Emerging role of glutamate in the pathophysiology of major depressive disorder

Benzotriazinone and benzopyrimidinone derivatives as potent positive allosteric AMPA receptor modulators

AMPA receptors (AMPARs) have been demonstrated to be an important therapeutic CNS target. A series of substituted benzotriazinone and benzopyrimidinone derivatives were prepared with the aim to improve in vivo activity over the previously reported bis-benzoxazinone based AMPAKINE series from our laboratory. These compounds were shown to be potent, positive allosteric AMPAR modulators that have better in vivo activity and improved metabolic stability over the analogous benzoxazinone derivatives.

Helplessness: a systematic translational review of theory and evidence for its relevance to understanding and treating depression

Helplessness is a major concept in depression and a major theme in preclinical and clinical depression research. For example, in rodents and humans, the learned helplessness (LH) effect describes a specific deficit in behaviour to control aversive stimuli that is induced by prior exposure to uncontrollable aversive stimuli. The LH effect is objective and valid in that the cause of the behavioural deficit, namely uncontrollability, is clear; furthermore, the deficit induced is underlain by emotional, motivational and cognitive processes that are relevant to depression psychopathology. As a further example, helplessness, hopelessness, external locus of control and causal attribution are inter-related and major themes in psychological theories (primarily cognitive theories) of depression. Despite this broad interest in helplessness, it can be argued that its potential usefulness as a scientific and clinical concept has so far not been investigated optimally, including with respect to its application in research aimed at development of improved anti-depressant pharmacotherapy. The first aim of this review was to describe and integrate the psychological evidence and the neurobiological evidence for the LH effect in rodents and healthy humans and for helplessness in depressed patients. The second aim was to conduct three systematic reviews, namely of rodent studies of the LH effect, rodent studies of effects of psychopharmacological agents on the LH effect, and human studies of efficacy of pharmacotherapeutic and psychotherapeutic treatment on helplessness in depressed patients. With respect to the first aim, the major findings are: the specificity of the LH effect in otherwise non-manipulated rodents and healthy humans has been under-estimated, and the LH effect is a specific learned aversive uncontrollability (LAU) effect. There is theoretical and empirical support for a model in which a specific LAU effect induced by a life event of major emotional significance can function as an aetiological factor for generalised helplessness which can in turn function as an aetiological and maintenance factor for depression. However, to date such models have focused on cognitive mediating processes whereas it is emotional-motivational-cognitive processes (as proposed for the LAU effect) that need to be invoked and understood. The evidence is for analogous neural processes underlying the LAU effect in rodents and healthy humans and helplessness in depression, with the ventro-medial prefrontal cortex exhibiting aversive uncontrollability-dependent activity. With respect to the second aim, the major findings are: the LAU effect is demonstrated quite consistently using a number of different paradigms in rat but is poorly studied in mouse. The rat LAU effect can be reversed by chronic administration of monoamine reuptake inhibitors. The effects of antidepressants on human helplessness have been scarcely studied to-date. The major conclusion is that the LAU effect and generalised helplessness constitute major neuropsychological concepts of high value to future translational research aimed at increased understanding of depression and development of novel, improved antidepressant treatments.

Involvement of nitric oxide-cGMP pathway in the antidepressant-like effect of ascorbic acid in the tail suspension test

Clinical and preclinical data reported that ascorbic acid has antidepressant properties. The present study was designed to investigate the participation of l-arginine-NO-cGMP pathway in the antidepressant-like effect of ascorbic acid in the tail suspension test (TST) in mice. The antidepressant-like effect of ascorbic acid (1mg/kg, p.o.) in the TST was prevented by the pre-treatment of mice with NMDA (0.1pmol/site, i.c.v.), l-arginine (750mg/kg, i.p., a substrate for nitric oxide synthase) or sildenafil (5mg/kg, i.p., a phosphodiesterase 5 inhibitor). The administration of MK-801 (0.001mg/kg, i.p), 7-nitroindazole (25mg/kg, i.p., a neuronal nitric oxide synthase inhibitor) or ODQ (30pmol/site i.c.v., a soluble guanylate cyclase inhibitor) in combination with a sub-effective dose of ascorbic acid (0.1mg/kg, p.o.) reduced the immobility time in the TST test when compared with either drug alone. None of the results in the TST appears to be due to a nonspecific locomotor effect. Our findings provide evidence that the effect of ascorbic acid in the TST involve an interaction with NMDA receptors and l-arginine-NO-cGMP pathway, contributing to the understanding of the mechanisms underlying the antidepressant-like effect of this vitamin.

Reduced metabotropic glutamate receptor 5 density in major depression determined by [(11)C]ABP688 PET and postmortem study

OBJECTIVE

Clinical and preclinical evidence suggests a hyperactive glutamatergic system in clinical depression. Recently, the metabotropic glutamate receptor 5 (mGluR5) has been proposed as an attractive target for novel therapeutic approaches to depression. The goal of this study was to compare mGluR5 binding (in a positron emission tomography [PET] study) and mGluR5 protein expression (in a postmortem study) between individuals with major depressive disorder and psychiatrically healthy comparison subjects.

METHOD

Images of mGluR5 receptor binding were acquired using PET with [(11)C]ABP688, which binds to an allosteric site with high specificity, in 11 unmedicated individuals with major depression and 11 matched healthy comparison subjects. The amount of mGluR5 protein was investigated using Western blot in postmortem brain samples of 15 depressed individuals and 15 matched comparison subjects.

RESULTS

The PET study revealed lower levels of regional mGluR5 binding in the prefrontal cortex, the cingulate cortex, the insula, the thalamus, and the hippocampus in the depression group relative to the comparison group. Severity of depression was negatively correlated with mGluR5 binding in the hippocampus. The postmortem study showed lower levels of mGluR5 protein expression in the prefrontal cortex (Brodmann's area 10) in the depression group relative to the comparison group, while prefrontal mGluR1 protein expression did not differ between groups.

CONCLUSIONS

The lower levels of mGluR5 binding observed in the depression group are consonant with the lower levels of protein expression in brain tissue in the postmortem depression group. Thus, both studies suggest that basal or compensatory changes in excitatory neurotransmission play roles in the pathophysiology of major depression.

Role of the mTOR signaling pathway in the rapid antidepressant action of ketamine

Some patients with major depressive disorder remain resistant to antidepressant medication. A randomized, placebo-controlled, double-blind trial demonstrated that a single subanesthetic dose (0.5 mg/kg) of the N-methyl-D-aspartate receptor antagonist ketamine caused a rapid antidepressant effect within hours in treatment-resistant patients with major depressive disorder. However, the precise cellular mechanisms underlying ketamine's rapid antidepressant actions were unclear, although it is proposed that the α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor might be involved in these mechanisms. Recently, Li et al. reported the role of the mammalian target of rapamycin (mTOR) signaling pathway, a ubiquitous protein kinase involved in protein synthesis and synaptic plasticity, in ketamine's rapid antidepressant effects. Here, these findings are put into context and their significance is discussed.

Social isolation-induced aggression potentiates anxiety and depressive-like behavior in male mice subjected to unpredictable chronic mild stress

Background

Accumulating epidemiological evidence shows that life event stressors are major vulnerability factors for psychiatric diseases such as major depression. It is also well known that social isolation in male mice results in aggressive behavior. However, it is not known how social isolation-induced aggression affects anxiety and depressive-like behavior in isolated male mice subjected to unpredictable chronic mild stress (CMS), an animal model of depression.

Methodology/Principal Findings

C57/B6 male mice were divided into 3 groups; non-stressed controls, in Group I; isolated mice subjected to the CMS protocol in Group II and aggression by physical contact in socially isolated mice subjected to the CMS protocol in Group III. In the sucrose intake test, ingestion of a 1% sucrose solution by mice in Groups II and III was significantly lower than in Group I. Furthermore, intake of this solution in Group III mice was significantly lower than in Group II mice. In the open field test, mice in Group III, showed reduced locomotor activity and reduced entry and retention time in the central zone, compared to Groups I and II mice. Moreover, the distances moved in 1 hour by Group III mice did not differ between night and morning. In the light/black box test, Groups II and III animals spent significantly less time in the light box compared to Group I animals. In the tail suspension test (TST) and forced swimming test (FST), the immobility times of Group II and Group III mice were significantly longer than in Group I mice. In addition, immobility times in the FST were significantly longer in Group III than in Group II mice.

Conclusions/Significance

These findings show that social isolation-induced aggression could potentiate anxiety and depressive -like behaviors in isolated male mice subjected to CMS.

Region and state specific glutamate downregulation in major depressive disorder: a meta-analysis of (1)H-MRS findings

For major depressive disorder (MDD), magnetic resonance spectroscopy ((1)H-MRS) studies of glutamate, glutamine and Glx (the composite measure of mainly glutamate and glutamine) have yielded inconclusive or seemingly inconsistent results. We therefore systematically reviewed whether in vivo concentrations of glutamate, glutamine and Glx measured with (1)H-MRS differ between MDD patients and controls. Meta-analysis including meta-regression, sensitivity, statistical heterogeneity, and publication bias analyses were conducted. Glutamate and Glx concentrations were found to be lower in the anterior cingulate cortex (ACC) in patients compared to controls (standardized mean difference (SMD) for glutamate with 95% CIs: -0.86, -1.55 to -0.17; and for Glx: -1.15, -1.86 to -0.44). In addition, Glx was decreased in all brain regions together in current episode patients (SMD: -0.62, -1.17 to -0.07). We conclude that in MDD, glutamate and possibly glutamine are downregulated primarily in the ACC and during depressive states. These results fit the central role of the ACC in depressive symptomatology and suggest that in MDD changes in glutamatergic neurotransmission are state-dependent.

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.

Discovering imaging endophenotypes for major depression

Psychiatry research lacks an in-depth understanding of mood disorders phenotypes, leading to limited success of genetics studies of major depressive disorder (MDD). The dramatic progress in safe and affordable magnetic resonance-based imaging methods has the potential to identify subtle abnormalities of neural structures, connectivity and function in mood disordered subjects. This review paper presents strategies to improve the phenotypic definition of MDD by proposing imaging endophenotypes derived from magnetic resonance spectroscopy measures, such as cortical gamma-amino butyric acid (GABA) and glutamate/glutamine concentrations, and from measures of resting-state activity and functional connectivity. The proposed endophenotypes are discussed regarding specificity, mood state-independence, heritability, familiarity, clinical relevance and possible associations with candidate genes. By improving phenotypic definitions, the discovery of new imaging endophenotypes will increase the power of candidate gene and genome-wide associations studies. It will also help to develop and evaluate novel therapeutic treatments and enable clinicians to apply individually tailored therapeutic approaches. Finally, improvements of the phenotypic definition of MDD based on neuroimaging measures will contribute to a new classification system of mood disorders based on etiology and pathophysiology.

Substituted benzoxazinones as potent positive allosteric AMPA receptor modulators: part II

AMPA receptors (AMPARs) are an important therapeutic target in the CNS. A series of substituted benzoxazinone derivatives with good to very good in vitro activity as positive allosteric AMPAR modulators was synthesized and evaluated. The appropriate substituent choice on the benzoxazinone fragment improved the affinity towards the AMPA receptor significantly in comparison to our lead molecule CX614.

Benzoxazinones as potent positive allosteric AMPA receptor modulators: part I

AMPA receptors (AMPARs) are an increasingly important therapeutic target in the CNS. Aniracetam, the first identified potentiator of AMPARs, led to the rigid and more potent CX614. This lead molecule was optimized in order to increase affinity towards the AMPA receptor. The substitution of the dioxine with a benzoxazinone ring system increased the activity and allowed further investigation of the sidechain SAR.

Glucocorticoids, cytokines and brain abnormalities in depression

Major depression (MD) is a common psychiatric disorder with a complex and multifactor aetiology. Potential mechanisms associated with the pathogenesis of this disorder include monoamine deficits, hypothalamic-pituitary-adrenal (HPA) axis dysfunctions, inflammatory and/or neurodegenerative alterations. An increased secretion and reactivity of cortisol together with an altered feedback inhibition are the most widely observed HPA abnormalities in MD patients. Glucocorticoids, such as cortisol, are vital hormones that are released in response to stress, and regulate metabolism and immunity but also neuronal survival and neurogenesis. Interestingly depression is highly prevalent in infectious, autoimmune and neurodegenerative diseases and at the same time, depressed patients show higher levels of pro-inflammatory cytokines. Since communication occurs between the endocrine, immune and central nervous system, an activation of the inflammatory responses can affect neuroendocrine processes, and vice versa. Therefore, HPA axis hyperactivity and inflammation might be part of the same pathophysiological process: HPA axis hyperactivity is a marker of glucocorticoid resistance, implying ineffective action of glucocorticoid hormones on target tissues, which could lead to immune activation; and, equally, inflammation could stimulate HPA axis activity via both a direct action of cytokines on the brain and by inducing glucocorticoid resistance. In addition, increased levels of pro-inflammatory cytokines also induce the production of neurotoxic end products of the tryptophan-kynurenine pathway. Although the evidence for neurodegeneration in MD is controversial, depression is co-morbid with many other conditions where neurodegeneration is present. Since several systems seem to be involved interacting with each other, we cannot unequivocally accept the simple model that glucocorticoids induce neurodegeneration, but rather that elevated cytokines, in the context of glucocorticoid resistance, are probably the offenders. Chronic inflammatory changes in the presence of glucocorticoid resistance may represent a common feature that could be responsible for the enhanced vulnerability of depressed patients to develop neurodegenerative changes later in life. However, further studies are needed to clarify the relative contribution of glucocorticoids and inflammatory signals to MD and other 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.

The metabotropic glutamate receptor 7 allosteric modulator AMN082: a monoaminergic agent in disguise?

Metabotropic glutamate receptor 7 (mGluR7) remains the most elusive of the eight known mGluRs primarily because of the limited availability of tool compounds to interrogate its potential therapeutic utility. The discovery of N,N'-dibenzhydrylethane-1,2-diamine dihydrochloride (AMN082) as the first orally active, brain-penetrable, mGluR7-selective allosteric agonist by Mitsukawa and colleagues (Proc Natl Acad Sci USA 102:18712-18717, 2005) provides a means to investigate this receptor system directly. AMN082 demonstrates mGluR7 agonist activity in vitro and interestingly has a behavioral profile that supports utility across a broad spectrum of psychiatric disorders including anxiety and depression. The present studies were conducted to extend the in vitro and in vivo characterization of AMN082 by evaluating its pharmacokinetic and metabolite profile. Profiling of AMN082 in rat liver microsomes revealed rapid metabolism (t(1/2) < 1 min) to a major metabolite, N-benzhydrylethane-1,2-diamine (Met-1). In vitro selectivity profiling of Met-1 demonstrated physiologically relevant transporter binding affinity at serotonin transporter (SERT), dopamine transporter (DAT), and norepinephrine transporter (NET) (323, 3020, and 3410 nM, respectively); whereas the parent compound AMN082 had appreciable affinity at NET (1385 nM). AMN082 produced antidepressant-like activity and receptor occupancy at SERT up to 4 h postdose, a time point at which AMN082 is significantly reduced in brain and plasma while the concentration of Met-1 continues to increase in brain. Acute Met-1 administration produced antidepressant-like activity as would be expected from its in vitro profile as a mixed SERT, NET, DAT inhibitor. Taken together, these data suggest that the reported in vivo actions of AMN082 should be interpreted with caution, because they may involve other mechanisms in addition to mGluR7.

To use or not to use: an update on licit and illicit ketamine use

Ketamine, a derivative of phencyclidine that was developed in the 1960s, is an anesthetic and analgesic with hallucinogenic effects. In this paper, the pharmacological and toxicological effects of ketamine are briefly reviewed. Ketamine possesses a wide safety margin but such a therapeutic benefit is somewhat offset by its emergence phenomenon (mind-body dissociation and delirium) and hallucinogenic effects. The increasing abuse of ketamine, initially predominantly in recreational scenes to experience a “k-hole” and other hallucinatory effects but more recently also as a drug abused during the workday or at home, has further pushed governments to confine its usage in many countries. Recently, urinary tract dysfunction has been associated with long-term ketamine use. In some long-term ketamine users, such damage can be irreversible and could result in renal failure and dialysis. Although ketamine has not yet been scheduled in the United Nations Conventions, previous studies using different assessment parameters to score the overall harms of drugs indicated that ketamine may cause more harm than some of the United Nations scheduled drugs. Some countries in Southeast and East Asia have reported an escalating situation of ketamine abuse. Dependence, lower urinary tract dysfunction, and sexual impulse or violence were the most notable among the ketamine-associated symptoms in these countries. These results implied that the danger of ketamine may have been underestimated previously. Therefore, the severity levels of the ketamine-associated problems should be scrutinized more carefully and objectively. To prevent ketamine from being improperly used and evolving into an epidemic, a thorough survey on the prevalence and characteristics of illicit ketamine use is imperative so that suitable policy and measures can be taken. On the other hand, recent findings that ketamine could be useful for treating major depressive disorder has given this old drug a new impetus. If ketamine is indeed a remedy for treating depression, more research on the risks and benefits of its clinical use will be indispensable.

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.

Antidepressants act on glial cells: SSRIs and serotonin elicit astrocyte calcium signaling in the mouse prefrontal cortex

One important target in the treatment of major depressive disorder (MDD) is the serotonin (5-hydroxytryptamine, 5-HT) system. Selective serotonin reuptake inhibitors (SSRI) are used to treat MDD. Yet, the mode of action of these drugs is not completely understood. There is evolving evidence for a role of glutamate in mood disorder and its signaling. Astrocytes are involved in glutamate metabolism and play an active role in memory processing but their role in mood disorders is still largely unknown. A modulation of astrocytic signaling by SSRIs or 5-HT has not been investigated up to now. We investigated astrocytic calcium signaling with the calcium indicator dye Fluo-4. Using a confocal microscope, we imaged astrocytes in the medial prefrontal cortex of acute mouse brain slices after the application of the SSRIs citalopram and fluoxetine. In the same way, we studied the effects of serotonin and the modulation of this signaling by glutamate in astrocytes. We found that astrocyte calcium signaling can be elicited by 5-HT. Also, the SSRIs citalopram and fluoxetine induce calcium signals in about 1/3 of all astrocytes, even when neuronal signal propagation is inhibited. Astrocytic responses to 5-HT have a unique pattern and they could mostly not be evoked twice. We determined that glutamate is a substance that can interfere with 5-HT-induced calcium signals in astrocytes since after stimulation by glutamate, astrocytes did not show a response to 5-HT. Astrocytic calcium signaling is elicited by SSRIs and 5-HT. They may serve as integrators, linking the serotonergic and glutamatergic signaling pathways.

Role of immune molecules in the establishment and plasticity of glutamatergic synapses

An increasing number of studies support an unexpected role for immune molecules in regulating healthy brain functions during development and in adulthood. Here we review the roles of specific immune molecules (including cytokines, components of the complement cascade, and members of the major histocompatibility complex class I family and their receptors) in the formation and plasticity of glutamatergic synapses. These findings add a new dimension to our understanding of neural-immune interactions, and suggest novel molecular mechanisms that may underlie the modification of glutamatergic synapses in both normal and pathological states.

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.

Minocycline attenuates subjective rewarding effects of dextroamphetamine in humans

RATIONALE

Minocycline, a tetracycline antibiotic, interacts with brain glutamate and dopamine neurotransmission. In preclinical studies, minocycline attenuated amphetamine-induced acute dopamine release and subsequent behavioral sensitization. The goal of this study was to determine minocycline's effects on the acute physiological, behavioral, and subjective responses to dextroamphetamine (DAMP) in healthy volunteers.

METHODS

Ten healthy volunteers participated in an outpatient double-blind, placebo-controlled, crossover study. Subjects had a 5-day treatment period with either minocycline (200 mg/day) or placebo and then were crossed over for 5 days of the other treatment. After 2 days of taking the study medication, on days 3 and 4, subjects were randomly assigned to double-blind acute challenge with either 20 mg/70 kg DAMP or placebo DAMP (randomly labeled as drug A or B) and then crossed over to the other challenge. On day 5 (experimental session 3), subjects had the opportunity to self-administer either placebo or DAMP capsules by working on a progressive ratio computer task.

RESULTS

Minocycline attenuated DAMP-induced subjective rewarding effects but did not change DAMP choice behavior. Minocycline treatment speeded reaction times on a Go No-Go task and reduced plasma cortisol levels.

CONCLUSIONS

These findings warrant further studies examining the potential use of minocycline for stimulant addiction.

Frontocingulate dysfunction in depression: toward biomarkers of treatment response

Increased rostral anterior cingulate cortex (rACC) activity has emerged as a promising predictor of treatment response in depression, but neither the reliability of this relationship nor the mechanisms supporting it have been thoroughly investigated. This review takes a three-pronged approach to these issues. First, I present a meta-analysis demonstrating that the relationship between resting rACC activity and treatment response is robust. Second, I propose that the rACC plays a key role in treatment outcome because of its 'hub' position in the default network. Specifically, I hypothesize that elevated resting rACC activity confers better treatment outcomes by fostering adaptive self-referential processing and by helping to recalibrate relationships between the default network and a 'task-positive network' that comprises dorsolateral prefrontal and dorsal cingulate regions implicated in cognitive control. Third, I support this hypothesis by reviewing neuropsychological, electrophysiological, and neuroimaging data on frontocingulate dysfunction in depression. The review ends with a discussion of the limitations of current work and future directions.

A novel target of action of minocycline in NGF-induced neurite outgrowth in PC12 cells: translation initiation [corrected] factor eIF4AI

BACKGROUND

Minocycline, a second-generation tetracycline antibiotic, has potential activity for the treatment of several neurodegenerative and psychiatric disorders. However, its mechanisms of action remain to be determined.

METHODOLOGY/PRINCIPAL FINDINGS

We found that minocycline, but not tetracycline, significantly potentiated nerve growth factor (NGF)-induced neurite outgrowth in PC12 cells, in a concentration dependent manner. Furthermore, we found that the endoplasmic reticulum protein inositol 1,4,5-triphosphate (IP3) receptors and several common signaling molecules (PLC-γ, PI3K, Akt, p38 MAPK, c-Jun N-terminal kinase (JNK), mammalian target of rapamycin (mTOR), and Ras/Raf/ERK/MAPK pathways) might be involved in the active mechanism of minocycline. Moreover, we found that a marked increase of the eukaryotic translation initiation factor eIF4AI protein by minocycline, but not tetracycline, might be involved in the active mechanism for NGF-induced neurite outgrowth.

CONCLUSIONS/SIGNIFICANCE

These findings suggest that eIF4AI might play a role in the novel mechanism of minocycline. Therefore, agents that can increase eIF4AI protein would be novel therapeutic drugs for certain neurodegenerative and psychiatric diseases.

The NADPH oxidase NOX2 controls glutamate release: a novel mechanism involved in psychosis-like ketamine responses

Subanesthetic doses of NMDA receptor antagonist ketamine induce schizophrenia-like symptoms in humans and behavioral changes in rodents. Subchronic administration of ketamine leads to loss of parvalbumin-positive interneurons through reactive oxygen species (ROS), generated by the NADPH oxidase NOX2. However, ketamine induces very rapid alterations, in both mice and humans. Thus, we have investigated the role of NOX2 in acute responses to subanesthetic doses of ketamine. In wild-type mice, ketamine caused rapid (30 min) behavioral alterations, release of neurotransmitters, and brain oxidative stress, whereas NOX2-deficient mice did not display such alterations. Decreased expression of the subunit 2A of the NMDA receptor after repetitive ketamine exposure was also precluded by NOX2 deficiency. However, neurotransmitter release and behavioral changes in response to amphetamine were not altered in NOX2-deficient mice. Our results suggest that NOX2 is a major source of ROS production in the prefrontal cortex controlling glutamate release and associated behavioral alterations after acute ketamine exposure. Prolonged NOX2-dependent glutamate release may lead to neuroadaptative downregulation of NMDA receptor subunits.

Chronic corticosterone administration down-regulates metabotropic glutamate receptor 5 protein expression in the rat hippocampus

Several lines of evidence suggest a dysfunctional glutamate system in major depressive disorder (MDD). Recently, we reported reduced levels of metabotropic glutamate receptor subtype 5 (mGluR5) in postmortem brains in MDD, however the neurobiological mechanisms that induce these abnormalities are unclear. In the present study, we examined the effect of chronic corticosterone (CORT) administration on the expression of mGluR5 protein and mRNA in the rat frontal cortex and hippocampus. Rats were injected with CORT (40 mg/kg s.c.) or vehicled once daily for 21 days. The expression of mGluR5 protein and mRNA was assessed by Western blotting and quantitative real-time PCR (qPCR). In addition, mGluR1 protein was measured in the same animals. The results revealed that while there was a significant reduction (-27%, P=0.0006) in mGluR5 protein expression in the hippocampus from CORT treated rats, mRNA levels were unchanged. Also unchanged were mGluR5 mRNA and protein levels in the frontal cortex and mGluR1 protein levels in both brain regions. Our findings provide the first evidence that chronic CORT exposure regulates the expression of mGluR5 and are in line with previous postmortem and imaging studies showing reduced mGluR5 in MDD. Our findings suggest that elevated levels of glucocorticoids may contribute to impairments in glutamate neurotransmission in MDD.

Therapeutic armamentarium for treating depression

Depressive disorders are highly prevalent and are a leading cause of disability, morbidity, and mortality worldwide; however, they often remain undertreated or untreated. This article provides a broad overview of the many strategies for treating depression. More than 24 antidepressant medications and depression-focused psychotherapies are available as first-choice options for treating depression. When patients have not had a satisfactory treatment response, the 2 main strategies are switching to an alternative antidepressant therapy or adding a second antidepressant therapy. A large number of medication combinations have been reported in the literature, and some have been shown to be effective in controlled studies. Nonstandard alternatives to conventional antidepressant treatments include exercise, light therapy, sleep deprivation, and various complementary and alternative therapies. For more chronic and refractory forms of depression, various neuromodulation therapies are available or are being investigated. Because depressive disorders are common in primary care and other medical settings, medical practitioners should be aware of the therapeutic armamentarium available for treating depression.

Rapid antidepressant effect of ketamine anesthesia during electroconvulsive therapy of treatment-resistant depression: comparing ketamine and propofol anesthesia

BACKGROUND

Reports of the superiority of the antidepressant effect of ketamine during the conduct of electroconvulsive therapy (ECT) have been limited. We conducted an open-label trial of ketamine to determine whether ketamine as the anesthetic during ECT would provide a greater antidepressant effect than the antidepressant effect obtained with propofol.

METHODS

Between April 2006 and April 2007, 31 inpatients with treatment-resistant depression gave written consent for ECT and to participate in this study. An anesthesiologist who was unaware of the mental symptoms of the subjects assigned them to receive propofol or ketamine anesthetic according to the preferences of the patients, and the patients underwent 8 ECT sessions for 4 weeks. The Hamilton Depression Rating Scale (HDRS) was valuated before ECT and after the completion of the second, fourth, sixth, and eighth ECT sessions.

RESULTS

The HDRS scores improved earlier in the ketamine group, with decreases in HDRS scores that were significantly greater in the ketamine group.

CONCLUSIONS

The results suggested that it is possible to improve symptoms of depression earlier by using ketamine anesthesia.

Brain-derived neurotrophic factor as a biomarker for mood disorders: an historical overview and future directions

Mood disorders, such as major depressive disorder (MDD) and bipolar disorder (BPD), are the most prevalent psychiatric conditions, and are also among the most severe and debilitating. However, the precise neurobiology underlying these disorders is currently unknown. One way to combat these disorders is to discover novel biomarkers for them. The development of such biomarkers will aid both in the diagnosis of mood disorders and in the development of effective psychiatric medications to treat them. A number of preclinical studies have suggested that the brain-derived neurotrophic factor (BDNF) plays an important role in the pathophysiology of MDD. In 2003, we reported that serum levels of BDNF in antidepressant-naive patients with MDD were significantly lower than those of patients medicated with antidepressants and normal controls, and that serum BDNF levels were negatively correlated with the severity of depression. Additionally, we found that decreased serum levels of BDNF in antidepressant-naive patients recovered to normal levels associated with the recovery of depression after treatment with antidepressant medication. This review article will provide an historical overview of the role played by BDNF in the pathophysiology of mood disorders and in the mechanism of action of therapeutic agents. Particular focus will be given to the potential use of BDNF as a biomarker for mood disorders. BDNF is initially synthesized as a precursor protein proBDNF, and then proBDNF is proteolytically cleaved to the mature BDNF. Finally, future perspectives on the use of proBDNF as a novel biomarker for mood disorders will be discussed.

Channelopathies linked to plasma membrane phosphoinositides

The plasma membrane phosphoinositide phosphatidylinositol 4,5-bisphosphate (PIP2) controls the activity of most ion channels tested thus far through direct electrostatic interactions. Mutations in channel proteins that change their apparent affinity to PIP2 can lead to channelopathies. Given the fundamental role that membrane phosphoinositides play in regulating channel activity, it is surprising that only a small number of channelopathies have been linked to phosphoinositides. This review proposes that for channels whose activity is PIP2-dependent and for which mutations can lead to channelopathies, the possibility that the mutations alter channel-PIP2 interactions ought to be tested. Similarly, diseases that are linked to disorders of the phosphoinositide pathway result in altered PIP2 levels. In such cases, it is proposed that the possibility for a concomitant dysregulation of channel activity also ought to be tested. The ever-growing list of ion channels whose activity depends on interactions with PIP2 promises to provide a mechanism by which defects on either the channel protein or the phosphoinositide levels can lead to disease.

Glycine transport inhibitors for the treatment of schizophrenia

Multiple lines of evidence indicate that hypofunction of glutamatergic neurotransmission via N-methyl-D-aspartate (NMDA) receptors might be implicated in the pathophysiology of schizophrenia, suggesting that increasing NMDA receptor function via pharmacological manipulation could provide a new strategy for the management of schizophrenia. Currently, the glycine modulatory sites on NMDA receptors present the most attractive therapeutic targets for the treatment of schizophrenia. One means of enhancing NMDA receptor neurotransmission is to increase the availability of the obligatory co-agonist glycine at modulatory sites on the NMDA receptors through the inhibition of glycine transporter-1 (GlyT-1) on glial cells. Clinical studies have demonstrated that the GlyT-1 inhibitor sarcosine (N-methyl glycine) shows antipsychotic activity in patients with schizophrenia. Accordingly, a number of pharmaceutical companies have developed novel and selective GlyT-1 inhibitors for the treatment of schizophrenia. This paper provides an overview of the various GlyT-1 inhibitors and their therapeutic potential.

Sigma receptors: potential targets for a new class of antidepressant drug

Despite the widespread and devastating impact of depression on society, our current understanding of its pathogenesis is limited. Likewise, existing treatments are inadequate, providing relief to only a subset of people suffering from depression. The search for more effective antidepressant drugs includes the investigation of new molecular targets. Among them, current data suggests that sigma receptors are involved in multiple processes effecting antidepressant-like actions in vivo and in vitro. This review summarizes accumulated evidence supporting a role for sigma receptors in antidepressant effects and provides a conceptual framework for delineating their potential roles over the course of antidepressant treatment.

The AMPA receptor as a therapeutic target: current perspectives and emerging possibilities

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) is a subtype of the ionotropic glutamate receptors that plays a prominent role in neurotransmission and is widespread throughout the CNS. Because of this, its malfunction can result in a multitude of nervous system diseases. This review looks at compounds that are able to modulate AMPAR function by binding to one of several sites on the receptor that either downregulate its function (competitive, noncompetitive and uncompetitive antagonists) or upregulate its function (positive modulators). It will also give an account of the various diseases that have implicated AMPAR dysfunction and how specific types of AMPAR modulator may be beneficial in their treatment. The AMPAR remains an unexploited but important therapeutic target.

Elevated level of metabotropic glutamate receptor 2/3 in the prefrontal cortex in major depression

Clinical, postmortem and preclinical research strongly implicates dysregulation of glutamatergic neurotransmission in major depressive disorder (MDD). Recently, metabotropic glutamate receptors (mGluRs) have been proposed as attractive targets for the discovery of novel therapeutic approaches against depression. The aim of this study was to examine mGluR2/3 protein levels in the prefrontal cortex (PFC) from depressed subjects. In addition, to test whether antidepressants influence mGluR2/3 expression we also studied levels of mGluR2/3 in fluoxetine-treated monkeys. Postmortem human prefrontal samples containing Brodmann's area 10 (BA10) were obtained from 11 depressed and 11 psychiatrically healthy controls. Male rhesus monkeys were treated chronically with fluoxetine (dose escalated to 3mg/kg, p.o.; n=7) or placebo (n=6) for 39 weeks. The mGluR2/3 immunoreactivity was investigated using Western blot method. There was a robust (+67%) increase in the expression of the mGlu2/3 protein in the PFC of depressed subjects relative to healthy controls. The expression of mGlu2/3 was unchanged in the PFC of monkeys treated with fluoxetine. Our findings provide the first evidence that mGluR2/3 is elevated in the PFC in MDD. This observation is consistent with reports showing that mGluR2/3 antagonists exhibit antidepressant-like activity in animal models and demonstrates that these receptors are promising targets for the discovery of novel antidepressants.

Peripheral and cerebral metabolic abnormalities of the tryptophan-kynurenine pathway in a murine model of major depression

Occurring both peripherally and centrally, the kynurenine pathway (KP) - an alternative pathway to 5-HT synthesis from tryptophan (TRP) - could be of particular value to better understand the link between peripheral changes of circulating levels of glucocorticoids (GC)/proinflammatory cytokines and altered neurotransmission observed in depressed patients. Indeed, it is activated by these mediators of stress and can produce several neuroactive compounds like quinolinic acid (QUIN) and kynurenic acid (KYNA) that can respectively increase and decrease glutamate concentration in brain. In order to characterize the role of both the peripheral and cerebral KP in the pathophysiology of depressive disorders, we used the Unpredictable Chronic Mild Stress (UCMS) to induce a depressive-like syndrome and we then measured the level of relevant TRP-KYN pathway metabolites: KYN, 3-hydroxykynurenine (3HK; precursor of QUIN) and KYNA. We also measured TRP-5HT pathway metabolites: TRP, 5-HT, 5-HIAA. We showed that UCMS increased TRP catabolism along the KP in the periphery. 5-HT and KYN were found to be strongly negatively correlated in all brain structures of control mice and of UCMS mice except in the hippocampus. More importantly we found that KYN was preferentially metabolized along the QUIN pathway at the subcortical level (amygdala/striatum) whereas, at the cortical level (cingulate cortex), the QUIN pathway was reduced. Considering the role of these metabolites on the glutamatergic neurotransmission, we propose that such KP alterations could participate to the cortical/subcortical glutamatergic alterations reported in depressed patients.

Open questions in current models of antidepressant action

Research on depression and antidepressant drugs is necessary, as many patients display poor response to therapy. Different symptomatic and pathophysiological features have been proposed as end points of the depressive phenotype and of the antidepressant action, including anhedonia, depressed mood, alterations in morphology and activity of some brain areas (amygdala, nucleus accumbens, hippocampus, prefrontal cortex and cingulate cortex), modifications in the connectivity between brain structures, changes in neurotransmitters (serotonin, noradrenaline, glutamate and neuropeptides), brain plasticity (neurogenesis, neurotrophins) and abnormal function of the hypothalamic-pituitary adrenal axis. However, few models have been proposed to describe how these end points could induce the depressive phenotype and are involved in the mechanism of action of antidepressants. Here we propose a connectionist-inspired network of depression and antidepressant action, in which the different aetiological factors participating in the release of a depressive episode are represented by input nodes, the different symptomatic as well as pathophysiological end points are represented by an intermediate layer, and the onset of depression or of comorbid disease is represented by the output node. The occurrence of depression and the mechanism of the antidepressant action thus depend upon the weight of the interactions between the different end points, none of them being per se crucial to the onset of a depressive phenotype or to the antidepressant action. This model is heuristic to draw future lines of research concerning new antidepressant therapies, designing new animal models of depression and for a better understanding of the depressive pathology and of its comorbid pathology such as anxiety disorders.

New approaches to the pharmacological management of major depressive disorder

Despite effective and safe therapies for major depressive disorder (MDD), the current arsenal of antidepressant therapies does not fully satisfy the needs of patients or physicians. Many patients are only partial responders or are treatment resistant and side effects interfere with compliance. The majority of antidepressants directly affect monoamine neurotransmission within the central nervous system. Moving beyond this mechanism has been a challenge because of the lack of knowledge about the underlying etiology and pathophysiology of MDD. Provided in this report is a review of some of the major new advances in MDD research that suggest the possibility of novel and improved future therapeutic options. Emphasis is placed on studies of unipolar, but not bipolar, depression. New therapies include dual and triple monoamine uptake inhibitors, non-conventional antidepressants such as tianeptine, and a number of augmentation strategies. In addition, studies are underway on a number of mechanisms of action that might yield the next therapeutic advance. These include agents that interact with endocannabiniod systems, examination of natural products, and compounds that influence neuropeptide systems such as galanin and melanin-concentrating hormone, and growth and neurotrophic factors. Epigenetic mechanisms involving histone modification are also being explored. An area of intensive investigation is glutamate neurotransmission. Data support the hypothesis that NMDA receptor antagonists are effective in MDD individuals resistant to conventional therapies. The potential of metabotropic glutamate receptors as novel targets is also discussed. Accumulating evidence supports the idea that amplification of AMPA receptor function is a critical link in the transduction processes involved antidepressant effects.

Effect of antidepressant drugs on veratridine-evoked glutamate and aspartate release in rat prefrontal cortex

In vivo microdialysis in conscious rats was used to evaluate the effect of local application, through a microdialysis probe, of desipramine (DMI), imipramine and citalopram (CIT), on veratridine-evoked glutamate and aspartate release in rat prefrontal cortex (PFCx). All antidepressant drugs (ADs), given at a concentration of 0.1 mM, significantly inhibited glutamate release, while aspartate release was affected only by DMI and CIT. In contrast, local administration of ADs markedly potentiated veratridine-evoked dopamine and noradrenaline release. Perfusion of clonidine, quinpirole and 1-[3-(trifluoro-methyl)phenyl]-piperazine (TFMPP) at 0.1 mM concentration also diminished, evoked release of glutamate and aspartate. The regulation of amino acid release in rat PFCx may be achieved by direct effect of ADs on Na+ channels or indirectly, by involvement of D2/D3, alpha 2 or 5-HT1B heteroceptors activated by the increased level of monoamines in response to the blockade of respective transporters.