Dendritic Branch Intersections Are Structurally Regulated Targets for Efficient Axonal Wiring and Synaptic Clustering

Synaptic clustering on dendritic branches enhances plasticity, input integration and neuronal firing. However, the mechanisms guiding axons to cluster synapses at appropriate sites along dendritic branches are poorly understood. We searched for such a mechanism by investigating the structural overlap between dendritic branches and axons in a simplified model of neuronal networks - the hippocampal cell culture. Using newly developed software, we converted images of meshes of overlapping axonal and dendrites into topological maps of intersections, enabling quantitative study of overlapping neuritic geometry at the resolution of single dendritic branch-to-branch and axon-to-branch crossings. Among dendro-dendritic crossing configurations, it was revealed that the orientations through which dendritic branches cross is a regulated attribute. While crossing angle distribution among branches thinner than 1 µm appeared to be random, dendritic branches 1 µm or wider showed a preference for crossing each other at angle ranges of either 50°–70° or 80°–90°. It was then found that the dendro-dendritic crossings themselves, as well as their selective angles, both affected the path of axonal growth. Axons displayed 4 fold stronger tendency to traverse within 2 µm of dendro-dendritic intersections than at farther distances, probably to minimize wiring length. Moreover, almost 70% of the 50°–70° dendro-denritic crossings were traversed by axons from the obtuse angle’s zone, whereas only 15% traversed through the acute angle’s zone. By contrast, axons showed no orientation restriction when traversing 80°–90° crossings. When such traverse behavior was repeated by many axons, they converged in the vicinity of dendro-dendritic intersections, thereby clustering their synaptic connections. Thus, the vicinity of dendritic branch-to-branch crossings appears to be a regulated structure used by axons as a target for efficient wiring and as a preferred site for synaptic clustering. This synaptic clustering mechanism may enhance synaptic co-activity and plasticity.     

Independently Outgrowing Neurons and Geometry-Based Synapse Formation Produce Networks with Realistic Synaptic Connectivity

Neuronal signal integration and information processing in cortical networks critically depend on the organization of synaptic connectivity. During development, neurons can form synaptic connections when their axonal and dendritic arborizations come within close proximity of each other. Although many signaling cues are thought to be involved in guiding neuronal extensions, the extent to which accidental appositions between axons and dendrites can already account for synaptic connectivity remains unclear. To investigate this, we generated a local network of cortical L2/3 neurons that grew out independently of each other and that were not guided by any extracellular cues. Synapses were formed when axonal and dendritic branches came by chance within a threshold distance of each other. Despite the absence of guidance cues, we found that the emerging synaptic connectivity showed a good agreement with available experimental data on spatial locations of synapses on dendrites and axons, number of synapses by which neurons are connected, connection probability between neurons, distance between connected neurons, and pattern of synaptic connectivity. The connectivity pattern had a small-world topology but was not scale free. Together, our results suggest that baseline synaptic connectivity in local cortical circuits may largely result from accidentally overlapping axonal and dendritic branches of independently outgrowing neurons.     

Wnt Signaling in the Vertebrate Central Nervous System: From Axon Guidance to Synaptic Function

Regulation of cell signaling by Wnt proteins is critical for the formation of neuronal circuits. Wnts modulate axon pathfinding, dendritic development, and synaptic assembly. Through different receptors, Wnts activate diverse signaling pathways that lead to local changes on the cytoskeleton or global cellular changes involving nuclear function. Recently, a link between neuronal activity, essential for the formation and refinement of neuronal connections, and Wnt signaling has been uncovered. Indeed, neuronal activity regulates the release of Wnt and the localization of their receptors. Wnts mediate synaptic structural changes induced by neuronal activity or experience. New emerging evidence suggests that dysfunction in Wnt signaling contributes to neurological disorders. In this article, the attention is focused on the function of Wnt signaling in the formation of neuronal circuits in the vertebrate central nervous system.The formation of neuronal connections requires the navigation of axons to their appropriate synaptic targets, the formation of terminal branches, and the assembly of functional synapses. These processes greatly depend on the proper dialogue between axons and their environment as they navigate to their target, and between axons and their postsynaptic dendrites during synapse assembly. A combination of secreted molecules and transmembrane proteins modulates these processes. Studies over the last 10 years have revealed an essential role for Wnt signaling in axon pathfinding, dendritic development, and synapse assembly in both central and peripheral nervous systems. Wnts also modulate basal synaptic transmission and the structural and functional plasticity of synapses in the central nervous system. Studies of Wnts in the nervous system have significantly contributed to our current understanding of the molecular mechanisms that control neuronal circuit assembly. These studies have also shed light into fundamental aspects of cell signaling such as novel mechanisms of protein secretion (Korkut et al. 2009) and receptor dynamics (Sahores et al. 2010). Here I review the mechanisms by which Wnts modulate axon guidance and synapse formation in the vertebrate central nervous system. I also discuss the increasing evidence in support for a role of Wnts in basal synaptic transmission, synaptic plasticity, and neurological disorders.     

In vivo time-lapse imaging and serial section electron microscopy reveal developmental synaptic rearrangements

Dendrites, axons, and synapses are dynamic during circuit development; however, changes in microcircuit connections as branches stabilize have not been directly demonstrated. By combining in?vivo time-lapse imaging of Xenopus tectal neurons with electron microscope reconstructions of imaged neurons, we report the distribution and ultrastructure of synapses on individual vertebrate neurons and relate these synaptic properties to dynamics in dendritic and axonal arbor structure over hours or?days of imaging. Dynamic dendrites have a high density of immature synapses, whereas stable dendrites have sparser, mature synapses. Axons initiate contacts from multisynapse boutons on stable branches. Connections are refined by decreasing convergence from multiple inputs to postsynaptic dendrites and by decreasing divergence from multisynapse boutons to postsynaptic sites. Visual deprivation or NMDAR antagonists decreased synapse maturation and elimination, suggesting that coactive input activity promotes microcircuit development by concurrently regulating synapse elimination and maturation of remaining contacts.     

Homozygous SLC6A17 Mutations Cause Autosomal-Recessive Intellectual Disability with Progressive Tremor,Speech Impairment,and Behavioral Problems

We report on Dutch and Iranian families with affected individuals who present with moderate to severe intellectual disability and additional phenotypes including progressive tremor, speech impairment, and behavioral problems in certain individuals. A combination of exome sequencing and homozygosity mapping revealed homozygous mutations c.484G>A (p.Gly162Arg) and c.1898C>G (p.Pro633Arg) in SLC6A17. SLC6A17 is predominantly expressed in the brain, encodes a synaptic vesicular transporter of neutral amino acids and glutamate, and plays an important role in the regulation of glutamatergic synapses. Prediction programs and 3D modeling suggest that the identified mutations are deleterious to protein function. To directly test the functional consequences, we investigated the neuronal subcellular localization of overexpressed wild-type and mutant variants in mouse primary hippocampal neuronal cells. Wild-type protein was present in soma, axons, dendrites, and dendritic spines. p.Pro633Arg altered SLC6A17 was found in soma and proximal dendrites but did not reach spines. p.Gly162Arg altered SLC6A17 showed a normal subcellular distribution but was associated with an abnormal neuronal morphology mainly characterized by the loss of dendritic spines. In summary, our genetic findings implicate homozygous SLC6A17 mutations in autosomal-recessive intellectual disability, and their pathogenic role is strengthened by genetic evidence and in silico and in vitro functional analyses.     

Patterning and organization of motor neuron dendrites in the Drosophila larva

Precise patterns of motor neuron connectivity depend on the proper establishment and positioning of the dendritic arbor. However, how different motor neurons orient their dendrites to selectively establish synaptic connectivity is not well understood. The Drosophila neuromuscular system provides a simple model to investigate the underlying organizational principles by which distinct subclasses of motor neurons orient their dendrites within the central neuropil. Here we used genetic mosaic techniques to characterize the diverse dendritic morphologies of individual motor neurons from five main nerve branches (ISN, ISNb, ISNd, SNa, and SNc) in the Drosophila larva. We found that motor neurons from different nerve branches project their dendrites to largely stereotyped mediolateral domains in the dorsal region of the neuropil providing full coverage of the receptive territory. Furthermore, dendrites from different motor neurons overlap extensively, regardless of subclass, suggesting that repulsive dendrite-dendrite interactions between motor neurons do not influence the mediolateral positioning of dendritic fields. The anatomical data in this study provide important information regarding how different subclasses of motor neurons organize their dendrites and establishes a foundation for the investigation of the mechanisms that control synaptic connectivity in the Drosophila motor circuit.     

A Highlights from MBoC Selection: γ-Tubulin controls neuronal microtubule polarity independently of Golgi outposts

Neurons have highly polarized arrangements of microtubules, but it is incompletely understood how microtubule polarity is controlled in either axons or dendrites. To explore whether microtubule nucleation by γ-tubulin might contribute to polarity, we analyzed neuronal microtubules in Drosophila containing gain- or loss-of-function alleles of γ-tubulin. Both increased and decreased activity of γ-tubulin, the core microtubule nucleation protein, altered microtubule polarity in axons and dendrites, suggesting a close link between regulation of nucleation and polarity. To test whether nucleation might locally regulate polarity in axons and dendrites, we examined the distribution of γ-tubulin. Consistent with local nucleation, tagged and endogenous γ-tubulins were found in specific positions in dendrites and axons. Because the Golgi complex can house nucleation sites, we explored whether microtubule nucleation might occur at dendritic Golgi outposts. However, distinct Golgi outposts were not present in all dendrites that required regulated nucleation for polarity. Moreover, when we dragged the Golgi out of dendrites with an activated kinesin, γ-tubulin remained in dendrites. We conclude that regulated microtubule nucleation controls neuronal microtubule polarity but that the Golgi complex is not directly involved in housing nucleation sites.     

Development of the dendritic branching pattern of the Medial Giant Interneuron in the grasshopper embryo

The embryonic development of the grasshopper's Medial Giant Interneuron (MGI) was examined by injecting the cell with the fluorescent dye Lucifer Yellow at a series of stages in its growth. Particular attention was given to the way in which this neuron constructs its stereotyped dendritic branching pattern. The MGI's dendrites originate as secondary processes which sprout at characteristic points along the neurite after the primary growth cone has passed. These processes then arborize to form a miniature version of their adult branching pattern before the end of embryonic life. While growing, the dendritic branches are covered with a radiant profusion of filopodia; however, these filopodia are ephemeral structures and disappear once the cell matures. By contrast there is no significant reduction in either the number or the spatial extent of the actual dendrites at any embryonic stage. This implies that the stereotyped branching pattern of the mature MGI is primarily determined by a precise pattern of initial growth, and that secondary pruning of branches does not play an important role in shaping the final form of this cell. The coordinate ingrowth of the first cercal sensory axons was examined by cobalt filling the embryonic nerve, and the means by which these sensory axons make their initial contacts with the MGI's dendrites is herein discussed. The following paper considers the degree to which this sensory innervation regulates dendritic growth and branching.     

Synaptic activity controls autophagic vacuole motility and function in dendrites

Macroautophagy (hereafter “autophagy”) is a lysosomal degradation pathway that is important for learning and memory, suggesting critical roles for autophagy at the neuronal synapse. Little is known, however, about the molecular details of how autophagy is regulated with synaptic activity. Here, we used live-cell confocal microscopy to define the autophagy pathway in primary hippocampal neurons under various paradigms of synaptic activity. We found that synaptic activity regulates the motility of autophagic vacuoles (AVs) in dendrites. Stimulation of synaptic activity dampens AV motility, whereas silencing synaptic activity induces AV motility. Activity-dependent effects on dendritic AV motility are local and reversible. Importantly, these effects are compartment specific, occurring in dendrites and not in axons. Most strikingly, synaptic activity increases the presence of degradative autolysosomes in dendrites and not in axons. On the basis of our findings, we propose a model whereby synaptic activity locally controls AV dynamics and function within dendrites that may regulate the synaptic proteome.     

Quantification of Filamentous Actin (F-actin) Puncta in Rat Cortical Neurons

Filamentous actin protein (F-actin) plays a major role in spinogenesis, synaptic plasticity, and synaptic stability. Changes in dendritic F-actin rich structures suggest alterations in synaptic integrity and connectivity. Here we provide a detailed protocol for culturing primary rat cortical neurons, Phalloidin staining for F-actin puncta, and subsequent quantification techniques. First, the frontal cortex of E18 rat embryos are dissociated into low-density cell culture, then the neurons grown in vitro for at least 12-14 days. Following experimental treatment, the cortical neurons are stained with AlexaFluor 488 Phalloidin (to label the dendritic F-actin puncta) and microtubule-associated protein 2 (MAP2; to validate the neuronal cells and dendritic integrity). Finally, specialized software is used to analyze and quantify randomly selected neuronal dendrites. F-actin rich structures are identified on second order dendritic branches (length range 25-75 µm) with continuous MAP2 immunofluorescence. The protocol presented here will be a useful method for investigating changes in dendritic synapse structures subsequent to experimental treatments.     

Role of Electrical Activity in Horizontal Axon Growth in the Developing Cortex: A Time-Lapse Study Using Optogenetic Stimulation

During development, layer 2/3 neurons in the neocortex extend their axons horizontally, within the same layers, and stop growing at appropriate locations to form branches and synaptic connections. Firing and synaptic activity are thought to be involved in this process, but how neuronal activity regulates axonal growth is not clear. Here, we studied axonal growth of layer 2/3 neurons by exciting cell bodies or axonal processes in organotypic slice cultures of the rat cortex. For neuronal stimulation and morphological observation, plasmids encoding channelrhodopsin-2 (ChR2) and DsRed were coelectroporated into a small number of layer 2/3 cells. Firing activity induced by photostimulation (475 nm) was confirmed by whole-cell patch recording. Axonal growth was observed by time-lapse confocal microscopy, using a different excitation wavelength (560 nm), at 10–20-min intervals for several hours. During the first week in vitro, when spontaneous neuronal activity is low, DsRed- and ChR2-expressing axons grew at a constant rate. When high-frequency photostimulation (4 or 10 Hz) for 1 min was applied to the soma or axon, most axons paused in their growth. In contrast, lower-frequency stimulation did not elicit this pause behavior. Moreover, in the presence of tetrodotoxin, even high-frequency stimulation did not cause axonal growth to pause. These results indicate that increasing firing activity during development suppresses axon growth, suggesting the importance of neuronal activity for the formation of horizontal connections.     

Afferent innervation shapes the dendritic branching pattern of the Medial Giant Interneuron in grasshopper embryos raised in culture

In the adult grasshopper the Medial Giant Interneuron (MGI) receives synaptic input from the peripheral sensory neurons of the cercus. We prevented this innervation in grasshopper embryos by cutting off one or both cerci at a stage when the first sensory axons are just beginning to reach the central nervous system (CNS), and the MGI has not yet formed its mature branching pattern. Following this operation the embryos were raised in vitro for 3–9 days, and the MGI injected with the fluorescent dye Lucifer Yellow to determine its morphology. The development of the deprived cells was then compared to that of the normal MGI (described in M. Shankland and C. S. Goodman, 1982, Develop. Biol., 92, 483–500) and of cultured, but unoperated, controls to ascertain whether these presynaptic axons influence the embryonic growth and branching of the MGI's dendrites. The results of these experiments show that dendrite formation is enhanced in regions of the neuropil containing sensory axon terminals and that the afferents exert their influence locally on restricted portions of the branching structure. The enhanced growth of innervated dendrites appears to occur at the expense of dendritic outgrowth elsewhere, suggesting that the growing dendrites may be competing for a limited supply of some cellular component necessary for continued growth. Thus, the MGI's final branching pattern is at least partially dictated by the spatial distribution of presynaptic axons within the embryonic nervous system.     

Glutamate-Bound NMDARs Arising from In Vivo-like Network Activity Extend Spatio-temporal Integration in a L5 Cortical Pyramidal Cell Model

In vivo, cortical pyramidal cells are bombarded by asynchronous synaptic input arising from ongoing network activity. However, little is known about how such ‘background’ synaptic input interacts with nonlinear dendritic mechanisms. We have modified an existing model of a layer 5 (L5) pyramidal cell to explore how dendritic integration in the apical dendritic tuft could be altered by the levels of network activity observed in vivo. Here we show that asynchronous background excitatory input increases neuronal gain and extends both temporal and spatial integration of stimulus-evoked synaptic input onto the dendritic tuft. Addition of fast and slow inhibitory synaptic conductances, with properties similar to those from dendritic targeting interneurons, that provided a ‘balanced’ background configuration, partially counteracted these effects, suggesting that inhibition can tune spatio-temporal integration in the tuft. Excitatory background input lowered the threshold for NMDA receptor-mediated dendritic spikes, extended their duration and increased the probability of additional regenerative events occurring in neighbouring branches. These effects were also observed in a passive model where all the non-synaptic voltage-gated conductances were removed. Our results show that glutamate-bound NMDA receptors arising from ongoing network activity can provide a powerful spatially distributed nonlinear dendritic conductance. This may enable L5 pyramidal cells to change their integrative properties as a function of local network activity, potentially allowing both clustered and spatially distributed synaptic inputs to be integrated over extended timescales.     

Heterogeneous synaptic covering and differential charge transfer sensitivity among the dendrites of a reconstructed abducens motor neurone: Correlations between electron microscopic and computer simulation data

Ultrastructural studies on the synaptology of dendritic arborizations of motoneurones have been problematic because dendrites are very thin in relation to their great length, and most of the studies on this topic have therefore dealt with only small parts of the dendritic tree. Here we compared the ultrastructural characteristics of the axon terminals distributed along the various dendrites of a single motoneurone. For this purpose, the light microscopic 3D reconstruction of the dendritic arborization of an intracellularly labelled abducens motoneurone was combined with an electron microscopic analysis of its synaptic contacts. Dendritic profiles were randomly sampled along the various dendrites and the axon terminals they received were classified on the basis of their ultrastructural features and their GABA-immunoreactivity. It emerged that the various dendrites differed according to the type and local arrangement of their synaptic inputs. Our second aim was to incorporate the morphological data obtained into a model giving the charge transfer effectiveness T(x) of the dendritic sites. The sensitivity S(x) of T(x) to changes in the membrane resistivity (Rm) simulating various levels of tonic synaptic activity was calculated. It turned out that both the proximal and distal regions of the dendritic arborization have a dense synaptic covering and a weak sensitivity to changes in the Rm, whereas the intermediate dendrites have a sparse synaptic covering and a high sensitivity to changes in tonic synaptic activity. This pattern of organisation might mediate the “gating” of a population of synapses covering some dendritic regions in a state-dependent fashion.     

A new case for a presynaptic role of dendrites: An immunocytochemical study of the N. Raphé Dorsalis

The distribution of serotonin (5-HT) was determined by the application of the prembedding peroxidase-anti-peroxidase (PAP) technique in vibratome and ultrathin sections of the brain stem. The antiserum stained the neuronal groups B₁ to B₉. Somata, dendrites and axons of multipolar and bipolar neurons were recognized in the usual locations. The most commonly found profiles in the area of the n.raphe dorsalis were dendrites. The search for axon terminals was unsuccesful. The labeled dendrites appear in synaptic contact with unlabeled endings containing round or pleomorphic vesicles, and occasionally some large dense core vesicles. Contacts between two labeled dendrites or processes were not found. Occasionally a dendrodendritic junction between a 5-HT labeled dendrite and an unlabeled dendrite has been found. There are areas of the dendritic membrane free of synaptic junctions and free of glial insulation. Results are discussed in relation with the previously proposed presynaptic role of the dendrites in the neuronal circuitry of then. raphé dorsalis.Special Issue dedicated to Prof. Eduardo De Robertis.Research supported by grants from the CONICET and SECYT, Argentina.     

Dendritic Targeting in the Leg Neuropil of Drosophila: The Role of Midline Signalling Molecules in Generating a Myotopic Map

Neural maps are emergent, highly ordered structures that are essential for organizing and presenting synaptic information. Within the embryonic nervous system of Drosophila motoneuron dendrites are organized topographically as a myotopic map that reflects their pattern of innervation in the muscle field. Here we reveal that this fundamental organizational principle exists in adult Drosophila, where the dendrites of leg motoneurons also generate a myotopic map. A single postembryonic neuroblast sequentially generates different leg motoneuron subtypes, starting with those innervating proximal targets and medial neuropil regions and producing progeny that innervate distal muscle targets and lateral neuropil later in the lineage. Thus the cellular distinctions in peripheral targets and central dendritic domains, which make up the myotopic map, are linked to the birth-order of these motoneurons. Our developmental analysis of dendrite growth reveals that this myotopic map is generated by targeting. We demonstrate that the medio-lateral positioning of motoneuron dendrites in the leg neuropil is controlled by the midline signalling systems Slit-Robo and Netrin-Fra. These results reveal that dendritic targeting plays a major role in the formation of myotopic maps and suggests that the coordinate spatial control of both pre- and postsynaptic elements by global neuropilar signals may be an important mechanism for establishing the specificity of synaptic connections.     

Midline Signalling Systems Direct the Formation of a Neural Map by Dendritic Targeting in the Drosophila Motor System

A fundamental strategy for organising connections in the nervous system is the formation of neural maps. Map formation has been most intensively studied in sensory systems where the central arrangement of axon terminals reflects the distribution of sensory neuron cell bodies in the periphery or the sensory modality. This straightforward link between anatomy and function has facilitated tremendous progress in identifying cellular and molecular mechanisms that underpin map development. Much less is known about the way in which networks that underlie locomotion are organised. We recently showed that in the Drosophila embryo, dendrites of motorneurons form a neural map, being arranged topographically in the antero-posterior axis to represent the distribution of their target muscles in the periphery. However, the way in which a dendritic myotopic map forms has not been resolved and whether postsynaptic dendrites are involved in establishing sets of connections has been relatively little explored. In this study, we show that motorneurons also form a myotopic map in a second neuropile axis, with respect to the ventral midline, and they achieve this by targeting their dendrites to distinct medio-lateral territories. We demonstrate that this map is “hard-wired”; that is, it forms in the absence of excitatory synaptic inputs or when presynaptic terminals have been displaced. We show that the midline signalling systems Slit/Robo and Netrin/Frazzled are the main molecular mechanisms that underlie dendritic targeting with respect to the midline. Robo and Frazzled are required cell-autonomously in motorneurons and the balance of their opposite actions determines the dendritic target territory. A quantitative analysis shows that dendritic morphology emerges as guidance cue receptors determine the distribution of the available dendrites, whose total length and branching frequency are specified by other cell intrinsic programmes. Our results suggest that the formation of dendritic myotopic maps in response to midline guidance cues may be a conserved strategy for organising connections in motor systems. We further propose that sets of connections may be specified, at least to a degree, by global patterning systems that deliver pre- and postsynaptic partner terminals to common “meeting regions.”     

Protein tyrosine phosphatase PTPRT as a regulator of synaptic formation and neuronal development

PTPRT/RPTPρ is the most recently isolated member of the type IIB receptor-type protein tyrosine phosphatase family and its expression is restricted to the nervous system. PTPRT plays a critical role in regulation of synaptic formation and neuronal development. When PTPRT was overexpressed in hippocampal neurons, synaptic formation and dendritic arborization were induced. On the other hand, knockdown of PTPRT decreased neuronal transmission and attenuated neuronal development. PTPRT strengthened neuronal synapses by forming homophilic trans dimers with each other and heterophilic cis complexes with neuronal adhesion molecules. Fyn tyrosine kinase regulated PTPRT activity through phosphorylation of tyrosine 912 within the membrane-proximal catalytic domain of PTPRT. Phosphorylation induced homophilic cis dimerization of PTPRT and resulted in the inhibition of phosphatase activity. BCR-Rac1 GAP and Syntaxin-binding protein were found as new endogenous substrates of PTPRT in rat brain. PTPRT induced polymerization of actin cytoskeleton that determined the morphologies of dendrites and spines by inhibiting BCR-Rac1 GAP activity. Additionally, PTPRT appeared to regulate neurotransmitter release through reinforcement of interactions between Syntaxin-binding protein and Syntaxin, a SNARE protein. In conclusion, PTPRT regulates synaptic function and neuronal development through interactions with neuronal adhesion molecules and the dephosphorylation of synaptic molecules. [BMB Reports 2015; 48(5): 249-255]     

In vivo imaging reveals dendritic targeting of laminated afferents by zebrafish retinal ganglion cells

Targeting of axons and dendrites to particular synaptic laminae is an important mechanism by which precise patterns of neuronal connectivity are established. Although axons target specific laminae during development, dendritic lamination has been thought to occur largely by pruning of inappropriately placed arbors. We discovered by in vivo time-lapse imaging that retinal ganglion cell (RGC) dendrites in zebrafish show growth patterns implicating dendritic targeting as a mechanism for contacting appropriate synaptic partners. Populations of RGCs labeled in transgenic animals establish distinct dendritic strata sequentially, predominantly from the inner to outer retina. Imaging individual cells over successive days confirmed that multistratified RGCs generate strata sequentially, each arbor elaborating within a specific lamina. Simultaneous imaging of RGCs and subpopulations of presynaptic amacrine interneurons revealed that RGC dendrites appear to target amacrine plexuses that had already laminated. Dendritic targeting of prepatterned afferents may thus be a novel mechanism for establishing proper synaptic connectivity.     

Formation of precise connections in the olfactory bulb occurs in the absence of odorant-evoked neuronal activity

Olfactory neurons expressing the same odorant receptor converge to a small number of glomeruli in the olfactory bulb. In turn, mitral and tufted cells receive and relay this information to higher cortical regions. In other sensory systems, correlated neuronal activity is thought to refine synaptic connections during development. We asked whether the pattern of connections between olfactory sensory axons and mitral cell dendrites is affected when odor-evoked signaling is eliminated in mice lacking functional olfactory cyclic nucleotide-gated (CNG) channels. We demonstrate that olfactory sensory axons converge normally in the CNG channel mutant background. We further show that the pruning of mitral cell dendrites, although slowed during development, is ultimately unperturbed in mutant animals. Thus, the olfactory CNG channel-and by inference correlated neural activity--is not required for generating synaptic specificity in the olfactory bulb.