Some of the same mutations that cause epilepsy in humans can also cause epilepsy in zebrafish; we can use molecular genetic tools to investigate cellular mechanisms of disease; and we can now record whole-brain images of brain function at single-cell resolution. using light-sheet microscopy in zebrafish with epilepsy-causing mutations for this research stream. This allows us to record activity during and between seizures and attempt to reconstruct functional connections between different brain areas and even groups of neurons. Using a variety of cutting-edge network neuroscience tools, we will link the abnormal neuronal dynamics caused by the mutations to the type of EEG dynamic abnormalities seen in patients with corresponding epilepsies. This study aims to connect this novel powerful animal model to our understanding of epilepsy in patients in order to develop new strategies for seizure control.

Epilepsy is a neurological disorder characterised by abnormal, hypersynchronous electrical brain activity that results in abnormal movements or cognitive states. Epilepsy affects more than 50 million people worldwide, causing mortality, disability, and social and behavioural stigma. Despite the availability of more than 25 anti-epileptic drugs (AEDs), treatment response is frequently unpredictable, and approximately one-third of patients fail to achieve complete seizure control with pharmacotherapy alone. Epilepsy can be caused by a variety of factors, each with a different prognosis and treatment outcome, such as metabolic disorders, brain lesions, and autoimmune causes, as well as ‘idiopathic’ cases. However, the role of genetic mutations is now increasingly recognised in both (1) severe epilepsies of early childhood and infancy, particularly the early infantile epileptic encephalopathies (EIEEs)] and (2)’idiopathic’ generalised epilepsy. Consistently, in a recent nomenclature update, the ‘idiopathic’ generalised epilepsy syndromes were renamed genetic generalised epilepsies (GGEs). Neurotransmitters are chemical agents that carry signals across synapses in the brain, and the main inhibitory neurotransmitter in the brain is gamma aminobutyric acid (GABA).

Zebrafish, a well-known small vertebrate model in developmental biology, is now widely recognised as an important organism for modelling human diseases. Zebrafish, in particular, has emerged as a novel and promising model in the fields of epilepsy and drug discovery in the last decade. We hope to provide a comprehensive overview of the success and potential of zebrafish epilepsy research in this chapter. We describe zebrafish models of epilepsy and epileptic seizures caused by various pharmacological and genetic manipulations, with an emphasis on current methods for monitoring and characterising seizure-related activities. Furthermore, future directions in zebrafish epilepsy modelling and epilepsy research are discussed. Finally, we discuss the challenges and strategies for future translational research in the field of zebrafish epilepsy.

Despite its many advantages, zebrafish has some limitations in neuroscientific research that should be considered. To begin with, the benefit of using zebrafish as a medium- to high-throughput model is limited to embryos and larvae, and to a lesser extent juveniles. Due to their large size, adults are exempt. Second, while zebrafish are genetically similar to humans, they are evolutionarily more distinct than rodents. These distinctions can result in the so-called zebrafish annotation problem. The annotated activity of small molecules and/or targets is frequently based on mammalian studies, and while it is assumed that it will be similar in zebrafish, this is not always the case. These distinctions can result in the so-called zebrafish annotation problem. The annotated activity of small molecules and/or targets is frequently based on mammalian studies, and while it is assumed that it will be similar in zebrafish, this is not always the case. Furthermore, not all brain regions, such as the cortex, are as developed as they are in mammals. However, the lower complexity can be advantageous in unravelling mechanisms of action. Furthermore, because the blood–brain barrier (BBB) of zebrafish is only mature at 10 dpf, using larvae at 5–7 dpf for drug discovery can result in false positives. In terms of drug discovery, certain compounds can be difficult to administer to zebrafish, such as water immersion of highly water-insoluble compounds or uptake of compounds with low bioavailability. This can be avoided by solubilizing the drug or using different administration routes, such as injection (into the yolk sac during the embryonic and larval stages, or intraperitoneally or subcutaneously in adult fish), or oral. Nonetheless, due to water insolubility, certain active compounds are unlikely to be identified in a screening setting, which could lead to false negative results.

In terms of drug discovery, certain compounds can be difficult to administer to zebrafish, such as water immersion of highly water-insoluble compounds or uptake of compounds with low bioavailability. This can be avoided by solubilizing the drug or using different administration routes, such as injection (into the yolk sac during the embryonic and larval stages, or intraperitoneally or subcutaneously in adult fish), or oral. Nonetheless, due to water insolubility, certain active compounds are unlikely to be identified in a screening setting, which could lead to false negative results.

Since the first pioneering study on zebrafish as a new paradigm for seizure investigations, several groups have set out to engineer and explore epilepsy-related models in zebrafish using various chemical and genetic approaches. As a result, the zebrafish epileptic seizure and epilepsy model is rapidly progressing from “emerging” to “established.” This is not to say that zebrafish models are now replacing rodent models, as the latter are still considered the standard model organisms for understanding epilepsy and testing new medications. However, the similarity between human and zebrafish epilepsy-related genes and potential drug targets, as demonstrated by recent studies and discovery work, places zebrafish inevitably in a prominent position in preclinical research. Furthermore, no other organism has the ability to combine large-scale testing of numerous compounds on an intact and human-relevant whole organism, as well as the ability to rapidly investigate the function of new epilepsy genes in vivo. Such research is important in the short term for gaining a better understanding of the disease, but it can also help to improve patient therapy in the long run. Indeed, it is expected that the refractory background of human disease can be introduced in zebrafish models, allowing for the screen-based discovery of efficacious medication. In fact, there is already a first example of this hyphenated genetic/discovery approach. The use of the scn1lab mutant zebrafish model, which mimics DS, a severe treatment-resistant epilepsy syndrome that begins within the first year of life, has already resulted in the discovery of clemizole as a potential treatment. Furthermore, similar research has confirmed the activity of fenfluramine, a compound that, as an add-on treatment in a recent clinical study, resulted in seizure freedom in 7 of 10 DS children.