Correlation between plasma levels of glutamate, alanine and serine with severity of depression

Blood glutamate scavenging: insight into neuroprotection

Brain insults are characterized by a multitude of complex processes, of which glutamate release plays a major role. Deleterious excess of glutamate in the brain's extracellular fluids stimulates glutamate receptors, which in turn lead to cell swelling, apoptosis, and neuronal death. These exacerbate neurological outcome. Approaches aimed at antagonizing the astrocytic and glial glutamate receptors have failed to demonstrate clinical benefit. Alternatively, eliminating excess glutamate from brain interstitial fluids by making use of the naturally occurring brain-to-blood glutamate efflux has been shown to be effective in various animal studies. This is facilitated by gradient driven transport across brain capillary endothelial glutamate transporters. Blood glutamate scavengers enhance this naturally occurring mechanism by reducing the blood glutamate concentration, thus increasing the rate at which excess glutamate is cleared. Blood glutamate scavenging is achieved by several mechanisms including: catalyzation of the enzymatic process involved in glutamate metabolism, redistribution of glutamate into tissue, and acute stress response. Regardless of the mechanism involved, decreased blood glutamate concentration is associated with improved neurological outcome. This review focuses on the physiological, mechanistic and clinical roles of blood glutamate scavenging, particularly in the context of acute and chronic CNS injury. We discuss the details of brain-to-blood glutamate efflux, auto-regulation mechanisms of blood glutamate, natural and exogenous blood glutamate scavenging systems, and redistribution of glutamate. We then propose different applied methodologies to reduce blood and brain glutamate concentrations and discuss the neuroprotective role of blood glutamate scavenging.

Synergist effects of n-acetylcysteine and deferoxamine treatment on behavioral and oxidative parameters induced by chronic mild stress in rats

A growing body of evidence has pointed to a relationship between oxidative stress and depression. Thus, the present study was aimed at evaluating the effects of the antioxidants n-acetylcysteine (NAC), deferoxamine (DFX) or their combination on sweet food consumption and oxidative stress parameters in rats submitted to 40days of exposure to chronic mild stress (CMS). Our results showed that in stressed rats treated with saline, there was a decrease in sweet food intake and treatment with NAC or NAC in combination with DFX reversed this effect. Treatment with NAC and DFX decreased the oxidative damage, which include superoxide and TBARS production in submitochondrial particles, and also thiobarbituric acid reactive substances (TBARS) levels and carbonyl proteins in the prefrontal cortex, amygdala and hippocampus. Treatment with NAC and DFX also increased the activity of the antioxidant enzymes, superoxide dismutase and catalase in the same brain areas. Even so, a combined treatment with NAC and DFX produced a stronger increase of antioxidant activities in the prefrontal cortex, amygdala and hippocampus. The results described here indicate that co-administration may induce a more pronounced antidepressant activity than each treatment alone. In conclusion, these results suggests that treatment with NAC or DFX alone or in combination on oxidative stress parameters could have positive effects against neuronal damage caused by oxidative stress in major depressive 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.

Region-specific glutamate changes in patients with unipolar depression

The present study aimed to investigate glutamate concentrations in patients with unipolar depression in the midcingulate cortex (MCC) as compared to the left dorsolateral prefrontal cortex (DLPFC). We hypothesized a dissociation of glutamate levels with unchanged levels in DLPFC and abnormally changed levels in MCC as well as differential effects of antidepressant pharmacotherapy. Glutamate was determined using magnetic resonance spectroscopy at 3 T in DLPFC and MCC in fourteen depressed patients and matched healthy volunteers. A follow-up measurement was performed after 4 weeks of antidepressant treatment. The main finding is a region-specific pattern of glutamate concentrations with increased MCC glutamate concentrations and no significant differences in DLPFC glutamate concentrations in unipolar depressive patients compared to healthy controls. Response and non-response to antidepressant pharmacotherapy were predicted by high glutamate at baseline in DLPFC and MCC, respectively. In addition, treatment responders showed a further increase in DLPFC glutamate levels after successful antidepressant treatment. Findings indicate altered region-specific glutamate concentrations in DLPFC and MCC that are predictive of response and non-response, respectively, to antidepressant pharmacotherapy. These findings might serve as a starting point for future studies in which the value of this metabolite pattern for treatment response prediction should be investigated.

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.

Memantine treatment reverses anhedonia, normalizes corticosterone levels and increases BDNF levels in the prefrontal cortex induced by chronic mild stress in rats

Memantine is a N-methyl-D-aspartate (NMDA) receptor antagonist and several studies have pointed to the NMDA receptor antagonists as a potential therapeutic target for the treatment of depression. The present study was aimed to evaluate the behavioral and physiological effects of administration of memantine in rats exposed to the chronic mild stress (CMS) model. To this aim, after 40 days of exposure to CMS procedure, rats were treated with memantine (20 mg/kg) for 7 days. In this study, sweet food consumption, adrenal gland weight, corticosterone levels, and brain-derived-neurotrophic factor (BDNF) protein levels in the prefrontal cortex, hippocampus and amygdala were assessed. Our results demonstrated that chronic stressful situations induced anhedonia, hypertrophy of adrenal gland weight, and an increase of corticosterone levels in rats, but did not alter BDNF protein levels in the rat brain. Memantine treatment reversed anhedonia and the increase of adrenal gland weight, normalized corticosterone levels and increased BDNF protein levels in the prefrontal cortex in stressed rats. Finally, these findings further support the hypothesis that NMDA receptor antagonists such as memantine could be helpful in the pharmacological treatment of depression.

The potential role of glutamate in the current diabetes epidemic

In the present article, we propose the perspective that abnormal glutamate homeostasis might contribute to diabetes pathogenesis. Previous reports and our recent data indicate that chronically high extracellular glutamate levels exert direct and indirect effects that might participate in the progressive loss of β-cells occurring in both T1D and T2D. In addition, abnormal glutamate homeostasis may impact all the three accelerators of the "accelerator hypothesis" and could partially explain the rising frequency of T1D and T2D.

Depressive-like behavior in adrenocorticotropic hormone-treated rats blocked by memantine

Hyperactivity of the hypothalamic pituitary-adrenal (HPA) axis plays a role in the pathophysiology of major depressive disorder (MDD). Recent studies suggest the role of the glutamatergic system in the pathophysiology of MDD, and N-methyl-D-aspartate (NMDA) receptor antagonists have shown antidepressant effects in both preclinical and clinical studies. However, little is known about the role of adrenocorticotropic hormone (ACTH) specifically in the glutamatergic response to HPA axis activation. Glutamate is an NMDA receptor agonist, and glycine and D-serine act as co-agonists. Here, we measured brain concentrations of these amino acids in rats given repeated administration of ACTH (100 μg/rat/day, sc, for 14 days). Further, we also evaluated behavioral effects of memantine, a non-competitive NMDA antagonist, on immobility time in the forced swimming test and on locomotor activity in ACTH-treated rats. Compared with control rats, glutamine, glycine, L-serine, and D-serine levels were increased in the hippocampus of ACTH-treated rats; glutamate, glutamine, glycine, L-serine, and D-serine were increased in the cerebellum; and glutamine and glycine were increased in the frontal cortex and striatum, all with statistical significance. Remarkably, these increases in agonists and co-agonists might have led to the augmentation of NMDA receptor activity. ACTH treatment increased immobility time in the forced swimming test and decreased locomotor activity in rats. On the contrary, memantine (10 mg/kg, ip) significantly decreased immobility time in the forced swimming test and increased locomotor activity in ACTH-treated rats. Furthermore, imipramine (15 mg/kg, ip) did not alter immobility time in the forced swimming test whereas this drug significantly decreased locomotor activity in ACTH-treated rats. These results suggest that depressive-like behaviors by chronic ACTH treatment could be blocked by memantine.

Negative allosteric modulation of metabotropic glutamate receptor 5 results in broad spectrum activity relevant to treatment resistant depression

Evidence suggests that 30-50% of patients suffering from major depressive disorder (MDD) are classified as suffering from treatment resistant depression (TRD) as they have an inadequate response to standard antidepressants. A key feature of this patient population is the increased incidence of co-morbid symptoms like anxiety and pain. Recognizing that current standards of care are largely focused on monoaminergic mechanisms of action (MOAs), innovative approaches to drug discovery for TRD are targeting glutamate hyperfunction. Here we describe the in vitro and in vivo profile of GRN-529, a novel negative allosteric modulator (NAM) of metabotropic glutamate receptor 5 (mGluR5). In cell based pharmacology assays, GRN-529 is a high affinity (Ki 5.4 nM), potent (IC50 3.1 nM) and selective (>1000-fold selective vs mGluR1) mGluR5 NAM. Acute administration of GRN-529 (0.1-30 mg/kg p.o.) had dose-dependent efficacy across a therapeutically relevant battery of animal models, comprising depression (decreased immobility time in tail suspension and forced swim tests) and 2 of the co-morbid symptoms overrepresented in TRD, namely anxiety (attenuation of stress-induced hyperthermia, and increased punished crossings in the four plate test) and pain (reversal of hyperalgesia due to sciatic nerve ligation or inflammation). The potential side effect liability of GRN-529 was also assessed using preclinical models: GRN-529 had no effect on rat sexual behavior or motor co-ordination (rotarod), however it impaired cognition in mice (social odor recognition). Efficacy and side effects of GRN-529 were compared to standard of care agents (antidepressant, anxiolytic or analgesics) and the tool mGluR5 NAM, MTEP. To assess the relationship between target occupancy and efficacy, ex vivo receptor occupancy was measured in parallel with efficacy testing. This revealed a strong correlation between target engagement, exposure and efficacy across behavioral endpoints, which supports the potential translational value of PET imaging to dose selection in patients. Collectively this broad spectrum profile of efficacy of GRN-529 supports our hypothesis that negative allosteric modulation of mGluR5 could represent an innovative therapeutic approach to the treatment of TRD. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

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.

Plasma metabonomics as a novel diagnostic approach for major depressive disorder

Major depressive disorder (MDD) is a socially detrimental psychiatric disorder, contributing to increased healthcare expenditures and suicide rates. However, no empirical laboratory-based tests are available to support the diagnosis of MDD. In this study, a NMR-based plasma metabonomic method for the diagnosis of MDD was tested. Proton nuclear magnetic resonance ((1)H NMR) spectra of plasma sampled from first-episode drug-naı̈ve depressed patients (n = 58) and healthy controls (n = 42) were recorded and analyzed by orthogonal partial least-squares discriminant analysis (OPLS-DA). The OPLS-DA score plots of the spectra demonstrated that the depressed patient group was significantly distinguishable from the healthy control group. Moreover, the method accurately diagnosed blinded samples (n = 26) in an independent replication cohort with a sensitivity and specificity of 92.8% and 83.3%, respectively. Taken together, NMR-based plasma metabonomics may offer an accurate empirical laboratory-based method applicable to the diagnosis of MDD.

AMPA receptor expression is increased post-mortem samples of the anterior cingulate from subjects with major depressive disorder

BACKGROUND

Glutamate is thought to be involved in the pathophysiology of major depressive disorder and bipolar disorder; however, the molecular changes underlying abnormal glutamatergic signalling remain poorly understood. Whilst previous studies have suggested that the NMDA receptor may be involved in the pathophysiology of mood disorders, it is unclear whether the non-NMDA receptors are also involved. Therefore, we sought to examine whether the expression of the non-NMDA, ionotropic glutamate receptors, AMPA receptor and kainate receptor, is altered in mood disorders.

METHODS

We used [3H]AMPA and [3H]kainate to measure the levels of AMPA and kainate receptor, respectively, in the anterior cingulate (BA 24) and dorsolateral prefrontal cortex (BA 46) from post-mortem CNS in 10 subjects with major depressive disorder, 10 subjects with bipolar disorder and 10 control subjects.

RESULTS

A 20.7% to 27.7% increase in [3H]AMPA binding density was seen in BA 24 (p<0.05) but not BA 46 (p>0.05) in major depressive disorder compared to control levels. [3H]AMPA binding density was not changed in bipolar disorder in either BA 24 or BA 46 (p>0.05) compared to controls. [3H]Kainate binding was not changed in either BA 24 or BA 46 in either disorder compared to controls (p>0.05).

LIMITATIONS

Small sample sizes (n=10) were used in this study. The subjects were not drug naïve.

CONCLUSIONS

Our data suggests increased in AMPA receptor levels in the anterior cingulate are involved in the pathophysiology of major depressive disorder. This data has relevance for the development of new anti-depressant drugs targeted towards the AMPA receptors.

Genetic ablation of the GluK4 kainate receptor subunit causes anxiolytic and antidepressant-like behavior in mice

There is a clear link between dysregulation of glutamatergic signaling and mood disorders. Genetic variants in the glutamate receptor gene GRIK4, which encodes the kainate receptor subunit GluK4, alter the susceptibility for depression, bipolar disorder and schizophrenia. Here we demonstrate that Grik4(-/-) mice have reduced anxiety and an antidepressant-like phenotype. In the elevated zero-maze, a test for anxiety and risk taking behavior, Grik4(-/-) mice spent significantly more time exploring the open areas of the maze. In anxiogenic tests of marble-burying and novelty-induced suppression of feeding, anxiety-like behavior was consistently reduced in knockout animals. In the forced swim test, a test of learned helplessness that is used to determine depression-like behavior, knockout mice demonstrated significantly less immobility suggesting that Grik4 ablation has an antidepressant-like effect. Finally, in the sucrose preference test, a test for anhedonia in rodents, Grik4(-/-) mice demonstrated increased sucrose preference. Expression of the GluK4 receptor subunit in the forebrain is restricted to the CA3 region of the hippocampus and dentate gyrus regions where KARs are known to modulate synaptic plasticity. We tested whether Grik4 ablation had effects on mossy fiber (MF) plasticity and found there to be a significant impairment in LTP likely through a loss of KAR modulation of excitability of the presynaptic MF axons. These studies demonstrate a clear anxiolytic and antidepressant phenotype associated with ablation of Grik4 and a parallel disruption in hippocampal plasticity, providing support for the importance of this receptor subunit in 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'.

Resting glutamate levels and rapid glutamate transients in the prefrontal cortex of the Flinders Sensitive Line rat: a genetic rodent model of depression

Despite the numerous drugs targeting biogenic amines for major depressive disorder (depression), the search for novel therapeutics continues because of their poor response rates (~30%) and slow onset of action (2-4 weeks). To better understand role of glutamate in depression, we used an enzyme-based microelectrode array (MEA) that was selective for glutamate measures with fast temporal (2 Hz) and high spatial (15 × 333 μm) resolution. These MEAs were chronically implanted into the prefrontal cortex of 3- to 6-month-old and 12- to 15-month-old Flinders Sensitive Line (FSL) and control Flinders Resistant Line (FRL) rats, a validated genetic rodent model of depression. Although no changes in glutamate dynamics were observed between 3 and 6 months FRL and FSL rats, a significant increase in resting glutamate levels was observed in the 12- to 15-month-old FSL rats compared with the 3- to 6-month-old FSL and age-matched FRL rats on days 3-5 post-implantation. Our MEA also recorded, for the first time, a unique phenomenon in all the four rat groups of fluctuations in resting glutamate, which we have termed glutamate transients. Although these events lasted only for seconds, they did occur throughout the testing paradigm. The average concentration of these glutamate-burst events was significantly increased in the 12- to 15-month-old FSL rats compared with 3- to 6-month-old FSL and age-matched FRL rats. These studies lay the foundation for future studies of both tonic and phasic glutamate signaling in rat models of depression to better understand the potential role of glutamate signaling in depression.

Late-life depression and Alzheimer's disease: the glutamatergic system inside of this mirror relationship

Late-life depressive syndromes often arise in the context of predementia, dementia syndromes, and Alzheimer's disease (AD). Conversely, patients with a history of mood disorders are at higher risk of developing cognitive impairment. The high rate of co-occurrence of these two disorders is becoming a major health problem in older subjects for both their epidemiological impact and the negative outcomes in terms of disability and increased mortality. In this perspective, it is possible to speculate on the presence of a mirror relationship between depressive and cognitive disorders in late-life. Indeed, although a causal contribution of genetic, environmental, and social factors is widely recognized in these disorders, the neurobiological links still remain largely unknown. l-glutamic acid and γ-aminobutyric acid are the principal excitatory and inhibitory neurotransmitters in the central nervous system, respectively, and increasing evidence suggests that alterations in this neurotransmitter system may contribute to the neurobiology linking depression and cognitive impairment. In the present review article, we examined the neurobiological bases of the relationship between late-life depressive syndromes and AD, with a particular attention to glutamatergic pathway signalling like a bridge connecting these two conditions. In addition, attempts have been made to explain changes in glutamatergic pathway, depression in older age, and dementia through the analysis of signal transduction mechanisms associated with these disabling disorders.

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.

Individual stress vulnerability is predicted by short-term memory and AMPA receptor subunit ratio in the hippocampus

Increased vulnerability to aversive experiences is one of the main risk factors for stress-related psychiatric disorders as major depression. However, the molecular bases of vulnerability, on the one hand, and stress resilience, on the other hand, are still not understood. Increasing clinical and preclinical evidence suggests a central involvement of the glutamatergic system in the pathogenesis of major depression. Using a mouse paradigm, modeling increased stress vulnerability and depression-like symptoms in a genetically diverse outbred strain, and we tested the hypothesis that differences in AMPA receptor function may be linked to individual variations in stress vulnerability. Vulnerable and resilient animals differed significantly in their dorsal hippocampal AMPA receptor expression and AMPA receptor binding. Treatment with an AMPA receptor potentiator during the stress exposure prevented the lasting effects of chronic social stress exposure on physiological, neuroendocrine, and behavioral parameters. In addition, spatial short-term memory, an AMPA receptor-dependent behavior, was found to be predictive of individual stress vulnerability and response to AMPA potentiator treatment. Finally, we provide evidence that genetic variations in the AMPA receptor subunit GluR1 are linked to the vulnerable phenotype. Therefore, we propose genetic variations in the AMPA receptor system to shape individual stress vulnerability. Those individual differences can be predicted by the assessment of short-term memory, thereby opening up the possibility for a specific treatment by enhancing AMPA receptor function.

Glycine and a glycine dehydrogenase (GLDC) SNP as citalopram/escitalopram response biomarkers in depression: pharmacometabolomics-informed pharmacogenomics

Major depressive disorder (MDD) is a common psychiatric disease. Selective serotonin reuptake inhibitors (SSRIs) are an important class of drugs used in the treatment of MDD. However, many patients do not respond adequately to SSRI therapy. We used a pharmacometabolomics-informed pharmacogenomic research strategy to identify citalopram/escitalopram treatment outcome biomarkers. Metabolomic assay of plasma samples from 20 escitalopram remitters and 20 nonremitters showed that glycine was negatively associated with treatment outcome (P = 0.0054). This observation was pursued by genotyping tag single-nucleotide polymorphisms (SNPs) for genes encoding glycine synthesis and degradation enzymes, using 529 DNA samples from SSRI-treated MDD patients. The rs10975641 SNP in the glycine dehydrogenase (GLDC) gene was associated with treatment outcome phenotypes. Genotyping for rs10975641 was carried out in 1,245 MDD patients in the Sequenced Treatment Alternatives to Relieve Depression (STAR*D) study, and its presence was significant (P = 0.02) in DNA taken from these patients. These results highlight a possible role for glycine in SSRI response and illustrate the use of pharmacometabolomics to "inform" pharmacogenomics.

Protective effects of paeoniflorin against glutamate-induced neurotoxicity in PC12 cells via antioxidant mechanisms and Ca(2+) antagonism

Preclinical and clinical investigations have shown hippocampal neuronal atrophy and destruction were observed in patients with depression, which could be ameliorated by the treatment with antidepressants. Therefore, neuroprotection has been proposed to be one of the acting mechanisms of antidepressant. Paeoniflorin, a monoterpene glycoside, has been reported to display antidepressant-like effects in animal models of behavioral despair. The present study aimed to examine the protective effect of paeoniflorin on glutamate-induced neurotoxicity in cultured rat pheochromocytoma (PC12) cells. The results showed that pretreatment with paeoniflorin elevated cell viability, inhibited apoptosis, decreased levels of intracellular reactive oxygen species and malondialdehyde, and enhanced activity of superoxide dismutase in glutamate-treated PC12 cells. Pretreatment with paeoniflorin also reversed the increased intracellular Ca(2+) concentration and the reduced Calbindin-D28K mRNA level caused by glutamate in PC12 cells. The results suggest that paeoniflorin exerts a neuroprotective effect on glutamate-induced neurotoxicity in PC12 cells, at least in part, via inhibiting oxidative stress and Ca(2+) overload. This neuroprotective effect may be one of the action pathways accounting for the in vivo antidepressant activity of paeoniflorin.

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.

Quantification of neurotransmitter amino acids by capillary electrophoresis laser-induced fluorescence detection in biological fluids

The role of neurotransmitter amino acids (NAAs) in the functioning of the nervous system has been the focus of increasingly intense research over the past several years. Among the various amino acids that have important roles as neurotransmitters, there are alanine (Ala), glutamic acid (Glu), aspartic acid (Asp), serine (Ser), taurine (Tau) and glycine (Gly). NAAs are present in plasma, cells and--at trace levels--in all biological fluids, but complex components in biological matrices make it difficult to determine them in biological samples. We describe a new capillary electrophoresis (CE) method with laser-induced fluorescence detection by which analytes are resolved in less than 12 minutes in a 18 mmol/L phosphate run buffer at pH 11.6. The use of elevated temperatures during sample derivatization leads to a drastic reduction in the reaction time, down to 20 min, compared to the 6-14 h usually described for reactions between FITC and amino acids at room temperature. In order to demonstrate its wide range of applications, the method was applied to the analysis of NAA in human plasma and in other sample types, such as red blood cells, urine, cultured cells, cerebrospinal fluid, saliva and vitreous humor, thus avoiding the typical limitations of other methods, which are normally suitable for use with only one or two matrix types.

Serum glutamine, set-shifting ability and anorexia nervosa

BACKGROUND

Set-shifting is impaired in people with anorexia nervosa (AN), but the underlying physiological and biochemical processes are unclear. Animal studies have established that glutamatergic pathways in the prefrontal cortex play an important role in set-shifting ability. However, it is not yet understood whether levels of serum glutamatergic amino acids are associated with set-shifting performance in humans. The aim of this study was to determine whether serum concentrations of amino acids related to glutamatergic neurotransmission (glutamine, glutamate, glycine, l-serine, d-serine) are associated with set-shifting ability in people with acute AN and those after recovery.

METHODS

Serum concentrations of glutamatergic amino acids were measured in 27 women with current AN (AN group), 18 women recovered from AN (ANRec group) and 28 age-matched healthy controls (HC group). Set-shifting was measured using the Wisconsin Card Sorting Test (WCST) and the Trail Making Task (TMT). Dimensional measures of psychopathology were used, including the Eating Disorder Examination Questionnaire (EDEQ), the Maudsley Obsessive-Compulsive Inventory (MOCI) and the Hospital Anxiety and Depression Scale (HADS).

RESULTS

Serum glutamine concentrations in the AN group (1,310.2 +/- 265.6 muM, mean +/- SD) were significantly higher (by approximately 20%) than those in the HC group (1,102.9 +/- 152.7 muM, mean +/- SD) (F(2, 70) = 6.3, P = 0.003, 95% CI 61.2 to 353.4). Concentrations of serum glutamine were positively associated with markers of the illness severity: a negative correlation was present between serum glutamine concentrations and body mass index (BMI) and lowest BMI and a positive correlation was found between duration of illness and EDEQ. The AN group showed significantly impaired set shifting in the WCST, both total errors, and perseverative errors. In the AN group, there were no correlations between serum glutamine concentrations and set shifting.

CONCLUSIONS

Serum concentrations of glutamine may be a biomarker of illness severity in people with AN. It does not appear to be directly associated with changes in executive function.

Brain glutamatergic characteristics of pediatric offspring of parents with bipolar disorder

We wished to determine whether decreases in prefrontal glutamate concentrations occur in offspring of parents with bipolar disorder with and at high risk for mania. Sixty children and adolescents, 9-18 years old, of parents with bipolar I or II disorder (20 offspring with established history of mania, "BD", 20 offspring with symptoms subsyndromal to mania, "SS", and 20 healthy controls "HC") were examined using proton magnetic resonance spectroscopy at 3T to study glutamatergic metabolite concentrations in the anterior cingulate cortex (ACC). A signal for reductions in absolute glutamate concentrations in the ACC was seen in the BD compared with HC and SS groups. No other statistically significant differences among groups were found. Offspring of parents with BD with prior histories of mania may have disruptions in glutamatergic function compared with HC or children at risk for BD who have not yet developed mania. Longitudinal studies are necessary to confirm whether prefrontal glutamate decreases only after the onset of full mania.

Metabotropic glutamate receptors: potential drug targets for psychiatric disorders

Metabotropic glutamate receptors (mGlu receptors) have emerged as new therapeutic targets for psychiatric disorders, such as schizophrenia, depression and anxiety with their regulatory roles in glutamatergic transmissions. To date, several ligands selective for each mGlu receptor have been synthesized, and pharmacological significances of these ligands have been demonstrated in animal models. Among them, mGlu2/3 receptor agonists have been proven to be effective for treating schizophrenia and anxiety disorders in clinical studies, which may prove utilities of mGlu receptor ligands for the treatment of psychiatric disorders. This article reviews recent advances in development of each mGlu receptor ligands and their therapeutic potential.

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.

Depression and antidepressants: molecular and cellular aspects

Clinical depression is viewed as a physical and psychic disease process having a neuropathological basis, although a clear understanding of its ethiopathology is still missing. The observation that depressive symptoms are influenced by pharmacological manipulation of monoamines led to the hypothesis that depression results from reduced availability or functional deficiency of monoaminergic transmitters in some cerebral regions. However, there are limitations to current monoamine theories related to mood disorders. Recently, a growing body of experimental data has showed that other classes of endogenous compounds, such as neuropeptides and amino acids, may play a significant role in the pathophysiology of affective disorders. With the development of neuroscience, neuronal networks and intracellular pathways have been identified and characterized, describing the existence of the interaction between monoamines and receptors in turn able to modulate the expression of intracellular proteins and neurotrophic factors, suggesting that depression/antidepressants may be intermingled with neurogenesis/neurodegenerative processes.

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.

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.

The role of the tripartite glutamatergic synapse in the pathophysiology and therapeutics of mood disorders

Bipolar disorder and major depressive disorder are common, chronic, and recurrent mood disorders that affect the lives of millions of individuals worldwide. Growing evidence suggests that glutamatergic system dysfunction is directly involved in mood disorders. This article describes the role of the "tripartite glutamatergic synapse," comprising presynaptic and postsynaptic neurons and glial cells, in the pathophysiology and therapeutics of mood disorders. Glutamatergic neurons and glia directly control synaptic and extrasynaptic glutamate levels/ release through integrative effects that target glutamate excitatory amino acid transporters, postsynaptic density proteins, ionotropic receptors (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid [AMPA], N-methyl-D-aspartate [NMDA], and kainate), and metabotropic receptors. This article also explores the glutamatergic modulators riluzole and ketamine, which are considered valuable proof-of-concept agents for developing the next generation of antidepressants and mood stabilizers. In therapeutically relevant paradigms, ketamine preferentially targets postsynaptic AMPA/NMDA receptors, and riluzole preferentially targets presynaptic voltage-operated channels and glia.

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.

The change in plasma GABA, glutamine and glutamate levels in fluoxetine- or S-citalopram-treated female patients with major depression

OBJECTIVE

The aim of this study was to investigate the relationship between plasma glutamate, glutamine and gamma-aminobutyric acid (GABA) levels in female patients with major depression treated with S-citalopram or fluoxetine.

METHODS

The patients were assigned into S-citalopram (10 mg/day) or fluoxetine (20 mg/day) groups (n = 15 per group). The Hamilton and Beck Depression Inventory Scales were performed on all study participants, and blood samples were collected. The same procedures were repeated 10 days following the onset of therapy. Fifteen female healthy volunteers were also included in the study for the evaluation of normal plasma levels.

RESULTS

The plasma GABA levels of the healthy volunteers were higher whereas those for glutamate and glutamine were lower than the day zero samples of the patients. An increase in plasma GABA levels and a decrease in glutamate and glutamine levels were observed on the 10th day of treatment. No difference was detected between the drug treatments.

CONCLUSION

Our findings may suggest that GABA, glutamate and glutamine play a role in depression and that plasma GABA may be used as a biomarker for treatment control.

Targeting the glutamatergic system to develop novel, improved therapeutics for mood disorders

Mood disorders are common, chronic, recurrent mental illnesses that affect the lives of millions of individuals worldwide. To date, the monoaminergic systems (serotonergic, noradrenergic and dopaminergic) in the brain have received the greatest attention in neurobiological studies of mood disorders, and most therapeutics target these systems. However, there is growing evidence that the glutamatergic system is central to the neurobiology and treatment of these disorders. Here, we review data supporting the involvement of the glutamatergic system in mood-disorder pathophysiology as well as the efficacy of glutamatergic agents in mood disorders. We also discuss exciting new prospects for the development of improved therapeutics for these devastating disorders.

Increased plasma glutamate, glycine, and arginine levels in complex regional pain syndrome type 1

BACKGROUND

Various inflammatory mediators have been identified as potential contributors to complex regional pain syndrome type 1 (CRPS1), but these mediators do not entirely explain certain manifestations of the syndrome, such as pain. The objective of this study was to investigate the role of amino acids in the pathogenesis of CRPS1.

METHODS

We used HPLC to determine plasma concentrations of 16 amino acids, especially those related to the NMDA receptor (e.g., glutamate and glycine) and nitric oxide (NO) synthesis (e.g., arginine and citrulline) in patients with CRPS1 (n=64) and age- and sex-matched healthy controls (n=51). Patients rated pain intensity (visual analog scale) and the subjective experience of pain intensity (McGill Pain Questionnaire). Psychological dysfunction was assessed using the SCL-90.

RESULTS

Relative to controls, in CRPS1 patients, plasma levels of glutamate, arginine, taurine, and glycine were increased, and plasma levels of glutamine and the ratio of citrulline to arginine were decreased. Remarkably, in CRPS1 patients there was a highly significant inverse correlation between glutamine and glutamate, although the sum of molar concentrations of glutamate and glutamine remains unchanged. Subjective measures of pain and indicators of psychoneuroticism and emotional instability did not correlate with amino acid levels.

CONCLUSION

This study shows for the first time a pronounced increase in amino acid levels in this chronic pain syndrome. The marked differences in glutamate, glutamine, glycine, taurine and arginine levels between patients and controls suggest the involvement of both the NDMA receptor and the endothelium-dependent arginine-NO system in CRPS1.

The role of glutamate in mood disorders: results from the ketamine in major depression study and the presumed cellular mechanism underlying its antidepressant effects

In this article, we first review a study showing that the N-methyl-D-aspartate (NMDA) receptor antagonist ketamine leads to rapid, robust, and relatively sustained antidepressant effects in patients with treatment-resistant major depression. We then discuss our hypothesis that the therapeutic effects of monoaminergic antidepressants and ketamine may be mediated by increased AMPA-to-NMDA glutamate receptor throughput in critical neuronal circuits. We hypothesize that ketamine directly mediates this throughput, whereas monoaminergic antidepressants work indirectly and gradually; this may explain, in part, the lag of onset of several weeks to months that is observed with traditional antidepressants.

Single voxel proton magnetic resonance spectroscopy in post-stroke depression

Mood disorders are associated with structural, metabolic and spectroscopic changes in prefrontal regions. In the case of depression associated with stroke, there is little information about the biochemical profile of these regions, as assessed by proton magnetic resonance spectroscopy ((1)H-MRS). In a group of first-ever stroke patients, we studied the association between post-stroke depression and (1)H-MRS measurements in unaffected frontal lobes. Twenty-six patients with a first ischemic stroke located outside the frontal lobes were included in the study. Single voxel proton magnetic resonance spectroscopy ((1)H-MRS) was performed to assess N-acetylaspartate/creatine (NAA)/Cr, glutamate+glutamine (Glx)/Cr, choline (Cho)/Cr and myo-inositol (mI)/Cr ratios. Patients were assessed within the first 10 days after stroke and again four months later. The diagnosis of depression was made on the basis of clinical observation, interview and Hamilton Depression Rating Scale scores. In a group of 26 patients, eight (31%) met criteria for depression at the first assessment, and nine (35%) met criteria for depression at follow-up. Patients with depression in the immediate post-stroke phase had significantly higher Glx/Cr ratios in the contralesional hemisphere than non-depressive patients. No biochemical differences were found between the groups at 4-month follow-up. These findings suggest that post-stroke depression is accompanied by changes in frontal lobe glutamate/glutamine levels, perhaps reflecting abnormalities in glutamatergic transmission in the immediate post-stroke period.