Revisiting the Serotonin Hypothesis: Implications for Major Depressive Disorders
TL;DR: The overarching goal of this review is to present recent findings from studies examining the serotonergic pathway in MDD, with a focus on SERT and the serotonin 1A (5-HT1A), serotonin 1B (5 -HT1B), and serotonin 2A ( 5-HT2A) receptors.
Abstract: Major depressive disorder (MDD) is a heritable neuropsychiatric disease associated with severe changes at cellular and molecular levels. Its diagnosis mainly relies on the characterization of a wide range of symptoms including changes in mood and behavior. Despite the availability of antidepressant drugs, 10 to 30 % of patients fail to respond after a single or multiple treatments, and the recurrence of depression among responsive patients is very high. Evidence from the past decades suggests that the brain neurotransmitter serotonin (5-HT) is incriminated in MDD, and that a dysfunction of 5-HT receptors may play a role in the genesis of this disease. The 5-HT membrane transporter protein (SERT), which helps regulate the serotonergic transmission, is also implicated in MDD and is one of the main targets of antidepressant therapy. Although a number of behavioral tests and animal models have been developed to study depression, little is known about the neurobiological bases of MDD. Understanding the role of the serotonergic pathway will significantly help improve our knowledge of the pathophysiology of depression and may open up avenues for the development of new antidepressant drugs. The overarching goal of this review is to present recent findings from studies examining the serotonergic pathway in MDD, with a focus on SERT and the serotonin 1A (5-HT1A), serotonin 1B (5-HT1B), and serotonin 2A (5-HT2A) receptors. This paper also describes some of the main molecules involved in the internalization of 5-HT receptors and illustrates the changes in 5-HT neurotransmission in knockout mice and animal model of depression.
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TL;DR: In this article, individual subject data of six acute tryptophan depletion (ATD) studies were pooled (mega-analysis) in order to investigate the mediating role of clinical, demographic and biochemical characteristics in the mood response to ATD.
160 citations
TL;DR: This study is the first to globally evaluate multiple neurotransmitters in MDD plasma and finds that the altered plasma neurotransmitter metabolite profile has potential differential diagnostic value for MDD.
Abstract: Major depressive disorder (MDD) is a debilitating psychiatric illness. However, there is currently no objective laboratory-based diagnostic tests for this disorder. Although, perturbations in multiple neurotransmitter systems have been implicated in MDD, the biochemical changes underlying the disorder remain unclear, and a comprehensive global evaluation of neurotransmitters in MDD has not yet been performed. Here, using a GC-MS coupled with LC-MS/MS-based targeted metabolomics approach, we simultaneously quantified the levels of 19 plasma metabolites involved in GABAergic, catecholaminergic, and serotonergic neurotransmitter systems in 50 first-episode, antidepressant drug-naive MDD subjects and 50 healthy controls to identify potential metabolite biomarkers for MDD (training set). Moreover, an independent sample cohort comprising 49 MDD patients, 30 bipolar disorder (BD) patients and 40 healthy controls (testing set) was further used to validate diagnostic generalizability and specificity of these candidate biomarkers. Among the 19 plasma neurotransmitter metabolites examined, nine were significantly changed in MDD subjects. These metabolites were mainly involved in GABAergic, catecholaminergic and serotonergic systems. The GABAergic and catecholaminergic had better diagnostic value than serotonergic pathway. A panel of four candidate plasma metabolite biomarkers (GABA, dopamine, tyramine, kynurenine) could distinguish MDD subjects from health controls with an AUC of 0.968 and 0.953 in the training and testing set, respectively. Furthermore, this panel distinguished MDD subjects from BD subjects with high accuracy. This study is the first to globally evaluate multiple neurotransmitters in MDD plasma. The altered plasma neurotransmitter metabolite profile has potential differential diagnostic value for MDD.
124 citations
University of Birmingham1, University of Melbourne2, Georgetown University Medical Center3, French Institute of Health and Medical Research4, University of Montpellier5, Mayo Clinic6, University of Texas Medical Branch7, University of Nottingham8, Columbia University9, University of Malta10, University of Toronto11, University of California, San Diego12, Albany Medical College13, École Polytechnique Fédérale de Lausanne14, University of Bari15, Oslo University Hospital16, University of Cambridge17, University of Paris18, Grünenthal GmbH19, University of Washington20, Dana Corporation21, University of Grenoble22, Centre national de la recherche scientifique23, Commissariat à l'énergie atomique et aux énergies alternatives24, University of North Carolina at Chapel Hill25, Queen Mary University of London26, University of Oxford27, CINVESTAV28, Michigan State University29, Florey Institute of Neuroscience and Mental Health30, Scripps Research Institute31
TL;DR: In this article, a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit, is provided, where appropriate, the utility of therapeutics targeting these receptors.
Abstract: 5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.
89 citations
TL;DR: A classification model to discriminate depressive patients with or without SI by machine learning technique is successfully created and a pilot algorithm to predict a grade of SI with citrate and kynurenine is produced.
Abstract: Evaluating the severity of depression (SOD), especially suicidal ideation (SI), is crucial in the treatment of not only patients with mood disorders but also psychiatric patients in general. SOD has been assessed on interviews such as the Hamilton Rating Scale for Depression (HAMD)-17, and/or self-administered questionnaires such as the Patient Health Questionnaire (PHQ)-9. However, these evaluation systems have relied on a person's subjective information, which sometimes lead to difficulties in clinical settings. To resolve this limitation, a more objective SOD evaluation system is needed. Herein, we collected clinical data including HAMD-17/PHQ-9 and blood plasma of psychiatric patients from three independent clinical centers. We performed metabolome analysis of blood plasma using liquid chromatography mass spectrometry (LC-MS), and 123 metabolites were detected. Interestingly, five plasma metabolites (3-hydroxybutyrate (3HB), betaine, citrate, creatinine, and gamma-aminobutyric acid (GABA)) are commonly associated with SOD in all three independent cohort sets regardless of the presence or absence of medication and diagnostic difference. In addition, we have shown several metabolites are independently associated with sub-symptoms of depression including SI. We successfully created a classification model to discriminate depressive patients with or without SI by machine learning technique. Finally, we produced a pilot algorithm to predict a grade of SI with citrate and kynurenine. The above metabolites may have strongly been associated with the underlying novel biological pathophysiology of SOD. We should explore the biological impact of these metabolites on depressive symptoms by utilizing a cross species study model with human and rodents. The present multicenter pilot study offers a potential utility for measuring blood metabolites as a novel objective tool for not only assessing SOD but also evaluating therapeutic efficacy in clinical practice. In addition, modification of these metabolites by diet and/or medications may be a novel therapeutic target for depression. To clarify these aspects, clinical trials measuring metabolites before/after interventions should be conducted. Larger cohort studies including non-clinical subjects are also warranted to clarify our pilot findings.
83 citations
TL;DR: The development of drugs that specifically target 5-HT2C receptors will allow for better understanding of their involvement in the pathophysiology of psychiatric disorders including schizophrenia, anxiety, and depression and the development of agonists that could activate signal transduction pathways leading to better therapeutic efficiency with fewer adverse effects.
Abstract: 5-HT2Rs have a different genomic organization from other 5-HT2Rs. 5HT2CR undergoes post-transcriptional pre-mRNA editing generating diversity among RNA transcripts. Selective post-transcriptional editing could be involved in the pathophysiology of psychiatric disorders through impairment in G-protein interactions. Moreover, it may influence the therapeutic response to agents such as atypical antipsychotic drugs. Additionally, 5-HT2CR exhibits alternative splicing. Central serotonergic and dopaminergic systems interact to modulate normal and abnormal behaviors. Thus, 5HT2CR plays a crucial role in psychiatric disorders. 5HT2CR could be a relevant pharmacological target in the treatment of neuropsychiatric disorders. The development of drugs that specifically target 5-HT2C receptors will allow for better understanding of their involvement in the pathophysiology of psychiatric disorders including schizophrenia, anxiety, and depression. Among therapeutic means currently available, most drugs used to treat highly morbid psychiatric diseases interact at least partly with 5-HT2CRs. Pharmacologically, 5HT2CRs, have the ability to generate differentially distinct response signal transduction pathways depending on the type of 5HT2CR agonist. Although this receptor property has been clearly demonstrated, in vitro, the eventual beneficial impact of this property opens new perspectives in the development of agonists that could activate signal transduction pathways leading to better therapeutic efficiency with fewer adverse effects.
82 citations
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TL;DR: Evidence of a gene-by-environment interaction is provided, in which an individual's response to environmental insults is moderated by his or her genetic makeup.
Abstract: In a prospective-longitudinal study of a representative birth cohort, we tested why stressful experiences lead to depression in some people but not in others. A functional polymorphism in the promoter region of the serotonin transporter (5-HT T) gene was found to moderate the influence of stressful life events on depression. Individuals with one or two copies of the short allele of the 5-HT T promoter polymorphism exhibited more depressive symptoms, diagnosable depression, and suicidality in relation to stressful life events than individuals homozygous for the long allele. This epidemiological study thus provides evidence of a gene-by-environment interaction, in which an individual's response to environmental insults is moderated by his or her genetic makeup.
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TL;DR: Another previously unappreciated strategy used by the receptors to regulate intracellular signaling pathways is indicated, which regulates aspects of cell motility, chemotaxis, apoptosis, and likely other cellular functions through a rapidly expanding list of signaling pathways.
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TL;DR: A mouse model of an anxiety/depressive-like state induced by chronic corticosterone treatment is described, and mice deficient in one of these genes, beta-arrestin 2, displayed a reduced response to fluoxetine in multiple tasks, suggesting that beta-Arrestin signaling is necessary for the antidepressant effects of fluoxettine.
1,133 citations
"Revisiting the Serotonin Hypothesis..." refers background in this paper
...Moreover, a study showed that mice deficient in βarrestin-2 display a reduced response to antidepressants, supporting the fact that β-arrestins are important for the regulation of drug-based antidepressant action [74]....
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TL;DR: Overexpression of β-arrestins in human embryonic kidney cells rescued the sequestration of β2-adrenergic receptor (β2AR) mutants defective in their ability to sequester, an effect enhanced by simultaneous overexpressionof β-adenergic receptor kinase 1 and two β-Arrestin mutants.
Abstract: β-Arrestins are proteins that bind phosphorylated heterotrimeric GTP-binding protein (G protein)-coupled receptors (GPCRs) and contribute to the desensitization of GPCRs by uncoupling the signal transduction process Resensitization of GPCR responsiveness involves agonist-mediated receptor sequestration Overexpression of β-arrestins in human embryonic kidney cells rescued the sequestration of β 2 -adrenergic receptor (β 2 AR) mutants defective in their ability to sequester, an effect enhanced by simultaneous overexpression of β-adrenergic receptor kinase 1 Wild-type β 2 AR sequestration was inhibited by the overexpression of two β-arrestin mutants These findings suggest that β-arrestins play an integral role in GPCR internalization and thus serve a dual role in the regulation of GPCR function
962 citations