Kynurenergic manipulations influence excitatory synaptic function and excitotoxic vulnerability in the rat hippocampus in vivo

Vimentin single cysteine residue acts as a tunable sensor for network organization and as a key for actin remodeling in response to oxidants and electrophiles

Cysteine residues can undergo multiple posttranslational modifications with diverse functional consequences, potentially behaving as tunable sensors. The intermediate filament protein vimentin has important implications in pathophysiology, including cancer progression, infection, and fibrosis, and maintains a close interplay with other cytoskeletal structures, such as actin filaments and microtubules. We previously showed that the single vimentin cysteine, C328, is a key target for oxidants and electrophiles. Here, we demonstrate that structurally diverse cysteine-reactive agents, including electrophilic mediators, oxidants and drug-related compounds, disrupt the vimentin network eliciting morphologically distinct reorganizations. As most of these agents display broad reactivity, we pinpointed the importance of C328 by confirming that local perturbations introduced through mutagenesis provoke structure-dependent vimentin rearrangements. Thus, GFP-vimentin wild type (wt) forms squiggles and short filaments in vimentin-deficient cells, the C328F, C328W, and C328H mutants generate diverse filamentous assemblies, and the C328A and C328D constructs fail to elongate yielding dots. Remarkably, vimentin C328H structures resemble the wt, but are strongly resistant to electrophile-elicited disruption. Therefore, the C328H mutant allows elucidating whether cysteine-dependent vimentin reorganization influences other cellular responses to reactive agents. Electrophiles such as 1,4-dinitro-1H-imidazole and 4-hydroxynonenal induce robust actin stress fibers in cells expressing vimentin wt. Strikingly, under these conditions, vimentin C328H expression blunts electrophile-elicited stress fiber formation, apparently acting upstream of RhoA. Analysis of additional vimentin C328 mutants shows that electrophile-sensitive and assembly-defective vimentin variants permit induction of stress fibers by reactive species, whereas electrophile-resistant filamentous vimentin structures prevent it. Together, our results suggest that vimentin acts as a break for actin stress fibers formation, which would be released by C328-aided disruption, thus allowing full actin remodeling in response to oxidants and electrophiles. These observations postulate C328 as a "sensor" transducing structurally diverse modifications into fine-tuned vimentin network rearrangements, and a gatekeeper for certain electrophiles in the interplay with actin.

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.

Cellular Localization of Kynurenine 3-Monooxygenase in the Brain: Challenging the Dogma

Kynurenine 3-monooxygenase (KMO), a key player in the kynurenine pathway (KP) of tryptophan degradation, regulates the synthesis of the neuroactive metabolites 3-hydroxykynurenine (3-HK) and kynurenic acid (KYNA). KMO activity has been implicated in several major brain diseases including Huntington’s disease (HD) and schizophrenia. In the brain, KMO is widely believed to be predominantly localized in microglial cells, but verification in vivo has not been provided so far. Here, we examined KP metabolism in the brain after depleting microglial cells pharmacologically with the colony stimulating factor 1 receptor inhibitor PLX5622. Young adult mice were fed PLX5622 for 21 days and were euthanized either on the next day or after receiving normal chow for an additional 21 days. Expression of microglial marker genes was dramatically reduced on day 22 but had fully recovered by day 43. In both groups, PLX5622 treatment failed to affect Kmo expression, KMO activity or tissue levels of 3-HK and KYNA in the brain. In a parallel experiment, PLX5622 treatment also did not reduce KMO activity, 3-HK and KYNA in the brain of R6/2 mice (a model of HD with activated microglia). Finally, using freshly isolated mouse cells ex vivo, we found KMO only in microglia and neurons but not in astrocytes. Taken together, these data unexpectedly revealed that neurons contain a large proportion of functional KMO in the adult mouse brain under both physiological and pathological conditions.

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.

Kynurenines in the Pathogenesis of Multiple Sclerosis: Therapeutic Perspectives

Over the past years, an increasing amount of evidence has emerged in support of the kynurenine pathway’s (KP) pivotal role in the pathogenesis of several neurodegenerative, psychiatric, vascular and autoimmune diseases. Different neuroactive metabolites of the KP are known to exert opposite effects on neurons, some being neuroprotective (e.g., picolinic acid, kynurenic acid, and the cofactor nicotinamide adenine dinucleotide), while others are toxic to neurons (e.g., 3-hydroxykynurenine, quinolinic acid). Not only the alterations in the levels of the metabolites but also disturbances in their ratio (quinolinic acid/kynurenic acid) have been reported in several diseases. In addition to the metabolites, the enzymes participating in the KP have been unearthed to be involved in modulation of the immune system, the energetic upkeep of neurons and have been shown to influence redox processes and inflammatory cascades, revealing a sophisticated, intertwined system. This review considers various methods through which enzymes and metabolites of the kynurenine pathway influence the immune system, the roles they play in the pathogenesis of neuroinflammatory diseases based on current evidence with a focus on their involvement in multiple sclerosis, as well as therapeutic approaches.

Long-term systemic administration of kynurenic acid brain region specifically elevates the abundance of functional CB1 receptors in rats

Kynurenic acid (KYNA) is one of the most significant metabolite of the kynurenine pathway both in terms of functional and potential therapeutic value. It is an N-methyl-D-aspartate (NMDA) receptor antagonist, but it can also activate the G-protein coupled receptor 35 (GPR35), which shares several structural and functional properties with cannabinoid receptors. Previously our group demonstrated that systemic chronic KYNA treatment altered opioid receptor G-protein activity. Opioid receptors also overlap in many features with cannabinoid receptors. Thus, our aim was to examine the direct in vitro and systemic, chronic in vivo effect of KYNA on type 1 cannabinoid receptor (CB₁R) binding and G-protein activity. Based on competition and [³⁵S]GTPγS G-protein binding assays in rat brain, KYNA alone did not show significant binding towards the CB₁R, nor did it alter CB₁R ligand binding and agonist activity in vitro. When rats were chronically treated with KYNA (single daily, i.p., 128 mg/kg for 9 days), the KYNA plasma and cerebrospinal fluid levels significantly increased compared to vehicle treated group. Furthermore, in G-protein binding assays, in the whole brain the amount of G-proteins in basal and in maximum activity coupled to the CB₁R also increased due to the treatment. At the same time, the overall stimulatory properties of the receptor remained unaltered in vehicle and KYNA treated samples. Similar observations were made in rat hippocampus, but not in the cortex and brainstem. In saturation binding assays the density of CB₁Rs in rat whole brain and hippocampus were also significantly enhanced after the same treatment, without significantly affecting ligand binding affinity. Thus, KYNA indirectly and brain region specifically increases the abundance of functional CB₁Rs, without modifying the overall binding and activity of the receptor. Supposedly, this can be a compensatory mechanism on the part of the endocannabinoid system induced by the long-term KYNA exposure.

Neurotransmitter and tryptophan metabolite concentration changes in the complete Freund's adjuvant model of orofacial pain

BACKGROUND

The neurochemical background of the evolution of headache disorders, still remains partially undiscovered. Accordingly, our aim was to further explore the neurochemical profile of Complete Freund's adjuvant (CFA)-induced orofacial pain, involving finding the shift point regarding small molecule neurotransmitter concentrations changes vs. that of the previously characterized headache-related neuropeptides. The investigated neurotransmitters consisted of glutamate, γ-aminobutyric acid, noradrenalin and serotonin. Furthermore, in light of its influence on glutamatergic neurotransmission, we measured the level of kynurenic acid (KYNA) and its precursors in the kynurenine (KYN) pathway (KP) of tryptophan metabolism.

METHODS

The effect of CFA was evaluated in male Sprague Dawley rats. Animals were injected with CFA (1 mg/ml, 50 μl/animal) into the right whisker pad. We applied high-performance liquid chromatography to determine the concentrations of the above-mentioned compounds from the trigeminal nucleus caudalis (TNC) and somatosensory cortex (ssCX) of rats. Furthermore, we measured some of these metabolites from the cerebrospinal fluid and plasma as well. Afterwards, we carried out permutation t-tests as post hoc analysis for pairwise comparison.

RESULTS

Our results demonstrated that 24 h after CFA treatment, the level of glutamate, KYNA and that of its precursor, KYN was still elevated in the TNC, all diminishing by 48 h. In the ssCX, significant concentration increases of KYNA and serotonin were found.

CONCLUSION

This is the first study assessing neurotransmitter changes in the TNC and ssCX following CFA treatment, confirming the dominant role of glutamate in early pain processing and a compensatory elevation of KYNA with anti-glutamatergic properties. Furthermore, the current findings draw attention to the limited time interval where medications can target the glutamatergic pathways.

Targeted plasma metabolome response to variations in dietary glycemic load in a randomized, controlled, crossover feeding trial in healthy adults

Low versus high glycemic load (GL) diet patterns are inversely associated with obesity and chronic diseases such as cancer and cardiovascular disease. These associations persist beyond the protection afforded by increased fiber alone, representing an important gap in our understanding of the metabolic effects of GL. We conducted a randomized, controlled, crossover feeding trial of two 28-day diet periods of high and low GL. Using LC-MS, targeted metabolomics analysis of 155 metabolites was performed on plasma samples from 19 healthy adults aged 18-45 years. Fourteen metabolites differed significantly between diets (P < 0.05), with kynurenate remaining significant after Bonferroni correction (P < 4 × 10(-4)). Metabolites with the largest difference in abundance were kynurenate and trimethylamine-N-oxide (TMAO), both significantly higher after consumption of the low GL diet. Partial least squares-discriminant analysis showed clear separation between the two diets; however no specific pathway was identified in pathway analyses. We found significant differences in 14 plasma metabolites suggesting a differing metabolic response to low and high GL diets. Kynurenate is associated with reduced inflammation, and may be one mechanism through which protective effects of a low GL diet are manifested and warrants further evaluation. This trial was registered at clinicaltrials.gov as NCT00622661.

Systemic L-Kynurenine sulfate administration disrupts object recognition memory, alters open field behavior and decreases c-Fos immunopositivity in C57Bl/6 mice

L-Kynurenine (L-KYN) is a central metabolite of tryptophan degradation through the kynurenine pathway (KP). The systemic administration of L-KYN sulfate (L-KYNs) leads to a rapid elevation of the neuroactive KP metabolite kynurenic acid (KYNA). An elevated level of KYNA may have multiple effects on the synaptic transmission, resulting in complex behavioral changes, such as hypoactivity or spatial working memory deficits. These results emerged from studies that focused on rats, after low-dose L-KYNs treatment. However, in several studies neuroprotection was achieved through the administration of high-dose L-KYNs. In the present study, our aim was to investigate whether the systemic administration of a high dose of L-KYNs (300 mg/bwkg; i.p.) would produce alterations in behavioral tasks (open field or object recognition) in C57Bl/6j mice. To evaluate the changes in neuronal activity after L-KYNs treatment, in a separate group of animals we estimated c-Fos expression levels in the corresponding subcortical brain areas. The L-KYNs treatment did not affect the general ambulatory activity of C57Bl/6j mice, whereas it altered their moving patterns, elevating the movement velocity and resting time. Additionally, it seemed to increase anxiety-like behavior, as peripheral zone preference of the open field arena emerged and the rearing activity was attenuated. The treatment also completely abolished the formation of object recognition memory and resulted in decreases in the number of c-Fos-immunopositive-cells in the dorsal part of the striatum and in the CA1 pyramidal cell layer of the hippocampus. We conclude that a single exposure to L-KYNs leads to behavioral disturbances, which might be related to the altered basal c-Fos protein expression in C57Bl/6j mice.

Endonuclease G mediates α-synuclein cytotoxicity during Parkinson's disease

Malfunctioning of the protein α-synuclein is critically involved in the demise of dopaminergic neurons relevant to Parkinson's disease. Nonetheless, the precise mechanisms explaining this pathogenic neuronal cell death remain elusive. Endonuclease G (EndoG) is a mitochondrially localized nuclease that triggers DNA degradation and cell death upon translocation from mitochondria to the nucleus. Here, we show that EndoG displays cytotoxic nuclear localization in dopaminergic neurons of human Parkinson-diseased patients, while EndoG depletion largely reduces α-synuclein-induced cell death in human neuroblastoma cells. Xenogenic expression of human α-synuclein in yeast cells triggers mitochondria-nuclear translocation of EndoG and EndoG-mediated DNA degradation through a mechanism that requires a functional kynurenine pathway and the permeability transition pore. In nematodes and flies, EndoG is essential for the α-synuclein-driven degeneration of dopaminergic neurons. Moreover, the locomotion and survival of α-synuclein-expressing flies is compromised, but reinstalled by parallel depletion of EndoG. In sum, we unravel a phylogenetically conserved pathway that involves EndoG as a critical downstream executor of α-synuclein cytotoxicity.

Post-ischemic treatment with L-kynurenine sulfate exacerbates neuronal damage after transient middle cerebral artery occlusion

Since brain ischemia is one of the leading causes of adult disability and death, neuroprotection of the ischemic brain is of particular importance. Acute neuroprotective strategies usually have the aim of suppressing glutamate excitotoxicity and an excessive N-methyl-d-aspartate (NMDA) receptor function. Clinically tolerated antagonists should antagonize an excessive NMDA receptor function without compromising the normal synaptic function. Kynurenic acid (KYNA) an endogenous metabolite of the tryptophan metabolism, may be an attractive neuroprotectant in this regard. The manipulation of brain KYNA levels was earlier found to effectively enhance the histopathological outcome of experimental ischemic/hypoxic states. The present investigation of the neuroprotective capacity of L-kynurenine sulfate (L-KYNs) administered systemically after reperfusion in a novel distal middle cerebral artery occlusion (dMCAO) model of focal ischemia/reperfusion revealed that in contrast with earlier results, treatment with L-KYNs worsened the histopathological outcome of dMCAO. This contradictory result indicates that post-ischemic treatment with L-KYNs may be harmful.

Administration of kynurenine during adolescence, but not during adulthood, impairs social behavior in rats

Kynurenic acid (KYNA) is a tryptophan metabolite that is present at high concentrations in the brains of persons with schizophrenia. This study tested the hypothesis that treatment with L-kynurenine (L-KYN), which increases KYNA concentration, would produce deficits in social behavior similar to those associated with schizophrenia. Rats treated with L-KYN throughout adolescence exhibited decreased social interaction when tested drug-free as adults. In contrast, neither acute nor chronic treatment during adulthood affected social behavior. These findings demonstrate that increases in KYNA concentration produce deficits in social behavior and that the adolescent brain is particularly susceptible to the effects of high KYNA concentration.

Behavioural studies with a newly developed neuroprotective KYNA-amide

The neuroactive properties and neuroprotective potential of endogenous L: -kynurenine, kynurenic acid (KYNA) and its derivatives are well established. KYNA acts as an antagonist on the obligatory co-agonist glycine site, and has long been at the focus of neuroprotective trials. Unfortunately, KYNA is barely able to cross the blood-brain barrier. Accordingly, the development and synthesis of KYNA analogs which can readily cross the BBB have been at the focus of research interest with the aim of neuroprotection. Earlier we reported a new KYNA-amide crosses the BBB and proved neuroprotective in several experiments. In the present study, we investigated the locomotor activity, working memory performance, and also the long-lasting, consolidated reference memory of animals treated intraperitoneally (i.p.) with the novel analog. The effects of the novel analog on the spatial orientation and learning ability of rats were assessed in the Morris water maze (MWM) paradigm. The effects on locomotor activity of mice was assessed in the open field (OF) paradigm, and those on the spatial orientation and learning ability of mice were investigated in the radial arm maze (RAM) paradigm. It emerged that there is a dose of this KYNA-amide which is neuroprotective, but does not worsen the cognitive function of the brain. This result is significant in that a putative neuroprotectant without adverse cognitive side-effects is of great benefit.

Neuroprotection with a new kynurenic acid analog in the four-vessel occlusion model of ischemia

Global forebrain ischemia results in damage to the pyramids in the CA1 hippocampal subfield, which is particularly vulnerable to excitotoxic processes. Morphological and functional disintegration of this area leads to a cognitive dysfunction and neuropsychiatric disorders. Treatment with N-methyl-d-aspartate receptor antagonists is a widely accepted method with which to stop the advance of excitotoxic processes and concomitant neuronal death. From a clinical aspect, competitive glycine- and polyamine-site antagonists with relatively low affinity and moderate side-effects are taken into account. Endogenous kynurenic acid acts as an antagonist on the obligatory co-agonist glycine site, and has long been at the focus of neuroprotective trials. In the present study, we estimated the neuroprotective capability of a novel kynurenic acid analog in transient global forebrain ischemia, measuring the rate of hippocampal CA1 pyramidal cell loss and the preservation of long-term potentiation at Schaffer collateral-CA1 synapses. The neuroprotective potential was reflected by a significantly diminished hippocampal CA1 cell loss and preserved long-term potentiation expression. The neuroprotective effect was robust in the event of pretreatment, and also when the drug was administered at the time of reperfusion. This result is beneficial since a putative neuroprotectant proven to be effective as post-treatment is of much greater benefit.

The involvement of neuroinflammation and kynurenine pathway in Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder characterised by loss of dopaminergic neurons and localized neuroinflammation occurring in the midbrain several years before the actual onset of symptoms. Activated microglia themselves release a large number of inflammatory mediators thus perpetuating neuroinflammation and neurotoxicity. The Kynurenine pathway (KP), the main catabolic pathway for tryptophan, is one of the major regulators of the immune response and may also be implicated in the inflammatory response in parkinsonism. The KP generates several neuroactive compounds and therefore has either a neurotoxic or neuroprotective effect. Several of these molecules produced by microglia can activate the N-methyl-D-aspartate (NMDA) receptor-signalling pathway, leading to an excitotoxic response. Previous studies have shown that NMDA antagonists can ease symptoms and exert a neuroprotective effect in PD both in vivo and in vitro. There are to date several lines of evidence linking some of the KP intermediates and the neuropathogenesis of PD. Moreover, it is likely that pharmacological modulation of the KP will represent a new therapeutic strategy for PD.

A biological pathway linking inflammation and depression: activation of indoleamine 2,3-dioxygenase

This article highlights the evidence linking depression to increased inflammatory drive and explores putative mechanisms for the association by reviewing both preclinical and clinical literature. The enzyme indoleamine 2,3-dioxygenase is induced by proinflammatory cytokines and may form a link between immune functioning and altered neurotransmission, which results in depression. Increased indoleamine 2,3-dioxygenase activity may cause both tryptophan depletion and increased neurotoxic metabolites of the kynurenine pathway, two alterations which have been hypothesized to cause depression. The tryptophan-kynurenine pathway is comprehensively described with a focus on the evidence linking metabolite alterations to depression. The use of immune-activated groups at high risk of depression have been used to explore these hypotheses; we focus on the studies involving chronic hepatitis C patients receiving interferon-alpha, an immune activating cytokine. Findings from this work have led to novel strategies for the future development of antidepressants including inhibition of indoleamine 2,3-dioxygenase, moderating the cytokines which activate it, or addressing other targets in the kynurenine pathway.

Of mice, rats and men: Revisiting the quinolinic acid hypothesis of Huntington's disease

The neurodegenerative disease Huntington's disease (HD) is caused by an expanded polyglutamine (polyQ) tract in the protein huntingtin (htt). Although the gene encoding htt was identified and cloned more than 15 years ago, and in spite of impressive efforts to unravel the mechanism(s) by which mutant htt induces nerve cell death, these studies have so far not led to a good understanding of pathophysiology or an effective therapy. Set against a historical background, we review data supporting the idea that metabolites of the kynurenine pathway (KP) of tryptophan degradation provide a critical link between mutant htt and the pathophysiology of HD. New studies in HD brain and genetic model organisms suggest that the disease may in fact be causally related to early abnormalities in KP metabolism, favoring the formation of two neurotoxic metabolites, 3-hydroxykynurenine and quinolinic acid, over the related neuroprotective agent kynurenic acid. These findings not only link the excitotoxic hypothesis of HD pathology to an impairment of the KP but also define new drug targets and therefore have direct therapeutic implications. Thus, pharmacological normalization of the imbalance in brain KP metabolism may provide clinical benefits, which could be especially effective in early stages of the disease.

Managing depression during hepatitis C treatment

OBJECTIVE

The prevalence of hepatitis C virus (HCV) infection in Canada is estimated to be 1% and expected to increase during the next decade. Mental illness, particularly depression, is common among HCV-infected patients and remains an obstacle to interferon-alpha (IFN-alpha) treatment. We summarize the risk factors for interferon-alpha-induced major depressive disorder (IFN-alpha-MDD) in HCV patients and the evidence for antidepressant prophylaxis and symptomatic antidepressant treatment of depression.

METHODS

We searched MEDLINE, EMBASE, and CINAHL for randomized controlled or quasi-experimental trials evaluating antidepressant prophylactic and symptomatic treatment approaches for depression emerging during IFN-alpha treatment. Manual searches of references listed in review articles, case series, and anecdotal reports supplemented our literature search.

RESULTS

A total of 9 trials involving prophylactic and symptomatic treatment approaches for IFN-alpha-MDD are summarized in our review. Antidepressant pretreatment is beneficial for patients with elevated baseline depressive symptoms and a preexisting history of IFN-alpha-MDD. Although limited evidence exists for several antidepressant agents, much of the evidence suggests that selective serotonin reuptake inhibitors (SSRIs) are safe and efficacious in treating depressive symptoms secondary to IFN-alpha therapy.

CONCLUSION

Both antidepressant pretreatment and symptomatic treatment are viable strategies for treating IFN-alpha-MDD. Improved treatment outcomes and early identification of depression during HCV treatment can be achieved using an integrated medical and mental health treatment approach.

Pharmacological manipulation of kynurenic acid: potential in the treatment of psychiatric disorders

The kynurenine pathway constitutes the main route of tryptophan degradation and generates the production of several neuroactive compounds; quinolinic acid is an excitotoxic NMDA receptor agonist, 3-hydroxykynurenine is a free-radical generator and kynurenic acid (KYNA) is an antagonist at glutamate and nicotinic receptors. In low micromolar concentrations, KYNA blocks the glycine site of the NMDA receptor and the nicotinic alpha(7) acetylcholine receptor. Knowledge regarding kynurenine metabolites and their involvement in neurophysiological processes has increased dramatically in recent years. In particular, endogenous KYNA appears to tightly control firing of midbrain dopamine neurons and to be involved in cognitive functions. Thus, decreased endogenous levels of rat brain KYNA have been found to reduce firing of these neurons, and mice with a targeted deletion of kynurenine aminotransferase II display low endogenous brain KYNA levels concomitant with an increased performance in cognitive tests. It is also suggested that kynurenines participate in the pathophysiology of psychiatric disorders. Thus, elevated levels of KYNA have been found in the CSF as well as in the post-mortem brain of patients with schizophrenia. Advantages in understanding how kynurenines can be pharmacologically manipulated may provide new possibilities in the treatment of psychiatric disorders, such as schizophrenia.

Effects of Kynurenine Pathway Metabolites on Intracellular NAD Synthesis and Cell Death in Human Primary Astrocytes and Neurons

The kynurenine pathway (KP) is a major route of L-tryptophan catabolism resulting in the production of the essential pyridine nucleotide nicotinamide adenine dinucleotide, (NAD⁺). Up-regulation of the KP during inflammation leads to the release of a number of biologically active metabolites into the brain. We hypothesised that while some of the extracellular KP metabolites may be beneficial for intracellular NAD⁺ synthesis and cell survival at physiological concentrations, they may contribute to neuronal and astroglial dysfunction and cell death at pathophysiological concentrations. In this study, we found that treatment of human primary neurons and astrocytes with 3-hydroxyanthranilic acid (3-HAA), 3-hydroxykynurenine (3-HK), quinolinic acid (QUIN), and picolinic acid (PIC) at concentrations below 100 nM significantly increased intracellular NAD⁺ levels compared to non-treated cells. However, a dose dependent decrease in intracellular NAD⁺ levels and increased extracellular LDH activity was observed in human astrocytes and neurons treated with 3-HAA, 3-HK, QUIN and PIC at concentrations >100 nM and kynurenine (KYN), at concentrations above 1 μM. Intracellular NAD⁺ levels were unchanged in the presence of the neuroprotectant, kynurenic acid (KYNA), and a dose dependent increase in intracellular NAD⁺ levels was observed for TRP up to 1 mM. While anthranilic acid (AA) increased intracellular NAD⁺ levels at concentration below 10 μM in astrocytes. NAD⁺ depletion and cell death was observed in AA treated neurons at concentrations above 500 nM. Therefore, the differing responses of astrocytes and neurons to an increase in KP metabolites should be considered when assessing KP toxicity during neuroinflammation.

L-kynurenine treatment alters contextual fear conditioning and context discrimination but not cue-specific fear conditioning

The glia-derived molecule kynurenic acid (KYNA) is an antagonist of alpha7 nicotinic acetylcholine receptors and the glycine(B) binding site on NMDA receptors. KYNA levels are elevated in the brain and cerebrospinal fluid of persons with schizophrenia and Alzheimer's disease, both of which are characterized by deficits in contextual learning and memory. The present study tested the hypothesis that increases in KYNA concentration would impair contextual fear conditioning but spare cue-specific fear conditioning. Rats in each experiment received injections of vehicle solution or l-kynurenine (L-KYN, 100mg/kg), the precursor for KYNA. Administration of L-KYN has been shown to produce clinically relevant increases in KYNA concentration. In Experiment 1, L-KYN-treated rats exhibited impaired contextual fear memory compared to control rats, while fear conditioning to a discrete auditory cue was unaffected. In Experiment 2, rats were trained to discriminate between two different training environments, one in which foot shock was delivered and one that was not paired with foot shock. Although both groups of rats eventually learned the discrimination, learning was slower in L-KYN-treated rats. The results of Experiment 3 demonstrated that the deficits in context discrimination could not be explained by the preferential use of an elemental learning strategy by L-KYN-treated rats. Together, these findings indicate that elevated concentration of endogenous KYNA interferes with contextual learning and memory and support the notion that increased concentration of KYNA may contribute to cognitive dysfunction. In addition, these data provide new insight into how novel 'gliotransmitters' may modulate neuronal function and behavior.

Perinatal kynurenine 3-hydroxylase inhibition in rodents: pathophysiological implications

The kynurenine pathway (KP) of tryptophan degradation contains three neuroactive metabolites: the neuroinhibitory agent kynurenic acid (KYNA) and, in a competing branch, the free radical generator 3-hydroxykynurenine (3-HK) and the excitotoxin quinolinic acid (QUIN). These three "kynurenines" derive from a common precursor, L-kynurenine, and are recognized for their role in brain physiology and pathophysiology. Inhibition of kynurenine 3-hydroxylase, the enzyme responsible for 3-HK formation, shifts KP metabolism in the mature brain toward enhanced KYNA formation. We now tested the cerebral effects of kynurenine 3-hydroxylase inhibition in immature rodents. Rat pups treated with the kynurenine 3-hydroxylase inhibitor UPF 648 (30 mg/kg, i.p.) 10 min after birth showed substantial increases in cerebral and liver kynurenine and KYNA levels up to 24 hr later, whereas 3-HK and QUIN levels were simultaneously decreased. Administered to pregnant rats or mice on the last day of gestation, UPF 648 (50 mg/kg, i.p.) produced qualitatively similar changes (i.e., large increases in kynurenine and KYNA and reductions in 3-HK and QUIN) in the brain and liver of the offspring. Rat pups delivered by UPF 648-treated mothers and immediately exposed to neonatal asphyxia showed further enhanced brain KYNA levels. These studies demonstrate that acute kynurenine 3-hydroxylase inhibition effectively shifts cerebral KP metabolism in neonatal rodents toward increased KYNA formation. Selective inhibitors of this enzyme may therefore provide neuroprotection in newborns and will also be useful for the experimental evaluation of the long-term effects of perinatal KP impairment.

Elevations of endogenous kynurenic acid produce spatial working memory deficits

Kynurenic acid (KYNA) is a tryptophan metabolite that is synthesized and released by astrocytes and acts as a competitive antagonist of the glycine site of N-methyl-D-aspartate receptors at high concentrations and as a noncompetitive antagonist of the alpha7-nicotinic acetylcholine receptor at low concentrations. The discovery of increased cortical KYNA levels in schizophrenia prompted the hypothesis that elevated KYNA concentration may underlie the working memory dysfunction observed in this population that has been attributed to altered glutamatergic and/or cholinergic transmission. The present study investigated the effect of elevated endogenous KYNA on spatial working memory function in rats. Increased KYNA levels were achieved with intraperitoneal administration of kynurenine (100 mg/kg), the precursor of KYNA synthesis. Rats were treated with either kynurenine or a vehicle solution prior to testing in a radial arm maze task at various delays. Elevations of endogenous KYNA resulted in increased errors in the radial arm maze. In separate experiments, assessment of locomotor activity in an open field and latency to retrieve food reward from one of the maze arms ruled out the possibility that deficits in the maze were attributable to altered locomotor activity or motivation to consume food. These results provide evidence that increased KYNA levels produce spatial working memory deficits and are among the first to demonstrate the influence of glia-derived molecules on cognitive function. The implications for psychopathological conditions such as schizophrenia are discussed.

Mitochondria, metabolic disturbances, oxidative stress and the kynurenine system, with focus on neurodegenerative disorders

The mitochondria have several important functions in the cell. A mitochondrial dysfunction causes an abatement in ATP production, oxidative damage and the induction of apoptosis, all of which are involved in the pathogenesis of numerous disorders. This review focuses on mitochondrial dysfunctions and discusses their consequences and potential roles in the pathomechanism of neurodegenerative disorders. Other pathogenetic factors are also briefly surveyed. The second part of the review deals with the kynurenine metabolic pathway, its alterations and their potential association with cellular energy impairment in certain neurodegenerative diseases. During energy production, most of the O(2) consumed by the mitochondria is reduced fully to water, but 1-2% of the O(2) is reduced incompletely to give the superoxide anion (O(2)(-)). If the function of one or more respiratory chain complexes is impaired for any reason, the enhanced production of free radicals further worsens the mitochondrial function by causing oxidative damage to macromolecules, and by opening the mitochondrial permeability transition pores thereby inducing apoptosis. These high-conductance pores offer a pathway which can open in response to certain stimuli, leading to the induction of the cells' own suicide program. This program plays an essential role in regulating growth and development, in the differentiation of immune cells, and in the elimination of abnormal cells from the organism. Both failure and exaggeration of apoptosis in a human body can lead to disease. The increasing amount of superoxide anions can react with nitric oxide to yield the highly toxic peroxynitrite anion, which can destroy cellular macromolecules. The roles of oxidative, nitrative and nitrosative damage are discussed. Senescence is accompanied by a higher degree of reactive oxygen species production, and by diminished functions of the endoplasmic reticulum and the proteasome system, which are responsible for maintenance of the normal protein homeostasis of the cell. In the event of a dysfunction of the endoplasmic reticulum, unfolded proteins aggregate in it, forming potentially toxic deposits which tend to be resistant to degradation. Cells possess adaptive mechanisms with which to avoid the accumulation of incorrectly folded proteins. These involve molecular chaperones that fold proteins correctly, and the ubiquitin proteasome system which degrades misfolded, unwanted proteins. Both the endoplasmic reticulum and the ubiquitin proteasome system fulfill cellular protein quality control functions. The kynurenine system: Tryptophan is metabolized via several pathways, the main one being the kynurenine pathway. A central compound of the pathway is kynurenine (KYN), which can be metabolized in two separate ways: one branch furnishing kynurenic acid, and the other 3-hydroxykynurenine and quinolinic acid, the precursors of NAD. An important feature of kynurenic acid is the fact that it is one of the few known endogenous excitatory amino acid receptor blockers with a broad spectrum of antagonistic properties in supraphysiological concentrations. One of its recently confirmed sites of action is the alpha7-nicotinic acetylcholine receptor and interestingly, a more recently identified one is a higher affinity positive modulatory binding site at the AMPA receptor. Kynurenic acid has proven to be neuroprotective in several experimental settings. On the other hand, quinolinic acid is a specific agonist at the N-methyl-d-aspartate receptors, and a potent neurotoxin with an additional and marked free radical-producing property. There are a number of neurodegenerative disorders whose pathogenesis has been demonstrated to involve multiple imbalances of the kynurenine pathway metabolism. These changes may disturb normal brain function and can add to the pathomechanisms of the diseases. In certain disorders, there is a quinolinic acid overproduction, while in others the alterations in brain kynurenic acid levels are more pronounced. A more precise knowledge of these alterations yields a basis for getting better therapeutic possibilities. The last part of the review discusses metabolic disturbances and changes in the kynurenine metabolic pathway in Parkinson's, Alzheimer's and Huntington's diseases.

N-methyl-D-aspartate autoreceptors respond to low and high agonist concentrations by facilitating, respectively, exocytosis and carrier-mediated release of glutamate in rat hippocampus

Presynaptic NMDA autoreceptors regulating glutamate release have rarely been investigated. High-micromolar N-methyl-D-aspartate (NMDA) was reported to elicit glutamate release from hippocampal synaptosomes in a Ca(2+)-independent manner by reversal of excitatory amino acid transporters. The aim of this work was to characterize excitatory amino acid release evoked by low-micromolar NMDA from glutamatergic axon terminals. Purified rat hippocampal synaptosomes were prelabelled with [(3)H]D-aspartate ([(3)H]D-ASP) and exposed in superfusion to varying concentrations of NMDA in the presence of 1 microM glycine. The release of [(3)H]D-ASP and also that of endogenous glutamate provoked by 10 microM NMDA were external Ca(2+) dependent and sensitive to the NMDA channel blocker MK-801 but insensitive to the glutamate transporter inhibitor DL-TBOA, which, on the contrary, prevented the Ca(2+)-independent release evoked by 100 microM NMDA. The NMDA (10 microM) response was blocked by 1 nM Zn(2+) and 1 microM ifenprodil, compatible with the involvement of a NR1/NR2A/NR2B assembly, although the presence of two separate receptor populations, i.e., NR1/NR2A and NR1/NR2B, cannot be excluded. This response was strongly antagonized by submicromolar (0.01-1 microM) concentrations of kynurenic acid and was mimicked by quinolinic acid (1-100 microM) plus 1 microM glycine. Finally, the HIV-1 protein gp120 potently mimicked the NMDA co-agonists glycine and D-serine, being significantly effective at 30 pM. In conclusion, glutamatergic nerve terminals possess NMDA autoreceptors mediating different types of release when activated by different agonist concentrations: low-micromolar glutamate would potentiate glutamate exocytosis, whereas higher glutamate concentrations would also provoke carrier-mediated release.

Pneumococcal meningitis causes accumulation of neurotoxic kynurenine metabolites in brain regions prone to injury

Pneumococcal meningitis (PM) is characterized by an intense inflammatory host reaction that contributes to the development of cortical necrosis and hippocampal apoptosis. Inflammatory conditions in the brain are known to induce tryptophan degradation along the kynurenine pathway, resulting in accumulation of neurotoxic metabolites. In the present study, we investigated the contribution of the kynurenine pathway to brain injury in experimental PM by measuring the concentration of its metabolites and the enzymatic activities and mRNA levels of its major enzymes in the vulnerable brain regions. In the late phase of acute PM, we found a significant transcriptional upregulation of kynurenine-3-hydroxylase and an accumulation of the neurotoxic metabolites 3-hydroxykynurenine (3-HKYN) and 3-hydroxyanthranilic acid in cortex and hippocampus. The positive correlation between the concentration of 3-HKYN and the extent of hippocampal apoptosis adds support to the concept that 3-HKYN contributes to brain injury in PM.

Kynurenine Pathway Enzymes in Dendritic Cells Initiate Tolerogenesis in the Absence of Functional IDO

Dendritic cell (DC) tryptophan catabolism has emerged in recent years as a major mechanism of peripheral tolerance. However, there are features of this mechanism, initiated by IDO, that are still unclear, including the role of enzymes that are downstream of IDO in the kynurenine pathway and the role of the associated production of kynurenines. In this study, we provide evidence that 1) murine DCs express all enzymes necessary for synthesis of the downstream product of tryptophan breakdown, quinolinate; 2) IFN-gamma enhances transcriptional expression of all of these enzymes, although posttranslational inactivation of IDO may prevent metabolic steps that are subsequent and consequent to IDO; 3) overcoming the IDO-dependent blockade by provision of a downstream quinolinate precursor activates the pathway and leads to the onset of suppressive properties; and 4) tolerogenic DCs can confer suppressive ability on otherwise immunogenic DCs across a Transwell in an IDO-dependent fashion. Altogether, these data indicate that kynurenine pathway enzymes downstream of IDO can initiate tolerogenesis by DCs independently of tryptophan deprivation. The paracrine production of kynurenines might be one mechanism used by IDO-competent cells to convert DCs lacking functional IDO to a tolerogenic phenotype within an IFN-gamma-rich environment.

Effect of Amantadine on Depressive Symptoms in Chronic Hepatitis C Patients Treated with Pegylated Interferon

OBJECTIVE

Assess the effect of amantadine on depressive symptoms during interferon alfa therapy for hepatitis C virus infection.

METHODS

We performed a randomized, controlled trial with 14 hepatitis C virus-infected patients, treated with pegylated interferon alfa-2a 180 microg/wk plus ribavirin 1.200 mg/d. Eight patients were randomized to receive amantadine 200 mg/d, and 6 other individuals were randomized to control group without amantadine. Severity of depression and anxiety was measured using the Hospital Anxiety and Depression Scale, before starting re-treatment, and 4, 12, and 24 weeks after immunotherapy with pegylated interferon alfa-2a. Evaluations were performed by psychiatrists blinded in reference to the state of amantadine treatment. Friedman chi(2) test for repeated measures and Mann-Whitney test for nonparametric data were used to assess significant differences.

RESULTS

From baseline to follow-up, no significant increase in mean Hospital Anxiety and Depression Scale scores of depressive symptoms were seen in amantadine group (P = 0.142), meanwhile there was a statistical increase of depression scores in the control group (P = 0.001).

CONCLUSIONS

Our randomized pilot study, though small, clearly indicates that interferon alfa-induced depressive symptoms can be prevented by the use of amantadine. However, double-blind placebo-controlled trials with a higher sample size are required to confirm these preliminary findings.

Kynurenines, Parkinson's disease and other neurodegenerative disorders: preclinical and clinical studies

The kynurenine pathway is the main pathway of tryptophan metabolism. L-kynurenine is a central compound of this pathway since it can change to the neuroprotective agent kynurenic acid or to the neurotoxic agent quinolinic acid. The break-up of these endogenous compounds' balance can be observable in many disorders. It can be occur in neurodegenerative disorders, such as Parkinson's disease, Huntington's and Alzheimer's disease, in stroke, in epilepsy, in multiple sclerosis, in amyotrophic lateral sclerosis, and in mental failures, such as schizophrenia and depression. The increase of QUIN concentration or decrease of KYNA concentration could enhance the symptoms of several diseases. According to numerous studies, lowered KYNA level was found in patients with Parkinson's disease. It can be also noticeable that KYNA-treatment prevents against the QUIN-induced lesion of rat striatum in animal experiments. Administrating of KYNA can be appear a promising therapeutic approach, but its use is limited because of its poorly transport across the blood-brain barrier. The solution may be the development of KYNA analogues (e.g. glucoseamine-kynurenic acid) which can pass across this barrier and disengaging in the brain, then KYNA can exert its neuroprotective effects binding at the excitatory glutamate receptors, in particular the NMDA receptors. Furthermore, it seems hopeful to use kynurenine derivatives (e.g. 4-chloro-kynurenine) or enzyme inhibitors (e.g. Ro-61-8048) to ensure an increased kynurenic acid concentration in the central nervous system.

Effect of kynurenine 3-hydroxylase inhibition on the dyskinetic and antiparkinsonian responses to levodopa in Parkinsonian monkeys

Homeostatic interactions between dopamine and glutamate are central to the normal physiology of the basal ganglia. This relationship is altered in Parkinsonism and in levodopa-induced dyskinesias (LID), resulting in an upregulation of corticostriatal glutamatergic function. Kynurenic acid (KYNA), a tryptophan metabolite with antagonist activity at ionotropic glutamate receptors and the capability to inhibit glutamate release presynaptically, might therefore be of therapeutic value in LID. To evaluate this hypothesis, we used a pharmacological tool, the kynurenine 3-hydroxylase inhibitor Ro 61-8048, which raises KYNA levels acutely. Ro 61-8048 was tested in MPTP cynomolgus monkeys with a stable parkinsonian syndrome and reproducible dyskinesias after each dose of levodopa. Serum and CSF concentrations of KYNA and its precursor kynurenine increased dose-dependently after Ro 61-8048 administration, alone or in combination with levodopa. Coadministration of Ro 61-8048 with levodopa produced a moderate but significant reduction in the severity of dyskinesias while maintaining the motor benefit. These results suggest that elevation of KYNA levels through inhibition of kynurenine 3-hydroxylase constitutes a promising novel approach for managing LID in Parkinson's disease.

Kynurenic Acid Inhibits the Release of the Neurotrophic Fibroblast Growth Factor (FGF)-1 and Enhances Proliferation of Glia Cells, in vitro

1. Kynurenic (KYNA) and quinolinic (QUIN) acids are neuroactive tryptophan metabolites formed along the kynurenine pathway: the first is considered a non-competitive antagonist and the second an agonist of glutamate receptors of NMDA type. The affinity of these compounds for glutamate receptors is, however, relatively low and does not explain KYNA neuroprotective actions in models of post-ischemic brain damage. 2. We evaluated KYNA effects on the release of fibroblast growth factor (FGF)-1, a potent neurotrophic cytokine. Because KYNA exhibits a neuroprotective profile in vitro and in vivo, we anticipated that it could function as an autocrine/paracrine inducer of FGF-1 release. Studies were performed in several models of FGF-1 secretion (FGF-1 transfected NIH 3T3 cells exposed to heat shock, A375 melanoma cells exposed to serum starvation, growth factor deprived human endothelial cells). To our surprise, KYNA, at low concentration, inhibited FGF-1 release in all cellular models. QUIN, a compound having opposite effects on glutamate receptors, also reduced this release, but its potency was significantly lower than that of KYNA. 3. KYNA and QUIN also displayed a major stimulatory effect on the proliferation rate of mouse microglia and human glioblastoma cells, in vitro. 4. Our data suggest that minor changes of local KYNA concentration may modulate FGF-1 release, cell proliferation, and ultimately tissue damage in different pathological conditions.

Subchronic treatment with kynurenine and probenecid: effects on prepulse inhibition and firing of midbrain dopamine neurons

Acute elevation of the endogenous NMDA-receptor antagonist kynurenic acid (KYNA) is associated with an increased neuronal activity of rat ventral tegmental area (VTA) dopamine (DA) neurons and disruption in prepulse inhibition (PPI). In the present study, the effects of subchronic exposure to kynurenine and probenecid (20 mg/kg/day and 10 mg/kg/day, respectively for 14 days), aiming at increasing brain KYNA turnover, on rat VTA dopaminergic firing and on PPI were investigated. This treatment increased neuronal firing of VTA DA neurons, changed the response of these neurons to systemically administered nicotine (3-400 microg/kg, i.v.) and tended to disrupt PPI. Present results show that the effect on firing of VTA DA neurons by acutely elevated levels of brain KYNA also persists following subchronic exposure. In addition, no adaptive changes seem to occur with regard to the electrophysiological effects of KYNA on VTA DA neurons following subchronic treatment with kynurenine and probenecid.

Kynurenine 3-mono-oxygenase inhibitors reduce glutamate concentration in the extracellular spaces of the basal ganglia but not in those of the cortex or hippocampus

Kynurenine 3-mono-oxygenase (KMO, kynurenine hydroxylase) inhibitors increase brain kynurenic acid (KYNA) synthesis and cause pharmacological actions possibly mediated by a reduced activity of excitatory synapses. We used in vivo microdialysis and passive avoidance to study the effects of local KYNA or systemic KMO inhibitor administration on glutamate (GLU) neurotransmission. Local application of KYNA (30-100 nM) through reverse microdialysis reduced GLU content in caudate and cortical dialysates by 75 and 55%, respectively. No changes were found in the hippocampus. Systemic administration of Ro 61-8048 (4-40 mg/kg) increased KYNA levels in dialysates obtained from the cortex (from 10.3 +/- 1.9 to 45.5 +/- 15 nM), caudate (from 2.4 +/- 0.8 to 9.5 +/- 0.9 nM) and hippocampus (from 7.7 +/- 1.7 to 19.2 +/- 3.5 nM). It also caused a parallel robust decrease in GLU levels in the dialysates collected from the caudate (from 2.2 +/- 0.5 to 0.63 +/- 0.05 microM) but not in those collected from the parietal cortex or the hippocampus. In a passive avoidance paradigm, the administration of the NMDA receptor antagonist MK-801 (0.1 mg/kg) reduced, while Ro 61-8048 (4-80 mg/kg) did not change the latency time of entering into the dark compartment on the recall trial. Our data show that KMO inhibitors increase brain KYNA synthesis and selectively reduce GLU extracellular concentration in the basal ganglia.

IDO and interferon-alpha-induced depressive symptoms: a shift in hypothesis from tryptophan depletion to neurotoxicity

Studies show that administration of interferon (IFN)-alpha causes a significant increase in depressive symptoms. The enzyme indoleamine 2,3-dioxygenase (IDO), which converts tryptophan (TRP) into kynurenine (KYN) and which is stimulated by proinflammatory cytokines, may be implicated in the development of IFN-alpha-induced depressive symptoms, first by decreasing the TRP availability to the brain and second by the induction of the KYN pathway resulting in the production of neurotoxic metabolites. Sixteen patients with chronic hepatitis C, free of psychiatric disorders and eligible for IFN-alpha treatment, were recruited. Depressive symptoms were measured using the Montgomery Asberg Depression Rating Scale (MADRS). Measurements of TRP, amino acids competing with TRP for entrance through the blood-brain barrier, KYN and kynurenic acid (KA), a neuroprotective metabolite, were performed using high-performance liquid chromatography. All assessments were carried out at baseline and 1, 2, 4, 8, 12 and 24 weeks after treatment was initiated. The MADRS score significantly increased during IFN-alpha treatment as did the KYN/TRP ratio, reflecting IDO activity, and the KYN/KA ratio, reflecting the neurotoxic challenge. The TRP/CAA (competing amino acids) ratio, reflecting TRP availability to the brain, did not significantly change during treatment. Total MADRS score was significantly associated over time with the KYN/KA ratio, but not with the TRP/CAA ratio. Although no support was found that IDO decreases TRP availability to the brain, this study does support a role for IDO activity in the pathophysiology of IFN-alpha-induced depressive symptoms, through its induction of neurotoxic KYN metabolites.

Prolonged nicotine administration results in biphasic, brain-specific changes in kynurenate levels in the rat

The content of the endogenous NMDA and alpha7 nicotinic acetylcholine receptor antagonist kynurenate (KYNA) is increased in the cerebral cortex and cerebrospinal fluid of patients with schizophrenia. In view of the very high incidence of smoking in schizophrenic individuals, a study was designed to examine the effect of acute and prolonged nicotine administration on brain KYNA levels in experimental animals. Adult male rats received subcutaneous nicotine injections twice daily for up to 10 days, and animals were routinely killed 1 h after the last injection. Neither acute treatment nor a 2-day regimen with 1 mg/kg nicotine (= 0.35 mg/kg pure base) caused changes in cerebral KYNA levels. Four- or 6 day-treatment with this dose resulted in 20-40% decreases in cerebral KYNA content. Animals treated with 1 or 10 mg/kg nicotine for 10 days showed dose-dependent, significant increases in KYNA in hippocampus, striatum, and cortex, but not in the serum. Discontinuation of nicotine treatment for 7 days restored brain KYNA to control levels. Separate animals, implanted with osmotic minipumps delivering 2 mg/kg of nicotine/day for 10 days also showed significant elevations in brain KYNA. Hippocampal microdialysis, performed in animals receiving nicotine (1 mg/kg) for 10 days, revealed a significant increase in basal extracellular KYNA levels compared to controls, whereas acute treatment with this dose produced no such change. Measurements of KYNA's bioprecursor kynurenine in brain or blood did not reveal any nicotine-induced changes. These results indicate that nicotine has a brain-specific, biphasic effect on the transamination of kynurenine to KYNA. Such nicotine-induced fluctuations in brain KYNA may cause functional changes in processes that regulate glutamatergic and cholinergic neurotransmission in the normal and diseased brain.

Cysteine and keto acids modulate mosquito kynurenine aminotransferase catalyzed kynurenic acid production

Kynurenine aminotransferase (KAT) catalyzes the formation of kynurenic acid (KYNA), the natural antagonist of ionotropic glutamate receptors. This study tests potential substrates and assesses the effects of amino acids and keto acids on the activity of mosquito KAT. Various keto acids, when simultaneously present in the same reaction mixture, display a combined effect on KAT catalyzed KYNA production. Moreover, methionine and glutamine show inhibitory effects on KAT activity, while cysteine functions as either an antagonist or an inhibitor depending on the concentration. Therefore, the overall level of keto acids and cysteine might modulate the KYNA synthesis. Results from this study will be useful in the study of KAT regulation in other animals.

Comparative study on the effects of kynurenic acid and glucosamine-kynurenic acid

Kynurenic acid (KYNA) is the only known endogenous N-methyl-D-aspartate (NMDA) receptor inhibitor and might therefore come into consideration as a therapeutic agent in certain neurobiological disorders. However, its use as a neuroprotective compound is practically excluded because KYNA does not readily cross the blood-brain barrier (BBB). We recently synthetized a new compound, glucosamine-kynurenic acid (KYNA-NH-GLUC), which is presumed to cross the BBB more easily. In this study, the effects of KYNA and KYNA-NH-GLUC on behavior and cortical activity were investigated in adult rats. The results show that (1) on intracerebroventricular application, the behavioral changes induced by KYNA and by KYNA-NH-GLUC are quite similar; (2) on intravenous administration, KYNA (25 mg/kg) has no effect on the somatosensory-evoked cortical potentials, whereas KYNA-NH-GLUC (25 mg/kg) causes transient but appreciable reductions in the amplitudes of the evoked responses within 5 min after application; and (3) the results of in vitro studies demonstrated that both KYNA and KYNA-NH-GLUC reduced the amplitudes of the field excitatory postsynaptic potentials (fEPSPs). These observations suggest that the two compounds have similar effects, but that KYNA-NH-GLUC passes the BBB much more readily than does KYNA. These results imply that the conjugated NH-GLUC is of importance in the passage across the BBB.

The kynurenine 3-hydroxylase inhibitor Ro 61-8048 improves dystonia in a genetic model of paroxysmal dyskinesia

The effects of the novel kynurenine 3-hydroxylase inhibitor 3,4-dimethoxy-N-[4-(3-nitrophenyl)thiazol-2-yl]benzenesulfonamide (Ro 61-8048) on severity of dystonia were examined in dt(sz) mutant hamsters, an animal model of paroxysmal dystonia, in which stress precipitates dystonic episodes. Ro 61-8048 (50, 100 and 150 mg/kg i.p.) significantly reduced the severity of dystonia in dt(sz) hamsters without leading to marked central side effects. Determinations of kynurenic acid concentrations in brain homogenates demonstrated that Ro 61-8048 (100 mg/kg i.p.) provoked a two- to threefold increase of the endogeneous broad spectrum glutamate receptor antagonist kynurenic acid in the striatum, cerebellum and brainstem of mutant hamsters. The antidystonic efficacy of Ro 61-8048 at well-tolerated doses suggests that kynurenine 3-hydroxylase inhibitors should be considered as new therapeutic candidates for the treatment of dyskinesias.

Analysis of the wild-type and mutant genes encoding the enzyme kynurenine monooxygenase of the yellow fever mosquito, Aedes aegypti

Kynurenine 3-monooxygenase (KMO) catalyses the hydroxylation of kynurenine to 3-hydroxykynurenine. KMO has a key role in tryptophan catabolism and synthesis of ommochrome pigments in mosquitoes. The gene encoding this enzyme in the yellow fever mosquito, Aedes aegypti, is called kynurenine hydroxylase (kh) and a mutant allele that produces white eyes has been designated khw. A number of cDNA clones representative of wild-type and mutant genes were isolated. Sequence analyses of the wild-type and mutant cDNAs revealed a deletion of 162 nucleotides in the mutant gene near the 3'-end of the deduced coding region. RT-PCR analyses confirm the transcription of a truncated mRNA in the mutant strain. The in-frame deletion results in a loss of 54 amino acids, which disrupts a major alpha-helix and which probably accounts for the loss of activity of the enzyme. Recombinant Ae. aegypti KMO showed high substrate specificity for kynurenine with optimum activity at 40 degrees C and pH = 7.5. Kinetic parameters and inhibition of KMO activity by Cl- and pyridoxal-5-phosphate were determined.

Micromolar brain levels of kynurenic acid are associated with a disruption of auditory sensory gating in the rat

Brain levels of kynurenic acid (KYNA), an endogenous antagonist of glycine(B)/NMDA and alpha-7 nicotinic acetylcholine receptors, are elevated in individuals with schizophrenia. Both receptors are broadly implicated in the pathophysiology of this disease, particularly in the deficits many patients show in filtering the sensorium. In the present study, we sought to determine whether elevated brain levels of KYNA disrupt auditory gating in anesthetized rats. A mid-latency evoked potential was recorded from the hippocampus in response to a pair of auditory tones. Gating was assessed by determining the ratio of the amplitude of test and conditioning responses (T/C ratio) in rats that had received KYNA's precursor L-kynurenine (KYN; 150 mg/kg, i.p.) together with probenecid (PBCD; 200 mg/kg, i.p.) 2 h prior to the start of the recording session. KYNA levels in the hippocampus of KYN+PBCD-treated rats were increased 500-fold, and accompanied by a significant increase in T/C ratio consistent with a disruption in sensory gating. PBCD alone increased hippocampal KYNA 12-fold, but did not significantly elevate T/C ratio. L-701,324 (3-30 mg/kg, i.v.), a centrally acting glycine(B) site antagonist, also failed to disrupt gating; however, large quantities of the competitive NMDA receptor antagonist DL-2-amino-5-phosphopentanoate (200 nmol, i.c.v.) markedly increased T/C ratio. Thus, while total blockade of NMDA receptors disrupts auditory gating, partial blockade achieved by antagonism of its glycine coagonist binding site does not. These observations indicate that the disruption in auditory processing in rats with greatly elevated KYNA levels is not attributable to the compound's antagonist actions at the glycine(B) receptor.

3-Hydroxykynurenine and Quinolinate: Pathogenic Synergism in Early Grade Huntington’s Disease?

Huntington's Disease (HD), an inherited neurodegenerative disorder, is caused by an abnormal polyglutamine extension of a protein named huntingtin. This genetic defect is believed to result in heightened neuronal susceptibility to excitotoxic injury, a likely mechanism of neurodegeneration in HD. Two neuroactive kynurenine pathway metabolites, quinolinate (QUIN) and kynurenate (KYNA), have been proposed to play critical roles in the precipitation and prevention, respectively, of excitotoxic neuron death in HD. We now provide evidence that a third kynurenine pathway metabolite, 3-hydroxykynurenine (3-HK), should also be considered a pathogen in HD. The brain levels of this free radical generator are increased 5-10-fold in early stage (Grade 1) HD patients. In the same brains, QUIN levels are also significantly elevated in the cortex and in the neostriatum, but not in the cerebellum. In contrast, brain 3-HK and QUIN levels are either unchanged or reduced in Grade 2 and end stage (Grade 3-4) HD patients. Brain KYNA levels are moderately increased during the early disease stages and decrease as the illness progresses. In rats, 3-HK potentiates striatal QUIN toxicity, and this pro-excitotoxic effect can be prevented by free radical scavengers. Taken together, these studies provide further evidence for an involvement of kynurenine pathway metabolites in the early phases of HD neuropathology and suggest novel therapeutic strategies for the disease.

Drosophila mutants of the kynurenine pathway as a model for ageing studies

A search for Drosophila mutants with phenotypes similar to human diseases might help to unravel evolutionary conserved genes implicated in polygenic human disorders. Among these are neurodegenerative diseases, characterized by a late onset disturbance of memory, structural brain impairments and altered content of the intermediates of the kynurenine pathway. The ratio between kynurenate (KYNA) and 3-hydroxykynurenine (3-HOK) in the brain is a critical determinant of neuronal viability. Therefore, the Drosophila mutants cinnabar (KYNA excess) and cardinal (3-HOK excess) allow an evaluation of the specific roles of these metabolites which present in physiologic concentrations and mimic systemic administration. Previously we have demonstrated that the mutant cardinal can serve as a model for dementia and can help to unravel the earliest manifestations of brain dysfunction. Here we show that a state of the brain control of locomotor coordination characterized by the parameters of sound production in males results from the neuroprotective and neurotoxic effects of KYNA and 3-HOK accumulated in young and aged Drosophila mutants. The high instability of 1) cycle form and number in pulses; 2) of pulse amplitude and 3) rhythm in the courtship song of aged cardinal males are similar to the alterations in mutants with defective central complex of the brain. The cardinal mutants demonstrate apoptosis in the brain after stress treatment. This might reflect the misbalance in the content of excitatory amino acids' and the glycine site agonists revealed by HPLC-determination. The mutant cinnabar proved to be normal in respect of the parameters studied.

Manipulation of brain kynurenines: glial targets, neuronal effects, and clinical opportunities

Degradation of the essential amino acid tryptophan along the kynurenine pathway (KP) yields several neuroactive intermediates, including the free radical generator 3-hydroxykynurenine, the excitotoxic N-methyl-D-aspartate (NMDA) receptor agonist quinolinic acid, and the NMDA and alpha7 nicotinic acetylcholine receptor antagonist kynurenic acid. The ambient levels of these compounds are determined by several KP enzymes, which in the brain are preferentially localized in astrocytes and microglial cells. Normal fluctuations in the brain levels of neuroactive KP intermediates might modulate several neurotransmitter systems. Impairment of KP metabolism is functionally significant and occurs in a variety of diseases that affect the brain. Pharmacological agents targeting specific KP enzymes are now available to manipulate the concentration of neuroactive KP intermediates in the brain. These compounds can be used to normalize KP defects, show remarkable efficacy in animal models of central nervous system disorders, and offer novel therapeutic opportunities.

Kynurenine 3-mono-oxygenase inhibitors attenuate post-ischemic neuronal death in organotypic hippocampal slice cultures

Kynurenine 3-mono-oxygenase (KMO) inhibitors reduce 3-hydroxykynurenine (3-HK) and quinolinic acid (QUIN) neosynthesis and facilitate kynurenine metabolism towards kynurenic acid (KYNA) formation. They also reduce tissue damage in models of focal or transient global cerebral ischemia in vivo. We used organotypic hippocampal slice cultures exposed to oxygen and glucose deprivation (OGD) to investigate KMO mechanism(s) of neuroprotective activity. Exposure of the slices to 30 min of OGD caused CA1 pyramidal cell death and significantly decreased the amount of KYNA released in the incubation medium. The KMO inhibitors (m-nitrobenzoyl)-alanine (30-100 micro m) or 3,4-dimethoxy-[-N-4-(nitrophenyl)thiazol-2yl]-benzenesulfonamide (1-10 micro m) reduced post-ischemic neuronal death and increased KYNA concentrations in slice incubation media. The maximal concentration of KYNA detected in the incubation media of slices treated with KMO inhibitors was approximately 50 nm and was too low to efficiently interact with alpha7 nicotinic acetylcholine receptors or with the glycineb site of N-methyl-d-aspartate (NMDA) receptors. On the other hand, the addition of either 3-HK or QUIN (1-10 micro m) to OGD-exposed hippocampal slices prevented the neuroprotective activity of KMO inhibitors. Our results suggest that KMO inhibitors reduce the neuronal death found in the CA1 region of organotypic hippocampal slices exposed to 30 min of OGD by decreasing the local synthesis of 3-HK and QUIN.

Endogenous kynurenines as targets for drug discovery and development

The kynurenine pathway is the main pathway for tryptophan metabolism. It generates compounds that can modulate activity at glutamate receptors and possibly nicotinic receptors, in addition to some as-yet-unidentified sites. The pathway is in a unique position to regulate other aspects of the metabolism of tryptophan to neuroactive compounds, and also seems to be a key factor in the communication between the nervous and immune systems. It also has potentially important roles in the regulation of cell proliferation and tissue function in the periphery. As a result, the pathway presents a multitude of potential sites for drug discovery in neuroscience, oncology and visceral pathology.

Presynaptic kynurenate-sensitive receptors inhibit glutamate release

Kynurenic acid is a tryptophan metabolite provided with antagonist activity on ionotropic glutamate and alpha7 nicotinic acetylcholine receptors. We noticed that in rats with a dialysis probe placed in the head of their caudate nuclei, local administration of kynurenic acid (30-100 nM) significantly reduced glutamate output. Qualitatively and quantitatively similar effects were observed after systemic administration of kynurenine hydroxylase inhibitors, a procedure able to increase brain kynurenate concentrations. Interestingly, in microdialysis studies, methyllycaconitine (0.3-10 nM), a selective alpha7 nicotinic receptor antagonist, also reduced glutamate output. In isolated superfused striatal synaptosomes, kynurenic acid (100 nM), but not methyllycaconitine, inhibited the depolarization (KCl 12.5 mM)-induced release of transmitter or previously taken-up [3H]-D-aspartate. This inhibition was not modified by glycine, N-methyl-D-aspartate or subtype-selective kainate receptor agents, while CNQX or DNQX (10 microM), two AMPA and kainate receptor antagonists, reduced kynurenic acid effects. Low concentrations of kynurenic acid, however, did not modify [3H]-kainate (high and low affinity) or [3H]-AMPA binding to rat brain membranes. Finally, because metabotropic glutamate (mGlu) receptors modulate transmitter release in striatal preparations, we evaluated, with negative results, kynurenic acid (1-100 nM) effects in cells transfected with mGlu1, mGlu2, mGlu4 or mGlu5 receptors. In conclusion, our data show that kynurenate-induced inhibition of glutamate release is not mediated by glutamate receptors. Nicotinic acetylcholine receptors, however, may contribute to the inhibitory effects of kynurenate found in microdialysis studies, but not in those found in isolated synaptosomes.