Pretreatment with L-kynurenine, the precursor to the excitatory amino acid antagonist kynurenic acid, suppresses epileptiform activity in combined entorhinal/hippocampal slices

The effects of vagus nerve stimulation on tryptophan metabolites in children with intractable epilepsy

BACKGROUND

The mechanism of action of vagus nerve stimulation (VNS) in intractable epilepsy is not entirely clarified. It is believed that VNS causes alterations in cytokines, which can lead to rebalancing the release of neurotoxic and neuroprotective tryptophan metabolites. We aimed to evaluate VNS effects on tryptophan metabolites and on epileptic seizures and investigated whether the antiepileptic effectiveness correlated with changes in tryptophan metabolism.

METHODS

Forty-one children with intractable epilepsy were included in a randomized, active-controlled, double-blind study. After a baseline period of 12 weeks, all children underwent implantation of a vagus nerve stimulator and entered a blinded active-controlled phase of 20 weeks. Half of the children received high-output (therapeutic) stimulation (n=21), while the other half received low-output (active control) stimulation (n=20). Subsequently, all children received high-output stimulation for another 19 weeks (add-on phase). Tryptophan metabolites were assessed in plasma and cerebrospinal fluid (CSF) by use of liquid chromatography-tandem mass spectrometry (LC-MS/MS) and compared between high- and low-output groups and between the end of both study phases and baseline. Seizure frequency was recorded using seizure diaries. Mood was assessed using Profile of Mood States (POMS) questionnaires.

RESULTS

Regarding tryptophan metabolites, anthranilic acid (AA) levels were significantly higher at the end of the add-on phase compared with baseline (p=0.002) and correlated significantly with improvement of mood (τ=-0.39, p=0.037) and seizure frequency reduction (τ=-0.33, p<0.01). No significant changes were found between high- and low-output groups regarding seizure frequency.

CONCLUSION

Vagus nerve stimulation induces a consistent increase in AA, a neuroprotective and anticonvulsant tryptophan metabolite. Moreover, increased AA levels are associated with improvement in mood and reduction of seizure frequency.

Entorhinal cortical defects in Tg2576 mice are present as early as 2-4 months of age

The entorhinal cortex (EC) is one of the first brain areas to display neuropathology in Alzheimer's disease. A mouse model which simulates amyloid-β (Aβ) neuropathology, the Tg2576 mouse, was used to address these early changes. Here, we show EC abnormalities occur in 2- to 4-month-old Tg2576 mice, an age before Aβ deposition and where previous studies suggest that there are few behavioral impairments. First we show, using a sandwich enzyme-linked immunosorbent assay, that soluble human Aβ40 and Aβ42 are detectable in the EC of 2-month-old Tg2576 mice before Aβ deposition. We then demonstrate that 2- to 4-month-old Tg2576 mice are impaired at object placement, an EC-dependent cognitive task. Next, we show that defects in neuronal nuclear antigen expression and myelin uptake occur in the superficial layers of the EC in 2- to 4-month-old Tg2576 mice. In slices from Tg2576 mice that contained the EC, there were repetitive field potentials evoked by a single stimulus to the underlying white matter, and a greater response to reduced extracellular magnesium ([Mg(2+)]o), suggesting increased excitability. However, deep layer neurons in Tg2576 mice had longer latencies to antidromic activation than wild type mice. The results show changes in the EC at early ages and suggest that altered excitability occurs before extensive plaque pathology.

A novel kynurenic acid analog (SZR104) inhibits pentylenetetrazole-induced epileptiform seizures. An electrophysiological study : special issue related to kynurenine

The concentration of kynurenic acid (KYNA) in the cerebrospinal fluid, which is in the nanomolar range, is known to decrease in epilepsy. The experimental data suggest that treatment with L: -KYN dose dependently increases the concentration of the neuroprotective KYNA in the brain, which itself hardly crosses the blood-brain barrier. However, it is suggested that new synthetic KYNA analogs may readily cross the blood-brain barrier. In this study, we tested the hypothesis that a new KYNA analog administered systemically in a sufficient dose results in a decreased population spike activity recorded from the pyramidal layer of area CA1 of the hippocampus, and also provides protection against pentylenetetrazole-induced epileptiform seizures.

α-methyl-L-tryptophan: mechanisms for tracer localization of epileptogenic brain regions

The purpose of this paper is to discuss the mechanisms of α-[(11)C]methyl-L-tryptophan (AMT) PET as an in vivo biomarker for detection of epileptogenic cortex. AMT was originally designed as a tracer to measure the serotonin synthesis rate. This tracer was first applied in patients with medically refractory epilepsy in an attempt to detect changes in serotonin synthesis based upon reports of increased serotonergic innervation in cortical specimens obtained following epilepsy surgery. The first group of epilepsy patients undergoing AMT PET scans were patients with tuberous sclerosis complex. Studies of brain tissue subsequent to epilepsy surgery in these patients with tuberous sclerosis complex implicated the kynurenine pathway of tryptophan metabolism as a primary mechanism of increased brain tissue retention of AMT in epileptogenic brain regions, rather than alterations in serotonin synthesis. Kinetic analyses of AMT in brain tumors indicate changes in tryptophan transport and tissue retention in other pools as well. These studies indicate that AMT PET may be a biomarker of immune activation in the epileptogenic process.

Neuroprotection with a new kynurenic acid analog in the four-vessel occlusion model of ischemia

Global forebrain ischemia results in damage to the pyramids in the CA1 hippocampal subfield, which is particularly vulnerable to excitotoxic processes. Morphological and functional disintegration of this area leads to a cognitive dysfunction and neuropsychiatric disorders. Treatment with N-methyl-d-aspartate receptor antagonists is a widely accepted method with which to stop the advance of excitotoxic processes and concomitant neuronal death. From a clinical aspect, competitive glycine- and polyamine-site antagonists with relatively low affinity and moderate side-effects are taken into account. Endogenous kynurenic acid acts as an antagonist on the obligatory co-agonist glycine site, and has long been at the focus of neuroprotective trials. In the present study, we estimated the neuroprotective capability of a novel kynurenic acid analog in transient global forebrain ischemia, measuring the rate of hippocampal CA1 pyramidal cell loss and the preservation of long-term potentiation at Schaffer collateral-CA1 synapses. The neuroprotective potential was reflected by a significantly diminished hippocampal CA1 cell loss and preserved long-term potentiation expression. The neuroprotective effect was robust in the event of pretreatment, and also when the drug was administered at the time of reperfusion. This result is beneficial since a putative neuroprotectant proven to be effective as post-treatment is of much greater benefit.

Imaging of serotonin mechanisms in epilepsy

Advances in positron emission tomography (PET) techniques have allowed the measurement and imaging of neurotransmitter synthesis, transport, and receptor binding to be performed in vivo. With regard to epileptic disorders, imaging of neurotransmitter systems not only assists in the identification of epileptic foci for surgical treatment, but also provides insights into the basic mechanisms of human epilepsy. Recent investigative interest in epilepsy has focused on PET imaging of tryptophan metabolism, via the serotonin and kynurenine pathways, as well as on imaging of serotonin receptors. This review summarizes advances in PET imaging and how these techniques can be applied clinically for epilepsy treatment.

Time course of changes in the concentration of kynurenic acid in the brain of pentylenetetrazol-kindled rats

The time response of changes in the brain concentration of kynurenic acid (KYNA) was examined in rats subjected to the pentylenetetrazol (PTZ)-induced kindling of seizures (n=32). The development of seizures was accompanied by a progressive decrease in KYNA concentration in the caudate putamen, entorhinal cortex, piriform cortex, amygdala and hippocampus. A single injection of PTZ (35 mg/kg i.p.--the dose used in the kindling experiment, n=7) caused a much less pronounced KYNA depletion, with different structures affected: the nucleus accumbens, piriform cortex and amygdala. The comparison of KYNA concentration in rats subjected to the kindling of seizures with that in animals given a single, proconvulsive, dose of PTZ (55 mg/kg, n=7) showed that the kindling itself, rather than the occurrence of a fit of seizures, was responsible for the depletion of KYNA in the hippocampus and caudate putamen. Another control experiment showed that neither single nor repeated saline injections caused significant changes in KYNA concentration. The data indicate that changes in the brain concentration of an endogenous inhibitory neurotransmitter, KYNA, undergo selective modulation in the course of a kindling of seizures. This suggests that the depletion of KYNA within the hippocampus may be directly related to the development of kindled seizures in this model of epilepsy.

Kynurenines, Parkinson's disease and other neurodegenerative disorders: preclinical and clinical studies

The kynurenine pathway is the main pathway of tryptophan metabolism. L-kynurenine is a central compound of this pathway since it can change to the neuroprotective agent kynurenic acid or to the neurotoxic agent quinolinic acid. The break-up of these endogenous compounds' balance can be observable in many disorders. It can be occur in neurodegenerative disorders, such as Parkinson's disease, Huntington's and Alzheimer's disease, in stroke, in epilepsy, in multiple sclerosis, in amyotrophic lateral sclerosis, and in mental failures, such as schizophrenia and depression. The increase of QUIN concentration or decrease of KYNA concentration could enhance the symptoms of several diseases. According to numerous studies, lowered KYNA level was found in patients with Parkinson's disease. It can be also noticeable that KYNA-treatment prevents against the QUIN-induced lesion of rat striatum in animal experiments. Administrating of KYNA can be appear a promising therapeutic approach, but its use is limited because of its poorly transport across the blood-brain barrier. The solution may be the development of KYNA analogues (e.g. glucoseamine-kynurenic acid) which can pass across this barrier and disengaging in the brain, then KYNA can exert its neuroprotective effects binding at the excitatory glutamate receptors, in particular the NMDA receptors. Furthermore, it seems hopeful to use kynurenine derivatives (e.g. 4-chloro-kynurenine) or enzyme inhibitors (e.g. Ro-61-8048) to ensure an increased kynurenic acid concentration in the central nervous system.

Neuroepileptic correlates of autistic symptomatology in tuberous sclerosis

Tuberous sclerosis is a genetic condition that is strongly associated with the development of an autism spectrum disorder. However, there is marked variability in expression, and only a subset of children with tuberous sclerosis develop autism spectrum disorder. Clarification of the mechanisms that underlie the association and variability in expression will potentially throw light on the biological processes involved in the etiology of idiopathic forms of autism spectrum disorder. Current evidence indicates that the likelihood of a child with tuberous sclerosis developing an autism spectrum disorder is greater if the child has a mutation in the TSC2 gene, although autism can and does develop in children with TSC1 mutations. The likelihood is also greater if the child has early-onset infantile spasms that are difficult to control, especially if there is an epileptiform focus in the temporal lobes. The emerging evidence is consistent with the notion that early onset electrophysiological disturbances within the temporal lobes (and perhaps other locations) has a deleterious effect on the development and establishment of key social cognitive representations concerned with processing social information, perhaps especially from faces. However, alternative mechanisms to account for the findings cannot yet be ruled out. Future research will have to employ prospective longitudinal designs and treatment trials to clarify the processes involved.

Kynurenine administered together with probenecid markedly inhibits pentylenetetrazol-induced seizures. An electrophysiological and behavioural study

The kynurenine pathway converts tryptophan into various compounds, including l-kynurenine, which in turn can be converted to the excitatory amino acid receptor antagonist kynurenic acid, which may therefore serve as a protective agent in such neurological disorders as epileptic seizures. Kynurenic acid, however, has a very limited ability to cross the blood-brain barrier, whereas kynurenine passes the barrier easily. In this study, we tested the hypothesis that kynurenine administered systemically together with probenecid, which inhibits kynurenic acid excretion from the cerebrospinal fluid, results in an increased level of kynurenic acid in the brain that is sufficiently high to provide protection against the development of pentylentetrazol-induced epileptic seizures. CA3 stimulation-evoked population spike activity was recorded from the pyramidal layer of area CA1 of the rat hippocampus, and in another series of behavioural experiments, water maze and open-field studies were carried out to test the presumed protective effect of kynurenine + probenecid pre-treatment against pentylenetetrazol-induced seizures. This study has furnished the first electrophysiological proof that systemic kynurenine (300 mg/kg, i.p.) and probenecid (200 mg/kg, i.p.) administration protects against pentylenetetrazol-induced (60 mg/kg, i.p.) epileptic seizures.

6-Hydroxykynurenic acid and kynurenic acid differently antagonise AMPA and NMDA receptors in hippocampal neurones

6-Hydroxykynurenic acid (6-HKA), a derivative of kynurenic acid (KYNA) extracted from Ginkgo biloba leaves, was tested for its putative glutamate receptor (GluR) antagonism in comparison to the scaffold substance. The patch-clamp method together with fast-application techniques were used to estimate inhibition by 6-HKA and KYNA of agonist binding at NMDA and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (NMDARs and AMPARs) of CA1 pyramidal neurones. 6-Hydroxykynurenic acid proved to be a low-affinity antagonist. When comparing with KYNA, 6-HKA was less potent at NMDARs (IC(50) = 136 versus 59 microM), but showed a higher affinity to AMPARs (K(B) = 22 versus 172 microM). The replacement of 6-HKA and KYNA by glutamate was investigated on outside-out patches. Both antagonists competitively inhibited AMPAR responses and displayed fast unbinding kinetics, but the derivative was significantly slower displaced than KYNA (tau = 1.63 versus 1.22 ms). Our findings demonstrate that 6-hydroxylation considerably changes the pharmacological profile of KYNA. Among the 6-derivatives of KYNA, 6-HKA shows the highest affinity to AMPARS: Despite its relatively low lipophily, these properties might be of clinical relevance under conditions that compromise the integrity of the blood-brain barrier. Furthermore, 6-HKA should be a useful tool to analyse glutamate-mediated synaptic responses.

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.

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.

Tryptophan metabolism and brain function: focus on kynurenine and other indole metabolites

The synthesis of NAD (or NADP) from tryptophan involves a series of enzymes and the formation of a number of intermediates which are collectively called 'kynurenines.' In the late 1970s and early 1980s, it became clear that intraventricular administration of several 'kynurenines' could cause convulsions and that one of the 'kynurenines,' quinolinic acid, was an agonist of a sub-population of NMDA receptors and caused excitotoxic neuronal death. A related metabolite, kynurenic acid, could, on the other hand, reduce excitotoxin-induced neuronal death by antagonising ionotropic glutamate receptors. Since then, modifications in quinolinic and kynurenic acid synthesis have been proposed as a pathogenetic mechanism in Huntington's chorea and epilepsy. It was subsequently shown that a robust activation of the kynurenine pathway and a large accumulation of quinolinic acid in the central nervous system occurred in several inflammatory neurological disorders. More recently, it has been shown that 3OH-kynurenine or 3OH-anthranilic acid, two other kynurenine metabolites, may cause either apoptotic or necrotic neuronal death in cultures and that inhibitors of kynurenine hydroxylase may reduce neuronal death in in vitro and in vivo models of brain ischaemia or excitotoxicity. Finally, it has been reported that indole metabolites, indirectly linked to the kynurenine pathway, are able to modify neuronal function and animal behaviour by interacting with voltage-dependent Na+ channels. Oxindole, one of these metabolites, has sedative and anticonvulsant properties and accumulates in the blood and brain when liver function is impaired. In conclusion, a number of metabolites affecting brain function originate from tryptophan metabolism. Selective inhibitors of their forming enzymes may be useful to understand their role in physiology or as therapeutic agents in pathology.

Interference with cellular energy metabolism reduces kynurenic acid formation in rat brain slices: reversal by lactate and pyruvate

This study was designed to investigate the role of cellular energy metabolism in the de novo formation of the endogenous excitatory amino acid receptor antagonist, kynurenic acid. Using rat cortical tissue slices, the roles of glucose transport, glycolysis, tricarboxylic acid cycle intermediates and oxidative phosphorylation were studied. Inhibition of glucose utilization resulted in quantitatively similar decreases in kynurenine uptake, kynurenic acid production and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction, a marker of mitochondrial activity. The end product of glycolysis, pyruvate, as well as lactate, attenuated all three deficits. Pyruvate also significantly increased kynurenic acid formation in normal brain slices without affecting kynurenine uptake. Oxaloacetate and alpha-ketoglutarate (tricarboxylic acid cycle intermediates) were the only compounds tested which were capable of duplicating the effects of pyruvate, indicating that 2-oxoacids can stimulate kynurenic acid synthesis by acting as aminoacceptors in the enzymatic transamination of kynurenine. When the mitochondrial electron transport chain was blocked by specific inhibitors, coincubation with succinate restored the rate of MTT formazan formation to normal (except in the case of 3-nitropropionic acid), yet failed to prevent the resulting reduction in kynurenic acid synthesis. Conversely, pyruvate increased kynurenic acid production in the presence of all inhibitors (except cyanide), but did not attenuate the reduction in kynurenine uptake and MTT formazan formation. Taken together, these results demonstrate that interference with cellular energy metabolism causes mechanistically diverse, pronounced reductions in the cerebral neosynthesis of kynurenic acid, and that 2-oxoacids and lactate can effectively reverse most of these detrimental effects.

Acute ammonia treatment in vitro and in vivo inhibits the synthesis of a neuroprotectant kynurenic acid in rat cerebral cortical slices

The synthesis of kynurenic acid (KYNA) from kynurenine was measured in the cerebral cortical slices. In vitro, ammonium acetate at the subtoxic to toxic concentration range from 1 mM to 10 mM dose-dependently inhibited KYNA synthesis (IC50=2.99 mM). Ammonia treatment in vivo decreased KYNA synthesis by 30%. These results suggest that impaired neuroprotection exerted by KYNA might be a potential contributor to the glutamate receptor-mediated aspect of acute ammonia neurotoxicity.

Tryptophan and its metabolite, kynurenine, stimulate expression of nerve growth factor in cultured mouse astroglial cells

Effects of L-tryptophan and its metabolites were evaluated on expression of nerve growth factor (NGF) in primary culture of mouse astroglial cells. L-Tryptophan produced concentration-dependent increases in accumulation of NGF transcripts in the cells. L-Kynurenine, a metabolite of the kynurenine pathway, markedly increased the levels of mRNAs for NGF, the maximal increases (4-5 fold) occurred at its dose of 1 microM. Kynurenine-induced increase in mRNA levels for NGF occurred as early as 1 h after the addition of the compound, peaked at 4 h and declined thereafter. In contrast to kynurenine, other tryptophan metabolites such as quinolinic acid, kynurenic acid and serotonin had little effect on the levels of NGF mRNA.