GDNF and major depressive disorder: This multi-target approach extends to depression (regulating Bacteroidetes/Firmicutes ratio, neurotransmitters, β-CaMKII/NMDAR/BDNF) [58], IBS (via GDNF signaling), tumor progression (reducing inflammation, enhancing immunity) [59], obesity, and cerebral ischemia, often involving normalization of specific bacterial genera, metabolites like SCFAs, and metabolic pathways (amino acid, purine metabolism) [60], highlighting EA’s broad efficacy through gut–microbiota–brain axis modulation.