mGluR Type Research Articles

Evolutionary and functional analysis of metabotropic glutamate receptors in lampreys.

The metabotropic glutamate receptor (mGluR, GRM) family is involved in multiple signaling pathways and regulates neurotransmitter release. However, the evolutionary history, distribution, and function of the mGluRs family in lampreys have not been determined. Therefore, we identified the mGluRs gene family in the genome of Lethenteron reissneri, which has been conserved throughout vertebrate evolution. We confirmed that Lr-GRM3, Lr-GRM5, and Lr-GRM7 encode three types of mGluRs in lamprey. Additionally, we investigated the distribution of Lr-GRM3 within this species by qPCR and Western blotting. Furthermore, we conducted RNA sequencing to investigate the molecular function of Lr-GRM3 in lamprey. Our gene expression profile revealed that, similar to that in jawed vertebrates, Lr-GRM3 participates in multiple signal transduction pathways and influences synaptic excitability in lampreys. Moreover, it also affects intestinal motility and the inflammatory response in lampreys. This study not only enhances the understanding of mGluRs' gene evolution but also highlights the conservation of GRM3's role in signal transduction while expanding our knowledge of its functions specifically within lampreys. In summary, our experimental findings provide valuable insights for studying both the evolution and functionality of the mGluRs family.

Subsynaptic mobility of presynaptic mGluR types is differentially regulated by intra- and extracellular interactions.

Presynaptic metabotropic glutamate receptors (mGluRs) are essential for the control of synaptic transmission. However, how the subsynaptic dynamics of these receptors is controlled and contributes to synaptic signaling remain poorly understood quantitatively. Particularly, since the affinity of individual mGluR subtypes for glutamate differs considerably, the activation of mGluR subtypes critically depends on their precise subsynaptic distribution. Here, using superresolution microscopy and single-molecule tracking, we unravel novel molecular mechanisms that control the nanoscale distribution and mobility of presynaptic mGluRs in hippocampal neurons. We demonstrate that the high-affinity group II receptor mGluR2 localizes diffusely along the axon, and is highly mobile, while the low-affinity group III receptor mGluR7 is stably anchored at the active zone. We demonstrate that intracellular interactions modulate surface diffusion of mGluR2, while immobilization of mGluR7 at the active zone relies on its extracellular domain. Receptor activation or increases in synaptic activity do not alter the surface mobility of presynaptic mGluRs. Finally, computational modeling of presynaptic mGluR activity revealed that this particular nanoscale arrangement directly impacts their ability to modulate neurotransmitter release. Altogether, this study demonstrates that distinct mechanisms control surface mobility of presynaptic mGluRs to contribute differentially to glutamatergic synaptic transmission.

Abstract 9101: RVLM Microinjection to Explain the Role of Metabotropic Glutamate Receptors in the Regulation of Baseline Blood Pressure and Heart Rate in SHRSP

Introduction: Glutamate (Glu) is activated by the stimulation of its receptors. The microinjection of Glu in rostral ventrolateral medulla (RVLM) elicited a rapid and large increase in blood pressure (BP) and heart rate (HR), which is inhibited with kynurenic acid (KYN), a selective inhibitor of ionotropic glutamate receptors (iGluR). Further, we showed that the response to Glu was much greater in stroke-prone spontaneously hypertensive rats (SHRSP) than that in Wistar Kyoto (WKY) rats, indicating that iGluR in RVLM played an important role to maintain high BP & HR in SHRSP. Whereas, the role of metabotropic glutamate receptors (mGluR) in RVLM were not fully elucidated in SHRSP yet. Hypotheses: Since the different mGluR types, e.g., group I (mGluR1, mGluR5), group II (mGluR2, mGluR3) & group III (mGluR4, mGluR6, mGluR7, mGluR8), are expressed in the various regions of rat’s brain, and several studies reported that mGluR5 is largely expressed in RVLM of rats; in this study we evaluated the pathophysiological role(s) of mGluR5, expecting that mGluR5 in RVLM keeps high baseline (i) BP and (ii) HR in SHRSP. Method: MTEP (10 nmole, a highly selective mGluR5 antagonist) and KYN (4 nmole) were microinjected unilaterally in RVLM in SHRSP (n=10) and WKY (n=10) to determine the effects on baseline BP & HR. In addition, the effects of two inhibitors on response to Glu (5 nmole) microinjection were observed. Results: MTEP reduced baseline BP & HR significantly in SHRSP when compared to those in WKY (ΔMAP: -30±3 vs. -3±2 mmHg, ΔHR: -35±3 vs. -4±2 bpm, respectively, p< 0.05), but failed to attenuate a response to Glu microinjection. In contrast, KYN had no significant effects on baseline BP & HR in the two strains, whereas it showed an inhibitory effect on the response to microinjected Glu, which was significantly greater in SHRSP than that in WKY. Conclusions: This study suggested that mGluR5 in RVLM played a major role in the maintenance of high baseline BP & HR in SHRSP.

Localization of group II and III metabotropic glutamate receptors at pre- and postsynaptic sites of inner hair cell ribbon synapses.

Glutamate is the major excitatory neurotransmitter in the CNS binding to a variety of glutamate receptors. Metabotropic glutamate receptors (mGluR1 to mGluR8) can act excitatory or inhibitory, depending on associated signal cascades. Expression and localization of inhibitory acting mGluRs at inner hair cells (IHCs) in the cochlea are largely unknown. Here, we analyzed expression of mGluR2, mGluR3, mGluR4, mGluR6, mGluR7, and mGluR8 and investigated their localization with respect to the presynaptic ribbon of IHC synapses. We detected transcripts for mGluR2, mGluR3, and mGluR4 as well as for mGluR7a, mGluR7b, mGluR8a, and mGluR8b splice variants. Using receptor-specific antibodies in cochlear wholemounts, we found expression of mGluR2, mGluR4, and mGluR8b close to presynaptic ribbons. Super resolution and confocal microscopy in combination with 3-dimensional reconstructions indicated a postsynaptic localization of mGluR2 that overlaps with postsynaptic density protein 95 on dendrites of afferent type I spiral ganglion neurons. In contrast, mGluR4 and mGluR8b were expressed at the presynapse close to IHC ribbons. In summary, we localized in detail 3 mGluR types at IHC ribbon synapses, providing a fundament for new therapeutical strategies that could protect the cochlea against noxious stimuli and excitotoxicity.-Klotz, L., Wendler, O., Frischknecht, R., Shigemoto, R., Schulze, H., Enz, R. Localization of group II and III metabotropic glutamate receptors at pre- and postsynaptic sites of inner hair cell ribbon synapses.

Neuropharmacological Screening of Chiral and Non-chiral Phthalimide- Containing Compounds in Mice: in vivo and in silico Experiments.

Thalidomide, the first synthesized phthalimide, has demonstrated sedative- hypnotic and antiepileptic effects on the central nervous system. N-substituted phthalimides have an interesting chemical structure that confers important biological properties. Non-chiral (ortho and para bis-isoindoline-1,3-dione, phthaloylglycine) and chiral phthalimides (N-substituted with aspartate or glutamate) were synthesized and the sedative, anxiolytic and anticonvulsant effects were tested. Homology modeling and molecular docking were employed to predict recognition of the analogues by hNMDA and mGlu receptors. The neuropharmacological activity was tested with the open field test and elevated plus maze (EPM). The compounds were tested in mouse models of acute convulsions induced either by pentylenetetrazol (PTZ; 90 mg/kg) or 4-aminopyridine (4-AP; 10 mg/kg). The ortho and para non-chiral compounds at 562.3 and 316 mg/kg, respectively, decreased locomotor activity. Contrarily, the chiral compounds produced excitatory effects. Increased locomotor activity was found with S-TGLU and R-TGLU at 100, 316 and 562.3 mg/kg, and S-TASP at 316 and 562.3 mg/kg. These molecules showed no activity in the EPM test or PTZ model. In the 4-AP model, however, S-TGLU (237.1, 316 and 421.7 mg/kg) as well as S-TASP and R-TASP (316 mg/kg) lowered the convulsive and death rate. The chiral compounds exhibited a non-competitive NMDAR antagonist profile and the non-chiral molecules possessed selective sedative properties. The NMDAR exhibited stereoselectivity for S-TGLU while it is not a preference for the aspartic derivatives. The results appear to be supported by the in silico studies, which evidenced a high affinity of phthalimides for the hNMDAR and mGluR type 1.

Intra-accumbal administration of AMN082, a metabotropic glutamate receptor type 7 allosteric agonist, inhibits the acquisition but not the expression of morphine-induced conditioned place preference in rats

The nucleus accumbens (NAc) plays a primary role in opioid reward. The actions of glutamate are mediated by the activation of ionotropic and metabotropic glutamate receptors (mGluRs). Previous documents have shown the extensive distributions of the different types of mGluRs, including mGluR7, in regions that are involved in opioid reward, such as the NAc. In this study, seventy male Wistar rats were used to investigate the role of mGluR7 receptors in the NAc on the acquisition and expression of morphine-induced conditioned place preference (CPP). In Experiment 1, to determine the effect of AMN082, a selective mGluR7 allosteric agonist, on the acquisition of morphine-induced conditioned place preference (CPP), the rats bilaterally received AMN082 (1, 3 and 5 μg/0.5 μL DMSO) during three-day conditioning by morphine (5 mg/kg). In Experiment 2, the rats bilaterally received AMN082 (5 μg/0.5 μL DMSO) 5 min prior to the post-conditioning test to investigate the effect of AMN082 on the expression of morphine-induced CPP. The results showed that the intra-accumbal injection of AMN082 prevents the acquisition of morphine-induced CPP in a dose-dependent manner. However, intra-accumbal injection of AMN082 had no effect on the expression of morphine-induced CPP. The findings propose that the mGluR7 in the NAc inhibits the acquisition of morphine-induced CPP that could be mediated by inhibition of NMDA receptors in the NAc.

AMN082—a metabotropic glutamate receptor type 7 allosteric agonist in the NAc facilitates extinction and inhibits the reinstatement of morphine-induced conditioned place preference in male rats

Nucleus accumbens (NAc) plays a primary role in opioid reward. The actions of glutamate (which is the most extensive excitatory neurotransmitter in the mammalian central nervous system) are mediated through the activation of the ionotropic and metabotropic glutamate receptors (mGluRs). Previous studies have shown the extensive distributions of the different types of mGluRs, including mGluR7, in regions that are involved in opioid reward, such as NAc. In this study, CPP was used to investigate the effect of mGluR7 on the extinction period, and the reinstatement of morphine. The animals received bilaterally microinjections of AMN082, a selective mGluR7 allosteric agonist, into the NAc. In Experiment 1, the rats received AMN082 (1 and 5 μg/0.5 μl) during the extinction period. In Experiment 2, the CPP morphine-extinguished rats received AMN082 (1, 3 and 5 μg/0.5 μl) five minutes prior to the administration of an ineffective dosage of morphine (1 mg/kg) in order to reinstate the extinguished morphine. The results of the recorded conditioning scores in this study showed that the intra-accumbal administration of AMN08 reduced the extinction period of morphine. Moreover, the administration of AMN082 into the NAc dose-dependently inhibited the reinstatement of morphine. The findings suggested that the mGluR7 in the NAc facilitates the extinction and inhibits the reinstatement of the morphine-induced CPP that could have been mediated by an increase in the release of extracellular glutamate.

AZD8529, a positive allosteric modulator at the mGluR2 receptor, does not improve symptoms in schizophrenia: A proof of principle study

IntroductionActivation of metabotropic glutamate (mGluR2/3) receptors has been proposed as an alternative mechanism to dopaminergic-based antipsychotics to correct glutamatergic deficits hypothesized to underlie schizophrenia symptoms. This study investigates the efficacy and safety of AZD8529, a selective positive allosteric modulator (PAM) at the mGlu2 receptor, in symptomatic patients with schizophrenia. MethodsPatients were randomized to receive AZD8529 40mg, risperidone 4mg, or placebo as monotherapy. Treatment lasted for 28days, and clinical efficacy was assessed using Positive and Negative Syndrome Scale (PANSS) and Clinical Global Impression (CGI) scores. ResultsThere were no significant differences between patients treated with AZD8529 versus placebo in change from baseline to endpoint in PANSS total, negative and positive symptom subscale, or CGI-S scores. In contrast, risperidone demonstrated significant efficacy relative to placebo. ConclusionThese results do not support a role for the mGluR-2 PAM AZD8529 as an antipsychotic and indicate that positive modulation of mGluR type 2 receptors alone is not sufficient for antipsychotic effects in acutely ill schizophrenia patients.

New caged neurotransmitter analogs selective for glutamate receptor sub-types based on methoxynitroindoline and nitrophenylethoxycarbonyl caging groups

Photolysis is widely used in experimental neuroscience to isolate post-synaptic receptor activation from presynaptic processes, to determine receptor mechanisms in situ, for pharmacological dissection of signaling pathways, or for photostimulation/inhibition in neural networks. We have evaluated new caged neuroactive amino acids that use 4-methoxy-7-nitroindolinyl- (MNI) or 1-(2-nitrophenyl)ethoxycarbonyl (NPEC) photoprotecting groups to make caged ligands specific for glutamate receptor sub-types. Each was tested for interference with synaptic transmission and excitability and for receptor-specific actions in slice preparations. No adverse effects were found at glutamate receptors. At high concentration, MNI-caged, but not NPEC-caged ligands, interfered with GABA-ergic transmission.MNI-caged amino acids have sub-microsecond release times suitable for investigating mechanisms at fast synaptic receptors in situ. MNI-NMDA and MNI-kainate were synthesized and tested. MNI-NMDA showed stoichiometric release of chirally pure NMDA. Wide-field photolysis in cerebellar interneurons produced a fast-rising sustained activation of NMDA receptors, and localized laser photolysis gave a fast, transient response. Photolysis of MNI-kainate to release up to 4 μM kainate generated large inward currents at resting membrane potential in Purkinje neurons. Application of GYKI 53655 indicated that 40% of the current was due to AMPA receptor activation by kainate. Signaling via metabotropic glutamate receptors (mGluR) does not require fast release rates. NPEC cages are simpler to prepare but have slower photorelease. Photolysis of NPEC-ACPD or NPEC-DHPG in Purkinje neurons generated slow inward currents blocked by the mGluR type 1 antagonist CPCCOEt similar to the slow sEPSC seen with parallel fiber burst stimulation. NPEC-AMPA was also tested in Purkinje neurons and showed large sustained inward currents selective for AMPA receptors with little activation of kainate receptors. MNI-caged l-glutamate, NMDA and kainate inhibit GABA-A receptors with IC50 concentrations close to the maximum concentrations useful in receptor signaling experiments.

Measures of anxiety, sensorimotor function, and memory in male and female mGluR4−/− mice

Metabotropic glutamate receptors (mGluRs) are coupled to second messenger pathways via G proteins and modulate synaptic transmission. Of the eight different types of mGluRs (mGluR1–mGluR8), mGluR4, mGluR6, mGluR7, and mGluR8 are members of group III. Group III receptors are generally located presynaptically, where they regulate neurotransmitter release. Because of their role in modulating neurotransmission, mGluRs are attractive targets for therapies aimed at treating anxiety disorders. Previously we showed that the mGluR4-selective allosteric agonist VU 0155041 reduces anxiety-like behavior in wild-type male mice. Here, we explore the role of mGluR4 in adult (6-month old) and middle-aged (12-month old) male and female mice lacking this receptor. Compared to age- and sex-matched wild-type mice, middle-aged mGluR4−/− male mice showed increased measures of anxiety in the open field and elevated zero maze and impaired sensorimotor function on the rotarod. These changes were not seen in adult 6-month-old male mice. In contrast to the male mice, mGluR4−/− female mice showed reduced measures of anxiety in the open field and elevated zero maze and enhanced rotarod performance. During the hidden platform training sessions of the water maze, mGluR4−/− mice swam farther away from the platform than wild-type mice at 6, but not at 12, months of age. mGluR4−/− mice also showed enhanced amygdala-dependent cued fear conditioning. No genotype differences were seen in hippocampus-dependent contextual fear conditioning. These data indicate that effects of mGluR4 on sensorimotor function and measures of anxiety, but not cued fear conditioning, are critically modulated by sex and age.

Structure of metabotropic glutamate receptor C-terminal domains in contact with interacting proteins

Metabotropic glutamate receptors (mGluRs) regulate intracellular signal pathways that control several physiological tasks, including neuronal excitability, learning, and memory. This is achieved by the formation of synaptic signal complexes, in which mGluRs assemble with functionally related proteins such as enzymes, scaffolds, and cytoskeletal anchor proteins. Thus, mGluR associated proteins actively participate in the regulation of glutamatergic neurotransmission. Importantly, dysfunction of mGluRs and interacting proteins may lead to impaired signal transduction and finally result in neurological disorders, e.g., night blindness, addiction, epilepsy, schizophrenia, autism spectrum disorders and Parkinson's disease. In contrast to solved crystal structures of extracellular N-terminal domains of some mGluR types, only a few studies analyzed the conformation of intracellular receptor domains. Intracellular C-termini of most mGluR types are subject to alternative splicing and can be further modified by phosphorylation and SUMOylation. In this way, diverse interaction sites for intracellular proteins that bind to and regulate the glutamate receptors are generated. Indeed, most of the known mGluR binding partners interact with the receptors' C-terminal domains. Within the last years, different laboratories analyzed the structure of these domains and described the geometry of the contact surface between mGluR C-termini and interacting proteins. Here, I will review recent progress in the structure characterization of mGluR C-termini and provide an up-to-date summary of the geometry of these domains in contact with binding partners.

Simulation of Postsynaptic Glutamate Receptors Reveals Critical Features of Glutamatergic Transmission

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

Enhanced binding of metabotropic glutamate receptor type 5 (mGluR5) PET tracers in the brain of parkinsonian primates

The interplay between dopamine and glutamate in the basal ganglia regulates critical aspects of motor learning and behavior. Metabotropic glutamate receptors (mGluR) are increasingly regarded as key modulators of neuroadaptation in these circuits, in normal and disease conditions. Using PET, we demonstrate a significant upregulation of mGluR type 5 in the striatum of MPTP-lesioned, parkinsonian primates, providing the basis for therapeutic exploration of mGluR5 antagonists in Parkinson disease.

Mechanisms Underlying Type I mGluR-Induced Activation of Lobster Gastric Mill Neurons

In addition to ionotropic effects, glutamate and acetylcholine have metabotropic modulatory effects on many neurons. Here we show that in the stomatogastric ganglion of the lobster, glutamate, one of the main ionotropic neurotransmitters, modulates the excitability of gastric mill neurons. The neurons in this well-studied system produce rhythmic output to a subset of lobster foregut muscles. Recently, metabotropic glutamate receptor (mGluR) agonists were suggested as modulators of the rhythmic output, in addition to the previously described muscarinic modulation by acetylcholine. However, the cellular mechanisms responsible for these effects on the pattern are not known. Using intracellular recording methods and calcium imaging, we show that glutamate has an excitatory effect on specific neurons in the stomatogastric ganglion, which is mediated by mGluRs. Responses to the application of mGluR type I agonists are transient oscillations in the system, probably arising from network interactions. We show that the excitatory effect is sensitive to phospholipase-C and IP(3) and is G-protein dependent. The G-protein dependency was demonstrated by GDPbetaS and GTPgammaS injection into identified neurons. The depolarizations and oscillations were accompanied by an increase of intracellular Ca(2+) levels and correlated Ca(2+) oscillations. By using cyclopiazonic acid, an endoreticular Ca(2+) uptake inhibitor, we show that some internal calcium release may augment the response, but is not crucial for its production. Interestingly, although Ca(2+) concentration increase is typically associated with the phosphoinositide pathway, in the lobster, the Ca(2+) concentration increase-either voltage dependent or independent-cannot account for the observed depolarization.

Electrophysiological effects of three groups of glutamate metabotropic receptors in rat piriform cortex.

1. The effects of three metabotropic glutamate receptor (mGluR) agonists were tested in two pathways of rat piriform cortex. The group I, II and III mGluR agonists used were RS-3,5-dihydroxyphenenylglycine (DHPG) (10-100 microM), (2S,1'S,2'S)-2-Carboxycyclopropyl (L-CCG) (20-100 microM) and L(+)-2-amino-4-phosphonobutyric acid (L-AP4) (5-500 microM), respectively. 2. The effects of the three groups of agonists on synaptic transmission in the two piriform cortex pathways also were examined. All three agonists reduced the amplitude of the monosynaptic EPSPs generated by stimulation of the lateral olfactory tract (LOT) or of the association fiber pathway (ASSN). This was always accompanied by an increase in paired pulse facilitation. 3. Group I and II mGluR agonists had similar synaptic effects on the two pathways, while the group III mGluR agonist suppressed the LOT pathway more than the association pathway. 4. The group II and III mGluR agonists had no effect on passive membrane properties of pyramidal neurons. Group I agonists depolarized the pyramidal neuron membrane potential, and enhanced both membrane resistance and noise. 5. Our data suggest that all three types of mGluRs modulate synaptic transmission in both of these pathways in piriform cortex. Only group I agonists alter post-synaptic membrane properties, while all three types of receptor regulate synaptic transmission. Groups I and II are equally potent in the LOT and association fiber pathways, while group III receptors are more potent in the LOT than the association fiber pathways.

Specific Association of Glutamate Receptor and Ca 2+ Channel Subtypes

Neurotransmitter release depends on the expression and organization of various molecules, including calcium (Ca 2+ ) channel and neurotransmitter receptor subtypes, in the presynaptic nerve terminal. Group III metabotropic glutamate receptors (mGluRs), which act as autoreceptors on glutamatergic nerve terminals, inhibit transmitter release largely through inhibiting Ca 2+ influx. Millán et al. used Ca 2+ imaging of rat brain synaptosomes, in combination with pharmacological and immunofluorescent analysis, to investigate the distribution of mGluRs and showed differential localization of two types of mGluRs with different Ca 2+ channel subtypes. The authors used the mGluR inhibitor L(+)-2-amino-4-phosphonobutyrate (L-AP4) to implicate two populations of mGluRs in inhibiting glutamate release after depolarization with potassium chloride. Immunocytochemical analysis with antibodies against mGluRs identified the mGluRs as mGlu4 and mGluR7; and labeling for the synaptic vesicle marker synaptophysin, vesicular glutamate transporters, and a γ-aminobutyric acid-synthetic enzyme indicated that most nerve terminals in the synaptosomal preparation were glutamatergic. Individual fura-2 loaded synaptosomes immobilized on coverslips showed differential responses to L-AP4 inhibition of Ca 2+ influx, suggesting segregation of the two subtypes of mGluRs in different populations of nerve terminals. Ca 2+ imaging experiments with Ca 2+ channel blockers indicated a heterogeneous distribution among different subpopulations of nerve terminals, with one population carrying only N-type channels and another carrying both N- and P/Q-type channels. A comparison of the responses of individual synaptosomes to Ca 2+ channel blockers and L-AP4 indicated that mGluRs with low affinity for L-AP4 (mGluR7) were largely associated with N-type channels, whereas mGluRs with high affinity for L-AP4 (mGluR4) were mostly found in the synaptosomes containing both N- and P/Q-type channels. C. Millán, R. Luján, R. Shigemoto, J. Sánchez-Prieto, Subtype-specific expression of group III metabotropic glutamate receptors and Ca 2+ channels in single nerve terminals. J. Biol. Chem. 277 , 47796-47803 (2002). [Abstract] [Full Text]

Glutamate receptors on the somata of dorsal unpaired median neurons in cockroach, Periplaneta americana, thoracic ganglia.

Effects of application of glutamate and glutamatergic ligands were studied to characterize the receptors for glutamate present on the soma membrane of the dorsal unpaired median (DUM) neurons in the thoracic ganglia of the cockroach, Periplaneta americana, using the intracellular recording technique. Application of L-glutamate did not block the GABA-response, and application of beta-guanidino-propionic acid, a competitive antagonist for GABA, failed to block the response to L-glutamate. These results indicate that most of L-glutamate action may not be mediated by a GABA-activated channel. To examine glutamate receptor types on the DUM neurons, glutamate receptor agonists were applied. The ionotropic glutamate receptor (iGluR) agonists evoked depolarizations with the following relative rank of order of potency: kainate > AMPA > quisqualate. Metabotropic glutamate receptor (mGluR) agonists also elicited membrane depolarizations or hyperpolarizations associated with an increase in membrane conductance. The mGluR agonists evoked depolarizations or hyperpolarizations with the following relative rank of order: L-CCG-1 > 1S, 3R-ACPD > L-AP4. Depolarization of the same DUM neuron was detected following exposure of kainate and L-CCG-I, suggesting the coexistence of distinct iGluR and mGluR types. A membrane permeable cAMP analog, CPT-cAMP, could not mimic the effect of mGluR agonists. The mGluR selective antagonists, MCCG and MCPG, failed to antagonize the response to mGluR agonists. The involvement of cAMP in the mGluR response was not confirmed in DUM neurons. Although the functional roles of these receptors are unknown, it might be possible then that these extrasynaptic receptors have a modulatory effect on the excitability of the DUM neurons.

Polymorphisms in the genes for mGluR types 7 and 8: association studies with schizophrenia

A dysfunctional glutamatergic system has been implicated in the pathophysiology of schizophrenia. The group III metabotropic glutamate receptor (mGluR) types 7 and 8 presynaptically inhibit glutamate release, thereby modulating glutamatergic transmission in the brain. We conducted association studies to investigate the novel Tyr433Phe (mGluR7) variant and the 2846-C/T (mGluR8) polymorphism in schizophrenia. Both variants, present at high frequencies, failed to demonstrate any significant association with schizophrenia (mGluR7 [Tyr433Phe] allele: P=0.33; genotype: P=0.63; mGluR8 [2846-C/T] allele: P=0.72; genotype: P=0.63).

Potentiation of NMDA and AMPA responses by the specific mGluR 5 agonist CHPG in spinal cord motoneurons

The specific metabotropic glutamate receptor (mGluR) 5 agonist ( RS)-2-chloro-5-hydroxyphenylglycine (CHPG) is able to potentiate NMDA and AMPA responses recorded from ventral roots of the isolated hemisected baby rat spinal cord. Previously we have demonstrated that activation of group I mGluRs (mGluR 1 and mGluR 5) with the broad spectrum mGluR agonist 1 S,3 R-1-amino-1,3-cyclopentanedicarboxylate (ACPD) produced potentiation of ionotropic glutamate responses. In contrast to ACPD-induced potentiation, however, no evidence for an involvement of protein kinase C (PKC) is found in the CHPG-induced potentiation of both NMDA and AMPA depolarization because the PKC blockers chelerythrine chloride or calphostin C did not antagonize this effect. Moreover, in the absence of Ca 2+ in the perfusing medium or depleting intracellular Ca 2+ stores with thapsigargin or dantrolene did not modify the CHPG-induced enhancement of NMDA depolarizations. Phorbol-12,13-diacetate (PDA), on the other hand, was able to attenuate this effect, which was reversed by chelerythrine chloride. These results suggest that both mGluR 5 and mGluR 1 may act to enhance ionotropic glutamate responses but the two types of mGluRs may have different intracellular mechanisms of action.

Heterogeneity of metabotropic glutamate receptors in autonomic cell groups of the medulla oblongata of the rat

Metabotropic glutamate receptors (mGluRs) in the medulla oblongata have been suggested to be involved in the regulation of autonomic function. The aim of the present study was to examine the localization and expression of four types of mGluRs: mGluRla, mGluR2/3, mGluR5, and mGluR7 in the dorsal and ventral autonomic nuclei of the medulla of the rat. The four mGluR subtypes studied were differentially distributed in distinct subnuclei in the nucleus of the solitary tract (NTS). mGluRla immunoreactivity was identified in cell bodies, dendrites, and axonal processes in the intermediate, dorsal lateral, and interstitial subnuclei of the NTS. No mGluRla immunoreactivity was observed in the commissural or medial NTS subnuclei. Immunoreactivity for mGluR2/3 and mGluR5 as observed in fibers and putative axonal processes in the interstitial, intermediate, and dorsolateral subnuclei of the NTS. In contrast, mGluR7 was expressed primarily in fibers and terminals in the central and commissural NTS subnuclei. Expression of mGluR2/3 was clearly evident in cell bodies, dendrites, and axonal processes within the area postrema. The vagal outflow nuclei were also studied. The dorsal motor nucleus of the vagus (DMN) contained mGluRla cell bodies, dendrites, and axonal fibers and light mGluR2/3 processes. Throughout the rostral-caudal extent of the compact and semicompact formation nucleus ambiguus, mGluRla was found in cell bodies and fibers. Within the caudal and rostral regions of the ventral lateral medulla, mGluRla was observed in cell bodies and fibers. Cell bodies containing mGluRla were found adjacent to cells staining positive for tyrosine hydroxylase (TH) in these regions but were not colocalized with the TH staining. However, mGluRla-expressing neurons in the ventral lateral medulla did appear to receive innervation from TH-containing fibers. These results suggest that the mGluRla-expressing neurons within the ventral lateral medulla are predominantly not catecholaminergic but may be innervated by catecholamine-containing fibers. These data are the first to provide a mapping of the different mGluR subtypes within the medulla and may facilitate predictions regarding the function of L-glutamate neurotransmission in these regions.