Onset and time course of apoptosis in the developing zebrafish retina

In mammalian development, apoptosis spreads over the retina in consecutive waves and induces a remarkable amount of cell loss. No evidence for such consecutive waves has been revealed in the fish retina so far. As the zebrafish is of growing importance as a model for retinal development and for degenerative retinal diseases, we examined the onset and time course of apoptosis in the developing zebrafish retina and in adult fish. We found that apoptosis peaked in the ganglion cell layer (GCL) and inner nuclear layer (INL) in early developmental stages (3-4 days post-fertilization; dpf) followed by a second, but clearly smaller wave at 6-7dpf. Apoptosis in the outer nuclear layer (ONL) started at 5dpf and peaked at 7dpf. This late-onset high peak of apoptosis of photoreceptors is different from that of all other species examined to date. With 1.09% of cells in the GCL and 1.10% in the ONL being apoptotic, the rate of apoptosis in the developing zebrafish retina was conspicuously lower than that observed in other vertebrates (up to 50% in GCL). During development (2-21dpf), apoptotic waves were most obvious in the central retina, whereas in the periphery near the marginal zone (MZ), apoptosis was much lower; in adult animals, practically no apoptosis was present in the central retina but it still occurred near the MZ. Our data show that the onset and time course of apoptosis in the GCL and INL of the zebrafish is comparable with other vertebrates; however, the amount of apoptosis is clearly reduced. Thus, apoptosis in the zebrafish retina may serve more as a mechanism for the fine tuning of the retinal neuronal network after mitotic waves during development or in remaining mitotic areas than as a mechanism for eliminating large numbers of excess cells.     

Zebrafish intestinal fatty acid binding protein (I-FABP) gene promoter drives gut-specific expression in stable transgenic fish

Mammalian intestinal fatty acid-binding protein (I-FABP) is a small cytosolic protein and is thought to play a crucial role of intracellular fatty acid trafficking and metabolism in gut. To establish an in vivo system for investigating its tissue-specific regulation during zebrafish intestinal development, we isolated 5'-flanking sequences of the zebrafish L-FABP gene and used a transgenic strategy to generate gut-specific transgenic zebrafish with green/red fluorescent intestine. The 4.5-kb 5'-flanking sequence of zebrafish I-FABP gene was sufficient to direct fluorescent expression in intestinal tube, first observed in 3 dpf embryos and then continuously to the adult stage. This pattern of transgenic expression is consistent with the expression pattern of the endogenous gene. In all five transgenic lines 45-52% of the F2 inheritance rates were consistent with the ratio of Mendelian segregation. These fish can also provide a valuable resource of labeled adult intestinal cells for in vivo or in vitro studies. Finally, it is possible to establish an in vivo system using these fish for screening genes required for gut development. genesis 38:26-31, 2004.     

Expressions of Raldh3 and Raldh4 during zebrafish early development

Retinoic acid (RA) plays crucial roles in vertebrate embryogenesis. Four retinal dehydrogenases (Raldhs) that are responsible for RA synthesis have been characterized in mammals. However, only Raldh2 ortholog is identified in zebrafish. Here, we report the identification of raldh3 and raldh4 genes in zebrafish. The predicted proteins encoded by zebrafish raldh3 and raldh4 exhibit 70.0% and 73.5% amino acid identities with mouse Raldh3 and Raldh4, respectively. RT-PCR analyses reveal that both raldh3 and raldh4 mRNAs are present in early development. However, whole mount in situ hybridization shows that raldh3 mRNA is first expressed in the developing eye region of zebrafish embryos at 10-somite stage. At 24 hpf (hours post fertilization), raldh3 mRNA is expressed in the ventral retina of eye. At 36 hpf, the mRNA is also expressed in otic vesicle in addition to ventral retina, and it maintains its expression pattern till 2 dpf (days post fertilization). At 3 dpf, raldh3 mRNA becomes very weak in ventral retina but is present in otic vesicle at a high level. At 5 dpf and 7 dpf, raldh3 is no longer expressed in eyes but is expressed in otic vesicles at a very low level. raldh4 mRNA is initially detected in developing liver and intestine regions at 2 dpf embryos. Its expression level becomes very high in these two regions of embryos from 3 dpf to 5 dpf. Analysis on the sections of 5 dpf embryos reveals that raldh4 is expressed in the epithelium of intestine. At 7 dpf, raldh4 mRNA is only weakly expressed in the epithelium of intestinal bulb.     

Ontogeny of excitatory and inhibitory control of gastrointestinal motility in the African clawed frog, Xenopus laevis

The transparent body wall of Xenopus laevis larvae during the first developmental stages allows in vivo studies of gastrointestinal tract activity. The purpose of this study was to chart the ontogeny of gut motility in Xenopus larvae and to identify the most important control systems during the first developmental stages. Coordinated descending contraction waves first occurred in the gut at Nieuwkoop and Faber stage 43 [0.8 +/- 0.1 contractions/min (cpm)] and increased to 4.9 +/- 0.1 cpm at stage 47. The cholinergic receptor agonist carbachol (5-10 microM) increased contraction frequency already at stage 43, as did neurokinin A (NKA, 0.3-1 microM). The muscarinic antagonist atropine (100 microM) first affected contraction frequency at stage 45, which coincides with the onset of feeding. The tachykinin antagonist MEN-10,376 (6 microM) blocked NKA-induced contractions but not spontaneous motility. Both sodium nitroprusside [nitric oxide (NO) donor, 1-10 microM] and vasoactive intestinal peptide (VIP, 0.1-1 microM) inhibited contractions from the earliest stage onward. Blocking NO synthesis using NG-nitro-L-arginine methyl ester (100 microM) had no effect per se, but antagonized VIP evoked inhibition at stage 47. We conclude that gastrointestinal motility is well developed in the Xenopus laevis larvae before the onset of feeding. Functional muscarinic and tachykinin receptors are present already at the onset of motility, whereas a cholinergic tone develops around the onset of feeding. No endogenous tachykinin tone was found. Functional VIP receptors mediate inhibition at the onset of motility. NO seems to mediate the VIP effect at later stages.     

Melatonin effects on gut motility are independent of the relaxation mediated by the nitrergic system in the goldfish

Melatonin is a key neuroendocrine transducer in the circadian organization of vertebrates. However, its role in gastrointestinal physiology has not been explored in depth. In goldfish, a role for melatonin as a modulator of intestinal motility has been reported, whereby it attenuates the cholinergic contraction. The aim of the present work was to investigate this relaxation induced by melatonin in the gut smooth muscle of the goldfish, studying the possible involvement of nitric oxide. An in vitro model of isolated goldfish intestine was used to test the effects on intestinal motility. The addition of melatonin (10 pM-100 μM) to the organ bath relaxed acetylcholine- and serotonin-stimulated gut strips, but no effect was observed on KCl-contracted preparations. The addition of L-NAME (nitric oxide synthase inhibitor) increased the amplitude of the spontaneous slow waves, while sodium nitroprusside (SNP, nitric oxide donor) abolished them. All these results support a role for the nitrergic system in goldfish gut motility. However, neither L-NAME, nor SNP nor the nitric oxide precursor, l-arginine, modified the melatonin relaxing effect. These results highlight the existence of a basal nitrergic tone in the gut of goldfish, where melatonin would exert a calcium-dependent, nitric oxide-independent relaxing effect on serotonergic and cholinergic contraction.     

Expression of neuropeptides and anoctamin 1 in the embryonic and adult zebrafish intestine, revealing neuronal subpopulations and ICC-like cells

This immunohistochemical study in zebrafish aims to extend the neurochemical characterization of enteric neuronal subpopulations and to validate a marker for identification of interstitial cells of Cajal (ICC). The expression of neuropeptides and anoctamin 1 (Ano1), a selective ICC marker in mammals, was analyzed in both embryonic and adult intestine. Neuropeptides were present from 3 days postfertilization (dpf). At 3 dpf, galanin-positive nerve fibers were found in the proximal intestine, while calcitonin gene-related peptide (CGRP)- and substance P-expressing fibers appeared in the distal intestine. At 5 dpf, immunoreactive fibers were present along the entire intestinal length, indicating a well-developed peptidergic innervation at the onset of feeding. In the adult intestine, vasoactive intestinal peptide (VIP), pituitary adenylate cyclase-activating peptide (PACAP), galanin, CGRP and substance P were detected in nerve fibers. Colchicine pretreatment enhanced only VIP and PACAP immunoreactivity. VIP and PACAP were coexpressed in enteric neurons. Colocalization stainings revealed three neuronal subpopulations expressing VIP and PACAP: a nitrergic noncholinergic subpopulation, a serotonergic subpopulation and a subpopulation expressing no other markers. Ano1-immunostaining revealed a 3-dimensional network in the adult intestine containing multipolar cells at the myenteric plexus and bipolar cells interspersed between circular smooth muscle cells. Ano1 immunoreactivity first appeared at 3 dpf, indicative of the onset of proliferation of ICC-like cells. It is shown that the Ano1 antiserum is a selective marker of ICC-like cells in the zebrafish intestine. Finally, it is hypothesized that ICC-like cells mediate the spontaneous regular activity of the embryonic intestine.     

Synaptic plasticity and functionality at the cone terminal of the developing zebrafish retina

Previous studies have analyzed photoreceptor development, some inner retina cell types, and specific neurotransmitters in the zebrafish retina. However, only minor attention has been paid to the morphology of the synaptic connection between photoreceptors and second order neurons even though it represents the transition from the light sensitive receptor to the neuronal network of the visual system. Here, we describe the appearance and differentiation of pre- and postsynaptic elements at cone synapses in the developing zebrafish retina together with the maturation of the directly connecting second order neurons and a dopaminergic third order feedback-neuron from the inner retina. Zebrafish larvae were examined at developmental stages from 2 to 7dpf (days postfertilization) and in the adult. Synaptic maturation at the photoreceptor terminals was examined with antibodies against synapse associated proteins. The appearance of synaptic plasticity at the so-called spinule-type synapses between cones and horizontal cells was assessed by electron microscopy, and the maturation of photoreceptor downstream connection was identified by immunocytochemistry for GluR4 (AMPA-type glutamate receptor subunit), protein kinase beta(1) (mixed rod-cone bipolar cells), and tyrosine hydroxylase (dopaminergic interplexiform cells). We found that developing zebrafish retinas possess first synaptic structures at the cone terminal as early as 3.5dpf. Morphological maturation of these synapses at 3.5-4dpf, together with the presence of synapse associated proteins at 2.5dpf and the maturation of second order neurons by 5dpf, indicate functional synaptic connectivity and plasticity between the cones and their second order neurons already at 5dpf. However, the mere number of spinules and ribbons at 7dpf still remains below the adult values, indicating that synaptic functionality of the zebrafish retina is not entirely completed at this stage of development.     

Ontogeny of gonadotropin-releasing hormone (GnRH) neurons in hybrid striped bass Morone sp.: whole-mount in situ hybridization analysis

The ontogeny of gonadotropin-releasing hormone (GnRH) mRNA-producing neurons was studied in developing hybrid striped bass (white bass Morone chrysops female × striped bass Morone saxatilis male), 1–55 days post-fertilization (dpf), by whole-mount in situ hybridization. Neurons that produce salmon (s) GnRH were first detected at 32 h post-fertilization in the olfactory placodes. These neurons migrated posteriorly during development and reached their final position at the olfactory bulbs-telencephalon boundary, possibly the terminal nerve ganglion (TNg) by 11 dpf. First signal of chicken (c) GnRH-II neurons appeared in the midbrain 2 dpf and remained there throughout development. A signal of seabream (sb) GnRH mRNA was first detected at 21 dpf in the preoptic area (POA) and as a bilateral continuum along the ventral telencephalon at 32–55 dpf. The expression of all three forms of GnRH increased throughout development. These results suggest that cGnRH-II neurons originate in the mid-brain, and that sGnRH neurons originate in the olfactory placodes and migrate caudally to the TNg. Neurons that express sbGnRH seem to originate at the preoptic area and then to migrate anteriorly along the ventral telencephalon. An olfactory placodal origin of these neurons, however, cannot be ruled out.     

Calbindin immunoreactivity in the enteric nervous system of larval and adult zebrafish (Danio rerio)

Calbindin is a calcium-binding protein, commonly found in certain subpopulations of the enteric nervous system in mammals. Recently, calbindin-immunoreactive enteric neurons have also been demonstrated in shorthorn sculpin (Myoxocephalus scorpius). In the present study, calbindin immunoreactivity has been investigated in the gut of adult and larval zebrafish (Danio rerio) and differences and similarities between the two species are discussed. Calbindin immunoreactivity is present in 40%?C50% of all enteric neurons in adult zebrafish. It first appears at 3?days post-fertilisation (dpf) and is present in all regions of the gut by 13 dpf. Calbindin-immunoreactive nerve cell bodies do not differ in size from calbindin-negative cells. Zebrafish calbindin-immunoreactive neurons are serotonin-negative, with at least some being choline acetyltransferase (ChAT)-positive, in contrast to the sculpin in which cells are generally smaller than the average enteric neuron and are serotonin-positive and ChAT-negative. These findings further emphasise the importance of comparative studies for understanding the diversity of chemical coding in the enteric nervous system of fish and other vertebrates. Improved knowledge of the role of the enteric nervous system is also essential for future studies of gut activity with regard to zebrafish being used as a model organism.     

The aPKC/Par3/Par6 Polarity Complex and Membrane Order Are Functionally Interdependent in Epithelia During Vertebrate Organogenesis

The differential distribution of lipids between apical and basolateral membranes is necessary for many epithelial cell functions, but how this characteristic membrane organization is integrated within the polarity network during ductal organ development is poorly understood. Here we quantified membrane order in the gut, kidney and liver ductal epithelia in zebrafish larvae at 3–11 days post fertilization (dpf) with Laurdan 2‐photon microscopy. We then applied a combination of Laurdan imaging, antisense knock‐down and analysis of polarity markers to understand the relationship between membrane order and apical‐basal polarity. We found a reciprocal relationship between membrane order and the cell polarity network. Reducing membrane condensation by exogenously added oxysterol or depletion of cholesterol reduced apical targeting of the polarity protein, aPKC. Conversely, using morpholino knock down in zebrafish, we found that membrane order was dependent upon the Crb3 and Par3 polarity protein expression in ductal epithelia. Hence our data suggest that the biophysical property of membrane lipid packing is a regulatory element in apical basal polarity.     

The Role of DNA Methylation Reprogramming During Sex Determination and Transition in Zebrafish

DNA methylation is a prevalent epigenetic modification in vertebrates, and it has been shown to be involved the regulation of gene expression and embryo development. However, it remains unclear how DNA methylation regulates sexual development, especially in species without sex chromosomes. To determine this, we utilized zebrafish to investigate DNA methylation reprogramming during juvenile germ cell development and adult female-to-male sex transition.We reveal that primordial germ cells(PGCs) undergo significant DNA methylation reprogramming during germ cell development, and the methylome of PGCs is reset to an oocyte/ovary-like pattern at 9 days post fertilization(9 dpf). When DNA methyltransferase(DNMT) activity in juveniles was blocked after 9 dpf, the zebrafish developed into females. We also show that Tet3 is involved in PGC development. Notably, we find that DNA methylome reprogramming during adult zebrafish sex transition is similar to the reprogramming during the sex differentiation from 9 dpf PGCs to sperm. Furthermore, inhibiting DNMT activity can prevent the female-to-male sex transition, suggesting that methylation reprogramming is required for zebrafish sex transition. In summary, DNA methylation plays important roles in zebrafish germ cell development and sexual plasticity.     

Modeling human muscle disease in zebrafish

Zebrafish reproduce in large quantities, grow rapidly, and are transparent early in development. For these reasons, zebrafish have been used extensively to model vertebrate development and disease. Like mammals, zebrafish express dystrophin and many of its associated proteins early in development and these proteins have been shown to be vital for zebrafish muscle stability. In dystrophin-null zebrafish, muscle degeneration becomes apparent as early as 3 days post-fertilization (dpf) making the zebrafish an excellent organism for large-scale screens to identify other genes involved in the disease process or drugs capable of correcting the disease phenotype. Being transparent, developing zebrafish are also an ideal experimental model for monitoring the fate of labeled transplanted cells. Although zebrafish dystrophy models are not meant to replace existing mammalian models of disease, experiments requiring large numbers of animals may be best performed in zebrafish. Results garnered from using this model could lead to a better understanding of the pathogenesis of the muscular dystrophies and the development of future therapies.     

Cloning and expression of zebrafish neuronal nicotinic acetylcholine receptors

We propose to use the zebrafish (Danio rerio) as a vertebrate model to study the role of neuronal nicotinic acetylcholine receptors (nAChR) in development. As a first step toward using zebrafish as a model, we cloned three zebrafish cDNAs with a high degree of sequence similarity to nAChR beta3, alpha2 and alpha7 subunits expressed in other species. RT-PCR was used to show that the beta3 and alpha2 subunit RNAs were present in zebrafish embryos only 2-5hours post-fertilization (hpf) while alpha7 subunit RNA was not detected until 8hpf, supporting the differential regulation of nAChRs during development. In situ hybridization was used to localize zebrafish beta3, alpha2, and alpha7 RNA expression. nAChR binding techniques were used to detect the early expression of two high-affinity [3H]-epibatidine binding sites in 2 days post-fertilization (dpf) zebrafish embryos with IC(50) values of 28.6pM and 29.7nM and in 5dpf embryos with IC(50) values of 28.4pM and 8.9nM. These studies are consistent with the involvement of neuronal nAChRs in early zebrafish development.     

Mortality Caused by Bath Exposure of Zebrafish (Danio rerio) Larvae to Nervous Necrosis Virus Is Limited to the Fourth Day Postfertilization

Nervous necrosis virus (NNV) is a member of the Betanodavirus genus that causes fatal diseases in over 40 species of fish worldwide. Mortality among NNV-infected fish larvae is almost 100%. In order to elucidate the mechanisms responsible for the susceptibility of fish larvae to NNV, we exposed zebrafish larvae to NNV by bath immersion at 2, 4, 6, and 8 days postfertilization (dpf). Here, we demonstrate that developing zebrafish embryos are resistant to NNV at 2 dpf due to the protection afforded by the egg chorion and, to a lesser extent, by the perivitelline fluid. The zebrafish larvae succumbed to NNV infection during a narrow time window around the 4th dpf, while 6- and 8-day-old larvae were much less sensitive, with mortalities of 24% and 28%, respectively.     

氯丙嗪在斑马鱼胚胎和早期幼鱼发育过程中的神经毒性作用

目的 采用模式动物斑马鱼作为研究对象,观察氯丙嗪（chlorpromazine,CPZ）暴露对胚胎和幼鱼早期神经发育的影响.方法 在一般毒性评价的基础上,通过整体胚胎细胞凋亡检测和脑组织病理学检查,了解CPZ对神经发育的器质性改变;采用神经行为学方法,包括幼鱼触动逃避反应、自发运动以及惊恐逃避反射等,研究氯丙嗪暴露所致的神经发育功能性障碍.结果斑马鱼胚胎受精后6 h（6 hpf）～72 hpf暴露于CPZ（≥5 mg/L）可引起胚胎和幼鱼死亡、致畸和幼鱼孵化延迟,并呈浓度和时间依赖性;采用吖啶橙染色检测36 hpf整体胚胎凋亡细胞,发现凋亡细胞主要集中在胚胎中脑、后脑、丘脑以及中后脑连接区、脊索和尾部等处;脑组织病理学检测发现,7dpf幼鱼颅腔增大、脑体积减小、脑细胞缩小且细胞间隙增宽.6～72 hpf CPZ（≥0.0625 mg/L）暴露后,幼鱼神经行为学研究发现,CPZ（≥0.125 mg/L）可引起3dpf幼鱼触觉运动能力下降;CPZ（≥0 5 mg/L）可浓度依赖性地抑制幼鱼自发运动,并出现僵直不动、震颤或快速刻板式转圈运动等行为改变;光惊恐实验中,暗环境下各暴露组幼鱼对突发强光刺激均表现为惊跳逃避,并且暗-光交替期运动加速度变化与对照组无显著差异;在撤除光源后,1mg/L和2 mg/L暴露组幼鱼暗适应时程缩短,而0.125 mg/L和0.25 mg/L暴露组暗适应时程延长,提示CPZ对外界刺激引发的幼鱼活跃游动有抑制和促进双重毒性作用.结论 CPZ暴露对斑马鱼胚胎和幼鱼具有明显的神经发育毒性作用.模式动物斑马鱼作为一种高通量筛选模型在外源性化合物神经发育毒性评价中具有较好的应用前景.     

A novel transgenic zebrafish model for blood-brain and blood-retinal barrier development

Background

Development and maintenance of the blood-brain and blood-retinal barrier is critical for the homeostasis of brain and retinal tissue. Despite decades of research our knowledge of the formation and maintenance of the blood-brain (BBB) and blood-retinal (BRB) barrier is very limited. We have established an in vivo model to study the development and maintenance of these barriers by generating a transgenic zebrafish line that expresses a vitamin D-binding protein fused with enhanced green fluorescent protein (DBP-EGFP) in blood plasma, as an endogenous tracer.

Results

The temporal establishment of the BBB and BRB was examined using this transgenic line and the results were compared with that obtained by injection of fluorescent dyes into the sinus venosus of embryos at various stages of development. We also examined the expression of claudin-5, a component of tight junctions during the first 4 days of development. We observed that the BBB of zebrafish starts to develop by 3 dpf, with expression of claudin-5 in the central arteries preceding it at 2 dpf. The hyaloid vasculature in the zebrafish retina develops a barrier function at 3 dpf, which endows the zebrafish with unique advantages for studying the BRB.

Conclusion

Zebrafish embryos develop BBB and BRB function simultaneously by 3 dpf, which is regulated by tight junction proteins. The Tg(l-fabp:DBP-EGFP) zebrafish will have great advantages in studying development and maintenance of the blood-neural barrier, which is a new application for the widely used vertebrate model.     

Analysis of spatiotemporal pattern and quantification of gastrointestinal slow waves caused by anticholinergic drugs

Anticholinergic drugs are well-known to cause adverse effects, such as constipation, but their effects on baseline contractile activity in the gut driven by slow waves is not well established. In a video-based gastrointestinal motility monitoring (GIMM) system, a mouse's small intestine was placed in Krebs solution and recorded using a high definition camera. Untreated controls were recorded for each specimen, then treated with a therapeutic concentration of the drug, and finally, treated with a supratherapeutic dose of the drug. Next, the video clips showing gastrointestinal motility were processed, giving us the segmentation motions of the intestine, which were then converted via Fast Fourier Transform (FFT) into their respective frequency spectrums. These contraction quantifications were analyzed from the video recordings under standardised conditions to evaluate the effect of drugs. Six experimental trials were included with benztropine and promethazine treatments. Only the supratherapeutic dose of benztropine was shown to significantly decrease the amplitude of contractions; at therapeutic doses of both drugs, neither frequency nor amplitude was significantly affected. We have demonstrated that intestinal slow waves can be analyzed based on the colonic frequency or amplitude at a supratherapeutic dose of the anticholinergic medications. More research is required on the effects of anticholinergic drugs on these slow waves to ascertain the true role of ICC in neurologic control of gastrointestinal motility.     

肌间刺缺失对斑马鱼骨骼发育的影响

利用斑马鱼(Danio rerio)野生型与肌间刺完全缺失突变型个体,从骨骼染色和骨骼发育相关基因表达两方面,初步评价了肌间刺缺失对斑马鱼骨骼发育的影响。通过骨骼染色对比观察了两种肌间刺表型个体受精后8dpf(days post fertilization, dpf)到56dpf的骨骼发育情况,结果显示,两种肌间刺表型除肌间刺外,其他骨骼发育基本同步。此外,通过qRT-PCR实验检测分析了6个骨骼发育相关基因(bmp2a、bmp4、smad1、smad4a、runx2a和sp7)在不同肌间刺表型5个胚胎发育时期(3hpf囊胚期、6hpf原肠胚期、12hpf体节期、24hpf咽囊期和72hpf孵化期)和5个胚后生长阶段(15、30、45、60和75dpf)的表达情况。结果显示:在胚胎发育时期,野生型和突变型个体中bmp2a、bmp4、smad1、smad4a基因和突变型个体中sp7基因的表达均呈现先升后降的变化趋势,且在体节期达到最高表达水平;野生型和突变型个体中runx2a基因和野生型个体中sp7基因则表现为逐渐上升的趋势。6个基因在囊胚期和原肠胚期表达量无显著差异, bmp2a的表达...     

Steps during the development of the zebrafish locomotor network.

This review summarizes recent data from our lab concerning the development of motor activities in the developing zebrafish. The zebrafish is a leading model for studies of vertebrate development because one can obtain a large number of transparent, externally and rapidly developing embryos with motor behaviors that are easy to assess (e.g. for mutagenic screens). The emergence of embryonic motility was studied behaviorally and at the cellular level. The embryonic behaviors appear sequentially and include an early, transient period of spontaneous, alternating tail coilings, followed by responses to touch, and swimming. Patch clamp recording in vivo revealed that an electrically coupled network of a subset of spinal neurons generates spontaneous tail coiling, whereas a chemical (glutamatergic and glycinergic) synaptic drive underlies touch responses and swimming and requires input from the hindbrain. Swimming becomes sustained in larvae once serotonergic neuromodulatory effects are integrated. We end with a brief overview of the genetic tools available for the study of the molecular determinants implicated in locomotor network development in the zebrafish. Combining genetic, behavioral and cellular experimental approaches will advance our understanding of the general principles of locomotor network assembly and function.