DPP4 and depressive disorder: T2DM - Type 2 Diabetes Mellitus; AD -Alzheimer's Disease; SU-TZD - Sulfonylurea-Thiazolidinediones; TZD - Thiazolidinediones; VD - Vascular Dementia; GLP-1 - Glucagon-Like Peptide 1; MMSE - Mini Mental Status Exam; DLPFC - Dorsolateral Prefrontal Cortex; HAM-D - Hamilton Depression Rating Scale; SU - Sulfonylurea; PD - Parkinson's Disease; cDDD - Cumulative Defined Daily Dose; DDD - Defined Daily Dose; DPP-4i - Dipeptidyl Peptidase 4 inhibitor; SGL-T2i - Sodium Glucose Cotransporter 2 Inhibitor.