GCG and diabetes mellitus: Exendin-4 (Ex-4) is a long-acting glucagon-like peptide-1 (GLP-1) agonist approved for the treatment of type 2 diabetes mellitus (T2DM) that not only effectively induces glucose-dependent insulin secretion to regulate blood glucose levels but also reduces apoptotic cell death of pancreatic β-cells [15,16].In addition to its presence in the pancreas, the GLP-1 receptor (GLP-1R) is expressed on neurons throughout the brain, which GLP-1 and Ex-4 readily enters [17].