The cloning of the melanocyte-stimulating hormone (MSH) and adrenocorticotropic hormone (ACTH) receptors (MC1-R and MC2-R, respectively) recently has led to the identification of three additional melanocortin receptors, MC3-R, MC4-R, and MC5-R. The MC2 receptor primarily recognizes only ACTH peptides, but the other four receptors all recognize alpha-melanocyte-stimulating hormone (alpha-MSH) and potent alpha-MSH agonists such as [Nle(4),D-Phe(7)]alpha-MSH-NH2 and Ac-Nle(4)-c[Asp(5),D-Phe(7),Lys(10)]alpha-MSH-(4-10)-NH2 as well as ACTH. The absence of any known physiological role for these new receptors, expressed both in the brain (MC3-R and MC4-R) and throughout a number of peripheral tissues (MC5-R), has necessitated a search for potent and receptor selective agonists and antagonists. We report here that analogues of the superpotent cyclic agonist analogue Ac-Nle(4)-c[Asp(5),D-Phe(7),Lys(10)]alpha-MSH-(4-10)NH2, in which a bulky aromatic amino acid is substituted in the 7-position, can produce potent and selective antagonists for melanocortin receptors. Thus, the D-p-iodophenylalanine(7)-containing analogue Ac-Nle(4)-c[Asp(5),D-Phe(pI)(7),Lys(10)]alpha-MSH-(4-10-NH2 is a potent antagonist (pA(2) = 10.3) in the classical frog skin (Rana pipiens) assay (MC1-R), as is the D-2'-naphthylalanine(7) (D-Nal(2)(7))-containing analogue Ac-Nle(4)-c[Asp(5),D-Nal(2)(7), Lys(10)]alpha-MSH-(4-10)-NH2 (pA(2) > 10.3). Interestingly, the D-p-chloro- and D-p-fluorophenylalanine(7)-containing analogues lacked antagonist activities at all melanotropin receptors, and both exhibited full agonist potency in the frog skin assay. The activity of these analogues also was examined at four mammalian melanocortin receptors. Interestingly, Ac-Nle(4)-c[Asp(5),(D-Nal(2)(7), Lys(10)]alpha-MSH-(4-10)-NH2 was found to be a potent antagonist of the MC4-R (pA(2) 9.3) with minimal agonist activity, a less potent antagonist of the MC3-R (pA(2) = 8.3) with minimal agonist activity, and a full agonist of the MC1 and MC5 receptors. Surprisingly, Nle(4)-c[Asp(5),D-Phe(pI)(7),Lys(10)]alpha-MSH was found to be a potent agonist at the cloned human MC1-R (EC(50) = 0.055 nM) and mouse MC1-R (EC(50) = 0.19 nM) but had potent antagonist activities at the human MC4-R (pA(2) = 9.7) and human MC3-R (pA(2) = 8.3) with significant partial agonist activities (EC(50) = 0.57 and 0.68 nM, respectively) as well. Thus, highly potent and receptor selective antagonist analogues can arise from substitution of the D-Phe(7) residue with a bulky aromatic amino acid. These analogues can be used to help determine the functional roles of these receptors.
