Melanin concentrating hormone analogues: Contraction of the cyclic structure. II. Antagonist activity

Asp-Thr-Met-Arg-Cys-Met-Val-Gly-Arg-Val-Tyr-Arg-Pro-Cys-Trp-Glu-Val, melanin concentrating hormone (MCH), is a cyclic hormone possessing both MCH-like (melanin granule aggregating effect) and melanocyte stimulating hormone (MSH)-like (melanin granule dispersing effect) activities. Nine ring-contracted analogues were synthesized and characterized for their melanotropic activity on the fish (Synbranchus marmoratus) and frog (Rana pipiens) bioassays. In most cases, these analogues were totally devoid of MCH-like agonist activity, demonstrating the essential role of the disulfide bridge between residues 5 and 14 of the hormone. [Ala⁵, Cys¹⁰]MCH, for example, was totally devoid of MCH-like activity. This analogue, like α-MSH, however, antagonized the melanosome aggregating actions of MCH on fish melanocytes. The antagonistic activity of the analogue, like that of α-MSH, was Ca²⁺-dependent. Evidence suggested that this antagonism of MCH activity was related to the intrinsic MSH-like activity of the analogue. These results suggest that MCH and α-MSH may be structurally and, therefore, evolutionarily related.