BDNF and major depressive disorder: The therapeutic mechanisms of CHM metabolites in IBS-D with concurrent anxiety and depression involve complex signaling pathways, including NLRP3 inflammatory vesicle, cAMP/PKA, BDNF-TrkB, PI3K/AKT/NF-κB, MEK/ERK, and FoxG1/CREB/BDNF pathways.