6,7-Dichloro-3-hydroxy-2-quinoxalinecarboxylic acid is a relatively potent antagonist at NMDA and kainate receptors

A study of the molecular structure and spectroscopic properties of 3-hydroxy-2-quinoxalinecarboxylic acid by experimental methods and quantum chemical calculations

The mid-IR and Raman spectra of 3-hydroxy-2-quinoxalinecarboxylic acid (3HQC) were recorded. These spectra were interpreted with the help of B3LYP/6-311++G(d,p) calculations and potential energy distribution (PED) analysis. As a result of the calculations, seven tautomers were determined among many stable conformations. The experimental spectra were concordant with the theoretical data of two tautomers. In the functional group region overtone and combination bands were detected and assigned. In addition, because of several peaks in the IR spectrum, it was proposed that the 3HQC exhibits dimerization in condensed phase. Possible dimeric forms of 3HQC were evaluated at the same level of theory, and it has been seen that the calculation results confirm the above proposal. 1H and 13C NMR chemical shifts of 3HQC have been calculated, and compared with the experimental data. The frontier molecular orbital properties and the atomic charges were also theoretically obtained and presented.

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.

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.

Screening of receptor antagonists using agonist-activated patch clamp detection in chemical separations

We present a capillary electrophoresis-patch clamp detection system optimized for screening of antagonists and inhibitors of ligand-gated ion channels. In this system, highly selective receptor agonists are delivered through the electrophoresis capillary to the cell surface where they continuously activate a receptor, resulting in increased steady-state transmembrane currents. Thus, receptor selection and biosensor functionality is simply achieved by selection of an appropriate agonist. The antagonists are fractionated in the same electrophoresis capillary and inhibit the agonist-evoked response, resulting in transiently decreased steady-state transmembrane currents. Specifically, a mixture containing 6-cyano-7-nitroquinoxaline-2,3-dione, that reversibly blocks alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate and kainate receptors, and 6,7-dichloro-3-hydroxy-2-quinoxaline-carboxylate, a broad-spectrum glutamate receptor antagonist, were separated and detected by kainate-activated patch-clamped interneurons freshly dissociated from rat brain olfactory bulb. In addition, Mg2+ that reversibly blocks the N-methyl-D-aspartate receptor in a voltage-dependent way was detected using the same cell detector system when activated by N-methyl-D-aspartate and the co-agonist glycine. The presented method offers new possibilities for drug screening and for identifying endogenous receptor antagonists and to determine their mode of action on any ionotropic receptor system of interest.

A comparison of non-NMDA receptor channels in type-2 astrocytes and granule cells from rat cerebellum

1. Patch-clamp recording methods have been used to compare the pharmacological properties and single-channel characteristics of non-NMDA receptor channels in cerebellar type-2 astrocytes and granule cells. 2. In type-2 astrocytes whole-cell concentration-response curves for glutamate, quisqualate, AMPA and kainate gave EC50 values of 5.8, 3.8, 7.6 and 160 microM and Hill slopes of 1.65, 1.18, 1.64 and 1.65, respectively, resembling estimates for granule cell receptors. 3. The non-NMDA receptor antagonists CNQX and diCl-HQC (see Methods) inhibited whole-cell kainate currents in both cell types. The IC50 for CNQX antagonism of the kainate response was 536 nM in type-2 astrocytes, and 500 nM in granule cells. The IC50 for diCl-HQC was 3.5 microM in astrocytes and 3.7 microM in granule cells. 4. CNQX acted as a competitive antagonist of whole-cell kainate responses in type-2 astrocytes and granule cells giving Schild plots with a slope near 1. The equilibrium constant, K, for CNQX binding was 524 nM in astrocytes and 489 nM in granule cells. 5. Quisqualate and AMPA responses showed rapid desensitization in type-2 astrocytes with a ratio of steady-state to peak response of 0.09. Concanavalin A reduced this desensitization. 6. Non-NMDA channels in type-2 astrocytes and granule cells showed a low permeability to Ca2+ ions with a reversal potential, for kainate-activated whole-cell currents in isotonic Ca2+, of approximately -25 mV for astrocytes and -45 mV for granule cells. 7. Outside-out patches from type-2 astrocytes exhibited a range of single-channel conductances that were superficially similar to the glutamate-activated conductances in granule cells. However, the type-2 astrocytes were devoid of NMDA receptors, hence all of these conductances originated from non-NMDA channels. Their slope conductances were approximately 11, 21, 32, 42 and 52 pS. Amplitudes were verified with mean low-variance plots and single-channel current-voltage curves, which were linear. 8. There was also evidence of lower conductance kainate-activated channels in astrocyte patches. From noise analysis their estimated mean conductance was 1.9 pS, as described for the 'low-conductance' type kainate responses in cerebellar neurones. 9. Apparent open times, shut times and burst lengths of AMPA-activated (3-10 microM) channels were examined in patches from type-2 astrocytes, and kinetic properties of the 40 and 50 pS levels were compared with the lower levels. 10. Our results indicate some marked pharmacological similarities between non-NMDA receptor channels in type-2 astrocytes and granule cells.(ABSTRACT TRUNCATED AT 400 WORDS)

CNQX increases spontaneous inhibitory input to CA3 pyramidal neurones in neonatal rat hippocampal slices

Whole-cell recordings were made from immature CA3 pyramidal neurones in the rat hippocampal slice. The addition of the glutamate receptor antagonist, CNQX, caused a robust increase in the frequency of spontaneous inhibitory post-synaptic currents (IPSC) concomitant with the expected reduction of excitatory drive to these neurones. This effect of CNQX was not shared by structurally related quinoxalinediones or kynurenic acid, which are also antagonists of non-NMDA glutamate receptors. This effect of CNQX was abolished by tetrodotoxin suggesting that an increase in interneurone spiking was responsible for the IPSCs. Recordings from stratum radiatum interneurones of CA3 confirmed this suggestion, since some interneurones were directly depolarized by CNQX. The excitation by CNQX of a small population of stratum radiatum interneurones of CA3 complicates interpretation of experiments designed to assess the consequences of blocking excitatory transmission with this drug.

Competitive inhibition by NBQX of kainate/AMPA receptor currents and excitatory synaptic potentials: importance of 6-nitro substitution

We evaluated the inhibitory potencies at excitatory amino acid receptors of 2,3-dihydroxy-7-sulfamoyl-benzo[f]quinoxaline (BQX) and its 6-nitro derivative, NBQX. Currents activated by kainate or (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) in two-electrode voltage-clamp recordings of Xenopus oocytes injected with rat cortex mRNA were inhibited by BQX and NBQX: the apparent Ki values versus kainate were 14 microM and 78 nM, respectively, and versus AMPA were 23 microM and 63 nM, respectively. Thus, to a degree even more marked than with other quinoxalinedione derivatives, 6-nitro substitution of BQX to yield NBQX increases potency (200-fold) at the non-NMDA ionotropic receptor, but does not confer selectivity for kainate or AMPA. Schild analysis of the NBQX inhibition of the kainate and AMPA currents yielded pA2 values of 7.17 +/- 0.05 and 7.05 +/- 0.10, respectively, and slopes near unity confirming the competitive nature of the inhibition. Neither BQX nor NBQX significantly inhibited the current activated by glycine plus NMDA. The selectivity ratio of NBQX (greater than 5000-fold) is by far the greatest of any quinoxalinedione derivative antagonist of the kainate/AMPA receptor. BQX and NBQX also inhibited the excitatory postsynaptic field potentials mediated by kainate/AMPA receptors in the CA1 region of hippocampal slices after stimulation of the Schaffer collateral-commissural pathways with IC50 values of 130 and 0.90 microM, respectively. The 10-fold differences between the IC50 values in hippocampal slices and the Ki values in Xenopus oocytes correlate closely with data for other quinoxalinedione derivative antagonists.

Effects of glycine antagonists on Mg(2+)- and glycine-induced [3H]N-(1-[2-thienyl]cyclohexyl)-3,4-piperidine binding

We investigated the effects of glycine antagonists, 3-amino-1-hydroxy-2- pyrrolidone (HA-966), 7-chlorokynurenic acid (7-Cl-KYNA), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 6,7-dichloro-3-hydroxy-2-quinoxalinecarboxylic acid (DHQXC), 6,7-dichloroquinoxaline-2,3-dione (DCQX), and 5-chloro-indole-2-carboxylic acid (5-Cl-I2CA), on Mg(2+)- and glycine-induced [3H]N-(1-[2-thienyl]cyclohexyl)-3,4-piperidine ([3H]TCP) binding to well-washed rat cortical membranes. Except for 5-Cl-I2CA, all the glycine antagonists completely inhibited not only glycine- but also Mg(2+)-induced [3H]TCP binding in a concentration-dependent manner. Out of all the glycine antagonists examined DHQXC most selectively inhibited Mg(2+)-induced [3H]TCP binding, while DCQX was the most selective for inhibiting glycine-induced [3H]TCP binding.

Reorganization of the Cytoskeleton in Rat Neurons Following Stimulation With Excitatory Amino Acids In Vitro

The state of neuronal microtubule polymerization is influenced by microtubule-associated proteins such as MAP2, which is specifically localized within neuronal dendrites and cell bodies. We have demonstrated that stimulation of spinal cord or cortical neurons in vitro with excitatory amino acids results in a dramatic modification of the neuronal cytoskeleton as monitored with antibodies against MAP2 and tubulin. Stimulation of cultures with glutamate receptor agonists induced a reorganization of MAP2 immunoreactivity into a distinctive network of bundles within certain neuronal cell bodies and their proximal neurites. The effect was not abolished by depolymerizing drugs such as nocodazole, or protein synthesis inhibitors. The effect was dependent upon the entry of sodium following depolarization and was not associated with neuronal damage. We suggest that in neurons the state of the neuronal cytoskeleton can be modulated by glutamate receptor activation acting through MAP2.

Identification of kainic and quisqualic acid receptors on inner retinal cells of the salamander Ambystoma mexicanum

The presence of kainic (KA) and quisqualic acid (QA) receptors on inner retinal neurones of the axolotl Ambystoma mexicanum has been studied using intracellular recording techniques. In the presence of CoCl2, which blocks neurotransmitter release, KA and QA depolarized the membrane. The minimum concentration of KA that induced a response was 1 microM and a maximum response was obtained with 10 microM (EC50: 3 microM). The operating range of QA was between 0.5 and 5 microM with an EC50 of 1 microM. These data show that inner retinal cells of the axolotl are sensitive to KA and QA. Cis-2,3-piperidine dicarboxylic acid (PDA, 3 mM) completely blocked responses to 5 microM KA, but not those induced by 2 microM QA. This suggests that the KA- and QA-sensitive receptors on inner retinal cells of the salamander are pharmacologically different and that PDA can be a valuable tool in distinguishing KA- and QA-sensitive receptors on these neurones.

6,7-Dinitroquinoxaline-2,3-dione blocks the cytotoxicity of N-methyl-D-aspartate and kainate, but not quisqualate, in cortical cultures

Based on radioligand binding and electrophysiological studies, quinoxalinediones such as 6,7-dinitroquinoxaline-2,3-dione (DNQX) have been shown to be potent competitive antagonists at the quisqualate and kainate subtypes of the glutamate receptor. In this report we have examined the effects of DNQX on excitatory amino acid neurotoxicity and evoked neurotransmitter release. DNQX was found to be a potent neuroprotective agent against glutamate and N-methyl-D-aspartate (NMDA) neurotoxicity. The data suggest that this neuroprotective activity of DNQX is due to its antagonism of the coagonist activity of glycine at the NMDA receptor-channel complex. The specificity of DNQX for the glycine site associated with the NMDA receptor-channel complex was confirmed in radioligand binding and neurotransmitter release studies. DNQX also prevented kainate neurotoxicity and kainate-evoked neurotransmitter release, presumably by direct competition for the kainate receptor. DNQX, however, did not prevent quisqualate neurotoxicity, suggesting that a novel quisqualate-preferring receptor insensitive to DNQX may mediate quisqualate toxicity.

Experiments with kainate and quisqualate agonists and antagonists in relation to the sub-classification of 'non-NMDA' receptors

1) The KD values for a range of antagonists including DGG, pCB-PzDA, pBB-PzDA, HDC-QXCA, DNQX and CNQX have been determined using a series of non-NMDA receptor agonists in the isolated spinal cord of the neonatal rat. 2) CNQX and DNQX, and, to a lesser extent, DCH-QXCA, were by far the most potent antagonists, although the degree of selectivity did not vary much throughout the whole range of antagonists used. 3) AMPA and domoate were the most and least sensitive agonists, respectively, to the action of all the antagonists. Ionophoretic experiments in the cat spinal cord in vivo confirmed this order of susceptibility in the case of the antagonists CNQX and pCB-PzDA. 4) Acromelic acid A was a more AMPA-like than domoate-like agonist. 5) The results suggest that two receptors contribute to the responses induced by several of the agonists, and that quisqualate and kainate are less selective agonists at these receptors than are AMPA and domoate, respectively.

Quinoxalines interact with the glycine recognition site of NMDA receptors: studies in guinea-pig myenteric plexus and in rat cortical membranes

1. The effects of several quinoxalines, including 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 6,7,dinitroquinoxaline-2,3-dione (DNQX), and of two kynurenates, kynurenate (KYNA) and 7-Clkynurenate (7-Cl-KYNA), have been evaluated on the N-methyl-D-aspartate (NMDA) receptors present in the guinea-pig ileum myenteric plexus preparation and on the strychnine-insensitive [3H]-glycine binding sites of cortical membranes. 2. Quinoxalines and kynurenates antagonized in a non-competitive manner L-glutamate-induced contraction. Their IC50s were (in microM): 5 for 7-Cl-KYNA, 7.5 for 6,7-Cl-3-hydroxy-2-quinoxaline carboxylate (6,7-Cl-HQCA), 20 for DNQX, 50 for CNQX, 76 for KYNA and 125 for 3-hydroxy-2-quinoxaline carboxylate (HQCA). 3. Glycine (5-50 microM) completely reversed the antagonism displayed by both quinoxalines and kynurenates. The interaction between glycine and the tested compounds appeared to be competitive in nature. 4. Quinoxalines and kynurenates displaced, in a concentration-dependent manner, [3H]-glycine from its strychnine-insensitive binding sites present in rat cortical membranes. Their IC50s for this action were (in microM): 0.45 for 7-Cl-KYNA, 0.6 for 6,7-Cl-HQCA, 2.4 for DNQX, 3.5 for CNQX, 20 for KYNA and 40 for HQCA. 5. When the IC50s for the displacement effect of [3H]-glycine binding were plotted against the IC50s obtained in the myenteric plexus, a significant correlation was found. 6. These data show that quinoxalines and kynurenates may antagonize the responses to L-glutamate by interacting with the glycine recognition sites of the NMDA receptor ion channel complex.

Quinoxaline derivatives are high-affinity antagonists of the NMDA receptor-associated glycine sites

Membranes from rat telencephalon contain strychnine-insensitive glycine binding sites associated with NMDA receptors. Three quinoxaline derivatives, among them the high-affinity AMPA receptor antagonists CNQX and DNQX, were found to inhibit [3H]glycine binding to these sites with micromolar affinities. Binding of these compounds to the glycine site also inhibited glutamate-stimulated association and dissociation of [3H]TCP. This suggests that these AMPA antagonists, like the structurally related compound kynurenate, act as glycine site antagonists.

Antagonist profile of 6,7-dichloro-3-hydroxy-2-quinoxalinecarboxylate at excitatory amino acid receptors in the neonatal rat spinal cord

In the neonatal rat spinal cord, 6,7-dichloro-3-hydroxy-2- quinoxalinecarboxylate antagonised responses mediated at both N-methyl-D-aspartate (NMDA) and non-NMDA receptors. The antagonism of responses to NMDA was unsurmountable and mediated via an antagonist action at the allosterically-linked strychnine-insensitive glycine site. At non-NMDA receptors, 6,7-dichloro-3-hydroxy-2-quinoxalinecarboxylate appeared to act as a competitive antagonist at low concentrations and a non-competitive antagonist at higher concentrations. In contrast to published data, this antagonist did not distinguish between the responses mediated by DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) and kainate.