Sulfate esters of hydroxy amino acids as stereospecific glutamate receptor agonists

An efficient synthesis of cis-4-hydroxyphosphonic and cis-4-hydroxyphosphinic analogs of pipecolic acid from cyclic enaminones

An expedient synthetic entry to cis-4-hydroxyphosphonic and cis-4-hydroxyphosphinic analogs of cis-4-hydroxypipecolic acid is presented in this paper. The main feature of this methodology is the highly regioselective addition of silyl phosphites or phosphonites to cyclic 1-benzyloxycarbonyl enaminones. Interestingly, the hydride reduction of the resulting 2-phospho-4-oxopiperidine proceeds with high diastereofacial preference using NaBH₄. In the last step, the cleavage of N-Cbz group under hydrogenolysis followed by the hydrolysis of the phosphonate or phosphinate functionalities, led to the target cis-4-hydroxyphosphonic and cis-4-hydroxyphosphinic acids, respectively.

Synthesis of novel enantiopure 4-hydroxypipecolic acid derivatives with a bicyclic beta-lactam structure from a common 3-azido-4-oxoazetidine-2-carbaldehyde precursor

Two different stereocontrolled accesses to new 4-hydroxypipecolic acid analogues with a bicyclic beta-lactam structure have been developed by using intramolecular reductive amination or allenic hydroamination reactions in 2-azetidinone-tethered azides. The access to the cyclization precursors was achieved from 3-azido-4-oxoazetidine-2-carbaldehyde via metal-mediated carbonyl-allenylation in aqueous environment or by organocatalytic direct aldol reaction. The tin hydride-promoted cyclization of the 2-azetidinone-tethered azidoallene is totally regioselective for the central allenic carbon providing a fused piperidine.

Synthesis of Enantiopure 4-Hydroxypipecolate and 4-Hydroxylysine Derivatives from a Common 4,6-Dioxopiperidinecarboxylate Precursor

tert-Butyl 2-substituted 4,6-dioxo-1-piperidinecarboxylates 4 have been prepared in good yield starting from Boc-Asp-O(t)Bu and other beta-amino acids. By analogy with chiral tetramic acids, their reduction by NaBH(4) in CH(2)Cl(2)/AcOH afforded the corresponding cis-4-hydroxy delta-lactams in good yield and stereoselectivity (68-98% de). In the absence of the A(1,3) strain (reduction of 6-substituted 2,4-dioxo-1-piperidines 7), the cis-4-hydroxy isomer was still obtained as the major product but the de values were consistently lower. 4-Hydroxy-6-oxo-1,2-piperidinedicarboxylate 2a, readily accessible from Boc-Asp-O(t)Bu (three steps, 63% overall yield), has proven to be an excellent building block for the synthesis of cis- and trans-4-hydroxypipecolates 17 and 24 (52 and 36% overall yield, respectively) and for the synthesis of a protected 4-hydroxylysine derivative 29 (41% overall yield).

Isolations of N-methyl-D-aspartic acid-type glutamate receptor ligands from Micronesian sponges

The bioassay-guided fractionation of the water-soluble extract of the marine sponge Cribrochalina olemda collected in Palau resulted in the isolation of a new amino acid cribronic acid (1): (2S,4R,5R)-5-hydroxy-4-sulfooxypiperidine-2-carboxylic acid. However, aqueous extracts of Stylotella aurantium and Axinella carteri collected in Yap State, Micronesia, afforded a known N-methyl-d-aspartic acid (NMDA)-type glutamate receptor agonist, (2S,4S)-4-sulfooxypiperidine-2-carboxylic acid (2), as a common active principle. Both 1 and 2 induced convulsive behaviors in mice upon intracerebroventricular (icv) injection with ED(50) values of 29 +/- 3.0 and 20 +/- 2.8 pmol/mouse, respectively. Radioligand binding assay using rat cerebrocortical membrane demonstrated that 1 and 2 inhibit the binding of the labeled NMDA receptor ligand [(3)H]CGP39653 at IC(50) values of 83 +/- 15 and 214 +/- 20 nM, respectively. However, 1 and 2 did not displace [(3)H]kainic acid or [(3)H]AMPA. These data indicated that 1 is a selective NMDA-type glutamate receptor ligand with potent convulsant activity in mice.

5-hydroxyindole causes convulsions and increases transmitter release in the CA1 region of the rat hippocampus

1 5-hydroxyindole (5-OHi) is a proposed tryptophan metabolite able to cause convulsions when systemically injected into rodents. We studied its effects using microdialysis in vivo and electrophysiological approaches in vitro. 2 Local administration of 5-OHi into the CA1 region of the rat hippocampus, via a microdialysis probe, significantly increased glutamate concentrations in the dialysates. 3 In rat hippocampal slices, using extracellular recordings in the CA1 region, 5-OHi (30-300 microM) increased the amplitude of population spikes and fEPSPs. 4 In the same preparation, using intracellular recordings in CA1 pyramidal neurons, 5-OHi reduced the latency of firing induced by direct depolarization and increased both evoked excitatory and slow inhibitory postsynaptic potential amplitudes, without affecting the resting membrane potential, the after-hyperpolarization or the neuronal input resistance. It also altered GABA(A)-mediated neurotransmission by increasing the frequency and the amplitude of pharmacologically isolated spontaneous inhibitory postsynaptic currents (sIPSC). 5 In separate experiments, performed by measuring AMPA or NMDA-induced depolarization in cortical wedges, 5-OHi did not modify glutamate receptor agonist responses. 6 Our results show that 5-OHi causes convulsions, modifies the properties and the function of the hippocampal circuitry, and facilitates the output of both excitatory and inhibitory transmitters.

The novel and systemically active metabotropic glutamate 1 (mGlu1) receptor antagonist 3-MATIDA reduces post-ischemic neuronal death

We examined the pharmacological properties of 3-methyl-aminothiophene dicarboxylic acid (3-MATIDA) by measuring second messenger responses in baby hamster kidney cells stably transfected with mGlu1a, mGlu2, mGlu4a or mGlu5a receptors and ionotropic glutamate receptor agonist-induced depolarizations in mouse cortical wedges. 3-MATIDA was a potent (IC(50)=6.3 microM, 95% confidence limits 3-15) and relatively selective mGlu1 receptor antagonist. When tested on mGlu2, mGlu4 or mGlu5 receptors its IC(50) was >300 microM. When tested in cortical wedges, however, 3-MATIDA was also able to antagonize AMPA or NMDA responses with an IC(50) of 250 microM. When present in the incubation medium of cultured murine cortical cells, 3-MATIDA (1-100 microM) significantly reduced the death of neurons induced by 60 min of oxygen and glucose deprivation (OGD), even when added up to 60 min after OGD. A similar neuroprotective activity was observed when 3-MATIDA was present at 10-100 microM in the medium of rat organotypic hippocampal slice cultures exposed to 30 min OGD. Systemic administration of 3-MATIDA (3-10 mg/kg, immediately and 1 h after the onset of ischemia) reduced the volume of brain infarcts following permanent middle cerebral artery occlusion in rats. Our results show that 3-MATIDA is a potent and possibly selective mGlu 1 receptor antagonist that may be considered as a novel prototype neuroprotective agent.

Alkyl Substituent Effects on Pipecolyl Amide Isomer Equilibrium: Efficient Methodology for Synthesizing Enantiopure 6-Alkylpipecolic Acids and Conformational Analysis of Their N-Acetyl N'-Methylamides

Enantiopure 6-alkylpipecolic acid hydrochlorides 1a-e were synthesized in five steps and 15-59% overall yields from alpha-tert-butyl beta-methyl N-(PhF)aspartate (3) via an approach featuring selective hydride reduction to the corresponding aspartate beta-aldehyde 2, aldol condensations with the enolates of various methyl alkyl ketones, and diastereoselective intramolecular reductive aminations. The influence of the 6-position substituent on the equilibrium and the energy barrier for isomerization of the amide N-terminal to pipecolate was then explored via the synthesis of N-acetyl N'-methylpipecolinamide (16) and its (2S,6R)-6-tert-butylpipecolinamide counterpart 17, and their conformational analysis by proton NMR spectroscopy and coalescence experiments. The presence of the tert-butyl substituent augmented the population of the amide cis-isomer and lowered the barrier for pipecolyl amide isomerization in water. Compared with the results from our previous examination of N-acetyl-5-tert-butylproline N'-methylamides (Beausoleil, E.; Lubell, W. D. J. Am. Chem. Soc. 1996, 118, 12902), the consequences of the bulky 6-alkyl substituent on the acetamide geometry and isomerization barrier were less pronounced in the pipecolate series relative to the respective proline amides.

Electrophysiological studies on oxindole, a neurodepressant tryptophan metabolite

1. The aim of the present work was to investigate the electrophysiological effects of oxindole, a tryptophan metabolite present in rat blood and brain, and recently proposed as a contributing factor in the pathogenesis of hepatic encephalopathy. 2. Using rat hippocampal slices in vitro and extra- or intracellular recordings, we evaluated oxindole effects on the neurotransmission of the CA1 region following orthodromic stimulation of the Schaffer collaterals. 3. Oxindole (0.3-3 mM) decreased the amplitude of population spikes extracellularly recorded at the somatic level and of the fEPSPs recorded at the dendritic level. In intracellular recordings, oxindole (0.1-3 mM) did not affect the resting membrane potential or the neuronal input resistance, but reduced the probability of firing action potentials upon either synaptic or direct activation of the pyramidal cells. 4. Oxindole (0.3-3 mM) increased the threshold and the latency of firing action potentials elicited by depolarizing steps without changing the duration or the peak amplitude of the spikes. It also significantly increased the spike frequency adaptation induced by long lasting (400 ms) depolarizing stimuli. 5. In separate experiments, performed by measuring AMPA or NMDA-induced responses in cortical slices, oxindole (1-3 mM) did not modify glutamate receptor agonist responses. 6. Our results show that concentrations of oxindole which may be reached in pathological conditions, significantly decrease neuronal excitability by modifying the threshold of action potential generation.

Glutamatergic enteric neurons

We tested the hypothesis that glutamate, the major excitatory neurotransmitter of the CNS, is also an excitatory neurotransmitter in the enteric nervous system (ENS). Glutamate immunoreactivity was found in cholinergic enteric neurons, many of which were identified as sensory by their co-storage of substance P and/or calbindin. Glutamate immunoreactivity was concentrated in terminal varicosities with a majority of small clear synaptic vesicles. The immunoreactivities of both AMPA and NMDA receptor subunits were also detected on neurons in both submucosal and myenteric plexuses. The immunoreactivity of the EAAC1 neuronal glutamate transporter was widespread in both plexuses. Glutamate evoked depolarizing responses in myenteric neurons that had fast and slow components. The fast component was mimicked by AMPA, and the slow component was mimicked by NMDA. The fast component and the response to AMPA mimicked fast EPSPs evoked in 2/AH neurons; moreover, fast EPSPs as well as fast glutamate and AMPA responses were blocked by selective AMPA antagonists and potentiated by the glutamate uptake inhibitor L-(-)-threo-3-hydroxyaspartic acid. These observations demonstrate, for the first time, the presence of glutamatergic neurons and glutamate-mediated neurotransmission in the ENS.

Pharmacological characterization of metabotropic glutamate receptors potentiating NMDA responses in mouse cortical wedge preparations

1. Mouse cortical wedge preparations were used in order to study the effects of metabotropic glutamate receptor (mGluR) agonists and antagonists on the depolarization induced by N-methyl-D-aspartate (NMDA) or by (S)-alpha-amino-4-bromo-3-hydroxy-5-isoxazolepropionic acid (AMPA). 2. (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) (30-300 microM) significantly potentiated the depolarizations induced by NMDA, leaving unchanged those mediated by AMPA. This potentiation developed slowly and lasted for up to 60 min provided that the slices were continuously perfused with the mGluR agonist. 3. Concentration-response curves to NMDA in the absence and in the presence of 1S,3R-ACPD (100 microM) indicated that the potentiation was due to increased affinity of the NMDA receptor complex for its agonist. The maximal responses to NMDA were not potentiated. 4. Selective agonists of group 1 mGluR such as quisqualate (Quis) (30 microM) or (RS)-3,5-dihydroxyphenylglycine (DHPG) (300 microM) did not potentiate NMDA responses. Similarly, selective agonists of group 2 mGluRs, such as (2S,3S,4S)-alpha-carboxycyclopropyl-glycine (L-CCG-I) (3-30 microM), and of group 3, such as L-2-amino-4-phosphonobutyric acid (L-AP4) (100 microM) were inactive in our test. A number of other putative mGluR agents having partial agonist activity on mGluRs in brain slices and in expression systems, such as 1R,3S-ACPD (500 microM), DL-2-amino-3-phosphonopropionic acid (DL-AP3) (300 microM) and (S)-4-carboxy-3-hydroxyphenylglycine (S-4C3HPG; 500 microM), when placed in the experimental protocol we used, did not change NMDA responses. 5. Available mGluR antagonists, such as DL-AP3 (1 mM), (+)-alpha-methyl-4-carboxyphenylglycine (MCPG) (500 microM), S-4-carboxyphenylglycine (4CPG; 500 microM) and S-4-carboxy-3-hydroxyphenylglycine (S-4C3HPG; 500 microM), did not reduce 1S,3R-ACPD potentiation of NMDA responses. 6. It is concluded that the potentiation of NMDA currents induced by the mGluR agonist 1S,3R-ACPD, in mouse cortical wedges, has a pharmacological profile which is different from that of the three mGluR groups so far described in expression systems.

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.