An antagonist of the metabotropic glutamate receptor prevents LTP in the dentate gyrus of freely moving rats

Glutamate receptors and synaptic plasticity: The impact of Evans and Watkins

On the occasion of the 40 year anniversary of the hugely impactful review by Richard (Dick) Evans and Jeff Watkins, we describe how their work has impacted the field of synaptic plasticity. We describe their influence in each of the major glutamate receptor subtypes: AMPARs, NMDARs, KARs and mGluRs. Particular emphasis is placed on how their work impacted our own studies in the hippocampus. For example, we describe how the tools and regulators that they identified for studying NMDARs (e.g., NMDA, D-AP5 and Mg²⁺) led to the understanding of the molecular basis of the induction of LTP. We also describe how other tools that they introduced (e.g., (1S,3R)-ACPD and MCPG) helped lead to the concept of metaplasticity.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

The late maintenance of hippocampal LTP: Requirements, phases, ‘synaptic tagging’, ‘late-associativity’ and implications

Our review focuses on the mechanisms which enable the late maintenance of hippocampal long-term potentiation (LTP; >3h), a phenomenon which is thought to underlie prolonged memory. About 20 years ago we showed for the first time that the maintenance of LTP - like memory storage--depends on intact protein synthesis and thus, consists of at least two temporal phases. Here we concentrate on mechanisms required for the induction of the transient early-LTP and of the protein synthesis-dependent late-LTP. Our group has shown that the induction of late-LTP requires the associative activation of heterosynaptic inputs, i.e. the synergistic activation of glutamatergic and modulatory, reinforcing inputs within specific, effective time windows. The induction of late-LTP is characterized by novel, late-associative properties such as 'synaptic tagging' and 'late-associative reinforcement'. Both phenomena require the associative setting of synaptic tags as well as the availability of plasticity-related proteins (PRPs) and they are restricted to functional dendritic compartments, in general. 'Synaptic tagging' guarantees input specificity and thus the specific processing of afferent signals for the establishment of late-LTP. 'Late-associative reinforcement' describes a process where early-LTP by the co-activation of modulatory inputs can be transformed into late-LTP in activated synapses where a tag is set. Recent evidence from behavioral experiments, which studied processes of emotional and cognitive reinforcement of LTP, point to the physiological relevance of the above mechanisms during cellular and system's memory formation.

The metabotropic glutamate receptor 5 is necessary for late-phase long-term potentiation in the hippocampal CA1 region

Selective antagonists of the metabotropic receptors 1 (mGluR1), +/-2-methyl-4-carboxyphenylglycine (LY367385), and mGluR5, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), were used to investigate the role of group I metabotropic receptors in late-phase long-term potentiation (L-LTP) at Schaffer collateral/commissural fiber-CA1 synapses in rat hippocampal slices. L-LTP was induced with three trains of tetanization of 1 s duration at 100 Hz separated by 10-min intervals. Neither LY367385 nor MPEP affected basal synaptic responses at the doses used (200 and 10 microM, respectively) and only the mGluR5 inhibitor MPEP blocked L-LTP. However, in agreement with previous mouse mutant studies, we found that both LY367385 and MPEP inhibited the induction of an LTP obtained with a single train of tetanization of 1 s duration at 100 Hz. MPEP's ability to disrupt L-LTP was not due to an effect on NMDA responses since it did not affect pharmacologically isolated N-methyl-D-aspartate (NMDA) excitatory postsynaptic potentials (EPSPs). However, MPEP prevented the increased phosphorylation in dendrites of p70 S6 kinase (p70(S6K)) at Thr3889, a major regulator of translation required for the induction of protein synthesis-dependent forms of LTP.

Group I metabotropic glutamate receptor signaling via Galpha q/Galpha 11 secures the induction of long-term potentiation in the hippocampal area CA1

Heterotromeric G-proteins of the Gq family are thought to transduce signals from group I metabotropic glutamate receptors (mGluRs) in central neurons. We investigated roles of this cascade in hippocampal long-term potentiation (LTP) by using null-mutant mice lacking the alpha subunit of Gq (Galphaq) or G11 (Galpha11). We found no obvious abnormalities in the morphology, layer structure, expression of NMDA receptors, and basic parameters of excitatory synaptic transmission in the hippocampus of Galphaq mutant mice. We used theta burst stimulation (TBS) (3-10 burst trains at 5 Hz; each train consisted of five stimuli at 100 Hz) to induce LTP at Schaffer collateral to CA1 pyramidal cell synapses. Conventional TBS with 10 burst trains induced robust LTP in wild-type, Galphaq mutant, and Galpha11 mutant mice. Weak TBS with three burst trains consistently induced LTP in wild-type mice. In contrast, the same weak TBS was insufficient to induce LTP in Galphaq and Galpha11 mutant mice. In wild-type mice, the LTP by weak TBS was abolished by inhibiting group I mGluR or protein kinase C (PKC) but not by blocking muscarinic acetylcholine receptors. Prior activation of group I mGluR by an agonist significantly enhanced the LTP by weak TBS in wild-type mice. However, this priming effect was absent in Galphaq mutant mice. These results indicate that the signaling from group I mGluR to PKC involving Galphaq/Galpha11 does not constitute the main pathway for LTP, but it secures LTP induction by lowering its threshold in the hippocampal area CA1.

Metabotropic glutamate receptor-mediated hippocampal phosphoinositide turnover is blunted in spatial learning-impaired aged rats

Maximal phosphoinositide (PI) turnover was examined in the hippocampus of young and aged Long-Evans rats that were behaviorally characterized for spatial learning in the Morris water maze. The type 1 metabotropic glutamate receptor (mGluR) agonist 1S,3R ACPD was used to stimulate PI turnover and to determine the E(MAX) for each rat. Protein levels in hippocampus for type 1 mGluRs, Galphaq11, and phospholipase Cbeta-1 (PLCbeta-1) were also measured by quantitative Western blotting. The results show that PI turnover mediated by the mGluRs was blunted in the aged rats. The magnitude of the decrement in PI turnover was also significantly correlated with age-related spatial memory decline. The decrease in mGluR-mediated PI turnover occurred without changes in the protein level of either the mGluRs or the G-protein coupled to those receptors, Galphaq11. A significant decrease in the immunoreactivity of PLCbeta-1, however, was observed in the hippocampus of aged rats; PLCbeta-1 immunoreactivity was significantly correlated with spatial learning only when the young and aged rats were considered together. The decrement in mGluR-mediated signal transduction in the hippocampus that is related to cognitive impairment in aging may be attributable, at least in part, to a deficiency in the enzyme PLCbeta-1. That deficiency may also contribute to a blunted response in muscarinic stimulation of hippocampal PI turnover that we previously found in this same study population. An age-related alteration in this signal transduction system may provide a functional basis for cognitive decline independent of any loss of neurons in the hippocampus.

Transient translocation of protein kinase Cgamma in hippocampal long-term potentiation depends on activation of metabotropic glutamate receptors

Protein kinase C has been implicated in long-term regulation of cellular functions including induction and maintenance of hippocampal long-term potentiation. In the present study the time-course of long-term potentiation-induced translocation of Ca(2+)-dependent protein kinase C isoenzymes (PKCalpha/beta and PKCgamma) was investigated. Quantitative immunoblot analysis was used to measure translocation of these isoenzymes between cytosolic, membrane-associated and membrane-inserted fraction at 5, 15 and 60 min after induction of long-term potentiation in the dentate gyrus in vivo. To investigate the involvement of metabotropic glutamate receptors in protein kinase C regulation during long-term potentiation induction, additional animals were treated before tetanization with (R,S)-alpha-methyl-4-carboxyphenylglycine, an antagonist of metabotropic glutamate receptors. Brief tetanic stimulation of the perforant path resulted in a 100-150% increase in the population spike amplitude in response to test stimuli 5, 15 or 60 min after stimulation in both untreated and (R,S)-alpha-methyl-4-carboxyphenylglycine-treated animals. Only those rats showing clear potentiation were selected for further biochemical analysis of the potentiated dentate gyrus. Five minutes after high-frequency stimulation the subcellular distribution of all studied protein kinase C isoenzymes was unchanged compared with controls. PKC-gamma translocated into the cytosol 15 min after tetanization and this redistribution was blocked by (R,S)-alpha-methyl-4-carboxyphenylgly-cine pretreatment. By contrast, PKC alpha/beta levels increased in the cytosolic fraction only 60 min after tetanization, but in a (R,S)-alpha-methyl-4-carboxyphenylglycine-independent manner. In an additional set of experiments it was shown that (R,S)-alpha-methyl-4-carboxyphenylglycine alone applied intraventricularly had no effect on the subcellular distribution of the studied isoenzymes. The data suggest that PKCalpha/beta and PKCgamma are activated during different post-tetanic phases and metabotropic glutamate receptor activation might be essential for tetanus-induced translocation of postsynaptic PKCgamma only.

Roles of metabotropic glutamate receptors in LTP and LTD in the hippocampus

Metabotropic L-glutamate receptors are involved in various forms of synaptic plasticity in the hippocampus. The use of a new antagonist (LY341495) that blocks all known metabotropic L-glutamate receptors in the brain, together with subtype-selective antagonists, has identified multiple roles both for cloned and novel metabotropic L-glutamate receptors in hippocampal long-term potentiation and long-term depression.

Metabotropic glutamate receptor activation and blockade: their role in long-term potentiation, learning and neurotoxicity

Metabotropic glutamate receptors represent a fairly recent addition to the family of glutamate receptors. These receptors have the distinguishing feature of being coupled to G-proteins rather than ion channels and they appear to have a variety of functional characteristics. These receptors play a vital role, for example, in the induction and maintenance of long-term potentiation, the most popular current model of the biological correlates of learning and memory. Blockade of metabotropic glutamate receptors prevents long-term potentiation induction and learning in a variety of tasks in different species. Chronic metabotropic glutamate receptor activation is also associated with neurodegeneration and selective neuronal loss when agonists of these receptors are injected in high concentrations directly into the brain. Metabotropic glutamate receptors also play a role in the normal development of the nervous system and these sites within the central nervous system offer possible routes for drug therapies; selective receptor antagonists, for example, may prove to have the very desirable feature of endowing neuroprotection during ischaemic episodes whilst allowing normal excitatory neurotransmission to occur.

Metabotropic glutamate receptors: electrophysiological properties and role in plasticity

Electrophysiological research on mGluRs is now very extensive, and it is clear that activation of mGluRs results in a large number of diverse cellular actions. Studies of mGluRs and on ionic channels has clearly demonstrated that mGluR activation has a widespread and potent inhibitory action on both voltage-gated Ca2+ channels and K+ channels. Inhibition of N-type Ca2+ channels, and inhibition of Ca(++)-dependent K+ current, IAHP, and IM being particularly prominent. Potentiation of activation of both Ca2+ and K+ channels has also been observed, although less prominently than inhibition, but mGluR-mediated activation of non-selective cationic channels is widespread. In a small number of studies, generation of an mGluR-mediated slow excitatory postsynaptic potential has been demonstrated as a consequence of the effect of mGluR activation on ion channels, such as activation of a non-selective cationic channels. Although certain mGluR-modulation of channels is a consequence of direct G-protein-linked action, for example, inhibition of Ca2+ channels, many other effects occur as a result of activation of intracellular messenger pathways, but at present, little progress has been made on the identification of the messengers. The field of study of the involvement of mGluRs in synaptic plasticity is very large. Evidence for the involvement of mGluRs in one form of LTD induction in the cerebellum and hippocampus is now particularly impressive. However, the role of mGluRs in LTP induction continues to be a source of dispute, and resolution of the question of the exact involvement of mGluRs in the induction of LTP will have to await the production of more selective ligands and of selective gene knockouts.

Selective Abolition of the NMDA Component of Long-Term Potentiation in Mice Lacking mGluR5

The mechanisms underlying the differential expression of long-term potentiation (LTP) by AMPA and NMDA receptors, are unknown, but could involve G-protein-linked metabotropic glutamate receptors. To investigate this hypothesis we created mutant mice that expressed no metabotropic glutamate receptor 5 (mGluR5), but showed normal development. In an earlier study of these mice we analyzed field-excitatory postsynaptic potential (fEPSPs) in CA1 region of the hippocampus and found a small decrease; possibly arising from changes in the NMDAR-mediated component of synaptic transmission. In the present study we used whole-cell patch clamp recordings of evoked excitatory postsynaptic currents (EPSCs) in CA1 pyramidal neurons to identify the AMPAR- and NMDAR-mediated components of LTP. Recordings from control mice following tetanus, or agonist application (IS, 3R-1-amino-cyclopentane 1,3-dicarboxylic acid) (ACPD), revealed equal enhancement of the AMPA and NMDA receptor-mediated components. In contrast, CA1 neurons from mGluR5-deficient mice showed a complete loss of the NMDA-receptor-mediated component of LTP (LTPNMDA), but normal LTP of the AMPA-receptor-mediated component (LTPAMPA). This selective loss of LTPNMDA was seen in three different genotypic backgrounds and was apparent at all holding potentials (−70 mV to +20 mV). Furthermore, the LTPNMDA deficit in mGluR5 mutant mice could be rescued by stimulating protein kinase C (PKC) with 4β-phorbol-12,13-dibutyrate (PDBu). These results suggest that PKC may couple the postsynaptic mGluR5 to the NMDA-receptor potentiation during LTP, and that this signaling mechanism is distinct from LTPAMPA. Differential enhancement of AMPAR and NMDA receptors by mGluR5 also supports a postsynaptic locus for LTP.

Analysis of the interaction between arachidonic acid and metabotropic glutamate receptor activation reveals that phospholipase C acts as a coincidence detector in the expression of long-term potentiation in the rat dentate gyrus

We have reported that arachidonic acid and the metabotropic glutamate receptor agonist, trans-1-amino-cyclopentyl-1,3-dicarboxylate (ACPD), act in synergy to increase release of glutamate from synaptosomes prepared from rat dentate gyrus. The observation that prior induction of LTP in perforant path-granule cell synapses occluded this synergism suggested that the interaction between arachidonic acid and ACPD might trigger the increase in glutamate release that accompanies LTP in dentate gyrus. Our objective was to identify the mechanism underlying the synergism between arachidonic acid and ACPD in LTP. The data indicate that both agents activate phospholipase C(PLC); the arachidonic acid-induced increase in phospholipase C activation was inhibited by the tyrosine kinase inhibitor, genistein, suggesting that PLCgamma, which is stimulated by tyrosine phosphorylation may be activated by arachidonic acid. The ACPD-induced increase was inhibited by neomycin, indicating the involvement of a G-protein and suggesting that PLCbeta may be activated by ACPD. We report that arachidonic acid stimulated phosphorylation of the specific tyrosine kinase substrate, poly(Glu80,Tyr20) and direct analysis indicated that arachidonic acid increased phosphorylation of PLCgamma. PLCgamma phosphorylation was assessed in control dentate gyrus and dentate gyrus in which LTP was induced in vivo. We report that the tyrosine kinase inhibitor, genistein, blocked expression of LTP and also blocked the associated increase in phosphorylation of PLCgamma. The data presented here indicate that tyrosine phosphorylation of PLCgamma was significantly enhanced following induction of LTP, but in separate experiments, in which LTP was inhibited by intraventricular injection of genistein, phosphorylation of PLCgamma was inhibited. The evidence presented is consistent with the hypothesis that PLC acts as a coincidence detector in LTP. The data indicate that PLCbeta is activated by ACPD, PLCgamma is activated by arachidonic acid, and coincident activation of both isoforms is necessary to stimulate an increase in glutamate release.

(+)-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.

The ability of aged rats to sustain long-term potentiation is restored when the age-related decrease in membrane arachidonic acid concentration is reversed

The ability of aged rats to sustain long-term potentiation in the dentate gyrus of the hippocampus is impaired and this impairment correlates with decreased release of glutamate and a decrease in membrane arachidonic acid concentration. Twenty-two-month-old rats receiving a diet supplemented with arachidonic acid and its precursor, gamma-linolenic acid, sustained long-term potentiation in a manner indistinguishable from four-month-old controls. Dietary supplementation also restored arachidonic acid concentrations in membranes prepared from hippocampus of these aged animals to levels observed in hippocampus of four-month-old rats. Glutamate release stimulated by depolarization was similar in dentate gyrus prepared from young rats and aged rats which received the experimental diet, but was markedly reduced in aged animals which received the control diet. In addition, the synergism between arachidonic acid and the metabotropic glutamate receptor agonist, trans-1-amino-cyclopentyl-1,3-dicarboxylate, on glutamate release, which was observed in hippocampal synaptosomes prepared from four-month-old rats, was also observed in hippocampal preparations obtained from aged rats which had been fed with the experimental diet, but was absent in hippocampal preparations obtained from aged animals which were fed with control diet. Thus, reversing the age-related decrease in membrane arachidonic acid concentration restored ability of aged animals to sustain long-term potentiation and reversed age-related changes in glutamate release.

Chronic intraventricular infusion with NGF improves LTP in old cognitively-impaired rats

Aged (21 months) cognitively-impaired male Sprague-Dawley rats received intraventricular infusion of nerve growth factor (NGF) or cytochrome C (Cit C) for 14 or 28 days using miniosmotic pumps and were evaluated either 1 week or 3 months after treatment. Groups of untreated young, aged-impaired and aged non-impaired rats were also evaluated. Under narcose recording and stimulating electrodes were stereotactically implanted in the dentate gyrus and the perforant path. The stimulation intensity was individually adjusted to obtain a half-maximal population spike (P) for test stimuli and a quarter-maximal for tetanization. The amplitude and latency of P and the slope (S) of the field EPSP were determined before and at 2, 5, 15, 30 and 60 min after tetanization at 400 Hz. Paired stimuli at 30 ms interval were also applied before and after tetanization. Aged, cognitively impaired rats showed an absent S potentiation and a delayed P potentiation, both in amplitude and latency, while non-impaired rats behaved like the young controls. Paired pulse inhibition showed no difference among groups before or after tetanization suggesting that the impaired potentiation is not due to an increased retroactive inhibition. NGF treatment ameliorates LTP deficits to levels equivalent to non-impaired rats, while Cit C controls showed no improvement. No differences appear among NGF treated groups, but evidence suggest that the animals evaluated 3 months after treatment developed a stronger potentiation.

Mice lacking metabotropic glutamate receptor 5 show impaired learning and reduced CA1 long-term potentiation (LTP) but normal CA3 LTP

Class I metabotropic glutamate receptors (mGluRs) have been postulated to play a role in synaptic plasticity. To test the involvement of one member of this class, we have recently generated mutant mice that express no mGluR5 but normal levels of other glutamate receptors. The CNS revealed normal development of gross anatomical features. To examine synaptic functions we measured evoked field EPSPs in the hippocampal slice. Measures of presynaptic function, such as paired pulse facilitation in mutant CA1 neurons, were normal. The response of mutant CA1 neurons to low concentrations of (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid (ACPD) was missing, which suggests that mGluR5 may be the primary high affinity ACPD receptor in these neurons. Long-term potentiation (LTP) in mGluR5 mutants was significantly reduced in the NMDA receptor (NMDAR)-dependent pathways such as the CA1 region and dentate gyrus of the hippocampus, whereas LTP remained intact in the mossy fiber synapses on the CA3 region, an NMDAR-independent pathway. Some of the difference in CA1 LTP could lie at the level of expression, because the reduction of LTP in the mutants was no longer observed 20 min after tetanus in the presence of 2-amino-5-phosphonopentanoate. We propose that mGluR5 plays a key regulatory role in NMDAR-dependent LTP. These mutant mice were also impaired in the acquisition and use of spatial information in both the Morris water maze and contextual information in the fear-conditioning test. This is consistent with the hypothesis that LTP in the CA1 region may underlie spatial learning and memory.

Regulation of synaptic plasticity by mGluR1 studied in vivo in mGluR1 mutant mice

The role of the metabotropic glutamate receptor 1 (mGluR1) in synaptic plasticity was investigated in vivo in the intact hippocampus of mutant mice lacking this receptor. In a previous study we showed reduced long-term potentiation (LTP) in the dentate gyrus of mGluR1 -/- mice in vivo, but not when LTP was studied in a slice preparation. A possible explanation of this difference is that dentate neurons receive more inhibitory synaptic drive in vivo than in slice preparation where many inhibitory axon collaterals are lost. We report here that another form of synaptic plasticity, paired-pulse depression of the population spike, is also abnormal in the dentate gyrus of mGluR1-deficient mice when tested in vivo. In wild-type mice, stimulation of the medial perforant path produced paired-pulse depression of inter-pulse intervals (IPIs) up to 30 ms. Mutant mGluR1, on the other hand, showed a significantly longer IPI depression, up to 50 ms. Paired-pulse depression results from the activation of inhibitory interneurons. The GABA(B) agonist baclofen, acting presynaptically on the GABA interneurons, attenuated paired-pulse depression and allowed for a normal and stable LTP in mGluR1 mutant mice. These findings suggest an indirect role for mGluR1 in synaptic plasticity via a regulation of GABA inhibition.

Metabotropic glutamate receptor mediated long-term depression in developing hippocampus

The effects of bath application of the metabotropic glutamate receptor (mGluR) agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD, 10 microM) were studied at the Schaffer collateral-CA1 synapse in hippocampal slices from rats of 8-33 days postnatal age. In immature animals (8-12 days) ACPD induced a biphasic response characterized by an acute decrease in field EPSP slope (approximately 50-60% of baseline) in the presence of the agonist, followed by long-term depression (LTD, approximately 75-80% of baseline) after washout. In animals older than 20 days, ACPD induced a slow onset potentiation or minimal change. Both the acute depression and LTD were blocked by the mGluR antagonist alpha-methyl-4-carboxyphenyl glycine (MCPG). ACPD-induced LTD was blocked by the N-methyl-D-aspartate receptor (NMDAR) antagonists D(-)-2-amino-5 phosphopentanoic acid (AP5) and dizocilpine maleate (MK-801), and by ethanol. Glutamic pyruvic transaminase, an enzyme that selectively metabolizes endogenous extracellular glutamate, also blocked LTD suggesting that the requisite NMDA currents were tonically activated by extracellular rather than synaptically released glutamate. ACPD-induced LTD was blocked by staurosporine, indicating a requirement for serinethreonine kinase activation, and was unaffected by the L-type voltage sensitive calcium channel blocker nitrendipine and the A1 adenosine receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT). Because mGluR-mediated LTD was observed only in immature CA1, mGluRs may play a role in hippocampal development, perhaps by contributing to synapse pruning in a temporally restricted fashion.

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.

Long-term depression at the medial perforant path-granule cell synapse in developing rat dentate gyrus

Long-term depression (LTD) is a decrease in synaptic efficacy that may model the elimination of inappropriate synapses during brain development. LTD might therefore be expected to be prominent in the juvenile hippocampal dentate gyrus (DG), where the majority of neuronogenesis and excitatory synapse production and pruning occur in the first postnatal month. Thus far, however, LTD in immature DG remains unexplored. Low-frequency stimulus induced homosynaptic LTD was studied at the medial perforant path-granule cell synapse in rats 8-30 days of age. LTD was most consistent and was of greatest magnitude in the youngest animals, and was more robust in response to stimulation at 1 Hz than at 3 or 5 Hz. LTD was saturable by repetitive delivery of low-frequency stimulation, and reversible by tetanic stimulation that induced long-term potentiation (LTP). LTD of the field EPSP was not prevented by bath application of the NMDA receptor antagonist AP5, the mGluR antagonist MCPG, or the L-type voltage sensitive calcium channel antagonist nitrendipine. In whole cell recordings LTD induction was blocked by hyperpolarization of the postsynaptic neuron but not by calcium chelation with BAPTA. Calcium chelation blocked LTP and simultaneously unmasked tetanus induced LTD. These data demonstrate that LTD is prominent in immature DG, that LTP and LTD are complementary processes, and that LTD is likely to be induced postsynaptically because it is voltage dependent, although the mechanism of voltage dependence remains to be elucidated.

Perisynaptic location of metabotropic glutamate receptors mGluR1 and mGluR5 on dendrites and dendritic spines in the rat hippocampus

Ionotropic and metabotropic (mGluR1a) glutamate receptors were reported to be segregated from each other within the postsynaptic membrane at individual synapses. In order to establish whether this pattern of distribution applies to the hippocampal principal cells and to other postsynaptic metabotropic glutamate receptors, the mGluR1a/b/c and mGluR4 subtypes were localized by immunocytochemistry. Principal cells in all hippocampal fields were reactive for mGluR5, the strata oriens and radiatum of the CA1 area being most strongly immunolabelled. Labelling for mGluR1b/c was strongest on some pyramids in the CA3 area, weaker on granule cells and absent on CA1 pyramids. Subpopulations of non-principal cells showed strong mGluR1 or mGluR5 immunoreactivity. Electron microscopic pre-embedding immunoperoxidase and both pre- and postembedding immunogold methods consistently revealed the extrasynaptic location of both mGluRs in the somatic and dendritic membrane of pyramidal and granule cells. The density of immunolabelling was highest on dendritic spines. At synapses, immunoparticles for both mGluR1 and mGluR5 were found always outside the postsynaptic membrane specializations. Receptors were particularly concentrated in a perisynaptic annulus around type 1 synaptic junctions, including the invaginations at 'perforated' synapses. Measurements of immunolabelling on dendritic spines showed decreasing levels of receptor as a function of distance from the edge of the synaptic specialization. We propose that glutamergic synapses with an irregular edge develop in order to increase the circumference of synaptic junctions leading to an increase in the metabotropic to ionotropic glutamate receptor ratio at glutamate release sites. The perisynaptic position of postsynaptic metabotropic glutamate receptors appears to be a general feature of glutamatergic synaptic organization and may apply to other G-protein-coupled receptors.

Comparing the role of metabotropic glutamate receptors in long-term potentiation and in learning and memory

1. Neuronal plasticity has been suggested to be the physical substrate for changes underlying the expression of memory. One model which has attracted wide attention as a possible candidate of such neuronal plasticity is long-term potentiation (LTP), mainly investigated in the hippocampus of rodents. Moreover, various processes with different time constants may underlie LTP, and these phases show striking correspondence to different phases of memory. 2. Pharmacological evidence strongly implicates that the neurotransmitter glutamate plays a major role in LTP. Although the involvement of ionotropic glutamate receptors has been proven, the role of the newly discovered metabotropic glutamate receptors is still uncertain. 3. Metabotropic glutamate receptors (mGluRs) comprise a whole family with currently eight members grouped into three classes according to their amino acid sequence identity and pharmacological profile. They are G-protein coupled, either positively linked to phospholipase C (class I) or negatively linked to adenylate cyclase (class II and III), and among other effects are known to induce phosphorylation of ionotropic glutamate receptors as well as modulate the excitability of neurons. Finally, they are heterogeneously distributed throughout the brain. 4. In hippocampal slice preparations, mGluRs have been shown to be involved in the induction of LTP in CA1 and dentate gyrus by some investigators, but others have failed to reproduce such experiments, leaving the question: what are the appropriate conditions for mGluR-mediated LTP? 5. In vivo, metabotropic receptor antagonists have been shown to block, and agonists to facilitate, induction and maintenance of LTP, mainly at perforant path/dentate granule cell synapses. As demonstrated in behavioral investigations, mGluRs apparently play an important part in hippocampus-dependent learning paradigms. As in LTP, antagonists block memory formation; in contrast to LTP, agonists also prevent memory formation. In memory recall metabotropic receptors seem to play no role. 6. Based on current information the authors develop models for a role of mGluRs in both LTP and memory formation. Activation of metabotropic receptors plays a particular modulatory role when high frequency stimulation is weak. Strong tetanization may bypass mGluRs by stimulating other systems leading to, at least phenomenologically, similar LTP, Behaviorally, mGluRs possibly set the signal to noise ratio of the hippocampal circuit.

The synergism between metabotropic glutamate receptor activation and arachidonic acid on glutamate release is occluded by induction of long-term potentiation in the dentate gyrus

In synaptosomes prepared from dentate gyrus, activation of the metabotropic glutamate receptor by the specific agonist, trans-1-amino-cyclopentyl-1,3-dicarboxylate, increases release of glutamate in the presence of a low concentration of arachidonic acid. A similar interaction between trans-1-amino-cyclopentyl-1,3-dicarboxylate and arachidonic acid is observed on inositol phospholipid turnover and on protein kinase C activity. We report here that when long-term potentiation is induced in the dentate gyrus by high frequency tetanic stimulation to the perforant path, the synergism between arachidonic acid and trans-1-amino-cyclopentyl-1,3-dicarboxylate is occluded. The occlusion of the synergistic action between arachidonic acid and trans-1-amino-cyclopentyl-1,3-dicarboxylate on glutamate release extended to occlusion of the effect in inositol phospholipid turnover and protein kinase C activation in synaptosomes prepared from dentate gyrus in which long-term potentiation was induced in vivo. One interpretation of the results presented here is that tetanic stimulation is followed by stimulation of metabotropic glutamate receptors at a time when arachidonic acid concentration in the synaptic region is elevated, and that this interaction triggers the presynaptic changes required for expression of long-term potentiation.

Physiological and pharmacological profile of trans-azetidine-2,4-dicarboxylic acid: metabotropic glutamate receptor agonism and effects on long-term potentiation

In this study, we biochemically analysed the effects of the novel metabotropic glutamate receptor agonist trans-azetidine-2,4-dicarboxylic acid and examined its role in hippocampal long-term potentiation. In cell lines expressing metabotropic receptor 1 or 5 subtypes, the compound stimulated phosphoinositide hydrolysis with EC50 values of 189.4 +/- 6.4 and 32.2 +/- 8.3 microM, respectively. In hippocampal slices, trans-azetidine-2,4-dicarboxylic acid also increased phosphoinositide hydrolysis, yet failed to show any effect on forskolin-stimulated formation of cyclic AMP, even if 1 mM azetidine was applied. Since trans-azetidine-2,4-dicarboxylic acid (20 mM in 5 microliters) injected cerebroventricularly prolongs long-term potentiation induced by weak tetanization, a possible interaction with N-methyl-D-aspartate receptors was investigated using patch-clamp techniques. Neither facilitation of N-methyl-D-aspartate (500 microM) currents nor induction of non-specific currents was observed in the presence of 50 and 500 microM azetidine. Strong tetanus-induced long-term potentiation in the dentate gyrus of freely moving rats was not influenced by azetidine. In combination with the antagonist (R,S)-alpha-methyl-4-carboxyphenylglycine (200 mM in 5 microliters), however, the potentiation was attenuated and returned to baseline within 90 min. Blockade of N-methyl-D-aspartate receptors using 2-amino-5-phosphonopentanoate (20 mM in 5 microliters) prevented the potentiation in controls, but not in the azetidine group, where normal potentiation was observed for both the population spike amplitude and the excitatory postsynaptic potential. These data suggest that (i) trans-azetidine-2,4- dicarboxylic acid is an agonist at glutamate metabotropic receptors; (ii) a facilitation of induction and maintenance of long-term potentiation via N-methyl-D-aspartate receptors seems unlikely; and (iii) pharmacological activation of metabotropic receptors prior to tetanization appears to bypass the N-methyl-D-aspartate receptor dependence of the potentiation. In conclusion, a role for metabotropic glutamate receptors in both short-term and long-term potentiation is indicated by these data.

A metabotropic glutamate receptor agonist regulates neurotrophin messenger RNA in rat forebrain

We have examined the role of metabotropic glutamate receptor activation in regulating neurotrophin messenger RNA levels in the brain with the use of the selective agonist (1S,3R)-1-aminocy-clopentane-1,3-dicarboxylic acid. Intracerebroventricular injection of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid into adult adult rats resulted in increased expression of nerve growth factor and brain-derived neurotrophic factor messenger RNA in the hippocampal and pyriform cortex and decreased levels of neurotrophin-3 messenger RNA in the hippocampal dentate gyrus granule cell layer. C-fos messenger RNA levels were also increased throughout hippocampal and cortical subfields following (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid administration. (1S,3R)-1-Aminocyclopentane-1,3-dicarboxylic acid-induced changes in messenger RNA levels occurred without behavioral seizures, yet these changes were similar in magnitude and time course to early changes in neurotrophin and c-fos messenger RNA levels observed following recurrent limbic seizures. In contrast quisqualate, a potent agonist of metabotropic as well as ionotropic kainate/alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors, was only capable of inducing increased expression of brain-derived neurotrophic factor messenger RNA at doses which produced recurrent motor seizures, and both effects were completely inhibited by the non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. Neurotrophin messenger RNA changes induced by (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid were also partially susceptible to 6-cyano-7-nitroquinoxaline-2,3-dione antagonism, as well as the specific N-methyl-D-aspartate receptor antagonist (+)-5-methyl-10,11-dihydroxy-5H-dibenzo(a,d)-cyclohepten-5,10- iminedizoleipine. These results suggest that (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid-sensitive metabotropic glutamate receptors can dramatically increase the expression of neurotrophin and c-fos messenger RNAs in rat forebrain without producing significant behavioral trauma and that these influences may involve ionotropic glutamate receptors in certain brain regions.

Effects of the metabotropic glutamate receptor antagonist MCPG on spatial and context-specific learning

The effects of the metabotropic glutamate receptor antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG) on performance in a water maze and in context-specific associative learning were examined in rats previously implanted with cannulae. MCPG (20.8 micrograms) injected intraventricularly (i.c.v.) before testing impaired the performance of rats in the spatial version of the Morris water maze, but 1/10 of this dose did not. Memory retention, evaluated 24 hr post-training, was also affected by the high dose of MCPG. However, performance in a cued version of the water maze was not impaired by the high dose, excluding effects of the drug on perceptual faculties. The effects of the MCPG were further characterized on performance in another hippocampus-dependent spatial learning task, the context-dependent fear conditioning task. MCPG (20.8 micrograms, i.c.v.) did not interfere with conditioned freezing to context in this task. For comparison, a group of rats was injected with the NMDA receptor blocker MK801. MK801 at a dose that disrupted the performance in the spatial version of the Morris water maze (0.08 mg/kg), significantly reduced freezing compared to controls. These experiments indicate that MCPG-sensitive metabotropic receptors may be required for only a restricted subset of spatial learning tasks, while NMDA receptors may play an integral role in all spatial learning.

Concurrent blockade of hippocampal metabotropic glutamate and N-methyl-D-aspartate receptors disrupts working memory in the rat

In order to clarify the roles of hippocampal metabotropic glutamate and N-methyl-D-aspartate receptors in working and reference memory performance of rats, the effects of intrahippocampal administration of selective antagonists for both receptors on these behaviours were examined with a three-panel runway task. In the working memory task, the potent and competitive N-methyl-D-aspartate receptor antagonist, (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points), when injected bilaterally at 10 and 32 ng/side into the dorsal hippocampus. Intrahippocampal injection of CPP at a dose of 3.2 ng/side had no effect on the number of working memory errors. The metabotropic glutamate receptor antagonist, (+)-alpha-methyl-4-carboxyphenylglycine (+)-(MCPG), injected into the hippocampus at doses up to 3.2 micrograms/side, did not significantly affect the number of working memory errors. Combined administration of (+)-MCPG (3.2 micrograms/side) and CPP (3.2 ng/side) into the hippocampus, neither of which had an individual effect on errors, significantly increased the number of working memory errors. However, intrahippocampal administration of the relatively inactive isomer, (-)-MCPG, at 3.2 micrograms/side did not affect working memory errors, whether given independently or concurrently with the behaviourally ineffective dose of CPP (3.2 ng/side). In the reference memory task, intrahippocampal injection of CPP at doses up to 32 ng/side had no effect on the number of errors. Intrahippocampal (+)-MCPG at doses up to 3.2 micrograms/side did not affect the number of reference memory errors, whether administered alone or together with 3.2 ng/side of CPP. These results indicate that blockade of hippocampal metabotropic glutamate receptors aggravates impairment of working memory resulting from deficiency of N-methyl-D-aspartate receptor-mediated glutamatergic neurotransmission, suggesting that mechanisms regulated by co-activation of hippocampal metabotropic glutamate and N-methyl-D-aspartate receptors are involved in working memory performance of rats.

Effects of the selective metabotropic glutamate receptor agonist, L-CCG-I, on acquisition of a Morris task by rats

L-Glutamate, a major excitatory neurotransmitter in the central nervous system, plays an important role in a variety of neuronal events associated with learning and memory, neuronal plasticity, neurotoxicity, and neurodegeneration. We assessed the effects of L-CCG-I ((2S,3S,4S)-alpha-(carboxycyclopropyl)glycine), a conformationally restricted glutamate analogue, in a standard Morris water escape task with young adult rats. L-CCG-I is considered to be a selective agonist of the metabotropic glutamate receptor. Vehicle, 5, 50, or 500 nmol L-CCG-I was injected intra-cerebroventricularly (i.c.v.) into the right lateral ventricle 30 min before the start of each of five daily acquisition sessions. The data indicate that L-CCG-I had a centrally mediated mode of action; rats treated with 500 nmol L-CCG-I were clearly impaired in acquiring the standard Morris water escape task. The no-effect dose was 5 nmol.

Excitatory amino acid-, except 1S,3R-ACPD, induced transient high stimulation of phosphoinositide metabolism during hippocampal neuron development

Rat hippocampal neurons in culture extended their neurites until day 5 in vitro (DIV). Then, the mean neuritic length slightly decreased. Excitatory amino acid (EAA)-elicited inositol phosphate (IP) formation increased from 0.5 to 2 DIV, reached a plateau between 2 and 4-5 DIV, and then gradually decreased until 10 DIV. This decrease was likely not due to neuronal death. This developmental pattern was observed for N-methyl-D-aspartate, kainate, glutamate, ibotenate and quisqualate (QA). Interestingly, the 1S,3R-aminocyclopentane dicarboxylate (1S,3R-ACPD) response slightly increased during neuronal culture development. At 3 DIV, the ionotropic antagonists 6,7-dinitro-quinoxalin-2,3-dion and D-2-amino-5-phosphonopentanoate efficiently blocked N-methyl-D-aspartate and kainate-elicited IP formation, and partially inhibited glutamate and ibotenate responses. QA and 1S,3R-ACPD responses were not affected, suggesting a metabotropic action for these two compounds. Furthermore, QA and 1S,3R-ACPD potencies significantly increased between 3 and 10 DIV. The transient high activity periods induced by EAA, except for 1S,3R-ACPD, are not observed for norepinephrine, carbachol and potassium chloride responses. Taken together, these data suggest that: (i) QA and 1S,3R-ACPD can act on two different glutamate metabotropic receptors subtypes during development; and (ii) the EAA-induced transient peaks of IP stimulation, which are specific with respect to other neuroactive substances profiles, could be involved in the development of hippocampal neurons. Indeed, these transient high activities take place when the neuritic length regularly increases in vitro.

Metabotropic glutamate receptors and visual cortical synaptic plasticity

Two forms of use-dependent synaptic plasticity, called long-term potentiation (LTP) and long-term depression (LTD), can be elicited in the visual cortex following different paradigms of electrophysiological stimulation. These neurobiological phenomena often are considered as necessary components of models for the alteration in function of the nervous system that must occur at some level for the establishment and (or) maintenance of memory engrams, for learning processes, or for the consolidation of active neural connections and regression of inactive contacts in the developing brain. It has been postulated that for LTP and LTD to be produced in the hippocampus, activation of a particular subtype of excitatory amino acid receptor, the metabotropic receptor, is a critical requirement. Only recently has it become possible to test this hypothesis directly, as a new compound, (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG), has been introduced and the suggestion made that it selectively antagonizes the metabotropic receptor. This substance has been tested in the present study on responses recorded from slices of rat visual cortex and has been found both to block the activation of the metabotropic receptor and to interfere selectively with the form of synaptic plasticity called LTD. It thus appears from the experiments reported in this paper as though the metabotropic receptor subtype that is blocked by MCPG is required for the expression of LTD but not for the expression of LTP, in the visual cortex of adult rats.

Metabotropic glutamate receptors in spatial and nonspatial learning in rats studied by means of agonist and antagonist application

We examined the effects of both the metabotropic glutamate receptor (mGluR) antagonist MCPG and the agonist tADA in two behavioral paradigms in rats: (1) brightness discrimination and (2) spatial alternation. Compounds were applied intracerebroventricularly at different times, either 30 min prior to training or immediately after training, and rats were tested for retention 24 hr later in the same paradigms. Both MCPG and tADA caused amnesia in the spatial alternation test, when applied pretraining, but no effect was obtained in the brightness discrimination paradigm. Drug-induced amnesia was shown not to be attributable to state-dependent effects of MCPG or tADA. Moreover, the memory inhibiting effect of MCPG was dose dependent, with a low dose (20 mM/5 ml) having no effect on learning and memory and a 10 times higher concentration (200 mM/5 ml) causing complete amnesia. Application of both saline and MCPG immediately post-training prevented memory formation, which may be attributable to an interference by the injection procedure with the process of memory formation. The mGluR agonist tADA, however, facilitated memory formation in the spatial alternation task, when injected immediately after training. Post-training application of the compounds had no effect on retention in the brightness discrimination task. On the basis of these data we conclude that (1) mGluRs are of particular importance for spatial learning and play no role in visual discrimination; (2) both the block and the activation of mGluRs inhibit spatial learning, suggesting that saturated activation prevents further modulation of mGluRs, which may be required during learning or memory formation; and (3) mGluR agonist tADA may be memory facilitating when applied after training, thus enhancing the establishment of the memory trace.

Presynaptic metabotropic glutamate receptors modulate omega-conotoxin-GVIA-insensitive calcium channels in the rat medulla

We have previously demonstrated that the metabotropic glutamate receptor (mGluR) agonist (1S,3R)-1 aminocyclopentane-1,3-dicarboxylate (ACPD) presynaptically inhibits evoked glutamatergic EPSCs and GABAergic IPSCs in patch clamped rat nucleus tractus solitarius (NTS) neurons recorded in this slices. The present study investigated the pharmacology of the presynaptic mGluRs, the the voltage dependent Ca2+ channel (VDCC) subtypes supporting neurotransmitter release, and possible interactions between the two. Monosynaptic EPSCs or IPSCs were evoked by electrical stimulation in the region of the tractus solitarius (TS). The effects of the mGluR agonists ACPD, (2S,3S,4S)-alpha-(carboxycyclopropyl)glycine (L-CCG-I) and L-2-amino-4-phosphonobutyrate (AP4) were examined upon EPSCs. The effects of the above compounds and quisqualate (QUIS) were examined upon IPSCs. L-CCG-I proved the most potent inhibitor of EPSCs and IPSCs. The VDCC blockers omega-AGA-IVA (AGA), omega-conotoxin GVIA (GVIA), omega-conotoxin MVIIC (MVIIC) and nimodipine (NIM) were assessed for their ability to inhibit monosynaptic EPSCs and IPSCs. EPSCs were inhibited by GVIA >> AGA > or = MVIIC. IPSCs were inhibited by AGA > or = MVIIC >> GVIA. NIM was without effect on the EPSC or IPSC. The potency of mGluR inhibition of evoked synaptic transmission was assessed in the absence and following treatment with VDCC blockers. mGluR agonists blocked a greater percentage of the EPSC or IPSC following treatment with GVIA, but not the other VDCC antagonists, than under control conditions. We have previously demonstrated that the postsynaptic inhibitory effects of mGluR activation upon GABAA mediated currents can be mimicked by cyclic guanosine monophosphate (cGMP) analogs. The cGMP-dependent protein kinase (PKG) inhibitors H8 and Rp-8-4-chlorophenylthio-guanosine-3',5'-cyclic monophosphorothioate (Rp-cG) blocked mGluR inhibition of GABAA mediated currents without blocking the ability of mGluR agonists to inhibit the IPSC. The effect of L-CCGI was enhanced following treatment with GVIA in the presence of Rp-cG, confirming a presynaptic locus of mGluR mediated inhibition of the IPSC. In contrast, cGMP analogues potentiate postsynaptic responses to glutamate agonists but depress the EPSC. As with the mGluR agonists, the inhibition of the EPSC by cGMP was potentiated following treatment with GVIA. These results suggest that presynaptic mGluR reduce both glutamate release from afferent fibers and GABA release from inhibitory interneurons following electrical stimulation in the region of the TS. Although different VDCCs support the majority of glutamate and GABA release and mGluR effects on release appear to utilize differing intracellular pathways, presynaptic GVIA-insensitive VDCCs are favorably targeted for inhibition by mGluR agonists.

Antagonists of the metabotropic glutamate receptor do not prevent induction of long-term potentiation in the dentate gyrus of rats

The effects of two competitive metabotropic glutamate (mGlu) receptor antagonists, (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG) and (S)-4-carboxylphenylglycine (4CPG), were studied on long-term potentiation in the dentate gyrus of rats under urethane anaesthesia. Intracerebroventricular (i.c.v.) injection of MCPG or 4CPG 30 min prior to tetanic stimulation of the perforant path in rats did not affect the induction of long-term potentiation measured by extracellular recording. As a control, i.c.v. injections of the NMDA receptor antagonist, dl(-)-2-amino-5-phosphonopentanoic (dl-AP5), effectively blocked long-term potentiation. These results suggest that the mGlu receptor subtype blocked by MCPG and 4CPG is not involved in long-term potentiation in the dentate gyrus.

Motor deficit and impairment of synaptic plasticity in mice lacking mGluR1

Metabotropic glutamate receptor 1 (mGluR1) is a member of a large family of G-protein-coupled glutamate receptors, the physiological functions of which are largely unknown. Mice deficient in mGluR1 have severe motor coordination and spatial learning deficits. They have no gross anatomical or basic electrophysiological abnormalities in either the cerebellum or hippocampus, but they show impaired cerebellar long-term depression and hippocampal mossy fibre long-term potentiation. mGluR1-deficient mice should therefore be valuable models for studying synaptic plasticity.

Phenylglycine derivatives as antagonists of metabotropic glutamate receptors

Metabotropic glutamate receptors represent a family of G protein-coupled receptors that can be activated by L-glutamate, the principal excitatory neurotransmitter in the brain. Until recently, progress in identifying the physiological and pathological roles of metabotropic glutamate receptors has been hampered by the lack of selective antagonists. In this article, Jeff Watkins and Graham Collingridge describe the pharmacology of, and initial physiological studies using, certain phenylglycine derivatives and related substances--the first definitive antagonists of metabotropic glutamate receptors.

I.c.v. infusions of ACPD(1S,3R) attenuate learning in a Morris water maze paradigm

The functional role of metabotropic glutamate receptors is beginning to be understood. Cognitive effects of metabotropic receptor activation were examined in the present study in a Morris water maze following intraventricular (i.c.v.) infusions of ACPD(1S,3R). I.c.v. infusions of ACPD substantially increased the amount of time animals took to find a hidden platform in a water maze. Current results demonstrate that subtoxic doses of ACPD can produce in vivo effects with functional significance. Spatial learning abilities are impaired following activation of metabotropic receptors with ACPD(1S,3R). Negative implications are provided for the use of cognitive enhancing compounds that stimulate metabotropic glutamate receptors.

Activation of metabotropic glutamate receptors is necessary for long-term potentiation in the dentate gyrus and for spatial learning

We have examined the effects of the metabotropic glutamate receptor antagonist (RS)-a-methyl-4-carboxyphenylglycine (MCPG) on performance in the water maze, and on LTP in the dentate gyrus, MCPG (5 mM) reversibly blocked the induction of LTP in the perforant path-granule cell projection when perfused into the dentate gyrus of the anaesthetized rat. When injected bilaterally into the lateral ventricles, MCPG (20 mM) disrupted the performance of rats in a spatial learning version of the water maze task. In a terminal experiment, when tetanic stimulation was given to the perforant path, LTP, was found to be significantly reduced in MCPG-injected rats compared to control rats injected with vehicle. These experiments indicate that activation of MCPG-sensitive metabotropic glutamate receptors is necessary both for the full expression of LTP and spatial learning, and supply further evidence for the hypothesis that LTP provides synaptic mechanism for certain forms of learning.

LTP in hippocampal CA1 of urethane-narcotized rats requires stronger tetanization parameters

Rats with chronically implanted electrodes in the hippocampal CA1 region were tested in their capacity to express and maintain long-term potentiation (LTP) of the population spike (PS) or of the field excitatory postsynaptic potential (fEPSP). Two different states were compared: a) freely moving animals; b) urethane-anesthetized animals (1 g/kg, IP). We found that a short, high-frequency tetanus (six bursts of 15 pulses; 200 Hz; double-pulse width; interburst interval 10 s) increased PS amplitudes and fEPSP slopes up to 300% in response to test stimuli in double-pulse width; interburst interval 10 s) increased PS amplitudes and fEPSP slopes up to 300% in response to test stimuli in the awake rat. The PS amplitude slowly decreased in time, returning to baseline levels 4 h post-tetanically, whereas the fEPSP slope remained at higher values for 24 h. Urethane injection reduced the fEPSP slope and abolished the PS to normal test pulses. We thus increased the strength of the test stimuli until we again recorded magnitudes of PSs and fEPSPs comparable to those in the awake animal. In conjunction with these stronger stimuli, tetanus-induced LTP was elicited that for the PS was increased in magnitude and prolonged in duration compared to the untreated control group. Although, stronger tetanic stimuli were applied to the narcotized fEPSP group too, no difference was found compared to controls. These results suggest that urethane narcosis influences the sensitivity of CA1 neurons to express LTP. Stronger stimulation was required to induce and maintain a long-lasting potentiation of the fEPSP slope and PS amplitude.

(RS)-alpha-methyl-4-carboxyphenylglycine (MCPG) does not block theta burst-induced long-term potentiation in area CA1 of rat hippocampal slices

We have used the selective metabotropic glutamate receptor antagonist (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG) to investigate in the CA1 hippocampal subregion in vitro whether coactivation of N-methyl-D-aspartate (NMDA) and metabotropic glutamate receptors is necessary for the induction of long-term potentiation (LTP) when LTP is induced by theta burst stimulation (TBS). When MCPG (500 microM) was bath applied 14-30 min prior to a triple high-frequency tetanization (100 Hz, 1 s) and washed out immediately afterwards the potentiation of the extracellularly recorded field potentials decayed gradually to baseline (P < 0.05) over 2-3 h. However, when MCPG was applied in the same manner before a triple TBS (10 bursts at 5 Hz, 100 Hz within the bursts) the resulting potentiation remained stable for at least 4 h. MCPG had no effect on baseline synaptic transmission or post-tetanic potentiation. These results demonstrate a clear difference in the mechanisms underlying these two different forms of LTP.

(R,S)-alpha-methyl-4-carboxyphenylglycine (MCPG) blocks spatial learning in rats and long-term potentiation in the dentate gyrus in vivo

Recently, it was demonstrated by the use of the competitive and selective antagonist (R,S)-alpha-methyl-4-carboxyphenylglycine (MCPG) that metabotropic glutamate receptor (mGluR) activation is required to induce long-term potentiation (LTP) in the hippocampus. Accordingly, we investigated whether MCPG also inhibits spatial learning. Rats were trained on a spatial alternation task in a Y-maze with footshock reinforcement, and MCPG (0.0208 mg) was injected intracerebroventricularly prior to training and/or retention test. Animals injected pre-training are clearly impaired in retention, whereas preretention application was without effect. A state dependency could be excluded. Additionally, MCPG at the same concentration completely blocks a potentiation at perforant path/dentate gyrus synapses in vivo. These results strongly implicate a role of mGluRs in spatial learning and LTP.