Distinct structure and activity of monoamine oxidase in the brain of zebrafish (Danio rerio)

Concerted monoamine oxidase activity following exposure to di-2-ethylhexyl phthalate is associated with aggressive neurobehavioral response and neurodegeneration in zebrafish brain

Di-2-ethylhexyl phthalate (DEHP) is the most commonly preferred synthetic organic chemical in plastics and its products for making them ductile, flexible and durable. As DEHP is not chemically bound to the macromolecular polymer of plastics, it can be easily leached out to accumulate in food and environment. Our recent report advocated that exposure to DEHP significantly transformed the innate bottom-dwelling and scototaxis behaviour of zebrafish. Our present study aimed to understand the possible role of DEHP exposure pertaining towards the development of aggressive behaviour and its association with amplified monoamine oxidase activity and neurodegeneration in the zebrafish brain. As heightened monoamine oxidase (MAO) is linked with genesis of aggressive behaviour, our observation also coincides with DEHP-persuaded aggressive neurobehavioral transformation in zebrafish. Our preliminary findings also showed that DEHP epitomized as a prime factor in transforming native explorative behaviour and genesis of aggressive behaviour through oxidative stress induction and changes in the neuromorphology in the periventricular grey zone (PGZ) of the zebrafish brain. With the finding demarcating towards heightened chromatin condensation in the PGZ of zebrafish brain, our further observation by immunohistochemistry showed a profound augmentation in apoptotic cell death marker cleaved caspase 3 (CC3) expression following exposure to DEHP. Our further observation by immunoblotting study also demarcated a temporal augmentation in CC3 and tyrosine hydroxylase expression in the zebrafish brain. Therefore, the gross findings of the present study delineate the idea that chronic exposure to DEHP is associated with MAO-instigated aggressive neurobehavioral transformation and neurodegeneration in the zebrafish brain.

Alterations in the brain serotonin system and serotonin-regulated behavior during aging in zebrafish males and females

The brain serotonin (5-HT) system performs a neurotrophic function and supports the plasticity of the nervous system, while its age-related changes can increase the risk of senile neurodegeneration. Zebrafish brain is highly resistant to damage and neurodegeneration due to its high regeneration potential and it is a promising model object in searching for molecular factors preventing age-related neurodegeneration. In the present study alterations in 5-HT-related behavior in the home tank and the novel tank diving test, as well as 5-HT, 5-HIAA levels, tryptophan hydroxylase (TPH), monoamine oxidase (MAO) activity and the expression of genes encoding TPH, MAO, 5-HT transporter and 5-HT receptors in the brain of 6, 12, 24 and 36 month old zebrafish males and females are investigated. Marked sexual dimorphism in the locomotor activity in the novel tank test is revealed: females of all ages move slower than males. No sexual dimorphism in 5-HT-related traits is observed. No changes in 5-HT and 5-HIAA levels in zebrafish brain during aging is observed. At the same time, the aging is accompanied by a decrease in the locomotor activity, TPH activity, tph2 and htr1aa genes expression as well as an increase in the MAO activity and slc6a4a gene expression in their brain. These results indicate that the brain 5-HT system in zebrafish is resistant to age-related alterations.

Fenpropathrin causes alterations in locomotion and social behaviors in zebrafish (Danio rerio)

Fenpropathrin is one of the widely used pyrethroid pesticides in agriculture and is frequently detected in the environment, groundwater, and food. While fenpropathrin was found to have neurotoxic effects in mammals, it remains unclear whether it has similar effects on fish. Here, we used adult zebrafish to investigate the impacts of fenpropathrin on fish social behaviors and neural activity. Exposure of adult zebrafish to 500 ppb of fenpropathrin for 72 h increased anxiety levels but decreased physical fitness, as measured by a novel tank diving test and swimming tunnel test. Fish exposed to fenpropathrin appeared to spend more time in the conspecific zone of the tank, possibly seeking greater comfort from their companions. Although learning, memory, and aggressive behavior did not change, fish exposed to fenpropathrin appeared to have shorter fighting durations. The immunocytochemical results showed the tyrosine hydroxylase antibody-labeled dopaminergic neurons in the teleost posterior tuberculum decreased in the zebrafish brain. According to a quantitative polymerase chain reaction (qPCR) analysis of the brain, exposure to fenpropathrin resulted in a decrease in the messenger (m)RNA expression of monoamine oxidase (mao), an enzyme that facilitates the deamination of dopamine. In contrast, the mRNA expression of the sncga gene, which may trigger Parkinson's disease, was found to have increased. There were no changes observed in expressions of genes related to antioxidants and apoptosis between the control and fenpropathrin-exposed groups. We provide evidence to demonstrate the defect of the neurotoxicity of fenpropathrin toward dopaminergic neurons in adult zebrafish.

An Astyanax mexicanus mao knockout line uncovers the developmental roles of monoamine homeostasis in fish brain

Monoaminergic systems are conserved in vertebrates, yet they present variations in neuroanatomy, genetic components and functions across species. MonoAmine Oxidase, or MAO, is the enzyme responsible for monoamine degradation. While mammals possess two genes, MAO-A and MAO-B, fish possess one single mao gene. To study the function of MAO and monoamine homeostasis on fish brain development and physiology, here we have generated a mao knockout line in Astyanax mexicanus (surface fish), by CRISPR/Cas9 technology. Homozygote mao knockout larvae died at 13 days post-fertilization. Through a time-course analysis, we report that hypothalamic serotonergic neurons undergo fine and dynamic regulation of serotonin level upon loss of mao function, in contrast to those in the raphe, which showed continuously increased serotonin levels - as expected. Dopaminergic neurons were not affected by mao loss-of-function. At behavioral level, knockout fry showed a transient decrease in locomotion that followed the variations in the hypothalamus serotonin neuronal levels. Finally, we discovered a drastic effect of mao knockout on brain progenitors proliferation in the telencephalon and hypothalamus, including a reduction in the number of proliferative cells and an increase of the cell cycle length. Altogether, our results show that MAO has multiple and varied effects on Astyanax mexicanus brain development. Mostly, they bring novel support to the idea that serotonergic neurons in the hypothalamus and raphe of the fish brain are different in nature and identity, and they unravel a link between monoaminergic homeostasis and brain growth.

Toward the use of novel alternative methods in epilepsy modeling and drug discovery

Epilepsy is a chronic brain disease and, considering the amount of people affected of all ages worldwide, one of the most common neurological disorders. Over 20 novel antiseizure medications (ASMs) have been released since 1993, yet despite substantial advancements in our understanding of the molecular mechanisms behind epileptogenesis, over one-third of patients continue to be resistant to available therapies. This is partially explained by the fact that the majority of existing medicines only address seizure suppression rather than underlying processes. Understanding the origin of this neurological illness requires conducting human neurological and genetic studies. However, the limitation of sample sizes, ethical concerns, and the requirement for appropriate controls (many patients have already had anti-epileptic medication exposure) in human clinical trials underscore the requirement for supplemental models. So far, mammalian models of epilepsy have helped to shed light on the underlying causes of the condition, but the high costs related to breeding of the animals, low throughput, and regulatory restrictions on their research limit their usefulness in drug screening. Here, we present an overview of the state of art in epilepsy modeling describing gold standard animal models used up to date and review the possible alternatives for this research field. Our focus will be mainly on ex vivo, in vitro, and in vivo larval zebrafish models contributing to the 3R in epilepsy modeling and drug screening. We provide a description of pharmacological and genetic methods currently available but also on the possibilities offered by the continued development in gene editing methodologies, especially CRISPR/Cas9-based, for high-throughput disease modeling and anti-epileptic drugs testing.

Suppression of Chronic Unpredictable Stress-Persuaded Increased Monoamine Oxidase Activity by Taurine Promotes Significant Neuroprotection in Zebrafish Brain

Neuropsychiatric upshots following chronic exposure to unpredictable adverse stressors have been well documented in the literature. Considering the significant impact of chronic unpredictable stress (CUS), the literature is elusive regarding the neuroprotective efficacy of taurine against CUS-induced oxidative stress and chromatin condensation in the zebrafish brain. In this study, to ameliorate CUS-persuaded neurological outcomes, waterborne treatment of taurine as a prophylactic intervention was undertaken. Further, our approach also focused on the gross neurobehavioral response of zebrafish, oxidative stress indices and neuromorphology of the zebrafish brain following CUS exposure with taurine treatment. Because taurine provides significant neuroprotection against oxidative insult, the cytosolic level of monoamine oxidase (MAO) in the zebrafish brain following CUS exposure is worth investigating. Further, as heightened MAO activity is associated with augmented oxidative and chromatin condensation, the focus of this study was on whether taurine provides neuroprotection by downregulating MAO levels in the brain. Our findings show that CUS-persuaded altered neurobehavioral response was significantly rescued by taurine. Moreover, our findings firmly support the hypothesis that taurine acts as a radical neuroprotector by restoring glutathione biosynthesis in the zebrafish brain subsequent to CUS exposure. Additionally, the rising level of brain MAO following chronic exposure to CUS is ameliorated by taurine treatment. These findings strongly advocate the role of taurine as a natural MAO inhibitor through the neuroprotection it provides against CUS-instigated oxidative stress in zebrafish. However, the fundamental neuroprotective mechanism of such natural compounds needs to be elucidated to determine their neuroprotective efficacy against stress regimens.

Abnormal brain development of monoamine oxidase mutant zebrafish and impaired social interaction of heterozygous fish

Monoamine oxidase (MAO) deficiency and imbalanced levels of brain monoamines have been associated with developmental delay, neuropsychiatric disorders and aggressive behavior. Animal models are valuable tools to gain mechanistic insight into outcomes associated with MAO deficiency. Here, we report a novel genetic model to study the effects of mao loss of function in zebrafish. Quantitative PCR, in situ hybridization and immunocytochemistry were used to study neurotransmitter systems and expression of relevant genes for brain development in zebrafish mao mutants. Larval and adult fish behavior was evaluated through different tests. Stronger serotonin immunoreactivity was detected in mao^+/− and mao^−/− larvae compared with their mao^+/+ siblings. mao^−/− larvae were hypoactive, and presented decreased reactions to visual and acoustic stimuli. They also had impaired histaminergic and dopaminergic systems, abnormal expression of developmental markers and died within 20 days post-fertilization. mao^+/− fish were viable, grew until adulthood, and demonstrated anxiety-like behavior and impaired social interactions compared with adult mao^+/+ siblings. Our results indicate that mao^−/− and mao^+/− mutants could be promising tools to study the roles of MAO in brain development and behavior.

This article has an associated First Person interview with the first author of the paper.

Summary: We assessed developmental, neurochemical and behavioral alterations displayed by mao^+/− and mao^−/− zebrafish, establishing that these model organisms are promising tools to study the consequences of MAOA/B deficiency.

Influence of Incorporation of Different dn-Electron Metal Cations into Biologically Active System on Its Biological and Physicochemical Properties

Three new compounds, namely [HL]₂⁺[CuCl₄]^2-, [HL]₂⁺[ZnCl₄]^2-, and [HL]₂⁺[CdCl₄]^2- (where L: imipramine) were synthesized and their physicochemical and biological properties were thoroughly investigated. All three compounds form isostructural, crystalline systems, which have been studied using Single-Crystal X-ray diffraction analysis (SC-XRD) and Fourier-transform infrared spectroscopy (FTIR). The thermal stability was investigated using thermogravimetric analysis (TGA) and melting points for all compounds have been determined. Magnetic measurements were performed in order to study the magnetic properties of the compounds. The above mentioned techniques allowed us to comprehensively examine the physicochemical properties of the newly obtained compounds. The biological activity was investigated using the number of Zebrafish tests, as it is one of the most common models for studying the impact of newly synthesized compounds on the central nervous system (CNS), since this model is very similar to the human CNS.

Deletion of lrrk2 causes early developmental abnormalities and age-dependent increase of monoamine catabolism in the zebrafish brain

LRRK2 gain-of-function is considered a major cause of Parkinson's disease (PD) in humans. However, pathogenicity of LRRK2 loss-of-function in animal models is controversial. Here we show that deletion of the entire zebrafish lrrk2 locus elicits a pleomorphic transient brain phenotype in maternal-zygotic mutant embryos (mzLrrk2). In contrast to lrrk2, the paralog gene lrrk1 is virtually not expressed in the brain of both wild-type and mzLrrk2 fish at different developmental stages. Notably, we found reduced catecholaminergic neurons, the main target of PD, in specific cell populations in the brains of mzLrrk2 larvae, but not adult fish. Strikingly, age-dependent accumulation of monoamine oxidase (MAO)-dependent catabolic signatures within mzLrrk2 brains revealed a previously undescribed interaction between LRRK2 and MAO biological activities. Our results highlight mzLrrk2 zebrafish as a tractable tool to study LRRK2 loss-of-function in vivo, and suggest a link between LRRK2 and MAO, potentially of relevance in the prodromic stages of PD.

Taurine modulates behavioral effects of intermittent ethanol exposure without changing brain monoamine oxidase activity in zebrafish: Attenuation of shoal- and anxiety-like responses, and abolishment of memory acquisition deficit

Prolonged alcohol consumption has been considered as an important risk factor for various diseases. Chronic ethanol (EtOH) intake is associated with deleterious effects on brain functions culminating in robust behavioral changes. Notably, drugs available to treat the effects of EtOH have low therapeutic efficacy so far. Taurine (TAU) appears as a promising neuroprotective molecule due to its pleiotropic action in the brain. Here, we investigated whether TAU plays a beneficial role in different behavioral domains of zebrafish submitted to an intermittent EtOH exposure model, specially focusing on social behavior, anxiety-like responses, and memory. Moreover, since monoamines play a role in EtOH-mediated responses, we also evaluated the influence of both TAU and EtOH exposures on brain monoamine oxidase (Z-MAO) activity. Fish were exposed to non-chlorinated water or 1% EtOH for 8 consecutive days (20 min per day). From the 5th day until the end of the experimental period (8th day), animals were kept in the absence or presence of TAU (42, 150, or 400 mg/L) 1 h per day immediately after EtOH exposure. Behavioral measurements started 24 h after the last EtOH exposure. We observed that TAU showed modest attenuating effects on shoaling behavior and anxiety-like responses, while 42 and 150 mg/L TAU abolished the memory acquisition deficit in the inhibitory avoidance task. Biochemical analysis revealed that TAU did not modulate EtOH-induced increase on brain Z-MAO activity. Collectively, our novel data show a potential beneficial effect of TAU in an intermittent EtOH exposure model in zebrafish. Moreover, these findings foster the growing utility of this aquatic species to investigate the neurobehavioral basis of EtOH- and TAU-mediated responses in vertebrates.

Androgenic activation, impairment of the monoaminergic system and altered behavior in zebrafish larvae exposed to environmental concentrations of fenitrothion

Fenitrothion is an organophosphorus insecticide usually found in aquatic ecosystems at concentrations in the range of low ng/L. In this manuscript we show that 24 h exposure to environmental concentrations of fenitrothion, from ng/L to low μg/L, altered basal locomotor activity, visual-motor response and acoustic/vibrational escape response of zebrafish larvae. Furthermore, fenitrothion and expression of gap43a, gfap, atp2b1a, and mbp exhibited a significant non-monotonic concentration-response relationship. Once determined that environmental concentrations of fenitrothion were neurotoxic for zebrafish larvae, a computational analysis identified potential protein targets of this compound. Some of the predictions, including interactions with acetylcholinesterase, monoamine-oxidases and androgen receptor (AR), were experimentally validated. Binding to AR was the most suitable candidate for molecular initiating event, as indicated by both the up-regulation of cyp19a1b and sult2st3 and the non-monotonic relationship found between fenitrothion and the observed responses. Finally, when the integrity of the monoaminergic system was evaluated, altered levels of L-DOPA, DOPAC, HVA and 5-HIAA were found, as well as a significant up-regulation of slc18a2 expression at the lowest concentrations of fenitrothion. These data strongly suggest that concentrations of fenitrothion commonly found in aquatic ecosystems present a significant environmental risk for fish communities.

Pharmacological Modulation of Serotonin Levels in Zebrafish Larvae: Lessons for Identifying Environmental Neurotoxicants Targeting the Serotonergic System

This study examines the effects of acute pharmacological modulation of the serotonergic system over zebrafish larvae’s cognitive, basic, and defense locomotor behaviors, using a medium to high throughput screening assay. Furthermore, the relationship between behavior, enzyme activity related to neurotransmitter metabolism, neurotransmitter levels, and gene expression was also determined. Modulation of larvae serotonergic system was accomplished by 24 h exposure to single and opposite pharmacodynamics co-exposure to three model psychopharmaceuticals with antagonistic and agonistic serotonin signaling properties: 2.5 mM 4-Chloro-DL-phenylalanine (PCPA) and 5 µM deprenyl and 0.5 µM fluoxetine, respectively. Similar behavioral outcome was observed for deprenyl and fluoxetine, which was reflected as hypolocomotion, decrease in larvae defensive responses, and cognitive impairment. Contrarily, PCPA induced hyperlocomotion and increase in larvae escape response. Deprenyl exposure effects were more pronounced at a lower level of organization than fluoxetine, with complete inhibition of monoamine oxidase (MAO) activity, dramatic increase of 5-HT and dopamine (DA) levels, and downregulation of serotonin synthesis and transporter genes. PCPA showed mainly effects over serotonin and dopamine’s main degradation metabolites. Finally, co-exposure between agonistic and antagonist serotonin signaling drugs reviled full recovery of zebrafish impaired locomotor and defense responses, 5-HT synthesis gene expression, and partial recovery of 5-HT levels. The findings of this study suggest that zebrafish larvae can be highly sensitive and a useful vertebrate model for short-term exposure to serotonin signaling changes.

Differential Modulation of the Central and Peripheral Monoaminergic Neurochemicals by Deprenyl in Zebrafish Larvae

Zebrafish embryos and larvae are vertebrate models increasingly used in translational neuroscience research. Behavioral impairment induced by the exposure to neuroactive or neurotoxic compounds is commonly linked to changes in modulatory neurotransmitters in the brain. Although different analytical methods for determining monoaminergic neurochemicals in zebrafish larvae have been developed, these methods have been used only on whole larvae, as the dissection of the brain of hundreds of larvae is not feasible. This raises a key question: Are the changes in the monoaminergic profile of the whole larvae predictive of the changes in the brain? In this study, the levels of ten monoaminergic neurotransmitters were determined in the head, trunk, and the whole body of zebrafish larvae in a control group and in those treated for 24 h with 5 M deprenyl, a prototypic monoamine-oxidase B inhibitor, eight days post-fertilization. In control larvae, most of the monoaminergic neurochemicals were found at higher levels in the head than in the trunk. Significant changes were found in the distribution of some neurochemicals after deprenyl-treatment, with serotonin and norepinephrine increasing in both the head and the trunk, whereas dopamine, L-DOPA, and homovanillic acid levels were only modulated in the head. In fact, the highly significant increase in dopamine levels observed in the head after deprenyl-treatment was not detected in the whole-body analysis. These results indicate that the analysis of neurotransmitters in the zebrafish larvae whole-body should not be used as a general surrogate of the brain.

Zygotic Venlafaxine Exposure Impacts Behavioral Programming by Disrupting Brain Serotonin in Zebrafish

The antidepressant venlafaxine, a selective serotonin and norepinephrine reuptake inhibitor, is present in surface waters downstream of wastewater treatment plants. We previously showed that zygotic venlafaxine deposition alters larval behavior in zebrafish (Danio rerio), but the mechanisms were unknown. Here we tested the hypothesis that venlafaxine disrupts central serotonergic development, leading to impaired behavioral responses in zebrafish larvae. This was tested by microinjecting embryos with venlafaxine immediately after fertilization and performing spatial distribution of serotonin immunoreactivity, as well as characterizing target genes involved in serotonin turnover in the zebrafish brain. We provide evidence that venlafaxine exposure reduces serotonin immunoreactivity and tyrosine hydroxylase-positive cell populations in specific larval brain regions, and this corresponded with reduced larval activity observed in the drug-exposed group. Lowered serotonin was not due to either reduced synthesis or increased breakdown capacity. However, co-injection of serotonin alongside venlafaxine in embryos recovered brain serotonin immunoreactivity, tyrosine hydroxylase-positive cell populations, and rescued venlafaxine-mediated behavioral changes. Overall, our results demonstrate for the first time that early life exposure to venlafaxine perturbs brain development, which may be due to reduced serotonin, leading to altered larval behavior in zebrafish.

The role of monoamine oxidase A in the neurobiology of aggressive, antisocial, and violent behavior: A tale of mice and men

Over the past two decades, research has revealed that genetic factors shape the propensity for aggressive, antisocial, and violent behavior. The best-documented gene implicated in aggression is MAOA (Monoamine oxidase A), which encodes the key enzyme for the degradation of serotonin and catecholamines. Congenital MAOA deficiency, as well as low-activity MAOA variants, has been associated with a higher risk for antisocial behavior (ASB) and violence, particularly in males with a history of child maltreatment. Indeed, the interplay between low MAOA genetic variants and early-life adversity is the best-documented gene × environment (G × E) interaction in the pathophysiology of aggression and ASB. Additional evidence indicates that low MAOA activity in the brain is strongly associated with a higher propensity for aggression; furthermore, MAOA inhibition may be one of the primary mechanisms whereby prenatal smoke exposure increases the risk of ASB. Complementary to these lines of evidence, mouse models of Maoa deficiency and G × E interactions exhibit striking similarities with clinical phenotypes, proving to be valuable tools to investigate the neurobiological mechanisms underlying antisocial and aggressive behavior. Here, we provide a comprehensive overview of the current state of the knowledge on the involvement of MAOA in aggression, as defined by preclinical and clinical evidence. In particular, we show how the convergence of human and animal research is proving helpful to our understanding of how MAOA influences antisocial and violent behavior and how it may assist in the development of preventative and therapeutic strategies for aggressive manifestations.

Zebrafish models of impulsivity and impulse control disorders

Impulse control disorders (ICDs) are characterized by generalized difficulty controlling emotions and behaviors. ICDs are a broad group of the central nervous system (CNS) disorders including conduct disorder, intermittent explosive, oppositional-defiant disorder, antisocial personality disorder, kleptomania, pyromania and other illnesses. Although they all share a common feature (aberrant impulsivity), their pathobiology is complex and poorly understood. There are also currently no ICD-specific therapies to treat these illnesses. Animal models are a valuable tool for studying ICD pathobiology and potential therapies. The zebrafish (Danio rerio) has become a useful model organism to study CNS disorders due to high genetic and physiological homology to mammals, and sensitivity to various pharmacological and genetic manipulations. Here, we summarize experimental models of impulsivity and ICD in zebrafish and highlight their growing translational significance. We also emphasize the need for further development of zebrafish ICD models to improve our understanding of their pathogenesis and to search for novel therapeutic treatments.

Nitric oxide interacts with monoamine oxidase to modulate aggression and anxiety-like behaviour

Nitric oxide (NO) is a gaseous neurotransmitter that has important behavioural functions in the vertebrate brain. In this study we compare the impact of decreased nitric NO signalling upon behaviour and neurobiology using both zebrafish and mouse. nitric oxide synthase mutant (nos1^-/-) zebrafish show significantly reduced aggression and an increase in anxiety-like behaviour without altered production of the stress hormone cortisol. Nos1^-/- mice also exhibit decreased aggression and are hyperactive in an open field test. Upon reduction of NO signalling, monoamine neurotransmitter metabolism is reduced as a consequence of decreased Monoamine oxidase activity. Treatment of nos1^-/- zebrafish with the 5-HT receptor 1A agonist 8-OH-DPAT rescues aggression and some aspects of anxiety-like behaviour. Taken together, the interplay between NO and 5-HT appears to be critical to control behaviour. Our cross-species approach challenges the previous notion that reduced neuronal NOS leads to increased aggression. Rather, Nos1 knock-out can also lead to decreased aggression in some situations, a finding that may have implications for future translational research.

Fgf3 is crucial for the generation of monoaminergic cerebrospinal fluid contacting cells in zebrafish

In most vertebrates, including zebrafish, the hypothalamic serotonergic cerebrospinal fluid-contacting (CSF-c) cells constitute a prominent population. In contrast to the hindbrain serotonergic neurons, little is known about the development and function of these cells. Here, we identify fibroblast growth factor (Fgf)3 as the main Fgf ligand controlling the ontogeny of serotonergic CSF-c cells. We show that fgf3 positively regulates the number of serotonergic CSF-c cells, as well as a subset of dopaminergic and neuroendocrine cells in the posterior hypothalamus via control of proliferation and cell survival. Further, expression of the ETS-domain transcription factor etv5b is downregulated after fgf3 impairment. Previous findings identified etv5b as critical for the proliferation of serotonergic progenitors in the hypothalamus, and therefore we now suggest that Fgf3 acts via etv5b during early development to ultimately control the number of mature serotonergic CSF-c cells. Moreover, our analysis of the developing hypothalamic transcriptome shows that the expression of fgf3 is upregulated upon fgf3 loss-of-function, suggesting activation of a self-compensatory mechanism. Together, these results highlight Fgf3 in a novel context as part of a signalling pathway of critical importance for hypothalamic development.

Summary: This study highlights Fgf3 in a novel context where it is part of a signalling pathway of critical importance for development of hypothalamic monoaminergic cells in zebrafish.

Social Defeat Stress Decreases mRNA for Monoamine Oxidase A and Increases 5-HT Turnover in the Brain of Male Nile Tilapia (Oreochromis niloticus)

Stress induces various neurobiological responses and causes psychiatric disorders, including depression. Monoamine oxidase A (MAO-A) plays an important role in various functions of the brain, such as regulation of mood, anxiety and aggression, and dysregulation of MAO-A is observed in stress-related psychiatric disorders. This study addressed the question whether acute social stress induces changes to transcriptional and/or post-transcriptional regulation of MAO-A expression in the brain. Using male Nile tilapia (Oreochromis niloticus), we investigated whether acute social stress, induced by the presence of a dominant male fish, changes the expression of MAO-A. We measured gene expression of MAO-A by quantitative PCR, enzymatic activity of MAO-A by the luminescent method, and 5-HT and 5-HIAA levels by liquid chromatography–mass spectrometry in the brain of socially stressed and control fish. Socially stressed males showed decreased MAO-A mRNA levels, consistent MAO-A enzymatic activity, increased 5-HT turnover in the brain, and elevated plasma cortisol levels, compared to controls. Our results suggest that acute social stress suppresses the transcription of MAO-A gene, enhances 5-HT metabolism but does not affect the production of MAO-A protein.

Maternal Exposure to Dietary Selenium Causes Dopaminergic Hyperfunction and Cognitive Impairment in Zebrafish Offspring

Maternal exposure to environmental contaminants is a predisposing factor for neurodevelopmental disorders with associated cognitive and social deficits in offspring. In this study, we investigated the effects of maternal exposure to selenium (Se), a contaminant of potential environmental concern in aquatic ecosystems, on cognitive performance and the underlying mechanisms in F1-generation adult zebrafish. Adult female zebrafish were exposed to different concentrations of dietary Se (3.5, 11.1, or 27.4 μg Se/g dry weight) for a period of 60 days. Fish were subsequently bred, and their offspring were collected and raised for 6 months on a normal diet. Maternal exposure to all concentrations of dietary Se induced learning impairment in F1-zebrafish tested in a latent learning task. The results also showed a hyperfunctioning dopaminergic system in fish exhibiting the learning deficit. The hyperfunction of the dopaminergic system was associated with enhanced oxidative stress and alterations in the mRNA abundance of several immediate early and late response genes in the zebrafish brain. Taken together, these results suggest that maternal exposure to dietary Se via alterations in the dopaminergic system leads to persistent neurobehavioral deficits in F1-generation adult zebrafish.

Zebrafish: an emerging real-time model system to study Alzheimer's disease and neurospecific drug discovery

Zebrafish (Danio rerio) is emerging as an increasingly successful model for translational research on human neurological disorders. In this review, we appraise the high degree of neurological and behavioural resemblance of zebrafish with humans. It is highly validated as a powerful vertebrate model for investigating human neurodegenerative diseases. The neuroanatomic and neurochemical pathways of zebrafish brain exhibit a profound resemblance with the human brain. Physiological, emotional and social behavioural pattern similarities between them have also been well established. Interestingly, zebrafish models have been used successfully to simulate the pathology of Alzheimer’s disease (AD) as well as Tauopathy. Their relatively simple nervous system and the optical transparency of the embryos permit real-time neurological imaging. Here, we further elaborate on the use of recent real-time imaging techniques to obtain vital insights into the neurodegeneration that occurs in AD. Zebrafish is adeptly suitable for Ca²⁺ imaging, which provides a better understanding of neuronal activity and axonal dystrophy in a non-invasive manner. Three-dimensional imaging in zebrafish is a rapidly evolving technique, which allows the visualisation of the whole organism for an elaborate in vivo functional and neurophysiological analysis in disease condition. Suitability to high-throughput screening and similarity with humans makes zebrafish an excellent model for screening neurospecific compounds. Thus, the zebrafish model can be pivotal in bridging the gap from the bench to the bedside. This fish is becoming an increasingly successful model to understand AD with further scope for investigation in neurodevelopment and neurodegeneration, which promises exciting research opportunities in the future.

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

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

From aggression to autism: new perspectives on the behavioral sequelae of monoamine oxidase deficiency

The two monoamine oxidase (MAO) enzymes, A and B, catalyze the metabolism of monoamine neurotransmitters, such as serotonin, norepinephrine, and dopamine. The phenotypic outcomes of MAO congenital deficiency have been studied in humans and animal models, to explore the role of these enzymes in behavioral regulation. The clinical condition caused by MAOA deficiency, Brunner syndrome, was first described as a disorder characterized by overt antisocial and aggressive conduct. Building on this discovery, subsequent studies were focused on the characterization of the role of MAOA in the neurobiology of antisocial conduct. MAO A knockout mice were found to display high levels of intermale aggression; however, further analyses of these mutants unveiled additional behavioral abnormalities mimicking the core symptoms of autism-spectrum disorder. These findings were strikingly confirmed in newly reported cases of Brunner syndrome. The role of MAOB in behavioral regulation remains less well-understood, even though Maob-deficient mice have been found to exhibit greater behavioral disinhibition and risk-taking responses, supporting previous clinical studies showing associations between low MAO B activity and impulsivity. Furthermore, lack of MAOB was found to exacerbate the severity of psychopathological deficits induced by concurrent MAOA deficiency. Here, we summarize how the convergence of clinical reports and behavioral phenotyping in mutant mice has helped frame a complex picture of psychopathological features in MAO-deficient individuals, which encompass a broad spectrum of neurodevelopmental problems. This emerging knowledge poses novel conceptual challenges towards the identification of the endophenotypes shared by autism-spectrum disorder, antisocial behavior and impulse-control problems, as well as their monoaminergic underpinnings.

Effects of alprazolam treatment on anxiety-like behavior induced by color stimulation in adult zebrafish

It has been reported that the use of certain stimuli can lead to anxiety-like behavior in zebrafish. Moreover, visual stimulation of zebrafish is becoming a popular tool. Here we evaluated the effects of six colors combinations and alprazolam, a benzodiazepine which is widely used in the treatment of anxiety disorders, on the behavior of adult zebrafish in a two-chambered apparatus, which chambers were illuminated by red/yellow, green/blue, red/green, green/yellow, red/blue and blue/yellow light. The following parameters were measured: time spent in the zone, number of entries to the zone, time of freezing, distance traveled and average speed in the zone. We report that the adult zebrafish spent more time in the red zone compared to yellow or green as well as in the yellow or blue compared to green. The zebrafish displayed a concomitant increase in time freezing in the red zone compared to yellow or green as well as in the yellow or blue compared to green. Moreover, average speed was decreased in the red zone compared to yellow or green and in the yellow zone compared to green. Treatment with alprazolam significantly affected the behavior of the zebrafish, e.g., following alprazolam administration time spent in the zone and time freezing were longer in the green zone than in red. Based on these observations, we suggest that light color combinations could be effective to manipulate zebrafish behavior and could be useful in neuropsychopharmacological studies, perhaps to study anxiety-like behavior and the effects of anxiolytic agents.

Serotonin Coordinates Responses to Social Stress-What We Can Learn from Fish

Social interaction is stressful and subordinate individuals are often subjected to chronic stress, which greatly affects both their behavior and physiology. In teleost fish the social position of an individual may have long-term effects, such as effects on migration, age of sexual maturation or even sex. The brain serotonergic system plays a key role in coordinating autonomic, behavioral and neuroendocrine stress responses. Social subordination results in a chronic activation of the brain serotonergic system an effect, which seems to be central in the subordinate phenotype. However, behavioral effects of short-term acute activation of the serotonergic system are less obvious. As in other vertebrates, divergent stress coping styles, often referred to as proactive and reactive, has been described in teleosts. As demonstrated by selective breeding, stress coping styles appear to be partly heritable. However, teleost fish are characterized by plasticity, stress coping style being affected by social experience. Again, the brain serotonergic system appears to play an important role. Studies comparing brain gene expression of fish of different social rank and/or displaying divergent stress coping styles have identified several novel factors that seem important for controlling aggressive behavior and stress coping, e.g., histamine and hypocretin/orexin. These may also interact with brain monoaminergic systems, including serotonin.

Zebrafish: A promising in vivo model for assessing the delivery of natural products, fluorescence dyes and drugs across the blood-brain barrier

The blood brain barrier (BBB) is the network of capillaries that controls the passage of substances from the blood into the brain and other parts of the central nervous system (CNS). As this barrier is the major obstacle for drug delivery into CNS, a credible BBB model is very necessary to assess the BBB permeability of novel neuroactive compounds including thousands of bioactive compounds which have been extracted from medicinal plants and have the potential for the treatment of CNS diseases. Increasing reports indicated that zebrafish has emerged as a timely, reproducible model for BBB permeability assessment. In this review, the development and functions of the BBB in zebrafish, such as its anatomical morphology, tight junctions, drug transporters and enzyme expression, are compared with those in mammals. The studies outlined in this review describe the utilization of the zebrafish as a BBB model to investigate the permeability and distribution of fluorescent dyes and drugs. Particularly, this review focuses on the use of zebrafish to evaluate the delivery of natural products and nanosized drug delivery systems across the BBB. Due to the highly conserved nature of both the structure and function of the BBB between zebrafish and mammals, zebrafish has the potential to be developed as a model for assessing and predicting the permeability of BBB to novel compounds.

Distribution, properties, and inhibitor sensitivity of zebrafish catechol-O-methyl transferases (COMT)

Catechol-O-methyltransferase (COMT; EC 2.1.1.6) is an enzyme with multiple functions in vertebrates. COMT methylates and thus inactivates catecholamine neurotransmitters and metabolizes xenobiotic catechols. Gene polymorphism rs4680 that influences the enzymatic activity of COMT affects cognition and behavior in humans. The zebrafish is widely used as an experimental animal in many areas of biomedical research, but most aspects of COMT function in this species have remained uncharacterized. We hypothesized that both comt genes play essential roles in zebrafish. Both comt-a and comt-b were widely expressed in zebrafish tissues, but their relative abundance varied considerably. Homogenates of zebrafish organs, including the brain, showed enzymatic COMT activity that was the highest in the liver and kidney. Treatment of larval zebrafish with the COMT inhibitor Ro41-0960 shifted the balance of catecholamine metabolic pathways towards increased oxidative metabolism. Whole-body concentrations of dioxyphenylacetic acid (DOPAC), a product of dopamine oxidation, were increased in the inhibitor-treated larvae, although the dopamine levels were unchanged. Thus, COMT is likely to participate in the processing of catecholamine neurotransmitters in the zebrafish, but the inhibition of COMT in larval fish is compensated efficiently and does not have pronounced effects on dopamine levels.

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

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

Selective toxicity of L-DOPA to dopamine transporter-expressing neurons and locomotor behavior in zebrafish larvae

Dopamine signaling is conserved across all animal species and has been implicated in the disease process of many neurological disorders, including Parkinson's disease (PD). The primary neuropathology in PD involves the death of dopaminergic cells in the substantia nigra (SN), an anatomical region of the brain implicated in dopamine production and voluntary motor control. Increasing evidence suggests that the neurotransmitter dopamine may have a neurotoxic metabolic product (DOPAL) that selectively damages dopaminergic cells. This study was designed to test this theory of oxidative damage in an animal model of Parkinson's disease, using a transgenic strain of zebrafish with fluorescent labeling of cells that express the dopamine transporter. The pretectum and ventral diencephalon exhibited reductions in cell numbers due to L-DOPA treatment while reticulospinal neurons that do not express the DAT were unaffected, and this was partially rescued by monoamine oxidase inhibition. Consistent with the MPTP model of PD in zebrafish larvae, spontaneous locomotor behavior in L-DOPA treated animals was depressed following a 24-h recovery period, while visually-evoked startle response rates and latencies were unaffected.

Zebrafish models of major depressive disorders

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

Interactions between serotonin and glutamate-nitric oxide pathways in zebrafish scototaxis

NMDA receptors have been implicated in the acute response to stress, possibly mediated the nitric oxide pathway; serotonin has also been implicated in these responses, and has recently been shown to modulate the nitric oxide pathway via 5-HT1 and 5-HT2 receptors. In this work, we compare the effects of NMDA and a 5-HT1A receptor ligands on light/dark preference in adult zebrafish, and investigate whether nitric oxide mediates the effects of such drugs. The noncompetitive NMDA receptor antagonist MK-801 decreased dark preference (scototaxis), while NMDA increased it; the effects of NMDA were completely blocked by pretreatment with the nitric oxide synthase (NOS) antagonist L-NAME. SNP, a nitric oxide donor, produced a bell-shaped dose-response profile on scototaxis. Treatment with 5-HTP increased scototaxis, an effect which was potentiated by pre-treatment with NMDA, but not MK-801, and partially blocked by L-NAME. The 5-HT1A receptor antagonist WAY 100,635 decreased scototaxis, an effect which was completely blocked by L-NAME. These results suggest that tonic NOS inhibition is an important downstream effector of 5-HT1A receptors in the regulation of dark preference behavior in zebrafish, and that NOS is also under phasic independent control by NMDA receptors.

Serotonergic modulation of zebrafish behavior: towards a paradox

Due to the fish-specific genome duplication event (~320-350 mya), some genes which code for serotonin proteins were duplicated in teleosts; this duplication event was preceded by a reorganization of the serotonergic system, with the appearance of the raphe nuclei (dependent on the isthmus organizer) and prosencephalic nuclei, including the paraventricular and pretectal complexes. With the appearance of amniotes, duplicated genes were lost, and the serotonergic system was reduced to a more complex raphe system. From a comparative point of view, then, the serotonergic system of zebrafish and that of mammals shows many important differences. However, many different behavioral functions of serotonin, as well as the effects of drugs which affect the serotonergic system, seem to be conserved among species. For example, in both zebrafish and rodents acute serotonin reuptake inhibitors (SSRIs) seem to increase anxiety-like behavior, while chronic SSRIs decrease it; drugs which act at the 5-HT1A receptor seem to decrease anxiety-like behavior in both zebrafish and rodents. In this article, we will expose this paradox, reviewing the chemical neuroanatomy of the zebrafish serotonergic system, followed by an analysis of the role of serotonin in zebrafish fear/anxiety, stress, aggression and the effects of psychedelic drugs.

Neurotransmitter map of the asymmetric dorsal habenular nuclei of zebrafish

The role of the habenular nuclei in modulating fear and reward pathways has sparked a renewed interest in this conserved forebrain region. The bilaterally paired habenular nuclei, each consisting of a medial/dorsal and lateral/ventral nucleus, can be further divided into discrete subdomains whose neuronal populations, precise connectivity, and specific functions are not well understood. An added complexity is that the left and right habenulae show pronounced morphological differences in many non-mammalian species. Notably, the dorsal habenulae of larval zebrafish provide a vertebrate genetic model to probe the development and functional significance of brain asymmetry. Previous reports have described a number of genes that are expressed in the zebrafish habenulae, either in bilaterally symmetric patterns or more extensively on one side of the brain than the other. The goal of our study was to generate a comprehensive map of the zebrafish dorsal habenular nuclei, by delineating the relationship between gene expression domains, comparing the extent of left-right asymmetry at larval and adult stages, and identifying potentially functional subnuclear regions as defined by neurotransmitter phenotype. Although many aspects of habenular organization appear conserved with rodents, the zebrafish habenulae also possess unique properties that may underlie lateralization of their functions.

Zebrafish as an emerging model for studying complex brain disorders

The zebrafish (Danio rerio) is rapidly becoming a popular model organism in pharmacogenetics and neuropharmacology. Both larval and adult zebrafish are currently used to increase our understanding of brain function, dysfunction, and their genetic and pharmacological modulation. Here we review the developing utility of zebrafish in the analysis of complex brain disorders (including, e.g., depression, autism, psychoses, drug abuse, and cognitive deficits), also covering zebrafish applications towards the goal of modeling major human neuropsychiatric and drug-induced syndromes. We argue that zebrafish models of complex brain disorders and drug-induced conditions are a rapidly emerging critical field in translational neuroscience and pharmacology research.

Chronic and acute alcohol administration induced neurochemical changes in the brain: comparison of distinct zebrafish populations

The zebrafish is increasingly utilized in the analysis of the effects of ethanol (alcohol) on brain function and behavior. We have shown significant population-dependent alcohol-induced changes in zebrafish behavior and have started to analyze alterations in dopaminergic and serotoninergic responses. Here, we analyze the effects of alcohol on levels of selected neurochemicals using a 2 × 3 (chronic × acute) between-subject alcohol exposure paradigm randomized for two zebrafish populations, AB and SF. Each fish first received the particular chronic treatment (0 or 0.5 vol/vol% alcohol) and subsequently the acute exposure (0, 0.5 or 1.0% alcohol). We report changes in levels of dopamine, DOPAC, serotonin, 5HIAA, glutamate, GABA, aspartate, glycine and taurine as quantified from whole brain extracts using HPLC. We also analyze monoamine oxidase and tyrosine hydroxylase enzymatic activity. The results demonstrate that compared to SF, AB is more responsive to both acute alcohol exposure and acute alcohol withdrawal at the level of neurochemistry, a finding that correlates well with prior behavioral observations and one which suggests the involvement of genes in the observed alcohol effects. We discuss correlations between the current results and prior behavioral findings, and stress the importance of characterization of zebrafish strains for future behavior genetic and psychopharmacology studies.

Probing the diversity of serotonin neurons

The serotonin (5-HT) system is generally considered as a single modulatory system, with broad and diffuse projections. However, accumulating evidence points to the existence of distinct cell groups in the raphe. Here, we review prior evidence for raphe cell heterogeneity, considering different properties of 5-HT neurons, from metabolism to anatomy, and neurochemistry to physiology. We then summarize more recent data in mice and zebrafish that support a genetic diversity of 5-HT neurons, based on differential transcription factor requirements for the acquisition of the 5-HT identity. In both species, PET1 plays a major role in the acquisition and maintenance of 5-HT identity in the hindbrain, although some 5-HT neurons do not require PET1 for their differentiation, indicating the existence of several transcriptional routes to become serotoninergic. In mice, both PET1-dependent and -independent 5-HT neurons are located in the raphe, but have distinct anatomical features, such as the morphology of axon terminals and projection patterns. In zebrafish, all raphe neurons express pet1, but Pet1-independent 5-HT cell groups are present in the forebrain. Overall, these observations support the view that there are a number of distinct 5-HT subsystems, including within the raphe nuclei, with unique genetic programming and functions.

Evolutionary shift from fighting to foraging in blind cavefish through changes in the serotonin network

BACKGROUND

Within the species Astyanax mexicanus, there are several interfertile populations of river-dwelling sighted fish and cave-dwelling blind fish which have evolved morphological and behavioral adaptations, the origins of which are unknown. Here, we have investigated the neural, genetic, and developmental bases for the evolution of aggressive behavior in this teleost.

RESULTS

We used an intruder-resident behavioral assay to compare aggressiveness quantitatively (attack counts) and qualitatively (pattern and nature of attacks) between the surface and cave populations of Astyanax. Using this paradigm, we characterize aggressive behavior in surface fish, bring support for the genetic component of this trait, and show that it is controlled by raphe serotonergic neurons and that it corresponds to the establishment of dominance between fish. Cavefish have completely lost such aggressive/dominance behavior. The few attacks performed by cavefish during the behavioral test instead correspond to food-seeking behavior, driven by the developmental evolution of their hypothalamic serotonergic paraventricular neurons, itself due to increased Sonic Hedgehog signaling during early forebrain embryogenesis.

CONCLUSIONS

We propose that during evolution and adaptation to their cave habitat, cavefish have undergone a behavioral shift, due to modifications of their serotonergic neuronal network. They have lost the typical aggressive behavior of surface fish and evolved a food-seeking behavior that is probably more advantageous to surviving in the dark. We have therefore demonstrated a link between the development of a neuronal network and the likely adaptive behaviors it controls.

Dopamine D2 receptor activity modulates Akt signaling and alters GABAergic neuron development and motor behavior in zebrafish larvae

An imbalance in dopamine-mediated neurotransmission is a hallmark physiological feature of neuropsychiatric disorders, such as schizophrenia. Recent evidence demonstrates that dopamine D(2) receptors, which are the main target of antipsychotics, modulate the activity of the protein kinase Akt, which is known to be downregulated in the brain of patients with schizophrenia. Akt has an important role in the regulation of cellular processes that are critical for neurodevelopment, including gene transcription, cell proliferation, and neuronal migration. Thus, it is possible that during brain development, altered Akt-dependent dopamine signaling itself may lead to defects in neural circuit formation. Here, we used a zebrafish model to assess the direct impact of altered dopamine signaling on brain development and larval motor behavior. We demonstrate that D(2) receptor activation acutely suppresses Akt activity by decreasing the level of pAkt(Thr308) in the larval zebrafish brain. This D(2)-dependent reduction in Akt activity negatively regulates larval movement and is distinct from a D(1)-dependent pathway with opposing affects on motor behavior. In addition, we show that D(2)-dependent suppression of Akt activity causes a late onset change in GSK3b activity, a known downstream target of Akt signaling. Finally, altered D(2) receptor signaling, or direct inhibition of Akt activity, causes a significant decrease in the size of the GABAergic neuron population throughout most of the brain. Our observations suggest that D(2) receptor signaling suppresses Akt-GSK3b activity, which regulates GABAergic neuron development and motor behavior.

The serotonergic system in fish

Neurons using serotonin (5-HT) as neurotransmitter and/or modulator have been identified in the central nervous system in representatives from all vertebrate clades, including jawless, cartilaginous and ray-finned fishes. The aim of this review is to summarize our current knowledge about the anatomical organization of the central serotonergic system in fishes. Furthermore, selected key functions of 5-HT will be described. The main focus will be the adult brain of teleosts, in particular zebrafish, which is increasingly used as a model organism. It is used to answer not only genetic and developmental biology questions, but also issues concerning physiology, behavior and the underlying neuronal networks. The many evolutionary conserved features of zebrafish combined with the ever increasing number of genetic tools and its practical advantages promise great possibilities to increase our understanding of the serotonergic system. Further, comparative studies including several vertebrate species will provide us with interesting insights into the evolution of this important neurotransmitter system.

Differential expression of dopaminergic cell markers in the adult zebrafish forebrain

Although the simultaneous presence of tyrosine hydroxylase (TH), aromatic amino acid decarboxylase (AADC), dopamine transporter (DAT), and vesicular monoamine transporter 2 (VMAT2) is considered as a phenotypic signature of dopamine (DA) neurons, it has been suggested that they are not uniformly expressed in all dopaminergic brain nuclei. Moreover, in nonmammalian vertebrates, two tyrosine hydroxylase genes (TH1 and TH2) are found, and they exhibit different expression patterns in zebrafish brains. Here we present a detailed description of the distribution of TH1, TH2, AADC, DAT, and VMAT2 transcripts, in relation to TH and DA immunoreactivity to better characterize dopaminergic nuclei in the adult zebrafish forebrain. TH2-positive cells in the hypothalamus are strongly DA immunoreactive (DAir), providing direct evidence that they are dopaminergic. DAir cells are also found in most TH1-positive or TH-immunoreactive (THir) nuclei. However, the DAir signal was weaker than THir in the olfactory bulb, telencephalon, ventral thalamus, pretectum, and some posterior tubercular and preoptic nuclei. These cell populations also exhibited low levels of VMAT2 transcripts, suggesting that low DA is due to a lower vesicular DA accumulation. In contrast, cell populations with low levels of AADC did not always have low levels of DA. DAT transcripts were abundantly expressed in most of the TH1- or TH2-positive territories. In addition, DAT and/or VMAT2 transcripts were found in some periventricular cell populations such as in the telencephalon without TH1 or TH2 expression. Thus, expression patterns of dopaminergic cell markers are not homogeneous, suggesting that the gene regulatory logic determining the dopaminergic phenotype is unexpectedly complex.

Pharmacological analysis of zebrafish (Danio rerio) scototaxis

The scototaxis test has been introduced recently to assess anxiety-like phenotypes in fish, including zebrafish. Parametric analyses suggest that scototaxis represents an approach-avoidance conflict, which hints at anxiety. In this model, white avoidance represents anxiety-like behavior, while the number of shuttling events represents activity. Acute or chronic fluoxetine, buspirone, benzodiazepines, ethanol, caffeine and dizocilpine were assessed using the light-dark box (scototaxis) test in zebrafish. Acute fluoxetine treatment did not alter white avoidance, but altered locomotion in the higher dose; chronic treatment (2 weeks), on the other hand, produced an anxiolytic effect with no locomotor outcomes. The benzodiazepines produced a hormetic (inverted U-shaped) dose-response profile, with intermediate doses producing anxiolysis and no effect at higher doses; clonazepam, a high-potency benzodiazepine agonist, produced a locomotor impairment at the highest dose. Buspirone produced an anxiolytic profile, without locomotor impairments. Moclobemide did not produce behavioral effects. Ethanol also produced a hormetic profile in white avoidance, with locomotor activation in 0.5% concentration. Caffeine produced an anxiogenic profile, without locomotor effects. These results suggest that the light-dark box is sensitive to anxiolytic and anxiogenic drugs in zebrafish.

The evolution of dopamine systems in chordates

Dopamine (DA) neurotransmission in the central nervous system (CNS) is found throughout chordates, and its emergence predates the divergence of chordates. Many of the molecular components of DA systems, such as biosynthetic enzymes, transporters, and receptors, are shared with those of other monoamine systems, suggesting the common origin of these systems. In the mammalian CNS, the DA neurotransmitter systems are diversified and serve for visual and olfactory perception, sensory–motor programming, motivation, memory, emotion, and endocrine regulations. Some of the functions are conserved among different vertebrate groups, while others are not, and this is reflected in the anatomical aspects of DA systems in the forebrain and midbrain. Recent findings concerning a second tyrosine hydroxylase gene (TH2) revealed new populations of DA-synthesizing cells, as evidenced in the periventricular hypothalamic zones of teleost fish. It is likely that the ancestor of vertebrates possessed TH2 DA-synthesizing cells, and the TH2 gene has been lost secondarily in placental mammals. All the vertebrates possess DA cells in the olfactory bulb, retina, and in the diencephalon. Midbrain DA cells are abundant in amniotes while absent in some groups, e.g., teleosts. Studies of protochordate DA cells suggest that the diencephalic DA cells were present before the divergence of the chordate lineage. In contrast, the midbrain cell populations have probably emerged in the vertebrate lineage following the development of the midbrain–hindbrain boundary. The functional flexibility of the DA systems, and the evolvability provided by duplication of the corresponding genes permitted a large diversification of these systems. These features were instrumental in the adaptation of brain functions to the very variable way of life of vertebrates.

Catalytic and inhibitor binding properties of zebrafish monoamine oxidase (zMAO): comparisons with human MAO A and MAO B

A comparative investigation of substrate specificity and inhibitor binding properties of recombinant zebrafish (Danio rerio) monoamine oxidase (zMAO) with those of recombinant human monoamine oxidases A and B (hMAO A and hMAO B) is presented. zMAO oxidizes the neurotransmitter amines (serotonin, dopamine and tyramine) with k(cat) values that exceed those of hMAO A or of hMAO B. The enzyme is competitively inhibited by hMAO A selective reversible inhibitors with the exception of d-amphetamine where uncompetitive inhibition is exhibited. The enzyme is unreactive with most MAO B-specific reversible inhibitors with the exception of chlorostyrylcaffeine. zMAO catalyzes the oxidation of para-substituted benzylamine analogs exhibiting (D)k(cat) and (D)(k(cat)/K(m)) values ranging from 2 to 8. Structure-activity correlations show a dependence of log k(cat) with the electronic factor σ(p) with a ρ value of +1.55±0.34; a value close to that for hMAO A but not with MAO B. zMAO differs from hMAO A or hMAO B in benzylamine analog binding correlations where an electronic effect (ρ=+1.29±0.31) is observed. These data demonstrate zMAO exhibits functional properties similar to hMAO A as well as exhibits its own unique behavior. These results should be useful for studies of MAO function in zebrafish models of human disease states.

Behavioral outcomes of monoamine oxidase deficiency: preclinical and clinical evidence

Monoamine oxidase (MAO) isoenzymes A and B are mitochondrial-bound proteins, catalyzing the oxidative deamination of monoamine neurotransmitters as well as xenobiotic amines. Although they derive from a common ancestral progenitor gene, are located at X-chromosome and display 70% structural identity, their substrate preference, regional distribution, and physiological role are divergent. In fact, while MAO-A has high affinity for serotonin and norepinephrine, MAO-B primarily serves the catabolism of 2-phenylethylamine (PEA) and contributes to the degradation of other trace amines and dopamine. Convergent lines of preclinical and clinical evidence indicate that variations in MAO enzymatic activity--due to either genetic or environmental factors--can exert a profound influence on behavioral regulation and play a role in the pathophysiology of a large spectrum of mental and neurodegenerative disorders, ranging from antisocial personality disorder to Parkinson's disease. Over the past few years, numerous advances have been made in our understanding of the phenotypical variations associated with genetic polymorphisms and mutations of the genes encoding for both isoenzymes. In particular, novel findings on the phenotypes of MAO-deficient mice are highlighting novel potential implications of both isoenzymes in a broad spectrum of mental disorders, ranging from autism and anxiety to impulse-control disorders and ADHD. These studies will lay the foundation for future research on the neurobiological and neurochemical bases of these pathological conditions, as well as the role of gene × environment interactions in the vulnerability to several mental disorders.