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

Regioselective Synthesis of α-Quaternary Amino Acid Derivatives Enabled by Photoinduced Energy Transfer

α-Quaternary amino acids have found application in many biologically relevant compounds and pharmaceuticals. Although there are many methods for the synthesis of α-quaternary amino acids, most of them are mainly realized with the aid of transition metals and complex ligands. We present herein a 2,7-Br-4CzIPN catalyzed regioselective alkylation of azlactones with redox-active esters via radical-radical couplings. Strikingly, this approach is devoid of any metal or additive and shows broad scope and superior sensitive functional group compatibility.

Selective and Catalytic Hydrocarboxylation of Enamides and Imines with CO2 to Generate α,α-Disubstituted α-Amino Acids

The first catalytic hydrocarboxylation of enamides and imines with CO₂ to generate valuable α,α-disubstituted α-amino acids is reported. Notably, excellent chemo- and regio-selectivity are achieved, significantly different from previous reports on β-carboxylation of enamides, homocoupling or reduction of imines. Moreover, this transition-metal-free procedure exhibits low loading of an inexpensive catalyst, easily available substrates, mild reaction conditions, high efficiency, facile scalability and easy product derivatization, providing great potential for application in organic synthesis, pharmaceutical chemistry, and biochemistry.

Phenylglycine analogs are inhibitors of the neutral amino acid transporters ASCT1 and ASCT2 and enhance NMDA receptor-mediated LTP in rat visual cortex slices

The N-methyl-d-aspartate receptor (NMDA) co-agonist d-serine is a substrate for the neutral amino acid transporters ASCT1 (SLC1A4) and ASCT2 (SLC1A5). We identified l-phenylglycine (PG) and its analogs as inhibitors of ASCT1 and ASCT2. PG analogs were shown to be non-substrate inhibitors of ASCT1 and ASCT2 with a range of activities relative to other amino acid transport systems, including sodium-dependent glutamate transporters, the sodium-independent d-serine transporter asc-1 and system L. L-4-chloroPG was the most potent and selective ASCT1/2 inhibitor identified. The PG analogs facilitated theta-burst induced long-term potentiation in rat visual cortex slices in a manner that was dependent on extracellular d-serine. For structurally-related PG analogs, there was an excellent correlation between ASCT1/2 transport inhibition and enhancement of LTP which was not the case for inhibition of asc-1 or system L. The ability of PG analogs to enhance LTP is likely due to inhibition of d-serine transport by ASCT1/2, leading to elevated extracellular levels of d-serine and increased NMDA receptor activity. These results suggest that ASCT1/2 may play an important role in regulating extracellular d-serine and NMDA receptor-mediated physiological effects and that ASCT1/2 inhibitors have the potential for therapeutic benefit.

Group II metabotropic glutamate receptor-mediated activation of G-proteins in rat hippocampal and striatal membranes

Stimulation of G-proteins coupled with metabotropic glutamate receptors (mGlu receptors) was investigated by means of guanosine-5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPγS) binding assay in rat hippocampal and striatal membranes. The endogenous ligand l-glutamate increased specific [(35)S]GTPγS binding in a concentration-dependent manner with a mean EC50 values of around 10μM in both brain regions, The maximal % increase over the respective basal binding was highest in cerebral cortex, intermediate in hippocampus, and lowest in striatum. The pharmacological profiles of the responses investigated with a series of glutamatergic agonists and antagonists clearly indicated that they were mediated through group II mGlu receptors, particularly mGlu2 subtype, in both brain regions. The pEC50 and relative %Emax values for a series of agonists were essentially identical in both brain regions that were also correlated with those previously reported in cerebral cortical membranes. The selective allosteric potentiator of mGlu2 receptor subtype, LY487379, potentiated the increasing effects of l-glutamate at a maximally effective concentration of 1mM on specific [(35)S]GTPγS binding, without altering the basal unstimulated binding. It is concluded that [(35)S]GTPγS binding assay is applicable to rat hippocampal and striatal membranes to detect functional activation of Gαi/o proteins coupled with mGlu2 receptors.

Metabotropic glutamate receptors inhibit microglial glutamate release

Pro-inflammatory stimuli evoke an export of glutamate from microglia that is sufficient to contribute to excitotoxicity in neighbouring neurons. Since microglia also express various glutamate receptors themselves, we were interested in the potential feedback of glutamate on this system. Several agonists of mGluRs (metabotropic glutamate receptors) were applied to primary rat microglia, and the export of glutamate into their culture medium was evoked by LPS (lipopolysaccharide). Agonists of group-II and -III mGluR ACPD [(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid] and L-AP4 [L-(+)-2-amino-4-phosphonobutyric acid] were both capable of completely blocking the glutamate export without interfering with the production of NO (nitric oxide); the group-I agonist tADA (trans-azetidine-2,4-dicarboxylic acid) was ineffective. Consistent with the possibility of feedback, inhibition of mGluR by MSPG [(R,S)-α-2-methyl-4sulfonophenylglycine] potentiated glutamate export. As the group-II and -III mGluR are coupled to Gα_i-containing G-proteins and the inhibition of adenylate cyclase, we explored the role of cAMP in this effect. Inhibition of cAMP-dependent protein kinase [also known as protein kinase A (PKA)] by H89 mimicked the effect of ACPD, and the mGluR agonist had its actions reversed by artificially sustaining cAMP through the PDE (phosphodiesterase) inhibitor IBMX (isobutylmethylxanthine) or the cAMP mimetic dbcAMP (dibutyryl cAMP). These data indicate that mGluR activation attenuates a potentially neurotoxic export of glutamate from activated microglia and implicate cAMP as a contributor to this aspect of microglial action.

A novel assay of Gi/o-linked G protein-coupled receptor coupling to potassium channels provides new insights into the pharmacology of the group III metabotropic glutamate receptors

The group III metabotropic glutamate receptors (mGluRs) represent a family of presynaptically expressed G-protein-coupled receptors (GPCRs) with enormous therapeutic potential; however, robust cellular assays to study their function have been difficult to develop. We present here a new assay, compatible with traditional high-throughput screening platforms, to detect activity of pharmacological ligands interacting with G(i/o)-coupled GPCRs, including the group III mGluRs 4, 7, and 8. The assay takes advantage of the ability of the Gbetagamma subunits of G(i) and G(o) heterotrimers to interact with G-protein regulated inwardly rectifying potassium channels (GIRKs), and we show here that we are able to detect the activity of multiple types of pharmacophores including agonists, antagonists, and allosteric modulators of several distinct GPCRs. Using GIRK-mediated thallium flux, we perform a side-by-side comparison of the activity of a number of commercially available compounds, some of which have not been extensively evaluated because of the previous lack of robust assays at each of the three major group III mGluRs. It is noteworthy that several compounds previously considered to be general group III mGluR antagonists have very weak activity using this assay, suggesting the possibility that these compounds may not effectively inhibit these receptors in native systems. We anticipate that the GIRK-mediated thallium flux strategy will provide a novel tool to advance the study of G(i/o)-coupled GPCR biology and promote ligand discovery and characterization.

mGluR7 inhibits glutamate release through a PKC-independent decrease in the activity of P/Q-type Ca2+ channels and by diminishing cAMP in hippocampal nerve terminals

The modulation of calcium channels by metabotropic glutamate receptors (mGluRs) is a key event in the fine-tuning of neurotransmitter release. Here we report that, in hippocampal nerve terminals from adult rats, the inhibition of glutamate release by the group III mGluR agonist L-2-amino-4-phosphonobutyrate (L-AP4) is largely mediated by mGluR7. In this preparation, P/Q-type Ca(2+) channels support the major component of glutamate release while the remaining release is supported by N-type Ca(2+) channels. The release associated with P/Q channels was modulated by mGluR7, either in the presence of omega-conotoxin-GVIA or after decreasing the extracellular Ca(2+) concentration [Ca(2+)](o) to abolish the contribution of N-type Ca(2+) channels. Under these conditions, L-AP4 (1 mm) reduced the evoked glutamate release by 35 +/- 2%. This inhibition was largely prevented by pertussis toxin, but it was insensitive to inhibitors of protein kinase C (bisindolylmaleimide) and protein kinase A (H-89). Furthermore, this inhibition was associated with a reduction in the Ca(2+) influx mediated by P/Q channels in the absence of any detectable change in cAMP levels. However, L-AP4 decreased the levels of cAMP in the presence of forskolin. The activation of this additional signalling pathway was very efficient in counteracting the facilitation of glutamate release induced by forskolin. Thus, mGluR7 mediates the inhibition of glutamate release at hippocampal nerve terminals primarily by inhibiting P/Q-type Ca(2+) channels, although augmenting the levels of cAMP reveals the ability of the receptor to decrease cAMP.

Group III metabotropic glutamate receptors and exocytosed protons inhibit L-type calcium currents in cones but not in rods

Light responses of photoreceptors (rods and cones) are transmitted to the second-order neurons (bipolar cells and horizontal cells) via glutamatergic synapses located in the outer plexiform layer of the retina. Although it has been well established that postsynaptic group III metabotropic glutamate receptors (mGluRs) of ON bipolar cells contribute to generating the ON signal, presynaptic roles of group III mGluRs remain to be elucidated at this synaptic connection. We addressed this issue by applying the slice patch-clamp technique to the newt retina. OFF bipolar cells and horizontal cells generate a steady inward current in the dark and a transient inward current at light offset, both of which are mediated via postsynaptic non-NMDA receptors. A group III mGluR-specific agonist, L-2-amino-4-phosphonobutyric acid (L-AP-4), inhibited both the steady and off-transient inward currents but did not affect the glutamate-induced current in these postsynaptic neurons. L-AP-4 inhibited the presynaptic L-type calcium current (ICa) in cones by shifting the voltage dependence of activation to more positive membrane potentials. The inhibition of ICa was most prominent around the physiological range of cone membrane potentials. In contrast, L-AP-4 did not affect L-type ICa in rods. Paired recordings from photoreceptors and the synaptically connected second-order neurons confirmed that L-AP-4 inhibited both ICa and glutamate release in cones but not in rods. Furthermore, we found that exocytosed protons also inhibited ICa in cones but not in rods. Selective modulation of ICa in cones may help broaden the dynamic range of synaptic transfer by controlling the amount of transmitter release from cones.

Metabotropic glutamate receptor antagonists but not NMDA antagonists affect conditioned taste aversion acquisition in the parabrachial nucleus of rats

The effect of glutamate receptor antagonists on conditioned taste aversion (CTA) was studied in rats. The association of the short-term memory of a gustatory conditioned stimulus (CS) with visceral malaise (unconditioned stimulus, US) in the CTA paradigm takes place in the parabrachial nuclei (PBN) of the brainstem. The first direct evidence of participation of glutamatergic neurotransmission in the PBN during CTA demonstrated that the extracellular level of glutamate rises during saccharin drinking (Bielavska et al. in Brain Res 887:413-417, 2000). Our results show an effect of microdialysis administration of selective GluR antagonists into the PBN on the formation of CTA engram. We used four glutamate receptor (GluR) antagonists of different types (D-AP5, MK-801 as antagonists of ionotropic GluR and L-AP3, MSPG as antagonists of metabotropic GluR). The disruptive effect of MK-801 on CTA formation in the PBN is concentration-dependent, with the greatest inhibition under the higher concentrations eliciting significant disruption. The application of D-AP5 (0.1, 1, 5 mM) did not elicit a statistically significant blockade of CTA acquisition. This indicates that the association of the US-CS in the PBN is not dependent on NMDA receptors. On the contrary, application of L-AP3 (0.1, 1, 5 mM) blocked the CS-US association.

Down-regulation of the glial glutamate transporter GLT-1 in rat hippocampus and striatum and its modulation by a group III metabotropic glutamate receptor antagonist following transient global forebrain ischemia

Our goals were to identify biochemical markers for transient global ischemia-induced delayed neuronal death and test possible drug therapies against this neuronal damage. Four-vessel occlusion (4-VO) for 20 min was used as a rat model. The temporal expression profiles of three glutamate transporters (GLT-1, GLAST and EAAC1) were evaluated in the CA1 region of the hippocampus and the striatum. The protein levels of the GLT-1 were significantly down-regulated between 3 and 6 h after ischemia-reperfusion in the CA1 region and striatum, returned to the control (2-VO) levels 24 h after reperfusion and remained unchanged for up to 7 days. The levels of GLAST in the CA1 region and striatum, and EAAC1 in the CA1 region did not change after ischemia from 1 h to 7 days. Pretreatment with group III metabotropic glutamate receptor antagonist s-alpha-MCPA (20 microg/5 microl) 30 min prior to 4-VO significantly restored the GLT-1 levels in the CA1 region caused by global ischemia at both 3 and 6 h after reperfusion. The loss of pyramidal neurons in the CA1 region due to ischemia-reperfusion could also be prevented by intraventricular pretreatment with s-alpha-MCPA. The current findings pinpoint the significance of GLT-1 during ischemia/reperfusion and suggest a potential application of group III metabotropic glutamate receptor antagonist against ischemic/hypoxic neuronal damage.

Melatonin decreases calcium levels in retinotectal axons of Xenopus laevis by indirect activation of group III metabotropic glutamate receptors

Melatonin is a neuromodulator that binds to receptors in the retinotectal laminae of the amphibian optic tectum. The effect of melatonin on calcium dynamics in Xenopus retinotectal axons was investigated by imaging retinotectal axons labeled with the fluorescent indicator Fluo-4. Melatonin exerted an inhibitory influence on depolarization-evoked calcium increases, and the melatonin receptor antagonist 4-P-PDOT blocked this effect. Blockade of group III metabotropic receptors (mGluRs) counteracted the effect of melatonin on retinotectal axons. Application of the group II/group III mGluR antagonist MSPG or the group III-selective antagonist MSOP abolished the effect of melatonin. Conversely, this effect was not significantly affected by the group I mGluR antagonist LY367385 nor by EGLU or LY341495 at concentrations that specifically inhibit group II mGluRs. Furthermore, a higher concentration of LY341495 that affects group III mGluRs inhibited the effect of melatonin. The data therefore support the hypothesis that, in retinotectal axons, melatonin reduces cAMP levels, thereby relieving PKA-induced inhibition of group III mGluRs; the newly activated mGluRs in turn inhibit voltage-sensitive calcium channels, leading to a decrease in Ca2+ concentrations. The role of GABA(C) receptors in retinotectal responses was also evaluated. GABA(C) receptor antagonists did not block the effects of melatonin but instead were additive. Moreover, while other studies have shown that in Xenopus tectal cells, GABA(C) receptors mediate inhibition, in retinotectal axons, the opposite appears to occur since depolarization-evoked calcium rises in retinotectal axons were inhibited by GABA(C) receptor blockade. This result suggests that activation of GABA(C) receptors produces an increase in the synaptic excitability of retinotectal axon terminals.

Distinct mechanisms of presynaptic inhibition at GABAergic synapses of the rat substantia nigra pars compacta

We investigated the mechanisms of presynaptic inhibition of GABAergic neurotransmission by group III metabotropic glutamate receptors (mGluRs) and GABA(B) receptors, in dopamine (DA) neurons of the substantia nigra pars compacta (SNc). Both the group III mGluRs agonist L-(+)-2-amino-4-phosphonobutyric acid (AP4, 100 microM) and the GABA(B) receptor agonist baclofen (10 microM) reversibly depressed the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) to 48.5 +/- 2.7 and 79.3 +/- 1.6% (means +/- SE) of control, respectively. On the contrary, the frequency of action potential-independent miniature IPSCs (mIPSCs), recorded in tetrodotoxin (TTX, 1 microM) and cadmium (100 microM) were insensitive to AP4 but were reduced by baclofen to 49.7 +/- 8.6% of control. When the contribution of voltage-dependent calcium channels (VDCCs) to synaptic transmission was boosted with external barium (1 mM), AP4 became effective in reducing TTX-resistant mIPSCs to 65.4 +/- 3.9% of control, thus confirming a mechanism of presynaptic inhibition involving modulation of VDCCs. Differently from AP4, baclofen inhibited to 58.5 +/- 6.7% of control the frequency mIPSCs recorded in TTX and the calcium ionophore ionomycin (2 microM), which promotes Ca2+-dependent, but VDCC-independent, transmitter release. Moreover, in the presence of alpha-latrotoxin (0.3 nM), to promote a Ca2+-independent vesicular release of GABA, baclofen reduced mIPSC frequency to 48.1 +/- 3.2% of control, while AP4 was ineffective. These results indicate that group III mGluRs depress GABA release to DA neurons of the SNc through inhibition of presynaptic VDCCs, while presynaptic GABA(B) receptors directly impair transmitter exocytosis.

Immunoreactivity for the group III receptor subtype mGluR4a in the visual layers of the rat superior colliculus

Several studies indicate that metabotropic glutamate receptors (mGluRs) participate in the transmission of visual stimuli in optic layers of the superior colliculus (SC). We examined the cellular and subcellular distribution of the group III mGluR4a in superficial layers of the rat SC by means of a specific antiserum and a preembedding immunogold method for electron microscopy. Deposits of mGluR4a immunoparticles were mostly observed on presynaptic membranes of large synaptic terminals, which made asymmetrical synapses and contained abundant spherical, clear synaptic vesicles and numerous electron translucent mitochondria. These characteristic ultrastructural features correspond to retinocollicular synaptic terminals. Also, chains of synaptic retinal terminals along dendrites were labeled for mGluR4a. About 70% of morphologically identified retinal terminals were mGluR4a immunopositive. Furthermore, mGluR4a immunoreactivity in SC greatly disappeared following retinal ablation. About 28% of cortical terminals identified by anterograde tracing showed mGluR4a labeling, whereas only 2% of collicular GABAergic profiles were labeled for mGluR4a. These results reveal that retinal terminals are the major contributors to the mGluR4a immunoreactivity observed in the superior collicular circuitry.

Comparison of Metabotropic Glutamate Receptor Responses at Segmental and Descending Inputs to Motoneurons in Neonatal Rat Spinal Cord

We compared the contribution of metabotropic glutamate receptors (mGluRs) to the generation and modulation of synaptic responses elicited in intracellularly recorded L5 motoneurons from neonatal rats by segmental and descending fibers. Dorsal root (DR) stimulation at high intensity (C-fiber strength) evoked long latency (2-5-s) depolarization in addition to early monosynaptic and polysynaptic responses. Stimulation of the descending ventrolateral funiculus (VLF) failed to evoke a late response in the same motoneuron. The mGluR antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG; 0.4 mM) selectively blocked the long latency DR response. This mGluR-mediated response persisted in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate or N-methyl-d-aspartate (NMDA) antagonists, but not both, suggesting that glutamate transmission (either AMPA/kainate or NMDA) is required for mGluR-mediated inputs from small diameter sensory afferents to affect the motoneuron. Although MCPG inhibited the long latency DR response, it induced moderate facilitation of monosynaptic DR and VLF responses. The mGluR agonist 1s3r-ACPD induced motoneuron depolarization and depressed the monosynaptic DR and VLF responses. MCPG also facilitated the neurotrophin-3 and brain-derived neurotrophic factor induced strengthening of the monosynaptic DR responses (but only before P6, since neurotrophins are ineffective later at DR synapses and never at VLF synapses after birth). Our results suggest that mGluRs are involved in synaptic pathways to motoneurons made by DR but not VLF fibers. MCPG-induced facilitation of monosynaptic AMPA/kainate DR and VLF responses suggests the possibility of tonic mGluR-mediated inhibition of DR and VLF responses. We speculate that MCPG facilitates neurotrophin-induced strengthening of monosynaptic DR responses by reducing this tonic inhibition.

N-acetylaspartylglutamate and β-NAAG protect against injury induced by NMDA and hypoxia in primary spinal cord cultures

The acidic dipeptide N-acetylaspartylglutamate (NAAG) is the most prevalent peptide in the central nervous system. NAAG is a low potency agonist at the NMDA receptor, and hydrolysis of NAAG yields the more potent excitatory amino acid neurotransmitter glutamate. beta-NAAG is a competitive inhibitor of the NAAG hydrolyzing enzyme N-acetylated alpha-linked acidic dipeptidase (NAAG peptidase activity) or glutamate carboxypeptidase II, and may also act as a NAAG-mimetic at some of the sites of NAAG pharmacological activity. Since NAAG has been shown to have neuroprotective characteristics in a number of experimental preparations, it is the purpose of the present study to specifically evaluate the possible efficacy of NAAG and beta-NAAG against NMDA- and hypoxia-induced injury to spinal cord mixed neuronal and glial cell cultures. NAAG (500-1000 microM) protected against NMDA- or hypoxia-induced injuries to spinal cord cultures, and the nonhydrolyzable analog beta-NAAG (250-1000 microM) completely eliminated the loss of viability caused by either insult. Both peptides also attenuated NMDA-induced increases in intraneuronal Ca(2+). Nonspecific mGluR antagonists, pertussis toxin, a stable cAMP analog, and manipulation of NAAG peptidase activity did not by themselves alter cell damage and did not influence the neuroprotective effects of NAAG. NAAG was not protective against kainate- or AMPA-induced cellular injury, while beta-NAAG was partially neuroprotective against both insults. At 2 mM, NAAG and beta-NAAG reduced neuronal survival and increased intraneuronal Ca(2+); these effects were only marginally attenuated by dizocilpine and APV. The results indicate that NAAG and beta-NAAG protect against excitotoxic and hypoxic injury to spinal cord neurons, and do so predominantly by interactions with NMDA and not mGluR receptors.

Phenylglycine derivatives as antagonists of group III metabotropic glutamate receptors expressed on neonatal rat primary afferent terminals

1. Three novel phenylglycine analogues; (RS)-alpha-methyl-3-chloro-4-phosphonophenylglycine (UBP1110), (RS)-alpha-methyl-3-methoxy-4-phosphonophenylglycine (UBP1111) and (RS)-alpha-methyl-3-methyl-4-phosphonophenylglycine (UBP1112) antagonised the depression of the fast component of the dorsal root-evoked ventral root potential induced by (S)-AP4 with apparent K(D) values of: 7.4+/-2.3, 5.4+/-0.6 and 5.1+/-0.3 micro M (all n=3), respectively. 2. A Schild analysis of the antagonism of (S)-AP4 induced depression of synaptic transmission by UBP1112 revealed a pA(2) value of 5.3 and a slope of 0.81+/-0.26 (n=9). 3. None of the phenylglycines tested were potent antagonists of responses mediated by group II mGlu receptors (apparent K(D) values >480 micro M). UBP1112 when tested at a concentration of 1 mM had little or no activity on (S)-3,5-DHPG-, NMDA-, AMPA- or kainate-induced responses on motoneurones. 4. UBP1110, UBP1111 and UBP1112 are at least 100-fold selective for group III over group I and II mGlu receptors expressed in the spinal cord making them the most potent, selective, antagonists yet tested at (S)-AP4 sensitive receptors in the spinal cord.

Different metabotropic glutamate receptors play opposite roles in synaptic plasticity of the rat medial vestibular nuclei

In the medial vestibular nuclei (MVN) of rat brainstem slices, the role of group II and III metabotropic glutamate receptors (mGluRs) and of the subtypes of group I mGluRs: mGluR1, mGluR5, was investigated in basal synaptic transmission and in the induction and maintenance of long-term potentiation (LTP). We used selective antagonists and agonists for mGluRs and we analysed the field potentials evoked by vestibular afferent stimulation before and after high-frequency stimulation (HFS) to induce LTP. The group II and III mGluR antagonist, (R,S)-alpha-2-methyl-4sulphonophenylglycine (MSPG), induced LTP per se and caused a reduction of the paired-pulse facilitation (PPF) ratio indicating an enhancement of glutamate release. This suggests that group II and III mGluRs are activated under basal conditions to limit glutamate release. Both the group II and III mGluR selective antagonists, 2S-2-amino-2-(1S,2S-2-carboxycycloprop-1-yl)-3-(xanth-9-yl)propanoate (LY341495) and (R,S)-alpha-methylserine-O-phosphate (MSOP), induced LTP, and the selective agonists, (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC) and L(+)-2-amino-4-phosphonobutyric acid (L-AP4) depressed the field potentials and prevented HFS-LTP, with a prevailing contribution of group II mGluRs over that of group III mGluRs. The mGluR1 antagonist, 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) prevented the full development and maintenance of HFS-LTP. By contrast, the mGluR5 antagonist, 2-methyl-6-phenylethynylpyridine (MPEP) induced LTP per se, which was impeded by CPCCOEt, and it had no effect on LTP once induced by HFS. The PPF analysis showed an enhancement of glutamate release during MPEP potentiation. The group I mGluR agonist, (R,S)-3,5-dihydroxyphenylglycine (DHPG) induced LTP per se, which was blocked by CPCCOEt. By contrast the mGluR5 agonist, (R,S)-2-chloro-5-hydroxypheylglycine (CHPG) prevented LTP elicited by HFS and DHPG as well. In conclusion vestibular LTP is inhibited by group II and III mGluRs during the early induction phase while it is facilitated by mGluR1 for achieving its full expression and consolidation. An additional inhibitory control is exerted by mGluR5 at the level of this facilitatory phase.

Interaction of group I mGlu and NMDA receptor agonists within the dorsal horn of the spinal cord of the juvenile rat

1. The modulatory effects of mGlu receptors on NMDA-induced potential changes in spinal motoneurones were studied in vitro. 2. Selective activation of mGlu5 receptors by 10 microM (RS)-2-Chloro-5-hydroxyphenylglycine (CHPG; EC(50)=280 +/- 24 microM) did not produce any change in the ventral root potential. However, the same concentration of CHPG (10 min perfusion) significantly attenuated the NMDA-induced ventral root depolarization (VRD). The effect persisted for 10 min after washout. NMDA-induced responses returned to control in 30 min. Brief co-application of CHPG and NMDA did not alter the NMDA-induced response indicating lack of direct receptor interaction. 3. The attenuating effect of CHPG on the NMDA-induced VRD was inhibited by the mGluR5 receptor antagonist, 2-methyl-6-phenyl-ethynylpyridine (MPEP). 4. In the presence of CGP56433A, a GABA(B) receptor antagonist, the NMDA-induced VRD was unchanged. However, NMDA-induced responses were potentiated after 10 min co-application of CHPG and CGP56433A. 5. (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate ((2R,4R)-APDC), a group II mGlu receptor agonist did not attenuate the NMDA-induced response. 6. Under normal physiological conditions group I mGlu receptor agonists activate at least two populations of neurones: (1) GABA-ergic cells, which could release GABA and inhibit dorsal horn neurones, and (2) deep dorsal horn neurones/motoneurones which express NMDA receptors. Therefore, activation of mGlu5 receptors located on GABA-ergic interneurones could influence any direct potentiating interaction between mGlu5 and NMDA receptors in spinal cord and result in depression of the VRD. In the presence of a GABA(B) receptor antagonist, the direct synergistic interaction is unmasked. These data suggest that group I mGlu receptors provide a complex modulation of spinal synaptic processes.

The inhibition of glutamate release by metabotropic glutamate receptor 7 affects both [Ca2+]c and cAMP: evidence for a strong reduction of Ca2+ entry in single nerve terminals

Metabotropic glutamate receptors (mGluRs) from group III reduce glutamate release. Because these receptors reduce cAMP levels, we explored whether this signaling pathway contributes to release inhibition caused by mGluRs with low affinity for L-2-amino-4-phosphonobutyrate (L-AP4). In biochemical experiments with the population of cerebrocortical nerve terminals we find that L-AP4 (1 mm) inhibited the Ca(2+)-dependent-evoked release of glutamate by 25%. This inhibitory effect was largely prevented by the pertussis toxin but was insensitive to inhibitors of protein kinase C bisindolylmaleimide and protein kinase A H-89. Furthermore, this inhibition was associated with reduction in N-type Ca(2+) channel activity in the absence of any detectable change in cAMP levels. In the presence of forskolin, however, L-AP4 decreased the levels of cAMP. The activation of this additional signaling pathway was very efficient in counteracting the facilitation of glutamate release induced either by forskolin or the beta-adrenergic receptor agonist isoproterenol. Imaging experiments to measure Ca(2+) dynamics in single nerve terminals showed that L-AP4 strongly reduced the Ca(2+) response in 28% of the nerve terminals. Moreover, immunochemical experiments showed that 25-35% of the nerve terminals that were immunopositive to synaptophysin were also immunoreactive to the low affinity L-AP4-sensitive mGluR7. Then, mGluR7 mediates the inhibition of glutamate release caused by 1 mm L-AP4, primarily by a strong inhibition of Ca(2+) channels, although high cAMP uncovers the receptor ability to decrease cAMP.

Effects of excitatory amino acid receptor antagonists on pulpal blood flow of the rat mandibular incisor

Pulpal blood flow (PBF) changes have been monitored by laser Doppler flowmetry on rat mandibular incisors. Electrical stimulation (10 sec, 50 microA, 2 ms, 20 Hz) of one incisor induced a blood flow decrease followed by a blood flow increase. The effect of intravenous administration of antagonists of ionotropic glutamate receptors (iGluRs) and antagonists of metabotropic glutamate receptors (mGluRs) was compared with that of those obtained from animals treated with the vehicle alone. No long-term effect on basal PBF was observed, except a remaining increase of 34.5% (p < 0.05, n = 5) for ketamine (10 mg/kg), an iGluR antagonist, and of 37% (p < 0.05, n = 5) for MCPG (7.5 mg/kg), an mGluR antagonist. In animals treated with iGluR antagonists, acute changes in PBF after stimulation were not significantly different from those observed with vehicle. In animals treated with mGluR antagonists, the blood flow decrease was significantly enhanced in amplitude and duration for MCPG (7.5 mg/kg), respectively, +73% and +92% (p < 0.05, n = 5). These results suggest that Group I mGluRs participate in the regulation of the immediate pulpal blood flow decrease induced by electrical stimulation of the lower incisor in the rat.

Synthesis of phenylglycine derivatives as potent and selective antagonists of group III metabotropic glutamate receptors

The syntheses of a range of ring and alpha-substituted 4-phosphonophenylglycines are described. A brief discussion of the antagonist activities of compounds 4-10 on group I, II and III metabotropic glutamate (mGlu) receptors expressed in the neonatal rat spinal cord is included.

Pharmacological profiles of the metabotropic glutamate receptor ligands

Metabotropic glutamate receptors (mGluRs) are a family of G-protein coupled receptors that are expressed in the central and peripheral nervous systems. The purpose of this study was to compare the ligand binding selectivity profiles of the mGluR agonist [(3)H]L-AP4 and the novel radiolabeled phenylglycine antagonist [(3)H]CPPG at all eight rat mGluR subtypes expressed in transfected human embryonic kidney cells. At a concentration of 30 nM [(3)H]L-AP4, no specific binding was detected in membranes expressing the group I receptors mGluR1a or mGluR5a, or in membranes expressing the group II mGluRs, mGluR2 and mGluR3. Among the group III mGluRs, specific [(3)H]L-AP4 binding was detected in cells expressing mGluR4a and mGluR8a but not in cells expressing mGluR6 or mGluR7a. The binding of [(3)H]CPPG showed an exceptional pattern of selectivity amongst the mGluR subtypes; at a concentration of 20 nM [(3)H]CPPG, a high level of specific binding was seen in membranes containing mGluR8a but not in any of the other mGluR subtypes. The affinity constant (K(D)) calculated for [(3)H]CPPG binding to mGluR8a was 183 nM. In competition experiments, the phosphono-substituted phenylglycine congeners including MPPG, (RS)-PPG, and unlabeled CPPG were the most potent inhibitors of [(3)H]CPPG binding while non-phosphonated compounds such as L-glutamate and MCPG were substantially less potent. These results demonstrate that [(3)H]L-AP4 and [(3)H]CPPG can be used as probes to selectively label group III mGluRs and that CPPG and related phenylglycine derivatives are useful for studying differences in the ligand recognition sites of highly homologous mGluRs.

Group II and group III metabotropic glutamate receptor agonists depress synaptic transmission in the rat spinal cord dorsal horn

The effects of group II and group III metabotropic glutamate receptor agonists on synaptic responses evoked by primary afferent stimulation in the dorsal horn, but mostly substantia gelatinosa, neurons were studied in the spinal cord slice preparation using conventional intracellular recording technique. Bath application of a potent metabotropic glutamate receptor 2- and 3-selective agonist (2S,1'R,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl) glycine reversibly suppressed monosynaptic and polysynaptic excitatory postsynaptic potentials evoked by A primary afferent fibers stimulation, the effect likely mediated by mGlu3 receptor subtype. This suppressing effect of (2S,1'R,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl) glycine on primary afferent neurotransmission was dose dependent and reduced by (S)-alpha-ethylglutamate, a group II metabotropic glutamate receptor antagonist. (2S,1'R,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl) glycine suppressed excitatory postsynaptic potentials without inducing detectable changes of postsynaptic membrane potential and neuronal input resistance in dorsal horn neurons. The paired-pulse depression at excitatory synapses between primary afferent fibers and dorsal horn neurons was reduced by (2S,1'R,2'R,3'R)-2-(2', 3'-dicarboxycyclopropyl) glycine application, suggesting a presynaptic site of action. The selective group III metabotropic glutamate receptor agonist (S)-2-amino-4-phosphonobutanoate also depressed A afferent fibers-evoked monosynaptic and polysynaptic excitatory postsynaptic potentials in a dose-dependent and reversible manner. The concentration-dependence of (S)-2-amino-4-phosphonobutanoate-mediated depression was most consistent with activation of mGlu receptor subtypes 4 and 7. However, on the basis of anatomical distribution of mGlu 4 and 7 subtypes, it is also possible that the (S)-2-amino-4-phosphonobatanoate effect is due to interaction with mGlu 7 receptor alone. (RS)-alpha-cyclopropyl-4-phosphonophenylglycine a preferential antagonist at group III metabotropic glutamate receptors, completely reversed the depressant effects of (S)-2-amino-4-phosphonobutanoate on both monosynaptic and polysynaptic responses. (S)-2-amino-4-phosphonobutanoate reduced the paired-pulse depression at excitatory synapses between primary afferent fibers and dorsal horn neurons, but did not alter their postsynaptic membrane potential and input resistance. A clear facilitation of the (S)-2-amino-4-phosphonobutanoate-induced depression of monosynaptic and polysynaptic excitatory postsynaptic potentials in the absence of gamma-aminobutyric acid-subtype A receptor- and glycine-mediated synaptic inhibition was shown. Besides the depressant effect on excitatory synaptic transmission, inhibitory actions of group II and III metabotropic glutamate receptor agonists on the inhibitory postsynaptic potentials evoked by primary afferent stimulation in dorsal horn neurons were observed. These results suggest that group II and group III metabotropic glutamate receptors are expressed at primary afferent synapses in the dorsal horn region, and activation of the receptors suppresses synaptic transmission by an action on the presynaptic site.

Immunoreactivity for the group III metabotropic glutamate receptor subtype mGluR4a in the superficial laminae of the rat spinal dorsal horn

Studies indicate that metabotropic glutamate receptors (mGluRs) may play a role in spinal sensory transmission. We examined the cellular and subcellular distribution of the mGluR subtype 4a in spinal tissue by means of a specific antiserum and immunocytochemical techniques for light and electron microscopy. A dense plexus of mGluR4a-immunoreactive elements was seen in the dorsal horn, with an apparent accumulation in lamina II. The immunostaining was composed of sparse immunoreactive fibres and punctate elements. No perikaryal staining was seen. Immunostaining for mGluR4a was detected in small to medium-sized cells but not in large cells in dorsal root ganglia. At the electron microscopic level, superficial dorsal horn laminae demonstrated numerous immunoreactive vesicle-containing profiles. Labelling was present in the cytoplasmic matrix, but accretion of immunoreaction product to presynaptic specialisations was commonly observed. Axolemmal labelling was confirmed by using a preembedding immunogold technique, which revealed distinctive deposits of gold immunoparticles along presynaptic thickenings with an average centre-to-centre distance of 41 nm (41.145 +/- 13.59). Immunoreactive terminals often formed synaptic contacts with dendritic profiles immunonegative for mGluR4a. Immunonegative dendritic profiles were observed in apposition to both mGluR4a-immunoreactive and immunonegative terminals. Diffuse immunoperoxidase reaction product was also detected in dendritic profiles, some of which were contacted by mGluR4a-immunoreactive endings, but only occasionally were they observed to accumulate immunoreaction product along the postsynaptic density. Terminals immunoreactive for mGluR4a also formed axosomatic contacts. The present results reveal that mGluR4a subserves a complex spinal circuitry to which the primary afferent system seems to be a major contributor.

Metabotropic glutamate autoreceptors of the mGlu(5) subtype positively modulate neuronal glutamate release in the rat forebrain in vitro

In the present study we have examined the role of presynaptic group I metabotropic glutamate (mGlu) receptors in the control of neuronal glutamate release using rat forebrain slices pre-loaded with [(3)H]D-aspartate. We have also addressed the question of which group I mGlu receptor subtype, mGlu(1) or mGlu(5), mediates the facilitatory response observed by the use of a range of established and some more novel agonists and antagonists showing selectivity for these receptors. The electrically-stimulated release of pre-loaded [(3)H]D-aspartate from rat forebrain slices was markedly potentiated by the potent group I mGlu receptor agonist, L-quisqualic acid (L-QUIS), in a concentration-dependent manner (EC(50) 17.31 microM). This response was inhibited by the mGlu receptor antagonists (S)-MCPG (100 microM) and (RS)-MTPG (100 microM) but not by the AMPA-type ionotropic glutamate receptor antagonist, NBQX (100 microM). The selective group I mGlu receptor agonist (S)-3, 5-dihydroxyphenylglycine ((S)-DHPG) also enhanced electrically-stimulated efflux of label, although responses diminished with high (10-100 microM) concentrations of the agonist. Maximum responses were fully restored when (S)-DHPG (10 microM) was applied in the presence of the proposed mGlu(5) receptor desensitization inhibitor, cyclothiazide (10 microM). The positive modulatory response to (S)-DHPG (1 microM) was powerfully inhibited by (S)-MCPG (IC(50) 0.08 microM) but was resistant to the mGlu(1) receptor antagonists, (RS)-AIDA (1-500 microM), CPCCOEt (0.1-100 microM) and (+)-2-methyl-4-carboxyphenylglycine (LY367385) (0.1-10 microM). The recently developed, selective mGlu(5) receptor agonist (RS)-2-chloro-5-hydroxyphenylglycine ((RS)-CHPG) enhanced electrically-stimulated [(3)H]D-aspartate efflux from rat forebrain slices with a similar concentration-response profile to that of (S)-DHPG. Responses to this receptor subtype-selective agonist were also blocked by (S)-MCPG (IC(50) 1.13 microM) but were unaffected by (RS)-AIDA (500 microM), CPCCOEt (100 microM) or LY367385 (10 microM). These results indicate that the positive modulation of neuronal glutamate release seen in the rat forebrain in the presence of group I mGlu receptor agonists is mediated by presynaptically located mGlu(5) glutamate autoreceptors. The pharmacological profile of these receptors appears to be distinct from that of postsynaptic mGlu receptors. Novel antagonists acting at these presynaptic receptors may provide new drugs for the experimental therapy of a range of acute or chronic neurodegenerative disorders.

(+)-4-phosphonophenylglycine (PPG) a new group III selective metabotropic glutamate receptor agonist

A new synthesis of (R,S)-PPG (4-phosphonophenylglycine) and the separation of the protected enantiomers leading after deprotection to (+)- and (-)-PPG are described. Pharmacological characterization at the group III metabotropic glutamate receptors hmGluR4a and hmGluR7b revealed (+)-PPG as the active enantiomer.

Conjugate additions of 1-propenylphosphonates to metalated Schöllkopf's bis-lactim ether: stereocontrolled access to 2-amino-3-methyl-4-phosphonobutanoic acids

Diastereoselectivity in the conjugate addition of metalated Schöllkopf's bis-lactim ethers 5a-e to (E)- and (Z)-1-propenylphosphonates 4a,b was studied experimentally and theoretically and utilized to achieve a direct and stereocontrolled synthesis of all four diastereoisomers of 2-amino-3-methyl-4-phosphonobutanoic acid, 6a,b and their enantiomers. The relative stereochemistry was assigned from an NMR study of cyclic derivatives 13a,b. According to semiempirical calculations, both in vacuo (PM3) or a dielectric continuum (PM3/COSMO), initial lithium-phosphoryl coordination, without an energy barrier, to form a solvated chelate complex is followed by the rate-determining reorganization to the 1,4-addition product through an eight-membered transition state. The translation of the Z,E geometry into a syn, anti configuration at the adducts originates from an orientational preference in the transition state for a compact disposition of the reaction partners.

Pharmacological characterization of the rat metabotropic glutamate receptor type 8a revealed strong similarities and slight differences with the type 4a receptor

In the brain, group-III metabotropic glutamate (mGlu) receptors mGlu(4), mGlu(7) and mGlu(8) receptors play a critical role in controlling the release process at many glutamatergic synapses. The pharmacological profile of mGlu(4) receptor has been studied extensively, allowing us to propose a pharmacophore model for this receptor subtype. Surprisingly, the activity of only a few compounds have been reported on mGlu(7) and mGlu(8) receptors. In order to identify new possibilities for the design of selective compounds able to discriminate between the members of the group-III mGlu receptors, we have undertaken a complete pharmacological characterization of mGlu(8) receptor and compared it with that of mGlu(4) receptor, using the same expression system, and the same read out. The activities of 32 different molecules revealed that these two mGlu receptors subtypes share a similar pharmacological profile. Only small differences were noticed in addition to that previously reported with S-carboxyglutamate (S-Gla) being a partial agonist at mGlu(4) receptor and a full antagonist at mGlu(8) receptor. These include: a slightly higher relative potency of the agonists 1S,3R and 1S,3S-aminocyclopentane-1,3-dicarboxylic acid (ACPD), S-4-carboxyphenylglycine (S-4CPG) and S-4-carboxy-3-hydroxyphenylglycine (S-4C3HPG), and a slightly higher potency of the antagonists 2-aminobicyclo[3.1.0]hexane-2, 6-dicarboxylic acid (LY354740) and RS-alpha-methyl-4-phosphonophenylglycine (MPPG) on mGlu(8) receptor. When superimposed on the mGlu(4) receptor pharmacophore model, these molecules revealed three regions that may be different between the ligand binding sites of mGlu(8) and mGlu(4) receptors.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Evaluation of the activity of a novel metabotropic glutamate receptor antagonist (+/-)-2-amino-2-(3-cis and trans-carboxycyclobutyl-3-(9-thioxanthyl)propionic acid) in the in vitro neonatal spinal cord and in an in vivo pain model

The cyclobutylglycine (+/-)-2-amino-2-(3-cis and trans-carboxycyclobutyl-3-(9-thioxanthyl)propionic acid) (LY393053) has been identified as a functionally potent metabotropic glutamate receptor antagonist. It is most potent on the two group I metabotropic glutamate receptors, 1alpha and 5alpha, with IC50 values of 1.0+/-0.4 microM and 1.6+/-1.4 microM, respectively. In this study, LY393053 has also been evaluated electrophysiologically on native group I metabotropic glutamate receptors in an in vitro spinal cord preparation as well as behaviourally, in a mouse model of visceral pain. LY393053 dose-dependently antagonised group I agonist, (RS)-3, 5-dihydroxyphenylglycine, or a broad-spectrum agonist (1S,3R)-amino-1,3-cyclopentanedicarboxylic acid-induced depolarisation of spinal motoneurons. The apparent Kd values were estimated to be 0.3 microM against (RS)-3, 5-dihydroxyphenylglycine-induced depolarisation and 0.5 microM against (1S,3R)-amino-1,3-cyclopentanedicarboxylic acid-induced depolarisation, respectively. On the other hand, the dorsal root-ventral root potential elicited at 8 x threshold was depressed by LY393053 with IC50 values of 9.0+/-0.7 microM and 12.7+/-1.7 microM on monosynaptic and polysynaptic responses, respectively. When investigated using the mouse acetic acid writhing test, LY393053 showed significant analgesic effects at doses of 1-10 mg/kg intraperitoneally. An ED50 value of 6.0 mg/kg was obtained in this test. By revealing a potent effect of LY393053 in antagonising the native group I metabotropic receptor-mediated responses in the spinal cord in rodents, and an antinociceptive efficacy in a mouse visceral pain model, these results, therefore, provide additional evidence in support of the analgesic potential of metabotropic glutamate receptor antagonists.

Selective activation of group II mGluRs with LY354740 does not prevent neuronal excitotoxicity

Recent reports have suggested a role for group II metabotropic glutamate receptors (mGluRs) in the attenuation of excitotoxicity. Here we examined the effects of the recently available group II agonist (+)-2-Aminobicyclo[3.1.0]hexane-2-6-dicarboxylic acid (LY354740) on N-methyl-D-aspartate (NMDA)-induced excitotoxic neuronal death, as well as on hypoxic-ischemic neuronal death both in vitro and in vivo. At concentrations shown to be selective for group II mGluRs expressed in cell lines (0.1-100 nM), LY354740 did not attenuate NMDA-mediated neuronal death in vitro or in vivo. Furthermore, LY354740 did not attenuate oxygen-glucose deprivation-induced neuronal death in vitro or ischemic infarction after transient middle cerebral artery occlusion in rats. In addition, the neuroprotective effect of another group II agonist, (S)-4-carboxy-3-phenylglycine (4C3HPG), which has shown injury attenuating effects both in vitro and in vivo, was not blocked by the group II antagonists (2 S)-alpha-ethylglutamic acid (EGLU), (RS)-alpha-methyl-4-sulphonophenylglycine (MSPG), or the group III antagonist (S)-alpha-methyl-3-carboxyphenylalanine (MCPA), suggesting that this neuroprotection may be mediated by other effects such as upon group I mGluRs.

Pharmacological agents acting at subtypes of metabotropic glutamate receptors

Metabotropic (G-protein-coupled) glutamate (mGlu) receptors have now emerged as a recognized, but still relatively new area of excitatory amino acid research. Current understanding of the roles and involvement of mGlu receptor subtypes in physiological/pathophysiological functions of the central nervous system has been recently propelled by the emergence of various structurally novel, potent, and mGlu receptor selective pharmacological agents. This article reviews the evolution of pharmacological agents that have been reported to target mGlu receptors, with a focus on the known receptor subtype selectivities of current agents.

Metabotropic glutamate receptor activation and blockade: their role in long-term potentiation, learning and neurotoxicity

Metabotropic glutamate receptors represent a fairly recent addition to the family of glutamate receptors. These receptors have the distinguishing feature of being coupled to G-proteins rather than ion channels and they appear to have a variety of functional characteristics. These receptors play a vital role, for example, in the induction and maintenance of long-term potentiation, the most popular current model of the biological correlates of learning and memory. Blockade of metabotropic glutamate receptors prevents long-term potentiation induction and learning in a variety of tasks in different species. Chronic metabotropic glutamate receptor activation is also associated with neurodegeneration and selective neuronal loss when agonists of these receptors are injected in high concentrations directly into the brain. Metabotropic glutamate receptors also play a role in the normal development of the nervous system and these sites within the central nervous system offer possible routes for drug therapies; selective receptor antagonists, for example, may prove to have the very desirable feature of endowing neuroprotection during ischaemic episodes whilst allowing normal excitatory neurotransmission to occur.

Induction of long-lasting depression by (+)-alpha-methyl-4-carboxyphenylglycine and other group II mGlu receptor ligands in the dentate gyrus of the hippocampus in vitro

Application of several well characterized group II mGlu receptor ligands was found to induce a long-lasting depression of synaptic transmission in the medial perforant path of the dentate gyrus. These ligands were N-acetylaspartylglutamate (NAAG), which is a dipeptide located in the brain and possibly functioning as a neurotransmitter, two agents widely used previously as mGluR antagonists, (+)-alpha-methyl-4-carboxyphenylglycine (MCPG), and (S)-alpha-ethylglutamate (EGLU), and the well characterized group II mGluR agonist (2S,1R,2R,3R)-2-(2S,1'R,2'R,3'R)-2(2'3'-dicarboxycyclopropyl)glyci ne (DCG-IV). It is postulated that all these ligands induced the long-lasting depression by an agonist/partial agonist action at group II mGlu receptor. The long-lasting depression induced by the ligands showed mutual occlusion with low frequency stimulation-induced long-term depression, demonstrating common induction or maintenance mechanisms. The induction of the long-lasting depression by the mGlu receptor ligands are suggested to occur postsynaptically as the induction was not associated with a change in paired pulse depression of excitatory postsynaptic potentials (EPSPs).

Asymmetric Strecker-Type Reaction of alpha-Aryl Ketones. Synthesis of (S)-alphaM4CPG, (S)-MPPG, (S)-AIDA, and (S)-APICA, the Antagonists of Metabotropic Glutamate Receptors

Heating a mixture of alpha-aryl ketone with (R)-phenylglycinol produces a mixture of imine and 1,3-dioxazolidine. Treatment of this mixture with trimethylsilyl cyanide followed by transformation of nitrile to ester gives Strecker-type reaction products. The diastereoselectivity of the generated alpha-amino esters is from 2/1 to 7/1, and the (R,S)isomer is found as the major product. The (R,S) and (R,R)isomers can be separated by conversion to their N-Cbz or cyclization derivatives. Using this methodology, four antagonists of metabotropic glutamate receptors, (S)-alphaM4CPG, (S)-MPPG, (S)-AIDA, and (S)-APICA, are synthesized.

Metabotropic glutamate receptors: electrophysiological properties and role in plasticity

Electrophysiological research on mGluRs is now very extensive, and it is clear that activation of mGluRs results in a large number of diverse cellular actions. Studies of mGluRs and on ionic channels has clearly demonstrated that mGluR activation has a widespread and potent inhibitory action on both voltage-gated Ca2+ channels and K+ channels. Inhibition of N-type Ca2+ channels, and inhibition of Ca(++)-dependent K+ current, IAHP, and IM being particularly prominent. Potentiation of activation of both Ca2+ and K+ channels has also been observed, although less prominently than inhibition, but mGluR-mediated activation of non-selective cationic channels is widespread. In a small number of studies, generation of an mGluR-mediated slow excitatory postsynaptic potential has been demonstrated as a consequence of the effect of mGluR activation on ion channels, such as activation of a non-selective cationic channels. Although certain mGluR-modulation of channels is a consequence of direct G-protein-linked action, for example, inhibition of Ca2+ channels, many other effects occur as a result of activation of intracellular messenger pathways, but at present, little progress has been made on the identification of the messengers. The field of study of the involvement of mGluRs in synaptic plasticity is very large. Evidence for the involvement of mGluRs in one form of LTD induction in the cerebellum and hippocampus is now particularly impressive. However, the role of mGluRs in LTP induction continues to be a source of dispute, and resolution of the question of the exact involvement of mGluRs in the induction of LTP will have to await the production of more selective ligands and of selective gene knockouts.

Adenosine A2A and group I metabotropic glutamate receptors synergistically modulate the binding characteristics of dopamine D2 receptors in the rat striatum

There is experimental evidence for the existence of interactions between metabotropic glutamate (mGlu), adenosine and dopamine receptors in the striatum. In membrane preparations from rat striatum the group I and II mGlu receptor agonist 1-aminocyclopentane-1S-3R-dicarboxylic acid (1S-3R-ACPD) was found to modulate the binding characteristics of D2 receptors in a similar manner as the A2A receptor agonist 2-[p-(2-carboxyethyl)phenthylamino]-5'-N-ethylcarboxamidoadenosine (CGS 21680), with a significant decrease in the affinity of the high-affinity state of D2 receptors for dopamine. The effect of 1S-3R-ACPD was mimicked by (+/-)-trans-ACPD (t-ACPD; a racemic mixture of 1S-3R-ACPD and its inactive isomer 1R-3S-ACPD) and by the selective group I mGlu receptor agonist 3,5-dihydroxyphenylglycine (DHPG) and it was counteracted by the selective group I mGlu receptor antagonist 1-aminoindan-1,5-dicarboxilic acid (AIDA), but not by the the group II and III mGlu receptor antagonist (RS)-alpha-methyl-4-tetrazolylphenylglycine (MTPG) or the adenosine receptor antagonist 8-phenyltheophylline. Furthermore, a strong synergistic effect was observed when the striatal membranes were exposed to both CGS 21680 and 1S-3R-ACPD. In agreement with the biochemical results, in unilaterally 6-OH-dopamine lesioned rats 1S-3R-ACPD counteracted the turning behaviour induced by the D2 receptor agonist quinpirole, but not by the D1 receptor agonist SKF 38393, and it synergistically potentiated the antagonistic effect of CGS 21680 on quinpirole-induced turning behaviour.