Potent antagonists at the L-AP4- and (1S,3S)-ACPD-sensitive presynaptic metabotropic glutamate receptors in the neonatal rat spinal cord

Amygdala group II mGluRs mediate the inhibitory effects of systemic group II mGluR activation on behavior and spinal neurons in a rat model of arthritis pain

The amygdala plays a critical role in emotional-affective aspects of behaviors and pain modulation. The central nucleus of amygdala (CeA) serves major output functions, and neuroplasticity in the CeA is linked to pain-related behaviors in different models. Activation of G_i/o-coupled group II metabotropic glutamate receptors (mGluRs), which consist of mGluR2 and mGluR3, can decrease neurotransmitter release and regulate synaptic plasticity. Group II mGluRs have emerged as targets for neuropsychiatric disorders and can inhibit pain-related processing and behaviors. Surprisingly, site and mechanism of antinociceptive actions of systemically applied group II mGluR agonists are not clear. Our previous work showed that group II mGluR activation in the amygdala inhibits pain-related CeA activity, but behavioral and spinal consequences remain to be determined. Here we studied the contribution of group II mGluRs in the amygdala to the antinociceptive effects of a systemically applied group II mGluR agonist (LY379268) on behavior and spinal dorsal horn neuronal activity, using the kaolin/carrageenan-induced knee joint arthritis pain model. Audible and ultrasonic vocalizations (emotional responses) and mechanical reflex thresholds were measured in adult rats with and without arthritis (5-6 h postinduction). Extracellular single-unit recordings were made from spinal dorsal horn wide dynamic range neurons of anesthetized (isoflurane) rats with and without arthritis (5-6 h postinduction). Systemic (intraperitoneal) application of a group II mGluR agonist (LY379268) decreased behaviors and activity of spinal neurons in the arthritis pain model but not under normal conditions. Stereotaxic administration of LY379268 into the CeA mimicked the effects of systemic application. Conversely, stereotaxic administration of a group II mGluR antagonist (LY341495) into the CeA reversed the effects of systemic application of LY379268 on behaviors and dorsal horn neuronal activity in arthritic rats. The data show for the first time that the amygdala is the critical site of action for the antinociceptive behavioral and spinal neuronal effects of systemically applied group II mGluR agonists.

Metabotropic glutamate receptors are involved in the detection of IMP and L-amino acids by mouse taste sensory cells

G-protein-coupled receptors are thought to be involved in the detection of umami and L-amino acid taste. These include the heterodimer taste receptor type 1 member 1 (T1r1)+taste receptor type 1 member 3 (T1r3), taste and brain variants of mGluR4 and mGluR1, and calcium sensors. While several studies suggest T1r1+T1r3 is a broadly tuned lLamino acid receptor, little is known about the function of metabotropic glutamate receptors (mGluRs) in L-amino acid taste transduction. Calcium imaging of isolated taste sensory cells (TSCs) of T1r3-GFP and T1r3 knock-out (T1r3 KO) mice was performed using the ratiometric dye Fura 2 AM to investigate the role of different mGluRs in detecting various L-amino acids and inosine 5' monophosphate (IMP). Using agonists selective for various mGluRs such as (RS)-3,5-dihydroxyphenylglycine (DHPG) (an mGluR1 agonist) and L-(+)-2-amino-4-phosphonobutyric acid (l-AP4) (an mGluR4 agonist), we evaluated TSCs to determine if they might respond to these agonists, IMP, and three L-amino acids (monopotassium L-glutamate, L-serine and L-arginine). Additionally, we used selective antagonists against different mGluRs such as (RS)-L-aminoindan-1,5-dicarboxylic acid (AIDA) (an mGluR1 antagonist), and (RS)-α-methylserine-O-phosphate (MSOP) (an mGluR4 antagonist) to determine if they can block responses elicited by these L-amino acids and IMP. We found that L-amino acid- and IMP-responsive cells also responded to each agonist. Antagonists for mGluR4 and mGluR1 significantly blocked the responses elicited by IMP and each of the L-amino acids. Collectively, these data provide evidence for the involvement of taste and brain variants of mGluR1 and mGluR4 in L-amino acid and IMP taste responses in mice, and support the concept that multiple receptors contribute to IMP and L-amino acid taste.

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

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

Progress toward advanced understanding of metabotropic glutamate receptors: structure, signaling and therapeutic indications

The metabotropic glutamate (mGlu) receptors are a group of Class C seven-transmembrane spanning/G protein-coupled receptors (7TMRs/GPCRs). These receptors are activated by glutamate, one of the standard amino acids and the major excitatory neurotransmitter. By activating G protein-dependent and non-G protein-dependent signaling pathways, mGlus modulate glutamatergic transmission both in the periphery and throughout the central nervous system. Since the discovery of the first mGlu receptor, and especially during the last decade, a great deal of progress has been made in understanding the signaling, structure, pharmacological manipulation and therapeutic indications of the 8 mGlu members.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

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

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

mGlu3 receptor and astrocytes: partners in neuroprotection

Astrocytes are currently studied intensively because of their now highlighted relevance as key players with neurons that modulate a wide range of central functions, from synaptic plasticity and synaptogenesis to regulation of metabolic and neuroinflammatory processes. Since the discovery of mGlu3 receptors on astrocytes, accumulating evidence supports a role of these receptors not only in maintaining synaptic homeostasis and treating psychiatric disorders but also in promoting astrocyte survival in several pathologic conditions. This review focuses on providing up-to-date knowledge regarding effects of activating astroglial mGlu3 receptors on psychiatric disorders, astrocyte and neuronal survival, and neurodegenerative diseases. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Functional coupling between metabotropic glutamate receptors and G-proteins in rat cerebral cortex assessed by guanosine-5'-O-(3-[(35)S]thio)triphosphate binding assay

Stimulation of specific guanosine-5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPγS) binding by l-glutamate was pharmacologically characterized in rat cerebral cortical membranes. Optimization of the experimental conditions with respect to the concentrations of GDP, MgCl(2) and NaCl in assay buffer prompted us to adopt the incubation of rat cerebral cortical membranes with 0.2 nM [(35)S]GTPγS at 30°C for 60 min. in the presence of 20 μM GDP, 5 mM MgCl(2) and 100 mM NaCl as a standard condition. Specific [(35)S]GTPγS binding was stimulated by l-glutamate in a concentration-dependent manner but not by ionotropic glutamate receptor agonists. The stimulatory responses were also elicited by many agonists for metabotropic glutamate (mGlu) receptor, with (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylic acid (LY379268) being the most potent. l-glutamate-stimulated [(35)S]GTPγS binding was inhibited by several mGlu antagonists, with (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (LY341495) being the most potent. The pharmacological properties of a series of agonists and antagonists indicated the involvement of group II mGlu receptors, especially mGlu2. Supportive of this notion was the finding that l-glutamate-stimulated specific [(35)S]GTPγS binding was augmented by 2,2,2-trifluoro-N-[4-(2-methoxyphenoxy)phenyl]-N-(3-pyridinylmethyl)ethanesulphonamide hydrochloride (LY487379), a reportedly selective allosteric positive modulator for mGlu2, by means of upward and leftward shift of the concentration-response curve. In addition, LY487379 per se stimulated [(35)S]GTPγS binding, though, through a mechanism different from the stimulation by l-glutamate. Pre-treatment of the membranes with N-ethylmaleimide (NEM) cancelled l-glutamate-stimulated [(35)S]GTPγS binding in a concentration- and incubation time-dependent manner. Taken altogether, l-glutamate-stimulated [(35)S]GTPγS binding serves as a useful functional assay for the activation of NEM-sensitive G(i/o) -mediated group II mGlu receptors in rat cerebral cortical membranes.

Functional plasticity of group II metabotropic glutamate receptors in regulating spinal excitatory and inhibitory synaptic input in neuropathic pain

Metabotropic glutamate receptors (mGluRs) are involved in the modulation of synaptic transmission and plasticity. Group II mGluRs in the spinal cord regulate glutamatergic input, but their functional changes in neuropathic pain are not clear. In this study, we determined the plasticity of spinal group II mGluRs in controlling excitatory and inhibitory synaptic transmission and nociception in neuropathic pain. Neuropathic pain was induced by spinal nerve ligation in rats, and whole-cell voltage-clamp recordings of glutamatergic excitatory postsynaptic currents (EPSCs) and spontaneous and miniature GABAergic and glycinergic inhibitory postsynaptic currents (sIPSCs and mIPSCs, respectively) were performed in spinal cord slices. The specific group II mGluR agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) had a similar inhibitory effect on monosynaptic EPSCs evoked from the dorsal root in sham and nerve-injured rats. However, DCG-IV produced a greater inhibitory effect on evoked polysynaptic EPSCs and the frequency of spontaneous EPSCs in nerve-injured rats than in control rats. Although DCG-IV similarly reduced the frequency of GABAergic sIPSCs and mIPSCs in both groups, it distinctly inhibited the frequency of glycinergic sIPSCs and mIPSCs only in nerve-injured rats. The DCG-IV effect was blocked by the group II mGluR antagonist but not by the N-methyl-D-aspartate receptor antagonist. Strikingly, intrathecal injection of DCG-IV dose-dependently attenuated allodynia and hyperalgesia in nerve-injured rats but produced hyperalgesia in control rats. Our study provides new information that nerve injury up-regulates group II mGluRs present on glutamatergic and glycinergic interneurons in the spinal cord. Activation of group II mGluRs reduces neuropathic pain probably by attenuating glutamatergic and glycinergic input to spinal dorsal horn neurons.

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

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

Neuroprotection and Functional Recovery Associated with Decreased Microglial Activation Following Selective Activation of mGluR2/3 Receptors in a Rodent Model of Parkinson's Disease

Clinical trials have demonstrated positive proof of efficacy of dual metabotropic glutamate receptor 2/3 (mGluR2/3) agonists in both anxiety and schizophrenia. Importantly, evidence suggests that these drugs may also be neuroprotective against glutamate excitotoxicity, implicated in the pathogenesis of Parkinson's disease (PD). However, whether this neuroprotection also translates into functional recovery is unclear. In the current study, we examined the neuroprotective efficacy of the dual mGluR2/3 agonist, 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (2R,4R-APDC), and whether this is accompanied by behavioral recovery in a rodent 6-hydroxydopamine (6-OHDA) model of PD. We now report that delayed post lesion treatment with 2R,4R-APDC (10 nmol), results in robust neuroprotection of the nigrostriatal system, which translated into functional recovery as measured by improved forelimb use asymmetry and reduced (+)-amphetamine-induced rotation compared to vehicle treated animals. Interestingly, these beneficial effects were associated with a decrease in microglial markers in the SNc, which may suggest an antiinflammatory action of this drug.

Involvement of Group II mGluRs in mossy fiber LTD

Mossy fiber long-term depression (LTD) has been shown to be triggered by either pharmacological or synaptic activation of Group II metabotropic glutamate receptors (mGluRs) whereas other studies indicate that synaptic activation of mGluRs is very limited. Therefore, we reexamined the role of Group II mGluRs for the induction of mossy fiber LTD. The complete depression of field potentials (fEPSPs) by 1 microM (2S,2'R,3'R)-2-(2',3'-Dicarboxycyclopropyl)glycine (DCG-IV) only partially reversed upon removal of the drug but fEPSPs were completely restored by the Group II antagonist 2S-2-amino-2-(1S,2S-2-carboxycyclopropyl-1-yl)-3-(xanth-9-yl)propanoic acid (LY341495) (3 microM). In contrast, fEPSPs returned back to baseline within 30 min after a brief application of 0.2 microM DCG-IV suggesting that the incomplete reversal of higher concentrations may be due to a residual receptor occupancy rather than to an induction of LTD. LY341495 itself did not increase fEPSPs and also blocked the inhibition of (2S,1'S,2'S)-2-(2-carboxycyclopropyl)glycine (L-CCG-I) (20 microM) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) (10 microM) and its effect was mimicked by CPPG (50 microM). Furthermore, stimulation at 1 Hz for 15 min induced an LTD of 81% +/- 3% and 80% +/- 4% in the absence and presence of LY341495, respectively (n = 7, 5). Finally, we found that synaptic activation of Group II mGluRs during 15 min of 1-Hz stimulation only produces an inhibition of release by 8% +/- 1% (30 degrees C, n = 3). Our data suggests that pharmacological activation of Group II mGluRs is fully reversible per se and does not produce a long lasting depression and that activation of Group II mGluRs is neither necessary nor sufficient for the induction of mossy fiber LTD.

Glutamate induces neurotrophic factor production from microglia via protein kinase C pathway

Microglia are intrinsic immune cells in the central nervous system and play key roles in the pathogenesis of various central nervous system disorders. Microglia have been shown to attack damaged neurons by secreting a variety of neurotoxic factors including inflammatory cytokines, reactive oxygen species and glutamate. On the other hand, they can produce neurotrophic factors (NTFs) which support neuronal survival and growth. However, the precise mechanism that regulates microglial NTF production is not fully understood, and the relation between glutamate and NTFs remains unclear. In the present study, we show that glutamate significantly induces microglial NTF production by the activation of N-methyl-d-aspartate (NMDA) receptors, group III metabotropic glutamate receptors, and glutamate transporters. Activation of NMDA receptors and group III metabotropic glutamate receptors induces intracellular Ca(2+) release from the endoplasmic reticulum. Further, stimulation of glutamate transporters leads to influx of extracellular Ca(2+) in a Na(+)-dependent manner. This intracellular Ca(2+) elevation activates the protein kinase C pathway which induces microglial NTF expression and production. These results suggest that microglia play a neuroprotective role during the excitotoxic state in neurodegenerative diseases.

The effect of N-acetyl-aspartyl-glutamate and N-acetyl-aspartate on white matter oligodendrocytes

Elevations of the levels of N-acetyl-aspartyl-glutamate (NAAG) and N-acetyl-aspartate (NAA) are associated with myelin loss in the leucodystrophies Canavan's disease and Pelizaeus-Merzbacher-like disease. NAAG and NAA can activate and antagonize neuronal N-methyl-D-aspartate (NMDA) receptors, and also act on group II metabotropic glutamate receptors. Oligodendrocytes and their precursors have recently been shown to express NMDA receptors, and activation of these receptors in ischaemia leads to the death of oligodendrocyte precursors and the loss of myelin. This raises the possibility that the failure to develop myelin, or demyelination, occurring in the leucodystrophies could reflect an action of NAAG or NAA on oligodendrocyte NMDA receptors. However, since the putative subunit composition of NMDA receptors on oligodendrocytes differs from that of neuronal NMDA receptors, the effects of NAAG and NAA on them are unknown. We show that NAAG, but not NAA, evokes an inward membrane current in cerebellar white matter oligodendrocytes, which is reduced by NMDA receptor block (but not by block of metabotropic glutamate receptors). The size of the current evoked by NAAG, relative to that evoked by NMDA, was much smaller in oligodendrocytes than in neurons, and NAAG induced a rise in [Ca²⁺]_i in neurons but not in oligodendrocytes. These differences in the effect of NAAG on oligodendrocytes and neurons may reflect the aforementioned difference in receptor subunit composition. In addition, as a major part of the response in oligodendrocytes was blocked by tetrodotoxin (TTX), much of the NAAG-evoked current in oligodendrocytes is a secondary consequence of activating neuronal NMDA receptors. Six hours exposure to 1 mM NAAG did not lead to the death of cells in the white matter. We conclude that an action of NAAG on oligodendrocyte NMDA receptors is unlikely to be a major contributor to white matter damage in the leucodystrophies.

Sophisticated temporal pattern recognition in retinal ganglion cells

Pattern recognition is one of the most important tasks of the visual system, and uncovering the neural mechanisms underlying recognition phenomena has been a focus of researchers for decades. Surprisingly, at the earliest stages of vision, the retina is capable of highly sophisticated temporal pattern recognition. We stimulated the retina of tiger salamander (Ambystoma tigrinum) with periodic dark flash sequences and found that retinal ganglion cells had a wide variety of different responses to a periodic flash sequence with many firing when a flash was omitted. The timing of the omitted stimulus response (OSR) depended on the period, with individual cells tracking the stimulus period down to increments of 5 ms. When flashes occurred earlier than expected, cells updated their expectation of the next flash time by as much as 50 ms. When flashes occurred later than expected, cells fired an OSR and reset their temporal expectation to the average time interval between flashes. Using pharmacology to investigate the retinal circuitry involved, we found that inhibitory transmission from amacrine cells was not required, but on bipolar cells were required. The results suggest a mechanism in which the intrinsic resonance of on bipolars leads to the OSR in ganglion cells. We discuss the implications of retinal pattern recognition on the neural code of the retina and visual processing in general.

Increased C-fiber nociceptive input potentiates inhibitory glycinergic transmission in the spinal dorsal horn

Glycine is an important inhibitory neurotransmitter in the spinal cord, but it also acts as a coagonist at the glycine site of N-methyl-d-aspartate (NMDA) receptors to potentiate nociceptive transmission. However, little is known about how increased nociceptive inflow alters synaptic glycine release in the spinal dorsal horn and its functional significance. In this study, we performed whole-cell recordings in rat lamina II neurons to record glycinergic spontaneous inhibitory postsynaptic currents (sIPSCs). The transient receptor potential vanilloid receptor 1 agonist capsaicin caused a prolonged increase in the frequency of sIPSCs in 17 of 25 (68%) neurons tested. The potentiating effect of capsaicin on sIPSCs was blocked by ionotropic glutamate receptor antagonists or tetrodotoxin in most lamina II neurons examined. In contrast, the P2X agonist alphabeta-methylene-ATP increased sIPSCs in only two of 16 (12.5%) neurons. The glutamate transporter inhibitor l-trans-pyrrolidine-2,4-dicarboxylic acid either increased or reduced the basal frequency of sIPSCs but did not significantly alter the potentiating effect of capsaicin on sIPSCs. Furthermore, the groups II and III metabotropic glutamate receptor antagonists had no significant effect on the capsaicin-induced increase in the sIPSC frequency. Although capsaicin reduced the amplitude of evoked excitatory postsynaptic currents at high stimulation currents, it did not change the ratio of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/NMDA currents. This study provides the important new information that increased nociceptive inflow augments synaptic glycine release to spinal dorsal horn neurons through endogenous glutamate release. Potentiation of inhibitory glycinergic tone by stimulation of nociceptive primary afferents may function as a negative feedback mechanism to attenuate nociceptive transmission at the spinal level.

Antagonism of group III metabotropic glutamate receptors results in impairment of LTD but not LTP in the hippocampal CA1 region, and prevents long-term spatial memory

Recently it has emerged that hippocampal long-term depression (LTD) may play an important role in the acquisition and storage of spatial memories. This form of synaptic plasticity is tightly regulated by metabotropic glutamate receptors (mGluRs) that negatively couple to adenylyl cyclase. Activation of group III mGluRs is necessary for persistent hippocampal LTD, but is not required for depotentiation or long-term potentiation (LTP) in the dentate gyrus in vivo. In the CA1 region antagonism of group III mGluRs prevents LTD in vivo. Effects on LTP in vivo are as yet unknown. We investigated the effects of group III mGluR antagonism on LTP and LTD at Schaffer collateral-CA1 synapses, and on spatial learning in the eight-arm radial maze. Daily application of the group III mGluR antagonist (R,S)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) resulted in impairment of long-term (reference) memory, with effects becoming apparent 4 days after training and drug treatment began. Short-term (working) memory was unaffected throughout the 10-day study. Application of CPPG prevented LTD, but not LTP, in the CA1 region. These data suggest that activation of group III mGluRs is required for the establishment of spatial long-term memory. Their exclusive role in mediating hippocampal LTD provides correlational evidence for a role for LTD in the type of spatial learning studied.

Modulation of pain transmission by G-protein-coupled receptors

The heterotrimeric G-protein-coupled receptors (GPCR) represent the largest and most diverse family of cell surface receptors and proteins. GPCR are widely distributed in the peripheral and central nervous systems and are one of the most important therapeutic targets in pain medicine. GPCR are present on the plasma membrane of neurons and their terminals along the nociceptive pathways and are closely associated with the modulation of pain transmission. GPCR that can produce analgesia upon activation include opioid, cannabinoid, alpha2-adrenergic, muscarinic acetylcholine, gamma-aminobutyric acidB (GABAB), groups II and III metabotropic glutamate, and somatostatin receptors. Recent studies have led to a better understanding of the role of these GPCR in the regulation of pain transmission. Here, we review the current knowledge about the cellular and molecular mechanisms that underlie the analgesic actions of GPCR agonists, with a focus on their effects on ion channels expressed on nociceptive sensory neurons and on synaptic transmission at the spinal cord level.

Group II metabotropic glutamate receptors reduce excitatory but not inhibitory neurotransmission in rat barrel cortex in vivo

Group II metabotropic (mGlu) receptors are known to play an important role in regulating the release of excitatory transmitter in a number of brain areas. Previous experiments demonstrated that (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) depressed excitatory transmission in the adult rat barrel cortex. Here we show, using in vivo extracellular single unit recordings and iontophoretic application of drugs, that selective activation of Group II mGlu receptors depresses excitatory but not inhibitory transmission. The selective Group II receptor agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (2R,4R-APDC) had similar depressant effects to 1S,3R-ACPD on tactile evoked responses of rapidly adapting neurons. The depressant effects were seen on shorter latency (<12 ms) responses, were most pronounced in layers 3-4 (and 5b for 2R,4R-APDC only), and were reversibly antagonized by the Group II receptor antagonist (2S)-alpha-ethylglutamic acid (EGLU) relative to depressions produced by iontophoretic GABA. Where 1S,3R-ACPD and 2R,4R-APDC depressed excitatory transmission, there was little or no effect on postsynaptic excitations produced by iontophoretic AMPA--a result that supports a presynaptic location of Group II receptors on excitatory terminals. To assess the possible involvement of Group II mGlu receptors in the modulation of inhibition, we studied the effect of iontophoretic 1S,3R-ACPD in a condition-test protocol. The results contrasted markedly from those previously observed using the Group III agonist L(+)-2-amino-4-phosphonobutyric acid in that activation of Group II receptors using 1S,3R-ACPD did not modulate inhibition. Therefore our results show that Group II mGlu receptors play an important role in modulating excitatory, but not inhibitory, transmission. We propose that the Group II mGlu receptors are located on excitatory terminals, and act as autoreceptors. Their role appears to be important in the early stages of cortical processing, by keeping excitatory inputs within specified physiological limits, and possibly by mediating depression evidenced during synaptic plasticity.

Activation of group II metabotropic glutamate receptors reduces directional selectivity in retinal ganglion cells

In the mammalian retina, high levels of the group II metabotropic glutamate receptor (mGluR) subtype are expressed in starburst amacrine cells. A prominent role of starburst amacrine cells is the generation of directional selectivity in ON-OFF directionally selective retinal ganglion cells (DS RGCs). Extracellular microelectrodes were used to study the effects of activation of group II mGluRs on the responses of rabbit ON-OFF DS RGCs to a moving light stimulus. Directionally selective responses in these RGCs were substantially reduced by the selective group II mGluR agonist DCG-IV. DCG-IV brought out a response to movement in the null direction that was similar in magnitude and time course to the response to movement in the preferred direction. This effect of DCG-IV was reversed by the group II mGluR antagonist EGLU. Application of EGLU alone failed to alter directional selectivity in the RGCs but did reduce the response to movement in the preferred direction. To determine whether group II mGluRs modulate the release of the neurotransmitter acetylcholine from starburst amacrine cells, the effect of DCG-IV on ON-OFF DS RGCs was examined in the presence of ambenonium, an acetylcholinesterase inhibitor. When applied alone, ambenonium greatly prolonged the responses of ON-OFF DS RGCs to a light stimulus. This effect of ambenonium was completely abolished upon application of DCG-IV. Overall, the results suggest that postsynaptic group II mGluRs have the potential to influence directional selectivity in RGCs by inhibiting transmitter release from starburst amacrine cells.

A temporally distinct role for group I and group II metabotropic glutamate receptors in object recognition memory

Recognition memory, involving the ability to discriminate between a novel and familiar object, depends on the integrity of the perirhinal cortex (PRH). Glutamate, the main excitatory neurotransmitter in the cortex, is essential for many types of memory processes. Of the subtypes of glutamate receptor, metabotropic receptors (mGluRs) have received less study than NMDA receptors; thus, the reported experiments examined the role of mGluRs in familiarity discrimination in the rat PRH. Experiments 1 and 2 assessed the effects of systemic administration of MPEP, a group I mGluR (specifically mGluR5) antagonist, and/or LY341495, a group II mGluR antagonist, on a spontaneous object novelty preference task. Simultaneous antagonism of both group I and II mGluRs impaired familiarity discrimination following a 24-h but not a 15-min delay, while antagonism of either mGluR subtype alone had no effect at either delay. The impairment was in acquisition, as in Experiment 3 coadministration of MPEP and LY341495 did not affect recognition memory performance when administered either after the sample phase or prior to test. The impairment in long-term recognition memory was mediated by mGluRs in the PRH, as localized intracortical antagonism of group I and II mGluRs also produced a deficit (Experiment 4). No evidence was found for an involvement of group III mGluRs in the acquisition of long-term familiarity discrimination (Experiment 5). These findings establish that glutamatergic neurotransmission in the PRH via group I and II mGluRs is crucial for the acquisition, but not for the consolidation or retrieval of long-term object recognition memory.

Place preference induced by nucleus accumbens amphetamine is impaired by local blockade of Group II metabotropic glutamate receptors in rats

Background

The nucleus accumbens (NAc) plays a critical role in amphetamine-produced conditioned place preference (CPP). In previous studies, NAc basal and amphetamine-produced DA transmission was altered by Group II mGluR agents. We tested whether NAc amphetamine CPP depends on Group II mGluR transmission.

Results

NAc injections (0.5 μl/side) of the Group II mGluR antagonist (2 S)- a-ethylglutamic acid (EGLU: 0.01–0.8 μg but not 0.001 μg) impaired CPP. The drug did not block the acute locomotor effect of amphetamine.

Conclusion

Results suggest that Group II mGluRs may be necessary for the establishment of NAc amphetamine-produced CPP. These receptors may also mediate other forms of reward-related learning dependent on this structure.

Enhanced group II mGluR-mediated inhibition of pain-related synaptic plasticity in the amygdala

Background

The latero-capsular part of the central nucleus of the amygdala (CeLC) is the target of the spino-parabrachio-amygdaloid pain pathway. Our previous studies showed that CeLC neurons develop synaptic plasticity and increased neuronal excitability in the kaolin/carrageenan model of arthritic pain. These pain-related changes involve presynaptic group I metabotropic glutamate receptors (mGluRs) and postsynaptic NMDA and calcitonin gene-related peptide (CGRP1) receptors. Here we address the role of group II mGluRs.

Results

Whole-cell current- and voltage-clamp recordings were made from CeLC neurons in brain slices from control rats and arthritic rats (>6 h postinjection of kaolin/carrageenan into the knee). Monosynaptic excitatory postsynaptic currents (EPSCs) were evoked by electrical stimulation of afferents from the pontine parabrachial (PB) area. A selective group II mGluR agonist (LY354740) decreased the amplitude of EPSCs more potently in CeLC neurons from arthritic rats (IC₅₀ = 0.59 nM) than in control animals (IC₅₀ = 15.0 nM). The inhibitory effect of LY354740 was reversed by a group II mGluR antagonist (EGLU) but not a GABA_A receptor antagonist (bicuculline). LY354740 decreased frequency, but not amplitude, of miniature EPSCs in the presence of TTX. No significant changes of neuronal excitability measures (membrane slope conductance and action potential firing rate) were detected.

Conclusion

Our data suggest that group II mGluRs act presynaptically to modulate synaptic plasticity in the amygdala in a model of arthritic pain.

Activation of receptors negatively coupled to adenylate cyclase is required for induction of long-term synaptic depression at Schaffer collateral-CA1 synapses

Chemical LTD (CLTD) of synaptic transmission is triggered by simultaneously increasing presynaptic [cGMP] while inhibiting PKA. Here, we supply evidence that class II, but not III, metabotropic glutamate receptors (mGluRs), and A1 adenosine receptors, both negatively coupled to adenylate cyclase, play physiologic roles in providing PKA inhibition necessary to promote the induction of LTD at Schaffer collateral-CA1 synapses in hippocampal slices. Simultaneous activation of group II mGluRs with the selective agonist (2S,2'R,3'R)-2-(2',3'-dicarboxy-cyclopropyl) glycine (DCGIV; 5 microM), while raising [cGMP] with the type V phosphodiesterase inhibitor, zaprinast (20 microM), resulted in a long-lasting depression of synaptic strength. When zaprinast (20 microM) was combined with a cell-permeant PKA inhibitor H-89 (10 microM), the need for mGluR IIs was bypassed. DCGIV, when combined with a "submaximal" low frequency stimulation (1 Hz/400 s), produced a saturating LTD. The mGluR II selective antagonist, (2S)-alpha-ethylglutamic acid (EGLU; 5 microM), blocked induction of LTD by prolonged low frequency stimulation (1 Hz/900 s). In contrast, the mGluR III selective receptor blocker, (RS)-a-Cyclopropyl-[3- 3H]-4-phosphonophenylglycine (CPPG; 10 microM), did not impair LTD. The selective adenosine A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 100 nM), also blocked induction of LTD, while the adenosine A1 receptor agonist N6-cyclohexyl adenosine (CHA; 50 nM) significantly enhanced the magnitude of LTD induced by submaximal LFS and, when paired with zaprinast (20 microM), was sufficient to elicit CLTD. Inhibition of PKA with H-89 rescued the expression of LTD in the presence of either EGLU or DPCPX, confirming the hypothesis that both group II mGluRs and A1 adenosine receptors enhance the induction of LTD by inhibiting adenylate cyclase and reducing PKA activity.

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

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

Pharmacological blockade of group II metabotropic glutamate receptors reduces the growth of glioma cells in vivo

U87MG human glioma cells in cultures expressed metabotropic glutamate (mGlu) receptors mGlu2 and mGlu3. Addition of the mGlu2/3 receptor antagonist LY341495 to the cultures reduced cell growth, expression of cyclin D1/2, and activation of the MAP kinase and phosphatidylinositol-3-kinase pathways. This is in line with the evidence that activation of mGlu2/3 receptors sustains glioma cell proliferation. U87MG cells were either implanted under the skin (1x10(6) cells/0.5 ml) or infused into the caudate nucleus (0.5x10(6) cells/5 microl) of nude mice. Animals were treated for 28 days with mGlu receptor antagonists by means of subcutaneous osmotic minipumps. Treatments with LY341495 or (2S)-alpha-ethylglutamate (both infused at a rate of 1 mg/kg per day) reduced the size of tumors growing under the skin. Infusion of LY341495 (10 mg/kg per day) also reduced the growth of brain tumors, as assessed by magnetic resonance imaging analysis carried out every seven days. The effect of drug treatment was particularly evident during the exponential phase of tumor growth, that is, between the third and the fourth week following cell implantation. Immunohistochemical analysis showed that U87MG cells retained the expression of mGlu2/3 receptors when implanted into the brain of nude mice. These data suggest that mGlu2/3 receptor antagonists are of potential use in the experimental treatment of malignant gliomas.

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

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

Activation of group II metabotropic glutamate receptors induces depotentiation in amygdala slices and reduces fear-potentiated startle in rats

There is a close correlation between long-term potentiation (LTP) in the synapses of lateral amygdala (LA) and fear conditioning in animals. We predict that reversal of LTP (depotentiation) in this area of the brain may ameliorate conditioned fear. Activation of group II metabotropic glutamate receptors (mGluR II) with DCG-IV induces depotentiation in the LA. The induction of depotentiation is independent of NMDA receptors, L-type Ca++ channels, and calcineurin activity, but requires presynaptic activity and extracellular Ca++. (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) depotentiation is accompanied by a decrease in the frequency but not the amplitude of miniature excitatory post-synaptic currents (mEPSCs) and could be mimicked by endogenously released glutamate. DCG-IV inhibited the release of glutamate evoked by 4-AP but not that evoked by ionomycin, suggesting that the effect of DCG-IV is not mediated by an action downstream of Ca++ entry. Intra-amygdala infusion of mGluR II agonist blocks the consolidation of fear memory measured with fear-potentiated startle. Taken together, the present results characterize the properties of DCG-IV depotentiation and reveal a close parallel between depotentiation in the amygdala slice and the reduction of conditioned fear in animals.

The metabotropic glutamate receptor mGluR3 is critically required for hippocampal long-term depression and modulates long-term potentiation in the dentate gyrus of freely moving rats

Group II metabotropic glutamate receptors (mGluRs) play an important role in the regulation of hippocampal synaptic plasticity in vivo: long-term potentiation (LTP) is inhibited and long-term depression (LTD) is enhanced by activation of these receptors. The contribution, in vivo, of the individual group II mGluR subtypes has not been characterized. We analysed the involvement of the subtype mGluR3 in LTD and LTP. Rats were implanted with electrodes to enable chronic measurement of evoked potentials from medial perforant path-dentate gyrus synapses. Neither the selective mGluR3 agonist, N-acetylaspartylglutamate (NAAG), nor the antagonist beta-NAAG, given intracerebrally, affected basal synaptic transmission. beta-NAAG significantly inhibited LTD expression. NAAG exhibited transient inhibitory effects on the intermediate phase of LTD. Whereas NAAG altered paired-pulse responses, beta-NAAG had no effect, suggesting that antagonism of mGluR3 prevents LTD via a postsynaptic mechanism, whereas agonist activation of mGluR3 modulates LTD at a presynaptic locus. NAAG impaired the expression of LTP, whereas beta-NAAG had no effect. NAAG effects on LTP were blocked by EGLU, a selective group II mGluR antagonist. Our data suggest an essential role for mGluR3 in LTD, and a modulatory role for mGluR3 in LTP, with effects being mediated by distinct pre- and post-synaptic loci.

Group II mGluR-induced long term depression in the dentate gyrus in vivo is NMDA receptor-independent and does not require protein synthesis

Long term depression (LTD) can be induced by low frequency stimulation (LFS) as well as by agonist activation of neurotransmitter receptors. Group II metabotropic glutamate receptors (mGluRs) play an essential role in the regulation of electrically-induced LTD in the hippocampus in vivo: LTD is inhibited by antagonists, and enhanced by agonists of group II mGluRs. Here we investigated induction of LTD by activation of group II mGluRs as well as the cellular mechanisms which might mediate group II mGluR-induced LTD. Rats were implanted with electrodes to enable chronic measurement of evoked potentials from medial perforant path-dentate gyrus synapses. Drug application was made through a cannula implanted into the ipsilateral cerebral ventricle. LTD could be induced by agonist activation of either group II mGluRs, or the group II mGluR subtype, mGluR3. Both, group II mGluR-induced LTD and mGluR3-induced LTD were not abolished by mRNA/protein synthesis inhibition. Furthermore, mGluR3-induced LTD was not inhibited by NMDA receptor antagonists or altered by L-type voltage-gated calcium channel blockers. Our data suggest that sole activation of group II mGluRs can mediate LTD in vivo. Intriguingly, this form of LTD is not dependent on protein synthesis or activation of NMDA receptors.

Group I and II metabotropic glutamate receptors alter brain cortical metabolic and glutamate/glutamine cycle activity: a 13C NMR spectroscopy and metabolomic study

Metabotropic glutamate receptors (mGluR) modulate neuronal function. Here, we tested the effect on metabolism of a range of Group I and II mGluR ligands in Guinea pig brain cortical tissue slices, applying 13C NMR spectroscopy and metabolomic analysis using multivariate statistics. The effects of Group I agonists (S)-3,5-dihydroxyphenylglycine (DHPG) and (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) depended upon concentration and were mostly stimulatory, increasing both net metabolic flux through the Krebs cycle and glutamate/glutamine cycle activity. Only the higher (50 microm) concentrations of CHPG had the opposite effect. The Group I antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA), consistent with its neuroprotective role, caused significant decreases in metabolism. With principal components analysis of the metabolic profiles generated by these ligands, the effects could be separated by two principal components. Agonists at Group II mGluR [(2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) and 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (APDC)] generally stimulated metabolism, including glutamate/glutamine cycling, although this varied with concentration. The antagonist (2S)-alpha-ethylglutamic acid (EGLU) stimulated astrocyte metabolism with minimal impact on glutamate/glutamine cycling. (RS)-1-Aminophosphoindan-1-carboxylic acid (APICA) decreased metabolism at 5 microm but had a stimulatory effect at 50 microm. All ligand effects were separated from control and from each other using two principal components. The ramifications of these findings are discussed.

Glutamate transporters and metabotropic receptors regulate excitatory neurotransmission in the medial entorhinal cortex of the rat

In layer III of the medial entorhinal cortex (mEC), a region that is especially prone to cell damage in Alzheimer's disease, schizophrenia and epilepsy, effects of blocking glutamate uptake on excitatory synaptic transmission were studied. Two competitive glutamate transporter antagonists, TBOA and tPDC, reduced the amplitude of pharmacologically isolated AMPAR and NMDAR mediated EPSPs/EPSCs without changing the time course of the events. This effect was mimicked by tACPD, an agonist of groups I and II metabotropic glutamate receptors (mGluRs). The competitive groups I and II mGluR antagonist MCPG blocked the depression of the EPSC amplitude induced by tACPD and also prevented the effect of either TBOA or tPDC. Furthermore, EGLU, which selectively antagonizes group II mGluRs, blocked the effect of tPDC and LY3414965, a specific group I mGluR antagonist, abolished the reduction of amplitude caused by TBOA. Additionally, application of TBOA increased the paired-pulse index, suggesting a presynaptic mechanism for the depression of EPSP/EPSC amplitude. The present data suggest that glutamate transporters and group I/II mGluRs regulate excitatory synaptic transmission in the mEC. Presynaptic mGluRs may limit excessive glutamate accumulation if uptake becomes compromised.

Activation of Group II and Group III metabotropic glutamate receptors by endogenous ligand(s) and the modulation of synaptic transmission in the superficial superior colliculus

Previous work from this laboratory indicates that Group II/III metabotropic glutamate (mGlu) receptors modulate responses of SC neurones to visual stimuli in vivo. It is thought that tonic levels of glutamate may be sufficient to activate some mGlu receptors. We wished to investigate if these receptors are activated under ambient conditions in SC. Field excitatory postsynaptic potentials (fEPSPs) evoked by optic tract stimulation were recorded from 300 microm slices of the adult pigmented rat superior colliculus at 34 degrees C. The Group II receptor selective agonist LY354740 (100-300 nM) had no significant effect on the peak amplitude of the fEPSP, although it did enhance the late phase of the fEPSP. In order to test for activation of Group II receptors by endogenous ligand, the selective antagonists LY341495 (50 nM) or EGLU (200 microM) were applied: these either enhanced or reduced the fEPSP amplitude. In similar experiments carried out at 22 degrees C, no effect was seen. The fEPSP enhancements, but not the fEPSP reductions, could be occluded by GABA antagonists. Application of higher concentrations of LY341495 (300, 600 nM-known to also affect Group III receptors, particularly mGlu8), or co-application of 50 nM LY341495 and the Group III-selective antagonist CPPG (100 microM) produced enhancements of responses, or counteracted response reductions over those seen with 50 nM LY341495 alone. The predominant Group II receptor in SC is mGlu3. It is known that this can be located presynaptically on GABAergic and glutamatergic terminals, postsynaptically, and on glia. Our results indicate that such receptors are tonically activated by endogenous transmitter, have distinct effects, and influence retino-collicular transmission. Furthermore, there is a segregation of effects where receptors exert some of their effects via modulation of GABAergic circuitry.

Distinct roles of group III metabotropic glutamate receptors in control of nociception and dorsal horn neurons in normal and nerve-injured Rats

Increased glutamatergic input to spinal dorsal horn neurons constitutes an important mechanism for neuropathic pain. However, the role of group III metabotropic glutamate receptors (mGluRs) in regulation of nociception and dorsal horn neurons in normal and neuropathic pain conditions is not fully known. In this study, we determined the effect of the group III mGluR specific agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) on nociception and dorsal horn projection neurons in normal rats and a rat model of neuropathic pain. Tactile allodynia was induced by ligation of L5/L6 left spinal nerves in rats. Allodynia was determined by von Frey filaments in nerve-injured rats. The nociceptive threshold was tested using a radiant heat and a Randall-Selitto pressure device in normal rats. Single-unit activity of ascending dorsal horn neurons was recorded from the lumbar spinal cord in anesthetized rats. An intrathecal (5-30 microg) L-AP4 dose-dependently attenuated allodynia in nerve-injured rats but had no antinociceptive effect in normal rats. Topical spinal application of 5 to 50 microM L-AP4 also significantly inhibited the evoked responses of ascending dorsal horn neurons in nerve-ligated but not normal rats. Furthermore, blockade of spinal group III mGluRs significantly decreased the withdrawal threshold and increased the evoked responses of dorsal horn neurons in normal but not nerve-injured rats. These data suggest that group III mGluRs play distinct roles in regulation of nociception and dorsal horn neurons in normal and neuropathic pain states. Activation of spinal group III mGluRs suppresses allodynia and inhibits the hypersensitivity of dorsal horn projection neurons associated with neuropathic pain.

Group I and II metabotropic glutamate receptors are necessary for the activity-dependent regulation of ribosomes in chick auditory neurons

Elimination of eighth-nerve activity results in the death of 30% of the neurons in the chick cochlear nucleus, nucleus magnocellularis (NM). One early event in this cell death cascade is the disruption of ribosomes in NM neurons which can be observed within 1 h following deafferentation. These rapid changes in ribosomes can be visualized using Y10B, a monoclonal antibody that recognizes ribosomal RNA. Previous studies using a brain slice preparation of the avian brain stem auditory system have shown that activation of metabotropic glutamate receptors (mGluRs) is necessary for the activity-dependent maintenance of Y10B antigenicity. The purpose of the present study was to determine if group I and/or II mGluRs are necessary for this activity-dependent regulation. This was accomplished by selectively blocking group I or II receptors while unilaterally stimulating the auditory nerve in vitro. In normal media, unilateral stimulation of the auditory nerve resulted in darker Y10B immunolabeling of NM neurons on the stimulated side of the slice. The group I antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) and the group II antagonists LY341495 and (S)-alpha-ethylglutamic acid (EGLU) all prevented the activity-dependent difference in Y10B immunolabeling. These data suggest that both group I and II mGluRs play vital roles in the activity-dependent regulation of ribosomes in NM.

Activation of group III metabotropic glutamate receptors presynaptically reduces both GABAergic and glutamatergic transmission in the rat globus pallidus

To investigate the role of group III metabotropic glutamate receptors (mGluRs) in the globus pallidus (GP), whole-cell recordings were performed using rat brain slice preparations. Application of the group III mGluRs specific agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) suppressed the amplitude of striatal stimulation-induced IPSCs and internal capsule stimulation-induced EPSCs in most of the GP neurons that were capable of generating repetitive firing without spike accommodation. The suppression of IPSCs and EPSCs was accompanied by an increase in the paired-pulse ratio. The L-AP4 effects were antagonized by (R,S)-alpha-cyclopropyl-4-phosphophenylglycine, a blocker for group II/III mGluRs. L-AP4 reduced the frequency of mIPSCs and mEPSCs without changing their amplitude distribution. L-AP4 failed to change iontophoretic glutamate induced responses. These results suggest that the subthalamo-pallidal glutamatergic input might homo- and hetero-synaptically control GABAergic and glutamatergic transmission in the GP.

Role of the group III metabotropic glutamate receptor in LTP, depotentiation and LTD in the dentate gyrus of freely moving rats

We investigated whether group III metabotropic glutamate (mGlu) receptors are critically involved in the expression of long-term potentiation (LTP), depotentiation, or long-term depression (LTD) in the dentate gyrus of freely moving rats. Male Wistar rats (7 8 weeks) underwent implantation of stimulating and recording electrodes in the medial perforant path and dentate gyrus granule cell layer, respectively. A cannula was permanently implanted into the ipsilateral cerebral ventricle to enable drug administration. Intracerebral injection of the group III mGlu receptor agonist, L(+)-2-amino-4-phosphonobutanoic acid (AP4), significantly inhibited LTP at a concentration which unaffects basal synaptic transmission. Depotentiation. short-term depression (STD) and LTDwere unaffected by the agonist. The antagonist. (R.S)-r-cyclopropyl-4-phosphonophenylglycine (CPPG), inhibited agonist effects. but had no independent effects on basal synaptic transmission. CPPG did not affect the profile of LTP, depotentiation or STD elicited by low frequency stimulation (LFS) at 0.5 or 3 Hz. but significantly impaired LTD expression (at I Hz) and STD elicited at 5 Hz. These findings suggest that activation of group III mGlu receptors is critically required for LTD. but not LTP or depotentiation in the dentate gyrus and provide evidence for the involvement of separate mechanisms underlying LTD and depotentiation.

Large releasable pool of synaptic vesicles in chick cochlear hair cells

Hearing requires the hair cell synapse to maintain notable temporal fidelity (< or =1 ms) while sustaining neurotransmitter release for prolonged periods of time (minutes). Here we probed the properties and possible anatomical substrate of prolonged neurotransmitter release by using electrical measures of cell surface area as a proxy for neurotransmitter release to study hair cell exocytosis evoked by repetitive stimuli. We observed marked depression of exocytosis by chick tall hair cells. This exocytic depression cannot be explained by calcium current inactivation, presynaptic autoinhibition by metabotropic glutamate receptors, or postsynaptic receptor desensitization. Rather, cochlear hair cell exocytic depression resulted from the exhaustion of a functional vesicle pool. This releasable vesicle pool is large, totaling approximately 8,000 vesicles, and is nearly 10 times greater than the number of vesicles tethered to synaptic ribbons. Such a large functional pool suggests the recruitment of cytoplasmic vesicles to sustain exocytosis, important for maintaining prolonged, high rates of neural activity needed to encode sound.

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

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

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

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

Baseline glutamate levels affect group I and II mGluRs in layer V pyramidal neurons of rat sensorimotor cortex

Possible functional roles for glutamate that is detectable at low concentrations in the extracellular space of intact brain and brain slices have not been explored. To determine whether this endogenous glutamate acts on metabotropic glutamate receptors (mGluRs), we obtained whole cell recordings from layer V pyramidal neurons of rat sensorimotor cortical slices. Blockade of mGluRs with (+)-alpha-amino-4-carboxy-alpha-methyl-benzeacetic acid (MCPG, a general mGluR antagonist) increased the mean amplitude of spontaneous excitatory postsynaptic currents (sEPSCs), an effect attributable to a selective increase in the occurrence of large amplitude sEPSCs. 2S-2-amino-2-(1S,2S-2-carboxycyclopropyl-1-yl)-3-(xanth-9-yl)propanoic acid (LY341495, a group II antagonist) increased, but R(-)-1-amino-2,3-dihydro-1H-indene-1,5-dicarboxylic acid (AIDA) and (RS)-hexyl-HIBO (group I antagonists) decreased sEPSC amplitude, and (R,S)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG, a group III antagonist) did not change it. The change in sEPSCs elicited by MCPG, AIDA, and LY341495 was absent in tetrodotoxin, suggesting that it was action potential-dependent. The increase in sEPSCs persisted in GABA receptor antagonists, indicating that it was not due to effects on inhibitory interneurons. AIDA and (S)-3,5-dihydroxyphenylglycine (DHPG, a group I agonist) elicited positive and negative shifts in holding current, respectively. LY341495 and (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV, a group II agonist) elicited negative and positive shifts in holding current, respectively. The AIDA and LY341495 elicited currents persisted in TTX. Finally, in current clamp, LY341495 depolarized cells by approximately 2 mV and increased the number of action potentials to a given depolarizing current pulse. Thus ambient levels of glutamate tonically activate mGluRs and regulate cortical excitability.