MCPG antagonizes metabotropic glutamate receptors but not long-term potentiation in the hippocampus

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.

Transient Switching of NMDA-Dependent Long-Term Synaptic Potentiation in CA3-CA1 Hippocampal Synapses to mGluR1-Dependent Potentiation After Pentylenetetrazole-Induced Acute Seizures in Young Rats

The mechanisms of impairment in long-term potentiation after status epilepticus (SE) remain unclear. We investigated the properties of LTP induced by theta-burst stimulation in hippocampal slices of rats 3 h and 1, 3, and 7 days after SE. Seizures were induced in 3-week old rats by a single injection of pentylenetetrazole (PTZ). Only animals with generalized seizures lasting more than 30 min were included in the experiments. The results revealed that LTP was strongly attenuated in the CA1 hippocampal area after PTZ-induced SE as compared with that in control animals. Saturation of synaptic responses following epileptic activity does not explain weakening of LTP because neither the quantal size of the excitatory responses nor the slopes of the input-output curves for field excitatory postsynaptic potentials changed in the post-SE rats. After PTZ-induced SE, NMDA-dependent LTP was suppressed, and LTP transiently switched to the mGluR₁-dependent form. This finding does not appear to have been reported previously in the literature. An antagonist of NMDA receptors, D-2-amino-5-phosphonovalerate, did not block LTP induction in 3-h and 1-day post-SE slices. An antagonist of mGluR₁, FTIDS, completely prevented LTP in 1-day post-SE slices; whereas it did not affect LTP induction in control and post-SE slices at the other studied times. mGluR₁-dependent LTP was postsynaptically expressed and did not require NMDA receptor activation. Recovery of NMDA-dependent LTP occurred 7 day after SE. Transient switching between NMDA-dependent LTP and mGluR1-dependent LTP could play a role in the pathogenesis of acquired epilepsy.

The Role of mGlu Receptors in Hippocampal Plasticity Deficits in Neurological and Psychiatric Disorders: Implications for Allosteric Modulators as Novel Therapeutic Strategies

Long-term potentiation (LTP) and long-term depression (LTD) are two distinct forms of synaptic plasticity that have been extensively characterized at the Schaffer collateral-CA1 (SCCA1) synapse and the mossy fiber (MF)-CA3 synapse within the hippocampus, and are postulated to be the molecular underpinning for several cognitive functions. Deficits in LTP and LTD have been implicated in the pathophysiology of several neurological and psychiatric disorders. Therefore, there has been a large effort focused on developing an understanding of the mechanisms underlying these forms of plasticity and novel therapeutic strategies that improve or rescue these plasticity deficits. Among many other targets, the metabotropic glutamate (mGlu) receptors show promise as novel therapeutic candidates for the treatment of these disorders. Among the eight distinct mGlu receptor subtypes (mGlu1-8), the mGlu1,2,3,5,7 subtypes are expressed throughout the hippocampus and have been shown to play important roles in the regulation of synaptic plasticity in this brain area. However, development of therapeutic agents that target these mGlu receptors has been hampered by a lack of subtype-selective compounds. Recently, discovery of allosteric modulators of mGlu receptors has provided novel ligands that are highly selective for individual mGlu receptor subtypes. The mGlu receptors modulate the multiple forms of synaptic plasticity at both SC-CA1 and MF synapses and allosteric modulators of mGlu receptors have emerged as potential therapeutic agents that may rescue plasticity deficits and improve cognitive function in patients suffering from multiple neurological and psychiatric disorders.

Temporal phases of long-term potentiation (LTP): myth or fact?

Long-term potentiation (LTP) remains the most widely accepted model for learning and memory. In accordance with this belief, the temporal differentiation of LTP into early and late phases is accepted as reflecting the differentiation of short-term and long-term memory. Moreover, during the past 30 years, protein synthesis inhibitors have been used to separate the early, protein synthesis-independent (E-LTP) phase and the late, protein synthesis-dependent (L-LTP) phase. However, the role of these proteins has not been formally identified. Additionally, several reports failed to show an effect of protein synthesis inhibitors on LTP. In this review, a detailed analysis of extensive behavioral and electrophysiological data reveals that the presumed correspondence of LTP temporal phases to memory phases is neither experimentally nor theoretically consistent. Moreover, an overview of the time courses of E-LTP in hippocampal slices reveals a wide variability ranging from <1 h to more than 5 h. The existence of all these conflictual findings should lead to a new vision of LTP. We believe that the E-LTP vs. L-LTP distinction, established with protein synthesis inhibitor studies, reflects a false dichotomy. We suggest that the duration of LTP and its dependency on protein synthesis are related to the availability of a set of proteins at synapses and not to the de novo synthesis of plasticity-related proteins. This availability is determined by protein turnover kinetics, which is regulated by previous and ongoing electrical activities and by energy store availability.

Coordinated activation of distinct Ca(2+) sources and metabotropic glutamate receptors encodes Hebbian synaptic plasticity

At glutamatergic synapses, induction of associative synaptic plasticity requires time-correlated presynaptic and postsynaptic spikes to activate postsynaptic NMDA receptors (NMDARs). The magnitudes of the ensuing Ca²⁺ transients within dendritic spines are thought to determine the amplitude and direction of synaptic change. In contrast, we show that at mature hippocampal Schaffer collateral synapses the magnitudes of Ca²⁺ transients during plasticity induction do not match this rule. Indeed, LTP induced by time-correlated pre- and postsynaptic spikes instead requires the sequential activation of NMDARs followed by voltage-sensitive Ca²⁺ channels within dendritic spines. Furthermore, LTP requires inhibition of SK channels by mGluR1, which removes a negative feedback loop that constitutively regulates NMDARs. Therefore, rather than being controlled simply by the magnitude of the postsynaptic calcium rise, LTP induction requires the coordinated activation of distinct sources of Ca²⁺ and mGluR1-dependent facilitation of NMDAR function.

During STDP, the magnitude of postsynaptic Ca²⁺ transients is hypothesized to determine the strength of synaptic plasticity. Here, the authors find that STDP in mature hippocampal synapses does not obey this rule but instead relies on the coordinated activation of NMDARs and VGCCs and their regulation by mGluRs and SK channels.

An interchangeable role for kainate and metabotropic glutamate receptors in the induction of rat hippocampal mossy fiber long-term potentiation in vivo

The roles of both kainate receptors (KARs) and metabotropic glutamate receptors (mGluRs) in mossy fiber long-term potentiation (MF-LTP) have been extensively studied in hippocampal brain slices, but the findings are controversial. In this study, we have addressed the roles of both mGluRs and KARs in MF-LTP in anesthetized rats. We found that MF-LTP could be induced in the presence of either GluK1-selective KAR antagonists or group I mGluR antagonists. However, LTP was inhibited when the group I mGluRs and the GluK1-KARs were simultaneously inhibited. Either mGlu1 or mGlu5 receptor activation is sufficient to induce this form of LTP as selective inhibition of either subtype alone, together with the inhibition of KARs, did not inhibit MF-LTP. These data suggest that mGlu1 receptors, mGlu5 receptors, and GluK1-KARs are all engaged during high-frequency stimulation, and that the activation of any one of these receptors alone is sufficient for the induction of MF-LTP in vivo.

Functional coupling of the metabotropic glutamate receptor, InsP3 receptor and L-type Ca2+ channel in mouse CA1 pyramidal cells

Activity-dependent regulation of calcium dynamics in neuronal cells can play significant roles in the modulation of many cellular processes such as intracellular signalling, neuronal activity and synaptic plasticity. Among many calcium influx pathways into neurons, the voltage-dependent calcium channel (VDCC) is the major source of calcium influx, but its modulation by synaptic activity has still been under debate. While the metabotropic glutamate receptor (mGluR) is supposed to modulate L-type VDCCs (L-VDCCs), its reported actions include both facilitation and suppression, probably reflecting the uncertainty of both the molecular targets of the mGluR agonists and the source of the recorded calcium signal in previous reports. In this study, using subtype-specific knockout mice, we have shown that mGluR5 induces facilitation of the depolarization-evoked calcium current. This facilitation was not accompanied by the change in single-channel properties of the VDCC itself; instead, it required the activation of calcium-induced calcium release (CICR) that was triggered by VDCC opening, suggesting that the opening of CICR-coupled cation channels was essential for the facilitation. This facilitation was blocked or reduced by the inhibitors of both L-VDCCs and InsP3 receptors (InsP3Rs). Furthermore, L-VDCCs and mGluR5 were shown to form a complex by coimmunoprecipitation, suggesting that the specific functional coupling between mGluR5, InsP3Rs and L-VDCCs played a pivotal role in the calcium-current facilitation. Finally, we showed that mGluR5 enhanced VDCC-dependent long-term potentiation (LTP) of synaptic transmission. Our study has identified a novel mechanism of the interaction between the mGluR and calcium signalling, and suggested a contribution of mGluR5 to synaptic plasticity.

Synergistic interactions between kainate and mGlu receptors regulate bouton Ca signalling and mossy fibre LTP

It is currently unknown why glutamatergic presynaptic terminals express multiple types of glutamate receptors. We have addressed this question by studying both acute and long-term regulation of mossy fibre function in the hippocampus. We find that inhibition of both mGlu₁ and mGlu₅ receptors together can block the induction of mossy fibre LTP. Furthermore, mossy fibre LTP can be induced by the pharmacological activation of either mGlu₁ or mGlu₅ receptors, provided that kainate receptors are also stimulated. Like conventional mossy fibre LTP, chemically-induced mossy fibre LTP (chem-LTPm) depends on Ca²⁺ release from intracellular stores and the activation of PKA. Similar synergistic interactions between mGlu receptors and kainate receptors were observed at the level of Ca²⁺ signalling in individual giant mossy fibre boutons. Thus three distinct glutamate receptors interact, in both an AND and OR gate fashion, to regulate both immediate and long-term presynaptic function in the brain.

Group I metabotropic glutamate receptors are involved in TEA-induced long-term potentiation at mossy fiber-CA3 synapses in the rat hippocampus

Gq-protein-coupled Group I metabotropic glutamate receptors (mGluR) reportedly activate phospholipase C (PLC), leading to Ca(2+) release from intracellular stores and the formation of diacylglycerol (DAG). We electrophysiologically examined the involvement of the Group I mGluR in tetraethylammonium (TEA)-induced long-term potentiation (LTP) at mossy fiber (MF)-CA3 synapses in the rat hippocampus. TEA-induced LTP was almost completely blocked under the selective blockade of either mGluR1 or mGluR5, both of which are Group I mGluR. This result was supported by the blockade of TEA-induced LTP even in the absence of these blockers under low temperature conditions, in which the activation of Group I mGluR is thought not to be fully effective. In addition, the blockade of mGluR1 resulted in lower short-term potentiation (STP) during TEA application compared with the blockade of mGluR5. These results demonstrate the crucial roles of Group I mGluR in the TEA-induced LTP at MF-CA3 synapses and the different contributions of mGluR1 and mGluR5 to the initial component of plasticity.

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.

MGluR5 mediates the interaction between late-LTP, network activity, and learning

Hippocampal synaptic plasticity and learning are strongly regulated by metabotropic glutamate receptors (mGluRs) and particularly by mGluR5. Here, we investigated the mechanisms underlying mGluR5-modulation of these phenomena. Prolonged pharmacological blockade of mGluR5 with MPEP produced a profound impairment of spatial memory. Effects were associated with 1) a reduction of mGluR1a-expression in the dentate gyrus; 2) impaired dentate gyrus LTP; 3) enhanced CA1-LTP and 4) suppressed theta (5–10 Hz) and gamma (30–100 Hz) oscillations in the dentate gyrus. Allosteric potentiation of mGluR1 after mGluR5 blockade significantly ameliorated dentate gyrus LTP, as well as suppression of gamma oscillatory activity. CA3-lesioning prevented MPEP effects on CA1-LTP, suggesting that plasticity levels in CA1 are driven by mGluR5-dependent synaptic and network activity in the dentate gyrus. These data support the hypothesis that prolonged mGluR5-inactivation causes altered hippocampal LTP levels and network activity, which is mediated in part by impaired mGluR1-expression in the dentate gyrus. The consequence is impairment of long-term learning.

Metabotropic glutamate receptor 1 (mGluR1) and 5 (mGluR5) regulate late phases of LTP and LTD in the hippocampal CA1 region in vitro

The group I metabotropic glutamate receptors, mGluR1 and mGluR5, exhibit differences in their regulation of synaptic plasticity, suggesting that these receptors may subserve separate functional roles in information storage. In addition, although effects in vivo are consistently described, conflicting reports of the involvement of mGluRs in hippocampal synaptic plasticity in vitro exist. We therefore addressed the involvement of mGluR1 and mGluR5 in long-term potentiation (LTP) and long-term depression (LTD) in the hippocampal CA1 region of adult male rats in vitro. The mGluR1 antagonist (S)-(+)-α-amino-4-carboxy-2-methylbenzene-acetic acid (LY367385) impaired both induction and late phases of both LTP and LTD, when applied before high-frequency tetanization (HFT; 100 Hz) or low-frequency stimulation (LFS; 1 Hz), respectively. Application after either HFT or LFS had no effect. The mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP), when given before HFT, inhibited both the induction and late phases of LTP. When given after HFT, late LTP was inhibited. MPEP, given prior to LFS, impaired LTD induction, although stable LTD was still expressed. Application after LFS significantly impaired late phases of LTD. Activation of protein synthesis may comprise a key mechanism underlying the group I mGluR contribution to synaptic plasticity. The mGluR5 agonist (R,S)-2-chloro-5-hydroxyphenylglycine (CHPG) converted short-term depression into LTD. Effects were prevented by application of the protein synthesis inhibitor anisomycin, suggesting that protein synthesis is triggered by group I mGluR activation to enable persistency of synaptic plasticity. Taken together, these data support the notion that both mGluR1 and mGluR5 are critically involved in bidirectional synaptic plasticity in the CA1 region and may enable functional differences in information encoding through LTP and LTD.

Modulation by group I mGLU receptor activation and group III mGLU receptor blockade of locomotor responses induced by D1-like and D2-like receptor agonists in the nucleus accumbens

Evidence for functional motor interactions between group I and group III metabotropic glutamatergic (mGlu) receptors and dopamine neurotransmission is now clearly established [David, H.N., Abraini, J.H., 2001a. The group I metabotropic glutamate receptor antagonist S-4-CPG modulates the locomotor response produced by the activation of D1-like, but not D2-like, dopamine receptors in the rat nucleus accumbens. Eur. J. Neurosci. 15, 2157-2164, David, H.N., Abraini, J.H., 2002. Group III metabotropic glutamate receptors and D1-like and D2-like dopamine receptors interact in the rat nucleus accumbens to influence locomotor activity. Eur. J. Neurosci. 15, 869-875]. Nevertheless, whether or not and how, activation of group I and blockade of group III mGlu receptors modulate the motor responses induced by the activation of dopaminergic receptors in the NAcc still remains unknown. Answering this question needs to be assessed since functional interactions between neurotransmitters in the NAcc are well known to depend upon the level of activation of glutamatergic and/or dopaminergic receptors and because the effects of glutamatergic receptor agonists and antagonists on dopaminergic receptor-mediated locomotor responses are not always reciprocal as shown in previous studies. Our results show that activation of group I mGlu receptors by DHPG in the NAcc potentiated the locomotor response induced by intra-NAcc activation of D1-like receptors and blocked those induced by D2-like presynaptic or postsynaptic receptors. Alternatively, blockade of group III mGlu receptors by MPPG in the NAcc potentiated the locomotor responses mediated by D1-like receptors and by D2-like postsynaptic receptors and inhibited that induced by D2-like presynaptic receptors. These results compiled with previous data demonstrate that group I mGlu receptors and group III mGlu receptors can modulate the locomotor responses produced by D1-like and/or D2-like receptor agonists in a complex phasic and tonic fashion.

Evidence for a role for the group I metabotropic glutamate receptor in the inhibitory effect of tumor necrosis factor-α on long-term potentiation

Pro-inflammatory cytokines are known to be elevated in several neuropathological states that are associated with learning and memory. We have previously demonstrated in our laboratory that the inhibition of long-term potentiation (LTP) in the dentate gyrus region of the rat hippocampus, by tumor necrosis factor (TNF)-alpha, represents a biphasic response, an early phase dependent on p38 mitogen activated protein kinase (MAPK) activation and a later phase, possible dependent on protein synthesis. Many of the factors involved in the early modulation of LTP by TNF-alpha have yet to be elucidated. This study investigated if metabotropic glutamate receptors (mGluRs) are functionally linked to the inhibitory effect of TNF-alpha on LTP in the rat dentate gyrus in vitro. We report that the impairment of early-LTP by TNF-alpha is significantly attenuated by prior application of the group I/II mGluR antagonist MCPG and more specifically the mGluR5 antagonist MPEP. Since TNF-alpha is now known to cause transient increases in intracellular Ca(2+) levels from ryanodine-sensitive stores, we explored the possibility that disruption of intracellular Ca(2+) homeostasis could be involved. Ryanodine was found to significantly reverse the inhibition of LTP by TNF-alpha. From these studies we propose that the TNF-alpha inhibition of LTP is dependent upon the activation of TNFR1 and mGlu5-receptors. Importantly this study provides the first proof of the involvement of ryanodine-sensitive intracellular Ca(2+) stores in TNF-alpha mediated inhibition of LTP.

Synaptic plasticity at hippocampal mossy fibre synapses

The dentate gyrus provides the main input to the hippocampus. Information reaches the CA3 region through mossy fibre synapses made by dentate granule cell axons. Synaptic plasticity at the mossy fibre-pyramidal cell synapse is unusual for several reasons, including low basal release probability, pronounced frequency facilitation and a lack of N-methyl-D-aspartate receptor involvement in long-term potentiation. In the past few years, some of the mechanisms underlying the peculiar features of mossy fibre synapses have been elucidated. Here we describe recent work from several laboratories on the various forms of synaptic plasticity at hippocampal mossy fibre synapses. We conclude that these contacts have just begun to reveal their many secrets.

Metabotropic glutamate receptor antagonist AIDA blocks induction of mossy fiber-CA3 LTP in vivo

Metabotropic glutamate receptors (mGluR) are implicated in long-term memory storage. mGluR-I and mGluR-II antagonists impede various forms of learning and long-term potentiation (LTP) in animals. Despite the evidence linking mGluR to learning mechanisms, their role in mossy fiber-CA3 long-term potentiation (LTP) is not yet clear. To explain the involvement of mGluR-I in memory mechanisms, we examined the function of the mGluR-I antagonist 1-aminoindan-1, 5-dicarboxylic acid (AIDA) on the induction of mossy fiber-CA3 LTP in vivo in male Sprague Dawley and Fischer 344 (F344) rats. Acute extracellular mossy fiber (MF) responses were evoked by stimulation of the MF bundle and recorded in the stratum lucidum of CA3. The excitatory postsynaptic potential (EPSP) magnitude was measured by using the initial slope of the field EPSP slope measured 2-3 ms after response onset. After collection of baseline MF-CA3 responses at 0.05 Hz, animals received either ((+/-))-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (N-methyl-D-aspartate-R antagonist, 10 mg/kg ip), naloxone (opioid-R antagonist, 10 mg/kg ip), or AIDA (mGluR antagonist, 1 mg/kg ip or 37.5 nmol ic). LTP was induced by two 100-Hz trains at the intensity sufficient to evoke 50% of the maximal response. Responses were collected for an additional 1 h. AIDA blocked induction of LTP in the mossy fiber pathway (P < 0.05) in both strains of rats after systemic and in Sprague Dawley rats after intrahippocampal injection.

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.

Substrates for coincidence detection and calcium signaling for induction of synaptic potentiation in the neonatal visual cortex

Regulation of the efficacy of synaptic transmission by activity-dependent processes has been implicated in learning and memory as well as in developmental processes. We previously described transient potentiation of excitatory synapses onto layer 2/3 pyramidal neurons in the visual cortex that is induced by coincident presynaptic stimulation and postsynaptic depolarization. In the adult visual cortex, activation of N-methyl-d-aspartate (NMDA) glutamate receptors is necessary to induce this plasticity. These receptors act as coincidence detectors, sensing presynaptic glutamate release and postsynaptic depolarization, and cause an influx of Ca(2+) that is necessary for the potentiation. In the neurons of the neonatal visual cortex, on the other hand, coincident presynaptic stimulation and postsynaptic depolarization induce stable long-term potentiation (LTP). In addition, reduced but significant LTP can be induced in many neurons in the presence of the NMDA receptor (NMDAR) antagonist, 2-amino-5-phosphonovaleric acid despite the Ca(2+) requirement. Therefore there must be an alternative postsynaptic Ca(2+) source and coincidence detection mechanism linked to the LTP induction mechanism in the neonatal cortex operating in addition to NMDARs. In this study, we find that in layer 2/3 pyramidal neurons, release of Ca(2+) from inositol trisphosphate (InsP(3)) receptor-mediated intracellular stores and influx through voltage-gated Ca(2+) channels (VGCCs) provide alternative postsynaptic Ca(2+) sources. We hypothesize that InsP(3)Rs are coincidence detectors, sensing presynaptic glutamate release through linkage with group I metabotropic glutamate receptors (mGluRs), and depolarization, through VGCCs. We also find that the downstream protein kinases, PKA and PKC, have a role in potentiation in layer 2/3 pyramidal neurons of the neonatal visual cortex.

Group I metabotropic glutamate receptors regulate the frequency-response function of hippocampal CA1 synapses for the induction of LTP and LTD

Synaptically released glutamate binds to ionotropic or metabotropic glutamate receptors. Metabotropic glutamate receptors (mGluRs) are G-protein-coupled receptors and can be divided into three subclasses (Group I-III) depending on their pharmacology and coupling to signal transduction cascades. Group I mGluRs are coupled to phospholipase C and are implicated in several important physiological processes, including activity-dependent synaptic plasticity, but their exact role in synaptic plasticity remains unclear. Synaptic plasticity can manifest itself as an increase or decrease of synaptic efficacy, referred to as long-term potentiation (LTP) and long-term depression (LTD). The likelihood, degree and direction of the change in synaptic efficacy depends on the history of the synapse and is referred to as 'metaplasticity'. We provide direct experimental evidence for an involvement of group I mGluRs in metaplasticity in CA1 hippocampal synapses. Bath application of a low concentration of the specific group I agonist 3,5-dihydroxyphenylglycine (DHPG), which does not affect basal synaptic transmission, resulted in a leftward shift of the frequency-response function for the induction of LTD and LTP in naïve synapses. DHPG resulted in the induction of LTP at frequencies which induced LTD in control slices. These alterations in the induction of LTD and LTP resemble the metaplastic changes observed in previously depressed synapses. In addition, in the presence of DHPG additional potentiation could be induced after LTP had apparently been saturated. These findings provide strong evidence for an involvement of group I mGluRs in the regulation of metaplasticity in the CA1 field of the hippocampus.

Blockade of the locomotor stimulant effects of amphetamine by group I, group II, and group III metabotropic glutamate receptor ligands in the rat nucleus accumbens: possible interactions with dopamine receptors

Previous investigations have shown that mGlu receptors would be involved in the amphetamine-induced motor response. However, data are somewhat controversial across studies where methodological protocols vary. The aim of the present study was to determine the involvement of mGlu receptors in the NAcc in the locomotor-activating properties of amphetamine in rats well habituated to their experimental environment, a condition known to modulate the motor response to amphetamine. Focal infusion of the group I mGlu receptor antagonist S-4-CPG, which has no effect on basal motor activity, virtually suppressed the locomotor response to amphetamine, while infusion of the group II mGlu receptor antagonist LY 341495 or the group III mGlu receptor agonist AP4, at the minimal dose that produces locomotor activation, reduced it by approximately a half. These effects were blocked by the group I mGlu receptor agonist DHPG, the group II mGlu receptor agonist APDC, and the group III mGlu receptor antagonist MPPG, respectively. These data confirm that mGlu receptors in the NAcc contribute to the psychostimulant motor effect of amphetamine. Results are discussed from the view of recent neuropharmacological studies that have defined the effects of these mGlu receptor ligands on basal motor activity and DA receptor agonists-induced locomotor responses in rats exposed to similar experimental procedures (Eur J Neuroscience 13 (2001) 2157; Neuropharmacology 41 (2001) 454; Eur J Neuroscience 13 (2001) 869). It is suggested that the contribution of mGlu receptors to the amphetamine-induced motor response may result mainly from their functional, either direct or indirect, interactions with D1-like receptors in the NAcc.

Potassium-evoked glutamate release liberates arachidonic acid from cortical neurons

Brain cells in situ contain low concentrations of free polyunsaturated fatty acids such as arachidonic acid (AA) that are released following pathological insults. As a large rise in extracellular [K(+)] accompanies cerebral ischemia, we explored whether this was a stimulus for cellular AA release employing a murine mixed cortical cell culture preparation radiolabeled with AA. Elevating the [K(+)](o) from 5 to 52 mm induced a time-dependent increase in [(3)H]AA release, which reached a plateau after 15 min. Removal of [Ca(2+)](o) or addition of CdCl(2) (100 microm) diminished the net high K(+)-induced AA release, as did treatment of the cultures with tetanus toxin (300 ng/ml) to block endogenous neurotransmitter release. Pharmacological antagonism of both ionotropic and metabotropic glutamate receptors completely prevented high K(+)-evoked AA release, indicating that glutamate was the neurotransmitter in question. Addition of exogenous glutamate mimicked precisely the characteristics of AA release that followed increases in [K(+)](o). Finally, glutamate and AA were released solely from neurons as tetanus toxin did not cleave astrocytic synaptobrevin-2, nor was AA released from pure astrocyte cultures using the same stimuli that were effective in mixed cultures. Taken in toto, our data are consistent with the following scenario: high [K(+)](o) depolarizes neurons, causing an influx of Ca(2+) via voltage-gated Ca(2+) channels. This Ca(2+) influx stimulates the release of glutamate into the synaptic cleft, where it activates postsynaptic glutamate receptors. Events likely converge on the activation of a phospholipase A(2) family member and possibly the enzymes diacylglycerol and monoacylglycerol lipases to yield free AA.

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.

In vivo effects of intracortical administration of NMDA and metabotropic glutamate receptors antagonists on neocortical long-term potentiation and conditioned taste aversion

It has been proposed that long-term potentiation (LTP), a form of activity-dependent modification of synaptic efficacy, may be a synaptic mechanism for certain types of learning. Recent studies on the insular cortex (IC), a region of the temporal cortex implicated in the acquisition and storage of conditioned taste aversion (CTA), have demonstrated that tetanic stimulation of the basolateral nucleus of the amygdala (Bla) induce an N-methyl-D-aspartate (NMDA) dependent LTP in the IC of adult rats in vivo. Here we present experimental data showing that intracortical administration of the NMDA receptor competitive antagonists CPP (-3(-2 carboxipiperazin-4-yl)-propyl-1-phosphonic acid, 0.03 microg per hemisphere) and AP-5 (D(-)-2-amino-5-phosphonopentanoic, 2.5 microg per hemisphere) disrupt the acquisition of conditioned taste aversion, as well as IC-LTP induction in vivo. In contrast, administration of the metabotropic glutamate receptor antagonist MCPG ((RS)-alpha-methyl-4-carboxyphenylglycine, 2.5 microg per hemisphere) does not disrupt the acquisition of CTA nor IC-LTP induction. These findings are of particular interest since they provide support for the view that the neural mechanisms underlying NMDA-dependent neocortical LTP constitute a possible mechanism for the learning-related functions performed by the IC.

Activation of group II metabotropic glutamate receptors results in long-term potentiation following preconditioning stimulation in the dentate gyrus

The role of group II metabotropic glutamate receptors in the induction/expression of long-term potentiation has been investigated in the medial perforant path of the outer (infrapyramidal) blade of the rat dentate gyrus in vitro. Activation of group II metabotropic glutamate receptors by perfusion of the selective agonist LY354740 did not induce long-term potentiation or long-term depression in control. However, LY354740, applied following the induction of long-term potentiation by high frequency stimulation, resulted in additional long-term potentiation. LY354740 was only found to cause additional long-term potentiation if the pre-existing high frequency stimulation-induced long-term potentiation was sub-maximal. Although activation of metabotropic glutamate receptors was not required for induction of high frequency stimulation-induced long-term potentiation, activation of both group I and group II metabotropic glutamate receptors was required during high frequency stimulation-induced long-term potentiation in order for subsequent application of LY354740 to result in additional long-term potentiation. Thus, the long-term potentiation caused by application of LY354740 following high frequency-induced long-term potentiation was prevented if the high frequency stimulation was given in the presence of (S)-alpha-methyl-4-carboxyphenylglycine or the selective group I or group II metabotropic glutamate receptor antagonists 1-aminoindan-1,5-dicarboxylic acid or (2S)-alpha-ethylglutamic acid respectively. The long-term potentiation caused by LY354740 was also dependent upon activation of N-methyl-D-aspartate receptors during the high frequency stimulation, being blocked if high frequency stimulation was given in the presence of the N-methyl-D-aspartate receptor antagonist, D(-)-2-amino-5-phosphonopentanoic acid. The long-term potentiation resulting from activation of group II metabotropic glutamate receptors could be due either to the enhancement of the expression level of the high frequency stimulation-induced long-term potentiation, or alternatively, to a direct novel induction of long-term potentiation. In either theory, the long-term potentiation resulting from activation of group II metabotropic glutamate receptors is dependent upon prestimulation of group I and group II metabotropic glutamate receptors and N-methyl-D-aspartate receptors during the 'preconditioning high frequency stimulation'.

Differential modulation of the D1-like- and D2-like dopamine receptor-induced locomotor responses by group II metabotropic glutamate receptors in the rat nucleus accumbens

There is strong evidence for the existence of functional interactions between metabotropic glutamate receptors and dopamine transmission in the nucleus accumbens. In the present study, we investigated the interactions between group II mGlu receptors and D1-like- and D2-like receptors in the rat nucleus accumbens. Administration of the selective group II metabotropic glutamate receptor agonist APDC, which had no effect when injected alone, potentiated the locomotor response produced by the selective D1-like receptor agonist SKF 38393 but had no effect on those induced by the selective D2-like receptor agonist quinpirole (also known as LY 171555)--a compound believed to act only at D2-like presynaptic receptors when injected alone--or co-administration of SKF 38393+quinpirole--a pharmacological condition thought to stimulate both D1-like receptors and presynaptic and postsynaptic D2-like receptors. In contrast, the selective group II mGlu receptor antagonist LY 341495, which induced an increase in basal locomotor activity, showed no effect on the SKF 38393-induced locomotor response, but abolished that produced by quinpirole or SKF 38393+quinpirole. The present findings demonstrate that stimulation of group II mGlu receptors has a cooperative and potentiating action on the locomotor response induced by D1-like receptor activation, whereas blockade of group II mGlu receptors has an antagonist action on the locomotor responses induced by activation of D2-like receptors. Although these data are consistent from a pharmacological point of view, as the effects of the group II mGlu receptor antagonist LY 341495 were blocked by the group II mGlu receptor agonist APDC and conversely, the subtle neurochemical crosstalks underlying such a differential effect of group II mGlu receptors on D1-like- and D2-like DA receptors remain to be elucidated.

Synaptic plasticity in the medial vestibular nuclei: role of glutamate receptors and retrograde messengers in rat brainstem slices

The analysis of cellular-molecular events mediating synaptic plasticity within vestibular nuclei is an attempt to explain the mechanisms underlying vestibular plasticity phenomena. The present review is meant to illustrate the main results, obtained in vitro, on the mechanisms underlying long-term changes in synaptic strength within the medial vestibular nuclei. The synaptic plasticity phenomena taking place at the level of vestibular nuclei could be useful for adapting and consolidating the efficacy of vestibular neuron responsiveness to environmental requirements, as during visuo-vestibular recalibration and vestibular compensation. Following a general introduction on the most salient features of vestibular compensation and visuo-vestibular adaptation, which are two plastic events involving neuronal circuitry within the medial vestibular nuclei, the second and third sections describe the results from rat brainstem slice studies, demonstrating the possibility to induce long-term potentiation and depression in the medial vestibular nuclei, following high frequency stimulation of the primary vestibular afferents. In particular the mechanisms sustaining the induction and expression of vestibular long-term potentiation and depression, such as the role of various glutamate receptors and retrograde messengers have been described. The relevant role of the interaction between the platelet-activating factor, acting as a retrograde messenger, and the presynaptic metabotropic glutamate receptors, in determining the full expression of vestibular long-term potentiation is also underlined. In addition, the mechanisms involved in vestibular long-term potentiation have been compared with those leading to long-term potentiation in the hippocampus to emphasize the most significant differences emerging from vestibular studies. The fourth part, describes recent results demonstrating the essential role of nitric oxide, another retrograde messenger, in the induction of vestibular potentiation. Finally the fifth part suggests the possible functional significance of different action times of the two retrograde messengers and metabotropic glutamate receptors, which are involved in mediating the presynaptic mechanism sustaining vestibular long-term potentiation.

The group I metabotropic glutamate receptor antagonist S-4-CPG modulates the locomotor response produced by the activation of D1-like, but not D2-like, dopamine receptors in the rat nucleus accumbens

Functional interactions between dopamine (DA) and glutamate neurotransmissions in both the dorsal and the ventral striatum have been described for long time. However, there is much controversy as to whether glutamate transmission stimulates or attenuates DA release and locomotor activity. We investigated the functional interactions on locomotor activity between group I metabotropic glutamatergic receptors (mGlu receptors) and both D1-like and D2-like DA receptors in the rat nucleus accumbens. Intra-accumbens administration of the selective group I mGlu receptor antagonist S-4-CPG (0.2 or 2 microg per side), which had no effect when injected alone, prevented the increase in locomotor activity produced by the selective D1-like receptor agonist SKF 38393 (1 microg per side). Co-administration with S-4-CPG of the group I mGlu receptor agonist DHPG, but not of the group II mGlu receptor agonist APDC or the group III mGlu receptor agonist AP4, reversed the antagonistic effect of S-4-CPG on the SKF 38393-induced increase in locomotor activity. This indicates that the antagonistic effect of S-4-CPG could result from an action at the group I mGlu receptors. In contrast, administration of S-4-CPG showed no effect on the locomotor responses produced by either the selective D2-like receptor agonist LY 171555 (1 microg per side) or a mixed solution of SKF 38393 + LY 171555 (1 microg per side each). Altogether, these results confirm that glutamate transmission may control locomotor function through mGlu receptors in a DA-dependent manner, and further indicate that group I mGlu receptors would interact with D1-like receptors, but not D2-like receptors, to modulate DA transmission and locomotor activity.

Neuronal plasticity in hippocampal mossy fiber–CA3 synapses of mice lacking the inositol-1,4,5-trisphosphate type 1 receptor

In the present study, we used inositol-1,4,5-trisphosphate (IP3) type 1 receptor (IP3R1) knockout mice to examine the role of this receptor in the induction of LTP, LTD, and DP at mossy fiber-CA3 synapses. No difference in synaptically induced field-EPSPs was seen between the wild-type (IP3R1(+/+)) and IP3R1 knockout mice (IP3R1(-/-)), showing that basic synaptic transmission does not involve IP3R1 activation. Tetanus induced LTP in both wild-type and IP(3)R1(-/-) mice, but the magnitude of LTP was significantly greater in IP3R1(-/-) mice (149.8+/-3.5%, mean+/-S.E.M., n=15) than in wild-type mice (132.4+/-1.5%, n=17), suggesting that the IP3R1 has a suppressive effect on LTP induction. To determine whether this effect involved N-methyl-D-aspartate receptor (NMDAR)-dependent LTP, the effect of tetanus was tested in the present of the NMDAR antagonist, D,L-AP5 (50 microM); under these conditions, the LTP in both IP3R1(-/-) and IP3R1(+/+) mice was not significantly reduced. In addition, group I mGluR activation was shown to be necessary for LTP induction, as the LTP was almost blocked by the group I mGluR antagonist, RS-4CPG (500 microM) in both IP3R1(-/-) (117.6+/-1.7%, n=8) and IP3R1(+/+) (116.9+/-1.8%, n=5) mice. The IP3R1 also plays an essential role in LTD induction, as low-frequency stimulation (LFS) failed to induce LTD in the mutant mice (104.5+/-2.1%, n=10). DP was induced in both IP3R1(-/-) and wild-type mice.

Postsynaptic Modulation of AMPA Receptor-Mediated Synaptic Responses and LTP by the Type 3 Ryanodine Receptor

The precise function of ryanodine receptors (RyRs) in synaptic transmission is unknown, but three of their subtypes are expressed in the brain. We examined the roleof RyRs in excitatory synaptic transmission in hippocampal slices, using type 3 RyR (RyR3)-deficient mice. The alpha-amino-3-hydroxy-5-methyl-4-isoxozolepropionic acid (AMPA) receptor-mediated basal synaptic responses in the CA1 region of mutant mice were smaller than those of wild-type mice, while there was no difference in N-methyl-d-aspartate receptor-mediated responses, suggesting selective postsynaptic modification of AMPA receptors by RyR3. The expression of synaptic AMPA receptor subunits examined by Western blotting or immunohistochemistry was indistinguishable, suggesting that the smaller AMPA synaptic responses in mutant mice were not due to the reduced number of synaptic AMPA receptors. Although the initial potentiation following tetanic stimulation of afferent fibers was similar, long-term potentiation (LTP) was smaller in mutant mice. There were no differences in presynaptic electrophysiological properties. We thus conclude that RyR3 postsynaptically regulates the properties of AMPA receptors and LTP.

Arachidonic acid rescues hippocampal long-term potentiation blocked by group I metabotropic glutamate receptor antagonists

Although there is evidence that group I metabotropic glutamate receptors participate in long-term potentiation, the role of these receptors remains unclear. Among antagonists of group I metabotropic glutamate receptors, the mGluR5-selective 6-methyl-2-(phenylethynyl)-pyridine inhibited long-term potentiation in the CA1 region of hippocampal slices from 30-day-old rats, whereas (RS)-1-aminoindan-1,5-dicarboxylic acid and cyclopropan[b]chromen-1a-carboxylic acid ethylester, which are more selective for mGluR1, failed to inhibit long-term potentiation. Evidence also indicates that arachidonic acid is required for long-term potentiation, as inhibition of phospholipase A(2) blocks long-term potentiation. Administration of arachidonic acid immediately after tetanic stimulation restored long-term potentiation that had been inhibited by group I antagonists. Furthermore, arachidonic acid overcame inhibition of long-term potentiation by xestospongin C, an inositol triphosphate receptor channel blocker, or by thapsigargin, an agent that depletes intracellular calcium stores. However, arachidonic acid did not restore long-term potentiation blocked by N-methyl-D-aspartate receptor antagonists. Although it has been assumed that the source of the arachidonic acid necessary for long-term potentiation is N-methyl-D-aspartate receptor activation, our results suggest that during long-term potentiation group I metabotropic glutamate receptors cause arachidonic acid release by mobilization of intracellular calcium.

Kainate receptors are involved in short- and long-term plasticity at mossy fiber synapses in the hippocampus

Kainate receptors alter the excitability of mossy fiber axons and have been reported to play a role in the induction of long-term potentiation (LTP) at mossy fiber synapses in the hippocampus. These previous studies have relied primarily on the use of compounds whose selectivity is unclear. In this report, we investigate short- and long-term facilitation of mossy fiber synaptic transmission in kainate receptor knockout mice. We find that LTP is reduced in mice lacking the GluR6, but not the GluR5, kainate receptor subunit. Additionally, short-term synaptic facilitation is impaired in GluR6 knockout mice, suggesting that kainate receptors act as presynaptic autoreceptors on mossy fiber terminals to facilitate synaptic transmission. These data demonstrate that kainate receptors containing the GluR6 subunit are important modulators of mossy fiber synaptic strength.

Synaptic plasticity and memory: an evaluation of the hypothesis

Changing the strength of connections between neurons is widely assumed to be the mechanism by which memory traces are encoded and stored in the central nervous system. In its most general form, the synaptic plasticity and memory hypothesis states that "activity-dependent synaptic plasticity is induced at appropriate synapses during memory formation and is both necessary and sufficient for the information storage underlying the type of memory mediated by the brain area in which that plasticity is observed." We outline a set of criteria by which this hypothesis can be judged and describe a range of experimental strategies used to investigate it. We review both classical and newly discovered properties of synaptic plasticity and stress the importance of the neural architecture and synaptic learning rules of the network in which it is embedded. The greater part of the article focuses on types of memory mediated by the hippocampus, amygdala, and cortex. We conclude that a wealth of data supports the notion that synaptic plasticity is necessary for learning and memory, but that little data currently supports the notion of sufficiency.