Anatomy and Connectivity of the Torus Longitudinalis of the Adult Zebrafish

Brain-wide circuitry underlying altered auditory habituation in zebrafish models of autism

Auditory processing is widely understood to occur differently in autism, though the patterns of brain activity underlying these differences are not well understood. The diversity of autism also means brain-wide networks may change in various ways to produce similar behavioral outputs. We used larval zebrafish to investigate auditory habituation in four genetic lines relevant to autism: fmr1, mecp2, scn1lab and cntnap2. In free-swimming behavioral tests, we found each line had a unique profile of auditory hypersensitivity and/or delayed habituation. Combining the optical transparency of larval zebrafish with genetically encoded calcium indicators and light-sheet microscopy, we then observed brain-wide activity at cellular resolution during auditory habituation. As with behavior, each line showed unique alterations in brain-wide spontaneous activity, auditory processing, and adaptation in response to repetitive acoustic stimuli. We also observed commonalities in activity across our genetic lines that indicate shared circuit changes underlying certain aspects of their behavioral phenotypes. These were predominantly in regions involved in sensory integration and sensorimotor gating rather than primary auditory areas. Overlapping phenotypes include differences in the activity and functional connectivity of the telencephalon, thalamus, dopaminergic regions, and the locus coeruleus, and excitatory/inhibitory imbalance in the cerebellum. Unique phenotypes include loss of activity in the habenula in scn1lab, increased activity in auditory regions in fmr1, and differences in network activity over time in mecp2 and cntnap2. Comparing these distinct but overlapping brain-wide auditory networks furthers our understanding of how diverse genetic factors can produce similar behavioral effects through a range of circuit- and network-scale mechanisms.

Specific biomarkers and neurons distribution of different brain regions in largemouth bass (Micropterus salmoides)

The brain regulates multiple physiological processes in fish. Despite this, knowledge about the basic structure and function of distinct brain regions in non-model fish species remains limited due to their diversity and the scarcity of common biomarkers. In the present study, four major brain parts, the telencephalon, diencephalon, mesencephalon and rhombencephalon, were isolated in largemouth bass, Micropterus salmoides. Within these parts, nine brain regions and 74 nuclei were further identified through morphological and cytoarchitectonic analysis. Transcriptome analysis revealed a total of 7153 region-highly expressed genes and 176 region-specifically expressed genes. Genes related to growth, reproduction, emotion, learning, and memory were significantly overexpressed in the olfactory bulb and telencephalon (OBT). Feeding and stress-related genes were in the hypothalamus (Hy). Visual system-related genes were predominantly enriched in the optic tectum (OT), while vision and hearing-related genes were widely expressed in the cerebellum (Ce) region. Sensory input and motor output-related genes were in the medulla oblongata (Mo). Osmoregulation, stress response, sleep/wake cycles, and reproduction-related genes were highly expressed in the remaining brain (RB). Three candidate marker genes were further identified for each brain regions, such as neuropeptide FF (npff) for OBT, pro-melanin-concentrating hormone (pmch) for Hy, vesicular inhibitory amino acid transporter (viaat) for OT, excitatory amino acid transporter 1 (eaat1) for Ce, peripherin (prph) for Mo, and isotocin neurophysin (itnp) for RB. Additionally, the distribution of seven neurotransmitter-type neurons and five types of non-neuronal cells across different brain regions were analyzed by examining the expression of their marker genes. Notably, marker genes for glutamatergic and GABAergic neurons showed the highest expression levels across all brain regions. Similarly, the marker gene for radial astrocytes exhibited high expression compared to other markers, while those for microglia were the least expressed. Overall, our results provide a comprehensive overview of the structural and functional characteristics of distinct brain regions in the largemouth bass, which offers a valuable resource for understanding the role of central nervous system in regulating physiological processes in teleost.

Visual shape discrimination in goldfish, modelled with the neural circuitry of optic tectum and torus longitudinalis

There is no satisfactory neurally-based theory as to how vertebrates that lack a neocortex discriminate even simple geometric shapes. In fishes, an intact optic tectum is necessary for such discriminations, but physiological studies of it have found nothing like the hierarchically arranged feature detecting neurons of mammalian visual cortex. Here, a neural model attempts a solution by basing shape discrimination upon the responses of only those elementary detectors (e.g. of size) that are within a focus of attention, formed by a winner-take-all arrangement of retinotopically mapped units representing tectal pyramidal cells. While this relatively primitive mechanism could recognize an object irrespective of position in space, it fails to distinguish patterns that differ only in their features' spatial relationships. The model's solution - imitating goldfish that naturally attend to the top of shapes - is to shift attention to the edges of a shape by spatially offsetting inputs to the pyramidal neurons, effected by the torus longitudinalis and its prolific synapses on pyramidal dendrites. The model's shape discrimination was compared to an extensive behavioral study using shapes with points and projections. In one test series fish were sensitive to the relative number of points on the tops of shapes. In another, fish were trained to discriminate points on the sides. By using different offset connections and only one elementary feature detector for small dark spots, the model successfully emulated the two sets of goldfish data, as judged by significant correlations between model response and fish discrimination.

Effects of sodium arsenite exposure on behavior, ultrastructure and gene expression of brain in adult zebrafish (Danio rerio)

Arsenic, a common metal-like substance, has been demonstrated to pose potential health hazards and induce behavioral changes in humans and rodents. However, the chronic neurotoxic effects of arsenic on aquatic animals are still not fully understood. This study aimed to investigate the effects of arsenic exposure on adult zebrafish by subjecting 3-month-old zebrafish to three different sodium arsenite water concentrations: 0 μg/L (control group), 50 μg/L, and 500 μg/L, over a period of 30 days. To assess the risk associated with arsenic exposure in the aquatic environment, behavior analysis, transmission electron microscopy techniques, and quantitative real-time PCR were employed. The behavior of adult zebrafish was evaluated using six distinct tests: the mirror biting test, shoaling test, novel tank test, social preference test, social recognition test, and T maze. Following the behavioral tests, the brains of zebrafish were dissected and collected for ultrastructural examination and gene expression analysis. The results revealed that sodium arsenite exposure led to a significant reduction in aggression, cohesion, social ability, social cognition ability, learning, and memory capacity of zebrafish. Furthermore, ultrastructure and genes regulating behavior in the zebrafish brain were adversely affected by sodium arsenite exposure.

Neural connections of the torus semicircularis in the adult Zebrafish

The torus semicircularis (TS) of teleosts is a key midbrain center of the lateral line and acoustic sensory systems. To characterize the TS in adult zebrafish, we studied their connections using the carbocyanine tracers applied to the TS and to other related nuclei and tracts. Two main TS nuclei, central and ventrolateral, were differentiable by their afferent connections. From central TS, (TSc) numerous toropetal cells were labeled bilaterally in several primary octaval nuclei (anterior, magnocellular, descending, and posterior octaval nuclei), in the secondary octaval nucleus, in the caudal octavolateralis nucleus, and in the perilemniscular region. In the midbrain, numerous toropetal cells were labeled in the contralateral TSc. In the diencephalon, toropetal cells labeled from the TSc were observed ipsilaterally in the medial prethalamic nucleus and the periventricular posterior tubercle nucleus. TSc toropetal neurons were also labeled bilaterally in the hypothalamic anterior tuberal nucleus (ATN) and ipsilaterally in the parvicellular preoptic nucleus but not in the telencephalon. Tracer application to the medial octavolateralis nucleus revealed contralateral projections to the ventrolateral TS (TSvl), whereas tracer application to the secondary octaval nucleus labeled fibers bilaterally in TSc and neurons in rostral TSc. The TSc sends ascending fibers to the ipsilateral lateral preglomerular region that, in turn, projects to the pallium. Application of DiI to the optic tectum labeled cells and fibers in the TSvl, whereas application of DiI to the ATN labeled cells and fibers in the TSc. These results reveal that the TSvl and TSc are mainly related with the mechanosensory lateral line and acoustic centers, respectively, and that they show different higher order connections.

A synaptic corollary discharge signal suppresses midbrain visual processing during saccade-like locomotion

In motor control, the brain not only sends motor commands to the periphery, but also generates concurrent internal signals known as corollary discharge (CD) that influence sensory information processing around the time of movement. CD signals are important for identifying sensory input arising from self-motion and to compensate for it, but the underlying mechanisms remain unclear. Using whole-cell patch clamp recordings from neurons in the zebrafish optic tectum, we discovered an inhibitory synaptic signal, temporally locked to spontaneous and visually driven locomotion. This motor-related inhibition was appropriately timed to counteract visually driven excitatory input arising from the fish's own motion, and transiently suppressed tectal spiking activity. High-resolution calcium imaging revealed localized motor-related signals in the tectal neuropil and the upstream torus longitudinalis, suggesting that CD enters the tectum via this pathway. Together, our results show how visual processing is suppressed during self-motion by motor-related phasic inhibition. This may help explain perceptual saccadic suppression observed in many species.

GABA-immunoreactive neurons in the Central Nervous System of the viviparous teleost Poecilia sphenops

Gamma-aminobutyric acid (GABA) functions as the primary inhibitory neurotransmitter within the central nervous system (CNS) of vertebrates. In this study, we examined the distribution pattern of GABA-immunoreactive (GABA-ir) cells and fibres in the CNS of the viviparous teleost Poecilia sphenops using immunofluorescence method. GABA immunoreactivity was seen in the glomerular, mitral, and granular layers of the olfactory bulbs, as well as in most parts of the dorsal and ventral telencephalon. The preoptic area consisted of a small cluster of GABA-ir cells, whereas extensively labelled GABA-ir neurons were observed in the hypothalamic areas, including the paraventricular organ, tuberal hypothalamus, nucleus recessus lateralis, nucleus recessus posterioris, and inferior lobes. In the thalamus, GABA-positive neurons were only found in the ventral thalamic and central posterior thalamic nuclei, whereas the dorsal part of the nucleus pretectalis periventricularis consisted of a few GABA-ir cells. GABA-immunoreactivity was extensively seen in the alar and basal subdivisions of the midbrain, whereas in the rhombencephalon, GABA-ir cells and fibres were found in the cerebellum, motor nucleus of glossopharyngeal and vagal nerves, nucleus commissuralis of Cajal, and reticular formation. In the spinal cord, GABA-ir cells and fibres were observed in the dorsal horn, ventral horn, and around the central canal. Overall, the extensive distribution of GABA-ir cells and fibres throughout the CNS suggests several roles for GABA, including the neuroendocrine, viscerosensory, and somatosensory functions, for the first time in a viviparous teleost.

Behavior and neural activation patterns of nonredundant visual and acoustic signaling during courtship in an African cichlid fish

Animals evolve mechanisms to send and receive communication signals through multiple sensory channels during crucial behavioral contexts like aggression and reproduction. This assures the transmission of important context-dependent signals that supply either the same (redundant) or different (nonredundant) information to the receiver. Despite the importance of multimodal communication, there are relatively few species in which information on sender signals and receiver responses are known. Further, little is known about where context-dependent unimodal and multimodal information is processed in the brain to produce adaptive behaviors. We used the African cichlid, Astatotilapia burtoni, to investigate how unimodal and multimodal signals are processed within the female brain in a reproductive context. During courtship, dominant males produce low frequency sounds in conjunction with visual displays (quivers) directed towards receptive gravid females. We compared affiliation behaviors and neural activation patterns in gravid females exposed to visual, acoustic, and visual-acoustic signals from courting dominant males. Females displayed reduced affiliation in auditory only conditions, but similar affiliation during visual and visual-acoustic conditions, demonstrating that visual-acoustic signaling from males is nonredundant but vision dominates. Using the neural activation marker cfos, we identified differential activation in specific socially-relevant brain nuclei between unimodal and multimodal conditions and distinct neural co-activation networks associated with each sensory context. Combined with our previous work on chemosensory signaling, we propose that A. burtoni represents a valuable vertebrate model for studying context-dependent behavioral and neural decision making associated with nonredundant multimodal communication.

A genetic labeling system to study dendritic spine development in zebrafish models of neurodevelopmental disorders

Dendritic spines are the principal site of excitatory synapse formation in the human brain. Several neurodevelopmental disorders cause spines to develop abnormally, resulting in altered spine number and morphology. Although spine development has been thoroughly characterized in the mammalian brain, spines are not unique to mammals. We have developed a genetic system in zebrafish to enable high-resolution in vivo imaging of spine dynamics during larval development. Although spiny neurons are rare in the larval zebrafish, pyramidal neurons (PyrNs) of the zebrafish tectum form an apical dendrite containing a dense array of dendritic spines. To characterize dendritic spine development, we performed mosaic genetic labeling of individual PyrNs labeled by an id2b:gal4 transgene. Our findings identify a developmental period during which PyrN dendrite growth is concurrent with spine formation. Throughout this period, motile, transient filopodia gradually transform into stable spines containing postsynaptic specializations. The utility of this system to study neurodevelopmental disorders was validated by examining spine development in fmr1 mutant zebrafish, a model of fragile X syndrome. PyrNs in fmr1 mutants exhibited pronounced defects in dendrite growth and spine stabilization. Taken together, these findings establish a genetic labeling system to study dendritic spine development in larval zebrafish. In the future, this system could be combined with high-throughput screening approaches to identify genes and drug targets that regulate spine formation.

Summary: We have developed a genetic labeling system in zebrafish to enable high-resolution in vivo imaging of dendritic spine dynamics during larval development.

Distribution of neurogranin-like immunoreactivity in the brain and sensory organs of the adult zebrafish

We studied the expression of neurogranin in the brain and some sensory organs (barbel taste buds, olfactory organs, and retina) of adult zebrafish. Database analysis shows zebrafish has two paralog neurogranin genes (nrgna and nrgnb) that translate into three peptides with a conserved IQ domain, as in mammals. Western blots of zebrafish brain extracts using an anti-neurogranin antiserum revealed three separate bands, confirming the presence of three neurogranin peptides. Immunohistochemistry shows neurogranin-like expression in the brain and sensory organs (taste buds, neuromasts and olfactory epithelium), not being able to discern its three different peptides. In the retina, the most conspicuous positive cells were bipolar neurons. In the brain, immunopositive neurons were observed in all major regions (pallium, subpallium, preoptic area, hypothalamus, diencephalon, mesencephalon and rhombencephalon, including the cerebellum), a more extended distribution than in mammals. Interestingly, dendrites, cell bodies and axon terminals of some neurons were immunopositive, thus zebrafish neurogranins may play presynaptic and postsynaptic roles. Most positive neurons were found in primary sensory centers (viscerosensory column and medial octavolateral nucleus) and integrative centers (pallium, subpallium, optic tectum and cerebellum), which have complex synaptic circuitry. However, we also observed expression in areas not related to sensory or integrative functions, such as in cerebrospinal fluid-contacting cells associated with the hypothalamic recesses, which exhibited high neurogranin-like immunoreactivity. Together, these results reveal important differences with the patterns reported in mammals, suggesting divergent evolution from the common ancestor.

Developmental neuroanatomy of the rosy bitterling Rhodeus ocellatus (Teleostei: Cypriniformes)-A microCT study

Bitterlings are carp-like teleost fish (Cypriniformes: Acheilanathidae) known for their specialized brood parasitic lifestyle. Bitterling embryos, in fact, develop inside the gill chamber of their freshwater mussel hosts. However, little is known about how their parasitic lifestyle affects brain development in comparison to nonparasitic species. Here, we document the development of the brain of the rosy bitterling, Rhodeus ocellatus, at four embryonic stages of 165, 185, 210, 235 hours postfertilization (hpf) using micro-computed tomography (microCT). Focusing on developmental regionalization and brain ventricular organization, we relate the development of the brain divisions to those described for zebrafish using the prosomeric model as a reference paradigm. Segmentation and three-dimensional visualization of the ventricular system allowed us to identify changes in the longitudinal brain axis as a result of cephalic flexure during development. The results show that during early embryonic and larval development, histological differentiation, tissue boundaries, periventricular proliferation zones, and ventricular spaces are all detectable by microCT. The results of this study visualized with differential CT profiles are broadly consistent with comparable histological studies, and with the genoarchitecture of teleosts like the zebrafish. Compared to the zebrafish, our study identifies distinct developmental heterochronies in the rosy bitterling, such as a precocious development of the inferior lobe.

Input from torus longitudinalis drives binocularity and spatial summation in zebrafish optic tectum

Background

A continued effort in neuroscience aims to understand the way brain circuits consisting of diverse neuronal types generate complex behavior following sensory input. A common feature of vertebrate visual systems is that lower-order and higher-order visual areas are reciprocally connected. Feedforward projections confer visual responsiveness to higher-order visual neurons while feedback projections likely serve to modulate responses of lower-order visual neurons in a context-dependent manner. Optic tectum is the largest first-order visual brain area in zebrafish and is reciprocally connected with the torus longitudinalis (TL), a second-order visual brain area that does not receive retinal input. A functional role for feedback projections from TL to tectum has not been identified. Here we aim to understand how this feedback contributes to visual processing.

Results

In this study, we demonstrate that TL feedback projections to tectum drive binocular integration and spatial summation in a defined tectal circuit. We performed genetically targeted, cell type-specific functional imaging in tectal pyramidal neurons (PyrNs) and their two input neuron populations: retinal ganglion cells (RGCs) and neurons in TL. We find that PyrNs encode gradual changes in scene luminance using a complement of three distinct response classes that encode different light intensity ranges. Functional imaging of RGC inputs to tectum suggest that these response classes originate in the retina and RGC input specifies PyrN functional classes. In contrast, TL input serves to endow PyrNs with large, compound receptive fields that span both retinal hemifields.

Conclusions

These findings reveal a novel role for the zebrafish TL in driving binocular integration and spatial summation in tectal PyrNs. The neural circuit we describe generates a population of tectal neurons with large receptive fields tailored for detecting changes in the visual scene.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01222-x.

Electrophysiological and pharmacological characterization of spreading depolarization in the adult zebrafish tectum

Spreading depolarization (SD) is a slowly propagating wave of neuronal and glial depolarization. A growing number of studies show that SD and SD-like phenomena play a role in neurological disorders such as migraine, stroke, and traumatic brain injury. Despite the clinical importance of SD, its underlying molecular and cellular mechanisms remain elusive, possibly because of insufficient animal model allowing genetic manipulation. Such a model would also allow high-throughput screening for SD-suppressing drug development. To address this, we developed a novel experimental system to study SD using zebrafish. Electrophysiological recordings in the immobilized adult zebrafish revealed that increasing extracellular potassium concentration elicited SD with a large and long-lasting negative shift of direct current (DC) potential in the optic tectum. It also reduced the oscillatory activity in the extracellular field potential and increased the expression of the immediate early gene c-fos. Pharmacological blocking of the N-methyl-d-aspartate (NMDA) glutamate receptor attenuated the propagation of SD, suggesting that glutamatergic neurotransmission mediated tectal SD in zebrafish. Our analyses revealed that the zebrafish tectum and rodent cortex had similar SD kinetics. The current study provides electrophysiological and pharmacological evidence that zebrafish SD and mammal SD are comparable. This zebrafish SD model is suitable for genetic manipulation and cost-effective high-throughput screening. It could pave the way to novel diagnostic and therapeutic methods applicable to SD-associated neurological disorders.NEW & NOTEWORTHY Previous studies have implicated spreading depolarization (SD) in stroke and migraine. Here, we demonstrate SD, for the first time, in the adult zebrafish tectum showing waveform kinetics, c-fos expression, and attenuation by N-methyl-d-aspartate glutamate receptor blocker as observed in the rodent cortex. Since the zebrafish is an animal model amenable to genetic manipulation and chemical screening, this result could pave the way to novel diagnostic and therapeutic methods applicable to SD-associated neurological disorders.

A novel inhibitor rescues cerebellar defects in a zebrafish model of Down syndrome-associated kinase Dyrk1A overexpression

The highly conserved dual-specificity tyrosine phosphorylation-regulated kinase 1A (Dyrk1A) plays crucial roles during central nervous system development and homeostasis. Furthermore, its hyperactivity is considered responsible for some neurological defects in individuals with Down syndrome. We set out to establish a zebrafish model expressing human Dyrk1A that could be further used to characterize the interaction between Dyrk1A and neurological phenotypes. First, we revealed the prominent expression of dyrk1a homologs in cerebellar neurons in the zebrafish larval and adult brains. Overexpression of human dyrk1a in postmitotic cerebellar Purkinje neurons resulted in a structural misorganization of the Purkinje cells in cerebellar hemispheres and a compaction of this cell population. This impaired Purkinje cell organization was progressive, leading to an age-dependent dispersal of Purkinje neurons throughout the cerebellar molecular layer with larval swim deficits resulting in miscoordination of swimming and reduced exploratory behavior in aged adults. We also found that the structural misorganization of the larval Purkinje cell layer could be rescued by pharmacological treatment with Dyrk1A inhibitors. We further reveal the in vivo efficiency of a novel selective Dyrk1A inhibitor, KuFal194. These findings demonstrate that the zebrafish is a well-suited vertebrate organism to genetically model severe neurological diseases with single cell type specificity. Such models can be used to relate molecular malfunction to cellular deficits, impaired tissue formation, and organismal behavior and can also be used for pharmacological compound testing and validation.

Algorithms underlying flexible phototaxis in larval zebrafish

To thrive, organisms must maintain physiological and environmental variables in suitable ranges. Given that these variables undergo constant fluctuations over varying time scales, how do biological control systems maintain control over these values? We explored this question in the context of phototactic behavior in larval zebrafish. We demonstrate that larval zebrafish use phototaxis to maintain environmental luminance at a set point, that the value of this set point fluctuates on a time scale of seconds when environmental luminance changes, and that it is determined by calculating the mean input across both sides of the visual field. These results expand on previous studies of flexible phototaxis in larval zebrafish; they suggest that larval zebrafish exert homeostatic control over the luminance of their surroundings, and that feedback from the surroundings drives allostatic changes to the luminance set point. As such, we describe a novel behavioral algorithm with which larval zebrafish exert control over a sensory variable.

Pyramidal Neurons of the Zebrafish Tectum Receive Highly Convergent Input From Torus Longitudinalis

The torus longitudinalis (TL) is a midbrain structure unique to ray finned fish. Although previously implicated in orienting behaviors elicited by changes in ambient lighting, the role of TL in visual processing is not well-understood. TL is reciprocally connected to tectum and is the only known source of synaptic input to the stratum marginalis (SM) layer of tectal neuropil. Conversely, tectal pyramidal neurons (PyrNs) are the only identified tectal neuron population that forms a dendrite in SM. In this study we describe a zebrafish gal4 transgenic that labels TL neurons that project to SM. We demonstrate that the axonal TL projection to SM in zebrafish is glutamatergic. Consistent with these axons synapsing directly onto PyrNs, SM-targeted dendrites of PyrNs contain punctate enrichments of the glutamatergic post-synaptic marker protein PSD95. Sparse genetic labeling of individual TL axons and PyrN dendrites enabled quantitative morphometric analysis that revealed (1) large, sparsely branched TL axons in SM and (2) small, densely innervated PyrN dendrites in SM. Together this unique combination of morphologies support a wiring diagram in which TL inputs to PyrNs exhibit a high degree of convergence. We propose that this convergence functions to generate large, compound visual receptive fields in PyrNs. This quantitative anatomical data will instruct future functional studies aimed at identifying the precise contribution of TL-PyrN circuitry to visual behavior.