N-(1-methyl-5-indolyl)-N'-(3-methyl-5-isothiazolyl)urea: a novel, high-affinity 5-HT2B receptor antagonist

Development of a Peripherally Restricted 5-HT2B Partial Agonist for Treatment of Pulmonary Arterial Hypertension

Ligands for the serotonin 2B receptor (5-HT2B) have shown potential to treat pulmonary arterial hypertension in preclinical models but cannot be used in humans because of predicted off-target neurological effects. The aim of this study was to develop novel systemically restricted compounds targeting 5-HT2B. Here, we show that mice treated with VU6047534 had decreased RVSP compared with control treatment in both the prevention and intervention studies using Sugen-hypoxia. VU6047534 is a novel 5-HT2B partial agonist that is peripherally restricted and able to both prevent and treat Sugen-hypoxia-induced pulmonary arterial hypertension. We have synthesized and characterized a structurally novel series of 5-HT2B ligands with high potency and selectivity for the 5-HT2B receptor subtype. Next-generation 5-HT2B ligands with similar characteristics, and predicted to be systemically restricted in humans, are currently advancing to investigational new drug-enabling studies.

2B Determined: The Future of the Serotonin Receptor 2B in Drug Discovery

The cardiotoxicity associated with des-ethyl-dexfenfluramine (norDF) and related agonists of the serotonin receptor 2B (5-HT2B) has solidified the receptor's place as an "antitarget" in drug discovery. Conversely, a growing body of evidence has highlighted the utility of 5-HT2B antagonists for the treatment of pulmonary arterial hypertension (PAH), valvular heart disease (VHD), and related cardiopathies. In this Perspective, we summarize the link between the clinical failure of fenfluramine-phentermine (fen-phen) and the subsequent research on the role of 5-HT2B in disease progression, as well as the development of drug-like and receptor subtype-selective 5-HT2B antagonists. Such agents represent a promising class for the treatment of PAH and VHD, but their utility has been historically understudied due to the clinical disasters associated with 5-HT2B. Herein, it is our aim to examine the current state of 5-HT2B drug discovery, with an emphasis on the receptor's role in the central nervous system (CNS) versus the periphery.

The serotonin transporter sustains human brown adipose tissue thermogenesis

Activation of brown adipose tissue (BAT) in humans is a strategy to treat obesity and metabolic disease. Here we show that the serotonin transporter (SERT), encoded by SLC6A4, prevents serotonin-mediated suppression of human BAT function. RNA sequencing of human primary brown and white adipocytes shows that SLC6A4 is highly expressed in human, but not murine, brown adipocytes and BAT. Serotonin decreases uncoupled respiration and reduces uncoupling protein 1 via the 5-HT2B receptor. SERT inhibition by the selective serotonin reuptake inhibitor (SSRI) sertraline prevents uptake of extracellular serotonin, thereby potentiating serotonin's suppressive effect on brown adipocytes. Furthermore, we see that sertraline reduces BAT activation in healthy volunteers, and SSRI-treated patients demonstrate no 18F-fluorodeoxyglucose uptake by BAT at room temperature, unlike matched controls. Inhibition of BAT thermogenesis may contribute to SSRI-induced weight gain and metabolic dysfunction, and reducing peripheral serotonin action may be an approach to treat obesity and metabolic disease.

β Cell mass expansion during puberty involves serotonin signaling and determines glucose homeostasis in adulthood

Puberty is associated with transient insulin resistance that normally recedes at the end of puberty; however, in overweight children, insulin resistance persists, leading to an increased risk of type 2 diabetes. The mechanisms whereby pancreatic β cells adapt to pubertal insulin resistance, and how they are affected by the metabolic status, have not been investigated. Here, we show that puberty is associated with a transient increase in β cell proliferation in rats and humans of both sexes. In rats, β cell proliferation correlated with a rise in growth hormone (GH) levels. Serum from pubertal rats and humans promoted β cell proliferation, suggesting the implication of a circulating factor. In pubertal rat islets, expression of genes of the GH/serotonin (5-hydroxytryptamine [5-HT]) pathway underwent changes consistent with a proliferative effect. Inhibition of the pro-proliferative 5-HT receptor isoform HTR2B blocked the increase in β cell proliferation in pubertal islets ex vivo and in vivo. Peripubertal metabolic stress blunted β cell proliferation during puberty and led to altered glucose homeostasis later in life. This study identifies a role of GH/GH receptor/5-HT/HTR2B signaling in the control of β cell mass expansion during puberty and identifies a mechanistic link between pubertal obesity and the risk of developing type 2 diabetes.

5-HT2A, 5-HT1B/D, 5HT3 and 5-HT7 receptors as mediators of serotonin-induced direct contractile response of bovine airway smooth muscle

Background: Serotonin (5-hydroxytryptamine; 5-HT) performs a variety of functions in the body including the modulation of muscle tone in respiratory airways. Several studies indicate a possible role of 5-HT in the pathophysiology of bronchial hyperresponsiveness. However, the receptors and the molecular mechanisms by which 5-HT acts on airway smooth muscle (ASM) continue to be controversial. Most of the evidence suggests the participation of different subtypes of receptors in an indirect response. This study supports the proposal that 5-HT directly contracts ASM and characterizes pharmacologically the subtypes of serotonergic receptors involved. The characterization was carried out by using selective antagonists in an organ bath model allowing study of the smooth muscle of segments of bovine trachea. Results: The results obtained show that 5-HT_2A receptors are the main mediators of the direct contractile response of bovine ASM, with the cooperation of the 5-HT₇, 5-HT₃ and 5-HT_1B/D receptors. Also, it was observed that the muscle response to serotonin is developed more slowly and to a lesser extent in comparison with the response to cholinergic stimulation. Conclusion: Overall, the receptors that mediate the direct serotonergic contraction of the smooth muscle of the bovine trachea are 5-HT_2A, 5-HT₇, 5-HT₃ and 5-HT_1B/D receptors.

Low Basicity as a Characteristic for Atypical Ligands of Serotonin Receptor 5-HT2

Serotonin receptors are extensively examined by academic and industrial researchers, due to their vital roles, which they play in the organism and constituting therefore important drug targets. Up to very recently, it was assumed that the basic nitrogen in compound structure is a necessary component to make it active within this receptor system. Such nitrogen interacts in its protonated form with the aspartic acid from the third transmembrane helix (D3x32) forming a hydrogen bond tightly fitting the ligand in the protein binding site. However, there are several recent studies that report strong serotonin receptor affinity also for compounds without a basic moiety in their structures. In the study, we carried out a comprehensive in silico analysis of the low-basicity phenomenon of the selected serotonin receptor ligands. We focused on the crystallized representatives of the proteins of 5-HT_1B, 5-HT_2A, 5-HT_2B, and 5-HT_2C receptors, and examined the problem both from the ligand- and structure-based perspectives. The study was performed for the native proteins, and for D3x32A mutants. The investigation resulted in the determination of nonstandard structural requirements for activity towards serotonin receptors, which can be used in the design of new nonbasic ligands.

Pharmacologic inhibition of serotonin htr2b ameliorates hyperglycemia and the altered expression of hepatic FGF21, Sdf2l1, and htr2a in db/db mice and KKAy mice

Plasma fibroblast growth factor 21 (FGF21) levels and hepatic FGF21, serotonin 2a receptor (htr2a), and stromal cell-derived factor 2 like 1 (Sdf2l1) expression are increased in insulin-resistant C57BL6J mice fed a high-fat diet. Here we show that plasma FGF21 levels and hepatic FGF21, Sdf2l1, and htr2a expression were decreased in 6-week-old db/db mice compared with C57BL6J mice, whereas they were increased in 6-week-old KKA^y mice compared with KK mice. Expression of hepatic htr2b was increased in db/db mice and KKA^y mice compared with controls. Treatment with the selective htr2b antagonist SB204741 suppressed the hyperglycemia in either db/db mice or KKA^y mice. Treatment with SB20471 reversed the decreases in plasma FGF21 levels and hepatic FGF21, Sdf2l1, and htr2a expression in db/db mice, whereas it suppressed the increases in plasma FGF21 levels and hepatic FGF21, Sdf2l1, and htr2a expression in KKA^y mice. Moreover, treatment with SB204741 increased plasma FGF21 levels and expression of hepatic FGF21, htr2a, and Sdf2l1 in C57BL6J mice, whereas it decreased plasma FGF21 levels and hepatic FGF21 expression in KK mice. These findings suggest that pharmacologic inhibition of htr2b ameliorates the hyperglycemia and altered expression of hepatic FGF21, Sdf2l1 and htr2a in obese and diabetic db/db and KKA^y mice.

Genetic ablation of serotonin receptor 2B improves aortic valve hemodynamics of Notch1 heterozygous mice in a high-cholesterol diet model

Calcific aortic valve disease (CAVD) is a deadly disease that is rising in prevalence due to population aging. While the disease is complex and poorly understood, one well-documented driver of valvulopathy is serotonin agonism. Both serotonin overexpression, as seen with carcinoid tumors and drug-related agonism, such as with Fenfluramine use, are linked with various diseases of the valves. Thus, the objective of this study was to determine if genetic ablation or pharmacological antagonism of the 5-HT_2B serotonin receptor (gene: Htr2b) could improve the hemodynamic and histological progression of calcific aortic valve disease. Htr2b mutant mice were crossed with Notch1^+/- mice, an established small animal model of CAVD, to determine if genetic ablation affects CAVD progression. To assess the effect of pharmacological inhibition on CAVD progression, Notch1^+/- mice were treated with the 5-HT_2B receptor antagonist SB204741. Mice were analyzed using echocardiography, histology, immunofluorescence, and real-time quantitative polymerase chain reaction. Htr2b mutant mice showed lower aortic valve peak velocity and mean pressure gradient–classical hemodynamic indicators of aortic valve stenosis–without concurrent left ventricle change. 5-HT_2B receptor antagonism, however, did not affect hemodynamic progression. Leaflet thickness, collagen density, and CAVD-associated transcriptional markers were not significantly different in any group. This study reveals that genetic ablation of Htr2b attenuates hemodynamic development of CAVD in the Notch1^+/- mice, but pharmacological antagonism may require high doses or long-term treatment to slow progression.

Serotonin (5-HT) Shapes the Macrophage Gene Profile through the 5-HT2B-Dependent Activation of the Aryl Hydrocarbon Receptor

Macrophages can either promote or resolve inflammatory responses, and their polarization state is modulated by peripheral serotonin (5-hydroxytryptamine [5-HT]). In fact, pro- and anti-inflammatory macrophages differ in the expression of serotonin receptors, with 5-HT_2B and 5-HT₇ expression restricted to M-CSF-primed monocyte-derived macrophages (M-MØ). 5-HT₇ drives the acquisition of profibrotic and anti-inflammatory functions in M-MØ, whereas 5-HT_2B prevents the degeneration of spinal cord mononuclear phagocytes and modulates motility of murine microglial processes. Because 5-HT_2B mediates clinically relevant 5-HT-related pathologies (valvular heart disease, pulmonary arterial hypertension) and is an off target of anesthetics, antiparkinsonian drugs, and selective serotonin reuptake inhibitors, we sought to determine the transcriptional consequences of 5-HT_2B engagement in human macrophages, for which 5-HT_2B signaling remains unknown. Assessment of the effects of specific agonists and antagonist revealed that 5-HT_2B engagement modifies the cytokine and gene signature of anti-inflammatory M-MØ, upregulates the expression of aryl hydrocarbon receptor (AhR) target genes, and stimulates the transcriptional activation of AhR. Moreover, we found that 5-HT dose dependently upregulates the expression of AhR target genes in M-MØ and that the 5-HT-mediated activation of AhR is 5-HT_2B dependent because it is abrogated by the 5-HT_2B-specific antagonist SB204741. Altogether, our results demonstrate the existence of a functional 5-HT/5-HT_2B/AhR axis in human macrophages and indicate that 5-HT potentiates the activity of a transcription factor (AhR) that regulates immune responses and the biological responses to xenobiotics.

Serotonergic regulation of insulin secretion

The exact physiological role for the monoamine serotonin (5-HT) in modulation of insulin secretion is yet to be fully understood. Although the presence of this monoamine in islets of Langerhans is well established, it is only with recent advances that the complex signalling network in islets involving 5-HT is being unravelled. With more than fourteen different 5-HT receptors expressed in human islets and receptor-independent mechanisms in insulin-producing β-cells, our understanding of 5-HT's regulation of insulin secretion is increasing. It is now widely accepted that failure of the pancreatic β-cell to release sufficient amounts of insulin is the main cause of type 2 diabetes (T2D), an ongoing global epidemic. In this context, 5-HT signalling may be of importance. In fact, 5-HT may serve an essential role in regulating the release of insulin and glucagon, the two main hormones that control glucose and lipid homoeostasis. In this review, we will discuss past and current understanding of 5-HT's role in the endocrine pancreas.

Interaction of hepatic stellate cells with neutrophils and macrophages in the liver following oncogenic kras activation in transgenic zebrafish

Activation of hepatic stellate cells (HSC) plays a crucial role in the liver disease progression from liver fibrosis/cirrhosis to cancer. Here, we found a rapid change of microenvironment after kras^V12-induction in zebrafish liver with progressively increased stromal cell number and enlarged liver size. Neutrophils and macrophages exhibited a faster response than HSCs. By manipulating the numbers of neutrophils and macrophages through morpholino knockdown, we found that macrophages contributed to both HSC survival and activation while neutrophils appear to be only required for HSC activation. Serotonin, which is essential for HSC survival and activation, was found up-regulated in hepatocytes and macrophages, but not in neutrophils after kras^V12 induction. Serotonin receptor was highly expressed in HSCs; increase of the receptor activity by an agonist stimulated HSCs and oncogenic growth of the liver while an opposite effect was observed with an antagonist. Activated HSCs promoted the pro-tumorigenesis functions of neutrophils and macrophages through secretion of Tgfb1. Overall, these observations elucidated a cellular interaction in microenvironment where that upregulated serotonin in hepatocytes and macrophages activated HSCs. Since the microenvironment crosstalk plays a vital role in manipulation of liver carcinogenesis, the underlying mechanism may provide potential therapeutic targets for liver diseases.

Serotonin drives the acquisition of a profibrotic and anti-inflammatory gene profile through the 5-HT7R-PKA signaling axis

Peripheral serotonin (5-hydroxytryptamine, 5-HT) regulates cell growth and differentiation in numerous cell types through engagement of seven types of cell surface receptors (HTR1-7). Deregulated 5-HT/HTR levels contribute to pathology in chronic inflammatory diseases, with macrophages being relevant targets for the physio-pathological effects of 5-HT. In fact, 5-HT skews human macrophage polarization through engagement of 5-HT2BR and 5-HT7R receptors. We now report that 5-HT primes macrophages for reduced pro-inflammatory cytokine production and IFN type I-mediated signaling, and promotes an anti-inflammatory and pro-fibrotic gene signature in human macrophages. The acquisition of the 5-HT-dependent gene profile primarily depends on the 5-HT7R receptor and 5-HT7R-initiated PKA-dependent signaling. In line with the transcriptional results, 5-HT upregulates TGFβ1 production by human macrophages in an HTR7- and PKA-dependent manner, whereas the absence of Htr7 in vivo results in diminished macrophage infiltration and collagen deposition in a mouse model of skin fibrosis. Our results indicate that the anti-inflammatory and pro-fibrotic activity of 5-HT is primarily mediated through the 5-HT7R-PKA axis, and that 5-HT7R contributes to pathology in fibrotic diseases.

Control of sensory neuron excitability by serotonin involves 5HT2C receptors and Ca(2+)-activated chloride channels

Serotonin (5HT) is a constituent of the so-called “inflammatory soup” that sensitizes nociceptors during inflammation. Nevertheless, receptors and signaling mechanisms that mediate an excitation of dorsal root ganglion (DRG) neurons by 5HT remained controversial. Therefore, capsaicin-sensitive nociceptive neurons dissociated from rat DRGs were used to investigate effects of 5HT on membrane excitability and currents through ligand- as well as voltage-gated ion channels. In 58% of the neurons tested, 5HT increased action potential firing, an effect that was abolished by the 5HT2 receptor antagonist ritanserin, but not by the 5HT3 antagonist tropisetron. Unlike other algogenic mediators, such as PGE2 and bradykinin, 5HT did not affect currents through TTX-resistant Na⁺ channels or Kv7 K⁺ channels. In all neurons investigated, 5HT potentiated capsaicin-evoked currents through TRPV1 channels, an effect that was attenuated by antagonists at 5HT2A (4 F 4 PP), 5HT2B (SB 204741), as well as 5HT2C (RS 102221) receptors. 5HT triggered slowly arising inward Cl⁻ currents in 53% of the neurons. This effect was antagonized by the 5HT2C receptor blocker only, and the current was prevented by an inhibitor of Ca²⁺-activated chloride channels (CaCC). The 5HT-induced increase in action potential firing was also abolished by this CaCC blocker and by the TRPV1 inhibitor capsazepine. Amongst the subtype selective 5HT2 antagonists, only RS 102221 (5HT2C-selectively) counteracted the rise in action potential firing elicited by 5HT. These results show that 5HT excites DRG neurons mainly via 5HT2C receptors which concomitantly mediate a sensitization of TRPV1 channels and an opening of CaCCs.

5-HT Receptor Antagonism Attenuates the Ischemia-Reperfusion Injury After Rabbit Lung Preservation

The success of lung transplantation is threatened by the appearance of ischemia-reperfusion injury, which is characterized by increased vascular permeability. 5-Hydroxytryptamine (5-HT; serotonin) is known to produce microvascular leakage in the systemic circulation, but its possible role in ischemia-reperfusion injury after lung preservation has not been reported. In this work we measured the release of 5-HT during a 24-hour rabbit lung preservation, and the effect of methiothepin (antagonist of the majority of 5-HT receptors) and SB204741 (antagonist of 5-HT2B/2C receptors) on the modified capillary filtration coefficient (mKf,c) was evaluated at the end of this period. Our results showed that the highest release rate of 5-HT occurred during the first 15 minutes after the lung harvesting and progressively decreased in the following time intervals. The baseline mKf,c greatly increased after 24 hours of lung preservation, and this increment was partially reduced by methiothepin and even more by SB204741. We concluded that 5-HT may play an important role in the ischemia-reperfusion process after lung preservation.

Serotonin 2B Receptor Antagonism Prevents Heritable Pulmonary Arterial Hypertension

Serotonergic anorexigens are the primary pharmacologic risk factor associated with pulmonary arterial hypertension (PAH), and the resulting PAH is clinically indistinguishable from the heritable form of disease, associated with BMPR2 mutations. Both BMPR2 mutation and agonists to the serotonin receptor HTR2B have been shown to cause activation of SRC tyrosine kinase; conversely, antagonists to HTR2B inhibit SRC trafficking and downstream function. To test the hypothesis that a HTR2B antagonist can prevent BMRP2 mutation induced PAH by restricting aberrant SRC trafficking and downstream activity, we exposed BMPR2 mutant mice, which spontaneously develop PAH, to a HTR2B antagonist, SB204741, to block the SRC activation caused by BMPR2 mutation. SB204741 prevented the development of PAH in BMPR2 mutant mice, reduced recruitment of inflammatory cells to their lungs, and reduced muscularization of their blood vessels. By atomic force microscopy, we determined that BMPR2 mutant mice normally had a doubling of vessel stiffness, which was substantially normalized by HTR2B inhibition. SB204741 reduced SRC phosphorylation and downstream activity in BMPR2 mutant mice. Gene expression arrays indicate that the primary changes were in cytoskeletal and muscle contractility genes. These results were confirmed by gel contraction assays showing that HTR2B inhibition nearly normalizes the 400% increase in gel contraction normally seen in BMPR2 mutant smooth muscle cells. Heritable PAH results from increased SRC activation, cellular contraction, and vascular resistance, but antagonism of HTR2B prevents SRC phosphorylation, downstream activity, and PAH in BMPR2 mutant mice.

5-HT(2B) antagonism arrests non-canonical TGF-β1-induced valvular myofibroblast differentiation

Transforming growth factor-β1 (TGF-β1) induces myofibroblast activation of quiescent aortic valve interstitial cells (AVICs), a differentiation process implicated in calcific aortic valve disease (CAVD). The ubiquity of TGF-β1 signaling makes it difficult to target in a tissue specific manner; however, the serotonin 2B receptor (5-HT(2B)) is highly localized to cardiopulmonary tissues and agonism of this receptor displays pro-fibrotic effects in a TGF-β1-dependent manner. Therefore, we hypothesized that antagonism of 5-HT(2B) opposes TGF-β1-induced pathologic differentiation of AVICs and may offer a druggable target to prevent CAVD. To test this hypothesis, we assessed the interaction of 5-HT(2B) antagonism with canonical and non-canonical TGF-β1 pathways to inhibit TGF-β1-induced activation of isolated porcine AVICs in vitro. Here we show that AVIC activation and subsequent calcific nodule formation is completely mitigated by 5-HT(2B) antagonism. Interestingly, 5-HT(2B) antagonism does not inhibit canonical TGF-β1 signaling as identified by Smad3 phosphorylation and activation of a partial plasminogen activator inhibitor-1 promoter (PAI-1, a transcriptional target of Smad3), but prevents non-canonical p38 MAPK phosphorylation. It was initially suspected that 5-HT(2B) antagonism prevents Src tyrosine kinase phosphorylation; however, we found that this is not the case and time-lapse microscopy indicates that 5-HT(2B) antagonism prevents non-canonical TGF-β1 signaling by physically arresting Src tyrosine kinase. This study demonstrates the necessity of non-canonical TGF-β1 signaling in leading to pathologic AVIC differentiation. Moreover, we believe that the results of this study suggest 5-HT(2B) antagonism as a novel therapeutic approach for CAVD that merits further investigation.

Disubstituted thiourea derivatives and their activity on CNS: synthesis and biological evaluation

A series of new thiourea derivatives of 1,2,4-triazole have been synthesized. The difference in structures of obtained compounds are directly connected with the kind of isothiocyanate (aryl/alkyl). The (1)H NMR, (13)C NMR, MS methods were used to confirm structures of obtained thiourea derivatives. The molecular structure of (1, 17) was determined by an X-ray analysis. Two of the new compounds (8 and 14) were tested for their pharmacological activity on animal central nervous system (CNS) in behavioural animal tests. The results presented in this work indicate the possible involvement of the serotonergic system in the activity of 8 and 14. In the case of 14 is also a possible link between its activity and the endogenous opioid system. All obtained compounds were tested for antibacterial activity against gram-positive cocci, gram-negative rods and antifungal activity. Compounds (1, 2, 5, 7, 9) showed significant inhibition against gram-positive cocci. Microbiological evaluation was carried out over 20 standard strains and 30 hospital strains. Selected compounds (1-13) were examined for cytotoxicity, antitumor, and anti-HIV activity.

Stimulating healthy tissue regeneration by targeting the 5-HT₂B receptor in chronic liver disease

Tissue homeostasis requires an effective, limited wound-healing response to injury. In chronic disease, failure to regenerate parenchymal tissue leads to the replacement of lost cellular mass with a fibrotic matrix. The mechanisms that dictate the balance of cell regeneration and fibrogenesis are not well understood. Here we report that fibrogenic hepatic stellate cells (HSCs) in the liver are negative regulators of hepatocyte regeneration. This negative regulatory function requires stimulation of the 5-hydroxytryptamine 2B receptor (5-HT(2B)) on HSCs by serotonin, which activates expression of transforming growth factor β1 (TGF-β1), a powerful suppressor of hepatocyte proliferation, through signaling by mitogen-activated protein kinase 1 (ERK) and the transcription factor JunD. Selective antagonism of 5-HT(2B) enhanced hepatocyte growth in models of acute and chronic liver injury. We also observed similar effects in mice lacking 5-HT(2B) or JunD or upon selective depletion of HSCs in wild-type mice. Antagonism of 5-HT(2B) attenuated fibrogenesis and improved liver function in disease models in which fibrosis was pre-established and progressive. Pharmacological targeting of 5-HT(2B) is clinically safe in humans and may be therapeutic in chronic liver disease.

Serotonin modulates the population activity profile of olfactory bulb external tufted cells

Serotonergic neurons in the raphe nuclei constitute one of the most prominent neuromodulatory systems in the brain. Projections from the dorsal and median raphe nuclei provide dense serotonergic innervation of the glomeruli of olfactory bulb. Odor information is initially processed by glomeruli, thus serotonergic modulation of glomerular circuits impacts all subsequent odor coding in the olfactory system. The present study discloses that serotonin (5-HT) produces excitatory modulation of external tufted (ET) cells, a pivotal neuron in the operation of glomerular circuits. The modulation is due to a transient receptor potential (TRP) channel-mediated inward current induced by activation of 5-HT(2A) receptors. This current produces membrane depolarization and increased bursting frequency in ET cells. Interestingly, the magnitude of the inward current and increased bursting inversely correlate with ET cell spontaneous (intrinsic) bursting frequency: slower bursting ET cells are more strongly modulated than faster bursting cells. Serotonin thus differentially impacts ET cells such that the mean bursting frequency of the population is increased. This centrifugal modulation could impact odor processing by: 1) increasing ET cell excitatory drive on inhibitory neurons to increase presynaptic inhibition of olfactory sensory inputs and postsynaptic inhibition of mitral/tufted cells; and/or 2) coordinating ET cell bursting with exploratory sniffing frequencies (5-8 Hz) to facilitate odor coding.

Central 5-HT(2A) receptors modulate the vagal bradycardia in response to activation of the von Bezold-Jarisch reflex in anesthetized rats

Activation of 5-hydroxytryptamine (5-HT) 5-HT(1A), 5-HT(2C), 5-HT(3), and 5-HT(7) receptors modulates the excitability of cardiac vagal motoneurones, but the precise role of 5-HT(2A/2B) receptors in these phenomena is unclear. We report here the effects of intracisternal (ic) administration of selective 5-HT(2A/2B) antagonists on the vagal bradycardia elicited by activation of the von Bezold-Jarisch reflex with phenylbiguanide. The experiments were performed on urethane-anesthetized male Wistar rats (250-270 g, N = 7-9 per group). The animals were placed in a stereotaxic frame and their atlanto-occipital membrane was exposed to allow ic injections. The rats received atenolol (1 mg/kg, iv) to block the sympathetic component of the reflex bradycardia; 20-min later, the cardiopulmonary reflex was induced with phenylbiguanide (15 µg/kg, iv) injected at 15-min intervals until 3 similar bradycardias were obtained. Ten minutes after the last pre-drug bradycardia, R-96544 (a 5-HT(2A) antagonist; 0.1 µmol/kg), SB-204741 (a 5-HT(2B) antagonist; 0.1 µmol/kg) or vehicle was injected ic. The subsequent iv injections of phenylbiguanide were administered 5, 20, 35, and 50 min after the ic injection. The selective 5-HT(2A) receptor antagonism attenuated the vagal bradycardia and hypotension, with maximal effect at 35 min after the antagonist (pre-drug = -200 ± 11 bpm and -42 ± 3 mmHg; at 35 min = -84 ± 10 bpm and -33 ± 2 mmHg; P < 0.05). Neither the 5-HT(2B) receptor antagonists nor the vehicle changed the reflex. These data suggest that central 5-HT(2A) receptors modulate the central pathways of the parasympathetic component of the von Bezold-Jarisch reflex.

A selective, high affinity 5-HT 2B receptor antagonist inhibits visceral hypersensitivity in rats

BACKGROUND

RS-127445 is a selective, high affinity 5-HT(2B)receptor antagonist. We investigated whether 5-HT(2B)receptor antagonists can reduce colonic visceral hypersensitivity caused by restraint stress or by proximal colonic inflammation.

METHODS

Visceral hypersensitivity was induced in rats by either restraint stress or injection of 2, 4, 6-trinitrobenzene sulfonic acid (TNBS) into the proximal colon. Restraint stress produced a significant increase in numbers of abdominal contractions evoked by colorectal distension (CRD), measured as a quantitative index of visceral nociception in rats. Seven days after TNBS injection, the pain threshold to CRD at the non-inflamed distal colon, that was determined as the minimum pressure required to evoke abdominal cramp, was significantly decreased. The effect of RS-127445 on visceral hypersensitivity was assessed in either naïve or TNBS-treated rats.

KEY RESULTS

Oral administration of a selective, high affinity 5-HT(2B)receptor antagonist, RS-127445, significantly inhibited visceral hypersensitivity provoked by restraint stress (35 to 74% inhibition at 1 to 10 mg kg(-1)). Oral RS-127445 produced a significant suppression of TNBS-induced visceral hypersensitivity (15 to 62% inhibition at 3 to 30 mg kg(-1)), although it was without significant effect on the visceral nociceptive threshold of naïve rats. RS-127445 (1 to 30 mg kg(-1), p.o.) also dose-dependently reduced the restraint stress-induced defecation in naïve and TNBS-treated rats.

CONCLUSIONS & INFERENCES

These results suggest that 5-HT(2B)receptors are involved in signaling from the colon in rats in which there is visceral hypersensitivity and that a selective 5-HT(2B)receptor antagonist could have therapeutic potential for the treatment of gut disorders characterized by visceral hypersensitivity.

Role of monoamine oxidases in the exaggerated 5-hydroxytryptamine-induced tension development of human isolated preeclamptic umbilical artery

We investigated the role(s) of monoamine oxidases (MAOs) on the altered 5-hydroxytryptamine (5-HT, serotonin)-induced tension development of the isolated umbilical artery of preeclamptic pregnancy of Chinese women. An enhanced 5-HT-induced tension development of the umbilical artery of preeclamptic pregnancy was observed when compared with that of normal pregnancy. The enhanced component of 5-HT-induced tension development was eradicated by clorgyline (a MAO-A inhibitor). Blockade of eNOS (endothelial isoform nitric oxide synthase) (N(omega)-nitro-L-arginine methyl ester), 5-HT transporter (citalopram), 5-HT receptor subtypes (5HT2B, SB 204741; 5-HT2C, RS 102221; 5-HT7, SB 269970), and endothelium denudation of the umbilical artery of normal pregnancy mimicked the enhanced 5-HT-induced tension development as observed in the preeclamptic tissues. In contrast, no apparent changes in 5-HT-induced tension development of the umbilical artery of preeclamptic pregnancy were observed with the same pharmacological manipulations. A decreased protein expression levels of MAO-A and eNOS (no iNOS and MAO-B expression was detected) and no change in caveolin-1 and 5-HT transporter expression were demonstrated in the umbilical artery (endothelium intact) lysate of preeclamptic pregnancy, compared to that of the umbilical artery of normal pregnancy. Thus, in the umbilical artery of preeclamptic pregnancy, a decrease of MAO-A and eNOS protein expression levels are probably associated with, or responsible for, the exaggerated 5-HT-induced tension development.

Formation of fluorine-18 labeled diaryl ureas--labeled VEGFR-2/PDGFR dual inhibitors as molecular imaging agents for angiogenesis

Urea subunits are common components of various pharmaceuticals' core structure. Since in most cases the design and development of PET biomarkers is based on approved or potential drugs, there is a growing need for a general labeling methodology of urea-containing pharmacophores. As a part of research in the field of molecular imaging of angiogenic processes, we synthesized several highly potent VEGFR-2/PDGFR dual inhibitors as potential PET biomarkers. The structure of these inhibitors is based on the N-phenyl-N'-{4-(4-quinolyloxy)phenyl}urea skeleton. A representative inhibitor was successfully labeled with fluorine-18 by a three-step process. Initially, a two-step radiosynthesis of 4-[(18)F]fluoro-aniline from 1,4-dinitrobenzene (60min, EOB decay corrected yield: 63%) was performed. At the third and final step, the 4-[(18)F]fluoro-aniline synthon reacted for 30min at room temperature with 4-(2-fluoro-4-isocyanato-phenoxy)-6,7-dimethoxy-quinoline to give complete conversion of the labeled synthon to 1-[4-(6,7-dimethoxy-quinolin-4-yloxy)-3-fluoro-phenyl]-3-(4-[(18)F]fluoro-phenyl)-urea. The desired labeled product was obtained after total radiosynthesis time of 3h including HPLC purification with 46+/-1% EOB decay corrected radiochemical yield, 99% radiochemical purity, 99% chemical purity, and a specific activity of 400+/-37GBq/mmol (n=5).

Synthesis and Pharmacological Activity of Urea and Thiourea Derivatives of 4-Azatricyclo[5.2.2.02,6]undec-8-ene-3,5-dione

A series of nineteen new thiourea and urea derivatives of 10-isopropyl-8-methyl-4-azatricyclo[5.2.2.0(2,6)]undec-8-ene-3,5-dione, 1-isopropyl-7-methyl-4-azatricyclo[5.2.2.0(2,6)]undec-8-ene-3,5-dione and 1,7,8,9,10-pentamethyl-4-azatricyclo[5.2.1.0(2,6)]dec-8-ene-3,5-dione have been prepared and studied by (1)H-NMR. The compound k1a (1-(1,7,8,9,10-pentamethyl-3,5-dioxo-4-aza-tricyclo[5.2.1.0(2,6)]dec-8-en-4-yl)-3-phenyl-urea) was tested for pharmacological activity on animal central nervous system (CNS). The activities of synthesized compounds were evaluated for their cytotoxicity and anti-HIV-1 activity in MT-4 cells. Antimicrobial activity of the newly obtained derivatives was tested against some Gram-positive and Gram-negative bacteria and fungi of the Candida species.

Contractile effects of 5-hydroxytryptamine and 5-carboxamidotryptamine in the equine jejunum

The use of human prokinetic drugs in colic horses leads to inconsistent results. This might be related to differences in gastrointestinal receptor populations. The motor effects of 5-hydroxytryptamine (5-HT; serotonin) on the equine mid-jejunum were therefore studied. Longitudinal muscle preparations were set up for isotonic measurement. 5-HT induced tonic contractions with superimposed phasic activity; these responses were not influenced by tetrodotoxin and atropine, suggesting a non-neurogenic, non-cholinergic pathway. The 5-HT receptor antagonists GR 127935 (5-HT(1B,D)), ketanserin (5-HT(2A)), SB 204741 (5-HT(2B)), RS 102221 (5-HT(2C)), granisetron (5-HT(3)), GR 113808 (5-HT(4)) and SB 269970 (5-HT(7)) had no influence on the 5-HT-induced response; the 5-HT(1A) receptor antagonists NAN 190 (pK(b)=8.13+/-0.06) and WAY 100635 (pK(b)=8.69+/-0.07), and the 5-HT(1,2,5,6,7) receptor antagonist methysergide concentration-dependently inhibited the 5-HT-induced contractile response. The 5-HT(1,7) receptor agonist 5-carboxamidotryptamine (5-CT) induced a contractile response similar to that of 5-HT; its effect was not influenced by tetrodotoxin and atropine, and SB 269970, but antagonised by WAY 100635. 8-OHDPAT, buspiron and flesinoxan, which are active at rat and human 5-HT(1A) receptors, had no contractile influence. These results suggest that the contractile effect of 5-HT in equine jejunal longitudinal muscle is due to interaction with muscular 5-HT receptors, which cannot be characterised between the actually known classes of 5-HT receptors.

Contribution of serotonin (5-HT) 5-HT2 receptor subtypes to the discriminative stimulus effects of cocaine in rats

RATIONALE

The serotonin (5-hydroxytryptamine; 5-HT) 5-HT2 receptor (5-HT2R) family is an important regulator of the behavioral responsiveness to cocaine.

OBJECTIVE

The present study is an analysis of the role of the 5-HT2R subtypes (5-HT2AR, 5-HT2BR, and 5-HT2CR) in the discriminative stimulus effects of cocaine.

METHODS

Male Wistar rats were trained to discriminate cocaine (10 mg/kg) from saline in a two-lever, water-reinforced FR 20 task, and we investigated the ability of the 5-HT2AR antagonist 1(Z)-[2-(dimethylamino)ethoxyimino]-1(2-fluorophenyl)-3-(4-hydroxyphenyl)-2(E)-propene (SR 46349B), the 5-HT2BR antagonist N-(1-methyl-5-indolyl)-N'-(3-methyl-5-isothiazolyl) urea (SB 204741), and the 5-HT2CR antagonist [(+)-cis-4,5,7a,8,9,10,11,11a-octahydro-7H-10-methylindolo(1,7-bC)(2,6)naphthyridine (SDZ SER-082) to substitute for or to modulate the stimulus effects of cocaine.

RESULTS

Pretreatment with SR 46349B (0.5-1 mg/kg) resulted in a rightward shift of the cocaine dose-response curve, while SDZ SER-082 (1 mg/kg) shifted the dose-response for cocaine to the left; SB 204741 (1-3 mg/kg) was inactive.

CONCLUSIONS

Our pharmacological analyses of selective antagonists of 5-HT2AR, 5-HT2BR, and 5-HT2CR indicate oppositional influence of 5-HT2AR and 5-HT2CR on the stimulus effects of cocaine and exclude a role for the 5-HT2BR. These data suggest that 5-HT2AR and 5-HT2CR may be important in modulating the subjective effects of cocaine in humans.

Functional 5-HT receptors in human occipital artery

5-HT receptors were studied in human occipital arteries, obtained from patients during neurosurgery. We detected mRNA for the following receptors (incidence): 5-HT(1B) (14/18), 5-HT(1D) (15/18), 5-HT(2A) (16/18), 5-HT(2B) (8/8), 5-HT(4(a)) (13/18), 5-HT(4(b)) (5/18), 5-HT(4(g)) (7/18), 5-HT(4(i)) (1/18), 5-HT(7(a/b)) (10/18) and 5-HT(7(d)) (12/18). 5-HT contracted and relaxed arterial rings at low (-logEC(50) M=7.0) and high (-logEC(50) M=4.2) concentrations, respectively. 5-HT-evoked contractions were antagonized partially by both 5-HT(1B)-selective SB224289 (200 nM) and 5-HT(2A)-selective ketanserin (1 microM) but not by 5-HT(1D)-selective BRL15572 (500 nM) or prazosin (1 microM). Sumatriptan caused contractions (-logEC(50) M=6.8, intrinsic activity with respect to 5-HT=0.3). Sumatriptan-evoked contractions were antagonized by SB224289 with high potency (pK(B)=9.4) but not by BRL15572. 5-HT-induced relaxations were resistant to blockade by 5-HT(1B)-selective SB224289 (1 microM), 5-HT(1D)-selective BRL15572, 5-HT(2B)-selective SB204741 (1 microM), 5-HT(4)-selective GR113808 (100 nM) and 5-HT(7)-selective SB269970 (1 microM), and a combination of SB204741 and SB269970, inconsistent with an involvement of 5-HT(1B), 5-HT(1D), 5-HT(2B), 5-HT(4) and 5-HT(7) receptors. Triton X-100 treatment of the arteries abolished acetylcholine-induced relaxations of rings precontracted by prostaglandin F(2alpha), but a reduction of the relaxant effects of 5-HT did not reach significance. Nitro-L-arginine (1 mM) reduced 5-HT-induced relaxations, suggesting a contribution of nitric oxide released from endothelial cells. Ketanserin (1 microM) prevented the relaxant effects of 5-HT. We conclude that 5-HT contracts human occipital artery through 5-HT(1B) receptors at low concentrations and through 5-HT(2A) receptors at high concentrations. Sumatriptan contracts mostly through 5-HT(1B) receptors. These results are consistent with the 5-HT(1B) and 5-HT(2A) mRNA data. 5-HT-induced relaxation is mediated, in part, through ketanserin-sensitive receptors, but 5-HT(1B), 5-HT(1D), 5-HT(2B), 5-HT(4) and 5-HT(7) receptors appear not to be involved.

m-CPP-induced self-grooming is mediated by 5-HT2C receptors

m-Chlorophenylpiperazine (m-CPP), a potent 5-HT receptor agonist, is known to induce self-grooming in rats and exacerbate symptoms in patients with obsessive-compulsive disorder (OCD). To characterise the possible role, 5-HT(2B) and 5-HT(2C) receptors play in m-CPP-induced self-grooming, subtype-selective receptor antagonists were used. m-CPP significantly increased the amount of self-grooming in male Sprague-Dawley rats. This effect followed a bell-shaped dose-response curve with a peak at 0.6 mg/kg, i.p. Pretreatment with SB-242084, a subtype-selective 5-HT(2C) receptor antagonist (0.1-0.5 mg/kg, i.p.), reversed m-CPP-induced self-grooming. In contrast, pretreatment with the subtype-selective 5-HT(2B) receptor antagonist SB-215505 (1 mg/kg, i.p) did not block the effect of m-CPP. Two days after depletion of brain 5-HT by p-chlorophenylalanine (p-CPA, 2 x 50, 2 x 100 mg/kg, i.p.) m-CPP-induced responses were significantly enhanced compared to controls. Our studies provide evidence that direct activation of 5-HT(2C) receptors mediate m-CPP-induced self-grooming and the depletion of brain 5-HT sensitizes these receptors.

Smooth muscle 5-HT2A receptors mediating contraction of porcine isolated proximal stomach strips

1. The aim of this study was to characterize the 5-HT receptors involved in the 5-HT-induced contraction of longitudinal muscle (LM) strips of porcine proximal stomach. This was done in a classical organ bath set-up for isotonic measurement. 2. The concentration-contraction curve to 5-HT was not modified by 5-HT(3) and 5-HT(4) receptor antagonism. Methysergide, ketanserin and mesulergine antagonized the curve to 5-HT. Concomitantly, increasing concentrations of ketanserin and mesulergine progressively revealed a biphasic nature of the 5-HT curve. Ketanserin antagonized the low-affinity receptor while it did not modify the high-affinity receptor. 3. Tetrodotoxin did not influence the concentration-contraction curve to 5-HT neither in the absence nor presence of ketanserin, indicating that nerves are not involved. 4. Ketanserin competitively antagonized the monophasic concentration-response curve to alpha-Methyl-5-HT, yielding a Schild slope that was not significantly different from unity. After constraining the Schild slope to unity, a pK(B) estimate of 8.23+/-0.90 was obtained. This affinity estimate of ketanserin closely approximates previously reported affinities at 5-HT(2A) receptors. 5. In the presence of ketanserin (0.1 microM; exposing the high-affinity receptor), a wide range of 5-HT receptor antagonists covering all 5-HT receptors known, was tested. Only methysergide and ritanserin inhibited the response to 5-HT, thus expressing affinity for the high-affinity receptor. This did not reveal the identity of the receptor involved. 6 It can be concluded that 5-HT induces pig proximal stomach (LM) contraction via 5-HT(2A) receptors located on smooth muscle. A ketanserin-insensitive phase of contractions could not be characterized between the actually known classes of 5-HT receptors with the pharmacological tools that were used.

Iodine(V) reagents in organic synthesis. Part 2. Access to complex molecular architectures via Dess-Martin periodinane-generated o-imidoquinones

o-Imidoquinones, a rather rare class of compounds, are prepared from anilides by the action of Dess-Martin periodinane (DMP) and water. Their chemistry has been extensively investigated and found to lead to p-quinones and polycyclic systems of diverse molecular architectures. Applications of this methodology to the total synthesis of the naturally occurring compounds, epoxyquinomycin B and BE-10988, are described. Finally, another rare chemical entity, the ketohydroxyamide moiety, has been accessed through this DMP-based synthetic technology, and its reactivity has been studied. Among its most useful reactions is a set of cascade heterocyclic annulations leading to a variety of polycyclic systems of possible biological relevance.

Characterization of the receptors involved in the 5-HT-induced excitation of canine antral longitudinal muscle

1. We aimed to characterize the 5-HT receptors involved in the 5-HT-induced effect on electrically induced contractions of dog antrum longitudinal muscle in vitro. 2. In the presence of L-NOARG (0.1 mM), electrical field stimulation (EFS) induced atropine- and tetrodotoxin-sensitive contractions. Tetrodotoxin or atropine left any agonist tested ineffective. These EFS-induced contractions were on average enhanced by 5-HT (0.3 microM), however, pronounced variation in the response to 5-HT was observed. There were non-significant trends of the selective 5-HT3 receptor antagonist granisetron (1 microM), and methysergide (1 microM; preventing interactions of 5-HT with 5-HT1, 5-HT2, 5-ht5, 5-HT6 and 5-HT7 receptors) to increase the response to 5-HT. The selective 5-HT4 receptor antagonist GR 113808 (0.1 microM) displayed a non-significant trend to inhibit the 5-HT-induced increase. 3. Combination experiments with methysergide (1 microM), granisetron (1 microM) and GR 113808 (0.1 microM) revealed that the 5-HT (0.3 microM)-induced response consisted of (1) an excitatory component blocked by GR 113808, (2) excitatory and inhibitory components both blocked by methysergide. 4. The selective 5-HT4 receptor agonist prucalopride (0.3 microM) increased EFS-induced contractions, an effect prevented by GR 113808 (0.1 microM). 5. The increase of EFS-induced contractions by the preferential 5-HT2 receptor agonist alpha-Me-5-HT (0.3 microM) was antagonized by 5-HT2B receptor antagonists. 6. The 5-HT1/5-HT7 receptor agonist 5-carboxamidotryptamine (5-CT; 0.3 microM) inhibited EFS-induced contractions. This was prevented by methysergide (1 microM), the 5-HT7 receptor antagonist mesulergine (0.3 microM) and the selective 5-HT7 receptor antagonist SB-269970 (0.3 microM). 7. In the presence of GR 113808 (0.1 microM), alpha-Me-5-HT (1 microM) increased EFS-induced contractions. The 5-HT (0.3 microM)-induced inhibition of the stimulation by alpha-Me-5-HT was prevented by SB-269970 (0.3 microM). 8. In conclusion, dog antral longitudinal muscle is endowed with (1) excitatory neuronal 5-HT4 receptors and 5-HT2B receptors and (2) inhibitory smooth muscle 5-HT7 receptors.

Reciprocal autoreceptor and heteroreceptor control of serotonergic, dopaminergic and noradrenergic transmission in the frontal cortex: relevance to the actions of antidepressant agents

The frontal cortex (FCX) plays a key role in processes that control mood, cognition and motor behaviour, functions which are compromised in depression, schizophrenia and other psychiatric disorders. In this regard, there is considerable evidence that a perturbation of monoaminergic input to the FCX is involved in the pathogenesis of these states. Correspondingly, the modulation of monoaminergic transmission in the FCX and other corticolimbic structures plays an important role in the actions of antipsychotic and antidepressant agents. In order to further understand the significance of monoaminergic systems in psychiatric disorders and their treatment, it is essential to characterize mechanisms underlying their modulation. Within this framework, the present commentary focuses on our electrophysiological and dialysis analyses of the complex and reciprocal pattern of auto- and heteroreceptor mediated control of dopaminergic, noradrenergic and serotonergic transmission in the FCX. The delineation of such interactions provides a framework for an interpretation of the influence of diverse classes of antidepressant agent upon extracellular levels of dopamine, noradrenaline and serotonin in FCX. Moreover, it also generates important insights into strategies for the potential improvement in the therapeutic profiles of antidepressant agents.

Serotonin(2C) receptors tonically suppress the activity of mesocortical dopaminergic and adrenergic, but not serotonergic, pathways: a combined dialysis and electrophysiological analysis in the rat

The present study evaluated, via a combined electrophysiological and dialysis approach, the potential influence of serotonin (5-HT)(2C) as compared to 5-HT(2A) and 5-HT(2B) receptors on dopaminergic, adrenergic, and serotonergic transmission in frontal cortex (FCX). Whereas the selective 5-HT(2A) antagonist MDL100,907 failed to modify extracellular levels of dopamine (DA), noradrenaline (NA) or 5-HT simultaneously quantified in single dialysate samples of freely-moving rats, the 5-HT(2B)/5-HT(2C) antagonist SB206,553 dose-dependently increased levels of DA and NA without affecting those of 5-HT. This action was attributable to 5-HT(2C) receptor blockade inasmuch as the selective 5-HT(2C) antagonist SB242,084 likewise increased FCX levels of DA and NA, whereas the selective 5-HT(2B) antagonist SB204,741 was ineffective. Further, the preferential 5-HT(2C) receptor agonist Ro60-0175 dose-dependently depressed FCX levels of DA. The suppressive influence of 5-HT(2C) receptors on DA release was also expressed on mesolimbic and nigrostriatal dopaminergic pathways, in that levels of DA in nucleus accumbens and striatum were likewise reduced by Ro60-0175 and elevated, though less markedly, by SB206,553. In line with the above findings, Ro60-0175 dose-dependently decreased the firing rate of ventrotegmental dopaminergic and locus coeruleus (LC) adrenergic perikarya, whereas their activity was dose-dependently enhanced by SB206,553. Furthermore, SB206,553 transformed the firing pattern of ventrotegmental dopaminergic neurons into a burst mode. In contrast to SB206,553, MDL100,907 had little affect on the firing rate of dopaminergic or adrenergic neurons. In conclusion, as compared to 5-HT(2A) and 5-HT(2B) receptors, 5-HT(2C) receptors exert a tonic, suppressive influence on the activity of mesocortical - as well as mesolimbic and nigrostriatal - dopaminergic pathways, likely via indirect actions expressed at the level of their cell bodies. Frontocortical adrenergic, but not serotonergic, transmission is also tonically suppressed by 5-HT(2C) receptors.

Evidence that activation of central 5-HT(2B) receptors causes renal sympathoexcitation in anaesthetized rats

The effects of injections i.c.v. of alpha-methyl-5-(2-thienylmethoxy)-1H-indole-3-ethanamine (BW723C86; 0.02 - 2 micromol kg(-1)), a 5-HT(2B) receptor agonist, on renal sympathetic and phrenic nerve activity, mean arterial blood pressure and heart rate were investigated in alpha-chloralose anaesthetized rats pretreated with a peripherally acting 5-HT(2) receptor antagonist. BW723C86 i.c.v. caused a dose-related increase in renal nerve activity reaching a maximum of 67+/-6%, which at the highest dose was associated with a small and maintained fall in mean arterial blood pressure of 7+/-3 mmHg. These changes were not associated with any significant changes in heart rate or phrenic nerve activity. BW723C86-evoked increases in renal nerve activity and hypotension were attenuated by pretreatment (i.c.v.) with SB204741 (300 nmol kg(-1); a 5-HT(2B) receptor antagonist) but not by the same dose (i.c.v.) of ketanserin (a 5-HT(2A) receptor antagonist) or RS102221 (a 5-HT(2C) receptor antagonist). None of these antagonists alone had any effect on the variables being measured. It is concluded that central 5-HT(2B) receptors may play a selective role in the control of sympathetic supply to the kidney, which could be important in the central mechanisms involved in blood volume regulation. British Journal of Pharmacology (2000) 129, 177 - 183

Evidence for a role for central 5-HT2B as well as 5-HT2A receptors in cardiovascular regulation in anaesthetized rats

1. The effects of injections i.c.v. of quipazine, (2 micromol kg-1) and 1-(2,5-di-methoxy-4-iodophenyl)-2-aminopropane (DOI; 2 micromol kg-1) on renal sympathetic and phrenic nerve activity, mean arterial blood pressure (MAP) and heart rate were investigated in alpha-chloralose anaesthetized rats pretreated with a peripherally acting 5-HT2 receptor antagonist. 2. Quipazine or DOI caused a rise in MAP which was associated with a tachycardia and renal sympathoinhibition in rats pretreated (i.c.v.) with the antagonist vehicle 10% PEG. These effects of quipazine were completely blocked by pretreatment with cinanserin (a 5-HT2 receptor antagonist) and attenuated by spiperone (a 5-HT2A receptor antagonist). However, pretreatment with SB200646A (a 5-HT2B/2C receptor antagonist) only blocked the sympathoinhibition, while pretreatment with SB204741 (a 5-HT2B receptor antagonist) reversed the sympathoinhibition to excitation as it also did for DOI. Quipazine also caused renal sympathoexcitation in the presence (i.v.) of a vasopressin V1 receptor antagonist. 3. Injection (i.v.) of the V1 receptor antagonist at the peak pressor response evoked by quipazine alone and in the presence of SB204741 caused an immediate fall in MAP. For quipazine alone the renal sympathoinhibition was slowly reversed to an excitation, while the renal sympathoexcitation observed in the presence of SB204741 was potentiated. In both, the quipazine-evoked tachycardia was unaffected. 4. The data indicate that cardiovascular responses caused by i.c.v. quipazine and DOI are primarily due to activation of central 5-HT2A receptors, which causes the release of vasopressin and a tachycardia. This released vasopressin appears to suppress a 5-HT2A receptor-evoked central increase in sympathetic outflow, which involves the activation of central 5-HT2B receptors indirectly by the released vasopressin.

RS-127445: a selective, high affinity, orally bioavailable 5-HT2B receptor antagonist

Efforts to define precisely the role of 5-HT2B receptors in normal and disease processes have been hindered by the absence of selective antagonists. To address this deficiency, we developed a series of naphthylpyrimidines as potentially useful 5-HT2B receptor antagonists. RS-127445 (2-amino-4-(4-fluoronaphth-1-yl)-6-isopropylpyrimidine) was found to have nanomolar affinity for the 5-HT2B receptor (pKi = 9.5+/-0.1) and 1,000 fold selectivity for this receptor as compared to numerous other receptor and ion channel binding sites. In cells expressing human recombinant 5-HT2B receptors, RS-127445 potently antagonized 5-HT-evoked formation of inositol phosphates (pK(B) = 9.5+/-0.1) and 5-HT-evoked increases in intracellular calcium (pIC50 = 10.4+/-0.1). RS-127445 also blocked 5-HT-evoked contraction of rat isolated stomach fundus (pA2 = 9.5+/-1.1) and (+/-)alpha-methyl-5-HT-mediated relaxation of the rat jugular vein (pA2 = 9.9+/-0.3). RS-127445 had no detectable intrinsic activity in these assays. In rats, the fraction of RS-127445 that was bioavailable via the oral or intraperitoneal routes was 14 and 60% respectively. Intraperitoneal administration of RS-127445 (5 mg kg(-1)) produced plasma concentrations predicted to fully saturate accessible 5-HT2B receptors for at least 4 h. In conclusion, RS-127445 is a selective, high affinity 5-HT2B receptor antagonist suitable for use is vivo. The therapeutic potential of this molecule is being further evaluated.

Discriminative stimulus properties of the novel serotonin (5-HT)2C receptor agonist, RO 60-0175: a pharmacological analysis

Employing a Fixed-Ratio 10, food-reinforced protocol, rats were trained to recognize the discriminative stimulus (DS) properties of the novel, potent, 5-HT2C agonist, Ro 60-0175 (2.5 mg/kg, i.p.). This schedule generated appropriate drug versus vehicle responding after 50 + 5 training sessions and Ro 60-0175 elicited full (100%) drug selection with an effective dose50 (ED50) of 0.6 mg/kg, i.p.. The 5-HT2C receptor agonists, mCPP and MK 212, fully generalized to Ro 60-0175 with ED50s of 0.8 and 0.4 mg/kg, s.c., respectively, whereas the preferential 5-HT2B agonist, BW 723C86 ( > 10.0 mg/kg, s.c.) and the 5-HT2A agonist, DOI ( > 2.5 mg/kg, s.c.), were ineffective. The 5-HT2A/2B/2C receptor antagonist, mianserin, dose-dependently blocked the DS properties of Ro 60-0175 with an ED50 of 0.7 mg/kg, s.c. This action was mimicked by the novel, 5-HT2B/2C antagonist, SB 206,553 (ED50 = 0.3 mg/kg, s.c.), whereas the selective 5-HT2A antagonist, MDL 100,907 ( > 0.63 mg/kg, s.c.), was ineffective. Further, the selective 5-HT2C antagonist, SB 242,084, dose-dependently and fully blocked drug selection (ED50 = 0.2 mg/kg, i.p.), whereas the selective 5-HT2B antagonist, SB 204,741, was not active ( > 0.63 mg/kg, i.p.). In conclusion, these data demonstrate that Ro 60-0175 generates a robust DS and suggest that activation of 5-HT2C receptors is the principal mechanism underlying its DS properties.

Serotonin in migraine: theories, animal models and emerging therapies

A role for serotonin in migraine has been supported by changes in circulating levels of serotonin and its metabolites during the phases of a migraine attack, along with the ability of serotonin-releasing agents to induce migraine-like symptoms. The development of serotonin receptor agonists with efficacy in the clinic for the alleviation of migraine pain further implicates serotonin as a key molecule in migraine. Several theories regarding the etiology of migraine have been proposed. The vasodilatory theory of migraine suggested that extracranial arterial dilation during an attack was related to migraine pain; a theory supported when vasoconstrictors such as sumatriptan alleviated migraine pain. The neurological theory of migraine proposed that migraine resulted from abnormal firing in brain neurons. Cortical spreading depression, one facet of the neurological theory, could explain the prodrome of migraine. The neurogenic dural inflammation theory of migraine supposed that the dural membrane surrounding the brain became inflamed and hypersensitive due to release of neuropeptides from primary sensory nerve terminals. Substance P, calcitonin gene related peptide and nitric oxide are all though to play a role in the dural inflammatory cascade. Animal models of migraine have been utilized to study the physiology of migraine and develop new pharmaceutical therapies. One model measures the shunting of blood to arteriovenous anastomoses based on a proposal that migraine primarily involves cranial arteriovenous vasodilation. Another model utilizes electrical stimulation of the trigeminal ganglion to induce neurogenic dural inflammation quantified by the resulting extravasation of proteins. Pharmacological agents such as meta-chlorophenylpiperazine (mCPP) and nitroglycerin have also been used to induce dural extravasation in animals. Both compounds also induce migraine attacks in individuals with a history of migraine. In addition, Fos, a protein produced by activation of the c-fos gene, has been measured as an index of migraine-like pain transmission to the CNS following chemical or electrical stimulation of the trigeminal nerve. A role for serotonin in migraine is further supported by the efficacy of serotonin receptor ligands. Sumatriptan is an agonist at 5-HT1D and 5-HT1B receptor subtypes, and effective in treating migraine pain and associated symptoms. Recently, selective 5-HT1F agonists have been proposed for the treatment of migraine, without the side effects associated with the present 5-HT1D and 5-HT1B receptor agonists. A role for 5-HT2B receptors has also been suggested the initiation of migraine, supporting use of selective 5-HT2B receptor antagonists in migraine. Thus, agents that modulate 5-HT1B, 5-HT1D, 5-HT1F and 5-HT2B receptors either have or may have clinical utility in the therapy of migraine headache.

Selective blockade of serotonin2C/2B receptors enhances dopamine release in the rat nucleus accumbens

The effects of mesulergine (100 and 200 microg/kg s.c.), SB 206553 (1 and 2.5 mg/kg i.p.), RP 62203 (2.5 and 4 mg/kg i.p.) and ritanserin (630 microg/kg i.p.) were studied on the extracellular concentration of dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) in the nucleus accumbens of chloral hydrate-anesthetized rats, using intracerebral microdialysis. Mesulergine, a non selective serotonin2C/2B/2A (5-HT2C/2B/2A) receptor antagonist, significantly increased DA release, which reached a peak level (+ 20%) 60 min after drug injection and slowly returned back to baseline values. Mesulergine also caused a dose-dependent increase in DOPAC outflow. Pretreatment with mesulergine (200 microg/kg) did not change the inhibition of DA release induced by apomorphine (100 microg/kg), whereas it prevented the reduction of DOPAC outflow induced by apomorphine (100 microg/kg). Administration of SB 206553, a selective blocker of 5-HT2C/2B receptors, dose-dependently increased DA outflow. The dose of 2.5 mg/kg SB 206553 caused a linear increase of DA output which reached a peak (+75%) 40 min after injection, while 1 mg/kg induced a more gradual increase of DA release which peaked (+54%) 60 min after administration of the drug. Treatment with RP 62203, a selective 5-HT2A receptor antagonist, did not produce any significant effect on DA outflow. Administration of ritanserin, a mixed 5-HT2A/2C receptor antagonist, did not cause any significant change of DA and DOPAC outflow. Taken together, these data indicate that selective blockade of 5-HT2/2B receptor subtypes increases DA release in the rat nucleus accumbens.

5-HT2C receptors mediate penile erections in rats: actions of novel and selective agonists and antagonists

The mixed 5-HT2A/5-HT2B/5-HT2C receptor agonist, m-(chlorophenyl)piperazine (mCPP), elicited penile erections in rats, an action mimicked by the selective 5-HT2C receptor agonist, RO 60-0175 (S)-2-(6-chloro-5-fluoroindol-1-yl)-1-methylethylamine), whereas the preferential 5-HT2B receptor agonist, BW 723C86 (1-[5-(thienylmethoxy)-1H-3-indoyl] propan-2-amine) was ineffective. The actions of mCPP and RO 60-0175 were dose-dependently abolished by the novel 5-HT2B/5-HT2C receptor antagonists, SB 200,646 (1-(1-methylindol-5-yl)-3-(3-pyridyl) urea) and SB 206,553 (5 methyl-1-(3-pyridil-carbamoyl)-1,2,3,5-tetrahydropyrrolo[2,3 -f]indole). In contrast, penile erections were not significantly affected by the selective 5-HT2B receptor antagonist, SB 204,741 (1-(1-methylindol-5-yl)-3-(3-methylisothiazol-5-yl)-urea) nor by the selective 5-HT2A receptor antagonist, MDL 100,907 ([R(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-p iperidine-methanol]). These data provide rigorous pharmacological evidence that activation of 5-HT2C receptor elicits penile erections in the rat. This model should, thus, be of use for characterising novel ligands at this site.

Characterization of the contraction to 5-HT in the canine colon longitudinal muscle

1. Although conscious dogs have often been used for colonic motility studies with 5-hydroxytryptamine (5-HT), the effects of 5-HT on the isolated colon have not been thoroughly characterized yet. The current study was undertaken to characterize the response to 5-HT of the canine isolated colon longitudinal muscle. 2. Longitudinal strips of canine midcolon deprived of (sub)mucosa were prepared for isotonic measurement. 5-HT induced contractions from 3 nM onwards, which were not affected by selective inhibition of 5-HT re-uptake, monoamine oxidase or blockade of alpha-adrenoceptors. Tetrodotoxin (0.3 microM) did not affect the responses to 5-HT, suggesting that smooth muscle 5-HT receptors are involved. The selective 5-HT4 receptor antagonist SB 204070 (10 nM) slightly enhanced contractions to 5-HT and therefore it was included in the organ bath solution in all further experiments. The 5-HT1 and 5-HT2 receptor antagonist methysergide (0.1 microM) depressed the curve to 5-HT, but the selective 5-HT3 receptor antagonist granisetron (0.3 microM) had no effect. 3. Besides 5-HT, alpha-methyl-5-HT (alpha-Me-5-HT), 5-methoxytryptamine (5-MeOT), 2-methyl-5-HT (2-Me-5-HT) and 5-carboxamidotryptamine (5-CT) also induced contractions, with the following rank order of potency (pEC50 values in parentheses): 5-HT (6.9) = alpha-methyl-5-HT (6.9) > 2-Me-5-HT (5.8) = 5-MeOT (5.7) = 5-CT (5.6), indicative of 5-HT2 receptor involvement, alpha-Me-5-HT produced a bell-shaped curve, which was not affected by alpha-adrenoceptor blockade. 5-HT, 5-MeOT, 2-Me-5-HT and 5-CT produced a monophasic concentration-response curve, consistent with an interaction with a single receptor site. 8-Hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and tryptamine only induced contractions at a concentration exceeding 1 microM. 4. The selective 5-HT2B receptor antagonist SB 204741 (0.3 microM) did not affect the curve to 5-HT. Ketanserin, cisapride and spiroxatrine behaved as competitive antagonists with pKb values of, respectively, 8.4, 8.1 and 6.7. Spiroxatrine (1 microM) shifted the curve to 5-MeOT rightward yielding an apparent pA2 of 7.1. Other antagonists at 5-HT2A receptors also surmountably inhibited the contractions to 5-HT (apparent pA2 value in parentheses): mesulergine (8.2), cinanserin (8.2), yohimbine (6.2) and mianserin (8.6). However, as well as a rightward shift, methiothepin (8.3), pizotifen (8.6) and spiperone (8.8) also caused a depression of the curve, indicative of "pseudo-irreversible' antagonism. Taken together, the above mentioned affinity estimates most closely corresponded to literature affinity values for 5-HT2A receptors. 5. It was concluded that 5-HT induces contractions of the canine midcolon longitudinal muscle primarily by stimulation of smooth muscle 5-HT2A receptors. The presence of inhibitory 5-HT4 receptors cannot be ruled out.