N-Acetylcysteine Inhibits Kynurenine Aminotransferase II.

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

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.

Oral administration of a specific kynurenic acid synthesis (KAT II) inhibitor attenuates evoked glutamate release in rat prefrontal cortex.

Cognitive deficits represent core symptoms in schizophrenia (SZ) and predict patient outcome; however, they remain poorly treated by current antipsychotic drugs. Elevated levels of the endogenous alpha7 nicotinic receptor negative allosteric modulator and NMDA receptor antagonist, kynurenic acid (KYNA), are commonly seen in post-mortem tissue and cerebrospinal fluid of patients with SZ. When acutely or chronically elevated in rodents, KYNA produces cognitive deficits similar to those seen in the disease, making down-regulation of KYNA, via inhibition of kynurenine aminotransferase II (KAT II), a potential treatment strategy. We determined, in adult Wistar rats, if the orally available KAT II inhibitor BFF816 a) prevents KYNA elevations in prefrontal cortex (PFC) after a systemic kynurenine injection and b) reverses the kynurenine-induced attenuation of evoked prefrontal glutamate release caused by stimulation of the nucleus accumbens shell (NAcSh). Systemic injection of kynurenine (25 or 100 mg/kg, i.p.) increased KYNA levels in PFC (532% and 1104% of baseline, respectively). NMDA infusions (0.15 µg/0.5 µL) into NAcSh raised prefrontal glutamate levels more than 30-fold above baseline. The two doses of kynurenine reduced evoked glutamate release in PFC (by 43% and 94%, respectively, compared to NMDA alone). Co-administration of BFF816 (30 or 100 mg/kg, p.o.) with kynurenine (25 mg/kg, i.p.) attenuated the neosynthesis of KYNA and dose-dependently restored NMDA-stimulated glutamate release in the PFC (16% and 69%, respectively). The ability to prevent KYNA neosynthesis and to normalize evoked glutamate release in PFC justifies further development of KAT II inhibitors for the treatment of cognitive deficits in SZ.

Kynurenines and Glutamate: Multiple Links and Therapeutic Implications.

Glutamate is firmly established as the major excitatory neurotransmitter in the mammalian brain and is actively involved in most aspects of neurophysiology. Moreover, glutamatergic impairments are associated with a wide variety of dysfunctional states, and both hypo- and hyperfunction of glutamate have been plausibly linked to the pathophysiology of neurological and psychiatric diseases. Metabolites of the kynurenine pathway (KP), the major catabolic route of the essential amino acid tryptophan, influence glutamatergic activity in several distinct ways. This includes direct effects of these "kynurenines" on ionotropic and metabotropic glutamate receptors or vesicular glutamate transport, and indirect effects, which are initiated by actions at various other recognition sites. In addition, some KP metabolites affect glutamatergic functions by generating or scavenging highly reactive free radicals. This review summarizes these phenomena and discusses implications for brain physiology and pathology.

Evaluation of kynurenine pathway metabolism in Toxoplasma gondii-infected mice: implications for schizophrenia.

Toxoplasma gondii, an intracellular protozoan parasite, is a major cause of opportunistic infectious disease affecting the brain and has been linked to an increased incidence of schizophrenia. In murine hosts, infection with T. gondii stimulates tryptophan degradation along the kynurenine pathway (KP), which contains several neuroactive metabolites, including 3-hydroxykynurenine (3-HK), quinolinic acid (QUIN) and kynurenic acid (KYNA). As these endogenous compounds may provide a mechanistic connection between T. gondii and the pathophysiology of schizophrenia, we measured KP metabolites in both the brain and periphery of T. gondii-treated C57BL/6 mice 8 and 28 days post-infection. Infected mice showed early decreases in the levels of tryptophan in the brain and serum, but not in the liver. These reductions were associated with elevated levels of kynurenine, KYNA, 3-HK and QUIN in the brain. In quantitative terms, the most significant increases in these KP metabolites were observed in the brain at 28 days post-infection. Notably, the anti-parasitic drugs pyrimethamine and sulfadiazine, a standard treatment of toxoplasmosis, significantly reduced 3-HK and KYNA levels in the brain of infected mice when applied between 28 and 56 days post-infection. In summary, T. gondii infection, probably by activating microglia and astrocytes, enhances the production of KP metabolites in the brain. However, during the first two months after infection, the KP changes in these mice do not reliably duplicate abnormalities seen in the brain of individuals with schizophrenia.

Early developmental elevations of brain kynurenic acid impair cognitive flexibility in adults: reversal with galantamine.

Levels of kynurenic acid (KYNA), an endogenous α7 nicotinic acetylcholine receptor (α7nAChR) antagonist, are elevated in the brain of patients with schizophrenia (SZ) and might contribute to the pathophysiology and cognitive deficits seen in the disorder. As developmental vulnerabilities contribute to the etiology of SZ, we determined, in rats, the effects of perinatal increases in KYNA on brain chemistry and cognitive flexibility. KYNA's bioprecursor l-kynurenine (100mg/day) was fed to dams from gestational day 15 to postnatal day 21 (PD21). Offspring were then given regular chow until adulthood. Control rats received unadulterated mash. Brain tissue levels of KYNA were measured at PD2 and PD21, and extracellular levels of KYNA and glutamate were determined by microdialysis in the prefrontal cortex in adulthood (PD56-80). In other adult rats, the effects of perinatal l-kynurenine administration on cognitive flexibility were assessed using an attentional set-shifting task. l-Kynurenine treatment raised forebrain KYNA levels ∼3-fold at PD2 and ∼2.5-fold at PD21. At PD56-80, extracellular prefrontal KYNA levels were moderately but significantly elevated (+12%), whereas extracellular glutamate levels were not different from controls. Set-shifting was selectively impaired by perinatal exposure to l-kynurenine, as treated rats acquired the discrimination and intra-dimensional shift at the same rate as controls, yet exhibited marked deficits in the initial reversal and extra-dimensional shift. Acute administration of the α7nAChR-positive modulator galantamine (3.0mg/kg, i.p.) restored performance to control levels. These results validate early developmental exposure to l-kynurenine as a novel, naturalistic animal model for studying cognitive deficits in SZ.

Cortical kynurenic acid bi-directionally modulates prefrontal glutamate levels as assessed by microdialysis and rapid electrochemistry.

Using two in vivo methods, microdialysis and rapid in situ electrochemistry, this study examined the modulation of extracellular glutamate levels by endogenously produced kynurenic acid (KYNA) in the prefrontal cortex (PFC) of awake rats. Measured by microdialysis, i.p. administration of KYNA's bioprecursor L-kynurenine dose-dependently elevated extracellular KYNA and reduced extracellular glutamate (nadir after 50 mg/kg kynurenine: 60% decrease from baseline values). This dose-dependent decrease in glutamate levels was also seen using a glutamate-sensitive microelectrode array (MEA) (31% decrease following 50 mg/kg kynurenine). The kynurenine-induced reduction in glutamate was blocked (microdialysis) or attenuated (MEA) by co-administration of galantamine (3 mg/kg i.p.), a drug that competes with KYNA at an allosteric potentiating site of the alpha 7 nicotinic acetylcholine receptor. In separate experiments, extracellular glutamate levels were measured by MEA following the local perfusion (45 min) of the PFC with kynurenine (2.5 microM) or the selective KYNA biosynthesis inhibitor S-ethylsulfonylbenzoylalanine (S-ESBA; 5 mM). In agreement with previous microdialysis studies, local kynurenine application produced a reversible reduction in glutamate (nadir: -29%), whereas perfusion with S-ESBA increased glutamate levels reversibly (maximum: +38%). Collectively, these results demonstrate that fluctuations in the biosynthesis of KYNA in the PFC bi-directionally modulate extracellular glutamate levels, and that qualitatively very similar data are obtained by microdialysis and MEA. Since KYNA levels are elevated in the PFC of individuals with schizophrenia, and since prefrontal glutamatergic and nicotinic transmission mediate cognitive flexibility, normalization of KYNA levels in the PFC may constitute an effective treatment strategy for alleviating cognitive deficits in schizophrenia.

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

We tested the hypothesis that fluctuations in the levels of kynurenic acid (KYNA), an endogenous antagonist of the alpha7 nicotinic acetylcholine (ACh) receptor, modulate extracellular ACh levels in the medial prefrontal cortex in rats. Decreases in cortical KYNA levels were achieved by local perfusion of S-ESBA, a selective inhibitor of the astrocytic enzyme kynurenine aminotransferase II (KAT II), which catalyses the formation of KYNA from its precursor L-kynurenine. At 5 mm, S-ESBA caused a 30% reduction in extracellular KYNA levels, which was accompanied by a two-threefold increase in basal cortical ACh levels. Co-perfusion of KYNA in the endogenous range (100 nm), which by itself tended to reduce basal ACh levels, blocked the ability of S-ESBA to raise extracellular ACh levels. KYNA perfusion (100 nm) also prevented the evoked ACh release caused by d-amphetamine (2.0 mg/kg). This effect was duplicated by the systemic administration of kynurenine (50 mg/kg), which resulted in a significant increase in cortical KYNA formation. Jointly, these data indicate that astrocytes, by producing and releasing KYNA, have the ability to modulate cortical cholinergic neurotransmission under both basal and stimulated conditions. As cortical KYNA levels are elevated in individuals with schizophrenia, and in light of the established role of cortical ACh in executive functions, our findings suggest that drugs capable of attenuating the production of KYNA may be of benefit in the treatment of cognitive deficits in schizophrenia.

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.

Dopamine receptor activation reveals a novel, kynurenate-sensitive component of striatal N-methyl-D-aspartate neurotoxicity.

The N-methyl-d-aspartate (NMDA) subtype of glutamate receptors plays an important role in brain physiology, but excessive receptor stimulation results in seizures and excitotoxic nerve cell death. NMDA receptor-mediated neuronal excitation and injury can be prevented by high, non-physiological concentrations of the neuroinhibitory tryptophan metabolite kynurenic acid (KYNA). Here we report that endogenous KYNA, which is formed in and released from astrocytes, controls NMDA receptors in vivo. This was revealed with the aid of the dopaminergic drugs d-amphetamine and apomorphine, which cause rapid, transient decreases in striatal KYNA levels in rats. Intrastriatal injections of the excitotoxins NMDA or quinolinate (but not the non-NMDA receptor agonist kainate) at the time of maximal KYNA reduction resulted in two- to threefold increases in excitotoxic lesion size. Pre-treatment with a kynurenine 3-hydroxylase inhibitor or with dopamine receptor antagonists, i.e., two classes of pharmacological agents that prevented the reduction in brain KYNA caused by dopaminergic stimulation, abolished the potentiation of neurotoxicity. Thus, the present study identifies a previously unappreciated role of KYNA as a functional link between dopamine receptor stimulation and NMDA neurotoxicity in the striatum.

Kynurenic acid leads, dopamine follows: a new case of volume transmission in the brain?

Intrastriatal infusion of nanomolar concentrations of kynurenic acid (KYNA), an astrocyte-derived neuroinhibitory tryptophan metabolite, reduces basal extracellular dopamine (DA) levels in the rat striatum. This effect is initiated by the inhibition of alpha7 nicotinic acetylcholine receptors (alpha7nAChRs) on glutamatergic afferents. The present study was designed to further investigate this functional link between KYNA and DA using striatal microdialysis in awake animals. In rats, increases in KYNA, caused by intrastriatal infusions of KYNA itself (100 nM) or of KYNA's bioprecursor L-kynurenine (2 microM), were associated with substantial reductions in DA. Co-infusion of KYNA with the alpha7nAChR agonist galantamine (5 microM), but not with the NMDA receptor agonist D-serine (100 nM), prevented this effect. Moreover, KYNA also reduced DA levels in the NMDA-lesioned striatum. Conversely, extracellular DA levels were enhanced when KYNA formation was compromised, either by astrocyte poisoning with fluorocitrate or by perfusion with aminooxyacetic acid (AOAA; 5 mM), a non-specific inhibitor of KYNA synthesis. Notably, this effect of AOAA was prevented by co-perfusion with 100 nM KYNA. In the striatum of 21 day-old mice with a targeted deletion of kynurenine aminotransferase II, extracellular KYNA levels were reduced by 67 +/- 6%, while extracellular DA levels were simultaneously increased by 170 +/- 14%. Taken together, a picture emerges where fluctuations in the astrocytic production of KYNA, possibly through volume transmission, inversely regulate dopaminergic tone. This newly uncovered mechanism may profoundly influence DA function under physiological and pathological conditions.

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.

Kynurenate production by cultured human astrocytes.

In the rodent brain, astrocytes are known to be the primary source of kynurenate (KYNA), an endogenous antagonist of both the glycine(B) and the alpha7 nicotinic acetylcholine receptor. In the present study, primary human astrocytes were used to examine the characteristics and regulation of de novo KYNA synthesis in vitro. To this end, cells were exposed to KYNA's bioprecursor L-kynurenine, and newly formed KYNA was recovered from the extracellular milieu. The production of KYNA was stereospecific and rose with increasing L-kynurenine concentrations, reaching a plateau in the high microM range. In an analogous experiment, astrocytes also readily produced and liberated the potent, specific glycine(B) receptor antagonist 7-chlorokynurenate from L-4-chlorokynurenine. KYNA synthesis was dose-dependently reduced by L-leucine or L-phenylalanine, two amino acids that compete with L-kynurenine for cellular uptake, and by aminooxyacetate, a non-specific aminotransferase inhibitor. In contrast, KYNA formation was stimulated by 5 mM pyruvate or oxaloacetate, which act as co-substrates of the transamination reaction. Aglycemic or depolarizing (50 mM KCl or 100 microM veratridine) conditions had no effect on KYNA synthesis. Subsequent studies using tissue homogenate showed that both known cerebral kynurenine aminotransferases (KAT I and KAT II) are present in astrocytes, but that KAT II appears to be singularly responsible for KYNA formation under physiological conditions. Taken together with previous results, these data suggest that very similar mechanisms control KYNA synthesis in the rodent and in the human brain. These regulatory events are likely to influence the neuromodulatory effects of astrocyte-derived KYNA in the normal and diseased human brain.

Effect of systemic L-DOPA administration on extracellular kynurenate levels in the rat striatum.

Dopaminergic stimulation, caused by direct dopamine receptor agonists or by indirectly acting agents such as d-amphetamine or methylphenidate, causes a functionally significant decrease in the concentration of kynurenate (KYNA), an astrocyte-derived endogenous antagonist of both N-methyl-D-aspartate (NMDA) and alpha7 nicotinic acetylcholine receptors, in the rat brain. The present study was designed to examine if this effect can be duplicated by the systemic administration of dopamine's bioprecursor L-DOPA. Experiments were conducted in unanesthetized rats, and the extracellular levels of KYNA were determined in striatal microdialysate samples. L-DOPA caused a dose-dependent, transient reduction in striatal KYNA, reaching a nadir of -37.5% 1.5 h after the administration of 200 mg/kg of the drug. This effect was abolished in animals with a 6-hydroxydopamine-induced lesion of the nigrostriatal pathway, but was not influenced by a prior striatal quinolinate lesion. These data confirm the dopaminergic control of striatal KYNA formation and suggest that the interactions are mediated by astrocytic dopamine receptors. This modulation of striatal KYNA levels might provide a link between dopaminergic, glutamatergic and cholinergic neurotransmission in the normal and diseased striatum.

Disruption of inhibition in area CA1 of the hippocampus in a rat model of temporal lobe epilepsy.

Previous studies have revealed a loss of neurons in layer III of the entorhinal cortex (EC) in patients with temporal lobe epilepsy. These neurons project to the hippocampus and may activate inhibitory interneurons, so that their loss could disrupt inhibitory function in the hippocampus. The present study evaluates this hypothesis in a rat model in which layer III neurons were selectively destroyed by focal injections of the indirect excitotoxin, aminooxyacetic acid (AOAA). Inhibitory function in the hippocampus was assessed by evaluating the discharge of CA1 neurons in response to stimulation of afferent pathways in vivo. In control animals, stimulation of the temporo-ammonic pathway leads to heterosynaptic inhibition of population spikes generated by subsequent stimulation of the commissural projection to CA1. This heterosynaptic inhibition was substantially reduced in animals that had received AOAA injections 1 mo previously. Stimulation of the commissural projection also elicited multiple population spikes in CA1 in AOAA-injected animals, and homosynaptic inhibition in response to paired-pulse stimulation of the commissural projection was dramatically diminished. These results suggest a disruption of inhibitory function in CA1 in AOAA-injected animals. To determine whether the disruption of inhibition occurred selectively in CA1, we assessed paired-pulse inhibition in the dentate gyrus. Both homosynaptic inhibition generated by paired-pulse stimulation of the perforant path, and heterosynaptic inhibition produced by activation of the commissural projection to the dentate gyrus appeared largely comparable in AOAA-injected and control animals; thus abnormalities in inhibitory function following AOAA injections occurred relatively selectively in CA1. Electrolytic lesions of the EC did not cause the same loss of inhibition as seen in animals with AOAA injections, indicating that the loss of inhibition in CA1 is not due to the loss of excitatory driving of inhibitory interneurons. Also, electrolytic lesions of the EC in animals that had been injected previously with AOAA had little effect on the abnormal physiological responses in CA1, suggesting that most alterations in inhibition in CA1 are not due to circuit abnormalities within the EC. Comparisons of control and AOAA-injected animals in a hippocampal kindling paradigm revealed that the duration of afterdischarges elicited by high-frequency stimulation of CA3, and the number of stimulations required to elicit kindled seizures were comparable. Taken together, our results reveal that the selective loss of layer III neurons induced by AOAA disrupts inhibitory function in CA1, but this does not create a circuit that is more prone to at least one form of kindling.

Immunohistochemical visualization of newly formed quinolinate in the normal and excitotoxically lesioned rat striatum.

Intracerebral infusion of 3-hydroxyanthranilate (3HANA) rapidly increases the brain content of the endogenous excitotoxin quinolinate (QUIN). QUIN formation from 3HANA is readily prevented by coadministration of the specific 3-hydroxyanthranilate oxygenase inhibitor 4-chloro-3HANA (4-Cl-3HANA). This experimental paradigm was used to identify the cell populations which are responsible for the rapid de novo production of QUIN in the rat striatum in vivo. Rats received an intrastriatal infusion of 3HANA, alone or together with equimolar 4-Cl-3HANA, for 1 h. Striatal QUIN immunoreactivity (ir) was assessed immunohistochemically, using an antibody against protein-conjugated QUIN. This antibody displayed no significant crossreactivity with compounds structurally or functionally related to QUIN. QUIN-ir cells were detected after infusion with > or =300 microM 3HANA, but not in naïve striata or after co-infusion of 4-Cl-3HANA. Cellular staining was also abolished by preabsorption of the antibody with protein-conjugated QUIN. In the normal striatum, QUIN-ir was detected exclusively in cells of an apparent microglial morphology. When examined in the excitotoxically lesioned striatum, 3HANA-induced QUIN-ir localized exclusively to OX42-ir cells of an activated microglial/macrophage morphology. These data indicate that microglia and macrophages are the major source of QUIN in the rat striatum when hyperphysiological concentrations of 3HANA are used to drive QUIN synthesis. Comparison with earlier biochemical and immunohistochemical studies suggests that the enzyme responsible for microglial QUIN production is a distinct 3-hydroxyanthranilate oxygenase with high capacity and low affinity for 3HANA.

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.

The brain metabolite kynurenic acid inhibits alpha7 nicotinic receptor activity and increases non-alpha7 nicotinic receptor expression: physiopathological implications.

The tryptophan metabolite kynurenic acid (KYNA) has long been recognized as an NMDA receptor antagonist. Here, interactions between KYNA and the nicotinic system in the brain were investigated using the patch-clamp technique and HPLC. In the electrophysiological studies, agonists were delivered via a U-shaped tube, and KYNA was applied in admixture with agonists and via the background perfusion. Exposure (>/=4 min) of cultured hippocampal neurons to KYNA (>/=100 nm) inhibited activation of somatodendritic alpha7 nAChRs; the IC(50) for KYNA was approximately 7 microm. The inhibition of alpha7 nAChRs was noncompetitive with respect to the agonist and voltage independent. The slow onset of this effect could not be accounted for by an intracellular action because KYNA (1 mm) in the pipette solution had no effect on alpha7 nAChR activity. KYNA also blocked the activity of preterminal/presynaptic alpha7 nAChRs in hippocampal neurons in cultures and in slices. NMDA receptors were less sensitive than alpha7 nAChRs to KYNA. The IC(50) values for KYNA-induced blockade of NMDA receptors in the absence and presence of glycine (10 microm) were approximately 15 and 235 microm, respectively. Prolonged (3 d) exposure of cultured hippocampal neurons to KYNA increased their nicotinic sensitivity, apparently by enhancing alpha4beta2 nAChR expression. Furthermore, as determined by HPLC with fluorescence detection, repeated systemic treatment of rats with nicotine caused a transient reduction followed by an increase in brain KYNA levels. These results demonstrate that nAChRs are targets for KYNA and suggest a functionally significant cross talk between the nicotinic cholinergic system and the kynurenine pathway in the brain.

Ibotenate injections into the pre- and parasubiculum provide partial protection against kainate-induced epileptic damage in layer III of rat entorhinal cortex.

PURPOSE: A loss of neurons in layer III of the entorhinal cortex (EC) is often observed in patients with temporal lobe epilepsy and in animal models of the disorder. We hypothesized that the susceptibility of layer III of the EC to prolonged seizure activity might be mediated by excitatory afferents originating in the presubiculum. METHODS: Experiments were designed to ablate the presubiculum unilaterally by focal ibotenate injections and to evaluate the effect of this deafferentation on the vulnerability of EC layer III neurons to the chemoconvulsant kainate (injected systemically 5 days later). RESULTS: After treatment with kainate, 11 of the 15 rats preinjected with ibotenate showed clear-cut, partial neuroprotection in layer III of the EC ipsilateral to the ibotenate lesion. Serial reconstruction of the ibotenate-induced primary lesion revealed that entorhinal neurons were protected only in animals that had lesions in the pre- and parasubiculum, especially in the deep layers (IV-VI). CONCLUSIONS: The deep layers of the pre- and parasubiculum appear to control the seizure-induced damage of EC layer III. This phenomenon may be of relevance for epileptogenesis and for the pathogenesis of temporal lobe epilepsy.

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.