Control of calcium oscillations by phosphorylation of metabotropic glutamate receptors

Distinct roles of M1 and M3 muscarinic acetylcholine receptors controlling oscillatory and non-oscillatory [Ca2+]i increase

We examined ACh-induced [Ca2+]i dynamics in pancreatic acinar cells prepared from mAChR subtype-specific knockout (KO) mice. ACh did not induce any [Ca2+]i increase in the cells isolated from M1/M3 double KO mice. In the cells from M3KO mice, ACh (0.3-3 μM) caused a monotonic [Ca2+]i increase. However, we found characteristic oscillatory [Ca2+]i increases in cells from M1KO mice in lower concentrations of ACh (0.03-0.3 μM). We investigated the receptor specific pattern of [Ca2+]i increase in COS-7 cells transfected with M1 or M3 receptors. ACh induced the oscillatory [Ca2+]i increase in M3 expressing cells, but not in cells expressing M1, which exhibited monotonic [Ca2+]i increases. IP3 production detected in fluorescent indicator co-transfected cells was higher in M1 than in M3 expressing cells. From the examination of four types of M1/M3 chimera receptors we found that the carboxyl-terminal region of M3 was responsible for the generation of Ca2+ oscillations. The present results suggest that the oscillatory Ca2+ increase in response to M3 stimulation is dependent upon a moderate IP3 increase, which is suitable for causing Ca(2+)-dependent IP3-induced Ca2+ release. The C-terminal domain of M3 may contribute as a regulator of the efficiency of Gq and PLC cooperation.

Group 1 metabotropic glutamate receptor function and its regulation of learning and memory in the aging brain

Normal aging is generally characterized by a slow decline of cognitive abilities albeit with marked individual differences. Several animal models have been studied to explore the molecular and cellular mechanisms underlying this phenomenon. The excitatory neurotransmitter glutamate and its receptors have been closely linked to spatial learning and hippocampus-dependent memory processes. For decades, ionotropic glutamate receptors have been known to play a critical role in synaptic plasticity, a form of adaptation regulating memory formation. Over the past 10 years, several groups have shown the importance of group 1 metabotropic glutamate receptor (mGluR) in successful cognitive aging. These G-protein-coupled receptors are enriched in the hippocampal formation and interact physically with other proteins in the membrane including glutamate ionotropic receptors. Synaptic plasticity is crucial to maintain cognitive abilities and long-term depression (LTD) induced by group 1 mGluR activation, which has been linked to memory in the aging brain. The translation and synthesis of proteins by mGluR-LTD modulate ionotropic receptor trafficking and expression of immediate early genes related to cognition. Fragile X syndrome, a genetic form of autism characterized by memory deficits, has been associated to mGluR receptor malfunction and aberrant activation of its downstream signaling pathways. Dysfunction of mGluR could also be involved in neurodegenerative disorders like Alzheimer’s disease (AD). Indeed, beta-amyloid, the main component of insoluble senile plaques and one of the hallmarks of AD, occludes mGluR-dependent LTD leading to diminished functional synapses. This review highlights recent findings regarding mGluR signaling, related synaptic plasticity, and their potential involvement in normal aging and neurological disorders.

Computational model of neuron-astrocyte interactions during focal seizure generation

Empirical research in the last decade revealed that astrocytes can respond to neurotransmitters with Ca²⁺ elevations and generate feedback signals to neurons which modulate synaptic transmission and neuronal excitability. This discovery changed our basic understanding of brain function and provided new perspectives for how astrocytes can participate not only to information processing, but also to the genesis of brain disorders, such as epilepsy. Epilepsy is a neurological disorder characterized by recurrent seizures that can arise focally at restricted areas and propagate throughout the brain. Studies in brain slice models suggest that astrocytes contribute to epileptiform activity by increasing neuronal excitability through a Ca²⁺-dependent release of glutamate. The underlying mechanism remains, however, unclear. In this study, we implemented a parsimonious network model of neurons and astrocytes. The model consists of excitatory and inhibitory neurons described by Izhikevich's neuron dynamics. The experimentally observed Ca²⁺ change in astrocytes in response to neuronal activity was modeled with linear equations. We considered that glutamate is released from astrocytes above certain intracellular Ca²⁺ concentrations thus providing a non-linear positive feedback signal to neurons. Propagating seizure-like ictal discharges (IDs) were reliably evoked in our computational model by repeatedly exciting a small area of the network, which replicates experimental results in a slice model of focal ID in entorhinal cortex. We found that the threshold of focal ID generation was lowered when an excitatory feedback-loop between astrocytes and neurons was included. Simulations show that astrocytes can contribute to ID generation by directly affecting the excitatory/inhibitory balance of the neuronal network. Our model can be used to obtain mechanistic insights into the distinct contributions of the different signaling pathways to the generation and propagation of focal IDs.

Characterization of preptin-induced insulin secretion in pancreatic β-cells

We aimed to characterize the effects of preptin on insulin secretion at the single-cell level, as well as the mechanisms underlying these changes, with respect to regulation by intracellular Ca(2+) [Ca(2+)](i) mobilization. This study assessed the effect of preptin on insulin secretion and investigated the link between preptin and the phospholipase C (PLC)/protein kinase C (PKC) pathway at the cellular level using fura-2 pentakis(acetoxymethyl) ester-loaded insulin-producing cells (Min 6 cells). Our results demonstrate that preptin promotes insulin secretion in a concentration-dependent manner. Using a PLC inhibitor (chelerythrine) or a PKC inhibitor (U73122) resulted in a concentration-dependent decrease in insulin secretion. Also, preptin mixed with IGF2 receptor (IGF2R) antibodies suppressed insulin secretion in a dose-dependent manner, which indicates that activation of IGF2R is mediated probably because preptin is a type of proIGF2. In addition, preptin stimulated insulin secretion to a similar level as did glibenclamide. The activation of PKC/PLC by preptin stimulation is highly relevant to the potential mechanisms for increase in insulin secretion. Our results provide new insight into the insulin secretion of preptin, a secreted proIGF2-derived peptide that can induce greater efficacy of signal transduction resulting from PLC and PKC activation through the IGF2R.

Investigating metabotropic glutamate receptor 5 allosteric modulator cooperativity, affinity, and agonism: enriching structure-function studies and structure-activity relationships

Drug discovery programs increasingly are focusing on allosteric modulators as a means to modify the activity of G protein-coupled receptor (GPCR) targets. Allosteric binding sites are topographically distinct from the endogenous ligand (orthosteric) binding site, which allows for co-occupation of a single receptor with the endogenous ligand and an allosteric modulator that can alter receptor pharmacological characteristics. Negative allosteric modulators (NAMs) inhibit and positive allosteric modulators (PAMs) enhance the affinity and/or efficacy of orthosteric agonists. Established approaches for estimation of affinity and efficacy values for orthosteric ligands are not appropriate for allosteric modulators, and this presents challenges for fully understanding the actions of novel modulators of GPCRs. Metabotropic glutamate receptor 5 (mGlu(5)) is a family C GPCR for which a large array of allosteric modulators have been identified. We took advantage of the many tools for probing allosteric sites on mGlu(5) to validate an operational model of allosterism that allows quantitative estimation of modulator affinity and cooperativity values. Affinity estimates derived from functional assays fit well with affinities measured in radioligand binding experiments for both PAMs and NAMs with diverse chemical scaffolds and varying degrees of cooperativity. We observed modulation bias for PAMs when we compared mGlu(5)-mediated Ca(2+) mobilization and extracellular signal-regulated kinase 1/2 phosphorylation data. Furthermore, we used this model to quantify the effects of mutations that reduce binding or potentiation by PAMs. This model can be applied to PAM and NAM potency curves in combination with maximal fold-shift data to derive reliable estimates of modulator affinities.

Guidepost neurons for the lateral olfactory tract: expression of metabotropic glutamate receptor 1 and innervation by glutamatergic olfactory bulb axons

The guidepost neurons for the lateral olfactory tract, which are called lot cells, are the earliest-generated neurons in the neocortex. They migrate tangentially and ventrally further down this tract, and provide scaffolding for the olfactory bulb axons projecting into this pathway. The molecular profiles of the lot cells are largely uncharacterized. We found that lot cells specifically express metabotropic glutamate receptor subtype-1 at a very early stage of development. This receptor is functionally competent and responds to a metabotropic glutamate receptor agonist with a transient increase in the intracellular calcium ion concentration. When the glutamatergic olfactory bulb axons were electrically stimulated, lot cells responded to the stimulation with a calcium increase mainly via ionotropic glutamate receptors, suggesting potential neurotransmission between the axons and lot cells during early development. Together with the finding that lot cells themselves are glutamatergic excitatory neurons, our results provide another notable example of precocious interactions between the projecting axons and their intermediate targets.

Signalling routes and developmental regulation of group I metabotropic glutamate receptors in rat auditory midbrain neurons

Group I metabotropic glutamate receptors (mGluRs) are linked to intracellular Ca(2+) signalling and play important roles related to synaptic plasticity and development. In neurons from the central nucleus of the inferior colliculus (CIC), the activation of these receptors evokes large [Ca(2+) ](i) responses. By using optical imaging of the fluorescent Ca(2+) -sensitive dye Fura-2, we have explored which [Ca(2+) ](i) routes are triggered by group I mGluR activation in young CIC neurons and whether mGluR-induced [Ca(2+) ](i) responses are regulated during postnatal development. In addition, real-time quantitative RT-PCR was used to study the developmental expression of both group I mGluR subtypes, mGluR1 and mGluR5. Application of DHPG, a specific agonist of group I mGluRs, was used on CIC slices from young rats to elicit [Ca(2+) ](i) responses. A majority of responses consisted of an initial thapsigargin-sensitive Ca(2+) peak, related to store depletion, followed by a plateau phase, sensitive to the store-operated Ca(2+) entry blocker 2-APB. During postnatal development, from P6 to P17, DHPG-induced [Ca(2+) ](i) responses changed. The largest Ca(2+) responses were reached at P6, whereas lower peak and plateau responses were found after hearing onset, at P13-P14 and P17. qRT-PCR analysis also revealed important differences in the expression of both mGluR1 and mGluR5 subtypes during development, with the highest levels of both subtypes at P7 and a developmental decrease of both transcripts. Our results suggest both intra- and extracellular routes for [Ca(2+) ](i) increases linked to group I mGluRs in CIC neurons and a regulation of group I mGluR activity and expression during auditory development.

Receptor-selective diffusion barrier enhances sensitivity of astrocytic processes to metabotropic glutamate receptor stimulation

Metabotropic glutamate receptor (mGluR)-dependent calcium ion (Ca²+) signaling in astrocytic processes regulates synaptic transmission and local blood flow essential for brain function. However, because of difficulties in imaging astrocytic processes, the subcellular spatial organization of mGluR-dependent Ca²+ signaling is not well characterized and its regulatory mechanism remains unclear. Using genetically encoded Ca²+ indicators, we showed that despite global stimulation by an mGluR agonist, astrocyte processes intrinsically exhibited a marked enrichment of Ca²+ responses. Immunocytochemistry indicated that these polarized Ca²+ responses could be attributed to increased density of surface mGluR5 on processes relative to the soma. Single-particle tracking of surface mGluR5 dynamics revealed a membrane barrier that blocked the movement of mGluR5 between the processes and the soma. Overexpression of mGluR or expression of its carboxyl terminus enabled diffusion of mGluR5 between the soma and the processes, disrupting the polarization of mGluR5 and of mGluR-dependent Ca²+ signaling. Together, our results demonstrate an mGluR5-selective diffusion barrier between processes and soma that compartmentalized mGluR Ca²+ signaling in astrocytes and may allow control of synaptic and vascular activity in specific subcellular domains.

Defining protein kinase/phosphatase isoenzymic regulation of mGlu₅ receptor-stimulated phospholipase C and Ca²⁺ responses in astrocytes

BACKGROUND AND PURPOSE

Cyclical phosphorylation and dephosphorylation of a key residue within the C-terminal domain of the activated type 5 metabotropic glutamate (mGlu₅) receptor is believed to cause the synchronous, oscillatory changes in inositol 1,4,5-trisphosphate and Ca²⁺ levels observed in a variety of cell types. Here, we have attempted to better define the kinase and phosphatase enzymes involved in this modulation.

EXPERIMENTAL APPROACH

Ca²⁺ and [³H]inositol phosphate ([³H]IP(x) ) measurements in astrocyte preparations have been used to evaluate the effects of pharmacological inhibition of protein kinase C (PKC) and protein phosphatase activities and small interfering RNA-mediated specific PKC isoenzymic knock-down on mGlu₅ receptor signalling.

KEY RESULTS

Ca²⁺ oscillation frequency or [³H]IP(x) accumulation in astrocytes stimulated by mGlu₅ receptors, was concentration-dependently decreased by protein phosphatase-1/2A inhibition or by PKC activation. PKC inhibition also increased [³H]IP(x) accumulation two- to threefold and changed the Ca²⁺ response into a peak-plateau response. However, selective inhibition of conventional PKC isoenzymes or preventing changes in [Ca²⁺](i) concentration by BAPTA-AM loading was without effect on mGlu₅ receptor-stimulated [³H]IP(x) accumulation. Selective knock-down of PKCδ was without effect on glutamate-stimulated Ca²⁺ responses; however, selective PKCε knock-down in astrocytes changed Ca²⁺ responses from oscillatory into peak-plateau type.

CONCLUSION AND IMPLICATIONS

These data confirm the acute regulation of mGlu₅ receptor signalling by protein kinases and protein phosphatases and provide novel data pinpointing the isoenzymic dependence of this regulation in the native mGlu₅ receptor-expressing rat cortical astrocyte. These data also highlight a potential alternative mechanism by which mGlu₅ receptor signalling might be therapeutically manipulated.

P2X7 receptor-pannexin 1 hemichannel association: effect of extracellular calcium on membrane permeabilization

Activation of P2X(7) receptor (P2X(7)R) and pannexin have been implicated in membrane permeabilization associated with ischemic cell death and many other inflammatory processes. P2X(7)R has a unique property of forming large pore upon repeated or prolonged application of agonist like ATP or 2', 3'-(4-benzoyl) benzoyl ATP. It has been proposed that pannexin 1 (panx1) hemichannel associates with P2X(7)R to form large pore, though the actual mechanism is not yet understood. Calcium concentration in extracellular milieu drops in many patho-physiological conditions, e.g. ischemia, when P2X(7)R/pannexin is also known to be activated. Therefore, we hypothesize that extracellular calcium ([Ca(2+)](o)) plays an important role in the coupling of P2X(7)R-panx1 and subsequent membrane permeabilization. In this study we show that membrane permeability of the P2X(7)R and panx1 expressing N2A cell increases in ([Ca(2+)](o))-free solution. In [Ca(2+)](o)-free solution, fluorescent dye calcein trapped cells exhibited time-dependent dye leakage resulting in about 50% decrease of fluorescence intensity in 30 min. Control cells in 2 mM [Ca(2+)](o) did not show such leakage. Like N2A cells, mixed culture of neuron and glia, derived from hippocampal progenitor cells showed similar dye leakage. Dye leakage was blocked either by pannexin-specific blocker, carbenoxolone or P2X(7)R antagonists, Brilliant Blue G, and oxidized ATP. Furthermore P2X(7)R and panx1 were co-immunoprecipitated. The amount of P2X(7)R protein pulled-down with panx1, increased by twofold when cells were incubated 30 min in [Ca(2+)](o)-free buffer. Taken together, the results of this study demonstrate the activation and association of P2X(7)R-panx1, triggered by the removal of [Ca(2+)](o).

Expression of the metabotropic glutamate receptor 5 (mGluR5) induces melanoma in transgenic mice

Glutamate is the major excitatory neurotransmitter in the mammalian CNS and mediates fast synaptic transmission upon activation of glutamate-gated ion channels. In addition, glutamate modulates a variety of other synaptic responses and intracellular signaling by activating metabotropic glutamate receptors (mGluRs), which are G protein-coupled receptors. The mGluRs are also expressed in nonneuronal tissues and are implicated in a variety of normal biological functions as well as diseases. To study mGluR-activated calcium signaling in neurons, we generated mGluR5 transgenic animals using a Thy1 promoter to drive expression in the forebrain, and one founder unexpectedly developed melanoma. To directly investigate the role of mGluR5 in melanoma formation, we generated mGluR5 transgenic lines under a melanocyte-specific promoter, tyrosinase-related protein 1. A majority of the founders showed a severe phenotype with early onset. Hyperpigmentation of the pinnae and tail could be detected as early as 3-5 d after birth for most of the mGluR5 transgene-positive mice. There was 100% penetrance in the progeny from the tyrosinase-related protein 1-mGluR5 lines generated from founders that developed melanoma. Expression of mGluR5 was detected in melanoma samples by RT-PCR, immunoblotting, and immunohistochemistry. We evaluated the expression of several cancer-related proteins in tumor samples and observed a dramatic increase in the phosphorylation of ERK, implicating ERK as a downstream effector of mGluR5 signaling in tumors. Our findings show that mGluR5-mediated glutamatergic signaling can trigger melanoma in vivo. The aggressive growth and severe phenotype make these mouse lines unique and a potentially powerful tool for therapeutic studies.

Differential binding of calmodulin to group I metabotropic glutamate receptors regulates receptor trafficking and signaling

Metabotropic glutamate receptors (mGluRs) are G-protein-coupled receptors that modulate excitatory neurotransmission and synaptic plasticity. The group I mGluRs (mGluR1 and mGluR5) have long intracellular C-terminal domains, which interact with many proteins. Our previous studies identified calmodulin (CaM) as a strong regulator of mGluR5 trafficking and mGluR5-induced calcium signaling. Although it has been accepted that both mGluR1 and mGluR5 interact with CaM, we now show that CaM specifically binds mGluR5 and not mGluR1. We have identified a single critical residue in mGluR5 (L896) that is required for CaM binding. In mGluR1, mutation of the corresponding residue, V909, to leucine is sufficient to confer CaM binding to mGluR1. To investigate the functional effects of CaM binding, we examined the surface expression of mGluR1 and mGluR5 in hippocampal neurons. The mutation in mGluR1 (V909L) that confers CaM binding dramatically increases mGluR1 surface expression, whereas the analogous mutation in mGluR5 that disrupts CaM binding (L896V) decreases mGluR5 surface expression. In addition, the critical residue that alters CaM binding regulates mGluR internalization. Furthermore, we find that mGluR-mediated AMPA receptor endocytosis is enhanced by CaM binding to group I mGluRs. Finally, we show that calcium responses evoked by group I mGluRs are modulated by these mutations, which regulate CaM binding. Our findings elucidate a critical mechanism that specifically affects mGluR5 trafficking and signaling, and distinguishes mGluR1 and mGluR5 regulation.

A single amino acid residue controls Ca2+ signaling by an octopamine receptor from Drosophila melanogaster

Rhythmic activity of cells and cellular networks plays an important role in physiology. In the nervous system oscillations of electrical activity and/or second messenger concentrations are important to synchronize neuronal activity. At the molecular level, rhythmic activity can be initiated by different routes. We have recently shown that an octopamine-activated G-protein-coupled receptor (GPCR; DmOctα1Rb, CG3856) from Drosophila initiates Ca(2+) oscillations. Here, we have unraveled the molecular basis of cellular Ca(2+) signaling controlled by the DmOctα1Rb receptor using a combination of pharmacological intervention, site-directed mutagenesis, and functional cellular Ca(2+) imaging on heterologously expressed receptors. Phosphorylation of a single amino acid residue in the third intracellular loop of the GPCR by PKC is necessary and sufficient to desensitize the receptor. From its desensitized state, DmOctα1Rb is resensitized by dephosphorylation, and a new Ca(2+) signal occurs on octopamine stimulation. Our findings show that transient changes of the receptor's surface profile have a strong effect on its physiological signaling properties. We expect that the detailed knowledge of DmOctα1Rb-dependent signal transduction fosters the identification of specific drugs that can be used for GPCR-mediated pest control, since octopamine serves important physiological and behavioral functions in arthropods.

Post-translational modification biology of glutamate receptors and drug addiction

Post-translational covalent modifications of glutamate receptors remain a hot topic. Early studies have established that this family of receptors, including almost all ionotropic and metabotropic glutamate receptor subtypes, undergoes active phosphorylation at serine, threonine, or tyrosine residues in their intracellular domains. Recent evidence identifies several glutamate receptor subtypes to be direct substrates for palmitoylation at cysteine residues. Other modifications such as ubiquitination and sumoylation at lysine residues also occur to certain glutamate receptors. These modifications are dynamic and reversible in nature and are regulatable by changing synaptic inputs. The regulated modifications significantly impact the receptor in many ways, including interrelated changes in biochemistry (synthesis, subunit assembling, and protein–protein interactions), subcellular redistribution (trafficking, endocytosis, synaptic delivery, and clustering), and physiology, usually associated with changes in synaptic plasticity. Glutamate receptors are enriched in the striatum and cooperate closely with dopamine to regulate striatal signaling. Emerging evidence shows that modification processes of striatal glutamate receptors are sensitive to addictive drugs, such as psychostimulants (cocaine and amphetamine). Altered modifications are believed to be directly linked to enduring receptor/synaptic plasticity and drug-seeking. This review summarizes several major types of modifications of glutamate receptors and analyzes the role of these modifications in striatal signaling and in the pathogenesis of psychostimulant addiction.

Quantitative analysis reveals multiple mechanisms of allosteric modulation of the mGlu5 receptor in rat astroglia

Positive and negative allosteric modulators (PAMs and NAMs, respectively) of the type 5 metabotropic glutamate (mGlu5) receptor have demonstrable therapeutic potential in an array of neurological and psychiatric disorders. Here, we have used rat cortical astrocytes to investigate how PAMs and NAMs mediate their activity and reveal marked differences between PAMs with respect to their modulation of orthosteric agonist affinity and efficacy. Affinity cooperativity factors (α) were assessed using [(3)H]2-methyl-6-(phenylethynyl)-pyridine (MPEP)-PAM competition binding in the absence and presence of orthosteric agonist, whereas efficacy cooperativity factors (β) were calculated from net affinity/efficacy cooperativity parameters (αβ) obtained from analyses of the abilities of PAMs to potentiate [(3)H]inositol phosphate accumulation in astrocytes stimulated with a submaximal (EC(20)) concentration of orthosteric agonist. We report that whereas 3,3'-difluorobenzaldazine (DFB) and 3-cyano-N-(1,3-diphenyl-1H-prazol-5-yl)benzamide (CDPPB) primarily exert their allosteric modulatory effects through modifying the apparent orthosteric agonist affinity at the astrocyte mGlu5 receptor, the effects of S-(4-fluoro-phenyl)-{3-[3-(4-fluoro-phenyl)-[1,2,4]oxadiazol-5-yl]-piperidinl-1-yl}-methanone (ADX47273) are mediated primarily via efficacy-driven modulation. In [(3)H]MPEP-NAM competition binding assays, both MPEP and 2-(2-(3-methoxyphenyl)ethynyl)-5-methylpyridine (M-5MPEP) defined similar specific binding components, with affinities that were unaltered in the presence of orthosteric agonist, indicating wholly negative efficacy-driven modulations. It is noteworthy that whereas M-5MPEP only partially inhibited orthosteric agonist-stimulated [(3)H]inositol phosphate accumulation in astrocytes, it could completely suppress Ca(2+) oscillations stimulated by quisqualate or (S)-3,5-dihydroxyphenylglycine. In contrast, MPEP was fully inhibitory with respect to both functional responses. The finding that M-5MPEP has different functional effects depending on the endpoint measured is discussed as a possible example of permissive allosteric antagonism.

Parasagittally aligned, mGluR1-dependent patches are evoked at long latencies by parallel fiber stimulation in the mouse cerebellar cortex in vivo

The parallel fibers (PFs) in the cerebellar cortex extend several millimeters along a folium in the mediolateral direction. The PFs are orthogonal to and cross several parasagittal zones defined by the olivocerebellar and corticonuclear pathways and the expression of molecular markers on Purkinje cells (PCs). The functions of these two organizations remain unclear, including whether the bands respond similarly or differentially to PF input. By using flavoprotein imaging in the anesthetized mouse in vivo, this study demonstrates that high-frequency PF stimulation, which activates a beamlike response at short latency, also evokes patches of activation at long latencies. These patches consist of increased fluorescence along the beam at latencies of 20-25 s with peak activation at 35 s. The long-latency patches are completely blocked by the type 1 metabotropic glutamate receptor (mGluR(1)) antagonist LY367385. Conversely, the AMPA and NMDA glutamate receptor antagonists DNQX and APV have little effect. Organized in parasagittal bands, the long-latency patches align with zebrin II-positive PC stripes. Additional Ca(2+) imaging demonstrates that the patches reflect increases in intracellular Ca(2+). Both the PLCβ inhibitor U73122 and the ryanodine receptor inhibitor ryanodine completely block the long-latency patches, indicating that the patches are due to Ca(2+) release from intracellular stores. Robust, mGluR(1)-dependent long-term potentiation (LTP) of the patches is induced using a high-frequency PF stimulation conditioning paradigm that generates LTP of PF-PC synapses. Therefore, the parasagittal bands, as defined by the molecular compartmentalization of PCs, respond differentially to PF inputs via mGluR(1)-mediated release of internal Ca(2+).

Metabotropic glutamate receptors: from the workbench to the bedside

Metabotropic glutamate (mGlu) receptors were discovered in the mid 1980s and originally described as glutamate receptors coupled to polyphosphoinositide hydrolysis. Almost 6500 articles have been published since then, and subtype-selective mGlu receptor ligands are now under clinical development for the treatment of a variety of disorders such as Fragile-X syndrome, schizophrenia, Parkinson's disease and L-DOPA-induced dyskinesias, generalized anxiety disorder, chronic pain, and gastroesophageal reflux disorder. Prof. Erminio Costa was linked to the early times of the mGlu receptor history, when a few research groups challenged the general belief that glutamate could only activate ionotropic receptors and all metabolic responses to glutamate were secondary to calcium entry. This review moves from those nostalgic times to the most recent advances in the physiology and pharmacology of mGlu receptors, and highlights the role of individual mGlu receptor subtypes in the pathophysiology of human disorders. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Decoding cytosolic Ca2+ oscillations

A rise in cytosolic Ca(2+) concentration is used as a universal signalling mechanism to control biological processes as diverse as exocytosis, contraction, cell growth and cell death. Ca(2+) signals are often presented to cells in the form of Ca(2+) oscillations, with signalling information encoded in both amplitude and frequency of the Ca(2+) spikes. Recent studies have revealed that the sub-cellular spatial profile of the Ca(2+) oscillation is also important in activating cellular responses, thereby suggesting a new mechanism for extracting information from the ubiquitous Ca(2+) oscillation.

Increased receptor stimulation elicits differential calcium-sensing receptor(T888) dephosphorylation

The calcium-sensing receptor (CaR) elicits oscillatory Ca(2+)(i) mobilization associated with dynamic, inhibitory protein kinase C-mediated phosphorylation of CaR(T888). While modest CaR stimulation elicits Ca(2+)(i) oscillations, greater stimulation either increases oscillation frequency or elicits sustained responses by an unknown mechanism. Here, moderate CaR stimulation (2.5 mm Ca(2+)(o), 10 min) increased CaR(T888) phosphorylation (160-kDa mature receptor) 5-fold in CaR stably transfected HEK-293 cells, whereas 3-5 mm Ca(2+)(o) treatments were without apparent effect. Treatment with 2 mm Ca(2+)(o) caused sustained CaR(T888) phosphorylation (> or = 20 min) and oscillatory Ca(2+)(i) mobilization. However, 5 mm Ca(2+)(o) increased CaR(T888) phosphorylation only briefly while eliciting sustained Ca(2+)(i) mobilization, suggesting that greater CaR activation induces rapid CaR(T888) dephosphorylation, thus permitting sustained Ca(2+)(i) responses. Indeed, 5 mm Ca(2+)(o) stimulated protein phosphatase 2A activity and induced CaR(T888) dephosphorylation following acute phorbol ester pretreatment, the latter effect being mimicked by CaR-positive allosteric modulators (NPS-R467 and l-Phe). Finally, the phosphatase inhibitor calyculin-A reversed CaR-induced inhibition of parathyroid hormone secretion from bovine parathyroid slices and normal human parathyroid cells, demonstrating the physiological importance of phosphorylation status on parathyroid function. Therefore, high Ca(2+)(o)-stimulated protein kinase C acts in concert with high Ca(2+)(o)-induced phosphatase activity to generate and maintain CaR-induced Ca(2+)(i) oscillations via the dynamic phosphorylation and dephosphorylation of CaR(T888).

Norbin is an endogenous regulator of metabotropic glutamate receptor 5 signaling

Metabotropic glutamate receptor 5 (mGluR5) is highly expressed in the mammalian central nervous system (CNS). It is involved in multiple physiological functions and is a target for treatment of various CNS disorders, including schizophrenia. We report that Norbin, a neuron-specific protein, physically interacts with mGluR5 in vivo, increases the cell surface localization of the receptor, and positively regulates mGluR5 signaling. Genetic deletion of Norbin attenuates mGluR5-dependent stable changes in synaptic function measured as long-term depression or long-term potentiation of synaptic transmission in the hippocampus. As with mGluR5 knockout mice or mice treated with mGluR5-selective antagonists, Norbin knockout mice showed a behavioral phenotype associated with a rodent model of schizophrenia, as indexed by alterations both in sensorimotor gating and psychotomimetic-induced locomotor activity.

Effects of positive allosteric modulators on single-cell oscillatory Ca2+ signaling initiated by the type 5 metabotropic glutamate receptor

Agonist stimulation of the type 5 metabotropic glutamate (mGlu5) receptor initiates robust oscillatory changes in cytosolic Ca2+ concentration ([Ca2+]i) in single cells by rapid, repeated cycles of phosphorylation/dephosphorylation of the mGlu5 receptor, involving protein kinase C and as-yet-unspecified protein phosphatase activities. An emergent property of this type of Ca2+ oscillation-generating mechanism (termed "dynamic uncoupling") is that once a threshold concentration has been reached to initiate the Ca2+ oscillation, its frequency is largely insensitive to further increases in orthosteric agonist concentration. Here, we report the effects of positive allosteric modulators (PAMs) on the patterns of single-cell Ca2+ signaling in recombinant and native mGlu5 receptor-expressing systems. In a Chinese hamster ovary cell-line (CHO-lac-mGlu5a), none of the mGlu5 receptor PAMs studied [3,3'-difluorobenzaldazine (DFB), N-{4-chloro-2-[(1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl) methyl]phenyl}-2-hydroxy-benzamide (CPPHA), 3-cyano-N-(1, 3-diphenyl-1H-prazol-5-yl)benzamide (CDPPB), S-(4-fluoro-phenyl)-{3-[3-(4-fluoro-phenyl)-[1,2,4]oxadiazol-5-yl]-piperidinl-1-yl}-methanone (ADX47273)], stimulated a Ca2+ response when applied alone, but each PAM concentration-dependently increased the frequency, without affecting the amplitude, of Ca2+ oscillations induced by glutamate or quisqualate. Therefore, PAMs can cause graded increases (and negative allosteric modulator-graded decreases) in the Ca2+ oscillation frequency stimulated by orthosteric agonist. Initial data in rat cerebrocortical astrocytes demonstrated that similar effects of PAMs could be observed in a native cell background, although at high orthosteric agonist concentrations, PAM addition could much more often be seen to drive rapid Ca2+ oscillations into peak-plateau responses. These data demonstrate that allosteric modulators can "tune" the Ca2+ oscillation frequency initiated by mGlu5 receptor activation, and this might allow pharmacological modification of the downstream processes (e.g., transcriptional regulation) that is unachievable through orthosteric ligand interactions.

Activation of metabotropic glutamate receptors induces periodic burst firing and concomitant cytosolic Ca2+ oscillations in cerebellar interneurons

Little is known about the generation of slow rhythms in brain neuronal circuits. Nevertheless, a few studies, both from reconstituted systems and from hippocampal slices, indicate that activation of metabotropic glutamate receptors (mGluRs) could generate such rhythms. Here we show in rat cerebellar slices that after either release of glutamate by repetitive stimulation, or direct stimulation of type 1 mGluRs, molecular layer interneurons exhibit repetitive slow Ca(2+) transients. By combining cell-attached patch-clamp recording with Ca(2+) imaging, we show that the regular Ca(2+) transients (mean frequency, 35 mHz induced by 2 microm quisqualate in the presence of ionotropic glutamate receptor blockers) are locked with bursts of action potentials. Nevertheless, the Ca(2+) transients are not blocked by tetrodotoxin, indicating that firing is not necessary to entrain oscillations. The first Ca(2+) transient within a train is different in several ways from subsequent transients. It is broader than the subsequent transients, displays a different phase relationship to associated spike bursts, and exhibits a distinct sensitivity to ionic and pharmacological manipulations. Whereas the first transient appears to involve entry of Ca(2+) ions through transient receptor potential channel-like channels and secondarily activated L-type Ca(2+) channels, subsequent transients rely mostly on an exchange of Ca(2+) ions between the cytosol and D-myo-inositol-1,4,5-triphosphate-sensitive intracellular Ca(2+) stores. The slow, highly regular oscillations observed in the present work are likely to drive pauses in postsynaptic Purkinje cells, and could play a role in coordinating slow oscillations involving the cerebello-olivar circuit loop.

Glutamate regulation of calcium and IP3 oscillating and pulsating dynamics in astrocytes

Recent years have witnessed an increasing interest in neuron-glia communication. This interest stems from the realization that glia participate in cognitive functions and information processing and are involved in many brain disorders and neurodegenerative diseases. An important process in neuron-glia communications is astrocyte encoding of synaptic information transfer-the modulation of intracellular calcium (Ca(2+)) dynamics in astrocytes in response to synaptic activity. Here, we derive and investigate a concise mathematical model for glutamate-induced astrocytic intracellular Ca(2+) dynamics that captures the essential biochemical features of the regulatory pathway of inositol 1,4,5-trisphosphate (IP(3)). Starting from the well-known two-variable (intracellular Ca(2+) and inactive IP(3) receptors) Li-Rinzel model for calcium-induced calcium release, we incorporate the regulation of IP(3) production and phosphorylation. Doing so, we extend it to a three-variable model (which we refer to as the ChI model) that could account for Ca(2+) oscillations with endogenous IP(3) metabolism. This ChI model is then further extended into the G-ChI model to include regulation of IP(3) production by external glutamate signals. Compared with previous similar models, our three-variable models include a more realistic description of IP(3) production and degradation pathways, lumping together their essential nonlinearities within a concise formulation. Using bifurcation analysis and time simulations, we demonstrate the existence of new putative dynamical features. The cross-couplings between IP(3) and Ca(2+) pathways endow the system with self-consistent oscillatory properties and favor mixed frequency-amplitude encoding modes over pure amplitude-modulation ones. These and additional results of our model are in general agreement with available experimental data and may have important implications for the role of astrocytes in the synaptic transfer of information.

Phosphorylation of the homer-binding domain of group I metabotropic glutamate receptors by cyclin-dependent kinase 5

Phosphorylation of neurotransmitter receptors can modify their activity and regulate neuronal excitability. Cyclin-dependent kinase 5 (cdk5) is a proline-directed serine/threonine kinase involved not only in neuronal development, but also in synaptic function and plasticity. Here we demonstrate that group I metabotropic glutamate receptors (mGluRs), which modulate post-synaptic signaling by coupling to intracellular signal transduction pathways, are phosphorylated by cdk5. In vitro kinase assays reveal that cdk5 phosphorylates mGluR5 within the domain of the receptor that interacts with the scaffolding protein homer. Using a novel phosphospecific mGluR antibody, we show that the homer-binding domain of both mGluR1 and mGluR5 are phosphorylated in vivo, and that inhibition of cdk5 with siRNA decreases the amount of phosphorylated receptor. Furthermore, kinetic binding analysis, by surface plasmon resonance, indicates that phosphorylation of mGluR5 enhances its association with homer. Homer protein complexes in the post-synaptic density, and their disruption by an activity-dependent short homer 1a isoform, have been shown to regulate the trafficking and signaling of the mGluRs and impact many neuroadaptive processes. Phosphorylation of the mGluR homer-binding domain, in contrast to homer 1a induction, provides a novel mechanism for potentially regulating a subset of homer interactions.

Identification of potential pharmacological targets by analysis of the comprehensive G protein-coupled receptor repertoire in the four cardiac chambers

Cardiac function is regulated by many hormones and neurotransmitters that exert their physiological effects through the activation of G protein-coupled receptors (GPCRs). Identification of new GPCRs that might display a specific pattern of expression within the heart and differentially regulate specific cardiac functions represents an important issue for the development of new drugs. Indeed, highly targeted receptors represent only a small percentage of known GPCRs. Here, we quantified the expression of 395 endoGPCRs (all GPCRs excluding taste and odorant receptors) in male mouse right and left atria and ventricles by using high-throughput real-time reverse-transcriptase polymerase chain reaction (RT-PCR) and focused on the 135 most highly expressed transcripts. No cardiac functional data are available for almost half of these receptors; therefore, linking GPCR expression patterns to cardiac function has allowed us to provide new insights into the possible function of some of these receptors. Indeed, ventricles and atria are both contractile; however, the latter, and especially the right atrium, are central to the generation and regulation of cardiac rhythm. Accordingly, the right atrium exhibited the most specific signature, whereas the majority of GPCRs found in ventricles were evenly expressed in both the right and left chambers. RT-PCR data were confirmed at the protein level for six selected transcripts. Furthermore, we provide new data showing that, as suggested by our repertoire, the metabotropic glutamate receptor 1b is expressed and is functional in ventricular cardiac myocytes. This is the first report describing GPCRs in the four cardiac chambers and may assist in the identification of therapeutic targets.

Ciglitazone induces apoptosis via activation of p38 MAPK and AIF nuclear translocation mediated by reactive oxygen species and Ca(2+) in opossum kidney cells

We have previously demonstrated that the synthetic peroxisome proliferator-activated receptor-gamma (PPARgamma) agonist ciglitazone induces apoptosis accompanied by activation of p38 mitogen-activated protein kinase (MAPK) and nuclear translocation of apoptosis inducing factor (AIF) in opossum kidney (OK) renal epithelial cells. However, the precise mechanism by which ciglitazone induces activation of p38 MAPK and the role of AIF in the induction of the apoptosis are not defined. This study was therefore undertaken to determine whether the roles of reactive oxygen species (ROS) generation and intracellular Ca(2+) in the ciglitazone-induced activation of p38 MAPK and whether AIF nuclear translocation is responsible for the ciglitazone-induced apoptosis in OK renal epithelial cells. Ciglitazone caused generation of ROS and an increase in intracellular Ca(2+). Ciglitazone-induced cell death was reduced by the antioxidant Trolox, the Ca(2+) chelator EGTA, and the store-operated Ca(2+) channels (SOCC) blocker lanthanum chloride (La(3+)), indicating involvement of ROS and Ca(2+) in the ciglitazone-induced cell death. Ciglitazone-induced intracellular Ca(2+) increase was decreased by Trolox, while ROS generation was not affected by EGTA and La(3+), suggesting that ROS generation promote the increase of intracellular Ca(2+). Transfection of small interfering RNA (siRNA) of p38 MAPK or vector expressing microRNA (miRNA) of AIF prevented the ciglitazone-induced cell death. Activation of p38 MAPK, mitochondrial membrane depolarization, and AIF nuclear translocation induced by ciglitazone were inhibited by Trolox, EGTA and La(3+). Taken together, these results suggest that ROS-dependent intracellular Ca(2+) increase is responsible for activation of p38 MAPK and nuclear translocation of AIF by ciglitazone.

Calmodulin dynamically regulates the trafficking of the metabotropic glutamate receptor mGluR5

Metabotropic glutamate receptors (mGluRs) 1-8 are G protein-coupled receptors (GPCRs) that modulate excitatory neurotransmission, neurotransmitter release, and synaptic plasticity. PKC regulates many aspects of mGluR function, including protein-protein interactions, Ca(2+) signaling, and receptor desensitization. However, the mechanisms by which PKC regulates mGluR function are poorly understood. We have now identified calmodulin (CaM) as a dynamic regulator of mGluR5 trafficking. We show that the major PKC phosphorylation site on the intracellular C terminus of mGluR5 is serine 901 (S901), and phosphorylation of this residue is up-regulated in response to both receptor and PKC activation. In addition, S901 phosphorylation inhibits mGluR5 binding to CaM, decreasing mGluR5 surface expression. Furthermore, blocking PKC phosphorylation of mGluR5 on S901 dramatically affects mGluR5 signaling by prolonging Ca(2+) oscillations. Thus, our data demonstrate that mGluR5 activation triggers phosphorylation of S901, thereby directly linking PKC phosphorylation, CaM binding, receptor trafficking, and downstream signaling.

Group I metabotropic glutamate receptors control proliferation, survival and differentiation of cultured neural progenitor cells isolated from the subventricular zone of adult mice

Neural progenitor cells (NPCs) are found in the subventricular zone (SVZ) of the adult brain, a specialized neurogenic niche that might provide a substrate for brain repair after injury. The incomplete knowledge of how NPCs in the niche respond to local signals limits the use of cultured NPCs in the development of cell transplantation strategies. We show that neurospheres obtained from the SVZ of the adult mouse expressed functional mGlu1 and mGlu5 metabotropic glutamate receptors. Pharmacological blockade of mGlu5 receptors promoted the apoptotic death of progenitors undergoing differentiation into neurons (PSA/NCAM+ cells for the most part), whereas blockade of mGlu1 receptors reduced the proliferation rate of NPCs, and promoted their differentiation towards the neuronal lineage. We conclude that endogenous activation of mGlu5 receptors might support specifically the survival of neuronal-restricted precursors, whereas endogenous activation of mGlu1 receptors might sustain the proliferation of earlier progenitors. Moreover, mGlu1 receptor antagonists increased the survival of NPCs, suggesting that endogenously activated mGlu1 receptors might play a role in the natural cell loss regulating the number or the type of progenitors.

Phosphorylation of group I metabotropic glutamate receptors (mGluR1/5) in vitro and in vivo

Group I metabotropic glutamate receptors (mGluR1 and mGluR5 subtypes) are densely expressed in mammalian brain. They are actively involved in the regulation of normal cellular activity and synaptic plasticity, and are frequently linked to the pathogenesis of various mental illnesses. Like ionotropic glutamate receptors, group I mGluRs are subject to the regulation by protein phosphorylation. Accumulative data demonstrate sufficient phosphorylation of the intracellular mGluR1/5 domains at specific serine/threonine sites by protein kinase C in heterologous cells or neurons, which serves as an important mechanism for regulating the receptor signaling and desensitization. Emerging evidence also shows the significant involvements of G protein-coupled receptor kinases, Ca2+/calmodulin-dependent protein kinase II, tyrosine kinases, and protein phosphatases in controlling the phosphorylation status of group I mGluRs. This review analyzes the recent data concerning group I mGluR phosphorylation and the phosphorylation-dependent regulation of group I mGluR function. Future research directions in this area with newly available high throughput and proteomic approaches are also discussed in the end.

Essential role for epidermal growth factor receptor in glutamate receptor signaling to NF-kappaB

Glutamate is a critical neurotransmitter of the central nervous system (CNS) and also an important regulator of cell survival and proliferation. The binding of glutamate to metabotropic glutamate receptors induces signal transduction cascades that lead to gene-specific transcription. The transcription factor NF-kappaB, which regulates cell proliferation and survival, is activated by glutamate; however, the glutamate receptor-induced signaling pathways that lead to this activation are not clearly defined. Here we investigate the glutamate-induced activation of NF-kappaB in glial cells of the CNS, including primary astrocytes. We show that glutamate induces phosphorylation, nuclear accumulation, DNA binding, and transcriptional activation function of glial p65. The glutamate-induced activation of NF-kappaB requires calcium-dependent IkappaB kinase alpha (IKKalpha) and IKKbeta activation and induces p65-IkappaBalpha dissociation in the absence of IkappaBalpha phosphorylation or degradation. Moreover, glutamate-induced IKK preferentially targets the phosphorylation of p65 but not IkappaBalpha. Finally, we show that the ability of glutamate to activate NF-kappaB requires cross-coupled signaling with the epidermal growth factor receptor. Our results provide insight into a glutamate-induced regulatory pathway distinct from that described for cytokine-induced NF-kappaB activation and have important implications with regard to both normal glial cell physiology and pathogenesis.

Inhibition of mGluR1 and IP3Rs impairs long-term memory formation in young chicks

Calcium (Ca(2+)) is involved in a myriad of cellular functions in the brain including synaptic plasticity. However, the role of intracellular Ca(2+) stores in memory processing remains poorly defined. The current study explored a role for glutamate-dependent intracellular Ca(2+) release in memory processing via blockade of metabotropic glutamate receptor subtype 1 (mGluR1) and inositol (1,4,5)-trisphosphate receptors (IP(3)Rs). Using a single-trial discrimination avoidance task developed for the young chick, administration of the specific and potent mGluR1 antagonist JNJ16259685 (500nM, immediately post-training, ic), or the IP(3)R antagonist Xestospongin C (5microM, immediately post-training, ic), impaired retention from 90min post-training. These findings are consistent with mGluR1 activating IP(3)Rs to release intracellular Ca(2+) required for long-term memory formation and have been interpreted within an LTP2 model. The consequences of different patterns of retention loss following ryanodine receptor (RyR) and IP(3)R inhibition are discussed.

Effects of subtype-selective group I mGluR antagonists on synchronous activity induced by 4-aminopyridine/CGP 55845 in adult guinea pig hippocampal slices

Co-application of the convulsant 4-aminopyridine (4-AP) and the GABA(B) receptor antagonist CGP 55845 to adult guinea pig hippocampal slices elicits giant GABA-mediated postsynaptic potentials (GPSPs) and epileptiform discharges. Here we tested the effects of the group I metabotropic glutamate receptor (mGluR) subtype-selective antagonists LY 367385 (mGlu1, 100 microM), MPEP (mGlu5, 10 microM), and MTEP (mGlu5, 500 nM) on this synchronous activity. Electrophysiological field recordings were performed in the CA3 region of hippocampal slices from adult guinea pigs. The mGlu5 receptor antagonists increased GPSP rate, but the mGlu1 receptor antagonist did not. This ability of mGlu5 receptor antagonists to increase the rate of GPSPs indicates that enough endogenous glutamate is released under these conditions to activate group I mGluR; nevertheless, co-application of a mGlu1 receptor antagonist (LY 367385 or JNJ 16259685) and MPEP did not decrease pre-existing epileptiform activity. Furthermore, co-application of LY 367,385 and MPEP did not prevent the emergence of epileptiform activity. When ionotropic glutamate receptor (iGluR) antagonists were present, neither MPEP nor the group I mGluR agonist DHPG changed GPSP rate, suggesting that pyramidal cell-to-interneuron iGluR-mediated synaptic connections are involved in the rate change mechanism. In contrast to the lack of effect of group I mGluR antagonists on epileptiform activity in the 4-AP/CGP 55845 model, group I mGluR antagonists blocked the emergence of longer epileptiform events and decreased the overall amount of synchronous activity in the GABA(A) antagonist/4-AP model. In conclusion, in the 4-AP/CGP 55845 model, enough glutamate was released to activate group I mGluRs and affect GPSP rate via mGlu5 receptors; however, this group I mGluR activation was not required for the generation of the epileptiform activity.

Metabotropic glutamate receptors: phosphorylation and receptor signaling

Metabotropic glutamate receptors (mGluRs) play important roles in neurotransmission, neuronal development, synaptic plasticity, and neurological disorders. Recent studies have revealed a sophisticated interplay between mGluRs and protein kinases: activation of mGluRs regulates the activity of a number of kinases, and direct phosphorylation of mGluRs affects receptor signaling, trafficking, and desensitization. Here we review the emerging literature on mGluR phosphorylation, signaling, and synaptic function.

Group I metabotropic glutamate receptors: a role in neurodevelopmental disorders?

Group I metabotropic glutamate receptors (mGlu1 and mGlu5) are coupled to polyphosphoinositide hydrolysis and are involved in activity-dependent forms of synaptic plasticity, both during development and in the adult life. Group I mGlu receptors can also regulate proliferation, differentiation, and survival of neural stem/progenitor cells, which further support their role in brain development. An exaggerated response to activation of mGlu5 receptors may underlie synaptic dysfunction in Fragile X syndrome, the most common inherited form of mental retardation. In addition, group I mGlu receptors are overexpressed in dysplastic neurons of focal cortical dysplasia and hemimegaloencephaly, which are disorders of cortical development associated with chronic epilepsy. Drugs that block the activity of group I mGlu receptors (in particular, mGlu5 receptors) are potentially helpful for the treatment of Fragile X syndrome and perhaps other neurodevelopmental disorders.

Group II metabotropic glutamate receptors in anxiety circuitry: correspondence of physiological response and subcellular distribution

Activation of group II metabotropic glutamate receptors (mGluR2/3) in the amygdala plays a critical role in the regulation of fear and anxiety states. Previous studies using nonselective agonists have suggested this action can result from activation of either pre- or postsynaptic mGluR2/3. Here, we have used a combination of whole-cell patch clamp recording with highly selective agonists (LY354740 and LY379268) and immunoelectron microscopy to examine structure-function relationships for mGluR2/3 in the basolateral amygdala (BLA) and bed nucleus of the stria terminalis (BNST). Stimulation of mGluR2/3 evoked a direct, TTX-insensitive membrane hyperpolarization in all BLA projection neurons tested, but only about half of BNST neurons. The membrane hyperpolarization was mediated by activation of an outward potassium current or blockade of a tonically active inward I(h) current in different groups of BLA neurons. In both regions, mGluR2/3 caused a long-lasting reduction of glutamate release from presynaptic afferent terminals even at concentrations that failed to elicit a direct postsynaptic response. The localization of mGluR2/3 differed regionally, with postsynaptic labeling significantly more common in BLA than BNST, corresponding to the strength of postsynaptic responses recorded there. Our results demonstrate a complex role for mGluR2/3 receptors in modulating anxiety circuitry, including direct inhibition and reduction of excitatory drive. The combination of direct inhibition of projection neurons within the BLA and suppression of excitatory neurotransmission in the BNST may be responsible for the anxiolytic actions of group II mGluR agonists.

Neuroprotective effects of the selective type 1 metabotropic glutamate receptor antagonist YM-202074 in rat stroke models

We describe in vitro properties and in vivo neuroprotective effects of a newly synthesized, high-affinity, selective allosteric metabotropic glutamate receptor type 1 (mGluR(1)) antagonist, N-cyclohexyl-6-{[(2-methoxyethyl)(methyl)amino]methyl}-N-methylthiazolo[3,2-a]benzimidazole-2-carboxamide (YM-202074). YM-202074 bound an allosteric site of rat mGluR(1) with a K(i) value of 4.8+/-0.37 nM. YM-202074 also inhibited the mGluR(1)-mediated inositol phosphates production in rat cerebellar granule cells with an IC(50) value of 8.6+/-0.9 nM, while showing selectivity over mGluR(2-7). When YM-202074 was infused intravenously at an initial dose of 20 mg/kg/h for 0.5 h followed by a dose of 5 mg/kg/h for 7.5 h, the free concentration of YM-202074 in the brain rapidly (<12 min) reached approximately 0.3 microM, reaching a steady-state phase within 1.5 h. We first treated rats such that they developed transient middle cerebral artery (MCA) occlusion. Results clearly demonstrate a dose-dependent improvement of neurological deficit and reduction of the infarct volume in both the hemisphere and cortex when YM-202074 was infused intravenously immediately after occlusion at a dose of 10 or 20 mg/kg/h for 0.5 h followed by a dose of 2.5 or 5 mg/kg/h for 23.5 h, respectively. Significant neuroprotection was maintained even when the administration of drugs was delayed by up to 2 h following the onset of ischemia. Furthermore, the improvement of neurological deficit and the reduction of infarct volume were sustained for 1 week following the onset of ischemia. These results suggest that YM-202074 exhibits great potential as a novel neuroprotective agent for the treatment of stroke.

Antinociceptive profile of a selective metabotropic glutamate receptor 1 antagonist YM-230888 in chronic pain rodent models

Metabotropic glutamate receptor 1 (mGlu(1) receptor) has been suggested to play an important role in pain transmission. In this study, the effects of a newly-synthesized mGlu(1) receptor antagonist, (R)-N-cycloheptyl-6-({[(tetrahydro-2-furyl)methyl]amino}methyl)thieno[2,3-d]pyrimidin-4-ylamine (YM-230888), were examined in a variety of rodent chronic pain models in order to characterize the potential analgesic profile of mGlu(1) receptor blockade. YM-230888 bound an allosteric site of mGlu(1) receptor with a K(i) value of 13+/-2.5 nM and inhibited mGlu(1)-mediated inositol phosphate production in rat cerebellar granule cells with an IC(50) value of 13+/-2.4 nM. It showed selectivity for mGlu(1) versus mGlu(2)-mGlu(7) subtypes and ionotropic glutamate receptors. YM-230888 recovered mechanical allodynia with an ED(50) value of 8.4 mg/kg p.o. in L5/L6 spinal nerve ligation models. It also showed antinociceptive response at doses of 10 and 30 mg/kg p.o. in streptozotocin-induced hyperalgesia models. In addition, it significantly reduced pain parameters at a dose of 30 mg/kg p.o. in complete Freund's adjuvant-induced arthritic pain models. Although YM-230888 showed no significant effect on rotarod performance time at doses of 10 or 30 mg/kg p.o., it significantly decreased it at a dose of 100 mg/kg p.o. On the other hand, YM-230888 showed no significant sedative effect in locomotor activity measurement up to 100 mg/kg p.o. These results suggest that the blockade of mGlu(1) receptors is an attractive target for analgesics. YM-230888 has potential as a new analgesic agent for the treatment of various chronic pain conditions. In addition, YM-230888 may be a useful tool for the investigation of mGlu(1) receptors.

Metabotropic glutamate receptors in stem/progenitor cells

Functional mGlu receptor subtypes are found in stem/progenitor cells, and regulate proliferation, differentiation, and survival of these cells. Activation of mGlu5 receptors supports self-renewal of embryonic stem cells, which are pluripotent cells isolated from the blastocyst capable of generating all the body's cell lineages, including germ cells. Differentiation of embryonic stem cells into embryoid bodies is associated with the induction of mGlu4 receptors, the activation of which drives cell differentiation towards the mesoderm and endoderm lineages. Different mGlu receptor subtypes, mGlu3 and mGlu5 receptors in particular, are found in neural stem cells (stem cells resident in the CNS that give rise to neurons, astrocytes or oligodendrocytes) isolated from the developing brain or from regions of persistent neurogenesis of the adult brain (e.g. the subventricular zone lining the wall of the lateral ventricles). The evidence that activation of mGlu3 and mGlu5 receptors stimulates proliferation of these cells is particularly interesting because of the similarities between neural stem cells and putative cancer stem cells that support the growth of malignant gliomas. A link among mGlu receptors, stem cells and cancer is supported by the finding that mGlu4 receptors are expressed by cerebellar granule cell neuroprogenitors, which are the putative cells of origin of medulloblastomas. The study of mGlu receptors in stem/progenitor cells has potential applications in the optimisation of protocols of cell expansion and differentiation aimed at cell replacement strategies, and may gain new insights into the pathophysiology of neurodevelopmental disorders and brain tumours.

Metabotropic glutamate receptors: new targets for the control of tumor growth?

Cancer stem cells are currently a target for the treatment of malignant tumors. Transformed neural stem-progenitor cells of the brain subventricular zone and the external granular layer of the cerebellum are the putative cells of origin of malignant gliomas and medulloblastomas, which are the most frequent malignant brain tumors in adults and children, respectively. The proliferation of neural stem-progenitor cells is regulated by metabotropic glutamate (mGlu) receptors, which are G-protein-coupled receptors that are activated by glutamate, the major excitatory neurotransmitter of the CNS. At least two receptor subtypes - mGlu(3) and mGlu(4) receptors - control the proliferation of brain tumor cells, whereas mGlu(1) receptors have been implicated in the development of melanomas. We believe that individual mGlu receptor subtypes represent new potential targets for the treatment of several malignant tumors, including brain tumors.

Nuclear localization of functional metabotropic glutamate receptor mGlu1 in HEK293 cells and cortical neurons: role in nuclear calcium mobilization and development

The Group I metabotropic glutamate receptor (mGlu1) plays an important role in neuromodulation, development, and synaptic plasticity. Using immunocytochemistry, subcellular fractionation, and western blot analysis, the present study shows that mGlu1a receptors are present on nuclear membranes in stably transfected human embryonic kidney 293 (HEK293) cells as well as being endogenously expressed on rat cortical nuclei. Both glutamate and the group I agonist, quisqualate, directly activate nuclear mGlu1 receptors leading to a characteristic oscillatory pattern of calcium flux in isolated HEK nuclei and a slow rise to plateau in isolated cortical nuclei. In either case calcium responses could be terminated upon application of the mGlu1-selective antagonist, 7-(hydroxyamino)cyclopropa[b]chromen-1a-carboxylate ethyl ester. Responses could also be blocked by ryanodine and inositol 1,4,5-triphosphate receptor inhibitors, demonstrating the involvement of these calcium channels. Agonist activation of intracellular receptors was driven by Na(+)-dependent and -independent processes in nuclei isolated from either HEK or cortical neurons. Finally, mGlu1 nuclear receptors were dramatically up-regulated in the course of post-natal development. Therefore, like the other Group I receptor, mGlu5, mGlu1 can function as an intracellular receptor, suggesting a more encompassing role for nuclear G protein-coupled receptors and downstream signaling elements in the regulation of nuclear events.

Kinetic diversity in G-protein-coupled receptor signalling

The majority of intracellular signalling cascades in higher eukaryotes are initiated by GPCRs (G-protein-coupled receptors). Hundreds of GPCRs signal through a handful of trimeric G-proteins, raising the issue of signal specificity. In the present paper, we illustrate a simple kinetic model of G-protein signalling. This model shows that stable production of significant amounts of free Galpha(GTP) (GTP-bound Galpha subunit) and betagamma is only one of multiple modes of behaviour of the G-protein system upon activation. Other modes, previously uncharacterized, are sustained production of betagamma without significant levels of Galpha(GTP) and transient production of Galpha(GTP) with sustained betagamma. The system can flip between different modes upon changes in conditions. This model demonstrates further that the negative feedback of receptor uncoupling or internalization, when combined with a positive feedback within the G-protein cycle, under a broad range of conditions results not in termination of the response but in relaxed oscillations in GPCR signalling. This variety of G-protein responses may serve to encode signal specificity in GPCR signal transduction.