POMC and obesity due to melanocortin 4 receptor deficiency: Because serotonin can regulate appetite by activating pro-opiomelanocortin (POMC) neurons, which play a key role in the regulation of feeding by sending anorectic signals to the periventricular nucleus (PVN) and other brain areas associated with energy homeostasis [17], serotonin as well as related genes are often tested for association with weight gain and obesity.