The in vitro effects of synthetic salmon melanin-concentrating hormone (MCH) on chromatophores of four teleost species were studied. In the erythrophores of the platyfish (Xiphophorus maculatus) and the swordtail (Xiphophorus helleri), and in the xanthophores and amelanotic melanophores of the medaka (Oryzias latipes), pigment aggregation took place in response to MCH even in the absence of Ca2+. In contrast to this, the leucophores of the medaka responded to MCH by the pigment dispersion but only when Ca2+ was present. The motile iridophores of the blue damselfish (Chrysiptera cyanea), which play a predominant role in coloration and its changes, were not affected by the hormone. Pharmacological studies employing various blocking agents suggest that the pigment-aggregating action of MCH is probably mediated through specific receptors possessed by the erythrophores, xanthophores, or amelanotic melanophores, while the pigment-dispersing action on the leucophores might be revealed through the receptors for melanophore-stimulating hormone (MSH).
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