NEOPLASTIC RESPONSE IN JAPANESE MEDAKA AND CHANNEL CATFISH EXPOSED TON-METHYL-N'-NITRO-N-NITROSOGUANIDINE

Japanese medaka (Oryzias latipes) and channel catfish (Ictalurus punct atus) were investigated for carcinogenic response following a 28-day, 3x/wk pulse exposure to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). F ive-wk-old medaka were exposed at concentrations of 0, 0.5, and 1.0 mg /L, and 5-mo-old catfish at concentrations of 0, 0.1, and 0.5 mg/L. In medaka, a total of 19 tumors including 2 branchioblastomas, 6 thyroid follicular adenomas and 1 adenocarcinoma and 11 subcutaneous fibrosar comas were observed in 16 of 96 MNNG-exposed fish. In catfish, a total of 37 tumors including 4 squamous cell carcinomas and 16 papillomas, 3 lipomas, 1 fibroma, 1 osteosarcoma, 4 branchioblastomas, 6 thymic ep ithelial tumors, and 2 generalized lymphosarcomas were observed in 34 of 172 MNNG-exposed fish. The induction of neoplasms in medaka was pri marily in the gill, thyroid, and subcutis of the cervical and trunk re gions, whereas in catfish skin, thymus, ore-pharynx, and hemopoietic t issues were also commonly affected. In both species, the neoplastic re sponse was considered to be related to direct exposure of the tissues to MNNG. Some of these tumors have not been reported in the literature in either natural or experimental fish. The results also suggest spec ies-specific differences in carcinogenic response following MNNG expos ure.