Acute neuroactive drug exposures alter locomotor activity in larval zebrafish

rbfox1 loss of function in zebrafish leads to dysregulation in bdnf/trkb2 and pac1a expression resulting in HPI axis hyperactivation, altered stress response and allostatic overload

RBFOX1 regulates transcriptional networks linked to synaptic transmission and neurodevelopment. Mutations in the RBFOX1 gene are associated with psychiatric disorders but how RBFOX1 influences psychiatric disorder vulnerability remains unclear. Recent studies showed that RBFOX1 mediates the alternative splicing of PAC1, a critical HPA axis activator. Further, RBFOX1 dysfunction is linked to dysregulation of BDNF/TrkB, a pathway promoting neuroplasticity, neuronal survival, and stress resilience. Hence, RBFOX1 dysfunction may increase psychiatric disorder vulnerability via HPA axis dysregulation, leading to disrupted development and allostatic overload. To test this hypothesis, we generated a zebrafish rbfox1 loss of function (LoF) line and examined behavioural and molecular effects during development. In larvae and adults, rbfox1 LoF resulted in hyperactivity, impulsivity and hyperarousal, and alterations in proliferation, fertility and survival, traits associated with allostatic overload. In larvae, rbfox1 LoF disrupted expression of pac1a, bdnf, trkb2, and HPI axis genes. These latter were restored after chronic TrkB agonist/antagonist treatment. In adults, bdnf/trkb2 and HPI axes dysregulation was only seen following acute stress. Our findings revealed a strict interplay between RBFOX1 and BDNF/TrkB in stress resilience and suggest that RBFOX1 LoF predisposes to psychiatric diseases through HPA axis hyperactivation during development, impairing adaptation and heightening vulnerability to allostatic overload.

Prolonged exposure to the synthetic glucocorticoid dexamethasone induces brain damage via oxidative stress and apoptotic response in adult Daniorerio

Due to its extensive use as a painkiller, anti-inflammatory, and immune modulatory agent, as well as its effectiveness in treating severe COVID-19, dexamethasone, a synthetic glucocorticoid, has gained attention not only for its impact on public health but also for its environmental implications. Various studies have reported its presence in aquatic environments, including urban waters, surface samples, sediments, drinking water, and wastewater effluents. However, limited information is available regarding its toxic effects on nontarget aquatic organisms. Therefore, this study aimed to investigate the mechanism of toxicity underlying dexamethasone-induced brain damage in the bioindicator Danio rerio following long-term exposure. Adult zebrafish were treated with environmentally relevant concentrations of dexamethasone (20, 40, and 60 ng L-1) for 28 days. To elucidate the possible mechanisms involved in the toxicity of the pharmaceutical compound, we conducted a behavioral test battery (Novel Tank and Light and Dark tests), oxidative stress biomarkers, acetylcholinesterase enzyme activity quantification, histopathological analysis, and gene expression analysis using qRT-PCR (p53, bcl-2, bax, caspase-3, nrf1, and nrf2).The results revealed that the pharmaceutical compound could produce anxiety-like symptoms, increase the oxidative-induced stress response, decrease the activity of acetylcholinesterase enzyme, and cause histopathological alterations, including perineuronal vacuolization, granular and molecular layers deterioration, cell swallowing and intracellular spaces. The expression of genes involved in the apoptotic process (p53, bax, and casp-3) and antioxidant defense (nrf1 and nrf2) was upregulated in response to oxidative damage, while the expression of the anti-apoptotic gene bcl-2 was down-regulated indicating that the environmental presence of dexamethasone may pose a threat to wildlife and human health.

A unified approach to investigating 4 dpf zebrafish larval behaviour through a standardised light/dark assay

Zebrafish are a dynamic research model in the domains of neuropsychopharmacology, biological psychiatry and behaviour. Working with larvae ≤4 days post-fertilisation (dpf) offers an avenue for high-throughput investigation whilst aligning with the 3Rs principles of animal research. The light/dark assay, which is the most widely used behavioural assay for larval neuropharmacology research, lacks experimental reliability and standardisation. This study aimed to formulate a robust, reproducible and standardised light/dark behavioural assay using 4 dpf zebrafish larvae. Considerable between-batch and inter-individual variability was found, which we rectified with a normalisation approach to ensure a reliable foundation for analysis. We then identified that 5-min light/dark transition periods are optimal for locomotor activity. We also found that a 30-min acclimation in the light was found to produce significantly increased dark phase larval locomotion. Next, we confirmed the pharmacological predictivity of the standardised assay using ethanol which, as predicted, caused hyperlocomotion at low concentrations and hypolocomotion at high concentrations. Finally, the assay was validated by assessing the behavioural phenotype of hyperactive transgenic (adgrl3.1-/-) larvae, which was rescued with psychostimulant medications. Our standardised assay not only provides a clear experimental and analytical framework to work with 4 dpf larvae, but also facilitates between-laboratory collaboration using our normalisation approach.

Ecotoxicity Assessment of α-Amino Acid-Derived Polyamidoamines Using Zebrafish as a Vertebrate Model

The aquatic ecotoxicity of three α-amino acid-derived polyamidoamines (PAAs) was studied using zebrafish embryos as a viable vertebrate model organism. The PAAs examined were water-soluble amphoteric polyelectrolytes with a primarily negative charge, which were efficient flame retardants for cotton. The fish embryo acute toxicity test performed with PAA water solutions using 1.5-500 mg L-1 concentrations showed that toxicity did not statistically differ from the control. The survival rates were indeed >90%, even at the highest concentration; the hatching rates were >80%; and the numbers of morphological defects were comparable to those of the control. Tests using transgenic zebrafish lines indicated that the numbers of microscopic vascular and musculoskeletal defects were comparable to the control, with one random concentration showing doubled alterations. Sensory-motor tests in response to visual and tactile stimuli were also performed. In the presence of PAAs, embryos exposed to alternating light/dark cycles showed an insignificant mobility reduction during the dark phase. Touch-evoked response tests revealed a mild effect of PAAs on the neuromotor system at concentrations > 10 mg L-1. The cystine/glycine copolymer at 100 mg L-1 exhibited the greatest effect. Overall, the studied PAAs showed a minimal impact on aquatic systems and should be further considered as promising ecofriendly materials.

Advancing statistical treatment of photolocomotor behavioral response study data

Fish photolocomotor behavioral response (PBR) studies have become increasingly prevalent in pharmacological and toxicological research to assess the environmental impact of various chemicals. There is a need for a standard, reliable statistical method to analyze PBR data. The most common method currently used, univariate analysis of variance (ANOVA), does not account for temporal dependence in observations and leads to incomplete or unreliable conclusions. Repeated measures ANOVA, another commonly used method, has drawbacks in its interpretability for PBR study data. Because each observation is collected continuously over time, we instead consider each observation to be a function and apply functional ANOVA (FANOVA) to PBR data. Using the functional approach not only accounts for temporal dependency but also retains the full structure of the data and allows for straightforward interpretation in any subregion of the domain. Unlike the traditional univariate and repeated measures ANOVA, the FANOVA that we propose is nonparametric, requiring minimal assumptions. We demonstrate the disadvantages of univariate and repeated measures ANOVA using simulated data and show how they are overcome by applying FANOVA. We then apply one-way FANOVA to zebrafish data from a PBR study and discuss how those results can be reproduced for future PBR studies.

Multilevel assessment of carbamazepine effects: An integrative approach using zebrafish early-life stages

Carbamazepine (CBZ) is the most commonly used drug in epilepsy treatment, and its metabolites are commonly detected among persistent pharmaceuticals in the aquatic environment. This study aimed to investigate CBZ effects on early-life-stage zebrafish (Danio rerio) (from 2 to 168 hpf) by employing of an integrative approach linking endpoints from molecular to individual level: (i) development; (ii) locomotor activity; (iii) biochemical markers (lactate dehydrogenase, glutathione-S-transferase, acetylcholinesterase and catalase) and (iv) transcriptome analysis using microarray. A 168 h - LC50 of 73.4 mg L-1 and a 72 h - EC50 of 66.8 mg L-1 for hatching were calculated while developmental effects (oedemas and tail deformities) were observed at CBZ concentrations above 37.3 mg L-1. At the biochemical level, AChE activity proved to be the most sensitive parameter, as evidenced by its decrease across all concentrations tested (∼25% maximum reduction, LOEC (lowest observed effect concentration) < 0.6 μg L-1). Locomotor behaviour seemed to be depressed by CBZ although this effect was only evident at the highest concentration tested (50 mg L-1). Molecular analysis revealed a dose-dependent effect of CBZ on gene expression. Although only 25 genes were deregulated in organisms exposed to CBZ when compared to controls, both 0.6 and 2812 μg L-1 treatments impaired gene expression related to development (e.g. crygmxl1, org, klf2a, otos, stx16 and tob2) and the nervous system (e.g. Rtn3, Gdf10, Rtn3), while activated genes were associated with behavioural response (e.g. prlbr and taar). Altogether, our results indicate that environmentally relevant CBZ concentrations might affect biochemical and genetic traits of fish. Thus, the environmental risk of CBZ cannot be neglected, especially in a realistic scenario of constant input of domestic effluents into aquatic systems.

Testing of putative antiseizure medications in a preclinical Dravet syndrome zebrafish model

Dravet syndrome is a severe genetic epilepsy primarily caused by de novo mutations in a voltage-activated sodium channel gene (SCN1A). Patients face life-threatening seizures that are largely resistant to available anti-seizure medications. Preclinical Dravet syndrome animal models are a valuable tool to identify candidate anti-seizure medications for these patients. Among these, scn1lab mutant zebrafish, exhibiting spontaneous seizure-like activity, are particularly amenable to large-scale drug screening. Thus far, we have screened more than 3000 drug candidates in scn1lab zebrafish mutants, identifying valproate, stiripentol, and fenfluramine e.g. Food and Drug Administration-approved drugs, with clinical application in the Dravet syndrome population. Successful phenotypic screening in scn1lab mutant zebrafish is rigorous and consists of two stages: (i) a locomotion-based assay measuring high-velocity convulsive swim behaviour and (ii) an electrophysiology-based assay, using in vivo local field potential recordings, to quantify electrographic seizure-like events. Historically, nearly 90% of drug candidates fail during translation from preclinical models to the clinic. With such a high failure rate, it becomes necessary to address issues of replication and false positive identification. Leveraging our scn1lab zebrafish assays is one approach to address these problems. Here, we curated a list of nine anti-seizure drug candidates recently identified by other groups using preclinical Dravet syndrome models: 1-Ethyl-2-benzimidazolinone, AA43279, chlorzoxazone, donepezil, lisuride, mifepristone, pargyline, soticlestat and vorinostat. First-stage locomotion-based assays in scn1lab mutant zebrafish identified only 1-Ethyl-2-benzimidazolinone, chlorzoxazone and lisuride. However, second-stage local field potential recording assays did not show significant suppression of spontaneous electrographic seizure activity for any of the nine anti-seizure drug candidates. Surprisingly, soticlestat induced frank electrographic seizure-like discharges in wild-type control zebrafish. Taken together, our results failed to replicate clear anti-seizure efficacy for these drug candidates highlighting a necessity for strict scientific standards in preclinical identification of anti-seizure medications.

The concentration of dissolved organic matter impacts the neurobehavior in female zebrafish exposed to cyclophosphamide

Cyclophosphamide (CP) is a broad-spectrum anticancer drug for various cancers and frequently detected in aquatic environments, reaching concentrations up to 22 μg/L. However, there is limited understanding of the toxicities of CP with the presence of dissolved organic matter, a ubiquitous component in aquatic environments, in fish. In this study, we investigated the behaviors, morphological alterations of retina, and related gene transcripts in zebrafish exposed to CP (0 and 50 μg/L) and Humic acid (HA, a main component of DOM) at concentrations of 0, 3, 10, and 30 mg-C/L for 30 days. The results showed that, relative to the zebrafish in CP treatment, HA at 30 mg-C/L increased the locomotion (12.1 % in the light and 7.2 % in the dark) and startle response (9.7 %), while inhibiting the anxiety (12.5 %) and cognition of female zebrafish (24.6 %). The levels of transcripts of neurotransmitter- (tph1b and ache), neuroinflammation-(il-6 and tnfα) and antioxidant-(gpx) related genes in the brain of female adult were also altered by CP with the presence of HA. In addition, HA promoted the transgenerational effects of CP on the neurobehaviors. Therefore, HA can enhance potential neurotoxicity of CP in female fish through alteration neurotransmission related genes. Our findings provide new insights into the toxicity and underlying mechanisms of CP with the presence of dissolved organic matter, thereby contribute to a deeper understanding of the risks posed by CP in aquatic ecosystems.

Carbon nanotubes and nanofibers seen as emerging threat to fish: Historical review and trends

Since the discovery of the third allotropic carbon form, carbon-based one-dimensional nanomaterials (1D-CNMs) became an attractive and new technology with different applications that range from electronics to biomedical and environmental technologies. Despite their broad application, data on environmental risks remain limited. Fish are widely used in ecotoxicological studies and biomonitoring programs. Thus, the aim of the current study was to summarize and critically analyze the literature focused on investigating the bioaccumulation and ecotoxicological impacts of 1D-CNMs (carbon nanotubes and nanofibers) on different fish species. In total, 93 articles were summarized and analyzed by taking into consideration the following aspects: bioaccumulation, trophic transfer, genotoxicity, mutagenicity, organ-specific toxicity, oxidative stress, neurotoxicity and behavioral changes. Results have evidenced that the analyzed studies were mainly carried out with multi-walled carbon nanotubes, which were followed by single-walled nanotubes and nanofibers. Zebrafish (Danio rerio) was the main fish species used as model system. CNMs' ecotoxicity in fish depends on their physicochemical features, functionalization, experimental design (e.g. exposure time, concentration, exposure type), as well as on fish species and developmental stage. CNMs' action mechanism and toxicity in fish are associated with oxidative stress, genotoxicity, hepatotoxicity and cardiotoxicity. Overall, fish are a suitable model system to assess the ecotoxicity of, and the environmental risk posed by, CNMs.

Preclinical toxicological assessment of polydatin in zebrafish model

Polydatin (3,4',5-trihydroxystilbene-3-β-D-glucoside, piceid), a natural stilbenoid found in different plant sources, has gained increasing attention for its potential health benefits. However, prior to its widespread adoption in human therapeutics and consumer products, a comprehensive investigation of its toxicological effects is crucial. In this study, the toxicity of polydatin was investigated in a developmental toxicity test using zebrafish (Danio rerio) as a valuable model for preclinical assessments. We employed the Fish Embryo Test (FET test - OECD n°236) to investigate the effects of polydatin on survival, hatchability, development, and behavior of zebrafish embryo-larval stage. Remarkably, the results demonstrated that polydatin up to 435 μM showed no toxicity. Throughout the exposure period, zebrafish embryos exposed to polydatin exhibited normal development, with no significant mortality observed. Furthermore, hatching success and heartbeat rate were unaffected, and no morphological abnormalities were identified, signifying a lack of teratogenic effects and cardiotoxicity. Locomotion activity assessment revealed normal swimming patterns and response to stimuli, indicating no neurotoxic effects. Our study provides valuable insights into the toxicological profile of polydatin, suggesting that it may offer potential therapeutic benefits under a considerable concentration range. In addition, zebrafish model proves to be an efficient system for early-stage toxicological screening, guiding further investigations into the secure utilization of polydatin for human health and wellness.

Effects of curcumin to counteract levodopa-induced toxicity in zebrafish

Parkinson's disease (PD) is a neurodegenerative disorder characterized by motor dysfunction due to the death of dopaminergic neurons in the substantia nigra pars compacta. Currently, treatment of PD has focused on increasing dopamine levels, using a dopamine precursor, levodopa (L-DOPA) or stimulation of dopaminergic receptors. Prolonged use of L-DOPA is associated with the occurrence of motor complications and dyskinesia, attributed to neurotoxic effects of this drug. The aim of this study was to investigate the effects of curcumin (CUR), a lipophilic polyphenol, to counteract L-DOPA induced toxicity. Zebrafish larvae were pre-treated with CUR (0.05 µM) or vehicle dimethyl sulfoxide (DMSO) for 24 hr and subsequently exposed to L-DOPA (1 mM) or vehicle. Immediately and 24 hr after L-DOPA exposure, spontaneous swimming and dark/light behavioral tests were performed. In addition, levels of reactive oxygen species (ROS) and lipid peroxidation products were determined at the end of treatment. CUR significantly improved the motor impairment induced by 24 hr L-DOPA treatment, and reduced levels of ROS and lipoperoxidation products in zebrafish larvae. In conclusion, our results suggest that CUR acts as a neuroprotector against toxicity initiated by L-DOPA. Evidence suggests the observed effects of CUR are associated with its antioxidant properties.

Differential Developmental Neurotoxicity and Tissue Uptake of the Per- and Polyfluoroalkyl Substance Alternatives, GenX and PFBS

Per- and polyfluoroalkyl substances (PFAS) are a class of synthetic chemicals with several applications. Multiple adverse health effects are reported for longer carbon chain (≤C8) PFAS. Shorter carbon chain PFAS, [e.g., hexafluoropropylene oxide dimer acid (HFPO-DA; GenX) and perfluorobutanesulfonic acid (PFBS)] were introduced as alternatives. Past studies indicate that longer-chain PFAS are neurotoxic targeting the dopamine pathway, but it is not known if shorter-chain PFAS act similarly. This study aimed to evaluate developmental neurotoxicity and tissue uptake of GenX and PFBS using the zebrafish (Danio rerio). First, acute toxicity was assessed by measuring LC50 at 120 h postfertilization (hpf). Body burden was determined after embryonic exposure (1-72 hpf) to sublethal concentrations of GenX or PFBS by LC-ESI-MS/MS. Locomotor activity using a visual motor response assay at 120 hpf and dopamine levels at 72 hpf was assessed after embryonic exposure. PFBS was more acutely toxic and bioaccumulative than GenX. GenX and PFBS caused hyperactivity at 120 hpf, but stronger behavioral alterations were observed for PFBS. An increase in whole organism dopamine occurred at 40 ppb of GenX, while a decrease was observed at 400 ppb of PFBS. Differences detected in dopamine for these two PFAS indicate differential mechanisms of developmental neurotoxicity.

Artificial Neural Network (ANN)-Based Pattern Recognition Approach Illustrates a Biphasic Behavioral Effect of Ethanol in Zebrafish: A High-Throughput Method for Animal Locomotor Analysis

Variations in stress responses between individuals are linked to factors ranging from stress coping styles to the sensitivity of neurotransmitter systems. Many anxiolytic compounds can increase stressor engagement through the modulation of neurotransmitter systems and are used to investigate stress response mechanisms. The effect of such modulation may vary in time depending on concentration or environment, but those effects are hard to dissect because of the slow transition. We investigated the temporal effect of ethanol and found that ethanol-treated individual zebrafish larvae showed altered behavior that is different between drug concentrations and decreases with time. We used an artificial neural network approach with a time-dependent method for analyzing long (90 min) experiments on zebrafish larvae and found that individuals from the 0.5% group begin to show locomotor activity corresponding to the control group starting from the 60th minute. The locomotor activity of individuals from the 2% group after the 80th minute is classified as the activity of individuals from the 1.5% group. Our method shows three clusters of different concentrations in comparison with two clusters, which were obtained with the usage of a statistical approach for analyzing just the speed of fish movements. In addition, we show that such changes are not explained by basic behavior statistics such as speed and are caused by shifts in locomotion patterns.

Ephedrine and cocaine cause developmental neurotoxicity and abnormal behavior in zebrafish

Ephedrine (EPH) and cocaine (COC) are illegal stimulant drugs, and have been frequently detected in aquatic environments. EPH and COC have negative effects on the nervous system and cause abnormal behaviors in mammals and fish at high concentrations, but their mechanisms of neurotoxicity remain unclear in larvae fish at low concentrations. To address this issue, zebrafish embryos were exposed to EPH and COC for 14 days post-fertilization (dpf) at 10, 100, and 1000 ng L-1. The bioaccumulation, development, behavior, cell neurotransmitter levels and apoptosis were detected to investigate the developmental neurotoxicity (DNT) of EPH and COC. The results showed that EPH decreased heart rate, while COC increased heart rate. EPH caused cell apoptosis in the brain by AO staining. In addition, behavior analysis indicated that EPH and COC affected spontaneous movement, touch-response, swimming activity and anxiety-like behaviors. EPH and COC altered the levels of the neurotransmitters dopamine (DA) and γ-aminobutyric acid (GABA) with changes of the transcription of genes related to the DA and GABA pathways. These findings indicated that EPH and COC had noticeable DNT in the early stage of zebrafish at environmentally relevant concentrations.

Behavioural, developmental and biochemical effects in zebrafish caused by ibuprofen, irgarol and terbuthylazine

The increasing use of chemicals and their release into aquatic ecosystems are harming aquatic biota. Despite extensive ecotoxicological research, many environmental pollutants' ecological effects are still unknown. This study examined the spatial avoidance, behavioural and biochemical impacts of ibuprofen, irgarol, and terbuthylazine on the early life stages of zebrafish (Danio rerio) under a range of ecologically relevant concentrations (0-500 μg/L). Embryos were exposed following the OECD guideline "fish embryo toxicity test" complemented with biochemical assessment of AChE activity and behavioural analyses (swimming activity) using the video tracking system Zebrabox. Moreover, spatial avoidance was assessed by exposing 120 hpf-old larvae of D. rerio to a gradient of each chemical, by using the heterogeneous multi-habitat assay system (HeMHAS). The results obtained revealed that the 3 compounds delayed hatching at concentrations of 50 and 500 μg/L for both ibuprofen and irgarol and 500 μg/L for terbuthylazine. Moreover, all chemicals elicited a dose-dependent depression of movement (swimming distance) with LOEC values of 5, 500 and 50 μg/L for ibuprofen, irgarol and terbuthylazine, respectively. Zebrafish larvae avoided the three chemicals studied, with 4 h-AC50 values for ibuprofen, irgarol, and terbuthylazine of 64.32, 79.86, and 131.04 μg/L, respectively. The results of the HeMHAS assay suggest that larvae may early on avoid (just after 4 h of exposure) concentrations of the three chemicals that may later induce, apical and biochemical effects. Findings from this study make clear some advantages of using HeMHAS in ecotoxicology as it is: ecologically relevant (by simulating a chemically heterogeneous environmental scenario), sensitive (the perception of chemicals and the avoidance can occur at concentrations lower than those producing lethal or sublethal effects) and more humane and refined approach (organisms are not mandatorily exposed to concentrations that can produce individual toxicity).

Testing of putative antiseizure drugs in a preclinical Dravet syndrome zebrafish model

Dravet syndrome (DS) is a severe genetic epilepsy primarily caused by de novo mutations in a voltage-activated sodium channel gene (SCN1A). Patients face life-threatening seizures that are largely resistant to available anti-seizure medications (ASM). Preclinical DS animal models are a valuable tool to identify candidate ASMs for these patients. Among these, scn1lab mutant zebrafish exhibiting spontaneous seizure-like activity are particularly amenable to large-scale drug screening. Prior screening in a scn1lab mutant zebrafish line generated using N-ethyl-Nnitrosourea (ENU) identified valproate, stiripentol, and fenfluramine e.g., Federal Drug Administration (FDA) approved drugs with clinical application in the DS population. Successful phenotypic screening in scn1lab mutant zebrafish consists of two stages: (i) a locomotion-based assay measuring high-velocity convulsive swim behavior and (ii) an electrophysiology-based assay, using in vivo local field potential (LFP) recordings, to quantify electrographic seizure-like events. Using this strategy more than 3000 drug candidates have been screened in scn1lab zebrafish mutants. Here, we curated a list of nine additional anti-seizure drug candidates recently identified in preclinical models: 1-EBIO, AA43279, chlorzoxazone, donepezil, lisuride, mifepristone, pargyline, soticlestat and vorinostat. First-stage locomotion-based assays in scn1lab mutant zebrafish identified only 1-EBIO, chlorzoxazone and lisuride. However, second-stage LFP recording assays did not show significant suppression of spontaneous electrographic seizure activity for any of the nine anti-seizure drug candidates. Surprisingly, soticlestat induced frank electrographic seizure-like discharges in wild-type control zebrafish. Taken together, our results failed to replicate clear anti-seizure efficacy for these drug candidates highlighting a necessity for strict scientific standards in preclinical identification of ASMs.

Development of a trace quantitative method to investigate caffeine distribution in the Yellow and Bohai Seas, China, and assessment of its potential neurotoxic effect on fish larvae

Caffeine is an emerging contaminant in aquatic environments. The study utilized a validated method to investigate the presence and distribution of caffeine in the surface water of the Yellow and Bohai Seas, urban rivers, and the Yantai estuary area. The analytical method conforms to EPA guidelines and exhibits a limit of quantification that is 200 times lower than that of prior investigations. The study revealed that the highest concentration of 1436.4 ng/L was found in convergence of ocean currents in the Yellow and Bohai Seas. The presence of larger populations and the process of urban industrialization have been observed to result in elevated levels of caffeine in offshore regions, confirming that caffeine can serve as a potential indicator of anthropogenic contamination. Fish larvae exhibited hypoactivity in response to caffeine exposure at environmentally relevant concentrations. The study revealed that caffeine pollution can have adverse effects on marine and offshore ecosystems. This emphasizes the importance of decreasing neurotoxic pollution in the aquatic environment.

Developmental Exposure to Kynurenine Affects Zebrafish and Rat Behavior

Proper nutrition and supplementation during pregnancy and breastfeeding are crucial for the development of offspring. Kynurenine (KYN) is the central metabolite of the kynurenine pathway and a direct precursor of other metabolites that possess immunoprotective or neuroactive properties, with the ultimate effect on fetal neurodevelopment. To date, no studies have evaluated the effects of KYN on early embryonic development. Thus, the aim of our study was to determine the effect of incubation of larvae with KYN in different developmental periods on the behavior of 5-day-old zebrafish. Additionally, the effects exerted by KYN administered on embryonic days 1-7 (ED 1-7) on the behavior of adult offspring of rats were elucidated. Our study revealed that the incubation with KYN induced changes in zebrafish behavior, especially when zebrafish embryos or larvae were incubated with KYN from 1 to 72 h post-fertilization (hpf) and from 49 to 72 hpf. KYN administered early during pregnancy induced subtle differences in the neurobehavioral development of adult offspring. Further research is required to understand the mechanism of these changes. The larval zebrafish model can be useful for studying disturbances in early brain development processes and their late behavioral consequences. The zebrafish-medium system may be applicable in monitoring drug metabolism in zebrafish.

Influence of Methylene Blue or Dimethyl Sulfoxide on Larval Zebrafish Development and Behavior

The use of larval zebrafish developmental testing and assessment, specifically larval zebrafish locomotor activity, has been recognized as a higher throughput testing strategy to identify developmentally toxic and neurotoxic chemicals. There are, however, no standardized protocols for this type of assay, which could result in confounding variables being overlooked. Two chemicals commonly employed during early-life stage zebrafish assays, methylene blue (antifungal agent) and dimethyl sulfoxide (DMSO, a commonly used vehicle) have been reported to affect the morphology and behavior of freshwater fish. In this study, we conducted developmental toxicity (morphology) and neurotoxicity (behavior) assessments of commonly employed concentrations for both chemicals (0.6-10.0 μM methylene blue; 0.3%-1.0% v/v DMSO). A light-dark transition behavioral testing paradigm was applied to morphologically normal, 6 days postfertilization (dpf) zebrafish larvae kept at 26°C. Additionally, an acute DMSO challenge was administered based on early-life stage zebrafish assays typically used in this research area. Results from developmental toxicity screens were similar between both chemicals with no morphological abnormalities detected at any of the concentrations tested. However, neurodevelopmental results were mixed between the two chemicals of interest. Methylene blue resulted in no behavioral changes up to the highest concentration tested, 10.0 μM. By contrast, DMSO altered larval behavior following developmental exposure at concentrations as low as 0.5% (v/v) and exhibited differential concentration-response patterns in the light and dark photoperiods. These results indicate that developmental DMSO exposure can affect larval zebrafish locomotor activity at routinely used concentrations in developmental neurotoxicity assessments, whereas methylene blue does not appear to be developmentally or neurodevelopmentally toxic to larval zebrafish at routinely used concentrations. These results also highlight the importance of understanding the influence of experimental conditions on larval zebrafish locomotor activity that may ultimately confound the interpretation of results.

Flight for fish in drug discovery: a review of zebrafish-based screening of molecules

Human disease and biological practices are modelled in zebrafish (Danio rerio) at various phases of drug development as well as toxicity evaluation. The zebrafish is ideal for in vivo pathological research and high-resolution investigation of disease progress. Zebrafish has an advantage over other mammalian models, it is cost-effective, it has external development and embryo transparency, easy to apply genetic manipulations, and open to both forward and reverse genetic techniques. Drug screening in zebrafish is suitable for target identification, illness modelling, high-throughput screening of compounds for inhibition or prevention of disease phenotypes and developing new drugs. Several drugs that have recently entered the clinic or clinical trials have their origins in zebrafish. The sophisticated screening methods used in zebrafish models are expected to play a significant role in advancing drug development programmes. This review highlights the current developments in drug discovery processes, including understanding the action of drugs in the context of disease and screening novel candidates in neurological diseases, cardiovascular diseases, glomerulopathies and cancer. Additionally, it summarizes the current techniques and approaches for the selection of small molecules and current technical limitations on the execution of zebrafish drug screening tests.

Optimizing assays of zebrafish larvae swimming performance for drug discovery

INTRODUCTION

Zebrafish larvae are one of the few vertebrates amenable to large-scale drug discovery screens. Larval swimming behavior is often used as an outcome variable and many fields of study have developed assays for evaluating swimming performance. An unintended consequence of this wide interest is that details related to assay methodology and interpretation become scattered across the literature. The aim of this review is to consolidate this information, particularly as it relates to high-throughput approaches.

AREAS COVERED

The authors describe larval swimming behaviors as this forms the basis for understanding their experimentally evoked swimming or spontaneous activity. Next, they detail how swimming activity can serve as an outcome variable, particularly in the multi-well formats used in large-scale screening studies. They also highlight biological and technical factors that can impact the sensitivity and variability of these measurements.

EXPERT OPINION

Careful attention to animal husbandry, experimental design, data acquisition, and interpretation of results can improve screen outcomes by maximizing swimming activity while minimizing intra- and inter-larval variability. The development of more sensitive, quantitative methods of assessing swimming performance that can be incorporated into high-throughput workflows will be important in order to take full advantage of the zebrafish model.

Caloric state modulates locomotion, heart rate and motor neuron responses to acute administration of d-amphetamine in zebrafish larvae

Psychostimulant drugs increase behavioral, cardiac and brain responses in humans and other animals. Acute food deprivation or chronic food restriction potentiates the stimulatory effects of abused drugs and increases the propensity for relapse to drug seeking in drug-experienced animals. The mechanisms by which hunger affects cardiac and behavioral activities are only beginning to be elucidated. Moreover, changes in motor neuron activities at the single neuron level induced by psychostimulants, and their modulation by food restriction, remain unknown. Here we investigated how food deprivation affects responses to d-amphetamine by measuring locomotor activity, cardiac output, and individual motor neuron activity in zebrafish larvae. We used wild-type larval zebrafish to record behavioral and cardiac responses and the larvae of Tg(mnx1:GCaMP5) transgenic zebrafish to record motor neuron responses. Physiological state gated responses to d-amphetamine. That is, d-amphetamine evoked significant increases in motor behavior (swimming distances), heart rate and motor neuron firing frequency in food-deprived but not fed zebrafish larvae. The results extend the finding that signals arising from food deprivation are a key potentiator of the drug responses induced by d-amphetamine to the zebrafish model. The larval zebrafish is an ideal model to further elucidate this interaction and identify key neuronal substrates that may increase vulnerability to drug reinforcement, drug-seeking and relapse.

High-fat diet feeding triggers a regenerative response in the adult zebrafish brain

Non-alcoholic fatty liver disease (NAFLD) includes a range of liver conditions ranging from excess fat accumulation to liver failure. NAFLD is strongly associated with high-fat diet (HFD) consumption that constitutes a metabolic risk factor. While HFD has been elucidated concerning its several systemic effects, there is little information about its influence on the brain at the molecular level. Here, by using a high-fat diet (HFD)-feeding of adult zebrafish, we first reveal that excess fat uptake results in weight gain and fatty liver. Prolonged exposure to HFD induces a significant increase in the expression of pro-inflammation, apoptosis, and proliferation markers in the liver and brain tissues. Immunofluorescence analyses of the brain tissues disclose stimulation of apoptosis and widespread activation of glial cell response. Moreover, glial activation is accompanied by an initial decrease in the number of neurons and their subsequent replacement in the olfactory bulb and the telencephalon. Long-term consumption of HFD causes activation of Wnt/β-catenin signaling in the brain tissues. Finally, fish fed an HFD induces anxiety, and aggressiveness and increases locomotor activity. Thus, HFD feeding leads to a non-traumatic brain injury and stimulates a regenerative response. The activation mechanisms of a regeneration response in the brain can be exploited to fight obesity and recover from non-traumatic injuries.

Effects of acute metaphedrone exposure on the development, behaviour, and DNA integrity of zebrafish (Danio rerio)

The presence of new psychoactive substances (NPS), like metaphedrone (3-MMC), in aquatic environments raises concern about the potential negative effects on ichthyofauna. Therefore, the aim of this study was to evaluate the potential effects of 3-MMC on zebrafish embryonic development, behaviour, and DNA integrity. For that, embryos were exposed during 96 h post-fertilization to 3-MMC (0.1, 1, 10, and 100 µg/L). Overall, an increase in the eye area of zebrafish larvae was observed for the concentrations of 1 μg/L (increase of 24%) and 100 μg/L (increase of 25%) in comparison with the control group. Genetic damage was noted at the highest concentration (100 µg/L) with an increase of DNA damage (increase of 48%) and hyperactivity and disorganised swimming pattern characterised by an increase in speed (increase of 49%), total distance moved (increase of 53%), and absolute turn angle (increase of 48%) of zebrafish larvae. These findings pointed that, at environmental low levels, 3-MMC harmful effects are not expected to occur during critical development life stages of fish.

Zebrafish (Danio rerio) as a model to assess the effects of cocaine as a drug of abuse and its environmental implications

Cocaine (COC) use concerns are on the increase for both authorities and civil society. Despite this, it is important to investigate COC effects or those of its main metabolite, belzoylecgonine (BE), in consolidated aquatic model organisms, such as the zebrafish (Danio rerio). This (mini) review consists in an assessment regarding toxicological studies carried out employing zebrafish (embryos, larvae or adults) exposed to COC and/or BE indexed at the SCOPUS and Web of Science databases. Ten different endpoints were analyzed in both embryos and larvae, whereas only four were analyzed in adults. Of the 23 studies, only five investigated COC and/or BE effects following an environmental approach when exposing zebrafish, while most (18 studies) analyzed COC effects under a drug of abuse approach. Cocaine exposure was noted as altering the expression of several genes, such as those linked to COC transport proteins, dopamine receptors, SP substance production, the tachykinin system, and the tyrosine hydroxylase enzyme. BE exposure resulted in more oxidative and proteomic effects than COC in embryos. Cocaine abstinence resulted in hyperactivity associated with stereotypy in adult fish, in addition to reduced responses to visual stimuli to red light and neuronal development pattern alterations. Cocaine was noted as accumulating in zebrafish eyes, possibly due to melanin binding, and causing dose-response cardiac effects in both embryos and adults. Despite the different effects addressed by our survey, we emphasize the lack of COC and BE exposure assessments in zebrafish employing an environmental point of view.

Developmental Neurotoxicity Screen of Psychedelics and Other Drugs of Abuse in Larval Zebrafish (Danio rerio)

In recent years, psychedelics have garnered significant interest as therapeutic agents for treating diverse neuropsychiatric disorders. However, the potential for these compounds to produce developmental neurotoxicity has not been rigorously assessed, and much of the available safety data is based on epidemiological studies with limited experimental testing in laboratory animal models. Moreover, the experimental safety data available thus far have focused on adult organisms, and the few studies conducted using developing organisms have tested a limited number of compounds, precluding direct comparisons between various chemical scaffolds. In the present study, 13 psychoactive compounds of different chemical or pharmacological classes were screened in a larval zebrafish model for teratological and behavioral abnormalities following acute and chronic developmental exposures. We found that the psychedelic tryptamines and ketamine were less neurotoxic to larval zebrafish than LSD and psychostimulants. Our work, which leverages the advantage of using zebrafish for higher throughput toxicity screening, provides a robust reference database for comparing the neurotoxicity profiles of novel psychedelics currently under development for therapeutic applications.

Zebrafish as a Mainstream Model for In Vivo Systems Pharmacology and Toxicology

Pharmacology and toxicology are part of a much broader effort to understand the relationship between chemistry and biology. While biomedicine has necessarily focused on specific cases, typically of direct human relevance, there are real advantages in pursuing more systematic approaches to characterizing how health and disease are influenced by small molecules and other interventions. In this context, the zebrafish is now established as the representative screenable vertebrate and, through ongoing advances in the available scale of genome editing and automated phenotyping, is beginning to address systems-level solutions to some biomedical problems. The addition of broader efforts to integrate information content across preclinical model organisms and the incorporation of rigorous analytics, including closed-loop deep learning, will facilitate efforts to create systems pharmacology and toxicology with the ability to continuously optimize chemical biological interactions around societal needs. In this review, we outline progress toward this goal.

Assessment of unique behavioral, morphological, and molecular alterations in the comparative developmental toxicity profiles of PFOA, PFHxA, and PFBA using the zebrafish model system

Perfluoroalkyl substances (PFAS) are a class of synthetic chemicals that are persistent in the environment. Due to adverse health outcomes associated with longer chain PFAS, shorter chain chemicals were used as replacements, but developmental toxicity assessments of the shorter chain chemicals are limited. Toxicity of three perfluoroalkyl acids (PFAAs) [perfluorooctanoic acid (PFOA), composed of 8 carbon (C8), perfluorohexanoic acid (PFHxA, C6), and perfluorobutanoic acid (PFBA, C4)] was compared in developing zebrafish (Danio rerio). LC50s at 120 h post fertilization (hpf) assessed potency of each PFAA by exposing developing zebrafish (1-120 hpf) to range of concentrations. Zebrafish were then exposed to sublethal concentrations (0.4-4000 ppb, µg/L) throughout embryogenesis (1-72 hpf). Effects of the embryonic exposure on locomotor activities was completed with the visual motor response test at 120 hpf. At 72 hpf, morphological changes (total body length, head length, head width) and transcriptome profiles to compare altered molecular and disease pathways were determined. The LC50 ranking followed trend as expected based on chain length. PFOA caused hyperactivity and PFBA hypoactivity, while PFHxA did not change behavior. PFOA, PFHxA, and PFBA caused morphological and transcriptomic alterations that were unique for each chemical and were concentration-dependent indicating different toxicity mechanisms. Cancer was a top disease for PFOA and FXR/RXR activation was a top canonical pathway for PFBA. Furthermore, comparison of altered biological and molecular pathways in zebrafish exposed to PFOA matched findings reported in prior epidemiological studies and other animal models, supporting the predictive value of the transcriptome approach and for predicting adverse health outcomes associated with PFHxA or PFBA exposure.

Benzoresorcinol induces developmental neurotoxicity and injures exploratory, learning and memorizing abilities in zebrafish

Benzophenones (BPs) are a class of UV absorber commonly used in skin care products like sunscreens. With its wide range of application, its environmental and human hazards have received much attention in recent days. Previous studies on the toxicity of BPs mainly focused on its endocrine-disrupting effects, but there are limited studies on its neurodevelopment and neurotoxicity. Herein, using the zebrafish model we studied the neurodevelopmental- and neuro-toxicity of benzophenone 1 (BP1) (0.8, 1.0, 1.2, 1.6, and 2.4 μg/mL). As a result, BP1 led to an increase of embryo mortality, a decrease in hatching rate, and an increase in the rate of developmental abnormalities in a concentration-dependent manner. BP1 also caused developmental defects in the central nervous system (CNS) and dopaminergic (DA) neurons. Accordingly, BP1 injured larval zebrafish general locomotion and response to stimuli in light/dark challenge. In adult zebrafish, BP1 exposure (1, 10, 100, 1000 μg/L) caused inhibition of learning and memory abilities in the T-maze tests, and inhibited exploratory behavior and activity in the novel tank diving tests. Further, transcription levels of genes related to neurotoxicity, neurodevelopment, and anxiety revealed that BP1 may affect the development and function of the myelin sheath, inducing structural and functional defects of CNS, manifested as abnormal behaviors such as anxiety. Hence, the current study revealed the neurodevelopmental toxicity and neurotoxicity of BP1, expanded our knowledge about the toxic effects of BP1 on organisms, posing a possible threat to the environment and human health.

A comparative analysis of Danionella cerebrum and zebrafish (Danio rerio) larval locomotor activity in a light-dark test

The genus Danionella comprises some of the smallest known vertebrate species and is evolutionary closely related to the zebrafish, Danio rerio. With its optical translucency, rich behavioral repertoire, and a brain volume of just 0.6 mm³, Danionella cerebrum (Dc) holds great promise for whole-brain in vivo imaging analyses with single cell resolution of higher cognitive functions in an adult vertebrate. Little is currently known, however, about the basic locomotor activity of adult and larval Danionella cerebrum and how it compares to the well-established zebrafish model system. Here, we provide a comparative developmental analysis of the larval locomotor activity of Dc and AB wildtype as well as crystal zebrafish in a light-dark test. We find similarities but also differences in both species, most notably a striking startle response of Dc following a sudden dark to light switch, whereas zebrafish respond most strongly to a sudden light to dark switch. We hypothesize that the different startle responses in both species may stem from their different natural habitats and could represent an opportunity to investigate how neural circuits evolve to evoke different behaviors in response to environmental stimuli.

Effects of di-(2-ethylhexyl) phthalate (DEHP) on behavior and dopamine signaling in zebrafish (Danio rerio)

Di (2-ethylhexyl) phthalate (DEHP) is a widely used plasticizer, also known as a developmental toxicant, but its neurobehavioral toxicity remains elusive. This study evaluated the neurobehavioral toxicity and its possible mechanism in larval zebrafish. Embryos at gastrula period (~6 h post fertilization, hpf) were exposure to DEHP (0, 1, 2.5, 5 and 10 mg/L) for 7 days. Spontaneous tail movement in embryos and swimming activity in larvae were monitored. Alterations in the mRNA expression of genes involved in dopamine signaling and apoptosis pathway were assessed. In situ apoptotic cells were assessed by Acridine orange staining, and oxidative damage were measured using enzymatic assay. The behavior results showed that DEHP inhibited spontaneous tail movement and decreased locomotor activities in the light/dark behavioral test. Meanwhile, behavioral changes were accompanied by increased apoptosis and malondialdehyde (MDA) content, decreased superoxide dismutase (SOD) activity and dopamine (DA) content, and perturbed the expression of genes associated with the synthesis (th), reuptake (dat) and metabolism (mao) of DA, with dopamine receptors (DRs), and with the apoptosis pathway (p53, bax, bcl2, caspase-3, caspase-8, caspase-9). The findings will help to illuminate the possible neurobehavioral toxicity mechanisms of organism exposure to DEHP.

Larval Zebrafish as a Model for Mechanistic Discovery in Mental Health

Animal models are essential for the discovery of mechanisms and treatments for neuropsychiatric disorders. However, complex mental health disorders such as depression and anxiety are difficult to fully recapitulate in these models. Borrowing from the field of psychiatric genetics, we reiterate the framework of 'endophenotypes' - biological or behavioral markers with cellular, molecular or genetic underpinnings - to reduce complex disorders into measurable behaviors that can be compared across organisms. Zebrafish are popular disease models due to the conserved genetic, physiological and anatomical pathways between zebrafish and humans. Adult zebrafish, which display more sophisticated behaviors and cognition, have long been used to model psychiatric disorders. However, larvae (up to 1 month old) are more numerous and also optically transparent, and hence are particularly suited for high-throughput screening and brain-wide neural circuit imaging. A number of behavioral assays have been developed to quantify neuropsychiatric phenomena in larval zebrafish. Here, we will review these assays and the current knowledge regarding the underlying mechanisms of their behavioral readouts. We will also discuss the existing evidence linking larval zebrafish behavior to specific human behavioral traits and how the endophenotype framework can be applied. Importantly, many of the endophenotypes we review do not solely define a diseased state but could manifest as a spectrum across the general population. As such, we make the case for larval zebrafish as a promising model for extending our understanding of population mental health, and for identifying novel therapeutics and interventions with broad impact.

Toxicological assessment of cadmium-containing quantum dots in developing zebrafish: Physiological performance and neurobehavioral responses

The present research investigated the effects of exposure to sublethal concentrations of cadmium selenide/zinc sulfide (CdSe_core/ZnS_shell)-containing quantum dots (QDs; 0 - 100 µg/L QDs) on the neurophysiological performance of developing zebrafish (Danio rerio). The results suggested that exposure to CdSe QDs for 5 days increased the whole-body content of Cd without affecting the general physiological conditions of larvae. Interestingly, CdSe QD exposure reduced swimming distance but increased swimming velocity of larvae, suggesting that the exposure may lead to burst/episodic swimming. The findings also suggested that CdSe QD exposure reduced the wall-hugging behavior of larvae during a sudden light-to-dark transition test, and that the exposure significantly decreased the locomotor activity of fish during the dark period. On the other hand, control larvae displayed a dark avoidance behavior, whereas CdSe QD-exposed larvae exhibited an increase in the time spent in the dark zone, providing further support that CdSe QDs inhibited anxiety-related responses in larvae. Additional analysis with droplet digital PCR revealed that CdSe QD exposure altered the mRNA levels of genes that are associated with dopamine signaling and oxidative stress response. Collectively, our findings suggested that CdSe QD exposure may induce neurobehavioural toxicity and alters the mRNA abundance of dopamine- and oxidative stress-related genes in developing animals.

Developmental Neurotoxicity and Behavioral Screening in Larval Zebrafish with a Comparison to Other Published Results

With the abundance of chemicals in the environment that could potentially cause neurodevelopmental deficits, there is a need for rapid testing and chemical screening assays. This study evaluated the developmental toxicity and behavioral effects of 61 chemicals in zebrafish (Danio rerio) larvae using a behavioral Light/Dark assay. Larvae (n = 16-24 per concentration) were exposed to each chemical (0.0001-120 μM) during development and locomotor activity was assessed. Approximately half of the chemicals (n = 30) did not show any gross developmental toxicity (i.e., mortality, dysmorphology or non-hatching) at the highest concentration tested. Twelve of the 31 chemicals that did elicit developmental toxicity were toxic at the highest concentration only, and thirteen chemicals were developmentally toxic at concentrations of 10 µM or lower. Eleven chemicals caused behavioral effects; four chemicals (6-aminonicotinamide, cyclophosphamide, paraquat, phenobarbital) altered behavior in the absence of developmental toxicity. In addition to screening a library of chemicals for developmental neurotoxicity, we also compared our findings with previously published results for those chemicals. Our comparison revealed a general lack of standardized reporting of experimental details, and it also helped identify some chemicals that appear to be consistent positives and negatives across multiple laboratories.

Micronized Curcumin Causes Hyperlocomotion in Zebrafish Larvae

Zebrafish larvae have been widely used in neuroscience and drug research and development. In the larval stage, zebrafish present a broad behavioral repertoire and physiological responses similar to adults. Curcumin (CUR), a major component of Curcuma longa L. (Zingiberaceae), has demonstrated the ability to modulate several neurobiological processes relevant to mental disorders in animal models. However, the low bioavailability of this compound can compromise its in vivo biological potential. Interestingly, it has been shown that micronization can increase the biological effects of several compounds. Thus, in this study, we compared the effects of acute exposure for 30 min to the following solutions: water (control), 0.1% DMSO (vehicle), 1 μM CUR, or 1 μM micronized curcumin (MC) in zebrafish larvae 7 days post-fertilization (dpf). We analyzed locomotor activity (open tank test), anxiety (light/dark test), and avoidance behavior (aversive stimulus test). Moreover, we evaluated parameters of oxidative status (thiobarbituric acid reactive substances and non-protein thiols levels). MC increased the total distance traveled and absolute turn angle in the open tank test. There were no significant differences in the other behavioral or neurochemical outcomes. The increase in locomotion induced by MC may be associated with a stimulant effect on the central nervous system, which was evidenced by the micronization process.

Getting more out of the zebrafish light dark transition test

In (eco-)toxicological studies the light/dark transition (LDT) test is one of the most frequently used behaviour assays with zebrafish eleutheroembryos. However, study results vary regarding data presentation and analysis and mostly focus on a limited amount of the recorded data. In this study, we investigated whether monitoring two behavioural outcomes (time and distance moved) together with analysing multiple parameters can improve test sensitivity and data interpretation. As a proof of principle 5-day old zebrafish (Danio rerio) eleutheroembryos exposed to either endocrine disruptors (EDs) or acetylcholine esterase (AChE) inhibitors were investigated. We analysed conventional parameters such as mean and sum and implemented additional endpoints such as minimum or maximum distance moved and new parameters assessing the bursting response of eleutheroembryos. Furthermore, changes in eleutheroembryonic behaviour during the moment of the light to dark transition were added. To improve data presentation control-normalised results were displayed in radar charts, enabling the simultaneous presentation of different parameters in relation to each other. This enabled us to identify parameters most relevant to a certain behavioural response. A cut off threshold using control data was applied to identify parameters that were altered in a biological relevant manner. Our approach was able to detect effects on different parameters that remained undetected when analysis was done using conventional bar graphs on - in most cases analysed - averaged, mean distance moved values. By combining the radar charts with additional parameters and by using control-based thresholds, we were able to increase the test sensitivity and promote a deeper understanding of the behaviour response of zebrafish eleutheroembryos in the LDT test and thereby increased its usability for behavioural toxicity studies.

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.

The psychoactive effects of Bryophyllum pinnatum (Lam.) Oken leaves in young zebrafish

Bryophyllum pinnatum (Lam.) Oken (BP) is a plant that is used worldwide to treat inflammation, infections, anxiety, restlessness, and sleep disorders. While it is known that BP leaves are rich in flavonoids, the extent of the beneficial and toxic effects of its crude extracts remains unclear. Although some neurobehavioral studies using leaf extracts have been conducted, none has examined the effects of water-extracted leaf samples. The zebrafish is a powerful animal model used to gain insights into the efficacy and toxicity profiles of this plant due to its high fecundity, external development, and ease of performing behavioral assays. In this study, we performed behavioral testing after acute exposure to different concentrations of aqueous extract from leaves of B. pinnatum (LABP) on larval zebrafish, investigating light/dark preference, thigmotaxis, and locomotor activity parameters under both normal and stressed conditions. LABP demonstrated dose-and time-dependent biphasic effects on larval behavior. Acute exposure (25 min) to 500 mg/L LABP resulted in decreased locomotor activity. Exposure to 300 mg/L LABP during the sleep cycle decreased dark avoidance and thigmotaxis while increasing swimming velocity. After sleep deprivation, the group treated with 100 mg/L LABP showed decreased dark avoidance and increased velocity. After a heating stressor, the 30 mg/L and 300 mg/L LABP-treated groups showed decreased dark avoidance. These results suggest both anxiolytic and psychoactive effects of LABP in a dose-dependent manner in a larval zebrafish model. These findings provide a better understanding of the mechanisms underlying relevant behavioral effects, consequently supporting the safe and effective use of LABP for the treatment of mood disorders.

Early detection of cyanide, organophosphate and rodenticide pollution based on locomotor activity of zebrafish larvae

Cyanide, organophosphate and rodenticides are highly toxic substances widely used in agriculture and industry. These toxicants are neuro- and organotoxic to mammals at low concentrations, thus early detection of these chemicals in the aqueous environment is of utmost importance. Here, we employed the behavioral toxicity test with wildtype zebrafish larvae to determine sublethal concentrations of the above mentioned common environmental pollutants. After optimizing the test with cyanide, nine rodenticides and an organophosphate were successfully tested. The compounds dose-dependently initially (0-60-min exposure) stimulated locomotor activity of larvae but induced toxicity and reduced swimming during 60-120-min exposure. IC₅₀ values calculated based on swimming distance after 2-h exposure, were between 0.1 and 10 mg/L for both first-generation and second-generation anticoagulant rodenticides. Three behavioral characteristics, including total distance travelled, sinuosity and burst count, were quantitatively analyzed and compared by hierarchical clustering of the effects measured by each three parameters. The toxicity results for all three behavioral endpoints were consistent, suggesting that the directly measured parameter of cumulative swimming distance could be used as a promising biomarker for the aquatic contamination. The optimized method herein showed the potential for utilization as part of a monitoring system and an ideal tool for the risk assessment of drinking water in the military and public safety.

In Silico Docking of Novel Phytoalkaloid Camalexin in the Management of Benomyl Induced Parkinson's Disease and its In Vivo Evaluation by Zebrafish Model

BACKGROUND

Parkinson's Disease (PD) exhibits the extrapyramidal symptoms caused due to the dopaminergic neuronal degeneration in the substantia nigra of the brain and depletion of Aldehyde Dehydrogenase (ALDH) enzyme.

OBJECTIVE

This study was designed to enlighten the importance of the Aldehyde dehydrogenase enzyme in protecting the dopamine levels in a living system. Camalexin, a potentially active compound, has been evaluated for its dopamine enhancing and aldehyde dehydrogenase protecting role in pesticide-induced Parkinson's disease.

METHODS

AutoDock 4.2 software was employed to perform the docking simulations between the ligand camalexin and standard drugs Alda-1, Ropirinole with three proteins 4WJR, 3INL, 5AER. Consequently, the compound was evaluated for its in vivo neuroprotective role in the zebrafish model by attaining Institutional Animal Ethical Committee permission. The behavioral assessments and catecholamine analysis in zebrafish were performed.

RESULTS

The Autodock result shows that the ligand camalexin has a lower binding energy (-3.84) that indicates a higher affinity with the proteins when compared to the standard drug of proteins (-3.42). In the zebrafish model, behavioral studies provided evidence that camalexin helps in the improvement of motor functions and cognition. The catecholamine assay has proved that there is an enhancement in dopamine levels, as well as an improvement in aldehyde dehydrogenase enzyme.

CONCLUSION

The novel compound, camalexin, offers a protective role in Parkinson's disease model by its interaction with neurochemical proteins and also in alternative in vivo model.

Neuromotor activity inhibition in zebrafish early-life stages after exposure to environmental relevant concentrations of caffeine

Caffeine (CAF), a neuroactive compound, has been found in surface waters at concentrations ranging from few nanograms up to micrograms and may induce adverse effects in aquatic vertebrates. Thus, the aim of this study was to evaluate the potential of CAF in affecting fish early-life stages in a wide concentration range, including occurring levels in surface waters. Specimens of zebrafish in early-life stages were exposed to CAF for 168 h and survival, developmental alterations, locomotor activity and acetylcholinesterase activity were evaluated. CAF induced mortality in embryos unable to hatch or in larvae after hatching (LC₅₀ - 168 h = 283.2 mg/L). Tail deformities were observed in organisms exposed to concentrations ≥ 40 mg/L, while edemas were found at concentrations of 100 mg/L. CAF also decreased the total swimming time and distance moved of exposed organisms (LOEC = 0.0006 mg/L). Locomotor inhibition may be associated with an acetylcholinesterase inhibition observed at concentration ≥ 0.0088 mg/L. Therefore, the hazard of CAF for fish populations deserves further attention since unexpected effects on neuro-behavioral parameters occurs at concentrations often detected in natural aquatic ecosystems.

Exploring Neurobehaviour in Zebrafish Embryos as a Screening Model for Addictiveness of Substances

Tobacco use is the leading cause of preventable death worldwide and is highly addictive. Nicotine is the main addictive compound in tobacco, but less is known about other components and additives that may contribute to tobacco addiction. The zebrafish embryo (ZFE) has been shown to be a good model to study the toxic effects of chemicals on the neurological system and thus may be a promising model to study behavioral markers of nicotine effects, which may be predictive for addictiveness. We aimed to develop a testing protocol to study nicotine tolerance in ZFE using a locomotion test with light-dark transitions as behavioral trigger. Behavioral experiments were conducted using three exposure paradigms: (1) Acute exposure to determine nicotine’s effect and potency. (2) Pre-treatment with nicotine dose range followed by a single dose of nicotine, to determine which pre-treatment dose is sufficient to affect the potency of acute nicotine. (3) Pre-treatment with a single dose combined with acute exposure to a dose range to confirm the hypothesized decreased potency of the acute nicotine exposure. These exposure paradigms showed that (1) acute nicotine exposure decreased ZFE activity in response to dark conditions in a dose-dependent fashion; (2) pre-treatment with increasing concentrations dose-dependently reversed the effect of acute nicotine exposure; and (3) a fixed pre-treatment dose of nicotine induced a decreased potency of the acute nicotine exposure. This effect supported the induction of tolerance to nicotine by the pre-treatment, likely through neuroadaptation. The interpretation of these effects, particularly in view of prediction of dependence and addictiveness, and suitability of the ZFE model to test for such effects of other compounds than nicotine, are discussed.

Effects of conventionally-treated and ozonated wastewater on mortality, physiology, body length, and behavior of embryonic and larval zebrafish (Danio rerio)

To date, micropollutants from anthropogenic sources cannot be completely removed from effluents of wastewater treatment plants and therefore enter freshwater systems, where they may impose adverse effects on aquatic organisms, for example, on fish. Advanced treatment such as ozonation aims to reduce micropollutants in wastewater effluents and, thus, to mitigate adverse effects on the environment. To investigate the impact and efficiency of ozonation, four different water types were tested: ozonated wastewater (before and after biological treatment), conventionally-treated wastewater, and water from a river (River Ruhr, Germany) upstream of the wastewater treatment plant effluent. Zebrafish (Danio rerio) embryos were used to study lethal and sublethal effects in a modified fish early life-stage test. Mortality occurred during exposure in the water samples from the wastewater treatment plant and the river in the first 24 h post-fertilization, ranging from 12% (conventional wastewater) to 40% (river water). Regarding sublethal endpoints, effects compared to the negative control resulted in significantly higher heart rates (ozonated wastewater), and significantly reduced swimming activity (highly significant in ozonated wastewater and ozone reactor water, significant in only the last time interval in river water). Moreover, the respiration rates were highly increased in both ozonated wastewater samples in comparison to the negative control. Significant differences between the ozonated wastewater samples occurred in the embryonic behavior and heart rates, emphasizing the importance of subsequent biological treatment of the ozonated wastewater. Only the conventionally-treated wastewater sample did not elicit negative responses in zebrafish, indicating that the discharge of conventional wastewater poses no greater risk to embryonic and larval zebrafish than water from the river Ruhr itself. The sublethal endpoints embryonic- and larval behavior, heart rates, and respiration were found to be the most sensitive endpoints in this fish early life-stage test and can add valuable information on the toxicity of environmental samples.

Tuning the Reduction of Graphene Oxide Nanoflakes Differently Affects Neuronal Networks in the Zebrafish

The increasing engineering of biomedical devices and the design of drug-delivery platforms enriched by graphene-based components demand careful investigations of the impact of graphene-related materials (GRMs) on the nervous system. In addition, the enhanced diffusion of GRM-based products and technologies that might favor the dispersion in the environment of GRMs nanoparticles urgently requires the potential neurotoxicity of these compounds to be addressed. One of the challenges in providing definite evidence supporting the harmful or safe use of GRMs is addressing the variety of this family of materials, with GRMs differing for size and chemistry. Such a diversity impairs reaching a unique and predictive picture of the effects of GRMs on the nervous system. Here, by exploiting the thermal reduction of graphene oxide nanoflakes (GO) to generate materials with different oxygen/carbon ratios, we used a high-throughput analysis of early-stage zebrafish locomotor behavior to investigate if modifications of a specific GRM chemical property influenced how these nanomaterials affect vertebrate sensory-motor neurophysiology-exposing zebrafish to GO downregulated their swimming performance. Conversely, reduced GO (rGO) treatments boosted locomotor activity. We concluded that the tuning of single GRM chemical properties is sufficient to produce differential effects on nervous system physiology, likely interfering with different signaling pathways.

Standardized Method for the Assessment of Behavioral Responses of Zebrafish Larvae

Zebrafish are easy to breed in a laboratory setting as they are extremely fertile and produce dozens of eggs per set. Because zebrafish eggs and the skin of the early-stage larvae are transparent, their embryos and the hearts and muscles of their larvae can be easily observed. Multiple rapid analyses of heart rate and behavior can be performed on these larvae simultaneously, enabling investigation of the influence of neuroactive substances on abnormal behavior, death, and associated pathogenetic mechanisms. Zebrafish larvae are becoming increasingly popular among researchers and are used in laboratories worldwide to study various vertebrate life phenomena; more experimental systems using zebrafish will undoubtedly be developed in the future. However, based on the available literature, we believe that the conceptualization of a protocol based on scientific evidence is necessary to achieve standardization. We exposed zebrafish larvae at 6–7 days post-fertilization to 50 repeated light–dark stimuli at either 15-min or 5-min intervals. We measured the traveled distance and habituation time through a video tracking apparatus. The traveled distance stabilized after the 16th repetition when the zebrafish were exposed to light–dark stimuli at 15-min intervals and after the 5th repetition when exposed at 5-min intervals. Additionally, at 15-min intervals, the peak of the traveled distance was reached within the first minute in a dark environment, whereas at 5-min intervals, it did not reach the peak even after 5 min. The traveled distance was more stable at 5-min intervals of light/dark stimuli than at 15-min intervals. Therefore, if one acclimatizes zebrafish larvae for 1 h and collects data from the 5th repetition of light/dark stimuli at intervals of 5 min in the light/dark test, a stable traveled distance result can be obtained. The establishment of this standardized method would be beneficial for investigating substances of unknown lethal concentration.

The evaluation of zebrafish cardiovascular and behavioral functions through microfluidics

This study proposed a new experimental approach for the vascular and phenotype evaluation of the non-anesthetized zebrafish with representative imaging orientations for heart, pectoral fin beating, and vasculature views by means of the designed microfluidic device through inducing the optomotor response and hydrodynamic pressure control. In order to provide the visual cues for better positioning of zebrafish, computer-animated moving grids were generated by an in-house control interface which was powered by the larval optomotor response, in conjunction with the pressure suction control. The presented platform provided a comprehensive evaluation of internal circulation and the linked external behaviors of zebrafish in response to the cardiovascular parameter changes. The insights from these imaging sections was extended to identify the linkage between the cardiac parameters and behavioral endpoints. In addition, selected chemicals such as ethanol and caffeine were employed for the treatment of zebrafish. The obtained findings can be applicable for future investigation in behavioral drug screening serving as the forefront in psychopharmacological and cognition research.

The possible hormetic effects of fluorene-9-bisphenol on regulating hypothalamic-pituitary-thyroid axis in zebrafish

Fluorene-9-bisphenol (BHPF) is a bisphenol A substitute, which has been introduced for the production of so-called 'bisphenol A (BPA)-free' plastics. However, it has been reported that BHPF can enter living organisms through using commercial plastic bottles and cause adverse effects. To date, the majority of the toxicologic study of BHPF focused on investigating its doses above the toxicological threshold. Here, we studied the effects of BHPF on development, locomotion, neuron differentiation of the central nervous system (CNS), and the expression of genes in the hypothalamic-pituitary-thyroid (HPT) axis in zebrafish exposed to different doses of BHPF ranging from 1/5 of LD1 to LD50 (300, 500, 750, 1500, 3000, and 4500 nM). As a result, the possible hormetic effects of BHPF on regulating the HPT axis were revealed, in which low-dose BHPF positively affected the HPT axis while this regulation was inhibited as the dose increased. Underlying mechanism investigation suggested that BHPF disrupted myelination through affecting HPT axis including related genes expression and TH levels, thus causing neurotoxic characteristics. Collectively, this study provides the full understanding of the environmental impact of BHPF and its toxicity on living organisms, highlighting a substantial and generalized ongoing dose-response relationship with great implications for the usage and risk assessment of BHPF.

Behavioural effects of early-life exposure to parabens in zebrafish larvae

Parabens are classified as endocrine disrupting chemicals due to their ability to activate several nuclear receptors causing changes in hormones-dependent signalling pathways. Central nervous system of developing organisms is particularly vulnerable to changes in hormonal pathways, which could lead to altered brain function, abnormal behaviour and even diseases later in life. The aim of the present study was to investigate the effects of exposure to butylparaben (BuP), ethylparaben (EtP) and methylparaben (MeP) during early development on nervous system using zebrafish larvae's behavioural models. Zebrafish were exposed until 4 days post fertilization (dpf) to three concentrations of each paraben chosen considering the environmentally realistic concentrations of human exposure and the benchmark-dose lower bound calculated for zebrafish larvae (BuP: 5, 50 and 500 μg/L; EtP: 50, 500 and 5000 μg/L; MeP: 100, 1000 and 10,000 μg/L). Activity in novel and in familiar environment, thigmotaxis, visual startle response and photic synchronization of the behavioural circadian rhythms were analysed at 4, 5 and 6 dpf. Zebrafish larvae exposed to BuP 500 μg/L and EtP 5000 μg/L revealed increased anxiety-like behaviour in novel environment. Larvae treated with 500 μg/L of BuP showed reduced activity in familiar and marginally in unfamiliar environment, and larvae exposed to 5000 μg/L of EtP exhibited hyperactivity in familiar environment. Parabens exposure did not influence the visual startle response and the photic synchronization of circadian rhythms in zebrafish larvae. This research highlighted as the exposure to parabens has the potential to interfere with behavioural development of zebrafish.

Approaches to Test the Neurotoxicity of Environmental Contaminants in the Zebrafish Model: From Behavior to Molecular Mechanisms

The occurrence of neuroactive chemicals in the aquatic environment is on the rise and poses a potential threat to aquatic biota of currently unpredictable outcome. In particular, subtle changes caused by these chemicals to an organism's sensation or behavior are difficult to tackle with current test systems that focus on rodents or with in vitro test systems that omit whole-animal responses. In recent years, the zebrafish (Danio rerio) has become a popular model organism for toxicological studies and testing strategies, such as the standardized use of zebrafish early life stages in the Organisation for Economic Co-operation and Development's guideline 236. In terms of neurotoxicity, the zebrafish provides a powerful model to investigate changes to the nervous system from several different angles, offering the ability to tackle the mechanisms of action of chemicals in detail. The mechanistic understanding gained through the analysis of this model species provides a good basic knowledge of how neuroactive chemicals might interact with a teleost nervous system. Such information can help infer potential effects occurring to other species exposed to neuroactive chemicals in their aquatic environment and predicting potential risks of a chemical for the aquatic ecosystem. In the present article, we highlight approaches ranging from behavioral to structural, functional, and molecular analysis of the larval zebrafish nervous system, providing a holistic view of potential neurotoxic outcomes. Environ Toxicol Chem 2021;40:989-1006. © 2020 SETAC.