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The serotonin hypothesis of pulmonary arterial hypertension (PAH) arose owing to anorexigens, acting as indirect serotinergic agonists, causing PAH. However, it is now thought that serotonin plays an important role in the pathobiology of PAH per se. The rate-limiting enzyme in the synthesis of peripheral serotonin is tryptophan hydroxylase 1 (TPH1), serotonin can mediate pulmonary arterial smooth muscle cell proliferation via the serotonin transporter (SERT) and serotonin can induce pulmonary vasoconstriction via the 5-HT1B receptor in man. There is evidence that TPH1, SERT and 5-HT1B expression/activity can be upregulated in clinical PAH. This review discusses recent evidence implicating serotonin in the development of experimental and clinical PAH and suggests potential therapeutic targets.