Molecular pharmacology of neural melanocortin receptors

The cloning of melanocortin receptors opened new avenues to identify selective ligands for this receptor family. γ-MSH was characterized as a melanocortin-3 receptor selective agonist. [D-Arg⁸]ACTH-(4-10) and [Pro^8,10,Gly⁹]ACTH-(4-10) were characterized as melanocortin-4 receptor antagonists. The application of these ligands in vivo revealed that melanocortin-4 receptors mediate melanocortin-induced grooming behaviour in the rat. Since we still lack potent and selective melanocortin receptor ligands, we performed homology modelling and site directed mutagenesis of the melanocortin-4 receptor, in order to understand how melanocortins bind melanocortin receptors. A histidine at position 260 in the melanocortin-4 receptor is important for normal receptor function. However this residue is not forming a salt bridge with a glutamate at position 92 to keep the receptor in an inactive conformation, nor with the glutamate in the melanocortin peptides as had been suggested before.