Metabotropic glutamate receptor subtype 7 ablation causes deficit in fear response and conditioned taste aversion

Therapeutic Potential for Metabotropic Glutamate Receptor 7 Modulators in Cognitive Disorders

Metabotropic glutamate receptor 7 (mGlu7) is the most highly conserved and abundantly expressed mGlu receptor in the human brain. The presynaptic localization of mGlu7, coupled with its low affinity for its endogenous agonist, glutamate, are features that contribute to the receptor's role in modulating neuronal excitation and inhibition patterns, including long-term potentiation, in various brain regions. These characteristics suggest that mGlu7 modulation may serve as a novel therapeutic strategy in disorders of cognitive dysfunction, including neurodevelopmental disorders that cause impairments in learning, memory, and attention. Primary mutations in the GRM7 gene have recently been identified as novel causes of neurodevelopmental disorders, and these patients exhibit profound intellectual and cognitive disability. Pharmacological tools, such as agonists, antagonists, and allosteric modulators, have been the mainstay for targeting mGlu7 in its endogenous homodimeric form to probe effects of its function and modulation in disease models. However, recent research has identified diversity in dimerization, as well as trans-synaptic interacting proteins, that also play a role in mGlu7 signaling and pharmacological properties. These novel findings represent exciting opportunities in the field of mGlu receptor drug discovery and highlight the importance of further understanding the functions of mGlu7 in complex neurologic conditions at both the molecular and physiologic levels. SIGNIFICANCE STATEMENT: Proper expression and function of mGlu7 is essential for learning, attention, and memory formation at the molecular level within neural circuits. The pharmacological targeting of mGlu7 is undergoing a paradigm shift by incorporating an understanding of receptor interaction with other cis- and trans- acting synaptic proteins, as well as various intracellular signaling pathways. Based upon these new findings, mGlu7's potential as a drug target in the treatment of cognitive disorders and learning impairments is primed for exploration.

Proteomic Analysis of the Amygdala Reveals Dynamic Changes in Glutamate Transporter-1 During Progression of Complete Freund's Adjuvant-Induced Pain Aversion

Pain sufferer usually show an aversion to the environment associated with pain, identified as pain aversion. The amygdala, an almond-shaped limbic structure in the medial temporal lobe, exerts a critical effect on emotion and pain formation. However, studies on inflammatory pain-induced aversion are still relatively limited, and the available evidence is not enough to clarify its inherent mechanisms. Proteomics is a high-throughput, comprehensive, and objective study method that compares the similarities and differences of protein expression under different conditions to screen potential targets. The current study aimed to identify potential pivotal proteins in the amygdala of rats after complete Freund's adjuvant (CFA)-induced pain aversion via proteomics analysis. Immunohistochemistry was performed to confirm the expression of glutamate transporter-1 (GLT-1) in the amygdala during different periods of pain aversion. Thirteen proteins were found to be different between the day 2 and day 15 groups. Among the 13 differentially expressed proteins, Q8R64 denotes GLT-1, which utilises synaptic glutamate to remain optimal extracellular glutamic levels, thereby preventing accumulation in the synaptic cleft and consequent excitotoxicity. The variation in GLT-1 expression was correlated with the variation tendency of pain aversion, which implies a potential link between the modulation of pain aversion and the excitability of glutamatergic neurons. This study demonstrated that exposure to inflammatory pain results in aversion induced from pain, leading to extensive biological changes in the amygdala.

Differential Activity of Orthosteric Agonists and Allosteric Modulators at Metabotropic Glutamate Receptor 7

Metabotropic glutamate receptor 7 (mGlu7) is a G protein coupled receptor that has demonstrated promise as a therapeutic target across a number of neurologic and psychiatric diseases. Compounds that modulate the activity of mGlu7, such as positive and negative allosteric modulators, may represent new therapeutic strategies to modulate receptor activity. The endogenous neurotransmitter associated with the mGlu receptor family, glutamate, exhibits low efficacy and potency in activating mGlu7, and surrogate agonists, such as the compound L-(+)-2-Amino-4-phosphonobutyric acid (L-AP4), are often used for receptor activation and compound profiling. To understand the implications of the use of such agonists in the development of positive allosteric modulators (PAMs), we performed a systematic evaluation of receptor activation using a system in which mutations can be made in either protomer of the mGlu7 dimer; we employed mutations that prevent interaction with the orthosteric site as well as the G-protein coupling site of the receptor. We then measured increases in calcium levels downstream of a promiscuous G protein to assess the effects of mutations in one of the two protomers in the presence of two different agonists and three positive allosteric modulators. Our results reveal that distinct PAMs, for example N-[3-Chloro-4-[(5-chloro-2-pyridinyl)oxy]phenyl]-2-pyridinecarboxamide (VU0422288) and 3-(2,3-Difluoro-4-methoxyphenyl)-2,5-dimethyl-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidine (VU6005649), do exhibit different maximal levels of potentiation with L-AP4 versus glutamate, but there appear to be common stable receptor conformations that are shared among all of the compounds examined here. SIGNIFICANCE STATEMENT: This manuscript describes the systematic evaluation of the mGlu7 agonists glutamate and L-(+)-2-Amino-4-phosphonobutyric acid (L-AP4) in the presence and absence of three distinct potentiators examining possible mechanistic differences. These findings demonstrate that mGlu7 potentiators display subtle variances in response to glutamate versus L-AP4.

GRM7 gene mutations and consequences for neurodevelopment

The metabotropic glutamate receptor 7 (mGlu7), encoded by the GRM7 gene in humans, is a presynaptic, G protein-coupled glutamate receptor that is essential for modulating neurotransmission. Mutations in or reduced expression of GRM7 have been identified in different genetic neurodevelopmental disorders (NDDs), and rare biallelic missense variants have been proposed to underlie a subset of NDDs. Clinical GRM7 variants have been associated with a range of symptoms consistent with neurodevelopmental molecular features, including hypomyelination, brain atrophy and defects in axon outgrowth. Here, we review the newest findings regarding the cellular and molecular defects caused by GRM7 variants in NDD patients.

Spinal Transection Switches the Effect of Metabotropic Glutamate Receptor Subtype 7 from the Facilitation to Inhibition of Ejaculation

Metabotropic glutamate receptor subtype 7 (mGluR7) is a member of the group III mGluRs, which localize to presynaptic active zones of the central nervous system. We previously reported that mGluR7 knockout (KO) mice exhibit ejaculatory disorders, although they have normal sexual motivation. We hypothesized that mGluR7 regulates ejaculation by potentiating the excitability of the neural circuit in the lumbosacral spinal cord, because administration of the mGluR7-selective antagonist into that region inhibits drug-induced ejaculation. In the present study, to elucidate the mechanism of impaired ejaculation in mGluR7 KO mice, we eliminated the influence of the brain by spinal transection (spinalization). Unexpectedly, sexual responses of male mGluR7 KO mice were stronger than those of wild-type mice after spinalization. Histological examination indicated that mGluR7 controls sympathetic neurons as well as parasympathetic neurons. In view of the complexity of its synaptic regulation, mGluR7 might control ejaculation by multi-level and multi-modal mechanisms. Our study provides insight into the mechanism of ejaculation as well as a strategy for future therapies to treat ejaculatory disorders in humans.

Persistent challenges in the development of an mGlu7 PAM in vivo tool compound: The discovery of VU6046980

Herein, we report on the further chemical optimization of the first reported mGlu7 positive allosteric modulator (PAM), VU6027459. Replacement of the quinoline core by a cinnoline scaffold increased mGlu7 PAM potency by ∼ 10-fold, and concomitant introduction of a chiral tricyclic motif led to potent mGlu7 PAMs with enantioselective mGlu receptor selectivity profiles. Of these, VU6046980 emerged as a putative in vivo tool compound with excellent CNS penetration (Kp = 4.1; Kp,uu = 0.7) and efficacy in preclinical models. However, either off-target activity at the sigma-1 receptor or activity at a target not elucidated by large ancillary pharmacology panels led to sedation not driven by activation of mGlu7 (validated in Grm7 knockout mice). Thus, despite a significant advance, a viable mGlu7 PAM in vivo tool remains elusive.

MGluR7 is a presynaptic metabotropic glutamate receptor at ribbon synapses of inner hair cells

Glutamate is the most pivotal excitatory neurotransmitter in the central nervous system. Metabotropic glutamate receptors (mGluRs) dimerize and can couple to inhibitory intracellular signal cascades, thereby protecting glutamatergic neurons from excessive excitation and cell death. MGluR7 is correlated with age-related hearing deficits and noise-induced hearing loss; however its exact localization in the cochlea is unknown. Here, we analyzed the expression and localization of mGluR7a and mGluR7b in mouse cochlear wholemounts in detail, using confocal microscopy and 3D reconstructions. We observed a presynaptic localization of mGluR7a at inner hair cells (IHCs), close to the synaptic ribbon. To detect mGluR7b, newly generated antibodies were characterized and showed co-localization with mGluR7a at IHC ribbon synapses. Compared to the number of synaptic ribbons, the numbers of mGluR7a and mGluR7b puncta were reduced at higher frequencies (48 to 64 kHz) and in older animals (6 and 12 months). Previously, we reported a presynaptic localization of mGluR4 and mGluR8b at this synapse type. This enables the possibility for the formation of homo- and/or heterodimeric receptors composed of mGluR4, mGluR7a, mGluR7b and mGluR8b at IHC ribbon synapses. These receptor complexes might represent new molecular targets suited for pharmacological concepts to protect the cochlea against noxious stimuli and excitotoxicity.

The G Protein-Coupled Glutamate Receptors as Novel Molecular Targets in Schizophrenia Treatment-A Narrative Review

Schizophrenia is a severe neuropsychiatric disease with an unknown etiology. The research into the neurobiology of this disease led to several models aimed at explaining the link between perturbations in brain function and the manifestation of psychotic symptoms. The glutamatergic hypothesis postulates that disrupted glutamate neurotransmission may mediate cognitive and psychosocial impairments by affecting the connections between the cortex and the thalamus. In this regard, the greatest attention has been given to ionotropic NMDA receptor hypofunction. However, converging data indicates metabotropic glutamate receptors as crucial for cognitive and psychomotor function. The distribution of these receptors in the brain regions related to schizophrenia and their regulatory role in glutamate release make them promising molecular targets for novel antipsychotics. This article reviews the progress in the research on the role of metabotropic glutamate receptors in schizophrenia etiopathology.

Synthesis and SAR of a series of mGlu7 NAMs based on an ethyl-8-methoxy-4-(4-phenylpiperazin-1-yl)quinoline carboxylate core

A High-Throughput Screening (HTS) campaign identified a fundamentally new mGlu7 NAM chemotype, based on an ethyl-8-methoxy-4-(4-phenylpiperazin-1-yl)quinolone carboxylate core. The initial hit, VU0226390, was a potent mGlu7 NAM (IC50 = 647 nM, 6% L-AP4 min) with selectivity versus the other group III mGlu receptors (>30 μM vs. mGlu4 and mGlu8). A multi-dimensional optimization effort surveyed all regions of this new chemotype, and found very steep SAR, reminiscent of allosteric modulators, and unexpected piperazine mimetics (whereas classical bioisosteres failed). While mGlu7 NAM potency could be improved (IC50s ~ 350 nM), the necessity of the ethyl ester moiety and poor physiochemical and DMPK properties precluded optimization towards in vivo tool compounds or clinical candidates. Still, this hit-to-lead campaign afforded key medicinal chemistry insights and new opportunities.

Taste perception and lifestyle: insights from phenotype and genome data among Africans and Asians

Taste is essential for the interaction of animals with their food and has co-evolved with diet. Humans have peopled a large range of environments and present a wide range of diets, but little is known about the diversity and evolution of human taste perception. We measured taste recognition thresholds across populations differing in lifestyles (hunter gatherers and farmers from Central Africa, nomad herders, and farmers from Central Asia). We also generated genome-wide genotype data and performed association studies and selection scans in order to link the phenotypic variation in taste sensitivity with genetic variation. We found that hunter gatherers have lower overall sensitivity as well as lower sensitivity to quinine and fructose than their farming neighbors. In parallel, there is strong population divergence in genes associated with tongue morphogenesis and genes involved in the transduction pathway of taste signals in the African populations. We find signals of recent selection in bitter taste-receptor genes for all four populations. Enrichment analysis on association scans for the various tastes confirmed already documented associations and revealed novel GO terms that are good candidates for being involved in taste perception. Our framework permitted us to gain insight into the genetic basis of taste sensitivity variation across populations and lifestyles.

Discovery of VU6027459: A First-in-Class Selective and CNS Penetrant mGlu7 Positive Allosteric Modulator Tool Compound

Herein, we report the discovery of the first selective and CNS penetrant mGlu₇ PAM (VU6027459) derived from a "molecular switch" within a selective mGlu₇ NAM chemotype. VU6027459 displayed CNS penetration in both mice (K_p = 2.74) and rats (K_p= 4.78), it was orally bioavailable in rats (%F = 69.5), and undesired activity at DAT was ablated.

Metabotropic Glutamate Receptor Subtype 7 Is Essential for Ejaculation

Metabotropic glutamate receptor subtype 7 (mGluR7) is a member of the group III mGluRs, which are negatively coupled to adenylate cyclase via Gi/Go proteins and localized to presynaptic active zones of the mammalian central nervous system (CNS). To elucidate the mechanism of impaired reproductivity of mGluR7 knockout (KO) mice, we investigated sexual behavior in this line, which exhibits ejaculatory disorder, although with normal sexual motivation and erectile function. To identify the site of action within the CNS responsible for the effect of mGluR7 on ejaculation, we then used a para-chloroamphetamine (PCA)-induced ejaculation model. Intrathecal administration of the mGluR7-selective antagonist 6-(4-methoxyphenyl)-5-methyl-3-pyridin-4-ylisoxazolo[4,5-c]pyridin-4(5H)-one (MMPIP) into the lumbosacral spinal cord inhibited PCA-induced ejaculation. Immunohistochemistry revealed mGluR7-like immunoreactivity (LI) expressed in the same area where lumbar spinothalamic (LSt) cells regulate the parasympathetic ejaculatory pathway. At high magnification, the apposition of mGluR7-LI puncta and neuronal nitric oxide synthase (nNOS)-LI-positive putative parasympathetic preganglionic neurons was evident. These results indicate that mGluR7 in the lumbosacral spinal cord regulates ejaculation by potentiating the excitability of parasympathetic preganglionic neurons. The ejaculatory disorder is a major issue in the field of male reproductive function. Erectile dysfunction (ED) can be treated by phosphodiesterase type 5 inhibitors like sildenafil (Viagra®), but the ejaculatory disorder cannot. Lack of understanding of the ejaculatory mechanism hinders the development of therapies for ejaculatory problems. This study is the first to demonstrate that mGluR7 regulates ejaculation and the results provide insight into the mechanism of ejaculation as well as a strategy for future therapies to treat ejaculatory disorders in humans.

The Modulation of Pain by Metabotropic Glutamate Receptors 7 and 8 in the Dorsal Striatum

The dorsal striatum, apart from controlling voluntary movement, displays a recently demonstrated pain inhibition. It is connected to the descending pain modulatory system and in particular to the rostral ventromedial medulla through the medullary dorsal reticular nucleus. Diseases of the basal ganglia, such as Parkinson's disease, in addition to being characterized by motor disorders, are associated with pain and hyperactivation of the excitatory transmission. A way to counteract glutamatergic hyperactivation is through the activation of group III metabotropic glutamate receptors (mGluRs), which are located on presynaptic terminals inhibiting neurotransmitter release. So far the mGluRs of group III have been the least investigated, owing to a lack of selective tools. More recently, selective ligands for each mGluR of group III, in particular positive and negative allosteric modulators, have been developed and the role of each subtype is starting to emerge. The neuroprotective potential of group III mGluRs in pathological conditions, such as those characterized by elevate glutamate, has been recently shown. In the dorsal striatum, mGluR7 and mGluR8 are located at glutamatergic corticostriatal terminals and their stimulation inhibits pain in pathological conditions such as neuropathic pain. The two receptors in the dorsal striatum have instead a different role in pain control in normal conditions. This review will discuss recent results focusing on the contribution of mGluR7 and mGluR8 in the dorsal striatal control of pain. The role of mGluR4, whose antiparkinsonian activity is widely reported, will also be addressed.

Phenotypic profiling of mGlu7 knockout mice reveals new implications for neurodevelopmental disorders

Neurodevelopmental disorders are characterized by deficits in communication, cognition, attention, social behavior and/or motor control. Previous studies have pointed to the involvement of genes that regulate synaptic structure and function in the pathogenesis of these disorders. One such gene, GRM7, encodes the metabotropic glutamate receptor 7 (mGlu₇ ), a G protein-coupled receptor that regulates presynaptic neurotransmitter release. Mutations and polymorphisms in GRM7 have been associated with neurodevelopmental disorders in clinical populations; however, limited preclinical studies have evaluated mGlu₇ in the context of this specific disease class. Here, we show that the absence of mGlu₇ in mice is sufficient to alter phenotypes within the domains of social behavior, associative learning, motor function, epilepsy and sleep. Moreover, Grm7 knockout mice exhibit an attenuated response to amphetamine. These findings provide rationale for further investigation of mGlu₇ as a potential therapeutic target for neurodevelopmental disorders such as idiopathic autism, attention deficit hyperactivity disorder and Rett syndrome.

Biallelic GRM7 variants cause epilepsy, microcephaly, and cerebral atrophy

Objective

Defects in ion channels and neurotransmitter receptors are implicated in developmental and epileptic encephalopathy (DEE). Metabotropic glutamate receptor 7 (mGluR7), encoded by GRM7, is a presynaptic G‐protein‐coupled glutamate receptor critical for synaptic transmission. We previously proposed GRM7 as a candidate disease gene in two families with neurodevelopmental disorders (NDDs). One additional family has been published since. Here, we describe three additional families with GRM7 biallelic variants and deeply characterize the associated clinical neurological and electrophysiological phenotype and molecular data in 11 affected individuals from six unrelated families.

Methods

Exome sequencing and family‐based rare variant analyses on a cohort of 220 consanguineous families with NDDs revealed three families with GRM7 biallelic variants; three additional families were identified through literature search and collaboration with a clinical molecular laboratory.

Results

We compared the observed clinical features and variants of 11 affected individuals from the six unrelated families. Identified novel deleterious variants included two homozygous missense variants (c.2671G>A:p.Glu891Lys and c.1973G>A:p.Arg685Gln) and one homozygous stop‐gain variant (c.1975C>T:p.Arg659Ter). Developmental delay, neonatal‐ or infantile‐onset epilepsy, and microcephaly were universal. Three individuals had hypothalamic–pituitary–axis dysfunction without pituitary structural abnormality. Neuroimaging showed cerebral atrophy and hypomyelination in a majority of cases. Two siblings demonstrated progressive loss of myelination by 2 years in both and an acquired microcephaly pattern in one. Five individuals died in early or late childhood.

Conclusion

Detailed clinical characterization of 11 individuals from six unrelated families demonstrates that rare biallelic GRM7 pathogenic variants can cause DEEs, microcephaly, hypomyelination, and cerebral atrophy.

Internal Clocks, mGluR7 and Microtubules: A Primer for the Molecular Encoding of Target Durations in Cerebellar Purkinje Cells and Striatal Medium Spiny Neurons

The majority of studies in the field of timing and time perception have generally focused on sub- and supra-second time scales, specific behavioral processes, and/or discrete neuronal circuits. In an attempt to find common elements of interval timing from a broader perspective, we review the literature and highlight the need for cell and molecular studies that can delineate the neural mechanisms underlying temporal processing. Moreover, given the recent attention to the function of microtubule proteins and their potential contributions to learning and memory consolidation/re-consolidation, we propose that these proteins play key roles in coding temporal information in cerebellar Purkinje cells (PCs) and striatal medium spiny neurons (MSNs). The presence of microtubules at relevant neuronal sites, as well as their adaptability, dynamic structure, and longevity, makes them a suitable candidate for neural plasticity at both intra- and inter-cellular levels. As a consequence, microtubules appear capable of maintaining a temporal code or engram and thereby regulate the firing patterns of PCs and MSNs known to be involved in interval timing. This proposed mechanism would control the storage of temporal information triggered by postsynaptic activation of mGluR7. This, in turn, leads to alterations in microtubule dynamics through a "read-write" memory process involving alterations in microtubule dynamics and their hexagonal lattice structures involved in the molecular basis of temporal memory.

Metabotropic glutamate receptor subtype 7 controls maternal care, maternal motivation and maternal aggression in mice

The group III metabotropic glutamate receptor subtype 7 (mGlu7) is an important regulator of glutamatergic and GABAergic neurotransmission and known to mediate emotionality and male social behavior. However, a possible regulatory role in maternal behavior remains unknown to date. Adequate expression of maternal behavior is essential for successful rearing and healthy development of the young. By understanding genetic and neural mechanisms underlying this important prosocial behavior, we gain valuable insights into possible dysregulations. Using genetic ablation as well as pharmacological modulation, we studied various parameters of maternal behavior in two different mouse strains under the influence of mGlu7. We can clearly show a regulatory role of mGlu7 in maternal behavior. Naïve virgin female C57BL/6 mGlu7 knockout mice showed more often nursing postures and less spontaneous maternal aggression compared to their heterozygous and wildtype littermates. In lactating C57BL/6 wildtype mice, acute central activation of mGlu7 by the selective agonist AMN082 reduced arched back nursing and accelerated pup retrieval without affecting maternal aggression. In addition, in lactating CD1 wildtype mice the selective mGlu7 antagonist XAP044 increased both pup retrieval and maternal aggression. With respect to receptor expression levels, mGlu7 mRNA expression was higher in lactating vs virgin C57BL/6 mice in the prefrontal cortex, but not hypothalamus or hippocampus. In conclusion, these findings highlight a significant role of the mGlu7 receptor subtype in mediating maternal behavior in mice. Region-dependent studies are warranted to further extend our knowledge on the specific function of the brain glutamate system in maternal behavior.

Conformational pathway provides unique sensitivity to a synaptic mGluR

Metabotropic glutamate receptors (mGluRs) are dimeric G-protein-coupled receptors that operate at synapses. Macroscopic and single molecule FRET to monitor structural rearrangements in the ligand binding domain (LBD) of the mGluR7/7 homodimer revealed it to have an apparent affinity ~4000-fold lower than other mGluRs and a maximal activation of only ~10%, seemingly too low for activation at synapses. However, mGluR7 heterodimerizes, and we find it to associate with mGluR2 in the hippocampus. Strikingly, the mGluR2/7 heterodimer has high affinity and efficacy. mGluR2/7 shows cooperativity in which an unliganded subunit greatly enhances activation by agonist bound to its heteromeric partner, and a unique conformational pathway to activation, in which mGluR2/7 partially activates in the Apo state, even when its LBDs are held open by antagonist. High sensitivity and an unusually broad dynamic range should enable mGluR2/7 to respond to both glutamate transients from nearby release and spillover from distant synapses.

Genetic Access to Gustatory Disgust-Associated Neurons in the Interstitial Nucleus of the Posterior Limb of the Anterior Commissure in Male Mice

Orofacial and somatic disgust reactions are observed in rats following intraoral infusion of not only bitter quinine (innate disgust) but also sweet saccharin previously paired with illness (learned disgust). It remains unclear, however, whether these innate and learned disgust reactions share a common neural basis and which brain regions, if any, host it. In addition, there is no established method to genetically access neurons whose firing is associated with disgust (disgust-associated neurons). Here, we examined the expression of cFos and Arc, two markers of neuronal activity, in the interstitial nucleus of the posterior limb of the anterior commissure (IPAC) of male mice that showed innate disgust and mice that showed learned disgust. Furthermore, we used a targeted recombination in active populations (TRAP) method to genetically label the disgust-associated neurons in the IPAC with YFP. We found a significant increase of both cFos-positive neurons and Arc-positive neurons in the IPAC of mice that showed innate disgust and mice that showed learned disgust. In addition, TRAP following quinine infusion (Quinine-TRAP) resulted in significantly more YFP-positive neurons in the IPAC, compared to TRAP following water infusion. A significant number of the YFP-positive neurons following Quinine-TRAP were co-labeled with Arc following the second quinine infusion, confirming that Quinine-TRAP preferentially labeled quinine-activated neurons in the IPAC. Our results suggest that the IPAC activity is associated with both innate and learned disgust and that disgust-associated neurons in the IPAC are genetically accessible by TRAP.

Discovery of an Orally Bioavailable and Central Nervous System (CNS) Penetrant mGlu7 Negative Allosteric Modulator (NAM) in Vivo Tool Compound: N-(2-(1 H-1,2,4-triazol-1-yl)-5-(trifluoromethoxy)phenyl)-4-(cyclopropylmethoxy)-3-methoxybenzamide (VU6012962)

Herein, we report the discovery of a new, orally bioavailable and CNS-penetrant metabotropic glutamate receptor 7 (mGlu₇) negative allosteric modulator (NAM) that achieves exposure in cerebral spinal fluid (CSF) 2.5× above the in vitro IC₅₀ at minimum effective doses (MEDs) of 3 mg/kg in preclinical anxiety models.

Metabotropic Glutamate Receptor 7: A New Therapeutic Target in Neurodevelopmental Disorders

Neurodevelopmental disorders (NDDs) are characterized by a wide range of symptoms including delayed speech, intellectual disability, motor dysfunction, social deficits, breathing problems, structural abnormalities, and epilepsy. Unfortunately, current treatment strategies are limited and innovative new approaches are sorely needed to address these complex diseases. The metabotropic glutamate receptors are a class of G protein-coupled receptors that act to modulate neurotransmission across many brain structures. They have shown great promise as drug targets for numerous neurological and psychiatric diseases. Moreover, the development of subtype-selective allosteric modulators has allowed detailed studies of each receptor subtype. Here, we focus on the metabotropic glutamate receptor 7 (mGlu₇) as a potential therapeutic target for NDDs. mGlu₇ is expressed widely throughout the brain in regions that correspond to the symptom domains listed above and has established roles in synaptic physiology and behavior. Single nucleotide polymorphisms and mutations in the GRM7 gene have been associated with idiopathic autism and other NDDs in patients. In rodent models, existing literature suggests that decreased mGlu₇ expression and/or function may lead to symptoms that overlap with those of NDDs. Furthermore, potentiation of mGlu₇ activity has shown efficacy in a mouse model of Rett syndrome. In this review, we summarize current findings that provide rationale for the continued development of mGlu₇ modulators as potential therapeutics.

Genetic Reduction or Negative Modulation of mGlu7 Does Not Impact Anxiety and Fear Learning Phenotypes in a Mouse Model of MECP2 Duplication Syndrome

Rett syndrome and MECP2 Duplication syndrome are neurodevelopmental disorders attributed to loss-of-function mutations in, or duplication of, the gene encoding methyl-CpG-binding protein 2 (MeCP2), respectively. We recently reported decreased expression and function of the metabotropic glutamate receptor 7 (mGlu₇) in a mouse model of Rett syndrome. Positive allosteric modulation of mGlu₇ activity was sufficient to improve several disease phenotypes including cognition. Here, we tested the hypothesis that mGlu₇ expression would be reciprocally regulated in a mouse model of MECP2 Duplication syndrome, such that negative modulation of mGlu₇ activity would exert therapeutic benefit. To the contrary, we report that mGlu₇ is not functionally increased in mice overexpressing MeCP2 and that neither genetic nor pharmacological reduction of mGlu₇ activity impacts phenotypes that are antiparallel to those observed in Rett syndrome model mice. These data expand our understanding of how mGlu₇ expression and function is affected by changes in MeCP2 dosage and have important implications for the therapeutic development of mGlu₇ modulators.

New evidences for a role of mGluR7 in astrocyte survival: Possible implications for neuroprotection

A specific activation of metabotropic glutamate receptor 7 (mGluR7) has been shown to be neuroprotective in various models of neuronal cell damage, however, its role in glia cell survival has not been studied, yet. Thus, we performed comparative experiments estimating protective effects of the mGluR7 allosteric agonist AMN082 in glia, neuronal and neuronal-glia cell cultures against various harmful stimuli. First, the transcript levels of mGluR7 and other subtypes of group II and III mGluRs in cortical neuronal, neuronal-glia and glia cell cultures have been measured by qPCR method. Next, we demonstrated that AMN082 with similar efficiency attenuated the glia cell damage evoked by staurosporine (St) and doxorubicin (Dox). The AMN082-mediated glioprotection was mGluR7-dependent and associated with decreased DNA fragmentation without involvement of caspase-3 inhibition. Moreover, the inhibitors of PI3K/Akt and MAPK/ERK1/2 pathways blocked the protective effect of AMN082. In neuronal and neuronal-glia cell cultures in the model of glutamate (Glu)- but not St-evoked cell damage, we showed a significant glia contribution to mGluR7-mediated neuroprotection. Finally, by using glia and neuronal cells derived from mGluR7+/+ and mGluR7-/- mice we demonstrated a higher cell-damaging effect of St and Dox in mGluR7-deficient glia but not in neurons (cerebellar granule cells). Our present data showed for the first time a glioprotective potential of AMN082 underlain by mechanisms involving the activation of PI3K/Akt and MAPK/ERK1/2 pathways and pro-survival role of mGluR7 in glia cells. These findings together with the confirmed neuroprotective properties of AMN082 justify further research on mGluR7-targeted therapies for various CNS disorders.

Metabotropic glutamate receptor subtype 7 has critical roles in regulation of the endocrine system and social behaviours

Metabotropic glutamate receptor subtype 7 (mGluR7) is one of the group III mGluRs, which are negatively coupled to adenylate cyclase via Gi/Go proteins and localised to presynaptic active zones of the mammalian central nervous system. We previously reported that mGluR7 is essential for intermale aggression and amygdala-dependent fear learning. To elucidate the role of mGluR7 in the neuroendocrine system, we performed biochemical analyses and found a significant reduction of testosterone levels in mGluR7 knockout (KO) mice. Testosterone replacement restored intermale aggressive behaviour in castrated wild-type mice to the level of gonadally intact wild-type mice. However, given the same dosage of testosterone replacement, mGluR7 KO mice showed almost no aggressive behaviour. These results indicate that reduction of plasma testosterone is unrelated to the deficit in intermale aggression in mGluR7 KO mice. Social investigating behaviour of intact mGluR7 KO mice also differed from that of wild-type mice; e.g. the KO mice showing less frequent anogenital sniffing and more frequent grooming behaviour. Testosterone replacement increased anogenital sniffing and grooming behaviour in castrated mGluR7 KO mice, while the differences were still present between castrated wild-type mice and KO mice after both underwent testosterone replacement. These results imply that reduction of plasma testosterone may partially inhibit social investigating behaviours in intact mGluR7 KO mice. Furthermore, castrated mGluR7 KO mice have smaller seminal vesicles than those of castrated wild-type mice, although seminal vesicle weights were normal in intact mice. These observations suggest that, besides testicular testosterone, some other hormone levels may be dysregulated in mGluR7 KO mice, and indicate a critical role of mGluR7 in the endocrine system. Taken together, our findings demonstrate that mGluR7 is essential for the regulation of the endocrine system, in addition to innate behaviours such as intermale aggression and fear response.

VU6010608, a Novel mGlu7 NAM from a Series of N-(2-(1H-1,2,4-Triazol-1-yl)-5-(trifluoromethoxy)phenyl)benzamides

Herein, we report the structure-activity relationships within a series of mGlu₇ NAMs based on an N-(2-(1H-1,2,4-triazol-1-yl)-5-(trifluoromethoxy)phenyl)benzamide core with excellent CNS penetration (K_p 1.9-5.8 and K_p,uu 0.4-1.4). Analogues in this series displayed steep SAR. Of these, VU6010608 (11a) emerged with robust efficacy in blocking high frequency stimulated long-term potentiation in electrophysiology studies.

Discovery of VU6005649, a CNS Penetrant mGlu7/8 Receptor PAM Derived from a Series of Pyrazolo[1,5-a]pyrimidines

Herein, we report the structure-activity relationships within a series of mGlu₇ PAMs based on a pyrazolo[1,5-a]pyrimidine core with excellent CNS penetration (K_ps > 1 and K_p,uus > 1). Analogues in this series proved to display a range of Group III mGlu receptor selectivity, but VU6005649 emerged as the first dual mGlu_7/8 PAM, filling a void in the Group III mGlu receptor PAM toolbox and demonstrating in vivo efficacy in a mouse contextual fear conditioning model.

Targeting metabotropic glutamate receptors for novel treatments of schizophrenia

Support for the N-methyl-D-aspartate receptor (NMDAR) hypofunction hypothesis of schizophrenia has led to increasing focus on restoring proper glutamatergic signaling as an approach for treatment of this devastating disease. The ability of metabotropic glutamate (mGlu) receptors to modulate glutamatergic neurotransmission has thus attracted considerable attention for the development of novel antipsychotics. Consisting of eight subtypes classified into three groups based on sequence homology, signal transduction, and pharmacology, the mGlu receptors provide a wide range of targets to modulate NMDAR function as well as glutamate release. Recently, allosteric modulators of mGlu receptors have been developed that allow unprecedented selectivity among subtypes, not just groups, facilitating the investigation of the effects of subtype-specific modulation. In preclinical animal models, positive allosteric modulators (PAMs) of the group I mGlu receptor mGlu5 have efficacy across all three symptom domains of schizophrenia (positive, negative, and cognitive). The discovery and development of mGlu5 PAMs that display unique signal bias suggests that efficacy can be retained while avoiding the neurotoxic effects of earlier compounds. Interestingly, mGlu1 negative allosteric modulators (NAMs) appear efficacious in positive symptom models of the disease but are still in early preclinical development. While selective group II mGlu receptor (mGlu2/3) agonists have reached clinical trials but were unsuccessful, specific mGlu2 or mGlu3 receptor targeting still hold great promise. Genetic studies implicated mGlu2 in the antipsychotic effects of group II agonists and mGlu2 PAMs have since entered into clinical trials. Additionally, mGlu3 appears to play an important role in cognition, may confer neuroprotective effects, and thus is a promising target to alleviate cognitive deficits in schizophrenia. Although group III mGlu receptors (mGlu4/6/7/8) have attracted less attention, mGlu4 agonists and PAMs appear to have efficacy across all three symptoms domains in preclinical models. The recent discovery of heterodimers comprising mGlu2 and mGlu4 may explain the efficacy of mGlu4 selective compounds but this remains to be determined. Taken together, compounds targeting mGlu receptors, specifically subtype-selective allosteric modulators, provide a compelling alternative approach to fill the unmet clinical needs for patients with schizophrenia.

The Effects of Magnesium Supplementation on Subjective Anxiety and Stress-A Systematic Review

BACKGROUND

Anxiety related conditions are the most common affective disorders present in the general population with a lifetime prevalence of over 15%. Magnesium (Mg) status is associated with subjective anxiety, leading to the proposition that Mg supplementation may attenuate anxiety symptoms. This systematic review examines the available evidence for the efficacy of Mg supplementation in the alleviation of subjective measures of anxiety and stress.

METHODS

A systematic search of interventions with Mg alone or in combination (up to 5 additional ingredients) was performed in May 2016. Ovid Medline, PsychInfo, Embase, CINAHL and Cochrane databases were searched using equivalent search terms. A grey literature review of relevant sources was also undertaken.

RESULTS

18 studies were included in the review. All reviewed studies recruited samples based upon an existing vulnerability to anxiety: mildly anxious, premenstrual syndrome (PMS), postpartum status, and hypertension. Four/eight studies in anxious samples, four/seven studies in PMS samples, and one/two studies in hypertensive samples reported positive effects of Mg on subjective anxiety outcomes. Mg had no effect on postpartum anxiety. No study administered a validated measure of subjective stress as an outcome.

CONCLUSIONS

Existing evidence is suggestive of a beneficial effect of Mg on subjective anxiety in anxiety vulnerable samples. However, the quality of the existing evidence is poor. Well-designed randomised controlled trials are required to further confirm the efficacy of Mg supplementation.

Relief from detrimental consequences of chronic psychosocial stress in mice deficient for the metabotropic glutamate receptor subtype 7

Chronic stress-related psychiatric conditions and comorbid somatic pathologies are an enormous public health concern in modern society. The etiology of these disorders is complex, with stressors holding a chronic and psychosocial component representing the most acknowledged risk factor. During the last decades, research on the metabotropic glutamate receptor (mGlu) system advanced dramatically and much attention was given to the role of the metabotropic glutamate receptor subtype 7 (mGlu7) in acute stress-related behavior and physiology. However, virtually nothing is known about the potential involvement of mGlu7 in chronic psychosocial stress-related conditions. Using the chronic subordinate colony housing (CSC, 19 days) in male mice, we addressed whether central mGlu7 is altered upon chronic psychosocial stressor exposure and whether genetic ablation of mGlu7 interferes with the multitude of chronic stress-induced alterations. CSC exposure resulted in a downregulation of mGlu7 mRNA transcript levels in the prefrontal cortex, a brain region relevant for stress-related behaviors and physiology. Interestingly, mGlu7 deficiency relieved multiple chronic stress-induced alterations including the CSC-induced anxiety-prone phenotype; mGlu7 ablation also ameliorated CSC-induced physiological and immunological consequences such as hypothalamo-pituitary-adrenal (HPA) axis dysfunctions and colonic inflammation, respectively. Together, our findings provide first evidence for the involvement of mGlu7 in a wide range of behavioral and physiological alterations in response to chronic psychosocial stressor exposure. Moreover, the stress-protective phenotype of genetic mGlu7 ablation suggests mGlu7 pharmacological blockade to be a relevant option for the treatment of chronic stress-related emotional and somatic dysfunctions. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

MMPIP, an mGluR7-selective negative allosteric modulator, alleviates pain and normalizes affective and cognitive behavior in neuropathic mice

This study investigated the effects of a single administration of 6-(4-methoxyphenyl)-5-methyl-3-pyridinyl-4-isoxazolo[4,5-c]pyridin-4(5H)-one (MMPIP), a negative allosteric modulator (NAM) of metabotropic glutamate receptor 7 (mGluR7), on pain and on affective and cognitive behavior in neuropathic mice. The activity of pyramidal neurons in the prelimbic cortex (PLC), which respond to stimulation of the basolateral amygdala (BLA) with either excitation or inhibition, was also investigated. The spared nerve injury (SNI) of the sciatic nerve induced, 14 days after surgery, thermal hyperalgesia and mechanical allodynia, reduced open-arm choice in the elevated plus-maze, increased time of immobility in the tail suspension, and increased digging and burying in the marble burying test. Cognitive performance was also significantly compromised in the SNI mice. Spared nerve injury induced phenotypic changes on pyramidal neurons of the PLC; excitatory responses increased, whereas inhibitory responses decreased after BLA stimulation. mGluR7 expression, mainly associated with vesicular glutamate transporter, increased in the hippocampus and decreased in the BLA, PLC, and dorsal raphe in SNI mice. MMPIP increased thermal and mechanical thresholds and open-arm choice. It reduced the immobility in the tail suspension test and the number of marbles buried and of digging events in the marble burying test. MMPIP also improved cognitive performance and restored the balance between excitatory and inhibitory responses of PLC neurons in SNI mice. 7-hydroxy-3-(4-iodophenoxy)-4H-chromen-4-one, XAP044, another selective mGluR7 NAM, reproduced the effects of MMPIP on thermal hyperalgesia, mechanical allodynia, tail suspension, and marble burying test. Altogether, these findings show that mGluR7 NAMs reduce pain responses and affective/cognitive impairments in neuropathic pain conditions.

Fear potentiated startle increases phospholipase D (PLD) expression/activity and PLD-linked metabotropic glutamate receptor mediated post-tetanic potentiation in rat amygdala

Long-term memory (LTM) of fear stores activity dependent modifications that include changes in amygdala signaling. Previously, we identified an enhanced probability of release of glutamate mediated signaling to be important in rat fear potentiated startle (FPS), a well-established translational behavioral measure of fear. Here, we investigated short- and long-term synaptic plasticity in FPS involving metabotropic glutamate receptors (mGluRs) and associated downstream proteomic changes in the thalamic-lateral amygdala pathway (Th-LA). Aldolase A, an inhibitor of phospholipase D (PLD), expression was reduced, concurrent with significantly elevated PLD protein expression. Blocking the PLD-mGluR signaling significantly reduced PLD activity. While transmitter release probability increased in FPS, PLD-mGluR agonist and antagonist actions were occluded. In the unpaired group (UNP), blocking the PLD-mGluR increased while activating the receptor decreased transmitter release probability, consistent with decreased synaptic potentials during tetanic stimulation. FPS Post-tetanic potentiation (PTP) immediately following long-term potentiation (LTP) induction was significantly increased. Blocking PLD-mGluR signaling prevented PTP and reduced cumulative PTP probability but not LTP maintenance in both groups. These effects are similar to those mediated through mGluR7, which is co-immunoprecipitated with PLD in FPS. Lastly, blocking mGluR-PLD in the rat amygdala was sufficient to prevent behavioral expression of fear memory. Thus, our study in the Th-LA pathway provides the first evidence for PLD as an important target of mGluR signaling in amygdala fear-associated memory. Importantly, the PLD-mGluR provides a novel therapeutic target for treating maladaptive fear memories in posttraumatic stress and anxiety disorders.

Metabotropic Glutamate Receptor Subtype 7 in the Bed Nucleus of the Stria Terminalis is Essential for Intermale Aggression

Metabotropic glutamate receptor subtype 7 (mGluR7) is a member of group III mGluRs, which localize to the presynaptic active zones of the mammalian central nervous system. Although histological, genetic, and electrophysiological studies ensure the importance of mGluR7, its roles in behavior and physiology remain largely unknown. Using a resident-intruder paradigm, we found a severe reduction in intermale aggressive behavior in mGluR7 knockout (KO) mice. We also found alterations in other social behaviors in male mGluR7 KO mice, including sexual behavior toward male intruders. Because olfaction is critical for rodent social behavior, including aggression, we performed an olfaction test, finding that mGluR7 KO mice failed to show interest in the smell of male urine. To clarify the olfactory deficit, we then exposed mice to urine and analyzed c-Fos-immunoreactivity, discovering a remarkable reduction in neural activity in the bed nucleus of the stria terminalis (BNST) of mGluR7 KO mice. Finally, intra-BNST administration of the mGluR7-selective antagonist 6-(4-methoxyphenyl)-5-methyl-3-pyridin-4-ylisoxazolo[4,5-c]pyridin-4(5H)-one (MMPIP) also reproduced the phenotype of mGluR7 KO mice, including reduced aggression and altered social interaction. Thus mGluR7 may work as an 'enhancer of neural activity' in the BNST and is important for intermale aggression. Our findings demonstrate that mGluR7 is essential for social behavior and innate behavior. Our study on mGluR7 in the BNST will shed light on future therapies for emotional disorders in humans.

Rare de novo deletion of metabotropic glutamate receptor 7 (GRM7) gene in a patient with autism spectrum disorder

GRM7, the gene encoding metabotropic glutamate receptor 7 (mGluR7), have been implicated in multiple neuropsychiatric disorders and shown to mediate excitatory synaptic neurotransmitter signaling and plasticity in the mammalian brain. Here we report a 303 kb de novo deletion at band 3p26.1, disrupting five coding exons of GRM7 in a proband with autism spectrum disorder, and hyperactivity. Our exon transcriptome-mutation contingency index method shows that three of the exons within the breakpoint boundaries are under purifying selection and highly expressed in prenatal brain regions. Based on our results and a thorough review of the literature, we propose that haploinsufficiency of the GRM7 product (mGluR7) contributes to autism spectrum disorders and hyperactivity phenotype as seen in the patient described here.

Functional Dissociation of Group III Metabotropic Glutamate Receptors Revealed by Direct Comparison between the Behavioral Profiles of Knockout Mouse Lines

BACKGROUND

Group III metabotropic glutamate receptors (mGlu4, mGlu7, mGlu8) display differential brain distribution, which suggests different behavioral functions. However, comparison across the available animal studies remains methodologically hazardous and controversial. The present report directly compares knockouts for each group III receptor subtype using a single behavioral test battery and multivariate analysis.

METHODS

The behavioral phenotypes of C57BL/6J mice lacking mGlu4, mGlu7, or mGlu8 and their respective littermates were examined using a multimetric test battery, which included elements of neuromotor performance, exploratory behavior, and learning and memory. Multivariate statistical methods were used to identify subtype-specific behavioral profiles and variables that distinguished between these mouse lines.

RESULTS

It generally appears that mGlu7 plays a significant role in hippocampus-dependent spatial learning and in some fear-related behaviors, whereas mGlu4 is most clearly involved in startle and motivational processes. Excepting its influence on body weight, the effect of mGlu8 deletion on behavior appears more subtle than that of the other group III receptors. These receptors have been proposed as potential drug targets for a variety of psychopathological conditions.

CONCLUSION

On the basis of these controlled comparisons, we presently conclude that the different group III receptors indeed have quite distinct behavioral functions.

Negative allosteric modulation of the mGlu7 receptor reduces visceral hypersensitivity in a stress-sensitive rat strain

Glutamate, the main excitatory neurotransmitter in the central nervous system, exerts its effect through ionotropic and metabotropic receptors. Of these, group III mGlu receptors (mGlu 4, 6, 7, 8) are among the least studied due to a lack of pharmacological tools. mGlu7 receptors, the most highly conserved isoform, are abundantly distributed in the brain, especially in regions, such as the amygdala, known to be crucial for the emotional processing of painful stimuli. Visceral hypersensitivity is a poorly understood phenomenon manifesting as an increased sensitivity to visceral stimuli. Glutamate has long been associated with somatic pain processing leading us to postulate that crossover may exist between these two modalities. Moreover, stress has been shown to exacerbate visceral pain. ADX71743 is a novel, centrally penetrant, negative allosteric modulator of mGlu7 receptors. Thus, we used this tool to explore the possible involvement of this receptor in the mediation of visceral pain in a stress-sensitive model of visceral hypersensitivity, namely the Wistar Kyoto (WKY) rat. ADX71743 reduced visceral hypersensitivity in the WKY rat as exhibited by increased visceral sensitivity threshold with concomitant reductions in total number of pain behaviours. Moreover, AD71743 increased total distance and distance travelled in the inner zone of the open field. These findings show, for what is to our knowledge, the first time, that mGlu7 receptor signalling plays a role in visceral pain processing. Thus, negative modulation of the mGlu7 receptor may be a plausible target for the amelioration of stress-induced visceral pain where there is a large unmet medical need.

Blocking metabotropic glutamate receptor subtype 7 (mGlu7) via the Venus flytrap domain (VFTD) inhibits amygdala plasticity, stress, and anxiety-related behavior

The metabotropic glutamate receptor subtype 7 (mGlu7) is an important presynaptic regulator of neurotransmission in the mammalian CNS. mGlu7 function has been linked to autism, drug abuse, anxiety, and depression. Despite this, it has been difficult to develop specific blockers of native mGlu7 signaling in relevant brain areas such as amygdala and limbic cortex. Here, we present the mGlu7-selective antagonist 7-hydroxy-3-(4-iodophenoxy)-4H-chromen-4-one (XAP044), which inhibits lateral amygdala long term potentiation (LTP) in brain slices from wild type mice with a half-maximal blockade at 88 nm. There was no effect of XAP044 on LTP of mGlu7-deficient mice, indicating that this pharmacological effect is mGlu7-dependent. Unexpectedly and in contrast to all previous mGlu7-selective drugs, XAP044 does not act via the seven-transmembrane region but rather via a binding pocket localized in mGlu7's extracellular Venus flytrap domain, a region generally known for orthosteric agonist binding. This was shown by chimeric receptor studies in recombinant cell line assays. XAP044 demonstrates good brain exposure and wide spectrum anti-stress and antidepressant- and anxiolytic-like efficacy in rodent behavioral paradigms. XAP044 reduces freezing during acquisition of Pavlovian fear and reduces innate anxiety, which is consistent with the phenotypes of mGlu7-deficient mice, the results of mGlu7 siRNA knockdown studies, and the inhibition of amygdala LTP by XAP044. Thus, we present an mGlu7 antagonist with a novel molecular mode of pharmacological action, providing significant application potential in psychiatry. Modeling the selective interaction between XAP044 and mGlu7's Venus flytrap domain, whose three-dimensional structure is already known, will facilitate future drug development supported by computer-assisted drug design.

Differential roles of mGlu(7) and mGlu(8) in amygdala-dependent behavior and physiology

Glutamate transmission and synaptic plasticity in the amygdala are essential for the learning and expression of conditioned fear. Glutamate activates both ionotropic glutamate receptors and eight subtypes of metabotropic glutamate receptors (mGlu1-8). In the present study, we investigated the roles of mGlu7 and mGlu8 in amygdala-dependent behavior and synaptic plasticity. We show that ablation of mGlu7 but not mGlu8 attenuates long-term potentiation (LTP) at thalamo-lateral amygdala (LA) synapses where a strong association between LTP and learning has been demonstrated. mGlu7-deficient mice express a general deficit in conditioned fear whereas mGlu8-deficient mice show a dramatic reduction in contextual fear. The mGlu7 agonist AMN082 reduced thalamo-LA LTP and intra-amygdala administration blocked conditioned fear learning. In contrast, the mGlu8 agonist DCPG decreased synaptic transmission but not LTP at thalamo-LA synapses. Intra-amygdala DCPG selectively reduced the expression of contextual fear but did not affect the acquisition and expression of cued fear. Taken together, these data revealed very different roles for mGlu7 and mGlu8 in amygdala synaptic transmission, fear learning and its expression. These receptors seem promising targets for treating anxiety disorders with different underlying pathologies with exaggerated fear learning (mGlu7) or contextual fear (mGlu8).

Adult siRNA-induced knockdown of mGlu7 receptors reduces anxiety in the mouse

Our knowledge regarding the molecular pathophysiology underlying anxiety disorders remains incomplete. Increasing evidence points to a role of glutamate in anxiety. The group III metabotropic glutamate receptors (mGlu4, mGlu6, mGlu7 and mGlu8 receptors) remain the least investigated glutamate receptor subtypes partially due to a delay in the development of specific pharmacological tools. Early work using knockout animals and pharmacological tools aimed at investigating the role of mGlu7 receptor in the pathophysiology of anxiety disorders has yielded exciting yet not always consistent results. To further investigate the role this receptor plays in anxiety-like behaviour, we knocked down mGlu7 receptor mRNA levels in the adult mouse brain using siRNA delivered via an osmotic minipump. This reduced anxiety-like behaviour in the light-dark box coupled with an attenuation of stress-induced hyperthermia (SIH) and a reduction of the acoustic startle response (ASRs) in the fear-potentiated startle paradigm (FPS). These effects on anxiety-like behaviour were independent of any impairment of locomotor activity and surprisingly, no behavioural changes were observed in the forced swim test (FST), which is in contrast to mGlu7 receptor knockout animals. Furthermore, the previously reported epilepsy-prone phenotype seen in mGlu7 receptor knockout animals was not observed following siRNA-induced knockdown of the receptor. These data suggest targeting mGlu7 receptors with selective antagonist drugs may be an effective and safe strategy for the treatment of anxiety disorders.

Functional monoclonal antibody acts as a biased agonist by inducing internalization of metabotropic glutamate receptor 7

BACKGROUND AND PURPOSE

The mGlu(7) receptors are strategically located at the site of vesicle fusion where they modulate the release of the main excitatory and inhibitory neurotransmitters. Consequently, they are implicated in the underlying pathophysiology of CNS diseases such as epilepsy and stress-related psychiatric disorders. Here, we characterized a selective, potent and functional anti-mGlu(7) monoclonal antibody, MAB1/28, that triggers receptor internalization.

EXPERIMENTAL APPROACH

MAB1/28's activity was investigated using Western blot and direct immunofluorescence on live cells, in vitro pharmacology by functional cAMP and [(35) S]-GTPγ binding assays, the kinetics of IgG-induced internalization by image analysis, and the activation of the ERK1/2 by elisa.

KEY RESULTS

mGlu(7) /mGlu(6) chimeric studies located the MAB1/28 binding site at the extracellular amino-terminus of mGlu(7) . MAB1/28 potently antagonized both orthosteric and allosteric agonist-induced inhibition of cAMP accumulation. The potency of the antagonistic actions was similar to the potency in triggering receptor internalization. The internalization mechanism occurred via a pertussis toxin-insensitive pathway and did not require Gα(i) protein activation. MAB1/28 activated ERK1/2 with potency similar to that for receptor internalization. The requirement of a bivalent receptor binding mode for receptor internalizations suggests that MAB1/28 modulates mGlu(7) dimers.

CONCLUSIONS AND IMPLICATIONS

We obtained evidence for an allosteric-biased agonist activity triggered by MAB1/28, which activates a novel IgG-mediated GPCR internalization pathway that is not utilized by small molecule, orthosteric or allosteric agonists. Thus, MAB1/28 provides an invaluable biological tool for probing mGlu(7) function and selective activation of its intracellular trafficking.

Association between the GRM7 rs3792452 polymorphism and attention deficit hyperacitiveity disorder in a Korean sample

Background

The purpose of this study was to investigate the association between the ionotropic and glutamate receptors, N-methyl D-asparate 2A (GRIN2A) and 2B (GRIN2B), and the metabotropic glutamate receptor mGluR7 (GRM7) gene polymorphisms and attention-deficit hyperactivity disorder (ADHD) in Korean population.

Methods

We conducted a case–control analysis of 202 ADHD subjects and 159 controls, performed a transmission disequilibrium test (TDT) on 149 trios, and compared scores from the continuous performance test (CPT), the Children’s Depression Inventory (CDI), and the State-Trait Anxiety Inventory for Children (STAIC) according to the genotype of the glutamate receptor genes.

Results

There were no significant differences in the genotype or allele frequencies of the GRIN2A rs8049651, GRIN2B rs2284411, or GRM7 rs37952452 polymorphisms between the ADHD and control groups. For 148 ADHD trios, the TDT analysis also showed no preferential transmission of the GRIN2A rs8049651 or GRIN2B rs2284411 polymorphisms. However, the TDT analysis of the GRM7 rs3792452 polymorphism showed biased transmission of the G allele (χ2 = 4.67, p = 0.031). In the ADHD probands, the subjects with GG genotype in the GRM7 rs37952452 polymorphism had higher mean T-scores for omission errors on the CPT than did those with the GA or AA genotype (t = 3.38, p = 0.001). In addition, the ADHD subjects who were homozygous for the G allele in the GRM7 rs37952452 polymorphism had higher STAIC-T (t = 5.52, p < 0.001) and STAIC-S (t = 2.74, p = 0.007) scores than did those with the GA or AA genotype.

Conclusions

These results provide preliminary evidence of an association between the GRM7 rs37952452 polymorphism and selective attention deficit and anxiety found within the Korean ADHD population.

ADX71743, a potent and selective negative allosteric modulator of metabotropic glutamate receptor 7: in vitro and in vivo characterization

Metabotropic glutamate receptor 7 (mGlu(7)) has been suggested to be a promising novel target for treatment of a range of disorders, including anxiety, post-traumatic stress disorder, depression, drug abuse, and schizophrenia. Here we characterized a potent and selective mGlu(7) negative allosteric modulator (NAM) (+)-6-(2,4-dimethylphenyl)-2-ethyl-6,7-dihydrobenzo[d]oxazol-4(5H)-one (ADX71743). In vitro, Schild plot analysis and reversibility tests at the target confirmed the NAM properties of the compound and attenuation of L-(+)-2-amino-4-phosphonobutyric acid-induced synaptic depression confirmed activity at the native receptor. The pharmacokinetic analysis of ADX71743 in mice and rats revealed that it is bioavailable after s.c. administration and is brain penetrant (cerebrospinal fluid concentration/total plasma concentration ratio at C(max) = 5.3%). In vivo, ADX71743 (50, 100, 150 mg/kg, s.c.) caused no impairment of locomotor activity in rats and mice or activity on rotarod in mice. ADX71743 had an anxiolytic-like profile in the marble burying and elevated plus maze tests, dose-dependently reducing the number of buried marbles and increasing open arm exploration, respectively. Whereas ADX71743 caused a small reduction in amphetamine-induced hyperactivity in mice, it was inactive in the mouse 2,5-dimethoxy-4-iodoamphetamine-induced head twitch and the rat conditioned avoidance response tests. In addition, the compound was inactive in the mouse forced swim test. These data suggest that ADX71743 is a suitable compound to help unravel the physiologic role of mGlu(7) and to better understand its implication in central nervous system diseases. Our in vivo tests using ADX71743, reported here, suggest that pharmacological inhibition of mGlu(7) is a valid approach for developing novel pharmacotherapies to treat anxiety disorders, but may not be suitable for treatment of depression or psychosis.