Acrylamide neuropathy and changes in the axonal transport and muscular content of the molecular forms of acetylcholinesterase

Acrylamide Neurotoxicity as a Possible Factor Responsible for Inflammation in the Cholinergic Nervous System

Acrylamide (ACR) is a chemical compound that exhibits neurotoxic and genotoxic effects. It causes neurological symptoms such as tremors, general weakness, numbness, tingling in the limbs or ataxia. Numerous scientific studies show the effect of ACR on nerve endings and its close connection with the cholinergic system. The cholinergic system is part of the autonomic nervous system that regulates higher cortical functions related to memory, learning, concentration and attention. Within the cholinergic system, there are cholinergic neurons, anatomical cholinergic structures, the neurotransmitter acetylcholine (ACh) and cholinergic receptors. Some scientific reports suggest a negative effect of ACR on the cholinergic system and inflammatory reactions within the body. The aim of the study was to review the current state of knowledge on the influence of acrylamide on the cholinergic system and to evaluate its possible effect on inflammatory processes. The cholinergic anti-inflammatory pathway (CAP) is a neuroimmunomodulatory pathway that is located in the blood and mucous membranes. The role of CAP is to stop the inflammatory response in the appropriate moment. It prevents the synthesis and the release of pro-inflammatory cytokines and ultimately regulates the local and systemic immune response. The cellular molecular mechanism for inhibiting cytokine synthesis is attributed to acetylcholine (ACh), the major vagal neurotransmitter, and the α7 nicotinic receptor (α7nAChR) subunit is a key receptor for the cholinergic anti-inflammatory pathway. The combination of ACh with α7nAChR results in inhibition of the synthesis and release of pro-inflammatory cytokines. The blood AChE is able to terminate the stimulation of the cholinergic anti-inflammatory pathway due to splitting ACh. Accordingly, cytokine production is essential for pathogen protection and tissue repair, but over-release of cytokines can lead to systemic inflammation, organ failure, and death. Inflammatory responses are precisely regulated to effectively protect against harmful stimuli. The central nervous system dynamically interacts with the immune system, modulating inflammation through the humoral and nervous pathways. The stress-induced rise in acetylcholine (ACh) level acts to ease the inflammatory response and restore homeostasis. This signaling process ends when ACh is hydrolyzed by acetylcholinesterase (AChE). There are many scientific reports indicating the harmful effects of ACR on AChE. Most of them indicate that ACR reduces the concentration and activity of AChE. Due to the neurotoxic effect of acrylamide, which is related to the disturbance of the secretion of neurotransmitters, and its influence on the disturbance of acetylcholinesterase activity, it can be concluded that it disturbs the normal inflammatory response.

Effects of Acrylamide-Induced Vasorelaxation and Neuromuscular Blockage: A Rodent Study

Acrylamide (ACR), which is formed during the Maillard reaction, is used in various industrial processes. ACR accumulation in humans and laboratory animals results in genotoxicity, carcinogenicity, neurotoxicity, and reproductive toxicity. In this study, we investigated the mechanisms by which ACR may induce vasorelaxation and neuromuscular toxicity. Vasorelaxation was studied using an isolated rat aortic ring model. The aortic rings were divided into the following groups: with or without endothelium, with nitric oxide synthase (NOS) inhibition, with acetylcholine receptor inhibition, and with extracellular calcium inhibition. Changes in tension were used to indicate vasorelaxation. Neuromuscular toxicity was assessed using a phrenic nerve-diaphragm model. Changes in muscle contraction stimulated by the phrenic nerve were used to indicate neuromuscular toxicity. ACR induced the vasorelaxation of phenylephrine-precontracted aortic rings, which could be significantly attenuated by NOS inhibitors. The results of the phrenic nerve-diaphragm experiments revealed that ACR reduced muscle stimulation and contraction through nicotinic acetylcholine receptor (AChR). ACR-induced vasotoxicity was regulated by NOS through the aortic endothelium. Nicotinic AChR regulated ACR-induced neuromuscular blockage.

Structural and ultrastructural evidence of neurotoxic effects of fried potato chips on rat postnatal development

OBJECTIVE

Acrylamide (ACR), a proved rodent carcinogen and neurotoxic agent, is present in significant quantities in commonly consumed foods such as fried potato chips (FPC) and French fries, raising a health concern worldwide. We investigated and compared the neurotoxic effects of ACR and FPC on postnatal development.

METHODS

Female rats were treated with ACR (30 mg/kg of body weight), fed a diet containing approximately 30% of FPC during pregnancy, or fed a standard diet (control) and their offspring were examined.

RESULTS

Female rats treated with ACR or fed a diet containing FPC during pregnancy gave birth to litters with delayed growth and decreased body and brain weights. Light microscopic studies of the cerebellar cortex of treated animals revealed drastic decreases in Purkinje cells and internal granular layers. Different patterns of cell death were detected in Purkinje cells and neurons in the brains of pups born to treated mothers. Ultrastructural analysis of Purkinje cells revealed changes in the endoplasmic reticulum, loss of the normal arrangement of polyribosomes, swollen mitochondria with abnormally differentiated cristae, and an abnormal Golgi apparatus. The gastrocnemius muscle in the ACR and FPC groups showed extensive degeneration of myofibrils as evidenced by poorly differentiated A, H, and Z bands.

CONCLUSION

The present study reveals for the first time that rat fetal exposure to ACR, as a pure compound or from a maternal diet of FPC, causes cerebellar cortical defects and myodegeneration of the gastrocnemius muscle during the postnatal development of pups. These results warrant a systematic study of the health effects of the consumption of FPC and French fries in the general population.

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.

An in vitro model for toxicological investigations of environmental neurotoxins in primary neuronal cell cultures

Currently, most neurotoxicological investigations are still conducted using various animal models (e.g. chickens, rodents). In this report, alternative strategies of testing were examined to detect the neurotoxic potency of foreign compounds. Primary neuronal cell cultures from fetal rats are already an accepted model for mechanistic and pharmacological studies in drug research. Their suitability for neurotoxicological studies was examined by using industrial model compounds, which are well-known inductors of neuropathies: acrylamide, hexachlorophene, paraquat, n-hexane, and its neurotoxic metabolites acetylaceton and 2,5-hexandione. As a control compound, the nonneurotoxic solvent n-heptane was used. General cytotoxicity and the intracellular content of glial fibrillary acid protein, neuron-specific enolase, and neurofilaments were measured. n-Heptane induced an acute cytotoxicity and acrylamide and 2,5-hexandione produced a delayed cytotoxicity in primary neuronal cells, whereas the others showed no cytotoxic potency in the tested concentration range. These results were in agreement with the quantification of neurons by neuron-specific enolase. In contrast, with the exception of acetylaceton, glia cells were significantly affected by all neurotoxins at the later time. Signs of axonopathies were demonstrated for acrylamide, n-hexane and its metabolites, as well as for hexachlorophene and paraquat in vitro, by determining the intracellular neurofilament level. Therefore, the determination of cell-specific end points is necessary to detect the neurotoxic potency and quality of a compound, whereas the cytotoxicity assay limited the tested concentration range.

Time-course of nociceptive disorders induced by chronic loose ligatures of the rat sciatic nerve and changes of the acetylcholinesterase transport along the ligated nerve

Changes in the axonal transport of acetylcholinesterase (AChE) were studied in the painful mononeuropathy induced by setting 4 loose ligatures around the right sciatic nerve of the rat. Since changes in the axonal transport of AChE can be used to assess axonal degeneration/regeneration, we used this marker to investigate whether the time course of pain-related behavioral disorders observed following chronic constriction injury (CCI) to the sciatic nerve are related to the time course of the regeneration of the injured axons. In addition, a comparison was made between changes in AChE observed in this model of nerve injury and those observed after sciatic nerve crush. The rats were examined for pain-related disorders daily during the first postoperative week then at 7, 14 and 21 days after nerve ligation. The pain-related disorders, only detected from 7 days after ligation, were maximal at 14 days postinjury, and began to lessen at the end of the 3rd postoperative week. Within the first 3 days after loose ligation, the AChE transport dropped to 40% of its normal value, but recovered rapidly during the 3rd week post-surgery, indicating that most of the injured neurons were reconnecting their target cells. Thus, the injury produced by the loose ligatures was registered by the neurons several days before the first nociceptive manifestations of the injury, and the pain-related disorders lasted after most of the re-elongating axons had reconnected their target.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of organophosphates on a chicken brain or sea urchin egg kinesin-driven microtubule motility assay

The effect of neuropathic and non-neuropathic organophosphates (OPs) and acrylamide on an in vitro kinesin-driven microtubule (MT) motility assay was compared. The goal of the study was to determine whether this in vitro assay could confirm that a mechanism of action of neuropathic OPs was to impair kinesin activity and, therefore, possibly fast axonal anterograde transport (FAAT) in vivo. For our study, kinesin from chicken brain (CK) and sea urchin egg (SUK) was initially purified. Western immunoblotting confirmed the close antigenic homology between CK and SUK, using a mouse monoclonal sea urchin kinesin heavy chain-specific antibody (SUK 4). In the presence of microtubules (MTs) and MgATP, both CK- and SUK-driven MT movement was measured using a video-enhanced differential interference contrast microscope system with computer-assisted analysis. Using this assay system, we then tested separately the effect of two neuropathic OPs (diisopropylfluorophosphate (DFP) and phenyl saligenin phosphate (PSP)) and a non-neuropathic OP (paraoxon (PO)) each at a concentration of 10(-2) M at 27 degrees C. Additionally, we tested acrylamide (10(-2) M), since it is one of the best-characterized neurotoxins impairing FAAT in vivo. Our results demonstrated that none of these compounds significantly affected kinesin-driven MT motility in vitro compared to the standard controls. Further, this assay system was thus not able to discriminate between the neuropathic and non-neuropathic effect of these OPs.

Neurotoxic acrylamide and neurotrophic melanocortin peptides--can contrasting actions provide clues about modes of action?

Experimental acrylamide neuropathy has been studied as a model of degenerative neurological disorders of the 'dying-back' type for over 30 years. Functional, histological, ultrastructural, electrophysiological and biochemical aspects of acrylamide neuropathy have been described and several hypotheses concerning the mode of action proposed. However, the mechanism whereby acrylamide causes axonal degeneration and inhibits nerve sprouting remains unknown. By analogy with agonist/antagonist comparisons used by the pharmacologist, we have reconsidered the acrylamide problem in the light of the opposite effects summarized in Table 1, of neurotrophic peptides related to ACTH/MSH (collectively termed melanocortins). The contrasting effects on sprouting and the eventual quality of repair of mechanically lesioned nerves have suggested a mechanism whereby sprouting may regulate perikaryal adjustments to injury. We have also posed the question as to whether a common biochemical mechanism, namely selective proteolysis of neurofilament protein may underlie the opposing effects of acrylamide and melanocortins on nerve sprouting. This possibility implies a hitherto unknown role for neurofilament protein turnover in neuronal maintenance and repair, a suggestion that may provoke further research and discussion.

Synaptic terminal degeneration and remodeling at the rat neuromuscular junction resulting from a single exposure to acrylamide

Repetitive exposure to low doses of acrylamide results in extensive pathological changes at the neuromuscular junction (NMJ), but it remains undetermined if a single exposure to a larger dose will produce a similar neuropathological outcome. In the present study, morphometric and ultrastructural analyses of rat soleus NMJ were performed to determine early pathological effects of an intraperitoneal injection of 100 mg/kg acrylamide. Widespread nerve terminal degeneration, terminal sprouting, and endplate lengthening were evident as early as 4 days after injection. Degenerating terminal branches were swollen and exhibited enhanced argyrophilia. Ultrastructurally, the majority of terminals exhibited axolemmal abnormalities, neurofilament accumulations, and a paucity of synaptic vesicles; occasional swollen terminals lacked neurofilaments but contained increased numbers of tubulovesicular profiles. This early morphological pattern of nerve terminal changes suggests that acrylamide may disrupt both synaptic vesicle recycling and neurofilament degradation. These findings indicate that a single high dose of acrylamide triggers pathological lesions and remodeling in motor nerve terminals virtually identical to those resulting from multiple low doses.

Reduced axonal transport of the G4 molecular form of acetylcholinesterase in the rat sciatic nerve during aging

Aging in the sciatic nerve of the rat is characterized by various alterations, mainly cytoskeletal impairment, the presence of residual bodies and glycogen deposits, and axonal dystrophies. These alterations could form a mechanical blockade in the axoplasm and disturb the axoplasmic transports. However, morphometric studies on the fiber distribution indicate that the increase of the axoplasmic compartment during aging could obviate this mechanical blockade. Analysis of the axoplasmic transport, using acetylcholinesterase (AChE) molecular forms as markers, demonstrates a reduction in the total AChE flow rate, which is entirely accounted for by a significant bidirectional 40-60% decrease in the rapid axonal transport of the G4 molecular form. However, the slow axoplasmic flow of G1 + G2 forms, as well as the rapid transport of the A12 form of AChE, remain unchanged. Our results support the hypothesis that the alterations observed in aged nerves might be related either to the impairment in the rapid transport of specific factor(s) or to modified exchanges between rapidly transported and stationary material along the nerves, rather than to a general defect in the axonal transport mechanisms themselves.

Cholinesterases in blood plasma and tissues of rats treated with n-hexane or with its neurotoxic metabolite 2,5-hexanedione

Adult male rats were subjected to 4 weeks' respiratory treatment with n-hexane (5000 ppm, 16h/day, 6 days/week); motor conduction velocity was significantly decreased in tail nerves at all weekly intervals and did not approach normal values in the 4 weeks following interruption of treatment. Plasma acetylcholinesterase (AChE) levels were significantly increased at all weekly intervals during treatment (25-40%); 2 weeks after the end of treatment they had returned to baseline. Oral treatment with 2,5-hexanedione (HD) (1% in drinking water) caused a similar increase in plasma levels; this increase was statistically significant also when compared with pair-fed (PF) control rats. A sucrose density gradient analysis showed only one peak of AChE activity at approximately 10 S (as in normal plasma). The levels of butyrylcholinesterase were unaltered in plasma of both n-hexane-and HD-treated rats. Both the fast-contracting EDL and the slow-contracting soleus muscles lost weight in HD-treated rats with respect to free-fed (AL) and PF controls. AChE levels responded differently to HD treatment in the two muscle types: in EDL total extracts, AChE activity increased considerably with respect to AL controls (+ 70%, p less than 0.001), while the levels of the 16 S and 4 S molecular forms were unaltered. The increased levels of AChE found in plasma of rats intoxicated with n-hexane or with its metabolite HD may originate from muscle and correspond to an increased secretion of this molecular form.

Acrylamide neuropathy: changes in the composition of proteins of fast axonal transport resemble those observed in regenerating axons

Proteins conveyed by fast axonal transport along sensory and motor axons of rat sciatic nerve were labelled with L-[35S]methionine and characterized by one- and two-dimensional electrophoresis on polyacrylamide gels, followed by fluorography. Nerves from normal or bis-acrylamide-treated animals were compared with nerves from acrylamide-treated animals and nerves regenerating after a crush axotomy. In both sensory and motor axons significant changes in the pattern of labelled bands on one-dimensional gels occurred after 10 days of acrylamide treatment (50 mg/kg daily, i.p.). These changes resembled those seen in regenerating axons, but were less pronounced. No changes were detectable after shorter periods of treatment, even though the onset of the neuropathy, assessed by a behavioral test, occurred on days 4-6 of treatment. Two-dimensional separations of the labelled proteins revealed increased labelling of growth-associated protein 43 in acrylamide-treated animals, but again this was less pronounced than in regenerating nerves. Acrylamide treatment induces changes in composition of fast-transported protein that are qualitatively similar to those seen after axotomy. Since these changes are not detectable until the neuropathy is advanced, it is unlikely that they are causative factors. Instead, they are most likely a result of the cell body reaction previously observed in acrylamide intoxication, a reaction that resembles that produced by axotomy.

Biochemical effects and monitoring of exposure of rats to 4-methylcyclohexyl-1,6-dicarboxylic acid anhydride vapour

Male Wistar rats exposed to 5, 10 or 20 p.p.m. 4-methylcyclohexyl-1,6-dicarboxylic acid anhydride vapour for 2-11 weeks showed dose-dependent blood anhydride concentrations. The maximum concentrations were found after two weeks. At five exposure weeks, they were smaller but they did not decrease further thereafter. The change was coincident with increased epoxide hydrase activity in liver. The excretion of 4-methylcyclohexyl-1,6-dicarboxylic acid was detected in the urine. The excretion was linearly related to the exposure in specimens collected during the 10th and 11th week. Brain acetylcholinesterase activity was transiently inhibited by the exposure at the two higher doses while decreased enzyme activity in the isolated spinal cord axons was only found after 11 weeks. This change was simultaneous with increased creatine kinase activity in the cerebellar homogenate. In the occupational setting, acetylcholinesterase activity might only be significantly inhibited in lungs because of the low exposure levels.

Slow and fast axonal transport of acetylcholinesterase molecular forms in polyarthritic rats

Acetylcholinesterase (AChE) activity and its distribution among different molecular forms were studied in the sciatic nerve of normal and polyarthritic rats. Axonal transport of each form was investigated on the basis of its accumulation on both sides of a transection. Although an increase in total AChE activity could be detected in the sciatic nerves of polyarthritic animals, both anterograde and retrograde axonal transport of all the molecular forms investigated were similar in normal and polyarthritic rats. This suggests that neither slow nor fast axonal transport is impaired in polyarthritic rats. Hence, the neurophysiological modifications observed at the spinal, thalamic and cortical levels of the CNS are presumably not a consequence of peripheral axonal disability.

Rapid axonal transport of three molecular forms of acetylcholinesterase in the frog sciatic nerve

Acetylcholinesterase occurs in the frog sciatic nerve under five stable molecular forms with distinct sedimentation coefficients in sucrose gradients: 3 globular forms (3.6S, 6S and 10.5S) and two asymmetric ones (14S and 18S). Whereas in birds and mammals, the asymmetric tailed forms of acetylcholinesterase are present in trace amounts in peripheral nerves and account for only a small part of the enzyme activity submitted to a rapid axonal transport, the two asymmetric 14S and 18S forms represent nearly 50% of total activity in the frog sciatic nerve and account for 60-70% of the acetylcholinesterase activity accumulated at both sides of a nerve transection, the rest being due to an accumulation of globular molecules. We showed that the three forms, 10.5S, 14S and 18S, are all carried with the fast phase of axonal transport at a velocity of 100-120 mm/day in the anterograde direction and 20-30 mm/day in the retrograde direction. The velocity of transport for the light molecular forms 3.6S and 6S could not be calculated. In addition, we observed that large amounts not only of the 10.5S but also of the asymmetric 14S and 18S forms appear to be stationary along the frog sciatic nerve, contrary to the situation described for peripheral nerves in birds or mammals. Our results thus reveal that some axonal transport parameters for the asymmetric forms of acetylcholinesterase greatly differ in the peripheral nerves of amphibians on the one hand and of birds and mammals on the other, suggesting that these heavy molecular forms might have distinct functions in the nerves of lower and higher vertebrates.

Morpholine vapour inhalation and interactions of simultaneous nitrite intake. Biochemical effects on rat spinal cord axons and skeletal muscle

Male Wistar rats exposed intermittently to 300 ppm (12.5 mumol/l) morpholine vapour 5 days a week for 6 h daily during 4-15 weeks displayed an increasing brain solvent content analyzed by a sensitive gas chromatographic method. Rats drinking water which contained 150 mg NaNO2/l during the vapour exposure period showed initially larger brain solvent concentrations which began to decrease after 8 weeks. Fat solvent concentrations were a fraction of those detected in brain of the solvent-exposed animals or of those in the combined exposure. Nitrite exposure alone caused decreased spinal cord axon acetylcholine esterase activity after 8 weeks while this effect was noted in the combined exposure only at 8 weeks disappearing later on. Axonal succinate dehydrogenase activity was below the controls in the combined exposure throughout the study, and combination also caused persistent increase in the muscle creatine kinase activity. The morpholine-induced effects were less remarkable. The results point at pharmacokinetic interaction between the solvent and nitrite with its own effects on energy metabolism. No N-nitrosomorpholine was found although other metabolic interactions could not be excluded.

Axonal transport of the molecular forms of acetylcholinesterase in developing and regenerating peripheral nerve

In chick sciatic nerve, acetylcholinesterase (AChE) occurs in four main molecular forms characterized by their sedimentation coefficients in sucrose gradients, referred to as G1 (5S), G2 (7.5S), G4 (11S), and A12 (20S). Under normal conditions, we previously showed by accumulation technique that the G4 and A12 forms are rapidly transported along the axons, whereas G1 and G2 are carried much more slowly. Here, we used to the same technique to study the anterograde axonal transport of these different AChE forms during normal axonal growth and experimental regeneration. During the first 2 months after hatching, G4 and A12 transport virtually doubled, whereas G1 + G2 transport increased only slightly. After nerve cutting, crushing, or freezing, the flow rates of G1 + G2 and G4 in the regenerating proximal stump decreased by 75% at 4 to 7 days compared with control values and that of A12, by 90 to 95%. In crushed and frozen nerves the transport of all four AChE forms slowly recovered thereafter, but failed to attain control values even after 7 weeks. In cut nerves, on the contrary, no significant recovery of G1 + G2, or G4 transport occurred, but A12 transport began to recover by day 7. Taken together, our results show that axonal transport of G1 + G2, G4, and A12 is selectively regulated in chick sciatic nerve, and suggest that the A12 form of AChE might have a special role and/or destination in regenerating axons.

Early and dose-dependent decrease of retrograde axonal transport in acrylamide-intoxicated rats

The effect of retrograde axonal transport of doses of acrylamide ranging from 50 to 500 mg/kg was studied in sensory nerve of rats. Accumulation of trichloroacetic acid-phosphotungstic acid-insoluble label was measured in a collection segment distal to a double ligature placed on the sciatic nerve at intervals 9-15 h and 9-24 h following injection into the dorsal root ganglion of the fifth lumbar root of [35S]methionine and [3H]fucose. After a dose of 100 mg/kg of acrylamide no neurological signs of neuropathy had yet appeared, but retrograde buildup of protein label was significantly reduced for the long interval (2.20 +/- 0.49 arbitrary units (AU) (mean +/- SD) versus 2.81 +/- 0.57 AU in controls, 2p = 0.034). No abnormality of the short interval appeared before a dose of 500 mg/kg was reached. The retrograde transport abnormality was dose-related (r = -0.85, n = 28, and 2p = 1.2 x 10(-8)), as was the degree of neuropathy evaluated by "blind" neurological scoring (r = 0.88, n = 14, and 2p = 2.8 x 10(-5)). After a dose of 500 mg/kg, when the rats were severely disabled with almost total incoordination of the hindlegs, the retrograde accumulation of the long interval was profoundly depressed (1.08 +/- 0.28 AU versus 2.81 +/- 0.57 AU in controls, 2p = 1.2 x 10(-7)). Similar changes were seen in accumulation of glycoprotein label. After the rats had recovered for 4-10 weeks neurological signs of neuropathy had disappeared and the transport abnormality had improved. To test the specificity of acrylamide on the retrograde transport defect N-hydroxymethylacrylamide and methylene-bisacrylamide, which do not induce neuropathy, were studied. None of these related compounds influenced the transport. These observations imply that in acrylamide intoxication a defect in the amount of material carried by retrograde axonal transport rather than in "turnaround" time or in transport velocity is present, that the transport abnormality precedes the development of neuropathy, and that it is related to the degree of the neurological disability. We suggest that the retention of protein in the distal axons in the functional counterpart of the well-known accumulation of vesicular organelles in the preterminals.