Organization and physiology of posterior lateral line afferent neurons in larval zebrafish

Underwater Biomimetic Lateral Line Sensor Based on Triboelectric Nanogenerator for Dynamic Pressure Monitoring and Trajectory Perception

Developing desirable sensors is crucial for underwater perceptions and operations. The perceiving organs of marine creatures have greatly evolved to react accurately and promptly underwater. Inspired by the fish lateral line, this study proposes a triboelectric dynamic pressure sensor for underwater perception. The biomimetic lateral line sensor (BLLS) has high sensitivity to the disturbance amplitude/frequency, good adaptability to underwater environments and (relative) low cost. The sensors are deployed at the bottom of the test basin to perceive various moving objects, such as a robotic fish, robotic seal, etc. By analyzing the electrical signal of the sensor, the motion parameters of the objects passed over can be obtained. By monitoring signal variations across multiple sensors, the ability to sense different disturbance movement trajectories, including linear and angular trajectories, is achievable. The study will prove significant in forming an unconventional underwater perceiving method, which can back-up the sonic/optical sensors when are impaired in complex underwater environments.

Functional and ultrastructural analysis of reafferent mechanosensation in larval zebrafish

All animals need to differentiate between exafferent stimuli, which are caused by the environment, and reafferent stimuli, which are caused by their own movement. In the case of mechanosensation in aquatic animals, the exafferent inputs are water vibrations in the animal's proximity, which need to be distinguishable from the reafferent inputs arising from fluid drag due to locomotion. Both of these inputs are detected by the lateral line, a collection of mechanosensory organs distributed along the surface of the body. In this study, we characterize in detail how hair cells-the receptor cells of the lateral line-in zebrafish larvae discriminate between such reafferent and exafferent signals. Using dye labeling of the lateral line nerve, we visualize two parallel descending inputs that can influence lateral line sensitivity. We combine functional imaging with ultra-structural EM circuit reconstruction to show that cholinergic signals originating from the hindbrain transmit efference copies (copies of the motor command that cancel out self-generated reafferent stimulation during locomotion) and that dopaminergic signals from the hypothalamus may have a role in threshold modulation, both in response to locomotion and salient stimuli. We further gain direct mechanistic insight into the core components of this circuit by loss-of-function perturbations using targeted ablations and gene knockouts. We propose that this simple circuit is the core implementation of mechanosensory reafferent suppression in these young animals and that it might form the first instantiation of state-dependent modulation found at later stages in development.

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.

Embryonic Exposure to Ethanol Increases Anxiety-Like Behavior in Fry Zebrafish

AIMS

Fetal alcohol spectrum disorder (FASD) is an umbrella term to describe the effects of ethanol (Eth) exposure during embryonic development, including several conditions from malformation to cognitive deficits. Zebrafish (Danio rerio) are a translational model popularly applied in brain disorders and drug screening studies due to its genetic and physiology homology to humans added to its transparent eggs and fast development. In this study, we investigated how early ethanol exposure affects zebrafish behavior during the initial growth phase.

METHODS

Fish eggs were exposed to 0.0 (control), 0.25 and 0.5% ethanol at 24 h post-fertilization. Later, fry zebrafish (10 days old) were tested in a novel tank task and an inhibitory avoidance protocol to inquire about morphology and behavioral alterations.

RESULTS

Analysis of variance showed that ethanol doses of 0.25 and 0.5% do not cause morphological malformations and did not impair associative learning but increased anxiety-like behavior responses and lower exploratory behavior when compared to the control.

CONCLUSION

Our results demonstrate that one can detect behavioral abnormalities in the zebrafish induced by embryonic ethanol as early as 10 days post-fertilization and that alcohol increases anxious behavior during young development in zebrafish.

Multimodal mechanosensing enables treefrog embryos to escape egg-predators

Mechanosensory-cued hatching (MCH) is widespread, diverse and important for survival in many animals. From flatworms and insects to frogs and turtles, embryos use mechanosensory cues and signals to inform hatching timing, yet mechanisms mediating mechanosensing in ovo are largely unknown. The arboreal embryos of red-eyed treefrogs, Agalychnis callidryas, hatch prematurely to escape predation, cued by physical disturbance in snake attacks. When otoconial organs in the developing vestibular system become functional, this response strengthens, but its earlier occurrence indicates another sensor must contribute. Post-hatching, tadpoles use lateral line neuromasts to detect water motion. We ablated neuromast function with gentamicin to assess their role in A. callidryas' hatching response to disturbance. Prior to vestibular function, this nearly eliminated the hatching response to a complex simulated attack cue, egg jiggling, revealing that neuromasts mediate early MCH. Vestibular function onset increased hatching, independent of neuromast function, indicating young embryos use multiple mechanosensory systems. MCH increased developmentally. All older embryos hatched in response to egg jiggling, but neuromast function reduced response latency. In contrast, neuromast ablation had no effect on the timing or level of hatching in motion-only vibration playbacks. It appears only a subset of egg-disturbance cues stimulate neuromasts; thus, embryos in attacked clutches may receive unimodal or multimodal stimuli. Agalychnis callidryas embryos have more neuromasts than described for any other species at hatching, suggesting precocious sensory development may facilitate MCH. Our findings provide insight into the behavioral roles of two mechanosensory systems in ovo and open possibilities for exploring sensory perception across taxa in early life stages.

Unraveling the Molecular Players at the Cholinergic Efferent Synapse of the Zebrafish Lateral Line

The lateral line (LL) is a sensory system that allows fish and amphibians to detect water currents. LL responsiveness is modulated by efferent neurons that aid in distinguishing between external and self-generated stimuli, maintaining sensitivity to relevant cues. One component of the efferent system is cholinergic, the activation of which inhibits afferent activity. LL hair cells (HCs) share structural, functional, and molecular similarities with those of the cochlea, making them a popular model for studying human hearing and balance disorders. Because of these commonalities, one could propose that the receptor at the LL efferent synapse is a α9α10 nicotinic acetylcholine receptor (nAChR). However, the identities of the molecular players underlying ACh-mediated inhibition in the LL remain unknown. Surprisingly, through the analysis of single-cell expression studies and in situ hybridization, we describe that α9, but not the α10, subunits are enriched in zebrafish HCs. Moreover, the heterologous expression of zebrafish α9 subunits indicates that homomeric receptors are functional and exhibit robust ACh-gated currents blocked by α-bungarotoxin and strychnine. In addition, in vivo Ca²⁺ imaging on mechanically stimulated zebrafish LL HCs show that ACh elicits a decrease in evoked Ca²⁺ signals, regardless of HC polarity. This effect is blocked by both α-bungarotoxin and apamin, indicating coupling of ACh-mediated effects to small-conductance Ca²⁺-activated potassium (SKs) channels. Our results indicate that an α9-containing (α9*) nAChR operates at the zebrafish LL efferent synapse. Moreover, the activation of α9* nAChRs most likely leads to LL HC hyperpolarization served by SK channels.SIGNIFICANCE STATEMENT The fish lateral line (LL) mechanosensory system shares structural, functional, and molecular similarities with those of the mammalian cochlea. Thus, it has become an accessible model for studying human hearing and balance disorders. However, the molecular players serving efferent control of LL hair cell (HC) activity have not been identified. Here we demonstrate that, different from the hearing organ of vertebrate species, a nicotinic acetylcholine receptor composed only of α9 subunits operates at the LL efferent synapse. Activation of α9-containing receptors leads to LL HC hyperpolarization because of the opening of small-conductance Ca²⁺-activated potassium channels. These results will further aid in the interpretation of data obtained from LL HCs as a model for cochlear HCs.

Brain-Wide Mapping of Water Flow Perception in Zebrafish

Information about water flow, detected by lateral line organs, is critical to the behavior and survival of fish and amphibians. While certain aspects of water flow processing have been revealed through electrophysiology, we lack a comprehensive description of the neurons that respond to water flow and the network that they form. Here, we use brain-wide calcium imaging in combination with microfluidic stimulation to map out, at cellular resolution, neuronal responses involved in perceiving and processing water flow information in larval zebrafish. We find a diverse array of neurons responding to head-to-tail (h-t) flow, tail-to-head (t-h) flow, or both. Early in this pathway, in the lateral line ganglia, neurons respond almost exclusively to the simple presence of h-t or t-h flow, but later processing includes neurons responding specifically to flow onset, representing the accumulated displacement of flow during a stimulus, or encoding the speed of the flow. The neurons reporting on these more nuanced details are located across numerous brain regions, including some not previously implicated in water flow processing. A graph theory-based analysis of the brain-wide water flow network shows that a majority of this processing is dedicated to h-t flow detection, and this is reinforced by our finding that details like flow velocity and the total accumulated flow are only encoded for the h-t direction. The results represent the first brain-wide description of processing for this important modality, and provide a departure point for more detailed studies of the flow of information through this network.SIGNIFICANCE STATEMENT In aquatic animals, the lateral line is important for detecting water flow stimuli, but the brain networks that interpret this information remain mysterious. Here, we have imaged the activity of individual neurons across the entire brains of larval zebrafish, revealing all response types and their brain locations as water flow processing occurs. We find neurons that respond to the simple presence of water flow, and others attuned to the direction, speed, and duration of flow, or the accumulated displacement of water that has passed during the stimulus. With this information, we modeled the underlying network, describing a system that is nuanced in its processing of water flow simulating head-to-tail motion but rudimentary in processing flow in the tail-to-head direction.

Canal neuromasts enhance foraging in zebrafish (Danio rerio)

Aquatic animals commonly sense flow using superficial neuromasts (SNs), which are receptors that extend from the body's surface. The lateral line of fishes is unique among these systems because it additionally possesses receptors, the canal neuromasts (CNs), that are recessed within a channel. The lateral line has inspired the development of engineered sensors and concepts in the analysis of flow fields for submersible navigation. The biophysics of CNs are known to be different from the SNs and thereby offer a distinct submodality. However, it is generally unclear whether CNs play a distinct role in behavior. We therefore tested whether CNs enhance foraging in the dark by zebrafish (Danio rerio), a behavior that we elicited with a vibrating rod. We found that juvenile fish, which have only SNs, bite at this rod at about one-third the rate and from as little as one-third the distance of adults for a high-frequency stimulus (50  <  f   <  100 Hz). We used novel techniques for manipulating the lateral line in adults to find that CNs offered only a modest benefit at a lower frequency (20 Hz) and that foraging was mediated entirely by cranial neuromasts. Consistent with our behavioral results, biophysical models predicted CNs to be more than an order of magnitude more sensitive than SNs at high frequencies. This enhancement helps to overcome the rapid spatial decay in high-frequency components in the flow around the stimulus. These findings contrast what has been previously established for fishes that are at least ten-times the length of zebrafish, which use trunk CNs to localize prey. Therefore, CNs generally enhance foraging, but in a manner that varies with the size of the fish and its prey. These results have the potential to improve our understanding of flow sensing in aquatic animals and engineered systems.

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.

Sensory Feedback and Animal Locomotion: Perspectives from Biology and Biorobotics: An Introduction to the Symposium

The successful completion of many behaviors relies on sensory feedback. This symposium brought together researchers using novel techniques to study how different stimuli are encoded, how and where multimodal feedback is integrated, and how feedback modulates motor output in diverse modes of locomotion (aerial, aquatic, and terrestrial) in a diverse range of taxa (insects, fish, tetrapods), and in robots. Similar to biological organisms, robots can be equipped with integrated sensors and can rely on sensory feedback to adjust the output signal of a controller. Engineers often look to biology for inspiration on how animals have evolved solutions to problems similar to those experienced in robotic movement. Similarly, biologists too must proactively engage with engineers to apply computer and robotic models to test hypotheses and answer questions on the capacity and roles of sensory feedback in generating effective movement. Through a diverse group of researchers, including both biologists and engineers, the symposium attempted to catalyze new interdisciplinary collaborations and identify future research directions for the development of bioinspired sensory control systems, as well as the use of robots to test hypotheses in neuromechanics.

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.

Principles Governing Locomotion in Vertebrates: Lessons From Zebrafish

Locomotor behaviors are critical for survival and enable animals to navigate their environment, find food and evade predators. The circuits in the brain and spinal cord that initiate and maintain such different modes of locomotion in vertebrates have been studied in numerous species for over a century. In recent decades, the zebrafish has emerged as one of the main model systems for the study of locomotion, owing to its experimental amenability, and work in zebrafish has revealed numerous new insights into locomotor circuit function. Here, we review the literature that has led to our current understanding of the neural circuits controlling swimming and escape in zebrafish. We highlight recent studies that have enriched our comprehension of key topics, such as the interactions between premotor excitatory interneurons (INs) and motoneurons (MNs), supraspinal and spinal circuits that coordinate escape maneuvers, and developmental changes in overall circuit composition. We also discuss roles for neuromodulators and sensory inputs in modifying the relative strengths of constituent circuit components to provide flexibility in zebrafish behavior, allowing the animal to accommodate changes in the environment. We aim to provide a coherent framework for understanding the circuitry in the brain and spinal cord of zebrafish that allows the animal to flexibly transition between different speeds, and modes, of locomotion.

Hair cell identity establishes labeled lines of directional mechanosensation

Directional mechanoreception by hair cells is transmitted to the brain via afferent neurons to enable postural control and rheotaxis. Neuronal tuning to individual directions of mechanical flow occurs when each peripheral axon selectively synapses with multiple hair cells of identical planar polarization. How such mechanosensory labeled lines are established and maintained remains unsolved. Here, we use the zebrafish lateral line to reveal that asymmetric activity of the transcription factor Emx2 diversifies hair cell identity to instruct polarity-selective synaptogenesis. Unexpectedly, presynaptic scaffolds and coherent hair cell orientation are dispensable for synaptic selectivity, indicating that epithelial planar polarity and synaptic partner matching are separable. Moreover, regenerating axons recapitulate synapses with hair cells according to Emx2 expression but not global orientation. Our results identify a simple cellular algorithm that solves the selectivity task even in the presence of noise generated by the frequent receptor cell turnover. They also suggest that coupling connectivity patterns to cellular identity rather than polarity relaxes developmental and evolutionary constraints to innervation of organs with differing orientation.

Mechanosensory systems are essential for maintaining posture, gaze, and body orientation during locomotion. Such stability requires a coherent representation in the brain of the location and movement of mechanical stimuli. In fishes, mechanical stimuli at a given position activate direction-sensitive receptors called hair cells that are oriented with polarized directionality. These hair cells stimulate neurons that selectively connect with them based on polarity. We have addressed how neurons target hair cells based on polarity during development of the mechanosensory lateral line system in zebrafish. We show that neurons selectively connect based on the expression pattern of the transcription factor Emx2 in hair cells. We find that the lateral line can maintain directionality after damage and regeneration. Our data suggest a cellular mechanism that controls the formation, maintenance, and regeneration of labeled lines to enable directional mechanosensation.

Connectomics of the zebrafish's lateral-line neuromast reveals wiring and miswiring in a simple microcircuit

The lateral-line neuromast of the zebrafish displays a restricted, consistent pattern of innervation that facilitates the comparison of microcircuits across individuals, developmental stages, and genotypes. We used serial blockface scanning electron microscopy to determine from multiple specimens the neuromast connectome, a comprehensive set of connections between hair cells and afferent and efferent nerve fibers. This analysis delineated a complex but consistent wiring pattern with three striking characteristics: each nerve terminal is highly specific in receiving innervation from hair cells of a single directional sensitivity; the innervation is redundant; and the terminals manifest a hierarchy of dominance. Mutation of the canonical planar-cell-polarity gene vangl2, which decouples the asymmetric phenotypes of sibling hair-cell pairs, results in randomly positioned, randomly oriented sibling cells that nonetheless retain specific wiring. Because larvae that overexpress Notch exhibit uniformly oriented, uniformly innervating hair-cell siblings, wiring specificity is mediated by the Notch signaling pathway.

Rapid whole brain imaging of neural activity in freely behaving larval zebrafish (Danio rerio)

The internal brain dynamics that link sensation and action are arguably better studied during natural animal behaviors. Here, we report on a novel volume imaging and 3D tracking technique that monitors whole brain neural activity in freely swimming larval zebrafish (Danio rerio). We demonstrated the capability of our system through functional imaging of neural activity during visually evoked and prey capture behaviors in larval zebrafish.

Rapid whole brain imaging of neural activity in freely behaving larval zebrafish (Danio rerio)

The internal brain dynamics that link sensation and action are arguably better studied during natural animal behaviors. Here, we report on a novel volume imaging and 3D tracking technique that monitors whole brain neural activity in freely swimming larval zebrafish (Danio rerio). We demonstrated the capability of our system through functional imaging of neural activity during visually evoked and prey capture behaviors in larval zebrafish.

How do neurons in the brain process information from the senses and drive complex behaviors? This question has fascinated neuroscientists for many years. It is currently not possible to record the electrical activities of all of the 100 billion neurons in a human brain. Yet, in the last decade, it has become possible to genetically engineer some neurons in animals to produce fluorescence reporters that change their brightness in response to brain activity and then monitor them under a microscope. In small animals such as zebrafish larvae, this method makes it possible to monitor the activities of all the neurons in the brain if the animal’s head is held still. However, many behaviors – for example, catching prey – require movement, and no existing technique could image brain activity in enough detail if the animal’s head was moving.

Cong, Wang, Chai, Hang et al. have now made progress towards this goal by developing a new technique to image neural activity across the whole brain of a zebrafish larva as it swims freely in a small water-filled chamber. The technique uses high-speed cameras and computer software to track the movements of the fish in three dimensions, and then automatically moves the chamber under the microscope such that the animal’s brain is constantly kept in focus. The newly developed microscope can capture changes in neural activity across a large volume all at the same time. It is then further adapted to overcome problems caused by sudden or swift movements, which would normally result in motion blur. With this microscope set up, Cong et al. were able to capture, for the first time, activity from all the neurons in a zebrafish larva’s brain as it pursued and caught its prey.

This technique provides a new window into how brain activity changes when animals are behaving naturally. In the future, this technique could help link the activities of neurons to different behaviors in several popular model organisms including fish, worms and fruit flies.

Intensity-dependent timing and precision of startle response latency in larval zebrafish

KEY POINTS

Using high-speed videos time-locked with whole-animal electrical recordings, simultaneous measurement of behavioural kinematics and field potential parameters of C-start startle responses allowed for discrimination between short-latency and long-latency C-starts (SLCs vs. LLCs) in larval zebrafish. Apart from their latencies, SLC kinematics and SLC field potential parameters were intensity independent. Increasing stimulus intensity increased the probability of evoking an SLC and decreased mean SLC latencies while increasing their precision; subtraction of field potential latencies from SLC latencies revealed a fixed time delay between the two measurements that was intensity independent. The latency and the precision in the latency of the SLC field potentials were linearly correlated to the latencies and precision of the first evoked action potentials (spikes) in hair-cell afferent neurons of the lateral line. Together, these findings indicate that first spike latency (FSL) is a fast encoding mechanism that can serve to precisely initiate startle responses when speed is critical for survival.

ABSTRACT

Vertebrates rely on fast sensory encoding for rapid and precise initiation of startle responses. In afferent sensory neurons, trains of action potentials (spikes) encode stimulus intensity within the onset time of the first evoked spike (first spike latency; FSL) and the number of evoked spikes. For speed of initiation of startle responses, FSL would be the more advantageous mechanism to encode the intensity of a threat. However, the intensity dependence of FSL and spike number and whether either determines the precision of startle response initiation is not known. Here, we examined short-latency startle responses (SLCs) in larval zebrafish and tested the hypothesis that first spike latencies and their precision (jitter) determine the onset time and precision of SLCs. We evoked startle responses via activation of Channelrhodopsin (ChR2) expressed in ear and lateral line hair cells and acquired high-speed videos of head-fixed larvae while simultaneously recording underlying field potentials. This method allowed for discrimination between primary SLCs and less frequent, long-latency startle responses (LLCs). Quantification of SLC kinematics and field potential parameters revealed that, apart from their latencies, they were intensity independent. We found that increasing stimulus intensity decreased SLC latencies while increasing their precision, which was significantly correlated with corresponding changes in field potential latencies and their precision. Single afferent neuron recordings from the lateral line revealed a similar intensity-dependent decrease in first spike latencies and their jitter, which could account for the intensity-dependent changes in timing and precision of startle response latencies.

Physiological recordings from the zebrafish lateral line

During sensory transduction, external physical stimuli are translated into an internal biological signal. In vertebrates, hair cells are specialized mechanosensory receptors that transduce sound, gravitational forces, and head movements into electrical signals that are transmitted with remarkable precision and efficiency to afferent neurons. Hair cells have a conserved structure between species and are also found in the lateral line system of fish, including zebrafish, which serve as an ideal animal model to study sensory transmission in vivo. In this chapter, we describe the methods required to investigate the biophysical properties underlying mechanosensation in the lateral line of the zebrafish in vivo from microphonic potentials and single hair cell patch-clamp recordings to single afferent neuron recordings. These techniques provide real-time measurements of hair-cell transduction and transmission following delivery of controlled and defined stimuli and their combined use on the intact zebrafish provides a powerful platform to investigate sensory encoding in vivo.

Converging axons collectively initiate and maintain synaptic selectivity in a constantly remodeling sensory organ

Sensory receptors are the functional link between the environment and the brain. The repair of sensory organs enables animals to continuously detect environmental stimuli. However, receptor cell turnover can affect sensory acuity by changing neural connectivity patterns. In zebrafish, two to four postsynaptic lateralis afferent axons converge into individual peripheral mechanosensory organs called neuromasts, which contain hair cell receptors of opposing planar polarity. Yet, each axon exclusively synapses with hair cells of identical polarity during development and regeneration to transmit unidirectional mechanical signals to the brain. The mechanism that governs this exceptionally accurate and resilient synaptic selectivity remains unknown. We show here that converging axons are mutually dependent for polarity-selective connectivity. If rendered solitary, these axons establish simultaneous functional synapses with hair cells of opposing polarities to transmit bidirectional mechanical signals. Remarkably, nonselectivity by solitary axons can be corrected upon the reintroduction of additional axons. Collectively, our results suggest that lateralis synaptogenesis is intrinsically nonselective and that interaxonal interactions continuously rectify mismatched synapses. This dynamic organization of neural connectivity may represent a general solution to maintain coherent synaptic transmission from sensory organs undergoing frequent variations in the number and spatial distribution of receptor cells.

Frequency response properties of primary afferent neurons in the posterior lateral line system of larval zebrafish

The ability of fishes to detect water flow with the neuromasts of their lateral line system depends on the physiology of afferent neurons as well as the hydrodynamic environment. Using larval zebrafish (Danio rerio), we measured the basic response properties of primary afferent neurons to mechanical deflections of individual superficial neuromasts. We used two types of stimulation protocols. First, we used sine wave stimulation to characterize the response properties of the afferent neurons. The average frequency-response curve was flat across stimulation frequencies between 0 and 100 Hz, matching the filtering properties of a displacement detector. Spike rate increased asymptotically with frequency, and phase locking was maximal between 10 and 60 Hz. Second, we used pulse train stimulation to analyze the maximum spike rate capabilities. We found that afferent neurons could generate up to 80 spikes/s and could follow a pulse train stimulation rate of up to 40 pulses/s in a reliable and precise manner. Both sine wave and pulse stimulation protocols indicate that an afferent neuron can maintain their evoked activity for longer durations at low stimulation frequencies than at high frequencies. We found one type of afferent neuron based on spontaneous activity patterns and discovered a correlation between the level of spontaneous and evoked activity. Overall, our results establish the baseline response properties of lateral line primary afferent neurons in larval zebrafish, which is a crucial step in understanding how vertebrate mechanoreceptive systems sense and subsequently process information from the environment.

There and back again: development and regeneration of the zebrafish lateral line system

The zebrafish lateral line is a sensory system used to detect changes in water flow. It is comprised of clusters of mechanosensory hair cells called neuromasts. The lateral line is initially established by a migratory group of cells, called a primordium, that deposits neuromasts at stereotyped locations along the surface of the fish. Wnt, FGF, and Notch signaling are all important regulators of various aspects of lateral line development, from primordium migration to hair cell specification. As zebrafish age, the organization of the lateral line becomes more complex in order to accommodate the fish's increased size. This expansion is regulated by many of the same factors involved in the initial development. Furthermore, unlike mammalian hair cells, lateral line hair cells have the capacity to regenerate after damage. New hair cells arise from the proliferation and differentiation of surrounding support cells, and the molecular and cellular pathways regulating this are beginning to be elucidated. All in all, the zebrafish lateral line has proven to be an excellent model in which to study a diverse array of processes, including collective cell migration, cell polarity, cell fate, and regeneration.

Afferent and motoneuron activity in response to single neuromast stimulation in the posterior lateral line of larval zebrafish

The lateral line system of fishes contains mechanosensory receptors along the body surface called neuromasts, which can detect water motion relative to the body. The ability to sense flow informs many behaviors, such as schooling, predator avoidance, and rheotaxis. Here, we developed a new approach to stimulate individual neuromasts while either recording primary sensory afferent neuron activity or swimming motoneuron activity in larval zebrafish (Danio rerio). Our results allowed us to characterize the transfer functions between a controlled lateral line stimulus, its representation by primary sensory neurons, and its subsequent behavioral output. When we deflected the cupula of a neuromast with a ramp command, we found that the connected afferent neuron exhibited an adapting response which was proportional in strength to deflection velocity. The maximum spike rate of afferent neurons increased sigmoidally with deflection velocity, with a linear range between 0.1 and 1.0 μm/ms. However, spike rate did not change when the cupula was deflected below 8 μm, regardless of deflection velocity. Our findings also reveal an unexpected sensitivity in the larval lateral line system: stimulation of a single neuromast could elicit a swimming response which increased in reliability with increasing deflection velocities. At high deflection velocities, we observed that lateral line evoked swimming has intermediate values of burst frequency and duty cycle that fall between electrically evoked and spontaneous swimming. An understanding of the sensory capabilities of a single neuromast will help to build a better picture of how stimuli are encoded at the systems level and ultimately translated into behavior.

Behavioral changes of zebrafish according to cisplatin-induced toxicity of the balance system

Background and objectives:

Zebrafish are commonly used as experimental animals in otolaryngology studies. However, the behavioral characteristics of these fish are not well known, especially those related to the vestibular system. The goal of this study was to evaluate behavioral changes in zebrafish due to toxicity in the balance system.

Materials and methods:

Zebrafish were exposed to 1000 μM cisplatin for 6 h. We, then, periodically monitored swimming depth, total swimming distance, peak swimming velocity, and mean swimming velocity of the fish for approximately 21 days.

Results:

Total swimming distance ( p < 0.0001), peak swimming velocity ( p = 0.0063), and mean swimming velocity ( p < 0.0001) in the cisplatin-administered group were significantly decreased when compared with control fish.

Conclusion:

Our findings demonstrate that cisplatin can alter the locomotion behavior of zebrafish.

Developmental and architectural principles of the lateral-line neural map

The transmission and central representation of sensory cues through the accurate construction of neural maps is essential for animals to react to environmental stimuli. Structural diversity of sensorineural maps along a continuum between discrete- and continuous-map architectures can influence behavior. The mechanosensory lateral line of fishes and amphibians, for example, detects complex hydrodynamics occurring around the animal body. It triggers innate fast escape reactions but also modulates complex navigation behaviors that require constant knowledge about the environment. The aim of this article is to summarize recent work in the zebrafish that has shed light on the development and structure of the lateralis neural map, which is helping to understand how individual sensory modalities generate appropriate behavioral responses to the sensory context.

Heterogeneity and dynamics of lateral line afferent innervation during development in zebrafish (Danio rerio)

The lateral line system of larval zebrafish is emerging as a model to study a range of topics in neurobiology, from hair cell regeneration to sensory processing. However, despite numerous studies detailing the patterning and development of lateral line neuromasts, little is known about the organization of their connections to afferent neurons and targets in the hindbrain. We found that as fish grow and neuromasts proliferate over the body surface, the number of afferent neurons increases linearly. The number of afferents innervating certain neuromasts increases over time, while it decreases for other neuromasts. The ratio of afferent neurons to neuromasts differs between the anterior and posterior lateral line system, suggesting potential differences in sensitivity threshold or spatial resolution. A single afferent neuron routinely contacts a group of neuromasts, suggesting that different afferent neurons can convey information about receptive fields along the body. When afferent projections are traced into the hindbrain, where a distinct somatotopy has been previously described, we find that this general organization is absent at the Mauthner cell. We speculate that directional input from the lateral line is less important at an early age, whereas the speed of the escape response is paramount, and that directional responses arise later in development. By quantifying morphological connections in the lateral line system, this study provides a detailed foundation to understand how hydrodynamic information is processed and ultimately translated into appropriate motor behaviors.

Neuronal birth order identifies a dimorphic sensorineural map

Spatially distributed sensory information is topographically mapped in the brain by point-to-point correspondence of connections between peripheral receptors and central target neurons. In fishes, for example, the axonal projections from the mechanosensory lateral line organize a somatotopic neural map. The lateral line provides hydrodynamic information for intricate behaviors such as navigation and prey detection. It also mediates fast startle reactions triggered by the Mauthner cell. However, it is not known how the lateralis neural map is built to subserve these contrasting behaviors. Here we reveal that birth order diversifies lateralis afferent neurons in the zebrafish. We demonstrate that early- and late-born lateralis afferents diverge along the main axes of the hindbrain to synapse with hundreds of second-order targets. However, early-born afferents projecting from primary neuromasts also assemble a separate map by converging on the lateral dendrite of the Mauthner cell, whereas projections from secondary neuromasts never make physical contact with the Mauthner cell. We also show that neuronal diversity and map topology occur normally in animals permanently deprived of mechanosensory activity. We conclude that neuronal birth order correlates with the assembly of neural submaps, whose combination is likely to govern appropriate behavioral reactions to the sensory context.

Physiology of afferent neurons in larval zebrafish provides a functional framework for lateral line somatotopy

Fishes rely on the neuromasts of their lateral line system to detect water flow during behaviors such as predator avoidance and prey localization. Although the pattern of neuromast development has been a topic of detailed research, we still do not understand the functional consequences of its organization. Previous work has demonstrated somatotopy in the posterior lateral line, whereby afferent neurons that contact more caudal neuromasts project more dorsally in the hindbrain than those that contact more rostral neuromasts (Gompel N, Dambly-Chaudiere C, Ghysen A. Development 128: 387-393, 2001). We performed patch-clamp recordings of afferent neurons that contact neuromasts in the posterior lateral line of anesthetized, transgenic larval zebrafish (Danio rerio) to show that larger cells are born earlier, have a lower input resistance, a lower spontaneous firing rate, and tend to contact multiple neuromasts located closer to the tail than smaller neurons, which are born later, have a higher input resistance, a higher spontaneous firing rate, and tend to contact single neuromasts. We suggest that early-born neurons are poised to detect large stimuli during the initial stages of development. Later-born neurons are more easily driven to fire and thus likely to be more sensitive to local, weaker flows. Afferent projections onto identified glutamatergic regions in the hindbrain lead us to hypothesize a novel mechanism for lateral line somatotopy. We show that afferent fibers associated with tail neuromasts respond to stronger stimuli and are wired to dorsal hindbrain regions associated with Mauthner-mediated escape responses and fast, avoidance swimming. The ability to process flow stimuli by circumventing higher-order brain centers would ease the task of processing where speed is of critical importance. Our work lays the groundwork to understand how the lateral line translates flow stimuli into appropriate behaviors at the single cell level.

Particle motion is broadly represented in the vestibular medulla of the bullfrog across larval development

In their shallow-water habitats, bullfrog (Rana catesbeiana) tadpoles are exposed to both underwater and airborne sources of acoustic stimulation. We probed the representation of underwater particle motion throughout the tadpole's dorsal medulla to determine its spatial extent over larval life. Using neurobiotin-filled micropipettes, we recorded neural activity to z-axis particle motion (frequencies of 40-200 Hz) in the medial vestibular nucleus, lateral vestibular nucleus, dorsal medullary nucleus (DMN), and along the dorsal arcuate pathway. Sensitivity was comparable in the medial and lateral vestibular nuclei, with estimated thresholds between 0.016 and 12.5 μm displacement. Neither best responding frequency nor estimated threshold varied significantly over larval stage. Transport of neurobiotin from active recording sites was also stable over development. The DMN responded poorly to z-axis particle motion, but did respond to low-frequency pressure stimulation. These data suggest that particle motion is represented widely and stably in the tadpole's vestibular medulla. This is in marked contrast to the representation of pressure stimulation in the auditory midbrain, where a transient "deaf period" of non-responsiveness and decreased connectivity occurs immediately prior to metamorphic climax. We suggest that, in bullfrogs, sensitivity to particle motion and to pressure follows different developmental trajectories.

A moving topic: control and dynamics of animal locomotion

Animal locomotion arises from complex interactions among sensory systems, processing of sensory information into patterns of motor output, the musculo-skeletal dynamics that follow motor stimulation, and the interaction of appendages and body parts with the environment. These processes conspire to produce motions and forces that permit stunning manoeuvres with important ecological and evolutionary consequences. Thus, the habitats that animals may exploit, their ability to escape predators or attack prey, their capacity to manoeuvre and turn, or the use of their available energy all depend upon the processes that determine locomotion. Here, we summarize a series of 10 papers focused on this integrative research topic.