A hybrid electrical/chemical circuit in the spinal cord generates a transient embryonic motor behavior

Mechanisms of surface groups regulating developmental toxicity of graphene-based nanomaterials via glycerophospholipid metabolic pathway

Surface modification of graphene-based nanomaterials (GBNs) may occur in aquatic environment and during intentional preparation. However, the influence of the surface groups on the developmental toxicity of GBNs has not been determined. In this study, we evaluated the developmental toxicity of three GBNs including GO (graphene oxide), RGO (reduced GO) and RGO-N (aminated RGO) by employing zebrafish embryos at environmentally relevant concentrations (1-100 μg/L), and the underlying metabolic mechanisms were explored. The results showed that both GO and RGO-N disturbed the development of zebrafish embryos, and the adverse effect of GO was greater than that of RGO-N. Furthermore, the oxygen-containing groups of GBNs play a more important role in inducing developmental toxicity compared to size, defects and nitrogen-containing groups. Specifically, the epoxide and hydroxyl groups of GBNs increased their intrinsic oxidative potential, promoted the generation of ROS, and caused lipid peroxidation. Moreover, a significant decrease in guanosine and abnormal metabolism of multiple glycerophospholipids were observed in all three GBN-treated groups. Nevertheless, GO exposure triggered more metabolic activities related to lipid peroxidation than RGO or RGO-N exposure, and the disturbance intensity of the same metabolite was greater than that of the other two agents. These findings reveal underlying metabolic mechanisms of GBN-induced developmental toxicity.

Design of mechanosensory feedback during undulatory locomotion to enhance speed and stability

Undulatory locomotion relies on the propagation of a wave of excitation in the spinal cord leading to consequential activation of segmental skeletal muscles along the body. Although this process relies on self-generated oscillations of motor circuits in the spinal cord, mechanosensory feedback is crucial to entrain the underlying oscillatory activity and thereby, to enhance movement power and speed. This effect is achieved through directional projections of mechanosensory neurons either sensing stretching or compression of the trunk along the rostrocaudal axis. Different mechanosensory feedback pathways act in concert to shorten and fasten the excitatory wave propagating along the body. While inhibitory mechanosensory cells feedback inhibition on excitatory premotor interneurons and motor neurons, excitatory mechanosensory cells feedforward excitation to premotor excitatory interneurons. Together, diverse mechanosensory cells coordinate the activity of skeletal muscles controlling the head and tail to optimize speed and stabilize balance during fast locomotion.

Integrating morphological, biochemical, behavioural, and molecular approaches to investigate developmental toxicity triggered by tebuthiuron in zebrafish (Danio rerio)

Tebuthiuron (TBU), a phenylurea herbicide, is widely applied in agricultural and non-agricultural soils. Because TBU resists degradation, it can contaminate water and reach the biota once it is released into the environment. However, the potential toxic effects of TBU on aquatic developing organisms have been poorly studied. By taking advantage of the early-life stages of zebrafish (Danio rerio), we have combined morphological, biochemical, behavioural, and molecular approaches to investigate the developmental toxicity triggered by environmentally relevant concentrations (from 0.1 to 1000 μg/L) of TBU. Exposure to TBU did not elicit morphological abnormalities but it significantly delayed hatching. In addition, TBU altered the frequency of tail coils in one-day post-fertilization (dpf) old embryos. Moreover, TBU exposure during four days significantly inhibited the whole body AChE activity of larvae. At the molecular level, TBU did not significantly affect the mRNA levels of four genes (elavl3, gfap, gap43, and shha) that play key roles during the neurodevelopment of zebrafish. By assessing the motor responses to repeated light-dark stimuli, 6 dpf larvae exposed to TBU displayed hyperactivity, showing greater travelling distance during the dark periods. Our categorization of swimming speed revealed an interesting finding - after the light was turned off, the exposed larvae abandoned the freezing mode (<2 mm/s) and travelled mainly at cruising speed (2-20 mm/s), showing that the larval hyperactivity did not translate into higher swimming velocity. Overall, our results offer new insights into the TBU toxicity to developing organisms, namely effects in AChE activity and hyperactivity, providing support data for future studies considering environmental risk assessment of this herbicide.

Permissible concentration of mancozeb in Brazilian drinking water elicits oxidative stress and bioenergetic impairments in embryonic zebrafish

Mancozeb (MZ) is widely used as a fungicide in Brazil due to its effectiveness in combating fungal infections in plantations. However, its toxicity to non-target organisms, including aquatic organisms, has been reported in the literature. Recently, Brazilian legislation was updated to allow a concentration of 8 μg/L of MZ in drinking water (Ordinance GM/MS nº 888, of May 4, 2021). However, the safety of this concentration for aquatic organisms has not yet been put to the test. To address this gap, we conducted a study using zebrafish (Danio rerio) embryos at 4 hpf exposed to MZ at the concentration allowed by law, as well as slightly higher sublethal concentrations (24, 72, and 180 μg/L), alongside a control group. We evaluated various morphophysiological markers of toxicity, including survival, spontaneous movements, heart rate, hatching rate, body axis distortion, total body length, total yolk sac area, and total eye area. Additionally, we measured biochemical biomarkers such as reactive oxygen species (ROS) levels, lipid peroxidation, non-protein thiols (NPSH), and mitochondrial bioenergetic parameters. Our results showed that the concentration of 8 μg/L, currently permitted in drinking water according to Brazilian legislation, increased ROS production levels and caused alterations in mitochondrial physiology. Among the markers assessed, mitochondrial bioenergetic function appeared to be the most sensitive indicator of MZ embryotoxicity, as a decrease in complex I activity was observed at concentrations of 8 and 180 μg/L. Furthermore, concentrations higher than 8 μg/L impaired morphophysiological markers. Based on these findings, we can infer that the concentration of MZ allowed in drinking water by Brazilian environmental legislation is not safe for aquatic organisms. Our study provides evidence that this fungicide is a potent embryotoxic agent, highlighting the potential risks associated with its exposure.

Cdkl5 mutant zebrafish shows skeletal and neuronal alterations mimicking human CDKL5 deficiency disorder

CDKL5 deficiency disorder (CDD) is a rare neurodevelopmental condition characterized primarily by seizures and impairment of cognitive and motor skills. Additional phenotypes include microcephaly, dysmorphic facial features, and scoliosis. Mutations in cyclin-dependent kinase-like 5 (CDKL5) gene, encoding a kinase essential for normal brain development and function, are responsible for CDD. Zebrafish is an accepted biomedical model for the study of several genetic diseases and has many advantages over other models. Therefore, this work aimed to characterize the phenotypic, behavioral, and molecular consequences of the Cdkl5 protein disruption in a cdkl5 mutant zebrafish line (sa21938). cdkl5sa21938 mutants displayed a reduced head size, suggesting microcephaly, a feature frequently observed in CDD individuals. Double staining revealed shorter craniofacial cartilage structures and decrease bone mineralization in cdkl5 homozygous zebrafish indicating an abnormal craniofacial cartilage development and impaired skeletal development. Motor behavior analysis showed that cdkl5sa21938 embryos had less frequency of double coiling suggesting impaired glutamatergic neurotransmission. Locomotor behavior analysis revealed that homozygous embryos swim shorter distances, indicative of impaired motor activity which is one of the main traits of CCD. Although no apparent spontaneous seizures were observed in these models, upon treatment with pentylenetetrazole, seizure behavior and an increase in the distance travelled were observed. Quantitative PCR showed that neuronal markers, including glutamatergic genes were dysregulated in cdkl5sa21938 mutant embryos. In conclusion, homozygous cdkl5sa21938 zebrafish mimic several characteristics of CDD, thus validating them as a suitable animal model to better understand the physiopathology of this disorder.

Modeling spinal locomotor circuits for movements in developing zebrafish

Many spinal circuits dedicated to locomotor control have been identified in the developing zebrafish. How these circuits operate together to generate the various swimming movements during development remains to be clarified. In this study, we iteratively built models of developing zebrafish spinal circuits coupled to simplified musculoskeletal models that reproduce coiling and swimming movements. The neurons of the models were based upon morphologically or genetically identified populations in the developing zebrafish spinal cord. We simulated intact spinal circuits as well as circuits with silenced neurons or altered synaptic transmission to better understand the role of specific spinal neurons. Analysis of firing patterns and phase relationships helped to identify possible mechanisms underlying the locomotor movements of developing zebrafish. Notably, our simulations demonstrated how the site and the operation of rhythm generation could transition between coiling and swimming. The simulations also underlined the importance of contralateral excitation to multiple tail beats. They allowed us to estimate the sensitivity of spinal locomotor networks to motor command amplitude, synaptic weights, length of ascending and descending axons, and firing behavior. These models will serve as valuable tools to test and further understand the operation of spinal circuits for locomotion.

The Temporal Mechanisms Guiding Interneuron Differentiation in the Spinal Cord

Neurogenesis timing is an essential developmental mechanism for neuronal diversity and organization throughout the central nervous system. In the mouse spinal cord, growing evidence is beginning to reveal that neurogenesis timing acts in tandem with spatial molecular controls to diversify molecularly and functionally distinct post-mitotic interneuron subpopulations. Particularly, in some cases, this temporal ordering of interneuron differentiation has been shown to instruct specific sensorimotor circuit wirings. In zebrafish, in vivo preparations have revealed that sequential neurogenesis waves of interneurons and motor neurons form speed-dependent locomotor circuits throughout the spinal cord and brainstem. In the present review, we discuss temporal principals of interneuron diversity taken from both mouse and zebrafish systems highlighting how each can lend illuminating insights to the other. Moving forward, it is important to combine the collective knowledge from different systems to eventually understand how temporally regulated subpopulation function differentially across speed- and/or state-dependent sensorimotor movement tasks.

Ecotoxicity evaluation of polymeric nanoparticles loaded with ascorbic acid for fish nutrition in aquaculture

Background

Ascorbic acid (AA) is a micronutrient essential for the mechanisms of reproduction, growth, and defense in fish. However, the biosynthesis of this micronutrient does not occur in fish, so it must be supplied with food. A difficulty is that plain AA is unstable, due to the effects of light, high temperature, and oxygen, among others. The use of nanoencapsulation may provide protection and preserve the physicochemical characteristics of AA for extended periods of time, decreasing losses due to environmental factors.

Method

This study evaluated the protective effect of nanoencapsulation in polymeric nanoparticles (chitosan and polycaprolactone) against AA degradation. Evaluation was made of the physicochemical stability of the nanoformulations over time, as well as the toxicological effects in zebrafish (Danio rerio), considering behavior, development, and enzymatic activity. For the statistical tests, ANOVA (two-way, significance of p < 0.05) was used.

Results

Both nanoparticle formulations showed high encapsulation efficiency and good physicochemical stability during 90 days. Chitosan (CS) and polycaprolactone (PCL) nanoparticles loaded with AA had mean diameters of 314 and 303 nm and polydispersity indexes of 0.36 and 0.28, respectively. Both nanosystems provided protection against degradation of AA exposed to an oxidizing agent, compared to plain AA. Total degradation of AA was observed after 7, 20, and 480 min for plain AA, the CS nanoparticle formulation, and the PCL nanoparticle formulation, respectively. For zebrafish larvae, the LC₅₀ values were 330.7, 57.4, and 179.6 mg/L for plain AA, the CS nanoparticle formulation, and the PCL nanoparticle formulation, respectively. In toxicity assays using AA at a concentration of 50 mg/L, both types of nanoparticles loaded with AA showed lower toxicity towards the development of the zebrafish, compared to plain AA at the same concentration. Although decreased activity of the enzyme acetylcholinesterase (AChE) did not affect the swimming behavior of zebrafish larvae in the groups evaluated, it may have been associated with the observed morphometric changes, such as curvature of the tail.

Conclusions

This study showed that the use of nanosystems is promising for fish nutritional supplementation in aquaculture. In particular, PCL nanoparticles loaded with AA seemed to be most promising, due to higher protection against AA degradation, as well as lower toxicity to zebrafish, compared to the chitosan nanoparticles. The use of nanotechnology opens new perspectives for aquaculture, enabling the reduction of feed nutrient losses, leading to faster fish growth and improved sustainability of this activity.

Graphic Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-021-00910-8.

The Downregulation of c19orf12 Negatively Affects Neuronal and Musculature Development in Zebrafish Embryos

Mitochondrial membrane Protein Associated Neurodegeneration (MPAN) is a rare genetic disorder due to mutations in C19orf12 gene. In most cases, the disorder is transmitted as an autosomal recessive trait and the main clinical features are progressive spastic para/tetraparesis, dystonia, motor axonal neuropathy, parkinsonisms, psychiatric symptoms, and optic atrophy. Besides iron accumulation in the globus pallidus and substantia nigra, the neuropathology shows features also observed in Parkinson’s Disease brains, such as α-synuclein-positive Lewy bodies and hyperphosphorylated tau. Mutations in the gene have been found in other neurodegenerative disorders, including PD, hereditary spastic paraplegia, pallido-pyramidal syndrome, and amyotrophic lateral sclerosis. The biological function of C19orf12 gene is poorly defined. In humans, it codes for two protein isoforms: the longer one is present in mitochondria, endoplasmic reticulum, and contact regions between mitochondria and ER. Mutations in the gene appear to be linked to defects in mitochondrial activity, lipid metabolism and autophagy/mitophagy. To increase the available tools for the investigation of MPAN pathogenesis, we generated a new animal model in zebrafish embryos. The zebrafish genome contains four co-orthologs of human C19orf12. One of them, located on chromosome 18, is expressed at higher levels at early stages of development. We downregulated its expression by microinjecting embryos with a specific ATG-blocking morpholino, and we analyzed embryonal development. Most embryos showed morphological defects such as unsettled brain morphology, with smaller head and eyes, reduced yolk extension, tilted and thinner tail. The severity of the defects progressively increased and all injected embryos died within 7 days post fertilization. Appropriate controls confirmed the specificity of the observed phenotype. Changes in the expression and distribution of neural markers documented a defective neuronal development, particularly evident in the eyes, the optic tectum, the midbrain-hindbrain boundary; Rohon Beard and dorsal root ganglia neurons were also affected. Phalloidin staining evidenced a significant perturbation of musculature formation that was associated with defective locomotor behavior. These data are consistent with the clinical features of MPAN and support the validity of the model to investigate the pathogenesis of the disease and evaluate molecules with potential therapeutic effect.

Behavioral changes occur earlier than redox alterations in developing zebrafish exposed to Mancozeb

As agriculture expands to provide food and wellbeing to the world's growing population, there is a simultaneous increasing concern about the use of agrochemicals, which can harm non-target organisms, mainly in the aquatic environment. The fungicide Mancozeb (MZ) has been used on a large-scale and is a potent inducer of oxidative stress. Therefore, there is an urgent need for the development of more sensitive biomarkers designed to earlier biomonitoring of this compound. Here we tested the hypothesis that behavioral changes induced by sublethal MZ concentrations would occur first as compared to biochemical oxidative stress markers. Embryos at 4 h post-fertilization (hpf) were exposed to Mancozeb at 5, 10 and 20 μg/L. Controls were kept in embryo water only. Behavioral and biochemical parameters were evaluated at 24, 28, 72, and 168 hpf after MZ exposure. The results showed that MZ significantly altered spontaneous movement, escape responses, swimming capacity, and exploratory behavior at all exposure times. However, changes in ROS steady-stead levels and the activity of antioxidant enzymes were observable only at 72 and 168 hpf. In conclusion, behavioral changes occurred earlier than biochemical alterations in zebrafish embryos exposed to MZ, highlighting the potential of behavioral biomarkers as sensitive tools for biomonitoring programs.

Electromagnetized-Nanoparticle-Modulated Neural Plasticity and Recovery of Degenerative Dopaminergic Neurons in the Mid-Brain

The degeneration of dopaminergic neurons is a major contributor to the pathogenesis of mid-brain disorders. Clinically, cell therapeutic solutions, by increasing the neurotransmitter dopamine levels in the patients, are hindered by low efficiency and/or side effects. Here, a strategy using electromagnetized nanoparticles to modulate neural plasticity and recover degenerative dopamine neurons in vivo is reported. Remarkably, electromagnetic fields generated by the nanoparticles under ultrasound stimulation modulate intracellular calcium signaling to influence synaptic plasticity and control neural behavior. Dopaminergic neuronal functions are reversed by upregulating the expression tyrosine hydroxylase, thus resulting in ameliorating the neural behavioral disorders in zebrafish. This wireless tool can serve as a viable and safe strategy for the regenerative therapy of the neurodegenerative disorders.

Exposure to low doses of inorganic arsenic induces transgenerational changes on behavioral and epigenetic markers in zebrafish (Danio rerio)

The ability of environmental pollutants to alter the epigenome with resultant development of behavioral alterations has received more attention in recent years. These alterations can be transmitted and affect later generations that have not been directly in contact with the contaminant. Arsenic (As) is a neurotoxicant and potent epigenetic disruptor that is widespread in the environment; however, the precise potential of As to produce transgenerational effects is unknown. Our study focused on the possible transgenerational effects on behavior by ancestral exposure to doses relevant to the environment of As, and the epigenetic mechanisms that could be involved. Embryos of F0 (ancestral generation) were directly exposed to 50 or 500 ppb of As for 150 days. F0 adults were raised to produce the F1 generation (intergeneration) and subsequently the F2 generation (transgeneration). We evaluated motor and cognitive behavior, neurodevelopment-related genes, and epigenetic markers on the F0 and F2 generation. As proposed in our hypothesis, ancestral arsenic exposure altered motor activity through the development and increased anxiety-like behaviors which were transmitted to the F2 generation. Additionally, we found a reduction in brain-derived neurotrophic factor expression between the F0 and F2 generation, and an increase in methylation on histone H3K4me3 in the nervous system.

Spatiotemporal Transition in the Role of Synaptic Inhibition to the Tail Beat Rhythm of Developing Larval Zebrafish

Significant maturation of swimming in zebrafish (Danio rerio) occurs within the first few days of life when fish transition from coiling movements to burst swimming and then to beat-and-glide swimming. This maturation occurs against a backdrop of numerous developmental changes - neurogenesis, a transition from predominantly electrical to chemical-based neurotransmission, and refinement of intrinsic properties. There is evidence that spinal locomotor circuits undergo fundamental changes as the zebrafish transitions from burst to beat-and-glide swimming. Our electrophysiological recordings confirm that the operation of spinal locomotor circuits becomes increasingly reliant on glycinergic neurotransmission for rhythmogenesis governing the rhythm of tail beats. This transition occurred at the same time that we observed a change in rhythmicity of synaptic inhibition to spinal motoneurons (MNs). When we examined whether the transition from weakly to strongly glycinergic dependent rhythmogenesis occurred at a uniform pace across the length of the spinal cord, we found that this transition occurred earlier at caudal segments than at rostral segments of the spinal cord. Furthermore, while this rhythmogenic transition occurred when fish transition from burst swimming to beat-and-glide swimming, these two transitions were not interdependent. These results suggest that there is a developmental transition in the operation of spinal locomotor circuits that is gradually set in place in the spinal cord in a caudo-rostral temporal sequence.

Analysis of tail coiling activity of zebrafish (Danio rerio) embryos allows for the differentiation of neurotoxicants with different modes of action

In (eco)toxicology, there is a critical need for efficient methods to evaluate the neurotoxic potential of environmental chemicals. Recent studies proposed analysis of early coiling activity in zebrafish embryos as a powerful tool for the identification of neurotoxic compounds. In order to demonstrate that the analysis of early tail movements of zebrafish embryos allows for the discrimination of neurotoxicants acting via different mechanisms, the present study investigated the effects of four different neurotoxicants on the embryogenesis (fish embryo toxicity test) and early tail coiling movements of zebrafish embryos. Cadmium predominantly increased the frequency of tail coiling at the late pharyngula stage. Dichlorvos delayed embryonic development and caused convulsive tail movements resulting in prolonged duration of tail coils. Embryos exposed to teratogenic concentrations of fluoxetine and citalopram displayed absence of spontaneous tail movements at 24 h post-fertilization. In contrast, a non-teratogenic test concentration of citalopram decreased coiling frequency at multiple time points. Results demonstrated that the analysis of tail coiling movements of zebrafish embryos has the potential to discriminate neurotoxic compounds with different primary modes of action. In addition, chemical-induced effects on coiling activity were shown to potentially overlap with effects on embryogenesis. Further studies are needed to clarify the interplay of unspecific developmental toxicity of neurotoxic chemicals and effects resulting from specific neurotoxic mechanisms.

Time-course of coiling activity in zebrafish (Danio rerio) embryos exposed to ethanol as an endpoint for developmental neurotoxicity (DNT) - Hidden potential and underestimated challenges

Detection of developmental neurotoxicity (DNT) has been recognized as a major challenge by regulatory bodies and science. In search of sensitive and specific test methods, spontaneous tail coiling of embryonic zebrafish has been recommended as a promising tool for identification of DNT-inducing chemicals. The present study was designed to develop a protocol for a prolonged test to study neurotoxicity during the entire development of coiling movement in zebrafish embryos. Ambient illumination was found to modulate coiling activity from the very onset of tail movements representing the earliest behavioral response to light possible in zebrafish. In the dark, embryos displayed increased coiling activity in a way known from photokinesis, a stereotypical element of the visual motor response. Elevated coiling activity during dark phases allows for the development of test strategies that integrate later coiling movements under the control of a further developed nervous system. Furthermore, zebrafish embryos were exposed to ethanol, and coiling activity was analyzed according to the new test protocol. Exposure of embryos to non-teratogenic concentrations of ethanol (0.4-1%) resulted in a delay of the onset of coiling activity and heartbeat. Moreover, ethanol produced a dose-dependent increase in coiling frequency at 26 h post-fertilization, indicating the involvement of neurotoxic mechanisms. Analysis of coiling activity during prolonged exposure allowed for (1) attributing effects on coiling activity to different mechanisms and (2) preventing false interpretation of results. Further research is needed to verify the potential of this test protocol to distinguish between different mechanisms of neurotoxicity.

Effects of acetochlor on neurogenesis and behaviour in zebrafish at early developmental stages

The herbicide acetochlor is used in most parts of the world and is frequently detected in agricultural land and surface water; however, knowledge on the neurotoxicity of acetochlor is limited. Here, to test the effects of acetochlor on zebrafish development and behaviour, zebrafish embryos were exposed to acetochlor from 6 h post-fertilization (hpf) to 24 hpf, and larvae at 6 days post-fertilization (dpf) were exposed to acetochlor for 24 h. Both were exposed to 5, 10, or 20 mg/L acetochlor. We found that acetochlor induced developmental abnormalities, locomotion variations and changes in the physiology and gene expression in the embryos and larvae. The abnormalities included spinal curvature, brain abnormalities, and the decreased formation of newborn neurons. Larval locomotion was decreased with increases in the absolute turn angle and sinuosity. Acetylcholinesterase activity reduced in both embryos and larvae, and the expression of genes that are involved in neurodevelopment and the neurotransmitter system altered. Acetochlor increased the production of ROS and the accumulation of MDA but decreased CAT activity in the embryonic brain. Additionally, acetochlor induced cell death in the brain and tail spinal cord, and the expression of the apoptosis-related genes Bcl2 and caspase 3 were significantly upregulated. Collectively, this is the first study to examine the molecular and physiological effects of acetochlor on neuronal development, and the potential mechanisms appear to be associated with oxidative stress and decreased AChE activity, which disrupt the expression of nervous system genes and apoptosis-related genes and finally lead to apoptosis and morphological malformations.

Development of vestibular behaviors in zebrafish

Most animals orient their bodies with respect to gravity to facilitate locomotion and perception. The neural circuits responsible for these orienting movements have long served as a model to address fundamental questions in systems neuroscience. Though postural control is vital, we know little about development of either balance reflexes or the neural circuitry that produces them. Recent work in a genetically and optically accessible vertebrate, the larval zebrafish, has begun to reveal the mechanisms by which such vestibular behaviors and circuits come to function. Here we highlight recent work that leverages the particular advantages of the larval zebrafish to illuminate mechanisms of postural development, the role of sensation for balance circuit development, and the organization of developing vestibular circuits. Further, we frame open questions regarding the developmental mechanisms for functional circuit assembly and maturation where studying the zebrafish vestibular system is likely to open new frontiers.

TPC2-mediated Ca2+ signaling is required for the establishment of synchronized activity in developing zebrafish primary motor neurons

During the development of the early spinal circuitry in zebrafish, spontaneous Ca²⁺ transients in the primary motor neurons (PMNs) are reported to transform from being slow and uncorrelated, to being rapid, synchronized and patterned. In this study, we demonstrated that in intact zebrafish, Ca²⁺ release via two-pore channel type 2 (TPC2) from acidic stores/endolysosomes is required for the establishment of synchronized activity in the PMNs. Using the SAIGFF213A;UAS:GCaMP7a double-transgenic zebrafish line, Ca²⁺ transients were visualized in the caudal PMNs (CaPs). TPC2 inhibition via molecular, genetic or pharmacological means attenuated the CaP Ca²⁺ transients, and decreased the normal ipsilateral correlation and contralateral anti-correlation, indicating a disruption in normal spinal circuitry maturation. Furthermore, treatment with MS-222 resulted in a complete (but reversible) inhibition of the CaP Ca²⁺ transients, as well as a significant decrease in the concentration of the Ca²⁺ mobilizing messenger, nicotinic acid adenine diphosphate (NAADP) in whole embryo extract. Together, our new data suggest a novel function for NAADP/TPC2-mediated Ca²⁺ signaling in the development, coordination, and maturation of the spinal network in zebrafish embryos.

Behavioral screening of the LOPAC1280 library in zebrafish embryos

Spontaneous activity represents an early, primitive form of motor activity within zebrafish embryos, providing a potential readout for identification of neuroactive compounds. However, despite use as an endpoint in chemical screens around the world, the predictive power and limitations of assays relying on spontaneous activity remain unclear. Using an improved high-content screening assay that increased throughput from 384 to 3072 wells per week, we screened a well-characterized library of 1280 pharmacologically active compounds (LOPAC¹²⁸⁰) - 612 of which target neurotransmission - to identify which targets are detected using spontaneous activity as a readout. Results from this screen revealed that (1) 8% of the LOPAC¹²⁸⁰ library was biologically active; (2) spontaneous activity was affected by compounds spanning a broad array of targets; (3) only 4% of compounds targeting neurotransmission impacted spontaneous activity; and (4) hypoactivity was observed for 100% of hits detected, including those that exhibit opposing mechanisms of action for the same target. Therefore, while this assay was able to rapidly identify potent neuroactive chemicals, these data suggest that spontaneous activity may lack the ability to discriminate modes of action for compounds interfering with neurotransmission, an issue that may be due to systemic uptake following waterborne exposure, persistent control variation, and/or interference with non-neurotransmission-related mechanisms.

Position- and quantity-dependent responses in zebrafish turning behavior

Neural reflexes are stereotypical automatic responses often modulated by both intrinsic and environmental factors. We report herein that zebrafish larval C-shaped turning is modulated by the stimulated position of Rohon-Beard (RB) neurons. Targeted stimulation of more anterior RB neurons produces larger trunk flexion, which anticipates adult escape behavior by coordinated turning toward the appropriate direction. We also demonstrated that turning laterality varies with the numbers of stimulated neurons. Multi-cell stimulation of RB neurons elicits contralateral turning, as seen in the touch response to physical contact, while minimum input from single-cell stimulation induces ipsilateral turning, a phenomenon not previously reported. This ipsilateral response, but not the contralateral one, is impaired by transecting the ascending neural tract known as the dorsolateral fascicule (DLF), indicating that two, distinct neural circuits trigger these two responses. Our results suggest that RB neurons transmit the position and quantity of sensory information, which are then processed separately to modulate behavioral strength and to select turning laterality.

Neuronal labeling patterns in the spinal cord of adult transgenic Zebrafish

We describe neuronal patterns in the spinal cord of adult zebrafish. We studied the distribution of cells and processes in the three spinal regions reported in the literature: the 8th vertebra used as a transection injury site, the 15th vertebra mainly used for motor cell recordings and also for crush injury, and the 24th vertebra used to record motor nerve activity. We used well-known transgenic lines in which expression of green fluorescent protein (GFP) is driven by promoters to hb9 and isl1 in motoneurons, alx/chx10 and evx1 interneurons, ngn1 in sensory neurons and olig2 in oligodendrocytes, as well as antibodies for neurons (HuC/D, NF and SV2) and glia (GFAP). In isl1:GFP fish, GFP-positive processes are retained in the upper part of ventral horns and two subsets of cell bodies are observed. The pattern of the transgene in hb9:GFP adults is more diffuse and fibers are present broadly through the adult spinal cord. In alx/chx10 and evx1 lines we respectively observed two and three different GFP-positive populations. Finally, the ngn1:GFP transgene identifies dorsal root ganglion and some cells in dorsal horns. Interestingly some GFP positive fibers in ngn1:GFP fish are located around Mauthner axons and their density seems to be related to a rostrocaudal gradient. Many other cell types have been described in embryos and need to be studied in adults. Our findings provide a reference for further studies on spinal cytoarchitecture. Combined with physiological, histological and pathological/traumatic approaches, these studies will help clarify the operation of spinal locomotor circuits of adult zebrafish.

Sensory gating of an embryonic zebrafish interneuron during spontaneous motor behaviors

In all but the simplest monosynaptic reflex arcs, sensory stimuli are encoded by sensory neurons that transmit a signal via sensory interneurons to downstream partners in order to elicit a response. In the embryonic zebrafish (Danio rerio), cutaneous Rohon-Beard (RB) sensory neurons fire in response to mechanical stimuli and excite downstream glutamatergic commissural primary ascending (CoPA) interneurons to produce a flexion response contralateral to the site of stimulus. In the absence of sensory stimuli, zebrafish spinal locomotor circuits are spontaneously active during development due to pacemaker activity resulting in repetitive coiling of the trunk. Self-generated movement must therefore be distinguishable from external stimuli in order to ensure the appropriate activation of touch reflexes. Here, we recorded from CoPAs during spontaneous and evoked fictive motor behaviors in order to examine how responses to self-movement are gated in sensory interneurons. During spontaneous coiling, CoPAs received glycinergic inputs coincident with contralateral flexions that shunted firing for the duration of the coiling event. Shunting inactivation of CoPAs was caused by a slowly deactivating chloride conductance that resulted in lowered membrane resistance and increased action potential threshold. During spontaneous burst swimming, which develops later, CoPAs received glycinergic inputs that arrived in phase with excitation to ipsilateral motoneurons and provided persistent shunting. During a touch stimulus, short latency glutamatergic inputs produced cationic currents through AMPA receptors that drove a single, large amplitude action potential in the CoPA before shunting inhibition began, providing a brief window for the activation of downstream neurons. We compared the properties of CoPAs to those of other spinal neurons and propose that glycinergic signaling onto CoPAs acts as a corollary discharge signal for reflex inhibition during movement.