Larval Zebrafish Use Olfactory Detection of Sodium and Chloride to Avoid Salt Water

The preoptic area and dorsal habenula jointly support homeostatic navigation in larval zebrafish

Animals must maintain physiological processes within an optimal temperature range despite changes in their environment. Through behavioral assays, whole-brain functional imaging, and neural ablations, we show that larval zebrafish, an ectothermic vertebrate, achieves thermoregulation through homeostatic navigation-non-directional and directional movements toward the temperature closest to its physiological setpoint. A brain-wide circuit encompassing several brain regions enables this behavior. We identified the preoptic area of the hypothalamus (PoA) as a key brain structure in triggering non-directional reorientation when thermal conditions are worsening. This result shows an evolutionary conserved role of the PoA as principal thermoregulator of the brain also in ectotherms. We further show that the habenula (Hb)-interpeduncular nucleus (IPN) circuit retains a short-term memory of the sensory history to support the generation of coherent directed movements even in the absence of continuous sensory cues. We finally provide evidence that this circuit may not be exclusive for temperature but may convey a more abstract representation of relative valence of physiologically meaningful stimuli regardless of their specific identity to enable homeostatic navigation.

Environmental salinity modulates olfactory sensitivity in the euryhaline European seabass, Dicentrarchus labrax, acclimated to seawater and brackish water

The olfactory epithelium of fish is - of necessity - in intimate contact with the surrounding water. In euryhaline fish, movement from seawater to freshwater (and vice versa) exposes the epithelium to massive changes in salinity and ionic concentrations. How does the olfactory system function in the face of such changes? The current study compared olfactory sensitivity in seawater- (35‰) and brackish water-adapted seabass (5‰) using extracellular multi-unit recording from the olfactory nerve. Seawater-adapted bass had higher olfactory sensitivity to amino acid odorants when delivered in seawater than in freshwater. Conversely, brackish water-adapted bass had largely similar sensitivities to the same odorants when delivered in seawater or freshwater, although sensitivity was still slightly higher in seawater. The olfactory system of seawater-adapted bass was sensitive to decreases in external [Ca2+], whereas brackish water-adapted bass responded to increases in [Ca2+]; both seawater- and brackish water-adapted bass responded to increases in external [Na+] but the sensitivity was markedly higher in brackish water-adapted bass. In seawater-adapted bass, olfactory sensitivity to l-alanine depended on external Ca2+ ions, but not Na+; brackish water-adapted bass did respond to l-alanine in the absence of Ca2+, albeit with lower sensitivity, whereas sensitivity was unaffected by removal of Na+ ions. A possible adaptation of the olfactory epithelium was the higher number of mucous cells in brackish water-adapted bass. The olfactory system of seabass is able to adapt to low salinities, but this is not immediate; further studies are needed to identify the processes involved.

3D printed porous membrane integrated devices to study the chemoattractant induced behavioural response of aquatic organisms

Biological models with genetic similarities to humans are used for exploratory research to develop behavioral screening tools and understand sensory-motor interactions. Their small, often mm-sized appearance raises challenges in the straightforward quantification of their subtle behavioral responses and calls for new, customisable research tools. 3D printing provides an attractive approach for the manufacture of custom designs at low cost; however, challenges remain in the integration of functional materials like porous membranes. Nanoporous membranes have been integrated with resin exchange using purpose-designed resins by digital light projection 3D printing to yield functionally integrated devices using a simple, economical and semi-automated process. Here, the impact of the layer thickness and layer number on the porous properties - parameters unique for 3D printing - are investigated, showing decreases in mean pore diameter and porosity with increasing layer height and layer number. From the same resin formulation, materials with average pore size between 200 and 600 nm and porosity between 45% and 61% were printed. Membrane-integrated devices were used to study the chemoattractant induced behavioural response of zebrafish embryos and planarians, both demonstrating a predominant behavioral response towards the chemoattractant, spending >85% of experiment time in the attractant side of the observation chamber. The presented 3D printing method can be used for printing custom designed membrane-integrated devices using affordable 3D printers and enable fine-tuning of porous properties through adjustment of layer height and number. This accessible approach is expected to be adopted for applications including behavioural studies, early-stage pre-clinical drug discovery and (environmental) toxicology.

Environmental and molecular control of tissue-specific ionocyte differentiation in zebrafish

Organisms adjust their physiology to cope with environmental fluctuations and maintain fitness. These adaptations occur via genetic changes over multiple generations or through acclimation, a set of reversible phenotypic changes that confer resilience to the individual. Aquatic organisms are subject to dramatic seasonal fluctuations in water salinity, which can affect the function of lateral line mechanosensory hair cells. To maintain hair cell function when salinity decreases, ion-regulating cells, Neuromast-associated ionocytes (Nm ionocytes), increase in number and invade lateral line neuromasts. How environmental changes trigger this adaptive differentiation of Nm ionocytes and how these cells are specified is still unknown. Here, we identify Nm ionocyte progenitors as foxi3a/foxi3b-expressing skin cells and show that their differentiation is associated with sequential activation of different Notch pathway components, which control ionocyte survival. We demonstrate that new Nm ionocytes are rapidly specified by absolute salinity levels, independently of stress response pathways. We further show that Nm ionocyte differentiation is selectively triggered by depletion of specific ions, such as Ca2+ and Na+/Cl-, but not by low K+ levels, and is independent of media osmolarity. Finally, we demonstrate that hair cell activity plays a role in Nm ionocyte recruitment and that systemic factors are not necessary for Nm ionocyte induction. In summary, we have identified how environmental changes activate a signaling cascade that triggers basal skin cell progenitors to differentiate into Nm ionocytes and invade lateral line organs. This adaptive behavior is an example of physiological plasticity that may prove essential for survival in changing climates.

Olfactory detection of viruses shapes brain immunity and behavior in zebrafish

Olfactory sensory neurons (OSNs) are constantly exposed to pathogens, including viruses. However, serious brain infection via the olfactory route rarely occurs. When OSNs detect a virus, they coordinate local antiviral immune responses to stop virus progression to the brain. Despite effective immune control in the olfactory periphery, pathogen-triggered neuronal signals reach the CNS via the olfactory bulb (OB). We hypothesized that neuronal detection of a virus by OSNs initiates neuroimmune responses in the OB that prevent pathogen invasion. Using zebrafish ( Danio rerio ) as a model, we demonstrate viral-specific neuronal activation of OSNs projecting into the OB, indicating that OSNs are electrically activated by viruses. Further, behavioral changes are seen in both adult and larval zebrafish after viral exposure. By profiling the transcription of single cells in the OB after OSNs are exposed to virus, we found that both microglia and neurons enter a protective state. Microglia and macrophage populations in the OB respond within minutes of nasal viral delivery followed decreased expression of neuronal differentiation factors and enrichment of genes in the neuropeptide signaling pathway in neuronal clusters. Pituitary adenylate-cyclase-activating polypeptide ( pacap ), a known antimicrobial, was especially enriched in a neuronal cluster. We confirm that PACAP is antiviral in vitro and that PACAP expression increases in the OB 1 day post-viral treatment. Our work reveals how encounters with viruses in the olfactory periphery shape the vertebrate brain by inducing antimicrobial programs in neurons and by altering host behavior.

An optofluidic platform for interrogating chemosensory behavior and brainwide neural representation in larval zebrafish

Studying chemosensory processing desires precise chemical cue presentation, behavioral response monitoring, and large-scale neuronal activity recording. Here we present Fish-on-Chips, a set of optofluidic tools for highly-controlled chemical delivery while simultaneously imaging behavioral outputs and whole-brain neuronal activities at cellular resolution in larval zebrafish. These include a fluidics-based swimming arena and an integrated microfluidics-light sheet fluorescence microscopy (µfluidics-LSFM) system, both of which utilize laminar fluid flows to achieve spatiotemporally precise chemical cue presentation. To demonstrate the strengths of the platform, we used the navigation arena to reveal binasal input-dependent behavioral strategies that larval zebrafish adopt to evade cadaverine, a death-associated odor. The µfluidics-LSFM system enables sequential presentation of odor stimuli to individual or both nasal cavities separated by only ~100 µm. This allowed us to uncover brainwide neural representations of cadaverine sensing and binasal input summation in the vertebrate model. Fish-on-Chips is readily generalizable and will empower the investigation of neural coding in the chemical senses.

Neuromasts and Olfactory Organs of Zebrafish Larvae Represent Possible Sites of SARS-CoV-2 Pseudovirus Host Cell Entry

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the acute respiratory disease coronavirus disease 2019 (COVID-19), which has resulted in millions of deaths globally. Here, we explored the mechanism of host cell entry of a luciferase-ZsGreen spike (SARS-CoV-2)-pseudotyped lentivirus using zebrafish embryos/larvae as an in vivo model. Successful pseudovirus entry was demonstrated via the expression of the luciferase (luc) gene, which was validated by reverse transcription-PCR (RT-PCR). Treatment of larvae with chloroquine (a broad-spectrum viral inhibitor that blocks membrane fusion) or bafilomycin A1 (a specific inhibitor of vacuolar proton ATPases, which blocks endolysosomal trafficking) significantly reduced luc expression, indicating the possible involvement of the endolysosomal system in the viral entry mechanism. The pharmacological inhibition of two-pore channel (TPC) activity or use of the tpcn2^dhkz1a mutant zebrafish line also led to diminished luc expression. The localized expression of ACE2 and TPC2 in the anterior neuromasts and the forming olfactory organs was demonstrated, and the occurrence of endocytosis in both locations was confirmed. Together, our data indicate that zebrafish embryos/larvae are a viable and tractable model to explore the mechanism of SARS-CoV-2 host cell entry, that the peripheral sense organs are a likely site for viral host cell entry, and that TPC2 plays a key role in the translocation of the virus through the endolysosomal system. IMPORTANCE Despite the development of effective vaccines to combat the COVID-19 pandemic, which help prevent the most life-threatening symptoms, full protection cannot be guaranteed, especially with the emergence of new viral variants. Moreover, some resistance to vaccination remains in certain age groups and cultures. As such, there is an urgent need for the development of new strategies and therapies to help combat this deadly disease. Here, we provide compelling evidence that the peripheral sensory organs of zebrafish possess several key components required for SARS-CoV-2 host cell entry. The nearly transparent larvae provide a most amenable complementary platform to investigate the key steps of viral entry into host cells, as well as its spread through the tissues and organs. This will help in the identification of key viral entry steps for therapeutic intervention, provide an inexpensive model for screening novel antiviral compounds, and assist in the development of new and more effective vaccines.

A scalable assay for chemical preference of small freshwater fish

Sensing the chemical world is of primary importance for aquatic organisms, and small freshwater fish are increasingly used in toxicology, ethology, and neuroscience by virtue of their ease of manipulation, tissue imaging amenability, and genetic tractability. However, precise behavioral analyses are generally challenging to perform due to the lack of knowledge of what chemical the fish are exposed to at any given moment. Here we developed a behavioral assay and a specific infrared dye to probe the preference of young zebrafish for virtually any compound. We found that the innate aversion of zebrafish to citric acid is not mediated by modulation of the swim but rather by immediate avoidance reactions when the product is sensed and that the preference of juvenile zebrafish for ATP changes from repulsion to attraction during successive exposures. We propose an information-based behavioral model for which an exploration index emerges as a relevant behavioral descriptor, complementary to the standard preference index. Our setup features a high versatility in protocols and is automatic and scalable, which paves the way for high-throughput preference compound screening at different ages.

Hydric status influences salinity-dependent water selection in frogs from coastal wetlands

The environment is heterogeneous across spatial and temporal scales, and the behavioural responses required to adjust individuals' needs to resource availability across such variable environments should be under selective pressure. Coastal wetlands are characterized by a diversity of habitats ranging from fresh- to salt water; and individuals occurring in such complex habitats need to adjust their habitat use based on their osmotic status. In this study, we experimentally tested whether an amphibian species (Pelophylax sp.) occurring in coastal wetlands was able to discriminate and select between different salinity concentrations (0, 4, 8 and 12 g.l^-1) and whether hydric status (hydrated versus dehydrated) influenced salinity-dependent water selection. We found that frogs selected water based on salinity differentially between hydrated and dehydrated individuals, with the later favoring lower salinities likely to improve their osmotic status. Interestingly, we highlighted the ability of frogs to select lower salinity before having access to water, suggesting that frogs can assess water salinity without actual contact. In coastal wetlands where salinity of water bodies can dynamically vary through space and time, such behavioural osmoregulation process is potentially a key factor affecting individual movements, habitat choice and thus species distribution. Our study further highlights the importance of salinity-dependent habitat heterogeneity and especially the presence of freshwater environments as structuring factors for the amphibian community.

Social isolation modulates appetite and avoidance behavior via a common oxytocinergic circuit in larval zebrafish

Animal brains have evolved to encode social stimuli and transform these representations into advantageous behavioral responses. The commonalities and differences of these representations across species are not well-understood. Here, we show that social isolation activates an oxytocinergic (OXT), nociceptive circuit in the larval zebrafish hypothalamus and that chemical cues released from conspecific animals are potent modulators of this circuit's activity. We delineate an olfactory to subpallial pathway that transmits chemical social cues to OXT circuitry, where they are transformed into diverse outputs simultaneously regulating avoidance and feeding behaviors. Our data allow us to propose a model through which social stimuli are integrated within a fundamental neural circuit to mediate diverse adaptive behaviours.

A Model of Discovery: The Role of Imaging Established and Emerging Non-mammalian Models in Neuroscience

Rodents have been the dominant animal models in neurobiology and neurological disease research over the past 60 years. The prevalent use of rats and mice in neuroscience research has been driven by several key attributes including their organ physiology being more similar to humans, the availability of a broad variety of behavioral tests and genetic tools, and widely accessible reagents. However, despite the many advances in understanding neurobiology that have been achieved using rodent models, there remain key limitations in the questions that can be addressed in these and other mammalian models. In particular, in vivo imaging in mammals at the cell-resolution level remains technically difficult and demands large investments in time and cost. The simpler nervous systems of many non-mammalian models allow for precise mapping of circuits and even the whole brain with impressive subcellular resolution. The types of non-mammalian neuroscience models available spans vertebrates and non-vertebrates, so that an appropriate model for most cell biological questions in neurodegenerative disease likely exists. A push to diversify the models used in neuroscience research could help address current gaps in knowledge, complement existing rodent-based bodies of work, and bring new insight into our understanding of human disease. Moreover, there are inherent aspects of many non-mammalian models such as lifespan and tissue transparency that can make them specifically advantageous for neuroscience studies. Crispr/Cas9 gene editing and decreased cost of genome sequencing combined with advances in optical microscopy enhances the utility of new animal models to address specific questions. This review seeks to synthesize current knowledge of established and emerging non-mammalian model organisms with advances in cellular-resolution in vivo imaging techniques to suggest new approaches to understand neurodegeneration and neurobiological processes. We will summarize current tools and in vivo imaging approaches at the single cell scale that could help lead to increased consideration of non-mammalian models in neuroscience research.

The Olfactory Tract: Basis for Future Evolution in Response to Rapidly Changing Ecological Niches

Within the forebrain the olfactory sensory system is unique from other sensory systems both in the projections of the olfactory tract and the ongoing neurogenic potential, characteristics conserved across vertebrates. Olfaction plays a crucial role in behaviors such as mate choice, food selection, homing, escape from predators, among others. The olfactory forebrain is intimately associated with the limbic system, the region of the brain involved in learning, memory, and emotions through interactions with the endocrine system and the autonomic nervous system. Previously thought to lack a limbic system, we now know that teleost fishes process emotions, have exceptional memories, and readily learn, behaviors that are often associated with olfactory cues. The association of neuromodulatory hormones, and more recently, the immune system, with odor cues underlies behaviors essential for maintenance and adaptation within natural ecological niches. Increasingly anthropogenic perturbations affecting ecosystems are impacting teleost fishes worldwide. Here we examine the role of the olfactory tract as the neural basis for the integration of environmental cues and resulting behaviors necessary for the regulation of biotic interactions that allow for future adaptation as the climate spins out of control.

Hypothalamic Galanin-producing neurons regulate stress in zebrafish through a peptidergic, self-inhibitory loop

Animals possess neuronal circuits inducing stress to avoid or cope with threats present in their surroundings, for instance, by promoting behaviors, such as avoidance and escape. However, mechanisms must exist to tightly control responses to stressors, since overactivation of stress circuits is deleterious for the wellbeing of an organism. The underlying neuronal dynamics responsible for controlling behavioral responses to stress have remained unclear. Here, we describe a neuronal circuit in the hypothalamus of zebrafish larvae that inhibits stress-related behaviors and prevents excessive activation of the neuroendocrine pathway hypothalamic-pituitary-interrenal axis. Central components of this circuit are neurons secreting the neuropeptide Galanin, as ablation of these neurons led to abnormally high levels of stress. Surprisingly, we found that Galanin has a self-inhibitory action on Galanin-producing neurons. Our results suggest that hypothalamic Galanin-producing neurons play an important role in fine-tuning stress responses by preventing potentially harmful overactivation of stress-regulating circuits.

Evolution of sensory systems

Sensory systems evolve and enable organisms to perceive their sensory Umwelt, the unique set of cues relevant for their survival. The multiple components that comprise sensory systems - the receptors, cells, organs, and dedicated high-order circuits - can vary greatly across species. Sensory receptor gene families can expand and contract across lineages, resulting in enormous sensory diversity. Comparative studies of sensory receptor function have uncovered the molecular basis of receptor properties and identified novel sensory receptor classes and noncanonical sensory strategies. Phylogenetically informed comparisons of sensory systems across multiple species can pinpoint when sensory changes evolve and highlight the role of contingency in sensory system evolution.

Effects of Sodium Chloride on Algae and Crustaceans—The Neighbouring Links of the Water Trophic Chain

Salinity limits the habitable living environment for aquatic organisms. Algae and crustaceans are widely used as bioindicators in freshwater environmental risk assessments. This study aimed to use biotests (Algaltoxkit and Daphtoxkit) to determine the effect of sodium chloride (NaCl) on algae Pseudokirchneriella subcapitata and crustaceans Daphnia magna. Standard biotests were extended to include NaCl effects on algal chlorophyll fluorescence and crustaceans swimming and heart rate. It was found that after 7 days, a 0.24 M of NaCl reduced the growth rate of the algae by 50% (EC50). A NaCl of 0.27 M inhibited the minimum (Fo), maximum (Fm) and variable (Fv) fluorescence by 50%, on average. The crustaceans also responded to NaCl. Those exposed to 0.19 M NaCl during 15 min swam slower by 50% and a 0.27 M immobilised three organisms (EC50). The crustacean immobilisation was less modified by NaCl than swimming. To determine the lethal effect in non-swimming organisms, the heart rate was examined. At 0.35 M of NaCl, all organisms were dead after 30 min, as their hearts did not beat. These studies suggest that physiological and behavioural features are sensitive indicators of the toxic effects of NaCl in algae and crustaceans, before morphological changes are observed.

Sub-Lethal Peak Exposure to Insecticides Triggers Olfaction-Mediated Avoidance in Zebrafish Larvae

In agricultural areas, insecticides inevitably reach water bodies via leaching or run-off. While designed to be neurotoxic to insects, insecticides have adverse effects on a multitude of organisms due to the high conservation of the nervous system among phyla. To estimate the ecological effects of insecticides, it is important to investigate their impact on non-target organisms such as fish. Using zebrafish as the model, we investigated how different classes of insecticides influence fish behavior and uncovered neuronal underpinnings of the associated behavioral changes, providing an unprecedented insight into the perception of these chemicals by fish. We observed that zebrafish larvae avoid diazinon and imidacloprid while showing no response to other insecticides with the same mode of action. Moreover, ablation of olfaction abolished the aversive responses, indicating that fish smelled the insecticides. Assessment of neuronal activity in 289 brain regions showed that hypothalamic areas involved in stress response were among the regions with the largest changes, indicating that the observed behavioral response resembles reactions to stimuli that threaten homeostasis, such as changes in water chemistry. Our results contribute to the understanding of the environmental impact of insecticide exposure and can help refine acute toxicity assessment.

Sensory Neuroscience: Smelling Salts Lead Fish to Safety

Animals use their sensory systems to detect danger in their environments. New research shows that larval zebrafish navigate away from dangerous salt water by using their olfactory systems to detect the presence of both sodium and chloride ions.