Effects Of Metabotropic Glutamate Receptor Research Articles

Effects of metabotropic glutamate receptor 2/3 agonism and antagonism on schizophrenia-like cognitive deficits induced by phencyclidine in rats

Dysregulation of glutamate neurotransmission may play a role in cognitive deficits in schizophrenia. Manipulation of glutamate signaling using drugs acting at metabotropic glutamate receptors has been suggested as a novel approach to treating schizophrenia-related cognitive dysfunction. We examined how the metabotropic glutamate receptor 2/3 agonist LY379268 and the metabotropic glutamate receptor 2/3 antagonist LY341495 altered phencyclidine-induced disruptions in performance in the 5-choice serial reaction time task. This test assesses multiple cognitive modalities characteristically impaired in schizophrenia that are disrupted by phencyclidine administration. Acute LY379268 alone did not affect 5-choice serial reaction time task performance, except for nonspecific response suppression at high doses. Acute LY379268 administration exacerbated phencyclidine-induced disruption of attentional performance in this task, while acute LY341495 did not alter 5-choice serial reaction time task performance during phencyclidine exposure. Chronic LY341495 impaired attentional performance in the 5-choice serial reaction time task by itself, but attenuated phencyclidine-induced excessive timeout responding. The mixed effects of metabotropic glutamate receptor 2/3 agonism and antagonism on cognitive performance under baseline conditions and after disruption with phencyclidine demonstrate that different aspects of cognition may respond differently to a given pharmacological manipulation, indicating that potential antipsychotic or pro-cognitive medications need to be tested for their effects on a range of cognitive modalities. Our findings also suggest that additional mechanisms, besides cortical glutamatergic transmission, may be involved in certain cognitive dysfunctions in schizophrenia.

EFFECTS OF NAAG AND MPEP ON RAT CORTICAL SPREADING DEPRESSION

Cortical spreading depression (CSD) is a pathophysiological phenomenon. There are sufficient evidences to prove that CSD plays an important role in some neurological disorders. However, exact mechanisms of its initiation and propagation are still unclear. Previous studies showed that glutamate receptors could be concerned with CSD, but those studies were mostly performed oriented to ionotropic glutamate receptors (iGluRs). There is relatively little report about effects of metabotropic glutamate receptors (mGluRs) on CSD. Here, we applied optical intrinsic signal imaging (OISI) combined with direct current (DC) potential recording to examine influences of some mGluRs antagonist (or agonist) on CSD propagation in rat's brain, to indirectly validate actions of some mGluRs on CSD. We found that N-acetyl-L-aspartyl-L-glutamate (NAAG, an agonist at mGluR3) inhibited the propagation of CSD, and the inhibition was gradually developed with time. However, 6-methyl-2-phenylethynyl-pyridine (MPEP, an antagonist of mGluR5) did not produce any significant alterations with the CSD propagation. Our findings suggest that mGluR3 could play an important role in the CSD propagation, but the activity of mGluR5 was comparatively weak. These findings can help to understand the propagation mechanism of CSD, and consider the therapy of some neurological diseases involved with CSD.

Effects of metabotropic glutamate receptor 5 on latent inhibition in conditioned taste aversion

Latent inhibition (LI) is a phenomenon by which pre-exposure of a conditioned stimulus (CS) prior to the CS-unconditioned stimulus (US) pairings retards conditioned responding (CR). LI has been demonstrated in a variety of learning tasks including conditioned taste aversion (CTA). Earlier work has shown that systemic administration of 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a selective metabotropic glutamate receptor 5 (mGlu5) antagonist, is able to disrupt classical conditioning in CTA. The present study investigated the involvement of mGlu5 receptors in LI using a CTA procedure. In the first experiment, rats received either water (non-pre-exposed, NPE) or a saccharin solution (pre-exposed, PE) on 2 consecutive days. The animals then received conditioning in which a fixed amount of saccharin was paired with lithium chloride and then the CR to the taste was tested. Either MPEP (3, 6, 12 mg/kg) or vehicle was injected intraperitoneally prior to taste pre-exposure or testing. Animals in the vehicle control groups displayed LI. MPEP injections before pre-exposure trials attenuated LI but also reduced consumption during pre-exposure, which obscured interpretation of the LI effect. The second experiment used four pre-exposure trials and controlled access to fixed amount of the solutions during the pre-exposure as well as the conditioning trials. Rats were injected before pre-exposure trials but not before the test trial. The results found that MPEP attenuates latent inhibition suggesting that the mGlu5 receptor exerts an influence on the processes that underlie the effects of taste pre-exposure on conditioning.

Multiple metabotropic glutamate receptor subtypes modulate GABAergic neurotransmission in rat periaqueductal grey neurons in vitro

The effect of metabotropic glutamate receptor (mGluR) activation on GABAergic synaptic transmission in rat periaqueductal grey (PAG) neurons was examined using whole-cell patch-clamp recordings in brain slices. The selective groups I, II and III mGluR agonists DHPG (10–30 μM), DCG-IV (1–3 μM) and L-AP4 (10–30 μM) inhibited electrically evoked GABA A mediated inhibitory postsynaptic currents (IPSCs) in all PAG neurons tested. DCG-IV and L-AP4 also reduced the frequency of spontaneous IPSCs, while DHPG produced both increases and decreases in spontaneous IPSC frequency in a dose dependent manner. In the presence of TTX, DHPG, DCG-IV and L-AP4 all reduced the frequency of spontaneous miniature IPSCs, but had no effect on their amplitudes. The DHPG, DCG-IV and L-AP4 effects on miniature IPSCs were dose dependent (EC 50s=1.4, 0.055 and 0.52 μM, respectively) and were reduced by the selective mGluR antagonists MCPG, EGLU and MSOP, respectively. These results indicate that GABAergic synaptic transmission within the PAG is reduced by groups I, II and III mGluR activation via a presynaptic mechanism and is increased by group I mGluR activation via an action potential dependent mechanism. The finding of convergent groups I, II and III mGluR-mediated inhibition of synaptic transmission is novel and indicates that all groups of mGluRs have the potential to modulate the constellation of analgesic, behavioural and autonomic functions within the PAG.

Dual modulation of gabaergic transmission by metabotropic glutamate receptors in rat ventral tegmental area

The effects of metabotropic glutamate receptor (mGluR) activation on non-dopamine (putative GABAergic) neurons and inhibitory synaptic transmission in the ventral tegmental area were examined using intracellular recordings from rat midbrain slices. Perfusion of (±)-1-aminocyclopentane- trans-1,3-dicarboxylic acid ( t-ACPD; agonist for group I and II mGluRs), but not l-amino-4-phosphonobutyric acid (L-AP4; agonist for group III mGluRs), produced membrane depolarization (current clamp) and inward current (voltage clamp) in non-dopamine neurons. The t-ACPD-induced depolarization was concentration-dependent (concentration producing 50% maximal depolarization [EC 50]=6.1±2.5 μM), and was blocked by the antagonist (±)-α-methyl-4-carboxyphenylglycine, but not by tetrodotoxin and ionotropic glutamate-receptor antagonists. The t-ACPD-evoked responses were mimicked comparably by selective group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG). Furthermore, the DHPG-induced depolarization in non-dopamine neurons was greatly reduced by mGluR1-specific antagonist 7(hydroxyimino)cyclopropachromen-1a-carboxylate ethyl ester. When recorded in dopamine neurons, the frequency of spontaneous GABA A receptor-mediated inhibitory postsynaptic potentials was increased by t-ACPD but not L-AP4. However, the amplitude of evoked inhibitory postsynaptic currents in dopamine neurons was reduced by all three group mGluR agonists. These results reveal a dual modulation of mGluR activation on inhibitory transmission in midbrain ventral tegmental area: enhancing putative GABAergic neuronal excitability and thus potentiating tonic inhibitory synaptic transmission while reducing evoked synaptic transmission at inhibitory terminals.

Group I mGluRs modulate the pattern of non-synaptic epileptiform activity in the hippocampus

The hippocampus is well known for its susceptibility to epileptic seizures, in part because of its neuronal architecture that facilitates synchronization. Although synaptic networks are important for the genesis and spread of epileptiform activity, synchronization of neuronal activity can occur when action potential-dependent chemical synaptic transmission is absent. In particular, it is possible to induce epileptiform activity by perfusing hippocampal slices with a low-Ca 2+/high-K + mediums. Using extracellular recording in area CA1 we have characterized the effects of metabotropic glutamate receptor (mGluR) activation on this non-synaptic bursting activity. Under control conditions, bursting occurred at intervals of 14–86 s with each burst comprising a long (up to 44 s) negative-going field potential of 2 to 13 mV superimposed upon which was sustained firing of population spikes. Activation of group I mGluRs by ( S)-3,5-dihydroxyphenylglycine (DHPG) (25 μM) caused a dramatic increase in burst frequency (up to five-fold), which was accompanied by a decrease in the duration and amplitude of bursts. The selective mGluR 1 antagonist 2-methyl-4-carboxyphenylglycine (LY367385) and the selective mGluR 5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) both restricted the increase in burst frequency induced by DHPG. However, only LY367385 inhibited the decrease in burst duration and amplitude. Combined application of both antagonists prevented all DHPG-induced changes in bursting activity. These data provide evidence for a role of both mGluR 1 and mGluR 5 subtypes in changing the frequency of non-synaptic bursting, with mGluR 1 alone causing alterations in burst duration and amplitude. These effects are likely to contribute to the group I mGluR-induced changes in synaptic epileptic activity that are already well documented.

The activation of metabotropic glutamate receptors differentially affects GABA and α-melanocyte stimulating hormone release from the hypothalamus and the posterior pituitary of male rats

The aim of the present study was to investigate the effect of metabotropic glutamate receptor (mGluR) activation on gamma-aminobutyric acid (GABA) and α-melanocyte stimulating hormone (α-MSH) release from hypothalamic fragments and posterior pituitaries. The actions of a number of subtype-selective mGluR agonists were monitored. A group I mGluR agonist, ( S)-3-hydroxyphenylglycine (3-HPG; 0.5 mM), decreased K +-induced hypothalamic GABA release. ( RS)-1-Aminoindan-1,5-dicarboxylic acid (AIDA), a specific group I mGluR antagonist (0.2 mM), blocked the effect of 3-HPG. (2 S, 1′ S, 2′ S)-2-(Carboxycyclopropyl) glycine (L-CCG-I) and l-serine- O-phosphate (L-SOP; 0.01–1 mM), agonists of group II and III mGluRs, respectively, did not modify hypothalamic evoked GABA release. Group I mGluR activation decreased, whereas group III increased and group II induced no changes in GABA release from the posterior pituitary. 3-HPG (1 mM) and L-CCG-I (0.1 mM) decreased, whereas L-SOP (0.01–0.1 mM) did not change α-MSH release from hypothalamic fragments. No agonists of the three mGluR groups modified α-MSH release from the posterior pituitary. These results indicate that activation of mGluRs differentially affects GABA and α-MSH release from the hypothalamus and the posterior pituitary.

Modulation of dendritic release of dopamine by metabotropic glutamate receptors in rat substantia nigra

A superfusion system was used to study the effects of metabotropic glutamate receptor (mGluR) ligands upon the release of [ 3 H ]dopamine ([ 3 H ]DA) previously taken up by rat substantia nigra (SN) slices. trans-(±)-1-Amino-(1 S,3 R)-cyclopentane dicarboxylic acid ( trans-ACPD; 100 and 600 μM), a group I and II mGluR agonist, evoked the release of [ 3 H ]DA from nigral slices. This last effect was reduced significantly by (2 S,3 S,4 S)-2-methyl-2-(carboxycyclopropyl)-glycine (MCCG; 300 μM), an antagonist of group II mGluR, or by the addition of tetrodotoxin ( d-APV; 1 μM) to the superfusion medium. d-(−)-2-Amino-5-phosphono-valeric acid (100 μM), an N-methyl- d-aspartate receptor antagonist, or the presence of Mg 2+ (1.2 mM) in the superfusion medium did not modify trans-ACPD-induced [ 3 H ]DA release. In addition, a group II mGluR agonist such as (2 S,1′ R,2′ R,3′ R)-2-(2′,3′-dicarboxycyclopropyl)-glycine (DCG-IV; 100 μM) significantly induced the release of [ 3 H ]DA from nigral slices, whereas a group I mGluR agonist such as ( RS)-3,5-dihydroxyphenylglycine (DHPG; 50 and 100 μM) did not modify the release of the [ 3 H ]-amine. Further experiments showed that the NMDA (100 μM)-evoked release of [ 3 H ]DA was decreased significantly by prior exposure of SN slices to trans-ACPD. Finally, partial denervation of the DA nigro-striatal pathway with 6-hydroxydopamine (6-OH-DA) increased trans-ACPD-induced release of [ 3 H ]DA, whereas it decreased trans-ACPD inhibitory effects on NMDA-evoked release of [ 3 H ]DA from nigral slices. The present results suggest that the dendritic release of DA in the SN is regulated by mGluR activation. Such nigral mGluR activation may produce opposite effects upon basal and NMDA-evoked release of DA in the SN. In addition, such mGluR-induced effects in the SN are modified in response to partial denervation of the DA nigro-striatal pathway.

Role of metabotropic glutamate receptors in vasopressin and oxytocin release.

The effect of metabotropic glutamate receptor (mGluR) activation on vasopressin (VP) and oxytocin (OT) release was evaluated using explants of the hypothalamoneurohypophysial system. (+/-)-1-Aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD), an agonist at groups I and II mGluRs, increased VP and OT release in a concentration-dependent manner. A role for group I mGluRs in VP and OT release was demonstrated by the ability of a group I-specific mGluR antagonist, 1-aminoindan-1,5-idicarboxylic acid (AIDA), to block the effect of t-ACPD and the ability of a group I-specific agonist, (R,S)-3,5-dihydroxyphenylglycine, to significantly increase both VP (P = 0.0029) and OT (P = 0.0032) release. However, AIDA did not alter VP or OT release induced by a ramp increase in osmolality of the perifusion medium. The role of group III mGluRs was examined using L(+)-2-amino-4-phosphonobutyric acid (L-AP4), an agonist of these receptors. L-AP4 did not change basal release of VP or OT and did not prevent osmotically stimulated hormone release. Thus mGluR activation stimulates VP and OT release, but it is not required for osmotic stimulation of hormone release.

Cell-type specificity of mGluR activation in striatal neuronal subtypes.

The effects of metabotropic glutamate receptor (mGluR) activation were studied in medium spiny neurons and large aspiny (LA) interneurons by means of electrophysiological and optical recordings. DCG-IV and L-SOP, agonists for group II and III mGluRs, respectively, produced a presynaptic inhibitory effect on corticostriatal glutamatergic excitatory postsynaptic potentials (EPSPs) in both spiny and LA cells. Activation of group I mGluRs by the selective agonist 3,5-DHPG produced no effect on membrane properties and glutamatergic transmission in spiny neurons, whereas it did cause a membrane depolarization in LA interneurons coupled to increased input resistance. In combined optical and electrophysiological experiments, in spiny neurons 3,5-DHPG enhanced membrane depolarization and intracellular calcium (Ca2+) levels induced by NMDA applications, but not in LA interneurons. These data suggest the existence of a positive interaction between NMDA and group I mGlu receptors only in medium spiny cells which might, at least partially, account for the differential vulnerability to excitotoxic damage observed in striatal neuronal subtypes.

Modulation of a Slowly Inactivating Potassium Current,ID, by Metabotropic Glutamate Receptor Activation in Cultured Hippocampal Pyramidal Neurons

I(D) is a slowly inactivating 4-aminopyridine (4-AP)-sensitive potassium current of hippocampal pyramidal neurons and other CNS neurons. Although I(D) exerts multifaceted influence on CNS excitability, whether I(D) is subject to modulation by neurotransmitters or neurohormones has not been clear. We report here that one prominent effect of metabotropic glutamate receptor (mGluR) activation by short (3 min) exposure to 1S, 3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) (100 microM) is suppression of I(D) by acceleration of its inactivation. I(D) was identified as a target of mGluR-mediated modulation because inactivation of a component of outward current sensitive to 100-200 microM 4-AP was accelerated by 1S,3R-ACPD, and because 4-AP occluded any further actions of 1S,3R-ACPD. Enhancement of I(D) inactivation was induced by the group I-preferring agonist RS-3, 5-dihydroxyphenylglycine (3,5-DHPG) and the group II-preferring agonist 2S,2'R,3'R)-2-(2',3'dicarboxycyclopropyl)-glycine (DCG-IV), but not by the group III-preferring agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4); it was blocked by the broadly acting mGluR antagonist S-alpha-methyl-4-carboxyphenylglycine (S-MCPG). Furthermore, inactivation of I(D) was enhanced by inclusion of GTPgammaS in the internal solution and blocked by inclusion of GDPbetaS. Metabotropic GluR-induced suppression of I(D) was manifest in three aspects of excitability previously linked to I(D) by their sensitivity to 4-AP: reduction in input conductance and enhanced excitability at voltages just positive to the resting potential, reduced delay to action potential firing during depolarizing current injections, and delayed action potential repolarization. We suggest that mGluR-induced suppression of I(D) could contribute to enhancement of hippocampal neuron excitability and synaptic connections.

Metabotropic glutamate receptors and cell-type-specific vulnerability in the striatum: implication for ischemia and Huntington's disease.

Differential sensitivity to glutamate has been proposed to contribute to the cell-type-specific vulnerability observed in neurological disorders affecting the striatum such as Huntington's disease (HD) and global ischemia. Under these pathological conditions striatal spiny neurons are selectively lost while large aspiny (LA) cholinergic interneurons are spared. We studied the electrophysiological effects of metabotropic glutamate receptor (mGluR) activation in striatal spiny neurons and LA interneurons in order to define the role of these receptors in the pathophysiology of the striatum. DCG-IV and L-SOP, agonists for group II and III mGluRs respectively, produced a presynaptic inhibitory effect on corticostriatal glutamatergic excitatory synaptic potentials in both spiny neurons and LA interneurons. Activation of group I mGluRs by the selective agonist 3,5-DHPG produced no detectable effects on membrane properties and glutamatergic synaptic transmission in spiny neurons while it caused a slow membrane depolarization in LA interneurons coupled to increased input resistance. In combined electrophysiological and microfluorometric recordings, 3,5-DHPG strongly enhanced membrane depolarizations and intracellular Ca2+ accumulation induced by NMDA applications in spiny neurons but not in LA interneurons. Activation of protein kinase C (PKC) by phorbol 12,13-diacetate mimicked this latter action of 3,5-DHPG while the facilitatory effect of 3,5-DHPG was prevented by calphostin C, an inhibitor of PKC. These data indicate that a positive interaction between NMDA receptors and group I mGluRs, via PKC activation, is differently expressed in these two neuronal subtypes. Our data also suggest that differential effects of the activation of group I mGluRs, but not of group II and III mGluRs, might partially account for the selective vulnerability to excitotoxic damage observed within the striatum.

Metabotropic glutamate receptor agonists reduce glutamate release from cultured astrocytes

Astrocytes are thought to control extracellular glutamate concentrations ([Glu]o) in the brain, thereby protecting neurons from excitotoxic injury. We investigated the effects of metabotropic glutamate receptor (mGluR) agonists on glutamate transport and [Glu]o in primary hippocampal astrocytic cultures. Acute or chronic exposure of astrocytes to the mGluR agonist trans-1-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) or its active isomer 1S,3R-ACPD reduced [Glu]o in a time- and dose-dependent manner (44.5 ± 3.6% reductions of [Glu]o in astrocytes from P0–P10 rats and 65.9 ± 4.1 % from rats P20 by 100 μM 1S,3R-ACPD, EC50 ∼ 5 μM). 1S,3R-ACPD effects developed slowly (median effective at ∼60 min) and persisted for several hours after agonist removal. ACPD-pretreated astrocytes established lower steady-state [Glu]o levels. ACPD effects persisted in the presence of the glutamate uptake inhibitors D,L-threo-β-hydroxyaspartate (THA) and L-trans-pyrrolidine-2,4-dicarboxylate (PDC) but were impaired by disruption of the transmembrane Na+, K+, or H+ gradients. In addition, 1S,3R-ACPD had no effects on intracellular glutamate content and did not directly block glutamate transport. Furthermore, ACPD effects could be mimicked by glutamate per se and several other compounds presumed to be mGluR agonists, although (S)-3,5-dihydroxyphenylglycine (DHPG), (2S,2R,3R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV), and L-(+)-2-amino-4-phosphonobutyric acid (L-AP4) were without effect. These data suggest that glutamate and certain mGluR agonists may regulate [Glu]o by modulating the transmembrane equilibrium of glutamate transport, especially by attenuating glutamate release. GLIA 25:270–281, 1999. © 1999 Wiley-Liss, Inc.

Metabotropic glutamate receptor agonists reduce glutamate release from cultured astrocytes

Astrocytes are thought to control extracellular glutamate concentrations ([Glu]o) in the brain, thereby protecting neurons from excitotoxic injury. We investigated the effects of metabotropic glutamate receptor (mGluR) agonists on glutamate transport and [Glu]o in primary hippocampal astrocytic cultures. Acute or chronic exposure of astrocytes to the mGluR agonist trans-1-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) or its active isomer 1S,3R-ACPD reduced [Glu]o in a time- and dose-dependent manner (44.5 +/- 3.6% reductions of [Glu]o in astrocytes from P0-P10 rats and 65.9 +/- 4.1% from rats P20 by 100 microM 1S,3R-ACPD, EC50 approximately 5 microM). 1S,3R-ACPD effects developed slowly (median effective at approximately 60 min) and persisted for several hours after agonist removal. ACPD-pretreated astrocytes established lower steady-state [Glu]o levels. ACPD effects persisted in the presence of the glutamate uptake inhibitors D,L-threo-beta-hydroxyaspartate (THA) and L-trans-pyrrolidine-2,4-dicarboxylate (PDC) but were impaired by disruption of the transmembrane Na+, K+, or H+ gradients. In addition, 1S,3R-ACPD had no effects on intracellular glutamate content and did not directly block glutamate transport. Furthermore, ACPD effects could be mimicked by glutamate per se and several other compounds presumed to be mGluR agonists, although (S)-3,5-dihydroxyphenylglycine (DHPG), (2S,2R,3R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV), and L-(+)-2-amino-4-phosphonobutyric acid (L-AP4) were without effect. These data suggest that glutamate and certain mGluR agonists may regulate [Glu]o by modulating the transmembrane equilibrium of glutamate transport, especially by attenuating glutamate release.

Endogenous activation of metabotropic glutamate receptors contributes to burst frequency regulation in the lamprey locomotor network.

The effect of metabotropic glutamate receptor (mGluR) agonists and antagonists on the spinal cord network underlying locomotion in the lamprey has been analysed. The specific group I mGluR agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) and the broad-spectrum mGluR agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) both increased the burst frequency of N-methyl-D-aspartic acid (NMDA)-induced fictive locomotion and depolarized grey matter neurons. The burst frequency increase induced by the mGluR agonists was counteracted by the mGluR antagonists (+)-alpha-methyl-4-carboxyphenylglycine ((+)-MCPG), cyclopropan[b]chromen-1a-carboxylic acid ethylester (CPCCOEt) and (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA). Application of CPCCOEt alone reduced the locomotor burst frequency, indicating that mGluRs are endogenously activated during fictive locomotion. The mGluR antagonist CPCCOEt had no effect on NMDA-, or (S)-alpha-amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (AMPA)-induced depolarizations. The mGluR agonists 1S,3R-ACPD and DHPG increased the amplitude of NMDA-induced depolarizations, a mechanism which could account for the increase in burst frequency. The group III mGluR agonist L-2-amino-4-phosphonobutyric acid reduced intraspinal synaptic transmission and burst frequency.

Activation of group I mGluRs increases spontaneous IPSC frequency in rat frontal cortex.

The effect of metabotropic glutamate receptor (mGluR) activation on inhibitory synaptic transmission was examined by using whole cell patch-clamp recordings. Spontaneous (s) and miniature (m) inhibitory postsynaptic currents (IPSCs) were recorded from visually identified layer II/III pyramidal neurons in rat neocortex in vitro. Excitatory postsynaptic currents (EPSCs) were blocked by using bath application of 20 microM D(-)2-amino-5-phosphonovaleric acid and 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione. In the presence of 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (30-100 microM), Lp4-quisqualate (5 microM), and the group I selective mGluR agonist (S)-3,5-dihydroxyphenylglycine (100 microM), the frequency of sIPSCs was increased. Decay kinetics of sIPSCs were unaffected. No enhancement of mIPSCs was observed. Bath application of group II (2S,3S,4S-alpha-carboxycyclopropyl-glycine; 5 microM) and group III selective mGluR agonists (L-2-amino-4-phosphonobutyric acid; 100 microM) had no detectable effects on the frequency or amplitude of sIPSCs. These findings indicate that activation of group I mGluRs (mGluR1 and/or mGluR5) enhances gamma-aminobutyric acid-mediated synaptic inhibition in layer II/III pyramidal neurons in neocortex. The lack of effect on mIPSCs suggests a presynaptic action via excitation of inhibitory interneurons.

(+)-MCPG blocks induction of LTP in CA1 of rat hippocampus via agonist action at an mGluR group II receptor.

We investigated the effect of metabotropic glutamate receptor (mGluR) ligands on the induction of long-term potentiation (LTP) of field excitatory postsynaptic potentials (EPSPs) in CA1 of rat hippocampus, in particular the manner by which the nonsubtype selective mGluR ligand alpha-methyl-4-carboxyphenylglycine [(+)-MCPG] blocks LTP induction. Normalized control LTP was blocked by (+)-MCPG (250 microM), but not by the mGluRI selective antagonist (S)-4-carboxyphenylglycine (4-CPG), the mGluRII selective antagonist 1/(2S,3S, 4S)-2-methyl-2-(carboxycyclopropyl) glycine (MCCG), or the mGluRIII antagonist (S)-2-amino-2-methyl-4-phosphonobutanoic acid/alpha-methyl (MAP4). In contrast the mGluRII agonist ((1S, 3S)-1-aminocyclopentante-1,3-dicarboxylic acid -(1S,3S)-ACPD-; 10 or 25 microM) completely and consistently blocked LTP. The block of LTP by both (1S,3S)-ACPD and (+)-MCPG could be prevented by preincubation with the mGluRII antagonist MCCG. These studies demonstrate that (+)-MCPG blocks LTP induction through an agonist action at an mGluRII receptor and not through a nonselective antagonist action.

Metabotropic glutamate receptor-mediated excitation and inhibition of sympathetic preganglionic neurones

The effects of metabotropic glutamate receptor (mGluR) subtype selective compounds on the excitability of sympathetic preganglionic neurones (SPNs) were investigated. Non-selective mGluR agonists (1 S,3 R)-aminocyclopentane-1,3-dicarboxylic acid and (2 S,1′ S,2′ S)-2-(carboxycyclopropyl)glycine, induced dose-dependent depolarisations in 96 and 75% of SPNs, respectively and hyperpolarisations in 2 and 21% of SPNs. Both agonists could induce subthreshold membrane potential oscillations in previously non-oscillating SPNs and either increased or reduced the frequency of spontaneously occurring oscillations. A selective group I mGluR agonist, 3,5-dihydroxyphenylglycine, depolarised all SPNs tested, induced oscillations in membrane potential of otherwise non-oscillating SPNs and increased the frequency of spontaneous oscillations. Agonists with selectivity for group II mGluRs (1 S, 3 S)-aminocyclopentane-1,3-dicarboxylic acid and ( S)-4-carboxy-3-hydroxy-phenylglycine (( S)-4C3HPG) did not induce depolarising responses. However ( S)-4C3HPG induced hyperpolarising responses associated with a reduction in the frequency of spontaneous oscillations in two of six SPNs tested. Depolarising and hyperpolarising responses were maintained in the presence of tetrodotoxin indicating a direct action of the agonists upon SPNs. In individual SPNs responses of opposite polarity could be induced from the same initial membrane potential using different agonists, indicating that the opposing responses involved different ionic mechanisms. The broad spectrum mGluR antagonist ( S)- α-methyl-4-carboxyphenylglycine and the selective group I mGluR antagonist ( S)-4-carboxyphenylglycine reversibly depressed mGluR agonist induced depolarisations. These results indicate that SPNs express two mGluR populations with opposing actions on neuronal excitability: group I mGluRs depolarise SPNs and can drive oscillatory membrane potential activity; a minority of SPNs express group II mGluRs which mediate membrane hyperpolarisations and reduce the frequency of membrane potential oscillations.

Enhancement of dopamine release from the striatum through metabotropic glutamate receptor activation in methamphetamine sensitized rats

An intracerebral microdialysis technique was applied to study the effect of metabotropic glutamate receptor (mGluR) agonist on dopamine release in the striatum of methamphetamine (MAP)-sensitized rats. Rats were treated with MAP (I mg/kg, i.p.) once daily for 6 consecutive days, followed by a 6-day withdrawal. Perfusion of 0.l mM (1S,3R)-1-aminocyclopentane- trans-1,3-dicarboxylic acid through a microdialysis probe placed in the striatum enhanced the extracellular dopamine level, and induced stereotyped behavior in MAP-sensitized rats. The enhancement of dopamine release and the stereotyped behavior were attenuated by co-perfusion of 0.4 mM RS-α-methyl-4-carboxyphenyl-glycine, a mGluR antagonist. The present results suggest that mGluRs may be involved in the expression of MAP-induced sensitization.

Pharmacological characterization of metabotropic glutamate receptors potentiating NMDA responses in mouse cortical wedge preparations.

1. Mouse cortical wedge preparations were used in order to study the effects of metabotropic glutamate receptor (mGluR) agonists and antagonists on the depolarization induced by N-methyl-D-aspartate (NMDA) or by (S)-alpha-amino-4-bromo-3-hydroxy-5-isoxazolepropionic acid (AMPA). 2. (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) (30-300 microM) significantly potentiated the depolarizations induced by NMDA, leaving unchanged those mediated by AMPA. This potentiation developed slowly and lasted for up to 60 min provided that the slices were continuously perfused with the mGluR agonist. 3. Concentration-response curves to NMDA in the absence and in the presence of 1S,3R-ACPD (100 microM) indicated that the potentiation was due to increased affinity of the NMDA receptor complex for its agonist. The maximal responses to NMDA were not potentiated. 4. Selective agonists of group 1 mGluR such as quisqualate (Quis) (30 microM) or (RS)-3,5-dihydroxyphenylglycine (DHPG) (300 microM) did not potentiate NMDA responses. Similarly, selective agonists of group 2 mGluRs, such as (2S,3S,4S)-alpha-carboxycyclopropyl-glycine (L-CCG-I) (3-30 microM), and of group 3, such as L-2-amino-4-phosphonobutyric acid (L-AP4) (100 microM) were inactive in our test. A number of other putative mGluR agents having partial agonist activity on mGluRs in brain slices and in expression systems, such as 1R,3S-ACPD (500 microM), DL-2-amino-3-phosphonopropionic acid (DL-AP3) (300 microM) and (S)-4-carboxy-3-hydroxyphenylglycine (S-4C3HPG; 500 microM), when placed in the experimental protocol we used, did not change NMDA responses. 5. Available mGluR antagonists, such as DL-AP3 (1 mM), (+)-alpha-methyl-4-carboxyphenylglycine (MCPG) (500 microM), S-4-carboxyphenylglycine (4CPG; 500 microM) and S-4-carboxy-3-hydroxyphenylglycine (S-4C3HPG; 500 microM), did not reduce 1S,3R-ACPD potentiation of NMDA responses. 6. It is concluded that the potentiation of NMDA currents induced by the mGluR agonist 1S,3R-ACPD, in mouse cortical wedges, has a pharmacological profile which is different from that of the three mGluR groups so far described in expression systems.