SLC6A4 and Anxiety: Notably, the study revealed that Bifidobacterium dentium changed the expression of key intestinal serotonin receptors, particularly isoforms 2a (Htr2a) and 4 (Htr4), and the 5-HT transporter, a serotonin transporter (Sert), upregulating the expression of Htr2a in the hippocampus and normalizing anxiety like-behaviors in mice [305].