Action of melanin-concentrating hormone (MCH) on teleost chromatophores

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 Ca 2+. In contrast to this, the leucophores of the medaka responded to MCH by the pigment dispersion but only when Ca 2+ 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).