For example, a recent study by Xie et al. (2020) has reported that oral treatment with Lactobacillus reuteri 3 has anti-depressive effects through increasing the expression of enzymes involved in 5-HT biosynthesis, but inhibiting that of the KP enzymes, including IDO in the colon and PFC in mice with depression-like symptoms induced by chronic social defeat stress. Here, NPPA is linked to major depressive disorder.