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

Fluorene-9-bisphenol acts on the gut-brain axis by regulating oxytocin signaling to disturb social behaviors in zebrafish

Previous studies have identified the exposure to ubiquitous environmental endocrine disruptors may be a risk factor of neurological disorders. However, the effects of fluorene-9-bisphenol (BHPF) in environmental exposure concentrations associated with these disorders are poorly understood. In this study, classic light-dark and social behavior tests were performed on zebrafish larvae and adults exposed BHPF exposure to evaluate social behavioral disorders and the microbiota-gut-brain axis was assessed to reveal the potential mechanisms underlying the behavioral abnormalities observed. Our results demonstrated that zebrafish larvae exposed to an environmentally relevant concentration (0.1 nM) of BHPF for 7 days showed a diminished response to external environmental factors (light or dark). Zebrafish larvae exposed to BHPF for 7 days or adults exposed to BHPF for 30 days at 1 μM displayed significant behavioral inhibition and altered social behaviors, including social recognition, social preference, and social fear contagion, indicating autism-like behaviors were induced by the exposure. BHPF exposure reduced the distribution of Nissl bodies in midbrain neurons and significantly reduced 5-hydroxytryptamine signaling. Oxytocin (OXT) levels and expression of its receptor oxtra in the gut and brain were down-regulated by BHPF exposure. In addition, the expression levels of genes related to the excitation-inhibitory balance of synaptic transmission changed. Microbiomics revealed increased community diversity and altered abundance of some microflora, such as an elevation in Bacillota and Bacteroidota and a decline in Mycoplasmatota in zebrafish guts, which might contribute to the abnormal neural circuits and autism-like behaviors induced by BHPF. Finally, the rescue effect of exogenous OXT on social behavioral defects induced by BHPF exposure was verified in zebrafish, highlighting the crucial role of OXT signaling through gut-brain axis in the regulatory mechanisms of social behaviors affected by BHPF. This study contributes to understanding the effects of environmental BHPF exposure on neuropsychiatric disorders and attracts public attention to the health risks posed by chemicals in aquatic organisms. The potential mental disorders should be considered in the safety assessments of environmental pollutants.

Chemical and Visual Cues as Modulators of the Stress Response to Social Isolation in the Marine Medaka, Oryzias melastigma

The marine medaka is emerging as a potential behavioral model organism for ocean studies, namely on marine ecotoxicology. However, not much is known on the behavior of the species and behavioral assays lack standardization. This study assesses the marine medaka as a potential model for chemical communication. We investigated how short exposure to visual and chemical cues mediated the stress response to social isolation with the light/dark preference test (LDPT) and the open field test (OFT). After a 5-day isolation period, and 1 h before testing, isolated fish were randomly assigned to one of four groups: (1) placed in visual contact with conspecifics; (2) exposed to a flow of holding water from a group of conspecifics; (3) exposed to both visual and chemical cues from conspecifics; or (4) not exposed to any stimuli (controls). During the LDPT, the distance traveled and transitions between zones were more pronounced in animals exposed to the conspecific's chemical stimuli. The time spent in each area did not differ between the groups, but a clear preference for the bright area in all animals indicates robust phototaxis. During the OFT, animals exposed only to chemical cues initially traveled more than those exposed to visual or both stimuli, and displayed lower thigmotaxis. Taken together, results show that chemical cues play a significant role in exploratory behavior in this species and confirm the LDPT and OFT as suitable tests for investigating chemical communication in this species.

Molecular organization of neuronal cell types and neuromodulatory systems in the zebrafish telencephalon

The function of neuronal networks is determined not only by synaptic connectivity but also by neuromodulatory systems that broadcast information via distributed connections and volume transmission. To understand the molecular constraints that organize neuromodulatory signaling in the telencephalon of adult zebrafish, we used transcriptomics and additional approaches to delineate cell types, to determine their phylogenetic conservation, and to map the expression of marker genes at high granularity. The combinatorial expression of GPCRs and cell-type markers indicates that all neuronal cell types are subject to modulation by multiple monoaminergic systems and distinct combinations of neuropeptides. Individual cell types were associated with multiple (typically >30) neuromodulatory signaling networks but expressed only a few diagnostic GPCRs at high levels, suggesting that different neuromodulatory systems act in combination, albeit with unequal weights. These results provide a detailed map of cell types and brain areas in the zebrafish telencephalon, identify core components of neuromodulatory networks, highlight the cell-type specificity of neuropeptides and GPCRs, and begin to decipher the logic of combinatorial neuromodulation.

Multisensory navigational strategies of hatchling fish for dispersal

Animals influence how they disperse in the environment by sensing local cues and adapting how they move. However, controlling dispersal can present a particular challenge early in life when animals tend to be more limited in their capacities to sense and move. To what extent and by what mechanisms can newly hatched fish control how they disperse? Here, we reveal hatchling sensorimotor mechanisms for controlling dispersal by combining swim tracking and precise sensory manipulations of a model species, zebrafish. In controlled laboratory experiments, if we physically constrained hatchlings or blocked sensations of motion through vision and the lateral line, hatchlings responded by elevating their buoyancy and passively moving with faster surface currents. Complementarily, in stagnant water, hatchlings covered more ground using hyperstable swimming, strongly orienting based on graviception. Using experimentally calibrated hydrodynamic simulations, we show that these hatchling behaviors nearly tripled diffusivity and made dispersal robust to local conditions, suggesting this multisensory strategy may provide important advantages for early life in a variable environment.

Bisphenol analogues induced social defects and neural impairment in zebrafish

There is evidence in humans that endocrine disrupting chemicals exposure, such as bisphenol A (BPA), is tied to social behavior impacts when evaluated in early life stage. However, the potential social impact of BPA alternatives and its association with central nervous system (CNS) is poorly understood. Here, we performed behavioral test for zebrafish that are continuously exposed to environmental relevant concentrations (5 and 500 ng/L) of BPA, BPF, and BPAF since embryonic stage. Surprisingly, significant social behavior defects, including increased social distance and decreased contact time, were identified in zebrafish treated by 500 ng/L BPAF and BPA. These behavioral changes were accompanied by apparent histological injury, microglia activation, enhanced apoptosis and neuron loss in brain. The gut-brain transcriptional profile showed that genes involved in neuronal development pathways were up-regulated in all bisphenol analogs treatments, indicating a protective phenotype of CNS; however, these pathways were inhibited in gut. Besides, a variety of key regulators in the gut-brain regulation were identified based on protein interaction prediction, such as rac1-limk1, insrb1 and fosab. These findings implicated that the existence of bisphenol analogues in water would influence the social life of fish, and revealed a potential role of gut-brain transcriptional alteration in mediating this effect.

Subpopulations of hypocretin/orexin neurons differ in measures of their cell proliferation, dynorphin co-expression, projections, and response to embryonic ethanol exposure

Numerous studies in animals demonstrate that embryonic exposure to ethanol (EtOH) at low-moderate doses stimulates neurogenesis and increases the number of hypothalamic neurons expressing the peptide, hypocretin/orexin (Hcrt). A recent study in zebrafish showed that this effect on the Hcrt neurons in the anterior hypothalamus (AH) is area specific, evident in the anterior (aAH) but not posterior (pAH) part of this region. To understand specific factors that may determine the differential sensitivity to EtOH of these Hcrt subpopulations, we performed additional measures in zebrafish of their cell proliferation, co-expression of the opioid dynorphin (Dyn), and neuronal projections. In association with the increase in Hcrt neurons in the aAH but not pAH, EtOH significantly increased only in the aAH the proliferation of Hcrt neurons and their number lacking Dyn co-expression. The projections of these subpopulations differed markedly in their directionality, with those from the pAH primarily descending to the locus coeruleus and those from the aAH ascending to the subpallium, and they were both stimulated by EtOH, which induced specifically the most anterior subpallium-projecting Hcrt neurons to become ectopically expressed beyond the aAH. These differences between the Hcrt subpopulations suggest they are functionally distinct in their regulation of behavior.

Role of Chemokine Cxcl12a in Mediating the Stimulatory Effects of Ethanol on Embryonic Development of Subpopulations of Hypocretin/Orexin Neurons and Their Projections

Studies in zebrafish and rats show that embryonic ethanol exposure at low-moderate concentrations stimulates hypothalamic neurons expressing hypocretin/orexin (Hcrt) that promote alcohol consumption, effects possibly involving the chemokine Cxcl12 and its receptor Cxcr4. Our recent studies in zebrafish of Hcrt neurons in the anterior hypothalamus (AH) demonstrate that ethanol exposure has anatomically specific effects on Hcrt subpopulations, increasing their number in the anterior AH (aAH) but not posterior AH (pAH), and causes the most anterior aAH neurons to become ectopically expressed further anterior in the preoptic area (POA). Using tools of genetic overexpression and knockdown, our goal here was to determine whether Cxcl12a has an important function in mediating the specific effects of ethanol on these Hcrt subpopulations and their projections. The results demonstrate that the overexpression of Cxcl12a has stimulatory effects similar to ethanol on the number of aAH and ectopic POA Hcrt neurons and the long anterior projections from ectopic POA neurons and posterior projections from pAH neurons. They also demonstrate that knockdown of Cxcl12a blocks these effects of ethanol on the Hcrt subpopulations and projections, providing evidence supporting a direct role of this specific chemokine in mediating ethanol's stimulatory effects on embryonic development of the Hcrt system.

Heat induces multiomic and phenotypic stress propagation in zebrafish embryos

Heat alters biology from molecular to ecological levels, but may also have unknown indirect effects. This includes the concept that animals exposed to abiotic stress can induce stress in naive receivers. Here, we provide a comprehensive picture of the molecular signatures of this process, by integrating multiomic and phenotypic data. In individual zebrafish embryos, repeated heat peaks elicited both a molecular response and a burst of accelerated growth followed by a growth slowdown in concert with reduced responses to novel stimuli. Metabolomes of the media of heat treated vs. untreated embryos revealed candidate stress metabolites including sulfur-containing compounds and lipids. These stress metabolites elicited transcriptomic changes in naive receivers related to immune response, extracellular signaling, glycosaminoglycan/keratan sulfate, and lipid metabolism. Consequently, non-heat-exposed receivers (exposed to stress metabolites only) experienced accelerated catch-up growth in concert with reduced swimming performance. The combination of heat and stress metabolites accelerated development the most, mediated by apelin signaling. Our results prove the concept of indirect heat-induced stress propagation toward naive receivers, inducing phenotypes comparable with those resulting from direct heat exposure, but utilizing distinct molecular pathways. Group-exposing a nonlaboratory zebrafish line, we independently confirm that the glycosaminoglycan biosynthesis-related gene chs1 and the mucus glycoprotein gene prg4a, functionally connected to the candidate stress metabolite classes sugars and phosphocholine, are differentially expressed in receivers. This hints at the production of Schreckstoff-like cues in receivers, leading to further stress propagation within groups, which may have ecological and animal welfare implications for aquatic populations in a changing climate.

Effects of different acute stressors on the regulation of appetite genes in the carp (Cyprinus carpio L.) brain

Our understanding of the timing of stress responses and specific roles of different regulatory pathways that drive stress responses is incomplete. In particular, the regulation of appetite genes as a consequence of exposure to different stressors has not been studied in sufficient detail in fish. Therefore, a stress trial was conducted with koi carp, aiming at identifying typical effects of stress on regulation of appetite genes. The stressors tank manipulation, air exposure and feed rewarding were chosen. The responses to these stressors were evaluated 10, 30 and 60 min after the stressors were applied. Orexigenic and anorexigenic genes were investigated in four different brain regions (telencephalon, hypothalamus, optic tectum and rhombencephalon). The results show that, apart from the typical appetite regulation in the hypothalamus, the different brain regions also display pronounced responses of appetite genes to the different stressors. In addition, several genes in the serotonergic, dopaminergic and gaba-related pathways were investigated. These genes revealed that rearing in pairs of two and opening of the tank lid affected anorexigenic genes, such as cart and cck, which were not changed by air exposure or feed rewarding. Moreover, distress and eustress led to limited, but distinguishable gene expression pattern changes in the investigated brain regions.

Light conditions during Atlantic salmon embryogenesis affect key neuropeptides in the melanocortin system during transition from endogenous to exogenous feeding

During the first feeding period, fish will adapt to exogenous feeding as their endogenous source of nutrients is depleted. This requires the development of a functional physiological system to control active search for food, appetite, and food intake. The Atlantic salmon (Salmo salar) melanocortin system, a key player in appetite control, includes neuronal circuits expressing neuropeptide y (npya), agouti-related peptide (agrp1), cocaine- and amphetamine-regulated transcript (cart), and proopiomelanocortin (pomca). Little is known about the ontogeny and function of the melanocortin system during early developmental stages. Atlantic salmon [0-730 day degrees (dd)] were reared under three different light conditions (DD, continuous darkness; LD, 14:10 Light: Dark; LL, continuous light) before the light was switched to LD and the fish fed twice a day. We examined the effects of different light conditions (DD _LD , LD _LD , and LL _LD ) on salmon growth, yolk utilization, and periprandial responses of the neuropeptides npya1, npya2, agrp1, cart2a, cart2b, cart4, pomca1, and pomca2. Fish were collected 1 week (alevins, 830 dd, still containing yolk sac) and 3 weeks (fry, 991 dd, yolk sac fully consumed) into the first feeding period and sampled before (-1 h) and after (0.5, 1.5, 3, and 6 h) the first meal of the day. Atlantic salmon reared under DD _LD , LD _LD , and LL _LD had similar standard lengths and myotome heights at the onset of first feeding. However, salmon kept under a constant light condition during endogenous feeding (DD _LD and LL _LD ) had less yolk at first feeding. At 830 dd none of the neuropeptides analyzed displayed a periprandial response. But 2 weeks later, and with no yolk remaining, significant periprandial changes were observed for npya1, pomca1, and pomca2, but only in the LD _LD fish. This suggests that these key neuropeptides serve an important role in controlling feeding once Atlantic salmon need to rely entirely on active search and ingestion of exogenous food. Moreover, light conditions during early development did not affect the size of salmon at first feeding but did affect the mRNA levels of npya1, pomca1, and pomca2 in the brain indicating that mimicking natural light conditions (LD _LD ) better stimulates appetite control.

The microbiota promotes social behavior by modulating microglial remodeling of forebrain neurons

Host-associated microbiotas guide the trajectory of developmental programs, and altered microbiota composition is linked to neurodevelopmental conditions such as autism spectrum disorder. Recent work suggests that microbiotas modulate behavioral phenotypes associated with these disorders. We discovered that the zebrafish microbiota is required for normal social behavior and reveal a molecular pathway linking the microbiota, microglial remodeling of neural circuits, and social behavior in this experimentally tractable model vertebrate. Examining neuronal correlates of behavior, we found that the microbiota restrains neurite complexity and targeting of forebrain neurons required for normal social behavior and is necessary for localization of forebrain microglia, brain-resident phagocytes that remodel neuronal arbors. The microbiota also influences microglial molecular functions, including promoting expression of the complement signaling pathway and the synaptic remodeling factor c1q. Several distinct bacterial taxa are individually sufficient for normal microglial and neuronal phenotypes, suggesting that host neuroimmune development is sensitive to a feature common among many bacteria. Our results demonstrate that the microbiota influences zebrafish social behavior by stimulating microglial remodeling of forebrain circuits during early neurodevelopment and suggest pathways for new interventions in multiple neurodevelopmental disorders.

Reproductive roles of the vasopressin/oxytocin neuropeptide family in teleost fishes

The vertebrate nonapeptide families arginine vasopressin (AVP) and oxytocin (OXT) are considered to have evolved from a single vasopressin-like peptide present in invertebrates and termed arginine vasotocin in early vertebrate evolution. Unprecedented genome sequence availability has more recently allowed new insight into the evolution of nonapeptides and especially their receptor families in the context of whole genome duplications. In bony fish, nonapeptide homologues of AVP termed arginine vasotocin (Avp) and an OXT family peptide (Oxt) originally termed isotocin have been characterized. While reproductive roles of both nonapeptide families have historically been studied in several vertebrates, their roles in teleost reproduction remain much less understood. Taking advantage of novel genome resources and associated technological advances such as genetic modifications in fish models, we here critically review the current state of knowledge regarding the roles of nonapeptide systems in teleost reproduction. We further discuss sources of plasticity of the conserved nonapeptide systems in the context of diverse reproductive phenotypes observed in teleost fishes. Given the dual roles of preoptic area (POA) synthesized Avp and Oxt as neuromodulators and endocrine/paracrine factors, we focus on known roles of both peptides on reproductive behaviour and the regulation of the hypothalamic-pituitary-gonadal axis. Emphasis is placed on the identification of a gonadal nonapeptide system that plays critical roles in both steroidogenesis and gamete maturation. We conclude by highlighting key research gaps including a call for translational studies linking new mechanistic understanding of nonapeptide regulated physiology in the context of aquaculture, conservation biology and ecotoxicology.

Osmotic Stress Uncovers Correlations and Dissociations Between Larval Zebrafish Anxiety Endophenotypes

Larval zebrafish are often used to model anxiety disorders. However, since it is impossible to recapitulate the full complexity and heterogeneity of anxiety in this model, examining component endophenotypes is key to dissecting the mechanisms underlying anxiety. While individual anxiety endophenotypes have been examined in zebrafish, an understanding of the relationships between them is still lacking. Here, we investigate the effects of osmotic stress on a range of anxiety endophenotypes such as thigmotaxis, dark avoidance, light-dark transitions, sleep, night startle, and locomotion. We also report a novel assay for stress-induced anorexia that extends and improves on previously reported food intake quantification methods. We show that acute <30 min osmotic stress decreases feeding but has no effect on dark avoidance. Further, acute osmotic stress dose-dependently increases thigmotaxis and freezing in a light/dark choice condition, but not uniform light environmental context. Prolonged >2 h osmotic stress has similar suppressive effects on feeding while also significantly increasing dark avoidance and sleep, with weaker effects on thigmotaxis and freezing. Notably, the correlations between anxiety endophenotypes were dependent on both salt and dark exposure, with increased dissociations at higher stressor intensities. Our results demonstrate context-dependent effects of osmotic stress on diverse anxiety endophenotypes, and highlight the importance of examining multiple endophenotypes in order to gain a more complete understanding of anxiety mechanisms.

Larval Zebrafish as a Model for Mechanistic Discovery in Mental Health

Animal models are essential for the discovery of mechanisms and treatments for neuropsychiatric disorders. However, complex mental health disorders such as depression and anxiety are difficult to fully recapitulate in these models. Borrowing from the field of psychiatric genetics, we reiterate the framework of 'endophenotypes' - biological or behavioral markers with cellular, molecular or genetic underpinnings - to reduce complex disorders into measurable behaviors that can be compared across organisms. Zebrafish are popular disease models due to the conserved genetic, physiological and anatomical pathways between zebrafish and humans. Adult zebrafish, which display more sophisticated behaviors and cognition, have long been used to model psychiatric disorders. However, larvae (up to 1 month old) are more numerous and also optically transparent, and hence are particularly suited for high-throughput screening and brain-wide neural circuit imaging. A number of behavioral assays have been developed to quantify neuropsychiatric phenomena in larval zebrafish. Here, we will review these assays and the current knowledge regarding the underlying mechanisms of their behavioral readouts. We will also discuss the existing evidence linking larval zebrafish behavior to specific human behavioral traits and how the endophenotype framework can be applied. Importantly, many of the endophenotypes we review do not solely define a diseased state but could manifest as a spectrum across the general population. As such, we make the case for larval zebrafish as a promising model for extending our understanding of population mental health, and for identifying novel therapeutics and interventions with broad impact.

Collective behavior emerges from genetically controlled simple behavioral motifs in zebrafish

It is not understood how changes in the genetic makeup of individuals alter the behavior of groups of animals. Here, we find that, even at early larval stages, zebrafish regulate their proximity and alignment with each other. Two simple visual responses, one that measures relative visual field occupancy and one that accounts for global visual motion, suffice to account for the group behavior that emerges. Mutations in genes known to affect social behavior in humans perturb these simple reflexes in individual larval zebrafish and change their emergent collective behaviors in the predicted fashion. Model simulations show that changes in these two responses in individual mutant animals predict well the distinctive collective patterns that emerge in a group. Hence, group behaviors reflect in part genetically defined primitive sensorimotor “motifs,” which are evident even in young larvae.