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R Schwarcz

Publications and source records attributed to R Schwarcz.

At least 37 records · Page 2Linked to original sources
PubMed2000

Perinatal kynurenine pathway metabolism in the normal and asphyctic rat brain.

The kynurenine pathway of tryptophan degradation contains several metabolites which may influence brain physiology and pathophysiology. The brain content of one of these compounds, kynurenic acid (KYNA), decreases precipitously around the time of birth, possibly to avoid deleterious N-methyl-D-aspartate (NMDA) receptor blockade during the perinatal period. The present study was designed to determine the levels of KYNA, the free radical generator 3-hydroxykynurenine (3-HK), and their common precursor L-kynurenine (L-KYN) between gestational day 16 and adulthood in rat brain and liver. The cerebral activities of the biosynthetic enzymes of KYNA and 3-HK, kynurenine aminotransferases (KATs) I and II and kynurenine 3-hydroxylase, respectively, were measured at the same ages. Additional studies were performed to assess whether and to what extent kynurenines in the immature brain derive from the mother, and to examine the short-term effects of birth asphyxia on brain KYNA and 3-HK levels. The results revealed that 1) the brain and liver content of L-KYN, KYNA and 3-HK is far higher pre-term than postnatally; 2) KAT I and kynurenine 3-hydroxylase activities are quite uniform between E-16 and adulthood, whereas KAT II activity rises sharply after postnatal day 14; 3) during the perinatal period, KYNA, but not L-KYN, may originate in part from the maternal circulation; and 4) oxygen deprivation at birth affects the brain content of both KYNA and 3-HK 1 h but not 24h later.

Animals↗
PubMed2000

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

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

3-Hydroxyanthranilic Acid↗
PubMed2000

Some metabotropic glutamate receptor ligands reduce kynurenate synthesis in rats by intracellular inhibition of kynurenine aminotransferase II.

Some metabotropic glutamate receptor (mGluR) ligands, such as quisqualate, L-(+)-2-amino-4-phosphonobutyric acid (L-AP4), 4-carboxy-3-hydroxyphenylglycine (4C3HPG), and L-serine-O:-phosphate (L-SOP), reduced the formation of the endogenous excitatory amino acid receptor antagonist kynurenate in brain and liver slices. The use of novel, subtype-selective mGluR agonists and antagonists excluded a role for any known mGluR subtype in this effect. The reduction of kynurenate formation was no longer observed when slices were incubated with the active mGluR ligands in the absence of extracellular Na(+). trans-Pyrrolidine-2,4-dicarboxylate (trans-PDC), a broad-spectrum ligand of Na(+)-dependent glutamate transporters, was also able to reduce kynurenate formation. Quisqualate, 4C3HPG, L-AP4, and L-SOP did not further reduce kynurenate formation in the presence of trans-PDC, suggesting that the two classes of drugs may share the same mechanism of action. Hence, we hypothesized that the active mGluR ligands are transported inside the cell and act intracellularly to reduce kynurenate synthesis. We examined this possibility by assessing the direct effect of mGluR ligands on the activity of kynurenine aminotransferases (KATs) I and II, the enzymes that transaminate kynurenine to kynurenate. In brain tissue homogenates, KAT II (but not KAT I) activity was inhibited by quisqualate, 4C3HPG, L-AP4, L-SOP, and trans-PDC. Drugs that were unable to reduce kynurenate formation in tissue slices were inactive. We conclude that some mGluR ligands act intracellularly, inhibiting KAT II activity and therefore reducing kynurenate formation. This effect should be taken into consideration when novel mGluR ligands are developed for the treatment of neurological and psychiatric diseases.

Aminobutyrates↗
PubMed2000

Neurons in layer III of the entorhinal cortex. A role in epileptogenesis and epilepsy?

A preferential lesion of neurons in layer III of the entorhinal cortex (EC) is often observed in patients suffering from temporal lobe epilepsy and in several animal models of the disease. This lesion is duplicated in rats by a focal, intra-entorhinal injection of the "indirect" excitotoxin aminooxyacetic acid (AOAA), providing a model that can be used to study the mechanisms underlying seizure-induced cell death and epilepsy. Doomed neurons in the EC and in several associated limbic structures show pathological changes within hours after the AOAA injection, but GABAergic neurons in layer III of the EC are quite resistant. This pattern of neuron loss eventually results in hippocampal and entorhinal hyperexcitability. Notably, the seizure-induced death of layer III neurons in the EC can be attenuated by eliminating the prominent excitatory input from the presubiculum. Taken together, these results suggest opportunities to target parahippocampal structures for the treatment of temporal lobe epilepsy.

Aminooxyacetic Acid↗
PubMed1999

3-Hydroxyanthranilic acid accumulation following administration of the 3-hydroxyanthranilic acid 3,4-dioxygenase inhibitor NCR-631.

In the kynurenine pathway of tryptophan metabolism, 3-hydroxyanthranilic acid is the substrate for formation of the excitotoxin quinolinic acid by 3-hydroxyanthranilic acid 3, 4-dioxygenase. This study was designed to characterize the effects on 3-hydroxyanthranilic acid after treatment with the 3-hydroxyanthranilic acid 3,4-dioxygenase inhibitor 4, 6-di-bromo-3-hydroxyanthranilic acid (NCR-631) in Sprague-Dawley rats. The blood plasma and brain concentrations of 3-hydroxyanthranilic acid were found to increase rapidly in a dose-dependent manner after gavage administration of NCR-631. However, the effect was relatively transient, with a decline in 3-hydroxyanthranilic acid levels already at 1h after NCR-631 treatment. Similar increases in plasma levels of 3-hydroxyanthranilic acid were observed following either gavage or parenteral (i.v. or s.c.) administration of NCR-631 (25 mg/kg). Only a minor enhancement of the NCR-631-induced increase in plasma 3-hydroxyanthranilic acid levels was found after sub-chronic treatment (25 mg/kg by gavage; 7 days, b.i.d.), suggesting a low propensity for altered 3-hydroxyanthranilic acid 3,4-dioxygenase activity following repeated inhibition. Administration of [14C]NCR-631 suggested 20 min initial plasma half life and an oral absorption around 50%. A dose of 250 mg/kg [14C]NCR-631 given by gavage provided plasma levels of almost 2 micromol/ml and a brain concentration of approximately 16 nmol/g, when analyzed 15 min after administration. Neither acute nor sub-chronic administration of NCR-631 caused any substantial effects on quinolinic acid levels in plasma or brain. Also, the plasma levels of kynurenic acid, another neuroactive kynurenine pathway metabolite, were unaffected by acute NCR-631 treatment. Moreover, the brain levels of the major cerebral tryptophan metabolites 5-hydroxytryptamine and 5-hydroxyindoleacetic acid remained unchanged following administration of NCR-631. Although reversible inhibition of 3-hydroxyanthranilic acid 3, 4-dioxygenase with NCR-631 in normal rats is insufficient to cause substantial changes in the levels of quinolinic acid or other important tryptophan metabolites, it causes a major accumulation of the substrate 3-hydroxyanthranilic acid.

3-Hydroxyanthranilate 3,4-Dioxygenase↗
PubMed1999

Quantitative differences in the effects of de novo produced and exogenous kynurenic acid in rat brain slices.

Kynurenic acid (KYNA) is an antagonist of (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors and it blocks the glycine site of the NMDA receptor preferentially (IC50 = 7.9 microM). KYNA is produced endogenously by transamination of its precursor L-kynurenine (L-KYN). We tested the hypothesis that effects of endogenous, de novo produced KYNA, following bath-application of L-KYN to slices, would be different than effects of commercially-synthesized (exogenous) KYNA. The ability to block spontaneous epileptiform activity, induced by lowering extracellular magnesium, was examined in area CA3 of hippocampus and the entorhinal cortex. At a concentration of 200 microM L-KYN, which produced 0.89 +/- 0.20 microM KYNA, there were fewer slices with spontaneous epileptiform activity than slices exposed to 2 microM exogenous KYNA. The results indicate a more potent neuromodulatory action of endogenous KYNA than has been previously realized.

Animals↗
PubMed1999

Indole-3-propionate: a potent hydroxyl radical scavenger in rat brain.

The hydroxyl radical scavenging activity of indole-3-propionate was evaluated by kinetic competition studies with the hydroxyl radical trapping reagent 2,2'-azino-bis-(3-ethyl-benz-thiazoline-6-sulfonic acid) (ABTS) and by measuring hydroxyl radical-initiated lipid peroxidation in the rat striatum. Using ABTS, the indole was shown to act as a potent hydroxyl radical scavenger with a rate constant of 7.8x1010 mol l-1 s-1. Hydroxyl radical-initiated lipid peroxidation, determined by measuring tissue malondialdehyde formation, was inhibited dose-dependently both in vitro and in vivo. Indole-3-propionate reacts with hydroxyl radicals at a diffusion controlled rate and can thereby provide on-site protection against the oxidative damage of biomolecules induced by these highly reactive and toxic oxygen intermediates. While it remains to be established if endogenous brain tissue levels of indole-3-propionate are sufficiently high to have a significant impact on total antioxidative capacity, the compound itself or a structurally related agent may be useful as an antioxidant adjuvant to combat hydroxyl radical-mediated oxidative stress.

Animals↗
PubMed1999

Modulation by cytokines of induction of oral tolerance to type II collagen.

OBJECTIVE: To determine whether the simultaneous administration of drugs and/or cytokines such as transforming growth factor beta (TGFbeta) can render oral tolerance to type II collagen (CII) more effective in causing resistance to collagen-induced arthritis (CIA) in mice, and to investigate whether oral tolerance can still be induced when high levels of anti-CII are present. METHODS: Tolerance was induced by intragastric feeding of low-dose CII to DBA/1 mice during a 2-week period, either before immunization with CII in Freund's complete adjuvant or after initiation of arthritis. Some mice were simultaneously injected with TGFbeta1 or with the H2 receptor agonist dimaprit. RESULTS: Both TGFbeta1 and dimaprit increased the degree of oral tolerance obtained. TGFbeta1 augmented the induction of immunoregulatory CD8 T cells, which transferred the resistance to CIA induction to normal recipients. Feeding of CII for 2 weeks, starting after the onset of arthritis, still significantly ameliorated the course of CIA. CONCLUSION: Administration of TGFbeta1 or dimaprit, both of which are believed to promote the development of immunoregulatory T cells, may reinforce induction of oral tolerance, even after the onset of arthritis.

Adjuvants, Immunologic↗
PubMed1999

Modulation and function of kynurenic acid in the immature rat brain.

Using in vivo and in vitro paradigms, the regulation and function of the brain metabolite kynurenic acid (KYNA) was examined in rats on postnatal days (PND) 7 and 14. As shown previously in adult rats, glucose removal and d-amphetamine (d-Amph) administration caused decreases in KYNA formation, while exposure to pyruvate up-regulated KYNA synthesis. The effect of glucose deprivation was substantially blunted in immature animals. In PND 14 rats, d-Amph pre-treatment exacerbated the excitotoxic effects of an intrastriatal N-methyl-D-aspartate (NMDA) injection. This potentiation was prevented by m-nitrobenzoylalanine, a kynurenine 3-hydroxylase inhibitor that also antagonized the KYNA reduction caused by d-Amph. These and additional experiments with the competitive NMDA receptor antagonist CGP 40116 indicate the existence of a functionally significant, novel high-affinity receptor for KYNA in the brain.

2-Amino-5-phosphonovalerate↗
PubMed1999

Ultrastructure and immunocytochemical distribution of GABA in layer III of the rat medial entorhinal cortex following aminooxyacetic acid-induced seizures.

Layer III of the entorhinal cortex (EC) is lesioned in patients with temporal lobe epilepsy (TLE). A similar neuropathology is also present in different animal models of TLE. For example, injection of the "indirect" excitotoxin aminooxyacetic acid (AOAA) into the EC of rats causes behavioral seizures and preferential loss of neurons in layer III of the medial EC. The animals also develop hyperexcitability of the EC and the hippocampal region CA1. To further explore the neuropathological changes within the EC, the ultrastructure and distribution of GABA-like immunoreactivity were assessed in layer III, 28 days after an intraentorhinal AOAA injection. At this time point, light microscopic preparations revealed that a large proportion of pyramidal (putative excitatory) neurons in layer III of the medial EC had degenerated, whereas GABA-immunoreactive neurons had survived. In immunogold-labeled ultrathin sections, the lesioned neuropil was found to contain morphologically intact GABA-containing neurons and nerve terminals. Pathologically swollen dendrites and electron-dense neuronal profiles were present in the lesioned sector as well. The majority of the electron-dense profiles was identified as degenerating dendritic spines that were closely apposed to strongly glutamate-immunopositive axon terminals. Thus, the entorhinal chemoarchitecture is dramatically altered following an episode of AOAA-induced epileptic seizures. One possible consequence of this pathology is a reduced "drive" of the surviving layer III GABA neurons, which in turn may cause hyperexcitability of the EC and the hippocampus. These findings may be of relevance for the genesis and spread of temporal lobe seizures.

Aminooxyacetic Acid↗
PubMed1999

Acute and chronic changes in kynurenate formation following an intrastriatal quinolinate injection in rats.

Intrastriatal injection of the endogenous excitotoxin quinolinate in experimental animals causes a lesion which duplicates many features of Huntington's disease (HD). This lesion can be prevented by a related metabolite, kynurenate. Since kynurenate levels are reduced in the HD neostriatum, a deficiency in brain kynurenate may be the cause of neuron loss in HD. In order to investigate the relationship between excitotoxic neurodegeneration and kynurenate formation, effects of a unilateral quinolinate injection on several measures of kynurenate metabolism were studied in the rat striatum and substantia nigra. Within 2 hours, quinolinate caused an approximately 100% increase in striatal kynurenate levels in the absence of changes in its bioprecursor L-kynurenine or its biosynthetic enzymes kynurenine aminotransferases (KATs) I and II. This increase was more dramatic after 2 days (+735%) and was accompanied by an increase in L-kynurenine (+182%). No change or a slight decrease in enzyme activities were detected at this time-point. More chronic excitotoxic lesions produced a substantial increase in kynurenate levels (by approximately 2-, 4- and 4-fold, respectively, after 7 days, 1 and 5 months). Lesion-induced changes in KAT II activity essentially paralleled those seen with kynurenate, whereas KAT I remained slightly decreased at all timepoints. Nigral KAT II activity was increased ipsilaterally 2 days, 1 and 5 months after the striatal quinolinate injection. Kinetic analyses, performed in the striatum 5 months after the quinolinate injection, showed an almost 3-fold decrease in Km values for KAT II in the absence of v(max) changes. These findings indicate that 1) different mechanisms regulate kynurenate production at different stages after an intrastriatal quinolinate injection; 2) an increased substrate affinity to KAT II is responsible for the elevation of kynurenate in the chronically lesioned rat striatum; and 3) qualitative differences in kynurenate metabolism exist between the HD neostriatum and the excitotoxin-lesioned rat striatum, supporting the idea that (a decrease in) kynurenate tone may play a primary role in the pathophysiology of HD.

Animals↗
PubMed1999

3-Hydroxykynurenine potentiates quinolinate but not NMDA toxicity in the rat striatum.

L-3-Hydroxykynurenine (L-3-HK) and quinolinate (QUIN) are two metabolites of the kynurenine pathway, the major route of tryptophan degradation in mammals. L-3-HK is a known generator of highly reactive free radicals, whereas QUIN is an endogenous excitotoxin acting specifically at N-methyl-D-aspartate (NMDA) receptors. This study was designed to examine possible synergistic interactions between L-3-HK and QUIN in the rat brain in vivo. Intrastriatal coinjection of 5 nmol L-3-HK and 15 nmol QUIN, i.e. doses which caused no or minimal neurodegeneration on their own, resulted in substantial neuronal loss, determined both behaviourally (apomorphine-induced rotations) and histologically (quantitative assessment of lesion size). The excitotoxic nature of the lesion was verified by tyrosine hydroxylase immunohistochemistry, showing the survival of dopaminergic striatal afferents. There was also a relative sparing of large striatal neurons, and neurodegeneration was prevented both by NMDA receptor blockade (using CGP 40116) and free radical scavenging [using N-tert-butyl-alpha-(2-sulphophenyl)-nitrone, S-PBN]. The pro-excitotoxic features of L-3-HK were especially pronounced at low QUIN doses and were not observed when QUIN was substituted by NMDA. Notably, the effect of L-3-HK was not due to its intracerebral conversion to QUIN and was duplicated by equimolar D,L-3-HK. These data indicate that an elevation of L-3-HK levels constitutes a significant hazard in situations of excitotoxic injury. Pharmacological interventions aimed at decreasing L-3-HK formation may therefore be particularly useful for the treatment of neurological diseases which are associated with an abnormally enhanced flux through the kynurenine pathway.

2-Amino-5-phosphonovalerate↗
PubMed1999

2-Oxoacids regulate kynurenic acid production in the rat brain: studies in vitro and in vivo.

This study was designed to examine the role of 2-oxoacids in the enzymatic transamination of L-kynurenine to the excitatory amino acid receptor antagonist, kynurenate, in the rat brain. In brain tissue slices incubated in Krebs-Ringer buffer with a physiological concentration of L-kynurenine, pyruvate, and several other straight- and branched-chain 2-oxoacids, substantially restored basal kynurenate production in a dose-dependent manner without increasing the intracellular concentration of L-kynurenine. All 2-oxoacids tested also reversed or attenuated the hypoglycemia-induced decrease in kynurenate synthesis, but only pyruvate and oxaloacetate also substantially restored intracellular L-kynurenine accumulation. Thus, 2-oxoacids increase kynurenate formation in the brain primarily by functioning as co-substrates of the transamination reaction. This was supported further by the fact that the nonspecific kynurenine aminotransferase inhibitors (aminooxy)acetic acid and dichlorovinylcysteine prevented the effect of pyruvate on kynurenate production in a dose-dependent manner. Moreover, all 2-oxoacids tested attenuated or prevented the effects of veratridine, quisqualate, or L-alpha-aminoadipate, which reduce the transamination of L-kynurenine to kynurenate. Finally, dose-dependent increases in extracellular kynurenate levels in response to an intracerebral perfusion with pyruvate or alpha-ketoisocaproate were demonstrated by in vivo microdialysis. Taken together, these data show that 2-oxoacids can directly augment the de novo production of kynurenate in several areas of the rat brain. 2-Oxoacids may therefore provide a novel pharmacological approach for the manipulation of excitatory amino acid receptor function and dysfunction.

2-Aminoadipic Acid↗
PubMed1998

Systemic d-amphetamine administration causes a reduction of kynurenic acid levels in rat brain.

Tissue levels of the endogenous excitatory amino acid receptor antagonist kynurenic acid (KYNA) and of its bioprecursor L-kynurenine were measured in rats of different ages after d-amphetamine administration. In adult animals, extracellular KYNA concentrations were also determined in vivo by hippocampal microdialysis. In the adult brain, d-amphetamine caused a transient, dose-dependent decrease in tissue content and extracellular levels of KYNA, reaching a nadir of approximately 70% of control values after 1 h at 5 mg/kg. Quantitatively similar decrements were observed in four different brain regions. Seven, 14 and 28-day-old pups were particularly sensitive to the drug, showing a reduction in forebrain KYNA levels to 25%, 40% and 35% of control values, respectively, 1 h after the administration of 5 mg/kg d-amphetamine. Notably, no changes in brain L-kynurenine levels and in liver L-kynurenine and KYNA concentrations were found after d-amphetamine administration. Thus, endogenous monoamines released by d-amphetamine may interfere with the transamination of L-kynurenine to KYNA specifically in the brain. These results suggest that d-amphetamine increases excitatory amino acid receptor function temporarily by reducing the levels of endogenous KYNA.

Animals↗
PubMed1998

Focal microinjection of gamma-acetylenic GABA into the rat entorhinal cortex: behavioral and electroencephalographic abnormalities and preferential neuron loss in layer III.

Neuron loss in layer III of the entorhinal cortex (EC) occurs in patients with temporal lobe epilepsy and in several animal models of the disease and may play a role in the development of spontaneously recurring seizures. This damage can be reproduced in rats by a focal microinjection of the indirect excitotoxin aminooxyacetic acid into the EC (Neurosci. Lett., 147: 185, 1992). We have now examined a similar but approximately 20 times more potent toxin, gamma-acetylenic GABA (GAG), for its ability to produce seizures and neurodegeneration in the rat EC. EEG activity was recorded continuously for 48 h after a focal injection of 4 micrograms GAG into the rat EC. Seizure episodes, spiking, and other irregularities occurred with a latency of 150 min. Behavioral abnormalities were observed in all animals and were always accompanied by EEG seizures. The behavioral changes subsided gradually, but EEG seizures continued up to 24 h after GAG treatment. Nissl and silver-stained tissue sections obtained 2-3 days after the injection of 4 micrograms GAG revealed neuron loss which preferentially affected the medial part of layer III of the EC, and caused a modest lesion in the hilar region of the ventral hippocampus. The neurodegenerative potency of GAG, in contrast to the effects of aminooxyacetic acid, was not influenced by the depth of anesthesia during surgery. A slight increase in the dose of GAG (to 5 micrograms) resulted in more severe behavioral seizures, causing generalized convulsions with salivation and loss of righting posture in 3 of 13 rats. These animals also showed a marked enlargement of the lesioned area, with substantial neuronal loss occurring in layer III of the EC, in the hilus of the dentate gyrus, and occasionally also in homotopic structures of the contralateral hemisphere. Seizure activity and lesions induced by 4 micrograms GAG were prevented by the NMDA receptor antagonist Dizolcipine (MK-801) (4 mg/kg, i.p., 10 min before and 12 h after GAG). These data support the notion of a close correlation between the occurrence of seizures and neuronal loss in layer III of the EC. Taken together, the study suggests that intraentorhinal injections of GAG may provide an advantageous model for the study of epileptogenic and epileptic mechanisms.

4-Aminobutyrate Transaminase↗
PubMed1998

Regulation of kynurenic acid levels in the developing rat brain.

Several brain-specific mechanisms control the formation of the endogenous excitatory amino acid receptor antagonist kynurenic acid (KYNA) in the adult rat brain. Two of these, dopaminergic neurotransmission and cellular energy metabolism, were examined in the brain of immature (postnatal day 7) rats. The results indicate that during the early postnatal period cerebral KYNA synthesis is exceptionally amenable to modulation by dopaminergic mechanisms but rather insensitive to fluctuations in cellular energy status. These findings may be of relevance for the role of KYNA in the function and dysfunction of the developing brain.

Animals↗