RESUMO
The serotonin type 3 receptor (5-HT3) is a ligand-gated ion channel that converts the binding of the neurotransmitter serotonin (5-HT) into a transient cation current that mediates fast excitatory responses in peripheral and central nervous systems. Information regarding the activation and modulation of the human 5-HT3 type A receptor has been based only on macroscopic current measurements because of its low ion conductance. By constructing a high-conductance human 5-HT3A receptor, we here revealed mechanistic information regarding the orthosteric activation by 5-HT and by the partial agonist tryptamine, and the allosteric activation by the terpenoids, carvacrol, and thymol. Terpenoids potentiated macroscopic currents elicited by the orthosteric agonist and directly elicited currents with slow-rising phases and submaximal amplitudes. At the single-channel level, activation by orthosteric and allosteric agonists appeared as openings in quick succession (bursts) that showed no ligand concentration dependence. Bursts were grouped into long-duration clusters in the presence of 5-HT and even longer in the presence of terpenoids, whereas they remained isolated in the presence of tryptamine. Kinetic analysis revealed that allosteric and orthosteric activation mechanisms can be described by the same scheme that includes transitions of the agonist-bound receptor to closed intermediate states before opening (priming). Reduced priming explained the partial agonism of tryptamine; however, equilibrium constants for gating and priming were similar for 5-HT and terpenoid activation. Thus, our kinetic analysis revealed that terpenoids are efficacious agonists for 5-HT3A receptors. These findings not only extend our knowledge about the human 5-HT3A molecular function but also provide novel insights into the mechanisms of action of allosteric ligands, which are of increasing interest as therapeutic drugs in all the superfamily.
Assuntos
Agonistas do Receptor 5-HT3 de Serotonina , Serotonina , Regulação Alostérica , Humanos , Cinética , Receptores 5-HT3 de Serotonina/metabolismoRESUMO
INTRODUCTION: Chemotherapy-induced nausea and vomiting are adverse effects responsible for worsening quality of life in cancer patients. To assess the efficacy, safety and effectiveness of serotonin receptor antagonist in cancer patients undergoing chemotherapy, comparing ondansetron with granisetron, dolasetron, tropisetron and palonosetron. AREAS COVERED: Systematic review and meta-analysis. The data were collected using CINAHL; CENTRAL; MEDLINE/PubMed; and LILACS databases; grey literature; and manual search. The methodological quality was assessed using the modified Jadad scale; Cochrane Collaboration's tool for assessing risk of bias in randomized clinical trials and the Newcastle-Ottawa Scale for observational studies. The search was completed in November, 2015. 26 studies were included in the meta-analysis. Ondansetron exhibited similar efficacy than granisetron and tropisetron, as well as greater efficacy than dolasetron for acute vomiting. Palonosetron exhibited greater efficacy than ondansetron for delayed nausea and acute and delayed vomiting. The comparison of granisetron with ondansetron in the cohort studies showed no difference. Expert commentary: In this review, palonosetron had increased efficiency compared with ondansetron, except in the subgroup analysis and acute nausea. Few cohort studies have been published addressing this topic.
Assuntos
Náusea/tratamento farmacológico , Ondansetron/uso terapêutico , Vômito/tratamento farmacológico , Antieméticos/efeitos adversos , Antieméticos/farmacologia , Antieméticos/uso terapêutico , Antineoplásicos/efeitos adversos , Antineoplásicos/uso terapêutico , Humanos , Náusea/induzido quimicamente , Neoplasias/tratamento farmacológico , Ondansetron/efeitos adversos , Ondansetron/farmacologia , Qualidade de Vida , Ensaios Clínicos Controlados Aleatórios como Assunto , Agonistas do Receptor 5-HT3 de Serotonina/efeitos adversos , Agonistas do Receptor 5-HT3 de Serotonina/farmacologia , Agonistas do Receptor 5-HT3 de Serotonina/uso terapêutico , Vômito/induzido quimicamenteRESUMO
Partial agonists have emerged as attractive therapeutic molecules. 2-Me-5HT and tryptamine have been defined as partial agonists of 5-HT3 receptors on the basis of macroscopic measurements. Because several mechanisms may limit maximal responses, we took advantage of the high-conductance form of the mouse serotonin type 3A (5-HT3A) receptor to understand their molecular actions. Individual 5-HT-bound receptors activate in long episodes of high open probability, consisting of groups of openings in quick succession. The activation pattern is similar for 2-Me-5HT only at very low concentrations since profound channel blockade takes place within the activating concentration range. In contrast, activation episodes are significantly briefer in the presence of tryptamine. Generation of a full activation scheme reveals that the fully occupied receptor overcomes transitions to closed preopen states (primed states) before opening. Reduced priming explains the partial agonism of tryptamine. In contrast, 2-Me-5HT is not a genuine partial agonist since priming is not dramatically affected and its low apparent efficacy is mainly due to channel blockade. The analysis also shows that the first priming step is the rate-limiting step and partial agonists require an increased number of priming steps for activation. Molecular docking suggests that interactions are similar for 5-HT and 2-Me-5HT but slightly different for tryptamine. Our study contributes to understanding 5-HT3A receptor activation, extends the novel concept of partial agonism within the Cys-loop family, reveals novel aspects of partial agonism, and unmasks molecular actions of classically defined partial agonists. Unraveling mechanisms underlying partial responses has implications in the design of therapeutic compounds.
Assuntos
Agonismo Parcial de Drogas , Receptores 5-HT3 de Serotonina/metabolismo , Agonistas do Receptor 5-HT3 de Serotonina/metabolismo , Animais , Feminino , Células HEK293 , Humanos , Masculino , Camundongos , Camundongos Transgênicos , Ligação Proteica/fisiologia , Estrutura Secundária de Proteína , Receptores 5-HT3 de Serotonina/química , Agonistas do Receptor 5-HT3 de Serotonina/químicaRESUMO
The effects of the selective 5-HT(3) receptor agonist m-chlorophenylbiguanide (m-CPBG), and of the NMDA (N-methyl-D-aspartate) and AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionate)/kainate antagonists AP-5 [(±)-2-amino-5-phosphono-pentanoic acid] and CNQX (6-cyano-7-nitroquinoxaline-2,3-dione), respectively, were studied in adult male Wistar rats implanted for chronic sleep recordings. The compounds were microinjected directly into the dorsal raphe nucleus (DRN) during the light period of the 12-h light/12-h dark cycle. Infusion of m-CPBG (2 and 4mM) into the DRN induced a significant reduction of rapid-eye-movement sleep (REMS) and of the number of REM periods. Local infusion of AP-5 (0.5-1 mM) and CNQX (2 mM) significantly increased slow wave sleep (SWS). Pretreatment with AP-5 (0.5 mM) or CNQX (0.5 mM) antagonized the m-CPBG-induced suppression of REMS. It is proposed that the reduction of REMS after microinjection of m-CPBG into de DRN is related to the activation of glutamatergic interneurons that express the 5-HT(3) receptor and make synaptic contacts with serotonergic cells. The resultant increase of serotonin release at postsynaptic sites involved in the induction of REMS would provoke the suppression of the behavioral state. Our findings provide, in addition, new details concerning the pharmacology of DRN serotonergic neurons in the rat that may become relevant to the development of drugs for enhancing cortical and subcortical serotonergic neurotransmission.
Assuntos
Biguanidas/farmacologia , Núcleos da Rafe/efeitos dos fármacos , Agonistas do Receptor 5-HT3 de Serotonina/farmacologia , Sono REM/efeitos dos fármacos , 2-Amino-5-fosfonovalerato/farmacologia , 6-Ciano-7-nitroquinoxalina-2,3-diona/farmacologia , Animais , Relação Dose-Resposta a Droga , Eletroencefalografia/efeitos dos fármacos , Eletromiografia/efeitos dos fármacos , Antagonistas de Aminoácidos Excitatórios/farmacologia , Masculino , N-Metilaspartato/metabolismo , Núcleos da Rafe/metabolismo , Núcleos da Rafe/fisiologia , Ratos , Ratos Wistar , Receptores de Ácido Caínico/antagonistas & inibidores , Receptores 5-HT3 de Serotonina , Sono/efeitos dos fármacos , Sono/fisiologia , Privação do Sono , Sono REM/fisiologia , Ácido alfa-Amino-3-hidroxi-5-metil-4-isoxazol Propiônico/metabolismoRESUMO
The aim of the present work was to investigate the role of brain µ, κ and δ opioid receptors in the central serotonergic mechanisms regulating blood pressure in rats. The data obtained show that: (1) pharmacological activation of central 5-HT(3) receptors yields a significant decrease in blood pressure; (2) the blockade of those receptors by a selective antagonist induces an acute hypertensive response; (3) the pharmacological blockade of central opioid receptors by three different opioid antagonists exhibiting variable degrees of selectivity to µ, κ and δ opioid receptors always suppressed the hypotensive response induced by central 5-HT(3) receptor stimulation; (4) the blockade of opioid receptors by the same opioid antagonists that impaired the hypotensive effect of central 5-HT(3) receptor stimulation failed to modify blood pressure in animals not submitted to pharmacological manipulations of central 5-HT(3) receptor function. It is shown that a 5-HT(3) receptor-dependent mechanism seems to be part of the brain serotonergic system that contributes to cardiovascular regulation since the hypertensive response observed after ondansetron administration indicates that central 5-HT(3) receptors exert a tonic inhibitory drive on blood pressure. Furthermore, the data obtained here clearly indicate that the hypotensive response observed after pharmacological stimulation of central 5-HT(3) receptors depends on the functional integrity of brain µ, κ and δ opioid receptors, suggesting that a functional interaction between serotonergic and opiatergic pathways in the brain is part of the complex, multifactorial system that regulates blood pressure in the central nervous system.
Assuntos
Pressão Sanguínea/efeitos dos fármacos , Hipotensão/metabolismo , Receptores Opioides/metabolismo , Receptores 5-HT3 de Serotonina/metabolismo , Animais , Biguanidas/farmacologia , Frequência Cardíaca/efeitos dos fármacos , Masculino , Naloxona/farmacologia , Naltrexona/análogos & derivados , Naltrexona/farmacologia , Antagonistas de Entorpecentes/farmacologia , Ondansetron/farmacologia , Ratos , Ratos Wistar , Agonistas do Receptor 5-HT3 de Serotonina/farmacologia , Antagonistas do Receptor 5-HT3 de Serotonina/farmacologiaRESUMO
The present study investigated a possible antidepressant-like activity of bis selenide using two predictive tests for antidepressant effect on rodents: the forced swimming test (FST) and the tail suspension test (TST). Bis selenide (0.5-5 mg/kg, p.o.) decreased the immobility time in the mouse FST and TST. The anti-immobility effect of bis selenide (1 mg/kg, p.o.) in the TST was prevented by the pretreatment of mice with p-chlorophenylalanine methyl ester (PCPA; 100 mg/kg, i.p., an inhibitor of serotonin synthesis), ketanserin (1 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist), and ondasentron (1 mg/kg, i.p., a 5-HT(3) receptor antagonist). Pretreatment of mice with prazosin (1 mg/kg, i.p., an alpha(1)-adrenoceptor antagonist), yohimbine (1 mg/kg, i.p., an alpha(2)-adrenoceptor antagonist), propranolol (2 mg/kg, i.p., a beta-adrenoceptor antagonist), SCH23390 (0.05 mg/kg, s.c., a dopamine D(1) receptor antagonist), sulpiride (50 mg/kg, i.p., a dopamine D(2) receptor antagonist), or WAY 100635 (0.1 mg/kg, s.c., a selective 5-HT(1A) receptor antagonist) did not block the antidepressant-like effect of bis selenide (1 mg/kg, p.o.) in the TST. Administration of bis selenide (0.1 mg/kg, p.o.) and fluoxetine (1 mg/kg), at subeffective doses, produced an antidepressant-like effect in the TST. Bis selenide did not alter Na(+) K(+) ATPase, MAO-A and MAO-B activities in whole brains of mice. Bis selenide produced an antidepressant-like effect in the mouse TST and FST, which may be related to the serotonergic system (5-HT(2A/2C) and 5-HT(3) receptors).
Assuntos
Antidepressivos/farmacologia , Depressão/tratamento farmacológico , Compostos Organosselênicos/farmacologia , Receptores 5-HT2 de Serotonina/metabolismo , Receptores 5-HT3 de Serotonina/metabolismo , Administração Oral , Antagonistas Adrenérgicos alfa/farmacologia , Antagonistas Adrenérgicos beta/farmacologia , Análise de Variância , Animais , Antidepressivos/administração & dosagem , Depressão/diagnóstico , Depressão/fisiopatologia , Modelos Animais de Doenças , Dopaminérgicos/farmacologia , Relação Dose-Resposta a Droga , Interações Medicamentosas , Comportamento Exploratório/efeitos dos fármacos , Elevação dos Membros Posteriores/métodos , Elevação dos Membros Posteriores/fisiologia , Resposta de Imobilidade Tônica/efeitos dos fármacos , Masculino , Camundongos , Monoaminoxidase/metabolismo , Compostos Organosselênicos/administração & dosagem , Propranolol/farmacologia , Agonistas do Receptor 5-HT2 de Serotonina , Antagonistas do Receptor 5-HT2 de Serotonina , Agonistas do Receptor 5-HT3 de Serotonina , Antagonistas do Receptor 5-HT3 de Serotonina , Serotoninérgicos/farmacologia , Simportadores de Cloreto de Sódio-Potássio/metabolismo , Natação/psicologia , Fatores de Tempo , Ioimbina/farmacologiaRESUMO
The 5-HT(3)A receptor is a member of the Cys-loop family of ligand-gated ion channels. To perform kinetic analysis, we mutated the 5-HT3A subunit to obtain a high-conductance form so that single-channel currents can be detected. At all 5-HT concentrations (> 0.1 microM), channel activity appears as openings in quick succession that form bursts, which coalesce into clusters. By combining single-channel and macroscopic data, we generated a kinetic model that perfectly describes activation, deactivation, and desensitization. The model shows that full activation arises from receptors with three molecules of agonist bound. It reveals an earlier conformational change of the fully liganded receptor that occurs while the channel is still closed. From this pre-open closed state, the receptor enters into an open-closed cycle involving three open states, which form the cluster whose duration parallels the time constant of desensitization. A similar model lacking the pre-open closed state can describe the data only if the opening rates are fixed to account for the slow activation rate. The application of the model to M4 mutant receptors shows that position 10' contributes to channel opening and closing rates. Thus, our kinetic model provides a foundation for understanding structural bases of activation and drug action.
Assuntos
Modelos Biológicos , Receptores 5-HT3 de Serotonina/metabolismo , Animais , Linhagem Celular , Simulação por Computador , Humanos , Cinética , Potenciais da Membrana/fisiologia , Camundongos , Mutação , Técnicas de Patch-Clamp , Conformação Proteica , Receptores 5-HT3 de Serotonina/genética , Serotonina/metabolismo , Agonistas do Receptor 5-HT3 de Serotonina , TransfecçãoRESUMO
In the present study, we investigated the effect of central serotonergic pathway activation achieved through third ventricle injections of quipazine, a serotonergic agonist, on plasma glucose levels of fasted and fed adult Wistar male rats, whose third ventricles were canulated 7 days before the experiments. Central quipazine administration induced a significant increase in plasma glucose levels in fasted animals, but was unable to modify plasma glucose concentrations in fed rats. Pretreatment with alpha-helical CRH, a CRH antagonist, significantly attenuated quipazine-induced hyperglycemia. Pretreatment with two different 5-HT3 receptor antagonists, LY-278,584 and ondansetron, was also able to produce a significant reduction in the hyperglycemic response evoked by central administration of quipazine. None of the antagonists used was capable of modifying plasma glucose concentrations when injected alone into the third ventricle. Quipazine-treated, hyperglycemic animals did not show any increase in plasma insulin levels. We conclude that acute pharmacological serotonergic stimulation by quipazine produces hyperglycemia by mechanisms that require the functional integrity of both CRH and 5-HT3 receptors, and that impairment in insulin secretion and/or activity may explain hyperglycemia induced by third ventricle injections of quipazine.
Assuntos
Hiperglicemia/sangue , Agonistas do Receptor 5-HT3 de Serotonina , Antagonistas do Receptor 5-HT3 de Serotonina , Antagonistas da Serotonina/administração & dosagem , Agonistas do Receptor de Serotonina/administração & dosagem , Animais , Glicemia/efeitos dos fármacos , Hormônio Liberador da Corticotropina/administração & dosagem , Hiperglicemia/induzido quimicamente , Indazóis/administração & dosagem , Insulina/sangue , Masculino , Ondansetron/administração & dosagem , Quipazina/administração & dosagem , Ratos , Ratos Wistar , Tropanos/administração & dosagemRESUMO
Neurotransmitter receptors from the Cys-loop superfamily couple the binding of agonist to the opening of an intrinsic ion pore in the final step in rapid synaptic transmission. Although atomic resolution structural data have recently emerged for individual binding and pore domains, how they are linked into a functional unit remains unknown. Here we identify structural requirements for functionally coupling the two domains by combining acetylcholine (ACh)-binding protein, whose structure was determined at atomic resolution, with the pore domain from the serotonin type-3A (5-HT3A) receptor. Only when amino-acid sequences of three loops in ACh-binding protein are changed to their 5-HT3A counterparts does ACh bind with low affinity characteristic of activatable receptors, and trigger opening of the ion pore. Thus functional coupling requires structural compatibility at the interface of the binding and pore domains. Structural modelling reveals a network of interacting loops between binding and pore domains that mediates this allosteric coupling process.