6-methyl-2-(phenylethynyl)pyridine and alpha-methyl-4-carboxyphenylglycine

The present study examined whether a single or multiple episode(s) of status epilepticus induced with kainic acid (KA) during the first 3 weeks of postnatal (P) development would aberrantly stimulate proliferation zones that alters migration to potentially injured areas and whether they would be blocked by selective Group I mGluR antagonists. mGluR1α (LY367385) and mGluR5 (MPEP) antagonists were administered 2h following KA-induced status epilepticus and animals were examined after 7days. Proliferating cells of the subventricular zone (SVZ), third ventricle, hippocampus, amygdala cortical complex were analyzed with the proliferative marker, Ki67; and two complementary retrograde dye tracers. Proliferation increased in extrahippocampal limbic structures when KA was administered on P13 or P20 which correlated with number of injured cells at the older age. LY367385 post-treatment caused striking decreases in proliferation in all limbic structures in the presence and absence of injury, whereas a reduction with MPEP was observed only within the amygdala cortical complex (Amg/ERcx) in the presence of multiple seizures (3×KA). After 3×KA and LY367385 post-treatments, diminished co-staining of dye tracers with Ki67 was observed within the Amg/ERcx despite high levels of progenitors marked by the retrograde tracers in this region. This indicates that not only was local proliferation within the SVZ and distant structures inhibited, but also that migration itself was reduced indirectly since there were less cells to migrate from the SVZ. Co-labeling with biomarkers provided evidence for neuronal differentiation suggesting potential aberrant integration may occur in distant locations, and that targeting of mGluR1α receptors may be a potential therapeutic strategy for future development.

Topics: Age Factors; Animals; Animals, Newborn; Benzoates; Brain; Cell Cycle; Cell Movement; Cell Proliferation; Disease Models, Animal; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Female; Gene Expression Regulation, Developmental; Glycine; Kainic Acid; Lateral Ventricles; Male; Pregnancy; Pyridines; Rats; Rats, Sprague-Dawley; Receptors, Metabotropic Glutamate; Rhodamines; Status Epilepticus; Time Factors

The hypothalamic paraventricular nucleus (PVN) is critically involved in elevated sympathetic output and the development of hypertension. However, changes in group I metabotropic glutamate receptors (mGluR1 and mGluR5) and their relevance to the hyperactivity of PVN presympathetic neurons in hypertension remain unclear. Here, we found that selectively blocking mGluR5 significantly reduced the basal firing activity of spinally projecting PVN neurons in spontaneously hypertensive rats (SHRs), but not in normotensive Wistar-Kyoto (WKY) rats. However, blocking mGluR1 had no effect on the firing activity of PVN neurons in either group. The mRNA and protein levels of mGluR5 in the PVN and rostral ventrolateral medulla were significantly higher in SHRs than in WKY rats. The group I mGluR selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) similarly increased the firing activity of PVN neurons in WKY rats and SHRs. In addition, blocking NMDA receptors (NMDARs) through bath application or intracellular dialysis not only decreased the basal firing in SHRs, but also eliminated DHPG-induced excitation of spinally projecting PVN neurons. DHPG significantly increased the amplitude of NMDAR currents without changing their decay kinetics. Interestingly, DHPG still increased the amplitude of NMDAR currents and caused reappearance of functional NMDAR channels after initially blocking NMDARs. In addition, protein kinase C (PKC) inhibition or intracellular dialysis with synaptosomal-associated protein of 25 kDa (SNAP-25)-blocking peptide abolished DHPG-induced increases in NMDAR currents of PVN neurons in SHRs. Our findings suggest that mGluR5 in the PVN is upregulated in hypertension and contributes to the hyperactivity of PVN presympathetic neurons through PKC- and SNAP-25-mediated surface expression of NMDARs.

Topics: Animals; Benzoates; Blood Pressure; Excitatory Amino Acid Antagonists; Glycine; Male; Medulla Oblongata; Membrane Potentials; Neurons; Paraventricular Hypothalamic Nucleus; Pyridines; Rats; Rats, Inbred SHR; Rats, Inbred WKY; Receptor, Metabotropic Glutamate 5; Receptors, N-Methyl-D-Aspartate; Up-Regulation

Homocysteine (Hcy), a neurotoxic amino acid, is a risk factor for neurodegenerative diseases. Previous in vitro studies have demonstrated that group I metabotropic glutamate receptors along with N-methyl-D-aspartic acid (NMDA) receptors participate in acute and chronic aspects of Hcy-induced neuronal damage. In the present study, we examined whether the same mechanism may be involved in homocysteine neurotoxicity in vivo. Memantine, MPEP, and LY367385 were used as NMDA, mGlu5, and mGlu1 antagonists, respectively. Repeated i.c.v injection of Hcy was performed for three consecutive days. Neuronal loss in different zones of the hippocampus was assessed by Nissl, Fluoro-Jade B, and TUNEL staining. Neuronal degeneration was observed in both types of apoptosis and necrosis. All glutamate receptor antagonists, even when given alone, provided some degree of neuroprotection. The degree of protection was dependent on the area of the hippocampus. While memantine was more potent against Hcy-induced apoptosis, the potency of mGluR antagonists in neuronal protection against apoptosis and necrosis was almost equal. No more protection was observed when all three antagonists were used simultaneously. It seems that Fluoro-Jade could be a useful marker of apoptotic cell death. Taken together, results demonstrate that, in vivo, Hcy neurotoxicity is mediated mainly by the NMDA receptors and group I mGluRs.

Topics: Animals; Apoptosis; Benzoates; Cell Survival; Excitatory Amino Acid Antagonists; Glycine; Hippocampus; Homocysteine; Male; Memantine; Necrosis; Neuroprotective Agents; Pyridines; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate

As a classic neuromodulator, dopamine has long been thought to modulate, rather than trigger, synaptic plasticity. In contrast, our present results demonstrate that within the parallel projections of dopaminergic and GABAergic terminals from the ventral tegmental area to the nucleus accumbens core (NAcCo), action-potential-activated release of dopamine heterosynaptically triggers LTD at GABAergic synapses, which is likely mediated by activating presynaptically located dopamine D1 class receptors and expressed by inhibiting presynaptic release of GABA. Moreover, this dopamine-mediated heterosynaptic LTD is abolished after withdrawal from cocaine exposure. These results suggest that action-potential-dependent dopamine release triggers very different cellular consequences from those induced by volume release or pharmacological manipulation. Activation of the ventral tegmental area to NAcCo projections is essential for emotional and motivational responses. This dopamine-mediated LTD allows a flexible output of NAcCo neurons, whereas disruption of this LTD may contribute to the rigid emotional and motivational state observed in addicts during cocaine withdrawal.

Topics: Analysis of Variance; Animals; Benzoates; Channelrhodopsins; Cocaine; Dopamine; Dopamine Uptake Inhibitors; Electric Stimulation; GABAergic Neurons; Genetic Vectors; Glycine; In Vitro Techniques; Inhibitory Postsynaptic Potentials; Long-Term Synaptic Depression; Male; Nucleus Accumbens; Optogenetics; Phosphinic Acids; Photic Stimulation; Propanolamines; Pyridines; Quinoxalines; Rats; Rats, Sprague-Dawley; Synapses; Time Factors; Transduction, Genetic; Tyrosine 3-Monooxygenase; Ventral Tegmental Area

Group I mGluRs (metabotropic glutamate receptors), including mGluR1 and mGluR5, are GPCRs (G-protein coupled receptors) and play important roles in physiology and pathology. Studies on their role in cerebral ischaemia have provided controversial results. In this study, we used a PT (photothrombosis)-induced ischaemia model to investigate whether antagonists to the group I mGluRs may offer acute and long-term protective effects in adult mice. Our results demonstrated that administration with mGluR5 antagonist MPEP [2-methyl-6-(phenylethynyl)-pyridine] or mGluR1 antagonist LY367385 by intraperitoneal injection at 3 h after PT decreased brain infarct volume evaluated one day after ischaemia. Additive effects on infarct volume were observed upon co-injection with MPEP and LY367385. These antagonists also significantly alleviated neurodegeneration and apoptosis in the penumbra. In addition, when evaluated 2 weeks after PT, they reduced infarct volume and tissue loss, attenuated glial scar formation, and inhibited cell proliferation in the penumbra. Importantly, co-injection with MPEP and LY367385 reduced the expression levels of calpain, a Ca2+-activated protease known to mediate ischaemia-induced neuronal death. Injection of calpeptin, a calpain inhibitor, could inhibit neuronal death and brain damage after PT but injection of calpeptin together with MPEP and LY367385 did not further improve the protective effects mediated by MPEP and LY367385. These results suggest that inhibition of group I mGluRs is sufficient to protect ischaemic damage through the calpain pathway. Taken together, our results demonstrate that inhibition of group I mGluRs can mitigate PT-induced brain damage through attenuating the effects of calpain, and improve long-term histological outcomes.

Topics: Animals; Antimetabolites, Antineoplastic; Astrocytes; Benzoates; Blotting, Western; Brain; Brain Ischemia; Bromodeoxyuridine; Cell Death; Cell Proliferation; Cerebral Infarction; Excitatory Amino Acid Antagonists; Glycine; Immunohistochemistry; In Situ Nick-End Labeling; Intracranial Thrombosis; Male; Mice; Mice, Inbred C57BL; Nerve Degeneration; Neuroglia; Pyridines; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate

Homer1 belongs to a family of scaffolding proteins that interact with various post-synaptic density proteins including group I metabotropic glutamate receptors (mGluR1/5). Previous research in our laboratory implicates the Homer1c isoform in spatial learning. Homer1 knockout mice (H1-KO) display cognitive impairments, but their synaptic plasticity properties have not been described. Here, we investigated the role of Homer1 in long-term potentiation (LTP) in the hippocampal CA1 region of H1-KO mice in vitro. We found that late-phase LTP elicited by high frequency stimulation (HFS) was impaired, and that the induction and maintenance of theta burst stimulation (TBS) LTP were reduced in H1-KO. To test the hypothesis that Homer1c was sufficient to rescue these LTP deficits, we delivered Homer1c to the hippocampus of H1-KO using recombinant adeno-associated virus (rAAV). We found that rAAV-Homer1c rescued HFS and TBS-LTP in H1-KO animals. Next, we tested whether the LTP rescue by Homer1c was occurring via mGluR1/5. A selective mGluR5 antagonist, but not an mGluR1 antagonist, blocked the Homer1c-induced recovery of late-LTP, suggesting that Homer1c mediates functional effects on plasticity via mGluR5. To investigate the role of Homer1c in spatial learning, we injected rAAV-Homer1c to the hippocampus of H1-KO. We found that rAAV-Homer1c significantly improved H1-KO performance in the Radial Arm Water Maze. These results point to a significant role for Homer1c in synaptic plasticity and learning.

Topics: Animals; Behavior, Animal; Benzoates; Carrier Proteins; Gene Transfer Techniques; Glycine; Hippocampus; Homer Scaffolding Proteins; Maze Learning; Mice; Mice, Knockout; Neuronal Plasticity; Pyridines; Receptors, Metabotropic Glutamate; Recognition, Psychology; Social Behavior; Synapses

Group I metabotropic glutamate receptors (mGluRs) are involved in memory formation. The Ca2+ signal derived from stimulation of IP3 receptors (IP3Rs) via mGluRs, initiates protein synthesis that is required for memory consolidation and reconsolidation. However it has been suggested that different mechanisms are triggered by mGluR1/5 activation in these two processes. It is also not clear whether the transient amnesia observed after blockade of group I mGluRs after a reminder, results from disturbance of memory reconsolidation or temporal impairment of recall. The aim of this study was to examine more closely the role of mGluR1 in memory consolidation and reconsolidation and to detect differences in the participation of mGluR1 and mGluR5 in memory retrieval after initial training and after the remainder of the task. Our results demonstrate, that in chicks performing a one-trial passive avoidance task, antagonists of mGluR1, mGluR5 and IP3R significantly disturb memory consolidation and reconsolidation. Inhibition of mGluR5 and IP3R also impairs memory recall, whereas mGluR1 do not seem to participate in this process. The presented data suggest that activation of mGluR1 and mGluR5 is necessary for the correct course of memory consolidation and reconsolidation, whereas mGluR5 are additionally involved in retrieval processes dependent on Ca2+ release from IP3 activated intracellular stores.

Topics: Animals; Animals, Newborn; Avoidance Learning; Benzoates; Boron Compounds; Chickens; Glycine; Male; Memory; Pyridines; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate

The globus pallidus plays a critical role in movement regulation. Morphological studies have shown that group I mGluRs including mGluR1 and mGluR5 are expressed in the globus pallidus. Up to now, little is known about the in vivo electrophysiological effects of group I mGluRs on the pallidal neurons. The present study investigated the electrophysiological effects of group I mGluRs on the firing rate of pallidal neurons in anesthetized rats. Single unit in vivo extracellular recordings showed that micropressure ejection of group I mGluRs agonist, 3,5-dihydroxyphenylglycine (DHPG), increased the spontaneous firing rate of pallidal neurons. DHPG-induced excitation could be blocked by mGluR1 antagonist, (S)-(+)-α-amino-4-carboxy-2-methylb-enzeneacetic acid (LY367385), but not mGluR5 antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP). LY367385 alone had no effect but MPEP alone increased the excitability of pallidal neurons. Unilateral microinjection of DHPG into the globus pallidus induced a contralateral dystonic posturing in the presence of systemic haloperidol administration and this effect could be blocked by LY367385 but not MPEP. The present in vivo electrophysiological and behavioral studies indicate that group I mGluRs could produce excitatory effect on pallidal neurons via mGluR1, and blockade of mGluR5 also has an excitatory effect on pallidal neurons. Our findings suggest that the effects of globus pallidus in movement regulation is partly mediated by group I mGluRs.

Topics: Action Potentials; Animals; Benzoates; Dopamine Antagonists; Drug Interactions; Excitatory Amino Acid Antagonists; Globus Pallidus; Glycine; Haloperidol; Male; Methoxyhydroxyphenylglycol; Movement; Neurons; Pyridines; Rats; Rats, Wistar; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate

The prion protein (PrP(C)) is highly expressed in the nervous system, and its abnormal conformer is associated with prion diseases. PrP(C) is anchored to cell membranes by glycosylphosphatidylinositol, and transmembrane proteins are likely required for PrP(C)-mediated intracellular signaling. Binding of laminin (Ln) to PrP(C) modulates neuronal plasticity and memory. We addressed signaling pathways triggered by PrP(C)-Ln interaction in order to identify transmembrane proteins involved in the transduction of PrP(C)-Ln signals. The Ln γ1-chain peptide, which contains the Ln binding site for PrP(C), induced neuritogenesis through activation of phospholipase C (PLC), Ca(2+) mobilization from intracellular stores, and protein kinase C and extracellular signal-regulated kinase (ERK1/2) activation in primary cultures of neurons from wild-type, but not PrP(C)-null mice. Phage display, coimmunoprecipitation, and colocalization experiments showed that group I metabotropic glutamate receptors (mGluR1/5) associate with PrP(C). Expression of either mGluR1 or mGluR5 in HEK293 cells reconstituted the signaling pathways mediated by PrP(C)-Ln γ1 peptide interaction. Specific inhibitors of these receptors impaired PrP(C)-Ln γ1 peptide-induced signaling and neuritogenesis. These data show that group I mGluRs are involved in the transduction of cellular signals triggered by PrP(C)-Ln, and they support the notion that PrP(C) participates in the assembly of multiprotein complexes with physiological functions on neurons.

Topics: Animals; Benzoates; Calcium; Cells, Cultured; Female; Glycine; HEK293 Cells; Humans; Immunoblotting; Laminin; Male; Mice; Mice, 129 Strain; Mice, Inbred C57BL; Mice, Knockout; Mitogen-Activated Protein Kinase 1; Mitogen-Activated Protein Kinase 3; Neurites; Neurons; Protein Binding; PrPC Proteins; Pyridines; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Signal Transduction; Type C Phospholipases

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

Astrocytes show a complex structural and physiological interplay with neurons and respond to neuronal activation in vitro and in vivo with intracellular calcium elevations. These calcium changes enable astrocytes to modulate synaptic transmission and plasticity through various mechanisms. However, the response pattern of astrocytes to single neuronal depolarization events still remains unresolved. This information is critical for fully understanding the coordinated network of neuron-glial signaling in the brain. To address this, we developed a system to map astrocyte calcium responses along apical dendrites of CA1 pyramidal neurons in hippocampal slices using single-neuron stimulation with channelrhodopsin-2. This technique allowed selective neuronal depolarization without invasive manipulations known to alter calcium levels in astrocytes. Light-evoked neuronal depolarization was elicited and calcium events in surrounding astrocytes were monitored using the calcium-sensitive dye Calcium Orange. Stimulation of single neurons caused calcium responses in populations of astrocytes along the apical axis of CA1 cell dendrites. Calcium responses included single events that were synchronized with neuronal stimulation and poststimulus changes in calcium event frequency, both of which were modulated by glutamatergic and purinergic signaling. Individual astrocytes near CA1 cells showed low ability to respond to repeated neuronal depolarization events. However, the response of the surrounding astrocyte population was remarkably accurate. Interestingly, the reliability of responses was graded with respect to astrocyte location along the CA1 cell dendrite, with astrocytes residing in the primary dendrite subregion being most responsive. This study provides a new perspective on the dynamic response property of astrocyte ensembles to neuronal activity.

Topics: Action Potentials; Analysis of Variance; Animals; Animals, Newborn; Aspartic Acid; Astrocytes; Bacterial Proteins; Benzoates; Benzoxazines; Calcium; Calcium Channel Blockers; Carbenoxolone; Channelrhodopsins; Electric Stimulation; Excitatory Amino Acid Antagonists; Female; Glial Fibrillary Acidic Protein; Glycine; Green Fluorescent Proteins; Hippocampus; In Vitro Techniques; Luminescent Proteins; Male; Methoxyhydroxyphenylglycol; Mice; Mice, Inbred C57BL; Models, Biological; Morpholines; Naphthalenes; Neurons; Patch-Clamp Techniques; Peptide Fragments; Phosphopyruvate Hydratase; Photic Stimulation; Piperidines; Pyrazoles; Pyridines; Sodium Channel Blockers; Statistics, Nonparametric; Tetanus Toxin; Tetrodotoxin; Transduction, Genetic

Examination of critical subreceptors in the seizure controlling perirhinal cortex has revealed that microinfusion of ionotropic glutamatergic antagonists can exert anticonvulsant efficacy against soman-induced seizures. The purpose of the present study was to investigate whether modulators of metabotropic glutamate (mGlu) receptors may ensure anticonvulsant effects when microinfused into the perirhinal cortex. The results showed that the mGlu5 receptor antagonist MPEP hydrochloride (2-Methyl-6-(phenylethynyl)pyridine hydrochloride) and the mGlu2/3 receptor agonist DCG-IV ((2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine) caused full protection against seizures or increased latency to onset of seizures, whereas the mGlu1 receptor antagonist LY367385 ((S)-(+)-alpha-Amino-4-carboxy-2-methylbenzeneacetic acid) did not produce anticonvulsant efficacy in response to systemically administered soman (1.3 x LD(50)). Low doses of the above modulators had no anticonvulsant effects, whereas too high dose of MPEP resulted in proconvulsant effects. The results suggest that the perirhinal cortex is a likely site of cholinergic recruitment of glutamatergic hyperactivity after exposure to a convulsant dose of soman. Modulators of mGlu receptors may represent an alternative or supplement to ionotropic glutamate antagonists as anticonvulsants against nerve agent-evoked seizures.

Topics: Animals; Anticonvulsants; Benzoates; Cyclopropanes; Dose-Response Relationship, Drug; Glycine; Infusion Pumps; Male; Microinjections; Pyridines; Rats; Rats, Wistar; Receptors, Metabotropic Glutamate; Seizures; Soman; Temporal Lobe; Time Factors

Endocannabinoids and their receptor CB1 play key roles in brain function. Astrocytes express CB1Rs that are activated by endocannabinoids released by neurons. However, the consequences of the endocannabinoid-mediated neuron-astrocyte signaling on synaptic transmission are unknown. We show that endocannabinoids released by hippocampal pyramidal neurons increase the probability of transmitter release at CA3-CA1 synapses. This synaptic potentiation is due to CB1R-induced Ca(2+) elevations in astrocytes, which stimulate the release of glutamate that activates presynaptic metabotropic glutamate receptors. While endocannabinoids induce synaptic depression in the stimulated neuron by direct activation of presynaptic CB1Rs, they indirectly lead to synaptic potentiation in relatively more distant neurons by activation of CB1Rs in astrocytes. Hence, astrocyte calcium signal evoked by endogenous stimuli (neuron-released endocannabinoids) modulates synaptic transmission. Therefore, astrocytes respond to endocannabinoids that then potentiate synaptic transmission, indicating that astrocytes are actively involved in brain physiology.

Topics: Animals; Animals, Newborn; Astrocytes; Benzoates; Biophysics; Calcium; Cannabinoid Receptor Modulators; Chelating Agents; Drug Interactions; Egtazic Acid; Electric Stimulation; Endocannabinoids; Enzyme Inhibitors; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Glycine; Hippocampus; In Vitro Techniques; Mice; Mice, Inbred C57BL; Mice, Knockout; Patch-Clamp Techniques; Photolysis; Piperidines; Pyramidal Cells; Pyrazoles; Pyridines; Receptor, Cannabinoid, CB1; Resorcinols; Synaptic Transmission; Thapsigargin

Previous studies have implicated that long-term depression (LTD) was developmentally regulated since LTD can be readily induced by low frequency stimulation (LFS) in acute hippocampal slices prepared from juvenile but not adult animals. Here, we have examined the LTD induced by LFS (1Hz, 900 pulses) paired with a certain pattern at the Schaffer collateral-CAl synapse in adult hippocampal slices. We found that, in the 90-day-old rat hippocampus, LTD could be induced reliably by LFS paired with stronger stimulus intensity than that used during baseline recording. However, this synaptic depression could be completely abolished by application of metabotropic glutamate receptor (mGluR) antagonist (S)-amethyl-4-carboxyphenylglycine (MCPG) which had no effect on that induced by the same protocol in the 16-day-old rat hippocampus. Furthermore, preincubation with group I mGluR antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP) and (S)-2-methyl-4-carboxyphenylglycine (LY367385), also completely prevented the LFS-induced LTD. In contrast, group II mGluR antagonist (2S)-a-ethylglutamic acid (EGLU), N-methyl-d-aspartate (NMDA) receptor antagonist APV and voltage-gated calcium channel antagonist nimodipine had no effect on the LFS-induced LTD. Taken together, these observations suggest that LFS paired with strong stimulus strength can efficiently induce group I mGluR-dependent LTD in the adult hippocampal CA1 region, proving insight into the functional significance of hippocampal mGluR-mediated LTD in learning and memory.

Topics: Age Factors; Aging; Animals; Axons; Benzoates; Electric Stimulation; Excitatory Amino Acid Antagonists; Glutamic Acid; Glycine; Hippocampus; Long-Term Synaptic Depression; Neural Inhibition; Neural Pathways; Neurons; Organ Culture Techniques; Pyridines; Rats; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Synaptic Membranes; Synaptic Transmission

There is evidence suggesting that protein kinase C (PKC) activation can prevent the enhanced network excitability associated with status epilepticus and group I metabotropic glutamate receptor (mGluR)-induced epileptogenesis. However, we observed no suppression of mGluR-induced burst prolongation in the guinea pig hippocampal slice when applied in the presence of the PKC activator phorbol-12,13-dibutyrate (PDBu). Furthermore, PDBu alone converted picrotoxin-induced interictal bursts into ictal-length discharges ranging from 2 to 6s in length. This effect could not be elicited by the inactive analog 4-alpha-PDBu and was suppressed with the PKC inhibitor chelerythrine, indicating PKC dependence. PKC activation can enhance neurotransmitter release, and both glutamate and acetylcholine are capable of eliciting similar prolonged synchronized discharges. However, neither mGluR1 nor NMDA receptor antagonist suppressed PDBu-driven burst prolongation, suggesting that increased glutamate release alone is unlikely to account for the PKC-induced expression of ictaform discharges. Similarly, atropine, a broad-spectrum muscarinic receptor antagonist, had no effect on PKC-induced burst prolongation. By contrast, AMPA/kainate receptor antagonist abolished PKC-induced burst prolongation, and mGluR5 antagonist significantly blunted the maximum burst length induced by PKC. These data suggest that PKC-induced prolongation of epileptiform bursts is dependent on changes specific to mGluR5 and AMPA/kainate receptors and not mediated simply by a generalized increase in transmitter release.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Acetylcholine; Action Potentials; Animals; Atropine; Benzoates; Benzophenanthridines; Enzyme Activation; Epilepsy; Glutamic Acid; Glycine; Guinea Pigs; Hippocampus; In Vitro Techniques; Neurotransmitter Agents; Phorbol 12,13-Dibutyrate; Picrotoxin; Protein Kinase C; Pyridines; Receptor, Metabotropic Glutamate 5; Receptors, Kainic Acid; Receptors, Metabotropic Glutamate; Signal Transduction

Synaptic plasticity in inhibitory interneurons is essential to maintain a proper equilibrium between excitation and inhibition in hippocampal network. Recent studies showed that theta-burst-induced long-term potentiation (LTP) at excitatory synapses of oriens/alveus (O/A) interneurons in CA1 hippocampal region required the activation of metabotropic glutamate receptor (mGluR) 1. However these interneurons also express mGluR5 and the contribution of this receptor subtype in interneuron synaptic plasticity remains unexplored. We combined pharmacological and transgenic approaches to examine the relative contribution of mGluR1/5 in LTP at excitatory synapses on O/A interneurons. Bath-application of the selective mGluR1/5 agonist (s)-3,5-dihydroxyphenylglycine (DHPG) induced LTP of compound excitatory postsynaptic potentials. DHPG-induced LTP was not prevented by application of either mGluR1 or mGluR5 antagonists, was still present in mGluR1 knockout mice, but was blocked by co-application of both antagonists. These results indicate that LTP can be induced at O/A interneuron synapses by either mGluR1 or mGluR5 activation. As previously reported for mGluR1-dependent LTP, the mGluR5-dependent LTP was independent of N-methyl-d-aspartate receptors. Pairing DHPG application with postsynaptic depolarization induced mGluR1- and mGluR5-dependent LTP of minimally-evoked excitatory postsynaptic currents, which were composed of calcium-permeable AMPA receptor and presynaptically modulated by group II mGluRs, hence confirming that both forms of LTP occurred directly at interneuron excitatory synapses. These findings uncover a new mGluR5-dependent form of LTP at O/A interneuron synapses and indicate that activation of mGluR1 or mGluR5 is sufficient to induce LTP at these synapses. Thus, a rich repertoire of adaptive changes may take place at these interneuron synapses to regulate hippocampal feedback inhibition.

Topics: Animals; Benzoates; Electrophysiology; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Glycine; Hippocampus; Interneurons; Long-Term Potentiation; Methoxyhydroxyphenylglycol; Mice; Mice, Knockout; Patch-Clamp Techniques; Pyridines; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Synapses

Long-term changes in the efficacy of glutamatergic synaptic transmission in the striatal complex are proposed to underlie motor learning and neuroadaptations leading to addiction. Dopamine and glutamate play key roles in the induction of long-term potentiation (LTP) and long-term depression (LTD) in the dorsal striatum, but their contribution to synaptic plasticity in the ventral striatum (nucleus accumbens, NAc) has been less extensively studied. We have examined the role of dopamine, glutamate and GABA in the induction of LTP in mouse brain slices containing the NAc. High-frequency stimulation of glutamatergic inputs elicited LTP of field excitatory postsynaptic potentials/population spikes (fEPSP/PSs) in the core region of the NAc. GABA did not seem to participate in LTP induction because LTP was not altered in the presence of either a GABA(A)- (bicuculline) or a GABA(B)- (CGP 55845) receptor antagonist. However, the dopamine D1 receptor antagonist SCH 23390, but not the dopamine D2 receptor antagonist sulpiride, impaired LTP. The dopamine reuptake blocker nomifensine also inhibited LTP induction. We found that group I metabotropic glutamate receptors (mGluRs) contribute to LTP induction because the mGluR1 antagonist LY 367385, or the mGluR5 antagonist MPEP, blocked LTP induction. Furthermore, the glutamate reuptake blocker DL-TBOA also impaired LTP. The present results demonstrate that dopamine and glutamate play critical roles in the mechanisms of induction of LTP in the NAc through the activation of dopamine D1 receptors and group I mGluRs. However, LTP is negatively regulated when endogenous levels of dopamine or glutamate are elevated.

Topics: Animals; Aspartic Acid; Benzazepines; Benzoates; Bicuculline; Dopamine Antagonists; Dose-Response Relationship, Radiation; Electric Stimulation; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; GABA Antagonists; Glycine; In Vitro Techniques; Long-Term Potentiation; Male; Mice; Mice, Inbred C57BL; Nucleus Accumbens; Phosphinic Acids; Propanolamines; Pyridines; Receptors, Dopamine D1; Receptors, Metabotropic Glutamate; Sulpiride

Group I metabotropic glutamate receptors (mGlu1 and 5) have been implicated in synaptic plasticity and learning and memory. However, much of our understanding of how these receptors in different brain regions contribute to distinct memory stages in different learning tasks remains incomplete. The present study investigated the effects of the mGlu5 receptor antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), and mGlu1 receptor antagonist, (S)-(+)-alpha-amino-4-carboxy-2-methylbenzene-acetic acid (LY 367385) in the dorsal hippocampus on the consolidation and extinction of memory for inhibitory avoidance learning. Male, Sprague-Dawley rats were trained in a single-trial step-down inhibitory avoidance task. MPEP, LY 367385 or saline were infused bilaterally into the CA1 region immediately after training or immediately after the first retention test which was given 24h after training. Rats receiving MPEP (1.5 or 5.0 microg/side) or LY 367385 (0.7 or 2.0 microg/side) infusion exhibited a dose-dependent decrease in retention when tested 24h later. MPEP was ineffective while LY 367385 significantly attenuated extinction when injected after the first retention test using an extinction procedure. These findings indicate a selective participation of hippocampal group I mGlu receptors in memory processing in this task.

Topics: Animals; Avoidance Learning; Benzoates; Excitatory Amino Acid Antagonists; Extinction, Psychological; Glycine; Hippocampus; Male; Neural Inhibition; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Retention, Psychology

Evidence suggests that increased glutamatergic input to the substantia nigra pars compacta as a result of hyperactivity of subthalalmic nucleus output pathways may contribute to the progressive degeneration of nigral dopaminergic neurones in Parkinson's disease (PD), a debilitating neurodegenerative disorder which affects approximately 1% of people aged over 65. Substantial electrophysiological evidence suggests that the excitation of nigral dopaminergic neurones is regulated by the activation of Group I metabotropic glutamate receptors (mGluR), comprising mGluR1 and mGluR5 subtypes. As activation of these receptors by endogenous glutamate may promote multiple cascades leading to excitotoxic neuronal death, it may be hypothesised that functional antagonism of Group I mGluR should be neuroprotective and could form the basis of a novel neuroprotective treatment for PD. To investigate this hypothesis, the neuroprotective potential of the selective competitive mGlu1 antagonist (+)-2-methyl-4-carboxyphenylglycine ((S)-(+)-alpha-amino-4-carboxy-2-methlybenzeneacetic acid; LY367385) and the selective allosteric mGlu5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) was tested in a rodent 6-hydroxydopamine (6-OHDA) model of PD in vivo. Both acute and subchronic intranigral administration of either LY367385 or MPEP resulted in significant neuroprotection of nigral tyrosine hydroxylase immunoreactive cell bodies, which correlated closely with prevention of striatal monoamine depletion following 6-OHDA lesioning. This neuroprotective action of LY367385 and MPEP displayed a clear concentration-dependent effect, suggesting a receptor-mediated mechanism of action. LY367385 produced robust neuroprotection at all concentrations tested (40, 200 and 1000 nmol in 4 microL), whilst MPEP displayed a bell-shaped neuroprotective profile with significant neuroprotection at low concentrations (2 and 10 nmol in 4 microL) but not at higher concentrations (50 nmol). Importantly, subchronic intranigral administration of MPEP and LY367385 appeared to slow the degeneration of remaining nigral dopaminergic neurones and prevented further striatal dopamine depletion in animals with established 6-OHDA induced nigrostriatal lesions, suggesting that these compounds may significantly influence disease progression in this model.

Topics: Animals; Benzoates; Corpus Striatum; Dopamine; Dose-Response Relationship, Drug; Excitatory Amino Acid Antagonists; Glutamic Acid; Glycine; Male; Molecular Structure; Nerve Degeneration; Neural Pathways; Neuroprotective Agents; Neurotoxins; Oxidopamine; Parkinsonian Disorders; Pyridines; Rats; Rats, Sprague-Dawley; Receptors, Metabotropic Glutamate; Substantia Nigra; Treatment Outcome; Tyrosine 3-Monooxygenase

Hippocampal inhibitory interneurones demonstrate pathway- and synapse-specific rules of transmission and plasticity, which are key determinants of their role in controlling pyramidal cell excitability. Mechanisms underlying long-term changes at interneurone excitatory synapses, despite their importance, remain largely unknown. We use two-photon calcium imaging and whole-cell recordings to determine the Ca2+ signalling mechanisms linked specifically to group I metabotropic glutamate receptors (mGluR1alpha and mGluR5) and their role in hebbian long-term potentiation (LTP) in oriens/alveus (O/A) interneurones. We demonstrate that mGluR1alpha activation elicits dendritic Ca2+ signals resulting from Ca2+ influx via transient receptor potential (TRP) channels and Ca2+ release from intracellular stores. By contrast, mGluR5 activation produces dendritic Ca2+ transients mediated exclusively by intracellular Ca2+ release. Using Western blot analysis and immunocytochemistry, we show mGluR1alpha-specific extracellular signal-regulated kinase (ERK1/2) activation via Src in CA1 hippocampus and, in particular, in O/A interneurones. Moreover, we find that mGluR1alpha/TRP Ca2+ signals in interneurone dendrites are dependent on activation of the Src/ERK cascade. Finally, this mGluR1alpha-specific Ca2+ signalling controls LTP at interneurone synapses since blocking either TRP channels or Src/ERK and intracellular Ca2+ release prevents LTP induction. Thus, our findings uncover a novel molecular mechanism of interneurone-specific Ca2+ signalling, critical in regulating synaptic excitability in hippocampal networks.

Topics: Animals; Benzoates; Calcium; Calcium Signaling; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glycine; Hippocampus; In Vitro Techniques; Interneurons; Long-Term Potentiation; Membrane Potentials; Mitogen-Activated Protein Kinase 1; Mitogen-Activated Protein Kinase 3; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Resorcinols; src-Family Kinases; Synapses; Synaptic Transmission; TRPC Cation Channels

Genetic deletion of fragile X mental retardation protein (FMRP) has been shown to enhance mGluR-dependent long-term depression (LTD). Herein, we demonstrate that mGluR-LTD induces a transient, translation-dependent increase in FMRP that is rapidly degraded by the ubiquitin-proteasome pathway. Moreover, proteasome inhibitors abolished mGluR-LTD, and LTD was absent in mice that overexpress human FMRP. Neither translation nor proteasome inhibitors blocked the augmentation of mGluR-LTD in FMRP-deficient mice. In addition, mGluR-LTD is associated with rapid increases in the protein levels of FMRP target mRNAs in wild-type mice. Interestingly, the basal levels of these proteins were elevated and their synthesis was improperly regulated during mGluR-LTD in FMRP-deficient mice. Our findings indicate that hippocampal mGluR-LTD requires the rapid synthesis and degradation of FMRP and that mGluR-LTD triggers the synthesis of FMRP binding mRNAs. These findings indicate that the translation, ubiquitination, and proteolysis of FMRP functions as a dynamic regulatory system for controlling synaptic plasticity.

Topics: Animals; Animals, Newborn; Anisomycin; Benzoates; Blotting, Western; Cysteine Proteinase Inhibitors; Dose-Response Relationship, Drug; Drug Interactions; Excitatory Amino Acid Antagonists; Fluorescent Antibody Technique; Fragile X Mental Retardation Protein; Glycine; In Vitro Techniques; Leupeptins; Long-Term Synaptic Depression; Male; Methoxyhydroxyphenylglycol; Mice; Mice, Knockout; Microtubule-Associated Proteins; Models, Biological; Proteasome Endopeptidase Complex; Protein Biosynthesis; Protein Synthesis Inhibitors; Pyridines; Receptors, Metabotropic Glutamate; RNA, Messenger; Signal Transduction

The present study investigated the role of peripheral group I metabotropic glutamate receptors (mGluRs) in MO-induced nociceptive behaviour and inflammation in the masseter muscles of lightly anesthetized rats. Experiments were carried out on male Sprague-Dawley rats weighing 300-400 g. After initial anesthesia with sodium pentobarbital (40 mg/kg, i.p.), one femoral vein was cannulated and connected to an infusion pump for intravenous infusion of sodium pentobarbital. The rate of infusion was adjusted to provide a constant level of anesthesia. Mustard oil (MO, 30 microl) was injected into the mid-region of the left masseter muscle via a 30-gauge needle over 10s. After 30 microl injection of 5, 10, 15, or 20% MO into the masseter muscle, the total number of hindpaw shaking behaviour and extravasated Evans' blue dye concentration in the masseter muscle were significantly higher in the MO-treated group in a dose-dependent manner compared with the vehicle (mineral oil)-treated group. Intramuscular pretreatment with 3 or 5% lidocaine reduced MO-induced hindpaw shaking behaviour and increases in extravasated Evans' blue dye concentration. Intramuscular pretreatment with 5 mM MCPG, non-selective group I/II mGluR antagonist, or MPEP, a selective group I mGluR5 antagonist, produced a significant attenuation of MO-induced hindpaw shaking behaviour and increases in extravasated Evans' blue dye concentration in the masseter muscle while LY367385, a selective group I mGluR1 antagonist, did not affect MO-induced nociceptive behaviour and inflammation in the masseter muscle. These results indicate that peripheral mGluR5 plays important role in mediating MO-induced nociceptive behaviour and inflammation in the craniofacial muscle.

Topics: Anesthetics, Local; Animals; Behavior, Animal; Benzoates; Dose-Response Relationship, Drug; Drug Interactions; Excitatory Amino Acid Antagonists; Facial Muscles; Facial Pain; Functional Laterality; Glycine; Inflammation; Lidocaine; Male; Mustard Plant; Pain Measurement; Plant Oils; Pyridines; Rats; Rats, Sprague-Dawley

This study was undertaken to analyze the involvement of periaqueductal gray (PAG) cannabinoid or group I metabotropic glutamate receptors in the formalin-induced changes on the rostral ventromedial medulla (RVM) ON- and OFF-cells activities. S.c. injection of formalin into the hind paw produced a transient decrease (4-6 min) followed by a longer increase (25-35 min) in tail flick latencies. Formalin also increased basal activity in RVM ON-cells (42+/-7%) and decreased it in OFF-cells (35+/-4%). Intra-PAG microinjection of (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl) pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate (WIN 55,212-2) (2 nmol/rat), a cannabinoid receptor agonist, prevented the formalin-induced changes in RVM cell activities. Higher dosages of WIN 55,212-2 (4-8 nmol/rat) increased the tail flick latencies, delayed the tail flick-related onset to ON-cell burst, and decreased the duration of OFF-cell pause. Furthermore, WIN 55,212-2 at a dosage of 8 nmol/rat decreased RVM ON-cell (57+/-7%) and increased OFF-cell ongoing activities (26+/-4%). These effects were prevented by N-piperidino-5-(4-chlorophenyl)-1-(2,4dichlorophenyl)-4-methyl-3-pyrazolecarboxamide SR141716A, (1 pmol/rat), a CB1 cannabinoid receptor antagonist, or by 2-methyl-6-(phenylethynyl)pyridine (MPEP 20 nmol/rat), a selective mGlu5 glutamate receptor antagonist. T7-(hydroxyimino) cyclopropa[b]chromen-1alpha-carboxylate ethyl ester (CPCOOE/50 nmol/rat) and (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid (LY367385, 20 nmol/rat), selective mGlu1 glutamate receptor antagonists, were ineffective in preventing the WIN-induced effects. This study suggests that s.c. injection of formalin modifies RVM neuronal activities and this effect is prevented by PAG cannabinoid receptor stimulation. Moreover, the physiological stimulation of PAG mGlu5, but not mGlu1 glutamate receptors, seems to be required for the cannabinoid-mediated effect.

Topics: Action Potentials; Analgesics; Animals; Benzoates; Benzoxazines; Chromones; Dose-Response Relationship, Drug; Drug Interactions; Excitatory Amino Acid Antagonists; Formaldehyde; Glycine; Injections, Subcutaneous; Male; Medulla Oblongata; Morpholines; Naphthalenes; Neurons; Pain Measurement; Periaqueductal Gray; Piperidines; Pyrazoles; Pyridines; Rats; Rats, Wistar; Reaction Time; Receptor, Cannabinoid, CB1; Receptors, Metabotropic Glutamate; Rimonabant

Kindling induced by repeated application of the convulsant pentylenetetrazole (PTZ) is a validated model of epilepsy and epilepsy-related neuromorphological, neurophysiological and behavioural alterations. In this study, we examined whether kindling-induced long-term aberrations in hippocampal synaptic plasticity can be prevented by application of group I mGluR antagonists. Kindling resulted in a higher magnitude of long-term potentiation (LTP) induced by a strong high-frequency stimulation in the hippocampal CA1 region in vitro. When the specific mGluR1 antagonist LY 367385 (0.40 microMol) or the specific mGluR5 inhibitor MPEP (0.06 microMol) were given 30 min prior to PTZ, this kindling-induced enhancement of LTP was almost completely prevented. In addition, application of MPEP led to an impaired maintenance of population spike LTP in kindled animals. LY 367385 applied to unkindled control animals caused a reduction of the initial magnitude of population spike LTP. MPEP, in contrast, left the initial magnitude untouched but resulted in a faster decay of potentiation. A single administration of LY 367385 (200 microM) and MPEP (50 microM), respectively, directly into the bath had almost no effect. Our data suggest that the long-lasting aberrations of hippocampal synaptic plasticity induced by the repeated occurrence of generalized epileptic seizures ultimately require a concurrent operation of mGluR1 and mGluR5.

Topics: Analysis of Variance; Animals; Benzoates; Dose-Response Relationship, Radiation; Drug Interactions; Electric Stimulation; Excitatory Postsynaptic Potentials; Glycine; Hippocampus; Long-Term Potentiation; Male; Neuronal Plasticity; Pentylenetetrazole; Pyridines; Rats; Rats, Wistar; Receptors, Metabotropic Glutamate; Seizures; Time Factors

Activation of glycogen synthase kinase 3beta (Gsk3beta) has been shown to be a key component in signaling pathways that underlie neurodegeneration and neurodegenerative disease. Conversely, inactivation of Gsk3beta by phosphoinositide 3-kinase (PI3K)/Akt is an important neuroprotective mechanism. Previous studies have shown that agonist activation of group I metabotropic glutamate receptors (mGluRs) can increase neuronal survival and prevent apoptosis. However, little is known about the signaling pathways that couple mGluR5 to neuroprotection. In this report, we investigated whether activation of the PI3K/Akt/Gsk3beta pathway, which has been shown to have an important neuroprotective mechanism, is required for mGluR5 activation mediated neuroprotection against beta-amyloid. We found that brief incubations of mouse hippocampal slices with (R,S)-3,5-dihydroxyphenylglycine (DHPG) resulted in increased phosphorylation of Akt and Gsk3beta. The PI3K inhibitors, LY294002 and wortmannin, blocked the DHPG-induced increased phosphorylation of Akt and Gsk3beta. Similar results were observed in rat primary hippocampal cultures. Finally, we found that the PI3K inhibitor LY294002 can block (R,S)-2-chloro-5-hydroxyphenylglycine (CHPG) mediated neuroprotection against beta-amyloid. Thus, these findings suggest that mGluR5 can modulate the PI3K/Akt/Gsk3beta pathway in the hippocampus, and that modulation of this signaling pathway can reverse beta-amyloid-induced neuronal toxicity.

Topics: Amyloid beta-Peptides; Animals; Benzoates; Cells, Cultured; Chromones; Dose-Response Relationship, Drug; Drug Interactions; Enzyme Inhibitors; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glycine; Glycogen Synthase Kinase 3; Glycogen Synthase Kinase 3 beta; Hippocampus; In Vitro Techniques; Indoles; Methoxyhydroxyphenylglycol; Mice; Mice, Inbred C57BL; Microtubule-Associated Proteins; Morpholines; Neurons; Oncogene Protein v-akt; Peptide Fragments; Phosphorylation; Pyridines; Rats; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Resorcinols; Serine; Signal Transduction

The effects of several metabotropic receptor (mGluR) ligands on baseline hippocampal glutamate and GABA overflow in conscious rats and the modulation of limbic seizure activity by these ligands were investigated. Intrahippocampal mGluR group I agonist perfusion via a microdialysis probe [1 mm (R,S)-3,5-dihydroxyphenylglycine] induced seizures and concomitant augmentations in amino acid dialysate levels. The mGlu1a receptor antagonist LY367385 (1 mm) decreased baseline glutamate but not GABA concentrations, suggesting that mGlu1a receptors, which regulate hippocampal glutamate levels, are tonically activated by endogenous glutamate. This decrease in glutamate may contribute to the reported LY367385-mediated anticonvulsant effect. The mGlu5 receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine (50 mg/kg) also clearly abolished pilocarpine-induced seizures. Agonist-mediated actions at mGlu2/3 receptors by LY379268 (100 microm, 10 mg/kg intraperitoneally) decreased basal hippocampal GABA but not glutamate levels. This may partly explain the increased excitation following systemic LY379268 administration and the lack of complete anticonvulsant protection within our epilepsy model with the mGlu2/3 receptor agonist. Group II selective mGluR receptor blockade with LY341495 (1-10 microm) did not alter the rats' behaviour or hippocampal amino acid levels. These data provide a neurochemical basis for the full anticonvulsant effects of mGlu1a and mGlu5 antagonists and the partial effects observed with mGlu2/3 agonists in vivo.

Topics: Amino Acids; Animals; Anticonvulsants; Benzoates; Bridged Bicyclo Compounds, Heterocyclic; Cyclopropanes; Disease Models, Animal; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Extracellular Fluid; gamma-Aminobutyric Acid; Glutamic Acid; Glycine; Hippocampus; Ligands; Limbic System; Male; Microdialysis; Pilocarpine; Pyridines; Rats; Rats, Wistar; Receptors, Metabotropic Glutamate; Seizures

LY 367385 (mGluR1) and MPEP (mGluR5), which are group I metabotropic glutamate receptor (mGluR) antagonists, were used to investigate their effects on pentylenetetrazole (PTZ) seizures, kindling, and kindling-related learning deficits. Both substances showed anticonvulsant efficacy against seizures induced by lower doses of PTZ (40 mg/kg), but they were ineffective in counteracting seizures evoked by higher PTZ doses. When these substances were given in the course of kindling induction, LY significantly depressed the progression of kindled seizure severity. In contrast, MPEP was ineffective in this experiment. Treatment with either LY or MPEP did not modify the reaction to challenge dose of PTZ. Kindling results in a worsening of shuttle-box learning. LY improved shuttle-box learning when administered in the course of kindling development or when given prior to the learning experiment. This suggests protective and restorative effectiveness. In contrast, MPEP was only effective on the learning performance of kindled rats when given prior to the shuttle-box experiment, which demonstrates restorative effectiveness. Kindling is associated with an increase in glutamate binding. LY counteracted this increase whereas MPEP was ineffective. It was concluded that mGluR1 and mGluR5 play a specific role in the convulsive component of kindling. The beneficial action of the antagonists on kindling-induced impairments in shuttle-box learning may be associated with their effect on glutamatergic synaptic activity.

Topics: Animals; Avoidance Learning; Behavior, Animal; Benzoates; Binding Sites; Dose-Response Relationship, Drug; Drug Administration Schedule; Epilepsies, Myoclonic; Glycine; Injections, Intraperitoneal; Injections, Intraventricular; Kindling, Neurologic; Learning Disabilities; Male; Pentylenetetrazole; Pyridines; Rats; Rats, Wistar; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Time Factors

Hippocampal long-term depression (LTD) is a long-lasting decrease in synaptic strength that is most commonly studied at glutamatergic inputs to pyramidal cells in hippocampal area CA1. Activation of G-protein-coupled group I (including types 1 and 5) metabotropic glutamate receptors (mGluRs) by the pharmacological agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) elicits LTD in area CA1 of the hippocampus. Recent reports have shown that de novo protein synthesis is necessary for DHPG-induced LTD. However, relatively little is known about the signaling pathways that couple mGluRs to translation initiation. In this study, we investigated whether the activation of the phosphoinositide 3-kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) pathway, which has been shown to regulate translation initiation, is necessary for mGluR-LTD induced by DHPG. We found that brief incubations of mouse hippocampal slices with DHPG resulted in increased phosphorylation of Akt and mTOR in hippocampal area CA1. Two structurally unrelated PI3K inhibitors, LY294002 and wortmannin, blocked the DHPG-induced increases in phosphorylation of Akt and mTOR. Biochemical fractionation studies showed that the DHPG-induced increase in the phosphorylation of Akt and mTOR could be detected in synaptoneurosome preparations, and immunohistochemical analysis revealed that similar increases could be detected in both stratum pyramidale and stratum radiatum in area CA1. Finally, we observed that both PI3K inhibitors and rapamycin, an mTOR inhibitor, prevented mGluR-LTD induced by DHPG. Together, our findings indicate that activation of the PI3K-Akt-mTOR signaling cascade is required for mGluR-LTD and suggest that this pathway may couple group I mGluRs to translation initiation in hippocampal area CA1.

Topics: 3-Phosphoinositide-Dependent Protein Kinases; Androstadienes; Animals; Benzoates; Chromones; Dendrites; Enzyme Inhibitors; Excitatory Amino Acid Agonists; Glycine; Hippocampus; Long-Term Synaptic Depression; Male; Mice; Mice, Inbred C57BL; Morpholines; Phosphatidylinositol 3-Kinases; Phosphoinositide-3 Kinase Inhibitors; Phosphorylation; Protein Kinases; Protein Processing, Post-Translational; Protein Serine-Threonine Kinases; Proto-Oncogene Proteins; Proto-Oncogene Proteins c-akt; Pyramidal Cells; Pyridines; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Resorcinols; Signal Transduction; Sirolimus; Synaptosomes; Tacrolimus; TOR Serine-Threonine Kinases; Wortmannin

Selective antagonists of the metabotropic receptors 1 (mGluR1), +/-2-methyl-4-carboxyphenylglycine (LY367385), and mGluR5, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), were used to investigate the role of group I metabotropic receptors in late-phase long-term potentiation (L-LTP) at Schaffer collateral/commissural fiber-CA1 synapses in rat hippocampal slices. L-LTP was induced with three trains of tetanization of 1 s duration at 100 Hz separated by 10-min intervals. Neither LY367385 nor MPEP affected basal synaptic responses at the doses used (200 and 10 microM, respectively) and only the mGluR5 inhibitor MPEP blocked L-LTP. However, in agreement with previous mouse mutant studies, we found that both LY367385 and MPEP inhibited the induction of an LTP obtained with a single train of tetanization of 1 s duration at 100 Hz. MPEP's ability to disrupt L-LTP was not due to an effect on NMDA responses since it did not affect pharmacologically isolated N-methyl-D-aspartate (NMDA) excitatory postsynaptic potentials (EPSPs). However, MPEP prevented the increased phosphorylation in dendrites of p70 S6 kinase (p70(S6K)) at Thr3889, a major regulator of translation required for the induction of protein synthesis-dependent forms of LTP.

Topics: Animals; Animals, Newborn; Benzoates; Dendrites; Dose-Response Relationship, Drug; Dose-Response Relationship, Radiation; Electric Stimulation; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Glycine; Hippocampus; Immunohistochemistry; In Vitro Techniques; Long-Term Potentiation; Neurons; Phosphorylation; Pyridines; Rats; Rats, Wistar; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Ribosomal Protein S6 Kinases, 70-kDa; Threonine; Time Factors

Metabotropic glutamate receptors (mGluRs) have been demonstrated to play a role in synaptic plasticity. It has been recently shown that mGluR1 is involved in corticostriatal long-term depression, by means of pharmacological approach and by using mGluR1-knockout mice. Here, we report that both mGluR1 and mGluR5 are involved in corticostriatal long-term potentiation (LTP). In particular, the mGluR1 antagonist LY 367385, as well as the mGluR5 antagonist MPEP, reduce LTP amplitude. Moreover, blockade of both mGluR1 and mGluR5 by LY 367385 and MPEP co-administration fully suppresses LTP. Accordingly, group II and group III mGluRs antagonists fail to affect LTP induction. Interestingly, LTP amplitude is also significantly reduced in both mGluR1- and mGluR5-knockout mice. The differential function of mGluR1 and mGluR5 in corticostriatal synaptic plasticity may play a role in the modulation of the motor activity mediated by the basal ganglia, thus providing a substrate for the pharmacological treatment of motor disorders involving the striatum.

Topics: Animals; Benzoates; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Glycine; In Vitro Techniques; Long-Term Potentiation; Male; Membrane Potentials; Mice; Mice, Knockout; Neocortex; Neostriatum; Patch-Clamp Techniques; Pyridines; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate

We have previously shown that the release of central neurotransmitters can be modulated by the activation of Group I and Group II subtypes of G-protein-linked metabotropic glutamate (mGlu) receptors. To date, however, very little is known about the regulation of serotonergic neurotransmission by these receptor subtypes. In the present study, we have utilized in vivo intracerebral microdialysis to elucidate the roles of Group I and Group II mGlu receptors in the regulation of neuronal 5-hydroxytryptamine (5-HT) release in the frontal cortex of conscious, freely moving rats. Dialysate 5-HT was of neuronal origin with basal release showing strong calcium dependency and tetrodotoxin sensitivity and marked elevation following K(+)-induced depolarization. The broad-spectrum mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD; 1-3 mM] did not significantly modify basal cerebrocortical 5-HT release. Similarly, the Group I mGlu receptor-specific agonist (RS)-3,5-dihydroxyphenylglycine [(RS)-3,5-DHPG; 1-3 mM] showed no marked effect on cortical dialysate 5-HT levels. To eliminate the possibility that these findings were the result of receptor desensitization, the effects of lower concentrations of (RS)-DHPG (100-300 microM) and shorter ligand exposure time (15 min) were also evaluated. Dialysate 5-HT levels remained unmodified by these manipulations. In comparison, the Group II mGlu receptor agonist, (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-1; 500 microM), evoked a marked facilitation of release (approximately 150% of basal) which was fully reversed by the Group I/II antagonist, (S)-alpha-methyl-4-carboxyphenylglycine [(S)-MCPG; 3 mM]. The modulatory action of L-CCG-1 showed a bell-shaped concentration-response relationship. (S)-MCPG (3 mM) and the potent and selective mGlu(5) receptor antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP; 100 microM), when given alone, did not significantly modify 5-HT levels.The current data provide strong evidence to suggest that while the release of neuronal 5-HT in the rat frontal cortex is not subject to regulation by facilitatory Group I mGlu receptors, it may be positively modulated by activation of Group II mGlu receptors. Taken together with data from other studies, the present investigation lends emphasis to the notion that neuromodulation by mGlu receptors is a region-specific phenomenon and also proposes that the heterogeneous distribution of these receptors is neurone-specific

Topics: Amino Acids, Dicarboxylic; Analysis of Variance; Anesthetics, Local; Animals; Benzoates; Calcium; Cerebral Cortex; Cycloleucine; Dose-Response Relationship, Drug; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glycine; Ligands; Male; Microdialysis; Potassium Chloride; Pyridines; Rats; Rats, Wistar; Receptors, Metabotropic Glutamate; Serotonin; Synaptic Transmission; Tetrodotoxin

The idea that excitatory amino acid transporters (EAATs) can control the activation of specific metabotropic glutamate receptors (mGluRs) was investigated in rat hippocampal slices. Using the accumulation of inositol phosphates as a measure of group I mGluR activity, we have shown that the broad spectrum, non-transportable EAAT blocker, TBOA, produces a significant shift to the left of agonist concentration-response curves. Moreover, this increase in potency did not occur if endogenous glutamate was enzymatically removed, suggesting a glutamate-dependent mechanism. This shift in potency was shown to be NMDA and group II mGlu receptor independent. Additionally, experiments with selective antagonists indicated that the group I receptor responsible for the stimulation of inositol phosphate production in this preparation is likely to be mGluR5. Inhibition of forskolin-stimulated cyclic AMP (cAMP) production was used as an index of group II/III mGluR activity. TBOA produced a rightward shift of the forskolin concentration-response curve. A group III, but not a group II, mGluR agonist also produced this effect, suggesting that the TBOA-mediated increase in glutamate activates a receptor, which appears to be a member of the group III mGluR subset. This was confirmed by the observation that an antagonist of group III mGluRs, prevented the TBOA-induced rightward shift in forskolin potency. These results provide evidence of a role for EAATs in the regulation of mGluR5 and group III mGluRs in the rat hippocampus, which may have therapeutic implications.

Topics: Animals; Aspartic Acid; Benzoates; Colforsin; Cyclic AMP; Excitatory Amino Acid Antagonists; Excitatory Amino Acid Transporter 1; Excitatory Amino Acid Transporter 2; Glycine; Hippocampus; In Vitro Techniques; Inositol 1,4,5-Trisphosphate; Male; Methoxyhydroxyphenylglycol; Pyridines; Quisqualic Acid; Rats; Rats, Sprague-Dawley; Receptors, Metabotropic Glutamate

We evaluated the role of spinal glutamate and substance P receptors in noxious stimulus-induced antinociception (NSIA). NSIA was produced by subdermal capsaicin administration in the hind paw of the rat and measured as attenuation of the jaw-opening reflex. NSIA was completely blocked by spinal intrathecal administration of the selective NMDA receptor antagonist LY235959 as well as the mGluR5 antagonists MPEP and SIB-1757 and partially attenuated by the selective AMPA/kainate receptor antagonist NBQX; however, neither the mGluR1 receptor antagonist LY367385 nor the NK1 antagonist L-703,606 affected NSIA. These results suggest that NSIA depends on glutamate, released from the central terminals of the primary afferent nociceptors, acting primarily on NMDA and mGluR5 receptors. Although substance P is also known to be released by similar stimuli, NK1 receptors do not appear to play a role in NSIA. The implications of these findings in the context of a proposed spinal circuit that mediates NSIA are discussed.

Topics: Animals; Benzoates; Capsaicin; Excitatory Amino Acid Antagonists; Glutamic Acid; Glycine; Isoquinolines; Male; Nociceptors; Pyridines; Quinoxalines; Quinuclidines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Receptors, N-Methyl-D-Aspartate; Receptors, Neurokinin-1; Spinal Cord; Stimulation, Chemical; Substance P

Application of group I metabotropic glutamate receptor (mGluR) agonists elicits seizure discharges in vivo and prolonged ictal-like activity in in vitro brain slices. In this study we examined 1) if group I mGluRs are activated by synaptically released glutamate during epileptiform discharges induced by convulsants in hippocampal slices and, if so, 2) whether the synaptically activated mGluRs contribute to the pattern of the epileptiform discharges. The GABA(A) receptor antagonist bicuculline (50 microM) was applied to induce short synchronized bursts of approximately 250 ms in mouse hippocampal slices. Addition of 4-aminopyridine (4-AP; 100 microM) prolonged these bursts to 0.7-2 s. The mGluR1 antagonist (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid (LY 367385; 25-100 microM) and the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP; 10-50 microM), applied separately, significantly reduced the duration of the synchronized discharges. The effects of these antagonists were additive when applied together, suggesting that mGluR1 and mGluR5 exert independent actions on the epileptiform bursts. In phospholipase C beta1 (PLCbeta1) knockout mice, bicuculline and 4-AP elicited prolonged synchronized discharges of comparable duration as those observed in slices from wild-type littermates. Furthermore, mGluR1 and mGluR5 antagonists reduced the duration of the epileptiform discharges to the same extent as they did in the wild-type preparations. The results suggest that mGluR1 and mGluR5 are activated synaptically during prolonged epileptiform discharges induced by bicuculline and 4-AP. Synaptic activation of these receptors extended the duration of synchronized discharges. In addition, the data indicate that the synaptic effects of the group I mGluRs on the duration of epileptiform discharges were mediated by a PLCbeta1-independent mechanism.

Topics: 4-Aminopyridine; Action Potentials; Animals; Benzoates; Bicuculline; Epilepsy; Excitatory Amino Acid Antagonists; GABA Antagonists; Glycine; Hippocampus; Isoenzymes; Mice; Mice, Inbred C57BL; Mice, Knockout; Organ Culture Techniques; Phospholipase C beta; Potassium Channel Blockers; Pyridines; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Synapses; Type C Phospholipases

Transient activation of group I metabotropic glutamate receptors (mGluRs) with the selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) produces persistent prolongation of epileptiform bursts in guinea pig hippocampal slices, the maintenance of which can be reversibly suppressed with group I mGluR antagonists. To determine the relative roles of mGluR1 and mGluR5 in these group I mGluR-dependent induction and maintenance processes, subtype-selective antagonists were utilized. In the presence of picrotoxin, DHPG (50 microM, 20-45 min) converted interictal bursts into 1- to 3-s discharges that persisted for hours following washout of the mGluR agonist. 2-methyl-6-(phenylethynyl)-pyridine (MPEP, an mGluR5 antagonist; 25 microM) and (+)-2-methyl-4-carboxyphenylglycine (LY367385, an mGluR1 antagonist; 20-25 microM) each significantly suppressed the ongoing expression of the mGluR-induced prolonged bursts. However, LY367385 was more effective, reducing the burst prolongation by nearly 90%; MPEP only produced a 64% reduction in burst prolongation. Nevertheless, MPEP was more effective at preventing the induction of the burst prolongation; all 10 slices tested failed to express prolonged bursts both during and after co-application of DHPG with MPEP. Co-application of DHPG with LY367385, in contrast, resulted in significant burst prolongation (in 68% of slices tested) that was revealed on washout of the two agents. These results suggest that while both receptor subtypes participate in both the induction and maintenance of mGluR-mediated burst prolongation, mGluR1 activation plays a greater role in sustaining the expression of prolonged bursts, whereas mGluR5 activation may be a more critical contributor to the induction process underlying this type of epileptogenesis.

Topics: Animals; Benzoates; Convulsants; Dose-Response Relationship, Drug; Epilepsy; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; GABA Antagonists; Glycine; Guinea Pigs; Hippocampus; In Vitro Techniques; Membrane Potentials; Picrotoxin; Pyridines; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Resorcinols; Time Factors

Spinal cord injury (SCI) initiates a cascade of biochemical events that leads to an increase in extracellular excitatory amino acid (EAA) concentrations, which results in glutamate receptor-mediated excitotoxic events. An important division of these glutamate receptors is the metabotropic glutamate receptor (mGluR) class, which is divided into three groups. Of these three groups, group I (mGluR1 and mGluR5) activation can initiate a number of intracellular pathways that lead to increased extracellular EAA concentrations. To evaluate subtypes of group I mGluRs in SCI, we administered AIDA (group I antagonist), LY 367385 (mGluR1 specific antagonist), or MPEP (mGluR5 specific antagonist) by interspinal injection to adult male Sprague-Dawley rats (175-200 g) immediately following injury at T10 with an NYU impactor (12.5-mm drop, 10-g rod, 2 mm in diameter). AIDA- and LY 367385-treated subjects had improved locomotor scores and demonstrated an attenuation in the development of mechanical allodynia as measured by von Frey stimulation of the forelimbs; however, LY 367385 potentiated the development of thermal hyperalgesia. MPEP had no effect on locomotor recovery or mechanical allodynia, but attenuated the development of thermal hyperalgesia. AIDA and LY 367385 treatment resulted in a significant increase in tissue sparing compared to the vehicle-treated group at 4 weeks following SCI. These results suggest that mGluRs play an important role in EAA toxicity and have different acute pathophysiological roles following spinal cord injury.

Topics: Animals; Benzoates; Chronic Disease; Excitatory Amino Acid Antagonists; Glycine; Hyperalgesia; Locomotion; Male; Neuroprotective Agents; Neurotoxins; Pain; Pain Threshold; Physical Stimulation; Pyridines; Rats; Rats, Sprague-Dawley; Receptors, Metabotropic Glutamate; Spinal Cord Injuries

Although metabotropic glutamate receptors (mGluRs) have been proposed to play a role in corticostriatal long-term depression (LTD), the specific receptor subtype required for this form of synaptic plasticity has not been characterized yet. Thus, we utilized a corticostriatal brain slice preparation and intracellular recordings from striatal spiny neurons to address this issue. We observed that both AIDA (100 microM) and LY 367385 (30 microM), two blockers of mGluR1s, were able to fully prevent the induction of this form of synaptic plasticity, whereas MPEP (30 microM), a selective antagonist of the mGluR5 subtype, did not significantly affect the amplitude and time-course of corticostriatal LTD. Both AIDA and LY 367385 were ineffective on LTD when applied after its induction. The critical role of mGluR1s in the formation of corticostriatal LTD was confirmed in experiments performed on mice lacking mGluR1s. In these mice, in fact, a significant reduction of the LTD amplitude was observed in comparison to the normal LTD measured in their wild-type counterparts. We found that neither acute pharmacological blockade of mGluR1s nor the genetic disruption of these receptors affected the presynaptic modulation of corticostriatal excitatory postsynapic potentials (EPSPs) exerted by DCG-IV and L-SOP, selective agonists of group II and III mGluRs, respectively. Our data show that the induction of corticostriatal LTD requires the activation of mGluR1 but not mGluR5. mGluR1-mediated control of this form of synaptic plasticity may play a role in the modulatory effect exerted by mGluRs in the basal ganglia-related motor activity.

Topics: Animals; Benzoates; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Glycine; Male; Mice; Mice, Knockout; Neocortex; Neostriatum; Neuronal Plasticity; Pyridines; Rats; Rats, Wistar; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate

The activation of group I metabotropic glutamate receptors (mGluRs) produces a variety of actions that lead to alterations in excitability and synaptic transmission in the CA1 region of the hippocampus. The group I mGluRs, mGluR1 and mGluR5, are activated selectively by (S)-3,5-dihydroxyphenylglycine (DHPG). To identify which of these mGluR subtypes are responsible for the various actions of DHPG in area CA1, we took advantage of two novel subtype-selective antagonists. (S)-(+)-alpha-amino-a-methylbenzeneacetic acid (LY367385) is a potent competitive antagonist that is selective for mGluR1, whereas 2-methyl-6-(phenylethynyl)-pyridine (MPEP) is a potent noncompetitive antagonist that is selective for mGluR5. The use of these compounds in experiments with whole-cell patch-clamp recording and Ca(2+)-imaging techniques revealed that each group I mGluR subtype plays distinct roles in regulating the function of CA1 pyramidal neurons. The block of mGluR1 by LY367385 suppressed the DHPG-induced increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) and the direct depolarization of CA1 hippocampal neurons. In addition, the increase in the frequency of spontaneous IPSCs (sIPSCs) caused by the DHPG-induced depolarization of inhibitory interneurons also was blocked by LY367385, as was the DHPG-induced inhibition of transmission at the Schaffer collateral-->CA1 synapse. On the other hand, the block of mGluR5 by MPEP antagonized the DHPG-induced suppression of the Ca(2+)-activated potassium current (I(AHP)) and potentiation of the NMDA receptor. Finally, antagonism of the DHPG-induced suppression of evoked IPSCs required the blockade of both mGluR1 and mGluR5. These data suggest that mGluR1 and mGluR5 play distinct roles in the regulation of the excitability of hippocampal CA1 pyramidal neurons.

Topics: Animals; Benzoates; Calcium; Electric Stimulation; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Fluorescent Dyes; Glycine; Hippocampus; In Vitro Techniques; Interneurons; Neural Inhibition; Patch-Clamp Techniques; Potassium Channels; Pyramidal Cells; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Receptors, N-Methyl-D-Aspartate; Resorcinols; Synaptic Transmission

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

We have applied subtype-selective antagonists of metabotropic glutamate (mGlu) receptors mGlu1 or mGlu5 [7-(hydroxy-imino) cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) or 2-methyl-6-(phenylethynyl)pyridine (MPEP)] to mixed rat cerebellar cultures containing both Purkinje and granule cells. The action of these two drugs on neuronal survival was cell specific. Although CPCCOEt (1, 10, 30 microm) reduced the survival of Purkinje cells, MPEP (3 or 30 microm) selectively reduced the survival of granule cells. Both effects required an early exposure of cultures to antagonists [from 3 to 6 d in vitro (DIV) for CPCCOEt, and from 3 to 6 or 6 to 9 DIV for MPEP]. Addition of MPEP from 6 to 9, 9 to 13, or 13 to 17 DIV also induced profound morphological changes in the dendritic tree and dendritic spines of Purkinje cells, suggesting that endogenous activation of mGlu5 receptors is required for the age-dependent refinement of Purkinje cell phenotype. In in vivo studies, an early blockade of mGlu1 receptors induced in rats by local injections of LY367385 (20 nmol/2 microl), local injections of mGlu1 antisense oligonucleotides (12 nmol/2 microl), or systemic administration of CPCCOEt (5 mg/kg, s.c.) from postnatal day (P) 3 to P9 reduced the number and dramatically altered the morphology of cerebellar Purkinje cells. In contrast, mGlu5 receptor blockade induced by local injections of antisense oligonucleotides reduced the number of granule cells but also produced substantial morphological changes in the dendritic tree of Purkinje cells. These results provide the first evidence that the development of cerebellar neurons is under the control of mGlu1 and mGlu5 receptors, i.e., the two mGlu receptor subtypes coupled to polyphosphoinositide hydrolysis.

Topics: Animals; Animals, Newborn; Benzoates; Cell Differentiation; Cell Survival; Cells, Cultured; Cerebellum; Chromones; Dendrites; Drug Administration Routes; Excitatory Amino Acid Antagonists; Glycine; Immunohistochemistry; Oligonucleotides, Antisense; Purkinje Cells; Pyridines; Rats; Rats, Sprague-Dawley; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Thiophenes; Time Factors

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

Stimulation of group I metabotropic glutamate receptors (mGluR 1 and 5) activates G-protein coupled-phospholipase C (PLC) to release 1,2-diacylglycerol (DAG) and arachidonic acid (ArAc). To elucidate the role of group I mGluR, we tested the effects of (S)-alpha-methyl-4-carboxy-phenylglycine (MCPG, mGluR 1 and 5 antagonist), 1-aminoindan-1,5-dicarboxylic acid (AIDA, mGluR 1a specific antagonist) and 2-methyl-6-(phenylethynyl) pyridine (MPEP, mGluR 5 antagonist) on ArAc release and neuronal survival after transient forebrain ischemia in gerbils. Ischemia resulted in (a) significant release of ArAc at 1-day reperfusion and (b) significant neuronal death in the hippocampal CA1 subfield after 6-day reperfusion. MCPG and MPEP decreased ArAc release and also significantly increased neuronal survival. AIDA was less effective in decreasing ArAc release and had no effect on neuronal death. These results suggest that activation of mGluR 5 may be an important pathway in ArAc release and neuronal death after transient ischemia.

Topics: Animals; Arachidonic Acid; Benzoates; Brain; Brain Ischemia; Cell Survival; Excitatory Amino Acid Antagonists; Gerbillinae; Glycine; Indans; Male; Neurons; Neuroprotective Agents; Prosencephalon; Pyridines; Receptors, Metabotropic Glutamate