Prepontine non-giant neurons drive flexible escape behavior in zebrafish.

Many species execute ballistic escape reactions to avoid imminent danger. Despite fast reaction times, responses are often highly regulated, reflecting a trade-off between costly motor actions and perceived threat level. However, how sensory cues are integrated within premotor escape circuits remains poorly understood. Here, we show that in zebrafish, less precipitous threats elicit a delayed escape, characterized by flexible trajectories, which are driven by a cluster of 38 prepontine neurons that are completely separate from the fast escape pathway. Whereas neurons that initiate rapid escapes receive direct auditory input and drive motor neurons, input and output pathways for delayed escapes are indirect, facilitating integration of cross-modal sensory information. These results show that rapid decision-making in the escape system is enabled by parallel pathways for ballistic responses and flexible delayed actions and defines a neuronal substrate for hierarchical choice in the vertebrate nervous system.

Brain-wide cellular resolution imaging of Cre transgenic zebrafish lines for functional circuit-mapping.

Decoding the functional connectivity of the nervous system is facilitated by transgenic methods that express a genetically encoded reporter or effector in specific neurons; however, most transgenic lines show broad spatiotemporal and cell-type expression. Increased specificity can be achieved using intersectional genetic methods which restrict reporter expression to cells that co-express multiple drivers, such as Gal4 and Cre. To facilitate intersectional targeting in zebrafish, we have generated more than 50 new Cre lines, and co-registered brain expression images with the Zebrafish Brain Browser, a cellular resolution atlas of 264 transgenic lines. Lines labeling neurons of interest can be identified using a web-browser to perform a 3D spatial search (zbbrowser.com). This resource facilitates the design of intersectional genetic experiments and will advance a wide range of precision circuit-mapping studies.

Presynaptic Inhibition Selectively Gates Auditory Transmission to the Brainstem Startle Circuit.

Filtering mechanisms prevent a continuous stream of sensory information from swamping perception, leading to diminished focal attention and cognitive processing. Mechanisms for sensory gating are commonly studied using prepulse inhibition, a paradigm that measures the regulated transmission of auditory information to the startle circuit; however, the underlying neuronal pathways are unresolved. Using large-scale calcium imaging, optogenetics, and laser ablations, we reveal a cluster of 30 morphologically identified neurons in zebrafish that suppress the transmission of auditory signals during prepulse inhibition. These neurons project to a key sensorimotor interface in the startle circuit-the termination zone of auditory afferents on the dendrite of a startle command neuron. Direct measurement of auditory nerve neurotransmitter release revealed selective presynaptic inhibition of sensory transmission to the startle circuit, sparing signaling to other brain regions. Our results provide the first cellular resolution circuit for prepulse inhibition in a vertebrate, revealing a central role for presynaptic gating of sensory information to a brainstem motor circuit.

Morphometric analysis and neuroanatomical mapping of the zebrafish brain.

Large-scale genomic studies have recently identified genetic variants causative for major neurodevelopmental disorders, such as intellectual disability and autism. However, determining how underlying developmental processes are affected by these mutations remains a significant challenge in the field. Zebrafish is an established model system in developmental neurogenetics that may be useful in uncovering the mechanisms of these mutations. Here we describe the use of voxel-intensity, deformation field, and volume-based morphometric techniques for the systematic and unbiased analysis of gene knock-down and environmental exposure-induced phenotypes in zebrafish. We first present a computational method for brain segmentation based on transgene expression patterns to create a comprehensive neuroanatomical map. This map allowed us to disclose statistically significant changes in brain microstructure and composition in neurodevelopmental models. We demonstrate the effectiveness of morphometric techniques in measuring changes in the relative size of neuroanatomical subdivisions in atoh7 morphant larvae and in identifying phenotypes in larvae treated with valproic acid, a chemical demonstrated to increase the risk of autism in humans. These tools enable rigorous evaluation of the effects of gene mutations and environmental exposures on neural development, providing an entry point for cellular and molecular analysis of basic developmental processes as well as neurodevelopmental and neurodegenerative disorders.

High-precision registration between zebrafish brain atlases using symmetric diffeomorphic normalization.

Atlases provide a framework for spatially mapping information from diverse sources into a common reference space. Specifically, brain atlases allow annotation of gene expression, cell morphology, connectivity, and activity. In larval zebrafish, advances in genetics, imaging, and computational methods now allow the collection of such information brain-wide. However, due to technical considerations, disparate datasets may use different references and may not be aligned to the same coordinate space. Two recent larval zebrafish atlases exemplify this problem: Z-Brain, containing gene expression, neural activity, and neuroanatomical segmentations, was acquired using immunohistochemical stains, while the Zebrafish Brain Browser (ZBB) was constructed from live scans of fluorescent reporters in transgenic larvae. Although different references were used, the atlases included several common transgenic patterns that provide potential "bridges" for transforming each into the other's coordinate space. We tested multiple bridging channels and registration algorithms and found that the symmetric diffeomorphic normalization algorithm improved live brain registration precision while better preserving cell morphology than B-spline-based registrations. Symmetric diffeomorphic normalization also corrected for tissue distortion introduced during fixation. Multi-reference channel optimization provided a transformation that enabled Z-Brain and ZBB to be co-aligned with precision of approximately a single cell diameter and minimal perturbation of cell and tissue morphology. Finally, we developed software to visualize brain regions in 3 dimensions, including a virtual reality neuroanatomy explorer. This study demonstrates the feasibility of integrating whole brain datasets, despite disparate reference templates and acquisition protocols, when sufficient information is present for bridging. Increased accuracy and interoperability of zebrafish digital brain atlases will facilitate neurobiological studies.

Genetic Ablation, Sensitization, and Isolation of Neurons Using Nitroreductase and Tetrodotoxin-Insensitive Channels.

Advances in genetic technologies enable the highly selective expression of transgenes in targeted neuronal cell types. Transgene expression can be used to noninvasively ablate, silence or activate neurons, providing a tool to probe their contribution to the control of behavior or physiology. Here, we describe the use of the tetrodotoxin (TTX)-resistant voltage-gated sodium channel Nav1.5 for either sensitizing neurons to depolarizing input, or isolating targeted neurons from surrounding neural activity, and methods for selective neuronal ablation using the bacterial nitroreductase NfsB.

A 3D Searchable Database of Transgenic Zebrafish Gal4 and Cre Lines for Functional Neuroanatomy Studies.

Transgenic methods enable the selective manipulation of neurons for functional mapping of neuronal circuits. Using confocal microscopy, we have imaged the cellular-level expression of 109 transgenic lines in live 6 day post fertilization larvae, including 80 Gal4 enhancer trap lines, 9 Cre enhancer trap lines and 20 transgenic lines that express fluorescent proteins in defined gene-specific patterns. Image stacks were acquired at single micron resolution, together with a broadly expressed neural marker, which we used to align enhancer trap reporter patterns into a common 3-dimensional reference space. To facilitate use of this resource, we have written software that enables searching for transgenic lines that label cells within a selectable 3-dimensional region of interest (ROI) or neuroanatomical area. This software also enables the intersectional expression of transgenes to be predicted, a feature which we validated by detecting cells with co-expression of Cre and Gal4. Many of the imaged enhancer trap lines show intrinsic brain-specific expression. However, to increase the utility of lines that also drive expression in non-neuronal tissue we have designed a novel UAS reporter, that suppresses expression in heart, muscle, and skin through the incorporation of microRNA binding sites in a synthetic 3' untranslated region. Finally, we mapped the site of transgene integration, thus providing molecular identification of the expression pattern for most lines. Cumulatively, this library of enhancer trap lines provides genetic access to 70% of the larval brain and is therefore a powerful and broadly accessible tool for the dissection of neural circuits in larval zebrafish.

Increased functional protein expression using nucleotide sequence features enriched in highly expressed genes in zebrafish.

Many genetic manipulations are limited by difficulty in obtaining adequate levels of protein expression. Bioinformatic and experimental studies have identified nucleotide sequence features that may increase expression, however it is difficult to assess the relative influence of these features. Zebrafish embryos are rapidly injected with calibrated doses of mRNA, enabling the effects of multiple sequence changes to be compared in vivo. Using RNAseq and microarray data, we identified a set of genes that are highly expressed in zebrafish embryos and systematically analyzed for enrichment of sequence features correlated with levels of protein expression. We then tested enriched features by embryo microinjection and functional tests of multiple protein reporters. Codon selection, releasing factor recognition sequence and specific introns and 3' untranslated regions each increased protein expression between 1.5- and 3-fold. These results suggested principles for increasing protein yield in zebrafish through biomolecular engineering. We implemented these principles for rational gene design in software for codon selection (CodonZ) and plasmid vectors incorporating the most active non-coding elements. Rational gene design thus significantly boosts expression in zebrafish, and a similar approach will likely elevate expression in other animal models.

Direct activation of the Mauthner cell by electric field pulses drives ultrarapid escape responses.

Rapid escape swims in fish are initiated by the Mauthner cells, giant reticulospinal neurons with unique specializations for swift responses. The Mauthner cells directly activate motoneurons and facilitate predator detection by integrating acoustic, mechanosensory, and visual stimuli. In addition, larval fish show well-coordinated escape responses when exposed to electric field pulses (EFPs). Sensitization of the Mauthner cell by genetic overexpression of the voltage-gated sodium channel SCN5 increased EFP responsiveness, whereas Mauthner ablation with an engineered variant of nitroreductase with increased activity (epNTR) eliminated the response. The reaction time to EFPs is extremely short, with many responses initiated within 2 ms of the EFP. Large neurons, such as Mauthner cells, show heightened sensitivity to extracellular voltage gradients. We therefore tested whether the rapid response to EFPs was due to direct activation of the Mauthner cells, bypassing delays imposed by stimulus detection and transmission by sensory cells. Consistent with this, calcium imaging indicated that EFPs robustly activated the Mauthner cell but only rarely fired other reticulospinal neurons. Further supporting this idea, pharmacological blockade of synaptic transmission in zebrafish did not affect Mauthner cell activity in response to EFPs. Moreover, Mauthner cells transgenically expressing a tetrodotoxin (TTX)-resistant voltage-gated sodium channel retained responses to EFPs despite TTX suppression of action potentials in the rest of the brain. We propose that EFPs directly activate Mauthner cells because of their large size, thereby driving ultrarapid escape responses in fish.

Transgenic quail as a model for research in the avian nervous system: a comparative study of the auditory brainstem.

Research performed on transgenic animals has led to numerous advances in biological research. However, using traditional retroviral methods to generate transgenic avian research models has proved problematic. As a result, experiments aimed at genetic manipulations on birds have remained difficult for this popular research tool. Recently, lentiviral methods have allowed the production of transgenic birds, including a transgenic Japanese quail (Coturnix coturnix japonica) line showing neuronal specificity and stable expression of enhanced green fluorescent protein (eGFP) across generations (termed here GFP quail). To test whether the GFP quail may serve as a viable alternative to the popular chicken model system, with the additional benefit of genetic manipulation, we compared the development, organization, structure, and function of a specific neuronal circuit in chicken (Gallus gallus domesticus) with that of the GFP quail. This study focuses on a well-defined avian brain region, the principal nuclei of the sound localization circuit in the auditory brainstem, nucleus magnocellularis (NM), and nucleus laminaris (NL). Our results demonstrate that structural and functional properties of NM and NL neurons in the GFP quail, as well as their dynamic properties in response to changes in the environment, are nearly identical to those in chickens. These similarities demonstrate that the GFP quail, as well as other transgenic quail lines, can serve as an attractive avian model system, with the advantage of being able to build on the wealth of information already available from the chicken.

Tonotopic organization of the superior olivary nucleus in the chicken auditory brainstem.

Topographic maps are salient features of neuronal organization in sensory systems. Inhibitory components of neuronal circuitry are often embedded within this organization, making them difficult to isolate experimentally. The auditory system provides opportunities to study the topographic organization of inhibitory long-range projection nuclei, such as the superior olivary nucleus (SON). We analyzed the topographic organization of response features of neurons in the SON of chickens. Quantitative methods were developed to assess and communicate this organization. These analyses led to three main conclusions: 1) sound frequency is linearly arranged from dorsal (low frequencies) to ventral (high frequencies) in SON; 2) this tonotopic organization is less precise than the organization of the excitatory nuclei in the chicken auditory brainstem; and 3) neurons with different response patterns to pure tone stimuli are interspersed throughout the SON and show similar tonotopic organizations. This work provides a predictive model to determine the optimal stimulus frequency for a neuron from its spatial location in the SON.

Topography and morphology of the inhibitory projection from superior olivary nucleus to nucleus laminaris in chickens (Gallus gallus).

The avian nucleus laminaris (NL) is involved in computation of interaural time differences (ITDs) that encode the azimuthal position of a sound source. Neurons in NL are bipolar, with dorsal and ventral dendritic arbors receiving input from separate ears. NL neurons act as coincidence detectors that respond maximally when input from each ear arrives at the two dendritic arbors simultaneously. Computational and physiological studies demonstrated that the sensitivity of NL neurons to coincident inputs is modulated by an inhibitory feedback circuit via the superior olivary nucleus (SON). To understand the mechanism of this modulation, the topography of the projection from SON to NL was mapped, and the morphology of the axon terminals of SON neurons in NL was examined in chickens (Gallus gallus). In vivo injection of AlexaFluor 568 dextran amine into SON demonstrated a coarse topographic projection from SON to NL. Retrogradely labeled neurons in NL were located within the zone of anterogradely labeled terminals, suggesting a reciprocal projection between SON to NL. In vivo extracellular physiological recording further demonstrated that this topography is consistent with tonotopic maps in SON and NL. In addition, three-dimensional reconstruction of single SON axon branches within NL revealed that individual SON neurons innervate a large area of NL and terminate on both dorsal and ventral dendritic arbors of NL neurons. The organization of the projection from SON to NL supports its proposed functions of controlling the overall activity level of NL and enhancing the specificity of frequency mapping and ITD detection.