Physical Activity, Energy Expenditure, and Defense of Body Weight in Melanocortin 4 Receptor-Deficient Male Rats

Melanocortin 4 receptor (MC4R) variants contribute to human obesity, and rats lacking functional MC4R (Mc4r(K314X/K314X)) are obese. We investigated the hypothesis that low energy expenditure (EE) and physical activity contribute to this obese phenotype in male rats, and determined whether lack of functional MC4R conferred protection from weight loss during 50% calorie restriction. Though Mc4r(K314X/K314X) rats showed low brown adipose Ucp1 expression and were less physically active than rats heterozygous for the mutation (Mc4r(+/K314X)) or wild-type (Mc4r(+/+)) rats, we found no evidence of lowered EE in Mc4r(K314X/K314X) rats once body weight was taken into account using covariance. Mc4r(K314X/K314X) rats had a significantly higher respiratory exchange ratio. Compared to Mc4r(+/+) rats, Mc4r(K314X/K314X) and Mc4r(+/K314X) rats lost less lean mass during calorie restriction, and less body mass when baseline weight was accounted for. Limited regional overexpression of Mc3r was found in the hypothalamus. Although lower physical activity levels in rats with nonfunctional MC4R did not result in lower total EE during free-fed conditions, rats lacking one or two functional copies of Mc4r showed conservation of mass, particularly lean mass, during energy restriction. This suggests that variants affecting MC4R function may contribute to individual differences in the metabolic response to food restriction.