6,7-Dichloroquinoxaline-2,3-dione is a competitive antagonist specific to strychnine-insensitive [3H]glycine binding sites on the N-methyl-D-aspartate receptor complex

Optimization and validation of two miniaturized glucocerebrosidase enzyme assays for high throughput screening

Glucocerebrosidase (GC) catalyzes the hydrolysis of beta-glucocerebroside to glucose and ceramide in lysosomes. Mutations in the glucocerebrosidase gene (GBA) result in Gaucher disease, an autosomal recessive lysosomal storage disorder. Many of the mutations encountered in patients with Gaucher disease are missense alterations that may cause misfolding, decreased stability and/or mistrafficking of this lysosomal protein. Some inhibitors of GC have been shown to act as chemical chaperones, stabilizing the conformation of mutant proteins and thus restoring their function. High throughput screening (HTS) of small molecule libraries for such compounds with potential for chaperone therapy requires an accurate, reproducible and sensitive assay method. We have adapted and optimized two fluorogenic GC enzyme assays and miniaturized them into the 1536-well plate format for HTS. The two substrates, 4-methylumbelliferyl beta-D-glucopyranoside and resorufin beta-D-glucopyranoside, have K(m) values of 768 microM and 33 microM, respectively, and different emission spectra. Paired screening with the two assays helps to eliminate false inference of activity due to autofluorescence or fluorescence quenching by the screened compounds. Test screens with the LOPAC library indicated that both assays were robust for HTS, and gave comparable results for GC inhibitor activities. These two assays can be used to identify both GC activators and inhibitors with potential therapeutic value.

Prevention of cocaine-induced convulsions and lethality in mice: effectiveness of targeting different sites on the NMDA receptor complex

N-methyl-D-aspartate (NMDA) receptors appear to be involved in the behavioral toxic effects of cocaine. Therefore, different classes of NMDA receptor antagonists were compared for their ability to attenuate cocaine-induced convulsions and lethality in male, Swiss Webster mice. The mice were pre-treated (i.p.) with vehicle or an antagonist from one of the following classes: NMDA/glycine site antagonist (7-chlorokynurenic acid, ACEA-1021, ACEA-1031, ACEA-1328, DCQX, R(+)-HA-966), competitive antagonist (CPP, D-AP7), channel blocker (MK-801, memantine), or allosteric modulator (ifenprodil, CP-101,606, Co 101022, haloperidol). After a 15 min pre-treatment period, the mice were administered a convulsive (60 mg/kg, i.p.) or lethal (125 mg/kg, i.p.) dose of cocaine, equivalent to the calculated ED/LD97 values. Pre-treatment with competitive or NMDA/glycine site antagonists dose-dependently attenuated cocaine-induced convulsions and lethality (P<0.05). Pre-treatment with channel blockers or allosteric modulators of the NMDA receptor protected against cocaine-induced convulsions (P<0.05), but were ineffective or less effective than the competitive and glycine site antagonists in preventing death. The glutamate release inhibitor riluzole failed to prevent both the convulsions and lethality induced by cocaine. Significantly, post-treatment with NMDA/glycine site antagonists (ACEA-1021, ACEA-1031, ACEA-1328) after a cocaine overdose prevented death in a significant number of animals. The data suggest that NMDA receptors are involved in the pathophysiology of a cocaine overdose.

Pregnenolone sulfate potentiation of NMDA-mediated increases in intracellular calcium in cultured chick cortical neurons

Pregnenolone sulfate (PS) has been reported to selectively augment glutamate-induced depolarizations mediated by the NMDA subtype of the glutamate receptor. The present study examines the ability of this neuroactive steroid to potentiate NMDA-mediated increases in intracellular calcium in cultured chick cortical neurons using the fluorescent dye Fura2. PS, in the absence of NMDA and glycine, significantly elevated intracellular calcium at 250 and 500 microM. This increase in free calcium was significantly attenuated at 250 microM PS by the prior addition of 50 microM CNQX, 10 microM dizocilpine or 1 microM nimodipine. NMDA and glycine, when added to the cells in saturating concentrations of 500 and 50 microM, respectively, consistently increased intracellular free calcium over baseline levels. In the presence of NMDA and glycine, both 50 and 100 microM PS produced a further significant rise in intracellular free calcium. The prior addition of CNQX, dizocilpine or both compounds together significantly inhibited this elevation in free calcium. The application of the endogenous polyamine spermine (250 microM) significantly potentiated the response of chick cortical neuronal cells to NMDA and glycine. PS, in the presence of NMDA, glycine and spermine, produced a further increase in intracellular free calcium at concentrations of 50 and 100 microM. The prior application of CNQX, dizocilpine or both compounds together significantly attenuated this rise in free calcium. These data confirm that PS is a positive allosteric modulator of the NMDA receptor and provide evidence that this neurosteroid does not interact with the polyamine modulatory site.

Differential control of vascular tone and heart rate by different amino acid neurotransmitters in the rostral ventrolateral medulla of the rat

1. To test the hypothesis that a central mechanism may play a role in the minimal reflex tachycardia noted in response to peripheral converting enzyme inhibition, we compared the effects of intravenous (i.v.) ceronapril (CER) with nitroglycerin (NTG) on neurotransmitter release in the rostral ventrolateral medulla (RVLM), using an in vivo microdialysis method in pentobarbital anaesthetized rats. 2. CER (0.1 mg/kg, i.v.) caused a progressive decrease in glutamate (GLU) release (CER 65 +/- 7% vs NTG 83 +/- 3% of each baseline at 140 min, P < 0.05) and attenuated the increase in glycine (GLY) release (CER 100 +/- 8% vs NTG 122 +/- 9%, P < 0.05). 3. Prevention of blood pressure reduction due to i.v. CER by concomitant infusion of a subpressor dose of angiotensin II (AII) attenuated the progressive reduction of GLU release (87 +/- 4%, P < 0.05 compared with NTG group), whereas GLY release was not affected (106 +/- 5%, NS compared with NTG group). 4. Perfusion of GLU into this area at approximately physiological concentrations resulted in a sustained tachycardia with an attenuation of the depressor effect of i.v. CER and perfusion of GLY solely lowered blood pressure. 5. These results demonstrate that i.v. converting enzyme inhibitor reduces the release of GLU in the RVLM, which was specifically caused by reducing circulating AII, without any effect on GLY release, thus resulting in the reduction of blood pressure with minimal effect on the heart rate.

Differential profiles of binding of a radiolabeled agonist and antagonist at a glycine recognition domain on the N-methyl-D-aspartate receptor ionophore complex in rat brain

Addition of several polyamines, including spermidine and spermine, was effective in inhibiting binding of the antagonist ligand [3H]5,7-dichlorokynurenic acid ([3H]-DCKA) a Gly recognition domain on the N-methyl-D-aspartic acid (NMDA) receptor ionophore complex in rat brain synaptic membranes. In contrast, [3H]DCKA binding was significantly potentiated by addition of proposed polyamine antagonists, such as ifenprodil and (+/-)-alpha-(4-chlorophenyl)-4-[(4-fluorophenyl) methyl]-1-piperidine ethanol, with [3H]Gly binding being unchanged. The inhibition by spermidine was significantly prevented by inclusion of ifenprodil. In addition, spermidine significantly attenuated the abilities of four different antagonists at the Gly domain to displace [3H]DCKA binding virtually without affecting those of four different agonists. Phospholipases A2 and C and p-chloromercuribenzosulfonic acid were invariably effective in significantly inhibiting [3H]DCKA binding with [3H]Gly binding being unaltered. Moreover, the densities of [3H]DCKA binding were not significantly different from those of [3H]-Gly binding in the hippocampus and cerebral cortex, whereas the cerebellum had more than a fourfold higher density of [3H]Gly binding than of [3H]DCKA binding. These results suggest that the Gly domain may have at least two different forms based on the preference to agonists and antagonists in the rodent brain.

Strychnine-insensitive glycine binding to cerebral cortical membranes in developing and ageing mice

The strychnine-insensitive binding of [3H]glycine was characterized in purified cerebral cortical membranes from mice aged from 7 days to 22 months. The binding was saturable, exhibiting only one component during the whole life-span studied. The binding constant KD did not change during development and ageing, whereas the maximal binding capacity Bmax, calculated per protein content, increased up to the age of two weeks and then again in ageing animals (18- and 22-month-olds). The binding was similarly inhibited by the antagonists 7-chlorokynurenate, 3-amino-1-hydroxypyrrolidin-2-one (HA-966) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) in 7-day-, 3-month- and 12-month-old mice. The inhibition caused by glycine, L-serine and beta-alanine also remained unaltered during the whole life-span. beta-Alanine was a noncompetitive inhibitor. The alterations in the maximal binding capacities during development and ageing could be of importance in the regulation of NMDA receptors, which have been implicated in synaptic potentiation, developmental processes and various pathological conditions.

Support for radiolabeling of a glycine recognition domain on the N-methyl-D-aspartate receptor ionophore complex by 5,7-[3H]dichlorokynurenate in rat brain

Pretreatment with Triton X-100 more than doubled the binding of radiolabeled 5,7-dichlorokynurenic acid (DCKA), a proposed antagonist at a glycine (Gly) recognition domain on the N-methyl-D-aspartate (NMDA) receptor ionophore complex, in rat brain synaptic membranes. The binding exhibited an inverse temperature dependency, reversibility, and saturability, the binding sites consisting of a single component with a high affinity (27.5 nM) and a relatively low density (2.87 pmol/mg of protein). The binding of both [3H]DCKA and [3H]Gly was similarly displaced by numerous putative agonists and antagonists at the Gly domain in a concentration-dependent manner at a concentration range of 100 nM to 0.1 mM. Among the 24 putative ligands tested, DCKA was the second most potent displacer of the binding of both radioligands with no intrinsic affinity for the binding of [3H]kainic acid and alpha-amino-3-hydroxy-5- [3H]methylisoxazole-4-propionic acid (AMPA) to the non-NMDA receptors. In contrast, the other proposed potent Gly antagonist, 5,7-dinitroquinoxaline-2,3-dione, was active in displacing the binding of [3H]glutamic ([3H]Glu) and D,L-(E)-2-amino-4-[3H]propyl-5-phosphono-3-pentenoic acids to the NMDA recognition domain with a relatively high affinity for the non-NMDA receptors. In addition, the proposed antagonist at the AMPA-sensitive receptor, 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline, not only displaced weakly the binding of both [3H]-Gly and [3H]DCKA, but also inhibited the binding of (+)-5-[3H]methyl-10,11- dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine ([3H]MK-801) to an ion channel associated with the NMDA-sensitive receptor in the presence of added Glu alone in a manner sensitive to antagonism by further added Gly. Clear correlations were seen between potencies of the displacers to displace [3H]DCKA binding and [3H]Gly binding, in addition to between the potencies to displace [3H]-DCKA or [3H]Gly binding and to potentiate or inhibit [3H]MK-801 binding. All quinoxalines tested were invariably more potent displacers of [3H]DCKA binding than [3H]Gly binding, whereas kynurenines were similarly effective in displacing the binding of both [3H]Gly and [3H]DCKA. These results undoubtedly give support to the proposal that [3H]DCKA is one useful radioligand available in terms of its high selectivity and affinity for the Gly domain in the brain. Possible multiplicity of the Gly domain is suggested by the differential pharmacological profiles between the binding of [3H]Gly and [3H]DCKA.

Binding of [3H]MK-801, NMDA-displaceable [3H]glutamate, [3H]glycine, [3H]spermidine, [3H]kainate and [3H]AMPA to regionally discrete brain membranes of the gerbil: a biochemical study

Bindings of glutamate receptor agonists and related modulators were investigated in 10 discrete tissues from gerbil brain using a biochemical technique. There appeared considerable discrepancies, in respect of intrahippocampal profiles, from reported data by autoradiography on rat brain. In the gerbil, an almost equivalent level of N-methyl-D-aspartate (NMDA)-displaceable [3H]glutamate binding was found in field CA1 and the dentate gyrus, while approx 30% less in field CA3, a profile which was strikingly similar to that of (+)-5-methyl-10,11-dihydro-5H-dibenzo-[a,d]-cycloheptene-5,10-imin e maleate (MK-801) or of [3H]glycine. [3H]Kainate binding was highest in the dentate gyrus followed by field CA3 and then field CA1, the ratio of the highest to the lowest being 3 to 2. Binding of [3H]DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) differed, to a certain extent, from that of [3H]kainate and showed the dentate gyrus followed by field CA1 and then field CA3 in the rank order of decreasing binding. Taking together, intrahippocampal localization of glutamate receptor subtypes in the gerbil, when analyzed with a biochemical binding assay, looks to be less region selective than the distribution obtained on autoradiography in the rat. Thus, it is likely that these different distribution profiles show different status of receptor function respectively, or are due merely to species difference.

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.

Antagonism of non-NMDA receptors augments the neuroprotective effect of NMDA receptor blockade in cortical cultures subjected to prolonged deprivation of oxygen and glucose

A 30-60 min period of oxygen and glucose deprivation induced widespread degeneration of cultured murine neocortical neurons. Neuronal degeneration could be blocked by adding the selective NMDA antagonist MK-801 to the bathing medium; however, if the deprivation period was prolonged to 90-105 min, the neuroprotective effect of MK-801 was overcome. The non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) at 1-100 microM concentrations also failed to protect neurons against this prolonged insult, but the combination of CNQX with either MK-801 or D-APV produced marked neuroprotection. This synergistic action of CNQX was not due to enhanced blockade of NMDA receptors, as it was not mimicked by combining MK-801 with D-APV or 7-chlorokynurenate. These observations support the idea that combined NMDA and non-NMDA receptor blockade may have value in ameliorating the neuronal loss associated with prolonged ischemic insults in vivo.

Neurochemical aspects of the N-methyl-D-aspartate receptor complex

The N-methyl-D-aspartic acid (NMDA)-sensitive subclass of brain excitatory amino acid receptors is supposed to be a receptor-ionophore complex consisting of at least 3 different major domains including an NMDA recognition site, glycine (Gly) recognition site and ion channel site. Biochemical labeling of the NMDA domain using [3H]L-glutamic acid (Glu) as a radioactive ligand often meets with several critical methodological pitfalls and artifacts that cause a serious misinterpretation of the results. Treatment of brain synaptic membranes with a low concentration of Triton X-100 induces a marked disclosure of [3H]Glu binding sensitive to displacement by NMDA with a concomitant removal of other several membranous constituents with relatively high affinity for the neuroactive amino acid. The NMDA site is also radiolabeled by the competitive antagonist (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid that reveals possible heterogeneity of the site. The Gly domain is sensitive to D-serine and D-alanine but insensitive to strychnine, and this domain seems to be absolutely required for an opening of the NMDA channels by agonists. The ionophore domain is radiolabeled by a non-competitive type of NMDA antagonist that is only able to bind to the open but not closed channels. The binding of these allosteric antagonists is markedly potentiated by NMDA agonists in a manner sensitive to antagonism by isosteric antagonists in brain synaptic membranes and additionally enhanced by further inclusion of Gly agonists through the Gly domain. Furthermore, physiological and biochemical responses mediated by the NMDA receptor complex are invariably potentiated by several endogenous polyamines, suggesting a novel polyamine site within the complex. At any rate, activation of the NMDA receptor complex results in a marked influx of Ca2+ as well as Na+ ions, which subsequently induces numerous intracellular metabolic alterations that could be associated with neuronal plasticity or excitotoxicity. Therefore, any isosteric and allosteric antagonists would be of great benefit for the therapy and treatment of neurodegenerative disorders with a risk of impairing the acquisition and formation process of memories.

Solubilization of spermidine-sensitive (+)-[3H]5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine ([3H]MK-801) binding activity from rat brain

The receptor-ionophore complex of the N-methyl-D-aspartate (NMDA)-sensitive receptor was solubilized by deoxycholic acid from rat brain using (+)-[3H]5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne ([3H]MK-801) binding as a marker for the receptor. Gel filtration of the solubilized preparations on a Sephadex G-25 column revealed significant [3H]MK-801 binding sensitive to potentiation by glutamate and glutamate/glycine, which was prevented by competitive antagonists for the NMDA and strychnine-insensitive glycine (GlyB) sites. In contrast to NMDA and glycine, spermidine markedly potentiated the amount of [3H]MK-801 binding in solubilized preparations by increasing the apparent affinity of the ligand. In the presence of all three stimulants, the solubilized preparations exhibited pharmacological profiles similar to those in the membrane preparations. These results clearly indicate that the whole macromolecular NMDA receptor-ionophore complex is solubilized under the experimental conditions used.

Competitive inhibition of NMDA-mediated responses by guanine nucleotides in brain synaptic membranes treated with Triton X-100

The effect of guanine nucleotides on physiological responses mediated by the N-methyl-D-aspartate (NMDA)-sensitive subclass of brain excitatory amino acid receptors was examined by using NMDA-sensitive [3H]L-glutamic acid (Glu) binding as well as [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne (MK-801) binding in rat brain synaptic membranes treated with a low concentration of Triton X-100. The NMDA-sensitive [3H]Glu binding was significantly inhibited by the addition of some guanine nucleotides such as GTP, GDP, 5'-guanylylimidodiphosphate and guanosine 5'-O-(3-thiotriphosphate), but not by other nucleotides or nucleosides such as guanosine, cyclic GMP, adenosine, AMP, ADP, ATP, CTP, ITP and UTP. Inclusion of GTP not only attenuated the ability of NMDA to displace [3H]Glu binding in a concentration-dependent manner, but also lowered the affinity of the binding sites for [3H]Glu without altering their densities. The inhibitory potency of an antagonist highly selective to the NMDA receptors (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate on [3H]Glu binding also deteriorated with GTP at concentrations above 10 microM. Addition of Glu induced a concentration-dependent potentiation of [3H]MK-801 binding through an activation of the NMDA-sensitive receptors, and the potency of Glu to potentiate the binding was markedly reduced by the afore-mentioned positive guanine nucleotides in a competitive manner. In contrast, GTP at 0.1 mM non-competitively weakened the stimulatory property of glycine to additionally enhance the binding found in the presence of Glu alone. These results suggest that some guanine nucleotides may have a relatively high affinity for NMDA recognition sites within the NMDA receptor complex in the brain.

6,7-Dinitroquinoxaline-2,3-dione and 6-nitro,7-cyanoquinoxaline-2,3-dione antagonize responses mediated by N-methyl-D-aspartate and NMDA-associated glycine recognition sites in vivo: measurements of cerebellar cyclic-GMP

Direct intracerebellar administration of quisqualate resulted in marked increases in levels of cGMP in the cerebellum of the mouse, with a Hill number of 2.0. Quinoxalinediones, DNQX (6,7-dinitroquinoxaline-2,3-dione) and CNQX (6-nitro,7-cyanoquinoxaline-2,3-dione) attenuated the quisqualate-induced response. 6,7-Dinitroquinoxaline-2,3-dione also attenuated the D-serine-induced increases in levels of cGMP in a competitive manner. Intracerebellar injection of DNQX also antagonized the response to parenterally-administered harmaline. Similar results were also obtained with CNQX. These results indicate that these quinoxalinediones can attenuate the responses, mediated through the NMDA-associated glycine recognition sites, as well as the NMDA receptor complex. However, the glycine antagonist HA-966 (3-amino-1-hydroxypyrrolidone-2), at doses which completely reversed the increases induced by D-serine, failed to alter the response to quisqualate, indicating a lack of effect of glycine antagonists on quisqualate-mediated synaptic events. These results further support the interaction of the quinoxalinediones, DNQX and CNQX, with the NMDA receptor complex as established in receptor binding and electrophysiological studies.