Metabotropic glutamate receptors are involved in the control of breathing at the medulla oblongata level of anaesthetized rats

The goal of the present study was to identify sites in the medulla oblongata where metabotropic glutamate receptors are involved in regulating respiration. Unilateral microinjections (50 nl) of L-glutamate (L-glu) (10-25-50 mM) into the nucleus tractus solitarii (NTS) of anaesthetized rats elicited apnea (8.6 +/- 0.3 sec; 21.3 +/- 3.6 sec; 66.3 +/- 16.5 sec respectively; N = 6) and arterial hypotension (7.3 +/- 2.4 mmHg; 10.1 +/- 2.3 mmHg; 35.3 +/- 7.5 mmHg respectively; N = 6). Similarly, in other rats 1-aminocyclopentane-1, 3-dicarboxylic acid (ACPD) (1-5-10 mM), a selective agonist of metabotrophic glutamate receptors, also induced apnea (22.4 +/- 2.5 sec; 32.5 +/- sec; 92.5 +/- 1.4 sec respectively; N = 6) and arterial hypotension (12.7 +/- 2.2 mmHg; 19.6 +/- 4.3 mmHg; 26.5 +/- 1.5 mmHg respectively; N = 6). Paired experiments showed that unilateral microinjections of L-glu (50 mM) and ACPD (1 mM) into the nucleus retroambigualis (NRA) of anaesthetized rats elicited apnea (20.2 +/- 2.6 sec and 33.8 +/- 3.2 sec respectively; N = 6) and arterial hypotension (15.7 +/- 3.7 mmHg and 22.5 +/- 4.5 mmHg respectively; N = 6). The ACPD effects on apnea and hypotension in NTS and NRA were not prevented by a 3 min pretreatment with L-AP3 (30 mM), a putative antagonist of metabotropic glutamate receptors (19.5 +/- 1.4 sec; 12.3 +/- 3.2 mmHg and 30.6 +/- 2.9 sec; 23.4 +/- 3.8 mmHg respectively; N = 6). These data suggest that metabotropic glutamate receptors are involved in NTS and NRA regulation of cardiorespiratory functions.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of metabotropic glutamate receptors differentially affects two classes of hippocampal interneurons and potentiates excitatory synaptic transmission

Based on responses to metabotropic glutamate receptor (mGluR) activation, we have characterized two distinct classes of interneuron in stratum (st.) oriens of the CA1 region of hippocampus. One type of interneuron was strongly excited by 1S,3R-aminocyclopentane dicarboxylic acid (ACPD), responding with a large inward current accompanied by increased baseline noise and prominent current oscillations. A second interneuron population responded with a modest inward current with no changes in baseline noise. These two classes of responses persisted in the presence of tetrodotoxin and antagonists of ionotropic glutamate and GABA receptors, suggesting that the inward currents result from mGluRs on the interneurons themselves. The two physiologically defined cell types correspond to two distinct morphological cell types in st. oriens/alveus, distinguished by very different patterns of local axonal connections. Large oscillatory inward current responses were recorded predominantly from an interneuron type whose axons heavily innervated st. lacunosum. The more modest inward current response was generally found in interneurons whose axons innervated the somata and proximal dendrites of CA1 pyramidal neurons. These differences in physiology and local circuitry imply that activation of mGluRs in st. oriens will cause very strong excitation of interneurons synapsing in st. lacunosum, and weaker excitation of interneurons innervating pyramidal cells at the soma and proximal dendrites. These data suggest that each interneuron population has a specific role in hippocampal function, and that mGluR activation will affect the local circuit differently for each interneuron type. Metabotropic GluR activation also markedly enhanced the amplitudes of the evoked and spontaneous EPSCs received by all interneurons in the region, independent of changes in the postsynaptic holding current and with no change in the kinetics of the EPSC. In contrast to the enhancement of evoked and spontaneous EPSCs, miniature EPSCs recorded in the presence of tetrodotoxin were not increased. These data suggest that ACPD acts at a presynaptic site to potentiate the EPSC. Taken together, these results highlight an important modulatory role for metabotropic receptors located at sites both pre- and postsynaptic to CA1 st. oriens interneurons.

Glutamate metabotropic receptor inhibition of voltage-gated calcium currents in visceral sensory neurons

1. Metabotropic glutamate receptors (mGluRs) have been suggested to modulate neurotransmission of glutamatergic pathways via autoreceptive action. Visceral sensory afferents and baroreceptor afferents in particular are thought to utilize L-glutamate (L-glu) as a primary neurotransmitter. The purpose of this study was to investigate whether visceral sensory afferents possess a mGluR and determine the effect of mGluR activation on voltage-gated calcium currents in these neurons. 2. Activation of mGluRs by the selective agonist trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD) reversibly suppressed the voltage-gated calcium currents in visceral sensory afferents of the nodose ganglion. Concentrations of t-ACPD ranging from 50 to 1,000 microM consistently decreased the evoked calcium current with a maximum suppression of the peak current of 25-30%. This response was repeatable and reversible within a given cell. 3. Metabotropic GluR activation selectively decreased the high-threshold calcium current evoked from step potentials greater than -30 mV and had no effect on the low-threshold calcium current. The inhibitory effects of t-ACPD on the high-threshold channel was partially blocked by omega-conotoxin (omega-CTx-GVIA) suggesting that at least part of the effects of mGluR inhibition of the voltage-gated calcium current is because of a modulation of the omega-CTx-GVIA sensitive high-threshold current. 4. Finally, the inhibitory effects of quisqualate (quis) on the high-threshold calcium current were blocked by pretreatment of the neurons with pertussis toxin (PTX). These results suggest that visceral sensory afferents do possess a PTX-sensitive mGluR and activation of this receptor results in the inhibition of a omega-CTx-GVIA sensitive high-threshold calcium channel.

Pharmacological similarity between the retinal APB receptor and the family of metabotropic glutamate receptors

1. We performed current-clamp and voltage-clamp experiments in the amphibian retina to examine the effects of 1-amino-1,3-cyclopentanedicarboxylic acid (1S,3R ACPD), which is a selective agonist for the family of metabotropic glutamate receptors. 2. 1S,3R ACPD was found to selectively block the light responses of ON bipolar cells. It did not suppress the responses of horizontal cells of OFF bipolar cells. It blocked ON but not OFF responses of third-order retinal neurons. 3. 1S,3R ACPD mimicked the effect of the photoreceptor transmitter at the ON bipolar synapse. It reduced an inward current by a decrease in conductance. 4. The action of 1S,3R ACPD was very similar to that of 2-amino-4-phosphonobutyrate (APB) both in terms of effects on the ON bipolar cell potential and conductance and in terms of the retinal network. This suggests that the APB receptor is a predominant synaptic metabotropic glutamate receptor in the retina. 5. The rank-order potency at the retinal APB receptor is APB > 1S,3R ACPD > ibotenate. Quisqualate appears to be inactive at this receptor. The pharmacology of the retinal APB receptor matches that of the cloned mGluR4 and mGluR6 metabotropic glutamate receptors. On the basis of the in situ localization of mGluR6 to the inner nuclear layer of the retina, the retinal APB receptor may be this cloned receptor protein. 6. The effects of the three other ACPD stereo isomers were examined. 1S,3S ACPD was a weak agonist at the APB receptor. 1R,3R ACPD was a potent agonist in the inner retina, but inactive in the outer retina. This fits the profile of N-methyl-D-aspartate agonists. 1R,3S ACPD was inactive.

Glutamate-induced calcium signaling in astrocytes

Astrocytes respond to the excitatory neurotransmitter glutamate with dynamic spatio-temporal changes in intracellular calcium [Ca2+]i. Although they share a common wave-like appearance, the different [Ca2+]i changes--an initial spike, sustained elevation, oscillatory intracellular waves, and regenerative intercellular waves--are actually separate and distinct phenomena. These separate components of the astrocytic Ca2+ response appear to be generated by two different signal transduction pathways. The metabotropic response evokes an initial spatial Ca2+ spike that can propagate rapidly from cell to cell and appears to involve IP3. The metabotropic response can also produce oscillatory intracellular waves of various amplitudes and frequencies that propagate within cells and are sustained only in the presence of external Ca2+. The ionotropic response, however, evokes a sustained elevation in [Ca2+]i associated with receptor-mediated Na+ and Ca2+ influx, depolarization, and voltage-dependent Ca2+ influx. In addition, the ionotropic response can lead to regenerative intercellular waves that propagate smoothly and nondecrementally from cell to cell, possibly involving Na+/Ca2+ exchange. All these astrocytic [Ca2+]i changes tend to appear wave-like, traveling from region to region as a transient rise in [Ca2+]i. Nevertheless, as our understanding of the cellular events that underlie these [Ca2+]i changes grows, it becomes increasingly clear that glutamate-induced Ca2+ signaling is a composite of separate and distinct phenomena, which may be distinguished not based on appearance alone, but rather on their underlying mechanisms.

Synaptic plasticity: LTP and LTD

Long-term potentiation (LTP) is a synaptic enhancement that follows brief, high-frequency electrical stimulation in the hippocampus and neocortex. Recent evidence suggests that induction of LTP may require, in addition to postsynaptic Ca2+ entry, activation of metabotropic glutamate receptors and the generation of diffusible intercellular messengers. A new form of synaptic plasticity, homosynaptic long-term depression (LTD) has also recently been documented, which, like LTP, requires Ca2+ entry through the NMDA receptor. Current work suggests that this LTD is a reversal of LTP, and vice versa, and that the mechanisms of LTP and LTD may converge at the level of specific phosphoproteins.

The depolarization-induced outflow of D-[3H]aspartate from rat brain slices is modulated by metabotropic glutamate receptors

Rat brain slices were used to study the effects of different metabotropic glutamate receptor ligands on (i) the depolarization (30 mM KCl)-induced outflow of previously taken up D-[3H]aspartate; (ii) the inhibition of forskolin (30 microM)-induced cyclic AMP accumulation; and (iii) the hydrolysis of phosphoinositides. In addition, the localization of mRNAs coding for different metabotropic glutamate receptor subtypes was detected using in situ hybridization. (1S-3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (30-300 microM), a non selective metabotropic glutamate receptor agonist, significantly increased the KCl-induced output of radioactivity from cortical slices, whereas it inhibited the output from striatal slices. Conversely, (1S,3S,4S)-carboxycyclopropylglycine (0.1-1 microM), a relatively selective agonist of the mGluR2 metabotropic glutamate receptor subtype, had an inhibitory effect on the output of D-[3H]aspartate from both cortical and striatal slices and proved to be the most potent metabotropic glutamate receptor agonist in inhibiting cyclic AMP accumulation, but not in stimulating phosphoinositide hydrolysis. Since 2-amino-4-phosphonobutyrate (a mGluR4, mGluR6 and mGluR7 agonist) was not active in any of the assays tested, we hypothesized that the mGluR2 subtype could be involved in these events. Accordingly, mGluR2 mRNA expression was abundant in cortical neurons projecting to the striatum. Our experiments suggest that the stimulation of metabotropic glutamate receptors may either decrease or increase transmitter release depending on the subtype that prevails in the region under study.

Glutamate stimulates the phosphorylation of glial fibrillary acidic protein in slices of immature rat hippocampus via a metabotropic receptor

Phosphorylation of the astrocyte cell marker glial fibrillary acidic protein (GFAP) in hippocampal slices from immature rats (10-16 days postnatal) was strongly stimulated by glutamate in the presence of Ca2+. This effect apparently occurred via a metabotropic receptor since the specific agonist of metabotropic glutamate receptors, 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), stimulated GFAP phosphorylation by 173% whilst the mixed agonists, ibotenate and quisqualate, stimulated to a lesser extent. Ionotropic agonists were mainly ineffective. The action of 1S,3R-ACPD was blocked by L(+)-2-amino-3-phosphonopropionic acid (L-AP3) a specific antagonist of the metabotropic glutamate receptor coupled to the hydrolysis of phosphoinositides and was reduced by 70% by preincubation of the slices with pertussis toxin. In contrast to these results with immature animals glutamate had little or no effect on the phosphorylation of GFAP in hippocampal slices from adult rats.

Is the heterologous expression of metabotropic glutamate receptors (mGluRs) an appropriate method to study the mGluR function? Experience with human embryonic kidney 293 cells transfected with mGluR1

The cloning of metabotropic glutamate receptors (mgluRs) has initiated a new approach to the study of their function: the introduction of mGluR cDNA into cells that do not normally express mGluRs, thus allowing the heterologous receptor expression. We have transfected human embryonic kidney (HEK) 293 cells with the full length mGluR1a cDNA and with its truncated variant which encodes the receptor termed mGluR1T (a receptor lacking the long intracellular domain and similar to the splice variant mGluR1c). Transient transfection of HEK-293 cells with mGluR1a, but not the mGluR1T cDNA, resulted in a significant increase in inositol phosphate (IP) formation in absence of any mGluR agonists. This effect was completely dependent on the presence of extracellular calcium, and unlike the agonist-stimulated IP formation it was insensitive to pertussis toxin. The prolonged activation of IP formation might affect the cell physiology. In an attempt to obtain stably transfected cells, we transfected about 1.5 x 10(6) HEK-293 cells with the plasmid conveying the full-length mGluR1a cDNA and the neomycin-resistance gene. Only 12 clones survived the antibiotic selection, and only one of these 12 clones continued to divide. The size of mRNA from the clone was smaller than the full-length mGluR1a mRNA. The shortened mRNA, revealed in the clone, apparently encoded a functional mGluR that was sensitive to glutamate, but unlike the mGluR1a, it did not respond to 1S,3R-ACPD (1S,3R-aminocyclopentane-1,3-dicarboxylic acid). A prudent use of the heterologous cell transfection technique is necessary in studying the function and the pharmacology of mGluRs.

Interaction between metabotropic receptors and purinergic transmission in rat hippocampal slices

Inhibition of forskolin-stimulated cAMP formation by (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) in rat hippocampal slices was partially obliterated by the adenosine-depleting enzyme, adenosine deaminase, or by the adenosine receptor agonist, 5'-(N-ethylcarboxamido)-adenosine, suggesting that activation of metabotropic glutamate receptors (mGluRs) modulates the release of endogenous adenosine. Consistent with this hypothesis, forskolin stimulated the release of purines from rat hippocampal slices, and this effect was reduced by 1S,3R-ACPD. To establish which transduction pathway is involved in the modulation of forskolin-stimulated purine release, we have tested the novel mGluR2 agonist, (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV), which reduced forskolin-stimulated cAMP formation but, as opposed to 1S,3R-ACPD, did not stimulate polyphosphoinositide hydrolysis. DCG-IV was highly potent and more efficacious than 1S,3R-ACPD in inhibiting forskolin-stimulated purine release. Neither DCG-IV nor 1S,3R-ACPD reduced the release of purines stimulated by depolarizing concentrations of K+, suggesting that their effect was stimulus-specific. These results indicate that, in rat hippocampal slices, activation of mGluR2 receptors attenuates the release of purines induced by forskolin, a process that amplifies the final effect of forskolin on cAMP formation as a result of A2 purinergic receptor activation. Thus, the final effect of mGluR agonists on forskolin-stimulated cAMP formation in hippocampal slices depends on both a direct inhibition of adenylyl cyclase and the inhibition of adenosine release.

Analysis of agonist and antagonist activities of phenylglycine derivatives for different cloned metabotropic glutamate receptor subtypes

The metabotropic glutamate receptors (mGluRs) consist of at least seven different subtypes and are coupled to intracellular signal transduction via G proteins. However, the lack of specific antagonists for the mGluRs limited the precise characterization of the role of the individual mGluRs. In this study, we investigated the agonist and antagonist activities of a series of phenylglycine derivatives for the mGluRs by examining their effects on the signal transduction of representative mGluR1, mGluR2, and mGluR4 subtypes expressed individually in Chinese hamster ovary cells. The phenylglycine derivatives examined included (S)- and (R)-forms of 3-hydroxyphenylglycine (3HPG), 4-carboxy-phenylglycine (4CPG), 4-carboxy-3-hydroxyphenylglycine (4C3HPG), 3-carboxy-4-hydroxyphenylglycine (3C4HPG), and (+)- and (-)-alpha-methyl-4-carboxyphenylglycine (alpha M4CPG). Among these 10 compounds, (S)-3HPG acted as an agonist for mGluR1, while (S)-4C3HPG, (S)-3C4HPG, and (S)-4CPG served as effective agonists for mGluR2. The rank order of agonist potencies for mGluR2 was L-glutamate > (S)-4C3HPG > (S)-3C4HPG > (S)-4CPG. No other phenylglycine derivatives showed any definite agonist activity on either mGluR1 or mGluR2. Among the phenylglycine derivatives with no mGluR1 agonist activity, (S)-4C3HPG, (S)-3C4HPG, (S)-4CPG, and (+)-alpha M4CPG effectively antagonized the action of L-glutamate on mGluR1. The rank order of antagonist potencies was (S)-4C3HPG > or = (S)-4CPG > or = (+)-alpha M4CPG > (S)-3C4HPG. The Schild plot analysis indicated that (RS)-4C3HPG, (S)-4CPG, and (+)-alpha M4CPG all act as competitive antagonists for mGluR1 with pA2 values of 4.38, 4.46, and 4.38, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Potentiation of a metabotropic glutamatergic response following NMDA receptor activation in rat hippocampus

Interactions between metabotropic glutamate and N-methyl-D-aspartate (NMDA) receptor-mediated responses were investigated in hippocampal CA3 cells using the single electrode voltage-clamp method. Bath application (2.5-10 microM, 30 s) or iontophoresis of 1-amino-cyclopentyl-trans-1S,3R-dicarboxylate (ACPD), a selective agonist for metabotropic glutamate receptors, resulted in an inward current associated with a decrease in membrane conductance. Following transient bath application of NMDA (5-10 microM, 30-60 s), the ACPD-induced inward current was potentiated for a period of up to 25 min (by 61 +/- 8% with bath application, by 32 +/- 15% with iontophoresis). Transient application of NMDA did not result in a potentiation of ionotropic RS-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or metabotropic muscarinic responses. ACPD responses were not potentiated following transient AMPA application. Intracellular buffering of calcium with tetrapotassium bis(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA) prevented potentiation by NMDA in all cells. Bath application of arachidonic acid did not mimic the NMDA-induced potentiation. These results demonstrate that activation of NMDA receptors can specifically induce a long-lasting potentiation of a metabotropic glutamatergic response in hippocampal CA3 pyramidal cells. The characterization of this interaction may contribute to the elucidation of the physiological significance of metabotropic glutamate receptors.

Novel functions for subtypes of metabotropic glutamate receptors

Metabotropic or "G-protein coupled" glutamate receptors (mGluRs) were discovered and established as a new type of excitatory amino acid receptor by their unique coupling mechanism (phosphoinositide hydrolysis) and pharmacological characteristics. Recently, the cloning of mGluRs and the availability of selective pharmacological agents has greatly increased knowledge of these receptors. It is now recognized that mGluRs are a highly heterogenous family of glutamate receptors with novel molecular structure that are linked to multiple second messenger pathways. Members of this family have unique pharmacological properties and function to modulate the presynaptic release of glutamate and the post-synaptic sensitivity of the cell to glutamate excitation. New information on mGluRs is elucidating the functions of mGluR subtypes in normal and pathological aspects of neuronal transmission. Basic knowledge of the role of specific mGluRs in CNS function and pathologies will further expand in the near future. This knowledge is providing the framework for the discovery of novel pharmacological approaches to modulate excitatory amino acid neuronal transmission.

The function of metabotropic excitatory amino acid receptors in synaptic transmission in the thalamus: studies with novel phenylglycine antagonists

The phenylglycines 3-hydroxyphenylglycine, 4-carboxy-3-hydroxy-phenylglycine (4C3HPG), 4-carboxyphenylglycine (4CPG) and alpha-methyl-4-carboxyphenylglycine (MCPG) were evaluated as putative selective antagonists of metabotropic glutamate receptors on single neurones of the ventrobasal thalamus of rats, with a view to using these compounds as tools to elucidate synaptic mechanisms in this brain area. The S-isomers of the latter three compounds were found to reduce excitations evoked by iontophoretically applied 1S,3R-ACPD, but not those evoked by ionotropic excitatory amino receptor agonists. When the antagonists were tested against sensory synaptic responses of ventrobasal neurones, it was found that responses evoked by noxious thermal stimulation of the peripheral receptive field were reduced in parallel with responses to 1S,3R-ACPD. In contrast, responses of neurones evoked by non-noxious (air-jet) stimuli were not reduced by the phenylglycine antagonists and 4C3HPG was found to enhance such responses, possibly by a presynaptic action mediated via mGluR2 receptors. The reductions of nociceptive responses are discussed in the context of antagonism of mGluR1 receptors, which are known to be numerous in the thalamus and located on post-synaptic dendrites. The involvement of such receptors in the nociceptive responses of thalamic neurones may be of considerable functional significance.

A molecular switch activated by metabotropic glutamate receptors regulates induction of long-term potentiation

Pharmacological studies of long-term potentiation (LTP) in the hippocampus are starting to provide a molecular understanding of synaptic plastic processes which are believed to be important for learning and memory in vertebrates. In the CA1 region of the hippocampus, the synaptic activation of glutamate receptors of the N-methyl-D-aspartate (NMDA) subtype is necessary for the induction of LTP under most experimental conditions. The synaptic activation of metabotropic glutamate receptors (mGluRs) is also needed for the induction of LTP. We now show that the role of mGluRs in the induction of LTP is fundamentally different from that of NMDA receptors. NMDA receptors initiate a molecular event that needs to be triggered each time a tetanus is delivered to induce LTP. In contrast, mGluRs activate a molecular switch which then negates the need for mGluR stimulation during the induction of LTP. This mGluR-activated switch is input-specific and can be turned off by a train of low-frequency stimulation. The molecular switch is a new feature of LTP which has fundamental consequences for our understanding of synaptic plastic mechanisms.

Induction of cerebellar long-term depression requires activation of glutamate metabotropic receptors

In rat cerebellar slices, 500 microM (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG) reversibly inhibited both dendritic and somatic increases in FLUO-3 fluorescence intensity induced by bath applications of 50-100 microM (+-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD). No effect of MCPG was observed on dendritically recorded excitatory postsynaptic potentials evoked by synaptic activation of either parallel or climbing fibres. Long-term depression of parallel fibre-Purkinje cell transmission, induced either by conjunctive activation of parallel and climbing fibres or by pairing parallel fibre stimulation with intradendritic injections of 8-BrcGMP, was not only prevented in the presence of MCPG but a robust long-term potentiation of responses consistently occurred. These data show that metabotropic glutamate receptor activation is necessary for the induction of LTD.

Metabotropic glutamate response in acutely dissociated hippocampal CA1 pyramidal neurones of the rat

1. The metabotropic glutamate (mGlu) response was investigated in dissociated rat hippocampal CA1 pyramidal neurones using conventional and nystatin-perforated whole-cell modes of the patch recording configuration. 2. In the perforated patch recording configuration, the application of glutamate (Glu), quisqualate (QA), aspartate (Asp) and N-methyl-D-aspartate (NMDA) induced a slow outward current superimposed on a fast ionotropic inward current, whereas alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and kainate (KA) induced only an ionotropic inward current at a holding potential (VH) of -20 mV. A specific agonist of the mGlu receptor (mGluR), trans-1-aminocyclopentane-1,3-dicarboxylate (tACPD), induced an outward current in approximately 80% of the neurones tested. Asp- and NMDA-induced outward currents were antagonized by D-2-amino-5-phosphonopentanoate (D-AP5) whereas Glu-, QA- and tACPD-induced outward currents were not antagonized by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 6,7-dinitroquinoxaline-2,3-dione (DNQX) and D-AP5, indicating that the mGlu response is an outward current component. 3. L-2-Amino-3-phosphonopropionate (L-AP3) and DL-2-amino-4-phosphonobutyrate (AP4) did not block the mGlu response. 4. The relative potencies of mGlu agonists were QA > Glu > tACPD. The threshold and EC50 values of metabotropic outward currents were 10-100 times lower than those of the ionotropic inward current (iGlu response). 5. The reversal potential of the mGlu response (EmGlu) was close to EK (K+ equilibrium potential), and it shifted 59.5 mV for a tenfold change in extracellular K+ concentration. 6. In Ca(2+)-free external solution, the mGlu response was elicited by an initial application of Glu, but subsequent applications failed to induce the response. There was also an increase in the intracellular free Ca2+ concentration ([Ca2+]i) during the application of Glu and QA but not of AMPA, indicating Ca2+ release from an intracellular Ca2+ store. 7. During the activation of a Ca(2+)-dependent K+ current (IK(Ca)) by inositol trisphosphate (IP3) in the internal solution, the mGlu response was suppressed. Addition of GDP-beta-S, neomycin or heparin to the internal solution also suppressed the mGlu response, but staurosporine had no effect. The mGlu response was abolished by pretreatment with either caffeine or ryanodine, but treatment with pertussis toxin (IAP) for 6-8 h had no effect. 8. The mGlu response was suppressed by tetraethylammonium, but not by either apamin or iberiotoxin, suggesting that intermediate-conductance Ca(2+)-dependent K+ (KCa+) channels are involved.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation by metabotropic glutamate receptor of long-term potentiation of population spikes in the dentate gyrus in vivo

The effect of 2-amino-3-phosphonopropionate (AP3), a metabotropic glutamate receptor antagonist, on long-term potentiation (LTP) of evoked potentials in the dentate gyrus was investigated using anesthetized rats. Intracerebroventricular injection of AP3 (50 nmol) prior to application of tetanic stimulation did not significantly affect LTP induced by 100-pulse, 100-Hz tetanus, but inhibited the formation of LTP by weaker tetanus (30 pulses at 60 Hz). When AP3 was injected after tetanus, it did not significantly affect the established LTP. These results suggest that metabotropic glutamate receptors play a role in facilitation of the formation of LTP in the dentate gyrus in vivo.

Evidence for a role of metabotropic glutamate receptors in sustained nociceptive inputs to rat dorsal horn neurons

Several antagonists at metabotropic glutamate (mGlu) receptors, when applied ionophoretically, inhibited the excitation of single dorsal horn neurons elicited by cutaneous administration of the C fibre-selective algogen, mustard oil. The selectivity and stereospecificity of AP3 isomers at mGlu, compared to NMDA receptors was confirmed on responses to agonists and matched by their effects on mustard oil-evoked activity.

Neuron damage induced by some potent kainoids and neuroprotective action of new agonists for metabotropic glutamate receptors

Selective and characteristic neuron damage induced by acromelic acid, a potent kainate analogue, was investigated in comparison to a kainate-induced one. A single systemic injection of acromelic acid A caused behavioral and pathological effects distinct from those seen after systemic kainate. There was an initial marked tonic extension of the rat hindlimb, often followed by convulsions and, in surviving rats, by a transient flaccid paralysis and ultimately, a persistent spastic paraplegia. Pathological examination suggested specific lesions of interneurons in the lower spinal cord with little or no damage to the hippocampal neurons preferentially affected by systemic kainate. Another agonist for kainate-type receptors, which is not a kainoid, demonstrated neurological symptoms and neuron damage quite similar to those of kainate. Pharmacological actions of our newly developed agonists for metabotropic glutamate receptors were described with special reference to kainate excitotoxicity. Intraventricular DCG-IV, a new agonist, caused selective neuron damage in the cingulate cortex and the hippocampal subiculum at relatively high doses, but other agonists did not cause neuron damage in the rat. DCG-IV considerably alleviated the kainate-induced limbic seizures. At relatively low doses, DCG-IV protected some kinds of neurons in the hippocampal CA3 and the amygdala against kainate neurotoxicity, when intraventricularly injected to the rat. These new agonists would provide useful probe for elucidating the mechanism underlying neuron damage induced by kainate-type agonists.

Molecular characterization of NMDA and metabotropic glutamate receptors

Our molecular studies have revealed the existence of a large number of different subunits or subtypes for the NMDA and metabotropic glutamate receptors. The individual receptors show functional variabilities and distinct expression patterns in the CNS. The NMDA receptors belong to the ligand-gated ion channel family and consist of a key subunit NMDAR1 and four accessory subunits NMDAR2A-NMDAR2D. The combination of NMDAR1 and NMDAR2 in heteromeric configurations potentiates glutamate response and produces a functional variability. All the NMDAR subunits have an asparagine residue at the corresponding position of the second transmembrane segments, and these residues are thought to be responsible for controlling Ca2+ permeation and the channel blockade by Mg2+ and cationic channel blockers. Individual NMDAR subunit mRNAs are different in their expression patterns during development and in the adult brain. The mGluR family consists of at least six different subtypes. These subtypes are divided into three subgroups according to their sequence similarities, signal transduction mechanisms, and pharmacological properties. Although their physiological roles largely remain to be elucidated, the retinal L-AP4-sensitive mGluR may have a specific function that mediates excitatory neurotransmission in the visual system. It is thus undoubtedly important to investigate specific functions of different combinations of the NMDA receptor subunits and different subtypes of mGluRs and to explore the molecular mechanisms of glutamate receptor-mediated neuronal plasticity and neurotoxicity.