Anion efflux mediates transduction in the hair cells of the zebrafish lateral line.

Hair cells are the principal sensory receptors of the vertebrate auditory system, where they transduce sounds through mechanically gated ion channels that permit cations to flow from the surrounding endolymph into the cells. The lateral line of zebrafish has served as a key model system for understanding hair cell physiology and development, often with the belief that these hair cells employ a similar transduction mechanism. In this study, we demonstrate that these hair cells are exposed to an unregulated external environment with cation concentrations that are too low to support transduction. Our results indicate that hair cell excitation is instead mediated by a substantially different mechanism involving the outward flow of anions. Further investigation of hair cell transduction in a diversity of sensory systems and species will likely yield deep insights into the physiology of these unique cells.

Tadpoles rely on mechanosensory stimuli for communication when visual capabilities are poor.

The ways in which animals sense the world changes throughout development. For example, young of many species have limited visual capabilities, but still make social decisions, likely based on information gathered through other sensory modalities. Poison frog tadpoles display complex social behaviors that have been suggested to rely on vision despite a century of research indicating tadpoles have poorly-developed visual systems relative to adults. Alternatively, other sensory modalities, such as the lateral line system, are functional at hatching in frogs and may guide social decisions while other sensory systems mature. Here, we examined development of the mechanosensory lateral line and visual systems in tadpoles of the mimic poison frog (Ranitomeya imitator) that use vibrational begging displays to stimulate egg feeding from their mothers. We found that tadpoles hatch with a fully developed lateral line system. While begging behavior increases with development, ablating the lateral line system inhibited begging in pre-metamorphic tadpoles, but not in metamorphic tadpoles. We also found that the increase in begging and decrease in reliance on the lateral line co-occurs with increased retinal neural activity and gene expression associated with eye development. Using the neural tracer neurobiotin, we found that axonal innervations from the eye to the brain proliferate during metamorphosis, with few retinotectal connections in recently-hatched tadpoles. We then tested visual function in a phototaxis assay and found tadpoles prefer darker environments. The strength of this preference increased with developmental stage, but eyes were not required for this behavior, possibly indicating a role for the pineal gland. Together, these data suggest that tadpoles rely on different sensory modalities for social interactions across development and that the development of sensory systems in socially complex poison frog tadpoles is similar to that of other frog species.

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.

The role of vision and lateral line sensing for schooling in giant danios (Devario aequipinnatus).

Schooling is a collective behavior that relies on a fish's ability to sense and respond to the other fish around it. Previous work has identified 'rules' of schooling - attraction to neighbors that are far away, repulsion from neighbors that are too close and alignment with neighbors at the correct distance - but we do not understand well how these rules emerge from the sensory physiology and behavior of individual fish. In particular, fish use both vision and their lateral lines to sense each other, but it is unclear how much they rely on information from these sensory modalities to coordinate schooling behavior. To address this question, we studied how the schooling of giant danios (Devario aequipinnatus) changes when they are unable to see or use their lateral lines. We found that giant danios were able to school without their lateral lines but did not school in darkness. Surprisingly, giant danios in darkness had the same attraction properties as fish in light when they were in close proximity, indicating that they could sense nearby fish with their lateral lines. However, they were not attracted to more distant fish, suggesting that long-distance attraction through vision is important for maintaining a cohesive school. These results help us expand our understanding of the roles that vision and the lateral line play in the schooling of some fish species.

Efferent axons in the zebrafish lateral line degenerate following sensory hair cell ablation.

The zebrafish lateral line is a frequently used model to study the mechanisms behind peripheral neuronal innervation of sensory organs and the regeneration thereof. The lateral line system consists of neuromasts, a cluster of protruding hair cells, which are innervated by sensory afferent and modulatory efferent neurons. These flow-sensing hair cells are similar to the hair cells in the mammalian ear. Though, while hair cell loss in humans is irreversible, the zebrafish neuromasts are regarded as the fastest regenerating structure in vertebrates, making them an ideal model to study regeneration. However, one component of the lateral line system, the efferent projections, has largely been omitted in regenerative studies. Here, for the first time, we bring insights into the fate of efferent axons during ablation and regeneration of the hair cells in the zebrafish lateral line. Our behavioral analysis showed functional recovery of hair cells and sensory transmission within 48 h and their regeneration were in line with previous studies. Analysis of the inhibitory efferent projections revealed that in approximately half the cases the inhibitory efferent axons degenerated, which was never observed for the sensory afferent axons. Quantification of hair cells following ablation suggests that the presence of mature hair cells in the neuromast may prevent axon degeneration. Within 120 h, degenerated efferent axons regenerated along the axonal tract of the lateral line. Reanalysis of published single cell neuromast data hinted to a role for Bdnf in the survival of efferent axons. However, sequestering Bdnf, blocking the Trk-receptors, and inhibiting the downstream ERK-signaling, did not induce axon degeneration, indicating that efferent survival is not mediated through neurotrophic factors. To further explore the relation between hair cells and efferent projections, we generated atoh1a mutants, where mature hair cells never form. In larvae lacking hair cells, inhibitory efferent projections were still present, following the tract of the sensory afferent without displaying any innervation. Our study reveal the fate of efferent innervation following hair cell ablation and provide insights into the inherent differences in regeneration between neurons in the peripheral and central nervous system.

Structural and functional evolution of the mechanosensory lateral line system of fishesa).

The mechanosensory lateral line system is the flow sensing system present in all 34 000+ species of fishes. Its neuromast receptor organs, located on the skin or in bony canals on the head and tubed scales on the trunk, respond to the near field component of acoustic stimuli as well as short range, low frequency (0-200 Hz) water flows of biotic and abiotic origin. Here, I discuss the genesis of my research career and its focus on the structural and functional evolution of the lateral line system among a wide taxonomic range of fishes including those from different aquatic habitats (tropical lakes to coral reefs and the deep sea). I discuss the importance of investigating structure before function, using investigations in my laboratory that had unexpected outcomes, as well as the role of serendipity in the evolution of a career and in the nature of scientific discovery.

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.

The early development and physiology of Xenopus laevis tadpole lateral line system.

Xenopus laevis has a lateral line mechanosensory system throughout its full life cycle, and a previous study on prefeeding stage tadpoles revealed that it may play a role in motor responses to both water suction and water jets. Here, we investigated the physiology of the anterior lateral line system in newly hatched tadpoles and the motor outputs induced by its activation in response to brief suction stimuli. High-speed videoing showed tadpoles tended to turn and swim away when strong suction was applied close to the head. The lateral line neuromasts were revealed by using DASPEI staining, and their inactivation with neomycin eliminated tadpole motor responses to suction. In immobilized preparations, suction or electrically stimulating the anterior lateral line nerve reliably initiated swimming but the motor nerve discharges implicating turning was observed only occasionally. The same stimulation applied during ongoing fictive swimming produced a halting response. The anterior lateral line nerve showed spontaneous afferent discharges at rest and increased activity during stimulation. Efferent activities were only recorded during tadpole fictive swimming and were largely synchronous with the ipsilateral motor nerve discharges. Finally, calcium imaging identified neurons with fluorescence increase time-locked with suction stimulation in the hindbrain and midbrain. A cluster of neurons at the entry point of the anterior lateral line nerve in the dorsolateral hindbrain had the shortest latency in their responses, supporting their potential sensory interneuron identity. Future studies need to reveal how the lateral line sensory information is processed by the central circuit to determine tadpole motor behavior.NEW & NOTEWORTHY We studied Xenopus tadpole motor responses to anterior lateral line stimulation using high-speed videos, electrophysiology and calcium imaging. Activating the lateral line reliably started swimming. At high stimulation intensities, turning was observed behaviorally but suitable motor nerve discharges were seen only occasionally in immobilized tadpoles. Suction applied during swimming produced a halting response. We analyzed afferent and efferent activities of the tadpole anterior lateral line nerve and located sensory interneurons using calcium imaging.

The Transfer Characteristics of Hair Cells Encoding Mechanical Stimuli in the Lateral Line of Zebrafish.

Hair cells transmit mechanical information by converting deflection of the hair bundle into synaptic release of glutamate. We have investigated this process in the lateral line of larval zebrafish (male and female) to understand how stimuli are encoded within a neuromast. Using multiphoton microscopy in vivo, we imaged synaptic release of glutamate using the reporter iGluSnFR as well as deflections of the cupula. We found that the neuromast is composed of a functionally diverse population of hair cells. Half the hair cells signaled cupula motion in both directions from rest, either by increasing glutamate release in response to a deflection in the positive direction or by reducing release in the negative direction. The relationship between cupula deflection and glutamate release demonstrated maximum sensitivity at displacements of just ∼40 nm in the positive direction. The remaining hair cells only signaled motion in one direction and were less sensitive, extending the operating range of the neuromast beyond 1 µm. Adaptation of the synaptic output was also heterogeneous, with some hair cells generating sustained glutamate release in response to a steady deflection of the cupula and others generating transient outputs. Finally, a distinct signal encoded a return of the cupula to rest: a large and transient burst of glutamate release from hair cells unresponsive to the initial stimulus. A population of hair cells with these different sensitivities, operating ranges, and adaptive properties will allow the neuromast to encode weak stimuli while maintaining the dynamic range to signal the amplitude and duration of stronger deflections.SIGNIFICANCE STATEMENT Hair cells transmit information about mechanical stimuli by converting very small deflections of their hair bundle into changes in the release of the neurotransmitter glutamate. We have measured this input/output relation in the live fish using a fluorescent protein and find that different hair cells vary in their mechanical sensitivity and the time course of their response. These variations will allow the fish to sense the timing and duration of both very weak stimuli (∼40 nm deflections) and strong stimuli (∼1 µm), underlying the ability of the fish to avoid predators and maintain its body position in flowing water.

A cerebellum-like circuit in the lateral line system of fish cancels mechanosensory input associated with its own movements.

An animal's own movement exerts a profound impact on sensory input to its nervous system. Peripheral sensory receptors do not distinguish externally generated stimuli from stimuli generated by an animal's own behavior (reafference) - although the animal often must. One way that nervous systems can solve this problem is to provide movement-related signals (copies of motor commands and sensory feedback) to sensory systems, which can then be used to generate predictions that oppose or cancel out sensory responses to reafference. Here, we studied the use of movement-related signals to generate sensory predictions in the lateral line medial octavolateralis nucleus (MON) of the little skate. In the MON, mechanoreceptive afferents synapse on output neurons that also receive movement-related signals from central sources, via a granule cell parallel fiber system. This parallel fiber system organization is characteristic of a set of so-called cerebellum-like structures. Cerebellum-like structures have been shown to support predictive cancellation of reafference in the electrosensory systems of fish and the auditory system of mice. Here, we provide evidence that the parallel fiber system in the MON can generate predictions that are negative images of (and therefore cancel) sensory input associated with respiratory and fin movements. The MON, found in most aquatic vertebrates, is probably one of the most primitive cerebellum-like structures and a starting point for cerebellar evolution. The results of this study contribute to a growing body of work that uses an evolutionary perspective on the vertebrate cerebellum to understand its functional diversity in animal behavior.

Evolutionary shift towards lateral line dependent prey capture behavior in the blind Mexican cavefish.

Feeding strategies are dependent on multi-modal sensory processing, that integrates visual, chemosensory, and mechanoreceptive cues. In many fish species, local environments and food availability dramatically influence the evolution of sensory and morphological traits that underlie feeding. The Mexican cavefish, Astyanax mexicanus, have developed robust changes in sensory-dependent behaviors, but the impact on prey detection and feeding behavior is not known. In the absence of eyes, cavefish have evolved enhanced sensitivity of the lateral line, comprised of mechanosensory organs that sense water flow and detect prey. Here, we identify evolved differences in prey capture behavior of larval cavefish that are dependent on lateral line sensitivity. Under lighted conditions, cavefish strike Artemia prey at a wider angle than surface fish; however, this difference is diminished under dark conditions. In addition, the strike distance is greater in cavefish than surface fish, revealing an ability to capture, and likely detect, prey at greater distances. Experimental ablation of the lateral line disrupts prey capture in cavefish under both light and dark conditions, while it only impacts surface fish under dark conditions. Together, these findings identify an evolutionary shift towards a dependence on the lateral line for prey capture in cavefish, providing a model for investigating how loss of visual cues impacts multi-modal sensory behaviors.

Efferent modulation of spontaneous lateral line activity during and after zebrafish motor commands.

Accurate sensory processing during movement requires the animal to distinguish between external (exafferent) and self-generated (reafferent) stimuli to maintain sensitivity to biologically relevant cues. The lateral line system in fishes is a mechanosensory organ that experiences reafferent sensory feedback, via detection of fluid motion relative to the body generated during behaviors such as swimming. For the first time in larval zebrafish (Danio rerio), we employed simultaneous recordings of lateral line and motor activity to reveal the activity of afferent neurons arising from endogenous feedback from hindbrain efferent neurons during locomotion. Frequency of spontaneous spiking in posterior lateral line afferent neurons decreased during motor activity and was absent for more than half of swimming trials. Targeted photoablation of efferent neurons abolished the afferent inhibition that was correlated to swimming, indicating that inhibitory efferent neurons are necessary for modulating lateral line sensitivity during locomotion. We monitored calcium activity with Tg(elav13:GCaMP6s) fish and found synchronous activity between putative cholinergic efferent neurons and motor neurons. We examined correlates of motor activity to determine which may best predict the attenuation of afferent activity and therefore what components of the motor signal are translated through the corollary discharge. Swim duration was most strongly correlated to the change in afferent spike frequency. Attenuated spike frequency persisted past the end of the fictive swim bout, suggesting that corollary discharge also affects the glide phase of burst and glide locomotion. The duration of the glide in which spike frequency was attenuated increased with swim duration but decreased with motor frequency. Our results detail a neuromodulatory mechanism in larval zebrafish that adaptively filters self-generated flow stimuli during both the active and passive phases of locomotion.NEW & NOTEWORTHY For the first time in vivo, we quantify the endogenous effect of efferent activity on afferent gain control in a vertebrate hair cell system during and after locomotion. We believe that this pervasive effect has been underestimated when afferent activity of octavolateralis systems is characterized in the current literature. We further identify a refractory period out of phase with efferent control and place this gain mechanism in the context of gliding behavior of freely moving animals.

The use of evoked potentials to determine sensory sub-modality contributions to acoustic and hydrodynamic sensing.

Both the lateral line and the inner ear contribute to near-field dipole source detection in fish. The precise roles these two sensory modalities provide in extracting information about the flow field remain of interest. In this study, evoked potentials (EP, 30-200 Hz) for blind Mexican cavefish were measured in response to a dipole source. Greatest sensitivity was observed at the lower and upper ends of the tested frequency range. To evaluate the relative contributions of the lateral line and inner ear, we measured the effects of neomycin on EP response characteristics at 40 Hz, and used the vital dye DASPEI to verify neuromast ablation. Neomycin increased the latency of the EP response up until 60 min post-treatment. DASPEI results confirmed that neuromast hair cell death was significant in treated fish over this timeframe. These results indicate that the inner ear, whether it is sound pressure or particle motion detection, makes a significant contribution to the dipole-induced EP in blind cavefish at near-field low frequencies where the lateral line contribution would be expected to be strongest. The results from this study imply that under some circumstances, lateral line function could be complemented by the inner ear.

Lateral line sensitivity in free-swimming toadfish Opsanus tau.

A longstanding question in aquatic animal sensory physiology is the impact of self-generated movement on lateral line sensitivity. One hypothesis is that efferent modulation of the sensory hair cells cancels self-generated noise and allows fish to sample their surroundings while swimming. In this study, microwire electrodes were chronically implanted into the anterior lateral line nerve of oyster toadfish and neural activity was monitored during forward movement. Fish were allowed to freely swim or were moved by a tethered sled. In all cases, neural activity increased during movement with no evidence of efferent modulation. The anterior lateral line of moving fish responded to a vibrating sphere or the tail oscillations of a robotic fish, indicating that the lateral line also remains sensitive to outside stimulus during self-generated movement. The results suggest that during normal swim speeds, lateral line neuromasts are not saturated and retain the ability to detect external stimuli without efferent modulation.

Assessing the use of vibrations and strobe lights at fish screens as enhanced deterrents for two estuarine fishes.

The role of vision and the lateral-line system in fish-screen avoidance behaviours was investigated in shiner surfperch Cymatogaster aggregata and staghorn sculpin Leptocottus armatus. Avoidance experiments were conducted in front of water-diversion-type wedge-wire fish screens in a laboratory flume with a 0.3 m s-1 water velocity. Fish contacted the screens less frequently during the day than night, indicating that fish screen avoidance is visually mediated during the day. Input from the fishes' lateral-line systems was also blocked with streptomycin-sulphate treatments during the night to determine if these fishes use mechanoreceptive cues to guide screen avoidance in darkened conditions. Streptomycin-treated and untreated fish had similar contact rates suggesting that mechanoreceptive guidance was not increasing the fishes' abilities to avoid contact with non-vibrating screens at night. Fishes were stained with 2-(4-(dimethylamino)styryl)-N-ethylpyridinium iodide (DASPEI) to assess the streptomycin treatment's effectiveness. We also tested the fishes' ability to avoid contact with the screens at night, when a strobe light or industrial vibrator was operated on the screens, to respectively increase the screen's visual and mechanoreceptory guidance potential. Cymatogaster aggregata contacted the screens significantly less frequently when they were vibrating, compared with their night-time controls, suggesting useful mechanoreceptive guidance. Leptocottus armatus contacted the screens significantly less frequently under strobe-light illumination, compared with their night-time controls, suggesting useful visual guidance. This research should benefit fishery and water-resource managers, regarding the development of future fish-protection decisions at screened water diversions.

Mechanosensory system of the lateral line in the subantarctic Patagonian blenny Eleginops maclovinus.

This study describes the cephalic and trunk lateral line systems in Patagonian blenny Eleginops maclovinus juveniles, providing morphological details for pores, canals and neuromasts. Eleginops maclovinus juveniles possess a complete laterodorsal lateral line that extends from the upper apex of the gill opening along the trunk as far as the caudal fin. The lateral line was ramified through pores and canals. The following pores were recorded: four supraorbital pores, with two along the eye border and two on the snout; seven infraorbital pores, with three on the lacrimal bone and four being infraorbital; five postorbital pores, with three along the preopercular border (upper preoperculum branch) and two on the bone curvature (inferior preoperculum branch); and four mandibular pores aligned along the jaw. Furthermore, five narrow-simple and interconnected canals were found (i.e. preopercular, mandibular, supraorbital and infraorbital canals). Histologically, the dorsal lateral line presented thin neuromasts (350 µm) with short hair cells. By contrast, the cranial region presented long, thick neuromasts. Infraorbital and mandibular neuromasts had a major axis length of 260 µm and respective average diameters of 200 and 185 µm. Sensory system variations would be due to a greater concentration of neuromasts in the cranial region, allowing for a greater perception of changes in water pressure. Scarce morphological information is available for the lateral sensory system in Eleginopsidae, particularly compared to Channichthyidae, Bovichthydae, Artedidraconidae and Bathydraconidae. Therefore, the presented results form a fundamental foundation of knowledge for the lateral-line system in juvenile E. maclovinus and provide a basis for future related research in this taxon as well as within the Notothenioidei suborder.

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.

Responses of medullary lateral line units of the rudd, Scardinius erythrophthalmus, and the nase, Chondrostoma nasus, to vortex streets.

Fish use their mechanosensory lateral line amongst others for the detection of vortices shed by an upstream object and/or for the detection of vortices caused by the tail fin movements of another fish. Thus, vortices are one type of hydrodynamic stimuli to which fish are exposed in their natural environment. We investigated the responses of medullary lateral line units of common rudd, Scardinius erythrophthalmus, and common nase, Chondrostoma nasus (Cyprinidae), to water flow (9.5-13.3 cm-1) that contained vortices (a Kármán vortex street) shed by an upstream cylinder (diameter 2 cm). The distance between the cylinder and the tip of the fish's snout varied between 8 and 24 cm. 21 out of 42 units (S. erythrophthalmus), respectively, 9 out of 39 units (Chondrostoma nasus) responded to the vortices shed by the cylinder. Up to a cylinder distance of 24 cm, interburst intervals revealed the vortex shedding frequency, i.e., burst frequency was similar to or identical with the vortex shedding frequency.

Potential role of the anterior lateral line in sound localization in toadfish (Opsanus tau).

Male oyster toadfish (Opsanus tau) acoustically attract females to nesting sites using a boatwhistle call. The rapid speed of sound underwater combined with the close proximity of the otolithic organs makes inner ear interaural time differences an unlikely mechanism to localize sound. To determine the role that the mechanosensory lateral line may play in sound localization, microwire electrodes were bilaterally implanted into the anterior lateral line nerve to record neural responses to vibrational stimuli. Highest spike rates and strongest phase-locking occurred at distances close to the fish and decreased as the stimulus was moved further from the fish. Bilateral anterior lateral line neuromasts displayed differential directional sensitivity to incoming vibrational stimuli, which suggests the potential for the lateral line to be used for sound localization in the near field. The present study also demonstrates that the spatially separated neuromasts of the toadfish may provide sufficient time delays between sensory organs for determining sound localization cues. Multimodal sensory input processing through both the inner ear (far field) and lateral line (near field) may allow for effective sound localization in fish.