Actions of two new antagonists showing selectivity for different sub-types of metabotropic glutamate receptor in the neonatal rat spinal cord

Role of glutamatergic neurotransmission in the enteric nervous system and brain-gut axis in health and disease

Several studies have been carried out in the last 30 years in the attempt to clarify the possible role of glutamate as a neurotransmitter/neuromodulator in the gastrointestinal tract. Such effort has provided immunohistochemical, biomolecular and functional data suggesting that the entire glutamatergic neurotransmitter machinery is present in the complex circuitries of the enteric nervous system (ENS), which participates to the local coordination of gastrointestinal functions. Glutamate is also involved in the regulation of the brain-gut axis, a bi-directional connection pathway between the central nervous system (CNS) and the gut. The neurotransmitter contributes to convey information, via afferent fibers, from the gut to the brain, and to send appropriate signals, via efferent fibers, from the brain to control gut secretion and motility. In analogy with the CNS, an increasing number of studies suggest that dysregulation of the enteric glutamatergic neurotransmitter machinery may lead to gastrointestinal dysfunctions. On the whole, this research field has opened the possibility to find new potential targets for development of drugs for the treatment of gastrointestinal diseases. The present review analyzes the more recent literature on enteric glutamatergic neurotransmission both in physiological and pathological conditions, such as gastroesophageal reflux, gastric acid hypersecretory diseases, inflammatory bowel disease, irritable bowel syndrome and intestinal ischemia/reperfusion injury.

Metabotropic glutamate receptors promote disinhibition of olfactory bulb glomeruli that scales with input strength

Increasing evidence indicates that the neural circuitry within glomeruli of the olfactory bulb plays a major role in affecting information flow between olfactory sensory neurons (OSNs) and output mitral cells (MCs). Glutamatergic external tufted (ET) cells, located at glomeruli, can act as intermediary cells in excitation between OSNs and MCs, whereas activation of MCs by OSNs is, in turn, suppressed by inhibitory synapses onto ET cells. In this study, we used patch-clamp recordings in rat olfactory bulb slices to examine the function of metabotropic glutamate receptors (mGluRs) in altering these glomerular signaling mechanisms. We found that activation of group II mGluRs profoundly reduced inhibition onto ET cells evoked by OSN stimulation. The mGluRs that mediated disinhibition were located on presynaptic GABAergic periglomerular cells and appeared to be activated by glutamate transients derived from dendrites in glomeruli. In terms of glomerular output, the mGluR-mediated reduction in GABA release led to a robust increase in the number of action potentials evoked by OSN stimulation in both ET cells and MCs. Importantly, however, the enhanced excitation was specific to when a glomerulus was strongly activated by OSN inputs. By being selective for strong vs. weak glomerular activation, mGluR-mediated disinhibition provides a mechanism to enhance the contrast in odor signals that activate OSN inputs into a single glomerulus at varying intensities.

Group III metabotropic glutamate receptors: pharmacology, physiology and therapeutic potential

Glutamate, the primary excitatory neurotransmitter in the central nervous system (CNS), exerts neuromodulatory actions via the activation of metabotropic glutamate (mGlu) receptors. There are eight known mGlu receptor subtypes (mGlu1-8), which are widely expressed throughout the brain, and are divided into three groups (I-III), based on signalling pathways and pharmacological profiles. Group III mGlu receptors (mGlu4/6/7/8) are primarily, although not exclusively, localised on presynaptic terminals, where they act as both auto- and hetero-receptors, inhibiting the release of neurotransmitter. Until recently, our understanding of the role of individual group III mGlu receptor subtypes was hindered by a lack of subtype-selective pharmacological tools. Recent advances in the development of both orthosteric and allosteric group III-targeting compounds, however, have prompted detailed investigations into the possible functional role of these receptors within the CNS, and revealed their involvement in a number of pathological conditions, such as epilepsy, anxiety and Parkinson's disease. The heterogeneous expression of group III mGlu receptor subtypes throughout the brain, as well as their distinct distribution at glutamatergic and GABAergic synapses, makes them ideal targets for therapeutic intervention. This review summarises the advances in subtype-selective pharmacology, and discusses the individual roles of group III mGlu receptors in physiology, and their potential involvement in disease.

Characterisation of an mGlu8 receptor-selective agonist and antagonist in the lateral and medial perforant path inputs to the dentate gyrus

Since its characterisation in 2001, the mGlu8-selective agonist DCPG has been widely used to explore the potential functional role of this group III mGlu receptor within the central nervous system. This research has implicated mGlu8 receptors in a number of disease states and conditions such as epilepsy and anxiety, suggesting that mGlu8-selective ligands may hold important therapeutic potential. However, there is evidence that DCPG exerts off-target effects at higher concentrations, limiting its use as an mGlu8-selective agonist. Here, we have used field recordings in rat hippocampal slices to investigate the effects of DCPG in the lateral perforant path (LPP), a pathway known to express high levels of mGlu8. We show that DCPG does inhibit excitatory transmission in this pathway, but produces a biphasic concentration-response curve suggesting activation of two distinct receptor types. The putative mGlu8-selective antagonist MDCPG antagonises the high, but not the low, potency component of this concentration-response curve. In addition, higher concentrations of DCPG also depress excitatory transmission in the medial perforant path (MPP), a pathway expressing very low levels of mGlu8 receptors. Experiments in slices from mice lacking mGlu8 receptors indicate that concentrations of DCPG >1 μM produce large non-selective effects in both the LPP and MPP. Further experiments in slices from mGlu2, 4 and 7 knock-out mice, as well as in an mGlu2-deficient substrain of Wistar rat, reveal that these non-selective effects are mediated primarily by mGlu2 receptors. Taken together, our results confirm the mGlu8-selectivity of DCPG at submicromolar concentrations, but suggest that care must be taken when employing higher concentrations of the agonist, which may additionally activate mGlu2 receptors, especially at synapses where their expression is high. MDCPG may be a useful tool in determining whether observable DCPG effects are attributable to mGlu8, versus mGlu2, receptor activation.

Metabotropic glutamate receptor mGlu2 is resistant to homologous agonist-induced desensitization but undergoes protein kinase C-mediated heterologous desensitization

To investigate the susceptibility of the group II metabotropic glutamate receptor mGlu2 to agonist-induced desensitization, the receptor was stably expressed in Chinese hamster ovary (CHO-mGlu2) or C6 glioma cells (C6-mGlu2). Exposure of CHO-mGlu2 cells to the group II mGlu receptor agonist (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (LCCG-1; 10 μM) for up to 15 h did not affect the subsequent ability of LCCG-1 to inhibit forskolin-stimulated cAMP accumulation. Similarly, in C6-mGlu2 cells, prolonged exposure to LCCG-1 also did not affect the subsequent ability of LCCG-1 to inhibit cAMP formation. In contrast, exposure of CHO-mGlu2 cells to the protein kinase C activator phorbol myristate acetate (PMA) suppressed the ability of LCCG-1 to inhibit cAMP formation. Using an in vitro model of group II mGlu receptor activity, the hemisected neonatal rat spinal cord preparation, the ability of the selective group II agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate ((2R,4R)-APDC) to depress the fast component of the dorsal root-evoked ventral root potential (fDR-VRP) did not desensitize when applied for up to 2 h. Together these results indicate that in contrast to most G protein-coupled receptors, the mGlu2 receptor is resistant to agonist-induced homologous desensitization, and that in vitro data suggests that resistance to desensitization is a physiologically relevant property of this mGlu receptor subtype.

Effects of amino acid antagonists on spontaneous dorsal root activity and evoked dorsal horn field potentials in an isolated preparation of rat spinal cord

Fast and slow dorsal horn field potentials and spontaneous dorsal root activity were recorded from 19-23-day-old rat isolated spinal cord preparations. The effects of GABA, glycine, and glutamate antagonists were tested on these recordings. CNQX, an AMPA/kainate antagonist, reduced all 3 components of the dorsal horn field potential whereas MK801, an NMDA ion channel antagonist, reduced the fast S2 component and the slow wave. Both reduced spontaneous dorsal root activity. NMDA antagonists, D-AP5, 7-chlorokynurenic acid and arcaine, and the metabotropic glutamate antagonists L-AP3 and ethylglutamic acid, while having little effect on the fast components of the field potential, all reduced the slow component. The GABA antagonist, bicuculline, and the glycine antagonist, strychnine, while having no effect on the fast S1 and slow components of the field potential, reduced both the fast S2 component of the field potential and spontaneous dorsal root activity. These results suggest that non-NMDA glutamate receptors are involved in low and high threshold transmission to dorsal horn neurones while NMDA and metabotropic glutamate receptors are primarily involved in high threshold transmission and both GABA and glycine have roles in the transmission or modulation of sensory information within the dorsal horn of the cord.

Regional Anesthesia and Pain

PURPOSE

Not all bone cancer pain can be effectively treated with current therapies. In the present study, the effects of ip administration of alpha-2 agonists (dexmedetomidine and clonidine), N-methyl-D-aspartate (NMDA) antagonists (MK-801 and ketamine), an N-acetylaspartylglutamate peptidase inhibitor (ZJ-43), and morphine were examined in a mouse bone cancer pain model.

METHODS

A bone cancer pain model was produced by injection of murine sarcoma cells into the medullary cavity of the distal femur. To estimate the level of bone cancer pain, the number of pain-related behaviours induced by repeated applications of a von Frey monofilament (0.166 g) to the site of tumour cells implantation was counted. Drugs were administered two weeks after the implantation.

RESULTS

Morphine produced a significant analgesic effect (P < 0.001). The alpha-2 agonists produced analgesic effects (P < 0.001) with an efficacy similar to that of morphine, but only at doses that produced severe sedation. MK-801 had only limited analgesic effects, while ketamine produced an analgesic effect (P < 0.001) with the same efficacy as morphine. ZJ-43 (100 mg.kg(-1)) had a significant analgesic effect (P < 0.05) and the effect of ZJ-43 was antagonized by the selective group II metabotropic glutamate receptor (mGluR) antagonist.

CONCLUSION

These data suggest that alpha-2 agonists produce an analgesic effect only at a sedative dose and that ketamine, but not MK-801, is associated with an analgesic response without overt side effects. The effect of ZJ-43 is mediated by activating group II mGluRs.

Metabotropic glutamate receptors

Metabotropic glutamate receptors (mGlus) are a family of G-protein-coupled receptors activated by the neurotransmitter glutamate. Molecular cloning has revealed eight different subtypes (mGlu1-8) with distinct molecular and pharmacological properties. Multiplicity in this receptor family is further generated through alternative splicing. mGlus activate a multitude of signalling pathways important for modulating neuronal excitability, synaptic plasticity and feedback regulation of neurotransmitter release. In this review, we summarize anatomical findings (from our work and that of other laboratories) describing their distribution in the central nervous system. Recent evidence regarding the localization of these receptors in peripheral tissues will also be examined. The distinct regional, cellular and subcellular distribution of mGlus in the brain will be discussed in view of their relationship to neurotransmitter release sites and of possible functional implications.

Prodrugs of 3-(3,4-dichlorobenzyloxy)-2-amino-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (MGS0039): A potent and orally active group II mGluR antagonist with antidepressant-like potential

3-(3,4-Dichlorobenzyloxy)-2-amino-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid 5 (MGS0039) is a highly selective and potent group II metabotropic glutamate receptor (mGluR) antagonist (antagonist activities for mGluR2; IC50=20.0 nM, mGluR3; IC50=24.0 nM) and is detected in both plasma (492 ng/mL) and brain (13.2 ng/g) at oral administration of 10 ng/mL [J. Med. Chem.2004, 47, 4750], but the oral bioavailability of 5 was 10.9%. In order to improve the oral bioavailability of 5, prodrugs of 5 were discovered by esterification of carboxyl group on C6-position of bicyclo[3.1.0]hexane ring. Among these compounds, 6-alkyl esters exhibited approximately 10-fold higher concentrations of 5 in the plasma and brain of rats after oral administration (e.g., ethyl ester of 5; plasma, Cmax=20.7+/-1.3 microM) compared to oral administration of 5 (plasma, Cmax=2.46+/-0.62 microM). 3-(3,4-Dichlorobenzyloxy)-2-amino-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid 6-heptyl ester (7ao), a prodrug of MGS0039, showed antidepressant-like effects in rat forced swimming test and mouse tail suspension test following oral administration. Moreover, following oral administration of 7ao in mice, high concentrations of MGS0039 were detected in both the brain and plasma, while 7ao was barely detected. In this paper, we report the synthesis, in vitro metabolic stabilities, and pharmacokinetic profiles of the prodrugs of 5, and the antidepressant-like effects of 7ao.

Stimulation of microglial metabotropic glutamate receptor mGlu2 triggers tumor necrosis factor alpha-induced neurotoxicity in concert with microglial-derived Fas ligand

Activated microglia may be detrimental to neuronal survival in a number of neurodegenerative diseases. Thus, strategies that reduce microglial neurotoxicity may have therapeutic benefit. Stimulation of group II metabotropic glutamate (mGlu) receptors on rat primary microglia with the specific group II agonist 2S,2'R,3'R-2-(2',3'-dicarboxy-cyclopropyl)glycine for 24 h induced microglial activation and resulted in a neurotoxic microglial phenotype. These effects were attributable to preferential mGlu2 stimulation, because N-acetyl-L-aspartyl-L-glutamate, a specific mGlu3 agonist, did not induce microglial activation or neurotoxicity. Stimulation of microglial mGlu2 but not mGlu3 induced caspase-3 activation in cerebellar granule neurons in culture, using microglial-conditioned media as well as cocultures. Stimulation of microglial mGlu2 induced tumor necrosis factor-alpha (TNFalpha) release, which contributed to microglial neurotoxicity mediated via neuronal TNF receptor 1 and caspase-3 activation. Stimulation of microglial group I or III mGlu receptors did not induce TNFalpha release. TNFalpha was only neurotoxic in the presence of microglia or microglial-conditioned medium. The toxicity of TNFalpha could be prevented by coexposure of neurons to conditioned medium from microglia stimulated by the specific group III agonist L-2-amino-4-phosphono-butyric acid. The neurotoxicity of TNFalpha derived from mGlu2-stimulated microglia was potentiated by microglial-derived Fas ligand (FasL), the death receptor ligand. FasL was constitutively expressed in microglia and shed after mGlu2 stimulation. Our data suggest that selective and inverse modulation of microglial mGlu2 and mGlu3 may prove a therapeutic target in neuroinflammatory diseases such as Alzheimer's disease and multiple sclerosis.

Activation of group I metabotropic glutamate receptors depresses recurrent inhibition of motoneurons in the neonatal rat spinal cord in vitro

This study examined whether activation of group I metabotropic glutamate receptors (mGluRs) could modulate synaptic inhibition of spinal motoneurons in the neonatal rat isolated spinal cord. Recurrent inhibitory postsynaptic potentials (IPSPs) generated by Renshaw cells were evoked via antidromic stimulation of motor axon collaterals and recorded intracellularly from lumbar motoneurons. The selective agonist of group I mGluRs DHPG (5 micromol L-1) depressed the recurrent IPSP, an effect prevented by the selective antagonist AIDA (500 micromol L-1). The depression by DHPG was use-independent and could be partly counteracted by increasing stimulus strength. Paired pulse depression observed at <or=50-ms intervals was blocked by DHPG in an AIDA-sensitive manner. These results suggest that, in the presence of DHPG, smaller recurrent IPSPs can contribute to the excitatory action of mGluR activation on spinal networks, including the generation of synchronous oscillations recorded from motoneurons.

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.

Differential expression of central metabotropic glutamate receptor (mGluR) subtypes in a clinical model of post-surgical pain

Tissue damage during surgery can induce 'central sensitization' and the development of pain and hyperalgesia post-operatively. Metabotropic glutamate receptors (mGluRs) contribute to nociception, inflammatory pain and hyperalgesia. This study characterized the temporal expression of group I (mGluR(1), mGluR(5)) and II (mGluR(2), mGluR(3)) mGluRs in spinal cord following abdominal surgery. Lumbar spinal cord was recovered from adult sheep euthanased 5 h, 1, 2, 3 and 6 days after undergoing a midline laparotomy, and processed for mGluR mRNA (real-time PCR, in situ hybridization) and protein (Western blotting). mGluR(5) mRNA was up-regulated 5 h and 1 day post-surgery in laminae I-II of the spinal cord dorsal horn. mGluR(5) protein was increased 1 day post-surgery. A delayed induction of mGluR(2) and mGluR(3) mRNAs and mGluR(2/3) protein occurred in spinal cord 3 days after surgery. By 6 days, mGluR(2) mRNA levels had returned to normal, however, mGluR(3) mRNA and mGluR(2/3) protein remained elevated. No change was detected in mGluR(1). These results demonstrate that mGluRs are differentially regulated following surgery and support a link between mGluR-mediated activity and post-surgical pain.

Asymmetric hydrogenation of alpha,beta-unsaturated phosphonates with Rh-BisP* and Rh-MiniPHOS catalysts: scope and mechanism of the reaction

Optically active 1,2-bis(alkylmethylphosphino)ethanes and bis(alkylmethylphosphino)methanes are unique diphosphine ligands combining the simple molecular structure and P-stereogenic asymmetric environment. This work shows that these ligands exhibit excellent enantioselectivity in rhodium-catalyzed asymmetric hydrogenation of alpha,beta-unsaturated phosphonic acid derivatives. The enantioselective hydrogenation mechanism elucidated by NMR study is also described.

Distinct subtypes of group I metabotropic glutamate receptors on rat spinal neurons mediate complex facilitatory and inhibitory effects

While group I glutamate metabotropic (mGlu) receptors show discrete neuronal distribution in the neonatal rat spinal cord, the functional role of their distinct receptor subtypes remains uncertain. Intracellular recording from lumbar motoneurons together with extracellular recording of ventral root (VR) responses was used to investigate the differential contribution by mGlu receptor subtypes to cell excitability and network activity. The group I agonist DHPG evoked motoneuron depolarization (via the AIDA or CPCCOEt-sensitive mGlu receptor subtype 1) mainly at network level and generated sustained, network-dependent oscillations (via the MPEP-sensitive mGlu receptor subtype 5). DHPG also decreased the peak amplitude of synaptic responses induced by dorsal root stimuli, an effect unrelated to depolarization and dependent on glycinergic transmission. Synaptic responses were insensitive to AIDA or MPEP. The present results can be explained by assuming excitation of discrete classes of interneurons by group I mGlu receptor activity. Thus, the cellular distribution of those mGlu receptors at strategic circuit connections may determine the functional outcome of the network in terms of excitation or inhibition. Even if there was insufficient activation by endogenous glutamate of mGlu receptors during synaptic activity evoked by DR stimuli, it is apparent that such receptors are important pharmacological targets for powerful and rapid up- or down-regulation of spinal signal processing at network level, providing a rationale for the proposed use of mGlu receptor agonists in a variety of spinal pathological conditions.

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.

Actions of LY341495 on metabotropic glutamate receptor-mediated responses in the neonatal rat spinal cord

1. The group II metabotropic glutamate (mGlu) receptor antagonist (2S,1'S,2'S)-2-(2-carboxycyclopropyl)-2-(9H-xanthen-9-yl)glycine (LY341495) also has activity at group I and III mGlu receptors at higher concentrations and can be used to discriminate between mGlu receptor subtypes. We report the antagonist action of LY341495 on glutamate receptors expressed in the neonatal rat spinal cord preparation and the use of this antagonist to investigate the group III mGlu receptor subtypes responsible for mediating the depression of synaptic transmission in the spinal cord mediated by the group III mGlu receptor agonists (S)-2-amino-4-phosphonobutanoic acid ((S)-AP4) and (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid (ACPT-I). 2. LY341495 antagonised mGlu receptor agonist-induced responses in the spinal cord with a rank order of potency of group II > group III > group I, which is the same as that observed in human cloned mGlu receptor cell lines. Antagonism of group II and III mGlu receptor-mediated effects were time dependent when low-nanomolar concentrations of LY341495 were used. Although the rank order of potency of LY341495 was the same on native rat and cloned human mGlu receptors, there was a compression in the selectivity between group II and III mGlu receptors, expressed in the spinal cord. 3. In agreement with a previous study on cloned ionotropic glutamate receptors 100 microM LY341495 had little or no effect on N-methyl-D-aspartate, (S)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) propionic acid or kainate receptor-mediated responses on motoneurones. 4. LY341495 exhibited low-nanomolar potency antagonist activity against (S)-AP4 and ACPT-I suggesting that these agonists are activating predominantly mGlu8 and that mGlu4 receptors do not play a role in modulating synaptic transmission in the pathways stimulated in the experiments described here.

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 , 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) is a potent agonist at, primarily, group II metabotropic glutamate receptors, alkylation of at the alpha-carbon notoriously result in group II mGluR antagonists, of which the most potent compound described so far, LY341495, 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 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.

The novel antagonist 3-CBW discriminates between kainate receptors expressed on neonatal rat motoneurones and those on dorsal root C-fibres

1. The natural product willardiine is a selective AMPA receptor agonist. We report that an N(3)-substituted analogue of willardiine, (S)-3-(4-carboxybenzyl)willardiine 3-CBW, antagonizes AMPA and kainate receptors expressed on motoneurones and dorsal root C-fibres, respectively. 2. Reduction of the fast component of the dorsal root-evoked ventral root potential (fDR-VRP) has been used as a novel method to compare AMPA receptor antagonists. 3-CBW, NBQX and GYKI53655 depressed the fDR-VRP with IC(50) values of 10.3+/-2.4, 0.214+/-0.043 and 4.03+/-0.31 micro M, respectively. That 3-CBW depressed the fDR-VRP by acting at AMPA and not metabotropic glutamate receptors was demonstrated by the lack of effect of LY341495 (100 micro M). 3. The Schild plot for antagonism of responses to (S)-5-fluorowillardiine on motoneurones by 3-CBW had a slope of 1.11+/-0.13 giving a pA(2) value of 4.48. The Schild plot for antagonism of kainate responses on the dorsal root by 3-CBW had a slope of 1.05+/-0.05 giving a pA(2) value of 4.96. 4. On neonatal rat motoneurones 3-CBW (200 micro M) almost completely abolished responses to AMPA while responses to NMDA, kainate and DHPG were 101.6+/-11.6%, 39.4+/-5.8% and 110.5+/-9.0% of control, respectively. 3-CBW can therefore be used to isolate kainate receptor responses from those mediated by AMPA receptors. 5 3-CBW antagonized kainate-induced responses on dorsal root C-fibres with a pA(2) value of 4.96 whereas kainate receptor mediated responses (isolated by including GYKI53655 in the medium) on motoneurones were not completely blocked by 200 micro M 3-CBW, substantiating evidence that kainate receptors on neonatal rat motoneurones differ from those on dorsal root C-fibres.

DCG-IV but not other group-II metabotropic receptor agonists induces microglial BDNF mRNA expression in the rat striatum. Correlation with neuronal injury

We have previously described a neuroprotective action of (2S,2'R,3'R)-2-(2'3'-dicarboxycyclopropyl)glycine (DCG-IV), an agonist for group-II metabotropic receptors, on dopaminergic nerve terminals against the degeneration induced by 1-methyl-4-phenylpyridinium (MPP+). This effect was accompanied by an up-regulation of brain-derived neurotrophic factor (BDNF) mRNA expression in the rat striatum. We have now analyzed the phenotypic nature of the BDNF mRNA-expressing cells in response to intrastriatal injection of DCG-IV. Dual in situ hybridization and immunohistochemistry revealed that microglial cells but not astrocytes were responsible for this induction. Subsequent analysis demonstrated that this effect was accompanied by striking loss of striatal glutamic acid decarboxylase (GAD) mRNA and massive appearance of internucleosomal DNA fragmentation, a hallmark of apoptosis. A dose-response study demonstrated that doses of DCG-IV as low as 5 nmol was very toxic in terms GAD mRNA and apoptosis. 0.5 nmol of DCG-IV did not induce toxicity at all in terms of GAD mRNA and apoptosis. Activation of group-II metabotropic receptors in striatum with N-Acetyl-Asp-Glu (NAAG; a mGlu3 agonist) and (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (a mGlu2 and mGlu3 agonist) did not induce neither loss of GAD mRNA nor appearance of apoptosis (doses up to 20 nmol). In additional experiments, NAAG, in contrast to DCG-IV, failed to protect the striatal dopaminergic system against the degeneration induced by MPP+ as studied by microdialysis. Finally, we studied the mechanism by which DCG-IV is highly toxic. For that, selective antagonists of either metabotropic--(R,S)-alpha-methyl-4-carboxyphenylglycine and LY 341495--or ionotropic (N-methyl-D-aspartate, NMDA)--DL-2-amino-5-phosphonovaleric acid (AP-5) glutamate receptors --were co-administered with DCG-IV. Only AP-5 highly protected the striatum against the degeneration induced by DCG-IV. Since DCG-IV also activates the NMDA receptor at concentrations higher than 3 microM, it is conceivable that a intrastriatal concentration equal or higher than 3 microM after a single striatal injection of 5-20 nmol of DCG-IV. Our findings suggest that much caution must be exerted when testing the numerous neuroprotective effects ascribed to group-II metabotropic receptor activation, in particular when using DCG-IV. We conclude that the neuroprotectant capability of a given compound on a specific system does not exclude the possibility of inducing toxicity on a different one.

Localization of metabotropic glutamate receptors 2/3 on primary afferent axons in the rat

The goal of the present study is to determine the relationship of metabotropic glutamate receptors 2/3 (mGluR2/3) to dorsal root ganglion cells, peripheral primary afferent fibers in digital nerves and central primary afferent fibers in the spinal cord. We demonstrate that approximately 40% of L4 and L5 dorsal root ganglion cells contain mGluR2/3-like immunoreactivity. These mGluR2/3-positive cells are small in diameter (23 microm) and 76% stain for the isolectin Griffonia simplicifolia (I-B4), while 67% of I-B4 cells have mGluR2/3-like immunoreactivity. Electron microscopic analyses of mGluR2/3-like immunoreactivity in axons in digital nerves indicate that 32% of unmyelinated and 28% of myelinated axons are labeled. In the lumbar dorsal horn, mGluR2/3-like immunoreactivity is localized preferentially in lamina IIi with lighter staining in laminae III and IV. The dense mGluR2/3-like immunoreactivity in lamina IIi is consistent with the localization of these receptors in I-B4-labeled dorsal root ganglion cells. Elimination of primary afferent input following unilateral dorsal rhizotomies significantly decreases the mGluR2/3-like immunoreactivity density in the dorsal horn although some residual staining does remain, suggesting that many but not all of these receptors are located on primary afferent processes. The finding that mGluR2/3s are located on peripheral sensory axons suggests that they are involved in peripheral sensory transduction and can modulate transmission of sensory input before it reaches the spinal cord. This offers the possibility of altering sensory input, particularly noxious input, at a site that would avoid CNS side effects. Since many but not all of these receptors are located on primary afferent terminals, these receptors may also influence primary afferent transmission in the dorsal horn through presynaptic mechanisms and glutamatergic transmission in general through both presynaptic and postsynaptic mechanisms. Since these receptors are concentrated in lamina IIi and also largely co-localized with I-B4, they may have considerable influence on nociceptive processing by what are considered to be non-peptidergic primary afferent neurons.

Design, synthesis and preliminary evaluation of novel 3'-substituted carboxycyclopropylglycines as antagonists at group 2 metabotropic glutamate receptors

Two novel 3'-substituted carboxycylopropylglycines, (2S,1'S,2'S,3'R)-2-(3'-xanthenylmethyl-2'-carboxycyclopropyl)glycine (8a) and (2S,1'S,2'S,3'R)-2-(3'-xanthenylethyl-2'-carboxycyclopropyl)glycine (8b), were synthesized and evaluated as mGluR ligands. Compound 8b showed to be a potent group II antagonist with submicromolar activity.

Glutamate receptor involvement in dentate granule cell epileptiform activity evoked by mossy fiber stimulation

In many persons with temporal lobe epilepsy, dentate granule cells form an interconnected synaptic network. This recurrent mossy fiber circuit mediates reverberating excitation that may facilitate seizure propagation by synchronizing granule cell discharge. The involvement of specific glutamate receptors in granule cell epileptiform activity evoked by stimulating the mossy fibers was investigated with use of rat hippocampal slices superfused with bicuculline, with or without increasing [K+](o) to 6 mM. The occurrence of short-latency mossy fiber-evoked granule cell epileptiform activity in slices from pilocarpine-treated rats correlated with the presence and extent of recurrent mossy fiber growth. Blockade of AMPA receptors nearly abolished the orthodromic component of the response; subsequent antagonism of kainate receptors as well appeared to have no further action. Antagonism of NMDA receptors reduced the duration of epileptiform discharge, but increased the amplitude of population spikes within the evoked burst. Thus AMPA and NMDA, but perhaps not kainate, receptors play an important role in this type of epileptiform activity. Activation of type II metabotropic glutamate receptors, which inhibits the release of glutamate from mossy fiber boutons, reduced the magnitude of epileptiform discharge. This action was reversed by a partial agonist of these receptors. However, neither an agonist nor an antagonist of type III metabotropic glutamate receptors significantly altered the response. Considering the importance of synchronous granule cell discharge for seizure propagation from the entorhinal cortex to the hippocampus, agonists of type II metabotropic glutamate receptors may be useful in suppressing such discharge both experimentally and clinically.

Metabotropic glutamate receptor subtypes as targets for neuroprotective drugs

Metabotropic glutamate (mGlu) receptors have been considered as potential targets for neuroprotective drugs, but the lack of specific drugs has limited the development of neuroprotective strategies in experimental models of acute or chronic central nervous system (CNS) disorders. The advent of potent and centrally available subtype-selective ligands has overcome this limitation, leading to an extensive investigation of the role of mGlu receptor subtypes in neurodegeneration during the last 2 years. Examples of these drugs are the noncompetitive mGlu1 receptor antagonists, CPCCOEt and BAY-36-7620; the noncompetitive mGlu5 receptor antagonists, 2-methyl-6-(phenylethynyl)pyridine, SIB-1893, and SIB-1757; and the potent mGlu2/3 receptor agonists, LY354740 and LY379268. Pharmacologic blockade of mGlu1 or mGlu5 receptors or pharmacologic activation of mGlu2/3 or mGlu4/7/8 receptors produces neuroprotection in a variety of in vitro or in vivo models. MGlu1 receptor antagonists are promising drugs for the treatment of brain ischemia or for the prophylaxis of neuronal damage induced by synaptic hyperactivity. MGlu5 receptor antagonists may limit neuronal damage induced by a hyperactivity of N-methyl-d-aspartate (NMDA) receptors, because mGlu5 and NMDA receptors are physically and functionally connected in neuronal membranes. A series of observations suggest a potential application of mGlu5 receptor antagonists in chronic neurodegenerative disorders, such as amyotrophic lateral sclerosis and Alzheimer disease. MGlu2/3 receptor agonists inhibit glutamate release, but also promote the synthesis and release of neurotrophic factors in astrocytes. These drugs may therefore have a broad application as neuroprotective agents in a variety of CNS disorders. Finally, mGlu4/7/8 receptor agonists potently inhibit glutamate release and have a potential application in seizure disorders. The advantage of all these drugs with respect to NMDA or AMPA receptor agonists derives from the evidence that mGlu receptors do not "mediate," but rather "modulate" excitatory synaptic transmission. Therefore, it can be expected that mGlu receptor ligands are devoid of the undesirable effects resulting from the inhibition of excitatory synaptic transmission, such as sedation or an impairment of learning and memory.

Spinal N-acetyl-alpha-linked acidic dipeptidase (NAALADase) inhibition attenuates mechanical allodynia induced by paw carrageenan injection in the rat

N-Acetylated-alpha-linked acidic dipeptidase (NAALADase) hydrolyzes N-acetyl-aspartyl-glutamate (NAAG) to liberate N-acetyl-aspartate and glutamate. NAAG is a putative neurotransmitter and acts as a mixed agonist/antagonist on N-methyl-D-aspartate (NMDA) receptors and acts as an agonist on the metabotropic glutamate receptor 3 (mGluR3). In the present study, we examined the role of spinal NAALADase in the maintenance of mechanical allodynia induced by carrageenan injection, skin incision and mild thermal injury using 2-(phosphonomethyl)pentanedioic acid (2-PMPA), a specific NAALADase inhibitor, in rats. Mechanical allodynia was induced by injection of 2 mg carrageenan into the paw (carrageenan model), by creating a 1-cm longitudinal skin incision of the plantar aspect of the foot (post-operative model), or by application of thermal stimulation (52.5 degrees C) for 45 s to the hind paw (mild thermal injury model). 2-PMPA was administered intrathecally at the time when the maximum mechanical allodynia occurred. Mechanical allodynia was assessed by the measurement of mechanical threshold using von Frey filaments. The mechanical threshold was measured 5, 15, 30, 60 and 90 min after the drug administration. In the carrageenan model, 100 microg of 2-PMPA attenuated the level of mechanical allodynia. 2-PMPA had no effect on the level of mechanical allodynia in both the post-operative pain model and the mild thermal injury model. These data suggested that the inhibition of spinal NAALADase alleviated mechanical allodynia induced by paw carrageenan injection.

Changes in metabotropic glutamate receptor expression following spinal cord injury

Spinal cord injury (SCI) initiates biochemical events that lead to an increase in extracellular excitatory amino acid concentrations, resulting in glutamate receptor-mediated excitotoxic events. These receptors include the three groups of metabotropic glutamate receptors (mGluRs). Group I mGluR activation can initiate a number of intracellular pathways that increase neuronal excitability. Group II and III mGluRs may function as autoreceptors to modulate neurotransmission. Thus, all three groups may contribute to the mechanisms of central sensitization and chronic central pain. To begin evaluating mGluRs in SCI, we quantified the changes in mGluR expression after SCI in control (naive), sham, and impact injured adult male Sprague-Dawley rats (200-250 g). SCI was produced at spinal segment T10 with a New York University impactor (12.5-mm drop, 10-g rod of 2-mm diameter). Expression levels were determined by Western blot and immunohistochemistry analyses at the epicenter of injury, as well as segments rostral and caudal. The group I subtype mGluR1 was increased over control levels in segments rostral and caudal by postsurgical day (PSD) 7 and remained elevated through PSD 60. The group I subtype mGluR5 was unchanged in all segments rostral and caudal to the injury at every time point measured. Group II mGluRs were decreased compared to control levels from PSD 7 through PSD 60 in all segments. These results suggest that different subtypes of mGluRs have different spatial and temporal expression patterns following SCI. The expression changes in mGluRs parallel the development of mechanical allodynia and thermal hyperalgesia following SCI; therefore, understanding the expression of mGluRs after SCI may give insight into mechanisms underlying the development of chronic central pain.

Elevated levels of group-III metabotropic glutamate receptors in the inferior colliculus of genetically epilepsy-prone rats following intracollicular administration of L-serine-O-phosphate

The selective group-III metabotropic glutamate receptor agonist, L-serine-O-phosphate (L-SOP), when injected bilaterally into the inferior colliculus of the sound sensitive genetically epilepsy-prone (GEP) rats produces a short proconvulsant excitation followed by a long phase of protection against sound-induced seizures lasting for 2-4 days. We have studied this prolonged suppression of audiogenic seizures using pharmacological and molecular biological approaches including semiquantitative RT-PCR and western blotting. The intracerebroventricular injection of the protein synthesis inhibitor cycloheximide (120 microg) 30 min beforehand significantly reduces the proconvulsant seizure activity and the prolonged anticonvulsant effect of intracollicular L-SOP (500 nmol/side). The sensitive semiquantitative RT-PCR revealed a significant up-regulation in mGlu(4) and mGlu(7) mRNA levels in the inferior colliculus at 2 days (maximum suppression of audiogenic seizures) after intracollicular L-SOP injection compared with the non-injected, 2-day post-vehicle treated and 7-day (return to expressing audiogenic seizures) post-drug or vehicle-treated groups. No significant changes were observed in mGlu(6) or mGlu(8) mRNA expression levels in drug-treated compared with control groups. Examination of mGlu(4a) and mGlu(7a) protein levels using western blotting showed a significant increase in mGlu(7a) but no significant change in mGlu(4a) protein levels 2 days after L-SOP treatment compared with the control groups (non-injected and 2-day vehicle-injected group). These results suggest that up-regulation of mGlu(7) receptors is involved in the prolonged anticonvulsant effect of L-SOP against sound-induced seizures in GEP rats. The potential use of mGlu(7) agonists as novel anti-epileptic agents merits investigation.

Interactions between LY354740, a group II metabotropic agonist and the GABA(A)-benzodiazepine receptor complex in the rat elevated plus-maze

Flumazenil, a benzodiazepine (BZ) receptor antagonist, and naloxone, a non-selective mu-receptor antagonist, were used to investigate whether the anxiolytic action of LY354740 [1S,2S,5R,6S-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylate monohydrate], a Group II metabotropic glutamate receptor agonist, was mediated through the benzodiazepine binding site on the GABA(A) receptor and opioid pathways. LY354740 (1.0-10.0 mg/kg i.p.) induced dose-dependent anxiolytic-like effects in the rat elevated plus-maze. The anxiolytic-like effects of LY354740 (10.0 mg/kg) and the benzodiazepine receptor agonist, chlordiazepoxide (CDP, 5.0 mg/kg i.p.) were blocked by flumazenil (15.0 mg/kg i.p.). By contrast, naloxone (10.0 mg/kg i.p.) failed to affect the anxiolytic-like effects of either LY354740 or CDP. The behaviour of animals treated with flumazenil or naloxone alone did not significantly differ from that of animals treated with vehicle alone. This study suggests that the anxiolytic-like effects of LY354740 on the elevated plus-maze may be directly or indirectly mediated by the benzodiazepine binding site on the GABA(A) receptor complex.

Inhibition of spinal N-acetylated-alpha-linked acidic dipeptidase produces an antinociceptive effect in the rat formalin test

N-acetyl-aspartyl-glutamate is a putative neurotransmitter and acts as a weak agonist at the N-methyl-D-aspartate receptor. N-acetyl-aspartyl-glutamate also acts as an agonist at the metabotropic glutamate receptor 3. N-acetyl-aspartyl-glutamate is hydrolyzed by N-acetylated-alpha-linked acidic dipeptidase to liberate N-acetyl-aspartate and glutamate. Recently, a specific inhibitor of N-acetylated-alpha-linked acidic dipeptidase, 2-(phosphonomethyl)pentanedioic acid, has been reported. In the present study, we examined the effect of i.t. administered 2-(phosphonomethyl)pentanedioic acid in the rat formalin test (a model of inflammatory pain) and the rat hot plate test. In the formalin test, drugs were administered 10min before (pre-treatment study) or 7min after (post-treatment study) the formalin injection. The paw formalin injection induces biphasic flinching (phase 1: 0-2min; phase 2: 10-60min) of the injected paw. In the pre-treatment study, i.t. administered 2-(phosphonomethyl)pentanedioic acid depressed both phases 1 and 2 flinching behavior in a dose-dependent manner but 2-(phosphonomethyl)pentanedioic acid had no effect on the flinching behavior in the post-treatment study. In the pre-treatment study, the potency of 2-(phosphonomethyl)pentanedioic acid in depressing the phase 2 response is greater than that in depressing the phase 1 response. Intrathecal injection of 2-(phosphonomethyl)pentanedioic acid had no effect in the hot plate test. We suggest that N-acetylated-alpha-linked acidic dipeptidase plays an important role in spinal nociceptive transmission and that inhibition of spinal N-acetylated-alpha-linked acidic dipeptidase produces an antinociceptive effect during the rat formalin test but not during the hot plate test.

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.

(S)-3,4-DCPG, a potent and selective mGlu8a receptor agonist, activates metabotropic glutamate receptors on primary afferent terminals in the neonatal rat spinal cord

(S)-3,4-Dicarboxyphenylglycine (DCPG) has been tested on cloned human mGlu1-8 receptors individually expressed in AV12-664 cells co-expressing a rat glutamate/aspartate transporter and shown to be a potent and selective mGlu8a receptor agonist (EC(50) value 31+/-2 nM, n=3) with weaker effects on the other cloned mGlu receptors (EC(50) or IC(50) values >3.5 microM on mGlu1-7). Electrophysiological characterisation on the neonatal rat spinal cord preparation revealed that (S)-3,4-DCPG depressed the fast component of the dorsal root-evoked ventral root potential (fDR-VRP) giving a biphasic concentration-response curve showing EC(50) values of 1.3+/-0.2 microM (n=17) and 391+/-81 microM (n=17) for the higher and lower affinity components, respectively. The receptor mediating the high-affinity component was antagonised by 200 microM (S)-alpha-methyl-2-amino-4-phosphonobutyrate (MAP4, K(D) value 5.4+/-1.5 microM (n=3)), a group III metabotropic glutamate (mGlu) receptor antagonist. The alpha-methyl substituted analogue of (S)-3,4-DCPG, (RS)-3,4-MDCPG (100 microM), antagonised the effects of (S)-3,4-DCPG (K(D) value 5.0+/-0.4 microM, n=3) in a similar manner to MAP4. (S)-3,4-DCPG-induced depressions of the fDR-VRP in the low-affinity range of the concentration-response curve were potentiated by 200 microM (S)-alpha-ethylglutamate (EGLU), a group II mGlu receptor antagonist, and were relatively unaffected by MAP4 (200 microM). However, depressions of the fDR-VRP mediated by the AMPA selective antagonist (R)-3,4-DCPG were not potentiated by EGLU, suggesting that the low-affinity component of the concentration-response curve for (S)-3,4-DCPG is not due to antagonism of postsynaptic AMPA receptors. It is suggested that the receptor responsible for mediating the high-affinity component is mGlu8. The receptor responsible for mediating the low-affinity effect of (S)-3,4-DCPG has yet to be identified but it is unlikely to be one of the known mGlu receptors present on primary afferent terminals or an ionotropic glutamate receptor of the AMPA or NMDA subtype.

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.

Dual modulation of excitatory synaptic transmission by agonists at group I metabotropic glutamate receptors in the rat spinal dorsal horn

The effects of group I metabotropic glutamate (mGlu) receptors on excitatory transmission in the rat dorsal horn, but mostly substantia gelatinosa, neurons were investigated using conventional intracellular recording in slices. The broad spectrum mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S, 3R-ACPD), the group I mGlu receptor selective agonist (S)-3, 5-dihydroxyphenylglycine (DHPG), and the selective mGlu subtype 5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), all induce long-lasting depression of A primary afferent fibers-mediated monosynaptic excitatory postsynaptic potential (EPSP), and long-lasting potentiation of polysynaptic EPSP, and EPSP in cells receiving C-afferent fiber input. The DHPG potentiation of polysynaptic EPSP was partially or fully reversed by (S)-4-carboxyphenylglycine (S-4CPG), the mGlu subtype 1 preferring antagonist. 2-Methyl-6-(phenylethynyl)-pyridine, the potent and selective mGlu subtype 5 antagonist, partially reversed the CHPG potentiation of polysynaptic EPSP. The effects of DHPG on monosynaptic and polysynaptic EPSPs were reduced, or abolished, by the N-methyl-D-aspartate (NMDA) receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (AP5). A clear and pronounced facilitation of the expression of DHPG- and CHPG-induced enhancement of polysynaptic EPSP, and EPSP evoked at C-fiber strength, was seen in the absence of gamma-aminobutyric acid subtype A receptor- and glycine-mediated synaptic inhibition. Besides dual modulation of excitatory synaptic transmission, DHPG induces depression of inhibitory postsynaptic potentials evoked by primary afferent stimulation in dorsal horn neurons. In addition, group I mGlu receptor agonists produced a direct persistent excitatory postsynaptic effect consisting of a slow membrane depolarization, an increase in input resistance, and an intense neuronal discharge. Cyclothiazide and (S)-4-CPG, the mGlu receptor subtype 1 preferring antagonists, significantly attenuated the DHPG-induced depolarization. These results demonstrate that the pharmacological activation of group I metabotropic glutamate receptors induces long-term depression (LTD) and long-term potentiation (LTP) of synaptic transmission in the spinal dorsal horn. These types of long-term synaptic plasticity may play a functional role in the generation of post-injury hypersensitivity (LTP) or antinociception (LTD).

Slow synaptic inhibition mediated by metabotropic glutamate receptor activation of GIRK channels

Glutamate is the predominant excitatory neurotransmitter in the vertebrate CNS. Ionotropic glutamate receptors mediate fast excitatory actions whereas metabotropic glutamate receptors (mGluRs) mediate a variety of slower effects. For example, mGluRs can mediate presynaptic inhibition, postsynaptic excitation, or, more rarely, postsynaptic inhibition. We previously described an unusually slow form of postsynaptic inhibition in one class of projection neuron in the song-control nucleus HVc of the songbird forebrain. These neurons, which participate in a circuit that is essential for vocal learning, exhibit an inhibitory postsynaptic potential (IPSP) that lasts several seconds. Only a portion of this slow IPSP is mediated by GABA(B) receptors. Since these cells are strongly hyperpolarized by agonists of mGluRs, we used intracellular recording from brain slices to investigate the mechanism of this hyperpolarization and to determine whether mGluRs contribute to the slow synaptic inhibition. We report that mGluRs hyperpolarize these HVc neurons by activating G protein-coupled, inwardly-rectifying potassium (GIRK) channels. MGluR antagonists blocked this response and the slow synaptic inhibition. Thus, glutamate can combine with GABA to mediate slow synaptic inhibition by activating GIRK channels in the CNS.

Developmental regulation of hippocampal excitatory synaptic transmission by metabotropic glutamate receptors

The aims of this study were, to use agonists selective for the 3 mGlu receptor groups to identify developmental changes in their effects, and to assess the usefulness of proposed selective antagonists as pharmacological tools. Hippocampal slices (400 microm) were prepared from neonate (9 - 14 days) and young adult (5 - 7 weeks) Sprague-Dawley rats. Field excitatory postsynaptic potentials (fEPSP) were recorded from CA1. DHPG (100 microM), a group I agonist, produced a slowly developing enhancement of fEPSP slope in slices from adults. In slices from neonates, DHPG (75 microM) depressed fEPSP slope. DCG-IV (500 nM), a group II agonist, did not affect the fEPSP recorded from slices from adults whereas perfusion in neonate slices produced a sustained depression. The group III agonist L-AP4 (50 microM) was ineffective in adult slices but depressed fEPSP slope in slices prepared from neonates. DHPG-induced depression of fEPSP slope was inhibited by 4-CPG (400 microM), a group I antagonist, but was unaffected by MCCG (500 microM) and MAP4 (500 microM), group II and III receptor antagonists respectively. MCCG but not MAP4 antagonized the effects of DCG-IV with 4-CPG producing variable effects. The effect of L-AP4 was unaffected by MCCG, blocked by MAP4, and enhanced by 4-CPG. The results show that the effects of the agonists for all groups of mGlu receptors are developmentally regulated. Furthermore, MCCG and MAP4 behave as effective and selective antagonists for group II and group III mGlu receptors respectively, whereas the usefulness of 4-CPG as a group I antagonist may be limited.

Behavioural evidence supporting a differential role for group I and II metabotropic glutamate receptors in spinal nociceptive transmission

Metabotropic glutamate receptors (mGluRs) have been shown to contribute to nociceptive processing in spinal cord. This study examined the effects of intrathecal treatment with group I and II mGluR compounds on withdrawal thresholds to noxious mechanical stimuli, in the absence of tissue damage or inflammation, in adult female sheep. Both the group I/II mGluR agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD; 5.2-520 nmol) and the group II agonist (2S,1S, 2S)-2-(carboxycyclopropyl)glycine (L-CCG-I; 620 nmol) significantly increased mechanical withdrawal thresholds between 5-15 min post-injection. These anti-nociceptive effects were blocked by co-administration of the mGluR antagonist (2S)-alpha-ethylglutamate (EGLU; 570 nmol; group II), but not (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; 450 nmol; group I). Intrathecal administration of the group I-specific agonist (S)-3,5-dihydroxyphenylglycine ((S)-3,5-DHPG; 50 nmol) produced a significant reduction in mechanical thresholds, which was blocked by co-administration of the group I antagonist AIDA. In contrast, the highest dose of (S)-3,5-DHPG tested, 5 micromol, significantly elevated response thresholds. These results demonstrate that both group I and II mGluRs play crucial, but contrasting roles in mediating acute mechanical nociceptive events in spinal cord.

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.

Role of metabotropic glutamate receptor subclasses in modulation of adenylyl cyclase activity by a nootropic NS-105

The involvement of metabotropic glutamate (mGlu) receptors in the modulatory actions of a novel cognition enhancer, (+)-5-oxo-D-prolinepiperidinamide monohydrate (NS-105), on adenylyl cyclase activity in rat cerebrocortical membranes and primary neuronal cultures was investigated using selective antagonists and antisense oligodeoxynucleotides for mGlu receptor subclasses. In rat cerebrocortical membranes, the inhibitory action of NS-105 (0.1 microM) on forskolin-stimulated cAMP formation was blocked by a group II mGlu receptor antagonist, (+/-)-alpha-ethylglutamic acid, and by a group III antagonist, (+)-2-amino-2-methyl-4-phosphonobutanoic acid (MAP-4), but not by a group I antagonist, (+/-)-1-aminoindan-1,5-dicarboxylic acid (AIDA), whereas the facilitation of cAMP formation by NS-105 (1 microM) in pertussis toxin-pretreated membranes was abolished by AIDA but not by (+/-)-alpha-ethylglutamic acid or MAP-4. In primary cultured neurons of mouse cerebral cortex, the inhibitory action of NS-105 on adenylyl cyclase activity disappeared after treatment with antisense oligodeoxynucleotides for group II (mGlu(2) and mGlu(3) receptors) and group III (mGlu(4) and mGlu(7) receptors) but not group I (mGlu(5) receptor) mGlu receptor subclasses. These findings suggest that the inhibitory action of NS-105 on adenylyl cyclase activity is mediated through group II and group III mGlu receptor subclasses while the facilitatory action is dependent on the group I mGlu receptor subclass.

Coincidence detection and changes of synaptic efficacy in spiny stellate neurons in rat barrel cortex

Paired whole-cell voltage recordings were made from synaptically connected spiny stellate neurons in layer 4 of the barrel field in young (P14) rat somatosensory cortex. When postsynaptic action potentials (APs) followed each of 5 presynaptic APs in a 10- or 20-Hz train by less than 25 ms, subsequent unitary EPSP amplitudes were persistently reduced. Induction of long-term depression (LTD) depended on activation of group II metabotropic glutamate receptors, but not on NMDA or AMPA receptors. Reducing postsynaptic increases in intracellular calcium ([Ca2+]i) by intracellular loading with a fast- (BAPTA) or a slow- (EGTA) acting Ca2+ buffer blocked synaptic depression. Analysis of EPSP failures suggested mediation of LTD by a reduction in release probability. We propose a mechanism by which coincident activity results in long-lasting reduction of synaptic efficacy between synaptically connected neurons.

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.