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

Recent Advances in the Modulation of Pain by the Metabotropic Glutamate Receptors

Metabotropic glutamate receptors (mGluR or mGlu) are G-protein coupled receptors activated by the binding of glutamate, the main classical neurotransmitter of the nervous system. Eight different mGluR subtypes (mGluR1-8) have been cloned and are classified in three groups based on their molecular, pharmacological and signaling properties. mGluRs mediate several physiological functions such as neuronal excitability and synaptic plasticity, but they have also been implicated in numerous pathological conditions including pain. The availability of new and more selective allosteric modulators together with the canonical orthosteric ligands and transgenic technologies has led to significant advances in our knowledge about the role of the specific mGluR subtypes in the pathophysiological mechanisms of various diseases. Although development of successful compounds acting on mGluRs for clinical use has been scarce, the subtype-specific-pharmacological manipulation might be a compelling approach for the treatment of several disorders in humans, including pain; this review aims to summarize and update on preclinical evidence for the roles of different mGluRs in the pain system and discusses knowledge gaps regarding mGluR-related sex differences and neuroimmune signaling in pain.

Clinical investigations of compounds targeting metabotropic glutamate receptors

Pharmacological modulation of glutamate has long been considered to be of immense therapeutic utility. The metabotropic glutamate receptors (mGluRs) are potential targets for safely altering glutamate-driven excitation. Data support the potential therapeutic use of mGluR modulators in the treatment of anxiety, depression, schizophrenia, and other psychiatric disorders, pain, epilepsy, as well as neurodegenerative and neurodevelopmental disorders. For each of the three mGluR groups, compounds have been constructed that produce either potentiation or functional blockade. PET ligands for mGlu5Rs have been studied in a range of patient populations and several mGlu5R antagonists have been tested for potential efficacy in patients including mavoglurant, diploglurant, basimglurant, GET 73, and ADX10059. Efficacy with mGlu5R antagonists has been reported in trials with patients with gastroesophageal reflux disease; data from patients with Parkinson's disease or Fragile X syndrome have not been as robust as hoped. Fenobam was approved for use as an anxiolytic prior to its recognition as an mGlu5R antagonist. mGlu2/3R agonists (pomaglumated methionil) and mGlu2R agonists (JNJ-40411813, AZD 8529, and LY2979165) have been studied in patients with schizophrenia with promising but mixed results. Antagonists of mGlu2/3Rs (decoglurant and TS-161) have been studied in depression where TS-161 has advanced into a planned Phase 2 study in treatment-resistant depression. The Group III mGluRs are the least developed of the mGluR receptor targets. The mGlu4R potentiator, foliglurax, did not meet its primary endpoint in patients with Parkinson's disease. Ongoing efforts to develop mGluR-targeted compounds continue to promise these glutamate modulators as medicines for psychiatric and neurological disorders.

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 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.

Metabolome and microbiome profiling of a stress-sensitive rat model of gut-brain axis dysfunction

Stress negatively impacts gut and brain health. Individual differences in response to stress have been linked to genetic and environmental factors and more recently, a role for the gut microbiota in the regulation of stress-related changes has been demonstrated. However, the mechanisms by which these factors influence each other are poorly understood, and there are currently no established robust biomarkers of stress susceptibility. To determine the metabolic and microbial signatures underpinning physiological stress responses, we compared stress-sensitive Wistar Kyoto (WKY) rats to the normo-anxious Sprague Dawley (SD) strain. Here we report that acute stress-induced strain-specific changes in brain lipid metabolites were a prominent feature in WKY rats. The relative abundance of Lactococcus correlated with the relative proportions of many brain lipids. In contrast, plasma lipids were significantly elevated in response to stress in SD rats, but not in WKY rats. Supporting these findings, we found that the greatest difference between the SD and WKY microbiomes were the predicted relative abundance of microbial genes involved in lipid and energy metabolism. Our results provide potential insights for developing novel biomarkers of stress vulnerability, some of which appear genotype specific.

Impact of the metabotropic glutamate receptor7 (mGlu7) allosteric agonist, AMN082, on fear learning and memory and anxiety-like behavior

The present study was conducted to evaluate the influence of AMN082, the metabotropic glutamate receptor subtype 7 (mGlu₇) allosteric agonist on different stages of memory processes connected with fear conditioning in the passive avoidance (PA) learning task in mice and negative emotional state (anxiety-like) induced by ethanol- and morphine-withdrawal in the elevated plus maze (EPM) test in rats. To perform the PA test, AMN082 (1.25, 2.5 and 5 mg/kg, i. p.) was injected to interfere with (or inhibit) acquisition, consolidation, and retrieval processes. The retention latency in each group was recorded using a step-through passive avoidance task 24 h after training. In turn, in ethanol- and morphine-withdrawal rats, the influence of AMN082 on anxiety-like behavior was estimated in the EPM test 24 h- (ethanol) and 72- h (morphine) after the last dose of repeated drug administrations. In all experimental groups, AMN082 at the dose of 5 mg/kg significantly decreased the step-through latency of long-term memory in the PA task. These AMN082 effects were reversed by MMPIP (10 mg/kg), the antagonist of mGlu₇ receptor. AMN082 (2.5 and 5 mg/kg) also decreased ethanol- and morphine withdrawal-induced anxiety-like behavior in the EPM test, and this AMN082 (5 mg/kg) effect was counteracted by MMPIP pretreatment. Taken together, the results show that mGlu₇ is involved in fear learning to the context and anxiety-like state connected with unpleasant experiences after ethanol- and morphine withdrawal in rodents. However, it appears that functional dissociation exists between these two AMN082 effects.

The mGlu7 receptor provides protective effects against epileptogenesis and epileptic seizures

Finding new targets to control or reduce seizure activity is essential to improve the management of epileptic patients. We hypothesized that activation of the pre-synaptic and inhibitory metabotropic glutamate receptor type 7 (mGlu7) reduces spontaneous seizures. We tested LSP2-9166, a recently developed mGlu7/4 agonist with unprecedented potency on mGlu7 receptors, in two paradigms of epileptogenesis. In a model of chemically induced epileptogenesis (pentylenetetrazole systemic injection), LSP2-9166 induces an anti-epileptogenic effect rarely observed in preclinical studies. In particular, we found a bidirectional modulation of seizure progression by mGlu4 and mGlu7 receptors, the latter preventing kindling. In the intra-hippocampal injection of kainic acid mouse model that mimics the human mesial temporal lobe epilepsy, we found that LSP2-9166 reduces seizure frequency and hippocampal sclerosis. LSP2-9166 also acts as an anti-seizure drug on established seizures in both models tested. Specific modulation of the mGlu7 receptor could represent a novel approach to reduce pathological network remodeling.

Glutamatergic Signaling Along The Microbiota-Gut-Brain Axis

A complex bidirectional communication system exists between the gastrointestinal tract and the brain. Initially termed the “gut-brain axis” it is now renamed the “microbiota-gut-brain axis” considering the pivotal role of gut microbiota in maintaining local and systemic homeostasis. Different cellular and molecular pathways act along this axis and strong attention is paid to neuroactive molecules (neurotransmitters, i.e., noradrenaline, dopamine, serotonin, gamma aminobutyric acid and glutamate and metabolites, i.e., tryptophan metabolites), sustaining a possible interkingdom communication system between eukaryota and prokaryota. This review provides a description of the most up-to-date evidence on glutamate as a neurotransmitter/neuromodulator in this bidirectional communication axis. Modulation of glutamatergic receptor activity along the microbiota-gut-brain axis may influence gut (i.e., taste, visceral sensitivity and motility) and brain functions (stress response, mood and behavior) and alterations of glutamatergic transmission may participate to the pathogenesis of local and brain disorders. In this latter context, we will focus on two major gut disorders, such as irritable bowel syndrome and inflammatory bowel disease, both characterized by psychiatric co-morbidity. Research in this area opens the possibility to target glutamatergic neurotransmission, either pharmacologically or by the use of probiotics producing neuroactive molecules, as a therapeutic approach for the treatment of gastrointestinal and related psychiatric disorders.

Increased amygdalar metabotropic glutamate receptor 7 mRNA in a genetic mouse model of impaired fear extinction

RATIONALE

Post-traumatic stress disorder (PTSD) is a devastating anxiety-related disorder which develops subsequent to a severe psychologically traumatic event. Only ~ 9% of people who experience such a trauma develop PTSD. It is clear that a number of factors, including genetics, influence whether an individual will develop PTSD subsequent to a trauma. The 129S1/SvImJ (S1) inbred mouse strain displays poor fear extinction and may be useful to model this specific aspect of PTSD. The metabotropic glutamate receptor 7 (mGlu₇ receptor) has previously been shown to be involved in cognitive processes and anxiety-like behaviour placing it in a key position to regulate fear extinction processes. We sought to compare mGlu₇ receptor mRNA levels in the S1 strain with those in the robustly extinguishing C57BL/6J (B6) inbred strain using in situ hybridisation (ISH) in three brain regions associated with fear extinction: the amygdala, hippocampus and prefrontal cortex (PFC).

RESULTS

Compared to the B6 strain, S1 mice had increased mGlu₇ receptor mRNA levels in the lateral amygdala (LA) and basolateral amygdala (BLA) subdivisions. An increase was also seen in the hippocampal CA1 and CA3 subregions of S1 mice. No difference in mGlu₇ receptor levels were seen in the central nucleus (CeA) of the amygdala, dentate gyrus (DG) of the hippocampus or prefrontal cortex.

CONCLUSIONS

These data show altered mGlu₇ receptor expression in key brain regions associated with fear extinction in two different inbred mouse strains which differ markedly in their fear extinction behaviour. Altered mGlu₇ receptor levels may contribute to the deficit fear extinction processes seen in fear extinction in the S1 strain.

Genotype-dependent responsivity to inflammatory pain: A role for TRPV1 in the periaqueductal grey

Negative affective state has a significant impact on pain, and genetic background is an important moderating influence on this interaction. The Wistar-Kyoto (WKY) inbred rat strain exhibits a stress-hyperresponsive, anxiety/depressive-like phenotype and also displays a hyperalgesic response to noxious stimuli. Transient receptor potential subfamily V member 1 (TRPV1) within the midbrain periaqueductal grey (PAG) plays a key role in regulating both aversive and nociceptive behaviour. In the present study, we investigated the role of TRPV1 in the sub-columns of the PAG in formalin-evoked nociceptive behaviour in WKY versus Sprague-Dawley (SD) rats. TRPV1 mRNA expression was significantly lower in the dorsolateral (DL) PAG and higher in the lateral (L) PAG of WKY rats, compared with SD counterparts. There were no significant differences in TRPV1 mRNA expression in the ventrolateral (VL) PAG between the two strains. TRPV1 mRNA expression significantly decreased in the DLPAG and increased in the VLPAG of SD, but not WKY rats upon intra-plantar formalin administration. Intra-DLPAG administration of either the TRPV1 agonist capsaicin, or the TRPV1 antagonist 5'-Iodoresiniferatoxin (5'-IRTX), significantly increased formalin-evoked nociceptive behaviour in SD rats, but not in WKY rats. The effects of capsaicin were likely due to TRPV1 desensitisation, given their similarity to the effects of 5'-IRTX. Intra-VLPAG administration of capsaicin or 5'-IRTX reduced nociceptive behaviour in a moderate and transient manner in SD rats, and similar effects were seen with 5'-IRTX in WKY rats. Intra-LPAG administration of 5'-IRTX reduced nociceptive behaviour in a moderate and transient manner in SD rats, but not in WKY rats. These results indicate that modulation of inflammatory pain by TRPV1 in the PAG occurs in a sub-column-specific manner. The data also provide evidence for differences in the expression of TRPV1, and differences in the effects of pharmacological modulation of TRPV1 in specific PAG sub-columns, between WKY and SD rats, suggesting that TRPV1 expression and/or functionality in the PAG plays a role in hyper-responsivity to noxious stimuli in a genetic background prone to negative affect.