It has been proven that AhR signaling pathways (especially after activation by such endogenous AhR ligands as tryptophan metabolites) are implicated in neurodegenerative diseases, in particular in well-known age-related brain diseases (Parkinson’s disease, Alzheimer’s disease, multiple sclerosis, and amyotrophic lateral sclerosis) and other diseases of the central nervous system [229,230,231,232,233]. This evidence concerns the gene AHR and Parkinson disease.