Neuroprotective effects of a postischemic treatment with a bradykinin B2 receptor antagonist in a rat model of temporary focal cerebral ischemia

Recurrent mossy fiber synapses in the dentate gyrus of epileptic brain facilitate the synchronous firing of granule cells and may promote seizure propagation. Mossy fiber terminals contain and release zinc. Released zinc inhibits the activation of NMDA receptors and may therefore oppose the development of granule cell epileptiform activity. Hippocampal slices from rats that had experienced pilocarpine-induced status epilepticus and developed a recurrent mossy fiber pathway were used to investigate this possibility. Actions of released zinc were inferred from the effects of chelation with 1 mM calcium disodium EDTA (CaEDTA). When granule cell population bursts were evoked by mossy fiber stimulation in the presence of 6 mM K(+) and 30 microM bicuculline, CaEDTA slowed the rate at which evoked bursting developed, but did not change the magnitude of the bursts once they had developed fully. The effects of CaEDTA were then studied on the pharmacologically isolated NMDA receptor- and AMPA/kainate receptor-mediated components of the fully developed bursts. CaEDTA increased the magnitude of NMDA receptor-mediated bursts and reduced the magnitude of AMPA/kainate receptor-mediated bursts. CaEDTA did not affect the granule cell bursts evoked in slices from untreated rats by stimulating the perforant path in the presence of bicuculline and 6 mM K(+). These results suggest that zinc released from the recurrent mossy fibers serves mainly to facilitate the recruitment of dentate granule cells into population bursts.     

Plasticity-dependent changes in metabotropic glutamate receptor expression at excitatory hippocampal synapses

Group I metabotropic glutamate receptors, mGluR1 and mGluR5, modulate NMDA receptor-mediated synaptic transmission and plasticity and mediate mGluR-dependent plasticity. Here we report that the synaptic expression of mGluRs can be regulated by NMDA receptor-dependent synaptic plasticity, but that this is dependent on the subtype of mGluR. Silent synapses, but not active synapses, were found to lack Group I mGluRs showing that mGluRs must be inserted into synapses after they are unsilenced. The induction of LTP resulted in an increased synaptic expression of mGluR1 in an NMDA receptor-dependent manner. mGluR1 is internalized from synapses via NMDA receptor-dependent LTD. Interestingly we found no evidence for the regulation of mGluR5 by NMDA receptor-dependent plasticity. This regulation of Group I mGluRs will determine the ability of synapses to undergo mGluR-dependent modulation of synaptic transmission and plasticity, providing a mechanism for metaplasticity and state-dependent plasticity at hippocampal synapses.     

Calcium-dependent mechanisms involved in presynaptic long-term depression at the hippocampal mossy fibre-CA3 synapse

Long-term potentiation (LTP) and long-term depression (LTD) are induced presynaptically at the hippocampal mossy fibre-CA3 synapse. Activation of presynaptic metabotropic glutamate receptors (mGluRs) is necessary, but not sufficient for the LTD induction. Using mouse hippocampal slices, we attempted to identify additional presynaptic factors involved in the induction of mossy fibre LTD. Suppression of a rise in the presynaptic intracellular Ca2+ concentration ([Ca2+]i) with a membrane-permeable Ca2+ chelator, 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester (BAPTA-AM), reduced the magnitude of LTD, whereas an increase in Ca2+ influx induced LTD, suggesting that an elevation of presynaptic [Ca2+]i is crucial for the LTD induction. A broad-spectrum protein kinase inhibitor, H-7, blocked LTD without affecting a presynaptic inhibition induced by an mGluR agonist. Furthermore, LTD was reduced by an inhibitor of calmodulin or Ca2+/calmodulin-dependent protein kinases. Thus, we conclude that mossy fibre LTD requires an increase in presynaptic [Ca2+]i and subsequent activation of Ca2+/calmodulin-dependent protein kinases. Because mossy fibre LTP may also require a rise in presynaptic [Ca2+]i, bidirectional long-term plasticity at the mossy fibre synapse is likely to be regulated by presynaptic Ca2+-dependent processes.     

Involvement of Group II mGluRs in mossy fiber LTD

Mossy fiber long‐term depression (LTD) has been shown to be triggered by either pharmacological or synaptic activation of Group II metabotropic glutamate receptors (mGluRs) whereas other studies indicate that synaptic activation of mGluRs is very limited. Therefore, we reexamined the role of Group II mGluRs for the induction of mossy fiber LTD. The complete depression of field potentials (fEPSPs) by 1 μM (2S,2′R,3′R)‐2‐(2′,3′‐Dicarboxycyclopropyl)glycine (DCG‐IV) only partially reversed upon removal of the drug but fEPSPs were completely restored by the Group II antagonist 2S‐2‐amino‐2‐(1S,2S‐2‐carboxycyclopropyl‐1‐yl)‐3‐(xanth‐9‐yl)propanoic acid (LY341495) (3 μM). In contrast, fEPSPs returned back to baseline within 30 min after a brief application of 0.2 μM DCG‐IV suggesting that the incomplete reversal of higher concentrations may be due to a residual receptor occupancy rather than to an induction of LTD. LY341495 itself did not increase fEPSPs and also blocked the inhibition of (2S,1′S,2′S)‐2‐(2‐carboxycyclopropyl)glycine (L‐CCG‐I) (20 μM) and (1S,3R)‐1‐aminocyclopentane‐1,3‐dicarboxylic acid (ACPD) (10 μM) and its effect was mimicked by CPPG (50 μM). Furthermore, stimulation at 1 Hz for 15 min induced an LTD of 81% ± 3% and 80% ± 4% in the absence and presence of LY341495, respectively (n = 7, 5). Finally, we found that synaptic activation of Group II mGluRs during 15 min of 1‐Hz stimulation only produces an inhibition of release by 8% ± 1% (30°C, n = 3). Our data suggests that pharmacological activation of Group II mGluRs is fully reversible per se and does not produce a long lasting depression and that activation of Group II mGluRs is neither necessary nor sufficient for the induction of mossy fiber LTD. Synapse 63:1060–1068, 2009. © 2009 Wiley‐Liss, Inc.     

A novel form of low-frequency hippocampal mossy fiber plasticity induced by bimodal mGlu1 receptor signaling

Mossy fiber synapses act as the critical mediators of highly dynamic communication between hippocampal granule cells in the dentate gyrus and CA3 pyramidal neurons. Excitatory synaptic strength at mossy fiber to CA3 pyramidal cell synapses is potentiated rapidly and reversibly by brief trains of low-frequency stimulation of mossy fiber axons. We show that slight modifications to the pattern of stimulation convert this short-term potentiation into prolonged synaptic strengthening lasting tens of minutes in rodent hippocampal slices. This low-frequency potentiation of mossy fiber EPSCs requires postsynaptic mGlu1 receptors for induction but is expressed presynaptically as an increased release probability and therefore impacts both AMPA and NMDA components of the mossy fiber EPSC. A nonconventional signaling pathway initiated by mGlu1 receptors contributes to induction of plasticity, because EPSC potentiation was prevented by a tyrosine kinase inhibitor and only partially reduced by guanosine 5'-O-(2-thiodiphosphate). A slowly reversible state of enhanced synaptic efficacy could serve as a mechanism for altering the integrative properties of this synapse within a relatively broad temporal window.     

Long- and short-term plasticity at mossy fiber synapses on mossy cells in the rat dentate gyrus

Mossy cells give rise to the commissural and associational pathway of the dentate gyrus, and receive their major excitatory inputs from the mossy fibers of granule cells. Through these feed-back excitatory connections, mossy cells have been suggested to play important roles in both normal signal processing in learning and memory, as well as in seizure propagation. However, the nature of the activity-dependent modifications of the mossy fiber inputs to mossy hilar cells is not well understood. We studied the long- and short-term plasticity properties of the mossy fiber-mossy cell synapse, using the minimal stimulation technique in slices in whole cell recorded mossy cells retrogradely prelabeled with the fluorescent dye DiO from the contralateral dentate gyrus. Following tetanic stimulation, mossy fiber synapses showed significant NMDA receptor-independent long-term potentiation (LTP), associated with increased excitatory postsynaptic currents (EPSC) amplitude and decreased failure rates. Coefficient of variance and failure rate analyses suggested a presynaptic locus of LTP induction. Mossy fiber synapses on mossy cells also showed activity-dependent short-term modification properties, including both frequency-dependent facilitation (stimuli at higher frequencies evoked larger EPSCs with lower failure rates) and burst facilitation (each EPSC in a burst had a larger amplitude and higher probability of occurrence than the preceding EPSCs within the burst). The data show that mossy fiber-mossy cell synapses exhibit both long- and short-term plasticity phenomena that are generally similar to the mossy fiber synapses on CA3 pyramidal cells.     

Synaptically-released zinc inhibits N-methyl-D-aspartate receptor activation at recurrent mossy fiber synapses

Hippocampal slices from pilocarpine-treated rats were used to explore the effect of zinc released at mossy fiber synapses on dentate granule cells. Chelation of zinc enhanced the N-methyl-D-aspartate (NMDA) receptor-mediated component of the excitatory postsynaptic current (EPSC), but did not affect the AMPA/kainate receptor-mediated component. Its effect was detectable only at negative membrane potentials and was pathway specific. Thus corelease of zinc reduces the ability of glutamate to activate postsynaptic NMDA receptors. Through this action, zinc would be expected to attenuate granule cell epileptiform activity supported by the recurrent mossy fiber pathway.     

Cajal-Retzius cells in early postnatal mouse cortex selectively express functional metabotropic glutamate receptors

Glutamate receptors have been linked to the regulation of several developmental events in the CNS. By using cortical slices of early postnatal mice, we show that in layer I cells, glutamate produces intracellular calcium ([Ca(2+)](i)) elevations mediated by ionotropic and metabotropic glutamate receptors (mGluRs). The contribution of mGluRs to these responses was demonstrated by application of tACPD, an agonist to groups I and II mGluRs, which evoked [Ca(2+)](i) increases that could be reversibly blocked by MCPG, an antagonist to groups I and II mGluRs. In the absence of extracellular Ca(2+), repetitive applications of tACPD or quisqualate, an agonist to group I mGluRs, elicited decreasing [Ca(2+)](i) responses that were restored by refilling a thapsigargin-sensitive Ca(2+) store. The use of specific group I mGluR agonists CHPG and DHPG indicated that the functional mGluR in layer I was of the mGluR1 subtype. Subtype specific antibodies confirmed the presence of mGlur1 alpha, but not mGluR5, in Cajal-Retzius (Reelin-immunoreactive) neurons.     

Group I metabotropic glutamate receptor actions in oriens/alveus interneurons of rat hippocampal CA1 region

Group I metabotropic glutamate receptors (mGluRs) are important for hippocampal interneuron function. We used whole-cell recording and confocal imaging to characterize group I mGluR actions in CA1 oriens/alveus interneurons in slices. In tetrodotoxin and ionotropic glutamate receptor antagonists, the group I mGluR specific agonist DHPG increased intradendritic Ca(2+) levels and depolarized interneurons, whereas the group II mGluR specific agonist DCG-IV and the group III mGluR specific agonist L-AP4 did not. DHPG-induced depolarizing and Ca(2+) responses were antagonized by the group I mGluR antagonist 4CPG, but only Ca(2+) responses were significantly inhibited by the mGluR1 antagonist CPCCOEt. DHPG-induced depolarizing responses were not blocked by the inositol-1,4,5-trisphosphate (IP(3)) receptor inhibitor heparin, the protein kinase C (PKC) antagonists GF-109203X, or the inhibitor of phospholipase C (PLC) U73122. Thus, these responses to DHPG may not be transduced by the PLC-->IP(3)/diacylglycerol (DAG) pathway classically linked to group I mGluRs. DHPG-induced depolarizations were not blocked by intracellular GDP beta S or bath-application of N-ethylmaleimide (NEM), suggesting the involvement of a G protein-independent pathway. Our findings indicate that group I mGluRs induce a depolarization of oriens/alveus interneurons via a G protein-independent mechanism different from their classic signalling pathway. Since depolarizations are associated with intracellular Ca(2+) rises, these actions may be important for their synaptic plasticity and vulnerability to excitotoxicity.     

Metabotropic glutamate receptors modulate [(3)H]acetylcholine release from cultured amacrine-like neurons.

Retinal amacrine cells express metabotropic glutamate receptors (mGluRs), but their physiological role is unknown. We investigated the effect of mGluR on [(3)H]acetylcholine release ([(3)H]ACh) from cultured chick amacrine-like neurons. Activation of group III mGluR with the agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) inhibited [(3)H]ACh release evoked by 25 mM KCl in a dose-dependent manner, and this effect was sensitive to pertussis toxin. In contrast, activation of group I or II mGluR with (S)-3, 5-dihydroxyphenylglycine (DHPG) and (2S,2'R,3'R)-2-(2', 3'-dicarboxycyclopropyl)glycine (DCG-IV), respectively, did not affect significantly [(3)H]ACh release. The effect of L-AP4 on [(3)H]ACh release was sensitive to nitrendipine, suggesting that it is, at least in part, due to inhibition of L-type Ca(2+) channels. Activation of group III mGluR also partly inhibited omega-conotoxin GVIA-sensitive Ca(2+) channels, coupled to [(3)H]ACh release. The L-AP4 did not affect the cAMP levels measured in amacrine-like neurons depolarized with 25 mM KCl or stimulated with forskolin, indicating that the effect of group III mGluR on [(3)H]ACh release is not due to inhibition of adenylyl cyclase activity. Inhibition of protein kinase A with KT-5720 was without effect on [(3)H]ACh release evoked by 25 mM KCl, further indicating that the effect of group III mGluR on [(3)H]ACh release cannot be attributed to the inhibition of the kinase. The effect of L-AP4 on [(3)H]ACh release was reversed by DHPG or by DCG-IV, and activation of group II mGluR also partially inhibited cAMP production stimulated by forskolin. Taken together, our results show that the effect of group III mGluR on [(3)H]ACh release may be due to a direct inhibition of L- and N-type Ca(2+) channels and is modulated by group I and group II mGluR.     

Insulin induces a novel form of postsynaptic mossy fiber long-term depression in the hippocampus

The mechanisms of induction and the site of expression of long-term depression (LTD) at the hippocampal mossy fiber-CA3 synapses are not clear. Here, we show that a brief bath application of insulin induces a novel form of mossy fiber LTD. This insulin-LTD is (1) induced and expressed postsynaptically, (2) entirely independent of synaptic stimulation during insulin application, (3) involving a rise in postsynaptic [Ca(2+)](i) and L-type voltage-activated Ca(2+) channel activation, (4) mechanistically distinct from low-frequency stimulation-induced LTD, (5) dependent on phosphatidylinositol 3-kinase signaling, and (6) associated with a clathrin-mediated endocytotic removal of surface 3-hydroxy-5-methylisoxazole-4-propionic acid receptors from the postsynaptic neurons. Moreover, insulin-LTD is specific to mossy fibers to CA3 pyramidal cell synapses, and is not present at associational commissural synapses. These findings not only support a postsynaptic locus of mossy fiber LTD, but also provide a further link between the AMPA receptor trafficking and the bidirectional expression of long-term synaptic plasticity.     

Developmental profile of the changing properties of NMDA receptors at cerebellar mossy fiber-granule cell synapses.

During cerebellar development, granule cells display well characterized changes in the expression of NMDA receptor (NMDAR) NR2 subunits, switching from NR2B to NR2A and NR2C in mature cells. Although various studies, including experiments on mutant mice with one or more NR2 subunit types deleted, suggest that NR2A, NR2B, and NR2C subunits contribute to synaptic NMDARs, changes in the properties of the mossy fiber EPSC during development have not been fully evaluated. In particular, information on NMDAR EPSCs in mature animals is lacking. We have examined pharmacological and kinetic properties of NMDARs at mossy fiber-granule cell synapses from their formation to maturity [postnatal day 7 (P7)-P40 rats]. Significant changes were seen in the relative amplitudes of the non-NMDAR- and NMDAR-mediated components of the evoked EPSC and in the decay kinetics of the latter. The NMDA/non-NMDA ratio was similar at P7, P21, and P40, but showed a clear peak at P12. This change coincided with a speeding of the NMDAR EPSC decay, accompanied by a decrease in sensitivity to ifenprodil (selective NR2B-antagonist). By P21, sensitivity of the NMDAR EPSC to Mg(2+) was approximately threefold less than that at P12 (IC(50), 76 vs 28 microm), suggesting incorporation of the NR2C subunit. However, the predicted slowing of decay kinetics to a value more characteristic of NR2C deactivation, was not seen until P40. Our data are consistent with the known switch from NR2B to NR2A subunits during the first two postnatal weeks, but suggest a gradual incorporation of the NR2C subunit that modifies Mg(2+) sensitivity and only later influences EPSC kinetics.     

Plasticity of Metabotropic Glutamate Receptor-Dependent Long-Term Depression in the Anterior Cingulate Cortex after Amputation

Long-term depression (LTD) is a key form of synaptic plasticity important in learning and information storage in the brain. It has been studied in various cortical regions, including the anterior cingulate cortex (ACC). ACC is a crucial cortical region involved in such emotion-related physiological and pathological conditions as fear memory and chronic pain. In the present study, we used a multielectrode array system to map cingulate LTD in a spatiotemporal manner within the ACC. We found that low-frequency stimulation (1 Hz, 15 min) applied onto deep layer V induced LTD in layers II/III and layers V/VI. Cingulate LTD requires activation of metabotropic glutamate receptors (mGluRs), while L-type voltage-gated calcium channels and NMDA receptors also contribute to its induction. Peripheral amputation of the distal tail impaired ACC LTD, an effect that persisted for at least 2 weeks. The loss of LTD was rescued by priming ACC slices with activation of mGluR1 receptors by coapplying (RS)-3,5-dihydroxyphenylglycine and MPEP, a form of metaplasticity that involved the activation of protein kinase C. Our results provide in vitro evidence of the spatiotemporal properties of ACC LTD in adult mice. We demonstrate that tail amputation causes LTD impairment within the ACC circuit and that this can be rescued by activation of mGluR1.     

Reversal of glutamate excitotoxicity by activation of PKC-associated metabotropic glutamate receptors in cerebellar granule cells relies on NR2C subunit expression.

Stimulation of metabotropic glutamate receptors (mGluRs) belonging to group I has been found to reduce N-methyl-D-aspartate (NMDA) receptor function in terms of both intracellular calcium concentration ([Ca2+]i) rise and neurotoxicity in cultured cerebellar granule cells. In the present study, we investigated whether the mGluR-elicited modulation of glutamate responses might rely on the heteromeric composition of NMDA receptor channel. NMDA receptors consist of two distinct groups of subunits: NR1, that is ubiquitously in the receptor complexes; and NR2A-D, that differentiate and potentiate NMDA receptor responses by assembling with NR1. Among NR2 subunits, only NR2A and NR2C mRNAs and relative proteins are detected in cerebellar granule cells at 10 days in vitro. To dissect the involvement of the two different subunits in making the NMDA receptor channel sensitive to modulation by group I mGluR agonists, expression of the NR2C subunit was prevented by treating the cells with specific antisense oligodeoxynucleotide (ODN). The capability of the mGluR agonists, trans-1-amino-cyclopentane-1,3-dicarboxylic acid (tACPD, 100 microM) or 3 hydroxyphenylglycine (3HPG, 100 microM), and the protein kinase C (PKC) activator, 4beta-phorbol-12,13-dibutyrate (PDBu, 1 microM), to inhibit the function of resultant NMDA receptors was then evaluated. We found that depletion of the NR2C subunit abolished the inhibitory effect of group I mGluR stimulation on glutamate-induced [Ca2+]i rise and neurotoxicity. The antisense ODN treatment also prevented the inhibitory effect of PDBu on glutamate responses. Conversely, in NR2C-lacking neurons, both group I mGluRs and PKC stimulation enhanced NMDA receptor-mediated effects. The present findings indicate that the capability of PKC-associated mGluRs to modulate native NMDA receptor function relies on the heteromeric configuration of the receptor-channel complex. Particularly, expression of the NR2C subunit is required to make the NMDA receptor sensitive to inhibitory modulation by mGluRs or PKC activation.     

High affinity group III mGluRs regulate mossy fiber input to CA3 interneurons

Stratum lacunosum‐moleculare interneurons (L‐Mi) in hippocampal area CA3 target the apical dendrite of pyramidal cells providing feedforward inhibition. Here we report that selective activation of group III metabotropic glutamate receptors (mGluRs) 4/8 with L(+)‐2‐amino‐4‐phosphnobytyric acid (L‐AP4; 10 μM) decreased the probability of glutamate release from the mossy fiber (MF) terminals synapsing onto L‐Mi. Consistent with this interpretation, application of L‐AP4 in the presence of 3 mM strontium decreased the frequency of asynchronous MF EPSCs in L‐Mi. Furthermore, the dose response curve showed that L‐AP4 at 400 μM produced no further decrease in MF EPSC amplitude compared with 20 μM L‐AP4, indicating the lack of mGluRs 7 at these MF terminals. We also found that one mechanism of mGluRs 4/8‐mediated inhibition of release is linked to N‐type voltage gated calcium channels at MF terminals. Application of the group III mGluR antagonist MSOP (100 μM) demonstrated that mGluRs 4/8 are neither tonically active nor activated by low and moderate frequencies of activity. However, trains of stimuli to the MF at 20 and 40 Hz delivered during the application of MSOP revealed a relief of inhibition of transmitter release and an increase in the overall probability of action potential firing in the postsynaptic L‐Mi. Interestingly, the time to first action potential was significantly shorter in the presence of MSOP, indicating that mGluR 4/8 activation delays L‐Mi firing in response to MF activity. Taken together, our data demonstrate that the timing and probability of action potentials in L‐Mi evoked by MF synaptic input is regulated by the activation of presynaptic high affinity group III mGluRs. © 2010 Wiley Periodicals, Inc.     

Stereologic estimation of hippocampal GluR2/3- and calretinin-immunoreactive hilar neurons (presumptive mossy cells) in two mouse models of temporal lobe epilepsy

Purpose: Hippocampal mossy cells receive dense innervation from dentate granule cells and, in turn, mossy cells innervate both granule cells and interneurons. Mossy cell loss is thought to trigger granule cell mossy fiber sprouting, which may affect granule cell excitability. The aim of this study was to quantify mossy cell loss in two animal models of temporal lobe epilepsy, and determine whether there exists a relationship between mossy cell loss, mossy fiber sprouting, and granule cell dispersion. Methods: Representative hippocampal sections from p35 knockout mice and mice with unilateral intrahippocampal kainate injection were immunolabeled for GluR2/3, two subunits of the amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionate (AMPA) receptor and calretinin to identify mossy cells. Mossy fibers were immunostained against synaptoporin. Key Findings: p35 Knockout mice showed no hilar cell death, but moderate mossy fiber sprouting and granule cell dispersion. In the kainate‐injected hippocampus, there was an 80% and 85% reduction of GluR2/3‐ and GluR2/3/calretinin‐positive hilar neurons, respectively, and dense mossy fiber sprouting and significant granule cell dispersion. In the contralateral hippocampus there was a 52% loss of GluR2/3‐, but only a 20% loss of GluR2/3‐calretinin‐immunoreactive presumptive mossy cells, and granule cell dispersion; no mossy fiber sprouting was observed. Significance: These results indicate a probable lack of causality between mossy cell death and mossy fiber sprouting.     

Roles of glutamate receptors in long-term potentiation at hippocampal mossy fiber synapses.

We have investigated the roles of glutamate (Glu) receptors in the mechanism of long-term potentiation observed in rat hippocampal mossy fiber synapses (MF-LTP). The mossy fiber responses were almost completely suppressed by ionotropic Glu receptor (iGluR) antagonists both before and after the induction of LTP. However, tetanic stimulation produced robust LTP even when the synaptic transmission was blocked postsynaptically by iGluR antagonists. In contrast, when the transmission was blocked presynaptically by Ca(2+)-free media, tetanic stimulation produced no LTP. D,L-2-amino-3-phosphono-propionate(D,L-AP3), a metabotropic Glu receptor (mGluR) antagonist, inhibited the induction of MF-LTP. Perfusion with ibotenate, a mGluR agonist, induced long-lasting enhancement of the mossy fiber responses without tetanic stimulation, and this ibotenate-induced potentiation was antagonized by D,L-AP3.     

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

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

Postsynaptic activity of metabotropic glutamate receptors in the piriform cortex

Effects of some glutamate receptor antagonists on signal propagation elicited by stimulation of association fibers in guinea pig piriform cortex slices were investigated using optical imaging. During simultaneous application of both NMDA and non-NMDA receptor antagonists (D-2-amino-5-phosphonopentanoic acid and 6-cyano-7-nitroquinoxaline-2,3-dione, respectively) the postsynaptic activity was largely suppressed, and a weak although distinct late propagation component was found to survive. This latter component was reversibly suppressed by application of low Ca(2+) solution or a group I/II specific metabotropic glutamate receptor (mGluR) antagonist (+)-alpha-methyl-4-carboxyphenylglycine. These results suggest that mGluRs mediate postsynaptic excitation, which would play a crucial role in activating the reverberating positive feedback circuit effectively.