Effect of 3 Euthanasia Methods on Serum Yield and Serum Cortisol Concentration in Zebrafish (Danio rerio)

Tricaine, eugenol and etomidate for repetitive procedural anesthesia in adult zebrafish, Danio rerio: effect on stress and behavior

Zebrafish has emerged as a popular animal model in biomedical research. Numerous procedures and interventions require occasionally repetitive anesthesia. Tricaine is the most frequently used anesthetic for zebrafish and its efficacy is well established. However, the safety and efficacy of other anesthetics used for zebrafish require further examination, especially regarding repetitive anesthesia. Hence, we compare three anesthetics: tricaine (150 mg/l), eugenol (55 mg/l) and etomidate (4 mg/l) in wildtype adult zebrafish with and without interventions in the form of intraperitoneal injections. Groups of fish receiving the injections are named as (+ injection). We quantify anesthesia induction and recovery times as well as swimming behavior and cortisol levels as indicators of stress. Swimming behavior is quantified with the novel tank method as tank preference and number of turnings. Adult zebrafish are randomly divided into seven groups; tricaine (n = 15), tricaine (+injection; n = 15), eugenol (n = 15), eugenol (+injection; n = 14), etomidate (n = 15), etomidate (+injection; n = 15) and sham (n = 10), and anesthetized until they reach stage 4 anesthesia, daily for 10 days. Following anesthesia induction, injection groups are given daily intraperitoneal injections with 0.9% saline (4 ml/kg) before transfer to a recovery tank to study the effect of a painful procedure (the intraperitoneal injection) during anesthesia on stress. The novel tank method is used for analyzing behavior at day 2 (beginning), day 5/6 (middle) and day 10 (end). Chronic stress is evaluated by whole-body cortisol measurement at the end of the 10-day experiment. Additionally, acute stress is evaluated by whole-body cortisol measurement 30 min after single anesthesia in five groups: tricaine (n = 5), eugenol (n = 5), etomidate (n = 5), sham (n = 5), and untreated controls (n = 5). We find that fish anesthetized with tricaine recover fast (~ 1.5 min) and show normal swimming behavior. Fish anesthetized with eugenol show recovery time (~ 2.5 min) and swimming behavior similar to that of fish receiving tricaine. Fish anesthetized with etomidate have the longest recovery time (~ 5.5 min) and exhibit stressed swimming behavior. Cortisol levels remain at similar levels. Our study supports the use of tricaine as the anesthetic-of-choice for repetitive anesthesia of short duration in zebrafish, followed by eugenol.

Evaluating the Toxicity Induced by Bisphenol F in Labeo rohita Fish Using Multiple Biomarker Approach

Bisphenol F (BPF) is an emerging contaminant extensively used in the pharmaceutical, chemical, and food industries, exerting deleterious effects on human and wildlife health. Therefore, the current study was conducted to assess the toxicity induced by BPF in rohu Labeo rohita using multiple biomarkers such as oxidative stress, activity of antioxidant enzymes, biochemical parameters, histology, and genotoxicity. Fish were separated into four groups (T1-T4). Group T1 served as a control (0 μg/L), while Groups T2, T3, and T4 were exposed to BPF concentrations of 600 μg/L, 1200 μg/L, and 1800 μg/L, respectively, for 21 days. Results showed a significant (p < 0.05) increase in oxidative biomarkers (thiobarbituric acid reactive substance [TBARS] and reactive oxygen species [ROS]), while the concentration of antioxidant biomarkers (peroxidase [POD], superoxide dismutase [SOD], reduced glutathione [GSH], and catalase) was significantly (p < 0.05) decreased with the rising concentration of BPF in the liver, gills, and kidney of fish. Significant reduction (p < 0.05) in biochemical parameters was measured from collected whole blood, including red blood cells (RBCs), hemoglobin (HGB), mean corpuscular HGB (MCH), MC volume (MCV), hematocrit (HCT), MC HGB concentration (MCHC), platelets, low-density lipoprotein (LDL), cholesterol, high-density lipoprotein (HDL), total proteins, very LDL (VLDL), albumin and globulin, while white blood cells (WBCs), neutrophils, triglycerides, aspartate aminotransferase (AST), blood glucose, and alanine transaminase (ALT) levels were increased significantly (p < 0.05). Comet assay showed the DNA damage potential of BPF in erythrocytes. Histological examination showed that exposure to BPF causes several degenerative effects in the soft tissues (gills, liver, and kidney) of treated fish. It is concluded that BPF induces deleterious effects via disruptions in histological, genotoxic, and biochemical alterations in several organs of exposed fish.

Tributyltin causes generational neurodevelopmental toxicity and the protective effect of folic acid in zebrafish

Tributyltin (TBT), a common organotin environmental pollutant, may pose a threat to human development during critical early-life periods. We aimed to assess the neurodevelopmental intergenerational toxicity of early-life exposure to TBT and the protective effect of DNA methyl donor folic acid (FA). Specifically, after early-life exposure (1-21 days post-fertilization, dpf) to TBT (0, 1, 10 and 100 ng/L), zebrafish (Danio rerio) were cultured in clean medium until sexual maturity. The exposed females were mated with unexposed males to produce embryos (F1). The F1 generation were cultured (4-120 hours post-fertilization, hpf) with and without 1 mmol/L FA. The neurotoxic effects of early-life TBT exposure for zebrafish and their offspring (F1) were significantly enhanced anxiety and reduced aggression, decreased gene expression of DNA methyltransferase in the brain and increased serotonin levels in the body. Moreover, the intergenerational neurodevelopmental toxicity, as manifested in the F1 generation, was attenuated by FA supplementation. In summary, early-life TBT exposure led to intergenerational neurodevelopmental deficits in zebrafish, and DNA methyl donors had a protective effect on F1 neurodevelopment, which can inform the prevention and treatment of intergenerational neurotoxicity due to organotin pollutants.

EVALUATION OF PLASMA CORTISOL AND LACTATE IN KOI (CYPRINUS CARPIO) EUTHANIZED BY IMMERSION IN TRICAINE METHANESULFONATE, CLOVE OIL, OR CARBON DIOXIDE GAS DISSOLVED IN WATER

Little research has taken place on the effect of euthanasia methods on biophysical and biochemical changes at the time of euthanasia in fish. These changes are used in multiple species to determine stress levels before death. Koi (Cyprinus carpio) are an important fish species often used in laboratory research, kept in backyard ponds, and managed in zoological and aquarium collections. The current study evaluated euthanasia of koi by immersion in 0.5 g/L tricaine methanesulfonate (MS-222) (n = 10), 0.5 g/L clove oil (n = 8), 1 g/L clove oil (n = 10), and CO2 (n = 7) on time to cessation of opercular movement, plasma lactate levels, and plasma cortisol levels. CO2 had the longest mean time to cessation of opercular movement, and MS-222 had the shortest (mean CO2: 24.9 min, range 13.18-31.35 min; MS-222: 2.68 min, range 1.33-4.5 min). The difference was not significant between any of the groups for plasma cortisol or lactate levels. MS-222 demonstrated the highest cortisol levels, and CO2 had the lowest (mean CO2: 108.7 ng/ml, range 33.9-195.8 ng/ml; MS-222: 650.6 ng/ml, range 77.3-2374.9 ng/ml). Average lactate levels were highest for 1 g/L clove oil and lowest for 0.5 g/L clove oil (mean 0.5 g/L clove oil: 5.1 mmol/L, range 1.8-8.1 mmol/L; 1 g/L clove oil: 7.4 mmol/L, range 5.6-10.5 mmol/L).

Binaprofen induces zebrafish liver injury via the mitochondrial pathway

Binaprofen (C18H23NO5) is a drug not commercially available that causes liver injury; however, the underlying mechanism is unknown. The aim of the present study was to determine the mechanism underlying binaprofen‑induced liver injury at the genetic level. Zebrafish were treated with binaprofen. Serum biomarkers [alanine transaminase (ALT), aspartate transaminase (AST) and lactate dehydrogenase (LDH)], malondialdehyde (MDA) and glutathione (GSH) content analysis, liver cell morphology examination, DAPI staining, electron microscopy, microarray analysis and reverse transcription‑quantitative (RT‑q)PCR were performed 12, 24 and 48 h post‑treatment to analyze the mechanism underlying binaprofen‑induced liver injury. Following exposure to binaprofen, zebrafish serum levels of ALT, AST and LDH increased; MDA content of liver tissue increased and GSH content decreased. Liver cells exhibited mild to moderate vacuolization and mitochondria exhibited vacuolization and disrupted cristae. Liver cell apoptosis rate increased. There were 190 common differentially expressed genes at 12, 24 and 48 h. Gene Ontology analysis showed that the function of downregulated genes was primarily associated with 'DNA replication', 'DNA metabolic process', 'cell cycle', 'cell redox homeostasis', 'mitochondrion' and 'lipid transport'. The function of upregulated genes was primarily associated with 'peroxisome proliferator', 'oxidation activity', 'peroxisome' and 'apoptosis'. Pathway analysis showed that downregulated genes were those pertaining to 'cell cycle', 'DNA replication', 'ribosome', 'spliceosome', 'pyrimidine metabolism', 'purine metabolism', upregulated genes were those pertaining to 'PPAR signaling pathway', 'p53 signaling pathway'; RT‑qPCR assay supported the microarray results. The mechanism underlying binaprofen‑induced liver injury was associated with lipid peroxidation and apoptosis. Binaprofen downregulated genes associated with lipid transport and anti‑apoptosis genes, upregulated pro‑apoptosis genes and induces liver cell injury via the mitochondrial signaling pathway.

Anesthesia Overdose Versus Rapid Cooling for Euthanasia of Adult Zebrafish

The rapid increase in zebrafish use needs to be accompanied by research into the refinement of procedures. The European (EU) Directive lists three possible euthanasia methods for fish: anesthetic overdose, electrical stunning, and concussion. However, for small fish such as zebrafish, concussion and electrical stunning are difficult to perform, leaving anesthetic overdose as the most used method. Our aim was to test the efficacy and side effects of anesthesia overdose using different anesthetics and the rapid cooling method to euthanize adult zebrafish. Adult mixed-sex AB zebrafish were randomly assigned to: 250 mg/L MS222; 20 mg/L propofol +100 mg/L lidocaine; 6 mg/L etomidate; 50 mg/L clove oil; and rapid cooling (water at 2°C-4°C). Two minutes after opercular movement ceased, animals were transferred into clean water for 20 min and recovery assessed, or decapitated and used for biochemical analysis of the gills, muscle, liver, and brain; for the histological analysis of the gills and muscle; or for the assessment of cortisol levels. No animal recovered; rapid cooling was the quickest and etomidate overdose was the slowest method to cease the opercular movements. There were no major differences between euthanasia methods regarding the biochemical or histological data. Cortisol levels were higher in the rapid cooling group, but only when compared with the propofol/lidocaine group. The use of a physical method of euthanasia, such as rapid cooling, is essential when chemicals, such as anesthetics, may interfere with postmortem analyses. Although anesthetic overdose can be used without major effects on the analyses conducted in this work, rapid cooling can be another option with the advantage of being simple to administer, easily available, affordable, and very quick; this decreases the potential duration of suffering, being more humane. Therefore, a change in EU legislation should be considered to include additional humane options for euthanasia, such as rapid cooling, for zebrafish and other small tropical fish.

Propofol Immersion As a Euthanasia Method for Adult Zebrafish (Danio Rerio)

The exponential rise of the zebrafish (Danio rerio) as a model organism in biomedical research has far outstripped our un- derstanding of basic husbandry and welfare for this species. As a case in point, here we investigate the efficacy and welfare impact of different euthanasia methods for zebrafish. Not only is a humane death central to welfare and the 3Rs, but stress during euthanasia can change scientific outcomes. However, the most frequently used methods of euthanasia have multiple shortcomings with regard to animal welfare and human safety. In this study, we propose the use of propofol for immersion euthanasia of adult zebrafish. Propofol has been known to rapidly induce anesthesia in many species, including zebrafish, but its efficacy as a euthanasia agent for zebrafish has not fully been explored. In this study, adult zebrafish were euthanized by immersion on one of 5 different preparations: ice bath, 250 ppm MS222, 600 ppm lidocaine hydrochloride, 100 ppm propofol, or 150 ppm propofol for 20 or 30 min. Display of aversive behaviors, time to loss of righting reflex, time to cessation of opercular movement, and time to recovery after transfer to clean tank water were assessed and recorded. Propofol at both concentrations induced loss of righting reflex and loss of opercular movement more quickly than did MS222 or lidocaine hydrochloride and caused no display of aversive behaviors as seen with ice bath or lidocaine exposure. However, fish exposed to propofol at either concentration for 20 min sometimes recovered, whereas a 30-min exposure was sufficient for euthanasia of all fish tested. These findings suggest that exposure to propofol for a duration of at least 30 min quickly and effectively euthanizes adult zebrafish without compromising end-of-life welfare.

Comparative transcriptome analysis reveals ectopic delta-5 and delta-6 desaturases enhance protective gene expression upon Vibrio vulnificus challenge in Tilapia (Oreochromis niloticus)

Background

Tilapia (Oreochromis niloticus) cultures are frequently infected by Vibrio vulnificus, causing major economic losses to production units. Previously, tilapia expressing recombinant delta-5 desaturase and delta-6 desaturase (D56) were found to be resistant to V. vulnificus infection. In this report, we profile the D56-mediated molecular changes underlying this resistance in tilapia. A comparative transcriptome analysis was performed on V. vulnificus-infected wild-type and D56-transgenic tilapia using Illumina’s sequencing-by-synthesis approach. Gene enrichment analysis on differentially expressed unigenes was performed, and the expression patterns were validated by real-time PCR.

Results

Comparative transcriptome analysis was performed on RNA-sequence profiles obtained from wild-type and D56-transgenic tilapia at 0, 6 and 24 h post-infection with V. vulnificaus. GO and KEGG gene enrichment analyses showed that D56 regulates several pathways and genes, including fatty acid (FA) metabolism associated, and inflammatory and immune response. Expression of selected FA metabolism-associated, inflammatory and immune responsive genes was validated by qPCR. The inflammatory and immune responsive genes that are modulated by FA-associated D56 likely contribute to the enhanced resistance against V. vulnificus infection in Tilapia.

Conclusions

Transcriptome profiling and filtering for two-fold change variation showed that 3795 genes were upregulated and 1839 genes were downregulated in D56-transgenic tilapia. These genes were grouped into pathways, such as FA metabolism, FA elongation, FA biosynthesis, biosynthesis of unsaturated FA, FA degradation, inflammation, immune response, and chemokines. FA-associated genes and immune-related genes were modulated by D56 at 6 h and 24 h post infection with V. vulnificus. The expression patterns of FA-related genes, inflammatory genes, antimicrobial peptide genes and immune responsive genes at 0, 3, 6, 12, 24 and 48 h post-infection suggests these genes are involved in the enhanced resistance of D56 transgenic tilapia to V. vulnificus.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07521-5.

Kisspeptin-1 regulates forebrain dopaminergic neurons in the zebrafish

The habenula is a phylogenetically conserved epithalamic structure, which conveys negative information via inhibition of mesolimbic dopamine neurons. We have previously shown the expression of kisspeptin (Kiss1) in the habenula and its role in the modulation of fear responses in the zebrafish. In this study, to investigate whether habenular Kiss1 regulates fear responses via dopamine neurons in the zebrafish, Kiss1 peptides were intracranially administered close to the habenula, and the expression of dopamine-related genes (th1, th2 and dat) were examined in the brain using real-time PCR and dopamine levels using LC–MS/MS. th1 mRNA levels and dopamine levels were significantly increased in the telencephalon 24-h and 30-min after Kiss1 administration, respectively. In fish administered with Kiss1, expression of neural activity marker gene, npas4a and kiss1 gene were significantly decreased in the ventral habenula. Application of neural tracer into the median raphe, site of habenular Kiss1 neural terminal projections showed tracer-labelled projections in the medial forebrain bundle towards the telencephalon where dopamine neurons reside. These results suggest that Kiss1 negatively regulates its own neuronal activity in the ventral habenula via autocrine action. This, in turn affects neurons of the median raphe via interneurons, which project to the telencephalic dopaminergic neurons.