Structure of the Zebrafish Locomotor Repertoire Revealed with Unsupervised Behavioral Clustering

An interhemispheric neural circuit allowing binocular integration in the optic tectum

Binocular stereopsis requires the convergence of visual information from corresponding points in visual space seen by two different lines of sight. This may be achieved by superposition of retinal input from each eye onto the same downstream neurons via ipsi- and contralaterally projecting optic nerve fibers. Zebrafish larvae can perceive binocular cues during prey hunting but have exclusively contralateral retinotectal projections. Here we report brain activity in the tectal neuropil ipsilateral to the visually stimulated eye, despite the absence of ipsilateral retinotectal projections. This activity colocalizes with arbors of commissural neurons, termed intertectal neurons (ITNs), that connect the tectal hemispheres. ITNs are GABAergic, establish tectal synapses bilaterally and respond to small moving stimuli. ITN-ablation impairs capture swim initiation when prey is positioned in the binocular strike zone. We propose an intertectal circuit that controls execution of the prey-capture motor program following binocular localization of prey, without requiring ipsilateral retinotectal projections.

Zebrafish larvae can binocularly detect prey objects in order to strike but lack ipsilateral retinotectal fibers for binocular superposition of visual information. Here the authors describe commissural intertectal neurons and show that they are required for the initiation of capture strikes.

Nested Neuronal Dynamics Orchestrate a Behavioral Hierarchy across Timescales

Classical and modern ethological studies suggest that animal behavior is organized hierarchically across timescales, such that longer-timescale behaviors are composed of specific shorter-timescale actions. Despite progress relating neuronal dynamics to single-timescale behavior, it remains unclear how different timescale dynamics interact to give rise to such higher-order behavioral organization. Here, we show, in the nematode Caenorhabditis elegans, that a behavioral hierarchy spanning three timescales is implemented by nested neuronal dynamics. At the uppermost hierarchical level, slow neuronal population dynamics spanning brain and motor periphery control two faster motor neuron oscillations, toggling them between different activity states and functional roles. At lower hierarchical levels, these faster oscillations are further nested in a manner that enables flexible behavioral control in an otherwise rigid hierarchical framework. Our findings establish nested neuronal activity patterns as a repeated dynamical motif of the C. elegans nervous system, which together implement a controllable hierarchical organization of behavior.

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Slow dynamics across brain and motor circuits drive upper-hierarchy motor states

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Fast dynamics in motor circuits drive lower-hierarchy movements within these states

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Slower dynamics tightly constrain the state and function of faster ones

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This rigid hierarchy nevertheless enables flexible behavioral control

Volume EM Reconstruction of Spinal Cord Reveals Wiring Specificity in Speed-Related Motor Circuits

Spinal interneurons coordinate the activity of motoneurons to generate the spatiotemporal patterns of muscle contractions required for vertebrate locomotion. It is controversial to what degree the orderly, gradual recruitment of motoneurons is determined by biophysical differences among them rather than by specific connections from presynaptic interneurons to subsets of motoneurons. To answer this question, we mapped all connections from two types of interneurons onto all motoneurons in a larval zebrafish spinal cord hemisegment, using serial block-face electron microscopy (SBEM). We found specific synaptic connectivity from dorsal but not from ventral excitatory ipsilateral interneurons, with large motoneurons, active only when strong force is required, receiving specific inputs from dorsally located interneurons, active only during fast swims. By contrast, the connectivity between inhibitory commissural interneurons and motoneurons lacks any discernible pattern. The wiring pattern is consistent with a recruitment mechanism that depends to a considerable extent on specific connectivity.

A primal role for the vestibular sense in the development of coordinated locomotion

Mature locomotion requires that animal nervous systems coordinate distinct groups of muscles. The pressures that guide the development of coordination are not well understood. To understand how and why coordination might emerge, we measured the kinematics of spontaneous vertical locomotion across early development in zebrafish (Danio rerio) . We found that zebrafish used their pectoral fins and bodies synergistically during upwards swims. As larvae developed, they changed the way they coordinated fin and body movements, allowing them to climb with increasingly stable postures. This fin-body synergy was absent in vestibular mutants, suggesting sensed imbalance promotes coordinated movements. Similarly, synergies were systematically altered following cerebellar lesions, identifying a neural substrate regulating fin-body coordination. Together these findings link the vestibular sense to the maturation of coordinated locomotion. Developing zebrafish improve postural stability by changing fin-body coordination. We therefore propose that the development of coordinated locomotion is regulated by vestibular sensation.

Pretectal neurons control hunting behaviour

For many species, hunting is an innate behaviour that is crucial for survival, yet the circuits that control predatory action sequences are poorly understood. We used larval zebrafish to identify a population of pretectal neurons that control hunting. By combining calcium imaging with a virtual hunting assay, we identified a discrete pretectal region that is selectively active when animals initiate hunting. Targeted genetic labelling allowed us to examine the function and morphology of individual cells and identify two classes of pretectal neuron that project to ipsilateral optic tectum or the contralateral tegmentum. Optogenetic stimulation of single neurons of either class was able to induce sustained hunting sequences, in the absence of prey. Furthermore, laser ablation of these neurons impaired prey-catching and prevented induction of hunting by optogenetic stimulation of the anterior-ventral tectum. We propose that this specific population of pretectal neurons functions as a command system to induce predatory behaviour.

The Zebrafish Visual System: From Circuits to Behavior

Visual stimuli can evoke complex behavioral responses, but the underlying streams of neural activity in mammalian brains are difficult to follow because of their size. Here, I review the visual system of zebrafish larvae, highlighting where recent experimental evidence has localized the functional steps of visuomotor transformations to specific brain areas. The retina of a larva encodes behaviorally relevant visual information in neural activity distributed across feature-selective ganglion cells such that signals representing distinct stimulus properties arrive in different areas or layers of the brain. Motor centers in the hindbrain encode motor variables that are precisely tuned to behavioral needs within a given stimulus setting. Owing to rapid technological progress, larval zebrafish provide unique opportunities for obtaining a comprehensive understanding of the intermediate processing steps occurring between visual and motor centers, revealing how visuomotor transformations are implemented in a vertebrate brain.

The zebrafish subcortical social brain as a model for studying social behavior disorders

Social behaviors are essential for the survival and reproduction of social species. Many, if not most, neuropsychiatric disorders in humans are either associated with underlying social deficits or are accompanied by social dysfunctions. Traditionally, rodent models have been used to model these behavioral impairments. However, rodent assays are often difficult to scale up and adapt to high-throughput formats, which severely limits their use for systems-level science. In recent years, an increasing number of studies have used zebrafish (Danio rerio) as a model system to study social behavior. These studies have demonstrated clear potential in overcoming some of the limitations of rodent models. In this Review, we explore the evolutionary conservation of a subcortical social brain between teleosts and mammals as the biological basis for using zebrafish to model human social behavior disorders, while summarizing relevant experimental tools and assays. We then discuss the recent advances gleaned from zebrafish social behavior assays, the applications of these assays to studying related disorders, and the opportunities and challenges that lie ahead.

Bumblebees learn foraging routes through exploitation-exploration cycles

How animals explore and acquire knowledge from the environment is a key question in movement ecology. For pollinators that feed on multiple small replenishing nectar resources, the challenge is to learn efficient foraging routes while dynamically acquiring spatial information about new resource locations. Here, we use the behavioural mapping t-Stochastic Neighbouring Embedding algorithm and Shannon entropy to statistically analyse previously published sampling patterns of bumblebees feeding on artificial flowers in the field. We show that bumblebees modulate foraging excursions into distinctive behavioural strategies, characterizing the trade-off dynamics between (i) visiting and exploiting flowers close to the nest, (ii) searching for new routes and resources, and (iii) exploiting learned flower visitation sequences. Experienced bees combine these behavioural strategies even after they find an optimal route minimizing travel distances between flowers. This behavioural variability may help balancing energy costs-benefits and facilitate rapid adaptation to changing environments and the integration of more profitable resources in their routes.

Nucleus Isthmi Is Required to Sustain Target Pursuit during Visually Guided Prey-Catching

Animals must frequently perform a sequence of behaviors to achieve a specific goal. However, the neural mechanisms that promote the continuation and completion of such action sequences are not well understood. Here, we characterize the anatomy, physiology, and function of the nucleus isthmi (NI), a cholinergic nucleus thought to modulate tectal-dependent, goal-directed behaviors. We find that the larval zebrafish NI establishes reciprocal connectivity with the optic tectum and identify two distinct types of isthmic projection neuron that either connect ipsilaterally to retinorecipient laminae of the tectum and pretectum or bilaterally to both tectal hemispheres. Laser ablation of NI caused highly specific deficits in tectally mediated loom-avoidance and prey-catching behavior. In the context of hunting, NI ablation did not affect prey detection or hunting initiation but resulted in larvae failing to sustain prey-tracking sequences and aborting their hunting routines. Moreover, calcium imaging revealed elevated neural activity in NI following onset of hunting behavior. We propose a model in which NI provides state-dependent feedback facilitation to the optic tectum and pretectum to potentiate neural activity and increase the probability of consecutive prey-tracking maneuvers during hunting sequences.

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Nucleus isthmi contains two types of neuron with distinct (pre)-tectal connectivity

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Neural activity in nucleus isthmi is recruited at onset of hunting behavior

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Nucleus isthmi is required for maintenance, but not initiation, of hunting routines

Learning: Complexities of Habituation in Escaping Zebrafish Larvae

Animals decrease responses to repeating stimuli through habituation. New research has revealed independent tuning of multiple parameters of zebrafish escape behavior during habituation.

Distributed Plasticity Drives Visual Habituation Learning in Larval Zebrafish

Habituation is a simple form of learning where animals learn to reduce their responses to repeated innocuous stimuli [1]. Habituation is thought to occur via at least two temporally and molecularly distinct mechanisms, which lead to short-term memories that last for seconds to minutes and long-term memories that last for hours or longer [1, 2]. Here, we focus on long-term habituation, which, due to the extended time course, necessitates stable alterations to circuit properties [2-4]. In its simplest form, long-term habituation could result from a plasticity event at a single point in a circuit, and many studies have focused on identifying the site and underlying mechanism of plasticity [5-10]. However, it is possible that these individual sites are only one of many points in the circuit where plasticity is occurring. Indeed, studies of short-term habituation in C. elegans indicate that in this paradigm, multiple genetically separable mechanisms operate to adapt specific aspects of behavior [11-13]. Here, we use a visual assay in which larval zebrafish habituate their response to sudden reductions in illumination (dark flashes) [14, 15]. Through behavioral analyses, we find that multiple components of the dark-flash response habituate independently of one another using different molecular mechanisms. This is consistent with a modular model in which habituation originates from multiple independent processes, each adapting specific components of behavior. This may allow animals to more specifically or flexibly habituate based on stimulus context or internal states.

Chronology-based architecture of descending circuits that underlie the development of locomotor repertoire after birth

The emergence of new and increasingly sophisticated behaviors after birth is accompanied by dramatic increase of newly established synaptic connections in the nervous system. Little is known, however, of how nascent connections are organized to support such new behaviors alongside existing ones. To understand this, in the larval zebrafish we examined the development of spinal pathways from hindbrain V2a neurons and the role of these pathways in the development of locomotion. We found that new projections are continually layered laterally to existing neuropil, and give rise to distinct pathways that function in parallel to existing pathways. Across these chronologically layered pathways, the connectivity patterns and biophysical properties vary systematically to support a behavioral repertoire with a wide range of kinematics and dynamics. Such layering of new parallel circuits equipped with systematically changing properties may be central to the postnatal diversification and increasing sophistication of an animal’s behavioral repertoire.

Newborn babies have limited abilities. Indeed, most of our actions shortly after birth are the result of reflexes that serve our most basic need: to stay alive. As we get older, however, our behaviour gradually becomes more sophisticated. During this time, the billions of cells in our brain form new connections to build intricate ‘circuits’ of neurons that allow for more complicated thoughts and actions.

It is clear that the brain circuits that support new behaviours must develop in a way that does not interfere with the existing circuits that are vital for survival. However, the challenge has been to find a way to peer into a brain as it develops to see how these new circuits form.

In recent years, zebrafish have revolutionised research into neuronal circuits in animals. Developing over the course of a few days, these small transparent fish provide a window into the brain during the earliest stages of development. Indeed, the circuits of neurons that descend from the brain and connect to the spinal cord have already been mapped in these animals. Now, Pujala and Koyama have begun to follow the careful development of these ‘descending’ neurons, and relate it to the appearance of new behaviours in young zebrafish.

Time-lapse imaging with a fluorescent protein that is active only in specific descending neurons revealed that new circuits are laid down over existing ones, like the growth rings in a tree. Next, at different timepoints in zebrafish development, Pujala and Koyama traced these neurons backwards from the spine to the brain to identify which connections formed first. This showed that the spinal connections develop one after the other, in the same order that the neurons mature.

Next, Pujala and Koyama asked how the activity of neurons that mature early or late in development relates to specific behaviours in young zebrafish. Early-born circuits connect to neurons that produce powerful, reflex-driven, whole-body movements such as an escape response. The later circuits connect to different neurons through slower, less direct pathways; the late-born neurons also generate the refined movements that are acquired later in a zebrafish’s development and help the fish to explore its environment.

These findings show that descending circuits in zebrafish run parallel to each other, but with distinct connections and properties that allow them to control different kinds of movements. While this study was conducted using an animal model, a better understanding of how such circuits develop and the movements they control may one day aid the treatment of patients with neurodegenerative diseases or injuries where connections have been lost.

Fishing forward and reverse: Advances in zebrafish phenomics

Understanding how the genome instructs the phenotypic characteristics of an organism is one of the major scientific endeavors of our time. Advances in genetics have progressively deciphered the inheritance, identity and biological relevance of genetically encoded information, contributing to the rise of several, complementary omic disciplines. One of them is phenomics, an emergent area of biology dedicated to the systematic multi-scale analysis of phenotypic traits. This discipline provides valuable gene function information to the rapidly evolving field of genetics. Current molecular tools enable genome-wide analyses that link gene sequence to function in multi-cellular organisms, illuminating the genome-phenome relationship. Among vertebrates, zebrafish has emerged as an outstanding model organism for high-throughput phenotyping and modeling of human disorders. Advances in both systematic mutagenesis and phenotypic analyses of embryonic and post-embryonic stages in zebrafish have revealed the function of a valuable collection of genes and the general structure of several complex traits. In this review, we summarize multiple large-scale genetic efforts addressing parental, embryonic, and adult phenotyping in the zebrafish. The genetic and quantitative tools available in the zebrafish model, coupled with the broad spectrum of phenotypes that can be assayed, make it a powerful model for phenomics, well suited for the dissection of genotype-phenotype associations in development, physiology, health and disease.

Neuroplasticity in the acoustic startle reflex in larval zebrafish

Learning is essential for animal survival under changing environments. Even in its simplest form, learning involves interactions between a handful of neuronal circuits, hundreds of neurons and many thousand synapses. In this review I will focus on habituation - a form of non-associative learning during which organisms decrease their response to repetitions of identical sensory stimuli. I will discuss how recent studies of the acoustic startle reflex mediated by the Mauthner cell in the zebrafish larva are helping to understand the neuroplastic processes that underlie habituation. In addition to being a fascinating biological process, habituation is clinically relevant because it is affected in various neuropsychiatric disorders in humans, including autism, schizophrenia, Fragile-X and Tourette's syndromes.

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.

Principles of Functional Circuit Connectivity: Insights From Spontaneous Activity in the Zebrafish Optic Tectum

The brain is continuously active, even in the absence of external stimulation. In the optic tectum of the zebrafish larva, this spontaneous activity is spatially organized and reflects the circuit's functional connectivity. The structure of the spontaneous activity displayed patterns associated with aspects of the larva's preferences when engaging in complex visuo-motor behaviors, suggesting that the tectal circuit is adapted for the circuit's functional role in detecting visual cues and generating adequate motor behaviors. Further studies in sensory deprived larvae suggest that the basic structure of the functional connectivity patterns emerges even in the absence of retinal inputs, but that its fine structure is affected by visual experience.

Fish learn collectively, but groups with differing personalities are slower to decide and more likely to split

We tested zebrafish shoals to examine whether groups exhibit collective spatial learning and whether this relates to the personality of group members. To do this we trained shoals to associate a collective spatial decision with a reward and tested whether shoals could reorient to the learned location from a new starting point. There were strong indications of collective learning and collective reorienting, most likely by memorising distal cues, but these processes were unrelated to personality differences within shoals. However, there was evidence that group decisions require agreement between differing personalities. Notably, shoals with more boldness variation were more likely to split during training trials and took longer to reach a collective decision. Thus cognitive tasks, such as learning and cue memorisation, may be exhibited collectively, but the ability to reach collective decisions is affected by the personality composition of the group. A likely outcome of the splitting of groups with very disparate personalities is the formation of groups with members more similar in their personality.

Summary: Zebrafish groups were trained to make collective decisions and memorised locations by using cues. However, collective decisions were made slower by the splitting of groups comprising individuals of dissimilar personality.

Origin and circuitry of spinal locomotor interneurons generating different speeds

The spinal circuitry governing the undulatory movements of swimming vertebrates consist of excitatory and commissural inhibitory interneurons and motor neurons. This locomotor network generates the rhythmic output, coordinate left/right alternation, and permit communication across segments. Through evolution, more complex movement patterns have emerged, made possible by sub-specialization of neural populations within the spinal cord. Walking tetrapods use a similar basic circuitry, but have added layers of complexity for the coordination of intralimbic flexor and extensor muscles as well as interlimbic coordination between the body halves and fore/hindlimbs. Although the basics of these circuits are known there is a gap in our knowledge regarding how different speeds and gaits are coordinated. Analysing subpopulations among described neuronal populations may bring insight into how changes in locomotor output are orchestrated by a hard-wired network.

Active mechanosensory feedback during locomotion in the zebrafish spinal cord

The investigation of mechanosensory feedback to locomotion has been hindered by the challenge of recording neurons in motion. Genetic accessibility and optical transparency of zebrafish larvae provide means to revisit this question. Glutamatergic Rohon-Beard (RB) and GABAergic CSF-contacting neurons (CSF-cNs) are spinal mechanosensory neurons. Recent studies combining bioluminescence, silencing and optogenetic activation show that mechanosensory neurons enhance speed and stabilize posture during locomotion. RB neurons can modulate speed by projecting onto glutamatergic premotor V2a interneurons during fast swimming, while CSF-cNs inhibit V0-v interneurons sustaining slow swimming. Sensory gating, either through inhibition of sensory interneurons (CoPA) or though the direct inhibition of primary motor neurons by CSF-cNs, mediates postural control. Advanced optical methods have shed light on the dynamics of sensorimotor integration during active locomotion unraveling implications for translational research.