On the pattern of changes in the rat nervous system produced by 2,5 hexanediol. A topographical study by light microscopy

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.

The reversibility of neurofilaments decline induced by 2,5-hexanedione in rat nerve tissues

To investigate the reversibility of the neuropathy induced by 2,5-HD, adult male rats were administered at a dosage of 400 mg/kg/day 2,5-HD (five times per week) for 2, 4, and 8 weeks, respectively. After stopping HD exposure, half of 8-week treated animals were allowed to naturally recover for 16 weeks. The relative levels of NF-H, NF-M, and NF-L in spinal cords and sciatic nerves of rats were determined by immunoblotting during the HD neuropathy. The results showed that NFs content in nerve tissues demonstrated a progressive decline as the intoxication continued. Furthermore, after a recovery of 16 weeks, the levels of three NF subunits in spinal cords of treated rats returned to normal while those in sciatic nerves displayed an inconsistent reversal. Among them, the level of NF-H in sciatic nerves returned to normal completely, and NF-L also showed a significant improvement, whereas NF-M did not demonstrate an obvious reversal. These findings suggest that HD-induced NFs decline is at least partially irreversible within the time frame of this study, which might be associated with the incomplete recovery of neurological dysfunctions of HD-treated rats.

Time-dependent Alteration of Cytoskeletal Proteins in Cerebral Cortex of Rat During 2,5-Hexanedione-induced Neuropathy

To investigate the mechanisms of the axonopathy induced by 2,5-hexanedione (2,5-HD), male Wistar rats were administered at a dosage of 400 mg/kg/day 2,5-HD (five times per week). The rats produced a slightly, moderately, or severely abnormal neurological changes, respectively, after 2, 4, or 8 weeks of treatment. The cerebrums were Triton-extracted and ultracentrifuged to yield a pellet fraction and a corresponding supernatant fraction. The relative levels of six cytoskeletal proteins (NF-L, NF-M, NF-H, alpha-tubulin, beta-tubulin, and beta-actin) in both fractions were determined by immunoblotting. The results showed that NFs content in HD-treated rats demonstrated a progressive decline as the intoxication of HD continued. As for microtubule proteins, the levels of alpha-tubulin and beta-tubulin demonstrated some inconsistent changes. The content of alpha-tubulin kept unchangeable, while the content of beta-tubulin increased significantly at the late stage of HD exposure. Furthermore, the content of beta-actin in both fractions remained unaffected throughout the study. These findings suggest that HD intoxication resulted in a progressive decline of NFs, which was highly correlated with the development of HD-induced neuropathy.

Fast axonal transport: a site of acrylamide neurotoxicity?

The cellular and molecular site and mode of action of acrylamide (ACR) leading to neurotoxicity has been investigated for four decades, without resolution. Although fast axonal transport compromise has been the central theme for several hypotheses, the results of many studies appear contradictory. Our analysis of the literature suggests that differing experimental designs and parameters of measurement are responsible for these discrepancies. Further investigation has demonstrated consistent inhibition of the quantity of bi-directional fast transport following single ACR exposures. Repeated compromise in fast anterograde transport occurs with each exposure. Modification of neurofilaments, microtubules, energy-generating metabolic enzymes and motor proteins are evaluated as potential sites of action causing the changes in fast transport. Supportive and contradictory data to the hypothesis that deficient delivery of fast-transported proteins to the axon causes, or contributes to, neurotoxicity are critically summarized. A hypothesis of ACR action is presented as a framework for future investigations.

Effects of 2,5-hexanedione on calpain-mediated degradation of human neurofilaments in vitro

2,5-Hexanedione (2,5-HD), the neurotoxic metabolite of n-hexane, can structurally modify neurofilaments (NF) by pyrrole adduct formation and subsequent covalent cross-linking. 2,5-HD also induces accumulations of NF within the pre-terminal axon. We examined whether exposure of NF to 2,5-HD affected NF degradation. Two different models were used: (1) NF-enriched cytoskeletons isolated from human sciatic nerve were incubated with 2,5-HD in vitro and (2) differentiated human neuroblastoma cells (SK-N-SH) were exposed to 2, 5-HD in culture prior to isolation of cytoskeletal proteins. The cytoskeletal preparations were subsequently incubated with calpain II. The amount of NF-H and NF-L remaining after proteolysis was determined by SDS-PAGE and quantitative immunoblotting. NF-M proteolysis could not be quantified. Incubation of sciatic nerve cytoskeletal preparations with 2,5-HD resulted in cross-linking of all three NF proteins into high molecular weight (HMW) material with a range of molecular weights. Proteolysis of the NF-H and NF-L polypeptides was not affected by 2,5-HD-exposure. Degradation of the HMW material containing NF-H or NF-L was retarded when comparing with degradation of the NF-H and NF-L polypeptides, respectively, from control samples, but not as compared to the corresponding NF polypeptides from 2,5-HD-treated samples. Exposure of SK-N-SH cells to 2,5-HD also resulted in considerable cross-linking of NF. No differences were found between the proteolytic rates of NF-L and NF-H from exposed cells as compared with those subunits from control cells. Moreover, degradation of cross-linked NF-H was not different from monomeric NF-H. In conclusion, whether 2,5-HD affects calpain-mediated degradation of cross-linked NF proteins will depend on which model better reflects NF cross-linking as occurring in 2, 5-HD-induced axonopathy. However, with both models it was demonstrated that exposure of NF proteins to 2,5-HD without subsequent cross-linking is not adequate to inhibit NF proteolysis in vitro by added calpain.

Pathogenetic studies of hexane and carbon disulfide neurotoxicity

Two commonly employed solvents, n-hexane and carbon disulfide (CS2), although chemically dissimilar, result in identical neurofilament-filled swellings of the distal axon in both the central and peripheral nervous systems. Whereas CS2 is itself a neurotoxicant, hexane requires metabolism to the gamma-diketone, 2,5-hexanedione (HD). Both HD and CS2 react with protein amino functions to yield initial adducts (pyrrolyl or dithiocarbamate derivatives, respectively), which then undergo oxidation or decomposition to an electrophile (oxidized pyrrole ring or isothiocyanate), that then reacts with protein nucleophiles to result in protein cross-linking. It is postulated that progressive cross-linking of the stable neurofilament during its anterograde transport in the longest axons ultimately results in the accumulation of neurofilaments within axonal swellings. Reaction with additional targets appears to be responsible for the degeneration of the axon distal to the swellings.

Function of the auditory and visual systems, and of peripheral nerve, in rats after long-term combined exposure to n-hexane and methylated benzene derivatives. I. Toluene

Rats were exposed to n-hexane, toluene, or toluene together with n-hexane, each solvent 1000 p.p.m. (1000 + 1000 p.p.m. in combined exposure), 21 hr/day, 7 days/week during 28 days. Neurophysiological recordings were made 2 days, 3 months, and one year after end of exposure. A reduction in auditory sensitivity, recorded by click evoked auditory brainstem response, was observed 2 days after exposure to toluene alone, or to toluene together with n-hexane, but not after exposure to n-hexane alone. The reduction lasted one year after the exposure. Three months after combined exposure, a synergistic enhancement of loss of auditory sensitivity was observed in the mixed exposure group. One amplitude in the flash evoked potentials was lowered in the n-hexane exposed group 2 days after exposure. No such reduction was seen after simultaneous exposure to toluene. Exposure to n-hexane alone caused a marked decrease in peripheral nerve conduction velocity 2 days and 3 months after exposure, while exposure to n-hexane together with toluene had only a small effect on this velocity.

Effect of exposure to 2,5-hexanediol in light or darkness on the retina of albino and pigmented rats. I. Morphology

Male albino (Sprague Dawley) and pigmented (Norwegian Brown) rats received 1% 2,5-hexanediol (H) in their drinking water for 5 or 8 weeks, respectively. The rats were housed either in 12 h light (average 30 cd/cm2 inside cage) and 12 h darkness (group LH) or in total darkness (group DH). Two control groups (Light only, LC; Darkness only, DC) were studied in parallel under identical conditions. The animals were sacrificed at the end of H exposure or after an ensuing 13-week period without H but under the same lighting conditions. The retinas of albino rats in the LH group showed a reduction (compared to the LC, DH and DC groups) in the number of nuclei per unit area of the outer nuclear layer (ONL; p < 0.05) and degeneration of the outer segment and the inner segment layers (photoreceptor cells). A less pronounced loss of nuclei was seen in the LC group. No decrease in the number of nuclei, or signs of degeneration, were demonstrated in the albino DH or DC groups. Thirteen weeks after exposure to H, the albino LH rats had lost about 50% of the nuclei in the ONL (p < 0.05) and the outer plexiform layer (OPL) had almost disappeared. At the corresponding time, in the pigmented rats the LH and DH groups differed from the LC and DC groups. The degenerative process resulted in no inflammatory changes in the retina. The results imply an interaction exceeding simple summation after exposure to light and H, in destroying photoreceptors and OPL (p < 0.001) in albino rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Axonal swellings in human intramuscular nerves

The presence, morphology, distribution, and abundance of axonal swellings in intramuscular nerves were evaluated. Axonal swellings were present in intramuscular nerves in 42% of 127 muscle biopsies from patients with a variety of conditions. The incidence was highest in muscle from patients with peripheral neuropathy, but swellings were present in muscle from patients with motor neuron disease, primary muscle diseases, and some individuals without clinical or histological evidence of neuromuscular disease. The greatest number of swellings in intramuscular nerves was in muscle from patients with chronic inflammatory demyelinating neuropathy. Swellings were spherical or elliptical, 4-20 microns in diameter, 5-30 microns in length, and composed of neurofilaments. Swellings were present only in myelinated axons of intramuscular nerves, proximal to nodes of Ranvier or in internodal regions. Swellings were not associated with axonal degeneration. They were probably not transported. The formation or accumulation of swellings may reflect altered axonal dynamics common to a number of disease processes.

The use of experimental studies to reveal suspected neurotoxic chemicals as occupational hazards: acute and chronic exposures to organic solvents

The nervous system differs from many other body organs by its central control of vital functions and its low regeneration capacity. Organic solvents have, as a group, been suspected to have neurotoxic effects. Because of their similar physical properties and the fact that in industrial uses, they are often present in various mixtures, organic solvents have also been regarded, unfortunately, to induce common neurotoxic effects. However, it is evident from experimental studies using specified exposure conditions that different organic solvents have very diverse neurotoxic effects and also that the toxic mechanism may differ between acute and chronic exposure. No specific method used to describe a neurotoxic effect or single toxic response can be used for the overall occupational risk assessment of all organic solvents. Each solvent has to be considered as having its own unique toxic effects.

Total number and mean cell volume of neocortical neurons in rats exposed to 2,5-hexanedione with and without acetone

The toxicological effects of 2,5-hexanedione (2,5-HD) alone and combined with acetone on the number and size of neurons in the cerebral cortex of rats were evaluated with stereological techniques. Thirty rats were equally divided into three groups: One control, one receiving 0.5% 2,5-HD, and one receiving 0.5% 2,5-HD and 0.5% acetone in the drinking water for seven weeks. Unbiased estimates of the total number of neocortical neurons, as well as the mean neuronal nuclei and cell body volumes were obtained from systematically sampled 3.5-microns sections. The total number of neurons in the 2,5-HD group was significantly smaller than the control group (p less than 0.05, one-tailed t-test). Both test groups showed significant changes in the mean cell body volume: Compared with the control group, animals exposed to 2,5-HD had 11% smaller cell body volumes while animals exposed to 2,5-HD and acetone had 13% larger cell body volumes. These data represent the first unbiased estimation of mean cell volume in toxicology. We propose the nucleator method as an efficient and accurate tool for estimating quantitative changes in toxicological research.

Giant axonopathy characterized by intermediate location of axonal enlargements and acceleration of neurofilament transport

It has previously been shown that 2,5-hexanedione (2,5-HD) and its 3,4-dimethyl derivative (3,4-DMHD) induce neurofilamentous accumulations at prenodal sites in distal and proximal, respectively, regions of peripheral axons. For 2,5-HD, neurofilament (NF) transport is accelerated and this is thought to be directly related to the appearance of the axonal enlargements. For 3,4-DMHD, however, the rate of NF transport cannot be assessed owing to the very proximal position of NF accumulation. In the present study, it is shown that administration to rats of 3-methyl-2,5-hexanedione, the structural 'average' of 2,5-HD and 3,4-DMHD, induces NF accumulations at midway axonal positions of the sciatic and optic systems, and results in acceleration of NF in the sections of optic axons proximal to the enlargements. These results suggest that a common mechanism underlies all gamma-diketone neuropathies, and that the proximodistal pattern of axonal enlargements represents pharmacokinetic variables rather than differences in mode of action. The neurotoxicity of gamma-diketones probably arises from pyrrolation of lysine epsilon-amino groups in crucial regions of NF or related proteins responsible for maintaining the proper supramolecular organization of the cytoskeleton. Acceleration of NF transport appears to be a common characteristic of chemically induced axonopathies, regardless of location, and this is contrary to the theory that gamma-diketone-induced NF accumulation results primarily from a progressive cross-linking of NF occurring subsequent to pyrrole formation.

Testicular atrophy and loss of nerve growth factor-immunoreactive germ cell line in rats exposed to n-hexane and a protective effect of simultaneous exposure to toluene or xylene

Testicular and germ cell line morphology in rats were studied 2 weeks, 10 months and 14 months after cessation of a 61-day inhalation exposure to 1000 ppm n-hexane. Androgen biosynthetic capacity of testis, testosterone blood concentration, vas deferens morphology and noradrenaline (NA) concentration, epididymal sperm morphology, and fertility were also studied. Severe testicular atrophy involving the seminiferous tubules with loss of the nerve growth factor (NGF) immunoreactive germ cell line was found. Total loss of the germ cell line was found in a fraction of animals up to 14 months post-exposure, indicating permanent testicular damage. No impairment of androgen synthesis or androgen dependent accessory organs was observed. Simultaneous administration of 1000 ppm n-hexane and 1000 ppm toluene, or 1000 ppm n-hexane and 1000 ppm xylene, did not cause germ cell line alterations or testicular atrophy. Toluene and xylene were thus found to protect from n-hexane induced testicular atrophy.

Subchronic dosing of macaques with 2,5-hexanedione causes long-lasting motor dysfunction but reversible visual loss

Visual sensitivity and neurological status were monitored in four female macaque monkeys dosed orally with 2.5-hexanedione (0.64 mM/kg, 5 days per week) for 15 or 17 weeks. The first sign of toxicity was intention tremor seen after 3 months of dosing. This was followed, a week later, by a moderate decrease in visual contrast sensitivity, which returned to baseline 6 to 11 weeks after the end of dosing. Acuity and flicker resolution were not disrupted. A progressive general weakness ensued for 5 to 7 weeks after dosing had ended. Some slow recovery was seen, although the animals remained severely disabled 20 weeks after dosing was discontinued. Pathologic changes were examined 3 weeks (one monkey) and 20 weeks (three monkeys) after dosing. Soon after the end of dosing, axonal swellings were present throughout the distal optic tracts, peripheral nerves, and long tracts of the spinal cord. Twenty weeks after dosing, there was no indication of degeneration in the retinocortical pathway. Peripheral nerves showed widespread axonal loss, residual ongoing degeneration, and only slight regeneration. Axon loss and gliosis were evident in distal dorsal columns, and to a lesser extent, dorsal spinocerebellar and corticospinal tracts. These effects of 2,5-hexanedione on the macaque differ from those found previously for two other axonotoxic compounds, acrylamide monomer and carbon disulfide, which caused marked permanent degeneration that was most prominent in the visual system.

Cytoskeletal changes induced in vitro by 2,5-hexanedione: an immunocytochemical study

The effects of 2,5-hexanedione, the main metabolite of the solvents hexane and methyl butyl ketone, have been explored in different in vitro epithelial (CG5 and HEp-2) and melanoma (JR8) cells by means of immunochemistry and electron microscopy. The administration of the toxicant to the cell monolayers at noncytolytic concentrations for 24 and 48 hr exerted several effects on the cell lines studied. Most epithelial and melanoma cells detached from the substrate were in the mitotic phase, whereas cells adhering to the substrate showed time-dependent organelle changes. In fact, after treatment with 2,5-hexanedione, mitochondria appeared swollen, with distorted cristae and rarefied matrix; changes in intracytoplasmic vesicles were also detected. Cytoskeletal components were also investigated. A remarkable rearrangement of microfilaments and intermediate filaments (keratin and vimentin) was detected in a time-dependent manner. In particular, actin ruffles and intermediate filament aggregates were observed. Furthermore, the microtubular apparatus seemed to be less affected. The results here reported seem to indicate cytoskeletal components as probable targets of 2,5-hexanedione cytotoxicity in cultured cells.

Morphologic changes induced in vitro by 2,5 hexanedione

The effects of 2,5 hexanedione (2,5 HD), a metabolite of n-hexane, were investigated in different in vitro systems. A human mammary carcinoma cell line, a human melanoma cell line, and fetal mouse neuronal cells in primary culture were considered. Light and electron microscopic observations demonstrated clearly that changes in cell proliferation can be detected. Furthermore, morphologic differentiative phenomena characterized by a noticeable increase in cell protrusions and dendriticlike processes can occur. Differences in the features of these processes were also detected between the different cell lines. These data can indicate non-neuronal cells as possible further targets of the toxicant. The possibility could be hypothesized that toxic neuropathies are generalized disorders, being neuronal system exceptionally vulnerable to 2,5 HD. Moreover, results obtained suggest that the sensitivity of in vitro systems could represent a useful tool in studying the mechanisms of action of the neurotoxicant 2,5 HD.

Modification of the cell surface expression of histocompatibility antigens induced by the neurotoxin 2,5 hexanedione

Class I histocompatibility antigens (HLA) are expressed on the surface of almost all nucleated mammalian cells; the expression of this surface antigenic molecule may be changed or abrogated by several factors. In this paper, a modification in HLA expression in a human carcinoma cell line following exposure to the neurotoxicant 2,5 hexanedione is reported. This compound is known to produce a wide spectrum of subcellular pathological events; in this study, we describe an effect on the surface and cytoplasmic distribution of both light and heavy subunits of HLA antigens, demonstrated by immunocytochemical and immunoelectron microscopy techniques. Human carcinoma cells, which under normal growing conditions express the HLA, abrogate the surface expression of this glycoprotein after exposure to 2,5 hexanedione and an intracytoplasmic accumulation seems to occur. Several possibilities are discussed, such as an effect of the toxicant on the transport of the nascent glycoprotein.

Effects of respiratory treatment with N-hexane on rat testis morphology. I. A light microscopic study

Testicular damage was induced in rats by respiratory treatment with n-hexane at a concentration of 5000 ppm. The earliest lesions were observed immediately after 24 hr of continuous treatment, and involved primary spermatocytes from the leptotene to the middle pachitene stages and spermatids at late stages of maturation; at the same time numerous exfoliated, injured germ cells reached the epididymis. After the 24-hr treatment was suspended, damage to the seminiferous epithelium increased for the first 7 days, while the epididymis showed also focal infiltration by inflammatory cells; recovery was completed from Days 14 to 30. Intermittent treatment (16 hr/day, 6 days/week) at the same concentration of 5000 ppm for up to 6 weeks induced progressive increases in testicular and epididymal lesions, which, after 5 weeks (when most animals began to show clinical symptoms of polyneuropathy), reached aplasia of the germinal epithelium involving also the spermatogonia. Recovery from clinical symptoms was not paralleled by a regression of testicular pathology. On the contrary, after interruption of the treatment, the testicular lesions became increasingly severe, up to complete atrophy of the seminiferous tubules, suggesting an irreversible sterility of the treated animals. Pair-fed controls did not show histological alterations of the testis or epididymis.

In vitro effects of 2,5 hexanedione on a melanoma cell line: a morphological study

The effect of 2,5 hexanedione (2,5 HD) on a cultured human melanoma cell line (JR8) was explored. The addition of the toxicant at noncytolitic concentrations (0.08-0.16%) to the monolayers for 24 and 48 h, resulted in an irreversible inhibition of cell proliferation. Cessation of melanoma cell proliferation was accompanied by wide changes in morphological features of cells still adhering to the substrate. Incubation with the toxicant seemed to induce a differentiative process characterized mainly by a significant increase in cell protrusions. Melanoma cells, losing their bipolar appearance, often increased cell size and developed long dendritic and axon-like processes sometimes ramified in distal portions. Electron microscopic observations established that a change in the polarized appearance of control cells often occurred with 2,5 HD treatment and that a regular arrangement of organelles and cytoskeletal elements was detectable within these dendritic and axon-like protrusions. Furthermore, immunocytochemical studies confirmed an involvement of microtubules and actin network within cell prolongations. After the differentiative process a necrotizing effect occurred, inducing a progressive loss of viable, dendritic cells after 4 or 5 days. Incubation with cyclic AMP was ineffective in control cells while after 2,5 HD treatment seemed to increase the survival rate of neuronal-like cells. Possible mechanisms for the growth inhibitory and differentiative effects of 2,5 HD were discussed.

Uncoupling of cerebral glucose supply and utilization after hexane-2,5-dione intoxication in the rat

Chronic administration of hexane-2,5-dione (2,5-HD) to rats causes an accumulation of neurofilaments within axons that may lead to their degeneration. This occurs in both the CNS and PNS. It has been suggested that one of the effects of 2,5-HD is an impairment of glucose utilization arising from an inhibition of specific glycolytic enzymes. This hypothesis is based principally on evidence obtained in vitro. In the present study, glucose utilization, glucose transport across the blood-brain barrier, and blood flow have been measured in vivo in brain regions of control rats and in three groups of rats treated with 2,5-HD as (a) a single intragastric dose (500 mg/kg of body weight), (b) high chronic doses of 500 mg/kg of body weight for 15 days, or (c) low chronic doses of 250 mg/kg of body weight for 21 days. Group b showed overt signs of neuropathy, whereas groups a and c did not. The results indicate two independent effects of 2,5-HD in the CNS: a dose-dependent inhibition of glucose utilization and an effect on glucose supply and transport across the blood-brain barrier, which is apparent only after chronic treatment.

2,5-Hexanedione neuropathy is associated with the covalent crosslinking of neurofilament proteins

Protein composition in different segments along nerves from rats intoxicated with 2,5-hexanedione (HD) was analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis and by immunoblotting, using monoclonal antibodies specific for each of the three neurofilament polypeptide components (H, M, and L). Comparison with nerve protein extracts from normal (control) rats revealed a disappearance of the largest neurofilament polypeptide (H), accompanied by accumulation of higher-molecular-weight products that were immunoreactive with H-specific antibodies. Both the extent of this crosslinking and its localization in particular portions of peripheral nerves showed a correlation with HD dosage and with the known progression of ultrastructural features during HD-induced neuropathy. Similar changes were not detected for the M and L neurofilament components.

Anterograde components of axonal transport in motor and sensory nerves in experimental 2,5-hexanedione neuropathy

Anterograde slow and fast axonal transport was examined in rats intoxicated with 2,5-hexanedione (1 g/kg/week) for 8 weeks. Distribution of radioactivity was measured in 3-mm segments of the sciatic nerve after labelling of proteins with [35S]methionine or [3H]leucine and glycoproteins with [3H]fucose. The axonal transport of the anterograde slow components was examined after 25 (SCa) and 10 days (SCb), in motor and sensory nerves. SCa showed an increased transport velocity in motor (1.25 +/- 0.08 mm/day versus 1.01 +/- 0.05 mm/day) and in sensory nerves (1.21 +/- 0.13 mm/day versus 1.06 +/- 0.07 mm/day). The relative amount of labelled protein in the SCa wave in both fiber systems was also increased. SCb showed unchanged transport velocity in motor as well as in sensory nerves, whereas the amount of label was decreased in the motor system. Anterograde fast transport in motor nerves was examined after intervals of 3 and 5 h, whereas intervals of 2 and 4 h were used for sensory nerves. Velocities and amounts of labelled proteins of the anterograde fast component remained normal. We suggest that the increase in protein transport in SCa reflects axonal regeneration.

Structural basis of gamma-diketone neurotoxicity: non-neurotoxicity of 3,3-dimethyl-2,5-hexanedione, a gamma-diketone incapable of pyrrole formation

The chronic exposure to gamma-diketones results in the formation of giant neurofilament (NF)-containing axonal enlargements, followed by axonal degeneration in peripheral axons. Based on the specific ability of gamma-diketones to react with primary amino groups to form pyrroles, and the observation of such reaction with NF protein in vitro and with other proteins in vivo, it has been proposed that pyrrole formation at primary amino groups of NF protein is responsible for the neurotoxicity of gamma-diketones. We have tested this hypothesis through an investigation of the neurotoxicity in rats of 3,3-dimethyl-2,5-hexanedione (3,3-DMHD), a gamma-diketone which is incapable of forming pyrroles. 3,3-DMHD was found to produce only a slight alteration of axonal caliber and no clinical neurotoxicity after up to 12 weeks of administration, at a dose over 20 times that for which its isomer 3,4-dimethyl-2,5-hexanedione (3,4-DMHD) produced massive focal NF-containing axonal enlargements and complete paralysis in 4 weeks. These results support the view that the pyrrole-forming capability of gamma-diketones is the initial molecular event in the pathogenesis of gamma-diketone neurotoxicity.

Pathogenesis of experimental giant neurofilamentous axonopathies: a unified hypothesis based on chemical modification of neurofilaments

This review summarizes current evidence suggesting that the pathogenetic basis of giant axonal neuropathies induced by neurotoxic chemicals involves a direct chemical modification of neurofilaments (NF) and/or related cytoskeletal proteins. Chemical modification of NF is believed to disrupt the normal cytoskeletal organization, which results in an alteration in NF transport rate and accumulation of NF at prenodal sites along the axon. The exact location at which axonal enlargements occur appears to be a continuous function, dependent on both the structure and dosage schedule of the chemical toxin. A unified hypothesis for the neuropathologic effect of the diverse spectrum of toxic chemicals known to induce giant axonopathies is presented, based on recently published data on the structure of NF protein. Neurotoxic chemicals are believed to alter the charge balance of highly ionic domains of NF proteins which are thought to enter into intermolecular coulombic interactions in forming the supramolecular cytoskeletal framework.

Molecular mechanisms of diketone neurotoxicity

The important industrial and commercial solvents n-hexane and methyl n-butyl ketone undergo metabolic conversion in experimental animals and man to the neurotoxic gamma-diketone 2,5-hexanedione. Several molecular mechanisms of action have been proposed to explain the pathogenesis of gamma-diketone neuropathy. Such a mechanism must account for the target organ specificity, neurofilament accumulation, structure/activity relationships, in vivo covalent binding, and apparent direct axonal toxicity encountered in this syndrome. It has been proposed that the gamma-diketones exert their effects by reaction with sulfhydryl moieties of energy-producing axonal glycolytic enzymes, with resultant disruption of axoplasmic transport. Others have suggested that reaction instead occurs with lysine moieties of axonal cytoskeletal proteins to form alkyl pyrrole adducts, leading to damaging physicochemical changes in these proteins. Additional hypotheses involve inhibition of axonal sterologenesis, alterations in nerve membrane properties, and reduced neurofilament proteolysis within the nerve terminal. Although a comprehensive mechanism of action for the gamma-diketones remains to be demonstrated, much progress has been made toward this goal. Ultimate success awaits elucidation of the interactions of the neurotoxic diketones with axonal components at the molecular level. Previous reviews have addressed the historical, pharmacokinetic, and neuropathological aspects of this neuropathy. The present critique will examine proposed molecular mechanisms for the gamma-diketones with regard to theoretical considerations and experimental evidence.

Effects of IDPN-induced axonal swellings on conduction in motor nerve fibers

Paranodal demyelination produces a reduction of conduction velocity and conduction block. The relative proportions of these changes appear to vary among different demyelinating disorders. In this study we have examined the effects on conduction of paranodal demyelination produced by giant axonal swellings. The axonal swellings were induced in rats by administration of beta, beta'-iminodipropionitrile (IDPN). In this experimental model synchronous axonal swellings occur in the proximal region of virtually every alpha-motorneuron without evidence of segmental demyelination or fiber loss. Conduction across the motor neuron was evaluated by two methods: a monosynaptic reflex pathway and intracellular recording from single motor neurons. Increases in the delay across the central region of the monosynaptic reflex pathway began between 2 and 4 days after toxin administration. Intracellular studies confirmed that the slowing occurred across the proximal regions of the motor axons; more distal regions of the motor axons were unaffected. The substantial reduction in conduction velocity over the swollen segment occurs with only moderate evidence of conduction block, as assayed by a reduction in the H-reflex/M-response amplitude ratio. Parallel morphological studies showed that in the enlarged fibers the myelin terminal loops maintained contact with the axon but were displaced from the paranodal region into the internode. The appearance of this "passive" paranodal demyelination correlated closely with the increase in conduction delay. We suggest that the contact maintained by the displaced myelin terminal loops with the axolemma allows saltatory conduction to continue, and explains the paucity of conduction block in this model despite the prominent conduction slowing.

In vitro evidence that covalent crosslinking of neurofilaments occurs in gamma-diketone neuropathy

We have postulated that the toxic neuropathies associated with neurofilament-filled axonal swellings have a common pathogenesis, the covalent crosslinking of neurofilaments during anterograde transport. The newly described gamma-diketone, 3,4-dimethyl-2,5-hexanedione (DMHD), is a more potent analogue of the toxic metabolite of n-hexane, 2,5-hexanedione. The axonal swellings observed in DMHD toxicity are in the proximal axon, as seen in intoxication with beta, beta'-iminodipropionitrile, rather than in the distal axon, where neurofilamentous swellings are observed in n-hexane, carbon disulfide, and acrylamide neurotoxicity. In these studies, 14C-labeled DMHD and 2-butanone were synthesized and allowed to react with peripheral nerve. Only 14C-labeled DMHD resulted in stable radiolabeled protein polymers, which were retained by nitrocellulose filters with pore sizes as large as 12 microns. More specific evidence for covalent crosslinking of neurofilaments was obtained when DMHD was allowed to react with peripheral nerve in which the neurofilaments had been pulse-labeled with L-[35S]methionine.

Changes in terminal sprout formation in rat sternocostalis muscle during chronic intoxication with 2,5 hexanedione

Qualitative and quantitative morphological studies of the sternocostalis muscle innervation were made on rats chronically intoxicated with 2,5 hexanedione (2,5 HD) using the zinc iodide-osmium (ZIO) technique. Two distinct phases were seen in the events at the motor endplate. First, the number of motor endplates forming spontaneous terminal sprouts was found to increase linearly with time and, from the third week onward, the sprouts appeared to become progressively elongated. This latter change was associated with the appearance of swollen axons within intramuscular nerve bundles. Second, from the sixth week onward, wallerian degeneration of nerve fibers was seen and terminal sprouts began to make new arborizations on muscle fibers. By the eighth week, this occurred in as many as 66% of the rats, and collateral sprouting was also observed at this time. The occurrence of increased spontaneous terminal sprouting due to altered neuromuscular function is discussed in the light of axonal changes resulting from neurofilament accumulation following 2,5 HD intoxication.

The effect of 3,4-dimethyl substitution on the neurotoxicity of 2,5-hexanedione. I. Accelerated clinical neuropathy is accompanied by more proximal axonal swellings

The neurotoxicity of the gamma-diketone, 3,4-dimethyl-2,5-hexanedione, was studied in rats and compared to the known neurotoxicity of the parent compound, 2,5-hexanedione. The test compound was found to be 20 to 30 times more potent on a molar basis than hexanedione. In addition, unlike the distal axonal changes associated with hexanedione, the neurofilamentous swellings following exposure to the dimethyl analog occurred more proximally in the axon, with a preponderance in the anterior horn and lateral tracts of the spinal cord, and in the anterior roots. Since alkyl substitution causes branched-chain compounds to cyclize more rapidly than unbranched analogs, the greater neurotoxicity of the dimethyl compound implicates pyrrole formation in the pathogenesis of n-hexane neuropathy. Furthermore, the location of the axonal swellings induced with 3,4-dimethyl 2,5-hexanedione suggests that there is a common mechanism of injury for the entire class of neurofilament neuropathies, providing a continuum between the intraspinal swellings of beta, beta'-iminodipropionitrile (IDPN) and the distal axonopathies of 2,5-hexanedione, carbon disulfide, and acrylamide. In addition, lower doses of 3,4-dimethyl-2,5-hexanedione for longer periods of time led to a shift in the location of the axonal swellings to include more distal sites. These observations support the hypothesis that covalent crosslinking of the stable neurofilament is the primary event in the molecular pathogenesis of these toxic neuropathies, and that the rate of crosslinking of neurofilaments determines the proximodistal location of the axonal swelling.

Cytoskeletal disorganization induced by local application of beta, beta'-iminodipropionitrile and 2,5-hexanedione

Beta, beta'-Iminodipropionitrile and 2,5-hexanedione are neurotoxins that produce neurofilamentous axonal swellings. The swellings produced experimentally with these agents are similar in structure but different in distribution. Neither the relationships between these agents nor the mechanisms of action are known. In this study local effects on nerve fibers were compared following injection of beta, beta'-iminodipropionitrile and 2,5-hexanedione beneath the perineurium of rat sciatic nerves. Soon after injection, 2,5-hexanedione reproduced the distinctive cytoskeletal disorganization previously described with beta, beta'-iminodipropionitrile: microtubules collected into a central channel, with neurofilaments segregated in a surrounding subaxolemmal ring. Later, the beta, beta'-iminodipropionitrile-injected nerves developed local neurofilaments accumulations, reproducing the neurofilamentous axonal swellings characteristic of systemic intoxication with these agents. The results indicate that both these agents have direct local effects on the axonal cytoskeleton and probably are similar in mechanism of action. Both these agents appear to segregate neurofilaments from the rest of the axonal cytoskeleton. This segregation may prevent the normal proximal-to-distal transport of neurofilaments, resulting in the formation of neurofilamentous axonal swellings.

Cytochemical staining characteristics of peripheral nodes of Ranvier in hexacarbon intoxication

Changes in the distribution of stainable gap substance and subaxolemmal density at peripheral nodes of Ranvier in hexacarbon-intoxicated rats have been studied by light and electron microscopy. Cupric ion binding to the nodal gap substance was seen in normal nodes as a discrete annulus by the formation with ferrocyanide of Hatchett's Brown reaction product. Staining of osmicated fibres with ferrocyanide ions alone gave a deposit of black reaction product at the subaxolemmal region at nodes of Ranvier. Paranodal distension by neurofilamentous masses and separation of the terminal myelin loops in the early phase of paranodal dilatation produced no change in the distribution of the two kinds of stainable material. Paranodal myelin retraction with increases both in nodal gap width and nodal axon diameter resulted in displacement and attenuation of both stained regions. Axonal protrusion at the nodal region tended only to displace the stained gap substance, but sometimes it resulted in its attenuation. Occasionally loss of subaxolemmal staining was found. The possible functional relevance of these abnormal findings is discussed in relation to changes in conduction in affected nerves.

Distortions of the nodes of Ranvier from axonal distension by filamentous masses in hexacarbon intoxication

A study has been made of the structural changes of nodal and paranodal regions of the nodes of Ranvier of peripheral nerves of rats in which marked accumulations of neurofilaments have occurred within axons under the influence of 2,5-hexanediol over 10 weeks. The neurofilamentous masses caused distension of the axon at two points of apparent weakness as they attempted to slide through the axonal constriction at the nodes. Principally, a spiral axonal protrusion pushed into the zone of unattached myelin loops in the proximal paranodal spinous bracelet of Nageotte. This led to a conical widening of the paranodal constriction and considerable attenuation of the overlying myelin. No degeneration of the myelin occurred however. Alternatively, or additionally, a protrusion occurred of the axon at the nodal region which increased the nodal gap width and occasionally compressed and displaced the adjacent distal paranodal constriction which could have led to some obstruction of axoplasmic flow. Swelling of distal paranodal regions occurred later and was usually associated with proximal swelling. It was also accompanied by evidence suggesting transnodal passage of filamentous material. Sometimes, however, striking nodal constriction occurred in association with symmetrical paranodal swelling. These observations suggest that the spiral glial-axonal relationships at nodes of Ranvier are capable of marked deformation that might allow the intra-axonal neurofilamentous masses to move distally. These findings are discussed in relation to the structural features of the paranodal constrictions.

IDPN neuropathy in the cat: coexistence of proximal and distal axonal swellings

Administration of beta,beta'-iminodipropionitrile (IDPN) to rodents has previously been shown to produce neurofilament-filled axonal swellings in the proximal regions of motor and sensory nerve fibers. Because of the distinctive distribution of these swellings, IDPN has been classed as a proximal axonopathy and thereby distinguished from other disorders in which similar axonal swellings occur in the distal parts of the axon (distal axonopathies). This report describes the pathology in the peripheral nerves of cats which received intermittent injections of IDPN and calls attention to two previously undescribed pathological changes. First, in addition to the typical proximal swellings associated with IDPN, these animals developed numerous axonal swellings within the distal branches of the sciatic nerve. Distal swellings were present as early as 23 days after initiation of intoxication, indicating that they formed locally (rather than developing in the proximal axon and undergoing transport into the distal regions). The second finding was Wallerian-like degeneration within the affected nerve branches. These changes in the distal sciatic nerve and its branches closely resembled the pathology of the distal axonopathies produced by agents such as the neurotoxic hexacarbons and carbon disulfide. The pathological similarities suggest that IDPN may share with these agents pathogenetic mechanisms to an extent not previously suspected.

The pathokinetics of acrylamide intoxication: a reassessment of the problem

Acrylamide was given intraperitoneally to rats (30 mg/kg/day, five times/week) for 3 weeks, and the nervous and muscle tissues were examined by conventional methods over 5 weeks. Three striking cellular changes were observed. 1 Scattered degeneration of many Purkinje cells from 5 days onwards. 2 Widespread swelling and argyrophilia of nerve terminals from 10 days in both PNS and CNS. Motor and sensory endings were equally affected in all muscles examined. Synaptic and preterminal swelling also occurred in spinal cord, brain stem, and in certain cerebellar terminals. Degeneration occasionally followed this change, particularly in sensory nerve fibres, but not necessarily. 3 Chromatolysis in spinal ganglion cells and occasionally in anterior horn cells from 7 days onwards before the onset of axonal degeneration. This unique sequence of events is discussed in the light of the metabolic and other changes described by earlier authors.

The early evolution of neurofilamentous accumulations due to 2,5-hexanediol in the optic pathways of the rat

Rats were given 2,5-hexanediol in their water for more than 5 weeks. The changes in the optic pathways were studied both qualitatively and quantitatively. Increase in 10-nm filaments within axons was noticeable from 10 days onwards in the superior colliculus and in the brachium of the superior colliculus. From then onwards there was a steady increase in the number of affected axons, and their gross enlargement occurred. The proportion of affected fibres in the brachia was in fact few, but all fibres were of retinotectal origin. Less than half of those in the superior colliculus that were swollen were of retinal origin. Measurement of axon diameter versus myelin sheath thickness showed gross relative thinning of the latter. Axon degeneration did not occur but there was increasing ultrastructural evidence of impairment of transport of organelles both centrifugally and centripetally as the filamentous masses accumulated.

Recovery from 2,5-hexanediol intoxication of the retinotectal tract of the rat. An ultrastructural study

Rats were given 2,5-hexanediol, a metabolite of n-hexane, in the drinking water until they developed a marked degree of paresis over about 7 weeks and were then allowed to recover naturally. The time course and the manner of removal of the neurofilamentous masses accumulated within axons caused by the intoxication were followed by electron microscopy over the subsequent 8 weeks. The neurofilamentous masses slowly disappeared completely from the axons of this tract, without there being any degeneration, over 6-7 weeks. They disappeared first from the fibres in the brachium of the superior colliculus, perhaps by transport towards the terminals, and later from the axons within the superior colliculus itself. Particularly in preterminal fibres in the superior colliculus the filamentous accumulations became permeated by a network of smooth endoplasmic reticulum which may have played a part in the removal of the filaments. Accumulations of mitochondria and dense bodies in preterminal regions, presumed to be caused by obstruction to retrograde transport, disappeared pari passu with loss of the filaments. The significance of these events in relation to neurofilament metabolism is discussed.

The pattern of recovery of axons in the nervous system of rats following 2,5-hexanediol intoxication: a question of rheology?

Rats have been dosed with 2,5-hexanediol for 48 days and then allowed to recover. The changes in the accumulations of neurofilamentous masses in various pathways in CNS and PNS have been followed by light microscopy over the subsequent 9 weeks. It was found that many CNS pathways allow the argyrophilic masses to pass to their terminals from whence they subsequently disappear, usually over 5-6 weeks. Little or no axonal degeneration is seen where this happens. The same occurs in many peripheral nerves, particularly cranial nerves. However, in many tracts in the spinal cord and in many axons in the longer peripheral nerves, filamentous masses remain and becomes associated with axon degeneration, and, in tracts, gliosis. The importance of paranodal constrictions at nodes of Ranvier which tend to be greater in larger diameter axons is emphasized as a likely mechanism for the axon degeneration which largely took place during the recovery period.