Pathophysiology of depression and innovative treatments: remodeling glutamatergic synaptic connections

General Anesthetics to Treat Major Depressive Disorder: Clinical Relevance and Underlying Mechanisms

Major depressive disorder is a frequent and devastating psychological condition with tremendous public health impact. The underlying pathophysiological mechanisms involve abnormal neurotransmission and a relatedly impaired synaptic plasticity. Since general anesthetics are potent modulators of neuronal activity and, thereby, can exert long-term context-dependent impact on neural networks, an intriguing hypothesis is that these drugs could enhance impaired neural plasticity associated with certain psychiatric diseases. Clinical observations over the past few decades appear to confirm this possibility. Indeed, equipotency of general anesthesia alone in comparison with electroconvulsive therapy under general anesthesia has been demonstrated in several clinical trials. Importantly, in the past 15 years, intravenous administration of subanesthetic doses of ketamine have also been demonstrated to have rapid antidepressant effects. The molecular, cellular, and network mechanisms underlying these therapeutic effects have been partially identified. Although several important questions remain to be addressed, the ensemble of these experimental and clinical observations opens new therapeutic possibilities in the treatment of depressive disorders. Importantly, they also suggest a new therapeutic role for anesthetics that goes beyond their principal use in the perioperative period to facilitate surgery.

Gut Microbiota and the Neuroendocrine System

The microbial ecosystem that inhabits the gastrointestinal tract of all mammals-the gut microbiota-has been in a symbiotic relationship with its hosts over many millennia. Thanks to modern technology, the myriad of functions that are controlled or modulated by the gut microbiota are beginning to unfold. One of the systems that is emerging to closely interact with the gut microbiota is the body's major neuroendocrine system that controls various body processes in response to stress, the hypothalamic-pituitary-adrenal (HPA) axis. This interaction is of pivotal importance; as various disorders of the microbiota-gut-brain axis are associated with dysregulation of the HPA axis. The present contribution describes the bidirectional communication between the gut microbiota and the HPA axis and delineates the potential underlying mechanisms. In this regard, it is important to note that the communication between the gut microbiota and the HPA axis is closely interrelated with other systems, such as the immune system, the intestinal barrier and blood-brain barrier, microbial metabolites, and gut hormones, as well as the sensory and autonomic nervous systems. These communication pathways will be exemplified through preclinical models of early life stress, beneficial roles of probiotics and prebiotics, evidence from germ-free mice, and antibiotic-induced modulation of the gut microbiota.

Treatment resistant depression: A multi-scale, systems biology approach

An estimated 50% of depressed patients are inadequately treated by available interventions. Even with an eventual recovery, many patients require a trial and error approach, as there are no reliable guidelines to match patients to optimal treatments and many patients develop treatment resistance over time. This situation derives from the heterogeneity of depression and the lack of biomarkers for stratification by distinct depression subtypes. There is thus a dire need for novel therapies. To address these known challenges, we propose a multi-scale framework for fundamental research on depression, aimed at identifying the brain circuits that are dysfunctional in several animal models of depression as well the changes in gene expression that are associated with these models. When combined with human genetic and imaging studies, our preclinical studies are starting to identify candidate circuits and molecules that are altered both in models of disease and in patient populations. Targeting these circuits and mechanisms can lead to novel generations of antidepressants tailored to specific patient populations with distinctive types of molecular and circuit dysfunction.

Engaging homeostatic plasticity to treat depression

Major depressive disorder (MDD) is a complex and heterogeneous mood disorder, making it difficult to develop a generalized, pharmacological therapy that is effective for all who suffer from MDD. Through the fortuitous discovery of N-methyl-D-aspartate receptor (NMDAR) antagonists as effective antidepressants, we have gained key insights into how antidepressant effects can be produced at the circuit and molecular levels. NMDAR antagonists act as rapid-acting antidepressants such that relief from depressive symptoms occurs within hours of a single injection. The mode of action of NMDAR antagonists seemingly relies on their ability to activate protein-synthesis-dependent homeostatic mechanisms that restore top-down excitatory connections. Recent evidence suggests that NMDAR antagonists relieve depressive symptoms by forming new synapses resulting in increased excitatory drive. This event requires the mammalian target of rapamycin complex 1 (mTORC1), a signaling pathway that regulates synaptic protein synthesis. Herein, we review critical studies that shed light on the action of NMDAR antagonists as rapid-acting antidepressants and how they engage a neuron's or neural network's homeostatic mechanisms to self-correct. Recent studies notably demonstrate that a shift in γ-amino-butyric acid receptor B (GABA_BR) function, from inhibitory to excitatory, is required for mTORC1-dependent translation with NMDAR antagonists. Finally, we discuss how GABA_BR activation of mTORC1 helps resolve key discrepancies between rapid-acting antidepressants and local homeostatic mechanisms.

ZL006, a small molecule inhibitor of PSD-95/nNOS interaction, does not induce antidepressant-like effects in two genetically predisposed rat models of depression and control animals

N-methyl-D-aspartate receptor (NMDA-R) antagonists and nitric oxide inhibitors have shown promising efficacy in depression but commonly induce adverse events. To circumvent these, a more indirect disruption of the nitric oxide synthase/postsynaptic density protein 95 kDa complex at the NMDA-R has been proposed. This disruption can be achieved using small molecule inhibitors such as ZL006, which has attracted attention as ischemic stroke therapy in rodents and has been proposed as a potential novel treatment for depression. Based on this, our aim was to translate these findings to animal models of depression to elucidate antidepressant-like properties in more detail. In the present study, we administered ZL006 to two established animal models of depression and control rodents. Following treatment, we measured locomotion in the Open Field and depressive-like behavior in the Forced Swim Test and Tail Suspension Test. Our experimental designs included the use of different species (rats, mice), strains (Flinders Sensitive Line rats, Flinders Resistant Line rats, Wistar Kyoto rats, Wistar Hanover rats, Sprague Dawley rats, B6NTac mice), routes of administration (intraperitoneal, intracerebroventricular), times of administration (single injection, repeated injections), treatment regimens (acute, sustained), and doses (5, 10, 15, 50 mg/kg). ZL006 did not affect behavior in any of the described settings. On a molecular level, ZL006 significantly reduced total nitrate/nitrite concentrations in the cerebellum, supporting that it is capable of reducing nitric oxide metabolites in the brain. Future studies using different experimental parameters are needed to further investigate the behavioral profile of ZL006.

Hippocampal BDNF in physiological conditions and social isolation

Exposure of an organism to chronic psychosocial stress may affect brain-derived neurotrophic factor (BDNF) expression that has been implicated in the etiology of psychiatric disorders, such as depression. Given that depression in humans has been linked with social stress, the chronic social stress paradigms for modeling psychiatric disorders in animals have thus been developed. Chronic social isolation in animal models generally causes changes in hypothalamic-pituitary-adrenal axis functioning, associated with anxiety- and depressive-like behaviors. Also, this chronic stress causes downregulation of BDNF protein and mRNA in the hippocampus, a stress-sensitive brain region closely related to the pathophysiology of depression. In this review, we discuss the current knowledge regarding the structure, function, intracellular signaling, inter-individual differences and epigenetic regulation of BDNF in both physiological conditions and depression and changes in corticosterone levels, as a marker of stress response. Since BDNF levels are age dependent in humans and rodents, this review will also highlight the effects of adolescent and adult chronic social isolation models of both genders on the BDNF expression.

Loneliness in Relation to Depression: The Moderating Influence of a Polymorphism of the Brain Derived Neurotrophic Factor Gene on Self-efficacy and Coping Strategies

Disturbances of brain derived neurotrophic factor (BDNF) signaling, which may occur among those with a polymorphism of the Val66Met gene, comprising a Met substitution for the Val allele, may be associated with depressive cognitions. However, presumed elevated BDNF levels among individuals with the Val/Val genotype, might confer increased responsivity to contextual challenges, thus fostering vulnerability to depression. In Study 1, among undergraduate students (N = 252), increased loneliness perceptions were accompanied with depressive symptoms. This relationship was moderated by self-efficacy and BDNF genotype, such that when individuals appraised high self-efficacy, those with the Val/Val genotype, compared to Met carriers, reported greater depression scores when they perceived feeling lonely. Study 2 revealed that among undergraduate students (N = 178), lower depressive scores were associated with increased problem-focused coping among Val/Val individuals, but not Met carriers. Moreover, with increased perceived loneliness, Val/Val carriers endorsed lower problem-focused coping. Findings suggest that Val/Val individuals may have adverse neurocognitive vulnerability to loneliness experiences.

Investigational drugs in recent clinical trials for treatment-resistant depression

INTRODUCTION

The authors describe the medications for treatment-resistant depression (TRD) in phase II/III of clinical development in the EU and USA and provide an opinion on how current treatment can be improved in the near future. Areas covered: Sixty-two trials were identified in US and EU clinical trial registries that included six investigational compounds in recent phase III development and 12 others in recent phase II clinical trials. Glutamatergic agents have been the focus of many studies. A single intravenous dose of the glutamatergic modulator ketamine produces a robust and rapid antidepressant effect in persons with TRD; this effect continues to remain significant for 1 week. This observation was a turning point that opened the way for other, more selective glutamatergic modulators (intranasal esketamine, AVP-786, AVP-923, AV-101, and rapastinel). Of the remaining compounds, monoclonal antibodies open highly innovative therapeutic options, based on new pathophysiological approaches to depression. Expert commentary: Promising new agents are emerging for TRD treatment. Glutamatergic modulators likely represent a very promising alternative to monoaminergic antidepressant monotherapy. We could see the arrival of the first robust and rapid acting antidepressant drug in the near future, which would strongly facilitate the ultimate goal of recovery in persons with TRD.

Illness severity and biomarkers in depression: Using a unidimensional rating scale to examine BDNF

BACKGROUND

Numerous studies have reported reduced peripheral brain-derived neurotrophic factor (BDNF) in major depression (MD). However, most of these studies used multidimensional depression rating scales, and failed to identify a relationship between BDNF levels and depression severity. Unidimensional scales are a more valid measure of syndrome severity. In these scales, items are ordered in increasing severity, so that as scores increase, syndrome severity increases; thus, each item adds unique information, and items can be totaled to a meaningful sum. The current study used the HAM-D6, a unidimensional measure of depression, to examine if it could identify a correlation between serum BDNF and depression severity.

METHODS

Serum BDNF levels and symptom severity were assessed in 163 depressed patients, including those with both unipolar (84.0%) and bipolar (16.0%) depression. The evaluation of depression severity included the total HAM-D17 and 3 subscales, including the HAM-D6.

RESULTS

On average, patients presented moderate to severe depression (HAM-D17=21.2±5.5). Overall BDNF levels were 60.4±22.6ng/mL. The correlation between serum BDNF and depression severity was modest and not different when assessed by the HAM-D6 subscale or the HAM-D17 as a whole (z=0.951; p=0.341), despite being statistically significant for the HAM-D6 (r=-0.185; p=0.019; 95% CI: -0.335 to -0.033), but not for the entire HAM-D17 (r=-0.127; p=0.108; 95% CI: -0.272 to 0.027).

CONCLUSION

We could not identify a strong relationship between serum BDNF levels and depression severity using the HAM-D6. This is in concordance with results of previous studies that reported no correlation between these variables, and indicates that the properties of the clinical measures used cannot explain the results these studies.

Ketamine Treatment and Global Brain Connectivity in Major Depression

Capitalizing on recent advances in resting-state functional connectivity magnetic resonance imaging (rs-fcMRI) and the distinctive paradigm of rapid mood normalization following ketamine treatment, the current study investigated intrinsic brain networks in major depressive disorder (MDD) during a depressive episode and following treatment with ketamine. Medication-free patients with MDD and healthy control subjects (HC) completed baseline rs-fcMRI. MDD patients received a single infusion of ketamine and underwent repeated rs-fcMRI at 24 h posttreatment. Global brain connectivity with global signal regression (GBCr) values were computed as the average of correlations of each voxel with all other gray matter voxels in the brain. MDD group showed reduced GBCr in the prefrontal cortex (PFC) but increased GBCr in the posterior cingulate, precuneus, lingual gyrus, and cerebellum. Ketamine significantly increased GBCr in the PFC and reduced GBCr in the cerebellum. At baseline, 2174 voxels of altered GBCr were identified, but only 310 voxels significantly differed relative to controls following treatment (corrected α<0.05). Responders to ketamine showed increased GBCr in the lateral PFC, caudate, and insula. Follow-up seed-based analyses illustrated a pattern of dysconnectivity between the PFC/subcortex and the rest of the brain in MDD, which appeared to normalize postketamine. The extent of the functional dysconnectivity identified in MDD and the swift and robust normalization following treatment suggest that GBCr may serve as a treatment response biomarker for the development of rapid acting antidepressants. The data also identified unique prefrontal and striatal circuitry as a putative marker of successful treatment and a target for antidepressants' development.

Effects of escitalopram and paroxetine on mTORC1 signaling in the rat hippocampus under chronic restraint stress

Background

Recent studies have suggested that the activation of mammalian target of rapamycin (mTOR) signaling may be related to antidepressant action. Therefore, the present study evaluated whether antidepressant drugs would exert differential effects on mTOR signaling in the rat hippocampus under conditions of chronic restraint stress. Male Sprague–Dawley rats were subjected to restraint stress for 6 h/days for 21 days with either escitalopram (10 mg/kg) or paroxetine (10 mg/kg) administered after the chronic stress procedure. Western blot analyses were used to assess changes in the levels of phospho-Ser²⁴⁴⁸-mTOR, phospho-Thr^37/46-4E-BP-1, phospho-Thr³⁸⁹-p70S6 K, phospho-Ser⁴²²-eIF4B, phospho-Ser^240/244-S6, phospho-Ser⁴⁷³-Akt, and phospho-Thr²⁰²/Tyr²⁰⁴-ERK in the hippocampus.

Results

Chronic restraint stress significantly decreased the levels of phospho-mTOR complex 1 (mTORC1), phospho-4E-BP-1, phospho-p70S6 K, phospho-eIF4B, phospho-S6, phospho-Akt, and phospho-ERK (p < 0.05); the administration of escitalopram and paroxetine increased the levels of all these proteins (p < 0.05 or 0.01). Additionally, chronic restraint stress reduced phospho-mTORC1 signaling activities in general, while escitalopram and paroxetine prevented these changes in phospho-mTORC1 signaling activities.

Conclusion

These findings provide further data that contribute to understanding the possible relationships among mTOR activity, stress, and antidepressant drugs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12868-017-0357-0) contains supplementary material, which is available to authorized users.

Changes in the Prefrontal Glutamatergic and Parvalbumin Systems of Mice Exposed to Unpredictable Chronic Stress

The prefrontal cortex (PFC) is highly sensitive to the effects of stress, a known risk factor of mood disorders including anxiety and depression. Abnormalities in PFC functioning have been well described in humans displaying stress-induced depressive symptoms, and hypoactivity of the PFC is now recognized to be a key feature of the depressed brain. However, little is known about the causes and mechanisms leading to this altered prefrontal functional activity in the context of stress-related mood disorders. We previously showed that unpredictable chronic mild stress (UCMS) in mice increases prefrontal expression of parvalbumin (PV), an activity-dependent calcium-binding albumin protein expressed in a specific subtype of GABAergic neurons, highlighting a potential mechanism through which chronic stress leads to hypofunction of the PFC. In this study, we aimed to investigate the mechanisms by which chronic stress alters the prefrontal GABA system. We hypothesized that chronic stress-induced enhancement of glutamatergic transmission in the PFC is a crucial contributing factor to changes within the prefrontal GABAergic and, specifically, PV system. BALB/c male and female mice were exposed to daily handling (control) or 2 or 4 weeks of UCMS. Female mice displayed a more severe altered phenotype than males, as shown by increased anxiety- and depressive-like behaviors and deficits in PFC-dependent cognitive abilities, particularly after exposure to 2 weeks of UCMS. This behavioral phenotype was paralleled by a large increase in prefrontal PV messenger RNA (mRNA) and number of PV-expressing neurons, supporting our previous findings. We further showed that the expression of pre- and postsynaptic markers of glutamatergic transmission (VGlut1 presynaptic terminals and pERK1/2, respectively) onto PV neurons was increased by 2 weeks of UCMS in a sex-specific manner; this was associated with sex-specific changes in the mRNA expression of the NR2B subunit of the NMDA receptor. These findings provide evidence of increased glutamatergic transmission onto prefrontal PV neurons, particularly in female mice, which could potentially contribute to their increased PV expression and the extent of their behavioral impairment following UCMS. Finally, our analysis of activity of subcortical regions sending glutamatergic afferents to the PFC reveals that glutamatergic neurons from the basolateral amygdala might be specifically involved in UCMS-induced changes in prefrontal glutamatergic transmission.

Genetic Studies on the Tripartite Glutamate Synapse in the Pathophysiology and Therapeutics of Mood Disorders

Both bipolar disorder (BD) and major depressive disorder (MDD) have high morbidity and share a genetic background. Treatment options for these mood disorders are currently suboptimal for many patients; however, specific genetic variables may be involved in both pathophysiology and response to treatment. Agents such as the glutamatergic modulator ketamine are effective in treatment-resistant mood disorders, underscoring the potential importance of the glutamatergic system as a target for improved therapeutics. Here we review genetic studies linking the glutamatergic system to the pathophysiology and therapeutics of mood disorders. We screened 763 original genetic studies of BD or MDD that investigated genes encoding targets of the pathway/mediators related to the so-called tripartite glutamate synapse, including pre- and post-synaptic neurons and glial cells; 60 papers were included in this review. The findings suggest the involvement of glutamate-related genes in risk for mood disorders, treatment response, and phenotypic characteristics, although there was no consistent evidence for a specific gene. Target genes of high interest included GRIA3 and GRIK2 (which likely play a role in emergent suicidal ideation after antidepressant treatment), GRIK4 (which may influence treatment response), and GRM7 (which potentially affects risk for mood disorders). There was stronger evidence that glutamate-related genes influence risk for BD compared with MDD. Taken together, the studies show a preliminary relationship between glutamate-related genes and risk for mood disorders, suicide, and treatment response, particularly with regard to targets on metabotropic and ionotropic receptors.

What is the mechanism of Ketamine's rapid-onset antidepressant effect? A concise overview of the surprisingly large number of possibilities

WHAT IS KNOWN AND OBJECTIVE

Abundant clinical data now confirm that ketamine produces a remarkable rapid-onset antidepressant effect - hours or days - in contrast to the delayed onset (typically weeks) of current antidepressant drugs. This surprising and revolutionary finding may lead to the development of life-saving pharmacotherapy for depressive illness by reducing the high suicide risk associated with the delayed onset of effect of current drugs. As ketamine has serious self-limiting drawbacks that restrict its widespread use for this purpose, a safer alternative is needed. Our objective is to review the proposed mechanism(s) of ketamine's rapid-onset antidepressant action for new insights into the physiological basis of depressive illness that may lead to new and novel targets for antidepressant drug discovery.

METHODS

A search was conducted on published literature (e.g. PubMed) and Internet sources to identify information relevant to ketamine's rapid-acting antidepressant action and, specifically, to the possible mechanism(s) of this action. Key search words included 'ketamine', 'antidepressant', 'mechanism of action', 'depression' and 'rapid acting', either individually or in combination. Information was sought that would include less well-known, as well as well-known, basic pharmacologic properties of ketamine and that identified and evaluated the several hypotheses about ketamine's mechanism of antidepressant action.

RESULTS

Whether the mechanistic explanation for ketamine's rapid-onset antidepressant action is related to its well-known antagonism of the NMDA (N-Methyl-d-aspartate) subtype of glutamate receptor or to something else has not yet been fully elucidated. The evidence from pharmacologic, medicinal chemistry, animal model and drug-discovery sources reveals a wide variety of postulated mechanisms.

WHAT IS NEW AND CONCLUSION

The surprising discovery of ketamine's rapid-onset antidepressant effect is a game-changer for the understanding and treatment of depressive illness. There is some convergence on NMDA receptor antagonism as a likely, but to date unproven, common mechanism. The surprising number of other mechanisms, and the several novel biochemical aetiologies of depression proposed, suggests exciting new drug-discovery targets.

The effects of adolescent methylphenidate exposure on the behavioral and brain-derived neurotrophic factor response to nicotine

This study analyzed the interaction of adolescent methylphenidate on the behavioral response to nicotine and the effects of these drug treatments on brain-derived neurotrophic factor in the nucleus accumbens and hippocampus in male and female Sprague-Dawley rats. Animals were intraperitoneal administered 1 mg/kg methylphenidate or saline using a "school day" regimen (five days on, two days off) beginning on postnatal day (P)28 and throughout behavioral testing. In Experiment 1, animals were intraperitoneal administered 0.5 mg/kg (free base) nicotine or saline every second day for 10 days from P45-P63 and tested after a three-day drug washout on the forced swim stress task on P67-P68. Results revealed that adolescent methylphenidate blunted nicotine behavioral sensitization. However, methylphenidate-treated rats given saline during sensitization demonstrated decreased latency to immobility and increased immobility time on the forced swim stress task in males that was reduced by nicotine. In Experiment 2, a different set of animals were conditioned to nicotine (0.6 mg/kg free base) or saline using the conditioned place preference behavioral paradigm from P44-P51, and given a preference test on P52. On P53, the nucleus accumbens and hippocampus were analyzed for brain-derived neurotrophic factor. Methylphenidate enhanced nicotine-conditioned place preference in females and nicotine produced conditioned place preference in males and females pre-exposed to saline in adolescence. In addition, methylphenidate and nicotine increased nucleus accumbens brain-derived neurotrophic factor in females and methylphenidate enhanced hippocampus brain-derived neurotrophic factor in males and females. Methylphenidate adolescent exposure using a clinically relevant dose and regimen results in changes in the behavioral and brain-derived neurotrophic factor responses to nicotine in adolescence that are sex-dependent.

Inflammation, Glutamate, and Glia: A Trio of Trouble in Mood Disorders

Increasing data indicate that inflammation and alterations in glutamate neurotransmission are two novel pathways to pathophysiology in mood disorders. The primary goal of this review is to illustrate how these two pathways may converge at the level of the glia to contribute to neuropsychiatric disease. We propose that a combination of failed clearance and exaggerated release of glutamate by glial cells during immune activation leads to glutamate increases and promotes aberrant extrasynaptic signaling through ionotropic and metabotropic glutamate receptors, ultimately resulting in synaptic dysfunction and loss. Furthermore, glutamate diffusion outside the synapse can lead to the loss of synaptic fidelity and specificity of neurotransmission, contributing to circuit dysfunction and behavioral pathology. This review examines the fundamental role of glia in the regulation of glutamate, followed by a description of the impact of inflammation on glial glutamate regulation at the cellular, molecular, and metabolic level. In addition, the role of these effects of inflammation on glia and glutamate in mood disorders will be discussed along with their translational implications.

Inflammation Effects on Brain Glutamate in Depression: Mechanistic Considerations and Treatment Implications

There has been increasing interest in the role of glutamate in mood disorders, especially given the profound effect of the glutamate receptor antagonist ketamine in improving depressive symptoms in patients with treatment-resistant depression. One pathway by which glutamate alterations may occur in mood disorders involves inflammation. Increased inflammation has been observed in a significant subgroup of patients with mood disorders, and inflammatory cytokines have been shown to influence glutamate metabolism through effects on astrocytes and microglia. In addition, the administration of the inflammatory cytokine interferon-alpha has been shown to increase brain glutamate in the basal ganglia and dorsal anterior cingulate cortex as measured by magnetic resonance spectroscopy (MRS). Moreover, MRS studies in patients with major depressive disorder have revealed that increased markers of inflammation including C-reactive protein correlate with increased basal ganglia glutamate, which in turn was associated with anhedonia and psychomotor retardation. Finally, human and laboratory animal studies have shown that the response to glutamate antagonists such as ketamine is predicted by increased inflammatory cytokines. Taken together, these data make a strong case that inflammation may influence glutamate metabolism to alter behavior, leading to depressive symptoms including anhedonia and psychomotor slowing.

Is There a Link Between Cholesterol Level and the Risk of Developing Depression?

Depressive disorders and hypercholesterolemia are serious and common global problems. Many researchers have attempted to demonstrate association between concentration of the lipid fractions, depression as well as the rate of suicide, but the results are inconclusive. To overcome this lack of knowledge, we have summarized the studies concerning this relationship and published in recent years. All of articles included were published in peer reviewed journals and were identified through systematic query of PubMed with follow-up manual searches.

Concentrations of total cholesterol – TC, cholesterol – LDL-C, cholesterol - HDL-C other triglycerides and others atherogenic index and comorbidity with depressive disorders have been considered. While both positive and negative association between lipid fractions and depression have been shown in many studies, some data revealed no relationships between the two factors. In addition, we have also summarized the usage of statins and the occurrence of depressive disorders, and the results remain inconclusive.

In conclusion, our review did not reveal a clear relationship between lipid fractions and depression. Additional studies are needed as increased incidence of comorbidity between mood disorders and lipid disorders have been observed.

Emotional and cognitive dysregulation in schizophrenia and depression: understanding common and distinct behavioral and neural mechanisms

Emerging behavioral and neuroimaging studies in schizophrenia (SCZ) and major depressive disorder (MD) are mapping mechanisms of co-occurring and distinct affective disturbances across these disorders. This constitutes a critical goal towards developing rationally guided therapies for upstream neural pathways that contribute to comorbid symptoms across disorders. We highlight the current state of the art in our understanding of emotional dysregulation in SCZ versus MD by focusing on broad domains of behavioral function that can map onto underlying neural systems, namely deficits in hedonics, anticipatory behaviors, computations underlying value and effort, and effortful goal-directed behaviors needed to pursue rewarding outcomes. We highlight unique disturbances in each disorder that may involve dissociable neural systems, but also possible interactions between affect and cognition in MD versus SCZ. Finally, we review computational and translational approaches that offer mechanistic insight into how cellular-level disruptions can lead to complex affective disturbances, informing development of therapies across MD and SCZ.

Adverse childhood experiences associate to reduced glutamate levels in the hippocampus of patients affected by mood disorders

Adverse childhood experiences (ACE) can possibly permanently alter the stress response system, affect the glutamatergic system and influence hippocampal volume in mood disorders. The aim of the study is to investigate the association between glutamate levels in the hippocampus, measured through single proton magnetic resonance spectroscopy (1H-MRS), and ACE in patients affected by mood disorders and healthy controls. Higher levels of early stress associate to reduced levels of Glx/Cr in the hippocampus in depressed patients but not in healthy controls. Exposure to stress during early life could lead to a hypofunctionality of the glutamatergic system in the hippocampus of depressed patients. Abnormalities of glutamatergic signaling could then possibly underpin the structural and functional abnormalities observed in patients affected by mood disorders.

Behavioral and biochemical effects of ketamine and dextromethorphan relative to its antidepressant-like effects in Swiss Webster mice

Ketamine has been shown to produce rapid and robust antidepressant effects in depressed individuals; however, its abuse potential and adverse psychotomimetic effects limit its widespread use. Dextromethorphan (DM) may serve as a safer alternative on the basis of pharmacodynamic similarities to ketamine. In this proof-of-concept study, behavioral and biochemical analyses were carried out to evaluate the potential involvement of brain-derived neurotrophic factor (BDNF) in the antidepressant-like effects of DM in mice, with comparisons to ketamine and imipramine. Male Swiss, Webster mice were injected with DM, ketamine, or imipramine and their behaviors were evaluated in the forced-swim test and the open-field test. Western blots were used to measure BDNF and its precursor, pro-BDNF, protein expression in the hippocampus and the frontal cortex of these mice. Our results show that both DM and imipramine reduced immobility time in the forced-swim test without affecting locomotor activity, whereas ketamine reduced immobility time and increased locomotor activity. Ketamine also rapidly (within 40 min) increased pro-BDNF expression in an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor-dependent manner in the hippocampus, whereas DM and imipramine did not alter pro-BDNF or BDNF levels in either the hippocampus or the frontal cortex within this timeframe. These data show that DM shares some features with both ketamine and imipramine. Additional studies examining DM may aid in the development of more rapid, safe, and efficacious antidepressant treatments.

Metabolomic signatures of drug response phenotypes for ketamine and esketamine in subjects with refractory major depressive disorder: new mechanistic insights for rapid acting antidepressants

Ketamine, at sub-anesthetic doses, is reported to rapidly decrease depression symptoms in patients with treatment-resistant major depressive disorder (MDD). Many patients do not respond to currently available antidepressants, (for example, serotonin reuptake inhibitors), making ketamine and its enantiomer, esketamine, potentially attractive options for treatment-resistant MDD. Although mechanisms by which ketamine/esketamine may produce antidepressant effects have been hypothesized on the basis of preclinical data, the neurobiological correlates of the rapid therapeutic response observed in patients receiving treatment have not been established. Here we use a pharmacometabolomics approach to map global metabolic effects of these compounds in treatment-refractory MDD patients upon 2 h from infusion with ketamine (n=33) or its S-enantiomer, esketamine (n=20). The effects of esketamine on metabolism were retested in the same subjects following a second exposure administered 4 days later. Two complementary metabolomics platforms were used to provide broad biochemical coverage. In addition, we investigated whether changes in particular metabolites correlated with treatment outcome. Both drugs altered metabolites related to tryptophan metabolism (for example, indole-3-acetate and methionine) and/or the urea cycle (for example, citrulline, arginine and ornithine) at 2 h post infusion (q<0.25). In addition, we observed changes in glutamate and circulating phospholipids that were significantly associated with decreases in depression severity. These data provide new insights into the mechanism underlying the rapid antidepressant effects of ketamine and esketamine, and constitute some of the first detailed metabolomics mapping for these promising therapies.

Electron Microscopic Analysis of Hippocampal Axo-Somatic Synapses in a Chronic Stress Model for Depression

Stress can alter the number and morphology of excitatory synapses in the hippocampus, but nothing is known about the effect of stress on inhibitory synapses. Here, we used an animal model for depression, the chronic mild stress model, and quantified the number of perisomatic inhibitory neurons and their synapses. We found reduced density of parvalbumin‐positive (PV+) neurons in response to stress, while the density of cholecystokinin‐immunoreactive (CCK+) neurons was unaffected. We did a detailed electron microscopic analysis to quantify the frequency and morphology of perisomatic inhibitory synapses in the hippocampal CA1 area. We analyzed 1100 CA1 pyramidal neurons and 4800 perisomatic terminals in five control and four chronically stressed rats. In the control animals we observed the following parameters: Number of terminals/soma = 57; Number of terminals/100 µm cell perimeter = 10; Synapse/terminal ratio = 32%; Synapse number/100 terminal = 120; Average terminal length = 920nm. None of these parameters were affected by the stress exposure. Overall, these data indicate that despite the depressive‐like behavior and the decrease in the number of perisomatic PV+ neurons in the light microscopic preparations, the number of perisomatic inhibitory synapses on CA1 pyramidal cells was not affected by stress. In the electron microscope, PV+ neurons and the axon terminals appeared to be normal and we did not find any apoptotic or necrotic cells. This data is in sharp contrast to the remarkable remodeling of the excitatory synapses on spines that has been reported in response to stress and depressive‐like behavior. © 2016 The Authors Hippocampus Published by Wiley Periodicals, Inc.

Antinociceptive and Anti-Inflammatory Effects of Ketamine and the Relationship to Its Antidepressant Action and GSK3 Inhibition

Ketamine (KET), a NMDA antagonist, exerts an antidepressant effect at subanaesthetic doses and possesses analgesic and anti-inflammatory activities. We evaluated the involvement of KET antinociceptive and anti-inflammatory effects with its antidepressant action. Male Swiss mice were subjected to formalin, carrageenan-induced paw oedema and forced swimming tests, for assessing antinociceptive, anti-inflammatory and antidepressant effects. The treatment groups were as follows: control, KET (2, 5 and 10 mg/kg), lithium (LI: 5 mg/kg) and KET2 + LI5 combination. Immunohistochemistry analyses (TNF-α, iNOS, COX-2 and GSK3) in oedematous paws were performed. KET5 and KET10 reduced licking times in neurogenic (22 and 38%) and inflammatory (67 and 78%) phases of the formalin test, respectively, as related to controls. While LI5 inhibited the second phase by 24%, the licking time was inhibited by 26 and 59% in the KET2 + LI5 group (first and second phases). Furthermore, oedema volumes were reduced by 37 and 45% in the KET5 and KET10 groups, respectively. Oedema reductions were 29% in the LI5 group and 48% in the KET2 + LI5 group. In the forced swimming test, there were 23, 38 and 53% decreases in the immobility time in KET2, KET5 and KET10 groups, respectively. While LI5 caused no significant effect, decreases of 52% were observed with KET2 + LI5. KET also decreased TNF-α, iNOS, COX-2 and GSK3 immunostainings in oedematous paws, effects intensified with KET2 + LI5. We showed that KET presents antinociceptive and anti-inflammatory effects associated with its antidepressant response. Furthermore, our results indicate the close involvement of GSK3 inhibition and blockade of inflammatory responses, in the antidepressant drug effect.

Differential Expression of FosB Proteins and Potential Target Genes in Select Brain Regions of Addiction and Depression Patients

Chronic exposure to stress or drugs of abuse has been linked to altered gene expression throughout the body, and changes in gene expression in discrete brain regions are thought to underlie many psychiatric diseases, including major depressive disorder and drug addiction. Preclinical models of these disorders have provided evidence for mechanisms of this altered gene expression, including transcription factors, but evidence supporting a role for these factors in human patients has been slow to emerge. The transcription factor ΔFosB is induced in the prefrontal cortex (PFC) and hippocampus (HPC) of rodents in response to stress or cocaine, and its expression in these regions is thought to regulate their "top down" control of reward circuitry, including the nucleus accumbens (NAc). Here, we use biochemistry to examine the expression of the FosB family of transcription factors and their potential gene targets in PFC and HPC postmortem samples from depressed patients and cocaine addicts. We demonstrate that ΔFosB and other FosB isoforms are downregulated in the HPC but not the PFC in the brains of both depressed and addicted individuals. Further, we show that potential ΔFosB transcriptional targets, including GluA2, are also downregulated in the HPC but not PFC of cocaine addicts. Thus, we provide the first evidence of FosB gene expression in human HPC and PFC in these psychiatric disorders, and in light of recent findings demonstrating the critical role of HPC ΔFosB in rodent models of learning and memory, these data suggest that reduced ΔFosB in HPC could potentially underlie cognitive deficits accompanying chronic cocaine abuse or depression.

Duality of Antidepressants and Neuroprotectants

The co-morbidity of neuropsychiatric disorders, particularly major depressive disorder (MDD) with neurodegenerative diseases, in particular Parkinson's disease (PD) is now well recognized. Indeed, it is suggested that depressive disorders, especially in late life, may be an indication of latent neurodegeneration. Thus, it is not unreasonable to expect that deterrents of MDD may also deter the onset and/or progression of the neurodegenerative diseases including PD. In this review, examples of neuroprotective efficacy of established as well as prospective antidepressants are provided. Conversely, mood-regulating effects of some neuroprotective drugs are also presented. Thus, in addition to currently used antidepressants, ketamine, nicotine, curcumin, and resveratrol are discussed for their dual efficacy. In addition, potential neurobiological substrates for their actions are presented. It is concluded that pharmacological developments of mood-regulating or neuroprotective drugs can have cross benefit in co-morbid conditions of neuropsychiatric and neurodegenerative disorders and that inflammatory and neurotrophic factors play important roles in both conditions.

Chronic stress, hippocampus and parvalbumin-positive interneurons: what do we know so far?

The hippocampus is a brain structure involved in the regulation of hypothalamic-pituitary-adrenal (HPA) axis and stress response. It plays an important role in the formation of declarative, spatial and contextual memory, as well as in the processing of emotional information. As a part of the limbic system, it is a very susceptible structure towards the effects of various stressors. The molecular mechanisms of structural and functional alternations that occur in the hippocampus under chronic stress imply an increased level of circulating glucocorticoids (GCs), which is an HPA axis response to stress. Certain data show that changes induced by chronic stress may be independent from the GCs levels, opening the possibility of existence of other poorly explored mechanisms and pathways through which stressors act. The hippocampal GABAergic parvalbumin-positive (PV+) interneurons represent an especially vulnerable population of neurons in chronic stress, which may be of key importance in the development of mood disorders. However, cellular and molecular hippocampal changes that arise as a consequence of chronic stress still represent a large and unexplored area. This review discusses the current knowledge about the PV+ interneurons of the hippocampus and the influence of chronic stress on this intriguing population of neurons.

Rapamycin blocks the antidepressant effect of ketamine in task-dependent manner

OBJECTIVE

The aim of our study was to test whether ketamine produces an antidepressant effect in animal model of olfactory bulbectomy and assess the role of mammalian target of rapamycin (mTOR) pathway in ketamine's antidepressant effect.

METHODS

Bulbectomized (OBX) rats and sham controls were assigned to four subgroups according to the treatment they received (ketamine, saline, ketamine + rapamycin, and saline + rapamycin). The animals were subjected to open field (OF), elevated plus maze (EPM), passive avoidance (PA), Morris water maze (MWM), and Carousel maze (CM) tests. Blood samples were collected before and after drug administration for analysis of phosphorylated mTOR level. After behavioral testing, brains were removed for evaluation of brain-derived neurotrophic factor (BDNF) in prefrontal cortex (PFC) and hippocampus.

RESULTS

Ketamine normalized hyperactivity of OBX animals in EPM and increased the time spent in open arms. Rapamycin pretreatment resulted in elimination of ketamine effect in EPM test. In CM test, ketamine + rapamycin administration led to cognitive impairment not observed in saline-, ketamine-, or saline + rapamycin-treated OBX rats. Prefrontal BDNF content was significantly decreased, and level of mTOR was significantly elevated in OBX groups.

CONCLUSIONS

OBX animals significantly differed from sham controls in most of the tests used. Treatment had more profound effect on OBX phenotype than controls. Pretreatment with rapamycin eliminated the anxiolytic and antidepressant effects of ketamine in task-dependent manner. The results indicate that ketamine + rapamycin application resulted in impaired stress responses manifested by cognitive deficits in active place avoidance (CM) test. Intensity of stressor (mild vs. severe) used in the behavioral tests had opposite effect on controls and on OBX animals.

Agmatine produces antidepressant-like effects by activating AMPA receptors and mTOR signaling

The activation of AMPA receptors and mTOR signaling has been reported as mechanisms underlying the antidepressant effects of fast-acting agents, specially the NMDA receptor antagonist ketamine. In the present study, oral administration of agmatine (0.1mg/kg), a neuromodulator that has been reported to modulate NMDA receptors, caused a significant reduction in the immobility time of mice submitted to the tail suspension test (TST), an effect prevented by the administration of DNQX (AMPA receptor antagonist, 2.5μg/site, i.c.v.), BDNF antibody (1μg/site, i.c.v.), K-252a (TrkB receptor antagonist, 1μg/site, i.c.v.), LY294002 (PI3K inhibitor, 10nmol/site, i.c.v.) or rapamycin (selective mTOR inhibitor, 0.2nmol/site, i.c.v.). Moreover, the administration of lithium chloride (non-selective GSK-3β inhibitor, 10mg/kg, p.o.) or AR-A014418 (selective GSK-3β inhibitor, 0.01μg/site, i.c.v.) in combination with a sub-effective dose of agmatine (0.0001mg/kg, p.o.) reduced the immobility time in the TST when compared with either drug alone. Furthermore, increased immunocontents of BDNF, PSD-95 and GluA1 were found in the prefrontal cortex of mice just 1h after agmatine administration. These results indicate that the antidepressant-like effect of agmatine in the TST may be dependent on the activation of AMPA and TrkB receptors, PI3K and mTOR signaling as well as inhibition of GSK-3β, and increase in synaptic proteins. The results contribute to elucidate the complex signaling pathways involved in the antidepressant effect of agmatine and reinforce the pivotal role of these molecular targets for antidepressant responses.

Behavioral effects and CRF expression in brain structures of high- and low-anxiety rats after chronic restraint stress

The aim of our study was to investigate the influence of chronic restraint stress (5 weeks, 3h/day) on behavior and central corticotropin-releasing factor (CRF) expression in rats selected for high (HR) and low anxiety (LR). The conditioned freezing response was used as a discriminating variable. Moreover, we assessed the influence of acute restraint on CRF expression in the brain in HR and LR rats. We found that chronic restraint induced symptoms of anhedonia (decreased consumption of 1% sucrose solution) in HR rats. In addition, HR restraint rats showed an increased learned helplessness behavior (immobility time in the Porsolt test) as well as neophobia in the open field test vs. LR restraint and HR control rats. These behavioral changes were accompanied by a decreased expression of CRF in the paraventricular nucleus of the hypothalamus (pPVN) and the dentate gyrus of the hippocampus (DG) compared to the HR control and LR restraint rat groups, respectively. The acute restraint condition increased the expression of CRF in the pPVN of HR rats compared to the HR control group, and enhanced the expression of CRF in the CA1 area and DG of LR restraint animals compared to the HR restraint and LR control rats, respectively. The present results indicate that chronic restraint stress in high anxiety rats attenuated CRF expression in the pPVN and DG, which was probably due to detrimental actions on the hippocampus-hypothalamus-pituitary-adrenal gland feedback mechanism, thus modulating the stress response and inducing anhedonia and depressive-like symptoms.

Pharmacological approaches to the challenge of treatment-resistant depression

Although monoaminergic antidepressants revolutionized the treatment of Major Depressive Disorder (MDD) over a half-century ago, approximately one third of depressed patients experience treatment-resistant depression (TRD). Such patients account for a disproportionately large burden of disease, as evidenced by increased disability, cost, human suffering, and suicide. This review addresses the definition, causes, evaluation, and treatment of unipolar TRD, as well as the major treatment strategies, including optimization, augmentation, combination, and switch therapies. Evidence for these options, as outlined in this review, is mainly focused on large-scale trials or meta-analyses. Finally, we briefly review emerging targets for antidepressant drug discovery and the novel effects of rapidly acting antidepressants, with a focus on ketamine.

Involvement of M1 and CB₁ receptors in the anxiogenic-like effects induced by neostigmine injected into the rat prelimbic medial prefrontal cortex

The prelimbic (PL) medial prefrontal cortex is a brain region highly involved in the control of emotional responses, being modulated by several neurotransmitter systems, including the cholinergic and endocannabinoid. Activation of muscarinic type 1 (M1) receptors in the brain induces retrograde suppression of inhibition through the induction of endocannabinoid release, which, in turn, activates cannabinoid type 1 (CB1) receptors. No study so far, however, has been conducted to investigate if the cholinergic and endocannabinoid systems interact in the PL to modulate anxiety-related behaviors. Thus, the present work aimed at verifying if intra-PL administration of neostigmine, an acetylcholinesterase inhibitor, would produce changes in anxiety-like behavior and if these effects are mediated by M1 and CB1 receptor activation. Independent groups of animals received bilateral injections of vehicle, the M1 receptor antagonist pirenzepine (0.06, 0.6, and 6 nmol), the CB1 receptor antagonist AM251 (0.1 nmol), or the fatty acid amide hydrolase (FAAH) enzyme inhibitor URB597 (1, 3, and 10 pmol), followed by vehicle or neostigmine (0.01, 0.1, and 1 nmol), and were submitted to the elevated plus-maze (EPM) test. Neostigmine (1 nmol) decreased open arm exploration of the maze. This anxiogenic-like effect was reproduced in another anxiety-related animal model, the light-dark box. Previous injection of pirenzepine or AM251 abolished this response in the EPM, whereas URB597 had no effect. These results suggest that CB1 and M1 receptors interact in the PL to control anxiety-like behaviors.

ERK-dependent brain-derived neurotrophic factor regulation by hesperidin in mice exposed to chronic mild stress

A previous study found that the antidepressant-like effects of ethanolic extracts from Hemerocallis citrina are predominantly related to the flavonoid, hesperidin. The study herein aimed to explore the antidepressant-like mechanism of hesperidin in mice induced by chronic mild stress (CMS). The results indicated that hesperidin reversed the reduction of sucrose preference and the elevation of immobility time in mice induced by CMS. In addition, the increase in serum corticosterone levels and decrease in hippocampal extracellular signal-regulated kinase (ERK) phosphorylation and brain-derived neurotrophic factor (BDNF) levels in CMS mice were also ameliorated by hesperidin treatment. In contrast, improvement by hesperidin was suppressed by pretreatment with ERK inhibitor SL327. Taken together, our findings confirmed the antidepressant-like effect of hesperidin and indicated that hesperidin-induced BDNF up-regulation was mediated in an ERK-dependent manner.

Brain-gut-microbiota axis: challenges for translation in psychiatry

PURPOSE

The accruing data linking the gut microbiome to the development and function of the central nervous system has been proposed as a paradigm shift in neuroscience. The gut microbiota can communicate with the brain via neuroimmune, neuroendocrine, and neural pathways comprising the brain-gut-microbiota axis. Dysfunctional neuroimmune pathways are implicated in stress-related psychiatric disorders.

METHODS

Using depression as our primary example, we review both the preclinical and clinical evidence supporting the possible role played by the gut microbiota in stress-related psychiatric disorders. We consider how this can inform future treatment strategies and outline the challenges and necessary studies for moving the field forward.

RESULTS

The role played by the gut microbiota has not been fully elucidated in psychiatric populations. Although tempting to speculate that psychiatric patients may benefit from therapeutic modulation of the brain-gut-microbiota axis, the translational applications of the results obtained in rodent studies have yet to be demonstrated.

CONCLUSIONS

Evidence of altered gut microbiota composition and function in psychiatric patients is limited and cannot be regarded as proven. Moreover the efficacy of targeting the gut microbiota has not yet been established, and needs further investigation.

Comorbidity Factors and Brain Mechanisms Linking Chronic Stress and Systemic Illness

Neuropsychiatric symptoms and mental illness are commonly present in patients with chronic systemic diseases. Mood disorders, such as depression, are present in up to 50% of these patients, resulting in impaired physical recovery and more intricate treatment regimen. Stress associated with both physical and emotional aspects of systemic illness is thought to elicit detrimental effects to initiate comorbid mental disorders. However, clinical reports also indicate that the relationship between systemic and psychiatric illnesses is bidirectional, further increasing the complexity of the underlying pathophysiological processes. In this review, we discuss the recent evidence linking chronic stress and systemic illness, such as activation of the immune response system and release of common proinflammatory mediators. Altogether, discovery of new targets is needed for development of better treatments for stress-related psychiatric illnesses as well as improvement of mental health aspects of different systemic diseases.

Emerging treatment mechanisms for depression: focus on glutamate and synaptic plasticity

Major depression is a chronic and debilitating illness that effects approximately 1 in 5 people, but currently available treatments are limited by low rates of efficacy, therapeutic time lag, and undesirable side effects. Recent efforts have been directed towards investigating rapid-acting agents that reverse the behavioral and neuronal deficits of chronic stress and depression, notably the glutamate NMDA receptor antagonist ketamine. The cellular mechanisms underlying the rapid antidepressant actions of ketamine and related agents are discussed, as well as novel, selective glutamatergic receptor targets that are safer and have fewer side effects.

MMPIP, an mGluR7-selective negative allosteric modulator, alleviates pain and normalizes affective and cognitive behavior in neuropathic mice

This study investigated the effects of a single administration of 6-(4-methoxyphenyl)-5-methyl-3-pyridinyl-4-isoxazolo[4,5-c]pyridin-4(5H)-one (MMPIP), a negative allosteric modulator (NAM) of metabotropic glutamate receptor 7 (mGluR7), on pain and on affective and cognitive behavior in neuropathic mice. The activity of pyramidal neurons in the prelimbic cortex (PLC), which respond to stimulation of the basolateral amygdala (BLA) with either excitation or inhibition, was also investigated. The spared nerve injury (SNI) of the sciatic nerve induced, 14 days after surgery, thermal hyperalgesia and mechanical allodynia, reduced open-arm choice in the elevated plus-maze, increased time of immobility in the tail suspension, and increased digging and burying in the marble burying test. Cognitive performance was also significantly compromised in the SNI mice. Spared nerve injury induced phenotypic changes on pyramidal neurons of the PLC; excitatory responses increased, whereas inhibitory responses decreased after BLA stimulation. mGluR7 expression, mainly associated with vesicular glutamate transporter, increased in the hippocampus and decreased in the BLA, PLC, and dorsal raphe in SNI mice. MMPIP increased thermal and mechanical thresholds and open-arm choice. It reduced the immobility in the tail suspension test and the number of marbles buried and of digging events in the marble burying test. MMPIP also improved cognitive performance and restored the balance between excitatory and inhibitory responses of PLC neurons in SNI mice. 7-hydroxy-3-(4-iodophenoxy)-4H-chromen-4-one, XAP044, another selective mGluR7 NAM, reproduced the effects of MMPIP on thermal hyperalgesia, mechanical allodynia, tail suspension, and marble burying test. Altogether, these findings show that mGluR7 NAMs reduce pain responses and affective/cognitive impairments in neuropathic pain conditions.

Acute Footshock Stress Induces Time-Dependent Modifications of AMPA/NMDA Protein Expression and AMPA Phosphorylation

Clinical studies on patients with stress-related neuropsychiatric disorders reported functional and morphological changes in brain areas where glutamatergic transmission is predominant, including frontal and prefrontal areas. In line with this evidence, several preclinical works suggest that glutamate receptors are targets of both rapid and long-lasting effects of stress. Here we found that acute footshock- (FS-) stress, although inducing no transcriptional and RNA editing alterations of ionotropic AMPA and NMDA glutamate receptor subunits, rapidly and transiently modulates their protein expression, phosphorylation, and localization at postsynaptic spines in prefrontal and frontal cortex. In total extract, FS-stress increased the phosphorylation levels of GluA1 AMPA subunit at Ser⁸⁴⁵ immediately after stress and of GluA2 Ser⁸⁸⁰ 2 h after start of stress. At postsynaptic spines, stress induced a rapid decrease of GluA2 expression, together with an increase of its phosphorylation at Ser⁸⁸⁰, suggesting internalization of GluA2 AMPA containing receptors. GluN1 and GluN2A NMDA receptor subunits were found markedly upregulated in postsynaptic spines, 2 h after start of stress. These results suggest selected time-dependent changes in glutamatergic receptor subunits induced by acute stress, which may suggest early and transient enhancement of AMPA-mediated currents, followed by a transient activation of NMDA receptors.

A New Perspective on the Anti-Suicide Effects With Ketamine Treatment: A Procognitive Effect

Available evidence indicates that a single, low-dose administration of ketamine is a robust, rapid-onset intervention capable of mitigating depressive symptoms in adults with treatment-resistant mood disorders. Additional evidence also suggests that ketamine may offer antisuicide effects. Herein, we propose that the antidepressant effects reported with ketamine administration are mediated, in part, by targeting neural circuits that subserve cognitive processing relevant to executive function and cognitive emotional processing. Empirical support for the conceptual framework of the cognitive domain as a critical target of ketamine's action is the additional observation that pretreatment cognitive function predicts treatment outcomes with ketamine administration. The proposal that beneficial effects on cognitive function may be, in some individuals, the proximate mechanism mitigating symptom relief in mood disorders exists alongside the well-established deleterious effect of ketamine on cognitive function. During the past 5 years, there have been several reviews and meta-analyses concluding that ketamine has possible clinical benefits in refractory mood disorders. We introduce the conceptual framework that ketamine's salutary effects, notably in suicidality, may in part be via procognitive mechanisms.

Regulation of glutamate transporter 1 via BDNF-TrkB signaling plays a role in the anti-apoptotic and antidepressant effects of ketamine in chronic unpredictable stress model of depression

RATIONALE

Growing evidence suggests that downregulated clearance of glutamate and signaling pathways involving brain-derived neurotrophic factor (BDNF) and its receptor TrkB play a role in morphological changes in the hippocampus of depressed patients. The N-methyl-D-aspartate (NMDA) receptor antagonist ketamine is the most attractive antidepressant, although precise mechanisms are unknown.

OBJECTIVE

In this study, we examined whether hippocampal BDNF-TrkB signaling underlies the antidepressant effects of ketamine via upregulating glutamate transporter 1 (GLT-1) in rats, subjected to the chronic unpredictable stress (CUS) for 42 days. The rats received a single injection of ketamine (10 mg/kg, i.p.) and/or a TrkB inhibitor, K252a (1 μl, 2 mM, intracerebroventicular (i.c.v.)) on day 43. Behavioral tests and brain sample collection were evaluated 24 h later.

RESULTS

The CUS-exposed rats exhibited depression- and anxiety-like behaviors; decreased number of glial fibrillary acidic protein (GFAP)-positive (but not NeuN-positive) cells in the dentate gyrus (DG), CA1, and CA3 areas; increased number of cleaved caspase-3-positive astrocytes; reduced spine density; lower ratio of Bcl2 to Bax; and decreased levels of BDNF, phosphorylated cAMP response element binging protein (CREB), GLT-1, and postsynaptic density 95 (PSD95) proteins in the hippocampus. Ketamine alleviated the CUS-induced abnormalities. The effects of ketamine were antagonized by pretreatment with K252a.

CONCLUSIONS

Our findings suggest that regulation of GLT-1 on astrocytes, responsible for 90 % of glutamate reuptake from the synapse, through BDNF-TrkB signaling is involved in mediation of the therapeutic effects of ketamine on behavioral abnormalities and morphological changes in the hippocampus of the CUS-exposed rats.

Apigenin reverses depression-like behavior induced by chronic corticosterone treatment in mice

Previous researches found that apigenin exerted antidepressant-like effects in rodents. However, it is unclear whether the neurotrophic system is involved in the antidepressant-like effects of apigenin. Our present study aimed to explore the neurotrophic related mechanism of apigenin in depressive-like mice induced by chronic corticosterone treatment. Mice were repeatedly injected with corticosterone (40 mg/kg) subcutaneously (s.c) once daily for consecutive 21 days. Apigenin (20 and 40 mg/kg) and fluoxetine (20 mg/kg) were administered 30 min prior to the corticosterone injection. The behavioral tests indicated that apigenin reversed the reduction of sucrose preference and the elevation of immobility time in mice induced by chronic corticosterone treatment. In addition, the increase in serum corticosterone levels and the decrease in hippocampal brain-derived neurotrophic factor (BDNF) levels in corticosterone-treated mice were also ameliorated by apigenin administration. Taken together, our findings intensively confirmed the antidepressant-like effects of apigenin and indicated that the antidepressant-like mechanism of apigenin was mediated, at least partly by up-regulation of BDNF levels in the hippocampus.

Dendritic Spines in Depression: What We Learned from Animal Models

Depression, a severe psychiatric disorder, has been studied for decades, but the underlying mechanisms still remain largely unknown. Depression is closely associated with alterations in dendritic spine morphology and spine density. Therefore, understanding dendritic spines is vital for uncovering the mechanisms underlying depression. Several chronic stress models, including chronic restraint stress (CRS), chronic unpredictable mild stress (CUMS), and chronic social defeat stress (CSDS), have been used to recapitulate depression-like behaviors in rodents and study the underlying mechanisms. In comparison with CRS, CUMS overcomes the stress habituation and has been widely used to model depression-like behaviors. CSDS is one of the most frequently used models for depression, but it is limited to the study of male mice. Generally, chronic stress causes dendritic atrophy and spine loss in the neurons of the hippocampus and prefrontal cortex. Meanwhile, neurons of the amygdala and nucleus accumbens exhibit an increase in spine density. These alterations induced by chronic stress are often accompanied by depression-like behaviors. However, the underlying mechanisms are poorly understood. This review summarizes our current understanding of the chronic stress-induced remodeling of dendritic spines in the hippocampus, prefrontal cortex, orbitofrontal cortex, amygdala, and nucleus accumbens and also discusses the putative underlying mechanisms.

In Sickness and in Health: Perineuronal Nets and Synaptic Plasticity in Psychiatric Disorders

Rapidly emerging evidence implicates perineuronal nets (PNNs) and extracellular matrix (ECM) molecules that compose or interact with PNNs, in the pathophysiology of several psychiatric disorders. Studies on schizophrenia, autism spectrum disorders, mood disorders, Alzheimer's disease, and epilepsy point to the involvement of ECM molecules such as chondroitin sulfate proteoglycans, Reelin, and matrix metalloproteases, as well as their cell surface receptors. In many of these disorders, PNN abnormalities have also been reported. In the context of the “quadripartite” synapse concept, that is, the functional unit composed of the pre- and postsynaptic terminals, glial processes, and ECM, and of the role that PNNs and ECM molecules play in regulating synaptic functions and plasticity, these findings resonate with one of the most well-replicated aspects of the pathology of psychiatric disorders, that is, synaptic abnormalities. Here we review the evidence for PNN/ECM-related pathology in these disorders, with particular emphasis on schizophrenia, and discuss the hypothesis that such pathology may significantly contribute to synaptic dysfunction.