A detailed experimental protocol is presented in this paper for the evaluation of neurobehavioral toxicity of environmental pollutants using a zebrafish larvae model, including the exposure process and tests for neurobehavioral indicators.
Recent years more and more environmental pollutants have been proved neurotoxic, especially at the early development stages of organisms. Zebrafish larvae are a preeminent model for the neurobehavioral study of environmental pollutants. Here, a detailed experimental protocol is provided for the evaluation of the neurotoxicity of environmental pollutants using zebrafish larvae, including the collection of the embryos, the exposure process, neurobehavioral indicators, the test process, and data analysis. Also, the culture environment, exposure process, and experimental conditions are discussed to ensure the success of the assay. The protocol has been used in the development of psychopathic drugs, research on environmental neurotoxic pollutants, and can be optimized to make corresponding studies or be helpful for mechanistic studies. The protocol demonstrates a clear operation process for studying neurobehavioral effects on zebrafish larvae and can reveal the effects of various neurotoxic substances or pollutants.
In recent years more and more environmental pollutants have been proved neurotoxic1,2,3,4. However, the assessment of neurotoxicity in vivo after exposure to environmental pollutants is not as easy as that of endocrine disruption or developmental toxicity. In addition, early exposure to pollutants, especially at environmentally relevant doses, has attracted increasing attention in toxicity studies5,6,7,8.
Zebrafish is being established as an animal model fit for neurotoxicity studies during early development after exposure to environmental pollutants. Zebrafish are vertebrates that develop faster than other species after fertilization. The larvae do not need to be fed because the nutrients in the chorion are enough for sustain them for 7 days postfertilization (dpf)9. Larvae come out from the chorion at ~2 dpf and develop behaviors such as swimming and turning that can be observed, tracked, quantified, and analyzed automatically using behavior instruments10,11,12,13 starting at 3-4 dpf14,15,16,17,18. In addition, high-throughput tests can also be realized by behavior instruments. Thus, zebrafish larvae are an outstanding model for the neurobehavioral study of environmental pollutants19. Here, a protocol is offered using high-throughput monitoring to study the neurobehavioral toxicity of environmental pollutants on zebrafish larvae under light stimuli.
Our lab has studied the neurobehavioral toxicity of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47)20,21, 6'-Hydroxy/Methoxy-2,2',4,4'-tetrabromodiphenyl ether (6-OH/MeO-BDE-47)22, deca-brominated diphenyl ether (BDE-209), lead, and commercial chlorinated paraffins23 using the presented protocol. Many labs also use the protocol to study the neurobehavioral effects of other pollutants on larvae or adult fish24,25,26,27. This neurobehavioral protocol was used to help provide mechanistic support showing that low-dose exposure to bisphenol A and replacement bisphenol S induced premature hypothalamic neurogenesis in embryonic zebrafish27. In addition, some researchers optimized the protocol to perform corresponding studies. A recent study eliminated the toxicity of amyloid beta (Aβ) in an easy, high-throughput zebrafish model using casein-coated gold nanoparticles (βCas AuNPs). It showed that βCas AuNPs in systemic circulation translocated across the blood-brain barrier of zebrafish larvae and sequestered intracerebral Aβ42, eliciting toxicity in a nonspecific, chaperone-like manner, which was supported by behavioral pathology28.
Locomotion, path angle, and social activity are three neurobehavioral indicators used to study the neurotoxicity effects of zebrafish larvae after exposure to pollutants in the presented protocol. Locomotion is measured by the swimming distance of larvae and can be damaged after exposure to pollutants. Path angle and social activity are more closely related with the function of the brain and the central nervous system29. The path angle refers to the angle of the path of animal motion relative to the swimming direction30. Eight angle classes from ~-180°-~+180° are set in the system. To simplify the comparison, six classes in the final outcome are defined as routine turns (-10° ~0°, 0° ~+10°), average turns (-10° ~-90°, +10° ~+90°), and responsive turns (-180° ~-90°, +90° ~+180°) according to our previous studies21,22. Two-fish social activity is fundamental of group shoaling behavior; here a distance of < 0.5 cm between two larvae valid is defined as social contact.
The protocol presented here demonstrates a clear process for studying neurobehavioral effects on zebrafish larvae and provides a way to reveal the neurotoxicity effects of various substances or pollutants. The protocol will benefit researchers interested in studying the neurotoxicity of environmental pollutants.
The protocol is in accordance with guidelines approved by the Animal Ethics Committee of Tongji University.
1. Zebrafish embryo collection
2. Preparation before exposure
3. Exposure of embryos
4. Preparation for the behavior test
5. Behavioral test
6. Data analysis
Here, we describe a protocol for studying the neurobehavioral effects of environmental pollutants using zebrafish larvae under light stimuli. The locomotion, path angle, and social activity tests are defined in the introduction. The setup of the microplates in the locomotion and path angle tests and the images of the software are shown below. In addition, our own research results are presented as examples. Two studies present the locomotion and path angle effects after exposure to BDE-47 and 6-OH/MeO-BDE-47. The third study presents the effects of four commercial chlorinated paraffins on social behavior.
The setup of the 48 well microplate and the movement locus of the larvae in the locomotion and path angle test.
Three groups, including one control group and two treatment groups, were used in the protocol. Because every group can have 16 animals, the system can be used to perform high-throughput tests of locomotion and path angle in one microplate. Figure 1 shows one larva treated with the control solution, 5 µg/L solution, and 50 µg/L solution in each well of the first, middle, and last two rows, respectively.
Figure 1 also shows all movement loci of the larvae in the locomotion and path angle tests. The system tracked the locomotion of the larvae and calculated the swimming distance at different speed classes. The system calculated the path angle numbers of larvae at different path angle classes. Researchers can analyze the data recorded by the system in their own ways.
Figure 1: The setup of the 48 well microplate and the movement loci of the larvae in the locomotion and path angle test. A1-A8, B1-B8 = the control group; C1-C8, D1-D8 = the 5 µg/L group; E1-E8, F1-F8 = the 50 µg/L group. The black color tracking line means inactivity or small movements; the green color tracking line means normal movements; and the red color tracking line means large movements. Please click here to view a larger version of this figure.
The 6 well microplate in the social activity test.
Figure 2 shows a 6 well microplate in the social activity testing process. Every well had two larvae, and the system recorded the distance between the two larvae during the whole testing process. The system recorded the social activity numbers and duration in the set testing time (1 min in this protocol).
Figure 2: The 6 well microplate in the social activity test. Every well had two larvae. The yellow line means the distance between two animals is < 0.5 cm; the red line means the distance between two animals is > 0.5 cm. Please click here to view a larger version of this figure.
BDE-47 exposure affected locomotion in zebrafish larvae at 5 dpf.
As shown in Figure 3, the highest concentration group of BDE-47 produced pronounced hypoactivity during the dark period. However, there were no observed changes due to BDE-47 exposure during the light periods.
Figure 3: Effects of BDE-47 exposure on locomotion of larval zebrafish at 5 dpf. Locomotion (distance moved measured in cm) was recorded in alternating periods of darkness and light for a total duration of 70 min. Solid and open bars at the bottom indicate dark and light periods, respectively. Data are presented as mean ± SEM (*p < 0.05 compared with the control group). This figure has been modified from Zhao et al.17 with permission. Please click here to view a larger version of this figure.
6-OH/MeO-BDE-47 exposure affected the path angles of zebrafish larvae at 5 dpf.
As shown in Figure 4, the high concentration group of 6-OH-BDE-47 performed fewer routine turns and average turns at 5 dpf. However, more responsive turns were induced by 6-MeO-BDE-47 exposure groups.
Figure 4: Effects of 6-OH/MeO-BDE-47 on the path angle of larval zebrafish during the dark period. Data are presented as the mean ± SEM (*p < 0.05 compared with control). This figure has been modified from Zhang et al.18 with permission. Please click here to view a larger version of this figure.
CPs exposure affected social activity of zebrafish larvae at 5 dpf.
As shown in Figure 5, the social behaviors of zebrafish larvae were influenced by three CP products. The social activity was stimulated by CP-70 and the short-chain CP-52b. The long chain CP-52a shortened the duration per contact of the larvae.
Figure 5: Effects of CPs on the average social duration per contact in different light/dark periods. (A) CP-42, (B) CP-52a, (C) CP-52b, (D) CP-70. The data are presented as the mean ± SEM (*p < 0.05 compared with the control). This figure has been modified from Yang et al.19 with permission. Please click here to view a larger version of this figure.
This work provides a detailed experimental protocol to evaluate the neurotoxicity of environmental pollutants using zebrafish larvae. Zebrafish go through the process from embryos to larvae during the exposure period, which means that good care of the embryos and larvae is essential. Anything that affects the development of the embryos and larvae can influence the final result. Here the culture environment, exposure process, and experimental conditions are discussed to ensure the success of the whole assay.
For the culture environment, zebrafish embryos and larvae live under a stable temperature of ~28 °C. In this work, a light incubator that can set the light conditions automatically and keep the temperature stable is used to house the embryos and larvae. The embryos do not come out from the chorion at 1 dpf and 2 dpf, so care should be taken to avoid damaging the unhatched embryos when renewing the exposure solution. Also, the ratio of DMSO in the solution should be under 0.1%34,35, and the fresh exposure solution should be at 28 °C before it is used for renewal.
The process of selecting embryos before exposure is also a key factor for the success of the experiment. Choosing healthy embryos developing concurrently for every group guarantees the accuracy of toxicity assessment. Zebrafish can live without food during the first 7 days after fertilization, so it is best to not feed the embryos or larvae during the whole exposure period because food could influence the final result. Also, it is best to prepare the exposure solution fresh when needed.
During the behavior test, it is essential to offer the larvae enough time to adapt to the environment of the high-throughput monitoring enclosure. Before the test, every step of the tested protocol should be checked carefully, including the light condition, testing time, etc. The testing room should be kept completely quiet and dark in order to not disturb the animals.
The protocol presented offers a fundamental frame to study the neurobehavioral toxicity of environmental pollutants. There are also other types of behaviors used when studying neurobehavioral effects, such as color-preference tests36, bottom dwelling tests37, light/dark preference tests38,39, etc. However, these tests mainly use adult zebrafish, which are not fit for high-throughput tests. In addition, Weichert et al. videotaped to the behavior of spontaneous tail movements which could be quantified just after 24 h exposure40. The evaluation of neurobehavioral toxicity also includes mechanism studies on the function of the brain and the central nervous system. The fundamental neurobehavioral indicators are introduced here and can form the basis for more complex indicators using other behavior instruments. Ultimately, the development of new neurobehavioral indicators accompanied with this study mechanism can be used in future studies.
The authors have nothing to disclose.
The authors are grateful for the financial support by the National Natural Science Foundation of China (21876135 and 21876136), the National Major Science and Technology Project of China (2017ZX07502003-03, 2018ZX07701001-22), the Foundation of MOE-Shanghai Key Laboratory of Children's Environmental Health (CEH201807-5), and Swedish Research Council (No. 639-2013-6913).
48-well-microplate | Corning | 3548 | Embyros housing |
6-well-microplate | Corning | 3471 | Embyros housing |
BDE-47 | AccuStandard | 5436-43-1 | Pollutant |
DMSO | Sigma | 67-68-5 | Cosolvent |
Microscope | Olympus | SZX 16 | Observation instrument |
Pipette | Eppendorf | 3120000267 | Transfer solution |
Zebrabox | Viewpoint | ZebraBox | Behavior instrument |
Zebrafish | Shanghai FishBio Co., Ltd. | Tubingen | Zebrafish supplier |
ZebraLab | Viewpoint | ZebraLab | Behavior software |