Serum levels of excitatory amino acids, serine, glycine, histidine, threonine, taurine, alanine and arginine in treatment-resistant depression: modulation by treatment with antidepressants and prediction of clinical responsivity

Astroglia, Glutamatergic Transmission and Psychiatric Diseases

Astrocytes are primary homeostatic cells of the central nervous system. They regulate glutamatergic transmission through the removal of glutamate from the extracellular space and by supplying neurons with glutamine. Glutamatergic transmission is generally believed to be significantly impaired in the contexts of all major neuropsychiatric diseases. In most of these neuropsychiatric diseases, astrocytes show signs of degeneration and atrophy, which is likely to be translated into reduced homeostatic capabilities. Astroglial glutamate uptake/release and glutamate homeostasis are affected in all forms of major psychiatric disorders and represent a common mechanism underlying neurotransmission disbalance, aberrant connectome and overall failure on information processing by neuronal networks, which underlie pathogenesis of neuropsychiatric diseases.

Decreased quinolinic acid in the hippocampus of depressive patients: evidence for local anti-inflammatory and neuroprotective responses?

Disturbances of glutamatergic neurotransmission and mononuclear phagocyte system activation have been described uni- and bipolar depression (UD/BD). Linking the glutamate and immune hypotheses of depression, quinolinic acid (QUIN) is synthesized by activated microglia and acts as an endogenous N-methyl-D-aspartate glutamate receptor (NMDA-R) agonist with neurotoxic properties. Recently, we observed an increased microglial QUIN expression in the subgenual and supracallosal, but not in the pregenual part of the anterior cingulate cortex in postmortem brains of suicide cases with severe depression. Since several hints point to a role of the hippocampus in depression, we extended our study and addressed the question whether microglial QUIN is also changed in subregions of the hippocampus (CA1 and CA2/3 areas) in these patients. Postmortem brains of 12 acutely depressed patients (UD, n = 6; BD, n = 6) and 10 neuropsychiatric healthy age- and gender-matched control subjects were analyzed using QUIN-immunohistochemistry. Hippocampal volumes were determined in order to assess possible neurotoxic or neurodegenerative aspects. Microglial QUIN expression in the whole group of depressed patients was either comparable (left CA1, right CA2/3) or decreased (right CA1: p = 0.004, left CA2/3: p = 0.044) relative to controls. Post hoc tests showed that QUIN was reduced both in UD and BD in the right CA1 field (UD, p = 0.048; BD, p = 0.031). No loss of hippocampal volume was detected. Our data indicate that UD and BD are associated with a local reduction in QUIN-immunoreactive microglia in the hippocampus and underline the importance of the NMDA-R signaling in depressive disorders.

A brief history of the development of antidepressant drugs: from monoamines to glutamate

Major depressive disorder (MDD) is a chronic, recurring, and debilitating mental illness that is the most common mood disorder in the United States. It has been almost 50 years since the monoamine hypothesis of depression was articulated, and just over 50 years since the first pharmacological treatment for MDD was discovered. Several monoamine-based pharmacological drug classes have been developed and approved for the treatment of MDD; however, remission rates are low (often less than 60%) and there is a delayed onset before remission of depressive symptoms is achieved. As a result of a "proof-of-concept" study in 2000 with the noncompetitive NMDA antagonist ketamine, a number of studies have examined the glutamatergic systems as viable targets for the treatment of MDD. This review will provide a brief history on the development of clinically available antidepressant drugs, and then review the possible role of glutamatergic systems in the pathophysiology of MDD. Specifically, the glutamatergic review will focus on the N-methyl-D-aspartate (NMDA) receptor and the efficacy of drugs that target the NMDA receptor for the treatment of MDD. The noncompetitive NMDA receptor antagonist ketamine, which has consistently produced rapid and sustained antidepressant effects in MDD patients in a number of clinical studies, has shown the most promise as a novel glutamatergic-based treatment for MDD. However, compounds that target other glutamatergic mechanisms, such as GLYX-13 (a glycine-site partial agonist at NMDA receptors) appear promising in early clinical trials. Thus, the clinical findings to date are encouraging and support the continued search for and the development of novel compounds that target glutamatergic mechanisms.

Ketamine and other potential glutamate antidepressants

The need for rapid acting antidepressants is widely recognised. There has been much interest in glutamate mechanisms in major depressive disorder (MDD) as a promising target for the development of new antidepressants. A single intravenous infusion of ketamine, a N-methyl-d-aspartate (NMDA) receptor antagonist anaesthetic agent, can alleviate depressive symptoms in patients within hours of administration. The mechanism of action appears to be in part through glutamate release onto non-NMDA receptors including α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and metabotropic receptors. However these are also reported effects on 5-HT, dopamine and intracellular effects on the mammalian target of rapamycin (mTOR) pathway. The effects of SSRI (Selective Serotonin Reuptake Inhibitor) antidepressants may also involve alterations in NMDA function. The article reviews the effect of current antidepressants on NMDA and examines the efficacy and mechanism of ketamine. Response to ketamine is also discussed and comparison with other glutamate drugs including lamotrigine, amantadine, riluzole, memantine, traxoprodil, GLYX-13, MK-0657, RO4917523, AZD2066 and Coluracetam. Future studies need to link the rapid antidepressant effects seen with ketamine to inflammatory theories in MDD.

Reduced cerebrospinal fluid ethanolamine concentration in major depressive disorder

Amino acids play key roles in the function of the central nervous system, and their alterations are implicated in psychiatric disorders. In the search for a biomarker for major depressive disorder (MDD), we used high-performance liquid chromatography to measure amino acids and related molecules in the cerebrospinal fluid (CSF) of 52 patients with MDD (42 depressed and 10 remitted; DSM-IV) and 54 matched controls. Significant differences were found in four amino acid concentrations between the depressed patients and controls. After Bonferroni correction, only ethanolamine (EA) levels remained significantly reduced in depressed patients (nominal P = 0.0000011). A substantial proportion of the depressed patients (40.5%) showed abnormally low CSF EA levels (<12.1 μM) (P = 0.000033; OR = 11.6, 95% CI: 3.1–43.2). When patients with low EA and those with high EA levels were compared, the former had higher scores for overall depression severity (P = 0.0033) and ‘Somatic Anxiety’ symptoms (P = 0.00026). In unmedicated subjects, CSF EA levels showed a significant positive correlation with levels of homovanillic acid (P = 0.0030) and 5-hydroxyindoleacetic acid (P = 0.019). To our knowledge, this is the first study showing that patients with MDD have significantly lower CSF EA concentrations compared with control subjects. CSF EA could be a state-dependent biomarker for a subtype of MDD.

GABAergic control of depression-related brain states

The GABAergic deficit hypothesis of major depressive disorders (MDDs) posits that reduced γ-aminobutyric acid (GABA) concentration in brain, impaired function of GABAergic interneurons, altered expression and function of GABA(A) receptors, and changes in GABAergic transmission dictated by altered chloride homeostasis can contribute to the etiology of MDD. Conversely, the hypothesis posits that the efficacy of currently used antidepressants is determined by their ability to enhance GABAergic neurotransmission. We here provide an update for corresponding evidence from studies of patients and preclinical animal models of depression. In addition, we propose an explanation for the continued lack of genetic evidence that explains the considerable heritability of MDD. Lastly, we discuss how alterations in GABAergic transmission are integral to other hypotheses of MDD that emphasize (i) the role of monoaminergic deficits, (ii) stress-based etiologies, (iii) neurotrophic deficits, and (iv) the neurotoxic and neural circuit-impairing consequences of chronic excesses of glutamate. We propose that altered GABAergic transmission serves as a common denominator of MDD that can account for all these other hypotheses and that plays a causal and common role in diverse mechanistic etiologies of depressive brain states and in the mechanism of action of current antidepressant drug therapies.

Markers of glutamate signaling in cerebrospinal fluid and serum from patients with bipolar disorder and healthy controls

Glutamate is the major excitatory neurotransmitter in the brain. Aberrations in glutamate signaling have been linked to the pathophysiology of mood disorders. Increased plasma levels of glutamate as well as higher glutamine+glutamate levels in the brain have been demonstrated in patients with bipolar disorder as compared to healthy controls. In this study, we explored the glutamate hypothesis of bipolar disorder by examining peripheral and central levels of amino acids related to glutamate signaling. A total of 215 patients with bipolar disorder and 112 healthy controls from the Swedish St. Göran bipolar project were included in this study. Glutamate, glutamine, glycine, L-serine and D-serine levels were determined in serum and in cerebrospinal fluid using high performance liquid chromatography with fluorescence detection. Serum levels of glutamine, glycine and D-serine were significantly higher whereas L-serine levels were lower in patients with bipolar disorder as compared to controls. No differences between the patient and control group in amino acid levels were observed in cerebrospinal fluid. The observed differences in serum amino acid levels may be interpreted as a systemic aberration in amino acid metabolism that affects several amino acids related to glutamate signaling.

D-serine plasma concentration is a potential biomarker of (R,S)-ketamine antidepressant response in subjects with treatment-resistant depression

RATIONALE

(R,S)-ketamine is a rapid and effective antidepressant drug that produces a response in two thirds of patients with treatment-resistant depression (TRD). The underlying biochemical differences between a (R,S)-ketamine responder (KET-R) and non-responder (KET-NR) have not been definitively identified but may involve serine metabolism.

OBJECTIVES

The aim of the study was to examine the relationship between baseline plasma concentrations of D-serine and its precursor L-serine and antidepressant response to (R,S)-ketamine in TRD patients.

METHODS

Plasma samples were obtained from 21 TRD patients at baseline, 60 min before initiation of the (R,S)-ketamine infusion. Patients were classified as KET-Rs (n = 8) or KET-NRs (n = 13) based upon the difference in Montgomery-Åsberg Depression Rating Scale (MADRS) scores at baseline and 230 min after infusion, with response defined as a ≥50 % decrease in MADRS score. The plasma concentrations of D-serine and L-serine were determined using liquid chromatography-mass spectrometry.

RESULTS

Baseline D-serine plasma concentrations were significantly lower in KET-Rs (3.02 ± 0.21 μM) than in KET-NRs (4.68 ± 0.81 μM), p < 0.001. A significant relationship between baseline D-serine plasma concentrations and percent change in MADRS at 230 min was determined using a Pearson correlation, r = 0.77, p < 0.001, with baseline D-serine explaining 60 % of the variance in (R,S)-ketamine response. The baseline concentrations of L-serine (L-Ser) in KET-Rs were also significantly lower than those measured in KET-NRs (66.2 ± 9.6 μM vs 242.9 ± 5.6 μM, respectively; p < 0.0001).

CONCLUSIONS

The results demonstrate that the baseline D-serine plasma concentrations were significantly lower in KET-Rs than in KET-NRs and suggest that this variable can be used to predict an antidepressant response following (R,S)-ketamine administration.

Sex-specific urinary biomarkers for diagnosing bipolar disorder

Sex-based differences are prominent in affective disorders, but there are no biomarkers available to support sex-specific, laboratory-based diagnostics for male and female bipolar disorder (BD) patients. Here, a NMR-based metabonomic approach was used to preliminarily identify sex-specific urinary metabolite biomarkers for diagnosing male and female BD patients. A male-specific biomarker panel consisting of four metabolites (α-hydroxybutyrate, choline, formate, and N-methylnicotinamide) effectively discriminated between male BD and healthy controls (HC) subjects, achieving an area under the receiver operating characteristic curve (AUC) of 0.942. A female-specific biomarkers panel consisting of four metabolites (α-hydroxybutyrate, oxalacetate, acetone, and N-methylnicotinamide) effectively discriminated between female BD and HC subjects, achieving an AUC of 0.909. The male-specific biomarker panel displayed low discriminatory power in the female group, and the female-specific biomarker panel displayed low discriminatory power in the male group. Moreover, several other metabolites showed different trends between male and female BD subjects. These findings suggest that male and female BD patients have distinct biomarker fingerprints and that these two sex-specific biomarker panels may serve as effective diagnostic tools in distinguishing male and female BD patients from their healthy counterparts. Our work may provide a window into the mechanisms underlying the pathoetiology of BD in both men and women.

The role of NMDA receptors in the pathophysiology and treatment of mood disorders

Mood disorders such as major depressive disorder and bipolar disorder are chronic and recurrent illnesses that cause significant disability and affect approximately 350 million people worldwide. Currently available biogenic amine treatments provide relief for many and yet fail to ameliorate symptoms for others, highlighting the need to diversify the search for new therapeutic strategies. Here we present recent evidence implicating the role of N-methyl-D-aspartate receptor (NMDAR) signaling in the pathophysiology of mood disorders. The possible role of NMDARs in mood disorders has been supported by evidence demonstrating that: (i) both BPD and MDD are characterized by altered levels of central excitatory neurotransmitters; (ii) NMDAR expression, distribution, and function are atypical in patients with mood disorders; (iii) NMDAR modulators show positive therapeutic effects in BPD and MDD patients; and (iv) conventional antidepressants/mood stabilizers can modulate NMDAR function. Taken together, this evidence suggests the NMDAR system holds considerable promise as a therapeutic target for developing next generation drugs that may provide more rapid onset relief of symptoms. Identifying the subcircuits involved in mood and elucidating the role of NMDARs subtypes in specific brain circuits would constitute an important step toward the development of more effective therapies with fewer side effects.

Antidepressant-like effect of the mGluR5 antagonist MTEP in an astroglial degeneration model of depression

The glutamatergic predominance in the excitatory-inhibitory balance is postulated to be involved in the pathogenesis of depression. Such imbalance may be induced by astrocyte ablation which reduces glutamate uptake and increases glutamate level in the synaptic cleft. In the present study, we tried to ascertain whether astroglial degeneration in the prefrontal cortex could serve as an animal model of depression and whether inhibition of glutamatergic transmission by the mGluR5 antagonist MTEP could have antidepressant potential. Astrocytic toxins l-or dl-alpha-aminoadipic acid (AAA), 100μg/2μl, were microinjected, bilaterally into the rat medial prefrontal cortex (PFC) on the first and second day of experiment. MTEP (10mg/kg) or imipramine (30mg/kg) were administered on the fifth day. Following administration of MTEP or imipramine the forced swim test (FST) was performed for assessment of depressive-like behavior. The brains were taken out for analysis on day eight. The astrocytic marker, glial fibrillary acidic protein (GFAP) was quantified in PFC by Western blot method and by stereological counting of immunohistochemically stained sections. Both l-AAA and dl-AAA induced a significant increase in immobility time in the FST. This effect was reversed by imipramine, which indicates depressive-like effects of these toxins. A significant decrease in GFAP (about 50%) was found after l-AAA. Both the behavioral and GFAP level changes were prevented by MTEP injection. The obtained results indicate that the degeneration of astrocytes in the PFC by l-AAA may be a useful animal model of depression and suggest antidepressant potential of MTEP.

Serotonergic modulation of glutamate neurotransmission as a strategy for treating depression and cognitive dysfunction

Monoamine-based treatments for depression have evolved greatly over the past several years, but shortcomings such as suboptimal efficacy, treatment lag, and residual cognitive dysfunction are still significant. Preclinical and clinical studies using compounds directly targeting glutamatergic neurotransmission present new opportunities for antidepressant treatment, with ketamine having a surprisingly rapid and sustained antidepressant effect that is presumably mediated through glutamate-dependent mechanisms. While direct modulation of glutamate transmission for antidepressant and cognition-enhancing actions may be hampered by nonspecific effects, indirect modulation through the serotonin (5-HT) system may be a viable alternative approach. Based on localization and function, 5-HT can modulate glutamate neurotransmission at least through the 5-HT1A, 5-HT1B, 5-HT3, and 5-HT7 receptors, which presents a rational pharmacological opportunity for modulating glutamatergic transmission without the direct use of glutamatergic compounds. Combining one or more of these glutamate-modulating 5-HT targets with 5-HT transporter inhibition may offer new therapeutic opportunities. The multimodal compounds vortioxetine and vilazodone are examples of this approach with diverse mechanisms, and their different clinical effects will provide valuable insights into serotonergic modulation of glutamate transmission for the potential treatment of depression and associated cognitive dysfunction.

Immunology of major depression

High levels of several proinflammatory components of the immune system, such as interleukin-6, C-reactive protein, tumor necrosis factor (TNF)-α, or neopterin in patients suffering from major depression (MD) point to the involvement of an inflammatory process in the pathophysiology of MD. The direct and indirect effects of cytokines on neurotransmitter storage and release - mediated by microglia cells and astrocytes - are discussed. The tryptophan/kynurenine metabolism is one of the indirect mechanisms because the enzyme indoleamine 2,3-dioxygenase - a key enzyme of this metabolism in the central nervous system - is driven by pro- and anti-inflammatory cytokines and degrades serotonin. Moreover, neuroactive kynurenines such as kynurenic acid and quinolinic acid act on the glutamatergic neurotransmission as N-methyl-D-aspartate antagonists and agonists, respectively. Alterations of the serotonergic, noradrenergic and glutamatergic neurotransmission have been shown with low-level neuroinflammation and may be involved in symptom generation. Epidemiological and clinical studies show a role for inflammation as a risk factor for MD. A large-scale epidemiological study in MD clearly demonstrates that severe infections and autoimmune disorders are lifetime risk factors for MD. The vulnerability-stress-inflammation model matches with this view as stress may increase proinflammatory cytokines and even contribute to a lasting proinflammatory state. Further support comes from the therapeutic benefit of anti-inflammatory medications such as the cyclo-oxygenase-2 inhibitors, TNF-α antagonists and others, and the anti-inflammatory and immunomodulatory intrinsic effects of antidepressants.

Glutamate transporters: a key piece in the glutamate puzzle of major depressive disorder

Glutamatergic therapies are emerging as the new path for the treatment of Major Depression Disorder. Recent reports reviewing the use of glutamate activity modulators in the treatment of resistant depression advocate the importance of understanding the alterations of the diverse components of this complex system in mood disorders. In this postmortem study we used in situ hybridization and microarray analysis to evaluate the gene expression of the membrane transporters SLC1A2 and SLCA3 and the vesicular transporter SLCA17A7 in the hippocampus of Major Depressive Disorder (MDD) and Bipolar Disorder (BPD) subjects. Samples from 8 controls, 11 MDD and 6 BPD subjects were processed for in situ hybridization using cRNA probes for SLC1A2, SLC1A3 and SLC17A7. Laser capture microdissection was used to collect tissue from adjacent sections for microarray analysis. The results showed that the expression of the membrane transporters SLC1A2 and SLC1A3 was diminished in the MDD group compared to controls. The expression of the vesicular glutamate transporter SLC17A7 on the other hand was increased in MDD subjects. As for the BPD group, all three transporters showed trends similar to those observed in MDD, but the changes observed did not reach significance. We hypothesize that the decreased expression of the membrane glutamate transporters and the increased expression of the vesicular transporter in the hippocampus would affect the balance of the glutamatergic circuitry of the hippocampus, and that this effect may be a major contributor to depressive symptoms.

Differential levels of brain amino acids in rat models presenting learned helplessness or non-learned helplessness

RATIONALE

Glutamatergic and γ-aminobutyric acid (GABA)ergic abnormalities have recently been proposed to contribute to depression. The learned helplessness (LH) paradigm produces a reliable animal model of depression that expresses a deficit in escape behavior (LH model); an alternative phenotype that does not exhibit LH is a model of resilience to depression (non-LH model).

OBJECTIVES

We measured the contents of amino acids in the brain to investigate the mechanisms involved in the pathology of depression.

METHODS

LH and non-LH models were subjected to inescapable electric footshocks at random intervals following a conditioned avoidance test to determine acquirement of predicted escape deficits. Tissue amino acid contents in eight brain regions were measured via high-performance liquid chromatography.

RESULTS

The non-LH model showed increased GABA levels in the dentate gyrus and nucleus accumbens and increased glutamine levels in the dentate gyrus and the orbitofrontal cortex. The LH model had reduced glutamine levels in the medial prefrontal cortex. Changes in the ratios of GABA, glutamine, and glutamate were detected in the non-LH model, but not in the LH model. Reductions in threonine levels occurred in the medial prefrontal cortex in both models, whereas elevated alanine levels were detected in the medial prefrontal cortex in non-LH animals.

CONCLUSIONS

The present study demonstrates region-specific compensatory elevations in GABA levels in the dentate gyrus and nucleus accumbens of non-LH animals, supporting the implication of the GABAergic system in the recovery of depression.

Glutamate system as target for development of novel antidepressants

Depression is a common psychiatric condition characterized by affective, cognitive, psychomotor, and neurovegetative symptoms that interfere with a person's ability to work, study, deal with interpersonal relationships, and enjoy once-pleasurable activities. After the serendipitous discovery of the first antidepressants, for years the only pharmacodynamic mechanisms explored in the search of novel antidepressants were those related to the 3 main monoamines: serotonin, norepinephrine, and dopamine. New-generation monoaminergic antidepressants, such as selective-serotonin and dual-acting serotonin/norepinephrine reuptake inhibitors, improved treatment and quality of life of depressed patients. Nevertheless, there are still important clinical limitations: the long latency of onset of the antidepressant action; side effects, which can lead to early discontinuation; low rate of response; and high rate of relapse/recurrence. Therefore, in the last several years, the focus of research has moved from monoamines toward other molecular mechanisms, including glutamatergic (Glu) neurotransmission. This review provides a comprehensive overview of the current knowledge on the Glu system and on its relationships with mood disorders. Up to now, N-methyl-D-aspartate (NMDA) receptor antagonists, in particular ketamine, provided the most promising results in preclinical studies and produced a consistent and rapid, although transient, antidepressant effect with a good tolerability profile in humans. Although data are encouraging, more double-blind, randomized, placebo-controlled trials are needed to clarify the real potentiality of ketamine, and of the other Glu modulators, in the treatment of unipolar and bipolar depression.

Selective breeding for helplessness in rats alters the metabolic profile of the hippocampus and frontal cortex: a 1H-MRS study at 9.4 T

In humans metabolic changes, particularly in frontal areas of the brain, accompany depressive disorders, but few studies were conducted in animal models of depression. We used hydrogen-1 magnetic resonance spectroscopy at 9.4 T to measure the metabolic profiles of the hippocampus and frontal cortex in congenital learned helpless (cLH) and wild-type (WT) rats. The learned helplessness model of depression exposes animals to uncontrollable stress to induce changes in emotion, cognition and behaviour, but cLH rats were selectively bred to show changes in behaviour even without exposure to uncontrollable stress. Experimentally naive male 8- to 10-wk-old cLH (n = 10) and WT rats (n = 22) underwent spectroscopy and were exposed to uncontrollable stress 1 wk after the scan. We found that cLH compared to WT rats had lower levels of glutamate in the hippocampus and lower levels of choline-containing compounds in the hippocampus and frontal cortex, but higher levels of taurine and phosphocreatine in these regions, pointing to compensatory efforts of the brain to reduce excitotoxic potential and to increase neuroprotection and energy, possibly as a result of cellular stress and damage. The reduction in choline-containing phospholipids might represent a source or correlate of such stress. Overall, the results indicate that metabolic abnormalities are present in animals with a predisposition to helplessness even without exposure to explicit stress and may help identify non-invasive biomarkers in individuals who are prone to depression.

Targeting the glutamatergic system to treat major depressive disorder: rationale and progress to date

Major depressive disorder (MDD) is a severe, debilitating medical illness that affects millions of individuals worldwide. The young age of onset and chronicity of the disorder has a significant impact on the long-term disability that affected individuals face. Most existing treatments have focused on the 'monoamine hypothesis' for rational design of compounds. However, patients continue to experience low remission rates, residual subsyndromal symptoms, relapses and overall functional impairment. In this context, growing evidence suggests that the glutamatergic system is uniquely central to the neurobiology and treatment of MDD. Here, we review data supporting the involvement of the glutamatergic system in the pathophysiology of MDD, and discuss the efficacy of glutamatergic agents as novel therapeutics. Preliminary clinical evidence has been promising, particularly with regard to the N-methyl-D-aspartate (NMDA) antagonist ketamine as a 'proof-of-concept' agent. The review also highlights potential molecular and inflammatory mechanisms that may contribute to the rapid antidepressant response seen with ketamine. Because existing pharmacological treatments for MDD are often insufficient for many patients, the next generation of treatments needs to be more effective, rapid acting and better tolerated than currently available medications. There is extant evidence that the glutamatergic system holds considerable promise for developing the next generation of novel and mechanistically distinct agents for the treatment of MDD.

Low plasma levels of L-arginine, impaired intraplatelet nitric oxide and platelet hyperaggregability: implications for cardiovascular disease in depressive patients

BACKGROUND

Major depression (MD) is an independent cardiovascular risk factor, but the exact mechanisms are not clear. In this study we have investigated the intraplatelet L-arginine-nitric oxide (NO) pathway and platelet function in depressive patients.

METHODS

Nineteen unmedicated patients with MD (34±4years) and 19 control subjects (CS, 34±3years) were included. L-[(3)H]-arginine influx, NO synthase (NOS) activity and intracellular cGMP levels were evaluated in platelets, as well as the expression of eNOS, iNOS, arginase and soluble guanylate cyclase (sGC), platelet aggregation and the systemic amino acid profile in MD patients and CS.

RESULTS

L-arginine influx (pmol/10(9)cells/min) in platelets was reduced from 46.2±9.5 to 20.02±2.12 in depression. NOS activity (pmol/10(8) cells) was diminished in MD patients (0.09±0.01) compared to CS (0.17±0.01). Intracellular cGMP levels were also impaired in MD patients associated with hyperaggregability. Moreover, the concentration of plasma L-arginine was reduced by 20% in MD patients. The expression of eNOS, iNOS, arginase II and sGC in platelet lysates was not affected by MD.

LIMITATIONS

Small number of patients in the study.

CONCLUSIONS

This study has demonstrated an impairment of L-arginine-NO signaling in platelets from MD patients, suggesting a role in platelet activation and cardiovascular events.

Enhancing synaptic plasticity and cellular resilience to develop novel, improved treatments for mood disorders

There is mounting evidence that recurrent mood disorders - once considered “good prognosis diseases”- are, in fact, often very severe and life-threatening illnesses. Furthermore, although mood disorders have traditionally been conceptualized as neurochemical disorders, there is now evidence from a variety of sources demonstrating regional reductions in central nervous system (CNS) volume, as well as reductions in the numbers and/or sizes ofglia and neurons in discrete brain areas. Although the precise cellular mechanisms underlying these morphometric changes remain to be fully elucidated, the data suggest that mood disorders are associated with impairments of synaptic plasticity and cellular resilience. In this context, it is noteworthy that there is increasing preclinical evidence that antidepressants regulate the function of the glutamatergic system. Moreover, although clearly preliminary, the available clinical data suggest that attenuation of N-methyl-D-aspartate (NMDA) function has antidepressant effects. Recent preclinical and clinical studies have shown that signaling pathways involved in regulating cell survival and cell death are long-term targets for the actions of antidepressant agents. Antidepressants and mood stabilizers indirectly regulate a number of factors involved in cell survival pathways, including cyclic adenosine monophosphate (cAMP) response element binding protein (CREB), brain-derived neurotrophic factor (BDNF), the antiapoptotic protein bcl-2, and mitogen-activated protein (MAP) kinases, and may thus bring about some of their delayed long-term beneficial effects via underappreciated neurotrophic effects. There is much promise for the future development of treatments that more directly target molecules in critical CNS signaling pathways regulating synaptic plasticity and cellular resilience. These will represent improved long-term treatments for mood disorders.

Plasma amino acid levels discriminate between control subjects and mildly depressed elderly women

BACKGROUND AND AIMS

Depression is the most common of all psychiatric disorders and the role of amino acid transmitters in this pathology has been recently studied. We undertook this study to investigate if the plasma levels of L-arginine, L-citrulline, L-glutamic acid, L-glutamine, L-asparagine and other amino acids, the L-citrulline/L-arginine and the L-tyrosine/L-phenylalanine ratios, and the nitrite levels are modified in mildly depressed women and if such changes are related to olfactory dysfunction.

METHODS

Plasma samples were obtained from elderly female subjects (n = 21) with mild depression and (n = 48) controls. Amino acids were analyzed with high-performance liquid chromatography, plasma nitrite levels were measured using the Griess method, and olfactory performance was assessed by the combined testing of odor identification, odor discrimination, odor recognition, and the olfactory threshold.

RESULTS

Compared to controls, depressed patients had a significantly higher concentration of L-arginine and a significantly lower L-citrulline/L-arginine ratio when the effect of other variables is not taken into account. A logistic regression model allowed us to identify two risk factors for mild depression, L-arginine and L-glutamic acid, and two protective factors, L-asparagine and the L-tyrosine/L-phenylalanine ratio. Additionally, a significant increase in nitrite levels in depressed women was found. No significant differences were found between the percentage of depressed and control women that identified the odors.

CONCLUSIONS

We identified that the amino acids L-arginine and L-glutamic acid are risk factors for mild depression, whereas L-asparagine and the L-tyrosine/L-phenylalanine ratio are protective factors.

Targeting the glutamatergic system to treat major depressive disorder: rationale and progress to date

Major depressive disorder (MDD) is a severe, debilitating medical illness that affects millions of individuals worldwide. The young age of onset and chronicity of the disorder has a significant impact on the long-term disability that affected individuals face. Most existing treatments have focused on the 'monoamine hypothesis' for rational design of compounds. However, patients continue to experience low remission rates, residual subsyndromal symptoms, relapses and overall functional impairment. In this context, growing evidence suggests that the glutamatergic system is uniquely central to the neurobiology and treatment of MDD. Here, we review data supporting the involvement of the glutamatergic system in the pathophysiology of MDD, and discuss the efficacy of glutamatergic agents as novel therapeutics. Preliminary clinical evidence has been promising, particularly with regard to the N-methyl-D-aspartate (NMDA) antagonist ketamine as a 'proof-of-concept' agent. The review also highlights potential molecular and inflammatory mechanisms that may contribute to the rapid antidepressant response seen with ketamine. Because existing pharmacological treatments for MDD are often insufficient for many patients, the next generation of treatments needs to be more effective, rapid acting and better tolerated than currently available medications. There is extant evidence that the glutamatergic system holds considerable promise for developing the next generation of novel and mechanistically distinct agents for the treatment of MDD.

Taurine and glutathione levels in plasma before and after ECT treatment

Taurine has been shown to be elevated in plasma and lymphocytes of depressed patients, but the level normalises after successful drug therapy. During depression, levels of glutathione (GSH) are decreased in the plasma and blood. This study was performed to examine taurine and GSH levels in depressed patients before and after electroconvulsive therapy (ECT). Fasting blood samples were collected from 23 patients before the first and after the third ECT treatment. The severity of depression was estimated with the Montgomery-Åsberg Depression Rating Scale (MADRS). We analysed GSH in blood and the levels of taurine and total GSH in plasma. After three ECTs, a significant decrease in MADRS scores was found for the entire group. Simultaneously, the decrease in the plasma taurine levels was significant for the seven responders but not for the sixteen non-responders. We observed no differences in blood or plasma GSH levels after three ECT treatments when compared to values before the therapy. Plasma taurine levels decrease significantly after three ECT treatments in patients who respond to treatment. GSH levels were not affected by ECT treatment. The results indicate that taurine may play a role in the pathophysiology of depression.

A possible participation of transient receptor potential vanilloid type 1 channels in the antidepressant effect of fluoxetine

The present study investigated the influence of transient receptor vanilloid type 1 (TRPV1) channel agonist (capsaicin) and antagonist (capsazepine) either alone or in combination with traditional antidepressant drug, fluoxetine; or a serotonin hydroxylase inhibitor, para-chlorophenylalanine; or a glutamate N-methyl-D-aspartate (NMDA) receptor agonist, NMDA on the forced swim test and tail suspension test using male Swiss mice. Results revealed that intracerebroventricular injections of capsaicin (200 and 300 μg/mouse) and capsazepine (100 and 200 μg/mouse) reduced the immobility time, exhibiting antidepressant-like activity that was comparable to the effects of fluoxetine (2.5-10 μg/mouse) in both the tests. However, in the presence of inactive dose (10 μg/mouse) of capsazepine, capsaicin (300 μg/mouse) had no influence on the indices of both tests, signifying that the effects are TRPV1-mediated. Further, the antidepressant-like effects of both the TRPV1 ligands were neutralized in mice-pretreated with NMDA (0.1 μg/mouse), suggestive of the fact that decreased glutamatergic transmission might contribute to the antidepressant-like activity. In addition, co-administration of sub-threshold dose of capsazepine (10 μg/mouse) and fluoxetine (1.75 μg/mouse) produced a synergistic effect in both the tests. In contrast, inactive doses of capsaicin (10 and 100 μg/mouse) partially abolished the antidepressant effect of fluoxetine (10 μg/mouse), while its effect was potentiated by active dose of capsaicin (200 μg/mouse). Moreover, pretreatment of mice with para-chlorophenylalanine (300 mg/kg/day × 3 days, i.p.) attenuated the effects of capsaicin and capsazepine, demonstrating a probable interplay between serotonin and TRPV1, at least in parts. Thus, our data indicate a possible role of TRPV1 in depressive-like symptoms.

Polymorphisms within the metabotropic glutamate receptor 1 gene are associated with depression phenotypes

OBJECTIVES

Glutamate has been implicated in the pathophysiology and treatment of mood disorders possibly by affecting the regulation of the hypothalamus-pituitary-adrenocortical (HPA) axis. Growing evidence suggests an important role of the metabotropic glutamate receptor 1 (mGlu1) in depression-related phenotypes. To test whether these findings can also be supported by human genetics data, we explored polymorphisms within the metabotropic glutamate receptor 1 gene (GRM1) for their association with unipolar depression (UPD) as well as with biological phenotypes of this disorder.

METHODS

We first tested the association of 43 tag-SNPs covering the GRM1 locus with UPD in 350 patients and 370 matched controls. We then investigated the effects of the associated SNPs on hippocampal glutamate levels estimated using ¹H-MR-spectroscopy (¹H-MRS) and on endocrine measures from the combined dexamethasone-suppression/CRH stimulation (dex/CRH) test.

RESULTS

Within the GRM1 locus, 22 SNPs showed nominally significant association with UPD, of which 6 withstood corrections for multiple testing (rs2268666 with best allelic p=7.0×10⁻⁵). Supportive evidence for an association with UPD was gained from a second independent sample with 904 patients and 1012 controls. Furthermore, patients homozygous for the non-risk genotypes showed reduced hippocampal glutamate levels as measured by ¹H-MRS, a more pronounced normalization of HPA-axis hyperactivity as well as a better antidepressant treatment outcome.

CONCLUSIONS

These results suggest that the combination of genetic and biological markers may allow to subgroup patients into etiopathogenetically more relevant subcategories which could guide clinicians in their antidepressant treatment choices.

Reduced metabotropic glutamate receptor 5 in the Flinders Sensitive Line of rats, an animal model of depression: an autoradiographic study

Depression is a brain disorder and there is still only a partial understanding of its underlying pathophysiology. Antidepressant medications with a fast onset have not yet been developed. In addition to the monoaminergic systems, the brain glutaminergic system has been implicated in the etiology of depression. Animal studies of depression have gained importance because they permit a more invasive manipulation of the subjects than human studies. In the present study, we measured the densities of the brain regional metabotropic glutaminergic receptor 5 (mGluR5) in the Flinders Sensitive Line (FSL) rat model of depression and two groups of control rats, the Flinders Resistant Line (FRL) and Sprague Dawley (SPD), the parent strain for both the FSL and FRL rats. The FSL rats showed lower densities of mGluR5 in many brain regions compared to either the SPD and/or FRL rats. In addition, the densities in the FRL rats were larger than in the SPD rats, suggesting possible problems in using FRL rats as controls. The presented data suggest that mGluR5 is lower in animal models of depression which could be related to the cognitive and emotional dysfunctions in the FSL rat model of depression and could be relevant to a better understanding of depression in humans.

Review of pharmacological treatment in mood disorders and future directions for drug development

After a series of serendipitous discoveries of pharmacological treatments for mania and depression several decades ago, relatively little progress has been made for novel hypothesis-driven drug development in mood disorders. Multifactorial etiologies of, and lack of a full understanding of, the core neurobiology of these conditions clearly have contributed to these development challenges. There are, however, relatively novel targets that have raised opportunities for progress in the field, such as glutamate and cholinergic receptor modulators, circadian regulators, and enzyme inhibitors, for alternative treatment. This review will discuss these promising new treatments in mood disorders, the underlying mechanisms of action, and critical issues of their clinical application. For these new treatments to be successful in clinical practice, it is also important to design innovative clinical trials that identify the specific actions of new drugs, and, ideally, to develop biomarkers for monitoring individualized treatment response. It is predicted that future drug development will identify new agents targeting the molecular mechanisms involved in the pathophysiology of mood disorders.

The role of glutamate on the action of antidepressants

Major depressive disorder (MDD) is a common, chronic, recurrent mental illness that affects millions of individuals worldwide. Currently available antidepressants are known to affect the monoaminergic (e.g., serotonin, norepinephrine, and dopamine) systems in the brain. Accumulating evidence suggests that the glutamatergic neurotransmission via the excitatory amino acid glutamate also plays an important role in the neurobiology and treatment of this disease. Clinical studies have demonstrated that the non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist ketamine has rapid antidepressant effects in treatment-resistant patients with MDD, suggesting the role of glutamate in the pathophysiology of treatment-resistant MDD. Furthermore, a number of preclinical studies demonstrated that the agents which act at glutamate receptors such as NMDA receptors, α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors and metabotropic glutamate receptors (mGluRs) might have antidepressant-like activities in animal models of depression. In this article, the author reviews the role of glutamate in the neuron-glia communication induced by potential antidepressants.

Towards a glutamate hypothesis of depression: an emerging frontier of neuropsychopharmacology for mood disorders

Half a century after the first formulation of the monoamine hypothesis, compelling evidence implies that long-term changes in an array of brain areas and circuits mediating complex cognitive-emotional behaviors represent the biological underpinnings of mood/anxiety disorders. A large number of clinical studies suggest that pathophysiology is associated with dysfunction of the predominant glutamatergic system, malfunction in the mechanisms regulating clearance and metabolism of glutamate, and cytoarchitectural/morphological maladaptive changes in a number of brain areas mediating cognitive-emotional behaviors. Concurrently, a wealth of data from animal models have shown that different types of environmental stress enhance glutamate release/transmission in limbic/cortical areas and exert powerful structural effects, inducing dendritic remodeling, reduction of synapses and possibly volumetric reductions resembling those observed in depressed patients. Because a vast majority of neurons and synapses in these areas and circuits use glutamate as neurotransmitter, it would be limiting to maintain that glutamate is in some way 'involved' in mood/anxiety disorders; rather it should be recognized that the glutamatergic system is a primary mediator of psychiatric pathology and, potentially, also a final common pathway for the therapeutic action of antidepressant agents. A paradigm shift from a monoamine hypothesis of depression to a neuroplasticity hypothesis focused on glutamate may represent a substantial advancement in the working hypothesis that drives research for new drugs and therapies. Importantly, despite the availability of multiple classes of drugs with monoamine-based mechanisms of action, there remains a large percentage of patients who fail to achieve a sustained remission of depressive symptoms. The unmet need for improved pharmacotherapies for treatment-resistant depression means there is a large space for the development of new compounds with novel mechanisms of action such as glutamate transmission and related pathways. This article is part of a Special Issue entitled 'Anxiety and Depression'.

The folate hydrolase 1561C>T polymorphism is associated with depressive symptoms in Puerto Rican adults

OBJECTIVE

To examine the associations between variants of genes involved in the uptake, retention, and metabolism of folate and depressive symptoms and to analyze whether such associations are direct or through mediation by folate or homocysteine.

METHODS

We performed a cross-sectional analysis of data from 976 Puerto Rican adults, aged 45 to 75 years, residing in the greater Boston area, Massachusetts. Twelve single nucleotide polymorphisms (SNPs) in genes involved in folate uptake, retention, and metabolism were investigated. These include FOLH1 (folate hydrolase), FPGS (folate polyglutamate synthase), GGH (γ-glutamyl hydrolase), MTHFR (methylenetetrahydrofolate reductase), MTR (methionine synthase), PCFT (proton-coupled folate transporter), and RFC1 (reduced folate carrier 1). The Center for Epidemiologic Studies Depression Scale (CES-D) was used to measure depressive symptoms.

RESULTS

The FOLH1 rs61886492 C>T (or 1561C>T) polymorphism was significantly associated with lower CES-D score (p = .0025) after adjusting for age, sex, population admixture, smoking, and educational attainment. Individuals with the TT and TC genotypes were 49% less likely (odds ratio = 0.51, 95% confidence interval = 0.29-0.89) to report mild depressive symptoms (CES-D score ≥16 and ≤26) and 64% less likely (odds ratio = 0.36, 95% confidence interval = 0.18-0.69) to report moderate to severe depressive symptoms (CES-D score >26), compared with those with the CC genotype. No significant mediation effects by plasma folate or homocysteine on the associations between this single nucleotide polymorphism and CES-D score were observed.

CONCLUSIONS

The FOLH1 1561C>T polymorphism may be associated with the risk of depressive symptoms.

Roles of glutamate signaling in preclinical and/or mechanistic models of depression

Accumulating evidence suggests that the glutamatergic system plays important roles in the pathophysiology and treatment of major depressive disorder (MDD). Abnormalities in the glutamatergic system are definitely observed in this disorder, and certain glutamatergic agents exhibit antidepressant effects in patients with MDD. In this review, we summarize the preclinical findings suggesting the involvement of glutamate signaling in the pathophysiology and treatment of MDD. Preclinical animal models for depression are often characterized by changes in molecules related to glutamatergic signaling. Some antidepressants exert their effects by affecting glutamatergic system components in animals. Animals with genetically modified glutamatergic function exhibit depression-like behaviors or anti-depressive behavior. In addition, several types of glutamatergic agents have shown antidepressant-like effects in preclinical models for depression. Many types of glutamate receptors (NMDA, AMPA, and metabotropic glutamate receptors) or transporters appear to be involved in the etiology of depression or in the mechanisms of action of antidepressants. These functional proteins related to glutamate signal transduction are potential targets for a new generation of antidepressants with fast-onset effects, such as the NMDA antagonist ketamine.

Inflammation and depression: why poststroke depression may be the norm and not the exception

Ischaemic stroke often precedes the appearance of clinical depression. Poststroke depression in turn influences the prognostic outcome. In the interest of advancing our understanding of the biological mechanisms underlying the development of poststroke depression, this systematic review explores the immunological processes driving the development of inflammation-related cell death in mood-related brain regions. Particular attention has been paid to cytokine-driven intrinsic apoptosis factors, including intracellular calcium, glutamate excitotoxicity and free radicals that appear in the brain following ischaemic damage and whose presence significantly increases the likelihood of clinically defined depression.

Protective effect of gan mai da zao decoction in unpredictable chronic mild stress-induced behavioral and biochemical alterations

AIM

Growing evidence indicates that the glutamatergic system, especially the abnormalities of glutamate and N-methyl-D-aspartate (NMDA) receptors contribute to the pathophysiology and possibly the pathogenesis of major depressive disorders. This study is to evaluate the effect of gan mai da zao (GMDZ) decoction on glutamate and NMDA receptor in unpredictable chronic mild stress (UCMS) rats.

MATERIALS AND METHODS

Sucrose preference test and open field test were used to estimate the depressive-like behaviors of UCMS rats. Glutamate levels and NMDA receptor subunits (NR1, NR2A and NR2B) in the frontal cortex and hippocampus were determined by HPLC-FLD and by western-blot respectively.

RESULTS

32 days UCMS induced depressive-like behaviors, increased glutamate concentration and decreased NMDA receptor subunits NR2A and NR2B in the frontal cortex and hippocampus of rats. However, NR1 expression remained constant in stressed rats compared with normal. The GMDZ decoction alleviated the depressive-like behavior, decreased glutamate level, and increased expression of NMDA receptor subunit NR2A and NR2B in the frontal cortex and hippocampus of stressed rats.

CONCLUSIONS

These results suggest that GMDZ treatment reversed chronic unpredictable stress-induced depressive-like behaviors in UCMS rats, possibly via reducing glutamate levels and increasing the NMDA receptor subunits NR2A and NR2B in frontal cortex and hippocampus.

Magnetic resonance spectroscopy studies of glutamate-related abnormalities in mood disorders

In mood disorders, there is growing evidence for glutamatergic abnormalities derived from peripheral measures of glutamatergic metabolites in patients, postmortem studies on glutamate-related markers, and animal studies on the mechanism of action of available treatments. Magnetic resonance spectroscopy (MRS) has the potential to corroborate and extend these findings with the advantage of in vivo assessment of glutamate-related metabolites in different disease states, in response to treatment, and in relation with functional imaging data. In this article, we first review the biological significance of glutamate, glutamine, and Glx (composed mainly of glutamate and glutamine). Next, we review the MRS literature in mood disorders, examining these glutamate-related metabolites. Here, we find a highly consistent pattern of Glx-level reductions in major depressive disorder and elevations in bipolar disorder. In addition, studies of depression, regardless of diagnosis, provide suggestive evidence for reduced glutamine/glutamate ratio and in mania for elevated glutamine/glutamate ratio. These patterns suggest that the glutamate-related metabolite pool (not all of it necessarily relevant to neurotransmission) is constricted in major depressive disorder and expanded in bipolar disorder. Depressive and manic episodes may be characterized by modulation of the glutamine/glutamate ratio in opposite directions, possibly suggesting reduced versus elevated glutamate conversion to glutamine by glial cells, respectively. We discuss the implications of these results for the pathophysiology of mood disorders and suggest future directions for MRS studies.

An update on the role of glutamate in the pathophysiology of depression

OBJECTIVE

To review the literature on the involvement of glutamate (Glu), including its interactions with other neurochemical systems, in the pathophysiology of depression.

METHOD

A MEDLINE search using the terms glutamate, depression and major depressive disorder, was performed.

RESULTS

Alterations in proteins involved in glutamatergic signalling are implicated in variations in behaviour in animal models of depression. Drugs acting at Glu receptors appear to have antidepressant-like effects in these models, and traditional antidepressant pharmacotherapies act on the glutamatergic system. Recent evidence from genetic studies and in vivo spectroscopy also correlate glutamatergic dysfunction with depression. Trials of N-methyl-d-aspartate receptor antagonists in humans have provided mixed results.

CONCLUSION

A growing body of evidence indicates that the glutamatergic system is involved in the pathophysiology of depression, and may represent a target for intervention.

Dietary taurine intake, nutrients intake, dietary habits and life stress by depression in Korean female college students: a case-control study

Background

The purpose of this study was to investigate the dietary taurine intake, nutrients intake, dietary habits and life stress by depression in Korean female college students.

Methods

In this study, research data were collected in March 2009 and 65 patients with depression and 65 controls without depression participated. The CES-D (Center for Epidemiologic Studies Depression) scale was used for depression measure and controls were matched for age. A 3-day recall method was used for dietary assessment (2 weekdays and 1 weekend day).

Results

Average height, weight and body mass index (BMI) were 161.3±0.5cm, 55.3±1.0kg and 21.2±0.4kg/m² for depression patients and those of control group were 161.4±0.7cm, 53.1±0.8kg and 20.3±0.2kg/m², respectively. Average dietary taurine intakes of depression patients and control group were 89.1 and 88.0 mg/day, respectively. There was no significant difference in dietary taurine intake between depression patients and control group. The average intakes of vitamin A (p<0.05), β-carotene (p<0.01), vitamin C (p<0.05), folic acid (p<0.05) and fiber (p<0.05) of depression patients were significantly lower compared to control group. The average total dietary habit score of depression patients (47.2) was significantly lower than that of control group (51.3) (p<0.01). The average dietary habit scores of “eating meals at regular times” (p<0.05), “eating adequate amount of meals” (p<0.05), “having meals with diverse foods” (p<0.05), “avoiding eating spicy foods” (p<0.01) and “eating protein foods such as meat, fish, eggs, beans more than 2 times a day” (p<0.05) were significantly lower in depression patients compare to control group. The average scores of total life stress (p<0.001) and all stress categories of depression patients were significantly higher than those of control group except faculty problem score.

Conclusions

These results show that depression patients have poor dietary habits and unbalanced nutrition status. Also depression patients have higher life stress score.

Therefore, continuous nutrition education and counselling for good dietary habits and balanced nutrition status are needed to prevent depression in Korean college students.

Inflammatory biomarkers and depression

Antidepressants, predominantly serotonin- and/or noradrenaline reuptake inhibiting drugs have several shortcomings. The exact pathophysiological mechanisms leading to serotonergic-, noradrenergic- or dopaminergic dysfunction are still unclear. An inflammatory mechanism has been postulated and will be discussed here including possible therapeutic advantages of cyclooxygenase-2 (COX-2) inhibitors. Differences in the activation of the enzyme indoleamine 2,3-dioxygenase (IDO) and in the tryptophan-kynurenine metabolism resulting in an increased tryptophan and serotonin degradation and probably in an increased production of quinolinic acid might play a key role in major depression (MD). These differences are associated with an imbalance in the glutamatergic neurotransmission, which may contribute to an overweight of N-methyl-D: -aspartate agonism in MD. The immunological imbalance results in an increased prostaglandin E₂ production and probably also in an increased COX-2 expression. Although there is strong evidence for the view that the interactions of the immune system, IDO, the serotonergic system and the glutamatergic neurotransmission play a key role in MD, several gaps, e.g. the roles of genetics, disease course, sex, different psychopathological states, etc., have to be bridged by intense further research. There were already hints that anti-inflammatory therapy might have beneficial effects in MD. COX-2 inhibitors, however, have been tested in animal models and in preliminary clinical studies showing favourable effects compared to placebo in MD. The effects of COX-2 inhibition in the CNS as well as the different components of the inflammatory system, the kynurenine-metabolism and the glutamatergic neurotransmission, however, still need careful further scientific evaluation including clinical studies in bigger samples of patients.

Hippocampal astrocytes are necessary for antidepressant treatment of learned helplessness rats

The astrocyte is a major component of the neural network and plays a role in brain function. Previous studies demonstrated changes in the number of astrocytes in depression. In this study, we examined alterations in the number of astrocytes in the learned helplessness (LH) rat, an animal model of depression. The numbers of activated and nonactivated astrocytes in the dentate gyrus (molecular layer, subgranular zone, and hilus), and CA1 and CA3 regions of the hippocampus were significantly increased 2 and 8 days after attainment of LH. Subchronic treatment with imipramine showed a tendency (although not statistically significant) to decrease the LH-induced increment of activated astrocytes in the CA3 region and dentate gyrus. Furthermore, subchronic treatment of naïve rats with imipramine did not alter the numbers of activated and nonactivated astrocytes. However, the antidepressant-like effects of imipramine in the LH paradigm were blocked when fluorocitrate (a reversible inhibitor of astrocyte function) was injected into the dentate gyrus or CA3 region. Injection of fluorocitrate into naive rats failed to induce behavioral deficits in the conditioned avoidance test. These results indicate that astrocytes are responsive to the antidepressant-like effect of imipramine in the dentate gyrus and CA3 region of the hippocampus.

Neurochemistry of the nucleus accumbens and its relevance to depression and antidepressant action in rodents

There is accumulating evidence that the nucleus accumbens (NAc) plays an important role in the pathophysiology of depression. Given that clinical depression is marked by anhedonia (diminished interest or pleasure), dysfunction of the brain reward pathway has been suggested as contributing to the pathophysiology of depression.Since the NAc is the center of reward and learning, it is hypothesized that anhedonia might be produced by hampering the function of the NAc. Indeed, it has been reported that stress, drug exposure and drug withdrawal, all of which produce a depressive-phenotype, alter various functions within the NAc, leading to inhibited dopaminergic activity in the NAc.In this review, we describe various factors as possible candidates within the NAc for the initiation of depressive symptoms. First, we discuss the roles of several neurotransmitters and neuropeptides in the functioning of the NAc, including dopamine, glutamate, gamma-aminobutyric acid (GABA), acetylcholine, serotonin, dynorphin, enkephaline, brain-derived neurotrophic factor (BDNF), cAMP response element-binding protein (CREB), melanin-concentrating hormone (MCH) and cocaine- and amphetamine-regulated transcript (CART). Second, based on previous studies, we propose hypothetical relationships among these substances and the shell and core subregions of the NAc.

Metabolomic differences in heart failure patients with and without major depression

Metabolomics is an emerging technology that allows researchers to characterize hundreds of small molecules that comprise the metabolome. We sought to determine metabolic differences in depressed and nondepressed participants. The sample consisted of a depressed group of patients with heart failure enrolled in an NIMH-supported clinical trial of sertraline versus placebo in depressed heart failure patients, and a nondepressed comparator group of heart failure patients. Plasma was obtained from blood samples provided by participants at baseline, and samples were profiled on GC-MS and LC-MS metabolomics platforms for biochemical content. A number of biochemicals were significantly different between groups, with depressed participants showing higher concentrations of several amino acids and dicarboxylic fatty acids. These results are consistent with prior findings where changes in neurotransmitter systems and fatty acid metabolism were shown to associate with the depressed state. It is unclear what role heart failure may have played in these differing concentrations.

Corticotropin-Releasing Hormone, Glutamate, and γ-Aminobutyric Acid in Depression

Stress response and depression have a significant impact on modern society. Although the symptoms are well characterized, the molecular mechanisms underlying depression are largely unknown. The monoamine hypothesis, which postulates dysfunctional noradrenergic and serotonergic systems as the underlying primary cause of depression, has been valuable for the development of conventional antidepressants, which can reverse these dysfunctional states to some degree. However, recent data from various neuroscience disciplines have questioned the major role of amines in the pathogenesis of depression. A considerable amount of evidence has accumulated that suggests that normalization of the hypothalamo—pituitary—adrenal (HPA) system might be the final step necessary for a remission of depression. In addition, an increasing body of clinical and postmortem evidence is pointing to a role played by γ-aminobutyric acid (GABA) and glutamate in the etiology of depression. This review examines the evidence, mainly obtained from clinical studies or from postmortem brain material, for a major role of the HPA axis, glutamatergic, and GABAergic systems in the pathogenesis of major and bipolar depression. The authors hope that these insights will stimulate further studies with the final aim of developing new types of antidepressants that combine increased efficacy with a shorter delay of the onset of action and reduced side-effect profiles.

Possible Role of Platelet GluR1 Receptors in Comorbid Depression and Cardiovascular Disease

The exact nature of the comorbidity between cardiovascular disease (CVD) and major depressive disorder (MDD) is poorly understood. The proposed mechanisms include various biochemical and molecular pathways as well as health behaviors such as physical inactivity. One possible link between MDD and CVD is increased platelet activity and blood viscosity. Recently, it was discovered that platelets express functional subtype of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, for example, glutamate receptor 1 (GluR1). Here, I propose that this type of AMPA receptor could play a role in comorbid MDD and CVD, and antidepressants may interfere with platelet activation via direct or indirect effects on platelet GluR1 phosphorylation. Testing this hypothesis could provide a novel view on the pathobiological mechanisms of comorbid MDD and CVD. With respect to the recently discovered role of AMPA receptors in regulating platelet activation and thrombosis, it appears that the information about the putative effects of psychoactive AMPA-modifying drugs on platelet AMPA receptors would be critical in evaluating the putative effects of such drugs on CVD.

Ketamine and the next generation of antidepressants with a rapid onset of action

Existing treatments for major depressive disorder (MDD) usually take weeks to months to achieve their antidepressant effects, and a significant number of patients do not have adequate improvement even after months of treatment. In addition, increased risk of suicide attempts is a major public health concern during the first month of standard antidepressant therapy. Thus, improved therapeutics that can exert their antidepressant effects within hours or a few days of their administration are urgently needed, as is a better understanding of the presumed mechanisms associated with these rapid antidepressant effects. In this context, the N-methyl-D-aspartate (NMDA) antagonist ketamine has consistently shown antidepressant effects within a few hours of its administration. This makes it a valuable research tool to identify biomarkers of response in order to develop the next generation of fast-acting antidepressants. In this review, we describe clinical, electrophysiological, biochemical, and imaging correlates as relevant targets in the study of the antidepressant response associated with ketamine, and their implications for the development of novel, fast-acting antidepressants. We also review evidence that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) to NMDA throughput may represent a convergent mechanism for the rapid antidepressant actions of ketamine. Overall, understanding the molecular basis of this work will likely lead to the ultimate development of improved therapeutics for MDD.

Emerging role of glutamate in the pathophysiology of major depressive disorder

Major depressive disorder (MDD) is a common, chronic, recurrent mental illness that affects millions of individuals worldwide. To date, the monoaminergic systems (serotonin, norepinephrine, and dopamine) have received the most attention in the neurobiology of MDD, and all classes of antidepressants target these monoaminergic systems. Accumulating evidence suggests that the glutamatergic system plays an important role in the neurobiology and treatment of this disease. Some clinical studies have demonstrated that the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist ketamine has rapid antidepressant effects in treatment-resistant patients with MDD. Here, the author reviews the recent findings on the role of the glutamatergic system in the neurobiology of MDD and in new potential therapeutic targets (NMDA receptors, AMPA receptors, metabotropic glutamate receptors, ceftriaxone, minocycline, N-acetyl-L-cysteine) for MDD.

Is major depressive disorder a metabolic encephalopathy?

Metabolic encephalopathy is an acute disturbance in cellular metabolism in the brain evoked by conditions of hypoxia, hypoglycaemia, oxidative stress and/or inflammation. It usually develops acutely or subacutely and is reversible if the systemic disorder is treated. If left untreated, however, metabolic encephalopathy may result in secondary structural damage to the brain. Most encephalopathies are present with neuropsychiatric symptoms, one in particular being depression. However, mood disorders are often co-morbid with cardiovascular, liver, kidney and endocrine disorders, while increasing evidence concurs that depression involves inflammatory and neurodegenerative processes. This would suggest that metabolic disturbances resembling encephalopathy may underscore the basic neuropathology of depression at a far deeper level than currently realized. Viewing depression as a form of encephalopathy, and exploiting knowledge gleaned from our understanding of the neurochemistry and treatment of metabolic encephalopathy, may assist in our understanding of the neurobiology of depression, but also in realizing new ideas in the pharmacotherapy of mood disorders.