Response of zebrafish larvae to mild electrical stimuli: A 96-well setup for behavioural screening

Treatment of Pain in Fish

This chapter provides an overview of our current understanding of clinical analgesic use in fish. Recently, the efficacy and pharmacokinetics of several analgesic drugs for use in fish have been investigated, and the most important data indicates that μ-opioid agonist drugs (e.g, morphine) are consistently effective as analgesics across fish species. In addition, bath application of some analgesic drugs may be useful, which affords multiple methods for delivering analgesics to fish. Although few published studies of non-steroidal anti-inflammatory drugs administered to fish show promise, we have much to learn about the analgesic efficacy of most drugs in this class.

Behavioural responses of zebrafish with sound stimuli in microfluidics

Neuronal activities of the human brain responsible for cognitive features have been theorized through several animal models that exhibited various complementary spatial learning modes by generating a flexible repertoire of behavioral strategies. However, for such studies associated with a neurodegenerative disease, which can be further manipulated to provide therapeutic strategies, the animal models employed in their developmental stages have been preferred over the adult ones. This pilot work was incepted to underscore the spatial memory capabilities that strengthened the intricate mechanism of memory acquisition potential in one of the low-order evolutionarily conserved species, such as zebrafish larvae. Initially, a reliable and more easily reproducible microfluidic platform integrating simple and intricate paths was designed to learn and test the spatial information in zebrafish larvae of 4-6 d.p.f. under non-invasive acoustic stimuli. Further, to acquire spatial information as the representation of spatial memory formation in zebrafish larvae, the acoustic startle responses were evaluated by quantifying various dynamic behaviors under distinct operating parameters. After significant conditioning sessions, the spatial memory was tested by employing variable 'freezing'. By the end of the 30 min-long test session, 6 d.p.f. larvae were found to exhibit the highest value of freezing of approximately 43% and 20% in the short and long paths, respectively. Even though a substantial rate of memory loss was observed, it can be envisaged to serve several behavioral strategies that process the dynamic cognitive memory among distinct spatiotemporal environments. Further, the proposed behavioral paradigm had the advantage of being more adaptable and reliably replicable by other researchers. As a consequence, different hypotheses can be readily tested to generate more reproducible findings towards distinct neurobehavioral characteristics. Therefore, the proposed paradigm for the consolidation of spatial memory based on the non-invasive spatial avoidance strategies could provide an enduring framework of reference for behavioral studies using zebrafish larvae.

Dopaminergic signaling regulates zebrafish larvae's response to electricity

Electrical stimulation of brain or muscle activities has gained attention for studying the molecular and cellular mechanisms involved in electric-induced responses. We recently showed zebrafish's response to electricity. Here, we hypothesized that this response is affected by the dopaminergic signaling pathways. The effects of multiple dopamine agonists and antagonists on the electric response of 6 days-postfertilization zebrafish larvae were investigated using a microfluidic device with enhanced control of experimentation and throughput. All dopamine antagonists decreased locomotor activities, while dopamine agonists did not induce similar behaviors. The D2-selective dopamine agonist quinpirole enhanced the movement. Exposure to nonselective and D1-selective dopamine agonists apomorphine and SKF-81297 caused no significant change in the electric response. Exposing larvae that were pretreated with nonselective and D2-selective dopamine antagonists butaclamol and haloperidol to apomorphine and quinpirole, respectively, restored the electric locomotion. These results reveal a correlation between electric response and dopamine signaling pathway. Furthermore, they demonstrate that electric-induced zebrafish larvae locomotion can be conditioned by modulating dopamine receptor functions. Our electrofluidic assay has profound application potential for fundamental electric-induced response research and brain disorder studies especially those related to the dopamine imbalance and as a chemical screening method when investigating biological pathways and behaviors. This article is protected by copyright. All rights reserved.

Behavioral effects of acute ethanol in larval zebrafish (D. rerio) depend on genotype and volume of experimental well

Ethanol consumption is a worldwide problem. Sensitivity to acute effects of ethanol influences the development of chronic ethanol abuse and ethanol dependence. Environmental and genetic factors have been found to contribute to differential effects of acute ethanol. Animal models have been employed to investigate these factors. An increasingly frequently utilized animal model in ethanol research is the zebrafish. A large proportion of ethanol studies with zebrafish have been conducted with adult zebrafish. However, high throughput drug and mutation screens are particularly well adapted to larval zebrafish. These studies are often carried out using the 96-well-plate that allows monitoring large numbers of fish efficiently. Here, we investigate the effects of acute (30 min long) ethanol exposure in 8-day post-fertilization (dpf) old zebrafish. We compare four genetically distinct populations (strains) of zebrafish, measuring numerous parameters of their swim path in two well sizes, i.e., in the 96-well-plate (small volume wells) and in the 6-well-plate (large volume wells). In general, we found that the highest dose of ethanol (1% vol/vol) reduced swim speed, increased duration of immobility, increased turn angle, and increased intra-individual variance of turn angle, while the intermediate dose (0.5%) had a less strong effect, compared to control. However, we also found that these ethanol effects were strain dependent and, in general, were better detected in the larger volume well. We conclude that larval zebrafish are appropriate for quantification of acute ethanol effects and also for the analysis of environmental and genetic factors that influence these effects. We also speculate that using larger wells will likely increase sensitivity of detection and precision in screening applications.

Current Methods to Investigate Nociception and Pain in Zebrafish

Pain is an unpleasant, negative emotion and its debilitating effects are complex to manage. Mammalian models have long dominated research on nociception and pain, but there is increasing evidence for comparable processes in fish. The need to improve existing pain models for drug research and the obligation for 3R refinement of fish procedures facilitated the development of numerous new assays of nociception and pain in fish. The zebrafish is already a well-established animal model in many other research areas like toxicity testing, as model for diseases or regeneration and has great potential in pain research, too. Methods of electrophysiology, molecular biology, analysis of reflexive or non-reflexive behavior and fluorescent imaging are routinely applied but it is the combination of these tools what makes the zebrafish model so powerful. Simultaneously, observing complex behavior in free-swimming larvae, as well as their neuronal activity at the cellular level, opens new avenues for pain research. This review aims to supply a toolbox for researchers by summarizing current methods to study nociception and pain in zebrafish. We identify treatments with the best algogenic potential, be it chemical, thermal or electric stimuli and discuss options of analgesia to counter effects of nociception and pain by opioids, non-steroidal anti-inflammatory drugs (NSAIDs) or local anesthetics. In addition, we critically evaluate these practices, identify gaps of knowledge and outline potential future developments.

Prolonged exposure to stressors suppresses exploratory behavior in zebrafish larvae

Environmental stressors induce rapid physiological and behavioral shifts in vertebrate animals. However, the neurobiological mechanisms responsible for stress-induced changes in behavior are complex and not well understood. Similar to mammalian vertebrates, zebrafish adults display a preference for dark environments that is associated with predator avoidance, enhanced by stressors, and broadly used in assays for anxiety-like behavior. Although the larvae of zebrafish are a prominent model organism for understanding neural circuits, few studies have examined the effects of stressors on their behavior. This study examines the effects of noxious chemical and electric shock stressors on locomotion and light preference in zebrafish larvae. We found that both stressors elicited similar changes in behavior. Acute exposure induced increased swimming activity, while prolonged exposure depressed activity. Neither stressor produced a consistent shift in light-dark preference, but prolonged exposure to these stressors resulted in a pronounced decrease in exploration of different visual environments. We also examined the effects of exposure to a noxious chemical cue using whole-brain calcium imaging, and identified neural correlates in the area postrema, an area of the hindbrain containing noradrenergic and dopaminergic neurons. Pharmaceutical blockade experiments showed that α-adrenergic receptors contribute to the behavioral response to an acute stressor but are not necessary for the response to a prolonged stressor. These results indicate that zebrafish larvae have complex behavioral responses to stressors comparable to those of adult animals, and also suggest that these responses are mediated by similar neural pathways.

A vertebrate model to reveal neural substrates underlying the transitions between conscious and unconscious states

The field of neuropharmacology has not yet achieved a full understanding of how the brain transitions between states of consciousness and drug-induced unconsciousness, or anesthesia. Many small molecules are used to alter human consciousness, but the repertoire of underlying molecular targets, and thereby the genes, are incompletely understood. Here we describe a robust larval zebrafish model of anesthetic action, from sedation to general anesthesia. We use loss of movement under three different conditions, spontaneous movement, electrical stimulation or a tap, as a surrogate for sedation and general anesthesia, respectively. Using these behavioral patterns, we find that larval zebrafish respond to inhalational and IV anesthetics at concentrations similar to mammals. Additionally, known sedative drugs cause loss of spontaneous larval movement but not to the tap response. This robust, highly tractable vertebrate model can be used in the detection of genes and neural substrates involved in the transition from consciousness to unconsciousness.

Neural circuit reorganisation after spinal cord injury in zebrafish

Spinal cord injuries disrupt signalling from the brain leading to loss of limb, locomotion, sexual and bladder function, usually irreversible in humans. In zebrafish, recovery of function occurs in a few days for larvae or a few weeks for adults due to regrowth of axons and de novo neurogenesis. Together with its genetic amenability and optical clarity, this makes zebrafish a powerful animal model to study circuit reorganisation after spinal cord injuries. With the fast evolution of techniques, we can forecast significative improvements of our knowledge of the mechanisms leading to successful or failed recovery of spinal cord function. We review here the present knowledge on the subject, the new technological approaches and we propose future directions of research.

Phenotypic chemical and mutant screening of zebrafish larvae using an on-demand response to electric stimulation

Behavioral responses of zebrafish larvae to environmental cues are important functional readouts that should be evoked on-demand and studied phenotypically in behavioral, genetical and developmental investigations. Very recently, it was shown that zebrafish larvae execute a voluntary and oriented movement toward the positive electrode of an electric field along a microchannel. Phenotypic characterization of this response was not feasible due to larva's rapid movement along the channel. To overcome this challenge, a microfluidic device was introduced to partially immobilize the larva's head while leaving its mid-body and tail unrestrained in a chamber to image motor behaviors in response to electric stimulation, hence achieving quantitative phenotyping of the electrically evoked movement in zebrafish larvae. The effect of electric current on the tail-beat frequency and response duration of 5-7 days postfertilization zebrafish larvae was studied. Investigations were also performed on zebrafish exposed to neurotoxin 6-hydroxydopamine and larvae carrying a pannexin1a (panx1a) gene knockout, as a proof of principle applications to demonstrate on-demand movement behavior screening in chemical and mutant assays. We demonstrated for the first time that 6-hydroxydopamine leads to electric response impairment, levodopa treatment rescues the response and panx1a is involved in the electrically evoked movement of zebrafish larvae. We envision that our technique is broadly applicable as a screening tool to quantitatively examine zebrafish larvae's movements in response to physical and chemical stimulations in investigations of Parkinson's and other neurodegenerative diseases, and as a tool to combine recent advances in genome engineering of model organisms to uncover the biology of electric response.