Discriminative stimulus effects of excitatory amino acid agonists in rats.

Sixteen male Sprague-Dawley rats were trained to discriminate 30 mg/kg i.p. NMDA from saline using a 2-lever operant procedure. Responding was maintained under a FR 32 schedule of food reinforcement. Substitution tests were completed with NMDA (3-56 mg/kg) and other putative excitatory amino acids, L-glutamate (30-560 mg/kg), L-aspartate (30-300 mg/kg), L-homocysteic acid (100-1500 mg/kg), L-cysteine (30-1000 mg/kg), monosodium glutamate (100-3000 mg/kg), kainic acid (0.1-3 mg/kg) and the selective NMDA receptor agonist, D,L-(tetrazol-5-yl)glycine (LY 285265) (0.01-1.0 mg/kg). LY 285265 fully substituted for NMDA and was approx 100-fold more potent than NMDA for producing NMDA-like discriminative stimulus effects. Partial substitution occurred with monosodium glutamate, L-glutamate and L-homocysteic acid, resulting in mean maximum levels of 49-59% NMDA-lever responding, however response rate decreases were only obtained with 3000 mg/kg monosodium glutamate, suggesting that behaviorally active doses of the other compounds may not have been fully studied. L-Cysteine, kainic acid and L-aspartate failed to substitute for NMDA or produce decreases in response rates. Unlike with other excitatory agonists tested, full substitution occurred only with LY 285265, providing evidence that selective NMDA receptor activation is the basis for the NMDA discriminative stimulus. These results also suggest that LY 285265 is a potent, systemically active, selective agonist for the NMDA receptor.

Electronic structures of cephalosporins and penicillins. 9. Departure of a leaving group in cephalosporins.

Molecular orbital calculations by the CNDO/2 method are used to study the potential energy surface for the stretching and rupturing of the CH2-OAc bond in a model cephalosporin structure, 7-amino-3-(acetoxymethyl)-3-cephem. The bond is easier to stretch and break when a nucleophilic group is in the vicinity of or attached to the beta-lactam carbonyl carbon (C8). The rate of acylation by a beta-lactam antibiotic at the receptor sites in bacterial cell-wall enzymes will be enhanced by a suitable leaving group at the 3' position. An orientational specificity is predicted for the direction of departure of the leaving group. Regardless of the direction the nucleophile approaches C8, the CH2-OAc bond is easiest to break when the acetate group departs from the alpha face of the molecule.

DL-tetrazol-5-ylglycine, a highly potent NMDA agonist: its synthesis and NMDA receptor efficacy.

At physiological pH, the spatial arrangement of the three charges of DL-tetrazol-5-ylglycine (5) could be viewed as similar to those found in certain conformations of the two excitatory amino acids (EAAs)--aspartic and glutamic acids. Given significant binding to one or more EAA receptors, 5 would offer unique modeling and perhaps biological opportunities. We have previously shown it to be the most potent NMDA agonist known, with a unique and marked in vitro neutrotoxicity at depolarizing concentrations. Now we report the details required for its synthesis, together with its potency and efficacy in two assays of functional activation of the NMDA receptor, namely agonist-influenced [3H]MK801 binding and agonist-induced release of the neurotransmitter [3H]-norepinephrine from brain slices. In both these assays DL-tetrazol-5-ylglycine proved to be more potent and efficacious than NMDA and cis-methanoglutamate. It was more potent than, and equally efficacious to, L-glutamate in [3H]MK801 binding. The structural features of 5 may well reflect optimal agonist interaction at the NMDA receptor site. (We considered the possibility that some decarboxylation of DL-tetrazol-5-ylglycine may have occurred during testing. This would give 5-(aminomethyl)tetrazole (13), the tetrazole acid analog of glycine; and glycine is involved in NMDA receptor activation. Compound 13 does not affect [3H]glycine binding at the strychnine-insensitive glycine binding site, and [3H]MK801 binding studies showed that the (aminomethyl)-tetrazole, even if is formed, would probably have no effect on the activity of tetrazol-5-ylglycine at the NMDA receptor.

The NMDA receptor agonist DL-(tetrazol-5-yl)glycine is a highly potent excitotoxin.

DL-(Tetrazol-5-yl)glycine is a highly selective N-methyl-D-aspartate (NMDA) receptor agonist with nanomolar in vitro potency. Previous work showed that DL-(tetrazol-5-yl)glycine has greater affinity and efficacy at NMDA receptors than other NMDA receptor agonists such as cis-methanoglutamate and NMDA. In this study, the in vivo excitotoxic potency of DL-(tetrazol-5-yl)glycine was compared to cis-methanoglutamate and NMDA. Adult (250-300 g) and neonatal (7-day-old) rats were anesthetized and compounds were unilaterally injected into the striatum. In adult rats DL-(tetrazol-5-yl)glycine (0.3-1.0 nmol/microliters) produced highly significant losses of striatal gamma-aminobutyric acid neurons (as indexed by glutamic acid decarboxylase activity) and cholinergic neurons (as indexed by choline acetyltransferase activity). Dose-response showed that DL-(tetrazol-5-yl)glycine was about 100 and 500 times more potent than cis-methanoglutamate and NMDA, respectively. In neonatal rats, DL-(tetrazol-5-yl)glycine (0.1-0.3 nmol/microliters) produced significant brain damage as indicated by brain weight losses 5 days later. DL-(Tetrazol-5-yl)glycine was about 50 and 150 times more potent than cis-methanoglutamate and NMDA, respectively, in the neonate. The excitotoxic potency of DL-(tetrazol-5-yl)glycine is likely due to its greater efficacy and potency at the NMDA receptor, when compared to other NMDA receptor agonists. The remarkable in vivo potency of DL-(tetrazol-5-yl)glycine in producing excitotoxic lesions makes it a useful agent to further probe NMDA receptor mediated excitotoxicity in brain pathologies.

D,L-(tetrazol-5-yl) glycine: a novel and highly potent NMDA receptor agonist.

This paper describes the pharmacological activity of D,L-(tetrazol-5-yl)glycine, a structurally novel and highly potent agonist at the N-methyl-D-aspartate (NMDA) subtype of excitatory amino acid receptor. D,L-(Tetrazol-5-yl)glycine potently displaced NMDA receptor binding to rat brain membranes as measured using [3H]CGS19755 (IC50 = 98 +/- 7 nM) and [3H]glutamate (IC50 = 36 +/- 18 nM) as ligands. D,L-(Tetrazol-5-yl)glycine did not appreciably inhibit the binding of D,L-alpha-[5-methyl-3H] amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), [3H]kainate, or [3H]glycine (IC50s greater than 30,000 nM). D,L-(Tetrazol-5-yl)glycine was more potent than NMDA or cis-methanoglutamate as a depolarizing agent in the rat cortical slice, and unlike these other agents induced rapid receptor-mediated neurotoxicity. Depolarization by D,L-(tetrazol-5-yl)glycine was antagonized by LY233053, a selective NMDA receptor antagonist. D,L-(Tetrazol-5-yl)glycine was a highly potent convulsant when given to neonatal rats (ED50 = 0.071 mg/kg i.p.). Convulsions in neonatal rats or lethality in mice induced by D,L-(tetrazol-5-yl)glycine were selectively antagonized by competitive and non-competitive NMDA receptor antagonists. D,L-(Tetrazol-5-yl)glycine is a structurally novel (tetrazole-substituted) compound that is a highly potent and selective NMDA receptor agonist. D,L-(Tetrazol-5-yl)glycine could be used to probe further NMDA receptor function in vitro and in vivo.

Mercapturic acid formation from 2-aminopyrazine in rats.

1. The compound 2-aminopyrazine was given by oral gavage to normal rats and their urine collected. 2. A mercapturic acid containing the 2-aminopyrazine moiety was isolated from this urine. This represents the first example of this type of a metabolite from a pyrazine.

Heteroatom-substitution as a strategy for increasing the potency of competitive NMDA antagonists.

We report the synthesis and characterization of compounds that are competitive NMDA receptor antagonists. Significant increases in affinity and potency were obtained by incorporation of a heteroatom into the substructure of the tetrazole-substituted amino acid LY233053.