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

Simulation of postsynaptic glutamate receptors reveals critical features of glutamatergic transmission

Activation of several subtypes of glutamate receptors contributes to changes in postsynaptic calcium concentration at hippocampal synapses, resulting in various types of changes in synaptic strength. Thus, while activation of NMDA receptors has been shown to be critical for long-term potentiation (LTP) and long term depression (LTD) of synaptic transmission, activation of metabotropic glutamate receptors (mGluRs) has been linked to either LTP or LTD. While it is generally admitted that dynamic changes in postsynaptic calcium concentration represent the critical elements to determine the direction and amplitude of the changes in synaptic strength, it has been difficult to quantitatively estimate the relative contribution of the different types of glutamate receptors to these changes under different experimental conditions. Here we present a detailed model of a postsynaptic glutamatergic synapse that incorporates ionotropic and mGluR type I receptors, and we use this model to determine the role of the different receptors to the dynamics of postsynaptic calcium with different patterns of presynaptic activation. Our modeling framework includes glutamate vesicular release and diffusion in the cleft and a glutamate transporter that modulates extracellular glutamate concentration. Our results indicate that the contribution of mGluRs to changes in postsynaptic calcium concentration is minimal under basal stimulation conditions and becomes apparent only at high frequency of stimulation. Furthermore, the location of mGluRs in the postsynaptic membrane is also a critical factor, as activation of distant receptors contributes significantly less to calcium dynamics than more centrally located ones. These results confirm the important role of glutamate transporters and of the localization of mGluRs in postsynaptic sites in their signaling properties, and further strengthen the notion that mGluR activation significantly contributes to postsynaptic calcium dynamics only following high-frequency stimulation. They also provide a new tool to analyze the interactions between metabotropic and ionotropic glutamate receptors.

Extracellular glutamate: functional compartments operate in different concentration ranges

Extracellular glutamate of glial origin modulates glial and neuronal glutamate release and synaptic plasticity. Estimates of the tonic basal concentration of extracellular glutamate range over three orders of magnitude (0.02-20 μM) depending on the technology employed to make the measurement. Based upon binding constants for glutamate receptors and transporters, this range of concentrations translates into distinct physiological and pathophysiological roles for extracellular glutamate. Here we speculate that the difference in glutamate measurements can be explained if there is patterned membrane surface expression of glutamate release and transporter sites creating extracellular subcompartments that vary in glutamate concentration and are preferentially sampled by different technologies.

New medications for drug addiction hiding in glutamatergic neuroplasticity

The repeated use of drugs that directly or indirectly stimulate dopamine transmission carry addiction liability and produce enduring pathological changes in the brain circuitry that normally regulates adaptive behavioral responding to a changing environment. This circuitry is rich in glutamatergic projections, and addiction-related behaviors in animal models have been linked to impairments in excitatory synaptic plasticity. Among the best-characterized glutamatergic projection in this circuit is the prefrontal efferent to the nucleus accumbens. A variety of molecular adaptations have been identified in the prefrontal glutamate synapses in the accumbens, many of which are induced by different classes of addictive drugs. Based largely on work with cocaine, we hypothesize that the drug-induced adaptations impair synaptic plasticity in the cortico-accumbens projection, and thereby dysregulate the ability of addicts to control their drug-taking habits. Accordingly, we go on to describe the literature implicating the drug-induced changes in protein content or function that impinge upon synaptic plasticity and have been targeted in preclinical models of relapse and, in some cases, in pilot clinical trials. Based upon modeling drug-induced impairments in neuroplasticity in the cortico-accumbens pathway, we argue for a concerted effort to clinically evaluate the hypothesis that targeting glial and neuronal proteins regulating excitatory synaptic plasticity may prove beneficial in treating addiction.

Impaired long-term depression in schizophrenia: a cathodal tDCS pilot study

BACKGROUND

Neural plasticity involves the reorganization of synaptic connections and represents the ability of the brain to adjust its function in response to challenge. Disturbed cortical plasticity has been linked to the pathophysiology of schizophrenia, with indirect evidence for disturbed plasticity in the disease state having been provided by postmortem studies and various animal models. However, glutamate-dependent long-term depression (LTD)-like cortical plasticity has not yet been investigated.

OBJECTIVE

To investigate LTD-like cortical plasticity after transcranial direct current stimulation (tDCS) in schizophrenia patients.

METHODS

Using excitability-diminishing cathodal tDCS, we performed the first in vivo assessment of glutamate-dependent LTD-like cortical plasticity in 21 schizophrenia patients and 21 matched healthy control subjects. To reveal the physiologic basis of the hypothesized plasticity deficits, we tested different inhibitory and excitatory neuronal circuits with transcranial magnetic stimulation (TMS).

RESULTS

Cathodal tDCS failed to reduce motor-evoked potential amplitudes in schizophrenia patients, indicating abolished LTD-like plasticity. Furthermore, schizophrenia patients had a prolonged GABA(B)-dependent cortical silent period (CSP) at baseline and tDCS failed to modulate the duration of CSP in the patient group. Finally, schizophrenia patients presented an elevated resting-motor threshold at baseline in comparison to healthy controls.

CONCLUSIONS

The pattern of our results provides evidence for a specific plasticity deficit in schizophrenia patients, which might be associated with a hyperglutamatergic state. These findings may reflect a reduced signal-to-noise ratio and a disturbed filter function in schizophrenia patients. An increase of GABA(B)-activity may be a compensatory mechanism to dysfunctional LTD-like plasticity in schizophrenia.

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.

Long-term potentiation in cultured hippocampal neurons

Studies performed on low-density primary neuronal cultures have enabled dissection of molecular and cellular changes during N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP). Various electrophysiological and chemical induction protocols were developed for the persistent enhancement of excitatory synaptic transmission in hippocampal neuronal cultures. The characterisation of these plasticity models confirmed that they share many key properties with the LTP of CA1 neurons, extensively studied in hippocampal slices using electrophysiological techniques. For example, LTP in dissociated hippocampal neuronal cultures is also dependent on Ca(2+) influx through post-synaptic NMDA receptors, subsequent activation and autophosphorylation of the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and an increase in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor insertion at the post-synaptic membrane. The availability of models of LTP in cultured hippocampal neurons significantly facilitated the monitoring of changes in endogenous postsynaptic receptor proteins and the investigation of the associated signalling mechanisms that underlie LTP. A central feature of LTP of excitatory synapses is the recruitment of AMPA receptors at the postsynaptic site. Results from the use of cell culture-based models started to establish the mechanism by which synaptic input controls a neuron's ability to modify its synapses in LTP. This review focuses on key features of various LTP induction protocols in dissociated hippocampal neuronal cultures and the applications of these plasticity models for the investigation of activity-induced changes in native AMPA receptors.

Glutamatergic targets for enhancing extinction learning in drug addiction

The persistence of the motivational salience of drug-related environmental cues and contexts is one of the most problematic obstacles to successful treatment of drug addiction. Behavioral approaches to extinguishing the salience of drug-associated cues, such as cue exposure therapy, have generally produced disappointing results which have been attributed to, among other things, the context specificity of extinction and inadequate consolidation of extinction learning. Extinction of any behavior or conditioned response is a process of new and active learning, and increasing evidence suggests that glutamatergic neurotransmission, a key component of the neural plasticity that underlies normal learning and memory, is also involved in extinction learning. This review will summarize findings from both animal and human studies that suggest that pharmacological enhancement of glutamatergic neurotransmission facilitates extinction learning in the context of drug addiction. Pharmacological agents that have shown potential efficacy include NMDA partial agonists, mGluR5 receptor positive allosteric modulators, inhibitors of the GlyT1 glycine transporter, AMPA receptor potentiators, and activators of the cystine-glutamate exchanger. These classes of cognition-enhancing compounds could potentially serve as novel pharmacological adjuncts to cue exposure therapy to increase success rates in attenuating cue-induced drug craving and relapse.

The FRAXopathies: definition, overview, and update

The fragile X syndrome, fragile X tremor ataxia syndrome, and premature ovarian insufficiency are conditions related to the X chromosome folate-sensitive fragile site FRAXA. Therefore, we propose that they are considered as a family of disorders under the general designation of FRAXopathies. The present review will outline the main clinical and molecular features of these disorders, with special emphasis on the pathogenic mechanisms that lead to distinct phenotypes, starting from related mutations. The understanding of these mechanisms is already generating promising therapeutic approaches.

Indirect modulation of neuronal excitability and synaptic transmission in the hippocampus by activation of proteinase-activated receptor-2

BACKGROUND AND PURPOSE

Proteinase-activated receptor-2 (PAR2) is widely expressed in the CNS under normal physiological conditions. However, its potential role in modulating neuronal excitability and synaptic transmission remains to be determined. Here, we have investigated whether PAR2 activation modulates synaptic activity in the hippocampus.

EXPERIMENTAL APPROACH

PAR2 activation and its effect on the hippocampus were examined in rat primary cultures and acute slices using whole cell patch clamp and standard extracellular recordings, respectively.

KEY RESULTS

PAR2 activation leads to a depolarization of hippocampal neurones and a paradoxical reduction in the occurrence of synaptically driven spontaneous action potentials (APs). PAR2-induced neuronal depolarization was abolished following either the inhibition of astrocytic function or antagonism of ionotropic glutamate receptors whilst the PAR2-induced decrease in AP frequency was also reduced when astrocytic function was inhibited. Furthermore, when examined in acute hippocampal slices, PAR2 activation induced a profound long-term depression of synaptic transmission that was dependent on NMDA receptor activation and was sensitive to disruption of astrocytic function.

CONCLUSIONS AND IMPLICATIONS

These novel findings show that PAR2 activation indirectly inhibits hippocampal synaptic activity and indicate that these receptors may play an active role in modulating normal physiological CNS function, in addition to their role in pathophysiological disorders.

Toward fulfilling the promise of molecular medicine in fragile X syndrome

Fragile X syndrome (FXS) is the most common inherited form of mental retardation and a leading known cause of autism. It is caused by loss of expression of the fragile X mental retardation protein (FMRP), an RNA-binding protein that negatively regulates protein synthesis. In neurons, multiple lines of evidence suggest that protein synthesis at synapses is triggered by activation of group 1 metabotropic glutamate receptors (Gp1 mGluRs) and that many functional consequences of activating these receptors are altered in the absence of FMRP. These observations have led to the theory that exaggerated protein synthesis downstream of Gp1 mGluRs is a core pathogenic mechanism in FXS. This excess can be corrected by reducing signaling by Gp1 mGluRs, and numerous studies have shown that inhibition of mGluR5, in particular, can ameliorate multiple mutant phenotypes in animal models of FXS. Clinical trials based on this therapeutic strategy are currently under way. FXS is therefore poised to be the first neurobehavioral disorder in which corrective treatments have been developed from the bottom up: from gene identification to pathophysiology in animals to novel therapeutics in humans. The insights gained from FXS and other autism-related single-gene disorders may also assist in identifying molecular mechanisms and potential treatment approaches for idiopathic autism.

Long-term depression in the CNS

Long-term depression (LTD) in the CNS has been the subject of intense investigation as a process that may be involved in learning and memory and in various pathological conditions. Several mechanistically distinct forms of this type of synaptic plasticity have been identified and their molecular mechanisms are starting to be unravelled. Most studies have focused on forms of LTD that are triggered by synaptic activation of either NMDARs (N-methyl-D-aspartate receptors) or metabotropic glutamate receptors (mGluRs). Converging evidence supports a crucial role of LTD in some types of learning and memory and in situations in which cognitive demands require a flexible response. In addition, LTD may underlie the cognitive effects of acute stress, the addictive potential of some drugs of abuse and the elimination of synapses in neurodegenerative diseases.

Transcranial magnetic stimulation in Alzheimer's disease: a neurophysiological marker of cortical hyperexcitability

Recently, neuropathological studies have shown an important motor cortex involvement in Alzheimer's disease (AD), even in its early stages, despite the lack of clinically evident motor deficit. Transcranial magnetic stimulation (TMS) studies have demonstrated that cortical excitability is enhanced in AD patients. This cortical hyperexcitability is believed to be a compensatory mechanism to execute voluntary movements, despite the progressive impairment of associative cortical areas. At present, it is not clear if these motor cortex excitability changes might be the expression of an involvement of intracortical excitatory glutamatergic circuits or an impairment of inhibitory cholinergic and, to a lesser extent, gabaergic activity. Although the main hypothesis for the pathogenesis of AD remains the degeneration of the basal forebrain cholinergic neurons, the development of specific TMS protocols, such as the paired-pulse TMS and the study of the short-latency afferent inhibition, points out the role of other neurotransmitters, such as gamma-amino-butyric acid, glutamate and dopamine. The potential therapeutic effect of repetitive TMS in restoring or compensating damaged cognitive functions, might become a possible rehabilitation tool in AD patients. Based on different patterns of cortical excitability, TMS may be useful in discriminating between physiological brain aging, mild cognitive impairment, AD and other dementing disorders. The present review provides a perspective of these TMS techniques by further understanding the role of different neurotransmission pathways and plastic remodelling of neuronal networks in the pathogenesis of AD.

Postsynaptic and presynaptic group II metabotropic glutamate receptor activation reduces neuronal excitability in rat midline paraventricular thalamic nucleus

Drugs that interact with group II metabotropic glutamate receptors (mGluRs) are presently being evaluated for a role in the treatment of anxiety disorders and symptoms of schizophrenia. Their mechanism of action is believed to involve a reduction in excitatory neurotransmission in limbic and forebrain regions commonly associated with these mental disorders. In rodents, the glutamatergic neurons in the midline paraventricular thalamic nucleus (PVT) provide excitatory inputs to the limbic system and forebrain. PVT also displays a high density of group II mGluRs, predominantly the metabotropic glutamate 2 receptor (mGluR2). Because the role of group II mGluRs in regulating cellular and synaptic excitability in this location has yet to be determined, we used whole-cell patch-clamp recording and acute rat brain slice preparations to evaluate PVT neuron responses to a selective group II mGluR agonist, (1R,4R,5S,6R)-4-amino-2-oxabicyclo[3.1.0]hexane-4,6-dicarboxylic acid (LY 379268). LY 379268 consistently induced membrane hyperpolarization and suppressed firing by postsynaptic receptor-mediated activation of a barium-sensitive background K(+) conductance. This effect could be blocked by (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl)propanoic acid (LY 341495), a selective group II mGluR antagonist. In addition, LY 379268 acted at presynaptic receptors to reduce ionotropic glutamate receptor-mediated excitatory synaptic transmission. An mGluR2-positive allosteric modulator, 2,2,2-trifluoro-N-[4-(2-methoxyphenoxy)phenyl]-N-(3-pyridinylmethyl)ethanesulfonamide hydrochloride (LY 487379), resulted in leftward shifts of the LY 379268 dose-response curve for both postsynaptic and presynaptic actions. The data demonstrate that activation of postsynaptic and presynaptic group II (presumably mGluR2) mGluRs reduces neuronal excitability in midline thalamus, an action that may contribute to the effectiveness of mGluR2-activating drugs in rodent models of anxiety and psychosis.

Transcranial magnetic stimulation in Alzheimer's disease: a neurophysiological marker of cortical hyperexcitability

Recently, neuropathological studies have shown an important motor cortex involvement in Alzheimer's disease (AD), even in its early stages, despite the lack of clinically evident motor deficit. Transcranial magnetic stimulation (TMS) studies have demonstrated that cortical excitability is enhanced in AD patients. This cortical hyperexcitability is believed to be a compensatory mechanism to execute voluntary movements, despite the progressive impairment of associative cortical areas. At present, it is not clear if these motor cortex excitability changes might be the expression of an involvement of intracortical excitatory glutamatergic circuits or an impairment of inhibitory cholinergic and, to a lesser extent, gabaergic activity. Although the main hypothesis for the pathogenesis of AD remains the degeneration of the basal forebrain cholinergic neurons, the development of specific TMS protocols, such as the paired-pulse TMS and the study of the short-latency afferent inhibition, points out the role of other neurotransmitters, such as gamma-amino-butyric acid, glutamate and dopamine. The potential therapeutic effect of repetitive TMS in restoring or compensating damaged cognitive functions, might become a possible rehabilitation tool in AD patients. Based on different patterns of cortical excitability, TMS may be useful in discriminating between physiological brain aging, mild cognitive impairment, AD and other dementing disorders. The present review provides a perspective of these TMS techniques by further understanding the role of different neurotransmission pathways and plastic remodelling of neuronal networks in the pathogenesis of AD.

Interactive effects of mGlu5 and 5-HT2A receptors on locomotor activity in mice

RATIONALE

Metabotropic glutamate (mGlu) receptors have been suggested to play a role in neuropsychiatric disorders including schizophrenia, drug abuse, and depression. Because serotonergic hallucinogens increase glutamate release and mGlu receptors modulate the response to serotonin (5-HT)(2A) activation, the interactions between serotonin 5-HT(2A) receptors and mGlu receptors may prove to be important for our understanding of these diseases.

OBJECTIVE

We tested the effects of the serotonergic hallucinogen and 5-HT(2A) agonist, 2,5-dimethoxy-4-methylamphetamine (DOM), and the selective 5-HT(2A) antagonist, M100907, on locomotor activity in the mouse behavioral pattern monitor (BPM) in mGlu5 wild-type (WT) and knockout (KO) mice on a C57 background.

RESULTS

Both male and female mGlu5 KO mice showed locomotor hyperactivity and diminished locomotor habituation compared with their WT counterparts. Similarly, the mGlu5-negative allosteric modulator 2-methyl-6-(phenylethynyl)pyridine (MPEP) also increased locomotor hyperactivity, which was absent in mGlu5 KO mice. The locomotor hyperactivity in mGlu5 receptor KO mice was potentiated by DOM (0.5 mg/kg, subcutaneously (SC)) and attenuated by M100907 (1.0 mg/kg, SC). M100907 (0.1 mg/kg, SC) also blocked the hyperactivity induced by MPEP.

CONCLUSIONS

These studies demonstrated that loss of mGlu5 receptor activity either pharmacologically or through gene deletion leads to locomotor hyperactivity in mice. Additionally, the gene deletion of mGlu5 receptors increased the behavioral response to the 5-HT(2A) agonist DOM, suggesting that mGlu5 receptors either mitigate the behavioral effects of 5-HT(2A) hallucinogens or that mGlu5 KO mice show an increased sensitivity to 5-HT(2A) agonists. Taken together, these studies indicate a functional interaction between mGlu5 and 5-HT(2A) receptors.

Induction and expression rules of synaptic plasticity in hippocampal interneurons

The knowledge that excitatory synapses on aspiny hippocampal interneurons can develop genuine forms of activity-dependent remodeling, independently from the surrounding network of principal cells, is a relatively new concept. Cumulative evidence has now unequivocally demonstrated that, despite the absence of specialized postsynaptic spines that serve as compartmentalized structure for intracellular signaling in principal cell plasticity, excitatory inputs onto interneurons can undergo forms of synaptic plasticity that are induced and expressed autonomously from principal cells. Yet, the rules for induction and expression of interneuron plasticity are much more heterogeneous than in principal neurons. Long-term plasticity in interneurons is not necessarily dependent upon postsynaptic activation of NMDA receptors nor relies on the same postsynaptic membrane potential requirements as principal cells. Plasticity in interneurons rather requires activation of Ca(2+)-permeable AMPA receptors and/or metabotropic glutamate receptors and is triggered by postsynaptic hyperpolarization. In this review we will outline these distinct features of interneuron plasticity and identify potential critical candidate molecules that might be important for sustaining long-lasting changes in synaptic strength at excitatory inputs onto interneurons. This article is part of a Special Issue entitled 'Synaptic Plasticity & Interneurons'.

Short- and long-term depression at glutamatergic synapses on hippocampal interneurons by group I mGluR activation

Group I metabotropic glutamate receptors (mGluRs) are expressed by many interneurons of the hippocampus. Although they have been implicated in short- and long-term synaptic plasticity of glutamatergic transmission, their roles in modulating transmission to interneurons are incompletely understood. The selective group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) acutely depressed transmission at synapses in the feed-forward inhibitory pathway made by Schaffer collaterals on interneurons in the rat hippocampal CA1 sub-field. DHPG elicited a qualitatively similar depression at synapses made by pyramidal neuron axon collaterals on interneurons in the feedback circuit in stratum oriens. Selective blockers revealed a link from mGluR1 to reversible, and mGluR5 to long-lasting, depression. The acute DHPG-induced depression was consistently accompanied by an elevation in paired-pulse ratio, implying a presynaptic decrease in release probability. However, it was also attenuated by blocking G-protein and Ca(2+) signalling within the postsynaptic neuron, arguing for a retrograde signalling cascade. The DHPG-evoked depression was unaffected by antagonists of CB1 and GABA(B) receptors but was occluded when presynaptic P/Q-type Ca(2+) channels were blocked. Finally, high-frequency stimulation delivered to an independent conditioning pathway evoked a heterosynaptic reversible depression, which was sensitive to group I mGluR antagonists. Group I mGluRs thus powerfully modulate synaptic excitation of hippocampal interneurons and mediate inter-synaptic cross-talk. This article is part of a Special Issue entitled 'Synaptic Plasticity & Interneurons'.

The BDNF Val66Met polymorphism impairs NMDA receptor-dependent synaptic plasticity in the hippocampus

The Val66Met polymorphism in the brain-derived neurotrophic factor (BDNF) gene results in a defect in regulated release of BDNF and affects episodic memory and affective behaviors. However, the precise role of the BDNF Val66Met polymorphism in hippocampal synaptic transmission and plasticity has not yet been studied. Therefore, we examined synaptic properties in the hippocampal CA3-CA1 synapses of BDNF(Met/Met) mice and matched wild-type mice. Although basal glutamatergic neurotransmission was normal, both young and adult mice showed a significant reduction in NMDA receptor-dependent long-term potentiation. We also found that NMDA receptor-dependent long-term depression was decreased in BDNF(Met/Met) mice. However, mGluR-dependent long-term depression was not affected by the BDNF Val66Met polymorphism. Consistent with the NMDA receptor-dependent synaptic plasticity impairment, we observed a significant decrease in NMDA receptor neurotransmission in the CA1 pyramidal neurons of BDNF(Met/Met) mice. Thus, these results show that the BDNF Val66Met polymorphism has a direct effect on NMDA receptor transmission, which may account for changes in synaptic plasticity in the hippocampus.

BRAGging about mechanisms of long-term depression

The mechanisms of long-term depression (LTD) underlie various aspects of normal brain function. Therefore, it is important to understand the signaling that underpins LTD. The study by Scholz et al. in this issue of Neuron describes how BRAG2, mGluRs, and AMPARs come together to produce LTD through AMPAR internalization.

Cognitive effects of Group I metabotropic glutamate receptor ligands in the context of drug addiction

Glutamate plays a pivotal role in regulating drug self-administration and drug-seeking behavior, and the past decade has witnessed a substantial surge of interest in the role of Group I metabotropic glutamate receptors (mGlu(1) and mGlu(5) receptors) in mediating these behaviors. As will be reviewed here, Group I mGlu receptors are involved in normal and drug-induced synaptic plasticity, drug reward, reinforcement and relapse-like behaviors, and addiction-related cognitive processes such as maladaptive learning and memory, behavioral inflexibility, and extinction learning. Animal models of addiction have revealed that antagonists of Group I mGlu receptors, particularly the mGlu(5) receptor, reduce self-administration of virtually all drugs of abuse. Since inhibitors of mGlu5 receptor function have now entered clinical trials for other medical conditions and appear to be well-tolerated, a key question that remains unanswered is - what changes in cognition are produced by these compounds that result in reduced drug intake and drug-seeking behavior? Finally, in contrast to mGlu(5) receptor antagonists, recent studies have indicated that positive allosteric modulation of mGlu(5) receptors actually enhances synaptic plasticity and improves various aspects of cognition, including spatial learning, behavioral flexibility, and extinction of drug-seeking behavior. Thus, while inhibition of Group I mGlu receptor function may reduce drug reward, reinforcement, and relapse-related behaviors, positive allosteric modulation of the mGlu5 receptor subtype may actually enhance cognition and potentially reverse some of the cognitive deficits associated with chronic drug use.

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.