Metabolism of [5-(3)H]kynurenine in the developing rat brain in vivo: effect of intrastriatal ibotenate injections

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.

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'.

NMDA antagonists as Parkinson’s disease therapy: disseminating the evidence

SUMMARY Oral levodopa is the current baseline therapy in the management of Parkinson’s disease, but nonmotor complications and therapy-related dyskinesias pose an important challenge for clinicians. Glutamate receptors have been implicated in the neurodegenerative process of Parkinson’s disease and also in the development of levodopa-induced dyskinesias. This article discusses the role of NMDA receptors in Parkinson’s disease and their modulation as a possible therapeutic approach.

3-Hydroxykynurenine: an intriguing molecule exerting dual actions in the central nervous system

Kynurenine pathway is gaining attention due to the many metabolic processes in which it has been involved. The tryptophan conversion into several other metabolites through this pathway provides neuronal and redox modulators useful for maintenance of major functions in the brain. However, when physiopathological conditions prevail - i.e. oxidative stress, excitotoxicity, and inflammation - preferential formation and accumulation of toxic metabolites could trigger factors for degeneration in neurological disorders. 3-Hydroxykynurenine has been largely described as one of these toxic metabolites capable of inducing oxidative damage and cell death; consequently, this metabolite has been hypothesized to play a pivotal role in different neurological and psychiatric disorders. Supporting evidence has shown altered 3-hydroxykynurenine levels in samples of patients from several disorders. In contrast, some experimental studies have provided evidence of antioxidant and scavenging properties inherent to this molecule. In this review, we explored most of literature favoring one or the other concept, in order to provide an accurate vision on the real participation of this tryptophan metabolite in both experimental paradigms and human brain pathologies. Through this collected evidence, we provide an integrative hypothesis on how 3-hydroxykynurenine is exerting its dual actions in the central nervous system and what will be the course of investigations in this field for the next years.

Kynurenic acid and kynurenine aminotransferases in retinal aging and neurodegeneration

The kynurenine aminotransferases (KATs) KAT I and KAT II are pivotal to the synthesis of kynurenic acid (KYNA), the only known endogenous glutamate receptor antagonist and neuroprotectant. KAT I and II have been found in avian, rodent, and human retina. Expression of KAT I in Müller cell endfeet and KAT II in retinal ganglion cells has been documented. Developmental changes in KAT expression and KYNA concentration in the avian and rodent retina have also been found. Studies of retinal neurodegeneration have shown alterations in KYNA synthesis in the retina in response to retinal ganglion cell loss. In DBA/2J mice, a model of ocular hypertension, an age-dependent decrease of retinal KYNA and KATs was found. In the corpora amylacea in the human retina intensive KAT I and II immunoreactivity was demonstrated. In summary, these findings point to the potential involvement of KYNA in the mechanisms of retinal aging and neurodegeneration.

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.

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.

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.

Alterations of kynurenic acid content in the retina in response to retinal ganglion cell damage

The present study is the first to examine the modulation of retinal kynurenic acid (KYNA) content in response to N-methyl-D-aspartate (NMDA)-induced cell death in adult rat retinal ganglion cells (RGC). Adult Brown Norway rats were intravitreally injected with NMDA or PBS. Surviving RGC were retrogradely labeled with fluorogold and counted in wholemounts of retinas 2, 7 and 14 days after injection. Retinal KYNA content was measured by HPLC at the same time points. RGC numbers decreased significantly 2, 7 and 14 days after NMDA injection if compared to control retinas. KYNA concentration increased significantly two days after NMDA-injection. However, 7 and 14 days after injection retinal KYNA content was found markedly decreased in NMDA-treated eyes as compared to controls. It is conceivable that KYNA deficiency is causally related to the pathology of excitotoxic retinal diseases.

Neonatal asphyxia in rats: acute effects on cerebral kynurenine metabolism

Two tryptophan metabolites, the anti-excitotoxic N-methyl-D-aspartate (NMDA) receptor antagonist kynurenic acid (KYNA) and the free radical generator 3-hydroxykynurenine (3-HK), have been proposed to influence neuronal viability in the mammalian brain. In rats, the brain content of both KYNA and 3-HK decreases immediately after birth, possibly to ensure normal postnatal functioning of NMDA receptors. Because complications of birth asphyxia have been suggested to be associated with anomalous NMDA receptor function, we examined the acute effects of an asphyctic insult on the brain levels of KYNA and 3-HK in neonatal rats. Asphyxia was induced in animals delivered by cesarean section on the last day of gestation, using the procedure introduced by Bjelke et al. (Brain Res 543: 1-9, 1991). KYNA and 3-HK levels were determined in the brain at seven time points between 10 min and 24 h after asphyxia. Up to 6 h, asphyxia caused 160-267% increases in KYNA levels. In the same tissues, 3-HK levels decreased (significantly at five of the seven time points), demonstrating an asphyxia-induced shift in kynurenine pathway metabolism toward the neuroprotectant KYNA. This shift might constitute the brain's attempt to counter the ill effects of birth asphyxia. Furthermore, the transient increase in the brain KYNA/3-HK ratio in these animals might be causally related to the well-documented detrimental long-term effects of asphyxia.

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.

Excitotoxicity of quinolinic acid: modulation by endogenous antagonists

Quinolinic acid (QUIN), a product of tryptophan metabolism by the kynurenine pathway, produces excitotoxicity by activation of NMDA receptors. Focal injections of QUIN can deplete the biochemical markers for dopaminergic, cholinergic, gabaergic, enkephalinergic and NADPH diaphorase neurons, which differ in their sensitivity to its neurotoxic action. This effect of QUIN differs from that of other NMDA receptor agonists in terms of its dependency on the afferent glutamatergic input and its sensitivity to the receptor antagonists. The enzymatic pathway yielding QUIN produces metabolites that inhibit QUIN-induced neurotoxicity. The most active of these metabolites, kynurenic acid (KYNA), blocks NMDA and non-NMDA receptor activity. Treatment with kynurenine hydroxylase and kynureinase inhibitors increases levels of endogenous KYNA in the brain and protects against QUIN-induced neurotoxicity. Other neuroprotective strategies involve reduction in QUIN synthesis from its immediate precursor, or endogenous synthesis of 7-chloro-kynurenic acid, a NMDA antagonist, from its halogenated precursor. Several other tryptophan metabolites--quinaldic acid, hydroxyquinaldic acid and picolinic acid--also inhibit excitotoxic damage but their presence in the brain is uncertain. Picolinic acid is of interest since it inhibits excitotoxic but not neuroexcitatory responses. The mechanism of its anti-excitotoxic action is unclear but might involve zinc chelation. Neurotoxic actions of QUIN are modulated by nitric oxide (NO). Treatment with inhibitors of NO synthase can augment QUIN toxicity in some models of excitotoxicity suggesting a neuroprotective potential of endogenous NO. In recent studies, certain nitroso compounds which could be NO donors, have been reported to reduce the NMDA receptor-mediated neurotoxicity. The existence of endogenous compounds which inhibit excitotoxicity provides a basis for future development of novel and effective neuroprotectants.

Kynurenic acid in brain and cerebrospinal fluid of fetal, newborn, and adult sheep and effects of placental embolization

Concentrations of the endogenous glutamate receptor antagonist kynurenic acid (KA) were measured in various brain regions and in cisternal cerebrospinal fluid of fetal, newborn, and adult sheep. KA concentrations were significantly higher in the fetal brain and cerebrospinal fluid at 90 and 140 d gestation compared with postnatal ages. In fetuses of 132-139 d gestation, KA concentrations in cerebrospinal fluid collected by drainage from an indwelling cisternal catheter increased significantly after infusion of the organic acid transport inhibitor probenecid (100 or 200 mg/kg, i.v.) indicating active transport of KA out of the fetal brain. In fetuses in which the umbilical circulation had been chronically restricted from 120 to 140 d gestation by partial embolization of the placenta, plasma concentrations of the KA precursor kynurenine were significantly lower than in control fetuses, and KA concentrations in the hypothalamus and hippocampus were significantly reduced; other brain regions were not affected. These results indicate that the production of KA is higher in the fetal brain compared with the newborn and adult brain. Because KA diminishes the risk of excitotoxic neuronal damage under hypoxic-ischemic conditions, the high levels of KA in the brain before birth may have a neuroprotective function. The decrease of KA concentrations in the hypothalamus and hippocampus after umbilical embolization suggests that, after chronic hypoxia in utero, these regions of the brain may become more vulnerable to subsequent episodes of acute hypoxia or ischemia encountered in late gestation or during parturition.