TDO2 and Anxiety: Tryptophan–kynurenine metabolism can convert tryptophan into kynurenine through tryptophan 2,3-dioxygenase or indoleamine 2,3-dioxygenase [71], eventually forming neurologically relevant metabolites, such as 3-hydroxykynurenine, quinolinic acid, niacin, and nicotinamide adenine dinucleotide [72], which regulate anxiety through neural control.