Human 5-HT1B receptor stimulated inositol phospholipid hydrolysis in CHO cells: synergy with Gq-coupled receptors

Serotonin and Pulmonary Hypertension; Sex and Drugs and ROCK and Rho

Serotonin is often referred to as a "happy hormone" as it maintains good mood, well-being, and happiness. It is involved in communication between nerve cells and plays a role in sleeping and digestion. However, too much serotonin can have pathogenic effects and serotonin synthesis is elevated in pulmonary artery endothelial cells from patients with pulmonary arterial hypertension (PAH). PAH is characterized by elevated pulmonary pressures, right ventricular failure, inflammation, and pulmonary vascular remodeling; serotonin has been shown to be associated with these pathologies. The rate-limiting enzyme in the synthesis of serotonin in the periphery of the body is tryptophan hydroxylase 1 (TPH1). TPH1 expression and serotonin synthesis are elevated in pulmonary artery endothelial cells in patients with PAH. The serotonin synthesized in the pulmonary arterial endothelium can act on the adjacent pulmonary arterial smooth muscle cells (PASMCs), adventitial macrophages, and fibroblasts, in a paracrine fashion. In humans, serotonin enters PASMCs cells via the serotonin transporter (SERT) and it can cooperate with the 5-HT1B receptor on the plasma membrane; this activates both contractile and proliferative signaling pathways. The "serotonin hypothesis of pulmonary hypertension" arose when serotonin was associated with PAH induced by diet pills such as fenfluramine, aminorex, and chlorphentermine; these act as indirect serotonergic agonists causing the release of serotonin from platelets and cells through the SERT. Here the role of serotonin in PAH is reviewed. Targeting serotonin synthesis or signaling is a promising novel alternative approach which may lead to novel therapies for PAH. © 2022 American Physiological Society. Compr Physiol 12: 1-16, 2022.

Serotonin: an overlooked regulator of endocytosis and endosomal sorting?

Serotonin is a neurotransmitter and a hormone that is typically associated with regulating our mood. However, the serotonin transporter and receptors are expressed throughout the body, highlighting the much broader, systemic role of serotonin in regulating human physiology. A substantial body of data strongly implicates serotonin as a fundamental regulator of endocytosis and endocytic sorting. Serotonin has the potential to enhance endocytosis through three distinct mechanisms - serotonin signalling, serotonylation and insertion into the plasma membrane - although the interplay and relationship between these mechanisms has not yet been explored. Endocytosis is central to the cellular response to the extracellular environment, controlling receptor distribution on the plasma membrane to modulate signalling, neurotransmitter release and uptake, circulating protein and lipid cargo uptake, and amino acid internalisation for cell proliferation. Uncovering the range of cellular and physiological circumstances in which serotonin regulates endocytosis is of great interest for our understanding of how serotonin regulates mood, and also the fundamental understanding of endocytosis and its regulation throughout the body. This article has an associated Future Leader to Watch interview with the first author of the paper.

Direct Delivery of MicroRNA96 to the Lungs Reduces Progression of Sugen/Hypoxia-Induced Pulmonary Hypertension in the Rat

The 5HT1B receptor (5HT1BR) contributes to the pathogenic effects of serotonin in pulmonary arterial hypertension. Here, we determine the effect of a microRNA96 (miR96) mimic delivered directly to the lungs on development of severe pulmonary hypertension in rats. Female rats were dosed with sugen (30 mg/kg) and subjected to 3 weeks of hypobaric hypoxia. In normoxia, rats were dosed with either a 5HT1BR antagonist SB216641 (7.5 mg/kg/day for 3 weeks), miR96, or scramble sequence (50 μg per rat), delivered by intratracheal (i.t) administration, once a week for 3 weeks. Cardiac hemodynamics were determined, pulmonary vascular remodeling was assessed, and gene expression was assessed by qRT-PCR, and in situ hybridization and protein expression were assessed by western blot and ELISA. miR96 expression was increased in pulmonary arteries and associated with a downregulation of the 5HT1BR protein in the lung. miR96 reduced progression of right ventricular systolic pressure, pulmonary arterial remodeling, right ventricular hypertrophy, and the occurrence of occlusive pulmonary lesions. Importantly, miR96 had no off-target effects and did not affect fibrotic markers of liver and kidney function. In conclusion, direct delivery of miR96 to the lungs was effective, reducing progression of sugen/hypoxia-induced pulmonary hypertension with no measured off-target effects. miR96 may be a novel therapy for pulmonary arterial hypertension, acting through downregulation of 5HT1BR.

Thyme (Thymus vulgaris [Lamiaceae]) Leaves Inhibit Contraction of the Nonpregnant Mouse Uterus

Dysmenorrhea is painful menstrual periods, which affects 25% of women within reproductive age and has a prevalence of 67.2-90.0%. Current treatment has several adverse effects and can be ineffective once the pain is initiated. Thymus vulgaris traditionally used for pain management was investigated in this study for its activity on uterine contraction in the nonpregnant uterus, as a parameter for dysmenorrhea. The dried leaves of T. vulgaris were macerated in water, and the resulting aqueous extract was investigated on the isolated mouse uterus. Parameters investigated included spontaneous contractions, oxytocin-induced contractions, and high potassium chloride (KCl; 80 mM)-induced tonic contractions. Mass spectrometric analysis of the thyme extract was also performed using liquid chromatography-high-resolution Fourier Transform mass spectrometry. Thyme extract inhibited the amplitude and frequency of spontaneous and oxytocin-induced uterine contractions. It also inhibited KCl-induced tonic contractions. The activities observed suggest that T. vulgaris inhibits uterine contractions through blockade of extracellular voltage-gated calcium channels. Secondary metabolites detected included compounds belonging to chlorogenic phytochemical class and flavonoids, which are known to have activities on extracellular calcium blockade. This study has shown that aqueous T. vulgaris extract, also known as thyme, inhibits contractions of the nonpregnant uterus and can be a lead plant in the drug discovery process for the management of dysmenorrhea.

The Trace Amine-Associated Receptor 1 Agonist 3-Iodothyronamine Induces Biased Signaling at the Serotonin 1b Receptor

Trace amine-associated receptors (TAARs) belong to the class A G-protein-coupled receptors (GPCR) and are evolutionary related to aminergic receptors. TAARs have been identified to mediate effects of trace amines. TAAR1 signaling is mainly mediated via activation of the G_s/adenylyl cyclase pathway. In addition to classical trace amines, TAAR1 can also be activated by the thyroid hormone derivative 3-iodothyronamine (3-T1AM). Pharmacological doses of 3-T1AM induced metabolic and anapyrexic effects, which might be centrally mediated in the hypothalamus in rodents. However, the observed anapyrexic effect of 3-T1AM persists in Taar1 knock-out mice which raises the question whether further GPCRs are potential targets for 3-T1AM and mediate the observed physiological effect. Anapyrexia has been observed to be related to action on aminergic receptors such as the serotonin receptor 1b (5-HT1b). This receptor primarily activates the G_i/o mediated pathway and PLC signaling through the G_βγ of G_i/o. Since the expression profiles of TAAR1 and 5-HT1b overlap, we questioned whether 3-T1AM may activate 5-HT1b. Finally, we also evaluated heteromerization between these two GPCRs and tested signaling under co-expressed conditions. In this study, we showed, that 3-T1AM can induce G_i/o signaling through 5-HT1b in a concentration of 10 μM. Strikingly, at 5-HT1b the ligand 3-T1AM only activates the G_i/o mediated reduction of cAMP accumulation, but not PLC activation. Co-stimulation of 5-HT1b by both ligands did not lead to additive or synergistic signaling effects. In addition, we confirmed the capacity for heteromerization between TAAR1 and 5-HT1b. Under co-expression of TAAR1 and HTR1b, 3-T1AM action is only mediated via TAAR1 and activation of 5-HT1b is abrogated. In conclusion, we found evidence for 5-HT1b as a new receptor target for 3-T1AM, albeit with a different signaling effect than the endogenous ligand. Altogether, this indicates a complex interrelation of signaling effects between the investigated GPCRs and respective ligands.

The serotonin hypothesis in pulmonary hypertension revisited: targets for novel therapies (2017 Grover Conference Series)

Increased synthesis of serotonin and/or activity of serotonin in pulmonary arteries has been implicated in the pathobiology of pulmonary arterial hypertension (PAH). The incidence of PAH associated with diet pills such as aminorex, fenfluramine, and chlorphentermine initially led to the “serotonin hypothesis of pulmonary hypertension.” Over the last couple of decades there has been an accumulation of convincing evidence that targeting serotonin synthesis or signaling is a novel and promising approach to the development of novel therapies for PAH. Pulmonary endothelial serotonin synthesis via tryptophan hydroxlase 1 (TPH1) is increased in patients with PAH and serotonin can act in a paracrine fashion on underlying pulmonary arterial smooth muscle cells (PASMCs), In humans, serotonin can enter PASMCs via the serotonin transporter (SERT) or activate the 5-HT1B receptor; 5-HT1B activation and SERT activity cooperate to induce PASMC contraction and proliferation via activation of downstream proliferative and contractile signaling pathways. Here we will review the current status of the serotonin hypothesis and discuss potential and novel therapeutic targets.

A Sex-Specific MicroRNA-96/5-Hydroxytryptamine 1B Axis Influences Development of Pulmonary Hypertension

RATIONALE

Females are predisposed to pulmonary arterial hypertension (PAH); evidence suggests that serotonin, mutations in the bone morphogenetic protein receptor (BMPR) II gene, and estrogens influence development of PAH. The 5-hydroxytryptamine 1B receptor (5-HT1BR) mediates human pulmonary artery smooth muscle cell (hPASMC) proliferation.

OBJECTIVES

We aimed to determine whether selected microRNAs (miRNAs) expressed in PASMCs are influenced by sex, BMPR-II mutations, and estrogens, and contribute to PASMC proliferation in PAH.

METHODS

Expression levels of miRNAs targeting genes related to PAH, estrogen, and serotonin were determined by quantitative RT-PCR in hPASMCs and mouse PASMCs harboring a heterozygous mutation in BMPR-II (BMPR-II(R899X+/-) PASMCs). miRNA-96 targets 5-HT1BR and was selected for further investigation. miRNA target validation was confirmed by luciferase reporter assay. Precursor miRNA-96 was transfected into hPASMCs to examine effects on proliferation and 5-HT1BR expression. The effect of a miRNA-96 mimic on the development of hypoxic pulmonary hypertension in mice was also assessed.

MEASUREMENTS AND MAIN RESULTS

miRNA-96 expression was reduced in BMPR-II(R899X+/-) PASMCs from female mice and hPASMCs from female patients with PAH; this was associated with increased 5-HT1BR expression and serotonin-mediated proliferation. 5-HT1BR was validated as a target for miRNA-96. Transfection of precursor miRNA-96 into hPASMCs reduced 5-HT1BR expression and inhibited serotonin-induced proliferation. Restoration of miRNA-96 expression in pulmonary arteries in vivo via administration of an miRNA-96 mimic reduced the development of hypoxia-induced pulmonary hypertension in the mouse.

CONCLUSIONS

Increased 5-HT1BR expression may be a consequence of decreased miRNA-96 expression in female patient PASMCs, and this may contribute to the development of PAH.

Serotonin (5-HT) activation of immortalized hypothalamic neuronal cells through the 5-HT1B serotonin receptor

Serotonin [or 5-hydroxytryptamine or (5-HT)] has been implicated as a key modulator in energy homeostasis and a primary focus in the treatment of obesity. There is growing evidence that 5-HT, acting through the 5-HT 1B receptor (5-HT(1B)R) in the paraventricular nucleus of the hypothalamus (PVN), is important to this regulation. However, there is some contention as to whether 5-HT(1B)R action occurs directly on PVN neurons or indirectly via inhibitory inputs into the PVN. To address these questions, we used a novel clonal, hypothalamic neuronal cell model, adult mouse hypothalamic-2/30 (mHypoA-2/30), expressing a PVN-specific marker, single-minded homolog 1, as well as a complement of PVN neuropeptides, including TRH, vasopressin, ghrelin, nucleobindin-2, and galanin. Adult mouse hypothalamic-2/30 neurons were also found to express the 5-HT(1B)R and 5-HT 6 receptor, but not 2C, all previously linked to feeding regulation. Direct serotonergic stimulation (100 nm to 10 μm) of these neurons resulted in dose-dependent cFos activation. 5-HT (10 μm) suppressed forskolin-induced cAMP levels and induced a rise in intracellular Ca(2+) through ER Ca(2+) release, effects that were mimicked by the 5-HT(1B)R agonists, CGS12066B and CP93129, and that were attenuated in the presence of the 5-HT(1B)R-specific inhibitors, GR55562 and isamoltane hemifumarate. Modest transcriptional changes in ghrelin and nucleobindin-2 were also observed in response to 100 nm and 10 μm 5-HT, respectively. These findings support the model wherein 5-HT action through the 1B receptor subtype occurs directly on PVN neurons, leading to potential modification of neuronal transcriptional and secretory machinery.

An essential role for adenosine signaling in alcohol abuse

In the central nervous system (CNS), adenosine plays an important role in regulating neuronal activity and modulates signaling by other neurotransmitters, including GABA, glutamate, and dopamine. Adenosine suppresses neurotransmitter release, reduces neuronal excitability, and regulates ion channel function through activation of four classes of G protein-coupled receptors, A(1), A(2A), A(2B), and A(3). Central adenosine are largely controlled by nucleoside transporters, which transport adenosine levels across the plasma membrane. Adenosine has been shown to modulate cortical glutamate signaling and ventral-tegmental dopaminergic signaling, which are involved in several aspects of alcohol use disorders. Acute ethanol elevates extracellular adenosine levels by selectively inhibiting the type 1 equilibrative nucleoside transporter, ENT1. Raised adenosine levels mediate the ataxic and sedative/hypnotic effects of ethanol through activation of A(1) receptors in the cerebellum, striatum, and cerebral cortex. Recently, we have shown that pharmacological inhibition or genetic deletion of ENT1 reduces the expression of excitatory amino acid transporter 2 (EAAT2), the primary regulator of extracellular glutamate, in astrocytes. These lines of evidence support a central role for adenosine-mediated glutamate signaling and the involvement of astrocytes in regulating ethanol intoxication and preference. In this paper, we discuss recent findings on the implication of adenosine signaling in alcohol use disorders.

Full and partial agonists of muscarinic M3 receptors reveal single and oscillatory Ca2+ responses by beta 2-adrenoceptors

Under physiological circumstances, cellular responses often reflect integration of signaling by two or more different receptors activated coincidentally or sequentially. In addition to heterologous desensitization, there are examples in which receptor activation either reveals or potentiates signaling by a different receptor type, although this is perhaps less well explored. Here, we characterize one such interaction between endogenous receptors in human embryonic kidney 293 cells in which Galpha(q/11)-coupled muscarinic M(3) receptors facilitate Ca(2+) signaling by Galpha(s)-coupled beta(2)-adrenoceptors. Measurement of changes in intracellular [Ca(2+)] demonstrated that noradrenaline released Ca(2+) from thapsigargin-sensitive intracellular stores only during activation of muscarinic receptors. Agonists with low efficacy for muscarinic receptor-mediated Ca(2+) responses facilitated cross-talk more effectively than full agonists. The cross-talk required Galpha(s) and was dependent upon intracellular Ca(2+) release channels, particularly inositol (1,4,5)-trisphosphate receptors. However, beta(2)-adrenoceptor-mediated Ca(2+) release was independent of measurable increases in phospholipase C activity and resistant to inhibitors of protein kinases A and C. Interestingly, single-cell imaging demonstrated that particularly lower concentrations of muscarinic receptor agonists facilitated marked oscillatory Ca(2+) signaling to noradrenaline. Thus, activation of muscarinic M(3) receptors profoundly influences the magnitude and oscillatory behavior of intracellular Ca(2+) signaling by beta(2)-adrenoceptors. Although these receptor subtypes are often coexpressed and mediate contrasting acute physiological effects, altered oscillatory Ca(2+) signaling suggests that cross-talk could influence longer term events through, for example, regulating gene transcription.

Phosphorylated FTY720 promotes astrocyte migration through sphingosine-1-phosphate receptors

Sphingosine-1-phosphate (S1P) receptors are widely expressed in the central nervous system where they are thought to regulate glia cell function. The phosphorylated version of fingolimod/FTY720 (FTY720P) is active on a broad spectrum of S1P receptors and the parent compound is currently in phase III clinical trials for the treatment of multiple sclerosis. Here, we aimed to identify which cell type(s) and S1P receptor(s) of the central nervous system are targeted by FTY720P. Using calcium imaging in mixed cultures from embryonic rat cortex we show that astrocytes are the major cell type responsive to FTY720P in this assay. In enriched astrocyte cultures, we detect expression of S1P1 and S1P3 receptors and demonstrate that FTY720P activates Gi protein-mediated signaling cascades. We also show that FTY720P as well as the S1P1-selective agonist SEW2871 stimulate astrocyte migration. The data indicate that FTY720P exerts its effects on astrocytes predominantly via the activation of S1P1 receptors, whereas S1P signals through both S1P1 and S1P3 receptors. We suggest that this distinct pharmacological profile of FTY720P, compared with S1P, could play a role in the therapeutic effects of FTY720 in multiple sclerosis.

The effect of serotonin and serotonin antagonists on bladder cancer cell proliferation

OBJECTIVE

To investigate the role of serotonin (5-hydroxytryptamine, 5HT) and its antagonists in the proliferation of high-grade bladder cancer cells (HT1376), as high-grade bladder cancer has a rapid rate of progression, invasion and recurrence, and 5HT antagonists inhibit the growth of the prostate cancer cell line (PC3).

MATERIALS AND METHODS

HT1376 (human grade III transitional cell carcinoma) cells were incubated with either 5HT or 5HT antagonists (5HT(1A), 5HT(1B), 5HT(1D), 5HT(2), 5HT(3) and 5HT(4)). After 72 h, cell viability was assessed using the crystal violet assay. The presence of 5HT receptor subtypes on HT1376 cells and sections of human bladder cancer tissue was determined by immunohistochemistry and Western blot analysis.

RESULTS

5HT caused a dose-dependent increase in the proliferation of HT1376 cells. The maximum increase in cell proliferation (12%; 12 samples, P < 0.001) was at 10(-8)m as compared to the control at 72 h. At 10(-4)m, 5HT(1A) antagonist (NAN-190 hydrobromide) and 5HT(1B) antagonist (SB224289 hydrochloride) had a 10% (12 samples, P < 0.001) and 93% (12, P < 0.001) inhibitory effect on HT1376 cell growth, respectively, compared to the control at 72 h. There was immunostaining for 5HT(1A) and 5HT(1B) receptors in HT1376 cells and malignant bladder tissue, confirming the presence of these two receptor subtypes. Western blot analysis showed the presence of 5HT(1A) and 5HT(1B) receptor proteins with bands of 46 kDa and 43 kDa, respectively.

CONCLUSION

5HT(1A) and to a greater extent 5HT(1B) antagonists significantly inhibit bladder cancer cell growth. This effect is probably mediated via the 5HT(1A) and 5HT(1B) receptors. These results highlight the potential use of 5HT(1A) and 5HT(1B) antagonists in the treatment of bladder cancer.

Constitutively active Gq/11-coupled receptors enable signaling by co-expressed G(i/o)-coupled receptors

Co-expression of guanine nucleotide-binding regulatory (G) protein-coupled receptors (GPCRs), such as the G(i/o)-coupled human 5-hydroxytryptamine receptor 1B (5-HT(1B)R), with the G(q/11)-coupled human histamine 1 receptor (H1R) results in an overall increase in agonist-independent signaling, which can be augmented by 5-HT(1B)R agonists and inhibited by a selective inverse 5-HT(1B)R agonist. Interestingly, inverse H1R agonists inhibit constitutively H1R-mediated as well as 5-HT(1B)R agonist-induced signaling in cells co-expressing both receptors. This phenomenon is not solely characteristic of 5-HT(1B)R; it is also evident with muscarinic M2 and adenosine A1 receptors and is mimicked by mastoparan-7, an activator of G(i/o) proteins, or by over-expression of Gbetagamma subunits. Likewise, expression of the G(q/11)-coupled human cytomegalovirus (HCMV)-encoded chemokine receptor US28 unmasks a functional coupling of G(i/o)-coupled CCR1 receptors that is mediated via the constitutive activity of receptor US28. Consequently, constitutively active G(q/11)-coupled receptors, such as the H1R and HCMV-encoded chemokine receptor US28, constitute a regulatory switch for signal transduction by G(i/o)-coupled receptors, which may have profound implications in understanding the role of both constitutive GPCR activity and GPCR cross-talk in physiology as well as in the observed pathophysiology upon HCMV infection.

Mechanisms of cross-talk between G-protein-coupled receptors resulting in enhanced release of intracellular Ca2+

Alteration in [Ca(2+)](i) (the intracellular concentration of Ca(2+)) is a key regulator of many cellular processes. To allow precise regulation of [Ca(2+)](i) and a diversity of signalling by this ion, cells possess many mechanisms by which they are able to control [Ca(2+)](i) both globally and at the subcellular level. Among these are many members of the superfamily of GPCRs (G-protein-coupled receptors), which are characterized by the presence of seven transmembrane domains. Typically, those receptors able to activate PLC (phospholipase C) enzymes cause release of Ca(2+) from intracellular stores and influence Ca(2+) entry across the plasma membrane. It has been well documented that Ca(2+) signalling by one type of GPCR can be influenced by stimulation of a different type of GPCR. Indeed, many studies have demonstrated heterologous desensitization between two different PLC-coupled GPCRs. This is not surprising, given our current understanding of negative-feedback regulation and the likely shared components of the signalling pathway. However, there are also many documented examples of interactions between GPCRs, often coupling preferentially to different signalling pathways, which result in a potentiation of Ca(2+) signalling. Such interactions have important implications for both the control of cell function and the interpretation of in vitro cell-based assays. However, there is currently no single mechanism that adequately accounts for all examples of this type of cross-talk. Indeed, many studies either have not addressed this issue or have been unable to determine the mechanism(s) involved. This review seeks to explore a range of possible mechanisms to convey their potential diversity and to provide a basis for further experimental investigation.

Conditional rescue of protein kinase C epsilon regulates ethanol preference and hypnotic sensitivity in adult mice

Conventional gene targeting is a powerful tool to study the influence of specific genes on behavior. However, conclusions relevant for adult animals are limited by consequences of gene loss during development. Mice lacking protein kinase C epsilon (PKCepsilon) consume less alcohol and show greater acute sensitivity to alcohol than do wild-type mice. There are no selective inhibitors of PKCepsilon that can be administered systemically and cross the blood-brain barrier to test whether these phenotypes result from loss of PKCepsilon during development or in adulthood. Here we used conditional expression of PKCepsilon in the basal forebrain, amygdala, and cerebellum to rescue wild-type responses to alcohol in adult PKCepsilon(-/-) mice. Subsequent suppression of transgenic PKCepsilon restored PKCepsilon(-/-) behaviors. These findings establish that PKCepsilon signaling in the adult brain regulates alcohol consumption and sensitivity. If this extends to humans, then PKCepsilon inhibitors might prove useful as novel therapeutics for alcoholism.

Involvement of 5-HT1B and 5-HT1D receptors in sumatriptan mediated vasocontractile response in rabbit common carotid artery

1. In this study we examined the involvement of 5-HT(1B) and 5-HT(1D) receptors in the vasocontractile response induced by 5-HT(1B/D)-receptor agonist sumatriptan in rabbit common carotid artery (CCA). 2. Immunoblotting experiments using specific antisera against 5-HT(1B) or 5-HT(1D) receptors revealed the presence of one weak (at 93 kD for 5-HT(1B) or at 105 kD for 5-HT(1D)) and one strong band (at 46 kD for 5-HT(1B) or at 52 kD for 5-HT(1D)) in CCA. 3. Sumatriptan-mediated vasocontractile response was antagonized by SB216641 with an apparent pKb value of 8.6, which was consistent with its affinity for 5-HT(1B) receptor. Antagonism by BRL15572 was weak and calculated apparent pKb (6.0) value was consistent with its affinity for 5-HT(1B) subtype (but not for 5-HT(1D) subtype). This result indicates insignificant or no involvement of 5-HT(1D) receptor in the vasocontractile response. 4. The vasocontractile response induced by sumatriptan was highly sensitive to pertussis toxin treatment of CCA. Nicardipine, a calcium channel blocker, also potently antagonized vasocontractile response induced by sumatriptan. 5. 5-HT, but not sumatriptan, stimulated inositol phosphate accumulation in CCA. 6. These results indicate that stimulation of 5-HT(1B) subtype activate a pertussis toxin (PTX) sensitive G protein (Go/Gi) and mediate vasocontraction, in which L-type voltage dependent calcium channels are involved.

Specific labelling of serotonin 5-HT(1B) receptors in rat frontal cortex with the novel, phenylpiperazine derivative, [3H]GR125,743. A pharmacological characterization

Although several tritiated agonists have been used for radiolabelling serotonin (5-hydroxytryptamine, 5-HT)(1B) receptors in rats, data with a selective, radiolabelled antagonist have not been presented. Inasmuch as [3H]GR125,743 specifically labels cloned, human and native guinea pig 5-HT(1B) receptors and has been employed for characterization of cerebral 5-HT(1B) receptor in the latter species [Eur. J. Pharmacol. 327 (1997) 247.], the present study evaluated its utility for characterization of native, cerebral 5-HT(1B) sites in the rat. In homogenates of frontal cortex, [3H]GR125,743 (0.8 nM) showed rapid association (t(1/2)=3.4 min), >90% specific binding and high affinity (K(d)=0.6 nM) for a homogeneous population of receptors with a density (B(max)) of 160 fmol/mg protein. In competition binding studies, affinities were determined for 15 chemically diverse 5-HT(1B) agonists, including 2-[5-[3-(4-methylsulphonylamino)benzyl-1,2,4-oxadiazol-5-yl]-1H-indole-3-yl]ethylamine (L694,247; pK(i), 10.4), 5-carboxamidotryptamine (5-CT; 9.7), 3-[3-(2-dimethylamino-ethyl)-1H-indol-6-yl]-N-(4-methoxybenzyl)acrylamide (GR46,611; 9.6), 5-methoxy-3-(1,2,5,6-tetrahydro-4-pyridinyl)-1H-indole (RU24,969; 9.5), dihydroergotamine (DHE; 8.6), 5-H-pyrrolo[3,2-b]pyridin-5-one,1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl (CP93,129; 8.4), anpirtoline (7.9), sumatriptan (7.4), 1-[2-(3-fluorophenyl)ethyl]-4-[3-[5-(1,2,4-triazol-4-yl)-1H-indol-3-yl]propyl]piperazine (L775,606; 6.4) and (minus sign)-1(S)-[2-[4-(4-methoxyphenyl)piperazin-1-yl]ethyl]-N-methyl-3,4-dihydro-1H-2-benzopyran-6-carboxamide (PNU109,291; <5.0). Similarly, affinities were established for 13 chemically diverse antagonists, including N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-3-methyl-4-(4-pyridyl)benzamide (GR125,743; pK(i), 9.1), (-)cyanopindolol (9.0), (-)-tertatolol (8.2), N-(4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiozol-3-yl)biphenyl-4-carboxamide (GR127,935; 8.2), N-[3-(1,4-benzodioxan-5-yl)piperidin-4-yl]N-(indan-2yl)amine (S18127; 7.9), metergoline (7.8), (-)-pindolol (7.6), 1'-methyl-5-[2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-biphenyl-4-ylcarbonyl]-2,3,6,7-tetrahydro-5H-spiro[furo[2,3-f]indole-3,4'-piperidine] (SB224,289; 7.5) and ketanserin (<5.0). These rank orders of affinity correspond to the binding profile of 5-HT(1B) rather than 5-HT(1D) receptors. The low affinities of L775,066 and PNU109,291 versus L694,247 should be noted, as well as the low affinity of ketanserin as compared to SB224,289. Finally, in line with species differences, the affinities of several ligands including CP93,129, RU24,969, (-)-pindolol and (-)-propanolol in rat 5-HT(1B) sites were markedly different to guinea pig 5-HT(1B) sites labelled with [3H]GR125,743. In conclusion, [3H]GR125,743 is an appropriate tool for the radiolabelling of native, rat 5-HT(1B) receptors and permitted determination of the affinities of an extensive series of ligands at these sites.

Multiplicity of mechanisms of serotonin receptor signal transduction

The serotonin (5-hydroxytryptamine, 5-HT) receptors have been divided into 7 subfamilies by convention, 6 of which include 13 different genes for G-protein-coupled receptors. Those subfamilies have been characterized by overlapping pharmacological properties, amino acid sequences, gene organization, and second messenger coupling pathways. Post-genomic modifications, such as alternative mRNA splicing or mRNA editing, creates at least 20 more G-protein-coupled 5-HT receptors, such that there are at least 30 distinct 5-HT receptors that signal through G-proteins. This review will focus on what is known about the signaling linkages of the G-protein-linked 5-HT receptors, and will highlight some fascinating new insights into 5-HT receptor signaling.

Coincident signalling between the Gi/Go-coupled delta-opioid receptor and the Gq-coupled m3 muscarinic receptor at the level of intracellular free calcium in SH-SY5Y cells

In SH-SY5Y cells, activation of delta-opioid receptors with [D-Pen(2,5)]-enkephalin (DPDPE; 1 microM) did not alter the intracellular free Ca(2+) concentration [Ca(2+)](i). However, when DPDPE was applied during concomitant Gq-coupled m3 muscarinic receptor stimulation by carbachol or oxotremorine-M, it produced an elevation of [Ca(2+)](i). The DPDPE-evoked increase in [Ca(2+)](i) was abolished when the carbachol-sensitive intracellular Ca(2+) store was emptied. There was a marked difference between the concentration-response relationship for the elevation of [Ca(2+)](i) by carbachol (EC(50) 13 microM, Hill slope 1) and the concentration-response relationship for carbachol's permissive action in revealing the delta-opioid receptor-mediated elevation of [Ca(2+)] (EC(50) 0.7 mM; Hill slope 1.8). Sequestration of free G protein beta gamma dimers by transient transfection of cells with a beta gamma binding protein (residues 495-689 of the C terminal tail of G protein-coupled receptor kinase 2) reduced the ability of delta opioid receptor activation to elevate [Ca(2+)](i). However, DPDPE did not elevate either basal or oxotremorine-M-evoked inositol phosphate production indicating that delta-opioid receptor activation did not stimulate phospholipase C. Furthermore, delta-opioid receptor activation did not result in the reversal of muscarinic receptor desensitization, membrane hyperpolarization or stimulation of sphingosine kinase. There was no coincident signalling between the delta-opioid receptor and the lysophosphatidic acid receptor which couples to elevation of [Ca(2+)](i) in SH-SY5Y cells by a PLC-independent mechanism. In SH-SY5Y cells the coincident signalling between the endogenously expressed delta-opioid and m3 muscarinic receptors appears to occur in the receptor activation-Ca(2+) release signalling pathway at a step after the activation of phospholipase C.

Collar-induced elevation of mRNA and functional activity of 5-HT(1B) receptor in the rabbit carotid artery

Hypersensitivity to serotonin (5-HT) develops in rabbit collared carotid arteries. Previous data demonstrated the involvement of 5-HT(1)-like receptors which are not active in normal carotid arteries. This study investigated the interaction in the rabbit carotid artery between 5-HT and a moderate tone as this can uncover functional 5-HT(1)-like receptors. Furthermore, the expression of messenger RNA (mRNA) and protein of 5-HT(1B), 5-HT(1D) and 5-HT(2A) receptors was addressed. Silicone collars were placed around the carotid arteries of male New Zealand White rabbits for 1 week. Rings from inside (=collar) and outside (=sham) the collar were either mounted in isolated organ baths for isometric force measurements or frozen in liquid nitrogen to isolate total RNA or proteins which were subsequently analysed by respectively reverse transcriptase-polymerase chain reaction and Western blot analysis. In sham and collared rings concentration-response curves (CRC's) to 5-HT were monophasic. Only in collared segments the presence of a 5-HT(2A) antagonist (spiperone or ketanserin, 0.1 microM) revealed a biphasic CRC which was even more pronounced when a moderate tone was induced by KCl pointing to functional 5-HT(1)-like receptors. The rabbit carotid artery constitutively expressed 5-HT(1B) and 5-HT(2A) mRNA, not 5-HT(1D) mRNA. Manipulation of the carotid artery increased the 5-HT(1B) mRNA level. Collar placement raised it even further. The 5-HT(2A) mRNA level remained unchanged. All the anti-5-HT receptor antibodies tested resulted in variable, non specific patterns with multiple bands. In conclusion, collar placement elevates mRNA expression and activity of the 5-HT(1B) receptor in the rabbit carotid artery.

5-HT-moduline controls serotonergic activity: implication in neuroimmune reciprocal regulation mechanisms

The serotonergic neurotransmission is known as a neuromodulatory system exerting its activity in the central nervous system (CNS) as well as at the periphery. The anatomical and morphological organization of the system based on a marked centralization of the cellular bodies and the large, almost ubiquitary, presence of axonal projections of the neurons is in good agreement with this modulatory role. Furthermore, a very high number of varicosities located along the axonal branches are capable of releasing serotonin (5-HT). The amine stimulates a number of different specific receptor types which allows 5-HT to exert different activities on its various cellular targets. Among these receptors, the 5-HT1B subtypes play a particular role as they are autoreceptors located on 5-HT neurons terminals and heteroreceptors located on non-serotonergic terminals where they control the release of the neurotransmitter. 5-HT-moduline, an endogenous tetrapeptide, regulates the efficacy of these 5-HT1B receptors, hence, is able to control the serotonergic activity in a synchronous manner for the various varicosities from a single neuron and thus may favour the differential effect of that neuron on distinct cerebral functions. Accordingly, the peptide allows the 'fine tuning' of the cerebral activity by the serotonergic system to elaborate the response given by the brain to a particular stimulus, that is, stress situations. At the periphery, the serotonergic system also appears to possess a regulatory activity via 5-HT1B receptors. In particular, the receptors located on immunocompetent cells control their activity and are themselves regulated by 5-HT-moduline likely originating from adrenal medulla and released after acute stress. The serotonergic system appears to play a major role in the reciprocal signalling existing between the neuronal and the immune system. The participation of 5-HT-moduline is likely in physiological functions as well as in pathological disorders affecting central and peripheral activities.

Synergy between thrombin and serotonin in inducing vascular smooth muscle cell proliferation

Previous studies have indicated that apart from playing an important role in hemostasis and thrombosis, thrombin may also contribute to the development of postangioplasty restenosis caused by the stimulation of vascular smooth muscle cell (VSMC) proliferation. Because thrombin generation in vivo is accompanied by platelet activation and release of smooth muscle cell (SMC) growth factors such as serotonin, we examined the possible interaction between these two compounds on VSMC proliferation. Thrombin (0.01 to 100 nmol/L), thrombin receptor-activating peptide (0.1 to 1000 micromol/L), and serotonin (5HT; 0.1 to 1000 micromol/L) increased tritiated thymidine incorporation into the DNA of canine aortic VSMCs in a dose-dependent manner. When thrombin and 5HT were added together at sub-threshold concentrations, they acted synergistically in inducing tritiated thymidine incorporation. These findings were paralleled by a 90%+/-5% increase in the cell number at 48 hours, as compared with a 37%+/-2% increase with 50 micromol/L serotonin and a 13%+/-3% increase with 0.1 nmol/L thrombin. We also demonstrated that a brief exposure to thrombin (1 hour) is sufficient to show its potentiating effect on serotonin. The mitogenic effect of serotonin and its synergistic interaction with thrombin on VSMC proliferation was abolished by serotonin type 2 receptor antagonist LY281067. Similarly, gamma-hirudin--a direct thrombin inhibitor--blocked the mitogenic effect of thrombin and its synergistic interaction with serotonin. When LY281067 and gamma-hirudin were used together, they abolished the mitogenic effects of both the agonists. Because clot-bound active thrombin can escape inactivation by anti-thrombin, this thrombin may potentiate the mitogenic effect of serotonin and keep the SMCs in a proliferative state for a long period of time. These findings support the use of 5HT2 receptor antagonists in combination with thrombin inhibitors in the prevention of SMC proliferation after coronary angioplasty.

5-HT-moduline, a 5-HT(1B/1D) receptor endogenous modulator, interacts with dopamine release measured in vivo by microdialysis

5-Hydroxytryptamine-moduline (5-HT-moduline) is an endogenous tetrapeptide (Leu-Ser-Ala-Leu) recently isolated and characterized from mammalian brain. This compound interacts with 5-HT1B receptors as a non-competitive, high-affinity antagonist and has the properties of an allosteric modulator. 5-HT-moduline could play an important role in the regulation of serotonergic transmission and also, through heteroreceptors, dopaminergic transmission. The aim of this work was to examine the potential ability of 5-HT-moduline to modify the basal extracellular concentration of dopamine and its metabolites (3-methoxytyramine, dihydroxyphenylacetic acid and homovanillic acid), in the rat striatum and to determine its potential interaction with the stimulating activity of a specific 5-HT1B receptor agonist, 3-(1,2,5,6-tetrahydropyrid-4-yl) pyrrolo [3,2-b] pyrid-5-one (CP-93,129), on the release of dopamine. The technique is based on in vivo microdialysis using probes implanted in the striatum of the conscious rat. Results showed that the perfusion of 5-HT-moduline directly into this structure (1.25 mM) increased the striatal level of dopamine by two-fold (104% of the absolute basal release values, P = 0.0015) and that of 3-methoxytyramine by 3-fold (293%, P = 0.0001) without any change in the terminal metabolite concentrations. The intrastriatal administration of CP-93,129 induced a statistically significant, dose-dependent increase of dopamine levels (P < 0.0001). Coperfusion of 5-HT-moduline did not significantly alter the effect of CP-93,129 at 0.1 and 0.5 mM, but appeared to have an additive effect on the lowest dose (P = 0.0406). The results obtained show that 5-HT-moduline directly administered into the striatum increases the release of dopamine in this area. Presumably, this effect results from the desensitization of 5-HT1B receptors located on dopamine terminals. However, the fact that a 5-HT1B receptor agonist (CP-93,129) also increased the release of dopamine in the striatum and that 5-HT-moduline exhibited a slight additive effect with that of a low concentration of CP-93,129 suggests that the two substances interact with different mechanisms.