The kynurenine pathway in adolescent depression: preliminary findings from a proton MR spectroscopy study

C-Reactive Protein Does Not Predict Future Depression Onset in Adolescents: Preliminary Findings from a Longitudinal Study

Introduction: Neuroinflammatory processes have been extensively implicated in the underlying neurobiology of numerous neuropsychiatric disorders. Elevated C-reactive protein (CRP), an indicator of nonspecific inflammation commonly utilized in clinical practice, has been associated with depression in adults. In adolescents, our group previously found CRP to be associated with altered neural reward function but not with mood and anxiety symptoms assessed cross-sectionally. We hypothesized that the distinct CRP findings in adolescent versus adult depression may be due to chronicity, with neuroinflammatory effects on psychiatric disorders gradually accumulating over time. Here, we conducted a longitudinal study to evaluate if CRP levels predicted future onset or progression of depression in adolescents. Methods: Participants were 53 adolescents (age = 14.74 ± 1.92 years, 35 female), 40 with psychiatric symptoms and 13 healthy controls. At baseline, participants completed semistructured diagnostic evaluations; dimensional assessments for anxiety, depression, anhedonia, and suicidality severity; and bloodwork to quantify CRP levels. Clinical assessments were repeated at longitudinal follow-up after ∼1.5 years. Spearman's correlation between CRP levels and follow-up symptom severity were controlled for body mass index, age, sex, and follow-up interval and considered significant at the two-tailed, Bonferroni-adjusted p < 0.05 level. Results: After correction for multiple comparisons, no relationships were identified between baseline CRP levels and follow-up symptom severity. Conclusion: CRP levels were not significantly associated with future psychiatric symptoms in adolescents in this preliminary analysis. This may suggest that CRP is not a useful biomarker for adolescent depression and anxiety. However, future longitudinal studies with larger sample sizes and incorporating additional indicators of neuroinflammation are needed.

Immune and Neural Response to Acute Social Stress in Adolescent Humans and Rodents

Studies in adults have linked stress-related activation of the immune system to the manifestation of psychiatric conditions. Using a translational design, this study aimed to examine the impact of social stress on immune activity in adolescents and on neuronal activity in a preclinical mouse model. Participants were 31 adolescents (ages 12-19), including 25 with mood and anxiety symptoms. Whole-blood samples were collected before and after the Trier Social Stress Test (TSST), a stress-inducing public speaking task, then cultured for 6 hours in the presence and absence of the inflammatory endotoxin lipopolysaccharide (LPS). Effects of TSST and LPS on 41 immune biomarkers were examined using repeated-measures analysis of variance. Separately, juvenile (8-week-old) male mice were non-stressed or exposed to reminder social defeat then intraperitoneally injected with saline or LPS (n = 6/group). Brains were perfused and collected for immunohistochemistry and confocal microscopy at 0, 1, 6, and 24 hours post-injection. Activity was determined by the density of cFos-positive neurons in the paraventricular hypothalamus, paraventricular thalamus, and basolateral amygdala, regions known to show sustained activation to immunological challenge. Analyses in the adolescent study indicated a strong effect of LPS but no effects of TSST or TSST×LPS interaction on immune biomarkers. Similarly, reminder social defeat did not induce sustained neuronal activity changes comparable to LPS immunological challenge in juvenile mice. Our convergent findings across species suggest that the acute immune response to stress documented in adults is not present in youth. Thus, aging and chronicity effects may play an important role in the inflammatory response to acute psychosocial stress.

The Neurochemistry of Depression: The Good, The Bad and The Ugly

A large constellation of experimental evidence suggests that neuroinflammation is involved in the onset of depression and neurodegenerative disorders. Many studies have shown impairments in tryptophan metabolism, the major pathway for the synthesis of serotonin, the mood regulating neurotransmitter. This article reviews the various metabolites generated in the competing pathways of tryptophan metabolism including the kynurenine pathway. Increased synthesis of the neurotoxic compound quinolinic acid occurs at the expense of the synthesis of the neuroprotective metabolite kynurenic acid. This shift in equilibrium plays a critical role in the induction of oxidative stress, neuroinflammation, and neurotoxicity. Sufficient protein intake with adequate amounts of tryptophan along with dietary antioxidants and flavonoids may offer protection against major depressive and neurodegenerative disorders.

C-reactive protein does not predict future depression onset in adolescents: preliminary findings from a longitudinal study

Introduction

Neuroinflammatory processes have been extensively implicated in the underlying neurobiology of numerous neuropsychiatric disorders. Elevated C-reactive protein (CRP), an indicator of non-specific inflammation commonly utilized in clinical practice, has been associated with depression in adults. In adolescents, our group previously found CRP to be associated with altered neural reward function but not with mood and anxiety symptoms assessed cross-sectionally. We hypothesized that the distinct CRP findings in adolescent vs. adult depression may be due to chronicity, with neuroinflammatory effects on psychiatric disorders gradually accumulating over time. Here, we conducted a longitudinal study to evaluate if CRP levels predicted future onset or progression of depression in adolescents.

Methods

Participants were 53 adolescents (ages 14.74 ± 1.92, 35 female), 40 with psychiatric symptoms and 13 healthy controls. At baseline, participants completed semi-structured diagnostic evaluations; dimensional assessments for anxiety, depression, anhedonia, and suicidality severity; and bloodwork to quantify CRP levels. Clinical assessments were repeated at longitudinal follow-up after approximately 1.5 years. Spearman's correlation between CRP levels and follow-up symptom severity were controlled for BMI, age, sex, and follow-up interval and considered significant at the two-tailed, Bonferroni-adjusted p < 0.05 level.

Results

After correction for multiple comparisons, no relationships were identified between baseline CRP levels and follow-up symptom severity.

Conclusion

CRP levels were not significantly associated with future psychiatric symptoms in adolescents in this preliminary analysis. This may suggest that CRP is not a useful biomarker for adolescent depression and anxiety. However, future longitudinal studies with larger sample sizes and incorporating additional indicators of neuroinflammation are needed.

Associations between the kynurenine pathway and the brain in patients with major depressive disorder-A systematic review of neuroimaging studies

Previous studies have indicated that an imbalance in the kynurenine (KYN) pathway is an important pathophysiological mechanism of depression. Several studies have reported that an imbalance in the KYN pathway and its metabolites is associated with abnormalities in cerebral structure and function in depression, but the available evidence has been inconsistent. In this review, we systematically reviewed and integrated the findings concerning the associations between the KYN pathway and the brain in patients with major depressive disorder (MDD). A total of 22 neuroimaging studies were ultimately included in the present study. The neuroimaging modalities used in the studies included structural magnetic resonance imaging (MRI), diffusion tensor imaging, functional MRI, magnetic resonance spectroscopy, arterial spin labelling and positron emission tomography. The results revealed that an imbalance in the KYN pathway was associated with structural and functional abnormalities in several brain regions in patients with MDD. The brain regions most frequently associated with an imbalance in the KYN pathway were cortical regions (i.e., anterior cingulate cortex and orbitofrontal cortex), subcortical regions (i.e., striatum, thalamus and amygdala) and white matter fibres (i.e., inner capsule and left superior longitudinal tract). Our study provides robust evidence that cerebral abnormalities associated with the KYN pathway may be the underlying pathophysiological mechanisms of MDD. Future prospective studies are needed to further elucidate the causal relationships between the imbalanced KYN pathway and cerebral abnormalities in patients with MDD.

Changes in Tryptophan-Kynurenine Metabolism in Patients with Depression Undergoing ECT-A Systematic Review

The kynurenine pathway of tryptophan (Trp) metabolism generates multiple biologically active metabolites (kynurenines) that have been implicated in neuropsychiatric disorders. It has been suggested that modulation of kynurenine metabolism could be involved in the therapeutic effect of electroconvulsive therapy (ECT). We performed a systematic review with aims of summarizing changes in Trp and/or kynurenines after ECT and assessing methodological issues. The inclusion criterium was measures of Trp and/or kynurenines before and after ECT. Animal studies and studies using Trp administration or Trp depletion were excluded. Embase, MEDLINE, PsycInfo and PubMed were searched, most recently in July 2022. Outcomes were levels of Trp, kynurenines and ratios before and after ECT. Data on factors affecting Trp metabolism and ECT were collected for interpretation and discussion of the reported changes. We included 17 studies with repeated measures for a total of 386 patients and 27 controls. Synthesis using vote counting based on the direction of effect found no evidence of effect of ECT on any outcome variable. There were considerable variations in design, patient characteristics and reported items. We suggest that future studies should include larger samples, assess important covariates and determine between- and within-subject variability. PROSPERO (CRD42020187003).

Clenbuterol attenuates immune reaction to lipopolysaccharide and its relationship to anhedonia in adolescents

While inflammation has been implicated in psychopathology, relationships between immune-suppressing processes and psychiatric constructs remain elusive. This study sought to assess whether β2-agonist clenbuterol (CBL) would attenuate immune activation in adolescents with mood and anxiety symptoms following ex vivo exposure of whole blood to lipopolysaccharide (LPS). Our focus on adolescents aimed to target a critical developmental period when psychiatric conditions often emerge and prior to chronicity effects. To capture a diverse range of immunologic and symptomatologic phenotypes, we included 97 psychotropic-medication free adolescents with mood and anxiety symptoms and 33 healthy controls. All participants had comprehensive evaluations and dimensional assessments of psychiatric symptoms. Fasting whole-blood samples were collected and stimulated with LPS in the presence and absence of CBL for 6 hours, then analyzed for 41 cytokines, chemokines, and hematopoietic growth factors. Comparison analyses used Bonferroni-corrected nonparametric tests. Levels of nine immune biomarkers-including IL-1RA, IL-1β, IL-6, IP-10, MCP-1, MIP-1α, MIP-1β, TGF-α, and TNF-α-were significantly reduced by CBL treatment compared to LPS alone. Exploratory factor analysis reduced 41 analytes into 5 immune factors in each experimental condition, and their relationships with psychiatric symptoms were examined as a secondary aim. CBL + LPS Factor 4-comprising EGF, PDGF-AA, PDGF-AB/BB, sCD40L, and GRO-significantly correlated with anticipatory and consummatory anhedonia, even after controlling for depression severity. This study supports the possible inhibitory effect of CBL on immune activation. Using a data-driven method, distinctive relationships between CBL-affected immune biomarkers and dimensional anhedonia were reported, further elucidating the role of β2-agonism in adolescent affective symptomatology.

Correlations between facial emotion processing and biochemical abnormalities in untreated adolescent patients with major depressive disorder: A proton magnetic resonance spectroscopy study

BACKGROUND

Studies show that disturbances of the fronto-striato-thalamic-cerebellar circuit could be correlated to facial emotion processing (FEP) biases in major depressive disorder (MDD). Nevertheless, the underlying mechanism of natural metabolism-emotion relationships in adolescent MDD remains unclear.

METHODS

Thirty-seven adolescent patients with MDD and 30 healthy controls completed FEP tasks using the Chinese Facial Affective Picture System (CAFPS). Proton magnetic resonance spectroscopy (¹H-MRS) was also used to obtain ratios of N-acetylaspartate (NAA) /creatine (Cr) and choline (Cho) /Cr ratios in the prefrontal cortex (PFC), anterior cingulate cortex (ACC), putamen, thalamus and cerebellum. Correlations between abnormal neurometabolic ratios and FEP were also computed.

RESULTS

Compared with the control group, the MDD group had significantly lower accuracy and perception intensity of happiness, and significantly higher accuracy of disgust and perception intensity of sad and fearful faces in FEP tasks. Compared to healthy controls, adolescent patients with MDD showed significantly lower NAA/Cr ratios in the left PFC, higher NAA/Cr ratios in the right thalamus, and higher Cho/Cr ratios in the right putamen, although there were no significant differences in metabolites in the ACC and cerebellum between two groups. In the MDD group, NAA/Cr ratios of the right thalamus were negatively correlated with happy reaction time and positively correlated with sad, anger, and fear intensity; Cho/Cr ratios in the right putamen were positively correlated with fear reaction time.

CONCLUSIONS

Our findings suggest that FEP bias may exist in adolescents with MDD, while the impairment of FEP may be associated with abnormal metabolites in the fronto-striato-thalamic circuit.

Brain Versus Blood: A Systematic Review on the Concordance Between Peripheral and Central Kynurenine Pathway Measures in Psychiatric Disorders

Objective

Disturbances in the kynurenine pathway have been implicated in the pathophysiology of psychotic and mood disorders, as well as several other psychiatric illnesses. It remains uncertain however to what extent metabolite levels detectable in plasma or serum reflect brain kynurenine metabolism and other disease-specific pathophysiological changes. The primary objective of this systematic review was to investigate the concordance between peripheral and central (CSF or brain tissue) kynurenine metabolites. As secondary aims we describe their correlation with illness course, treatment response, and neuroanatomical abnormalities in psychiatric diseases.

Methods

We performed a systematic literature search until February 2021 in PubMed. We included 27 original research articles describing a correlation between peripheral and central kynurenine metabolite measures in preclinical studies and human samples from patients suffering from neuropsychiatric disorders and other conditions. We also included 32 articles reporting associations between peripheral KP markers and symptom severity, CNS pathology or treatment response in schizophrenia, bipolar disorder or major depressive disorder.

Results

For kynurenine and 3-hydroxykynurenine, moderate to strong concordance was found between peripheral and central concentrations not only in psychiatric disorders, but also in other (patho)physiological conditions. Despite discordant findings for other metabolites (mainly tryptophan and kynurenic acid), blood metabolite levels were associated with clinical symptoms and treatment response in psychiatric patients, as well as with observed neuroanatomical abnormalities and glial activity.

Conclusion

Only kynurenine and 3-hydroxykynurenine demonstrated a consistent and reliable concordance between peripheral and central measures. Evidence from psychiatric studies on kynurenine pathway concordance is scarce, and more research is needed to determine the validity of peripheral kynurenine metabolite assessment as proxy markers for CNS processes. Peripheral kynurenine and 3-hydroxykynurenine may nonetheless represent valuable predictive and prognostic biomarker candidates for psychiatric disorders.

Lack of Associations Between C-Reactive Protein and Mood and Anxiety Symptoms in Adolescents

Introduction: Increased peripheral inflammation has been consistently documented in both adult and pediatric depression. However, elevated levels of C-reactive protein (CRP), a nonspecific biomarker for inflammation, have been primarily reported in adults; whether CRP plays a similar role in adolescent depression has not been conclusively established. In our prior work, we identified relationships between CRP and reward neurocircuitry in adolescents with psychiatric symptoms (N = 64) but not with depressive symptoms. Extending this work, we sought to examine CRP across the full range of mood and anxiety symptom severity in a larger, clinically diverse cohort of psychotropic medication-free adolescents and healthy controls (HCs). Methods: Subjects were adolescents (N = 127, age: 15.17 ± 2.19 years, 78 female) with psychiatric symptoms (n = 96, including previous cohort of 64) and HC (n = 31). All completed a semi-structured psychiatric evaluation and dimensional assessments for depression, anxiety, anhedonia, and suicidality. Group-comparison and correlation analyses utilized nonparametric statistics controlled for body mass index, sex, and age at p_FWE < 0.05. Results: No group differences were identified in CRP levels between the clinical cohort and HCs. In addition, correlations between CRP and clinical symptomatology were not significant in either the whole sample or the psychiatric group. Conclusions: We found that, unlike in adults, CRP was not associated with depressive symptoms. This suggests that inflammation in pediatric depression is more narrowly delimited at the onset of psychiatric symptoms and may only become systemic with chronicity.

Multimodal Investigations of Reward Circuitry and Anhedonia in Adolescent Depression

Depression is a highly prevalent condition with devastating personal and public health consequences that often first manifests during adolescence. Though extensively studied, the pathogenesis of depression remains poorly understood, and efforts to stratify risks and identify optimal interventions have proceeded slowly. A major impediment has been the reliance on an all-or-nothing categorical diagnostic scheme based solely on whether a patient endorses an arbitrary number of common symptoms for a sufficiently long period. This approach masks the well-documented heterogeneity of depression, a disorder that is highly variable in presentation, severity, and course between individuals and is frequently comorbid with other psychiatric conditions. In this targeted review, we outline the limitations of traditional diagnosis-based research and instead advocate an alternative approach centered around symptoms as unique dimensions of clinical dysfunction that span across disorders and more closely reflect underlying neurobiological abnormalities. In particular, we highlight anhedonia-the reduced ability to anticipate and experience pleasure-as a specific, quantifiable index of reward dysfunction and an ideal candidate for dimensional investigation. Anhedonia is a core symptom of depression but also a salient feature of numerous other conditions, and its severity varies widely within clinical and even healthy populations. Similarly, reward dysfunction is a hallmark of depression but is evident across many psychiatric conditions. Reward function is especially relevant in adolescence, a period characterized by exaggerated reward-seeking behaviors and rapid maturation of neural reward circuitry. We detail extensive work by our research group and others to investigate the neural and systemic factors contributing to reward dysfunction in youth, including our cumulative findings using multiple neuroimaging and immunological measures to study depressed adolescents but also trans-diagnostic cohorts with diverse psychiatric symptoms. We describe convergent evidence that reward dysfunction: (a) predicts worse clinical outcomes, (b) is associated with functional and chemical abnormalities within and beyond the neural reward circuitry, (c) is linked to elevated peripheral levels of inflammatory biomarkers, and (d) manifests early in the course of illness. Emphasis is placed on high-resolution neuroimaging techniques, comprehensive immunological assays, and data-driven analyses to fully capture and characterize the complex, interconnected nature of these systems and their contributions to adolescent reward dysfunction.

Fronto-limbic neuroimaging biomarkers for diagnosis and prediction of treatment responses in major depressive disorder

The neuroimaging is an important tool for understanding the biomarkers and predicting treatment responses in major depressive disorder (MDD). The potential biomarkers and prediction of treatment response in MDD will be addressed in the review article. The brain regions of cognitive control and emotion regulation, such as the frontal and limbic regions, might represent the potential targets for MDD biomarkers. The potential targets of frontal lobes might include anterior cingulate cortex (ACC), dorsolateral prefrontal cortex (DLPFC) and orbitofrontal cortex (OFC). For the limbic system, hippocampus and amygdala might be the potentially promising targets for MDD. The potential targets of fronto-limbic regions have been found in the studies of several major neuroimaging modalities, such as the magnetic resonance imaging, near-infrared spectroscopy, electroencephalography, positron emission tomography, and single-photon emission computed tomography. Additional regions, such as brainstem and midbrain, might also play a part in the MDD biomarkers. For the prediction of treatment response, the gray matter volumes, white matter tracts, functional representations and receptor bindings of ACC, DLPFC, OFC, amygdala, and hippocampus might play a role in the prediction of antidepressant responses in MDD. For the response prediction of psychotherapies, the fronto-limbic, reward regions, and insula will be the potential targets. For the repetitive transcranial magnetic stimulation, the DLPFC, ACC, limbic, and visuospatial regions might represent the predictive targets for treatment. The neuroimaging targets of MDD might be focused in the fronto-limbic regions. However, the neuroimaging targets for the prediction of treatment responses might be inconclusive and beyond the fronto-limbic regions.

Correlates of C-reactive protein with neural reward circuitry in adolescents with psychiatric symptoms

INTRODUCTION

Increased inflammation has been implicated in many psychiatric conditions across ages. We previously reported relationships between blood cytokine levels and anhedonia, the decreased capacity to experience pleasure, as well as with reward brain activation in adolescents with psychiatric symptoms. Here, we sought to extend this work in a larger cohort of adolescents with psychiatric symptoms and assess the relationships of C-Reactive Protein (CRP, inflammation biomarker) with clinical symptoms and reward-related brain activation.

METHODS

Subjects were 64 psychotropic-medication-free adolescents with psychiatric symptoms (ages: 15.17 ± 2.10, 44 female). All had psychiatric evaluations and dimensional assessments for anxiety, depression, anhedonia, and suicidality. Neuroimaging included the Reward Flanker fMRI Task examining brain activation during reward anticipation, attainment and positive prediction error. Both whole-brain and ROI analyses focusing on reward circuitry were performed. All analyses were controlled for BMI, age, and sex at pFWE < 0.05.

RESULTS

No relationships were identified between CRP and clinical symptom severity. CRP was positively associated with brain activation during reward attainment in regions of the visual and dorsal attention networks, as well as during positive prediction error in the cerebellum. In ROI analyses, CRP was negatively correlated with brain activation during reward anticipation in dorsal anterior cingulate cortex. When subject with high CRP was excluded, CRP was also positively correlated with positive predication error activation in nucleus accumbens.

CONCLUSION

Despite lack of associations of CRP with clinical symptomatology, our fMRI findings suggest a relationship between inflammation and brain function early course of psychiatric conditions.

Associations between tryptophan and iron metabolism observed in individuals with and without iron deficiency

Current literature proposes associations between tryptophan metabolism and anaemia. However, study cohorts are rather small and final conclusions are still lacking. Here, we evaluated potential associations of tryptophan, kynurenine, and kynurenic acid with indicators of iron metabolism (i.e., mean corpuscular volume, mean corpuscular haemoglobin, ferritin, transferrin saturation, serum iron, transferrin, soluble transferrin receptor, reticulocyte haemoglobin) and haemoglobin in 430 individuals grouped by the presence or absence of iron deficiency or anaemia. Indicators of tryptophan metabolism were positively correlated with haemoglobin and markers of iron metabolism (p-values: <0.001–0.038; r-values: 0.100–0.305). The strongest correlation was observed between tryptophan and haemoglobin (p < 0.001, r = 0.305). The cubic regression model yielded the highest R-square values between haemoglobin and tryptophan markers. Overall, 115 patients with iron deficiency showed lower tryptophan and kynurenic acid concentrations compared to 315 individuals without iron deficiency. Six patients with anaemia of chronic disease were observed with the lowest serum tryptophan levels and the highest kynurenine/tryptophan ratio compared to 11 individuals with iron deficiency anaemia and 413 non-anaemic patients. This study showed little/moderate associations between haemoglobin, biomarkers of iron metabolism and tryptophan markers. Further studies are needed to get better insight in the causality of these findings.

Promising Neuroimaging Biomarkers in Depression

The neuroimaging has been applied in the study of pathophysiology in major depressive disorder (MDD). In this review article, several kinds of methodologies of neuroimaging would be discussed to summarize the promising biomarkers in MDD. For the magnetic resonance imaging (MRI) and magnetoencephalography field, the literature review showed the potentially promising roles of frontal lobes, such as anterior cingulate cortex (ACC), dorsolateral prefrontal cortex (DLPFC) and orbitofrontal cortex (OFC). In addition, the limbic regions, such as hippocampus and amygdala, might be the potentially promising biomarkers for MDD. The structures and functions of ACC, DLPFC, OFC, amygdala and hippocampus might be confirmed as the biomarkers for the prediction of antidepressant treatment responses and for the pathophysiology of MDD. The functions of cognitive control and emotion regulation of these regions might be crucial for the establishment of biomarkers. The near-infrared spectroscopy studies demonstrated that blood flow in the frontal lobe, such as the DLPFC and OFC, might be the biomarkers for the field of near-infrared spectroscopy. The electroencephalography also supported the promising role of frontal regions, such as the ACC, DLPFC and OFC in the biomarker exploration, especially for the sleep electroencephalogram to detect biomarkers in MDD. The positron emission tomography (PET) and single-photon emission computed tomography (SPECT) in MDD demonstrated the promising biomarkers for the frontal and limbic regions, such as ACC, DLPFC and amygdala. However, additional findings in brainstem and midbrain were also found in PET and SPECT. The promising neuroimaging biomarkers of MDD seemed focused in the fronto-limbic regions.

Values of Cytokines and Tryptophan Metabolites over a 12 Weeks Time Course in Patients with Depression and Somatoform Disorder

Objective

Previous studies have suggested alterations in the kynurenine pathway as a major link between cytokine and neurotransmitter abnormalities in psychiatric disorders. Most of these studies used a cross-sectional case-control study design. However, knowledge is still lacking regarding the stability over time of kynurenine pathway metabolites and the functionally related cytokines. Therefore, we studied the stability of cytokines and tryptophan (TRP) parameters over a period of 12 weeks.

Methods

A total of 117 participants-39 with major depression, 27 with somatoform disorder, and 51 healthy controls were enrolled. Four evaluations, including blood withdrawal and psychometric testing, were performed over a period of 12 weeks. We used ELISA to measure interleukin (IL)-6, IL-1 receptor antagonist (RA) and tumor necrosis factor α (TNF α). High-performance liquid chromatography was used to analyze neurotransmitter variables, i.e. TRP, 5-hydroxyindoleacetic acid (5-HIAA), kynurenine (KYN), 3-OH-kynurenine (3-HK), and kynurenic acid (KYNA).

Results

We found no significant fluctuations of TRP, its metabolites (5-HIAA, KYN, KYNA, and 3-HK), or the cytokines (IL-1RA, IL-6, and TNF α) in any of the groups over the 12 weeks.

Conclusion

To our knowledge, this is the first longitudinal study performed in psychiatric patients to verify the stability and consequently the reliability of the biological parameters we investigated. Our data indicate that TRP metabolites and cytokines are reliable biological parameters in psychiatric research because they do not fluctuate significantly over time.

Anhedonia as a clinical correlate of inflammation in adolescents across psychiatric conditions

Objectives: Peripheral inflammation has been associated with multiple psychiatric disorders, particularly with depression. However, findings remain inconsistent and unreproducible, most likely due to the disorder's heterogeneity in phenotypic presentation. Therefore, in the present study, in an effort to account for inter-individual differences in symptom severity, we utilised a dimensional approach to assess the relationships between a broad panel of inflammatory cytokines and key psychiatric symptoms (i.e. depression, anhedonia, anxiety, fatigue and suicidality) in adolescents across psychiatric disorders. We hypothesised that only anhedonia (reflecting deficits of reward function) will be associated with inflammation.Methods: Participants were 54 psychotropic medication-free adolescents with diverse psychiatric conditions and 22 healthy control (HC) adolescents, aged 12-20. We measured 41 cytokines after in vitro lipopolysaccharide stimulation. Mann-Whitney U and Spearman correlation tests examined group comparison and associations, respectively, while accounting for multiple comparisons and confounds, including depression severity adolescent.Results: There were no group differences in cytokine levels. However, as hypothesised, within the psychiatric group, only anhedonia was associated with 19 cytokines, including haematopoietic growth factors, chemokines, pro-inflammatory cytokines, and anti-inflammatory cytokines.Conclusions: Our findings suggest that general inflammation may induce reward dysfunction, which plays a salient role across psychiatric conditions, rather than be specific to one categorical psychiatric disorder.

A Double-Blind Placebo-Controlled Trial of Omega-3 Fatty Acids as a Monotherapy for Adolescent Depression

OBJECTIVE

Reports are mixed on the efficacy of omega-3 fatty acids (O3FA) for the treatment of major depressive disorder (MDD), with only limited data in adolescents. The present trial aimed to investigate systematically the efficacy of O3FA as a monotherapy, compared to a placebo, in adolescents with MDD. Secondarily, we explored O3FA effects on anhedonia, irritability, and suicidality-all key features of adolescent MDD.

METHODS

Fifty-one psychotropic medication-free adolescents with DSM-IV-TR diagnoses of MDD (aged 12-19 years; 57% female) were randomized to receive O3FA or a placebo for 10 weeks. Data were collected between January 2006 and June 2013. O3FA and a placebo were administered on a fixed-flexible dose titration schedule based on clinical response and side effects. The initial dose of 1.2 g/d was increased 0.6 g/d every 2 weeks, up to a maximum of 3.6 g/d. Clinician-rated and self-rated depression severity, along with treatment response, served as primary outcome measures. Additionally, we examined O3FA effects on depression-related symptoms, including anhedonia, irritability, and suicidality. Treatment differences were analyzed via intent-to-treat analyses.

RESULTS

O3FA were not superior to a placebo on any clinical feature, including depression severity and levels of anhedonia, irritability, or suicidality. Additionally, response rates were comparable between treatment groups. Within-treatment analyses indicated that both treatments were associated with significant improvement in depression severity on self- (O3FA: t = -4.38, P < .001; placebo: t = -3.52, P = .002) and clinician (O3FA: t = -6.47, P < .001; placebo: t = -8.10, P < .001) ratings.

CONCLUSIONS

In adolescents with MDD, O3FA do not appear to be superior to placebo.

TRIAL REGISTRATION

ClinicalTrials.gov identifier: NCT00962598.

Assessment of tryptophan metabolism and signs of depression in individuals with carbohydrate malabsorption

This prospective cross-sectional study aimed to investigate the potential association between primary-adult lactose malabsorption, fructose malabsorption, tryptophan (TRP) metabolism and the presence of depressive signs. Overall 251 patients, who were referred for lactase gene C/T_-13910 polymorphism genotyping and fructose hydrogen/methane breath testing, were included. All participants filled out the Beck Depression Inventory (BDI II). Serum concentrations of tryptophan (TRP), kynurenine (KYN), kynuric acid (KYNA), and TRP competing amino acids (leucine, isoleucine, valine, phenylalanine, tyrosine) were measured by high-pressure liquid-chromatography. Logistic regression analysis was performed with lactose malabsorption, fructose malabsorption and all potential biomarkers of TRP metabolism to assess the effect on signs of depression, defined as a BDI II score > 13. Primary-adult lactose malabsorption and fructose malabsorption was detected in 65 (25.90%) and 65 (25.90%) patients, respectively. Fructose malabsorption was significantly associated with BDI II score, whereas no such relationship was found for lactose malabsorption. Serum levels of TRP and TRP metabolites were no predictors of depression. The authors suggest to conduct further prospective longitudinal studies in order to get further insight of associations between carbohydrate malabsorption, biomarkers and mood disorders.

Brain choline in major depression: A review of the literature

The focus of this review is to provide a synthesis of the current literature on the role of brain choline, as measured by proton magnetic resonance spectroscopy (1H-MRS), in major depressive disorder (MDD). The most recent ¹H-MRS literature review took place over 10 years ago and, reflecting the high level of research on this topic, much has been learned since then. Higher brain choline levels have been linked to an increase in depression, and a cholinergic model for MDD development has been postulated. However, current ¹H-MRS studies have been inconclusive regarding the role of choline in depression. Data from eighty-six peer-reviewed studies were analyzed for a random-effects model meta-analysis. Two significant findings are reported. Papers that did not report segmentation had a significant, moderate effect size. Higher choline concentrations in the frontal lobe were found in depressed patients, both in those who responded to treatment and those who did not, after treatment with psychiatric medication, repetitive transcranial magnetic stimulation, or electroconvulsive therapy. Findings from this review may add to existing information regarding the role of brain choline in MDD. This may provide a future target for treatment and drug development. It also may serve as a biomarker for treatment progress.

A pilot study of cortical glutathione in youth with depression

AIM

This study used proton magnetic resonance spectroscopy (¹H MRS) to measure in vivo brain glutathione (GSH) in adolescents with major depressive disorder (MDD), and explored the relationship between GSH and illness severity and chronicity. Secondarily, associations between GSH and anhedonia, a key symptom of MDD in adolescents, were investigated.

METHODS

Occipital cortex GSH levels were obtained in 19 psychotropic medication-free adolescents with MDD (ages 12-21) and compared to those in eight healthy control adolescents. Correlations between GSH levels and anhedonia severity were examined both in the full participant sample and within the MDD group. Within the MDD group, correlations between GSH levels and illness severity and chronicity were assessed.

RESULTS

Occipital GSH levels were lower in adolescents with MDD compared to controls, but did not correlate with anhedonia (either within the MDD group or the full sample), MDD severity, or onset. There were also no group differences in levels of total choline, creatine, and N-acetylaspartate - all neurometabolites that were simultaneously detected with ¹H MRS.

CONCLUSIONS

Although preliminary, findings add new data to support the role of oxidative stress in MDD and suggest that lower GSH may be a potential marker of MDD early on in the course of illness.

Suicidality and Activation of the Kynurenine Pathway of Tryptophan Metabolism

A recent report by the World Health Organization declared suicide to be a major global problem. With more than 800,000 lives lost each year, suicide is calculated to be the 14th leading cause of death around the world. While the biological mechanisms causing suicidal ideation and behavior are not fully understood, increased levels of inflammation, arising from various sources, have been detected in the central nervous system and the peripheral blood of suicidal patients and suicide completers. Inflammation induces the kynurenine pathway of tryptophan metabolism, which generates a range of metabolites with potent effects on neurotransmitter systems as well as on inflammation. Recent evidence indicates that a dysregulation of the enzymes in the kynurenine pathway may be present in suicidal patients, with a resulting imbalance of metabolites that modulate glutamate neurotransmission and neuroinflammation. As the body of research in these areas grows, targeting the kynurenine pathway enzymes and metabolites may provide novel therapeutic opportunities for detection, treatment, and ultimately prevention of suicidal behavior.

Reduced kynurenine pathway metabolism and cytokine expression in the prefrontal cortex of depressed individuals

BACKGROUND

Neuroinflammatory processes are increasingly believed to participate in the pathophysiology of a number of major psychiatric diseases, including depression. Immune activation stimulates the conversion of the amino acid tryptophan to kynurenine, leading to the formation of neuroactive metabolites, such as quinolinic acid and kynurenic acid. These compounds affect glutamatergic neurotransmission, which plays a prominent role in depressive pathology. Increased tryptophan degradation along the kynurenine pathway (KP) has been proposed to contribute to disease etiology.

METHODS

We used postmortem brain tissue from the ventrolateral prefrontal cortex (VLPFC) to assess tissue levels of tryptophan and KP metabolites, the expression of several KP enzymes and a series of cytokines as well as tissue pathology, including microglial activation. Tissue samples came from nonpsychiatric controls (n = 36) and individuals with depressive disorder not otherwise specified (DD-NOS, n = 45) who died of natural causes, homicide, accident, or suicide.

RESULTS

We found a reduction in the enzymatic conversion of tryptophan to kynurenine, determined using the kynurenine:tryptophan ratio, and reduced messenger RNA expression of the enzymes indoleamine-2,3-dioxygenase 1 and 2 and tryptophan-2,3-dioxygenase in depressed individuals irrespective of the cause of death. These findings correlated with reductions in the expression of several cytokines, including interferon-γ and tumour necrosis factor-α. Notably, quinolinic acid levels were also lower in depressed individuals than controls.

LIMITATIONS

Information on the use of antidepressants and other psychotropic medications was insufficient for statistical comparisons.

CONCLUSION

Contrary to expectations, the present results indicate that depression, in the absence of medical illness or an overt inflammatory process, is associated with compromised, rather than increased, KP metabolism in the VLPFC.

IgA/IgM responses to tryptophan and tryptophan catabolites (TRYCATs) are differently associated with prenatal depression, physio-somatic symptoms at the end of term and premenstrual syndrome

There is some evidence that lowered tryptophan and an activated tryptophan catabolite (TRYCAT) pathway play a role in depression, somatoform disorder, and postpartum blues. The aim of this study is to delineate the associations between the TRYCAT pathway and premenstrual syndrome (PMS) and perinatal depressive and physio-somatic symptoms. We examine the associations between end of term serum IgM and IgA responses to tryptophan and 9 TRYCATs in relation to zinc, C-reactive protein (CRP), and haptoglobin and prenatal physio-somatic (previously known as psychosomatic) symptoms (fatigue, back pain, muscle pain, dyspepsia, obstipation) and prenatal and postnatal depression and anxiety symptoms as measured using the Edinburgh Postnatal Depression Scale (EPDS), Hamilton Depression Rating Scale (HAMD), and Spielberger's State Anxiety Inventory (STAI). We included pregnant females with (n = 24) and without depression (n = 25) and 24 non-pregnant females. There were no significant associations between the IgA/IgM responses to tryptophan and TRYCATs and prenatal and postnatal depression/anxiety symptoms, except for lowered IgA responses to anthranilic acid in prenatal depression. A large part of the variance in IgA responses to most TRYCATs was explained by PMS and haptoglobin (positively) and CRP (inversely) levels. The IgA responses to TRYCATs were significantly increased in PMS, in particular picolinic, anthranilic, xanthurenic and kynurenic acid, and 3OH-kynurenine. Variance (62.5%) in physio-somatic symptoms at the end of term was explained by PMS, previous depressions, zinc (inversely), CRP and haptoglobin (both positively), and the IgM responses to quinolinic acid (positively), anthranilic acid, and tryptophan (both negatively). The results suggest that mucosa-derived TRYCAT pathway activation is significantly associated with PMS, but not with perinatal depression/anxiety symptoms. Physio-somatic symptoms in pregnancy have an immune-inflammatory pathophysiology. Induction of the TRYCAT pathway appears to be more related to physio-somatic than to depression symptoms.

Increased ventricular cerebrospinal fluid lactate in depressed adolescents

BACKGROUND

Mitochondrial dysfunction has been increasingly examined as a potential pathogenic event in psychiatric disorders, although its role early in the course of major depressive disorder (MDD) is unclear. Therefore, the purpose of this study was to investigate mitochondrial dysfunction in medication-free adolescents with MDD through in vivo measurements of neurometabolites using high-spatial resolution multislice/multivoxel proton magnetic resonance spectroscopy.

METHODS

Twenty-three adolescents with MDD and 29 healthy controls, ages 12-20, were scanned at 3T and concentrations of ventricular cerebrospinal fluid lactate, as well as N-acetyl-aspartate (NAA), total creatine (tCr), and total choline (tCho) in the bilateral caudate, putamen, and thalamus were reported.

RESULTS

Adolescents with MDD exhibited increased ventricular lactate compared to healthy controls [F(1,41)=6.98, P=0.01]. However, there were no group differences in the other neurometabolites. Dimensional analyses in the depressed group showed no relation between any of the neurometabolites and symptomatology, including anhedonia and fatigue.

CONCLUSIONS

Increased ventricular lactate in depressed adolescents suggests mitochondrial dysfunction may be present early in the course of MDD; however it is still not known whether the presence of mitochondrial dysfunction is a trait vulnerability of individuals predisposed to psychopathology or a state feature of the disorder. Therefore, there is a need for larger multimodal studies to clarify these chemical findings in the context of network function.

Kynurenine pathway metabolites and suicidality

Suicide is a major global problem, claiming more than 800,000 lives annually. The neurobiological changes that underlie suicidal ideation and behavior are not fully understood. Suicidal patients have been shown to display elevated levels of inflammation both in the central nervous system and the peripheral blood. A growing body of evidence suggests that inflammation is associated with a dysregulation of the kynurenine pathway in suicidal patients, resulting in an imbalance of neuroactive metabolites. Specifically, an increase in the levels of the NMDA receptor agonist quinolinic acid and a simultaneous decrease in neuroprotective metabolites have been observed in suicidal patients, and may contribute to the development of suicidality via changes in glutamate neurotransmission and neuroinflammation. The cause of the dysregulation of kynurenine metabolites in suicidality is not known, but is likely due to differential activity of the involved enzymes in patients. As knowledge in these areas is rapidly growing, targeting the kynurenine pathway enzymes may provide novel therapeutic approaches for managing suicidal behavior. This article is part of the Special Issue entitled 'The Kynurenine Pathway in Health and Disease'.

Smaller Dentate Gyrus and CA2 and CA3 Volumes Are Associated with Kynurenine Metabolites in Collegiate Football Athletes

An imbalance in kynurenine pathway metabolism is hypothesized to be associated with dysregulated glutamatergic neurotransmission, which has been proposed as a mechanism underlying the hippocampal volume loss observed in a variety of neurological disorders. Pre-clinical models suggest that the CA2-3 and dentate gyrus hippocampal subfields are particularly susceptible to excitotoxicity after experimental traumatic brain injury. We tested the hypothesis that smaller hippocampal volumes in collegiate football athletes with (n = 25) and without (n = 24) a concussion history would be most evident in the dentate gyrus and CA2-3 subfields relative to nonfootball healthy controls (n = 27). Further, we investigated whether the concentration of peripheral levels of kynurenine metabolites are altered in football athletes. Football athletes with and without a self-reported concussion history had smaller dentate gyrus (p < 0.05, p < 0.10) and CA2-3 volumes (p's < 0.05) relative to healthy controls. Football athletes with and without a concussion history had a trend toward lower (p < 0.10) and significantly lower (p < 0.05) kynurenine levels compared with healthy controls, while athletes with a concussion history had greater levels of quinolinic acid compared with athletes without a concussion history (p < 0.05). Finally, plasma levels of 3-hydroxykynurenine inversely correlated with bilateral hippocampal volumes in football athletes with a concussion history (p < 0.01), and left hippocampal volume was correlated with the ratio of kynurenic acid to quinolinic acid in football athletes without a concussion history (p < 0.05). Our results raise the possibility that abnormalities of the kynurenine metabolic pathway constitute a mechanism for hippocampal volume differences in the context of sports-related brain injury.

Kynurenine pathway dysfunction in the pathophysiology and treatment of depression: Evidences from animal and human studies

Treatment-resistant depression affects up to 20% of individuals suffering from major depressive disorder (MDD). The medications currently available to treat depression, including serotonin re-uptake inhibitors (SSRIs), monoamine oxidase inhibitors (MAOIs) and tricyclic antidepressants (TCAs), fail to produce adequate remission of depressive symptoms for a large number of patients. The monoamine hypothesis upon which these medications are predicated should be expanded and revised as research elucidates alternative mechanisms of depression and effective methods to treat the underlying pathologic consequences. Research into the role of tryptophan degradation and the kynurenine pathway in the setting of inflammation has brought new insight into potential etiologies of MDD. Further investigation into the connection between inflammatory mediators, tryptophan degradation, and MDD can provide many targets for novel antidepressant therapies. Thus, this review will highlight the role of the kynurenine pathway in the pathophysiology of depression, as well as a novel therapeutic target to classic and new modulators to treat depression based on findings from preclinical and clinical studies.

The role of the kynurenine pathway in suicidality in adolescent major depressive disorder

The neuroimmunological kynurenine pathway (KP) has been implicated in major depressive disorder (MDD) in adults and adolescents, most recently in suicidality in adults. The KP is initiated by the enzyme indoleamine 2,3-dioxygenase (IDO), which degrades tryptophan (TRP) into kynurenine (KYN) en route to neurotoxins. Here, we examined the KP in 20 suicidal depressed adolescents-composed of past attempters and those who expressed active suicidal intent-30 non-suicidal depressed youth, and 22 healthy controls (HC). Plasma levels of TRP, KYN, 3-hydroxyanthranilic acid (3-HAA), and KYN/TRP (index of IDO) were assessed. Suicidal adolescents showed decreased TRP and elevated KYN/TRP compared to both non-suicidal depressed adolescents and HC. Findings became more significantly pronounced when excluding medicated participants, wherein there was also a significant positive correlation between KYN/TRP and suicidality. Finally, although depressed adolescents with a history of suicide attempt differed from acutely suicidal adolescents with respect to disease severity, anhedonia, and suicidality, the groups did not differ in KP measures. Our findings suggest a possible specific role of the KP in suicidality in depressed adolescents, while illustrating the clinical phenomenon that depressed adolescents with a history of suicide attempt are similar to acutely suicidal youth and are at increased risk for completion of suicide.

Fatigue in immune thrombocytopenia

Fatigue is an important aspect of health-related quality of life from the patient perspective and can have significant socio-economic consequences. It is a common feature of chronic illnesses and a significant number of both adults and children with immune thrombocytopenia (ITP) suffer from fatigue. Reliable, validated fatigue scales have been developed for use in ITP. These will facilitate future investigation of its pathogenesis and the effectiveness of intervention. Acute inflammation acts on neural and endocrine systems resulting in 'sickness behaviour', an adaptive response to infection and injury. Inflammation is also thought to cause fatigue in chronic disease and immune dysregulation in ITP appears to have a number of pro-inflammatory components. Clinicians should consider fatigue when assessing the burden of disease. Although effective ITP-directed therapy can improve fatigue, a number of fatigue-directed strategies may also need to be considered.

Unraveling the miswired connectome: a developmental perspective

The vast majority of mental illnesses can be conceptualized as developmental disorders of neural interactions within the connectome, or developmental miswiring. The recent maturation of pediatric in vivo brain imaging is bringing the identification of clinically meaningful brain-based biomarkers of developmental disorders within reach. Even more auspicious is the ability to study the evolving connectome throughout life, beginning in utero, which promises to move the field from topological phenomenology to etiological nosology. Here, we scope advances in pediatric imaging of the brain connectome as the field faces the challenge of unraveling developmental miswiring. We highlight promises while also providing a pragmatic review of the many obstacles ahead that must be overcome to significantly impact public health.

The development of an RDoC-based treatment program for adolescent depression: "Training for Awareness, Resilience, and Action" (TARA)

Major depressive disorder (MDD) is one of the current leading causes of disability worldwide. Adolescence is a vulnerable period for the onset of depression, with MDD affecting 8–20% of all youth. Traditional treatment methods have not been sufficiently effective to slow the increasing prevalence of adolescent depression. We therefore propose a new model for the treatment of adolescent depression – Training for Awareness, Resilience, and Action (TARA) – that is based on current understanding of developmental and depression neurobiology. The TARA model is aligned with the Research Domain Criteria (RDoC) of the National Institute of Mental Health. In this article, we first address the relevance of RDoC to adolescent depression. Second, we identify the major RDoC domains of function involved in adolescent depression and organize them in a way that gives priority to domains thought to be driving the psychopathology. Third, we select therapeutic training strategies for TARA based on current scientific evidence of efficacy for the prioritized domains of function in a manner that maximizes time, resources, and feasibility. The TARA model takes into consideration the developmental limitation in top-down cognitive control in adolescence and promotes bottom-up strategies such as vagal afference to decrease limbic hyperactivation and its secondary effects. The program has been informed by mindfulness-based therapy and yoga, as well as modern psychotherapeutic techniques. The treatment program is semi-manualized, progressive, and applied in a module-based approach designed for a group setting that is to be conducted one session per week for 12 weeks. We hope that this work may form the basis for a novel and more effective treatment strategy for adolescent depression, as well as broaden the discussion on how to address this challenge.

In vivo (1)H MRS study of potential associations between glutathione, oxidative stress and anhedonia in major depressive disorder

Inflammation and oxidative stress are important mechanisms that have been implicated in the pathophysiology of major depressive disorder (MDD). Glutathione (GSH) is the most abundant antioxidant in human tissue, and a key index of antioxidant capacity and, hence, of oxidative stress. The aims of this investigation were to examine possible relationships between occipital GSH and dimensional measures of depressive symptom severity, including anhedonia - the reduced capacity to experience pleasure - and fatigue. We hypothesized that the magnitude of anhedonia and fatigue will be negatively correlated with occipital GSH levels in subjects with MDD and healthy controls (HC). Data for eleven adults with MDD and ten age- and sex-matched HC subjects were included in this secondary analysis of data from a previously published study. In vivo levels of GSH in a 3cm×3cm×2cm voxel of occipital cortex were obtained by proton magnetic resonance spectroscopy ((1)H MRS) on a 3T MR system, using the standard J-edited spin-echo difference technique. Anhedonia was assessed by combining interest items from depression and fatigue rating scales, and fatigue by use of the multidimensional fatigue inventory. Across the full sample of participants, anhedonia severity and occipital GSH levels were negatively correlated (r=-0.55, p=0.01). No associations were found between fatigue severity and GSH in this sample. These preliminary findings are potentially consistent with a pathophysiological role for GSH and oxidative stress in anhedonia and MDD. Larger studies in anhedonic depressed patients are indicated.

Kynurenines in CNS disease: regulation by inflammatory cytokines

The kynurenine pathway (KP) metabolizes the essential amino acid tryptophan and generates a number of neuroactive metabolites collectively called the kynurenines. Segregated into at least two distinct branches, often termed the “neurotoxic” and “neuroprotective” arms of the KP, they are regulated by the two enzymes kynurenine 3-monooxygenase and kynurenine aminotransferase, respectively. Interestingly, several enzymes in the pathway are under tight control of inflammatory mediators. Recent years have seen a tremendous increase in our understanding of neuroinflammation in CNS disease. This review will focus on the regulation of the KP by inflammatory mediators as it pertains to neurodegenerative and psychiatric disorders.

Kynurenine and depressive symptoms in a poststroke population

BACKGROUND AND PURPOSE

Depression is a commonly occurring and persistent sequel of stroke affecting approximately 29% of patients. An immunological hypothesis has been put forward, and synthesis of kynurenine from tryptophan has been proposed to link inflammatory activity with neurotoxicity and neurotransmitter dysfunction. This study assessed the relationship between peripheral blood kynurenine and poststroke depressive symptoms.

PATIENTS AND METHODS

This was a multisite cross-sectional observational cohort study of patients with ischemic stroke. Depressive symptoms were assessed using the Center for Epidemiological Studies Depression (CES-D) scale and divided into high, medium, and low depressive symptom tertiles. Concentrations of kynurenine and tryptophan were assayed from fasting serum samples, and the kynurenine/tryptophan ratio was compared between tertiles. Serum cytokine concentrations were assayed in a subgroup of patients, and the ratio of proinflammatory (IL-6, IL-18, IFNγ, TNF, IL-1β) to anti-inflammatory (IL-10) cytokines compared.

NLM IDENTIFIER

NCT00254020.

RESULTS

In these patients (n=86, 52.3% male, mean age 71.7±14.2 years), there were no differences in kynurenine/tryptophan ratios between CES-D scale tertiles (F 2,76=0.04, P=0.96) controlling for relevant covariates. For cytokines (n=53), serum IL-1β concentrations (F 2,52=3.55, P=0.037) and serum ratios of IL-18/IL-10 (F 2,52=3.30, P=0.046), IFNγ/IL-10 (F 2,52=4.02, P=0.025), and IL-1β/IL-10 (F 2,52=4.34, P=0.019) were elevated in the middle CES-D tertile. Post hoc analyses suggested that serum ratios of IL-18/IL-10 (ρ=0.28, P=0.04), and IL-1β/IL-10 (ρ=0.43, P=0.001), as well as IL-1β (ρ=0.29, P=0.04), were significantly associated with fatigue.

CONCLUSION

Peripheral kynurenine/tryptophan ratios were not associated with depressive symptoms in a poststroke population. However, in exploratory analyses a proinflammatory bias was identified specifically in patients with mild depressive symptoms and associated with poststroke fatigue, suggesting an avenue for future research.

Kynurenines: from the perspective of major psychiatric disorders

Psychiatric disorders are documented to be associated with a mild pro-inflammatory state. Pro-inflammatory mediators could activate the tryptophan breakdown and kynurenine pathway with a shift toward the neurotoxic arm where excitotoxic N-methyl-D-aspartate receptor agonist quinolinic acid is formed. An unbalanced metabolism in terms of neuroprotective and neurotoxic effects, such as reduced kynurenic acid to kynurenine ratio, has been demonstrated in the major psychiatric disorders such as unipolar depression, bipolar manic-depressive disorder and schizophrenia, and in drug-induced neuropsychiatric side effects such as interferon-α treated patients. The changes in serum or plasma are shown to be associated with central changes such as in the cerebrospinal fluid and certain brain areas. While currently available antidepressants and mood stabilizers could not efficiently improve these neurochemical changes within the same period that could induce clinical improvement, some antipsychotic treatments could reverse certain metabolic imbalances. Some of these changes were tested also in animal models. In this review the role of this unbalanced kynurenine metabolism through interactions with other neurochemicals is discussed as a major contributing pathophysiological mechanism in psychiatric disorders. Moreover, the biomarker role of kynurenine metabolites and future therapeutic opportunities are also discussed.

Sub-chronic dietary tryptophan depletion--an animal model of depression with improved face and good construct validity

Sub-chronic tryptophan depletion (SCTD) is proposed as an animal model for depression. Aims were to test the hypothesis and optimise the time of SCTD-induced depression-related behaviour and associated biochemical changes. Sprague Dawley rats were treated with a low tryptophan (TRP) containing diet for 0, 7 or 14 days. Peripheral and central neurochemical markers were measured. SCTD-induced depression-related behaviour was assessed by the forced swim test (FST). Model sensitivity to antidepressants was tested by concomitant treatment with paroxetine. SCTD-induced significant reductions in weight gain and measures of peripheral and central TRP. Corticosterone, aldosterone and kynurenine (K), increased whilst kynurenic acid (KA), an NMDA antagonist decreased. 5-HT(2) receptor binding Bmax was enhanced but was reversed by paroxetine. Corticosterone and aldosterone were significantly negatively-correlated to weight gain. SCTD increased floating time and reduced swimming time in the FST but were reversed by paroxetine. Aldosterone was increased at 7 and 14 days, whereas other changes maximised at 14 days. Aldosterone may be an early marker or causal link for depression development. Increased corticosterone and brain tissue 5-HT-receptor density may be correlates of depressive behaviour. Consequential increases in NMDA signalling through increased K/KA ratios suggest the model may be useful for testing novel antidepressants.

The possible role of the kynurenine pathway in anhedonia in adolescents

To address the heterogeneous nature of adolescent major depression (MDD), we investigated anhedonia, a core symptom of MDD. We recently reported activation of the kynurenine pathway (KP), a central neuroimmunological pathway which metabolizes tryptophan (TRP) into kynurenine (KYN) en route to several neurotoxins, in a group of highly anhedonic MDD adolescents. In this study, we aimed to extend our prior work and examine the relationship between KP activity and anhedonia, measured quantitatively, in a group of MDD adolescents and in a combined group of MDD and healthy control adolescents. Thirty-six adolescents with MDD (22 medication-free) and 20 controls were included in the analysis. Anhedonia scores were generated based on clinician- and subject-rated assessments and a semi-structured clinician interview. Blood KP metabolites, collected in the AM after an overnight fast, were measured using high-performance liquid chromatography. The rate-limiting enzyme of the KP, indoleamine 2,3-dioxygenase (IDO), was estimated by the ratio of KYN/TRP. Pearson correlation tests were used to assess correlations between anhedonia scores and KP measures while controlling for MDD severity. IDO activity and anhedonia scores were positively correlated in the group psychotropic medication-free adolescents with MDD (r = 0.42, P = 0.05) and in a combined group of MDD subjects and healthy controls (including medicated patients: r = 0.30, P = 0.02; excluding medicated patients: r = 0.44, P = 0.004). In conclusions, our findings provide further support for the role for the KP, particularly IDO, in anhedonia in adolescent MDD. These results emphasize the importance of dimensional approaches in the investigation of psychiatric disorders.

Review: magnetic resonance spectroscopy studies of pediatric major depressive disorder

Introduction. This paper focuses on the application of Magnetic Resonance Spectroscopy (MRS) to the study of Major Depressive Disorder (MDD) in children and adolescents. Method. A literature search using the National Institutes of Health's PubMed database was conducted to identify indexed peer-reviewed MRS studies in pediatric patients with MDD. Results. The literature search yielded 18 articles reporting original MRS data in pediatric MDD. Neurochemical alterations in Choline, Glutamate, and N-Acetyl Aspartate are associated with pediatric MDD, suggesting pathophysiologic continuity with adult MDD. Conclusions. The MRS literature in pediatric MDD is modest but growing. In studies that are methodologically comparable, the results have been consistent. Because it offers a noninvasive and repeatable measurement of relevant in vivo brain chemistry, MRS has the potential to provide insights into the pathophysiology of MDD as well as the mediators and moderators of treatment response.

The possible role of the kynurenine pathway in adolescent depression with melancholic features

BACKGROUND

Although adolescent major depressive disorder (MDD) is acknowledged to be a heterogeneous disorder, no studies have reported on biological correlates of its clinical subgroups. This study addresses this issue by examining whether adolescent MDD with and without melancholic features (M-MDD and NonM-MDD) have distinct biological features in the kynurenine pathway (KP). The KP is initiated by pro-inflammatory cytokines via induction of the enzyme indoleamine 2,3-dioxygenase (IDO), which degrades tryptophan (TRP) into kynurenine (KYN). KYN is further metabolized into neurotoxins linked to neuronal dysfunction in MDD. Hypotheses were that, compared to healthy controls and to NonM-MDD adolescents, adolescents with M-MDD would exhibit: (i) increased activation of the KP [i.e., increased KYN and KYN/TRP (reflecting IDO activity)]; (ii) greater neurotoxic loads [i.e., increased 3-hydroxyanthranilic acid (3-HAA, neurotoxin) and 3-HAA/KYN (reflecting production of neurotoxins)]; and (iii) decreased TRP. We also examined relationships between severity of MDD and KP metabolites.

METHODS

Subjects were 20 adolescents with M-MDD, 30 adolescents with NonM-MDD, and 22 healthy adolescents. MDD episode duration had to be >or= 6 weeks and Children's Depression Rating Scale-Revised (CDRS-R) scores were >or= 36. Blood samples were collected at AM after an overnight fast and analyzed using high-performance liquid chromatography. Group contrasts relied on analysis of covariance based on ranks, adjusted for age, gender, and CDRS-R scores. Analyses were repeated excluding medicated patients. Fisher's protected least significant difference was used for multiple comparisons.

RESULTS

As hypothesized, KYN/TRP ratios were elevated and TRP concentrations were reduced in adolescents with M-MDD compared to NonM-MDD adolescents (p = .001 and .006, respectively) and to healthy controls (p = .008 and .022, respectively). These findings remained significant when medicated patients were excluded from the analyses. Significant correlations were obtained exclusively in the M-MDD group between KYN and 3-HAA/KYN and CDRS-R.

CONCLUSIONS

Findings support the notion that adolescent M-MDD may represent a biologically distinct clinical syndrome.