Analyses of visual pigments and G proteins coupled with them in light-sensitive chromatophores of teleost fishes

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1. In Nile tilapia erythrophores, pigment aggregation is caused by light at 400 and 600 nm and dispersion occurred in response to light at 500 nm. We found the expression of mRNAs of cone opsins, red, green and blue, in the chromatophores. Further neon tetra iridophores present in the lateral stripe skin are also light-sensitive, and light near 500 nm is most effective. RT-PCR showed the expression of mRNAs encoding rhodopsin and two kinds of cone opsins (Pi-green 1 and Pi green 2) in neon tetra skin where the iridophores exist. These mRNAs are also expressed in the lateral eyes. The cone opsin genes, Pi-green 1 and Pi-green 2, show high similarity with the g101 and g102 genes of unique green cone opsins (belonging to the MWS/LWS group) of the blind Mexican cavefish The identity of amino acid sequences between Pi-green 1 and Pi-green 2 is considered to be more than 90% The red opsin gene was not expressed in neon tetra skin.2. Using anti-opsin (MWS/LWS) and anti-rhodopsin antibodies, western blotting analysis also demonstrated the presence of opsins in the skin where iridophores exist, and the localization of the opsins in iridophores was confirmed by immunohistochemical analysis.3. The involvement of cyclic AMP in the phototransduction was indicated using various inhibitors and activators of enzymes participating in the transduction. That is, light stimulation was accompanied by the decrease in cAMP level in the light-sensitive chromatophores. The treatment of Nile tilapia etythrophores and neon tetra iridophores with bacteria toxins also suggested the important role of Gi in the phototransduction.4. We also confirmed the expression of Gi mRNA and Gi protein in neon tetra iridophores by RT-PCR and western blotting respectively. Although partial expression of Gt mRNA was confirmed, Gt protein was not detected in the iridophores of neon tetra.