Insulin signaling inhibits the 5-HT2C receptor in choroid plexus via MAP kinase.
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BACKGROUND: G protein-coupled receptors (GPCRs) interact with heterotrimeric GTP-binding proteins (G proteins) to modulate acute changes in intracellular messenger levels and ion channel activity. In contrast, long-term changes in cellular growth, proliferation and differentiation are often mediated by tyrosine kinase receptors and certain GPCRs by activation of mitogen-activated protein (MAP) kinases. Complex interactions occur between these signaling pathways, but the specific mechanisms of such regulatory events are not well-understood. In particular it is not clear whether GPCRs are modulated by tyrosine kinase receptor-MAP kinase pathways. RESULTS: Here we describe tyrosine kinase receptor regulation of a GPCR via MAP kinase. Insulin reduced the activity of the 5-HT2C receptor in choroid plexus cells which was blocked by the MAP kinase kinase (MEK) inhibitor, PD 098059. We demonstrate that the inhibitory effect of insulin and insulin-like growth factor type 1 (IGF-1) on the 5-HT2C receptor is dependent on tyrosine kinase, RAS and MAP kinase. The effect may be receptor-specific: insulin had no effect on another GPCR that shares the same G protein signaling pathway as the 5-HT2C receptor. This effect is also direct: activated MAP kinase mimicked the effect of insulin, and removing a putative MAP kinase site from the 5-HT2C receptor abolished the effect of insulin. CONCLUSION: These results show that insulin signaling can inhibit 5-HT2C receptor activity and suggest that MAP kinase may play a direct role in regulating the function of a specific GPCR. (+info)
Assessment of affinities and dissociation potencies of several 5-HT2 antagonists to and from M2 muscarinic receptor in rat heart membranes.
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The aim of the present study was to investigate the binding affinities and dissociation potencies of several 5-HT(2) antagonists in M(2) muscarinic receptor of rat heart membranes using [(3)H]QNB as a radioligand. The 5-HT(2) antagonists used in this study were sarpogrelate, ketanserin and cyproheptadine. The results showed that sarpogrelate and ketanserin had very weak binding affinities to M(2) muscarinic receptor, whereas cyproheptadine had higher binding affinity to this receptor than the muscarinic receptor antagonist, atropine. All of these three 5-HT(2) antagonists as well as muscarinic receptor antagonists (atropine and pirenzepine) were readily dissociated from M(2) muscarinic receptor in rat heart membranes after washing. Therefore, the findings of the present investigation suggest that the dissociation potencies of neither 5-HT(2) antagonists nor muscarinic antagonists used correlate with their binding affinities to M(2) muscarinic receptors in rat heart. (+info)
Synergism interaction between arachidonic acid by 5-hydroxytryptamine in human platelet aggregation is mediated through multiple signalling pathways.
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AIM: To examine the signalling mechanisms involved in the synergistic interaction of 5-hydroxytryptamine (5-HT) and arachidonic acid (AA) in human platelet aggregation. METHODS: Blood was obtained from healthy human subjects, mixed with 3.8 % sodium citrate (9:1), and centrifuged to prepare platelet rich plasma (PRP). Aggregation was monitored using a Dual-channel Lumi-aggregometer. The agonist-induced influx of Ca2+ was measured using Fura-2 AM. TXA2 formation was studied using radiochemical method. RESULTS: Subthreshold concentration of 5-HT (2 micromol/L) potentiated the effect of low dose of AA (0.2 mmol/L) in human platelets. This synergistic effect was blocked by 5-HT2 receptor antagonist (methysergide IC(50)=5.2 nmol/L; cyproheptadine IC(50)=0.6 nmol/L), and thromboxane A2 receptor antagonist (SQ 29 548; IC(50)=30 nmol/L), showing that the effect is receptor-mediated. To examine the down-stream signalling pathways, we found that such an interaction was inhibited by calcium channel blockers (diltiazem; IC(50)=3 micromol/L and verapamil; IC(50)=5 micromol/L), phospholipase C (PLC) inhibitor (U73122; IC50=4 micromol/L), cyclooxygenase inhibitor, (indomethacin; IC(50)=0.2 micromol/L) and mitogen-activated protein (MAP) kinase inhibitor (PD98059; IC(50)=3 micromol/L). The effect was also inhibited by a specific tyrosine light chain kinase (TLCK) inhibitor, herbimycin A with IC(50) value of 5 micromol/L. Pretreatment of platelet with 5-HT and AA induced rise in intracellular calcium and this effect was blocked by verapamil. CONCLUSION: The synergism between 5-HT and AA in platelet aggregation involves activation of PLC/Ca2+, COX, and MAP kinase pathways. (+info)
Role of the serotonin 5-HT(2A) receptor in learning.
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This study reviews the role of the serotonin 5-HT2A receptor in learning as measured by the acquisition of the rabbit's classically conditioning nictitating membrane response, a component of the eyeblink response. Agonists at the 5-HT2A receptor including LSD (d-lysergic acid diethylamide) enhanced associative learning at doses that produce cognitive effects in humans. Some antagonists such as BOL (d-bromolysergic acid diethylamide), LY53,857, and ketanserin acted as neutral antagonists in that they had no effect on learning, whereas others (MDL11,939, ritanserin, and mianserin) acted as inverse agonists in that they retarded learning through an action at the 5-HT2A receptor. These results were placed in the context of what is known concerning the anatomical distribution and electrophysiological effects of 5-HT2A receptor activation in frontal cortex and hippocampus, as well as the role of cortical 5-HT2A receptors in schizophrenia. It was concluded that the 5-HT2A receptor demonstrates constitutive activity, and that variations in this activity can produce profound alterations in cognitive states. (+info)
In vivo evidence that 5-HT2C receptor antagonist but not agonist modulates cocaine-induced dopamine outflow in the rat nucleus accumbens and striatum.
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During recent years, much attention has been devoted at investigating the modulatory role of central 5-HT(2C) receptors on dopamine (DA) neuron activity, and it has been proposed that these receptors modulate selectively DA exocytosis associated with increased firing of DA neurons. In the present study, using in vivo microdialysis in the nucleus accumbens (NAc) and the striatum of halothane-anesthetized rats, we addressed this hypothesis by assessing the ability of 5-HT(2C) agents to modulate the increase in DA outflow induced by haloperidol and cocaine, of which the effects on DA outflow are associated or not with an increase in DA neuron firing, respectively. The intraperitoneal administration of cocaine (10-30 mg/kg) induced a dose-dependent increase in DA extracellular levels in the NAc and the striatum. The effect of 15 mg/kg cocaine was potentiated by the mixed 5-HT(2C/2B) antagonist SB 206553 (5 mg/kg i.p.) and the selective 5-HT(2C) antagonist SB 242084 (1 mg/kg i.p.) in both brain regions. The mixed 5-HT(2C/2B) agonist, Ro 60-0175 (1 mg/kg i.p.), failed to affect cocaine-induced DA outflow, but reduced significantly the increase in DA outflow induced by the subcutaneous administration of 0.1 mg/kg haloperidol. The obtained results provide evidence that 5-HT(2C) receptors exert similar effects in both the NAc and the striatum, and they modulate DA exocytosis also when its increase occurs independently from an increase in DA neuron impulse activity. Furthermore, they show that 5-HT(2C) agonists, at variance with 5-HT(2C) antagonists, exert a preferential control on the impulse-stimulated release of DA. (+info)
Selective interaction of ARF1 with the carboxy-terminal tail domain of the 5-HT2A receptor.
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The 5-hydroxytryptamine 2A receptor (5-HT2AR) is a member of the class I family of rhodopsin-related G protein-coupled receptors. The receptor is known to activate phospholipase C via the heterotrimeric G proteins Gq/11, but we showed previously that it can also signal through the phospholipase D (PLD) pathway in an ADP-ribosylation factor (ARF)-dependent manner that seems to be independent of Gq/11 (Mitchell et al., 1998). Both coimmunoprecipitation experiments and the effects of negative mutant ARF constructs on 5-HT2AR-induced PLD activation here suggested that ARF1 may play a greater role than ARF6 in the function of this receptor. Furthermore, we demonstrated using glutathione S-transferase (GST)-fusion proteins of receptor domains that ARF1 and ARF6 bind to the third intracellular loop (i3) and the carboxy terminal tail (ct) of the 5-HT2AR. The association of ARF1 with the ct domain of the receptor was stronger than its interaction with i3, or the interactions of ARF6 with either construct. Experiments using ARF mutants that are deficient in GTP loading, and the in vitro addition of GTPgammaS suggested that GTP loading enhances ARF1 binding to the receptor. The N376PxxY motif in the transmembrane 7 domain of the receptor (rather than a N376DPxxY mutant form) was shown to be essential for ARF-dependent PLD signaling and ARF1 coimmunoprecipitation. In GST-fusion proteins of the 5-HT2AR ct domain, mutation of Asn376 to Asp also markedly reduced ARF1-HA binding, although additional motifs in the Asn376-Asn384 sequence and to a lesser extent elsewhere, seem also to contribute to the interaction. (+info)
The selective serotonin-2A receptor antagonist M100907 reverses behavioral deficits in dopamine transporter knockout mice.
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A hyperdopaminergic state in humans has been hypothesized to contribute to the pathology of a number of psychiatric illnesses, including schizophrenia, bipolar disorder, and attention deficit hyperactivity disorder. Mice that display elevated synaptic levels of dopamine due to a genetically engineered deletion of the dopamine transporter (DAT) model behavioral deficits that simulate the above conditions. As novel treatment strategies for these disorders have focused on the serotonin (5-HT) 2A receptor, we determined the capacity of the highly selective 5-HT(2A) receptor antagonist M100907 to reverse behavioral deficits in DAT knockout (KO) mice. Prior to drug treatment, DAT KO mice exhibited increased levels of locomotor activity and highly linearized movement in a novel environment, as well as reduced prepulse inhibition (PPI) of acoustic startle, compared to wild-type littermates. Treatment with M100907 (0.3-1.0 mg/kg, but not 0.1 mg/kg) reversed locomotor deficits in DAT KO mice. Similarly, treatment with 1.0 mg/kg M100907 reversed the PPI deficits in DAT KO mice. These data indicate that selective 5-HT(2A) receptor antagonists, such as M100907, may represent a class of drugs that can be used to treat conditions in which a chronic, elevated dopaminergic tone is present and contributes to abnormal behavior and sensorimotor gating deficits. (+info)
Serotonin receptor antagonist inhibits monocrotaline-induced pulmonary hypertension and prolongs survival in rats.
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OBJECTIVES: It has been reported that serotonin (5-HT) is involved in the development of pulmonary arterial hypertension (PAH) with pulmonary vascular remodeling. The purpose of the present study was to examine the role of a 5-HT2A receptor antagonist, sarpogrelate hydrochroride, in preventing or reversing monocrotaline (MCT)-induced PAH in rats. METHODS: Rats were injected with 40 mg/kg of MCT subcutaneously and randomized to either sarpogrelate (50 mg/kg, intraperitoneally) or placebo for 3 weeks. Animals treated with MCT and survived for 3 weeks were assigned to either sarpogrelate (50 mg/kg, intraperitoneally) or placebo for next 3 weeks. The animals had pressure measurement of the pulmonary artery, and then underwent histologic, immunohistochemical, and Western blot analyses of the lung tissue. Survival rate was also assessed after treatment. RESULTS: Sarpogrelate immediately following MCT injection suppressed PAH with severe pulmonary vascular remodeling and right-sided heart failure. The survival rate was significantly increased in the sarpogrelate-treated group compared with the placebo group (71% vs. 44%, p<0.05). Intense expression of P-selectin was found on the endothelium of the pulmonary arteries in the placebo group, and it was markedly attenuated in the sarpogrelate-treated group. The numbers of the CD45-positive cells and those of the proliferating cell nuclear antigen (PCNA)-positive cells in the lung tissue were significantly increased in the placebo group, and the increases in these cells were prevented by sarpogrelate. Endothelial nitric oxide synthase (eNOS) expression in the lung tissue was markedly decreased in the placebo group, but it was prevented by sarpogrelate (p<0.001). In contrast, late treatment with sarpogrelate failed to reverse established PAH. CONCLUSIONS: Specific 5-HT2A receptor blockade with sarpogrelate immediately after MCT inhibited PAH and prolongs survival in rats. These effects were accompanied by anti-inflammatory and anti-proliferative effects in the lung tissue and marked improvement of pulmonary vascular endothelial dysfunction and activation. (+info)