TPH1 and Arthritis: Finally, to explore the role of 5-HIAA in the ability of butyrate supplementation to suppress arthritis, mice were treated with the tryptophan hydrolase (TPH) inhibitor L-para-chlorophenylalanine (PCPA), which is known to reduce 5-HIAA and serotonin biosynthesis (Welford et al., 2016).