Sensitivity of hippocampal neurones to kainic acid, and antagonism by kynurenate

Questioning Glutamate Excitotoxicity in Acute Brain Damage: The Importance of Spreading Depolarization

BACKGROUND

Within 2 min of severe ischemia, spreading depolarization (SD) propagates like a wave through compromised gray matter of the higher brain. More SDs arise over hours in adjacent tissue, expanding the neuronal damage. This period represents a therapeutic window to inhibit SD and so reduce impending tissue injury. Yet most neuroscientists assume that the course of early brain injury can be explained by glutamate excitotoxicity, the concept that immediate glutamate release promotes early and downstream brain injury. There are many problems with glutamate release being the unseen culprit, the most practical being that the concept has yielded zero therapeutics over the past 30 years. But the basic science is also flawed, arising from dubious foundational observations beginning in the 1950s METHODS: Literature pertaining to excitotoxicity and to SD over the past 60 years is critiqued.

RESULTS

Excitotoxicity theory centers on the immediate and excessive release of glutamate with resulting neuronal hyperexcitation. This instigates poststroke cascades with subsequent secondary neuronal injury. By contrast, SD theory argues that although SD evokes some brief glutamate release, acute neuronal damage and the subsequent cascade of injury to neurons are elicited by the metabolic stress of SD, not by excessive glutamate release. The challenge we present here is to find new clinical targets based on more informed basic science. This is motivated by the continuing failure by neuroscientists and by industry to develop drugs that can reduce brain injury following ischemic stroke, traumatic brain injury, or sudden cardiac arrest. One important step is to recognize that SD plays a central role in promoting early neuronal damage. We argue that uncovering the molecular biology of SD initiation and propagation is essential because ischemic neurons are usually not acutely injured unless SD propagates through them. The role of glutamate excitotoxicity theory and how it has shaped SD research is then addressed, followed by a critique of its fading relevance to the study of brain injury.

CONCLUSIONS

Spreading depolarizations better account for the acute neuronal injury arising from brain ischemia than does the early and excessive release of glutamate.

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 ( approximately 100 microm) were analyzed in anesthetized newborn piglets. Topical application of NMDA (10(-4) M) or KAIN (5 x 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% CO2. 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.

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.

Development and therapeutic potential of kynurenic acid and kynurenine derivatives for neuroprotection

Manipulation of the kynurenine pathway of tryptophan metabolism has yielded a plethora of agents that are now being developed as neuroprotectants and anticonvulsants. This pathway is involved in the production of the excitotoxin quinolinic acid and the neuroprotectant kynurenic acid. Approaches used in the development of therapeutic agents include production of analogues or pro-drugs of kynurenic acid and inhibitors of the enzyme responsible for the synthesis of quinolinic acid. Indeed, analogues of the amino acid receptor antagonist kynurenic acid are now in, or are about to enter, clinical trials for stroke and related disorders. This review summarizes the mechanism of action of these various agents, the development of glutamate receptor antagonists from kynurenic acid and the range of their potential uses in neurology and psychiatry.

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.

7-Nitroindazole, a selective inhibitor of nNOS, increases hippocampal extracellular glutamate concentration in status epilepticus induced by kainic acid in rats

The glutamate extracellular concentration is controlled by metabolic and neuronal pathways via release and uptake mechanisms. Stimulation of glutamate receptors induces neuronal nitric oxide (NO) release, which in turn modulates glutamate transmission. In this study, the influence of neuronally derived NO on hippocampal glutamate extracellular concentration was investigated in conditions of intense metabolic activation, i.e., during status epilepticus induced by systemic kainic acid (KA). Glutamate, arginine and citrulline concentrations were measured by microdialysis coupled to HPLC. Experiments were performed in conscious rats implanted with a microdialysis probe within the hippocampal CA3 area. Three groups were used: (1) rats treated with KA i.p. (12 mg/kg) and vehicle locally, via the microdialysis probe (n = 9); (2) rats given KA i.p. and a selective inhibitor of neuronal NO synthase, 7-nitroindazole (7-NI, 1.25 mM) locally (n = 13); (3) rats treated with saline i.p. and 7-NI locally (n = 7). Infusion of 7-NI or vehicle was performed throughout the second hour of status epilepticus. In groups 1 and 3, no significant modifications of extracellular glutamate, arginine and citrulline concentrations were measured. In group 2, the local application of 7-NI in the hippocampus during status epilepticus significantly increased extracellular glutamate and arginine concentrations, whereas citrulline concentration remained constant. The concomitant increases of extracellular glutamate and arginine concentrations under local 7-NI perfusion in seizure conditions, suggest that glutamate and arginine are linked in a common metabolic pathway and/or that glutamate is involved in the cross-talk between glia and neurons. A cerebrovascular effect of 7-NI which triggers glutamate release may also occur.

Prevention by a purine analogue of kainate-induced neuropathology in rat hippocampus

Systemic injection of kainic acid produces a characteristic regional and cellular pattern of neuronal loss in the central nervous system by mechanisms which may be relevant to an understanding of neurodegenerative disorders. It has previously been found, by measuring the binding of a glial marker ligand, that analogues of adenosine, such as R-N6-phenylisopropyladenosine (R-PIA), can prevent kainate-induced damage of the hippocampus at doses as low as 10 micrograms/kg, i.p. The use of gliotic markers, however, is open to misinterpretation, and the present work was designed to re-examine purine protection against kainate using histological methods. The results show that R-PIA, at a dose of 25 micrograms/kg i.p. in rats, can protect against the neuronal damage caused by kainate and that this protection could be completely prevented by the simultaneous administration of 1,3-dipropyl-8-cyclopentylxanthine, indicating the involvement of adenosine A1 receptors in the protection.

Effect of inhibiting NO synthesis on hippocampal extracellular glutamate concentration in seizures induced by kainic acid

It has been suggested that nitric oxide (NO) interferes with both glutamatergic neurotransmission and the regulation of cerebral blood flow in epileptic seizures. This study examines the effect of an inhibitor of NO synthesis, NG-nitro-L-arginine methyl ester (L-NAME, 20 mg/kg), on the extracellular concentration of glutamate during seizures induced by kainic acid (KA; 10 mg/kg), both drugs being administered systemically. L-NAME was injected 40 min before KA. The extracellular glutamate concentration was measured in the hippocampus of awake, spontaneously breathing rats using microdialysis combined with HPLC. The arterial blood gases and glycemia were periodically checked. The arterial blood pressure, the electrocorticogram and the body temperature were continuously monitored. In basal conditions, the systemic injection of L-NAME increased arterial blood pressure but did not significantly change the hippocampal glutamate level. In seizure conditions, the hippocampal glutamate concentration was either slightly increased or not significantly changed in saline-treated rats (n = 6) but it was decreased in L-NAME-treated rats (n = 6). At all times after KA injection, the hippocampal glutamate concentration was significantly lower in L-NAME-treated rats than in saline-treated rats. Unlike saline-treated rats, L-NAME-treated rats died during status epilepticus. This study shows that acute systemic injection of L-NAME reduces the extracellular concentration of glutamate in the rat hippocampus during seizures induced by KA.

Non-steroidal anti-inflammatory drugs and spinal nociceptive processing

Non-steroidal anti-inflammatory drugs have a direct action on spinal nociceptive processing in vivo with a relative order of potency which correlates with their capacity as inhibitors of cyclooxygenase activity. However, recent clinical surveys and new in vivo evidence strongly suggest that for some of these agents, centrally mediated analgesia may also be achieved by additional mechanisms, which are independent of prostaglandin synthesis inhibition. In this review we explore the likelihood for such mechanisms following an extensive survey of existing data. The implications of these mechanisms are discussed in the light of our current understanding of spinal nociceptive processing.

The spinal actions of nonsteroidal anti-inflammatory drugs and the dissociation between their anti-inflammatory and analgesic effects

The traditional classification of nonsteroidal anti-inflammatory drugs (NSAIDs) as exclusively 'peripherally acting' agents is no longer valid. For many of these agents there is a growing body of evidence in favour of an additional central mechanism for their anti-inflammatory and analgesic effects. This view is further supported by the recent discovery that a substantial component of the hyperalgesia and allodynia that characterise postinjury hypersensitivity occurs in the CNS, notably the spinal dorsal horn. An important corollary is that inhibition of central nociceptive processing may represent an important analgesic mode of action for those NSAIDs that are effective in the management of pain after tissue injury. Historically, attempts to group this heterogeneous class of compounds into a single entity are largely derived from the observation that the majority of clinically useful NSAIDs are weak organic acids (pKa 3 to 5), bind extensively to plasma albumin (= 99%), and inhibit (to varying degrees) prostaglandin synthesis. However, the significance of these various unifying features is becoming increasingly obscure. While inhibition of prostaglandin synthesis apparently remains an important analgesic mode of action for NSAIDs both in the periphery and the CNS, other mechanisms should be considered. Some NSAIDs, in addition to their effects on prostaglandin synthesis, also affect the synthesis and activity of other neuroactive substances believed to have key roles in processing nociceptive input within the dorsal horn. It has been argued that these other actions, in conjunction with inhibition of prostaglandin synthesis, may synergistically augment the effects of NSAIDs on spinal nociceptive processing. Despite much effort, it remains a formidable task to assess the significance of these differential mechanisms upon clinical pain states. In the meantime, however, it may be possible, on the basis of in vivo studies, to evaluate the impact of putative spinal analgesic mechanisms that are unrelated to inhibition of prostaglandin synthesis. This approach has recently been extended to include the identification of pharmacokinetic and clinical correlates of these derived in vivo parameters, and in this way attempt to demonstrate clinical relevance.

Kainate toxicity in energy-compromised rat hippocampal slices: differences between oxygen and glucose deprivation

The effects of kainate (KA) on the recovery of neuronal function in rat hippocampal slices after hypoxia or glucose deprivation (GD) were investigated and compared to those of (R,S)-alpha-amino-3-hydroxy-5-methyl-4- isoxazoleproprionate (AMPA). KA and AMPA were found to be more toxic than either N-methyl-D-aspartate (NMDA), quinolinate, or glutamate, both under normal conditions and under states of energy deprivation. Doses as low as 1 microM KA or AMPA were sufficient to significantly reduce the recovery rate of neuronal function in slices after a standardized period of hypoxia or GD. The enhancement of hypoxic neuronal damage by both agonists could be partially blocked by the antagonist kynurenate, by the NMDA competitive antagonist AP5, and by elevating [Mg2+] in or by omitting Ca2+ from the perfusion medium. The AMPA antagonist glutamic acid diethyl ester was ineffective in preventing the enhanced hypoxic neuronal damage by either KA or AMPA. The antagonist of the glycine modulatory site on the NMDA receptor, 7-chlorokynurenate, did not block the KA toxicity but was able to block the toxicity of AMPA. 2,3-Dihydroxyquinoxaline completely blocked the KA- and AMPA-enhanced hypoxic neuronal damage. The KA-enhanced, GD-induced neuronal damage was prevented by Ca2+ depletion and partially antagonized by kynurenate but not by AP5 or elevated [Mg2+]. The results of the present study indicate that the KA receptor is involved in the mechanism of neuronal damage induced by hypoxia and GD, probably allowing Ca2+ influx and subsequent intracellular Ca2+ overload.(ABSTRACT TRUNCATED AT 250 WORDS)

Methohexitone antagonises kainate and epileptiform activity in rat neocortical slices

Using a grease-seal technique on cortical slices, methohexitone (10-316 microM) dose dependently and reversibly reduced depolarising responses to kainate more than those to alpha-amino-3-hydroxy-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate (NMDA). The respective pA2 values were 4.9 +/- 0.07, 3.6 +/- 0.03 and 4.0 +/- 0.05 whereas, for 6-nitro,7-sulphamoylbenz[F]quinoxalinedione (NBQX), they were 5.8 +/- 0.06, 6.7 +/- 0.05 and < 4.0. Methohexitone was also more effective than NBQX in reducing the spontaneous epileptiform activity occurring in these cortical slices. Thus 10 and 20 microM of this short-acting barbiturate reduced afterpotentials and burst frequencies respectively by about 50% whereas NBQX 10 microM only reduced burst frequency by some 15%. The results are discussed with respect to a putative methohexitone- and kainate-sensitive autoreceptor which facilitates presynaptic glutamate release.

Pharmacological characterization of non-NMDA subtypes of glutamate receptor in the neonatal rat hemisected spinal cord in vitro

1. A grease-gap technique was used to record depolarizing responses to alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA), kainate and N-methyl-D-aspartate (NMDA) in the hemisected spinal cord of the neonatal rat. The pharmacology of non-NMDA subtypes of glutamate receptor was investigated with the novel quinoxalinedione, 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo (F)-quinoxaline (NBQX) and with a series of barbiturates. 2. NBQX antagonized AMPA- and kainate-, but not NMDA- induced depolarizations. The near parallel shifts of the major part of the dose-response curves for AMPA and kainate by NBQX gave pA2 values (+/- s.e.) of 6.7 +/- 0.2 and 6.8 +/- 0.2 respectively, consistent with a common site of action for these two agonists. 3. Below the 50% level at which these pA2 values were calculated, however, an NBQX-resistant plateau was seen within the kainate, but not the AMPA, dose-response curve. 4. In decreasing order of potency, methohexitone, secobarbitone, thiopentone, pentobarbitone and phenobarbitone preferentially reduced kainate-, rather than AMPA- and NMDA-, induced depolarizations. Methohexitone was also the most selective with IC50S against kainate, AMPA and NMDA of 31 +/- 7, 172 +/- 47 and greater than 200 microM respectively. 5. The NBQX-resistant plateau seen within the kainate dose-response curve was reduced by methohexitone. Kainate antagonism by methohexitone was not reduced by 50 microM picrotoxin. 6. We conclude that, while mixed agonist actions may hamper demonstration of antagonist selectivity, depolarizations induced by the non-NMDA ionotropic agonists, AMPA and kainate, are mediated in part via distinct receptors.

Nicotinylalanine increases cerebral kynurenic acid content and has anticonvulsant activity

1. Nicotinylalanine is an analogue of kynurenine which has been reported to inhibit the enzymes kynurenine hydroxylase and kynureninase. 2. In the present study rats were given a tryptophan load together with nicotinylalanine two hours before killing, and the brain, liver and kidneys analysed by HPLC for their kynurenic acid content. 3. Tryptophan alone produced a significant elevation of kynurenate but with the additional administration of nicotinylalanine, levels rose dramatically, including a 19-fold increase in brain. 4. In mice the same dose of nicotinylalanine reduced the incidence of seizures induced by leptazol or electroshock treatment. 5. Since kynurenic acid is an antagonist at excitatory amino acid receptors the results may herald a new approach to producing a pharmacological blockade of amino acid receptors in the brain.