A Three-Layer Network Model of Direction Selective Circuits in the Optic Tectum

Prolonged exposure to the synthetic glucocorticoid dexamethasone induces brain damage via oxidative stress and apoptotic response in adult Daniorerio

Due to its extensive use as a painkiller, anti-inflammatory, and immune modulatory agent, as well as its effectiveness in treating severe COVID-19, dexamethasone, a synthetic glucocorticoid, has gained attention not only for its impact on public health but also for its environmental implications. Various studies have reported its presence in aquatic environments, including urban waters, surface samples, sediments, drinking water, and wastewater effluents. However, limited information is available regarding its toxic effects on nontarget aquatic organisms. Therefore, this study aimed to investigate the mechanism of toxicity underlying dexamethasone-induced brain damage in the bioindicator Danio rerio following long-term exposure. Adult zebrafish were treated with environmentally relevant concentrations of dexamethasone (20, 40, and 60 ng L-1) for 28 days. To elucidate the possible mechanisms involved in the toxicity of the pharmaceutical compound, we conducted a behavioral test battery (Novel Tank and Light and Dark tests), oxidative stress biomarkers, acetylcholinesterase enzyme activity quantification, histopathological analysis, and gene expression analysis using qRT-PCR (p53, bcl-2, bax, caspase-3, nrf1, and nrf2).The results revealed that the pharmaceutical compound could produce anxiety-like symptoms, increase the oxidative-induced stress response, decrease the activity of acetylcholinesterase enzyme, and cause histopathological alterations, including perineuronal vacuolization, granular and molecular layers deterioration, cell swallowing and intracellular spaces. The expression of genes involved in the apoptotic process (p53, bax, and casp-3) and antioxidant defense (nrf1 and nrf2) was upregulated in response to oxidative damage, while the expression of the anti-apoptotic gene bcl-2 was down-regulated indicating that the environmental presence of dexamethasone may pose a threat to wildlife and human health.

The Visual Systems of Zebrafish

The zebrafish visual system has become a paradigmatic preparation for behavioral and systems neuroscience. Around 40 types of retinal ganglion cells (RGCs) serve as matched filters for stimulus features, including light, optic flow, prey, and objects on a collision course. RGCs distribute their signals via axon collaterals to 12 retinorecipient areas in forebrain and midbrain. The major visuomotor hub, the optic tectum, harbors nine RGC input layers that combine information on multiple features. The retinotopic map in the tectum is locally adapted to visual scene statistics and visual subfield-specific behavioral demands. Tectal projections to premotor centers are topographically organized according to behavioral commands. The known connectivity in more than 20 processing streams allows us to dissect the cellular basis of elementary perceptual and cognitive functions. Visually evoked responses, such as prey capture or loom avoidance, are controlled by dedicated multistation pathways that-at least in the larva-resemble labeled lines. This architecture serves the neuronal code's purpose of driving adaptive behavior.

From perception to behavior: The neural circuits underlying prey hunting in larval zebrafish

A key challenge for neural systems is to extract relevant information from the environment and make appropriate behavioral responses. The larval zebrafish offers an exciting opportunity for studying these sensing processes and sensory-motor transformations. Prey hunting is an instinctual behavior of zebrafish that requires the brain to extract and combine different attributes of the sensory input and form appropriate motor outputs. Due to its small size and transparency the larval zebrafish brain allows optical recording of whole-brain activity to reveal the neural mechanisms involved in prey hunting and capture. In this review we discuss how the larval zebrafish brain processes visual information to identify and locate prey, the neural circuits governing the generation of motor commands in response to prey, how hunting behavior can be modulated by internal states and experience, and some outstanding questions for the field.

Automated synapse-level reconstruction of neural circuits in the larval zebrafish brain

Dense reconstruction of synaptic connectivity requires high-resolution electron microscopy images of entire brains and tools to efficiently trace neuronal wires across the volume. To generate such a resource, we sectioned and imaged a larval zebrafish brain by serial block-face electron microscopy at a voxel size of 14 × 14 × 25 nm3. We segmented the resulting dataset with the flood-filling network algorithm, automated the detection of chemical synapses and validated the results by comparisons to transmission electron microscopic images and light-microscopic reconstructions. Neurons and their connections are stored in the form of a queryable and expandable digital address book. We reconstructed a network of 208 neurons involved in visual motion processing, most of them located in the pretectum, which had been functionally characterized in the same specimen by two-photon calcium imaging. Moreover, we mapped all 407 presynaptic and postsynaptic partners of two superficial interneurons in the tectum. The resource developed here serves as a foundation for synaptic-resolution circuit analyses in the zebrafish nervous system.

Panx1b Modulates the Luminance Response and Direction of Locomotion in the Zebrafish

Pannexin1 (Panx1) can form ATP-permeable channels that play roles in the physiology of the visual system. In the zebrafish two ohnologs of Panx1, Panx1a and Panx1b, have unique and shared channel properties and tissue expression patterns. Panx1a channels are located in horizontal cells of the outer retina and modulate light decrement detection through an ATP/pH-dependent mechanisms and adenosine/dopamine signaling. Here, we decipher how the strategic localization of Panx1b channels in the inner retina and ganglion cell layer modulates visually evoked motor behavior. We describe a panx1b knockout model generated by TALEN technology. The RNA-seq analysis of 6 days post-fertilization larvae is confirmed by real-time PCR and paired with testing of locomotion behaviors by visual motor and optomotor response tests. We show that the loss of Panx1b channels disrupts the retinal response to an abrupt loss of illumination and it decreases the larval ability to follow leftward direction of locomotion in low light conditions. We concluded that the loss of Panx1b channels compromises the final output of luminance as well as motion detection. The Panx1b protein also emerges as a modulator of the circadian clock system. The disruption of the circadian clock system in mutants suggests that Panx1b could participate in non-image forming processes in the inner retina.

Reduction of visual stimulus artifacts using a spherical tank for small, aquatic animals

Delivering appropriate stimuli remains a challenge in vision research, particularly for aquatic animals such as zebrafish. Due to the shape of the water tank and the associated optical paths of light rays, the stimulus can be subject to unwanted refraction or reflection artifacts, which may spoil the experiment and result in wrong conclusions. Here, we employ computer graphics simulations and calcium imaging in the zebrafish optic tectum to show, how a spherical glass container optically outperforms many previously used water containers, including Petri dish lids. We demonstrate that aquatic vision experiments suffering from total internal reflection artifacts at the water surface or at the flat container bottom may result in the erroneous detection of visual neurons with bipartite receptive fields and in the apparent absence of neurons selective for vertical motion. Our results and demonstrations will help aquatic vision neuroscientists on optimizing their stimulation setups.

Functional, molecular and morphological heterogeneity of superficial interneurons in the larval zebrafish tectum

The superficial interneurons, SINs, of the zebrafish tectum, have been implicated in a range of visual functions, including size discrimination, directional selectivity, and looming-evoked escape. This raises the question if SIN subpopulations, despite their morphological similarities and shared anatomical position in the retinotectal processing stream, carry out diverse, task-specific functions in visual processing, or if they have simple tuning properties in common. Here we have further characterized the SINs through functional imaging, electrophysiological recordings, and neurotransmitter typing in two transgenic lines, the widely used Gal4s1156t and the recently reported LCRRH2-RH2-2:GFP. We found that about a third of the SINs strongly responded to changes in whole-field light levels, with a strong preference for OFF over ON stimuli. Interestingly, individual SINs were selectively tuned to a diverse range of narrow luminance decrements. Overall responses to whole-field luminance steps did not vary with the position of the SIN cell body along the depth of the tectal neuropil or with the orientation of its neurites. We ruled out the possibility that intrinsic photosensitivity of Gal4s1156t+ SINs contribute to the measured visual responses. We found that, while most SINs express GABAergic markers, a substantial minority express an excitatory neuronal marker, the vesicular glutamate transporter, expanding the possible roles of SIN function in the tectal circuitry. In conclusion, SINs represent a molecularly, morphologically, and functionally heterogeneous class of interneurons, with subpopulations that detect a range of specific visual features, to which we have now added narrow luminance decrements.

Retinotectal circuitry of larval zebrafish is adapted to detection and pursuit of prey

Retinal axon projections form a map of the visual environment in the tectum. A zebrafish larva typically detects a prey object in its peripheral visual field. As it turns and swims towards the prey, the stimulus enters the central, binocular area, and seemingly expands in size. By volumetric calcium imaging, we show that posterior tectal neurons, which serve to detect prey at a distance, tend to respond to small objects and intrinsically compute their direction of movement. Neurons in anterior tectum, where the prey image is represented shortly before the capture strike, are tuned to larger object sizes and are frequently not direction-selective, indicating that mainly interocular comparisons serve to compute an object's movement at close range. The tectal feature map originates from a linear combination of diverse, functionally specialized, lamina-specific, and topographically ordered retinal ganglion cell synaptic inputs. We conclude that local cell-type composition and connectivity across the tectum are adapted to the processing of location-dependent, behaviorally relevant object features.

On the organization of receptive fields of retinal spot detectors projecting to the fish tectum: Analogies with the local edge detectors in frogs and mammals

Responses of ON- and OFF-ganglion cells (GCs) were recorded extracellularly from their axon terminals in the medial sublamina of tectal retino-recipient layer of immobilized cyprinid fish (goldfish and carp). These units were recorded deeper than direction selective (DS) ones and at the same depth where responses of orientation selective (OS) GCs were recorded. Prominent responses of these units are evoked by small contrast spots flickering within or moving across their visual field. They are not selective either to the direction of motion or to the orientation of stimuli and are not characterized by any spontaneous spike activity. We refer to these fish GCs as spot detectors (SDs) by analogy with the frog SD. Receptive fields (RFs) of SDs are organized concentrically: the excitatory center (about 4.5°) is surrounded by opponent periphery. Study of interactions in the RF has shown that inhibitory influences are generated already inside the central RF area. This fact suggests that RFs of SDs cannot be defined as homogeneous sensory zone driven by a linear mechanism of response generation. Physiological properties of fish SDs are compared with the properties of frog SDs and analogous mammalian retinal GCs-local edge detectors (LEDs). The potential role of the SDs in visually guided fish behavior is discussed.

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.

Putative targets of direction-selective retinal ganglion cells in the tectum opticum of cyprinid fish

Responses of direction selective (DS) units of retinal and tectal origin were recorded extracellularly from the tectum opticum (TO) of immobilized fish. The data were collected from three cyprinid species - goldfish, carp and roach. Responses of the retinal DS ganglion cells (GCs) were recorded from their axon terminals in the superficial layers of TO. According to their preferred directions DS GCs, characterized by small receptive fields (3-8°), can be divided in three distinct groups, each group containing ON and OFF subtypes approximately in equal quantity. Conversely, direction-selective tectal neurons (DS TNs), recorded at two different tectal levels deeper than the zone of retinal DS afferents, are characterized by large receptive fields (up to 60°) and are indifferent to any sign of contrast i.e. can be considered as ON-OFF type units. Fish DS TNs unlike the retinal DS GCs, select four preferred directions. Three types of tectal DS units prefer practically the same directions as those already selected on the retinal level - caudo-rostral, dorso-ventral and ventro-dorsal. The fact that three preferred directions of DS GCs and DS TNs coincide allows us to assume that three types of DS GCs are input neurons for corresponding types of DS TNs. The fourth group of DS TNs has the emergent rostro-caudal preference not explicitly present in any of the DS GC inputs. These units are recorded in deep TO layers exclusively. Receptive fields of these DS neurons could be entirely formed on the tectal level. Possible interrelations between retinal and tectal DS units are discussed.

Selective processing of all rotational and translational optic flow directions in the zebrafish pretectum and tectum

Background

The processing of optic flow in the pretectum/accessory optic system allows animals to stabilize retinal images by executing compensatory optokinetic and optomotor behavior. The success of this behavior depends on the integration of information from both eyes to unequivocally identify all possible translational or rotational directions of motion. However, it is still unknown whether the precise direction of ego-motion is already identified in the zebrafish pretectum or later in downstream premotor areas.

Results

Here, we show that the zebrafish pretectum and tectum each contain four populations of motion-sensitive direction-selective (DS) neurons, with each population encoding a different preferred direction upon monocular stimulation. In contrast, binocular stimulation revealed the existence of pretectal and tectal neurons that are specifically tuned to only one of the many possible combinations of monocular motion, suggesting that further downstream sensory processing might not be needed to instruct appropriate optokinetic and optomotor behavior.

Conclusion

Our results suggest that local, task-specific pretectal circuits process DS retinal inputs and carry out the binocular sensory computations necessary for optokinetic and optomotor behavior.

Electronic supplementary material

The online version of this article (10.1186/s12915-019-0648-2) contains supplementary material, which is available to authorized users.

Probabilistic Encoding Models for Multivariate Neural Data

A key problem in systems neuroscience is to characterize how populations of neurons encode information in their patterns of activity. An understanding of the encoding process is essential both for gaining insight into the origins of perception and for the development of brain-computer interfaces. However, this characterization is complicated by the highly variable nature of neural responses, and thus usually requires probabilistic methods for analysis. Drawing on techniques from statistical modeling and machine learning, we review recent methods for extracting important variables that quantitatively describe how sensory information is encoded in neural activity. In particular, we discuss methods for estimating receptive fields, modeling neural population dynamics, and inferring low dimensional latent structure from a population of neurons, in the context of both electrophysiology and calcium imaging data.

Optic tectal superficial interneurons detect motion in larval zebrafish

Detection of moving objects is an essential skill for animals to hunt prey, recognize conspecifics and avoid predators. The zebrafish, as a vertebrate model, primarily uses its elaborate visual system to distinguish moving objects against background scenes. The optic tectum (OT) receives and integrates inputs from various types of retinal ganglion cells (RGCs), including direction-selective (DS) RGCs and size-selective RGCs, and is required for both prey capture and predator avoidance. However, it remains largely unknown how motion information is processed within the OT. Here we performed in vivo whole-cell recording and calcium imaging to investigate the role of superficial interneurons (SINs), a specific type of optic tectal neurons, in motion detection of larval zebrafish. SINs mainly receive excitatory synaptic inputs, exhibit transient ON- or OFF-type of responses evoked by light flashes, and possess a large receptive field (RF). One fifth of SINs are DS and classified into two subsets with separate preferred directions. Furthermore, SINs show size-dependent responses to moving dots. They are efficiently activated by moving objects but not static ones, capable of showing sustained responses to moving objects and having less visual adaptation than periventricular neurons (PVNs), the principal tectal cells. Behaviorally, ablation of SINs impairs prey capture, which requires local motion detection, but not global looming-evoked escape. Finally, starvation enhances the gain of SINs' motion responses while maintaining their size tuning and DS. These results indicate that SINs serve as a motion detector for sensing and localizing sized moving objects in the visual field.