BDNF and Anxiety: Compared to conventional specific pathogen-free (SPF) mice, adult germ-free (GF) mice demonstrate reduced anxiety-like behavior in the elevated plus maze, exhibit elevated expression of brain-derived neurotrophic factor (BDNF), and show decreased expression of serotonin (5-hydroxytryptamine) receptors in the brain (Neufeld et al., 2011b; Chen et al., 2013).