HTR5A and metabolic dysfunction-associated steatotic liver disease: For this purpose, in this study, we used melanocortin 4 receptor (MC4R) knockout (KO) mice as a model to test the central melanocortin pathway in NAFLD, as MC4R expression is limited in the brain, and 5-HT and its receptor HTR2C in the brain inhibit appetites via the MC4R signaling pathway (Lam et al., 2010).