Long-term depression induced by postsynaptic group II metabotropic glutamate receptors linked to phospholipase C and intracellular calcium rises in rat prefrontal cortex

Sexually Dimorphic Adolescent Trajectories of Prefrontal Endocannabinoid Synaptic Plasticity Equalize in Adulthood, Reflected by Endocannabinoid System Gene Expression

Introduction: How sex influences prefrontal cortexes (PFCs) synaptic development through adolescence remains unclear. Materials and Methods: In this study we describe sex-specific cellular and synaptic trajectories in the rat PFC from adolescence to adulthood. Results: The excitability of PFC layer 5 pyramidal neurons was lower in adult females compared with other developmental stages. The developmental course of endocannabinoid-mediated long-term depression (eCB-LTD) was sexually dimorphic, unlike long-term potentiation or mGluR3-LTD. eCB-LTD was expressed in juvenile females but appeared only at puberty in males. Endovanilloid TRPV1R or eCB receptors were engaged during LTD in a sequential and sexually dimorphic manner. Gene expression of the eCB/vanilloid systems was sequential and sex specific. LTD-incompetent juvenile males had elevated expression levels of the CB1R-interacting inhibitory protein cannabinoid receptor interacting protein 1a and of the 2-arachidonoylglycerol-degrading enzyme ABHD6. Pharmacological inhibition of ABHD6 or MAGL enabled LTD in young males, whereas inhibition of anandamide degradation was ineffective. Conclusions: These results reveal sex differences in the maturational trajectories of the rat PFC.

GPCR interactions involving metabotropic glutamate receptors and their relevance to the pathophysiology and treatment of CNS disorders

Cellular responses to metabotropic glutamate (mGlu) receptor activation are shaped by mechanisms of receptor-receptor interaction. mGlu receptor subtypes form homodimers, intra- or inter-group heterodimers, and heteromeric complexes with other G protein-coupled receptors (GPCRs). In addition, mGlu receptors may functionally interact with other receptors through the βγ subunits released from G proteins in response to receptor activation or other mechanisms. Here, we discuss the interactions between (i) mGlu₁ and GABA_B receptors in cerebellar Purkinje cells; (ii) mGlu₂ and 5-HT_2Aserotonergic receptors in the prefrontal cortex; (iii) mGlu₅ and A_2A receptors or mGlu₅ and D₁ dopamine receptors in medium spiny projection neurons of the indirect and direct pathways of the basal ganglia motor circuit; (iv) mGlu₅ and A_2A receptors in relation to the pathophysiology of Alzheimer's disease; and (v) mGlu₇ and A₁ adenosine or α- or β₁ adrenergic receptors. In addition, we describe in detail a novel form of non-heterodimeric interaction between mGlu₃ and mGlu₅ receptors, which appears to be critically involved in mechanisms of activity-dependent synaptic plasticity in the prefrontal cortex and hippocampus. Finally, we highlight the potential implication of these interactions in the pathophysiology and treatment of cerebellar disorders, schizophrenia, Alzheimer's disease, Parkinson's disease, l-DOPA-induced dyskinesias, stress-related disorders, and cognitive dysfunctions.

Long-term memory, synaptic plasticity and dopamine in rodent medial prefrontal cortex: Role in executive functions

Mnemonic functions, supporting rodent behavior in complex tasks, include both long-term and (short-term) working memory components. While working memory is thought to rely on persistent activity states in an active neural network, long-term memory and synaptic plasticity contribute to the formation of the underlying synaptic structure, determining the range of possible states. Whereas, the implication of working memory in executive functions, mediated by the prefrontal cortex (PFC) in primates and rodents, has been extensively studied, the contribution of long-term memory component to these tasks received little attention. This review summarizes available experimental data and theoretical work concerning cellular mechanisms of synaptic plasticity in the medial region of rodent PFC and the link between plasticity, memory and behavior in PFC-dependent tasks. A special attention is devoted to unique properties of dopaminergic modulation of prefrontal synaptic plasticity and its contribution to executive functions.

Frontal cortex genetic ablation of metabotropic glutamate receptor subtype 3 (mGlu3) impairs postsynaptic plasticity and modulates affective behaviors

Clinical and translational studies suggest that prefrontal cortex (PFC) dysregulation is a hallmark feature of several affective disorders. Thus, investigating the mechanisms involved in the regulation of PFC function and synaptic plasticity could aid in developing new medications. In recent years, the mGlu2 and mGlu3 subtypes of metabotropic glutamate (mGlu) receptors have emerged as exciting potential targets for the treatment of affective disorders, as mGlu2/3 antagonists exert antidepressant-like effects across many rodent models. Several recent studies suggest that presynaptic mGlu2 receptors may contribute to these effects by regulating excitatory transmission at synapses from the thalamus to the PFC. Interestingly, we found that mGlu3 receptors also inhibit excitatory drive to the PFC but act by inducing long-term depression (LTD) at amygdala-PFC synapses. It remains unclear, however, whether blockade of presynaptic, postsynaptic, or glial mGlu3 receptors contribute to long-term effects on PFC circuit function and antidepressant-like effects of mGlu2/3 antagonists. To address these outstanding questions, we leveraged transgenic Grm3fl/fl mice and viral-mediated gene transfer to genetically ablate mGlu3 receptors from pyramidal cells in the frontal cortex of adult mice of all sexes. Consistent with a role for mGlu3 in PFC pyramidal cells, mGlu3-dependent amygdala-cortical LTD was eliminated following mGlu3 receptor knockdown. Furthermore, knockdown mice displayed a modest, task-specific anxiolytic phenotype and decreased passive coping behaviors. These studies reveal that postsynaptic mGlu3 receptors are critical for mGlu3-dependent LTD and provide convergent genetic evidence suggesting that modulating cortical mGlu3 receptors may provide a promising new approach for the treatment of mood disorders.

International Union of Basic and Clinical Pharmacology. CXI. Pharmacology, Signaling, and Physiology of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors respond to glutamate, the major excitatory neurotransmitter in the mammalian brain, mediating a modulatory role that is critical for higher-order brain functions such as learning and memory. Since the first mGlu receptor was cloned in 1992, eight subtypes have been identified along with many isoforms and splice variants. The mGlu receptors are transmembrane-spanning proteins belonging to the class C G protein-coupled receptor family and represent attractive targets for a multitude of central nervous system disorders. Concerted drug discovery efforts over the past three decades have yielded a wealth of pharmacological tools including subtype-selective agents that competitively block or mimic the actions of glutamate or act allosterically via distinct sites to enhance or inhibit receptor activity. Herein, we review the physiologic and pathophysiological roles for individual mGlu receptor subtypes including the pleiotropic nature of intracellular signal transduction arising from each. We provide a comprehensive analysis of the in vitro and in vivo pharmacological properties of prototypical and commercially available orthosteric agonists and antagonists as well as allosteric modulators, including ligands that have entered clinical trials. Finally, we highlight emerging areas of research that hold promise to facilitate rational design of highly selective mGlu receptor-targeting therapeutics in the future. SIGNIFICANCE STATEMENT: The metabotropic glutamate receptors are attractive therapeutic targets for a range of psychiatric and neurological disorders. Over the past three decades, intense discovery efforts have yielded diverse pharmacological tools acting either competitively or allosterically, which have enabled dissection of fundamental biological process modulated by metabotropic glutamate receptors and established proof of concept for many therapeutic indications. We review metabotropic glutamate receptor molecular pharmacology and highlight emerging areas that are offering new avenues to selectively modulate neurotransmission.

Perinatal THC exposure via lactation induces lasting alterations to social behavior and prefrontal cortex function in rats at adulthood

Cannabis is the world's most widely abused illicit drug and consumption amongst women during and surrounding the period of pregnancy is increasing. Previously, we have shown that cannabinoid exposure via lactation during the early postnatal period disrupts early developmental trajectories of prefrontal cortex maturation and induces behavioral abnormalities during the first weeks of life in male and female rat progeny. Here, we investigated the lasting consequences of this postnatal cannabinoid exposure on synaptic and behavioral parameters in the adult offspring of ∆9-tetrahydrocannabinol (THC)-treated dams. At adulthood, these perinatally THC-exposed rats exhibits deficits in social discrimination accompanied by an overall augmentation of social exploratory behavior. These behavioral alterations were further correlated with multiple abnormalities in synaptic plasticity in the prefrontal cortex, including lost endocannabinoid-mediated long-term depression (LTD), lost long-term potentiation and augmented mGlu2/3-LTD. Finally, basic parameters of intrinsic excitability at prefrontal cortex pyramidal neurons were similarly altered by the perinatal THC exposure. Thus, perinatal THC exposure via lactation induces lasting deficits in behavior and synaptic function which persist into adulthood life in male and female progeny.

Metabotropic glutamate receptor subtype 3 gates acute stress-induced dysregulation of amygdalo-cortical function

Stress can precipitate or worsen symptoms of many psychiatric disorders by dysregulating glutamatergic function within the prefrontal cortex (PFC). Previous studies suggest that antagonists of group II metabotropic glutamate (mGlu) receptors (mGlu₂ and mGlu₃) reduce stress-induced anhedonia through actions in the PFC, but the mechanisms by which these receptors act are not known. We now report that activation of mGlu₃ induces long-term depression (LTD) of excitatory transmission in the PFC at inputs from the basolateral amygdala. Our data suggest mGlu₃-LTD is mediated by postsynaptic AMPAR internalization in PFC pyramidal cells, and we observed a profound impairment in mGlu₃-LTD following a single, 20-min restraint stress exposure. Finally, blocking mGlu₃ activation in vivo prevented the stress-induced maladaptive changes to amydalo-cortical physiology and motivated behavior. These data demonstrate that mGlu₃ mediates stress-induced physiological and behavioral impairments and further support the potential for mGlu₃ modulation as a treatment for stress-related psychiatric disorders.

Synaptic activity induces input-specific rearrangements in a targeted synaptic protein interaction network

Cells utilize dynamic, network-level rearrangements in highly interconnected protein interaction networks to transmit and integrate information from distinct signaling inputs. Despite the importance of protein interaction network dynamics, the organizational logic underlying information flow through these networks is not well understood. Previously, we developed the quantitative multiplex co-immunoprecipitation platform, which allows for the simultaneous and quantitative measurement of the amount of co-association between large numbers of proteins in shared complexes. Here, we adapt quantitative multiplex co-immunoprecipitation to define the activity-dependent dynamics of an 18-member protein interaction network in order to better understand the underlying principles governing glutamatergic signal transduction. We first establish that immunoprecipitation detected by flow cytometry can detect activity-dependent changes in two known protein-protein interactions (Homer1-mGluR5 and PSD-95-SynGAP). We next demonstrate that neuronal stimulation elicits a coordinated change in our targeted protein interaction network, characterized by the initial dissociation of Homer1 and SynGAP-containing complexes followed by increased associations among glutamate receptors and PSD-95. Finally, we show that stimulation of distinct glutamate receptor types results in different modular sets of protein interaction network rearrangements, and that cells activate both modules in order to integrate complex inputs. This analysis demonstrates that cells respond to distinct types of glutamatergic input by modulating different combinations of protein co-associations among a targeted network of proteins. Our data support a model of synaptic plasticity in which synaptic stimulation elicits dissociation of pre-existing multiprotein complexes, opening binding slots in scaffold proteins and allowing for the recruitment of additional glutamatergic receptors. Open Science: This manuscript was awarded with the Open Materials Badge. For more information see: https://cos.io/our-services/open-science-badges/.

Functional partnership between mGlu3 and mGlu5 metabotropic glutamate receptors in the central nervous system

mGlu5 receptors are involved in mechanisms of activity-dependent synaptic plasticity, and are targeted by drugs developed for the treatment of CNS disorders. We report that mGlu3 receptors, which are traditionally linked to the control of neurotransmitter release, support mGlu5 receptor signaling in neurons and largely contribute to the robust mGlu5 receptor-mediated polyphosphoinositide hydrolysis in the early postnatal life. In cortical pyramidal neurons, mGlu3 receptor activation potentiated mGlu5 receptor-mediated somatic Ca²⁺ mobilization, and mGlu3 receptor-mediated long-term depression in the prefrontal cortex required the endogenous activation of mGlu5 receptors. The interaction between mGlu3 and mGlu5 receptors was also relevant to mechanisms of neuronal toxicity, with mGlu3 receptors shaping the influence of mGlu5 receptors on excitotoxic neuronal death. These findings shed new light into the complex role played by mGlu receptors in physiology and pathology, and suggest reconsideration of some of the current dogmas in the mGlu receptor field.

Presynaptic G Protein-Coupled Receptors: Gatekeepers of Addiction?

Drug abuse and addiction cause widespread social and public health problems, and the neurobiology underlying drug actions and drug use and abuse is an area of intensive research. Drugs of abuse alter synaptic transmission, and these actions contribute to acute intoxication as well as the chronic effects of abused substances. Transmission at most mammalian synapses involves neurotransmitter activation of two receptor subtypes, ligand-gated ion channels that mediate fast synaptic responses and G protein-coupled receptors (GPCRs) that have slower neuromodulatory actions. The GPCRs represent a large proportion of neurotransmitter receptors involved in almost all facets of nervous system function. In addition, these receptors are targets for many pharmacotherapeutic agents. Drugs of abuse directly or indirectly affect neuromodulation mediated by GPCRs, with important consequences for intoxication, drug taking and responses to prolonged drug exposure, withdrawal and addiction. Among the GPCRs are several subtypes involved in presynaptic inhibition, most of which are coupled to the Gi/o class of G protein. There is increasing evidence that these presynaptic Gi/o-coupled GPCRs have important roles in the actions of drugs of abuse, as well as behaviors related to these drugs. This topic will be reviewed, with particular emphasis on receptors for three neurotransmitters, Dopamine (DA; D1- and D2-like receptors), Endocannabinoids (eCBs; CB1 receptors) and glutamate (group II metabotropic glutamate (mGlu) receptors). The focus is on recent evidence from laboratory animal models (and some evidence in humans) implicating these receptors in the acute and chronic effects of numerous abused drugs, as well as in the control of drug seeking and taking. The ability of drugs targeting these receptors to modify drug seeking behavior has raised the possibility of using compounds targeting these receptors for addiction pharmacotherapy. This topic is also discussed, with emphasis on development of mGlu2 positive allosteric modulators (PAMs).

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.

An animal model of female adolescent cannabinoid exposure elicits a long-lasting deficit in presynaptic long-term plasticity

Cannabis continues to be the most accessible and popular illicit recreational drug. Whereas current data link adolescence cannabinoid exposure to increased risk for dependence on other drugs, depression, anxiety disorders and psychosis, the mechanism(s) underlying these adverse effects remains controversial. Here we show in a mouse model of female adolescent cannabinoid exposure deficient endocannabinoid (eCB)-mediated signaling and presynaptic forms of long-term depression at adult central glutamatergic synapses in the prefrontal cortex. Increasing endocannabinoid levels by blockade of monoacylglycerol lipase, the primary enzyme responsible for degrading the endocannabinoid 2-arachidonoylglycerol (2-AG), with the specific inhibitor JZL 184 ameliorates eCB-LTD deficits. The observed deficit in cortical presynaptic signaling may represent a neural maladaptation underlying network instability and abnormal cognitive functioning. Our study suggests that adolescent cannabinoid exposure may permanently impair brain functions, including the brain's intrinsic ability to appropriately adapt to external influences.

Group II mGluRs modulate baseline and arthritis pain-related synaptic transmission in the rat medial prefrontal cortex

The medial prefrontal cortex (mPFC) serves executive control functions that are impaired in neuropsychiatric disorders and pain. Therefore, restoring normal synaptic transmission and output is a desirable goal. Group II metabotropic glutamate receptors mGluR2 and mGluR3 are highly expressed in the mPFC, modulate synaptic transmission, and have been targeted for neuropsychiatric disorders. Their pain-related modulatory effects in the mPFC remain to be determined. Here we evaluated their ability to restore pyramidal output in an arthritis pain model. Whole-cell patch-clamp recordings of layer V mPFC pyramidal cells show that a selective group II mGluR agonist (LY379268) decreased synaptically evoked spiking in brain slices from normal and arthritic rats. Effects were concentration-dependent and reversed by a selective antagonist (LY341495). LY379268 decreased monosynaptic excitatory postsynaptic currents (EPSCs) and glutamate-driven inhibitory postsynaptic currents (IPSCs) in the pain model. Effects on EPSCs preceded those on IPSCs and could explain the overall inhibitory effect on pyramidal output. LY379268 decreased frequency, but not amplitude, of miniature EPSCs without affecting miniature IPSCs. LY341495 alone increased synaptically evoked spiking under normal conditions and in the pain model. In conclusion, group II mGluRs act on glutamatergic synapses to inhibit direct excitatory transmission and feedforward inhibition onto pyramidal cells. Their net effect is decreased pyramidal cell output. Facilitatory effects of a group II antagonist suggest the system may be tonically active to control pyramidal output. Failure to release the inhibitory tone and enhance mPFC output could be a mechanism for the development or persistence of a disease state such as pain.

Metabotropic glutamate receptor 3 activation is required for long-term depression in medial prefrontal cortex and fear extinction

Clinical studies have revealed that genetic variations in metabotropic glutamate receptor 3 (mGlu3) affect performance on cognitive tasks dependent upon the prefrontal cortex (PFC) and may be linked to psychiatric conditions such as schizophrenia, bipolar disorder, and addiction. We have performed a series of studies aimed at understanding how mGlu3 influences PFC function and cognitive behaviors. In the present study, we found that activation of mGlu3 can induce long-term depression in the mouse medial PFC (mPFC) in vitro. Furthermore, in vivo administration of a selective mGlu3 negative allosteric modulator impaired learning in the mPFC-dependent fear extinction task. The results of these studies implicate mGlu3 as a major regulator of PFC function and cognition. Additionally, potentiators of mGlu3 may be useful in alleviating prefrontal impairments associated with several CNS disorders.

The function of metabotropic glutamate receptors in thalamus and cortex

Metabotropic glutamate receptors (mGluRs) are found throughout thalamus and cortex and are clearly important to circuit behavior in both structures, and so considering only participation of ionotropic glutamate receptors (e.g., [R,S]-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA] and N-methyl-d-aspartate receptors [NMDA] receptors) in glutamatergic processing would be an unfortunate oversimplification. These mGluRs are found both postsynaptically, on target cells of glutamatergic afferents, and presynaptically, on various synaptic terminals themselves, and when activated, they produce prolonged effects lasting at least hundreds of msec to several sec and perhaps longer. Two main types exist: activation of group I mGluRs causes postsynaptic depolarization, and group II, hyperpolarization. Both types are implicated in synaptic plasticity, both short term and long term. Their evident importance in functioning of thalamus and cortex makes it critical to develop a better understanding of how these receptors are normally activated, especially because they also seem implicated in a wide range of neurological and cognitive pathologies.

Long-term depression of synaptic transmission in the adult mouse insular cortex in vitro

The insular cortex (IC) is known to play important roles in higher brain functions such as memory and pain. Activity-dependent long-term depression (LTD) is a major form of synaptic plasticity related to memory and chronic pain. Previous studies of LTD have mainly focused on the hippocampus, and no study in the IC has been reported. In this study, using a 64-channel recording system, we show for the first time that repetitive low-frequency stimulation (LFS) can elicit frequency-dependent LTD of glutamate receptor-mediated excitatory synaptic transmission in both superficial and deep layers of the IC of adult mice. The induction of LTD in the IC required activation of the N-methyl-d-aspartate (NMDA) receptor, metabotropic glutamate receptor (mGluR)5, and L-type voltage-gated calcium channel. Protein phosphatase 1/2A and endocannabinoid signaling are also critical for the induction of LTD. In contrast, inhibiting protein kinase C, protein kinase A, protein kinase Mζ or calcium/calmodulin-dependent protein kinase II did not affect LFS-evoked LTD in the IC. Bath application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine produced another form of LTD in the IC, which was NMDA receptor-independent and could not be occluded by LFS-induced LTD. Our studies have characterised the basic mechanisms of LTD in the IC at the network level, and suggest that two different forms of LTD may co-exist in the same population of IC synapses.

Improved spatial learning is associated with increased hippocampal but not prefrontal long-term potentiation in mGluR4 knockout mice

Although much information about metabotropic glutamate receptors (mGluRs) and their role in normal and pathologic brain function has been accumulated during the last decades, the role of group III mGluRs is still scarcely documented. Here, we examined mGluR4 knockout mice for types of behavior and synaptic plasticity that depend on either the hippocampus or the prefrontal cortex (PFC). We found improved spatial short- and long-term memory in the radial arm maze, which was accompanied by enhanced long-term potentiation (LTP) in hippocampal CA1 region. In contrast, LTP in the PFC was unchanged when compared with wild-type controls. Changes in paired-pulse facilitation that became overt in the presence of the GABAA antagonist picrotoxin indicated a function of mGluR4 in maintaining the excitation/inhibition balance, which is of crucial importance for information processing in the brain and the deterioration of these processes in neuropsychological disorders such as autism, epilepsy and schizophrenia.

SNARE proteins are essential in the potentiation of NMDA receptors by group II metabotropic glutamate receptors

The group II metabotropic glutamate receptors (group II mGluRs) have emerged as the new drug targets for the treatment of mental disorders like schizophrenia. To understand the potential mechanisms underlying the antipsychotic effects of group II mGluRs, we examined their impact on NMDA receptors (NMDARs), since NMDAR hypofunction has been implicated in schizophrenia. The activation of group II mGluRs caused a significant enhancement of NMDAR currents in cortical pyramidal neurons, which was associated with increased NMDAR surface expression and synaptic localization. We further examined whether these effects of group II mGluRs are through the regulation of NMDAR exocytosis via SNARE proteins, a family of proteins involved in vesicle fusion. We found that the enhancing effect of APDC, a selective agonist of group II mGluRs, on NMDAR currents was abolished when botulinum toxin was delivered into the recorded neurons to disrupt the SNARE complex. Inhibiting the function of two key SNARE proteins, SNAP-25 and syntaxin 4, also eliminated the effect of APDC on NMDAR currents. Moreover, the application of APDC increased the activity of Rab4, a small Rab GTPase mediating fast recycling from early endosomes to the plasma membrane, and enhanced the interaction between syntaxin 4 and Rab4. Knockdown of Rab4 or expression of dominant-negative Rab4 attenuated the effect of APDC on NMDAR currents. Taken together, these results have identified key molecules involved in the group II mGluR-induced potentiation of NMDAR exocytosis and function.

Group II metabotropic glutamate receptor agonist LY379268 regulates AMPA receptor trafficking in prefrontal cortical neurons

Group II metabotropic glutamate receptor (mGluR) agonists have emerged as potential treatment drugs for schizophrenia and other neurological disorders, whereas the mechanisms involved remain elusive. Here we examined the effects of LY379268 (LY37) on the expression and trafficking of the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor subunits GluA1 and GluA2 in prefrontal neurons. We show that LY37 significantly increased the surface and total expression of both GluA1 and GluA2 subunits in cultured prefrontal neurons and in vivo. This effect was mimicked by the selective mGluR2 agonist LY395756 and was blocked by mGluR2/3 antagonist LY341495. Moreover, we found that both GluA1 and GluA2 subunits were colocalized with PSD95 but not synapsin I, suggesting a postsynaptic localization. Consistently, treatment with LY37 significantly increased the amplitude, but not frequency, of miniature excitatory postsynaptic currents. Further, actinomycin-D blocked LY37's effects, suggesting a transcriptional regulation. In addition, application of glycogen synthase kinase-3beta (GSK-3β) inhibitor completely blocked LY37's effect on GluA2 surface expression, whereas GSK-3β inhibitor itself induced decreases in the surface and total protein levels of GluA1, but not GluA2 subunits. This suggests that GSK-3β differentially mediates GluA1 and GluA2 trafficking. Further, LY37 significantly increased the phosphorylation, but not total protein, of extracellular signal-regulated kinase 1/2 (ERK1/2). Neither ERK1/2 inhibitor PD98059 alone nor PD98059 combined with LY37 treatment induced changes in GluA1 or GluA2 surface expression or total protein levels. Our data thus suggest that mGluR2/3 agonist regulates postsynaptic AMPA receptors by affecting the synaptic trafficking of both GluA1 and GluA2 subunits and that the regulation is likely through ERK1/2 signaling in GluA1 and/or both ERK1/2 and GSK-3β signaling pathways in the GluA2 subunit.

Antagonists reversibly reverse chemical LTD induced by group I, group II and group III metabotropic glutamate receptors

Metabotropic glutamate (mGlu) receptors are implicated in many neurological and psychiatric diseases and are the targets of therapeutic agents currently in clinical development. Their activation has diverse effects in the central nervous system (CNS) that includes an involvement in synaptic plasticity. We previously reported that the brief exposure of hippocampal slices to dihydroxyphenylglycine (DHPG) can result in a long-term depression (LTD) of excitatory synaptic transmission. Surprisingly, this LTD could be fully reversed by mGlu receptor antagonists in a manner that was itself fully reversible upon washout of the antagonist. Here, 15 years after the discovery of DHPG-LTD and its reversible reversibility, we summarise these initial findings. We then present new data on DHPG-LTD, which demonstrates that evoked epileptiform activity triggered by activation of group I mGlu receptors can also be reversibly reversed by mGlu receptor antagonists. Furthermore, we show that the phenomenon of reversible reversibility is not specific to group I mGlu receptors. We report that activation of group II mGlu receptors in the temporo-ammonic pathway (TAP) and mossy fibre pathway within the hippocampus and in the cortical input to neurons of the lateral amygdala induces an LTD that is reversed by LY341495, a group II mGlu receptor antagonist. We also show that activation of group III mGlu8 receptors induces an LTD at lateral perforant path inputs to the dentate gyrus and that this LTD is reversed by MDCPG, an mGlu8 receptor antagonist. In conclusion, we have shown that activation of representative members of each of the three groups of mGlu receptors can induce forms of LTD than can be reversed by antagonists, and that in each case washout of the antagonist is associated with the re-establishment of the LTD. This article is part of the Special Issue entitled 'Glutamate Receptor-Dependent Synaptic Plasticity'.

Interactions between the cannabinoid and dopaminergic systems: evidence from animal studies

There is a prominent role of the cannabinoid system to control basal ganglia function, in respect to reward, psychomotor function and motor control. Cannabinoid dysregulations might have a pathogenetic role in dopamine- and basal ganglia related neuropsychiatric disorders, such as drug addiction, psychosis, Parkinson's disease and Huntington's disease. This review highlights interactions between cannabinoids, and dopamine, to modulate neurotransmitter release and synaptic plasticity in the context of drug addiction, psychosis and cognition. Modulating endocannabinoid function, as a plasticity based therapeutic strategy, in the above pathologies with particular focus on cannabinoid receptor type 1 (CB1 receptor) antagonists/inverse agonists, is discussed. On the basis of the existing literature and of new experimental evidence presented here, CB1 receptor antagonists might be beneficial in disease states associated with hedonic dysregulation, and with cognitive dysfunction in particular in the context of psychosis. It is suggested that this effects might be mediated via a hyperglutamatergic state through metabotropic glutamate activation. Indications for endocannabinoid catabolism inhibitors in psychiatric disorders, that might be CB1 receptor independent and might involve TRPV1 receptors, are also discussed.

Selective activation of either mGlu2 or mGlu3 receptors can induce LTD in the amygdala

Group II metabotropic glutamate (mGlu) receptors are known to induce a long-term depression (LTD) of synaptic transmission in many brain regions including the amygdala. However the roles of the individual receptor subtypes, mGlu2 and mGlu3, in LTD are not well understood. In particular, it is unclear whether activation of mGlu3 receptors is sufficient to induce LTD at synapses in the CNS. In the present study, advantage was taken of a Wistar rat strain not expressing mGlu2 receptors (Ceolin et al., 2011) to investigate the function of mGlu3 receptors in the amygdala. In this preparation, the group II agonist, DCG-IV induced an LTD of the cortical, but not the intra-nuclear, synaptic input to the lateral amygdala. This LTD was concentration dependent and was blocked by the group II mGlu receptor antagonist, LY341495. To investigate further the role of mGlu3 receptors, we used LY395756 (an mGlu2 agonist and mGlu3 antagonist), which acts as a pure mGlu3 receptor antagonist in this rat strain. This compound alone had no effect on basal synaptic transmission, but blocked the LTD induced by DCG-IV. Furthermore, we found that DCG-IV also induces LTD in mGlu2 receptor knock-out (KO) mice to a similar extent as in wild-type mice. This confirms that the activation of mGlu3 receptors alone is sufficient to induce LTD at this amygdala synapse. To address whether mGlu2 activation alone is also sufficient to induce LTD at this synapse we used LY541850 (the active enantiomer of LY395756) in wild-type mice. LY541850 induced a substantial LTD showing that either receptor alone is capable of inducing LTD in this pathway. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Activation of group II metabotropic glutamate receptors induces long-term depression of excitatory synaptic transmission in the substantia nigra pars reticulata

Activation of group II metabotropic glutamate receptors (mGlu2 and mGlu3) has been implicated as a potential therapeutic strategy for treating both motor symptoms and progressive neurodegeneration in Parkinson's disease (PD). Modulation of excitatory transmission in the basal ganglia represents a possible mechanism by which group II mGlu agonists could exert antiparkinsonian effects. Previous studies have identified reversible effects of mGlu2/3 activation on excitatory transmission at various synapses in the basal ganglia, including the excitatory synapse between the subthalamic nucleus (STN) and the substantia nigra pars reticulata (SNr). Using whole-cell patch clamp studies of GABAergic SNr neurons in rat midbrain slices, we have found that a prolonged activation of group II mGlus by the selective agonist LY379268 induces a long-term depression (LTD) of evoked excitatory postsynaptic current (EPSC) amplitude. Bath application of LY379268 (100nM, 10min) induced a marked reduction in EPSC amplitude, and excitatory transmission remained depressed for at least 40min after agonist washout. The effect of LY379268 was concentration-dependent and was completely blocked by the group II mGlu-preferring antagonist LY341495 (500nM). To determine the relative contributions of mGlu2 and mGlu3 to the LTD induced by LY379268, we tested the ability of LY379268 (100nM) to induce LTD in wild type mice and mice lacking mGlu2 or mGlu3. LY379268 induced similar LTD in wild type mice and mGlu3 knockout mice, whereas LTD was absent in mGlu2 knockout mice, indicating that mGlu2 activation is necessary for the induction of LTD in the SNr. These studies suggest a novel role for mGlu2 in the long-term regulation of excitatory transmission in the SNr and invite further exploration of mGlu2 as a therapeutic target for treating the motor symptoms of PD.

The role of tonic and phasic dopamine for long-term synaptic plasticity in the prefrontal cortex: a computational model

This work presents a computational model of dopamine (DA) influence on long-term potentiation (LTP) and long-term depression (LTD) in the prefrontal cortex. Distinct properties of the model are a DA-concentration-dependent switch from depression to potentiation during induction of plasticity, and an inverted-U-shaped dependence of protein synthesis on the level of background DA. Protein synthesis is responsible for the maintenance of LTP/LTD in the model. Our simulations suggest that in vitro experimental data on prefrontal plasticity, induced by high-frequency stimulation, may be accounted for by a single synaptic mechanism that is slowly (on the timescale of minutes) activated in the presence of DA in a concentration-dependent manner. The activation value determines the direction of plasticity during induction, while it also modulates the magnitude of plasticity during maintenance. More generally, our results support the hypothesis that phasic release of endogenous DA is necessary for the maintenance of long-term changes in synaptic efficacy, while the concentration of tonic DA determines the direction and magnitude of these changes in the PFC.

Long-term depression at parallel fiber to Golgi cell synapses

Golgi cells (GoCs) are the primary inhibitory interneurons of the granular layer of the cerebellum. Their inhibition of granule cells is central to operate the relay of excitatory inputs to the cerebellar cortex. Parallel fibers (PFs) establish synapses to the GoCs in the molecular layer; these synapses contain AMPA, N-methyl-D-aspartate (NMDA), and mostly group II metabotropic glutamate receptors. Long-term changes in the efficacy of synaptic transmission at the PF-GoC synapse have not been described previously. We used whole cell patch-clamp recordings of GoCs in acute rat cerebellar slices to study synaptic plasticity. We report that high-frequency burst stimulation of PFs, using a current-clamp or voltage-clamp induction protocol, gave rise to long-term depression (LTD) at the PF-GoC synapse. This form of LTD was not associated with persistent changes of paired-pulse ratio, suggesting a postsynaptic origin. Furthermore, LTD induction was not dependent on activation of NMDA receptors. PF-GoC LTD does require activation of specifically group II metabotropic glutamate receptors and of protein kinase A.

Metabotropic glutamate2/3 (mGlu2/3) receptors, schizophrenia and cognition

Recently, a metabotropic glutamate(2/3) (mGlu(2/3)) receptor agonist prodrug was found to improve both the positive and negative symptoms of schizophrenic patients. Thus far, however, definitive data directly describing the effects of mGlu(2/3) receptor agonists on cognition in schizophrenic patients is lacking. In this review, we will first describe the location of mGlu(2) and mGlu(3) receptors with respect to cellular compartments in cortical circuits of both the prefrontal cortex and the hippocampal formation. We will then address the function of mGlu(2) and mGlu(3) receptors in both macrocircuits and microcircuits involving the prefrontal cortex and hippocampal formation. Imbalance within and between macrocircuits, including the re-entrant cortico-striatal-thalamic loops; the trisynaptic organization of the hippocampal formation; and the ascending reticular activating system/monoaminergic brainstem nuclei projecting throughout the neural axis, appear central to understanding both the pathophysiology and therapeutic approaches for treating the pervasive cognitive dysfunction associated with schizophrenia. Understanding the function of mGlu(2) and mGlu(3) receptors in these macrocircuits also may provide answers to currently conflicting data between some preclinical studies and the clinical studies seemingly predicting impairment and improvement in cognitive function with activation of mGlu(2) and mGlu(3) receptors.

Functional and dysfunctional synaptic plasticity in prefrontal cortex: roles in psychiatric disorders

Prefrontal cortex (PFC) mediates an assortment of cognitive functions including working memory, behavioral flexibility, attention, and future planning. Unlike the hippocampus, where induction of synaptic plasticity in the network is well-documented in relation to long-term memory, cognitive functions mediated by the PFC have been thought to be independent of long-lasting neuronal adaptation of the network. Nonetheless, accumulating evidence suggests that prefrontal cortical neurons possess the cellular machinery of synaptic plasticity and exhibit lasting changes of neural activity associated with various cognitive processes. Moreover, deficits in the mechanisms of synaptic plasticity induction in the PFC might be involved in the pathophysiology of psychiatric and neurological disorders such as schizophrenia, drug addiction, mood disorders, and Alzheimer's disease.

Sensitivity to MK-801 in phospholipase C-β1 knockout mice reveals a specific NMDA receptor deficit

Phospholipase C-β1 (PLC-β1) is a critical component of multiple signalling pathways downstream of neurotransmitter receptors. Mice lacking this enzyme display a striking behavioural phenotype with relevance to human psychiatric disease. Glutamatergic dysfunction is strongly associated with several abnormal behavioural states and may underlie part of the phenotype of the phospholipase C-β1 knockout (KO) mouse. A heightened response to glutamatergic psychotomimetic drugs is a critical psychosis-related endophenotype, and in this study it was employed as a correlate of glutamatergic dysfunction. Control (n=8) and PLC-β1 KO mice (n=6) were treated with MK-801, a NMDA receptor (NMDAR) antagonist, following either standard housing or environmental enrichment, and the motor function and locomotor activity thus evoked was assessed. In addition, MK-801 binding to the NMDAR was evaluated through radioligand autoradiography in post-mortem tissue (on a drug-naive cohort). We have demonstrated a significantly increased sensitivity to the effects of the NMDA antagonist MK-801 in the PLC-β1 KO mouse. In addition, we found that this mouse line displays reduced hippocampal NMDAR expression, as measured by radioligand binding. We previously documented a reversal of specific phenotypes in this mouse line following housing in an enriched environment. Enrichment did not alter this heightened MK-801 response, nor NMDAR expression, indicating that this therapeutic intervention works on specific pathways only. These findings demonstrate the critical role of the glutamatergic system in the phenotype of the PLC-β1 KO mouse and highlight the role of these interconnected signalling pathways in schizophrenia-like behavioural disruption. These results also shed further light on the capacity of environmental factors to modulate subsets of these phenotypes.

Emerging role of glutamate in the pathophysiology of major depressive disorder

Major depressive disorder (MDD) is a common, chronic, recurrent mental illness that affects millions of individuals worldwide. To date, the monoaminergic systems (serotonin, norepinephrine, and dopamine) have received the most attention in the neurobiology of MDD, and all classes of antidepressants target these monoaminergic systems. Accumulating evidence suggests that the glutamatergic system plays an important role in the neurobiology and treatment of this disease. Some clinical studies have demonstrated that the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist ketamine has rapid antidepressant effects in treatment-resistant patients with MDD. Here, the author reviews the recent findings on the role of the glutamatergic system in the neurobiology of MDD and in new potential therapeutic targets (NMDA receptors, AMPA receptors, metabotropic glutamate receptors, ceftriaxone, minocycline, N-acetyl-L-cysteine) for MDD.

Effects of repeated exposure to cocaine on group II metabotropic glutamate receptor function in the rat medial prefrontal cortex: behavioral and neurochemical studies

RATIONALE

Repeated exposure to cocaine progressively increases drug-induced locomotor activity, which is termed behavioral sensitization. Enhanced excitatory output from the medial prefrontal cortex (mPFC), which can be modulated by group II metabotropic glutamate receptors (mGluR), is thought to play a key role in the development of sensitization to cocaine.

OBJECTIVES

The present studies were designed to determine whether the ability of intra-mPFC injections of the group II mGluR agonist 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (APDC) to inhibit cocaine-induced motor activity and dopamine release in the nucleus accumbens is reduced in sensitized animals.

RESULTS

Initial studies demonstrated that injection of APDC (0.015-15 nmol/side) into the mPFC dose dependently reduced cocaine-induced (15 mg/kg, i.p.) motor activity. The lowest dose in the present studies that significantly reduced the acute motor-stimulant response to cocaine was 1.5 nmol/side. The specificity of the effects of APDC was confirmed by demonstrating that intra-mPFC co-injection of LY341495 (1.5 nmol/side), a group II mGluR antagonist, prevented the inhibitory actions of APDC. Finally, it was shown that intra-mPFC injection of APDC was able to prevent the initiation of behavioral and neurochemical sensitization to cocaine. Intra-mPFC APDC was also observed to block the expression of cocaine-induced sensitization after short (1 day), but not prolonged (7 and 30 days), abstinence from cocaine.

CONCLUSIONS

Taken together, these data suggest that mPFC group II mGluR function is reduced following extended abstinence from repeated cocaine.

Plasticity and memory in the prefrontal cortex

Neuropsychological and neuroimaging studies in humans have shown that the prefrontal cortex (PFC) is involved in long-term memory functioning. In general, the participation of the PFC in long-term memory has been attributed to its role in executive control rather than information storage. Accumulating data from recent animal studies, however, suggest the possible role of the PFC in the storage of long-term memory. In support of this view, there is evidence that various projection systems in the PFC support long-term synaptic plasticity. Recording studies have further demonstrated neural correlates of learning in various animal species. Lastly, behavioral and physiological studies indicate that the PFC is critically involved in memory consolidation, retrieval and extinction processes. These studies then suggest that the PFC is an integral part of the neural network where long-term memory trace is stored and retrieved. Though decisive evidence is still lacking at present, we propose here to assign a term 'control memory' (i.e., memory for top-down control processes) as a new type of memory function for the PFC. This new principle of PFC-long-term memory can help organize existing data and provide novel insights into future empirical studies.

Blockade of glutamate transporters leads to potentiation of NMDA receptor current in layer V pyramidal neurons of the rat prefrontal cortex via group II metabotropic glutamate receptor activation

Membrane currents of layer V pyramidal cells in slices of the rat prefrontal cortex (PFC) were recorded with the patch-clamp technique. In an Mg2+-free superfusion medium l-trans-pyrrolidine-2,4-dicarboxylic acid (PDC), a preferential blocker of astrocytic glutamate transporters, caused inward current due to the activation of NMDA receptors. The blockade of conducted action potentials by tetrodotoxin did not interfere with this effect. ATP was inactive when given alone and potentiated the NMDA-induced current in an Mg2+-containing but not Mg2+-free superfusion medium. Agonists of group I ((S)-3,5-dihydroxyphenylglycine; DHPG) and II ((1R,4R,5S,6R)-4-amino-2-oxabicyclo[3.1.0]hexane-4,6-dicarboxylic acid; LY 379268) metabotropic glutamate receptors (mGluRs) also potentiated responses to NMDA, whereas the group III mGluR agonist L-(+)-2-amino-4-phosphonobutyric acid (L-AP4) did not affect them. In contrast to ATP, PDC evoked inward current in the absence but not in the presence of external Mg2+, when given alone, and facilitated the NMDA effect Mg2+-independently. The PDC-induced facilitation of NMDA responses was blocked by group II ((2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid; LY 341495), but not group I ((RS)-1-aminoindan-1,5-dicarboxylic acid; AIDA) or III (alpha-methyl-3-methyl-4-phosphonophenylglycine; UBP 1112) mGluR antagonists. In conclusion, the blockade of astrocytic glutamate uptake by PDC may lead to a stimulation of group II mGluRs, while the triggering of exocytotic glutamate release from astrocytes by ATP may cause activation of group I mGluRs, both situated postsynaptically at layer V PFC pyramidal cells. Either group of mGluRs may interact with NMDA receptors in a positive manner.

The role of NMDA receptors in regulating group II metabotropic glutamate receptor-mediated long-term depression in rat medial prefrontal cortex

Previous work has shown that brief application of group II metabotropic glutamate receptor (mGluR) agonist (2S,2'R,3'R)-2-(2',3'-dicarbox-ycyclopropyl) glycine (DCG-IV) can induce long-term depression (LTD) of excitatory transmission on layer V pyramidal neurons of rat medial prefrontal cortex (mPFC). An unusual feature of this LTD is that it relies on activation of both group II mGluRs and N-methyl-D-aspartate receptors (NMDARs). However, it is not known whether other specific group II mGluR agonists also induce LTD and whether they depend on the conjoint activation of group II mGluRs and NMDARs. We show here that the ability of DCG-IV to induce LTD was mimicked by a more selective group II mGluR agonist, LY379268. The induction of LTD by a lower concentration of DCG-IV (0.2 microM) or LY379268 (0.03 microM) was blocked by the NMDAR antagonist APV or the interruption of synaptic stimulation during drug application. In contrast, application of a higher concentration of DCG-IV (1 microM) or LY379268 (0.1 microM) can induce LTD that was independent of synaptic NMDAR activation. These results suggest that although molecular cooperation between group II mGluRs and synaptic NMDARs may facilitate the induction of group II mGluR-mediated LTD at excitatory synapses onto mPFC layer V pyramidal neurons, enhancing group II mGluR activation may remove NMDAR involvement in this form of synaptic plasticity.

Dopamine D1/5 receptor-mediated long-term potentiation of intrinsic excitability in rat prefrontal cortical neurons: Ca2+-dependent intracellular signaling

Prefrontal cortex (PFC) dopamine D1/5 receptors modulate long- and short-term neuronal plasticity that may contribute to cognitive functions. Synergistic to synaptic strength modulation, direct postsynaptic D1/5 receptor activation also modulates voltage-dependent ionic currents that regulate spike firing, thus altering the neuronal input-output relationships in a process called long-term potentiation of intrinsic excitability (LTP-IE). Here, the intracellular signals that mediate this D1/5 receptor-dependent LTP-IE were determined using whole cell current-clamp recordings in layer V/VI rat pyramidal neurons from PFC slices. After blockade of all major amino acid receptors (V(hold) = -65 mV) brief tetanic stimulation (20 Hz) of local afferents or application of the D1 agonist SKF81297 (0.2-50 microM) induced LTP-IE, as shown by a prolonged (>40 min) increase in depolarizing pulse-evoked spike firing. Pretreatment with the D1/5 antagonist SCH23390 (1 microM) blocked both the tetani- and D1/5 agonist-induced LTP-IE, suggesting a D1/5 receptor-mediated mechanism. The SKF81297-induced LTP-IE was significantly attenuated by Cd(2+), [Ca(2+)](i) chelation, by inhibition of phospholipase C, protein kinase-C, and Ca(2+)/calmodulin kinase-II, but not by inhibition of adenylate cyclase, protein kinase-A, MAP kinase, or L-type Ca(2+) channels. Thus this form of D1/5 receptor-mediated LTP-IE relied on Ca(2+) influx via non-L-type Ca(2+) channels, activation of PLC, intracellular Ca(2+) elevation, activation of Ca(2+)-dependent CaMKII, and PKC to mediate modulation of voltage-dependent ion channel(s). This D1/5 receptor-mediated modulation by PKC coexists with the previously described PKA-dependent modulation of K(+) and Ca(2+) currents to dynamically regulate overall excitability of PFC neurons.

Group I metabotropic glutamate receptor-induced Ca(2+)-gradients in rat superficial spinal dorsal horn neurons

Here, we investigated changes in the free cytosolic Ca(2+) concentration ([Ca(2+)](i)), induced by the pharmacological activation of metabotropic glutamate receptors (mGluRs), in nociceptive neurons of the superficial spinal dorsal horn. Microfluorometric Ca(2+) measurements with fura-2 in a lumbar spinal cord slice preparation from young rats were used. Bath application of the specific group I mGluR agonist (S)-3,5-dihydroxyphenylglycine ((S)-3,5-DHPG) resulted in a distinct increase of [Ca(2+)](i) in most of the neurons in superficial dorsal horn. In contrast, activation of groups II or III mGluRs by DCG-IV or l-AP4, respectively, failed to evoke any significant change in [Ca(2+)](i). The effect of (S)-3,5-DHPG was mediated by both group I subtypes mGluR1 and mGluR5, since combined pre-treatment with the subtype antagonists (S)-4-CPG and MPEP was necessary to abolish the [Ca(2+)](i) increase. Depleting intracellular Ca(2+) stores with CPA or inhibiting IP(3)-receptors with 2-APB, respectively, reduced the (S)-3,5-DHPG-evoked [Ca(2+)](i) increase significantly. Inhibition of voltage-dependent L-type Ca(2+) channels (VDCCs) by verapamil or nicardipine reduced the (S)-3,5-DHPG-induced [Ca(2+)](i) rise likewise. Thus, in rat spinal cord, (S)-3,5-DHPG enhances Ca(2+) signalling in superficial dorsal horn neurons, mediated by the release of Ca(2+) from IP(3)-sensitive intracellular stores and by an influx through L-type VDCCs. This may be relevant to the processing of nociceptive information in the spinal cord.

Muscarinic acetylcholine receptor knockout mice show distinct synaptic plasticity impairments in the visual cortex

In the present report, we focused our attention on the role played by the muscarinic acetylcholine receptors (mAChRs) in different forms of long-term synaptic plasticity. Specifically, we investigated long-term potentiation (LTP) and long-term depression (LTD) expression elicited by theta-burst stimulation (TBS) and low-frequency stimulation (LFS), respectively, in visual cortical slices obtained from different mAChR knockout (KO) mice. A normal LTP was evoked in M(1)/M(3) double KO mice, while LTP was impaired in the M(2)/M(4) double KO animals. On the other hand, LFS induced LTD in M(2)/M(4) double KO mice, but failed to do so in M(1)/M(3) KO mice. Interestingly, LFS produced LTP instead of LTD in M(1)/M(3) KO mice. Analysis of mAChR single KO mice revealed that LTP was affected only by the simultaneous absence of both M(2) and M(4) receptors. A LFS-dependent shift from LTD to LTP was also observed in slices from M(1) KO mice, while LTD was simply abolished in slices from M(3) KO mice. Using pharmacological tools, we showed that LTP in control mice was blocked by pertussis toxin, an inhibitor of G(i/o) proteins, but not by raising intracellular cAMP levels. In addition, the inhibition of phospholipase C by U73122 induced the same shift from LTD to LTP after LFS observed in M(1) single KO and M(1)/M(3) double KO mice. Our results indicate that different mAChR subtypes regulate different forms of long-term synaptic plasticity in the mouse visual cortex, activating specific G proteins and downstream intracellular mechanisms.

Metabotropic glutamate receptors

Metabotropic glutamate receptors (mGlus) are a family of G-protein-coupled receptors activated by the neurotransmitter glutamate. Molecular cloning has revealed eight different subtypes (mGlu1-8) with distinct molecular and pharmacological properties. Multiplicity in this receptor family is further generated through alternative splicing. mGlus activate a multitude of signalling pathways important for modulating neuronal excitability, synaptic plasticity and feedback regulation of neurotransmitter release. In this review, we summarize anatomical findings (from our work and that of other laboratories) describing their distribution in the central nervous system. Recent evidence regarding the localization of these receptors in peripheral tissues will also be examined. The distinct regional, cellular and subcellular distribution of mGlus in the brain will be discussed in view of their relationship to neurotransmitter release sites and of possible functional implications.

Group-II metabotropic glutamate receptors negatively modulate NMDA transmission at striatal cholinergic terminals: Role of P/Q-type high voltage activated Ca++ channels and endogenous dopamine

Striatal cholinergic nerve terminals express functional group-II metabotropic (mGlu) and NMDA glutamate receptors. To investigate whether these receptors interact to regulate ACh release, LY354740 (a group-II mGlu receptor agonist) and NMDA were co-applied in striatal synaptosomes and slices. LY354740 prevented the NMDA-evoked [3H]-choline release from synaptosomes and ACh release from slices. In synaptosomes, this modulation was prevented by omega-agatoxin IVA, suggesting that it was mediated by P/Q-type high voltage activated Ca++ channels. In slices, LY341495 (a group-II mGlu receptor antagonist) enhanced the NMDA-induced ACh release, suggesting that group-II mGlu receptor activation by endogenous glutamate inhibits NMDA transmission. Co-immunoprecipitation studies excluded direct group-II mGlu-NMDA receptor interactions. Finally, group-II mGlu negative modulation of NMDA transmission was abolished in dopamine-depleted synaptosomes and slices, suggesting that it relied on endogenous dopamine. We conclude that group-II mGlu receptors attenuate NMDA inputs at striatal cholinergic terminals via Ca++ channel modulation and dopamine-sensitive pathways.

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.

Calcium From Internal Stores Triggers GABA Release From Retinal Amacrine Cells

The Ca2+ that promotes transmitter release is generally thought to enter presynaptic terminals through voltage-gated Ca2+channels. Using electrophysiology and Ca2+ imaging, we show that, in amacrine cell dendrites, at least some of the Ca2+ that triggers transmitter release comes from endoplasmic reticulum Ca2+ stores. We show that both inositol 1,4,5-trisphosphate receptors (IP3Rs) and ryanodine receptors (RyRs) are present in these dendrites and both participate in the elevation of cytoplasmic [Ca2+] during the brief depolarization of a dendrite. Only the Ca2+ released through IP3Rs, however, seems to promote the release of transmitter. Antagonists for the IP3R reduced transmitter release, whereas RyR blockers had no effect. Application of an agonist for metabotropic glutamate receptor, known to liberate Ca2+ from internal stores, enhanced both spontaneous and evoked transmitter release.

The role of metabotropic glutamate receptors for the generation of calcium oscillations in rat hippocampal astrocytes in situ

Ca2+ oscillations are part of the intra- and intercellular signalling in many cell types. We have studied Ca2+ oscillations in astrocytes in acute brain slices of the hippocampus of juvenile rats (postnatal 8-14 days old), using confocal laser scanning microscopy and bulk-loading of the Ca2+ -sensitive dye Fluo-4. Astrocytes were identified morphologically in the stratum radiatum, and by their Ca2+ response in the absence of external K+. Thirty-five per cent of astrocytes (43 slices) showed spontaneous Ca2+ oscillations, with a frequency of 1.26 +/- 0.11 transients/min (n = 366). These Ca2+ signals were unaffected by tetrodotoxin (0.5 microM) and Ni2+ (2 mM), but were sensitive to interference with the phospholipase C-mediated Ca2+ release from intracellular stores. Spontaneous Ca2+ oscillations were reduced or suppressed by antagonists of metabotropic glutamate receptors (mGluRs) of groups I and II, but not affected by antagonists of group III. Glutamate (1-100 microM) and specific agonists of mGluR groups I and II evoked concentration-dependent Ca2+ signals, which were oscillatory at intermediate concentrations (e.g. at 10 microM glutamate). Our results indicate that mGluRs of both groups I and II are involved in mediating Ca2+ oscillations in astrocytes, which might be glial responses to micromolar changes of glutamate in the extracellular spaces.

Postsynaptic IP3 receptor-mediated Ca2+ release modulates synaptic transmission in hippocampal neurons

Ca(2+)-dependent mechanisms are important in regulating synaptic transmission. The results herein indicate that whole-cell perfusion of inositol 1,4,5-trisphosphate receptor (IP(3)R) agonists greatly enhanced excitatory postsynaptic current (EPSC) amplitudes in postsynaptic hippocampal CA1 neurons. IP(3)R agonist-mediated increases in synaptic transmission changed during development and paralleled age-dependent increases in hippocampal type-1 IP(3)Rs. IP(3)R agonist-mediated increases in EPSC amplitudes were inhibited by postsynaptic perfusion of inhibitors of Ca(2+)/calmodulin, PKC and Ca(2+)/calmodulin-dependent protein kinase II. Postsynaptic perfusion of inhibitors of smooth endoplasmic reticulum (SER) Ca(2+)-ATPases, which deplete intracellular Ca(2+) stores, also enhanced EPSC amplitudes. Postsynaptic perfusion of the IP(3)R agonist adenophostin (AdA) during subthreshold stimulation appeared to convert silent to active synapses; synaptic transmission at these active synapses was completely blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Postsynaptic IP(3)R-mediated Ca(2+) release also produced a significant increase in spontaneous EPSC frequency. These results indicate that Ca(2+) release from intracellular stores play a key role in regulating the function of postsynaptic AMPARs.

Group II and III metabotropic glutamate receptors suppress excitatory synaptic transmission in the dorsolateral bed nucleus of the stria terminalis

Conditions such as anxiety, drug abuse, and post-traumatic stress disorder are thought to reflect alterations in central nervous system stress and reward circuitry. Recent evidence suggests a key component of this circuitry is the bed nucleus of the stria terminalis (BNST). In particular, regulation of glutamatergic transmission in the BNST plays a critical role in animal performance on anxiety tasks. Metabotropic glutamate receptors (mGluRs) have been implicated in stress and drug addiction and are known to regulate glutamatergic transmission in many brain regions. We have utilized both extracellular field potential and whole-cell patch-clamp recording in an in vitro slice preparation of mouse dorsal anterolateral BNST to determine whether G(i/o)-linked mGluRs modulate excitatory transmission in this region. We find that activation of group II and group III mGluRs in an in vitro slice preparation of the dBNST causes a depression of excitatory transmission. The depression evoked by group II mGluR activation may represent a form of synaptic plasticity as prolonged activation of the receptor produces a long-term depression of glutamatergic transmission. Based on paired-pulse ratio analysis, initiation of depression by group II and group III mGluR subfamilies appears to, at least in part, involve decreased glutamate release. In total, our data suggest a plausible site of action for some of the anxiolytic effects of group II and group III mGluR agonists.

Activation of group II metabotropic glutamate receptors induces depotentiation in amygdala slices and reduces fear-potentiated startle in rats

There is a close correlation between long-term potentiation (LTP) in the synapses of lateral amygdala (LA) and fear conditioning in animals. We predict that reversal of LTP (depotentiation) in this area of the brain may ameliorate conditioned fear. Activation of group II metabotropic glutamate receptors (mGluR II) with DCG-IV induces depotentiation in the LA. The induction of depotentiation is independent of NMDA receptors, L-type Ca++ channels, and calcineurin activity, but requires presynaptic activity and extracellular Ca++. (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) depotentiation is accompanied by a decrease in the frequency but not the amplitude of miniature excitatory post-synaptic currents (mEPSCs) and could be mimicked by endogenously released glutamate. DCG-IV inhibited the release of glutamate evoked by 4-AP but not that evoked by ionomycin, suggesting that the effect of DCG-IV is not mediated by an action downstream of Ca++ entry. Intra-amygdala infusion of mGluR II agonist blocks the consolidation of fear memory measured with fear-potentiated startle. Taken together, the present results characterize the properties of DCG-IV depotentiation and reveal a close parallel between depotentiation in the amygdala slice and the reduction of conditioned fear in animals.

Enhanced cocaine responsiveness and impaired motor coordination in metabotropic glutamate receptor subtype 2 knockout mice

Extensive pharmacological studies have recently emerged indicating that group 2 metabotropic glutamate receptors (mGluRs) comprising mGluR2 and mGluR3 subtypes are associated with several neurological and psychiatric disorders. mGluR2 is widely distributed both presynaptically and postsynaptically in a variety of neuronal cells, but the physiological role of mGluR2 in brain function is poorly understood. This investigation involves a comprehensive behavioral analysis of mGluR2-/- knockout (KO) mice to explore the physiological role of mGluR2 in brain function. Although, under general observation, mGluR2-/- KO mice appeared to have no behavioral abnormalities, they exhibited several lines of behavioral alterations in the enforcing and defined behavioral tests. They showed a significant increase in locomotor sensitization and conditioned place preference in association with repeated cocaine administration, indicating that mGluR2 contributes to behavioral responses implicated in reinforcement and addiction of cocaine. Upon in vivo microdialysis analysis after cocaine administration, not only did extracellular levels of dopamine increase but also the response pattern of glutamate release markedly changed in the nucleus accumbens of mGluR2-/- KO mice. The mGluR2-/- KO mice also showed significant impairment in motor coordination in the accelerating rota-rod test and exhibited hyperlocomotion in novel environmental and stressful conditions, when assessed by the open-field and forced-swim tests. These results indicate that the inhibitory mGluR2 plays a pivotal role in synaptic regulation of glutamatergic transmission in the neural network.