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

AIMS: The family of kynurenine pathway (KP) metabolites includes compounds produced along two arms of the path and acting in clearly opposite ways. The equilibrium between neurotoxic kynurenines, such as 3-hydroxykynurenine (3-HK) or quinolinic acid (QUIN), and neuroprotective kynurenic acid (KYNA) profoundly impacts the function and survival of neurons. This comprehensive review summarizes accumulated evidence on the role of KYNA in Alzheimer's, Parkinson's and Huntington's diseases, and discusses future directions of potential pharmacological manipulations aimed to modulate brain KYNA. DISCUSSION: The synthesis of specific KP metabolites is tightly regulated and may considerably vary under physiological and pathological conditions. Experimental data consistently imply that shift of the KP to neurotoxic branch producing 3-HK and QUIN formation, with a relative or absolute deficiency of KYNA, is an important factor contributing to neurodegeneration. Targeting specific brain regions to maintain adequate KYNA levels seems vital; however, it requires the development of precise pharmacological tools, allowing to avoid the potential cognitive adverse effects. CONCLUSIONS: Boosting KYNA levels, through interference with the KP enzymes or through application of prodrugs/analogs with high bioavailability and potency, is a promising clinical approach. The use of KYNA, alone or in combination with other compounds precisely influencing specific populations of neurons, is awaiting to become a significant therapy for neurodegenerative disorders.

Examination of Kynurenine Toxicity on Corneal and Conjunctival Epithelium: In vitro and in vivo Studies.

Kynurenine (KYN) is a metabolite of tryptophan, proposed for the treatment of corneal diseases. Our goal was to evaluate the effects of KYN on normal human corneal and conjunctival epithelial cells in vitro and the re-epithelization of corneal erosion in rabbits. In our study, we used corneal (10.014 pRSV-T) and conjunctival (HC0597) epithelium cell cultures. KYN was applied at a concentration range of 1-100 µM for 24 and 48 h. We examined the effects on cellular metabolism, viability, interleukin-1ß (IL-1ß), IL-6, IL-10 secretion, cytoskeleton organization and transwell migration ability. Following a bilateral corneal de-epithelialization, the rabbits received drops containing 1% KYN and a saline solution to the contralateral control eye, 5 times daily. Digital images were analyzed using the EPCO 2000 software. The metabolic activity of cells was slightly decreased by KYN in the corneal but not in the conjunctival epithelium. The viability of both epithelia was improved by KYN; it caused alterations in the secretion of IL-6 and IL-10 but not IL-1ß. It had no impact on both epithelia morphology and the organization of the cellular cytoskeleton. KYN stimulated the formation of pseudopodia projections in both epithelia in vitro, which may be important in terms of wound healing. However, there were no differences in the re-epithelization rate in vivo. At the tested concentrations, KYN was not toxic for the corneal and the conjunctival epithelium in vitro and did not affect corneal re-epithelization in rabbits in vivo. Our results suggest that KYN may be taken into consideration for the treatment of ocular disorders.

Inhibition of kynurenine aminotransferase II attenuates hippocampus-dependent memory deficit in adult rats treated prenatally with kynurenine.

A combination of genetic and environmental factors contributes to schizophrenia (SZ), a catastrophic psychiatric disorder with a hypothesized neurodevelopmental origin. Increases in the brain levels of the tryptophan metabolite kynurenic acid (KYNA), an endogenous antagonist of α7 nicotinic acetylcholine and NMDA receptors, have been implicated specifically in the cognitive deficits seen in persons with SZ. Here we evaluated this role of KYNA by adding the KYNA precursor kynurenine (100 mg/day) to chow fed to pregnant rat dams from embryonic day (ED) 15 to ED 22 (control: ECon; kynurenine treated: EKyn). Upon termination of the treatment, all rats received normal rodent chow until the animals were evaluated in adulthood (postnatal days 56-85). EKyn treatment resulted in increased extracellular KYNA and reduced extracellular glutamate in the hippocampus, measured by in vivo microdialysis, and caused impairments in hippocampus-dependent learning in adult rats. Acute administration of BFF816, a systemically active inhibitor of kynurenine aminotransferase II (KAT II), the major KYNA-synthesizing enzyme in the brain, normalized neurochemistry and prevented contextual memory deficits in adult EKyn animals. Collectively, these results demonstrate that acute inhibition of KYNA neosynthesis can overcome cognitive impairments that arise as a consequence of elevated brain KYNA in early brain development.

Enzymatic and non-enzymatic pathways of kynurenines' dimerization: the molecular factors for oxidative stress development.

Kynurenines, the products of tryptophan oxidative degradation, are involved in multiple neuropathologies, such as Huntington's chorea, Parkinson's disease, senile dementia, etc. The major cause for hydroxykynurenines's neurotoxicity is the oxidative stress induced by the reactive oxygen species (ROS), the by-products of L-3-hydroxykynurenine (L-3HOK) and 3-hydroxyanthranilic acid (3HAA) oxidative self-dimerization. 2-aminophenol (2AP), a structural precursor of L-3HOK and 3HAA, undergoes the oxidative conjugation to form 2-aminophenoxazinone. There are several modes of 2AP dimerization, including both enzymatic and non-enzymatic stages. In this study, the free energies for 2AP, L-3HOK and 3HAA dimerization stages have been calculated at B3LYP/6-311G(d,p)//6-311+(O)+G(d) level, both in the gas phase and in heptane or water solution. For the intermediates, ionization potentials and electron affinities were calculated, as well as free energy and kinetics of molecular oxygen interaction with several non-enzymatically formed dimers. H-atom donating power of the intermediates increases upon the progress of the oxidation, making possible generation of hydroperoxyl radical or hydrogen peroxide from O2 at the last stages. Among the dimerization intermediates, 2-aminophenoxazinole derivatives have the lowest ionization potential and can reduce O2 to superoxide anion. The rate for O-H homolytic bond dissociation is significantly higher than that for C-H bond in non-enzymatic quinoneimine conjugate. However, the last reaction passes irreversibly, reducing O2 to hydroperoxyl radical. The inorganic ferrous iron and the heme group of Drosophila phenoxazinone synthase significantly reduce the energy cost of 2AP H-atom abstraction by O2. We have also shown experimentally that total antioxidant capacity decreases in Drosophila mutant cardinal with L-3HOK excess relative to the wild type Canton-S, and lipid peroxidation decreases in aged cardinal. Taken together, our data supports the conception of hydroxykynurenines' dual role in neurotoxicity: serving as antioxidants themselves, blocking lipid peroxidation by H-atom donation, they also can easily generate ROS upon dimerization, leading to the oxidative stress development.

Toxicity and cardiac effects of acute exposure to tryptophan metabolites on the kynurenine pathway in early developing zebrafish (Danio rerio) embryos.

Defects in tryptophan metabolism on the l-kynurenine pathway (KP) are implicated in a number of human diseases, including chronic kidney disease, brain edema or injury, tuberculosis and malaria - as well as cancer, neurodegenerative and autoimmune disorders. However, it is unclear to what extent detrimental effects of exposure to tryptophan metabolites might impact the early development of organism. Thus, this study examined the effects of KP exposure in zebrafish embryos starting at the blastula period (4hpf) and the segmentation stage (24hpf). 24-hour EC50 and LC50 values were determined in 4hpf embryos as: 26.74 and 331.6µM for anthranilic acid (AA), 62.88 and 616.4µM for quinolinic acid (QUIN), and EC50 - 96.10µM for picolinic acid (PA) and LC50 - 400µM in kynurenic acid (KYNA). In addition, treatment with nanomolar concentrations of KYNA (50nM, 48 and 72hpf embryos) caused a dose-dependent increase in heartbeat. The increase was also seen with l-kyn treatment (50µM, 72hpf), which was the opposite of other applied l-kyn metabolites. A significant drop in heartbeat was observed after a 20-min acute exposure to 626µM PA, 594µM XA and 499µM QUIN, and complete recovery was seen only when PA had been removed. Concentrations of KP metabolites reached in people with different pathological conditions did not exert toxicity to zebrafish embryos and seems to be safe for developing embryos and therefore, the risk of developing impairments in pregnancy of women carrying KP-associated pathologies is initially low.

Prenatal kynurenine treatment in rats causes schizophrenia-like broad monitoring deficits in adulthood.

RATIONALE: Elevated brain kynurenic acid (KYNA) levels are implicated in the pathology and neurodevelopmental pathogenesis of schizophrenia. In rats, embryonic treatment with kynurenine (EKyn) causes elevated brain KYNA levels in adulthood and cognitive deficits reminiscent of schizophrenia. OBJECTIVES: Growing evidence suggests that people with schizophrenia have a narrowed attentional focus, and we aimed at establishing whether these abnormalities may be related to KYNA dysregulation. METHODS: To test whether EKyn rats display broad monitoring deficits, kynurenine was added to the chow of pregnant Wistar dams on embryonic days 15-22. As adults, 20 EKyn and 20 control rats were trained to stable performance on the five-choice serial reaction time task, requiring the localization of 1-s light stimuli presented randomly across five apertures horizontally arranged along a curved wall, equating the locomotor demands of reaching each hole. RESULTS: EKyn rats displayed elevated omission errors and reduced anticipatory responses relative to control rats, indicative of a lower response rate, and showed reduced locomotor activity. The ability to spread attention broadly was measured by parsing performance by stimulus location. Both groups displayed poorer stimulus detection with greater target location eccentricity, but this effect was significantly more pronounced in the EKyn group. Specifically, the groups differed in the spatial distribution of correct but not incorrect responses. This pattern cannot be explained by differences in response rate and is indicative of a narrowed attentional focus. CONCLUSIONS: The findings suggest a potential etiology of broad monitoring deficits in schizophrenia, which may constitute a core cognitive deficit.

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

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

Neurotoxic kynurenine metabolism is increased in the dorsal hippocampus and drives distinct depressive behaviors during inflammation.

The kynurenine pathway of tryptophan metabolism has an important role in mediating the behavioral effects of inflammation, which has implications in understanding neuropsychiatric comorbidity and for the development of novel therapies. Inhibition of the rate-limiting enzyme, indoleamine 2,3-dioxygenase (IDO), prevents the development of many of these inflammation-induced preclinical behaviors. However, dysregulation in the balance of downstream metabolism, where neuroactive kynurenines are generated, is hypothesized to be a functionally important pathogenic feature of inflammation-induced depression. Here we utilized two novel transgenic mouse strains to directly test the hypothesis that neurotoxic kynurenine metabolism causes depressive-like behavior following peripheral immune activation. Wild-type (WT) or kynurenine 3-monooxygenase (KMO)-deficient (KMO-/-) mice were administered either lipopolysaccharide (LPS, 0.5 mg kg-1) or saline intraperitoneally. Depressive-like behavior was measured across multiple domains 24 h after immune challenge. LPS precipitated a robust depressive-like phenotype, but KMO-/- mice were specifically protected from LPS-induced immobility in the tail suspension test (TST) and reduced spontaneous alternations in the Y-maze. Direct administration of 3-hydroxykynurenine, the metabolic product of KMO, caused a dose-dependent increase in depressive-like behaviors. Mice with targeted deletion of 3-hydroxyanthranilic acid dioxygenase (HAAO), the enzyme that generates quinolinic acid, were similarly challenged with LPS. Similar to KMO-/- mice, LPS failed to increase immobility during the TST. Whereas kynurenine metabolism was generally increased in behaviorally salient brain regions, a distinct shift toward KMO-dependent kynurenine metabolism occurred in the dorsal hippocampus in response to LPS. Together, these results demonstrate that KMO is a pivotal mediator of hippocampal-dependent depressive-like behaviors induced by peripheral LPS challenge.

Overexpression of human kynurenine-3-monooxygenase protects against 3-hydroxykynurenine-mediated apoptosis through bidirectional nonlinear feedback.

Kynurenine 3-monooxygenase (KMO) is a critical regulator of inflammation. The preferred KMO substrate, kynurenine, is converted to 3-hydroxykynurenine (3HK), and this product exhibits cytotoxicity through mechanisms that culminate in apoptosis. Here, we report that overexpression of human KMO with orthotopic localisation to mitochondria creates a metabolic environment during which the cell exhibits increased tolerance for exogenous 3HK-mediated cellular injury. Using the selective KMO inhibitor Ro61-8048, we show that KMO enzyme function is essential for cellular protection. Pan-caspase inhibition with Z-VAD-FMK confirmed apoptosis as the mode of cell death. By defining expression of pathway components upstream and downstream of KMO, we observed alterations in other key kynurenine pathway components, particularly tryptophan-2,3-dioxygenase upregulation, through bidirectional nonlinear feedback. KMO overexpression also increased expression of inducible nitric oxide synthase (iNOS). These changes in gene expression are functionally relevant, because siRNA knockdown of the pathway components kynureninase and quinolinate phosphoribosyl transferase caused cells to revert to a state of susceptibility to 3HK-mediated apoptosis. In summary, KMO overexpression, and importantly KMO activity, have metabolic repercussions that fundamentally affect resistance to cell stress.

Mitochondrial dysfunction related to cell damage induced by 3-hydroxykynurenine and 3-hydroxyanthranilic acid: Non-dependent-effect of early reactive oxygen species production.

The kynurenines 3-hydroxyanthranilic acid (3-HANA) and its precursor 3-hydroxykynurenine (3-HK) are metabolites derived from tryptophan degradation. 3-HK, has been related to diverse neurodegenerative diseases including Huntington's, Alzheimer's and Parkinson's diseases that share mitochondrial metabolic dysregulation. Nevertheless, the direct effect of these kynurenines on mitochondrial function has not been investigated despite it could be regulated by their redox properties that are controversial. A body of literature has suggested a ROS mediated cell death induced by 3-HK and 3-HANA. On the other hand, some works have supported that both kynurenines have antioxidant effects. Therefore, the aim of this study was to investigate 3-HK and 3-HANA effects on mitochondrial and cellular function in rat cultured cortical astrocytes (rCCA) and in animals intrastriatally injected with these kynurenines as well as to determinate the ROS role on these effects. First, we evaluated 3-HK and 3-HANA effect on cellular function, ROS production and mitochondrial membrane potential in vivo and in vitro in rCCA. Our results show that both kynurenines decreased MTT reduction in a concentration-dependent manner together with mitochondrial membrane potential. These observations were accompanied with increased cell death in rCCA and in circling behavior and morphological changes of injected animals. Interestingly, we found that ROS production was not increased in both in vitro and in vivo experiments, and accordingly lipid peroxidation (LP) was neither increased in striatal tissue of animals injected with both kynurenines. The lack of effect on these oxidative markers is in agreement with the ·OH and ONOO(-) scavenging capacity of both kynurenines detected by chemical combinatorial assays. Altogether, these data indicate that both kynurenines exert toxic effects through mechanisms that include impairment of cellular energy metabolism which are not related to early ROS production.

Changing the face of kynurenines and neurotoxicity: therapeutic considerations.

Kynurenines are the products of tryptophan metabolism. Among them, kynurenine and kynurenic acid are generally thought to have neuroprotective properties, while 3-hydroxykynurenine, 3-hydroxyanthranilic acid and quinolinic acid are considered neurotoxic. They participate in immunoregulation and inflammation and possess pro- or anti-excitotoxic properties, and their involvement in oxidative stress has also been suggested. Consequently, it is not surprising that kynurenines have been closely related to neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis and multiple sclerosis. More information about the less-known metabolites, picolinic and cinnabarinic acid, evaluation of new receptorial targets, such as aryl-hydrocarbon receptors, and intensive research on the field of the immunomodulatory function of kynurenines delineated the high importance of this pathway in general homeostasis. Emerging knowledge about the kynurenine pathway provides new target points for the development of therapeutical solutions against neurodegenerative diseases.

Elevated levels of kynurenic acid during gestation produce neurochemical, morphological, and cognitive deficits in adulthood: implications for schizophrenia.

The levels of kynurenic acid (KYNA), an endogenous negative modulator of alpha7 nicotinic acetylcholine receptors (α7nAChRs), are elevated in the brains of patients with schizophrenia (SZ). We reported that increases of brain KYNA in rats, through dietary exposure to its precursor kynurenine from embryonic day (ED)15 to postnatal day (PD) 21, result in neurochemical and cognitive deficits in adulthood. The present experiments focused on the effects of prenatal exposure to elevated kynurenine on measures of prefrontal excitability known to be impaired in SZ. Pregnant dams were fed a mash containing kynurenine (100 mg/day; progeny = EKYNs) from ED15 until ED22. Controls were fed an unadulterated mash (progeny = ECONs). The dietary loading procedure elevated maternal and fetal plasma kynurenine (2223% and 693% above controls, respectively) and increased fetal KYNA (forebrain; 500% above controls) on ED21. Elevations in forebrain KYNA disappeared after termination of the loading (PD2), but KYNA levels in the prefrontal cortex (PFC) were unexpectedly increased again when measured in adults (PD56-80; 75% above controls). We also observed changes in several markers of prefrontal excitability, including expression of the α7nAChR (22% and 17% reductions at PD2 and PD56-80), expression of mGluR2 (31% and 24% reductions at ED21 and PD56-80), dendritic spine density (11-14% decrease at PD56-80), subsensitive mesolimbic stimulation of glutamate release in PFC, and reversal/extra-dimensional shift deficits in the prefrontally-mediated set-shifting task. These results highlight the deleterious impact of elevated KYNA levels during sensitive periods of early development, which model the pathophysiological and cognitive deficits seen in SZ.

The Janus faces of 3-hydroxykynurenine: dual redox modulatory activity and lack of neurotoxicity in the rat striatum.

3-Hydroxykynurenine (3-HK), an intermediate metabolite of the kynurenine pathway, has been largely hypothesized as a neurotoxic molecule contributing to neurodegeneration in several experimental and clinical conditions. Interestingly, the balance in literature points to a dual role of this molecule in the CNS: in vitro studies describe neurotoxic and/or antioxidant properties, whereas in vivo studies suggest a role of this metabolite as a weak neurotoxin. This work was designed to investigate, under different experimental conditions, whether or not 3-HK is toxic to cells, and if the redox activity exerted by this molecule modulates its actions in the rat striatum. In order to evaluate these effects, 3-HK was administered in vitro to isolated striatal slices, and in vivo to the striatum of rats. In striatal slices, 3-HK exerted a concentration- and time-dependent effect on lipid peroxidation, inducing both pro-oxidant actions at low (5-20) micromolar concentrations, and antioxidant activity at a higher concentration (100µM). Interestingly, while 3-HK was unable to induce mitochondrial dysfunction in slices, at the same range of concentrations it prevented the deleterious effects exerted by the neurotoxin and related metabolite quinolinic acid (QUIN), the mitochondrial toxin 3-nitropropionic acid, and the pro-oxidant compound iron sulfate. These protective actions were related to the stimulation of glutathione S-transferase (GST) and superoxide dismutase (SOD) activities. In addition, 3-HK stimulated the protein content of the transcription factor and antioxidant regulator Nrf2, and some of its related proteins. Accordingly, 3-HK, but not QUIN, exhibited reductive properties at high concentrations. The striatal tissue of animals infused with 3-HK exhibited moderate levels of lipid and protein oxidation at short times post-lesion (h), but these endpoints were substantially decreased at longer times (days). These effects were correlated with an early increase in glutathione reductase (GR) and GST activities. However, these changes were likely to be merely compensatory as 3-HK-infused animals did not display behavioral (rotation) alterations or morphological changes in their injected striata. Altogether, these findings suggest that, despite 3-HK might exert pro-oxidant actions under certain conditions, these changes serve to evoke a redox modulatory activity that, in turn, could decrease the risk of cell damage. In light of this evidence, 3-HK seems to be more a redox modulatory molecule than a neurotoxic metabolite.

Kynurenine production mediated by indoleamine 2,3-dioxygenase aggravates liver injury in HBV-specific CTL-induced fulminant hepatitis.

UNLABELLED: Indoleamine 2,3-dioxygenase (IDO), an enzyme that is ubiquitously distributed in mammalian tissues and cells, converts tryptophan to kynurenine, and is also known as a key molecule that promotes apoptosis in lymphocytes and neurons. In this study, we established hepatitis B virus (HBV)-transgenic (Tg)/IDO-knockout (KO) mice and examined the influence of IDO in a murine fulminant hepatitis model induced by HBV-specific cytotoxic T lymphocytes (CTL). An increase of IDO expression in the livers of HBV-Tg/IDO-wild-type (WT) mice administered HBV-specific CTL was confirmed by real-time polymerase chain reaction, western blotting, and evaluating IDO activity. Plasma alanine aminotransferase (ALT) levels in HBV-Tg/IDO-KO mice after HBV-specific CTL injection significantly decreased compared with those in HBV-Tg/IDO-WT mice. An inhibitor of IDO, 1-methyl-d-tryptophan (1-MT), could also attenuated the observed liver injury induced by this HBV-specific CTL. The expression levels of cytokine and chemokine mRNAs in the livers of HBV-Tg/IDO-WT mice were higher than those in the livers of HBV-Tg/IDO-KO mice. The administration of kynurenine aggravated the liver injury in HBV-Tg/IDO-KO mice injected with HBV-specific CTL. Simultaneous injection of recombinant murine interferon (IFN-γ) and kynurenine also increased the ALT levels in HBV-Tg/IDO-KO mice. The liver injury induced by IFN-γ and kynurenine was improved in HBV-Tg/tumor necrosis factor-α-KO mice. CONCLUSION: Kynurenine and IFN-γ induced by the administration with HBV-specific CTL are cooperatively involved in the progression of liver injury in acute hepatitis model. Our results may lead to a new therapy for the acute liver injury caused by HBV infection.

Continuous kynurenine administration during the prenatal period, but not during adolescence, causes learning and memory deficits in adult rats.

RATIONALE: Cognitive dysfunctions, including deficits in hippocampus-mediated learning and memory, are core features of the psychopathology of schizophrenia (SZ). Increased levels of kynurenic acid (KYNA), an astrocyte-derived tryptophan metabolite and antagonist of α7 nicotinic acetylcholine and N-methyl-D-aspartate receptors, have been implicated in these cognitive impairments. OBJECTIVES: Following recent suggestive evidence, the present study was designed to narrow the critical time period for KYNA elevation to induce subsequent cognitive deficits. METHODS: KYNA levels were experimentally increased in rats (1) prenatally (embryonic day (ED) 15 to ED 22) or (2) during adolescence (postnatal day (PD) 42 to PD 49). The KYNA precursor kynurenine was added daily to wet mash fed to (1) dams (100 mg/day; control: ECon; kynurenine-treated: EKyn) or (2) adolescent rats (300 mg/kg/day; control: AdCon; kynurenine-treated: AdKyn). Upon termination of the treatment, all animals were fed normal chow until biochemical analysis and behavioral testing in adulthood. RESULTS: On the last day of continuous kynurenine treatment, forebrain KYNA levels were significantly elevated (EKyn +472 %; AdKyn +470 %). KYNA levels remained increased in the hippocampus of adult EKyn animals (+54 %), but were unchanged in adult AdKyn rats. Prenatal, but not adolescent, kynurenine treatment caused significant impairments in two hippocampus-mediated behavioral tasks, passive avoidance and Morris water maze. CONCLUSIONS: Collectively, these studies provide evidence that a continuous increase in brain KYNA levels during the late prenatal period, but not during adolescence, induces hippocampus-related cognitive dysfunctions later in life. Such increases may play a significant role in illnesses with known hippocampal pathophysiology, including SZ.

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.

Low level of glutathione can intensify the toxic effect of salsolinol in SH-SY5Y neuroblastoma cell line.

There is increasing evidence that endogenously produced toxins may be involved in the development of a number of neurodegenerative diseases such as Alzheimer's, Parkinson's or Huntington's disease and that the mechanisms leading to cell loss are a combination of oxidative stress, mitochondrial dysfunction and a decrease in antioxidant defenses. The purpose of this study was to investigate the effects of glutathione on 3-hydroxykynurenine, 6-hydroxydopamine and salsolinol mediated neurotoxicity in the human neuroblastoma SH-SY5Y cell line in order to find a possible therapeutic application of this compound to neurodegenerative disorders. In this study, we tested the protective effect of glutathione on SH-SY5Y cells against 3-hydroxykynurenine, 6-hydroxydopamine and salsolinol induced cytotoxicity and demonstrated that glutathione inhibits cell death and adenosine-5'triphosphate depletion caused by 3-hydroxykynurenine and 6-hydroxydopamine. However, unexpectedly salsolinol neurotoxicity toward SH-SY5Y cells was potentiated during treatment with concentrations of glutathione below 250 µM, whereas glutathione concentrations above 250 µM resulted in protection against salsolinol induced neuronal cell death. We also report that the incubation of salsolinol and low concentrations of glutathione led to increased apoptosis. Hence, salsolinol in the presence of low glutathione concentration may be involved in neurodegeneration. These data may provide new promising insights into the pathophysiology of neudegenerative disorders such as Parkinson's disease.

Pre- and postnatal exposure to kynurenine causes cognitive deficits in adulthood.

Levels of kynurenic acid (KYNA), an endogenous product of tryptophan degradation, are elevated in the brain and cerebrospinal fluid of individuals with schizophrenia (SZ). This increase has been implicated in the cognitive dysfunctions seen in the disease, as KYNA is an antagonist of the α7 nicotinic acetylcholine receptor and the N-methyl-d-aspartate receptor, both of which are critically involved in cognitive processes and in a defining neurodevelopmental period in the pathophysiology of SZ. We tested the hypothesis that early developmental increases in brain KYNA synthesis might cause biochemical and functional impairments in adulthood. To this end, we stimulated KYNA formation by adding the KYNA precursor kynurenine (100 mg/day) to the chow fed to rat dams from gestational day 15 to postnatal day 21 (PD 21). This treatment raised brain KYNA levels in the offspring by 341% on PD 2 and 210% on PD 21. Rats were then fed normal chow until adulthood (PD 56-80). In the adult animals, basal levels of extracellular KYNA, measured in the hippocampus by in vivo microdialysis, were elevated (+12%), whereas extracellular glutamate levels were significantly reduced (-13%). In separate adult animals, early kynurenine treatment was shown to impair performance in two behavioral tasks linked to hippocampal function, the passive avoidance test and the Morris water maze test. Collectively, these studies introduce a novel, naturalistic rat model of SZ, and also suggest that increases in brain KYNA during a vulnerable period in brain development may play a significant role in the pathophysiology of the disease.

Acute elevations of brain kynurenic acid impair cognitive flexibility: normalization by the alpha7 positive modulator galantamine.

RATIONALE: Cognitive deficits represent a core symptom cluster in schizophrenia (SZ) that is predictive of outcome but not effectively treated by current antipsychotics. Thus, there is a need for validated animal models for testing potential pro-cognitive drugs. OBJECTIVE: As kynurenic acid levels are increased in prefrontal cortex (PFC) of individuals with SZ, we acutely increased brain levels of this astrocyte-derived, negative modulator of alpha7 nicotinic acetylcholine receptors (α7nAChRs) by administration of its bioprecursor kynurenine and measured the effects on extracellular kynurenic acid and glutamate levels in PFC and also performance in a set-shifting task. RESULTS: Injections of kynurenine (100 mg/kg, i.p.) increased extracellular kynurenic acid (1,500%) and decreased glutamate levels (30%) in PFC. Kynurenine also produced selective deficits in set-shifting. Saline- and kynurenine-treated rats similarly acquired the compound discrimination and intra-dimensional shift (saline, 7.0 and 6.3 trials, respectively; kynurenine, 8.0 and 6.7). Both groups required more trials to acquire the initial reversal (saline, 15.3; kynurenine, 22.2). Only kynurenine-treated rats were impaired in acquiring the extra-dimensional shift (saline, 8.2; kynurenine, 21.3). These deficits were normalized by administering the α7nAChR positive allosteric modulator galantamine (3.0 mg/kg, i.p) prior to kynurenine, as trials were comparable between galantamine + kynurenine (7.8) and controls (8.2). Bilateral local perfusion of the PFC with galantamine (5.0 µM) also attenuated kynurenine-induced deficits. CONCLUSIONS: These results validate the use of animals with elevated brain kynurenic acid levels in SZ research and support studies of drugs that normalize brain kynurenic acid levels and/or positively modulate α7nAChRs as pro-cognitive treatments for SZ.

Interleukin-1ß: a new regulator of the kynurenine pathway affecting human hippocampal neurogenesis.

Increased inflammation and reduced neurogenesis have been associated with the pathophysiology of major depression. Here, we show for the first time how IL-1ß, a pro-inflammatory cytokine shown to be increased in depressed patients, decreases neurogenesis in human hippocampal progenitor cells. IL-1ß was detrimental to neurogenesis, as shown by a decrease in the number of doublecortin-positive neuroblasts (-28%), and mature, microtubule-associated protein-2-positive neurons (-36%). Analysis of the enzymes that regulate the kynurenine pathway showed that IL-1ß induced an upregulation of transcripts for indolamine-2,3-dioxygenase (IDO), kynurenine 3-monooxygenase (KMO), and kynureninase (42-, 12- and 30-fold increase, respectively, under differentiating conditions), the enzymes involved in the neurotoxic arm of the kynurenine pathway. Moreover, treatment with IL-1ß resulted in an increase in kynurenine, the catabolic product of IDO-induced tryptophan metabolism. Interestingly, co-treatment with the KMO inhibitor Ro 61-8048 reversed the detrimental effects of IL-1ß on neurogenesis. These observations indicate that IL-1ß has a critical role in regulating neurogenesis whereas affecting the availability of tryptophan and the production of enzymes conducive to toxic metabolites. Our results suggest that inhibition of the kynurenine pathway may provide a new therapy to revert inflammatory-induced reduction in neurogenesis.