Acrylamide neuropathy. III. Spatiotemporal characteristics of nerve cell damage in forebrain

Repeated administration of acrylamide for 28 days suppresses adult neurogenesis of the olfactory bulb in young-adult rats

Acrylamide (AA) is a neurotoxicant that inhibits synaptic function in distal axons. We previously found that AA decreased neural cell lineages during late-stage differentiation of adult hippocampal neurogenesis and downregulated genes related to neurotrophic factor, neuronal migration, neurite outgrowth, and synapse formation in the hippocampal dentate gyrus in rats. To investigate whether olfactory bulb (OB)-subventricular zone (SVZ) neurogenesis is similarly affected by AA exposure, AA was administered to 7-week-old male rats via oral gavage at doses of 0, 5, 10, and 20 mg/kg for 28 days. Immunohistochemical analysis revealed that AA decreased the numbers of doublecortin-positive ⁽⁺⁾ cells and polysialic acid-neural cell adhesion molecule⁺ cells in the OB. On the other hand, the numbers of doublecortin⁺ cells and polysialic acid-neural cell adhesion molecule⁺ cells in the SVZ did not change with AA exposure, suggesting that AA impaired neuroblasts migrating in the rostral migratory stream and OB. Gene expression analysis in the OB revealed that AA downregulated Bdnf and Ncam2, which are related to neuronal differentiation and migration. These results suggest that AA decreased neuroblasts in the OB by suppressing neuronal migration. Thus, AA decreased neuronal cell lineages during late-stage differentiation of adult neurogenesis in the OB-SVZ, similar to the effect on adult hippocampal neurogenesis.

Subchronic Acrylamide Exposure Activates PERK-eIF2α Signaling Pathway and Induces Synaptic Impairment in Rat Hippocampus

Acrylamide (ACR), a well-documented neurotoxicant to humans, is extensively found in starchy foods. More than 30% of the typical daily calorie intake comes from ACR-containing foods. Epidemiological and toxicological studies have found that ACR exposure is associated with mild cognitive change in men and experimental animals. However, there is limited information on the mechanisms by which ACR exposure induces memory deficits. The aberrant activation of the PKR-like ER kinase (PERK)-eukaryotic initiation factor 2α (eIF2α) signaling pathway is emerging as a major common theme in cognitive decline. The present study is designed to explore the effect of subchronic ACR exposure on the PERK signaling and the synaptic impairment to elucidate the potential mechanism of ACR-induced cognitive dysfunction in rat. ACR exposure at 5 and 10 (mg/kg)/day by gavage for 14 weeks results in gait abnormality and cognitive impairment in rats, which were accompanied by neuronal loss, glial cell proliferation, and synaptic ultrastructure damage in the hippocampus. ACR reduced the expression of phosphorylated cAMP response element-binding protein (P-CREB), brain-derived neurotrophic factor (BDNF), and synaptic vesicle proteins synapsin-1 and synaptophysin synthesis. ACR also excessively activates the PERK-eIF2α signaling, resulting in overexpression of C/EBP homologous protein (CHOP) and activating transcription factor 4 (ATF4). This work helps to propose a possible mechanism of subchronic exposure of ACR-induced neurotoxicity.

Developmental and Neurotoxicity of Acrylamide to Zebrafish

Acrylamide is a commonly used industrial chemical that is known to be neurotoxic to mammals. However, its developmental toxicity is rarely assessed in mammalian models because of the cost and complexity involved. We used zebrafish to assess the neurotoxicity, developmental and behavioral toxicity of acrylamide. At 6 h post fertilization, zebrafish embryos were exposed to four concentrations of acrylamide (10, 30, 100, or 300 mg/L) in a medium for 114 h. Acrylamide caused developmental toxicity characterized by yolk retention, scoliosis, swim bladder deficiency, and curvature of the body. Acrylamide also impaired locomotor activity, which was measured as swimming speed and distance traveled. In addition, treatment with 100 mg/L acrylamide shortened the width of the brain and spinal cord, indicating neuronal toxicity. In summary, acrylamide induces developmental toxicity and neurotoxicity in zebrafish. This can be used to study acrylamide neurotoxicity in a rapid and cost-efficient manner.

Taurine attenuates acrylamide-induced axonal and myelinated damage through the Akt/GSK3β-dependent pathway

Acrylamide (ACR), formed during the Maillard reaction induced by high temperature in food processing, is one of the main causes of neurodegenerative diseases. Taurine, a free intracellular β-amino acid, is characterized by many functions, including antioxidation, anti-inflammatory, and neuroprotective properties. This promotes its application in the treatment of neurodegenerative diseases. In this study, the neuroprotective effects of taurine against ACR-induced neurotoxicity and the potential underlying mechanisms were explored. Rats were intoxicated with ACR and injected with taurine in different groups for totally 2 weeks between January and July 2017. Electron microscopic analysis was used to observe the changes in tissues of the rats. Meanwhile, the levels of proteins including p-Akt, p-GSK3β, SIM312, and MBP were detected by Western blot. Furthermore, the GSK3β phosphorylation in taurine-treated dorsal root ganglion (DRG) with ACR was examined in the presence of the Akt inhibitor, MK-2206. The analysis of behavioral performances and electron micrographs indicated that taurine treatment significantly attenuated the toxic manifestations induced by ACR and stimulated the growth of axons and the medullary sheath, which was associated with the activation of the Akt/GSK3β signaling pathway. Mechanistically, it was found that taurine activated GSK3β, leading to significant recovery of the damage in ACR-induced sciatic nerves. Furthermore, MK-2206, an inhibitor of Akt, was applied in DRG cells, suggesting that taurine-induced GSK3β phosphorylation was Akt dependent. Our findings demonstrated that taurine attenuated ACR-induced neuropathy in vivo, in an Akt/GSK3β-dependent manner. This confirmed the treatment with taurine to be a novel strategy against ACR-induced neurotoxicity.

Subchronic exposure to acrylamide leads to pancreatic islet remodeling determined by alpha cell expansion and beta cell mass reduction in adult rats

Acrylamide (AA) is a toxic substance, used to synthesize polymers for industrial and laboratory processes. Also, AA is a food contaminant formed during the high temperature preparation of carbohydrate-rich food. The main subject of this study was to examine effects of subchronic AA treatment on the islets of Langerhans of adult rats. Adult male Wistar rats were orally treated with 25 or 50 mg/kg bw of AA for 3 weeks. Qualitative and quantitative immunohistochemical evaluation of glucagon and insulin expression and stereological analyses of pancreatic alpha and beta cells were performed. Serum insulin and glucose levels were measured. Analysis of glucagon-immunostained sections revealed a dose-dependent increase of intensity of glucagon immunopositive signal, alpha cell surface and numerical densities, volume density of alpha cell nuclei and nucleocytoplasmic ratio in AA-treated groups compared to the control. In insulin-immunolabeled pancreatic sections in AA-treated animals was observed decrease of intensity of insulin immunopositive signal, beta cell surface, numerical and volume densities and volume density of beta cell cytoplasm. Serum insulin and glucose concentrations remained unchanged after both AA treatments. The number of islets of Langerhans was not affected by AA treatment. Our results suggest that AA subchronic treatment of adult rats leads to remodeling of islet of Langerhans characterized by alpha cell expansion and beta cell mass reduction.

Taurine attenuates acrylamide-induced apoptosis via a PI3K/AKT-dependent manner

As a potent neurotoxic agent, acrylamide (ACR) is formed in food processing at higher temperature. Taurine (TAU), a nonessential amino acid, is used to cure neurodegenerative disorders, followed by activation of the phosphatidylinositol 3-kinase/protein kinase B (PI3K/AKT) signaling pathway. In this article, we certified that antiapoptotic efficacy of TAU in vivo and vitro. ACR-treated rats received TAU by drinking water 2 weeks after ACR intoxication. The results showed that in treated rats, TAU alleviated ACR-induced neuronal apoptosis, which was associated with the activation of PI3K/AKT signaling pathway. TAU attenuated apoptosis caused by ACR through observing terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL)-positive cells, measure of protein expression of Bcl-2, Bax, and caspase 3 activity. TAU-induced antiapoptotic effect is PI3K/AKT-dependent, which was proved in ACR-intoxicated ventral spinal cord 4.1 cells in the presence of AKT inhibitor, MK-2206. Therefore, our results demonstrated that TAU-attenuated ACR-induced apoptosis in vivo through a PI3K/AKT-dependent manner provided new sights in the molecular mechanism of TAU protection against ACR-induced neurotoxicity.

Toxic effect of acrylamide on the development of hippocampal neurons of weaning rats

Although numerous studies have examined the neurotoxicity of acrylamide in adult animals, the effects on neuronal development in the embryonic and lactational periods are largely unknown. Thus, we examined the toxicity of acrylamide on neuronal development in the hippocampus of fetal rats during pregnancy. Sprague-Dawley rats were mated with male rats at a 1:1 ratio. Rats were administered 0, 5, 10 or 20 mg/kg acrylamide intragastrically from embryonic days 6–21. The gait scores were examined in pregnant rats in each group to analyze maternal toxicity. Eight weaning rats from each group were also euthanized on postnatal day 21 for follow-up studies. Nissl staining was used to observe histological change in the hippocampus. Immunohistochemistry was conducted to observe the condition of neurites, including dendrites and axons. Western blot assay was used to measure the expression levels of the specific nerve axon membrane protein, growth associated protein 43, and the presynaptic vesicle membrane specific protein, synaptophysin. The gait scores of gravid rats significantly increased, suggesting that acrylamide induced maternal motor dysfunction. The number of neurons, as well as expression of growth associated protein 43 and synaptophysin, was reduced with increasing acrylamide dose in postnatal day 21 weaning rats. These data suggest that acrylamide exerts dose-dependent toxic effects on the growth and development of hippocampal neurons of weaning rats.

Evaluation of cII gene mutation in the brains of Big Blue mice exposed to acrylamide and glycidamide in drinking water

Potential health risks for humans from dietary exposure to acrylamide (AA) and its reactive epoxide metabolite, glycidamide (GA), exist because substantial amounts of AA are found in a variety of fried and baked starchy foods. AA is tumorigenic in rodents, and a large number of studies indicate that AA is genotoxic in multiple organs of mice and rats. Although AA is neurotoxic, there are no reports on AA-induced gene mutations in the mouse brain. Therefore, to investigate if gene mutation can be induced by AA or its metabolite GA, we screened brains for cII mutant frequency (MF) and scored for mutation types in previously treated male and female Big Blue mice with 0, 1.4 mM, and 7.0 mM AA or GA in drinking water for up to 4 weeks. High doses of AA and GA induced similar cII MFs in males and females but only the induced cII MF in males was significantly higher than the corresponding male control MF (p < 0.05). Molecular analysis of the cII mutants from males showed that AA and GA each induced at least a 2.5-fold increase in the incidence of G:C → T:A, A:T → T:A, and A:T → C:G transversions compared to the vehicle controls, with similar mutational spectra observed when comparing AA with GA treatment. These results suggest that the MFs and types of mutations induced by AA and GA in the brain are consistent with AA exerting its genotoxicity via metabolism to GA.

Acrylamide alters glycogen content and enzyme activities in the liver of juvenile rat

Acrylamide (AA) is spontaneously formed in carbohydrate-rich food during high-temperature processing. It is neurotoxic and potentially cancer causing chemical. Its harmful effects on the liver, especially in a young organism, are still to be elucidated. The study aimed to examine main liver histology, its glycogen content and enzyme activities in juvenile rats treated with 25 or 50mg/kg bw of AA for 3 weeks. Liver samples were fixed in formalin, routinely processed for paraffin embedding, sectioning and histochemical staining. Examination of haematoxylin and eosin (H&E)-stained sections showed an increase in the volume of hepatocytes, their nuclei and cytoplasm in both AA-treated groups compared to the control. In Periodic acid-Schiff (PAS)-stained sections in low-dose group was noticed glycogen reduction, while in high-dose group was present its accumulation compared to the control, respectively. Serum analysis showed increased activity of aspartate aminotransferase (AST), and decreased activity of alanine aminotransferase (ALT) in both AA-treated groups, while the activity of alkaline phosphatase (ALP) was increased in low-dose, but decreased in high-dose group compared to the control, respectively. Present results suggest a prominent hepatotoxic potential of AA which might alter the microstructural features and functional status in hepatocytes of immature liver.

The Potential for Plant Derivatives against Acrylamide Neurotoxicity

Certain industrial chemicals and food contaminants have been demonstrated to possess neurotoxic activity and have been suspected to cause brain-related disorders in humans. Acrylamide (ACR), a confirmed neurotoxicant, can be found in trace amount in commonly consumed human aliments as a result of food processing or cooking. This discovery aroused a great concern in the public, and increasing efforts are continuously geared towards the resolution of this serious threat. The broad chemical diversity of plants may offer the resources for novel antidotes against neurotoxicants. With the goal of attenuating neurotoxicity of ACR, several plants extracts or derivatives have been employed. This review presents the plants and their derivatives that have been shown most active against ACR-induced neurotoxicity, with a focus on their origin, pharmacological activity, and antidote effects.

The impact of vitamin E against acrylamide induced toxicity on skeletal muscles of adult male albino rat tongue: Light and electron microscopic study

Acrylamide, one of the major environmental public health problems, results from its increased accumulation in the process of cooking food materials. This study aimed to demonstrate the light and electron microscopic structural effects of acrylamide on the skeletal muscle fibers of adult male albino rat tongue and to investigate the possible protective effect of vitamin E co-administration. Thirty adult male albino Sprague-Dawley rats were divided into 3 groups, each group included 10 rats. Group I (control), group II which was subdivided into two equal subgroups: subgroup IIa: included 5 rats that received acrylamide orally once daily for 20 days. Subgroup IIb: included 5 rats that received acrylamide orally once daily for 40 days. Group III was also subdivided into two equal subgroups: subgroup IIIa: included 5 rats that received acrylamide and vitamin E orally once daily for 20 days. Subgroup IIIb: included 5 rats that received acrylamide and vitamin E orally once daily for 40 days. At the end of the experiment the tongue was dissected out for histological and electron microscopic studies, another muscle sample was homogenized and processed for biochemical estimation of malondialdehyde (MDA) and total antioxidant capacity (TAC). Light microscopic study of tongue skeletal muscles in acrylamide exposed animals revealed abnormal wavy course and splitting of the muscle fibers with fatty infiltration in between. Moreover, pyknosis and remnants of nuclei were detected. EM revealed marked aggregation of mitochondria of different size and shape with giant cells formation, and partial loss of myofilaments. There were statistically significant increase in MDA and decrease in TAC indicating oxidative stress in acrylamide administrated groups (group II) than the control group which increased by prolonged duration (subgroup IIb versus subgroup IIa, p < 0.0001). This oxidative stress could explain the histological changes in tongue muscles of acrylamide exposed rats. Co-administration of vitamin E with acrylamide ameliorated most of the above mentioned histological changes in the animals used and signs of improvement that became better with prolonged administration of it (subgroup IIIb versus subgroup IIIa, p < 0.0001) were detected. It could be concluded that, chronic exposure to acrylamide might lead to skeletal muscle damage in rat tongue which becomes worth with prolonged duration of exposure. Acrylamide induced oxidative stress is the implicated mechanism of such histological changes. This toxic effect of acrylamide could be minimized when vitamin E is given concomitantly with it by its antioxidant effect.

Hazardous effects of fried potato chips on the development of retina in albino rats

OBJECTIVE

To evaluate the hazardous effects of fried potato chips upon the retina of two developmental stages of the albino rats aged 7 and 14 days from parturition.

METHODS

PREGNANT RATS WERE ARRANGED INTO TWO GROUPS: control pregnant rats and consequently their delivered newborns until reaching 7 and 14 days old from parturition and fried potato chips group in which pregnant rats at the 6th day of gestation maintained on diet formed of fried potato chips supplied from the market mixed with standard diet at a concentration of 50% per each till 7 and 14 post-partum. Three fold integrated approaches were adopted, namely, histological, ultrastructural and proteomic analysis.

RESULTS

Histological examination of the retina of the experimental offsprings revealed many histopathological changes, including massive degeneration, vacuolization and cell loss in the ganglion cell layer, as well as general reduction in retinal size. At the ultrastructural level, the retina of experimental offsprings exhibited number of deformities, including ill differentiated and degenerated nuclear layer, malformed and vacuolated pigment epithelium with vesiculated and fragmented rough endoplasmic reticulum, degenerated outer segment of photoreceptors, as well as swollen choriocapillaris and loss of neuronal cells. Proteomic analysis of retina of the two experimental developmental stages showed variations in the expressed proteins as a result of intoxication which illustrated the adverse toxic effects of fried potato chips upon the retina.

CONCLUSIONS

It can be concluded that the effect of fried potato chips on the development of retina in rats may be due to the presence of acrylamide or its metabolite.

Effectiveness of selenium on acrylamide toxicity to retina

AIM

To investigate the hematological parameters, biochemical and electrophysiological role of acrylamide (ACR) in the retina and to assess whether selenium (Se) has protective potential in experimental oral intoxication with ACR.

METHODS

Sixty Wistar age matched-albino rats (3mo) weighing 195-230 g comprised of both sex were divided into 4 groups. Group I served as the control one in which animals take saline; group II was animals administrated ACR in dose of 15 mg/kg body weight per day for 28d; group III was animals received ACR then additionally Se (0.1 mg/kg body weight) for 28d; and group IV was animals received Se only (0.1 mg/kg body weight) for 28d. Blood analysis and serum trace element levels (Fe, Cu, and Zn) were measured. The electroretinogram (ERG) was recorded, the levels of malondialdehyde (MDA) and glutathione peroxidase (GSH-Px) in the retinal tissues were determined. Moreover the regulation of ion channels such as calcium, sodium and potassium were studied. All measurements were done for all groups after 28d.

RESULTS

Administration of ACR in group II caused a significant decrease (P<0.05) in hemoglobin (Hb), red blood cells (RBCs), hematocrit (HCT), white blood cells (WBCs) and lymphocyte of rats. A significant decrease (P<0.05) in Zn level, and alkaline phosphatase enzyme was observed compared to control. ERG which is a reflection of the electric activity in the retina; a- and-b wave amplitudes in ACR group had a reduction of 40% and 20% respectively. These changes accompanied by significant increases (P<0.05) in MDA level in the ACR group, in contrast with GSH-Px which is significant decreased (P<0.05). Moreover sodium and calcium were significant increased but potassium was significant decreased (P<0.05) compared to control group. There were no significant differences between group III (treated with Se) and control in all hematological parameter. Also serum trace elements levels (Cu, Fe and Zn), alkaline phosphatase enzyme and electric activity of the retina didn't change compared to control due to Se treatment.

CONCLUSION

This study provides evidence for the protective effect of Se on acrylamide induced toxicity by reducing oxidative stress.

Molecular mechanism of acrylamide neurotoxicity: lessons learned from organic chemistry

BACKGROUND

Acrylamide (ACR) produces cumulative neurotoxicity in exposed humans and laboratory animals through a direct inhibitory effect on presynaptic function.

OBJECTIVES

In this review, we delineate how knowledge of chemistry provided an unprecedented understanding of the ACR neurotoxic mechanism. We also show how application of the hard and soft, acids and bases (HSAB) theory led to the recognition that the α,β-unsaturated carbonyl structure of ACR is a soft electrophile that preferentially forms covalent bonds with soft nucleophiles.

METHODS

In vivo proteomic and in chemico studies demonstrated that ACR formed covalent adducts with highly nucleophilic cysteine thiolate groups located within active sites of presynaptic proteins. Additional research showed that resulting protein inactivation disrupted nerve terminal processes and impaired neurotransmission.

DISCUSSION

ACR is a type-2 alkene, a chemical class that includes structurally related electrophilic environmental pollutants (e.g., acrolein) and endogenous mediators of cellular oxidative stress (e.g., 4-hydroxy-2-nonenal). Members of this chemical family produce toxicity via a common molecular mechanism. Although individual environmental concentrations might not be toxicologically relevant, exposure to an ambient mixture of type-2 alkene pollutants could pose a significant risk to human health. Furthermore, environmentally derived type-2 alkenes might act synergistically with endogenously generated unsaturated aldehydes to amplify cellular damage and thereby accelerate human disease/injury processes that involve oxidative stress.

CONCLUSIONS

These possibilities have substantial implications for environmental risk assessment and were realized through an understanding of ACR adduct chemistry. The approach delineated here can be broadly applied because many toxicants of different chemical classes are electrophiles that produce toxicity by interacting with cellular proteins.

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.

Elaidic acid enhanced the simultaneous neurotoxicity attributable to the cerebral pathological lesion resulted from oxidative damages induced by acrylamide and benzo(a)pyrene

Acrylamide (ACR), benzopyrene [B(a)P] and trans-fatty acids (TFA) could be found to co-exist in many foods processed by high temperature. Our study investigated the effects of elaidic acid (ELA), a predominant TFA, on neuropathology induced by simultaneous exposure of ACR and B(a)P to mice. Results showed ELA enhanced the decrease of weight gains induced by simultaneous exposure of ACR and B(a)P (AB). Moreover, ELA enhanced ACR-induced increase of gait abnormality, B(a)P-induced damage to learning and memory, and AB-induced both of the damage above. Meanwhile, ELA enhanced B(a)Pinduced axonal degeneration in hippocamp, ACR- and AB-induced up-regulating of abnormal cerebellar Purkinje cells. ELA enhanced ACR-induced up-regulating of MDA in cerebrum and 8-OHdG in cerebrum and cerebellum; ELA enhanced B(a)P-induced up-regulating of MDA in cerebrum, PCO in cerebellum and 8-OHdG in cerebrum and cerebellum. Meanwhile, the enhancing role of ELA, on ACR-induced reduction of SOD activity in cerebrum and cerebellum, on B(a)P-induced reduction of GPx activity in cerebrum were found. Results suggested that ELA play a enhancing role on ACR-induced and B(a)P-induced oxidative damage, which attributable to the cerebral pathological lesion, and subsequent effect on gait abnormality and deficit on learning and memory in mice.

Acrylamide-induced nerve terminal damage: relevance to neurotoxic and neurodegenerative mechanisms

Acrylamide (ACR) has demonstrable neurotoxic effects in animals and humans that stem from its chemical behavior as a soft electrophilic alpha,beta-unsaturated carbonyl compound. Evidence is presented that the nerve terminal is a primary site of ACR action and that inhibition of neurotransmission mediates the development of neurological deficits. At the mechanistic level, recent proteomic, neurochemical, and kinetic data are considered, which suggest that ACR inhibits neurotransmission by disrupting presynaptic nitric oxide (NO) signaling. Nerve-terminal damage likely mediates the neurological complications that accompany the occupational exposure of humans to ACR. In addition, the proposed molecular mechanism of synaptotoxicity has substantial implications for the pathogenesis of Alzheimer's disease and other neurodegenerative conditions that involve neuronal oxidative stress and the secondary endogenous generation of acrolein and other conjugated carbonyl chemicals.

Molecular mechanisms of the conjugated alpha,beta-unsaturated carbonyl derivatives: relevance to neurotoxicity and neurodegenerative diseases

Conjugated alpha,beta-unsaturated carbonyl derivatives such acrylamide, acrolein, and 4-hydroxy-2-nonenal (HNE) are members of a large class of chemicals known as the type-2 alkenes. Human exposure through diet, occupation, and pollution is pervasive and has been linked to toxicity in most major organs. Evidence suggests that these soft electrophiles produce toxicity by a common mechanism involving the formation of Michael-type adducts with nucleophilic sulfhydryl groups. In this commentary, the adduct chemistry of the alpha,beta-unsaturated carbonyls and possible protein targets will be reviewed. We also consider how differences in electrophilic reactivity among the type-2 alkenes impact corresponding toxicokinetics and toxicological expression. Whereas these concepts have mechanistic implications for the general toxicity of type-2 alkenes, this commentary will focus on the ability of these chemicals to produce presynaptic damage via protein adduct formation. Given the ubiquitous environmental presence of the conjugated alkenes, discussions of molecular mechanisms and possible neurotoxicological risks could be important. Understanding the neurotoxicodynamic of the type-2 alkenes might also provide mechanistic insight into neurodegenerative conditions where neuronal oxidative stress and presynaptic dysfunction are presumed initiating events. This is particularly germane to a recent proposal that lipid peroxidation and the subsequent liberation of acrolein and HNE in oxidatively stressed neurons mediate synaptotoxicity in brains of Alzheimer's disease patients. This endogenous neuropathogenic process could be accelerated by environmental type-2 alkene exposure because common nerve terminal proteins are targeted by alpha,beta-unsaturated carbonyl derivatives. Thus, the protein adduct chemistry of the conjugated type-2 alkenes offers a mechanistic explanation for the environmental toxicity induced by these chemicals and might provide insight into the pathogenesis of certain human neurodegenerative diseases.

Acrylamide: review of toxicity data and dose-response analyses for cancer and noncancer effects

Acrylamide (ACR) is used in the manufacture of polyacrylamides and has recently been shown to form when foods, typically containing certain nutrients, are cooked at normal cooking temperatures (e.g., frying, grilling or baking). The toxicity of ACR has been extensively investigated. The major findings of these studies indicate that ACR is neurotoxic in animals and humans, and it has been shown to be a reproductive toxicant in animal models and a rodent carcinogen. Several reviews of ACR toxicity have been conducted and ACR has been categorized as to its potential to be a human carcinogen in these reviews. Allowable levels based on the toxicity data concurrently available had been developed by the U.S. EPA. New data have been published since the U.S. EPA review in 1991. The purpose of this investigation was to review the toxicity data, identify any new relevant data, and select those data to be used in dose-response modeling. Proposed revised cancer and noncancer toxicity values were estimated using the newest U.S. EPA guidelines for cancer risk assessment and noncancer hazard assessment. Assessment of noncancer endpoints using benchmark models resulted in a reference dose (RfD) of 0.83 microg/kg/day based on reproductive effects, and 1.2 microg/kg/day based on neurotoxicity. Thyroid tumors in male and female rats were the only endpoint relevant to human health and were selected to estimate the point of departure (POD) using the multistage model. Because the mode of action of acrylamide in thyroid tumor formation is not known with certainty, both linear and nonlinear low-dose extrapolations were conducted under the assumption that glycidamide or ACR, respectively, were the active agent. Under the U.S. EPA guidelines (2005), when a chemical produces rodent tumors by a nonlinear or threshold mode of action, an RfD is calculated using the most relevant POD and application of uncertainty factors. The RfD was estimated to be 1.5 microg/kg/day based on the use of the area under the curve (AUC) for ACR hemoglobin adducts under the assumption that the parent, ACR, is the proximate carcinogen in rodents by a nonlinear mode of action. When the mode of action in assumed to be linear in the low-dose region, a risk-specific dose corresponding to a specified level of risk (e.g., 1 x 10-5) is estimated, and, in the case of ACR, was 9.5 x 10-2 microg ACR/kg/day based on the use of the AUC for glycidamide adduct data. However, it should be noted that although this review was intended to be comprehensive, it is not exhaustive, as new data are being published continuously.

Assessment of nociception in acrylamide-induced neuropathy in rats

Acrylamide was intraperitoneally administered to male Sprague-Dawley rats at four different doses (5, 10, 20 and 30 mg/kg) three times a week for 5 consecutive weeks. Because of motor dysfunction, the 30 mg/kg dose was not used for behavioral pain tests. Clinical status remained good throughout the experiment and no motor deficit was observed at the other doses. We showed that acrylamide administration at low doses and cumulative dose (CD) range of 35-140 mg/kg produced mechanical allodynia and rapid, marked heat (42 degrees C) and cold (10 degrees C) allodynia after tail immersion test. Mechanical and thermal hyperalgesia appeared after higher cumulative doses (70-280 mg/kg), except for cold (4 degrees C) hyperalgesia (20-80 mg/kg). All the modifications persisted throughout all study, except the mechanical hyperalgia. All the cumulative doses tested were lower than those generally reported to induce motor dysfunction (CD>250 mg/kg), confirming that CD may be considered to be a suitable index in assessing neurological signs and suggesting that early detection of acrylamide neurotoxicity would be possible using the sensory tests, especially those for detecting allodynia thresholds.

Developmental and behavioral effects of acrylamide in Fischer 344 rats

Human exposures to acrylamide (ACR), a known neurotoxicant, can occur via a variety of substances, including cigarette smoke and the ingestion of certain carbohydrate-based foods cooked at high temperatures. In this study, Fischer 344 sperm plug-positive female rats were treated daily with ACR (0, 0.5, 1.0, 2.5, 5.0 or 10.0 mg/kg/day) by gavage beginning on gestation day 7. Dosing of dams ended when litters were born; pups received daily gavage at the same dose as their dam from postnatal day (PND) 1 through PND22. Pups were tested using a battery of behavioral assessments from PNDs 4-22. Statistically significant decreases in body weight were observed in pups exposed to ACR at doses as low as 1.0 mg/kg/day (treatmentxday; repeated measures ANOVA, P<0.0001). No statistically significant differences among treatment groups were observed in righting reflex, forelimb hang, or open field measures of activity. Statistically significant effects of ACR were observed at the 10 mg/kg/day dose on negative geotaxis performance (P<0.01) and a linear trend in fall-time latencies on Rotarod performance on PNDs 21-22 (P<0.05), with higher doses producing shorter latencies. These results suggest that ACR exposure produces deficits in development and motor coordination that are observable before weaning.

Effect of subchronic acrylamide exposure on the expression of neuronal and inducible nitric oxide synthase in rat brain

Acrylamide (ACR) is a known industrial neurotoxic chemical. Evidence suggests that ACR neurotoxic effect is related to brain neurotransmission disturbances. Since nitric oxide (NO) acts as a neurotransmission modulator and is produced by nitric oxide synthase (NOS), the neuronal NOS (nNOS) and inducible NOS (iNOS) expression pattern were determined in rat cerebral cortex and striatum after subchronic exposure to ACR. Using immunocytochemistry, the neuronal count of nNOS or optical density of iNOS from sections at three coronal levels, bregma 1.0, -0.4, and -2.3 mm, were compared between ACR-treated and control rats. At all three levels, nNOS expressions were uniformly decreased in most of the neocortical subregions following the treatment of ACR. At bregma level 1.0 mm, total numbers of nNOS expressing neurons were significantly decreased to 58.7% and 64.7% of the control in the cortex and striatum of ACR-treated rats, respectively. However, at the bregma level -2.3 mm, ACR treatment did not produce a significant difference in the numbers of nNOS expressing neurons both in the cortex and striatum. Contrary to nNOS, iNOS expressions were consistently increased to approximately 32% in the neocortex and 25% in the striatum, following the subchronic ACR treatment. These data suggest that subchronic ACR exposure involves compensatory mechanism on nNOS and iNOS expression to maintain the homeostasis of NO at the rostral part of the neocortex and the striatum. However, in the caudal brain, increased iNOS expression did not suppress nNOS expression. Therefore, the present study is consistent with the hypothesis that ACR toxicity is mediated through the disturbance to the NO signaling pathway and exhibits a rostrocaudal difference through the differential expressions of nNOS and iNOS in the neocortex and the striatum.

Proteomic analysis of acrylamide-protein adduct formation in rat brain synaptosomes

Evidence suggests that the neurological defects (gait abnormalities, foot splay, and skeletal muscle weakness) associated with acrylamide (ACR) intoxication are mediated by impaired neurotransmission at central and peripheral synapses. ACR can form adducts with nucleophilic residues on proteins and thereby alter corresponding structure and function. To evaluate protein adduction in nerve terminals as a possible mechanism of action, recombinant N-ethylmaleimide sensitive factor (NSF) was exposed in vitro to ACR (10 micromol) and mass spectrometry (MS) was used to identify adduct sites. MS analyses demonstrated that ACR formed adducts with sulfhydryl groups on cysteine residues (carbamoylethylcysteine, or CEC) of NSF. Ex vivo incubation of whole brain synaptosomes with ACR (0.001-1.0 M) produced concentration-dependent increases in CEC that were inversely correlated to reductions in neurotransmitter release that occurred over the same neurotoxicant concentration range. In synaptosomes isolated from rats intoxicated at a higher (50 mg/kg per day x 3, 5, 8, or 11 days) or a lower (21 mg/kg per day x 14, 21, or 28 day) ACR dose rate, CEC levels increased progressively up to a moderate level of neurotoxicity. To identify protein adducts, synaptosomal proteins labeled by ex vivo 14C-ACR exposure were separated by gel electrophoresis and probed by immunoblot analysis. Results showed that NSF and the SNARE protein, SNAP-25, were tentative ACR targets. Subsequent experiments indicated that ACR exposure increased synaptosomal levels of the 7S SNARE core complex, which is consistent with inhibition of NSF, SNAP-25 function, or both. These data suggest that adduction of cysteine residues on NSF and certain SNARE proteins might be causally involved in the nerve terminal dysfunction induced by ACR.

Suppressive silver methods—a tool for identifying axotomy-induced neuron degeneration

Suppressive silver methods evolved from empirical observations about 50 years ago that argyrophilia of normal nerve fibers can be suppressed by a short period of oxidation of tissue sections, whereas degenerating nerve fibers in the same preparations were still clearly visible. Based on this property, suppressive silver impregnation became the main technique for investigating pathways in the central nervous system until the early 1970s. Suppressive silver methods were also found to visualize degenerating nerve cell bodies, in addition to degenerating nerve fibers. This possibility has given these methods an important place among current tools for identifying neuronal degeneration in trauma, disease and toxicity. In this article we demonstrate and review the usefulness of suppressive silver methods in identifying neurons undergoing degeneration as a result of peripheral or central axon injury in immature animals. The documentation is based on previously published data from experiments in which silver impregnation was used to demonstrate degeneration of motoneurons following pure motor axon injury or mixed peripheral nerve injury, as well as on new results on degeneration-induced argyrophilia in the inferior olive following cerebellar lesions. We find that silver precipitates resulting from these injuries are localized either to the entire neuronal cytoplasm, to a few (typically two) intranuclear bodies, or to both sites. The findings are discussed in relation to morphological features of apoptosis, necrosis and retrograde neuronal responses. We suggest that suppressive silver methods allow visualization of different processes of neuronal degeneration, and therefore may be a useful adjunct for identifying axotomy-induced neuronal degeneration.

Acrylamide axonopathy revisited

Distal swelling and eventual degeneration of axons in the CNS and PNS have been considered to be the characteristic neuropathological features of acrylamide (ACR) neuropathy. These axonopathic changes have been the basis for classifying ACR neuropathy as a central-peripheral distal axonopathy and, accordingly, research over the past 30 years has focused on the primacy of axon damage and on deciphering underlying mechanisms. However, based on accumulating evidence, we have hypothesized that nerve terminals, and not axons, are the primary site of ACR action and that compromise of corresponding function is responsible for the autonomic, sensory, and motor defects that accompany ACR intoxication (NeuroToxicology 23 (2002) 43). In this paper, we provide a review of data from a recently completed comprehensive, longitudinal silver stain study of brain and spinal cord from rats intoxicated with ACR at two different daily dosing rates, i.e., 50 mg/kg/day, ip or 21 mg/kg/day, po. Results show that, regardless of dose-rate, ACR intoxication was associated with early, progressive nerve terminal degeneration in all CNS regions and with Purkinje cell injury in cerebellum. At the lower dose-rate, initial nerve terminal argyrophilia was followed by abundant retrograde axon degeneration in white matter tracts of spinal cord, brain stem, and cerebellum. The results support and extend our nerve terminal hypothesis and suggest that Purkinje cell damage also plays a role in ACR neurotoxicity. Substantial evidence now indicates that axon degeneration is a secondary effect and is, therefore, not pathophysiologically significant. These findings have important implications for future mechanistic research, classification schemes, and assessment of neurotoxicity risk.

Acrylamide neuropathy. II. Spatiotemporal characteristics of nerve cell damage in brainstem and spinal cord

Previous studies of acrylamide (ACR) neuropathy in rat PNS [Toxicol. Appl. Pharmacol. 151 (1998) 211] and cerebellum [NeuroToxicology 23 (2002) 397] have suggested that axon degeneration was not a primary effect and was, therefore, of unclear neurotoxicological significance. To continue morphological examination of ACR neurotoxicity in CNS, a cupric silver stain method was used to define spatiotemporal characteristics of nerve cell body, dendrite, axon and terminal degeneration in brainstem and spinal cord. Rats were exposed to ACR at a dose-rate of either 50 mg/kg per day (i.p.) or 21 mg/kg per day (p.o.), and at selected times brains and spinal cord were removed and processed for silver staining. Results show that intoxication at the higher ACR dose-rate produced a nearly pure terminalopathy in brainstem and spinal cord regions, i.e. widespread nerve terminal degeneration and swelling were present in the absence of significant argyrophilic changes in neuronal cell bodies, dendrites or axons. Exposure to the lower ACR dose-rate caused initial nerve terminal argyrophilia in selected brainstem and spinal cord regions. As intoxication continued, axon degeneration developed in white matter of these CNS areas. At both dose-rates, argyrophilic changes in brainstem nerve terminals developed prior to the onset of significant gait abnormalities. In contrast, during exposure to the lower ACR dose-rate the appearance of axon degeneration in either brainstem or spinal cord was relatively delayed with respect to changes in gait. Thus, regardless of dose-rate, ACR intoxication produced early, progressive nerve terminal degeneration. Axon degeneration occurred primarily during exposure to the lower ACR dose-rate and developed after the appearance of terminal degeneration and neurotoxicity. Spatiotemporal analysis suggested that degeneration began at the nerve terminal and then moved as a function of time in a somal direction along the corresponding axon. These data suggest that nerve terminals are a primary site of ACR action and that expression of axonopathy is restricted to subchronic dosing-rates.