Melanocyte-stimulating hormone facilitates hypermelanosis on the non-eyed side of the barfin flounder, a pleuronectiform fish

A pleuronectiform fish, the barfin flounder, Verasper moseri, is promising for aquaculture and resource enhancement in Northern Japan due to its high commercial value. Hypermelanosis of its non-eyed side, which frequently occurs under culture conditions, diminishes its commercial value. Two peptide hormones, melanin-concentrating hormone (MCH) and melanocyte-stimulating hormone (MSH), having opposing actions, are associated with the color changes of fish. We have previously reported the positive effect of MCH in preventing hypermelanosis. Here, we examined the effects of MSH on the occurrence of hypermelanosis. A single injection of Des-Ac-α-MSH [0.01 nmol/g–10 nmol/g (0.016 μg–16 μg/g)] did not change the eyed-side body color, while a single injection of MCH [0.1 nmol/g (0.21 μg/g)] made the eyed-side color paler. No difference was observed in eyed-side lightness between fish injected with MCH (0.1 nmol/g) and those receiving MCH (0.1 nmol/g) and an increased amount of Des-Ac-α-MSH (0.01 nmol/g–10 nmol/g) simultaneously. These results indicate that MSH does not suppress the in vivo body color-paling effects of MCH in barfin flounders. On the other hand, implantation of a cholesterol pellet containing Des-Ac-α-MSH (280 μg, twice at 29-day interval) increased hypermelanosis of the non-eyed side of barfin flounders compared to control fish. Eyed-side bodies of MSH-treated fish were darker than control fish; thus, MSH is involved in morphological color change including ectopic melanin synthesis in non-eyed-side skin.