Stabilization of dendritic arbor structure in vivo by CaMKII

Single-cell electroporation for gene transfer in vivo

We report an electroporation technique for targeting gene transfer to individual cells in intact tissue. Electrical stimulation through a micropipette filled with DNA or other macromolecules electroporates a single cell at the tip of the micropipette. Electroporation of a plasmid encoding enhanced green fluorescent protein (GFP) into the brain of intact Xenopus tadpoles or rat hippocampal slices resulted in GFP expression in single neurons and glia. In vivo imaging showed morphologies, dendritic arbor dynamics, and growth rates characteristic of healthy cells. Coelectroporation of two plasmids resulted in expression of both proteins, while electroporation of fluorescent dextrans allowed direct visualization of transfer of molecules into cells. This technique will allow unprecedented spatial and temporal control over gene delivery and protein expression.

Spaced stimuli stabilize MAPK pathway activation and its effects on dendritic morphology

Memory storage in mammalian neurons probably depends on both biochemical events and morphological alterations in dendrites. Here we report an activity-dependent stabilization of the MAP kinase (MAPK) pathway, prominent in hippocampal dendrites. The longevity of the signal in these dendrites was increased to hours when multiple spaced stimuli were used. Likewise, spaced stimuli and MAPK activation were critical for protrusion of new dendritic filopodia that also remained stable for hours. Our experiments define a new role for stimulus-specific responses of MAPK signaling in activity-dependent neuronal plasticity. The local biochemical signaling in dendrites complements MAPK signaling in gene expression. Together, these processes may support long-lasting behavioral changes.

Dendritic arbor development and synaptogenesis

In vivo studies indicate that synaptic activity promotes dendritic arbor elaboration at early stages of brain development. At later stages of development, synaptic activity stabilizes dendritic structure. The different roles of synaptic activity with respect to structural plasticity probably reflect the regulated spatiotemporal expression of key components within signaling pathways.

Propagation of action potentials in dendrites depends on dendritic morphology

Action potential propagation links information processing in different regions of the dendritic tree. To examine the contribution of dendritic morphology to the efficacy of propagation, simulations were performed in detailed reconstructions of eight different neuronal types. With identical complements of voltage-gated channels, different dendritic morphologies exhibit distinct patterns of propagation. Remarkably, the range of backpropagation efficacies observed experimentally can be reproduced by the variations in dendritic morphology alone. Dendritic geometry also determines the extent to which modulation of channel densities can affect propagation. Thus in Purkinje cells and dopamine neurons, backpropagation is relatively insensitive to changes in channel densities, whereas in pyramidal cells, backpropagation can be modulated over a wide range. We also demonstrate that forward propagation of dendritically initiated action potentials is influenced by morphology in a similar manner. We show that these functional consequences of the differences in dendritic geometries can be explained quantitatively using simple anatomical measures of dendritic branching patterns, which are captured in a reduced model of dendritic geometry. These findings indicate that differences in dendritic geometry act in concert with differences in voltage-gated channel density and kinetics to generate the diversity in dendritic action potential propagation observed between neurons. They also suggest that changes in dendritic geometry during development and plasticity will critically affect propagation. By determining the spatial pattern of action potential signaling, dendritic morphology thus helps to define the size and interdependence of functional compartments in the neuron.

Why do we sleep?

Slow-wave sleep consists in slowly recurring waves that are associated with a large-scale spatio-temporal synchrony across neocortex. These slow-wave complexes alternate with brief episodes of fast oscillations, similar to the sustained fast oscillations that occur during the wake state. We propose that alternating fast and slow waves consolidate information acquired previously during wakefulness. Slow-wave sleep would thus begin with spindle oscillations that open molecular gates to plasticity, then proceed by iteratively 'recalling' and 'storing' information primed in neural assemblies. This scenario provides a biophysical mechanism consistent with the growing evidence that sleep serves to consolidate memories.

delta Ca(2+)/Calmodulin-dependent protein kinase II isozyme-specific induction of neurite outgrowth in P19 embryonal carcinoma cells

Ca(2+)/calmodulin-dependent protein kinase II (CaMK-II) has been linked to the induction of differentiation in preneuronal cells. In these cells, delta isozymes represent the majority of CaMK-IIs expressed and are activated by differentiation stimuli. To determine whether delta CaMK-IIs are causative or coincident with in vitro differentiation, we overexpressed wild-type, constitutively active, and C-terminal domains of delta and gamma CaMK-II isozymes in mouse P19 and NIH/3T3 cells using high-efficiency transfections. At 1-2 days after transfection, only constitutively active delta CaMK-II isozymes induced branched cellular extensions in both cell types. In P19 cells, retinoic acid induced neurite extensions after 3-4 days; these extensions were coincident with a fourfold increase in endogenous CaMK-II activity. Extensions induced by both retinoic acid and delta CaMK-IIs contained class III beta-tubulin in a discontinuous or beaded pattern. C-terminal CaMK-II constructs disrupted the ability of endogenous CaMK-II to autophosphorylate and blocked retinoic acid-induced differentiation. delta CaMK-II was found along extensions, whereas gamma CaMK-II exhibited a more diffuse, cytosolic localization. These data not only support an extranuclear role for CaMK-II in promoting neurite outgrowth, but also demonstrate CaMK-II isozyme specificity in these early steps of neuronal differentiation.

Postsynaptic CPG15 promotes synaptic maturation and presynaptic axon arbor elaboration in vivo

The formation of CNS circuits is characterized by the coordinated development of neuronal structure and synaptic function. The activity-regulated candidate plasticity gene 15 (cpg15) encodes a glycosylphosphatidylinositol (GPI)-linked protein whose in vivo expression increases the dendritic arbor growth rate of Xenopus optic tectal cells. We now demonstrate that tectal cell expression of CPG15 significantly increases the elaboration of presynaptic retinal axons by decreasing rates of branch retractions. Whole-cell recordings from optic tectal neurons indicate that CPG15 expression promotes retinotectal synapse maturation by recruiting functional AMPA receptors to synapses. Expression of truncated CPG15, lacking its GPI anchor, does not promote axon arbor growth and blocks synaptic maturation. These results suggest that CPG15 coordinately increases the growth of pre- and postsynaptic structures and the number and strength of their synaptic contacts.

Control of dendritic field formation in Drosophila: the roles of flamingo and competition between homologous neurons

Neurons elaborate dendrites with stereotypic branching patterns, thereby defining their receptive fields. These branching patterns may arise from properties intrinsic to the neurons or competition between neighboring neurons. Genetic and laser ablation studies reported here reveal that different multiple dendritic neurons in the same dorsal cluster in the Drosophila embryonic PNS do not compete with one another for dendritic fields. In contrast, when dendrites from homologous neurons in the two hemisegments meet at the dorsal midline in larval stages, they appear to repel each other. The formation of normal dendritic fields and the competition between dendrites of homologous neurons require the proper expression level of Flamingo, a G protein-coupled receptor-like protein, in embryonic neurons. Whereas Flamingo functions downstream of Frizzled in specifying planar polarity, Flamingo-dependent dendritic outgrowth is independent of Frizzled.

Neural cell adhesion molecules, CaM kinase II and long-term memory in the chick

The intermediate and medial hyperstriatum ventrale (IMHV) of the chick brain is a site of recognition memory for filial imprinting. Previous results have demonstrated learning-related changes in the amounts of the three major isoforms of neural cell adhesion molecule (NCAM) in the left IMHV. The increases were present 24 h after training. The present study enquired whether the increases persisted and were present 48 h after training. The brain regions analysed were the left and right IMHV and the left and right hyperstriatum accessorium (HA), a visual projection area. The alpha-subunit of calcium/calmodulin protein kinase II (CaMKIIalpha) was also assayed. There were significant correlations between a measure of the strength of learning and the amount of NCAM 180 in the right IMHV (r = +0.65; p = 0.012) but not in the left, and in the left HA (r = -0.61; p = 0.02), but not in the right. There were no learning-related changes for CaMKIIalpha. We conclude that in IMHV the effects of imprinting on NCAM 180 are expressed mainly in the left IMHV 24 h after training, but 48 h after training are expressed mainly in the right IMHV.

Neuronal cell shape and neurite initiation are regulated by the Ndr kinase SAX-1, a member of the Orb6/COT-1/warts serine/threonine kinase family

The Caenorhabditis elegans sax-1 gene regulates several aspects of neuronal cell shape. sax-1 mutants have expanded cell bodies and ectopic neurites in many classes of neurons, suggesting that SAX-1 functions to restrict cell and neurite growth. The ectopic neurites in sensory neurons of sax-1 mutants resemble the defects caused by decreased sensory activity. However, the activity-dependent pathway, mediated in part by the UNC-43 calcium/calmodulin-dependent kinase II, functions in parallel with SAX-1 to suppress neurite initiation. sax-1 encodes a serine/threonine kinase in the Ndr family that is related to the Orb6 (Schizosaccharomyces pombe), Warts/Lats (Drosophila), and COT-1 (Neurospora) kinases that function in cell shape regulation. These kinases have similarity to Rho kinases but lack consensus Rho-binding domains. Dominant negative mutations in the C. elegans RhoA GTPase cause neuronal cell shape defects similar to those of sax-1 mutants, and genetic interactions between rhoA and sax-1 suggest shared functions. These results suggest that SAX-1/Ndr kinases are endogenous inhibitors of neurite initiation and cell spreading.

A working model of CaM kinase II activity in hippocampal long-term potentiation and memory

Recent advances in molecular genetics provide strong evidence for a relationship between hippocampal long-term potentiation (LTP) and hippocampus-dependent memory. The alpha-CaM kinase II knock-out mouse and transgenic mice expressing a mutant form of CaM kinase II clearly demonstrate that CaM kinase II plays a prominent role in hippocampal LTP and hippocampus-dependent memory. Furthermore, the observation that there is a diversity of silent as well as functional synapses has shed light on the molecular basis of learning and memory during development as well as in adult brain. Here we present a working model of CaM kinase II activity as a memory molecule in hippocampal LTP and describe molecular targets of CaM kinase II involved in the establishment of functional synapses following LTP induction.

Expression of GAP-43 and SCG10 mRNAs in lateral geniculate nucleus of normal and monocularly deprived macaque monkeys

We performed nonradioactive in situ hybridization histochemistry (ISH) in the lateral geniculate nucleus (LGN) of the macaque monkey to investigate the distribution of mRNA for two growth-associated proteins, GAP-43 and SCG10. GAP-43 and SCG10 mRNAs were coexpressed in most neurons of both magnocellular layers (layers I and II) and parvocellular layers (layers III-VI). Double-labeling using nonradioactive ISH and immunofluorescence revealed that both GAP-43 and SCG10 mRNAs were coexpressed with the alpha-subunit of type II calcium/calmodulin-dependent protein kinase, indicating that both mRNAs are expressed also in koniocellular neurons in the LGN. We also showed that GABA-immunoreactive neurons in the LGN did not contain GAP-43 and SCG10 mRNAs, indicating that neither GAP-43 nor SCG10 mRNAs were expressed in inhibitory interneurons in the LGN. GABA-immunoreactive neurons in the perigeniculate nucleus, however, contained both GAP-43 and SCG10 mRNAs, indicating that both mRNAs were expressed in inhibitory neurons in the perigeniculate nucleus, which project to relay neurons in the LGN. Furthermore, to determine whether the expression of GAP-43 and SCG10 mRNAs is regulated by visual input, we performed nonradioactive ISH in the LGN and the primary visual area of monkeys deprived of monocular visual input by intraocular injections of tetrodotoxin. Both mRNAs were downregulated in the LGN after monocular deprivation for 5 d or longer. From these results, we conclude that both GAP-43 and SCG10 mRNAs are expressed in the excitatory relay neurons of the monkey LGN in an activity-dependent manner.

Multicolor "DiOlistic" labeling of the nervous system using lipophilic dye combinations

We describe a technique for rapid labeling of a large number of cells in the nervous system with many different colors. By delivering lipophilic dye-coated particles to neuronal preparations with a "gene gun," individual neurons and glia whose membranes are contacted by the particles are quickly labeled. Using particles that are each coated with different combinations of various lipophilic dyes, many cells within a complex neuronal network can be simultaneously labeled with a wide variety of colors. This approach is most effective in living material but also labels previously fixed material. In living material, labeled neurons continue to show normal synaptic responses and undergo dendritic remodeling. This technique is thus useful for studying structural plasticity of neuronal circuits in living preparations. In addition, the Golgi-like labeling of neurons with many different colors provides a novel way to study neuronal connectivity.

Calcium-dependent persistent facilitation of spike backpropagation in the CA1 pyramidal neurons

Sodium-dependent action potentials initiated near the soma are known to backpropagate over the dendrites of CA1 pyramidal neurons in an activity-dependent manner. Consequently, later spikes in a train have smaller amplitude when recorded in the apical dendrites. We found that depolarization and resultant Ca(2+) influx into dendrites caused a persistent facilitation of spike backpropagation. Dendritic patch recordings were made from CA1 pyramidal neurons in mouse hippocampal slices under blockade of fast excitatory and inhibitory synaptic inputs. Trains of 10 backpropagating action potentials induced by antidromic stimulation showed a clear decrement in the amplitude of later spikes when recorded in the middle apical dendrites. After several depolarizing current pulses, the amplitude of later spikes increased persistently, and all spikes in a train became almost equal in size. BAPTA (10 mm) contained in the pipette or low-Ca(2+) perfusing solution abolished this depolarization-induced facilitation, indicating that Ca(2+) influx is required. This facilitation was present in Galpha(q) knock-out mice that lack the previously reported muscarinic receptor-mediated enhancement of spike backpropagation. Therefore, these two forms of facilitation are clearly distinct in their intracellular mechanisms. Intracellular injection of either calmodulin binding domain (100 micrometer) or Ca(2+)/calmodulin-kinase II (CaMKII) inhibitor 281-301 (10 micrometer) blocked the depolarization-induced facilitation. Bath application of a membrane-permeable CaMKII inhibitor KN-93 (10 micrometer) also blocked the facilitation, but KN-92 (10 micrometer), an inactive isomer of KN-93, had no effect. These results suggest that increases in [Ca(2+)](i) cause persistent facilitation of spike backpropagation in the apical dendrite of CA1 pyramidal neuron by CaMKII-dependent mechanisms.

Comparative analysis of dendritic architecture of identified neurons using the Hausdorff distance metric

Dendritic trees often are complex, three-dimensional structures. Comparative morphologic studies have not yet provided a reliable measure to analyze and compare the geometry of different dendritic trees. Therefore, it is important to develop quantitative methods for analyzing the three-dimensional geometry of these complex trees. The authors developed a comparison measure based on the Hausdorff distance for comparing quantitatively the three-dimensional structure of different neurons. This algorithm was implemented and incorporated into a new software package that the authors developed called NeuroComp. The authors tested this algorithm to study the variability in the three-dimensional structure of identified central neurons as well as measuring the structural differences between homologue neurons. They took advantage of the uniform dendritic morphology of identified interneurons of an insect, the giant interneurons of the cockroach. More specifically, after establishing a morphometric data base of these neurons, the authors found that the algorithm is a reliable tool for distinguishing between dendritic trees of different neurons, whereas conventional metric analysis often is inadequate. The authors propose to use this method as a quantitative tool for the investigation of the effects of various experimental paradigms on three-dimensional dendritic architecture.

The development of abnormal axon trajectories after rotation of one eye in Xenopus

The targeting of isthmotectal axons in the Xenopus binocular pathway is guided by both activity-dependent cues and activity-independent cues. Abnormal visual activity induced by unilateral eye rotation overrides activity-independent cues and causes isthmotectal axons to arborize at new locations during a critical period of development that ends approximately 3 months postmetamorphosis (PM). Horseradish peroxidase staining of isthmotectal axons reveals that they normally run rostrocaudally in the tectum; in contrast, those axons in animals with early eye rotation have circuitous trajectories. In this paper, by studying the trajectories and branching patterns of isthmotectal axons at different times after eye rotation, we aimed to investigate when and how activity cues determine the projection pattern of isthmotectal axons. As suggested by electrophysiological recording, isthmotectal axons initially grow normally and make arbors according to activity-independent cues despite the presence of abnormal visual input. Our findings demonstrate that the development of abnormal trajectories starts by 2 weeks PM in response to eye rotation and is a protracted process. It begins in the tectal regions in which the initial connections of isthmotectal axons are first formed according to activity-independent cues. At transitional stages (5 and 10 weeks), axons with arbors at two different locations are observed, with locations corresponding to the old and new termination sites, respectively. Later, at 10 weeks of age, the fainter horseradish peroxidase staining in arbors at old termination sites suggests that the older arbors are undergoing withdrawal.

Glutamate-induced changes in the pattern of hippocampal dendrite outgrowth: a role for calcium-dependent pathways and the microtubule cytoskeleton

Glutamate regulation of a variety of aspects of dendrite development may be involved in neuronal plasticity and neuropathology. In this study, we examine the calcium-dependent pathways and alterations in the microtubule (MT) cytoskeleton that may mediate glutamate-induced changes in the pattern of dendrite outgrowth. We used Fura-2 AM and inhibitors of the calcium-dependent proteins, calmodulin and calpain, to identify the role of specific calcium-dependent pathways in glutamate-regulated dendrite outgrowth. Additionally, we used a quantitative fluorescence technique to correlate changes in MT levels with glutamate-induced changes in dendrite outgrowth. We show that the intracellular calcium concentration ([Ca(2+)](i)) changes in a biphasic manner over a 12-h period in the presence of glutamate. A transient increase in [Ca(2+)](i) over the first hour of glutamate exposure correlated with a calmodulin-associated increase in the rate of dendrite outgrowth, whereas a sustained increase in [Ca(2+)](i) was correlated with calpain-associated dendrite retraction. Quantitative fluorescence measurements showed no net change in the level of MTs during calmodulin-associated increases in dendrite outgrowth, but showed a significant decline in the level of MTs during calpain-associated dendrite retraction. These findings provide insights into the intracellular mechanisms involved in activity-dependent regulation of dendrite morphology during development and after pathology.

Semaphorin 3A is a chemoattractant for cortical apical dendrites

The apical dendrites of pyramidal neurons integrate inputs from various cortical layers and are central to information processing. Here we show that the growth of apical dendrites towards the pial surface is regulated by a diffusible chemoattractant present at high levels near the marginal zone. A major component of this signal is semaphorin 3A (Sema3A), which was previously characterized as a chemorepellant for cortical axons. Soluble guanylate cyclase is asymmetrically localized to the developing apical dendrite, and is required for the chemoattractive effect of Sema3A. Thus the asymmetric localization of soluble guanylate cyclase confers distinct Sema3A responses to axons and dendrites. These observations reveal a mechanism by which a single chemotropic signal can pattern both axons and dendrites during development.

Driving AMPA receptors into synapses by LTP and CaMKII: requirement for GluR1 and PDZ domain interaction

To elucidate mechanisms that control and execute activity-dependent synaptic plasticity, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors (AMPA-Rs) with an electrophysiological tag were expressed in rat hippocampal neurons. Long-term potentiation (LTP) or increased activity of the calcium/calmodulin-dependent protein kinase II (CaMKII) induced delivery of tagged AMPA-Rs into synapses. This effect was not diminished by mutating the CaMKII phosphorylation site on the GluR1 AMPA-R subunit, but was blocked by mutating a predicted PDZ domain interaction site. These results show that LTP and CaMKII activity drive AMPA-Rs to synapses by a mechanism that requires the association between GluR1 and a PDZ domain protein.

Growth cone and dendrite dynamics in zebrafish embryos: early events in synaptogenesis imaged in vivo

We used time-lapse fluorescence microscopy to observe the growth of Mauthner cell axons and their postsynaptic targets, the primary motor neurons, in spinal cords of developing zebrafish embryos. Upon reaching successive motor neurons, the Mauthner growth cone paused briefly before continuing along its path. Varicosities formed at regular intervals and were preferentially associated with the target regions of the primary motor neurons. In addition, the postsynaptic motor neurons showed highly dynamic filopodia, which transiently interacted with both the growth cone and the axon. Both Mauthner cell and motor neurons were highly active, each showing motility sufficient to initiate synaptogenesis.

Rho GTPases regulate distinct aspects of dendritic arbor growth in Xenopus central neurons in vivo

The development and structural plasticity of dendritic arbors are governed by several factors, including synaptic activity, neurotrophins and other growth-regulating molecules. The signal transduction pathways leading to dendritic structural changes are unknown, but likely include cytoskeleton regulatory components. To test whether GTPases regulate dendritic arbor development, we collected time-lapse images of single optic tectal neurons in albino Xenopus tadpoles expressing dominant negative or constitutively active forms of Rac, Cdc42 or RhoA. Analysis of images collected at two-hour intervals over eight hours indicated that enhanced Rac activity selectively increased branch additions and retractions, as did Cdc42 to a lesser extent. Activation of endogenous RhoA decreased branch extension without affecting branch additions and retractions, whereas dominant-negative RhoA increased branch extension. Finally, we provide data suggesting that RhoA mediates the promotion of normal dendritic arbor development by NMDA receptor activation.

Vaccinia as a tool for functional analysis in regenerating limbs: ectopic expression of Shh

Axolotls, with their extensive abilities to regenerate as adults, provide a useful model in which to study the mechanisms of regeneration in a vertebrate, in hopes of understanding why other vertebrates cannot regenerate. Although the expression of many genes has been described in regeneration, techniques for functional analysis have so far been limited. In this paper we demonstrate a new method for efficient overexpression of foreign genes in axolotls. Using vaccinia virus expressing beta-galactosidase microinjected into regenerating limbs, we show that vaccinia can infect both dividing and nondividing limb cells. The site of infection remains discrete and there is no secondary spread of infection to nearby cells. beta-Gal is expressed at high levels in blastema cells for about a week and in differentiated cells for longer. Blastemas that have been injected with vaccinia at different stages regenerate normally. As a test of the utility of vaccinia for functional analysis in regeneration, we constructed a virus expressing Shh and injected it into the anterior of regenerating limbs. Ectopic Shh expression caused extra digits, carpals, and tarsals in the hands and feet of regenerating limbs, suggesting that despite differences in the timing of expression and the eventual pattern, the function of Shh appears to be similar to that in the developing limbs of other vertebrates. Our results demonstrate that vaccinia virus is an excellent vector for ectopically expressing genes for secreted proteins and is a useful tool to study the function of signaling molecules during the process of regeneration in urodeles.

Development of neuron-neuron synapses

Our understanding of neuronal synapse development has advanced in recent years. The development of glycinergic synapses appears to depend on gephyrin and glycine receptor activity. Molecular characterization of the structure and development of glutamatergic synapses is in progress, but the underlying mechanisms remain unclear. Activity-dependent mechanisms and specific molecules that regulate the morphological development of dendritic spines have recently been identified.

Kinetically distinguishable AMPA receptors in rat hippocampus are associated with the loss of glutamate-sensitive conformational transitions

We describe a stopped-flow method to study alpha-amino-7-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-kainate receptor-mediated Na+ ion flux through native membranes. Resealed plasmalemma vesicles and nerve endings from the rat hippocampus were mixed rapidly with a membrane impermeant form of the fluorescence indicator, sodium binding benzofurane oxazole and the changes in fluorescence intensity in response to various [Glu] on the time scale of 0.04 ms-10 s were monitored at a sampling rate of 6.55 kHz. Inhibitors like ouabain (1 mM) and 5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (dizocilpine, 50 microM) enhanced Na+ ion translocation under low-[Na+] and physiological conditions, respectively. Dependence of AMPA-kainate receptor kinetics on [Glu] was described in a model of channel activation by faster and slower desensitizing receptors. The model accounted for almost all of the Na+ ion flux activity in the 30 microM-10 mM range of [Glu]. We found that the values of the initial rate constant for Na+ ion influx, JA, and rate constant for desensitization, alpha, for the faster desensitizing receptor were dependent on data sampling rate, whereas the initial rate constant for Na+ ion flux through the slower desensitizing receptor, JB, varied much less with the sampling rate. These phenomena can be described by (1) a fractal model of short-lived AMPA-kainate receptor channel with many closely spaced states (fractal dimension approximately 1.8) and (2) a model of long-lived AMPA-kainate receptor channel with two discrete states.

Visualization of synaptic activity in hippocampal slices with FM1-43 enabled by fluorescence quenching

Fluorescence imaging of presynaptic uptake and release of styryl dyes such as FM1-43 has provided valuable insights into synaptic function. However, in studies of CNS neurons, the utility of these dyes has been severely limited by nonsynaptic background fluorescence. This has thwarted the use of FM dyes in systems more intact than dissociated neuronal cultures. Here, we describe an approach to selectively reduce undesired fluorescence through quenching of the surface-bound FM1-43 signal. The introduction of sulforhodamine, a fluorophore that is not taken up by synaptic vesicles, selectively reduced the nonsynaptic fluorescence in FM1-43-labeled hippocampal cultures. When applied to rat hippocampal slices, this procedure allowed us to observe activity-dependent staining and destaining of functional synapses. Extending the usefulness of styryl dyes to slice preparations may help make functional synaptic networks amenable to optical measurements.

Lesions of an avian forebrain nucleus that disrupt song development alter synaptic connectivity and transmission in the vocal premotor pathway

The avian forebrain nucleus, the lateral magnocellular nucleus of the anterior neostriatum (LMAN), is necessary for normal song development because LMAN lesions made in juvenile birds disrupt song production but do not disrupt song when made in adults. Although these age-limited behavioral effects implicate LMAN in song learning, a potential confound is that LMAN lesions could disrupt normal vocal motor function independent of any learning role by altering LMAN's premotor target, the song nucleus, the robust nucleus of the archistriatum (RA). To date, however, no studies have examined directly the effects of LMAN lesions on the circuitry of the RA. We report here that juvenile LMAN lesions rapidly and profoundly affect RA, altering synaptic connectivity within this nucleus, including descending inputs from the song nucleus HVc. Specifically, morphological assays of the dendritic spines of RA projection neurons and axon terminal boutons arising from HVc show a numerical decline in the density of connections in RA in LMAN-lesioned juveniles compared with controls. Concurrently, LMAN lesions alter excitatory transmission within the juvenile RA: after LMAN lesions, the stimulus-response relationship between HVc fibers and RA neurons steepens, and the amplitude of spontaneous monophasic EPSCs increases. Rather than arresting RA in a juvenile state, LMAN lesions transform the structure and function of RA and its connections, such that it is distinct from that of the normal juvenile. In many ways, RA circuitry in LMAN-lesioned juveniles resembles that of normal adults, suggesting that LMAN lesions induce a premature maturation of the vocal motor pathway, which may lead to a loss of behavioral plasticity and abnormal song development.

Behaviour of NMDA and AMPA receptor-mediated miniature EPSCs at rat cortical neuron synapses identified by calcium imaging

1. Simultaneous recording of intracellular calcium concentration at a synapse and synaptic currents from the cell body allows mapping of miniature excitatory postsynaptic currents (mEPSCs) to single synapses. 2. In the absence of extracellular Mg2+, 77 % of synapses had mEPSCs with fast and slow components, attributed to AMPA- and NMDA-type glutamate receptors, respectively. The remainder of synapses (23 %) had mEPSCs that lacked a fast component; these responses were attributed to NMDA receptors. 3. A strong positive correlation between the amplitude of the calcium transient and the NMDA receptor-mediated mEPSC was observed, indicating that the mEPSCs originate from an identified synapse. 4. At synapses that had both mEPSC components, the AMPA receptor component was positively correlated with charge influx mediated by NMDA receptors during repeated synaptic events. No periodic failure in the AMPA receptor mEPSC was observed at synapses expressing both receptor components. 5. A significant positive correlation between the mean amplitudes of NMDA and AMPA receptor components of mEPSCs is observed across different synapses. 6. We suggest that factors effecting both receptor classes, such as the amount of transmitter in synaptic vesicles, might contribute to the variation in mEPSC amplitude during repeated miniature events at a single synapse. Although the average postsynaptic response at different synapses can vary in amplitude, there appears to be a mechanism to keep the ratio of each receptor subtype within a narrow range.

CaMKII regulates the density of central glutamatergic synapses in vivo

Synaptic connections undergo a dynamic process of stabilization or elimination during development, and this process is thought to be critical in memory and learning and in establishing the specificity of synaptic connections. The type II calcium- and calmodulin-dependent protein kinase (CaMKII) has been proposed to be pivotal in regulating synaptic strength and in maturation of synapses during development. Here we describe how CaMKII regulates the formation of central glutamatergic synapses in Caenorhabditis elegans. During larval development, the density of ventral nerve cord synapses containing the GLR-1 glutamate receptor is held constant despite marked changes in neurite length. The coupling of synapse number to neurite length requires both CaMKII and voltage-gated calcium channels. CaMKII regulates GLR-1 by at least two distinct mechanisms: regulating transport of GLR-1 from cell bodies to neurites; and regulating the addition or maintenance of GLR-1 to postsynaptic elements.

Postsynaptic calcium/calmodulin-dependent protein kinase II is required to limit elaboration of presynaptic and postsynaptic neuronal arbors

Neuronal dendritic and axonal arbors grow to a characteristic size and then stabilize their structures. Activity-dependent stop-growing signals may limit neuronal process elaboration. We tested whether endogenous calcium/calmodulin-dependent protein kinase II (CaMKII) activity in postsynaptic optic tectal cells is required to restrict the elaboration of neuronal processes in the Xenopus tadpole retinotectal projection. Optic tectal cells were infected with vaccinia viruses that express CaMKII-specific inhibitory peptides. In vivo time-lapse imaging revealed that expression of CaMKII inhibitors blocked the growth restriction that normally occurs during maturation of tectal cell dendritic arbors. Postsynaptic CaMKII inhibition also increased the growth of presynaptic retinotectal axon arbors. The results indicate that endogenous postsynaptic CaMKII activity is required to limit the growth of presynaptic and postsynaptic arbor structures in vivo.

Genes regulating dendritic outgrowth, branching, and routing in Drosophila

Signaling between neurons requires highly specialized subcellular structures, including dendrites and axons. Dendrites exhibit diverse morphologies yet little is known about the mechanisms controlling dendrite formation in vivo. We have developed methods to visualize the stereotyped dendritic morphogenesis in living Drosophila embryos. Dendrite development is altered in prospero mutants and in transgenic embryos expressing a constitutively active form of the small GTPase cdc42. From a genetic screen, we have identified several genes that control different aspects of dendrite development including dendritic outgrowth, branching, and routing. These genes include kakapo, a large cytoskeletal protein related to plectin and dystrophin; flamingo, a seven-transmembrane protein containing cadherin-like repeats; enabled, a substrate of the tyrosine kinase Abl; and nine potentially novel loci. These findings begin to reveal the molecular mechanisms controlling dendritic morphogenesis.

Dendritic dynamics in vivo change during neuronal maturation

In vivo imaging of optic tectal neurons in the intact Xenopus tadpole permits direct observation of the structural dynamics that occur during dendritic arbor formation. Based on images of single DiI-labeled neurons collected at daily intervals over a period of 6 d, we divided tectal cell development into three phases according to the total length of the dendritic arbor. During phase 1, the cell differentiates from a neuroepithelial cell type and extends an axon out of the tectum. The total dendritic branch length (TDBL) is <100 micrometers. During phase 2, when TDBL is 100-400 micrometers, the dendritic arbor grows rapidly. During phase 3, when TDBL is >400 micrometers, the dendritic arbor grows slowly and appears stable. Neurons at different positions along the rostrocaudal developmental axis of the tectum were imaged at 2 hr intervals over 6 hr and at 24 hr intervals over several days. Images collected at 2 hr intervals were analyzed to determine rates of branch additions and retractions. Morphologically complex, phase 3 neurons show half the rate of branch additions and retractions as phase 2 neurons. Therefore, rapidly growing neurons have dynamic dendritic arbors, and slower-growing neurons are structurally stable. The change in growth rate and dendritic arbor dynamics from phase 2 to phase 3 correlates with the developmental increase in synaptic strength in neurons located along the rostrocaudal tectal axis. The data are consistent with the idea that strong synaptic inputs stabilize dendritic arbor structures and that weaker synaptic inputs are permissive for a greater degree of dynamic rearrangements and a faster growth rate in the dendritic arbor.

SNAP-25 requirement for dendritic growth of hippocampal neurons

Structure and dimension of the dendritic arbor are important determinants of information processing by the nerve cell, but mechanisms and molecules involved in dendritic growth are essentially unknown. We investigated early mechanisms of dendritic growth using mouse fetal hippocampal neurons in primary culture, which form processes during the first week in vitro. We detected a key component of regulated exocytosis, SNAP-25 (synaptosomal associated protein of 25 kDa), in axons and axonal terminals as well as in dendrites identified by the occurrence of the dendritic markers transferrin receptor and MAP2. Selective inactivation of SNAP-25 by botulinum neurotoxin A (BoNTA) resulted in inhibition of axonal growth and of vesicle recycling in axonal terminals. In addition, dendritic growth of hippocampal pyramidal and granule neurons was significantly inhibited by BoNTA. In contrast, cleavage of synaptobrevin by tetanus toxin had an effect on neither axonal nor dendritic growth. Our observations indicate that SNAP-25, but not synaptobrevin, is involved in constitutive axonal growth and dendrite formation by hippocampal neurons.

Assembly of signaling machinery at the postsynaptic membrane

The postsynaptic membrane and the subsynaptic cell compartment are specialized for inter- and intracellular signaling. Recent work has focused on the role of synaptic activity in regulating the surface distribution of neurotransmitter receptors. In addition, several components of secondary signaling pathways involved in the long-term regulation of synaptic efficacy have been identified.

Destabilization of cortical dendrites and spines by BDNF

Particle-mediated gene transfer and two-photon microscopy were used to monitor the behavior of dendrites of individual cortical pyramidal neurons coexpressing green fluorescent protein (GFP) and brain-derived neurotrophic factor (BDNF). While the dendrites and spines of neurons expressing GFP alone grew modestly over 24-48 hr, coexpressing BDNF elicited dramatic sprouting of basal dendrites, accompanied by a regression of dendritic spines. Compared to GFP-transfected controls, the newly formed dendrites and spines were highly unstable. Experiments utilizing Trk receptor bodies, K252a, and overexpression of nerve growth factor (NGF) demonstrated that these effects were mediated by secreted BDNF interacting with extracellular TrkB receptors. Thus, BDNF induces structural instability in dendrites and spines, which, when restricted to particular portions of a dendritic arbor, may help translate activity patterns into specific morphological changes.

Homeostatic plasticity in neuronal networks: the more things change, the more they stay the same

During learning and development, neural circuitry is refined, in part, through changes in the number and strength of synapses. Most studies of long-term changes in synaptic strength have concentrated on Hebbian mechanisms, where these changes occur in a synapse-specific manner. While Hebbian mechanisms are important for modifying neuronal circuitry selectively, they might not be sufficient because they tend to destabilize the activity of neuronal networks. Recently, several forms of homeostatic plasticity that stabilize the properties of neural circuits have been identified. These include mechanisms that regulate neuronal excitability, stabilize total synaptic strength, and influence the rate and extent of synapse formation. These forms of homeostatic plasticity are likely to go 'hand-in-glove' with Hebbian mechanisms to allow experience to modify the properties of neuronal networks selectively.

Retinal waves and visual system development

Many pathways in the developing visual system are restructured and become highly organized even before vision occurs. Yet the developmental processes underlying the remodeling of visual connectivity are crucially dependent on retinal activity. Surprisingly, the immature and light-insensitive retina spontaneously generates a pattern of rhythmic bursting activity during the period when the connectivity patterns of retinal ganglion cells are shaped. Spatially, the activity is seen to spread across the retina in the form of waves that bring into synchrony the bursts of neighboring cells. Waves are present in the developing retina of higher and lower vertebrates, which suggests that this form of activity may be a common and fundamental mechanism employed in the activity-dependent refinement of early patterns of visual connections. Unraveling the cues encoded by the waves promises to provide important insights into how interactions driven by specific patterns of activity could lead to the modification of connectivity during development.

Dynamic control of CaMKII translocation and localization in hippocampal neurons by NMDA receptor stimulation

Calcium-calmodulin-dependent protein kinase II (CaMKII) is thought to increase synaptic strength by phosphorylating postsynaptic density (PSD) ion channels and signaling proteins. It is shown that N-methyl-D-aspartate (NMDA) receptor stimulation reversibly translocates green fluorescent protein-tagged CaMKII from an F-actin-bound to a PSD-bound state. The translocation time was controlled by the ratio of expressed beta-CaMKII to alpha-CaMKII isoforms. Although F-actin dissociation into the cytosol required autophosphorylation of or calcium-calmodulin binding to beta-CaMKII, PSD translocation required binding of calcium-calmodulin to either the alpha- or beta-CaMKII subunits. Autophosphorylation of CaMKII indirectly prolongs its PSD localization by increasing the calmodulin-binding affinity.

The role of nitric oxide and NMDA receptors in the development of motor neuron dendrites

Nitric oxide (NO) has been implicated in the establishment of precise synaptic connectivity throughout the neuroaxis in several species. To determine the contribution of NO to NMDA receptor-dependent dendritic growth in motor neurons, we administered the NMDA antagonist MK-801 to wild-type mice and neuronal nitric oxide synthase (nNOS) knock-out mice between postnatal days 7 and 14. Compared to saline-treated wild-type animals the number of dendritic bifurcations was significantly reduced in nNOS knock-out animals and MK-801-treated wild-type animals. There was no significant difference in dendritic bifurcation between MK-801-treated wild-type, MK-801-treated nNOS knock-out, and saline-treated nNOS knock-out animals, suggesting that nNOS knock-out and NMDA receptor block had similar effects. The path of the longest dendrite and the number of primary dendrites was the same in all treatment groups, indicating an effect specific to bifurcation. Sholl analysis revealed that differences in bifurcation numbers occurred between 160 and 320 micrometers from the cell body, the distance at which second, third, and fourth order dendrites are most prevalent. Dendrite order analyses confirmed a significant reduction in numbers, but not lengths, of third and fourth order dendrites in nNOS knock-out and drug-treatment groups. Finally, immunohistochemical examination of the developing spinal cord indicated that NMDA receptors and nNOS are colocalized within interneurons surrounding the motor neuron pool. These results support the view that at least part of NMDA receptor-dependent arborization of motor neuron dendrites is mediated by the local production of NO within the developing spinal cord.

NMDA receptor activity stabilizes presynaptic retinotectal axons and postsynaptic optic tectal cell dendrites in vivo

To investigate the role of N-methyl-D-aspartate (NMDA) receptor activity in the stability of the presynaptic axon arbor and postsynaptic dendritic arbors in vivo, we took time-lapse confocal images of single DiI-labeled Xenopus retinotectal axons and optic tectal neurons in the presence and absence of the NMDA receptor antagonist, APV. Retinotectal axons or tectal neurons were imaged at 30-min intervals over 2 h, or twice over a 24-h period. Retinal axons in animals exposed to DL-APV (100 microM) showed an increase in rates of branch additions and a decrease in branch lifetimes over 2 h compared to untreated axons. Under the same experimental conditions, tectal neurons showed a decreased rate of branch tip additions and retractions. APV treatment over 24 h had no apparent effect on axon arbor morphology, but did decrease tectal cell dendritic arbor elaboration. These observations demonstrate that NMDA receptor activity in postsynaptic neurons stabilizes pre- and postsynaptic neuronal morphology in vivo.. However, when NMDA receptor activity is blocked, presynaptic retinal axons respond with increased arbor dynamics while postsynaptic tectal cell dendrites decrease arbor dynamics. Such differential responses of pre- and postsynaptic partners might increase the probability of coactive afferents converging onto a common target under conditions of lower NMDA receptor activity.

Light-induced calcium influx into retinal axons is regulated by presynaptic nicotinic acetylcholine receptor activity in vivo

Visual activity is thought to be a critical factor in controlling the development of central retinal projections. Neuronal activity increases cytosolic calcium, which was hypothesized to regulate process outgrowth in neurons. We performed an in vivo imaging study in the retinotectal system of albino Xenopus laevis tadpoles with the fluorescent calcium indicator calcium green 1 dextran (CaGD) to test the role of calcium in regulating axon arbor development. We find that visual stimulus to the retina increased CaGD fluorescence intensity in retinal ganglion cell (RGC) axon arbors within the optic tectum and that branch additions to retinotectal axon arbors correlated with a local rise in calcium in the parent branch. We find three types of responses to visual stimulus, which roughly correlate with the ON, OFF, and SUSTAINED response types of RGC reported by physiological criteria. Imaging in bandscan mode indicated that patterns of calcium transients were nonuniform throughout the axons. We tested whether the increase in calcium in the retinotectal axons required synaptic activity in the retina; intraocular application of tetrodotoxin (10 microM) or nifedipine (1 and 10 microM) blocked the stimulus-induced increase in RGC axonal fluorescence. A second series of pharmacological investigations was designed to determine the mechanism of the calcium elevation in the axon terminals within the optic tectum. Injection of bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid-AM (BAPTA-AM) (20 mM) into the tectal ventricle reduced axonal calcium levels, supporting the idea that visual stimulation increases axonal calcium. Injection of BAPTA (20 mM) into the tectal ventricle to chelate extracellular calcium also attenuated the calcium response to visual stimulation, indicating that calcium enters the axon from the extracellular medium. Caffeine (10 mM) caused a large increase in axonal calcium, indicating that intracellular stores contribute to the calcium signal. Presynaptic nicotinic acetylcholine receptors (nAChRs) may play a role in axon arbor development and the formation of the topographic retinotectal projection. Injection of nicotine (10 microM) into the tectal ventricle significantly elevated RGC axonal calcium levels, whereas application of the nAChR antagonist alphaBTX (100 nM) reduced the stimulus-evoked rise in RGC calcium fluorescence. These data suggest that light stimulus to the retina increases calcium in the axon terminal arbors through a mechanism that includes influx through nAChRs and amplification by calcium-induced calcium release from intracellular calcium stores. Such a mechanism may contribute to developmental plasticity of the retinotectal system by influencing both axon arbor elaboration and the strength of synaptic transmission.

TrkB signaling modulates spine density and morphology independent of dendrite structure in cultured neonatal Purkinje cells

Neurotrophins cooperate with neural activity to modulate CNS neuronal survival and dendritic differentiation. In a previous study, we demonstrated that a critical balance of neurotrophin and neural activity is required for Purkinje cell survival in cocultures of purified granule and Purkinje cells (Morrison and Mason, 1998). Here we investigate whether TrkB signaling regulates dendrite and spine development of Purkinje cells. BDNF treatment of purified Purkinje cells cultured alone did not elicit formation of mature dendrites or spines. In cocultures of granule and Purkinje cells, however, continuous treatment with BDNF over a 2 week postnatal culture period increased the density of Purkinje cell dendritic spines relative to controls without causing a shift in the proportions of headed and filopodia-like spines. The increase in spine number was blocked by adding TrkB-IgG to the medium together with BDNF. Although BDNF alone did not consistently modify the morphology of dendritic spines, treatment with TrkB-IgG alone yielded spines with longer necks than those in control cultures. None of these treatments altered Purkinje cell dendritic complexity. These analyses reveal a role for TrkB signaling in modulating spine development, consistent with recently reported effects of neurotrophins on synaptic function. Moreover, spine development can be uncoupled from dendrite outgrowth in this reductionist system of purified presynaptic and postsynaptic neurons.

Topographic maps: developing roles of synaptic plasticity

Recent studies have provided convincing evidence that cellular forms of synaptic plasticity that are thought to represent the building blocks of learning and memory are also used during development to establish organized sensory projections within the brain.

CPEB-mediated cytoplasmic polyadenylation and the regulation of experience-dependent translation of alpha-CaMKII mRNA at synapses

Long-term changes in synaptic efficacy may require the regulated translation of dendritic mRNAs. While the basis of such regulation is unknown, it seemed possible that some features of translational control in development could be recapitulated in neurons. Polyadenylation-induced translation of oocyte mRNAs requires the cis-acting CPE sequence and the CPE-binding protein CPEB. CPEB is also present in the dendritic layers of the hippocampus, at synapses in cultured neurons, and in postsynaptic densities of adult brain. alpha-CaMKII mRNA, which is localized in dendrites and is necessary for synaptic plasticity and LTP, contains two CPEs. These CPEs are bound by CPEB and mediate polyadenylation-induced translation in injected Xenopus oocytes. In the intact brain, visual experience induces alpha-CaMKII mRNA polyadenylation and translation, suggesting that this process likely occurs at synapses.

Glutamate receptor activity is required for normal development of tectal cell dendrites in vivo

Glutamatergic retinotectal inputs mediated principally by NMDA receptors can be recorded from optic tectal neurons early during their morphological development in Xenopus tadpoles. As tectal cell dendrites elaborate, retinotectal synaptic responses acquire an AMPA receptor-mediated synaptic component, in addition to the NMDA component. Here, we tested whether glutamatergic activity was required for the elaboration of dendritic arbors in Xenopus optic tectal neurons. In vivo time-lapse imaging of single DiI-labeled neurons shows that the NMDA receptor antagonist APV (100 microM) blocked the early development of the tectal cell dendritic arbor, whereas the AMPA receptor antagonist CNQX (20 microM) or the sodium channel blocker TTX (1 microM) did not. The decreased dendritic development is attributable to failure to add new branches and extend preexisting branches. These observations indicate that NMDA-type glutamatergic activity promotes the initial development of the dendritic arbor. At later stages of tectal neuron development when AMPA receptor-mediated synaptic transmission is strong, both APV and CNQX decrease dendritic arbor branch length, consistent with a role for glutamatergic synaptic transmission in maintaining dendritic arbor structure. These results indicate that AMPA and NMDA receptors can differentially influence dendritic growth at different stages of neuronal development, in correlation with changes in the relative contribution of the receptor subtype to synaptic transmission.

Promotion of dendritic growth by CPG15, an activity-induced signaling molecule

Activity-independent and activity-dependent mechanisms work in concert to regulate neuronal growth, ensuring the formation of accurate synaptic connections. CPG15, a protein regulated by synaptic activity, functions as a cell-surface growth-promoting molecule in vivo. In Xenopus laevis, CPG15 enhanced dendritic arbor growth in projection neurons, with no effect on interneurons. CPG15 controlled growth of neighboring neurons through an intercellular signaling mechanism that requires its glycosylphosphatidylinositol link. CPG15 may represent a new class of activity-regulated, membrane-bound, growth-promoting proteins that permit exquisite spatial and temporal control of neuronal structure.

CaMKIIbeta functions as an F-actin targeting module that localizes CaMKIIalpha/beta heterooligomers to dendritic spines

Ca2+/calmodulin-dependent protein kinase II (CaMKII) is a serine/threonine protein kinase that regulates long-term potentiation and other forms of neuronal plasticity. Functional differences between the neuronal CaMKIIalpha and CaMKIIbeta isoforms are not yet known. Here, we use green fluorescent protein-tagged (GFP-tagged) CaMKII isoforms and show that CaMKIIbeta is bound to F-actin in dendritic spines and cell cortex while CaMKIIalpha is largely a cytosolic enzyme. When expressed together, the two isoforms form large heterooligomers, and a small fraction of CaMKIIbeta is sufficient to dock the predominant CaMKIIalpha to the actin cytoskeleton. Thus, CaMKIIbeta functions as a targeting module that localizes a much larger number of CaMKIIalpha isozymes to synaptic and cytoskeletal sites of action.