2-chloro-5-hydroxyphenylglycine and 6-methyl-2-(phenylethynyl)pyridine

Within the hippocampus, different kinds of spatial experience determine the direction of change of synaptic weights. Synaptic plasticity resulting from such experience may enable memory encoding. The CA3 region is very striking in this regard: due to the distinct molecular properties of the mossy fiber (MF) and associational-commissural (AC) synapses, it is believed that they enable working memory and pattern completion. The question arises, however, as to how information reaching these synapses results in differentiated encoding. Given its crucial role in enabling persistent synaptic plasticity in other hippocampal subfields, we speculated that the metabotropic glutamate receptor mGlu5 may regulate information encoding at MF and AC synapses. Here, we show that antagonism of mGlu5 inhibits LTP, but not LTD at MF synapses of freely behaving adult rats. Conversely, mGlu5 antagonism prevents LTD but not LTP at AC-CA3 synapses. This suggests that, under conditions in which mGlu5 is activated, LTP may be preferentially induced at MF synapses, whereas LTD is favored at AC synapses. To assess this possibility, we applied 50 Hz stimulation that should generate postsynaptic activity that corresponds to θm, the activation threshold that lies between LTP and LTD. MGlu5 activation had no effect on AC responses but potentiated MF synapses. These data suggest that mGlu5 serves as a switch that alters signal-to-noise ratios during information encoding in the CA3 region. This mechanism supports highly tuned and differentiated information storage in CA3 synapses.

Topics: Animals; CA3 Region, Hippocampal; Electric Stimulation; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glycine; Long-Term Potentiation; Long-Term Synaptic Depression; Male; Mossy Fibers, Hippocampal; Neural Pathways; Neuronal Plasticity; Phenylacetates; Pyridines; Rats; Receptor, Metabotropic Glutamate 5; Synapses; Synaptic Transmission

Transient receptor potential channel 1 (TRPC1; a cation channel activated by store depletion and/or through an intracellular messenger) is expressed in a variety of tissues, including the brain. To study the physiological function of TRPC1, we investigated the role of endogenously expressed TRPC1 in glutamate-induced cell death, using the murine hippocampal cell line HT22. Knocking down TRPC1 mRNA using TRPC1-shRNA or blocking of TRPC channels using 2-APB (≥200 μM) robustly attenuated glutamate-induced cell death after 24 h of incubation with 5 mM glutamate. Glutamate toxicity in HT22 cells seems to involve metabotropic glutamate receptor mGluR5 since MPEP (2-methyl-6-(phenylethynyl)-pyridine), an mGluR5 antagonist (≥100 μM), abrogated glutamate toxicity. Furthermore, a direct activation of mGluR5 by CHPG [(RS)-chloro-5-hydroxyphenylglycine; 100 μM or 300 μM] promoted HT22 cell death. TRPC1 knock-down markedly reduced CHPG-induced cell death. These observations suggest that glutamate-induced cell death in HT22 cells activates mGluR5 receptors, which significantly increases Ca(2+) influx through TRPC1 channels. TRPC1 knock-down prevented glutamate- and CHPG-induced cell death, suggesting that glutamate-induced toxicity in HT22 cells is mediated through TRPC1 channels and an mGluR5-dependent pathway. Together, this work provides evidence for a novel receptor activation pathway of TRPC1 in glutamate-induced toxicity.

Topics: Animals; Cell Death; Cell Line; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glutamic Acid; Glycine; Hippocampus; Mice; Neurons; Phenylacetates; Pyridines; Receptor, Metabotropic Glutamate 5; RNA, Messenger; TRPC Cation Channels

Metabotropic glutamate receptors (mGluRs) regulate neurogenesis in the mammalian central nervous system during development and throughout adulthood. However, the mechanisms remain unknown. The present study was aimed at investigating the effect of mGluR5 on the differentiation of rat neural progenitor cells (NPCs) into neurons as well as the underlying molecular mechanisms. NPCs were treated with mGluR5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), mGluR5 siRNA, and antagonist 6-methyl-2-(phenylethynyl) pyridine hydrochloride (MPEP), respectively. Three different subtypes of neurons (cholinergic, GABAergic, and dopaminergic neurons) were evaluated, and the activation of signaling pathways of mitogen-activated protein kinases was determined. Results showed that CHPG caused rat NPCs to differentiate into neurons, whereas mGluR5 siRNA and MPEP inhibited the cell differentiation. The proportion of cholinergic neurons increased with CHPG treatment and decreased after siRNA or MPEP treatment, whereas there were no significant changes in the proportions of GABAergic and dopaminergic neurons after treatment. The phosphorylated ERK1/2 levels increased after CHPG treatment and decreased after siRNA or MPEP treatment. In conclusion, our findings showed that mGluR5 caused rat NPCs to differentiate into cholinergic neurons by activating ERKs, suggesting that mGluR5 may play a significant role in the mechanism and treatment of degenerative diseases such as Alzheimer's disease.

Topics: Animals; Cell Differentiation; Cells, Cultured; Cholinergic Neurons; Dopaminergic Neurons; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Extracellular Signal-Regulated MAP Kinases; GABAergic Neurons; Glycine; MAP Kinase Signaling System; Neural Stem Cells; Neurogenesis; Phenylacetates; Phosphorylation; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; RNA, Small Interfering

Adult hippocampal neural stem cells can be activated by hippocampal neural activities. When focal cerebral ischemia, known as middle cerebral artery occlusion (MCAO), occurs, neural stem cells are activated to promote their proliferation. However, the mechanism by which these cells are activated is still unclear. Here, we indicate the involvement of metabotropic glutamate receptor 5 (mGluR5) signaling in neural stem cells in their activity-related proliferation after MCAO. We found mGluR5 molecules on neural stem cells by using calcium imaging. We detected the activation of neural stem cells by adding the mGluR5 agonist (RS)-2-chloro-5-hydroxyphenylglycine. On a hippocampal slice, the activation of neural stem cells to promote their proliferation was initiated by theta-burst electrical stimulation at the perforant pathway, and this activation was significantly blocked by an mGluR5 antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP). In addition to this, the injection of the blood-brain barrier-permeable mGluR5 agonist 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide into live mice promoted the proliferation of neural stem cells. Moreover, in vivo theta-burst electrical stimulation induced proliferation of neural stem cells. A chronic field recording study showed that the activity of the hippocampal formation was elevated after MCAO. Finally, we observed that the mGluR5 antagonist MPEP significantly blocked the stimulated proliferation of neural stem cells induced by MCAO, by blocking mGluR5 signaling. Our results suggest that glutamates released by the elevated neural activities after MCAO may trigger mGluR5 signaling in neural stem cells to promote their proliferation.

Topics: Adult Stem Cells; Animals; Benzamides; Calcium Signaling; Cell Proliferation; Electric Stimulation; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glycine; Hippocampus; Infarction, Middle Cerebral Artery; Mice; Mice, Inbred ICR; Neural Stem Cells; Phenylacetates; Pyrazoles; Pyridines; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Signal Transduction; Theta Rhythm

Several subtypes of interneurons in the feedback circuit in stratum oriens of the hippocampus exhibit NMDA receptor-independent long-term potentiation (LTP) at glutamatergic synapses made by local pyramidal neurons. LTP has been reported with both "Hebbian" and "anti-Hebbian" induction protocols, where high-frequency presynaptic stimulation is paired with either postsynaptic depolarization or hyperpolarization. Do these phenomena represent distinct forms of plasticity, dependent on group I metabotropic receptors (mGluRs) and rectifying Ca2+ -permeable AMPA receptors, respectively? Blockade of either mGluR1 or mGluR5 prevented anti-Hebbian LTP induction in stratum oriens interneurons in rat hippocampal slices. Exogenous activation of group I mGluRs by the selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) was unable to induce LTP on its own, and instead depressed excitatory transmission. However, when paired with postsynaptic hyperpolarization, DHPG or the group I metabotropic receptor (mGluR5)-selective agonist (R,S)-2-chloro-5-hydroxyphenylglycine (CHPG) elicited a delayed long-lasting potentiation, which was accompanied by a decrease in paired-pulse facilitation. Anti-Hebbian LTP occluded the effect of DHPG paired with hyperpolarization, implying that the induction cascades triggered by both conjunctions of stimuli converge on common expression mechanisms.

Topics: Animals; Benzoates; Electric Stimulation; Electrodes, Implanted; Electrophysiological Phenomena; Excitatory Amino Acid Agonists; Excitatory Postsynaptic Potentials; Glycine; Hippocampus; Interneurons; Long-Term Potentiation; Male; Phenylacetates; Pyridines; Rats; Rats, Sprague-Dawley; Receptors, AMPA; Receptors, Metabotropic Glutamate; Resorcinols

We evaluated the ability of spinally administered 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a selective antagonist of the metabotropic glutamate receptor subtype 5 (mGluR5), and 2-chloro-5-hydroxyphenylglycine (CHPG), an mGluR5 agonist, to modulate the antinociceptive action and tolerance of intrathecal (i.t.) morphine infusion in rats, and assessed the expression of spinal nitric oxide synthase (NOS). MPEP co-infused with morphine not only preserved the analgesia and retarded the development of antinociceptive tolerance, but also partially inhibited the up-regulation of spinal nNOS protein. However, the loss of morphine antinociceptive effect and tolerance were accelerated when CHPG and morphine were co-infused, while spinal nNOS activity was significantly up-regulated. We hypothesize that activation of mGluR5 and NMDA receptors occurs after the appearance of antinociceptive tolerance to morphine. The activation of these receptors might stimulate an increased concentration of intracellular calcium and activation of PKC, which both play a vital role in the development of morphine antinociceptive tolerance and expression of spinal NOS. The synergistic effect which seems to exist between mGluRs and iGluRs may also contribute to this phenomenon.

Topics: Animals; Dose-Response Relationship, Drug; Drug Tolerance; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Gene Expression Regulation; Glycine; Male; Morphine; Narcotics; Nitric Oxide Synthase Type I; Pain Measurement; Pain Threshold; Phenylacetates; Pyridines; Rats; Rats, Sprague-Dawley; Spinal Cord; Time Factors

Prolonged intrathecal (i.t.) administration of morphine results in tolerance to morphine-induced antinociception. We found that co-administration of selective metabotropic glutamate receptor subtype 5 antagonist MPEP with morphine could suppress the loss of morphine-induced antinociception and inhibit the development of tolerance to morphine-induced antinociceptive effect. Whereas, the specific metabotropic glutamate receptor subtype 5 agonist CHPG does the opposite. As the activation of NMDA receptor after chronic morphine administration has been verified, we suppose there is an enhanced activation of mGluR5 during the development of tolerance to morphine-induced antinociception. Activation of mGluR5 may mobilize the release of intracellular Ca(2+) and activate PKC, leading to morphine-induced antinociception suppression. We conclude that mGluR5 contributes to the development of tolerance to morphine-induced antinociception after chronic morphine exposure.

Topics: Analgesics, Opioid; Animals; Calcium; Drug Administration Schedule; Drug Tolerance; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glutamic Acid; Glycine; Injections, Spinal; Male; Morphine; Neurons, Afferent; Nociceptors; Pain; Pain Threshold; Phenylacetates; Protein Kinase C; Pyridines; Rats; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Spinal Cord; Synaptic Transmission; Up-Regulation

Group I metabotropic glutamate receptors (mGluRs) have been reported to regulate N-methyl-d-aspartate (NMDA) receptor function in various brain regions. The selective mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) can potentiate NMDA antagonists such as PCP and MK-801-induced behavioural responses. In the present study, the role of group I mGluRs on ketamine- and propofol-induced general anaesthesia was examined.. Mice were pretreated with various doses of the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG), selective mGluR5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), mGluR1 antagonist 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) and mGluR5 antagonist MPEP followed by administration of ketamine (120 mg kg(-1)) or propofol (140 mg kg(-1)) to induce anaesthesia. The duration of loss of righting reflex was recorded.. DHPG and CHPG antagonized and MPEP potentiated ketamine-induced anaesthesia in a dose-dependent manner. CPCCOEt was ineffective. However, propofol-induced anaesthesia was not affected after manipulating mGluR1 and mGluR5 receptors.. mGluR5 receptors play an important role in modulation of anaesthesia induced by ketamine, but not propofol.

Topics: Anesthetics, Dissociative; Anesthetics, Intravenous; Animals; Chromones; Dose-Response Relationship, Drug; Glycine; Ketamine; Male; Methoxyhydroxyphenylglycol; Mice; Phenylacetates; Propofol; Pyridines; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Reflex

An increasing amount of evidence suggests that the family of nuclear factor kappaB (NF-kappaB) transcription factors plays an important role in synaptic plasticity and long-term memory formation. The present study investigated the regulation of NF-kappaB family members p50, p65/RelA, and c-Rel in the hippocampus in response to metabotropic glutamate receptor (mGluR) signaling. Activation of group I metabotropic glutamate receptors (GpI-mGluRs) with the agonist (S)-3,5-dihydroxyphenylglycine (DHPG) resulted in a time-dependent increase in DNA binding activity of p50, p65, and c-Rel in area CA1 of the hippocampus. An antagonist of mGluR5, 2-Methyl-6-(phenylethynyl)pyridine, inhibited the DHPG-induced activation of NF-kappaB, whereas an antagonist of mGluR1, (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid, did not. Using a series of inhibitors, we investigated the signaling pathways necessary for DHPG-induced activation of NF-kappaB and found that they included the phosphatidyl inositol 3-kinase, protein kinase C, mitogen-activated protein kinase kinase, and p38-mitogen-activated protein kinase pathways. To determine the functional significance of mGluR-induced regulation of NF-kappaB, we measured long-term depression (LTD) of Schaffer-collateral synapses in the hippocampus of c-Rel knock-out mice. Early phase LTD was normal in c-rel(-/-) mice. However, late-phase LTD (>90 min) was impaired in c-rel(-/-) mice. The observations of this deficit in hippocampal synaptic plasticity prompted us to further investigate long-term memory formation in c-rel(-/-) mice. c-rel(-/-) mice exhibited impaired performance in a long-term passive avoidance task, providing additional evidence for c-Rel in long-term memory formation. These results demonstrate that the NF-kappaB transcription factor family is regulated by GpI-mGluRs in the hippocampus and that the c-Rel transcription factor is necessary for long-term maintenance of LTD and formation of long-term memory.

Topics: Animals; Animals, Newborn; Avoidance Learning; Behavior, Animal; Blotting, Western; Dose-Response Relationship, Radiation; Electric Stimulation; Enzyme Activation; Enzyme Inhibitors; Enzyme-Linked Immunosorbent Assay; Excitatory Postsynaptic Potentials; Gene Expression; Glycine; Hippocampus; In Vitro Techniques; Long-Term Synaptic Depression; Methoxyhydroxyphenylglycol; Mice; Mice, Inbred C57BL; Mice, Knockout; NF-kappa B; Patch-Clamp Techniques; Phenylacetates; Protein Binding; Protein Subunits; Proto-Oncogene Proteins c-rel; Pyridines; Receptors, Metabotropic Glutamate; Reverse Transcriptase Polymerase Chain Reaction; RNA, Messenger; Signal Transduction; Synaptic Transmission; Time Factors

Glutamate is thought to be the excitatory neurotransmitter in the lateral geniculate nucleus (LGN) of the cat, mediating visual transmission from the retina via ionotropic receptors of both D,L-alpha-amino-3-hydroxy-5-alpha-methyl-4-isoxazolepropionate and N-methyl-D-aspartate subtypes. Moreover, glutamate also exerts an important modulatory influence on LGN cells, where metabotropic glutamate receptors (mGluRs) seem to play a crucial role. Here we show in anesthetized adult cats that iontophoretic application of the specific mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) produced two, distinctly different, effects on LGN neurons. Visual responses to flashing spots and drifting gratings were attenuated (decreased by an average of 59%) in 13 of 23 of the cells but augmented (increased by an average of 60%) in 10 of 23 of the cells. Further, in each case when the specific mGluR5 agonist (R,S)-2-chloro-5-hydroxyphenylglycine was applied, the effects obtained were the opposite to those of MPEP. Data obtained in a second group of experiments to determine a possible interaction between mGluR5 blockade by MPEP and glutamate ionotropic receptors show that, in the majority of neurons (11 of 15, 73%), the MPEP-mediated effects seem to be independent of N-methyl-D-aspartate and D,L-alpha-amino-3-hydroxy-5-alpha-methyl-4-isoxazolepropionate receptor activity. Our results demonstrate a physiological role for mGluR5 in controlling retinal input and show, in vivo, a more intricate scenario than previously suggested, highlighting the complexity of metabotropic receptor interactions with excitatory and inhibitory elements in the thalamus.

Topics: Action Potentials; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Analysis of Variance; Animals; Cats; Drug Interactions; Female; Geniculate Bodies; Glycine; Iontophoresis; Male; N-Methylaspartate; Neurons; Phenylacetates; Photic Stimulation; Pyridines; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate

1 Neuroprotection has been reported after either activation or blockade of the group I metabotropic glutamate receptor subtype 5 (mGluR5). However, some recent evidence suggests that protection provided by mGluR5 antagonists may reflect their ability to inhibit N-methyl-D-aspartate (NMDA) receptor activity. 2 Here, in both rat and mouse cortical neurons, we compare the neuroprotective actions of two mGluR5 antagonists: 2-methyl-6-(phenylethynyl)-pyridine (MPEP), which has been commonly used and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP), a more recently developed compound believed to have greater mGluR5 selectivity. We have previously shown that MPEP directly reduces single-channel NMDA receptor open time at the same concentrations (20 microM or greater) that show neuroprotection, whereas MPEP antagonizes mGluR5 agonist ((RS)-2-chloro-5-hydroxyphenylglycine (CHPG))-induced changes in inositol phosphates (IP) at concentrations as low as 0.2 microM. 3 In the present studies, MTEP significantly inhibited CHPG-mediated IP hydrolysis at concentrations as low as 0.02 microM. In contrast to MPEP, which significantly reduced glutamate- or NMDA-mediated cell death in primary rat neuronal cultures at a concentration of 20 microM, small neuroprotective effects were observed with MTEP only at a concentration of 200 microM. Neither MPEP- nor MTEP-mediated mGluR5 inhibition had any effect on etoposide-induced apoptotic cell death. In rat cortical neurons, the neuroprotective effects of MTEP at very high concentrations, like those of MPEP, reflect ability to directly reduce NMDA receptor peak and steady-state currents. 4 We also compared the effects of MPEP and MTEP in primary cortical neuronal cultures from parental and mGluR5 knockout mice. Both agents were neuroprotective, at high concentrations in normal as well as in the knockout cultures. In contrast to rat cortical neurons, neither MPEP nor MTEP appears to directly alter NMDA receptor activity. 5 Combined, these studies support the conclusion that MTEP has greater mGluR5 selectivity than MPEP, and that neuroprotection provided by either antagonist in neuronal cultures does not reflect inhibition of mGluR5 receptors.

Topics: Animals; Apoptosis; Cell Survival; Cells, Cultured; Cerebral Cortex; Dizocilpine Maleate; Dose-Response Relationship, Drug; Etoposide; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Female; Glutamic Acid; Glycine; Hydrolysis; Male; Membrane Potentials; Mice; Mice, Knockout; N-Methylaspartate; Neurons; Neuroprotective Agents; Phenylacetates; Phosphatidylinositols; Pyridines; Rats; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Receptors, N-Methyl-D-Aspartate; Thiazoles

This study was designed to test whether chronic treatment with the metabotropic glutamate receptor 5 (mGlu5R) antagonist MPEP showed antiparkinsonian effects in rats unilaterally lesioned with 6-hydroxydopamine (6-OHDA) (a "classic" model of Parkinson's disease, PD), and to evaluate whether chronic MPEP influenced the functional properties and/or the expression of striatal mGlu5Rs. Wistar rats were lesioned with 6-OHDA and then treated with MPEP (3 mg/kg/day, i.p.) or its vehicle over 2 weeks. Chronic MPEP did not induce measurable antiparkinsonian effects, since no differences were found between MPEP- and vehicle-treated animals in the pattern of L-DOPA-induced contralateral rotations. In corticostriatal slices taken from animals chronically treated with MPEP, the functional effects of the mGlu5R agonist CHPG were significantly reduced in the lesioned vs. the intact side, while no changes were found in slices taken from vehicle-treated rats. The binding of [3H]MPEP to striatal membranes showed that neither the maximal number of binding sites (Bmax) nor the dissociation constant (Kd) were changed by the lesion and/or by chronic MPEP. While chronic MPEP did not potentiate L-DOPA-induced turning in a classical model of PD, its ability to reduce mGlu5R-associated signal could help to explain the neuroprotective/antiparkinsonian effects observed in other models of PD.

Topics: Animals; Antiparkinson Agents; Behavior, Animal; Corpus Striatum; Denervation; Drug Interactions; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glycine; Levodopa; Male; Organ Culture Techniques; Oxidopamine; Parkinson Disease; Phenylacetates; Pyridines; Rats; Rats, Wistar; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Sympatholytics; Tritium

Hippocampal metabotropic glutamate 5 receptors (mGlu5Rs) regulate both physiological and pathological responses to glutamate. Because mGlu5R activation enhances NMDA-mediated effects, and given the role played by NMDA receptors in synaptic plasticity and excitotoxicity, modulating mGlu5R may influence both the physiological and the pathological effects elicited by NMDA receptor stimulation. We evaluated whether adenosine A2A receptors (A(2A)Rs) modulated mGlu5R-dependent effects in the hippocampus, as they do in the striatum. Co-application of the A(2A)R agonist CGS 21680 with the mGlu5R agonist (RS)-2-chloro-s-hydroxyphenylglycine(CHPG) synergistically reduced field excitatory postsynaptic potentials in the CA1 area of rat hippocampal slices. Endogenous tone at A(2A)Rs seemed to be required to enable mGlu5R-mediated effects, as the ability of CHPG to potentiate NMDA effects was antagonized by the selective A(2A)R antagonist ZM 241385 in rat hippocampal slices and cultured hippocampal neurons, and abolished in the hippocampus of A(2A)R knockout mice. Evidence for the interaction between A(2A)Rs and mGlu5Rs was further strengthened by demonstrating their co-localization in hippocampal synapses. This is the first evidence showing that hippocampal A(2A)Rs and mGlu5Rs are co-located and act synergistically, and that A(2A)Rs play a permissive role in mGlu5R receptor-mediated potentiation of NMDA effects in the hippocampus.

Topics: Adenosine; Adenosine A2 Receptor Agonists; Animals; Bicuculline; Blotting, Western; Colforsin; Disks Large Homolog 4 Protein; Dose-Response Relationship, Drug; Drug Interactions; Electric Stimulation; Embryo, Mammalian; Enzyme Inhibitors; Excitatory Amino Acid Agonists; Excitatory Postsynaptic Potentials; Female; Fluorescent Antibody Technique; GABA Antagonists; Glycine; Hippocampus; In Vitro Techniques; Intracellular Signaling Peptides and Proteins; Male; Membrane Proteins; N-Methylaspartate; Neurons; Patch-Clamp Techniques; Phenethylamines; Phenylacetates; Pregnancy; Presynaptic Terminals; Pyridines; Qa-SNARE Proteins; Rats; Rats, Wistar; Receptor, Metabotropic Glutamate 5; Receptors, Adenosine A2; Receptors, Metabotropic Glutamate; Synaptophysin; Vesicular Glutamate Transport Protein 1

The laterocapsular division of the central nucleus of the amygdala (CeA) is now defined as the "nociceptive amygdala" because of its high content of neurons that respond to painful stimuli. The majority of these neurons become sensitized in a model of arthritis pain. Here we address the role of G protein-coupled group I metabotropic glutamate receptor subtypes mGluR1 and mGluR5 in nociceptive processing under normal conditions and in pain-related sensitization. Extracellular single-unit recordings were made from 65 CeA neurons in anesthetized rats. Each neuron's responses to brief mechanical stimuli, background activity, receptive field size, and threshold were measured before and after induction of the kaolin/carrageenan mono-arthritis in one knee and before and during applications of agonists and antagonists into the CeA by microdialysis. All neurons received excitatory input from the knee(s) and responded most strongly to noxious stimuli. Before arthritis, a group I mGluR1 and mGluR5 agonist (DHPG, n = 10) potentiated the responses to innocuous and noxious stimuli. This effect was mimicked by an mGluR5 agonist (CHPG, n = 15). In the arthritis pain state (>6 h after induction), the facilitatory effects of DHPG (n = 9), but not CHPG (n = 7), increased. An mGluR1 antagonist (CPCCOEt) had no effect before arthritis (n = 12) but inhibited the responses of sensitized neurons in the arthritis pain state (n = 8). An mGluR5 antagonist (MPEP) inhibited brief nociceptive responses under normal conditions (n = 19) and prolonged nociception in arthritis (n = 8). These data suggest a change of mGluR1 function and activation in the amygdala in pain-related sensitization, whereas mGluR5 is involved in brief as well as prolonged nociception.

Topics: Action Potentials; Amygdala; Animals; Arthritis; Chromones; Dose-Response Relationship, Drug; Glycine; Kaolin; Male; Methoxyhydroxyphenylglycol; Neurons; Phenylacetates; Physical Stimulation; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate

Activation of metabotropic glutamate receptor 5 (mGluR5) has been shown to reduce caspase-dependent apoptosis in primary neuronal cultures induced by staurosporine and etoposide. beta-Amyloid (Abeta)-induced neurotoxicity in culture appears to be in part caspase mediated. In the present studies the effects of treatment with an mGluR5 agonist or antagonist on Abeta-induced neuronal apoptosis were examined in rat cortical neuronal cultures. Pretreatment with the selective mGluR5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) markedly reduced the number of apoptotic cells after exposure to Abeta (25-35), as well as associated LDH release. Blockade of mGluR5 by the selective antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP) attenuated these effects of CHPG. A similar neuroprotective effect of mGluR5 activation by CHPG was observed in cultures treated with full-length Abeta peptide (1-42). CHPG attenuated Abeta (25-35)-induced cytochrome c release and decreased levels of active caspase-3 protein. CHPG also reduced translocation of apoptosis-inducing factor (AIF) induced by Abeta (25-35). Thus, mGluR5 activation limits the release of mitochondrial proteins associated with induction of both caspase-dependent and -independent apoptosis.

Topics: Amyloid beta-Peptides; Animals; Apoptosis; Apoptosis Inducing Factor; Caspase 3; Caspases; Cells, Cultured; Cerebral Cortex; Cytochromes c; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Flavoproteins; Glycine; Membrane Proteins; Mitochondria; Neurons; Neuroprotective Agents; Peptide Fragments; Phenylacetates; Protein Transport; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Up-Regulation

The metabotropic glutamate receptors 5 (mGlu5Rs) and the adenosine A2A receptors (A2ARs) have been reported to functionally interact in the striatum. The aim of the present work was to verify the hypothesis that the state of activation of A2A Rs could influence mGlu5R-mediated effects in the striatum. In electrophysiological experiments (extracellular recording in rat corticostriatal slices), the ability of the selective mGlu5R agonist CHPG to potentiate the reduction of the field potential amplitude induced by NMDA was prevented not only by the selective mGlu5R antagonist MPEP, but also by the selective A2AR antagonist ZM 241385. Analogously, the application of CHPG potentiated NMDA-induced toxicity (measured by LDH release) in cultured striatal neurons, an effect that was abolished by both MPEP and ZM 241385. Finally, the A2AR agonist CGS 21680 potentiated CHGP effects, an action that was reproduced and abolished, respectively, by forskolin (an activator of the cAMP/protein kinase A, PKA, pathway) and KT 5720 (a PKA inhibitor). The results indicate that A2ARs exert a permissive role on mGlu5R-induced effects in the striatum. Such an interaction may represent an additional target for the development of therapeutic strategies towards striatal disorders.

Topics: Adenosine; Animals; Carbazoles; Cells, Cultured; Corpus Striatum; Dose-Response Relationship, Drug; Drug Interactions; Electrophysiology; Embryo, Mammalian; Enzyme Inhibitors; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Female; Glycine; In Vitro Techniques; Indoles; Male; Membrane Potentials; N-Methylaspartate; Phenethylamines; Phenylacetates; Pregnancy; Purinergic P1 Receptor Agonists; Pyridines; Pyrroles; Rats; Rats, Wistar; Receptor, Metabotropic Glutamate 5; Receptors, Adenosine A2; Receptors, Metabotropic Glutamate; Triazines; Triazoles

Group I metabotropic glutamate receptors (mGluR1 and mGluR5 subtypes) are positively coupled to phosphoinositide hydrolysis through G-proteins and are densely expressed in medium-sized projection neurons of striatum. Selective activation of Group I mGluRs upregulates preproenkephalin (PPE) mRNA expression in the rat dorsal striatum. This study investigated the role of one subtype of Group I receptors, mGluR5, in the regulation of PPE mRNA expression in the rat dorsal striatum using quantitative in situ hybridization. Unilateral injection of the mGluR5 selective agonist (RS)-2-Chloro-5-hydroxyphenylglycine (CHPG) into the dorsal striatum (caudoputamen) of chronically cannulated rats at doses of 50 and 200 nmol elevated basal levels of PPE mRNA in the injected dorsal striatum. The induction of PPE mRNA was evident at 1 h, remained at 3 h, and returned to normal level 6 h after CHPG injection. Pretreatment with an mGluR5 selective antagonist 2-methyl-6-(phenylethynyl) pyridine hydrochloride (MPEP) at a dose of 10 mg/kg (i.p.) blocked CHPG-stimulated PPE expression. MPEP also attenuated PPE expression induced by dopamine D(2) receptor blockade with eticlopride (0.5 mg/kg, i.p.). Administration of MPEP alone had no significant effects on basal levels of PPE mRNA in the striatum. The results from the present study demonstrate that glutamatergic tone on mGluR5 possesses the ability to positively regulate PPE gene expression in striatal neurons in vivo. Moreover, activation of mGluR5 participates in the mediation of D(2) antagonist-induced PPE expression.

Topics: Animals; Corpus Striatum; Dopamine Antagonists; Dopamine D2 Receptor Antagonists; Enkephalins; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Gene Expression Regulation; Glycine; In Situ Hybridization; Male; Phenylacetates; Protein Precursors; Pyridines; Rats; Rats, Wistar; Receptors, Dopamine D2; Receptors, Metabotropic Glutamate; RNA, Messenger; Salicylamides; Time Factors

In this study, we investigated the effects of group I metabotropic glutamate (mglu) receptor ligands on glutamate and gamma-aminobutyric acid (GABA) extracellular concentrations at the periaqueductal grey level by using in vivo microdialysis. An agonist of group I mglu receptors, (S)-3,5-dihydroxyphenylglycine [(S)-3,5-DHPG, 1 and 2 mM], as well as a selective agonist of mglu(5) receptors, (RS)-2-chloro-5-hydroxyphenylglycine (CHPG, 2 and 4 mM), both increased dialysate glutamate and GABA concentrations. 7-(Hydroxyimino)cyclopropa-[b]-chromen-1alpha-carboxylate ethyl ester (CPCCOEt, 1 mM), a selective mglu(1) receptor antagonist, and 2-methyl-6-(phenylethynyl)pyridine (MPEP, 0.5 mM), a selective mglu(5) receptor antagonist, perfused in combination with DHPG, antagonized the effect induced by DHPG on the extracellular glutamate and GABA concentrations. MPEP (0.5 mM), perfused in combination with CHPG, antagonized the increased glutamate and GABA extracellular levels induced by CHPG. MPEP (1 mM) decreased the extracellular concentrations of glutamate but did not modify the dialysate GABA concentrations. Moreover, as the intra-periaqueductal grey perfusion of (RS)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid [(RS)-CPP, 100 microM], a selective N-methyl-D-aspartate (NMDA) glutamate receptor antagonist, did not change the extracellular concentrations of glutamate, this suggests that the MPEP-induced decrease in glutamate is not a consequence of NMDA receptor blockade. These data show that group I mglu receptors in the periaqueductal grey may modulate the release of glutamate and GABA in awake, freely moving rats. In particular, mglu(5), but not mglu(1), receptors seem to be functionally active on glutamate terminals.

Topics: Animals; Chromones; Dendrites; Excitatory Amino Acid Agonists; gamma-Aminobutyric Acid; Glutamates; Glycine; Male; Microdialysis; Microscopy, Electron; Periaqueductal Gray; Phenylacetates; Piperazines; Pyridines; Rats; Rats, Wistar; Receptors, Metabotropic Glutamate; Receptors, N-Methyl-D-Aspartate; Resorcinols; Tetrodotoxin

Use-dependent N-methyl-d-aspartate receptor (NMDAR) antagonists produce behaviors in human volunteers that resemble schizophrenia and exacerbate those behaviors in schizophrenic patients, suggesting that hypofunction of NMDAR-mediated neuronal circuitry may be involved in the etiology of clinical schizophrenia. Activation of the metabotropic glutamate receptor subtype 5 (mGluR5) enhances NMDAR-mediated currents in vitro. Thus, activation of mGluR5 could potentiate hypofunctional NMDARs in neuronal circuitry relevant to schizophrenia. To further elucidate the role of mGluR5, the present study examined the effects of mGluR5 antagonist administration, with and without coadministration of the use-dependent NMDAR antagonist phencyclidine (PCP), on locomotor activity and prepulse inhibition (PPI) of the acoustic startle response in rodents. We further examined PPI in mGluR5 knockout mice. Finally, we examined PPI after administration of the mGluR5 agonist 2-chloro-5-hydroxyphenylglycine (CHPG) alone and in combination with amphetamine. The data indicate that the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine has no effect on locomotor activity or PPI by itself but does potentiate both PCP-induced locomotor activity and disruption of PPI. We further found that mGluR5 knockout mice display consistent deficits in PPI relative to their wild-type controls. Finally, the data indicate that CHPG has no effect on PPI by itself, but ameliorates amphetamine-induced disruption of PPI. Collectively, these data suggest that mGlu5 receptors play a modulatory role on rodent PPI and locomotor behaviors and are consistent with the hypothesis that mGlu5 agonist/potentiators may represent a novel approach for antipsychotic drug development.

Topics: Animals; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glycine; Humans; Male; Mice; Mice, Knockout; Motor Activity; Phencyclidine; Phenylacetates; Psychomotor Performance; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Receptors, N-Methyl-D-Aspartate

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.

Topics: Amino Acids; Animals; Chromones; Excitatory Amino Acid Antagonists; Glycine; Membrane Potentials; Neuronal Plasticity; Organ Culture Techniques; Phenylacetates; Pyridines; Rats; Rats, Wistar; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Resorcinols; Vestibular Nuclei; Xanthenes

From previous work, it appears that glutamate can activate the hypothalamic-pituitary-adrenocortical (HPA) axis by an interaction at either ionotopic or metabotropic (G-protein coupled) receptors. For example, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD), a metabotropic glutamate (mGlu) receptor agonist, has been shown to increase the levels of serum corticosterone in rats. The present study was undertaken to further characterize which of the mGlu receptors are substantially involved in control of the HPA axis. The group I mGlu receptor agonists, 3,5-dihydroxyphenylglycine (DHPG), 1S,3R-ACPD, and 2-chloro-5-hydroxyphenylglycine (CHPG) but not the inactive isomer 1R,3S-ACPD were found to dose-dependently increase serum corticosterone 1 h after intracerebroventricular (i.c.v.) injection in male rats. The relative potency, DHPG (EC50 = 520 nmol) > 1S,3R-ACPD (1.4 micromol) = CHPG (2.7 micromol) >> 1R,3S-ACPD (>> 3 micromol) is consistent with activation of group I (mGlu1/5) receptors. The effects of DHPG were long lasting with substantial elevations in corticosterone remaining for at least 3 h. In a similar manner, the group III mGlu receptor agonists, L-AP4 (4-phosphono-2-aminobutyric acid) and L-SOP (serine-O-phosphate), were found to increase serum corticosterone levels at 1 h. In contrast, the mGlu group II selective agonists LY354740 (10 mg/kg, i.p.) and subtype-selective doses of the group II antagonist LY341495 (1 mg/kg, i.p.) did not significantly elevate serum corticosterone. Given the group I agonists results, it was surprising to find that group I selective and mGlu1 selective antagonists given alone also increased serum corticosterone. As with the agonists, the rise in serum corticosterone with LY393675 (an mGlu1/5 antagonist, EC50 = 20 nmol, i.c.v.) and LY367385 (an mGlu1 antagonist, 325 nmol, i.c.v.) were dose-dependent and consistent with their relative affinity for the group I mGlu receptors. The selective mGlu5 antagonist MPEP [2-methyl-6-(phenylethylnyl)pyridine] increased serum corticosterone but only at high doses (> 30 mg/kg, i.p.). A model involving the high glutamatergic tone on GABAergic interneurons in the paraventricular nucleus of the hypothalamus is discussed as a possible explanation for these results.

Topics: Adrenalectomy; Adrenocorticotropic Hormone; Amino Acids; Animals; Benzoates; Bridged Bicyclo Compounds; Corticosterone; Cycloleucine; Dose-Response Relationship, Drug; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glutamic Acid; Glycine; Male; Neuroprotective Agents; Paraventricular Hypothalamic Nucleus; Phenylacetates; Propionates; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Resorcinols; Xanthenes

Medium spiny neurons were recorded from striatal slices obtained from mice lacking the group I metabotropic glutamate receptor (mGluR) subtype 1 or subtype 5. In wild-type animals, N-methyl-D-aspartate (NMDA)-induced membrane depolarization/inward currents were potentiated in the presence of both the group I mGluR agonist 3,5-dihydroxyphenylglycine (3,5-DHPG) and the mGluR5 selective agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG). Likewise, in mGluR1 knockout mice, both 3,5-DHPG and CHPG were able to potentiate NMDA responses. Conversely, in neurons recorded from mGluR5-deficient mice, the enhancement of NMDA responses by both 3,5-DHPG and CHPG was absent. Pharmacological analysis performed from rat slices confirmed the data obtained with mice. In the presence of the competitive mGluR1 antagonist LY367385, the NMDA responses were potentiated in the presence of CHPG, whereas the CHPG-induced enhancement was not observed in slices treated with the non-competitive mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine. As in wild-type mice, in neither of the mGluR1- and mGluR5-deficient mice did (2S,1'R,2'R,3'R)-2-(2,3-dicarboxylcyclopropyl)-glycine (1 microM), nor L-serine-O-phosphate (30 microM) (agonists for group II and III mGluRs, respectively) affect the NMDA-evoked responses. In striatal medium spiny neurons, NMDA responses are potentiated by endogenous acetylcholine via M1-like muscarinic receptors. Since the enhancement of NMDA responses by 3,5-DHPG and by M1-like muscarinic agonists was shown to share common post-receptor mechanisms, we verified whether the muscarinic potentiation of NMDA responses was affected in these group I mGluR-deficient mice. Both in mGluR1 and mGluR5 knockout animals, in the presence of either muscarine or the M1-like muscarinic receptor agonist McN-A-343, the positive modulation of the NMDA-induced membrane depolarization persisted.These results confirm the permissive role of group I mGluRs on NMDA responses in striatal neurons and reveal that this functional interplay occurs exclusively through the mGluR5 subtype. The NMDA-mGluR5 interaction might play an important modulatory role in the final excitatory drive from corticostriatal afferents and suggests that drugs acting at mGluR5 might prove useful for the treatment of movement disorders involving the striatum.

Topics: (4-(m-Chlorophenylcarbamoyloxy)-2-butynyl)trimethylammonium Chloride; Action Potentials; Animals; Anticonvulsants; Benzoates; Cyclopropanes; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glutamic Acid; Glycine; Mice; Mice, Knockout; Muscarine; Muscarinic Agonists; N-Methylaspartate; Neostriatum; Neurons; Phenylacetates; Pyridines; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Receptors, N-Methyl-D-Aspartate; Resorcinols; Synaptic Transmission

Activation of group I metabotropic glutamate receptors (mGluR) has been implicated in the pathophysiology of acute central nervous system injury. However, the relative roles of the two group I subtypes, mGluR1 or mGluR5, in such injury has not been well examined. We compared the effects of treatment with the newly developed, selective mGluR5 antagonist 2-methyl-6-phenylethynylpyridine (MPEP) and the selective mGluR5 agonist (R,S)-2-chloro-5-hydroxyphenylglycine (CHPG) in a rat intraluminal filament model of temporary middle cerebral artery occlusion (MCAo). Rats were administered MPEP or CHPG i.c.v. beginning 15 or 135 min after induction of ischemia for 2 h. Infarct size was measured after either 22 or 70 h of reperfusion, and neurological function was quantified at 2, 24, 48 and 72 h. Treatment with MPEP or CHPG at 15 min reduced 24 h infarct volume by 61 and 44%, respectively. The neuroprotective effects were dose dependent. Delaying MPEP treatment until 135 min eliminated the neuroprotective effects. In other studies, using early MPEP treatment (15 min) at optimal doses, infarct volume was reduced by 44% at 72 h and this was correlated with significant neurological recovery. These data suggest that both MPEP and CHPG are neuroprotective when administered after focal cerebral ischemia. In separate, recent studies we found that although MPEP does act as an mGluR5 antagonist and blocks agonist induced phosphoinositide hydrolysis, it also serves as a non-competitive NMDA antagonist; in contrast, other results indicate that CHPG mediated neuroprotection may reflect anti-apoptotic activity. Therefore, both types of compounds may prove to have therapeutic potential for the treatment of stroke.

Topics: Animals; Body Temperature; Body Weight; Brain Ischemia; Cell Survival; Cerebral Cortex; Disease Models, Animal; Dose-Response Relationship, Drug; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glycine; Infarction, Middle Cerebral Artery; Male; Neurons; Neuroprotective Agents; Phenylacetates; Pyridines; Rats; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Reperfusion Injury

1. The metabotropic glutamate receptors (mGluRs) are a family of G-protein linked receptors that can be divided into three groups (group I, II and III). A number of studies have implicated group I mGluR activation in acute neuronal injury, but until recently it was not possible to pharmacologically differentiate the roles of the two individual subunits (mGluR1 and mGluR5) in this group. 2. We investigated the role of mGluR5 in acute NMDA and glutamate mediated neurodegeneration in cultured rat cortical cells using the mGluR5 antagonists MPEP and SIB-1893, and found that they provide significant protection at concentrations of 20 or 200 microM. 3. These compounds act as effective mGluR5 antagonists in our cell culture system, as indicated by the ability of SIB-1893 to prevent phosphoinositol hydrolysis induced by the specific mGluR5 agonist, (RS)-2-chloro-5-hydroxyphenylglycine (CHPG). 4. However, they also significantly reduce NMDA evoked current recorded from whole cells voltage clamped at -60 mV, and significantly decrease the duration of opening of NMDA channels recorded in the outside out patch configuration. 5. This suggests that although MPEP and SIB-1893 are effective mGluR5 antagonists, they also act as noncompetitive NMDA receptor antagonists. Therefore, the neuroprotective effects of these compounds are most likely mediated through their NMDA receptor antagonist action, and caution should be exercised when drawing conclusions about the roles of mGluR5 based on their use.

Topics: Animals; Cell Death; Cells, Cultured; Dose-Response Relationship, Drug; Electrophysiology; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Female; Fetus; Glutamic Acid; Glycine; Hydrolysis; Membrane Potentials; N-Methylaspartate; Neurons; Patch-Clamp Techniques; Phenylacetates; Phosphatidylinositols; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Receptors, N-Methyl-D-Aspartate

The intrastriatal perfusion of the selective metabotropic glutamate (mGlu)5 receptor agonist (RS)-2-chloro-5-hydroxy-phenylglycine (CHPG, 1000 microM) significantly increased (approximately + 100%, p < 0.05) glutamate extracellular levels with respect to basal values. The potentiating effect of CHPG was prevented by the selective mGlu5 receptor antagonist 2-methyl-6(phenyl-ethynyl)-pyridine (MPEP, 250 microM)) and by the adenosine A2A receptor antagonist SCH 58261 (2 mg/kg, i.p.). The results show that mGlu5 receptors are involved in the regulation of striatal glutamate release and suggest an involvement of adenosine A2A receptors in mGlu5 receptor-mediated effects.

Topics: Animals; Corpus Striatum; Excitatory Amino Acid Antagonists; Glutamic Acid; Glycine; Male; Microdialysis; Neuroprotective Agents; Phenylacetates; Pyridines; Pyrimidines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Receptors, Purinergic P1; Triazoles

Our previous work has shown that Group I mGlu receptors participate in thalamic sensory processing in vivo. However, unequivocal demonstration of mGlu5 participation has not been possible due to the lack of specific ligands. We have therefore made a preliminary study of the in vivo actions of the agonist (R,S)-2-Chloro-5-hydroxyphenylglycine [CHPG] and the novel mGlu5 antagonist 6-methyl-2-(phenylethynyl)-pyridine [MPEP] in order to characterize their suitability for functional studies. Iontophoretically administered MPEP selectively antagonized excitatory responses of single rat thalamic neurones to CHPG compared to the broad-spectrum mGlu agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate. In contrast, the established mGlu1 and mGlu5 antagonist (S)-4-carboxyphenylglycine reduced responses to both agonists. These findings are the first demonstration of an in vivo action of CHPG and its antagonism by a selective mGlu5 antagonist. Furthermore MPEP appears to be a good tool for functional studies of mGlu5.

Topics: Animals; Cycloleucine; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glycine; Phenylacetates; Pyridines; Rats; Rats, Wistar; Receptors, Metabotropic Glutamate; Thalamus