Comparison of kynurenic acid and 2-APV suppression of epileptiform activity in rat hippocampal slices

An expanding range of targets for kynurenine metabolites of tryptophan

The kynurenine pathway of tryptophan metabolism accounts for most of the tryptophan that is not committed to protein synthesis and includes compounds active in the nervous and immune systems. Kynurenine acts on the aryl hydrocarbon receptor, affecting the metabolism of xenobiotics and promoting carcinogenesis. Quinolinic acid is an agonist at N-methyl-D-aspartate receptors (NMDARs), but is also pro-oxidant, has immunomodulatory actions, and promotes the formation of hyperphosphorylated tau proteins. Kynurenic acid blocks NMDARs and α7-homomeric nicotinic cholinoceptors and is also an agonist at the orphan G-protein-coupled receptor GPR35. 3-Hydroxykynurenine and 3-hydroxyanthranilic acid have pronounced redox activity and regulate T cell function. Cinnabarinic acid can activate metabotropic glutamate receptors. This review highlights the increasing range of molecular targets for components of the kynurenine pathway in both the nervous and immune systems in relation to their relevance to disease and drug development.

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.

Kynurenic acid blocks nicotinic synaptic transmission to hippocampal interneurons in young rats

The tryptophan metabolite kynurenic acid can block glutamate at ionotropic receptors, but recent evidence suggests a more potent antagonistic action at alpha7 nicotinic receptors for acetylcholine on cultured neurons. The present study examines activity of kynurenic acid at those nicotinic receptors, which mediate cholinergic neurotransmission onto interneurons in the rat hippocampus. Intracellular recordings were made from pyramidal cells and interneurons in the presence of atropine, bicuculline methobromide, (3-aminopropyl)(diethoxymethyl)-phosphinic acid [CGP35348, to block gamma-aminobutyric acid (GABA)(B) receptors] and 3-tropanyl-3,5-dichlorobenzoate (MDL 72222, to block 5-HT3 receptors). In the added presence of glutamate antagonists 2-amino-5-phosphono-pentanoic acid and 6-cyano-7-nitroquinoxaline-2,3-dione, interneurons exhibited a residual excitatory postsynaptic potential (EPSP) that could be blocked by the nicotinic alpha7 receptor blocker methyl-lycaconitine, but not by dihydro-beta-erythroidine which blocks alpha4beta2 receptors. Kynurenic acid reduced the amplitude of these EPSPs with an EC50 of 136 microM. The amplitudes of nicotinic spontaneous miniature EPSPs were also reduced by methyl-lycaconitine and kynurenic acid. The results show that kynurenic acid is more potent in blocking nicotinic EPSPs compared with the full, glutamate-mediated EPSPs, but it was substantially less potent than has been reported in cultures, possibly because of differences in the accessibility of synaptic and extrasynaptic receptors. It is suggested that blockade of nicotinic synaptic transmission may be relevant to the actions of kynurenic acid in the hippocampus, but that in the intact brain this activity is likely to be comparable in importance to the blockade of glutamate-mediated transmission.

Inhibition of calpain and caspase-3 prevented apoptosis and preserved electrophysiological properties of voltage-gated and ligand-gated ion channels in rat primary cortical neurons exposed to glutamate

Glutamate toxicity in traumatic brain injury, ischemia, and Huntington's disease causes cortical neuron death and dysfunction. We tested the efficacy of calpain and caspase-3 inhibitors alone and in combination to prevent neuronal death and preserve electrophysiological functions in rat primary cortical neurons following glutamate exposure. Cortical neurons exposed to 0.5 microM glutamate for 24 h committed mostly apoptotic death as determined by Wright staining and ApopTag assay. Levels of expression, formation of active forms, and activities of calpain and caspase-3 were increased following glutamate exposure. Also, in situ double labeling identified conformationally active caspase-3-p20 fragment and chromatin condensation in apoptotic neurons. Pretreatment of cortical neurons with 0.2 microM N-benzyloxylcarbonyl-Leu-Nle-aldehyde (calpain-specific inhibitor) and 100 microM N-benzyloxylcarbonyl-Asp(OCH3)-Glu(OCH3)-Val-Asp(OCH3)-fluoromethyl ketone (caspase-3-specific inhibitor) provided strong neuroprotection. Standard patch-clamp techniques were used to measure the whole-cell currents associated with Na+ channels, N-methyl-D-aspartate receptors, and kainate receptors. The lack of a change in capacitance indicated that neurons treated with inhibitor(s) plus glutamate did not undergo apoptotic shrinkage and maintained the same size as the control neurons. Whole-cell currents associated with Na+ channels, N-methyl-D-aspartate receptors, and kainate receptors were similar in amplitude and activation/inactivation kinetics for cells untreated and treated with inhibitor(s) and glutamate. Spontaneous synaptic activity as observed by miniature end-plate currents was also similar. Prevention of glutamate-induced apoptosis by calpain and caspase-3 inhibitors preserved normal activities of crucial ion channels such as Na+ channels, N-methyl-D-aspartate receptors, and kainate receptors in neurons. Our studies strongly imply that calpain and caspase-3 inhibitors may also provide functional neuroprotection in the animal models of traumatic brain injury and neurodegenerative diseases.

Endogenous kynurenines as targets for drug discovery and development

The kynurenine pathway is the main pathway for tryptophan metabolism. It generates compounds that can modulate activity at glutamate receptors and possibly nicotinic receptors, in addition to some as-yet-unidentified sites. The pathway is in a unique position to regulate other aspects of the metabolism of tryptophan to neuroactive compounds, and also seems to be a key factor in the communication between the nervous and immune systems. It also has potentially important roles in the regulation of cell proliferation and tissue function in the periphery. As a result, the pathway presents a multitude of potential sites for drug discovery in neuroscience, oncology and visceral pathology.

The pharmacological manipulation of glutamate receptors and neuroprotection

The overactivation of glutamate receptors is a major cause of Ca(2+) overload in cells, potentially leading to cell damage and death. There is an abundance of agents and mechanisms by which glutamate receptor activation can be prevented or modulated in order to control these effects. They include the well-established, competitive and non-competitive antagonists at the N-methyl-D-aspartate (NMDA) receptors and modulators of desensitisation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors. More recently, it has emerged that some compounds can act selectively at different subunits of glutamate receptors, allowing a differential blockade of subtypes. It is also becoming clear that a number of endogenous compounds, including purines, can modify glutamate receptor sensitivity. The kynurenine pathway is an alternative but distinct pathway to the generation of glutamate receptor ligands. The products of tryptophan metabolism via the kynurenine pathway include both quinolinic acid, a selective agonist at NMDA receptors, and kynurenic acid, an antagonist at several glutamate receptor subtypes. The levels of these metabolites change as a result of the activation of inflammatory processes and immune-competent cells, and may have a significant impact on Ca(2+) fluxes and neuronal damage. Drugs which target some of these various sites and processes, or which change the balance between the excitotoxin quinolinic acid and the neuroprotective kynurenic acid, could also have potential as neuroprotective drugs.

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.

Kynurenic acid antagonists and kynurenine pathway inhibitors

The kynurenine pathway accounts for the metabolism of around 80% of non-protein tryptophan metabolism. It includes both an agonist (quinolinic acid) at NMDA receptors and an antagonist (kynurenic acid). Since their discovery, there has been a major development of kynurenic acid analogues as neuroprotectants for the treatment of stroke and neurodegenerative disease. Several prodrugs of kynurenic acid or its analogues that can be hydrolysed within the CNS are also available. More recently, the pathway itself has proved to be a valuable drug target, affected by agents which reduce the synthesis of quinolinic acid and increase the formation of kynurenic acid. The change in the balance of these, away from the excitotoxin and towards the neuroprotectant, has anticonvulsant and neuroprotective properties.

Inhibitors of the kynurenine pathway

Strokes (intracranial thomboses or haemorrhaging) cause death and disability, but effective treatments are lacking. The metabolism of tryptophan leads to the generation of quinolinic acid, an agonist potentially neurotoxic at glutamate receptors, and kynurenic acid, an antagonist at the same population of receptors. The commercial development of the kynurenine pathway has included the use of analogues of kynurenic acid as antagonists at glutamate receptors. A second has been to use prodrugs of kynurenic acid or its analogues. Alternatively, it is proving possible to interfere directly with the kynurenine pathway to block the synthesis of quinolinic acid and promote the formation of kynurenic acid. This change yields neuroprotectant and anticonvulsant compounds.

Seizure activity causes elevation of endogenous extracellular kynurenic acid in the rat brain

This study was designed to examine the effects of several classic convulsants on the extracellular concentration of the anticonvulsant and neuroprotective brain metabolite kynurenic acid (KYNA) in the rat brain. Drug effects were investigated in vivo, mostly by unilateral microdialysis in the dorsal hippocampus. Systemic administration of pentylenetetrazole (60 mg/kg, SC), pilocarpine (325 mg/kg, SC), bicuculline (6 mg/kg, SC), or kainic acid (10 mg/kg, SC) caused characteristic clonic and/or tonic convulsions. In all seizure paradigms, KYNA levels in the dialysate began to rise within 1 h and gradually reached a plateau approximately 4 h after administration of the convulsants. Peak increases were 1.5-3-fold over basal levels. The duration of the elevation in KYNA levels was significantly prolonged following kainic acid application. In the kainic acid model, extracellular KYNA was also measured and found to be increased in the ventral hippocampus, piriform cortex, and striatum. Moreover, temporary intrahippocampal infusion of the KYN synthesis inhibitor aminooxyacetic acid (1 mM) in the kainic acid- and pentylenetetrazole models attenuated the increase in extracellular KYNA levels, demonstrating that de novo production of KYNA in the brain accounts for the seizure-induced KYNA overflow. A separate group of animals received a unilateral intrahippocampal injection of the endogenous convulsant excitotoxin quinolinic acid (120 nmol) and showed long-lasting (> 24 h) bilateral increases in extracellular KYNA levels. Taken together, these data indicate that an increase in extracellular KYNA may constitute a common occurrence in response to seizures and that KYNA elevations may signify the brain's attempt to counteract seizure activity.

Excitatory amino acid antagonists and pentylenetetrazol-induced seizures during ontogenesis: III. The action of kynurenic acid and glutamic acid diethylester

N-methyl-D-aspartate (NMDA) receptor antagonists are anticonvulsant drugs with specific activity against tonic-clonic pentylenetetrazol-induced seizures. However, they do not affect clonic seizures with preserved righting reflexes. In these experiments, we tested the anticonvulsant activity of strychnine-insensitive glycine receptor (at the NMDA site) antagonist kynurenic acid and nonspecific excitatory amino acid receptor antagonist glutamic acid diethylester (GDEE) in the pentylenetetrazol-induced seizure model in developing rats 7, 12, 18, 25, and 90 days old. Control rats were injected with pentylenetetrazol (100 mg/kg subcutaneously). Other rats were pretreated either with kynurenic acid (40, 80, or 240 mg/kg IP) or with GDEE (0.48-480 mg/kg IP), followed by pentylenetetrazol (100 mg/kg). In very young rats (7 and 12 days), both kynurenic acid and GDEE increased the incidence of clonic seizures, whereas the occurrence of tonic-clonic seizures was suppressed or delayed compared to controls. This effect is very similar to the anticonvulsant action of the competitive and noncompetitive NMDA receptor antagonists. In adult rats, the pretreatment with rather higher doses of kynurenic acid or GDEE suppressed or delayed clonic seizures as well as tonic-clonic seizures. Both drugs also induced behavioral side effects: repetitive orientation, wet dog shakes, and frequent jumping. Our data show that there are only weak and nonconsistent age-specific anticonvulsant effects resulting from the blockade of strychnine-insensitive glycine receptor often associated with serious side effects, thus decreasing chances to develop effective antiepileptic treatment in this drug class.

Kynurenine and glycine enhance neuronal sensitivity to N-methyl-D-aspartate

Neurones in rat hippocampal slices were excited by microiontophoretic applications of N-methyl-D-aspartate (NMDA) and kainate. Responses to NMDA were potentiated by glycine 300 microM or 1 mM in the perfusing medium. A small potentiation of kainate was not observed in the presence of the NMDA antagonist 2-amino-5-phosphonopentanoic acid (2AP5). The potentiation of NMDA responses by glycine was not prevented by strychnine 5 or 30 microM and was also shown by D-serine and L-kynurenine but not L-leucine. If sensitivity to NMDA was reduced by kynurenic acid, glycine and L-kynurenine produced a greater enhancement of NMDA. The requirement of NMDA receptor activation for the occupation of strychnine-resistant glycine sites can thus be demonstrated in complex systems such as brain slices. It is possible that L-kynurenine may also be an endogenous ligand capable of modulating NMDA sensitivity.

Modulation of the NMDA receptor by polyamines

Results of recent biochemical and electrophysiological studies have suggested that a recognition site for polyamines exists as part of the NMDA receptor complex. This site appears to be distinct from previously described binding sites for glutamate, glycine, Mg++,Zn++, and open-channel blockers such as MK-801. The endogenous polyamines spermine and spermidine increase the binding of open-channel blockers and increase NMDA-elicited currents in cultured neurons. These polyamines have been termed agonists at the polyamine recognition site. Studies of the effects of natural and synthetic polyamines on the binding of [3H]MK-801 and on NMDA-elicited currents in cultured neurons have led to the identification of compounds classified as partial agonists, antagonists, and inverse agonists at the polyamine recognition site. Polyamines have also been found to affect the binding of ligands to the recognition sites for glutamate and glycine. However, these effects may be mediated at a site distinct from that at which polyamines act to modulate the binding of open-channel blockers. Endogenous polyamines may modulate excitatory synaptic transmission by acting at the polyamine recognition site of the NMDA receptor. This site could represent a novel therapeutic target for the treatment of ischemia-induced neurotoxicity, epilepsy, and neurodegenerative diseases.

A role of excitatory amino acids in the activation of locus coeruleus neurons following cutaneous thermal stimuli

Previous electrophysiological experiments have shown that brain noradrenaline neurons in the locus ceruleus are activated by thermal cutaneous stimuli. In the present study a putative involvement of excitatory amino acids (EAA) in cutaneous LC activation was analyzed. Intraventricular administration of kynurenic acid (1 mumol), a broad spectrum EAA antagonist, or the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 0.1 mumol) as well as subcutaneous administration of the specific NMDA antagonist MK 801 (2 mg/kg) almost totally abolished the response of LC neurons to both non-noxious and noxious cutaneous sensory stimuli. We propose that the activation of LC neurons following thermal cutaneous stimuli is mediated via release of EAA from nerve terminals emanating from nucleus paragigantocellularis (PGi).

NMDA receptor antagonists attenuate a portion of the penicillin-induced epileptiform burst

In the CA1 region of the rat hippocampal slice, epileptiform activity was induced by the GABAA antagonist penicillin (PEN, 3.4 mM). The competitive N-methyl-D-aspartate (NMDA) receptor antagonists D-2-amino-7-phosphonoheptanoate (D-AP7) and D-2-amino-5-phosphonovolerate (D-AP5) attenuated extracellularly recorded evoked burst duration, the number of population spikes per evoked bursts and the frequency of spontaneously occurring bursts, but did not affect the sum of the population spike amplitudes or the evoked burst coastline measures due to increases in amplitude of the remaining secondary population spikes. Intracellular recordings showed that many of the secondary action potentials in the PEN burst were decreased in amplitude and broadened in duration, perhaps due to spike inactivation. D-AP7 allowed these secondary action potentials to increase in amplitude, which could explain the increases in secondary population spike amplitude seen extracellularly. Decrements in stimulus strength can mimic the effect of D-AP7 on PEN bursts. These data suggest that there is a portion of the PEN-induced epileptiform burst which is sensitive to NMDA antagonists.

Inhibitory influence of excitatory amino acid antagonists on penicillin-induced epileptiform bursting in rat hippocampal slices

The inhibitory influence of excitatory amino acid (E.A.A.) antagonists such as kynurenic acid, 2-amino-5-phosphonopentanoic acid (AP5), cis-2,3-piperidine dicarboxylic acid (cis-2,3 PDA) and (+)-5-methyl-10,11,-dihydro-5H-dibenzo(a,d)cyclo-hepten-5,10-imine maleate (MK 801), has been studied on the epileptiform activity elicited in rat hippocampal slices, bathed in penicillin (1 mM). The rank of the inhibitory potency was: MK 801 greater than kynurenic acid greater than cis 2,3 PDA greater than AP5. Moreover, only MK 801 was able to block the last population spike of the penicillin-induced epileptiform bursting in 100% of the experiments. The data indicate that the antiepileptic activity of E.A.A. antagonists on the penicillin epileptiform bursting in CA1 pyramidal cells is low and limited, indicating that the hippocampal area is not the primary site of the anticonvulsant activity of E.A.A. antagonists.

Local neuronal circuitry underlying cholinergic rhythmical slow activity in CA3 area of rat hippocampal slices

1. Intracellular and extracellular recordings were obtained from the CA3 area of rat hippocampal slices to study cellular and synaptic mechanisms underlying rhythmic slow activity (RSA). In all impaled CA3 pyramidal neurones, continuous applications of carbachol, a non-hydrolysable cholinergic agonist, induced first a brief non-rhythmic excitation and then periodic bursts of RSA which could persist for several hours. Each burst of RSA consisted of 4-10 Hz oscillatory depolarizations which had a rise time much slower than conventional EPSPs recorded in the same cell. 2. The carbachol-induced RSA was blocked by atropine; therefore the cholinergic stimulation involved muscarinic receptors. 3. Analyses of simultaneous recordings from pairs of neurones, or a neurone and a glial cell, or a neurone and the extracellular field, indicated that carbachol-induced RSA was synchronous in a large population of CA3 pyramidal neurones. 4. Complete removal of the dentate gyrus and CA1 region did not block carbachol-induced RSA in CA3, but applications of tetrodotoxin or inorganic Ca2+ channel blockers (Cd2+, Co2+ or Mn2+) abolished carbachol-induced RSA. This suggested that the RSA involved propagation of action potentials through a local synaptic network in the CA3 area. 5. Carbachol-induced RSA was reversibly blocked by a broad-spectrum excitatory amino acid antagonist (kynurenic acid), but not by two selective N-methyl-D-aspartate (NMDA) antagonists (DL-2-amino-7-phosphonoheptanoic acid or DL-2-amino-5-phosphonovaleric acid), a GABAA antagonist (bicuculline), or a GABAB antagonist (phaclofen), suggesting that carbachol-induced RSA involved primarily non-NMDA excitatory amino acid, but not GABAergic, synapses. 6. Raising extracellular [Ca2+] beyond 7 mM, which should significantly weaken the polysynaptic recurrent excitation among CA3 pyramidal neurones, abolished carbachol-induced RSA. This suggests that the recurrent excitation among CA3 pyramidal neurones is necessary for carbachol-induced RSA in the CA3 area. However, our experiments cannot clarify whether the recurrent excitation, alone, is sufficient for carbachol-induced RSA.