GCG and diabetes mellitus: We and others previously demonstrated that the endogenous incretin glucagon-like peptide-1 (GLP-1) as well as exendin-4 (also known as exenatide), a long-acting GLP-1 receptor (GLP-1R) agonist approved for the treatment of type 2 diabetes mellitus (T2DM) (Drucker, 2018; Gentilella et al., 2019), protect tyrosine hydroxylase immunoreactivity (TH-IR) in primary ventromesenchephalic neurons from 6-OHDA lesioning.