In vivo physiological recording from the lateral line of juvenile zebrafish

Longitudinal MRI brain studies in live adult zebrafish

Zebrafish (Danio rerio) has been successfully used for decades in developmental studies and disease modelling. The remarkable uptake of zebrafish as a model system is partly due to its transparency during the early weeks of its development, allowing in vivo imaging of cellular and molecular processes. However, this key advantage wears off when tissues become opaque as the animal reaches juvenile and adult stages, rendering access to tissues for live imaging and longitudinal studies difficult. Here we provide a novel approach to image and assess tissue integrity of adult zebrafish using MRI on live zebrafish suitable for longitudinal studies. We built a 3D-printed life support chamber and designed a protocol-directed sedation regime to recover adult zebrafish after scanning in a 9.4 T MRI scanner. Our life support chamber is cheap and easy to create using 3D printing, allowing other groups to copy our template for quick setup. Additionally, we optimized the delivery of contrast agent to enhance brain signals in order to refine current delivery, usually delivered intravenously in rodents. We show here that immersion in gadolinium was a viable alternative to intraperitoneal injection to reduce T₁ relaxation times. This resulted in protocol refinement as per the 3Rs guidelines and improved image contrast in adult zebrafish disease models. In conclusion, we provide here a detailed methodology to allow longitudinal studies of brain tissue integrity of adult zebrafish, combining safe and efficient delivery of contrast agent and live MRI. This technique can be used to bridge the gap between in vivo studies and longitudinal brain analysis in adult zebrafish, and can be applied to the ever-growing number of adult zebrafish models of ageing and neurodegenerative diseases.

Sensory deficit screen identifies nsf mutation that differentially affects SNARE recycling and quality control

The AAA+ NSF complex is responsible for SNARE complex disassembly both before and after membrane fusion. Loss of NSF function results in pronounced developmental and degenerative defects. In a genetic screen for sensory deficits in zebrafish, we identified a mutation in nsf, I209N, that impairs hearing and balance in a dosage-dependent manner without accompanying defects in motility, myelination, and innervation. In vitro experiments demonstrate that while the I209N NSF protein recognizes SNARE complexes, the effects on disassembly are dependent upon the type of SNARE complex and I209N concentration. Higher levels of I209N protein produce a modest decrease in binary (syntaxin-SNAP-25) SNARE complex disassembly and residual ternary (syntaxin-1A-SNAP-25-synaptobrevin-2) disassembly, whereas at lower concentrations binary disassembly activity is strongly reduced and ternary disassembly activity is absent. Our study suggests that the differential effect on disassembly of SNARE complexes leads to selective effects on NSF-mediated membrane trafficking and auditory/vestibular function.

Early life exposure to chronic unpredictable stress induces anxiety-like behaviors and increases the excitability of cerebellar neurons in zebrafish

Anxiety is a common emotional disorder in children. To understand its underlying mechanisms, chronic unpredictable stress (CUS) has been established as a stress model in zebrafish. By using the tall tank test, the stress response reliability could be improved in adult fish which has not been confirmed in larvae. In addition, the increasing evidences have shown that cerebellum plays important roles in anxiety. Whether CUS will affect cerebellar neuronal activity remains unknown. We found that CUS exposure to larvae (from 10 to 17 days post fertilization) induced anxiety-like behaviors and social cohesion impairments within 1-2 d after CUS, including a prolonged freezing time, an increased time spent at the bottom of tank, an increased thigmotaxis index, and an increased interindividual distance. Our results showed that the four behavioral tests were homogeneous, especially the tall tank test either anxiety-like behaviors or the basal locomotion. Furthermore, we found that CUS enhanced the excitability of cerebellar neurons, as the amplitude, frequency, time to peak and half-width of spontaneous firing significantly decreased, as well as the amplitude of excitatory post-synaptic current when compared with the control group. CUS also activated hyperpolarization-activated cyclic nucleotide-gated and potassium channels of cerebellar neurons. Multiple linear regression analysis showed that the total distance in bottom (tall tank test) was correlated positively with outward Na⁺-K⁺ currents (r = 0.848, P = 0.016), and the thigmotaxis index (open field test) correlated with action potential amplitude (r = 0.854, P = 0.030). Altogether, early life CUS transiently induced an anxiety-like behavior which could be more accurately assessed by combining the tall tank test with other behavior tests in young zebrafish. CUS increased the excitability of cerebellar neurons might provide new targets to treat emotional diseases such as anxiety.

Screening of Anaesthetics in Adult Zebrafish (Danio rerio) for the Induction of Euthanasia by Overdose

Simple Summary

Although zebrafish are used in vast numbers in laboratories all over the world, no consensus has been reached in the scientific community on a humane, consistent, and effective method for euthanasia of this species. Here, we screened commonly used anaesthetic drugs to see if an overdose could induce loss of reflexes of adult zebrafish in a rapid and reliable manner, and without causing distress. The tested anaesthetics were isoeugenol, clove oil, 2-phenoxyethanol, tricaine, benzocaine, lidocaine hydrochloride, and etomidate. We found that lidocaine hydrochloride, buffered with sodium bicarbonate and ethanol to increase its efficacy, induces loss of reflexes in a fast, predictable, and relatively peaceful manner. We recommend its use for adult zebrafish euthanasia.

Abstract

Zebrafish are often euthanized by overdose of anaesthesia. However, fish may have aversion towards some anaesthetics, and protocol efficacy varies between species. Using wild type adult Danio rerio, we assessed time to loss of opercular beat, righting, and startle reflexes during induction of anaesthetic overdose by either tricaine (0.5 g/L or 1 g/L), benzocaine (1 g/L), 2-phenoxyethanol (3 mL/L), clove oil (0.1%), isoeugenol (540 mg/L), lidocaine hydrochloride (1 g/L), or etomidate (50 mg/L). Initial screening demonstrated that benzocaine and buffered lidocaine hydrochloride achieved the fastest loss of reflexes. The rapid induction times were confirmed when retesting using larger batches of fish. The fastest induction was obtained with 1 g/L lidocaine hydrochloride buffered with 2 g/L NaHCO₃, in which all adult zebrafish lost reflexes in less than 2 min. Next, we monitored signs of distress during benzocaine or buffered lidocaine hydrochloride overdose induction. The results indicated that buffered lidocaine hydrochloride caused significantly less aversive behaviors than benzocaine. Finally, we tested several buffers to refine the lidocaine hydrochloride immersion. The most efficient buffer for euthanasia induction using 1g/L lidocaine hydrochloride was 2 g/L NaHCO₃ with 50 mL/L 96% ethanol, inducing immobility in less than 10 s and with only 2% of adult zebrafish displaying aversive behaviors during treatment.

The Cholinergic Lateral Line Efferent Synapse: Structural, Functional and Molecular Similarities With Those of the Cochlea

Vertebrate hair cell (HC) systems are innervated by efferent fibers that modulate their response to external stimuli. In mammals, the best studied efferent-HC synapse, the cholinergic medial olivocochlear (MOC) efferent system, makes direct synaptic contacts with HCs. The net effect of MOC activity is to hyperpolarize HCs through the activation of α9α10 nicotinic cholinergic receptors (nAChRs) and the subsequent activation of Ca²⁺-dependent SK2 potassium channels. A serious obstacle in research on many mammalian sensory systems in their native context is that their constituent neurons are difficult to access even in newborn animals, hampering circuit observation, mapping, or controlled manipulation. By contrast, fishes and amphibians have a superficial and accessible mechanosensory system, the lateral line (LL), which circumvents many of these problems. LL responsiveness is modulated by efferent neurons which aid to distinguish between external and self-generated stimuli. One component of the LL efferent system is cholinergic and its activation inhibits LL afferent activity, similar to what has been described for MOC efferents. The zebrafish (Danio rerio) has emerged as a powerful model system for studying human hearing and balance disorders, since LL HC are structurally and functionally analogous to cochlear HCs, but are optically and pharmacologically accessible within an intact specimen. Complementing mammalian studies, zebrafish have been used to gain significant insights into many facets of HC biology, including mechanotransduction and synaptic physiology as well as mechanisms of both hereditary and acquired HC dysfunction. With the rise of the zebrafish LL as a model in which to study auditory system function and disease, there has been an increased interest in studying its efferent system and evaluate the similarity between mammalian and piscine efferent synapses. Advances derived from studies in zebrafish include understanding the effect of the LL efferent system on HC and afferent activity, and revealing that an α9-containing nAChR, functionally coupled to SK channels, operates at the LL efferent synapse. In this review, we discuss the tools and findings of these recent investigations into zebrafish efferent-HC synapse, their commonalities with the mammalian counterpart and discuss several emerging areas for future studies.

Corollary discharge enables proprioception from lateral line sensory feedback

Animals modulate sensory processing in concert with motor actions. Parallel copies of motor signals, called corollary discharge (CD), prepare the nervous system to process the mixture of externally and self-generated (reafferent) feedback that arises during locomotion. Commonly, CD in the peripheral nervous system cancels reafference to protect sensors and the central nervous system from being fatigued and overwhelmed by self-generated feedback. However, cancellation also limits the feedback that contributes to an animal's awareness of its body position and motion within the environment, the sense of proprioception. We propose that, rather than cancellation, CD to the fish lateral line organ restructures reafference to maximize proprioceptive information content. Fishes' undulatory body motions induce reafferent feedback that can encode the body's instantaneous configuration with respect to fluid flows. We combined experimental and computational analyses of swimming biomechanics and hair cell physiology to develop a neuromechanical model of how fish can track peak body curvature, a key signature of axial undulatory locomotion. Without CD, this computation would be challenged by sensory adaptation, typified by decaying sensitivity and phase distortions with respect to an input stimulus. We find that CD interacts synergistically with sensor polarization to sharpen sensitivity along sensors' preferred axes. The sharpening of sensitivity regulates spiking to a narrow interval coinciding with peak reafferent stimulation, which prevents adaptation and homogenizes the otherwise variable sensor output. Our integrative model reveals a vital role of CD for ensuring precise proprioceptive feedback during undulatory locomotion, which we term external proprioception.

Functional development and regeneration of hair cells in the zebrafish lateral line

Hair cells are mechanosensory receptors responsible for transducing auditory and vestibular information into electrical signals, which are then transmitted with remarkable precision to afferent neurons. Different from mammals, the hair cells of lower vertebrates, including those present in the neuromasts of the zebrafish lateral line, regenerate following environmental or chemical insults. Here we investigate the time-course of regeneration of hair cells in vivo using electrophysiology, 2-photon imaging and immunostaining applied to wild-type and genetically-encoded fluorescent indicator zebrafish lines. Functional hair cells drive spontaneous action potentials in the posterior lateral line afferent fibres, the frequency of which progressively increases over the first 10-days post-fertilization (dpf). Higher firing-rate fibres are only observed from ~6 dpf. Following copper treatment, newly formed hair cells become functional and are able to drive APs in the afferent fibres within 48 hours in both early-larval (≤8 dpf) and late-larval (12-17 dpf) zebrafish. However, the complete functional regeneration of the entire neuromast is delayed in late-larval compared to early-larval zebrafish. We propose that while individual regenerating hair cells can rapidly become active, the acquisition of fully functional neuromasts progresses faster at early-larval stages, a time when hair cells are still under development. At both ages, the afferent terminals in the regenerating neuromast appear to make initial contact with supporting cells. The ablation of the lateral line afferent neurons prevents the timely regeneration of supporting cells and hair cells. These findings indicate that the afferent system is likely to facilitate or promote the neuromast regeneration process.

Sensory adaptation at ribbon synapses in the zebrafish lateral line

Adaptation is used by sensory systems to adjust continuously their sensitivity to match changes in environmental stimuli. In the auditory and vestibular systems, the release properties of glutamate-containing vesicles at the hair-cell ribbon synapses play a crucial role in sensory adaptation, thus shaping the neural response to sustained stimulation. How ribbon synapses regulate the release of glutamate and how they modulate afferent responses in vivo is still largely unknown. Here, we have used two-photon imaging and electrophysiology to investigate the synaptic transfer characteristics of the hair cells in the context of sensory adaptation in live zebrafish. Prolonged and repeated water-jet stimulation of the hair-cell stereociliary bundles caused adaptation of the action potential firing rate elicited in the afferent neurons. By monitoring glutamate at ribbon synapses using time-lapse imaging, we identified two kinetically distinct release components: a rapid response that was exhausted within 50-100 ms and a slower and sustained response lasting the entire stimulation. After repeated stimulations, the recovery of the fast component followed a biphasic time course. Depression of glutamate release was largely responsible for the rapid firing rate adaptation recorded in the afferent neurons. However, postsynaptic Ca²⁺ responses had a slower recovery time course than that of glutamate release, indicating that they are also likely to contribute to the afferent firing adaptation. Hair cells also exhibited a form of adaptation during inhibitory bundle stimulations. We conclude that hair cells have optimised their synaptic machinery to encode prolonged stimuli and to maintain their sensitivity to new incoming stimuli.

Cnr2 Is Important for Ribbon Synapse Maturation and Function in Hair Cells and Photoreceptors

The role of the cannabinoid receptor 2 (CNR2) is still poorly described in sensory epithelia. We found strong cnr2 expression in hair cells (HCs) of the inner ear and the lateral line (LL), a superficial sensory structure in fish. Next, we demonstrated that sensory synapses in HCs were severely perturbed in larvae lacking cnr2. Appearance and distribution of presynaptic ribbons and calcium channels (Ca_v1.3) were profoundly altered in mutant animals. Clustering of membrane-associated guanylate kinase (MAGUK) in post-synaptic densities (PSDs) was also heavily affected, suggesting a role for cnr2 for maintaining the sensory synapse. Furthermore, vesicular trafficking in HCs was strongly perturbed suggesting a retrograde action of the endocannabinoid system (ECs) via cnr2 that was modulating HC mechanotransduction. We found similar perturbations in retinal ribbon synapses. Finally, we showed that larval swimming behaviors after sound and light stimulations were significantly different in mutant animals. Thus, we propose that cnr2 is critical for the processing of sensory information in the developing larva.

Anaesthesia of laboratory, aquaculture and ornamental fish: Proceedings of the first LASA-FVS Symposium

Following on from the Annual Fish Veterinary Society Conference, this symposium was organised with the Laboratory Animal Science Association and brought together experts from ornamental (pond and aquarium) fish practice, aquaculture and aquatic-research facilities to discuss good practice of anaesthesia. This proceedings paper gives an overview of relevant experiences involving a range of immersion drugs including tricaine, benzocaine and isoeugenol, as well as a summary of the main topics of discussion. While fish anaesthesia is commonplace, administration methods, drugs and monitoring procedures may often be regarded as antiquated when compared with mammalian practice. These limitations notwithstanding, individual fish will benefit from good anaesthetic monitoring. Although the most common anaesthetic drugs may be perceived as equally efficacious and therefore interchangeable for different settings, challenges are different for the anaesthesia of grouped fish, when determining species-dependent anaesthetic dosing in a multi-species tank, or adapting to farming requirements, nationally licensed products, costs and withdrawal periods. The fish anaesthetic arsenal fails to address premedication, analgesia and issues of averseness. The two latter factors should be part of the evaluation of anaesthetic protocols; therefore, instructions for the analgesic provision of lidocaine to fin clipped zebrafish are proposed. Euthanasia practices could sometimes be refined too. Alternative physical methods such as electrical stunning are options to be considered.

Potential of mucoadhesive nanocapsules in drug release and toxicology in zebrafish

Mucoadhesive polymeric nanocapsules have attracted interest of researchers from different fields from natural sciences because of their ability to interact with the mucosa and increase drug permeation. Anesthesia by immersion causes absorption through the skin and gills of fish, so it is important to evaluate the exposure of these organs to drug nanosystems. Benzocaine (BENZ) is one of the most popular anesthetic agents used in fish anesthesia, but it has drawbacks because of its low bioavailability, resulting in weak absorption after immersion. Here we describe method developed for preparing and characterizing chitosan-coated PLGA mucoadhesive nanoparticles containing BENZ (NPMAs) for zebrafish immersion anesthesia. We determined the lowest effective concentration, characterized the interaction of the mucoadhesive system with fish, measured the anesthetic efficacy, and evaluated possible toxic effects in embryos and adults exposed to the nanoformulations. This study opens perspectives for using nanoformulations prepared with BENZ in aquaculture, allowing reduction of dosage as well as promoting more effective anesthesia and improved interaction with the mucoadhesive system of fish.

The Psychoactive Drug Escitalopram Affects Foraging Behavior in Zebrafish (Danio rerio)

Selective serotonin reuptake inhibitors (SSRIs) are psychoactive pharmaceuticals that have been detected intact in natural waters globally. Laboratory experiments have reported that several SSRIs inhibit fish foraging behavior, but data for the SSRI escitalopram are lacking. The objectives of the present study were to determine whether escitalopram affects feeding behavior in zebrafish and whether possible sex differences exist. We exposed female and male zebrafish (Danio rerio) to 0.00, 0.10, and 1.50 µg/L of escitalopram in flow-through tanks for a 3-wk exposure period. We used a video tracking system with high temporal and spatial resolution to collect data on zebrafish swimming patterns in test tanks containing a food source. The results show a more pronounced effect of escitalopram in males compared with females. At the assumed most environmentally relevant concentration (0.10 µg/L), male average feeding time/visit and maximum feeding duration were significantly reduced by 27 and 42%, respectively. In addition, male total feeding duration was also significantly reduced (by 73%) at the highest concentration (1.50 µg/L). In females, only the maximum feeding duration was significantly reduced (by 41%) in the 0.10 µg/L treatment group. Hence, we reject our initial hypothesis that female feeding behavior is more vulnerable to escitalopram. There was no effect of escitalopram on length or weight among the experimental groups. The present study demonstrates that escitalopram, like other SSRIs, can inhibit fish foraging behavior and therefore potentially disturb natural food chains. Finally, our study suggests that SSRIs can both be sex and behavior specific. Environ Toxicol Chem 2019;38:1902-1910. © 2019 SETAC.

Water Waves to Sound Waves: Using Zebrafish to Explore Hair Cell Biology

Although perhaps best known for their use in developmental studies, over the last couple of decades, zebrafish have become increasingly popular model organisms for investigating auditory system function and disease. Like mammals, zebrafish possess inner ear mechanosensory hair cells required for hearing, as well as superficial hair cells of the lateral line sensory system, which mediate detection of directional water flow. Complementing mammalian studies, zebrafish have been used to gain significant insights into many facets of hair cell biology, including mechanotransduction and synaptic physiology as well as mechanisms of both hereditary and acquired hair cell dysfunction. Here, we provide an overview of this literature, highlighting some of the particular advantages of using zebrafish to investigate hearing and hearing loss.

In Vivo Calcium Imaging of Lateral-line Hair Cells in Larval Zebrafish

Sensory hair cells are mechanoreceptors found in the inner ear that are required for hearing and balance. Hair cells are activated in response to sensory stimuli that mechanically deflect apical protrusions called hair bundles. Deflection opens mechanotransduction (MET) channels in hair bundles, leading to an influx of cations, including calcium. This cation influx depolarizes the cell and opens voltage-gated calcium channels located basally at the hair-cell presynapse. In mammals, hair cells are encased in bone, and it is challenging to functionally assess these activities in vivo. In contrast, larval zebrafish are transparent and possess an externally located lateral-line organ that contains hair cells. These hair cells are functionally and structurally similar to mammalian hair cells and can be functionally assessed in vivo. This article outlines a technique that utilizes a genetically encoded calcium indicator (GECI), GCaMP6s, to measure stimulus-evoked calcium signals in zebrafish lateral-line hair cells. GCaMP6s can be used, along with confocal imaging, to measure in vivo calcium signals at the apex and base of lateral-line hair cells. These signals provide a real-time, quantifiable readout of both mechanosensation- and presynapse-dependent calcium activities within these hair cells. These calcium signals also provide important functional information regarding how hair cells detect and transmit sensory stimuli. Overall, this technique generates useful data about relative changes in calcium activity in vivo. It is less well-suited for quantification of the absolute magnitude of calcium changes. This in vivo technique is sensitive to motion artifacts. A reasonable amount of practice and skill are required for proper positioning, immobilization, and stimulation of larvae. Ultimately, when properly executed, the protocol outlined in this article provides a powerful way to collect valuable information about the activity of hair-cells in their natural, fully integrated states within a live animal.

Behavior, Electrophysiology, and Robotics Experiments to Study Lateral Line Sensing in Fishes

The lateral line system is a sensory system unique to fishes and amphibians. It is composed of distributed mechanosensory hair cell organs on the head and body (neuromasts), which are sensitive to pressure gradients and water movements. Over the last decade, we have pursued an interdisciplinary approach by combining behavioral, electrophysiology, and robotics experiments to study this fascinating sensory system. In behavioral and electrophysiology experiments, we have studied the larval lateral line system in the model genetic organism, zebrafish (Danio rerio). We found that the lateral line system, even in 5-day-old larvae, is involved in an array of behaviors that are critical to survival, and the deflection of a single neuromast can elicit a swimming response. In robotics experiments, we used a range of physical models with distributed pressure sensors to better understand the hydrodynamic environments from the local perspective of a fish or robot. So far, our efforts have focused on extracting control-related information for a range of application scenarios including characterizing unsteady flows such as Kármán vortex streets for station holding. We also used robot models to test biological hypotheses on how morphology and movement of fishes affect lateral line sensing. Overall, with this review we aim to increase the visibility and accessibility of this multi-disciplinary research approach.

Transmission Disrupted: Modeling Auditory Synaptopathy in Zebrafish

Sensorineural hearing loss is the most common form of hearing loss in humans, and results from either dysfunction in hair cells, the sensory receptors of sound, or the neurons that innervate hair cells. A specific type of sensorineural hearing loss, referred to as auditory synaptopathy, occurs when hair cells are able to detect sound but fail to transmit sound stimuli at the hair-cell synapse. Auditory synaptopathy can originate from genetic alterations that specifically disrupt hair-cell synapse function. Additionally, environmental factors such as noise exposure can leave hair cells intact but result in loss of hair-cell synapses, and represent an acquired form of auditory synaptopathy. The zebrafish model has emerged as a valuable system for studies of hair-cell function, and specifically hair-cell synaptopathy. In this review, we describe the experimental tools that have been developed to study hair-cell synapses in zebrafish. We discuss how zebrafish genetics has helped identify and define the roles of hair-cell synaptic proteins crucial for hearing in humans, and highlight how studies in zebrafish have contributed to our understanding of hair-cell synapse formation and function. In addition, we also discuss work that has used noise exposure or pharmacological mimic of noise-induced excitotoxicity in zebrafish to define cellular mechanisms underlying noise-induced hair-cell damage and synapse loss. Lastly, we highlight how future studies in zebrafish could enhance our understanding of the pathological processes underlying synapse loss in both genetic and acquired auditory synaptopathy. This knowledge is critical in order to develop therapies that protect or repair auditory synaptic contacts.

Imaging Neuronal Activity in the Optic Tectum of Late Stage Larval Zebrafish

The zebrafish is an established model to study the development and function of visual neuronal circuits in vivo, largely due to their optical accessibility at embryonic and larval stages. In the past decade multiple experimental paradigms have been developed to study visually-driven behaviours, particularly those regulated by the optic tectum, the main visual centre in lower vertebrates. With few exceptions these techniques are limited to young larvae (7–9 days post-fertilisation, dpf). However, many forms of visually-driven behaviour, such as shoaling, emerge at later developmental stages. Consequently, there is a need for an experimental paradigm to image the visual system in zebrafish larvae beyond 9 dpf. Here, we show that using NBT:GCaMP3 line allows for imaging neuronal activity in the optic tectum in late stage larvae until at least 21 dpf. Utilising this line, we have characterised the receptive field properties of tectal neurons of the 2–3 weeks old fish in the cell bodies and the neuropil. The NBT:GCaMP3 line provides a complementary approach and additional opportunities to study neuronal activity in late stage zebrafish larvae.

The genetics of hair-cell function in zebrafish

Our ears are remarkable sensory organs, providing the important senses of balance and hearing. The complex structure of the inner ear, or 'labyrinth', along with the assorted neuroepithelia, have evolved to detect head movements and sounds with impressive sensitivity. The rub is that the inner ear is highly vulnerable to genetic lesions and environmental insults. According to National Institute of Health estimates, hearing loss is one of the most commonly inherited or acquired sensorineural diseases. To understand the causes of deafness and balance disorders, it is imperative to understand the underlying biology of the inner ear, especially the inner workings of the sensory receptors. These receptors, which are termed hair cells, are particularly susceptible to genetic mutations - more than two dozen genes are associated with defects in this cell type in humans. Over the past decade, a substantial amount of progress has been made in working out the molecular basis of hair-cell function using vertebrate animal models. Given the transparency of the inner ear and the genetic tools that are available, zebrafish have become an increasingly popular animal model for the study of deafness and vestibular dysfunction. Mutagenesis screens for larval defects in hearing and balance have been fruitful in finding key components, many of which have been implicated in human deafness. This review will focus on the genes that are required for hair-cell function in zebrafish, with a particular emphasis on mechanotransduction. In addition, the generation of new tools available for the characterization of zebrafish hair-cell mutants will be discussed.

Enlargement of Ribbons in Zebrafish Hair Cells Increases Calcium Currents But Disrupts Afferent Spontaneous Activity and Timing of Stimulus Onset

In sensory hair cells of auditory and vestibular organs, the ribbon synapse is required for the precise encoding of a wide range of complex stimuli. Hair cells have a unique presynaptic structure, the synaptic ribbon, which organizes both synaptic vesicles and calcium channels at the active zone. Previous work has shown that hair-cell ribbon size is correlated with differences in postsynaptic activity. However, additional variability in postsynapse size presents a challenge to determining the specific role of ribbon size in sensory encoding. To selectively assess the impact of ribbon size on synapse function, we examined hair cells in transgenic zebrafish that have enlarged ribbons, without postsynaptic alterations. Morphologically, we found that enlarged ribbons had more associated vesicles and reduced presynaptic calcium-channel clustering. Functionally, hair cells with enlarged ribbons had larger global and ribbon-localized calcium currents. Afferent neuron recordings revealed that hair cells with enlarged ribbons resulted in reduced spontaneous spike rates. Additionally, despite larger presynaptic calcium signals, we observed fewer evoked spikes with longer latencies from stimulus onset. Together, our work indicates that hair-cell ribbon size influences the spontaneous spiking and the precise encoding of stimulus onset in afferent neurons.

SIGNIFICANCE STATEMENT Numerous studies support that hair-cell ribbon size corresponds with functional sensitivity differences in afferent neurons and, in the case of inner hair cells of the cochlea, vulnerability to damage from noise trauma. Yet it is unclear whether ribbon size directly influences sensory encoding. Our study reveals that ribbon enlargement results in increased ribbon-localized calcium signals, yet reduces afferent spontaneous activity and disrupts the timing of stimulus onset, a distinct aspect of auditory and vestibular encoding. These observations suggest that varying ribbon size alone can influence sensory encoding, and give further insight into how hair cells transduce signals that cover a wide dynamic range of stimuli.

The Power of Zebrafish in Personalised Medicine

The goal of personalised medicine is to develop tailor-made therapies for patients in whom currently available therapeutics fail. This approach requires correlating individual patient genotype data to specific disease phenotype data and using these stratified data sets to identify bespoke therapeutics. Applications for personalised medicine include common complex diseases which may have multiple targets, as well as rare monogenic disorders, for which the target may be unknown. In both cases, whole genome sequence analysis (WGS) is discovering large numbers of disease associated mutations in new candidate genes and potential modifier genes. Currently, the main limiting factor is the determination of which mutated genes are important for disease progression and therefore represent potential targets for drug discovery. Zebrafish have gained popularity as a model organism for understanding developmental processes, disease mechanisms and more recently for drug discovery and toxicity testing. In this chapter, we will examine the diverse roles that zebrafish can make in the expanding field of personalised medicine, from generating humanised disease models to xenograft screening of different cancer cell lines, through to finding new drugs via in vivo phenotypic screens. We will discuss the tools available for zebrafish research and recent advances in techniques, highlighting the advantages and potential of using zebrafish for high throughput disease modeling and precision drug discovery.