Metabolism of [5-3H]kynurenine in the rat brain in vivo: evidence for the existence of a functional kynurenine pathway

Therapeutic potential of targeting kynurenine pathway in neurodegenerative diseases

Kynurenine pathway (KP), the primary pathway of L-tryptophan (Trp) metabolism in mammals, contains several neuroactive metabolites such as kynurenic acid (KA) and quinolinic acid (QA). Its imbalance involved in aging and neurodegenerative diseases (NDs) has attracted much interest in therapeutically targeting KP enzymes and KP metabolite-associated receptors, especially kynurenine monooxygenase (KMO). Currently, many agents have been discovered with significant improvement in animal models but only one aryl hydrocarbon receptor (AHR) agonist 30 (laquinimod) has entered clinical trials for treating Huntington's disease (HD). In this review, we describe neuroactive KP metabolites, discuss the dysregulation of KP in aging and NDs and summarize the development of KP regulators in preclinical and clinical studies, offering an outlook of targeting KP for NDs treatment in future.

Kynurenine Pathway Metabolites as Potential Clinical Biomarkers in Coronary Artery Disease

Coronary artery disease (CAD) is one of the leading cause of mortality worldwide. Several risk factors including unhealthy lifestyle, genetic background, obesity, diabetes, hypercholesterolemia, hypertension, smoking, age, etc. contribute to the development of coronary atherosclerosis and subsequent coronary artery disease. Inflammation plays an important role in coronary artery disease development and progression. Pro-inflammatory signals promote the degradation of tryptophan via the kynurenine pathway resulting in the formation of several immunomodulatory metabolites. An unbalanced kynurenic pathway has been implicated in the pathomechanisms of various diseases including CAD. Significant improvements in detection methods in the last decades may allow simultaneous measurement of multiple metabolites of the kynurenine pathway and such a thorough analysis of the kynurenine pathway may be a valuable tool for risk stratification and determination of CAD prognosis. Nevertheless, imbalance in the activities of different branches of the kynurenine pathway may require careful interpretation. In this review, we aim to summarize clinical evidence supporting a possible use of kynurenine pathway metabolites as clinical biomarkers in various manifestations of CAD.

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.

Serotonin is the main tryptophan metabolite associated with psychiatric comorbidity in abstinent cocaine-addicted patients

The lack of effective treatments and a high rate of relapse in cocaine addiction constitute a major health problem. The present study was conducted to examine the expression of tryptophan-derived metabolites in the context of cocaine addiction and psychiatric comorbidity, which is common in addicted subjects. Abstinent patients with cocaine use disorder (CUD) and control subjects were recruited for a cross-sectional study. Participants were assessed with a semi-structured diagnostic interview (PRISM) based on DSM-IV-TR for substance and mental disorders. Plasma concentrations of tryptophan metabolites and their association with relevant CUD-related variables and psychiatric comorbidity were explored. We observed decreased plasma kynurenic acid concentrations in the cocaine group, however no associations between CUD-related variables and tryptophan-derived metabolites were found. In contrast, 5-HT concentrations were increased in CUD-patients and the diagnosis of different psychiatric disorders in the cocaine group was related to higher plasma 5-HT concentrations compared with non-comorbid patients. Therefore, while changes in plasma kynurenic acid concentrations appear to be directly associated with lifetime CUD, changes in 5-HT concentrations are associated with psychiatric comorbidity. These results emphasize the need to find potential biomarkers for a better stratification of cocaine-addicted patients in order to develop therapeutic approaches to prevent cocaine relapse.

A brain-permeable inhibitor of the neurodegenerative disease target kynurenine 3-monooxygenase prevents accumulation of neurotoxic metabolites

Dysregulation of the kynurenine pathway (KP) leads to imbalances in neuroactive metabolites associated with the pathogenesis of several neurodegenerative disorders, including Huntington's disease (HD). Inhibition of the enzyme kynurenine 3-monooxygenase (KMO) in the KP normalises these metabolic imbalances and ameliorates neurodegeneration and related phenotypes in several neurodegenerative disease models. KMO is thus a promising candidate drug target for these disorders, but known inhibitors are not brain permeable. Here, 19 new KMO inhibitors have been identified. One of these (1) is neuroprotective in a Drosophila HD model but is minimally brain penetrant in mice. The prodrug variant (1b) crosses the blood-brain barrier, releases 1 in the brain, thereby lowering levels of 3-hydroxykynurenine, a toxic KP metabolite linked to neurodegeneration. Prodrug 1b will advance development of targeted therapies against multiple neurodegenerative and neuroinflammatory diseases in which KP likely plays a role, including HD, Alzheimer's disease, and Parkinson's disease.

IDO and TDO as a potential therapeutic target in different types of depression

Depression is highly prevalent worldwide and a leading cause of disabilty. However, the medications currently available to treat depression fail to adequately relieve depressive symptoms for a large number of patients. Research into the aberrant overactivation of the kynurenine pathway and the production of various active metabolites has brought new insight into the progression of depression. IDO and TDO are the first and rate-limiting enzymes in the kynurenine pathway and regulate the production of active metabolites. There is substantial evidence that TDO and IDO enzyme are activated during depression, and therefore, IDO and TDO inhibitors have been identified as ideal therapeutic targets for depressive disorder. Hence, this review will focus on the kynurenine branch of tryptophan metabolism and describe the role of IDO and TDO in the pathology of depression. In addition, this review will compare the relative imbalance between KYNA and neurotoxic kynurenine metabolites in different psychiatric disorders. Finally, this review is also directed toward assessing whether IDO and TDO are potential therapeutic target in depression associated with other diseases such as diabetes and/or cancer, as well as the development of potent IDO and TDO inhibitors.

Assessing and Modulating Kynurenine Pathway Dynamics in Huntington's Disease: Focus on Kynurenine 3-Monooxygenase

The link between disturbances in kynurenine pathway (KP) metabolism and Huntington's disease (HD) pathogenesis has been explored for a number of years. Several novel genetic and pharmacological tools have recently been developed to modulate key regulatory steps in the KP such as the reaction catalyzed by the enzyme kynurenine 3-monooxygenase (KMO). This insight has offered new options for exploring the mechanistic link between this metabolic pathway and HD, and provided novel opportunities for the development of candidate drug-like compounds. Here, we present an overview of the field, focusing on some novel approaches for interrogating the pathway experimentally.

Xanthurenic Acid Formation from 3-Hydroxykynurenine in the Mammalian Brain: Neurochemical Characterization and Physiological Effects

Xanthurenic acid (XA), formed from 3-hydroxykynurenine (3-HK) in the kynurenine pathway of tryptophan degradation, may modulate glutamatergic neurotransmission by inhibiting the vesicular glutamate transporter and/or activating Group II metabotropic glutamate receptors. Here we examined the molecular and cellular mechanisms by which 3-HK controls the neosynthesis of XA in rat, mouse and human brain, and compared the physiological actions of 3-HK and XA in the rat brain. In tissue homogenates, XA formation from 3-HK was observed in all three species and traced to a major role of kynurenine aminotransferase II (KAT II). Transamination of 3-HK to XA was also demonstrated using human recombinant KAT II. Neosynthesis of XA was significantly increased in the quinolinate-lesioned rat striatum, indicating a non-neuronal localization of the process. Studies using rat cortical slices revealed that newly produced XA is rapidly released into the extracellular compartment, and that XA biosynthesis can be manipulated experimentally in the same way as the production of kynurenic acid from kynurenine (omission of Na+ or glucose, depolarizing conditions, or addition of 2-oxoacids). The synthesis of XA from 3-HK was confirmed in vivo by striatal microdialysis. In slices from the rat hippocampus, both 3-HK and XA reduced the slopes of dentate gyrus field EPSPs. The effect of 3-HK was reduced in the presence of the KAT inhibitor aminooxyacetic acid. Finally, both 3-HK and XA reduced the power of gamma-oscillatory activity recorded from the hippocampal CA3 region. Endogenous XA, newly formed from 3-HK, may therefore play a physiological role in attentional and cognitive processes.

Serum kynurenic acid is reduced in affective psychosis

A subgroup of individuals with mood and psychotic disorders shows evidence of inflammation that leads to activation of the kynurenine pathway and the increased production of neuroactive kynurenine metabolites. Depression is hypothesized to be causally associated with an imbalance in the kynurenine pathway, with an increased metabolism down the 3-hydroxykynurenine (3HK) branch of the pathway leading to increased levels of the neurotoxic metabolite, quinolinic acid (QA), which is a putative N-methyl-d-aspartate (NMDA) receptor agonist. In contrast, schizophrenia and psychosis are hypothesized to arise from increased metabolism of the NMDA receptor antagonist, kynurenic acid (KynA), leading to hypofunction of GABAergic interneurons, the disinhibition of pyramidal neurons and striatal hyperdopaminergia. Here we present results that challenge the model of excess KynA production in affective psychosis. After rigorous control of potential confounders and multiple testing we find significant reductions in serum KynA and/or KynA/QA in acutely ill inpatients with major depressive disorder (N=35), bipolar disorder (N=53) and schizoaffective disorder (N=40) versus healthy controls (N=92). No significant difference was found between acutely ill inpatients with schizophrenia (n=21) and healthy controls. Further, a post hoc comparison of patients divided into the categories of non-psychotic affective disorder, affective psychosis and psychotic disorder (non-affective) showed that the greatest decrease in KynA was in the affective psychosis group relative to the other diagnostic groups. Our results are consistent with reports of elevations in proinflammatory cytokines in psychosis, and preclinical work showing that inflammation upregulates the enzyme, kynurenine mono-oxygenase (KMO), which converts kynurenine into 3-hydroxykynurenine and quinolinic acid.

The kynurenine pathway and the brain: Challenges, controversies and promises

Research on the neurobiology of the kynurenine pathway has suffered years of relative obscurity because tryptophan degradation, and its involvement in both physiology and major brain diseases, was viewed almost exclusively through the lens of the well-established metabolite serotonin. With increasing recognition that kynurenine and its metabolites can affect and even control a variety of classic neurotransmitter systems directly and indirectly, interest is expanding rapidly. Moreover, kynurenine pathway metabolism itself is modulated in conditions such as infection and stress, which are known to induce major changes in well-being and behaviour, so that kynurenines may be instrumental in the etiology of psychiatric and neurological disorders. It is therefore likely that the near future will not only witness the discovery of additional physiological and pathological roles for brain kynurenines, but also ever-increasing interest in drug development based on these roles. In particular, targeting the kynurenine pathway with new specific agents may make it possible to prevent disease by appropriate pharmacological or genetic manipulations. The following overview focuses on areas of kynurenine research which are either controversial, of major potential therapeutic interest, or just beginning to receive the degree of attention which will clarify their relevance to neurobiology and medicine. It also highlights technical issues so that investigators entering the field, and new research initiatives, are not misdirected by inappropriate experimental approaches or incorrect interpretations at this time of skyrocketing interest in the subject matter. This article is part of the Special Issue entitled 'The Kynurenine Pathway in Health and Disease'.

Tetrahydrobiopterin Biosynthesis as a Potential Target of the Kynurenine Pathway Metabolite Xanthurenic Acid

Tryptophan metabolites in the kynurenine pathway are up-regulated by pro-inflammatory cytokines or glucocorticoids, and are linked to anti-inflammatory and immunosuppressive activities. In addition, they are up-regulated in pathologies such as cancer, autoimmune diseases, and psychiatric disorders. The molecular mechanisms of how kynurenine pathway metabolites cause these effects are incompletely understood. On the other hand, pro-inflammatory cytokines also up-regulate the amounts of tetrahydrobiopterin (BH4), an enzyme cofactor essential for the synthesis of several neurotransmitter and nitric oxide species. Here we show that xanthurenic acid is a potent inhibitor of sepiapterin reductase (SPR), the final enzyme in de novo BH4 synthesis. The crystal structure of xanthurenic acid bound to the active site of SPR reveals why among all kynurenine pathway metabolites xanthurenic acid is the most potent SPR inhibitor. Our findings suggest that increased xanthurenic acid levels resulting from up-regulation of the kynurenine pathway could attenuate BH4 biosynthesis and BH4-dependent enzymatic reactions, linking two major metabolic pathways known to be highly up-regulated in inflammation.

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.

Early activation of the kynurenine pathway predicts early death and long-term outcome in patients resuscitated from out-of-hospital cardiac arrest

Background

The kynurenine pathway (KP) is the major route of tryptophan (TRP) catabolism and is activated by inflammation and after cardiac arrest in animals. We hypothesized that the KP activation level correlates with severity of post–cardiac arrest shock, early death, and long‐term outcome.

Methods and Results

Plasma was obtained from 245 patients enrolled in a prospective multicenter observational study in 21 intensive care units in Finland. Time to return of spontaneous circulation, lowest systolic arterial pressure, and bicarbonate during the first 24 hours were collected. A cerebral performance category of 3 to 5 defined 12‐month poor outcome. Plasma TRP and KP metabolites, kynurenine (KYN), kynurenic acid, 3‐hydroxyanthranilic acid, and the ratio of KYN to TRP were measured by liquid chromatography and mass spectrometry. All KP metabolites at intensive care unit admission were significantly higher in cardiac arrest patients with a nonshockable rhythm compared to those with a shockable rhythm, and kynurenic acid and 3‐hydroxyanthranilic acid correlated with time to return of spontaneous circulation. Patients with higher levels of KYN, KYN to TRP, kynurenic acid, and 3‐hydroxyanthranilic acid had lower 24‐hour systolic arterial pressure and bicarbonate. All KP metabolites and the ratio of KYN to TRP, but not TRP, were significantly higher in patients who died in the intensive care unit in comparison to those who survived. Multivariable logistic regression showed that high kynurenic acid (odds ratio: 1.004; 95% confidence interval: 1.001 to 1.008; P=0.014), and 3‐hydroxyanthranilic acid (odds ratio: 1.011; 95% confidence interval: 1.001 to 1.022; P=0.03) were independently associated with 12‐month poor outcome and significantly improved risk reclassification.

Conclusions

KP is activated early after cardiac arrest and is associated with severity of post–cardiac arrest shock, early death, and poor long‐term outcome.

Behavioral disturbances in adult mice following neonatal virus infection or kynurenine treatment--role of brain kynurenic acid

Exposure to infections in early life is considered a risk-factor for developing schizophrenia. Recently we reported that a neonatal CNS infection with influenza A virus in mice resulted in a transient induction of the brain kynurenine pathway, and subsequent behavioral disturbances in immune-deficient adult mice. The aim of the present study was to investigate a potential role in this regard of kynurenic acid (KYNA), an endogenous antagonist at the glycine site of the N-methyl-D-aspartic acid (NMDA) receptor and at the cholinergic α7 nicotinic receptor. C57BL/6 mice were injected i.p. with neurotropic influenza A/WSN/33 virus (2400 plaque-forming units) at postnatal day (P) 3 or with L-kynurenine (2×200 mg/kg/day) at P7-16. In mice neonatally treated with L-kynurenine prepulse inhibition of the acoustic startle, anxiety, and learning and memory were also assessed. Neonatally infected mice showed enhanced sensitivity to D-amphetamine-induced (5 mg/kg i.p.) increase in locomotor activity as adults. Neonatally L-kynurenine treated mice showed enhanced sensitivity to D-amphetamine-induced (5 mg/kg i.p.) increase in locomotor activity as well as mild impairments in prepulse inhibition and memory. Also, D-amphetamine tended to potentiate dopamine release in the striatum in kynurenine-treated mice. These long-lasting behavioral and neurochemical alterations suggest that the kynurenine pathway can link early-life infection with the development of neuropsychiatric disturbances in adulthood.

2-Aminonicotinic acid 1-oxides are chemically stable inhibitors of quinolinic acid synthesis in the mammalian brain: a step toward new antiexcitotoxic agents

3-Hydroxyanthranilic acid 3,4-dioxygenase (3-HAO) is the enzyme responsible for the production of the neurotoxic tryptophan metabolite quinolinic acid (QUIN). Elevated brain levels of QUIN are observed in several neurodegenerative diseases, but pharmacological investigation on its role in the pathogenesis of these conditions is difficult because only one class of substrate-analogue 3-HAO inhibitors, with poor chemical stability, has been reported so far. Here we describe the design, synthesis, and biological evaluation of a novel class of chemically stable inhibitors based on the 2-aminonicotinic acid 1-oxide nucleus. After the preliminary in vitro evaluation of newly synthesized compounds using brain tissue homogenate, we selected the most active inhibitor and showed its ability to acutely reduce the production of QUIN in the rat brain in vivo. These findings provide a novel pharmacological tool for the study of the mechanisms underlying the onset and propagation of neurodegenerative diseases.

Kynurenines in the mammalian brain: when physiology meets pathology

The essential amino acid tryptophan is not only a precursor of serotonin but is also degraded to several other neuroactive compounds, including kynurenic acid, 3-hydroxykynurenine and quinolinic acid. The synthesis of these metabolites is regulated by an enzymatic cascade, known as the kynurenine pathway, that is tightly controlled by the immune system. Dysregulation of this pathway, resulting in hyper-or hypofunction of active metabolites, is associated with neurodegenerative and other neurological disorders, as well as with psychiatric diseases such as depression and schizophrenia. With recently developed pharmacological agents, it is now possible to restore metabolic equilibrium and envisage novel therapeutic interventions.

Gas chromatography/tandem mass spectrometry detection of extracellular kynurenine and related metabolites in normal and lesioned rat brain

We describe here a gas chromatography-tandem mass spectrometry (GC/MS/MS) method for the sensitive and concurrent determination of extracellular tryptophan and the kynurenine pathway metabolites kynurenine, 3-hydroxykynurenine (3-HK), and quinolinic acid (QUIN) in rat brain. This metabolic cascade is increasingly linked to the pathophysiology of several neurological and psychiatric diseases. Methodological refinements, including optimization of MS conditions and the addition of deuterated standards, resulted in assay linearity to the low nanomolar range. Measured in samples obtained by striatal microdialysis in vivo, basal levels of tryptophan, kynurenine, and QUIN were 415, 89, and 8 nM, respectively, but 3-HK levels were below the limit of detection (<2 nM). Systemic injection of kynurenine (100 mg/kg, i.p.) did not affect extracellular tryptophan but produced detectable levels of extracellular 3-HK (peak after 2-3 h: ~50 nM) and raised extracellular QUIN levels (peak after 2h: ~105 nM). The effect of this treatment on QUIN, but not on 3-HK, was potentiated in the N-methyl-D-aspartate (NMDA)-lesioned striatum. Our results indicate that the novel methodology, which allowed the measurement of extracellular kynurenine and 3-HK in the brain in vivo, will facilitate studies of brain kynurenines and of the interplay between peripheral and central kynurenine pathway functions under physiological and pathological conditions.

Subchronic elevation of brain kynurenic acid augments amphetamine-induced locomotor response in mice

The neuromodulating tryptophan metabolite kynurenic acid (KYNA) is increased in the brain of patients with schizophrenia. In the present study we investigate the spontaneous locomotor activity as well as the locomotor response to d-amphetamine [5 mg/kg, administered intraperitoneal (i.p.)] after increasing endogenous levels of brain KYNA in mice by acute (10 mg/kg, i.p., 60 min) or subchronic (100 mg/kg i.p., twice daily for 6 days) pretreatment with the blood-brain crossing precursor, L: -kynurenine. We found that an acute increase in the brain KYNA levels caused increased corner time and percent peripheral activity but did not change the d-amphetamine-induced locomotor response. In contrast, subchronic elevation of KYNA did not change the spontaneous locomotor activity but produced an exaggerated d-amphetamine-induced hyperlocomotion. These results cohere with clinical studies of patients with schizophrenia, where a potentiated DA release associated with exacerbation of positive symptoms has been observed following d-amphetamine administration. Present results further underscore KYNA as a possible mediator of the aberrant dopaminergic neurotransmission seen in schizophrenia.

Inhibition of the kynurenine-NAD+ pathway leads to energy failure and exacerbates apoptosis in pneumococcal meningitis

Pneumococcal meningitis causes neurological sequelae, including learning and memory deficits in up to half of the survivors. In both humans and in animal models of the disease, there is apoptotic cell death in the hippocampus, a brain region involved in learning and memory function. We previously demonstrated that in an infant rat model of pneumococcal meningitis, there is activation of the kynurenine (KYN) pathway in the hippocampus, and that there was a positive correlation between the concentration of 3-hydroxykynurenine and the extent of hippocampal apoptosis. To clarify the role of the KYN pathway in the pathogenesis of hippocampal apoptosis in pneumococcal meningitis, we specifically inhibited 2 key enzymes of the KYN pathway and assessed hippocampal apoptosis, KYN pathway metabolites, and nicotinamide adenine dinucleotide (NAD) concentrations by high-performance liquid chromatography. Pharmacological inhibition of kynurenine 3-hydroxylase and kynureninase led to decreased cellular NAD levels and increased apoptosis in the hippocampus. The cerebrospinal fluid levels of tumor necrosis factor and interleukin-1α and -β were not affected. Our data suggest that activation of the KYN pathway in pneumococcal meningitis is neuroprotective by compensating for an increased NAD demand caused by infection and inflammation;this mechanism may prevent energy failure and apoptosis in the hippocampus.

Kynurenine metabolism in health and disease

Kynurenine is a small molecule derived from tryptophan when this amino acid is metabolised via the kynurenine pathway. The biological activity of kynurenine and its metabolites (kynurenines) is well recognised. Therefore, understanding the regulation of the subsequent biochemical reactions is essential for the design of therapeutic strategies which aim to interfere with the kynurenine pathway. However, kynurenine concentration in the body may not only be determined by the efficiency of kynurenine synthesis but also by the rate of kynurenine clearance. In this review, current knowledge about the mechanisms of kynurenine production and routes of its clearance is presented. In addition, the involvement of kynurenine and its metabolites in the biology of different T cell subsets (including Th17 cells and regulatory T cells) and neuronal cells is discussed.

Dysfunctional kynurenine pathway metabolism in the R6/2 mouse model of Huntington's disease

Elevated concentrations of neurotoxic metabolites of the kynurenine pathway (KP) of tryptophan degradation may play a causative role in Huntington's disease (HD). The brain levels of one of these compounds, 3-hydroxykynurenine (3-HK), are increased in both HD and several mouse models of the disease. In the present study, we examined this impairment in greater detail using the R6/2 mouse, a well-established animal model of HD. Initially, mutant and age-matched wild-type mice received an intrastriatal injection of (3)H-tryptophan to assess the acute, local de novo production of kynurenine, the immediate bioprecursor of 3-HK, in vivo. No effect of genotype was observed between 4 and 12 weeks of age. In contrast, intrastriatally applied (3)H-kynurenine resulted in significantly increased neosynthesis of (3)H-3-HK, but not other tritiated KP metabolites, in the R6/2 striatum. Subsequent ex vivo studies in striatal, cortical and cerebellar tissue revealed substantial increases in the activity of the biosynthetic enzyme of 3-HK, kynurenine 3-monooxygenase and significant reductions in the activity of its degradative enzyme, kynureninase, in HD mice starting at 4 weeks of age. Decreased kynureninase activity was most evident in the cortex and preceded the increase in kynurenine 3-monooxygenase activity. The activity of other KP enzymes showed no consistent brain abnormalities in the mutant mice. These findings suggest that impairments in its immediate metabolic enzymes jointly account for the abnormally high brain levels of 3-HK in the R6/2 model of HD.

Role of the NMDA-receptor in Prepulse Inhibition in the Rat

Kynurenic acid (KYNA) is an endogenous metabolite of tryptophan. Studies have revealed increased brain KYNA levels in patients with schizophrenia. Prepulse inhibition (PPI) is a behavioral model for sensorimotor gating and found to be reduced in schizophrenia. Previous studies have shown that pharmacologically elevated brain KYNA levels disrupt PPI in the rat. The aim of the present study was to investigate the receptor(s) involved in this effect. Rats were treated with different drugs selectively blocking each of the sites that KYNA antagonizes, namely the glutamate recognition site of the N-methyl-D-aspartate receptor (NMDAR), the α7* nicotinic acetylcholine receptor (α7nAChR) and the glycine site of the NMDAR. Kynurenine (200 mg/kg) was given to replicate the effects of increased levels of KYNA on PPI. In order to block the glutamate recognition site of the NMDAR, CGS 19755 (10 mg/kg) or SDZ 220–581 (2.5 mg/kg) were administered and to antagonize the α7nAChR methyllycaconitine (MLA; 6 mg/kg) was given. L-701,324 (1 and 4 mg/kg) or 4-Chloro-kynurenine (4-Cl-KYN; 25, 50 and 100 mg/kg), a drug in situ converted to 7-Chloro-kynurenic acid, were used to block the glycine-site of the NMDAR. Administration of SDZ 220-581 or CGS 19755 was associated with a robust reduction in PPI, whereas L-701,324, 4-Cl-KYN or MLA failed to alter PPI. Kynurenine increased brain KYNA levels 5-fold and tended to decrease PPI. The present study suggests that neither antagonism of the glycine-site of the NMDA receptor nor antagonism of the α7nAChR disrupts PPI, rather with regard to the effects of KYNA, blockade of the glutamate recognition-site is necessary to reduce PPI.

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.

Kynurenines in chronic neurodegenerative disorders: future therapeutic strategies

Parkinson's, Alzheimer's and Huntington's diseases are chronic neurodegenerative disorders of a progressive nature which lead to a considerable deterioration of the quality of life. Their pathomechanisms display some common features, including an imbalance of the tryptophan metabolism. Alterations in the concentrations of neuroactive kynurenines can be accompanied by devastating excitotoxic injuries and metabolic disturbances. From therapeutic considerations, possibilities that come into account include increasing the neuroprotective effect of kynurenic acid, or decreasing the levels of neurotoxic 3-hydroxy-L-kynurenine and quinolinic acid. The experimental data indicate that neuroprotection can be achieved by both alternatives, suggesting opportunities for further drug development in this field.

Protective actions of ovarian hormones in the serotonin system of macaques

The serotonin neurons of the dorsal and medial raphe nuclei project to all areas of the forebrain and play a key role in mood disorders. Hence, any loss or degeneration of serotonin neurons could have profound ramifications. In a monkey model of surgical menopause with hormone replacement and no neural injury, E and P decreased gene expression in the dorsal raphe nucleus of c-jun n-terminal kinase (JNK1) and kynurenine mono-oxygenase (KMO) that promote cell death. In concert, E and P increased gene expression of superoxide dismutase (SOD1), VEGF, and caspase inhibitory proteins that promote cellular resilience in the dorsal raphe nucleus. Subsequently, we showed that ovarian steroids inhibit pivotal genes in the caspase-dependent and caspase-independent pathways in laser-captured serotonin neurons including apoptosis activating factor (Apaf1), apoptosis-inducing factor (AIF) and second mitochondria-derived activator of caspases (Smac/Diablo). SOD1 was also increased specifically in laser-captured serotonin neurons. Examination of protein expression in the dorsal raphe block revealed that JNK1, phosphoJNK1, AIF and the translocation of AIF from the mitochondria to the nucleus decreased with hormone therapy, whereas pivotal execution proteins in the caspase pathway were unchanged. In addition, cyclins A, B, D1 and E were inhibited, which would prevent re-entry into the cell cycle and catastrophic death. These data indicated that in the absence of gross injury to the midbrain, ovarian steroids inhibit the caspase-independent pathway and cell cycle initiation in serotonin neurons. To determine if these molecular actions prevented cellular vulnerability or death, we examined DNA fragmentation in the dorsal raphe nucleus with the TUNEL assay (terminal deoxynucleotidyl transferase nick end labeling). Ovarian steroids significantly decreased the number of TUNEL-positive cells in the dorsal raphe. Moreover, TUNEL staining prominently colocalized with TPH immunostaining, a marker for serotonin neurons. In summary, ovarian steroids increase the cellular resilience of serotonin neurons and may prevent serotonin neuron death in women facing decades of life after menopause. The survival of serotonin neurons would support cognition and mental health.

On the relationship between the two branches of the kynurenine pathway in the rat brain in vivo

In the mammalian brain, kynurenine aminotransferase II (KAT II) and kynurenine 3-monooxygenase (KMO), key enzymes of the kynurenine pathway (KP) of tryptophan degradation, form the neuroactive metabolites kynurenic acid (KYNA) and 3-hydroxykynurenine (3-HK), respectively. Although physically segregated, both enzymes use the pivotal KP metabolite l-kynurenine as a substrate. We studied the functional consequences of this cellular compartmentalization in vivo using two specific tools, the KAT II inhibitor BFF 122 and the KMO inhibitor UPF 648. The acute effects of selective KAT II or KMO inhibition were studied using a radiotracing method in which the de novo synthesis of KYNA, and of 3-HK and its downstream metabolite quinolinic acid (QUIN), is monitored following an intrastriatal injection of (3)H-kynurenine. In naïve rats, intrastriatal BFF 122 decreased newly formed KYNA by 66%, without influencing 3-HK or QUIN production. Conversely, UPF 648 reduced 3-HK synthesis (by 64%) without affecting KYNA formation. Similar, selective effects of KAT II and KMO inhibition were observed when the inhibitors were applied acutely together with the excitotoxin QUIN, which impairs local KP metabolism. Somewhat different effects of KMO (but not KAT II) inhibition were obtained in rats that had received an intrastriatal QUIN injection 7 days earlier. In these neuron-depleted striata, UPF 648 not only decreased both 3-HK and QUIN production (by 77% and 66%, respectively) but also moderately raised KYNA synthesis (by 27%). These results indicate a remarkable functional segregation of the two pathway branches in the brain, boding well for the development of selective KAT II or KMO inhibitors for cognitive enhancement and neuroprotection, respectively.

Specific inhibition of kynurenate synthesis enhances extracellular dopamine levels in the rodent striatum

Fluctuations in the endogenous levels of kynurenic acid (KYNA), a potent alpha7 nicotinic and NMDA receptor antagonist, affect extracellular dopamine (DA) concentrations in the rat brain. Moreover, reductions in KYNA levels increase the vulnerability of striatal neurons to NMDA receptor-mediated excitotoxic insults. We now assessed the role of a key KYNA-synthesizing enzyme, kynurenine aminotransferase II (KAT II), in these processes in the rodent striatum, using KAT II KO mice-which have reduced KYNA levels-and the selective KAT II inhibitor (S)-4-(ethylsulfonyl)benzoylalanine (S-ESBA) as tools. S-ESBA (applied by reverse dialysis) raised extracellular DA levels in the striatum of KYNA-deficient mice threefold and caused a much larger, 15-fold increase in wild-type mice. In the rat striatum, S-ESBA produced a 35% reduction in extracellular KYNA, which was accompanied by a 270% increase in extracellular DA. The latter effect was abolished by co-infusion of 100 nM KYNA. Intrastriatal S-ESBA pre-treatment augmented the size of a striatal quinolinate lesion by 370%, and this potentiation was prevented by co-infusion of KYNA. In separate animals, acute inhibition of KAT II reduced the de novo synthesis of KYNA during an early excitotoxic insult without enhancing the formation of the related neurotoxic metabolites 3-hydroxykynurenine and quinolinate. Taken together, these results provide further support for the concept that KAT II is a critical determinant of functionally relevant KYNA fluctuations in the rodent striatum.

Xanthurenic acid distribution, transport, accumulation and release in the rat brain

Tryptophan metabolism through the kynurenine pathway leads to several neuroactive compounds, including kynurenic and picolinic acids. Xanthurenic acid (Xa) has been generally considered as a substance with no physiological role but possessing toxic and apoptotic properties. In the present work, we present several findings which support a physiological role for endogenous Xa in synaptic signalling in brain. This substance is present in micromolar amounts in most regions of the rat brain with a heterogeneous distribution. An active vesicular synaptic process inhibited by bafilomycin and nigericin accumulates xanthurenate into pre-synaptic terminals. A neuronal transport, partially dependant on adenosine 5'-triphosphate (ATP), sodium and chloride ions exists in NCB-20 neurons which could participate in the clearance of extracellular xanthurenate. Both transports (neuronal and vesicular) are greatly enhanced by the presence of micromolar amounts of zinc ions. Finally, electrical in vivo stimulation of A10-induced Xa release in the extracellular spaces of the rat prefrontal cortex. This phenomenon is reproduced by veratrine, K+ ions and blocked by EGTA and tetrodotoxin. These results strongly argue for a role for Xa in neurotransmission/neuromodulation in the rat brain, thus providing the existence of specific Xa receptors.

Mitochondrial aspartate aminotransferase: a third kynurenate-producing enzyme in the mammalian brain

The tryptophan metabolite kynurenic acid (KYNA), which is produced enzymatically by the irreversible transamination of l-kynurenine, is an antagonist of alpha7 nicotinic and NMDA receptors and may thus modulate cholinergic and glutamatergic neurotransmission. Two kynurenine aminotransferases (KAT I and II) are currently considered the major biosynthetic enzymes of KYNA in the brain. In this study, we report the existence of a third enzyme displaying KAT activity in the mammalian brain. The novel KAT had a pH optimum of 8.0 and a low capacity to transaminate glutamine or alpha-aminoadipate (the classic substrates of KAT I and KAT II, respectively). The enzyme was inhibited by aspartate, glutamate, and quisqualate but was insensitive to blockade by glutamine or anti-KAT II antibodies. After purification to homogeneity, the protein was sequenced and the enzyme was identified as mitochondrial aspartate aminotransferase (mitAAT). Finally, the relative contributions of KAT I, KAT II, and mitAAT to total KAT activity were determined in mouse, rat, and human brain at physiological pH using anti-mitAAT antibodies. KAT II was most abundant in rat and human brain, while mitAAT played the major role in mouse brain. It remains to be seen if mitAAT participates in cerebral KYNA synthesis under physiological and/or pathological conditions in vivo.

Astrocytic localization of kynurenine aminotransferase II in the rat brain visualized by immunocytochemistry

Kynurenic acid (KYNA), a metabolite of the kynurenine pathway of tryptophan degradation, is a neuroinhibitory agent present in the mammalian brain. Endogenous KYNA preferentially affects the alpha7 nicotinic acetylcholine (alpha7nACh) receptor and, possibly, the glycine co-agonist (glycineB) site of the NMDA receptor. Functionally relevant fluctuations in brain KYNA occur under both physiological and pathological conditions, affecting cholinergic and glutamatergic neurotransmission. Kynurenine aminotransferase II (KAT II), the major biosynthetic enzyme of KYNA in the rat brain, catalyzes the irreversible formation of KYNA from its immediate bioprecursor, kynurenine. We now purified rat kidney KAT II to homogeneity, generated a polyclonal rabbit anti-rat KAT II antibody, and purified the antibody using routine biochemical methods. The antibody selectively recognized KAT II by Western blot analysis and in immunotitration experiments. Used for immunocytochemistry, the antibody revealed discrete, specific staining of KAT II-positive astrocyte-like cells throughout the adult rat brain. The presence of KAT II in astrocytes was confirmed by double fluorescence immunostaining with an antibody against the astrocyte-specific marker glial fibrillary acidic protein (GFAP). No specific labeling was detected in neurons or microglia. However, KAT II-positive astrocytes were intimately associated with select neuron populations, supporting a neuromodulatory role of KYNA. Intense staining was frequently seen around brain capillaries, with astrocytic end feet contacting the capillary wall. This may explain the rapid access of blood-derived kynurenine to KAT II-containing astrocytes. The new anti-KAT II antibody should be useful in the further elucidation of the presumed role of KYNA in brain physiology and pathology.

Chronic neuroleptic treatment reduces endogenous kynurenic acid levels in rat brain

The brain and cerebrospinal fluid levels of kynurenic acid (KYNA), a metabolite of the kynurenine pathway of tryptophan degradation and antagonist of the glycine(B) receptor and the alpha7 nicotinic acetylcholine receptor, are elevated in persons with schizophrenia. To evaluate whether this increase is related to antipsychotic medication, we examined the effects of haloperidol (HAL), clozapine (CLOZ) or raclopride (RAC) on brain KYNA levels in rats. Animals received either acute drug injections or ingested the drugs chronically with the drinking water. Acute application or one-week drug exposure had no effect on brain KYNA levels. After one month, HAL, CLOZ and RAC all caused significant reductions in KYNA levels in striatum, hippocampus and frontal cortex. Quantitatively similar reductions in the brain tissue content of KYNA were observed after one year of HAL administration. All these effects were accompanied by equivalent decreases in the extracellular concentration of KYNA, measured by striatal microdialysis. Separate animals received an intrastriatal infusion of (3)H-kynurenine to probe the entire kynurenine pathway acutely in rats treated with HAL for one year. These animals showed reduced (3)H-KYNA production, but no changes in the formation of other kynurenine pathway metabolites. By enhancing glutamatergic and cholinergic neurotransmission, reduced brain KYNA levels may play a role in the clinical effects of prolonged antipsychotic medication.

Endogenous kynurenate controls the vulnerability of striatal neurons to quinolinate: Implications for Huntington's disease

Excessive activation of NMDA receptors results in excitotoxic nerve cell loss, which is believed to play a critical role in the pathophysiology of Huntington's disease (HD) and several other catastrophic neurodegenerative diseases. Kynurenic acid (KYNA), a neuroinhibitory tryptophan metabolite, has neuroprotective properties and may serve as an endogenous anti-excitotoxic agent. This hypothesis was tested in the striatum, using mice with a targeted deletion of kynurenine aminotransferase II (KAT II), a major biosynthetic enzyme of KYNA in the mammalian brain. On post-natal day (PND) 14, the striatum of mkat-2-/- mice showed a reduction in KYNA levels but contained normal concentrations of the metabolically related neurotoxins 3-hydroxykynurenine and quinolinic acid (QUIN). Intrastriatal injections of QUIN, a NMDA receptor agonist, caused significantly larger lesions in these immature mutant mice than in age-matched wild-type animals. This lesion enlargement was not observed when mkat-2-/- mice were acutely pre-treated with the kynurenine 3-hydroxylase inhibitor UPF 648, which counteracted the striatal KYNA deficit. Moreover, no increased vulnerability to QUIN was observed in 2-month-old mkat-2-/- mice, which present with normal brain KYNA levels. Intrastriatal injections of the non-NMDA receptor agonist kainate caused similar lesion sizes in both genotypes regardless of age. These results indicate that endogenous KYNA preferentially controls the vulnerability of striatal neurons to QUIN. Our data suggest that timely pharmacological interventions resulting in an up-regulation of brain KYNA levels may benefit patients suffering from HD or other neurodegenerative diseases.

Preliminary array analysis reveals novel genes regulated by ovarian steroids in the monkey raphe region

We hypothesize that ovarian hormones may improve serotonin neuron survival. We sought the effect of estradiol (E) and progesterone (P) on novel gene expression in the macaque dorsal raphe region with Affymetrix array analysis. Nine spayed rhesus macaques were treated with either placebo, E or E+P via Silastic implant for 1 month prior to euthanasia (n=3 per treatment). RNA was extracted from a small block of midbrain containing the dorsal raphe and examined on an Agilent Bioanalyzer. The RNA from each monkey was labeled and hybridized to an Affymetrix HG_U95AV Human GeneChip Array. After filtering and sorting, 25 named genes remained that were regulated by E, and 24 named genes remained that were regulated by supplemental P. These genes further sorted into functional categories that would promote neuronal plasticity, transmitter synthesis, and trafficking, as well as reduce apoptosis. The relative abundance of four pivotal genes was examined in all nine animals with quantitative RT-PCR and normalized by glyceraldehyde 3-phosphate dehydrogenase (GAPDH). E+/-P caused a significant threefold reduction in JNK-1 (a pro-apoptosis gene, p<0.007); and a significant sixfold decrease in kynurenine mono-oxygenase (produces neurotoxic quinolones, p<0.05). GABA-A receptor (alpha3 subunit; benzodiazepine site) and E2F1 (interferes with cytokine signaling) were unaffected by E, but increased sevenfold (p<0.02) and fourfold (p<0.009), respectively, upon treatment with P. In summary, subsets of genes related to tissue remodeling or apoptosis were up- or down-regulated by E and P in a tissue block containing the dorsal raphe. These changes could promote cellular resilience in the region where serotonin neurons originate.

Effect of systemic administration of L-kynurenine on corticocerebral blood flow under normal and ischemic conditions of the brain in conscious rabbits

Kynurenic acid, the only known endogenous antagonist of the excitatory amino acid receptors, exerts neuroprotective effect in focal cerebral ischemia. Kynurenic acid poorly while its bioprecursor, l-kynurenine (L-KYN) completely crosses the blood-brain barrier. The aim of our study was to investigate the effect of intravenous l-KYN (0.3, 1, and 3 mg/kg) on the normal and the unilateral carotid artery occlusion induced ischemic corticocerebral blood flow (cCBF) measured by hydrogen polarography in conscious rabbits. Administration of l-KYN produced a significant increase in the normal cCBF; the peak values were recorded at the dose of 1 mg/kg (187% at 120 and 150 mins. respectively). The cCBF-improving effect of l-KYN was immediate and highly pronounced also in rabbits with carotid occlusion (peak value was 192% at 120 mins. at the dose of 1 mg/kg). Pretreatment with either atropine or Nomega-nitro-L-arginine-methyl-ester (L-NAME) prevented the l-KYN induced enhancement of the normal and the ischemic cCBF alike. It is suggested that the cCBF-increasing effect of l-KYN might be mediated by activation of cholinergic and nitric oxide pathways.

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.

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.

Increased levels of 3-hydroxykynurenine in different brain regions of rats with chronic renal insufficiency

Tryptophan (TRP) metabolism via the kynurenine pathway leads to formations of neuroactive substances like kynurenine (KYN) and 3-hydroxykynurenine (3-HK), which may be involved in the pathogenesis of several human brain diseases. 3-Hydroxykynurenine especially is known to have strong neurotoxic properties. The generation of reactive oxygen species (ROS) leads to neuronal cell death with apoptotic features. Because the chronic renal insufficiency (CRI) results in disturbances in the functioning of the central nervous system (CNS), it is conceivable that the metabolism of some kynurenines may be altered and could play an important role in uremic encephalopathy. The levels of TRP, KYN and 3-HK were measured in the plasma and in different brain regions of uremic rats. The total plasma concentration of TRP as well as in all the studied brain samples was significantly diminished during uremia. Surprisingly, the level of KYN and 3-HK were elevated both in the plasma and different brain regions of CRI animals. KYN concentrations were approximately two times higher in the cerebellum, midbrain and cortex compared to the control group. The changes of 3-HK levels were more pronounced in the striatum and medulla than in other structures. This data suggests that CRI results in deep disturbances on the kynurenine pathway in CNS, which could be responsible for neurological abnormalities seen in uremia.

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.

Effects of systemic and central nervous system localized inflammation on the contributions of metabolic precursors to the L-kynurenine and quinolinic acid pools in brain

L-Kynurenine and quinolinic acid are neuroactive L-tryptophan-kynurenine pathway metabolites of potential importance in pathogenesis and treatment of neurologic disease. To identify precursors of these metabolites in brain, [(2)H(3) ]-L-kynurenine was infused subcutaneously by osmotic pump into three groups of gerbils: controls, CNS-localized immune-activated, and systemically immune-activated. The specific activity of L-kynurenine and quinolinate in blood, brain and systemic tissues at equilibrium was then quantified by mass spectrometry and the results applied to a model of metabolism to differentiate the relative contributions of various metabolic precursors. In control gerbils, 22% of L-kynurenine in brain was derived via local synthesis from L-tryptophan/formylkynurenine versus 78% from L-kynurenine from blood. Quinolinate in brain was derived from several sources, including: local tissue L-tryptophan/formylkynurenine (10%), blood L-kynurenine (35%), blood 3-hydroxykynurenine/3-hydroxyanthranilate (7%), and blood quinolinate (48%). After systemic immune-activation, however, L-kynurenine in brain was derived exclusively from blood, whereas quinolinate in brain was derived from three sources: blood L-kynurenine (52%), blood 3-hydroxykynurenine or 3-hydroxyanthranilate (8%), and blood quinolinate (40%). During CNS-localized immune activation, > 98% of both L-kynurenine and quinolinate were derived via local synthesis in brain. Thus, immune activation and its site determine the sources from which L-kynurenine and quinolinate are synthesized in brain. Successful therapeutic modulation of their concentrations must take into account the metabolic and compartment sources.

Increased cortical kynurenate content in schizophrenia

BACKGROUND

Metabolites of the kynurenine pathway of tryptophan degradation may play a role in the pathogenesis of several human brain diseases. One of the key metabolites in this pathway, kynurenine, is either transaminated to form the glutamate receptor antagonist, kynurenate, or hydroxylated to 3-hydroxykynurenine, which in turn is further degraded to the excitotoxic N-methyl-D-aspartate receptor agonist quinolinate. Because a hypoglutamatergic tone may be involved in the pathophysiology of schizophrenia, it is conceivable that alterations in kynurenine pathway metabolism may play a role in the disease.

METHODS

The tissue levels of kynurenine, kynurenate, and 3-hydroxykynurenine were measured in brain tissue specimens obtained from the Maryland Brain Collection. All three metabolites were determined in the same samples from three cortical brain regions (Brodmann areas 9, 10, and 19), obtained from 30 schizophrenic and 31 matched control subjects.

RESULTS

Kynurenate levels were significantly increased in schizophrenic cases in Brodmann area 9 (2.9 +/- 2.2 vs. 1.9 +/- 1.3 pmol/mg protein, p <.05), but not in Brodmann areas 10 and 19. Kynurenine levels were elevated in schizophrenic cases in Brodmann areas 9 (35.2 +/- 28.0 vs. 22.4 +/- 14.3 pmol/mg protein; p <.05) and 19 (40.3 +/- 23.4 vs. 30.9 +/- 10.8; p <.05). No significant differences in 3-hydroxykynurenine content were observed between the two groups. In both groups, significant (p <.05) correlations were found in all three brain areas between kynurenine and kynurenate, but not between kynurenine and 3-hydroxykynurenine (p >.05). In rats, chronic (6-months) treatment with haloperidol did not cause an increase in kynurenate levels in the frontal cortex, indicating that the elevation observed in schizophrenia is not due to antipsychotic medication.

CONCLUSIONS

The data demonstrate an impairment of brain kynurenine pathway metabolism in schizophrenia, resulting in elevated kynurenate levels and suggesting a possible concomitant reduction in glutamate receptor function.

Kynurenine pathway metabolism in human astrocytes: a paradox for neuronal protection

There is good evidence that the kynurenine pathway (KP) and one of its products, quinolinic acid (QUIN), play a role in the pathogenesis of neurological diseases, in particular AIDS dementia complex. Although QUIN has been shown to be produced in neurotoxic concentrations by macrophages and microglia, the role of astrocytes in QUIN production is controversial. Using cytokine-stimulated cultures of human astrocytes, we assayed key enzymes and products of the KP. We found that human astrocytes lack kynurenine hydroxylase so that large amounts of kynurenine and the QUIN antagonist kynurenic acid were produced. However, the amounts of QUIN that were synthesized were subsequently completely degraded. We then showed that kynurenine in concentrations comparable with those produced by astrocytes led to significant production of QUIN by macrophages. These results suggest that astrocytes alone are neuroprotective by minimizing QUIN production and maximizing synthesis of kynurenic acid. However, it is likely that, in the presence of macrophages and/or microglia, astrocytes become indirectly neurotoxic by the production of large concentrations of kynurenine that can be secondarily metabolized by neighbouring or infiltrating monocytic cells to form the neurotoxin QUIN.

Kynurenines in the CNS: from endogenous obscurity to therapeutic importance

In just under 20 years the kynurenine family of compounds has developed from a group of obscure metabolites of the essential amino acid tryptophan into a source of intensive research, with postulated roles for quinolinic acid in neurodegenerative disorders, most especially the AIDS-dementia complex and Huntington's disease. One of the kynurenines, kynurenic acid, has become a standard tool for use in the identification of glutamate-releasing synapses, and has been used as the parent for several groups of compounds now being developed as drugs for the treatment of epilepsy and stroke. The kynurenines represent a major success in translating a basic discovery into a source of clinical understanding and therapeutic application, with around 3000 papers published on quinolinic acid or kynurenic acid since the discovery of their effects in 1981 and 1982. This review concentrates on some of the recent work most directly relevant to the understanding and applications of kynurenines in medicine.

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

Competing enzymatic mechanisms degrade the tryptophan metabolite L-kynurenine to kynurenate, an inhibitory and neuroprotective compound, and to the neurotoxins 3-hydroxykynurenine and quinolinate. Kynurenine 3-hydroxylase inhibitors such as PNU 156561 shift metabolism towards enhanced kynurenate production, and this effect may underlie the recently discovered anticonvulsant and neuroprotective efficacy of these drugs. Using electrophysiological and neurotoxicological endpoints, we now used PNU 156561 as a tool to examine the functional interplay of kynurenate, 3-hydroxykynurenine and quinolinate in the rat hippocampus in vivo. First, population spike amplitude in area CA1 and the extent of quinolinate-induced excitotoxic neurodegeneration were studied in animals receiving acute or prolonged intravenous infusions of L-kynurenine, PNU 156561, (L-kynurenine+PNU 156561) or kynurenate. Only the latter two treatments, but not L-kynurenine or PNU 156561 alone, caused substantial inhibition of evoked responses in area CA1, and only prolonged (3h) infusion of (L-kynurenine+PNU 156561) or kynurenate was neuroprotective. Biochemical analyses in separate animals revealed that the levels of kynurenate attained in both blood and brain (hippocampus) were essentially identical in rats receiving extended infusions of L-kynurenine alone or (L-kynurenine+PNU 156561) (4 and 7microM, respectively, after an infusion of 90 or 180min). However, addition of the kynurenine 3-hydroxylase inhibitor resulted in a significant decrement in the formation of 3-hydroxykynurenine and quinolinate in both blood and brain. These data suggest that the ratio between kynurenate and 3-hydroxykynurenine and/or quinolinate in the brain is a critical determinant of neuronal excitability and viability. The anticonvulsant and neuroprotective potency of kynurenine 3-hydroxylase inhibitors may therefore be due to the drugs' dual action on both branches of the kynurenine pathway of tryptophan degradation.

Serotonergic and kynurenic pathways in rats exposed to foot shock

Electric foot shock was applied to rats and levels of tryptophan and its metabolites were measured in the plasma, central nervous system and peripheral tissues. Metabolites of tryptophan are the results of the enhancement of serotonergic and kynurenine pathways. Plasma levels of tryptophan increased significantly immediately after the foot shock and returned to normal values within 24 h. Tryptophan levels also increased in all the brain areas immediately after stress application and returned to normal values within 24 h. Foot shock elevated the levels of kynurenine in the plasma, liver, kidney and every parts of the brain. 3-Hydroxykynurenine and kynurenic acid levels were increased in the brain. The present observations suggest that stress activates not only serotonergic pathway but also kynurenine pathway in the central nervous system and periphery. Some metabolites of kynurenine pathway, such as 3-hydroxykynurenine, are neurotoxic while other metabolite, such as kynurenic acid, may be neuroprotective. Increase in serotonin level in the hypothalamus and midbrain stabilises emotion and prevents mood disorders. Therefore, some brain dysfunction resulting from stress may be prevented by the metabolites of tryptophan. The balance of these functions may be important in the maintenance of nerve integrity under stress conditions.

In situ produced 7-chlorokynurenate provides protection against quinolinate- and malonate-induced neurotoxicity in the rat striatum

Excitotoxic mechanisms may play a critical role in the pathophysiology of several neurological and psychiatric diseases. Excitatory amino acid receptor antagonists are therefore of great therapeutic interest, but untoward side effects often prevent their clinical use. Targeting the glycine coagonist site of the (NMDA) receptor may bypass these shortcomings. The present study was designed to evaluate the neuroprotective characteristics of l-4-chlorokynurenine (4-Cl-KYN), a synthetic compound which is enzymatically converted to the selective glycine/NMDA receptor antagonist 7-chlorokynurenate (7-Cl-KYNA). Using slow (2 h) intrastriatal infusions of the excitotoxins quinolinate (QUIN; 120 nmol) or malonate (6.8 micromol) as the experimental paradigm, the neuroprotective potency of 4-Cl-KYN was first compared with that of exogenous 7-Cl-KYNA, using glutamate decarboxylase activity as a lesion marker. One hundred and thirty-five nanomoles of the prodrug 4-Cl-KYN or 27 nmol 7-Cl-KYNA, the former used in a pre- and cotreatment regimen, were required to block QUIN or, less efficiently, malonate toxicity. In separate animals, the metabolic fate of this neuroprotective dose of 4-Cl-KYN was examined in vivo. In control striata, the treatment gave rise to 170 +/- 25 pmol 7-Cl-KYNA/mg protein, approximately six times less than an infusion of 27 nmol exogenous 7-Cl-KYNA, indicating greatly superior efficacy of the focally produced antagonist. Notably, the conversion of 4-Cl-KYN to 7-Cl-KYNA increased by 82% in the presence of QUIN. 4-Cl-KYN was also metabolized to 4-chloro-3-hydroxyanthranilate, an established, powerful inhibitor of QUIN synthesis. This unique pharmacological profile and the fact that the prodrug, unlike 7-Cl-KYNA, readily penetrates the blood-brain barrier suggest that 4-Cl-KYN may be exceptionally useful as an anti-excitotoxic agent.

Comparative effects of oxygen on indoleamine 2,3-dioxygenase and tryptophan 2,3-dioxygenase of the kynurenine pathway

Indoleamine 2,3-dioxygenase (IDO) reacts with either oxygen or superoxide and tryptophan (trp) or other indoleamines while tryptophan 2,3-dioxygenase (TDO) reacts with oxygen and is specific for trp. These enzymes catalyze the rate-limiting step in the kynurenine (KYN) pathway from trp to quinolinic acid (QA) with TDO in kidney and liver and IDO in many tissues, including brain where it is low but inducible. QA, which does not cross the blood-brain barrier, is an excitotoxin found in the CNS during various pathologies and is associated with convulsions. We proposed that HBO-induced convulsions result from increased flux through the KYN pathway via oxygen stimulation of IDO. To test this, TDO and IDO of liver and brain, respectively, of Sprague Dawley rats were assayed with oxygen from 0 to 6.2 atm HBO. TDO activity was appreciable at even 30 microM oxygen and rose steeply to a maximum at 40 microM. Conversely, IDO had almost no detectable activity at or below 100 microM oxygen and maximum activity was not reached until about 1150 microM. (Plasma contains about 215 microM oxygen and capillaries about 20 microM oxygen when rats breathe air.) KYN was 60% higher in brains of HBO-convulsed rats compared to rats breathing air. While the oxygen concentration inside cells of rats breathing air or HBO is not known precisely, it is clear that the rate-limiting, IDO-catalyzed step in the brain KYN pathway (but not liver TDO) can be greatly accelerated in rats breathing HBO.