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Serotonin is a very important neuromodulatory agent that affects many physiological and behavioral responses of both vertebrates and invertebrates. In helminths, especially parasitic ones, not much is known about the biosynthesis and mode of action of serotonin or any of the related biogenic amine neurotransmitters, such as catecholamines (dopamine and noradrenaline). In this study, we cloned two full length cDNAs from Schistosoma mansoni encoding tryptophan hydroxylase (TPH) and tyrosine hydroxylase (TH). TPH and TH catalyze the rate limiting steps in the biosynthesis of serotonin and catecholamines, respectively. Both enzymes were expressed in Escherichia coli and the purified proteins were shown to have TPH and TH activities. This indicates that S. mansoni, and possibly other parasitic helminths, may be capable of synthesizing serotonin and catecholamines endogenously. In the second part of our studies, we looked at the mode of action of serotonin in helminths, in particular the molecular properties of serotonergic G protein-coupled receptors (GPCR). We cloned two helminth GPCRs, one from the free living nematode Caenorhabditis elegans and the second from S. mansoni. The C. elegans receptor (5-HT2Ce) was shown to encode a functional serotonin receptor with structural and signaling properties similar to those of mammalian 5-HT2 receptors. However, its agonist I antagonist binding profile differed from previously characterized serotonin receptors. The cloned S. mansoni receptor (SmGPCRx) was found to represent a new structural class of receptor, which shared about the same level of amino acid sequence homology with various biogenic amines receptors, such as serotonin, catecholamines, and octopamine receptors. Additional sequence analysis and immunolocalization studies confirmed that SmGPCRx possesses structural characteristics of a GPCR. SmGPCRx is the first GPCR ever cloned from a parasitic flatworm. Taken together, these studies mark an important first step to