Hedgehog and Fgf signaling pathways regulate the development of tphR-expressing serotonergic raphe neurons in zebrafish embryos

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.

Effects of serta and sertb knockout on aggression in zebrafish (Danio rerio)

Zebrafish (Danio rerio) are unusual in having two paralogues of the serotonin re-uptake transporter (Sert), slc6a4a (serta) and slc6a4b (sertb), the transporter that serves in serotonin re-uptake from a synapse into the pre-synaptic cell or in serotonin uptake from the extracellular milieu into cells in the peripheral tissues. To address a knowledge gap concerning the specific roles of these paralogues, we used CRISPR/Cas9 technology to generate zebrafish knockout lines predicted to lack functional expression of Serta or Sertb. The consequences of loss-of-function of Serta or Sertb were assessed at the gene expression level, focusing on the serotonergic signalling pathway, and at the behaviour level, focusing on aggression. Whereas serta mRNA was expressed in all tissues examined, with high expression in the heart, gill and brain, only the brain displayed substantial sertb mRNA expression. In both serta-/- and sertb-/- fish, changes in transcript abundances of multiple components of the serotonin signalling pathway were detected, including proteins involved in serotonin synthesis (tph1a, tph1b, tph2, ddc), packaging (vmat2) and degradation (mao), and serotonin receptors (htr1aa, htr1ab). Using a mirror aggression test, serta-/- male but not female fish exhibited greater aggression than wildtype fish. However, both male and female sertb-/- fish displayed less aggression than their wildtype counterparts. These differences in behaviour between serta-/- and sertb-/- individuals hold promise for increasing our understanding of the neurophysiological basis of aggression in zebrafish.

Individual differences in the boldness of female zebrafish are associated with alterations in serotonin function

One of the most prevalent axes of behavioral variation in both humans and animals is risk taking, where individuals that are more willing to take risk are characterized as bold while those that are more reserved as shy. Brain monoamines (i.e., serotonin, dopamine, and norepinephrine) have been found to play a role in a variety of behaviors related to risk taking. Genetic variation related to monoamine function have also been linked to personality in both humans and animals. Using zebrafish, we investigated the relationship between monoamine function and boldness behavior during exploration of a novel tank. We found a sex-specific correlation between serotonin metabolism (5-HIAA:5-HT ratio) and boldness that was limited to female animals; there were no relationships between boldness and dopamine or norepinephrine. To probe differences in serotonergic function, we administered a serotonin reuptake inhibitor, escitalopram, to bold and shy fish, and assessed their exploratory behavior. We found that escitalopram had opposing effects on thigmotaxis in female animals with bold fish spending more time near the center of the tank and shy fish spent more time near the periphery. Taken together, our findings suggest that variation in serotonergic function makes sex-specific contributions to individual differences in risk taking behavior.

The zebrafish for preclinical psilocybin research

In this review, we discuss the possible utility of zebrafish in research on psilocybin, a psychedelic drug whose recreational use as well as possible clinical application are gaining increasing interest. First, we review behavioral tests with zebrafish, focussing on anxiety and social behavior, which have particular relevance in the context of psilocybin research. Next, we briefly consider methods of genetic manipulations with which psilocybin's phenotypical effects and underlying mechanisms may be investigated in zebrafish. We briefly review the known mechanisms of psilocybin, and also discuss what we know about its safety and toxicity profile. Last, we discuss examples of how psilocybin may be employed for testing treatment efficacy in preclinical research for affective disorders in zebrafish. We conclude that zebrafish has a promising future in preclinical research on psychedelic drugs.

Role of Sonic Hedgehog Signaling Activation in the Prevention of Neurological Abnormalities Associated with Obsessive-Compulsive Disorder

The smoothened sonic hedgehog (Smo-Shh) pathway is one mechanism that influences neurogenesis, including brain cell differentiation and development during childhood. Shh signaling dysregulation leads to decreased target gene transcription, which contributes to increased neuronal excitation, apoptosis, and neurodegeneration, eventually leading to neurological deficits. Neuropsychiatric disorders such as OCD and related neurological dysfunctions are characterized by neurotransmitter imbalance, neuroinflammation, oxidative stress, and impaired neurogenesis, disturbing the cortico-striato-thalamo-cortical (CSTC) link neuronal network. Despite the availability of several treatments, such as selective serotonin reuptake inhibitors, some individuals may not benefit much from them. Several trials on the use of antipsychotics in the treatment of OCD have also produced inadequate findings. This evidence-based review focuses on a potential pharmacological approach to alleviating OCD and associated neuronal deficits by preventing neurochemical alterations, in which sonic hedgehog activators are neuroprotective, lowering neuronal damage while increasing neuronal maintenance and survival. As a result, stimulating SMO-Shh via its potential activators may have neuroprotective effects on neurological impairment associated with OCD. This review investigates the link between SMO-Shh signaling and the neurochemical abnormalities associated with the progression of OCD and associated neurological dysfunctions. Role of Smo-Shh signaling in serotonergic neurogenesis and in maintaining their neuronal identity. The Shh ligand activates two main transcriptional factors known as Foxa2 and Nkx2.2, which again activates another transcriptional factor, GATA (GATA2 and GATA3), in post mitotic precursor cells of serotonergic neurons-following increased expression of Pet-1 and Lmx1b after GATA regulates the expression of many serotonergic enzymes such as TPH2, SERT, VMAT, slc6a4, Htr1a, Htr1b (Serotonin receptor enzymes), and MAO that regulate and control the release of serotonin and maintain their neuronal identity after their maturation. Abbreviation: Foxa2: Forkhead box; GATA: Globin transcription factor; Lmx1b: LIM homeobox transcription factor 1 beta; TPH2: Tryptophan hydroxylase 2; Htr1a: Serotonin receptor 1a; Htr1b: Serotonin receptor 1b; SERT: Serotonin transporter; VMAT: Vesicular monoamine transporter; MAO: Monoamine oxidase.

Pseudoephedrine hydrochloride causes hyperactivity in zebrafish via modulation of the serotonin pathway

This study aimed to explore behavioral changes of embryonic and larval zebrafish caused by pseudoephedrine hydrochloride (PSE) and its underlying mechanism. Zebrafish embryos were exposed to 0.5 µM, 2 µM, and 8 µM PSE at 4 h post-fertilization (4 hpf) or 22-23 hpf. Mortality, hatching rate, coiling frequency, heart rate, behavior changes, and related gene expression were observed at different developmental stages. PSE below 8 µM did not affect zebrafish mortality, hatching rate, and heart rate compared with the control group. For embryos, PSE caused an increase at 16-32 hpf in zebrafish coiling frequency which could be rescued by serotonin antagonist WAY100635. Similarly, PSE caused an increase in the swimming distance of zebrafish larvae at 120 hpf. PSE also elevated the expression of serotonin (5-HT)-related genes 5-htr1ab and tph2 and dopamine-related gene dbh. Behavioral changes in zebrafish embryos and larvae caused by PSE may be closely associated with increased expression of 5-HT and dopamine-related genes. This may be reflected that the behavioral changes in zebrafish are a possible PSE monitoring indicator.

Validation of Candidate Sleep Disorder Risk Genes Using Zebrafish

Sleep disorders and chronic sleep disturbances are common and are associated with cardio-metabolic diseases and neuropsychiatric disorders. Several genetic pathways and neuronal mechanisms that regulate sleep have been described in animal models, but the genes underlying human sleep variation and sleep disorders are largely unknown. Identifying these genes is essential in order to develop effective therapies for sleep disorders and their associated comorbidities. To address this unmet health problem, genome-wide association studies (GWAS) have identified numerous genetic variants associated with human sleep traits and sleep disorders. However, in most cases, it is unclear which gene is responsible for a sleep phenotype that is associated with a genetic variant. As a result, it is necessary to experimentally validate candidate genes identified by GWAS using an animal model. Rodents are ill-suited for this endeavor due to their poor amenability to high-throughput sleep assays and the high costs associated with generating, maintaining, and testing large numbers of mutant lines. Zebrafish (Danio rerio), an alternative vertebrate model for studying sleep, allows for the rapid and cost-effective generation of mutant lines using the CRISPR/Cas9 system. Numerous zebrafish mutant lines can then be tested in parallel using high-throughput behavioral assays to identify genes whose loss affects sleep. This process identifies a gene associated with each GWAS hit that is likely responsible for the human sleep phenotype. This strategy is a powerful complement to GWAS approaches and holds great promise to identify the genetic basis for common human sleep disorders.

Distribution of two isoforms of tryptophan hydroxylase in the brain of rainbow trout (Oncorhynchus mykiss). An in situ hybridization study

Serotonin (5-HT) is one of the principal neurotransmitters in the nervous system of vertebrates. It is initially synthesized by hydroxylation of tryptophan (Trp) by means of tryptophan hydroxylase or TPH which is the rate-limiting enzyme in the production of 5-HT. In most vertebrates, there are two isoforms of TPH present, TPH1 and TPH2, which exhibit different catalytic or substrate specificity as well as different expression domains. Studies carried out in mammals show that only tph2 is expressed in the brain whereas tph1-mRNA is primarily localized in the enterochromaffin cells and pineal gland. A large number of neurons are also considered to be serotonergic or "pseudo-serotonergic" as they accumulate and release 5-HT yet do not produce it as no amine-synthetic enzymes are expressed, yet a combination of 5-HT transporters is observed. Therefore, tph expression is considered to be the only specific marker of 5-HT-producing neurons that can discriminate true 5-HT from pseudo-serotonergic neurons. This work examined in situ hybridization to study the mRNA distribution of one paralogue for tph1 and tph2 in the central nervous system of rainbow trout. Results show a segregated expression for both paralogues that predominantly match previous immunocytochemical studies. This study thus adds valuable information to the scarce analyses focusing on the central distribution of the expression of serotonergic markers, particularly tphs, in the vertebrate brain thus characterizing the true serotonergic brain territories.

Zebrafish Minichromosome Maintenance Protein 5 Gene Regulates the Development and Migration of Facial Motor Neurons via Fibroblast Growth Factor Signaling

Minichromosome maintenance protein 5 (MCM5), a member of the microchromosomal maintenance protein family, plays an important role in the initiation and extension of DNA replication. However, its role in neural development in zebrafish remains unclear. Here, we used morpholino (MO) and CRISPR/Cas9 to knock down mcm5 and investigated the developmental features of facial motor neurons (FMNs) in the hindbrain of zebrafish. We found that knockdown of mcm5 using mcm5 MO resulted in a small head, small eyes, and a blurred midbrain-hindbrain boundary, while MO injection of mcm5 led to decrease in FMNs and their migration disorder. However, the mutant of mcm5 only resulted in the migration defect of FMNs rather than quantity change. We further investigated the underlying mechanism of mcm5 in the development of hindbrain using in situ hybridization (ISH) and fgfr1a mRNA co-injected with mcm5 MO. Results from ISH showed that the fibroblast growth factor (FGF) signaling pathway was changed when the MCM5 function was lost, with the decrease in fgfr1a and the increase in fgf8, while that of pea3 had opposite trend. FMN development defects were rescued by fgfr1a mRNA co-injected with mcm5 MO. Our results demonstrated that FGF signaling pathway is required for FMN development in zebrafish. Specifically, mcm5 regulates FMN development during zebrafish growing.

Aconitine disrupts serotonin neurotransmission via 5-hydroxytryptamine receptor in zebrafish embryo

Medicinal plants of the genus Aconitum are one of the most commonly used herbs in traditional medicine in East Asia to treat conditions related to the heart, pain, or inflammation. However, these herbs are also dangerous as accidental poisoning due to misuse is a recurring issue. These plants contain a number of diester-diterpenoid alkaloid compounds and aconitine is the most abundant and active one. This study investigated neurotoxicity of aconitine to zebrafish embryos in early development in relation to serotonin regulation. Experimental results showed that aconitine exposure (1, 10, and 100 μM) increased frequency of coiling behavior in zebrafish embryos in a dose-dependent manner and this effect can be triggered by either exposure to 5-hydroxytryptamine 1A (5-HT1A) receptor agonist (±)-8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT) or overexpression of serotonin receptor 5-htr1ab. At the same time, coiling behavior caused by aconitine exposure could be rescued by co-exposure to 5-HT1A receptor antagonist WAY-100635 Maleate (WAY100635) and knockdown of 5-htr1ab using morpholino. Exposure to aconitine also significantly increased serotonin receptor 5-htr1ab and 5-htr1bd gene expression at 24 h post fertilization (hpf), but decreased their expression and protein expression of the serotonin receptor at 96 hpf with the high dose. These results suggest that neurotoxicity caused by aconitine is mediated through the 5-HT receptor.

Reelin Signaling Controls the Preference for Social Novelty in Zebrafish

Reelin (Reln) is an extracellular glycoprotein that is important for brain patterning. During development Reln coordinates the radial migration of postmitotic cortical neurons, cerebellar and hippocampal neurons, whereas it promotes dendrite maturation, synaptogenesis, synaptic transmission, plasticity and neurotransmitter release in the postnatal and adult brain. Genetic studies of human patients have demonstrated association between the RELN locus and autism spectrum disorder, schizophrenia, bipolar disorder, and Alzheimer’s disease. In this study we have characterized the behavioral phenotype of reelin (reln) mutant zebrafish, as well as two canonical signaling pathway targets DAB adaptor protein 1a (dab1a) and the very low density lipoprotein receptor (vldlr). Zebrafish reln^–/– mutants display a selective reduction in preference for social novelty that is not observed in dab1a^–/– or vldlr^–/– mutant lines. They also exhibit an increase in 5-HT signaling in the hindbrain that parallels but does not underpin the alteration in social preference. These results suggest that zebrafish reln^–/– mutants can be used to model some aspects of human diseases in which changes to Reln signaling alter social behavior.

The Serotonergic Raphe Promote Sleep in Zebrafish and Mice

The role of serotonin (5-HT) in sleep is controversial: early studies suggested a sleep-promoting role, but eventually the paradigm shifted toward a wake-promoting function for the serotonergic raphe. Here, we provide evidence from zebrafish and mice that the raphe are critical for the initiation and maintenance of sleep. In zebrafish, genetic ablation of 5-HT production by the raphe reduces sleep, sleep depth, and the homeostatic response to sleep deprivation. Pharmacological inhibition or ablation of the raphe reduces sleep, while optogenetic stimulation increases sleep. Similarly, in mice, ablation of the raphe increases wakefulness and impairs the homeostatic response to sleep deprivation, whereas tonic optogenetic stimulation at a rate similar to baseline activity induces sleep. Interestingly, burst optogenetic stimulation induces wakefulness in accordance with previously described burst activity of the raphe during arousing stimuli. These results indicate that the serotonergic system promotes sleep in both diurnal zebrafish and nocturnal rodents. VIDEO ABSTRACT.

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.

Brain Aromatase Modulates Serotonergic Neuron by Regulating Serotonin Levels in Zebrafish Embryos and Larvae

Teleost fish are known to express two isoforms of P450 aromatase, a key enzyme for estrogen synthesis. One of the isoforms, brain aromatase (AroB), cyp19a1b, is highly expressed during early development of zebrafish, thereby suggesting its role in brain development. On the other hand, early development of serotonergic neuron, one of the major monoamine neurons, is considered to play an important role in neurogenesis. Therefore, in this study, we investigated the role of AroB in development of serotonergic neuron by testing the effects of (1) estradiol (E₂) exposure and (2) morpholino (MO)-mediated AroB knockdown. When embryos were exposed to E₂, the effects were biphasic. The low dose of E₂ (0.005 µM) significantly increased serotonin (5-HT) positive area at 48 hour post-fertilization (hpf) detected by immunohistochemistry and relative mRNA levels of tryptophan hydroxylase isoforms (tph1a, tph1b, and tph2) at 96 hpf measured by semi-quantitative PCR. To test the effects on serotonin transmission, heart rate and thigmotaxis, an indicator of anxiety, were analyzed. The low dose also significantly increased heart rate at 48 hpf and decreased thigmotaxis. The high dose of E₂ (1 µM) exhibited opposite effects in all parameters. The effects of both low and high doses were reversed by addition of estrogen receptor (ER) blocker, ICI 182,780, thereby suggesting that the effects were mediated through ER. When AroB MO was injected to fertilized eggs, 5-HT-positive area was significantly decreased, while the significant decrease in relative tph mRNA levels was found only with tph2 but not with two other isoforms. AroB MO also decreased heart rate and increased thigmotaxis. All the effects were rescued by co-injection with AroB mRNA and by exposure to E₂. Taken together, this study demonstrates the role of brain aromatase in development of serotonergic neuron in zebrafish embryos and larvae, implying that brain-formed estrogen is an important factor to sustain early development of serotonergic neuron.

Venlafaxine in Embryos Stimulates Neurogenesis and Disrupts Larval Behavior in Zebrafish

Venlafaxine, a widely prescribed antidepressant, is a selective serotonin and norepinephrine reuptake inhibitor in humans, and this drug is prevalent in municipal wastewater effluents. While studies have shown that this drug affects juvenile fish behavior, little is known about the developmental impact on nontarget aquatic animals. We tested the hypothesis that venlafaxine deposition in the egg, mimicking maternal transfer of this antidepressant, disrupts developmental programming using zebrafish (Danio rerio) as a model. Embryos (1-4 cell stage) were microinjected with either 1 or 10 ng venlafaxine, which led to a rapid reduction (90%) of this drug in the embryo at hatch. There was a concomitant increase in the concentration of the major metabolite o-desmethylvenlafaxine during the same period. Embryonic exposure to venlafaxine accelerated early development, increased hatching rate and produced larger larvae at 5 days post fertilization. Also, there was an increase in neuronal birth in the hypothalamus, dorsal thalamus, posterior tuberculum, and the preoptic region, and this corresponded with a higher spatial expression of nrd4, a key marker of neurogenesis. The venlafaxine-exposed larvae were less active and covered shorter distance in a light and dark behavioral test compared to the controls. Overall, zygotic exposure to venlafaxine disrupts early development, including brain function, and compromises larval behavior, suggesting impact of this drug on developmental programming in zebrafish.

Cadherin-13 Deficiency Increases Dorsal Raphe 5-HT Neuron Density and Prefrontal Cortex Innervation in the Mouse Brain

Background: During early prenatal stages of brain development, serotonin (5-HT)-specific neurons migrate through somal translocation to form the raphe nuclei and subsequently begin to project to their target regions. The rostral cluster of cells, comprising the median and dorsal raphe (DR), innervates anterior regions of the brain, including the prefrontal cortex. Differential analysis of the mouse 5-HT system transcriptome identified enrichment of cell adhesion molecules in 5-HT neurons of the DR. One of these molecules, cadherin-13 (Cdh13) has been shown to play a role in cell migration, axon pathfinding, and synaptogenesis. This study aimed to investigate the contribution of Cdh13 to the development of the murine brain 5-HT system.

Methods: For detection of Cdh13 and components of the 5-HT system at different embryonic developmental stages of the mouse brain, we employed immunofluorescence protocols and imaging techniques, including epifluorescence, confocal and structured illumination microscopy. The consequence of CDH13 loss-of-function mutations on brain 5-HT system development was explored in a mouse model of Cdh13 deficiency.

Results: Our data show that in murine embryonic brain Cdh13 is strongly expressed on 5-HT specific neurons of the DR and in radial glial cells (RGCs), which are critically involved in regulation of neuronal migration. We observed that 5-HT neurons are intertwined with these RGCs, suggesting that these neurons undergo RGC-guided migration. Cdh13 is present at points of intersection between these two cell types. Compared to wildtype controls, Cdh13-deficient mice display increased cell densities in the DR at embryonic stages E13.5, E17.5, and adulthood, and higher serotonergic innervation of the prefrontal cortex at E17.5.

Conclusion: Our findings provide evidence for a role of CDH13 in the development of the serotonergic system in early embryonic stages. Specifically, we indicate that Cdh13 deficiency affects the cell density of the developing DR and the posterior innervation of the prefrontal cortex (PFC), and therefore might be involved in the migration, axonal outgrowth and terminal target finding of DR 5-HT neurons. Dysregulation of CDH13 expression may thus contribute to alterations in this system of neurotransmission, impacting cognitive function, which is frequently impaired in neurodevelopmental disorders including attention-deficit/hyperactivity and autism spectrum disorders.

Cd36 is a candidate lipid sensor involved in the sensory detection of fatty acid in zebrafish

Recently more and more evidences raise the possibility for the taste system in the role of the perception of lipids in mammals, and the fatty acid receptor CD36 has been proved to be as an important candidate receptor of fat taste. Fish has different taste modality with mammals. No information was known about the function of cd36 in fish taste till now. Here, using in situ hybridization and immunofluorescence technologies, we showed that fish cd36/Cd36 localized in taste buds. Real-time PCR technology demonstrated that, in zebrafish cd36 (zcd36)-transfected cells, linoleic acid (LA) increased the expression level of tryptophan hydroxylase-1 (TPH-1), which encodes the enzyme involved in the biosynthesis of monoamine neurotransmitter of 5-HT. Moreover, the LA-induced up-regulation expression of TPH-1 was significantly curtailed by SSO, a specific inhibitor of LCFA binding to CD36, suggesting zCd36 is implicated in the LA-induced release of neurotransmitter. Importantly, we observed that zcd36 gene knockout zebrafish reduced the preference for LA contrast to wild-type zebrafish. Together, our findings indicate that Cd36 is a candidate lipid sensor involved in the sensory detection of fatty acid in zebrafish.

Good night, sleep tight: The effects of sleep deprivation on spatial associative learning in zebrafish

Learning and memory are vital to an animal's survival, and numerous factors can disrupt cognitive performance. Sleep is an evolutionarily conserved physiological process known to be important for the consolidation of learning and memory. The zebrafish has emerged as a powerful model organism sharing organizational and functional characteristics with other vertebrates, providing great translational relevance. In our study, we used a simple spatial associative learning task to quantify the effects of sleep deprivation (partial vs. total) on learning performance in zebrafish, using an animated conspecific shoal image as a reward. Control animals maintained on a regular light:dark cycle were able to acquire the association between the unconditioned and conditioned stimulus, reinforcing zebrafish as a valid and reliable model for appetitive conditioning tasks. Notably, sleep deprivation did not alter the perception of and response to the conspecific image. In contrast, although partial sleep deprivation did not impair cognitive performance, total sleep deprivation significantly impaired performance on the associative learning task. Our results suggest that sleep is important for learning and memory, and that the effects of sleep deprivation on these processes can be investigated in zebrafish.

Automated deep-phenotyping of the vertebrate brain

Here, we describe an automated platform suitable for large-scale deep-phenotyping of zebrafish mutant lines, which uses optical projection tomography to rapidly image brain-specific gene expression patterns in 3D at cellular resolution. Registration algorithms and correlation analysis are then used to compare 3D expression patterns, to automatically detect all statistically significant alterations in mutants, and to map them onto a brain atlas. Automated deep-phenotyping of a mutation in the master transcriptional regulator fezf2 not only detects all known phenotypes but also uncovers important novel neural deficits that were overlooked in previous studies. In the telencephalon, we show for the first time that fezf2 mutant zebrafish have significant patterning deficits, particularly in glutamatergic populations. Our findings reveal unexpected parallels between fezf2 function in zebrafish and mice, where mutations cause deficits in glutamatergic neurons of the telencephalon-derived neocortex.

The dual developmental origin of spinal cerebrospinal fluid-contacting neurons gives rise to distinct functional subtypes

Chemical and mechanical cues from the cerebrospinal fluid (CSF) can affect the development and function of the central nervous system (CNS). How such cues are detected and relayed to the CNS remains elusive. Cerebrospinal fluid-contacting neurons (CSF-cNs) situated at the interface between the CSF and the CNS are ideally located to convey such information to local networks. In the spinal cord, these GABAergic neurons expressing the PKD2L1 channel extend an apical extension into the CSF and an ascending axon in the spinal cord. In zebrafish and mouse spinal CSF-cNs originate from two distinct progenitor domains characterized by distinct cascades of transcription factors. Here we ask whether these neurons with different developmental origins differentiate into cells types with different functional properties. We show in zebrafish larva that the expression of specific markers, the morphology of the apical extension and axonal projections, as well as the neuronal targets contacted by CSF-cN axons, distinguish the two CSF-cN subtypes. Altogether our study demonstrates that the developmental origins of spinal CSF-cNs give rise to two distinct functional populations of sensory neurons. This work opens novel avenues to understand how these subtypes may carry distinct functions related to development of the spinal cord, locomotion and posture.

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.

Aggression in non-human vertebrates: Genetic mechanisms and molecular pathways

Aggression is an adaptive behavioral trait that is important for the establishment of social hierarchies and competition for mating partners, food, and territories. While a certain level of aggression can be beneficial for the survival of an individual or species, abnormal aggression levels can be detrimental. Abnormal aggression is commonly found in human patients with psychiatric disorders. The predisposition to aggression is influenced by a combination of environmental and genetic factors and a large number of genes have been associated with aggression in both human and animal studies. In this review, we compare and contrast aggression studies in zebrafish and mouse. We present gene ontology and pathway analyses of genes linked to aggression and discuss the molecular pathways that underpin agonistic behavior in these species. © 2015 Wiley Periodicals, Inc.

The neurotoxicity of DE-71: effects on neural development and impairment of serotonergic signaling in zebrafish larvae

The underlying mechanism of polybrominated diphenyl ether (PBDE)-induced neurotoxicity is still a major concern due to its ubiquitous nature and persistence. Here, zebrafish embryos (2 h postfertilization, hpf) were exposed to different concentrations of the commercial PBDE mixture DE-71 (0-100 µg l^-1 ) until 120 hpf, and the impact on neural development and serotonergic system was investigated. The in vivo results revealed significantly reduced transcription of genes involved in neurogenesis (fgf8, shha, wnt1), and contents of proteins in neuronal morphogenesis (myelin basic protein, synapsin IIa), suggesting an impairment of neural development in zebrafish embryos. Further results demonstrated a reduction of 5-hydroxytryptamine neuron and a dose-dependent decrease of whole-body serotonin levels, as well as the transcription of genes involved in serotonergic synthesis (tph1, tph2, trhr) and neurotransmission (serta/b, htr1aa/b). In addition, we predicted possible targets of PBDEs by molecular docking, and the results indicated that PBDE congeners showed high binding affinities with fibroblast growth factor 8 other than SHH and HTR1B. Taken together, this study demonstrated that PBDE exposure during embryogenesis could damage neural development and cause impairment of the serotonergic system as secondary effects in the zebrafish larvae. Copyright © 2016 John Wiley & Sons, Ltd.

A Serotonin Circuit Acts as an Environmental Sensor to Mediate Midline Axon Crossing through EphrinB2

Modulation of connectivity formation in the developing brain in response to external stimuli is poorly understood. Here, we show that the raphe nucleus and its serotonergic projections regulate pathfinding of commissural axons in zebrafish. We found that the raphe neurons extend projections toward midline-crossing axons and that when serotonergic signaling is blocked by pharmacological inhibition or by raphe neuron ablation, commissural pathfinding is disrupted. We demonstrate that the serotonin receptor htr2a is expressed on these commissural axons and that genetic knock-down of htr2a disrupts crossing. We further show that knock-down of htr2a or ablation of the raphe neurons increases ephrinB2a protein levels in commissural axons. An ephrinB2a mutant can rescue midline crossing when serotonergic signaling is blocked. Furthermore, we found that regulation of serotonin expression in the raphe neurons is modulated in response to the developmental environment. Hypoxia causes the raphe to decrease serotonin levels, leading to a reduction in midline crossing. Increasing serotonin in the setting of hypoxia restored midline crossing. Our findings demonstrate an instructive role for serotonin in axon guidance acting through ephrinB2a and reveal a novel mechanism for developmental interpretation of the environmental milieu in the generation of mature neural circuitry.

SIGNIFICANCE STATEMENT

We show here that serotonin has a novel role in regulating connectivity in response to the developmental environment. We demonstrate that serotonergic projections from raphe neurons regulate pathfinding of crossing axons. The neurons modulate their serotonin levels, and thus alter crossing, in response to the developmental environment including hypoxia. The findings suggest that modification of the serotonergic system by early exposures may contribute to permanent CNS connectivity alterations. This has important ramifications because of the association between premature birth and accompanying hypoxia, and increased risk of autism and evidence associating in utero exposure to some antidepressants and neurodevelopmental disorders. Finally, this work demonstrates that the vertebrate CNS can modulate its connectivity in response to the external environment.

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.

Intraspinal serotonergic neurons consist of two, temporally distinct populations in developing zebrafish

Zebrafish intraspinal serotonergic neuron (ISN) morphology and distribution have been examined in detail at different ages; however, some aspects of the development of these cells remain unclear. Although antibodies to serotonin (5-HT) have detected ISNs in the ventral spinal cord of embryos, larvae, and adults, the only tryptophan hydroxylase (tph) transcript that has been described in the spinal cord is tph1a. Paradoxically, spinal tph1a is only expressed transiently in embryos, which brings the source of 5-HT in the ISNs of larvae and adults into question. Because the pet1 and tph2 promoters drive transgene expression in the spinal cord, we hypothesized that tph2 is expressed in spinal cords of zebrafish larvae. We confirmed this hypothesis through in situ hybridization. Next, we used 5-HT antibody labeling and transgenic markers of tph2-expressing neurons to identify a transient population of ISNs in embryos that was distinct from ISNs that appeared later in development. The existence of separate ISN populations may not have been recognized previously due to their shared location in the ventral spinal cord. Finally, we used transgenic markers and immunohistochemical labeling to identify the transient ISN population as GABAergic Kolmer-Agduhr double-prime (KA″) neurons. Altogether, this study revealed a novel developmental paradigm in which KA″ neurons are transiently serotonergic before the appearance of a stable population of tph2-expressing ISNs.

Molecular psychiatry of zebrafish

Due to their well-characterized neural development and high genetic homology to mammals, zebrafish (Danio rerio) have emerged as a powerful model organism in the field of biological psychiatry. Here, we discuss the molecular psychiatry of zebrafish, and its implications for translational neuroscience research and modeling central nervous system (CNS) disorders. In particular, we outline recent genetic and technological developments allowing for in vivo examinations, high-throughput screening and whole-brain analyses in larval and adult zebrafish. We also summarize the application of these molecular techniques to the understanding of neuropsychiatric disease, outlining the potential of zebrafish for modeling complex brain disorders, including attention-deficit/hyperactivity disorder (ADHD), aggression, post-traumatic stress and substance abuse. Critically evaluating the advantages and limitations of larval and adult fish tests, we suggest that zebrafish models become a rapidly emerging new field in modern molecular psychiatry research.

Acute and chronic alcohol administration: effects on performance of zebrafish in a latent learning task

Alcohol abuse is a major medical problem. Zebrafish have been proposed to model alcohol related human disorders. Alcohol impairs learning and memory. Here, we analyze the effects of alcohol on performance of zebrafish in a recently developed latent learning paradigm. We employ a 2×3×2 experimental design (chronic×acute alcohol treatment×path blocked). The latent learning task had two phases: one, 30min long exploration trials (16 days, 1 trial/day) with left or right path of a complex maze blocked, and two, a subsequent probe trial with all paths open leading to a goal box that now contained stimulus fish. During the 16 days each fish received one of two chronic treatments: freshwater or 0.50% (v/v%) alcohol. Subsequently, fish were immersed for 1h in one of the following solutions: 0.00 (freshwater), 0.50% or 1.00% alcohol, the acute challenge. Behavior of fish was recorded during the probe trial that commenced immediately after the acute treatment. Path choices, latency to leave the start box and to enter the goal box, time spent in the goal box, distance traveled, and duration of freezing were quantified. We found that acute exposure to 1.00% alcohol after chronic freshwater disrupted learning performance, so did exposure to freshwater after chronic alcohol treatment (withdrawal). We also found exposure to chronic alcohol to diminish the effect of subsequent acute alcohol suggesting development of tolerance. Our results demonstrate that analysis of learning performance of zebrafish allows detection of alcohol-induced functional changes. The simplicity and scalability of the employed task also imply the utility of the zebrafish in high throughput drug screens.

Serotonin is required for pharyngeal arch morphogenesis in zebrafish

Serotonin (5-HT) is not only a neurotransmitter but also a mediator of developmental processes in vertebrates. In this study, we analyzed the importance of 5-HT during zebrafish development. The expression patterns of three zebrafish tryptophan hydroxylase isoforms (Tph1A, Tph1B, Tph2), the rate-limiting enzymes in 5-HT synthesis, were analyzed and compared to the appearance and distribution of 5-HT. 5-HT was found in the raphe nuclei correlating with tph2 expression and in the pineal gland correlating with tph1a and tph2 expression. tph2 deficient fish generated with antisense morpholino oligonucleotides exhibited morphogenesis defects during pharyngeal arch development. The correct specification of neural crest cells was not affected in tph2 morphants as shown by the expression of early markers, but the survival and differentiation of pharyngeal arch progenitor cells were impaired. An organizing role of 5-HT in pharyngeal arch morphogenesis was suggested by a highly regular pattern of 5-HT positive cells in this tissue. Moreover, the 5-HT2B receptor was expressed in the pharyngeal arches and its pharmacological inhibition also induced defects in pharyngeal arch morphogenesis. These results support an important role of Tph2-derived serotonin as a morphogenetic factor in the development of neural crest derived tissues.

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.

Valproic acid silencing of ascl1b/Ascl1 results in the failure of serotonergic differentiation in a zebrafish model of fetal valproate syndrome

Fetal valproate syndrome (FVS) is caused by in utero exposure to the drug sodium valproate. Valproate is used worldwide for the treatment of epilepsy, as a mood stabiliser and for its pain-relieving properties. In addition to birth defects, FVS is associated with an increased risk of autism spectrum disorder (ASD), which is characterised by abnormal behaviours. Valproate perturbs multiple biochemical pathways and alters gene expression through its inhibition of histone deacetylases. Which, if any, of these mechanisms is relevant to the genesis of its behavioural side effects is unclear. Neuroanatomical changes associated with FVS have been reported and, among these, altered serotonergic neuronal differentiation is a consistent finding. Altered serotonin homeostasis is also associated with autism. Here we have used a chemical-genetics approach to investigate the underlying molecular defect in a zebrafish FVS model. Valproate causes the selective failure of zebrafish central serotonin expression. It does so by downregulating the proneural gene ascl1b, an ortholog of mammalian Ascl1, which is a known determinant of serotonergic identity in the mammalian brainstem. ascl1b is sufficient to rescue serotonin expression in valproate-treated embryos. Chemical and genetic blockade of the histone deacetylase Hdac1 downregulates ascl1b, consistent with the Hdac1-mediated silencing of ascl1b expression by valproate. Moreover, tonic Notch signalling is crucial for ascl1b repression by valproate. Concomitant blockade of Notch signalling restores ascl1b expression and serotonin expression in both valproate-exposed and hdac1 mutant embryos. Together, these data provide a molecular explanation for serotonergic defects in FVS and highlight an epigenetic mechanism for genome-environment interaction in disease.

Development of hypothalamic serotoninergic neurons requires Fgf signalling via the ETS-domain transcription factor Etv5b

Serotonin is a monoamine neurotransmitter that is involved in numerous physiological functions and its dysregulation is implicated in various psychiatric diseases. In all non-placental vertebrates, serotoninergic (5-HT) neurons are present in several regions of the brain, including the hypothalamus. In placental mammals, however, 5-HT neurons are located in the raphe nuclei only. In all species, though, 5-HT neurons constitute a functionally and molecularly heterogeneous population. How the non-raphe 5-HT populations are developmentally encoded is unknown. Using the zebrafish model we show that, in contrast to the raphe populations, hypothalamic 5-HT neurons are generated independently of the ETS-domain transcription factor Pet1 (Fev). By applying a combination of pharmacological tools and gene knockdown and/or overexpression experiments, we demonstrate that Fgf signalling acts via another ETS-domain transcription factor, Etv5b (Erm), to induce hypothalamic 5-HT neurons. We provide evidence that Etv5b exerts its effects by regulating cell cycle parameters in 5-HT progenitors. Our results highlight a novel role for Etv5b in neuronal development and provide support for the existence of a developmental heterogeneity among 5-HT neurons in their requirement for ETS-domain transcription factors.

The dorsal raphe modulates sensory responsiveness during arousal in zebrafish

During waking behavior, animals adapt their state of arousal in response to environmental pressures. Sensory processing is regulated in aroused states, and several lines of evidence imply that this is mediated at least partly by the serotonergic system. However, there is little information directly showing that serotonergic function is required for state-dependent modulation of sensory processing. Here we find that zebrafish larvae can maintain a short-term state of arousal during which neurons in the dorsal raphe modulate sensory responsiveness to behaviorally relevant visual cues. After a brief exposure to water flow, larvae show elevated activity and heightened sensitivity to perceived motion. Calcium imaging of neuronal activity after flow revealed increased activity in serotonergic neurons of the dorsal raphe. Genetic ablation of these neurons abolished the increase in visual sensitivity during arousal without affecting baseline visual function or locomotor activity. We traced projections from the dorsal raphe to a major visual area, the optic tectum. Laser ablation of the tectum demonstrated that this structure, like the dorsal raphe, is required for improved visual sensitivity during arousal. These findings reveal that serotonergic neurons of the dorsal raphe have a state-dependent role in matching sensory responsiveness to behavioral context.

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.

The ADHD-susceptibility gene lphn3.1 modulates dopaminergic neuron formation and locomotor activity during zebrafish development

Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterized by inattention, hyperactivity, increased impulsivity and emotion dysregulation. Linkage analysis followed by fine-mapping identified variation in the gene coding for Latrophilin 3 (LPHN3), a putative adhesion-G protein-coupled receptor, as a risk factor for ADHD. In order to validate the link between LPHN3 and ADHD, and to understand the function of LPHN3 in the etiology of the disease, we examined its ortholog lphn3.1 during zebrafish development. Loss of lphn3.1 function causes a reduction and misplacement of dopamine-positive neurons in the ventral diencephalon and a hyperactive/impulsive motor phenotype. The behavioral phenotype can be rescued by the ADHD treatment drugs methylphenidate and atomoxetine. Together, our results implicate decreased Lphn3 activity in eliciting ADHD-like behavior, and demonstrate its correlated contribution to the development of the brain dopaminergic circuitry.

Monoaminergic modulation of photoreception in ascidian: evidence for a proto-hypothalamo-retinal territory

Background

The retina of craniates/vertebrates has been proposed to derive from a photoreceptor prosencephalic territory in ancestral chordates, but the evolutionary origin of the different cell types making the retina is disputed. Except for photoreceptors, the existence of homologs of retinal cells remains uncertain outside vertebrates.

Methods

The expression of genes expressed in the sensory vesicle of the ascidian Ciona intestinalis including those encoding components of the monoaminergic neurotransmission systems, was analyzed by in situ hybridization or in vivo transfection of the corresponding regulatory elements driving fluorescent reporters. Modulation of photic responses by monoamines was studied by electrophysiology combined with pharmacological treatments.

Results

We show that many molecular characteristics of dopamine-synthesizing cells located in the vicinity of photoreceptors in the sensory vesicle of the ascidian Ciona intestinalis are similar to those of amacrine dopamine cells of the vertebrate retina. The ascidian dopamine cells share with vertebrate amacrine cells the expression of the key-transcription factor Ptf1a, as well as that of dopamine-synthesizing enzymes. Surprisingly, the ascidian dopamine cells accumulate serotonin via a functional serotonin transporter, as some amacrine cells also do. Moreover, dopamine cells located in the vicinity of the photoreceptors modulate the light-off induced swimming behavior of ascidian larvae by acting on alpha2-like receptors, instead of dopamine receptors, supporting a role in the modulation of the photic response. These cells are located in a territory of the ascidian sensory vesicle expressing genes found both in the retina and the hypothalamus of vertebrates (six3/6, Rx, meis, pax6, visual cycle proteins).

Conclusion

We propose that the dopamine cells of the ascidian larva derive from an ancestral multifunctional cell population located in the periventricular, photoreceptive field of the anterior neural tube of chordates, which also gives rise to both anterior hypothalamus and the retina in craniates/vertebrates. It also shows that the existence of multiple cell types associated with photic responses predates the formation of the vertebrate retina.

The sea lamprey tryptophan hydroxylase: new insight into the evolution of the serotonergic system of vertebrates

Recent research has shown that at least two tryptophan hydroxylase (Tph) genes are present in gnathostome vertebrates, but it is not known when the duplication of the ancestral Tph gene took place during evolution. By their position as an out-group of gnathostomes, lampreys (agnathans) are key models to understand molecular evolution in vertebrates. Here, we report the cloning of a Tph cDNA of the sea lamprey and the pattern of Tph mRNA expression in larval and postmetamorphic (young adult) sea lampreys using in situ hybridization. Phylogenetic analysis indicated that the lamprey Tph is an orthologue of Tphs of other vertebrates and suggested that the duplication of the ancestral Tph gene occurred before the separation of agnathans and gnathostomes, although alternative hypothesis are also discussed in the present study. In the sea lamprey brain, the Tph transcript was expressed in perikarya of the pineal organ, the retina, the diencephalic and rhombencephalic nuclei reported previously with serotonin immunohistochemistry and in small cells of the spinal cord, with a pattern similar to that observed with anti-serotonin antibodies. This suggests that expression of this Tph gene is shared by all lamprey serotonergic brain populations, unlike that reported in zebrafish and mammals for their different Tph genes. However, no Tph expression was observed in peripheral serotonergic cells, which, unlike in other vertebrates, are widely distributed in lampreys. Our results suggest that the selection of Tph2 to be expressed in raphe neurons may have occurred along the line leading to gnathostomes.

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.

Localized expression of urocortin genes in the developing zebrafish brain

The corticotropin-releasing hormone (CRH) family consists of four paralogous genes, CRH and urocortins (UCNs) 1, 2, and 3. In a previous study, we analyzed CRH in the teleost model organism zebrafish and its transcript distribution in the embryonic brain. Here, we describe full-length cDNAs encoding urotensin 1 (UTS1), the teleost UCN1 ortholog, and UCN3 of zebrafish. Major expression sites of uts1 in adult zebrafish are the caudal neurosecretory system and brain. By using RT-PCR analysis, we show that uts1 mRNA is also present in ovary, maternally contributed to the embryo, and expressed throughout embryonic development. Expression of ucn3 mRNA was detected in a range of adult tissues and during developmental stages from 24 hours post fertilization onward. Analysis of spatial transcript distributions by whole-mount in situ hybridization revealed limited forebrain expression of uts1 and ucn3 during early development. Small numbers of uts1-synthesizing neurons were found in subpallium, hypothalamus, and posterior diencephalon, whereas ucn3-positive cells were restricted to telencephalon and retina. The brainstem was the main site of uts1 and ucn3 synthesis in the embryonic brain. uts1 Expression was confined to the midbrain tegmentum; distinct hindbrain cell groups, including locus coeruleus and Mauthner neurons; and the spinal cord. ucn3 Expression was localized to the optic tectum, serotonergic raphe, and distinct rhombomeric cell clusters. The prominent expression of uts1 and ucn3 in brainstem is consistent with proposed roles of CRH-related peptides in stress-induced modulation of locomotor activity through monoaminergic brainstem neuromodulatory systems.

Neurogenesis

For more than a decade, the zebrafish has proven to be an excellent model organism to investigate the mechanisms of neurogenesis during development. The often cited advantages, namely external development, genetic, and optical accessibility, have permitted direct examination and experimental manipulations of neurogenesis during development. Recent studies have begun to investigate adult neurogenesis, taking advantage of its widespread occurrence in the mature zebrafish brain to investigate the mechanisms underlying neural stem cell maintenance and recruitment. Here we provide a comprehensive overview of the tools and techniques available to study neurogenesis in zebrafish both during development and in adulthood. As useful resources, we provide tables of available molecular markers, transgenic, and mutant lines. We further provide optimized protocols for studying neurogenesis in the adult zebrafish brain, including in situ hybridization, immunohistochemistry, in vivo lipofection and electroporation methods to deliver expression constructs, administration of bromodeoxyuridine (BrdU), and finally slice cultures. These currently available tools have put zebrafish on par with other model organisms used to investigate neurogenesis.

Axonal projections originating from raphe serotonergic neurons in the developing and adult zebrafish, Danio rerio, using transgenics to visualize raphe-specific pet1 expression

Serotonin is a major central nervous modulator of physiology and behavior and plays fundamental roles during development and plasticity of the vertebrate central nervous system (CNS). Understanding the developmental control and functions of serotonergic neurons is therefore an important task. In all vertebrates, prominent serotonergic neurons are found in the superior and inferior raphe nuclei in the hindbrain innervating most CNS regions. In addition, all vertebrates except for mammals harbor other serotonergic centers, including several populations in the diencephalon. This, in combination with the intricate and wide distribution of serotonergic fibers, makes it difficult to sort out serotonergic innervation originating from the raphe from that of other serotonergic cell populations. To resolve this issue, we isolated the regulatory elements of the zebrafish raphe-specific gene pet1 and used them to drive expression of an eGFP transgene in the raphe population of serotonergic neurons. With this approach together with retrograde tracing we 1) describe in detail the development, anatomical organization, and projection pattern of zebrafish pet1-positive neurons compared with their mammalian counterparts, 2) identify a new serotonergic population in the ventrolateral zebrafish hindbrain, and 3) reveal some extent of functional subdivisions within the zebrafish superior raphe complex. Together, our results reveal for the first time the specific innervation pattern of the zebrafish raphe and, thus, provide a new model and various tools to investigate further the role of raphe serotonergic neurons in vertebrates.

Comparative analysis of serotonin receptor (HTR1A/HTR1B families) and transporter (slc6a4a/b) gene expression in the zebrafish brain

In this study we analyze 5-hydroxytryptamine [5-HT]; serotonin) signaling in zebrafish, an increasingly popular vertebrate disease model. We compare and contrast expression of the 5-HT transporter genes slc6a4a and slc6a4b, which identify 5-HT-producing neurons and three novel 5-HT receptors, htr1aa, htr1ab, and htr1bd. slc6a4a and slc6a4b are expressed in the raphe nuclei, retina, medulla oblongata, paraventricular organ, pretectal diencephalic complex, and caudal zone of the periventricular hypothalamus, in line with the expression profiles of homologues from other vertebrates. Our analysis of serotonin transporter (SERT)-encoding genes also identifies parallel genetic pathways used to build the 5-HT system in zebrafish. In cells in which 5-HT is synthesized by tph1, slc6a4b is used for re-uptake, whereas tph2-positive cells utilize slc6a4a. The receptors htr1aa, htr1ab, and htr1bd also show widespread expression in both the larval and adult brain. Receptor expression is seen in the superior raphe nucleus, retina, ventral telencephalon, optic tectum, thalamus, posterior tuberculum, cerebellum, hypothalamus, and reticular formation, thus implicating 5-HT signaling in several neural circuits. We also examine larval brains double-labeled with 5-HTergic and dopaminergic pathway-specific antibodies, to uncover the identity of some 5-HTergic target neurons. Furthermore, comparison of the expression of transporter and receptor genes also allows us to map sites of autoreceptor activity within the brain. We detect autoreceptor activity in the pretectal diencephalic cluster (htr1aa-, htr1ab-, htr1bd-, and slc6a4a-positive), superior raphe nucleus (htr1aa-, htr1ab-, and slc6a4a-positive), paraventricular organ (htr1aa-, htr1ab-, htr1bd-, and slc6a4b-positive), and the caudal zone of the periventricular hypothalamus (htr1ab- and slc6a4b-positive).

Exposure to sodium metam during zebrafish somitogenesis results in early transcriptional indicators of the ensuing neuronal and muscular dysfunction

Exposures to sodium metam (NaM) within the developmental period of somitogenesis (10- to 18-h postfertilization [hpf]) results in easily detectable distortions of the notochord by 24 hpf in the developing zebrafish. We hypothesized that NaM-induced transcriptional changes during somitogenesis would reveal the major molecular targets in the zebrafish embryo. Embryos were exposed to NaM beginning at 4 hpf (1000 cells) and total RNA was isolated from embryos at the 3 somite (11 hpf), 10 somite (14 hpf), 18 somite (18 hpf), and larval (24 hpf) stages of development. Using the Affymetrix zebrafish gene array we observed relatively few mRNAs differentially regulated at least twofold at each time point (11 hpf, 101 genes; 14 hpf, 151; 18 hpf, 154; 24 hpf, 33). The transcriptional profiles reveal neurodevelopment and myogenesis as the two primary targets of NaM developmental exposure. Quantitative PCR of several muscle and neuronal genes confirmed the array response. We also followed the structural development of the peripheral nervous system under NaM exposure using antibodies against neuronal structural proteins. Although there was no change in the onset of antibody staining, profound alterations became apparent during the period in which the notochord becomes distorted (> 18 hpf). Motor neuron development observed with the Tg(NBT:MAPT-GFP)zc1 transgenic zebrafish and a primary motor neuron specific antibody showed similar timing in the structural alterations observed in these cell types. Further study of the interactions of dithiocarbamates with the regulatory elements of fast muscle development and neurodevelopment is warranted.