Acrylamide: A Neurotoxin and a Hazardous Waste

Abstract

Acrylamide is an organic water-soluble compound and a vinyl-substituted primary amide. It is well known for its toxic effects on humans. This chemical may lead to neurodegenerative disorders like Alzheimer’s and Parkinson’s. It is exposed to humans through diet, occupation, lifestyle and many environmental factors. Acrylamide is used in molecular laboratories and even in various manufacturing and processing industries. Acrylamide is formed in food cooked at high temperatures, and exposure to this chemical may cause damage to the nervous system. In this chapter the toxicity of acrylamide and its role as a hazardous waste are highlighted. The main topics of this study are occurrence, effects and toxicity caused by acrylamide and analysis of acrylamide induced neurotoxicity in rats. Furthermore, mitigation strategies involving acrylamide have been discussed.

1. Introduction

Acrylamide (ACR), a water-soluble vinyl monomer, is a by-product of foods rich in carbohydrates that are cooked at higher temperatures. It has been shown to evoke genotoxic, carcinogenic, and neurotoxic effects in various kinds of animal species [1]. When exposed to humans through lifestyle, diet, occupation, and various other environmental factors it can cause adverse neurotoxic effects like ataxia, peripheral neuropathy and may result in the pathogenesis of neurodegenerative diseases. ACR could be found as a result of the use of polyacrylamide (PAM) in the environment. It can be seen in ingredients that are eaten by humans daily, including biscuits, breakfast cereals, bread, and crackers [2]. They are also used in cosmetics and toiletries, paper and textile production, production of dyes and organic chemicals, sugar refining, etc. PAM depolymerizes when it is exposed to high temperatures or pH to form ACR causing contamination to the environment [3]. Some other ways of exposure to ACR are through oral, inhalation, and dermal routes. It is formed from the amino acid asparagine during high-temperature cooking like baking and frying. Studies have reported that ACR is obtained from reducing sugars and the amino acid asparagine through the Maillard reaction [2]. Asparagine has been considered to be a major precursor of ACR, and heating foods having high starch content such as potatoes can result in high levels of asparagine eventually resulting in high ACR formation [4]. Rats subjected to specific time and dose-dependent measures of ACR have shown decreased norepinephrine levels and density of noradrenergic axons in different parts of the brain showing morphological evidence. According to the US FDA, a survey of 2015 on ACR values in individual food product samples, 70 ppb of ACR level was found for bread and bakery products, 500 ppb for Nuts and Fruits, and 1030 ppb for French Fries and Other Potato Foods. The no-observed-adverse-effect limit (NOAEL) and lowest observed adverse effect level (LOAEL) for laboratory animals is 0.2–0.5 μg/kg/day and 2 μg/kg/day respectively whereas the mean dietary exposure estimated by the World Health Organisation (WHO) is 0.001 cmg/kg/day [5]. It is, therefore, crucial to identify the cause of and exposure to ACR, its ways of reduction, and the health risks that are involved to establish a safer environment. The European Food Safety Authority (EFSA) has estimated the Benchmark Dose Lower Confidence Limit (BMDL) for ACR. For tumours, experts chose a BMDL₁₀ of 0.17 mg/kg bw/day and for other effects, neurological alterations were seen to be closely related with a BMDL₁₀ of 0.43 mg/kg bw/day [6]. Currently, many mitigation strategies are being investigated for their therapeutic effects against ACR present in the environment. Due to the harmful effects of ACR, research focuses on human health risks, dietary exposure to ACR, and its limit in foods by modulating processing ACR [3, 4].

Although the exact mechanism of ACR toxicity is still under investigation, many studies have shown that an imbalance in the antioxidant system can be one of the major reasons [7]. However, the US FDA [8] suggests that the level of ACR used in laboratory studies is higher than what humans are exposed through food. Also, the study by [9] states that more investigation is required to establish occupational exposure levels of ACR.

This review chapter aims to throw light on ACR as a neurotoxin and hazardous waste by discussing various aspects like the occurrence of ACR as a hazardous waste, effects of ACR and its by-products, ACR induced neurotoxicity leading to neurodegenerative changes and the potential of different therapeutic strategies to mitigate the toxicity.

2. Occurrence of acrylamide as a hazardous waste

ACR is a monomer and may be found in the environment because of the use of PAM polymers. ACR and its derivatives are used as sewage-flocculating agents and mainly occur in mineral extraction and chemical and food processing industries [10]. PAMs are agents also used in soil conditioning and strengthening in paper manufacturing [11]. ACR contaminates water through the use of PAM polymers in a range of industries such as agricultural, oil drilling, cement, herbicide, paper production, cosmetics, soap, chalk, adhesives, dyes, explosives, printing inks, and latex. All of these PAM applications, particularly flocculants and soil stabilisers, are potential sources of PAM contamination in drinking water supplies. Toxicity testing with some PAM-sensitive aquatic organisms revealed that oil-based PAM was harmful, whereas water-based PAM products were not. The cationic PAM has a lethal concentration (LC50a) of 0.3–10 mg/L, and it adheres to fish gills, obstructing the osmoregulation system. Several studies have found that anionic PAM products are safer to use in environmental water than cationic and neutral PAM products. Rainbow trout, especially larger fish rather than fingerlings, have seen acute alterations in their gills at LC as a result of cationic PAM poisoning. Due to ACR exposure, goldfish developed acute tissue lesions in the pancreas and genotoxic damages in their erythrocytes, disrupting homeostasis and eventually having a carcinogenic effect. It is also noted that ACR is not accumulated in sludges produced by PAM flocculants [10]. At room temperature, ACR is a solid, however it is extremely soluble (2155 g L1 of water) and mobile in water [12]. The major source of ACR in drinking water is the residual monomer of PAM, which is released throughout the treatment process. PAMs can be a source of release to drinking water sources when used as a chemical grouting agent and soil stabiliser in the building of tunnels, sewers, wells, reservoirs, and dams. ACR is also released into water by plastic and dye industries. Because ACR does not participate in soil binding but is extremely soluble and mobile in water, it will travel quickly with seepage, increasing the risk of pollution of surface or groundwater [13]. The concentration of ACR in aquatic and terrestrial ecosystems around ACR or PAM using—industries was found to be 0.3 ppb to 5 ppm [14].

Individuals inhale ACR mostly through smoking [15]. The amount of haemoglobin adduct identified was precisely related to the amount of ACR inhaled from three cigarettes each day [16]. Continuous ACR exposure, particularly by blue-collar workers, has been linked to headaches, muscle weakness, increased sensitivity in their extremities, dyspnea, and in certain cases, balance impairment, paresthesia, discomfort, and truncal ataxia [13]. The average ACR level per cigarette is 679.3 ng, with a range of 455.0–822.5 ng per cigarette. Adult smokers in Poland are predicted to be exposed to 0.17 g/kg b.w. of ACR per day via tobacco smoke [17]. It has been stated that each cigarette contains an average of 1.2 μg of ACR. According to the study, smoking 20 cigarettes per day exposes the body to 0.5 μg/kg b.w. per day [18]. ACR poisoning causes rashes, peeling of the skin and hands, cramping, and sweating, among other symptoms. It’s also a skin irritant that causes peeling contact dermatitis on the palms, which can lead to neurologic conditions. In humans, dermal exposure can cause an exfoliative reddish rash [13, 19, 20]. In a study including two grouting workers, it was observed that one has experienced skin peeling after 2 weeks of exposure to high concentration of ACR and systemic neuropathy in the next 6 months whereas the other worker showed cerebellar dysfunctions, including gait ataxia and slurred speech after 1-month exposure [19]. The use of ACR in cosmetics is a risk to the population. Due to ACR toxicity, the initial approved dose of 100 mg.kg⁻¹ cosmetic product was reduced to 0.5 mg.kg⁻¹ therefore the daily exposure due to cosmetics has lowered to 0.7 g.kg⁻¹ b.w. per day.

3. Effects of acrylamide and its by-products

3.1 Reproductive toxicity

ACR can play a direct role in the toxicological effects of sperm morphology, motility, and production, as well as being an indirect cause of reproductive issues as shown by various studies done on ACR exposed male mice [16, 19]. Repeated injections of ACR (20 mg/kg) into male rats for 20 days resulted in dose-dependent reductions in testosterone and prolactin levels [21]. In another study it was observed, after a one-month experiment, ACR exposure at levels of 1.25–24 mg/kg/day in their drinking water lowered fertility rates and litter sizes in mice, while increasing morphological anomalies of sperm and embryo resorption rates. In animal toxicity experiments using ACR, decreased reproductive behaviour, testicular atrophy, aberrant spermatogenesis, and poor sperm quality are some of the symptoms [13]. Peripheral neuropathies caused by ACR, such as decreased hind-limb function, may impede copulatory behaviour, mounting responses, and intromission, eventually impact the sperm deposition in the vagina and uterus, as well as cause hormonal alterations. In terms of hormonal mechanisms of action, ACR decreased serum testosterone and prolactin levels, which could contribute to testicular shrinkage and sperm motility [22].

3.2 Genotoxicity, carcinogenicity and mutagenicity

When the nervous systems of humans and animals are exposed to excessive quantities of ACR, α, β unsaturated carbonyl molecule with strong chemical activity, it can cause cancer and neurotoxicity [23]. ACR and its metabolites have been shown to be both genotoxic and carcinogenic in various studies [24, 25]. When ACR enters the body, it is oxidised and transformed into the genotoxic metabolite glycidamide (GA) [26]. ACR is ingested in the digestive system and transported to the liver at a rate of 4 mol ACR per 1 mol haemoglobin, where it is processed and destroyed by two distinct routes. The carcinogenic action of ACR, which is metabolised to GA in the liver by CYP2E1, has a mutagenic effect in the brain, kidneys, lungs, uterus, and testis in several organisms that includes experimental animals as well [27]. It was discovered that the genotoxic effect on DNA was mostly caused by GA, an ACR metabolite, rather than ACR itself [28]. The conversion of ACR to GA was found to be quite common in rats and mice, and it’s mode of action included it’s interaction with purine bases in the liver, renal, and pulmonary DNAs of rats and mice, causing genotoxic impacts [24, 25]. ACR also induced gene mutations and chromosomal defects in cultured mouse embryonic fibroblast cells, according to in vitro experiments [29, 30].

In this chapter, we have focused on neurotoxic effects caused by ACR in rats. ACR is predominantly known as a neurotoxin in humans. In this chapter, we are discussing ACR induced neurotoxicity in rat models where extensive studies have been done.

4. Analysis of acrylamide induced neurotoxicity in rat models

4.1 Dose and time dependent response

Studies have been made to understand the relation between the dose response and effect of ACR in rats. ACR has shown remarkable toxic effects with acute doses from the very beginning [31]. The response has been found to vary with the dose of exposure. The different doses of ACR to which the rat model has been exposed to be discussed to understand the dose-effect of ACR includes 0.5–50 mg/kg [1, 7, 31, 32, 33, 34, 35, 36] (Table 1). In rats, the NOAEL for ACR induced neurotoxicity was 0.5 mg/kg body weight/day and the LOAEL was 2 mg/kg-day in F344 male rats for the most sensitive effect (microscopic nerve alterations).

No. of days of exposure	Dose of exposure (mg/kg)	Effects	References
24 hours	0.5, 2.5, 12.5	No apoptotic neuronal death, decrease in GSH.	[1]
10 days	38.27 (1/3rd dose of LD50)	Decreased in GSH, SOD, CAT and AChE activity and increase in LPO.	[36]
12 days	40	Weight loss, gait abnormality, Purkinje cell nuclear condensation, DNA damage in rat cerebellum were observed after the exposure period.	[1]
21 days (3 weeks)	25	A decrease in the haematological parameters, brain NT concentrations, AChE activity, antioxidant biomarkers. Elevation in the levels of oxidative stress biomarkers. Astrocytosis was also observed.	[7]
28 days (4 weeks)	5	Increase in weight but no neurotoxicity observed	[33, 35]
	15, 30	Uncoordinated motor movement, nervous function defects, increase in the quantity of abnormal neurons distributed in varied layers of the cerebral cortex and wide distribution of astrocytes in the brain was observed.
	40	A significant loss in body weight, continuing deficits in motor function, adverse pathological changes in the cortex and hippocampus of rats.
30 days	20	Impairment in motor performance and cognition, a decrease in brain GSH and SOD.	[32]

Table 1.

Dose and time dependent exposure of acrylamide in rats.

5. Role of oxidative stress in acrylamide induced neurotoxicity

The principle mechanism of ACR neurotoxicity is unknown, but some studies have linked it with the reduction in antioxidative capacity and inflammatory responses [7]. Oxidative stress which occurs due to the imbalance between the production and the removal of reactive oxygen species (ROS), free radicals and antioxidants, is evident in neurological disorders like Alzheimer’s disease (AD), Huntington’s disease (HD) and Parkinson’s disease (PD), ataxia, peripheral neuropathy, amyotrophic lateral sclerosis (ALS) and multiple sclerosis (MS) [37, 38, 39, 40]. Antioxidant enzymes like superoxide dismutase (SOD), catalase (CAT), glutathione S-transferase (GST), glutathione peroxidase (GSH-Px) neutralise the effects of free radicals but due to the pro-oxidant effect of ACR, the levels of these enzyme are decreased resulting in an imbalance between the production and removal of free radicals by elevating the oxidative stress markers like ROS and thereby inducing lipid peroxidation (LPO) [33, 41, 42, 43] (Figure 1). Like many other xenobiotic compounds, ACR is an electrophile that can interact with nucleophiles containing specific residues [43]. It reacts with molecules that consist of bisulfide (SH), azanide (NH2) or hydroxide (OH). Glutathione (GSH) is a thiol that is well known for its free radical and ROS scavenging property [7, 33, 34]. Previous research has established that rats administered with ACR have a significant decrease of GSH in brain tissue when compared to the control group [33, 34, 44, 45]. The reduction in GSH levels results in an increase in levels of ROS that accumulates and induces oxidative stress. GSH is a nonenzymatic antioxidant that also acts as a coenzyme for the peroxide decomposition enzyme GSH-Px [7]. Studies also suggest that LPO is an effect of low levels of GSH [46, 47, 48]. First-line defence enzymatic antioxidants like SOD, CAT and GSH-Px have reduced in ACR-treated rats [33, 42, 45, 47]. GST is an antioxidant enzyme used to maintain the free radical balance. ACR was shown to enhance GST activity, suggesting an increase in the synthesis of S-conjugates between ACR and GSH [43, 46]. In contrast, a study by [47] reported a decrease in GST activity while measuring ACR-treated rat brains. As a result of the changes in levels and activity of various antioxidant enzymes and molecules, the concentration of total oxidants and antioxidants are increased and decreased respectively [49]. LPO, protein damage and DNA damage are biomarkers for oxidative stress in neurodegenerative diseases [39, 50].

Figure 1.

Role of oxidative stress in ACR induced neurotoxicity.

5.1 Lipid peroxidation

LPO is the degradation of lipids by free radicals and is assessed by measuring the levels of its marker, malondialdehyde (MDA). Thiobarbituric acid reactive substance (TBARS) assay is used to measure MDA, where it reacts with TBA and produces a pink-coloured complex [51]. MDA is formed by the free radicals generated from LPO and causes protein oxidation. Several lines of evidence suggest that exposing rats to ACR have high contents of MDA in the brain when compared to control groups [33, 40, 44, 47].

5.2 Protein degradation

Protein oxidation is considered as the damage of proteins. To understand the level of extent of this effect, the protein carbonyl content is the marker to understand oxidative damage in rats that have been treated with ACR. The Dinitrophenylhydrazine (DNPH) assay is used to measure the levels of protein-hydrazone to quantify the protein carbonyl content [42].

5.3 DNA damage

Since GA binds to the DNA and causes detrimental effects, it is important to understand the genotoxicity of ACR on rats. A commonly used DNA damage marker for oxidative stress is 8-OHdG, which is quantified by ELISA kits [52]. The principle mechanism of ACR-induced neurotoxicity is widely accepted to be apoptosis induced by ACR in rats [33]. ROS induces cell death via apoptotic mechanisms that are either non-physiological or controlled [53]. Telomerase reverse transcriptase (TERT) is an apoptosis-related molecule and is influenced by oxidative stress because of its anti-apoptotic effect. When ACR was administered to rats TERT associated mRNA and protein expression was downregulated in the rat brain [33, 54]. The sensitivity of cells to various apoptotic stimuli is determined by the ratio of anti-apoptotic protein B-cell lymphoma 2 (Bcl2) to pro-cell death proteins such as Bax and Bad, these ratios and the relative density of caspase-3 and caspase-9 is higher in ACR treated rats [37, 41, 55]. Proteins involved in apoptosis signalling pathways and cellular functions are also influenced by the presence of ACR. An appropriate balance must be maintained within the mitogen activated protein kinases (MAPKs) for regulating apoptosis. But, when ACR is induced, due to excessive ROS production a reduction in P-ERK/ERK ratio and elevation in the P-JNK/JNK and P-P38/P38 is observed, this causes mitochondrial dysfunction [1, 41].

6. Mechanisms underlying neurodegenerative diseases

The main neurotoxic consequences of ACR are peripheral nervous system (PNS) degradation and degeneration in a brain area related to learning and memory function. Drowsiness, cerebellar ataxia, muscle atrophy, dysarthria, and sensory or motor peripheral polyneuropathy are common clinical symptoms [56, 57]. ACR-induced neurotoxicity is associated with symptoms like ataxia, hindfoot splay, skeletal muscle weakness, and numbness of the hands and feet [1, 58]. ACR-induced neurodegenerative diseases have been shown in various studies to be mediated by axon and medullary sheath destruction in the PNS [59]. Distal axon swelling and degeneration are the key pathological features of ACR exposure [56, 60]. Even though recent research studies report that ACR-induced neurotoxicity and neurodegenerative effects in humans and experimental animals are mediated by nerve terminal and axonal damage, the exact underlying mechanism remains unclear [58].

6.1 Acrylamide induced dopaminergic neuronal loss in rat striatum

Recent research has revealed that ACR-induced locomotor abnormalities and neurotoxicity are comparable to the effects seen in PD, as ACR can cause key parkinsonian pathology such as α- synuclein aggregation [61]. The prominent hallmark of PD is the depletion of monoamine neurotransmitters (NTs) known as dopamine (DA) and its associated loss of dopaminergic A9 neurons in the substantia nigra pars compacta and striatum [58, 62, 63]. Motor control, cognitive decline, muscular stiffness, body posture instability, and movement difficulties are symptoms associated with the loss of dopaminergic A9 neurons [62]. DA, a kind of catecholamine, is a NT that governs important functions like cognition, motor control, emotion, and neuroendocrine activity [1, 7, 63]. A massive proportion of DA-carrying nigrostriatal neurons can be found in the striatum, which is the largest integral processing unit present in the basal ganglia [1, 63]. Tyrosine hydroxylase (TH) is a rate-limiting enzyme that is accountable for the synthesis of DA. TH helps to convert tyrosine into 3,4-dihydroxyphenylalanine (DOPA). DOPA is further converted to DA by the action of the enzyme, aromatic amino acid decarboxylase. Cells that are TH-positive are represented as dopaminergic neurons [1, 63, 64]. ACR is most commonly administered to rats either through their oral gavage or through intraperitoneal injection. When ACR is injected in this form, it gets metabolised into GA because of chromosome P450-2E1 present in the liver microsomes. DNA adducts that are formed as a result of the interaction between GA and DNA are responsible for provoking modxicity and carcinogenicity. Since ACR-induced neurotoxicity is strongly associated with the monomer of ACR itself, intracerebroventricular injection aids in transmitting the ACR to the neurons without resulting in the formation of GA. Studies have reported that rats treated with ACR through intracerebroventricular injection have shown a serious decline in the protein expression of TH and the number of TH-positive cells belonging to the striatum [1].

6.2 Acrylamide induced neuronal apoptosis in the rat striatum

Neuronal apoptosis results in the death of neuronal cells gradually leading to the development of neurodegenerative diseases. Neuronal apoptosis, a definite form of cell death, has a pivotal function in ACR-induced neurotoxicity in rats [65]. Studies have reported that ACR can result in neuronal apoptosis of the striatum. Nissl body is a chromatophilic substance that is very specific and is found in the cytoplasm of neuronal dendrites. Besides protein synthesis, Nissl bodies are crucial for brain functions like memory and learning. Since protein synthesis is essential for proper neuronal function, the presence of the Nissl body is indispensable. Rats treated with ACR reported the presence of pyknotic nuclei and the disappearance of Nissl substance in the striatal neurons. Striatal neurons treated with ACR also appeared swollen with decreased cellular integrity and exhibited an irregular arrangement [60]. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) staining is generally performed to investigate the loss of neurons due to apoptosis. Recent studies have reported that ACR treatment in rats has significantly raised the levels of TUNEL-positive cells in the rat striatum, which suggests that ACR exposure can result in striatal dopaminergic neuronal apoptosis. Hence, these kinds of studies suggest that ACR can also be a significant environmental risk factor for diseases like PD [1].

7. Therapeutic agents against neurotoxicity of acrylamide

To be able to mitigate the neurotoxic effects of ACR, therapeutic agents of different types are used at different doses. Phytoconstituents have been widely studied for amelioration of neurotoxicity in rats but there are adequate studies on dietary supplements, drugs and probiotics (Figure 2).

Figure 2.

Effects of therapeutic agents on neurotoxicity caused by acrylamide.

7.1 Phytochemicals

Few phytochemicals are thymoquinone, curcumin and quercetin. The anti-apoptotic property of thymoquinone plays a crucial role in attenuating the toxicity induced by ACR in rats by mitigating oxidative stress, reducing Bax/Bcl ratio, maintaining the integrity of the blood brain barrier (BBB), decreasing the level of caspase 3 and 9 and reducing glial fibrillary acidic protein (GFAP) content which indicates astrocyte damage [41, 66]. Curcumin increased the number of TERT positive cells and decreased the number of TUNEL positive cells in the cortex of ACR treated rats. Additionally, curcumin can also cross the BBB and alleviate spatial memory damage induced by ACR [33, 67]. Quercetin enhanced DA and serotonin levels, reduced biomarkers of oxidative stress, restored acetylcholinesterase (AChE) activity in ACR-treated rats. It can move across the BBB and exhibit its therapeutic efficiency [36, 68, 69]. Other compounds like metformin, minocycline and zolpidem also show similar therapeutic effects to dietary supplements when administered to ACR-treated rats [70, 71, 72] (Table 2).

Therapeutic agents	Methods of exposure	Time exposure	Dose		References
			ACR (mg/kg)	Agent
Thymoquinone	Primary treatment with Agent and followed by a concomitant treatment (ACR + AGENT)	11 days	50	2.5, 5, 10 mg/kg	[41, 66]
	Concomitant (ACR + AGENT)	11 days	50	2.5, 5, 10 mg/kg
Curcumin	ACR + AGENT	4 weeks	40	50, 100 mg/kg	[33]
	Concomitant (ACR + AGENT)	7 weeks	10	90 mg/kg	[67]
Quercetin	Agent followed by ACR	5 days	50	10 mg/kg	[68]
	ACR followed by Agent	10 days	38.27	5, 10, 20, 40 mg/kg	[36]
	ACR followed by Agent	30 days	20	25, 50 mg/kg	[69]
Vitamin E	Concomitant (ACR + AGENT)	20 days	5	100 mg/kg	[49, 73]
	Concomitant (ACR + AGENT)	28 days	20	50 IU/kg
	ACR followed by Agent	42 days	20	50 IU/kg
Vitamin C	ACR + Agent	21 days	10	200 mg/kg	[74]
Vitamin F	ACR followed by agent	13 days	38.27	5, 10, 20, 40 mg/kg	[75]
Omega-3 Fish oil	Concomitant (ACR + Agent)	8 weeks	45	200 mg/kg	[65, 76, 77]
	Agent followed by ACR	30 days	30	0.5 ml/kg
Melatonin	ACR + Agent	21 days	50	10 mg/kg	[34, 78, 79]

Table 2.

Therapeutic agents that attenuate acrylamide neurotoxicity in rats by different methods of exposure at various doses and times.

7.2 Drugs and supplements

Vitamins have shown therapeutic effects when administered to ACR-induced rats by ameliorating their toxic effects. Vitamins like vitamin E, vitamin F, vitamin C and vitamin B6 have been studied for their ameliorative property on rats influenced by ACR neurotoxicity [37, 74, 75]. They are widely known for their powerful antioxidative property and are also used as positive control groups while evaluating the potential of other therapeutic agents against ACR toxicity in rats [41, 80]. Vitamin E is phospholipid soluble and a neuroprotective antioxidant. It elevated brain-derived neurotrophic factor levels and lessened oxidative stress through its sweeper effect and removed free radicals in the brain tissue of fetal rats [49]. It also attenuated inflammation, apoptosis and behavioural neurotoxic effects in rats [37, 73]. Linoleic Acid (LA) is an essential omega-6 fatty acid with antioxidative, anti-inflammatory and neuroprotective effects [75]. LA improved ACR oxidative effects by restoring the activities of antioxidant enzymes, reducing the generation of free radicals, preventing LPO and obstructing genotoxic damage by reducing GA. AchE activity was also ameliorated by restoring vacuolization loss by pyramidal cells and Purkinje cells [75]. Vitamin B6 was also able to attenuate the intensity of ACR effects by increasing the availability of energy to the neurons [81]. When administered to pregnant rats vitamin C lessens the effects in white matter volume, the volume of the cerebellar cortex, molecular and granular layer volume and cerebellum damage [74]. Omega-3 fatty acids have also been studied as a therapeutic agent that can attenuate neurotoxicity caused by ACR in rats. Fish oil was able to reduce the neurotoxic effects evoked by ACR in rats. It restored oxidative stress by improving MDA, GSH, LPO, protein carbonyl content, free radicals and antioxidant status [65, 76]. Omega-3 Polyunsaturated fatty acids (PUFAs) regulate neurotransmission by modulating the activity of NTs. They also attenuate apoptosis by increasing anti-apoptotic BCL-2, expressing Hsp27 and inducing oligodendrogenesis [77]. Fish oil mitigates inflammation and astrogliosis by reducing inflammatory cytokines and GFAP positive cells [76]. Melatonin (MT) alleviates DNA damage, levels of MDA, SOD, GSH-Px, GSH and nucleus concentration [34, 78]. It relieved weight loss and gait abnormality. MT shows an increase in the levels of brain NTs and a reduction in AchE activity, serum tumour necrosis factor (TNF)—α and cortical amyloid protein levels [79]. MT treatment restored ACR evoked oxidative stress by down-regulating Nrf2, nuclear factor kappa B (NF-kB) and Kelch-like ECH-associated protein 1 (Keap-1) activity (Table 2) [78].

7.3 Probiotics

Probiotics maintain the intestinal barrier by increasing the expression of tight junction proteins, they can even reduce toxic substance absorption in the gut and enhance angiogenic activities in the central nervous system (CNS) [45, 82]. Lactobacillus plantarum (L. plantarum) ATCC8014 was studied to understand how it attenuates ACR induced toxicity in rats. This strain of Lactic acid bacteria is chosen because it has a high absorption rate of ACR and antioxidant capacity [45]. At high doses, L. plantarum ATCC8014 increased the body’s weight growth in ACR-treated rats. Administration of L. plantarum ATCC8014 improved nerve tissue damage and elevated the antioxidant capacity of nerve tissue by preventing attacks because of its effective capacity to scavenge and reduce free radicals. Similarly, probiotic Enterococcus faecium NCIM 5593 showed improved protection against the neurodegenerative changes due to oxidative damage in adult mice [83]. Additionally, it is important to consider the effect of prebiotics while considering attenuating properties of probiotics. Prebiotics are fibre-rich foods that promote the growth of probiotic microorganisms in the gastrointestinal tract. The study by [84] explored the effects of oral prebiotic supplements containing fructo- and xylo-oligosaccharides on pregnant rats exposed to ACR. It was found that AChE activity was restored and DA levels increased in the cortex of rats after administering prebiotics. Also, spirulina is a prebiotic obtained from the blue-green algae, Arthrospira platensis. It is the dried biomass of this cyanobacterium. It contains many beneficial compounds like proteins, vitamins, phytochemicals, etc. [85]. Spirulina is also known for its antioxidant properties owing to the presence of compounds like C-phycocyanin, and ß-carotene [86]. The study by [86] showed that spirulina was effective in ameliorating the toxicity induced by ACR in rats in a dose-dependent manner. Administration of spirulina enhanced the antioxidant activity and reduced the levels of TNF-α, IL-1β, and IL-6 in the serum. This shows the enormous scope of studies with regard to probiotics and prebiotics and their ability to attenuate ACR-induced neurotoxicity. Further studies are required to understand the protective role of probiotics and prebiotics.

8. Management and mitigation of acrylamide and its by-product wastes

In many countries, the current standard for ACR concentration in drinking water is 0.25 g/litre. It is advised to maintain the level of ACR monomer at 0.05 percent in PAM used in wastewater treatment. A wide range of microbes can degrade ACR, but there exists a latent period before this occurs. However, in regions with low microbial activity, ACR may remain in the environment for days, weeks or even months. ACR contamination also occurs during sewage treatment. This can be mitigated by chemically decontaminating ACR containing effluents [87]. The limitation set by Food and Drug Administration is of 0.2 percent (2 g/kg) monomer in PAM for use in paper or food or cardboard. In the Federal Republic of Germany, the amount of PAM in packaged foods is regulated to 0.3 percent (3 g/kg) and the amount of residual ACR monomer is limited to 0.2 percent (2 g/kg). Various methods such as addition of divalent cations, replacement of reducing sugars with sucrose or addition of organic acids, addition of calcium salts, using glycine to dilute the asparagine level, reduction in the free asparagine concentration by asparaginase or substitution of ammonium salts with baking powder, are suggested in recent years to mitigate the formation of ACR in heat processed foods [87].

The study by [88] showed a decline in ACR content in baked corn chips and French fries by pre-treating the potato cuts with citric acid solution prior to frying. The citric acid solution was able to lower the pH and leach out the asparagine and reduce sugar from the potato cuts. Aiswarya and Baskar [89] showed that the pre-treatment of potato with asparaginase prior to frying was effective in reducing the ACR content in fried potato chips. The effects of NaCl and citric acid combined with asparaginase was also studied and it was found that the use of NaCl + asparaginase and citric acid + asparaginase was effective in reducing ACR levels. To prevent workers from absorbing more than 0.012 mg/kg body weight per day during their occupational exposure, preventive measures such as enclosing production activities and wearing protective garments should be implemented. In the workroom, the concentration of ACR in the air should not exceed 0.1 mg/m3. To avoid inhaling ACR, ventilated face masks can be worn. It’s likely that the underlying neurological disease and/or the administration of neuroactive treatments modify human sensitivity to ACR, but no particular recommendations could be given until there is evidence [87].

9. Conclusions

Many studies have identified the potential health risks of ACR and the ambiguity of the mechanisms underlying ACR induced neurotoxicity has gained interest. Current toxicological studies are insufficient to indicate that ACR amounts consumed in the normal diet are likely to result in adverse human health effects. ACR is considered to be a potential health hazard that can impact toxicity to humans. An overview of their occurrence and effects have been comprehended in this review chapter. The importance of oxidative stress, dose and time variations in exposed rat models is being used to comprehend the mechanisms and the neurotoxic effects induced by ACR in rat models. ACR, a toxic neurotoxin is associated with the pathogenesis of various neurodegenerative diseases and their effects on neuronal apoptosis are analysed. Various therapeutic agents against ACR induced neurotoxicity have been analysed to understand their ameliorative effect. This review would give an overall insight on the toxicological effects of acrylamide and provides a comprehensive approach about the recent findings on how to mitigate the formation of acrylamide by using effective therapeutic strategies. More research at the cellular level will aid in the identification of early biomarkers that can be utilised to detect, avoid or mitigate the effects of ACR induced neurotoxicity.

Acronyms and abbreviations

ACR: acrylamide; NOAEL: no-observed-adverse-effect limit; LOAEL: lowest observed adverse effect level; BMDL: benchmark dose lower confidence limit; PAM: polyacrylamide; CYPs: Cytochrome; GA: glycidamide; CNS: central nervous system; PNS: peripheral nervous system; MT: melatonin; GSH: glutathione; SOD: superoxide dismutase; CAT: catalase; SH: bisulfide; NH2: azanide; OH: hydroxide; NT: neurotransmitters; TBA: thiobarbituric acid; LA: linolenic acid; PUFA: polyunsaturated fatty acids; GST: glutathione S-transferase; MDA: malondialdehyde; DNPH: dinitrophenylhydrazine; TERT: telomerase reverse transcriptase; ROS: reactive oxygen species; GFAP: glial fibrillary acidic protein; TBARS: thiobarbituric acid reactive substance; TUNEL: terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling; AChE: acetylcholinesterase; DA: dopamine; Bcl2: B-cell lymphoma 2; MAPK: mitogen-activated protein kinase; TNF: tumour necrosis factor; LPO: lipid peroxidation; NF-kB: nuclear factor kappa B; Keap-1: Kelch-like ECH-associated protein 1; TH: tyrosine hydroxylase; DOPA: 3,4-dihydroxyphenylalanine; BBB: blood brain barrier; AD: Alzheimer’s disease; PD: Parkinson’s disease; HD: Huntington’s disease; ALS: amyotrophic lateral sclerosis; and MS: multiple sclerosis.