A Review of Monoaminergic Neuropsychopharmacology in Zebrafish

Towards zebrafish models to unravel translational insights of obsessive-compulsive disorder: A neurobehavioral perspective

Obsessive-compulsive disorder (OCD) is a chronic and debilitating illness that has been considered a polygenic and multifactorial disorder, challenging effective therapeutic interventions. Although invaluable advances have been obtained from human and rodent studies, several molecular and mechanistic aspects of OCD etiology are still obscure. Thus, the use of non-traditional animal models may foster innovative approaches in this field, aiming to elucidate the underlying mechanisms of disease from an evolutionary perspective. The zebrafish (Danio rerio) has been increasingly considered a powerful organism in translational neuroscience research, especially due to the intrinsic features of the species. Here, we outline target mechanisms of OCD for translational research, and discuss how zebrafish-based models can contribute to explore neurobehavioral aspects resembling those found in OCD. We also identify possible advantages and limitations of potential zebrafish-based models, as well as highlight future directions in both etiological and therapeutic research. Lastly, we reinforce the use of zebrafish as a promising tool to unravel the biological basis of OCD, as well as novel pharmacological therapies in the field.

Myclobutanil induces neurotoxicity by activating autophagy and apoptosis in zebrafish larvae (Danio rerio)

Myclobutanil (MYC), a typical broad-spectrum triazole fungicide, is often detected in surface water. This study aimed to explore the neurotoxicity of MYC and the underlying mechanisms in zebrafish and in PC12 cells. In this study, zebrafish embryos were exposed to 0, 0.5 and 1 mg/L of MYC from 4 to 96 h post fertilization (hpf) and neurobehavior was evaluated. Our data showed that MYC decreased the survival rate, hatching rate and heart rate, but increased the malformation rate and spontaneous movement. MYC caused abnormal neurobehaviors characterized by decreased swimming distance and movement time. MYC impaired cerebral histopathological morphology and inhibited neurogenesis in HuC:egfp transgenic zebrafish. MYC also reduced the activities of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), and downregulated neurodevelopment related genes (gfap, syn2a, gap43 and mbp) in zebrafish and PC12 cells. Besides, MYC activated autophagy through enhanced expression of the LC3-II protein and suppressed expression of the p62 protein and autophagosome formation, subsequently triggering apoptosis by upregulating apoptotic genes (p53, bax, bcl-2 and caspase 3) and the cleaved caspase-3 protein in zebrafish and PC12 cells. These processes were restored by the autophagy inhibitor 3-methyladenine (3-MA) both in vivo and in vitro, indicating that MYC induces neurotoxicity by activating autophagy and apoptosis. Overall, this study revealed the potential autophagy and apoptosis mechanisms of MYC-induced neurotoxicity and provided novel strategies to counteract its toxicity.

Chronic Behavioral and Neurochemical Effects of Four Novel N-Benzyl-2-phenylethylamine Derivatives Recently Identified as "Psychoactive" in Adult Zebrafish Screens

Potently affecting human and animal brain and behavior, hallucinogenic drugs have recently emerged as potentially promising agents in psychopharmacotherapy. Complementing laboratory rodents, the zebrafish (Danio rerio) is a powerful model organism for screening neuroactive drugs, including hallucinogens. Here, we tested four novel N-benzyl-2-phenylethylamine (NBPEA) derivatives with 2,4- and 3,4-dimethoxy substitutions in the phenethylamine moiety and the -F, -Cl, and -OCF3 substitutions in the ortho position of the phenyl ring of the N-benzyl moiety (34H-NBF, 34H-NBCl, 24H-NBOMe(F), and 34H-NBOMe(F)), assessing their behavioral and neurochemical effects following chronic 14 day treatment in adult zebrafish. While the novel tank test behavioral data indicate anxiolytic-like effects of 24H-NBOMe(F) and 34H-NBOMe(F), neurochemical analyses reveal reduced brain norepinephrine by all four drugs, and (except 34H-NBCl) - reduced dopamine and serotonin levels. We also found reduced turnover rates for all three brain monoamines but unaltered levels of their respective metabolites. Collectively, these findings further our understanding of complex central behavioral and neurochemical effects of chronically administered novel NBPEAs and highlight the potential of zebrafish as a model for preclinical screening of small psychoactive molecules.

The influence of acute dopamine transporter inhibition on manic-, depressive-like phenotypes, and brain oxidative status in adult zebrafish

Functional changes in dopamine transporter (DAT) are related to various psychiatric conditions, including bipolar disorder (BD) symptoms. In experimental research, the inhibition of DAT induces behavioral alterations that recapitulate symptoms found in BD patients, including mania and depressive mood. Thus, developing novel animal models that mimic BD-related conditions by pharmacologically modulating the dopaminergic signaling is relevant. The zebrafish (Danio rerio) has been considered a suitable vertebrate system for modeling BD-like responses, due to the well-characterized behavioral responses and evolutionarily conservation of the dopaminergic system of this species. Here, we investigate whether GBR 12909, a selective inhibitor of DAT, causes neurobehavioral alterations in zebrafish similar to those observed in BD patients. Behaviors were recorded after a single intraperitoneal (i.p.) administration of GBR 12909 at different doses (3.75, 7.5, 15 and 30 mg/kg). To observe temporal effects on behavior, swim path parameters were measured immediately after the administration period during 30 min. Locomotion, anxiety-like behavior, social preference, aggression, despair-like behavior, and oxidative stress-related biomarkers in the brain were measured 30 min post administration. GBR 12909 induced prominent effects on locomotor activity and vertical exploration during the 30-min period. Hyperactivity was observed in GBR 30 group after 25 min, while all doses markedly reduced vertical drifts. GBR 12909 elicited hyperlocomotion, anxiety-like behavior, decreased social preference, aggression, and induced depressive-like behavior in a behavioral despair task. Depending on the dose, GBR 12909 also decreased SOD activity and TBARS levels, as well as increased GR activity and NPSH content. Collectively, our novel findings show that a single GBR 12909 administration evokes neurobehavioral changes that recapitulate manic- and depressive-like states observed in rodents, fostering the use of zebrafish models to explore BD-like responses in translational neuroscience research.

Tryptophan and Cortisol Modulate the Kynurenine and Serotonin Transcriptional Pathway in the Kidney of Oncorhynchus kisutch

Aquaculture fish are kept for long periods in sea cages or tanks. Consequently, accumulated stress causes the fish to present serious problems with critical economic losses. Fish food has been supplemented to reduce this stress, using many components as amino acids such as tryptophan. This study aims to determine the transcriptional effect of tryptophan and cortisol on primary cell cultures of salmon head and posterior kidney. Our results indicate activation of the kynurenine pathway and serotonin activity when stimulated with tryptophan and cortisol. An amount of 95% of tryptophan is degraded by the kynurenine pathway, indicating the relevance of knowing how this pathway is activated and if stress levels associated with fish culture trigger its activation. Additionally, it is essential to know the consequence of increasing kynurenic acid "KYNA" levels in the short and long term, and even during the fish ontogeny.

MiR-133b as a crucial regulator of TCS-induced cardiotoxicity via activating β-adrenergic receptor signaling pathway in zebrafish embryos

As a commonly used antibacterial agent in daily consumer products, triclosan (TCS) has attracted significant attention due to its potential environmental risks. In this study, we investigated the toxic effects of TCS exposure (1.4 μM) on heart development in zebrafish embryos. Our findings revealed that TCS exposure caused significant cardiac dysfunction, characterized by pericardial edema, malformations in the heart structure, and a slow heart rate. Additionally, TCS exposure induced oxidative damage and abnormal apoptosis in heart cells through the up-regulation of β-adrenergic receptor (β-AR) signaling pathway genes (adrb1, adrb2a, arrb2b), similar to the effects induced by β-AR agonists. Notably, the adverse effects of TCS exposure were alleviated by β-AR antagonists. Using high-throughput transcriptome miRNA sequencing and targeted miRNA screening, we focused on miR-133b, which targets adrb1 and was down-regulated by TCS exposure, as a potential contributor to TCS-induced cardiotoxicity. Inhibition of miR-133b produced similar toxic effects as TCS exposure, while overexpression of miR-133b down-regulated the β-AR signaling pathway and rescued heart defects caused by TCS. In summary, our findings provide new insights into the mechanisms underlying the cardiotoxic effects of TCS. We suggest that targeting the β-AR pathway and miR-133b may be effective strategies for pharmacotherapy in cardiotoxicity induced by environmental pollutants such as TCS.

A comparative study to optimize experimental conditions of pentylenetetrazol and pilocarpine-induced epilepsy in zebrafish larvae

A common way to investigate epilepsy and the effect of antiepileptic pharmaceuticals is to analyze the movement patterns of zebrafish larvae treated with different convulsants like pentylenetetrazol (PTZ), pilocarpine, etc. Many articles have been written on this topic, but the research methods and exact settings are not sufficiently defined in most. Here we designed and executed a series of experiments to optimize and standardize the zebrafish epilepsy model. We found that during the light and the dark trials, the zebrafish larvae moved significantly more in the light, independent of the treatment, both in PTZ and pilocarpine-treated and the control groups. As expected, zebrafish larvae treated with convulsants moved significantly more than the ones in the control group, although this difference was higher between the individuals treated with PTZ than pilocarpine. When examining the optimal observation time, we divided the half-hour period into 5-minute time intervals, and between these, the first 5 minutes were found to be the most different from the others. There were fewer significant differences in the total movement of larvae between the other time intervals. We also performed a linear regression analysis with the cumulative values of the distance moved during the time intervals that fit the straight line. In conclusion, we recommend 30 minutes of drug pretreatment followed by a 10-minute test in light conditions with a 5-minute accommodation time. Our result paves the way toward improved experimental designs using zebrafish to develop novel pharmaceutical approaches to treat epilepsy.

Ethanol and caffeine age-dependently alter brain and retinal neurochemical levels without affecting morphology of juvenile and adult zebrafish (Danio rerio)

Adolescent alcohol exposure in humans is predictive of adult development of alcoholism. In rodents, caffeine pre-exposure enhances adult responsiveness to ethanol via a pathway targeted by both compounds. Embryonic exposure to either compound adversely affects development, and both compounds can alter zebrafish behaviors. Here, we evaluate whether co-exposure to caffeine and/or alcohol in adolescence exerts neurochemical changes in retina and brain. Zebrafish (Danio rerio) were given daily 20 min treatments to ethanol (1.5% v/v), caffeine (25-100 mg/L), or caffeine + ethanol for 1 week during mid-late adolescence (53-92 days post fertilization (dpf)) or early adulthood (93-142 dpf). Immediately after exposure, anatomical measurements were taken, including weight, heart rate, pigment density, length, girth, gill width, inner and outer eye distance. Brain and retinal tissue were subsequently collected either (1) immediately, (2) after a short interval (2-4d) following exposure, or (3) after a longer interval that included an acute 1.5% ethanol challenge. Chronic ethanol and/or caffeine exposure did not alter anatomical parameters. However, retinal and brain levels of tyrosine hydroxylase were elevated in fish sacrificed after the long interval following exposure. Protein levels of glutamic acid decarboxylase were also increased, with the highest levels observed in 70-79 dpf fish exposed to caffeine. The influence of ethanol and caffeine exposure on neurochemistry demonstrates specificity of their effects during postembryonic development. Using the zebrafish model to assess neurochemistry relevant to reward and anxiety may inform understanding of the mechanisms that reinforce co-addiction to alcohol and stimulants.

A novel small molecule, AS1, reverses the negative hedonic valence of noxious stimuli

BACKGROUND

Pain is the primary reason people seek medical care, with chronic pain affecting ~ 20% of people in the USA. However, many existing analgesics are ineffective in treating chronic pain, while others (e.g., opioids) have undesirable side effects. Here, we describe the screening of a small molecule library using a thermal place aversion assay in larval zebrafish to identify compounds that alter aversion to noxious thermal stimuli and could thus serve as potential analgesics.

RESULTS

From our behavioral screen, we discovered a small molecule, Analgesic Screen 1 (AS1), which surprisingly elicited attraction to noxious painful heat. When we further explored the effects of this compound using other behavioral place preference assays, we found that AS1 was similarly able to reverse the negative hedonic valence of other painful (chemical) and non-painful (dark) aversive stimuli without being inherently rewarding. Interestingly, targeting molecular pathways canonically associated with analgesia did not replicate the effects of AS1. A neuronal imaging assay revealed that clusters of dopaminergic neurons, as well as forebrain regions located in the teleost equivalent of the basal ganglia, were highly upregulated in the specific context of AS1 and aversive heat. Through a combination of behavioral assays and pharmacological manipulation of dopamine circuitry, we determined that AS1 acts via D1 dopamine receptor pathways to elicit this attraction to noxious stimuli.

CONCLUSIONS

Together, our results suggest that AS1 relieves an aversion-imposed "brake" on dopamine release, and that this unique mechanism may provide valuable insight into the development of new valence-targeting analgesic drugs, as well as medications for other valence-related neurological conditions, such as anxiety and post-traumatic stress disorder (PTSD).

Acute administration of a dopamine D2/D3 receptor agonist alters behavioral and neural parameters in adult zebrafish

The dopaminergic neurotransmitter system is implicated in several brain functions and behavioral processes. Alterations in it are associated with the pathogenesis of several human neurological disorders. Pharmacological agents that interact with the dopaminergic system allow the investigation of dopamine-mediated cellular and molecular responses and may elucidate the biological bases of such disorders. Zebrafish, a translationally relevant biomedical research organism, has been successfully employed in prior psychopharmacology studies. Here, we evaluated the effects of quinpirole (dopamine D2/D3 receptor agonist) in adult zebrafish on behavioral parameters, brain-derived neurotrophic factor (BDNF) and neurotransmitter levels. Zebrafish received intraperitoneal injections of 0.5, 1.0, or 2.0 mg/kg quinpirole or saline (control group) twice with an inter-injection interval of 48 h. All tests were performed 24 h after the second injection. After this acute quinpirole administration, zebrafish exhibited decreased locomotor activity, increased anxiety-like behaviors and memory impairment. However, quinpirole did not affect social and aggressive behavior. Quinpirole-treated fish exhibited stereotypic swimming, characterized by repetitive behavior followed by immobile episodes. Moreover, quinpirole treatment also decreased the number of BDNF-immunoreactive cells in the zebrafish brain. Analysis of neurotransmitter levels demonstrated a significant increase in glutamate and a decrease in serotonin, while no alterations were observed in dopamine. These findings demonstrate that dopaminergic signaling altered by quinpirole administration results in significant behavioral and neuroplastic changes in the central nervous system of zebrafish. Thus, we conclude that the use of quinpirole administration in adult zebrafish may be an appropriate tool for the analysis of mechanisms underlying neurological disorders related to the dopaminergic system.

Methamphetamine-induced lethal toxicity in zebrafish larvae

RATIONALE

The use of novel psychoactive substances has been steadily increasing in recent years. Given the rapid emergence of new substances and their constantly changing chemical structure, it is necessary to develop an efficient and expeditious approach to examine the mechanisms underlying their pharmacological and toxicological effects. Zebrafish (Danio rerio) have become a popular experimental subject for drug screening due to their amenability to high-throughput approaches.

OBJECTIVES

In this study, we used methamphetamine (METH) as an exemplary psychoactive substance to investigate its acute toxicity and possible underlying mechanisms in 5-day post-fertilization (5 dpf) zebrafish larvae.

METHODS

Lethality and toxicity of different concentrations of METH were examined in 5-dpf zebrafish larvae using a 96-well plate format.

RESULTS

METH induced lethality in zebrafish larvae in a dose-dependent manner, which was associated with initial sympathomimetic activation, followed by cardiotoxicity. This was evidenced by significant heart rate increases at low doses, followed by decreased cardiac function at high doses and later time points. Levels of ammonia in the excreted water were increased but decreased internally. There was also evidence of seizures. Co-administration of the glutamate AMPA receptor antagonist GYKI-52466 and the dopamine D2 receptor antagonist raclopride significantly attenuated METH-induced lethality, suggesting that this lethality may be mediated synergistically or independently by glutamatergic and dopaminergic systems.

CONCLUSIONS

These experiments provide a baseline for the study of the toxicity of related amphetamine compounds in 5-dpf zebrafish as well as a new high-throughput approach for investigating the toxicities of rapidly emerging new psychoactive substances.

Cardiotoxicity of Zebrafish Induced by 6-Benzylaminopurine Exposure and Its Mechanism

6-BA is a common plant growth regulator, but its safety has not been conclusive. The heart is one of the most important organs of living organisms, and the cardiogenesis process of zebrafish is similar to that of humans. Therefore, based on wild-type and transgenic zebrafish, we explored the development of zebrafish heart under 6-BA exposure and its mechanism. We found that 6-BA affected larval cardiogenesis, inducing defective expression of key genes for cardiac development (myl7, vmhc, and myh6) and AVC differentiation (bmp4, tbx2b, and notch1b), ultimately leading to weakened cardiac function (heart rate, diastolic speed, systolic speed). Acridine orange staining showed that the degree of apoptosis in zebrafish hearts was significantly increased under 6-BA, and the expression of cell-cycle-related genes was also changed. In addition, HPA axis assays revealed abnormally expressed mRNA levels of genes and significantly increased cortisol contents, which was also consistent with the observed anxiety behavior in zebrafish at 3 dpf. Transcriptional abnormalities of pro- and anti-inflammatory factors in immune signaling pathways were also detected in qPCR experiments. Collectively, we found that 6-BA induced cardiotoxicity in zebrafish, which may be related to altered HPA axis activity and the onset of inflammatory responses under 6-BA treatment.

Zebrafish Larvae Behavior Models as a Tool for Drug Screenings and Pre-Clinical Trials: A Review

To discover new molecules or review the biological activity and toxicity of therapeutic substances, drug development, and research relies on robust biological systems to obtain reliable results. Phenotype-based screenings can transpose the organism’s compensatory pathways by adopting multi-target strategies for treating complex diseases, and zebrafish emerged as an important model for biomedical research and drug screenings. Zebrafish’s clear correlation between neuro-anatomical and physiological features and behavior is very similar to that verified in mammals, enabling the construction of reliable and relevant experimental models for neurological disorders research. Zebrafish presents highly conserved physiological pathways that are found in higher vertebrates, including mammals, along with a robust behavioral repertoire. Moreover, it is very sensitive to pharmacological/environmental manipulations, and these behavioral phenotypes are detected in both larvae and adults. These advantages align with the 3Rs concept and qualify the zebrafish as a powerful tool for drug screenings and pre-clinical trials. This review highlights important behavioral domains studied in zebrafish larvae and their neurotransmitter systems and summarizes currently used techniques to evaluate and quantify zebrafish larvae behavior in laboratory studies.

Behavioral and neurochemical profile of MK-801 adult zebrafish model: Forebrain β2-adrenoceptors contribute to social withdrawal and anxiety-like behavior

Deficits in social communication and interaction are core clinical symptoms characterizing multiple neuropsychiatric conditions, including autism spectrum disorder (ASD) and schizophrenia. Interestingly, elevated anxiety levels are a common comorbid psychopathology characterizing individuals with aberrant social behavior. Despite recent progress, the underlying neurobiological mechanisms that link anxiety with social withdrawal remain poorly understood. The present study developed a zebrafish pharmacological model displaying social withdrawal behavior, following a 3-h exposure to 4 μΜ (+)-MK-801, a non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist, for 7 days. Interestingly, MK-801-treated zebrafish displayed elevated anxiety levels along with higher frequency of stereotypical behaviors, rendering this zebrafish model appropriate to unravel a possible link of catecholaminergic and ASD-like phenotypes. MK-801-treated zebrafish showed increased telencephalic protein expression of metabotropic glutamate 5 receptor (mGluR5), dopamine transporter (DAT) and β₂-adrenergic receptors (β₂-ARs), supporting the presence of excitation/inhibition imbalance along with altered dopaminergic and noradrenergic activity. Interestingly, β₂-ARs expression, was differentially regulated across the Social Decision-Making (SDM) network nodes, exhibiting increased levels in ventral telencephalic area (Vv), a key-area integrating reward and social circuits but decreased expression in dorso-medial telencephalic area (Dm) and anterior tuberal nucleus (ATN). Moreover, the co-localization of β₂-ARs with elements of GABAergic and glutamatergic systems, as well as with GAP-43, a protein indicating increased brain plasticity potential, support the key-role of β₂-ARs in the MK-801 zebrafish social dysfunctions. Our results highlight the importance of the catecholaminergic neurotransmission in the manifestation of ASD-like behavior, representing a site of potential interventions for amelioration of ASD-like symptoms.

Dopaminergic signaling regulates zebrafish larvae's response to electricity

Electrical stimulation of brain or muscle activities has gained attention for studying the molecular and cellular mechanisms involved in electric-induced responses. We recently showed zebrafish's response to electricity. Here, we hypothesized that this response is affected by the dopaminergic signaling pathways. The effects of multiple dopamine agonists and antagonists on the electric response of 6 days-postfertilization zebrafish larvae were investigated using a microfluidic device with enhanced control of experimentation and throughput. All dopamine antagonists decreased locomotor activities, while dopamine agonists did not induce similar behaviors. The D2-selective dopamine agonist quinpirole enhanced the movement. Exposure to nonselective and D1-selective dopamine agonists apomorphine and SKF-81297 caused no significant change in the electric response. Exposing larvae that were pretreated with nonselective and D2-selective dopamine antagonists butaclamol and haloperidol to apomorphine and quinpirole, respectively, restored the electric locomotion. These results reveal a correlation between electric response and dopamine signaling pathway. Furthermore, they demonstrate that electric-induced zebrafish larvae locomotion can be conditioned by modulating dopamine receptor functions. Our electrofluidic assay has profound application potential for fundamental electric-induced response research and brain disorder studies especially those related to the dopamine imbalance and as a chemical screening method when investigating biological pathways and behaviors. This article is protected by copyright. All rights reserved.

Zebrafish as a Useful Tool in the Research of Natural Products With Potential Anxiolytic Effects

Zebrafish (Danio rerio) is a popular and valuable species used in many different biomedical research areas. The complex behavior that fish exhibit in response to different stimuli allows researchers to explore the biological and pharmacological basis of affective and mood disorders. In this sense, anxiety is commonly studied in preclinical research with animal models in rodents. During the last decade, those models have been successfully adapted to zebrafish. Stressful stimuli, such as novel environments, chemical substances, light conditions, and predator images, can trigger defensive behaviors considered indicators of an anxiety-like state. In the first stage, models were adapted and validated with different stressors and anxiolytic drugs with promising results and are now successfully used to generate scientific knowledge. In that sense, zebrafish allows several routes of administration and other methodological advantages to explore the anxiolytic effects of natural products in behavioral tests as novel tank, light-dark chamber, and black/white maze, among others. The present work will review the main findings on preclinical research using adult zebrafish to explore anxiolytics effects of natural products as plant secondary metabolites such as flavonoids, alkaloids and terpenes or standardized extracts of plants, among others. Scientific literature confirms the utility of zebrafish tests to explore anxiety-like states and anxiolytic-like effects of plant secondary metabolites, which represent a useful and ethical tool in the first stages of behavioral.

Dopamine-neurotransmission and nociception in zebrafish: An anti-nociceptive role of dopamine receptor drd2a

Dopamine (DA) is an important modulator in nociception and analgesia. Spinal DA receptors are involved in descending modulation of the nociceptive transmission. Genetic variations within DA neurotransmission have been associated with altered pain sensitivity and development of chronic pain syndromes. The variant rs6277 in dopamine receptor 2 a (drd2a) has been associated with a decreased D₂ receptor availability and increased nociception. The aim of this study is to further characterize the role of DA neurotransmission in nociception and the anti-nociceptive function of drd2a. The phenotype caused by rs6277 was modelled in zebrafish larvae using morpholino's and the effect on nociception was tested using a validated behavioural assay. The anti-nociceptive role of drd2a was tested using pharmacological intervention of D₂ agonist Quinpirole. The experiments demonstrate that a decrease in drd2a expression results in a pro-nociceptive behavioural phenotype (P = 0.016) after a heat stimulus. Furthermore, agonism of drd2a with agonist Quinpirole (0.2 μM) results in dose-dependent anti-nociception (P = 0.035) after a heat stimulus. From these results it is concluded that the dopamine receptor drd2a is involved in anti-nociceptive behaviour in zebrafish. The model allows further screening and testing of genetic variation and treatment involved in nociception.

Neurochemical and Behavioral Consequences of Ethanol and/or Caffeine Exposure: Effects in Zebrafish and Rodents

Zebrafish are increasingly being utilized to model the behavioral and neurochemical effects of pharmaceuticals and, more recently, pharmaceutical interactions. Zebrafish models of stress establish that both caffeine and ethanol influence anxiety, though few studies have implemented co-administration to assess the interaction of anxiety and reward-seeking. Caffeine exposure in zebrafish is teratogenic, causing developmental abnormalities in the cardiovascular, neuromuscular, and nervous systems of embryos and larvae. Ethanol is also a teratogen and, as an anxiolytic substance, may be able to offset the anxiogenic effects of caffeine. Co-exposure to caffeine and alcohol impacts neuroanatomy and behavior in adolescent animal models, suggesting stimulant substances may moderate the impact of alcohol on neural circuit development. Here, we review the literature describing neuropharmacological and behavioral consequences of caffeine and/or alcohol exposure in the zebrafish model, focusing on neurochemistry, locomotor effects, and behavioral assessments of stress/anxiety as reported in adolescent/juvenile and adult animals. The purpose of this review is twofold: (1) describe the work in zebrafish documenting the effects of ethanol and/or caffeine exposure and (2) compare these zebrafish studies with comparable experiments in rodents. We focus on specific neurochemical pathways (dopamine, serotonin, adenosine, GABA, adenosine), anxiety-type behaviors (assessed with novel tank, thigmotaxis, shoaling), and locomotor changes resulting from both individual and co-exposure. We compare findings in zebrafish with those in rodent models, revealing similarities across species and identifying conservation of mechanisms that potentially reinforce co-addiction.

Developmental exposure to non-dioxin-like polychlorinated biphenyls promotes sensory deficits and disrupts dopaminergic and GABAergic signaling in zebrafish

The most abundant polychlorinated biphenyl (PCB) congeners found in the environment and in humans are neurotoxic. This is of particular concern for early life stages because the exposure of the more vulnerable developing nervous system to neurotoxic chemicals can result in neurobehavioral disorders. In this study, we uncover currently unknown links between PCB target mechanisms and neurobehavioral deficits using zebrafish as a vertebrate model. We investigated the effects of the abundant non-dioxin-like (NDL) congener PCB153 on neuronal morphology and synaptic transmission linked to the proper execution of a sensorimotor response. Zebrafish that were exposed during development to concentrations similar to those found in human cord blood and PCB contaminated sites showed a delay in startle response. Morphological and biochemical data demonstrate that even though PCB153-induced swelling of afferent sensory neurons, the disruption of dopaminergic and GABAergic signaling appears to contribute to PCB-induced motor deficits. A similar delay was observed for other NDL congeners but not for the potent dioxin-like congener PCB126. The effects on important and broadly conserved signaling mechanisms in vertebrates suggest that NDL PCBs may contribute to neurodevelopmental abnormalities in humans and increased selection pressures in vertebrate wildlife.

Functional brain imaging in larval zebrafish for characterising the effects of seizurogenic compounds acting via a range of pharmacological mechanisms

BACKGROUND AND PURPOSE

Functional brain imaging using genetically encoded Ca2+ sensors in larval zebrafish is being developed for studying seizures and epilepsy as a more ethical alternative to rodent models. Despite this, few data have been generated on pharmacological mechanisms of action other than GABAA antagonism. Assessing larval responsiveness across multiple mechanisms is vital to test the translational power of this approach, as well as assessing its validity for detecting unwanted drug-induced seizures and testing antiepileptic drug efficacy.

EXPERIMENTAL APPROACH

Using light-sheet imaging, we systematically analysed the responsiveness of 4 days post fertilisation (dpf; which are not considered protected under European animal experiment legislation) transgenic larval zebrafish to treatment with 57 compounds spanning more than 12 drug classes with a link to seizure generation in mammals, alongside eight compounds with no such link.

KEY RESULTS

We show 4dpf zebrafish are responsive to a wide range of mechanisms implicated in seizure generation, with cerebellar circuitry activated regardless of the initiating pharmacology. Analysis of functional connectivity revealed compounds targeting cholinergic and monoaminergic reuptake, in particular, showed phenotypic consistency broadly mapping onto what is known about neurotransmitter-specific circuitry in the larval zebrafish brain. Many seizure-associated compounds also exhibited altered whole brain functional connectivity compared with controls.

CONCLUSIONS AND IMPLICATIONS

This work represents a significant step forward in understanding the translational power of 4dpf larval zebrafish for use in neuropharmacological studies and for studying the events driving transition from small-scale pharmacological activation of local circuits, to the large network-wide abnormal synchronous activity associated with seizures.

Triadimefon triggers circling behavior and conditioned place preference/aversion in zebrafish in a dose dependent manner

Triadimefon (TDF) is a pesticide used in agricultural crops to control powdery mildews, rusts and other fungal pests. It exerts its fungicidal activity through the inhibition of ergosterol biosynthesis, impairing the formation of the cell membrane. For vertebrates, one of its side effects is the binding to the dopamine transporter increasing the levels of synaptic dopamine, similarly to cocaine. In addition, it has been demonstrated that TDF affects the abundance of other monoamines in the brain, specifically serotonin. It is well known that drugs which alter the dopaminergic and serotonergic systems produce behavioral changes and participate in the development of addictions in mammals. In this work we have used the conditioned place preference paradigm to assess, for the first time, the rewarding properties of TDF in zebrafish. We found out that TDF triggers both, preference and aversion depending on the dosage used during conditioning. We observed that 5 mg/L produced aversion to the pattern previously paired with TDF. However, 15 mg/L induced the opposite behavior, showing that zebrafish seek out those environments which had previously been paired with the higher dose of TDF. These results are congruent with our previous findings, where we showed that 5 mg/L reduced the levels of serotonin, usually linked to anxious behaviors (a negative cue), whereas higher concentrations of TDF increased extracellular dopamine, the main currency of the reward system. Interestingly, both doses of TDF induced circling behavior, a feature usually seen in glutamatergic antagonists.

Stimulants cocktail: Methylphenidate plus caffeine impairs memory and cognition and alters mitochondrial and oxidative status

Methylphenidate (MPH) is a psychostimulant widely misused to increase wakefulness by drivers and students. Also, MPH can be found in dietary supplements in a clandestine manner aiming to burst performance of physical exercise practitioners. The abusive use of high doses of caffeine (CAF) in these contexts is equally already known. Here, we demonstrate the behavioral, oxidative and mitochondrial effects after acute exposure to high doses of MPH (80 mg/L) and CAF (150 mg/L), alone or associated (80 mg/L + 150 mg/L, respectively). We used zebrafish as animal model due to its high translational relevance. We evaluated the behavioral effects using the Novel Tank Test (NTT), Social Preference Test (SPT) and Y-maze Task and analyzed biomarkers of oxidative stress and activity of mitochondrial respiratory chain complexes. MPH alone induced antisocial behavior. MPH inhibited lipid peroxidation. The association of MPH + CAF presented memory impairment and anxiogenic behavior. In oxidative status, it inhibited lipid peroxidation, increased protein carbonylation and mitochondrial complex II, III and IV activity. Our results demonstrate that MPH and CAF alone negatively impact the typical behavioral of zebrafish. When associated, changes in cognition, memory, oxidative and mitochondrial status are more relevant.

Exposure to tricyclic antidepressant nortriptyline affects early-life stages of zebrafish (Danio rerio)

Psychiatric drugs are among the leading medications prescribed for humans, with their presence in aquatic environments raising concerns relating to potentially harmful effects on non-target organisms. Nortriptyline (NTP) is a selective serotonin-norepinephrine reuptake inhibitor antidepressant, widely used in clinics and found in environmental water matrices. In this study, we evaluated the toxic effects of NTP on zebrafish (Danio rerio) embryos and early larval stages. Developmental and mortality analyses were performed on zebrafish exposed to NTP for 168 h at concentrations ranging from 500 to 46,900 µg/L. Locomotor behaviour and acetylcholinesterase (AChE) activity were evaluated by exposing embryos/larvae to lower NTP concentrations (0.006-500 µg/L). The median lethal NTP concentration after 168 h exposure was 2190 µg/L. Although we did not identify significant developmental changes in the treated groups, lack of equilibrium was already visible in surviving larvae exposed to ≥ 500 µg/L NTP. The behavioural analyses showed that NTP was capable of modifying zebrafish larvae swimming behaviour, even at extremely low (0.006 and 0.088 µg/L) environmentally relevant concentrations. We consistently observed a significant reduction in AChE activity in the animals exposed to 500 µg/L NTP. Our results highlight acute toxic effects of NTP on the early-life stages of zebrafish. Most importantly, exposure to environmentally relevant NTP concentrations may affect zebrafish larvae locomotor behaviour, which in turn could reduce the fitness of the species. More studies involving chronic exposure and sensitive endpoints are warranted to better understand the effect of NTP in a more realistic exposure scenario.

Time-Dependent Effects of Acute Handling on the Brain Monoamine System of the Salmonid Coregonus maraena

The immediate stress response involves the activation of the monoaminergic neurotransmitter systems including serotonin, dopamine and noradrenaline in particular areas of the fish brain. We chose maraena whitefish as a stress-sensitive salmonid species to investigate the influence of acute and chronic handling on the neurochemistry of monoamines in the brain. Plasma cortisol was quantified to assess the activation of the stress axis. In addition, we analyzed the expression of 37 genes related to the monoamine system to identify genes that could be used as markers of neurophysiological stress effects. Brain neurochemistry responded to a single handling (1 min netting and chasing) with increased serotonergic activity 3 h post-challenge. This was accompanied by a modulated expression of monoaminergic receptor genes in the hindbrain and a significant increase of plasma cortisol. The initial response was compensated by an increased monoamine synthesis at 24 h post-challenge, combined with the modulated expression of serotonin-receptor genes and plasma cortisol concentrations returning to control levels. After 10 days of repeated handling (1 min per day), we detected a slightly increased noradrenaline synthesis and a down-regulated expression of dopamine-receptor genes without effect on plasma cortisol levels. In conclusion, the changes in serotonergic neurochemistry and selected gene-expression profiles, together with the initial plasma cortisol variation, indicate an acute response and a subsequent recovery phase with signs of habituation after 10 days of daily exposure to handling. Based on the basal expression patterns of particular genes and their significant regulation upon handling conditions, we suggest a group of genes as potential biomarkers that indicate handling stress on the brain monoamine systems.

The zebrafish histamine H3 receptor modulates aggression, neural activity and forebrain functional connectivity

AIM

Aggression is a behavioural trait characterized by the intention to harm others for offensive or defensive purposes. Neurotransmitters such as serotonin and dopamine are important mediators of aggression. However, the physiological role of the histaminergic system during this behaviour is currently unclear. Here, we aimed to better understand histaminergic signalling during aggression by characterizing the involvement of the histamine H3 receptor (Hrh3).

METHODS

We have generated a novel zebrafish Hrh3 null mutant line using CRISPR-Cas9 genome engineering and investigated behavioural changes and alterations to neural activity using whole brain Ca2+ imaging in zebrafish larvae and ribosomal protein S6 (rpS6) immunohistochemistry in adults.

RESULTS

We show that genetic inactivation of the histamine H3 receptor (Hrh3) reduces aggression in zebrafish, an effect that can be reproduced by pharmacological inhibition. In addition, hrh3-/- zebrafish show behavioural impairments consistent with heightened anxiety. Larval in vivo whole brain Ca2+ imaging reveals higher neuronal activity in the forebrain of mutants, but lower activity in specific hindbrain areas and changes in measures of functional connectivity between subregions. Adult hrh3-/- zebrafish display brain region-specific neural activity changes in response to aggression of both key regions of the social decision-making network, and the areas containing histaminergic neurons in the zebrafish brain.

CONCLUSION

These results highlight the importance of zebrafish Hrh3 signalling for aggression and anxiety and uncover the brain areas involved. Targeting this receptor might be a potential novel therapeutic route for human conditions characterized by heightened aggression.

Characterization of monoaminergic neurochemicals in the different brain regions of adult zebrafish

Monoaminergic neurotransmitters are the main components that regulate of a lot of processes in the vertebrate brain. There is growing interest to monitor the changes produced in these neurochemicals due to the large number of exogenous agents, such as pharmaceuticals and drugs of abuse, targeting and affecting this system. Adult zebrafish (Danio rerio) shares the common neurotransmitter pathways and nervous system organization with mammals. Therefore, a method based on liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) has been developed for the first time to study the profile of ten monoaminergic neurochemicals in the anterior, middle and posterior brain regions of adult zebrafish. Moreover, the applied LC-MS/MS method has been studied in terms of quality such as linearity, sensitivity and intra- and inter-day precision. The analytical method based in LC-MS/MS has become a new source in neurotoxicology using adult zebrafish as research model. Significant differences on the levels of these neurotransmitters have been found between the different brain regions. CAPSULE: The profile of ten monoaminergic neurochemicals in the main three brain areas of adult zebrafish has been reported for the first time in this manuscript.

Serotonin distribution in the brain of the plainfin midshipman: Substrates for vocal-acoustic modulation and a reevaluation of the serotonergic system in teleost fishes

Serotonin (5-HT) is a modulator of neural circuitry underlying motor patterning, homeostatic control, and social behavior. While previous studies have described 5-HT distribution in various teleosts, serotonergic raphe subgroups in fish are not well defined and therefore remain problematic for cross-species comparisons. Here we used the plainfin midshipman fish, Porichthys notatus, a well-studied model for investigating the neural and hormonal mechanisms of vertebrate vocal-acoustic communication, to redefine raphe subgroups based on both stringent neuroanatomical landmarks as well as quantitative cell measurements. In addition, we comprehensively characterized 5-HT-immunoreactive (-ir) innervation throughout the brain, including well-delineated vocal and auditory nuclei. We report neuroanatomical heterogeneity in populations of the serotonergic raphe nuclei of the brainstem reticular formation, with three discrete subregions in the superior raphe, an intermediate 5-HT-ir cell cluster, and an extensive inferior raphe population. 5-HT-ir neurons were also observed within the vocal motor nucleus (VMN), forming putative contacts on those cells. In addition, three major 5-HT-ir cell groups were identified in the hypothalamus and one group in the pretectum. Significant 5-HT-ir innervation was found in components of the vocal pattern generator and cranial motor nuclei. All vocal midbrain nuclei showed considerable 5-HT-ir innervation, as did thalamic and hindbrain auditory and lateral line areas and vocal-acoustic integration sites in the preoptic area and ventral telencephalon. This comprehensive atlas offers new insights into the organization of 5-HT nuclei in teleosts and provides neuroanatomical evidence for serotonin as a modulator of vocal-acoustic circuitry and behavior in midshipman fish, consistent with findings in vocal tetrapods.

Understanding the neurobiological effects of drug abuse: Lessons from zebrafish models

Drug abuse and brain disorders related to drug comsumption are public health problems with harmful individual and social consequences. The identification of therapeutic targets and precise pharmacological treatments to these neuropsychiatric conditions associated with drug abuse are urgently needed. Understanding the link between neurobiological mechanisms and behavior is a key aspect of elucidating drug abuse-related targets. Due to various molecular, biochemical, pharmacological, and physiological features, the zebrafish (Danio rerio) has been considered a suitable vertebrate for modeling complex processes involved in drug abuse responses. In this review, we discuss how the zebrafish has been successfully used for modeling neurobehavioral phenotypes related to drug abuse and review the effects of opioids, cannabinoids, alcohol, nicotine, and psychedelic drugs on the central nervous system (CNS). Moreover, we summarize recent advances in zebrafish-based studies and outline potential advantages and limitations of the existing zebrafish models to explore the neurochemical bases of drug abuse and addiction. Finally, we discuss how the use of zebrafish models may present fruitful approaches to provide valuable clinically translatable data.

Anxiolytic-like effect of chalcone N-{4'[(2E)-3-(3-nitrophenyl)-1-(phenyl)prop-2-en-1-one]} acetamide on adult zebrafish (Danio rerio): Involvement of the 5-HT system

The action of anxiolytic compounds that act on selective serotonin receptors (SSRIs) have been scarcely evaluated. Serotonergic drugs have been shown to be effective in treating anxiety without presenting adverse effects as benzodiazepines. However, the anxiolytic effects take days to occur. This study aimed to evaluate the anxiolytic effect of the synthetic chalcone, 4'-[(2E) -3- (3-nitrophenyl) -1- (phenyl) prop-2-en-1-one] acetamide (PAAMNBA), and its possible mechanism of action in adult zebrafish (Danio rerio). PAAMNBA was synthesized with a yield of 51.3% and its chemical structure was determined by ¹H and ¹³C NMR. Initially, PAAPMNBA was intraperitoneally administered to zebrafish (n = 6/group) at doses of 4, 12, or 40 mg/kg, and the animals were subsequently subjected to acute and open field toxicity tests. PAAMNBA was administered to the other groups (n = 6/group) for analyzing its effect in the light and dark test. The involvement of the serotonergic (5HT) system was also evaluated using 5-HTR 1, 5-HTR 2A/2C, and 5-HTR 3A/3B receptor antagonists, namely, pizotifeo, granizetron, and ciproeptadina, respectively. Molecular coupling was performed using the 5-HT1 receptor. PAAMNBA was found to be non-toxic, reduced the locomotor activity, and had an anxiolytic effect in adult zebrafish. The effect was reduced by pretreatment with pizotifene and was not reversed by treatment with granizetron and cyproeptadine. A previous in vivo molecular coupling study indicated that chalcones interact with the 5-HT1 receptor. The results suggested that the chalcone, PAAPMNBA, has anxiolytic activity, that is mediated by the serotonergic system via the 5-HT1 receptor. The interaction of PAAPMNBA with the 5-HT1 receptor was confirmed by molecular docking studies.

A Novel Function of the Lysophosphatidic Acid Receptor 3 (LPAR3) Gene in Zebrafish on Modulating Anxiety, Circadian Rhythm Locomotor Activity, and Short-Term Memory

Lysophosphatidic acid (LPA) is a small lysophospholipid molecule that activates multiple cellular functions through pathways with G-protein-coupled receptors. So far, six LPA receptors (LPAR1 to LPAR6) have been discovered and each one of them can connect to the downstream cell message-transmitting network. A previous study demonstrated that LPA receptors found in blood-producing stem cells can enhance erythropoietic processes through the activation of LPAR3. In the current study, newly discovered functions of LPAR3 were identified through extensive behavioral tests in lpar3 knockout (KO) zebrafish. It was found that the adult lpar3 KO zebrafish display an abnormal movement orientation and altered exploratory behavior compared to that of the control group in the three-dimensional locomotor and novel tank tests, respectively. Furthermore, consistent with those results, in the circadian rhythm locomotor activity test, the lpar3 KO zebrafish showed a lower level of angular velocity and average speed during the light cycles, indicating an hyperactivity-like behavior. In addition, the mutant fish also exhibited considerably higher locomotor activity during the dark cycle. Supporting those findings, this phenomenon was also displayed in the lpar3 KO zebrafish larvae. Furthermore, several important behavior alterations were also observed in the adult lpar3 KO fish, including a lower degree of aggression, less interest in conspecific social interaction, and looser shoal formation. However, there was no significant difference regarding the predator avoidance behavior between the mutant and the control fish. In addition, lpar3 KO zebrafish displayed memory deficiency in the passive avoidance test. These in vivo results support for the first time that the lpar3 gene plays a novel role in modulating behaviors of anxiety, aggression, social interaction, circadian rhythm locomotor activity, and memory retention in zebrafish.

Role of the serotonergic system in ethanol-induced aggression and anxiety: A pharmacological approach using the zebrafish model

Acute ethanol (EtOH) consumption exerts a biphasic effect on behavior and increases serotonin levels in the brain. However, the molecular mechanisms underlying alcohol-mediated behavioral responses still remain to be fully elucidated. Here, we investigate pharmacologically the involvement of the serotonergic pathway on acute EtOH-induced behavioral changes in zebrafish. We exposed zebrafish to 0.25, 0.5, 1.0% (v/v) EtOH for 1 h and analyzed the effects on aggression, anxiety-like behaviors, and locomotion. EtOH concentrations that changed behavioral responses were selected to the subsequent experiments. As a pharmacological approach, we used pCPA (inhibitor of tryptophan hydroxylase), WAY100135 (5-HT_1A antagonist), buspirone (5-HT_1A agonist), CGS12066A and CGS12066B (5-HT_1B antagonist and agonist, respectively), ketanserin (5-HT_2A antagonist) and (±)-DOI hydrochloride (5-HT_2A agonist). All serotonergic receptors tested modulated aggression, with a key role of 5-HT_2A in aggressive behavior following 0.25% EtOH exposure. Because CGS12066B mimicked 0.5% EtOH anxiolysis, which was antagonized by CGS12066A, we hypothesized that anxiolytic-like responses are possibly mediated by 5-HT_1B receptors. Conversely, the depressant effects of EtOH are probably not related with direct changes on serotonergic pathway. Overall, our novel findings demonstrate a role of the serotonergic system in modulating the behavioral effects of EtOH in zebrafish. These data also reinforce the growing utility of zebrafish models in alcohol research and help elucidate the neurobiological mechanisms underlying alcohol abuse and associated complex behavioral phenotypes.

Low concentrations of the benzodiazepine drug oxazepam induce anxiolytic effects in wild-caught but not in laboratory zebrafish

Pollution by psychoactive pharmaceuticals has been found to disrupt anti-predator behaviors of wild fish. The challenge is now to identify which of the many psychoactive drugs pose the greatest threat. One strategy is to screen for behavioral effects of selected pharmaceuticals using a single, widely available fish species such as zebrafish. Here, we show that although such high-throughput behavioral screening might facilitate comparisons between pharmaceuticals, the choice of strain is essential. While wild-caught zebrafish exposed to concentrations of the anxiolytic drug oxazepam as low as 0.57 μg L^-1 showed a reduction in the response to conspecific alarm pheromone, laboratory strain AB did not respond to the alarm cue, and consequently, the anxiolytic effect of oxazepam could not be measured. Adaptation to the laboratory environment may have rendered laboratory strains unfit for use in some ecotoxicological and pharmacological studies, since the results might not translate to wild fish populations.

The Role of Zebrafish and Laboratory Rodents in Schizophrenia Research

Schizophrenia is a severe disorder characterized by positive, negative and cognitive symptoms, which are still not fully understood. The development of efficient antipsychotics requires animal models of a strong validity, therefore the aims of the article were to summarize the construct, face and predictive validity of schizophrenia models based on rodents and zebrafish, to compare the advantages and disadvantages of these models, and to propose future directions in schizophrenia modeling and indicate when it is reasonable to combine these models. The advantages of rodent models stem primarily from the high homology between rodent and human physiology, neurochemistry, brain morphology and circuitry. The advantages of zebrafish models stem in the high fecundity, fast development and transparency of the embryo. Disadvantages of both models originate in behavioral repertoires not allowing specific symptoms to be modeled, even when the models are combined. Especially modeling the verbal component of certain positive, negative and cognitive symptoms is currently impossible.

Behavioral effects of triadimefon in zebrafish are associated with alterations of the dopaminergic and serotonergic pathways

Triadimefon (TDF) is a triazole fungicide extensively used in agriculture that has been found as a pollutant in numerous water sources. In mammals, it inhibits monoamine uptake through binding to the dopamine transporter, with a mechanism of action similar to cocaine, resulting in higher levels of dopamine at the synapse. Dopamine is a neurotransmitter involved in a broad spectrum of processes such as locomotion, cognition, reward, and mental disorders. In this work we have studied, for the first time, the effects of TDF on behavior of both larval and adult zebrafish and its connection with changes in the dopaminergic and serotonergic systems. We evaluated the acute exposure of 5 dpf larvae to different concentrations of TDF, ranging from 5 mg/L to 35 mg/L. The lowest concentration does not alter neither locomotor activity nor dopamine levels but produced changes in the expression of two genes, tyrosine hydroxylase 1 (th1) and dopamine transporter (dat). Besides, it induced a reduction in extracellular serotonin and had an anxiolytic-like effect, supported by a decrease in cortisol production. On the other hand, a high concentration of TDF produced a dose-dependent reduction in locomotion, which was reversed or enhanced by D1 (SCH-23390) or D2 (Haloperidol) dopamine receptor antagonists, respectively. Using in vivo electrochemistry, we show that these changes could be associated with higher levels of dopamine in the brain. Thus, in adult zebrafish, though not in larvae, TDF exposure increases locomotor activity, anxiety and aggressiveness, which coincides with the behaviors observed in mammals.

Depression-like state behavioural outputs may confer beneficial outcomes in risky environments

Recent theories in evolutionary medicine have suggested that behavioural outputs associated with depression-like states (DLS) could be an adaptation to unpredictable and precarious situations. In animal models, DLS are often linked to diverse and unpredictable stressors or adverse experiences. Theoretically, there are a range of potential fitness benefits associated with behavioural inhibition (typical to DLS), as opposed to more active/aggressive responses to adverse or uncontrollable events. This stance of evolutionary medicine has to our knowledge not been tested empirically. Here we address a possible key benefit of behavioural inhibition in a comparative model for social stress (territorial rainbow trout). By treating fish with the fast-acting antidepressant ketamine, we reversed the behavioural inhibition (i.e. stimulated an increase in activity level) in subordinate fish. During confrontation with a previously unfamiliar larger, aggressive and dominant individual, this increase in activity led to higher amounts of received aggression compared to sham-treated subordinates. This suggests that the behavioural inhibition characterizing animal models of DLS is indeed an effective coping strategy that reduces the risk of injuries in vulnerable social situations.

Divergent action of fluoxetine in zebrafish according to responsivity to novelty

Here we show that the novel object recognition test can discriminate between high (HRN, neophobic) and low (LRN, neophilic) novelty responders in zebrafish populations. Especially when we observe the latency to the first entry in the novel object zone, zebrafish did not maintain these behavioral phenotypes in sequential tests and only the HRN group returned to their initial responsive behavior when exposed to fluoxetine. Our results have important implications for behavioral data analysis since such behavioral differences can potentially increase individual response variability and interfere with the outcomes obtained from various behavioral tasks. Our data reinforce the validity of personality determination in zebrafish since we show clear differences in behavior in response to fluoxetine.

Bold zebrafish (Danio rerio) express higher levels of delta opioid and dopamine D2 receptors in the brain compared to shy fish

Individual variation in coping with environmental challenges is a well-known phenomenon across vertebrates, including teleost fish. Dopamine is the major transmitter in the brain reward networks, and important for motivational processes and stress coping. Functions of the endogenous opioid system are not well studied in teleosts. However, in mammals the activity in the brain reward networks is regulated by the endogenous opioid system. This study aimed at investigating if there was a correlation between risk-taking behavior and the expression of dopamine and opioid receptors in the zebrafish (Danio rerio) brain. Risk-taking behavior was assessed in a novel tank diving test, and the most extreme high risk taking, i.e. bold, and low risk taking, i.e. shy, fish were sampled for qPCR analysis of whole brain gene expression. The expression analysis showed a significantly higher expression of the dopamine D2 receptors (drd2a and drd2b) and the delta opioid receptor (DOR; oprd1b) in bold compared to shy fish. Besides reward and reinforcing properties, DORs are also involved in emotional responses. Dopamine D2 receptors are believed to be important for active stress coping in rodents, and taken together the results of the current study suggest similar functions in zebrafish. However, additional experiments are required to clarify how dopamine and opioid receptor activation affect behavior and stress coping in this species.

Zebrafish as an Animal Model for Drug Discovery in Parkinson's Disease and Other Movement Disorders: A Systematic Review

Movement disorders can be primarily divided into hypokinetic and hyperkinetic. Most of the hypokinetic syndromes are associated with the neurodegenerative disorder Parkinson’s disease (PD). By contrast, hyperkinetic syndromes encompass a broader array of diseases, including dystonia, essential tremor, or Huntington’s disease. The discovery of effective therapies for these disorders has been challenging and has also involved the development and characterization of accurate animal models for the screening of new drugs. Zebrafish constitutes an alternative vertebrate model for the study of movement disorders. The neuronal circuitries involved in movement in zebrafish are well characterized, and most of the associated molecular mechanisms are highly conserved. Particularly, zebrafish models of PD have contributed to a better understanding of the role of several genes implicated in the disease. Furthermore, zebrafish is a vertebrate model particularly suited for large-scale drug screenings. The relatively small size of zebrafish, optical transparency, and lifecycle, are key characteristics that facilitate the study of multiple compounds at the same time. Several transgenic, knockdown, and mutant zebrafish lines have been generated and characterized. Therefore, it is central to critically analyze these zebrafish lines and understand their suitability as models of movement disorders. Here, we revise the pathogenic mechanisms, phenotypes, and responsiveness to pharmacotherapies of zebrafish lines of the most common movement disorders. A systematic review of the literature was conducted by including all studies reporting the characterization of zebrafish models of the movement disorders selected from five bibliographic databases. A total of 63 studies were analyzed, and the most relevant data within the scope of this review were gathered. The majority (62%) of the studies were focused in the characterization of zebrafish models of PD. Overall, the zebrafish models included display conserved biochemical and neurobehavioral features of the phenomenology in humans. Nevertheless, in light of what is known for all animal models available, the use of zebrafish as a model for drug discovery requires further optimization. Future technological developments alongside with a deeper understanding of the molecular bases of these disorders should enable the development of novel zebrafish lines that can prove useful for drug discovery for movement disorders.

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

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

Zebrafish models in neuropsychopharmacology and CNS drug discovery

Despite the high prevalence of neuropsychiatric disorders, their aetiology and molecular mechanisms remain poorly understood. The zebrafish (Danio rerio) is increasingly utilized as a powerful animal model in neuropharmacology research and in vivo drug screening. Collectively, this makes zebrafish a useful tool for drug discovery and the identification of disordered molecular pathways. Here, we discuss zebrafish models of selected human neuropsychiatric disorders and drug-induced phenotypes. As well as covering a broad range of brain disorders (from anxiety and psychoses to neurodegeneration), we also summarize recent developments in zebrafish genetics and small molecule screening, which markedly enhance the disease modelling and the discovery of novel drug targets.

Zebrafish Get Connected: Investigating Neurotransmission Targets and Alterations in Chemical Toxicity

Neurotransmission is the basis of neuronal communication and is critical for normal brain development, behavior, learning, and memory. Exposure to drugs and chemicals can alter neurotransmission, often through unknown pathways and mechanisms. The zebrafish (Danio rerio) model system is increasingly being used to study the brain and chemical neurotoxicity. In this review, the major neurotransmitter systems, including glutamate, GABA, dopamine, norepinephrine, serotonin, acetylcholine, histamine, and glutamate are surveyed and pathways of synthesis, transport, metabolism, and action are examined. Differences between human and zebrafish neurochemical pathways are highlighted. We also review techniques for evaluating neurological function, including the measurement of neurotransmitter levels, assessment of gene expression through transcriptomic analysis, and the recording of neurobehavior. Finally examples of chemical toxicity studies evaluating alterations in neurotransmitter systems in the zebrafish model are reviewed.

Inhibiting effects of rhynchophylline on zebrafish methamphetamine dependence are associated with amelioration of neurotransmitters content and down-regulation of TH and NR2B expression

Others and we have reported that rhynchophylline reverses amphetamine-induced conditioned place preference (CPP) effect which may be partly mediated by amelioration of central neurotransmitters and N-methyl-d-aspartate receptor 2B (NR2B) levels in the rat brains. The current study investigated the inhibiting effects of rhynchophylline on methamphetamine-induced (METH-induced) CPP in adult zebrafish and METH-induced locomotor activity in tyrosine hydroxylase-green fluorescent protein (TH-GFP) transgenic zebrafish larvae and attempted to confirm the hypothesis that these effects were mediated via regulation of neurotransmitters and dopaminergic and glutamatergic systems. After baseline preference test (on days 1-3), zebrafish were injected intraperitoneally METH (on days 4, 6 and 8) or the same volume of fish physiological saline (on days 5 and 7) and were immediately conditioned. Rhynchophylline was administered at 12h after injection of METH. On day 9, zebrafish were tested for METH-induced CPP. Results revealed that rhynchophylline (100mg/kg) significantly inhibited the acquisition of METH-induced CPP, reduced the content of dopamine and glutamate and down-regulated the expression of TH and NR2B in the CPP zebrafish brains. Furthermore, the influence of rhynchophylline on METH-induced locomotor activity was also observed in TH-GFP transgenic zebrafish larvae. Results showed that rhynchophylline (50mg/L) treatment led to a significant reduction on the locomotor activity and TH expression in TH-GFP transgenic zebrafish larvae. Taken together, these data indicate that the inhibition of the formation of METH dependence by rhynchophylline in zebrafish is associated with amelioration of the neurotransmitters dopamine and glutamate content and down-regulation of TH and NR2B expression.

Serotonergic Modulation as Effective Treatment for Dravet Syndrome in a Zebrafish Mutant Model

Dravet syndrome (DS) is a severe epilepsy syndrome that starts within the first year of life. In a clinical study, add-on treatment with fenfluramine, a potent 5-hydroxytryptamine (5-HT) releaser activating multiple 5-HT receptor subtypes, made 70% of DS children seizure free. Others and we recently confirmed the efficacy of fenfluramine as an antiepileptiform compound in zebrafish models of DS. By using a large set of subtype selective agonists, in this study we examined which 5-HT receptor subtypes can be targeted to trigger antiseizure effects in homozygous scn1Lab(-/-) mutant zebrafish larvae that recapitulate DS well. We also provide evidence that zebrafish larvae express the orthologues of all human 5-HT receptor subtypes. Using an automated larval locomotor behavior assay, we were able to show that selective 5-HT1D-, 5-HT1E-, 5-HT2A-, 5-HT2C-, and 5-HT7-agonists significantly decreased epileptiform activity in the mutant zebrafish at 7 days post fertilization (dpf). By measuring local field potentials in the zebrafish larval forebrain, we confirmed the antiepileptiform activity of the 5-HT1D-, 5-HT2C-, and especially the 5-HT2A-agonist. Interestingly, we also found a significant decrease of serotonin in the heads of homozygous scn1Lab(-/-) mutants as compared to the wild type zebrafish, which suggest that neurochemical defects might play a crucial role in the pathophysiology of DS. Taken together, our results emphasize the high conservation of the serotonergic receptors in zebrafish larvae. Modulating certain serotonergic receptors was shown to effectively reduce seizures. Our findings therefore open new avenues for the development of future novel DS therapeutics.

Behavioral Analysis of Dopaminergic Activation in Zebrafish and Rats Reveals Similar Phenotypes

Zebrafish is emerging as a complement to mammals in behavioral studies; however, there is a lack of comparative studies with rodents and humans to establish the zebrafish as a predictive translational model. Here we present a detailed phenotype evaluation of zebrafish larvae, measuring 300-3000 variables and analyzing them using multivariate analysis to identify the most important ones for further evaluations. The dopamine agonist apomorphine has previously been shown to have a complex U-shaped dose-response relationship in the variable distance traveled. In this study, we focused on breaking down distance traveled into more detailed behavioral phenotypes for both zebrafish and rats and identified in the multivariate analysis low and high dose phenotypes with characteristic behavioral features. Further analysis of single parameters also identified an increased activity at the lowest concentration indicative of a U-shaped dose-response. Apomorphine increased the distance of each swim movement (bout) at both high and low doses, but the underlying behavior of this increase is different; at high dose, both bout duration and frequency increased whereas bout max speed was higher at low dose. Larvae also displayed differences in place preference. The low dose phenotype spent more time in the center, indicative of an anxiolytic effect, while the high-dose phenotype had a wall preference. These dose-dependent effects corroborated findings in a parallel rat study and previous observations in humans. The translational value of pharmacological zebrafish studies was further evaluated by comparing the amino acid sequence of the dopamine receptors (D1-D4), between zebrafish, rats and humans. Humans and zebrafish share 100% of the amino acids in the binding site for D1 and D3 whereas D2 and D4 receptors share 85-95%. Molecular modeling of dopamine D2 and D4 receptors indicated that nonconserved amino acids have limited influence on important ligand-receptor interactions.

Zebrafish models of major depressive disorders

The zebrafish (Danio rerio) has emerged as a model species for translational research in various neuroscience areas, including depressive disorders. Because of their physiological (neuroanatomical, neuroendocrine, neurochemical) and genetic homology to mammals, robust phenotypes, and value in high-throughput genetic and chemical genetic screens, zebrafish are ideal for developing valid experimental models of major depression and discovering novel therapeutics. Behavioral testing approaches, such as approach-avoidance, cognitive, and social paradigms, are available in zebrafish and have utility in identifying depression-like indices in zebrafish in response to physiological, genetic, environmental, and/or psychopharmacological alterations. In addition, the high sensitivity of zebrafish to commonly prescribed psychotropic drugs supports the use of this model as an invaluable tool for pharmacological research and drug screening. This Review outlines the benefits of using the zebrafish model for depression studies and summarizes the current research in this field.