Academic literature on the topic 'Axon varicosity'

Axons in the neurohypophysis are known for their “beads on a string” morphology, with numerous in-line secretory swellings lined up along the axon cable. A significant fraction of these secretory swellings, called Herring bodies, is large enough to serve as an identifying feature of the neural lobe in histological sections. Little is known about the physiological role such large axonal swellings might play in neuroendocrine physiology. Using numerical simulations, we have investigated whether large in-line varicosities affect the waveform and propagation of action potentials (APs) along neurohypophysial axons. Due to the strong nonlinear dependence of calcium influx on AP waveforms, such modulation would inevitably affect neuroendocrine release. The parameters for our numerical simulations were matched to established properties of voltage-gated ion channels in neurohypophysial swellings. We find that even a single in-line varicosity can severely depress AP waveforms far upstream in the axonal cable. In contrast, AP depolarization within varicosities becomes amplified. Amplification within varicosities varies in a nontrivial manner with varicosity dimensions, and is most pronounced for diameters close to those of Herring bodies. Overall, we find that large axonal varicosities significantly modulate AP waveforms and their propagation, and do so over large distances. Varicosity size is the main determinant for the observed AP amplification, with the kinetics of voltage-gated ion channels playing a noticeable but secondary role. Our results imply that large varicosities are sites of enhanced hormone release, suggesting that small and large varicosities target different neurohypophysial structures.

Axonal ionotropic receptors are present in a variety of neuronal types, and their function has largely been associated with the modulation of axonal activity and synaptic release. It is usually assumed that activation of axonal GABAARs comes from spillover, but in cerebellar molecular layer interneurons (MLIs) the GABA source is different: in these cells, GABA release activates presynaptic GABAA autoreceptors (autoRs) together with postsynaptic targets, producing an autoR-mediated synaptic event. The frequency of presynaptic, autoR-mediated miniature currents is twice that of their somatodendritic counterparts, suggesting that autoR-mediated responses have an important effect on interneuron activity. Here, we used local Ca2+ photolysis in MLI axons of juvenile rats to evoke GABA release from individual varicosities to study the activation of axonal autoRs in single release sites. Our data show that single-site autoR conductances are similar to postsynaptic dendritic conductances. In conditions of high [Cl−]i, autoR-mediated conductances range from 1 to 5 nS; this corresponds to ∼30–150 GABAA channels per presynaptic varicosity, a value close to the number of channels in postsynaptic densities. Voltage responses produced by the activation of autoRs in single varicosities are amplified by a Nav-dependent mechanism and propagate along the axon with a length constant of 91 µm. Immunolabeling determination of synapse location shows that on average, one third of the synapses produce autoR-mediated signals that are large enough to reach the axon initial segment. Finally, we show that single-site activation of presynaptic GABAA autoRs leads to an increase in MLI excitability and thus conveys a strong feedback signal that contributes to spiking activity.

This study was conducted in order to investigate the structure of sensory nerve endings of the human epiglottis and substance P immunoreactive nerve fibers of the canine epiglottis in relationship to physiologic functions of the larynx. The human epiglottis was observed by light microscopy (silver impregnation) and electron microscopy, and the canine epiglottis was studied by peroxidase-anti-peroxidase (PAP) immunohistochemistry. The results are summarized as follows: (1) In the membranes of the epiglottis, we observed free endings of simple or complex tree shape, corpuscle endings with glomerular patterns, and taste-bud-like structures, and (2) electron microscopic studies revealed varicosity of the terminal axon with processes that contained small, clear and large, dense cored vesicles. Substance P was observed in these structures, and it was suggested that substance P was related to perception in the larynx.

Stanniocalcin is a polypeptide hormone that was first reported in fish as a regulator of mineral metabolism. Its recent identification in mammals has opened a new area of investigation in basic and clinical endocrinology. In the present study, regulation of the stanniocalcin (STC) and stanniocalcin related protein (STCrP) genes were investigated in mouse neuroblastoma cells (Neuro-2A) in relation to neuronal cell differentiation. Neuro-2A is an undifferentiated cell line that contains measurable levels of STCrP mRNA, but undetectable levels of STC mRNA. Treatment of the cells with either dbcAMP (1-4 mM) or 50 microM euxanthone (PW1) resulted in extensive differentiation and neurite outgrowth. However, only neurites of dbcAMP-treated cells developed varicosities, a phenotypic marker of axon formation. Furthermore, following differentiation induced by dbcAMP, there was an upregulation of STC and downregulation of STCrP mRNA levels. In the first 24 and 48 h of treatments, there was a maximum twofold induction and 1.5-fold reduction in STC and STCrP mRNAs respectively. Following 96 h of treatment, an additional 14-fold STC induction and 1.2-fold STCrP reduction were observed. The increase in STC mRNA levels was accompanied by a concomitant increase in axon-specific low molecular form microtubule-associated protein (MAP-2c) mRNA and varicosities on the neurites, suggesting a possible role for STC in axonogenesis. There was no induction of STC mRNA levels when PW1 was added into the culture media, whereas ionomycin (1-10 microM) had no observable effects on cell differentiation or STC/STCrP mRNA. Immunocytochemical staining of dbcAMP-treated cells revealed abundant levels of immunoreactive STC, particularly in the varicosities, with only weak staining in control, untreated cells. Antisense oligodeoxynucleotides transfection studies indicated that the expression of STC was a cause of varicosity formation and a consequence of cell differentiation. Our findings lend further support to the notion that STC is involved in the process of neural differentiation.

Little is known about mechanical regulation of morphological and functional polarity of central neurons. In this study, we report that mechanical stress specifically induces varicosities in the axons but not the dendrites of central neurons by activating TRPV4, a Ca2+/Na+-permeable mechanosensitive channel. This process is unexpectedly rapid and reversible, consistent with the formation of axonal varicosities in vivo induced by mechanical impact in a mouse model of mild traumatic brain injury. In contrast, prolonged stimulation of glutamate receptors induces varicosities in dendrites but not in axons. We further show that axonal varicosities are induced by persistent Ca2+ increase, disassembled microtubules (MTs), and subsequently reversible disruption of axonal transport, and are regulated by stable tubulin-only polypeptide, an MT-associated protein. Finally, axonal varicosity initiation can trigger action potentials to antidromically propagate to the soma in retrograde signaling. Therefore, our study demonstrates a new feature of neuronal polarity: axons and dendrites preferentially respond to physical and chemical stresses, respectively.

We have recently shown that membrane association of neuronal nitric oxide synthase-α (nNOSα) is critical in the regulation of synthesis of NO during nitrergic neurotransmission. The purpose of this study was to examine the role of the synapse-associated proteins (SAPs) in membrane association of nNOSα. Varicosities (swellings on terminal axons) were isolated from mice gastrointestinal tract and examined for nNOSα, postsynaptic density protein 95 (PSD95), and membrane interactions by coimmunoprecipitation and SDS-PAGE. Our results show that PSD95 protein was present in the membrane fraction of the nerve varicosity, whereas both PSD95 and SAP97 were present in the cytosol. nNOSα was associated with PSD95 but not SAP97. nNOSα-PSD95 complex was bound to the membrane via palmitoylation of PSD95. Depalmitoylation of PSD95 with 2-bromopalmitate dislocates nNOSα and PSD95 from the varicosity membrane and abolishes NO production. These studies show that palmitoylation of PSD95 anchors nNOSα to the varicosity membrane and that it is obligatory for NO production by the enzyme. Palmitoylation of PSD95 may provide a novel target for regulation of nitrergic neurotransmission.

Action potential (AP) propagation in presynaptic axons of the crayfish opener neuromuscular junction (NMJ) was investigated by simultaneously recording from a terminal varicosity and a proximal branch. Although orthodromically conducting APs could be recorded in terminals with amplitudes up to 70 mV, depolarizing steps in terminals to −20 mV or higher failed to fire APs. Patch-clamp recordings did detect Na+ current ( INa) in most terminals. The INa exhibited a high threshold and fast activation rate. Local perfusion of Na+-free saline showed that terminal INa contributed to AP waveform by slightly accelerating the rising phase and increasing the peak amplitude. These findings suggest that terminal INa functions to “touch up” but not to generate APs.

α-Herpesviruses constitute closely related neurotropic viruses, including herpes simplex virus in man and pseudorabies virus (PRV) in pigs. Peripheral sensory neurons, such as trigeminal ganglion (TG) neurons, are predominant target cells for virus spread and lifelong latent infections. We report that in vitro infection of swine TG neurons with the homologous swine α-herpesvirus PRV results in the appearance of numerous synaptophysin-positive synaptic boutons (varicosities) along the axons. Nonneuronal cells that were juxtaposed to these varicosities became preferentially infected with PRV, suggesting that varicosities serve as axonal exit sites for the virus. Viral envelope glycoprotein D (gD) was found to be necessary and sufficient for the induction of varicosities. Inhibition of Cdc42 Rho GTPase and p38 mitogen-activated protein kinase signaling pathways strongly suppressed gD-induced varicosity formation. These data represent a novel aspect of the cell biology of α-herpesvirus infections of sensory neurons, demonstrating that virus attachment/entry is associated with signaling events and neuronal changes that may prepare efficient egress of progeny virus.

ABSTRACT The features of herpes simplex virus 1 (HSV-1) strain 129 (H129), including natural neurotropism and anterograde transneuronal trafficking, make it a potential tool for anterograde neural circuitry tracing. Recently anterograde polysynaptic and monosynaptic tracers were developed from H129 and have been applied for the identification of novel connections and functions of different neural circuitries. However, how H129 viral particles are transported in neurons, especially those of the central nervous system, remains unclear. In this study, we constructed recombinant H129 variants with mCherry-labeled capsids and/or green fluorescent protein (GFP)-labeled envelopes and infected the cortical neurons to study axonal transport of H129 viral particles. We found that different types of viral particles were unevenly distributed in the nucleus, cytoplasm of the cell body, and axon. Most H129 progeny particles were unenveloped capsids and were transported as capsids rather than virions in the axon. Notably, capsids acquired envelopes at axonal varicosities and terminals where the sites forming synapses are connected with other neurons. Moreover, viral capsids moved more frequently in the anterograde direction in axons, with an average velocity of 0.62 ± 0.18 μm/s and maximal velocity of 1.80 ± 0.15 μm/s. We also provided evidence that axonal transport of capsids requires the kinesin-1 molecular motor. These findings support that H129-derived tracers map the neural circuit anterogradely and possibly transsynaptically. These data will guide future modifications and improvements of H129-based anterograde viral tracers. IMPORTANCE Anterograde transneuronal tracers derived from herpes simplex virus 1 (HSV-1) strain 129 (H129) are important tools for mapping neural circuit anatomic and functional connections. It is, therefore, critical to elucidate the transport pattern of H129 within neurons and between neurons. We constructed recombinant H129 variants with genetically encoded fluorescence-labeled capsid protein and/or glycoprotein to visualize viral particle movement in neurons. Both electron microscopy and light microscopy data show that H129 capsids and envelopes move separately, and notably, capsids are enveloped at axonal varicosity and terminals, which are the sites forming synapses to connect with other neurons. Superresolution microscopy-based colocalization analysis and inhibition of H129 particle movement by inhibitors of molecular motors support that kinesin-1 contributes to the anterograde transport of capsids. These results shed light into the mechanisms for anterograde transport of H129-derived tracer in axons and transmission between neurons via synapses, explaining the anterograde labeling of neural circuits by H129-derived tracers.

Les systèmes cholinergique et dopaminergique jouent un rôle prépondérant dans les fonctions cognitives. Ce rôle est exercé principalement grâce à leur action modulatrice de l’activité des neurones pyramidaux du cortex préfrontal. L’interaction pharmacologique entre ces systèmes est bien documentée mais les études de leurs interactions neuroanatomiques sont rares, étant donné qu’ils sont impliqués dans une transmission diffuse plutôt que synaptique. Ce travail de thèse visait à développer une expertise pour analyser ce type de transmission diffuse en microscopie confocale. Nous avons étudié les relations de microproximité entre ces différents systèmes dans le cortex préfrontal médian (mPFC) de rats et souris. En particulier, la densité des varicosités axonales en passant a été quantifiée dans les segments des fibres cholinergiques et dopaminergiques à une distance mutuelle de moins de 3 µm ou à moins de 3 µm des somas de cellules pyramidales. Cette microproximité était considérée comme une zone d’interaction probable entre les éléments neuronaux. La quantification était effectuée après triple-marquage par immunofluorescence et acquisition des images de 1 µm par microscopie confocale. Afin d’étudier la plasticité de ces relations de microproximité, cette analyse a été effectuée dans des conditions témoins, après une activation du mPFC et dans un modèle de schizophrénie par déplétion des neurones cholinergiques du noyau accumbens. Les résultats démontrent que 1. Les fibres cholinergiques interagissent avec des fibres dopaminergiques et ce sur les mêmes neurones pyramidaux de la couche V du mPFC. Ce résultat suggère différents apports des systèmes cholinergique et dopaminergique dans l’intégration effectuée par une même cellule pyramidale. 2. La densité des varicosités en passant cholinergiques et dopaminergiques sur des segments de fibre en microproximité réciproque est plus élevée comparé aux segments plus distants les uns des autres. Ce résultat suggère un enrichissement du nombre de varicosités axonales dans les zones d’interaction. 3. La densité des varicosités en passant sur des segments de fibre cholinergique en microproximité de cellules pyramidales, immunoúactives pour c-Fos après une stimulation visuelle et une stimulation électrique des noyaux cholinergiques projetant au mPFC est plus élevée que la densité des varicosités de segments en microproximité de cellules pyramidales non-activées. Ce résultat suggère un enrichissement des varicosités axonales dépendant de l’activité neuronale locale au niveau de la zone d'interaction avec d'autres éléments neuronaux. 4. La densité des varicosités en passant des fibres dopaminergiques a été significativement diminuée dans le mPFC de rats ayant subi une déplétion cholinergique dans le noyau accumbens, comparée aux témoins. Ces résultats supportent des interrelations entre la plasticité structurelle des varicosités dopaminergiques et le fonctionnement cortical. L’ensemble des donneès démontre une plasticité de la densité locale des varicosités axonales en fonction de l’activité neuronale locale. Cet enrichissement activité-dépendant contribue vraisemblablement au maintien d’une interaction neurochimique entre deux éléments neuronaux.
The cognitive functions of the rat medial prefrontal cortex (mPFC) are modulated by ascending modulatory systems such as the cholinergic and dopaminergic afferent systems. However, despite the well-documented pharmacological interactions between the cholinergic and dopaminergic afferents and pyramidal cells in the PFC, there is only scarce neuroanatomical data on the reciprocal interrelationships between these neuronal elements in the mPFC. This might be due to the diffuse rather than synaptic transmission mode of intercellular communication of the cholinergic system in the mPFC. For these reasons, the neuroanatomical relationships between the cholinergic and dopaminergic systems and pyramidal cells in the mPFC are examined, with an emphasis on the local density of the cholinergic and dopaminergic axon varicosities. To analyze the plasticity of these interrelationships, the two systems were examined in condition of increased neuronal activity in the mPFC, or of decrease dopaminergic activity in a model of schizophrenia. The microproximity relationships between cholinergic and dopaminergic fibers as well as with pyramidal cells were studied in the mPFC of rats and mice. In particular, the number of axon varicosities in cholinergic and dopaminergic fiber segments within 3 µm from each other or from pyramidal cells were quantified. This microproximity was considered as a possible interaction zone between two neuronal elements. Quantification was performed using triple immunofluorescence labeling and acquisition of 1 µm optic sections using confocal microscopy. To assess the plasticity of these relationships, the analysis has been performed in control condition as well as after a cortical activation or a decreased dopaminergic input in a schizophrenia model. Our results demonstrate a neuroanatomical convergence of cholinergic and dopaminergic fibers on the same pyramidal cell from layer V (output) of mPFC, suggestinggests the integration of different types of inputs by the same pyramidal cell, which may be transmitted to subcortical areas to execute prefrontal cognitive control. Close apposition between cholinergic and dopaminergic fibers could also be seen in the mPFC. There was an increase of the density of cholinergic and dopaminergic en passant varicosities on those fiber segments within microproximity of each other, compared to those outside the reciprocal microproximity, supporting functional importance of the close apposition between those two ascending neuromodulatory systems into the mPFC. There was enrichment of cholinergic en passant varicosities on the fiber segments within microproximity of c-Fos activated pyramidal cells in the mPFC of visually and HDB electrically stimulated rats, indicating association between axonal varicosity density and the local neuronal activity. There was decrease of dopaminergic en passant varicosities in the mPFC of rats with ChAT depletion in the N.Acc., compared to controls. This evidence supports the association between dopaminergic axonal varicosities and relevant neuronal activity in a complex neuronal network. This thesis shows that the density of cholinergic and dopaminergic axonal varicosity density in the mPFC is influenced by and contributes to the relevant local neuronal activity from the interactions of different transmitter systems. Such interactions of different systems in a complex and intricate prefrontal neuronal network endeavour to maintain the delicate balance for cognitive processes.

Traumatic brain injury (TBI) occurs in many blunt, ballistic and blast impact events. During trauma axons in the white matter are especially vulnerable to injury due to the rapid mechanical loading of brain. The axonal pathology leads to cytoskeletal failure and disconnection. The microtubules are one of major structural components of the cytoskeleton filamentous network. By bridging the macroscopic forces acting on the whole brain with the cellular and subcellular failure, the macro-micro computational models in both time and space can help us better understand the complex biophysics and elucidate the injury mechanism of both severe and mild TBI (concussion). At the macroscopic scale we developed the high-fidelity anatomical human body finite element model (FEM) to predict intracranial pressures and strain and strain rate fields of brain in the blast event. The macro-scale models and the coupled blast and biomechanics approach were validated against test data of shock wave interacting with a surrogate head in the shock tube. The mechanical deformation of brain tissue was mapped to the white matter tracts to obtain local axonal strain and strain rate for the micromechanical models. We developed the micromechanical FEM of myelinated axons interconnected with the oligodendrocyte by the processes, utilizing a novel beam element free of rotational degrees of freedom (DOFs). The numerical results reveal the possible mechanism of impact-induced axon injury including demyelination, breakup of processes, and axonal varicosity. We also investigate the dynamic response of microtubules bundles under traumatic loading. Different from the commonly discrete bead-spring models, a network of microtubules cross-linked with microtubule-associated-protein (MAP) tau proteins was modeled by the nonlinear beam model. Tau protein is modeled by the rate-dependent bar element for its complicated material behavior. The model considers the rupture of microtubule and the failure of tau-tau interface and tau-microtubule interface. The simulation result of the combined effects of the failure of the cross-linked architecture and elongation and bending of the bundle are possibly correlated to the axonal undulations following traumatic loading observed in the experiments. The developed macro-micro biomechanics models can be used as a starting point for modeling the neurobiology effects and guide the design of novel injury protection strategies.