In vivo Magnetic Resonance Microscopy and Hypothermic Anaesthesia of a Disease Model in Medaka

Individual identification of inbred medaka based on characteristic melanophore spot patterns on the head

With disease progression, individual differences appear, even in an animal disease model with genetic homogeneity. Therefore, non-invasive long term observation and individual identification is desirable for late-onset diseases. To this end, the natural markings used in ecological studies are preferable to the external invasive markings used in animal husbandry and fisheries management. Here, we propose using the distribution pattern of melanophore spots on the head of an inbred strain of medaka, a small fish model organism with monotonous pigmentation, as biometric identifier. Long term and variation analyses show different patterns whose characteristics can be attributed to individual animals. These findings were also valid in a non-inbred medaka strain and will help individual follow-up of late-onset disease medaka models for the elucidation of the pathogenesis and drug discovery.

Intact in vivo visualization of telencephalic microvasculature in medaka using optical coherence tomography

To date, various human disease models in small fish-such as medaka (Oryzias lapties)-have been developed for medical and pharmacological studies. Although genetic and environmental homogeneities exist, disease progressions can show large individual differences in animal models. In this study, we established an intact in vivo angiographic approach and explored vascular networks in the telencephalon of wild-type adult medaka using the spectral-domain optical coherence tomography. Our approach, which required neither surgical operations nor labeling agents, allowed to visualize blood vessels in medaka telencephala as small as about 8 µm, that is, almost the size of the blood cells of medaka. Besides, we could show the three-dimensional microvascular distribution in the medaka telencephalon. Therefore, the intact in vivo imaging via optical coherence tomography can be used to perform follow-up studies on cerebrovascular alterations in metabolic syndrome and their associations with neurodegenerative disease models in medaka.

Usefulness of adult medaka fish as a model for the evaluation of alcoholic fatty liver

Alcohol has long been acknowledged to be one of the main causes of hepatic disorders. In recent years, with the advancements in antiviral therapies, the relative proportion that alcoholic liver disease contributes among liver diseases has increased, necessitating the establishment of a useful model for the elucidation of the mechanism of its development. In this study, we developed a model of alcoholic liver disease using medaka, a small-sized fish known for its usefulness as a model organism. After rearing medaka in water containing ethanol for 2 months, fat deposition was observed in their livers. In addition, on the basis of the metabolomic analysis of the liver to evaluate metabolic changes resulting from ethanol administration, the increases in ethanol metabolites and changes in lipid metabolism were assessed. As minimally invasive evaluation methods, transparent medaka enabled the macroscopic evaluation of the progression of alcoholic fatty liver, while ultrasonography enabled the quantification of the fatty deposition of the liver. Furthermore, intestinal microbiota, the composition of which is important for the development of alcoholic liver disease, was evaluated. Microbiota changes similar to those of humans with alcoholic liver disease were observed. This study demonstrates that the development of liver disease and its amelioration through drugs can be easily evaluated using the present model or modifications thereof. Thus, this study is expected to be useful in the elucidation of liver disease development.

Assessment of high-fat-diet-induced fatty liver in medaka

Fatty liver, which has been continuously becoming more common in a number of patients, is the most common liver disease. For detailed analysis, a useful model for fatty liver is needed and fish are considered as a potential candidate. We assessed through direct observation of the liver, which is the most conventional method for non-invasive analysis of progression in fatty liver. By using transparent medaka (Oryzias latipes), we were able to observe changes in fat deposition in the liver. An analysis of the progression of fatty liver using ultrasound showed a significant increase in echo intensity, which indicates that this is a useful examination method. In addition, we clarified a metabolite profile in the medaka liver fed a high-fat diet (HFD), which had not previously been shown in detail. This medaka model, allowing non-invasive and repetitive assessment, is a useful model for the analysis of diseases that cause fatty liver in which changes in detailed metabolites are identified.

Summary: Our medaka model allows for non-invasive and repetitive assessment and is useful in the analysis of fatty liver in which changes in detailed metabolites are identified.