NMDA receptor agonists derived from ibotenic acid. Preparation, neuroexcitation and neurotoxicity

Tweaking Agonist Efficacy at N-Methyl-d-aspartate Receptors by Site-Directed Mutagenesis

The structural basis for partial agonism at N-methyl-D-aspartate (NMDA) receptors is currently unresolved. We have characterized several partial agonists at the NR1/NR2B receptor and investigated the mechanisms underlying their reduced efficacy by introducing mutations in the glutamate binding site. Key residues were selected for mutation based on ligand-protein docking studies using a homology model of NR2B-S1S2 built from the X-ray structure of NR1-S1S2 in complex with glycine. Wild-type and mutant forms of NR2B were coexpressed with NR1 in Xenopus laevis oocytes and characterized by two-electrode voltage-clamp electrophysiology. By combining mutagenesis of residues His486 or Val686 with activation by differently substituted partial agonists, we introduce varying degrees of steric clash between the ligand and the two binding domains S1 and S2. In cases where ligand-protein docking predicts increased steric clashes between agonists and the residues forming the S1-S2 interface, the agonists clearly show decreased relative efficacy. Furthermore, we demonstrate that the mutation S690A affects both potency and efficacy in an agonist-specific manner. The results indicate that essential residues in the ligand binding pocket of NR2B may adopt different conformations depending on the agonist bound. Together, these data indicate that agonist efficacy at the NR2B subunit can be controlled by the extent of steric clashes between the agonist and the ligand binding domains and by ligand-dependent arrangements of residues within the binding pocket.

The respective N-hydroxypyrazole analogues of the classical glutamate receptor ligands ibotenic acid and (RS)-2-amino-2-(3-hydroxy-5-methyl-4-isoxazolyl)acetic acid

We have determined the pharmacological activity of N-hydroxypyrazole analogues (3a and 4a) of the classical glutamate receptor ligands ibotenic acid and (RS)-2-amino-2-(3-hydroxy-5-methyl-4-isoxazolyl)acetic acid (AMAA), as well as substituted derivatives of these two compounds. The pharmacological profile of 3a is closer to that of thioibotenic acid rather than ibotenic acid, while 4a is a selective N-methyl-D-aspartic acid (NMDA) receptor agonist. Ring substitution of 3a and 4a leads to NMDA receptor antagonists. Whereas efficacy of 3a derivatives at mglu2 receptor decreases from agonism via partial agonism to antagonism with increasing substituent size, substitution abolishes affinity for mglu1 and mglu4 receptors. Ligand- and receptor-based modelling approaches assist in explaining these pharmacological trends among the metabotropic receptors and suggest a mechanism of partial agonism at mglu2 receptor similar to that proposed for the GluR2 glutamate receptor.

Structural basis for understanding structure-activity relationships for the glutamate binding site of the NMDA receptor

We present new homology-based models of the glutamate binding site (in closed and open forms) of the NMDA receptor NR2B subunit derived from X-ray structures of the water soluble AMPA sensitive glutamate receptor. The models were used for revealing binding modes of agonists and competitive antagonists, as well as for rationalizing known experimental facts concerning structure-activity relationships: (i) the switching between the agonist and the antagonist modes of action upon lengthening the chain between the distal acidic group and the amino acid moiety, (ii) the preference for the methyl group attached to the alpha-amino group of ligands, (iii) the preference for the D-configuration of agonists and antagonists, and (iv) the existence of "superacidic" agonists.

Synthesis and pharmacology of N-alkylated derivatives of the excitotoxin ibotenic acid

Three amino-alkylated derivatives of the naturally occurring excitatory amino acid (EAA) receptor agonist ibotenic acid (Ibo) have been synthesized and tested pharmacologically. N-Methyl-Ibo (1a) and N-ethyl-Ibo (1b) were shown to be agonists at NMDA receptors (EC50 = 140 and 320 microM, respectively), though with activities considerably lower than Ibo (EC50 = 9.6 microM). N-Benzyl-Ibo (1c) was inactive at ionotropic EAA receptors and all three compounds were, in contrast to Ibo, inactive at metabotropic EAA receptors. Molecular mechanics calculations have been performed on Ibo, 1a-c and the potent NMDA agonist 2-amino-2-(3-hydroxy-5-methyl-4-isoxazolyl)acetic acid (AMAA) in order to elucidate the observed structure-activity data.

Competitive antagonism of recombinant AMPA/kainate receptors by N-methyl-D-aspartate and analogues

The classical division of mammalian ionotropic L-glutamate (Glu) receptors into N-methyl-D-aspartate (NMDA) and non-NMDA classes is supported by a wealth of biochemical and molecular biological data. In binding studies, selective agonists for non-NMDA receptors such as L-kainate (KA), alpha-amino-3-hydroxy-5-methylisoxazole-propionate (AMPA) and L-domoate have submicromolar affinities; in contrast, a millimolar concentration of NMDA is required significantly to compete with the non-NMDA agonists. Despite the supposed clear-cut selectivities of these amino acids, interactions between the responses to submillimolar concentrations of NMDA and KA have been observed in cells expressing both classes of Glu receptor. We present here evidence that NMDA is a competitive antagonist of recombinant non-NMDA receptors. We also present preliminary data on competitive antagonism of recombinant NMDA receptors by KA. These antagonisms are inhibited non-competitively by cyclothiazide and a benzodiazipine.

A new highly selective metabotropic excitatory amino acid agonist: 2-amino-4-(3-hydroxy-5-methylisoxazol-4-yl)butyric acid

The homologous series of acidic amino acids, ranging from aspartic acid (1) to 2-aminosuberic acid (5), and the corresponding series of 3-isoxazolol bioisosteres of these amino acids, ranging from (RS)-2-amino-2-(3-hydroxy-5-methylisoxazol-4-yl)acetic acid (AMAA, 6) to (RS)-2-amino-6-(3-hydroxy-5-methylisoxazol-4-yl)hexanoic acid (10), were tested as ligands for metabotropic excitatory amino acid receptors (mGlu1 alpha, mGlu2, mGlu4a, and mGlu6). Whereas AMAA (6) and (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propinoic acid (AMPA, 7) are potent and highly selective agonists at N-methyl-D-aspartic acid (NMDA) and AMPA receptors, respectively, the higher homologue of AMPA (7), (RS)-2-amino-4-(3-hydroxy-5-methylisoxazol-4-yl)butyric acid (homo-AMPA, 8), is inactive at ionotropic excitatory amino acid receptors. Homo-AMPA (8), which is a 3-isoxazolol bioisostere of 2-aminoadipic acid (3), was, however, shown to be a specific and rather potent agonist at mGlu6, approximately 4 times weaker than the nonselective excitatory amino acid receptor agonist (S)-glutamic acid. 2-Aminoadipic acid (3), which shows a complex excitatory amino acid synaptic pharmacology, was an agonist at mGlu6 as well as mGlu2. AMPA (7) and the higher homologue of homo-AMPA (8), (RS)-2-amino-5-(3-hydroxy-5-methylisoxazol-4-yl)pentanoic acid (9), showed relatively weak agonist effects at mGlu6. It is concluded that homo-AMPA (8) is likely to be a useful tool for studies of the pharmacology and physiological role of mGlu6. We describe a new versatile synthesis of this homologue of AMPA and the synthesis of compound 10.

Different characteristics of AMPA receptor agonists acting at AMPA receptors expressed in Xenopus oocytes

A series of (RS)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) analogues were evaluated for activity at homo-oligomeric glutamate1-flop (Glu1-flop) receptors expressed in Xenopus oocytes, using the two-electrode voltage clamp technique. (RS)-2-Amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid (ACPA) (EC50, 2.4 microM), a homologue of AMPA having a carboxyl group as the terminal acidic functionality, was five times more potent than AMPA (EC50, 12 microM) and 20 times more potent than kainate (EC50, 46 microM). (RS)-2-Amino-3(3-hydroxy-5-trifluoromethyl-4-isoxazolyl)propionic acid (Tri-F-AMPA), in which an electronegative trifluoromethyl group is substituted for the methyl group on the isoxazole ring in the AMPA structure, was three times more potent than AMPA, whereas (RS)-3-hydroxy-4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-5-carboxylic acid (5-HPCA), a bicyclic analogue of AMPA with highly restricted conformational flexibility was 10 times less potent than AMPA. The limiting slope of log-log plots of Glu1-flop receptor currents versus low agonist concentrations had a value of 1.7 for ACPA and kainate compared to 1.5 for Tri-F-AMPA and 1.3 for 5-HPCA and AMPA. The amplitude of responses evoked by near saturating concentrations of the agonists varied more than 7-fold. The sequence of efficacy was ACPA = kainate > Tri-F-AMPA > AMPA > 5-HPCA. Moreover, when saturating concentrations of Tri-F-AMPA and kainate were co-applied, the response was significantly greater than when each of the agonists was applied separately. The potency of the antagonist 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline (NBQX) (estimated KB, approximately 200 nM), to block currents mediated by Glu1-flop receptors was similar for all of the agonists tested in this study. These results indicate that relatively minor changes in the molecular structure of AMPA are associated with marked effects on potency and efficacy. In particular, it is suggested that the acidity of the terminal group plays a major role in determining the degree of receptor activation in the steady state.

D, L-(tetrazol-5-yl)glycine stimulation of NMDA receptors in neonatal dissociated neurons: inhibition by ethanol

Ethanol inhibition of NMDA receptor stimulation by the high-affinity selective agonist D, L-(tetrazol-5-yl)glycine (T5G) was studied using acutely dissociated neonatal whole-brain neurons loaded with the fluorescent indicator fura-2. T5G induced a concentration-dependent increase in intracellular calcium with a maximal increase above basal of 70nM at 16 microM T5G (EC50 of 0.66 +/- 0.18 microM). T5G agonist specificity was verified using the NMDA antagonists MK-801 (40 nM), APV (100 microM), and Mg2+ (1 mM). The T5G stimulation of calcium entry was both blocked and reversed by these antagonists. Ethanol significantly inhibited the T5G-mediated increase in intracellular calcium only at concentrations > or = 100 mM. In addition, the effect of increasing concentrations of ethanol in the presence of the glycine-site antagonist 5, 7-dichlorokynurenic acid (DCKA, 0.37 microM) on T5G-stimulated calcium entry was examined. A significant inhibition of the T5G-stimulated response in the presence of DCKA was observed at ethanol concentrations as low as 20 mM. These results support previous findings that T5G is a potent agonist of the NMDA receptor and indicate that stimulation of calcium entry by this agonist is less sensitive to ethanol inhibition than stimulation by NMDA.

N-methyl-D-aspartic acid receptor agonists: resolution, absolute stereochemistry, and pharmacology of the enantiomers of 2-amino-2-(3-hydroxy-5-methyl-4-isoxazolyl)acetic acid

(R,S)-2-Amino-2-(3-hydroxy-5-methyl-4-isoxazolyl)acetic acid [(R,S)-AMAA, 4] is a potent and selective agonist at the N-methyl-D-aspartic acid (NMDA) subtype of excitatory amino acid receptors. Using the Ugi "four-component condensation" method, the two diastereomers (2R)- and (2S)-2-[3-(benzyloxy)-5-methyl-4-isoxazolyl]N-tert-butyl-2- [N-[(S)-1-phenylethyl]benzamido]-acetamide (16 and 17, respectively) were synthesized and separated chromatographically. The absolute stereochemistry of 16 was confirmed by an X-ray analysis. Deprotection of these intermediates did, however, provide (R)- (8) and (S)- (9) AMAA, respectively, in extensively racemized forms. N-BOC-protected (R,S)-AMAA (21) was successfully resolved via diastereomeric salt formation using cinchonidine. The stereochemical purity and stability of 8 and 9 obtained via this resolution were determined using chiral HPLC. (R)-AMAA (8) showed peak affinity for [3H]AMPA receptor sites (IC50 = 72 +/- 13 microM) and was shown to be a more potent inhibitor of [3H]CPP binding (IC50 = 3.7 +/- 1.5 microM) than (S)-AMAA (9) (IC50 = 61 +/- 6.4 microM). Neither enantiomer of AMAA affected [3H]kainic acid receptor binding significantly. In electrophysiological studies using rat brain tissue, 8 (EC50 = 7.3 +/- 0.3 microM) was 1 order of magnitude more potent than 9 (EC50 = 75 +/- 9 microM) as an NMDA receptor agonist.

Differentiation of in vivo effects of AMPA and NMDA receptor ligands using drug discrimination methods and convulsant/anticonvulsant activity

The discriminative stimulus properties of the AMPA ((RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid) receptor agonist ATPA ((RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propionic acid) and NMDA (N-methyl-D-aspartic acid) in rats have been characterized. It is suggested that the cues are mediated by separate mechanisms in the central nervous system. The ATPA cue is not mimicked by NMDA or an NMDA receptor agonist, and is inhibited by the AMPA receptor antagonist (R)-APPA ((R)-2-amino-3-(3-hydroxy-5-phenylisoxazol-4-yl)propionic acid) but not the AMPA receptor antagonist ATOA ((RS)-2-amino-3-(3-carboxymethoxy-5-tert-butylisoxazol-4-yl)propio nic acid) or the NMDA receptor antagonist CPP ((RS)-3-(2-carboxypiperazin-4-yl)propyl)phosphonic acid). The ATPA cue is not mimicked by AMPA which is believed not to penetrate the blood-brain barrier. In contrast, ATPA does not generalize to the NMDA cue, which is mimicked by some NMDA receptor agonists (tetrazol-5-yl-glycine and AMAA ((RS)-2-amino-2-(3-hydroxy-5-methylisoxazol-4-yl)acetic acid)) and is inhibited by the NMDA receptor antagonist CPP. Highly potent convulsant activity was demonstrated in mice with all AMPA and NMDA receptor agonists after intracerebroventricular (i.c.v.) injection, whereas weaker or no effects were found after subcutaneous (s.c.) or intravenous injection. Only (RS)-tetrazol-5-yl-glycine had a potent effect after s.c. administration. I.c.v. ATOA and CPP inhibited convulsions induced by i.c.v. injection of AMPA or NMDA, while (R)-APPA was ineffective. These results indicate that there are differences in the structure-activity relations in the drug discrimination and convulsant/anticonvulsant models, even when effects after i.c.v. and s.c. injection are taken into consideration. The convulsion models are rapid tests which can give an indication of central nervous system penetration, but are less pharmacologically specific with respect to differentiation between AMPA and NMDA ligands where cue models demonstrate clear differences in effects of ligands with selectivity for receptor subtypes.

Excitatory amino acid receptor ligands: asymmetric synthesis, absolute stereochemistry and pharmacology of (R)- and (S)-homoibotenic acid

The (R)- and (S)-forms of 2-amino-3-(3-hydroxyisoxazol-5-yl)propionic acid (homoibotenic acid, HIBO) were synthesized, using (S)-BOC-phenylalanine as a chiral auxiliary and their absolute stereochemistry correlated with that of (R)-Br-HIBO. The enantiomeric excesses for (R)-HIBO (1) (> 99.5%) and (S)-HIBO (2) (99.5%) were determined using chiral HPLC. Whereas compounds 1 and 2 were equipotent inhibitors of the binding of [3H]glutamic acid in the presence of calcium chloride, 2 showed AMPA agonist activity and 1 very weak NMDA agonist activity.

2-(Carboxycyclopropyl)glycines: binding, neurotoxicity and induction of intracellular free Ca2+ increase

The excitatory actions of the eight stereoisomers of 2-(carboxycyclopropyl)glycine (CCG), conformationally rigid glutamate analogues, were analyzed for the glutamate receptor subtypes by means of binding assays with rat brain membranes. All CCG isomers inhibited the binding of [3H]3-(2-carboxypiperazine-4-yl)propyl-1-phosphonic acid ([3H]CPP) to N-methyl-D-aspartate (NMDA) receptors. The (2S,3R,4S) isomer (L-CCG-IV) was the most potent agonist for the NMDA receptor and its binding potency was 17- and 790-fold higher than that of L-glutamate and NMDA, respectively. The (2S,3S,4R) isomer (L-CCG-III) showed a potent inhibitory activity for [3H]D-aspartate uptake. Further, L-CCG-IV caused a marked increase of intracellular free Ca2+ concentration [( Ca2+]i) and potent neurotoxicity in the single rat cerebral cortical neurons in vitro, and both were blocked effectively by the NMDA antagonists. Significant correlations were observed between neurotoxicity and the increase of [Ca2+]i and [3H]CPP binding affinity to the NMDA receptor.

Identification of a novel NMDA receptor in rat cerebellum

Specific binding sites for the noncompetitive N-methyl-D-aspartic acid (NMDA) receptor antagonist, [3H]MK-801, were identified in synaptic membranes isolated from rat cerebellum. The density of these sites (0.61 pmol/mg protein), derived from linear Scatchard plots, was lower than those measured in a number of forebrain regions (0.81-2.96 pmol/mg protein). The Kd value for cerebellar [3H]MK-801 binding sites (37.7 nM) was markedly higher than those (1.53-1.82 nM) detected in rat forebrain regions. Experiments were carried out in the presence of 30 microM L-glutamic acid and 1 microM glycine, and after a 210 min incubation [3H]MK-801 binding was maximally stimulated. The pharmacology of these cerebellar [3H]MK-801 binding sites was markedly different from that of [3H]MK-801 sites in the rat cortex. These data have highlighted a novel population of NMDA receptors, which are functionally coupled to an ion channel but exhibit remarkably weak affinity for MK-801.