Genoarchitectonics of the larval zebrafish diencephalon.

The brain is spatially organized into subdivisions, nuclei and areas, which often correspond to functional and developmental units. A segmentation of brain regions in the form of a consensus atlas facilitates mechanistic studies and is a prerequisite for sharing information among neuroanatomists. Gene expression patterns objectively delineate boundaries between brain regions and provide information about their developmental and evolutionary histories. To generate a detailed molecular map of the larval zebrafish diencephalon, we took advantage of the Max Planck Zebrafish Brain (mapzebrain) atlas, which aligns hundreds of transcript and transgene expression patterns in a shared coordinate system. Inspection and co-visualization of close to 50 marker genes have allowed us to resolve the tripartite prosomeric scaffold of the diencephalon at unprecedented resolution. This approach clarified the genoarchitectonic partitioning of the alar diencephalon into pretectum (alar part of prosomere P1), thalamus (alar part of prosomere P2, with habenula and pineal complex), and prethalamus (alar part of prosomere P3). We further identified the region of the nucleus of the medial longitudinal fasciculus, as well as the posterior and anterior parts of the posterior tuberculum, as molecularly distinct basal parts of prosomeres 1, 2, and 3, respectively. Some of the markers examined allowed us to locate glutamatergic, GABAergic, dopaminergic, serotoninergic, and various neuropeptidergic domains in the larval zebrafish diencephalon. Our molecular neuroanatomical approach has thus (1) yielded an objective and internally consistent interpretation of the prosomere boundaries within the zebrafish forebrain; has (2) produced a list of markers, which in sparse combinations label the subdivisions of the diencephalon; and is (3) setting the stage for further functional and developmental studies in this vertebrate brain.

A single-cell resolution gene expression atlas of the larval zebrafish brain.

The advent of multimodal brain atlases promises to accelerate progress in neuroscience by allowing in silico queries of neuron morphology, connectivity, and gene expression. We used multiplexed fluorescent in situ RNA hybridization chain reaction (HCR) technology to generate expression maps across the larval zebrafish brain for a growing set of marker genes. The data were registered to the Max Planck Zebrafish Brain (mapzebrain) atlas, thus allowing covisualization of gene expression, single-neuron tracings, and expertly curated anatomical segmentations. Using post hoc HCR labeling of the immediate early gene cfos, we mapped responses to prey stimuli and food ingestion across the brain of freely swimming larvae. This unbiased approach revealed, in addition to previously described visual and motor areas, a cluster of neurons in the secondary gustatory nucleus, which express the marker calb2a, as well as a specific neuropeptide Y receptor, and project to the hypothalamus. This discovery exemplifies the power of this new atlas resource for zebrafish neurobiology.

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.

The retina is the thin layer of tissue in the eye that can receive light stimuli and convert them into electric signals to be transmitted to the brain. The cells that sense fine detail cluster at the center of the retina while the motion-sensing cells that keep track of movement lie at the periphery. When zebrafish larvae hunt, their motion-sensing cells are triggered as a prey crosses their peripheral field of view. They then turn and swim towards it. As they approach, the prey image moves to the detail-sensing part of the retina and appears larger, filling more of the field of view at close range. The signals are then processed in defined parts of the brain, in particular in a region called the optic tectum. How this area is organized in response to the organization of the eye and the requirements of the hunt is still unclear. Förster et al. set out to explore how the hunting routine of zebrafish larvae shapes the arrangement of neurons in the optic tectum. The larvae were exposed to different images representing the various aspects of the prey capture process: small moving dots represented passing prey at a distance, while large moving dots stood for prey just before capture. Measuring activity in the neurons of the optic tectum revealed that, like the eye, different areas specialize in different tasks. The back of the tectum was frequently activated by small dots and worked out which direction they were moving in during the first hunting steps. The front of the tectum responded best to large dots, often ignoring their direction, and helped the larvae to track their prey straight ahead. To test these findings, Förster et al. destroyed the large object-responsive cells with a laser and watched the larvae hunting real prey. Without the cells, the fish found it much harder to track and catch their targets. These results shed light on the link between behavior and how neurons are arranged in the brain. Future work could explore how the different neurons in the optic tectum are connected, and the behaviors they trigger in the fish. This could help to reveal general principles about how sensory information guides behavior and how evolution has shaped the layout of the brain.

A Cellular-Resolution Atlas of the Larval Zebrafish Brain.

Understanding brain-wide neuronal dynamics requires a detailed map of the underlying circuit architecture. We built an interactive cellular-resolution atlas of the zebrafish brain at 6 days post-fertilization (dpf) based on the reconstructions of over 2,000 individually GFP-labeled neurons. We clustered our dataset in "morphotypes," establishing a unique database of quantitatively described neuronal morphologies together with their spatial coordinates in vivo. Over 100 transgene expression patterns were imaged separately and co-registered with the single-neuron atlas. By annotating 72 non-overlapping brain regions, we generated from our dataset an inter-areal wiring diagram of the larval brain, which serves as ground truth for synapse-scale, electron microscopic reconstructions. Interrogating our atlas by "virtual tract tracing" has already revealed previously unknown wiring principles in the tectum and the cerebellum. In conclusion, we present here an evolving computational resource and visualization tool, which will be essential to map function to structure in a vertebrate brain. VIDEO ABSTRACT.