BDNF and diabetes mellitus: Therefore, validated rat models of depression, such as streptozotocin (STZ)-induced diabetic rats [28,29,30,31], also include a component of anxiety in them as diabetes mellitus (DM) and MDD or anxiety are linked by several pathways, including the tryptophan catabolism (TRYCATs) pathway [32,33,34], advanced glycation end products (AGEs) [35], brain-derived neurotrophic factor (BDNF), and nuclear factor kappa B (NFKB) [36,37,38].