The neurotransmitter serotonin [5-hydroxytryptamine (5-HT)] is involved in multiple central nervous facets of mood control and in regulating sleep, anxiety, alcoholism, drug abuse, food intake, and sexual behavior. In peripheral tissues, serotonin is reportedly implicated in the regulation of vascular tone, gut motility, primary hemostasis, and cell-mediated immune responses. Walther, D. J., et al., Science 299:76 (2003).
The enzyme tryptophan hydroxylase (TPH) catalyzes the rate limiting step of the biosynthesis of serotonin. Two isoforms of TPH have been reported: TPH1, which is expressed in the periphery, primarily in the gastrointestinal (GI) tract; and TPH2, which is expressed in the brain. Id. The isoform TPH1 is encoded by the tph1 gene; TPH2 is encoded by the tph2 gene. Id.
Mice genetically deficient for the tph1 gene (“knockout mice”) have been reported. In one case, the mice reportedly expressed normal amounts of serotonin in classical serotonergic brain regions, but largely lacked serotonin in the periphery. Id. In another, the knockout mice exhibited abnormal cardiac activity, which was attributed to a lack of peripheral serotonin. Côté, F., et al., PNAS 100(23):13525-13530 (2003).
Because serotonin is involved in so many biochemical processes, drugs that affect serotonin levels are often attended by adverse effects. Thus, a need exists for new ways of treating diseases and disorders that are affected by serotonin.