Unconventional anxiety pharmacology in zebrafish: Drugs beyond traditional anxiogenic and anxiolytic spectra

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.

Probiotics as technological innovations in psychiatric disorders: patents and research reviews

Probiotics have shown promising results in treating anxiety and depression by modulating the gut-brain-microbiota axis using probiotics, which has motivated increasing commercial and academic interest in innovations and the probiotic market. This work explored innovation trends in the use of probiotics in the management of anxiety and depression, through a patent search performed in the Espacenet patent database. To expand the discussion, an additional search was performed on ClinicalTrials.gov and ScienceDirect. Recently probiotic innovations developed were deposited as pharmaceutical products (24.1%), functional foods (20.4%), or both (51.8%). Probiotic strains showed anxiolytic, antidepressant or both effects related to one or more mechanisms including modulation of neurotransmitters (61.1%), neuroendocrine mediators (35.2%) or neuroinflammation and oxidative stress (20.3%). The effects mainly were related to strains of the Lactobacillus (48.1%) and Bifidobacterium (38.9%) genera. In an additional search, 1,945 scientific publications and 11 clinical trials were found. Despite the efficacy observed in preclinical and clinical studies, transitioning from academic discoveries to patented innovations is not always straightforward. This review provides evidence for therapeutic applications of novel probiotic technologies in treating psychiatric disorders. It supports further studies exploring their benefits and highlights the need for greater investment in innovation in this area.

Trace amine signaling in zebrafish models: CNS pharmacology, behavioral regulation and translational relevance

Tyramine, β-phenylethylamine, octopamine and other trace amines are endogenous substances recently recognized as important novel neurotransmitters in the brain. Trace amines act via multiple selective trace amine-associated receptors (TAARs) of the G protein-coupled receptor family. TAARs are expressed in various brain regions and modulate neurotransmission, neuronal excitability, adult neurogenesis, cognition, mood, locomotor activity and olfaction. Disrupted trace amine circuits have been implicated in various clinical neuropsychiatric disorders, including schizophrenia, Parkinson's disease, addiction, depression and anxiety. Dysregulated TAAR signaling has been linked in rodents to altered dopamine and serotonin neurotransmission, known to be associated with these psychiatric conditions. Complementing rodent genetic and pharmacological evidence, zebrafish (Danio rerio) are rapidly becoming a novel powerful model system in translational neuropharmacology research. Here, we review trace amine/TAAR neurobiology in zebrafish and discuss their developing translational utility as pharmacological and genetic models for unraveling the role of trace amines in CNS processes and brain disorders.

Rearing in a Physically Enriched Environment Affects Shoaling and Stress Responses of Zebrafish (Danio rerio) Exposed to Novel Conditions

The impact of enrichment on stress reduction in zebrafish (Danio rerio) exposed to a novel environment was assessed. Four control shoals (CTRL) and five treated shoals (TRT), each with eight fish, were observed; in TRT tanks, a PVC pipe was included (three-way tube, 11.7 × 4 cm) as enrichment for 90 days. Subsequently, fish were moved to a new tank for a shoaling test, and behavior was evaluated over periods of 0'-5' and 5'-10'. Cortisol dissolved in water was measured before and after the test. No differences were found between the two groups in distance moved, swimming speed, or shoal acceleration. Both groups reduced interindividual distance in the second phase of the test (CTRL: t = 8.977, p ≤ 0.0001; TRT: t = 8.247, p ≤ 0.0001), though TRT fish maintained greater spacing (t = 2.292, p ≤ 0.05). TRT fish spent more time without contact during both phases (first: t = 2.645, p ≤ 0.05; second: t = 3.134, p ≤ 0.01), while CTRL fish reduced this time in the second phase (t = 2.991, p ≤ 0.05). Cortisol rose significantly in CTRL after the test (t = 2.452, p ≤ 0.05) but not in TRT fish. These results suggest that environmental enrichment mitigates stress, as seen by reduced cohesiveness and cortisol in TRT fish.

Anxiety modulators elicit different behavioral outcomes in adult zebrafish: Emphasis on homebase-related parameters and spatio-temporal exploration

Anxiety is an emotion that represents a crucial anticipatory reaction of aversive stimuli, with clinical relevance in cases of disproportional and severe occurrences. Although distinct animal models have contributed to elucidate anxiety-related mechanisms, the influence of anxiogenic and anxiolytic modulations on both locomotion and exploration-related parameters in the open field test (OFT) is not fully elucidated. Here, we aimed to assess the influence of anxiogenic and anxiolytic manipulations on the exploratory dynamics of adult zebrafish (Danio rerio) focusing on homebase-related behaviors. As anxiogenic manipulations, we used the morphine (1.5 mg/L) withdrawal protocol (MOR); 3.5 mL/L conspecific alarm substance (CAS) for 5 min; and 100 mg/L caffeine (CAF) for 15 min. To evoke anxiolytic-like responses, animals were acutely exposed to 0.5 % (v/v) ethanol (ETOH) for 1 h; 100 μg/L fluoxetine (FLU) for 15 min; and 0.006 mg/L clonazepam (CZP) for 10 min. Then, fish were individually exposed to the 30-min OFT trial, with posterior analysis of behavioral activity. While MOR induced hyperlocomotion and increased periphery occupancy, CAS and CAF groups showed higher immobility and increased latency to homebase formation, respectively. Conversely, ETOH and FLU reduced homebase occupancy, supporting anxiolytic-like behaviors, while CZP did not change zebrafish behavior in the OFT. Cluster analysis was used to reconfirm the remarkable similarities and discrepancies between treatments, thus contributing to characterize the distinct responses measured. Overall, our novel data show the relevance of homebase-related analysis as a sensitive tool to reflect affective-like states in zebrafish, providing innovative approaches to unravel the spatio-temporal dynamics of anxiety-like behaviors in vertebrates.

Ketamine modulates the exploratory dynamics and homebase-related behaviors of adult zebrafish

Anxiety can be a protective emotion when animals face aversive conditions, but is commonly associated with various neuropsychiatric disorders when pathologically exacerbated. Drug repurposing has emerged as a valuable strategy based on utilizing the existing pharmaceuticals for new therapeutic purposes. Ketamine, traditionally used as an anesthetic, acts as a non-competitive antagonist of the glutamate N-methyl-d-aspartate (NMDA) receptor, and shows potential anxiolytic and antidepressant effects at subanesthetic doses. However, the influence of ketamine on multiple behavioral domains in vertebrates is not completely understood. Here, we evaluated the potential modulatory effect of ketamine on the spatio-temporal exploratory dynamics and homebase-related behaviors in adult zebrafish using the open field test (OFT). Animals were exposed to subanesthetic concentrations of ketamine (0, 2, 20, and 40 mg/L) for 20 min and their locomotion-, exploration- and homebase-related behaviors were assessed in a single 30-min trial. Our data revealed that acute ketamine (20 and 40 mg/L) induced hyperlocomotion, as verified by the increased total distance traveled. All concentrations tested elicited circling behavior, a stereotyped-like response which gradually reduced across the periods of test. We also observed modulatory effects of ketamine on the spatio-temporal exploratory pattern, in which the reduced thigmotaxis and homebase activity, associated with the increased average length of trips, suggest anxiolytic-like effects. Collectively, our findings support the modulatory effects of ketamine on the spatio-temporal exploratory activity, and corroborate the utility of homebase-related measurements to evaluate the behavioral dynamics in zebrafish models.

Sex, drugs, and zebrafish: Acute exposure to anxiety-modulating compounds in a modified novel tank dive test

This study investigated the effects of anxiogenic and anxiolytic drugs on zebrafish (Danio rerio) behaviour using a modified novel tank dive test with higher walls and a narrower depth. Zebrafish were administered chondroitin sulfate, beta-carboline, delta-9-tetrahydrocannabinol (THC), ethanol, and beta-caryophyllene, and their behaviours were evaluated for geotaxis, swimming velocity, and immobility. Both anxiogenic and anxiolytic compounds generally increased bottom-dwelling behaviour, suggesting that the tank's modified dimensions significantly influence zebrafish responses. EC50 values for ethanol showed a lower threshold for velocity reduction compared to zone preference. Chondroitin sulfate uniquely caused a sex-specific increase in male swimming velocity, whereas no other sex-differences were observed with any compound. Interestingly, the presence of drug-treated fish did not alter the behaviour of observer fish, suggesting limited social buffering effects. The findings underscore the complexity of zebrafish behavioural phenotypes and highlight the need for considering tank dimensions and multiple behavioural parameters to accurately assess the effects of anxiety-modulating drugs. This study demonstrates the utility of the modified novel tank dive test in providing nuanced insights into the behavioural effects of different pharmacological agents in zebrafish.

A larval zebrafish model of cardiac physiological recovery following cardiac arrest and myocardial hypoxic damage

Contemporary cardiac injury models in zebrafish larvae include cryoinjury, laser ablation, pharmacological treatment and cardiac dysfunction mutations. Although effective in damaging cardiomyocytes, these models lack the important element of myocardial hypoxia, which induces critical molecular cascades within cardiac muscle. We have developed a novel, tractable, high throughput in vivo model of hypoxia-induced cardiac damage that can subsequently be used in screening cardioactive drugs and testing recovery therapies. Our potentially more realistic model for studying cardiac arrest and recovery involves larval zebrafish (Danio rerio) acutely exposed to severe hypoxia (PO2=5-7 mmHg). Such exposure induces loss of mobility quickly followed by cardiac arrest occurring within 120 min in 5 days post fertilization (dpf) and within 40 min at 10 dpf. Approximately 90% of 5 dpf larvae survive acute hypoxic exposure, but survival fell to 30% by 10 dpf. Upon return to air-saturated water, only a subset of larvae resumed heartbeat, occurring within 4 min (5 dpf) and 6-8 min (8-10 dpf). Heart rate, stroke volume and cardiac output in control larvae before hypoxic exposure were 188±5 bpm, 0.20±0.001 nL and 35.5±2.2 nL/min (n=35), respectively. After briefly falling to zero upon severe hypoxic exposure, heart rate returned to control values by 24 h of recovery. However, reflecting the severe cardiac damage induced by the hypoxic episode, stroke volume and cardiac output remained depressed by ∼50% from control values at 24 h of recovery, and full restoration of cardiac function ultimately required 72 h post-cardiac arrest. Immunohistological staining showed co-localization of Troponin C (identifying cardiomyocytes) and Capase-3 (identifying cellular apoptosis). As an alternative to models employing mechanical or pharmacological damage to the developing myocardium, the highly reproducible cardiac effects of acute hypoxia-induced cardiac arrest in the larval zebrafish represent an alternative, potentially more realistic model that mimics the cellular and molecular consequences of an infarction for studying cardiac tissue hypoxia injury and recovery of function.

Effect of Cinnamaldehyde Chalcone on Behavior in Adult Zebrafish (Danio rerio): In Silico Approach

The study focuses on the anxiolytic potential of chalcone (2E,4E)-1-(2-hydroxyphenyl)-5-phenylpenta-2,4-dien-1-one (CHALCNM) in adult zebrafish. Successfully synthesized in 58 % yield, CHALCNM demonstrated no toxicity after 96 h of exposure. In behavioral tests, CHALCNM (40 mg/kg) reduced locomotor activity and promoted less anxious behavior in zebrafish, confirmed by increased permanence in the light zone of the aquarium. Flumazenil reversed its anxiolytic effect, indicating interaction with GABAA receptors. Furthermore, CHALCNM (4 and 20 mg/kg) preserved zebrafish memory in inhibitory avoidance tests. Virtual screening and ADMET profile studies suggest high oral bioavailability, access to the CNS, favored by low topological polarity (TPSA≤75 Å2) and low incidence of hepatotoxicity, standing out as a promising pharmacological agent against the GABAergic system. In molecular coupling, CHALCNM demonstrated superior affinity to diazepam for the GABAA receptor. These results reinforce the therapeutic potential of CHALCNM in the treatment of anxiety, highlighting its possible future clinical application.

Jnk1 and downstream signalling hubs regulate anxiety-like behaviours in a zebrafish larvae phenotypic screen

Current treatments for anxiety and depression show limited efficacy in many patients, indicating the need for further research into the underlying mechanisms. JNK1 has been shown to regulate anxiety- and depressive-like behaviours in mice, however the effectors downstream of JNK1 are not known. Here we compare the phosphoproteomes from wild-type and Jnk1-/- mouse brains and identify JNK1-regulated signalling hubs. We next employ a zebrafish (Danio rerio) larvae behavioural assay to identify an antidepressant- and anxiolytic-like (AA) phenotype based on 2759 measured stereotypic responses to clinically proven antidepressant and anxiolytic (AA) drugs. Employing machine learning, we classify an AA phenotype from extracted features measured during and after a startle battery in fish exposed to AA drugs. Using this classifier, we demonstrate that structurally independent JNK inhibitors replicate the AA phenotype with high accuracy, consistent with findings in mice. Furthermore, pharmacological targeting of JNK1-regulated signalling hubs identifies AKT, GSK-3, 14-3-3 ζ/ε and PKCε as downstream hubs that phenocopy clinically proven AA drugs. This study identifies AKT and related signalling molecules as mediators of JNK1-regulated antidepressant- and anxiolytic-like behaviours. Moreover, the assay shows promise for early phase screening of compounds with anti-stress-axis properties and for mode of action analysis.

Assessing the exploratory profile of two zebrafish populations: influence of anxiety-like phenotypes and independent trials on homebase-related parameters and exploration

Anxiety is a protective behavior when animals face aversive conditions. The open field test (OFT) is used to assess the spatio-temporal dynamics of exploration, in which both homebase formation and recognition of environmental cues may reflect habituation to unfamiliar conditions. Because emotional- and affective-like states influence exploration patterns and mnemonic aspects, we aimed to verify whether the exploratory behaviors of two zebrafish populations showing distinct baselines of anxiety differ in two OFT sessions. Firstly, we assessed the baseline anxiety-like responses of short fin (SF) and leopard (LEO) populations using the novel tank test (NTT) and light-dark test (LDT) in 6-min trials. Fish were later tested in two consecutive days in the OFT, in which the spatial occupancy and exploratory profile were analyzed for 30min. In general, LEO showed pronounced diving behavior and scototaxis in the NTT and LDT, respectively, in which an "anxiety index" corroborated their exacerbated anxiety-like behavior. In the OFT, the SF population spent less time to establish the homebase in the 1^st trial, while only LEO showed a markedly reduction in the latency to homebase formation in the 2^nd trial. Both locomotion and homebase-related activities were decreased in the 2^nd trial, in which animals also revealed increased occupancy in the center area of the apparatus. Moreover, we verified a significant percentage of homebase conservation for both populations, while only SF showed reduced the number of trips and increased the average length of trips. Principal component analyses revealed that distinct factors accounted for total variances between trials for each population tested. While homebase exploration was reduced in the 2^nd trial for SF, an increased occupancy in the center area and hypolocomotion were the main factors that contribute to the effects observed in LEO during re-exposure to the OFT. In conclusion, our novel data support the homebase conservation in zebrafish subjected to independent OFT sessions, as well as corroborate a population-dependent effect on specific behavioral parameters related to exploration.

The anxiolytic and circadian regulatory effect of agarwood water extract and its effects on the next generation; zebrafish modelling

Anxiety is one of the most common psychiatric symptoms worldwide. Studies show that there is an increase of >25 % in the prevalence of anxiety with the onset of the COVID-19 pandemic process. Due to the various side effects of drugs used in the treatment of anxiety, interest in natural therapeutic alternatives has increased. Agarwood is a plant used as a natural therapeutic due to its sedative effect as well as many effects such as antioxidant and antibacterial. Although there are many studies with agarwood, comprehensive behavioral studies, including the next generation, are limited. In present study, zebrafish fed with diets containing 10-100 ppm water extract of Agarwood (AWE) for 3 and 8 weeks were exposed to predator stress using Oscar fish in order to test the potential anxiolytic effect of AWE. At the end of the period, zebrafish exposed to predator stress were subjected to anxiety and circadian tests. Histopathological evaluation and immunofluorescent analyzes of BDNF and 5HT4-R proteins were performed in the brains of zebrafish. The effects on the next generation were examined by taking offspring from zebrafish. According to the results, it was observed that AWE had a healing effect on anxiety-like behaviors and on the disrupted circadian rhythm triggered by the predatory stress it applied, especially in the 8 weeks 100 ppm group. Interestingly, it was also found to be effective in offspring of zebrafish fed diets with AWE.

Expanding the use of homebase-related parameters to investigate how distinct stressful conditions affect zebrafish behaviors

Stress is a physiological reaction that allows the organisms to cope with challenging situations daily. Thus, elucidating the behavioral outcomes following different stressors is of great importance in translational research. Here, we aimed to characterize the main factors which explain similarities and differences of two stress protocols on zebrafish exploratory activity. To answer this point, we performed behavioral analyses aiming to simplify the data structure associated with homebase-related measurements in an integrated manner. Adult zebrafish were exposed to conspecific alarm substance for 5 min (acute stress protocol - AS) or submitted to 7 days of unpredictable chronic stress (UCS). Immediately after AS or in the subsequent day following UCS (8^th day), fish were individually tested in the open field and the behaviors were recorded for 30 min to posterior identification of homebase locations. For both protocols, behavioral clustering revealed two major clusters, grouping homebase- and locomotor-related parameters, respectively. While AS increased both positive and negative correlations between exploratory and locomotor endpoints, a significant increase in negative correlations was found in UCS-challenged fish. Comparison of the principal component analyses data set revealed a reduced exploratory activity using the homebase in AS group, while decreased locomotion in the periphery and anxiety-like behaviors were evidenced in UCS fish. In conclusion, our findings revealed a different structure of behavior in zebrafish following AS and UCS protocols, supporting the existence of distinct behavioral strategies to cope with acute and chronic stress. Furthermore, we expand the use of homebase-related measurements as a valuable tool to investigate complex behavioral modulations in future translational neuropsychiatry research.

Understanding CNS Effects of Antimicrobial Drugs Using Zebrafish Models

Antimicrobial drugs represent a diverse group of widely utilized antibiotic, antifungal, antiparasitic and antiviral agents. Their growing use and clinical importance necessitate our improved understanding of physiological effects of antimicrobial drugs, including their potential effects on the central nervous system (CNS), at molecular, cellular, and behavioral levels. In addition, antimicrobial drugs can alter the composition of gut microbiota, and hence affect the gut-microbiota-brain axis, further modulating brain and behavioral processes. Complementing rodent studies, the zebrafish (Danio rerio) emerges as a powerful model system for screening various antimicrobial drugs, including probing their putative CNS effects. Here, we critically discuss recent evidence on the effects of antimicrobial drugs on brain and behavior in zebrafish, and outline future related lines of research using this aquatic model organism.

The Tapping Assay: A Simple Method to Induce Fear Responses in Zebrafish

The zebrafish is increasingly employed in behavioral neuroscience as a translationally relevant model organism for human central nervous system disorders. One of the most prevalent CNS disorders representing an unmet medical need is the disorder cluster defined under the umbrella term anxiety disorders. Zebrafish have been shown to respond to a variety of anxiety and fear inducing stimuli and have been suggested for modeling human anxiety. Here, we describe a simple method with which we intend to induce fear/anxiety responses in this species. The method allows us to deliver a visual and lateral line stimulus (vibration or "tapping") to the fish with the use of a moving object, a ball colliding with the side glass of the experimental tank. We describe the hardware construction of the apparatus and the procedure of the behavioral paradigm. We also present data on how zebrafish respond to the tapping. Our results demonstrate that the method induces significant fear/anxiety responses. We argue that the simplicity of the method and the efficiency of the paradigm should make it popular among those who plan to use zebrafish as a tool in anxiety research.