Trevor W. Stone

An iontophoretic investigation of the actions of convulsant kynurenines and their interaction with the endogenous excitant quinolinic acid

Abstract Kynurenine and related compounds are endogenous metabolites of tryptophan which have previously been found to be convulsant in animals. We now report that kynurenine, kynurenic acid, nicotinic acid do not excite neurones in the cerebral cortex, but that quinolinic acid is an effective excitant. Kynurenine had no effect on responses to quinolinic acid, glutamate or N-metyl- D -aspartate (NMDA) and did not block depressant responses to GABA, glycine or 5-hydroxytryptamine. Suprisingly kynurenic acid proved a powerful antagonist of quinolinic acid, NMDA and quisqualic acid, but showed no preferential antagonism to one agonist. It did, however, show a preference for reducing quinolinic acid responses compared to acetylcholine responses.

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.

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 Huntingtons 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.

NMDA receptors and ligands in the vertebrate CNS

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Antioxidants and fatty acids in the amelioration of rheumatoid arthritis and related disorders

The generation of reactive oxygen species (free radicals) is an important factor in the development and maintenance of rheumatoid arthritis in humans and animal models. One source of free radicals is nitric oxide produced within the synoviocytes and chondrocytes and giving rise to the highly toxic radical peroxynitrite. Several cytokines, including tumour necrosis factor-alpha (TNFalpha) are involved in the formation of free radicals, partly by increasing the activity of nitric oxide synthase. Indeed, nitric oxide may mediate some of the deleterious effects of cytokines on bone resorption. Aspirin, tetracyclines, steroids and methotrexate can suppress nitric oxide synthase. Dietary antioxidants include ascorbate and the tocopherols and beneficial effects of high doses have been reported especially in osteoarthritis. There is also evidence for beneficial effects of beta-carotene and selenium, the latter being a component of the antioxidant enzyme glutathione peroxidase. The polyunsaturated fatty acids (PUFA) include the n-3 compounds, some of which are precursors of eicosanoid synthesis, and the n-6 group which can increase formation of the pro-inflammatory cytokines TNFalpha and interleukin-6, and of reactive oxygen species. Some prostaglandins, however, suppress cytokine formation, so that n-3 PUFA often oppose the inflammatory effects of some n-6-PUFA. gamma-linolenic acid (GLA) is a precursor of prostaglandin E1, a fact which may account for its reported ability to ameliorate arthritic symptoms. Fish oil supplements, rich in n-3 PUFA such as eicosapentaenoic acid have been claimed as beneficial in rheumatoid arthritis, possibly by suppression of the immune system and its cytokine repertoire. Some other oils of marine origin (e.g. from the green-lipped mussel) and a range of vegetable oils (e.g. olive oil and evening primrose oil) have indirect anti-inflammatory actions, probably mediated via prostaglandin E1. Overall, there is a growing scientific rationale for the use of dietary supplements as adjuncts in the treatment of inflammatory disorders such as rheumatoid arthritis and osteoarthritis.

Tryptophan metabolism and oxidative stress in patients with Huntington's disease

Abnormalities in the kynurenine pathway may play a role in Huntingtons disease (HD). In this study, tryptophan depletion and loading were used to investigate changes in blood kynurenine pathway metabolites, as well as markers of inflammation and oxidative stress in HD patients and healthy controls. Results showed that the kynurenine : tryptophan ratio was greater in HD than controls in the baseline state and after tryptophan depletion, indicating increased indoleamine dioxygenase activity in HD. Evidence for persistent inflammation in HD was provided by elevated baseline levels of C‐reactive protein, neopterin and lipid peroxidation products compared with controls. The kynurenate : kynurenine ratio suggested lower kynurenine aminotransferase activity in patients and the higher levels of kynurenine in patients at baseline, after depletion and loading, do not result in any differences in kynurenic acid levels, providing no supportive evidence for a compensatory neuroprotective role for kynurenic acid. Quinolinic acid showed wide variations in blood levels. The lipid peroxidation data indicate a high level of oxidative stress in HD patients many years after disease onset. Levels of the free radical generators 3‐hydroxykynurenine and 3‐hydroxyanthranilic acid were decreased in HD patients, and hence did not appear to contribute to the oxidative stress. It is concluded that patients with HD exhibit abnormal handling of tryptophan metabolism and increased oxidative stress, and that these factors could contribute to ongoing brain dysfunction.

Oxidative stress as a mechanism for quinolinic acid-induced hippocampal damage: protection by melatonin and deprenyl.

There are differences between the excitotoxic actions of quinolinic acid and N‐methyl‐D‐aspartate (NMDA) which suggest that quinolinic acid may act by mechanisms additional to the activation of NMDA receptors. The present study was designed to examine the effect of a potent antioxidant, melatonin, and the potential neuroprotectant, deprenyl, as inhibitors of quinolinic acid‐induced brain damage. Injections were made into the hippocampus of anaesthetized rats, which were allowed to recover before the brains were taken for histology and the counting of surviving neurones. Quinolinic acid (120 nmols) induced damage to the pyramidal cell layer, which was prevented by the co‐administration of melatonin (5 nmols locally plus 2×20 mg kg−1 i.p.). This protective effect was not prevented by the melatonin receptor blocker luzindole. Neuronal damage produced by NMDA (120 nmols) was not prevented by melatonin. Quinolinic acid increased the formation of lipid peroxidation products from hippocampal tissue and this effect was prevented by melatonin. Deprenyl also prevented quinolinic acid‐induced damage at a dose of 50 nmols but not 10 nmols plus 2×1.0 mg kg−1 i.p. The non‐selective monoamine oxidase inhibitor nialamide (10 and 50 nmols plus 2×25 mg kg−1) did not afford protection. The results suggest that quinolinic acid‐induced neuronal damage can be prevented by a receptor‐independent action of melatonin and deprenyl, agents which can act as a potent free radical scavenger and can increase the activity of endogenous antioxidant enzymes respectively. This suggests that free radical formation contributes significantly to quinolinic acid‐induced damage in vivo.

ADENOSINE INHIBITION OF γ‐AMINOBUTYRIC ACID RELEASE FROM SLICES OF RAT CEREBRAL CORTEX

1 The effect of purine compounds on the potassium‐evoked release of 14C‐labelled γ‐aminobutyric acid (GABA) has been studied in 400 μm slices of rat cerebral cortex in vitro. 2 Adenosine and adenosine 5′ monophosphate (AMP) inhibited the release of GABA at 10−5 to 10−3 m. Adenosine triphosphate (ATP) produced a significant inhibition of release only at 10−3 m. 3 Theophylline 10−4 or 10−3 m reduced the inhibitory effect of adenosine, but did not change basal release of GABA. 4 Dipyridamole 10−5 m itself reduced evoked GABA release, but did not prevent the inhibitory effect of adenosine, implying that adenosine was acting at an extracellularly directed receptor. 5 Calcium removal or antagonism by verapamil reduced the evoked release of GABA, but adenosine did not produce any further reduction of the calcium‐independent release. This may indicate that the inhibitory effect of adenosine on GABA release results from interference with calcium influx or availability within the terminals.

Adenosine Receptors and Neurological Disease: Neuroprotection and Neurodegeneration

Adenosine receptors modulate neuronal and synaptic function in a range of ways that may make them relevant to the occurrence, development and treatment of brain ischemic damage and degenerative disorders. A(1) adenosine receptors tend to suppress neural activity by a predominantly presynaptic action, while A(2A) adenosine receptors are more likely to promote transmitter release and postsynaptic depolarization. A variety of interactions have also been described in which adenosine A(1) or A(2) adenosine receptors can modify cellular responses to conventional neurotransmitters or receptor agonists such as glutamate, NMDA, nitric oxide and P2 purine receptors. Part of the role of adenosine receptors seems to be in the regulation of inflammatory processes that often occur in the aftermath of a major insult or disease process. All of the adenosine receptors can modulate the release of cytokines such as interleukins and tumor necrosis factor-alpha from immune-competent leukocytes and glia. When examined directly as modifiers of brain damage, A(1) adenosine receptor (AR) agonists, A(2A)AR agonists and antagonists, as well as A(3)AR antagonists, can protect against a range of insults, both in vitro and in vivo. Intriguingly, acute and chronic treatments with these ligands can often produce diametrically opposite effects on damage outcome, probably resulting from adaptational changes in receptor number or properties. In some cases molecular approaches have identified the involvement of ERK and GSK-3beta pathways in the protection from damage. Much evidence argues for a role of adenosine receptors in neurological disease. Receptor densities are altered in patients with Alzheimers disease, while many studies have demonstrated effects of adenosine and its antagonists on synaptic plasticity in vitro, or on learning adequacy in vivo. The combined effects of adenosine on neuronal viability and inflammatory processes have also led to considerations of their roles in Lesch-Nyhan syndrome, Creutzfeldt-Jakob disease, Huntingtons disease and multiple sclerosis, as well as the brain damage associated with stroke. In addition to the potential pathological relevance of adenosine receptors, there are earnest attempts in progress to generate ligands that will target adenosine receptors as therapeutic agents to treat some of these disorders.

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.