Analyses of the regulatory mechanisms of motile activities by factors other than neurotransmitters and hormones in teleost fish chromatophores

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1.Nile tilapia erythrophores respond directly to light at 365 nm, 400nm and 600nm by pigment aggregation, while light at 500 nm induces pigment dispersion. Using cultured erythrophores isolated from tail fins, the effects of pertussis toxin, cholera toxin, Ca^<2+> and Mg^<2+>-free K solution etc. on the photoresponses were examined. Pigment aggregation and dispersion responses were suggested to couple to Gi and Gs, respectively. KT-PCR showed the expression of mRNAs encoding red and green opsins in tilapia fins, only where erythrophores exist.2.Light-sensitive iridophores (light-reflecting chromatophores) in the skin of neon tetra were also analyzed. The photoresponses of these cells depended on the light intensity, and light near 500 nm was most effective in inducing the shift in wavelengths of reflected light toward longer wavelengths. The decrease in cyclic AMP may be related in the photoresponse. RT-PCR showed the expression of rhodopsin mRNA in the skin of neontetra, in addition to retina. However, the mRNA expression was not detected in muscle tissues. Cloning of cDNA of neon tetra rhodopsin was also performed.3.Using split tail-fin preparations and cultured chromatophores, the effects of endothelin(ET) on fish chromatophores were examined. ET-1,2 and 3 were found to cause pigment aggregation in melanophores and the shift in wavelengths of reflected light toward longer wavelengths in iridophores, which may be triggered by the decrease in cAMP via Gi. In leucophores, dispersion responses were induced probably mediated by Gs. By pharmacological analyses and immunofluorescence technique, it was suggested that ET_B receptors were present in the chromatophore membrane.