Reproductive and Developmental Effects of Sex-Specific Chronic Exposure to Dietary Arsenic in Zebrafish (Danio rerio)

The present study investigated the reproductive and developmental effects of sex-specific chronic exposure to dietary arsenic in zebrafish. Adult zebrafish (Danio rerio) were exposed to environmentally realistic doses of arsenic via diet [0 (control; no added arsenic), 30 (low), 60 (medium), and 100 (high) μg/g dry weight, as arsenite] for 90 days. Following exposure, arsenic-exposed females from each dietary treatment were mated with control males, and similarly, arsenic-exposed males from each dietary treatment were mated with control females. In females, arsenic exposure resulted in a dose-dependent decrease in reproductive performance (fecundity, fertilization success, and hatching success). Moreover, a dose-dependent increase in developmental toxicity (larval deformities and larval mortality) was observed with maternal exposure to arsenic. In contrast, in males, arsenic exposure also induced similar reproductive and developmental toxicity; however, the adverse effects were mainly evident only in the medium and high dietary arsenic treatment groups. We also examined the sex-specific effects of dietary arsenic exposure on the expression of genes that regulate the hypothalamus-pituitary-gonadal-liver (HPG-L) axis in fish. The gene expression results indicated the downregulation of HPG-L axis genes in females irrespective of the arsenic treatment dose; however, the reduced expression of HPG-L axis genes in males was recorded only in the medium and high arsenic treatment groups. These observations suggest that chronic arsenic exposure in either females or males causes reproductive and developmental toxicity in zebrafish. However, these toxic effects are markedly higher in females than in males. Our results also suggest that arsenic can act as an endocrine disruptor and mediate reproductive and developmental toxicity by disrupting the HPG-L axis in zebrafish.

Maternal exposure to bisphenol S reduces anxiety and impairs collective antipredator behavior of male zebrafish (Danio rerio) offspring through dysregulation of their serotonergic system

Bisphenol S (BPS) is a common endocrine-disrupting chemical globally used in several consumer and industrial products. Although previous studies suggested that BPS induces multiple effects in exposed organisms, very little is known about its intergenerational effect on offspring behavior and/or the potential underlying mechanisms. To this end, adult female zebrafish Danio rerio were exposed to BPS (0, 10, 30 µg/L) and 1 µg/L of 17-β-estradiol (E2) as a positive control for 60 days. Afterwards, female fish were bred with untreated males, and their offspring were raised to 6 months old in control water. Maternal exposure to BPS decreased male offspring anxiety and antipredator behaviors while boldness remained unaffected. Specifically, maternal exposure to 10 and 30 µg/L BPS and 1 µg/L E2 were found to impact male offspring anxiety levels as they decreased the total time that individuals spent in the dark zone in the light/dark box test and increased the total track length in the center of the open field test. In addition, maternal exposure to all concentrations of BPS and E2 disrupted antipredator responses of male offspring by decreasing shoal cohesion in the presence of chemical alarm cues derived from conspecifics, which communicated high risk. To elucidate the possible molecular mechanism underlying these neuro-behavioral effects of BPS, we assessed the serotonergic system via changes in mRNA expression of serotonin receptors, including the 5-HT1A, 5-HT1B, and 5-HT1D subtypes, the serotonin transporter and monoamine oxidase (MAO). The impaired anxiety and antipredator responses were associated with reduced levels of 5-HT1A subtype and MAO mRNA expression within the brain of adult male offspring. Collectively, the results of this study demonstrate that maternal exposure to environmental concentrations of BPS can interfere with the serotonergic signaling pathway in the developing brain, subsequently leading to the onset of a suite of behavioral deficits in adult offspring.

Oral arsenite exposure induces inflammation and apoptosis in pulmonary tissue: acute and chronic evaluation in young and adult mice

Inorganic arsenic is a well-known environmental toxicant, and exposure to this metalloid is strongly linked with severe and extensive toxic effects in various organs including the lungs. In the present study, we aimed to investigate the acute and chronic effects of arsenite exposure on pulmonary tissue in young and adult mice. In brief, young and adult female Balb/C mice were exposed to 3 and 30 ppm arsenite daily via drinking water for 30 and 90 days. Subsequently, the animals were sacrificed and various histological and immunohistochemistry (IHC) analyses were performed using lung tissues. Our findings showed arsenite was found to cause dose-dependent pathological changes such as thickening of the alveolar septum, inflammatory cell infiltrations and lung fibrosis in young and adult mice. In addition, arsenite exposure significantly increased the expression of inflammatory markers NF-κB and TNF-α, indicating that arsenite-exposed mice suffered from severe lung inflammation. Moreover, the IHC analysis of fibrotic proteins demonstrated an increased expression of TGF-β1, α-SMA, vimentin and collagen-I in the arsenite-exposed mice compared to the control mice. This was accompanied by apoptosis, which was indicated by the upregulated expression of caspase-3 in arsenite-exposed mice compared to the control. Adult mice were generally found to be more prone to arsenite toxicity during chronic exposure relative to their younger counterparts. Overall, our findings suggest that arsenite in drinking water may induce dose-dependent and age-dependent structural and functional impairment in the lungs through elevating inflammation and fibrotic proteins.

Examining the subchronic (28-day) effects of aqueous Cd-BaP co-exposure on detoxification capacity and cardiac function in adult zebrafish (Danio rerio)

The present study aimed to examine the effects of environmentally relevant concentrations of cadmium (Cd) and Benzo[a]Pyrene (BaP) in the adult zebrafish (Danio rerio). To this end, fish were exposed to either 1 or 10 μg/L Cd or 0.1 or 1 μg/L BaP in isolation, or a co-exposure containing a mixture of the two toxicants. Our results showed extensive modulation of the expression of key antioxidant genes (GPx, SOD1, catalase), detoxifying genes (MT1, MT2, CYP1A1) and a stress biomarker (HSP70) differing between control, single toxicant groups and co-exposure groups. We additionally carried out histopathological analysis of the gills, liver, and hearts of exposed animals, noting no differences in tissue necrosis or apoptosis. Finally, we carried out ultrasonographic analysis of cardiac function, noting a significant decrease of E-wave peak velocity and end diastolic volume in exposed fish. This in turn was accompanied by a decrease in stroke volume and ejection fraction, but not cardiac output in co-exposed fish. The present study is the first to demonstrate that a subchronic aqueous exposure to a Cd-BaP mixture can extensively modulate detoxification capacity and cardiac function in adult zebrafish in a tissue-specific manner.

Chronic dietary exposure to arsenic at environmentally relevant concentrations impairs cognitive performance in adult zebrafish (Danio rerio) via oxidative stress and dopaminergic dysfunction

The current study was designed to evaluate the effects of chronic dietary arsenic exposure on the cognitive performance of adult zebrafish and uncover probable pathways by which arsenic mediates such neurotoxic effects. Adult zebrafish were treated with 3 different dietary arsenic concentrations (30, 60, and 100 μg/g dry weight (dw), as arsenite) in addition to control for 60 days. A latent learning paradigm, which employs a complex maze, was used to assess the cognitive performance of fish. Our results demonstrated that dietary treatment with arsenic, especially at medium (60 μg/g dw) and high (100 μg/g dw) exposure dose levels, significantly impaired the performance of fish in various latent learning tasks evaluated in the present study. Concomitant with cognitive dysfunction, chronic dietary exposure to arsenic was also found to increase arsenic accumulation and dopamine levels, and induce oxidative stress (reduced thiol redox, increased lipid peroxidation and expression of antioxidant enzyme genes) in the brain of zebrafish in a dose-dependent manner. Dopaminergic system in the brain is known to play a critical role in regulating cognitive behaviours in fish, and our observations suggested that chronic dietary treatment with medium and high arsenic doses leads to significant alterations in the expression of genes involved in dopamine signalling (dopamine receptors), synthesis (thyroxine hydroxylase) and metabolism (monoamine oxidase) in the zebrafish brain. Moreover, we also recorded significant downregulation of genes such as the brain-derived neurotrophic factor (BDNF) and ectonucleotidases (entpd2_mg, entpd2_mq, and 5'-nucleotidase), which are critical for learning and memory functions, in the zebrafish brain following chronic dietary exposure to arsenic. Overall, the present study suggests that chronic environmentally relevant dietary exposure to arsenic can impair the cognitive performance in zebrafish, essentially by inducing oxidative stress and disrupting the dopaminergic neurotransmission in the brain.

Investigating the combined effects of pH changes and UV radiation exposure on dissolved metal-humate complexes: an important process in aquatic systems

An in vitro study was carried out to examine the impact of UV exposure on metal-dissolved humic material (M-DHM) complexes in aqueous systems at different pH. Complexation reactions of dissolved M (Cu, Ni, and Cd) with DHM increased with the increasing pH of the solution. Kinetically inert M-DHM complexes dominated at higher pH in the test solutions. Exposure to UV radiation did affect the chemical speciation of M-DHM complexes at different pH of the systems. The overall observation suggests that exposure to increasing UV radiation increased the lability, mobility, and bioavailability of M-DHM complexes in aquatic environments. The dissociation rate constant of Cu-DHM was found to be slower than Ni-DHM and Cd-DHM complexes (both before and after UV exposure). At a higher pH range, Cd-DHM complexes dissociated after exposure to UV radiation and a part of this dissociated Cd precipitated out from the system. No change in the lability of the produced Cu-DHM and Ni-DHM complexes after UV radiation exposure was observed. They did not appear to form new kinetically inert complexes even after 12 h of exposure. The outcome of this research has important global implications. The results of this study helped to understand DHM leachability from soil and its effect on dissolved metal concentrations in the Northern Hemisphere water bodies. The results of this study also facilitated to comprehend the fate of M-DHM complexes at photic depths (where pH changes are accompanied by high UV radiation exposure) in tropical marine/freshwater systems during summer.

Modulation of Cd and BaP uptake rate during acute aqueous co-exposure in adult zebrafish (Danio rerio)

Cadmium and Benzo[a]pyrene are two toxicants of great environmental importance given their frequency and ability to cause extensive toxicity in aquatic organisms including fish. There is evidence that fish can modulate their respective uptake rate during simultaneous exposures, albeit the mechanism behind this is poorly understood. The present study aimed to examine this interaction by exposing adult zebrafish to either 89.3 nM Cd, 4.25 nM BaP or a combination of the two for 72 hrs prior to examining the uptake rate of either toxicant via short-term exposures (3-6 hrs) to radiotracers (¹⁰⁹Cd and ¹⁴C-BaP). Our results showed that Cd uptake rate increased significantly in the gills when animals were pre-exposed to both toxicants simultaneously, resulting in an increased maximum uptake rate (Jmax). The increased Cd uptake rate did not correspond to increased expression of gill Cd transporters such as the epithelium calcium channel (ECaC) or the divalent metal transporter 1 (DMT1). Furthermore, BaP uptake rate increased significantly at the whole-body level when animals were exposed to both 5.03 nM ¹⁴C-BaP and 89.3 nM Cd concurrently. Additionally, we ran a time-course and observed BaP uptake rate is highest in the 6-12 hrs following the beginning of the exposure. Our results provide evidence that the increased bioaccumulation of Cd and BaP observed during co-exposures is at least in part due to an increase in uptake rate and is driven by separate mechanisms.

Melatonin Attenuates Extracellular Matrix Accumulation and Cardiac Injury Manifested by Copper

Copper-induced cardiac injury is not widely reported in spite of its ability to cause oxidative damage and tissue injury. Structural and morphological changes in the cardiac tissue are triggered via oxidative stress and inflammatory responses following copper exposure. The varied and unavoidable exposure of copper through contaminated food and water warrants a safe and effective agent against its harmful effects. Since the heart is highly sensitive to changes in the redox balance, the present study was undertaken to examine the protective effects of melatonin against copper-induced cardiac injury. Sprague Dawley (SD) rats were exposed to 100 ppm of elemental copper via drinking water for 4 months. The cardiac tissue was evaluated for various biochemical, histological, and protein expression studies. Animals exposed to copper exhibited induced oxidative stress and cardiac injury compared to normal control. To this end, we found that melatonin treatment ameliorated copper-induced alterations in tissue oxidative variables like ROS, nitrate, MDA, and GSH. In addition, histological examinations unravelled decreased cardiac muscle dilation, atrophy, and cardiomyopathy in melatonin-treated rats. Furthermore, melatonin-treated rats were associated with reduced tissue copper levels, collagen deposition, α-SMA, and increased HO-1 expression as compared to rats exposed exclusively to copper. Moreover, the levels of NF-κB and cardiac markers such as CK-MB, cTnI, and cTnT were found to be decreased in the melatonin-treated animals. Altogether, melatonin-triggered increase in antioxidant capacity resulting in reduced aggregation of ECM components demonstrates the therapeutic potential of melatonin in the treatment of cardiac injury and tissue fibrosis.

Contemporary Comprehensive Review on Arsenic-Induced Male Reproductive Toxicity and Mechanisms of Phytonutrient Intervention

Arsenic (As) is a poisonous metalloid that is toxic to both humans and animals. Drinking water contamination has been linked to the development of cancer (skin, lung, urinary bladder, and liver), as well as other disorders such as diabetes and cardiovascular, gastrointestinal, neurological, and developmental damage. According to epidemiological studies, As contributes to male infertility, sexual dysfunction, poor sperm quality, and developmental consequences such as low birth weight, spontaneous abortion, and small for gestational age (SGA). Arsenic exposure negatively affected male reproductive systems by lowering testicular and accessory organ weights, and sperm counts, increasing sperm abnormalities and causing apoptotic cell death in Leydig and Sertoli cells, which resulted in decreased testosterone synthesis. Furthermore, during male reproductive toxicity, several molecular signalling pathways, such as apoptosis, inflammation, and autophagy are involved. Phytonutrient intervention in arsenic-induced male reproductive toxicity in various species has received a lot of attention over the years. The current review provides an in-depth summary of the available literature on arsenic-induced male toxicity, as well as therapeutic approaches and future directions.

Maternal exposure to bisphenol S induces neuropeptide signaling dysfunction and oxidative stress in the brain, and abnormal social behaviors in zebrafish (Danio rerio) offspring

Recent studies show that bisphenol S (BPS) induces multiple adverse effects in exposed organisms; however, the maternal effects of BPS exposure remain poorly understood. Here, we expose adult female zebrafish to environmentally relevant concentrations of BPS (0, 1, 10, 30 μg/L) and 1 μg/L of 17-β-estradiol (E2) as a positive control for 60 days. Females were then paired with BPS-unexposed males and their offspring were raised in control water for 6 months. Maternal exposure to BPS was found to alter social behavior and anxiety response in a dose-specific manner in male offspring. Group preferences and social cohesion were significantly reduced by maternal exposure to 1 and 10 μg/L BPS, respectively. Additionally, maternal exposure to 1 and 30 μg/L BPS and E2 decreased offspring stress responses during the novel tank test. The impaired social behavior was associated with elevated arginine-vasotocin (AVT) level as well as with the altered expression of genes involved in AVT signaling pathway (AVT, avpr1aa) and enzymatic antioxidant genes (cat and Mn-sod) in the brain. Collectively, these results suggest that maternal exposure to environmentally relevant concentrations of BPS alters social behavior in zebrafish offspring, which is likely mediated by oxidative stress and disruption of neuropeptide signaling pathways in the brain.

Proximate causes and ultimate effects of common antidepressants, fluoxetine and venlafaxine, on fish behavior

Antidepressant (AD) drugs are widely prescribed for the treatment of psychiatric disorders, including depression and anxiety disorders. The continuous use of ADs causes significant quantities of these bioactive chemicals to enter the aquatic ecosystems mainly through wastewater effluent discharge. This may result in many aquatic organisms being inadvertently affected by these drugs. Fluoxetine (FLX) and venlafaxine (VEN) are currently among the most widely detected ADs in aquatic systems. A growing body of experimental evidence demonstrates that FLX and VEN have a substantial capacity to induce neurotoxicity and cause behavioral dysfunctions in a wide range of teleost species. At the same time, these studies often report seemingly contradictory results that are confounding in nature. Hence, we clearly require comprehensive reviews that attempt to find overarching patterns and establish possible causes for these variable results. This review aims to explore the current state of knowledge regarding the neurobehavioral effects of FLX and VEN on fishes. This study also discusses the potential mechanistic linkage between the neurotoxicity of ADs and behavioral dysfunction and identifies key knowledge gaps and areas for future research.

Predictive biomarkers of cardiovascular disease in adult Canadian population

Background

Elevated levels of the enzymes gamma-glutamyltransferase (GGT), alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), and C-reactive protein (CRP) have been shown to be associated with increased risk of cardiovascular disease (CVD). Objective: To assess cross-sectional relationships between biomarkers GGT, ALT, AST, ALP and CVD in adult Canadian population.

Methods

The Canadian Health Measures Surveys (CHMSs) are a series of cross-sectional national surveys and collect information on indicators of general health and wellness of Canadians. The CHMS has four components. We used data from the first three components (for Study participants ≥ 20 years) from CHMS cycles 1 through 5.

RESULTS

Multivariable logistic regression revealed: immigration status [Odds ratio (OR)(95% Confidence Interval (95% CI)) = 0.67 (0.53-0.85), reference category (RC)-no-immigrant] education [1.38(1.10-1.75), RC- > secondary education]; smoking status [ex-smokers: 1.16(0.89-1.51); current smokers: 1.41(0.98-2.05), RC-non-smoker]; and income [middle income: 0.69(0.43-1.10); high income: 0.49(0.29-0.83); RC-lower income] were significantly associated with CVD prevalence.

Conclusion

The relationship of GGT with CVD prevalence changed among age groups and body mass index categories; was different for males and females; and diabetes was an effect modifier in the relationship between AST and CVD prevalence. Socio-economic factors were significantly associated with CVD prevalence.

Transgenerational effects of selenomethionine on behaviour, social cognition, and the expression of genes in the serotonergic pathway in zebrafish

Elevated levels of contaminants from human activities have become a major threat to animals, particularly within aquatic ecosystems. Selenium (Se) is a naturally occurring element with a narrow range of safe intake, but excessive Se has toxicological effects, as it can bioaccumulate and cause cognitive and behavioural impairments. In this study, we investigated whether exposure to Se would also have transgenerational effects, causing changes in the descendants of exposed individuals. We exposed adult female zebrafish to either a control diet or environmentally relevant concentrations of dietary Se-Met (3.6, 12.8, 34.1 μg Se/g dry weight) for 90 days. Then, females from each treatment group were bred with untreated males, and the offspring (F1-generation) were raised to adulthood (6 months old) without Se exposure. In behavioural tests, offspring that were maternally exposed to 34.1 μg Se/g showed signs of elevated stress, weaker group preferences, and impaired social learning. Maternal exposure to high levels of Se-Met also led to dysregulation of the serotonergic system via changes in mRNA expression of serotonin receptors, including the 5-HT1A, 5-HT1B, and 5-HT1D subtypes, the serotonin transporter, and monoamine oxidase (MAO). Such perturbations in the serotonergic system, thus, appear to underlie the neurobehavioural deficits that we observed. These findings suggest that Se contamination can have important transgenerational consequences on social behaviour and cognition.

Molecular Mechanism of Arsenic-Induced Neurotoxicity including Neuronal Dysfunctions

Arsenic is a key environmental toxicant having significant impacts on human health. Millions of people in developing countries such as Bangladesh, Mexico, Taiwan, and India are affected by arsenic contamination through groundwater. Environmental contamination of arsenic leads to leads to various types of cancers, coronary and neurological ailments in human. There are several sources of arsenic exposure such as drinking water, diet, wood preservatives, smoking, air and cosmetics, while, drinking water is the most explored route. Inorganic arsenic exhibits higher levels of toxicity compared its organic forms. Exposure to inorganic arsenic is known to cause major neurological effects such as cytotoxicity, chromosomal aberration, damage to cellular DNA and genotoxicity. On the other hand, long-term exposure to arsenic may cause neurobehavioral effects in the juvenile stage, which may have detrimental effects in the later stages of life. Thus, it is important to understand the toxicology and underlying molecular mechanism of arsenic which will help to mitigate its detrimental effects. The present review focuses on the epidemiology, and the toxic mechanisms responsible for arsenic induced neurobehavioral diseases, including strategies for its management from water, community and household premises. The review also provides a critical analysis of epigenetic and transgenerational modifications, mitochondrial oxidative stress, molecular mechanisms of arsenic-induced oxidative stress, and neuronal dysfunction.

Using zebrafish as a model to assess the individual and combined effects of sub-lethal waterborne and dietary zinc exposure during development

The present research used zebrafish (5-28 days post-fertilization; dpf) as a model organism to investigate the effects of chronic exposure to environmentally relevant sub-lethal concentrations of waterborne (261 μg/L) and dietary zinc (Zn) (1500 mg Zn/kg dw), either independently or simultaneously, during development. The results showed that whole body contents of Zn were increased in all Zn treatment groups, with the highest accumulation of Zn observed in larvae simultaneously exposed to elevated waterborne and dietary Zn. In addition, exposure to elevated levels of Zn, either through the water or the diet, led to a decrease in whole body calcium (Ca) contents at 28 dpf. The findings also suggested that exposure to elevated levels of Zn resulted in a significant reduction in whole body manganese (Mn) contents. More importantly, the magnitude of decrease in Mn contents by Zn exposure was markedly higher than that in Ca and appeared to mirror the increases in whole body Zn accumulation. These results indicate that Mn regulation is more sensitive than Ca to disruption by Zn exposure in developing fish. Further examination of the Zrt-Irt-Like Protein (ZIP) family of transporters using droplet digital PCR technologies revealed that several zip transporters exhibited temporal and exposure route-specific changes following Zn exposure. In particular, the level of zip4 was influenced by Zn exposure regardless of the exposure routes, while changes in zip7 and zip8 levels were predominantly driven by waterborne exposure. Overall, our findings demonstrated that zebrafish during the developmental periods are sensitive to elevated levels of Zn seen in the environment, particularly following co-exposures to waterborne and dietary Zn. Future toxicological assessment of elevated Zn exposure should consider both the exposure routes and the life stages of fish.

A comprehensive review on the neuropathophysiology of selenium

As an essential micronutrient, selenium (Se) exerts its biological function as a catalytic entity in a variety of enzymes. From a toxicological perspective, however, Se can become extremely toxic at concentrations slightly above its nutritional levels. Over the last few decades, there has been a growing level of concern worldwide regarding the adverse effects of both inorganic and organic Se compounds on a broad spectrum of neurological functions. A wealth of evidence has shown that exposure to excess Se may compromise the normal functioning of various key proteins, neurotransmitter systems (the glutamatergic, dopaminergic, serotonergic, and cholinergic systems), and signaling molecules involved in the control and regulation of cognitive, behavioral, and neuroendocrine functions. Elevated Se exposure has also been suspected to be a risk factor for the development of several neurodegenerative and neuropsychiatric diseases. Nonetheless, despite the various deleterious effects of excess Se on the central nervous system (CNS), Se neurotoxicity and negative behavioral outcomes are still disregarded at the expense of its beneficial health effects. This review focuses on the current state of knowledge regarding the neurobehavioral effects of Se and discusses its potential mode of action on different aspects of the central and peripheral nervous systems. This review also provides a brief history of Se discovery and uses, its physicochemical properties, biological roles in the CNS, environmental occurrence, and toxicity. We also review potential links between exposure to different forms of Se compounds and aberrant neurobehavioral functions in humans and animals, and identify key knowledge gaps and hypotheses for future research.

Interactive effects of cadmium and Benzo[a]pyrene in adult zebrafish (Danio rerio) during short-term aqueous co-exposure

Environmental water quality guidelines often work under the assumption that the toxicity of environmental pollutants is identical when present in isolation or in a complex chemical mixture. Thus, there is a crucial gap in our knowledge regarding how these toxicants interact and alter the toxicological effects in aquatic organisms. The present study examined the effects of acute (72-hr) aqueous exposures of Cadmium (Cd), a highly toxic non-essential trace metal, and Benzo[a]Pyrene (B[a]P), a prototypical polycyclic aromatic hydrocarbon (PAH) in adult zebrafish. Following a range-finding series of individual single-toxicant exposures, a second series was carried out using select concentrations in binary mixture exposures (using 5.8 or 22 μg/L for Cd; 0.44 or 1.07 μg/L for B[a]P). Our results demonstrated that tissue accumulation of both toxicants increased significantly in the presence of the second toxicant relative to single-toxicant exposures. Cd-only and B[a]P-only single toxicant exposures caused a significant downregulation of cytochrome p4501a (CYP1A1) and metallothionein-2 (MT2) mRNA in the gills, respectively, however binary co-exposures using both toxicants resulted in strong up-regulation of CYP1A1 and MT2. Additionally, co-exposures caused a strong induction of SOD1 and CAT mRNA transcript levels in the gill. The observed increase in body burden and transcript modulation did not translate into additive or more-than-additive toxic effects (oxidative stress) in zebrafish.

Differences in the subcellular distribution of cadmium and copper in the gills and liver of white sturgeon (Acipenser transmontanus) and rainbow trout (Oncorhynchus mykiss)

Recent studies have shown that white sturgeon (Acipenser transmontanus) are more resistant to cadmium (Cd) compared to rainbow trout (Oncorhynchus mykiss), whereas they are more sensitive than rainbow trout when exposed to copper (Cu). Differences in the subcellular distribution of metals among species could be one of the factors responsible for the differences in the sensitivity to metals. Although, subcellular distribution has been studied extensively in many species with many metals, its direct role in species-specific differences in the sensitivity has not been well studied. The objective of this study was to evaluate the role of subcellular distribution of metals in species-specific differences in the sensitivity to metals between sturgeon and trout. We compared the subcellular distribution of metals Cd and Cu in the cellular debris, heat-stable proteins, heat-denatured fraction, metal-rich granules, and organelles fractions from the gills and liver after exposure of juveniles of both species to 1.25 and 20 μg/L Cd and Cu for 8 days, respectively. Sturgeon diverted a higher amount of Cd towards biologically inactive metal pool (BIM) and a lower amount towards the biologically active metal pool (BAM) compared to trout in both tissues. This explained why sturgeon are able to tolerate a relatively higher exposure level to Cd compared to trout. For Cu, there was no statistically significant species-specific differences in the amounts diverted towards either BAM or BIM; hence, white sturgeon's greater sensitivity to Cu was not explained by its subcellular distribution strategies.

Skeletal pathology and bone mineral density changes in wild muskrats (Ondatra zibethicus) and red squirrels (Tamiasciurus hudsonicus) inhabiting arsenic polluted areas of Yellowknife, Northwest Territories (Canada): A radiographic densitometry study

The City of Yellowknife is a known hotspot of arsenic contamination and there is a growing body of evidence suggesting that local wildlife in the vicinity of the abandoned Giant Mine site may be at risk of decreased bone mineralization and various bone disorders. The purpose of this study was to preliminarily measure bone mineral density (BMD) changes and investigate the incidence, pattern, and severity of bone lesions in wild muskrats and red squirrels breeding in three (3) catchment areas at different distances from the Giant Mine Site in Yellowknife, Northwest Territories (Canada): ~2 km (location 1), ~18 km (location 2), and ~40-100 km (location 3). Full femoral bones of 15 muskrats and 15 red squirrels were collected from the three sampling locations (5 from each location) and subjected to radiographic analysis and densitometric measurements. The patterns and severities of bone lesions, including changes in bone mineral density, were evaluated and compared between groups. As levels were significantly higher in the bones of muskrats caught from location 1 and 2, relative to location 3. Further, As and Cd levels were significantly higher in the bones of squirrels caught from locations 1 and 2 relative to squirrels caught from location 3. The preliminary results from bones revealed that radiographic abnormalities such as bone rarefaction, osteopenia, and thinning of the femoral shafts with significant ossific cystic lesions and bowing were the most common skeletal pathologies found in bones of red squirrels from the three locations. Radiographic appearances of massive sclerosis and dysplasia, including severe osteocondensation and osteopathia striata-like abnormalities, were found in the bones of muskrats from all the sampling locations. Densitometric evaluation showed no significant differences between the three locations in the bone parameters measured. However, there was a statistically significant correlation between As content in the bones of muskrats and percent fat content in the femur samples, which suggests that accumulation of As could have been a causal factor for a change in percent fat in femurs of muskrats.

Chronic exposure to bisphenol S induces oxidative stress, abnormal anxiety, and fear responses in adult zebrafish (Danio rerio)

Bisphenol S (BPS) is increasingly used in a wide range of industrial and consumer products, resulting in its ubiquitous distribution across the environment, including aquatic ecosystems. Although it is commonly known as a weak/moderate estrogenic compound, there has been a growing acknowledgment of the potential of BPS to cause toxicity by inducing oxidative stress. Oxidative stress is a major participant in the development of anxiety-like behaviors in humans and animals. Therefore, the present study was designed to examine the impact of BPS on anxiety-like behavior and fear responses in adult zebrafish and also to elucidate the possible linkage between the BPS neurotoxicity and oxidative status of the brain. To this end, adult male and female zebrafish were exposed to 0 (control), 1, 10, and 30 μg/L of BPS and 1 μg/L of 17-β-estradiol (E2) for 75 days. Following exposure, changes in anxiety and fear-related responses were evaluated by applying a novel tank test and by exposing focal fish to chemical alarm cues. Additionally, we evaluated the expression of multiple antioxidant genes in the zebrafish brain. Our results indicate that BPS, irrespective of exposure concentration, and E2 significantly decreased bottom-dwelling behavior and the latency to enter the upper water column. Furthermore, exposure to the highest concentration of BPS and E2 induced a significant decrease in fear-related responses. The impaired anxiety and reduced fear-related responses were associated with a down-regulation in the transcription of genes involved in enzymatic antioxidant defense. Taken together, our results suggest that chronic exposure to BPS impairs anxiety and fear responses in adult zebrafish, possibly by inducing oxidative stress in the brain.

Investigating the mechanism of vanadium toxicity in freshwater organisms

Vanadium (V) could present a risk for aquatic organisms from the Alberta oil sands region, if present in high concentrations. An industry pilot project has used petroleum coke (PC) as a sorbent to remove organic toxicants from oil sands process-affected water (OSPW), but it also caused V to leach from PC into the OSPW, reaching concentrations of up to 7 mg V/L (a level known to be toxic to aquatic organisms). Vanadium is a transition metal with several oxidation states, which could potentially elicit its toxicity through either ion imbalance or oxidative stress. This study investigated the effect of V on Daphnia magna and Oncorhynchus mykiss. Daphinds and O. mykiss were exposed to concentrations of V up to their respective calculated median lethal concentration (LC₅₀): 3 mg V/L for D. magna and 7 mg V/L for O. mykiss. For both organisms, the influence of V on sodium flux and whole body sodium was evaluated. Its effect on whole body calcium and the oxidative stress responses in O. mykiss at the gill and liver levels was also studied. Results suggested that 3.1 mg V/L for D. magna and 6.8 mg V/L for O. mykiss caused an overall increase in sodium influx in both the daphnids and rainbow trout. However, concentrations of V ranging between 0.2 and 4 mg V/L for D. magna and 1.8 and 6 mg V/L for O. mykiss reduced whole body sodium in both organisms and whole body calcium in O. mykiss. Concentrations above 3.6 mg V/L caused significant lipid peroxidation in the gills and liver of rainbow trout, while 1.9 mg V/L produced a substantial decrease in the fish gill GSH:GSSG ratio, but no change in the ratio between these thiols in the liver. Concentrations of 6.62 mg V/L sharply increased catalase activity in the liver but not in the gills. Neither liver nor gill superoxide dismutase was altered by V. Overall, results suggest that both ion imbalance and oxidative stress are part of the mechanism of toxicity of V in D. magna and O. mykiss and that further research is warranted to fully elucidate the mechanism(s) of V toxicity in aquatic organisms.

Multiple Linear Regression Modeling Predicts the Effects of Surface Water Chemistry on Acute Vanadium Toxicity to Model Freshwater Organisms

Multiple linear regression (MLR) modeling has been successfully used to predict how water chemistry variables influence the toxicity of cationic metals to aquatic organisms, but no MLR model exists for vanadium (V). Recent research has indicated that an increase in pH (from 6 to 9), or high concentrations of sodium (473 mg Na⁺ /L), increase V toxicity to Daphnia pulex. In contrast, increases in alkalinity (>100 mg as CaCO₃ ) and sulfate (>100 mg SO₄^2- /L) reduce V toxicity. How these variables influence V toxicity to Oncorhynchus mykiss (rainbow trout) was still unknown. Our results show that increasing pH from 6.2 to 8.9 tended to decrease the 96-h median lethal concentration (LC50) for V toxicity to O. mykiss by 9.6 mg V/L. An alkalinity increase from 71 to 330 mg/L as CaCO₃ tended to increase the 96-h LC50 by 3.3 mg V/L, whereas when SO₄^2- rose from 150 to 250 mg/L, the LC50 significantly increased by 0.3 mg V/L followed by a significant decrease of 1 mg V/L when SO₄^2- was >250 mg/L. Sodium (between 100 and 336 mg/L) showed no effect on V toxicity to O. mykiss. The toxicity patterns for O. mykiss were similar to those observed for D. pulex, except for that of SO₄^2- , potentially indicating different mechanisms of V uptake or regulation in the 2 species. The LC50s and associated water chemistry were combined to develop an MLR model for O. mykiss and D. pulex. Alkalinity and pH modified V toxicity to both species, whereas SO₄^2- influenced V toxicity to D. pulex. Overall, MLR models should be considered for creating new local benchmarks or water quality guidelines for V. Environ Toxicol Chem 2020;39:1737-1745. © 2020 SETAC.

Regulation of metal homeostasis and zinc transporters in early-life stage zebrafish following sublethal waterborne zinc exposure

In the present research, the effects of exposure to a sublethal concentration of zinc (Zn) on metal and ion homeostasis, and the regulation and the localization of various Zn transporters (i.e., the Zrt-Irt Like Protein (ZIP) family of Zn transporters), were investigated in zebrafish (Danio rerio) during early development. Exposure to an elevated level of Zn [4 μM (high) vs. 0.25 μM (control)] from 0 day post-fertilization (dpf) resulted in a significant increase in the whole body content of Zn at 5 dpf. A transient decrease in the whole body calcium (Ca) level was observed in 3 dpf larvae exposed to high Zn. Similarly, whole body nickel (Ni) and copper (Cu) contents were also reduced in 3 dpf larvae exposed to high Zn. Importantly, the magnitude of reduction in whole body Ni and Cu contents following Zn exposure was markedly higher than that in Ca content, suggesting that internal Ni and Cu balance were likely more sensitive to Zn exposure in developing zebrafish. Exposure to high Zn altered the mRNA expression levels of specific zip transporters, with an increase in zip1 (at 3 dpf) and zip8 (at 5 dpf), and a decrease in zip4 (at 5 dpf). The expression levels of most zip transporters tended to decrease from 3 dpf to 5 dpf with the exception of zip4 and zip8. Results from in situ hybridization revealed that several zip transporters exhibited distinct spatial distribution (e.g., zip8 in the intestinal tract, zip14 in the pronephric tubules). Overall, our findings suggested that exposure to sublethal concentrations of Zn disrupts the homeostasis of essential metals during early development and that different ZIP transporters may play unique roles in regulating Zn homeostasis in various organs in developing zebrafish.

Effects of chronic exposure to bisphenol-S on social behaviors in adult zebrafish: Disruption of the neuropeptide signaling pathways in the brain

Bisphenol S (BPS), considered to be a safe alternative to Bisphenol A, is increasingly used in a wide variety of consumer and industrial products. However, mounting evidence suggests that BPS can act as a xenoestrogen targeting a wide range of neuro-endocrine functions in animals. At present, very little is known about the impacts of BPS on social behaviors and/or the potential underlying mechanisms. To this end, we exposed adult male and female zebrafish to environmentally relevant concentrations of BPS (0 (control), 1, 10, and 30 μg/L), as well as to 17β-estradiol (E2; 1 μg/L; as positive control) for 75 days. Subsequently, alterations in social behaviors were evaluated by measuring shoal cohesion, group preferences, and locomotor activity. Furthermore, to elucidate the possible molecular mechanism underlying the neuro-behavioral effects of BPS, we also quantified the changes in the mRNA abundance of arginine vasotocin (AVT), isotocin (IT), and their corresponding receptors in the zebrafish brain. The results showed that E2 and BPS (30 μg/L) decreased shoal cohesion in both males and females. Moreover, a marked decline in group preferences was observed in all treatment groups, while locomotor activity remained unaffected. Alterations in the social behaviors were associated with sex-specific changes in the mRNA expression of genes involved in IT and AVT signaling. Taken together, the results of this study suggest that chronic exposure to BPS can impair zebrafish social behaviors via disruption of isotocinergic and vasotocinergic neuro-endocrine systems.

Speciation and risk assessment of selected trace metals in bottom sediment of coral reef ecosystems of the Persian Gulf

To evaluate the hazard assessment of anthropogenic activities on coastal ecosystems, fractionation and bioaccumulation of trace metals were carried out for sediment and coral samples of three distinct habitats including petroleum exploration area of Kharg, the rural harbor of Chirouyeh, and unpopulated area of Hendorabi. Fractionation results suggested that Ni (~ 51%), Pb (~ 49%), and V (~ 45%) in Kharg; Ni (~ 46%), Pb (~ 84%), and Zn (~ 47%) in Chirouyeh; and Cd (~ 51%) in Hendorabi were the predominant metals in the non-resistant fractions of the sediment samples. Risk Assessment Code (RAC), individual contamination factor (ICF), and global contamination factor (GCF) were derived. The highest relative risk for Cd, Cu, and Ti was observed in Kharg, whereas the highest relative risk for Ni, Pb, V, and Zn was recorded in Chirouyeh. The lowest risk for almost all of the trace metals was observed in Hendorabi. Biota-sediment accumulation factor (BSAF) values of coral samples revealed that species examined in the present study are reliable biomonitors for Cd, Ni, Pb, and Zn contamination. Principal component analysis (PCA) and Pearson's correlation coefficient (PCC) suggested that Pb, Ni, and Zn can be considered the main pollutants of the Persian Gulf which originated mainly from petroleum industries. Furthermore, the metal uptake rates of coral samples seemed to be dependent on bioabsorption pathways and coral species. Overall, the present work constitutes a good basis for further studies on trace metal fractionation, risk assessment, and source apportionment in the Persian Gulf, which could contribute to more effective decisions for reducing the anthropogenic trace metal pollution.

Chronic exposure to environmentally relevant concentrations of bisphenol S differentially affects cognitive behaviors in adult female zebrafish

Evidence is emerging that environmental exposure to bisphenol S (BPS), a substitute for bisphenol A (BPA), to humans and wildlife is on the rise. However, research on the neurobehavioral effects of this endocrine disruptive chemical is still in its infancy. In this study, we aimed to investigate the effects of long-term exposure to environmentally relevant concentrations of BPS on recognition memory and its mechanism(s) of action, especially focusing on the glutamatergic/ERK/CREB pathway in the brain. Adult female zebrafish were exposed to the vehicle, 17β-estradiol (E2, 1 μg/L), or BPS (1, 10 and 30 μg/L) for 120 days. Fish were then tested in the object recognition (OR), object placement (OP), and social recognition tasks (SR). Chronic exposure to E2 and 1 μg/L of BPS improved fish performance in OP task. This was associated with an up-regulation in the mRNA expression of several subtypes of metabotropic and ionotropic glutamate receptors, an increase in the phosphorylation levels of ERK1/2 and CREB, and an elevated transcript abundance of several immediate early genes involved in synaptic plasticity and memory formation. In contrast, the exposure to 10 and 30 μg/L of BPS attenuated fish performance in all recognition memory tasks. The impairment of these memory functions was associated with a marked down-regulation in the expression and activity of genes and proteins involved in glutamatergic/ERK/CREB signaling cascade. Collectively, our study demonstrated that the long-term exposure to BPS elicits hermetic effects on the recognition memory in zebrafish. Furthermore, the effect of BPS on the recognition memory seems to be mediated by the glutamatergic/ERK/CREB signaling pathway.

Comparative study of arsenic toxicosis and ocular pathology in wild muskrats (Ondatra zibethicus) and red squirrels (Tamiasciurus hudsonicus) breeding in arsenic contaminated areas of Yellowknife, Northwest Territories (Canada)

The Giant Mine is an abandoned gold mine in Yellowknife, Northwest Territories, Canada. Throughout its operation from 1948 to 2004, the Giant Mine released heavy amounts of arsenic trioxide into the environment, thus contaminating the soil and surface water within and around the vicinity of the mine site. Chronic arsenic (As) poisoning negatively impacts wildlife health and can induce multi-organ damages including neurodegeneration and visual dysfunction depending on concentration and duration of exposure. The aim of the current study was to comparatively assess retina layer changes and prevalence of ocular lesions in wild rodent populations (i.e. muskrats and red squirrels) breeding in arsenic endemic areas of Yellowknife, near the vicinity of the abandoned Giant mine site (∼2 km radius), at an intermediate location (approximately 20 km from the mine area) as well as a reference location (spanning 52-105 km from the city of Yellowknife, Canada). Eye globes were removed from euthanized muskrats and squirrels from the three sampling locations with increasing distance from the Giant mine area. Optical Coherence Tomography (OCT) was used to attempt a pan-retinal layer assessment, and histologic examination was utilized for assessment and confirmation of ocular lesions. The retinal layers were measured and statistically compared between the groups based on sampling locations to enhance the scope of histologic evaluations. The preliminary results revealed that thicknesses of ganglion cell layer (GCL), retina nerve fibre layer (NFL), and inner retina layer (IR) were statistically reduced in the muskrats from arsenic endemic area, particularly near the vicinity of the Giant mine compared to the control group. Generalized ocular pathology was histologically confirmed in all the muskrats from the arsenic endemic areas with the manifestation of moderate to severe lymphocytic plasmacytic uveitis (LPU), keratitis and subcapsular cataracts. Inner retinal degeneration was also observed in all the muskrats from the arsenic endemic areas, while muskrats from the control group were predominantly normal. Three muskrats from the control group were noted to have a mild LPU and keratitis. Significant histopathologic changes were not detected in the squirrel eyes from the three groups except for incidental mild cornea scars from all the locations. In general, these preliminary findings confirm the presence of ocular lesions and retina abnormalities in wild muskrats in the Yellowknife area and provide the first evidence of visual dysfunction and impairment in wildlife inhabiting arsenic endemic areas of Canada.

Effects of chronic exposure to selenomethionine on social learning outcomes in zebrafish (Danio rerio): serotonergic dysregulation and oxidative stress in the brain

For many species, social learning is crucial for fitness-related activities, but human-induced environmental changes can impair such learning processes. For instance, mining can release the element, selenium (Se), that is vital for physiological functions but also has toxicological properties at elevated concentrations. In this study, we investigated the effects of chronic exposure to Se on social learning outcomes and potential underlying molecular mechanisms in adult zebrafish. After exposure to different levels of dietary selenomethionine (control, 3.6, 12.8, 34.1 μg Se/g dry weight) for 90 days, we examined the ability of observer fish to follow demonstrators (experienced individuals) in escaping an oncoming trawl. Social learning outcomes were then assessed in the absence of demonstrators. Our results indicated that fish in the highest exposure group (34.1 μg/g) displayed significantly slower escape responses compared to fish in the control and lower exposure groups (3.6 and 12.8 μg Se/g). This impaired behavior was associated with higher oxidative stress and dysregulation in genes that are key in the serotonergic pathway, indicating that oxidative stress and alteration in the serotonergic system lead to impairment of social learning.

Comparative analyses of oxidative stress response and metallothionein induction in white sturgeon and rainbow trout during acute waterborne copper exposure

Early life-stages of the endangered white sturgeon (Acipenser transmontanus) have been shown to be among the most sensitive fishes to aqueous copper (Cu) exposure. In a recent analogous study, we examined the role of whole-body Cu accumulation and Na homeostasis in species-specific differences between the sensitivity of white sturgeon and a common laboratory fish model, rainbow trout, to Cu. However, the potential roles of important mechanisms such as Cu-induced oxidative stress and/or metallothionein (MT) induction as potential drivers of sensitivity of white sturgeon to Cu have not been investigated to date. Here, rainbow trout and white sturgeon from three different early life-stages were exposed to waterborne Cu for 96 h, following which major antioxidant parameters, lipid peroxidation and MT gene expression were evaluated. Results indicated that during larval and swim-up life-stages, Cu induced oxidative damage in white sturgeon was greater than in rainbow trout. Moreover, baseline glutathione (GSH) was significantly greater in rainbow trout than white sturgeon. Observations also suggested that trout exceedingly relied on GSH to combat Cu-induced oxidative stress as they grew older. In contrast, sturgeon recruited an increasing level of MT to neutralize Cu-induced oxidative stress and/or Cu loading. In our recent study, we demonstrated that Na homeostasis is more susceptible to Cu in white sturgeon than in rainbow trout. Collectively, these findings indicate that the greater degree of oxidative damage in early life-stages, in addition to the higher magnitude of the disruption of Na homeostasis, contributes to the higher sensitivity of white sturgeon to Cu exposure.

Rapid effects of estradiol and its receptor agonists on object recognition and object placement in adult male zebrafish

In recent years, there has been a growing appreciation that 17β-estradiol (E2) can rapidly modulate learning and memory processes by binding to membrane estrogen receptors and cause the activation of a number of signaling cascades within the central nervous system. In this study, we sought to investigate the effects of post-training administration of E2 (100 ng/g, 1 μg/g, 10 μg/g) and involvement of the estrogen receptors (ERs) using selective ER agonists on the consolidation of object recognition (OR) and object placement memory (OP) in adult male zebrafish. The general activation of ERs with the highest E2 dose improved consolidation of memory in both learning tasks within 1.45 h of administration. Activation of classical ERs (ERα and ERβ) improved consolidation of OR memory, but had no effect on fish performance in OP task. On the other hand, activation of G protein-coupled ER1 impaired and enhanced consolidation of OR and OP memories, respectively. Memory improvement in both tasks was accompanied by a marked up-regulation in the expression of genes encoding ionotropic and metabotropic glutamate receptors in a task-dependent manner. In contrast, the down-regulation in the expression of certain ionotropic glutamate receptors was observed in fish with impaired OR memory. Moreover, our study also revealed an increase in the transcript abundance of genes associated with synaptic protein synthesis (brain-derived neurotrophic factor, synaptophysin, and the mechanistic target of rapamycin). These results suggest that E2 may affect consolidation of memory in zebrafish likely through rapid changes in synaptic morphology and function.

Altered neurotransmission and neuroimaging biomarkers of chronic arsenic poisoning in wild muskrats (Ondatra zibethicus) and red squirrels (Tamiasciurus hudsonicus) breeding near the City of Yellowknife, Northwest Territories (Canada)

Chronic arsenic poisoning has been shown to be a risk factor for the development of intellectual disability. Numerous human and animal studies have also confirmed that low-level arsenic exposure has deleterious effects on neurotransmission and brain structures which have been further linked to neurobehavioral disorders. The aim of this present work was to comparatively assess structural brain volume changes and alteration of two (2) neurotransmitters, specifically dopamine (DA) and serotonin (5-HT) in the brains of wild muskrats and squirrels breeding in arsenic endemic areas, near the vicinity of the abandoned Giant mine site in Yellowknife and in reference locations between 52 and 105 km from the city of Yellowknife. The levels of DA and 5-HT were measured in the brain tissues, and Magnetic Resonance Imaging (MRI) was used to attempt brain volume measurements. The results revealed that the concentrations of DA and 5-HT were slightly increased in the brains of squirrels from the arsenic endemic areas compared to the reference site. Further, DA and 5-HT were slightly reduced in the brains of muskrats from the arsenic endemic areas compared to the reference location. In general, no statistically significant neurotransmission changes and differences were observed in the brain tissues of muskrats and squirrels from both arsenic endemic areas and non-endemic sites. Although MRI results showed that the brain volumes of squirrels and muskrats were not statistically different between sites after multiple comparison correction; it was noted that core brain regions were substantially affected in muskrats, in particular the hippocampal memory circuit, striatum and thalamus. Squirrel brains showed more extensive neuroanatomical changes, likely due to their relatively smaller body mass, with extensive shrinkage of the core brain structures, and the cortex, even after accounting for differences in overall brain size. The results of this present study constitute the first observation of neuroanatomical changes in wild small mammal species breeding in arsenic endemic areas of Canada.

Neuropathological changes in wild muskrats (Ondatra zibethicus) and red squirrels (Tamiasciurus hudsonicus) breeding in arsenic endemic areas of Yellowknife, Northwest Territories (Canada): Arsenic and cadmium accumulation in the brain and biomarkers of oxidative stress

The brain is one of the critical organs particularly susceptible to the damaging effects of chronic arsenic poisoning and there is a growing body of evidence that suggest that oxidative stress plays a key role in the pathogenesis of neurodegenerative disorders. The aim of this present work was to comparatively assess biomarkers of oxidative stress and status of antioxidant enzyme activities in the brains of muskrats and squirrels breeding in arsenic endemic areas, specifically near the vicinity of the abandoned Giant mine site (~2 km radius), and an intermediate location approximately 20 km from the mine area and in reference locations spanning 52-105 km from the city of Yellowknife, Northwest Territories (Canada). Analysis included measurement of total arsenic and cadmium concentration in the nails, brain, and stomach content of muskrats and squirrels, in addition to biochemical evaluation of lipid peroxidation levels and antioxidant enzymes defense: catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GPx) in the brain tissues. The results revealed that arsenic concentration in the nails of muskrats collected closest to the vicinity of the mine area was in the range of 11 to 35.1 times higher than those from the reference site. The maximum concentration of arsenic in the nails of muskrats from the intermediate location was 47.6 times higher than the maximum concentration observed in the reference muskrats. Cadmium was generally undetected in the nails of muskrats and squirrels from the three sampling locations. Arsenic in the gut contents of muskrats from the arsenic affected area was 4.5 to 49.1 times higher than those from the reference site. Cadmium levels in the guts of muskrats from the mine area almost doubled those from the reference site. Arsenic accumulated in the nails of squirrels from the areas closest to the mine but was undetected in the squirrel nails from the reference location. The maximum arsenic levels in the stomach content of squirrels from the mine area was ~40 times higher than those from the reference site. Arsenic did not accumulate in the brains of muskrats, but cadmium was detected in a few brains of muskrats. Brains of squirrels from the mine area and intermediate locations accumulated both arsenic and cadmium. The brains of squirrels and muskrats from the arsenic affected area showed no evidence of increased lipid peroxidation compared to the animals from the reference site. However, SOD, CAT and GPx activities in the brains of animals from the arsenic endemic areas tended to be higher compared to the control sites. This is the first study documenting evidence of oxidative stress and altered antioxidant enzyme activities in brains of wild rodent population in arsenic endemic areas of Canada.

Sensitivity of white sturgeon and rainbow trout to waterborne copper exposure: A comparative study of copper-induced disruption of sodium homeostasis

Recent studies have demonstrated that white sturgeon are more sensitive to acute exposure to Cu than rainbow trout (Oncorhynchus mykiss), especially during early life-stages. However, the physiological mechanisms underlying this difference in sensitivity to Cu is not known. In the present study, we first confirmed the higher sensitivity (lower 96 h LC₅₀ values) of white sturgeon to Cu at three different life stages (larva, swim-up, and juvenile) relative to their counterparts in rainbow trout. We also demonstrated that acute exposure to Cu (50 μg/L for 4.5 h) caused a significantly greater reduction in the rate of waterborne Na uptake in white sturgeon relative to that in rainbow trout across all three life-stages. In agreement with this observation, we also found that acute exposure to Cu (20 μg/L for 48 h) elicits a significantly greater decrease in whole body Na level in all life stages of white sturgeon compared to rainbow trout. In contrast, white sturgeon demonstrated a higher or similar level of Cu body burden relative to rainbow trout during acute Cu exposure (20 μg/L for 24 h), thereby indicating that Cu bioaccumulation is not a good indicator of its toxicity in these species. Overall, our study demonstrated that the differences in sensitivity to acute Cu exposure between white sturgeon and rainbow trout can be explained on the basis of differential effects of Cu on Na homeostasis.

Small microplastic particles (S-MPPs) in sediments of mangrove ecosystem on the northern coast of the Persian Gulf

We present a study of small microplastic particles (S-MPPs) in the sediments of mangrove ecosystem of Khor-e- Khoran, a Ramsar site in Iran. The spatial distribution of S-MPPs (<1 mm) in mangrove surface sediments were investigated, which provided new insights into the detection and composition of S-MPPs in the study area. S-MPPs were extracted via the air-induced overflow (AIO) extraction procedure, and then they were counted and categorized according to the particle shape, color and size. The mean number of S-MPPs at the five sampling sites ranged from 19.5 to 34.5 particles per kg dry sediment in Bandar Gelkan and Bandar Lengeh, respectively. In general, microfibres followed by fragments were the most common type of S-MPPs isolated in each site (>56% and ~35%, respectively). Sewage discharge is probably the main source of extracted fibres in almost all the sites. The observed S-MPPs were classified into two size groups (10-300 μm and 300-1000 μm). The majority of S-MPPs fell into the smallest size group which accounted for 70-97% of the total S-MPPs. Fourier transform infrared (FTIR) analysis of some subsamples showed that polyethylene (PE) was the most common recovered polymer. Some non-plastic particles were also isolated from plastic-like particles of suspected S-MPPs in the mangrove sediments using a Scanning Electron Microscope (FE-SEM). This study provided the first evidence of S-MPPs contamination in the mangroves of the Iranian coast of the Persian Gulf. Long-term studies are required to understand, monitor and prevent further microplastics pollution in the region.

Species and life-stage specific differences in cadmium accumulation and cadmium induced oxidative stress, metallothionein and heat shock protein responses in white sturgeon and rainbow trout

Understanding the mechanistic basis of differences in the sensitivity of fishes to metals is important for developing informed ecological risk assessment approaches for metals. Whole body metal accumulation, metallothionein induction, oxidative stress and associated antioxidant response, as well as heat shock proteins (mainly HSP70) are known to play important roles in determining the toxicity of metals in fish. Hence, in this study we have cross-evaluated these parameters as a function of Cd exposure across different life-stages of two evolutionarily distinct fish species, namely rainbow (Oncorhynchus mykiss) trout and white sturgeon (Acipenser transmontanus). These two species have been shown to differ significantly in their physiological and apical responses to Cd exposure. The findings of the present study suggest that species-specific differences in the sensitivity to Cd could partially be explained by HSP70 gene response and oxidative damage biomarkers. However, not all the parameters studied in this study could explain the life-stage specific differences universally and were limited to only some life-stages. Based on the observations in the present study and other recent studies, it is apparent that species- and life-stage specific differences in the sensitivity to Cd and possibly other metals is a complex phenomenon and could be driven by multiple toxicokinetic and toxicodynamic factors.

Chronic exposure to dietary selenomethionine dysregulates the genes involved in serotonergic neurotransmission and alters social and antipredator behaviours in zebrafish (Danio rerio)

Selenium (Se) is a metalloid of potential interest from both a toxicological and nutritional perspective, having a range of safe intake. The adverse neuro-behavioural effects of Se have been investigated in both humans and fishes, but little is known about its effects on social behaviours or the serotonergic signaling pathway in the brain. In the present study, we investigated the effects of chorionic dietary exposure to Se (as selenomethionine) at different concentrations (control, 2.1, 11.6 or 31.5 μg/g dry wt.) on antipredator avoidance, shoaling behaviour, and social group preferences in adult zebrafish (Danio rerio). In addition, we also measured the expression of important genes in the serotonergic pathway that influence social behaviours. After 60 days of exposure, the highest dose (31.5 μg/g dry wt.) caused the highest level of baseline fear behaviour, with fish swimming lower in the water column and in tighter shoals compared to fish in the other treatments. With high levels of baseline fear, these fish did not significantly intensify fear behaviours in response to predation risk in the form of exposure to chemical alarm cues. When individual fish were given an opportunity to shoal with groups of differing sizes (3 vs. 4 individuals), fish exposed to the high dose spent less time with groups in general, and only control fish showed a significant preference for the larger group. In the zebrafish brain, we found significant upregulation in the mRNA expression of serotonin receptors (htr1aa and htr1b), a transporter (slc6a4a), and tryptophan hydroxylase-2 (tph2), whereas there was a downregulation of the monoamine oxidase (mao) gene. The results of this study suggest that disruption of serotonergic neurotransmission might have been responsible for Se-induced impairment of antipredator and social behaviour in zebrafish.

The effects of dietary selenomethionine on tissue-specific accumulation and toxicity of dietary arsenite in rainbow trout (Oncorhynchus mykiss) during chronic exposure

Selenomethionine facilitated arsenic deposition in the brain and likely in other tissues, possibly via bio-complexation. Elevated dietary selenomethionine can increase the tissue-specific accumulation and toxicity of As³⁺ in fish during chronic dietary exposure.

Maternal Exposure to Dietary Selenium Causes Dopaminergic Hyperfunction and Cognitive Impairment in Zebrafish Offspring

Maternal exposure to environmental contaminants is a predisposing factor for neurodevelopmental disorders with associated cognitive and social deficits in offspring. In this study, we investigated the effects of maternal exposure to selenium (Se), a contaminant of potential environmental concern in aquatic ecosystems, on cognitive performance and the underlying mechanisms in F1-generation adult zebrafish. Adult female zebrafish were exposed to different concentrations of dietary Se (3.5, 11.1, or 27.4 μg Se/g dry weight) for a period of 60 days. Fish were subsequently bred, and their offspring were collected and raised for 6 months on a normal diet. Maternal exposure to all concentrations of dietary Se induced learning impairment in F1-zebrafish tested in a latent learning task. The results also showed a hyperfunctioning dopaminergic system in fish exhibiting the learning deficit. The hyperfunction of the dopaminergic system was associated with enhanced oxidative stress and alterations in the mRNA abundance of several immediate early and late response genes in the zebrafish brain. Taken together, these results suggest that maternal exposure to dietary Se via alterations in the dopaminergic system leads to persistent neurobehavioral deficits in F1-generation adult zebrafish.

Investigating the Role of Ionoregulatory Processes in the Species- and Life-Stage-Specific Differences in Sensitivity of Rainbow Trout and White Sturgeon to Cadmium

There are huge variations in life-stage- and species-specific sensitivities among the fishes to the exposure with metals; however, the physiological mechanisms underlying these differences are not well understood to date. This study revealed significant life-stage-specific (larval, swim-up, and juvenile) and species-specific differences between two evolutionary distant species of fishes, rainbow trout ( Oncorhynchus mykiss) and white sturgeon ( Acipenser transmontanus), following acute exposures to Cd. Although the 96 h LC50 of Cd was similar in both species at the larval stage, trout demonstrated an increased sensitivity to Cd at later life stages as compared to sturgeon. Moreover, exposure to Cd disrupted calcium (Ca) uptake and whole body Ca levels in trout by a greater degree relative to that in sturgeon regardless of life stage. Finally, white sturgeon demonstrated a lower affinity for Cd uptake relative to the more sensitive rainbow trout. This infers a differential nature of the interaction between Cd and Ca transport pathways in the two species and partially explains the differences in Cd sensitivity between rainbow trout and white sturgeon described previously. Overall, our results suggest that species- and life-stage-specific differences in sensitivity to waterborne Cd in fish are likely a function of the interplay between Cd uptake and Cd-induced disruption of Ca homeostasis.

A FTIRM study of the interactive effects of metals (zinc, copper and cadmium) in binary mixtures on the biochemical constituents of the gills in rainbow trout (Oncorhynchus mykiss)

We employed Fourier Transform Infrared Microspectroscopy to examine, in situ, the effects of waterborne Cu, Cd and Zn, alone and in binary mixtures, during acute exposure on the integrity of major lipid and protein constituents of the gill of a model teleost species, rainbow trout (Oncorhynchus mykiss). Our findings demonstrated that acute exposure to metals, both individually and in binary mixture, resulted in the degradations of various components of proteins and lipids in the gill tissue. Generally, when comparing the effects of individual metals, Cu was found to induce the maximum adverse effects followed by Cd and Zn, respectively. Among the binary metal-mixture combinations, Cu and Cd produced additive effects on the degradation of major proteins and lipid moieties, whereas the co-exposure of Zn with Cd or Cu elicited ameliorative effects, indicating antagonistic (less than additive) interactions between Zn and Cd or Cu in the rainbow trout gill. Overall, the present study demonstrates that FTIRM can be a useful tool to gain novel mechanistic insights into the biochemical changes induced by metals in the fish gill, which could influence the overall toxicity of metals to fish.

Dopaminergic dysregulation and impaired associative learning behavior in zebrafish during chronic dietary exposure to selenium

A growing body of evidence indicates that exposure to selenium (Se) can cause neurotoxicity, and this can occur because of its interference with several neurotransmitter systems in humans and animals. Dopamine is a critical modulator of a variety of brain functions and a prime target for environmental neurotoxicants. However, effects of environmentally relevant concentrations of Se on dopaminergic system and its neurobehavioral effects are still largely unknown. For this purpose, we exposed zebrafish, a model organism, to different concentrations of dietary l-selenomethionine (control, 3.5, 11.1, 27.4, and 63.4 μg Se/g dry weight) for a period of 60 days. Cognitive performance of fish was evaluated using a plus maze associative learning paradigm. Oxidative stress, as the main driver of Se neurotoxicity, was assessed by measuring the ratio of reduced to oxidized glutathione (GSH:GSSG), lipid peroxidation (LPO) levels, and mRNA expression of several antioxidant enzymes in the zebrafish brain. Dopamine levels in the brain and the expression of genes involved in dopamine synthesis, storage, reuptake, metabolism, and receptor activation were examined. Moreover, transcription of several synaptic plasticity-related immediate-early and late response genes was determined. Overall, fish fed with the two highest concentrations of dietary Se displayed impaired associative learning. Se exposure also induced oxidative stress in the zebrafish brain, as indicated by a reduction in GSH:GSSG ratio, increased LPO levels, and up-regulation of antioxidant genes in fish treated with the two highest concentrations of Se. An increase in brain dopamine levels associated with altered expression of dopaminergic cell markers was evident in different treatment groups. Moreover, Se exposure led to the down-regulation of immediate-early and late response genes in fish that exhibiting learning impairment. Taken together, the results of this study imply that the induction of oxidative stress and dysregulation of dopaminergic neurotransmission may underlie Se-induced impairment of associative learning in zebrafish.

Interactive effects of chronic dietary selenomethionine and cadmium exposure in rainbow trout (Oncorhynchus mykiss): A preliminary study

The present study investigated the interactive effects of dietary cadmium (Cd) and selenium (Se) on the tissue-specific (liver, kidney, and muscle) accumulation of these two elements, hepatic oxidative stress response, and morphometrics in rainbow trout (Oncorhynchus mykiss) during chronic exposure. Fish were exposed to elevated dietary Cd (45 μg g^-1 dry wt.), and medium (10 μg g^-1 dry wt.) or high (45 μg g^-1 dry wt.) dietary selenium (added as selenomethionine), both alone and in combination, for 30 days. Exposure to dietary Cd alone caused oxidative stress in fish as reflected by reduced thiol redox (GSH:GSSG), increased lipid peroxidation, and induction of anti-oxidative enzymes (catalase, superoxide dismutase, and glutathione peroxidase) in the liver. Also, an increase in tissue-specific Cd burden and impaired morphometrics (hepato-somatic index and condition factor) were also recorded in fish following exposure to dietary Cd. In contrast, the dietary co-exposure to Cd and Se (at both medium and high doses) resulted in a decrease in Cd burden in the liver and kidney of fish. However, co-exposure to medium, but not high, dose of dietary Se completely alleviated Cd-induced oxidative stress and impaired morphometrics in fish, indicating that the reduced Cd tissue burden might not have been the primary factor behind the amelioration of Cd toxicity by Se. Overall, our study demonstrated that the protective effect of Se against the chronic Cd toxicity in fish is mainly mediated by the anti-oxidative properties of Se, but this protective effect is dose-specific and occurs only at a moderate exposure dose.

Distribution and speciation of zinc in the gills of rainbow trout (Oncorhynchus mykiss) during acute waterborne zinc exposure: Interactions with cadmium or copper

We utilized micro X-ray fluorescence imaging (μ-XFI) and micro X-ray absorption near-edge spectroscopy (μ-XANES), which are both synchrotron-based techniques to investigate Zn distribution profile, its co-localization patterns with Ca, S, and Fe and speciation in the gills of rainbow trout (RBT). Fish (~100 g) were exposed to acutely toxic levels of waterborne Zn alone and in combination with waterborne Cd or Cu for 24 h (each at 1 × 96 h LC₅₀). Gill sections were prepared and analyzed at the VESPERS beamline of the Canadian Light Source. The primary lamellae of the fish gill were found to be the primary area of Zn accumulation. These regions also correspond to the zones of mitochondria rich cells localization in fish gills, supporting the putative roles of these cells in metal uptake. Zn was also found to predominantly co-localize with Ca and S, but not with Fe, indicating the roles of Ca and S in intracellular Zn handling. Zn distribution in the gill was markedly reduced during co-exposure to Cd, but not to Cu, suggesting a competitive interaction between Zn and Cd for uptake. The speciation of Zn in the gill was dominated by Zn-phosphate, Zn-histidine and Zn-cysteine species; however, the interactions of Zn with Cd or Cu resulted in the loss of Zn-cysteine. Overall, our findings provide important novel insights into the interactions of Zn, Cd and Cu in the fish gill, which may ultimately help to explain the mechanisms underlying the acute toxicity of these metals in binary mixture to fish.

Chronic arsenicosis and cadmium exposure in wild snowshoe hares (Lepus americanus) breeding near Yellowknife, Northwest Territories (Canada), part 1: Evaluation of oxidative stress, antioxidant activities and hepatic damage

Previous gold mining activities and arsenopyrite ore roasting activities at the Giant mine site (1948 to 2004) resulted in the release of high amounts of arsenic and trace metals into the terrestrial and aquatic ecosystems of Yellowknife, Northwest Territories, Canada. While elevated levels of arsenic has been consistently reported in surface soils and vegetation near the vicinity of the Giant mine area and in surrounding locations, systematic studies evaluating the overall health status of terrestrial small mammals endemic to the area are lacking. The purpose of this present study was to evaluate and comparatively assess the biochemical responses and histopathological effects induced by chronic arsenic and cadmium exposure in wild snowshoe hares breeding near the city of Yellowknife, specifically around the vicinity of the abandoned Giant mine site and in reference locations. Analysis included measurement of total arsenic and cadmium concentration in nails, livers, kidneys, bones, stomach content of hares, in addition to histopathological evaluation of hepatic and ocular lesions. Biochemical responses were determined through measurement of lipid peroxidation levels and antioxidant enzymes activities (catalase, superoxide dismutase, glutathione peroxidase, and glutathione disulfide). The results revealed that arsenic concentration was 17.8 to 48.9 times higher in the stomach content, and in the range of 4 to 23 times elevated in the nails of hares from the mine area compared to the reference location. Arsenic and cadmium levels were also noted to be increased in the bones, renal and hepatic tissues of hares captured near the mine area compared to the reference site. Specifically, hares from the mine area showed nail cadmium levels that was 2.3 to 17.6 times higher than those from the reference site. Histopathological examination of the eyes revealed no specific ocular lesions, such as lens opacity (cataracts) or conjunctivitis; however, hares from both locations exhibited hepatic steatosis (fatty liver change). Lipid peroxidation levels were relatively increased and accompanied with reduced antioxidant enzyme activities in hares from the mine area compared to the hares from the reference site. The results of this preliminary study suggest that the snowshoe hares breeding near the vicinity of Yellowknife, including near the Giant mine area have been chronically exposed to elevated levels of arsenic and cadmium, which consequently led to the increased levels of oxidative stress and perturbation of antioxidant defense system in exposed animals. The results of this present study constitute the first observation of chronic arsenicosis in wild small mammal species in Canada.

Chronic arsenicosis and cadmium exposure in wild snowshoe hares (Lepus americanus) breeding near Yellowknife, Northwest Territories (Canada), part 2: Manifestation of bone abnormalities and osteoporosis

Various bone abnormalities, including osteoporosis, have been associated with chronic arsenic and cadmium exposure in experimental animal models, but information regarding the bone pathology of wild population of small mammals breeding in contaminated environment is limited. This present study was conducted to comparatively assess the prevalence and pattern of skeletal abnormalities in free ranging snowshoe hares inhabiting an area heavily contaminated by arsenic and other trace metals, near the vicinity of the abandoned Giant mine, and in a reference location approximately 20km from the city of Yellowknife, Northwest Territories, Canada. The femur and vertebrae of snowshoe hares from the mine area and reference location were subjected to bone densitometry examination and biomechanical testing using dual energy X-ray absorptiometry (DXA) and 3-point bending test. t-test results indicated that femoral densitometry parameters such as bone mineral density (BMD) (p=0.5), bone mineral content (BMC) (p=0.675), bone area (BA) (p=0.978) and tissue area (TA) (p=0.549) were not significantly different between locations. All densitometry parameters of the vertebrae (BMD, BA and TA) differed between locations (p<0.05), except for BMC (p=0.951) which showed no significant difference between the two locations. Vertebrae from the mine area also showed relatively lower BA and TA compared to the reference location. A constellation of skeletal abnormalities were also observed along the axial and appendicular bones respectively. Specifically, growth defects, osteoporosis, cortical fractures, sclerosis, and cyst like changes were commonly observed in the femurs and vertebrae of hares from both locations. With respect to biomechanical properties, only bone stiffness and peak load tended to be relatively reduced in specimens from the mine area, whereas work to failure was notably increased in specimens from the reference site compared to those from the mine area. Taken together, the results of this preliminary study suggest that chronic concomitant exposure to arsenic and cadmium may be involved in the etiology of various bone abnormalities, including osteoporosis in wild population of snowshoe hares from the Yellowknife area. The result presented in this study represent the first evaluation of osteological effects in free-ranging furbearers (snowshoe hares) diagnosed with arsenicosis, and concomitantly exposed to environmental levels of cadmium.

Chronic Dietary Selenomethionine Exposure Induces Oxidative Stress, Dopaminergic Dysfunction, and Cognitive Impairment in Adult Zebrafish (Danio rerio)

The present study was designed to investigate the effects of chronic dietary exposure to selenium (Se) on zebrafish cognition and also to elucidate possible mechanism(s) by which Se exerts its neurotoxicity. To this end, adult zebrafish were exposed to different concentrations of dietary l-selenomethionine (control, 2.3, 9.7, 32.5, or 57.7 μg Se/g dry weight) for 30 days. Cognitive performance of fish was tested using a latent learning paradigm in a complex maze. In addition, we also evaluated oxidative stress biomarkers and the expression of genes involved in dopaminergic neurotransmission in the zebrafish brain. Fish treated with higher dietary Se doses (32.5 and 57.5 μg Se/g) exhibited impaired performance in the latent learning task. The impaired learning was associated with the induction of oxidative stress and altered mRNA expression of dopamine receptors, tyrosine hydroxylase, and dopamine transporter genes in the zebrafish brain. Collectively, our results illustrate that cognitive impairment in zebrafish could be associated with Se-induced oxidative stress and altered dopaminergic neurotransmission in the brain.

Effects of chronic exposure to waterborne copper and nickel in binary mixture on tissue-specific metal accumulation and reproduction in fathead minnow (Pimephales promelas)

The current study evaluated the interactive effects of chronic waterborne copper (Cu) and nickel (Ni) exposure on tissue-specific metal accumulation and reproductive performance in fathead minnow (Pimephales promelas). Fish trios (1 male: 2 female; n = 5-6) were exposed for 21 days to: (i) control (no added Cu or Ni), (ii) waterborne Cu (45 μg/L), (iii) waterborne Ni (270 μg/L), and (iv) binary mixture of waterborne Cu and Ni (45 and 270 μg/L, respectively). Fish fecundity (cumulative egg production) was found to be the most sensitive reproductive endpoint, and the interaction of Cu and Ni elicited an additive effect on egg production. Tissue-specific accumulation of both metals was not influenced by the interaction of Cu and Ni, except an increased Cu and Ni burden in the carcass and ovary, respectively, were recorded. The expressions of hepatic estrogen receptor genes (ER-α and ER-β) and the circulating estradiol level in females were also not affected by the metal-mixture treatment. However, co-exposure to waterborne Cu and Ni resulted in a significant downregulation of the hepatic vitellogenin gene in females, which was associated with the maximum upregulation of the hepatic metallothionein gene. In addition, a significant alteration of ovarian histopathology (decreased abundance of post-vitellogenic follicles, and increased follicular atresia) was also observed only in females exposed to Cu and Ni in mixture. Collectively, these observations suggest that chronic waterborne exposure to Cu and Ni in binary mixture may impair fish reproductive capacity by inducing histopathological damage in ovarian tissue, and disrupting of energy homeostasis in fish.

Expression stability and selection of optimal reference genes for gene expression normalization in early life stage rainbow trout exposed to cadmium and copper

Gene expression analysis represents a powerful approach to characterize the specific mechanisms by which contaminants interact with organisms. One of the key considerations when conducting gene expression analyses using quantitative real-time reverse transcription-polymerase chain reaction (qPCR) is the selection of appropriate reference genes, which is often overlooked. Specifically, to reach meaningful conclusions when using relative quantification approaches, expression levels of reference genes must be highly stable and cannot vary as a function of experimental conditions. However, to date, information on the stability of commonly used reference genes across developmental stages, tissues and after exposure to contaminants such as metals is lacking for many vertebrate species including teleost fish. Therefore, in this study, we assessed the stability of expression of 8 reference gene candidates in the gills and skin of three different early life-stages of rainbow trout after acute exposure (24h) to two metals, cadmium (Cd) and copper (Cu) using qPCR. Candidate housekeeping genes were: beta actin (b-actin), DNA directed RNA polymerase II subunit I (DRP2), elongation factor-1 alpha (EF1a), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), glucose-6-phosphate dehydrogenase (G6PD), hypoxanthine phosphoribosyltransferase (HPRT), ribosomal protein L8 (RPL8), and 18S ribosomal RNA (18S). Four algorithms, geNorm, NormFinder, BestKeeper, and the comparative ΔCt method were employed to systematically evaluate the expression stability of these candidate genes under control and exposed conditions as well as across three different life-stages. Finally, stability of genes was ranked by taking geometric means of the ranks established by the different methods. Stability of reference genes was ranked in the following order (from lower to higher stability): HPRT<GAPDH<EF1a<G6PD<RPL8<DRP2<b-actin in gills of fish exposed to Cd; b-actin<GAPDH<G6PD<DRP2<RPL8<HPRT<EF1a in gills of fish exposed to Cu; RPL8<HPRT<GAPDH<G6PD<EF1a<DRP2<b-actin in the skin of fish exposed to Cd; and EF1a<GAPDH<RPL8<HPRT<G6PD<b-actin<DRP2 in the skin of fish exposed to Cu. The results demonstrated that the stability of reference genes depended on the metal, life-stage and/or organ in question. Thus, attention should be paid to these factors before selection of reference gene for relative quantification of the gene expressions.

Effects of chronic waterborne cadmium and zinc interactions on tissue-specific metal accumulation and reproduction in fathead minnow (Pimephales promelas)

The present study was designed to evaluate the interactive effects of chronic waterborne cadmium (Cd) and zinc (Zn) on tissue-specific metal accumulation and reproduction in fathead minnow (Pimephales promelas). Trios (1 male: 2 female; n=6-7) of fish were exposed for 21 days to: (i) control (no added Cd or Zn), (ii) waterborne Cd (7µg/L), (iii) waterborne Zn (170µg/L), and (iv) Cd and Zn in mixture (7 and 170µg/L, respectively). Exposure to Cd or Zn alone did not elicit any significant effect on reproductive output (cumulative egg production) relative to the control, however exposure to Cd and Zn in mixture resulted in a ~50% decrease in fish fecundity. Plasma estradiol in females was reduced by Cd and Zn exposures, both individually and in mixture, with the maximum reduction in the metal mixture exposure. The expression of hepatic estrogen receptor genes (ER-α and ER-β) in females was affected by exposure to Zn, alone and in mixture with Cd, but not to Cd alone, whereas hepatic vitellogenin gene expression was downregulated across all treatments. Increased follicular atresia in the ovary was also recorded, but only in fish exposed to Cd and Zn in mixture. The interactions of Cd and Zn in mixture decreased Cd accumulation in tissues (gill and liver), however no reciprocal reduction in tissue Zn accumulation was observed. In addition, the expression of the hepatic metallothionein gene was upregulated following exposure to Zn, alone and in combination with Cd, with no additive effects in the latter treatment. Overall, our findings suggest that chronic exposure to waterborne Cd and Zn in mixture may induce additive reproductive toxicity, essentially by disrupting estrogen-mediated functions in fish.