Methionine enkephalin-induced changes in pigmentation of zebrafish (Cyprinidae, Brachydanio rerio) and related species and varieties, measured videodensitometrically. I. Zebrafish.

The ability of MSH and of methionine enkephalin (met-E) to induce dispersion of pigment granules was examined in melanophores and in xanthophores of the zebrafish Brachydanio rerio using the melanophore index (MI) and videodensitometry. Both methods gave similar results. In B. rerio both MSH and met-E induced pigment dispersion in dermal melanophores, in fin and peritoneal melanophores, and in xanthophores. Darkening lasted a few hours. However, met-E-induced darkening developed 40-50 min later and faded more slowly than the effect of MSH. Both effects were dose related. Naloxone prevented met-E-induced darkening while it did not interfere with the MSH-induced effect. Epidermal melanophores did not react to either MSH or met-E. Thus met-E proved to induce changes of coloration when injected into a fish. Our data suggest a central mechanism involved in met-E-mediated change of coloration in zebrafish under the conditions examined. A new approach was suggested for objective measurement of the mean body darkness of the fish with the help of computational videodensitometry. Our fist results indicate a proportionality between the MI evaluation and videodensitometry.