Hajnalka Németh

Role of Kynurenines in the Central and Peripherial Nervous Systems

Kynurenine (KYN) is an intermediate in the pathway of the metabolism of tryptophan to nicotinic acid. KYN is formed in the mammalian brain (40%) and is taken up from the periphery (60%), indicating that it can be transported across the blood-brain barrier (BBB). In the brain, KYN can be converted to two other components of the pathway: the neurotoxic quinolinic acid (QUIN) and the neuroprotective kynurenic acid (KYNA). QUIN is probably the most widely studied metabolite of KYN, because it may cause excitotoxic neuronal cell loss and convulsions by interacting with the N-methyl-D-aspartate (NMDA) receptor complex, a type of glutamate receptor. KYNA is another metabolite of KYN; its synthesis is catalysed by KYN aminotransferases. This is the only known endogenous NMDA receptor inhibitor, which can act at the glycine site on the receptor complex. Furthermore, KYNA non-competitively inhibits alpha7 nicotinic acetylcholine presynaptic receptors (nAChRs), inhibiting glutamate release, and regulates the expression of alpha4beta2 nAChR. It is well-known that the activation of excitatory amino acid (EAA) receptors can play a role in a number of neurodegenerative disorders, such as Parkinsons disease, Alzheimers disease, stroke and epilepsy. Various studies have been made of whether the EAA receptor antagonist KYNA can exert a therapeutic effect in these neurological disorders. It has been established that KYNA has only a very limited ability to cross the BBB. Other KYNA derivatives have been synthesised (e.g. glucosamine-KYNA, 4-chloro-KYNA and 7-chloro-KYNA), which are well transported across the BBB and act on the glutamate receptors. Moreover, it has been demonstrated that probenecid, a known inhibitor of the transport of organic acids (e.g. KYNA), increases the cerebral concentration of KYNA. There is another new perspective to the maintenance of a high level of KYNA in the brain: the use of enzyme inhibitors, which can block the synthesis of the neurotoxic QUIN. These are some of the most promising possibilities as novel therapeutic strategies for the treatment of neurodegenerative diseases, in which the hyperactivation of amino acid receptors could be involved. The presence and importance of KYN derivatives in the periphery are also discussed in the light of recent publications.

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.

Kynurenine in combination with probenecid mitigates the stimulation-induced increase of c-fos immunoreactivity of the rat caudal trigeminal nucleus in an experimental migraine model

SummaryNitroglycerin, often used as a migraine model, results in increased number of c-fos immunoreactive secondary sensory neurons in the caudal trigeminal nucleus. Since synapses between first- and second-order trigeminal neurons are mediated by excitatory amino acids, NMDA receptors are presumably inhibited by kynurenic acid, the only known endogeneous NMDA receptor antagonist. Although kynurenic acid does not cross the BBB, its precursor, kynurenine, if combined with probenecid, crosses it readily. Systemic kynurenine + probenecid treatment significantly diminishes nitroglycerin-induced increase of c-fos immunoreactivity in the brainstem.

Hippocampal (CA1) activities in Wistar rats from different vendors Fundamental differences in acute ischemia

Two-vessel occlusion, a frequently used model of global cerebral ischemia in rats, results in a dysfunction predominantly within the CA1 field of the hippocampus; it induces many processes with different time-scales. However, the great divergence in the results of the studies reported in the literature suggests valuable differences in response to hypoperfusion-induced ischemia among the laboratory rats used in these studies. In the present work, the acute effects of two-carotid occlusion-induced global ischemia (2VO) on the CA3 stimulation-evoked population spike activity in the CA1 region of Wistar rats from different suppliers (Charles-River and Harlan) were compared. In the acute electrophysiological experiments, the hippocampal CA1 responses revealed that the Charles-River rats immediately compensated the 2VO much better than did the Harlan rats. However, 3 days later, no difference could be observed between the CA1 activities of these rats. The presented data show that the Wistar rats from different vendors represent an important source of variability in the results of acute experiments on the hippocampal ischemia. These observations draw attention to the importance of the careful choice of the laboratory rats (both strains and breeds) used in such experiments.

Prevention of electrical stimulation-induced increase of c-fos immunoreaction in the caudal trigeminal nucleus by kynurenine combined with probenecid

The systemic administration of nitroglycerine, regarded as a migraine model, was previously observed to result in an increased number of c-fos immunoreactive secondary sensory neurons in the caudal trigeminal nucleus, which forward nociceptive impulses to the thalamus. The present investigation tested the hypothesis of whether kynurenine in combination with systemically administered probenecid protects second-order trigeminal neurons against stimulation arriving via central processes of trigeminal ganglion cells. Electrical stimulation of the trigeminal ganglion, one of the experimental migraine models, is known to induce an increase in the number of c-fos immunoreactive second-order nerve cells projecting to the thalamus. Since the synapses between first- and second-order trigeminal neurons are presumed to be mediated by excitatory amino acids, postsynaptic NMDA receptors should be inhibited by kynurenic acid, an endogenous NMDA receptor antagonist. Kynurenic acid, however, does not cross the blood-brain barrier, and its use as a neuroprotective agent is therefore not feasible. In contrast, kynurenine, from which kynurenic acid is formed on the action of kynurenine aminotransferase, passes the blood-brain barrier without difficulty. After the i.p. injection of kynurenine combined with probenecid it was found that the stimulation-induced increase in the c-fos immunoreactivity of the secondary sensory neurons does not occur.

Behaviour changes in a transgenic model of Huntington's disease.

Huntingtons disease is an autosomal dominant inherited disorder, caused by an expanded polyglutamine region of a protein called huntingtin with unknown function. Transgenic mice expressing the N-terminal of huntingtin, containing 82 glutamines, exhibit a progressive disorder, which resembles to the human disease. In this study, we tested the longitudinal behaviour changes in this transgenic line in open-field and elevated-plus-maze tests. The motor performance deteriorated at 12 weeks of age and the disease progressed as indicated by the decreased total distance covered, the decreased mean velocity and the decreased exploratory behaviour. The level of anxiety was unchanged in transgenic mice as compared with their littermate controls. The motor deterioration was similar to that in other Huntingtons disease models, while the level of anxiety was different. These tests are suitable means of following the progression of the disease and useful for studies of the effects of therapeutic interventions.

Comparative study on the effects of kynurenic acid and glucosamine–kynurenic acid

Kynurenic acid (KYNA) is the only known endogenous N-methyl-D-aspartate (NMDA) receptor inhibitor and might therefore come into consideration as a therapeutic agent in certain neurobiological disorders. However, its use as a neuroprotective compound is practically excluded because KYNA does not readily cross the blood-brain barrier (BBB). We recently synthetized a new compound, glucosamine-kynurenic acid (KYNA-NH-GLUC), which is presumed to cross the BBB more easily. In this study, the effects of KYNA and KYNA-NH-GLUC on behavior and cortical activity were investigated in adult rats. The results show that (1) on intracerebroventricular application, the behavioral changes induced by KYNA and by KYNA-NH-GLUC are quite similar; (2) on intravenous administration, KYNA (25 mg/kg) has no effect on the somatosensory-evoked cortical potentials, whereas KYNA-NH-GLUC (25 mg/kg) causes transient but appreciable reductions in the amplitudes of the evoked responses within 5 min after application; and (3) the results of in vitro studies demonstrated that both KYNA and KYNA-NH-GLUC reduced the amplitudes of the field excitatory postsynaptic potentials (fEPSPs). These observations suggest that the two compounds have similar effects, but that KYNA-NH-GLUC passes the BBB much more readily than does KYNA. These results imply that the conjugated NH-GLUC is of importance in the passage across the BBB.

Kynurenines in the Central Nervous System: Recent Developments

The intermediates of the kynurenine pathway, called kynurenines, are derived directly or indirectly from the tryptophan metabolism. This metabolic pathway is responsible for nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate, which participate in basic cellular processes. It was discovered some thirty years ago that kynurenines have neuroactive properties. Kynurenine, the central compound of this pathway, can be converted to two other important agents: the neuroprotective kynurenic acid and the neurotoxic quinolinic acid. Kynurenic acid is an endogenous broad-spectrum antagonist of excitatory amino acid receptors, including the N-methyl- D-aspartate receptors. It can inhibit the overexcitation of these receptors and reduce the cell damage induced by excitotoxins. Moreover, kynurenic acid non-competitively blocks the α7-nicotinic acetylcholine receptors, thereby permitting modulation of the cholinergic and glutamatergic neurotransmission. Quinolinic acid is a selective N-methyl-D-aspartate receptor agonist which can cause lipid peroxidation, the generation of free radicals and apoptosis via the overexcitation of these receptors. Changes in the relative or absolute concentrations of the kynurenines have been found in several neurodegenerative disorders, such as Huntingtons disease and Parkinsons disease, stroke and epilepsy, in which the hyperactivation of amino acid receptors could be involved. Increase of the brain level of kynurenic acid seems to be a good therapeutic strategy; however, kynurenic acid can cross the blood-brain barrier only poorly. The latest findings provide promising opportunities involving the development of the analogues 4-chloro-kynurenine and glucoseamine-kynurenic acid, which can enter the brain and exert neuroprotective effects. Another recent possibility is the use of different enzyme inhibitors which can reduce the production of the neurotoxic quinolinic acid.