The effects of (RS)-alpha-cyclopropyl-4-phosphonophenylglycine ((RS)-CPPG), a potent and selective metabotropic glutamate receptor antagonist

mGlu4R, mGlu7R, and mGlu8R allosteric modulation for treating acute and chronic neurodegenerative disorders

Neuroprotection, defined as safeguarding neurons from damage and death by inhibiting diverse pathological mechanisms, continues to be a promising approach for managing a range of central nervous system (CNS) disorders, including acute conditions such as ischemic stroke and traumatic brain injury (TBI) and chronic neurodegenerative diseases like Parkinson's disease (PD), Alzheimer's disease (AD), and multiple sclerosis (MS). These pathophysiological conditions involve excessive glutamatergic (Glu) transmission activity, which can lead to excitotoxicity. Inhibiting this excessive Glu transmission has been proposed as a potential therapeutic strategy for treating the CNS disorders mentioned. In particular, ligands of G protein-coupled receptors (GPCRs), including metabotropic glutamatergic receptors (mGluRs), have been recognized as promising options for inhibiting excessive Glu transmission. This review discusses the complex interactions of mGlu receptors with their subtypes, including the formation of homo- and heterodimers, which may vary in function and pharmacology depending on their protomer composition. Understanding these intricate details of mGlu receptor structure and function enhances researchers' ability to develop targeted pharmacological interventions, potentially offering new therapeutic avenues for neurological and psychiatric disorders. This review also summarizes the current knowledge of the neuroprotective potential of ligands targeting group III mGluRs in preclinical cellular (in vitro) and animal (in vivo) models of ischemic stroke, TBI, PD, AD, and MS. In recent years, experiments have shown that compounds, especially those activating mGlu4 or mGlu7 receptors, exhibit protective effects in experimental ischemia models. The discovery of allosteric ligands for specific mGluR subtypes has led to reports suggesting that group III mGluRs may be promising targets for neuroprotective therapy in PD (mGlu4R), TBI (mGlu7R), and MS (mGlu8R).

Restoring glutamate receptor signaling in pancreatic alpha cells rescues glucagon responses in type 1 diabetes

Glucagon secretion from pancreatic alpha cells is crucial to prevent hypoglycemia. People with type 1 diabetes lose this glucoregulatory mechanism and are susceptible to dangerous hypoglycemia for reasons still unclear. Here we determine that alpha cells in living pancreas slices from donors with type 1 diabetes do not mount an adequate glucagon response and cannot activate the positive autocrine feedback mediated by AMPA/kainate glutamate receptors. This feedback is required to elicit full glucagon responses in the healthy state. Reactivating residual AMPA/kainate receptor function with positive allosteric modulators restores glucagon secretion in human slices from donors with type 1 diabetes as well as glucose counterregulation in non-obese diabetic mice. Our study thus identifies a defect in autocrine signaling that contributes to alpha cell failure. The use of positive allosteric modulators of AMPA/kainate receptors overcomes this deficiency and prevents hypoglycemia, an effect that could be used to improve the management of diabetes.

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

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

The Cold Case of Metabotropic Glutamate Receptor 6: Unjust Detention in the Retina?

It is a common opinion that metabotropic glutamate receptor subtype 6 (mGluR6) is expressed exclusively in the retina, and in particular in the dendrites of ON-bipolar cells. Glutamate released in darkness from photoreceptors activates mGluR6, which is negatively associated with a membrane non-selective cation channel, the transient receptor potential melanoma-related 1, TRPM1, resulting in cell hyperpolarization. The evidence that mGluR6 is expressed not only in the retina but also in other tissues and cell populations has accumulated over time. The expression of mGluR6 has been identified in microglia, bone marrow stromal and prostate cancer cells, B lymphocytes, melanocytes and keratinocytes and non-neural tissues such as testis, kidney, cornea, conjunctiva, and eyelid. The receptor also appears to be expressed in brain areas, such as the hypothalamus, cortex, hippocampus, nucleus of tractus solitarius, superior colliculus, axons of the corpus callosum and accessory olfactory bulb. The pharmacological activation of mGluR6 in the hippocampus produced an anxiolytic-like effect and in the periaqueductal gray analgesic potential. This review aims to collect all the evidence on the expression and functioning of mGluR6 outside the retina that has been accumulated over the years for a broader view of the potential of the receptor whose retinal confinement appears understimated.

Calbindin Deficits May Underlie Dissociable Effects of 5-HT6 and mGlu7 Antagonists on Glutamate and Cognition in a Dual-Hit Neurodevelopmental Model for Schizophrenia

Despite several compounds entering clinical trials for the negative and cognitive symptoms of schizophrenia, few have progressed beyond phase III. This is partly attributed to a need for improved preclinical models, to understand disease and enable predictive evaluation of novel therapeutics. To this end, one recent approach incorporates "dual-hit" neurodevelopmental insults like neonatal phencyclidine plus isolation rearing (PCP-Iso). Glutamatergic dysfunction contributes to schizophrenia pathophysiology and may represent a treatment target, so we used enzyme-based microsensors to evaluate basal- and drug-evoked glutamate release in hippocampal slices from rats that received neonatal PCP and/or isolation rearing. 5-HT₆ antagonist-evoked glutamate release (thought to be mediated indirectly via GABAergic disinhibition) was reduced in PCP-Iso, as were cognitive effects of a 5-HT₆ antagonist in a hippocampal glutamate-dependent novel object discrimination task. Yet mGlu₇ antagonist-evoked glutamatergic and cognitive responses were spared. Immunohistochemical analyses suggest these findings (which mirror the apparent lack of clinical response to 5-HT₆ antagonists in schizophrenia) are not due to reduced hippocampal 5-HT input in PCP-Iso, but may be explained by reduced calbindin expression. This calcium-binding protein is present in a subset of GABAergic interneurons receiving preferential 5-HT innervation and expressing 5-HT₆ receptors. Its loss (in schizophrenia and PCP-Iso) would be expected to reduce interneuron firing and potentially prevent further 5-HT₆ antagonist-mediated disinhibition, without impacting on responses of VIP-expressing interneurons to mGlu₇ antagonism. This research highlights the importance of improved understanding for selection of appropriate preclinical models, especially where disease neurobiology impacts on cells mediating the effects of potential therapeutics.

Inhibition of group-I metabotropic glutamate receptors protects against prion toxicity

Prion infections cause inexorable, progressive neurological dysfunction and neurodegeneration. Expression of the cellular prion protein PrPC is required for toxicity, suggesting the existence of deleterious PrPC-dependent signaling cascades. Because group-I metabotropic glutamate receptors (mGluR1 and mGluR5) can form complexes with the cellular prion protein (PrPC), we investigated the impact of mGluR1 and mGluR5 inhibition on prion toxicity ex vivo and in vivo. We found that pharmacological inhibition of mGluR1 and mGluR5 antagonized dose-dependently the neurotoxicity triggered by prion infection and by prion-mimetic anti-PrPC antibodies in organotypic brain slices. Prion-mimetic antibodies increased mGluR5 clustering around dendritic spines, mimicking the toxicity of Aβ oligomers. Oral treatment with the mGluR5 inhibitor, MPEP, delayed the onset of motor deficits and moderately prolonged survival of prion-infected mice. Although group-I mGluR inhibition was not curative, these results suggest that it may alleviate the neurological dysfunctions induced by prion diseases.

Metabotropic glutamate receptors are involved in the detection of IMP and L-amino acids by mouse taste sensory cells

G-protein-coupled receptors are thought to be involved in the detection of umami and L-amino acid taste. These include the heterodimer taste receptor type 1 member 1 (T1r1)+taste receptor type 1 member 3 (T1r3), taste and brain variants of mGluR4 and mGluR1, and calcium sensors. While several studies suggest T1r1+T1r3 is a broadly tuned lLamino acid receptor, little is known about the function of metabotropic glutamate receptors (mGluRs) in L-amino acid taste transduction. Calcium imaging of isolated taste sensory cells (TSCs) of T1r3-GFP and T1r3 knock-out (T1r3 KO) mice was performed using the ratiometric dye Fura 2 AM to investigate the role of different mGluRs in detecting various L-amino acids and inosine 5' monophosphate (IMP). Using agonists selective for various mGluRs such as (RS)-3,5-dihydroxyphenylglycine (DHPG) (an mGluR1 agonist) and L-(+)-2-amino-4-phosphonobutyric acid (l-AP4) (an mGluR4 agonist), we evaluated TSCs to determine if they might respond to these agonists, IMP, and three L-amino acids (monopotassium L-glutamate, L-serine and L-arginine). Additionally, we used selective antagonists against different mGluRs such as (RS)-L-aminoindan-1,5-dicarboxylic acid (AIDA) (an mGluR1 antagonist), and (RS)-α-methylserine-O-phosphate (MSOP) (an mGluR4 antagonist) to determine if they can block responses elicited by these L-amino acids and IMP. We found that L-amino acid- and IMP-responsive cells also responded to each agonist. Antagonists for mGluR4 and mGluR1 significantly blocked the responses elicited by IMP and each of the L-amino acids. Collectively, these data provide evidence for the involvement of taste and brain variants of mGluR1 and mGluR4 in L-amino acid and IMP taste responses in mice, and support the concept that multiple receptors contribute to IMP and L-amino acid taste.

Group III metabotropic glutamate receptors: pharmacology, physiology and therapeutic potential

Glutamate, the primary excitatory neurotransmitter in the central nervous system (CNS), exerts neuromodulatory actions via the activation of metabotropic glutamate (mGlu) receptors. There are eight known mGlu receptor subtypes (mGlu1-8), which are widely expressed throughout the brain, and are divided into three groups (I-III), based on signalling pathways and pharmacological profiles. Group III mGlu receptors (mGlu4/6/7/8) are primarily, although not exclusively, localised on presynaptic terminals, where they act as both auto- and hetero-receptors, inhibiting the release of neurotransmitter. Until recently, our understanding of the role of individual group III mGlu receptor subtypes was hindered by a lack of subtype-selective pharmacological tools. Recent advances in the development of both orthosteric and allosteric group III-targeting compounds, however, have prompted detailed investigations into the possible functional role of these receptors within the CNS, and revealed their involvement in a number of pathological conditions, such as epilepsy, anxiety and Parkinson's disease. The heterogeneous expression of group III mGlu receptor subtypes throughout the brain, as well as their distinct distribution at glutamatergic and GABAergic synapses, makes them ideal targets for therapeutic intervention. This review summarises the advances in subtype-selective pharmacology, and discusses the individual roles of group III mGlu receptors in physiology, and their potential involvement in disease.

Group III mGlu receptor agonist, ACPT-I, exerts potential neuroprotective effects in vitro and in vivo

Many evidence suggest that metabotropic glutamate receptors (mGluRs) may modulate glutamatergic transmission, hence, these receptors are regarded as potential targets for neuroprotective drugs. Since group III mGlu receptor agonists are known to reduce glutamatergic transmission by inhibiting glutamate release, we decided to investigate the neuroprotective potential of the group III mGlu receptor agonist, (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid (ACPT-I) against kainate (KA)-induced excitotoxicity in vitro and in vivo. In primary neuronal cell cultures ACPT-I (1-200 μM), applied 30 min-3 h after starting the exposure to KA (150 μM), significantly attenuated the KA-induced LDH release, increased cell viability, and inhibited caspase-3 activity both in cortical and hippocampal cell cultures. The effects were dose-, time- and structure-dependent. The neuroprotective effects of ACPT-I were reversed by (RS)-alpha-cyclopropyl-4-phosphonophenyl glycine, a group III mGluR antagonist. In the in vivo studies, KA (2.5 nmol/1 μl) was unilaterally injected into the rat dorsal CA1 hippocampal region and the size of degeneration was examined by stereological counting of surviving neurons in the CA pyramidal layer. It was found that ACPT-I (7.5 or 15 nmol/1 μl), injected into the dorsal hippocampus 30 min, 1 or 3 h after KA in dose-dependent manner prevented the KA-induced neuronal damage. Moreover, in vivo microdialysis studies in the rat hippocampus showed that ACPT-I (200 μM) given simultaneously with KA (50 μM) significantly diminished the KA-induced glutamate release in the hippocampus. This mechanism seems to play a role in mediating the neuroprotective effect of ACPT-I.

Supraspinal metabotropic glutamate receptor subtype 8: a switch to turn off pain

Glutamate is the main excitatory neurotransmitter in the central nervous system and as such controls the majority of synapses. Glutamatergic neurotransmission is mediated via ionotropic and metabotropic glutamate receptors (iGluRs and mGluRs). Signaling via mGluRs permits to finely tune, rather than turning on/off, the excitatory neurotransmission as the iGluRs do. Eight mGluRs (mGluR1-8) have been cloned so far, which have been divided into three groups based on sequence homology, pharmacological properties and second messenger signaling. mGluRs are widely expressed both on glia and neurons. On neurons they are located both at postsynaptic (group I) and presynaptic sites (group II and III). Group II and III mGluR stimulation reduces glutamate release, which can prove useful in pathological conditions characterized by elevated glutamatergic neurotransmission which include chronic pain. Indeed, mGluRs are widely distributed on pain neuraxis. The recent development of selective mGluR ligands has permitted investigating the individual role of each mGluR on pain control. The development of (S)-3,4-dicarboxyphenylglycine, a selective mGluR8 agonist, has revealed the mGluR8 role in inhibiting pain and its related affective consequences in chronic pain conditions. mGluR8 proved also to be overexpressed in pain controlling areas during pathological pain guaranteeing the availability of a switch for turning off abnormal pain. Thus, mGluR8 corresponds to an ideal target in designing novel analgesics. This review will focus on the novel insights into the mGluR8 role on pain control, with particular emphasis on the supraspinal descending pathway, an antinociceptive endogenous source, whose activation or disinhibition (via mGluR8) induces analgesia.

Astrocyte activation of presynaptic metabotropic glutamate receptors modulates hippocampal inhibitory synaptic transmission

In the CNS, fine processes of astrocytes often wrap around dendrites, axons and synapses, which provides an interface where neurons and astrocytes might interact. We have reported previously that selective Ca(2+) elevation in astrocytes, by photolysis of caged Ca(2+) by o-nitrophenyl-EGTA (NP-EGTA), causes a kainite receptor-dependent increase in the frequency of spontaneous inhibitory post-synaptic potentials (sIPSCs) in neighboring interneurons in hippocampal slices. However, tetrodotoxin (TTX), which blocks action potentials, reduces the frequency of miniature IPSCs (mIPSCs) in interneurons during Ca(2+) uncaging by an unknown presynaptic mechanism. In this study we investigate the mechanism underlying the presynaptic inhibition. We show that Ca(2+) uncaging in astrocytes is accompanied by a decrease in the amplitude of evoked IPSCs (eIPSCs) in neighboring interneurons. The decreases in eIPSC amplitude and mIPSC frequency are prevented by CPPG, a group II/III metabotropic glutamate receptor (mGluR) antagonist, but not by the AMPA/kainate and NMDA receptor antagonists CNQX/CPP. Application of either the group II mGluR agonist DCG IV or the group III mGluR agonist L-AP4 decreased the amplitude of eIPSCs by a presynaptic mechanism, and both effects are blocked by CPPG. Thus, activation of mGluRs mediates the effects of Ca(2+) uncaging on mIPSCs and eIPSCs. Our results indicate that Ca(2+)-dependent release of glutamate from astrocytes can activate distinct classes of glutamate receptors and differentially modulate inhibitory synaptic transmission in hippocampal interneurons.

Antiparkinsonian potential of targeting group III metabotropic glutamate receptor subtypes in the rodent substantia nigra pars reticulata

BACKGROUND AND PURPOSE

Increased firing of the glutamatergic pathway between the subthalamic nucleus and substantia nigra pars reticulata (SNpr) contributes to the abnormal firing of motor circuits and subsequent motor deficits seen in Parkinson's disease. Broad spectrum agonist-induced activation of presynaptic group III metabotropic glutamate (mGlu) receptors within the SNpr reduced glutamate release and reversed akinesia in the reserpine-treated rat model of Parkinson's disease. Here, we have sought to identify which subtypes of group III mGlu receptor in the SNpr were responsible for these beneficial effects.

EXPERIMENTAL APPROACH

The ability of the mGlu(4) positive allosteric modulator, N-phenyl-7-(hydroxyminocyclopropa[b]chromen-1a-carboxamide) (PHCCC), the mGlu(7) allosteric agonist, N,N'-dibenzhydrylethane-1,2-diamine dihydrochloride (AMN082) and the mGlu(8) -selective agonist (S)-3,4-dicarboxyphenylglycine [(S)-3,4-DCPG] to inhibit KCl-evoked [(3) H]-D-aspartate release was examined in vitro in rat nigral prisms. Reversal of akinesia in reserpine-treated rats was also assessed following intranigral injection of these agents.

KEY RESULTS

PHCCC and AMN082 inhibited [(3) H]-D-aspartate release by 42% and 53%, respectively when given alongside a sub-threshold concentration of the broad spectrum group III agonist, L-2-amino-4-phosphonobutyrate (L-AP4; 1 µM). In contrast (S)-3,4-DCPG failed to inhibit [(3) H]-D-aspartate release. All three agents also reversed reserpine-induced akinesia although only the effects of PHCCC and AMN082 were inhibited by pre-treatment with the group III antagonist (RS)-α-cyclopropyl-4-phosphonophenylglycine (CPPG).

CONCLUSIONS AND IMPLICATIONS

These findings reveal that targeting SNpr mGlu(4) or mGlu(7) receptors, but not mGlu(8) receptors, provided relief from akinesia in the reserpine-treated rat model of Parkinson's disease, most likely reflecting inhibition of excess glutamate release in this region.

Development of GPCR modulation of GABAergic transmission in chicken nucleus laminaris neurons

Neurons in the nucleus laminaris (NL) of birds act as coincidence detectors and encode interaural time difference to localize the sound source in the azimuth plane. GABAergic transmission in a number of CNS nuclei including the NL is subject to a dual modulation by presynaptic GABA_B receptors (GABA_BRs) and metabotropic glutamate receptors (mGluRs). Here, using in vitro whole-cell patch clamp recordings from acute brain slices of the chick, we characterized the following important but unknown properties pertaining to such a dual modulation: (1) emergence of functional GABA synapses in NL neurons; (2) the temporal onset of neuromodulation mediated by GABA_BRs and mGluRs; and (3) the physiological conditions under which GABA_BRs and mGluRs are activated by endogenous transmitters. We found that (1) GABA_AR-mediated synaptic responses were observed in about half of the neurons at embryonic day 11 (E11); (2) GABA_BR-mediated modulation of the GABAergic transmission was detectable at E11, whereas the modulation by mGluRs did not emerge until E15; and (3) endogenous activity of GABA_BRs was induced by both low- (5 or 10 Hz) and high-frequency (200 Hz) stimulation of the GABAergic pathway, whereas endogenous activity of mGluRs was induced by high- (200 Hz) but not low-frequency (5 or 10 Hz) stimulation of the glutamatergic pathway. Furthermore, the endogenous activity of mGluRs was mediated by group II but not group III members. Therefore, autoreceptor-mediated modulation of GABAergic transmission emerges at the same time when the GABA synapses become functional. Heteroreceptor-mediated modulation appears at a later time and is receptor type dependent in vitro.

The role of glutamate in neuronal ischemic injury: the role of spark in fire

Although being a physiologically important excitatory neurotransmitter, glutamate plays a pivotal role in various neurological disorders including ischemic neurological diseases. Its level is increased during cerebral ischemia with excessive neurological stimulation causing the glutamate-induced neuronal toxicity, excitotoxicity, and this is considered the triggering spark in the ischemic neuronal damage. The glutamatergic stimulation will lead to rise in the intracellular sodium and calcium, and the elevated intracellular calcium will lead to mitochondrial dysfunction, activation of proteases, accumulation of reactive oxygen species and release of nitric oxide. Interruption of the cascades of glutamate-induced cell death during ischemia may provide a way to prevent, or at least reduce, the ischemic damage. Various therapeutic options are suggested interrupting the glutamatergic pathways, e.g., inhibiting the glutamate synthesis or release, increasing its clearance, blocking of its receptors or preventing the rise in intracellular calcium. Development of these strategies may provide future treatment options in the management of ischemic stroke.

Metabotropic glutamate receptors: from the workbench to the bedside

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

Multiple receptor systems for umami taste in mice

Recent molecular studies proposed that the T1r1/T1r3 heterodimer, mGluR1 and mGluR4 might function as umami taste receptors in mice. However, the roles of each of these receptors in umami taste are not yet clear. In this paper, we summarize recent data for T1r3, mGluR1, and mGluR4 as umami taste receptors and discuss receptor systems responsible for umami detection in mice.

Multiple receptors underlie glutamate taste responses in mice

l-Glutamate is known to elicit a unique taste, umami, that is distinct from the tastes of sweet, salty, sour, and bitter. Recent molecular studies have identified several candidate receptors for umami in taste cells, such as the heterodimer T1R1/T1R3 and brain-expressed and taste-expressed type 1 and 4 metabotropic glutamate receptors (brain-mGluR1, brain-mGluR4, taste-mGluR1, and taste-mGluR4). However, the relative contributions of these receptors to umami taste reception remain to be elucidated. We critically discuss data from recent studies in which mouse taste cell, nerve fiber, and behavioral responses to umami stimuli were measured to evaluate whether receptors other than T1R1/T1R3 are involved in umami responses. We particularly emphasized studies of umami responses in T1R3 knockout (KO) mice and studies of potential effects of mGluR antagonists on taste responses. The results of these studies indicate the existence of substantial residual responses to umami compounds in the T1R3-KO model and a significant reduction of umami responsiveness after administration of mGluR antagonists. These findings thus provide evidence of the involvement of mGluRs in addition to T1R1/T1R3 in umami detection in mice and suggest that umami responses, at least in mice, may be mediated by multiple receptors.

Multiple receptor systems for glutamate detection in the taste organ

L-Glutamate and 5'-ribonucleotides such as guanosine-5'-monophosphate (GMP) and inosine-5'-monophosphate (IMP) elicit a unique taste called 'umami' that is distinct from the tastes of sweet, salty, sour, and bitter. For umami, like sweet and bitter compounds, taste signaling is initiated by binding of tastants to G-protein-coupled receptors (GPCR) in taste bud cells. To date, several GPCRs for umami compounds have been identified in taste cells, including the heterodimer T1R1/T1R3, and truncated type 1 and 4 metabotropic glutamate receptors missing most of the N-terminal extracellular domain (taste-mGluR4 and truncated-mGluR1). Apparently contradictory data in T1R3 knock-out (KO) mouse models have been reported. One study showed that behavioral preference and taste nerve responses to umami stimuli in T1R3-KO mice were totally abolished, suggesting that T1R1/T1R3 is a sole receptor for umami taste. The other reported reduced but not abolished responses to umami in T1R3-KO mice, suggesting existence of multiple receptors for umami taste. In this paper, we summarized the data from recent studies that further addressed this issue by using different experimental techniques. Some of the studies provided additional evidence for the existence of umami receptor systems mediated by mGluR1 and mGluR4 in addition to T1R1/T1R3. It is proposed that the signal mediated by the pathway involving T1R1/T1R3 may play a different role from that derived from the mGluRs. The former occurs mainly in the anterior tongue, and plays a major role in preference behavior, whereas the latter occurs mainly in the posterior tongue and contributes to behavioral discrimination between umami and other taste compounds.

AMN082, an allosteric mGluR7 agonist that inhibits afferent glutamatergic transmission in rat basolateral amygdala

Glutamatergic neurotransmission has been implicated in the pathophysiology of psychiatric disorders, such as anxiety and depression. The possible contribution of group III metabotropic glutamate receptors has been poorly investigated, due to the lack of selective pharmacological tools. However, a selective agonist of mGLUR7, AMN082 has been identified recently, and shown to act through an allosteric mechanism in recombinant cells expressing the receptor. Thus, using AMN082, we examined the role of mGLUR7 in modulating synaptic transmission in the rat basolateral amygdala (BLA), a brain region known to be important for the genesis of anxious states. We found that bath application of AMN082 (1-10microM) produced a concentration-dependent inhibition of synaptic transmission evoked at 2Hz, but had no effect on transmission evoked at 0.05Hz. However, at this lower frequency, AMN082 (10microM) significantly increased the synaptic inhibition produced by a group III mGLUR agonist, L-AP4 (100microM). This effect was blocked by pre-application of CPPG (500microM), a group III mGLUR-preferring antagonist, consistent with the involvement of mGLUR7. Thus, we have shown that AMN082 can modulate high frequency synaptic transmission in the BLA, in vitro, and appears to act on the receptor via an allosteric mechanism. These results suggest that mGLUR7 has a unique role in regulating neuronal activity in the BLA and may be a target for novel drugs for the treatment of anxiety.

Antagonism of group III metabotropic glutamate receptors results in impairment of LTD but not LTP in the hippocampal CA1 region, and prevents long-term spatial memory

Recently it has emerged that hippocampal long-term depression (LTD) may play an important role in the acquisition and storage of spatial memories. This form of synaptic plasticity is tightly regulated by metabotropic glutamate receptors (mGluRs) that negatively couple to adenylyl cyclase. Activation of group III mGluRs is necessary for persistent hippocampal LTD, but is not required for depotentiation or long-term potentiation (LTP) in the dentate gyrus in vivo. In the CA1 region antagonism of group III mGluRs prevents LTD in vivo. Effects on LTP in vivo are as yet unknown. We investigated the effects of group III mGluR antagonism on LTP and LTD at Schaffer collateral-CA1 synapses, and on spatial learning in the eight-arm radial maze. Daily application of the group III mGluR antagonist (R,S)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) resulted in impairment of long-term (reference) memory, with effects becoming apparent 4 days after training and drug treatment began. Short-term (working) memory was unaffected throughout the 10-day study. Application of CPPG prevented LTD, but not LTP, in the CA1 region. These data suggest that activation of group III mGluRs is required for the establishment of spatial long-term memory. Their exclusive role in mediating hippocampal LTD provides correlational evidence for a role for LTD in the type of spatial learning studied.

Targeting group III metabotropic glutamate receptors produces complex behavioral effects in rodent models of Parkinson's disease

Drugs activating group III metabotropic glutamate receptors (mGluRs) represent therapeutic alternatives to L-DOPA (L-3,4-dihydroxyphenylalanine) for the treatment of Parkinson's disease (PD). Their presynaptic location at GABAergic and glutamatergic synapses within basal ganglia nuclei provide a critical target to reduce abnormal activities associated with PD. The effects of selective group III mGluR agonists (1S,3R,4S)-1-aminocyclopentane-1,3,4-tricarboxylic acid (ACPT-I) and L-(+)-2-amino-4-phosphonobutyric acid (L-AP4) infused into the globus pallidus (GP) or the substantia nigra pars reticulata (SNr) were thus studied in rat models of PD. Bilateral infusions of ACPT-I (1, 2.5, and 5 nmol/microl) into the GP fully reverse the severe akinetic deficits produced by 6-hydroxydopamine nigrostriatal dopamine lesions in a reaction-time task without affecting the performance of controls. Similar results were observed after L-AP4 (1 nmol) or picrotoxin, a GABA(A) receptor antagonist, infused into the GP. In addition, intrapallidal ACPT-I counteracts haloperidol-induced catalepsy. This effect is reversed by concomitant administration of a selective group III receptor antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine. In contrast, ACPT-I (0.05, 0.1, and 0.25 nmol) infusions into the SNr enhance the lesion-induced akinetic deficits in control and lesioned rats and do not reverse haloperidol-induced catalepsy. L-AP4 (0.05 nmol) and picrotoxin in the SNr produce the same effects. Together, these results show that activation of group III mGluRs in the GP provides benefits in parkinsonian rats, presumably by modulating GABAergic neurotransmission. The opposite effects produced by group III mGluR activation in the SNr, also observed with a selective mGluR8 agonist, support the use of subtype-selective group III mGluR agonists as a potential antiparkinsonian strategy.

Endogenous mGluR activity suppresses GABAergic transmission in avian cochlear nucleus magnocellularis neurons

GABAergic transmission in the avian cochlear nucleus magnocellularis (NM) of the chick is subject to modulation by gamma-aminobutyric acid type B (GABA(B)) autoreceptors. Here, I investigated modulation of GABAergic transmission in NM by metabotropic glutamate receptors (mGluRs) with whole cell recordings in brain slice preparations. I found that tACPD, a nonspecific mGluR agonist, exerted dose-dependent suppression on evoked inhibitory postsynaptic currents (eIPSCs) in NM neurons. At concentrations of 100 or 200 microM, tACPD increased the failure rate of GABAergic transmission. Agonists for group I (3,5-DHPG, 200 microM), group II (DCG-IV, 2 microM), and group III (L-AP4, 10 microM) mGluRs produced a significant reduction in the amplitude of eIPSCs and a significant increase in failure rate, indicating the involvement of multiple mGluRs in this modulation. The frequency, but not the amplitude, of miniature IPSCs (mIPSCs) was decreased significantly by 3,5-DHPG or DCG-IV. Neither frequency nor amplitude of mIPSCs was affected by L-AP4. mGluR antagonists LY341495 (20 microM) plus CPPG (10 microM) significantly increased the amplitude of eIPSCs, indicating that endogenous mGluR activity suppresses GABA release to NM neurons. Furthermore, blockage of mGluRs increased GABA-evoked discharges recorded under physiological Cl(-) concentrations, whereas tACPD (100 microM) eliminated them. The results indicate that mGluRs play important roles in achieving balanced excitation and inhibition in NM and preserving fidelity of temporal information encoded by NM neurons.

Anxiolytic-like effect of group III mGlu receptor antagonist is serotonin-dependent

Literature data have provided evidence that antagonists of group I metabotropic glutamate receptors (mGluRs) and agonists of group II/III mGluRs show anxiolytic-like properties in preclinical studies. However data reporting anxiolytic-like action of group III mGlu receptor antagonists were also published. In the present paper we investigated the anxiolytic-like activity of the group III mGlu receptor antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG). To examine its anxiolytic-like effects, the basolateral amygdala was chosen as an injection site, as this brain region is involved in the regulation of anxiety-related behavior. To detect anxiolytic-like activity, the Vogel conflict-drinking test in rats was used. Intra-amygdalar injections of CPPG exhibited dose-dependent, potent anxiolytic-like action at a dose of 75 nmol, which was blocked by a concomitant administration of the group III mGlu receptor agonist CI (S,3R,4S)-1-aminocyclo-pentane-1,3,4-tricarboxylic acid (ACPT-I) at a dose of 7.5 nmol. The benzodiazepine receptor antagonist flumazenil (given intraperitoneally, 10 mg/kg) did not change the anxiolytic-like effect of CPPG, but that effect was abolished by the non-selective antagonist of 5-HT receptors metergoline and the antagonist of 5-HT2A/C receptors ritanserin (both given intraperitoneally at doses of 2 and 0.5 mg/kg, respectively). These findings suggest that the blockade of group III mGlu receptors in the amygdala is responsible for anxiolysis and that serotonergic, but not the benzodiazepine recognition site of the GABA-ergic system are involved in the anxiolytic-like response induced by group III mGlu antagonist.

Secretion of L-glutamate from osteoclasts through transcytosis

Osteoclasts are involved in the catabolism of the bone matrix and eliminate the resulting degradation products through transcytosis, but the molecular mechanism and regulation of transcytosis remain poorly understood. Upon differentiation, osteoclasts express vesicular glutamate transporter 1 (VGLUT1), which is essential for vesicular storage and subsequent exocytosis of glutamate in neurons. VGLUT1 is localized in transcytotic vesicles and accumulates L-glutamate. Osteoclasts secrete L-glutamate and the bone degradation products upon stimulation with KCl or ATP in a Ca2+-dependent manner. KCl- and ATP-dependent secretion of L-glutamate was absent in osteoclasts prepared from VGLUT1-/- knockout mice. Osteoclasts express mGluR8, a class III metabotropic glutamate receptor. Its stimulation by a specific agonist inhibits secretion of L-glutamate and bone degradation products, whereas its suppression by a specific antagonist stimulates bone resorption. Finally, it was found that VGLUT1-/- mice develop osteoporosis. Thus, in bone-resorbing osteoclasts, L-glutamate and bone degradation products are secreted through transcytosis and the released L-glutamate is involved in autoregulation of transcytosis. Glutamate signaling may play an important role in the bone homeostasis.

Pattern-specific synaptic mechanisms in a multifunctional network. II. Intrinsic modulation by metabotropic glutamate receptors

The in vitro respiratory network contained in the transverse brain stem slice of mice simultaneously generates fast (approximately 15 min(-1)) and slow ( approximately 0.5 min(-1)) rhythmic activities corresponding to fictive eupnea ("normal" breathing) and fictive sighs. We show that these two activity patterns are differentially controlled through the modulatory actions of metabotropic glutamate receptors (mGluRs). Sighs were selectively inhibited by agonists of the group III mGluRs according to a pharmacological profile most consistent with activation of mGluR8. Sighs were also blocked by the supposedly inactive L-isomer of the widely used N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonopentanoic acid (L-AP5, 5 microM), an effect that was abolished in the presence of group III mGluR antagonists. Excitatory postsynaptic potentials (EPSPs) were recorded in pre-Bötzinger Complex neurons after stimulation of the contralateral ventral respiratory group (VRG); evoked EPSP amplitude was variably reduced after bath application of the group III agonist L-serine-O-phosphate (L-SOP), with an average reduction of 15%. Therefore although group III mGluRs do play a role in regulating synapse strength, this seems to be only a minor factor in the regulation of synapses made by midline-crossing axons. Intrinsic modulation of the respiratory central pattern generator by mGluRs appears to be an essential component of the multifunctionality that characterizes this network.

Activation of receptors negatively coupled to adenylate cyclase is required for induction of long-term synaptic depression at Schaffer collateral-CA1 synapses

Chemical LTD (CLTD) of synaptic transmission is triggered by simultaneously increasing presynaptic [cGMP] while inhibiting PKA. Here, we supply evidence that class II, but not III, metabotropic glutamate receptors (mGluRs), and A1 adenosine receptors, both negatively coupled to adenylate cyclase, play physiologic roles in providing PKA inhibition necessary to promote the induction of LTD at Schaffer collateral-CA1 synapses in hippocampal slices. Simultaneous activation of group II mGluRs with the selective agonist (2S,2'R,3'R)-2-(2',3'-dicarboxy-cyclopropyl) glycine (DCGIV; 5 microM), while raising [cGMP] with the type V phosphodiesterase inhibitor, zaprinast (20 microM), resulted in a long-lasting depression of synaptic strength. When zaprinast (20 microM) was combined with a cell-permeant PKA inhibitor H-89 (10 microM), the need for mGluR IIs was bypassed. DCGIV, when combined with a "submaximal" low frequency stimulation (1 Hz/400 s), produced a saturating LTD. The mGluR II selective antagonist, (2S)-alpha-ethylglutamic acid (EGLU; 5 microM), blocked induction of LTD by prolonged low frequency stimulation (1 Hz/900 s). In contrast, the mGluR III selective receptor blocker, (RS)-a-Cyclopropyl-[3- 3H]-4-phosphonophenylglycine (CPPG; 10 microM), did not impair LTD. The selective adenosine A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 100 nM), also blocked induction of LTD, while the adenosine A1 receptor agonist N6-cyclohexyl adenosine (CHA; 50 nM) significantly enhanced the magnitude of LTD induced by submaximal LFS and, when paired with zaprinast (20 microM), was sufficient to elicit CLTD. Inhibition of PKA with H-89 rescued the expression of LTD in the presence of either EGLU or DPCPX, confirming the hypothesis that both group II mGluRs and A1 adenosine receptors enhance the induction of LTD by inhibiting adenylate cyclase and reducing PKA activity.

Group III metabotropic glutamate-receptor-mediated modulation of excitatory transmission in rodent substantia nigra pars compacta dopamine neurons

Glutamate plays an important role in the regulation of dopamine neuron activity. In particular, the glutamatergic input from the subthalamic nucleus is thought to provide control over dopamine neuron firing patterns. The degeneration of dopamine neurons in the substantia nigra pars compacta (SNc) observed in Parkinson's disease (PD) is believed to be due to a complex interplay of factors, including oxidative stress and mitochondrial dysfunction. Although glutamate is not the primary cause of cell death in PD, there is evidence suggesting excessive glutamate release onto dopamine neurons may play a role in continued degeneration. Although many studies have focused on the role of glutamate in the SNc, little work has been directed at exploring the modulatory control of glutamate release in this region. Previous studies have found a high-potency inhibitory effect of nonselective group III mGluR agonist on glutamatergic transmission in the SNc. Using whole-cell patch-clamp methods and novel pharmacological tools, we have determined that mGluR4 mediates the group III mGluR modulation of excitatory transmission in the rat SNc. The group III mGluR-selective agonist l-(+)-2-amino-4-phosphonobutyric acid inhibits excitatory transmission in the SNc at low micromolar concentrations with a maximal inhibition occurring at 3 muM. This effect was potentiated by the mGluR4-selective allosteric modulator N-phenyl-7-(hydroxymino)cyclopropa[b]chromen-1a-carboxamide and was not mimicked by the mGluR8-selective agonist (S)-3,4-dicarboxyphenylglycine. Interestingly, in an attempt to employ knockout mice to confirm the role of mGluR4, we discovered an apparent species difference suggesting that in mice, both mGluR4 and mGluR8 modulate excitatory transmission in the SNc.

Group III metabotropic glutamate receptors maintain tonic inhibition of excitatory synaptic input to hypocretin/orexin neurons

Hypocretin/orexin neurons play an important role in hypothalamic arousal. Synaptic glutamate input to hypocretin neurons regulates cell firing. We studied the actions of group III metabotropic glutamate receptors (mGluRs) in modulating the activity of hypocretin neurons using whole-cell voltage- and current-clamp recording in mouse whole hypothalamic slices or minislices consisting only of the lateral hypothalamus. Selective green fluorescent protein expression was used to detect live hypocretin neurons. The mGluR agonist l-(+)-2-amino-4-phosphonobutyric acid (l-AP-4) inhibited synaptic input to hypocretin neurons in a dose-dependent manner; both spontaneous glutamate and GABA-mediated synaptic currents were reduced in frequency. l-AP-4 also reduced the amplitude of postsynaptic potentials evoked by a stimulating electrode placed medial or lateral to the recorded cell. No postsynaptic effect of l-AP-4 was found relative to membrane potential, input resistance, or AMPA-evoked currents. l-AP-4 appeared to act by a presynaptic mechanism and reduced the frequency of both glutamate- and GABA-mediated miniature events recorded in the presence of tetrodotoxin, with no change in amplitude. (RS)-phosphonopentanoic acid (CPPG), a group III mGluR antagonist, suppressed the actions of l-AP-4. Of substantial interest, CPPG by itself increased synaptic activity recorded in hypocretin neurons, suggesting an ongoing inhibitory tone attributable to activation of group III mGluRs. Glutamatergic interneurons have been suggested to play a role in a positive feedback recruitment of hypocretin on hypocretin neurons. l-AP-4 blocked hypocretin-mediated increases in EPSCs and attenuated the hypocretin-mediated increase in spike frequency. Together, these data suggest that tonically active inhibitory mGluRs are expressed on local hypocretin-sensitive glutamate neurons within the lateral hypothalamus that modulate the output of the hypocretin arousal system.

Group III mGlu receptor agonists produce anxiolytic- and antidepressant-like effects after central administration in rats

It was well established that compounds which decrease glutamatergic transmission via blockade of NMDA or group I mGlu receptors produce anxiolytic- and antidepressant-like action in animal tests and models. Since group III metabotropic glutamate receptor (mGluR) agonists are known to reduce glutamatergic neurotransmission by the inhibition of glutamate release, we decided to investigate potential anxiolytic- and/or antidepressant-like effects of group III mGluR agonists, after central administration in rats. It was found that group III mGluR agonists, (1S,3R,4S)-1-aminocyclo-pentane-1,3,4-tricarboxylic acid (ACPT-I) and 2-amino-4-(3-hydroxy-5-methylisoxazol-4-yl)butyric acid (HomoAMPA), given intrahippocampally, produced a dose-dependent anxiolytic-like effect in the conflict drinking test. The effects of ACPT-I and HomoAMPA were reversed by (RS)-alpha-cyclopropyl-4-phosphonophenyl glycine (CPPG), group III mGluR antagonist. Moreover, a dose-dependent antidepressant-like action of group III mGluR agonists, ACPT-I and (RS)-4-phosphonophenylglycine (RS-PPG), but not HomoAMPA, was found in behavioral despair test, after intracerebroventricular injections, and the effect of ACPT-I was reversed by CPPG. The results obtained indicate that group III mGluR agonists produce anxiolytic- as well as antidepressant-like effects in behavioral tests, after central administration in rats. The reduction of glutamate release by group III mGluR activation may be a possible mechanism underlying anxiolytic- and antidepressant-like properties of the tested compounds. In conclusion, the results of our studies indicate that group III mGlu receptor agonists may play a role in the therapy of both anxiety and depression.

Metabotropic glutamate receptor type 4 is involved in autoinhibitory cascade for glucagon secretion by alpha-cells of islet of Langerhans

In islets of Langerhans, L-glutamate is stored in glucagon-containing secretory granules of alpha-cells and cosecreted with glucagon under low-glucose conditions. The L-glutamate triggers secretion of gamma-aminobutyric acid (GABA) from beta-cells, which in turn inhibits glucagon secretion from alpha-cells through the GABAA receptor. In the present study, we tested the working hypothesis that L-glutamate functions as an autocrine/paracrine modulator and inhibits glucagon secretion through a glutamate receptor(s) on alpha-cells. The addition of L-glutamate at 1 mmol/l; (R,S)-phosphonophenylglycine (PPG) and (S)-3,4-dicarboxyphenylglycine (DCPG), specific agonists for class III metabotropic glutamate receptor (mGluR), at 100 micromol/l; and (1S,3R,4S)-1-aminocyclopentane-1,3,4-tricarboxylic acid (ACPT-I) at 50 micromol/l inhibited the low-glucose-evoked glucagon secretion by 87, 81, 73, and 87%, respectively. This inhibition was dose dependent and was blocked by (R,S)-cyclopropyl-4-phosphonophenylglycine (CPPG), a specific antagonist of class III mGluR. Agonists of other glutamate receptors, including kainate and quisqualate, had little effectiveness. RT-PCR and immunological analyses indicated that mGluR4, a class III mGluR, was expressed and localized with alpha- and F cells, whereas no evidence for expression of other mGluRs, including mGluR8, was obtained. L-Glutamate, PPG, and ACPT-I decreased the cAMP content in isolated islets, which was blocked by CPPG. Dibutylyl-cAMP, a nonhydrolyzable cAMP analog, caused the recovery of secretion of glucagon. Pertussis toxin, which uncouples adenylate cyclase and inhibitory G-protein, caused the recovery of both the cAMP content and secretion of glucagon. These results indicate that alpha- and F cells express functional mGluR4, and its stimulation inhibits secretion of glucagon through an inhibitory cAMP cascade. Thus, L-glutamate may directly interact with alpha-cells and inhibit glucagon secretion.

Activation of group III metabotropic glutamate receptors presynaptically reduces both GABAergic and glutamatergic transmission in the rat globus pallidus

To investigate the role of group III metabotropic glutamate receptors (mGluRs) in the globus pallidus (GP), whole-cell recordings were performed using rat brain slice preparations. Application of the group III mGluRs specific agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) suppressed the amplitude of striatal stimulation-induced IPSCs and internal capsule stimulation-induced EPSCs in most of the GP neurons that were capable of generating repetitive firing without spike accommodation. The suppression of IPSCs and EPSCs was accompanied by an increase in the paired-pulse ratio. The L-AP4 effects were antagonized by (R,S)-alpha-cyclopropyl-4-phosphophenylglycine, a blocker for group II/III mGluRs. L-AP4 reduced the frequency of mIPSCs and mEPSCs without changing their amplitude distribution. L-AP4 failed to change iontophoretic glutamate induced responses. These results suggest that the subthalamo-pallidal glutamatergic input might homo- and hetero-synaptically control GABAergic and glutamatergic transmission in the GP.

Group III metabotropic glutamate receptor-mediated modulation of the striatopallidal synapse

The globus pallidus (GP) is a key GABAergic nucleus in the basal ganglia (BG). The predominant input to the GP is an inhibitory striatal projection that forms the first synapse in the indirect pathway. The GP GABAergic neurons project to the subthalamic nucleus, providing an inhibitory control of these glutamatergic cells. Given its place within the BG circuit, it is not surprising that alterations in GP firing pattern are postulated to play a role in both normal and pathological motor behavior. Because the inhibitory striatal input to the GP may play an important role in shaping these firing patterns, we set out to determine the role that the group III metabotropic glutamate receptors (GluRs) play in modulating transmission at the striatopallidal synapse. In rat midbrain slices, electrical stimulation of the striatum evoked GABA(A)-mediated IPSCs recorded in all three types of GP neurons. The group III mGluR-selective agonist L-(+)-2-amino-4-phosphonobutyric acid (L-AP4) inhibited these IPSCs through a presynaptic mechanism of action. L-AP4 exhibited high potency and a pharmacological profile consistent with mediation by mGluR4. Furthermore, the effect of L-AP4 on striatopallidal transmission was absent in mGluR4 knock-out mice, providing convincing evidence that mGluR4 mediates this effect. The finding that mGluR4 may selectively modulate striatopallidal transmission raises the interesting possibility that activation of mGluR4 could decrease the excessive inhibition of the GP that has been postulated to occur in Parkinson's disease. Consistent with this, we find that intracerebroventricular injections of L-AP4 produce therapeutic benefit in both acute and chronic rodent models of Parkinson's disease.

Group III metabotropic glutamate receptor-mediated modulation of the striatopallidal synapse

The globus pallidus (GP) is a key GABAergic nucleus in the basal ganglia (BG). The predominant input to the GP is an inhibitory striatal projection that forms the first synapse in the indirect pathway. The GP GABAergic neurons project to the subthalamic nucleus, providing an inhibitory control of these glutamatergic cells. Given its place within the BG circuit, it is not surprising that alterations in GP firing pattern are postulated to play a role in both normal and pathological motor behavior. Because the inhibitory striatal input to the GP may play an important role in shaping these firing patterns, we set out to determine the role that the group III metabotropic glutamate receptors (GluRs) play in modulating transmission at the striatopallidal synapse. In rat midbrain slices, electrical stimulation of the striatum evoked GABA(A)-mediated IPSCs recorded in all three types of GP neurons. The group III mGluR-selective agonist L-(+)-2-amino-4-phosphonobutyric acid (L-AP4) inhibited these IPSCs through a presynaptic mechanism of action. L-AP4 exhibited high potency and a pharmacological profile consistent with mediation by mGluR4. Furthermore, the effect of L-AP4 on striatopallidal transmission was absent in mGluR4 knock-out mice, providing convincing evidence that mGluR4 mediates this effect. The finding that mGluR4 may selectively modulate striatopallidal transmission raises the interesting possibility that activation of mGluR4 could decrease the excessive inhibition of the GP that has been postulated to occur in Parkinson's disease. Consistent with this, we find that intracerebroventricular injections of L-AP4 produce therapeutic benefit in both acute and chronic rodent models of Parkinson's disease.

A novel role of pedunculopontine tegmental kainate receptors: a mechanism of rapid eye movement sleep generation in the rat

Considerable evidence suggests that pedunculopontine tegmental cholinergic cells are critically involved in normal regulation of rapid eye movement sleep. The major excitatory input to the cholinergic cell compartment of the pedunculopontine tegmentum arises from glutamatergic neurons in the pontine reticular formation. Immunohistochemical studies reveal that both ionotropic and metabotropic receptors are expressed in pedunculopontine tegmental cells. This study aimed to identify the role of endogenous glutamate and its specific receptors in the pedunculopontine tegmentum in the regulation of physiological rapid eye movement sleep. To identify this physiological rapid eye movement sleep-inducing glutamate receptor(s) in the pedunculopontine tegmental cholinergic cell compartment, specific receptors were blocked differentially by local microinjection of selective glutamate receptor antagonists into the pedunculopontine tegmental cholinergic cell compartment while quantifying the effects on rapid eye movement sleep in freely moving chronically instrumented rats. By comparing the alterations in the patterns of rapid eye movement sleep following injections of control vehicle and selective glutamate receptor antagonists, contributions made by each receptor subtype in rapid eye movement sleep were evaluated. The results demonstrate that when kainate receptors were blocked by local microinjection of a kainate receptor selective antagonist, spontaneous rapid eye movement sleep was completely absent for the first 2 h, and for the next 2 h the total percentage of rapid eye movement sleep was significantly less compared to the control values. In contrast, when N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid, groups I, II, and III metabotropic receptors were blocked, total percentages of rapid eye movement sleep did not change compared to the control values. These findings suggest, for the first time, that the activation of kainate receptors within the cholinergic cell compartment of the pedunculopontine tegmentum is a critical step for the regulation of normal rapid eye movement sleep in the freely moving rat. The results also suggest that the different types of glutamate receptors within a small part of the brainstem may be involved in different types of physiological functions.

Cardiovascular responses to activation of metabotropic glutamate receptors in the nTS of the rat

Although several agonists and antagonists for different subtypes of metabotropic glutamate receptors (mGLURs) have become available in recent years, detailed information regarding their selectivity is not complete in the in vivo animal models. The purpose of the present investigation was to study the cardiovascular effects of microinjections of some of these mGLUR agonists and antagonists into the nucleus tractus solitarius (nTS). Microinjections (100 nl) of EC(50) concentrations of 3,5-DHPG (0.005 mM; mGLUR(1) agonist), APDC (17.3 mM; mGLUR(2/3) agonist), PPG (11.7 mM; mGLUR(8) agonist) and L-AP(4) (1 mM; mGLUR(4) agonist) into the nucleus tractus solitarius of urethane-anesthetized male Wistar rats elicited depressor and bradycardic responses which may be mediated by pre- and/or postsynaptic mechanisms. The blocking effect of mGLUR antagonists used here was not specific for any one type of glutamate receptors (GLURs). For example, AIDA (50 mM; mGLUR(1) antagonist) blocked the effects of EC(50) concentrations of 3,5-DHPG, and PPG. LY341495 (135 mM; mGLUR(2/3) antagonist) blocked all of the mGLURs and ionotropic GLURs. EGLU, APICA and MCCG (250 mM each; mGLUR(2/3) antagonists) blocked the effects of APDC, NMDA and AMPA. CPPG (80 mM) and MSOP (125 mM), mGLUR(4) antagonists, blocked the effects of 3,5-DHPG, PPG and L-AP(4.) D-AP7 (NMDA receptor antagonist) and NBQX (a non-NMDA receptor antagonist) did not alter the responses of any of the mGLUR agonists. The data presented may be useful in assessing the role of metabotropic and ionotropic GLURs in mediating different cardiovascular reflexes.

Localization and function of group III metabotropic glutamate receptors in rat pancreatic islets

Pancreatic islets contain ionotropic glutamate receptors that can modulate hormone secretion. The purpose of this study was to determine whether islets express functional group III metabotropic glutamate (mGlu) receptors. RT-PCR analysis showed that rat islets express the mGlu8 receptor subtype. mGlu8 receptor immunoreactivity was primarily displayed by glucagon-secreting alpha-cells and intrapancreatic neurons. By demonstrating the immunoreactivities of both glutamate and the vesicular glutamate transporter 2 (VGLUT2) in these cells, we established that alpha-cells express a glutamatergic phenotype. VGLUT2 was concentrated in the secretory granules of islet cells, suggesting that glutamate might play a role in the regulation of glucagon processing. The expression of mGlu8 by glutamatergic cells also suggests that mGlu8 may function as an autoreceptor to regulate glutamate release. Pancreatic group III mGlu receptors are functional because mGlu8 receptor agonists inhibited glucagon release and forskolin-induced accumulation of cAMP in isolated islets, and (R,S)-cyclopropyl-4-phosphonophenylglycine, a group III mGlu receptor antagonist, reduced these effects. Because excess glucagon secretion causes postprandial hyperglycemia in patients with type 2 diabetes, group III mGlu receptor agonists could be of value in the treatment of these patients.

Mechanism of NMDA receptor contribution to axon-to-glia signaling in the crayfish medial giant nerve fiber

Electrical stimulation of crayfish giant axons at high frequency activates group II metabotropic and NMDA glutamate receptors on adjacent glial cells via release of N-acetylaspartylglutamate and glutamate formed upon its hydrolysis. This produces a transient depolarization followed by a prolonged hyperpolarization of glial cells that involves nicotinic acetylcholine receptor activation. The hyperpolarization is nearly completely blocked by antagonists of metabotropic glutamate receptors but only slightly reduced by inhibition of NMDA receptors. We report that the NMDA-induced hyperpolarization of glial cells is reduced by decreased calcium in the solution bathing the giant nerve fiber, while removal of sodium ions or block of voltage-dependent calcium channels completely prevents the glial response to NMDA. Inhibition of nicotinic acetylcholine receptors or removal of extracellular Cl(-) converts the glial response from a hyperpolarization to a depolarization that is sensitive to NMDA receptor antagonist. We propose that NMDA receptor activation by glutamate, formed from extracellular N-acetylaspartylglutamate during nerve stimulation, contributes to glial hyperpolarization by increasing intracellular Ca(2+) via opening of voltage-sensitive Ca(2+) channels. Based on our previous work, we propose further that the added Ca(2+) supplements that produced by N-acetylaspartylglutamate and glutamate acting on group II metabotropic glutamate receptors to cause an increased release of acetylcholine and a larger hyperpolarization.

Activation of groups I or III metabotropic glutamate receptors inhibits excitatory transmission in the rat subthalamic nucleus

The subthalamic nucleus (STN) is a key nucleus in the basal ganglia motor circuit that provides the major glutamatergic excitatory input to the basal ganglia output nuclei. The STN plays an important role in the normal motor function, as well as in pathological conditions such as Parkinson's disease. Development of a complete understanding of the role of the STN in motor control will require a detailed understanding of the mechanisms involved in the regulation of excitatory and inhibitory synaptic transmission in this nucleus. Here, we report that activation of groups I or III metabotropic glutamate (mGlu) receptors, but not group II, causes a depression of excitatory transmission in the STN. In contrast, mGlu receptor activation has no effect on the inhibitory transmission in this nucleus. Further characterization of the group I mGlu receptor-induced effect on EPSCs suggests that this response is mediated by mGlu1 and not mGlu5. Further, paired pulse studies suggest that both the mGlu1 receptor and the group III mGlu receptor-mediated effects are due to a presynaptic mechanism. If these receptors are involved in endogenous synaptic transmission in the STN, these results raise the exciting possibility that selective agents targeting mGlu receptors may provide a novel approach for the treatment of motor disorders involving the STN.

Group II and III metabotropic glutamate receptors contribute to different aspects of visual response processing in the rat superior colliculus

1. Neurones in the superior colliculus (SC) respond to novel sensory stimuli and response habituation is a key feature of this. It is known that both ionotropic and metabotropic glutamate (mGlu) receptors participate in visual responses of superficial SC neurones. A feature of Group II and Group III mGlu receptors is that they may modulate specific neural pathways, possibly via presynaptic mechanisms. However, less is known about how this may relate to functions of systems in whole animals. We have therefore investigated whether these receptors affect specific attributes of visual responses in the superficial SC. 2. Recordings were made from visually responsive neurones in anaesthetised rats, and agonists and antagonists of Group II and III mGlu receptors were applied iontophoretically at the recording site. 3. We found that application of the Group III metabotropic glutamate receptor agonist L-2-amino-4-phosphonobutyric acid (L-AP4) produced an increase in visual response habituation, whilst Group III antagonists decreased habituation. These effects were independent of the response habituation mediated via GABA(B) receptors. In contrast, modulation of Group II mGlu receptors with the specific agonist LY354740 or the antagonist LY341495 did not affect response habituation, although these compounds did modulate visual responses. This suggests a specific role for Group III mGlu receptors in visual response habituation. 4. The magnitude of Group II effects was smaller during presentation of low contrast stimuli compared with high contrast stimuli. This suggests that activation of Group II receptors may be activity dependent and that these receptors can translate this into a functional effect in adapting to high contrast stimuli.

Activation of Group III mGluRs Inhibits GABAergic and Glutamatergic Transmission in the Substantia Nigra Pars Reticulata

The GABAergic projection neurons of the substantia nigra pars reticulata (SNr) exert an important influence on the initiation and control of movement. The SNr is a primary output nucleus of the basal ganglia (BG) and is controlled by excitatory inputs from the subthalamic nucleus (STN) and inhibitory inputs from the striatum and globus pallidus. Changes in the output of the SNr are believed to be critically involved in the development of a variety of movement disorders. Anatomical studies reveal that metabotropic glutamate receptors (mGluRs) are highly expressed throughout the BG. Interestingly, mRNA for group III mGluRs are highly expressed in STN, striatum, and globus pallidus, and immunocytochemical studies have shown that the group III mGluR proteins are present in the SNr. Thus it is possible that group III mGluRs play a role in the modulation of synaptic transmission in this nucleus. We performed whole cell patch-clamp recordings from nondopaminergic SNr neurons to investigate the effect of group III mGluR activation on excitatory and inhibitory transmission in the SNr. We report that activation of group III mGluRs by the selective agonist l(+)-2-amino-4-phosphonobutyric acid (l-AP4, 100 μM) decreases inhibitory synaptic transmission in the SNr. Miniature inhibitory postsynaptic currents studies and paired-pulse studies reveal that this effect is mediated by a presynaptic mechanism. Furthermore we found that l-AP4 (500 μM) also reduces excitatory synaptic transmission at the STN-SNr synapse by action on presynaptically localized group III mGluRs. The finding that mGluRs modulate the major inputs to SNr neurons suggests that these receptors may play an important role in motor function and could provide new targets for the development of pharmacological treatments of movement disorders.

Synthesis of phenylglycine derivatives as potent and selective antagonists of group III metabotropic glutamate receptors

The syntheses of a range of ring and alpha-substituted 4-phosphonophenylglycines are described. A brief discussion of the antagonist activities of compounds 4-10 on group I, II and III metabotropic glutamate (mGlu) receptors expressed in the neonatal rat spinal cord is included.

Pharmacological profiles of the metabotropic glutamate receptor ligands

Metabotropic glutamate receptors (mGluRs) are a family of G-protein coupled receptors that are expressed in the central and peripheral nervous systems. The purpose of this study was to compare the ligand binding selectivity profiles of the mGluR agonist [(3)H]L-AP4 and the novel radiolabeled phenylglycine antagonist [(3)H]CPPG at all eight rat mGluR subtypes expressed in transfected human embryonic kidney cells. At a concentration of 30 nM [(3)H]L-AP4, no specific binding was detected in membranes expressing the group I receptors mGluR1a or mGluR5a, or in membranes expressing the group II mGluRs, mGluR2 and mGluR3. Among the group III mGluRs, specific [(3)H]L-AP4 binding was detected in cells expressing mGluR4a and mGluR8a but not in cells expressing mGluR6 or mGluR7a. The binding of [(3)H]CPPG showed an exceptional pattern of selectivity amongst the mGluR subtypes; at a concentration of 20 nM [(3)H]CPPG, a high level of specific binding was seen in membranes containing mGluR8a but not in any of the other mGluR subtypes. The affinity constant (K(D)) calculated for [(3)H]CPPG binding to mGluR8a was 183 nM. In competition experiments, the phosphono-substituted phenylglycine congeners including MPPG, (RS)-PPG, and unlabeled CPPG were the most potent inhibitors of [(3)H]CPPG binding while non-phosphonated compounds such as L-glutamate and MCPG were substantially less potent. These results demonstrate that [(3)H]L-AP4 and [(3)H]CPPG can be used as probes to selectively label group III mGluRs and that CPPG and related phenylglycine derivatives are useful for studying differences in the ligand recognition sites of highly homologous mGluRs.

Group II and group III metabotropic glutamate receptor agonists depress synaptic transmission in the rat spinal cord dorsal horn

The effects of group II and group III metabotropic glutamate receptor agonists on synaptic responses evoked by primary afferent stimulation in the dorsal horn, but mostly substantia gelatinosa, neurons were studied in the spinal cord slice preparation using conventional intracellular recording technique. Bath application of a potent metabotropic glutamate receptor 2- and 3-selective agonist (2S,1'R,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl) glycine reversibly suppressed monosynaptic and polysynaptic excitatory postsynaptic potentials evoked by A primary afferent fibers stimulation, the effect likely mediated by mGlu3 receptor subtype. This suppressing effect of (2S,1'R,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl) glycine on primary afferent neurotransmission was dose dependent and reduced by (S)-alpha-ethylglutamate, a group II metabotropic glutamate receptor antagonist. (2S,1'R,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl) glycine suppressed excitatory postsynaptic potentials without inducing detectable changes of postsynaptic membrane potential and neuronal input resistance in dorsal horn neurons. The paired-pulse depression at excitatory synapses between primary afferent fibers and dorsal horn neurons was reduced by (2S,1'R,2'R,3'R)-2-(2', 3'-dicarboxycyclopropyl) glycine application, suggesting a presynaptic site of action. The selective group III metabotropic glutamate receptor agonist (S)-2-amino-4-phosphonobutanoate also depressed A afferent fibers-evoked monosynaptic and polysynaptic excitatory postsynaptic potentials in a dose-dependent and reversible manner. The concentration-dependence of (S)-2-amino-4-phosphonobutanoate-mediated depression was most consistent with activation of mGlu receptor subtypes 4 and 7. However, on the basis of anatomical distribution of mGlu 4 and 7 subtypes, it is also possible that the (S)-2-amino-4-phosphonobatanoate effect is due to interaction with mGlu 7 receptor alone. (RS)-alpha-cyclopropyl-4-phosphonophenylglycine a preferential antagonist at group III metabotropic glutamate receptors, completely reversed the depressant effects of (S)-2-amino-4-phosphonobutanoate on both monosynaptic and polysynaptic responses. (S)-2-amino-4-phosphonobutanoate reduced the paired-pulse depression at excitatory synapses between primary afferent fibers and dorsal horn neurons, but did not alter their postsynaptic membrane potential and input resistance. A clear facilitation of the (S)-2-amino-4-phosphonobutanoate-induced depression of monosynaptic and polysynaptic excitatory postsynaptic potentials in the absence of gamma-aminobutyric acid-subtype A receptor- and glycine-mediated synaptic inhibition was shown. Besides the depressant effect on excitatory synaptic transmission, inhibitory actions of group II and III metabotropic glutamate receptor agonists on the inhibitory postsynaptic potentials evoked by primary afferent stimulation in dorsal horn neurons were observed. These results suggest that group II and group III metabotropic glutamate receptors are expressed at primary afferent synapses in the dorsal horn region, and activation of the receptors suppresses synaptic transmission by an action on the presynaptic site.

Origin of transient and sustained responses in ganglion cells of the retina

Phasic and tonic light responses provide a fundamental division of visual information that is thought to originate in the inner retina. However, evidence presented here indicates that this duality originates in the outer retina. In response to a steady light stimulus, the temporal responses of On-bipolar cells fell into two groups. In one group, the light response peaked and then rapidly declined (tau approximately 400 msec) close to the resting membrane potential. At light offset, these cells exhibited a transient afterhyperpolarization. In the second group of On-bipolar cells, the light response declined 10-fold more slowly and reached a steady depolarization that was approximately 40% of the peak response. These neurons had a slowly decaying afterhyperpolarization at light offset. A metabotropic glutamate antagonist, (RS)-alpha-cyclopropyl-4-phosphonophenylyglycine (CPPG), blocked light responses in both types of On-bipolar cell. CPPG only slightly depolarized transient On-bipolar cells, whereas sustained On-bipolar cells were significantly depolarized. Inorganic calcium channel blockers disclosed that these distinct On-bipolar responses were inherent to the bipolar cell and not attributable to synaptic feedback. CPPG had distinct effects on sustained and transient ganglion cells, similar to its action on bipolar cells. The antagonist depolarized and blocked the light responses of sustained ganglion cells. In transient ganglion cells, CPPG suppressed the On light response but did not depolarize the cell or block the Off light response. These results suggest that transient and sustained light responses in ganglion cells result from selective bipolar cell input and that these two fundamental visual channels originate at the dendritic terminals of bipolar cells.