Graduate School of Science and Engineering, Senshu University of Ishinomaki, Miyagi 986-8580, Japan
Effects of cortisol and RU486 on the blind side pigmentation in the Japanese flounder, Paralichthys olivaceus (total length: 80-200 mm), were investigated. Two intraperitoneal injections of cortisol (100 mg/kg body weight) suppressed the blind side pigmentation. Plasma cortisol level was significantly increased in cortisol-treated fish. On the other hand, RU486 (Mifepristone) treatment, oral administration (100 mg/kg body weight/day) for 6 weeks or two intraperitoneal injections (100 mg/kg body weight), promoted the pigmentation. Plasma cortisol was increased by the treatment. These results suggest that the increases in plasma ACTH and αMSH caused by activation of the hypothalamus-pituitary-interrenal axis induce the blind side pigmentation in juvenile and young Japanese flounder.