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.

Transgenerational plasticity of exploratory behavior and a hidden cost of mismatched risk environments between parental sexes

We require a better understanding of the relative contribution of different modes of non-genetic inheritance in behavioral trait development. Thus, we investigate variation in exploratory behavior, which is ecologically relevant and a target of selection. The metabolic hypothesis predicts exploratory behavior to be size-dependent across taxa. This size-dependency is cancelled out under high perceived risk, allowing us to determine the transgenerationally integrated estimated level of risk. Using fathead minnows Pimephales promelas, we manipulated perceived risk in mothers, fathers, caring males and offspring through continuous exposure to either conspecific alarm cues or to a control water treatment. In 1000 four-month old offspring, we determined body sizes and exploratory behavior. Perceived high risk in mothers, followed by personal risk, was most effective in eliminating size-dependent behavior whereas effects of paternal risk on offspring behavioral development were substantially weaker. When maternal risk is high, environmental mismatches between parents prevented offspring from responding appropriately to personal high risk. The environment of the caring male also impacted offspring behavior to a greater extent than that of its genetic parents. Our study highlights the high relative importance of maternal, personal and caring male risk environments and showcases potential costs of an environmental mismatch between parental sexes.

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.

Effects of common antiepileptic drugs on teleost fishes

Antiepileptic drugs (AEDs) are globally prescribed to treat epilepsy and many other psychiatric disorders in humans. Their high consumption, low metabolic rate in the human body and low efficiency of wastewater treatment plants (WWTPs) in eliminating these chemicals results in the frequent occurrence of these pharmaceutical drugs in aquatic systems. Therefore, aquatic organisms, including ecologically and economically important teleost fishes, may be inadvertently exposed to these chemicals. Due to their physiological similarity with humans, fishes may be particularly vulnerable to AEDs. Almost all AED drugs are detectable in natural aquatic ecosystems, but diazepam (DZP) and carbamazepine (CBZ) are among the most widely detected AEDs to date. Recent studies suggest that these drugs have a substantial capacity to induce neurotoxicity and behavioral abnormality in fishes. Here we review the current state of knowledge regarding the potential mode of action of DZP and CBZ as well as that of some other AEDs on teleosts and put observable behavioral effects into a mechanistic context. We find that following their intended mode of action in humans, AEDs also disrupt the GABAergic, glutamatergic and serotonergic systems as well as parasympathetic neurotransmitters in fishes. Moreover, AEDs have non-specific modes of action in teleosts ranging from estrogenic activity to oxidative stress. These physiological changes are often accompanied by dose-dependent disruptions of anxiety, locomotor activity, social behaviors, food uptake, and learning and memory, but DZP and CBZ consistently induced anxiolytic effects. Thereby, AED exposure severely compromises individual fitness across teleost fish species, which may lead to population and ecosystem impairment. We also showcase promising avenues for future research by highlighting where we lack data when it comes to effects of certain AEDs, AED concentrations and behavioral endpoints.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.