Telotristat ethyl, a tryptophan hydroxylase inhibitor, enhances antitumor efficacy of standard chemotherapy in preclinical cholangiocarcinoma models.

Cholangiocarcinoma (CCA), an aggressive biliary tract cancer, carries a grim prognosis with a 5-year survival rate of 5%-15%. Standard chemotherapy regimens for CCA, gemcitabine plus cisplatin (GemCis) or its recently approved combination with durvalumab demonstrate dismal clinical activity, yielding a median survival of 12-14 months. Increased serotonin accumulation and secretion have been implicated in the oncogenic activity of CCA. This study investigated the therapeutic efficacy of telotristat ethyl (TE), a tryptophan hydroxylase inhibitor blocking serotonin biosynthesis, in combination with standard chemotherapies in preclinical CCA models. Nab-paclitaxel (NPT) significantly enhanced animal survival (60%), surpassing the marginal effects of TE (11%) or GemCis (9%) in peritoneal dissemination xenografts. Combining TE with GemCis (26%) or NPT (68%) further increased survival rates. In intrahepatic (iCCA), distal (dCCA) and perihilar (pCCA) subcutaneous xenografts, TE exhibited substantial tumour growth inhibition (41%-53%) compared to NPT (56%-69%) or GemCis (37%-58%). The combination of TE with chemotherapy demonstrated enhanced tumour growth inhibition in all three cell-derived xenografts (67%-90%). PDX studies revealed TE's marked inhibition of tumour growth (40%-73%) compared to GemCis (80%-86%) or NPT (57%-76%). Again, combining TE with chemotherapy exhibited an additive effect. Tumour cell proliferation reduction aligned with tumour growth inhibition in all CDX and PDX tumours. Furthermore, TE treatment consistently decreased serotonin levels in all tumours under all therapeutic conditions. This investigation decisively demonstrated the antitumor efficacy of TE across a spectrum of CCA preclinical models, suggesting that combination therapies involving TE, particularly for patients exhibiting serotonin overexpression, hold the promise of improving clinical CCA therapy.

Synthesis and biological evaluation of xanthine derivatives with phenacyl group as tryptophan hydroxylase 1 (TPH1) inhibitors for obesity and fatty liver disease.

Tryptophan hydroxylase 1 (TPH1) has emerged as a target for the treatment of metabolic diseases including obesity and fatty liver disease. A series of xanthine derivatives were synthesized and evaluated for their TPH1 inhibition. Among the synthesized compounds, compound 40 showed good in vitro activity and liver microsomal stability. Docking studies revealed that compound 40 showed better binding to TPH1 via key intermolecular interactions involving the xanthine scaffold, imidazo-thiazolyl ring, and hydroxyl-containing phenacyl moiety. In addition, compound 40 effectively suppressed the adipocyte differentiation of 3 T3-L1 cells.

Gastric Serotonin Biosynthesis and Its Functional Role in L-Arginine-Induced Gastric Proton Secretion.

Among mammals, serotonin is predominantly found in the gastrointestinal tract, where it has been shown to participate in pathway-regulating satiation. For the stomach, vascular serotonin release induced by gastric distension is thought to chiefly contribute to satiation after food intake. However, little information is available on the capability of gastric cells to synthesize, release and respond to serotonin by functional changes of mechanisms regulating gastric acid secretion. We investigated whether human gastric cells are capable of serotonin synthesis and release. First, HGT-1 cells, derived from a human adenocarcinoma of the stomach, and human stomach specimens were immunostained positive for serotonin. In HGT-1 cells, incubation with the tryptophan hydroxylase inhibitor p-chlorophenylalanine reduced the mean serotonin-induced fluorescence signal intensity by 27%. Serotonin release of 147 ± 18%, compared to control HGT-1 cells (set to 100%) was demonstrated after treatment with 30 mM of the satiating amino acid L-Arg. Granisetron, a 5-HT3 receptor antagonist, reduced this L-Arg-induced serotonin release, as well as L-Arg-induced proton secretion. Similarly to the in vitro experiment, human antrum samples released serotonin upon incubation with 10 mM L-Arg. Overall, our data suggest that human parietal cells in culture, as well as from the gastric antrum, synthesize serotonin and release it after treatment with L-Arg via an HTR3-related mechanism. Moreover, we suggest not only gastric distension but also gastric acid secretion to result in peripheral serotonin release.

Serotonin pathway in carcinoid syndrome: Clinical, diagnostic, prognostic and therapeutic implications.

Carcinoid syndrome represents the most common functional syndrome that affects patients with neuroendocrine neoplasms. Its clinical presentation is really heterogeneous, ranging from mild and often misdiagnosed symptoms to severe manifestations, that significantly worsen the patient's quality of life, such as difficult-to-control diarrhoea and fibrotic complications. Serotonin pathway alteration plays a central role in the pathophysiology of carcinoid syndrome, accounting for most clinical manifestations and providing diagnostic tools. Serotonin pathway is complex, resulting in production of biologically active molecules such as serotonin and melatonin, as well as of different intermediate molecules and final metabolites. These activities require site- and tissue-specific catalytic enzymes. Variable expression and activities of these enzymes result in different clinical pictures, according to primary site of origin of the tumour. At the same time, the biochemical diagnosis of carcinoid syndrome could be difficult even in case of typical symptoms. Therefore, the accuracy of the diagnostic methods of assessment should be improved, also attenuating the impact of confounding factors and maybe considering new serotonin precursors or metabolites as diagnostic markers. Finally, the prognostic role of serotonin markers has been only evaluated for its metabolite 5-hydroxyindole acetic acid but, due to heterogeneous and biased study designs, no definitive conclusions have been achieved. The most recent progress is represented by the new therapeutic agent telotristat, an inhibitor of the enzyme tryptophan hydroxylase, which blocks the conversion of tryptophan in 5-hydroxy-tryptophan. The present review investigates the clinical significance of serotonin pathway in carcinoid syndrome, considering its role in the pathogenesis, diagnosis, prognosis and therapy.

Effect of the Tryptophan Hydroxylase Inhibitor Telotristat on Growth and Serotonin Secretion in 2D and 3D Cultured Pancreatic Neuroendocrine Tumor Cells.

Serotonin, a biologically active amine, is related to carcinoid syndrome in functioning neuroendocrine tumors (NETs). Telotristat ethyl is a novel inhibitor of the tryptophan hydroxylase (TPH), a key enzyme in the production of serotonin. While its use in patients with carcinoid syndrome and uncontrolled diarrhea under somatostatin analogs (SSAs) has been recently approved, in vitro data evaluating its effectiveness are lacking. For this reason, we aimed to evaluate the effect of telotristat as monotherapy, and in combination with SSAs, on proliferation and secretion in a NET cell line model. The human pancreatic NET cell lines BON-1/QGP-1 were used as 2D and 3D cultured models; somatostatin receptor and TPH mRNA expression, as well as the potential autocrine effect of serotonin on tumor cell proliferation using a 3D culture system were evaluated. Telotristat decreased serotonin production in a dose-dependent manner at a clinically feasible concentration, without affecting cell proliferation. Its combination with pasireotide, but not with octreotide, had an additive inhibitory effect on serotonin secretion. The effect of telotristat was slightly less potent, when BON-1 cells were co-treated with octreotide. Octreotide and pasireotide had no effect on the expression of TPH. Telotristat did not have an effect on mRNA expression of somatostatin receptor subtypes. Finally, we showed that serotonin did not have an autocrine effect on NET cell proliferation on the 3D cell model. These results suggest that telotristat is an effective drug for serotonin inhibition, but the effectiveness of its combination with SST2 (somatostatin receptor subtype 2)-preferring SSA should be evaluated in more detail.

Effects of Thryptophan Hydroxylase Blockade by P-Chlorophenylalanine on Contextual Memory Reconsolidation after Training of Different Intensity.

The processes of memory formation and its storage are extremely dynamic. Therefore, the determination of the nature and temporal evolution of the changes that underlie the molecular mechanisms of retrieval and cause reconsolidation of memory is the key to understanding memory formation. Retrieval induces the plasticity, which may result in reconsolidation of the original memory and needs critical molecular events to stabilize the memory or its extinction. 4-Chloro-DL-phenylalanine (P-chlorophenylalanine-PCPA) depresses the most limiting enzyme of serotonin synthesis the tryptophan hydroxylase. It is known that PCPA reduces the serotonin content in the brain up to 10 times in rats (see Methods). We hypothesized that the PCPA could behave the similar way in snails and could reduce the content of serotonin in snails. Therefore, we investigated the effect of PCPA injection on contextual memory reconsolidation using a protein synthesis blocker in snails after training according to two protocols of different intensities. The results obtained in training according to the first protocol using five electrical stimuli per day for 5 days showed that reminding the training environment against the background of injection of PCPA led to a significant decrease in contextual memory. At the same time, the results obtained in training according to the second protocol using three electrical stimuli per day for 5 days showed that reminding the training environment against the injection of PCPA did not result in a significant change in contextual memory. The obtain results allowed us to conclude that the mechanisms of processes developed during the reconsolidation of contextual memory after a reminding depend both on the intensity of learning and on the state of the serotonergic system.

Peripartum dietary supplementation of a small-molecule inhibitor of tryptophan hydroxylase 1 compromises infant, but not maternal, bone.

Long-term effects of breastfeeding on maternal bone are not fully understood. Excessive maternal bone loss stimulated by serotonin signaling during lactation may increase bone fragility later in life. We hypothesized that inhibiting nonneuronal serotonin activity by feeding a small-molecule inhibitor of the rate-limiting enzyme in serotonin synthesis [tryptophan hydroxylase 1 (TPH1)] would preserve maternal bone postweaning without affecting neonatal bone. Chow supplemented with the small-molecule TPH1 inhibitor LP778902 (~100 mg/kg) or control chow was fed to C57BL/6 dams throughout pregnancy and lactation, and blood was collected on days 1 and 21 of lactation. Dams returned to a common diet postweaning and were aged to 3 or 9 mo postweaning. Pups were euthanized at weaning. The effect of TPH1 inhibition on dam and pup femoral bone was determined by micro-computed tomography. Peripartum dietary supplementation with LP778902 decreased maternal serum serotonin concentrations ( P = 0.0007) and reduced bone turnover, indicated by serum NH2-terminal propeptide of type I collagen ( P = 0.01) and COOH-terminal collagen cross-links ( P = 0.02) concentrations, on day 21 of lactation. Repressed bone turnover from TPH1 inhibition was not associated with structural changes in maternal femur at 3 or 9 mo postweaning. By contrast, neonates exposed to peripartum LP778902 demonstrated differences in trabecular and cortical femoral bone compared with pups from control dams, with fewer ( P = 0.02) and thinner ( P = 0.001) trabeculae as well as increased trabecular spacing ( P = 0.04). Additionally, cortical porosity was increased ( P = 0.007) and cortical tissue mineral density was decreased ( P = 0.005) in pups of LP778902-treated dams. Small-molecule TPH1 inhibitors should be carefully considered in pregnant and lactating women, given potential risks to neonatal bone development.

Crucial Roles of 5-HT and 5-HT2 Receptor in Diabetes-Related Lipid Accumulation and Pro-Inflammatory Cytokine Generation in Hepatocytes.

BACKGROUND/AIMS: Previously, we confirmed that liver-synthesized 5-HT rather than non-liver 5-HT, acting on the 5-HT2 receptor (5-HT2R), modulates lipid-induced excessive lipid synthesis (ELS). Here, we further revealed the effects of the hepatocellular 5-HT system in diabetes-related disorders. METHODS: Studies were conducted in male ICR mice, human HepG2 cells, and primary mouse hepatocytes (PMHs) under gene or chemical inhibition of the 5-HT system, key lipid metabolism, and inflammation-related factors. Protein and messenger RNA expression and levels of the factors were determined via western blotting, reverse transcription PCR, and quantitative assay kits, respectively. Hepatic steatosis with inflammation and fibrosis, intracellular lipid droplet accumulation (LDA), and reactive oxygen species (ROS) location were determined via hematoxylin and eosin, Masson's trichrome, Oil red O, and fluorescent-specific staining, respectively. RESULTS: Palmitic acid induced the activation of the 5-HT system: the activation of 5-HT2R, primarily 5-HT2AR, in addition to upregulating monoamine oxidase A (MAO-A) expression and 5-HT synthesis, by activating the G protein/ phospholipase C pathway modulated PKCε activation, resulting in ELS with LDA; the activation of NF-κB, which mediates the generation of pro-inflammatory cytokines, was primarily due to ROS generation in the mitochondria induced by MAO-A-catalyzed 5-HT degradation, and secondarily due to the activation of PKCε. These effects of the 5-HT system were also detected in palmitic acid- or high glucose-treated PMHs and regulated multiple inflammatory signaling pathways. In diabetic mice, co-treatment with antagonists of both 5-HT synthesis and 5-HT2R significantly abolished hepatic steatosis, inflammation, and fibrosis as well as hyperglycemia and dyslipidemia. CONCLUSION: Activation of the hepatocellular 5-HT system plays a crucial role in inducing diabetes-related hepatic dysfunction and is a potential therapeutic target.

Telotristat ethyl: a novel agent for the therapy of carcinoid syndrome diarrhea.

Carcinoid syndrome (CS), characterized by diarrhea and flushing, is present in 20% of patients with neuroendocrine tumors at diagnosis and becomes more frequent with progression. The diarrhea of CS is caused mainly by tumoral secretion of serotonin. It may not be fully controlled by somatostatin analogs, the currently indicated drugs for symptomatic relief. Telotristat ethyl is a novel inhibitor of tryptophan hydroxylase, the rate-limiting enzyme in serotonin biosynthesis. Administration of the drug decreases diarrhea in patients with CS. Telotristat ethyl was approved in February 2017 (USA) and September 2017 (European Commission) for the treatment of CS diarrhea in adults inadequately controlled by somatostatin analog alone. This drug is expected to greatly improve the health and quality of life of patients with CS diarrhea.

Enteric serotonin and oxytocin: endogenous regulation of severity in a murine model of necrotizing enterocolitis.

Necrotizing enterocolitis (NEC), a gastrointestinal inflammatory disease of unknown etiology that may also affect the liver, causes a great deal of morbidity and mortality in premature infants. We tested the hypothesis that signaling molecules, which are endogenous to the bowel, regulate the severity of intestinal and hepatic damage in an established murine NEC model. Specifically, we postulated that mucosal serotonin (5-HT), which is proinflammatory, would exacerbate experimental NEC and that oxytocin (OT), which is present in enteric neurons and is anti-inflammatory, would oppose it. Genetic deletion of the 5-HT transporter (SERT), which increases and prolongs effects of 5-HT, was found to increase the severity of systemic manifestations, intestinal inflammation, and associated hepatotoxicity of experimental NEC. In contrast, genetic deletion of tryptophan hydroxylase 1 (TPH1), which is responsible for 5-HT biosynthesis in enterochromaffin (EC) cells of the intestinal mucosa, and TPH inhibition with LP-920540 both decrease the severity of experimental NEC in the small intestine and liver. These observations suggest that 5-HT from EC cells helps to drive the inflammatory damage to the gut and liver that occurs in the murine NEC model. Administration of OT decreased, while the OT receptor antagonist atosiban exacerbated, the intestinal inflammation of experimental NEC. Data from the current investigation are consistent with the tested hypotheses-that the enteric signaling molecules, 5-HT (positively) and OT (negatively) regulate severity of inflammation in a mouse model of NEC. Moreover, we suggest that mucosally restricted inhibition of 5-HT biosynthesis and/or administration of OT may be useful in the treatment of NEC.NEW & NOTEWORTHY Serotonin (5-HT) and oxytocin reciprocally regulate the severity of intestinal inflammation and hepatotoxicity in a murine model of necrotizing enterocolitis (NEC). Selective depletion of mucosal 5-HT through genetic deletion or inhibition of tryptophan hydroxylase-1 ameliorates, while deletion of the 5-HT uptake transporter, which increases 5-HT availability, exacerbates the severity of NEC. In contrast, oxytocin reduces, while the oxytocin receptor antagonist atosiban enhances, NEC severity. Peripheral tryptophan hydroxylase inhibition may be useful in treatment of NEC.

Tryptophan hydroxylase 1 Inhibition Impacts Pulmonary Vascular Remodeling in Two Rat Models of Pulmonary Hypertension.

Pulmonary arterial hypertension (PAH) is a progressive disease defined by a chronic elevation in pulmonary arterial pressure with extensive pulmonary vascular remodeling and perivascular inflammation characterized by an accumulation of macrophages, lymphocytes, dendritic cells, and mast cells. Although the exact etiology of the disease is unknown, clinical as well as preclinical data strongly implicate a role for serotonin (5-HT) in the process. Here, we investigated the chronic effects of pharmacological inhibition of tryptophan hydroxylase 1 (TPH1), the rate-limiting enzyme in peripheral 5-HT biosynthesis, in two preclinical models of pulmonary hypertension (PH), the monocrotaline (MCT) rat and the semaxanib (SUGEN, Medinoah, Suzhou, China)-hypoxia rat. In both PH models, ethyl (S)-8-(2-amino-6-((R)-1-(5-chloro-[1,1'-biphenyl]-2-yl)-2,2,2-trifluoroethoxy)pyrimidin-4-yl)-2,8-diazaspiro[4.5]decane-3-carboxylate and ethyl (S)-8-(2-amino-6-((R)-1-(3',4'-dimethyl-3-(3-methyl-1 H-pyrazol-1-yl)-[1,1'-biphenyl]-4-yl)-2,2,2-trifluoroethoxy)pyrimidin-4-yl)-2,8-diazaspiro[4.5]decane-3-carboxylate, novel orally active TPH1 inhibitors with nanomolar in vitro potency, decreased serum, gut, and lung 5-HT levels in a dose-dependent manner and significantly reduced pulmonary arterial pressure, and pulmonary vessel wall thickness and occlusion in male rats. In the MCT rat model, decreases in lung 5-HT significantly correlated with reductions in histamine levels and mast cell number (P < 0.001, r2 = 0.88). In contrast, neither ambrisentan nor tadalafil, which are vasodilators approved for the treatment of PAH, reduced mast cell number or 5-HT levels, nor were they as effective in treating the vascular remodeling as were the TPH1 inhibitors. When administered in combination with ambrisentan, the TPH1 inhibitors showed an additive effect on pulmonary vascular remodeling and pressures. These data demonstrate that in addition to reducing vascular remodeling, TPH1 inhibition has the added benefit of reducing the perivascular mast cell accumulation associated with PH.

Optimization of spirocyclic proline tryptophan hydroxylase-1 inhibitors.

As a follow-up to the discovery of our spirocyclic proline-based TPH1 inhibitor lead, we describe the optimization of this scaffold. Through a combination of X-ray co-crystal structure guided design and an in vivo screen, new substitutions in the lipophilic region of the inhibitors were identified. This effort led to new TPH1 inhibitors with in vivo efficacy when dosed as their corresponding ethyl ester prodrugs. In particular, 15b (KAR5585), the prodrug of the potent TPH1 inhibitor 15a (KAR5417), showed robust reduction of intestinal serotonin (5-HT) levels in mice. Furthermore, oral administration of 15b generated high and sustained systemic exposure of the active parent 15a in rats and dogs. KAR5585 was selected for further pharmacological evaluation in disease models associated with a dysfunctional peripheral 5-HT system.

Discovery of acyl guanidine tryptophan hydroxylase-1 inhibitors.

An increasing number of diseases have been linked to a dysfunctional peripheral serotonin system. Given that tryptophan hydroxylase 1 (TPH1) is the rate limiting enzyme in the biosynthesis off serotonin, it represents an attractive target to regulate peripheral serotonin. Following up to our first disclosure, we report a new chemotype of TPH1 inhibitors where-by the more common central planar heterocycle has been replaced with an open-chain, acyl guanidine surrogate. Through our work, we found that compounds of this nature provide highly potent TPH1 inhibitors with favorable physicochemical properties that were effective in reducing murine intestinal 5-HT in vivo. Furthermore, we obtained a high resolution (1.90Å) X-ray structure crystal structure of one of these inhibitors (compound 51) that elucidated the active conformation along with revealing a dimeric form of TPH1 for the first time.

Discovery of spirocyclic proline tryptophan hydroxylase-1 inhibitors.

The central role of the biogenic monoamine serotonin (5-hydroxytryptamine, 5-HT) as a neurotransmitter with important cognitive and behavioral functions is well known. However, 5-HT produced in the brain only accounts for approximately 5% of the total amount of 5-HT generated in the body. At the onset of our work, it appeared that substituted phenylalanine derivatives or related aryl amino acids were required to produce potent inhibitors of TPH1, as significant losses of inhibitory activity were noted in the absence of this structural element. We disclose herein the discovery of a new class of TPH1 inhibitors that significantly lower peripherally 5-HT.

Blocking peripheral serotonin synthesis by telotristat etiprate (LX1032/LX1606) reduces severity of both chemical- and infection-induced intestinal inflammation.

Mucosal inflammation is accompanied by an alteration in 5-HT. Intestinal 5-HT synthesis is catalyzed by tryptophan hydroxylase 1 (Tph1) and we have shown that mice deficient in this rate-limiting enzyme have reduced severity of intestinal inflammation in models of chemical-induced experimental colitis. Here, we investigated the effect of blocking peripheral 5-HT synthesis in generation of intestinal inflammation by a using peripheral Tph inhibitor, telotristat etiprate (LX1606), in models of intestinal inflammation. LX1606 was given orally either prophylactically or therapeutically to mice with dextran sulfate sodium (DSS)-induced colitis or with infection with Trichuris muris. Severity of intestinal inflammation was measured by assessment of disease activity scores, histological damage, and MPO and inflammatory cytokine levels. LX1606 significantly reduced intestinal 5-HT levels and delayed onset and severity of DSS-induced acute and chronic colitis. This was associated with decreased MPO and proinflammatory cytokine levels compared with vehicle-treated controls. In the infection-induced inflammation model, treatment with LX1606 enhanced worm expulsion as well as increased IL-10 production and goblet cell numbers. LX1606-treated mice had significantly lower MPO and IL-1ß levels compared with controls postinfection. Our results demonstrate that peripheral 5-HT plays an important role in intestinal inflammation and in the generation of immune responses. Pharmacological reduction of peripheral 5-HT may serve as a potential strategy for modulating various intestinal inflammatory disorders.

Tetramethylpyrazine Produces Antidepressant-Like Effects in Mice Through Promotion of BDNF Signaling Pathway.

BACKGROUND: Current antidepressants are clinically effective only after several weeks of administration. Tetramethylpyrazine (TMP) is an identified component of Ligusticum wallichii with neuroprotective effects. Here, we investigated the antidepressant effects of TMP in mice models of depression. METHODS: Antidepressant effects of TMP were first detected in the forced swim test (FST) and tail suspension test (TST), and further assessed in the chronic social defeat stress (CSDS) model. Changes in the brain-derived neurotrophic factor (BDNF) signaling pathway and in hippocampal neurogenesis after CSDS and TMP treatment were then investigated. A tryptophan hydroxylase inhibitor and BDNF signaling inhibitors were also used to determine the mechanisms of TMP. RESULTS: TMP exhibited potent antidepressant effects in the FST and TST without affecting locomotor activity. TMP also prevented the CSDS-induced symptoms. Moreover, TMP completely restored the CSDS-induced decrease of BDNF signaling pathway and hippocampal neurogenesis. Furthermore, a blockade of the BDNF signaling pathway prevented the antidepressant effects of TMP, while TMP produced no influence on the monoaminergic system. CONCLUSIONS: In conclusion, these data provide the first evidence that TMP has antidepressant effects, and this was mediated by promoting the BDNF signaling pathway.

Pharmacological reduction of mucosal but not neuronal serotonin opposes inflammation in mouse intestine.

OBJECTIVE: Enterochromaffin cell-derived serotonin (5-HT) promotes intestinal inflammation. We tested hypotheses that peripheral tryptophan hydroxylase (TPH) inhibitors, administered orally, block 5-HT biosynthesis and deplete 5-HT from enterochromaffin cells sufficiently to ameliorate intestinal inflammation; moreover, peripheral TPH inhibitors fail to enter the murine enteric nervous system (ENS) or central nervous systems and thus do not affect constitutive gastrointestinal motility. DESIGN: Two peripheral TPH inhibitors, LP-920540 and telotristat etiprate (LX1032; LX1606) were given orally to mice. Effects were measured on 5-HT levels in the gut, blood and brain, 5-HT immunoreactivity in the ENS, gastrointestinal motility and severity of trinitrobenzene sulfonic acid (TNBS)-induced colitis. Quantitation of clinical scores, histological damage and intestinal expression of inflammation-associated cytokines and chemokines with focused microarrays and real-time reverse transcriptase PCR were employed to evaluate the severity of intestinal inflammation. RESULTS: LP-920540 and LX1032 reduced 5-HT significantly in the gut and blood but not in the brain. Neither LP-920540 nor LX1032 decreased 5-HT immunoreactive neurons or fibres in the myenteric plexus and neither altered total gastrointestinal transit time, colonic motility or gastric emptying in mice. In contrast, oral LP-920540 and LX1032 reduced the severity of TNBS-induced colitis; the expression of 24% of 84 genes encoding inflammation-related cytokines and chemokines was lowered at least fourfold and the reduced expression of 17% was statistically significant. CONCLUSIONS: Observations suggest that that peripheral TPH inhibitors uncouple the positive linkage of enterochromaffin cell-derived 5-HT to intestinal inflammation. Because peripheral TPH inhibitors evidently do not enter the murine ENS, they lack deleterious effects on constitutive intestinal motility in mice.

Inhibition of tryptophan hydroxylase abolishes fatigue induced by central tryptophan in exercising rats.

Fatigue during prolonged exercise is related to brain monoamines concentrations, but the mechanisms underlying this relationship have not been fully elucidated. We investigated the effects of increased central tryptophan (TRP) availability on physical performance and thermoregulation in running rats that were pretreated with parachlorophenylalanine (p-CPA), an inhibitor of the conversion of TRP to serotonin. On the 3 days before the experiment, adult male Wistar rats were treated with intraperitoneal (ip) injections of saline or p-CPA. On the day of the experiment, animals received intracerebroventricular (icv) injections of either saline or TRP (20.3 µM) and underwent a submaximal exercise test until fatigue. Icv TRP-treated rats that received ip saline presented higher heat storage rate and a 69% reduction in time to fatigue compared with the control animals. Pretreatment with ip p-CPA blocked the effects of TRP on thermoregulation and performance. Moreover, ip p-CPA administration accelerated cutaneous heat dissipation when compared with saline-pretreated rats. We conclude that an elevated availability of central TRP interferes with fatigue mechanisms of exercising rats. This response is modulated by serotonergic pathways, because TRP effects were blocked in the presence of p-CPA. Our data also support that a depletion of brain serotonin facilitates heat loss mechanisms during exercise.

Production of serotonin by tryptophan hydroxylase 1 and release via platelets contribute to allergic airway inflammation.

RATIONALE: 5-Hydroxytryptamine (5-HT) is involved in the pathogenesis of allergic airway inflammation (AAI). It is unclear, however, how 5-HT contributes to AAI and whether this depends on tryptophan hydroxylase (TPH) 1, the critical enzyme for peripheral 5-HT synthesis. OBJECTIVES: To elucidate the role of TPH1 and the peripheral source of 5-HT in asthma pathogenesis. METHODS: TPH1-deficient and TPH1-inhibitor-treated animals were challenged in ovalbumin and house dust mite models of AAI. Experiments with bone marrow chimera, mast cell-deficient animals, platelets transfusion, and bone marrow dendritic cells (BMDC) driven model of AAI were performed. 5-HT levels were measured in bronchoalveolar lavage fluid or serum of animals with AAI and in human asthma. MEASUREMENTS AND MAIN RESULTS: 5-HT levels are increased in bronchoalveolar lavage fluid of mice and people with asthma after allergen provocation. TPH1 deficiency and TPH1 inhibition reduced all cardinal features of AAI. Administration of exogenous 5-HT restored AAI in TPH1-deficient mice. The pivotal role of 5-HT production by structural cells was corroborated by bone marrow chimera experiments. Experiments in mast cell-deficient mice revealed that mast cells are not a source of 5-HT, whereas transfusion of platelets from wild-type and TPH1-deficient mice revealed that only platelets containing 5-HT enhanced AAI. Lack of endogenous 5-HT in vitro and in vivo was associated with an impaired Th2-priming capacity of BMDC. CONCLUSIONS: In summary, TPH1 deficiency or inhibition reduces AAI. Platelet- and not mast cell-derived 5-HT is pivotal in AAI, and lack of 5-HT leads to an impaired Th2-priming capacity of BMDC. Thus, targeting TPH1 could offer novel therapeutic options for asthma.

Irritable bowel syndrome: new findings in pathophysiological and therapeutic field.

Irritable bowel syndrome (IBS) is a high prevalence disease, whose symptoms are reported by a large number of young adults with significant effects on quality of life and social costs. Traditionally, IBS has been treated with dietary and lifestyle modification, fiber supplementation, psychological and pharmacological therapy. Since its complex and multifactorial etiopathogenesis is only partially known, therapeutic choices may be difficult and not always effective. New research efforts focused on the role of relationship between central nervous system and gut disorders (brain-gut axis), altered composition of gut microbiota (e.g. an eight times increased risk for IBS after Salmonella infection), immune activation with an increased number of T lymphocytes and mast cells associated with mucosa as well as an increased level of pro-inflammatory cytokines (IL-10 and IL-12, suggesting Th1 polarization), visceral hypersensitivity causing perception of pain even for minimal abdominal distension. Based on these findings, new possibilities of treatment are emerging with encouraging outcomes. Attention is directed to drugs that showed good tolerability profile and poor systemic absorption, which may make them suitable for repeated or long term treatments, as frequently required in patients with IBS. They have been successfully used drugs such as tachykinin receptors antagonists, tryptophan hydroxylase inhibitors, bile acid sequestrants, µ agonist and δ antagonist opioid receptors. Recent studies are discussed in this review, focusing both on new therapeutic approaches and innovative adaptation of previously available treatments.