Zebrafish and conditioned place preference: a translational model of drug reward

Extinction and reinstatement of methamphetamine-induced conditioned place preference in zebrafish

Conditioned place preference (CPP) paradigm in zebrafish has been used to measure drug reward, but there is limited research on CPP reinstatement to determine relapse vulnerability. The present study aimed to investigate extinction and reinstatement of methamphetamine (MA)-induced CPP in zebrafish and evaluate the model's predictive validity. Zebrafish received different doses of MA (0-60 mg/kg) during CPP training. The preferred dose of MA at 40 mg/kg was used for extinction via either confined or nonconfined procedures. The extinguished CPP was reinstated by administering a priming dose of MA (20 mg/kg) or various stressors. To assess persistent susceptibility to reinstatement, MA CPP and reinstatement were retested following 14 days of abstinence. In addition, the effects of SCH23390, naltrexone, and clonidine on MA CPP during acquisition, expression, or reinstatement phases were monitored. MA induced CPP in a dose-dependent manner. Both nonconfined and confined extinction procedures time-dependently reduced the time spent on the MA-paired side. A priming dose of MA, chasing stress, or yohimbine reinstated the extinguished CPP. After 14 days of abstinence, the MA CPP remained extinguished and was significantly reinstated by MA priming or chasing stress. Similar to the observations in rodents, SCH23390 suppressed the acquisition of MA CPP, naltrexone reduced the expression and MA priming-induced reinstatement, while clonidine prevented stress-induced reinstatement of MA CPP. This work expanded the zebrafish CPP paradigm to include extinction and reinstatement phases, demonstrating predictive validity and highlighting its potential as a valuable tool for exploring drug relapse.

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.

Chemo-kindling in adult zebrafish alters spatial cognition but not social novelty recognition

In the past decades, zebrafish have gathered immense attention and importance in the field of neurological sciences. In the case of epilepsy, zebrafish have appeared as a promising acute animal model for the screening and identification of potential antiepileptic molecules. However, the necessity for establishing competent chronic models of epilepsy in zebrafish is apparent. In this regard, recently we developed a chemo-kindling zebrafish model with a better clinical resemblance. In the present study, an attempt to examine the effect of pentylenetetrazole (PTZ)-induced kindling on the cognitive functions of zebrafish was made. In brief, adult zebrafish were repetitively given a sub-effective concentration of PTZ, till the onset of clonic-tonic seizures, entitled as kindled. Thereafter, T-maze test and social recognition memory test were conducted to evaluate spatial memory and social novelty recognition memory of the fish. At the end, both the groups were sacrificed and the brains were isolated to estimate neurotransmitter and gene expression levels. It was observed that PTZ kindling induced spatial cognition deficits and lower social exploration in zebrafish. However, it didn't change the novelty recognition memory of kindled zebrafish. The results of genes and neurotransmitters estimations in the brain also supported the behavioural findings. The results concluded that PTZ kindling alters spatial cognitive functions in adult zebrafish without affecting the social novelty recognition memory.

Evaluation of drug seeking behavior on nicotine conditioned place preference in zebrafish

Seeking of drugs is commonly evaluated in a specific environment for assessing drug preference. However, cognitive strategies involved in drug seeking are mostly unknown. To assess the strength of environmental cues that can be associated with nicotine in the zebrafish brain reward circuitry, we have designed herein a modified conditioned place preference (CPP) paradigm. This task was devised to identify salient environmental cues relevant for strong nicotine-environment association and drug seeking induction. During test sessions, background colors of the CPP tank chambers were shifted and preference for colors associated to nicotine was assessed. We have compared several tank designs and different compartment colors. Our findings indicated that zebrafish seeking behavior was strongly dependent on compartment color shades. Combination of red and yellow environments, which were preferred and avoided compartments, respectively, was the most effective design presenting the highest CPP-score. Interestingly, animals that stayed for longer periods in the environment conditioned to nicotine during a first testing interval were also able to follow the background color shade conditioned to nicotine to the other compartment immediately after background colors were relocated between compartments. During a second testing period, zebrafish also stayed for longer periods in the colored compartment paired to nicotine during conditioning. These findings suggest that under salient environmental conditions, zebrafish voluntarily followed a shifting visual cue previously associated with nicotine delivery. Furthermore, our findings indicate that zebrafish exhibit spatial associative learning and memory, which generates a repertoire of conspicuous locomotor behaviors induced by nicotine preference in the CPP task.

The development of behavioral sensitization induced by a single morphine exposure in adult zebrafish (Danio rerio)

BACKGROUND

Accumulating evidence suggest that behavioral sensitization is involved in the process of drug addiction. Zebrafish are sensitive to a variety of addictive drugs and are thus suitable for the study of behavioral sensitization. However, in contrast to mature rodent models of behavioral sensitization, how this phenomenon manifests in aquatic organisms, especially zebrafish, is largely unknown. In this study, we developed a morphine-induced behavioral sensitization adult zebrafish model and performed a preliminary investigation of the underlying mechanisms.

METHODS

Behavioral sensitization was established in zebrafish by observing their behavior after treatment and challenge with morphine. The effect of morphine was evaluated by a behavioral locomotor test. Different doses of morphine and withdrawal times were used to evaluate the establishment of the behavioral sensitization model.

RESULTS

Hyperlocomotion was induced after administration of morphine in adult zebrafish. After withdrawing the drug for a period, challenge with low-dose morphine evoked behavioral sensitization in zebrafish acutely pre-treated with morphine. Low-dose morphine failed to induce behavioral sensitization in zebrafish if the withdrawal time was less than 5 days or more than 7 days. Morphine induced behavioral sensitization in zebrafish may involve dopaminergic, glutamatergic and opioid systems.

CONCLUSION

A single low-dose of morphine could induce behavioral sensitization in zebrafish acutely pre-treated with morphine, and this phenomenon was highly correlated with drug dose and withdrawal time. These findings suggest that zebrafish is a suitable model for the study of behavioral sensitization.

Differences in the Alcohol Preference Assessment of Shy and Bold Zebrafish

Individuals differ in their preference for alcohol and propensity to develop alcoholism, where the behavioral profile, such as the bold-shy axis, plays an important role for such a difference. However, literature is limited and conflicting on the causes and consequences of this relationship. Translational studies using animal models, such as zebrafish, can help identify behavioral traits that predispose individuals to drink alcohol compulsively. Here, the preference for alcohol was investigated in two distinct traits in zebrafish: shy and bold. For this purpose, fish were separated into shy and bold traits and then a conditioned place preference paradigm was used, a strategy that allows the rewarding effects from alcohol to be assessed by the ability to enhance the animal’s preference for an environment that initially was not preferred. It was found that bold zebrafish actively searched for the environment that was paired to alcohol after one acute exposure, whereas, shy fish changed their place preference even without alcohol administration, showing that the conditioned place preference protocol, given the short amount time to assess place preference, is not ample enough for shy fish to choose. Our results show that behavioral profiles must be considered in further studies since differences between shy and bold individuals on preference behavior can strongly interfere in the assessment of drug preference, mainly when using the conditioned place preference paradigm.

An Overview of Zebrafish Modeling Methods in Drug Discovery and Development

Animal studies are recognized as a significant step forward in the bridging between drug discovery and clinical applications. Animal models, due to their relative genetic, molecular, physiological, and even anatomical similarities to humans, can provide a suitable platform for unraveling the mechanisms underlying human diseases and discovering new therapeutic approaches as well. Recently, zebrafish has attracted attention as a valuable experimental and pharmacological model in drug discovery and development studies due to its prominent characteristics such as the high degree of genetic similarity with humans, genetic manipulability, and prominent clinical features. Since advancing a theory to a valid and reliable observation requires the manipulation of animals, it is, therefore, essential to use efficient modeling methods appropriate to the different aspects of experimental conditions. In this context, applying several various approaches such as using chemicals, pathogens, and genetic manipulation approaches allows zebrafish development into a preferable model that mimics some human disease pathophysiology. Thus, such modeling approaches not only can provide a framework for a comprehensive understanding of the human disease mechanisms that have a counterpart in zebrafish but also can pave the way for discovering new drugs that are accompanied by higher amelioration effects on different human diseases.

Towards Modeling Anhedonia and Its Treatment in Zebrafish

Mood disorders, especially depression, are a major cause of human disability. The loss of pleasure (anhedonia) is a common, severely debilitating symptom of clinical depression. Experimental animal models are widely used to better understand depression pathogenesis and to develop novel antidepressant therapies. In rodents, various experimental models of anhedonia have already been developed and extensively validated. Complementing rodent studies, the zebrafish (Danio rerio) is emerging as a powerful model organism to assess pathobiological mechanisms of affective disorders, including depression. Here, we critically discuss the potential of zebrafish for modeling anhedonia and studying its molecular mechanisms and translational implications.

Long-lasting implications of embryonic exposure to alcohol: Insights from zebrafish research

The harmful consumption of ethanol is associated with significant health problems and social burdens. This drug activates a complex network of reward mechanisms and habit formation learning that is supposed to contribute to the consumption of increasingly high and frequent amounts, ultimately leading to addiction. In the context of fetal alcohol spectrum disorders, fetal alcohol syndrome (FAS) is a consequence of the harmful use of alcohol during pregnancy, which affects the embryonic development of the fetus. FAS can be easily reproduced in zebrafish by exposing the embryos to different concentrations of ethanol in water. In this regard, the aim of the present review is to discuss the late pathological implications in zebrafish exposed to ethanol at the embryonic stage, providing information in the context of human fetal alcoholic spectrum disorders. Experimental FAS in zebrafish is associated with impairments in the metabolic, morphological, neurochemical, behavioral, and cognitive domains. Many of the pathways that are affected by ethanol in zebrafish have at least one ortholog in humans, collaborating with the wider adoption of zebrafish in studies on alcohol disorders. In fact, zebrafish present validities required for the study of these conditions, which contributes to the use of this species in research, in addition to studies with rodents.

Epigenetic Mechanisms Mediate Nicotine-Induced Reward and Behaviour in Zebrafish

Nicotine induces long-term changes in the neural activity of the mesocorticolimbic reward pathway structures. The mechanisms involved in this process have not been fully characterized. The hypothesis discussed here proposed that epigenetic regulation participates in the installation of persistent adaptations and long-lasting synaptic plasticity generated by nicotine action on the mesolimbic dopamine neurons of zebrafish. The epigenetic mechanisms induced by nicotine entail histone and DNA chemical modifications, which have been described to lead to changes in gene expression. Among the enzymes that catalyze epigenetic chemical modifications, histone deacetylases (HDACs) remove acetyl groups from histones, thereby facilitating DNA relaxation and making DNA more accessible to gene transcription. DNA methylation, which is dependent on DNA methyltransferase (DNMTs) activity, inhibits gene expression by recruiting several methyl binding proteins that prevent RNA polymerase binding to DNA. In zebrafish, phenylbutyrate (PhB), an HDAC inhibitor, abolishes nicotine rewarding properties together with a series of typical reward-associated behaviors. Furthermore, PhB and nicotine alter long- and short-term object recognition memory in zebrafish, respectively. Regarding DNA methylation effects, a methyl group donor L-methionine (L-met) was found to dramatically reduce nicotine-induced conditioned place preference (CPP) in zebrafish. Simultaneous treatment with DNMT inhibitor 5-aza-2’-deoxycytidine (AZA) was found to reverse the L-met effect on nicotine-induced CPP as well as nicotine reward-specific effects on genetic expression in zebrafish. Therefore, pharmacological interventions that modulate epigenetic regulation of gene expression should be considered as a potential therapeutic method to treat nicotine addiction.

Acute ethanol induces behavioral changes and alters c-fos expression in specific brain regions, including the mammillary body, in zebrafish

Ethanol is one of the most commonly abused substances in the world, and ethanol abuse and dependence disorders represent major societal problems. However, appropriate treatment is lacking as we still do not fully understand the molecular bases of these disorders. The zebrafish is one of the model organisms utilized for studying such mechanisms. In this study, we examined the effects of acute ethanol administration on the behavior of zebrafish, and we also analyzed correlated gene expression changes using whole-mount in situ hybridization focusing on a number of genes associated with different neurotransmitter systems, stress response, and neuronal activity. We found ethanol treatment to result in hyperactivity and reduced shoal cohesion compared to control. Analysis of c-fos expression demonstrated altered activity patterns in certain brain regions, including intense activation of the mammillary body in zebrafish with acute ethanol treatment. We also found reduced level of gad1b expression in the cerebellum of ethanol treated fish compared to control. However, we could not detect significant changes in the expression level of other genes, including vglut2b, th, crh, hdc, avp, pomc, and galn in ethanol treated fish compared controls. Our results suggest that zebrafish is a promising animal model for the study of mechanisms underlying alcohol induced behavioral changes and alcohol related human disorders.

Methamphetamine pollution elicits addiction in wild fish

Illicit drug abuse presents pervasive adverse consequences for human societies around the world. Illicit drug consumption also plays an unexpected role in contamination of aquatic ecosystems that receive wastewater discharges. Here, we show that methamphetamine, considered as one of the most important global health threats, causes addiction and behavior alteration of brown trout Salmo trutta at environmentally relevant concentrations (1 µg l-1). Altered movement behavior and preference for methamphetamine during withdrawal were linked to drug residues in fish brain tissues and accompanied by brain metabolome changes. Our results suggest that emission of illicit drugs into freshwater ecosystems causes addiction in fish and modifies habitat preferences with unexpected adverse consequences of relevance at the individual and population levels. As such, our study identifies transmission of human societal problems to aquatic ecosystems.

Measurement of emotional states of zebrafish through integrated analysis of motion and respiration using bioelectric signals

Fear, anxiety, and preference in fish are generally evaluated by video-based behavioural analyses. We previously proposed a system that can measure bioelectrical signals, called ventilatory signals, using a 126-electrode array placed at the bottom of an aquarium and achieved cameraless real-time analysis of motion and ventilation. In this paper, we propose a method to evaluate the emotional state of fish by combining the motion and ventilatory indices obtained with the proposed system. In the experiments, fear/anxiety and appetitive behaviour were induced using alarm pheromone and ethanol, respectively. We also found that the emotional state of the zebrafish can be expressed on the principal component (PC) space extracted from the defined indices. The three emotional states were discriminated using a model-based machine learning method by feeding the PCs. Based on discrimination performed every 5 s, the F-score between the three emotional states were as follows: 0.84 for the normal state, 0.76 for the fear/anxiety state, and 0.59 for the appetitive behaviour. These results indicate the effectiveness of combining physiological and motional indices to discriminate the emotional states of zebrafish.

Test-dependent expression of behavioral syndromes: A study of aggressiveness, activity, and stress of chub

Aggressiveness has been one of the behavioral traits most examined with various standard testing methods. We used two distinct methods (the mirror and the real opponent tests) to evaluate individual aggression and relate it to the activity and individual stress of chub (Squalius cephalus L.). Three hypotheses were formulated and tested: (a) there is a significant positive relationship between the aggressiveness of individuals measured with the mirror and the real opponent tests, indicating their convergent validity; (b) the irregularities in response to the aggressiveness and activity tests lead to the context-specific expression of the behavioral syndromes; and (c) there is a significant positive relationship between the stress induced in individuals by both tests of aggressiveness, demonstrating individually consistent stress-coping strategies. The first and the second hypothesis were confirmed, while the third hypothesis was rejected. Our results suggest that particular tests of aggressiveness could act as a situation with high strength, leaving little variation between individual responses. Thus, we propose that for the proper interpretation of various studies using different tests to study identical behavioral traits, it is important to consider the convergent validity of not only the tested behavioral traits but also the individual stress responses. The chub also showed stress relieve through aggressiveness, suggesting the species as a prospective animal model to the study interaction between the stress and the aggressiveness. A detailed aggression ethogram of chub was provided to facilitate the use of this specie in future studies.

Neuroprotective effects of magnesium L-threonate in a hypoxic zebrafish model

BACKGROUND

Hypoxia inhibits the uptake of glutamate (a major neurotransmitter in the brain closely related to cognitive function) into brain cells, and the initial response of cells to cortical hypoxia depends on glutamate. Previous studies have suggested that magnesium may have protective effects against hypoxic injuries. In particular, magnesium L-threonate (MgT) may increase magnesium ion concentrations in the brain better than MgSO₄ and improve cognitive function.

METHODS

We evaluated cell viability under hypoxic conditions in the MgT- and MgSO₄-treated human SH-SY5Y neurons, in vivo behavior using the T-maze test following hypoxia in MgT-treated zebrafish, activity of brain mitochondrial dehydrogenase by 2,3,5-triphenyltetrazolium chloride (TTC) staining, and protein expression of the excitatory amino acid transporter (EAAT) 4 glutamate transporter by western blotting.

RESULTS

Among the groups treated with hypoxia, cell viability significantly increased when pre-treated with 1 or 10 mM MgT (p = 0.009 and 0.026, respectively). Despite hypoxic insult, MgT-treated zebrafish showed preferences for the red compartment (p = 0.025 for distance and p = 0.007 for frequency of entries), suggesting memory preservation. TTC staining showed reduced cerebral infarction and preserved absorbance in the MgT-treated zebrafish brain after hypoxia (p = 0.010 compared to the hypoxia group). In addition, western blot showed upregulation of EAAT4 protein in the MgT treated group.

CONCLUSIONS

Pre-treatment with MgT attenuated cell death and cerebral infarction due to hypoxia and protected cognitive function in zebrafish. In addition, MgT appeared to modulate expression of the glutamate transporter, EAAT4.

Inhibition of Lrrk2 reduces ethanol preference in a model of acute exposure in zebrafish

Due to its multifactorial and yet to be fully understood origin, ethanol addiction is a field that still requires studies for the elucidation of novel genes and pathways that potentially influence the establishment and maintenance of addiction-like phenotypes. In this context, the present study aimed to evaluate the role of the LRRK2 pathway in the modulation of ethanol preference behavior in Zebrafish (Danio rerio). Using the behavioral Conditioned Place Preference (CPP) paradigm, we accessed the preference of animals for ethanol. Next, we evaluated the transcriptional regulation of the gene lrrk2 and the receptors drd1, drd2, grin1a, gria2a, and gabbr1b in the zebrafish brain. Additionally, we used a selective inhibitor of Lrrk2 (GNE-0877) to assess the role of this gene in the preference behavior. Our results revealed four distinct ethanol preference phenotypes (Light, Heavy, Negative Reinforcement, and Inflexible), each showing different transcriptional regulation patterns of the drd1, drd2, grin1a, gria2a, and gabbr1b receptors. We showed that the lrrk2 gene was hyperregulated only in the brains of the animals with the Inflexible phenotype. Most importantly, we showed, for the first time in the context of preference for ethanol, that treatment with the GNE-0877 inhibitor modulates the transcription of the target receptor genes and reduces the preference for ethanol in the animals of the Inflexible group. This result corroborates the hypothesis that the LRRK2 pathway is involved in the inflexible preference for ethanol behavior. Lastly, we identified a possible pharmacological target for the treatment of abusive preference behavior for ethanol.

Design and development of a robotic predator as a stimulus in conditioned place aversion for the study of the effect of ethanol and citalopram in zebrafish

Zebrafish are becoming a species of choice in psychopharmacology, laying a promising path to refined pharmacological manipulations and high-throughput behavioral phenotyping. The field of robotics has the potential to accelerate progress along this path, by offering unprecedented means for the design and development of accurate and reliable experimental stimuli. In this work, we demonstrate, for the first time, the integration of robotic predators in place conditioning experiments. We hypothesized zebrafish to be capable of forming a spatial association under a simulated predation risk. We repeatedly exposed experimental subjects to a robotic heron impacting the water surface and then evaluated their spatial avoidance within the experimental tank in a subsequent predator-free test session. To pharmacologically validate the paradigm, we tested zebrafish in drug-free conditions (control groups) or in response to three different concentrations of citalopram (30, 50, and 100 mg/L) and ethanol (0.25, 0.50, and 1.00%). Experimental data indicate that, when tested in the absence of the conditioning stimulus, zebrafish displayed a marked preference for the bottom of the test tank, that is, the farthest location from the simulated attacks by the robotic heron. This conditioned geotaxis was reduced by the administration of citalopram in a linear dose-response curve and ethanol at the low concentration. Ultimately, our data demonstrate that robotic stimuli may represent valid conditioning tools and, thereby, aid the field of zebrafish psychopharmacology.

Maternal ethanol consumption before paternal fertilization: Stimulation of hypocretin neurogenesis and ethanol intake in zebrafish offspring

There are numerous clinical and pre-clinical studies showing that exposure of the embryo to ethanol markedly affects neuronal development and stimulates alcohol drinking and related behaviors. In rodents and zebrafish, our studies show that embryonic exposure to low-dose ethanol, in addition to increasing voluntary ethanol intake during adolescence, increases the density of hypothalamic hypocretin (hcrt) neurons, a neuropeptide known to regulate reward-related behaviors. The question addressed here in zebrafish is whether maternal ethanol intake before conception also affects neuronal and behavioral development, phenomena suggested by clinical reports but seldom investigated. To determine if preconception maternal ethanol consumption also affects these hcrt neurons and behavior in the offspring, we first standardized a method of measuring voluntary ethanol consumption in AB strain adult and larval zebrafish given gelatin meals containing 10% or 0.1% ethanol, respectively. We found the number of bites of gelatin to be an accurate measure of intake in adults and a strong predictor of blood ethanol levels, and also to be a reliable indicator of intake in larval zebrafish. We then used this feeding paradigm and live imaging to examine the effects of preconception maternal intake of 10% ethanol-gelatin compared to plain-gelatin for 14 days on neuronal development in the offspring. Whereas ethanol consumption by adult female HuC:GFP transgenic zebrafish had no impact on the number of differentiated HuC⁺ neurons at 28 h post-fertilization (hpf), preconception ethanol consumption by adult female hcrt:EGFP zebrafish significantly increased the number of hcrt neurons in the offspring, an effect observed at 28 hpf and confirmed at 6 and 12 days post-fertilization (dpf). This increase in hcrt neurons was primarily present on the left side of the brain, indicating asymmetry in ethanol's actions, and it was accompanied by behavioral changes in the offspring, including a significant increase in novelty-induced locomotor activity but not thigmotaxis measured at 6 dpf and also in voluntary consumption of 0.1% ethanol-gelatin at 12 dpf. Notably, these measures of ethanol intake and locomotor activity stimulated by preconception ethanol were strongly, positively correlated with the number of hcrt neurons. These findings demonstrate that preconception maternal ethanol consumption affects the brain and behavior of the offspring, producing effects similar to those caused by embryonic ethanol exposure, and they provide further evidence that the ethanol-induced increase in hcrt neurogenesis contributes to the behavioral disturbances caused by ethanol.

D-Amphetamine Exposure Differentially Disrupts Signaling Across Ontogeny in the Zebrafish

Background: Prescriptive and illicit amphetamine (AMPH) use continues to increase along with the likelihood that during an individual's lifetime, the drug deleteriously influences the growth and connectivity of behavior circuits necessary for survival. Throughout ontogeny, neural circuits underlying these behaviors grow in complexity, gradually integrating many sensory inputs that trigger higher order coordinated motor responses. In the present study, we examine how AMPH disrupts the establishment of these circuits at critical neurodevelopmental periods, as well as the communication among established survival circuits. Materials and Methods: Zebrafish embryos (from 1 hpf) were raised in AMPH solutions, growth parameters and escape behavior were assessed at 24 and 48 hpf, and spinal cord tissues analyzed for differences in excitatory-inhibitory signaling balance among treatments. Adult fish were fed an acute dosage of AMPH over an 11-day conditioned place preference (PP) paradigm during which behaviors were recorded and brain tissues analyzed for alterations in dopaminergic signaling. Results: AMPH negatively affects embryonic growth and slows the execution of escape behavior, suggesting an imbalance in locomotor signaling. Although local spinal circuits provide primary escape modulation, no differences in inhibitory glycinergic, and excitatory glutamatergic signaling were measured among spinal neurons. AMPH also influenced place preference in adult zebrafish and resulted in the increased expression of dopamine signaling proteins (DRD1) in brain areas governing survival behaviors.

Effects of hypoxic preconditioning on memory evaluated using the T-maze behavior test

Perioperative brain ischemia and stroke are leading causes of morbidity and mortality. Brief hypoxic preconditioning is known to have protective effects against hypoxic-ischemic insult in the brain. Current studies on the neuroprotective effects of ischemic preconditioning are based on histologic findings and biomarker changes. However, studies regarding effects on memory are rare. To precondition zebrafish to hypoxia, they were exposed to a dissolved oxygen (DO) concentration of 1.0 ± 0.5 mg/L in water for 30 s. The hypoxic zebrafish were then exposed to 1.0 ± 0.5 mg/L DO until the third stage of hypoxia, for 10 min ± 30 s. Zebrafish were assessed for memory retention after the hypoxic event. Learning and memory were tested using the T-maze, which evaluates memory based on whether or not zebrafish moves to the correct target compartment. In the hypoxic preconditioning group, infarct size was reduced compared with the hypoxic-only treated zebrafish group; memory was maintained to a degree similar to that in the hypoxia-untreated group. The hypoxic-only group showed significant memory impairments. In this study, we used a hypoxic zebrafish model and assessed the effects of ischemic preconditioning not only on histological damages but also on brain function, especially memory. This study demonstrated that a brief hypoxic event has protective effects in hypoxic brain damage and helped maintain memory in zebrafish. In addition, our findings suggest that the zebrafish model is useful in rapidly assessing the effects of ischemic preconditioning on memory.

Evaluation of the rewarding properties of nicotine and caffeine by implementation of a five-choice conditioned place preference task in zebrafish

The rewarding properties of drugs in zebrafish can be studied using the conditioned place preference (CPP) paradigm. Most devices that have been used for CPP consist of two-half tanks with or without a central chamber. Here we evaluated the rewarding effects of nicotine and caffeine using a tank with five arms distributed radially from a central chamber that we have denoted Fish Tank Radial Maze (FTRM). Zebrafish were trained to associate nicotine or caffeine with a coloured arm. In testing sessions to assess CPP induction, between two and five different arms were available to explore. We found that when offering the two arms, one of them associated to the drug mediating conditioning for 14 days, zebrafish showed nicotine-induced CPP but not caffeine-induced CPP. When zebrafish had the option to explore drug-paired arms together with new coloured arms as putative distractors, the nicotine-CPP strength was maintained for at least three days. The presence of novel environments induced caffeine-CPP, which was still positive after three days of testing sessions. Complementary behavioural data supported these findings. Nicotine-CPP was prevented by the histone deacetylase inhibitor phenylbutyrate administered during conditioning; however, there were no effects on caffeine-CPP. The specific acetylation of lysine 9 in histone 3 (H3-K9) was increased in nicotine-conditioned zebrafish brains. This study suggests that novel environmental cues facilitate drug-environment associations, and hence, the use of drugs of abuse.

Assessing the Value of the Zebrafish Conditioned Place Preference Model for Predicting Human Abuse Potential

Regulatory agencies recommend that centrally active drugs are tested for abuse potential before approval. Standard preclinical assessments are conducted in rats or non-human primates (NHPs). This study evaluated the ability of the zebrafish conditioned place preference (CPP) model to predict human abuse outcomes. Twenty-seven compounds from a variety of pharmacological classes were tested in zebrafish CPP, categorized as positive or negative, and analyzed using standard diagnostic tests of binary classification to determine the likelihood that zebrafish correctly predict robust positive signals in human subjective effects studies (+HSE) and/or Drug Enforcement Administration drug scheduling. Results were then compared with those generated for rat self-administration and CPP, as well as NHP self-administration, using this same set of compounds. The findings reveal that zebrafish concordance and sensitivity values were not significantly different from chance for both +HSE and scheduling. Although significant improvements in specificity and negative predictive values were observed for zebrafish relative to +HSE, specificity without sensitivity provides limited predictive value. Moreover, assessments in zebrafish provided no added value for predicting scheduling. By contrast, rat and NHP models generally possessed significantly improved concordance, sensitivity, and positive predictive values for both clinical measures. Although there may be predictive value with compounds from specific pharmacological classes (e.g., µ-opioid receptor agonists, psychostimulants) for zebrafish CPP, altogether these data highlight that using the current methodology, the zebrafish CPP model does not add value to the preclinical assessment of abuse potential.

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.

Abuse potential of methylenedioxymethamphetamine (MDMA) and its derivatives in zebrafish: role of serotonin 5HT2-type receptors

RATIONALE

The synthetic phenethylamines are recreational drugs known to produce psychostimulant effects. However, their abuse potential has not been widely studied.

OBJECTIVES

Here, we investigated the rewarding and the hallucinatory effects of 2,5-dimetoxy-4-bromo-amphetamine hydrobromide (DOB) and para-methoxyamphetamine (PMA) in comparison with the classical 3,4-methylenedioxymethamphetamine (MDMA). In addition, the role of serotonin 5-HT2-like receptor on the abovementioned effects was evaluated.

METHODS

Zebrafish were intramuscularly (i.m.) treated with a wide range of doses of DOB (0.1-20 mg/kg), PMA (0.0005-2 mg/kg), or MDMA (0.5-160 mg/kg). Animals were submitted to a conditioned place preference (CPP) task, to investigation of the rewarding properties, and to the evaluation of hallucinatory behavior in terms of appearance of a trance-like behavior. The serotonin 5-HT2 subtype receptor antagonist ritanserin (0.025-2.5 mg/kg) in association with the maximal effective dose of MDMA, DOB, and PMA was given i.m., and the effect on CPP or hallucinatory behavior was evaluated.

RESULTS

MDMA and its derivatives exhibited CPP in a biphasic fashion, being PMA the most potent. This effect was accompanied, for DOB (2 mg/kg) and PMA (0.1 mg/kg), by a trance-like hallucinatory behavior. MDMA at a high dose as 160 mg/kg did not induce any hallucinatory behavior. Ritanserin significantly blocked the rewarding and hallucinatory effects suggesting the involvement of serotonin 5HT2 subtype receptor.

CONCLUSION

Collectively, these findings demonstrate for the first time that the rewarding properties of DOB and PMA are accompanied by hallucinatory behavior through a serotonergic system and reinforce zebrafish as an emerging experimental model for screening new hallucinogens.

Analysis of Extracellular Nucleotide Metabolism in Adult Zebrafish After Embryological Exposure to Valproic Acid

Autism is a neurodevelopmental disorder characterized by symptoms related to stereotyped movements, deficits in social interaction, impaired communication, anxiety, hyperactivity, and the presence of restricted interests. Evidence indicates an important role of extracellular ATP and adenosine as signaling molecules in autism. ATP hydrolysis by ectonucleotidases is an important source of adenosine, and adenosine deaminase (ADA) contributes to the control of the nucleoside concentrations. Considering zebrafish is an animal model that may contribute towards to understanding the mechanisms that underlie social behavior, we investigated the purinergic signaling in a model of embryological exposure to valproic acid (VPA) that induces social interaction deficit in adult zebrafish. We demonstrated embryological exposure to VPA did not change ATP and ADP hydrolysis in zebrafish at 120 dpf, and the cytosolic (soluble) ADA activity was not altered. However, we observed an increase of AMP hydrolysis (12.5 %) whereas the ecto-ADA activity was decreased (19.2 %) in adult zebrafish submitted to embryological exposure to VPA. Quantitative reverse transcription PCR (RT-PCR) analysis showed changes on ntpd8, ADA 2.1, and A2a1 mRNA transcript levels. Brain ATP metabolism showed a rapid catabolism of ATP and ADP, whereas the extracellular metabolism of AMP and adenosine (ADO) occurred slowly. We demonstrated that embryological exposure to VPA altered biochemical and molecular parameters related to purinergic system in adult zebrafish. These findings indicate that the enzyme activities involved in the control of ATP and adenosine levels may be involved in the pathophysiological mechanisms of diseases related to the impairment of social interaction, such as autism.

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.

Sustained Effects of Developmental Exposure to Ethanol on Zebrafish Anxiety-Like Behaviour

In zebrafish developmentally exposed to ambient ethanol (20mM-50mM) 1–9 days post fertilization (dpf), the cortisol response to stress has been shown to be significantly attenuated in larvae, juveniles and 6 month old adults. These data are somewhat at variance with similar studies in mammals, which often show heightened stress responses. To test whether these cortisol data correlate with behavioural changes in treated animals, anxiety-like behaviour of zebrafish larvae (9dpf and 10dpf) and juveniles (23dpf) was tested in locomotor assays designed to this end. In open field tests treated animals were more exploratory, spending significantly less time at the periphery of the arena. Behavioural effects of developmental exposure to ethanol were sustained in 6-month-old adults, as judged by assessment of thigmotaxis, novel tank diving and scototaxis. Like larvae and juveniles, developmentally treated adults were generally more exploratory, and spent less time at the periphery of the arena in thigmotaxis tests, less time at the bottom of the tank in the novel tank diving tests, and less time in the dark area in scototaxis tests. The conclusion that ethanol-exposed animals showed less anxiety-like behaviour was validated by comparison with the effects of diazepam treatment, which in thigmotaxis and novel tank diving tests had similar effects to ethanol pretreatment. There is thus a possible link between the hypophyseal-pituitary-interrenal axis and the behavioural actions of developmental ethanol exposure. The mechanisms require further elucidation.

Zebrafish dives into food research: effectiveness assessment of bioactive compounds

Zebrafish ease of use and characteristics reveal it to be an interesting and underused model in food and nutrition research.

Zebrafish neurobehavioral phenomics for aquatic neuropharmacology and toxicology research

Zebrafish (Danio rerio) are rapidly emerging as an important model organism for aquatic neuropharmacology and toxicology research. The behavioral/phenotypic complexity of zebrafish allows for thorough dissection of complex human brain disorders and drug-evoked pathological states. As numerous zebrafish models become available with a wide spectrum of behavioral, genetic, and environmental methods to test novel drugs, here we discuss recent zebrafish phenomics methods to facilitate drug discovery, particularly in the field of biological psychiatry. Additionally, behavioral, neurological, and endocrine endpoints are becoming increasingly well-characterized in zebrafish, making them an inexpensive, robust and effective model for toxicology research and pharmacological screening. We also discuss zebrafish behavioral phenotypes, experimental considerations, pharmacological candidates and relevance of zebrafish neurophenomics to other 'omics' (e.g., genomic, proteomic) approaches. Finally, we critically evaluate the limitations of utilizing this model organism, and outline future strategies of research in the field of zebrafish phenomics.

Embryological exposure to valproic acid induces social interaction deficits in zebrafish (Danio rerio): A developmental behavior analysis

Changes in social behavior are associated with brain disorders, including mood disorders, stress, schizophrenia, Alzheimer's disease, and autism spectrum disorders (ASD). Autism is a complex neurodevelopmental disorder characterized by deficits in social interaction, impaired communication, anxiety, hyperactivity, and the presence of restricted interests. Zebrafish is one of the most social vertebrates used as a model in biomedical research, contributing to an understanding of the mechanisms that underlie social behavior. Valproic acid (VPA) is used as an anti-epileptic drug and mood stabilizer; however, prenatal VPA exposure in humans has been associated with an increased incidence of autism and it can also affect fetal brain development. Therefore, we conducted a behavioral screening at different periods of zebrafish development at 6, 30, 70, and 120dpf (days postfertilization) after VPA exposure in the early development stage to investigate social behavior, locomotion, aggression, and anxiety. VPA (48μM) exposure during the first 48hpf (hours postfertilization) did not promote changes on survival, morphology, and hatching rate at 24hpf, 48hpf, and 72hpf. The behavioral patterns suggest that VPA exposure induces changes in locomotor activity and anxiety at different developmental periods in zebrafish. Furthermore, a social interaction deficit is present at 70dpf and 120dpf. VPA exposure did not affect aggression in the adult stage at 70dpf and 120dpf. This is the first study that demonstrated zebrafish exposed to VPA during the first 48h of development exhibit deficits in social interaction, anxiety, and hyperactivity at different developmental periods.

Sustained action of developmental ethanol exposure on the cortisol response to stress in zebrafish larvae and adults

Background

Ethanol exposure during pregnancy is one of the leading causes of preventable birth defects, leading to a range of symptoms collectively known as fetal alcohol spectrum disorder. More moderate levels of prenatal ethanol exposure lead to a range of behavioural deficits including aggression, poor social interaction, poor cognitive performance and increased likelihood of addiction in later life. Current theories suggest that adaptation in the hypothalamo-pituitary-adrenal (HPA) axis and neuroendocrine systems contributes to mood alterations underlying behavioural deficits and vulnerability to addiction. In using zebrafish (Danio rerio), the aim is to determine whether developmental ethanol exposure provokes changes in the hypothalamo-pituitary-interrenal (HPI) axis (the teleost equivalent of the HPA), as it does in mammalian models, therefore opening the possibilities of using zebrafish to elucidate the mechanisms involved, and to test novel therapeutics to alleviate deleterious symptoms.

Results and Conclusions

The results showed that developmental exposure to ambient ethanol, 20mM-50mM 1-9 days post fertilisation, had immediate effects on the HPI, markedly reducing the cortisol response to air exposure stress, as measured by whole body cortisol content. This effect was sustained in adults 6 months later. Morphology, growth and locomotor activity of the animals were unaffected, suggesting a specific action of ethanol on the HPI. In this respect the data are consistent with mammalian results, although they contrast with the higher corticosteroid stress response reported in rats after developmental ethanol exposure. The mechanisms that underlie the specific sensitivity of the HPI to ethanol require elucidation.

Aquatic toxicology of fluoxetine: understanding the knowns and the unknowns

Fluoxetine is one of the most prescribed psychotropic medications, and is an agent of increasing interest for environmental toxicology. Fish and other aquatic organisms are excellent models to study neuroactive small molecules like fluoxetine. However, prone to variance due to experimental factors, data obtained in these models need to be interpreted with caution, using proper experimental protocols, study designs, validated endpoints as well as well-established models and tests. Choosing the treatment protocol and dose range for fluoxetine and other serotonergic drugs is critical for obtaining valid test results and correct data interpretation. Here we discuss the value of aquatic models to study fluoxetine effects, based on prior high-quality research, and outline the directions of future translational studies in the field. We review fluoxetine-evoked phenotypes in acute vs. chronic protocols, discussing them in the contact of complex role of serotonin in behavioral regulation. We conclude that zebrafish and other aquatic models represent a useful in-vivo tool for fluoxetine pharmacology and (eco)toxicology research.

Current approaches for the discovery of drugs that deter substance and drug abuse

INTRODUCTION

Much has been presented and debated on the topic of drug abuse and its multidimensional nature, including the role of society and its customs and laws, economical factors, and the magnitude and nature of the burden. Given the complex nature of the receptors and pathways implicated in regulation of the cognitive and behavioral processes associated with addiction, a large number of molecular targets have been interrogated during recent years to discover starting points for development of small-molecule interventions.

AREAS COVERED

This review describes recent developments in the field of early drug discovery for drug abuse interventions with an emphasis on the advances published during the 2012 - 2014 period.

EXPERT OPINION

Technologically, the processes/platforms utilized in drug abuse drug discovery are nearly identical to those used in the other disease areas. A key complicating factor in drug abuse research is the enormous biological complexity surrounding the brain processes involved and the associated difficulty in finding 'good' targets and achieving exquisite selectivity of treatment agents. While tremendous progress has been made during recent years to use the power of high-throughput technologies to discover proof-of-principle molecules for many new targets, next-generation models will be especially important in this field. Examples include: seeking advantageous drug-drug combinations, the use of automated whole-animal behavioral screening systems, advancing our understanding of the role of epigenetics in drug addiction and the employment of organoid-level 3D test platforms (also referred to as tissue-chip or organs-on-chip).