Additionally, serotonin and melatonin, which are derived from tryptophan through the serotonergic pathway and are reduced in patients with AD, play a protective role in AD by increasing brain-derived neurotrophic factor expression in the hippocampus and prefrontal cortex and reducing intraneuronal Aβ accumulation and hippocampal apoptosis15. This evidence concerns the gene BDNF and Alzheimer disease.