Melanocyte stimulating hormone and melanin concentrating hormone may be structurally and evolutionarily related

Two melanotropic peptides, melanin concentrating hormone (MCH) and α-melanocyte stimulating hormone (α-MSH), exert opposing actions on melanosome (melanin granule) movements within teleost pigment cells, melanocytes (melanophores). MCH stimulates melanosome aggregation to the cell center whereas α-MSH stimulates pigment organelle dispersion out into the dendritic processes of the melanocytes. The actions of α-MSH are dependent upon extracellular calcium (Ca²⁺), whereas those of MCH are actually enhanced in the absence of the cation. At high concentrations (10^-5-10^-8 M) MCH also exhibits MSH-like activity (autoantagonism), an effect which is abolished in the absence of Ca²⁺. Therefore, MCH exhibits MCH-like as well as MSH-like activity depending on the presence or absence of extracellular Ca²⁺. An analogue of MCH, [Ala⁵, Cys¹⁰]MCH, has been synthesized which is totally devoid of MCH activity but still exhibits MSH-like activity. These results suggest that the two melanotropic peptides share some component of structural similarity and may be evolutionarily related.