Severe depression is associated with increased microglial quinolinic acid in subregions of the anterior cingulate gyrus: evidence for an immune-modulated glutamatergic neurotransmission?

New concepts in the development of schizophrenia, autism spectrum disorders, and degenerative brain diseases based on chronic inflammation: A working hypothesis from continued advances in neuroscience research

This paper was written prompted by a poignant film about adolescent girl with schizophrenia who babysits for a younger girl in an isolated cabin. Schizophrenia is an illness that both authors are fascinated with and that they continue to study and investigate. There is now compelling evidence that schizophrenia is a very complex syndrome that involves numerous neural pathways in the brain, far more than just dopaminergic and serotonergic systems. One of the more popular theories in recent literature is that it represents a hypo glutaminergic deficiency of certain pathways, including thalamic ones. After much review of research and study in this area, we have concluded that most such theories contain a number of shortcomings. Most are based on clinical responses to certain drugs, particularly antipsychotic drugs affecting the dopaminergic neurotransmitters; thus, assuming dopamine release was the central cause of the psychotic symptoms of schizophrenia. The theory was limited in that dopamine excess could only explain the positive symptoms of the disorder. Antipsychotic medications have minimal effectiveness for the negative and cognitive symptoms associated with schizophrenia. It has been estimated that 20–30% of patients show either a partial or no response to antipsychotic medications. In addition, the dopamine hypothesis does not explain the neuroanatomic findings in schizophrenia.

Mitophagy in depression: Pathophysiology and treatment targets

Mitochondria, the 'powerhouse' of eukaryotic cells, play a key role in cellular homeostasis. However, defective mitochondria increase mitochondrial ROS (mtROS) production and cell-free mitochondrial DNA (mtDNA) release, leading to increased inflammation. Mitophagy is a vital pathway, which selectively removes defective mitochondria through the process of autophagy. Thus, an impairment in the mitophagy pathway might trigger the gradual accumulation of defective mitochondria. Accumulating evidence suggest that inflammation and mitochondrial dysfunction are linked to the pathogenesis of depression. In this article, we have reviewed the role of impaired mitophagy as a contributing factor in depression pathophysiology. Further, we have discussed the potential therapeutic interventions aimed at modulating mitophagy in depression.

The Kynurenine Pathway as a Potential Target for Neuropathic Pain Therapy Design: From Basic Research to Clinical Perspectives

Accumulating evidence suggests the key role of the kynurenine pathway (KP) of the tryptophan metabolism in the pathogenesis of several diseases. Despite extensive research aimed at clarifying the mechanisms underlying the development and maintenance of neuropathic pain, the roles of KP metabolites in this process are still not fully known. Although the function of the peripheral KP has been known for several years, it has only recently been acknowledged that its metabolites within the central nervous system have remarkable consequences related to physiology and behavior. Both the products and metabolites of the KP are involved in the pathogenesis of pain conditions. Apart from the neuroactive properties of kynurenines, the KP regulates several neurotransmitter systems in direct or indirect ways. Some neuroactive metabolites are known to have neuroprotective properties (kynurenic acid, nicotinamide adenine dinucleotide cofactor), while others are toxic (3-hydroxykynurenine, quinolinic acid). Numerous animal models show that modulation of the KP may turn out to be a viable target for the treatment of diseases. Importantly, some compounds that affect KP enzymes are currently described to possess analgesic properties. Additionally, kynurenine metabolites may be useful for assessing response to therapy or as biomarkers in therapeutic monitoring. The following review describes the molecular site of action and changes in the levels of metabolites of the kynurenine pathway in the pathogenesis of various conditions, with a particular emphasis on their involvement in neuropathy. Moreover, the potential clinical implications of KP modulation in chronic pain therapy as well as the directions of new research initiatives are discussed.

How Inflammation Affects the Brain in Depression: A Review of Functional and Structural MRI Studies

This narrative review discusses how peripheral and central inflammation processes affect brain function and structure in depression, and reports on recent peripheral inflammatory marker-based functional and structural magnetic resonance imaging (MRI) studies from the perspective of neural-circuit dysfunction in depression. Chronic stress stimulates the activity of microglial cells, which increases the production of pro-inflammatory cytokines in the brain. In addition, microglial activation promotes a shift from the synthesis of serotonin to the synthesis of neurotoxic metabolites of the kynurenine pathway, which induces glutamate-mediated excitotoxicity in neurons. Furthermore, the region specificity of microglial activation is hypothesized to contribute to the vulnerability of specific brain regions in the depression-related neural circuits to inflammation-mediated brain injury. MRI studies are increasingly investigating how the blood levels of inflammatory markers such as C-reactive protein, interleukin (IL)-1β, IL-6, and tumor necrosis factor-α are associated with functional and structural neuroimaging markers in depression. Functional MRI studies have found that peripheral inflammatory markers are associated with aberrant activation patterns and altered functional connectivity in neural circuits involved in emotion regulation, reward processing, and cognitive control in depression. Structural MRI studies have suggested that peripheral inflammatory markers are related to reduced cortical gray matter and subcortical volumes, cortical thinning, and decreased integrity of white matter tracts within depression-related neural circuits. These neuroimaging findings may improve our understanding of the relationships between neuroinflammatory processes at the molecular level and macroscale in vivo neuralcircuit dysfunction in depression.

Exploring the roles of tryptophan metabolism in MS beyond neuroinflammation and neurodegeneration: A paradigm shift to neuropsychiatric symptoms

The metabolism of tryptophan through the kynurenine pathway (KP) has been increasingly recognised in contributing to disease progression in the autoimmune and inflammatory disease multiple sclerosis (MS). In this review, the roles of inflammation and the KP are recontextualised to better understand the aetiology of the neuropsychiatric symptoms (depression, postpartum depression, suicidality, fatigue and cognitive dysfunction) in MS. These symptoms will be discussed in the context of cytokine-induced sickness behaviours, KP activation and levels of neurotoxicity and neuroprotection in MS. In particular, there will be emphasis on how neuropsychiatric symptoms in MS occur against the shared background of inflammation and KP dysregulation. The discourse of this review aims to promote future research in elucidating KP mechanisms in MS that would inevitably lead to more targeted treatment options for neuropsychiatric symptoms and disease progression.

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Research on tryptophan metabolism and neuroinflammation on neurodegeneration in multiple sclerosis (MS) is mounting.

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This review reframes the roles of neuroinflammation and tryptophan metabolism dysregulation on mental health issues in MS.

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The impact of neuroinflammation and tryptophan metabolism on depression, suicidality, fatigue, and cognitive impairment in MS are discussed.

Alterations of kynurenine pathway in alcohol use disorder and abstinence: a link with gut microbiota, peripheral inflammation and psychological symptoms

The gut-brain communication is mostly driven by the immune, metabolic and neural pathways which remained poorly explored in patients with alcohol use disorder (AUD). The metabolites arising from the tryptophan-kynurenine pathway have gained considerable attention since they are at the interface between intestinal bacteria, host immune response and brain functions. This study described the circulating levels of kynurenine metabolites in AUD patients, at the onset (T1) and end (T2) of a 3-week detoxification program, and tested correlations between those metabolites and inflammatory markers, the gut microbiota and the psychological symptoms. Increased concentration of the neurotoxic metabolite quinolinic acid (QUIN) and decreased levels of the neuroprotector metabolite kynurenic acid (KYNA) which both modulate glutamatergic neurotransmission were observed in AUD patients, particularly at T2. The inflammatory marker hsCRP was associated with several metabolic ratios of the kynurenine pathway. Tryptophan, KYNA and QUIN were correlated with depression, alcohol craving and reaction time, respectively. Analysis of gut microbiota revealed that bacteria known as short-chain fatty acid producers, as well as bacterial metabolites including butyrate and medium-chain fatty acids were associated with some metabolites of the tryptophan-kynurenine pathway. Targeting the glutamatergic neurotransmission through the modulation of the kynurenine pathway, by manipulating the gut microbiota, might represent an interesting alternative for modulating alcohol-related behavior.

Psychological Stresses in Children Trigger Cytokine- and Kynurenine Metabolite-Mediated Abdominal Pain and Proinflammatory Changes

Recurrent abdominal pain (RAP) is a common medically unexplained symptom among children worldwide. However, the biological mechanisms behind the development of functional and behavioral symptoms and changes in blood markers have not been well explored. This study aimed to assess changes in the concentrations of inflammatory markers, including cytokines and tryptophan catabolites, in the serum of children with RAP compared to those with subclinical infections. Children with RAP but without organic diseases were included, and those with asymptomatic intestinal parasitic infections were used as a subclinical infection cohort. Blood samples were collected and used to measure the cytokine profile using Multiplex Immunoassay and tryptophan catabolites using high performance liquid chromatography. Children with RAP showed significantly higher concentrations of serum tumor necrotic factor-α, p<0.05, but lower concentrations of IL-10, p<0.001, IL-6, p<0.001 and brain-derived neurotrophic factors (BDNF) p<0.01. In addition, a significant increase in the metabolite of the kynurenine pathway, 3-hydroxyanthranilic acid (3-HAA) p<0.01, a significant decrease in the concentrations of anthranilic acid (AA) p<0.001, together with an increased ratio of serum 3-HAA to AA (3-HAA/AA) p<0.001, was found in this cohort. These findings indicate the significant activation of the immune system and presence of inflammation in children with RAP than those with subclinical parasitic infections. Moreover, children with RAP tested with the Strengths and Difficulties Questionnaire (SDQ), displayed high psychological problems though these SDQ scores were not statistically associated with measured cytokines and kynurenine metabolites. We however could hypothesize that the pro-inflammatory state together with concomitant low concentrations of BDNF in those children with RAP could play a role in psychological stress and experiencing medically unexplained symptoms.

Is there a place for cellular therapy in depression?

Although efforts have been made to improve the pharmacological treatment of depression, approximately one-third of patients with depression do not respond to conventional therapy using antidepressants. Other potential non-pharmacological therapies have been studied in the last years, including the use of mesenchymal stem cell therapies to treat depression. These therapies are reviewed here since it is clinically relevant to develop innovative therapeutics to treat psychiatric patients. Experimental data corroborate that mesenchymal stem cell therapy could be considered a potential treatment for depression based on its anti-inflammatory and neurotrophic properties. However, some clinical trials involving treatment of depression with stem cells are in progress, but with no published results. These studies and other future clinical investigations will be crucial to define how much mesenchymal stem cells can effectively be used in psychiatric clinics as a strategy for supporting depression treatment.

Update on the Relationship Between Depression and Neuroendocrine Metabolism

Through the past decade of research, the correlation between depression and metabolic diseases has been noticed. More and more studies have confirmed that depression is comorbid with a variety of metabolic diseases, such as obesity, diabetes, metabolic syndrome and so on. Studies showed that the underlying mechanisms of both depression and metabolic diseases include chronic inflammatory state, which is significantly related to the severity. In addition, they also involve endocrine, immune systems. At present, the effects of clinical treatments of depression is limited. Therefore, exploring the co-disease mechanism of depression and metabolic diseases is helpful to find a new clinical therapeutic intervention strategy. Herein, focusing on the relationship between depression and metabolic diseases, this manuscript aims to provide an overview of the comorbidity of depression and metabolic.

The emerging tale of microglia in psychiatric disorders

As the professional phagocytes of the brain, microglia orchestrate the immunological response and play an increasingly important role in maintaining homeostatic brain functions. Microglia are activated by pathological events or slight alterations in brain homeostasis. This activation is dependent on the context and type of stressor or pathology. Through secretion of cytokines, chemokines and growth factors, microglia can strongly influence the response to a stressor and can, therefore, determine the pathological outcome. Psychopathologies have repeatedly been associated with long-lasting priming and sensitization of cerebral microglia. This review focuses on the diversity of microglial phenotype and function in health and psychiatric disease. We first discuss the diverse homeostatic functions performed by microglia and then elaborate on context-specific spatial and temporal microglial heterogeneity. Subsequently, we summarize microglia involvement in psychopathologies, namely major depressive disorder, schizophrenia and bipolar disorder, with a particular focus on post-mortem studies. Finally, we postulate microglia as a promising novel therapeutic target in psychiatry through antidepressant and antipsychotic treatment.

From psychiatry to neurology: Psychedelics as prospective therapeutics for neurodegenerative disorders

The studies of psychedelics, especially psychedelic tryptamines like psilocybin, are rapidly gaining interest in neuroscience research. Much of this interest stems from recent clinical studies demonstrating that they have a unique ability to improve the debilitating symptoms of major depressive disorder (MDD) long-term after only a single treatment. Indeed, the Food and Drug Administration (FDA) has recently designated two Phase III clinical trials studying the ability of psilocybin to treat forms of MDD with "Breakthrough Therapy" status. If successful, the use of psychedelics to treat psychiatric diseases like depression would be revolutionary. As more evidence appears in the scientific literature to support their use in psychiatry to treat MDD on and substance use disorders (SUD), recent studies with rodents revealed that their therapeutic effects might extend beyond treating MDD and SUD. For example, psychedelics may have efficacy in the treatment and prevention of brain injury and neurodegenerative diseases such as Alzheimer's Disease. Preclinical work has highlighted psychedelics' ability to induce neuroplasticity and synaptogenesis, and neural progenitor cell proliferation. Psychedelics may also act as immunomodulators by reducing levels of proinflammatory biomarkers, including IL-1β, IL-6, and tumor necrosis factor-α (TNF-α). Their exact molecular mechanisms, and induction of cellular interactions, especially between neural and glial cells, leading to therapeutic efficacy, remain to be determined. In this review, we discuss recent findings and information on how psychedelics may act therapeutically on cells within the central nervous system (CNS) during brain injuries and neurodegenerative diseases.

Doxycycline at subantimicrobial dose combined with escitalopram reverses depressive-like behavior and neuroinflammatory hippocampal alterations in the lipopolysaccharide model of depression

Doxycycline (DOXY) is a second-generation tetracycline with anti-inflammatory and neuroprotective effects. A proinflammatory profile seems to predict the severity of depressive symptoms. In the present study, we aimed at determining whether the anti-inflammatory action of subantimicrobial-dose doxycycline (SDD) (DOXY, 10mg/kg), alone or combined with the antidepressant escitalopram (ESC), could revert lipopolysaccharide-induced depressive-like alterations in mice. Male Swiss mice received saline or lipopolysaccharide (LPS) for ten consecutive days. From the 6^th day of LPS exposure, they were treated with DOXY 10 mg/kg, ESC 4 mg/kg, DOXY 10 mg/kg plus ESC 4 mg/kg (DOXY+ESC), or saline. On the 10^th day, we assessed behavioral despair (forced swimming test), anhedonia (sucrose preference test), brain oxidative stress markers, and inflammatory and protective pathways related to depression, such as NF-kB and phospho-CREB. Our results showed that DOXY alone or combined with ESC reduced hippocampal Iba-1 expression and interleukin (IL)-1β levels. Only DOXY+ESC successfully reversed the LPS-induced increase in NF-kBp65 expression and TNFα levels. DOXY caused a marked increase in the hippocampal expression of phospho-CREB and GSH concentrations. DOXY and DOXY+ESC showed a tendency to modulate the functional status of mitogen-activated kinase p42-44 (Phospho-p44/42 MAPK) and of the phosphorylated form of glycogen synthase kinase 3 beta (GSK3β), revealing a protective profile against inflammation. In conclusion, SDD, combined with ESC, seems to be a good strategy for reverting inflammatory changes and protecting against depression.

The kynurenine pathway and bipolar disorder: intersection of the monoaminergic and glutamatergic systems and immune response

Dysfunction in a wide array of systems-including the immune, monoaminergic, and glutamatergic systems-is implicated in the pathophysiology of depression. One potential intersection point for these three systems is the kynurenine (KYN) pathway. This study explored the impact of the prototypic glutamatergic modulator ketamine on the endogenous KYN pathway in individuals with bipolar depression (BD), as well as the relationship between response to ketamine and depression-related behavioral and peripheral inflammatory markers. Thirty-nine participants with treatment-resistant BD (23 F, ages 18-65) received a single ketamine infusion (0.5 mg/kg) over 40 min. KYN pathway analytes-including plasma concentrations of indoleamine 2,3-dioxygenase (IDO), KYN, kynurenic acid (KynA), and quinolinic acid (QA)-were assessed at baseline (pre-infusion), 230 min, day 1, and day 3 post-ketamine. General linear models with restricted maximum likelihood estimation and robust sandwich variance estimators were implemented. A repeated effect of time was used to model the covariance of the residuals with an unstructured matrix. After controlling for age, sex, and body mass index (BMI), post-ketamine IDO levels were significantly lower than baseline at all three time points. Conversely, ketamine treatment significantly increased KYN and KynA levels at days 1 and 3 versus baseline. No change in QA levels was observed post-ketamine. A lower post-ketamine ratio of QA/KYN was observed at day 1. In addition, baseline levels of proinflammatory cytokines and behavioral measures predicted KYN pathway changes post ketamine. The results suggest that, in addition to having rapid and sustained antidepressant effects in BD participants, ketamine also impacts key components of the KYN pathway.

Neuromodulation by the immune system: a focus on cytokines

Interactions between the immune system and the nervous system have been described mostly in the context of diseases. More recent studies have begun to reveal how certain immune cell-derived soluble effectors, the cytokines, can influence host behaviour even in the absence of infection. In this Review, we contemplate how the immune system shapes nervous system function and how it controls the manifestation of host behaviour. Interactions between these two highly complex systems are discussed here also in the context of evolution, as both may have evolved to maximize an organism's ability to respond to environmental threats in order to survive. We describe how the immune system relays information to the nervous system and how cytokine signalling occurs in neurons. We also speculate on how the brain may be hardwired to receive and process information from the immune system. Finally, we propose a unified theory depicting a co-evolution of the immune system and host behaviour in response to the evolutionary pressure of pathogens.

Statins: Neurobiological underpinnings and mechanisms in mood disorders

Statins (3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors) treat dyslipidaemia and cardiovascular disease by inhibiting cholesterol biosynthesis. They also have immunomodulatory and anti-inflammatory properties. Beyond cardiovascular disease, cholesterol and inflammation appear to be components of the pathogenesis and pathophysiology of neuropsychiatric disorders. Statins may therefore afford some therapeutic benefit in mood disorders. In this paper, we review the pathophysiology of mood disorders with a focus on pharmacologically relevant pathways, using major depressive disorder and bipolar disorder as exemplars. Statins are discussed in the context of these disorders, with particular focus on the putative mechanisms involved in their anti-inflammatory and immunomodulatory effects. Recent clinical data suggest that statins may have antidepressant properties, however given their interactions with many known biological pathways, it has not been fully elucidated which of these are the major determinants of clinical outcomes in mood disorders. Moreover, it remains unclear what the appropriate dose, or appropriate patient phenotype for adjunctive treatment may be. High quality randomised control trials in concert with complementary biological investigations are needed if the potential clinical effects of statins on mood disorders, as well as their biological correlates, are to be better understood.

Neuroinflammatory Basis of Depression: Learning From Experimental Models

The neuroinflammatory basis of depression encompasses the detrimental role of otherwise supportive non-neuronal cells and neuroinflammation in hampering neuronal function, leading to depressive behavior. Animals subjected to different stress paradigms show glial cell activation and a surge in proinflammatory cytokines in various brain regions. The concept of sterile inflammation observed in animal models of depression has intrigued many researchers to determine the possible triggers of central immune cell activation. Notably, microglial activation and subsequent phenotypic polarization in depression have been strongly advocated by the wealth of recent preclinical studies; however, findings from human studies have shown contradictory results. Despite intensive investigation, many research gaps still exist to elucidate the molecular mechanisms of neuroinflammatory cascades underlying the pathophysiology of depression. In this mini-review, recent progress in understanding neuroinflammatory mechanisms in light of experimental models of depression will be thoroughly discussed. The challenges of mirroring depression in animal and in vitro models will also be highlighted. Furthermore, prospects of targeting neuroinflammation to treat depressive disorder will be covered.

The Footprint of Kynurenine Pathway in Neurodegeneration: Janus-Faced Role in Parkinson's Disorder and Therapeutic Implications

Progressive degeneration of neurons and aggravation of dopaminergic neurons in the substantia nigra pars compacta results in the loss of dopamine in the brain of Parkinson's disease (PD) patients. Numerous therapies, exhibiting transient efficacy have been developed; however, they are mostly accompanied by side effects and limited reliability, therefore instigating the need to develop novel optimistic treatment targets. Significant therapeutic targets have been identified, namely: chaperones, protein Abelson, glucocerebrosidase-1, calcium, neuromelanin, ubiquitin-proteasome system, neuroinflammation, mitochondrial dysfunction, and the kynurenine pathway (KP). The role of KP and its metabolites and enzymes in PD, namely quinolinic acid (QUIN), kynurenic acid (KYNA), 3-hydroxykynurenine (3-HK), 3-hydroxyanthranillic acid (3-HAA), kunurenine-3-monooxygenase (KMO), etc. has been reported. The neurotoxic QUIN, N-methyl-D-aspartate (NMDA) receptor agonist, and neuroprotective KYNA-which antagonizes QUIN actions-primarily justify the Janus-faced role of KP in PD. Moreover, KP has been reported to play a biomarker role in PD detection. Therefore, the authors detail the neurotoxic, neuroprotective, and immunomodulatory neuroactive components, alongside the upstream and downstream metabolic pathways of KP, forming a basis for a therapeutic paradigm of the disease while recognizing KP as a potential biomarker in PD, thus facilitating the development of a suitable target in PD management.

Comprehensive Bibliometric Analysis of the Kynurenine Pathway in Mood Disorders: Focus on Gut Microbiota Research

Background: Emerging evidence implicates the dysregulated kynurenine pathway (KP), an immune-inflammatory pathway, in the pathophysiology of mood disorders (MD), including depression and bipolar disorder characterized by a low-grade chronic pro-inflammatory state. The metabolites of the KP, an important part of the microbiota-gut-brain axis, serve as immune system modulators linking the gut microbiota (GM) with the host central nervous system.

Aim: This bibliometric analysis aimed to provide a first glimpse into the KP in MD, with a focus on GM research in this field, to guide future research and promote the development of this field.

Methods: Publications relating to the KP in MD between the years 2000 and 2020 were retrieved from the Scopus and Web of Science Core Collection (WoSCC), and analyzed in CiteSpace (5.7 R5W), biblioshiny (using R-Studio), and VOSviewer (1.6.16).

Results: In total, 1,064 and 948 documents were extracted from the Scopus and WoSCC databases, respectively. The publications have shown rapid growth since 2006, partly owing to the largest research hotspot appearing since then, “quinolinic acid.” All the top five most relevant journals were in the neuropsychiatry field, such as Brain Behavior and Immunity. The United States and Innsbruck Medical University were the most influential country and institute, respectively. Journal co-citation analysis showed a strong tendency toward co-citation of research in the psychiatry field. Reference co-citation analysis revealed that the top four most important research focuses were “kynurenine pathway,” “psychoneuroimmunology,” “indoleamine 2,3-dioxygenase,” and “proinflammatory cytokines,” and the most recent focus was “gut-brain axis,” thus indicating the role of the KP in bridging the GM and the host immune system, and together reflecting the field’s research foundations. Overlap analysis between the thematic map of keywords and the keyword burst analysis revealed that the topics “Alzheimer’s disease,” “prefrontal cortex,” and “acid,” were research frontiers.

Conclusion: This comprehensive bibliometric study provides an updated perspective on research associated with the KP in MD, with a focus on the current status of GM research in this field. This perspective may benefit researchers in choosing suitable journals and collaborators, and aid in the further understanding of the field’s hotspots and frontiers, thus facilitating future research.

Neuroinflammation and the Kynurenine Pathway in CNS Disease: Molecular Mechanisms and Therapeutic Implications

Diseases of the central nervous system (CNS) remain a significant health, social and economic problem around the globe. The development of therapeutic strategies for CNS conditions has suffered due to a poor understanding of the underlying pathologies that manifest them. Understanding common etiological origins at the cellular and molecular level is essential to enhance the development of efficacious and targeted treatment options. Over the years, neuroinflammation has been posited as a common link between multiple neurological, neurodegenerative and neuropsychiatric disorders. Processes that precipitate neuroinflammatory conditions including genetics, infections, physical injury and psychosocial factors, like stress and trauma, closely link dysregulation in kynurenine pathway (KP) of tryptophan metabolism as a possible pathophysiological factor that 'fuel the fire' in CNS diseases. In this study, we aim to review emerging evidence that provide mechanistic insights between different CNS disorders, neuroinflammation and the KP. We provide a thorough overview of the different branches of the KP pertinent to CNS disease pathology that have therapeutic implications for the development of selected and efficacious treatment strategies.

Tryptophan-kynurenine metabolism: a link between the gut and brain for depression in inflammatory bowel disease

Inflammatory bowel disease (IBD), which mainly includes ulcerative colitis (UC) and Crohn's disease (CD), is a group of chronic bowel diseases that are characterized by abdominal pain, diarrhea, and bloody stools. IBD is strongly associated with depression, and its patients have a higher incidence of depression than the general population. Depression also adversely affects the quality of life and disease prognosis of patients with IBD. The tryptophan-kynurenine metabolic pathway degrades more than 90% of tryptophan (TRP) throughout the body, with indoleamine 2,3-dioxygenase (IDO), the key metabolic enzyme, being activated in the inflammatory environment. A series of metabolites of the pathway are neurologically active, among which kynerunic acid (KYNA) and quinolinic acid (QUIN) are molecules of great interest in recent studies on the mechanisms of inflammation-induced depression. In this review, the relationship between depression in IBD and the tryptophan-kynurenine metabolic pathway is overviewed in the light of recent publications.

Immunoregulation and antidepressant effect of ketamine

Major depressive disorder (MDD) is a common mental health disorder that brings severe disease burden worldwide. Traditional antidepressants are mainly targeted at monoamine neurotransmitters, with low remission rates and high recurrence rates. Ketamine is a noncompetitive glutamate N-methyl-d-aspartate receptor (NMDAR) antagonist, and its rapid and powerful antidepressant effects have come to light. Its antidepressant mechanism is still unclarified. Research found that ketamine had not only antagonistic effect on NMDAR but also strong immunomodulatory effect, both of which were closely related to the pathophysiology of MDD. Although there are many related studies, they are relatively heterogeneous. Therefore, this review mainly describes the immune mechanisms involved in MDD and how ketamine plays an antidepressant role by regulating peripheral and central immune system, including peripheral inflammatory cytokines, central microglia, and astrocytes. This review summarizes the related research, finds out the deficiencies of current research, and provides ideas for future research and the development of novel antidepressants.

The kynurenine pathway in major depression: What we know and where to next

Major depression is a serious psychiatric disorder, occurring in up to 20 % of the population. Despite its devastating burden, the neurobiological changes associated with depression are not fully understood. A growing body of evidence suggests the kynurenine pathway is implicated in the pathophysiology of depression. In this review, we bring together the literature examining elements of the kynurenine pathway in depression and explore the implications for the pathophysiology and treatment of depression, while highlighting the gaps in the current knowledge. Current research indicates an increased potential for neurotoxic activity of the kynurenine pathway in peripheral blood samples but an increased activation of the putative neuroprotective arm in some brain regions in depression. The disconnect between these findings requires further investigation, with a greater research effort on elucidating the central effects of the kynurenine pathway in driving depression symptomology. Research investigating the benefits of targeting the kynurenine pathway centred on human brain findings and the heterogenous subtypes of depression will help guide the identification of effective drug targets in depression.

Major Depression: One Brain, One Disease, One Set of Intertwined Processes

Major depressive disorder (MDD) is a heterogeneous disease affecting one out of five individuals and is the leading cause of disability worldwide. Presently, MDD is considered a multifactorial disease with various causes such as genetic susceptibility, stress, and other pathological processes. Multiple studies allowed the formulation of several theories attempting to describe the development of MDD. However, none of these hypotheses are comprehensive because none of them can explain all cases, mechanisms, and symptoms of MDD. Nevertheless, all of these theories share some common pathways, which lead us to believe that these hypotheses depict several pieces of the same big puzzle. Therefore, in this review, we provide a brief description of these theories and their strengths and weaknesses in an attempt to highlight the common mechanisms and relationships of all major theories of depression and combine them together to present the current overall picture. The analysis of all hypotheses suggests that there is interdependence between all the brain structures and various substances involved in the pathogenesis of MDD, which could be not entirely universal, but can affect all of the brain regions, to one degree or another, depending on the triggering factor, which, in turn, could explain the different subtypes of MDD.

Suicidality Associated With Deep Brain Stimulation in Extrapyramidal Diseases: A Critical Review and Hypotheses on Neuroanatomical and Neuroimmune Mechanisms

Deep brain stimulation (DBS) is a very well-established and effective treatment for patients with extrapyramidal diseases. Despite its generally favorable clinical efficacy, some undesirable outcomes associated with DBS have been reported. Among such complications are incidences of suicidal ideation (SI) and behavior (SB) in patients undergoing this neurosurgical procedure. However, causal associations between DBS and increased suicide risk are not demonstrated and they constitute a debated issue. In light of these observations, the main objective of this work is to provide a comprehensive and unbiased overview of the literature on suicide risk in patients who received subthalamic nucleus (STN) and internal part of globus pallidum (GPi) DBS treatment. Additionally, putative mechanisms that might be involved in the development of SI and SB in these patients as well as caveats associated with these hypotheses are introduced. Finally, we briefly propose some clinical implications, including therapeutic strategies addressing these potential disease mechanisms. While a mechanistic connection between DBS and suicidality remains a controversial topic that requires further investigation, it is of critical importance to consider suicide risk as an integral component of candidate selection and post-operative care in DBS.

The role of microglia in chronic pain and depression: innocent bystander or culprit?

Clinical evidence shows that chronic pain and depression often accompany each other, but the underlying pathogenesis of comorbid chronic pain and depression remains mostly undetermined. Biotechnology is gradually revealing the phenotype and function of microglia, with great progress regarding microglia's role in neurodegeneration, depression, chronic pain, and other conditions. This article summarizes the role of microglia in chronic pain, depression, and comorbidities, which is conducive to finding new targets to treat chronic pain and depression.

Brain-immune crosstalk in the treatment of major depressive disorder

A growing number of studies are pointing out the need for a conceptual shift from a brain-centered to a body-inclusive approach in mental health research. In this perspective, the link between the immune and the nervous system, which are deeply interconnected and continuously interacting, is one of the most important novel theoretical framework to investigate the biological bases of major depressive disorder and, more in general, mental illness. Indeed, depressed patients show high levels of inflammatory markers, administration of pro-inflammatory drugs triggers a depressive symptomatology and antidepressant efficacy is reduced by excessive immune system activation. A number of molecular and cellular mechanisms have been hypothesized to act as a link between the immune and brain function, thus representing potential pharmacologically targetable processes for the development of novel and effective therapeutic strategies. These include the modulation of the kynurenine pathway, the crosstalk between metabolic and inflammatory processes, the imbalance in acquired immune responses, in particular T cell responses, and the interplay between neural plasticity and immune system activation. In the personalized medicine approach, the assessment and regulation of these processes have the potential to lead, respectively, to novel diagnostic approaches for the prediction of treatment outcome according to the patient's immunological profile, and to improved efficacy of antidepressant compounds through immune modulation.

Predictive Genetic Variations in the Kynurenine Pathway for Interferon-α-Induced Depression in Patients with Hepatitis C Viral Infection

Importance: The high incidence of major depressive episodes during interferon-α (IFN-α) therapy is considered the most powerful supportive evidence for the inflammation theory of depression. As the kynurenine pathway plays an important role connecting inflammation and depression, it is plausible to investigate this pathway for predictive genetic markers for IFN-α-induced depression. Methods: In this prospective case-control study, we assessed 291 patients with chronic hepatitis C viral infection taking IFN-α therapy and analyzed the single nucleotide polymorphisms (SNPs) in genes in the kynurenine pathway. Our case group contained patients who developed IFN-α-induced depression during the treatment, and others were defined as the control group. Genomic DNA was extracted from leukocytes in the peripheral blood and analyzed by Affymetrix TWB array. We first tested allelic, dominant, and recessive models on each of our SNPs using Fisher’s exact test. We then conducted 5000 gene-wide max(T) permutations based on the best model of each SNP to provide strong gene-wide family-wise error rate control. Finally, we preformed logistic regression for the significant SNPs acquired in previous procedures, with sex and education level as covariates to build predictive models. Additional haplotype analyses were conducted with Haploview 4.2 to investigate the combining effect of multiple significant SNPs within a gene. Results: With sex and education level as covariates, rs8082252 (p = 0.0015, odds ratio = 2.716), rs8082142 (p = 0.0031, odds ratio = 2.499) in arylformamidase (AFMID), and rs12477181 (p = 0.0004, odds ratio = 0.3478) in kynureninase (KYNU) were significant in logistic regression models with dominant modes of inheritance. Haplotype analyses showed the two significant SNPs in AFMID to be in the same haploblock and highly correlated (r² = 0.99). There were two significant haplotypes (by the sequence of rs8082252, rs8082142): AT (χ2 = 7.734, p = 0.0054) and GC (χ2 = 6.874, p = 0.0087). Conclusions: This study provided supportive evidence of the involvement of the kynurenine pathway in IFN-α-induced depression. SNPs in this pathway were also predictive of this disease.

The neuroinflammatory component of negative affect in patients with chronic pain

Negative affect (NA) is a significant cause of disability for chronic pain patients. While little is known about the mechanism underlying pain-comorbid NA, previous studies have implicated neuroinflammation in the pathophysiology of both depression and chronic pain. Here, we tested the hypothesis that NA in pain patients is linked to elevations in the brain levels of the glial marker 18 kDa translocator protein (TSPO), and changes in functional connectivity. 25 cLBP patients (42.4 ± 13 years old; 13F, 12M) with chronic low back pain (cLBP) and 27 healthy control subjects (48.9 ± 13 years old; 14F, 13M) received an integrated (i.e., simultaneous) positron emission tomography (PET)/magnetic resonance imaging (MRI) brain scan with the second-generation TSPO ligand [¹¹C]PBR28. The relationship between [¹¹C]PBR28 signal and NA was assessed first with regression analyses against Beck Depression Inventory (BDI) scores in patients, and then by comparing cLBP patients with little-to-no, or mild-to-moderate depression against healthy controls. Further, the relationship between PET signal, BDI and frontolimbic functional connectivity was evaluated in patients with mediation models. PET signal was positively associated with BDI scores in patients, and significantly elevated in patients with mild-to-moderate (but not low) depression compared with controls, in anterior middle and pregenual anterior cingulate cortices (aMCC, pgACC). In the pgACC, PET signal was also associated with this region's functional connectivity to the dorsolateral PFC (pgACC-dlPFC), and mediated of the association between pgACC-dlPFC connectivity and BDI. These observations support a role for glial activation in pain-comorbid NA, identifying in neuroinflammation a potential therapeutic target for this condition.

Linking atypical depression and insulin resistance-related disorders via low-grade chronic inflammation: Integrating the phenotypic, molecular and neuroanatomical dimensions

Insulin resistance (IR) and related disorders, such as T2DM, increase the risk of major depressive disorder (MDD) and vice versa. Current evidence indicates that psychological stress and overeating can induce chronic low-grade inflammation that can interfere with glutamate metabolism in MDD as well as insulin signaling, particularly in the atypical subtype. Here we first review the interactive role of inflammatory processes in the development of MDD, IR and related metabolic disorders. Next, we describe the role of the anterior cingulate cortex in the pathophysiology of MDD and IR-related disorders. Furthermore, we outline how specific clinical features of atypical depression, such as hyperphagia, are more associated with inflammation and IR-related disorders. Finally, we examine the regional specificity of the effects of inflammation on the brain that show an overlap with the functional and morphometric brain patterns activated in MDD and IR-related disorders.

A randomized cross-over trial to define neurophysiological correlates of AV-101 N-methyl-D-aspartate receptor blockade in healthy veterans

The kynurenine pathway (KP) is a strategic metabolic system that combines regulation of neuronal excitability via glutamate receptor function and neuroinflammation via other KP metabolites. This pathway has great promise in treatment of depression and suicidality. The KP modulator AV-101 (4-chlorokynurenine, 4-Cl-KYN), an oral prodrug of 7-chlorokynurenic acid (7-Cl-KYNA), an N-methyl-D-aspartate receptor (NMDAR) glycine site antagonist, and of 4-chloro-3-hydroxyanthranilic acid (4-Cl-3-HAA), a suppressor of NMDAR agonist quinolinic acid (QUIN), is a promising potential antidepressant that targets glutamate functioning via the KP. However, a recent placebo-controlled clinical trial of AV-101 in depression found negative results. This raises the question of whether AV-101 can penetrate the brain and engage the NMDAR and KP effectively. To address this problem, ten healthy US military veterans (mean age = 32.6 years ± 6.11; 1 female) completed a phase-1 randomized, double-blind, placebo-controlled, crossover study to examine dose-related effects of AV-101 (720 and 1440 mg) on NMDAR engagement measured by γ-frequency band auditory steady-state response (40 Hz ASSR) and resting EEG. Linear mixed models revealed that 1440 mg AV-101, but not 720 mg, increased 40 Hz ASSR and 40 Hz ASSR γ-inter-trial phase coherence relative to placebo. AV-101 also increased 4-Cl-KYN, 7-Cl-KYNA, 4-Cl-3-HAA, 3-HAA, and KYNA in a dose-dependent manner, without affecting KYN and QUIN. AV-101 was safe and well tolerated. These results corroborate brain target engagement of 1440 mg AV-101 in humans, consistent with blockade of interneuronal NMDAR blockade. Future studies should test higher doses of AV-101 in depression. Suicidal behavior, which has been associated with high QUIN and low KYNA, is also a potential target for AV-101.

Central levels of tryptophan metabolites in subjects with bipolar disorder

The kynurenine pathway of tryptophan degradation produces several neuroactive metabolites such as kynurenic acid (KYNA), quinolinic acid (QUIN), and picolinic acid (PIC) thought to be involved in the pathophysiology of psychosis, major depression, and suicidal behavior. Here, we analyzed cerebrospinal fluid (CSF) concentrations of tryptophan, kynurenine, KYNA, QUIN, and PIC utilizing ultra-performance liquid chromatography - tandem mass spectrometry system (UPLC-MS/MS) in persons with bipolar disorder (n = 101) and healthy controls (n = 80) to investigate if the metabolites correlated with depressive symptoms or to the history of suicidal behavior. Furthermore, we analyzed if genetic variants of the enzyme amino-β-carboxymuconate-semialdehyde-decarboxylase (ACMSD) were associated with the CSF concentrations of PIC and QUIN. We found that CSF KYNA and PIC concentrations, as well as the kynurenine/tryptophan ratio were increased in bipolar disorder compared with controls. CSF PIC concentrations were lower in subjects with a history of suicidal behavior than those without, supporting the hypothesis that low CSF PIC is a marker of vulnerability for suicidality. Bipolar subjects taking antidepressants had higher CSF concentrations of kynurenine and KYNA than subjects not given these medications. A negative association was found between a genetic variant of ACMSD and the ratio of PIC/QUIN, indicating that a polymorphism in ACMSD is associated with excess of QUIN formation at the expense of PIC. The present results confirm that the kynurenine pathway is activated in bipolar disorder, and suggest that shifting the activity of the kynurenine pathway away from QUIN production towards a production of KYNA and PIC might be a beneficial therapeutic strategy.

A Modest Increase in 11C-PK11195-Positron Emission Tomography TSPO Binding in Depression Is Not Associated With Serum C-Reactive Protein or Body Mass Index

Background

Immune mechanisms have been implicated in the pathogenesis of depression. Translocator protein (TSPO)–targeted positron emission tomography (PET) has been used to assess neuroinflammation in major depressive disorder. We aimed to 1) test the hypothesis of significant case-control differences in TSPO binding in the anterior cingulate cortex, prefrontal cortex, and insula regions; and 2) explore the relationship between cerebral TSPO binding and peripheral blood C-reactive protein (CRP) concentration.

Methods

A total of 51 depressed subjects with Hamilton Depression Rating Scale score >13 (median 17; interquartile range, 16–22) and 25 healthy control subjects underwent dynamic brain ¹¹C-PK11195 PET and peripheral blood immune marker characterization. Depressed subjects were divided into high CRP (>3 mg/L; n = 20) and low CRP (<3 mg/L; n = 31).

Results

Across the three regions, TSPO binding was significantly increased in depressed versus control subjects (η²_p = .09; F_1,71 = 6.97, p = .01), which was not influenced by body mass index. The case-control difference was greatest in the anterior cingulate cortex (d = 0.49; t₇₄ = 2.00, p = .03) and not significant in the prefrontal cortex or insula (d = 0.27 and d = 0.36, respectively). Following CRP stratification, significantly higher TSPO binding was observed in low-CRP depression compared with controls (d = 0.53; t₅₄ = 1.96, p = .03). These effect sizes are comparable to prior major depressive disorder case-control TSPO PET data. No significant correlations were observed between TSPO and CRP measures.

Conclusions

Consistent with previous findings, there is a modest increase in TSPO binding in depressed patients compared with healthy control subjects. The lack of a significant correlation between brain TSPO binding and blood CRP concentration or body mass index poses questions about the interactions between central and peripheral immune responses in the pathogenesis of depression.

Time to re-engage psychiatric drug discovery by strengthening confidence in preclinical psychopharmacology

BACKGROUND

There is urgent need for new medications for psychiatric disorders. Mental illness is expected to become the leading cause of disability worldwide by 2030. Yet, the last two decades have seen the pharmaceutical industry withdraw from psychiatric drug discovery after costly late-stage trial failures in which clinical efficacy predicted pre-clinically has not materialised, leading to a crisis in confidence in preclinical psychopharmacology.

METHODS

Based on a review of the relevant literature, we formulated some principles for improving investment in translational neuroscience aimed at psychiatric drug discovery.

RESULTS

We propose the following 8 principles that could be used, in various combinations, to enhance CNS drug discovery: (1) consider incorporating the NIMH Research Domain Criteria (RDoC) approach; (2) engage the power of translational and systems neuroscience approaches; (3) use disease-relevant experimental perturbations; (4) identify molecular targets via genomic analysis and patient-derived pluripotent stem cells; (5) embrace holistic neuroscience: a partnership with psychoneuroimmunology; (6) use translational measures of neuronal activation; (7) validate the reproducibility of findings by independent collaboration; and (8) learn and reflect. We provide recent examples of promising animal-to-human translation of drug discovery projects and highlight some that present re-purposing opportunities.

CONCLUSIONS

We hope that this review will re-awaken the pharma industry and mental health advocates to the opportunities for improving psychiatric pharmacotherapy and so restore confidence and justify re-investment in the field.

Neurobiology of resilience in depression: immune and vascular insights from human and animal studies

Major depressive disorder (MDD) is a chronic and recurrent psychiatric condition characterized by depressed mood, social isolation and anhedonia. It will affect 20% of individuals with considerable economic impacts. Unfortunately, 30-50% of depressed individuals are resistant to current antidepressant treatments. MDD is twice as prevalent in women and associated symptoms are different. Depression's main environmental risk factor is chronic stress, and women report higher levels of stress in daily life. However, not every stressed individual becomes depressed, highlighting the need to identify biological determinants of stress vulnerability but also resilience. Based on a reverse translational approach, rodent models of depression were developed to study the mechanisms underlying susceptibility vs resilience. Indeed, a subpopulation of animals can display coping mechanisms and a set of biological alterations leading to stress resilience. The aetiology of MDD is multifactorial and involves several physiological systems. Exacerbation of endocrine and immune responses from both innate and adaptive systems are observed in depressed individuals and mice exhibiting depression-like behaviours. Increasing attention has been given to neurovascular health since higher prevalence of cardiovascular diseases is found in MDD patients and inflammatory conditions are associated with depression, treatment resistance and relapse. Here, we provide an overview of endocrine, immune and vascular changes associated with stress vulnerability vs. resilience in rodents and when available, in humans. Lack of treatment efficacy suggests that neuron-centric treatments do not address important causal biological factors and better understanding of stress-induced adaptations, including sex differences, could contribute to develop novel therapeutic strategies including personalized medicine approaches.

Depression, Estrogens, and Neuroinflammation: A Preclinical Review of Ketamine Treatment for Mood Disorders in Women

Ketamine has been shown to acutely and rapidly ameliorate depression symptoms and suicidality. Given that women suffer from major depression at twice the rate of men, it is important to understand how ketamine works in the female brain. This review explores three themes. First, it examines our current understanding of the etiology of depression in women. Second, it examines preclinical research on ketamine's antidepressant effects at a neurobiological level as well as how ovarian hormones present a unique challenge in interpreting these findings. Lastly, the neuroinflammatory hypothesis of depression is highlighted to help better understand how ovarian hormones might interact with ketamine in the female brain.

From "Leaky Gut" to Impaired Glia-Neuron Communication in Depression

In the last three decades, the robust scientific data emerged, demonstrating that the immune-inflammatory response is a fundamental component of the pathophysiology of major depressive disorder (MDD). Psychological stress and various inflammatory comorbidities contribute to such immune activation. Still, this is not uncommon that patients with depression do not have defined inflammatory comorbidities, and alternative mechanisms of immune activation need to take place. The gastrointestinal (GI) tract, along with gut-associated lymphoid tissue (GALT), constitutes the largest lymphatic organ in the human body and forms the biggest surface of contact with the external environment. It is also the most significant source of bacterial and food-derived antigenic material. There is a broad range of reciprocal interactions between the GI tract, intestinal microbiota, increased intestinal permeability, activation of immune-inflammatory response, and the CNS that has crucial implications in brain function and mental health. This intercommunication takes place within the microbiota-gut-immune-glia (MGIG) axis, and glial cells are the main orchestrator of this communication. A broad range of factors, including psychological stress, inflammation, dysbiosis, may compromise the permeability of this barrier. This leads to excessive bacterial translocation and the excessive influx of food-derived antigenic material that contributes to activation of the immune-inflammatory response and depressive psychopathology. This chapter summarizes the role of increased intestinal permeability in MDD and mechanisms of how the "leaky gut" may contribute to immune-inflammatory response in this disorder.

Microbiota metabolites modulate the T helper 17 to regulatory T cell (Th17/Treg) imbalance promoting resilience to stress-induced anxiety- and depressive-like behaviors

Chronic stress disrupts immune homeostasis while gut microbiota-derived metabolites attenuate inflammation, thus promoting resilience to stress-induced immune and behavioral abnormalities. There are both peripheral and brain region-specific maladaptations of the immune response to chronic stress that produce interrelated mechanistic considerations required for the design of novel therapeutic strategies for prevention of stress-induced psychological impairment. This study shows that a combination of probiotics and polyphenol-rich prebiotics, a synbiotic, attenuates the chronic-stress induced inflammatory responses in the ileum and the prefrontal cortex promoting resilience to the consequent depressive- and anxiety-like behaviors in male mice. Pharmacokinetic studies revealed that this effect may be attributed to specific synbiotic-produced metabolites including 4-hydroxyphenylpropionic, 4-hydroxyphenylacetic acid and caffeic acid. Using a model of chronic unpredictable stress, behavioral abnormalities were associated to strong immune cell activation and recruitment in the ileum while inflammasome pathways were implicated in the prefrontal cortex and hippocampus. Chronic stress also upregulated the ratio of activated proinflammatory T helper 17 (Th17) to regulatory T cells (Treg) in the liver and ileum and it was predicted with ingenuity pathway analysis that the aryl hydrocarbon receptor (AHR) could be driving the synbiotic's effect on the ileum's inflammatory response to stress. Synbiotic treatment indiscriminately attenuated the stress-induced immune and behavioral aberrations in both the ileum and the brain while in a gut-immune co-culture model, the synbiotic-specific metabolites promoted anti-inflammatory activity through the AHR. Overall, this study characterizes a novel synbiotic treatment for chronic-stress induced behavioral impairments while defining a putative mechanism of gut-microbiota host interaction for modulating the peripheral and brain immune systems.

Reduced ribosomal DNA transcription in the prefrontal cortex of suicide victims: consistence of new molecular RT-qPCR findings with previous morphometric data from AgNOR-stained pyramidal neurons

Prefrontal cortical regions play a key role in behavioural regulation, which is profoundly disturbed in suicide. The study was carried out on frozen cortical samples from the anterior cingulate cortex (dorsal and ventral parts, ACd and ACv), the orbitofrontal cortex (OFC), and the dorsolateral cortex (DLC) obtained from 20 suicide completers (predominantly violent) with unknown psychiatric diagnosis and 21 non-suicidal controls. The relative level of ribosomal RNA (rRNA) as a marker of the transcriptional activity of ribosomal DNA (rDNA) was evaluated bilaterally in prefrontal regions mentioned above (i.e. in eight regions of interest, ROIs) by reverse transcription and quantitative polymerase chain reaction (RT-qPCR). The overall statistical analysis revealed a decrease in rDNA activity in suicide victims versus controls, particularly in male subjects. Further ROI-specific post hoc analyses revealed a significant decrease in this activity in suicides compared to non-suicides in five ROIs. This effect was accentuated in the ACv, where it was observed bilaterally. Our findings suggest that decreased rDNA transcription in the prefrontal cortex plays an important role in suicide pathogenesis and corresponds with our previous morphometric analyses of AgNOR-stained neurons.

Toxoplasma gondii, Suicidal Behavior, and Intermediate Phenotypes for Suicidal Behavior

Within the general literature on infections and suicidal behavior, studies on Toxoplasma gondii (T. gondii) occupy a central position. This is related to the parasite's neurotropism, high prevalence of chronic infection, as well as specific and non-specific behavioral alterations in rodents that lead to increased risk taking, which are recapitulated in humans by T. gondii's associations with suicidal behavior, as well as trait impulsivity and aggression, mental illness and traffic accidents. This paper is a detailed review of the associations between T. gondii serology and suicidal behavior, a field of study that started 15 years ago with our publication of associations between T. gondii IgG serology and suicidal behavior in persons with mood disorders. This "legacy" article presents, chronologically, our primary studies in individuals with mood disorders and schizophrenia in Germany, recent attempters in Sweden, and in a large cohort of mothers in Denmark. Then, it reviews findings from all three meta-analyses published to date, confirming our reported associations and overall consistent in effect size [ranging between 39 and 57% elevation of odds of suicide attempt in T. gondii immunoglobulin (IgG) positives]. Finally, the article introduces certain links between T. gondii and biomarkers previously associated with suicidal behavior (kynurenines, phenylalanine/tyrosine), intermediate phenotypes of suicidal behavior (impulsivity, aggression) and state-dependent suicide risk factors (hopelessness/dysphoria, sleep impairment). In sum, an abundance of evidence supports a positive link between suicide attempts (but not suicidal ideation) and T. gondii IgG (but not IgM) seropositivity and serointensity. Trait impulsivity and aggression, endophenotypes of suicidal behavior have also been positively associated with T. gondii seropositivity in both the psychiatrically healthy as well as in patients with Intermittent Explosive Disorder. Yet, causality has not been demonstrated. Thus, randomized interventional studies are necessary to advance causal inferences and, if causality is confirmed, to provide hope that an etiological treatment for a distinct subgroup of individuals at an increased risk for suicide could emerge.

Probióticos na redução de sintomas de ansiedade e depressão: uma revisão integrativa

Objetivos: sumarizar estudos que avaliaram a suplementação de probióticos como estratégia terapêutica nos sintomas da ansiedade e depressão. Métodos: revisão integrativa de artigos indexados na base de dados PubMed, SciELO e Biblioteca Virtual em Saúde publicados de janeiro de 2010 a setembro de 2019. Para isso, utilizou-se a conjugação dos descritores: “intestino”, “cérebro”, “microbiota intestinal”, “ansiedade”, “depressão”, “probióticos”, nos idiomas português e inglês. Resultados: Após a aplicação dos critérios de inclusão e exclusão, 13 ensaios clínicos randomizados foram selecionados. O tempo de duração dos estudos, em sua maioria, foi de 8 ou 12 semanas (61,5%; n = 8), e a forma mais ofertada do suplemento foi o probiótico em pó (46,2%; n = 6) e em cápsula (30,8%; n = 4). Sobre a utilização de escalas como parâmetro de avaliação dos sintomas de ansiedade e depressão, 38,5% (n = 5) utilizaram apenas uma escala e 69,2% (n = 8) utilizaram a combinação de duas ou três escalas. Em relação ao gênero das bactérias, a maior parte dos estudos utilizou Lactobacillus e Bifidobacterium em conjunto (53,8%; n = 7). Apesar das limitações metodológicas e dos resultados inconsistentes, a maioria dos ensaios clínicos (76,9%; n = 10) evidenciaram uma redução significativa dos sintomas relacionados à ansiedade e depressão através da suplementação de probióticos. Conclusão: As evidências indicam que a suplementação com probióticos apresenta potencial promissor na redução dos sintomas de ansiedade e depressão, no entanto são necessárias pesquisas adicionais sobre essa estratégia como terapia adjuvante no tratamento efetivo para a saúde mental.

Kynurenine Pathway of Tryptophan Metabolism in Neuropsychiatric Disorders: Pathophysiologic and Therapeutic Considerations

Under physiological conditions 95% of the ingested essential amino acid tryptophan is metabolized by the kynurenine pathway (KP) to yield the ubiquitous co-enzyme nicotinamide adenine dinucleotide, fulfilling cellular energy require-ments. Importantly, the intermediaries of KP exert crucial effects throughout the body, including the central nervous system. Besides, KP metabolites are implicated in diverse disease processes such as inflammation/immune disorders, endocrine/metabolic conditions, cancers and neuropsychiatric diseases. A burgeoning body of research indicates that the KP plays a pathogenic role in major psychiatric diseases like mood disorders and schizophrenia. Triggered by inflammatory processes, the balance between neurotoxic and neuroprotective branches of the KP is disturbed. In preclinical models these discrepancies result in behaviors reminiscent of depression and psychosis. In clinical samples, recent studies are discovering key kynurenine pathway abnormalities which incriminate it in the pathogenesis of the main psychiatric disorders. Harnessing this knowledge has the potential to find disease biomarkers helpful in identifying and prognosticating neuropsychiatric disorders. Concurrently, earnest research efforts directed towards manipulating the KP hold the promise of discovering novel pharmacological agents that have therapeutic value. In this manuscript, an in-depth appraisal of the extant literature is done to understand the working of KP as this applies to neuropsychiatric disorders. It is concluded that this pathway plays an overarching role in the development of major psychiatric disorders, the KP metabolites have the potential to serve as disease markers and new medications based on KP modulation can bring lasting cures for patients suffering from these intractable conditions.

The anti-inflammatory role of SSRI and SNRI in the treatment of depression: a review of human and rodent research studies

RATIONALE

Depression has the topmost prevalence of all psychiatric diseases. It is characterized by a high recurrence rate, disability, and numerous and mostly unclear pathogenic mechanisms. Besides the monoamine or the neurotrophic hypothesis of depression, the inflammatory mechanism has begun to be supported by more and more evidence. At the same time, the current knowledge about the standard treatment of choice, the selective serotonin reuptake inhibitors (SSRIs) and serotonin and noradrenaline reuptake inhibitors (SNRIs), is expanding rapidly, adding more features to the initial ones.

OBJECTIVES

This review summarizes the in vivo anti-inflammatory effects of SSRIs and SNRIs in the treatment of depression and outlines the particular mechanisms of these effects for each drug separately. In addition, we provide an overview of the inflammation-related theory of depression and the underlying mechanisms.

RESULTS

SSRIs and SNRIs decrease the neuroinflammation through multiple mechanisms including the reduction of blood or tissue cytokines or regulating complex inflammatory pathways: nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), inflammasomes, Toll-like receptor 4 (TLR4), peroxisome proliferator-activated receptor gamma (PPARγ). Also, SSRIs and SNRIs show these effects in association with an antidepressant action.

CONCLUSIONS

SSRIs and SNRIs have an anti-neuroinflammatory role which might contribute the antidepressant effect.

Time-restricted feeding prevents depressive-like and anxiety-like behaviors in male rats exposed to an experimental model of shift-work

Individuals who regularly shift their sleep timing, like night and/or shift-workers suffer from circadian desynchrony and are at risk of developing cardiometabolic diseases and cancer. Also, shift-work is are suggested to be a risk factor for the development of mood disorders such as the burn out syndrome, anxiety, and depression. Experimental and clinical studies provide evidence that food intake restricted to the normal activity phase is a potent synchronizer for the circadian system and can prevent the detrimental health effects associated with circadian disruption. Here, we explored whether adult male Wistar rats exposed to an experimental model of shift-work (W-AL) developed depressive and/or anxiety-like behaviors and whether this was associated with neuroinflammation in brain areas involved with mood regulation. We also tested whether time-restricted feeding (TRF) to the active phase could ameliorate circadian disruption and therefore would prevent depressive and anxiety-like behaviors as well as neuroinflammation. In male Wistar rats, W-AL induced depressive-like behavior characterized by hypoactivity and anhedonia and induced increased anxiety-like behavior in the open field test. This was associated with increased number of glial fibrillary acidic protein and IBA-1-positive cells in the prefrontal cortex and basolateral amygdala. Moreover W-AL caused morphological changes in the microglia in the CA3 area of the hippocampus indicating microglial activation. Importantly, TRF prevented behavioral changes and decreased neuroinflammation markers in the brain. Present results add up evidence about the importance that TRF in synchrony with the light-dark cycle can prevent neuroinflammation leading to healthy mood states in spite of circadian disruptive conditions.

Molecular insights into the therapeutic promise of targeting HMGB1 in depression

Depression is a common psychiatric disorder, the exact pathogenesis of which is still elusive. Studies have proposed that immunity disproportion and enhancement in proinflammatory cytokines might be linked with the development of depression. HMGB1 (High-mobility group box (1) protein has obtained more interest as an essential factor in inherent immune reactions and a regulating factor in various inflammation-related diseases. HMGB1 is a ubiquitous chromatin protein and is constitutively expressed in nucleated mammalian cells. HMGB1 is released by glial cells and neurons upon inflammasome activation and act as a pro-inflammatory cytokine. HMGB1 is a late mediator of inflammation and has been indicated as a major mediator in various neuroinflammatory diseases. Microglia, which is the brain immune cell, is stimulated by HMGB1 and released inflammatory mediators and induces chronic neurodegeneration in the CNS (central nervous system). In the current review, we aimed to investigate the role of HMGB1 in the pathogenesis of depression. The studies found that HMGB1 functions as proinflammatory cytokines primarily via binding receptors like RAGE (receptor for advanced glycation end product), TLR2 and TLR4 (Toll-like receptor 2 and 4). Further, HMGB1 added to the preparing impacts of stress-pretreatment and assumed a major function in neurodegenerative conditions through moderating neuroinflammation. Studies demonstrated that neuroinflammation played a major role in the development of depression. The patients of depression generally exhibited an elevated amount of proinflammatory cytokines in the serum, microglia activation and neuronal deficit in the CNS.

Systemic neuro-dysregulation in depression: Evidence from genome-wide association

Depression is the world's leading cause of disability. Greater understanding of the neurobiological basis of depression is necessary for developing novel treatments with improved efficacy and acceptance. Recently, major advances have been made in the search for genetic variants associated with depression which may help to elucidate etiological mechanisms. The present review has two major objectives. First, we offer a brief review of two major biological systems with strong evidence for involvement in depression pathology: neurotransmitter systems and the stress response. Secondly, we provide a synthesis of the functions of the 269 genes implicated by the most recent genome-wide meta-analysis, supporting the importance of these systems in depression and providing insights into other possible mechanisms involving neurodevelopment, neurogenesis, and neurodegeneration. Our goal is to undertake a broad, preliminary stock-taking of the most recent hypothesis-free findings and examine the weight of the evidence supporting these existing theories and highlighting novel directions. This qualitative review and accompanying gene function table provides a valuable resource and guide for basic and translational researchers, with suggestions for future mechanistic research, leveraging genetics to prioritize studies on the neurobiological processes involved in depression etiology and treatment.

[The next psychoactive drugs: From imipramine to ketamine]

Depuis les années 1950, l’arsenal thérapeutique permettant de lutter contre la dépression s’est considérablement enrichi. De la découverte des inhibiteurs de la monoamine oxydase (IMAO) à celle de la kétamine, ces percées pharmacologiques ont marqué l’histoire de la psychiatrie et guidé la recherche sur la physiopathologie de la dépression, cette pathologie dévastatrice affectant entre 10 et 20 % de la population mondiale. Nous proposons dans cet article une courte revue historique des différentes options thérapeutiques développées au cours du siècle passé et des conséquences qu’ont eu ces innovations. Nous réalisons ensuite un état des lieux de la plus récente de ces découvertes, celle des effets antidépresseurs de la kétamine (et de son énantiomère S, l’eskétamine), spectaculaires de par leur délai d’apparition et leur efficacité même dans les formes les plus résistantes de dépression. De même que la découverte des IMAO et des tricycliques a permis de concevoir une théorie monoaminergique de la dépression, l’étude des mécanismes d’actions de la kétamine pourrait permettre de comprendre le rôle de la transmission glutamatergique ou de la neuro-inflammation dans la neurobiologie de cette pathologie, d’affiner nos connaissances sur sa physiopathologie cognitive ou encore de transformer en profondeur les représentations des cliniciens sur cette maladie.

Single-cell mass cytometry of microglia in major depressive disorder reveals a non-inflammatory phenotype with increased homeostatic marker expression

Stress-induced disturbances of brain homeostasis and neuroinflammation have been implicated in the pathophysiology of mood disorders. In major depressive disorder (MDD), elevated levels of proinflammatory cytokines and chemokines can be found in peripheral blood, but very little is known about the changes that occur directly in the brain. Microglia are the primary immune effector cells of the central nervous system and exquisitely sensitive to changes in the brain microenvironment. Here, we performed the first single-cell analysis of microglia from four different post-mortem brain regions (frontal lobe, temporal lobe, thalamus, and subventricular zone) of medicated individuals with MDD compared to controls. We found no evidence for the induction of inflammation-associated molecules, such as CD11b, CD45, CCL2, IL-1β, IL-6, TNF, MIP-1β (CCL4), IL-10, and even decreased expression of HLA-DR and CD68 in microglia from MDD cases. In contrast, we detected increased levels of the homeostatic proteins P2Y₁₂ receptor, TMEM119 and CCR5 (CD195) in microglia from all brain regions of individuals with MDD. We also identified enrichment of non-inflammatory CD206^hi macrophages in the brains of MDD cases. In sum, our results suggest enhanced homeostatic functions of microglia in MDD.

Affective Immunology: The Crosstalk Between Microglia and Astrocytes Plays Key Role?

Emerging evidence demonstrates the critical role of the immune response in the mechanisms relating to mood disorders, such as major depression (MDD) and bipolar disorder (BD). This has cast a spotlight on a specialized branch committed to the research of dynamics of the fine interaction between emotion (or affection) and immune response, which has been termed as “affective immunology.” Inflammatory cytokines and gut microbiota are actively involved in affective immunology. Furthermore, abnormalities of the astrocytes and microglia have been observed in mood disorders from both postmortem and molecular imaging studies; however, the underlying mechanisms remain elusive. Notably, the crosstalk between astrocyte and microglia acts as a mutual and pivotal intermediary factor modulating the immune response posed by inflammatory cytokines and gut microbiota. In this study, we propose the “altered astrocyte-microglia crosstalk (AAMC)” hypothesis which suggests that the astrocyte-microglia crosstalk regulates emotional alteration through mediating immune response, and thus, contributing to the development of mood disorders.