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Small teleost fish including zebrafish and medaka have been used as animal models in basic science research due to the relative ease of handling and transparency during embryogenesis. Current advances in genetic engineering and progress in disease genetics allowed utilization of these fish to study neurological diseases and psychiatric disorders. This review summarizes the advantages and disadvantages of using fish for neuropsychiatric research using primarily our own studies as examples. We discuss how fish belong to a class of vertebrates, are feasible for imaging, and include diverse species with multiple research possibilities yet to be discovered.

Parkinson's disease is a neurodegenerative disease associated with the degeneration of dopaminergic neurons in the substantia nigra. The PTEN-induced kinase 1 gene (PINK1) is responsible for recessive inherited familial Parkinson's disease (PARK6). Neither the function of PINK1 nor its role in the prevention of Parkinson's disease is fully understood. Gene disruption of PINK1 causes remarkably different phenotypes in animal models such as Drosophila melanogaster, zebrafish, and mouse, none of which recapitulate Parkinson's-disease-like symptoms. We established PINK1-gene-disrupted medaka fish. These mutant fish grew normally at first, then developed significant decrease in the frequency of spontaneous swimming movements in the late-adult stage. Although the mutants did not show any dopaminergic cell loss, the amount of 3,4-dihydroxyphenylacetic acid, a major metabolite of dopamine, decreased. Thus, PINK1 contributes to the maintenance of dopamine metabolism, even before the selective death of dopaminergic neurons. Our animal model is therefore a valuable tool to detect pathogenesis in Parkinson's patients in the early stages.

Parkinson's disease (PD) is the second most common neurodegenerative disease associated with the degeneration of dopaminergic neurons in the substantia nigra. To create a new model of PD, we used medaka (Oryzias latipes), a small teleost that has been used in genetics and environmental biology. We identified tyrosine hydroxylase (TH) immunopositive dopaminergic and noradrenergic fibers and neurons in the medaka brain. Following establishment of a method for counting the number of dopaminergic neurons and an assay for the evaluation of the medaka behavior, we exposed medaka to 1-methyl-4-phenyl-1,2,3,4-tetrahydropyridine (MPTP). The treatment of medaka at the larval stage, but not at adult stage, decreased the number of dopaminergic cells in the diencephalon and reduced spontaneous movement, which is reminiscent of human PD patients and other MPTP-induced animal PD models. Among TH(+) neurons in the medaka brain, only a specific cluster in the paraventricular area of the middle diencephalon was vulnerable to MPTP toxicity. Detailed examinations of medaka transiently exposed to MPTP at the larval stage revealed that the number of dopaminergic cells was not fully recovered at their adult stage. Moreover, the amounts of dopamine persistently decreased in the brain of these MPTP-treated fish. MPTP-treated medaka is valuable for modeling human PD.