The involvement of glutamate in the pathophysiology of depression

Mechanism of lily bulb and Rehmannia decoction in the treatment of lipopolysaccharide-induced depression-like rats based on metabolomics study and network pharmacology

CONTEXT

Lily bulb and Rehmannia decoction (LBRD), consisting of Lilium henryi Baker (Liliaceae) and Rehmannia glutinosa (Gaertn) DC (Plantaginaceae), is a specialized traditional Chinese medicine formula for treating depression. However, the underlying mechanisms, especially the relationship between LBRD efficacy and metabolomics, remains unclear.

OBJECTIVE

This study was aimed to investigate the metabolic mechanism of LBRD in treating depression.

MATERIALS AND METHODS

Network pharmacology was conducted using SwissTargetPrediction, DisGeNET, DrugBank, Metascape, etc., to construct component-target-pathway networks. The depression-like model was induced by intraperitoneal injection with lipopolysaccharide (LPS) (0.3 mg/kg) for 14 consecutive days. After the administration of LBRD (90 g/kg) and fluoxetine (2 mg/kg) for 14 days, we assessed behaviour and the levels of neurotransmitter, inflammatory cytokine and circulating stress hormone. Prefrontal metabolites of rats were detected by using liquid chromatography-mass spectrometry metabolomics method.

RESULTS

The results of network pharmacology showed that LBRD mainly acted on neurotransmitter and second messenger pathways. Compared to the model group, LBRD significantly ameliorated depressive phenotypes and increased the level of 5-HT (13.4%) and GABA (24.8%), as well as decreased IL-1β (30.7%), IL-6 (32.8%) and TNF-α (26.6%). Followed by LBRD treatment, the main metabolites in prefrontal tissue were contributed to retrograde endocannabinoid signalling, glycerophospholipid metabolism, glycosylphosphatidylinositol-anchor biosynthesis, autophagy signal pathway, etc.

DISCUSSION AND CONCLUSIONS

LBRD were effective at increasing neurotransmitter, attenuating proinflammatory cytokine and regulating glycerophospholipid metabolism and glutamatergic synapse, thereby ameliorating depressive phenotypes. This research will offer reference for elucidating the metabolomic mechanism underlying novel antidepressant agents contained LBRD formula.

Noise Induced Depression-Like Behavior, Neuroinflammation and Synaptic Plasticity Impairments: The Protective Effects of Luteolin

Noise is a kind of sound that causes agitation and harms human health. Studies have shown that noise can lead to neuroinflammation, damage to synaptic plasticity and altered levels of neurotransmitters that may result in depression. The present study demonstrated that luteolin exerted antidepressant-like effects by improving neuroinflammation in a mouse model of noise-induced depression. Luteolin significantly alleviated noise-induced depression-like behavior. Notably, luteolin treatment not only remarkably ameliorated noise-induced inflammation in the hippocampus and prefrontal cortex, but also increased synapsin. Furthermore, luteolin treatment significantly increased the contents of serum 5-hydroxytryptamine and norepinephrine in noise-induced mice. In sum, luteolin exerts antidepressant effects indepression-like mice caused by noise, which can serve as a potential agent for the treatment of chronic noise-induced depression.

Scrutinizing the Therapeutic Promise of Purinergic Receptors Targeting Depression

Antidepressant use has resulted in a variety of negative consequences, including permanent brain damage and erectile dysfunction. So, the purpose lies in developing something more productive with minimal side effects and consequently improved efficacy. A growing body of evidences indicated a remarkable purinergic signalling system, which helped in dealing with this complication. This has been found to be a powerful formula in dealing with psychiatric disorders. P1 (adenosine), P2X, and P2Y (ATP) are the receptors, involved in the pathology as well as exhibiting the therapeutic action by triggering the purinergic pathway. It was found that A2A and P2X7 receptors specifically were involved and recognized as possible targets for treating depression. Further, the development of biomarkers for the diagnosis of depression has also been attributed to accelerate the process. One such biomarker includes serum uric acid. Many clinical studies reveal the importance of antagonizing P2X7 and A2A receptors, for promising research in understanding the molecular premises of depression. However, further investigations are still needed to be done to open several unfolded mysteries for a better and safe upshot. The selective antagonists for A2A and P2X7 receptors may have antidepressant effects showing positive results, in agreement with non-clinical testing. In this review, efforts are being devoted to the targeted receptors in bringing out antidepressant effects with a possible link involving depression and defined purinergic signalling. Additionally, the overview of various receptors, including their functions and distribution, is being explored in a representative way along with the biomarkers involved.

Dissecting the shared genetic basis of migraine and mental disorders using novel statistical tools

Migraine is three times more prevalent in people with bipolar disorder or depression. The relationship between schizophrenia and migraine is less certain although glutamatergic and serotonergic neurotransmission are implicated in both. A shared genetic basis to migraine and mental disorders has been suggested but previous studies have reported weak or non-significant genetic correlations and five shared risk loci. Using the largest samples to date and novel statistical tools, we aimed to determine the extent to which migraine’s polygenic architecture overlaps with bipolar disorder, depression and schizophrenia beyond genetic correlation, and to identify shared genetic loci.

Summary statistics from genome-wide association studies were acquired from large-scale consortia for migraine (n cases = 59 674; n controls = 316 078), bipolar disorder (n cases = 20 352; n controls = 31 358), depression (n cases = 170 756; n controls = 328 443) and schizophrenia (n cases = 40 675, n controls = 64 643). We applied the bivariate causal mixture model to estimate the number of disorder-influencing variants shared between migraine and each mental disorder, and the conditional/conjunctional false discovery rate method to identify shared loci. Loci were functionally characterized to provide biological insights.

Univariate MiXeR analysis revealed that migraine was substantially less polygenic (2.8 K disorder-influencing variants) compared to mental disorders (8100–12 300 disorder-influencing variants). Bivariate analysis estimated that 800 (SD = 300), 2100 (SD = 100) and 2300 (SD = 300) variants were shared between bipolar disorder, depression and schizophrenia, respectively. There was also extensive overlap with intelligence (1800, SD = 300) and educational attainment (2100, SD = 300) but not height (1000, SD = 100). We next identified 14 loci jointly associated with migraine and depression and 36 loci jointly associated with migraine and schizophrenia, with evidence of consistent genetic effects in independent samples. No loci were associated with migraine and bipolar disorder. Functional annotation mapped 37 and 298 genes to migraine and each of depression and schizophrenia, respectively, including several novel putative migraine genes such as L3MBTL2, CACNB2 and SLC9B1. Gene-set analysis identified several putative gene sets enriched with mapped genes including transmembrane transport in migraine and schizophrenia.

Most migraine-influencing variants were predicted to influence depression and schizophrenia, although a minority of mental disorder-influencing variants were shared with migraine due to the difference in polygenicity. Similar overlap with other brain-related phenotypes suggests this represents a pool of ‘pleiotropic’ variants that influence vulnerability to diverse brain-related disorders and traits. We also identified specific loci shared between migraine and each of depression and schizophrenia, implicating shared molecular mechanisms and highlighting candidate migraine genes for experimental validation.

Association between thiopurine exposure and depression in patients with inflammatory bowel disease and rheumatoid arthritis

BACKGROUND

Ras-related C3 botulinum substrate 1 (Rac1) is a member of the small molecule family Rho guanosine triphosphate (GTP)ases. Recent findings reveal epigenetic downregulation of Rac1 is a mechanism of depression.

AIMS

The purpose of this study was to evaluate Rac1 as a therapeutic target for depression we examine the association between thiopurines, which inhibit Rac1, and the risk of depression among US veterans.

METHODS

This study uses data spanning January 2000-May 2019, comparing thiopurine exposure (no exposure, less than one year, 1-2.9 years, 3-5 years, and greater than five years) in two separate cohorts, a rheumatoid arthritis cohort and inflammatory bowel disease cohort. We estimate the hazard of depression using a time dependent cox proportional hazards model.

RESULTS

A total of 76,763 rheumatoid arthritis and 46,787 inflammatory bowel disease patients met all inclusion criteria. Patients exposed to thiopurines less than one year have a 27% (hazard ratio=1.272; 95% confidence interval=(1.038-1.559)) and 67% (hazard ratio=1.667 95% confidence interval=(1.501-1.850)) higher risk of depression in the rheumatoid arthritis and inflammatory bowel disease cohorts, respectively. In the inflammatory bowel disease cohort, we find the risk of depression is increased for up to five years of thiopurine exposure.

CONCLUSION

These results provide evidence that Rac1 regulation is a viable therapeutic target for depression. Further research into therapeutics targeting Rac1 for the treatment of depression is warranted.

Urine metabolomic responses to aerobic and resistance training in rats under chronic unpredictable mild stress

Background

It is known that bioenergetics of aerobic and resistance exercise are not the same but both can effectively improve depression. However, it is not clear whether and how different types of exercise can influence depression through the same metabolic regulatory system. Metabolomics provides a way to study the correlation between metabolites and changes in exercise and/or diseases through the quantitative analysis of all metabolites in the organism. The objective of this study was to investigate the effects of aerobic and resistance training on urinary metabolites by metabolomics analysis in a rodent model of depression.

Methods

Male Sprague-Dawley rats were given chronic unpredictable mild stress (CUMS) for eight weeks. The validity of the modeling was assessed by behavioral indices. After four weeks of CUMS, the rats that developed depression were randomly divided into a depression control group, an aerobic training group and a resistance training group. There was also a normal control group. From week 5, the rats in the exercise groups were trained for 30 min per day, five days per week, for four weeks. The urine samples were collected pre and post the training program, and analyzed by proton nuclear magnetic resonance (¹H-NMR) spectroscopy.

Results

Both types of training improved depression-like behavior in CUMS rats. Compared with normal control, 21 potential biomarkers were identified in the urine of CUMS rats, mainly involved in energy, amino acids and intestinal microbial metabolic pathways. Common responses to the training were found in the two exercise groups that the levels of glutamine, acetone and creatine were significantly recalled (all P<0.05) Aerobic training also resulted in changes in pyruvate and trigonelline, while resistance training modified α-Oxoglutarate, citric acid, and trimethylamine oxide (all P<0.05).

Conclusions

Aerobic and resistance training resulted in common effects on the metabolic pathways of alanine-aspartate-glutamate, TCA cycle, and butyric acid. Aerobic training also had effects on glycolysis or gluconeogenesis and pyruvate metabolism, while resistance training had additional effect on intestinal microbial metabolism.

Prophylactic effect of flavanol rich preparation metabolites in promoting resilience to a mouse model of social stress

Major depressive disorder (MDD) is a leading cause of disability, and there is an urgent need for new therapeutics. Stress-mediated induction of pro-inflammation in the periphery contributes to depression-like behaviors both in humans and in experimental models. Inflammatory cytokine interleukin-6 (IL-6) has emerged as a potential therapeutic target. Our studies demonstrated that metabolism of flavanol rich cocoa preparation (FRP) led to the accumulation of select phenolic acids that may contribute to its anti-inflammatory activity. Using a repeated social defeat stress (RSDS) model of depression, we showed that oral administration of FRP attenuates susceptibility to RSDS-mediated depression, supporting the further development of FRP as a novel therapeutic for the treatment of stress disorders and anxiety in humans.

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.

Epigenetic modulation of inflammation and synaptic plasticity promotes resilience against stress in mice

Major depressive disorder is associated with abnormalities in the brain and the immune system. Chronic stress in animals showed that epigenetic and inflammatory mechanisms play important roles in mediating resilience and susceptibility to depression. Here, through a high-throughput screening, we identify two phytochemicals, dihydrocaffeic acid (DHCA) and malvidin-3′-O-glucoside (Mal-gluc) that are effective in promoting resilience against stress by modulating brain synaptic plasticity and peripheral inflammation. DHCA/Mal-gluc also significantly reduces depression-like phenotypes in a mouse model of increased systemic inflammation induced by transplantation of hematopoietic progenitor cells from stress-susceptible mice. DHCA reduces pro-inflammatory interleukin 6 (IL-6) generations by inhibiting DNA methylation at the CpG-rich IL-6 sequences introns 1 and 3, while Mal-gluc modulates synaptic plasticity by increasing histone acetylation of the regulatory sequences of the Rac1 gene. Peripheral inflammation and synaptic maladaptation are in line with newly hypothesized clinical intervention targets for depression that are not addressed by currently available antidepressants.

Polyphenols have partial antidepressant effect without known mechanism. Here, the authors identify two phytochemicals from bioactive dietary polyphenols, show their antidepressant effect in a rodent model of depression, and that this effect is mediated by epigenetic and anti-inflammatory mechanisms.

Discovery, screening and evaluation of a plasma biomarker panel for subjects with psychological suboptimal health state using (1)H-NMR-based metabolomics profiles

Individuals in the state of psychological suboptimal health keep increasing, only scales and questionnaires were used to diagnose in clinic under current conditions, and symptoms of high reliability and accuracy are destitute. Therefore, the noninvasive and precise laboratory diagnostic methods are needed. This study aimed to develop an objective method through screen potential biomarkers or a biomarker panel to facilitate the diagnosis in clinic using plasma metabolomics. Profiles were based on H-nuclear magnetic resonance ((1)H-NMR) metabolomics techniques combing with multivariate statistical analysis. Furthermore, methods of correlation analysis with Metaboanalyst 3.0 for selecting a biomarker panel, traditional Chinese medicine (TCM) drug intervention for validating the close relations between the biomarker panel and the state and the receiver operating characteristic curves (ROC curves) analysis for evaluation of clinical diagnosis ability were carried out. 9 endogenous metabolites containing trimethylamine oxide (TMAO), glutamine, N-acetyl-glycoproteins, citrate, tyrosine, phenylalanine, isoleucine, valine and glucose were identified and considered as potential biomarkers. Then a biomarker panel consisting of phenylalanine, glutamine, tyrosine, citrate, N-acetyl-glycoproteins and TMAO was selected, which exhibited the highest area under the curve (AUC = 0.971). This study provided critical insight into the pathological mechanism of psychological suboptimal health and would supply a novel and valuable diagnostic method.

Metabolomic analysis reveals metabolic disturbances in the prefrontal cortex of the lipopolysaccharide-induced mouse model of depression

Major depressive disorder (MDD) is a debilitating illness. However, the underlying molecular mechanisms of depression remain largely unknown. Increasing evidence supports that inflammatory cytokine disturbances may be associated with the pathophysiology of depression in humans. Systemic administration of lipopolysaccharide (LPS) has been used to study inflammation-associated neurobehavioral changes in rodents, but no metabonomic study has been conducted to assess differential metabolites in the prefrontal cortex (PFC) of a LPS-induced mouse model of depression. Here, we employed a gas chromatography-mass spectrometry-based metabonomic approach in the LPS-induced mouse model of depression to investigate any significant metabolic changes in the PFC. Multivariate statistical analysis, including principal component analysis (PCA), partial least squares-discriminate analysis (PLS-DA), and pair-wise orthogonal projections to latent structures discriminant analysis (OPLS-DA), was implemented to identify differential PFC metabolites between LPS-induced depressed mice and healthy controls. A total of 20 differential metabolites were identified. Compared with control mice, LPS-treated mice were characterized by six lower level metabolites and 14 higher level metabolites. These molecular changes were closely related to perturbations in neurotransmitter metabolism, energy metabolism, oxidative stress, and lipid metabolism, which might be evolved in the pathogenesis of MDD. These findings provide insight into the pathophysiological mechanisms underlying MDD and could be of valuable assistance in the clinical diagnosis of MDD.

Plasma-metabolite-biomarkers for the therapeutic response in depressed patients by the traditional Chinese medicine formula Xiaoyaosan: A (1)H NMR-based metabolomics approach

BACKGROUND

Depression is one of the most prevalent and serious mental disorders. Xiaoyaosan, a well-known Chinese prescription, has been widely used for the treatment of depression in China. Both clinical studies and animal experiments indicate that Xiaoyaosan has an obvious antidepressant activity. Additionally, a large number of candidate biomarkers have emerged that can be used for early disease detection and for monitoring ongoing treatment response to therapy because of their correlations with the characteristics of the disease. However, there have been few reports on biomarkers that measure the treatment response to the clinical use of Xiaoyaosan using a metabolomics approach. The current study is aimed at discovering biomarkers and biochemical pathways to facilitate the diagnosis of depression and the efficient evaluation of Xiaoyaosan using plasma metabolomics profiles based on (1)H NMR.

METHODS

Sixteen depressed patients diagnosed by standard methods (HAMD and CGI-SI) and sixteen healthy volunteers were recruited. (1)H NMR-based metabolomics techniques and multivariate statistical methods were used to analyze the plasma metabolites of the depressed patients before and after treatment and to compare them with healthy controls.

RESULTS

The plasma levels of trimethylamine oxide, glutamine and lactate in depressed patients increased significantly (p≤0.05) compared with healthy controls, whereas the levels of phenylalanine, valine, alanine, glycine, leucine, citrate, choline, lipids and glucose decreased significantly (p≤0.05). Additionally, alanine, choline, trimethylamine oxide, glutamine, lactate and glucose were returned to normal levels after Xiaoyaosan treatment. These statistically significant perturbations are involved in energy metabolism, amino acid metabolism and gut microbiota metabolism.

LIMITATIONS

Additional experimentation with gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-mass spectrometry (LC-MS) is required to confirm our findings.

CONCLUSIONS

Application of these biomarkers in clinical practice may help to optimize the diagnosis of depression and to evaluate the efficacy of Xiaoyaosan. Metabolomics is promising as a biomarker discovery tool.

Doxepin and imipramine but not fluoxetine reduce the activity of the rat glutamate transporter EAAT3 expressed in Xenopus oocytes

Background

Many researchers have suggested that the glutamatergic system may be involved in the effects of antidepressant therapies. We investigated the effects of doxepin, imipramine, and fluoxetine on the excitatory amino acid transporter type 3 (EAAT3).

Methods

EAAT3 was expressed in Xenopus oocytes by injection of EAAT3 mRNA. Membrane currents were recorded after application of L-glutamate (30 μM) in the presence or absence of various concentrations of doxepin, imipramine, and fluoxetine. To study the effects of protein kinase C (PKC) activation on EAAT3 activity, oocytes were pre-incubated with phorbol 12-myristate-13-acetate (PMA) before application of imipramine and doxepin.

Results

Doxepin at 0.063–1.58 μM significantly decreased EAAT3 activity. Imipramine reduced EAAT3 activity in a concentration-dependent manner at 0.16–0.95 μM. However, fluoxetine did not affect EAAT3 activity, and PMA increased EAAT3 activity. At 0.32 μM, imipramine caused an equivalent decrease in EAAT3 activity in the presence or absence of PMA. However, 0.79 μM doxepin did not abolish the enhancement of EAAT3 activity by PMA.

Conclusions

We showed that doxepin and imipramine, but not fluoxetine, inhibited EAAT3 activity at clinically relevant concentrations. This reveals a novel mechanism of action for doxepin and imipramine; that they increase glutamatergic neurotransmission. PKC may be involved in the effects of doxepin on EAAT3, but is not involved in the effects of imipramine at the concentrations studied.

Corticosterone mediates the synaptic and behavioral effects of chronic stress at rat hippocampal temporoammonic synapses

Chronic stress is thought to impart risk for depression via alterations in brain structure and function, but contributions of specific mediators in generating these changes remain unclear. We test the hypothesis that stress-induced increases in corticosterone (CORT), the primary rodent glucocorticoid, are the key mediator of stress-induced depressive-like behavioral changes and synaptic dysfunction in the rat hippocampus. In rats, we correlated changes in cognitive and affective behavioral tasks (spatial memory consolidation, anhedonia, and neohypophagia) with impaired excitatory strength at temporoammonic-CA1 (TA-CA1) synapses, an archetypical stress-sensitive excitatory synapse. We tested whether elevated CORT was sufficient and necessary to generate a depressive-like behavioral phenotype and decreased excitatory signaling observed at TA-CA1 after chronic unpredictable stress (CUS). Chronic CORT administration induced an anhedonia-like behavioral state and neohypophagic behavior. Like CUS, chronic, but not acute, CORT generated an impaired synaptic phenotype characterized by reduced α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-preferring glutamate receptor-mediated excitation at TA-CA1 synapses, decreased AMPA-type glutamate receptor subunit 1 protein expression, and altered serotonin-1B receptor-mediated potentiation. Repeatedly blunting stress-induced increases of CORT during CUS with the CORT synthesis inhibitor metyrapone (MET) prevented these stress-induced neurobehavioral changes. MET also prevented the CUS-induced impairment of spatial memory consolidation. We conclude that corticosterone is sufficient and necessary to mediate glutamatergic dysfunction underlying stress-induced synaptic and behavioral phenotypes. Our results indicate that chronic excessive glucocorticoids cause specific synaptic deficits in the hippocampus, a major center for cognitive and emotional processing, that accompany stress-induced behavioral dysfunction. Maintaining excitatory strength at stress-sensitive synapses at key loci throughout corticomesolimbic reward circuitry appears critical for maintaining normal cognitive and emotional behavior.

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

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

Synthesis and SAR of substituted pyrazolo[1,5-a]quinazolines as dual mGlu(2)/mGlu(3) NAMs

Herein we report the design and synthesis of a series of substituted pyrazolo[1,5-a]quinazolin-5(4H)-ones as negative allosteric modulators of metabotropic glutamate receptors 2 and 3 (mGlu2 and mGlu3, respectively). Development of this series was initiated by reports that pyrazolo[1,5-a]quinazoline-derived scaffolds can yield compounds with activity at group II mGlu receptors which are prone to molecular switching following small structural changes. Several potent analogues, including 4-methyl-2-phenyl-8-(pyrimidin-5-yl)pyrazolo[1,5-a]quinazolin-5(4H)-one (10b), were discovered with potent in vitro activity as dual mGlu2/mGlu3 NAMs, with excellent selectivity versus the other mGluRs.

Do we need pharmacogenetics to personalize antidepressant therapy?

This review examines the role of drug metabolism and drug target polymorphism in determining the clinical response to antidepressants. Even though antidepressants are the most effective available treatment for depressive disorders, there is still substantial need for improvement due to the slow onset of appreciable clinical improvement and the association with side effects. Moreover, a substantial group of patients receiving antidepressant therapy does not achieve remission or fails to respond entirely. Even if the large variation in antidepressant treatment outcome across individuals remains poorly understood, one possible source of this variation in treatment outcome are genetic differences. The review focuses on a few polymorphisms which have been extensively studied, while reporting a more comprehensive reference to the existing literature in table format. It is relatively easy to predict the effect of polymorphisms in drug metabolizing enzymes, such as cytochromes P450 2D6 (CYP2D6) and cytochrome P450 2C19 (CYP2C19), which may be determined in the clinical context in order to explain or prevent serious adverse effects. The role of target polymorphism, however, is much more difficult to establish and may be more relevant for disease susceptibility and presentation rather than for response to therapy.

Stress- and antidepressant treatment-induced modifications of 5-HT₇ receptor functions in the rat brain

This paper summarizes a series of electrophysiological studies aimed at finding the effects of the activation of 5-HT(7) receptors on neuronal excitability as well as on excitatory and inhibitory synaptic transmission in the hippocampus and in the frontal cortex of the rat. These studies demonstrated that 5-HT(7) receptors play an important role in the modulation of the activity of the hippocampal network by regulating the excitability of pyramidal cells of the CA1 area, as well as via their effect on GABA and glutamatergic transmission. The reactivity of 5-HT(7) receptors in the hippocampus is decreased by repeated administration of antidepressant drugs and increased by a prolonged high level of corticosterone. More importantly, administration of antidepressant drug, imipramine, prevents the occurrence of corticosterone-induced changes in the function of hippocampal 5-HT(7) receptors. It has also been found that the blockade of 5-HT(7) receptors by the selective antagonist SB 269970, lasting for a few days, causes similar changes to those observed after long-term administration of antidepressants. Thus, it seems that the pharmacological blockade of 5-HT(7) receptors produces faster effects compared to classic antidepressant drugs. A similarity between the changes in the glutamatergic transmission induced by the blockade of 5 HT7 receptors and those caused by repeated administration of the antidepressant drug, imipramine, has also been found in the frontal cortex. It has also been shown that the changes in glutamatergic transmission and the impairment of long-term synaptic plasticity in the frontal cortex of animals subjected to repeated restraint stress are reversed by the blockade of 5-HT(7) receptors. Overall, these studies, together with the data provided by other investigators, support the hypothesis that 5-HT(7) receptor antagonists may become a prototype of a new class of antidepressant drugs. Such compounds will not function by blocking 5-HT reuptake, as many of the currently used drugs, but through a direct interaction with the 5-HT(7) receptor. This type of action is highly selective and usually does not require the occurrence of adaptive changes in neuronal functions, thus allowing for a much quicker therapeutic effect.

Epigenetic regulation of RAC1 induces synaptic remodeling in stress disorders and depression

Depression induces structural and functional synaptic plasticity in brain reward circuits, although the mechanisms promoting these changes and their relevance to behavioral outcomes are unknown. Transcriptional profiling of the nucleus accumbens (NAc) for Rho GTPase-related genes, which are known regulators of synaptic structure, revealed a sustained reduction in RAS-related C3 botulinum toxin substrate 1 (Rac1) expression after chronic social defeat stress. This was associated with a repressive chromatin state surrounding the proximal promoter of Rac1. Inhibition of class 1 histone deacetylases (HDACs) with MS-275 rescued both the decrease in Rac1 transcription after social defeat stress and depression-related behavior, such as social avoidance. We found a similar repressive chromatin state surrounding the RAC1 promoter in the NAc of subjects with depression, which corresponded with reduced RAC1 transcription. Viral-mediated reduction of Rac1 expression or inhibition of Rac1 activity in the NAc increases social defeat-induced social avoidance and anhedonia in mice. Chronic social defeat stress induces the formation of stubby excitatory spines through a Rac1-dependent mechanism involving the redistribution of synaptic cofilin, an actin-severing protein downstream of Rac1. Overexpression of constitutively active Rac1 in the NAc of mice after chronic social defeat stress reverses depression-related behaviors and prunes stubby spines. Taken together, our data identify epigenetic regulation of RAC1 in the NAc as a disease mechanism in depression and reveal a functional role for Rac1 in rodents in regulating stress-related behaviors.

Short- and long-term consequences of nicotine exposure during adolescence for prefrontal cortex neuronal network function

More than 70% of adolescents report to have smoked a cigarette at least once. At the adolescent stage the brain has not completed its maturation. The prefrontal cortex (PFC), the brain area responsible for executive functions and attention performance, is one of the last brain areas to mature and is still developing during adolescence. Smoking during adolescence increases the risk of developing psychiatric disorders and cognitive impairment in later life. In addition, adolescent smokers suffer from attention deficits, which aggravate with the years of smoking. Recent studies in rodents reveal the molecular changes induced by adolescent nicotine exposure that alter the functioning of synapses in the PFC and that underlie the lasting effects on cognitive function. Here we provide an overview of these recent findings.

Imipramine counteracts corticosterone-induced enhancement of glutamatergic transmission and impairment of long-term potentiation in the rat frontal cortex

The effects of corticosterone administration lasting for 7 and 21 days were studied ex vivo in rat frontal cortex slices prepared 48 h after the last dose of the hormone. In slices originating from corticosterone-treated animals, the amplitude of extracellular field potentials recorded in cortical layer II/III was increased. Corticosterone administration also resulted in an increase of the mean frequency, but not the mean amplitude, of spontaneous excitatory postsynaptic currents (sEPSCs) in layer II/III pyramidal neurons. These effects were accompanied by a reduced magnitude of long-term potentiation (LTP) of field potentials. In a separate set of experiments, rats were treated with corticosterone for 21 days and additionally with a tricyclic antidepressant, imipramine, beginning on the eighth day of corticosterone administration. In this experimental group, the amplitude of field potentials, the mean frequency of sEPSCs and the magnitude of LTP were not different from the control, indicating that corticosterone-induced modifications of basal glutamatergic transmission and synaptic plasticity were reversed by the antidepressant.

A critical review of the mechanism of action for the selective serotonin reuptake inhibitors: do these drugs possess anti-inflammatory properties and how relevant is this in the treatment of depression?

The selective serotonin reuptake inhibitors (SSRIs) are the most widely prescribed pharmacological treatment for depression. Since their introduction many have considered the primary mechanism by which the SSRIs produced therapeutic improvement in depression is their effect on monoaminergic signalling. In recent years, however, the credibility of the monoamine theory and the therapeutic efficacy of these compounds in the treatment of depression has been extensively criticized. In the current review the legitimacy of these criticisms is critically examined, in many instances the evidence base used to support these criticisms is found to be weak. Nevertheless, the apparent 'failure' of the monoamine theory has been of benefit in motivating research into alternative mechanisms through which the SSRIs may act. Given research demonstrating that depressive symptoms are intimately linked with disturbances in pro-inflammatory signalling, perhaps the most promising discovery has been the realisation that SSRIs posses significant anti-inflammatory properties. These recent findings are discussed and contextualised with respect to the neurogenic, neurotrophic and gluatamatergic effects that these drugs also possess.

Nicotine exposure during adolescence leads to short- and long-term changes in spike timing-dependent plasticity in rat prefrontal cortex

Adolescence is a critical period of brain development during which maturation of areas involved in cognitive functioning, such as the medial prefrontal cortex (mPFC), is still ongoing. Tobacco smoking during this age can compromise the normal course of prefrontal development and lead to cognitive impairments in later life. Recently, we reported that nicotine exposure during adolescence results in a short-term increase and lasting reduction in synaptic mGluR2 levels in the rat mPFC, causing attention deficits during adulthood. It is unknown how changed synaptic mGluR2 levels after adolescent nicotine exposure affect the ability of mPFC synapses to undergo long-term synaptic plasticity. Here, we addressed this question. To model nicotine exposure, adolescent (P34-P43) or adult (P60-P69) rats were treated with nicotine injections three times per day for 10 d. We found that, both during acute activation of nicotinic receptors in the adolescent mPFC as well as immediately following nicotine treatment during adolescence, long-term plasticity in response to timed presynaptic and postsynaptic activity (tLTP) was strongly reduced. In contrast, in the mPFC of adult rats 5 weeks after they received nicotine treatment during adolescence, but not during adulthood, tLTP was increased. Short- and long-term adaptation of mPFC synaptic plasticity after adolescent nicotine exposure could be explained by changed mGluR2 signaling. Blocking mGluR2s augmented tLTP, whereas activating mGluR2s reduced tLTP. Our findings suggest neuronal mechanisms by which exposure to nicotine during adolescence alters the rules for spike timing-dependent plasticity in prefrontal networks that may explain the observed deficits in cognitive performance in later life.

Characterization of an mGluR2/3 negative allosteric modulator in rodent models of depression

There is growing evidence suggesting that antagonists of group II metabotropic glutamate receptors (mGluR2/3) exhibit antidepressant-like properties in several preclinical models of depression. However, all those studies have been performed using competitive group II non-selective orthosteric antagonists. In this study we extensively characterized a group II selective negative allosteric modulator (4-[3-(2,6-Dimethylpyridin-4-yl)phenyl]-7-methyl-8-trifluoromethyl-1,3-dihydrobenzo[b][1,4]diazepin-2-one, namely RO4491533, Woltering et al., 2010) in several in vitro biochemical assays and in vivo models of depression. In vitro, RO4491533 completely blocked the glutamate-induced Ca(2+) mobilization and the glutamate-induced accumulation in [(35)S]GTP(γS) binding in cells expressing recombinant human or rat mGluR2 and in native tissues. Results from Schild plot experiments and reversibility test at the target on both cellular and membrane-based assays confirmed the negative allosteric modulator properties of the compound. RO4491533 was equipotent on mGluR2 and mGluR3 receptors but not active on any other mGluRs. RO4491533 has acceptable PK properties in mice and rats, is bioavailable following oral gavage (F = 30%) and brain-penetrant (CSF conc/total plasma conc ratio = 0.8%). RO4491533 appeared to engage the central mGluR2 and mGluR3 receptors since the compound reversed the hypolocomotor effect of an mGluR2/3 orthosteric agonist LY379268 in a target-specific manner, as did the group II orthosteric mGluR2/3 antagonist LY341495. RO4491533 and LY341495 dose-dependently reduced immobility time of C57Bl6/J mice in the forced swim test. Also, RO4491533 and LY341495 were active in the tail suspension test in a line of Helpless (H) mice, a putative genetic model of depression. These data suggest that mGluR2/3 receptors are viable targets for development of novel pharmacotherapies for depression.

Nicotine exposure during adolescence alters the rules for prefrontal cortical synaptic plasticity during adulthood

The majority of adolescents report to have smoked a cigarette at least once. Adolescence is a critical period of brain development during which maturation of areas involved in cognitive functioning, such as the medial prefrontal cortex (mPFC), is still ongoing. Tobacco smoking during this age may compromise the normal course of prefrontal development and lead to cognitive impairments in later life. In addition, adolescent smokers suffer from attention deficits, which progress with the years of smoking. Recent studies in rodents reveal the molecular changes induced by adolescent nicotine exposure that alter the functioning of synapses in the PFC and underlie the lasting effects on cognitive function. In particular, the expression and function of metabotropic glutamate receptors (mGluRs) are changed and this has an impact on short- and long-term plasticity of glutamatergic synapses in the PFC and ultimately on the attention performance. Here, we review and discuss these recent findings.

Pharmacological modulation of mGluR7 with AMN082 and MMPIP exerts specific influences on alcohol consumption and preference in rats

Growing evidence supports a role for the central nervous system (CNS) neurotransmitter L-glutamate and its metabotropic receptors (mGluRs) in drug addiction in general and alcohol-use disorders in particular. Alcohol dependence, for instance, has a genetic component, and the recent discovery that variations in the gene coding for mGluR7 modulate alcohol consumption further validates involvement of the L-glutamate system. Consequently, increasing interest emerges in developing L-glutamatergic therapies for the treatment of alcohol abuse and dependence. To this end, we performed a detailed behavioral pharmacology study to investigate the regulation of alcohol consumption and preference following administration of the mGluR7-selective drugs N,N'-dibenzyhydryl-ethane-1,2-diamine dihydrochloride (AMN082) and 6-(4-Methoxyphenyl)-5-methyl-3-(4-pyridinyl)-isoxazolo[4,5-c]pyridin-4(5H)-one hydrochloride (MMPIP). Upon administration of the allosteric agonist AMN082 (10 mg/kg, i.p.) in rats, there was a significant decrease in ethanol consumption and preference, without affecting ethanol blood metabolism. In contrast, mGluR7 blockade with MMPIP (10 mg/kg, i.p.) showed an increase in alcohol intake and reversed AMN082's effect on ethanol consumption and preference. Both mGluR7-directed pharmacological tools had no effect on total fluid intake, taste preference, or on spontaneous locomotor activity. In conclusion, these findings support a specific regulatory role for mGluR7 on alcohol drinking and preference and provide evidence for the use of AMN082-type drugs as potential new treatments for alcohol-use disorders in man.

Structural and synaptic plasticity in stress-related disorders

Abstract Stress can have a lasting impact on the structure and function of brain circuitry that results in long-lasting changes in the behavior of an organism. Synaptic plasticity is the mechanism by which information is stored and maintained within individual synapses, neurons, and neuronal circuits to guide the behavior of an organism. Although these mechanisms allow the organism to adapt to its constantly evolving environment, not all of these adaptations are beneficial. Under prolonged bouts of physical or psychological stress, these mechanisms become dysregulated, and the connectivity between brain regions becomes unbalanced, resulting in pathological behaviors. In this review, we highlight the effects of stress on the structure and function of neurons within the mesocorticolimbic brain systems known to regulate mood and motivation. We then discuss the implications of these spine adaptations on neuronal activity and pathological behaviors implicated in mood disorders. Finally, we end by discussing recent brain imaging studies in human depression within the context of these basic findings to provide insight into the underlying mechanisms leading to neural dysfunction in depression.

GluN2B-containing NMDA receptors as possible targets for the neuroprotective and antidepressant effects of fluoxetine

Accumulating evidence has indicated the involvement of glutamatergic neurotransmission in the pathophysiology of excitotoxicity and in the mechanism of action of antidepressants. We have previously shown that tricyclic desipramine and the selective serotonin reuptake inhibitor fluoxetine inhibit NMDA receptors (NMDARs) in the clinically relevant, low micromolar concentration range. As the different subtypes of NMDARs are markedly different in their physiological and pathological functions, our aim was to investigate whether the effect of antidepressants is subtype-specific. Using whole-cell patch-clamp recordings in rat cortical cell cultures, we studied the age-dependence of inhibition of NMDA-induced currents after treatment with desipramine and fluoxetine, as the expression profile of the NMDAR subtypes changes as a function of days in vitro. We also investigated the inhibitory effect of these antidepressants on NMDA-induced currents in HEK 293 cell lines that stably expressed rat recombinant NMDARs with GluN1a/GluN2A or GluN1a/GluN2B subunit compositions. The inhibitory effect of desipramine was not age-dependent, whereas fluoxetine displayed a continuously decreasing inhibitory profile, which was similar to the GluN1/GluN2B subtype-selective antagonist ifenprodil. In HEK 293 cells, desipramine equally inhibited NMDA currents in both cell lines, whereas fluoxetine showed an inhibitory effect only in cells that expressed the GluN1/GluN2B subtype. Our data show that fluoxetine is a selective inhibitor of GluN2B-containing NMDARs, whereas desipramine inhibits both GluN1/GluN2A and GluN1/GluN2B subtypes. As the clinical efficacy of these drugs is very similar, the putative NMDAR-associated therapeutic effect of antidepressants may be mediated only via inhibition of the GluN2B-containing subtype. The manifestation of the GluN1/GluN2B-selectivity of fluoxetine suggests the neuroprotective potential for this drug in both acute and chronic neurodegenerative disorders.

Immunoexcitotoxicity as a central mechanism in chronic traumatic encephalopathy-A unifying hypothesis

Some individuals suffering from mild traumatic brain injuries, especially repetitive mild concussions, are thought to develop a slowly progressive encephalopathy characterized by a number of the neuropathological elements shared with various neurodegenerative diseases. A central pathological mechanism explaining the development of progressive neurodegeneration in this subset of individuals has not been elucidated. Yet, a large number of studies indicate that a process called immunoexcitotoxicity may be playing a central role in many neurodegenerative diseases including chronic traumatic encephalopathy (CTE). The term immunoexcitotoxicity was first coined by the lead author to explain the evolving pathological and neurodevelopmental changes in autism and the Gulf War Syndrome, but it can be applied to a number of neurodegenerative disorders. The interaction between immune receptors within the central nervous system (CNS) and excitatory glutamate receptors trigger a series of events, such as extensive reactive oxygen species/reactive nitrogen species generation, accumulation of lipid peroxidation products, and prostaglandin activation, which then leads to dendritic retraction, synaptic injury, damage to microtubules, and mitochondrial suppression. In this paper, we discuss the mechanism of immunoexcitotoxicity and its link to each of the pathophysiological and neurochemical events previously described with CTE, with special emphasis on the observed accumulation of hyperphosphorylated tau.

Suicidal ideation during antidepressant treatment: do genetic predictors exist?

Suicidal thoughts during antidepressant treatment have recently been the focus of several candidate gene and genome-wide association studies. Although the clinical risk factors for such events are well known, unfortunately they do not help to predict who will have a suicidal event during antidepressant treatment and who will not. Pharmacogenomic studies have therefore attempted to use genetic variants to predict individual susceptibility to treatment-related suicidal ideation. In this perspective, several genetic predictors have been highlighted, the majority of which relate to common mechanisms of antidepressant action: genes involved in the neurotrophic and synaptic plasticity systems (CREB1, and BDNF and its receptor NTRK2), noradrenergic system (ADRA2A), glutamatergic system (GRIA3, GRIK2 and GDA), inflammatory and hypothalamic-pituitary-adrenal (HPA) axis systems (IL28RA and FKBP5) and in other brain functions (PAPLN, APOO, KCNIP4 and ELP3). Although some of these genes may be of interest in predicting antidepressant-induced suicidal ideation, they still need to be validated in better phenotypically designed samples. Several methodological factors are indeed responsible for the problems involved in implicating these findings in the causation of a clinically relevant phenotype. These include discrepancies between studies in defining phenotypes, with several different thresholds used to establish significant suicidal ideation; the use of scales not truly designed to measure suicidal ideation; and the paucity of true suicidal events (suicide attempts and/or completion) in pharmacogenomic studies. In conclusion, pharmacogenomic studies are far from fulfilling their promise. There is a need for future pharmacogenetic studies targeting events that are clinically significant in order to find associated variants that will help clinicians to improve their treatment strategies. While awaiting these genetic predictors, clinicians need to bear in mind that all studies in this field support a beneficial effect of antidepressants on suicidal ideation. This should therefore encourage them to prescribe antidepressant medication even in patients with suicidal ideation.

Oxidative stress and brain glutamate-mediated excitability in depressed patients

BACKGROUND

Several neuropsychiatric pathologies have been recently linked to oxidative stress. In this study, we investigated the relationship between depression, markers of oxidative stress and neurotransmission, as expressed by sensory cortex excitability.

METHODS

Serum levels of oxidative stress markers and somatosensory magnetic fields, evoked by external galvanic stimulation, were measured in 13 depressed patients and 13 controls.

RESULTS

Depressives had higher levels of total and free copper than controls and lower levels of transferrin. They also showed lower sensory cortex excitability, which correlated with copper levels in controls, but not in patients. Transferrin correlated with sensory cortex excitability in both patients and controls, although in opposite ways. Copper level results associated with the patients' clinical status.

LIMITATIONS

Small sample size and possible sampling bias in patient selection.

CONCLUSIONS

Pro-oxidant agents appear to affect neuronal excitability and clinical state of depressed patients, as free copper excess alters their cortical glutamatergic neurotransmission.

Serum zinc level in depressed patients during zinc supplementation of imipramine treatment

BACKGROUND

Recurrent major depression is associated with decreased blood zinc concentrations that may be increased by effective antidepressant therapy. Some clinical investigations point to alterations of the zinc level in blood as a potential marker of depression.

METHODS

A placebo-controlled, double blind study of zinc supplementation to imipramine therapy was conducted on sixty patients fulfilling the DSM-IV criteria for major depression (18-55 years old, 40 females, 20 males). Moreover, a group of 25 healthy volunteers was recruited (16 females, 9 males). Blood samples were drawn for the assay of serum zinc once from the control subjects and four times (before, and then 2, 6 and 12 weeks after the beginning of treatment) from the depressed subjects.

RESULTS

We report that: 1) the serum zinc level was significantly lower (by 22%) in depressed patients than in healthy volunteers, 2) all groups demonstrated a gradual increase in zinc concentrations over the period of imipramine treatment with or without zinc supplementation, 3) treatment-resistant patients demonstrated lower concentrations of zinc (by 14%) than treatment-non-resistant patients, 4) zinc concentrations were higher in zinc-supplemented patients than in placebo-supplemented patients, 5) zinc supplementation increased zinc concentrations over the period of treatment, and 6) at a 12-week imipramine treatment, a significant negative correlation was demonstrated between the Montgomery-Asberg Depression Rating Scale and the serum zinc level together with a concomitant increase in serum zinc in patients in remission.

CONCLUSIONS

Serum zinc is a state marker of depression.

Pharmacogenomics of suicidal events

Pharmacogenomic studies of antidepressant treatment-emergent suicidal events in depressed patients report associations with polymorphisms in genes involved in transcription (CREB1), neuroprotection (BDNF and NTRK2), glutamatergic and noradrenergic neurotransmission (GRIA3, GRIK2 and ADRA2A), the stress and inflammatory responses (FKBP5 and IL28RA), and the synthesis of glycoproteins (PAPLN). Nearly all of the reported events in these studies were modest one-time increases in suicidal ideation. In 3231 unique subjects across six studies, 424 (13.1%) patients showed increases in suicidal ideation, eight (0.25%) attempted suicide and four (0.12%) completed suicide. Systems related to most of these genes have also been implicated in studies of suicidal behavior irrespective of treatment. Future pharmacogenomic studies should target events that are clinically significant, related clinical phenotypes of response and medication side effects, and biological pathways that are involved in these outcomes in order to improve treatment approaches.

Antidepressants are a rational complementary therapy for the treatment of Alzheimer's disease

There is a high prevalence rate (30-50%) of Alzheimer's disease (AD) and depression comorbidity. Depression can be a risk factor for the development of AD or it can be developed secondary to the neurodegenerative process. There are numerous documented diagnosis and treatment challenges for the patients who suffer comorbidity between these two diseases. Meta analysis studies have provided evidence for the safety and efficacy of antidepressants in treatment of depression in AD patients. Preclinical and clinical studies show the positive role of chronic administration of selective serotonin reuptake inhibitor (SSRI) antidepressants in hindering the progression of the AD and improving patient performance. A number of clinical studies suggest a beneficial role of combinatorial therapies that pair antidepressants with FDA approved AD drugs. Preclinical studies also demonstrate a favorable effect of natural antidepressants for AD patients. Based on the preclinical studies there are a number of plausible antidepressants effects that may modulate the progression of AD. These effects include an increase in neurogenesis, improvement in learning and memory, elevation in the levels of neurotrophic factors and pCREB and a reduction of amyloid peptide burden. Based on this preclinical and clinical evidence, antidepressants represent a rational complimentary strategy for the treatment of AD patients with depression comorbidity.

Amitriptyline inhibits the activity of the rat glutamate transporter EAAT3 expressed in Xenopus oocytes

Objectives

Evidence suggests that glutamatergic systems may be involved in the pathophysiology of major depression and the mechanism of action of antidepressants. We have investigated the effects of amitriptyline, a tricyclic antidepressant, on the activity of the excitatory amino acid transporter type 3 (EAAT3), a protein that can regulate extracellular glutamate concentrations in the brain.

Methods

EAAT3 was expressed in Xenopus oocytes. Using a two-electrode voltage clamp, membrane currents were recorded after application of 30 μM l-glutamate in the presence or absence of various concentrations of amitriptyline or after application of various concentrations of l-glutamate in the presence or absence of 0.64 μM amitriptyline.

Key findings

Amitriptyline concentration-dependently reduced EAAT3 activity. This inhibition reached statistical significance at 0.38–1.27 μM amitriptyline. Amitriptyline 0.64 μM reduced the pharmacokinetic parameter Vmax, but did not affect the pharmacokinetic parameter Km, of EAAT3 for l-glutamate. The amitriptyline inhibition disappeared after a 4-min washout. Phorbol-12-myristate-13-acetate, a protein kinase C activator, increased EAAT3 activity. However, 0.64 μM amitriptyline induced a similar degree of decrease in EAAT3 activity in the presence or absence of phorbol-12-myristate-13-acetate.

Conclusions

Our results suggested that amitriptyline at clinically relevant concentrations reversibly reduced EAAT3 activity via decreasing its maximal velocity of glutamate transporting function. The effects of amitriptyline on EAAT3 activity may have represented a novel site of action for amitriptyline to increase glutamatergic neuro-transmission. Protein kinase C may not have been involved in the effects of amitriptyline on EAAT3.

Amitriptyline inhibits the activity of the rat glutamate transporter EAAT3 expressed in Xenopus oocytes

OBJECTIVES

Evidence suggests that glutamatergic systems may be involved in the pathophysiology of major depression and the mechanism of action of antidepressants. We have investigated the effects of amitriptyline, a tricyclic antidepressant, on the activity of the excitatory amino acid transporter type 3 (EAAT3), a protein that can regulate extracellular glutamate concentrations in the brain.

METHODS

EAAT3 was expressed in Xenopus oocytes. Using a two-electrode voltage clamp, membrane currents were recorded after application of 30 microM L-glutamate in the presence or absence of various concentrations of amitriptyline or after application of various concentrations of L-glutamate in the presence or absence of 0.64 microM amitriptyline.

KEY FINDINGS

Amitriptyline concentration-dependently reduced EAAT3 activity. This inhibition reached statistical significance at 0.38-1.27 microM amitriptyline. Amitriptyline 0.64 microM reduced the pharmacokinetic parameter Vmax, but did not affect the pharmacokinetic parameter Km, of EAAT3 for L-glutamate. The amitriptyline inhibition disappeared after a 4-min washout. Phorbol-12-myristate-13-acetate, a protein kinase C activator, increased EAAT3 activity. However, 0.64 microM amitriptyline induced a similar degree of decrease in EAAT3 activity in the presence or absence of phorbol-12-myristate-13-acetate.

CONCLUSIONS

Our results suggested that amitriptyline at clinically relevant concentrations reversibly reduced EAAT3 activity via decreasing its maximal velocity of glutamate transporting function. The effects of amitriptyline on EAAT3 activity may have represented a novel site of action for amitriptyline to increase glutamatergic neurotransmission. Protein kinase C may not have been involved in the effects of amitriptyline on EAAT3.

Cannabis and psychiatric disorders: it is not only addiction

Since the discovery of the endocannabinoid system, a growing body of psychiatric research has emerged focusing on the role of this system in major psychiatric disorders like schizophrenia (SCZ), bipolar disorder (BD), major depression and anxiety disorder. Continuing in the line of earlier epidemiological studies, recent replication studies indicate that frequent cannabis use doubles the risk for psychotic symptoms and SCZ. Further points of clinical research interest are alterations of endocannabinoids and their relation to symptoms as well as postmortem analyses of cannabinoid CB(1) receptor densities in SCZ. A possible neurobiological mechanism for the deleterious influence of cannabis use in SCZ has been suggested, involving the disruption of endogenous cannabinoid signaling and functioning. Even though the number of studies is still limited for affective and anxiety disorders, previous results suggest these diseases to be exciting objectives of cannabinoid-associated research. Therefore, it became apparent that cannabis use is not only frequent in patients suffering from BD, but that it also induces manic symptoms in this group. In addition, prior antipsychotic treatment decreased the numerical density of CB(1) immunoreactive glial cells in bipolar patients. Although the data on the influence of cannabis use on the development of major depression is controversial, cannabinoid compounds could display a new class of medication, as suggested by the antidepressive effects of the fatty acid amino hydrolase inhibitor URB597 in animal models. With numerous open questions and controversial results, further research is required to specify and extend the findings in this area, which provides a promising target for novel pharmacotherapeutic interventions.

Defective group-II metaboropic glutamate receptors in the hippocampus of spontaneously depressed rats

Spontaneously depressed flinders sensitive line (FSL) rats showed a reduced expression of mGlu2/3 metabotropic glutamate receptors in the hippocampus, as compared to "non-depressed" flinders resistant line (FRL) rats. No changes in mGlu2/3 receptor protein levels were found in other brain regions, including the amygdala, hypothalamus, and cerebral cortex. Biochemical analysis of receptor signalling supported the reduction of mGlu2/3 receptors in the hippocampus of FSL rats. Accordingly, the selective mGlu2/3 receptor agonist, LY379268 (1microM) reduced forskolin-stimulated cAMP formation by 56% and 32% in hippocampal slices from FRL and FSL rats, respectively. In addition, LY379268 enhanced 3,5-dihydroxyphenylglycine-stimulated inositol phospholipid hydrolysis from 65% to 215% in hippocampal slices from FRL rats, whereas it was inactive in slices from FRL rats. We also examined the behavioural response of FSL rats to systemic injection of LY379268 (0.5mg/kg, i.p., once a day for 1-21 days) by measuring the immobility time in the forced swim test, which is known to be increased in these rats. LY379268 was administered alone or combined with the classical antidepressant, chlorimipramine (10mg/kg, i.p.). LY379268 alone had no effect at any of the selected time-points, whereas chlorimipramine alone reduced the immobility time only after 21 days of treatment. In contrast, when combined with LY379268, chlorimipramine reduced the immobility time during the first 14 days of treatment. These data support the view that mGlu2/3 receptors might be involved in the pathophysiology of depressive disorders, and that pharmacological activation of these receptors may shorten the latency of antidepressant medication.

Repeated administration of imipramine attenuates glutamatergic transmission in rat frontal cortex

The effects of repeated administration of a tricyclic antidepressant, imipramine, lasting 14 days (10 mg/kg p.o., twice daily), were studied ex vivo in rat frontal cortex slices prepared 48 h after last dose of the drug. In slices prepared from imipramine-treated animals the mean frequency, and to a lesser degree the mean amplitude, of spontaneous excitatory postsynaptic currents recorded from layer II/III pyramidal neurons, were decreased. These effects were accompanied by a reduction of the initial slope ratio of pharmacologically isolated N-methyl-D-aspartate to AMPA/kainate receptor-mediated stimulation-evoked excitatory postsynaptic currents. Imipramine treatment also resulted in a decrease of extracellular field potentials evoked in layer II/III by stimulation of underlying sites in layer V. These results indicate that chronic treatment with imipramine results in an attenuation of the release of glutamate and an alteration in the postsynaptic reactivity of ionotropic glutamate receptors in rat cerebral cortex.

Direct inhibitory effect of fluoxetine on N-methyl-D-aspartate receptors in the central nervous system

BACKGROUND

Data accumulated in the last decade indicate that N-methyl-D-aspartate (NMDA) receptors might be involved in the pathophysiology of depression and the mechanism of action of antidepressants, although a direct inhibitory effect has been reported only in connection with tricyclic compounds, which interact with a wide range of receptors.

METHODS

Using whole-cell patch-clamp recording in rat cortical cell cultures, we investigated whether the selective serotonin reuptake inhibitor fluoxetine, which has a much better adverse effect profile, has a direct effect on NMDA receptors, and we compared its action to that of the tricyclic desipramine.

RESULTS

Both desipramine (concentration that causes 50% inhibition (IC(50)) = 3.13 microM) and fluoxetine (IC(50) = 10.51 microM) inhibited NMDA-evoked currents with similar efficacy in the clinically relevant low micromolar concentration range. However, in contrast to desipramine, the inhibition by fluoxetine was not voltage-dependent, and fluoxetine partially preserved its ability to associate with NMDA receptor in the presence of Mg(2+), suggesting different binding sites for the two drugs.

CONCLUSIONS

The fact that different classes of antidepressants were found to be low-affinity NMDA antagonists suggests that direct inhibition of NMDA receptors may contribute to the clinical effects of antidepressants.

The immune-mediated alteration of serotonin and glutamate: towards an integrated view of depression

Beside the well-known deficiency in serotonergic neurotransmission as pathophysiological correlate of major depression (MD), recent evidence points to a pivotal role of increased glutamate receptor activation as well. However, cause and interaction of these neurotransmitter alterations are not understood. In this review, we present a hypothesis integrating current concepts of neurotransmission and hypothalamus-pituitary-adrenal (HPA) axis dysregulation with findings on immunological alterations and alterations in brain morphology in MD. An immune activation including increased production of proinflammatory cytokines has repeatedly been described in MD. Proinflammatory cytokines such as interleukin-2, interferon-gamma, or tumor necrosis factor-alpha activate the tryptophan- and serotonin-degrading enzyme indoleamine 2,3-dioxygenase (IDO). Depressive states during inflammatory somatic disorders are also associated with increased proinflammatory cytokines and increased consumption of tryptophan via activation of IDO. An enhanced consumption of serotonin and its precursor tryptophan through IDO activation could well explain the reduced availability of serotonergic neurotransmission in MD. An increased activation of IDO and its subsequent enzyme kynurenine monooxygenase by proinflammatory cytokines, moreover, leads to an enhanced production of quinolinic acid, a strong agonist of the glutamatergic N-methyl-D-aspartate receptor. In inflammatory states of the central nervous system, IDO is mainly activated in microglial cells, which preferentially metabolize tryptophan to the NMDA receptor agonist quinolinic acid, whereas astrocytes - counteracting this metabolism due to the lack of an enzyme of this metabolism - have been observed to be reduced in MD. Therefore the type 1/type 2 immune response imbalance, associated with an astrocyte/microglia imbalance, leads to serotonergic deficiency and glutamatergic overproduction. Astrocytes are further strongly involved in re-uptake and metabolic conversion of glutamate. The reduced number of astrocytes could contribute to both, a diminished counterregulation of IDO activity in microglia and an altered glutamatergic neurotransmission. Further search for antidepressant agents should take into account anti-inflammatory drugs, for example, cyclooxygenase-2 inhibitors, might exert antidepressant effects by acting on serotonergic deficiency, glutamatergic hyperfunction and antagonizing neurotoxic effects of quinolinic acid.