LEP and obesity due to melanocortin 4 receptor deficiency: Krawczewski and co-authors found that voluntary exercise in a mouse strain susceptible to diet-induced obesity decreased fat mass & increased energy expenditure via activation of leptin receptor-positive neurons in the ventromedial nucleus; in this study, intracerebral leptin did not decrease body weight or food intake in sedentary mice fed high fat diets but did reduce body weight in exercising mice.