Effects of atropine on the melanophores and light-reflecting chromatophores of some teleost fishes

Abstract 1. The mechanism of the action of atropine, which is known to accelerate the dispersion response of fish melanophores, was examined by use of various receptor antagonists. 2. The effects of atropine were found to be independent of adenosine receptors, beta-adrenoceptors and MSH receptors on the melanophore membrane. 3. Analogs of atropine, such as scopolamine, also had a potent pigment-dispersing effect on melanophores, whereas the quaternary ammonium derivatives, which are positively charged molecules, had only a small effect. 4. These results suggest that the possible site of atropine action is within the chromatophores themselves. 5. In addition to the melanosome-dispersing effect, atropine caused a shift in the spectral peak of reflected light toward shorter wavelengths and the dispersion of leucosomes in the motile iridophores of the blue damselfish and in the leucophores of the medaka, respectively.