Pharmacological analyses of learning and memory in zebrafish (Danio rerio)

From land to water: "Sunken" T-maze for associated learning in cichlid fish

The study introduced and evaluated learning paradigms for Maylandia callainos cichlids using a modified version of the rodent T-maze, filled with tank water (the "sunken" modification). Both male and female fish underwent training in two distinct conditioning paradigms. Firstly, simple operant conditioning involved placing a food reward in either the right or left compartment. Cichlids demonstrated the ability to purposefully find the bait within 6 days of training, with a persistent place preference lasting up to 6 days. Additionally, the learning dynamics varied with sex: female cichlids exhibited reduction in latency to visit the target compartment and consume the bait, along with a decrease in the number of errors 3 and 4 days earlier than males, respectively. Secondly, visually-cued operant conditioning was conducted, with a food reward exclusively placed in the yellow compartment, randomly positioned on the left or right side of the maze during each training session. Visual learning persisted for 10 days until reaction time improvement plateaued. Color preference disappeared after 4 consecutive check-ups, with no sex-related interference. For further validation of visually-cued operant conditioning paradigm, drugs MK-801 (dizocilpine) and caffeine, known to affect performance in learning tasks, were administered intraperitoneally. Chronic MK-801 (0.17 mg/kg) impaired maze learning, resulting in no color preference development. Conversely, caffeine administration enhanced test performance, increasing precision in fish. This developed paradigm offers a viable approach for studying learning and memory and presents an effective alternative to rodent-based drug screening tools, exhibiting good face and predictive validity.

Development of CRID3-Based Anti-inflammatory Agents to Ameliorate Chronic Hypoxia-Induced Memory Impairment in Zebrafish Models

Chronic hypoxic exposure triggers the onset and progression of cognitive dysfunction; however, the mechanisms underlying chronic hypoxia-induced neuroinflammation and its contribution to cognitive dysfunction remain poorly understood. Although inflammation and hypoxia are interdependent, numerous recent studies have linked the development of various human diseases to hypoxia-induced inflammation. In this study, we focused on the NLRP3 inflammasome with novel analogues of cytokine release inhibitory drug 3 (CRID3), a class of small molecule inhibitors for the NLRP3 inflammasome, to investigate their potential contribution to alleviating chronic hypoxia-induced neuroinflammation using the zebrafish model. The designed CRID3 analogues 6a-q were prepared from 2-methyl furan-3-carboxylate, following a four-step reaction sequence and fully characterized by NMR and mass spectral analysis. The administration of CRID3 analogues 6a-q led to a notable reduction in neuroinflammation and an increase in glial proliferation markers in both sexes. In addition, we investigated the potential effects of CRID3 analogues 6a-q through various behavioral tasks to assess their role in ameliorating post-hypoxic behavioral deficits and cognitive impairment. Notably, the study revealed that post-chronic hypoxia, male zebrafish exhibited significantly higher levels of inflammatory marker expression than females. Furthermore, we observed that the neurogenic response to treatment with CRID3 derivative 6o varied depending on the sex, with females showing a sex-specific differential increase in neurogenesis compared to males. This work emphasizes the significance of considering sex differences into account in developing therapeutic strategies for neurological disorders, as shown by the sex-specific molecular and behavioral changes in zebrafish cognitive impairment and neuroinflammation.

One-trial odour recognition learning and its underlying brain areas in the zebrafish

Distinguishing familiar from novel stimuli is critical in many animals' activities, and procedures based on this ability are among the most exploited in translational research in rodents. However, recognition learning and the underlying brain substrates remain unclear outside a few mammalian species. Here, we investigated one-trial recognition learning for olfactory stimuli in a teleost fish using a behavioural and molecular approach. With our behavioural analysis, we found that zebrafish can learn to recognise a novel odour after a single encounter and then, discriminate between this odour and a different one provided that the molecular structure of the cues is relatively differentiated. Subsequently, by expression analysis of immediate early genes in the main brain areas, we found that the telencephalon was activated when zebrafish encountered a familiar odour, whereas the hypothalamus and the optic tectum were activated in response to the novel odour. Overall, this study provided evidence of single-trial spontaneous learning of novel odours in a teleost fish and the presence of multiple neural substrates involved in the process. These findings are promising for the development of zebrafish models to investigate cognitive functions.

Acute ethanol exposure leads to long-term effects on memory, behavior, and transcriptional regulation in the zebrafish brain

Alcohol consumption is associated with alterations in memory and learning processes in humans and animals. In this context, research models such as the zebrafish (Danio rerio) arise as key organisms in behavioral and molecular studies that attempt to clarify alterations in the Central Nervous System (CNS), like those related to alcohol use. Accordingly, we used the zebrafish as a model to evaluate the effects of ethanol on the learning and memory process, as well as its relationship with behavior and transcriptional regulation of lrfn2, lrrk2, grin1a, and bdnf genes in the brain. To this end, for the memory and learning evaluation, we conducted the Novel Object Recognition test (NOR); for behavior, the Novel Tank test; and for gene transcription, qPCR, after 2 h, 24 h, and 8 days of ethanol exposure. As a result, we noticed in the NOR that after 8 days of ethanol exposure, the control group spent more time exploring the novel object than when compared to 2 h post-exposure, indicating that naturally zebrafish remember familiar objects. In animals in the Treatment group, however, no object recognition behavior was observed, suggesting that alcohol affected the learning and memory processes of the animals and stimulated an anxiolytic effect in them. Regarding transcriptional regulation, 24 h after alcohol exposure, we found hyper-regulation of bdnf and, after 8 days, a hypo-regulation of lrfn2 and lrrk2. To conclude, we demonstrated that ethanol exposure may have influenced learning ability and memory formation in zebrafish, as well as behavior and regulation of gene transcription. These data are relevant for further understanding the application of zebrafish in research associated with ethanol consumption and behavior.

Zebrafish as a Potential Model for Neurodegenerative Diseases: A Focus on Toxic Metals Implications

In the last century, industrial activities increased and caused multiple health problems for humans and animals. At this moment, heavy metals are considered the most harmful substances for their effects on organisms and humans. The impact of these toxic metals, which have no biological role, poses a considerable threat and is associated with several health problems. Heavy metals can interfere with metabolic processes and can sometimes act as pseudo-elements. The zebrafish is an animal model progressively used to expose the toxic effects of diverse compounds and to find treatments for different devastating diseases that human beings are currently facing. This review aims to analyse and discuss the value of zebrafish as animal models used in neurological conditions, such as Alzheimer's disease (AD), and Parkinson's disease (PD), particularly in terms of the benefits of animal models and the limitations that exist.

Exposure to leucine alters glutamate levels and leads to memory and social impairment in zebrafish

Maple Syrup Urine Disease (MSUD) is a metabolic disorder characterized by high levels in blood and urine of branched-chain amino acids leucine, isoleucine, and valine and their alpha-ketoacids, by a partial or total blockade in the activity of branched-chain complex alpha-keto acids dehydrogenase. The main symptoms in MSUD occur in the central nervous system, including cognitive deficits, locomotor, poor feeding, seizures, psychomotor delay, and mental retardation, but the mechanisms of neurotoxicity and behavior alteration due to this disease are poorly understood, thus this study aimed at showing the effects of leucine exposure on glutamate levels and behavior in zebrafish. For this, we analyzed the behavior using the social preference test and novel object recognition test, moreover, we analyse the glutamate levels and uptake using scintillation and high-performance liquid chromatography methods. Our results demonstrated a decrease in glutamate levels and uptake, accompanied by memory and social impairment. In conclusion, these results suggest that alterations in glutamate levels can be associated with behavior impairment, however, more studies are necessary to understand the mechanisms for brain damage in MSUD.

Dopamine Release Impairments Accompany Locomotor and Cognitive Deficiencies in Rotenone-Treated Parkinson's Disease Model Zebrafish

In this work, we carried out neurochemical and behavioral analysis of zebrafish (Danio rerio) treated with rotenone, an agent used to chemically induce a syndrome resembling Parkinson's disease (PD). Dopamine release, measured with fast-scan cyclic voltammetry (FSCV) at carbon-fiber electrodes in acutely harvested whole brains, was about 30% of that found in controls. Uptake, represented by the first order rate constant (k) and the half-life (t1/2) determined by nonlinear regression modeling of the stimulated release plots, was also diminished. Behavioral analysis revealed that rotenone treatment increased the time required for zebrafish to reach a reward within a maze by more than 50% and caused fish to select the wrong pathway, suggesting that latent learning was impaired. Additionally, zebrafish treated with rotenone suffered from diminished locomotor activity, swimming shorter distances with lower mean velocity and acceleration. Thus, the neurochemical and behavioral approaches, as applied, were able to resolve rotenone-induced differences in key parameters. This approach may be effective for screening therapies in this and other models of neurodegeneration.

Pharmacological characterization of a novel putative nootropic beta-alanine derivative, MB-005, in adult zebrafish

BACKGROUND

Cognitive deficits represent an urgent biomedical problem, and are commonly reduced by nootropic drugs. Animal models, including both rodents and zebrafish, offer a valuable tool for studying cognitive phenotypes and screening novel nootropics. Beta-alanine and its derivatives have recently been proposed to exert nootropic activity.

AIMS

This study aimed to characterize putative nootropic profile of a novel β-alanine analogue, 1,3-diaminopropane (MB-005), in adult zebrafish.

METHODS

Nootropic profile of MB-005 was assessed in adult zebrafish in the novel tank and conditioned place aversion (CPA) tests acutely, and in cued-learning plus-maze (PMT) tests chronically.

RESULTS/OUTCOMES

MB-005 did not alter zebrafish anxiety-like behavior or monoamine neurochemistry acutely, improved short-term memory in the CPA test, but impaired cognitive performance in both CPA and PMT tests chronically.

CONCLUSIONS/INTERPRETATION

This study reveals high sensitivity of zebrafish cognitive phenotypes to MB-005, suggesting it as a potential novel cognitive enhancer acutely, but raises concerns over its cognitive (and, possibly, other) side-effects chronically.

The Brilliance of the Zebrafish Model: Perception on Behavior and Alzheimer's Disease

Alzheimer's disease (AD) has become increasingly prevalent in the elderly population across the world. It's pathophysiological markers such as overproduction along with the accumulation of amyloid beta (Aβ) plaques and neurofibrillary tangles (NFT) are posing a serious challenge to novel drug development processes. A model which simulates the human neurodegenerative mechanism will be beneficial for rapid screening of potential drug candidates. Due to the comparable neurological network with humans, zebrafish has emerged as a promising AD model. This model has been thoroughly validated through research in aspects of neuronal pathways analogous to the human brain. The cholinergic, glutamatergic, and GABAergic pathways, which play a role in the manifested behavior of the zebrafish, are well defined. There are several behavioral models in both adult zebrafish and larvae to establish various aspects of cognitive impairment including spatial memory, associative memory, anxiety, and other such features that are manifested in AD. The zebrafish model eliminates the shortcomings of previously recognized mammalian models, in terms of expense, extensive assessment durations, and the complexity of imaging the brain to test the efficacy of therapeutic interventions. This review highlights the various models that analyze the changes in the normal behavioral patterns of the zebrafish when exposed to AD inducing agents. The mechanistic pathway adopted by drugs and novel therapeutic strategies can be explored via these behavioral models and their efficacy to slow the progression of AD can be evaluated.

Zebrafish models of fetal alcohol spectrum disorders

Fetal alcohol spectrum disorder (FASD) describes a wide range of structural deficits and cognitive impairments. FASD impacts up to 5% of children born in the United States each year, making ethanol one of the most common teratogens. Due to limitations and ethical concerns, studies in humans are limited in their ability to study FASD. Animal models have proven critical in identifying and characterizing the mechanisms underlying FASD. In this review, we will focus on the attributes of zebrafish that make it a strong model in which to study ethanol-induced developmental defects. Zebrafish have several attributes that make it an ideal model in which to study FASD. Zebrafish produced large numbers of externally fertilized, translucent embryos. With a high degree of genetic amenability, zebrafish are at the forefront of identifying and characterizing the gene-ethanol interactions that underlie FASD. Work from multiple labs has shown that embryonic ethanol exposures result in defects in craniofacial, cardiac, ocular, and neural development. In addition to structural defects, ethanol-induced cognitive and behavioral impairments have been studied in zebrafish. Building upon these studies, work has identified ethanol-sensitive loci that underlie the developmental defects. However, analyses show there is still much to be learned of these gene-ethanol interactions. The zebrafish is ideally suited to expand our understanding of gene-ethanol interactions and their impact on FASD. Because of the conservation of gene function between zebrafish and humans, these studies will directly translate to studies of candidate genes in human populations and allow for better diagnosis and treatment of FASD.

Developmental exposure to a POPs mixture or PFOS increased body weight and reduced swimming ability but had no effect on reproduction or behavior in zebrafish adults

Complex mixtures of persistent organic pollutants (POPs) are regularly detected in the environment and animal tissues. Often these chemicals are associated with latent effects following early-life exposures, following the developmental origin of health and disease paradigm. We investigated the long-term effects of a human relevant mixture of 29 POPs on adult zebrafish following a developmental exposure, in addition to a single PFOS exposure for comparison, as it was the compound with the highest concentration within the mixture. Zebrafish embryos were exposed from 6 to 96 h post fertilization to x10 and x70 the level of POP mixture or PFOS (0.55 and 3.83 μM) found in human blood before being transferred to clean water. We measured growth, swimming performance, and reproductive output at different life stages. In addition, we assessed anxiety behavior of the adults and their offspring, as well as performing a transcriptomic analysis on the adult zebrafish brain, as the POP mixture and PFOS concentrations used are known to affect larval behavior. Exposure to POP mixture and PFOS reduced swimming performance and increased length and weight, compared to controls. No effect of developmental exposure was observed on reproductive output or anxiety behavior. Additionally, RNA-seq did not reveal pathways related to anxiety although pathways related to synapse biology were affected at the x10 PFOS level. Furthermore, pathway analysis of the brain transcriptome of adults exposed as larvae to the low concentration of PFOS revealed enrichment in pathways such as calcium, MAPK, and GABA signaling, all of which are important for learning and memory. Based on our results we can conclude that some effects on the endpoints measured were apparent, but if these effects lead to adversities at population levels remains elusive.

Subchronic effects of plant alkaloids on anxiety-like behavior in zebrafish

Zebrafish provide a valuable emerging complementary model for neurobehavioral research. They offer a powerful way to screen for the potential therapeutic effects of neuroactive drugs. A variety of behavioral tests for zebrafish have been developed and validated for assessing neurobehavioral function. The novel tank diving test is a straightforward, reproducible way of measuring anxiety-like behavior in zebrafish. When introduced into a novel tank, zebrafish normally dive to the bottom of the tank and then gradually explore the higher levels of the water column as time progresses. Buspirone is an effective anxiolytic drug in humans, which has been found, with acute administration, to reduce this anxiety-like response in zebrafish. The current study used the zebrafish model to evaluate the potential anxiolytic effects of alkaloids, commonly found in Solanaceae plants, with known neuropharmacology relevant to mood regulation. In line with previous findings, acute treatment with anxiolytic positive controls buspirone and the plant alkaloid nicotine reduced the anxiety-like diving response in the zebrafish novel tank diving test. Further, both buspirone and nicotine continued to produce anxiolytic-like effects in zebrafish after 5 days of exposure. In the same treatment paradigm, the effects of five other alkaloids-cotinine, anatabine, anabasine, harmane, and norharmane-were investigated. Cotinine, the major metabolite of nicotine, also caused anxiolytic-like effects, albeit at a dose higher than the effective dose of nicotine. Nicotine's anxiolytic-like effect was not shared by the other nicotinic alkaloids, anabasine and anatabine, or by the naturally present monoamine oxidase inhibitors harmane and norharmane. We conclude that nicotine uniquely induces anxiolytic-like effects after acute and subchronic treatment in zebrafish. The zebrafish model with the novel tank diving test could be a useful complement to rodent models for screening candidate compounds for anxiolytic effects in nonclinical studies.

Effects of the Hydroethanolic Extract of Lycopodium selago L. on Scopolamine-Induced Memory Deficits in Zebrafish

This scientific research focused on the production of hydroethanolic extract of the plant species Lycopodium selago L. (L. selago) by the ultrasound-assisted extraction (USAE) and the identification of biocompounds with high antioxidant activity is of interest for possible phytotherapeutic treatment against Alzheimer's disease (AD). The extract was phytochemically analyzed to investigate polyphenols, flavonoids, and identify the sesquiterpenoid alkaloid huperzine A (HupA), which is known in the literature for its great relevance in AD. Evaluation and comparison of the antioxidant activity of the extract were performed by four complementary spectrophotometric methods (DPPH, FRAP, ABTS, ORAC). In vitro tests of the extract showed an excellent reciprocal link between the concentration of polyphenols and the measurement of the antioxidant activity of the extract with the sesquiterpenoid HupA. To confirm the antioxidant activity, L. selago hydroethanolic extract was administered in vivo to zebrafish (Danio rerio) with a pattern of scopolamine-induced cognitive impairment. Moreover, this study explored a possible correlation between the expression of oxidative stress markers in the brain tissue with the behavior of the scopolamine zebrafish model. In vivo tests showed that this fern could be used as a nutritional supply and as a phytotherapeutic method to prevent or treat various neurodegenerative diseases that call for high-nutritive-value medications.

Chronic unpredictable early-life stress (CUELS) protocol: Early-life stress changes anxiety levels of adult zebrafish

Early-life stress can lead to two different behavioral responses: (1) increased susceptibility to psychiatric disorders or (2) resilience. Here, we created a chronic unpredictable early-life stress (CUELS) protocol to assess the effects of early experiences in adult zebrafish. Animals were exposed to mild stressors twice a day and the duration was varied between groups (0, 1, 3, 7 and 14 days of stress). The stressor consisted of light/dark cycle changes; social isolation; overcrowding; water changes; water cooling; mechanical stirring; water heating; and immersion in shallow water. Behavior was assessed at young stages (21 days post-fertilization - open field analysis) and adulthood (4-months-old - novel tank diving test, light/dark task, shoaling, free movement pattern Y-maze and Pavlovian fear conditioning). Cortisol levels were assessed to evaluate the impact of CUELS in the HPI axis. Zebrafish exposed to 7 days of CUELS showed a decreased anxiety-like phenotype in two behavioral tasks, presenting increased time spent in top and decreased time spent in the dark area. Animals exposed to 14 days of CUELS showed an opposite anxious phenotype compared to 3 and 7 days of CUELS. No significant changes were observed in memory and cognition, social behavior and cortisol levels. In general, 7 days of CUELS protocol decreased anxiety in young and adult zebrafish, and could be used to understand the mechanisms underlying early-life experiences-derived alterations in neural circuits of anxiety.

Embryonic ethanol exposure on zebrafish early development

INTRODUCTION

Embryonic exposure to ethanol leads to a condition of physical, behavioral, and cognitive deficiencies named fetal alcohol spectrum disorders (FASD). The most severe variations are in fetal alcohol syndrome (FAS), which is easier to diagnose and not studied in animal models. On the other side, the pFAS (partial fetal alcohol syndrome) includes cases of alcohol-related congenital disabilities and neurodevelopmental disorder with an inconclusive diagnosis. In recent years, the zebrafish has become a valuable model to study FASD and its variations.

METHODS

This study characterizes the zebrafish embryonic and larval development after low and moderate ethanol concentration exposure. Fish eggs were exposed to 0.0%, 0.25%, 0.5%, and 1.0% ethanol at 24 hr postfertilization, and embryonic development was observed every 8 hr up to 120 hpf. It evaluated movements, phenotypic abnormalities, hatching, cardiac function and heartbeat frequency, larvae length at 120 hpf, and the apoptotic cells' fluorescence stained with acridine orange.

RESULTS

Embryonic exposure to 0.5% and 1% ethanol presented reduced body size, decreased heartbeat rate, higher numbers of apoptotic cells, and hatching time differences.

CONCLUSIONS

Our results suggest any ethanol exposure during embryogenesis can be harmful and reinforces zebrafish as a suitable model for fetal alcohol spectrum disorders (FASD).

Red LED light treatment promotes cognitive learning through up-regulation of trpm4 in zebrafish

Although light emitting diodes (LEDs) are widely used in our daily lives, there is little research regarding LED light's possible effects on biological functions. We used a zebrafish animal model to investigate the long-term effects of white, blue and red LED lights on cognitive learning and memory recall. Our data suggest that these treatments had not only an impact on learning but also surprisingly long-lasting effects, particularly with regard to individuals treated with red light. The qPCR results revealed that the expression levels of trpm4, trpa1b, grin2aa and dlg4 in the skin were increased after monochromatic light treatment. Furthermore, the up-regulation of trpm4 in the brain may correlate to enhanced learning and memory following red-light treatment. Our results identify a light-based stimulation system for enhancing zebrafish learning, which has the potential to provide important insights into the relationship between LED lighting and animal behaviour.

CRISPR Meets Zebrafish: Accelerating the Discovery of New Therapeutic Targets

Bringing a new drug to the market costs an average of US$2.6 billion and takes more than 10 years from discovery to regulatory approval. Despite the need to reduce cost and time to increase productivity, pharma companies tend to crowd their efforts in the same indications and drug targets. This results in the commercialization of drugs that share the same mechanism of action (MoA) and, in many cases, equivalent efficacies among them-an outcome that helps neither patients nor the balance sheet of the companies trying to bring therapeutics to the same patient population. Indeed, the discovery of new therapeutic targets, based on a deeper understanding of the disease biology, would likely provide more innovative MoAs and potentially greater drug efficacies. It would also bring better chances for identifying appropriate treatments according to the patient's genetic stratification. Nowadays, we count with an enormous amount of unprocessed information on potential disease targets that could be extracted from omics data obtained from patient samples. In addition, hundreds of pharmacological and genetic screenings have been performed to identify innovative drug targets. Traditionally, rodents have been the animal models of choice to perform functional genomic studies. The high experimental cost, combined with the low throughput provided by those models, however, is a bottleneck for discovering and validating novel genetic disease associations. To overcome these limitations, we propose that zebrafish, in conjunction with the use of CRISPR/Cas9 genome-editing tools, could streamline functional genomic processes to bring biologically relevant knowledge on innovative disease targets in a shorter time frame.

Delayed behavioral and genomic responses to acute combined stress in zebrafish, potentially relevant to PTSD and other stress-related disorders: Focus on neuroglia, neuroinflammation, apoptosis and epigenetic modulation

Stress is a common trigger of stress-related illnesses, such as anxiety, phobias, depression and post-traumatic stress disorder (PTSD). Various animal models successfully reproduce core behaviors of these clinical conditions. Here, we develop a novel zebrafish model of stress (potentially relevant to human stress-related disorders), based on delayed persistent behavioral, endocrine and genomic responses to an acute severe 'combined' stressor. Specifically, one week after adult zebrafish were exposed to a complex combined 90-min stress, we assessed their behaviors in the novel tank and the light-dark box tests, as well as whole-body cortisol and brain gene expression, focusing on genomic biomarkers of microglia, astrocytes, neuroinflammation, apoptosis and epigenetic modulation. Overall, stressed fish displayed persistent anxiety-like behavior, elevated whole-body cortisol, as well as upregulated brain mRNA expression of genes encoding the glucocorticoid receptor, neurotrophin BDNF and its receptors (TrkB and P75), CD11b (a general microglial biomarker), COX-2 (an M1-microglial biomarker), CD206 (an M2-microglial biomarker), GFAP (a general astrocytal biomarker), C3 (an A1-astrocytal biomarker), S100α₁₀ (an A2-astrocytal biomarker), as well as pro-inflammatory cytokines IL-6, IL-1β, IFN-γ and TNF-α. Stress exposure also persistently upregulated the brain expression of several key apoptotic (Bax, Caspase-3, Bcl-2) and epigenetic genes (DNMT3a, DNMT3b, HAT1, HDAC4) in these fish. Collectively, the present model not only successfully recapitulates lasting behavioral and endocrine symptoms of clinical stress-related disorders, but also implicates changes in neuroglia, neuroinflammation, apoptosis and epigenetic modulation in long-term effects of stress pathogenesis in vivo.

The Environmental Geometry in Spatial Learning by Zebrafish (Danio rerio)

During navigation, disoriented animals learn to use the spatial geometry of rectangular environments to gain rewards. The length of macroscopic surfaces (metric: short/long) and their spatial arrangement (sense: left/right) are powerful cues that animals prove to encode for reorientation. The aim of this study was to investigate if zebrafish (Danio rerio) could take advantage of such geometric properties in a rewarded exit task, by applying a reference memory procedure. The experiment was performed in a rectangular arena having four white walls, where fish were required to choose the two geometrically equivalent exit corners lying on the reinforced diagonal. Results showed that zebrafish encoded the geometry of the arena during reorientation, solving the spatial task within the first 5 days of training. With the aim to avoid the possible influence of extravisual cues on the zebrafish success, we performed a geometric test in extinction of response after the learning day. At test, fish persisted in choosing the two correct corners, thus confirming that the navigation strategy used at training was based on geometric cues. This study adds evidence about the role of geometric frameworks in fish species, and it further validates an effective spatial learning paradigm for zebrafish.

High-glucose/high-cholesterol diet in zebrafish evokes diabetic and affective pathogenesis: The role of peripheral and central inflammation, microglia and apoptosis

Neuroinflammation and metabolic deficits contribute to the etiology of human affective disorders, such as anxiety and depression. The zebrafish (Danio rerio) has recently emerged as a powerful new model organism in CNS disease modeling. Here, we exposed zebrafish to 2% glucose and 10% cholesterol for 19 days to experimentally induce type 2 diabetes (DM) and to assess stress responses, microglia, inflammation and apoptosis. We analyzed zebrafish anxiety-like behavior in the novel tank and light-dark box (Days 15-16) tests, as well as examined their biochemical and genomic biomarkers (Day 19). Confirming DM-like state in zebrafish, we found higher whole-body glucose, triglyceride, total cholesterol, low-density lipoprotein levels and glucagon mRNA expression, and lower high-density lipoprotein levels. DM zebrafish also showed anxiety-like behavior, elevated whole-body cortisol and cytokines IFN-γ and IL-4, as well as higher brain mRNA expression of the glucocorticoid receptor, CD11b (a microglial biomarker), pro-inflammatory cytokines IL-6 and TNF-α (but not IL-1β or anti-inflammatory cytokines IL-4 and IL-10), GFAP (an astrocytal biomarker), neurotrophin BDNF, its receptors p75 and TrkB, as well as apoptotic Bax and Caspase-3 (but not BCl-2) genes. Collectively, this supports the overlapping nature of DM-related affective pathogenesis and emphasizes the role of peripheral and central inflammation and apoptosis in DM-related affective and neuroendocrine deficits in zebrafish.

Schinus terebinthifolius Essential Oil Attenuates Scopolamine-Induced Memory Deficits via Cholinergic Modulation and Antioxidant Properties in a Zebrafish Model

Schinus terebinthifolius is a plant well recognized for its therapeutic profile such as anti-inflammatory and antitumor activities, promoting antibacterial activity and antioxidant and antidiabetic properties. This study aimed at examining whether Schinus terebinthifolius memory-enhancing activities are mediated by cholinergic and brain antioxidant systems in a scopolamine zebrafish model. Schinus terebinthifolius essential oil (10, 25, and 50 μL/L) was delivered to zebrafish by immersion in water for 8 days. Memory deficits were induced by scopolamine (100 μM) administration. Zebrafish were divided into seven groups (n = 15/group): vehicle group, scopolamine (100 μM) group, Schinus terebinthifolius essential oil groups (STF; 10, 25, and 50 μL/L), the imipramine group (IMP; 20 mg/L, as the positive control in the NTT test), and the donepezil group (DP; 10 mg/L, as the positive control in the Y-maze test). Memory status was estimated by the novel tank diving test (NTT) and the Y-maze test and finally was validated by comparison with imipramine (20 mg/L) and donepezil (10 mg/L). Gas chromatography-mass spectrometry (GC-MS) was used to detect oil compounds. Brain levels of acetylcholinesterase (AChE) and antioxidant enzymes were measured. After being exposed to Schinus terebinthifolius essential oil, the scopolamine zebrafish exhibited an improvement of memory processes in the NTT and Y-maze tests. The essential oil attenuated the elevated level of AChE and brain oxidative stress. Schinus terebinthifolius essential oil was found to support memory formation through the inhibition of the AChE activity and decreasing oxidative stress in the scopolamine-treated zebrafish brains.

Linarin improves the dyskinesia recovery in Alzheimer's disease zebrafish by inhibiting the acetylcholinesterase activity

BACKGROUND

Due to complex pathogenesis of Alzheimer's disease (AD), currently there is no effective disease-modifying treatment. Acetylcholinesterase (AChE) has introduced itself as an important target for AD therapy. Linarin as the representative active ingredient of flavonoid glycoside in Flos chrysanthemi indici has been found to have anti-acetylcholinesterase effect.

AIMS

The present study intended to explore the potential effect of linarin for treatment of AD.

MAIN METHODS

In this study, molecular docking simulation was used to evaluate whether linarin could dock with AChE and decipher the mechanism of linarin as an AChE inhibitor. After molecular docking simulation, AlCl₃-induced Alzheimer's disease zebrafish model was established. Effects of linarin on treating AD zebrafish dyskinesia and AChE inhibition were compared with donepezil (DPZ) which was used as a positive control drug.

KEY FINDINGS

Molecular docking simulation showed that linarin plays a critical role in AChE inhibition by binding AChE active sites. The experiments illustrated that the dyskinesia recovery rate of AD zebrafish could be significantly improved by linarin. The dyskinesia recovery and AChE inhibition rate were 88.0% and 74.5% respectively, while those of DPZ were 79.3% and 43.6%.

SIGNIFICANCE

These findings provide evidences for supporting linarin to be developed into an AD drug by inhibiting the activity of AChE.

Understanding Central Nervous System Effects of Deliriant Hallucinogenic Drugs through Experimental Animal Models

Hallucinogenic drugs potently alter human behavior and have a millennia-long history of use for medicinal and religious purposes. Interest is rapidly growing in their potential as CNS modulators and therapeutic agents for brain conditions. Antimuscarinic cholinergic drugs, such as atropine and scopolamine, induce characteristic hyperactivity and dream-like hallucinations and form a separate group of hallucinogens known as "deliriants". Although atropine and scopolamine are relatively well-studied drugs in cholinergic physiology, deliriants represent the least-studied class of hallucinogens in terms of their behavioral and neurological phenotypes. As such, novel approaches and new model organisms are needed to investigate the CNS effects of these compounds. Here, we comprehensively evaluate the preclinical effects of deliriant hallucinogens in various animal models, their mechanisms of action, and potential interplay with other signaling pathways. We also parallel experimental and clinical findings on deliriant agents and outline future directions of translational research in this field.

The Visually Mediated Social Preference Test: A Novel Technique to Measure Social Behavior and Behavioral Disturbances in Zebrafish

Zebrafish are an emerging model in behavioral neuroscience. They display a wide range of measurable behaviors such as locomotion, aggression, anxiety, learning and memory, and social behavior. In addition, the relative ease of genetic manipulation and the increasing availability of disease models mean that zebrafish have gained in popularity as an animal model for various neurological and psychiatric diseases including autism spectrum disorder (ASD). In order to better characterize social behavior and behavioral abnormalities in zebrafish, we have developed the visually mediated social preference (VMSP) test, a novel assay to measure social preference and social novelty in two consecutive 5-min sessions. Using recording and video tracking, the time spent in different areas of the tank, the time spent immobile, swimming speed, and distance moved can be easily measured and analyzed. Untreated experimentally naive AB WT zebrafish typically show a strong preference for spending time near and interacting with a compartment containing unfamiliar conspecifics over the empty compartments during session 1 and a stronger preference for a group of unfamiliar zebrafish over familiar conspecifics from session 1, during session 2 of the test. Research in our lab has shown that the VMSP is suitable to measure the social behavior of individual zebrafish, to uncover social phenotypes of mutant strains, and to better understand animal models of disease that include impaired sociability such as ASD. The current paper provides a step-by-step guide on how to implement and perform this test and highlights important considerations for data acquisition, analysis, and interpretation.

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

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

zPACT: Tissue Clearing and Immunohistochemistry on Juvenile Zebrafish Brain

In studies of brain function, it is essential to understand the underlying neuro-architecture. Very young zebrafish larvae are widely used for neuroarchitecture studies, due to their size and natural transparency. However, this model system has several limitations, due to the immaturity, high rates of development and limited behavioral repertoire of the animals used. We describe here a modified version of the passive clearing technique (PACT) ( Chung et al., 2013 ; Tomer et al., 2014 ; Yang et al., 2014 ; Treweek et al., 2015) , which facilitates neuroanatomical studies on large specimens of aquatic species. This method was initially developed for zebrafish (Danio rerio) ( Frétaud et al., 2017 ; Mayrhofer et al., 2017 ; Xavier et al., 2017 ), but has also been successfully tested on other fish, such as medaka (Oryzias latipes) ( Dambroise et al., 2017 ), Mexican cave fish (Astyanax mexicaus) and African zebra mbuna (Metriaclima zebra), and on other aquatic species, such as Xenopus spp. (Xenopus laevis, Xenopus tropicalis) ( Fini et al., 2017 ) . This protocol, based on the CLARITY method developed and modified by Deisseroth's laboratory and others ( Chung et al., 2013 ; Tomer et al., 2014 ; Yang et al., 2014 ), was adapted for use in aquatic species, including zebrafish in particular (zPACT). This protocol is designed to render zebrafish specimens optically transparent while preserving the overall architecture of the tissue, through crosslinking in a polyacrylamide/formaldehyde mesh. Most of the lipids present in the specimen are then removed by SDS treatment, to homogenize the refractive index of the specimen by eliminating light scattering at the water/lipid interface, which causes opacity. The final clearing step, consists of the incubation of the specimen in a fructose-based mounting medium (derived from SeeDB) ( Ke et al., 2013 ) , with a refractive index matching that of the objective lens of the microscope. The combination of this technique with the use of genetically modified zebrafish in which green fluorescent protein (GFP) is expressed in specific cell populations provides opportunities to describe anatomical details not visible with other techniques.

Zebrafish have an ethanol-inducible hepatic 4-nitrophenol hydroxylase that is not CYP2E1-like

Zebrafish are an attractive model organism for toxicology; however, an important consideration in translating between species is xenobiotic metabolism/bioactivation. CYP2E1 metabolizes small hydrophobic molecules, e.g. ethanol, cigarette smoke, and diesel exhaust components. CYP2E1 is thought to only be conserved in mammals, but recent reports identified homologous zebrafish cytochrome P450s. Herein, ex vivo biochemical measurements show that unlike mammals, zebrafish possess a low-affinity 4-nitrophenol hydroxylase (K_m ∼0.6 mM) in hepatic microsomes and mitochondria that is inducible only 1.5- to 2-fold by ethanol and is insensitive to 4-methylpyrazole inhibition. In closing, we suggest creating improved models to study CYP2E1 in zebrafish.

Action sequencing in the spontaneous swimming behavior of zebrafish larvae - implications for drug development

All motile organisms need to organize their motor output to obtain functional goals. In vertebrates, natural behaviors are generally composed of a relatively large set of motor components which in turn are combined into a rich repertoire of complex actions. It is therefore an experimental challenge to investigate the organizational principles of natural behaviors. Using the relatively simple locomotion pattern of 10 days old zebrafish larvae we have here characterized the basic organizational principles governing the swimming behavior. Our results show that transitions between different behavioral states can be described by a model combining a stochastic component with a control signal. By dividing swimming bouts into a limited number of categories, we show that similar types of swimming behavior as well as stand-stills between bouts were temporally clustered, indicating a basic level of action sequencing. Finally, we show that pharmacological manipulations known to induce alterations in the organization of motor behavior in mammals, mainly through basal ganglia interactions, have related effects in zebrafish larvae. This latter finding may be of specific relevance to the field of drug development given the growing importance of zebrafish larvae in phenotypic screening for novel drug candidates acting on central nervous system targets.

Dopamine system, cerebellum, and nucleus ruber in fish and mammals

Small teleost fish including zebrafish and medaka have been used as animal models for research because of their small body size, vast amounts of eggs produced, their rapid development, low husbandry costs, and transparency during embryogenesis. Although the body size and appearance seem different, fish and mammals including human still possess anatomical and functional similarities in their brains. This review summarizes the similarities of brain structures and functions between teleost fish and mammalian brains, focusing on the dopamine system, functional regionalization of the cerebellum, and presence of the nucleus ruber.

Fishing for Fetal Alcohol Spectrum Disorders: Zebrafish as a Model for Ethanol Teratogenesis

Fetal Alcohol Spectrum Disorders (FASD) describes a wide array of ethanol-induced developmental defects, including craniofacial dysmorphology and cognitive impairments. It affects ∼1 in 100 children born in the United States each year. Due to the pleiotropic effects of ethanol, animal models have proven critical in characterizing the mechanisms of ethanol teratogenesis. In this review, we focus on the utility of zebrafish in characterizing ethanol-induced developmental defects. A growing number of laboratories have focused on using zebrafish to examine ethanol-induced defects in craniofacial, cardiac, ocular, and neural development, as well as cognitive and behavioral impairments. Growing evidence supports that genetic predisposition plays a role in these ethanol-induced defects, yet little is understood about these gene-ethanol interactions. With a high degree of genetic amenability, zebrafish is at the forefront of identifying and characterizing the gene-ethanol interactions that underlie FASD. Because of the conservation of gene function between zebrafish and humans, these studies will directly translate to studies of candidate genes in human populations and allow for better diagnosis and treatment of FASD.

Dopamine receptors participate in acquisition and consolidation of latent learning of spatial information in zebrafish (Danio rerio)

There is growing appreciation that various aspects of learning and memory are strongly influenced by dopamine neurotransmission, and that zebrafish hold particular promise in the study of neurotransmitter systems. In this study, we sought to investigate the effect of dopamine receptors on acquisition and consolidation of memory in zebrafish using a latent learning paradigm. To this end, fish were subjected to a 30 min training trial each day for 16 days during which fish were allowed to freely explore a complex maze with the left or right path blocked and without the presence of a reward. During 16 days fish were treated with dopaminergic agonists (apomorphine, SKF-38393, and quinpirole) and antagonists (SCH-23390 and eticlopride) before or after training trials. To assess cognitive performance of fish, a subsequent probe trial was performed on day 17 while all paths leading to a reward chamber were open and the maze now contained stimulus fish as a reward. Pre- and post-training exposure to apomorphine, SKF-38393, and quinpirole significantly impaired learning and memory in fish. In contrast, fish exposed to eticlopride before and after training exhibited improved performance in a latent learning task. Administration of SCH-23390 before training did not affect zebrafish learning ability, but produced significant memory enhancement when given after training trials. Taken together, these findings are the first indications that D1 and D2 receptors are critically involved in acquisition and consolidation of latent learning in zebrafish, with a more prominent role for D2 receptors. The current study opens the door to future studies to investigate the involvement of dopamine receptors in various aspects of cognitive processes.

Home tank water versus novel water differentially affect alcohol-induced locomotor activity and anxiety related behaviours in zebrafish

The zebrafish may be uniquely well suited for studying alcohol's mechanisms of action in vivo, since alcohol can be administered via immersion in a non-invasive manner. Despite the robust behavioural effects of alcohol administration in mammals, studies reporting the locomotor stimulant and anxiolytic effects of alcohol in zebrafish have been inconsistent. In the current study, we examined whether differences in the type of water used for alcohol exposure and behavioural testing contribute to these inconsistencies. To answer this question, we exposed zebrafish to either home water from their housing tanks or novel water from an isolated reservoir (i.e. water lacking zebrafish chemosensory and olfactory cues) with 0% or 1% v/v alcohol for 30 min, a 2 × 2 between subject experimental designs. Behavioural responses were quantified throughout the 30-minute exposure session via a video tracking system. Although control zebrafish exposed to home water and novel water were virtually indistinguishable in their behavioural responses, alcohol's effect on locomotor activity and anxiety-like behavioural responses were dependent on the type of water used for testing. Alcohol exposure in home tank water produced a mild anxiolytic and locomotor stimulant effect, whereas alcohol exposure in novel water produced an anxiogenic effect without altering locomotor activity. These results represent a dissociation between alcohol's effects on locomotor and anxiety related responses, and also illustrate how environmental factors, in this case familiarity with the water, may interact with such effects. In light of these findings, we urge researchers to explicitly state the type of water used.

Delayed effects of developmental exposure to low levels of the aryl hydrocarbon receptor agonist 3,3',4,4',5-pentachlorobiphenyl (PCB126) on adult zebrafish behavior

Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants. The most toxic PCBs are the non-ortho-substituted ("dioxin-like") congeners that act through the aryl hydrocarbon receptor (AHR) pathway. In humans, perinatal exposure to dioxin-like PCBs is associated with neurodevelopmental toxicity in children. Yet, the full potential for later-life neurobehavioral effects that result from early-life low level exposure to dioxin-like PCBs is not well understood. The objective of this study was to determine the effects of developmental exposure to low levels of dioxin-like PCBs on early- and later-life behavioral phenotypes using zebrafish as a model system. We exposed zebrafish embryos to either vehicle (DMSO) or low concentrations of PCB126 (0.3, 0.6, 1.2nM) for 20h (4-24h post fertilization), and then reared them to adulthood in clean water. Locomotor activity was tested at two larval stages (7 and 14 days post fertilization). Adult fish were tested for anxiety-related behavior using the novel tank and shoaling assays. Adult behavioral assays were repeated several times on the same group of fish and effects on intra- and inter-trial habituation were determined. While there was no effect of PCB126 on larval locomotor activity in response to changes in light conditions, developmental exposure to PCB126 resulted in impaired short- and long-term habituation to a novel environment in adult zebrafish. Cyp1a induction was measured as an indicator for AHR activation. Despite high induction at early stages, cyp1a expression was not induced in the brains of developmentally exposed adult fish that showed altered behavior, suggesting that AHR was not activated at this stage. Our results demonstrate the effectiveness of the zebrafish model in detecting subtle and delayed behavioral effects resulting from developmental exposure to an environmental contaminant.

Developmental exposure to organophosphate flame retardants causes behavioral effects in larval and adult zebrafish

BACKGROUND

Organophosphate flame retardants (OPFRs) have grown in usage since concerns about the health effects of the previously used polybrominated flame retardants led to their being phased out. The potential for OPFRs to cause adverse health effects of their own is still unexamined. Because of their structural similarities to organophosphate pesticides, which have themselves been heavily researched and shown to be neurobehavioral teratogens, we investigated the possibility that developmental exposure to two OPFRs, triphenyl phosphate (TPHP), and tris(1,3-dichloroisopropyl)phosphate (TDCIPP) might lead to behavioral impairment across the lifespan, as has been observed with the organophosphate pesticide chlorpyrifos.

METHODS

Zebrafish were exposed to 0.03 or 0.3 μM of TPHP, TDCIPP, or chlorpyrifos from 0 to 5 days post fertilization. Vehicle control consisted of 0.03% solution of DMSO. At 6 days post fertilization, larvae were tested on a locomotor assay. Separate cohorts of 6 day old larvae that were not tested on the larval assay were allowed to grow to adulthood. At 12 weeks post fertilization, these adult zebrafish were tested on a battery of behavioral assays that included tests of novel environment exploration, startle habituation, social affiliation, and predator escape.

RESULTS

Developmental exposure altered zebrafish behavior across the lifespan. Larval zebrafish exposed to the 0.03 μM doses of chlorpyrifos or TDCIPP exhibited significant (p<0.05) hyperactivity in the locomotor assay. Organophosphate exposure significantly (p<0.05) altered the time course of adult zebrafish behavior in the novel environment, startle habituation, and social affiliation assays. Predator escape behavior was significantly (p<0.05) reduced in fish exposed to the 0.3 μM dose of TDCIPP. Exposure also caused hyperactivity in adult fish, with fish exposed to the 0.3 μM dose of TDCIPP exhibiting significantly (p<0.05) elevated locomotor behavior in the novel environment assay.

DISCUSSION

Early developmental exposure to OPFRs produced behavioral impairment that persisted into adulthood. These findings support broader research investigating the role of organophosphate compounds, including the OPFRs used here, in developmental neurotoxicity.