number of processes and branches per cell. Interestingly, the BMAA-induced changes were mitotically heritable to daughter cells. The results suggest that early-life exposure to BMAA impairs neuronal stem cell programming, which is vital for development of the nervous system and may result in long-term consequences predisposing for both neurodevelopmental disorders and neurodegenerative disease later in life. More attention should be given to the potential adverse effects of BMAA exposure on brain development.

Inhibition of RhoA reduces propofol-mediated growth cone collapse, axonal transport impairment, loss of synaptic connectivity, and behavioural deficits

BACKGROUND

Exposure of the developing brain to propofol results in cognitive deficits. Recent data suggest that inhibition of neuronal apoptosis does not prevent cognitive defects, suggesting mechanisms other than neuronal apoptosis play a role in anaesthetic neurotoxicity. Proper neuronal growth during development is dependent upon growth cone morphology and axonal transport. Propofol modulates actin dynamics in developing neurones, causes RhoA-dependent depolymerisation of actin, and reduces dendritic spines and synapses. We hypothesised that RhoA inhibition prevents synaptic loss and subsequent cognitive deficits. The present study tested whether RhoA inhibition with the botulinum toxin C3 (TAT-C3) prevents propofol-induced synapse and neurite loss, and preserves cognitive function.

METHODS

RhoA activation, growth cone morphology, and axonal transport were measured in neonatal rat neurones (5-7 days in vitro) exposed to propofol. Synapse counts (electron microscopy), dendritic arborisation (Golgi-Cox), and network connectivity were measured in mice (age 28 days) previously exposed to propofol at postnatal day 5-7. Memory was assessed in adult mice (age 3 months) previously exposed to propofol at postnatal day 5-7.

RESULTS

Propofol increased RhoA activation, collapsed growth cones, and impaired retrograde axonal transport of quantum dot-labelled brain-derived neurotrophic factor, all of which were prevented with TAT-C3. Adult mice previously treated with propofol had decreased numbers of total hippocampal synapses and presynaptic vesicles, reduced hippocampal dendritic arborisation, and infrapyramidal mossy fibres. These mice also exhibited decreased hippocampal-dependent contextual fear memory recall. All anatomical and behavioural changes were prevented with TAT-C3 pre-treatment.

CONCLUSION

Inhibition of RhoA prevents propofol-mediated hippocampal neurotoxicity and associated cognitive deficits.

TDP-43 misexpression causes defects in dendritic growth

Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD) share overlapping genetic causes and disease symptoms, and are linked neuropathologically by the RNA binding protein TDP-43 (TAR DNA binding protein-43 kDa). TDP-43 regulates RNA metabolism, trafficking, and localization of thousands of target genes. However, the cellular and molecular mechanisms by which dysfunction of TDP-43 contributes to disease pathogenesis and progression remain unclear. Severe changes in the structure of neuronal dendritic arbors disrupt proper circuit connectivity, which in turn could contribute to neurodegenerative disease. Although aberrant dendritic morphology has been reported in non-TDP-43 mouse models of ALS and in human ALS patients, this phenotype is largely unexplored with regards to TDP-43. Here we have employed a primary rodent neuronal culture model to study the cellular effects of TDP-43 dysfunction in hippocampal and cortical neurons. We show that manipulation of TDP-43 expression levels causes significant defects in dendritic branching and outgrowth, without an immediate effect on cell viability. The effect on dendritic morphology is dependent on the RNA-binding ability of TDP-43. Thus, this model system will be useful in identifying pathways downstream of TDP-43 that mediate dendritic arborization, which may provide potential new avenues for therapeutic intervention in ALS/FTD.

Mechanisms of Hierarchical Cortical Maturation

Cortical information processing is structurally and functionally organized into hierarchical pathways, with primary sensory cortical regions providing modality specific information and associative cortical regions playing a more integrative role. Historically, there has been debate as to whether primary cortical regions mature earlier than associative cortical regions, or whether both primary and associative cortical regions mature simultaneously. Identifying whether primary and associative cortical regions mature hierarchically or simultaneously will not only deepen our understanding of the mechanisms that regulate brain maturation, but it will also provide fundamental insight into aspects of adolescent behavior, learning, neurodevelopmental disorders and computational models of neural processing. This mini-review article summarizes the current evidence supporting the sequential and hierarchical nature of cortical maturation, and then proposes a new cellular model underlying this process. Finally, unresolved issues associated with hierarchical cortical maturation are also addressed.

Expansion of mossy fibers and CA3 apical dendritic length accompanies the fall in dendritic spine density after gonadectomy in male, but not female, rats

Androgen loss is an important clinical concern because of its cognitive and behavioral effects. Changes in androgen levels are also suspected to contribute to neurological disease. However, the available data on the effects of androgen deprivation in areas of the brain that are central to cognition, like the hippocampus, are mixed. In this study, morphological analysis of pyramidal cells was used to investigate if structural changes could potentially contribute to the mixed cognitive effects that have been observed after androgen loss in males. Male Sprague-Dawley rats were orchidectomized or sham-operated. Two months later, their brains were Golgi-impregnated for morphological analysis. Morphological endpoints were studied in areas CA3 and CA1, with comparisons to females either intact or 2 months after ovariectomy. CA3 pyramidal neurons of orchidectomized rats exhibited marked increases in apical dendritic arborization. There were increases in mossy fiber afferent density in area CA3, as well as robust enhancements to dendritic structure in area CA3 of orchidectomized males, but not in CA1. Remarkably, apical dendritic length of CA3 pyramidal cells increased, while spine density declined. By contrast, in females overall dendritic structure was minimally affected by ovariectomy, while dendritic spine density was greatly reduced. Sex differences and subfield-specific effects of gonadal hormone deprivation on the hippocampal circuitry may help explain the different behavioral effects reported in males and females after gonadectomy, or other conditions associated with declining gonadal hormone secretion.

Synapse Loss and Regeneration: A Mechanism for Functional Decline and Recovery after Cerebral Ischemia?

Little is known of the mechanisms governing functional recovery after ischemic brain injury, and there is no clinical therapy established to restore neurologic function after ischemic injury is complete. Even so, pronounced spontaneous recovery of function is often observed in a subset of patients. Resolution of neurological deficits after ischemia must occur through replacement of lost tissue via production of new neurons, or through changes in the structure, function, or connectivity of surviving neurons. This review focuses on the neuronal synapse as a potential locus for functional recovery. Selective disruption of synaptic elements is a characteristic feature of hypoxic-ischemic brain injury, such as that seen in ischemic stroke or cardiac arrest. Ischemic damage to synapses occurs even in the absence of neuronal loss, and therefore might underlie the clinical disability observed in patients following mild or transient ischemia. We review evidence that recovery of lost synapses occurs after ischemic injury and that this recovery may be a necessary step for restoration of neurological function. The process of synapse loss and recovery can be examined in neuronal cultures and experimental stroke models. Such studies may help to gain a better understanding of the extracellular factors and intracellular cascades that facilitate recovery of synapses, and may result in therapeutic approaches to improve function after cerebral ischemia.

Somato-dendritic decoupling as a novel mechanism for protracted cortical maturation

BACKGROUND

Both human and animal data indicate that disruption of the endogenously slow maturation of temporal association cortical (TeA) networks is associated with abnormal higher order cognitive development. However, the neuronal mechanisms underlying the endogenous maturation delay of the TeA are poorly understood.

RESULTS

Here we report a novel form of developmental plasticity that is present in the TeA. It was found that deep layer TeA neurons, but not hippocampal or primary visual neurons, exist in a protracted 'embryonic-like' state through a mechanism involving reduced somato-dendritic communication and a non-excitable somatic membrane. This mechanism of neural inactivity is present in intact tissue and shows a remarkable transition into an active somato-dendritically coupled state. The quantity of decoupled cells diminishes in a protracted and age-dependent manner, continuing into adolescence.

CONCLUSIONS

Based on our data, we propose a model of neural plasticity through which protracted compartmentalization and decoupling in somato-dendritic signalling plays a key role in controlling how excitable neurons are incorporated into recurrent cortical networks independent of neurogenesis.

Neuronal connectivity as a convergent target of gene × environment interactions that confer risk for Autism Spectrum Disorders

Evidence implicates environmental factors in the pathogenesis of Autism Spectrum Disorders (ASD). However, the identity of specific environmental chemicals that influence ASD risk, severity or treatment outcome remains elusive. The impact of any given environmental exposure likely varies across a population according to individual genetic substrates, and this increases the difficulty of identifying clear associations between exposure and ASD diagnoses. Heritable genetic vulnerabilities may amplify adverse effects triggered by environmental exposures if genetic and environmental factors converge to dysregulate the same signaling systems at critical times of development. Thus, one strategy for identifying environmental risk factors for ASD is to screen for environmental factors that modulate the same signaling pathways as ASD susceptibility genes. Recent advances in defining the molecular and cellular pathology of ASD point to altered patterns of neuronal connectivity in the developing brain as the neurobiological basis of these disorders. Studies of syndromic ASD and rare highly penetrant mutations or CNVs in ASD suggest that ASD risk genes converge on several major signaling pathways linked to altered neuronal connectivity in the developing brain. This review briefly summarizes the evidence implicating dysfunctional signaling via Ca(2+)-dependent mechanisms, extracellular signal-regulated kinases (ERK)/phosphatidylinositol-3-kinases (PI3K) and neuroligin-neurexin-SHANK as convergent molecular mechanisms in ASD, and then discusses examples of environmental chemicals for which there is emerging evidence of their potential to interfere with normal neuronal connectivity via perturbation of these signaling pathways.

The Ca2+ and Rho GTPase signaling pathways underlying activity-dependent actin remodeling at dendritic spines

Most excitatory synapses reside on small protrusions located on the dendritic shaft of neurons called dendritic spines. Neuronal activity regulates the number and structure of spines in both developing and mature brains. Such morphological changes are mediated by the modification of the actin cytoskeleton, the major structural component of spines. Because the number and size of spines is tightly correlated with the strength of synaptic transmission, the activity-dependent structural remodeling of a spine plays an important role in the modulation of synaptic transmission. The regulation of spine morphogenesis utilizes multiple intracellular signaling pathways that alter the dynamics of actin remodeling. Here, we will review recent studies examining the signaling pathways underlying activity-dependent actin remodeling at excitatory postsynaptic neurons.

Persistent region-dependent neuroinflammation, NMDA receptor loss and atrophy in an animal model of penetrating brain injury

Dynamic changes in neuroinflammation and glutamate NMDA receptors (NMDAR) have been noted in traumatic and ischemic brain injury.

AIM

Here we investigate the time course and regional distribution of these changes and their relationship with atrophy in a rat model of penetrating brain injury.

MATERIALS METHODS

Quantitative autoradiography, with the neuroinflammation marker [³H]PK11195 and the NMDAR antagonist [¹²⁵I]iodoMK801, was performed on brains of animals subjected to a unilateral wireknife injury at the level of striatum and killed 3 - 60 days later. Regional atrophy was measured by morphometry.

RESULTS

The injury produced large increases in [³H]PK11195 binding density in cortical and septal regions adjacent to the knife track by day 7, with modest increases in the striatum. [¹²⁵I]iodoMK801 binding was reduced in cor tical and hippocampal regions showing marked neuroinflammation, which showed marked atrophy at subsequent time points.

CONCLUSION

These results indicate that neuroinflammaton and loss of NMDAR precede and predict tissue atrophy in cortical and hippocampal regions.

Dendritic structural plasticity

Dendrites represent the compartment of neurons primarily devoted to collecting and computating input. Far from being static structures, dendrites are highly dynamic during development and appear to be capable of plastic changes during the adult life of animals. During development, it is a combination of intrinsic programs and external signals that shapes dendrite morphology; input activity is a conserved extrinsic factor involved in this process. In adult life, dendrites respond with more modest modifications of their structure to various types of extrinsic information, including alterations of input activity. Here, the author reviews classical and recent evidence of dendrite plasticity in invertebrates and vertebrates and current progress in the understanding of the molecular mechanisms that underlie this plasticity. Importantly, some fundamental questions such as the functional role of dendrite remodeling and the causal link between structural modifications of neurons and plastic processes, including learning, are still open.

Facilitation of neocortical presynaptic terminal development by NMDA receptor activation

Background

Neocortical circuits are established through the formation of synapses between cortical neurons, but the molecular mechanisms of synapse formation are only beginning to be understood. The mechanisms that control synaptic vesicle (SV) and active zone (AZ) protein assembly at developing presynaptic terminals have not yet been defined. Similarly, the role of glutamate receptor activation in control of presynaptic development remains unclear.

Results

Here, we use confocal imaging to demonstrate that NMDA receptor (NMDAR) activation regulates accumulation of multiple SV and AZ proteins at nascent presynaptic terminals of visual cortical neurons. NMDAR-dependent regulation of presynaptic assembly occurs even at synapses that lack postsynaptic NMDARs. We also provide evidence that this control of presynaptic terminal development is independent of glia.

Conclusions

Based on these data, we propose a novel NMDAR-dependent mechanism for control of presynaptic terminal development in excitatory neocortical neurons. Control of presynaptic development by NMDARs could ultimately contribute to activity-dependent development of cortical receptive fields.

A new look at glutamate and ischemia: NMDA agonist improves long-term functional outcome in a rat model of stroke

Ischemic stroke triggers a massive, although transient, glutamate efflux and excessive activation of NMDA receptors (NMDARs), possibly leading to neuronal death. However, multiple clinical trials with NMDA antagonists failed to improve, or even worsened, stroke outcome. Recent findings of a persistent post-stroke decline in NMDAR density, which plays a pivotal role in plasticity and memory formation, suggest that NMDAR stimulation, rather than inhibition, may prove beneficial in the subacute period after stroke. AIM: This study aims to examine the effect of the NMDAR partial agonist d-cycloserine (DCS) on long-term structural, functional and behavioral outcomes in rats subjected to transient middle cerebral artery occlusion, an animal model of ischemic stroke. MATERIALS #ENTITYSTARTX00026; METHODS: Rats (n = 36) that were subjected to 90 min of middle cerebral artery occlusion were given a single injection of DCS (10 mg/kg) or vehicle (phosphate-buffered saline) 24 h after occlusion and followed up for 30 days. MRI (structural and functional) was used to measure infarction, atrophy and cortical activation due to electrical forepaw stimulation. Memory function was assessed on days 7, 21 and 30 postocclusion using the novel object recognition test. A total of 20 nonischemic controls were included for comparison. RESULTS: DCS treatment resulted in significant improvement of somatosensory and cognitive function relative to vehicle treatment. By day 30, cognitive performance of the DCS-treated animals was indistinguishable from nonischemic controls, while vehicle-treated animals demonstrated a stable memory deficit. DCS had no significant effect on infarction or atrophy. CONCLUSION: These results support a beneficial role for NMDAR stimulation during the recovery period after stroke, most likely due to enhanced neuroplasticity rather than neuroprotection.

From the periphery to the brain: Wiring the olfactory system

The olfactory system represents a perfect model to study the interactions between the central and peripheral nervous systems in order to establish a neural circuit during early embryonic development. In addition, another important feature of this system is the capability to integrate new cells generated in two neurogenic zones: the olfactory epithelium in the periphery and the wall of the lateral ventricles in the CNS, both during development and adulthood. In all these processes the combination and sequence of specific molecular signals plays a critical role in the wiring of the olfactory axons, as well as the precise location of the incoming cell populations to the olfactory bulb. The purpose of this review is to summarize recent insights into the cellular and molecular events that dictate cell settling position and axonal trajectories from their origin in the olfactory placode to the formation of synapses in the olfactory bulb to ensure rapid and reliable transmission of olfactory information from the nose to the brain.

Contacts among non-sister dendritic branches at bifurcations shape neighboring dendrites and pattern their synaptic inputs

The size and shape of neuronal dendritic arbors affect the number and pattern of synaptic inputs, as well as the complexity and function of brain circuits. However, the means by which different dendritic arbors take their final shape and how these shapes are associated with distinct synaptic patterns is still largely unknown. Dendritic ramification is influenced by dendrite-dendrite interactions that stabilize specific branching directions and ensure appropriate synaptic contacts. Yet, it is not clear by which mechanism these contacts are allocated. We found that stable dendro-dendritic contacts occur preferentially between non-sister dendritic branches at sites of bifurcations, and that this process is promoted by synaptic activity. Moreover, these contacts are associated with synaptic connections of higher density, higher level of synaptophysin, NR1, GluR2 subunits of glutamate receptors and elevated secretion capability than synaptic connections found on contacts made by non-bifurcating branches or along non-contacting parts of the dendrites. Thus, in cultured neurons, stabilization of hetero-neuronal dendro-dendritic contacts at bifurcations is a new mean to pattern and associate morphogenesis and synaptic input distribution in neighboring dendritic trees.

Evidence of altered calmodulin immunoreactivity in areas 9 and 32 of schizophrenic prefrontal cortex

Schizophrenia is a severe neuropsychiatric disorder. Previous studies have implicated the prefrontal cortex (PFC) [Harrison PJ. The neuropathology of schizophrenia a critical review of the data and their interpretation. Brain 1999;122:593-624; Jones LB. Recent cytoarchitectonic changes in the prefrontal cortex of schizophrenics. Frontiers of Bioscience 2001;6:E148-53]. Recent immunocytochemical studies have shown a dramatic decrease in MAP2 and neurogranin [Jones L, Johnson N, Byne W. Alterations in MAP2 staining in area 9 and 32 of schizophrenic prefrontal cortex. Psychiatry Research 2002;114:137-48; Broadbelt K, Pamprasaud A, Jones LB. Evidence of altered neurogranin immunoreactivity in areas 9 and 32 of schizophrenic prefrontal cortex. Schizophrenia Research 2006;87:6-14] a loss of either is suggestive of dendritic lesions [Li GL, Farooque M, Lewen A., Lennmyr F, Holtz A., Olsson Y. MAP2 and neurogranin as markers for dendritic lesions in cns injury an immunohistochemical study in the rat. APMIS 2002;108:98-106.]. Neurogranin is an upstream regulator of calcium and calmodulin [Prichard L, Deloulmes JC, Storm DR. Interactions between Neurogranin and Calmodulin in vivo. Journal of Biological Chemistry 1999;274:7689-94]. A direct action of this pathway is the phosphorylation of MAP2, which is required for microtubule stabilization. Because of the above findings as well as moropholigical alterations [Broadbelt K, Byne W, Jones LB. Evidence for a decrease in primary and secondary basilar dendrites on pyramidal cells in area 32 of schizophrenic prefrontal cortex. Schizophrenia Research 2002;58:75-81] we examined the expression of the active form of calmodulin in layers III and V of areas 9 and 32 in six controls and six schizophrenics matched for age, sex, and postmortem interval. Using area fraction analysis we quantified immunostaining and counted the number of immunopositive pyramidal cells and interneurons as well as immunonegative pyramidal cells. Area fraction analysis showed a significant decrease in immunostaining in area nine layers III (58%) and V (44%), area 32 layers III (51%) and V (32%). We found a significant reduction in the density of immunopositive pyramidal cells in area 9 (11%) layer III, (20%) layer V, area 32 (16%) layer III and (17%) layer V with no difference in immunopositive interneurons. These data suggest a loss of the active form of calmodulin with pyramidal cells being preferentially affected suggesting that the calcium calmodulin dependent pathway may be altered in the pyramidal cells in the PFC.

Morphology of pyramidal neurons in the rat prefrontal cortex: lateralized dendritic remodeling by chronic stress

The prefrontal cortex (PFC) plays an important role in the stress response. We filled pyramidal neurons in PFC layer III with neurobiotin and analyzed dendrites in rats submitted to chronic restraint stress and in controls. In the right prelimbic cortex (PL) of controls, apical and distal dendrites were longer than in the left PL. Stress reduced the total length of apical dendrites in right PL and abolished the hemispheric difference. In right infralimbic cortex (IL) of controls, proximal apical dendrites were longer than in left IL, and stress eliminated this hemispheric difference. No hemispheric difference was detected in anterior cingulate cortex (ACx) of controls, but stress reduced apical dendritic length in left ACx. These data demonstrate interhemispheric differences in the morphology of pyramidal neurons in PL and IL of control rats and selective effects of stress on the right hemisphere. In contrast, stress reduced dendritic length in the left ACx.

Activity-dependent synaptogenesis: regulation by a CaM-kinase kinase/CaM-kinase I/betaPIX signaling complex

Neuronal activity augments maturation of mushroom-shaped spines to form excitatory synapses, thereby strengthening synaptic transmission. We have delineated a Ca(2+)-signaling pathway downstream of the NMDA receptor that stimulates calmodulin-dependent kinase kinase (CaMKK) and CaMKI to promote formation of spines and synapses in hippocampal neurons. CaMKK and CaMKI form a multiprotein signaling complex with the guanine nucleotide exchange factor (GEF) betaPIX and GIT1 that is localized in spines. CaMKI-mediated phosphorylation of Ser516 in betaPIX enhances its GEF activity, resulting in activation of Rac1, an established enhancer of spinogenesis. Suppression of CaMKK or CaMKI by pharmacological inhibitors, dominant-negative (dn) constructs and siRNAs, as well as expression of the betaPIX Ser516Ala mutant, decreases spine formation and mEPSC frequency. Constitutively-active Pak1, a downstream effector of Rac1, rescues spine inhibition by dnCaMKI or betaPIX S516A. This activity-dependent signaling pathway can promote synapse formation during neuronal development and in structural plasticity.

Neuronal sandwiches: A method for rapid and controlled initiation of synapses

Synapse formation is a fast, dynamic process that involves the assembly of many molecules following axodendritic contact. Neuronal cultures are often used to study the insertion of fluorescently tagged pre- and postsynaptic molecules in vitro. However, this task still remains challenging, since the time-point and location of newly forming synapses are largely unpredictable and rely on random contact events. We developed a technique that controls the time-point of interaction between axons and dendrites, and thus the onset of synapse formation. Dissociated hippocampal neurons were cultivated on two different coverslips, allowing for the separate outgrowth of axonal networks and of neurons with sparsely innervated dendrites. Pre- and postsynaptic partners were brought in contact as coverslips were merged. Time-lapse imaging showed clustering of GFP/PSD-95 in postsynaptic neurons within 1-3h, indicating the rapid formation of new synaptic sites. Localization of DsRed, as a control protein, remained unchanged. Imaging of neuronal activity using calcium sensitive dyes revealed that in a number of cases neurons of the pre- and postsynaptic layer were synchronously active, suggesting the functionality of newly formed synapses across layers. Therefore, our new method is a valuable tool to control synapse formation and for investigating the temporal role of signaling molecules during this process.

Simulation of motoneuron morphology in three dimensions. II. Building complete neurons

By using dendrogram data from six adult cat alpha motoneurons, we have constructed computer simulations of these cells in three dimensions (3D) by "growing" their dendritic trees from stem branches that were oriented as in the original cells. Individual trees were simulated by using the algorithms and parameters discussed in the companion paper (Marks and Burke [2007] J. Comp. Neurol. 503:685-700). It was not possible to distinguish real from simulated motoneurons by visual inspection of 3D drawings. Simulated cells were compared quantitatively with their actual exemplars by using features that were measured in spherical shells at various radii centered on the soma. These included nearest neighbor distances (NNDs) between branches, the sizes and overlaps between the territories of individual dendrites measured as convex hulls (polygons that enclose all branches passing through a shell), and the sizes of circular zones that contained no branches. We also compared the 3D fractal dimensions and lacunarity (a measure of the 3D dispersion of branches) in actual cells and their simulations. The statistical properties of these quantitative measures were not significantly different, suggesting that the simulation algorithm was quite successful. However, there were three exceptions: 1) there were more NNDs at distances < 50 microm in simulated than in actual motoneurons; 2) average overlaps between the territories of different dendrites were almost twice as large in simulated compared with actual motoneurons; and 3) estimates of lacunarity were also larger in simulated cells. These exceptions suggest that dendritic branches in actual motoneurons tend to avoid one another. We discuss possible interpretations of these results.

Network activity-independent coordinated gene expression program for synapse assembly

Global biological datasets generated by genomics, transcriptomics, and proteomics provide new approaches to understanding the relationship between the genome and the synapse. Combined transcriptome analysis and multielectrode recordings of neuronal network activity were used in mouse embryonic primary neuronal cultures to examine synapse formation and activity-dependent gene regulation. Evidence for a coordinated gene expression program for assembly of synapses was observed in the expression of 642 genes encoding postsynaptic and plasticity proteins. This synaptogenesis gene expression program preceded protein expression of synapse markers and onset of spiking activity. Continued expression was followed by maturation of morphology and electrical neuronal networks, which was then followed by the expression of activity-dependent genes. Thus, two distinct sequentially active gene expression programs underlie the genomic programs of synapse function.

Metabolic and behavioral deficits following a routine surgical procedure in rats

To test the hypothesis that functional metabolic deficits observed following surgical brain injury are associated with changes in cognitive performance in rodents, we performed serial imaging studies in parallel with behavioral measures in control animals and in animals with surgical implants. Memory function was assessed using the novel object recognition (NOR) test, administered 3 days prior to and 3, 7, 14 and 56 days after surgery. At each time point, general locomotion was also measured. Metabolic imaging with 18F-fluorodeoxyglucose ([18F]FDG) occurred 28 and 58 days after surgery. Animals with surgical implants performed significantly worse on tests of object recognition, while general locomotion was unaffected by the implant. There was a significant decrease in glucose uptake after surgery in most of the hemisphere ipsilateral to the implant relative to the contralateral hemisphere. At both time points, the most significant metabolic deficits occurred in the primary motor cortex (-25%; p<0.001), sensory cortex (-15%, p<0.001) and frontal cortex (-12%; p<0.001). Ipsilateral areas further from the site of insertion became progressively worse, including the sensory cortex, dorsal striatum and thalamus. These data was supported by a voxel-based analysis of the PET data, which revealed again a unilateral decrease in [18F]FDG uptake that extended throughout the ipsilateral cortex and persisted for the duration of the 58-day study. Probe implantation in the striatum results in a widespread and long-lasting decline in cortical glucose metabolism together with a persistent, injury-related deficit in the performance of a cognitive (object recognition) task in rats.

Enhanced visual activity in vivo forms nascent synapses in the developing retinotectal projection

Patterned neural activity during development is critical for proper wiring of sensory circuits. Previous work demonstrated that exposing freely swimming Xenopus tadpoles to 4 h of enhanced visual stimulation accelerates the dendritic growth rate of optic tectal neurons in vivo. Here we test whether this same period of visual stimulation increased synaptic maturation and formation of new synapses in the retinotectal pathway. We assessed synaptic properties of stage 48 tadpoles that were exposed to a simulated-motion stimulus for 4-5 h. Based on our findings that immature retinotectal synapses have greater paired-pulse facilitation compared with more mature synapses, consistent with a lower release probability (Pr), we used a paired-pulse protocol to elicit responses selectively from nascent synapses with low Pr. Although AMPA/NMDA ratios for single and paired stimuli were the same in control tadpoles, visual stimulation caused a relative decrease in the AMPA/NMDA ratio of the paired response. We evoked retinotectal synaptic transmission in the presence of Sr(2+) to record asynchronous vesicle release. We compared evoked mEPSCs induced by single and paired stimuli and found that visual stimulation selectively enhances the amplitude and number of AMPA receptor (AMPAR)-mediated mEPSCs evoked by paired stimuli relative to those evoked by single stimuli. Together these results show that enhanced visual stimulation affects both AMPAR- and NMDAR-mediated responses in a population of synapses revealed by paired-pulse stimulation. This suggests that in vivo visual stimulation increases synapses that have a low Pr and that have properties consistent with immature synapses.

Time frame of mitral cell development in the mice olfactory bulb

Along with tufted cells, mitral cells are the principal projection neurons in the olfactory bulb (OB). During the development of the OB, mitral cells migrate from the ventricular zone to the intermediate zone, where they begin to send axons along the lateral olfactory tract (LOT) to the cortical olfactory zones. Subsequently, they lose their tangential orientation, enabling them to make contact with the axons of the olfactory sensory neurons (OSN) that innervate the whole OB. Here, we investigated the distinct morphological features displayed by developing mitral cells and analyzed the relationship between the changes undertaken by these neurons and the arrival of the OSN axons. Immunostaining for specific markers of developing axons and dendrites, coupled with the use of fluorescent tracers, revealed the morphological changes, the continuous reorientation, and the final refinement that these cells undergo. We found that some of these changes are dependent on the arrival of the OSN axons. Indeed, we identified three main chronological events: 1) newly generated neurons become established in the intermediate zone and project to the LOT; 2) the cells reorient and spread their dendrites at the same time as OSN axons penetrate the OB (this is a sensitive period between embryonic day (E)15-16, in which the arrival of afferents establishes a spatial and temporal gradient that facilitates protoglomerulus and glomerulus formation); and 3) final refinement of the radially orientated cells to adopt a mature morphology. These results suggest that both afferent inputs and intrinsic factors participate to produce the well-defined sensory system.

Plasticity of both excitatory and inhibitory synapses is associated with seizures induced by removal of chronic blockade of activity in cultured hippocampus

One factor common to many neurological insults that can lead to acquired epilepsy is a loss of afferent neuronal input. Neuronal activity is one cellular mechanism implicated in transducing deafferentation into epileptogenesis. Therefore the effects of chronic activity blockade on seizure susceptibility and its underlying mechanisms were examined in organotypic hippocampal slice cultures treated chronically with the sodium channel blocker, tetrodotoxin (TTX), or the N-methyl-D-aspartate receptor (NMDAR) antagonist, D-2-amino-5-phosphonovaleric acid (D-APV). Granule cell field potential recordings in physiological buffer revealed spontaneous electrographic seizures in 83% of TTX-, 9% of D-APV-, but 0% of vehicle-treated cultures. TTX-induced seizures were not associated with membrane property alterations that would elicit granule cell hyperexcitability. Seizures were blocked by glutamate receptor antagonists, suggesting that plasticity in excitatory synaptic circuits contributed to seizures. The morphology of granule cells and their mossy fiber axons remained largely unchanged, and the number of synapses onto granule cells measured immunohistochemically was not increased in TTX- or D-APV-treated cultures. However, voltage-clamp recordings revealed that miniature excitatory postsynaptic current frequency and kinetics were increased and miniature inhibitory postsynaptic current kinetics were decreased in D-APV- and TTX-treated cultures compared with vehicle. Changes were more profound and qualitatively different in TTX- compared with D-APV-treated cultures, consistent with the dramatic effects of TTX treatment on seizure expression. We propose that chronic blockade of action potentials by TTX induces homeostatic responses including plasticity of both excitatory and inhibitory synapses. Removal of TTX unmasks the impact of these synaptic plasticities on local circuit excitability, resulting in spontaneous seizures.

Effects of secreted oligomers of amyloid beta-protein on hippocampal synaptic plasticity: a potent role for trimers

The accumulation of amyloid beta-protein (Abeta) in brain regions serving memory and cognition is a central pathogenic feature of Alzheimer's disease (AD). We have shown that small soluble oligomers of human Abeta that are naturally secreted by cultured cells inhibit hippocampal long-term potentiation (LTP) in vitro and in vivo and transiently impair the recall of a complex learned behaviour in rats. These results support the hypothesis that diffusible oligomers of Abeta initiate a synaptic dysfunction that may be an early event in AD. We now report detailed electrophysiological analyses that define conditions under which acute application of soluble Abeta inhibits hippocampal synaptic plasticity in wild-type mice. To ascertain which Abeta assemblies contribute to the impairment of LTP, we fractionated oligomers by size-exclusion chromatography and found that Abeta trimers fully inhibit LTP, whereas dimers and tetramers have an intermediate potency. Natural Abeta oligomers are sensitive to heat denaturation, primarily inhibit the induction phase of LTP, and cause a sustained impairment of LTP even after extensive washout. We observed no effects of Abeta oligomers on presynaptic vesicle release. LTP in juvenile mice is resistant to the effects of Abeta oligomers, as is brain-derived-neurotrophic-factor-induced LTP in adult hippocampus. We conclude that specific assemblies, particularly timers, of naturally secreted Abeta oligomers are potent and selective inhibitors of certain forms of hippocampal LTP.

Dopamine profoundly suppresses excitatory transmission in neonatal rat hippocampus via phosphatidylinositol-linked D1-like receptor

Dopamine modulates synaptic transmission in various brain regions. The disorder of dopamine system may be related to neurodevelopmental dysfunction. However, the action of dopamine on synaptic transmission during development is largely unknown. We studied the effect of dopamine on GABAergic and glutamatergic transmission in neonatal rat hippocampus from the early period of synapse formation by whole-cell patch-clamp recordings from CA1 pyramidal cells. Dopamine (100 muM) profoundly decreased the amplitude of GABA(A) receptor-mediated postsynaptic currents (GABA(A)-PSCs) to 32.2+/-5.4% (mean+/-S.E.M., EC(50): 2.9 muM) in the first postnatal week, when GABA provides excitatory drive. Dopamine also decreased the amplitude of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated excitatory postsynaptic currents (EPSCs) to 29.1+/-2.7% (EC(50): 18.7 muM) in the second postnatal week, when glutamate responses first appear. The dopamine-induced inhibition declined after these periods and became only partial after postnatal day 30. Further we identified the receptor subtype involved in the dopamine-induced inhibition as phosphatidylinositol-linked D1-like receptor, since 6-chloro-2,3,4,5-tetrahydro-3-methyl-1-(3-methylphenyl)-1H-3-benzazepine-7,8-diol hydrobromide (SKF 83959), a selective agonist for phosphatidylinositol-linked D1-like receptor, clearly mimicked the action of dopamine, and 1-[6-[((17beta)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione (U-73122), an inhibitor of phospholipase C, significantly reduced the dopamine-induced inhibition. Dopamine did not change the response to puff-applied GABA or kainic acid, nor the amplitude of miniature GABA(A)-PSCs or miniature EPSCs. These results suggest that the activation of phosphatidylinositol-linked D1-like receptor profoundly suppresses the excitatory transmission during the early period of synapse formation in the developing hippocampus by presynaptic mechanisms. This study firstly demonstrates the effect of phosphatidylinositol-linked D1-like receptor on synaptic transmission.

Control of dendritic diversity

The dendritic trees of different neuronal types display an astonishing diversity in structure and function. How this diversity is generated remains incompletely understood. However, recent studies have revealed some of the underlying mechanisms by which intrinsic programs of cell-type specification and extrinsic factors exert their effects on the dendritic cytoskeleton to regulate patterns of growth and branching.

Synapse composition and organization following chronic activity blockade in cultured hippocampal neurons

Activity plays multiple roles in the expression of synaptic plasticity, and has been shown to regulate the localization of both neurotransmitter receptors and downstream signaling machinery. However, the role of activity in central synapse formation and organization is incompletely understood. Some studies indicate that synapse formation can occur in the absence of synaptic activity, while others indicate that activity is required for synapse maintenance and receptor recruitment. In addition, the effects of long-term blockade of transmission generally, rather than blockade of specific receptors, on postsynaptic protein complement has been poorly characterized. In order to address the role of activity in synapse formation and postsynaptic specialization, we used tetanus toxin to chronically cleave VAMP2 and inhibit SNARE-mediated neurotransmitter release in cultured hippocampal neurons. Although these neurons are deficient in synaptic release, they are of normal size and morphology. In addition, both excitatory and inhibitory synapses form along their processes with normal density. These synapses have a remarkably similar cellular and molecular organization compared to controls, and are capable of recruiting postsynaptic scaffolding proteins, GABA, and glutamate receptors. Subcellular enrichment of synaptic proteins into specialized domains also appears intact. These data indicate that global activity inhibition is insufficient to disrupt central synapse formation or organization.

Two populations of glutamatergic axons in the rat dorsal raphe nucleus defined by the vesicular glutamate transporters 1 and 2

Most glutamatergic neurons in the brain express one of two vesicular glutamate transporters, vGlut1 or vGlut2. Cortical glutamatergic neurons highly express vGlut1, whereas vGlut2 predominates in subcortical areas. In this study immunohistochemical detection of vGlut1 or vGlut2 was used in combination with tryptophan hydroxylase (TPH) to characterize glutamatergic innervation of the dorsal raphe nucleus (DRN) of the rat. Immunofluorescence labeling of both vGlut1 and vGlut2 was punctate and homogenously distributed throughout the DRN. Puncta labeled for vGlut2 appeared more numerous then those labeled for vGlut1. Ultrastructural analysis revealed axon terminals containing vGlut1 and vGlut2 formed asymmetric-type synapses 80% and 95% of the time, respectively. Postsynaptic targets of vGlut1- and vGlut2-containing axons differed in morphology. vGlut1-labeled axon terminals synapsed predominantly on small-caliber (distal) dendrites (42%, 46/110) or dendritic spines (46%, 50/110). In contrast, vGlut2-containing axons synapsed on larger caliber (proximal) dendritic shafts (> 0.5 microm diameter; 48%, 78/161). A fraction of both vGlut1- or vGlut2-labeled axons synapsed onto TPH-containing dendrites (14% and 34%, respectively). These observations reveal that different populations of glutamate-containing axons innervate selective dendritic domains of serotonergic and non-serotonergic neurons, suggesting they play different functional roles in modulating excitation within the DRN.

Dendritic plasticity in the adult rat following middle cerebral artery occlusion and Nogo-a neutralization

Our work has shown that following focal ischemic lesion in adult rats, neutralization of the axon growth inhibitor Nogo-A with the monoclonal antibody (mAb) IN-1 results in functional recovery. Furthermore, new axonal connections were formed from the contralesional cortex to subcortical areas corresponding to the observed functional recovery. The present study investigated whether dendritic changes, also known to subserve functional recovery, paralleled the axonal plasticity shown after ischemic lesion and treatment with mAb IN-1. Golgi-Cox-stained layer V pyramidal neurons in the contralesional sensorimotor cortex were examined for evidence of dendritic sprouting. Results demonstrated increased dendritic arborization and spine density in the mAb IN-1-treated animals with lesion. Interestingly, administration of mAb IN-1 without lesion resulted in transient dendritic outgrowth with no change in spine density. These results suggest a novel role for Nogo-A in limiting dendritic plasticity after stroke.

Dendritic spines and long-term plasticity

A recent flurry of time-lapse imaging studies of live neurons have tried to address the century-old question: what morphological changes in dendritic spines can be related to long-term memory? Changes that have been proposed to relate to memory include the formation of new spines, the enlargement of spine heads and the pruning of spines. These observations also relate to a more general question of how stable dendritic spines are. The objective of this review is to critically assess the new data and to propose much needed criteria that relate spines to memory, thereby allowing progress in understanding the morphological basis of memory.

Molecular mechanisms of dendritic spine development and remodeling

Dendritic spines are small protrusions that cover the surface of dendrites and bear the postsynaptic component of excitatory synapses. Having an enlarged head connected to the dendrite by a narrow neck, dendritic spines provide a postsynaptic biochemical compartment that separates the synaptic space from the dendritic shaft and allows each spine to function as a partially independent unit. Spines develop around the time of synaptogenesis and are dynamic structures that continue to undergo remodeling over time. Changes in spine morphology and density influence the properties of neural circuits. Our knowledge of the structure and function of dendritic spines has progressed significantly since their discovery over a century ago, but many uncertainties still remain. For example, several different models have been put forth outlining the sequence of events that lead to the genesis of a spine. Although spines are small and apparently simple organelles with a cytoskeleton mainly composed of actin filaments, regulation of their morphology and physiology appears to be quite sophisticated. A multitude of molecules have been implicated in dendritic spine development and remodeling, suggesting that intricate networks of interconnected signaling pathways converge to regulate actin dynamics in spines. This complexity is not surprising, given the likely importance of dendritic spines in higher brain functions. In this review, we discuss the molecules that are currently known to mediate the exquisite sensitivity of spines to perturbations in their environment and we outline how these molecules interface with each other to mediate cascades of signals flowing from the spine surface to the actin cytoskeleton.

Similar diffusibility of membrane proteins across the axon-soma and dendrite-soma boundaries revealed by a novel FRAP technique

In polarized mature neurons, the asymmetrical distribution of proteins between axonal and somatodendritic plasma membrane (PM) domains may be maintained by a diffusion barrier at the axon-soma boundary. At the boundary, a complex containing membrane-associated and cytoskeletal proteins is formed, anchoring axonal membrane proteins and indirectly hindering the diffusion of other membrane proteins. We examined the latter case, i.e., secondary diffusion impedance by comparing the mobility of fluorescently labeled membrane proteins within the axon-soma and dendrite-soma boundaries. We performed fluorescence recovery after photobleaching (FRAP) experiments using mature cultured hippocampal neurons that had been labeled specifically at their PMs with fluorescent proteins (FPs). The maturation of these neurons was confirmed by immunolocalization with Ankyrin-G, which is thought to participate in the creation of the diffusion barrier at the axon-soma boundary. We developed a wide-field microscope equipped with a device (digital micromirror device) composed of 1024 x 768 binary mirrors at the field-stop, allowing free control of the illumination area and intensity. After the FPs in peripheral processes were photobleached, nonbleached FPs diffused into all the processes at equivalent speeds. These results indicate that the secondary diffusion barrier to exogenously overexpressed membrane proteins is not specific to the axon-soma boundary.

Lack of TrkB and TrkC signaling alters the synaptogenesis and maturation of mossy fiber terminals in the hippocampus

We have studied the role of endogenous neurotrophins in the formation and maturation of intrinsic hippocampal connections in vivo and analyzed the dentate granule cell projections in both trkB-/- and trkC-/- mice. Immunohistochemistry against calbindin did not show major alterations in the distribution of granule cell axons, which were located exclusively in the hilus and the stratum lucidum. However, the thickness of the stratum lucidum (mossy fiber termination zone) and the density of mossy fiber terminals were reduced in the absence of TrkB signaling. Electron-microscopic analyses showed that the fine structure of mossy terminals was altered in both trkB-/- and trkC-/- mice. Mutant granule cell terminals were smaller than those in wild-type animals and showed a reduction in both the number of synaptic contacts and synaptic vesicles. Immunofluorescence assays demonstrated that the expression levels of most synaptic-associated proteins (v-SNAREs and t-SNAREs) were altered in the mossy fibers of trkB- and trkC-deficient mice. Our results therefore reveal that TrkB and TrkC signaling is required for the maturation of granule cell axons.

Age-dependence in the homeostatic upregulation of hippocampal dendritic spine number during blocked synaptic transmission

Homeostatic regulation of spine number in mature hippocampal neurons results in more dendritic spines when synaptic transmission is blocked, providing a mechanism to compensate for diminished synaptic input. It is unsettled whether blockade of synaptic transmission also elevates spine number during development. To address this question, synaptic transmission was blocked in rat hippocampal slices during critical developmental stages of spine formation at postnatal days (P) 6-P22 and compared to adults. CA1 pyramidal cells were labeled with DiI and maintained for 5 h in one of three conditions, control artificial cerebrospinal fluid (ACSF), block media containing synaptic transmission antagonists in ACSF, or block media containing synaptic transmission antagonists in a nominally calcium-free ACSF with high magnesium. Slices were fixed in mixed aldehydes, sectioned, and the lateral dendrites were imaged in stratum radiatum with confocal microscopy. Dendritic spine density was quantified per unit length of dendrite. At P6-7 there were only a few protrusions emerging from the dendrites, which were predominantly filopodia-like in appearance. At both P11-12 and P15-16 there was a mixture of dendritic spines and filopodia-like structures. By P20-22 dendritic spines predominated and spine density was about 82% of the adult level. Dendritic spine density increased during blocked synaptic transmission at P20-22 as in adults, but was unchanged during blockade at younger ages. When extracellular calcium was nominally zero, dendritic spine density further increased on P20-22 dendrites as in adults. In contrast, spine density decreased along P11-12 dendrites under the nominally zero calcium condition. Under control conditions, dendritic protrusions were longer at P6-7 than at all other ages, which did not differ from one another. When synaptic transmission was blocked, dendritic protrusions further elongated at P6-7 only. Under the nominally zero calcium condition with blocked synaptic transmission, dendritic protrusions shortened at P11-12 only. These findings reveal age-dependent changes in the manifestation of homeostatic control of dendritic spines that could be mediated by maturational changes in mechanisms regulating postsynaptic calcium.

Activity-dependent maturation of excitatory synaptic connections in solitary neuron cultures of mouse neocortex

Activity plays important roles in the formation and maturation of synaptic connections. We examined these roles using solitary neocortical excitatory neurons, receiving only self-generated synaptic inputs, cultured in a microisland with and without spontaneous spike activity. The amplitude of excitatory postsynaptic currents (EPSCs), evoked by applying brief depolarizing voltage pulses to the cell soma, continued to increase from 7 to 14 days in culture. Short-term depression of EPSCs in response to paired-pulse or 10-train-pulse stimulation decreased with time in culture. These developmental changes were prevented when neurons were cultured in a solution containing tetrodotoxin (TTX). The number of functional synapses estimated by recycled synaptic vesicles with FM4-64 was significantly smaller in TTX-treated than control neurons. However, the miniature EPSC amplitude remained unchanged during development, irrespective of activity. Transmitter release probability, assessed by use-dependent blockade of N-methyl-D-aspartate receptor-mediated EPSCs with MK-801, was higher in TTX-treated than control neurons. Therefore, the activity-dependent increase in EPSC amplitude was mainly ascribed to the increase in synapse number, while activity-dependent alleviation of short-term depression was mostly ascribed to the decrease in release probability. The effect of activity blockade on short-term depression, but not EPSC amplitude, was reversed after 4 days of TTX removal, indicating that synapse number and release probability are controlled by activity in very different ways. These results demonstrate that activity regulates the conversion of immature synapses transmitting low-frequency input signals preferentially to mature synapses transmitting both low- and high-frequency signals effectively, which may be necessary for information processing in mature cortex.

Modulation of dendritic differentiation by corticotropin-releasing factor in the developing hippocampus

The interplay of environmental and genetic factors in the developmental organization of the hippocampus has not been fully elucidated. The neuropeptide corticotropin-releasing factor (CRF) is released from hippocampal interneurons by environmental signals, including stress, to increase synaptic efficacy. In the early postnatal hippocampus, we have previously characterized a transient population of CRF-expressing Cajal-Retzius-like cells. Here we queried whether this stress-activated neuromodulator influences connectivity in the developing hippocampal network. Using mice deficient in the principal hippocampal CRF receptor [CRF(1)(-/-)] and organotypic cultures grown in the presence of synthetic CRF, or CRF receptor antagonists, we found robust effects of CRF on dendritic differentiation in hippocampal neurons. In CRF(1)(-/-) mice, the dendritic trees of hippocampal principal cells were exuberant, an effect that was induced in normal hippocampi in vitro by the presence of CRF(1) antagonists. In both cases, total dendritic length and dendritic branching were significantly increased. In contrast, exogenous synthetic CRF blunted the dendritic growth in hippocampal organotypic cultures. Taken together, these findings suggest that endogenous CRF, if released excessively by previous early postnatal stress, might influence neuronal connectivity and thus function of the immature hippocampus.

Accelerated dendritic development of rat cortical pyramidal cells and interneurons after biolistic transfection with BDNF and NT4/5

Neurotrophins are candidate molecules for regulating dendritogenesis. We report here on dendritic growth of rat visual cortex pyramidal and interneurons overexpressing 'brain-derived neurotrophic factor' BDNF and 'neurotrophin 4/5' NT4/5. Neurons in organotypic cultures were transfected with plasmids encoding either 'enhanced green fluorescent protein' EGFP, BDNF/EGFP or NT4/5/EGFP either at the day of birth with analysis at 5 days in vitro, or at 5 days in vitro with analysis at 10 days in vitro. In pyramidal neurons, both TrkB ligands increased dendritic length and number of segments without affecting maximum branch order and number of primary dendrites. In the early time window, only infragranular neurons were responsive. Neurons in layers II/III became responsive to NT4/5, but not BDNF, during the later time window. BDNF and NT4/5 transfectants at 10 days in vitro had still significantly shorter dendrites than adult pyramidal neurons, suggesting a massive growth spurt after 10 days in vitro. However, segment numbers were already in the range of adult neurons. Although this suggested a role for BDNF, long-term activity-deprived, and thus BDNF-deprived, pyramidal cells developed a dendritic complexity not different from neurons in active cultures except for higher spine densities on neurons of layers II/III and VI. Neutralization of endogenous NT4/5 causes shorter and less branched dendrites at 10 days in vitro suggesting an essential role for NT4/5. Neutralization of BDNF had no effect. Transfected multipolar interneurons became identifiable during the second time window. Both TrkB ligands significantly increased number of segments and branch order towards the adult state with little effects on dendritic length. The results suggested that early in development BDNF and NT4/5 probably accelerate dendritogenesis in an autocrine fashion. In particular, branch formation was advanced towards the adult pattern in pyramidal cells and interneurons.

Development of a sexually dimorphic neuromuscular system in male rats after spinal transection: morphologic changes and implications for estrogen sites of action

The lumbar spinal cord of rats contains the sexually dimorphic, steroid-sensitive spinal nucleus of the bulbocavernosus (SNB). In male rats, SNB motoneurons exhibit a biphasic pattern of dendritic growth, having an initial period of exuberant growth followed by a period of retraction to mature lengths by 7 weeks of age. This growth is steroid dependent: dendrites fail to grow after castration, but growth is supported in castrates treated with estradiol. In this experiment, we examined whether supraspinal afferent input by means of descending spinal tracts to the SNB was involved in the normal postnatal development of SNB motoneurons, and whether the effect of estradiol on SNB dendritic growth could be explained by an indirect action of estradiol on supraspinal afferents. Motoneuron morphology was assessed in normal males, early- or late-postnatally transected males, castrated males left untreated or treated with estradiol, and transected castrates treated with estradiol. SNB motoneurons were retrogradely labeled with cholera toxin-horseradish peroxidase during both the growth and retraction phases of dendritic development and reconstructed in three dimensions. The removal of supraspinal afferents resulted in extremely local effects within the developing SNB arbor, as well as transient alterations in somal growth. Furthermore, spinal transection did not block the trophic effect of estradiol on supporting SNB dendritic growth, indicating that estrogens do not act by means of supraspinal input to support SNB motoneuron development.

Local Effects of BDNF on Dendritic Growth

The development of dendrites is a crucial step in the formation of cortical circuitry. The morphogen brain-derived neurotrophic factor (BDNF) may mediate the effects of activity on dendritic morphology since its expression and release are thought to be activity-dependent. Using two-photon microscopy, the autocrine and paracrine effects of BDNF on dendritic morphology were assessed. Overexpression of BDNF profoundly altered the form and stability of basal dendritic arbors via an autocrine mechanism. Paracrine BDNF also altered dendritic branching, though in a highly local fashion. BDNF is capable of acting as an intercellular morphogen, and could hypothetically shape dendritic arbors to best fit the developing structure and function of the pre-synaptic circuit.

Decreased dendrite growth from cultured mouse cortical neurons surviving excitotoxic activation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate/kainate receptors

During cerebral ischemia, massive glutamate release leads to cell death through ionotropic glutamate receptor activation. An early consequence of this excitotoxicity is dendrite injury, which can precede cell death. We therefore tested whether cells that survived an excitotoxic insult triggered by overactivation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate (KA) subtype of ionotropic glutamate receptors displayed altered dendrite growth. We demonstrate that 24 h exposure of cultured cortical neurons to AMPA or KA dramatically reduced dendrite growth from surviving neurons. AMPA or KA exposure decreased primary dendrite number and length, and also reduced dendritic branching. The AMPA/KA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione blocked the deleterious effect of AMPA and KA on dendrite growth. These results suggest that AMPA/KA receptor overactivation may contribute to dendritic injury from neurons that survive an ischemic insult.

Brain-derived neurotrophic factor mediates activity-dependent dendritic growth in nonpyramidal neocortical interneurons in developing organotypic cultures

Brain-derived neurotrophic factor (BDNF) promotes postnatal maturation of GABAergic inhibition in the cerebral and cerebellar cortices, and its expression and release are enhanced by neuronal activity, suggesting that it acts in a feedback manner to maintain a balance between excitation and inhibition during development. BDNF promotes differentiation of cerebellar, hippocampal, and neostriatal inhibitory neurons, but its effects on the dendritic development of neocortical inhibitory interneurons remain unknown. Here, we show that BDNF mediates depolarization-induced dendritic growth and branching in neocortical interneurons. To visualize inhibitory interneurons, we biolistically transfected organotypic cortical slice cultures from neonatal mice with green fluorescent protein (GFP) driven by the glutamic acid decarboxylase (GAD)67 promoter. Nearly all GAD67-GFP-expressing neurons were nonpyramidal, many contained GABA, and some expressed markers of neurochemically defined GABAergic subtypes, indicating that GAD67-GFP-expressing neurons were GABAergic. We traced dendritic trees from confocal images of the same GAD67-GFP-expressing neurons before and after a 5 d growth period, and quantified the change in total dendritic length (TDL) and total dendritic branch points (TDBPs) for each neuron. GAD67-GFP-expressing neurons growing in control medium exhibited a 20% increase in TDL, but in 200 ng/ml BDNF or 10 mm KCl, this increase nearly doubled and was accompanied by a significant increase in TDBPs. Blocking action potentials with TTX did not prevent the BDNF-induced growth, but antibodies against BDNF blocked the growth-promoting effect of KCl. We conclude that BDNF, released by neocortical pyramidal neurons in response to depolarization, enhances dendritic growth and branching in nearby inhibitory interneurons.

Formation of dendritic spines in cultured striatal neurons depends on excitatory afferent activity

The role of afferent innervation in the formation of dendritic spines was studied in cultured rat striatum. The striatum is a unique structure in that it contains highly spiny GABAergic projection neurons, with no known local excitation. Grown alone in culture, striatal neurons did not express spontaneous network activity and were virtually devoid of dendritic spines. Adding GFP-expressing mouse cortical neurons to the striatal culture caused the appearance of spontaneous and evoked excitatory synaptic currents in the striatal neurons and a 10-fold increase in the density of spines on their dendrites. This effect was blocked by a continuous presence of TTX in the growth medium, while removal of the drug caused a rapid appearance of spines. Exposure to glutamate, or the presence of cortex-conditioned medium did not mimic the effect of cortical neurons on formation of spines in the striatal neurons. Also, the cortical innervation did not cause a selective enhancement of survival of specific subtypes of spiny striatal neurons. These experiments demonstrate that excitatory afferents are necessary for the formation of dendritic spines in striatal neurons.

Structural changes in AMPA-receptive neurons in the nucleus of the solitary tract of spontaneously hypertensive rats

The baroreceptor reflex is critical for homeostatic regulation of blood pressure and is initiated centrally by glutamate release from baroreceptive afferents onto neurons in the nucleus of the solitary tract that activates AMPA-type glutamate receptors. The GluR1 subunit of the AMPA receptor is located at postsynaptic sites within the nucleus of the solitary tract, particularly in dendritic spines, which are important sites for synaptic plasticity. We tested whether the distribution of GluR1 changes after sustained hypertension, which alters baroreceptor afferent activity. We examined the distribution of GluR1 in the nucleus of the solitary tract of both hypertensive (spontaneously hypertensive) and normotensive (Wistar-Kyoto) rats at the light microscopic and electron microscopic levels. There were more GluR1-containing dendritic spines in the nucleus of the solitary tract of hypertensive rats compared with normotensive rats, which was attributable to an increase in the proportion of dendritic spines containing GluR1 as well as an increase in the total number of dendritic spines. The differences were only seen after the development of hypertension and were not seen in rostral regions of the nucleus of the solitary tract. In the spontaneously hypertensive rat, many synapses on GluR1-containing dendrites had the morphological features of synapses undergoing dynamic changes, including the presence of perforated synapses. These results suggest that changes in afferent activity to the nucleus of the solitary tract during sustained hypertension alter both the dendritic structure and AMPA receptor content of some neurons. These structural changes may be a substrate for central resetting of the baroreceptor reflex.

Dendritic morphology is altered in hippocampal neurons following prenatal compromise

Chronic placental insufficiency (CPI), a known cause of intrauterine growth restriction, can lead to structural alterations in the developing brain that might underlie postnatal neurological deficits. We have previously demonstrated significant reductions in the volumes of hippocampal neuropil layers in fetal guinea pig brains following experimentally induced growth restriction. To determine the components of the neuropil affected in the brains of growth restricted (GR) fetuses, the dendritic morphology of CA1 pyramidal neurons and dentate granule cells was examined. CPI was induced by unilateral uterine artery ligation in pregnant guinea pigs at midgestation (term approximately 67 days). Hippocampi from control and GR fetuses were stained using the Rapid Golgi technique and the growth and branching of the dendritic arbors were quantified using the Sholl method. In addition, the density of dendritic spines was determined on the apical arbors of each population. In GR brains (n = 7) compared to controls (n = 7), there was a reduction in dendritic elongation (p < 0.005) and an alteration in the branch point distribution in CA1 basal arbors, and a reduction both in the outgrowth (p < 0.05) and branch point number (p < 0.05) of CA1 apical arbors. Dentate granule cells from GR brains also demonstrated reduced dendritic outgrowth (p < 0.05). There was an increase in dendritic spine density in both neuronal populations; this might be due either to altered synaptic pruning or as a compensatory mechanism for reduced dendritic length. These findings demonstrate that a chronic prenatal insult causes selective changes in the morphology of hippocampal cell dendrites and may lead to alterations in hippocampal function in the postnatal period.

Developmental relationship between cholinergic amacrine cell processes and ganglion cell dendrites of the mouse retina

Ganglion cells of the mammalian retina undergo structural remodeling before their dendrites are confined to functionally distinct laminas within the inner plexiform layer. It has been proposed that cholinergic amacrine cells provide laminar cues that remodel ganglion cell dendrites, because their processes stratify before those of the ganglion cells. To address this possibility, it is necessary to know whether cholinergic cells contact all or only some classes of ganglion cells during development. We, therefore, used two-photon microscopy to simultaneously reconstruct the dendritic arbors of different classes of ganglion cells and terminal processes of cholinergic cells in neonatal mouse retina. We determined that, after birth, cholinergic cells contacted only a subset of ganglion cells. Large bistratified cells (LBCs), resembling direction selective ganglion cells in other species, had dendrites that fasciculated with the cholinergic plexuses. The LBCs received numerous presynaptic cholinergic contacts shortly after birth. In contrast, large monostratified cells (LMCs), ramifying outside the cholinergic plexuses at maturity, received few, if any, cholinergic contacts even at early stages when their dendrites overlapped with the cholinergic processes. These observations suggest that cholinergic cells provide laminar cues for only subsets of ganglion cells. They also indicate that the synaptic organization between amacrine and ganglion cells may be specified early in development.