The problems of neurons with long axons

Actomyosin-II protects axons from degeneration induced by mild mechanical stress

Whether, to what extent, and how the axons in the central nervous system (CNS) can withstand sudden mechanical impacts remain unclear. By using a microfluidic device to apply controlled transverse mechanical stress to axons, we determined the stress levels that most axons can withstand and explored their instant responses at nanoscale resolution. We found mild stress triggers a highly reversible, rapid axon beading response, driven by actomyosin-II-dependent dynamic diameter modulations. This mechanism contributes to hindering the long-range spread of stress-induced Ca2+ elevations into non-stressed neuronal regions. Through pharmacological and molecular manipulations in vitro, we found that actomyosin-II inactivation diminishes the reversible beading process, fostering progressive Ca2+ spreading and thereby increasing acute axonal degeneration in stressed axons. Conversely, upregulating actomyosin-II activity prevents the progression of initial injury, protecting stressed axons from acute degeneration both in vitro and in vivo. Our study unveils the periodic actomyosin-II in axon shafts cortex as a novel protective mechanism, shielding neurons from detrimental effects caused by mechanical stress.

Basal forebrain cholinergic signalling: development, connectivity and roles in cognition

Acetylcholine plays an essential role in fundamental aspects of cognition. Studies that have mapped the activity and functional connectivity of cholinergic neurons have shown that the axons of basal forebrain cholinergic neurons innervate the pallium with far more topographical and functional organization than was historically appreciated. Together with the results of studies using new probes that allow release of acetylcholine to be detected with high spatial and temporal resolution, these findings have implicated cholinergic networks in 'binding' diverse behaviours that contribute to cognition. Here, we review recent findings on the developmental origins, connectivity and function of cholinergic neurons, and explore the participation of cholinergic signalling in the encoding of cognition-related behaviours.

An adult-stage transcriptional program for survival of serotonergic connectivity

Neurons must function for decades of life, but how these non-dividing cells are preserved is poorly understood. Using mouse serotonin (5-HT) neurons as a model, we report an adult-stage transcriptional program specialized to ensure the preservation of neuronal connectivity. We uncover a switch in Lmx1b and Pet1 transcription factor function from controlling embryonic axonal growth to sustaining a transcriptomic signature of 5-HT connectivity comprising functionally diverse synaptic and axonal genes. Adult-stage deficiency of Lmx1b and Pet1 causes slowly progressing degeneration of 5-HT synapses and axons, increased susceptibility of 5-HT axons to neurotoxic injury, and abnormal stress responses. Axon degeneration occurs in a die back pattern and is accompanied by accumulation of α-synuclein and amyloid precursor protein in spheroids and mitochondrial fragmentation without cell body loss. Our findings suggest that neuronal connectivity is transcriptionally protected by maintenance of connectivity transcriptomes; progressive decay of such transcriptomes may contribute to age-related diseases of brain circuitry.

Neuronal mTOR Outposts: Implications for Translation, Signaling, and Plasticity

The kinase mTOR is a signaling hub for pathways that regulate cellular growth. In neurons, the subcellular localization of mTOR takes on increased significance. Here, we review findings on the localization of mTOR in axons and offer a perspective on how these may impact our understanding of nervous system development, function, and disease. We propose a model where mTOR accumulates in local foci we term mTOR outposts, which can be found in processes distant from a neuron’s cell body. In this model, pathways that funnel through mTOR are gated by local outposts to spatially select and amplify local signaling. The presence or absence of mTOR outposts in a segment of axon or dendrite may determine whether regional mTOR-dependent signals, such as nutrient and growth factor signaling, register toward neuron-wide responses. In this perspective, we present the emerging evidence for mTOR outposts in neurons, their putative roles as spatial gatekeepers of signaling inputs, and the implications of the mTOR outpost model for neuronal protein synthesis, signal transduction, and synaptic plasticity.

The axonal radial contractility: Structural basis underlying a new form of neural plasticity

Axons are the longest cellular structure reaching over a meter in the case of human motor axons. They have a relatively small diameter and contain several cytoskeletal elements that mediate both material and information exchange within neurons. Recently, a novel type of axonal plasticity, termed axonal radial contractility, has been unveiled. It is represented by dynamic and transient diameter changes of the axon shaft to accommodate the passages of large organelles. Mechanisms underpinning this plasticity are not fully understood. Here, we first summarised recent evidence of the functional relevance for axon radial contractility, then discussed the underlying structural basis, reviewing nanoscopic evidence of the subtle changes. Two models are proposed to explain how actomyosin rings are organised. Possible roles of non-muscle myosin II (NM-II) in axon degeneration are discussed. Finally, we discuss the concept of periodic functional nanodomains, which could sense extracellular cues and coordinate the axonal responses. Also see the video abstract here: https://youtu.be/ojCnrJ8RCRc.

Integrated phylogeny of the human brain and pathobiology of Alzheimer's disease: A unifying hypothesis

The disproportionate evolutionary expansion of the human cerebral cortex with reinforcement of cholinergic innervations warranted a major rise in the functional and metabolic load of the conserved basal forebrain (BF) cholinergic system. Given that acetylcholine (ACh) regulates properties of the microtubule-associated protein (MAP) tau and promotes non-amyloidogenic processing of amyloid precursor protein (APP), growing neocortex predicts higher demands for ACh, while the emerging role of BF cholinergic projections in Aβ clearance infers greater exposure of source neurons and their innervation fields to amyloid pathology. The higher exposure of evolutionary most recent cortical areas to the amyloid pathology of Alzheimer's disease (AD) with synaptic impairments and atrophy, therefore, might involve attenuated homeostatic effects of BF cholinergic projections, in addition to fall-outs of inherent processes of expanding association areas. This unifying model, thus, views amyloid pathology and loss of cholinergic cells as a quid pro quo of the allometric evolution of the human brain, which in combination with increase in life expectancy overwhelm the fine homeostatic balance and trigger the disease process.

Single-cell proteomics: A treasure trove in neurobiology

Single-cell analysis came to change the way we look at cell populations. RNA sequencing of single cells allowed us to appreciate the diversity of cell types in the human brain in an unprecedented manner and its power to reveal cell-type specific changes in cell populations has just begun to be explored. In this context, looking at the proteome of single cells promises to bring functional information and contribute to completing the picture. The potential of single cell proteome, in developing a better understanding of the intricate connections between the very diverse cell populations in the brain, is huge. Whereas early approaches to address single-cell proteome have identified hundreds of proteins, today, techniques combining isobaric labelling and LC-MS can lead to the identification of thousands of proteins. In this review, we describe methods which have been used to identify and quantify proteins from single cells and propose that the application of isobaric labeling and label-free quantitative proteomics approach for single-cell analysis is ready to provide useful information for the neurobiology field.

Evolution of white matter damage in amyotrophic lateral sclerosis

OBJECTIVE

To characterize disease evolution in amyotrophic lateral sclerosis using an event-based model designed to extract temporal information from cross-sectional data. Conventional methods for understanding mechanisms of rapidly progressive neurodegenerative disorders are limited by the subjectivity inherent in the selection of a limited range of measurements, and the need to acquire longitudinal data.

METHODS

The event-based model characterizes a disease as a series of events, each comprising a significant change in subject state. The model was applied to data from 154 patients and 128 healthy controls selected from five independent diffusion MRI datasets acquired in four different imaging laboratories between 1999 and 2016. The biomarkers modeled were mean fractional anisotropy values of white matter tracts implicated in amyotrophic lateral sclerosis. The cerebral portion of the corticospinal tract was divided into three segments.

RESULTS

Application of the model to the pooled datasets revealed that the corticospinal tracts were involved before other white matter tracts. Distal corticospinal tract segments were involved earlier than more proximal (i.e., cephalad) segments. In addition, the model revealed early ordering of fractional anisotropy change in the corpus callosum and subsequently in long association fibers.

INTERPRETATION

These findings represent data-driven evidence for early involvement of the corticospinal tracts and body of the corpus callosum in keeping with conventional approaches to image analysis, while providing new evidence to inform directional degeneration of the corticospinal tracts. This data-driven model provides new insight into the dynamics of neuronal damage in amyotrophic lateral sclerosis.

Simple capacitor-switch model of excitatory and inhibitory neuron with all parts biologically explained allows input fire pattern dependent chaotic oscillations

Due to known information processing capabilities of the brain, neurons are modeled at many different levels. Circuit theory is also often used to describe the function of neurons, especially in complex multi-compartment models, but when used for simple models, there is no subsequent biological justification of used parts. We propose a new single-compartment model of excitatory and inhibitory neuron, the capacitor-switch model of excitatory and inhibitory neuron, as an extension of the existing integrate-and-fire model, preserving the signal properties of more complex multi-compartment models. The correspondence to existing structures in the neuronal cell is then discussed for each part of the model. We demonstrate that a few such inter-connected model units are capable of acting as a chaotic oscillator dependent on fire patterns of the input signal providing a complex deterministic and specific response through the output signal. The well-known necessary conditions for constructing a chaotic oscillator are met for our presented model. The capacitor-switch model provides a biologically-plausible concept of chaotic oscillator based on neuronal cells.

Tubulin polyglutamylation is a general traffic-control mechanism in hippocampal neurons

Neurons are highly complex cells that heavily rely on intracellular transport to distribute a range of functionally essential cargoes within the cell. Post-translational modifications of tubulin are emerging as mechanisms for regulating microtubule functions, but their impact on neuronal transport is only marginally understood. Here, we have systematically studied the impact of post-translational polyglutamylation on axonal transport. In cultured hippocampal neurons, deletion of a single deglutamylase, CCP1 (also known as AGTPBP1), is sufficient to induce abnormal accumulation of polyglutamylation, i.e. hyperglutamylation. We next investigated how hyperglutamylation affects axonal transport of a range of functionally different neuronal cargoes: mitochondria, lysosomes, LAMP1 endosomes and BDNF vesicles. Strikingly, we found a reduced motility for all these cargoes, suggesting that polyglutamylation could act as a regulator of cargo transport in neurons. This, together with the recent discovery that hyperglutamylation induces neurodegeneration, makes it likely that perturbed neuronal trafficking could be one of the central molecular causes underlying this novel type of degeneration.This article has an associated First Person interview with the first author of the paper.

Neurite orientation and dispersion density imaging (NODDI) detects cortical and corticospinal tract degeneration in ALS

BACKGROUND

Corticospinal tract (CST) degeneration and cortical atrophy are consistent features of amyotrophic lateral sclerosis (ALS). We hypothesised that neurite orientation dispersion and density imaging (NODDI), a multicompartment model of diffusion MRI, would reveal microstructural changes associated with ALS within the CST and precentral gyrus (PCG) 'in vivo'.

METHODS

23 participants with sporadic ALS and 23 healthy controls underwent diffusion MRI. Neurite density index (NDI), orientation dispersion index (ODI) and free water fraction (isotropic compartment (ISO)) were derived. Whole brain voxel-wise analysis was performed to assess for group differences. Standard diffusion tensor imaging (DTI) parameters were computed for comparison. Subgroup analysis was performed to investigate for NODDI parameter differences relating to bulbar involvement. Correlation of NODDI parameters with clinical variables were also explored. The results were accepted as significant where p<0.05 after family-wise error correction at the cluster level, clusters formed with p<0.001.

RESULTS

In the ALS group NDI was reduced in the extensive regions of the CST, the corpus callosum and the right PCG. ODI was reduced in the right anterior internal capsule and the right PCG. Significant differences in NDI were detected between subgroups stratified according to the presence or absence of bulbar involvement. ODI and ISO correlated with disease duration.

CONCLUSIONS

NODDI demonstrates that axonal loss within the CST is a core feature of degeneration in ALS. This is the main factor contributing to the altered diffusivity profile detected using DTI. NODDI also identified dendritic alterations within the PCG, suggesting microstructural cortical dendritic changes occur together with CST axonal damage.

General Approach to Peripheral Nerve Disorders

PURPOSE OF REVIEW

This article provides a conceptual framework for the evaluation of patients with suspected polyneuropathy to enhance the clinician's ability to localize and confirm peripheral nervous system pathology and, when possible, identify an etiologic diagnosis through use of rational clinical and judicious testing strategies.

RECENT FINDINGS

Although these strategies are largely time-honored, recent insights pertaining to the pathophysiology of certain immune-mediated neuropathies and to evolving genetic testing strategies may modify the way that select causes of neuropathy are conceptualized, evaluated, and managed.

SUMMARY

The strategies suggested in this article are intended to facilitate accurate bedside diagnosis in patients with suspected polyneuropathy and allow efficient and judicious use of supplementary testing and application of rational treatment when indicated.

Pathological classification of equine recurrent laryngeal neuropathy

Recurrent Laryngeal Neuropathy (RLN) is a highly prevalent and predominantly left-sided, degenerative disorder of the recurrent laryngeal nerves (RLn) of tall horses, that causes inspiratory stridor at exercise because of intrinsic laryngeal muscle paresis. The associated laryngeal dysfunction and exercise intolerance in athletic horses commonly leads to surgical intervention, retirement or euthanasia with associated financial and welfare implications. Despite speculation, there is a lack of consensus and conflicting evidence supporting the primary classification of RLN, as either a distal ("dying back") axonopathy or as a primary myelinopathy and as either a (bilateral) mononeuropathy or a polyneuropathy; this uncertainty hinders etiological and pathophysiological research. In this review, we discuss the neuropathological changes and electrophysiological deficits reported in the RLn of affected horses, and the evidence for correct classification of the disorder. In so doing, we summarize and reveal the limitations of much historical research on RLN and propose future directions that might best help identify the etiology and pathophysiology of this enigmatic disorder.

Abnormal Brain Connectivity Spectrum Disorders Following Thimerosal Administration: A Prospective Longitudinal Case-Control Assessment of Medical Records in the Vaccine Safety Datalink

BACKGROUND

Autism spectrum disorder (ASD), tic disorder (TD), and hyperkinetic syndrome of childhood (attention deficit disorder [ADD]/attention deficit hyperactivity disorder [ADHD]) are disorders recently defined as abnormal connectivity spectrum disorders (ACSDs) because they show a similar pattern of abnormal brain connectivity. This study examines whether these disorders are associated with exposure to thimerosal, a mercury (Hg)-based preservative.

METHODS

A hypothesis testing case-control study evaluated the Vaccine Safety Datalink for the potential dose-dependent odds ratios (ORs) for diagnoses of ASD, TD, and ADD/ADHD compared to controls, following exposure to Hg from thimerosal-containing Haemophilus influenzae type b vaccines administrated within the first 15 months of life. Febrile seizures, cerebral degeneration, and unspecified disorders of metabolism, which are not biologically plausibly linked to thimerosal, were examined as control outcomes.

RESULTS

On a per 25 μg Hg basis, cases diagnosed with ASD (OR = 1.493), TD (OR = 1.428), or ADD/ADHD (OR = 1.503) were significantly (P < .001) more likely than controls to have received increased Hg exposure. Similar relationships were observed when separated by gender. Cases diagnosed with control outcomes were no more likely than controls to have received increased Hg exposure.

CONCLUSION

The results suggest that Hg exposure from thimerosal is significantly associated with the ACSDs of ASD, TD, and ADD/ADHD.

Complete morphologies of basal forebrain cholinergic neurons in the mouse

The basal forebrain cholinergic system modulates neuronal excitability and vascular tone throughout the cerebral cortex and hippocampus. This system is severely affected in Alzheimer's disease (AD), and drug treatment to enhance cholinergic signaling is widely used as symptomatic therapy in AD. Defining the full morphologies of individual basal forebrain cholinergic neurons has, until now, been technically beyond reach due to their large axon arbor sizes. Using genetically-directed sparse labeling, we have characterized the complete morphologies of basal forebrain cholinergic neurons in the mouse. Individual arbors were observed to span multiple cortical columns, and to have >1000 branch points and total axon lengths up to 50 cm. In an AD model, cholinergic axons were slowly lost and there was an accumulation of axon-derived material in discrete puncta. Calculations based on published morphometric data indicate that basal forebrain cholinergic neurons in humans have a mean axon length of ∼100 meters.

DOI:http://dx.doi.org/10.7554/eLife.02444.001

The human brain is made up of roughly 80 to 100 billion neurons, organized into extensive networks. Each neuron consists of a number of components: a cell body, which contains the nucleus; numerous short protrusions from the cell body called dendrites; and a long thin structure called an axon that carries the electrical signals generated in the cell body and the dendrites to the next neuron in the network.

One of the most studied networks in the human brain is the basal forebrain network, which is made up of large neurons that communicate with one another using a chemical transmitter called acetylcholine. This network has a key role in cognition, and its neurons are among the first to degenerate in Alzheimer's disease. However, relatively little is known about the structure of these ‘cholinergic’ neurons because their large size makes them difficult to study using standard techniques.

Now, Wu et al. have visualized, for the first time, the complete 3D structure of cholinergic neurons in the mouse forebrain. The mice in question had been genetically modified so that only ten or so of their many thousands of cholinergic neurons expressed a distinctive ‘marker’ protein. This made it possible to distinguish these neurons from surrounding brain tissue in order to visualize their structures. The resulting pictures clearly illustrate the neurons' complexity, with individual axons in adult mice displaying up to 1000 branches.

Measurements showed that each cholinergic axon in the mouse brain is roughly 30 centimeters long, even though the brain itself is less than 2 centimeters from front to back. Based on measurements by other researchers, Wu et al. calculated that the axons of single cholinergic neurons in the human brain are about 100 meters long on average.

The extreme length and complex branching structure of cholinergic forebrain neurons helps to explain why each neuron is able to modulate the activity of many others in the network. It could also explain their vulnerability to degeneration, as the need to transport materials over such long distances may limit the ability of these neurons to respond to damage.

DOI:http://dx.doi.org/10.7554/eLife.02444.002

Vitamin E and neurological function

The clinical, neuropathological and electrophysiological evidence that vitamin E (alpha-tocopherol) is essential for normal neurological function will be reviewed. The possible reasons why neural tissues should be particularly affected by a deficiency of this fat-soluble vitamin and the mechanism(s) involved will be considered.

Disease mechanisms in hereditary sensory and autonomic neuropathies

Inherited peripheral neuropathies are common monogenically inherited diseases of the peripheral nervous system. In the most common variant, i.e., the hereditary motor and sensory neuropathies, both motor and sensory nerves are affected. In contrast, sensory abnormalities predominate or are exclusively present in hereditary sensory and autonomic neuropathies (HSAN). HSAN are clinically and genetically heterogeneous and are subdivided according to mode of inheritance, age of onset and clinical evolution. In recent years, 6 disease-causing genes have been identified for autosomal dominant and recessive HSAN. However, vesicular transport and axonal trafficking seem important common pathways leading to degeneration of sensory and autonomic neurons. This review discusses the HSAN-related genes and their biological role in the disease mechanisms leading to HSAN.

Influence of age of horse on results of quantitative electromyographic needle examination of skeletal muscles in Dutch Warmblood horses

OBJECTIVE

To determine the influence of age on results of quantitative analysis of electromyographic (EMG) needle examination in the subclavian, triceps, and lateral vastus muscles of Dutch Warmblood horses.

ANIMALS

7 healthy young Dutch Warmblood horses (range, 13 to 18 months old), 7 healthy adult Dutch Warmblood horses (range, 4 to 10 years old), and 7 healthy elderly Dutch Warmblood horses (range, 18 to 21 years old).

PROCEDURE

An EMG needle examination was performed to evaluate insertional activity, spontaneous activity, and motor unit action potential (MUAP) variables. Although all horses were conscious, young horses were sedated prior to examination.

RESULTS

Mean insertional activity in young horses was significantly lower than in elderly horses. Pathologic spontaneous activity was rarely found in young and adult horses but was frequently evident in all muscles in all elderly horses. The MUAP duration and amplitude were significantly lower in all muscles of young horses, compared with values for adult and elderly horses. The MUAP duration and number of phases and turns were significantly lower in adult horses than in elderly horses. Group differences for percentages of polyphasic and complex MUAPs were also found. The 95% confidence intervals for MUAP duration, MUAP amplitude, and number of phases and turns for the subclavian, triceps, and lateral vastus muscles were significantly lower in young horses than in adult or elderly horses.

CONCLUSIONS AND CLINICAL RELEVANCE

Age of the horse being examined should be considered when EMG examination is performed.

Effects of the new herbicide fentrazamide on the glucose utilization in neurons and erythrocytes in vitro

Treatment of rats with fentrazamide for 2 years at 3000 ppm (males) and 4000 ppm (females) led to an increased incidence and degree of axonal degeneration in sciatic nerve as well as to effects on red blood cells. The mechanism underlying these effects was investigated in vitro using various cell cultures (permanent rodent cell lines from the nervous system, liver, kidney, skeletal and heart muscle and fibroblasts, primary cortical neurons and erythrocytes from the rat). Added to cultured rat cortical neurons for 1 week, fentrazamide considerably decreased glucose consumption, ATP levels and mitochondrial membrane potential and lowered the GSH level, however, it had little impact on viability and neurofilaments and did not induce oxidative stress (ROS) over the first 2 h. After recovery for 1 week, in addition some destruction of neurofilaments had occurred probably secondary to the disturbance of energy production. These effects were prevented by pyruvate. Further studies indicated that fentrazamide primarily inhibited glucose utilization, most likely by interfering with glycolysis. Similar effects were found in erythrocytes treated with fentrazamide over a period of 7 days. Primarily, the glucose consumption was reduced after 1-day treatment, followed by a marked reduction of the energy supply at days 3 and 7. Comparable to the neurons, the GSH level was significantly reduced. A marked hemolysis of the red blood cells was then observed after prolonged treatment. The extensive energy demand and exclusive dependency on glucose utilization of neurons and erythrocytes may explain the specific vulnerability of motor neurons and erythrocytes. When comparing the concentrations necessary for inducing effects in vitro on neuronal cells and erythrocytes to the very low plasma concentrations of fentrazamide in treated rats it is suggested that only a small impact of fentrazamide on the energy status at high doses will occur in vivo. Therefore, aging of the rat as another factor compromising mitochondrial energy production in motor neurons must be considered as additional contribution for the induction of axonal degeneration. It is concluded that this effect of fentrazamide in rats poses no specific risk under the exposure conditions relevant to humans.

Neurotoxic mode of action of artemisinin

We recently described a screening system designed to detect neurotoxicity of artemisinin derivatives based on primary neuronal brain stem cell cultures (G. Schmuck and R. K. Haynes, Neurotoxicity Res. 2:37-49, 2000). Here, we probe possible mechanisms of this brain stem-specific neurodegeneration, in which artemisinin-sensitive neuronal brain stem cell cultures are compared with nonsensitive cultures (cortical neurons, astrocytes). Effects on the cytoskeleton of brain stem cell cultures, but not that of cortical cell cultures, were visible after 7 days. However, after a recovery period of 7 days, this effect also became visible in cortical cells and more severe in brain stem cell cultures. Neurodegeneration appears to be induced by effects on intracellular targets such as the cytoskeleton, modulation of the energy status by mitochondrial or metabolic defects, oxidative stress or excitotoxic events. Artemisinin reduces intracellular ATP levels and the potential of the inner mitochondrial membrane below the cytotoxic concentration range in all three cell cultures, with these effects being most dominant in the brain stem cultures. Surprisingly, there were substantial effects on cortical neurons after 7 days and on astrocytes after 1 day. Artemisinin additionally induces oxidative stress, as observed as an increase of reactive oxygen species and of lipid peroxidation in both neuronal cell types. Interestingly, an induction of expression of AOE was only seen in astrocytes. Here, manganese superoxide dismutase (MnSOD) expression was increased more than 3-fold and catalase expression was increased more than 1.5-fold. In brain stem neurons, MnSOD expression was dose dependently decreased. Copper-zinc superoxide dismutase and glutathione peroxidase, two other antioxidant enzymes that were investigated, did not show any changes in their mRNA expression in all three cell types after exposure to artemisinin.

Dysfunction of sensory nerves during attacks of hypokalemic periodic paralysis

Reversible electrophysiologic abnormalities of sensory nerve function were found by chance in three patients with hypokalemic periodic paralysis, a disorder previously considered to affect the function of muscle membranes only. A formal, prospective study was therefore conducted. Serial nerve conduction studies were done in ten additional patients. Amplitude of sensory action potentials was significantly smaller during paralytic attacks, but did not differ from controls after normalization of serum potassium concentration. These apparently novel findings might be explained by previous electrodiagnostic studies either not involving the testing of sensory nerves at all, or not being repeated after recovery from an attack. Involvement of sensory nerves in hypokalemic periodic paralysis is suggested to arise through dorsal root ganglia having an incomplete blood-nerve barrier and sensory neurons being particularly vulnerable to derangements affecting nerve cell metabolism. Neuronal inexcitability is postulated to occur consequent upon possible inactivation of the sodium-potassium pump by the low concentration of extracellular potassium. In patients with acute areflexic limb weakness, the diagnosis of hypokalemic periodic paralysis should not be excluded by abnormal results of sensory nerve conduction studies.

Orphan neurones and amine excess: the functional neuropathology of Parkinsonism and neuropsychiatric disease

The aetiology and treatment of Parkinsonism is currently conceptualised within a dopamine (DA) deficiency-repletion framework. Loss of striatal DA is thought to cause motor impairment of which tremor, bradykinaesia and rigidity are prominent features. Repletion of deficient DA should at least minimise parkinsonian signs and symptoms. In Section 2, based on extensive pre-clinical and clinical findings, the instability of this approach to Parkinsonism is scrutinised as the existing negative findings challenging the DA deficiency hypothesis are reviewed and reinterpreted. In Section 3 it is suggested that Parkinsonism is due to a DA excess far from the striatum in the area of the posterior lateral hypothalamus (PLH) and the substantia nigra (SN). This unique area, around the diencephalon/mesencephalon border (DCMCB), is packed with many ascending and descending fibres which undergo functional transformation during degeneration, collectively labelled 'orphan neurones'. These malformed cells remain functional resulting in pathological release of transmitter and perpetual neurotoxicity. Orphan neurone formation is commonly observed in the PLH of animals and in man exhibiting Parkinsonism. The mechanism by which orphan neurones impair motor function is analogous to that seen in the diseased human heart. From this perspective, to conceptualise orphan neurones at the DCMCB as 'Time bombs in the brain' is neither fanciful nor unrealistic [E.M. Stricker, M.J. Zigmond, Comments on effects of nigro-striatal dopamine lesions, Appetite 5 (1984) 266-267] as the DA excess phenomenon demands a different therapeutic approach for the management of Parkinsonism. In Section 4 the focus is on this novel concept of treatment strategies by concentrating on non-invasive, pharmacological and surgical modification of functional orphan neurones as they affect adjacent systems. The Orphan neurone/DA excess hypothesis permits a more comprehensive and defendable interpretation of the interrelationship between Parkinsonism and schizophrenia and other related disorders.

Sensory neuropathies including painful and toxic neuropathies

In most peripheral neuropathies, dysfunction of motor and sensory nerve fibres is present. However, in some of them either pattern may predominate or be exclusively present. In this review we describe the clinical characteristics of sensory neuropathies, with emphasis on their possible causes. Guidelines are given for the diagnostic approach in these patients and, where possible, suggestions are given for treatment, including symptomatic treatment of painful neuropathies.

Effect of chronic vitamin E deficiency on sympathetic and sensorimotor function in rat mesenteric arteries

1. Mesenteric arterial beds from male rats deprived of vitamin E for 12 months postweaning were isolated and perfused at 5 ml min-1. 2. The basal perfusion pressure of vitamin E-deficient preparations was significantly higher (34.0 +/- 1.9 mmHg, n = 15) than in age-matched controls (26.1 +/- 2 mmHg, n = 14; P < 0.01). 3. At basal tone, vasoconstrictor responses to electrical field stimulation (EFS) were not attenuated by vitamin E deficiency; at high stimulation frequencies, responses were enhanced. According to dose-response curves, exogenous noradrenaline was significantly more efficacious in preparations from vitamin E-deficient rats (P < 0.05). 4. In preparations with tone raised by methoxamine (6-20 microM) and in the presence of guanethidine (5 microM), EFS of perivascular sensorimotor nerves elicited frequency-dependent vasodilatation which was significantly attenuated by vitamin E deficiency. There was no difference in relaxation to calcitonin gene-related peptide (CGRP; 1.5 x 10(-11) mol), or to the sensory neurotoxin capsaicin (5 x 10(-11) mol). 5. Immunohistochemical analysis of CGRP-containing nerves in the superior mesenteric artery showed no differences in density of innervation. 6. In conclusion, chronic vitamin E deficiency impairs sensorimotor vasodilatation in rat mesenteric arteries; this does not appear to be due to changes in postjunctional receptors, or to a depletion of transmitter (CGRP) content of the superior mesenteric artery. Sensorimotor nerves appear to be more vulnerable than sympathetic nerves to chronic vitamin E deficiency.

The relation between conduction velocity and axonal length

Motor evoked potentials (MEPs) obtained by electrical root stimulation and F waves were used to examine the proximal nerve conduction velocity (CV) to tibialis anterior (TA), extensor digitorum brevis (EDB), flexor carpi radialis (FCR), and abductor pollicis brevis (APB) muscles in 40 humans. By subtracting motor latencies obtained by stimulating the peripheral nerve at the same point from the F-wave and MEP latencies, we could measure the CV over identical proximal segments. It was found that proximal CV to TA and FCR was significantly higher than to EDB and APB, respectively. Combining the data of the proximal CV to all four muscles in relation with axonal length resulted in a highly significant inverse relationship (r2 = 0.77). Thus the axonal length explained to a large extent the higher CV of the arm nerves and also the inverse relation between body height and CV. The distal CV was always lower than proximal CV; however, there was no support for an additional effect of this gradient in explaining the relationship between CV and height since it was constant for all body heights.

Excitatory amino acids, free radicals and the pathogenesis of motor neuron disease

The cause of motor neuron disease (MND) remains unknown, but the pathogenic involvement of excitatory amino acid (EAA) neurotransmitters and related exogenous compounds has been proposed. We discuss current concepts of the mechanisms of action of EAAs and the evidence for links between these neurotransmitters and free radical hypotheses of neuronal damage. These concepts are especially pertinent following reports of mutations in the gene encoding the free radical scavenging enzyme, copper-zinc superoxide dismutase, in familial MND. New approaches to treatment are suggested by advances in understanding of the disease.

Selective vulnerability in acute energy deprivation syndromes

The topography and cellular events in the experimental lesions caused by chlorosugars, 6-aminonicotinamide, dinitrobenzene and tribromoimidazole in animals are considered in relation to those features in human acute thiamine deficiency (Wernicke's) encephalopathy and for comparison in Leigh's disease. The topography and cellular changes when closely examined are different and particular to each condition, although there is a basic cellular process common to all. The pathogenesis of each condition must be considered as multifactorial and a search for the factors responsible for the neuronal and cellular selective vulnerability of different regions of the neuraxis will lead us to understanding the pathogenesis of the disease process in each instance. The experimental models offer much for the understanding of the human conditions, particularly in the search for satisfactory therapeutic strategies.

Giant axonal degeneration: determination of essential glycolytic enzymes

Aggregation and accumulation of intermediate filaments (IF) in different cell types is an ultrastructural hallmark of autosomal recessively inherited giant axonal degeneration (GAD) as well as of experimentally induced or occupationally acquired toxic polyneuropathies. Insufficient glycolysis caused by glycolytic enzyme deficiencies has been proposed as a likely cause of the IF accumulation. In this report data are presented on essential glycolytic enzymes in erythrocytes of a GAD patient and family. No significant changes were noted.

Pathological aspects of Australian Stringhalt

Nine horses with clinical signs of Australian Stringhalt were killed and tissues collected for a detailed pathological study. Lesions were limited to peripheral nerves and muscles. The most severely affected nerves were the superficial and deep peroneal, distal tibial, plantar digital, volar and recurrent laryngeal nerve with changes characterised by a selective loss of large diameter myelinated fibres with various degrees of demyelination, fibrosis, Schwann cell proliferation and onion-bulb formation. A routine evaluation of the brain and spinal cord by light microscopy failed to reveal any consistent abnormalities. Morphometric analysis of deep peroneal and recurrent laryngeal nerves confirmed the reduced number of large diameter myelinated axons. Teased fibre preparations of these nerves did not show any abnormalities in internodal distance. The most severe muscle lesions were in the long and lateral digital extensors, cranial tibial, dorsal cricoarytenoid, gracilis and lateral deep digital flexor with extensive atrophy of fibres and diffuse fibrosis. Histochemical evaluation of the long digital extensor from 3 affected horses showed an abnormally wide distribution in fibre size and a reduction in type II fibres compared with controls. These lesions are consistent with a distal axonopathy leading to neurogenic muscle atrophy. The distribution of neuromuscular lesions in Australian Stringhalt may be explained by the susceptibility of longer, larger myelinated nerve fibres to injury, but the cause for this distal axonopathy remains unknown.

Effects of thallium on primary cultures of testicular cells

The objective of this in vitro study was to examine the response of mixed cultures of Sertoli and germ cells to treatment with thallium (Tl) at the range of concentrations that, in previous studies, was shown in vivo to affect reproduction. Cultures were prepared from the testis of Sprague-Dawley rats. Cultures containing approximately 3.75 x 10(6) cells/ml were treated with Tl concentrations corresponding to 35, 7, and 1.4 micrograms Tl/g testis, estimated from protein content of cultures. Observations at 24, 48, and 72 h after treatment showed a significant release of germ cells into the culture medium that was both concentration and time dependent. Cultures treated with 35 micrograms Tl/g testis showed a threefold increase in germ-cell detachment compared with controls after only 24 h of exposure. As the treatment time increased to 48 h of exposure, even cultures exposed at the lowest Tl concentration (1.4 micrograms Tl/g testis) showed significant loss of germ cells. After 48 h, cultures exposed to 7 micrograms Tl/g testis exhibited a 2.5-fold increase in germ-cell detachment, and those exposed to 35 micrograms Tl/g testis exhibited a 10-fold increase over controls. Morphological investigations of cell cultures showed evident loss of germ cells with significant reduction in prepachytene and pachytene spermatocytes and changes in the shape of Sertoli cells. These results are in agreement with in vivo studies, in which thallium treatment at comparable exposure levels manifested its earliest toxic testicular effects in Sertoli and germ cells. They also demonstrate the usefulness of this in vitro culture technique to assess toxic testicular damage rapidly.

Idiopathic trigeminal sensory neuropathy

Idiopathic trigeminal sensory neuropathy is a rare disorder characterized by transient sensory disturbances in the territory of one or more branches of the trigeminal nerve. Acute and chronic forms of the disease have been recognized, but the boundaries between these groups are not clearly defined. Six cases of idiopathic trigeminal sensory neuropathy are reported. All patients presented with sensory abnormalities limited to the territory of one trigeminal nerve, in the absence of other clinical features. The symptoms were on the left side of the face in five cases and on the right side in one. No bilateral involvement was seen. There were no cases of muscle weakness, and the corneal reflex was always present. In two cases, sensory disturbances began in the tongue or lips and later spread through the entire territory of the three trigeminal branches. Both patients recovered completely after 3 months. The other four patients had involvement of the second and third branches (two cases), or the third branch alone (two cases), and the symptoms persisted unmodified for 2 to 4 years. There were no clinical variables at onset to enable one to predict an acute or chronic evolution of the disease.

Stimulation of rat sciatic nerve regeneration with pulsed electromagnetic fields

The effects of pulsed electromagnetic fields (PEMF) on rat sciatic nerve regeneration after a crush lesion were determined. The rats were placed between a pair of Helmholtz coils and exposed to PEMF of frequency 2 Hz and magnetic flux density of 0.3 mT. A 4 h/day treatment for 3-6 days increased the rate of nerve regeneration by 22%. This stimulatory effect was independent of the orientation of the coils. Exposure times of 1 h/day-10 h/day were equally effective in stimulating nerve regeneration. Rats exposed to PEMF for 4 h/day for 7 days before crush, followed by 3 days after crush without PEMF, also showed significantly increased regeneration. This pre-exposure 'conditioning' effect suggests that PEMF influences regeneration indirectly.

Peripheral nerve function in patients with chronic bronchitis receiving almitrine or placebo

A double blind prospective study of the effect of almitrine bismesylate and placebo on peripheral-nerve function was carried out in 12 patients with chronic bronchitis and arterial hypoxaemia (mean (SD) FEV1% predicted 38 (16), arterial oxygen tension (PaO2) 7.56 (0.76) kPa). Of the seven patients who took placebo, none developed symptoms or signs of peripheral neuropathy. One patient who had abnormal lower limb sensory nerve conduction initially showed improvement of sensory conduction but deterioration in motor conduction during the 12 month study period. Two further patients developed some slowing of motor conduction velocities in their right lateral popliteal nerve. Five patients received almitrine and all showed an improvement in PaO2 (mean from 7.0 to 7.9 kPa). None had symptoms or signs of peripheral neuropathy on entry to the study; one patient had evidence of impaired nerve conduction on electrophysiological testing. Three patients developed symptoms and signs of peripheral neuropathy during the 12 months of the study and a fourth developed peripheral neuropathy at 18 months, having continued to receive almitrine. Studies of nerve physiology showed abnormalities in the lower limbs of all four patients. Recovery was poor, possibly because of the long half life of almitrine. The studies suggest that almitrine may precipitate peripheral neuropathy in patients with chronic obstructive pulmonary disease. Patients should be warned of this potential complication so that the drug can be stopped as soon as symptoms develop.

Propylene oxide causes central-peripheral distal axonopathy in rats

In Wistar rats subjected daily to a 6-hr exposure of propylene oxide (PO) at a concentration of 1,500 ppm (5 times a wk for 7 wk), ataxia developed in the hindlegs. Myelinated fibers in hindleg nerves and in the fasciculus gracilis showed axonal degeneration, sparing the nerve cell body of the first sacral dorsal root ganglion and myelinated fibers of the first sacral dorsal and ventral roots. These pathologic findings are compatible with central-peripheral distal axonopathy. This is apparently the first animal model of PO neuropathy to be verified histologically.

Neurological and neurobehavioral development of the mutant 'twitcher' mouse

The present study described the neurological and locomotor development of the mutant 'twitcher' mouse (B57BL/6J-twi), an enzymatically authentic model of globoid cell (Krabbe) leukodystrophy. Comparisons were made on a neurological developmental battery and a series of behavioral tests, including open field, rotorod, and hangtime performance. Homozygous affected (twi/twi), heterozygous carriers (+/twi) and homozygous normals (+/+) were compared. Neurological development was slowed in twi/twi with some subtler differences between +/twi and normals. Twi/twi reached all functional milestones except grasp. There was a rapid deterioration of motor indices after 20 days of age. However, most sensory markers were preserved. On hangtime, there were significant differences from normal for both twi/twi and +/twi at 15 days of age and across the 15-30 day developmental stage, with the +/twi males slightly more impaired. On the rotorod, all animals were equally unable to stay on the rod at 15 days of age and neither male nor female twi/twi showed significant development. Male +/twi lagged significantly behind male +/+. In the open field, all groups were equally inactive at 13-15 days and showed similar increases in activity, rearing, and grooming until weaning. All groups peaked immediately after weaning and declined thereafter, with twi/twi showing the lowest activity. The data were discussed in terms of the relationship between the human disease and the animal model.

The pathogenesis of equine laryngeal hemiplegia — a review

Recent research on the muscular and nervous changes which occur in idiopathic equine laryngeal hemiplegia has indicated that many of the traditional concepts of the aetiology of this disease are erroneous. In light of the new knowledge gained, the various predispositions and possible causes of laryngeal hemiplegia are discussed, and it is suggested that the underlying mechanism of axonal damage in this neuropathy of horses may be related to abnormal energy metabolism in the axon.

Vitamin E--its role in neurological function

Studies in patients with abetalipoproteinaemia, other chronic and severe fat malabsorptive states and a selective defect in vitamin E absorption, together with neuropathological studies in the vitamin E deficient human, monkey and rat indicate that vitamin E is important for normal neurological function. Appropriate vitamin E supplementation is, therefore, advisable for all patients with chronic fat malabsorption who have low serum vitamin E concentrations. Serum vitamin E concentrations should also be measured in patients with spinocerebellar disorders, whatever the aetiology.

Axonal degeneration distal to the site of accumulation of vesicular profiles in the myelinated fiber axon in experimental isoniazid neuropathy

Morphometric sequential studies of pathologic changes were carried out on myelinated fibers in the lumbar ventral root of Sprague-Dawley rats administered with isoniazid, 1,500 mg/kg body weight, in a single dose. Accumulation of axoplasmic organelles with secondary paranodal retraction of myelin sheath occurred in the middle part of the ventral root as early as day 2 after the administration. On day 3, axonal degeneration started to occur, distal to the middle part, where the accumulation of axoplasmic organelles is prominent. Such accumulation with the possible blockade of the fast axoplasmic transport in the proximal axon may be directly responsible for the distal axonal degeneration. Alternatively such accumulation may be secondary to the distal axonal degeneration. The morphological sequential findings described clearly reflects the pathological events in isoniazid neuropathy.