AHR and multiple sclerosis: Considering the expression of IDO/TDO is regulated by activation of AHR;109,110 tryptophan catabolites via kynurenine pathway are involved in immune, metabolic and neural communication mechanisms in gut-brain axis, which is controlled by AHR activation.111 In addition, certain microbiota that produce indole and indole derivatives from dietary tryptophan can also control astrocyte activity to influence CNS inflammation.112 Another study reported AHR agonists in serum are altered during different stages of multiple sclerosis,113 the most common neurological autoimmune disease of CNS.