Demonstration of axonal flow by the movement of tritium-labeled protein in mature optic nerve fibers

A Systematic Review of Direct Outputs from the Cerebellum to the Brainstem and Diencephalon in Mammals

The cerebellum is involved in many motor, autonomic and cognitive functions, and new tasks that have a cerebellar contribution are discovered on a regular basis. Simultaneously, our insight into the functional compartmentalization of the cerebellum has markedly improved. Additionally, studies on cerebellar output pathways have seen a renaissance due to the development of viral tracing techniques. To create an overview of the current state of our understanding of cerebellar efferents, we undertook a systematic review of all studies on monosynaptic projections from the cerebellum to the brainstem and the diencephalon in mammals. This revealed that important projections from the cerebellum, to the motor nuclei, cerebral cortex, and basal ganglia, are predominantly di- or polysynaptic, rather than monosynaptic. Strikingly, most target areas receive cerebellar input from all three cerebellar nuclei, showing a convergence of cerebellar information at the output level. Overall, there appeared to be a large level of agreement between studies on different species as well as on the use of different types of neural tracers, making the emerging picture of the cerebellar output areas a solid one. Finally, we discuss how this cerebellar output network is affected by a range of diseases and syndromes, with also non-cerebellar diseases having impact on cerebellar output areas.

Papilledema: A review of etiology, pathophysiology, diagnosis, and management

Papilledema is optic nerve head edema secondary to raised intracranial pressure (ICP). It is distinct from other causes of optic disk edema in that visual function is usually normal in the acute phase. Papilledema is caused by transmission of elevated ICP to the subarachnoid space surrounding the optic nerve that hinders axoplasmic transport within ganglion cell axons. There is ongoing controversy as to whether axoplasmic flow stasis is produced by physical compression of axons or microvascular ischemia. The most common cause of papilledema, especially in patients under the age of 50, is idiopathic intracranial hypertension (IIH); however, conditions that decrease cerebrospinal fluid (CSF) outflow by either causing CSF derangements or mechanically blocking CSF outflow channels, and rarely conditions that increase CSF production, can be the culprit. When papilledema is suspected clinically, blood pressure should be measured, and pseudopapilledema should be ruled out. Magnetic resonance imaging of the brain and orbits with venography sequences is the preferred neuroimaging modality that should be performed next to look for indirect imaging signs of increased ICP and to rule out nonidiopathic causes. Lumbar puncture with measurement of opening pressure and evaluation of CSF composition should then be performed. In patients not in a typical demographic group for IIH, further investigations should be conducted to assess for underlying causes of increased ICP. Magnetic resonance imaging of the neck and spine, magnetic resonance angiography of the brain, computed tomography of the chest, complete blood count, and creatinine testing should be able to identify most secondary causes of intracranial hypertension. Treatment for patients with papilledema should be targeted toward the underlying etiology. Most patients with IIH respond to weight loss and oral acetazolamide. For patients with decreased central acuity and constricted visual fields at presentation, as well as patients who do not respond to treatment with acetazolamide, surgical treatments should be considered, with ventriculoperitoneal shunting being the typical procedure of choice.

A Model of In Vivo HSV-1 DNA Transport Using Murine Retinal Ganglion Cells

Mammalian nervous tissues are heterogeneous. The retina, brain, spinal cord, and peripheral sensory and autonomic ganglia are each composed of neuronal and glial cell partners embedded in a connective tissue bed and supplied by vascular and immune cells. This complicated structure presents many challenges to neuroscientists and cell biologists (e.g., how to carry out a quantitative study of neurons surrounded by the hormonal and immune stimuli of supporting cells). A reductionist view has led investigators to study tissue slices and cultures of isolated neurons in vitro, subtracting the immune and vascular components to simplify the problem. Recently, investigators have extended the approach and produced organoids which are derived from embryonic neurons from induced pluripotent stem cells (Muffat et al., Proc Natl Acad Sci U S A 115:7117-7122, 2018).Using this approach advances have been made in the study of viral infections of the nervous system. For example, by using a genetically modified carrier virus, they can compare the effect of different viral envelope proteins on viral tropism and viral response pathways. However, the timed delivery of hormonal stimuli and interactions with immune cells remain problematic.We present an alternative method for studying these issues using the axonal transport of Herpes simplex virus in mature retinal neurons in vivo. Using genetically identical mice and carefully controlling the delivery of virus, an investigator can obtain insight into the transport of virus to and from the neuron cell body within the cellular environment of an intact, mature animal. This allows confirmation and extension of results seen in vitro.

Can transsynaptic viral strategies be used to reveal functional aspects of neural circuitry?

Viruses have proved instrumental to elucidating neuronal connectivity relationships in a variety of organisms. Recent advances in genetic technologies have facilitated analysis of neurons directly connected to a defined starter population. These advances have also made viral transneuronal mapping available to the broader neuroscience community, where one-step rabies virus mapping has become routine. This method is commonly used to identify inputs onto defined cell populations, to demonstrate the quantitative proportion of inputs coming from specific brain regions, or to compare input patterns between two or more cell populations. Furthermore, the number of inputs labeled is often assumed to reflect the number of synaptic connections, and these viruses are commonly believed to label strong synapses more efficiently than weak synapses. While these maps are often interpreted to provide a quantitative estimate of the synaptic landscape onto starter cell populations, in fact very little is known about how transneuronal transmission takes place. We do not know how these viruses transmit between neurons, if they display biases in the cell types labeled, or even if transmission is synapse-specific. In this review, we discuss the experimental evidence against or in support of key concepts in viral tracing, focusing mostly on the use of one-step rabies input mapping and related methods. Does spread of these viruses occur specifically through synaptic connections, preferentially through synapses, or non-specifically? How efficient is viral transneuronal transmission, and is this efficiency equal in all cell types? And lastly, to what extent does viral labeling reflect functional connectivity?

Viral Vectors for Neural Circuit Mapping and Recent Advances in Trans-synaptic Anterograde Tracers

Viral tracers are important tools for neuroanatomical mapping and genetic payload delivery. Genetically modified viruses allow for cell-type-specific targeting and overcome many limitations of non-viral tracers. Here, we summarize the viruses that have been developed for neural circuit mapping, and we provide a primer on currently applied anterograde and retrograde viral tracers with practical guidance on experimental uses. We also discuss and highlight key technical and conceptual considerations for developing new safer and more effective anterograde trans-synaptic viral vectors for neural circuit analysis in multiple species.

Connectome verification: inter-rater and connection reliability of tract-tracing-based intrinsic hypothalamic connectivity

MOTIVATION

Structural connectomics supports understanding aspects of neuronal dynamics and brain functions. Conducting metastudies of tract-tracing publications is one option to generate connectome databases by collating neuronal connectivity data. Meanwhile, it is a common practice that the neuronal connections and their attributes of such retrospective data collations are extracted from tract-tracing publications manually by experts. As the description of tract-tracing results is often not clear-cut and the documentation of interregional connections is not standardized, the extraction of connectivity data from tract-tracing publications could be complex. This might entail that different experts interpret such non-standardized descriptions of neuronal connections from the same publication in variable ways. Hitherto, no investigation is available that determines the variability of extracted connectivity information from original tract-tracing publications. A relatively large variability of connectivity information could produce significant misconstructions of adjacency matrices with faults in network and graph analyzes. The objective of this study is to investigate the inter-rater and inter-observation variability of tract-tracing-based documentations of neuronal connections. To demonstrate the variability of neuronal connections, data of 16 publications which describe neuronal connections of subregions of the hypothalamus have been assessed by way of example.

RESULTS

A workflow is proposed that allows detecting variability of connectivity at different steps of data processing in connectome metastudies. Variability between three blinded experts was found by comparing the connection information in a sample of 16 publications that describe tract-tracing-based neuronal connections in the hypothalamus. Furthermore, observation scores, matrix visualizations of discrepant connections and weight variations in adjacency matrices are analyzed.

AVAILABILITY

The resulting data and software are available at http://neuroviisas.med.uni-rostock.de/neuroviisas.shtml.

Neuroanatomy goes viral!

The nervous system is complex not simply because of the enormous number of neurons it contains but by virtue of the specificity with which they are connected. Unraveling this specificity is the task of neuroanatomy. In this endeavor, neuroanatomists have traditionally exploited an impressive array of tools ranging from the Golgi method to electron microscopy. An ideal method for studying anatomy would label neurons that are interconnected, and, in addition, allow expression of foreign genes in these neurons. Fortuitously, nature has already partially developed such a method in the form of neurotropic viruses, which have evolved to deliver their genetic material between synaptically connected neurons while largely eluding glia and the immune system. While these characteristics make some of these viruses a threat to human health, simple modifications allow them to be used in controlled experimental settings, thus enabling neuroanatomists to trace multi-synaptic connections within and across brain regions. Wild-type neurotropic viruses, such as rabies and alpha-herpes virus, have already contributed greatly to our understanding of brain connectivity, and modern molecular techniques have enabled the construction of recombinant forms of these and other viruses. These newly engineered reagents are particularly useful, as they can target genetically defined populations of neurons, spread only one synapse to either inputs or outputs, and carry instructions by which the targeted neurons can be made to express exogenous proteins, such as calcium sensors or light-sensitive ion channels, that can be used to study neuronal function. In this review, we address these uniquely powerful features of the viruses already in the neuroanatomist's toolbox, as well as the aspects of their biology that currently limit their utility. Based on the latter, we consider strategies for improving viral tracing methods by reducing toxicity, improving control of transsynaptic spread, and extending the range of species that can be studied.

In vivo HSV-1 DNA transport studies using murine retinal ganglion cells

The mammalian retina, brain, spinal cord, and peripheral ganglia are all heterogeneous tissues. Each is composed of neuronal and glial cell partners embedded in a connective tissue bed and supplied by vascular and immune cells. This complicated structure presents many challenges to neuroscientists and cell biologists, e.g., how to carry out a quantitative study of neurons in a mature animal surrounded by the hormonal and immune stimuli. A reductionist view leads investigators to study single neurons in vitro, subtracting the immune and vascular components and simplifying the problem. While this has advantages, it limits relevance of the study. We present a method for studying the axonal transport of Herpes simplex virus in mature neurons in situ. Using genetically identical mice and carefully controlling the delivery of virus, an investigator can obtain insight into the transport of virus to and from the neuron cell body within the cellular environment of an intact animal.

Controlling feeding behavior by chemical or gene-directed targeting in the brain: what's so spatial about our methods?

Intracranial chemical injection (ICI) methods have been used to identify the locations in the brain where feeding behavior can be controlled acutely. Scientists conducting ICI studies often document their injection site locations, thereby leaving kernels of valuable location data for others to use to further characterize feeding control circuits. Unfortunately, this rich dataset has not yet been formally contextualized with other published neuroanatomical data. In particular, axonal tracing studies have delineated several neural circuits originating in the same areas where ICI injection feeding-control sites have been documented, but it remains unclear whether these circuits participate in feeding control. Comparing injection sites with other types of location data would require careful anatomical registration between the datasets. Here, a conceptual framework is presented for how such anatomical registration efforts can be performed. For example, by using a simple atlas alignment tool, a hypothalamic locus sensitive to the orexigenic effects of neuropeptide Y (NPY) can be aligned accurately with the locations of neurons labeled by anterograde tracers or those known to express NPY receptors or feeding-related peptides. This approach can also be applied to those intracranial "gene-directed" injection (IGI) methods (e.g., site-specific recombinase methods, RNA expression or interference, optogenetics, and pharmacosynthetics) that involve viral injections to targeted neuronal populations. Spatial alignment efforts can be accelerated if location data from ICI/IGI methods are mapped to stereotaxic brain atlases to allow powerful neuroinformatics tools to overlay different types of data in the same reference space. Atlas-based mapping will be critical for community-based sharing of location data for feeding control circuits, and will accelerate our understanding of structure-function relationships in the brain for mammalian models of obesity and metabolic disorders.

Evaluation of the visual system in a rat model of chronic glaucoma using manganese-enhanced magnetic resonance imaging

This study aims to employ in vivo manganese-enchanced MRI (MEMRI) to evaluate dynamically the Mn(2+) enhancements along the visual pathway following an induction of ocular hypertension in a rat model of chronic glaucoma. Results showed an accumulation of Mn(2+) ions in the vitreous humor of the glaucomatous eye, with no statistical changes in the total retinal thickness but a possible occlusion of the ions at the optic nerve head. Meanwhile, there was a reduction in Mn(2+) transport in the glaucomatous optic nerve in the later stage of our model. Fewer enhancements in the visual cortex projected from the glaucomatous eye were also detectable. These may help understand the disease mechanisms, monitor the effect of drug interventions to glaucoma models, and complement the conventional techniques in examining the visual components.

Imagining the brain cell: the neuron in visual culture

Images of the exquisitely formed apparatus of the nervous system have great potential to capture the imagination. However, the fascinating complexity and diversity of neuronal form has only rarely been celebrated in broader visual culture. We discuss how scientific and cultural practices at the time of the neuron's discovery generated a legacy of schematic and simplified popular neuronal imagery, which is only now being revised in the light of technological advances and a changing artistic climate.

Functional mapping of neural pathways in rodent brain in vivo using manganese-enhanced three-dimensional magnetic resonance imaging

This work presents three-dimensional MRI studies of rodent brain in vivo after focal and systemic administration of MnCl2. Particular emphasis is paid to the morphology and dynamics of Mn2+-induced MRI signal enhancements, and the physiological mechanisms underlying cerebral Mn2+ uptake and distribution. It turns out that intravitreal and intrahippocampal injections of MnCl2 emerge as useful tools for a delineation of major axonal connections in the intact central nervous system. Subcutaneous administrations may be exploited to highlight regions involved in fundamental brain functions such as the olfactory bulb, inferior colliculus, cerebellum and hippocampal formation. Specific insights into the processes supporting cerebral Mn2+ accumulation may be obtained by intraventricular MnCl2 injection as well as by pharmacologic modulation of, for example, hippocampal function. Taken together, Mn2+-enhanced MRI opens new ways for mapping functioning pathways in animal brain in vivo with applications ranging from assessments of transgenic animals to follow-up studies of animal models of human brain disorders.

Mapping of retinal projections in the living rat using high-resolution 3D gradient-echo MRI with Mn2+-induced contrast

This study describes the neuroaxonal tracing of the visual pathway in the living rat using high-resolution T1-weighted 3D gradient-echo MRI (195 x 195 x 125 microm3) at 8, 24, 48, and 72 h after intraocular Mn2+ injection (0.1 microl of 1 M aqueous MnCl2). Best results were obtained at 24 h postinjection, revealing a continuous pattern of anterograde labeling from the retina, optic nerve, and chiasm to the contralateral optic tract, the dorsal and ventral lateral geniculate nucleus, the superior colliculus and its brachium, the olivary pretectal nucleus, the nucleus of the optic tract, and the suprachiasmatic nucleus. These results underline the feasibility of repeated MRI tract tracing in living animals after a single injection of Mn2+. The approach is expected to advance studies of neuroaxonal function in behaving animals with special emphasis on applications in developmental neurobiology.

Analysis of neuronal networks: a review of techniques for labeling axonal projections

In order to analyze connections between neurons in the vertebrate central nervous system, methods have been developed to label a given population of axons of known origin so that they can be differentiated from other, non-labeled structures. Three such methods are reviewed here: experimentally induced orthograde (Wallerian) degeneration, axon transport of radioactive proteins demonstrated by autoradiography, and axon transport of macromolecules that can be reacted histochemically to yield a visible reaction product. Each of the methods has particular strengths and weaknesses. Degeneration methods may differentiate between different functional classes of axons which have different fiber diameters. However, degeneration distorts the morphology of axon terminals, making them more difficult to interpret, and degenerating terminals may be removed rapidly by phagocytosis. Autoradiography of radioactive terminals preserves normal fine structure, but the necessary exposure times extend the method by weeks or months, and care must be exercised to distinguish labeled axons from other structures exhibiting background or transneuronal radioactivity. Histochemical methods, such as those used to demonstrate horseradish peroxidase conjugated to wheat germ lectin (WGA-HRP), are sensitive and rapid, but the injection site must be carefully characterized, and the presence of transneuronal label may make interpretation of the results difficult. Experimental methods of axonal labeling have been invaluable in studying neuronal networks. Each of the methods described here may be of particular value, given the nature of the system to be analyzed.

Changes in axonally transported proteins in the rat visual system following systemic methyl mercury exposure

In an effort to understand the effect of methylmercury on protein synthesis and axonal transport, we have analysed the composition and rate of axonally transported proteins in the retinal ganglion cells of the mature rat. By means of scintillation spectrometry and autoradiography, it was established that systemic exposure to 4 mg Hg/kg/day for four to six days, or twelve days, resulted in an increased rate and volume of transported protein-bound radioactivity in the visual system of the mature rat. In an effort to characterize these changes, the composition of transported polypeptides was analysed by means of SDS polyacrylamide gel electrophoresis. Selective changes in the composition of transported polypeptides were evident. These changes of a small subset of proteins known as GAPs (growth-associated proteins) are consistent with the suggestion that they may have been involved in growth-specific functions during the early stages of methyl mercury exposure. We concluded that, during this period, retinal ganglion cells may express growth-related genes and engage in regenerative processes.

The ninth Frederick H. Verhoeff lecture. The life history of retinal cells

Images

Direct retinal projections to the hypothalamus, piriform cortex, and accessory optic nuclei in the golden hamster as demonstrated by a sensitive anterograde horseradish peroxidase technique

The central projections of the retinal ganglion cells of the golden hamster were examined using horseradish peroxidase (HRP) as the anterograde tracer molecule. Following monocular injections of HRP into the vitreous, retinofugal fibers were histochemically demonstrated using the chromagen tetramethylbenzidine. This procedure, being more sensitive than the 3H-amino acid radioautographic technique, provided a clear demonstration of previously controversial retinal projections, clearer definition of established projections, and the discovery of new retinal pathways. An inferior accessory optic system was shown to be unequivocally present in this species and to consist of both crossed and uncrossed components. A direct retinal projection to the suprachiasmatic nucleus (SCN) of the hypothalamus was confirmed in this study. But the distribution of terminals as seen by this procedure was substantially different than previously reported; both rostrocaudal and mediolateral asymmetries in the distribution of label between the ipsilateral and contralateral SCN were observed. Substantial differences in the retinal projection to the SCN in the hamster and the rat were also noted. It is suggested that these differences may reflect the different effects photic input has on the neuro-endocrine-gonadal axis in these two species. Finally, labeled retinal axons were followed leaving the optic tract and coursing anteriorly through the plexiform layer of the piriform cortex; other labeled fibers were seen to enter the septal region. The physiological significance of these previously undescribed retinal projections is not known.

A quantitative analysis of isotope concentration profiles and rapid transport velocities in the C-fibers of the garfish olfactory nerve

In the olfactory nerve of the long-nosed garfish (Lepisosteus osseus), unusually well-defined isotope concentration distributions can be established with the rapid transport process. Transport velocities of two profile loci can be accurately described and a quantitative profile analysis is possible after profile normalization. Results from such studies indicate that: (1) peak amplitudes decrease exponentially as a function of distance from the olfactory mucosa according to the equation p = 2130 exp (-0.109chi); (2) the wavefront base and the peak apex loci move at rates of 221 +/- 2 and 201 +/- 4 mm/day, respectively (at 23 degrees C), revealing a peak dispersion or broadening during transport; (3) the broadening is asymmetric with material shifting to the rear of the peak; (4) plateau regions are established behind the peak with material deposited by the peak; (5) only 20% of the total radioactivity in a cut nerve reaches the nerve terminals in the rapid transport peak while 80% is deposited along the axon; (6) profile areas from cut nerves decrease and lose 15% of their activity in 20 hr, while intact nerve profiles increase 10% in 16 hr due to continued somal contribution to the profile; (7) the displacement of the wavefront base (WFB) and peak apex (PA) profile loci can be described by the functions s(WFB) = (0.055T - 0.345)t - 1.43 s(PA) = (0.053T - 0.391)t - 2.71 (8) transport velocities are linear functions of temperature between 10 and 25 degrees C and increase 370% in that range. A linear extrapolation of the WFB and PA functions to 37 degrees C yields 410 and 377 mm/day, respectively.

The response of ventral horn neurons to axonal transection

The morphological changes induced in the frog ventral horn neurons by axonal transection have been studied with the electron microscope. During the first 2 wk after axotomy the neuronal nucleus becomes more translucent and the nucleolus becomes enlarged and less compact. The cisternae of the granular endoplasmic reticulum vesiculate and ribosomes dissociate from membranes. Free ribosomes and polysomes are dispersed in the cytoplasmic matrix. Neurofilaments and neurotubules are increased in number. These structures appear to be important in the regeneration of the axon. It is proposed that neurotubules, neurofilaments, and axoplasmic matrix are synthesized by the free polyribosomes in the chromatolytic neuron. By the fourth postoperative week, the neurons show evidence of recovery. The cytoplasm is filled with profiles of granular endoplasmic reticulum and many intercisternal polysomes. The substances being manufactured by the newly formed granular endoplasmic reticulum are not clearly defined, but probably include elements essential to electrical and chemical conduction of impulses. The significance of these observations in respect to recent studies of axoplasmic flow is discussed.

Synaptic transmission depressed by colchicine blockade of axoplasmic flow

Colchicine, which inhibits axoplasmic transport and induces organelle alterations in nerve terminals, was injected intraocularly in pigeons. Electrical stimulation of the optic nerve yielded normal evoked potentials in retinotectal fibers, whereas postsynaptic responses recorded in the tectum were reduced. Postsynaptic depression suggests a deficit of synaptic transmission, presumably dependent on colchicine interference with migrating material.

Transneuronal transfer of radioactivity in the central nervous system

After injection of tritiated amino acid into the mouse eye, radioactivity appeared in the contralateral visual cortex, indicating that some material had been transferred from optic axons to lateral geniculate neurons. The radioactivity in the cortex was about 2 percent of that arriving in the geniculate, and most of it was contained in material that appeared to be protein.