Modeling psychiatric comorbid symptoms of epileptic seizures in zebrafish

Characterization of the zebrafish gabra1sa43718/sa43718 germline loss of function allele confirms a function for Gabra1 in motility and nervous system development

Mutation of the GABRA1 gene is associated with neurodevelopmental defects and epilepsy. GABRA1 encodes for the α1 subunit of the γ-aminobutyric acid type A receptor (GABAAR), which regulates the fast inhibitory impulses of the nervous system. Multiple model systems have been developed to understand the function of GABRA1, but these models have produced complex and, at times, incongruent data. Thus, additional model systems are required to validate and substantiate previous results. We sought to provide initial phenotypic analysis of a novel germline mutant allele. Our analysis provides a solid foundation for the future use of this allele to characterize gabra1 functionally and pharmacologically using zebrafish. We investigated the behavioral swim patterns associated with a nonsense mutation of the zebrafish gabra1 (sa43718 allele) gene. The sa43718 allele causes a decrease in gabra1 mRNA expression, which is associated with light induced hypermotility, one phenotype previously associated with seizure like behavior in zebrafish. Mutation of gabra1 was accompanied by decreased mRNA expression of gabra2, gabra3, and gabra5, indicating a reduction in the expression of additional α sub-units of the GABAAR. Although multiple sub-units were decreased, larvae continued to respond to pentylenetetrazole (PTZ), indicating that a residual GABAAR exists in the sa43718 allele. Proteomics analysis demonstrated that mutation of gabra1 is associated with abnormal expression of proteins that regulate synaptic vesicle fusion, vesicle transport, synapse development, and mitochondrial protein complexes. These data support previous studies performed in a zebrafish nonsense allele created by CRISPR/Cas9 and validate that loss of function mutations in the gabra1 gene result in seizure-like phenotypes with abnormal development of the GABA synapse. Our results add to the existing body of knowledge as to the function of GABRA1 during development and validate that zebrafish can be used to provide complete functional characterization of the gene.

Assessment of the Novel Anti-Seizure Potential of Validamycin A Using Zebrafish Epilepsy Model

Epilepsy is a prevalent neurological disorder characterized by recurrent seizures. Validamycin A (VA) is an antibiotic fungicide that inhibits trehalase activity and is widely used for crop protection in agriculture. In this study, we identified a novel function of VA as a potential anti-seizure medication in a zebrafish epilepsy model. Electroencephalogram (EEG) analysis demonstrated that VA reduced pentylenetetrazol (PTZ)-induced seizures in the brains of larval and adult zebrafish. Moreover, VA reduced PTZ-induced irregular movement in a behavioral assessment of adult zebrafish. The developmental toxicity test showed no observable anatomical alteration when the zebrafish larvae were treated with VA up to 10 µM within the effective range. The median lethal dose of VA in adult zebrafish was > 14,000 mg/kg. These results imply that VA does not demonstrate observable toxicity in zebrafish at concentrations effective for generating anti-seizure activity in the EEG and alleviating abnormal behavior in the PTZ-induced epileptic model. Furthermore, the effectiveness of VA was comparable to that of valproic acid. These results indicate that VA may have a potentially safer anti-seizure profile than valproic acid, thus offering promising prospects for its application in agriculture and medicine.

Berberine and hesperidin prevent the memory consolidation impairment induced by pentylenetetrazole in zebrafish

This study verified the effects of the natural compounds berberine and hesperidin on seizure development and cognitive impairment triggered by pentylenetetrazole (PTZ) in zebrafish. Adult animals were submitted to a training session in the inhibitory avoidance test and, after 10 minutes, they received an intraperitoneal injection of 25, 50, or 100 mg/kg berberine or 100 or 200 mg/kg hesperidin. After 30 minutes, the animals were exposed to 7.5 mM PTZ for 10 minutes. Animals were submitted to the test session 24 h after the training session to verify their cognitive performance. Zebrafish larvae were exposed to 100 µM or 500 µM berberine or 10 µM or 50 µM hesperidin for 30 minutes. After, larvae were exposed to PTZ and had the seizure development evaluated by latency to reach the seizure stages I, II, and III. Adult zebrafish pretreated with 50 mg/kg berberine showed a longer latency to reach stage III. Zebrafish larvae pretreated with 500 µM berberine showed a longer latency to reach stages II and III. Hesperidin did not show any effect on seizure development both in larvae and adult zebrafish. Berberine and hesperidin pretreatments prevented the memory consolidation impairment provoked by PTZ-induced seizures. There were no changes in the distance traveled in adult zebrafish pretreated with berberine or hesperidin. In larval stage, berberine caused no changes in the distance traveled; however, hesperidin increased the locomotion. Our results reinforce the need for investigating new therapeutic alternatives for epilepsy and its comorbidities.

The influence of acute dopamine transporter inhibition on manic-, depressive-like phenotypes, and brain oxidative status in adult zebrafish

Functional changes in dopamine transporter (DAT) are related to various psychiatric conditions, including bipolar disorder (BD) symptoms. In experimental research, the inhibition of DAT induces behavioral alterations that recapitulate symptoms found in BD patients, including mania and depressive mood. Thus, developing novel animal models that mimic BD-related conditions by pharmacologically modulating the dopaminergic signaling is relevant. The zebrafish (Danio rerio) has been considered a suitable vertebrate system for modeling BD-like responses, due to the well-characterized behavioral responses and evolutionarily conservation of the dopaminergic system of this species. Here, we investigate whether GBR 12909, a selective inhibitor of DAT, causes neurobehavioral alterations in zebrafish similar to those observed in BD patients. Behaviors were recorded after a single intraperitoneal (i.p.) administration of GBR 12909 at different doses (3.75, 7.5, 15 and 30 mg/kg). To observe temporal effects on behavior, swim path parameters were measured immediately after the administration period during 30 min. Locomotion, anxiety-like behavior, social preference, aggression, despair-like behavior, and oxidative stress-related biomarkers in the brain were measured 30 min post administration. GBR 12909 induced prominent effects on locomotor activity and vertical exploration during the 30-min period. Hyperactivity was observed in GBR 30 group after 25 min, while all doses markedly reduced vertical drifts. GBR 12909 elicited hyperlocomotion, anxiety-like behavior, decreased social preference, aggression, and induced depressive-like behavior in a behavioral despair task. Depending on the dose, GBR 12909 also decreased SOD activity and TBARS levels, as well as increased GR activity and NPSH content. Collectively, our novel findings show that a single GBR 12909 administration evokes neurobehavioral changes that recapitulate manic- and depressive-like states observed in rodents, fostering the use of zebrafish models to explore BD-like responses in translational neuroscience research.

Effects of N-acetylcysteine and acetyl-L-carnitine on acute PTZ-induced seizures in larval and adult zebrafish

BACKGROUND

Epilepsy is a prevalent neurological disease, affecting approximately 1-2% of the global population. The hallmark of epilepsy is the occurrence of epileptic seizures, which are characterized by predictable behavioral changes reflecting the underlying neural mechanisms of the disease. Unfortunately, around 30% of patients do not respond to current pharmacological treatments. Consequently, exploring alternative therapeutic options for managing this condition is crucial. Two potential candidates for attenuating seizures are N-acetylcysteine (NAC) and Acetyl-L-carnitine (ALC), as they have shown promising neuroprotective effects through the modulation of glutamatergic neurotransmission.

METHODS

This study aimed to assess the effects of varying concentrations (0.1, 1.0, and 10 mg/L) of NAC and ALC on acute PTZ-induced seizures in zebrafish in both adult and larval stages. The evaluation of behavioral parameters such as seizure intensity and latency to the crisis can provide insights into the efficacy of these substances.

RESULTS

Our results indicate that both drugs at any of the tested concentrations were not able to reduce PTZ-induced epileptic seizures. On the other hand, the administration of diazepam demonstrated a notable reduction in seizure intensity and increased latencies to higher scores of epileptic seizures.

CONCLUSION

Consequently, we conclude that, under the conditions employed in this study, NAC and ALC do not exhibit any significant effects on acute seizures in zebrafish.

Chemically-induced epileptic seizures in zebrafish: A systematic review

The use of zebrafish as a model organism is gaining evidence in the field of epilepsy as it may help to understand the mechanisms underlying epileptic seizures. As zebrafish assays became popular, the heterogeneity between protocols increased, making it hard to choose a standard protocol to conduct research while also impairing the comparison of results between studies. We conducted a systematic review to comprehensively profile the chemically-induced seizure models in zebrafish. Literature searches were performed in PubMed, Scopus, and Web of Science, followed by a two-step screening process based on inclusion/exclusion criteria. Qualitative data were extracted, and a sample of 100 studies was randomly selected for risk of bias assessment. Out of the 1058 studies identified after removing duplicates, 201 met the inclusion criteria. We found that the most common chemoconvulsants used in the reviewed studies were pentylenetetrazole (n = 180), kainic acid (n = 11), and pilocarpine (n = 10), which increase seizure severity in a dose-dependent manner. The main outcomes assessed were seizure scores and locomotion. Significant variability between the protocols was observed for administration route, duration of exposure, and dose/concentration. Of the studies subjected to risk of bias assessment, most were rated as low risk of bias for selective reporting (94%), baseline characteristics of the animals (67%), and blinded outcome assessment (54%). Randomization procedures and incomplete data were rated unclear in 81% and 68% of the studies, respectively. None of the studies reported the sample size calculation. Overall, these findings underscore the need for improved methodological and reporting practices to enhance the reproducibility and reliability of zebrafish models for studying epilepsy. Our study offers a comprehensive overview of the current state of chemically-induced seizure models in zebrafish, highlighting the common chemoconvulsants used and the variability in protocol parameters. This may be particularly valuable to researchers interested in understanding the underlying mechanisms of epileptic seizures and screening potential drug candidates in zebrafish models.

Characterization of the zebrafish gabra1sa43718/sa43718 germline loss of function allele confirms a function for Gabra1 in motility and nervous system development

Mutation of the GABRA1 gene is associated with neurodevelopmental defects and epilepsy. GABRA1 encodes for the α1 subunit of the gamma-aminobutyric acid type A receptor (GABAAR), which regulates the fast inhibitory impulses of the nervous system. Multiple model systems have previously been developed to understand the function of GABRA1 during development, but these models have produced complex and at times incongruent data. Thus, additional model systems are required to validate and substantiate previously published results. We investigated the behavioral swim patterns associated with a nonsense mutation of the zebrafish gabra1 (sa43718 allele) gene. The sa43718 allele causes a decrease in gabra1 mRNA expression, which is associated with light induced hypermotility, one phenotype associated with seizure like behavior in zebrafish. Mutation of gabra1 was accompanied by decreased mRNA expression of gabra2, gabra3, and gabra5, indicating a reduction in the expression of additional alpha sub-units of the GABAAR. Although multiple sub-units were decreased in total expression, larvae continued to respond to pentylenetetrazole (PTZ) indicating that a residual GABAAR exists in the sa43718 allele. Proteomics analysis demonstrated that nonsense mutation of gabra1 is associated with abnormal expression of proteins that regulate proton transport, ion homeostasis, vesicle transport, and mitochondrial protein complexes. These data support previous studies performed in a zebrafish nonsense allele created by CRISPR/Cas9 and validate that loss of function mutations in the gabra1 gene result in seizure like phenotypes with abnormal function of inhibitory synapses.

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.

Trihydroxy piperlongumine protects aluminium induced neurotoxicity in zebrafish: Behavioral and biochemical approach

Aluminium (Al) is proven to be a potent environmental neurotoxin involved in progressive neurodegeneration. Al primarily induces oxidative stress by free radical generation in the brain, followed by neuronal apoptosis. Antioxidants are promising therapeutic options for Al toxicity. Piperlongumine is traditionally long known for its medicinal properties. Therefore, the present study has been designed to explore the antioxidant role of trihydroxy piperlongumine (THPL) against Al-induced neurotoxicity in the zebrafish model. Zebrafish exposed to AlCl₃ exhibited higher oxidative stress and altered locomotion. Adult fish displayed anxiety comorbid with depression phenotype. THPL increases antioxidant enzyme activity by quenching Al-induced free radicals and lipid peroxidation, thus minimizing oxidative damage in the brain. THPL rescues behavior deficits and improves anxiety-like phenotype in adult fish. Histological alterations caused by Al were also attenuated on administration with THPL. Results of the study demonstrate the neuroprotective role of THPL against Al-induced oxidative damage and anxiety, which could be exploited as a psychopharmacological drug.

The Effect of Adenosine Signaling on Memory Impairment Induced by Pentylenetetrazole in Zebrafish

Epilepsy is characterized by the manifestation of spontaneous and recurrent seizures. The high prevalence of comorbidities associated with epilepsy, such as cognitive dysfunction, affects the patients quality of life. Adenosine signaling modulation might be an effective alternative to control seizures and epilepsy-associated comorbidities. This study aimed to verify the role of adenosine modulation on the seizure development and cognitive impairment induced by pentylenetetrazole (PTZ) in zebrafish. At first, animals were submitted to a training session in the inhibitory avoidance test and, after 10 min, they received an intraperitoneal injection of valproate, adenosine A₁ receptor agonist cyclopentyladenosine (CPA), adenosine A₁ receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), adenosine A_2A receptor antagonist ZM 241385, adenosine deaminase inhibitor erythro-9-(2-hydroxy-3-nony1)-adenine hydrochloride (EHNA) or the nucleoside transporter inhibitor dipyridamole. Thirty min after the intraperitoneal injection, the animals were exposed to 7.5 mM PTZ for 10 min, where they were evaluated for latency to reach the seizure stages (I, II, and III). Finally, 24 h after the training session, the animals were submitted to the inhibitory avoidance test to verify their cognitive performance during the test session. Valproate, CPA, and EHNA showed antiseizure effects and prevented the memory impairment induced by PTZ exposure. DPCPX, ZM 241385, and dipyridamole pretreatments caused no changes in seizure development; however, these drugs prevented memory impairment without altering locomotion. Our results reinforce the antiseizure effects of adenosine signaling and support the idea that the involvement of adenosine in memory processes may be a target for preventive strategies against cognitive impairment associated with epilepsy.

Zebrafish - The Neurobehavioural Model in Trend

Zebrafish (Danio rerio) is currently in vogue as a prevalently used experimental model for studies concerning neurobehavioural disorders and associated fields. Since the 1960s, this model has succeeded in breaking most barriers faced in the hunt for an experimental model. From its appearance to its high parity with human beings genetically, this model renders itself as an advantageous experimental lab animal. Neurobehavioural disorders have always posed an arduous task in terms of their detection as well as in determining their exact etiology. They are still, in most cases, diseases of interest for inventing or discovering novel pharmacological interventions. Thus, the need for a harbinger experimental model for studying neurobehaviours is escalating. Ensuring the same model is used for studying several neuro-studies conserves the results from inter-species variations. For this, we need a model that satisfies all the pre-requisite conditions to be made the final choice of model for neurobehavioural studies. This review recapitulates the progress of zebrafish as an experimental model with its most up-to-the-minute advances in the area. Various tests, assays, and responses employed using zebrafish in screening neuroactive drugs have been tabulated effectively. The tools, techniques, protocols, and apparatuses that bolster zebrafish studies are discussed. The probable research that can be done using zebrafish has also been briefly outlined. The various breeding and maintenance methods employed, along with the information on various strains available and most commonly used, are also elaborated upon, supplementing Zebrafish's use in neuroscience.

A novel behavioral paradigm to measure anxiety-like behaviors in zebrafish by the concomitant assessment of geotaxis and scototaxis

Pathological anxiety is a set of diseases characterized by specific clinical manifestations and the use of alternative models may provide novel insights in translational neurobehavioral research. In zebrafish, the separate performance of novel tank and light dark tests in different order to assess anxiety using a same animal may provide conflicting data due to the battery effect and/or time-drug-response and variability across tests. To improve data reliability, we aimed to characterize a novel behavioral paradigm to measure geotaxis and scototaxis as anxiety-like responses in the same trial. The novel apparatus consisted of four colored-compartments, with specific white- and black sections delimited in both bottom and upper areas of the tank. The main baseline responses of zebrafish in the novel apparatus were measured and animals were further exposed to modulators of anxiety. Zebrafish showed robust habituation to novelty stress during the 6-min trial with preference for the black section while exploring the top area. Fluoxetine (100 μg/L, 15 min) reduced geotaxis and scototaxis and ketamine (20 mg/L, 20 min) decreased geotaxis and increased the distance traveled in the black section while exploring the top, possibly due to the increased circling behavior. As anxiogenic modulators, conspecific alarm substance (3.5 mL/L, 5 min) exacerbated risk assessment, geotaxis, and scototaxis, whereas caffeine (10 mg/L, 15 min) increased geotaxis and exploration in the black section of the top area. Since important correlations were also found for relevant anxiety-like behaviors, our findings support the predictive validity of this novel paradigm to simultaneously assess geotaxis and scototaxis in zebrafish. Moreover, it fully adheres to the 3Rs principle of animal experimentation of reducing the number of subjects tested, execution time, also minimizing a potential battery effect.

Zebrafish as a potential non-traditional model organism in translational bipolar disorder research: Genetic and behavioral insights

Bipolar disorder (BD) is a severe and debilitating illness that affects 1-2% of the population worldwide. BD is characterized by recurrent and extreme mood swings, including mania/hypomania and depression. Animal experimental models have been used to elucidate the mechanisms underlying BD and different strategies have been proposed to assess BD-like symptoms. The zebrafish (Danio rerio) has been considered a suitable vertebrate system for modeling BD-like responses, due to the genetic tractability, molecular/physiological conservation, and well-characterized behavioral responses. In this review, we discuss how zebrafish-based models can be successfully used to understand molecular, biochemical, and behavioral alterations paralleling those found in BD. We also outline some advantages and limitations of this aquatic species to examine BD-like phenotypes in translational neurobehavioral research. Overall, we reinforce the use of zebrafish as a promising tool to investigate the neural basis associated with BD-like behaviors, which may foster the discovery of novel pharmacological therapies.

Optimization of a locomotion-based zebrafish seizure model

BACKGROUND

Locomotor assays in zebrafish have emerged as a screening test in early drug discovery for antiseizure compounds. However, parameters differ considerably between published studies, which may explain some discrepant results with (candidate) antiseizure medications.

NEW METHOD

We optimized a locomotor-based seizure assay in zebrafish with pentylenetetrazol (PTZ) as the pharmacological proconvulsant to generate a therapeutic window in which proconvulsant-treated zebrafish larvae could be discriminated from a non-treated control. To generate a reliable control, exposure time and concentration of valproate (VPA, anticonvulsant) was optimized.

RESULTS

Wells with one or three larvae show a similar PTZ dose-dependent increase in locomotion with less variability in motility for the latter. Zebrafish immersed in 10 mM PTZ showed a significant increase in movement with a sustained effect, without any indication of toxicity. Animals treated with 3 mM VPA showed the strongest reduction of PTZ-induced movement without toxicity. The decrease in PTZ-induced locomotion was greater after 18 h versus 2 h.

COMPARISON WITH EXISTING METHOD(S)

For the larval zebrafish PTZ-induced seizure model, varying experimental parameters have been reported in literature. Our results show that PTZ is often used at toxic concentrations, and we provide instead reliable conditions to quantify convulsant behaviour using an infrared-beam motility assay.

CONCLUSIONS

We recommend using three zebrafish larvae per well to quantify locomotion in 96-multiwell plates. Larvae should preferably be exposed to 10 mM PTZ for 1 h, consisting of 30 min acclimation and 30 min subsequent recording. As positive control for anticonvulsant activity, we recommend exposure to 3 mM VPA for 18 h before administration of PTZ.

Learning Deficits and Attenuated Adaptive Stress Response After Early-Life Seizures in Zebrafish

Early-life seizures (ELS) are often associated with the development of cognitive deficits. However, methods to predict and prevent these deficits are lacking. To increase the range of research models available to study cognitive consequences of ELS, we investigated whether seizures in larval zebrafish (Danio rerio) lead to behavioral deficits later in life. We thus modified the existing pentylenetetrazole (PTZ)-induced seizure model in larval zebrafish, exposing zebrafish to PTZ daily from 5 to 7 days post-fertilization (dpf). We then compared later-life learning, social behavior (shoaling), and behavioral and chemical measures of anxiety in the PTZ-exposed zebrafish (PTZ group) to that of naïve clutchmates (untouched controls, UC) and to a second control group (handling control, HC) that experienced the same handling as the PTZ group, but without PTZ exposure. We observed that only the PTZ group displayed a significant deficit in a y-maze learning task, while only the HC group displayed a social deficit of decreased shoaling. HC fish also showed an increased frequency of behavioral freezing and elevated cortisol responses to netting, heightened stress responses not seen in the PTZ fish. Since mild stressors, such as the handling the HC fish experienced, can lead to learned, advantageous responses to stress later in life, we tested escape response in the HC fish using an acoustic startle stimulus. The HC group showed an enhanced startle response, swimming significantly farther than either the PTZ or UC group immediately after being startled. Taken together, these results indicate that seizures in larval zebrafish impair learning and the development of an adaptive, heightened stress response after early-life stress. These findings expand the behavioral characterization of the larval zebrafish seizure model, strengthening the power of this model for ELS research.

Taurine prevents MK-801-induced shoal dispersion and altered cortisol responses in zebrafish

Schizophrenia is a chronic neuropsychiatric disorder characterized by a shortened lifespan and significant impaired social and vocational functioning. Schizophrenic patients can present hypothalamic-pituitary-adrenal (HPA) axis dysfunctions and cortisol dysregulation, which play an important role on the etiology onset, exacerbation, and relapsing of symptoms. Based on its intrinsic neuroprotective properties, taurine is considered a promising substance with beneficial role on various brain disorders, including schizophrenia. Here, we evaluated the effects of taurine on shoaling behavior and whole-body cortisol levels in zebrafish treated with dizocilpine (MK-801), which elicits schizophrenia-like phenotypes in animal models. Briefly, zebrafish shoals (4 fish per shoal) were exposed to dechlorinated water or taurine (42, 150, or 400 mg/L) for 60 min. Then, saline (PBS, pH 7.4 or 2.0 mg/kg MK-801) were intraperitoneally injected and zebrafish behavior was recorded 15 min later. In general, MK-801 disrupted shoaling behavior and reduced whole-body cortisol levels in zebrafish. All taurine pretreatments prevented MK-801-induced increase in shoal area, while 400 mg/L taurine prevented the MK-801-induced alterations in neuroendocrine responses. Moreover, all taurine-pretreated groups showed increased geotaxis, supporting a modulatory role in the overall dispersion pattern of the shoal. Collectively, our novel findings show a potential protective effect of taurine on MK-801-induced shoal dispersion and altered neuroendocrine responses, fostering the use of zebrafish models to assess schizophrenia-like phenotypes.

Anticonvulsant activity of melatonin and its success in ameliorating epileptic comorbidity-like symptoms in zebrafish

Epilepsy is one of common neurological disorders, greatly distresses the well-being of the sufferers. Melatonin has been used in clinical anti-epileptic studies, but its effect on epileptic comorbidities is unknown, and the underlying mechanism needs further investigation. Herein, by generating PTZ-induced zebrafish seizure model, we carried out interdisciplinary research using neurobehavioral assays, bioelectrical detection, molecular biology, and network pharmacology to investigate the activity of melatonin as well as its pharmacological mechanisms. We found melatonin suppressed seizure-like behavior by using zebrafish regular locomotor assays. Zebrafish freezing and bursting activity assays revealed the ameliorative effect of melatonin on comorbidity-like symptoms. The preliminary screening results of neurobehavioral assays were further verified by the expression of key genes involved in neuronal activity, neurodevelopment, depression and anxiety, as well as electrical signal recording from the midbrain of zebrafish. Subsequently, network pharmacology was introduced to identify potential targets of melatonin and its pathways. Real-time qPCR and protein-protein interaction (PPI) were conducted to confirm the underlying mechanisms associated with glutathione metabolism. We also found that melatonin receptors were involved in this process, which were regulated in response to melatonin exposure before PTZ treatment. The antagonists of melatonin receptors affected anticonvulsant activity of melatonin. Overall, current study revealed the considerable ameliorative effects of melatonin on seizure and epileptic comorbidity-like symptoms and unveiled the underlying mechanism. This study provides an animal model for the clinical application of melatonin in the treatment of epilepsy and its comorbidities.

Predictable chronic stress modulates behavioral and neuroendocrine phenotypes of zebrafish: Influence of two homotypic stressors on stress-mediated responses

The zebrafish (Danio rerio) has been considered a suitable model organism to assess the evolutionarily conserved bases of behavioral and neuroendocrine responses to stress. Depending on the nature of the stressor, prolonged stress may elicit habituation or evoke long-term changes in the central nervous systems (CNS) often associated with various neuropsychiatric disorders. Conspecific alarm substance (CAS) and net chasing (NC) constitute chemical and physical stressors, respectively, which cause aversive behaviors and physiological changes in fishes. Here, we investigate whether predictable chronic stress (PCS) using two homotypic stressors differently modulates behavioral and physiological responses in zebrafish. PCS-CAS or PCS-NC were performed for 14 days, 2-times daily, while locomotion, exploratory activity, anxiety-like behaviors, and whole-body cortisol levels were measured on day 15. PCS-CAS reduced distance traveled, the number of transitions and time in top area, as well as increased the latency to enter the top in the novel tank test. In the light/dark test, CAS-exposed fish showed decreased time spent in lit area, shorter latency to enter the dark area, and increased risk assessments. PCS-CAS also increased whole-body cortisol levels in zebrafish. Although PCS-NC reduced the latency to enter the dark area, whole-body cortisol levels did not change. Moreover, acute experiments revealed that both CAS and NC promoted anxiogenesis and increased cortisol levels, suggesting habituation to stress following PCS-NC. Overall, our novel findings demonstrate that PCS induces behavioral and physiological changes in zebrafish depending on the nature of the stressor.

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

Anxiety is the most prevalent brain disorder and a common cause of human disability. Animal models are critical for understanding anxiety pathogenesis and its pharmacotherapy. The zebrafish (Danio rerio) is increasingly utilized as a powerful model organism in anxiety research and anxiolytic drug screening. High similarity between human, rodent and zebrafish molecular targets implies shared signaling pathways involved in anxiety pathogenesis. However, mounting evidence shows that zebrafish behavior can be modulated by drugs beyond conventional anxiolytics or anxiogenics. Furthermore, these effects may differ from human and/or rodent responses, as such 'unconventional' drugs may affect zebrafish behavior despite having no such profiles (or exerting opposite effects) in humans or rodents. Here, we discuss the effects of several putative unconventional anxiotropic drugs (aspirin, lysergic acid diethylamide (LSD), nicotine, naloxone and naltrexone) and their potential mechanisms of action in zebrafish. Emphasizing the growing utility of zebrafish models in CNS drug discovery, such unconventional anxiety pharmacology may provide important, evolutionarily relevant insights into complex regulation of anxiety in biological systems. Albeit seemingly complicating direct translation from zebrafish into clinical phenotypes, this knowledge may instead foster the development of novel CNS drugs, eventually facilitating innovative treatment of patients based on novel 'unconventional' targets identified in fish models.

Stress increases susceptibility to pentylenetetrazole-induced seizures in adult zebrafish

Stress is the body's reaction to any change that requires adaptive responses. In various organisms, stress is a seizure-related comorbidity. Despite the exposure to stressors eliciting aversive behaviors in zebrafish, there are no data showing whether stress potentiates epileptic seizures in this species. Here, we investigated whether a previous exposure to an intense acute stressor positively modulates the susceptibility to seizures in pentylenetetrazole (PTZ)-challenged zebrafish. The conspecific alarm substance (CAS) was used to elicit aversive responses (3.5 mL/L for 5 min), observed by increased bottom dwelling and erratic movements. Then, fish were immediately exposed to 7.5 mM PTZ for 10 min to induce seizure-like behaviors. Stress increased the seizure intensity, the number of clonic-like seizure behaviors (score 4), as well as facilitated the occurrence of score 4 episodes by decreasing the latency in which fish reached the score 4. Moreover, fish with heightened anxiety showed increased susceptibility to PTZ, since positive correlations between anxiety- and seizure-like behaviors were found. Overall, since CAS also increased whole-body cortisol levels in zebrafish, our novel findings show a prominent response to PTZ-induced seizures in previously stressed zebrafish. Moreover, we reinforce the growing utility of zebrafish models to assess seizure-related comorbidities aiming to elucidate how stress can affect epileptic seizures in vertebrates.

Comparison of three administration modes for establishing a zebrafish seizure model induced by N-Methyl-D-aspartic acid

BACKGROUND

Epilepsy is a complex neurological disorder characterized by recurrent, unprovoked seizures resulting from the sudden abnormal discharge of brain neurons. It leads to transient brain dysfunction, manifested by abnormal physical movements and consciousness. It can occur at any age, affecting approximately 65 million worldwide, one third of which are still estimated to suffer from refractory seizures. There is an urgent need for further establishment of seizure models in animals, which provides an approach to model epilepsy and could be used to identify novel anti-epileptic therapeutics in the future.

AIM

To compare three administration modes for establishing a seizure model caused by N-Methyl-D-aspartic acid (NMDA) in zebrafish.

METHODS

Three administration routes of NMDA, including immersion, intravitreal injection and intraperitoneal injection, were compared with regard to their effects on inducing seizure-like behaviors in adult zebrafish. We evaluated neurotoxicity by observing behavioral changes in zebrafish and graded those behaviors with a seizure score. In addition, the protective effects of MK-801 (Dizocilpine) and natural active constituent resveratrol against NMDA-induced alterations were studied.

RESULTS

The three NMDA-administration methods triggered different patterns of the epileptic process in adult zebrafish. Seizure scores were increased after increasing NMDA concentration regardless of the mode of administration. However, the curve of immersion continuously rose to a high plateau (after 50 min), while the curves of intravitreal injection and intraperitoneal injection showed a spike in the early stage (10-20 min) followed by a steady decrease in seizure scores. Furthermore, pretreatment with resveratrol and MK-801 significantly delayed seizure onset time and lowered seizure scores.

CONCLUSION

By comparing the three methods of administration, intravitreal injection of NMDA was the most suitable for establishing an acute epileptic model in zebrafish. Thus, intraperitoneal injection in zebrafish can be applied to simulate diseases such as epilepsy. In addition, NMDA immersion may be an appropriate method to induce persistent seizures. Moreover, MK-801 and resveratrol showed strong anti-epileptic effects; thus, both of them may be clinically valuable treatments for epilepsy.