Control of calcium oscillations by phosphorylation of metabotropic glutamate receptors

Cyclothiazide selectively inhibits mGluR1 receptors interacting with a common allosteric site for non-competitive antagonists

Metabotropic glutamate receptors mGluR1 and mGluR5 stimulate phospholipase C, leading to an increased inositol trisphosphate level and to Ca(2+) release from intracellular stores. Cyclothiazide (CTZ), known as a blocker of AMPA receptor desensitization, produced a non-competitive inhibition of [Ca(2+)](i) increases induced by mGluR agonists in HEK 293 cells transfected with rat mGluR1a but had no effect on the [Ca(2+)](i) signals in cells expressing rat mGluR5a. In cells expressing mGluR1, CTZ also inhibited phosphoinositide hydrolysis, as well as cAMP accumulation and arachidonic acid release induced by mGluR1 agonists, indicating a direct inhibition of the receptor and not of a particular signal transduction system. However, CTZ failed to antagonize cAMP inhibition stimulated by rat mGluR2, -3, -4, -6, -7 and -8 receptors confirming its selectivity for mGluR1. The use of chimeric receptors with substituted N-terminal domains showed that CTZ did not interact with the N-terminal mGluR1a domain. Instead, mutation analysis revealed that CTZ interacts with the Thr-815 and Ala-818 residues, located at the 7th transmembrane domain, similarly as the mGluR1-selective antagonist CPCCOEt. In primary cultures of cerebellar granule neurons, expressing native metabotropic and ionotropic glutamate receptors, the final outcome of CTZ effects depended on its combined ability to potentiate AMPA receptors and inhibit mGluR1 receptors.

Endogenously activated mGlu5 metabotropic glutamate receptors sustain the increase in c-Myc expression induced by leukaemia inhibitory factor in cultured mouse embryonic stem cells

We have shown that endogenous activation of type 5 metabotropic glutamate (mGlu5) receptors supports the maintenance of a pluripotent, undifferentiated state in D3 mouse embryonic stem cells cultured in the presence of leukaemia inhibitory factor (LIF). Here, we examined the interaction between LIF and mGlu5 receptors using as a read-out the immediate early gene, c-Myc. The selective mGlu5 receptor antagonist, 2-methyl-6-(phenylenthynyl)pyridine (MPEP; 1 mum), reduced the increase in c-Myc protein levels induced by LIF by enhancing c-Myc ubiquitination. A reduction in c-Myc levels was also observed following small interfering RNA-mediated mGlu5 receptor gene silencing. MPEP reduced glycogen synthase kinase-3beta phosphorylation on Ser9, but increased phosphorylation of the phosphatidylinositol-3-kinase (PI-3-K) substrate, AKT. In our hands, activated PI-3-K reduced the stability of c-Myc, because (i) the PI-3-K inhibitor, LY294002, prevented the reduction in c-Myc levels induced by MPEP; and (ii) over-expression of AKT promoted c-Myc ubiquitination. All effects of MPEP were mimicked by protein kinase C (PKC) inhibitors and reversed by the PKC activator, tetradecanoylphorbol-13-acetate. We conclude that endogenous activation of mGlu5 receptors sustains the increase in c-Myc induced by LIF in embryonic stem cells by inhibiting both glycogen synthase kinase-3beta and PI-3-K, both effects resulting from the activation of PKC.

Oligodendroglial metabotropic glutamate receptors are developmentally regulated and involved in the prevention of apoptosis

Oligodendrocytes (OLs) are responsible for axon myelination and are the principal cells targeted in preterm white matter injury. The cellular and molecular mechanisms involved in white matter development and immature OL injury are incompletely understood. Metabotropic glutamate receptors (mGluRs) modulate neuronal development and survival, and have recently been identified in oligodendrocyte progenitor cells (OPCs). Using the highly homogeneous CG-4 OPC line and O4 marker-immunoselected primary OLs, we established the differentiation stage-specific expression profile of mGluR3 and mGluR5 mRNAs and proteins in the oligodendroglial lineage and type-2-astrocytes (ASTs). Our quantitative analysis indicated no changes in mGluR3, but a significant down-regulation of mGluR5a mRNA and protein expression during differentiation of OPCs into OLs or ASTs. The down-regulation of mGluR5a had functional consequences, with significantly fewer OLs and ASTs than OPCs responding to the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine with intracellular Ca(2+) concentration oscillations. Neither stimulation nor inhibition of mGluR3 or mGluR5 altered OPC migration, suggesting that these receptors do not play prominent roles in the regulation of OPC motility. The activation of mGluR5 completely protected OPCs and substantially reduced staurosporine-induced apoptosis in OLs. This suggests that the down-regulation of mGluR5 in premyelinating OLs is likely to contribute to their increased vulnerability, and that the targeting of mGluR5 may be a potential therapeutic strategy for future development.

Context-dependent regulation of embryonic stem cell differentiation by mGlu4 metabotropic glutamate receptors

The mGlu5 receptor is the only metabotropic glutamate receptor subtype expressed by mouse embryonic stem (ES) cells grown under non-differentiating conditions [Cappuccio, I., Spinanti, P. Porcellini, A., Desiderati, F., De Vita, T., Storto, M., Capobianco, L., Battaglia, G., Nicoletti, F., Melchiorri, D., 2005. Endogenous activation of mGlu5 metabotropic glutamate receptors supports self-renewal of cultured mouse embryonic stem cells. Neuropharmacology 1, 196-205]. We now report that ES cells differentiating into embryoid bodies (EBs) progressively lose mGlu5 receptors and begin to express mGlu4 receptors at both mRNA and proteinc level. A 4-day treatment of EBs with the mGlu4 receptor agonist, L-2-amino-4-phosphonobutanoate (L-AP4), increased mRNA levels of the mesoderm marker, brachyury and the endoderm marker, H19, and decreased the expression of the transcript for the primitive ectoderm marker, fibroblast-growth factor-5 (FGF-5). These effects were prevented by the mGlu4 receptor antagonists, alpha-methylserine-O-phosphate (MSOP). Plating of EBs for 4 days in vitro in ITSFn medium induced cell differentiation towards a neural lineage, as reflected by the expression of the intermediate filament protein, nestin, and the homeobox protein, Dlx-2. Pharmacological activation of mGlu4 receptors during cell incubation in ITSFn medium increased the expression of both neural markers. Similar results were obtained when neural differentiation was induced by exposure of EBs to retinoic acid. These data suggest that differentiation of cultured ES cells is associated with changes in the expression pattern of mGlu receptors and that activation of mGlu4 receptors affects cell differentiation in a context-dependent manner.

Cytosolic inositol 1,4,5-trisphosphate dynamics during intracellular calcium oscillations in living cells

We developed genetically encoded fluorescent inositol 1,4,5-trisphosphate (IP₃) sensors that do not severely interfere with intracellular Ca²⁺ dynamics and used them to monitor the spatiotemporal dynamics of both cytosolic IP₃ and Ca²⁺ in single HeLa cells after stimulation of exogenously expressed metabotropic glutamate receptor 5a or endogenous histamine receptors. IP₃ started to increase at a relatively constant rate before the pacemaker Ca²⁺ rise, and the subsequent abrupt Ca²⁺ rise was not accompanied by any acceleration in the rate of increase in IP₃. Cytosolic [IP₃] did not return to its basal level during the intervals between Ca²⁺ spikes, and IP₃ gradually accumulated in the cytosol with a little or no fluctuations during cytosolic Ca²⁺ oscillations. These results indicate that the Ca²⁺-induced regenerative IP₃ production is not a driving force of the upstroke of Ca²⁺ spikes and that the apparent IP₃ sensitivity for Ca²⁺ spike generation progressively decreases during Ca²⁺ oscillations.

Astrocyte control of synaptic transmission and neurovascular coupling

From a structural perspective, the predominant glial cell of the central nervous system, the astrocyte, is positioned to regulate synaptic transmission and neurovascular coupling: the processes of one astrocyte contact tens of thousands of synapses, while other processes of the same cell form endfeet on capillaries and arterioles. The application of subcellular imaging of Ca2+ signaling to astrocytes now provides functional data to support this structural notion. Astrocytes express receptors for many neurotransmitters, and their activation leads to oscillations in internal Ca2+. These oscillations induce the accumulation of arachidonic acid and the release of the chemical transmitters glutamate, d-serine, and ATP. Ca2+ oscillations in astrocytic endfeet can control cerebral microcirculation through the arachidonic acid metabolites prostaglandin E2 and epoxyeicosatrienoic acids that induce arteriole dilation, and 20-HETE that induces arteriole constriction. In addition to actions on the vasculature, the release of chemical transmitters from astrocytes regulates neuronal function. Astrocyte-derived glutamate, which preferentially acts on extrasynaptic receptors, can promote neuronal synchrony, enhance neuronal excitability, and modulate synaptic transmission. Astrocyte-derived d-serine, by acting on the glycine-binding site of the N-methyl-d-aspartate receptor, can modulate synaptic plasticity. Astrocyte-derived ATP, which is hydrolyzed to adenosine in the extracellular space, has inhibitory actions and mediates synaptic cross-talk underlying heterosynaptic depression. Now that we appreciate this range of actions of astrocytic signaling, some of the immediate challenges are to determine how the astrocyte regulates neuronal integration and how both excitatory (glutamate) and inhibitory signals (adenosine) provided by the same glial cell act in concert to regulate neuronal function.

The PDZ scaffold NHERF-2 interacts with mGluR5 and regulates receptor activity

The two members of the group I metabotropic glutamate receptor family, mGluR1 and mGluR5, both couple to G(q) to mediate rises in intracellular calcium. The alternatively spliced C termini (CT) of mGluRs1 and 5are known to be critical for regulating receptor activity and to terminate in motifs suggestive of potential interactions with PDZ domains. We therefore screened the CTs of both mGluR1a and mGluR5 against a PDZ domain proteomic array. Out of 96 PDZ domains examined, the domain that bound most strongly to mGluR5-CT was the second PDZ domain of the Na(+)/H(+) exchanger regulatory factor 2 (NHERF-2). This interaction was confirmed by reverse overlay, and a single point mutation to the mGluR5-CT was found to completely disrupt the interaction. Full-length mGluR5 robustly associated with full-length NHERF-2 in cells, as assessed by co-immunoprecipitation and confocal microscopy experiments. In contrast, mGluR1a was found to bind NHERF-2 in vitro with a weaker affinity than mGluR5, and furthermore mGluR1a did not detectably associate with NHERF-2 in a cellular context. Immunohistochemical experiments revealed that NHERF-2 and mGluR5 exhibit overlapping patterns of expression in mouse brain, being found most abundantly in astrocytic processes and postsynaptic neuronal elements. In functional experiments, the interaction of NHERF-2 with mGluR5 in cells was found to prolong mGluR5-mediated calcium mobilization and to also potentiate mGluR5-mediated cell death, whereas coexpression of mGluR1a with NHERF-2 had no evident effects on mGluR1a functional activity. These observations reveal that NHERF-2 can selectively modulate mGluR5 signaling, which may contribute to cell-specific regulation of mGluR5 activity.

Generation of time delays: simplified models of intracellular signalling in cerebellar Purkinje cells

In many neuronal systems, information is encoded in temporal spike patterns. The recognition and storage of temporal patterns requires the generation and modulation of time delays between inputs and outputs. In cerebellar Purkinje cells, stimulation of metabotropic glutamate receptors (mGluRs) results in a delayed calcium and voltage response that has been implicated in classical conditioning and temporal pattern recognition. Here, we analyse and simplify a complex model of the intracellular signalling network that has been proposed as a substrate for this delayed response. We systematically simplify the original model, present a minimal model of time delay generation, and show that a delayed response can be produced by the combination of negative feedback and autocatalysis, without any intervening signalling steps that would contribute additive delays. The minimal model is analysed using phase plane methods, and classified as an excitable system. We discuss the implication of excitability for computations performed by intracellular signalling networks in general.

Regulation of metabotropic glutamate receptor signaling, desensitization and endocytosis

Metabotropic glutamate receptors (mGluRs) comprise a unique family of G protein-coupled receptors (GPCR) that can be classified into 3 groups based on G protein coupling specificity and sequence similarity. Group I mGluRs (mGluR1 and mGluR5) are coupled to the heterotrimeric G protein Galpha(q/11) and trigger the release of calcium from intracellular stores. In the present review, we discuss the molecular mechanisms involved in the desensitization and endocytosis of group I mGluRs. Group I mGluRs desensitize in response to both second-messenger-dependent protein kinases and G protein-coupled receptor kinases (GRK). However, GRK2-mediated mGluR1 desensitization appears to be both phosphorylation- and beta-arrestin-independent. In addition to GRK-mediated uncoupling of mGluRs from heterotrimeric G proteins, the huntingtin-interacting protein, optineurin, also contributes to mGluR1 and mGluR5 desensitization. The G protein-uncoupling activity of optineurin appears to be facilitated by the presence of polyglutamine-expanded mutant huntingtin but not wild-type huntingtin. Group I mGluRs also undergo both agonist-dependent and -independent endocytosis in both heterologous cell expression systems and primary neuronal cultures. The present review overviews the current understanding of the contribution of second messenger-dependent protein kinases, beta-arrestins and a novel Ral/phospholipase D2 (PLD2)-mediated endocytic pathway to the regulation of Group I mGluR endocytosis. Overall, the regulation of Group I mGluR desensitization and endocytosis appears to be mediated by the same molecular intermediates as have been described for more typical GPCR such as the beta(2)-adrenergic receptor. However, there appears to be subtle, but important, differences in the mechanisms by which these intermediates are employed to regulate Group I mGluR desensitization and endocytosis.

Comparative analysis of the subcellular and subsynaptic localization of mGluR1a and mGluR5 metabotropic glutamate receptors in the shell and core of the nucleus accumbens in rat and monkey

Group I metabotropic glutamate receptors (mGluRs) play critical roles in synaptic plasticity and drug addiction. To characterize potential sites whereby these receptors mediate their effects in the ventral striatum, we studied the subcellular and subsynaptic localization of mGluR1a and mGluR5 in the shell and core of the nucleus accumbens in rat and monkey. In both species, group I mGluRs are mainly postsynaptic in dendrites and spines, with rare presynaptic labeling in unmyelinated axons. Minor, yet significant, differences in proportions of specific immunoreactive elements were found between the accumbens shell and the accumbens core in monkey. At the subsynaptic level, significant differences were found in the proportion of plasma membrane-bound mGluR5 labeling between species. In dendrites, spines, and unmyelinated axons, a significantly larger proportion of mGluR5 labeling was bound to the plasma membrane in rats (50-70%) than in monkeys (30-50%). Conversely, mGluR1a displayed the same pattern of immunogold labeling in the two species. Electron microscopic colocalization studies revealed 30% colocalization of mGluR1a and mGluR5 in dendrites and as much as 50-65% in spines in both compartments of the rat accumbens. Both group I mGluRs were significantly expressed in D1-immunoreactive dendritic processes (60-75% colocalization) and spines (30-50%) of striatal projection neurons as well as dendrites of cholinergic (30-70%) and parvalbumin-containing (70-85%) interneurons. These findings highlight the widespread expression of group I mGluRs in projection neurons and interneurons of the shell and core of the nucleus accumbens, providing a solid foundation for regulatory and therapeutic functions of group I mGluRs in reward-related behaviors and drug addiction.

Loss of metabotropic glutamate receptor-mediated regulation of glutamate transport in chemically activated astrocytes in a rat model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by a selective loss of motor neurones accompanied by intense gliosis in lesioned areas of the brain and spinal cord. Glutamate-mediated excitotoxicity resulting from impaired astroglial uptake constitutes one of the current pathophysiological hypotheses explaining the progression of the disease. In this study, we examined the regulation of glutamate transporters by type 5 metabotropic glutamate receptor (mGluR5) in activated astrocytes derived from transgenic rats carrying an ALS-related mutated human superoxide dismutase 1 (hSOD1(G93A)) transgene. Cells from transgenic animals and wild-type littermates showed similar expression of glutamate-aspartate transporter and glutamate transporter 1 (GLT-1) after in vitro activation, whereas cells carrying the hSOD1 mutation showed a three-fold higher expression of functional mGluR5, as observed in the spinal cord of end-stage animals. In cells from wild-type animals, (S)-3,5-dihydroxyphenylglycine (DHPG) caused an immediate protein kinase C (PKC)-dependent up-regulation of aspartate uptake that reflected the activation of GLT-1. Although this effect was mimicked in both cultures by direct activation of PKC using phorbol myristate acetate, DHPG failed to up-regulate aspartate uptake in cells derived from the transgenic rats. The failure of activated mGluR5 to increase glutamate uptake in astrocytes derived from this animal model of ALS supports the theory of glutamate excitotoxicity in the pathogenesis of the disease.

Endogenous activation of mGlu5 metabotropic glutamate receptors supports self-renewal of cultured mouse embryonic stem cells

Cultured mouse embryonic stem (ES) cells maintained under undifferentiated conditions (i.e. grown in medium containing 15% FCS and leukemia inhibitory factor--LIF) expressed mGlu5 metabotropic glutamate receptors. Activation of these receptors with quisqualate increased [Ca2+]i but only when cultures were deprived of extracellular glutamate, indicating that the receptor was saturated by the endogenous glutamate. Pharmacological blockade of mGlu5 receptors with 2-methyl-6-(phenylethynyl)pyridine (MPEP) or antisense-induced knock-down of mGlu5 receptors decreased the expression of the two main transcription factors that sustain ES cell self-renewal, i.e. Oct-4 and Nanog, as assessed by real-time PCR and immunoblotting. Exposure of ES cell cultures to MPEP also reduced alkaline phosphatase activity, a marker of undifferentiated ES cells. These data support a critical role for mGlu receptors in early development showing that mGlu5 receptors are expressed by ES cells and their activation sustains ES cell self-renewal in culture.

Altered expression of G(q/11alpha) protein shapes mGlu1 and mGlu5 receptor-mediated single cell inositol 1,4,5-trisphosphate and Ca(2+) signaling

The metabotropic glutamate (mGlu) receptors mGlu1 and mGlu5 mediate distinct inositol 1,4,5-trisphosphate (IP(3)) and Ca(2+) signaling patterns, governed in part by differential mechanisms of feedback regulation after activation. Single cell imaging has shown that mGlu1 receptors initiate sustained elevations in IP(3) and Ca(2+), which are sensitive to agonist concentration. In contrast, mGlu5 receptors are subject to cyclical PKC-dependent uncoupling and consequently mediate coincident IP(3) and Ca(2+) oscillations that are largely independent of agonist concentration. In this study, we investigated the contribution of G(q/11)alpha protein expression levels in shaping mGlu1/5 receptor-mediated IP(3) and Ca(2+) signals, using RNA interference (RNAi). RNAi-mediated knockdown of G(q/11)alpha almost abolished the single-cell increase in IP(3) caused by mGlu1 and mGlu5 receptor activation. For the mGlu1 receptor, this unmasked baseline Ca(2+) oscillations that persisted even at maximal agonist concentrations. mGlu5 receptor-activated Ca(2+) oscillations were still observed but were only initiated at high agonist concentrations. Recombinant overexpression of G(q)alpha enhanced IP(3) signals after mGlu1 and mGlu5 receptor activation. It is noteworthy that although mGlu5 receptor-mediated IP(3) and Ca(2+) oscillations in control cells were largely insensitive to agonist concentration, increasing G(q)alpha expression converted these oscillatory signatures to sustained plateau responses in a high proportion of cells. In addition to modulating temporal Ca(2+) signals, up- or down-regulation of G(q/11)alpha expression alters the threshold for the concentration of glutamate at which a measurable Ca(2+) signal could be detected. These experiments indicate that altering G(q/11)alpha expression levels differentially affects spatiotemporal aspects of IP(3) and Ca(2+) signaling mediated by the mGlu1 and mGlu5 receptors.

Group I mGluR-induced epileptogenesis: distinct and overlapping roles of mGluR1 and mGluR5 and implications for antiepileptic drug design

The group I metabotropic glutamate receptor subtypes, mGluR1 and mGluR5, have both distinct and overlapping actions in epileptogenesis. Data are reviewed revealing how activation of these receptor subtypes participates in the induction and maintenance of the long-lasting epileptiform discharges elicited in the hippocampal circuit. Differences in the cellular actions and regional distributions of mGluR1 and mGluR5 provide hints regarding the potential usefulness and limitations of subtype-specific antagonists as antiepileptic agents.

Protein Kinase C Phosphorylation of the Metabotropic Glutamate ReceptormGluR5 on Serine 839 Regulates Ca2+Oscillations

The activation of Group 1 metabotropic glutamate receptors, mGluR5 and mGluR1alpha, triggers intracellular calcium release; however, mGluR5 activation is unique in that it elicits Ca2+ oscillations. A short region of the mGluR5 C terminus is the critical determinant and differs from the analogous region of mGluR1alpha by a single amino acid residue, Thr-840, which is an aspartic acid (Asp-854) in mGluR1alpha. Previous studies show that mGluR5-elicited Ca2+ oscillations require protein kinase C (PKC)-dependent phosphorylation and identify Thr-840 as the phosphorylation site. However, direct phosphorylation of mGluR5 has not been studied in detail. We have used biochemical analyses to directly investigate the phosphorylation of the mGluR5 C terminus. We showed that Ser-839 on mGluR5 is directly phosphorylated by PKC, whereas Thr-840 plays a permissive role. Although Ser-839 is conserved in mGluR1alpha (Ser-853), it is not phosphorylated, as the adjacent residue (Asp-854) is not permissive; however, mutagenesis of Asp-854 to a permissive alanine residue allows phosphorylation of Ser-853 on mGluR1alpha. We investigated the physiological consequences of mGluR5 Ser-839 phosphorylation using Ca2+ imaging. Mutations that eliminate Ser-839 phosphorylation prevent the characteristic mGluR5-dependent Ca2+ oscillations. However, mutation of Thr-840 to alanine, which prevents potential Thr-840 phosphorylation but is still permissive for Ser-839 phosphorylation, has no effect on Ca2+ oscillations. Thus, we showed that it is phosphorylation of Ser-839, not Thr-840, that is absolutely required for the unique Ca2+ oscillations produced by mGluR5 activation. The Thr-840 residue is important only in that it is permissive for the PKC-dependent phosphorylation of Ser-839.

Radioligand Binding Properties and Pharmacological Characterization of 6-Amino-N-cyclohexyl-N,3-dimethylthiazolo[3,2-a]benzimidazole-2-carboxamide (YM-298198), a High-Affinity, Selective, and Noncompetitive Antagonist of Metabotropic Glutamate Receptor Type 1

Metabotropic glutamate receptor type 1 (mGluR1) is thought to play important roles in the neurotransmission and pathogenesis of several neurological disorders. Here, we describe the radioligand binding properties and pharmacological effects of a newly synthesized, high-affinity, selective, and noncompetitive mGluR1 antagonist, 6-amino-N-cyclohexyl-N,3-dimethylthiazolo[3,2-a]benzimidazole-2-carboxamide (YM-298198). YM-298198 inhibited glutamate-induced inositol phosphate production in mGluR1-NIH3T3 cells with an IC50 of 16 +/- 5.8 nM in a noncompetitive manner. Its radiolabeled form, [3H]YM-298198, bound to mGluR1-NIH3T3 cell membranes with a KD of 32 +/- 8.5 nM and a Bmax of 2297 +/- 291 fmol/mg protein. In ligand displacement experiments using rat cerebellum membrane, an existing noncompetitive mGluR1 antagonist 7-(hydroxyimino)cyclo-propa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) competitively displaced [3H]YM-298198 binding, although glutamate and other mGluR1 ligands acting on a glutamate site failed to inhibit [3H]YM-298198 binding, suggesting that YM-298198 binds to CPCCOEt (allosteric) binding sites but not to glutamate (agonist) binding sites. Specificity was demonstrated for mGluR1 over mGluR subtypes 2 to 7, ionotropic glutamate receptors, and other receptor, transporter, and ion channel targets. In in vivo experiments, orally administered YM-298198 showed a significant analgesic effect in streptozotocin-induced hyperalgesic mice at doses (30 mg/kg) that did not cause Rotarod performance impairment, indicating that it is also useful even for in vivo experiments. In conclusion, YM-298198 is a newly synthesized, high-affinity, selective, and noncompetitive antagonist of mGluR1 that will be a useful pharmacological tool due to its highly active properties in vitro and in vivo. Its radiolabeled form [3H]YM-298198 will also be a valuable tool for future investigation of the mGluR1.

A family of octopamine [corrected] receptors that specifically induce cyclic AMP production or Ca2+ release in Drosophila melanogaster

In invertebrates, the biogenic-amine octopamine is an important physiological regulator. It controls and modulates neuronal development, circadian rhythm, locomotion, 'fight or flight' responses, as well as learning and memory. Octopamine mediates its effects by activation of different GTP-binding protein (G protein)-coupled receptor types, which induce either cAMP production or Ca(2+) release. Here we describe the functional characterization of two genes from Drosophila melanogaster that encode three octopamine receptors. The first gene (Dmoa1) codes for two polypeptides that are generated by alternative splicing. When heterologously expressed, both receptors cause oscillatory increases of the intracellular Ca(2+) concentration in response to applying nanomolar concentrations of octopamine. The second gene (Dmoa2) codes for a receptor that specifically activates adenylate cyclase and causes a rise of intracellular cAMP with an EC(50) of approximately 3 x 10(-8) m octopamine. Tyramine, the precursor of octopamine biosynthesis, activates all three receptors at > or = 100-fold higher concentrations, whereas dopamine and serotonin are non-effective. Developmental expression of Dmoa genes was assessed by RT-PCR. Overlapping but not identical expression patterns were observed for the individual transcripts. The genes characterized in this report encode unique receptors that display signature properties of native octopamine receptors.

FLR-4, a novel serine/threonine protein kinase, regulates defecation rhythm in Caenorhabditis elegans

The defecation behavior of the nematode Caenorhabditis elegans is controlled by a 45-s ultradian rhythm. An essential component of the clock that regulates the rhythm is the inositol trisphosphate receptor in the intestine, but other components remain to be discovered. Here, we show that the flr-4 gene, whose mutants exhibit very short defecation cycle periods, encodes a novel serine/threonine protein kinase with a carboxyl terminal hydrophobic region. The expression of functional flr-4::GFP was detected in the intestine, part of pharyngeal muscles and a pair of neurons, but expression of flr-4 in the intestine was sufficient for the wild-type phenotype. Furthermore, laser killing of the flr-4-expressing neurons did not change the defecation phenotypes of wild-type and flr-4 mutant animals. Temperature-shift experiments with a temperature-sensitive flr-4 mutant suggested that FLR-4 acts in a cell-functional rather than developmental aspect in the regulation of defecation rhythms. The function of FLR-4 was impaired by missense mutations in the kinase domain and near the hydrophobic region, where the latter allele seemed to be a weak antimorph. Thus, a novel protein kinase with a unique structural feature acts in the intestine to increase the length of defecation cycle periods.

Synthesis, in vitro pharmacology, structure-activity relationships, and pharmacokinetics of 3-alkoxy-2-amino-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid derivatives as potent and selective group II metabotropic glutamate receptor antagonists

Novel group II metabotropic glutamate receptor (mGluR) antagonists, 3-alkoxy-2-amino-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid derivatives 11 and 12, were discovered by the incorporation of a hydroxy or alkoxyl group onto the C-3 portion of selective and potent group II mGluR agonist 5, (1R,2S,5R,6R)-2-amino-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid. Among these compounds, (1R,2R,3R,5R,6R)-2-amino-3-(3,4-dichlorobenzyloxy)-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (-)-11be (MGS0039) was a highly selective and potent group II mGluR antagonist with the best pharmacokinetic profile. Compound (-)-11be exhibited high affinities for mGlu 2 (Ki = 2.38 +/- 0.40 nM) and mGlu 3 (4.46 +/- 0.31 nM) but low affinity for mGluR 7 (Ki = 664 +/- 106 nM), and potent antagonist activities for mGlu 2 (IC50 = 20.0 +/- 3.67 nM) and mGluR 3 (IC50 = 24.0 +/- 3.54 nM) but much less potent antagonist activities for mGlu 4 (IC50 = 1740 +/- 1080 nM), mGlu 6 (IC50 = 2060 +/- 1270 nM), mGlu 1 (IC50 = 93300 +/- 14600 nM), and mGluR 5 (IC(50) = 117000 +/- 38600 nM). No significant agonist activities of (-)-11be were found for mGluRs 2, 3, 4, 6, 1, and 5 (EC50 > 100,000 nM). Furthermore, (-)-11be exhibited dose-dependent oral absorption (plasma C(max): 214 +/- 56.7, 932 +/- 235, and 2960 +/- 1150 ng/mL for 3 mg/kg, 10 mg/kg, and 30 mg/kg, po, respectively) and acceptable blood-brain barrier penetration (brain C(max): 13.2 ng/mL for 10 mg/kg, p.o. 6 h). In this paper, we report the synthesis, in vitro pharmacological profile, and structure-activity relationships (SARs) of 3-alkoxy-2-amino-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid derivatives 11 and 12, and pharmacokinetic profiles of several typical compounds.

Altered agonist sensitivity and desensitization of neuronal mGluR1 responses in knock-in mice by a single amino acid substitution at the PKC phosphorylation site

mGluR1 and mGluR5 of the metabotropic glutamate receptor family are coupled to inositol trisphosphate-Ca2+ signal cascades and evoke distinct Ca2+ responses in neural cells and heterologously expressing cells. In heterologous cells, stimulation of recombinant mGluR1 evokes a single-peaked Ca2+ response whereas mGluR5 elicits an oscillatory Ca2+ response. The distinct Ca2+ responses are interchangeable by single amino substitution of aspartate for threonine at the corresponding position of the carboxy-terminal cytoplasmic regions of mGluR1 and mGluR5, respectively. In this investigation, we generated knock-in mice, termed mGluR1 D854T mice, in which aspartate of mGluR1 was replaced with threonine. We examined the effect of this D854T substitution on Ca2+ and current responses mediated by mGluR1 in cultured cerebellar Purkinje cells. Stimulation of mGluR1 D854T by a group 1 mGluR agonist, 3,5-dihydroxyphenylglycine (DHPG) evoked, as in wild-type mGluR1, only single-peaked Ca2+ responses as measured by Ca2+ fluorometric analysis. We then examined mGluR1-induced inward currents carried by nonselective cation channels during whole-cell recordings from cultured Purkinje cells. The mGluR1 D854T mutation abolished the responsiveness of mGluR1 to low concentrations of DHPG (0.5-500 nM) and reduced its desensitization during prolonged agonist application. mGluR1 D854T homozygous mutants showed no apparent behavioural abnormality as analysed by motor movement tests. These results indicate that, although additional modulatory mechanisms seem to be required to produce oscillatory Ca2+ responses of mGluR1, the single amino acid substitution at position 854 of mGluR1 is capable of influencing the kinetics of neuronal mGluR1 responses, most probably through PKC-mediated phosphorylation.

Interactions between NMDA receptors and mGlu5 receptors expressed in HEK293 cells

1 Ca2+ imaging was used to investigate interactions between responses induced by N-methyl-D-aspartate (NMDA; 15 microm) and (RS)-3,5-dihydroxyphenyl-glycine (DHPG; 30 microm) in human embryonic kidney (HEK) 293 cells, transiently transfected with rat recombinant NR1a, NR2A and mGlu5a cDNA. 2 Responses to NMDA were reversibly depressed by DHPG from 244+/-14 to 194+/-12% of baseline. Treatment with thapsigargin (1 microm, 10 min) prevented this effect. 3 After thapsigargin pretreatment, repeated applications of NMDA showed a gradual rundown in amplitude over a period of several hours, and were unaffected by DHPG. 4 Continuous perfusion with staurosporine (0.1 microm), after thapsigargin pretreatment, converted the run-down to a small increase in NMDA responses to 123+/-6 % of baseline. DHPG induced a further and sustained potentiation of NMDA responses to 174+/-12% of the initial baseline. 5 The protein tyrosine kinase (PTK) inhibitors genistein (50 microm) and 3-(4-chlorophenyl)1-(1,1-dimethylethyl)-1H-pyrazolo[3,4-d]pyrimidin-4-amine (PP2; 1 microm) inhibited the staurosporine- and DHPG-induced potentiation of NMDA responses. 6 The protein phosphatase (PTP) inhibitors orthovanadate (100 microm) and phenyl arsine oxide (PAO, 1 microm) facilitated the staurosporine-evoked potentiation of NMDA responses and occluded DHPG-induced potentiation. 7 In conclusion, complex interactions can be demonstrated between mGlu5 and NMDA receptors expressed in HEK293 cells. There is a negative inhibitory influence of Ca2+ release and PKC activation. Inhibition of these processes reveals a tonic, mGlu5 receptor and PTK-dependent potentiation of NMDA receptors that can be augmented by either stimulating mGlu5 receptors or by inhibiting PTPs.

Spontaneous synchronized calcium oscillations in neocortical neurons in the presence of physiological [Mg(2+)]: involvement of AMPA/kainate and metabotropic glutamate receptors

Primary cultures of neocortical neurons exhibit spontaneous Ca(2+) oscillations under zero or low extracellular [Mg(2+)] conditions. We find that mature murine neocortical neurons cultured for 9 days also produce spontaneous Ca(2+) oscillations in the presence of physiological [Mg(2+)]. These Ca(2+) oscillations were action potential mediated inasmuch as tetrodotoxin eliminated their occurrence. AMPA receptors were found to regulate the frequency of Ca(2+) oscillations. In contrast, Ca(2+) oscillations were independent of activation of L-type Ca(2+) channels, and NMDA receptors provided only a minor contribution. Release of intracellular Ca(2+) stores was involved in the oscillatory activity since thapsigargin reduced the amplitude and frequency of the oscillations. S-4-carboxyphenylglycine (S)-4CPG), an antagonist of group I metabotropic glutamate receptor (mGluR), also reduced the amplitude of oscillations. In addition, 1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD), a group I mGluR agonist, increased the oscillation frequency, suggesting a critical role for mGluR in the generation of Ca(2+) oscillations. The mGluR-mediated release of intracellular Ca(2+) stores appeared to be mediated by phospholipase C (PLC) since the PLC inhibitor U73122 eliminated the Ca(2+) oscillations. These results indicate that Ca(2+) oscillations in neocortical cultures in the presence of physiologic [Mg(2+)] are primarily initiated by excitatory input from AMPA receptors and involve mobilization of intracellular Ca(2+) stores following activation of mGluR.

Characterization of Novel L-threo-β-Benzyloxyaspartate Derivatives, Potent Blockers of the Glutamate Transporters

Nontransportable blockers of the glutamate transporters are important tools for investigating mechanisms of synaptic transmission. DL-threo-beta-Benzyloxyaspartate (DL-TBOA) is a potent blocker of all subtypes of the excitatory amino acid transporters (EAATs). We characterized novel L-TBOA analogs possessing a substituent on their respective benzene rings. The analogs significantly inhibited labeled glutamate uptake, the most potent of which was (2S,3S)-3-[3-[4-(trifluoromethyl)benzoylamino]benzyloxy]aspartate (TFB-TBOA). In an uptake assay using cells transiently expressing EAATs, the IC(50) values of TFB-TBOA for EAAT1, EAAT2, and EAAT3 were 22, 17, and 300 nM, respectively. TFB-TBOA was significantly more potent at inhibiting EAAT1 and EAAT2 compared with L-TBOA (IC(50) values for EAAT1-3 were 33, 6.2, and 15 microM, respectively). Electrophysiological analyses revealed that TBOA analogs block the transport-associated currents in all five EAAT subtypes and also block leak currents in EAAT5. The rank order of the analogs for potencies at inhibiting substrate-induced currents was identical to that observed in the uptake assay. However, the kinetics of TFBTBOA differed from the kinetics of L-TBOA, probably because of the strong binding affinity. Notably, TFB-TBOA did not affect other representative neurotransmitter transporters or receptors, including ionotropic and metabotropic glutamate receptors, indicating that it is highly selective for EAATs. Moreover, intracerebroventricular administration of the TBOA analogs induced severe convulsive behaviors in mice, probably because of the accumulation of glutamate. Taken together, these findings indicate that novel TBOA analogs, especially TFB-TBOA, should serve as useful tools for elucidating the physiological roles of the glutamate transporters.

Modulation of stretch-induced enhancement of neuronal NMDA receptor current by mGluR1 depends upon presence of glia

Stretching of cultured neurons has been used to model diffuse axonal injury associated with brain trauma. N-Methyl-D-aspartate receptor (NMDAR) activation and group I metabotropic glutamate receptors (mGluRs) are implicated in the pathophysiology of such injury. Here we detail the effects of culture condition and mGluR1 modulation on stretch-enhanced NMDA receptor activity, and show the presence of mGluR1 in addition to mGluR5 in glia. In cortical neurons grown in the absence (PN) or presence (NG) of a glial monolayer, stretch injury (5.7 mm) enhances NMDAR activity by increasing maximal NMDAR current, decreasing the voltage-dependent Mg(2+) block, and altering the kinetic behavior of these receptors. In PN cultures, activation of mGluR1 increases stretch-enhanced NMDAR activity, whereas in NG cultures, such activity is reduced. In contrast, inhibition of mGluR1 in PN cultures limits stretch-enhanced NMDAR activity, whereas in NG cultures activity is increased. MGluR1 modulate stretch-enhanced NMDAR activity through multiple mechanisms including: altering peak or steady state current, affecting Mg(2+) blockade of the NMDAR, or by changing NMDAR kinetics. The presence of glia significantly alters the nature of mGluR1-mediated modulation of NMDAR activity and stretch-induced injury. Together these data indicate a significant neuronal/glial interaction between glial mGluR1 and neuronal NMDA receptor activity.

Expression and functional properties of group I metabotropic glutamate receptors in bovine chromaffin cells

We demonstrate the presence and functional properties of Group I metabotropic glutamate receptors (mGluRs) expressed in chromaffin cells. Immunocytochemical techniques revealed that two mGluR subtypes (mGluR1alpha and mGluR5) are expressed in chromaffin cells, located in both the cytoplasmic membrane and the cytosol surrounding the nucleus. These mGluRs are functionally active on catecholamine (CA) secretion in chromaffin cells because both (1S, 3R)-1-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD) and the specific agonist of Group I mGluRs, (S)-3,5-dihydroxyphenylglycine (DHPG), were able to stimulate the release of CAs (adrenaline and noradrenaline) in a dose-response manner. These effects were specifically reversed by L-(+)-2-amino-3-phosphonopropionic acid (L-AP3), a selective antagonist of the Group I metabotropic glutamate receptors. t-ACPD induced an increase in CA secretion in both the presence and absence of extracellular calcium, the former effect being accompanied by cell membrane depolarization. Noradrenaline (NA) release was higher in the presence of extracellular calcium than in its absence, whereas adrenaline release was of the same order under both conditions. These results indicate that different subtypes of Group I mGluRs are present in noradrenergic and adrenergic cells. Fluorescence imaging techniques in single cells showed different t-ACPD-induced increases in intracellular calcium in different chromaffin cells: in chromaffin cells, 67% expressed functional metabotropic glutamate receptors and with nicotinic receptors, whereas the remaining 33% expressed only nicotinic receptors. In the absence of external calcium, only about 25% of cells responded to t-ACPD-increased intracellular calcium by increasing inositol 1,4,5-trisphosphate (IP(3)) concentration and subsequent calcium mobilization from intracellular stores, whereas the remaining 75% increased intracellular calcium by promoting Ca(2+) influx from the extracellular medium through L- and N- but not P/Q voltage-dependent calcium channels.

Isoform-specific phosphorylation of metabotropic glutamate receptor 5 by protein kinase C (PKC) blocks Ca2+ oscillation and oscillatory translocation of Ca2+-dependent PKC

Prolonged activation of metabotropic glutamate receptor 5a (mGluR5a) causes synchronized oscillations in intracellular calcium, inositol 1,4,5-trisphosphate production, and protein kinase C (PKC) activation. Additionally, mGluR5 stimulation elicited cyclical translocations of myristoylated alanine-rich protein kinase C substrate, which were opposite to that of gammaPKC (i.e. from plasma membrane to cytosol) and dependent on PKC activity, indicating that myristoylated alanine-rich protein kinase C substrate is repetitively phosphorylated by oscillating gammaPKC on the plasma membrane. Mutation of mGluR5 Thr(840) to aspartate abolished the oscillation of gammaPKC, but the mutation to alanine (T840A) did not. Cotransfection of gammaPKC with betaIIPKC, another Ca2+-dependent PKC, resulted in synchronous oscillatory translocation of both classical PKCs. In contrast, cotransfection of deltaPKC, a Ca2+-independent PKC, abolished the oscillations of both gammaPKC and inositol 1,4,5-trisphosphate. Regulation of the oscillations was dependent on deltaPKC kinase activity but not on gammaPKC. Furthermore, the T840A-mGluR5-mediated oscillations were not blocked by the deltaPKC overexpression. These results revealed that activation of mGluR5 causes translocation of both gammaPKC and deltaPKC to the plasma membrane. deltaPKC, but not gammaPKC, phosphorylates mGluR5 Thr(840), leading to the blockade of both Ca2+ oscillations and gammaPKC cycling. This subtype-specific targeting proposes the molecular basis of the multiple functions of PKC.

Cyclosporine-A treatment inhibits the expression of metabotropic glutamate receptors in rat thymus

We have evaluated the expression of metabotropic glutamate receptors (mGluR subtypes 2/3, 4 and 5) in rat thymus under normal and experimental conditions after 2 and 21 days of cyclosporine-A treatment. In normal rats, immunohistochemical analysis showed that expression of mGluRs was high in dendritic cells and lymphocytes of the medulla whereas it was weak in lymphocytes of the cortex. However, there were some differences in the expression of mGluRs subtypes. mGluR5 showed strong expression in lymphocytes of medulla and dendritic cells. mGluR2/3 and mGluR4 were moderately expressed in lymphocytes and dendritic cells of the medulla and weakly in cortical lymphocytes. Immunoblotting showed moderate levels of mGluR2/3 and mGluR4 and strong levels of mGluRS. After 2 days of cyclosporine-A treatment, we observed by immunohistochemistry and immunoblotting a distinct decrease in all mGluRs and their expression had almost completely disappeared after 21 days of treatment. The results clearly indicate that: 1) mGluR2/3, 4 and 5 are widely expressed in thymic cells; 2) the mGluR5 subtype is expressed most strongly in medullary cells; and 3) cyclosporine-A rapidly inhibits expression of all mGluR subtypes after 2 days of treatment and their complete disappearance after prolonged treatment. These findings may indicate a possible mechanism by which cyclosporine-A produces its immunosupressive effects.

mGluR1, but not mGluR5, mediates depolarization of spinal cord neurons by blocking a leak current

The modulation of neuronal excitability by group I metabotropic glutamate receptors (mGluRs) was studied in isolated lamprey spinal cord. At resting potential, application of the group I mGluR agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) slightly depolarized the cells. However, at depolarized membrane potentials, this agonist induced repetitive firing. When Na+ channels were blocked by TTX, DHPG induced a slight depolarization at rest that increased in amplitude as the neurons were held at more depolarized membrane potentials. In voltage-clamp conditions, DHPG application induced an inward current associated with a decrease in membrane conductance when cells were held at -40 mV. At resting membrane potential, no significant change in the current was induced by DHPG, although a decrease in membrane conductance was seen. The conductance blocked by DHPG corresponded to a leak current, since DHPG had no effect on the voltage-gated current elicited by a voltage step from -60 to -40 mV, when leak currents were subtracted. The leak current blocked by DHPG is mediated by fluxes of both K+ and Na+. The subtype of group I mGluR mediating the block of the leak current was characterized using specific antagonists for mGluR1 and mGluR5. The inhibition of the leak current was blocked by the mGluR1 antagonist LY 367385 but not by the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP). The DHPG-induced blockage of the leak current required phospholipase C (PLC)-activation and release of Ca2+ from internal stores as the effect of DHPG was suppressed by the PLC-blocker U-73122 and after depletion of intracellular Ca2+ pools by thapsigargin. Our results thus show that mGluR1 activation depolarizes spinal neurons by inhibiting a leak current. This will boost membrane depolarization and result in an increase in the excitability of spinal cord neurons, which could contribute to the modulation of the activity of the spinal locomotor network.

The distinct role of mGlu1 receptors in post-ischemic neuronal death

Metabotropic glutamate receptors of the mGlu(1) and mGlu(5) subtypes exhibit a high degree of sequence homology and are both coupled to phospholipase C and intracellular Ca(2+) mobilization. However, functional differences have been detected for these receptor subtypes when they are coexpressed in the same neuronal populations. Experimental evidence indicates that mGlu(1) and mGlu(5) receptors play a differential role in models of cerebral ischemia and that only mGlu(1) receptors are implicated in the pathways leading to post-ischemic neuronal injury. The localization of mGlu(1) receptors in GABA-containing interneurons rather than in hippocampal CA1 pyramidal cells that are vulnerable to ischemia has prompted studies that have provided a new viewpoint on the neuroprotective mechanism of mGlu(1) receptor antagonists. The hypothesis predicts that these pharmacological agents attenuate post-ischemic injury by enhancing GABA-mediated neurotransmission.

Molecular and functional characterization of an octopamine receptor from honeybee (Apis mellifera) brain

Biogenic amines and their receptors regulate and modulate many physiological and behavioural processes in animals. In vertebrates, octopamine is only found in trace amounts and its function as a true neurotransmitter is unclear. In protostomes, however, octopamine can act as neurotransmitter, neuromodulator and neurohormone. In the honeybee, octopamine acts as a neuromodulator and is involved in learning and memory formation. The identification of potential octopamine receptors is decisive for an understanding of the cellular pathways involved in mediating the effects of octopamine. Here we report the cloning and functional characterization of the first octopamine receptor from the honeybee, Apis mellifera. The gene was isolated from a brain-specific cDNA library. It encodes a protein most closely related to octopamine receptors from Drosophila melanogaster and Lymnea stagnalis. Signalling properties of the cloned receptor were studied in transiently transfected human embryonic kidney (HEK) 293 cells. Nanomolar to micromolar concentrations of octopamine induced oscillatory increases in the intracellular Ca2+ concentration. In contrast to octopamine, tyramine only elicited Ca2+ responses at micromolar concentrations. The gene is abundantly expressed in many somata of the honeybee brain, suggesting that this octopamine receptor is involved in the processing of sensory inputs, antennal motor outputs and higher-order brain functions.

Visualizing phosphoinositide signalling in single neurons gets a green light

There is now substantial evidence, from single-cell imaging, that complex patterns of release from Ca(2+) stores play an important role in regulating synaptic efficacy and plasticity. Moreover, the major mechanism of store release depends on the generation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] through the action of phospholipase(s) C on phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)], and several neurotransmitters can enhance receptor-mediated activation of this enzyme. The recent development of techniques to image real-time changes in PtdIns(4,5)P(2) hydrolysis according to generation of Ins(1,4,5)P(3) and diacylglycerol in single cells has significantly advanced our ability to investigate these signalling pathways, particularly in relation to single-cell Ca(2+) signals. This article reviews these new approaches and how they have provided novel insights into mechanisms underlying spatio-temporal Ca(2+) signals and phospholipase C activation in neurons.

Activation of metabotropic glutamate receptor mGlu5 on nuclear membranes mediates intranuclear Ca2+ changes in heterologous cell types and neurons

Nuclear Ca2+ plays a critical role in many cellular functions although its mode (s) of regulation is unclear. This study shows that the metabotropic glutamate receptor, mGlu5, mobilizes nuclear Ca2+ independent of cytosolic Ca2+ regulation. Immunocytochemical, ultrastructural, and subcellular fractionation techniques revealed that the metabotropic glutamate receptor, mGlu5, can be localized to nuclear membranes in heterologous cells as well as midbrain and cortical neurons. Nuclear mGlu5 receptors derived from HEK cells or cortical cell types bound [3H]quisqualate. When loaded with Oregon Green BAPTA, nuclei isolated from mGlu5-expressing HEK cells responded to the addition of glutamate with rapid, oscillatory [Ca2+] elevations that were blocked by antagonist or EGTA. In contrast, carbachol-activation of endogenous muscarinic receptors led to cytoplasmic but not nuclear Ca2+ responses. Similarly, activation of mGlu5 receptors expressed on neuronal nuclei led to sustained Ca2+ oscillatory responses. These results suggest mGlu5 may mediate intranuclear signaling pathways.

Neuroprotective activity of metabotropic glutamate receptor ligands

Metabotropic glutamate receptors form a family of currently eight subtypes (mGluR1-8), subdivided into three groups (I-III). Activation of group-II (mGluR2 and -3) or group-III metabotropic glutamate receptors (mGluR4, -6, -7 and -8) has been established to be neuroprotective in vitro and in vivo. In contrast, group-I mGluRs (mGluR1 and -5) need to be antagonized in order to evoke protection. Initially, all neuroprotective mGluR ligands were analogues of L-glutamate. Those compounds were valuable to demonstrate protection in vitro, but showed limited applicability in animal models, particularly in chronic tests, due to low blood-brain-barrier penetration. Recently, systemically active and more potent and selective ligands became available, e.g., the group-II mGluR agonists LY354740 and LY379268 or group-I antagonists like MPEP (mGluR5-selective) and BAY36-7620 (mGluR1-selective). This new generation of pharmacological agents allows a more stringent assessment of the role of individual mGluR-subtypes or groups of receptors in various nervous system disorders, including ischaemia-induced brain damage, traumatic brain injury, Huntington's and Parkinson's-like pathology or epilepsy. Moreover, the use of genetically modified animals (e.g., knock-out mice) is starting to shed light on specific functions of mGluR-subtypes in experimental neuropathologies.

Direct interaction and functional coupling between metabotropic glutamate receptor subtype 1 and voltage-sensitive Cav2.1 Ca2+ channel

Intracellular Ca2+ concentrations ([Ca2+]i) are regulated in a spatiotemporal manner via both entry of extracellular Ca2+ and mobilization of Ca2+ from intracellular stores. Metabotropic glutamate receptor subtype 1 (mGluR1) is a G protein-coupled receptor that stimulates the inositol 1,4,5-trisphosphate-Ca2+ signaling cascade, whereas Cav2.1 is a pore-forming channel protein of P/Q-type voltage-sensitive Ca2+ channels. In this investigation, we showed that mGluR1 and Cav2.1 are colocalized at dendrites of cerebellar Purkinje neurons and form the heteromeric assembly in both the brain and heterologously expressing COS-7 cells. This assembly occurs through the direct interaction between their carboxyl-terminal intracellular domains. Calcium imaging and whole-cell recording showed that mGluR1 inhibits Cav2.1-mediated [Ca2+]i increases and Ba2+ currents in HEK 293 cells expressing Cav2.1 with auxiliary alpha2/delta and beta1 subunits, respectively. This inhibition occurred in a ligand-independent manner and was enhanced by pre-activation of mGluR1 in a ligand-dependent manner. In contrast, simultaneous stimulation of mGluR1 and Cav2.1 induced large [Ca2+]i increases. Furthermore, the temporally regulated inhibition and stimulation of [Ca2+]i increases by mGluR1 and Cav2.1 were observed at dendrites but not soma of cultured Purkinje neurons. These data suggest that the assembly of mGluR1 and Cav2.1 provides the mechanism that ensures spatiotemporal regulation of [Ca2+]i in glutamatergic neurotransmission.

Calcium signalling: dynamics, homeostasis and remodelling

Ca2+ is a highly versatile intracellular signal that operates over a wide temporal range to regulate many different cellular processes. An extensive Ca2+-signalling toolkit is used to assemble signalling systems with very different spatial and temporal dynamics. Rapid highly localized Ca2+ spikes regulate fast responses, whereas slower responses are controlled by repetitive global Ca2+ transients or intracellular Ca2+ waves. Ca2+ has a direct role in controlling the expression patterns of its signalling systems that are constantly being remodelled in both health and disease.

Role of Ca2+ feedback on single cell inositol 1,4,5-trisphosphate oscillations mediated by G-protein-coupled receptors

The dynamics of inositol 1,4,5-trisphosphate (Ins (1,4,5)P3) production during periods of G-protein-coupled receptor-mediated Ca2+ oscillations have been investigated using the pleckstrin homology (PH) domain of phospholipase C (PLC) delta1 tagged with enhanced green fluorescent protein (eGFP-PHPLCdelta1). Activation of noradrenergic alpha1B and muscarinic M3 receptors recombinantly expressed in the same Chinese hamster ovary cell indicates that Ca2+ responses to these G-protein-coupled receptors are stimulus strength-dependent. Thus, activation of alpha1B receptors produced transient base-line Ca2+ oscillations, sinusoidal Ca2+ oscillations, and then a steady-state plateau level of Ca2+ as the level of agonist stimulation increased. Activation of M3 receptors, which have a higher coupling efficiency than alpha1B receptors, produced a sustained increase in intracellular Ca2+ even at low levels of agonist stimulation. Confocal imaging of eGFP-PHPLCdelta1 visualized periodic increases in Ins(1,4,5)P3 production underlying the base-line Ca2+ oscillations. Ins(1,4,5)P3 oscillations were blocked by thapsigargin but not by protein kinase C down-regulation. The net effect of increasing intracellular Ca2+ was stimulatory to Ins(1,4,5)P3 production, and dual imaging experiments indicated that receptor-mediated Ins(1,4,5)P3 production was sensitive to changes in intracellular Ca2+ between basal and approximately 200 nM. Together, these data suggest that alpha1B receptor-mediated Ins(1,4,5)P3 oscillations result from a positive feedback effect of Ca2+ onto phospholipase C. The mechanisms underlying alpha1B receptor-mediated Ca2+ responses are therefore different from those for the metabotropic glutamate receptor 5a, where Ins(1,4,5)P3 oscillations are the primary driving force for oscillatory Ca2+ responses (Nash, M. S., Young, K. W., Challiss, R. A. J., and Nahorski, S. R. (2001) Nature 413, 381-382). For alpha1B receptors the Ca2+-dependent Ins(1,4,5)P3 production may serve to augment the existing regenerative Ca2+-induced Ca2+-release process; however, the sensitivity to Ca2+ feedback is such that only transient base-line Ca2+ spikes may be capable of causing Ins(1,4,5)P3 oscillations.

[Analysis of PKC targeting mechanism using PKC fused with fluorescent proteins]

Protein kinase C (PKC) is a family, which consists of at least ten subtypes. To elucidate subtype-specific functions of PKC, we have developed the methods to observe PKC translocation in real time and in the living state using PKC fused with fluorescent proteins, including GFP and DsRed. Based on the live imaging of PKC translocation, we have demonstrated that PKC showed stimulus- and subtype-specific translocation, which can recognize its specific substrate and induce its specific cellular response (PKC targeting). These findings suggest that PKC targeting is the molecular basis underlying the diversity of PKC functions. Live imaging of PKC translocation has been proved to be a beneficial tool for understanding not only PKC functions, but also PKC-mediated signal transduction pathways. We have further analyzed PKC functions in the central nervous system using transgenic mice, which can express PKC-GFP in a brain-region-specific manner.

Evolution, structure, and activation mechanism of family 3/C G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) represent one of the largest gene families in the animal genome. These receptors can be classified into several groups based on the sequence similarity of their common heptahelical domain. The family 3 (or C) GPCRs are receptors for the main neurotransmitters glutamate and gamma-aminobutyric acid, for Ca(2+), for sweet and amino acid taste compounds, and for some pheromone molecules, as well as for odorants in fish. Although none of these family 3 receptors have been found in plants, members have been identified in ancient organisms, such as slime molds (Dictyostelium) and sponges. Like any other GPCRs, family 3 receptors possess a transmembrane heptahelical domain responsible for G-protein activation. However, most of these identified receptors also possess a large extracellular domain that is responsible for ligand recognition, is structurally similar to bacterial periplasmic proteins involved in the transport of small molecules, and is called a Venus Flytrap module. The recent resolution of the structure of this binding domain in one of these receptors, the metabotropic glutamate 1 receptor, together with the recent demonstration that these receptors are dimers, revealed a unique mechanism of activation for these GPCRs. Such data open new possibilities in the development of drugs aimed at modulating these receptors, and raise a number of interesting questions on the activation mechanism of the other GPCRs.

G protein-coupled receptor kinases regulate metabotropic glutamate receptor 5 function and expression

Metabotropic glutamate receptors (mGluR) serve important neuromodulatory roles at glutamatergic synapses to shape excitatory neurotransmission. Recent evidence indicates that the desensitization of mGluRs is an important determinant in regulating the functions of these receptors. The present results demonstrate that G protein-coupled receptor kinases (GRKs), which are known to regulate the desensitization of many G protein-coupled receptors, regulate both the expression and function of mGluR5 in a heterologous expression system. This regulatory event is limited to members of the GRK2 family since GRK4 family members do not elicit the same effects on mGluR5. Kinase activity is shown to be required for GRK-mediated regulation of mGluR5. Furthermore, the ability of GRK2 to regulate mGluR5 is dependent, at least in part, on the presence of threonine 840 in the carboxyl terminus of mGluR5. These studies identify novel roles for GRKs in regulating mGluR5 that may serve to further shape the function of these receptors in neurotransmission.

Functional metabotropic glutamate receptors are expressed in oligodendrocyte progenitor cells

We investigated the expression of metabotropic glutamate receptor (mGluR) isoforms in CG-4 rodent oligodendroglial progenitor cells (OPC) and rat brain oligodendrocytes. Our RT-PCR analysis detected mRNAs for mGluR3 and mGluR5 isoforms in OPCs. Although neurons express both mGluR5a and mGluR5b splice variants, only mGluR5a was identified in OPCs. Antibodies to mGluR2/3 and mGluR5 detected the corresponding receptor proteins in immunoblots of OPC membrane fractions. Furthermore, immunocytochemical analysis identified mGluR5 in oligodendrocyte marker O4-positive OPCs. The expression of mGluR5 was also demonstrated in oligodendrocyte marker (O4 and O1) positive cells in white matter of postnatal 4- and 7-day-old rat brain sections using immunofluorescent double labelling and confocal microscopy. The mGluR5 receptor function was assessed in CG-4 OPCs with fura-2 microfluorometry. Application of the mGluR1/5 specific agonist (S)-3,5-dihydroxyphenylglycine (DHPG) induced calcium oscillations, which were inhibited by the selective mGluR5 antagonist 2-methyl-6-(phenylethynyl) pyridine hydrochloride (MPEP). The DHPG induced calcium oscillations required Ca2+ release from intracellular stores. In OPCs the group II mGluR agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) decreased forskolin-stimulated cAMP synthesis, indicating the presence of functional mGluR3. The newly identified mGluR3 and mGluR5a may be involved in the differentiation of oligodendrocytes, myelination and the development of white matter damage.

Protein kinase C isoform-specific differences in the spatial-temporal regulation and decoding of metabotropic glutamate receptor1a-stimulated second messenger responses

Metabotropic glutamate receptors (mGluRs) coupled via Gq to the hydrolysis of phosphoinositides stimulate Ca(2+) and PKCbetaII oscillations in both excitable and non-excitable cells. In the present study, we show that mGluR1a activation stimulates the repetitive plasma membrane translocation of each of the conventional and novel, but not atypical, PKC isozymes. However, despite similarities in sequence and cofactor regulation by diacyglycerol and Ca(2+), conventional PKCs exhibit isoform-specific oscillation patterns. PKCalpha and PKCbetaI display three distinct patterns of activity: (1) agonist-independent oscillations, (2) agonist-stimulated oscillations, and (3) persistent plasma membrane localization in response to mGluR1a activation. In contrast, only agonist-stimulated PKCbetaII translocation responses are observed in mGluR1a-expressing cells. PKCbetaI expression also promotes persistent increases in intracellular diacyglycerol concentrations in response to mGluR1a stimulation without affecting PKCbetaII oscillation patterns in the same cell. PKCbetaII isoform-specific translocation patterns are regulated by specific amino acid residues localized within the C-terminal PKC V5 domain. Specifically, Asn-625 and Lys-668 localized within the V5 domain of PKCbetaII cooperatively suppress PKCbetaI-like response patterns for PKCbetaII. Thus, redundancy in PKC isoform expression and differential decoding of second messenger response provides a novel mechanism for generating cell type-specific responses to the same signal.

Selective blockade of mGlu5 metabotropic glutamate receptors is protective against acetaminophen hepatotoxicity in mice

BACKGROUND/AIMS

mGlu5 metabotropic glutamate receptor antagonists protect rat hepatocytes against hypoxic death. Here, we have examined whether mGlu5 receptor antagonists are protective against liver damage induced by oxidative stress.

METHODS

Toxicity of isolated hepatocytes was induced by tert-butylhydroperoxide (t-BuOOH) after pretreatment with the mGlu5 receptor antagonists, MPEP, SIB-1757 and SIB-1893. The effect of these drugs was also examined in mice challenged with toxic doses of acetaminophen.

RESULTS

Addition of tBuOOH (0.5 mM) to isolated hepatocytes induced cell death (70+/-5% at 3 h). Addition of MPEP or SIB-1893 to hepatocytes reduced both the production of reactive oxygen species (ROS) and cell toxicity induced by t-BuOOH (tBuOOH=70+/-5%; tBuOOH+MPEP=57+/-6%; tBuOOH+SIB-1893=40+/-4%). In mice, a single injection of acetaminophen (300 mg/kg, i.p.) induced centrilobular liver necrosis, which was detectable after 24 h. MPEP (20 mg/kg, i.p.) substantially reduced liver necrosis and the production of ROS, although it did not affect the conversion of acetaminophen into the toxic metabolite, N-acetylbenzoquinoneimine. MPEP, SIB-1893 and SIB-1757 (all at 20 mg/kg, i.p.) also reduced the increased expression and activity of liver iNOS induced by acetaminophen.

CONCLUSIONS

We conclude that pharmacological blockade of mGlu5 receptors might represent a novel target for the treatment of drug-induced liver damage.

Distinct functional roles of the metabotropic glutamate receptors 1 and 5 in the rat globus pallidus

Group I metabotropic glutamate receptors (mGluRs) 1 and 5 frequently colocalize in the same neurons throughout the CNS. Because both receptors can couple to the same effector systems, the purpose of their cellular coexpression remains unclear. Here, we report that group I mGluR1 and mGluR5 have distinct functional roles in type II neurons of the rat globus pallidus (GP). Type II GP neurons form a large population of GABAergic projection neurons that are characterized by the presence of inwardly rectifying current I(h), low-threshold voltage-activated calcium current I(t), and activity at rest. Although immunocytochemical analysis reveals a high degree of neuronal colocalization of the two group I mGluRs in the GP, activation of mGluR1 only directly depolarizes type II GP neurons. Interestingly, blockade of mGluR5 by a highly selective antagonist, methylphenylethynylpyridine, leads to the potentiation of the mGluR1-mediated depolarization in this neuronal subpopulation. Metabotropic GluR1 desensitizes during repeated activation with the agonist in type II GP neurons, and blocking mGluR5 prevents the desensitization of the mGluR1-mediated depolarization. Elimination of the activity of protein kinase C (PKC) by an application of 1 microm bisendolylmaleimide or 1 microm chelerythrine, both protein kinase C inhibitors, potentiates the mGluR1-mediated response and prevents the desensitization of mGluR1 in type II GP neurons, suggesting that the effect of mGluR5 on mGluR1 signaling may involve PKC. Together, these data illustrate a novel mechanism by which mGluR1 and mGluR5, members of the same family of G-protein-coupled receptors, can interact to modulate neuronal activity in the rat GP.

Identification and characterization of a novel splice variant of the metabotropic glutamate receptor 5 gene in human hippocampus and cerebellum

The G-protein coupled metabotropic glutamate receptor mGlu5 plays a pivotal role as a modulator of synaptic plasticity, ion channel activity and excitotoxicity. Two splice variants, hmGlu5a and -5b have been reported previously. During screening of a human brain cDNA library for hmGlu5a, we identified a novel variant (hmGlu5d) generated by alternative splicing at the C-terminal domain. The predicted hmGlu5d protein has a C-terminal 267 amino acid shorter than that of hmGlu5a. The pattern of mRNA expression of mGluR5 variants in human brain were analyzed by RT-PCR and in situ hybridization histochemistry. RT-PCR analysis demonstrated the presence of the hmGlu5d transcript, although at low level, in human whole brain, cerebellum, cerebral cortex and hippocampus. [3H]Quisqualate displayed similar affinity at the hmGlu5 splice variants (K(D) values of 80+/-8 and 54+/-17 nM for hmGlu5a and -5d receptors, respectively). For the five mGlu agonists studied, a similar rank order of potency was observed on both hmGlu5a and -5d receptors: quisqualate>glutamate>DHPG>L-CCGI approximately ACPD. MPEP inhibited the glutamate (2 microM)-induced [Ca(2+)](i) response in hmGlu5a and -5d-HEK293 cells also with similar potency (IC(50) values 25+/-1.5 and 20+/-1.4 nM, respectively). Therefore, the large truncation of the C-terminal tail of mGlu5 does not have any apparent major effect on the potency and efficacy of agonists as measured by the [Ca(2+)](i) responses or by activation of recombinant G-protein coupled inwardly rectifying K(+) (GIRK) channel currents. The only major functional difference is the increased sensitivity of hmGlu5d to protein kinase C (PKC)-mediated desensitization, relative to hmGlu5a.

Ca2+-stimulated Ca2+ oscillations produced by the Ca2+-sensing receptor require negative feedback by protein kinase C

We examined the role of protein kinase C (PKC) in the mechanism and regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) oscillations elicited by an increase in the extracellular concentration of Ca(2+) ([Ca(2+)](e)) in human embryonic kidney 293 cells expressing the Ca(2+)-sensing receptor (CaR). Exposure to the PKC inhibitors bisindolylmaleimide I (GF I) or Ro-31-8220 converted oscillatory responses to transient, non-oscillatory responses, significantly reducing the percentage of cells that showed [Ca(2+)](i) oscillations but without decreasing the overall response to increase in [Ca(2+)](e). Exposure to 100 nm phorbol 12,13-dibutyrate, a direct activator of PKC, eliminated [Ca(2+)](i) oscillations. Addition of phorbol 12,13-dibutyrate at lower concentrations (3 and 10 nm) did not eliminate the oscillations but greatly reduced their frequency in a dose-dependent manner. Co-expression of CaR with constitutively active mutants of PKC (either epsilon or beta(1) isoforms) also reduced [Ca(2+)](i) oscillation frequency. Expression of a mutant CaR in which the major PKC phosphorylation site is altered by substitution of alanine for threonine (T888A) eliminated oscillatory behavior, producing [Ca(2+)](i) responses almost identical to those produced by the wild type CaR exposed to PKC inhibitors. These results support a model in which phosphorylation of the CaR at the inhibitory threonine 888 by PKC provides the negative feedback needed to cause [Ca(2+)](i) oscillations mediated by this receptor.

YM872: a selective, potent and highly water-soluble alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonist

This review focuses on the in vitro and in vivo neuropharmacology of YM872, a potential neuroprotective agent currently undergoing clinical trials in the United States (trial name: AMPA Receptor Antagonist Treatment in Ischemic Stroke - ARTIST). Its neuroprotective properties in rats and cats with induced focal cerebral ischemia are described. YM872, [2,3-dioxo-7-(1H-imidazol-1-yl)-6-nitro-1,2,3,4-tetrahydroquinoxalin-1-yl]-acetic acid monohydrate, is a selective, potent and highly water-soluble competitive alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor antagonist. YM872 has a potent inhibitory effect on [(3)H]AMPA binding with a K(i) value of 0.096 microM. In contrast, YM872 has very low affinity for other ionotropic glutamate receptors. The solubility of YM872 is approximately 500 to 1000 times higher than that of the other competitive AMPA antagonists: YM90K, NBQX, or CNQX. The neuroprotective efficacy of YM872 was investigated in rats and cats subjected to permanent occlusion of the left middle cerebral artery. The animals were assessed either histologically or neurologically following ischemia. In rats with occluded middle cerebral artery (MCAO) YM872, by i.v. infusion, significantly reduced infarct volume measured at 24 h and 1 week after ischemia. Significant neuroprotection was maintained even when drug administration was delayed for up to 2 h after ischemia. In addition, YM872 significantly improved neurological deficit measured at 1 week after ischemia. In cats with MCAO YM872, by i.v. infusion, dose-dependently reduced infarct volume at 6 h after ischemia. YM872 produced no behavioral abnormalities and was not nephrotoxic. The evidence for the neuroprotective efficacy of YM872 suggests its therapeutic potential in the treatment of acute stroke in humans.

Mechanisms of metabotropic glutamate receptor desensitization: role in the patterning of effector enzyme activation

Metabotropic glutamate receptors (mGluRs) constitute an unique subclass of G protein-coupled receptors (GPCRs). These receptors are activated by the excitatory amino acid glutamate and play an essential role in regulating neural development and plasticity. In the present review, we overview the current understanding regarding the molecular mechanisms involved in the desensitization and endocytosis of Group 1 mGluRs as well as the relative contribution of desensitization to the spatial-temporal patterning of glutamate receptor signaling. Similar to what has been reported previously for prototypic GPCRs, mGluRs desensitization is mediated by second messenger-dependent protein kinases and GPCR kinases (GRKs). However, it remains to be determined whether mGluRs phosphorylation by GRKs and beta-arrestin binding are absolutely required for desensitization. Group 1 mGluRs endocytosis is both agonist-dependent and -independent. Agonist-dependent mGluRs internalization is mediated by a beta-arrestin- and dynamin-dependent clathrin-coated vesicle dependent endocytic pathway. The activation of Group 1 mGluRs also results in oscillatory Gq protein-coupling leading to the cyclical activation of phospholipase Cbeta thereby stimulating oscillations in both inositol 1,4,5-triphosphate formation and Ca(2+) release from intracellular stores. These glutamate receptor-stimulated Ca(2+) oscillations are translated into the synchronous activation of protein kinase C (PKC), which has led to the hypothesis that oscillatory mGluRs signaling involves the repetitive phosphorylation of mGluRs by PKC. However, recent experimental evidence suggests that oscillatory signaling is an intrinsic glutamate receptor property that is independent of feedback receptor phosphorylation by PKC. The challenge in the future will be to determine the structural determinants underlying mGluRs-mediated spatial-temporal signaling as well as to understand how complex signaling patterns can be interpreted by cells in both the developing and adult nervous systems.

Determinants of metabotropic glutamate receptor-5-mediated Ca2+ and inositol 1,4,5-trisphosphate oscillation frequency. Receptor density versus agonist concentration

Diverse patterns of Ca(2+)(i) release differentially regulate Ca(2+)-sensitive enzymes and gene transcription, and generally the extent of agonist activation of phospholipase C-linked G protein-coupled receptors determines the type of Ca(2+) signal. We have studied global Ca(2+) oscillations arising through activation of the metabotropic glutamate receptor mGluR5a expressed in Chinese hamster ovary cells and find that these oscillations are largely insensitive to agonist concentration. Using an inducible receptor expression system and a non-competitive antagonist, in conjunction with the translocation of eGFP-PH(PLCdelta) to monitor inositol 1,4,5-trisphosphate (InsP(3)) oscillations in single cells, we show that mGluR5a density determines the frequency of these oscillations. The predominant underlying mechanism resulted from a negative feedback loop whereby protein kinase C (PKC) inhibited InsP(3) generation. Down-regulation of PKC by prolonged exposure to phorbol ester revealed a second form of Ca(2+)(i) oscillation at low agonist concentrations. These Ca(2+)(i) signals showed features typical of classic repetitive Ca(2+)-induced Ca(2+) release and were sensitive to agonist concentration. Therefore, a single receptor can stimulate two types of InsP(3)-mediated Ca(2+) signal dependent upon feedback inhibition, producing two distinct means of controlling the final pattern of Ca(2+)(i) release. Our results have physiological implications for Ca(2+) signaling in general and emphasize the importance of mGluR5 surface expression for modulating synaptic plasticity.

Tyrosine phosphorylation of the metabotropic glutamate receptor mGluR5 in striatal neurons

Protein phosphorylation, controlled by the coordinated actions of phosphatases and kinases, is an important regulatory mechanism in synaptic transmission and other neurophysiological processes. Ionotropic glutamate receptors are known targets of phosphorylation on serine, threonine and tyrosine residues, with functional consequences for cell excitability, plasticity and toxicity. While phosphorylation of metabotropic glutamate receptors (mGluRs) also impacts critical cellular processes, there has been no evidence for direct tyrosine phosphorylation of mGluRs. In the present study, anti-phosphotyrosine and specific mGluR antibodies were used to detect tyrosine-phosphorylated mGluRs in rat brain. In particular, we found that mGluR5 is an abundant phosphotyrosine protein in vivo as well as in primary striatal neurons and tissue slices in vitro. The protein phosphatase inhibitor pervanadate robustly increased the amount of tyrosine-phosphorylated mGluR5, suggesting the receptor is subject to an endogenous, active cycle of phosphorylation and dephosphorylation. Furthermore, NMDA treatment also increased the amount of tyrosine-phosphorylated mGluR5, suggesting these endogenous phosphorylation regulatory mechanisms can be used to mediate crosstalk between synaptic glutamate receptors. While mGluR5-stimulated phosphoinositide hydrolysis appears to be unaltered by pervanadate treatment, tyrosine phosphorylation of mGluR5 may be important in trafficking, anchoring, or signaling of the receptor through G protein-independent pathways.