GCG and major depressive disorder: Despite this, class B GPCRs have been validated for the treatment of a number of disease conditions, including depression and anxiety (corticotropin-releasing factor (CRF) receptors), osteoporosis (calcitonin and parathyroid hormone receptors), and metabolic disorders (glucagon-like peptide-1 (GLP-1), gastric inhibitory polypeptide (GIP), glucagon (GCG), and amylin receptors) (4).