Furthermore, Ber can also increase 5-HT levels in the hippocampus by regulating enzymes such as tryptophan 5-hydroxylase-1 (TPH1) and indoleamine 2,3-dioxygenase-1 (IDO1), thus shifting the kynurenine (KYN) pathway in tryptophan metabolism more towards the 5-HT pathway for the treatment of depression (Wang et al., 2022c; Ge et al., 2023). Here, IDO1 is linked to major depressive disorder.