- The melanocortin pathway: effects of voluntary exercise on the melanocortin-4 receptor knockout mice and ACTH(1-24) ligand structure activity relationships at the melanocortin-2 receptor. Haskell-Luevano, C., Todorovic, A., Gridley, K., Sorenson, N., Irani, B., Xiang, Z. Endocr. Res. (2004)