Clinical studies implementing glutamate neurotransmission in mood disorders

Memantine and SKF82958 but not an enriched environment modulate naloxone-precipitated morphine abstinence syndrome without affecting hippocampal tPA mRNA levels in rats

There is accumulating evidence supporting the involvement of tissue-plasminogen activator (tPA) in the mechanisms underlying the effects of morphine and an enriched environment. This study was designed to investigate possible interactive roles of the glutamatergic and the dopaminergic systems regarding hippocampal tPA in the neurobiology of morphine dependence. For this purpose, Wistar albino rats, housed in either a standard- (SE) or an enriched environment (EE) were implanted subcutaneously with morphine (150 mg base) or placebo pellets. Behavioral and somatic signs of morphine abstinence precipitated by an opioid-receptor antagonist naloxone (1 mg/kg, i.p.) 72 h after the pellet implantation were observed individually for 15 min in all groups. Memantine (10 mg/kg i.p.), an antagonist of N-methyl-D-aspartic acid class of glutamatergic receptor-subtype decreased teeth-chattering, ptosis, diarrhea and the loss of body weight. SKF82958 (1 mg/kg, i.p.), a dopamine D1-receptor agonist decreased jumping and ptosis but increased rearing and loss of body weight. On the other hand, co-administration of SKF82958 with memantine prevented some of their effects that occur when administered alone at the same doses. Furthermore, the EE did not change the intensity of morphine abstinence. The level of hippocampal tPA mRNA was found to be lower in the SE morphine abstinence group than in the placebo group and close to the EE morphine abstinence group, whereas there was no significant alteration of its level in the memantine or SKF82958 groups. These findings suggest that the interaction between the glutamatergic and the dopaminergic systems may be an important component of the neurobiology of morphine dependence, and the role of tPA in this interaction should be further investigated.

Patients' recovery and non-recovery narratives after intravenous ketamine for treatment-resistant depression

BACKGROUND

Intravenous (IV) ketamine is an effective therapy for treatment-resistant depression. A large data base is confirmatory and steadily expanding. Qualitative studies can inform best practices and suggest new research directions. As part of a clinical trial designed to identify biomarkers of ketamine response, a qualitative study was conducted to characterize experiences with: receiving infusions; recovering or not recovering from depression; and beliefs about why ketamine worked or did not work.

METHODS

Adults with treatment-resistant depression received three IV ketamine infusions in a two-week period and were characterized as remitters or non-remitters via symptom reduction 24 h after the third infusion. Qualitative interviews of a subset of participants were audio recorded, transcribed verbatim, and coded using deductive and inductive methods. Themes were derived and compared across a broader construct of recovery status.

RESULTS

Of the 21 participants, nine (43 %) were characterized as having experienced remission and 12 (57 %) non-remission. Of the 12 non-remitters, five were characterized as having experienced partial recovery based on their subjective experiences, reporting substantial benefit from ketamine infusions despite non-remission status based on scale measurements. Attributions for ketamine's effects included biological and experiential mechanisms. Among non-remitters there was risk of disappointment when adding another failed treatment.

LIMITATIONS

A more diverse sample may have yielded different themes. Different patients had different amounts of time elapsed between ketamine infusions and qualitative interview.

CONCLUSIONS

Qualitative methods may enhance researchers' characterization of IV ketamine's impact on treatment-resistant depression. While requiring confirmation, patients may benefit from a preparatory milieu that prepares them for multiple recovery pathways; decouples the psychedelic experience from clinical outcomes; and addresses potential risks of another failed treatment.

The metabotropic glutamate receptor 5 as a biomarker for psychiatric disorders

The role of glutamate system in the etiology and pathophysiology of psychiatric disorders has gained considerable attention in the past two decades, including dysregulation of the metabotropic glutamatergic receptor subtype 5 (mGlu5). Thus, mGlu5 may represent a promising therapeutic target for psychiatric conditions, particularly stress-related disorders. Here, we describe mGlu5 findings in mood disorders, anxiety, and trauma disorders, as well as substance use (specifically nicotine, cannabis, and alcohol use). We highlight insights gained from positron emission tomography (PET) studies, where possible, and discuss findings from treatment trials, when available, to explore the role of mGlu5 in these psychiatric disorders. Through the research evidence reviewed in this chapter, we make the argument that, not only is dysregulation of mGlu5 evident in numerous psychiatric disorders, potentially functioning as a disease "biomarker," the normalization of glutamate neurotransmission via changes in mGlu5 expression and/or modulation of mGlu5 signaling may be a needed component in treating some psychiatric disorders or symptoms. Finally, we hope to demonstrate the utility of PET as an important tool for investigating mGlu5 in disease mechanisms and treatment response.

Is depression the missing link between inflammatory mediators and cancer?

Patients with cancer are at greater risk of developing depression in comparison to the general population and this is associated with serious adverse effects, such as poorer quality of life, worse prognosis and higher mortality. Although the relationship between depression and cancer is now well established, a common underlying pathophysiological mechanism between the two conditions is yet to be elucidated. Existing theories of depression, based on monoamine neurotransmitter system dysfunction, are insufficient as explanations of the disorder. Recent advances have implicated neuroinflammatory mechanisms in the etiology of depression and it has been demonstrated that inflammation at a peripheral level may be mirrored centrally in astrocytes and microglia serving to promote chronic levels of inflammation in the brain. Three major routes to depression in cancer in which proinflammatory mediators are implicated, seem likely. Activation of the kynurenine pathway involving cytokines, increases tryptophan catabolism, resulting in diminished levels of serotonin which is widely acknowledged as being the hallmark of depression. It also results in neurotoxic effects on brain regions thought to be involved in the evolution of major depression. Proinflammatory mediators also play a crucial role in impairing regulatory glucocorticoid mediated feedback of the hypothalamic-pituitary-adrenal axis, which is activated by stress and considered to be involved in both depression and cancer. The third route is via the glutamatergic pathway, whereby glutamate excitotoxicity may lead to depression associated with cancer. A better understanding of the mechanisms underlying these dysregulated and other newly emerging pathways may provide a rationale for therapeutic targeting, serving to improve the care of cancer patients.

Involvement of regulation of the excitation:inhibition functional balance in the mPFC in the antidepressant-anxiolytic effect of YL-IPA08, a novel TSPO ligand

TSPO, an 18 kDa translocator protein, has received increased attention due to its antidepressant-anxiolytic effects. The balance between glutamatergic and GABAergic (E: I) in the medial prefrontal cortex (mPFC) is crucial for antidepressant-anxiolytic effects. However, no evidence is available to clarify the relationship between TSPO and E:I balance. In the present study, we used the TSPO global-knockout (KO) and TSPO wild-type (WT) mice to assess the effects of TSPO on antidepressant-anxiolytic effects of YL-IPA08 (a novel TSPO ligand) and the underlying neurobiological mechanism. Additionally, a multichannel electrophysiological technique was used to explore the effects of YL-IPA08 on pyramidal neurons and interneurons in mPFC. Open field test (OFT) and elevated plus maze (EPM) test revealed that a single dose of YL-IPA08 (0.3 mg/kg, i.p.) exhibited significant anxiolytic actions in WT mice except in KO mice. In only WT mice, significant antidepressant effects were observed in tail suspension test (TST) and forced swim test (FST). The multichannel electrophysiological technique demonstrated that YL-IPA08 significantly increased the firing rates of pyramidal neurons and decreased those of interneurons. Further studies illustrated that the firing rates of glutamatergic might be antagonized by PK11195 (a classic TSPO antagonist). Our results suggest that YL-IPA08 might regulate the E:I balance in mPFC, mediated by TSPO. In summary, TSPO regulates E:I functional balance in mPFC, play a critical role in antidepressant-anxiolytic effects of YL-IPA08, and provide a potential target site for the development of antidepressant and anxiolytic drugs.

Group III metabotropic glutamate receptors as promising targets for neuroprotective therapy: Particular emphasis on the role of mGlu4 and mGlu7 receptors

There is still no effective treatment for central nervous system (CNS) pathologies, including cerebral ischemia, neurotrauma, and neurodegenerative diseases in which the Glu/GABA balance is disturbed with associated excitotoxicity. It is thus important to search for new efficacious therapeutic strategies. Preclinical studies on the role of metabotropic glutamate receptors (mGluRs) in neuroprotection conducted over the years show that these receptors may have therapeutic potential in these CNS disorders. However, clinical trials, especially for treating Parkinson's disease, have been unsatisfactory. This review focuses on the specific role of group III mGluRs in neuroprotection in experimental in vitro and in vivo models of excitotoxicity/neurotoxicity using neurotoxins as well as ischemia, traumatic brain injury, and neurodegenerative diseases such as Parkinson's disease, Alzheimer's diseases, and multiple sclerosis. The review highlights recent preclinical studies in which group III mGluR ligands (especially those acting at mGluR4 or mGluR7) were administered after damage, thus emphasizing the importance of the therapeutic time window in the treatment of ischemic stroke and traumatic brain injury. From a clinical standpoint, the review also highlights studies using group III mGluR agonists with favorable neuroprotective efficacy (histological and functional) in experimental ischemic stroke, including healthy normotensive and-hypertensive rats. This review also summarizes possible mechanisms underlying the neuroprotective activity of the group III mGluR ligands, which may be helpful in developing more effective and safe therapeutic strategies. Therefore, to fully assess the role of these receptors in neuroprotection, it is necessary to uncover new selective ligands, primarily those stimulating mGlu4 and mGlu7 receptors.

Sigma-1 Receptors in Depression: Mechanism and Therapeutic Development

Depression is the most common type of neuropsychiatric illness and has increasingly become a major cause of disability. Unfortunately, the recent global pandemic of COVID-19 has dramatically increased the incidence of depression and has significantly increased the burden of mental health care worldwide. Since full remission of the clinical symptoms of depression has not been achieved with current treatments, there is a constant need to discover new compounds that meet the major clinical needs. Recently, the roles of sigma receptors, especially the sigma-1 receptor subtype, have attracted increasing attention as potential new targets and target-specific drugs due to their translocation property that produces a broad spectrum of biological functions. Even clinical first-line antidepressants with or without affinity for sigma-1 receptors have different pharmacological profiles. Thus, the regulatory role of sigma-1 receptors might be useful in treating these central nervous system (CNS) diseases. In addition, long-term mental stress disrupts the homeostasis in the CNS. In this review, we discuss the topical literature concerning sigma-1 receptor antidepressant mechanism of action in the regulation of intracellular proteostasis, calcium homeostasis and especially the dynamic Excitatory/Inhibitory (E/I) balance in the brain. Furthermore, based on these discoveries, we discuss sigma-1 receptor ligands with respect to their promise as targets for fast-onset action drugs in treating depression.

A Novel Biomarker of Neuronal Glutamate Metabolism in Nonhuman Primates Using Localized 1H-Magnetic Resonance Spectroscopy: Development and Effects of BNC375, an α7 Nicotinic Acetylcholine Receptor Positive Allosteric Modulator

BACKGROUND

The development of treatments for cognitive deficits associated with central nervous system disorders is currently a significant medical need. Despite the great need for such therapeutics, a significant challenge in the drug development process is the paucity of robust biomarkers to assess target modulation and guide clinical decisions. We developed a novel, translatable biomarker of neuronal glutamate metabolism, the ¹³C-glutamate+glutamine (Glx) H3:H4 labeling ratio, in nonhuman primates using localized ¹H-magnetic resonance spectroscopy combined with ¹³C-glucose infusions.

METHODS

We began with numerical simulations in an established model of brain glutamate metabolism, showing that the ¹³C-Glx H3:H4 ratio should be a sensitive biomarker of neuronal tricarboxylic acid cycle activity, a key measure of overall neuronal metabolism. We showed that this biomarker can be measured reliably using a standard ¹H-magnetic resonance spectroscopy method (point-resolved spectroscopy sequence/echo time = 20 ms), obviating the need for specialized hardware and pulse sequences typically used with ¹³C-magnetic resonance spectroscopy, thus improving overall clinical translatability. Finally, we used this biomarker in 8 male rhesus macaques before and after administration of the compound BNC375, a positive allosteric modulator of the α7 nicotinic acetylcholine receptor that enhances glutamate signaling ex vivo and elicits procognitive effects in preclinical species.

RESULTS

The ¹³C-Glx H3:H4 ratios in the monkeys showed that BNC375 increases neuronal metabolism in nonhuman primates in vivo, detectable on an individual basis.

CONCLUSIONS

This study demonstrates that the ratio of ¹³C-Glx H3:H4 labeling is a biomarker that may provide an objective readout of compounds affecting glutamatergic neurotransmission and could improve decision making for the development of therapeutic agents.

Blood Glutamate Scavenging With Pyruvate as a Novel Preventative and Therapeutic Approach for Depressive-Like Behavior Following Traumatic Brain Injury in a Rat Model

Depression is a common and serious complication following traumatic brain injury (TBI). Both depression and TBI have independently been associated with pathologically elevated extracellular brain glutamate levels. In the setting of TBI, blood glutamate scavenging with pyruvate has been widely shown as an effective method to provide neuroprotection by reducing blood glutamate and subsequent brain glutamate levels. Here we evaluate pyruvate as a novel approach in the treatment and prevention of post-TBI depression-like behavior in a rat model. Rats were divided into five groups: (1) sham-operated control with pyruvate, (2) sham-operated control with placebo, (3) post-TBI with placebo, (4) post-TBI given preventative pyruvate, and (5) post-TBI treated with pyruvate. These groups had an equal number of females and males. Rats were assessed for depressive-like behavior, neurological status, and glutamate levels in the blood and brain. Post-TBI neurological deficits with concurrent elevations in glutamate levels were demonstrated, with peak glutamate levels 24 h after TBI. Following TBI, the administration of either prophylactic or therapeutic pyruvate led to reduced glutamate levels, improved neurologic recovery, and improved depressive-like behavior. Glutamate scavenging with pyruvate may be an effective prophylactic and therapeutic option for post-TBI depression by reducing associated elevations in brain glutamate levels.

Molecular characterization of depression trait and state

Major depressive disorder (MDD) is a brain disorder often characterized by recurrent episode and remission phases. The molecular correlates of MDD have been investigated in case-control comparisons, but the biological alterations associated with illness trait (regardless of clinical phase) or current state (symptomatic and remitted phases) remain largely unknown, limiting targeted drug discovery. To characterize MDD trait- and state-dependent changes, in single or recurrent depressive episode or remission, we generated transcriptomic profiles of subgenual anterior cingulate cortex of postmortem subjects in first MDD episode (n = 20), in remission after a single episode (n = 15), in recurrent episode (n = 20), in remission after recurring episodes (n = 15) and control subject (n = 20). We analyzed the data at the gene, biological pathway, and cell-specific molecular levels, investigated putative causal events and therapeutic leads. MDD-trait was associated with genes involved in inflammation, immune activation, and reduced bioenergetics (q < 0.05) whereas MDD-states were associated with altered neuronal structure and reduced neurotransmission (q < 0.05). Cell-level deconvolution of transcriptomic data showed significant change in density of GABAergic interneurons positive for corticotropin-releasing hormone, somatostatin, or vasoactive-intestinal peptide (p < 3 × 10^-3). A probabilistic Bayesian-network approach showed causal roles of immune-system-activation (q < 8.67 × 10^-3), cytokine-response (q < 4.79 × 10^-27) and oxidative-stress (q < 2.05 × 10^-3) across MDD-phases. Gene-sets associated with these putative causal changes show inverse associations with the transcriptomic effects of dopaminergic and monoaminergic ligands. The study provides first insights into distinct cellular and molecular pathologies associated with trait- and state-MDD, on plasticity mechanisms linking the two pathologies, and on a method of drug discovery focused on putative disease-causing pathways.

Bipolar spectrum disorders in neurologic disorders

Psychiatric symptoms frequently predate or complicate neurological disorders, such as neurodegenerative diseases. Symptoms of bipolar spectrum disorders (BSD), like mood, behavioral, and psychotic alterations, are known to occur - individually or as a syndromic cluster - in Parkinson's disease and in the behavioral variant of frontotemporal dementia (FTD). Nonetheless, due to shared pathophysiological mechanisms, or genetic predisposition, several other neurological disorders show significant, yet neglected, clinical and biological overlaps with BSD like neuroinflammation, ion channel dysfunctions, neurotransmission imbalance, or neurodegeneration. BSD pathophysiology is still largely unclear, but large-scale network dysfunctions are known to participate in the onset of mood disorders and psychotic symptoms. Thus, functional alterations can unleash BSD symptoms years before the evidence of an organic disease of the central nervous system. The aim of our narrative review was to illustrate the numerous intersections between BSD and neurological disorders from a clinical-biological point of view and the underlying predisposing factors, to guide future diagnostic and therapeutical research in the field.

The alterations of glutamate transporter 1 and glutamine synthetase in the rat brain of a learned helplessness model of depression

BACKGROUND

Although glutamate transmission via astrocytes has been proposed to contribute to the pathophysiology of depression, the precise mechanisms are unknown. Herein, we investigated the levels of glutamate transporter-1 (GLT-1) and glutamine synthetase (GS) of astrocytes in learned helplessness (LH) rats (an animal model of depression) and non-LH rats (an animal model of resilience).

METHODS

We administered inescapable mild electric shock to rats and then discriminated the LH and non-LH rats by a post-shock test. Almost 55% of the rats acquired LH. We then measured the expressions of GLT-1 and GS in several brain regions of LH and non-LH rats by Western blot analysis.

RESULTS

The levels of GLT-1 and GS in the CA-1, CA-3, dentate gyrus (DG), medial prefrontal cortex (mPF), and nucleus accumbens (NAc) of the LH group were significantly higher than those of the control group. The GS levels in the amygdala of the LH rats were significantly decreased compared to the controls. There were significant differences in GLT-1 and GS levels between the non-LH and LH rats in the CA-1 and CA-3.

CONCLUSIONS

These results suggest that the LH rats experienced up-regulations of GLT-1 and GS in the CA-1, CA-3, DG, mPF, and NAc and a down-regulation of GS in the amygdala. It is possible that the effects of the GLT-1 and GS levels on astrocytes in the CA-1 and CA-3 are critical for the differentiation of resilience from vulnerability.

Free L-glutamate-induced modulation in oxidative and neurochemical profile contributes to enhancement in locomotor and memory performance in male rats

Glutamate (Glu), the key excitatory neurotransmitter in the central nervous system, is considered essential for brain functioning and has a vital role in learning and memory formation. Earlier it was considered as a harmful agent but later found to be useful for many body functions. However, studies regarding the effects of free l-Glu administration on CNS function are limited. Therefore, current experiment is aimed to monitor the neurobiological effects of free l-Glu in male rats. l-Glu was orally administered to rats for 5-weeks and changes in behavioral performance were monitored. Thereafter, brain and hippocampus were collected for oxidative and neurochemical analysis. Results showed that chronic supplementation of free l-Glu enhanced locomotor performance and cognitive function of animals which may be attributed to the improved antioxidant status and cholinergic, monoaminergic and glutamatergic neurotransmission in brain and hippocampus. Current results showed that chronic supplementation of l-Glu affects the animal behaviour and brain functioning via improving the neurochemical and redox system of brain. Free l-Glu could be a useful therapeutic agent to combat neurological disturbances however this requires further targeted studies.

Blood glutamate scavenging as a novel glutamate-based therapeutic approach for post-stroke depression

Post-stroke depression (PSD) is a major complication of stroke that significantly impacts functional recovery and quality of life. While the exact mechanism of PSD is unknown, recent attention has focused on the association of the glutamatergic system in its etiology and treatment. Minimizing secondary brain damage and neuropsychiatric consequences associated with excess glutamate concentrations is a vital part of stroke management. The blood glutamate scavengers, oxaloacetate and pyruvate, degrade glutamate in the blood to its inactive metabolite, 2-ketoglutarate, by the coenzymes glutamate-oxaloacetate transaminase (GOT) and glutamate-pyruvate transaminase (GPT), respectively. This reduction in blood glutamate concentrations leads to a subsequent shift of glutamate down its concentration gradient from the blood to the brain, thereby decreasing brain glutamate levels. Although there are not yet any human trials that support blood glutamate scavengers for clinical use, there is increasing evidence from animal research of their efficacy as a promising new therapeutic approach for PSD. In this review, we present recent evidence in the literature of the potential therapeutic benefits of blood glutamate scavengers for reducing PSD and other related neuropsychiatric conditions. The evidence reviewed here should be useful in guiding future clinical trials.

Tianeptine, an atypical pharmacological approach to depression

The introduction of the first antidepressants in the 50s of the 20th century radically changed the treatment of depression, while providing information on pathophysiological aspects of this disease. New antidepressants drugs (agomelatine, tianeptine, vortioxetine) are providing data that give rise to pathophysiological hypotheses of depression that differ from the classic monoaminergic theory. In this sense, tianeptina, an atypical drug by its mechanism of differential action, contributes to clarify that in depression there is more than monoamines. Thus, tianeptine does not modify the rate of extracellular serotonin, so it does not increase or decrease the reuptake of serotonin. Chronic administration of tianeptine does not alter the density or affinity of more than a hundred classical receptors related to depression. Recently, a weak action of tianeptine on Mu opioid receptors has been described that could explain the release of dopamine in the limbic system and its participation in the modulation of glutamatergic mechanisms. These mechanisms support the hypothesis of the possible mechanism of action of this antidepressant. Tianeptine is an antidepressant, with anxiolytic properties, that can improve somatic symptoms. Tianeptine as a glutamatergic modulator, among other mechanisms, allows us to approach depression from a different point of view than other antidepressants.

International Union of Basic and Clinical Pharmacology CIV: The Neurobiology of Treatment-resistant Depression: From Antidepressant Classifications to Novel Pharmacological Targets

Major depressive disorder is one of the most prevalent and life-threatening forms of mental illnesses and a major cause of morbidity worldwide. Currently available antidepressants are effective for most patients, although around 30% are considered treatment resistant (TRD), a condition that is associated with a significant impairment of cognitive function and poor quality of life. In this respect, the identification of the molecular mechanisms contributing to TRD represents an essential step for the design of novel and more efficacious drugs able to modify the clinical course of this disorder and increase remission rates in clinical practice. New insights into the neurobiology of TRD have shed light on the role of a number of different mechanisms, including the glutamatergic system, immune/inflammatory systems, neurotrophin function, and epigenetics. Advances in drug discovery processes in TRD have also influenced the classification of antidepressant drugs and novel classifications are available, such as the neuroscience-based nomenclature that can incorporate such advances in drug development for TRD. This review aims to provide an up-to-date description of key mechanisms in TRD and describe current therapeutic strategies for TRD before examining novel approaches that may ultimately address important neurobiological mechanisms not targeted by currently available antidepressants. All in all, we suggest that drug targeting different neurobiological systems should be able to restore normal function but must also promote resilience to reduce the long-term vulnerability to recurrent depressive episodes.

Excitatory and inhibitory synapse reorganization immediately after critical sensory experience in a vocal learner

Excitatory and inhibitory synapses are the brain’s most abundant synapse types. However, little is known about their formation during critical periods of motor skill learning, when sensory experience defines a motor target that animals strive to imitate. In songbirds, we find that exposure to tutor song leads to elimination of excitatory synapses in HVC (used here as a proper name), a key song generating brain area. A similar pruning is associated with song maturation, because juvenile birds have fewer excitatory synapses, the better their song imitations. In contrast, tutoring is associated with rapid insertion of inhibitory synapses, but the tutoring-induced structural imbalance between excitation and inhibition is eliminated during subsequent song maturation. Our work suggests that sensory exposure triggers the developmental onset of goal-specific motor circuits by increasing the relative strength of inhibition and it suggests a synapse-elimination model of song memorization.

A wide range of species use complex sounds to communicate, including humans and songbirds like zebra finches. During a critical period of learning, infants and young animals learn how to remember and discriminate this ‘language’ from other sounds. However, the changes that happen in the brain during this learning period are not well understood.

The process of learning forms new connections between neurons in the brain and prunes away old connections. These connections, known as synapses, come in different types. Signals sent across excitatory synapses increase the activity of the receiving neuron, while signals sent across inhibitory synapses reduce neuron activity.

What happens to the synapses in the brain during the critical period? To find out, Huang et al. used electron microscopy to examine the brains of young zebra finches that either had never heard birdsong, or had just heard birdsong for the first time. A single day of hearing song dramatically shifted the balance of excitatory and inhibitory synapses in the main vocal control area of the young birds’ brains. The number of excitatory synapses decreased, and the number of inhibitory synapses increased.

The balance between excitation and inhibition is important for the brain to work correctly. Therefore, as well as helping us to understand how infants learn their first language, the results presented by Huang et al. could also help us to improve treatments for conditions where this balance goes wrong, such as mood disorders. For example, tailoring the time point of medication intake in combination with sensory exposure therapies could improve how effectively either one works.

Metabolic Changes Associated with a Rat Model of Diabetic Depression Detected by Ex Vivo 1H Nuclear Magnetic Resonance Spectroscopy in the Prefrontal Cortex, Hippocampus, and Hypothalamus

Diabetic patients often present with comorbid depression. However, the pathogenetic mechanisms underlying diabetic depression (DD) remain unclear. To explore the mechanisms underpinning the pathogenesis of the disease, we used ex vivo ¹H nuclear magnetic resonance spectroscopy and immunohistochemistry to investigate the main metabolic and pathological changes in various rat brain areas in an animal model of DD. Compared with the control group, rats in the DD group showed significant decreases in neurotransmitter concentrations of glutamate (Glu) and glutamine (Gln) in the prefrontal cortex (PFC), hippocampus, and hypothalamus and aspartate and glycine in the PFC and hypothalamus. Gamma-aminobutyric acid (GABA) was decreased only in the hypothalamus. Levels of the energy product, lactate, were higher in the PFC, hippocampus, and hypothalamus of rats with DD than those in control rats, while creatine was lower in the PFC and hippocampus, and alanine was lower in the hypothalamus. The levels of other brain metabolites were altered, including N-acetyl aspartate, taurine, and choline. Immunohistochemistry analysis revealed that expressions of both glutamine synthetase and glutaminase were decreased in the PFC, hippocampus, and hypothalamus of rats with DD. The metabolic changes in levels of Glu, Gln, and GABA indicate an imbalance of the Glu-Gln metabolic cycle between astrocytes and neurons. Our results suggest that the development of DD in rats may be linked to brain metabolic changes, including inhibition of the Glu-Gln cycle, increases in anaerobic glycolysis, and disturbances in the lactate-alanine shuttle, and associated with dysfunction of neurons and astrocytes.

Zdhhc13-dependent Drp1 S-palmitoylation impacts brain bioenergetics, anxiety, coordination and motor skills

Protein S-palmitoylation is a reversible post-translational modification mediated by palmitoyl acyltransferase enzymes, a group of Zn²⁺-finger DHHC-domain-containing proteins (ZDHHC). Here, for the first time, we show that Zdhhc13 plays a key role in anxiety-related behaviors and motor function, as well as brain bioenergetics, in a mouse model (luc) carrying a spontaneous Zdhhc13 recessive mutation. At 3 m of age, mutant mice displayed increased sensorimotor gating, anxiety, hypoactivity, and decreased motor coordination, compared to littermate controls. Loss of Zdhhc13 in cortex and cerebellum from 3- and 24 m old hetero- and homozygous male mutant mice resulted in lower levels of Drp1 S-palmitoylation accompanied by altered mitochondrial dynamics, increased glycolysis, glutaminolysis and lactic acidosis, and neurotransmitter imbalances. Employing in vivo and in vitro models, we identified that Zdhhc13-dependent Drp1 S-palmitoylation, which acting alone or in concert, enables the normal occurrence of the fission-fusion process. In vitro and in vivo direct Zdhhc13-Drp1 protein interaction was observed, confirming Drp1 as a substrate of Zdhhc13. Abnormal fission-fusion processes result in disrupted mitochondria morphology and distribution affecting not only mitochondrial ATP output but neurotransmission and integrity of synaptic structures in the brain, setting the basis for the behavioral abnormalities described in the Zdhhc13-deficient mice.

Efficacy of off-label augmentation in unipolar depression: A systematic review of the evidence

Treatment of unipolar depression with currently available antidepressants is still unsatisfactory. Augmentation with lithium or second generation antipsychotics is an established practice in non-responders to antidepressant monotherapy, but is also associated with a substantial non-response rate and with non-tolerance. Based on a systematic review of the literature, including meta-analyses, randomized controlled trials (RCTs), non-randomized comparative studies and case studies, off-label augmentation agents (administered in addition to an antidepressant, without FDA approval for treatment of MDD) were identified and evaluated regarding their efficacy using levels of evidence. The agents had to be added to an existing antidepressant regime with the aim of achieving an improved clinical response to an ongoing antidepressant treatment (augmentation) or an earlier onset of effect when starting antidepressant and augmentation agent simultaneously (acceleration). Five substances, modafinil, ketamine, pindolol, testosterone and estrogen (the latter two in hormone-deficient patients) were shown to be clinically effective in high evidence studies. For the six drugs dexamethasone, mecamylamine, riluzole, amantadine, pramipexole and yohimbine clear proof of efficacy was not possible due to low levels of evidence, small sample sizes or discordant results. For the two agents methylphenidate and memantine only studies with negative outcomes could be found. Overall, the quality of study designs was low and results were often contradictory. However, the use of pindolol, ketamine, modafinil, estrogen and testosterone might be an option for depressed patients who are not responding to antidepressant monotherapy or established augmentation strategies. Further high quality studies are necessary and warranted.

Multi-target therapeutics for neuropsychiatric and neurodegenerative disorders

Historically, neuropsychiatric and neurodegenerative disease treatments focused on the 'magic bullet' concept; however multi-targeted strategies are increasingly attractive gauging from the escalating research in this area. Because these diseases are typically co-morbid, multi-targeted drugs capable of interacting with multiple targets will expand treatment to the co-morbid disease condition. Despite their theoretical efficacy, there are significant impediments to clinical success (e.g., difficulty titrating individual aspects of the drug and inconclusive pathophysiological mechanisms). The new and revised diagnostic frameworks along with studies detailing the endophenotypic characteristics of the diseases promise to provide the foundation for the circumvention of these impediments. This review serves to evaluate the various marketed and nonmarketed multi-targeted drugs with particular emphasis on their design strategy.

A genome-wide association study of antidepressant response in Koreans

We conducted a three-stage genome-wide association study (GWAS) of response to antidepressant drugs in an ethnically homogeneous sample of Korean patients in untreated episodes of nonpsychotic unipolar depression, mostly of mature onset. Strict quality control was maintained in case selection, diagnosis, verification of adherence and outcome assessments. Analyzed cases completed 6 weeks of treatment with adequate plasma drug concentrations. The overall successful completion rate was 85.5%. Four candidate single-nucleotide polymorphisms (SNPs) on three chromosomes were identified by genome-wide search in the discovery sample of 481 patients who received one of four allowed selective serotonin reuptake inhibitor (SSRI) antidepressant drugs (Stage 1). In a focused replication study of 230 SSRI-treated patients, two of these four SNP candidates were confirmed (Stage 2). Analysis of the Stage 1 and Stage 2 samples combined (n = 711) revealed GWAS significance (P = 1.60 × 10(-8)) for these two SNP candidates, which were in perfect linkage disequilibrium. These two significant SNPs were confirmed also in a focused cross-replication study of 159 patients treated with the non-SSRI antidepressant drug mirtazapine (Stage 3). Analysis of the Stage 1, Stage 2 and Stage 3 samples combined (n = 870) also revealed GWAS significance for these two SNPs, which was sustained after controlling for gender, age, number of previous episodes, age at onset and baseline severity (P = 3.57 × 10(-8)). For each SNP, the response rate decreased (odds ratio=0.31, 95% confidence interval: 0.20-0.47) as a function of the number of minor alleles (non-response alleles). The two SNPs significantly associated with antidepressant response are rs7785360 and rs12698828 of the AUTS2 gene, located on chromosome 7 in 7q11.22. This gene has multiple known linkages to human psychological functions and neurobehavioral disorders. Rigorous replication efforts in other ethnic populations are recommended.

Stress as a one-armed bandit: Differential effects of stress paradigms on the morphology, neurochemistry and behavior in the rodent amygdala

Neuroplasticity may be defined as the ability of the central nervous system (CNS) to respond to changes in the internal and external environment and it is well established that some stimuli have the ability to facilitate or impair neuroplasticity depending on the pre-existing milieu. A classic example of a stimulus that can both facilitate and impair neuroplasticity is stress. Indeed, the ability of CNS to respond to acute stress is often dependent upon the prior stress history of the individual. While responses to acute stress are often viewed as adaptive in nature, stress reactivity in subjects with prior chronic stress experiences are often linked to neuropsychiatric disorders, including major depressive disorder, post-traumatic stress disorder (PTSD) and anxiety. In rodent studies, chronic stress exposure produces structural and functional alterations in the hippocampus and medial prefrontal cortex that are consistent across different types of stress paradigms. Conversely, the amygdala appears to exhibit differential structural and functional responses to stress that are dependent on a variety of factors, including the type of stressor performed and the duration of the stress paradigm. This is most evident in output measures including morphological analysis of amygdala neurons, measurement of glutamatergic tone in amygdalar subdivisions and the analysis of amygdala-centric behaviors. Accordingly, this review will provide an overview of the effects of stress on the structural and functional plasticity of the rodent amygdala, especially in relation to the differential effects of repeated or chronic stress paradigms on dendritic architecture, neurochemistry of the glutamatergic system and behavior.

DSR-98776, a novel selective mGlu5 receptor negative allosteric modulator with potent antidepressant and antimanic activity

Modulation of monoaminergic systems has been the main stream of treatment for patients with mood disorders. However, recent evidence suggests that the glutamatergic system plays an important role in the pathophysiology of these disorders. This study pharmacologically characterized a structurally novel metabotropic glutamate 5 (mGlu5) receptor negative allosteric modulator, DSR-98776, and evaluated its effect on rodent models of depression and mania. First, DSR-98776 in vitro profile was assessed using intracellular calcium and radioligand binding assays. This compound showed dose-dependent inhibitory activity for mGlu5 receptors by binding to the same allosteric site as 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a known mGlu5 inhibitor. The in vivo therapeutic benefits of DSR-98776 were evaluated in common rodent models of depression and mania. In the rat forced swimming test, DSR-98776 (1-3mg/kg) significantly reduced rats immobility time after treatment for 7 consecutive days, while paroxetine (3 and 10mg/kg) required administration for 2 consecutive weeks to reduce rats immobility time. In the mouse forced swimming test, acute administration of DSR-98776 (10-30 mg/kg) significantly reduced immobility time. This effect was not influenced by 4-chloro-DL-phenylalanine methyl ester hydrochloride-induced 5-HT depletion. Finally, DSR-98776 (30 mg/kg) significantly decreased methamphetamine/chlordiazepoxide-induced hyperactivity in mice, which reflects this compound antimanic-like effect. These results indicate that DSR-98776 acts as an orally potent antidepressant and antimanic in rodent models and can be a promising therapeutic option for the treatment of a broad range of mood disorders with depressive and manic states.

Dissociation of glutamate and cortical thickness is restricted to regions subserving trait but not state markers in major depressive disorder

BACKGROUND

The anterior cingulate cortex (ACC) plays an important role in the neuropathology of major depressive disorder (MDD). So far, the effect of local cortical alteration on metabolites in multiple subdivisions of ACC has not been studied. We aimed to investigate structural and biochemical changes and their relationship in the pregenual ACC (pgACC), dorsal ACC (dACC) in MDD.

METHODS

We obtained magnetic resonance spectroscopy (MRS) in two investigated regions for 24 depressed patients and matched controls. In each region, cortical thickness (CTh) was calculated within a template mask based on its MRS voxel. We investigated neurotransmitter concentrations of Glx, N-acetyl aspartate (NAA), and myo-inositol (m-Ins) in two investigated regions, as well as their relationships with CTh in depressed individuals and healthy controls.

RESULTS

Patients showed significantly lower cortical thickness in dACC compared to controls. Glx in dACC significantly correlated with CTh in healthy controls but not MDD patients, while NAA and CTh in dACC significantly correlated in both groups. A marginal decrease of Glx in pgACC was found in the subgroup of more severely depressive patients, compared to the mildly depressed patients.

LIMITATIONS

Modest sample size and lack of episodes of depression may limit the generalizability of our findings.

CONCLUSION

Our results indicate an abolished CTh-MRS relation in dACC-associated with structural decline-but not in pgACC, where acute MRS alterations prevailed. Our study provides the first evidence of a neurochemical basis explaining some of the inter-individual variability in CTh in MDD.

An update of the classical and novel methods used for measuring fast neurotransmitters during normal and brain altered function

To understand better the cerebral functions, several methods have been developed to study the brain activity, they could be related with morphological, electrophysiological, molecular and neurochemical techniques. Monitoring neurotransmitter concentration is a key role to know better how the brain works during normal or pathological conditions, as well as for studying the changes in neurotransmitter concentration with the use of several drugs that could affect or reestablish the normal brain activity. Immediate response of the brain to environmental conditions is related with the release of the fast acting neurotransmission by glutamate (Glu), γ-aminobutyric acid (GABA) and acetylcholine (ACh) through the opening of ligand-operated ion channels. Neurotransmitter release is mainly determined by the classical microdialysis technique, this is generally coupled to high performance liquid chromatography (HPLC). Detection of neurotransmitters can be done by fluorescence, optical density, electrochemistry or other detection systems more sophisticated. Although the microdialysis method is the golden technique to monitor the brain neurotransmitters, it has a poor temporal resolution. Recently, with the use of biosensor the drawback of temporal resolution has been improved considerably, however other inconveniences have merged, such as stability, reproducibility and the lack of reliable biosensors mainly for GABA. The aim of this review is to show the important advances in the different ways to measure neurotransmitter concentrations; both with the use of classic techniques as well as with the novel methods and alternant approaches to improve the temporal resolution.

Dysfunctional glutamatergic and γ-aminobutyric acidergic activities in prefrontal cortex of mice in social defeat model of depression

BACKGROUND

Depression is a complex neuropsychiatric syndrome that is often very severe and life threatening. In spite of the remarkable progress in understanding the neural biology, the etiopathophysiology of depression is still elusive. In this study, we have investigated molecular mechanisms in the prefrontal cortex of mice showing depression-like phenotype induced by chronic defeat stress.

METHODS

Depression-like phenotype was induced in C57BL/6 mice by subjecting them to a 10-day social defeat paradigm. The metabolic activity of excitatory (glutamatergic) and inhibitory (γ-aminobutyric acid [GABA]ergic) neurons of the prefrontal cortex was measured by (1)H-[(13)C]-nuclear magnetic resonance spectroscopy together with infusion of [1,6-(13)C2]glucose. In addition, the expression level of genes associated with glutamatergic and GABAergic pathways was monitored by quantitative polymerase chain reaction.

RESULTS

Mice showing depression-like phenotype exhibit significant reduction in the levels of glutamate, glutamine, N-acetyl aspartate, and taurine in the prefrontal cortex. Most importantly, findings of reduced (13)C labeling of glutamate-C4, glutamate-C3, and GABA-C2 from [1,6-(13)C2]glucose indicate decreased glutamatergic and GABAergic neuronal metabolism and neurotransmitter cycling in the depressed mice. The reduced glutamine-C4 labeling suggests decreased neurotransmitter cycling in depression. Quantitative polymerase chain reaction analysis revealed reduced transcripts of Gad1 and Eaat2 genes, which code for enzymes involved in the synthesis of GABA and the clearance of glutamate from synapses, respectively.

CONCLUSIONS

These data indicate that the activities of glutamatergic and GABAergic neurons are reduced in mice showing a depression-like phenotype, which is supported by molecular data for the expression of genes involved in glutamate and GABA pathways.

Oxidative & nitrosative stress in depression: why so much stress?

Many studies support a crucial role for oxidative & nitrosative stress (O&NS) in the pathophysiology of unipolar and bipolar depression. These disorders are characterized inter alia by lowered antioxidant defenses, including: lower levels of zinc, coenzyme Q10, vitamin E and glutathione; increased lipid peroxidation; damage to proteins, DNA and mitochondria; secondary autoimmune responses directed against redox modified nitrosylated proteins and oxidative specific epitopes. This review examines and details a model through which a complex series of environmental factors and biological pathways contribute to increased redox signaling and consequently increased O&NS in mood disorders. This multi-step process highlights the potential for future interventions that encompass a diverse range of environmental and molecular targets in the treatment of depression.

Glutamate and choline levels predict individual differences in reading ability in emergent readers

Reading disability is a brain-based difficulty in acquiring fluent reading skills that affects significant numbers of children. Although neuroanatomical and neurofunctional networks involved in typical and atypical reading are increasingly well characterized, the underlying neurochemical bases of individual differences in reading development are virtually unknown. The current study is the first to examine neurochemistry in children during the critical period in which the neurocircuits that support skilled reading are still developing. In a longitudinal pediatric sample of emergent readers whose reading indicators range on a continuum from impaired to superior, we examined the relationship between individual differences in reading and reading-related skills and concentrations of neurometabolites measured using magnetic resonance spectroscopy. Both continuous and group analyses revealed that choline and glutamate concentrations were negatively correlated with reading and related linguistic measures in phonology and vocabulary (such that higher concentrations were associated with poorer performance). Correlations with behavioral scores obtained 24 months later reveal stability for the relationship between glutamate and reading performance. Implications for neurodevelopmental models of reading and reading disability are discussed, including possible links of choline and glutamate to white matter anomalies and hyperexcitability. These findings point to new directions for research on gene-brain-behavior pathways in human studies of reading disability.

Disturbance of the glutamatergic system in mood disorders

The role of glutamatergic system in the neurobiology of mood disorders draws increasing attention, as disturbance of this system is consistently implicated in mood disorders including major depressive disorder and bipolar disorder. Thus, the glutamate hypothesis of mood disorders is expected to complement and improve the prevailing monoamine hypothesis, and may indicate novel therapeutic targets. Since the contribution of astrocytes is found to be crucial not only in the modulation of the glutamatergic system but also in the maintenance of brain energy metabolism, alterations in the astrocytic function and neuroenergetic environment are suggested as the potential neurobiological underpinnings of mood disorders. In the present review, the evidence of glutamatergic abnormalities in mood disorders based on postmortem and magnetic resonance spectroscopy (MRS) studies is presented, and disrupted energy metabolism involving astrocytic dysfunction is proposed as the underlying mechanism linking altered energy metabolism, perturbations in the glutamatergic system, and pathogenesis of mood disorders.

Zinc, magnesium and NMDA receptor alterations in the hippocampus of suicide victims

BACKGROUND

There is evidence for an association between suicidal behavior and depression. Accumulating data suggests that depression is related to a dysfunction of the brain's glutamatergic system, and that the N-methyl-d-aspartate (NMDA) receptor plays an important role in antidepressant activity. Zinc and magnesium, the potent antagonists of the NMDA receptor complex, are involved in the pathophysiology of depression and exhibit antidepressant activity.

METHODS

The present study investigated the potency of Zn(2+) and Mg(2+) to [(3)H] MK-801, which binds to the NMDA receptor channel in the hippocampus of suicide victims (n=17) and sudden death controls (n=6). Moreover, the concentrations of zinc and magnesium (by flame atomic absorption spectrometry) and levels of NMDA subunits (NR2A and NR2B) and PSD-95 protein (by Western blotting) were determined.

RESULTS

Our results revealed that there was a statistically significant decrease (by 29% and 40%) in the potency of zinc and magnesium (respectively) to inhibit [(3)H] MK-801 binding to NMDA receptors in the hippocampus in suicide tissue relative to the controls. These alterations were associated with increased NR2A (+68%) and decreases in both the NR2B (-46%) and PSD-95 (-35%) levels. Furthermore, lower concentrations (-9%) of magnesium (although not of zinc) were demonstrated in suicide tissue.

CONCLUSIONS

Our findings indicate that alterations in the zinc, magnesium and NMDA receptor complex in the hippocampus are potentially involved in the pathophysiology of suicide-related disorders (depression), which may lead to functional NMDA receptor hyperactivity.

Contribution of methotrexate in precipitation of manic episode in bipolar affective disorder explored: a case report

Bipolar affective disorder is characterized by recurring episodes of mania with or without, but commonly with, episodes of depression. It usually begins in adolescence and can cause enduring and substantial impairment if left untreated. It needs a long-term treatment with mood stabilizers to prevent relapses. Elevated or depressed mood relapses can be either primary or secondary. However, primary mood relapses can occur without a significant precipitating factor, more often tending to occur following stressful life events or discontinuation of mood stabilizer medications. Secondary mood relapses can be caused by many conditions, such as physical illnesses, substance misuse and medications. When a mental illness coexists with another physical illness and the treatment of one complicates the other, it adds complexity to the selection of appropriate pharmacological regime for either condition. In this paper, the authors present a case of bipolar affective disorder who had two episodes of mania likely precipitated by methotrexate, which were reversed by the withdrawal of the offending drug (methotrexate). To the best of the authors' knowledge, to date there have been no published reports in the literature in which methotrexate, an immunosuppressive and a cytotoxic drug, precipitated a manic episode in a patient with bipolar affective disorder.

From pathophysiology to novel antidepressant drugs: glial contributions to the pathology and treatment of mood disorders

Several structural and cellular changes, including marked glial anomalies, have been observed in association with major depressive disorder. Here we review these cellular alterations and highlight the importance of glial cell pathology, especially astroglial dysfunction, in the pathophysiology of neuropsychiatric disorders with a particular interest in major depressive disorder. The functional role of astrocytes in glutamate uptake and glutamate/glutamine cycling is discussed, as is the deleterious effects of chronic stress on glial cell function. Lastly, we discuss the effect of antidepressants on glial cell function and the possibility of targeting glial cells in the quest to develop novel therapeutics.

Dysfunctional astrocytic regulation of glutamate transmission in a rat model of depression

Depression is usually associated with alterations in the monoaminergic system. However, new evidences suggest the involvement of the glutamatergic system in the aetiology of depression. Here we explored the glutamatergic system in a rat model of depression (i.e., the flinders sensitive line (FSL)) to reveal the mechanism underlying the emotional and cognitive aspects associated with the disease. We showed a dramatically elevated level of baseline glutamatergic synaptic transmission by whole-cell recordings as well as impairment in long-term potentiation induced by high-frequency stimulation in hippocampal slices from FSL rats compared with Sprague-Dawley rats. At behavioural level, FSL rats displayed recognition memory impairment in the novel object recognition test. Enantioselective chromatography analysis revealed lower levels of D-serine in the hippocampus of FSL rats and both synaptic plasticity and memory impairments were restored by administration of D-serine. We also observed dysfunctional astrocytic glutamate regulation including downregulation of the glia glutamate transporter GLAST as shown by western blot. One possibility is that the dysfunctional astrocytic glutamate reuptake triggers a succession of events, including the reduction of D-serine production as a safety mechanism to avoid NMDA receptor overactivation, which in turn causes the synaptic plasticity and memory impairments observed. These findings open up new brain targets for the development of more potent and efficient antidepressant drugs.

Glutamine deficiency in the prefrontal cortex increases depressive-like behaviours in male mice

BACKGROUND

The brain levels of glutamate (Glu) and glutamine (Gln) are partially regulated through the Glu-Gln cycle. Astrocytes play a role in regulating the Glu-Gln cycle, and loss of astrocytes has been associated with depressive disorders. We hypothesized that levels of Glu and Gln would be affected by astrocyte loss and dysregulation of the Glu-Gln cycle and that depressive-like behaviours would be closely related to the level of changes in Glu and Gln.

METHODS

We used liquid chromatography to measure Glu and Gln concentrations in the prefrontal cortex of male mice infused with L-α aminoadipic acid (L-AAA), a specific astrocyte toxin, in the prelimbic cortex. Methionine sulfoximine, a Gln synthetase inhibitor, and α-methyl-amino-isobutyric acid, a blocker of neuronal Gln transporters, were used to disturb the Glu-Gln cycle. We assessed the behavioural change by drug infusion using the forced swim test (FST) and sucrose preference test.

RESULTS

The Glu and Gln levels were decreased on the fifth day after L-AAA infusion, and the infused mice showed longer durations of immobility in the FST and lower sucrose preference, indicative of depressive-like behaviour. Mice in which Gln synthetase or Gln transport were inhibited also exhibited increased immobility in the FST. Direct infusion of L-Gln reversed the increased immobility induced by astrocyte ablation and Glu-Gln cycle impairments.

LIMITATIONS

Genetically modified animal models and diverse behavioural assessments would have been helpful to solidify our conclusions.

CONCLUSION

Neuronal Gln deficiency in the prefrontal cortex may cause depressive behaviours.

The same antidepressant elicits contrasting patterns of synaptic changes in the amygdala vs hippocampus

As depression-like symptoms are often precipitated by some form of stress, animal models of stress have been used extensively to investigate cellular mechanisms of depression. Despite being implicated in the emotional symptoms of depression, the amygdala has received little attention compared to the hippocampus in the past studies of antidepressant action. Further, these investigations have not taken into account the contrasting effects of chronic stress on the hippocampus vs amygdala. If an antidepressant is to be equally effective in countering the differential effects of stress on both brain areas, then it is faced with the challenge of eliciting contrasting effects in these two structures. We tested this prediction by examining the impact of tianeptine, an antidepressant with proven clinical efficacy, on neurons of the lateral amygdala (LA) and hippocampal area CA1. Tianeptine reduces N-methyl-D-aspartate (NMDA)-receptor-mediated synaptic currents, without affecting α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) currents, in LA neurons. By contrast, tianeptine enhances both NMDA and AMPA currents in area CA1. Tianeptine also lowers action potential firing in LA neurons. As tianeptine modulates cellular metrics that, in addition to mediating amygdalar behavioral output, are also affected by stress, we tested if tianeptine succeeds in countering stress effects in the intact animal. We find that tianeptine prevents two important functional consequences of chronic stress-induced plasticity in the amygdala--dendritic growth and enhanced anxiety-like behavior. These results provide evidence for antidepressant action on amygdalar neurons that are not only distinct from the hippocampus, but also protect against the debilitating impact of stress on amygdalar structure and function.

Genetic association study of individual symptoms in depression

The heritability of some individual depressive symptoms has been well established. However, the causal genes related to individual depressive symptoms and genetic effects on the courses of individual depressive symptoms are still unclear. We examined these issues in 241 Korean patients who met the DSM-IV-TR criteria for major depression. Patients entered a 12-week clinical trial with antidepressants. A total of 1399 single-nucleotide polymorphisms (SNPs) of 79 candidate genes were assessed. The rs557762 and the TT haplotype in the 11th haplotype block of the GRIA3 gene were associated with feelings of guilt in females. The GGCCGGGC haplotype in the first haplotype block of TPH1 was significantly associated with middle insomnia. The ACAG haplotype in the 13th haplotype block of the GRIK2 gene was associated with somatic anxiety. Moreover, the effect of the rs557762 on guilt significantly varied across times. Our results indicate that there are associations between particular gene polymorphisms and some individual depressive symptoms. These results could contribute to understanding the biological mechanisms of depression.

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.

Glutamatergic and GABAergic metabolism in mouse brain under chronic nicotine exposure: implications for addiction

Background and Purpose

The effects of nicotine on cerebral metabolism and its influence on smoking behavior is poorly understood. An understanding of the chronic effects of nicotine on excitatory and inhibitory metabolic demand, and corresponding neurotransmission may provide clues for designing strategies for the optimal smoking cessation intervention. The objective of the current study was to investigate neuronal and astroglial metabolism in mice exposed to nicotine (0.5 and 2.0 mg/kg, sc) three times in a day for 4 weeks.

Experimental Approach/Principal Findings

Metabolic measurements were carried out by co-infusing [U-¹³C₆]glucose and [2-¹³C]acetate, and monitoring ¹³C labeling of amino acids in brain tissue extract using ¹H-[¹³C] and ¹³C-[¹H]-NMR spectroscopy. Concentration of ¹³C-labeled glutamate-C4 was increased significantly from glucose and acetate with chronic nicotine treatment indicating an increase in glucose oxidation by glutamatergic neurons in all brain regions and glutamate-glutamine neurotransmitter cycle in cortical and subcortical regions. However, chronic nicotine treatment led to increased labeling of GABA-C2 from glucose only in the cortical region. Further, increased labeling of glutamine-C4 from [2-¹³C]acetate is suggestive of increased astroglial activity in subcortical and cerebellum regions of brain with chronic nicotine treatment.

Conclusions and Significance

Chronic nicotine exposure enhanced excitatory activity in the majority of brain regions while inhibitory and astroglial functions were enhanced only in selected brain regions.

Fluvoxamine increased glutamate release by activating both 5-HT(3) and sigma-1 receptors in prelimbic cortex of chronic restraint stress C57BL/6 mice

Emerging evidence from therapeutic trials in humans and animal models suggests that in the treatment of depression, antidepressants play a role by targeting the glutamatergic system. Fluvoxamine is one of the widely used SSRIs which has been considered to target monoamine neurotransmitter reuptake mechanisms. However, whether fluvoxamine has an effect on the glutamate release is still unclear. The present experiment studied the effect of fluvoxamine on presynaptic glutamate release in prelimbic cortex, both in control C57BL/6 mice and chronic restraint stress C57BL/6 mice, and further investigated the mechanism underlying this effect by using patch clamp, on-line fluorimetry, pharmacological approaches combined with other techniques. The results showed that fluvoxamine increased the glutamate release in the depression model mice but it had no effect on the glutamate release in the control mice. The mechanism underlying these effects in depression model mice was that, fluvoxamine firstly activated presynaptic 5-HT(3) receptors, which transiently increased the Ca(2+) concentration. The increase of Ca(2+) concentration via 5-HT(3) receptors caused the activation of sigma-1 receptors, which were activated by fluvoxamine. The activation of sigma-1 receptors increased the intrasynaptosomal Ca(2+) concentration significantly through the outflow of endoplasmic reticulum calcium and finally activated PKC. These results suggested that fluvoxamine may have a selective effect and different mechanism based on the condition of animal.

The PSD-95/nNOS complex: new drugs for depression?

Drug treatment of major depressive disorder is currently limited to the use of agents which influence monoaminergic neuronal transmission including inhibitors of presynaptic transporters and monoamine oxidase. Typically improvement in depressive symptoms only emerges after several weeks of treatment, suggesting that downstream neuronal adaptations rather than the elevation in synaptic monoamine levels are responsible for antidepressant effects. In recent years, the NMDA receptor has emerged as a promising target for treating CNS disorders including stroke, pain and depression. In this review, we outline the molecular mechanisms underlying NMDA receptor signalling in neurons and in particular provide an overview of the role of the NMDAR/PSD-95/nNOS complex in CNS disorders. We discuss novel drug developments made that suggest the NMDAR/PSD-95/nNOS complex as a potential target for the treatment of depression. The review also provides examples of how PDZ-based protein-protein interactions can be exploited as novel drug targets for disease.

Investigational NMDA receptor modulators for depression

INTRODUCTION

With regards to depression, the role of N-methyl-D-aspartate receptor (NMDA) was pursued many years ago, mainly in the form of preclinical studies. Since then, there have been several clinical data in the literature indicating the efficacy of NMDA receptor antagonists of either stand-alone or as an adjunct therapy in depression and depression-related diseases.

AREAS COVERED

The present review focuses on clinical data of well-known and recently discovered NMDA receptor antagonists/modulators and their mechanisms of action.

EXPERT OPINION

Several NMDA receptor modulators have been tested in both human and animal studies to examine their potential antidepressant activity. Most of the compounds that exhibited beneficial properties in the animal tests and models of depression either have never been tested or did not show efficacy in humans. For some of them, such as ketamine, where a consistently reproducible antidepressant effect was found, clinical use is limited by a variety of adverse effects. However, ketamine has become a standard tool for identifying the biological factors associated with rapid antidepressant action and, as such, is a novel target for the development of new therapeutics.

Proton magnetic resonance spectroscopy in depression

Magnetic Resonance Spectroscopy (MRS) is a unique technique that can directly assess the concentration of various biochemical metabolites in the brain. Thus, it is used in the study of molecular pathophysiology of different neuropsychiatric disorders, such as, the major depressive disorder and has been an area of active research. We conducted a computer-based literature search using the Pubmed database with 'magnetic resonance spectroscopy', 'MRS', 'depression', and 'major depressive disorder' as the key words, supplemented by a manual search of bibliographic cross-referencing. Studies in depression report abnormalities in the frontal cortex, basal ganglia, hippocampus, anterior cingulate cortex, and the occipital cortex. These abnormalities improve after treatment with selective serotonin reuptake inhibitor, electroconvulsive therapy, and yoga, and thus, are possibly state-dependent. The findings are consistent with other morphometric and clinical studies and support the proposed pathophysiological theory of dysfunction in the neuronal circuits involving the frontal cortex, limbic cortex, and basal ganglia. Spectroscopy also has potential implications in predicting the response to treatment and formulating individualized pharmacotherapy.

Altered expression of glutamate signaling, growth factor, and glia genes in the locus coeruleus of patients with major depression

Several studies have proposed that brain glutamate signaling abnormalities and glial pathology have a role in the etiology of major depressive disorder (MDD). These conclusions were primarily drawn from post-mortem studies in which forebrain brain regions were examined. The locus coeruleus (LC) is the primary source of extensive noradrenergic innervation of the forebrain and as such exerts a powerful regulatory role over cognitive and affective functions, which are dysregulated in MDD. Furthermore, altered noradrenergic neurotransmission is associated with depressive symptoms and is thought to have a role in the pathophysiology of MDD. In the present study we used laser-capture microdissection (LCM) to selectively harvest LC tissue from post-mortem brains of MDD patients, patients with bipolar disorder (BPD) and from psychiatrically normal subjects. Using microarray technology we examined global patterns of gene expression. Differential mRNA expression of select candidate genes was then interrogated using quantitative real-time PCR (qPCR) and in situ hybridization (ISH). Our findings reveal multiple signaling pathway alterations in the LC of MDD but not BPD subjects. These include glutamate signaling genes, SLC1A2, SLC1A3 and GLUL, growth factor genes FGFR3 and TrkB, and several genes exclusively expressed in astroglia. Our data extend previous findings of altered glutamate, astroglial and growth factor functions in MDD for the first time to the brainstem. These findings indicate that such alterations: (1) are unique to MDD and distinguishable from BPD, and (2) affect multiple brain regions, suggesting a whole-brain dysregulation of such functions.