GLP1R and major depressive disorder: Extant literature pertaining to the administration of glucagon-like peptide-1 receptor agonists (GLP-1RAs) for Alzheimer’s disease, Parkinson’s disease, major depressive disorder, bipolar disorder, substance-, alcohol- and nicotine-use disorders, suggests promising efficacy beyond the current FDA-approved indications (e.g., type 2 diabetes mellitus, obesity).