Selective metabotropic receptor agonists distinguish non-ionotropic glutamate binding sites

Metabotropic glutamate receptors (mGluRs) are thought to mediate diverse processes in brain including synaptic plasticity and excitotoxicity. These receptors are often divided into three groups by their pharmacological profiles. [3H]Glutamate binding in the presence of compounds selective for ionotropic glutamate receptors can be used as a general assay for these receptors; subtypes of this non-ionotropic [3H]glutamate binding differ in both pharmacology and anatomical distribution, and are differentially sensitive to quisqualate. The characteristics of these binding sites are consistent with those of group 1 (high-affinity quisqualate) and group 2 (low-affinity quisqualate) mGluRs. Under our assay conditions, no [3H]glutamate binding to group 3-like (L-AP4 sensitive) sites could be demonstrated. We have attempted to characterize particular agents which may selectively measure [3H]glutamate binding to mGluR subtypes. We used two isomers of 2-(carboxycyclopropyl)glycine, L-CCG-I and L-CCG-II, and the (2S,1'R,2'R,3'R) isomer of 2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) as competitors of non-ionotropic [3H]glutamate binding sites. DCG-IV clearly distinguishes two binding sites. Quantitative levels of DCG-IV binding by anatomic region correlate with quisqualate-defined binding subtypes: high-affinity DCG-IV binding correlates with low-affinity quisqualate binding, whereas low-affinity DCG-IV binding correlates with high-affinity quisqualate binding. L-CCG-II displaces only one type of non-ionotropic [3H]glutamate binding, corresponding to high-affinity quisqualate binding. Therefore DCG-IV and L-CCG-II at appropriate concentrations appear to distinguish binding to putative group 2 vs. group 1 mGluRs. L-CCG-I displaces both high- and low-affinity quisqualate binding sites, but unlike the other two compounds, does not clearly distinguish between them.     

Expression of metabotropic glutamate receptor 1 isoforms in the substantia nigra pars compacta of the rat

Metabotropic glutamate receptors, which are linked via G-proteins to second messenger systems, have been implicated in the physiological regulation of dopaminergic neurons of the substantia nigra pars compacta as well as in neurodegeneration. Of the eight known metabotropic glutamate receptors, metabotropic glutamate receptor 1 is the most abundant subtype in the substantia nigra pars compacta. Metabotropic glutamate receptor 1 is alternatively spliced at the carboxy terminal region to yield five variants: 1a, 1b, 1c, 1d and a form recently identified in human brain, 1g. We used an antibody recognizing metabotropic glutamate receptor 1, and another recognizing the splice form la only, to study the localization of these receptors in dopaminergic neurons identified by the presence of tyrosine hydroxylase. Metabotropic glutamate receptor immunoreactivity was present within the somata, axons, and dendrites of substantia nigra pars compacta neurons. The 1a splice form specific antibody, however, did not label these cells, suggesting that they express a metabotropic glutamate receptor 1 splice form different from 1a. In situ hybridization with splice form-specific oligonucleotide probes was used to determine which of the other known metabotropic glutamate receptor 1 splice forms might be present in the substantia nigra pars compacta. Each probe produced a very distinct labelling pattern in the rat brain with the exception of the 1g specific probe which produced only background signal. Substantia nigra pars compacta neurons were most intensely labelled by the metabotropic glutamate receptor 1d splice form specific probe. Metabotropic glutamate receptor 1a was expressed weakly whereas there was no detectable 1b, c, or g signal in the substantia nigra pars compacta. These data demonstrate that metabotropic glutamate receptor 1 protein is present within the perikarya and processes of dopaminergic neurons in the substantia nigra pars compacta. The majority of this protein is not the 1a splice form, which is abundant in other brain regions, and may be the 1d isoform. Since splicing alters the carboxy terminus of the receptor, it is likely to affect the interaction of the receptor with intracellular signalling systems.     

Altered spatial learning and memory in mice lacking the mGluR4 subtype of metabotropic glutamate receptor.

The glutamate analog, L-2-amino-4-phosphonobutyric acid (L-AP4) is a selective agonist for several members of the metabotropic glutamate receptor (mGluR) family. Activation of presynaptic mGluRs by L-AP4 causes a suppression of synaptic transmission in the central nervous system. In this study, the role of 1 subtype of mGluR in the nervous system was investigated by analyzing mutant mice lacking the L-AP4-sensitive receptor, mGluR4. Experiments designed to probe hippocampal function showed no impairments in acquisition of spatial learning in the water maze task. However, in a spatial reversal learning task, the mutant mice exhibited significantly accelerated learning performance. Furthermore, in a probe trial administered 6 weeks posttraining, these mice showed impaired spatial accuracy. The results suggest that the mutant mice differed in their ability to learn and integrate new spatial information into previously formed memory traces and that their use of stored spatial information also was altered. Thus, the presynaptically expressed mGluR4 plays a role in the processing of spatial information. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Expression of an olfactory receptor in Escherichia coli: purification, reconstitution, and ligand binding

An olfactory receptor has been expressed in bacterial cells as a fusion protein with glutathione S-transferase (GST). Overexpression of receptor protein yielding about 10% of the cell protein was achieved with mutants lacking the N-terminus and the first transmembrane region or with mutants carrying three positively charged residues in the first intracellular loop. The overexpressed fusion protein accumulated in inclusion bodies and could be solubilized in detergent. It was purified by metal chelation chromatography based on a C-terminal 6-histidine tag, and the GST portion was removed after proteolytic cleavage. The purified receptor was reconstituted into lipid vesicles and specific binding of odor ligands was shown by photoaffinity labeling and tryptophan fluorescence measurements. Thus, for the first time, an odorant receptor/ligand pair becomes available in large amounts for biophysical and screening studies.     

Disruption of the first extracellular loop of thyrotropin receptor prevents ligand binding

In order to understand the function of the first extracellular loop of the human thyrotropin receptor (hTSHR), each of two peptides of nine amino acids was inserted into the first extracellular loop of hTSHR. hTSHR cDNA was subcloned into the eukaryotic expression vector, pRc/CMV (hTSHR/pRc/CMV). B-hTSHR/pRc/CMV, a mutant hTSHR cDNA which encodes a hydrophilic peptide insert (AGTTRRVAI) and C-hTSHR/pRc/CMV which encodes a hydrophobic peptide insert (ATVLVVPMI) between +486 Ileu and +487 Asp of hTSHR were transfected into Chinese hamster ovary cells to generate the B-1 and C-6 cell lines, respectively. Neither thyrotropin (TSH) nor thyroid stimulating antibody (TSAb) stimulated cAMP production by B-1 or C-6 cells. An 125I-TSH binding assay showed that neither cell line bound TSH. Our data demonstrated that these mutations impaired both TSH binding and cAMP production. This evidence suggests that the first extracellular loop of hTSHR may have a crucial role in the TSH- and TSAb-dependent signal transduction.     

Importance of extracellular domains for ligand binding in the thyrotropin-releasing hormone receptor

The role of putative extracellular sequences for ligand binding in the TRH receptor was examined using deletion or substitution mutations. Each mutant receptor was transiently expressed in TRH receptor-minus GH(1)2C(1)b rat pituitary cells, and binding of 4 Nu Mu [3H]pGlu-N(tau)-MeHis-Pro-NH2 ([3H] MeTRH) was measured. When binding was not detected, signal transduction at 10 microM MeTRH was measured to assess receptor expression. Deletion of most of the N-terminal sequences (Glu(2)-Leu(22)), including two potential glycosylation sites, had no effect on the affinity of the receptor for MeTRH. Segmental deletions or simultaneous substitution of multiple amino acid residues in the first, second, or third extracellular loop (EL1, EL2, or EL3) resulted, however, in total loss of [3H]MeTRH binding, suggesting important roles for the loop sequences in either receptor expression or ligand binding. Individual substitutions were made to test further the role of the specific extracellular loop sequences in TRH binding. In EL1, conversion of Tyr93 to Ala resulted in more than 20-fold decrease in affinity for MeTRH. In EL2 and the top portion of the fifth transmembrane helix, conversion of Tyr181 to Phe, Tyr188 to Ala, and Phe199 to Ala resulted in a large ( > 100-fold) decrease in affinity for MeTRH, and conversion of Tyr 188 to Phe and Phe196 to Ala caused an agonist-specific 4- to 5-fold decrease in affinity. In EL3, conversion of Asn289 to Ala and of Ser290 to Ala caused a large ( > 100-fold) decrease in affinity for MeTRH. These results suggest important roles for the extracellular loops in high affinity TRH binding and lead us to propose a model in which TRH binds to the extra-cellular domain of its receptor.     

Mapping of the sites for ligand binding and receptor dimerization at the extracellular domain of the vascular endothelial growth factor receptor FLT-1

The vascular endothelial growth factor (VEGF) receptor FLT-1 has been shown to be involved in vasculogenesis and angiogenesis. The receptor is characterized by seven Ig-like loops within the extracellular domain. Upon VEGF binding FLT-1 becomes phosphorylated, which has been thought to be preceded by receptor dimerization. To further investigate high affinity binding of VEGF to FLT-1 and ligand-induced receptor dimerization, we expressed in Sf9 cells the entire extracellular domain comprising all seven Ig-like loops: sFLT-1(7) and several truncated mutants consisting of loop one, one and two, one to three, one to four, and one to five. The corresponding proteins, named sFLT-1(1), (2), (3), (4), and (5) were purified. Only mutants sFLT-1(3) to (7) were able to bind 125I-VEGF with high affinity. No binding of VEGF was observed with sFLT-1(1) and sFLT-1(2), indicating that the first three Ig-like loops are involved in high affinity binding of VEGF. The binding of VEGF to sFLT-1(3) could be competed with placenta growth factor (PlGF), a VEGF-related ligand, suggesting that high affinity binding of VEGF and PlGF is mediated by the same or closely related contact sites on sFLT-1. Deglycosylation of the sFLT-1(3), (4), (5), and (7) did not abolish VEGF binding. Furthermore, unglycosylated sFLT-1(3), expressed in Escherichia coli, was able to bind VEGF with similar affinity as sFLT-1(3) or sFLT-1(7), both expressed in Sf9 cells. This indicates that receptor glycosylation is not essential for high affinity binding. Dimerization of the extracellular domains of FLT-1 upon addition of VEGF was detected with all mutants containing the Ig-like loop four. Although sFLT-1(3) was able to bind VEGF, dimerization of this mutant was inefficient, indicating that sites on Ig-like loop four are essential to stabilize receptor dimers.     

Reversible and non-competitive antagonist profile of CPCCOEt at the human type 1alpha metabotropic glutamate receptor

In transfected CHO cells expressing the human metabotropic glutamate receptor mGlu1alpha, 7-(hydroxyimino)cyclopropan[b]-chromen-1a-carboxylic acid ethylester (CPCCOEt) was found to antagonize L-quisqualate-induced phosphoinositide hydrolysis in a non-competitive and reversible manner (apparent pKi value, 4.76+/-0.18; n=3). This suggests that CPCCOEt antagonizes type 1alpha metabotropic glutamate receptor activation by interacting with a site distinct from the agonist binding site.     

Group 1 metabotropic glutamate receptors contribute to slow-onset potentiation in the rat CA1 region in vivo

It has been demonstrated in the CA1 region of the hippocampus in vitro, and in the dentate gyrus and CA1 region in vivo, that application of the metabotropic glutamate receptor (mGluR) agonist, 1S, 3R-amino cyclopentane 2,3-dicarboxylic acid triggers a slow-onset potentiation of synaptic transmission in the hippocampus. This study examined the involvement of group 1 and 2 mGluRs in this phenomenon in the CA1 region of freely moving rats. Drugs were applied via the lateral cerebral ventricle, and measurements were obtained from the CA1 region via permanently implanted electrodes. The group 1 mGluR agonists, 3,5-dihydroxyphenylglycine (DHPG, 20-100 nmol/5 microl) and trans-azetidine-2,4-dicarboxylic acid (ADA, 100 nmol-1 micromol/5 microl) induced a dose-dependent potentiation of basal synaptic transmission. The mGluR antagonist R,S-alpha-methyl-carboxyphenylglycine (MCPG, 1 micromol), and the group 1 mGluR antagonist, S-4-carboxyphenylglycine (4CPG, 100 nmol) completely inhibited the effects of both DHPG and ADA. The group 2 mGluR agonist, (S)-4-carboxy-3-hydroxy phenylglycine (4C3H-PG, 50-200 nmol/5 microl) induced a dose-dependent decrease of basal synaptic transmission. These results suggest that in the CA1 region in vivo, slow-onset potentiation may be mediated by group 1 mGluRs.     

Identification of the amino acid subsets accounting for the ligand binding specificity of a glutamate receptor

In a situation so far unique among neurotransmitter receptors, glutamate receptors share amino acid sequence similarities with the bacterial periplasmic binding proteins (PBPs). On the basis of the primary structure similarity of two bacterial periplasmic proteins (lysine/arginine/ornithine- and phosphate-binding proteins) with the chick cerebellar kainate-binding protein (KBP), a member of the ionotropic glutamate receptor family, we have generated a three-dimensional model structure of the KBP extracellular domain. By an interplay between homology modeling and site-directed mutagenesis, we have investigated the kainate binding properties of 55 different mutants (corresponding to 43 positions) and studied the interactions of some of these mutants with various glutamatergic ligands. As a result, we present here the subsets of amino acids accounting for the binding free energies and specificities of KBP for kainate, glutamate, and CNQX and propose a three-dimensional model, at the microarchitectural level, of the glutamatergic binding domain.     

A modulatory effect of extracellular Ca2+ on type 1alpha metabotropic glutamate receptor-mediated signalling

DiGeorge syndrome or anomaly consists of a developmental field defect which is characterized by congenital absence or hypoplasia of the thymus and parathyroids, as well as facial dysmorphism and congenital heart defects. Other congenital malformations may coexist, in particular, thyroid abnormalities. A case of congenital hypothyroidism and DiGeorge syndrome is reviewed. Necropsy, clinical, and experimental studies also show that thyroid abnormalities may be a feature of DiGeorge syndrome. Although this could be purely coincidental, our case suggests that thyroid gland dysgenesis may be more common than previously thought. Thus, children with the DiGeorge syndrome may be at higher risk for hypothyroidism. Because of this potential association, patients who are considered to have this anomaly should have early newborn thyroid screening.     

Molecular characterization and localization of human metabotropic glutamate receptor type 4

Oligonucleotides of consensus sequences from rat metabotropic glutamate receptor (mGluR) genes were synthesized and used to amplify human DNA by the polymerase chain reaction (PCR). Five unique human sequences homologous to these rat receptor genes were isolated including mGluR4. A human cerebellum cDNA library was screened using this amplified mGluR4 sequence as a probe and yielded clones which between them contained the complete coding sequence for human mGluR4. The coding sequence is very similar to the equivalent rat gene (90% DNA sequence identity and 97% predicted protein sequence identity). The mGluR4 cDNA was transfected in Chinese hamster ovary (CHO) cells and stable clonal cell lines were isolated. Stimulation of the expressed receptor by L-2-amino-4-phosphonobutyrate (L-AP4), L-glutamate or (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) resulted in a reduction of forskolin-stimulated cyclic AMP (cAMP) with EC50 values of 0.2, 13 and 90 microM respectively. Quisqualate had little effect at concentrations up to 1 mM. In Northern blots mGluR4 mRNA appears to be brain-specific, and shows a distinct distribution (excluding the cerebellum), being expressed in the thalamus, hypothalamus and caudate nucleus. In situ hybridization studies on human brain sections confirmed this general pattern of distribution. The strongest mGluR4 mRNA signal was found in the cerebellar granule cells consistent with the reported distribution of mGluR4 in the rat brain. The major difference from the rat brain is the presence in the human brain of mGluR4 mRNA in the caudate nucleus and putamen.     

Mutagenesis analysis of the membrane-proximal ligand binding site of the TGF-beta receptor type III extracellular domain

There are two TGF-beta binding subdomains in the extracellular domain of receptor type III (proximal and distal in relation to the transmembrane domain). Here we present an extension of our analysis of the proximal binding site of receptor type III. Due to the original deletion mutagenesis strategy, our proximal binding site contained 19 amino acids from the N-terminal part of the receptor. By deleting these, we demonstrated that they did not contribute to the binding ability of the proximal binding site. We also produced a soluble, secreted form of the proximal binding site and demonstrated that it was able to bind TGF-beta. Finally, we analyzed the role of the three asparagine residues (580, 591, 595) that are located in the region of the receptor that is necessary for expression of a functional proximal binding site, and found that mutation of these residues individually to alanine did not affect ligand binding.     

Differential localization of phosphoinositide-linked metabotropic glutamate receptor (mGluR1) and the inositol 1,4,5-trisphosphate receptor in rat brain

The type 1 metabotropic glutamate receptor (mGluR1) is through to act via the phosphoinositide (PI) system with the associated formation of inositol 1,4,5-trisphosphate (IP3) and Ca2+ release. Utilizing immunohistochemistry and in situ hybridization, we have localized protein and mRNA, respectively, for the mGluR1 and the IP3 receptor (IP3R). We have also localized glutamate-linked PI turnover by autoradiography with 3H-cytidine. We observe a striking contrast in localizations of mGluR1 and IP3R both for protein and mRNA. For instance, mGluR1 occurs in the apparent absence of IP3R in neurons of the stratum oriens of the CA1 hippocampus, islands of Calleja, anterodorsal nucleus of thalamus, lateral nucleus of hypothalamus, and the granular cell layer and the deep nuclei of cerebellum. mGluR1 actions in these brain regions may primarily be mediated through the protein kinase C limb of the PI system, as they contain moderate amounts of 3H-phorbol ester binding. The subthalamic nucleus, red nucleus, and Darkshevich's nucleus, which possess high levels of mGluR1, are devoid of both IP3R immunoreactivity and 3H-phorbol ester binding. These reciprocal localizations suggest that mGluR1 actions in many brain areas may not primarily involve IP3, reflecting instead influences on protein kinase C or other second messengers.     

Metabotropic glutamate receptor subtype mGluR1alpha stimulates the secretion of the amyloid beta-protein precursor ectodomain

To examine the effects of glutamatergic neurotransmission on amyloid processing, we stably expressed the metabotropic glutamate receptor subtype 1alpha (mGlu-R1alpha) in HEK 293 cells. Both glutamate and the selective metabotropic agonist 1-amino-1,3-cyclopentanedicarboxylic acid (ACPD) rapidly increased phosphatidylinositol (PI) turnover four- to fivefold compared with control cells that were transfected with the expression vector alone. Increased PI turnover was effectively blocked by the metabotropic antagonist alpha-methyl-4-carbophenylglycine (MCPG), indicating that heterologous expression of mGluR1alpha resulted in efficient coupling of the receptors to G protein and phospholipase C activation. Stimulation of mGluR1alpha with glutamate, quisqualate, or ACPD rapidly increased secretion of the APP ectodomain (APPs); these effects were blocked by MCPG. The metabotropic receptors were coupled to APP processing by protein kinases and by phospholipase A2 (PLA2), and melittin, a peptide that stimulates PLA2, potently increased APPs secretion. These data indicate that mGluR1alpha can be involved in the regulation of APP processing. Together with previous findings that muscarinic and serotonergic receptor subtypes can increase the secretion of the APP ectodomain, these observations support the concept that proteolytic processing of APP is under the control of several major neurotransmitters.     

Glutamate-induced swelling of cultured astrocytes is mediated by metabotropic glutamate receptor

The aim of this study was to define from a morpho-structural point of view, using scanning electron microscopy, the features of various types of disposable latex gloves commonly used in Italian dental practice (Biogel D, Trend, Pagni, J&J, Latechnics, Pehasoft, Bantex). None of the brands examined was free from morphological flaws; however, while in some of these only slight depressions were found (Biogel D, Trend), in others (Latechnics, Bantex) there was a marked lack of homogeneity in the latex structure or real holes (Pehasoft). This study emphasizes the current difficulties faced by dentists in the search for safe working conditions.     

Attenuation of morphine withdrawal symptoms by subtype-selective metabotropic glutamate receptor antagonists

1. We have previously shown that chronic antagonism of group I metabotropic glutamate receptors (mGluRs), in the brain, attenuates the precipitated morphine withdrawal syndrome in rats. In the present investigation we assessed the effects of chronic antagonism of group II and III mGluRs on the severity of withdrawal symptoms in rats treated chronically with subcutaneous (s.c.) morphine. 2. Concurrently with s.c. morphine we infused intracerebroventricularly (i.c.v.) one of a series of phenylglycine derivatives selective for specific mGluR subtypes. Group II mGluRs (mGluR2,3), which are negatively coupled to adenosine 3':5'-cyclic monophosphate (cyclic AMP) production, were selectively antagonized with 2s, 1's, 2's-2-methyl-2-(2'-carboxycyclopropyl) glycine (MCCG). Group III mGluRs (mGluR4,6,7 and 8), which are also negatively linked to cyclic AMP production, were selectively antagonized with alpha-methyl-L-amino-4-phosphonobutanoate (MAP4). The effects of MCCG and MAP4 were compared with alpha-methyl-4-carboxyphenylglycine (MCPG), which non-selectively antagonizes group II mGluRs, as well as group I mGluRs (mGluR1,5) which are positively coupled to phosphatidylinositol (PI) hydrolysis. 3. Chronic i.c.v. administration of both MCCG and MAP4 significantly decreased the time spent in withdrawal, MCPG and MCCG reduced the frequency of jumps and wet dog shakes and attenuated the severity of agitation. 4. Acute i.c.v. injection of mGluR antagonists just before the precipitation of withdrawal failed to decrease the severity of abstinence symptoms. Rather, acute i.c.v. injection of MCCG significantly increased the time spent in withdrawal. 5. Our results suggest that the development of opioid dependence is affected by mGluR-mediated PI hydrolysis and mGluR-regulated cyclic AMP production.     

Cloning, distribution and functional expression of the human mGlu6 metabotropic glutamate receptor

The levels of biogenic amines and of a number of the products of their metabolism were studied in the hypothalamic nuclei in Wistar and August rats, which have different levels of resistance to emotional stress; levels were also studied in structures functionally and anatomically associated with the hypothalamic nuclei, i.e., the reticular formation of the midbrain, the amygdaloid body, the septum, the locus ceruleus, the dorsal cervical nucleus, and the ventral region of the tegmentum. The genotype was found to determine the level of metabolism of biogenic amines in structures of the central nervous system in conditions of emotional stress. In August rats, the activities of the dopaminergic and serotoninergic systems, which are stress-limiting, decreased to a greater extent during 24-hour immobilization stress. Adrenaline levels in structures of the central nervous system in August rats were higher during stress. Changes in the contents of biogenic amines in the paraventricular and ventromedial nuclei of the hypothalamus in Wistar and August rats could affect the preganglionic neurons of the autonomic nervous system.     

Neuroadaptations in ionotropic and metabotropic glutamate receptor mRNA produced by cocaine treatment

The expression of glutamate receptor/subunit mRNAs was examined 3 weeks after discontinuing 1 week of daily injections of saline or cocaine. The level of mRNA for GluR1-4, NMDAR1, and mGluR5 receptors was measured with in situ hybridization and RT-PCR. In nucleus accumbens, acute cocaine treatment significantly reduced the mRNA level for GluR3, GluR4, and NMDAR1 subunits, whereas repeated cocaine reduced the level for GluR3 mRNA. Acute cocaine treatment also reduced the NMDAR1 mRNA level in dorsolateral striatum and ventral tegmental area. In prefrontal cortex, repeated cocaine treatment significantly increased the level of GluR2 mRNA. The GluR2 mRNA level was not changed by acute or repeated cocaine in any other brain regions examined. Repeated cocaine treatment also significantly increased mGluR5 mRNA levels in nucleus accumbens shell and dorsolateral striatum. Functional properties of the ionotropic glutamate receptors are determined by subunit composition. In addition, metabotropic glutamate receptors can modulate synaptic transmission and the response to stimulation of ionotropic receptors. Thus, the observed changes in levels of AMPA and NMDA receptor subunits and the mGluR5 metabotropic receptor may alter excitatory neurotransmission in the mesocorticolimbic dopamine system, which could play a significant role in the enduring biochemical and behavioral effects of cocaine.     

Evidence for metabotropic glutamate receptor activation in the induction of depolarization-induced suppression of inhibition in hippocampal CA1

Depolarization-induced suppression of inhibition (DSI) is a transient reduction of GABAA receptor-mediated IPSCs that is mediated by a retrograde signal from principal cells to interneurons. Using whole-cell recordings, we tested the hypothesis that mGluRs are involved in the DSI process in hippocampal CA1, as has been proposed for cerebellar DSI. Group II mGluR agonists failed to affect either evoked monosynaptic IPSCs or DSI, and forskolin, which blocks cerebellar DSI, did not affect CA1 DSI. Group I and group III mGluR agonists reduced IPSCs, but only group I agonists occluded DSI. (S)-MCPG blocked (1S,3R)-ACPD-induced IPSC suppression and markedly reduced DSI, whereas group III antagonists had no effect on DSI. Many other similarities between DSI and the (1S,3R)-ACPD-induced suppression of IPSCs also were found. Our data suggest that a glutamate-like substance released from pyramidal cells could mediate CA1 DSI by reducing GABA release from interneurons via the activation of group I mGluRs.