Levels Of Endogenous Kynurenic Acid Research Articles

2041-P: Genetic Intervention in Kynurenine Pathway Alters Glucose Metabolism in Mice

Most of tryptophan is metabolized to kynurenine (KYN) to synthesize NAD+, while a subsidiary pathway changes KYN to kynurenic acid (KYNA) by kynurenine aminoltransferases (KATs). We previously reported genetic association of a mutation in the gene encoding KAT-1, an isoform of KATs, with hypertension, adiposity and glucose intolerance in rats. Although it is widely known that KYNA negatively regulates central nervous system by antagonizing neuro-receptors, its physiological role in glucose metabolism remains unclear. To explore such role, first, we have established KAT-1 knockout mice that showed decreased levels of KAT activity in liver and brain (p<0.01) and KYNA levels decreased significantly in brain and blood (p<0.05) and tending to decrease in liver. The KAT-1 knockout mice also showed significantly deteriorated glucose tolerance (p<0.05 at 60min after glucose i.p.) and insulin sensitivity (p<0.05 at 45, 60, 75 and 90min after insulin i.p.) as compared with wild littermates. Next, to establish mice with endogenously increased levels of KYNA, the hepatic gene encoding kynurenine mono-oxygenase (KMO), a rate-limiting enzyme for conversion of KYN to NAD+, was knocked down by shRNA adenovirus. The hepatic KMO knockdown in normal mice markedly increased endogenous KYNA levels in liver, blood and several other tissues, whereas the levels of 3-hydroxykynurenine, a downstream product of KMO, were decreased in liver (p<0.001). Finally, the hepatic KMO knockdown in diet-induced obese mice significantly ameliorated glucose intolerance and improved insulin sensitivity and pyruvate tolerance independently of food intake or body weight. Among the enzymes related to glucose metabolism, both the gene expression (p<0.01) and the enzymatic activity (p<0.05) of glucokinase were increased in the liver of the KMO knockdown group as compared with controls. These results indicated that alterations in KYNA pathway influence glucose metabolism in vivo. Disclosure T. Yamamoto: None. T. Gotoda: None.

Astrocytic Mechanisms Involving Kynurenic Acid Control Δ9-Tetrahydrocannabinol-Induced Increases in Glutamate Release in Brain Reward-Processing Areas.

The reinforcing effects of Δ9-tetrahydrocannabinol (THC) in rats and monkeys, and the reinforcement-related dopamine-releasing effects of THC in rats, can be attenuated by increasing endogenous levels of kynurenic acid (KYNA) through systemic administration of the kynurenine 3-monooxygenase inhibitor, Ro 61-8048. KYNA is a negative allosteric modulator of α7 nicotinic acetylcholine receptors (α7nAChRs) and is synthesized and released by astroglia, which express functional α7nAChRs and cannabinoid CB1 receptors (CB1Rs). Here, we tested whether these presumed KYNA autoreceptors (α7nAChRs) and CB1Rs regulate glutamate release. We used in vivo microdialysis and electrophysiology in rats, RNAscope in situ hybridization in brain slices, and primary culture of rat cortical astrocytes. Acute systemic administration of THC increased extracellular levels of glutamate in the nucleus accumbens shell (NAcS), ventral tegmental area (VTA), and medial prefrontal cortex (mPFC). THC also reduced extracellular levels of KYNA in the NAcS. These THC effects were prevented by administration of Ro 61-8048 or the CB1R antagonist, rimonabant. THC increased the firing activity of glutamatergic pyramidal neurons projecting from the mPFC to the NAcS or to the VTA in vivo. These effects were averted by pretreatment with Ro 61-8048. In vitro, THC elicited glutamate release from cortical astrocytes (on which we demonstrated co-localization of the CB1Rs and α7nAChR mRNAs), and this effect was prevented by KYNA and rimonabant. These results suggest a key role of astrocytes in interactions between the endocannabinoid system, kynurenine pathway, and glutamatergic neurotransmission, with ramifications for the pathophysiology and treatment of psychiatric and neurodegenerative diseases.

Attenuating Nicotine Reinforcement and Relapse by Enhancing Endogenous Brain Levels of Kynurenic Acid in Rats and Squirrel Monkeys

The currently available antismoking medications have limited efficacy and often fail to prevent relapse. Thus, there is a pressing need for newer, more effective treatment strategies. Recently, we demonstrated that enhancing endogenous levels of kynurenic acid (KYNA, a neuroinhibitory product of tryptophan metabolism) counteracts the rewarding effects of cannabinoids by acting as a negative allosteric modulator of α7 nicotinic receptors (α7nAChRs). As the effects of KYNA on cannabinoid reward involve nicotinic receptors, in the present study we used rat and squirrel monkey models of reward and relapse to examine the possibility that enhancing KYNA can counteract the effects of nicotine. To assess specificity, we also examined models of cocaine reward and relapse in monkeys. KYNA levels were enhanced by administering the kynurenine 3-monooxygenase (KMO) inhibitor, Ro 61-8048. Treatment with Ro 61-8048 decreased nicotine self-administration in rats and monkeys, but did not affect cocaine self-administration. In rats, Ro 61-8048 reduced the ability of nicotine to induce dopamine release in the nucleus accumbens shell, a brain area believed to underlie nicotine reward. Perhaps most importantly, Ro 61-8048 prevented relapse-like behavior when abstinent rats or monkeys were reexposed to nicotine and/or cues that had previously been associated with nicotine. Ro 61-8048 was also effective in monkey models of cocaine relapse. All of these effects of Ro 61-8048 in monkeys, but not in rats, were reversed by pretreatment with a positive allosteric modulator of α7nAChRs. These findings suggest that KMO inhibition may be a promising new approach for the treatment of nicotine addiction.

Metabolic shift of the kynurenine pathway impairs alcohol and cocaine seeking and relapse.

The glutamatergic system plays a key role in the maintenance of drug use and development of drug-related conditioned behaviours. In particular, hyper-glutamatergic activity and N-methyl-D-aspartate receptor (NMDAR) activation may drive drug craving and relapse. Inhibition of kynurenine-3-monooxygenase (KMO) shifts the metabolic kynurenine pathway towards production of kynurenic acid, which leads to a reduction of glutamatergic/NMDAR activity via different mechanisms. In this study, we investigated whether drug-seeking and relapse behaviour could be modified by the metabolic shift of endogenous kynurenine pathway. An inhibitor of kynurenine-3-monooxygenase (KMO) Ro61-8048 (4 and 40mg/kg) and its prodrug JM6 (100 and 200mg/kg) were tested in two behavioural rat models for drug seeking and relapse-the alcohol deprivation effect (ADE) model in long-term alcohol-drinking rats and the model of cue-induced reinstatement of alcohol- and cocaine-seeking behaviour. Our results show that relapse-like alcohol drinking during the ADE was abolished by repeated intraperitoneal administration of Ro61-8048 and significantly reduced by its oral prodrug JM6. Cue-induced reinstatement of both alcohol- and cocaine-seeking behaviour was also abolished by administration of Ro61-8048. Pharmacological enhancement of endogenous kynurenic acid levels provides a novel treatment strategy to interfere with glutamatergic/NMDAR activity as well as with craving and relapse in alcohol-dependent patients and drug addicts.

Absence of aryl hydrocarbon receptors increases endogenous kynurenic acid levels and protects mouse brain against excitotoxic insult and oxidative stress.

L-kynurenine (Kyn) is a key element of tryptophan metabolism; it is enzymatically converted by kynurenine aminotransferase II (KAT II) to kynurenic acid (KYNA), which acts as an antagonist to the NMDA receptor-glycine site. Kyn is also an endogenous ligand of the aryl hydrocarbon receptor (AhR), a transcription factor that regulates the expression of a diverse set of genes. KYNA levels are reduced in several regions of the brain of Huntington's disease (HD) patients. The present work uses an AhR-null mouse and age-matched wild-type mice to determine the effect of the absence of AhR on KYNA availability. We found that, in AhR-null mice, there is an increase of KYNA levels in specific brain areas associated with higher expression of KAT II. Moreover, we induced an excitotoxic insult by intrastriatal administration of quinolinic acid, a biochemical model of HD, in both AhR-null and wild-type mice to evaluate the neurological damage as well as the oxidative stress caused by the lesion. The present work demonstrates that, in specific brain regions of AhR-null mice, the levels of KYNA are increased and that this induces a neuroprotective effect against neurotoxic insults. Moreover, AhR-null mice also show improved motor performance in the rotarod test, indicating a constitutive protection of striatal tissue.

Hypothermic Effects of Δ9‐THC and Nicotine in Kynurenine‐3‐Monooxygenase (KMO) Knockout Mice

Kynurenic acid, a metabolic product of the tryptophan‐kynurenine synthesis pathway, is both a competitive antagonist at the glycine co‐agonist site of NMDA receptors and a negative allosteric modulator at α7 nicotinic acetylcholine receptors (nAChRs). α7 nAChRs are important for mediating the effects of nicotine, but have also recently been implicated in the effects of Δ9‐tetrahydrocannabinol (Δ9‐THC), the primary psychoactive component of Cannabis. Transgenic mice lacking kynurenine‐3‐monooxygenase (KMO) exhibit increased levels of kynurenic acid and thus presumably decreased function of α7 nAChRs. The current study examined the extent to which the hypothermic effects of nicotine and Δ9‐THC are modified in KMO knockout mice (n=7) as compared to wild‐type C57BL/6J controls (n=7). Δ9‐THC (10 mg/kg) produced a maximum decrease in temperature of 3.0°C at 60 min; hypothermia was no longer evident at 240 min. Nicotine (1.78 mg/kg base) produced a maximum hypothermic effect of 7.2°C at 30 min; this effect was no longer evident at 90 min. The magnitude of effect and the time course of hypothermia produced by Δ9‐THC and nicotine were not significantly different between KMO knockout and wild‐type mice. These data suggest that at least some of the in vivo effects of Δ9‐THC and nicotine are not affected by differences in endogenous levels of kynurenic acid. Supported by USPHS grant DA25267 and MH090127.

Kynurenic acid, by targeting α7 nicotinic acetylcholine receptors, modulates extracellular GABA levels in the rat striatum in vivo

Kynurenic acid (KYNA) is an astrocyte-derived non-competitive antagonist of the α7 nicotinic acetylcholine receptor (α7nAChR) and inhibits the NMDA receptor (NMDAR) competitively. The main aim of the present study was to examine the possible effects of KYNA (30 - 1000nm), applied locally by reverse dialysis for 2h, on extracellular GABA levels in the rat striatum. KYNA concentration-dependently reduced GABA levels, with 300nm KYNA causing a maximal reduction to ~60% of baseline concentrations. The effect of KYNA (100nm) was prevented by co-application of galantamine (5μm), an agonist at a site of the α7nAChR that is very similar to that targeted by KYNA. Infusion of 7-chlorokynurenic acid (100nm), an NMDAR antagonist acting selectively at the glycineB site of the receptor, affected neither basal GABA levels nor the KYNA-induced reduction in GABA. Inhibition of endogenous KYNA formation by reverse dialysis of (S)-4-(ethylsulfonyl)benzoylalanine (ESBA; 1mm) increased extracellular GABA levels, reaching a peak of 156% of baseline levels after 1h. Co-infusion of 100nm KYNA abolished the effect of ESBA. Qualitatively and quantitatively similar, bi-directional effects of KYNA on extracellular glutamate were observed in the same microdialysis samples. Taken together, the present findings suggest that fluctuations in endogenous KYNA levels, by modulating α7nAChR function, control extracellular GABA levels in the rat striatum. This effect may be relevant for a number of physiological and pathological processes involving the basal ganglia.

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

Fluctuations in the endogenous levels of kynurenic acid (KYNA), a potent α7 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.

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.

Kynurenic acid attenuates NMDA-induced pial arteriolar dilation in newborn pigs

The excitatory amino acid glutamate is a potent vasodilator in the central nervous system. Glutamate-induced vasodilation is mediated primarily by N-methyl- d-aspartate (NMDA) and AMPA/kainate (KAIN) receptors. We have now tested whether two metabolites of the kynurenine pathway of tryptophan degradation acting at the NMDA receptor, the antagonist kynurenic acid (KYNA) and the agonist quinolinic acid (QUIN), are capable of modulating the dilation of pial arterioles. The closed cranial window technique was used, and changes in vessel diameter (∼100 μm) were analyzed in anesthetized newborn piglets. Topical application of NMDA (10 −4 M) or KAIN (5 × 10 −5 M) resulted in marked vasodilation (44 ± 5% and 39 ± 4%, respectively). Neither KYNA nor QUIN (both at 10 −5 to 10 −3 M) affected the vessel diameter when applied alone. Co-application of KYNA dose-dependently reduced the vasodilation caused by 10 −4 M NMDA and also attenuated the KAIN-induced response. Ten minutes of global cerebral ischemia did not modify the interaction between KAIN and KYNA. In contrast, KYNA did not affect vasodilation to hypercapnia, elicited by the inhalation of 10% CO 2. Moreover, endogenous levels of KYNA and QUIN in the cerebral cortex, hippocampus and thalamus were found to be essentially unchanged during the early reperfusion period (0.5–2 h) following an episode of cerebral ischemia. Our data are relevant for the use of drugs that target the kynurenine pathway for therapeutic interventions in cerebrovascular diseases.

PH dependence, substrate specificity and inhibition of human kynurenine aminotransferase I

Human kynurenine aminotransferase I/glutamine transaminase K (hKAT-I) is an important multifunctional enzyme. This study systematically studies the substrates of hKAT-I and reassesses the effects of pH, Tris, amino acids and alpha-keto acids on the activity of the enzyme. The experiments were comprised of functional expression of the hKAT-I in an insect cell/baculovirus expression system, purification of its recombinant protein, and functional characterization of the purified enzyme. This study demonstrates that hKAT-I can catalyze kynurenine to kynurenic acid under physiological pH conditions, indicates indo-3-pyruvate and cysteine as efficient inhibitors for hKAT-I, and also provides biochemical information about the substrate specificity and cosubstrate inhibition of the enzyme. hKAT-I is inhibited by Tris under physiological pH conditions, which explains why it has been concluded that the enzyme could not efficiently catalyze kynurenine transamination. Our findings provide a biochemical basis towards understanding the overall physiological role of hKAT-I in vivo and insight into controlling the levels of endogenous kynurenic acid through modulation of the enzyme in the human brain.

Endogenous kynurenic acid disrupts prepulse inhibition

Recent studies show that endogenous levels of kynurenic acid (KYNA) are increased in the cerebrospinal fluid of schizophrenic patients. Prepulse inhibition (PPI) of the acoustic startle reflex is an operational measure of sensorimotor gating that is reduced in neuropsychiatric disorders, such as schizophrenia. Previous studies show that administration of N-methyl-D-aspartate (NMDA) receptor antagonists, such as phencyclidine or MK-801, leads to deficits in sensorimotor gating that mimic those observed in schizophrenic patients. The present study examined the effects of the endogenous NMDA receptor antagonist KYNA on startle and PPI in rats. Elevation of endogenous brain levels of KYNA was achieved through intraperitoneal (IP) administration of kynurenine (100 mg/kg), the precursor of KYNA, or by intravenous administration of PNU 156561A (10 mg/kg). A fourfold increase in brain KYNA levels, as induced by kynurenine or PNU 156561A, significantly reduced PPI. There were no differences in startle magnitudes between control rats and drug-treated rats. The disruption of PPI was restored by administration of the antipsychotic drugs haloperidol (.2 mg/kg, IP) or clozapine (7.5 mg/kg, IP). The present results suggest that brain KYNA serves as an endogenous modulator of PPI and are consistent with the hypothesis that KYNA contributes to the pathophysiology of schizophrenia.

Inhibitory action of clozapine on rat ventral tegmental area dopamine neurons following increased levels of endogenous kynurenic acid.

The mode of action by which the atypical antipsychotic drug clozapine exerts its superior efficacy to ameliorate both positive and negative symptoms is still unknown. In the present in vivo electrophysiological study, we investigate the effects of haloperidol (a typical antipsychotic drug) and clozapine on ventral tegmental area (VTA) dopamine (DA) neurons in a situation of hyperdopaminergic activity in order to mimic tentatively a condition similar to that seen in schizophrenia. Increased DA transmission was induced by elevating endogenous levels of the N-methyl-D-aspartate receptor and alpha7(*) nicotinic receptor antagonist kynurenic acid (KYNA; by means of PNU 156561A, 40 mg /kg, i.v.). In control rats, i.v. administered haloperidol (0.05-0.8 mg/kg) or clozapine (1.25-10 mg/kg) was associated with increased firing rate and burst firing activity of VTA DA neurons. However, in rats displaying hyperdopaminergia (induced by elevated levels of KYNA), the effects of clozapine on VTA DA neurons were converted into pure inhibitory responses, including decrease in burst firing activity. In contrast, haloperidol still produced an excitatory action on VTA DA neurons in rats with elevated levels of endogenous brain KYNA. The results of the present study suggest that clozapine facilitates or inhibits VTA DA neurotransmission, depending on brain concentration of KYNA. Such an effect of clozapine may be related to its unique effect in also ameliorating negative symptoms of schizophrenia.

Increased phasic activity of dopaminergic neurones in the rat ventral tegmental area following pharmacologically elevated levels of endogenous kynurenic acid.

Kynurenic acid (KYNA) is an antagonist of ionotropic glutamate receptors, preferentially blocking the glycine-site of the N-methyl-D-aspartate (NMDA) receptor. In the present electrophysiological study, the firing pattern of dopamine (DA) neurones of rat ventral tegmental area (VTA) was investigated following pharmacologically elevated endogenous levels of KYNA by means of an inhibitor of kynurenine 3-hydroxylase (PNU 156561A). Pre-treatment with PNU 156561A (40 mg kg-1, i.v., 5-9 h) caused a threefold increase in endogenous KYNA in whole brain levels and also evoked a significant increase in firing rate and bursting activity of VTA DA neurones. Administration of D-cycloserine (2-128 mg kg-1, i.v.), a partial agonist at the glycine-site of the NMDA-receptor, was found to reverse the increase in firing rate and bursting activity as induced by elevated concentrations of KYNA. The electrophysiological effects of elevated KYNA levels were in all essential mimicked by administration of the NMDA-receptor antagonist MK 801 (0.05-1.6 mg kg-1, i.v.). Thus, the effects of elevated endogenous brain KYNA observed in the present study are likely to be carried out by NMDA receptor antagonism. In conclusion, this study shows that an increase in endogenous KYNA levels produces significant actions on the tonic afferent control of the firing pattern of VTA DA neurones. Given the psychotomimetic effects of NMDA-receptor antagonists, e.g. phencyclidine and ketamine, the state of hyperactivity of mesocorticolimbic DA system induced by elevated levels of KYNA may represent a pathophysiological condition analogous to that seen in schizophrenic patients.

Pharmacological elevation of endogenous kynurenic acid levels activates nigral dopamine neurons.

Inhibitors of kynurenine 3-hydroxylase have previously been used to increase endogenous levels of kynurenic acid, an excitatory amino acid receptor antagonist. In the present electrophysiological study PNU 156561A was utilized to elevate endogenous concentrations of kynurenic acid and subsequent effects on the firing pattern of dopamine (DA) neurons of rat substantia nigra (SN) were analyzed. Pretreatment with PNU 156561A (40 mg/kg, i.v., 5-7 h) caused a five-fold increase in endogenous kynurenic acid levels in whole brain five to seven hours after administration and also evoked a significant increase in firing rate and bursting activity of nigral DA neurons. The results of the present study show that a moderate increase in endogenous kynurenic acid levels produces significant actions on the tonic glutamatergic control of the firing pattern of nigral DA neurons, and implicate kynurenine 3-hydroxylase inhibitors as novel antiparkinsonian agents.

Pharmacologically elevated levels of endogenous kynurenic acid prevent nicotine-induced activation of nigral dopamine neurons.

Previous studies have shown that systemically administered nicotine is associated with an activation of rat midbrain dopamine neurons. The aim of the present electrophysiological study was to investigate if manipulation of brain kynurenic acid, an endogenous excitatory amino acid receptor antagonist, can affect the response of nigral dopamine neurons to nicotine. A potent inhibitor of kynurenine 3-hydroxylase, PNU 156561A (40 mg/kg, i.v., 4-7 h), was utilized to increase the levels of kynurenic acid in rat brain. This treatment, which caused a fourfold increase in brain kynurenic acid levels, abolished the increase in firing rate and burst activity of nigral dopamine neurons as induced by nicotine (25-400 microg/kg, i.v.). It is proposed that the excitation of dopamine neurons in the substantia nigra following nicotine administration is an indirect effect, mediated by glutamate release. In addition, our data highlight the role of brain kynurenic acid as a potentially important modulator of basic glutamatergic responses in brain.

Excitation of nigral dopamine neurons by the GABA A receptor agonist muscimol is mediated via release of glutamate

Previous electrophysiological studies have shown that the GABA A-receptor agonist muscimol is able to markedly increase the firing rate of rat nigral dopamine (DA) neurons. This action of the drug is paradoxical since local microiontophoretic application of the drug is associated with a clearcut inhibition of this neurons. In the present electrophysiological study, an attempt was made to analyze the mechanism of this action of the drug. Administration of muscimol (0.25–4.0 mg/kg, i.v.) was associated with a dose-dependent increase in firing rate as well as an increased bursting activity of the nigral DA neurons. Both these effects of muscimol were clearly antagonised by intravenous administration of the NMDA receptor antagonist MK 801(1 mg/kg) or by intracerebroventricular administration of the broadspectrum excitatory amino acid receptor antagonist kynurenic acid. Furthermore, pretreatment with PNU 156561A (40 mg/kg, i.v., 5–8h), a compound that raised endogenous kynurenic acid levels about 9 times, also clearly antagonised the actions of muscimol. Indeed, this treatment reversed the excitatory action of muscimol into an inhibitory effect on the nigral DA neurons. Here, we report that the excitatory action of muscimol is mediated indirectly by release of glutamate.

Nicotine‐induced excitation of locus coeruleus neurons is blocked by elevated levels of endogenous kynurenic acid

The present electrophysiological study shows that manipulation with endogenous brain kynurenic acid (KYNA) is able to affect the response of central noradrenergic neurons to nicotine. Previous studies have shown that systemically administered nicotine in low doses is associated with a marked, but short-lasting increase in the firing rate of rat noradrenergic neurons in the locus coeruleus (LC). This action of nicotine is of peripheral origin and finally mediated via a release of glutamate within the LC. KYNA is an endogenous glutamate receptor antagonist, which shows an uneven distribution in human brain. Previous studies have shown that a potent inhibitor of kynurenine 3-hydroxylase, PNU 156561A, is able to dose-dependently increase the levels of KYNA in brain. Anesthetized rats were given PNU 156561A in a dose that caused a 5-fold increase in brain KYNA levels after 3–6 hours (40 mg/kg, i.v.). This treatment was found to abolish the increase in firing rate of LC neurons induced by nicotine (25–200 μg/kg, i.v.). The results of the present study show that an increased concentration of endogenous brain KYNA is able to inhibit the activation of central noradrenergic neurons by nicotine. In addition, our results highlight the role of endogenous KYNA in brain as a potentially important modulator of brain glutamatergic responses. Synapse 37:104–108, 2000. © 2000 Wiley-Liss, Inc.

Role of kynurenines in the neurotoxic actions of kainic acid.

The neurotoxic actions of kainic acid can be partly suppressed by antagonists acting at N-methyl-D-aspartate (NMDA) receptors. The present study examined the possible role of endogenous components of the kynurenine pathway to this phenomenon. Administration of kainate (2 nmols) into the hippocampus of anaesthetized rats produced damage in the CA1 and CA3 regions. The involvement of NMDA receptors was confirmed by the ability of dizocilpine (1 mg kg(-1)) to reduce cell loss in the CA1 region from 92 to 42%. The co-administration of m-nitrobenzoylalanine (20 nmols into the hippocampus), an inhibitor of kynurenine hydroxylase and kynureninase, together with a systemic injection of the compound (100 mg kg(-1), i.p.), afforded some protection against kainate, reducing cell loss from 91 to 48%. Protection was not exerted against damage by quinolinic acid or NMDA, excluding a direct interaction between m-nitrobenzoylalanine and NMDA receptors. The protective effect of m-nitrobenzoylalanine was not prevented by glycine, which would be expected to reverse protection caused by an elevation in the levels of endogenous kynurenic acid, arguing against a major role for increased levels of kynurenic acid. The results indicate that inhibition of the kynurenine pathway offers protection against kainate-induced damage. One possible mechanism for the protection is that an increased production of quinolinic acid in the brain, possibly from glial cells and macrophages activated by the initial kainate insult, normally contributes to the local activation of NMDA receptors and thus to kainate-induced cerebral insults. This generation of endogenous quinolinic acid would be suppressed by m-nitrobenzoylalanine.

Nicotine-induced excitation of locus coeruleus neurons is blocked by elevated levels of endogenous kynurenic acid.

The present electrophysiological study shows that manipulation with endogenous brain kynurenic acid (KYNA) is able to affect the response of central noradrenergic neurons to nicotine. Previous studies have shown that systemically administered nicotine in low doses is associated with a marked, but short-lasting increase in the firing rate of rat noradrenergic neurons in the locus coeruleus (LC). This action of nicotine is of peripheral origin and finally mediated via a release of glutamate within the LC. KYNA is an endogenous glutamate receptor antagonist, which shows an uneven distribution in human brain. Previous studies have shown that a potent inhibitor of kynurenine 3-hydroxylase, PNU 156561A, is able to dose-dependently increase the levels of KYNA in brain. Anesthetized rats were given PNU 156561A in a dose that caused a 5-fold increase in brain KYNA levels after 3-6 hours (40 mg/kg, i.v. ). This treatment was found to abolish the increase in firing rate of LC neurons induced by nicotine (25-200 microg/kg, i.v.). The results of the present study show that an increased concentration of endogenous brain KYNA is able to inhibit the activation of central noradrenergic neurons by nicotine. In addition, our results highlight the role of endogenous KYNA in brain as a potentially important modulator of brain glutamatergic responses.