Complex Neuronal Networks Research Articles

Notum Homolog Plays a Novel Role in Primary Motor Innervation

To form complex neuronal networks, growth cones use intermediate targets as guideposts on the path to more distant targets. In the developing zebrafish (Danio rerio), the muscle pioneers (MPs) are intermediate targets for primary motor neurons (PMNs) that innervate the trunk musculature. The mechanisms regulating PMN axon guidance at the MPs are not fully understood. We have identified a new member of the Notum family in zebrafish, Notum 2, which is expressed exclusively in the MPs during primary motor innervation. While homologs of Notum, including zebrafish Notum 1a, negatively regulate the Wnt/β-catenin signaling pathway, we discovered a novel function of Notum 2 in regulating motor axon guidance. Knockdown of Notum 2 resulted in a failure of caudal primary (CaP) axons to migrate beyond the MPs, despite the proper specification of the intermediate target. In contrast, mosaic Notum 2 overexpression induced branching of PMN axons. This effect is specific to Notum 2, as overexpression of Notum 1a does not affect PMN axon trajectory. Ectopic expression of Notum 2 by cells contacting the growing CaP axon induced the highest frequency of branching, suggesting that localized Notum 2 expression affects axon behavior. We propose a model where Notum 2 expression at the MPs provides a cue to release CaP motor axons from their intermediate targets, allowing growth cones to proceed to secondary targets in the ventral muscle. This work demonstrates an unexpected role for a Notum homolog in regulating growth cone migration, separate from the well established functions of other Notum homologs in Wnt signaling.

EFFICACY OF A NEW CHARGE-BALANCED BIPHASIC ELECTRICAL STIMULUS IN THE ISOLATED SCIATIC NERVE AND THE HIPPOCAMPAL SLICE

Most deep brain stimulators apply rectangular monophasic voltage pulses. By modifying the stimulus shape, it is possible to optimize stimulus efficacy and find the best compromise between clinical effect, minimal side effects and power consumption of the stimulus generator. In this study, we compared the efficacy of three types of charge-balanced biphasic pulses (CBBPs, nominal duration 100 μs) in isolated sciatic nerves and in in vitro hippocampal brain slices of the rat. Using these two models, we tested the efficacy of several stimulus shapes exclusively on axons (in the sciatic nerve) and compared the effect with that of stimuli in the more complex neuronal network of the hippocampal slice by considering the stimulus-response relation. We showed that (i) adding an interphase gap (IPG, range 100-500 μs) to the CBBP enhances stimulus efficacy in the rat sciatic nerve and (ii) that this type of stimuli (CBBP with IPG) is also more effective in hippocampal slices. This benefit was similar for both models of voltage and current stimulation. In our two models, asymmetric CBBPs were less beneficial. Therefore, CBBPs with IPG appear to be well suited for application to DBS, since they enhance efficacy, extend battery life and potentially reduce harmful side effects.

Enhanced Survival and Neurite Network Formation of Human Umbilical Cord Blood Neuronal Progenitors in Three-Dimensional Collagen Constructs

Umbilical cord blood (CB) stem cells have been proposed for cell-based therapeutic applications for diverse diseases of the CNS. We hypothesized that tissue-engineering strategies may extend the efficacy of these approaches by improving the long-term viability and function of stem cell-derived neuronal progenitors. To test our hypothesis, we explored the survival and differentiation of human CB-derived neuronal progenitors (HUCBNP) in a three-dimensional (3D) collagen construct. In contrast to two-dimensional culture conditions, the cells survived in 3D for an extended period of time of more than 2months. Under 3D conditions, HUCBNP underwent spontaneous neuronal differentiation, which was further enhanced by treatment with neuronal conditioned medium (CM) and nerve growth factor (NGF). Neurite outgrowth, quantified by assessing the fractal dimension (D f) of the complex neuronal networks, was significantly enhanced under 3D conditions in the presence of CM/NGF, concomitant with a reduced expression of the early neuronal marker nestin (1.9-fold), and increased levels of mature neuronal markers such as MAP-2 (3.6-fold), β-tubulin (1.5-fold), and neuronal specific enolase (6.6-fold) and the appearance of the synaptic marker synaptophysin. To assess the feasibility for clinical usage, HUCBNP were also isolated from frozen CB samples and cultured under 3D conditions. The data indicate the essential complete preservation of neurotrophic (survival) and neurotropic (neurite outgrowth) properties. In conclusion, 3D culture conditions are proposed as an essential step for both maintenance of CB neuronal progenitors in vitro and for investigating specific features of neuronal differentiation towards future use in regenerative therapy.

Development and developmental disorders of the enteric nervous system

The enteric nervous system (ENS) arises from neural crest-derived cells that migrate into and along the gut, leading to the formation of a complex network of neurons and glial cells that regulates motility, secretion and blood flow. This Review summarizes the progress made in the past 5 years in our understanding of ENS development, including the migratory pathways of neural crest-derived cells as they colonize the gut. The importance of interactions between neural crest-derived cells, between signalling pathways and between developmental processes (such as proliferation and migration) in ensuring the correct development of the ENS is also presented. The signalling pathways involved in ENS development that were determined using animal models are also described, as is the evidence for the involvement of the genes encoding these molecules in Hirschsprung disease-the best characterized paediatric enteric neuropathy. Finally, the aetiology and treatment of Hirschsprung disease in the clinic and the potential involvement of defects in ENS development in other paediatric motility disorders are outlined.

Doubly stochastic coherence in complex neuronal networks

A system composed of coupled FitzHugh-Nagumo neurons with various topological structures is investigated under the co-presence of two independently additive and multiplicative Gaussian white noises, in which particular attention is paid to the neuronal networks spiking regularity. As the additive noise intensity and the multiplicative noise intensity are simultaneously adjusted to optimal values, the temporal periodicity of the output of the system reaches the maximum, indicating the occurrence of doubly stochastic coherence. The network topology randomness exerts different influences on the temporal coherence of the spiking oscillation for dissimilar coupling strength regimes. At a small coupling strength, the spiking regularity shows nearly no difference in the regular, small-world, and completely random networks. At an intermediate coupling strength, the temporal periodicity in a small-world neuronal network can be improved slightly by adding a small fraction of long-range connections. At a large coupling strength, the dynamical behavior of the neurons completely loses the resonance property with regard to the additive noise intensity or the multiplicative noise intensity, and the spiking regularity decreases considerably with the increase of the network topology randomness. The network topology randomness plays more of a depressed role than a favorable role in improving the temporal coherence of the spiking oscillation in the neuronal network research study.

The Role of octopamine and tyramine in Drosophila larval locomotion

The characteristic crawling behavior of Drosophila larvae consists of a series of rhythmic waves of peristalsis and episodes of head swinging and turning. The two biogenic amines octopamine and tyramine have recently been shown to modulate various parameters of locomotion, such as muscle contraction, the time spent in pausing or forward locomotion, and the initiation and maintenance of rhythmic motor patterns. By using mutants having altered octopamine and tyramine levels and by genetic interference with both systems we confirm that signaling of these two amines is necessary for larval locomotion. We show that a small set of about 40 octopaminergic/tyraminergic neurons within the ventral nerve cord is sufficient to trigger proper larval locomotion. Using single-cell clones, we describe the morphology of these neurons individually. Given various potential roles of octopamine and tyramine in the larval brain, such as locomotion, learning and memory, stress-induced behaviors or the regulation of the energy state, functions that are often not easy to discriminate, we dissect here for the first time a subset of this complex circuit that modulates specifically larval locomotion. Thus, these data will help to understand-for a given neuronal modulator-how specific behavioral functions are executed within distinct subcircuits of a complex neuronal network.

Kisspeptins and Reproduction: Physiological Roles and Regulatory Mechanisms

Procreation is essential for survival of species. Not surprisingly, complex neuronal networks have evolved to mediate the diverse internal and external environmental inputs that regulate reproduction in vertebrates. Ultimately, these regulatory factors impinge, directly or indirectly, on a final common pathway, the neurons producing the gonadotropin-releasing hormone (GnRH), which stimulates pituitary gonadotropin secretion and thereby gonadal function. Compelling evidence, accumulated in the last few years, has revealed that kisspeptins, a family of neuropeptides encoded by the Kiss1 gene and produced mainly by neuronal clusters at discrete hypothalamic nuclei, are pivotal upstream regulators of GnRH neurons. As such, kisspeptins have emerged as important gatekeepers of key aspects of reproductive maturation and function, from sexual differentiation of the brain and puberty onset to adult regulation of gonadotropin secretion and the metabolic control of fertility. This review aims to provide a comprehensive account of the state-of-the-art in the field of kisspeptin physiology by covering in-depth the consensus knowledge on the major molecular features, biological effects, and mechanisms of action of kisspeptins in mammals and, to a lesser extent, in nonmammalian vertebrates. This review will also address unsolved and contentious issues to set the scene for future research challenges in the area. By doing so, we aim to endow the reader with a critical and updated view of the physiological roles and potential translational relevance of kisspeptins in the integral control of reproductive function.

Lethality of complex neuronal network in Caenorhabditis elegans nervous system based on cell attacks

The aim of this study was to investigate changes in network structural properties and functional perturbations of the C. elegans network which were induced by simulated lesions of the neural network through removals of each single neuron (attacks). We analyzed complete neuronal wiring data (i.e. connectome) of the nematode C. elegans [1] consisting of 279 neurons (nodes) and their connections (edges). We constructed the circular wiring diagram of simply combined network of gap junctions and chemical synapses as shown Figure ​Figure1.1. Then, we measured several measures of complex network properties of directed weighted neuronal network of C. elegans to examine the effect of single node attack: the clustering coefficient, global efficiency, isolated nodes, and reachability [2]. We found that the deletions of motor neurons and interneurons were more effective to the clustering coefficient of the network than the sensory neurons. Eliminations of some interneurons mainly decreased global efficiency, and remarkably increased global efficiencies were induced by each removal of sensory neurons (see Table ​Table1).1). We suggest that this complex network analysis of the c. elegans connectome is helpful for understanding the potential functions of all neurons, and provide insight into which neurons are crucial for specific functions and which neurons are critical for lethality of the network information processing. Figure 1 Directed circular wiring diagram of the C. elegans combined network. The colors of the links showed the weights of each connection (Color: Weight ranges; light grey: 1-5, grey: 6-10, green: 11-15, blue: 16-20, orange: 21-25, pink: 26-30, and red: over ... Table 1 The average clustering coefficient and the average global efficiency when a target node deleted. A deletion of one of listed neurons induced remarkable changes in each measure.

Multi-scale modeling of complex neuronal networks: a view towards striatal cholinergic pattern formations

The phenomena related to brain function occur as the interplay of various modules at different spatial and temporal scales. Particularly, the integration of the dynamical behavior of cells within the complex brain topology reveals a heterogeneous multi-scale problem, which has, to date, mainly been addressed by methods of statistical physics such as mean-field approximations. In contrast, the present study introduces an abstract mathematical model of a deterministic nature that provides a robust integral transformation of the microscopic activities into macroscopic spatiotemporal patterns. The existence of the transformation operator is guaranteed by the convergence of a repetitive patching of the network domain with its fundamental domains that express the local topologies of the tissue. Depending on the choice of the local connectivity function, this framework represents a computationally efficient generalization of the classical Kirchhoff's, Hebbian, and Hopfield's approaches. The capabilities of this multi-scale method have been evaluated within the structure of the dorsal striatum of rats, a brain region with major involvement in motor and cognitive information processing. Numerical simulations suggest the formation of characteristic spatiotemporal patterns due to the activation of cholinergic interneurons.

Improved synapse detection for mGRASP-assisted brain connectivity mapping

Motivation: A new technique, mammalian green fluorescence protein (GFP) reconstitution across synaptic partners (mGRASP), enables mapping mammalian synaptic connectivity with light microscopy. To characterize the locations and distribution of synapses in complex neuronal networks visualized by mGRASP, it is essential to detect mGRASP fluorescence signals with high accuracy.Results: We developed a fully automatic method for detecting mGRASP-labeled synapse puncta. By modeling each punctum as a Gaussian distribution, our method enables accurate detection even when puncta of varying size and shape partially overlap. The method consists of three stages: blob detection by global thresholding; blob separation by watershed; and punctum modeling by a variational Bayesian Gaussian mixture models. Extensive testing shows that the three-stage method improved detection accuracy markedly, and especially reduces under-segmentation. The method provides a goodness-of-fit score for each detected punctum, allowing efficient error detection. We applied this advantage to also develop an efficient interactive method for correcting errors.Availability: The software is available on http://jinny.kist.re.krContact: tingzhao@gmail.com; kimj@kist.re.kr

Bifurcations of Emergent Bursting in a Neuronal Network

Complex neuronal networks are an important tool to help explain paradoxical phenomena observed in biological recordings. Here we present a general approach to mathematically tackle a complex neuronal network so that we can fully understand the underlying mechanisms. Using a previously developed network model of the milk-ejection reflex in oxytocin cells, we show how we can reduce a complex model with many variables and complex network topologies to a tractable model with two variables, while retaining all key qualitative features of the original model. The approach enables us to uncover how emergent synchronous bursting can arise from a neuronal network which embodies known biological features. Surprisingly, the bursting mechanisms are similar to those found in other systems reported in the literature, and illustrate a generic way to exhibit emergent and multiple time scale oscillations at the membrane potential level and the firing rate level.

Extrusion of misfolded and aggregated proteins – a protective strategy of aging neurons?

Cellular senescence is the consequence of repetitive exposures to oxidative stress, perturbed energy homeostasis, accumulation of damaged proteins and lesions in their nucleic acids. Whereas mitotic cells are equipped with efficient cell replacement strategies; postmitotic neurons have--with a few exceptions--no mechanism to substitute dysfunctional cells within a complex neuronal network. Here we propose a potential strategy by which aging neurons contend against abnormal accumulation of damaged/misfolded proteins. The suggested mechanism involves the formation of 'budding-like' extrusions and their subsequent clearance by glia. This hypothesis emerged from our previous investigations of the aged hippocampus revealing layer-specific accumulations of Reelin, a glycoprotein with fundamental roles during brain development and adult synaptic plasticity. We showed that Reelin deposits constitute a conserved neuropathological feature of aging, which is significantly accelerated in adult wild-type mice prenatally exposed to a viral-like infection. Here, we employed two- and three-dimensional immunoelectron microscopy to elucidate their morphological properties, localization and origin in immune challenged vs. control mice. In controls, Reelin-positive deposits were dispersed in the neuropil, some being engulfed by glia. In immune challenged mice, however, significantly more Reelin-immunoreactive deposits were associated with neuritic swellings containing mitochondria, vacuoles and cellular debris, pointing to their intracellular origin and suggesting that 'budding-like' neuronal extrusions of misfolded proteins and glial clearance may represent a protective strategy to counteract aging-associated impairments in proteosomal/lysosomal degradation. Neurons exposed to chronic neuroinflammation with increased levels of misfolded/damaged proteins, however, may fail to combat intraneuronal protein accumulations, a process probably underlying neuronal dysfunction and degeneration during aging.

IN-PHASE BURST SYNCHRONIZATION AND RHYTHM DYNAMICS OF COMPLEX NEURONAL NETWORKS

In-phase burst synchronization, spatiotemporal order and rhythm dynamics of a complex neuronal network with electrical or chemically excitatory synapses are studied in this paper. A quantitative characteristic, the width factor, is introduced to describe the rhythm dynamics of an individual neuron, and the average width factor is used to characterize the rhythm dynamics of a neural network. The in-phase burst synchronization is studied in terms of the burst phase order parameter. We also study the effects of the coupling schemes, the intrinsic neuronal property and the network topology on the rhythm dynamics of the network. It is found that the neuronal network with electrical coupling is easier to realize the in-phase burst synchronization than that with the chemically excitatory coupling. The bursting type of short bursting neuronal networks is unchanged for different coupling schemes with the coupling strength increasing. Moreover, the short bursting type is robust both to the coupling strength and the coupling scheme. As for the network topology, more links can only change the bursting type of long bursting neurons, but short bursting neurons are robust to the link numbers.

Neonatal hippocampal lesion alters the functional maturation of the prefrontal cortex and the early cognitive development in pre-juvenile rats

Mnemonic and executive performance is encoded into activity patterns of complex neuronal networks. Lesion studies revealed that adult recognition memory critically depends on the activation of the prefrontal cortex (PFC) and hippocampus (HP). However, its developmental profile remains poorly elucidated. We previously showed the rat PFC and HP are functionally coupled in theta- and gamma-band oscillations during neonatal [postnatal day (P) 5–8] and pre-juvenile (P10–15) stages of development. Here, we assess the behavioral readout of this early prefrontal–hippocampal activation by investigating the ontogeny and the mechanisms of novelty detection and recognition memory in relationship to the functional integrity of the PFC and HP. Excitotoxic lesion of the HP at birth led to abnormal oscillatory entrainment of the PFC throughout neonatal and pre-juvenile development. Although the onset of novelty detection correlated rather with the maturation of sensory perception and motor skills than with hippocampal integrity, the pre-juvenile performance in item, spatial and temporal order recognition memory significantly decreased after HP lesion at birth. This poorer performance does result neither from abnormal developmental milestones and locomotion nor from increased anxiety. Thus, novelty recognition in rat emerges during the second postnatal week and requires functional integrity of communication within neuronal networks including the PFC and HP.

Intentionality and free-will: the crucial effects of neuronal networks on human evolution

Human evolution can be divided into two distinct phases: (1) in the first, human evolution depends on the DNA-gene information based on direct and predictable processes, and (2) in the second, human evolution depends on the neuronal networks which generate complex and non-predictable effects. In the first phase, the information necessary for the generation of the neuronal cells depends on the DNA-gene information, while the second phase is connected with the formation of complex neuronal networks which is largely independent from DNA-gene information.

Opposite effects of fear conditioning and extinction on dendritic spine remodelling

It is generally believed that fear extinction is a form of new learning that inhibits rather than erases previously acquired fear memories. Although this view has gained much support from behavioural and electrophysiological studies, the hypothesis that extinction causes the partial erasure of fear memories remains viable. Using transcranial two-photon microscopy, we investigated how neural circuits are modified by fear learning and extinction by examining the formation and elimination of postsynaptic dendritic spines of layer-V pyramidal neurons in the mouse frontal association cortex. Here we show that fear conditioning by pairing an auditory cue with a footshock increases the rate of spine elimination. By contrast, fear extinction by repeated presentation of the same auditory cue without a footshock increases the rate of spine formation. The degrees of spine remodelling induced by fear conditioning and extinction strongly correlate with the expression and extinction of conditioned fear responses, respectively. Notably, spine elimination and formation induced by fear conditioning and extinction occur on the same dendritic branches in a cue- and location-specific manner: cue-specific extinction causes formation of dendritic spines within a distance of two micrometres from spines that were eliminated after fear conditioning. Furthermore, reconditioning preferentially induces elimination of dendritic spines that were formed after extinction. Thus, within vastly complex neuronal networks, fear conditioning, extinction and reconditioning lead to opposing changes at the level of individual synapses. These findings also suggest that fear memory traces are partially erased after extinction.

Neuronal Transplants Reconstitute Complex Neuronal Circuitry and Rescue Phenotype

Neuronal Transplants Reconstitute Complex Neuronal Circuitry and Rescue Phenotype

Balancing neural crest cell intrinsic processes with those of the microenvironment in Tcof1 haploinsufficient mice enables complete enteric nervous system formation

The enteric nervous system (ENS) comprises a complex neuronal network that regulates peristalsis of the gut wall and secretions into the lumen. The ENS is formed from a multipotent progenitor cell population called the neural crest, which is derived from the neuroepithelium. Neural crest cells (NCCs) migrate over incredible distances to colonize the entire length of the gut and during their migration they must survive, proliferate and ultimately differentiate. The absence of an ENS from variable lengths of the colon results in Hirschsprung's disease (HSCR) or colonic aganglionosis. Mutations in about 12 different genes have been identified in HSCR patients but the complex pattern of inheritance and variable penetrance suggests that additional genes or modifiers must be involved in the etiology and pathogenesis of this disease. We discovered that Tcof1 haploinsufficiency in mice models many of the early features of HSCR. Neuroepithelial apoptosis diminished the size of the neural stem cell pool resulting in reduced NCC numbers and their delayed migration along the gut from E10.5 to E14.5. Surprisingly however, we observe continued and complete colonization of the entire colon throughout E14.5-E18.5, a period in which the gut is considered to be non- or less-permissive to NCC. Thus, we reveal for the first time that reduced NCC progenitor numbers and delayed migration do not unequivocally equate with a predisposition for the pathogenesis of HSCR. In fact, these deficiencies can be overcome by balancing NCC intrinsic processes of proliferation and differentiation with extrinsic influences of the gut microenvironment.

Initiation and Propagation of Action Potentials in Gonadotropin-Releasing Hormone Neuron Dendrites

Gonadotropin-releasing hormone (GnRH) neurons are the final output neurons in a complex neuronal network that regulates fertility. The morphology of GnRH neuron dendrites is very different to other central neurons in that they are very long, thin, and unbranched. To study the function of these dendrites, we have used Na(+) and Ca(2+) imaging in combination with dual soma and dendrite electrical recordings in brain slices from GnRH-GFP mice. Here, we show that GnRH neurons actively propagate Na(+) spikes throughout their dendrites. Multisite dendritic recordings confirmed that these spikes were observed in one of the dendrites before the soma in the great majority of neurons tested. Na(+) imaging experiments revealed that the initial 150 μm of dendrite has a higher density of functional Na(+) channels than more distal regions, suggesting that this region of dendrite is highly excitable and may be the site of spike initiation. Finally, we show that the depolarization from dendritic spikes opens voltage-gated Ca(2+) channels giving rise to dendritic Ca(2+) transients. Together, these findings suggest that the proximal dendrites of GnRH neurons are highly excitable and are likely to be the site of action potential initiation in these neurons.

Genetic strategies to investigate neuronal circuit properties using stem cell-derived neurons

The mammalian brain is anatomically and functionally complex, and prone to diverse forms of injury and neuropathology. Scientists have long strived to develop cell replacement therapies to repair damaged and diseased nervous tissue. However, this goal has remained unrealized for various reasons, including nascent knowledge of neuronal development, the inability to track and manipulate transplanted cells within complex neuronal networks, and host graft rejection. Recent advances in embryonic stem cell (ESC) and induced pluripotent stem cell (iPSC) technology, alongside novel genetic strategies to mark and manipulate stem cell-derived neurons, now provide unprecedented opportunities to investigate complex neuronal circuits in both healthy and diseased brains. Here, we review current technologies aimed at generating and manipulating neurons derived from ESCs and iPSCs toward investigation and manipulation of complex neuronal circuits, ultimately leading to the design and development of novel cell-based therapeutic approaches.