Muscarinic acetylcholine receptors in the brain of the zebrafish (Danio rerio) measured by radioligand binding techniques

The Brilliance of the Zebrafish Model: Perception on Behavior and Alzheimer’s Disease

Alzheimer’s disease (AD) has become increasingly prevalent in the elderly population across the world. It’s pathophysiological markers such as overproduction along with the accumulation of amyloid beta (Aβ) plaques and neurofibrillary tangles (NFT) are posing a serious challenge to novel drug development processes. A model which simulates the human neurodegenerative mechanism will be beneficial for rapid screening of potential drug candidates. Due to the comparable neurological network with humans, zebrafish has emerged as a promising AD model. This model has been thoroughly validated through research in aspects of neuronal pathways analogous to the human brain. The cholinergic, glutamatergic, and GABAergic pathways, which play a role in the manifested behavior of the zebrafish, are well defined. There are several behavioral models in both adult zebrafish and larvae to establish various aspects of cognitive impairment including spatial memory, associative memory, anxiety, and other such features that are manifested in AD. The zebrafish model eliminates the shortcomings of previously recognized mammalian models, in terms of expense, extensive assessment durations, and the complexity of imaging the brain to test the efficacy of therapeutic interventions. This review highlights the various models that analyze the changes in the normal behavioral patterns of the zebrafish when exposed to AD inducing agents. The mechanistic pathway adopted by drugs and novel therapeutic strategies can be explored via these behavioral models and their efficacy to slow the progression of AD can be evaluated.

Zebrafish as a Promising Tool for Modeling Neurotoxin-Induced Alzheimer's Disease

Drug discovery and development for Alzheimer's disease (AD) are complex and challenging due to the higher failure rate in the drug development process. The overproduction and deposition of Aβ senile plaque and intracellular neurofibrillary tangle (NFT) formation are well-recognized diagnostic hallmarks of AD. Numerous transgenic models of Alzheimer's disease have restrictions on cost-effectiveness and time in the preclinical setup. Zebrafish has emerged as an excellent complementary model for neurodegenerative research due to simpler organisms with robust, clearly visible behavior forms. Glutaminergic and cholinergic pathways responsible for learning and memory are present in zebrafish and actively participate in the transmission process. Therefore, it is imperative to study neurotoxic agents' mechanisms that induce dysfunction of memory, learning, and neurons in the zebrafish. This review illustrates the in-depth molecular mechanism of several neurotoxic agents such as okadaic acid, cigarette smoke extract, and metals to produce cognitive deficits or neurodegeneration similar to mammals. These updates would determine an ideal and effective neurotoxic agent for producing AD pathophysiology in the zebrafish brain for preclinical screening.

The Role of Zebrafish and Laboratory Rodents in Schizophrenia Research

Schizophrenia is a severe disorder characterized by positive, negative and cognitive symptoms, which are still not fully understood. The development of efficient antipsychotics requires animal models of a strong validity, therefore the aims of the article were to summarize the construct, face and predictive validity of schizophrenia models based on rodents and zebrafish, to compare the advantages and disadvantages of these models, and to propose future directions in schizophrenia modeling and indicate when it is reasonable to combine these models. The advantages of rodent models stem primarily from the high homology between rodent and human physiology, neurochemistry, brain morphology and circuitry. The advantages of zebrafish models stem in the high fecundity, fast development and transparency of the embryo. Disadvantages of both models originate in behavioral repertoires not allowing specific symptoms to be modeled, even when the models are combined. Especially modeling the verbal component of certain positive, negative and cognitive symptoms is currently impossible.

Cholinergic Stimulation of the Adult Zebrafish Brain Induces Phosphorylation of Glycogen Synthase Kinase-3 β and Extracellular Signal-Regulated Kinase in the Telencephalon

The sequencing of the zebrafish genome, the emergence of powerful gene-editing tools, and the development of in vivo imaging techniques have propelled the economical zebrafish into prominence as a biomedical research model. Neurodegenerative disorders with a cholinergic component, such as Alzheimer's and Parkinson's diseases, are currently modeled using zebrafish. Still, the utility of zebrafish as a research model will not be fully realized until their neurophysiological properties are thoroughly characterized. In mammals, the coupling of cholinergic receptors to the phosphorylation of glycogen synthase kinase-3 β (GSK3β) and extracellular signal-regulated kinase 1/2 (ERK1/2) is of critical importance to cognitive processes and imparts protection against neuropathogenic events. Similarly, it is known that cholinergic receptors are required for learning and memory in zebrafish and that in vivo activation of cholinergic receptors induces transient changes in evoked synaptic transmission in the telencephalon. However, the intracellular events mediating cholinergic processes in zebrafish have yet to be elucidated. In the current study, an ex vivo drug treatment assay was used to demonstrate that carbachol (CCh)-mediated cholinergic stimulation of the intact adult zebrafish brain induces phosphorylation of GSK3β and ERK1/2 in the zebrafish telencephalon. These findings suggest GSK3β and ERK1/2 may underly cognitive processes in zebrafish.

Muscarinic acetylcholine receptor expression in brain and immune cells of Oreochromis niloticus

Nervous and immune systems maintain a bidirectional communication, expressing receptors for neurotransmitters and cytokines. Despite being well established in mammals, this has been poorly described in lower vertebrates as fishes. Experimental evidence shows that the neurotransmitter acetylcholine (ACh) regulates the immune response. In this research, we evaluated mRNA levels of muscarinic acetylcholine receptor (mAChR) in spleen mononuclear cells of Nile tilapia (Oreochromis niloticus) and compared the expression levels of immune cells with the brain. The mAChR subtypes (M2-M5A) were detected in both tissues, but mAChRs mRNA levels were higher in immune cells. This data have a potential use in biomedical and comparative immunology fields.

Understanding Central Nervous System Effects of Deliriant Hallucinogenic Drugs through Experimental Animal Models

Hallucinogenic drugs potently alter human behavior and have a millennia-long history of use for medicinal and religious purposes. Interest is rapidly growing in their potential as CNS modulators and therapeutic agents for brain conditions. Antimuscarinic cholinergic drugs, such as atropine and scopolamine, induce characteristic hyperactivity and dream-like hallucinations and form a separate group of hallucinogens known as "deliriants". Although atropine and scopolamine are relatively well-studied drugs in cholinergic physiology, deliriants represent the least-studied class of hallucinogens in terms of their behavioral and neurological phenotypes. As such, novel approaches and new model organisms are needed to investigate the CNS effects of these compounds. Here, we comprehensively evaluate the preclinical effects of deliriant hallucinogens in various animal models, their mechanisms of action, and potential interplay with other signaling pathways. We also parallel experimental and clinical findings on deliriant agents and outline future directions of translational research in this field.

Cholinergic Stimulation of the Adult Zebrafish Brain Induces Phosphorylation of Glycogen Synthase Kinase-3 β and Extracellular Signal-Regulated Kinase in the Telencephalon

The sequencing of the zebrafish genome, the emergence of powerful gene-editing tools, and the development of in vivo imaging techniques have propelled the economical zebrafish into prominence as a biomedical research model. Neurodegenerative disorders with a cholinergic component, such as Alzheimer's and Parkinson's diseases, are currently modeled using zebrafish. Still, the utility of zebrafish as a research model will not be fully realized until their neurophysiological properties are thoroughly characterized. In mammals, the coupling of cholinergic receptors to the phosphorylation of glycogen synthase kinase-3 β (GSK3β) and extracellular signal-regulated kinase 1/2 (ERK1/2) is of critical importance to cognitive processes and imparts protection against neuropathogenic events. Similarly, it is known that cholinergic receptors are required for learning and memory in zebrafish and that in vivo activation of cholinergic receptors induces transient changes in evoked synaptic transmission in the telencephalon. However, the intracellular events mediating cholinergic processes in zebrafish have yet to be elucidated. In the current study, an ex vivo drug treatment assay was used to demonstrate that carbachol (CCh)-mediated cholinergic stimulation of the intact adult zebrafish brain induces phosphorylation of GSK3β and ERK1/2 in the zebrafish telencephalon. These findings suggest GSK3β and ERK1/2 may underly cognitive processes in zebrafish.

Zebrafish Get Connected: Investigating Neurotransmission Targets and Alterations in Chemical Toxicity

Neurotransmission is the basis of neuronal communication and is critical for normal brain development, behavior, learning, and memory. Exposure to drugs and chemicals can alter neurotransmission, often through unknown pathways and mechanisms. The zebrafish (Danio rerio) model system is increasingly being used to study the brain and chemical neurotoxicity. In this review, the major neurotransmitter systems, including glutamate, GABA, dopamine, norepinephrine, serotonin, acetylcholine, histamine, and glutamate are surveyed and pathways of synthesis, transport, metabolism, and action are examined. Differences between human and zebrafish neurochemical pathways are highlighted. We also review techniques for evaluating neurological function, including the measurement of neurotransmitter levels, assessment of gene expression through transcriptomic analysis, and the recording of neurobehavior. Finally examples of chemical toxicity studies evaluating alterations in neurotransmitter systems in the zebrafish model are reviewed.

Behavioral screening for neuroactive drugs in zebrafish

The larval zebrafish has emerged asa vertebrate model system amenable to small molecule screens for probing diverse biological pathways. Two large-scale small molecule screens examined the effects of thousands of drugs on larval zebrafish sleep/wake and photomotor response behaviors. Both screens identified hundreds of molecules that altered zebrafish behavior in distinct ways. The behavioral profiles induced by these small molecules enabled the clustering of compounds according to shared phenotypes. This approach identified regulators of sleep/wake behavior and revealed the biological targets for poorly characterized compounds. Behavioral screening for neuroactive small molecules in zebrafish is an attractive complement to in vitro screening efforts, because the complex interactions in the vertebrate brain can only be revealed in vivo.

Zebrafish neurotransmitter systems as potential pharmacological and toxicological targets

Recent advances in neurobiology have emphasized the study of brain structure and function and its association with numerous pathological and toxicological events. Neurotransmitters are substances that relay, amplify, and modulate electrical signals between neurons and other cells. Neurotransmitter signaling mediates rapid intercellular communication by interacting with cell surface receptors, activating second messenger systems and regulating the activity of ion channels. Changes in the functional balance of neurotransmitters have been implicated in the failure of central nervous system function. In addition, abnormalities in neurotransmitter production or functioning can be induced by several toxicological compounds, many of which are found in the environment. The zebrafish has been increasingly used as an animal model for biomedical research, primarily due to its genetic tractability and ease of maintenance. These features make this species a versatile tool for pre-clinical drug discovery and toxicological investigations. Here, we present a review regarding the role of different excitatory and inhibitory neurotransmitter systems in zebrafish, such as dopaminergic, serotoninergic, cholinergic, purinergic, histaminergic, nitrergic, glutamatergic, glycinergic, and GABAergic systems, and emphasizing their features as pharmacological and toxicological targets. The increase in the global knowledge of neurotransmitter systems in zebrafish and the elucidation of their pharmacological and toxicological aspects may lead to new strategies and appropriate research priorities to offer insights for biomedical and environmental research.

Knockdown of zebrafish lumican gene (zlum) causes scleral thinning and increased size of scleral coats

The lumican gene (lum), which encodes one of the major keratan sulfate proteoglycans (KSPGs) in the vertebrate cornea and sclera, has been linked to axial myopia in humans. In this study, we chose zebrafish (Danio rerio) as an animal model to elucidate the role of lumican in the development of axial myopia. The zebrafish lumican gene (zlum) spans approximately 4.6 kb of the zebrafish genome. Like human (hLUM) and mouse (mlum), zlum consists of three exons, two introns, and a TATA box-less promoter at the 5'-flanking region of the transcription initiation site. Sequence analysis of the cDNA predicts that zLum encodes 344 amino acids. zLum shares 51% amino acid sequence identity with human lumican. Similar to hLUM and mlum, zlum mRNA is expressed in the eye and many other tissues, such as brain, muscle, and liver as well. Transgenic zebrafish harboring an enhanced GFP reporter gene construct downstream of a 1.7-kb zlum 5'-flanking region displayed enhanced GFP expression in the cornea and sclera, as well as throughout the body. Down-regulation of zlum expression by antisense zlum morpholinos manifested ocular enlargement resembling axial myopia due to disruption of the collagen fibril arrangement in the sclera and resulted in scleral thinning. Administration of muscarinic receptor antagonists, e.g. atropine and pirenzepine, effectively subdued the ocular enlargement caused by morpholinos in in vivo zebrafish larvae assays. The observation suggests that zebrafish can be used as an in vivo model for screening compounds in treating myopia.

The role of alpha7 and alpha4beta2 nicotinic receptors in the nicotine-induced anxiolytic effect in zebrafish

Zebrafish (Danio rerio) have been widely used to study the molecular mechanisms of development including neurodevelopment. More recently, they have begun to be used to study neuropharmacology and neurotoxicology. Critical for this line of research are methods to study behavioral function in zebrafish. Previous studies have compared zebrafish with mammalian models to determine similarities and differences in locomotor behavior, learning and memory. Relatively little research has been conducted on stress response and anxiety behavior as well as the pharmacologic response in zebrafish. We have developed a test for zebrafish to assess stress response and anxiety: the novel tank diving test. In this short test normally zebrafish dive to the bottom of a novel tank and then gradually over the 5-min test begin exploring higher levels of the tank. Nicotine, which has anxiolytic effects in rodents and humans was found to diminish this novel tank diving response in zebrafish. The current study examined the nicotinic receptor subtype selectivity involved in the actions of nicotine. Two nicotinic receptor subtype selective antagonists were used: MLA (an alpha7 antagonist) and DHbetaE (an alpha4beta2 antagonist). We replicated our previous finding of the anxiolytic effect of nicotine with significantly less bottom dwelling by the fish after nicotine treatment. This nicotine-induced anxiolytic effect was reversed by both MLA and DHbetaE, indicating that both nicotinic alpha7 and alpha4beta2 receptors are involved in the nicotinic effect on anxiety in zebrafish.

Non-associative learning in larval zebrafish

Habituation, where a response is reduced when exposed to a continuous stimulus is one of the simplest forms of non-associative learning and has been shown in a number of organisms from sea slugs to rodents. However, very little has been reported in the zebrafish, a model that is gaining popularity for high-throughput compound screens. Furthermore, since most of the studies involving learning and memory in zebrafish have been conducted in adults, we sought to determine if zebrafish larvae could display non-associative learning and whether it could be modulated by compounds identified in previous rodent studies. We demonstrated that zebrafish larvae (7 days post fertilization) exhibit iterative reduction in a startle response to a series of acoustic stimuli. Furthermore, this reduction satisfied criteria for habituation: spontaneous recovery, more rapid reductions in startle to shorter intertrial intervals and dishabituation. We then investigated the pathways mediating this behavior using established compounds in learning and memory. Administration of rolipram (PDE4 inhibitor), donepezil (acetylcholinesterase inhibitor), and memantine (N-methyl-D-aspartic acid (NMDA) receptor antagonist) all increased the acoustic startle response and decreased habituation in the larvae, similar to previous rodent studies. Further studies demonstrated that NMDA blocked the memantine response and the effect of donepezil was blocked by mecamylamine but not atropine suggesting that the donepezil response was mediated by nicotinic rather than muscarinic receptors. Zebrafish larvae possess numerous advantages for medium to high-throughput screening; the model described herein therefore offers the potential to screen for additional compounds for further study on cognition function.

Methanol alters ecto-nucleotidases and acetylcholinesterase in zebrafish brain

Methanol is a neurotoxic compound that is responsible for serious damage on CNS. Besides being found as an environmental contaminant, this alcohol is also employed as a component of cryoprotector solutions for zebrafish embryos. Here we tested the acute effect of methanol on ecto-nucleotidase (NTPDase, ecto-5'-nucleotidase) and acetylcholinesterase (AChE) activities in zebrafish brain. After acute treatment, there were significant decreases on ATP (26% and 45%) and ADP hydrolysis (26% and 30%) at 0.5% and 1.0%, respectively. However, no significant alteration on ecto-5'-nucleotidase activity was verified in zebrafish brain. A significant inhibition on AChE activity (39%, 33% and 30%) was observed at the range of 0.25% to 1.0% methanol exposure. Four NTPDase sequences were identified from phylogenetic analyses, which one is similar to NTPDase1 and the others to NTPDase2. Methanol was able to inhibit NTPDase1, two isoforms of NTPDase2 and AChE transcripts. To evaluate if methanol affects directly these enzymes activities, we have performed in vitro assays. ATP hydrolysis presented a significant inhibition (19% and 34%) at 1.5% and 3.0%, respectively, and ADP hydrolysis decreased only at 3.0% (29.2%). Nevertheless, AMP hydrolysis and AChE were not altered after in vitro exposure. The inhibitory effect observed on these enzymes could contribute to the neurodegenerative events promoted by methanol in zebrafish brain.