Metabotropic Excitatory Amino Acid Research Articles

Synthesis and Structure–Activity Relationship Studies of Novel 2-Diarylethyl Substituted (2-Carboxycycloprop-1-yl)glycines as High-Affinity Group II Metabotropic Glutamate Receptor Ligands

The major excitatory neurotransmitter in the central nervous system, ( S)-glutamic acid ( 1 ), activates both ionotropic and metabotropic excitatory amino acid receptors. Its importance in connection to neurological and psychiatric disorders has directed great attention to the development of compounds that modulate the effects of this endogenous ligand. Whereas l-carboxycyclopropylglycine ( l-CCG-1, 2 ) is a potent agonist at, primarily, group II metabotropic glutamate receptors, alkylation of 2 at the α-carbon notoriously result in group II mGluR antagonists, of which the most potent compound described so far, LY341495 ( 12 ), displays IC 50 values of 23 and 10 nM at the group II receptor subtypes mGlu2 and mGlu3, respectively. In this study we synthesized a series of structural analogues of 12 in which the xanthyl moiety is replaced by two substituted-phenyl groups. The pharmacological characterization shows that these novel compounds have very high affinity for group II mGluRs when tested as their racemates. The most potent analogues demonstrate K i values in the range of 5–12 nM, being thus comparable to LY341495 ( 12 ).

Ionotropic excitatory amino acid receptor ligands. Synthesis and pharmacology of a new amino acid AMPA antagonist

We have previously described the potent and selective ( RS)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor agonist, ( RS)-2-amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid (ACPA), and the AMPA receptor antagonist ( RS)-2-amino-3-[3-(carboxymethoxy)-5-methyl-4-isoxazolyl]propionic acid (AMOA). Using these AMPA receptor ligands as leads, a series of compounds have been developed as tools for further elucidation of the structural requirements for activation and blockade of AMPA receptors. The synthesized compounds have been tested for activity at ionotropic excitatory amino acid (EAA) receptors using receptor binding and electrophysiological techniques, and for activity at metabotropic EAA receptors using second messenger assays. Compounds 1 and 4 were essentially inactive. ( RS)-2-Amino-3-[3-(2-carboxyethyl)-5-methyl-4-isoxazolyl]propionic acid (ACMP, 2), on the other hand, was shown to be a selective AMPA receptor antagonist (IC 50 = 73 μM), more potent in electrophysiological experiments than AMOA (IC 50 = 320 μM). The isomeric analogue of 2, compound 5, did not show AMPA antagonist effects, but was a weak NMDA receptor antagonist (IC 50 = 540 μM). Finally, compound 3, which is an isomer of ACPA, turned out to be a very weak NMDA antagonist, and an AMPA receptor agonist approximately 1 000 times weaker than ACPA. None of the compounds showed agonist or antagonist effects at metabotropic EAA receptors.

3-Pyrazolone analogues of the 3-isoxazolol metabotropic excitatory amino acid receptor agonist homo-AMPA. Synthesis and pharmacological testing

We have previously shown that the higher homologue of ( S)-glutamic acid [( S)-Glu], ( S)-α-aminoadipic acid [( S)-α-AA] is selectively recognized by the mGlu 2 and mGlu 6 subtypes of the family of metabotropic glutamic acid (mGlu) receptors. Furthermore, a number of analogues of ( S)-α-AA, in which the terminal carboxyl group has been replaced by various bioisosteric groups, such as phosphonic acid or 3-isoxazolol groups, have been shown to interact selectively with different subtypes of mGlu receptors. In this paper we report the synthesis of the 3-pyrazolone bioisosteres of α-AA, compounds ( RS)-2-amino-4-(1,2-dihydro-5-methyl-3-oxo-3 H-pyrazol-4-yl)butyric acid ( 1) and ( RS)-2-amino-4-(1,2-dihydro-1,5-dimethyl-3-oxo-3 H-pyrazol-4-yl)butyric acid ( 2). At a number of steps in the reaction sequences used, the reactions took unexpected courses and provided products which could not be transformed into the target compounds, and attempts to synthesize the 2,5-dimethyl isomer of 2, compound 3, failed. An X-ray crystallographic analysis of the intermediate 1,2-dihydro-4-(2-hydroxyethyl)-2,5-dimethyl-3 H-pyrazol-3-one ( 5b) confirmed the expected regioselectivity of the reaction between methylhydrazine and α-acetylbutyrolactone ( 4). Neither 1 nor 2 showed significant effects at the different types of ionotropic glutamic acid receptors or at mGlu 1a (group I), mGlu 2 (group II), and mGlu 4a and mGlu 6 (group III) receptors, representing the three indicated groups of mGlu receptors.

Regulation of substantia nigra pars reticulata neuronal activity by excitatory amino acids.

Midbrain non-dopaminergic neurons of the substantia nigra pars reticulata play an important role in the basal ganglia circuitry. The regulation of their electrical activity by excitatory amino acid (EAA) inputs was investigated using in vivo electrophysiological methods in chloral hydrate-anaesthetized rats. We first determined the subtypes of EAA receptors present on reticulata neurons, using microiontophoretic application of selective agonists: kainic acid (KA), (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), N-methyl-D-aspartic acid (NMDA), and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (trans-ACPD). Each agonist activated reticulata neurons and the apparent rank order of efficacy was: KA> or =AMPA=NMDA>trans-ACPD. Using pressure or iontophoretic microejections of ionotropic and metabotropic receptor antagonists, we then investigated EAA receptor subtypes involved in the spontaneous firing rate of reticulata neurons. Kynurenic acid and (+/-)-2-amino-5-phosphonopentanoic acid (AP-5) markedly decreased the spontaneous firing rate of reticulata neurons, while 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) was much less effective. The metabotropic receptor antagonist (R,S)-alpha-methyl-4-carboxyphenylglycine (MCPG) failed to affect the spontaneous electrical activity. In contrast to CNQX, microapplications of AP-5 sometimes produced total inhibition. This powerful effect may reflect the potential importance of NMDA receptors in regulating the activity of some reticulata neurons. These results indicate that both functional ionotropic (NMDA and non-NMDA) and metabotropic EAA receptors are present on non-dopaminergic substantia nigra pars reticulata neurons. Moreover, in the anaesthetized animal, the spontaneous firing rate of these neurons, mediated by EAA inputs, seems mainly due to the tonic activation of ionotropic, but not metabotropic, receptors.

21] Signal transduction through ion channels associated with excitatory amino acid receptors

Excitatory amino acid (EAA) receptors are currently divided into two major categories according to their signal transduction processes in the mammalian central nervous system (CNS), including the ionotropic and metabotropic classes. The metabotropic EAA receptor family so far comprises at least eight different subunits (mGluR1-8) and is subclassified into three groups based on differences in structure and signal transduction systems. On the other hand, ionotropic EAA receptors linked to cation channels are pharmacologically and physiologically classified into two major categories according to differential sensitivities to N-methyl-D-aspartic acid (NMDA). The NMDA channel is highly permeable to Ca 2+ , with different ion channels coupled to non-NMDA receptors. This chapter discusses a receptor binding assay, an electrophoretic mobility shift assay (EMSA) and an immunoblotting analysis for evaluation of signal transduction from membranes to nuclei.

The metabotropic glutamate receptors of the vestibular organs

This research sought to test the presence and function of metabotropic excitatory amino acid receptors (mGluR) in the frog semicircular canal (SCC). The mGluR agonist ±1-aminocyclopentane- trans-1,3-dicarboxylate (ACPD) produced an increase in afferent firing rates of the ampullar nerve of the intact posterior canal. This increase was not due to a stimulation of cholinergic efferent terminals or the acetylcholine (ACh) receptor, since atropine, in concentrations which blocked the response to exogenous acetylcholine, did not affect the response to ACPD. Likewise, ACPD effects were not due to stimulation of postsynaptic NMDA receptors, since the NMDA antagonist d(−)-2-amino-5-phosphonopentanoate (AP-5) did not affect the response to ACPD, reinforcing the reported selectivity of ACPD for mGluRs. When the SCC was superfused with artificial perilymph known to inhibit hair cell transmitter release (i.e. low Ca-high Mg), ACPD failed to increase afferent firing. This suggests that the receptor activated by ACPD is located on the hair cell. Pharmacological evidence suggested that the mGluRs involved in afferent facilitation belong to Group I (i.e. subtypes 1 and 5). In fact, the Group III agonist AP-4 had no effect, and the ACPD facilitatory effect was blocked by the Group I mGluR antagonists ( S)-4-carboxyphenylglycine (CPG) and ( RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA). Additional pharmacological evidence supported the presence of Group I mGluRs. Interestingly, the mGluR antagonists, AIDA and 4CPG, by themselves did not affect the resting firing rates of ampullar afferents. This may suggest that the mGluRs are not involved in resting activity but perhaps only in evoked activity (as suggested in Guth et al. (1991) Hear. Res. 56, 69–78). In addition, the mRNA for the mGluR 1 has been detected in hair cells of both SCC, utricle, and saccule. In summary, the evidence points to an mGluR localized to the hair cell (i.e. an autoreceptor) which may be activated to produce a positive feedback augmentation of evoked but not resting transmitter release and thus affect afferent activity.

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.

Differential involvement of metabotropic and p75 neurotrophin receptors in effects of nerve growth factor and neurotrophin-3 on cultured Purkinje cell survival.

We have examined the role of the p75 neurotrophin receptor in survival-promoting effects of nerve growth factor (NGF) and neurotrophin-3 (NT-3) on cultured Purkinje cells. Previously, we showed that NGF promotes Purkinje cell survival in conjunction with (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), an agonist of metabotropic excitatory amino acid receptors, whereas NT-3 by itself increases cell number. We now present evidence that p75 plays different roles in Purkinje cell responses to the two neurotrophins. A metabotropic receptor of the mGluR1 subtype may interact with p75 function, so as to regulate Purkinje cell responsiveness to neurotrophins. When cerebellar cultures were grown for 6 days in the presence of ACPD and a mutant form of NGF that does not bind to p75, no increase in Purkinje cell number was observed. Moreover, the survival-promoting effect of wild-type NGF and ACPD could be inhibited by a neutralizing antiserum to p75 or by a pyrazoloquinazolinone inhibitor of neurotrophin binding to p75. In contrast, the response to NT-3 was potentiated by anti-p75 treatment and by the quinazolinone. These data indicate the mediation of p75 in the trophic response to NGF-ACPD and a negative modulatory role of p75 in the action of NT-3. To probe the role of ACPD in the p75-dependent response to NGF, metabotropic receptor subtype-specific ligands were tested. The pattern of agonist specificity implicated the mGluR1 subtype, a receptor that is expressed at high levels by Purkinje cells and linked to activation of protein kinase C (PKC). Down-regulation or blockade of PKC abolished the response to NGF-ACPD. Consistent with the opposite roles of p75 in effects of the two neurotrophins, blockade of mGluR1 or PKC potentiated the survival response elicited by NT-3. In sum, our data suggest that afferent excitatory transmitters activate specific metabotropic receptors to elicit a p75-mediated action of NGF. NT-3 acts on Purkinje cells by a different mechanism that is not absolutely p75-dependent and that is reduced by neurotrophin access to p75 and metabotropic receptor activity.

(S)-homo-AMPA, a specific agonist at the mGlu6 subtype of metabotropic glutamic acid receptors.

Our previous publication (J. Med. Chem. 1996, 39, 3188-3194) described (RS)-2-amino-4-(3-hydroxy-5-methylisoxazol-4-yl)butyric acid (Homo-AMPA) as a highly selective agonist at the mGlu6 subtype of metabotropic excitatory amino acid (EAA) receptors. Homo-AMPA has already become a standard agonist for the pharmacological characterization of mGlu6 (Trends Pharmacol. Sci. Suppl. 1997, 37-39), and we here report the resolution, configurational assignment, and pharmacology of (S)- (6) and (R)- (7) Homo-AMPA. Using the "Ugi four-component condensation", 3-(3-ethoxy-5-methylisoxazol-4-yl)propanal (10) was converted into the separable diastereomeric derivatives of 6 and 7, compounds 12 and 11, respectively. Deprotection of 12, in one or two steps, gave extensively racemized 6, which was converted in low yield into 6 (99.0% ee) through several crystallizations. 6 (99.7% ee) and 7 (99.9% ee) were finally obtained by preparative chiral HPLC. The configurational assignments of 6 and 7 were based on 1H NMR spectroscopic studies on 12 and 11, respectively, and circular dichroism studies on 6 and 7. Values of optical rotations using different solvents and the chiral HPLC elution order of 6 and 7 supported the results of the spectroscopic configurational assignments. The activities of 6 and 7 at ionotropic EAA (iGlu) receptors and at mGlu1-7 were studied. (S)-Homo-AMPA (6) was shown to be a specific agonist at mGlu6 (EC50 = 58 +/- 11 microM) comparable in potency with the endogenous mGlu agonist (S)-glutamic acid (EC50 = 20 +/- 3 microM). Although Homo-AMPA did not show significant effects at iGlu receptors, (R)-Homo-AMPA (7), which was inactive at mGlu1-7, turned out to be a weak N-methyl-D-aspartic acid (NMDA) receptor antagonist (IC50 = 131 +/- 18 microM).

Ontogeny of non-NMDA glutamate receptors in rat barrel field cortex: I. metabotropic receptors

The ontogeny of metabotropic excitatory amino acid receptors (mGluRs) in rat barrel field cortex was characterized by using receptor autoradiography and immunocytochemistry to test the hypothesis that changes in mGluR expression coincide with the emergence of somatotopic patterns in this region. On postnatal days 1 (P1) and 3, [3H]glutamate binding to mGluRs was not distributed in a somatotopic pattern. By P5, mGluRs exhibited a whisker-related pattern, with higher densities of mGluRs in barrel centers than in surrounding cortex. Between P5 and P14 and at P60, the overall binding density remained higher in barrels than in surrounding cortex. At P60, a somatotopic pattern of binding was not apparent. The majority of mGluR sites in the barrel field were blocked by the metabotropic agonist trans-1-aminocyclopentane-1,3-dicarboxylic acid but were not significantly displaced by quisqualate. Immunocytochemical studies of phosphoinositide-linked mGluRs, mGluR5 and mGluR1alpha, showed that the developmental expression of mGluR5 mirrored that of the pattern of autoradiographically labeled mGluRs. The immature barrel field (ages P5-P14) was enriched in mGluR5, with greater concentrations of mGluR5 immunoreactivity in barrels than in surrounding cortex. Within barrel centers, mGluR5 was localized within the neuropil, on the surfaces of cell bodies and dendrites in layer IV. A somatotopic pattern of mGluR5 immunoreactivity persisted into adulthood, although the pattern was less pronounced after P14. In contrast, mGluR1alpha was never localized in a somatotopic pattern in barrel field cortex. We conclude from the developmental localization of mGluRs that the spatiotemporal regulated expression of these receptors may influence barrel maturation and plasticity.

Molecular pharmacology of 4-substituted glutamic acid analogues at ionotropic and metabotropic excitatory amino acid receptors

The pharmacology of (2 S,4 R)-4-methylglutamic acid, (2 S,4 S)-4-methylglutamic acid and ( S)- and ( R)-4-methyleneglutamic acids (obtained in high chemical and enantiomeric purity from racemic 4-methyleneglutamic acid by chiral HPLC using a Crownpak CR(+) column), was examined in binding experiments using rat brain ionotropic glutamate receptors, and in functional assays using cloned metabotropic glutamate (mGlu) receptors. As a notable result of these studies, (2 S,4 R)-4-methylglutamic acid and (2 S,4 S)-4-methylglutamic acid were both shown to be selective for kainic acid receptors and mGlu receptors (subtypes 1 α and 2), respectively, whereas ( S)-4-methyleneglutamic acid showed high but rather non-selective affinity for the ( RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA), kainic acid, NMDA and mGlu receptors (subtypes 1 α and 2). Although none of the compounds were specific for any of the receptor subtypes, the results demonstrate that each of these structurally related compounds has a distinct pharmacological profile.

ChemInform Abstract: A New Highly Selective Metabotropic Excitatory Amino Acid Agonist: 2‐ Amino‐4‐(3‐hydroxy‐5‐methylisoxazol‐4‐yl)butyric Acid.

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Involvement of the glutamatergic metabotropic receptors in the regulation of glutamate uptake and extracellular excitatory amino acid levels in the striatum of chloral hydrate-anesthetized rats

The microdialysis technique was used to assess in vivo the putative functional role of metabotropic excitatory amino acid receptors in regulating extracellular levels of the excitatory amino acids glutamate and aspartate in the striatum of chloral hydrate-anesthetized rats. Addition of the metabotropic glutamate receptor antagonist (+)-α-methyl-4-carboxyphenylglycine (MCPG) (10−3 or 4 × 10−3 M) in the dialysis probe did not modify the basal extracellular levels of glutamate and aspartate but induced a significant dose-dependent decrease in the KCl-elicited elevation of glutamate and aspartate extracellular levels. The effect of MCPG on glutamate extracellular concentration under K+ stimulation was reduced by the simultaneous superfusion of the metabotropic glutamate receptor agonist (2S,3S,4S)-α-(carboxycyclopropyl)glycine (L-CCG) (10−3 M) which had no significant effect when tested alone. In contrast, L-CCG alone significantly potentiated the KCl-elicited elevation of aspartate extracellular concentrations but failed to modify the MCPG effect on this amino acid concentration. In a parallel series of experiments, high-affinity glutamate uptake was measured ex vivo 20 min after an in vivo injection of 10 pmol of MCPG in the striatum. MCPG was found unable to modify the glutamate uptake rate. In vitro, MCPG (1–1000 μM) again had no effect on glutamate transport rate. These data suggest that metabotropic excitatory amino acid receptors (1) may act to increase the extracellular levels of glutamate and aspartate under depolarizing conditions, and (2) may not have a major role in the regulation of high affinity glutamate uptake under basal conditions. In addition, it can be assumed that the control of glutamate and aspartate extracellular levels may involve different metabotropic receptors.

Activation of a Metabotropic Excitatory Amino Acid Receptor Potentiates Spike-Driven Calcium Increases in Neurons of the Dorsolateral Septum

(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), an agonist for metabotropic glutamate receptors (mGluRs), causes depolarization and burst firing in rat dorsolateral septal nucleus (DLSN) neurons and results in long-term potentiation (LTP) at DLSN synapses. In the present study, we investigated whether these actions of 1S,3R-ACPD are attributable to the release of calcium from an inositol triphosphate-sensitive store after activation of mGluRs coupled to phospholipase C. Our data demonstrated that the ACPD-induced depolarization was associated with a small but significant decrease, not an increase, in [Ca2+]i; however, changes of [Ca2+]i, during ACPD-induced bursting were up to seven times larger than those produced by regular firing. Depletion of internal calcium stores by thapsigargin or ryanodine had a small to insignificant effect on the maximum changes of [Ca2+]i, associated with ACPD-induced bursting. Thus, elevation of [Ca2+]i, during firing by 1S,3R-ACPD is likely attributable to enhancement of calcium influx through voltage-gated channels and not to calcium release from internal stores. ACPD-induced burst firing elevated somatic and dendritic calcium levels up to 3 and 6 microM, respectively. Such an increase may be the underlying mechanism for ACPD-induced LTP as well as ACPD-induced acute cell death in rat DLSN.

Excitatory Amino Acid Receptors in the Ventral Tegmental Area Regulate Dopamine Release in the Ventral Striatum

The role of excitatory amino acid (EAA) receptors located in the ventral tegmental area (VTA) in tonic and phasic regulation of dopamine release in the ventral striatum was investigated. Microdialysis in conscious rats was used to assess dopamine release primarily from the nucleus accumbens shell region of the ventral striatum while applying EAA antagonists or agonists to the VTA. Infusion of the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (25 and 100 microM) into the VTA did not affect dopamine release in the ventral striatum. In contrast, intra-VTA infusion of the NMDA receptor antagonist 2-amino-5-phosphopentanoic acid (100 and 500 microM) dose-dependently decreased the striatal release of dopamine. Intra-VTA application of the ionotropic EAA receptor agonists NMDA and AMPA dose-dependently (10 and 100 microM) increased dopamine efflux in the ventral striatum. However, infusion of 50 or 500 microM trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (ACPD), a metabotropic EAA receptor agonist, did not significantly affect these levels. These data suggest that NMDA receptors in the VTA exert a tonic excitatory influence on dopamine release in the ventral striatum. Furthermore, dopamine neurotransmission in this region may be enhanced by activation of NMDA and AMPA receptors, but not ACPD-sensitive metabotropic receptors, located in the VTA. These data further suggest that EAA regulation of dopamine release primarily occurs in the VTA as opposed to presynaptically at the terminal level.

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.

Agonists and Antagonists for Metabotropic Glutamate Receptors

<p>Glutamate (Glu) is a prominent excitatory amino acid (EAA) neurotransmitter in the central nervous system, and it exerts its actions at a number of different receptor subclasses. lonotropic Glu receptors, such as the N-methyl-D-aspartate (NMDA), 2- amino-3-(5-methyl-3-hydroxyisoxazol-4-yl)propanoic acid (AMPA), and kainic acid subtypes, are coupled to ion channels, and transduce signals by gating the flow of calcium and sodium ions into the cell. Metabotropic EAA receptors (mGluRs) are coupled through G-proteins to intracellular enzymes such as phospholipase C (PLC) and adenylate cyclase (AC). Agonist activation of mG!uRs coupled to PLC produces an increase in the intracellular concentrations of inositol triphosphate (IP3) and diacylglycerol (DAG), while agonist activation of mG!uRs coupled to AC produces a decrease in intracellular concentrations of cyclic­ adenosine monophosphate (cAMP). At least eight distinct metabotropic receptor proteins (mGluRl-8) have been identified, and these can be divided into three groups. Group 1 mG!uRs consist of mG!uRl and mG!uR5, and are PLC-coupled receptors. Group 2 mG!uRs include mG!uR2 and mG!uR3, and are negatively coupled to AC. </p><p> Group 3 mG!uRs consist of mGluR4, mG!uR6, mGluR 7and mGluR8, and they are also negatively coupled to AC. Many studies are currently underway to identify novel agonists and antagonists for these receptors. The discovery of such compounds will be an important step toward understanding the physiological and pathophysiological consequences of mG!uR activation in vivo, and will ultimately be essential in defining the therapeutic potential of selective mGluR agents. Novel aspects of the medicinal chemistry and pharmacology of metabotropic receptors are discussed in this review.

Modulation of sensory neurone excitatory and inhibitory responses in the ventrobasal thalamus by activation of metabotropic excitatory amino acid receptors

Several different types of metabotropic excitatory amino acid receptors (mGluRs) are present in the thalamus. We have previously shown that the agonists L-AP4 and CCG-I can have apparently presynaptic effects on GABAergic inhibitory transmission in the thalamus. In this study we attempted to characterize the different receptor types which may mediate these effects and the known post-synaptic excitatory actions of 1 S,3 R-ACPD in the ventrobasal thalamus, by using a number of agonists with different spectra of activity at the various mGluRs. Inhibitory responses in ventrobasal thalamic neurones of urethane-anaesthetized rats were evoked by air-jet stimuli to the vibrissae and extracellular recording methods were used to reveal inhibitory responses as an inhibition of excitatory responses in a condition-test paradigm. The Group II and Group III mGluR agonists L-AP4, CCG-I, DCG-IV, 1 S,3 R-ACPD and S-4C3HPG, applied in the vicinity of the recording site by iontophoresis, were found to reduce inhibitions revealed by the condition-test paradigm (by 67, 75, 50, 43 and 77% from control inhibitions, respectively). The endogenous mGluR agonist l-serine- O-phosphate (L-SOP) was found to have similar, although weaker, actions (31% reduction of inhibition), while the Group I agonist 3,5-DHPG had little effect in this test (9% reduction of inhibition). In contrast, both 3,5-DHPG and 1 S,3 R-ACPD had direct excitatory actions on VB neurones, and these could be antagonized by 4CPG. The effects of CCG-I in the condition-test paradigm could be antagonized by the antagonists MCCG, MCPG, but not MAP4. In contrast, we have previously shown that MAP4 was able to block the effects of L-AP4. These data suggest that there may be at least two types of receptor mediating the disinhibitory effects, which may belong to the Group II and Group III type of mGluR, whereas the postsynaptic excitatory responses to mGluR agonists are probably mediated via Group I receptors (possibly mGluR1).

Metabolic, hormonal, and hemodynamic changes induced by metabotropic excitatory amino acid agonist (1S,3R)-ACPD.

The purpose of the present study was to determine whether central administration of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), a selective metabotropic glutamate receptor agonist, would stimulate glucose metabolism, activate the hypothalamic-pituitary-adrenal axis, or influence pancreatic endocrine secretion. Intracerebroventricular injection of ACPD increased arterial glucose levels by 60% within 15 min, which were sustained throughout the 3-h experimental protocol. This hyperglycemia resulted from an early increase in hepatic glucose production (HGP, 88%) that exceeded the increase in glucose uptake by peripheral tissues (66%). Stimulation of glucose metabolism was associated with transient elevations in plasma insulin (145%) and glucagon (3-fold) levels and more sustained elevations in corticosterone (141%), epinephrine (3- to 5-fold), and norepinephrine (32-110%). Intravenous infusion of alpha- and beta-adrenergic antagonists prevented the ACPD-induced increase in glucose metabolism. Arterial blood pressure, cardiac index, and total peripheral resistance were not altered after ACPD. Overall, the changes in regional blood flow were unremarkable, although ACPD did increase blood flow to the liver (2-fold) and heart (48%) and decrease flow to the stomach (33%). These results indicate that central administration of ACPD 1) enhances HGP, which is primarily mediated by adrenergic stimulation; 2) increases glucose uptake by peripheral tissues, which appears to be mediated by both hyperinsulinemia and hyperglycemia; 3) stimulates pancreatic and adrenal hormone secretion independent of adrenergic activation; and 4) produces minimal changes in regional blood flow that cannot explain the glucose metabolic response produced by ACPD.(ABSTRACT TRUNCATED AT 250 WORDS)

Distinct presynaptic metabotropic receptors for l-AP4 and CCG1 on GABAergic terminals: Pharmacological evidence using novel α-methyl derivative mGluR antagonists, MAP4 and MCCG, in the rat thalamus in vivo

A variety of metabotropic excitatory amino acid receptors are present in the thalamus. We have investigated the possibility that some of these receptors may have presynaptic effects on GABAergic inhibitory transmission in the thalamus. Inhibitory responses in ventrobasal thalamic neurons of urethane-anaesthetized rats were evoked by either air-jet stimuli to the vibrissae or by electrical stimulation of the somatosensory cortex. Both intracellular and extracellular recording methods were used to reveal inhibitory responses, either as inhibitory postsynaptic potentials or inhibition of excitatory responses in a condition-test paradigm. The metabotropic glutamate receptor agonists (S)-2-amino-4-phosphonobutyrate (L-AP4) and (2S,3S,4S)-alpha-(carboxycyclopropyl)-glycine (CCG1), applied in the vicinity of the recording site by iontophoresis, were found to reduce the amplitudes of inhibitory postsynaptic potentials (to 76% and 63% of control amplitudes, respectively) and inhibitions revealed by the condition-test paradigm (to 33% and 28% of control inhibitions, respectively). As the inhibitory responses arise from the neurons of the nucleus reticularis thalami, some distance away from the site of recording and iontophoretic drug application, it is likely that the reduction of inhibition seen with L-AP4 and CCG1 is due to an action of these agonists on the terminals or axons of these inhibitory neurons. The novel antagonists of L-AP4 and CCG1, alpha-methyl-L-AP4 and alpha-methyl-CCG1, were found to block the disinhibitory actions of the agonists in a differential manner when applied iontophoretically. This suggests that there may be at least two types of receptor mediating the disinhibitory effects.(ABSTRACT TRUNCATED AT 250 WORDS)