Abnormally high activity of 3-hydroxyanthranilate 3,4-dioxygenase in brain of epilepsy-prone El mice

Expression of indoleamine 2,3-dioxygenase and production of quinolinic acid by human microglia, astrocytes, and neurons

There is good evidence that the kynurenine pathway (KP) and one of its end products, quinolinic acid (QUIN) play a role in the pathogenesis of several major neurological diseases. While QUIN has been shown to be produced in neurotoxic concentrations by macrophages and microglia, the capacity of astrocytes and neurons to produce QUIN is controversial. Using interferon gamma (IFN-gamma)-stimulated primary cultures of human mixed brain cells, we assayed expression of the KP regulatory enzyme indoleamine 2,3-dioxygenase (IDO) and QUIN production by immunocytochemistry. Using IFN-gamma-stimulated purified cultures of neurons, astrocytes, microglia and macrophages, we studied IDO expression by RT-PCR and production of QUIN using mass spectrometry. We found that astrocytes, neurons, and microglia expressed IDO but only microglia were able to produce detectable amounts of QUIN. However, astrocytes and neurons had the ability to catabolize QUIN. This study also provides the first evidence of IDO expression and lack of production of QUIN in culture of primary human neurons.

Kynurenines in the CNS: from endogenous obscurity to therapeutic importance

In just under 20 years the kynurenine family of compounds has developed from a group of obscure metabolites of the essential amino acid tryptophan into a source of intensive research, with postulated roles for quinolinic acid in neurodegenerative disorders, most especially the AIDS-dementia complex and Huntington's disease. One of the kynurenines, kynurenic acid, has become a standard tool for use in the identification of glutamate-releasing synapses, and has been used as the parent for several groups of compounds now being developed as drugs for the treatment of epilepsy and stroke. The kynurenines represent a major success in translating a basic discovery into a source of clinical understanding and therapeutic application, with around 3000 papers published on quinolinic acid or kynurenic acid since the discovery of their effects in 1981 and 1982. This review concentrates on some of the recent work most directly relevant to the understanding and applications of kynurenines in medicine.

Increased expression of 3-hydroxyanthranilate 3,4-dioxygenase gene in brain of epilepsy-prone El mice

The El mouse is an established animal model for human epilepsy. We previously reported that the level of quinolinic acid (QUIN), an excitotoxin, was high in the brain of epilepsy-prone El mice and that the increased production of QUIN was secondary to an increased activity of 3-hydroxyanthranilate 3,4-dioxygenase (3-HAO, EC 1.13.11. 6) in the brains of these mice. In this study, we cloned and sequenced the cDNA for 3-HAO and showed that its expression in the brain of El mice was higher than that of control ddY mice. These results suggest that a genetic defect leading to derepression of the 3-HAO gene expression in the brain may be involved in the pathogenesis for the epileptic diseases of El mice.

Paroxysmal discharges in the EL mouse, a genetic model of epilepsy

The EL/Suz (EL) mouse is a strain that is highly susceptible to convulsive seizures after repeated sensory stimulation. Its control strain, DDY/Jc1 (DDY), is less susceptible under similar conditions. The seizure prone phenotype is the result of differences at several genetic loci. In vivo electrical recordings from the seizure prone EL mouse brain have shown that the appearance of abnormal discharges in the hippocampus are critical to the onset of generalized seizures, indicating that the hippocampus plays an important role in EL mouse seizure activity. In the present study, electrophysiological differences between EL and DDY mice (9-15 weeks of age) were examined by comparing field potentials recorded from the dentate granule cell layer of hippocampal brain slices from mice that had not been stimulated to induce seizures. In control physiological solution, no significant differences were observed in characteristics of perforant path evoked field potentials or in paired pulse depression of evoked field potentials using 20 to 300 ms interstimulus intervals. After 60 min of disinhibition following bicuculline (10 microM) exposure, however, prolonged large amplitude potentials, paroxysmal discharges, were evoked by perforant path stimulation in the dentate gyrus of EL mice but were absent in the DDY strain. Paroxysmal discharges were curtailed by APV and were similar to responses recorded from the dentate gyrus in hippocampal brain slices from temporal lobe epileptic patients. The field response to hilar stimulation was identical in both strains and was composed of a single population spike before and after bicuculline exposure. Mossy fiber terminals were not present in the molecular layer of either strain. We propose that the mechanisms leading to a greater likelihood of paroxysmal discharge generation in EL mouse may be important in the development and/or generation of epileptic seizures in this mouse strain and may be a significant phenotypic difference between the EL mouse and its parent strain.

Peripheral-type benzodiazepine receptors in association with epileptic seizures in EL mice

Peripheral-type benzodiazepine receptors (PBR) in the brain were studied in association with epileptic seizures using EL mice, an animal model of epilepsy, and DDY mice as controls. Ro 5-4864 (i.p.), a specific agonist for PBR, elicited tonic-clonic convulsions in EL mice 2.6-times more potently than in DDY mice with CD50s of 11.9 and 31.2 mg/kg for EL and DDY mice, respectively. In contrast, pentylenetetrazole (i.p.) exerted convulsant actions on EL and DDY mice in a less differential way with CD50s of 29.2 and 48.1 mg/kg for EL and DDY mice, respectively. PK 11195 (i.v.), a specific antagonist for PBR, raised seizure thresholds of EL mice at a dose of 2 mg/kg. Binding assay revealed a 50% higher density of [3H]Ro 5-4864 binding sites in the mitochondrial fraction isolated from the cerebrum of EL mice in comparison with DDY mice. Similarly, a 40% higher density of [3H]flunitrazepam binding was observed in the mitochondrial fraction of EL mice. The results support the hypothesis that PBR, particularly those associated with mitochondria, are involved in the pathogenesis of epileptic seizures in EL mice.

Differential expression of the astrocytic enzymes 3-hydroxyanthranilic acid oxygenase, kynurenine aminotransferase and glutamine synthetase in seizure-prone and non-epileptic mice

Previous investigations in seizure-prone mice have suggested that an abnormally elevated production of the astrocyte-derived neuroexcitant, quinolinic acid (QUIN), plays a role in seizure susceptibility. In order to evaluate further the role of QUIN metabolism in genetic murine seizure models, the activities of its biosynthetic enzyme 3-hydroxyanthranilic acid oxygenase (3HAO), and of two other astrocytic enzymes, kynurenine aminotransferase (KAT) and glutamine synthetase (GS), were measured in the brains of seizure-prone EL and DBA/2 mice and two non-epileptic strains (BALB/c and Swiss-Webster). 3HAO activity was found to be markedly higher in both EL and DBA/2 mice than in the non-epileptic strains in all brain regions examined. The activity of 3HAO was not modified by the tossing procedure employed to promote seizures in EL mice. While some strain differences were noted in the activities of KAT and GS, these enzymes did not distinguish seizure-prone from the non-epileptic mice. In order to delineate better the relationship between glial activation and 3HAO, KAT and GS, further studies were performed in the ibotenate-lesioned hippocampus. In mice (but not in rats), the activity of 3HAO was selectively increased in gliotic tissue. These data demonstrate substantial species and strain differences in astroglial enzymes and in their response to brain injury. The observation of widespread abnormally high 3HAO activity in two distinct seizure-prone mouse strains strengthens the hypothesis that enhanced production of QUIN contributes to seizure susceptibility in mice.

Kynurenine pathway enzymes in brain: responses to ischemic brain injury versus systemic immune activation

Accumulation of L-kynurenine and quinolinic acid (QUIN) in the brain occurs after either ischemic brain injury or after systemic administration of pokeweed mitogen. Although conversion of L-[13C6]tryptophan to [13C6]-QUIN has not been demonstrated in brain either from normal gerbils or from gerbils given pokeweed mitogen, direct conversion in brain tissue does occur 4 days after transient cerebral ischemia. Increased activities of enzymes distal to indoleamine-2,3-dioxygenase may determine whether L-kynurenine is converted to QUIN. One day after 10 min of cerebral ischemia, the activities of kynureninase and 3-hydroxy-3,4-dioxygenase were increased in the hippocampus, but local QUIN levels and the activities of the indoleamine-2,3-dioxygenase and kynurenine-3-hydroxylase were unchanged. By days 2 and 4 after ischemia, however, the activities of all these enzymes in the hippocampus as well as QUIN levels were significantly increased. Kynurenine aminotransferase activity in the hippocampus was unchanged on days 1 and 2 after ischemia but was decreased on day 4, at a time when local kynurenic acid levels were unchanged. A putative precursor of QUIN, [13C6]anthranilic acid, was not converted to [13C6]QUIN in the hippocampus of either normal or 4-day post-ischemic gerbils. Gerbil macrophages stimulated by endotoxin in vitro converted L-[13C6]tryptophan to [13C6]QUIN. Kinetic analysis of kynurenine-3-hydroxylase activity in the cerebral cortex of postischemic gerbils showed that Vmax increased, without changes in Km. Systemic administration of pokeweed mitogen increased indoleamine-2,3-dioxygenase and kynureninase activities in the brain without significant changes in kynurenine-3-hydroxylase or 3-hydroxyanthranilate-3,4-dioxygenase activities. Increases in kynurenine-3-hydroxylase activity, in conjunction with induction of indoleamine-2,3-dioxygenase, kynureninase, and 3-hydroxyanthranilate-3,4-dioxygenase in macrophage infiltrates at the site of brain injury, may explain the ability of postischemic hippocampus to convert L-[13C6]tryptophan to [13C6]QUIN.

High levels of quinolinic acid in brain of epilepsy-prone E1 mice

Quinolinic acid (QUIN) may act.as an excitotoxin when it is abundant in the brain. We have shown previously that the activity of 3-hydroxyanthranilate 3,4-dioxygenase, a QUIN-synthesizing enzyme, was abnormally high in the brains of epilepsy-prone E1 mice as compared with that of ddY mice. Here, we estimated the QUIN contents in the brains of these mice. The results showed that the basal QUIN content in the cerebral cortex of E1 mice was twice as high as that of ddY mice. Systemic injection of 400 mumol/kg body weight of L-tryptophan (L-Trp) increased the cortical levels of QUIN in both E1 mice and ddY mice by 189% and 118%, respectively. Administration of 400 mumol/kg each of L-threonine and D,L-methionine had no appreciable effect on the L-Trp-caused increase in the cortical QUIN levels. Co-administration of 5-fluorotryptophan or 5-methyltryptophan, tryptophan analogs, with L-Trp did not reduce but rather enhanced the cortical QUIN levels (by 18% and 92%, respectively). No significant change in the cortical QUIN concentrations was observed with injection of 2 mg/kg body weight of E. coli lipopolysaccharide (LPS) in E1 mice. However, injection of L-Trp in the LPS-treated E1 mice produced a more marked increase in the cortical QUIN levels than that injected with L-Trp alone. These results suggest that the brain QUIN contents of E1 mice are dependent not only on the activity of QUIN-synthesizing enzyme but also on the rate of flux of its substrate, L-Trp or its metabolite(s), in the brain.

Kynurenine pathway enzymes in a rat model of chronic epilepsy: immunohistochemical study of activated glial cells

The kynurenine pathway metabolites quinolinic acid and kynurenic acid have been hypothetically linked to the occurrence of seizure phenomena. The present immunohistochemical study reports the activation of astrocytes containing three enzymes responsible for the metabolism of quinolinic acid and kynurenic acid in a rat model of chronic epilepsy. Rats received 90 min of patterned electrical stimulation through a bipolar electrode stereotaxically positioned in one hippocampus. This treatment induces non-convulsive limbic status epilepticus that leads to chronic, spontaneous, recurrent seizures. One month after the status epilepticus, the rats showed neuronal loss and gliosis in the piriform cortex, thalamus, and hippocampus, particularly on the side contralateral to the stimulation. Astrocytes containing the kynurenic acid biosynthetic enzyme (kynurenine aminotransferase) and the enzymes for the biosynthesis and degradation of quinolinic acid (3-hydroxyanthranilic acid oxygenase and quinolinic acid phosphoribosyltransferase, respectively) became highly hypertrophied in brain areas where neurodegeneration occurred. Detailed qualitative and quantitative analyses were performed in the hippocampus. In CA1 and CA3 regions, the immunostained surface area of reactive astrocytes increased up to five-fold as compared to controls. Enlarged cells containing the three enzymes were mainly observed in the stratum radiatum, whereas the stratum pyramidale, in which neuronal somata degenerated, showed relatively fewer reactive glial cells. Hypertrophied kynurenine aminotransferase- and 3-hydroxyanthranilic acid oxygenase-immunoreactive cells were comparable in their morphology and distribution pattern. In contrast, reactive quinolinic acid phosphoribosyl transferase-positive glial cells displayed diversified sizes and shapes. Some very large quinolinic acid phosphoribosyl transferase-immunoreactive cells were noticed in the molecular layer of the dentate gyrus. In the hippocampus, the number of immunoreactive glial cells increased in parallel to the hypertrophic responses. In addition, pronounced increases in immunoreactivities, associated with hypertrophied astrocytes, occurred around lesioned sites in the thalamus and piriform cortex. These findings indicate that kynurenine metabolites derived from glial cells may play a role in chronic epileptogenesis.