BDNF and Anxiety: Antibiotic treatment was shown to reduce the BDNF (brain-derived neurotrophic factor) level in serum and its mRNA expression in the hippocampus, which along with changes in the level of several other hormones and neurotransmitters including oxytocin, vasopressin, acetylcholine, noradrenaline, and serotonergic precursor tryptophan, underlie cognitive dysfunctions and anxiety [24,25,27,56,73].