Deciphering the mode of action of pollutants impairing the fish larvae escape response with the vibrational startle response assay

Short- and Long-Term Neurobehavioral Effects of Developmental Exposure to Valproic Acid in Zebrafish

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impairments in social interaction and communication, anxiety, hyperactivity, and interest restricted to specific subjects. In addition to the genetic factors, multiple environmental factors have been related to the development of ASD. Animal models can serve as crucial tools for understanding the complexity of ASD. In this study, a chemical model of ASD has been developed in zebrafish by exposing embryos to valproic acid (VPA) from 4 to 48 h post-fertilization, rearing them to the adult stage in fish water. For the first time, an integrative approach combining behavioral analysis and neurotransmitters profile has been used for determining the effects of early-life exposure to VPA both in the larval and adult stages. Larvae from VPA-treated embryos showed hyperactivity and decreased visual and vibrational escape responses, as well as an altered neurotransmitters profile, with increased glutamate and decreased acetylcholine and norepinephrine levels. Adults from VPA-treated embryos exhibited impaired social behavior characterized by larger shoal sizes and a decreased interest for their conspecifics. A neurotransmitter analysis revealed a significant decrease in dopamine and GABA levels in the brain. These results support the potential predictive validity of this model for ASD research.

Cardiac and neurobehavioral impairments in three phylogenetically distant aquatic model organisms exposed to environmentally relevant concentrations of boscalid

Boscalid (2-Chloro-N-(4'-chlorobiphenyl-2-yl) nicotinamide), a pyridine carboxamide fungicide, is an inhibitor of the complex II of the respiration chain in fungal mitochondria. As boscalid is only moderately toxic for aquatic organisms (LC50 > 1-10 mg/L), current environmental levels of this compound in aquatic ecosystems, in the range of ng/L-μg/L, are considered safe for aquatic organisms. In this study, we have exposed zebrafish (Danio rerio), Japanese medaka (Oryzias latipes) and Daphnia magna to a range of concentrations of boscalid (1-1000 μg/L) for 24 h, and the effects on heart rate (HR), basal locomotor activity (BLA), visual motor response (VMR), startle response (SR), and habituation (HB) to a series of vibrational or light stimuli have been evaluated. Moreover, changes in the profile of the main neurotransmitters have been determined. Boscalid altered HR in a concentration-dependent manner, leading to a positive or negative chronotropic effect in fish and D. magna, respectively. While boscalid decreased BLA and increased VMR in Daphnia, these behaviors were not altered in fish. For SR and HB, the response was more species- and concentration-specific, with Daphnia exhibiting the highest sensitivity. At the neurotransmission level, boscalid exposure decreased the levels of L-aspartic acid in fish larvae and increased the levels of dopaminergic metabolites in D. magna. Our study demonstrates that exposure to environmental levels of boscalid alters cardiac activity, impairs ecologically relevant behaviors, and leads to changes in different neurotransmitter systems in phylogenetically distinct vertebrate and invertebrate models. Thus, the results presented emphasize the need to review the current regulation of this fungicide.

Assessments of carbon nanotubes toxicities in zebrafish larvae using multiple physiological and molecular endpoints

In recent years, carbon nanotubes (CNTs) have become one of the most promising materials for the technology industry. However, due to the extensive usage of these materials, they may be released into the environment, and cause toxicities to the organism. Here, their acute toxicities in zebrafish embryos and larvae were evaluated by using various assessments that may provide us with a novel perspective on their effects on aquatic animals. Before conducting the toxicity assessments, the CNTs were characterized as multiwall carbon nanotubes (MWCNTs) functionalized with hydroxyl and carboxyl groups, which improved their solubility and dispersibility. Based on the results, abnormalities in zebrafish behaviors were observed in the exposed groups, indicated by a reduction in tail coiling frequency and alterations in the locomotion as the response toward photo and vibration stimuli that might be due to the disruption in the neuromodulatory system and the formation of reactive oxygen species (ROS) by MWCNTs. Next, based on the respiratory rate assay, exposed larvae consumed more oxygen, which may be due to the injuries in the larval gill by the MWCNTs. Finally, even though no irregularity was observed in the exposed larval cardiac rhythm, abnormalities were shown in their cardiac physiology and blood flow with significant downregulation in several cardiac development-related gene expressions. To sum up, although the following studies are necessary to understand the exact mechanism of their toxicity, the current study demonstrated the environmental implications of MWCNTs in particularly low concentrations and short-term exposure, especially to aquatic organisms.

Short-term exposure to environmental levels of nicotine and cotinine impairs visual motor response in zebrafish larvae through a similar mode of action: Exploring the potential role of zebrafish α7 nAChR

The current view is that environmental levels of nicotine and cotinine, commonly in the ng/L range, are safe for aquatic organisms. In this study, 7 days post-fertilization zebrafish embryos have been exposed for 24 h to a range of environmental concentrations of nicotine (2.0 ng/L-2.5 μg/L) and cotinine (50 pg/L-10 μg/L), as well as to a binary mixture of these emerging pollutants. Nicotine exposure led to hyperactivity, decreased vibrational startle response and increased non-associative learning. However, the more consistent effect found for both nicotine and cotinine was a significant increase in light-off visual motor response (VMR). The effect of both pollutants on this behavior occurred through a similar mode of action, as the joint effects of the binary mixture of both chemicals were consistent with the concentration addition concept predictions. The results from docking studies suggest that the effect of nicotine and cotinine on light-off VMR could be mediated by zebrafish α7 nAChR expressed in retina. The results presented in this study emphasize the need to revisit the environmental risk assessment of chemicals including additional ecologically relevant sublethal endpoints.

Impact of environmentally relevant concentrations of fluoxetine on zebrafish larvae: From gene to behavior

Fluoxetine is widely prescribed for the treatment of depressive states, acting at the level of the central nervous system, consequently affecting non-target organisms. This study aimed to investigate the influence of environmentally relevant fluoxetine concentrations (1-1000 ng/L) on Danio rerio development, assessing both embryotoxicity and behavior, antioxidant defense, gene expression and neurotransmitter levels at larval stage. Exposure to fluoxetine during early development was found to be able to accelerate embryo hatching in embryos exposed to 1, 10 and 100 ng/L, reduce larval size in 1000 ng/L, and increase heart rate in 10, 100 and 1000 ng/L exposed larvae. Behavioral impairments (decreased startle response and increased larvae locomotor activity) were associated with effects on monoaminergic systems, detected through the downregulation of key genes (vmat2, mao, tph1a and th2). In addition, altered levels of neurochemicals belonging to the serotonergic and dopaminergic systems (increased levels of tryptophan and norepinephrine) highlighted the sensitivity of early life stages of zebrafish to low concentrations of fluoxetine, inducing effects that may compromise larval survival. The obtained data support the necessity to test low concentrations of SSRIs in environmental risk assessment and the use of biomarkers at different levels of biological organization for a better understanding of modes of action.

Integrate transcriptomic and metabolomic analysis reveals the underlying mechanisms of behavioral disorders in zebrafish (Danio rerio) induced by imidacloprid

Imidacloprid, a widely used neonicotinoid insecticide, poses a significant threat to aquatic ecosystems. Behavior is a functional indicator of the net sensory, motor, and integrative processes of the nervous system and is presumed to be more sensitive in detecting toxicity. In the present study, we investigated the behavioral effects of imidacloprid at the level of environmental concentrations (1, 10 and 100 μg/L) for a constant exposure to zebrafish adults, and performed the integrated transcriptomic and metabolomic analysis to analyze the molecular mechanism underlying behavioral effects of imidacloprid. Our results show that imidacloprid exposure significantly induce behavioral disruptions characterized by anxiety, depression, and reduced physiological function including exploratory, decision, social interaction and locomotor activity. Integrated transcriptomic and metabolomic analysis indicate that the disruption of circadian rhythm, metabolic imbalance of arginine and proline, and neurotransmitter disorder are the underlying molecular mechanisms of behavioral impairment induced by imidacloprid. The "gene-metabolite-disease" network consisted by 11 metabolites and 15 genes is associated human disease Alzheimer's disease (AD) and schizophrenia. Our results confirm the behavioral impairment induced by imidacloprid at environmental concentrations for constant exposure. The identified genes and metabolites can be used not only to illustrate the underlying mechanisms, but also can be developed as biomarkers in determining the ecological risk of imidacloprid to aquatic organisms even Homo sapiens.

Combined effects of high temperature and pesticide mixture exposure on free-swimming behaviors and hepatic cytochrome P450 1A expression in goldfish, Carassius auratus

The synergy between multiple compounds and other stressors, including heat, creates volatility and greater unpredictability than standard single-chemical toxicity testing, especially in the case of pesticides and metabolites which might contain several noxious ingredients resulting in adverse ecological effects. To address this, the aim of this study was to examine the dose- and time-dependent effects of low- and high-dose pesticide mixture (metalachlor, linuron, isoproturon, tebucanazole, aclonifen, atrazine, pendimethalin, azinphos-methyl) and heat stress co-exposure (22°C control/32°C treatment for 4-week) on free-swimming behaviors and cumulative actionless time (CAT) of goldfish. Behavioral analysis showed a dose- and time-dependent decrease in distance swam, as well as a subsequent increase in CAT. Vertical and horizontal spatial behavioral use were affected under heat and pesticides co-exposure conditions. In 3- and 4-week(s) exposure groups, horizontal spatial behavioral use demonstrated elevated time spent in the lower third of the aquarium. Similarly, during 3- and 4-week(s) exposure (32°C control and 32°C high doses) vertical spatial behavioral use was found to increase time spent in the outermost edges of the aquarium. In all treatment groups, the final condition factor (KM) showed significant attenuation when compared to the initial KM. However, there was an unclear relationship between heat/pesticide co-exposure and growth most notably in 32°C high-dose groups. In addition, the expression of hepatic cytochrome P450 1A mRNA was significantly higher in pesticide-exposed groups. Taken together, data demonstrated that co-exposure with low- or high-dose pesticide mixture and heat stress significantly impacted natural swimming patterns, which over time might result in the broader population and ecological effects.

Mepanipyrim induces visual developmental toxicity and vision-guided behavioral alteration in zebrafish larvae

Mepanipyrim, an anilinopyrimidine fungicide, has been extensively used to prevent fungal diseases in fruit culture. Currently, research on mepanipyrim-induced toxicity in organisms is still very scarce, especially visual developmental toxicity. Here, zebrafish larvae were employed to investigate mepanipyrim-induced visual developmental toxicity. Intense light and monochromatic light stimuli-evoked escape experiments were used to investigate vision-guided behaviors. Meanwhile, transcriptomic sequencing and real-time quantitative PCR assays were applied to assess the potential mechanisms of mepanipyrim-induced visual developmental toxicity and vision-guided behavioral alteration. Our results showed that mepanipyrim exposure could induce retinal impairment and vision-guided behavioral alteration in larval zebrafish. In addition, the grk1b gene of the phototransduction signaling pathway was found to be a potential aryl hydrocarbon receptor (AhR)-regulated gene. Mepanipyrim-induced visual developmental toxicity was potentially related to the AhR signaling pathway. Furthermore, mepanipyrim-induced behavioral alteration was guided by the visual function, and the effects of mepanipyrim on long and middle wavelength light-sensitive opsins may be the main cause of vision-guided behavioral alteration. Our results provide insights into understanding the relationship between visual development and vision-guided behaviors induced by mepanipyrim exposure.

Characterization of locomotor phenotypes in zebrafish larvae requires testing under both light and dark conditions

Despite growing knowledge, much remains unknown regarding how signaling within neural networks translate into specific behaviors. To pursue this quest, we need better understanding of the behavioral output under different experimental conditions. Zebrafish is a key model to study the relationship between network and behavior and illumination is a factor known to influence behavioral output. By only assessing behavior under dark or light conditions, one might miss behavioral phenotypes exclusive to the neglected illumination setting. Here, we identified locomotor behavior, using different rearing regimes and experimental illumination settings, to showcase the need to assess behavior under both light and dark conditions. Characterization of free-swimming zebrafish larvae, housed under continuous darkness or a day/night cycle, did not reveal behavioral differences; larvae were most active during light conditions. However, larvae housed under a day/night cycle moved a shorter distance, had lower maximum velocity and maximum acceleration during the startle response under light conditions. Next, we explored if we could assess behavior under both dark and light conditions by presenting these conditions in sequence, using the same batch of larvae. Our experiments yielded similar results as observed for naïve larvae: higher activity during light conditions, regardless of order of illumination (i.e. dark-light or light-dark). Finally, we conducted these sequenced illumination conditions in an experimental setting by characterizing behavioral phenotypes in larvae following neuromast ablation. Depending on the illumination during testing, the behavioral phenotype following ablation was characterized differently. In addition, the results indicate that the order in which the light and dark conditions are presented has to be considered, as habituation may occur. Our study adds to existing literature on illumination-related differences in zebrafish behavior and emphasize the need to explore behavioral phenotypes under both light and dark condition to maximize our understanding of how experimental permutations affect behavior.

Morphological and cardiac alterations after crude oil exposure in the early-life stages of the tropical gar (Atractosteus tropicus)

Fish development can be affected by environmental pollutants such as crude oil (anthropogenic or natural sources), causing alterations especially in cardiac function and morphology. Most such studies have focused on saltwater species, whereas studies in freshwater fishes are scant. The objective of the current study was to evaluate the effects of crude oil exposure (as 0, 5, 10, 15, or 20% high-energy water accommodated fractions, HEWAF) on cardiac function and edema formation during two early periods of development (embryo and eleuteroembryo, 48 h each) individually using the tropical gar Atractosteus tropicus as a model. Embryos did not exhibit alterations in body mass, total length, condition factor, and cardiac function as a function of oil. In contrast, eleuteroembryos proved to be more sensitive and exhibited increased body mass, total length, and condition factor, decreased heart rate and phenotypic alterations such as cardiac dysmorphia (tubular hearts) and spine curvature at high concentrations of HEWAF. Moreover, edema formation was observed in both stages This study shows different functional responses of A. tropicus after crude oil exposure and provides useful information of the developmental impacts of these compounds on the early life stages of freshwater tropical fishes.

Behavioral changes and neurochemical responses in Chinese rare minnow exposed to four psychoactive substances

With the increase use of psychoactive pharmaceuticals, these substances and their metabolites are frequently detected in aquatic environment. However, there is still a knowledge gap in the neurotoxicity of these pollutants on aquatic organisms as well as related behavioral effects. In this study, the effects of four psychoactive substances alprazolam (ALPZ), lorazepam (LORZ), codeine (COD) and morphine (MOR) were investigated on 23 neurochemicals and 5 behaviors in Chinese rare minnow (Gobiocypris rarus). The comprehensive neurotoxicity was then evaluated at three levels of neurochemical, neurotransmitter system and comprehensive index. The results indicated that ALPZ and LORZ not only increased serotonin and dopamine along with the decrease of glutamic acid, but also depressed the locomotory activity of Chinese rare minnow although without significance. Exposure to COD and MOR increased acetylcholine, dopamine and adrenaline, and significantly increased anxiety-related behaviors of Chinese rare minnow. Comprehensive evaluation showed that COD has the lowest neurotoxic effect on Chinese rare minnow. LORZ shows a stronger neurotoxicity at low concentration of exposure than the other three substances. MOR has the highest neurotoxic effect at high concentration of exposure among the four drugs. The findings revealed the comprehensive neurotoxicity of these psychoactive substances in fish and suggested ecological risks of these pollutants in aquatic environment.

Aqueous stability and degradation of psychiatric and neuroactive compounds and its biological activity in Daphnia magna

Pharmaceuticals and other emerging contaminants are continuously released into the aquatic environment, considered as 'pseudo-persistent' pollutants. Many compounds degrade fast in the environment, but sometimes their transformation products (TPs) are equally or even more toxic than the parental compounds, raising concern about the potential risks to the environment. In this way, the crustacean Daphnia magna (D. magna) is one of the most widely used organisms in aquatic toxicology studies, since it is an interesting non-vertebrate model to study via neurotransmitters the toxicological consequences of contaminants. In this study, the stability in water of 17 neuroactive compounds using ultra-high-performance liquid chromatography (UHPLC) coupled to a MS/MS detector was evaluated. In order to assess the stability of the compounds, samples of 1 ng μL^-1 were analyzed at different times (0, 24 and 48 h). No degradation was observed for most of the studied compounds, except for apomorphine and 6-hydroxydopamine that were degraded completely in the first 24 h. The behavioral assay was based in the automatized delivery of visible light stimuli. Most of the tested compounds altered motile responses to light significantly. The pharmaceuticals memantine, imidacloprid, fluoxetine, deprenyl, diazepam, apomorphine and 6-hydroxydopamine decreased motile responses to light. Conversely, pilocarpine, scopolamine, nicotine and p-chlorophenylalanine increased motile responses. Despite the observed degradation of apomorphine and 6-hydroxydopamine, their degradation products (APO-TP1 and 6OH-TP1) were stable and so their effects on behavior. This study shows that a degradation or transformation of the main pollutant is not always linked to a decrease in its toxicity.

The role of auditory and vibration stimuli in zebrafish neurobehavioral models

Strongly affecting human and animal physiology, sounds and vibration are critical environmental factors whose complex role in behavioral and brain functions necessitates further clinical and experimental studies. Zebrafish are a promising model organism for neuroscience research, including probing the contribution of auditory and vibration stimuli to neurobehavioral processes. Here, we summarize mounting evidence on the role of sound and vibration in zebrafish behavior and brain function, and outline future directions of translational research in this field. With the growing environmental exposure to noise and vibration, we call for more active use of zebrafish models for probing neurobehavioral and bioenvironmental consequences of acute and long-term exposure to sounds and vibration in complex biological systems.

Can the insecticide Imidacloprid affect the health of the Neotropical freshwater fish Astyanax altiparanae (Teleostei: Characidae)?

Female juveniles of the Neotropical fish Astyanax altiparanae were exposed for 96 h to four treatments containing the active ingredient from Imidacloprid® commercial formulation (IMI 1, IMI 2, IMI 3, and IMI 4) and to a control treatment (only dechlorinated tap water). Glutathione content, glutathione S-transferase activity, lipid peroxidation (LPO) and protein carbonylation levels, acetylcholinesterase (AChE) activity, and frequency of micronuclei and erythrocyte nuclear abnormalities (ENA) were measured in the fish. The muscle and gills were the most affected organs; their antioxidant defense was not enough to prevent oxidative damage (LPO) in the IMI 2 and IMI 4 treatment fish. IMI also inhibited AChE activity in the muscle (IMI 3 and IMI 4) and increased ENA frequency (IMI 4). IMI can affect the health of A. altiparanae in environmentally relevant concentrations, causing oxidative damage in different organs, neurotoxic effects in the muscle, and genotoxicity.

Applications of advanced neuro-behavioral analysis strategies in aquatic ecotoxicology

Despite mounting evidence of pleiotropic ecological risks, the understanding of the eco-neurotoxic impact of most industrially relevant chemicals is still very limited. In particularly the acute and chronic exposures to industrial pollutants on nervous systems and thus potential alterations in ecological fitness remain profoundly understudied. Since the behavioral phenotype is the highest-level and functional manifestation of integrated neurological functions, the alterations in neuro-behavioral traits have been postulated as very sensitive and physiologically integrative endpoints to assess eco-neurotoxicological risks associated with industrial pollutants. Due to a considerable backlog of risk assessments of existing and new production chemicals there is a need for a paradigm shift from high cost, low throughput ecotoxicity test models to next generation systems amenable to higher throughput. In this review we concentrate on emerging aspects of laboratory-based neuro-behavioral phenotyping approaches that can be amenable for rapid prioritizing pipelines. We outline the importance of development and applications of innovative neuro-behavioral assays utilizing small aquatic biological indicators and demonstrate emerging concepts of high-throughput chemo-behavioral phenotyping. We also discuss new analytical approaches to effectively and rapidly evaluate the impact of pollutants on higher behavioral functions such as sensory-motor assays, decision-making and cognitive behaviors using innovative model organisms. Finally, we provide a snapshot of most recent analytical approaches that can be applied to elucidate mechanistic rationale that underlie the observed neuro-behavioral alterations upon exposure to pollutants. This review is intended to outline the emerging opportunities for innovative multidisciplinary research and highlight the existing challenges as well barriers to future development.

Characterization of neurotransmitters and related metabolites in Daphnia magna juveniles deficient in serotonin and exposed to neuroactive chemicals that affect its behavior: A targeted LC-MS/MS method

Neurotransmitters are endogenous metabolites that play a crucial role within an organism, at the chemical synapses. There is a growing interest in their analytical determination for understanding the neurotoxic effect of contaminants. Daphnia magna represents an excellent aquatic model for these environmental studies, due to its similarities with vertebrates in several neurotransmitters and related gene pathways and because of its wide application in ecotoxicological studies. Within this study, an accurate and sensible method of analysis of 17 neurotransmitters and related precursors and metabolites was developed. The method was validated in terms of sensitivity, reproducibility, precision, and accuracy, and also matrix effect was evaluated. As an independent probe of method validation and applicability, the method was applied to two different scenarios. First, it was used for the study of neurotransmitter levels in genetically mutated tryptophan hydrolase D. magna clones, confirming the absence of serotonin and its metabolite 5-HIAA. Additionally, the method was applied for determining the effects of chemical compounds known to affect different neurotransmitter systems and to alter Daphnia behavior. Significant changes were observed in 13 of the analyzed neurotransmitters across treatments, which were related to the neurotransmitter systems described as being affected by these neurochemicals. These two studies, which provide results on the ways in which the neurotransmitter systems in D. magna are affected, have corroborated the applicability of the presented method, of great importance due to the suitability of this organism for environmental neurotoxicity studies.

Glyphosate targets fish monoaminergic systems leading to oxidative stress and anxiety

Glyphosate is the active ingredient of some of the most highly produced and used herbicides worldwide. The intensive applications of glyphosate-based herbicides and its half-life in water lead to its presence in many aquatic ecosystems. Whereas recent studies have reported neurotoxic effects of glyphosate including autism-related effects, most of them used extremely high (mg/L to g/L) concentrations, so it is still unclear if chronic, low environmentally relevant concentrations of this compound (ng/L to μg/L) can induce neurotoxicity. In this study we analyzed the neurotoxicity of glyphosate in adult zebrafish after waterborne exposure to environmentally relevant concentrations (0.3 and 3 μg/L) for two weeks. Our data showed that exposed fish presented a significant impairment of exploratory and social behaviors consistent with increased anxiety. The anterior brain of the exposed fish presented a significant increase in dopamine and serotonin levels, as well as in the DOPAC/dopamine and homovanillic acid/dopamine turnover ratios. Moreover, the expression of genes involved in the dopaminergic system, as th1, th2, comtb, and scl6a3 was downregulated. Finally, the brain of exposed fish presented a significant increase in the catalase and superoxide dismutase activities, with a concomitant decrease of glutathione stores. These changes in the antioxidant defense system are consistent with the observed increase in oxidative stress, reflected by the increase in the levels of lipid peroxidation in the brain. The presented results show that current glyphosate concentrations commonly found in many aquatic ecosystems may have detrimental consequences on fish survival by decreasing exploration of the environment or altering social interactions. Furthermore, as zebrafish is also a vertebrate model widely used in human neurobehavioral studies, these results are relevant not only for environmental risk assessment, but also for understanding the risk of chronic low-dose exposures on human health.

Comparison of the Zebrafish Embryo Toxicity Assay and the General and Behavioral Embryo Toxicity Assay as New Approach Methods for Chemical Screening

The movement away from mammalian testing of potential toxicants and new chemical entities has primarily led to cell line testing and protein-based assays. However, these assays may not yet be sufficient to properly characterize the toxic potential of a chemical. The zebrafish embryo model is widely recognized as a potential new approach method for chemical testing that may provide a bridge between cell and protein-based assays and mammalian testing. The Zebrafish Embryo Toxicity (ZET) model is increasingly recognized as a valuable toxicity testing platform. The ZET assay focuses on the early stages of embryo development and is considered a more humane model compared to adult zebrafish testing. A complementary model has been developed that exposes larvae to toxicants at a later time point during development where body patterning has already been established. Here we compare the toxicity profiles of 20 compounds for this General and Behavioral Toxicity (GBT) assay to the ZET assay. The results show partially overlapping toxicity profiles along with unique information provided by each assay. It appears from this work that these two assays applied together can strengthen the use of zebrafish embryos/larvae as standard toxicity testing models.

A high-throughput assay for screening environmental pollutants and drugs impairing predator avoidance in Daphnia magna

This study addresses short-term habituation of the escape response in the aquatic crustacean Daphnia magna evoked by sudden changes in light intensity, using a high-throughput system. Daphnia magna exhibits a marked phototactic behaviour and swim away from light to avoid predation by fish. Currently, there is no information available on the habituation of this phototactic response. The Daphnia photomotor response assay (DPRA) measures the distance moved after a sudden increase in light intensity. Using DPRA, it is possible to determine not only the magnitude of the phototactic response, but also its habituation after repetitive cycles of light and darkness. The progressive reduction observed in response to a series of light stimuli in the proposed assay meet the criteria for habituation. Most cholinergic and serotonergic modulators enhanced photomotor responses and reduced habituation. Dopaminergic and histaminergic modulators also reduced habituation, whereas diazepam was the only compound that increased habituation. Imidacloprid, apomorphine, diphenhydramine, diazepam, and memantine decreased photomotor responses. Thus, the DPRA was also predictive in assessing the effects of neuroactive and neurotoxic environmental contaminants such as selective serotonin reuptake inhibitors, diazepam, organophosphorous, and neonicotinoid pesticides. We conclude that the proposed DPRA may be an effective screening tool for compounds that can impair predation avoidance behaviour in aquatic organisms.

Screening anti-predator behaviour in fish larvae exposed to environmental pollutants

Predation is one of the main sources of mortality for fish larvae. During evolution, they have developed different anti-predator behaviours, as the vibrational-evoked startle response and its habituation, for promoting survival to predator's strikes. Whereas these two behaviours can be altered by the exposure to some neurotoxicants, it is currently unknown if the exposure to environmentally relevant concentration (ERC) of neurotoxic pollutants could impair them. In this study thirty neurotoxic environmental pollutants from nine chemical groups, including: herbicides; carbamate, organophosphate (OP), organochlorine (OC), neonicotinoid and pyrethroid insecticides; toxins; metal and non-metal elements, have been screened at two concentrations, including one environmental relevant concentration (ERC), for adverse effects on anti-predator behaviours by using the Vibrational Startle Response Assay on zebrafish larvae. Significant effects over anti-predator responses were equally observed in both exposure concentrations. Focusing on the ERC scenario, it was found that the startle response was the less affected behaviour, where ten pollutants from all chemical groups except for organochlorine, neonicotinoid and pyrethroids, altered this response. Interestingly, organic and inorganic pollutants showed opposite effects on this response: whereas all organic pollutants decreased the startle response, the three remaining inorganic pollutants increased it. On the other hand, more pollutants affected habituation of the startle response of the larvae, where thirteen of the pollutants from all groups, except for herbicides, altered this behaviour at ERC, generally resulting in a faster habituation except for one OP and one marine toxin, which were able to delay this response. Ultimately, only one chemical from the OP, toxin, metal and non-metal element groups altered both the startle response and its habituation at both ERC and WSC. These results emphasize the environmental risk of the current levels of some neurotoxicants present in our aquatic ecosystems, as they are high enough to impair essential anti-predator behaviours in fish larvae.