SLC6A3 and obesity due to melanocortin 4 receptor deficiency: The expressions of dopamine receptors 1 (D1R) and 2 (D2R) are decreased, as well as the rate-limiting synthesizing enzyme (tyrosine hydroxylase, TH), whereas the dopamine transporter (DAT) is increased.8–12 This hypofunctioning of the dopamine pathway has been suggested to feed the vicious circle of weight gain since it leads to an increase in the meal size of fatty and sweet food in an attempt to feel the same rewarding effect as before the development of obesity.13 The gut microbiota plays a key role in the gut-to-brain axis influencing the food behavior.