TPH1 and progressive pseudorheumatoid arthropathy of childhood: In the rat PPD model, a decrease in the ratio of Firmicutes to Bacteroidetes as well as a decrease in the expression of tryptophan hydroxylase (TPH), a key enzyme in the 5-HT production pathway, resulted in a significant decrease in 5-HT, while in the mouse PPD model, the disruption of the gut microbiota was accompanied by a significant decrease in 4-aminobutyric acid [50, 54].