TPH1 and Anxiety: Though often thought of as a distinct neurotransmitter of the central nervous system (CNS), regulating mood, anxiety and behavior3, the bioamine 5-HT is synthesized in both the CNS and peripheral tissues, mostly within the gastrointestinal tract, via the rate-limiting enzyme Trp hydroxylase (Tph) the isoform 1 (Tph1) of which is expressed in gut neural and enterochromaffin cells while the isoform Tph2 is expressed predominantly in the CNS.