Astrocyte-derived kynurenic acid modulates basal and evoked cortical acetylcholine release

The Tryptophan Metabolite Indole-3-Propionic Acid Raises Kynurenic Acid Levels in the Rat Brain In Vivo

Alterations in the composition of the gut microbiota may be causally associated with several brain diseases. Indole-3-propionic acid (IPrA) is a tryptophan-derived metabolite, which is produced by intestinal commensal microbes, rapidly enters the circulation, and crosses the blood-brain barrier. IPrA has neuroprotective properties, which have been attributed to its antioxidant and bioenergetic effects. Here, we evaluate an alternative and/or complementary mechanism, linking IPrA to kynurenic acid (KYNA), another neuroprotective tryptophan metabolite. Adult Sprague-Dawley rats received an oral dose of IPrA (200 mg/kg), and both IPrA and KYNA were measured in plasma and frontal cortex 90 minutes, 6 or 24 hours later. IPrA and KYNA levels increased after 90 minutes and 6 hours (brain IPrA: ~56- and ~7-fold; brain KYNA: ~4- and ~3-fold, respectively). In vivo microdialysis, performed in the medial prefrontal cortex and in the striatum, revealed increased KYNA levels (~2.5-fold) following the administration of IPrA (200 mg/kg, p.o), but IPrA failed to affect extracellular KYNA when applied locally. Finally, treatment with 100 or 350 mg IPrA, provided daily to the animals in the chow for a week, resulted in several-fold increases of IPrA and KYNA levels in both plasma and brain. These results suggest that exogenously supplied IPrA may provide a novel strategy to affect the function of KYNA in the mammalian brain.

Reducing brain kynurenic acid synthesis precludes kynurenine-induced sleep disturbances

Patients with neurocognitive disorders often battle sleep disturbances. Kynurenic acid is a tryptophan metabolite of the kynurenine pathway implicated in the pathology of these illnesses. Modest increases in kynurenic acid, an antagonist at glutamatergic and cholinergic receptors, result in cognitive impairments and sleep dysfunction. We explored the hypothesis that inhibition of the kynurenic acid synthesising enzyme, kynurenine aminotransferase II, may alleviate sleep disturbances. At the start of the light phase, adult male and female Wistar rats received systemic injections of either: (i) vehicle; (ii) kynurenine (100 mg kg-1; i.p.); (iii) the kynurenine aminotransferase II inhibitor, PF-04859989 (30 mg kg-1; s.c.); or (iv) PF-04859989 and kynurenine in combination. Kynurenine and kynurenic acid levels were evaluated in the plasma and brain. Separate animals were implanted with electroencephalogram and electromyogram telemetry devices to record polysomnography, and evaluate the vigilance states wake, rapid eye movement sleep and non-rapid eye movement sleep following each treatment. Kynurenine challenge increased brain kynurenic acid and resulted in reduced rapid eye movement sleep duration, non-rapid eye movement sleep delta power and sleep spindles. PF-04859989 reduced brain kynurenic acid formation when given prior to kynurenine, prevented disturbances in rapid eye movement sleep and sleep spindles, and enhanced non-rapid eye movement sleep. Our findings suggest that reducing kynurenic acid in conditions where the kynurenine pathway is activated may serve as a potential strategy for improving sleep dynamics.

Modulation of Kynurenic Acid Production by N-acetylcysteine Prevents Cognitive Impairment in Adulthood Induced by Lead Exposure during Lactation in Mice

Lead (Pb2+) exposure during early life induces cognitive impairment, which was recently associated with an increase in brain kynurenic acid (KYNA), an antagonist of NMDA and alpha-7 nicotinic receptors. It has been described that N-acetylcysteine (NAC) favors an antioxidant environment and inhibits kynurenine aminotransferase II activity (KAT II, the main enzyme of KYNA production), leading to brain KYNA levels decrease and cognitive improvement. This study aimed to investigate whether the NAC modulation of the brain KYNA levels in mice ameliorated Pb2+-induced cognitive impairment. The dams were divided into four groups: Control, Pb2+, NAC, and Pb2++NAC, which were given drinking water or 500 ppm lead acetate in the drinking water ad libitum, from 0 to 23 postnatal days (PNDs). The NAC and Pb2++NAC groups were simultaneously fed NAC (350 mg/day) in their chow from 0 to 23 PNDs. At PND 60, the effect of the treatment with Pb2+ and in combination with NAC on learning and memory performance was evaluated. Immediately after behavioral evaluation, brain tissues were collected to assess the redox environment; KYNA and glutamate levels; and KAT II activity. The NAC treatment prevented the long-term memory deficit exhibited in the Pb2+ group. As expected, Pb2+ group showed redox environment alterations, fluctuations in glutamate levels, and an increase in KYNA levels, which were partially avoided by NAC co-administration. These results confirmed that the excessive KYNA levels induced by Pb2+ were involved in the onset of cognitive impairment and could be successfully prevented by NAC treatment. NAC could be a tool for testing in scenarios in which KYNA levels are associated with the induction of cognitive impairment.

IDO-Kynurenine pathway mediates NLRP3 inflammasome activation-induced postoperative cognitive impairment in aged mice

AIM

Postoperative cognitive dysfunction (POCD) is a common postoperative complication, especially in elderly patients. It extends hospital stay, increases the mortality rate and are heavy burdens to the family and society. Accumulating research has indicated that overactivation of pyrin domain-containing protein 3 (NLRP3) inflammasomes is related to POCD andplays a critical role in activating pro-inflammatory cytokines. According to existing studies, indoleamine 2,3-dioxygenase (IDO) is potently up-regulated by inflammatory factors, tryptophan in brain is mainly catalyzed by IDO to kynurenine (KYN), KYN metabolism may contribute to the development of depressive disorder and memory deficits. Hence, this study elucidated whether IDO-Kynurenine pathway mediates NLRP3 inflammasome activation-induced postoperative cognitive impairment in aged mice.

MATERIAL AND METHODS

POCD model was established in aged C57BL/6J mice by exploratory laparotomy under isoflurane anesthesia. Learning and memory were determined using Morris water maze.

RESULTS

The data showed that IDO and kynurenine aminotransferase-II (KAT-II) mRNA in hippocampus was up-regulated, and NLRP3, caspase recruitment domain (ASC), interleukin-1b (IL-1b) and IDO overexpressed, KYN levels increased after anesthesia and surgery. NLRP3 inflammasome inhibitor (MCC950) reversed NLRP3, ASC, IL-1b and IDO overexpression, and the elevation of KYN levels. To clarify the role of IDO-Kynurenine pathway in postoperative cognitive impairment, IDO inhibitor (1-methyl-Ltryptophan 1-MT) reduced the elevation of KYN and kynurenic acid (KYNA) levels, reduction of tryptophan (TRP), as well as improved learning and memory abilities. Finally, KAT-II inhibitor (PF-04859989) reduced brain KYNA levels and restored the cognitive impairment.

CONCLUSION

These results reveal that IDO-Kynurenine pathway mediates NLRP3 inflammasome activation-induced postoperative cognitive impairment.

Functional connectivity and glutamate levels of the medial prefrontal cortex in schizotypy are related to sensory amplification in a probabilistic reasoning task

The circular inference (CI) computational model assumes a corruption of sensory data by prior information and vice versa, leading at the extremes to 'see what we expect' (through prior amplification) and/or to 'expect what we see' (through sensory amplification). Although a CI mechanism has been reported in a schizophrenia population, it has not been investigated in individuals experiencing psychosis-like experiences, such as people with high schizotypy traits. Furthermore, the neurobiological basis of CI, such as the link between hierarchical amplifications, excitatory neurotransmission, and resting state functional connectivity (RSFC), remains untested. The participants included in the present study consisted of a subsample of those recruited in a study previously published by our group, Kozhuharova et al. (2021b). We included 36 participants with High (n=18) and Low (n=18) levels of schizotypy who completed a probabilistic reasoning task (the Fisher task) for which individual confidence levels were obtained and fitted to the CI model. Participants also underwent a 1H-Magnetic Resonance Spectroscopy (MRS) scan to measure medial prefrontal cortex (mPFC) glutamate metabolite levels, and a functional Magnetic Resonance Imaging (fMRI) scan to measure RSFC of the medial prefrontal cortex (mPFC). People with high levels of schizotypy exhibited changes in CI parameters, altered cortical excitatory neurotransmission and RSFC that were all associated with sensory amplification. Our findings capture a multimodal signature of CI that is observable in people early in the psychosis spectrum.

Maternal Inflammation with Elevated Kynurenine Metabolites Is Related to the Risk of Abnormal Brain Development and Behavioral Changes in Autism Spectrum Disorder

Several studies show that genetic and environmental factors contribute to the onset and progression of neurodevelopmental disorders. Maternal immune activation (MIA) during gestation is considered one of the major environmental factors driving this process. The kynurenine pathway (KP) is a major route of the essential amino acid L-tryptophan (Trp) catabolism in mammalian cells. Activation of the KP following neuro-inflammation can generate various endogenous neuroactive metabolites that may impact brain functions and behaviors. Additionally, neurotoxic metabolites and excitotoxicity cause long-term changes in the trophic support, glutamatergic system, and synaptic function following KP activation. Therefore, investigating the role of KP metabolites during neurodevelopment will likely promote further understanding of additional pathophysiology of neurodevelopmental disorders, including autism spectrum disorder (ASD). In this review, we describe the changes in KP metabolism in the brain during pregnancy and represent how maternal inflammation and genetic factors influence the KP during development. We overview the patients with ASD clinical data and animal models designed to verify the role of perinatal KP elevation in long-lasting biochemical, neuropathological, and behavioral deficits later in life. Our review will help shed light on new therapeutic strategies and interventions targeting the KP for neurodevelopmental disorders.

Dysregulation of tryptophan metabolism and distortion of cell signaling after oral exposure to ethanol and Kynurenic acid

Kynurenic acid (KYNA), an unavoidable tryptophan metabolite during fermentation is naturally blended with alcohol in all alcoholic beverages. Thus, alcohol drinking inevitably results in co-intake of KYNA. Effects of alcohol or KYNA on human health have been widely studied. However, the combined effects of both remain unknown. Here we report that alcohol and KYNA have a synergistic impact of on global gene expression, especially the gene sets related to tryptophan metabolism and cell signaling. Adult mice were exposed to alcohol (ethanol) and/or KYNA daily for a week. Transcriptomes of the brain, kidney and liver were profiled via bulk RNA sequencing. Results indicate that while KYNA alone largely promotes, and alcohol alone mostly inhibits gene expression, alcohol and KYNA co-administration has a stronger inhibition of global gene expression. Tryptophan metabolism is severely skewed towards kynurenine pathway by decreasing tryptophan hydroxylase 2 and increasing tryptophan dioxygenase. Quantification of tryptophan metabolic enzymes corroborates the transcriptional changes of these enzymes. Furthermore, the co-administration greatly enhances the GnRH signaling pathway. This research provides critical data to better understand the effects of alcohol and KYNA in mix on human health.

Lipopolysaccharide Exacerbates Ketamine-Induced Psychotic-Like Behavior, Oxidative Stress, and Neuroinflammation in Mice: Ameliorative Effect of Diosmin

Schizophrenia, a neuropsychiatric disorder has been associated with aberrant neurotransmission affecting behaviors, social preference, and cognition. Limitations in understanding its pathogenesis via the dopamine hypothesis have engendered other hypotheses such as the glutamate hypothesis. That antagonism of the N-methyl-D-aspartate receptor (NMDAR) elicits schizophrenia-like behaviors indistinguishable from the disorder in animal and human models. There are growing concerns that redox imbalance and neuro-immuno dysfunction may play role in aggravating the symptomologies of this disorder. This 14-day treatment study was designed to investigate the effect of diosmin on lipopolysaccharide (LPS) plus ketamine (NMDAR antagonist). Mice were divided into 4 groups (n = 6). Group 1 was administered 5% DMSO (10 mL/kg, i.p) while group 2-4 received LPS (0.1 mg/kg, i.p) daily for 14 days. Diosmin (50 mg/kg, i.p) and risperidone (0.5 mg/kg, i.p) were given to groups 3 and 4 respectively. Groups 2-4 were given KET (20 mg/kg, i.p.) daily from days 8-14. Behavioral tests were done 30 min after the last dose, and oxidative stress and neuroinflammatory maker were assayed. LPS plus ketamine-induced hyperlocomotion, stereotypy, decreased social preference, and memory impairment. Furthermore, LPS plus-ketamine-induced oxidative stress (reduced GSH, CAT, SOD, and increased MDA and nitrite levels) and marked pro-inflammatory cytokines TNF-α and IL-6 suggesting neuroinflammation. However, diosmin attenuated behavioral deficits and improved memory. Additionally, diosmin potentiated antioxidant level via increased GSH, CAT, and SOD while reducing MDA and nitrite levels. Finally, diosmin reduced TNF-α and IL-6 suggesting anti-neuro-immuno activity. Conclusively, diosmin attenuated LPS plus ketamine-induced behavioral deficits, oxidative stress, neuroinflammation, and improved memory.

Pentoxifylline as an adjunctive in treatment of negative symptoms in chronic schizophrenia: A double-blind, randomized, placebo-controlled trial

AIM

The aim of this study was to explore the effectiveness and safety of pentoxifylline as an adjuvant to risperidone in mitigating the negative symptoms in patients with chronic schizophrenia.

METHODS

In this randomized, placebo-controlled study, eighty outpatients with chronic schizophrenia were given risperidone for 8 weeks along with either pentoxifylline or a placebo. The positive and negative syndrome scale (PANSS) was used to assess patients at the start of the trial, as well as at 2, 4, 6, and 8 weeks. Pre- and posttreatment serum levels of cAMP, TNF-α-, and IL-6 were measured.

RESULTS

The pentoxifylline group revealed a significant effect for time-treatment interaction on PANSS-negative subscale scores (p < 0.001), PANSS general psychopathology subscale scores (p < 0.001), and PANSS total scores (p < 0.001), but not on PANSS-positive subscale scores (p = 0.169). Additionally, when compared to the placebo group, the pentoxifylline group demonstrated a statistically significant increase in cAMP serum level and a statistically significant decrease in TNF-α and IL-6 serum levels.

CONCLUSION

Pentoxifylline adjunctive therapy with risperidone for 8 weeks was found to be promising in mitigating the negative symptoms in patients with chronic schizophrenia.

TRIAL REGISTRATION NUMBER

NCT04094207.

Stress and Kynurenine-Inflammation Pathway in Major Depressive Disorder

Major depressive disorder (MDD) is one of the most prevalent disorders and causes severe damage to people's quality of life. Lifelong stress is one of the major villains in triggering MDD. Studies have shown that both stress and MDD, especially the more severe conditions of the disorder, are associated with inflammation and neuroinflammation and the relationship to an imbalance in tryptophan metabolism towards the kynurenine pathway (KP) through the enzymes indoleamine-2,3-dioxygenase (IDO), which is mainly stimulated by pro-inflammatory cytokines and tryptophan-2,3-dioxygenase (TDO) which is activated primarily by glucocorticoids. Considering that several pathophysiological mechanisms of MDD underlie or interact with biological processes from KP metabolites, this chapter addresses and discusses the function of these mechanisms. Activities triggered by stress and the hypothalamic-pituitary-adrenal (HPA) axis and immune and inflammatory processes, in addition to epigenetic phenomena and the gut-brain axis (GBA), are addressed. Finally, studies on the function and mechanisms of physical exercise in the KP metabolism and MDD are pointed out and discussed.

Binge-like mephedrone treatment induces memory impairment concomitant with brain kynurenic acid reduction in mice

While mephedrone (4-methylmethcathinone), a synthetic cathinone derivative, is widely abused by adolescents and young adults, the knowledge about its long-term effects on memory processes is limited. Kynurenic acid (KYNA) is a neuroactive metabolite of the kynurenine pathway of tryptophan degradation. KYNA is considered an important endogenous modulator influencing physiological and pathological processes, including learning and memory processes. The aim of this study was to determine whether (A) binge-like mephedrone administration (10.0 and 30.0 mg/kg, intraperitoneally, in 4 doses separated by 2 h) induces memory impairments, assessed 2, 8 and 15 days after mephedrone cessation in the passive avoidance test in mice, and whether (B) KYNA is involved in these memory processes. To clarify the role of KYNA in the mephedrone effects, its level in the murine brain in vivo, and in cortical slices in vitro, as well as the activities of kynurenine aminotransferases (KATs) I and II were assessed. Furthermore, cell line experiments were conducted to investigate the effects of mephedrone on normal human brain cells. Our results showed memory impairments 8 and 15 days after binge-like mephedrone administration. At the same time, reduction in the KYNA level in the murine brain was noted. In vitro studies showed no effect of mephedrone on the production of KYNA in cortical slices or on the activity of the KAT I and II enzymes. Finally, exposure of normal cells to mephedrone in vitro resulted in a modest reduction of cell viability and proliferation.

Kynurenine Pathway Metabolites as Biomarkers in Alzheimer’s Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that deteriorates cognitive function. Patients with AD generally exhibit neuroinflammation, elevated beta-amyloid (Aβ), tau phosphorylation (p-tau), and other pathological changes in the brain. The kynurenine pathway (KP) and several of its metabolites, especially quinolinic acid (QA), are considered to be involved in the neuropathogenesis of AD. The important metabolites and key enzymes show significant importance in neuroinflammation and AD. Meanwhile, the discovery of changed levels of KP metabolites in patients with AD suggests that KP metabolites may have a prominent role in the pathogenesis of AD. Further, some KP metabolites exhibit other effects on the brain, such as oxidative stress regulation and neurotoxicity. Both analogs of the neuroprotective and antineuroinflammation metabolites and small molecule enzyme inhibitors preventing the formation of neurotoxic and neuroinflammation compounds may have potential therapeutic significance. This review focused on the KP metabolites through the relationship of neuroinflammation in AD, significant KP metabolites, and associated molecular mechanisms as well as the utility of these metabolites as biomarkers and therapeutic targets for AD. The objective is to provide references to find biomarkers and therapeutic targets for patients with AD.

Kynurenic acid in neurodegenerative disorders-unique neuroprotection or double-edged sword?

AIMS

The family of kynurenine pathway (KP) metabolites includes compounds produced along two arms of the path and acting in clearly opposite ways. The equilibrium between neurotoxic kynurenines, such as 3-hydroxykynurenine (3-HK) or quinolinic acid (QUIN), and neuroprotective kynurenic acid (KYNA) profoundly impacts the function and survival of neurons. This comprehensive review summarizes accumulated evidence on the role of KYNA in Alzheimer's, Parkinson's and Huntington's diseases, and discusses future directions of potential pharmacological manipulations aimed to modulate brain KYNA.

DISCUSSION

The synthesis of specific KP metabolites is tightly regulated and may considerably vary under physiological and pathological conditions. Experimental data consistently imply that shift of the KP to neurotoxic branch producing 3-HK and QUIN formation, with a relative or absolute deficiency of KYNA, is an important factor contributing to neurodegeneration. Targeting specific brain regions to maintain adequate KYNA levels seems vital; however, it requires the development of precise pharmacological tools, allowing to avoid the potential cognitive adverse effects.

CONCLUSIONS

Boosting KYNA levels, through interference with the KP enzymes or through application of prodrugs/analogs with high bioavailability and potency, is a promising clinical approach. The use of KYNA, alone or in combination with other compounds precisely influencing specific populations of neurons, is awaiting to become a significant therapy for neurodegenerative disorders.

Glutamate and GABA in Microglia-Neuron Cross-Talk in Alzheimer's Disease

The physiological balance between excitation and inhibition in the brain is significantly affected in Alzheimer’s disease (AD). Several neuroactive compounds and their signaling pathways through various types of receptors are crucial in brain homeostasis, among them glutamate and γ-aminobutyric acid (GABA). Activation of microglial receptors regulates the immunological response of these cells, which in AD could be neuroprotective or neurotoxic. The novel research approaches revealed the complexity of microglial function, including the interplay with other cells during neuroinflammation and in the AD brain. The purpose of this review is to describe the role of several proteins and multiple receptors on microglia and neurons, and their involvement in a communication network between cells that could lead to different metabolic loops and cell death/survival. Our review is focused on the role of glutamatergic, GABAergic signaling in microglia–neuronal cross-talk in AD and neuroinflammation. Moreover, the significance of AD-related neurotoxic proteins in glutamate/GABA-mediated dialogue between microglia and neurons was analyzed in search of novel targets in neuroprotection, and advanced pharmacological approaches.

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.

Relationships and Interactions between Ionotropic Glutamate Receptors and Nicotinic Receptors in the CNS

Although ionotropic glutamate receptors and nicotinic receptors for acetylcholine (ACh) have usually been studied separately, they are often co-localized and functionally inter-dependent. The objective of this review is to survey the evidence for interactions between the two receptor families and the mechanisms underlying them. These include the mutual regulation of subunit expression, which change the NMDA:AMPA response balance, and the existence of multi-functional receptor complexes which make it difficult to distinguish between individual receptor sites, especially in vivo. This is followed by analysis of the functional relationships between the receptors from work on transmitter release, cellular electrophysiology and aspects of behavior where these can contribute to understanding receptor interactions. It is clear that nicotinic receptors (nAChRs) on axonal terminals directly regulate the release of glutamate and other neurotransmitters, α7-nAChRs generally promoting release. Hence, α7-nAChR responses will be prevented not only by a nicotinic antagonist, but also by compounds blocking the indirectly activated glutamate receptors. This accounts for the apparent anticholinergic activity of some glutamate antagonists, including the endogenous antagonist kynurenic acid. The activation of presynaptic nAChRs is by the ambient levels of ACh released from pre-terminal synapses, varicosities and glial cells, acting as a 'volume neurotransmitter' on synaptic and extrasynaptic sites. In addition, ACh and glutamate are released as CNS co-transmitters, including 'cholinergic' synapses onto spinal Renshaw cells. It is concluded that ACh should be viewed primarily as a modulator of glutamatergic neurotransmission by regulating the release of glutamate presynaptically, and the location, subunit composition, subtype balance and sensitivity of glutamate receptors, and not primarily as a classical fast neurotransmitter. These conclusions and caveats should aid clarification of the sites of action of glutamate and nicotinic receptor ligands in the search for new centrally-acting drugs.

Prenatal Kynurenine Elevation Elicits Sex-Dependent Changes in Sleep and Arousal During Adulthood: Implications for Psychotic Disorders

Dysregulation of the kynurenine pathway (KP) of tryptophan catabolism has been implicated in psychotic disorders, including schizophrenia and bipolar disorder. Kynurenic acid (KYNA) is a KP metabolite synthesized by kynurenine aminotransferases (KATs) from its biological precursor kynurenine and acts as an endogenous antagonist of N-methyl-D-aspartate and α7-nicotinic acetylcholine receptors. Elevated KYNA levels found in postmortem brain tissue and cerebrospinal fluid of patients are hypothesized to play a key role in the etiology of cognitive symptoms observed in psychotic disorders. Sleep plays an important role in memory consolidation, and sleep disturbances are common among patients. Yet, little is known about the effect of altered KP metabolism on sleep-wake behavior. We presently utilized a well-established experimental paradigm of embryonic kynurenine (EKyn) exposure wherein pregnant dams are fed a diet laced with kynurenine the last week of gestation and hypothesized disrupted sleep-wake behavior in adult offspring. We examined sleep behavior in adult male and female offspring using electroencephalogram and electromyogram telemetry and determined sex differences in sleep and arousal in EKyn offspring. EKyn males displayed reduced rapid eye movement sleep, while female EKyn offspring were hyperaroused compared to controls. We determined that EKyn males maintain elevated brain KYNA levels, while KYNA levels were unchanged in EKyn females, yet the activity levels of KAT I and KAT II were reduced. Our findings indicate that elevated prenatal kynurenine exposure elicits sex-specific changes in sleep-wake behavior, arousal, and KP metabolism.

Kynurenine 3-monooxygenase deficiency induces depression-like behavior via enhanced antagonism of α7 nicotinic acetylcholine receptors by kynurenic acid

Tryptophan (TRP) is metabolized via the kynurenine (KYN) pathway, which is related to the pathogenesis of major depressive disorder (MDD). Kynurenine 3-monooxygenase (KMO) is a pivotal enzyme in the metabolism of KYN to 3-hydroxykynurenine. In rodents, KMO deficiency induces a depression-like behavior and increases the levels of kynurenic acid (KA), a KYN metabolite formed by kynurenine aminotransferases (KATs). KA antagonizes α7 nicotinic acetylcholine receptor (α7nAChR). Here, we investigated the involvement of KA in depression-like behavior in KMO knockout (KO) mice. KYN, KA, and anthranilic acid but not TRP or 3-hydroxyanthranilic acid were elevated in the prefrontal cortex of KMO KO mice. The mRNA levels of KAT1 and α7nAChR but not KAT2-4, α4nAChR, or β2nAChR were elevated in the prefrontal cortex of KMO KO mice. Nicotine blocked increase in locomotor activity, decrease in social interaction time, and prolonged immobility in a forced swimming test, but it did not decrease sucrose preference in the KMO KO mice. Methyllycaconitine (an α7nAChR antagonist) antagonized the effect of nicotine on decreased social interaction time and prolonged immobility in the forced swimming test, but not increased locomotor activity. Galantamine (an α7nAChR allosteric agonist) blocked the increased locomotor activity and prolonged immobility in the forced swimming test, but not the decreased social interaction time in the KMO KO mice. In conclusion, elevation of KA levels contributes to depression-like behaviors in KMO KO mice by α7nAChR antagonism. The ameliorating effects of nicotine and galantamine on depression-like behaviors in KMO KO mice are associated with the activation of α7nAChR.

Effects of Sleep Deprivation on the Tryptophan Metabolism

Sleep has a regulatory role in maintaining metabolic homeostasis and cellular functions. Inadequate sleep time and sleep disorders have become more prevalent in the modern lifestyle. Fragmentation of sleep pattern alters critical intracellular second messengers and neurotransmitters which have key functions in brain development and behavioral functions. Tryptophan metabolism has also been found to get altered in SD and it is linked to various neurodegenerative diseases. The kynurenine pathway is a major regulator of the immune response. Adequate sleep alleviates neuroinflammation and facilitates the cellular clearance of metabolic toxins produced within the brain, while sleep deprivation activates the enzymatic degradation of tryptophan via the kynurenine pathway, which results in an increased accumulation of neurotoxic metabolites. SD causes increased production and accumulation of kynurenic acid in various regions of the brain. Higher levels of kynurenic acid have been found to trigger apoptosis, leads to cognitive decline, and inhibit neurogenesis. This review aims to link the impact of sleep deprivation on tryptophan metabolism and associated complication in the brain.

Evolutionary puzzle of Toxoplasma gondii with suicidal ideation and suicide attempts: An updated systematic review and meta-analysis

The World Health Organization has reported an annual global suicide rate of 14.5 per 100,000 people. On the other hand, it is estimated that approximately one-third of the global population are infected with Toxoplasma gondii (T. gondii) parasite. It is widely assumed that microbial pathogens, such as T. gondii, are probably associated with affective and behavioural modulation. The present article aimed to assess the proposed role of toxoplasmosis in raising the risk of suicidal ideation (SI) and suicide attempts (SA) using the available epidemiological data. Seven major electronic databases and the Internet search engine Google were searched for all the studies published between the 1st of January 1950 and 31st of October 2019. The heterogeneity and the risk of bias within and across studies were assessed. Following data extraction, pooled odds ratios (ORs) with 95% confidence interval (CI) across studies were calculated using the random-effects models. A total number of 9,696 articles were screened and 27 studies were regarded as eligible in our systematic review (SI with five papers and 22 papers on SA). A significant association was detected between antibodies against T. gondii with TA (ORs = 1.57; 95% confidence interval [CI] 1.23-2.00, p = .000). Exploration of the association between T. gondii and SA yielded a positive effect of seropositivity for IgG antibodies but not IgM. Despite the limited number of studies, a statistical association was detected between suicidal behaviours and infection with latent T. gondii.

N-Acetylcysteine Inhibits Kynurenine Aminotransferase II

The tryptophan metabolite kynurenic acid (KYNA) may play an important role in normal and abnormal cognitive processes, most likely by interfering with α7 nicotinic and NMDA receptor function. KYNA is formed from its immediate precursor kynurenine either by non-enzymatic oxidation or through irreversible transamination by kynurenine aminotransferases. In the mammalian brain, kynurenine aminotransferase II (KAT II) is the principal enzyme responsible for the neosynthesis of rapidly mobilizable KYNA, and therefore constitutes an attractive target for pro-cognitive interventions. N-acetylcysteine (NAC), a brain-penetrant drug with pro-cognitive efficacy in humans, has been proposed to exert its actions by increasing the levels of the anti-oxidant glutathione (GSH) in the brain. We report here that NAC, but not GSH, inhibits KAT II activity in brain tissue homogenates from rats and humans with IC₅₀ values in the high micromolar to low millimolar range. With similar potency, the drug interfered with the de novo formation of KYNA in rat brain slices, and NAC was a competitive inhibitor of recombinant human KAT II (Ki: 450 μM). Furthermore, GSH failed to S-glutathionylate recombinant human KAT II treated with the dithiocarbamate drug disulfiram. Shown by microdialysis in the prefrontal cortex of rats treated with kynurenine (50 mg/kg, i.p.), peripheral administration of NAC (500 mg/kg, i.p., 120 and 60 min before the application of kynurenine) reduced KYNA neosynthesis by ∼50%. Together, these results suggest that NAC exerts its neurobiological effects at least in part by reducing cerebral KYNA formation via KAT II inhibition.

Activation of alpha7 nicotinic and NMDA receptors is necessary for performance in a working memory task

RATIONALE

Working memory deficits are present in schizophrenia (SZ) but remain insufficiently resolved by medications. Similar cognitive dysfunctions can be produced acutely in animals by elevating brain levels of kynurenic acid (KYNA). KYNA's effects may reflect interference with the function of both the α7 nicotinic acetylcholine receptor (α7nAChR) and the glycineB site of the NMDA receptor.

OBJECTIVES

The aim of the present study was to examine, using pharmacological tools, the respective roles of these two receptor sites on performance in a delayed non-match-to-position working memory (WM) task (DNMTP).

METHODS

DNMTP consisted of 120 trials/session (5, 10, and 15 s delays). Rats received two doses (25 or 100 mg/kg, i.p.) of L-kynurenine (KYN; bioprecursor of KYNA) or L-4-chlorokynurenine (4-Cl-KYN; bioprecursor of the selective glycineB site antagonist 7-Cl-kynurenic acid). Attenuation of KYN- or 4-Cl-KYN-induced deficits was assessed by co-administration of galantamine (GAL, 3 mg/kg) or PAM-2 (1 mg/kg), two positive modulators of α7nAChR function. Reversal of 4-Cl-KYN-induced deficits was examined using D-cycloserine (DCS; 30 mg/kg), a partial agonist at the glycineB site.

RESULTS

Both KYN and 4-Cl-KYN administration produced dose-related deficits in DNMTP accuracy that were more severe at the longer delays. In KYN-treated rats, these deficits were reversed to control levels by GAL or PAM-2 but not by DCS. In contrast, DCS eliminated performance deficits in 4-Cl-KYN-treated animals.

CONCLUSIONS

These experiments reveal that both α7nAChR and NMDAR activity are necessary for normal WM accuracy. They provide substantive new support for the therapeutic potential of positive modulators at these two receptor sites in SZ and other major brain diseases.

Increased Plasma Kynurenic Acid Levels are Associated with Impaired Attention/Vigilance and Social Cognition in Patients with Schizophrenia

OBJECTIVE

Preclinical studies have reported that abnormal kynurenic acid (KYNA) may play a role in cognitive deficits. Schizophrenia (SCZ) is characterized by a wide range of cognitive deficits that may evolve from abnormal KYNA. This study aimed to explore the relationship between KYNA and cognitive impairment in SCZ, which has not yet been reported.

METHODS

We recruited 30 SCZ patients and 34 healthy controls, measured clinical symptoms by using the Positive and Negative Syndrome Scale and performed cognitive tests using the MATRICS Consensus Cognitive Battery (MCCB). Plasma levels of tryptophan, kynurenine, and KYNA were determined by high-performance liquid chromatography-tandem mass spectrometry.

RESULTS

We found that plasma KYNA levels were significantly higher in patients than in healthy controls (p=0.009). The cognitive performance of patients in the total MCCB scores and the scores of all subscales were significantly lower than those in healthy controls (all P < 0.01). Correlation analysis showed that KYNA levels were negatively correlated with attention/vigilance (r=-0.457, p=0.019) and social cognition (r=-0.481, p=0.013) only in SCZ patients.

CONCLUSION

Our results indicate that elevated plasma KYNA levels may serve as a biomarker of cognitive impairment in SCZ patients.

Possible Use of Blood Tryptophan Metabolites as Biomarkers for Coronary Heart Disease in Sudden Unexpected Death

Coronary heart disease (CHD) is the major cause of death in sudden unexpected death (SUD) cases. Tryptophan (TRP) and its metabolites are correlated with the CHD patient but less studies in the SUD. The aim of this study was to evaluate the relationship of TRP and its metabolites with the CHD in the SUD cases. Blood samples and heart tissues were collected from CHD subjects (n = 31) and the control group (n = 72). Levels of kynurenine (KYN), kynurenic acid (KYA), xanthurenic acid (XAN), 3-hydroxyanthranillic acid (HAA), quinolinic acid (QA), picolinic acid (PA) and 5-hydroxyindoleacetic acid (HIAA) were determined by HPLC-DAD. A severity of heart occlusion was categorized into four groups, and the relationship was measured with the TRP metabolites. The HIAA and The KYN levels significantly differed (p < 0.01) between the CHD group and the control group. Lower levels of QA/XAN, PA/KA, HAA/XAN, KYN/XAN and KYN/TRP were found in the CHD group. However, PA/HAA, PA/HIAA, PA/KYN and XAN/KA values in the CHD group were higher than the control group (p < 0.05). This study revealed that the values of PA/KA and PA/HAA provided better choices for a CHD biomarker in postmortem bodies.

Kynurenines and the Endocannabinoid System in Schizophrenia: Common Points and Potential Interactions

Schizophrenia, which affects around 1% of the world’s population, has been described as a complex set of symptoms triggered by multiple factors. However, the exact background mechanisms remain to be explored, whereas therapeutic agents with excellent effectivity and safety profiles have yet to be developed. Kynurenines and the endocannabinoid system (ECS) play significant roles in both the development and manifestation of schizophrenia, which have been extensively studied and reviewed previously. Accordingly, kynurenines and the ECS share multiple features and mechanisms in schizophrenia, which have yet to be reviewed. Thus, the present study focuses on the main common points and potential interactions between kynurenines and the ECS in schizophrenia, which include (i) the regulation of glutamatergic/dopaminergic/γ-aminobutyric acidergic neurotransmission, (ii) their presence in astrocytes, and (iii) their role in inflammatory mechanisms. Additionally, promising pharmaceutical approaches involving the kynurenine pathway and the ECS will be reviewed herein.

Kynurenines, Neuropsychiatric Symptoms, and Cognitive Prognosis in Patients with Mild Dementia

Introduction:

Circulating tryptophan (Trp) and its downstream metabolites, the kynurenines, are potentially neuroactive. Consequently, they could be associated with neuropsychiatric symptoms and cognitive prognosis in patients with dementia.

Objective:

The objective of this study was to assess associations between circulating kynurenines, cognitive prognosis, and neuropsychiatric symptoms.

Methods:

We measured baseline serum Trp, neopterin, pyridoxal 5′-phosphate (PLP), and 9 kynurenines in 155 patients with mild dementia (90 with Alzheimer’s disease, 65 with Lewy body dementia). The ratios between kynurenine and Trp and kynurenic acid (KA) to kynurenine (KKR) were calculated. The Mini-Mental State Examination (MMSE) and the Neuropsychiatric Inventory (NPI) were administered at baseline and annually over 5 years. Associations between baseline metabolite concentrations with MMSE and the NPI total score were assessed using a generalized structural equation model (mixed-effects multiprocess model), adjusted for age, sex, current smoking, glomerular filtration rate, and PLP. Post hoc associations between KKRs and individual NPI items were assessed using logistic mixed-effects models. False discovery rate (0.05)–adjusted P values (Q values) are reported.

Results:

Kynurenine had a nonlinear quadratic relationship with the intercept of the MMSE scores over 5 years (Q < 0.05), but not with the slope of MMSE decline. Kynurenine was associated with a higher NPI total score over time (Q < 0.001). Post hoc, both KKR and KA were associated with more hallucinations (Q < 0.05).

Conclusions:

Kynurenine has a complex relationship with cognition, where both low and high levels were associated with poor cognitive performance. A higher KKR indicated risk for neuropsychiatric symptoms, especially hallucinations.

Selective and competitive inhibition of kynurenine aminotransferase 2 by glycyrrhizic acid and its analogues

The enzyme kynurenine aminotransferase (KAT) catalyses the conversion of kynurenine (KYN) to kynurenic acid (KYNA). Although the isozymes KAT1–4 have been identified, KYNA is mainly produced by KAT2 in brain tissues. KNYA is an antagonist of N-methyl-D-aspartate and α-7-nicotinic acetylcholine receptors, and accumulation of KYNA in the brain has been associated with the pathology of schizophrenia. Therefore, KAT2 could be exploited as a therapeutic target for the management of schizophrenia. Although currently available KAT2 inhibitors irreversibly bind to pyridoxal 5′-phosphate (PLP), inhibition via this mechanism may cause adverse side effects because of the presence of other PLP-dependent enzymes. Therefore, we identified novel selective KAT2 inhibitors by screening approximately 13,000 molecules. Among these, glycyrrhizic acid (GL) and its analogues, glycyrrhetinic acid (GA) and carbenoxolone (CBX), were identified as KAT2 inhibitors. These compounds were highly selective for KAT2 and competed with its substrate KYN, but had no effects on the other 3 KAT isozymes. Furthermore, we demonstrated that in complex structures that were predicted in docking calculations, GL, GA and CBX were located on the same surface as the aromatic ring of KYN. These results indicate that GL and its analogues are highly selective and competitive inhibitors of KAT2.

Acute liver failure increases kynurenic acid production in rat brain via changes in tryptophan metabolism in the periphery

The tryptophan metabolite, kynurenic acid (KYNA), is a preferential antagonist of the α7 nicotinic acetylcholine receptor and N-methyl-d-aspartic acid receptor at endogenous brain concentrations. Recent studies have suggested that increased brain KYNA levels are involved in psychiatric disorders such as schizophrenia and depression. Most of the brain kynurenine (KYN), the KYNA precursor, comes from the periphery, and the liver has a central role in the peripheral tryptophan metabolism. In this study, the effect of acute liver failure (ALF) on brain KYNA production and on the peripheral tryptophan metabolism was investigated in rats. ALF was induced by administration of the hepatotoxin, thioacetamide (TAA). Brain KYNA levels were increased by TAA-induced ALF, and these increases were consistent with KYN levels in the brain, serum and liver. These results suggest that the ALF-induced increase in serum KYN contributes to the increase in brain KYNA via elevated KYN uptake within the brain. This increase in serum KYN level can be caused by the changes in tryptophan-2,3-dioxygenase activity in the liver and the immune-related activation of indoleamine-2,3-dioxygenase in extrahepatic tissues. These findings suggest that hepatic dysfunction may contribute to neurological and psychiatric diseases associated with increased KYNA levels.

Kynurenine pathway metabolic balance influences microglia activity: Targeting kynurenine monooxygenase to dampen neuroinflammation

Chronic stress or inflammation increases tryptophan metabolism along the kynurenine pathway (KP), and the generation of neuroactive kynurenine metabolites contributes to subsequent depressive-like behaviors. Microglia regulate KP balance by preferentially producing oxidative metabolites, including quinolinic acid. Research has focused on the interplay between cytokines and HPA axis-derived corticosteroids in regulating microglial activity and effects of KP metabolites directly on neurons; however, the potential role that KP metabolites have directly on microglial activity is unknown. Here, murine microglia were stimulated with lipopolysaccharide(LPS). After 6 h, mRNA expression of interleukin(IL)-1β, IL-6, tumor necrosis factor(TNF)-α and inducible nitric oxide synthase(iNOS) was dose-dependently increased along with the rate-limiting enzymes for oxidative KP metabolism, indoleamine-2,3-dioxygenase(IDO)-1 and kynurenine 3-monooxygenase(KMO). By 24 h post-LPS, kynurenine and quinolinic acid in the media was elevated. Inhibiting KMO with Ro 61-8048 during LPS challenge attenuated extracellular nitrite accumulation and expression of KMO and TNF-α in response to LPS. Similarly, primary microglia isolated from KMO^-/- mice exhibited a significantly reduced pro-inflammatory response to LPS compared to WT controls. To determine whether the substrate (kynurenine) or end product (quinolinic acid) of KMO-dependent metabolism modulates the LPS response, microglia were treated with increasing concentrations of L-kynurenine or quinolinic acid in combination with LPS or saline. Interestingly, quinolinic acid did not impact the microglial LPS response. However, L-kynurenine had dose-dependent inhibitory effect on the LPS response. These data are the first to show an anti-inflammatory effect of KMO inhibition on microglia during immune challenge and suggest that KP metabolic balance may play a direct role in regulating microglia activity.

Relevance of Alternative Routes of Kynurenic Acid Production in the Brain

The catabolism of tryptophan has gained great importance in recent years due to the fact that the metabolites produced during this process, with neuroactive and redox properties, are involved in physiological and pathological events. One of these metabolites is kynurenic acid (KYNA), which is considered as a neuromodulator since it can interact with NMDA, nicotinic, and GPR35 receptors among others, modulating the release of neurotransmitters as glutamate, dopamine, and acetylcholine. Kynureninate production is attributed to kynurenine aminotransferases. However, in some physiological and pathological conditions, its high production cannot be explained just with kynurenine aminotransferases. This review focuses on the alternative mechanism whereby KYNA can be produced, either from D-amino acids or by means of other enzymes as D-amino acid oxidase or by the participation of free radicals. It is important to mention that an increase in KYNA levels in processes as brain development, aging, neurodegenerative diseases, and psychiatric disorders, which share common factors as oxidative stress, inflammation, immune response activation, and participation of gut microbiota that can also be related with the alternative routes of KYNA production, has been observed.

Improvement of kynurenine aminotransferase-II inhibitors guided by mimicking sulfate esters

The mammalian kynurenine aminotransferase (KAT) enzymes are a family of related isoforms that are pyridoxal 5’-phosphate-dependent, responsible for the irreversible transamination of kynurenine to kynurenic acid. Kynurenic acid is implicated in human diseases such as schizophrenia where it is found in elevated levels and consequently KAT-II, as the isoform predominantly responsible for kynurenic acid production in the brain, has been targeted for the development of specific inhibitors. One class of compounds that have also shown inhibitory activity towards the KAT enzymes are estrogens and their sulfate esters. Estradiol disulfate in particular is very strongly inhibitory and it appears that the 17-sulfate makes a significant contribution to its potency. The work here demonstrates that the effect of this moiety can be mirrored in existing KAT-II inhibitors, from the development of two novel inhibitors, JN-01 and JN-02. Both inhibitors were based on NS-1502 (IC₅₀: 315 μM), but the deliberate placement of a sulfonamide group significantly improved the potency of JN-01 (IC₅₀: 73.8 μM) and JN-02 (IC₅₀: 112.8 μM) in comparison to the parent compound. This 3–4 fold increase in potency shows the potential of these moieties to be accommodated in the KAT-II active site and the effect they can have on improving inhibitors, and the environments in the KAT-II have been suitably modelled using docking calculations.

Acute Kynurenine Challenge Disrupts Sleep-Wake Architecture and Impairs Contextual Memory in Adult Rats

Study Objectives

Tryptophan metabolism via the kynurenine pathway may represent a key molecular link between sleep loss and cognitive dysfunction. Modest increases in the kynurenine pathway metabolite kynurenic acid (KYNA), which acts as an antagonist at N-methyl-d-aspartate and α7 nicotinic acetylcholine receptors in the brain, result in cognitive impairments. As glutamatergic and cholinergic neurotransmissions are critically involved in modulation of sleep, our current experiments tested the hypothesis that elevated KYNA adversely impacts sleep quality.

Methods

Adult male Wistar rats were treated with vehicle (saline) and kynurenine (25, 50, 100, and 250 mg/kg), the direct bioprecursor of KYNA, intraperitoneally at zeitgeber time (ZT) 0 to rapidly increase brain KYNA. Levels of KYNA in the brainstem, cortex, and hippocampus were determined at ZT 0, ZT 2, and ZT 4, respectively. Analyses of vigilance state-related parameters categorized as wake, rapid eye movement (REM), and non-REM (NREM) as well as spectra power analysis during NREM and REM were assessed during the light phase. Separate animals were tested in the passive avoidance paradigm, testing contextual memory.

Results

When KYNA levels were elevated in the brain, total REM duration was reduced and total wake duration was increased. REM and wake architecture, assessed as number of vigilance state bouts and average duration of each bout, and theta power during REM were significantly impacted. Kynurenine challenge impaired performance in the hippocampal-dependent contextual memory task.

Conclusions

Our results introduce kynurenine pathway metabolism and formation of KYNA as a novel molecular target contributing to sleep disruptions and cognitive impairments.

Fragment Screening of Human Kynurenine Aminotransferase-II

Kynurenine aminotransferase-II (KAT-II) is a pyridoxal 5'-phosphate (PLP)-dependent enzyme that acts in the tryptophan metabolic pathway by catalyzing the transamination of kynurenine into kynurenic acid (KYNA). It is one of four isoforms in the KAT family, of which it is the primary homologue responsible for KYNA production in the mammalian brain. KAT-II is targeted for inhibition as KYNA is implicated in diseases such as schizophrenia, where it is found in elevated concentrations. Previously, many different approaches have been taken to develop KAT-II inhibitors, and herein fragment-based drug design (FBDD) approaches have been exploited to provide further lead compounds that can be designed into novel inhibitors. Surface plasmon resonance (SPR) was used to screen a fragment library containing 1000 compounds, of which 41 hits were identified. These hits were further evaluated with SPR, and 18 were selected for inhibition studies. From these hits, two fragments, F6037-0164 and F0037-7280, were pursued and determined to have an IC₅₀ of 524.5 (± 25.6) μM and 115.2 (± 4.5) μM, respectively. This strategy shows the viability of using FBDD in gleaning knowledge about KAT-II inhibition and generating leads for the production of KAT-II inhibitors.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Inhibition of human kynurenine aminotransferase isozymes by estrogen and its derivatives

The kynurenine aminotransferase (KAT) enzymes are pyridoxal 5′-phosphate-dependent homodimers that catalyse the irreversible transamination of kynurenine into kynurenic acid (KYNA) in the tryptophan metabolic pathway. Kynurenic acid is implicated in cognitive diseases such as schizophrenia, and several inhibitors have been reported that selectively target KAT-II as it is primarily responsible for kynurenic acid production in the human brain. Not only is schizophrenia a sexually dimorphic condition, but women that have schizophrenia have reduced estrogen levels in their serum. Estrogens are also known to interact in the kynurenine pathway therefore exploring these interactions can yield a better understanding of the condition and improve approaches in ameliorating its effects. Enzyme inhibitory assays and binding studies showed that estradiol disulfate is a strong inhibitor of KAT-I and KAT-II (IC₅₀: 291.5 μM and 26.3 μM, respectively), with estradiol, estradiol 3-sulfate and estrone sulfate being much weaker (IC₅₀ > 2 mM). Therefore it is possible that estrogen levels can dictate the balance of kynurenic acid in the brain. Inhibition assay results and modelling suggests that the 17-sulfate moiety in estradiol disulfate is very important in improving its potency as an inhibitor, increasing the inhibition by approximately 10–100 fold compared to estradiol.

Glial and tissue-specific regulation of Kynurenine Pathway dioxygenases by acute stress of mice

Stressors activate the hypothalamic-pituitary-adrenal (HPA) axis and immune system eliciting changes in cognitive function, mood and anxiety. An important link between stress and altered behavior is stimulation of the Kynurenine Pathway which generates neuroactive and immunomodulatory kynurenines. Tryptophan entry into this pathway is controlled by rate-limiting indoleamine/tryptophan 2,3-dioxygenases (DOs: Ido1, Ido2, Tdo2). Although implicated as mediating changes in behavior, detecting stress-induced DO expression has proven inconsistent. Thus, C57BL/6J mice were used to characterize DO expression in brain-regions, astrocytes and microglia to characterize restraint-stress-induced DO expression. Stress increased kynurenine in brain and plasma, demonstrating increased DO activity. Of three Ido1 transcripts, only Ido1-v1 expression was increased by stress and within astrocytes, not microglia, indicating transcript- and glial-specificity. Stress increased Ido1-v1 only in frontal cortex and hypothalamus, indicating brain-region specificity. Of eight Ido2 transcripts, Ido2-v3 expression was increased by stress, again only within astrocytes. Likewise, stress increased Tdo2-FL expression in astrocytes, not microglia. Interestingly, Ido2 and Tdo2 transcripts were not correspondingly induced in Ido1-knockout (Ido1KO) mice, suggesting that Ido1 is necessary for the central DO response to acute stress. Unlike acute inflammatory models resulting in DO induction within microglia, only astrocyte DO expression was increased by acute restraint-stress, defining their unique role during stress-dependent activation of the Kynurenine Pathway.

Preferential Disruption of Prefrontal GABAergic Function by Nanomolar Concentrations of the α7nACh Negative Modulator Kynurenic Acid

Increased concentrations of kynurenic acid (KYNA) in the prefrontal cortex (PFC) are thought to contribute to the development of cognitive deficits observed in schizophrenia. Although this view is consistent with preclinical studies showing a negative impact of prefrontal KYNA elevation on executive function, the mechanism underlying such a disruption remains unclear. Here, we measured changes in local field potential (LFP) responses to ventral hippocampal stimulation in vivo and conducted whole-cell patch-clamp recordings in brain slices to reveal how nanomolar concentrations of KYNA alter synaptic transmission in the PFC of male adult rats. Our data show that prefrontal infusions of KYNA attenuated the inhibitory component of PFC LFP responses, a disruption that resulted from local blockade of α7-nicotinic acetylcholine receptors (α7nAChR). At the cellular level, we found that the inhibitory action exerted by KYNA in the PFC occurred primarily at local GABAergic synapses through an α7nAChR-dependent presynaptic mechanism. As a result, the excitatory-inhibitory ratio of synaptic transmission becomes imbalanced in a manner that correlates highly with the level of GABAergic suppression by KYNA. Finally, prefrontal infusion of a GABA_AR positive allosteric modulator was sufficient to overcome the disrupting effect of KYNA and normalized the pattern of LFP inhibition in the PFC. Thus, the preferential inhibitory effect of KYNA on prefrontal GABAergic transmission could contribute to the onset of cognitive deficits observed in schizophrenia because proper GABAergic control of PFC output is one key mechanism for supporting such cortical functions.SIGNIFICANCE STATEMENT Brain kynurenic acid (KYNA) is an astrocyte-derived metabolite and its abnormal elevation in the prefrontal cortex (PFC) is thought to impair cognitive functions in individuals with schizophrenia. However, the mechanism underlying the disrupting effect of KYNA remains unclear. Here we found that KYNA biases the excitatory-inhibitory balance of prefrontal synaptic activity toward a state of disinhibition. Such disruption emerges as a result of a preferential suppression of local GABAergic transmission by KYNA via presynaptic inhibition of α7-nicotinic acetylcholine receptor signaling. Therefore, the degree of GABAergic dysregulation in the PFC could be a clinically relevant contributing factor for the onset of cognitive deficits resulting from abnormal increases of cortical KYNA.

Kynurenic Acid in Schizophrenia: A Systematic Review and Meta-analysis

Kynurenic acid (KYNA) is an endogenous antagonist of N-methyl-D-aspartate and α7 nicotinic acetylcholine receptors that is derived from astrocytes as part of the kynurenine pathway of tryptophan degradation. Evidence suggests that abnormal KYNA levels are involved in the pathophysiology of schizophrenia. However, this has never been assessed through a meta-analysis. A literature search was conducted through Ovid using Embase, Medline, and PsycINFO databases (last search: December 2016) with the search terms: (kynuren* or KYNA) and (schizophreni* or psychosis). English language studies measuring KYNA levels using any method in patients with schizophrenia and healthy controls (HCs) were identified. Standardized mean differences (SMDs) were calculated to determine differences in KYNA levels between groups. Subgroup analyses were separately performed for nonoverlapping participant samples, KYNA measurement techniques, and KYNA sample source. The influences of patients' age, antipsychotic status (%medicated), and sex (%male) on study SMDs were assessed through a meta-regression. Thirteen studies were deemed eligible for inclusion in the meta-analysis. In the main analysis, KYNA levels were elevated in the patient group. Subgroup analyses demonstrated that KYNA levels were increased in nonoverlapping participant samples, and centrally (cerebrospinal fluid and brain tissue) but not peripherally. Patients' age, %medicated, and %male were each positively associated with study SMDs. Overall, KYNA levels are increased in patients with schizophrenia, specifically within the central nervous system. An improved understanding of KYNA in patients with schizophrenia may contribute to the development of novel diagnostic approaches and therapeutic strategies.

Attenuating Nicotine Reinforcement and Relapse by Enhancing Endogenous Brain Levels of Kynurenic Acid in Rats and Squirrel Monkeys

The currently available antismoking medications have limited efficacy and often fail to prevent relapse. Thus, there is a pressing need for newer, more effective treatment strategies. Recently, we demonstrated that enhancing endogenous levels of kynurenic acid (KYNA, a neuroinhibitory product of tryptophan metabolism) counteracts the rewarding effects of cannabinoids by acting as a negative allosteric modulator of α7 nicotinic receptors (α7nAChRs). As the effects of KYNA on cannabinoid reward involve nicotinic receptors, in the present study we used rat and squirrel monkey models of reward and relapse to examine the possibility that enhancing KYNA can counteract the effects of nicotine. To assess specificity, we also examined models of cocaine reward and relapse in monkeys. KYNA levels were enhanced by administering the kynurenine 3-monooxygenase (KMO) inhibitor, Ro 61-8048. Treatment with Ro 61-8048 decreased nicotine self-administration in rats and monkeys, but did not affect cocaine self-administration. In rats, Ro 61-8048 reduced the ability of nicotine to induce dopamine release in the nucleus accumbens shell, a brain area believed to underlie nicotine reward. Perhaps most importantly, Ro 61-8048 prevented relapse-like behavior when abstinent rats or monkeys were reexposed to nicotine and/or cues that had previously been associated with nicotine. Ro 61-8048 was also effective in monkey models of cocaine relapse. All of these effects of Ro 61-8048 in monkeys, but not in rats, were reversed by pretreatment with a positive allosteric modulator of α7nAChRs. These findings suggest that KMO inhibition may be a promising new approach for the treatment of nicotine addiction.

Systemic administration of l-kynurenine sulfate induces cerebral hypoperfusion transients in adult C57Bl/6 mice

The kynurenine pathway is a cascade of enzymatic steps generating biologically active compounds. l-kynurenine (l-KYN) is a central metabolite of tryptophan degradation. In the mammalian brain, l-KYN is partly converted to kynurenic acid (KYNA), which exerts multiple effects on neurotransmission. Recently, l-KYN or one of its derivatives were attributed a direct role in the regulation of the systemic circulation. l-KYN dilates arterial blood vessels during sepsis in rats, while it increases cerebral blood flow (CBF) in awake rabbits. Therefore, we hypothesized that acute elevation of systemic l-KYN concentration may exert potential effects on mean arterial blood pressure (MABP) and on resting CBF in the mouse brain. C57Bl/6 male mice were anesthetized with isoflurane, and MABP was monitored in the femoral artery, while CBF was assessed through the intact parietal bone with the aid of laser speckle contrast imaging. l-KYN sulfate (l-KYNs) (300mg/kg, i.p.) or vehicle was administered intraperitoneally. Subsequently, MABP and CBF were continuously monitored for 2.5h. In the control group, MABP and CBF were stable (69±4mmHg and 100±5%, respectively) throughout the entire data acquisition period. In the l-KYNs-treated group, MABP was similar to that, of control group (73±6mmHg), while hypoperfusion transients of 22±6%, lasting 7±3min occurred in the cerebral cortex over the first 60-120min following drug administration. In conclusion, the systemic high-dose of l-KYNs treatment destabilizes resting CBF by inducing a number of transient hypoperfusion events. This observation indicates the careful consideration of the dose of l-KYN administration by interpreting the effect of kynurenergic manipulation on brain function. By planning clinical trials basing on kynurenergic manipulation possible vascular side effects should also be considered.

Elevated kynurenine pathway metabolism during neurodevelopment: Implications for brain and behavior

The kynurenine pathway (KP) of tryptophan degradation contains several neuroactive metabolites that may influence brain function in health and disease. Mounting focus has been dedicated to investigating the role of these metabolites during neurodevelopment and elucidating their involvement in the pathophysiology of psychiatric disorders with a developmental component, such as schizophrenia. In this review, we describe the changes in KP metabolism in the brain from gestation until adulthood and illustrate how environmental and genetic factors affect the KP during development. With a particular focus on kynurenic acid, the antagonist of α7 nicotinic acetylcholine (α7nACh) and N-methyl-d-aspartate (NMDA) receptors, both implicated in modulating brain development, we review animal models designed to ascertain the role of perinatal KP elevation on long-lasting biochemical, neuropathological, and behavioral deficits later in life. We present new data demonstrating that combining perinatal choline-supplementation, to potentially increase activation of α7nACh receptors during development, with embryonic kynurenine manipulation is effective in attenuating cognitive impairments in adult rat offspring. With these findings in mind, we conclude the review by discussing the advancement of therapeutic interventions that would target not only symptoms, but potentially the root cause of central nervous system diseases that manifest from a perinatal KP insult. This article is part of the Special Issue entitled 'The Kynurenine Pathway in Health and Disease'.

The Kynurenine Pathway in the Acute and Chronic Phases of Cerebral Ischemia

Kynurenines are a wide range of catabolites which derive from tryptophan through the "Kynurenine Pathway" (KP). In addition to its peripheral role, increasing evidence shows a role of the KP in the central nervous system (CNS), mediating both physiological and pathological functions. Indeed, an imbalance in this route has been associated with several neurodegenerative disorders such as Alzheimer´s and Huntington´s diseases. Altered KP catabolism has also been described during both acute and chronic phases of stroke; however the contribution of the KP to the pathophysiology of acute ischemic damage and of post-stroke disorders during the chronic phase including depression and vascular dementia, and the exact mechanisms implicated in the regulation of the KP after stroke are not well established yet. A better understanding of the regulation and activity of the KP after stroke could provide new pharmacological tools in both acute and chronic phases of stroke. In this review, we will make an overview of CNS modulation by the KP. We will detail the KP contribution in the ischemic damage, how the unbalance of the KP might trigger an alteration of the cognitive function after stroke as well as potential targets for the development of new drugs.

Prenatal kynurenine exposure in rats: age-dependent changes in NMDA receptor expression and conditioned fear responding

RATIONALE

Levels of kynurenic acid (KYNA), an endogenous negative modulator of alpha 7 nicotinic acetylcholine receptors (α7nAChRs) and antagonist at glutamatergic N-methyl-D-aspartate receptors (NMDARs), are elevated in the brain of patients with schizophrenia (SZ). In rats, dietary exposure to KYNA's immediate precursor kynurenine during the last week of gestation produces neurochemical and cognitive deficits in adulthood that resemble those seen in patients with SZ.

OBJECTIVES

The present experiments examined whether prenatal kynurenine exposure results in age-dependent changes in the kynurenine pathway (KP), expression of selected receptors, and cognitive function.

METHODS

Pregnant dams were fed unadulterated mash (progeny = ECON) or mash containing kynurenine (100 mg/day; progeny = EKYN) from embryonic day (ED) 15 to 22. Male offspring were assessed as juveniles, i.e., prior to puberty (postnatal day [PD] 32), or as adults (PD70) for brain KYNA levels, α7nAChR and NMDAR gene expression, and performance on a trace fear conditioning (TFC) task.

RESULTS

KYNA levels were comparable between juvenile ECON and EKYN rats, whereas EKYN adults exhibited a ~3-fold increase in brain KYNA relative to ECONs. NR2A expression was persistently reduced (30-40 %) in EKYN rats at both ages. Compared to ECON adults, there was a 50 % reduction in NR1, and a trend toward decreased α7nAChR expression, in adult EKYN rats. Surprisingly, juvenile EKYN rats performed significantly better in the TFC paradigm than controls, whereas adult EKYN animals showed the predicted deficits.

CONCLUSIONS

Collectively, our results provide evidence that KP changes in the fetal brain alter neuronal development and cause age-dependent effects on neurochemistry and cognitive performance.

Astrocytic and neuronal localization of kynurenine aminotransferase-2 in the adult mouse brain

During catabolism of tryptophan through the kynurenine (KYN) pathway, several endogenous metabolites with neuromodulatory properties are produced, of which kynurenic acid (KYNA) is one of the highest significance. The causal role of altered KYNA production has been described in several neurodegenerative and neuropsychiatric disorders (e.g., Parkinson's disease, Huntington's disease, schizophrenia) and therefore kynurenergic manipulation with the aim of therapy has recently been proposed. Conventionally, KYNA is produced from its precursor L-KYN with the aid of the astrocytic kynurenine aminotransferase-2 (KAT-2) in the murine brain. Although the mouse is a standard therapeutic research organism, the presence of KAT-2 in mice has not been described in detail. This study demonstrates the presence of kat-2 mRNA and protein throughout the adult C57Bl6 mouse brain. In addition to the former expression data from the rat, we found prominent KAT-2 expression not only in the astrocyte, but also in neurons in several brain regions (e.g., hippocampus, substantia nigra, striatum, and prefrontal cortex). A significant number of the KAT-2 positive neurons were positive for GAD67; the presence of the KAT-2 enzyme we could also demonstrate in mice brain homogenate and in cells overexpressing recombinant mouse KAT-2 protein. This new finding attributes a new role to interneuron-derived KYNA in neuronal network operation. Furthermore, our results suggest that the thorough investigation of the spatio-temporal expression pattern of the relevant enzymes of the KYN pathway is a prerequisite for developing and understanding the pharmacological and transgenic murine models of kynurenergic manipulation.

Targeting inflammation to influence cognitive function following spinal cord injury: a randomized clinical trial

STUDY DESIGN

This study was a randomized, parallel-group, controlled clinical trial.

OBJECTIVES

The purpose of this study was to examine the efficacy of targeting inflammation as a means of improving cognitive function in individuals with spinal cord injury.

SETTING

Participants were recruited from the Niagara region of Ontario Canada and all testing occurred on-site at Brock University.

METHODS

Indices of memory and verbal learning were assessed by means of the California Verbal Learning Test (CVLT). Inflammation and concentrations of neuroactive compounds related to the kynurenine pathway were assessed via a number of pro- and anti-inflammatory cytokines, as well as tryptophan, kynurenine and several large neutral amino acids. All assessments were performed at baseline as well as at 1 month and 3 months during a 3-month intervention by means of an anti-inflammatory diet.

RESULTS

Despite a reduction in inflammation, all measures of the CVLT, including list A, trial 1 (P=0.48), learning slope (P=0.46), long delay free recall (P=0.83), intrusions (P=0.61) and repetitions (P=0.07), showed no significant group × time interaction.

CONCLUSION

It may be possible that the reduction in inflammation achieved in the current study was insufficient to induce substantial changes in indices of verbal learning and memory. Alternatively, as these participants likely underwent years of previous chronic inflammation, the underlying hippocampal damage may have negated potential improvements induced by acute reductions in inflammation.