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      A Neurobiological Hypothesis of Treatment-Resistant Depression – Mechanisms for Selective Serotonin Reuptake Inhibitor Non-Efficacy

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          Abstract

          First-line treatment of major depression includes administration of a selective serotonin reuptake inhibitor (SSRI), yet studies suggest that remission rates following two trials of an SSRI are <50%. The authors examine the putative biological substrates underlying “treatment resistant depression (TRD)” with the goal of elucidating novel rationales to treat TRD. We look at relevant articles from the preclinical and clinical literature combined with clinical exposure to TRD patients. A major focus was to outline pathophysiological mechanisms whereby the serotonin system becomes impervious to the desired enhancement of serotonin neurotransmission by SSRIs. A complementary focus was to dissect neurotransmitter systems, which serve to inhibit the dorsal raphe. We propose, based on a body of translational studies, TRD may not represent a simple serotonin deficit state but rather an excess of midbrain peri-raphe serotonin and subsequent deficit at key fronto-limbic projection sites, with ultimate compromise in serotonin-mediated neuroplasticity. Glutamate, serotonin, noradrenaline, and histamine are activated by stress and exert an inhibitory effect on serotonin outflow, in part by “flooding” 5-HT 1A autoreceptors by serotonin itself. Certain factors putatively exacerbate this scenario – presence of the short arm of the serotonin transporter gene, early-life adversity and comorbid bipolar disorder – each of which has been associated with SSRI-treatment resistance. By utilizing an incremental approach, we provide a system for treating the TRD patient based on a strategy of rescuing serotonin neurotransmission from a state of SSRI-induced dorsal raphe stasis. This calls for “stacked” interventions, with an SSRI base, targeting, if necessary, the glutamatergic, serotonergic, noradrenergic, and histaminergic systems, thereby successively eliminating the inhibitory effects each are capable of exerting on serotonin neurons. Future studies are recommended to test this biologically based approach for treatment of TRD.

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          Antidepressant effects of ketamine in depressed patients.

          A growing body of preclinical research suggests that brain glutamate systems may be involved in the pathophysiology of major depression and the mechanism of action of antidepressants. This is the first placebo-controlled, double-blinded trial to assess the treatment effects of a single dose of an N-methyl-D-aspartate (NMDA) receptor antagonist in patients with depression. Seven subjects with major depression completed 2 test days that involved intravenous treatment with ketamine hydrochloride (.5 mg/kg) or saline solutions under randomized, double-blind conditions. Subjects with depression evidenced significant improvement in depressive symptoms within 72 hours after ketamine but not placebo infusion (i.e., mean 25-item Hamilton Depression Rating Scale scores decreased by 14 +/- SD 10 points vs. 0 +/- 12 points, respectively during active and sham treatment). These results suggest a potential role for NMDA receptor-modulating drugs in the treatment of depression.
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            A molecular and cellular theory of depression.

            Recent studies have begun to characterize the actions of stress and antidepressant treatments beyond the neurotransmitter and receptor level. This work has demonstrated that long-term antidepressant treatments result in the sustained activation of the cyclic adenosine 3',5'-monophosphate system in specific brain regions, including the increased function and expression of the transcription factor cyclic adenosine monophosphate response element-binding protein. The activated cyclic adenosine 3',5'-monophosphate system leads to the regulation of specific target genes, including the increased expression of brain-derived neurotrophic factor in certain populations of neurons in the hippocampus and cerebral cortex. The importance of these changes is highlighted by the discovery that stress can decrease the expression of brain-derived neurotrophic factor and lead to atrophy of these same populations of stress-vulnerable hippocampal neurons. The possibility that the decreased size and impaired function of these neurons may be involved in depression is supported by recent clinical imaging studies, which demonstrate a decreased volume of certain brain structures. These findings constitute the framework for an updated molecular and cellular hypothesis of depression, which posits that stress-induced vulnerability and the therapeutic action of antidepressant treatments occur via intracellular mechanisms that decrease or increase, respectively, neurotrophic factors necessary for the survival and function of particular neurons. This hypothesis also explains how stress and other types of neuronal insult can lead to depression in vulnerable individuals and it outlines novel targets for the rational design of fundamentally new therapeutic agents.
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              Early-life blockade of the 5-HT transporter alters emotional behavior in adult mice.

              Reduced serotonin transporter (5-HTT) expression is associated with abnormal affective and anxiety-like symptoms in humans and rodents, but the mechanism of this effect is unknown. Transient inhibition of 5-HTT during early development with fluoxetine, a commonly used serotonin selective reuptake inhibitor, produced abnormal emotional behaviors in adult mice. This effect mimicked the behavioral phenotype of mice genetically deficient in 5-HTT expression. These findings indicate a critical role of serotonin in the maturation of brain systems that modulate emotional function in the adult and suggest a developmental mechanism to explain how low-expressing 5-HTT promoter alleles increase vulnerability to psychiatric disorders.
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                Author and article information

                Contributors
                URI : http://frontiersin.org/people/u/17988
                URI : http://frontiersin.org/people/u/142353
                URI : http://frontiersin.org/people/u/28752
                Journal
                Front Behav Neurosci
                Front Behav Neurosci
                Front. Behav. Neurosci.
                Frontiers in Behavioral Neuroscience
                Frontiers Media S.A.
                1662-5153
                20 May 2014
                2014
                : 8
                Affiliations
                [1] 1Division of Neuropsychopharmacology, Department of Psychiatry and Behavioral Science, State University of New York Downstate Medical Center , Brooklyn, NY, USA
                [2] 2Department of Psychiatry, Yale School of Medicine , New Haven, CT, USA
                [3] 3Clinical Neuroscience Division, National Center for PTSD , West Haven, CT, USA
                [4] 4State University of New York Downstate College of Medicine , Brooklyn, NY, USA
                Author notes

                Edited by: Francesca Cirulli, Istituto Superiore di Sanità, Italy

                Reviewed by: Osborne F. Almeida, Max Planck Institute of Psychiatry, Germany; Igor Branchi, Istituto Superiore di Sanità, Italy

                *Correspondence: Jeremy D. Coplan, SUNY-Downstate Medical Center, 450 Clarkson Avenue, Box No. 120, Brooklyn, NY 11203, USA e-mail: copstat00@ 123456aol.com

                This article was submitted to the journal Frontiers in Behavioral Neuroscience.

                Article
                10.3389/fnbeh.2014.00189
                4033019
                83516ac6-ae6d-49b6-94c9-cc3e969d21ce
                Copyright © 2014 Coplan, Gopinath, Abdallah and Berry.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

                Page count
                Figures: 4, Tables: 1, Equations: 0, References: 157, Pages: 16, Words: 13923
                Categories
                Neuroscience
                Review Article

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