Tyrosine dephosphorylation underlies DHPG-induced LTD

ArcKR expression modifies synaptic plasticity following epileptic activity: Differential effects with in vitro and in vivo seizure-induction protocols

OBJECTIVES

Pathological forms of neural activity, such as epileptic seizures, modify the expression pattern of multiple proteins, leading to persistent changes in brain function. One such protein is activity-regulated cytoskeleton-associated protein (Arc), which is critically involved in protein-synthesis-dependent synaptic plasticity underlying learning and memory. In the present study, we have investigated how the expression of ArcKR, a form of Arc in which the ubiquitination sites have been mutated, resulting in slowed Arc degradation, modifies group I metabotropic glutamate receptor-mediated long-term depression (G1-mGluR-LTD) following seizures.

METHODS

We used a knock-in mice line that express ArcKR and two hyperexcitation models: an in vitro model, where hippocampal slices were exposed to zero Mg2+, 6 mM K+; and an in vivo model, where kainic acid was injected unilaterally into the hippocampus. In both models, field excitatory postsynaptic potentials (fEPSPs) were recorded from the CA1 region of hippocampal slices in response to Schaffer collateral stimulation and G1-mGluR-LTD was induced chemically with the group 1 mGluR agonist DHPG.

RESULTS

In the in vitro model, ArcKR expression enhanced the effects of seizure activity and increased the magnitude of G1-mGluR LTD, an effect that could be blocked with the mGluR5 antagonist MTEP. In the in vivo model, fEPSPs were significantly smaller in slices from ArcKR mice and were less contaminated by population spikes. In this model, the amount of G1-mGluR-LTD was significantly less in epileptic slices from ArcKR mice as compared to wildtype (WT) mice.

SIGNIFICANCE

We have shown that expression of ArcKR, a form of Arc in which degradation is reduced, significantly modulates the magnitude of G1-mGluR-LTD following epileptic seizures. However, the effect of ArcKR on LTD depends on the epileptic model used, with enhancement of LTD in an in vitro model and a reduction in the kainate mouse model.

SHP2 regulates GluA2 tyrosine phosphorylation required for AMPA receptor endocytosis and mGluR-LTD

Posttranslational modifications regulate the properties and abundance of synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors that mediate fast excitatory synaptic transmission and synaptic plasticity in the central nervous system. During long-term depression (LTD), protein tyrosine phosphatases (PTPs) dephosphorylate tyrosine residues in the C-terminal tail of AMPA receptor GluA2 subunit, which is essential for GluA2 endocytosis and group I metabotropic glutamate receptor (mGluR)-dependent LTD. However, as a selective downstream effector of mGluRs, the mGluR-dependent PTP responsible for GluA2 tyrosine dephosphorylation remains elusive at Schaffer collateral (SC)-CA1 synapses. In the present study, we find that mGluR5 stimulation activates Src homology 2 (SH2) domain-containing phosphatase 2 (SHP2) by increasing phospho-Y542 levels in SHP2. Under steady-state conditions, SHP2 plays a protective role in stabilizing phospho-Y869 of GluA2 by directly interacting with GluA2 phosphorylated at Y869, without affecting GluA2 phospho-Y876 levels. Upon mGluR5 stimulation, SHP2 dephosphorylates GluA2 at Y869 and Y876, resulting in GluA2 endocytosis and mGluR-LTD. Our results establish SHP2 as a downstream effector of mGluR5 and indicate a dual action of SHP2 in regulating GluA2 tyrosine phosphorylation and function. Given the implications of mGluR5 and SHP2 in synaptic pathophysiology, we propose SHP2 as a promising therapeutic target for neurodevelopmental and autism spectrum disorders.

Functional Interaction between Adenosine A2A and mGlu5 Receptors Mediates STEP Phosphatase Activation and Promotes STEP/mGlu5R Binding in Mouse Hippocampus and Neuroblastoma Cell Line

(1) Background: Recently, we found that adenosine A2A receptor (A2AR) stimulation results in an increase in STEP phosphatase activity. In order to delve into the mechanism through which A2AR stimulation induced STEP activation, we investigated the involvement of mGlu5R since it is well documented that A2AR and mGlu5R physically and functionally interact in several brain areas. (2) Methods: In a neuroblastoma cell line (SH-SY5Y) and in mouse hippocampal slices, we evaluated the enzymatic activity of STEP by using a para-nitrophenyl phosphate colorimetric assay. A co-immunoprecipitation assay and a Western blot analysis were used to evaluate STEP/mGlu5R binding. (3) Results: We found that the A2AR-dependent activation of STEP was mediated by the mGlu5R. Indeed, the A2AR agonist CGS 21680 significantly increased STEP activity, and this effect was prevented not only by the A2AR antagonist ZM 241385, as expected, but also by the mGlu5R antagonist MPEP. In addition, we found that mGlu5R agonist DHPG-induced STEP activation was reversed not only by the mGlu5R antagonist MPEP but also by ZM 241385. Finally, via co-immunoprecipitation experiments, we found that mGlu5R and STEP physically interact when both receptors are activated (4) Conclusions: These results demonstrated a close functional interaction between mGlu5 and A2A receptors in the modulation of STEP activity.

Reelin Regulates Neuronal Excitability through Striatal-Enriched Protein Tyrosine Phosphatase (STEP61) and Calcium Permeable AMPARs in an NMDAR-Dependent Manner

Alzheimer's disease (AD) is a progressive neurodegenerative disease marked by the accumulation of amyloid-β (Aβ) plaques and neurofibrillary tangles. Aβ oligomers cause synaptic dysfunction early in AD by enhancing long-term depression (LTD; a paradigm for forgetfulness) via metabotropic glutamate receptor (mGluR)-dependent regulation of striatal-enriched tyrosine phosphatase (STEP61). Reelin is a neuromodulator that signals through ApoE (apolipoprotein E) receptors to protect the synapse against Aβ toxicity (Durakoglugil et al., 2009) Reelin signaling is impaired by ApoE4, the most important genetic risk factor for AD, and Aβ-oligomers activate metabotropic glutamate receptors (Renner et al., 2010). We therefore asked whether Reelin might also affect mGluR-LTD. To this end, we induced chemical mGluR-LTD using DHPG (Dihydroxyphenylglycine), a selective mGluR5 agonist. We found that exogenous Reelin reduces the DHPG-induced increase in STEP61, prevents the dephosphorylation of GluA2, and concomitantly blocks mGluR-mediated LTD. By contrast, Reelin deficiency increased expression of Ca2+-permeable GluA2-lacking AMPA receptors along with higher STEP61 levels, resulting in occlusion of DHPG-induced LTD in hippocampal CA1 neurons. We propose a model in which Reelin modulates local protein synthesis as well as AMPA receptor subunit composition through modulation of mGluR-mediated signaling with implications for memory consolidation or neurodegeneration.SIGNIFICANCE STATEMENT Reelin is an important neuromodulator, which in the adult brain controls synaptic plasticity and protects against neurodegeneration. Amyloid-β has been shown to use mGluRs to induce synaptic depression through endocytosis of NMDA and AMPA receptors, a mechanism referred to as LTD, a paradigm of forgetfulness. Our results show that Reelin regulates the phosphatase STEP, which plays an important role in neurodegeneration, as well as the expression of calcium-permeable AMPA receptors, which play a role in memory formation. These data suggest that Reelin uses mGluR LTD pathways to regulate memory formation as well as neurodegeneration.

Insight into the Role of the STriatal-Enriched Protein Tyrosine Phosphatase (STEP) in A2A Receptor-Mediated Effects in the Central Nervous System

The STriatal-Enriched protein tyrosine phosphatase STEP is a brain-specific tyrosine phosphatase that plays a pivotal role in the mechanisms of learning and memory, and it has been demonstrated to be involved in several neuropsychiatric diseases. Recently, we found a functional interaction between STEP and adenosine A_2A receptor (A_2AR), a subtype of the adenosine receptor family widely expressed in the central nervous system, where it regulates motor behavior and cognition, and plays a role in cell survival and neurodegeneration. Specifically, we demonstrated the involvement of STEP in A_2AR-mediated cocaine effects in the striatum and, more recently, we found that in the rat striatum and hippocampus, as well as in a neuroblastoma cell line, the overexpression of the A_2AR, or its stimulation, results in an increase in STEP activity. In the present article we will discuss the functional implication of this interaction, trying to examine the possible mechanisms involved in this relation between STEP and A_2ARs.

Differential ability of the dorsal and ventral rat hippocampus to exhibit group I metabotropic glutamate receptor-dependent synaptic and intrinsic plasticity

Background:

The hippocampus is critically involved in learning and memory processes. Although once considered a relatively homogenous structure, it is now clear that the hippocampus can be divided along its longitudinal axis into functionally distinct domains, responsible for the encoding of different types of memory or behaviour. Although differences in extrinsic connectivity are likely to contribute to this functional differentiation, emerging evidence now suggests that cellular and molecular differences at the level of local hippocampal circuits may also play a role.

Methods:

In this study, we have used extracellular field potential recordings to compare basal input/output function and group I metabotropic glutamate receptor-dependent forms of synaptic and intrinsic plasticity in area CA1 of slices taken from the dorsal and ventral sectors of the adult rat hippocampus.

Results:

Using two extracellular electrodes to simultaneously record field EPSPs and population spikes, we show that dorsal and ventral hippocampal slices differ in their basal levels of excitatory synaptic transmission, paired-pulse facilitation, and EPSP-to-Spike coupling. Furthermore, we show that slices taken from the ventral hippocampus have a greater ability than their dorsal counterparts to exhibit long-term depression of synaptic transmission and EPSP-to-Spike potentiation induced by transient application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine.

Conclusions:

Together, our results provide further evidence that the information processing properties of local hippocampal circuits differ in the dorsal and ventral hippocampal sectors, and that these differences may in turn contribute to the functional differentiation that exists along the hippocampal longitudinal axis.

Metabotropic glutamate receptor dependent long-term depression in the cortex

Metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD), a type of synaptic plasticity, is characterized by a reduction in the synaptic response, mainly at the excitatory synapses of the neurons. The hippocampus and the cerebellum have been the most extensively studied regions in mGluR-dependent LTD, and Group 1 mGluR has been reported to be mainly involved in this synaptic LTD at excitatory synapses. However, mGluR-dependent LTD in other brain regions may be involved in the specific behaviors or diseases. In this paper, we focus on five cortical regions and review the literature that implicates their contribution to the pathogenesis of several behaviors and specific conditions associated with mGluR-dependent LTD.

Hippocampal metabotropic glutamate receptor long-term depression in health and disease: focus on mitogen-activated protein kinase pathways

Group I metabotropic glutamate receptor (mGluR) dependent long-term depression (LTD) is a major form of synaptic plasticity underlying learning and memory. The molecular mechanisms involved in mGluR-LTD have been investigated intensively for the last two decades. In this 60th anniversary special issue article, we review the recent advances in determining the mechanisms that regulate the induction, transduction and expression of mGluR-LTD in the hippocampus, with a focus on the mitogen-activated protein kinase (MAPK) pathways. In particular we discuss the requirement of p38 MAPK and extracellular signal-regulated kinase 1/2 (ERK 1/2) activation. The recent advances in understanding the signaling cascades regulating mGluR-LTD are then related to the cognitive impairments observed in neurological disorders, such as fragile X syndrome and Alzheimer's disease. mGluR-LTD is a form of synaptic plasticity that impacts on memory formation. In the hippocampus mitogen-activated protein kinases (MAPKs) have been found to be important in mGluR-LTD. In this 60th anniversary special issue article, we review the independent and complementary roles of two classes of MAPK, p38 and ERK1/2 and link this to the aberrant mGluR-LTD that has an important role in diseases. This article is part of the 60th Anniversary special issue.

Rapid regulation of endoplasmic reticulum dynamics in dendritic spines by NMDA receptor activation

Endoplasmic reticulum (ER) is motile within dendritic spines, but the mechanisms underlying its regulation are poorly understood. To address this issue, we have simultaneously imaged morphology and ER content of dendritic spines in cultured dissociated mouse hippocampal neurons. Over a 10 min period, spines were highly dynamic, with spines both increasing and decreasing in volume. ER was present in approximately 50% of spines and was also highly dynamic, with a net increase over this period of time. Inhibition of the endogenous activation of NMDA receptors resulted in a reduction in ER growth. Conversely, augmentation of the synaptic activation of NMDA receptors, by elimination of striatal-enriched protein tyrosine phosphatase (STEP), resulted in enhanced ER growth. Therefore, NMDA receptors rapidly regulate spine ER dynamics.

Molecular changes associated with hippocampal long-lasting depression induced by the serine protease subtilisin-A

The serine protease subtilisin-A (SubA) induces a form of long-term depression (LTD) of synaptic transmission in the rat hippocampus, and molecular changes associated with SubA-induced LTD (SubA-LTD) were explored by using recordings of evoked postsynaptic potentials and immunoblotting. SubA-LTD was prevented by a selective inhibitor of SubA proteolysis, but the same inhibitor did not affect LTD induced by electrical stimulation or activation of metabotropic glutamate receptors. SubA-LTD was reduced by the protein kinase inhibitors genistein and lavendustin A, although not by inhibitors of p38 mitogen-activated protein kinase, glycogen synthase kinase-3, or protein phosphatases. It was also reduced by (RS)-α-methyl-4-carboxyphenylglycine, a broad-spectrum antagonist at metabotropic glutamate receptors. Inhibition of the Rho kinase enzyme Rho-associated coiled-coil kinase reduced SubA-LTD, although inhibitors of the RhoGTPase-activating enzymes farnesyl transferase and geranylgeranyl transferase did not. In addition, a late phase of SubA-LTD was dependent on new protein synthesis. There was a small, non-significant difference in SubA-LTD between wild-type and RhoB(-/-) mice. Marked decreases were seen in the levels of Unc-5H3, a protein that is intimately involved in the development and plasticity of glutamatergic synapses. Smaller changes were noted, at higher concentrations of SubA, in Unc-5H1, vesicle-associated membrane protein-1 (synaptobrevin), and actin, with no changes in the levels of synaptophysin, synaptotagmin, RhoA, or RhoB. None of these changes was associated with LTD induced electrically or by the metabotropic glutamate receptor agonist (RS)-3,5-dihydroxyphenylglycine. These results indicate that SubA induces molecular changes that overlap with other forms of LTD, but that the overall molecular profile of SubA-LTD is quite different.

The serine protease subtilisin suppresses epileptiform activity in rat hippocampal slices and neocortex in vivo

Serine proteases of the S8A family and those belonging to the subtilase group generate a long-lasting inhibition of hippocampal evoked potentials, which shows little recovery and resembles long-term depression. The present work investigates the effects of subtilisin A on epileptiform activity induced in hippocampal slices. Interictal bursts were generated by perfusion with 4-aminopyridine in magnesium-free medium, whereas ictal bursts were produced by the addition of baclofen. Subtilisin A superfused for 10 min at concentrations of 50 nM and above reduced the duration of ictal bursts, whereas higher concentrations reduced the frequency of interictal activity with little or no recovery, indicating similarity with the long-term depression reported previously. The anti-epileptiform activity was not prevented by inhibitors of phosphatases or several kinases, but the inhibition of ictal activity was selectively reduced by the tyrosine kinase inhibitor genistein. The rho-activated coiled-coil kinase (ROCK) inhibitor Y-27632 had no effect on the suppression of ictal or interictal bursts. Subtilisin applied at nanomolar concentrations to the surface of the cerebral cortex in vivo also suppressed epileptiform spikes induced by bicuculline. It is concluded that serine proteases of the subtilase group are highly potent inhibitors of epileptiform activity, especially ictal bursts, and that tyrosine kinases may be involved in that inhibition. The mechanism of inhibition is different from the long-lasting depression of evoked potentials, which is partly mediated via ROCK.

Reversible plasticity of fear memory-encoding amygdala synaptic circuits even after fear memory consolidation

It is generally believed that after memory consolidation, memory-encoding synaptic circuits are persistently modified and become less plastic. This, however, may hinder the remaining capacity of information storage in a given neural circuit. Here we consider the hypothesis that memory-encoding synaptic circuits still retain reversible plasticity even after memory consolidation. To test this, we employed a protocol of auditory fear conditioning which recruited the vast majority of the thalamic input synaptic circuit to the lateral amygdala (T-LA synaptic circuit; a storage site for fear memory) with fear conditioning-induced synaptic plasticity. Subsequently the fear memory-encoding synaptic circuits were challenged with fear extinction and re-conditioning to determine whether these circuits exhibit reversible plasticity. We found that fear memory-encoding T-LA synaptic circuit exhibited dynamic efficacy changes in tight correlation with fear memory strength even after fear memory consolidation. Initial conditioning or re-conditioning brought T-LA synaptic circuit near the ceiling of their modification range (occluding LTP and enhancing depotentiation in brain slices prepared from conditioned or re-conditioned rats), while extinction reversed this change (reinstating LTP and occluding depotentiation in brain slices prepared from extinguished rats). Consistently, fear conditioning-induced synaptic potentiation at T-LA synapses was functionally reversed by extinction and reinstated by subsequent re-conditioning. These results suggest reversible plasticity of fear memory-encoding circuits even after fear memory consolidation. This reversible plasticity of memory-encoding synapses may be involved in updating the contents of original memory even after memory consolidation.

Differential trafficking of AMPA receptors following activation of NMDA receptors and mGluRs

The removal of AMPA receptors from synapses is a major component of long-term depression (LTD). How this occurs, however, is still only partially understood. To investigate the trafficking of AMPA receptors in real-time we previously tagged the GluA2 subunit of AMPA receptors with ecliptic pHluorin and studied the effects of NMDA receptor activation. In the present study we have compared the effect of NMDA receptor and group I mGluR activation, using GluA2 tagged with super ecliptic pHluorin (SEP-GluA2) expressed in cultured hippocampal neurons. Surprisingly, agonists of the two receptors, which are both able to induce chemical forms of LTD, had clearly distinct effects on AMPA receptor trafficking. In agreement with our previous work we found that transient NMDA receptor activation results in an initial decrease in surface GluA2 from extrasynaptic sites followed by a delayed reduction in GluA2 from puncta (putative synapses). In contrast, transient activation of group I mGluRs, using DHPG, led to a pronounced but more delayed decrease in GluA2 from the dendritic shafts. Surprisingly, there was no average change in the fluorescence of the puncta. Examination of fluorescence at individual puncta, however, indicated that alterations did take place, with some puncta showing an increase and others a decrease in fluorescence. The effects of DHPG were, like DHPG-induced LTD, prevented by treatment with a protein tyrosine phosphatase (PTP) inhibitor. The electrophysiological correlate of the effects of DHPG in the SEP-GluA2 infected cultures was a reduction in mEPSC frequency with no change in amplitude. The implications of these findings for the initial mechanisms of expression of both NMDA receptor- and mGluR-induced LTD are discussed.

Calcium/calmodulin-dependent protein kinase II mediates group I metabotropic glutamate receptor-dependent protein synthesis and long-term depression in rat hippocampus

Activation of Group I metabotropic glutamate receptors (mGluRs) in rat hippocampus induces a form of long-term depression (LTD) that is dependent on protein synthesis. However, the intracellular mechanisms leading to the initiation of protein synthesis and expression of LTD after mGluR activation are only partially understood. We investigated the role of several pathways linked to mGluR activation, translation initiation, and induction of LTD. We found that Group I mGluR-dependent protein synthesis and associated LTD, as induced by the agonist (RS)-3,5-dihydrophenylglycine (DHPG) or paired-pulse synaptic stimulation, was dependent on activation of calcium/calmodulin-dependent protein kinase IIα (CaMKII). DHPG induced a transient increase in the level of phospho-CaMKII (phospho-CaMKII(T286)) in synaptoneurosomes prepared from whole hippocampus and in CA1 minislices. In synaptoneurosomes, DHPG also induced an increase in phosphorylation of eIF4E, and an increase in protein synthesis that was abolished by translation inhibitors and the CaMKII inhibitors 1-[N,O-bis(5-isoquinolinesulphonyl)-N-methyl-l-tyrosyl]-4-phenylpiperazine (KN62) and 2-[N-(2-hydroxyethyl)]-N-(4-methoxybenzenesulfonyl)amino-N-(4-chloro-cinnamyl)-N-methylbenzylamine (KN93). In field recordings from CA1, both the translation inhibitor cycloheximide and KN62 significantly reduced DHPG-induced LTD. Combined application did not further reduce the LTD, suggesting a common mechanism. In whole-cell recordings, a third CaMKII inhibitor, AIP (autocamtide-2-related inhibitory peptide), significantly reduced the DHPG-induced LTD of synaptic currents. Inhibition of the classical pathway mediating many Group I mGluR effects by blocking PKC (protein kinase C) or PLC (phospholipase C) did not impair DHPG-induced protein synthesis or LTD. Collectively, these findings demonstrate an important role for CaMKII in mediating the initiation of protein synthesis that then supports the postsynaptic expression of DHPG-induced LTD.

Synaptic activation of mGluR1 generates persistent depression of a fast after-depolarizing potential in CA3 pyramidal neurons

Burst firing is an important property of hippocampal pyramidal neurons. Group I metabotropic glutamate receptors (mGluRs) produce a multitude of effects on both the synaptic and intrinsic properties of neurons. We investigated whether brief activation of these receptors results in persistent modifications to the intrinsic excitability of rat hippocampal CA3 pyramidal cells (CA3-PCs). In whole-cell current-clamp recordings, current stimuli consisting of filtered, pseudo-random noise produced action potential firing with a mean frequency of ∼1.5-2 Hz. Analysis of spike intervals revealed that this firing included a substantial component (∼20%) of high-frequency (∼100 Hz) bursting activity. Activation of group I mGluRs with (S)-3,5-dihydroxyphenylglycine [(S)-DHPG] selectively eliminated the high-frequency bursts, an effect that persisted > 30 min after (S)-DHPG washout. The fast after-depolarizing potential (ADP) of CA3-PCs is known to be important for generating high-frequency action potential bursting. This ADP was persistently depressed following a short application of (S)-DHPG. This effect was blocked by the mGluR1 antagonist, (S)-(+)-α-amino-4-carboxy-2-methylbenzeneacetic acid (LY367385). In contrast, the depression was resistant to the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate, N-methyl-D-aspartate (NMDA) and γ-aminobutyric acid (GABA)(A) antagonists. Unlike other manipulations that generate persistent depression of the ADP in CA3-PCs, DHPG-mediated ADP depression was insensitive to the Kv7 channel inhibitor 10,10-bis(4-Pyridinylmethyl)-9(10H)-anthracenone dihydrochloride (XE991) and strong intracellular Ca(2+) buffering by 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). Synaptic activation of mGluRs in the associational-commissural pathway also resulted in persistent depression of the ADP in postsynaptic CA3-PCs, which was blocked by LY367385. These data represent the first evidence that synaptic activation of mGluR1 can modulate the intrinsic excitability properties of hippocampal neurons.

Interaction between Ephrins and mGlu5 metabotropic glutamate receptors in the induction of long-term synaptic depression in the hippocampus

We applied the group-I metabotropic glutamate (mGlu) receptor agonist, 3,5-dihydroxyphenylglycine (DHPG), to neonatal or adult rat hippocampal slices at concentrations (10 microM) that induced a short-term depression (STD) of excitatory synaptic transmission at the Schaffer collateral/CA1 synapses. DHPG-induced STD was entirely mediated by the activation of mGlu5 receptors because it was abrogated by the mGlu5 receptor antagonist, MPEP [2-methyl-6-(phenylethynyl)pyridine], but not by the mGlu1 receptor antagonist, CPCCOEt [7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester]. Knowing that ephrin-Bs functionally interact with group-I mGlu receptors (Calò et al., 2005), we examined whether pharmacological activation of ephrin-Bs could affect DHPG-induced STD. We activated ephrin-Bs using their cognate receptor, EphB1, under the form of a preclustered EphB1/Fc chimera. Addition of clustered EphB1/Fc alone to the slices induced a small but nondecremental depression of excitatory synaptic transmission, which differed from the depression induced by 10 microM DHPG. Surprisingly, EphB1/Fc-induced synaptic depression was abolished by MPEP (but not by CPCCOEt) suggesting that it required the endogenous activation of mGlu5 receptors. In addition, coapplication of DHPG and EphB1/Fc, resulted in a large and nondecremental long-term depression. The effect of clustered EphB1/Fc was specific because it was not mimicked by unclustered EphB1/Fc or clustered EphA1/Fc. These findings raise the intriguing possibility that changes in synaptic efficacy mediated by mGlu5 receptors are under the control of the ephrin/Eph receptor system, and that the neuronal actions of ephrins can be targeted by drugs that attenuate mGlu5 receptor signaling.

Group 1 mGluR-dependent synaptic long-term depression: mechanisms and implications for circuitry and disease

Many excitatory synapses express Group 1, or Gq coupled, metabotropic glutamate receptors (Gp1 mGluRs) at the periphery of their postsynaptic density. Activation of Gp1 mGluRs typically occurs in response to strong activity and triggers long-term plasticity of synaptic transmission in many brain regions, including the neocortex, hippocampus, midbrain, striatum, and cerebellum. Here we focus on mGluR-induced long-term synaptic depression (LTD) and review the literature that implicates Gp1 mGluRs in the plasticity of behavior, learning, and memory. Moreover, recent studies investigating the molecular mechanisms of mGluR-LTD have discovered links to mental retardation, autism, Alzheimer's disease, Parkinson's disease, and drug addiction. We discuss how mGluRs lead to plasticity of neural circuits and how the understanding of the molecular mechanisms of mGluR plasticity provides insight into brain disease.

Metabotropic glutamate receptor-mediated long-term depression: molecular mechanisms

The ability to modify synaptic transmission between neurons is a fundamental process of the nervous system that is involved in development, learning, and disease. Thus, synaptic plasticity is the ability to bidirectionally modify transmission, where long-term potentiation and long-term depression (LTD) represent the best characterized forms of plasticity. In the hippocampus, two main forms of LTD coexist that are mediated by activation of either N-methyl-d-aspartic acid receptors (NMDARs) or metabotropic glutamate receptors (mGluRs). Compared with NMDAR-LTD, mGluR-LTD is less well understood, but recent advances have started to delineate the underlying mechanisms. mGluR-LTD at CA3:CA1 synapses in the hippocampus can be induced either by synaptic stimulation or by bath application of the group I selective agonist (R,S)-3,5-dihydroxyphenylglycine. Multiple signaling mechanisms have been implicated in mGluR-LTD, illustrating the complexity of this form of plasticity. This review provides an overview of recent studies investigating the molecular mechanisms underlying hippocampal mGluR-LTD. It highlights the role of key molecular components and signaling pathways that are involved in the induction and expression of mGluR-LTD and considers how the different signaling pathways may work together to elicit a persistent reduction in synaptic transmission.

Mechanisms of Group I mGluR-Dependent Long-Term Depression of NMDA Receptor–Mediated Transmission at Schaffer Collateral–CA1 Synapses

The mechanisms underlying group I metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) of N-methyl-d-aspartate receptor (NMDAR)-mediated synaptic currents (EPSCsNMDAR) are poorly understood. Here we investigated the effects of ( R,S)-3,5-dihydroxyphenylglycine (DHPG), a selective agonist of group I mGluRs, on the EPSCsNMDAR in area CA1 of acute hippocampal slices from 6- to 8-wk Sprague-Dawley rats. DHPG acutely and persistently depressed the isolated EPSCNMDAR and transiently slowed its decay rate. Combined antagonism of mGluR1 and mGluR5 blocked the effects of DHPG. Strong calcium buffering with intracellular BAPTA did not reduce the acute depression or LTD, making the involvement of elevated postsynaptic calcium unlikely. The acute depression and LTD were not mediated by activation of tyrosine kinases or phosphatases, nor were they dependent on protein synthesis. However, the LTD was prevented by the intracellular actin-stabilizer jasplakinolide, raising the possibility that it was associated with a lateral movement of NMDARs. Supporting this hypothesis, when the effective spatial spread of synaptically released glutamate was increased using the glutamate transporter inhibitor TBOA, the resultant EPSCNMDAR did not undergo LTD in response to DHPG. Importantly, isolation of the extrasynaptic EPSCNMDAR by blockade of synaptic NMDARs with MK-801 showed that this was not due to a potentiation of the preexisting extrasynaptic component. These findings indicate that LTD of NMDAR-mediated synaptic transmission occurs via lateral movement of receptors away from the synapse.

Tyrosine dephosphorylation regulates AMPAR internalisation in mGluR-LTD

Long-term depression (LTD) can be induced at hippocampal CA1 synapses by activation of either NMDA receptors (NMDARs) or group I metabotropic glutamate receptors (mGluRs), using their selective agonists NMDA and (RS)-3,5-dihydroxyphenylglycine (DHPG), respectively. Recent studies revealed that DHPG-LTD is dependent on activation of postsynaptic protein tyrosine phosphatases (PTPs), which transiently dephosphorylate tyrosine residues in AMPA receptors (AMPARs). Here we show that while both endogenous GluR2 and GluR3 AMPAR subunits are tyrosine phosphorylated at basal activity, only GluR2 is dephosphorylated in DHPG-LTD. The tyrosine dephosphorylation of GluR2 does not occur in NMDA-LTD. Conversely, while NMDA-LTD is associated with the dephosphorylation of GluR1-serine-845, DHPG-LTD does not alter the phosphorylation of this site. The increased AMPAR endocytosis in DHPG-LTD is PTP-dependent and involves tyrosine dephosphorylation of cell surface AMPARs. Together, these results indicate that the subunit selective tyrosine dephosphorylation of surface GluR2 regulates AMPAR internalisation in DHPG-LTD but not in NMDA-LTD in the hippocampus.

The induction of long-term plasticity of non-synaptic, synchronized activity by the activation of group I mGluRs

It is well established that activation of group I metabotropic glutamate receptors (mGluRs) produces long-lasting alterations in synaptic efficacy. We now demonstrate that activation of mGluRs can also induce long-term alterations in synchronised network activity that are both induced and expressed in the absence of chemical synaptic transmission. Specifically, in hippocampal slices in which synaptic transmission was eliminated by perfusing with a Ca2+-free medium, the selective group I mGluR agonist 3,5-dihydroxyphenylglycine (DHPG) induced a persistent (>3h) enhancement (>2-fold) of the frequency of synchronised bursting activity. The underlying biochemical mechanism responsible for the induction of this form of plasticity was similar to that for DHPG-induced long-term depression (LTD) in that it required the activation of tyrosine phosphatases. Also, like DHPG-induced LTD, this form of neuronal plasticity could be reversed by application of the mGluR antagonist alpha-methyl-4-carboxyphenylglycine (MCPG). This unusual form of plasticity, which presumably also occurs when synaptic transmission is intact, could contribute to long-term alterations in synchronised activity in hippocampal neuronal networks.

Co-activation of p38 mitogen-activated protein kinase and protein tyrosine phosphatase underlies metabotropic glutamate receptor-dependent long-term depression

Long-term potentiation (LTP) and long-term depression (LTD) are forms of synaptic plasticity thought to contribute to learning and memory. Much is known about the mechanisms of NMDA receptor-dependent LTD in the CA1 region of rat hippocampus but there is still considerable uncertainty about the mechanisms of LTD induced by mGluR activation (mGluR-LTD). Furthermore, data on mGluR-LTD derives largely from studies using pharmacologically induced LTD. To investigate mGluR-LTD that is more physiologically relevant we have examined, in CA1 of adult rat hippocampus, mechanisms of synaptically induced mGluR-LTD. We provide the first demonstration that activation of protein tyrosine phosphatase (PTP) is essential for the induction of synaptically induced mGluR-LTD. In addition, we show that activation of p38 MAPK is also required for this form of LTD. Furthermore, LTD can be mimicked and occluded by activation of p38 MAPK, provided that protein tyrosine kinases (PTKs) are inhibited. These data therefore demonstrate that a novel combination of signalling cascades, requiring both activation of p38 MAPK and tyrosine de-phosphorylation, underlies the induction of synaptically induced mGluR-LTD.

Shift in induction mechanisms underlies an age-dependent increase in DHPG-induced synaptic depression at CA3 CA1 synapses

Several forms of log-term synaptic plasticity have been identified and the mechanisms for induction and expression of synaptic modifications change over development and maturation. The present study examines age-related changes in the induction of group I metabotropic receptor selective agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) induced long-term synaptic depression (DHPG-LTD) at CA3-CA1 synapses. The results demonstrate that the magnitude of DHPG-LTD is enhanced in male aged Fischer 344 rats compared with young adults. The role of mGluR1 in the induction of DHPG-LTD was increased with advanced age and, in contrast to young adults, induction involved a significant contribution of NMDA receptors and L-type Ca(2+) channels. Moreover, the protein tyrosine phosphatase inhibitor sodium orthovanadate significantly attenuated DHPG-LTD only in young adults. The expression of DHPG-LTD in aged animals was dependent on protein synthesis and the enhanced expression was associated with an increase in paired-pulse facilitation. The results provide evidence that DHPG-LTD is one of the few forms of synaptic plasticity that increases with advanced age and suggest that DHPG-LTD may contribute to age-related changes in hippocampal function.

Group I mGluRs and long-term depression: potential roles in addiction?

Addiction is an enormous societal problem. A number of recent studies have focused on adaptations at glutamatergic synapses that may play a role in the behavioral responses to drugs of abuse. These studies have largely focused on NMDA receptor-dependent forms of synaptic plasticity such as NMDA receptor-dependent long-term potentiation (LTP) and long-term depression (LTD). A growing body of evidence, however, suggests that metabotropic glutamate receptors (mGluRs) also play important roles in the behavioral responses to drugs of abuse and participate in producing synaptic plasticity at glutamate synapses. In this review, we focus first on the evidence supporting a role for mGluRs in addiction and then on the properties of mGluR-dependent forms of synaptic plasticity, focusing in particular on Gq-linked receptor-induced LTD.

Comparison of cellular mechanisms of long-term depression of synaptic strength at perforant path-granule cell and Schaffer collateral-CA1 synapses

This chapter compares the cellular mechanisms that have been implicated in the induction and expression of long-term depression (LTD) at Schaffer collateral-CA1 synapses to perforant path-dentate gyrus (DG) synapses. In general, Schaffer collateral LTD and long-term potentiation (LTP) both appear to be a complex combination of many alterations in synaptic transmission that occur at both presynaptic and postsynaptic sites, while at perforant path synapses, most evidence has focused on postsynaptic long-term alterations. Within the DG, the medial perforant path is far more studied than lateral perforant path synapses, where most evidence relates to the induction of heterosynaptic LTD at lateral perforant path synapses when LTP is induced in the medial perforant path. Of course, there remain many other classes of synapses in the DG where synaptic plasticity, including LTD, have been largely neglected. It is clear that a better understanding of the range of DG loci where long-lasting activity-dependent plasticity, both LTD and LTP, are expressed will be essential to improve our understanding of the cognitive roles of such DG plasticity.

The dynamics of recycled acetylcholine receptors at the neuromuscular junction in vivo

At the peripheral neuromuscular junction (NMJ), a significant number of nicotinic acetylcholine receptors (AChRs) recycle back into the postsynaptic membrane after internalization to intermingle with not-yet-internalized ;pre-existing' AChRs. However, the way in which these receptor pools are maintained and regulated at the NMJ in living animals remains unknown. Here, we demonstrate that recycled receptors in functional synapses are removed approximately four times faster than pre-existing receptors, and that most removed recycled receptors are replaced by new recycled ones. In denervated NMJs, the recycling of AChRs is significantly depressed and their removal rate increased, whereas direct muscle stimulation prevents their loss. Furthermore, we show that protein tyrosine phosphatase inhibitors cause the selective accumulation of recycled AChRs in the peri-synaptic membrane without affecting the pre-existing AChR pool. The inhibition of serine/threonine phosphatases, however, has no effect on AChR recycling. These data show that recycled receptors are remarkably dynamic, and suggest a potential role for tyrosine dephosphorylation in the insertion and maintenance of recycled AChRs at the postsynaptic membrane. These findings may provide insights into long-term recycling processes at less accessible synapses in the central nervous system in vivo.

Tyrosine phosphatases regulate AMPA receptor trafficking during metabotropic glutamate receptor-mediated long-term depression

Two forms of long-term depression (LTD), triggered by activation of NMDA receptors (NMDARs) and metabotropic glutamate receptors (mGluRs), respectively, can be induced at CA1 synapses in the hippocampus. Compared with NMDAR-LTD, relatively little is known about mGluR-LTD. Here, we show that protein tyrosine phosphatase (PTP) inhibitors, orthovanadate and phenylarsine oxide, selectively block mGluR-LTD induced by application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG-LTD), because NMDAR-LTD is unaffected by these inhibitors. Furthermore, DHPG-LTD measured using whole-cell recording is similarly blocked by either bath-applied or patch-loaded PTP inhibitors. These inhibitors also block the changes in paired-pulse facilitation and coefficient of variation that are associated with the expression of DHPG-LTD. DHPG treatment of hippocampal slices was associated with a decrease in the level of tyrosine phosphorylation of GluR2 AMPA receptor (AMPAR) subunits, an effect blocked by orthovanadate. Finally, in dissociated hippocampal neurons, orthovanadate blocked the ability of DHPG to reduce the number of AMPA receptor clusters on the surface of dendrites. Again, the effects of PTP blockade were selective, because NMDA-induced decreases in surface AMPAR clusters was unaffected by orthovanadate. Together, these data suggest that activation of postsynaptic PTP results in tyrosine dephosphorylation of AMPARs and their removal from the synapse.

Search for a two-input model for future investigations of ‘synaptic tagging’ in freely moving animals in vivo

Processes of "synaptic tagging" guarantee synaptic input specificity after the induction of a protein synthesis-dependent late long-term potentiation (late-LTP). Distinct high-frequency stimulation can set a transient "synaptic tag" at the activated synapses, which captures plasticity-related proteins (PRPs) synthesized synapse-non-specifically in dendritic branches/compartments or the somata. Thus, only those synapses, which expressed a "tag", are also able to express late-LTP. Additionally, it was shown that the synthesis of PRPs is triggered by heterosynaptic, non-glutamatergic requirements during LTP-induction in tissue from adult animals. All these experiments were performed in hippocampal slices in vitro so far. Two questions now arise: first, is it possible to describe processes of 'synaptic tagging' in the intact, freely moving animal and second, is the stimulation of glutamatergic inputs sufficient to induce 'tagging' or is the co-activation of a modulatory-heterosynaptic input, also required for the process? We have first developed a technique, which allows us now to induce distinct forms of LTP at the ipsilateral CA1 site by specifically stimulating glutamatergic hippocampal structures at the contralateral site in the intact, freely moving rat. Thus, the used stimulation protocol allowed us to activate two separate synaptic inputs to the same neuronal stimulation, a pre-requisite for tagging-experiments to be investigated in vivo.

A reduced number of metabotropic glutamate subtype 5 receptors are associated with constitutive homer proteins in a mouse model of fragile X syndrome

Fragile X (FRAX) syndrome is a common inherited form of mental retardation resulting from the lack of fragile X mental retardation protein (FMRP) expression. The consequences of FMRP absence in the mechanism underlying mental retardation are unknown. Here, we tested the hypothesis that glutamate receptor (GluR) expression might be altered in FRAX syndrome. Initial in situ hybridization and Western blotting experiments did not reveal differences in mRNA levels and protein expression of AMPA and NMDA subunits and metabotropic glutamate subtype 5 (mGlu5) receptors between control and Fmr1 knock-out (KO) mice during postnatal development. However, a detergent treatment (1% Triton X-100) revealed a selective reduction of mGlu5 receptor expression in the detergent-insoluble fraction of synaptic plasma membranes (SPMs) from KO mice, with no difference in the expression of NR2A, GluR1, GluR2/3, GluR4, and Homer proteins. mGlu5 receptor expression was also lower in Homer immunoprecipitates from Fmr1 KO SPMs. Homer, but not NR2A, mGlu5, and GluR1, was found to be less tyrosine phosphorylated in Fmr1 KO than control mice. Our data indicate that, in FRAX syndrome, a reduced number of mGlu5 receptors are tightly linked to the constituents of postsynaptic density and, in particular, to the constitutive forms of Homer proteins, with possible consequent alterations in synaptic plasticity.

Induction and expression mechanisms of postsynaptic NMDA receptor-independent homosynaptic long-term depression

The induction of long-term depression (LTD) can be divided into two main forms, one dependent upon activation of postsynaptic NMDAR, and another independent of postsynaptic NMDAR. Non-postsynaptic NMDAR-LTD (non-NMDAR-LTD) occurs in many regions of the brain, and encompasses a wide variety of induction and expression mechanisms. In this article, the induction and expression mechanisms of such LTD in over 10 brain regions are described, with a number of common mechanisms compared across a large range of types of LTD. The article describes the involvement of different presynaptic or postsynaptic receptors in the induction of non-NMDAR-LTD, especially metabotropic glutamate receptors, cannabinoid receptors and dopamine receptors. An increase in presynaptic or postsynaptic intracellular Ca concentration is a key event in induction, commonly followed by activation of certain kinases, especially PKC, p38 MAPK and ERK. Expression mechanisms are either presynaptic via a reduction in release probability, or postsynaptic involving a decrease in AMPAR via phosphorylation of a glutamate receptor subunit, especially GluR2, followed by clathrin-mediated endocytosis. Retrograde signalling from postsynaptic to presynaptic occurs when induction is postsynaptic and expression is presynaptic.

Sustained activation of metabotropic glutamate receptor 5 and protein tyrosine phosphatases mediate the expression of (S)-3,5-dihydroxyphenylglycine-induced long-term depression in the hippocampal CA1 region

Previous studies have shown that brief application of group I metabotropic glutamate receptor (mGluR) agonist (S)-3, 5-dihydroxyphenylglycine (DHPG) to hippocampal slices can induce a chemical form of long-term depression (DHPG-LTD) in the hippocampal CA1 region; however, the expression mechanisms of this LTD remain unclear. We show here that the expression of DHPG-LTD can be specifically reversed by application of the broad-spectrum mGluR antagonists, (S)-alpha-methyl-4-carboxyphenylglycine (MCPG) and LY341495, and mGluR5 antagonist, 2-methyl-6-(phenylethyl)pyridine, but not by NMDA receptor antagonist, D-2-amino-5-phosphonopentanoic acid, mGluR1 antagonist, LY367385, group II mGluR antagonist, (2S)-alpha-ethylglutamic acid, or group III mGluR antagonist, (S)-2-amino-2-methyl-4-phosphonobutanic acid (MAP4). In addition, the ability of MCPG to reverse DHPG-LTD was mimicked by the protein tyrosine phosphatase inhibitors, phenylarsine oxide and orthovanadate, but not phospholipase C inhibitor, U73122, protein kinase C inhibitor, bisindolylmaleimide 1, p38 mitogen-activated protein kinase inhibitor, SB203580, or protein phosphatases 1/2 A inhibitor, okadaic acid. Moreover, MCPG reversed the DHPG-LTD without affecting the paired-pulse facilitation. The expression of DHPG-LTD was associated with the reduction of both tyrosine phosphorylation and surface expression of AMPA receptor GluR2 subunits. Together, these results suggest that sustained activation of mGluR5 and in turn triggering a protein tyrosine phosphatase-dependent regulation of postsynaptic expression of AMPA receptors may contribute to the expression of DHPG-LTD.

Long-term potentiation persists in an occult state following mGluR-dependent depotentiation

Depotentiation, the reversal of long-term potentiation (LTP), can be induced by activation of metabotropic glutamate receptors (mGluRs) or NMDA receptors (NMDARs). Although NMDAR-dependent depotentiation is due to a protein phosphatase-dependent erasure of LTP, the notion that mGluR-dependent depotentiation also involves LTP erasure is controversial. To address this issue we used electrophysiological and biochemical approaches to investigate mGluR-dependent depotentiation in hippocampal slices. Activating group I mGluRs with (R,S)-3,5-dihydroxyphenylglycine (DHPG) induced robust depotentiation in both the CA1 and CA3 regions of hippocampal slices. Western immunoblotting of samples prepared from DHPG-treated slices revealed, however, that activation of group I mGluRs causes a transient increase in phosphorylation of AMPA receptor GluR1 subunits at sites crucial for LTP and under some conditions causes persistent activation of alphaCamKII. The paradoxical ability of DHPG to induce depotentiation while at the same time activating signaling pathways involved in LTP suggests that LTP might not be erased by mGluR-dependent depotentiation. Consistent with this, DHPG-induced depotentiation did not restore the ability of high-frequency stimulation to induce LTP at synapses that had previously undergone saturating levels of LTP. In addition, blocking the expression of DHPG-induced LTD revealed hidden LTP at depotentiated synapses. Our results indicate that LTP and mGluR-dependent LTD can co-exist at excitatory synapses.

Activation of the phosphoinositide 3-kinase-Akt-mammalian target of rapamycin signaling pathway is required for metabotropic glutamate receptor-dependent long-term depression

Hippocampal long-term depression (LTD) is a long-lasting decrease in synaptic strength that is most commonly studied at glutamatergic inputs to pyramidal cells in hippocampal area CA1. Activation of G-protein-coupled group I (including types 1 and 5) metabotropic glutamate receptors (mGluRs) by the pharmacological agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) elicits LTD in area CA1 of the hippocampus. Recent reports have shown that de novo protein synthesis is necessary for DHPG-induced LTD. However, relatively little is known about the signaling pathways that couple mGluRs to translation initiation. In this study, we investigated whether the activation of the phosphoinositide 3-kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) pathway, which has been shown to regulate translation initiation, is necessary for mGluR-LTD induced by DHPG. We found that brief incubations of mouse hippocampal slices with DHPG resulted in increased phosphorylation of Akt and mTOR in hippocampal area CA1. Two structurally unrelated PI3K inhibitors, LY294002 and wortmannin, blocked the DHPG-induced increases in phosphorylation of Akt and mTOR. Biochemical fractionation studies showed that the DHPG-induced increase in the phosphorylation of Akt and mTOR could be detected in synaptoneurosome preparations, and immunohistochemical analysis revealed that similar increases could be detected in both stratum pyramidale and stratum radiatum in area CA1. Finally, we observed that both PI3K inhibitors and rapamycin, an mTOR inhibitor, prevented mGluR-LTD induced by DHPG. Together, our findings indicate that activation of the PI3K-Akt-mTOR signaling cascade is required for mGluR-LTD and suggest that this pathway may couple group I mGluRs to translation initiation in hippocampal area CA1.

Metabotropic Glutamate Receptor-Mediated Depression of the Slow Afterhyperpolarization Is Gated by Tyrosine Phosphatases in Hippocampal CA1 Pyramidal Neurons

Group I metabotropic glutamate receptor (mGluR) agonists increase the excitability of hippocampal CAl pyramidal neurons via depression of the postspike afterhyperpolarization. In adult rats, this is mediated by both mGluR1 and -5, but the signal transduction processes involved are unknown. In this study, we investigated whether altered levels of tyrosine phosphorylation of proteins are involved in the depression of the slow-duration afterhyperpolarization (sAHP) by the Group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) in CA1 pyramidal neurons of rat hippocampal slices. Preincubation with the tyrosine kinase inhibitors lavendustin A or genistein, or the Src-specific inhibitor 3-(4-chlorophenyl) 1-(1,1-dimethylethyl)-1 H-pyrazolo[3,4-d]pyrimidin-4-amine (PP2), did not inhibit the DHPG-mediated depression of the sAHP. However, preincubation with the tyrosine phosphatase inhibitor orthovanadate reduced the effects of DHPG. This effect of orthovanadate was prevented by simultaneous inhibition of tyrosine kinases with lavendustin A. Selective activation of either mGluR1 or -5 by application of DHPG plus either the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) or the mGluR1 antagonist (S)-(+)-α-amino-4-carboxy-2-methylbenzeneacetic acid (LY367385) demonstrated that the effect of inhibiting tyrosine phosphatases is not specific to either subtype of mGluR. These results suggest that the depression of the sAHP induced by activation of mGluR1 and -5 is gated by a balance between tyrosine phosphorylation and dephosphorylation.

Extracellular signal-regulated protein kinase activation is required for metabotropic glutamate receptor-dependent long-term depression in hippocampal area CA1

Activation of group 1 metabotropic glutamate receptors (mGluRs) induces long-term depression (LTD) of synaptic transmission that relies on dendritic protein synthesis. We investigated the signal transduction pathways required for mGluR-LTD to identify candidate mechanisms for mGluR regulation of synaptic protein synthesis. Our results demonstrate a role for extracellular signal-regulated protein kinase (ERK), a subclass of the mitogen-activated protein kinases (MAPKs), in mGluR-LTD in area CA1 of the rat hippocampus. Inhibitors of the upstream kinase of ERK, MAP/ERK kinase significantly reduce mGluR-LTD induced by the group 1 agonist dihydroxyphenylglycine (DHPG) and synaptic stimulation but do not affect NMDA receptor-dependent LTD. In contrast, inhibitors of p38 MAPK were ineffective against DHPG-induced LTD. Consistent with the role of ERK in mGluR-LTD, we observed that DHPG treatment of hippocampal slices (isolated CA1), at concentrations that induce LTD, results in a robust phosphorylation of ERK but not of p38 MAPK. These results point to ERK as an important regulator of mGluR-LTD and a potential mechanism for mGluR regulation of synaptic protein synthesis.

Rap1-induced p38 Mitogen-activated Protein Kinase Activation Facilitates AMPA Receptor Trafficking via the GDI·Rab5 Complex

Recent evidence has emphasized the importance of p38 mitogen-activated protein kinase (MAPK) in the induction of metabotropic glutamate receptor (mGluR)-dependent long term depression (LTD) at hippocampal CA3-CA1 synapses. However, the cascade responsible of mGluR to activate p38 MAPK and the signaling pathway immediately downstream from it to induce synaptic depression is poorly understood. Here, we show that transient activation of group I mGluR with the selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) activates p38 MAPK through G protein betagamma-subunit, small GTPase Rap1, and MAPK kinase 3/6 (MKK3/6), thus resulting in mGluR5-dependent LTD. Furthermore, our data clearly show that an accelerating AMPA receptor endocytosis by stimulating the formation of guanyl nucleotide dissociation inhibitor-Rab5 complex is a potential downstream processing of p38 MAPK activation to mediate DHPG-LTD. These results suggest an important role for Rap1-MKK3/6-p38 MAPK pathway in the induction of mGluR-dependent LTD by directly coupling to receptor trafficking machineries to facilitate the loss of synaptic AMPA receptors.

Synaptically stimulated induction of group i metabotropic glutamate receptor-dependent long-term depression and depotentiation is inhibited by prior activation of metabotropic glutamate receptors and PROTEIN KINASE C

We have investigated metaplasticity of the group I metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) and depotentiation (DP) induced by physiological synaptic stimulation in the medial perforant path of the dentate gyrus in vitro. Group I mGluR-LTD/DP was inhibited by prior preconditioning brief high frequency stimulation (HFS) if the preconditioning HFS induced long-term potentiation (LTP) or if the induction of LTP was inhibited by an NMDA receptor antagonist. The inhibitory effect of the preconditioning HFS on LTD/DP was dependent upon activation of mGluRs, as it was blocked by the presence of the mGluR antagonist (S)-alpha-methyl-4-carboxyphenylglycine during the preconditioning stimulation. The inhibitory effect of the preconditioning HFS involved stimulation of PKC, as the presence of the PKC inhibitor bisindolylmaleimide (BIS) during the preconditioning stimulation prevented the inhibitory effect of such preconditioning stimulation. Activation of PKC was also necessary for the induction of mGluR-LTD itself, as the PKC inhibitor BIS prevented the induction of the mGluR-LTD. We suggest that the physiological stimulation of mGluRs by the preconditioning stimulation produces a PKC-dependent inactivation of subsequent group I mGluR functioning and thereby an inhibition of induction of group I mGluR-dependent LTD/DP induction.

Presynaptic group I metabotropic glutamate receptors modulate synaptic transmission in the rat superior colliculus via 4-AP sensitive K(+) channels

1. Group I metabotropic glutamate receptors (mGluRs) are thought to be important modulators of neuronal function in the superior colliculus (SC). Here, we investigated the pharmacology and signalling mechanisms underlying group I mGluR-mediated inhibition of neuronal excitability and synaptic transmission in the rat SC slice. 2. The group I agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) potently depressed synaptically evoked excitatory postsynaptic potentials (EPSPs), currents (EPSCs), and action potentials in a dose-dependent manner (IC50: 6.3 microm). This was strongly reduced by the broad-spectrum antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG, 1 mm, approximately 95% reduction), by the mGluR1 antagonist LY367385 (100 microm, approximately 80% reduction) but not by the mGluR5 antagonist 6-methyl-2-(phenylethynyl)-pyridine (MPEP, 1-100 microm). 3. The putative mGluR5-specific agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG, 500 microm) also inhibited EPSPs. Interestingly, CHPG's actions were not blocked by MPEP, but LY367385 (100 microm) reduced the effect of CHPG by 50%. 4. Inhibition induced by DHPG was independent of phospholipase C (PLC)/protein kinase C pathways, and did not require intact intracellular Ca2+ stores. It was not abolished but enhanced by the GABAA antagonist bicuculline (5 microm), suggesting that DHPG's action was not due to facilitated inhibition or changes in neuronal network activity. 5. The K+ channel antagonist 4-aminopyridine (4-AP, 50-100 microm) converted the inhibitory effect of DHPG into facilitation. Paired-pulse depression was strongly reduced by DHPG, an effect that was also prevented by 4-AP. 6. Our data indicate that group I agonists regulate transmitter release, presumably via an autoreceptor in the SC. This receptor may be involved in adaptation to repetitive stimulation via a non-PLC mediated pathway.

On long-term depression induced by activation of G-protein coupled receptors

In this mini-review I consider the mechanisms by which activation of glutamate and acetylcholine metabotropic receptors can result in the induction of long-term depression. Two regions of the CNS will receive particular attention; the CA1 region of hippocampus and the perirhinal cortex.

Cellular and molecular connections between sleep and synaptic plasticity

The hypothesis that sleep promotes learning and memory has long been a subject of active investigation. This hypothesis implies that sleep must facilitate synaptic plasticity in some way, and recent studies have provided evidence for such a function. Our knowledge of both the cellular neurophysiology of sleep states and of the cellular and molecular mechanisms underlying synaptic plasticity has expanded considerably in recent years. In this article, we review findings in these areas and discuss possible mechanisms whereby the neurophysiological processes characteristic of sleep states may serve to facilitate synaptic plasticity. We address this issue first on the cellular level, considering how activation of T-type Ca(2+) channels in nonREM sleep may promote either long-term depression or long-term potentiation, as well as how cellular events of REM sleep may influence these processes. We then consider how synchronization of neuronal activity in thalamocortical and hippocampal-neocortical networks in nonREM sleep and REM sleep could promote differential strengthening of synapses according to the degree to which activity in one neuron is synchronized with activity in other neurons in the network. Rather than advocating one specific cellular hypothesis, we have intentionally taken a broad approach, describing a range of possible mechanisms whereby sleep may facilitate synaptic plasticity on the cellular and/or network levels. We have also provided a general review of evidence for and against the hypothesis that sleep does indeed facilitate learning, memory, and synaptic plasticity.

Activation of mGlu receptors induces LTD without affecting postsynaptic sensitivity of CA1 neurons in rat hippocampal slices

Two forms of long-term depression (LTD) of excitatory synaptic transmission have been identified in the mammalian CNS, which are induced by the synaptic activation of N-methyl-D-aspartate (NMDA) and metabotropic glutamate (mGlu) receptors, respectively. The mGlu receptor-dependent form of LTD can be activated by application of 3,5-dihydroxyphenylglycine (DHPG), a group I selective mGlu receptor agonist. DHPG-induced LTD is increasingly being used to investigate the mechanisms of mGlu receptor-dependent LTD. However, recent experiments have argued for both a pre- and postsynaptic locus of expression of DHPG-induced LTD. In the present study we report that DHPG-induced LTD is not associated with changes in the sensitivity of CA1 neurons to bath applied AMPA. Furthermore, in contrast to homosynaptic LTD, DHPG-induced LTD is also not associated with changes in sensitivity to focally uncaged L-glutamate. These data do not support the notion that DHPG-induced LTD requires a modification of AMPA receptors, such as their internalisation, but are compatible with a presynaptic mechanism of expression.