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IntroductionPulmonary hypertension (PH) is characterized by an increase in pulmonary vascular resistance that impedes ejection of blood by the right ventricle and leads to right ventricular failure. Primary PH (PPH) is the clinical term used to describe a rare and fatal condition for which no underlying cause can be found (1). Its pathogenesis remains largely unknown, although recent reports of familial PPH associated with BMPR2 gene mutations suggest a role for genetic predisposition (2, 3). Histologically, the remodeled pulmonary arteries show various degrees of medial hypertrophy and intimal thickening that, ultimately, lead to obliteration of the vessels. Hyperplasia of pulmonary artery smooth muscle cells (PA-SMCs) is the main component of these changes (4). Its origin, however, remains unknown.Investigations on serotonin, 5-hydroxytryptamine (5-HT), and its transporter (5-HTT) in patients with PPH are of special interest because an increased risk of PPH has been reported in patients who used appetite suppressants interfering with 5-HT (5). In previous studies, we found that 5-HT promoted the development of hypoxic PH by stimulating PA-SMC growth (6). As shown in bovine and rat PA-SMCs, the mitogenic and comitogenic effects of 5-HT require internalization of indoleamine by a high-affinity and selective transporter (7,8). Exposure of PA-SMCs to hypoxia results in a rapid increase in 5-HTT expression and activity, together with a marked enhancement in the growth-promoting effect of 5-HT (7). Increased 5-HTT gene expression also occurs in remodeled pulmonary arteries from animals developing PH related to chronic hypoxia exposure (7). Moreover, mice with targeted disruption of the 5-HTT gene develop less severe hypoxic PH than wild-type controls (9), which is direct evidence that 5-HTT plays a key role in pulmonary vessel remodeling. Hyperplasia of pulmonary artery smooth muscle cells (PA-SMCs) is a hallmark pathological feature of primary pulmonary hypertension (PPH). Here we found that PA-SMCs from patients with PPH grow faster than PA-SMCs from controls when stimulated by serotonin or serum and that these effects are due to increased expression of the serotonin transporter (5-HTT), which mediates internalization of indoleamine. In the presence of 5-HTT inhibitors, the growth stimulatory effects of serum and serotonin were markedly reduced and the difference between growth of PA-SMCs from patients and controls was no longer observed. As compared with controls, the expression of 5-HTT was increased in cultured PA-SMCs as well as in platelets and lungs from patients with PPH where it predominated in the media of thickened pulmonary arteries and in onion-bulb lesions. The L-allelic variant of the 5HTT gene promoter, which is associated with 5-HTT overexpression and increased PA-SMC growth, was present in homozygous form in 65% of patients but in only 27% of controls. We conclude that 5-HTT activity plays a key role in the pathogenesis of PA-SMC proliferation in PPH and that a 5HTT polymorphism confers susceptibi...

Deficiency in the monoamine degradation enzyme monoamine oxidase A (MAOA) or prenatal exposure to the monoamine uptake inhibitor cocaine alters behavior in humans and rodents, but the mechanisms are unclear. In MAOA knock-out mice, inhibiting serotonin synthesis during development can prevent abnormal segregation of axons in the retinogeniculate and somatosensory thalamocortical systems. To investigate this effect, we crossed MAOA knock-outs with mice lacking the serotonin transporter 5-HTT or the 5-HT1B receptor, two molecules present in developing sensory projections. Segregation was abnormal in 5-HTT knock-outs and MAOA/5-HTT double knock-outs but was normalized in MAOA/5-HT1B double knock-outs and MAOA/5-HTT/5-HT1B triple knock-outs. This demonstrates that the 5-HT1B receptor is a key factor in abnormal segregation of sensory projections and suggests that serotonergic drugs represent a risk for the development of these projections. We also found that the 5-HT1B receptor has an adverse developmental impact on beam-walking behavior in MAOA knock-outs. Finally, because the 5-HT1B receptor inhibits glutamate release, our results suggest that visual and somatosensory projections must release glutamate for proper segregation.