Usefulness of the medaka, Oryzias latipes, as a test animal: DNA repair processes in medaka exposed to carcinogens.

Results of previous studies indicate that the medaka, Oryzias latipes, a small aquarium fish found in Japan and neighboring countries, is a suitable animal for use in cancer research. This fish showed a high incidence of liver tumors after a short induction period when kept in water containing diethylnitrosamine. Unscheduled DNA synthesis (UDS) was demonstrated in ganglion cells of the medaka in vivo by autoradiography. For this experiment, part of the bony skull of the medaka was removed surgically, and the living fish was kept in an isotonic solution containing a carcinogen and tritiated thymidine. Its brain was then examined histologically. By this method, UDS was clearly demonstrated over the nuclei of ganglion cells after exposure to a wide variety of chemical carcinogens. The abdominal cavity was opened and treated with carcinogens in the same way as the brain; however, no UDS was detected in liver or intestinal cells. We conducted comparative studies on DNA repair in the brains of 5 species of small fish from widely different families. These 5 species (blond cave fish, Oriental weatherfish, medaka, guppy, and Siamese fighting fish) were exposed to various concentrations of 4-hydroxyaminoquinoline 1-oxide or methyl methanesulfonate. We detected UDS in all 5 species after both treatments, although the extent varied in the different species.