The Olfactory System of Zebrafish as a Model for the Study of Neurotoxicity and Injury: Implications for Neuroplasticity and Disease

Structure, transduction pathway, behavior and toxicity of fish olfactory in aquatic environments

The olfactory system in teleost fish plays a vital role as chemosensory organ that directly interacts with the aquatic environment, exhibiting high sensitivity to chemical alteration in aquatic environments. However, despite its importance, there has been a lack of systematic reviews in the past decade on fish olfactory structure, transduction mechanisms, and the impact of environmental pollutants on olfactory toxicity. This study analyzed 272 relevant studies, focusing on the role of the olfactory system and the disruption of olfactory function by contaminants. Fish processes odors through olfactory receptor neurons, olfactory nerves, mitral/ruffed cells, glomeruli, and neurotransmitters, mediated by membrane potentials resulting from ion channels in the olfactory epithelium and olfactory bulb, which are then relayed to higher brain regions via the medial olfactory tracts and lateral olfactory tracts for further integration and modulation. This process minimizes the overlap between complex odor sets, ensuring distinct representation of each odor and eliciting appropriate olfactory-mediated behaviors, such as feeding, migration, alarm responses, and reproduction. Current research identifies four main types of contaminants affecting the fish olfactory system: heavy metals (51.60 %), organic contaminants (33.79 %), acidification (12.33 %), and salinity (5.94 %). The main mechanisms of impact are: morphological changes (21.19 %), alterations in olfactory receptors (29.24 %), damage to olfactory receptor neurons and neurotransmitters disruption (26.69 %), plasticity (2.97 %), and defense mechanisms (19.92 %). We also identify uncertainties and proposes future research directions on the effects of contaminants on fish olfactory. Overall, this review provides valuable insights into the toxicity of contaminants on fish olfactory.

Hexafluoropropylene oxide trimer acid induces olfactory toxicity to crucian carp by disrupting olfactory function and olfactory-mediated behavior

Hexafluoropropylene oxide trimer acid (HFPO-TA), an emerging substitute for perfluorooctanoic acid, has been widely detected in aquatic environment and received extensive attentions of research due to its potential toxicity. However, the olfactory toxicity of HFPO-TA remains unknown yet. In the study, the effects of HFPO-TA on the olfactory epithelium (OE) and olfactory bulb (OB) were investigated in crucian carp as well as the behavioral response to olfactory stimulants. The results show that exposure to HFPO-TA inhibited the expression of genes encoding olfactory G protein-coupled receptors (GPCRs), reduced the activities of ion transporter enzymes Na+/K+-ATPase and Ca2+-ATPase, as well as the activities of superoxide dismutase (SOD) and catalase (CAT), increasing the level of malondialdehyde (MDA), inducing the activities of caspase-1 and caspase-3, and causing tissue damage in the OE of crucian carp. Similarly, exposure to HFPO-TA also induced oxidative stress and apoptosis in the OB. Moreover, the levels of neurotransmitters including 5-hydroxytryptamine (5-HT), glutamate (Glu), acetylcholine (Ach), and the appetite regulator neuropeptide Y (NPY) were depressed in the OB. The behavioral test indicates that HFPO-TA altered the preference of crucian carp for food stimulants. Therefore, HFPO-TA poses olfactory toxicity to crucian carp by disrupting olfactory function and olfactory-mediated behavior.

Purslane leaf powder dietary supplementation rescues cadmium-induced disruption of behavior, antioxidant status, and expression of tight junction genes, in the brain of Nile tilapia (Oreochromis niloticus)

This study examined the effects of supplementing the Nile tilapia (Oreochromis niloticus) diet with purslane (Portulaca oleracea L.) leaf powder (PLNP, 10 g/kg) on neurobehavioral performance, brain oxidative status, tight junction mRNA expression, and brain histology in fish exposed to waterborne cadmium (Cd, 50 μg /L) for 60 days. Adding PLNP to the diet ameliorated the Cd-induced decline in ingestive behavior and swimming behavior, and reversed the Cd-induced increase in aggressive behavior. The significant decrease in the non-enzymatic (reduced glutathione) and enzymatic (catalase and superoxide dismutase) brain antioxidants detected in Cd-exposed fish was eliminated by dietary PLNP. PLNP supplementation also led to a decrease in brain malondialdehyde content, which was elevated by Cd exposure. In addition, dietary PLNP increased brain acetylcholinesterase content, upregulated mRNA expression of tight junction (zo-2, claudin-4, and zo-1) and oxidative stress genes (sod-2, gpx, and nrf-2), and downregulated apoptotic genes (p53, caspase-9, caspase-8, and caspase-3) in the brain, relative to the alterations in these parameters caused by Cd exposure. Furthermore, the Cd-induced histological changes in the Nile tilapia brain were ameliorated by PLNP dietary supplementation. In light of these findings, PLNP may be a useful dietary supplement for reducing the harmful effects of Cd on the brain and behavior of Nile tilapia.

Chronic fluoride induces neurotoxicity in zebrafish through the gut-brain axis

Fluoride (F) is a common pollutant in aquatic environment. Chronic exposure to fluoride can cause toxicity to nervous system and behavior of fish. However, the underlying mechanism is still unclear. This study is designed to explore the effects and potential mechanisms of chronic fluoride exposure on zebrafish behavior. Zebrafish were exposed to CK, LF, MF, and HF for 90 days. The results showed that chronic fluoride exposure caused damage to the brain tissues of adult zebrafish and significantly reduced the total distance of free swimming per min. Fluoride significantly reduced dopamine content and dopamine pathway related gene expression in brain. In addition, fluoride disrupted the intestinal barrier function, reduced the expression of tight junction proteins and mucin related genes in intestinal tissues, increased intestinal permeability. Males in the HF group were more sensitive than the corresponding females. Fluoride significantly decreased the dopamine content in the intestine, disturbed the gene expression of dopamine pathway related genes. Fluoride significantly increased the relative abundance of g_uncultured_bacterium_c_Gammaprotebacteria in three F groups, while significantly reduced the relative abundance of g_lactobacilli and g_Lachnospiraceae NK4A136_group. At the same time, metabolic pathways including amino acid metabolism, nucleotide metabolism, and carbohydrate metabolism were also affected. These indicate that fluoride can result in neurotoxicity and behavioral changes by disrupting intestinal permeability and causing intestinal damage caused by imbalance of gut microbiota nucleotide metabolism, and the abnormal expression of dopamine related genes in the gut in zebrafish. Male zebrafish are more sensitive than female zebrafish.

Nanoplastics transport in zebrafish brain: Molecular and phenotypic behavioral impacts

Nanoplastics (NPs) can accumulate in animal brain, but their kinetics and potential adverse impacts on the brain are not yet understood. This study compared the impacts of 200 nm (L-NPs) and 50 nm (S-NPs) NPs on the zebrafish (Danio rerio) brain at environmentally relevant concentration (500 μg L-1), exploring how these nanoparticles were accumulated and removed, as well as their subsequent adverse effects at various biological levels. The bioconcentration factor and biological half-time in brain of L-NPs were 11.8 L kg-1 and 1.54 d, while those for S-NPs were 18.9 L kg-1 and 2.48 d, respectively. S-NPs displayed a higher tendency to accumulate and a slower elimination with a higher potential for long-term brain accumulation. Proteomic analysis of fish brain revealed that S-NPs exposure exhibited a higher level of differential expression of vesicle transport proteins. Transcriptomic analysis suggested that both NPs exposure significantly impacted pathways linked to inflammation and locomotion, while S-NPs additionally leading to increased enrichment of pathways associated with photoreceptor and phototransduction disorders. Both L-NPs and S-NPs caused damages in brain regions and even led to heightened oxidative stress across the whole brain regions. At the behavioral level, NPs induced abnormal patterns and feeding responses, and S-NPs further heightened the zebrafish sensitivity to light stimulation. Our study provides strong evidence of the risk of neurological diseases stemming from the prolonged accumulation of NPs in the brain, especially for the smaller-sized NPs.

Zebrafish as a model for olfactory research: A systematic review from molecular mechanism to technology application

Zebrafish with unique biological traits can serve as an ideal model for studying olfactory mechanisms. This review analyzes their olfactory system, focusing on the regulation of receptor gene expression, mechanisms of odor recognition, and research methodologies including behavioral assays, molecular docking, and biotechnological approaches. Current limitations include predominantly qualitative data, insufficient cross-species comparisons, and unclear mechanisms of environmental modulation. Nevertheless, zebrafish models show significant potential in deciphering human olfaction and applications in neuroscience, biotechnology, healthcare, food safety, and environmental monitoring. Future research should establish cross-species olfactory databases, standardize behavioral assessments, and resolve technical bottlenecks to advance applications in precision medicine, food quality control, and pollutant detection.

An appetitive olfactory learning paradigm for zebrafish in their home tanks

Olfaction is a subject of increasing interest in fish biology, but there are few learning paradigms available to investigate olfactory behaviour. In the present study, groups of zebrafish were trained in their home tanks retrofitted for automated conditioning with a microprocessor-controlled syringe pump and feeder to deliver odourant and food, respectively, and responses recorded remotely to minimize researcher interference. Fish were presented with phenylethyl alcohol (PEA), and a food reward was given 15 seconds later for experimental groups or at a variably delayed interval for controls. This schedule continued for 48 trials over four days. Both groups showed initial attraction to PEA, but by the end of Day 2 all fish exhibited reduced interest in the odourant. Further experiments with different fish indicated that this attraction was dependent upon high food motivation, as starved animals reacted more than non-starved animals to the novel stimulus. On Day 3, experimental fish began once again to show attraction associated with the odourant, thus indicating that they formed an association between the PEA and food reward, which increased by the end of Day 4. Conversely, control fish showed little or no response to the odourant on Days 3 and 4. When exposed to a water-only trial, trained fish largely ignored the cue, indicating that odour and not turbulence was the main stimulus for learning. This experiment demonstrated that an appetitive learning paradigm, using olfactory cues presented in home tanks, is both feasible and cost-effective for testing olfactory behaviour in zebrafish.

Olfactory Dysfunction in a Novel Model of Prodromal Parkinson's Disease in Adult Zebrafish

Olfactory dysfunction is a clinical marker of prodromal Parkinson's disease (PD), yet the underlying mechanisms remain unclear. To explore this relationship, we developed a zebrafish model that recapitulates the olfactory impairment observed in prodromal PD without affecting motor function. We used zebrafish due to their olfactory system's similarity to mammals and their unique nervous system regenerative capacity. By injecting 6-hydroxydopamine (6-OHDA) into the dorsal telencephalic ventricle, we observed a significant loss of dopaminergic (DA) periglomerular neurons in the olfactory bulb (OB) and retrograde degeneration of olfactory sensory neurons (OSNs) in the olfactory epithelium (OE). These alterations impaired olfactory responses to cadaverine, an aversive odorant, while responses to alanine remained intact. 6-OHDA also triggered robust neuroinflammatory responses. By 7 days post-injection, dopaminergic synapses in the OB were remodeled, OSNs in the OE appeared recovered, and neuroinflammation subsided, leading to full recovery of olfactory responses to cadaverine. These findings highlight the remarkable neuroplasticity of zebrafish and suggest that this model of olfactory dysfunction associated with dopaminergic loss could provide valuable insights into some features of early PD pathology. Understanding the interplay between dopaminergic loss and olfactory dysfunction in a highly regenerative vertebrate may inform therapeutic strategies for individuals suffering from olfactory loss.

Olfactory GnRH3 crypt sensory neurons transduce sex pheromone signals to induce male courtship behavior in zebrafish

Olfactory activation of neuroendocrine pathways plays vital roles in many organisms for reproduction and survival. The importance of gonadotropin-releasing hormone (GnRH) neurons for reproduction is well-established but little is known about whether they can directly sense and transmit sex pheromone signals. We have uncovered the migration path and distribution pattern of a new GnRH neuronal population that fulfills this role. GnRH3 neurons arise from the region located beneath olfactory placode, undergo bidirectional migration along the olfactory nerve, and cell bodies lie within the olfactory epithelium, olfactory bulb and hypothalamus. These olfactory epithelial GnRH3 neurons express ora4, the olfactory receptor that detects pheromones. GnRH3-OB neurons with olfactory epithelial GnRH3 neurons ablation failed to respond to the waterborne post-ovulatory sex pheromone prostaglandin F2α (PGF2α). GnRH3 neurons in gnrh3-/- mutants have a reduced basal firing rate leading to abnormal responses to PGF2α. Male gnrh3-/- zebrafish exhibit deficiencies in courtship behavior and a decreased capacity to compete and spawn with females. These findings indicate that GnRH3-OE neurons function as crypt sensory neurons transducing sex pheromone-encoded information critical to reproductive success.

Intranasal delivery of drugs to the central nervous system of adult zebrafish

The small teleost zebrafish (Danio rerio) has become a critically important laboratory animal in biomedicine. One of their key practical advantages, the convenient method of small-molecule administration via water immersion, has certain problems with dosing precision and drug delivery. Here, we present a simple protocol for the intranasal delivery of neuroactive drugs in adult zebrafish using arecoline and nicotine, two well-studied reference neuroactive drugs chosen for the proof of concept. Adult fish received 1 μL water solution of arecoline (1 and 10 mg/mL) or nicotine tartrate (5 and 10 mg/mL) or the same volume of drug-free water (control) into both nostrils, followed by behavioral testing in the novel tank test 5 min later. Mass spectrometry analyses confirmed that both drugs rapidly reached the zebrafish brain following intranasal administration. Intranasally administered arecoline (10 mg/mL) and nicotine (5 and 10 mg/mL) demonstrated overt behavioral profiles, evoking characteristic anxiolytic-like effects in zebrafish similar to those observed here for a standard 20-min water immersion method (10 mg/L arecoline or 30 mg/L nicotine). Overall, we showed that neuroactive drugs can be delivered to adult zebrafish intranasally to exert central effects, which may (at least for some drugs) occur faster and can need smaller drug quantities than for the water immersion delivery.

Olfactory Impairment and Recovery in Zebrafish (Danio rerio) Following Cadmium Exposure

Anthropic activities have significantly elevated cadmium levels, making it a significant stressor in aquatic ecosystems. Present in high concentrations across water bodies, cadmium is known to bioaccumulate and biomagnify throughout the food chain. While the toxic effects of cadmium on the organs and tissues of aquatic species are well-documented, little is known about its impact on sensory systems crucial for survival. Consequently, this study investigated the impact of short-term exposure (96 h) to 25 µM cadmium chloride on the olfactory system of adult zebrafish. The research aimed to assess structural and functional changes in the zebrafish's olfactory lamellae, providing a deeper understanding of how cadmium affects the sense of smell in this aquatic species. After exposure, cyto-anatomical alterations in the lamellae were analysed using light microscopy and immunocytochemistry. They revealed severe lamellar edema, epithelial thickening, and an increased number of apoptotic and crypt cells. Rodlet and goblet cells also increased by 3.5- and 2.5-fold, respectively, compared to control lamellae, and collagen density in the lamina propria increased 1.7-fold. Cadmium upregulated metallothioneins and increased the number of PCNA-positive cells. The olfactory function was assessed through a behavioural odour recognition test, followed by a recovery phase in which zebrafish exposed to cadmium were placed in clean water for six days. The exposed fish performed poorly, failing to reach food in five consecutive trials. However, lamellar damage was reduced after the recovery period, and their performance improved, becoming comparable to the control group. These results suggest that cadmium disrupts the sense of smell, and that recovery is possible after short-term exposure. This evidence sheds light on aspects of animal survival that are often overlooked when assessing environmental pollution.

Long-term impact of embryonic exposure to ethinylestradiol and clotrimazole on behavior and neuroplasticity in zebrafish (Danio rerio)

Estrogen receptors (ER) are widely expressed in the brain of many species and experimental results highlighted the role of estradiol in neuronal plasticity and behavior. Consequently, the brain is therefore a prime target for endocrine disrupting chemicals (EDCs) interacting with estrogen signaling. Very little is known about the late effects of early disruption of estrogen signaling by EDCs. We focused on: ethinylestradiol (EE2; ER agonist) and clotrimazole (inhibitor of key steroidogenesis enzymes, including aromatases). Zebrafish eleutheroembryos were exposed (0-5 days) and then raised normally until adulthood. Several behavioral tests were performed in adults, then cell proliferation and dopaminergic neurons were quantified in several brain regions using immunostaining. Overall, a developmental exposure to EDCs stimulates cell proliferation in the dorsal telencephalon. At environmentally-relevant concentrations, male fish exposed to EE2 exhibited increased activity levels and decreased social behavior, posing a potential risk to population balance and health.

Development of an rpS6-Based Ex Vivo Assay for the Analysis of Neuronal Activity in Mouse and Human Olfactory Systems

Olfactory sensitivity to odorant molecules is a complex biological function influenced by both endogenous factors, such as genetic background and physiological state, and exogenous factors, such as environmental conditions. In animals, this vital ability is mediated by olfactory sensory neurons (OSNs), which are distributed across several specialized olfactory subsystems depending on the species. Using the phosphorylation of the ribosomal protein S6 (rpS6) in OSNs following sensory stimulation, we developed an ex vivo assay allowing the simultaneous conditioning and odorant stimulation of different mouse olfactory subsystems, including the main olfactory epithelium, the vomeronasal organ, and the Grueneberg ganglion. This approach enabled us to observe odorant-induced neuronal activity within the different olfactory subsystems and to demonstrate the impact of environmental conditioning, such as temperature variations, on olfactory sensitivity, specifically in the Grueneberg ganglion. We further applied our rpS6-based assay to the human olfactory system and demonstrated its feasibility. Our findings show that analyzing rpS6 signal intensity is a robust and highly reproducible indicator of neuronal activity across various olfactory systems, while avoiding stress and some experimental limitations associated with in vivo exposure. The potential extension of this assay to other conditioning paradigms and olfactory systems, as well as its application to other animal species, including human olfactory diagnostics, is also discussed.

Environmental exposure to cadmium induces olfactory neurotoxicity in fire ants and the molecular basis

Cadmium (Cd) exhibits widely olfactory toxicity to animals. We previously reported that Cd exposure induces the transcriptional dysregulation of olfactory marker proteins (OMPs) of the red imported fire ant Solenopsis invicta. However, it is still unclear how environmental Cd exposure-induced deregulation of OMPs affects the olfactory signal transduction and olfaction-driven social behavior of S. invicta. Here, we showed that S. invicta displayed dull sensory perception on bait in Cd-contaminated areas and dietary Cd ingestion by S. invicta reduced the bait search efficiency. We hypothesize that deregulation of OMPs by Cd exposure blocks the olfactory signal transduction in fire ants. Our results indicated the odor binding protein 14 (SiOBP14) was consistently inhibited in antennal sensilla of fire ants across Cd exposure at 0.5, 5 and 50 mg/kg. Function analysis in vitro and in vivo demonstrated that SiOBP14 is essential in perception of S. invicta to bait odorants. Cd-exposed fire ants showed weak odorant receptor neurons (ORNs) chemosensory signaling and electroantennogram (EAG) response. Moreover, Cd exposure repeals the preference of S. invicta to the active bait odorants, including 2-methyltetrahydrofuran-3-one, 2-methyl-3-furanthiol and 4,5-dimethylthiazole, and even triggers a behavioral transition from preference to repellence. These results indicate that Cd exposure inhibits the specific OMP expression and disrupts olfactory signal transduction, thereby inducing dull sensory perception of S. invicta to bait odorants. The findings provide new implications for monitoring and control of agricultural insect pests in heavy metal polluted areas.

The Promising Role of Synthetic Flavors in Advancing Fish Feeding Strategies: A Focus on Adult Female Zebrafish (Danio rerio) Growth, Welfare, Appetite, and Reproductive Performances

The present study aimed to test over a six-month period different synthetic flavors in zebrafish (Danio rerio) as an experimental model. Specifically, two attractive and one repulsive synthetic flavors were added (1% w/w) to a specific zebrafish diet, which was administered to the fish during the whole life cycle (from larvae to adults), to evaluate their physiological responses, emphasizing fish welfare, feed intake, growth, reward mechanisms, and reproductive performances. Fish welfare was not affected by all tested flavors, while both attractive flavors promoted fish feed ingestion and growth. The results were supported by both molecular and immunohistochemical analyses on appetite-regulating neurohormonal signals, along with the influence of the feed hedonic properties induced by the brain reward sensation, as demonstrated by the dopamine receptor gene expression. Finally, the present study demonstrated that a higher feed intake also had positive implications on fish reproductive performances, suggesting a promising role of synthetic flavors for the aquaculture industry. In conclusion, the results highlighted the potential of synthetic flavors to improve fish feeding strategies by providing a consistent and effective alternative to traditional stimulants, thereby reducing dependence on natural sources.

In-depth histological, lectin-histochemical, immunohistochemical and ultrastructural description of the olfactory rosettes and olfactory bulbs of turbot (Scophthalmus maximus)

Chemical communication through olfaction is crucial for fish behaviours, mediating in socio-sexual behaviours as reproduction. Turbot, a flatfish with significant aquaculture production, possesses a well-developed olfactory system from early developmental stages. After metamorphosis, flatfish acquire their characteristic bilateral asymmetry with an ocular side facing the open water column, housing the dorsal olfactory rosette, and a blind side in contact with the sea bottom where the ventral rosette is located. This study aimed to address the existing gap in specific histological, ultrastructural, lectin-histochemical and immunohistochemical studies of the turbot olfactory rosettes and olfactory bulbs. We examined microdissected olfactory organs of adult turbots and premetamorphic larvae by using routine histological staining techniques, and a wide array of lectins and primary antibodies against G-proteins and calcium-binding proteins. We observed no discernible structural variations in the olfactory epithelium between rosettes, except for the dorsal rosette being larger in size compared to the ventral rosette. Additionally, the use of transmission electron microscopy significantly improved the characterization of the adult olfactory epithelium, exhibiting high cell density, small cell size, and a wide diversity of cell types. Moreover, specific immunopositivity in sensory and non-sensory cells provided us of essential information regarding their olfactory roles. The results obtained significantly enriched the scarce morphological and neurochemical information available on the turbot olfactory system, revealing a highly complex olfactory epithelium with distinct features compared to other teleost species, especially with regard to olfactory cell distribution and immunolabelling patterns.

Development of a noninvasive olfactory stimulation fMRI system in marmosets

Olfactory dysfunction is associated with aging and the earliest stages of neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases; it is thought to be an early biomarker of cognitive decline. In marmosets, a small non-human primate model used in brain research, olfactory pathway activity during olfactory stimulation has not been well studied because of the difficulty in clearly switching olfactory stimuli inside a narrow MRI. Here, we developed an olfactory-stimulated fMRI system using a small-aperture MRI machine. The olfactory presentation system consisted of two tubes, one for supply and one for suction of olfactory stimulants and a balloon valve. A balloon valve installed in the air supply tube controlled the presentation of the olfactory stimulant, which enabled sharp olfactory stimulation within MRI, such as 30 s of stimulation repeated five times at five-minute intervals. The olfactory stimulation system was validated in vivo and in a simulated system. fMRI analysis showed a rapid increase in signal values within 30 s of olfactory stimulation in eight regions related to the sense of smell. As these regions include those associated with Alzheimer's and Parkinson's diseases, olfactory stimulation fMRI may be useful in clarifying the relationship between olfactory dysfunction and dementia in non-human primates.

The olfactory network of larval Xenopus laevis regenerates accurately after olfactory nerve transection

Across vertebrate species, the olfactory epithelium (OE) exhibits the uncommon feature of lifelong neuronal turnover. Epithelial stem cells give rise to new neurons that can adequately replace dying olfactory receptor neurons (ORNs) during developmental and adult phases and after lesions. To relay olfactory information from the environment to the brain, the axons of the renewed ORNs must reconnect with the olfactory bulb (OB). In Xenopus laevis larvae, we have previously shown that this process occurs between 3 and 7 weeks after olfactory nerve (ON) transection. In the present study, we show that after 7 weeks of recovery from ON transection, two functionally and spatially distinct glomerular clusters are reformed in the OB, akin to those found in non-transected larvae. We also show that the same odourant response tuning profiles observed in the OB of non-transected larvae are again present after 7 weeks of recovery. Next, we show that characteristic odour-guided behaviour disappears after ON transection but recovers after 7-9 weeks of recovery. Together, our findings demonstrate that the olfactory system of larval X. laevis regenerates with high accuracy after ON transection, leading to the recovery of odour-guided behaviour.

Mitral Cell Dendritic Morphology in the Adult Zebrafish Olfactory Bulb following Growth, Injury and Recovery

The role of afferent target interactions in dendritic plasticity within the adult brain remains poorly understood. There is a paucity of data regarding the effects of deafferentation and subsequent dendritic recovery in adult brain structures. Moreover, although adult zebrafish demonstrate ongoing growth, investigations into the impact of growth on mitral cell (MC) dendritic arbor structure and complexity are lacking. Leveraging the regenerative capabilities of the zebrafish olfactory system, we conducted a comprehensive study to address these gaps. Employing an eight-week reversible deafferentation injury model followed by retrograde labeling, we observed substantial morphological alterations in MC dendrites. Our hypothesis posited that cessation of injury would facilitate recovery of MC dendritic arbor structure and complexity, potentially influenced by growth dynamics. Statistical analyses revealed significant changes in MC dendritic morphology following growth and recovery periods, indicating that MC total dendritic branch length retained significance after 8 weeks of deafferentation injury when normalized to individual fish physical characteristics. This suggests that regeneration of branch length could potentially function relatively independently of growth-related changes. These findings underscore the remarkable plasticity of adult dendritic arbor structures in a sophisticated model organism and highlight the efficacy of zebrafish as a vital implement for studying neuroregenerative processes.

Molecular Characterization of Neurogranin (NRGN) Gene from Red‑Bellied Pacu (Piaractus brachypomus)

Neurogranin (NRGN) is a small brain protein expressed in various telencephalic areas and plays an essential role in synaptic plasticity by regulating the availability of calmodulin (CaM). The study aims to characterize the neurogranin gene in Colombian native fish, red-bellied pacu, Piaractus brachypomus, its basal tissue expression and differential expression in brain injury and sublethal toxicity by organophosphates. NRGN gene contains an open reading frame of 183 nucleotides encoding for 60 amino acids. Bioinformatics analysis showed an IQ motif necessary in the interaction with CaM. NRGN mRNA was detected in tissues with higher expression in brain, gills, and head kidney. In brain regions, NRGN showed high expression in the telencephalon (TE) and olfactory bulb (OB). In the sublethal toxicity experiment, NRGN mRNA was upregulated in individuals under organophosphate exposure in the OB and optic chiasm (OC). In brain injury experiment, NRGN showed upregulation at 14 days in OC and at 24 h and 7 days in TE. These findings demonstrate the differential expression of NRGN under different experimental conditions which make it a candidate for a biomarker in the brain of P. brachypomus.

Manganese Overexposure Alters Neurogranin Expression and Causes Behavioral Deficits in Larval Zebrafish

Manganese (Mn), a cofactor for various enzyme classes, is an essential trace metal for all organisms. However, overexposure to Mn causes neurotoxicity. Here, we evaluated the effects of exposure to Mn chloride (MnCl2) on viability, morphology, synapse function (based on neurogranin expression) and behavior of zebrafish larvae. MnCl2 exposure from 2.5 h post fertilization led to reduced survival (60%) at 5 days post fertilization. Phenotypical changes affected body length, eye and olfactory organ size, and visual background adaptation. This was accompanied by a decrease in both the fluorescence intensity of neurogranin immunostaining and expression levels of the neurogranin-encoding genes nrgna and nrgnb, suggesting the presence of synaptic alterations. Furthermore, overexposure to MnCl2 resulted in larvae exhibiting postural defects, reduction in motor activity and impaired preference for light environments. Following the removal of MnCl2 from the fish water, zebrafish larvae recovered their pigmentation pattern and normalized their locomotor behavior, indicating that some aspects of Mn neurotoxicity are reversible. In summary, our results demonstrate that Mn overexposure leads to pronounced morphological alterations, changes in neurogranin expression and behavioral impairments in zebrafish larvae.

Visualization of the chemical defense molecule formoside binding to sensory structures in a model fish predator

Sensory perception of chemical threats coming from an organism's environment relies on the coordination of numerous receptors and cell types. In many cases, the physiological processes responsible for driving behavioral responses to chemical cues are poorly understood. Here, we investigated the physiological response of fish to an unpalatable compound, formoside, which is employed as a chemical defense by marine sponges. Construction of fluorescent probe derivatives of formoside allowed visualization of this chemical defense molecule in vivo, interacting with the cells and tissues of the early larvae of a model predator, the zebrafish (Danio rerio). This revealed the precise chemosensory structures targeted by formoside to be in the taste buds and olfactory epithelium of developing zebrafish. Mechanosensory neuromasts were also targeted. This study supports the involvement of a previously identified co-receptor in detection of the chemical defense and provides a springboard for the long-term goal of identification of the cellular receptor of formoside. Extension of this approach to other predators and chemical defenses may provide insight into common mechanisms of chemoreception by predators as well as common strategies of chemical defense employed by prey.

Zinc Oxide Nanoparticles (ZnO-NPs) Induce Cytotoxicity in the Zebrafish Olfactory Organs via Activating Oxidative Stress and Apoptosis at the Ultrastructure and Genetic Levels

Nanotechnology has gained tremendous attention because of its crucial characteristics and wide biomedical applications. Although zinc oxide nanoparticles (ZnO-NPs) are involved in many industrial applications, researchers pay more attention to their toxic effects on living organisms. Since the olfactory epithelium is exposed to the external environment, it is considered the first organ affected by ZnO-NPs. Herein, we demonstrated the cytotoxic effect of ZnO-NPs on the olfactory organ of adult zebrafish after 60 days post-treatment. We opted for this period when fishes stop eating their diet from the aquarium, appear feeble, and cannot swim freely. Our study demonstrated that ZnO-NPs induced significant malformations of the olfactory rosettes at histological, ultrastructural, and genetic levels. At the ultrastructure level, the olfactory lamellae appeared collapsed, malformed, and twisted with signs of degeneration and loss of intercellular connections. In addition, ZnO-NPs harmed sensory receptor and ciliated cells, microvilli, rodlet, crypt, and Kappe cells, with hyper-activity of mucous secretion from goblet cells. At the genetic level, ZnO-NPs could activate the reactive oxygen species (ROS) synthesis expected by the down-regulation of mRNA expression for the antioxidant-related genes and up-regulation of DNA damage, cell growth arrest, and apoptosis. Interestingly, ZnO-NPs affected the odor sensation at 60 days post-treatment (60-dpt) more than at 30-dpt, severely damaging the olfactory epithelium and irreparably affecting the cellular repairing mechanisms. This induced a dramatically adverse effect on the cellular endoplasmic reticulum (ER), revealed by higher CHOP protein expression, that suppresses the antioxidant effect of Nrf2 and is followed by the induction of apoptosis via the up-regulation of Bax expression and down-regulation of Bcl-2 protein.

Type II Grass Carp Reovirus Rapidly Invades Grass Carp (Ctenopharyngodon idella) via Nostril-Olfactory System-Brain Axis, Gill, and Skin on Head

Type II grass carp reovirus (GCRV-II) with high pathogenicity and infectivity causes severe hemorrhagic disease, which leads to extensive death in the grass carp and black carp aquaculture. However, the early invasion portal remains unclear. In this study, we explored the invasion portal, time, and pathway of GCRV-II by immersion infection in grass carp. Through the detection of the infected grass carp external body surface tissues, most of them could be detected to carry GCRV-II within 45 min except for the skin covered by scales. Further shortening the duration of infection, we proved that GCRV-II rapidly invades through the nostril (especially), gill, and skin on head at only 5 min post-immersion, rather than merely by adhesion. Subsequently, visual localization investigations of GCRV-II were conducted on the nostril, olfactory system (olfactory bulb and olfactory tract), and brain via immunofluorescence microscopy and transmission electron microscopy. We found that few viruses were located in the nostril at 5 min post-immersion infection, while a significantly increased quantity of viruses were distributed in all of the examined tissues at 45 min. Furthermore, the semi-qRT-PCR and Western blotting results of different infection times confirmed that GCRV-II invades grass carp via the nostril-olfactory system-brain axis and then viral replication unfolds. These results revealed the infection mechanism of GCRV-II in terms of the invasion portal, time, and pathway in grass carp. This study aims to understand the invasion mode of GCRV-II in grass carp, thus providing theoretical support for the prevention and control strategies of hemorrhagic disease.

Olfactory receptor OR52N2 for PGE2 in mediation of guppy courtship behaviors

Prostaglandins (PGs) are a type of physiologically active unsaturated fatty acids. As an important sex pheromone, PGs play a vital role in regulating the reproductive behaviors of species by mediating nerve and endocrine responses. In this study, guppy (Poecilia reticulate) was used as the model specie to detect the function of PGE₂ in inducing the onset of courtship behaviors. Our results showed that adding PGE₂ into the water environment could activate the courtship behavior of male guppy, indicating that the peripheral olfactory system mediated the PGE₂ function. Thereafter, the open reading frame (ORF) of olfactory receptor or52n2 was cloned, which was 936 bp in length, coding 311 amino acids. As a typical G protein-coupled receptor, OR52N2 had a conservative seven α-helix transmembrane domains. To confirm the regulatory relationship between OR52N2 and PGE₂, dual-luciferase reporter assay was employed to verify the activation of downstream CREB signaling pathways. Results showed that PGE₂ significantly enhanced CRE promoter activity in or52n2 ORF transient transfected HEK-293 T cells. Finally, localization of or52n2 mRNA were observed in ciliated receptor cells of the olfactory epithelium using in situ hybridization. Our results provide a novel insight into sex pheromone signaling transduction in reproductive behavior.

Effects of carnosine on the embryonic development and TiO2 nanoparticles-induced oxidative stress on Zebrafish

Oxidative stress is due to an unbalance between pro-oxidants, such as reactive oxygen (ROS) and nitrogen (RNS) species, and antioxidants/antioxidant system. Under physiological conditions these species are involved in different cellular processes such as cellular homeostasis and immune response, while an excessive production of ROS/RNS has been linked to the development of various diseases such as cancer, diabetes, and Alzheimer's disease. In this context, the naturally occurring dipeptide carnosine has shown the ability to scavenge ROS, counteract lipid peroxidation, and inhibit proteins oxidation. Titanium dioxide nanoparticles (TiO2-NPs) have been widely used to produce cosmetics, in wastewater treatment, in food industry, and in healthcare product. As consequence, these NPs are often released into aquatic environments. The Danio rerio (commonly called zebrafish) embryos exposure to TiO2-NPs did not affect the hatching rate, but induced oxidative stress. According to this scenario, in the present study, we first investigated the effects of carnosine exposure and of a sub-toxic administration of TiO2-NPs on the development and survival of zebrafish embryos/larvae measured through the acute embryo toxicity test (FET-Test). Zebrafish larvae represent a useful model to study oxidative stress-linked disorders and to test antioxidant molecules, while carnosine was selected based on its well-known multimodal mechanism of action that includes a strong antioxidant activity. Once the basal effects of carnosine were assessed, we then evaluated its effects on TiO2-NPs-induced oxidative stress in zebrafish larvae, measured in terms of total ROS production (measured with 2,7-dichlorodihydrofluorescein diacetate probe) and protein expression by immunohistochemistry of two cellular stress markers, 70 kDa-heat shock protein (Hsp70) and metallothioneins (MTs). We demonstrated that carnosine did not alter the phenotypes of both embryos and larvae of zebrafish at different hours post fertilization. Carnosine was instead able to significantly decrease the enhancement of ROS levels in zebrafish larvae exposed to TiO2-NPs and its antioxidant effect was paralleled by the rescue of the protein expression levels of Hsp70 and MTs. Our results suggest a therapeutic potential of carnosine as a new pharmacological tool in the context of pathologies characterized by oxidative stress such as neurodegenerative disorders.

Fractionation of perfluoroalkyl acids (PFAAs) along the aquatic food chain promoted by competitive effects between longer and shorter chain PFAAs

Perfluoroalkyl acids (PFAAs) are proteinophilic pollutants. We hypothesized that fractionation of PFAAs may occur along a food chain. To testify this hypothesis, we investigated the bioconcentration, bioaccumulation, and fractionation of 11 kinds of PFAAs (C-F = 3-11) along an aquatic food chain consisting of D. magna, zebrafish, and cichlid. The results showed that the proportions of PFNA, PFOA, and all shorter chain PFAAs in the D. magna and fish tissues were lower than the ones in exposure water, opposing to the other longer chain PFAAs. Predation promoted such fractionation differences, and the proportions of PFNA, PFOA, and all shorter chain PFAAs in organisms decreased while those of the other longer chain PFAAs increased along the food chain. The results of isothermal titration calorimetry and molecular docking experiments showed that binding affinities of PFAAs and fish proteins increased with the number of perfluorinated carbons, resulting in a substitution of shorter chain PFAAs by their longer chain analogues. It also triggered the differences in the uptake and elimination of PFFAs and competitive bioaccumulation between longer and shorter chain PFAAs. This study suggests that fractionation should be considered in studying environmental behaviors and evaluating ecological risks of multiple PFAAs.

Corrigendum: The olfactory tract: Basis for future evolution in response to rapidly changing ecological niches

[This corrects the article DOI: 10.3389/fnana.2022.831602.].

Ontogenetic shifts in olfactory rosette morphology of the sockeye salmon, Oncorhynchus nerka

Sockeye salmon, Oncorhynchus nerka, are anadromous, semelparous fish that breed in freshwater-typically in streams, and juveniles in most populations feed in lakes for 1 or 2 years, then migrate to sea to feed for 2 or 3 additional years, before returning to their natal sites to spawn and die. This species undergoes important changes in behavior, habitat, and morphology through these multiple life history stages. However, the sensory systems that mediate these migratory patterns are not fully understood, and few studies have explored changes in sensory function and specialization throughout ontogeny. This study investigates changes in the olfactory rosette of sockeye salmon across four different life stages (fry, parr, smolt, and adult). Development of the olfactory rosette was assessed by comparing total rosette size (RS), lamellae number, and lamellae complexity from scanning electron microscopy images across life stages, as a proxy for olfactory capacity. Olfactory RS increased linearly with lamellae number and body size (p < .001). The complexity of the rosette, including the distribution of sensory and nonsensory epithelia and the appearance of secondary lamellar folding, varied between fry and adult life stages. These differences in epithelial structure may indicate variation in odor-processing capacity between juveniles imprinting on their natal stream and adults using those odor memories in the final stages of homing to natal breeding sites. These findings improve our understanding of the development of the olfactory system throughout life in this species, highlighting that ontogenetic shifts in behavior and habitat may coincide with shifts in nervous system development.

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.

Self-recovery study of fluoride-induced ferroptosis in the liver of zebrafish (Danio rerio)

Ferroptosis plays a key role in fluorosis in aquatic organisms, but whether it is involved in fluoride-induced liver damage remains unclear. Previous studies have indicated that fluoride toxicity has the reversible tendency, but the mechanism of self-recovery after fluorosis in aquatic animals has not been elucidated. In this study, adult zebrafish and embryos were exposed to 0, 20, 40, 80 mg/L of fluoride for 30, 60 and 90 d and 3, 4 and 5 d post-fertilization (dpf), respectively. After 90 d, adult zebrafish were transferred to clean water for self-recovery of 30 d. The results showed that fluoride induced the prominent histopathologial changes in liver of adults, and the developmental delay and dark liver area in larvae. Fluoride significantly increased the iron overload, while decreased the expression levels of transferrin (tf), transferrin receptor (tfr), ferroportin (fpn), membrane iron transporter (fpn), and ferritin heavy chain (fth) in adults and larvae. Fluoride also induced the oxidative stress in adults and larvae by increasing the levels of reactive oxygen species (ROS) and malondialdehyde (MDA), while decreasing the glutathione (GSH) content and the levels of glutathione peroxidase 4 (gpx4) and solute carrier family 7 member 11 (slc7a11). Self-recovery relieved fluoride-induced ferroptosis by reducing the histopathological damage and oxidative stress, reversing the expression levels of fth and slc7a11, Fe²⁺ metabolism and GSH synthesis. Lipid peroxidation and Fe²⁺ metabolism may be the key factor in alleviating effects of self-recovery on fluoride toxicity. Moreover, males are more sensitive than females. Our results provide a theoretical basis for studying the alleviating effects of self-recovery on fluoride toxicity and the underlying mechanism of its protective effect.

Changes in Dendritic Spine Morphology and Density of Granule Cells in the Olfactory Bulb of Anguilla anguilla (L., 1758): A Possible Way to Understand Orientation and Migratory Behavior

Simple Summary

The olfactory bulb can process odour cues through granular cells (GCs) and dendritic spines, changing their synaptic plasticity properties and their morphology. The GCs’ dendritic spines density and morphology were analysed in Anguilla anguilla, considering the olfaction as a driver involved in fish orientation and migration. For the head and neck morphology, spines were classified as mushroom, long thin, stubby, and filopodia. Spines’ density decreased from juvenile migrants to no-migrant stages and increased in the adult migrants. Spines’ density was comparable between glass and silver eels as an adaptation to migration, while at non-migrating phases, spines’ density decreased. For its phylogenetic Elopomorph attribution and its complex life cycle, A. anguilla could be recommended as a model species to study the development of dendritic spines in GCs of the olfactory bulb. Considering the role of olfaction in the orientation and migration of A. anguilla, the modification of environmental stimuli (ocean alterations and climate change) could represent contributing factors that threaten this critically endangered species.

Abstract

Olfaction could represent a pivotal process involved in fish orientation and migration. The olfactory bulb can manage olfactive signals at the granular cell (GC) and dendritic spine levels for their synaptic plasticity properties and changing their morphology and structural stability after environmental odour cues. The GCs’ dendritic spine density and morphology were analysed across the life stages of the catadromous Anguilla anguilla. According to the head and neck morphology, spines were classified as mushroom (M), long thin (LT), stubby (S), and filopodia (F). Total spines’ density decreased from juvenile migrants to no-migrant stages, to increase again in the adult migrant stage. Mean spines’ density was comparable between glass and silver eels as an adaptation to migration. At non-migrating phases, spines’ density decreased for M and LT, while M, LT, and S density increased in silver eels. A great dendritic spine development was found in the two migratory phases, regressing in trophic phases, but that could be recreated in adults, tracing the migratory memory of the routes travelled in juvenile phases. For its phylogenetic Elopomorph attribution and its complex life cycle, A. anguilla could be recommended as a model species to study the development of dendritic spines in GCs of the olfactory bulb as an index of synaptic plasticity involved in the modulation of olfactory stimuli. If olfaction is involved in the orientation and migration of A. anguilla and if eels possess a memory, these processes could be influenced by the modification of environmental stimuli (ocean alterations and rapid climate change) contributing to threatening this critically endangered species.

Amelioration of acrylamide induced neurotoxicity by benzo[b]thiophene analogs via glutathione redox dynamics in zebrafish larvae

Acrylamide is a thermal process contaminant, which gets global attention due to its neurotoxic nature and its omnipresence in carbohydrate-rich foods. Chronic exposure to acrylamide leads to neuronal deterioration and motor dysfunction. Acrylamide could severely affect the antioxidant defense system, especially in the developing brain leading to premature neurological disorders. Acrylamide forms adduct in presynaptic neurons leading to neuroinflammation which is also a factor to consider. In this present study, we have explored whether our benzo[b]thiophene analogs, 1-(3-hydroxybenzo[b]thiophen-2-yl) ethanone (BP) and 1-(3-hydroxybenzo[b]thiophen-2-yl) propan-1-one hydrate (EP) with antioxidant activity, could inhibit the acrylamide-induced neurotoxicity-like behavior in zebrafish larvae. The experiment was set up to expose 3 days post fertilized (dpf) larvae to acrylamide (0.75 mM) for 3 days with or without compounds (80 µM). Locomotion behavioral analysis, antioxidants, glutathione, and acetylcholineesterase activity in the head region were analyzed after one day of the experimental procedure. We witnessed a restoration effect on glutathione redox dynamics. Since glutathione plays a crucial role in the detoxification of acrylamide, it is necessary to maintain the glutathione redox cycle to eliminate acrylamide from the body. BP and EP reduced the pro-inflammatory transcript in the head, which correlates with the reduction in oxidative stress. Finally, BP and EP showed a positive effect on synaptic vesicle cycling transcript and partially restores the motor neuron response to stimuli. Findings in this study showed the ability of compound BP and EP possess therapeutic value in oxidative stress-associated neurological disorders.

Interactive effects of fluoride and seleno-l-methionine at environmental related concentrations on zebrafish (Danio rerio) liver via the gut-liver axis

Fluoride (F) is a ubiquitous aquatic environmental pollutant and co-exists with other pollutants to form combined pollution. Selenium (Se) is beneficial at low levels yet toxic at high levels and can interact with some metals. However, the interactive effects of F and Se on the liver in fish remains enigmatic. In this study, zebrafish (Danio rerio) were exposed to F (80 mg/L) and dietary seleno-l-methionine (Se-Met, 0.25, 0.5 and 1.0 μg/g dry weight) alone or in combination for 90 d. The results indicated that co-treatment to F and Se-Met attenuated the histopathological damage, oxidative stress, and inflammatory in the liver, compared with the F treatment alone. Meanwhile, dietary Se-Met treatment improved F-induced intestinal barrier dysfunction, increased the transcripts of tight junction proteins (ZO-1, Claudin-1 and Occludin), and restored the homeostasis of intestinal microbiota. Moreover, dietary Se-Met ameliorated F-induced intestinal and liver inflammation by inhibiting lipopolysaccharide (LPS) levels and transcripts of TLR4 and p65 in the intestine and liver. This study manifested that Se-Met alleviates F-induced liver and intestinal injury when both co-occur at specific concentrations, and that the gut-liver axis pathway may serve as a mechanistic base for these alleviative effects.

Beyond the behavioural phenotype: Uncovering mechanistic foundations in aquatic eco-neurotoxicology

During the last decade, there has been an increase in awareness of how anthropogenic pollution can alter behavioural traits of diverse aquatic organisms. Apart from understanding profound ecological implications, alterations in neuro-behavioural indices have emerged as sensitive and physiologically integrative endpoints in chemical risk assessment. Accordingly, behavioural ecotoxicology and broader eco-neurotoxicology are becoming increasingly popular fields of research that span a plethora of fundamental laboratory experimentations as well as applied field-based studies. Despite mounting interest in aquatic behavioural ecotoxicology studies, there is, however, a considerable paucity in deciphering the mechanistic foundations underlying behavioural alterations upon exposure to pollutants. The behavioural phenotype is indeed the highest-level integrative neurobiological phenomenon, but at its core lie myriads of intertwined biochemical, cellular, and physiological processes. Therefore, the mechanisms that underlie changes in behavioural phenotypes can stem among others from dysregulation of neurotransmitter pathways, electrical signalling, and cell death of discrete cell populations in the central and peripheral nervous systems. They can, however, also be a result of toxicity to sensory organs and even metabolic dysfunctions. In this critical review, we outline why behavioural phenotyping should be the starting point that leads to actual discovery of fundamental mechanisms underlying actions of neurotoxic and neuromodulating contaminants. We highlight potential applications of the currently existing and emerging neurobiology and neurophysiology analytical strategies that should be embraced and more broadly adopted in behavioural ecotoxicology. Such strategies can provide new mechanistic discoveries instead of only observing the end sum phenotypic effects.

Gene duplication, conservation, and divergence of activating transcription factor 5 gene in zebrafish

Activating transcription factor 5 (Atf5) is a member of the ATF/CREB family of transcription factors and involved in diverse cellular functions and diseases in mammals. However, the function of atf5 remains largely unknown in fish. Here, we report the expression pattern and function of duplicated atf5 genes in zebrafish. The results showed that the gene structures of zebrafish atf5a and atf5b were similar to their mammalian orthologs. Zebrafish Atf5a and Atf5b shared an amino acid sequence identity of 40.7%. Zebrafish atf5a and atf5b had maternal origin with dynamic expression during embryonic development. Zebrafish atf5a mRNA is mainly enriched in olfactory epithelium, midbrain, and hindbrain, while zebrafish atf5b mRNA is mainly detected in midbrain, hindbrain, and liver during embryogenesis. The results of acute hypoxia experiment showed that atf5a mRNA was significantly upregulated in the brain, liver, and muscle, while atf5b mRNA was just increased significantly in the brain. Functional analysis showed that knockdown of atf5a affects the development of the ciliated neurons in zebrafish embryos. The effect was enhanced when atf5a MO was co-injected with atf5b MO. The development of ciliated neurons in zebrafish embryos was not affected by injection of atf5b MO alone. atf5a knockdown also affects the development of early-born olfactory neurons. The effects caused by atf5a knockdown could be rescued by atf5b mRNA. These results suggest that the duplicated atf5 genes may have evolved divergently and play redundant biological roles in the development of olfactory sensory neurons in zebrafish.

Intranasal delivery of SARS-CoV-2 spike protein is sufficient to cause olfactory damage, inflammation and olfactory dysfunction in zebrafish

Anosmia, loss of smell, is a prevalent symptom of SARS-CoV-2 infection. Anosmia may be explained by several mechanisms driven by infection of non-neuronal cells and damage in the nasal epithelium rather than direct infection of olfactory sensory neurons (OSNs). Previously, we showed that viral proteins are sufficient to cause neuroimmune responses in the teleost olfactory organ (OO). We hypothesize that SARS-CoV-2 spike (S) protein is sufficient to cause olfactory damage and olfactory dysfunction. Using an adult zebrafish model, we report that intranasally delivered SARS-CoV-2 S RBD mostly binds to the non-sensory epithelium of the olfactory organ and causes severe olfactory histopathology characterized by loss of cilia, hemorrhages and edema. Electrophysiological recordings reveal impaired olfactory function to both food and bile odorants in animals treated intranasally with SARS-CoV-2 S RBD. However, no loss of behavioral preference for food was detected in SARS-CoV-2 S RBD treated fish. Single cell RNA-Seq of the adult zebrafish olfactory organ indicated widespread loss of olfactory receptor expression and inflammatory responses in sustentacular, endothelial, and myeloid cell clusters along with reduced numbers of T_regs. Combined, our results demonstrate that intranasal SARS-CoV-2 S RBD is sufficient to cause structural and functional damage to the zebrafish olfactory system. These findings may have implications for intranasally delivered vaccines against SARS-CoV-2.

Localization of BDNF and Calretinin in Olfactory Epithelium and Taste Buds of Zebrafish (Danio rerio)

Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family and it is involved in several fundamental functions in the central and peripheral nervous systems, and in sensory organs. BDNF regulates the chemosensory systems of mammals and is consistently expressed in those organs. In zebrafish, the key role of BDNF in the biology of the hair cells of the inner ear and lateral line system has recently been demonstrated. However, only some information is available about its occurrence in the olfactory epithelium, taste buds, and cutaneous isolated chemosensory cells. Therefore, this study was undertaken to analyze the involvement of BDNF in the chemosensory organs of zebrafish during the larval and adult stages. To identify cells displaying BDNF, we compared the cellular pattern of BDNF-displaying cells with those immunoreactive for calretinin and S100 protein. Our results demonstrate the localization of BDNF in the sensory part of the olfactory epithelium, mainly in the ciliated olfactory sensory neurons in larvae and adult zebrafish. Intense immunoreaction for BDNF was also observed in the chemosensory cells of oral and cutaneous taste buds. Moreover, a subpopulation of olfactory sensory neurons and chemosensory cells of olfactory rosette and taste bud, respectively, showed marked immunopositivity for calcium-binding protein S100 and calretinin. These results demonstrate the possible role of BDNF in the development and maintenance of olfactory sensory neurons and sensory cells in the olfactory epithelium and taste organs of zebrafish during all stages of development.

The BDNF/TrkB Neurotrophin System in the Sensory Organs of Zebrafish

The brain-derived neurotrophic factor (BDNF) was discovered in the last century, and identified as a member of the neurotrophin family. BDNF shares approximately 50% of its amino acid with other neurotrophins such as NGF, NT-3 and NT-4/5, and its linear amino acid sequences in zebrafish (Danio rerio) and human are 91% identical. BDNF functions can be mediated by two categories of receptors: p75NTR and Trk. Intriguingly, BDNF receptors were highly conserved in the process of evolution, as were the other NTs’ receptors. In this review, we update current knowledge about the distribution and functions of the BDNF-TrkB system in the sensory organs of zebrafish. In fish, particularly in zebrafish, the distribution and functions of BDNF and TrkB in the brain have been widely studied. Both components of the system, associated or segregated, are also present outside the central nervous system, especially in sensory organs including the inner ear, lateral line system, retina, taste buds and olfactory epithelium.

Immune responses in the injured olfactory and gustatory systems: a role in olfactory receptor neuron and taste bud regeneration?

Sensory cells that specialize in transducing olfactory and gustatory stimuli are renewed throughout life and can regenerate after injury unlike their counterparts in the mammalian retina and auditory epithelium. This uncommon capacity for regeneration offers an opportunity to understand mechanisms that promote the recovery of sensory function after taste and smell loss. Immune responses appear to influence degeneration and later regeneration of olfactory sensory neurons and taste receptor cells. Here we review surgical, chemical, and inflammatory injury models and evidence that immune responses promote or deter chemosensory cell regeneration. Macrophage and neutrophil responses to chemosensory receptor injury have been the most widely studied without consensus on their net effects on regeneration. We discuss possible technical and biological reasons for the discrepancy, such as the difference between peripheral and central structures, and suggest directions for progress in understanding immune regulation of chemosensory regeneration. Our mechanistic understanding of immune-chemosensory cell interactions must be expanded before therapies can be developed for recovering the sensation of taste and smell after head injury from traumatic nerve damage and infection. Chemosensory loss leads to decreased quality of life, depression, nutritional challenges, and exposure to environmental dangers highlighting the need for further studies in this area.

Damage to Olfactory Organs of Adult Zebrafish Induced by Diesel Particulate Matter

Particulate matter (PM) is an environmental hazard that is associated with various human health risks. The olfactory system is directly exposed to PM; therefore, the influence of PM exposure on olfactory function must be investigated. In this study, we propose a zebrafish olfactory model to evaluate the effects of exposure to diesel particulate matter (DPM), which was labeled Korean diesel particulate matter (KDP20). KDP20 comprises heavy metals and polycyclic aromatic hydrocarbons (PAHs). KDP20 exposed olfactory organs exhibited reduced cilia and damaged epithelium. Olfactory dysfunction was confirmed using an odor-mediated behavior test. Furthermore, the olfactory damage was analyzed using Alcian blue and anti-calretinin staining. KDP20 exposed olfactory organs exhibited histological damages, such as increased goblet cells, decreased cell density, and calretinin level. Quantitative real-time polymerase chain reaction (qRT-PCR) revealed that PAHs exposure related genes (AHR2 and CYP1A) were upregulated. Reactive oxidation stress (ROS) (CAT) and inflammation (IL-1B) related genes were upregulated. Furthermore, olfactory sensory neuron (OSN) related genes (OMP and S100) were downregulated. In conclusion, KDP20 exposure induced dysfunction of the olfactory system. Additionally, the zebrafish olfactory system exhibited a regenerative capacity with recovery conditions. Thus, this model may be used in future investigating PM-related diseases.

Developmental titanium dioxide nanoparticle exposure induces oxidative stress and neurobehavioral changes in zebrafish

The widespread commercial application of titanium dioxide nanoparticles (TiO₂ NPs) leads to ubiquitous presence of TiO₂ NPs in the aquatic environment, which highlights the necessity to determine their potential adverse effects on aquatic organisms. The developing nerve system is particularly susceptible to environment perturbation. However, few studies have explored the developmental neurobehavioral toxicity of TiO₂ NPs, especially at smaller particle size ranges (≤20 nm) that have relatively longer retention time in the water column. In this study, zebrafish embryos were exposed to non-teratogenic concentrations of 0.1 and 1 mg/L TiO₂ NPs (average size of 14-20 nm) from 8 to 108 h post-fertilization (hpf) followed by various assessments at different time points up to 12 days post-fertilization (dpf). Our findings revealed that 1 mg/L TiO₂ NPs perturbed the motor and social behaviors in larval zebrafish. These behavioral changes were characterized by decreased swimming speed in a locomotor response test at 5 dpf, increased travel distance in a flash stimulus test at 5 dpf, increased preference to the light zone in a light/dark preference test at 10 dpf, and increased mirror attack and percent time spent in the mirror zone in a mirror stimulus response assay at 12 dpf. Mechanistic examinations at 5 dpf revealed elevated cell apoptosis and oxidative stress. Cell apoptosis was characterized by increased acridine orange (AO) positive cells in the olfactory region and neuromasts of the lateral line system. Oxidative stress was characterized by increased lipid peroxidation, increased ROS production, and upregulated catalase (cat) gene expression. In addition, TiO₂ NP exposure also upregulated genes associated with the developmental nervous system such as the growth associated protein 43 (gap43) and proliferating cell nuclear antigen (pcna). Our results suggest that the neurobehavioral changes in larvae exposed to 1 mg/L TiO₂ NPs during early development may result from cell apoptosis and oxidative stress induced neuronal damages.

Chemosensory Dysfunction Induced by Environmental Pollutants and Its Potential As a Novel Neurotoxicological Indicator: A Review

Air pollution composed of the complex interactions among particular matter, chemicals, and pathogens is an emerging and global environmental issue that closely correlates with a variety of diseases and adverse health effects, especially increasing incidences of neurodegenerative diseases. However, as one of the prevalent health outcomes of air pollution, chemosensory dysfunction has not attracted enough concern until recently. During the COVID-19 pandemic, multiple scientific studies emphasized the plausibly essential roles of the chemosensory system in the airborne transmission airway of viruses into the human body, which can also be utilized by pollutants. In this Review, in addition to summarizing current progress regarding the contributions of traditional air pollutants to chemosensory dysfunction, we highlight the roles of emerging contaminants. We not only sum up clarified mechanisms, such as inflammation and apoptosis but also discuss some not yet completely identified mechanisms, e.g., disruption of olfactory signal transduction. Although the existing evidence is not overwhelming, the chemosensory system is expected to be a useful indicator in neurotoxicology and neural diseases based on accumulating studies that continually excavate the deep link between chemosensory dysfunction and neurodegenerative diseases. Finally, we argue the importance of studies concerning chemosensory dysfunction in understanding the health effects of air pollution and provide comments for some future directions of relevant research.

Principles of odor coding in vertebrates and artificial chemosensory systems

The biological olfactory system is the sensory system responsible for the detection of the chemical composition of the environment. Several attempts to mimic biological olfactory systems have led to various artificial olfactory systems using different technical approaches. Here we provide a parallel description of biological olfactory systems and their technical counterparts. We start with a presentation of the input to the systems, the stimuli, and treat the interface between the external world and the environment where receptor neurons or artificial chemosensors reside. We then delineate the functions of receptor neurons and chemosensors as well as their overall I-O relationships. Up to this point, our account of the systems goes along similar lines. The next processing steps differ considerably: while in biology the processing step following the receptor neurons is the "integration" and "processing" of receptor neuron outputs in the olfactory bulb, this step has various realizations in electronic noses. For a long period of time, the signal processing stages beyond the olfactory bulb, i.e., the higher olfactory centers were little studied. Only recently there has been a marked growth of studies tackling the information processing in these centers. In electronic noses, a third stage of processing has virtually never been considered. In this review, we provide an up-to-date overview of the current knowledge of both fields and, for the first time, attempt to tie them together. We hope it will be a breeding ground for better information, communication, and data exchange between very related but so far little connected fields.

Developmental co-exposure of TBBPA and titanium dioxide nanoparticle induced behavioral deficits in larval zebrafish

Both tetrabromobisphenol A (TBBPA) and titanium dioxide nanoparticle (TiO₂ NP) have widespread commercial applications, resulting in their ubiquitous co-presence in the environment and biota. Although environmental chemicals exist as mixtures, toxicity studies are nearly always conducted with single chemicals. Few studies explore potential interactions of different chemical mixtures. In this study, we employ the sensitive developing nerve system in zebrafish to assess the neurotoxicity of TBBPA/TiO₂ NP mixtures. Specifically, zebrafish embryos were exposed to solvent control (0.1% DMSO), 2 μM TBBPA, 0.1 mg/L TiO₂ NP, and their mixture from 8 to 120 h post fertilization (hpf), and motor/social behavioral assessments were conducted on embryos/larvae at different developmental stages. Our results showed that TBBPA/TiO₂ NP single or co-exposures increased spontaneous movement, decreased touch response and swim speed, and affected social behaviors of light/dark preference, shoaling, mirror attack and social contact. In particular, many of these phenotypes were manifested with higher magnitude of changes from the mixture exposure. These behavioral deficits were also accompanied with increased cell death in olfactory region and neuromasts in the lateral line system, increased ROS in gallbladder, pancreas, liver, and intestine, as well as increased lipid peroxidation and decreased ATP levels in whole larval tissue homogenates. Further, genes coding for key cell apoptosis marker and antioxidant enzyme were significantly upregulated by these two chemicals, in particular to their mixture. Interestingly, the co-presence of TBBPA also increased the mean particle size of TiO₂ NP in the exposure solutions and the TiO₂ NP content in larval tissue. Together, our analysis suggests that TBBPA/TiO₂ NP induced behavioral changes may be due to physical accumulation of these two chemicals in the target organs, and TiO₂ NP may serve as carriers for increased accumulation of TBBPA. To conclude, we demonstrated that TBBPA/TiO₂ NP together cause increased bioaccumulation of TiO₂, and heightened responses in behavior, cell apoptosis and oxidative stress. Our findings also highlight the importance of toxicity assessment using chemical mixtures.

Evolutionary dynamics of the OR gene repertoire in teleost fishes: evidence of an association with changes in olfactory epithelium shape

Teleost fishes perceive their environment through a range of sensory modalities, among which olfaction often plays an important role. Richness of the olfactory repertoire depends on the diversity of receptors coded by homologous genes classified into four families: OR, TAAR, VR1, and VR2. Herein, we focus on the OR gene repertoire. While independent large contractions of the OR gene repertoire associated with ecological transitions have been found in mammals, little is known about the diversity of the OR gene repertoire and its evolution in teleost fishes, a group that includes more than 34,000 living species. We analyzed genomes of 163 species representing diversity in this large group. We found a large range of variation in the number of functional OR genes, from 15 in the Broad-nose Pipefish Syngnathus typhle and the Ocean Sunfish Mola mola, to 429 in the Zig-zag Eel Mastacembelus armatus. The number of OR genes was higher in species when a multilamellar olfactory rosette was present. Moreover, the number of lamellae was correlated with the richness of the OR gene repertoire. While a slow and balanced birth-and-death process generally drives the evolution of the OR gene repertoire, we inferred several episodes of high rates of gene loss, sometimes followed by large gains in the number of OR genes. These gains coincide with morphological changes of the olfactory organ and suggest a strong functional association between changes in the morphology and the evolution of the OR gene repertoire.

A comprehensive structural, lectin and immunohistochemical characterization of the zebrafish olfactory system

Fish chemosensory olfactory receptors allow them to detect a wide range of water-soluble chemicals, that mediate fundamental behaviours. Zebrafish possess a well-developed sense of smell which governs reproduction, appetite, and fear responses. The spatial organization of functional properties within the olfactory epithelium and bulb are comparable to those of mammals, making this species suitable for studies of olfactory differentiation and regeneration and neuronal representation of olfactory information. The advent of genomic techniques has been decisive for the discovery of specific olfactory cell types and the identification of cell populations expressing vomeronasal receptors. These advances have marched ahead of morphological and neurochemical studies. This study aims to fill the existing gap in specific histological, lectin-histochemical and immunohistochemical studies on the olfactory rosette and the olfactory bulb of the zebrafish. Tissue dissection and microdissection techniques were employed, followed by histological staining techniques, lectin-histochemical labelling (UEA, LEA, BSI-B4) and immunohistochemistry using antibodies against G proteins subunits αo and αi2, growth-associated protein-43, calbindin, calretinin, glial-fibrillary-acidic-protein and luteinizing-hormone-releasing-hormone. The results obtained enrich the available information on the neurochemical patterns of the zebrafish olfactory system, pointing to a greater complexity than the one currently considered, especially when taking into account the peculiarities of the nonsensory epithelium.

Olfactory Rod Cells: A Rare Cell Type in the Larval Zebrafish Olfactory Epithelium With a Large Actin-Rich Apical Projection

We report the presence of a rare cell type, the olfactory rod cell, in the developing zebrafish olfactory epithelium. These cells each bear a single actin-rich rod-like apical projection extending 5–10 μm from the epithelial surface. Live imaging with a ubiquitous Lifeact-RFP label indicates that the olfactory rods can oscillate. Olfactory rods arise within a few hours of the olfactory pit opening, increase in numbers and size during larval stages, and can develop in the absence of olfactory cilia. Olfactory rod cells differ in morphology from the known classes of olfactory sensory neuron, but express reporters driven by neuronal promoters. A sub-population of olfactory rod cells expresses a Lifeact-mRFPruby transgene driven by the sox10 promoter. Mosaic expression of this transgene reveals that olfactory rod cells have rounded cell bodies located apically in the olfactory epithelium and have no detectable axon. We offer speculation on the possible function of these cells in the Discussion.

Multimodal Imaging and Analysis of the Neuroanatomical Organization of the Primary Olfactory Inputs in the Brownbanded Bamboo Shark, Chiloscyllium punctatum

There is currently a limited understanding of the morphological and functional organization of the olfactory system in cartilaginous fishes, particularly when compared to bony fishes and terrestrial vertebrates. In this fish group, there is a clear paucity of information on the characterization, density, and distribution of olfactory receptor neurons (ORNs) within the sensory olfactory epithelium lining the paired olfactory rosettes, and their functional implications with respect to the hydrodynamics of incurrent water flow into the nares. This imaging study examines the brownbanded bamboo shark Chiloscyllium punctatum (Elasmobranchii) and combines immunohistochemical labeling using antisera raised against five G-protein α-subunits (Gα_s/olf, Gα_q/11/14, Gα_i–1/2/3, Gα_i–3, Gα_o) with light and electron microscopy, to characterize the morphological ORN types present. Three main ORNs (“long”, “microvillous” and “crypt-like”) are confirmed and up to three additional microvilli-bearing types are also described; “Kappe-like” (potential or homologous “Kappe” as in teleosts), “pear-shaped” and “teardrop-shaped” cells. These morphotypes will need to be confirmed molecularly in the future. Using X-ray diffusible iodine-based contrast-enhanced computed tomography (diceCT), high-resolution scans of the olfactory rosettes, olfactory bulbs (OBs), peduncles, and telencephalon reveal a lateral segregation of primary olfactory inputs within the OBs, with distinct medial and lateral clusters of glomeruli, suggesting a potential somatotopic organization. However, most ORN morphotypes are found to be ubiquitously distributed within the medial and lateral regions of the olfactory rosette, with at least three microvilli-bearing ORNs labeled with anti-Gα_o found in significantly higher densities in lateral lamellae [in lateral lamellae] and on the anterior portion of lamellae (facing the olfactory cavity). These microvilli-bearing ORN morphotypes (microvillous, “Kappe-like,” “pear-shaped,” and “teardrop-shaped”) are the most abundant across the olfactory rosette of this species, while ciliated ORNs are less common and crypt cells are rare. Spatial simulations of the fluid dynamics of the incurrent water flow into the nares and within the olfactory cavities indicate that the high densities of microvilli-bearing ORNs located within the lateral region of the rosette are important for sampling incoming odorants during swimming and may determine subsequent tracking behavior.