David C. Sunter

Competitive antagonism at metabotropic glutamate receptors by (S) -4-carboxyphenylglycine and (RS) -α-methyl-4-carboxyphenylglycine

Two phenylglycine derivates, (S)-4-carboxyphenylglycine and (RS)-alpha-methyl-4-carboxyphenylglycine, competitively antagonised (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD)-stimulated phosphoinositide hydrolysis in rat cerebral cortical slices. The same phenylglycine derivatives selectively antagonized ACPD-induced depolarization in neonatal rat spinal motoneurones and rate thalamic neurones relative to depolarization or excitation induced by N-methyl-D-aspartate (NMDA) or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA). Both phenylglycine derivatives also selectively depressed synaptic excitation in thalamic neurones evoked by noxious thermal stimuli, without affecting the synaptic stimulation of the same cells by non-noxious stimuli.

1S,3R-ACPD stimulates and L-AP3 blocks Ca2+ mobilization in rat cerebellar neurons.

We have used digital fluorescence imaging to compare the ability of the separate enantiomers of trans-ACDP to mobilize intracellular free calcium in cerebellar granule cells, maintained in primary culture. In addition, we have examined the ability of the separate D and L enantiomers of AP3 to antagonise this calcium mobilizing response

Phenylglycine derivatives as new pharmacological tools for investigating the role of metabotropic glutamate receptors in the central nervous system.

The possible roles of G-protein coupled metabotropic glutamate receptors in central nervous function are currently the focus of intensive investigation. The complexity of effects produced by agonists at these receptors probably reflects the activity of a range of sub-types. The metabotropic glutamate receptors first described are linked to phospholipase C, mediating phosphoinositide hydrolysis and release of Ca2+ from intracellular stores. A substance generally considered to be a selective agonist for these receptors is (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD). This substance not only stimulates phosphoinositide hydrolysis, but also inhibits cyclic AMP formation. A family of metabotropic glutamate receptors, incorporating both phospholipase C- and adenylcyclase-linked sub-types has been cloned. Various effects of metabotropic glutamate receptor agonists on membrane ion fluxes and synaptic events have been reported, including neuronal depolarization and/or excitation, hyperpolarization, inhibition of Ca(2+)-dependent and voltage-gated K+ currents, potentiation of N-methyl-D-aspartate-induced responses, depression of synaptic excitation and either induction or augmentation of long-term potentiation. To clarify the role of metabotropic glutamate receptors in central nervous activity and to aid the characterization of the various receptor types that may be involved, a range of highly selective agonists and antagonists is required. To date, currently available antagonists such as L-2-amino-3-phosphonopropionate and L-aspartic acid-beta-hydroxamate appear to be unselective and insufficiently potent. We report here the actions of three phenylglycine derivatives, the particular agonist and/or antagonist properties of which may help to elucidate the roles of metabotropic glutamate receptors in central nervous activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabotropic glutamate receptors contribute to the induction of long-term depression in the CA1 region of the hippocampus.

Long-term depression of synaptic transmission was induced following the prior induction of long-term potentiation in the CA1 region of rat hippocampal slices. We show that the induction of this form of synaptic depression can be prevented by (+)-alpha-methyl-4-carboxyphenylglycine, a selective antagonist of metabotropic glutamate receptors.

New phenylglycine derivatives with potent and selective antagonist activity at presynaptic glutamate receptors in neonatal rat spinal cord

The depression of the monosynaptic excitation of neonatal rat motoneurones produced by the metabotropic glutamate receptor (mGluR) agonists (1S,3S)-1-aminocyclopentane-1, 3-dicarboxylate (ACPD) or L-2-amino-4-phosphonobutyrate (L-AP4) was antagonized by three novel phenylglycine analogues: (RS)-alpha-methyl-4-sulphonophenylglycine (MSPG), (RS)-alpha-methyl-4-phosphonophenylglycine (MPPG) and (RS)-alpha-methyl-4-tetrazolylphenylglycine (MTPG). The potencies of all the new compounds were greater than that of the previously reported (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG). For L-AP4-sensitive presynaptic mGluRs, the order of antagonist potency found was MPPG > MSPG > MTPG > MCPG. In contrast, the order of antagonist potency found for (1S,3S)-ACPD-sensitive presynaptic mGluRs was MTPG > MPPG > MSPG > MCPG. To date, MPPG (KD 9.2 microM) is the most potent L-AP4-sensitive receptor antagonist yet tested on the neonatal rat spinal cord. In addition, MTPG (KD 77 microM) is the most potent antagonist yet tested for (1S,3S)-ACPD-sensitive receptors in this preparation.

Antagonism of presynaptically mediated depressant responses and cyclic AMP-coupled metabotropic glutamate receptors

The depression of monosynaptic excitation of neonatal rat motoneurones by (1S,3S)- and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD) and by L-2-amino-4-phosphonobutyrate (L-AP4), which is probably presynaptically mediated, is antagonized by (+/-)- and (+)-alpha-methyl-4-carboxyphenylglycine (MCPG). The same phenylglycine derivatives also antagonize the depression of forskolin-stimulated cyclic AMP synthesis effected by the two ACPD stereoisomers and by L-AP4. These results support previous suggestions that presynaptic depression is mediated by metabotropic glutamate receptors negatively coupled to adenylyl cyclase activity. MCPG is the first antagonist to be reported for these receptors.

Stereospecific antagonism by (+)-α-methyl-4-carboxyphenylglycine (MCPG) of (1S,3R)-ACPD-induced effects in neonatal rat motoneurones and rat thalamic neurones

The (+)-enantiomer of alpha-methyl-4-carboxyphenylglycine (MCPG) stereoslectively antagonized the depolarization of neonatal rat motoneurones and the excitation of rat thalamic neurons induced by the specific metabotropic glutamate receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD). (+)-MCPG preferentially reduced (1S,3R)-ACPD-induced responses relative to responses induced by (S)-alpha-amino-3-hydorxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA).

Mediation of thalamic sensory responses in vivo by ACPD-activated excitatory amino acid receptors.

The existence of the so‐called metabotropic excitatory amino acid receptor has been known for some years. Various functions have been suggested for this receptor, but the lack of selective antagonists for (IS, 3R)‐aminocyclopentane dicarboxylic acid (ACPD) has precluded the direct demonstration of a functional role for this receptor in synaptic processes. We describe here a specific antagonism of the excitatory responses of thalamic neurons to ACPD by two novel antagonists, and a parallel antagonism by these compounds of sensory synaptic responses to noxious stimuli of the same neurons. This provides the first direct pharmacological evidence for a functional role of ACPD‐sensitive receptors in central neurotransmission, and indicates that these receptors may play an important part in central sensory processes.

The actions of phenylglycine derived metabotropic glutamate receptor antagonists on multiple (1S,3R)-ACPD responses in the rat nucleus of the tractus solitarius

The effects of the metabotropic glutamate receptor (mGluR) agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD] and a series of phenylglycine-derived putative mGluR antagonists were examined on electrophysiological responses mediated by glutamate and GABA receptors in the nucleus of the tractus solitarius (NTS) in transverse brainstem slices of the rat. Monosynaptic excitatory currents (EPSCs) evoked by electrical stimulation in the region of the tractus solitarius (TS) were reduced in the presence of (1S,3R)-ACPD in > 90% of neurons recorded in the dorsomedial subdivision of the NTS adjacent to the area postrema (AP). Monosynaptic evoked inhibitory currents (IPSCs) were similarly inhibited by (1S,3R)-ACPD. The inward current evoked by pressure application of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (IAMPA) was potentiated in the presence of (1S,3R)-ACPD, whereas the outward current evoked by the gamma-amino-butyric acid-A (GABA-A) receptor agonist muscimol (IMUSC) was inhibited. (1S,3R)-APCD also produced a postsynaptic inward current (IK(ACPD)) associated with a decrease in membrane conductance in approximately 50% of cells. The novel mGluR antagonists (S)-4-carboxy-3-hydroxy-phenylglycine (4C3H-PG), (R,S)-4-carboxy-phenylglycine (4C-PG) and (R,S)-alpha-methyl-4-carboxy-phenylglycine (alpha M4C-PG) reversibly antagonized the effects of (1S,3R)-ACPD on EPSCs IPSCs, IAMPA and IMUSC. The first two compounds also displayed weak agonist activity. However, none of the antagonists significantly inhibited IK(ACPD) at concentrations which blocked (1S,3R)-ACPD effects on synaptic transmission. These results suggest that pharmacologically distinct mGluRs may be present in the NTS.

Direct separation of amino acid enantiomers using a chiral crown ether stationary phase: Application to 2-amino-ω-phosphonoalkanoic acids

The resolution of a series of 2-amino-omega-phosphonoalkanoic acid enantiomers using a crown ether chiral stationary phase is described. The method is applicable to other primary amino acid and shows some advantages over chiral derivatization with fluorometric detection. Optical isomers of under 0.5% may be quantified.