BDNF and Anxiety: Several lines of compelling animal research supports this association: germ‐free (GF) mice exhibit altered expression of mRNA levels of brain‐derived neurotrophic factor (BDNF) and the 5HT1A receptor in the dentate gyrus, and the NR2B N‐methyl‐d‐aspartate (NMDA) receptor subunit in the amygdala (Neufeld et al., 2011), while administration of a specific bacterial strain to conventional mice for 28 days reduced anxiety‐like behavior and produced regional changes in brain gamma‐aminobutyric acid (GABA) receptor subtypes—effects that were abrogated by prior vagotomy (Bravo et al., 2011).