Inactivation of the mouse melanocortin-3 receptor results in increased fatmass and reduced lean body mass

Genetic(1-8) and pharmacological(6,9-12) studies have defined a role for th e melanocortin-4 receptor (Mc4r) in the regulation of energy homeostasis. T he physiological function of Mc3r, a melanocortin receptor expressed at hig h levels in the hypothalamus(13), has remained unknown. We evaluated the po tential role of Mc3r in energy homeostasis by studying Mc3r-deficient (Mc3r (-/-)) mice and compared the functions of Mc3r and Mc4r in mice deficient f or both genes. The 4-6-month Mc3r(-/-) mice have increased fat mass, reduce d lean mass and higher feed efficiency than wild-type littermates. despite being hypophagic and maintaining normal metabolic rates. (Feed efficiency i s the ratio of weight gain to food intake.) Consistent with increased fat m ass. Mc3r(-/-) mice are hyperleptinaemic and male Mc3r(-/-) mice develop mi ld hyperinsulinaemia. Mc3r(-/-) mice did not have significantly altered cor ticosterone or total thyroxine (T4) levels. Mice lacking both Mc3r and Mc4r become significantly heavier than Mc4r(-/-) mice. We conclude that Mc3r an d Mc4r serve nonredundant roles in the regulation of energy homeostasis.