GLP1R and type 2 diabetes mellitus: The role of the nervous system, genetics, and hormones involved in metabolic homeostasis (such as insulin, glucagon, GLP-1, and GIP), as well as glucotoxicity diets and feeding behaviors, sedentary lifestyle, altered islet cell behavior, altered extra pancreatic behavior, and risk factors (such as psychological stress), are part of the etiology and pathogenesis of T2DM.