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

Activation of metabotropic glutamate receptors does not alter the phosphorylation state of GluR1 AMPA receptor subunit at serine 845 in perirhinal cortical neurons

Activation of group I and group II metabotropic glutamate receptors (mGluRs) is thought to be required for long-term depression (LTD) of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor-mediated synaptic transmission in the perirhinal cortex. However, little is known about how activation of mGluRs leads to this form of synaptic plasticity. AMPA receptor phosphorylation has been implicated in several forms of modulation of synaptic transmission. In the CA1 area of the hippocampus, N-methyl-d-aspartate (NMDA) receptor-dependent LTD is associated with the reduced phosphorylation of the GluR1 AMPA receptor subunit at serine 845 (GluR1-S845). Immunoblot analysis of perirhinal cortical neurons using GluR1 and GluR1-S845 phosphorylation state specific antibodies showed that stimulation of adenylyl cyclase (AC) with forskolin (FSK) dramatically increased PKA-mediated phosphorylation of GluR1-S845. However, selective or simultaneous application of mGluR5 agonist (S)-3,5-dihydroxyphenylglycine (CHPG) and mGluR2/3 agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) did not produce detectable changes in GluR1-S845 phosphorylation. These results indicate that in the perirhinal cortex mGluR activation does not alter the phosphorylation state of GluR1-S845. Therefore, it is likely that the process involved in the modification of AMPA receptors in mGluR activation dependent LTD in the perirhinal cortex is mechanistically distinct from NMDA receptor-mediated LTD described in hippocampal neurons.

The principal features and mechanisms of dopamine modulation in the prefrontal cortex

Mesocortical [corrected] dopamine (DA) inputs to the prefrontal cortex (PFC) play a critical role in normal cognitive process and neuropsychiatic pathologies. This DA input regulates aspects of working memory function, planning and attention, and its dysfunctions may underlie positive and negative symptoms and cognitive deficits associated with schizophrenia. Despite intense research, there is still a lack of clear understanding of the basic principles of actions of DA in the PFC. In recent years, there has been considerable efforts by many groups to understand the cellular mechanisms of DA modulation of PFC neurons. However, the results of these efforts often lead to contradictions and controversies. One principal feature of DA that is agreed by most researchers is that DA is a neuromodulator and is clearly not an excitatory or inhibitory neurotransmitter. The present article aims to identify certain principles of DA mechanisms by drawing on published, as well as unpublished data from PFC and other CNS sites to shed light on aspects of DA neuromodulation and address some of the existing controversies. Eighteen key features about DA modulation have been identified. These points directly impact on the end result of DA neuromodulation, and in some cases explain why DA does not yield identical effects under all experimental conditions. It will become apparent that DA's actions in PFC are subtle and depend on a variety of factors that can no longer be ignored. Some of these key factors include distinct bell-shaped dose-response profiles of postsynaptic DA effects, different postsynaptic responses that are contingent on the duration of DA receptor stimulation, prolonged duration effects, bidirectional effects following activation of D1 and D2 classes of receptors and membrane potential state and history dependence of subsequent DA actions. It is hoped that these factors will be borne in mind in future research and as a result a more consistent picture of DA neuromodulation in the PFC will emerge. Based on these factors, a theory is proposed for DA's action in PFC. This theory suggests that DA acts to expand or contract the breadth of information held in working memory buffers in PFC networks.

Metabotropic glutamate receptors contribute to neocortical synaptic plasticity in vivo

Metabotropic glutamate receptors (mGluRs) have been shown to be important for hippocampus-dependent memory, as well as activity-dependent synaptic plasticity in the hippocampus. In this study, we examined the role of mGluRs in the induction of two forms of activity-dependent synaptic plasticity, long-term potentiation (LTP) and long-term depression (LTD), in the neocortex of awake, freely-moving rats. The mGluR antagonist AIDA was administered during the induction of LTP or LTD in the motor cortex. There was a 50% reduction of LTP induced in the early component of the evoked response, but there was no effect on the late component and no effect on the induction of LTD. Thus, mGluRs contribute to at least one form of activity dependent synaptic plasticity in the neocortex.

Inhibitory effects of group II mGluR-related drugs on memory performance in mice

The cAMP/protein kinase A signaling pathway is negatively modulated by group II metabotropic glutamate receptors (mGluRs), and the cross-talk that occurs between these receptors may modulate learning and memory. To examine the relationship among cAMP/PKA-signaling pathway activity, group II mGluRs, and learning and memory, mice were trained to perform a step-through-type passive avoidance task, and 10 min before each avoidance trial the following drugs were injected intracisternally (i.cist.): vehicle (0.05% dimethylsulfoxide); a specific group II mGluR agonist, DCG-IV (1-50 ng/mouse); a specific group II mGluR antagonist, LY341495 (10-300 ng); a selective inhibitor of cAMP-specific phosphodiesterase, rolipram (100-1000 ng); an activator of adenylyl cyclase, forskolin (25-250 ng); a specific inhibitor of PKA, H-89 (150 or 300 ng) or; an activator of protein kinase C, phorbol 12-myristate 13-acetate (PMA 200 ng). DCG-IV (25 and 50 ng) or LY341495 (150 and 300 ng) reduced the latency in the avoidance task. The reduction of latency by DCG-IV was not observed in mice coinjected with DCG-IV (50 ng) together with rolipram (500 ng) or forskolin (25 ng). Conversely, coinjection of LY341495 with 100 or 1000 ng rolipram, or with 25 or 250 ng forskolin tended to potentiate the LY341495-induced shortening of latency. In addition, the reduction of latency by DCG-IV (50 ng) was not observed in mice coinjected with DCG-IV and PMA together. However, the reduction of latency by LY341495 (300 ng) was potentiated when the drug was coadministered with PMA. These results suggest that changes in the cAMP/PKA-signaling pathway, mediated by group II mGluRs, influence memory in the passive avoidance task, and that both the excessive activation and deactivation of this pathway may induce the impairment of learning and memory.

Metabotropic glutamate receptor signalling in perirhinal cortical neurons

Long-term depression (LTD) induction relies upon receptor cross-talk between group I and group II metabotropic glutamate receptors (mGluRs) in perirhinal cortex. The molecular mechanism of this mGluR interplay is not clear. Here, we show that the mGluR subtypes postulated to be involved in this mechanism are developmentally regulated and mGluR2 has a preferential role over mGluR3 in the synergistic interaction with mGluR5. We have identified a >70% reduction in basal cAMP levels following mGluR2 stimulation, which could lead to increased mGluR5 function via reduced PKA mediated phosphorylation and decreased desensitisation of mGluR5. To further investigate the roles of mGluRs in downstream intracellular signalling, we have examined the effects of mGluRs on the phosphorylation state of cAMP response element-binding protein (CREB). Both group I and group II agonists increased the phosphorylation of CREB, which indicates a cAMP- and PKA-independent signalling mechanism. These results suggest a convergence of signalling mechanisms from surface mGluRs to CREB-mediated transcription.

Inositol 1,4,5-trisphosphate receptors as signal integrators

The inositol 1,4,5 trisphosphate (IP3) receptor (IP3R) is a Ca2+ release channel that responds to the second messenger IP3. Exquisite modulation of intracellular Ca2+ release via IP3Rs is achieved by the ability of IP3R to integrate signals from numerous small molecules and proteins including nucleotides, kinases, and phosphatases, as well as nonenzyme proteins. Because the ion conduction pore composes only approximately 5% of the IP3R, the great bulk of this large protein contains recognition sites for these substances. Through these regulatory mechanisms, IP3R modulates diverse cellular functions, which include, but are not limited to, contraction/excitation, secretion, gene expression, and cellular growth. We review the unique properties of the IP3R that facilitate cell-type and stimulus-dependent control of function, with special emphasis on protein-binding partners.

Evidence That Phospholipase D Activation Prevents Group I mGluR-Induced Persistent Prolongation of Epileptiform Bursts

Selective activation of group I metabotropic glutamate receptors (mGluRs) with ( S)-3,5-dihydroxyphenylglycine (DHPG) in guinea pig hippocampal slices converts 275- to 475-ms picrotoxin-induced interictal bursts into persistent seizure-length discharges typically over 1 s in duration. Here we report that l-cysteine sulfinic acid (CSA), a sulfur-containing amino acid, prevented the induction of this persistent group I mGluR-mediated epileptiform burst prolongation. However, CSA had no effect on baseline interictal bursting activity and failed to suppress the expression of the group I mGluR-induced persistent prolonged bursts once they were fully induced. (2 R,1′ S,2′ R,3′ S)-2-(2′-carboxy-3′-phenylcyclopropyl)glycine (PCCG-13), a selective antagonist at the phospholipase D (PLD)-coupled mGluR, had no effect of its own on DHPG-induced burst prolongation; however, CSA applied in the presence of PCCG-13 could no longer fully block the burst prolongation induced by DHPG, suggesting that CSA's antiepileptogenic effect is mediated by agonist action at this PLD-coupled receptor. These data parallel our previous data revealing that protein synthesis inhibitors prevent induction but not expression of group I mGluR-mediated persistent seizure-length discharges. Hence, PLD activation with CSA may prevent the synthesis of a protein critical for the induction of group I mGluR-mediated epileptogenesis.

Plasticity Mechanisms Underlying mGluR-Induced Epileptogenesis

Transient application of group I metabotropic glutamate receptor (mGluR) agonists to hippocampal slices produces ictal-like discharges that persist for hours after the removal of the agonist. This effect of group I mGluR stimulation--converting a 'normal' hippocampal slice into an 'epileptic-like' one--may represent a form of epileptogenesis. Because this epileptogenic process can be induced in vitro and it occurs within hours, it has been possible to examine the cellular and transduction processes underlying the generation and long-term maintenance of ictal-like bursts. ImGluR(V), a voltage-dependent depolarizing current activated by group I mGluR agonists, appears to play an important role in the expression of the ictal-like bursts. Long-term activation of ImGluR(V) following mGluR stimulation is a possible plastic change that enables the long-term maintenance of ictal discharges. Induction of ImGluR(V) may represent a cellular event underlying the mGluR-induced epileptogenesis.

Protein phosphatase 2C binds selectively to and dephosphorylates metabotropic glutamate receptor 3

Cell surface receptor membrane localization is strongly dependent on protein-protein interactions often involving regulation by phosphorylation/dephosphorylation of the intracellular domains of membrane proteins. The present study was carried out to identify metabotropic glutamate receptor (mGluR) 3 regulatory binding proteins. Using the yeast two-hybrid technique, we found that the 50-aa C-terminal cytoplasmic tail of mGluR3 interacts specifically with protein phosphatase 2Calpha (PP2Calpha). This interaction was confirmed by GST pull-down and coimmunoprecipitation assays. mGluR3 interacts with PP2Calpha, beta, gamma, and delta isoforms; however, among the mGluR family only mGluR3 interacted with PP2C. The minimal interacting domain of mGluR3 comprised residues 836-855. Alignment between mGluR3 and mGluR2, a closely related group II receptor, indicated that this domain is not conserved between the two receptors. The mGluR3 cytoplasmic C-terminal tail contains one phosphorylation site for protein kinase A (Ser-845), but the phosphatase that dephosphorylates this site has not been previously identified. We find that PP2C, but not PP1, PP2A, or PP2B, dephosphorylates the mGluR3 cytoplasmic tail in vitro. The dephosphorylated form of the mGluR3 cytoplasmic tail, but not the equivalent region of mGluR2, inhibited PP2C assayed by using [32P]casein as a substrate. However, phosphorylation of the mGluR3 cytoplasmic tail at Ser-845 inhibits the interaction with PP2C. These results indicate distinct functions for mGluR2 and mGluR3 and suggest a dynamic regulation of mGluR3 by PP2C.

Ionotropic and metabotropic glutamate receptor mediation of glucocorticoid-induced apoptosis in hippocampal cells and the neuroprotective role of synaptic N-methyl-D-aspartate receptors

Glutamate receptors have been proposed to mediate the apoptotic actions of glucocorticoids in hippocampal cells. To further analyze the role of glutamate receptors in this process, we pretreated primary hippocampal cells from neonatal (postnatal day 4) rats with antagonists of ionotropic glutamate receptor (iGluR) and metabotropic glutamate receptor (mGluR) antagonists before exposure to the specific glucocorticoid receptor agonist dexamethasone (DEX) at a dose of 1 microM. Dizocilpine (MK801; a general N-methyl-D-aspartic acid [NMDA] receptor antagonist, NMDAR antagonist) and ifenprodil (a specific ligand of the NMDAR 2B subunit, NR2B), were used to block iGluR; (RS)-alpha-ethyl-4-carboxyphenylglycine (E4CPG) and (RS)-alpha-cyclopropyl-4-phosphonophenyl-glycine (CPPG) were employed as I/II (E4CPG) and II/III (CPPG) mGluR antagonists. Blockade of iGluR resulted in a significant attenuation of DEX-induced cell death; the finding that ifenprodil exerted a similar potency to MK801 demonstrates the involvement of NR2B receptors in glucocorticoid-induced cell death. Apoptosis accounted for a significant amount of the cell loss observed, as detected by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling histochemistry for the in situ labeling of DNA breaks; apoptotic cells were distinguished from necrosis on the basis of morphological criteria, including chromatin condensation, membrane blebbing and presence of apoptotic bodies. Treatment with E4CPG and CPPG completely abolished the apoptotic response to DEX, thus showing the additional contribution of mGluR to the phenomenon. Further, dose-response studies with NMDA revealed that whereas high (10 microM) doses of NMDA themselves elicit cytotoxic responses, low (1-5 microM) concentrations of NMDA can effectively oppose DEX-induced cell death. Interestingly, the neuroprotective actions of low dose NMDA stimulation were abolished when either synaptic or extrasynaptic NMDA receptors were blocked with MK801 in combination with the GABA receptor antagonist bicuculline (synaptic) or ifenprodil (extrasynaptic). In summary, the present data show that both iGluR and mGluR mediate the neurotoxic effects of glucocorticoids on hippocampal cells and that pre-treatment with low doses of NMDA, by acting on synaptic and extrasynaptic receptors, render hippocampal cells less vulnerable to glucocorticoid insults.

Group II metabotropic glutamate receptors enhance NMDA receptor currents via a protein kinase C-dependent mechanism in pyramidal neurones of rat prefrontal cortex

The action of glutamate in CNS is mediated by the activation of metabotropic and ionotropic receptors. The metabotropic glutamate receptors (mGluRs) are highly enriched in prefrontal cortex (PFC) - a brain region critically involved in the regulation of cognition and emotion. Emerging evidence has suggested that mGluRs are viable drug targets for neuropsychiatric disorders associated with PFC dysfunction. However, the mGluR-mediated signalling in PFC remains unclear. To understand the physiological functions of postsynaptic group II mGluRs (mGluR2/3) in PFC neurones, we investigated the molecular and cellular mechanisms underlying the regulation of NMDA receptor channels by group II mGluRs. We found that APDC, a highly selective and potent group II mGluR agonist, reversibly increased NMDAR currents in acutely dissociated PFC pyramidal neurones. Selective group II mGluR antagonists, but not group I mGluR antagonists, blocked APDC-induced enhancement of NMDAR currents, suggesting the mediation by mGluR2/3 receptors. The APDC effect on NMDAR currents was independent of Mg(2+) block or membrane voltages, and primarily targeted NR2A subunits containing NMDARs. While changing protein kinase A levels was without effect, inhibiting protein kinase C (PKC) or dialysis with Ca(2+) chelators largely blocked the mGluR2/3 modulation of NMDAR currents. In contrast, inhibiting protein tyrosine kinases, cyclin-dependent kinase 5, Ca(2+)/calmodulin-dependent kinase II or the Ca(2+)/calmodulin-dependent phosphatase calcineurin failed to do so. Moreover, treatment of PFC slices with APDC significantly increased the PKC activity and PKC phosphorylation of NMDA receptors. These findings suggest that activation of mGluR2/3 receptors potentiates NMDAR channel functions in PFC through a PKC-dependent mechanism. This modulation may be relevant for developing novel mGluR-related pharmacological agents for the treatment of mental illnesses.

Glutamatergic calcium responses in the developing lateral superior olive: receptor types and their specific activation by synaptic activity patterns

The lateral superior olive (LSO) is a binaural auditory brain stem nucleus that plays a central role in sound localization. Survival and maturation of developing LSO neurons critically depend on intracellular calcium signaling. Here we investigated the mechanisms by which glutamatergic afferents from the cochlear nucleus increase intracellular calcium concentration in LSO neurons. Using fura-2 calcium imaging in slices prepared from neonatal mice, we found that cochlear nucleus afferents can activate all major classes of ionotropic and metabotropic glutamate receptors, each of which contributes to an increase in intracellular calcium. The specific activation of different glutamate receptor classes was dependent on response amplitudes and afferent stimulus patterns. Low-amplitude responses elicited by single stimuli were entirely mediated by calcium-impermeable AMPA/kainate receptors that activated voltage-gated calcium channels. Larger-amplitude responses elicited by either single stimuli or stimulus trains resulted in additional calcium influx through N-methyl-d-aspartate receptors. Finally, high-frequency stimulation also recruited group I and group II metabotropic glutamate receptors, both of which mobilized intracellular calcium. This calcium release in turn activated a strong influx of extracellular calcium through a membrane calcium channel that is distinct from voltage-gated calcium channels. Together, these results indicate that before hearing onset, distinct patterns of afferent activity generate qualitatively distinct types of calcium responses, which likely serve in guiding different aspects of LSO development.

Direct and indirect interactions between cannabinoid CB1 receptor and group II metabotropic glutamate receptor signalling in layer V pyramidal neurons from the rat prefrontal cortex

At proximal synapses from layer V pyramidal neurons from the rat prefrontal cortex, activation of group II metabotropic glutamate receptors (group II mGlu) by (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl) glycine (DCG IV) induced a long-lasting depression of excitatory postsynaptic currents. Paired-pulse experiments suggested that the depression was expressed presynaptically. Activation of type 1 cannabinoid receptors (CB1) by WIN 55,212-2 occluded the DCG IV-induced depression in a mutually occlusive manner. At the postsynaptic level, WIN 55,212-2 and DCG IV were also occlusive for the activation of extracellular signal-regulated kinase. The postsynaptic localization of active extracellular signal-regulated kinase was confirmed by immunocytochemistry after activation of CB1 receptors. However, phosphorylation of extracellular signal-regulated kinase in layer V pyramidal neurons was dependent on the activation of N-methyl-d-aspartate receptors, consequently to a release of glutamate in the local network. Group II mGlu were also shown to be involved in long-term changes in synaptic plasticity induced by high frequency stimulations. The group II mGlu antagonist (RS)-alpha-methylserine-O-phosphate monophenyl ester (MSOPPE) favoured long-term depression. However, no interaction was found between MSOPPE, WIN 55,212-2 and the CB1 receptor antagonist SR 141716A on the modulation of long-term depression or long-term potentiation and the effects of these drugs were rather additive. We suggest that CB1 receptor and group II mGlu signalling may interact through a presynaptic mechanism in the induction of a DCG IV-induced depression. Postsynaptically, an indirect interaction occurs for activation of extracellular signal-regulated kinase. However, none of these interactions seem to play a role in synaptic plasticities induced with high frequency stimulations.

Baseline glutamate levels affect group I and II mGluRs in layer V pyramidal neurons of rat sensorimotor cortex

Possible functional roles for glutamate that is detectable at low concentrations in the extracellular space of intact brain and brain slices have not been explored. To determine whether this endogenous glutamate acts on metabotropic glutamate receptors (mGluRs), we obtained whole cell recordings from layer V pyramidal neurons of rat sensorimotor cortical slices. Blockade of mGluRs with (+)-alpha-amino-4-carboxy-alpha-methyl-benzeacetic acid (MCPG, a general mGluR antagonist) increased the mean amplitude of spontaneous excitatory postsynaptic currents (sEPSCs), an effect attributable to a selective increase in the occurrence of large amplitude sEPSCs. 2S-2-amino-2-(1S,2S-2-carboxycyclopropyl-1-yl)-3-(xanth-9-yl)propanoic acid (LY341495, a group II antagonist) increased, but R(-)-1-amino-2,3-dihydro-1H-indene-1,5-dicarboxylic acid (AIDA) and (RS)-hexyl-HIBO (group I antagonists) decreased sEPSC amplitude, and (R,S)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG, a group III antagonist) did not change it. The change in sEPSCs elicited by MCPG, AIDA, and LY341495 was absent in tetrodotoxin, suggesting that it was action potential-dependent. The increase in sEPSCs persisted in GABA receptor antagonists, indicating that it was not due to effects on inhibitory interneurons. AIDA and (S)-3,5-dihydroxyphenylglycine (DHPG, a group I agonist) elicited positive and negative shifts in holding current, respectively. LY341495 and (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV, a group II agonist) elicited negative and positive shifts in holding current, respectively. The AIDA and LY341495 elicited currents persisted in TTX. Finally, in current clamp, LY341495 depolarized cells by approximately 2 mV and increased the number of action potentials to a given depolarizing current pulse. Thus ambient levels of glutamate tonically activate mGluRs and regulate cortical excitability.

Cellular and molecular connections between sleep and synaptic plasticity

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

Memory trace in prefrontal cortex: theory for the cognitive switch

The dorsolateral prefrontal cortex in human and non-human primates functions as the highest-order executor for the perception-action cycle. According to this view, when perceptual stimuli from the environment are novel or complex, the dorsolateral prefrontal cortex serves to set consciously a goal-directed scheme which broadly determines an action repertory to meet the particular demand from the environment. In this respect, the dorsolateral prefrontal cortex is a short-term activation device with the properties of a cognitive switch', because it couples a particular set of perceptual stimuli to a particular set of actions. Here, I suggest that, in order for the organism to react systematically to the environment, neural traces for the switch function must be stored in the brain. Thus, the highest-order, perception-action interface function of the dorsolateral prefrontal cortex per se depends on permanently stored neural traces in the dorsolateral prefrontal cortex and related structures. Such a memory system may be located functionally between two of the well-documented memory systems in the brain: the declarative memory system and the procedural memory system. Finally, based on available neurophysiological data, the possible mechanisms underlying the formation of cognitive switch traces are proposed.