Identification of olfactory alarm substances in zebrafish

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.

Courtship and spawning behaviour of medaka in a semi-outdoor environment initiating at midnight

Reproductive timing is a critical ecological trait that directly influences fitness. Medaka (Oryzias latipes), a small freshwater fish, is widely used as a model organism in various scientific fields. However, ecological studies conducted under (semi-) natural conditions remain limited. Although, spawning has been reported to occur within 1 h before and after sunrise, direct observations remain scarce. We investigated the timing of spawning initiation and associated courtship in medaka through 24-h observations using infrared cameras under semi-natural conditions. During the experiments, sunrise occurred at approximately 4:45. Observations of the 31 pairs revealed that spawning occurred between 1:05-9:48, with a peak at 2:00-4:00. Unlike previous reports but like the most recent fieldwork, only 26% of the total spawning events (8/31) were observed within 1 h before and after sunrise. Male courtship behaviours, including following females and quick circle displays, increased from midnight, peaking between 2:00-5:00. This study provides new insights into the natural reproductive timing of medaka, suggesting a possible adaptation to nocturnal spawning, likely as a strategy to reduce predation on both parents and eggs. It also underscores the importance of investigating the ecology of model organisms under (semi-)natural conditions to gain a more comprehensive understanding of biological phenomena observed in laboratory settings.

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.

The number of conspecific alarm substance donors notably influences the behavioural responses of zebrafish subjected to a traumatic stress procedure

Zebrafish (Danio rerio) represents a complementary pre-clinical model in stress and anxiety research. Conspecific alarm substance (CAS), an alarm pheromone secreted by injured fish, acts as a warning signal and modulates fear responses. Given their schooling nature and that injury precedes CAS release, varying fresh CAS concentrations extracted from different numbers of CAS-donating zebrafish may uniquely influence trauma-related behaviours. Thus, we investigated the behaviour of juvenile and adult zebrafish exposed to traumatic stress protocols, in the presence of CAS extracted from varying numbers of donating zebrafish. Juveniles were assessed for anxiety and boldness in the light-dark and open field tests (LDT and OFT), while adults were assessed in the novel tank test (NTT) and novel OFT (nOFT). We found that (1) trauma minimally impacted juvenile behaviour regardless of donor-derived CAS concentrations, (2) trauma-exposed adults displayed reduced exploration and heightened risk-taking behaviours in the NTT and nOFT compared to control-exposed fish, (3) NTT and nOFT freezing behaviours were distinctly emulated in adult fish and (4) post-trauma behaviour in adults was influenced by the number of donors. Therefore, CAS concentration as determined by donor number has age-related effects on anxiety- and risk-taking behaviours in trauma-exposed zebrafish, a valuable finding for studies utilising fresh CAS as a stress trigger. While we did not directly investigate CAS concentration through serial dilution, our data are of significant translational and ethological relevance, highlighting the importance of in-house method standardization in stress-related studies utilizing fresh CAS as an alarm cue.

Zebrafish (Danio rerio) Prefer Undisturbed Shoals over Shoals Exposed to the Synthetic Alarm Substance Hypoxanthine-3N-oxide (C5H4N4O2)

As an anti-predation behavior, shoaling enhances survival among prey species by reducing individual predation risk through mechanisms like the dilution effect and collective vigilance. Zebrafish-a highly social and genetically tractable species-are valuable for studying these behaviors. The present study examined zebrafish's social preferences in a 3-chamber open-tank free-swim task, assessing whether visual cues alone could distinguish between an intact and an alarmed shoal exposed to the synthetic alarm substance H3NO. Subjects were allowed to freely associate with either shoal while their behaviors were recorded and analyzed. The results reveal a significant preference for proximity to the intact shoal, indicating zebrafish's ability to visually discern threat levels. Subjects spent nearly twice as much time in the zone near the intact shoal, with reduced freezing and faster movement velocities compared to the alarmed shoal zone. Males exhibited more freezing behavior than females, consistent with sex-specific strategies in threat response. These findings underscore zebrafish's reliance on visual cues for social responding under predatory threat and highlight sex-based differences in threat perception. This research expands the understanding of zebrafish's social dynamics and provides a robust framework for future exploration of the neural mechanisms underlying social behavior and threat assessment in zebrafish.

Medaka (Oryzias latipes) initiate courtship and spawning late at night: Insights from field observations

Laboratory experiments were conducted using model organisms to elucidate biological phenomena. However, the natural habitats of organisms are inherently more complex than those found in the laboratory. To complement the laboratory experiments, we conducted field observations of the small freshwater fish medaka (Oryzias latipes), widely used as a model organism, to elucidate its ecology and behavior in natural environments. Our results showed that medaka initiated courtship and spawning late at night, much earlier than previously thought. Nocturnal video observations examining spawning time during the breeding season in Gifu, Japan (sunset: 19:00; sunrise: 5:00) revealed the presence of post-spawning medaka females around midnight. Behavioral analysis showed that the medaka was inactive until 23:00, with activity increasing from 0:00 and peaking from 1:00 to 3:00. Furthermore, a significant increase in male courtship was observed between 0:00 and 4:00. These findings provide the first empirical evidence that medaka mating begins significantly earlier than previously reported in the laboratory, as within an hour before or after light onset in the morning. This study highlights the importance of field observations in revealing critical aspects of organismal biology that may be overlooked in laboratory settings.

Single-Cell RNA-Sequencing of Zebrafish Olfactory Epithelium Identifies Odor-Responsive Candidate Olfactory Receptors

Single-cell RNA-sequencing (scRNA-seq) is a powerful method to comprehensively overlook gene expression profiles of individual cells in various tissues, providing fundamental datasets for classification of cell types and further functional analyses. Here we adopted scRNA-seq analysis for the zebrafish olfactory sensory neurons which respond to water-borne odorants and pheromones to elicit various behaviors crucial for survival and species preservation. Firstly, a single-cell dissociation procedure of the zebrafish olfactory rosettes was optimized by using cold-active protease, minimizing artifactual neuronal activation. Secondly, various cell types were classified into distinct clusters, based on the expressions of well-defined marker genes. Notably, we validated non-overlapping expressions of different families of olfactory receptors among the clusters of olfactory sensory neurons. Lastly, we succeeded in estimating candidate olfactory receptors responding to a particular odor stimulus by carefully scrutinizing correlated expressions of immediate early genes. Thus, scRNA-seq is a useful measure for the analysis of olfactory sensory neurons not only in classifying functional cell types but also in identifying olfactory receptor genes for given odorants and pheromones.

Schreckstoff: It takes two to panic

Schreckstoff (fear substance) is an alarm signal released by injured fish that induces a fear response. Its chemical nature has long been debated. A new study finds that zebrafish Schreckstoff is composed of at least three components, two of which elicit the fear response only in combination.