Neuroprotective effects of the selective type 1 metabotropic glutamate receptor antagonist YM-202074 in rat stroke models

Upregulation of Striatal Metabotropic Glutamate Receptor Subtype 1 (mGluR1) in Rats with Excessive Glutamate Release Induced by N-Acetylcysteine

The aim of this study is to investigate the changes in expression of metabotropic glutamate (Glu) receptor subtype 1 (mGluR1), a key molecule involved in neuroexcitetoxicity, during excessive Glu release in the brain by PET imaging. An animal model of excessive Glu release in the brain was produced by intraperitoneally implanting an Alzet osmotic pump containing N-acetylcysteine (NAC), an activator of the cysteine/Glu antiporter, into the abdomen of rats. Basal Glu concentration in the brain was measured by microdialysis, which showed that basal Glu concentration in NAC-treated rats (0.31 µM) was higher than that in saline-treated rats (0.17 µM) at day 7 after the implantation of the osmotic pump. Similarly, PET studies with [¹¹C]ITDM, a useful radioligand for mGluR1 imaging exhibited that the striatal binding potential (BP_ND) of [¹¹C]ITDM for mGluR1 in PET assessments was increased in NAC-treated animals at day 7 after implantation (2.30) compared with before implantation (1.92). The dynamic changes in striatal BP_ND during the experimental period were highly correlated with basal Glu concentration. In conclusion, density of mGluR1 is rapidly upregulated by increases in basal Glu concentration, suggesting that mGluR1 might to be a potential biomarker of abnormal conditions in the brain.

In vitro and In vivo Assessment of Suitable Reference Region and Kinetic Modelling for the mGluR1 Radioligand [11C]ITDM in Mice

PURPOSE

This study aimed at investigating binding specificity, suitability of reference region-based kinetic modelling, and pharmacokinetics of the metabotropic glutamate receptor 1 (mGluR1) radioligand [¹¹C]ITDM in mice.

PROCEDURES

We performed in vivo blocking as well as displacement of [¹¹C]ITDM during positron emission tomography (PET) imaging using the specific mGluR1 antagonist YM-202074. Additionally, we assessed in vitro blocking of [³H]ITDM at two different doses of YM-202074. As an alternative to reference region models, we validated the use of a noninvasive image-derived input function (IDIF) compared to an arterial input function measured with an invasive arteriovenous (AV) shunt using a population-based curve for radiometabolite correction and characterized the pharmacokinetic modelling of [¹¹C]ITDM in the mouse brain. Finally, we also assessed semi-quantitative approaches.

RESULTS

In vivo blocking with YM-202074 resulted in a decreased [¹¹C]ITDM binding, ranging from - 35.8 ± 8.0 % in pons to - 65.8 ± 3.0 % in thalamus. Displacement was also markedly observed in all tested regions. In addition, in vitro [³H]ITDM binding could be blocked in a dose-dependent manner. The volume of distribution (V_T) based on the noninvasive IDIF (V_{T (IDIF)}) showed excellent agreement with the V_T values based on the metabolite-corrected plasma input function regardless of the metabolite correction (r² > 0.943, p < 0.0001). Two-tissue compartmental model (2TCM) was found to be the preferred model and showed optimal agreement with Logan plot (r² > 0.960, p < 0.0001). A minimum scan duration of 80 min was required for proper parameter estimation. SUV was not reliable (r² = 0.379, p = 0.0011), unlike the SUV ratio to the SUV of the input function, which showed to be a valid approach.

CONCLUSIONS

No suitable reference region could be identified for [¹¹C]ITDM as strongly supported by in vivo and in vitro evidence of specific binding in all brain regions. However, by applying appropriate kinetic models, [¹¹C]ITDM PET imaging represents a promising tool to visualize mGluR1 in the mouse brain.

G-Protein-Coupled Receptors and Ischemic Stroke: a Focus on Molecular Function and Therapeutic Potential

In ischemic stroke, there is only one approved drug, tissue plasminogen activator, to be used in clinical conditions for thrombolysis. New neuroprotective therapies for ischemic stroke are desperately needed. Several targets and pathways have been shown to confer neuroprotective effects in ischemic stroke. G-protein-coupled receptors (GPCRs) are one of the most frequently targeted receptors for developing novel therapeutics for central nervous system disorders. GPCRs are a large family of cell surface receptors that response to a wide variety of extracellular stimuli. GPCRs are involved in a wide range of physiological and pathological processes. More than 90% of the identified non-sensory GPCRs are expressed in the brain, where they play important roles in regulating mood, pain, vision, immune responses, cognition, and synaptic transmission. There is also good evidence that GPCRs are implicated in the pathogenesis of stroke. This review narrates the pathophysiological role and possible targeted therapy of GPCRs in ischemic stroke.

Regulator of G-Protein Signaling 5 Maintains Brain Endothelial Cell Function in Focal Cerebral Ischemia

Background

Regulator of G‐protein signaling 5 (RGS5) is a negative modulator of G‐protein–coupled receptors. The role of RGS5 in brain endothelial cells is not known. We hypothesized that RGS5 in brain microvascular endothelial cells may be an important mediator of blood‐brain barrier function and stroke severity after focal cerebral ischemia.

Methods and Results

Using a transient middle cerebral artery occlusion model, we found that mice with global and endothelial‐specific deletion of Rgs5 exhibited larger cerebral infarct size, greater neurological motor deficits, and increased brain edema. In our in vitro models, we observed increased G_q activity and elevated intracellular Ca²⁺ levels in brain endothelial cells. Furthermore, the loss of endothelial RGS5 leads to decreased endothelial NO synthase expression and phosphorylation, relocalization of endothelial tight junction proteins, and increased cell permeability. Indeed, RGS5 deficiency leads to increased Rho‐associated kinase and myosin light chain kinase activity, which were partially reversed in our in vitro model by pharmacological inhibition of G_q, metabotropic glutamate receptor 1, and ligand‐gated ionotropic glutamate receptor.

Conclusions

Our findings indicate that endothelial RGS5 plays a novel neuroprotective role in focal cerebral ischemia. Loss of endothelial RGS5 leads to hyperresponsiveness to glutamate signaling pathways, enhanced Rho‐associated kinase– and myosin light chain kinase–mediated actin‐cytoskeleton reorganization, endothelial dysfunction, tight junction protein relocalization, increased blood‐brain barrier permeability, and greater stroke severity. These findings suggest that preservation of endothelial RGS5 may be an important therapeutic strategy for maintaining blood‐brain barrier integrity and limiting the severity of ischemic stroke.

Phased Treatment Strategies for Cerebral Ischemia Based on Glutamate Receptors

Extracellular glutamate accumulation following cerebral ischemia leads to overactivation of glutamate receptors, thereby resulting in intracellular Ca²⁺ overload and excitotoxic neuronal injury. Multiple attempts have been made to counteract such effects by reducing glutamate receptor function, but none have been successful. In this minireview, we present the available evidence regarding the role of all types of ionotropic and metabotropic glutamate receptors in cerebral ischemia and propose phased treatment strategies based on glutamate receptors in both the acute and post-acute phases of cerebral ischemia, which may help realize the clinical application of glutamate receptor antagonists.

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.

G-protein coupled receptors as therapeutic targets for neurodegenerative and cerebrovascular diseases

Neurodegenerative and cerebrovascular diseases are frequent in elderly populations and comprise primarily of dementia (mainly Alzheimer's disease) Parkinson's disease and stroke. These neurological disorders (NDs) occur as a result of neurodegenerative processes and represent one of the most frequent causes of death and disability worldwide with a significant clinical and socio-economic impact. Although NDs have been characterized for many years, the exact molecular mechanisms that govern these pathologies or why they target specific individuals and specific neuronal populations remain unclear. As research progresses, many similarities appear which relate these diseases to one another on a subcellular level. Discovering these similarities offers hope for therapeutic advances that could ameliorate the conditions of many diseases simultaneously. G-protein coupled receptors (GPCRs) are the most abundant receptor type in the central nervous system and are linked to complex downstream pathways, manipulation of which may have therapeutic application in many NDs. This review will highlight the potential use of neurotransmitter GPCRs as emerging therapeutic targets for neurodegenerative and cerebrovascular diseases.

Selective inhibition of metabotropic glutamate type 1 alpha receptor (mGluR1α) reduces cell proliferation and migration following status epilepticus in early development

The present study examined whether a single or multiple episode(s) of status epilepticus induced with kainic acid (KA) during the first 3 weeks of postnatal (P) development would aberrantly stimulate proliferation zones that alters migration to potentially injured areas and whether they would be blocked by selective Group I mGluR antagonists. mGluR1α (LY367385) and mGluR5 (MPEP) antagonists were administered 2h following KA-induced status epilepticus and animals were examined after 7days. Proliferating cells of the subventricular zone (SVZ), third ventricle, hippocampus, amygdala cortical complex were analyzed with the proliferative marker, Ki67; and two complementary retrograde dye tracers. Proliferation increased in extrahippocampal limbic structures when KA was administered on P13 or P20 which correlated with number of injured cells at the older age. LY367385 post-treatment caused striking decreases in proliferation in all limbic structures in the presence and absence of injury, whereas a reduction with MPEP was observed only within the amygdala cortical complex (Amg/ERcx) in the presence of multiple seizures (3×KA). After 3×KA and LY367385 post-treatments, diminished co-staining of dye tracers with Ki67 was observed within the Amg/ERcx despite high levels of progenitors marked by the retrograde tracers in this region. This indicates that not only was local proliferation within the SVZ and distant structures inhibited, but also that migration itself was reduced indirectly since there were less cells to migrate from the SVZ. Co-labeling with biomarkers provided evidence for neuronal differentiation suggesting potential aberrant integration may occur in distant locations, and that targeting of mGluR1α receptors may be a potential therapeutic strategy for future development.

Dynamic Changes in Striatal mGluR1 But Not mGluR5 during Pathological Progression of Parkinson's Disease in Human Alpha-Synuclein A53T Transgenic Rats: A Multi-PET Imaging Study

Parkinson's disease (PD) is a prevalent degenerative disorder affecting the CNS that is primarily characterized by resting tremor and movement deficits. Group I metabotropic glutamate receptor subtypes 1 and 5 (mGluR1 and mGluR5, respectively) are important targets for investigation in several CNS disorders. In the present study, we investigated the in vivo roles of mGluR1 and mGluR5 in chronic PD pathology by performing longitudinal positron emission tomography (PET) imaging in A53T transgenic (A53T-Tg) rats expressing an abnormal human α-synuclein (ASN) gene. A53T-Tg rats showed a dramatic decline in general motor activities with age, along with abnormal ASN aggregation and striatal neuron degeneration. In longitudinal PET imaging, striatal nondisplaceable binding potential (BPND) values for [(11)C]ITDM (N-[4-[6-(isopropylamino) pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methyl-4-[(11)C]methylbenzamide), a selective PET ligand for mGluR1, temporarily increased before PD symptom onset and dramatically decreased afterward with age. However, striatal BPND values for (E)-[(11)C]ABP688 [3-(6-methylpyridin-2-ylethynyl)-cyclohex-2-enone-(E)-O-[(11)C]methyloxime], a specific PET ligand for mGluR5, remained constant during experimental terms. The dynamic changes in striatal mGluR1 BPND values also showed a high correlation in pathological decreases in general motor activities. Furthermore, declines in mGluR1 BPND values were correlated with decreases in BPND values for [(18)F]FE-PE2I [(E)-N-(3-iodoprop-2E-enyl)-2β-carbo-[(18)F]fluoroethoxy-3β-(4-methylphenyl) nortropane], a specific PET ligand for the dopamine transporter, a biomarker for dopaminergic neurons. In conclusion, our results have demonstrated for the first time that dynamic changes occur in mGluR1, but not mGluR5, that accompany pathological progression in a PD animal model.

SIGNIFICANCE STATEMENT

Synaptic signaling by glutamate, the principal excitatory neurotransmitter in the brain, is modulated by group I metabotropic glutamate receptors, including the mGluR1 and mGluR5 subtypes. In the brain, mGluR1 and mGluR5 have distinct functional roles and regional distributions. Their roles in brain pathology, however, are not well characterized. Using longitudinal PET imaging in a chronic rat model of PD, we demonstrated that expression of mGluR1, but not mGluR5, dynamically changed in the striatum accompanying pathological PD progression. These findings imply that monitoring mGluR1 in vivo may provide beneficial information to further understand central nervous system disorders.

Improved Visualization and Specific Binding for Metabotropic Glutamate Receptor Subtype 1 (mGluR1) Using [11C]ITMM with Ultra-High Specific Activity in Small-Animal PET

Metabotropic glutamate receptor subtype 1 (mGluR1) is a crucial target in the development of new medications to treat central nervous system (CNS) disorders. Recently, we developed N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-4-[¹¹C]methoxy-N-methyl-benzamide ([¹¹C]ITMM) as a useful positron emission tomography (PET) probe for mGluR1 in clinical studies. Here, we aimed to improve visualization and threshold of specific binding for mGluR1 using [¹¹C]ITMM with ultra-high specific activity (SA) of > 3,500 GBq/μmol in rat brains. A two-tissue compartment model indicated large differences between the two SAs in the constants k₃ and k₄, representing binding ability for mGluR1, while constants K₁ and k₂ showed no differences. The total distribution volume (V_T) values of conventional and ultra-high SA were 9.1 and 11.2 in the thalamus, 7.7 and 9.7 in the striatum, and 6.4 and 8.5 mL/cm³ in the substantia nigra, respectively. The specific binding of [¹¹C]ITMM with ultra-high SA was significantly higher than the conventional SA, especially in the basal ganglia. Parametric PET images scaled with V_T of the ultra-high SA clearly identified regional differences in the rat brain. In conclusion, PET studies using [¹¹C]ITMM with ultra-high SA could sufficiently improve visualization and specific binding for mGluR1, which could help further understanding for mGluR1 functions in CNS disorders.

Allosteric Binding Site and Activation Mechanism of Class C G-Protein Coupled Receptors: Metabotropic Glutamate Receptor Family

Metabotropic glutamate receptors (mGluR) are mainly expressed in the central nervous system (CNS) and contain eight receptor subtypes, named mGluR1 to mGluR8. The crystal structures of mGluR1 and mGluR5 that are bound with the negative allosteric modulator (NAM) were reported recently. These structures provide a basic model for all class C of G-protein coupled receptors (GPCRs) and may aid in the design of new allosteric modulators for the treatment of CNS disorders. However, these structures are only combined with NAMs in the previous reports. The conformations that are bound with positive allosteric modulator (PAM) or agonist of mGluR1/5 remain unknown. Moreover, the structural information of the other six mGluRs and the comparisons of the mGluRs family have not been explored in terms of their binding pockets, the binding modes of different compounds, and important binding residues. With these crystal structures as the starting point, we built 3D structural models for six mGluRs by using homology modeling and molecular dynamics (MD) simulations. We systematically compared their allosteric binding sites/pockets, the important residues, and the selective residues by using a series of comparable dockings with both the NAM and the PAM. Our results show that several residues played important roles for the receptors' selectivity. The observations of detailed interactions between compounds and their correspondent receptors are congruent with the specificity and potency of derivatives or compounds bioassayed in vitro. We then carried out 100 ns MD simulations of mGluR5 (residue 26-832, formed by Venus Flytrap domain, a so-called cysteine-rich domain, and 7 trans-membrane domains) bound with antagonist/NAM and with agonist/PAM. Our results show that both the NAM and the PAM seemed stable in class C GPCRs during the MD. However, the movements of "ionic lock," of trans-membrane domains, and of some activation-related residues in 7 trans-membrane domains of mGluR5 were congruent with the findings in class A GPCRs. Finally, we selected nine representative bound structures to perform 30 ns MD simulations for validating the stabilities of interactions, respectively. All these bound structures kept stable during the MD simulations, indicating that the binding poses in this present work are reasonable. We provided new insight into better understanding of the structural and functional roles of the mGluRs family and facilitated the future structure-based design of novel ligands of mGluRs family with therapeutic potential.

Inducible glutamate oxaloacetate transaminase as a therapeutic target against ischemic stroke

SIGNIFICANCE

Glutamate serves multi-faceted (patho)physiological functions in the central nervous system as the most abundant excitatory neurotransmitter and under pathological conditions as a potent neurotoxin. Regarding the latter, elevated extracellular glutamate is known to play a central role in ischemic stroke brain injury.

RECENT ADVANCES

Glutamate oxaloacetate transaminase (GOT) has emerged as a new therapeutic target in protecting against ischemic stroke injury. Oxygen-sensitive induction of GOT expression and activity during ischemic stroke lowers glutamate levels at the stroke site while sustaining adenosine triphosphate levels in brain. The energy demands of the brain are among the highest of all organs underscoring the need to quickly mobilize alternative carbon skeletons for metabolism in the absence of glucose during ischemic stroke. Recent work builds on the important observation of Hans Krebs that GOT-mediated metabolism of glutamate generates tri-carboxylic acid (TCA) cycle intermediates in brain tissue. Taken together, outcomes suggest GOT may enable the transformative switch of otherwise excitotoxic glutamate into life-sustaining TCA cycle intermediates during ischemic stroke.

CRITICAL ISSUES

Neuroprotective strategies that focus solely on blocking mechanisms of glutamate-mediated excitotoxicity have historically failed in clinical trials. That GOT can enable glutamate to assume the role of a survival factor represents a paradigm shift necessary to develop the overall significance of glutamate in stroke biology.

FUTURE DIRECTIONS

Ongoing efforts are focused to develop the therapeutic significance of GOT in stroke-affected brain. Small molecules that target induction of GOT expression and activity in the ischemic penumbra are the focus of ongoing studies.

Thiazole: a promising heterocycle for the development of potent CNS active agents

Thiazole is a valuable scaffold in the field of medicinal chemistry and has accounted to display a variety of biological activities. Thiazole and its derivatives have attracted continuing interest to design various novel CNS active agents. In the past few decades, thiazoles have been widely used to develop a variety of therapeutic agents against numerous CNS targets. Thiazole containing drug molecules are currently being used in treatment of various CNS disorders and a number of thiazole derivatives are also presently in clinical trials. A lot of research has been carried out on thiazole and their analogues, which has proved their efficacy to overcome several CNS disorders in rodent as well as primate models. The aim of present review is to highlights diverse CNS activities displayed by thiazole and their derivatives. SAR of this nucleus has also been well discussed. This review covers the recent updates present in literature and will surely provide a greater insight for the designing and development of potent thiazole based CNS active agents in future.

Transcriptome analysis of CNS immediately before and after the detection of PrP(Sc) in SSBP/1 sheep scrapie

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Arrays and DGE-tags quantified gene expression in the CNS during sheep scrapie.

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Neurological receptors were increased with disease progression.

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Clues to basis of psychiatric changes.

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Step changes to gene expression after the detection of PrP^Sc in CNS.

Sheep scrapie is a transmissible spongiform encephalopathy (TSE), progressive and fatal neurodegenerative diseases of the central nervous system (CNS) linked to the accumulation of misfolded prion protein, PrP^Sc. New Zealand Cheviot sheep, homozygous for the VRQ genotype of the PRNP gene are most susceptible with an incubation period of 193 days with SSBP/1 scrapie. However, the earliest time point that PrP^Sc can be detected in the CNS is 125 days (D125). The aim of this study was to quantify changes to the transcriptome of the thalamus and obex (medulla) at times immediately before (D75) and after (D125) PrP^Sc was detected. Affymetrix gene arrays were used to quantify gene expression in the thalamus and Illumina DGE-tag profiling for obex. Ingenuity Pathway Analysis was used to help describe the biological processes of scrapie pathology.

Neurological disease and Cancer were common Bio Functions in each tissue at D75; inflammation and cell death were major processes at D125. Several neurological receptors were significantly increased at D75 (e.g. CHRNA6, GRM1, HCN2), which might be clues to the molecular basis of psychiatric changes associated with TSEs. No genes were significantly differentially expressed at both D75 and D125 and there was no progression of events from earlier to later time points. This implies that there is no simple linear progression of pathological or molecular events. There seems to be a step-change between D75 and D125, correlating with the detection of PrP^Sc, resulting in the involvement of different pathological processes in later TSE disease.

A glutamate receptor antagonist, S-4-carboxyphenylglycine (S-4-CPG), inhibits vasospasm after subarachnoid hemorrhage in haptoglobin 2-2 mice [corrected]

BACKGROUND

Vasospasm contributes to delayed cerebral ischemia after aneurysmal subarachnoid hemorrhage (SAH). Glutamate concentrations increase after SAH and correlate with vasospasm in experimental SAH. The haptoglobin (Hp) 2-2 genotype is associated with higher risk of vasospasm after SAH. We tested the efficacy of (S)-4-carboxyphenylglycine (S-4-CPG), a metabotropic glutamate receptor inhibitor, for the treatment of vasospasm after SAH in Hp 2-2 and Hp 1-1 mice.

OBJECTIVE

To evaluate the effect on vasospasm and neurobehavioral scores after SAH of systemic S-4-CPG, as well as its toxicity, and phosphorylation of vasodilator-stimulated phosphoprotein (VASP) in Hp 2-2 mice.

METHODS

Western blot was used to assess changes in VASP phosphorylation in response to glutamate with and without S-4-CPG. A pharmacokinetics study was done to evaluate S-4-CPG penetration through the blood-brain barrier in vivo. Toxicity was assessed by administering increasing S-4-CPG doses. Efficacy of S-4-CPG assessed the effect of S-4-CPG on lumen patency of the basilar artery and animal behavior after SAH in Hp 1-1 and Hp 2-2 mice. Immunohistochemistry was used to evaluate the presence of neutrophils surrounding the basilar artery after SAH.

RESULTS

Exposure of human brain microvascular endothelial cells to glutamate decreased phosphorylation of VASP, but glutamate treatment in the presence of S-4-CPG maintains phosphorylation of VASP. S-4-CPG crosses the blood-brain barrier and was not toxic to mice. S-4-CPG treatment significantly prevents vasospasm after SAH. S-4-CPG administered after SAH resulted in a trend toward improvement of animal behavior.

CONCLUSION

S-4-CPG prevents vasospasm after experimental SAH in Hp2-2 mice. S-4-CPG was not toxic and is a potential therapeutic agent for vasospasm after SAH.

Noninvasive quantification of metabotropic glutamate receptor type 1 with [¹¹C]ITDM: a small-animal PET study

Because of its role in multiple central nervous system (CNS) pathways, metabotropic glutamate receptor type 1 (mGluR1) is a crucial target in the development of pharmaceuticals for CNS disorders. N-[4-[6-(isopropylamino)-pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methyl-4-[(11)C]-methylbenzamide ([(11)C]ITDM) was recently developed as a positron emission tomography (PET) ligand for mGluR1. To devise a method for measurement of the binding potential (BPND) of [(11)C]ITDM to mGluR1, reference tissue methods aimed at replacing measurement of the arterial input function are desirable. In this study, we evaluated a noninvasive quantification method of mGluR1 with [(11)C]ITDM, demonstrating its accuracy using Huntington disease model R6/2 mice. The BPND measurements based on the Logan reference (Logan Ref) method have closely approximated that based on the arterial input method. We performed PET analysis with Logan Ref to assess its accuracy in quantifying the decline of mGluR1 expression in R6/2 mice. Significant decreases in BPND values in R6/2 mice were detected in cerebellum, thalamus, striatum, and cingulate cortex. We compared autoradiographs of R6/2 mouse brain sections with immunohistochemical images, and found a close correlation between changes in radioactive signal intensity and degree of mGluR1 expression. In conclusion, [(11)C]ITDM-PET is a promising tool for in vivo quantification of mGluR1 expression.

Inhibition of the group I mGluRs reduces acute brain damage and improves long-term histological outcomes after photothrombosis-induced ischaemia

Group I mGluRs (metabotropic glutamate receptors), including mGluR1 and mGluR5, are GPCRs (G-protein coupled receptors) and play important roles in physiology and pathology. Studies on their role in cerebral ischaemia have provided controversial results. In this study, we used a PT (photothrombosis)-induced ischaemia model to investigate whether antagonists to the group I mGluRs may offer acute and long-term protective effects in adult mice. Our results demonstrated that administration with mGluR5 antagonist MPEP [2-methyl-6-(phenylethynyl)-pyridine] or mGluR1 antagonist LY367385 by intraperitoneal injection at 3 h after PT decreased brain infarct volume evaluated one day after ischaemia. Additive effects on infarct volume were observed upon co-injection with MPEP and LY367385. These antagonists also significantly alleviated neurodegeneration and apoptosis in the penumbra. In addition, when evaluated 2 weeks after PT, they reduced infarct volume and tissue loss, attenuated glial scar formation, and inhibited cell proliferation in the penumbra. Importantly, co-injection with MPEP and LY367385 reduced the expression levels of calpain, a Ca2+-activated protease known to mediate ischaemia-induced neuronal death. Injection of calpeptin, a calpain inhibitor, could inhibit neuronal death and brain damage after PT but injection of calpeptin together with MPEP and LY367385 did not further improve the protective effects mediated by MPEP and LY367385. These results suggest that inhibition of group I mGluRs is sufficient to protect ischaemic damage through the calpain pathway. Taken together, our results demonstrate that inhibition of group I mGluRs can mitigate PT-induced brain damage through attenuating the effects of calpain, and improve long-term histological outcomes.

β-Estradiol unmasks metabotropic receptor-mediated metaplasticity of NMDA receptor transmission in the female rat dentate gyrus

Loss of estrogen in women following menopause is associated with increased risk for cognitive decline, dementia and depression, all of which can be prevented by estradiol replacement. The dentate gyrus plays an important role in cognition, learning and memory. The gatekeeping function of the dentate gyrus to filter incoming activity into the hippocampus is modulated by estradiol in a frequency-dependent manner and involves activation of metabotropic glutamate receptors (mGluR). In the present study, we investigated whether estradiol (EB) modulates the metaplastic effect of inducing synaptic long-term potentiation (LTP) on subsequent propensity for expression of LTP in the dentate gyrus. At medial perforant path-dentate granule cell synapses in hippocampal slices of ovariectomized female rats, EB replacement was critical for an initial induction of LTP to enhance the magnitude of subsequent LTP elicited by a second high-frequency stimulation, metaplasticity, which was not present in slices from oil-treated control animals. EB enhanced expression of group I mGluRs, and the metaplastic effect of EB on LTP required activation of group I mGluRs that led to Src-family tyrosine kinase-mediated phosphorylation of NR2B subunits of N-methyl-d-aspartate receptors (NMDAR) that enhanced the magnitude of NMDAR-dependent LTP. Our data show that EB effects on LTP in the hippocampal dentate gyrus require activation of group I mGluRs, which in turn leads to functional metaplastic regulation of NR2B subunit-containing NMDARs, as opposed to direct effects of EB on NMDARs.

Selective mGluR1 antagonist EMQMCM inhibits the kainate-induced excitotoxicity in primary neuronal cultures and in the rat hippocampus

Abundant evidence suggests that indirect inhibitory modulation of glutamatergic transmission, via metabotropic glutamatergic receptors (mGluR), may induce neuroprotection. The present study was designed to determine whether the selective antagonist of mGluR1 (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate (EMQMCM), showed neuroprotection against the kainate (KA)-induced excitotoxicity in vitro and in vivo. In in vitro studies on mouse primary cortical and hippocampal neuronal cultures, incubation with KA (150 μM) induced strong degeneration [measured as lactate dehydrogenase (LDH) efflux] and apoptosis (measured as caspase-3 activity). EMQMCM (0.1-100 μM) added 30 min to 6 h after KA, significantly attenuated the KA-induced LDH release and prevented the increase in caspase-3 activity in the cultures. Those effects were dose- and time-dependent. In in vivo studies KA (2.5 nmol/1 μl) was unilaterally injected into the rat dorsal CA1 hippocampal region. Degeneration was calculated by counting surviving neurons in the CA pyramidal layer using stereological methods. It was found that EMQMCM (5-10 nmol/1 μl) injected into the dorsal hippocampus 30 min, 1 h, or 3 h (the higher dose only) after KA significantly prevented the KA-induced neuronal degeneration. In vivo microdialysis studies in rat hippocampus showed that EMQMCM (100 μM) significantly increased γ-aminobutyric acid (GABA) and decreased glutamate release. When perfused simultaneously with KA, EMQMCM substantially increased GABA release and prevented the KA-induced glutamate release. The obtained results indicate that the mGluR1 antagonist, EMQMCM, may exert neuroprotection against excitotoxicity after delayed treatment (30 min to 6 h). The role of enhanced GABAergic transmission in the neuroprotection is postulated.

Neuroprotective effect of Buyang Huanwu decoction against focal cerebral ischemia/reperfusion injury in rats--time window and mechanism

ETHNOPHARMACOLOGICAL RELEVANCE

Buyang Huanwu Decoction, a traditional Chinese medicine, consists of different herbal medicines, and has been traditionally used for centuries to treat paralysis and stroke. However, its optimal therapeutic time window and the mechanism are still unclear.

AIM OF THE STUDY

This study was designed to explore the therapeutic time window and mechanism of Buyang Huanwu Decoction on transient focal cerebral ischemia/reperfusion injury.

MATERIALS AND METHODS

Middle cerebral artery occlusion was conducted in male Sprague-Dawley rats, and 40g/kg of Buyang Huanwu Decoction was intragastrically infused at different time points, and the same dose was infused every 24h for 3 days. The level of glutamate in cerebrospinal fluid and the expression of metabotropic glutamate receptor-1 RNA in striatum were detected before, during, and after ischemia/reperfusion. Neurological deficit scores and brain infarction volumes were measured at 72h after reperfusion.

RESULT

Cerebral ischemia/reperfusion resulted in significant neurological deficit and extensive cerebral infarct volume, associated with a large amount of glutamate in cerebrospinal fluid and elevation of metabotropic glutamate receptor-1 RNA expression. Buyang Huanwu Decoction significantly suppressed the release of glutamate, and reduced the expression of metabotropic glutamate receptor-1 RNA. The neurological defect score and infarction volume were significantly improved by administration of Buyang Huanwu Decoction, when compared with the Ischemia group.

CONCLUSIONS

Administration of Buyang Huanwu Decoction, within 4h of post-transient focal stroke, reduced significant cerebral ischemia/reperfusion damage. The neuroprotective mechanism of Buyang Huanwu Decoction is, in part, associated with the down-regulation of metabotropic glutamate receptor-1 RNA and inhibition of glutamate release resulting from cerebral ischemia.

Synthesis and evaluation of novel radioligands for positron emission tomography imaging of metabotropic glutamate receptor subtype 1 (mGluR1) in rodent brain

We designed three novel positron emission tomography ligands, N-(4-(6-(isopropylamino)pyrimidin-4-yl)-1,3-thiazol-2-yl)-4-[(11)C]methoxy-N-methylbenzamide ([(11)C]6), 4-[(18)F]fluoroethoxy-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ([(18)F]7), and 4-[(18)F]fluoropropoxy-N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ([(18)F]8), for imaging metabotropic glutamate receptor type 1 (mGluR1) in rodent brain. Unlabeled compound 6 was synthesized by benzoylation of 4-pyrimidinyl-2-methylaminothiazole 10, followed by reaction with isopropylamine. Removal of the methyl group in 6 gave phenol precursor 12 for radiosynthesis. Two fluoroalkoxy analogues 7 and 8 were prepared by reacting 12 with tosylates 13 and 14. Radioligands [(11)C]6, [(18)F]7, and [(18)F]8 were synthesized by O-[(11)C]methylation or [(18)F]fluoroalkylation of 12. Compound 6 showed high in vitro binding affinity for mGluR1, whereas 7 and 8 had weak affinity. Autoradiography using rat brain sections showed that [(11)C]6 binding is aligned with the reported distribution of mGluR1 with high specific binding in the cerebellum and thalamus. PET study with [(11)C]6 in rats showed high brain uptake and a similar distribution pattern to that in autoradiography, indicating the usefulness of [(11)C]6 for imaging brain mGluR1.

Metabotropic glutamate receptors in neurodegeneration/neuroprotection: still a hot topic?

Moving from early studies, we here review the most recent evidence linking metabotropic glutamate (mGlu) receptors to processes of neurodegeneration/neuroprotection. The use of knockout mice and subtype-selective drugs has increased our knowledge of the precise role played by individual mGlu receptor subtypes in these processes. Activation of mGlu1 and mGlu5 receptors may either amplify or reduce neuronal damage depending on the context and the nature of the toxic insults. In contrast, mGlu1 and mGlu5 receptors antagonists are consistently protective in in vitro and in vivo models of neuronal death. A series of studies suggest that mGlu1 receptor antagonists or negative allosteric modulators (NAMs) are promising candidates for the treatment of ischemic brain damage, whereas mGlu5 receptor NAMs, which have been clinically developed for the treatment of Parkinson's disease (PD) and l-DOPA-induced dyskinesias, protect nigro-striatal dopaminergic neurons against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity in mice and monkeys. Activation of glial mGlu3 receptors promotes the formation of various neurotrophic factors, such as transforming growth factor-β (TGF-β), glial-derived neurotrophic factor (GDNF), nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF). Hence, selective mGlu3 receptor agonists or positive allosteric modulators (PAMs) (not yet available) are potentially helpful in the treatment of chronic neurodegenerative disorders such as PD, Alzheimer's disease (AD), and amyotrophic lateral sclerosis. Selective mGlu2 receptor PAMs should be used with caution in AD patients because these drugs are shown to amplify β-amyloid neurotoxicity. Finally, mGlu4 receptor agonists/PAMs share with mGlu5 receptor NAMs the ability to improve motor symptoms associated with PD and attenuate nigro-striatal degeneration at the same time. No data are yet available on the role of mGlu7 and mGlu8 receptors in neurodegeneration/neuroprotection.

Metabotropic glutamate receptor-mediated signaling in neuroglia

Metabotropic glutamate (mGlu) receptors are G-protein-coupled receptors, which include eight subtypes that have been classified into three groups (I-III) based upon sequence homology, signal transduction mechanism and pharmacological profile. Although most studied with regard to neuronal function and modulation, mGlu receptors are also expressed by neuroglia-including astrocytes, microglia and oligodendrocytes. Activation of mGlu receptors on neuroglia under both physiologic and pathophysiologic conditions mediates numerous actions that are essential for intrinsic glial cell function, as well as for glial-neuronal interactions. Astrocyte mGlu receptors play important physiological roles in regulating neurotransmission and maintaining neuronal homeostasis. However, mGlu receptors on astrocytes and microglia also serve to modulate cell death and neurological function in a variety of pathophysiological conditions such as acute and chronic neurodegenerative disorders. The latter effects are complex and bi-directional, depending on which mGlu receptor sub-types are activated.

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.

Oxygen/glucose deprivation induces a reduction in synaptic AMPA receptors on hippocampal CA3 neurons mediated by mGluR1 and adenosine A3 receptors

Hippocampal CA1 pyramidal neurons are highly sensitive to ischemic damage, whereas neighboring CA3 pyramidal neurons are less susceptible. It is proposed that switching of AMPA receptor (AMPAR) subunits on CA1 neurons during an in vitro model of ischemia, oxygen/glucose deprivation (OGD), leads to an enhanced permeability of AMPARs to Ca(2+), resulting in delayed cell death. However, it is unclear whether the same mechanisms exist in CA3 neurons and whether this underlies the differential sensitivity to ischemia. Here, we investigated the consequences of OGD for AMPAR function in CA3 neurons using electrophysiological recordings in rat hippocampal slices. Following a 15 min OGD protocol, a substantial depression of AMPAR-mediated synaptic transmission was observed at CA3 associational/commissural and mossy fiber synapses but not CA1 Schaffer collateral synapses. The depression of synaptic transmission following OGD was prevented by metabotropic glutamate receptor 1 (mGluR1) or A(3) receptor antagonists, indicating a role for both glutamate and adenosine release. Inhibition of PLC, PKC, or chelation of intracellular Ca(2+) also prevented the depression of synaptic transmission. Inclusion of peptides to interrupt the interaction between GluA2 and PICK1 or dynamin and amphiphysin prevented the depression of transmission, suggesting a dynamin and PICK1-dependent internalization of AMPARs after OGD. We also show that a reduction in surface and total AMPAR protein levels after OGD was prevented by mGluR1 or A(3) receptor antagonists, indicating that AMPARs are degraded following internalization. Thus, we describe a novel mechanism for the removal of AMPARs in CA3 pyramidal neurons following OGD that has the potential to reduce excitotoxicity and promote neuroprotection.

Microglial activation in stroke: therapeutic targets

Microglial activation is an early response to brain ischemia and many other stressors. Microglia continuously monitor and respond to changes in brain homeostasis and to specific signaling molecules expressed or released by neighboring cells. These signaling molecules, including ATP, glutamate, cytokines, prostaglandins, zinc, reactive oxygen species, and HSP60, may induce microglial proliferation and migration to the sites of injury. They also induce a nonspecific innate immune response that may exacerbate acute ischemic injury. This innate immune response includes release of reactive oxygen species, cytokines, and proteases. Microglial activation requires hours to days to fully develop, and thus presents a target for therapeutic intervention with a much longer window of opportunity than acute neuroprotection. Effective agents are now available for blocking both microglial receptor activation and the microglia effector responses that drive the inflammatory response after stroke. Effective agents are also available for targeting the signal transduction mechanisms linking these events. However, the innate immune response can have beneficial as well deleterious effects on outcome after stoke, and a challenge will be to find ways to selectively suppress the deleterious effects of microglial activation after stroke without compromising neurovascular repair and remodeling.

Treadmill pre-training suppresses the release of glutamate resulting from cerebral ischemia in rats

This study was designed to investigate the neuroprotective effect of treadmill pre-training against the over-release of glutamate resulting from cerebral ischemia. Sprague-Dawley rats underwent 2 weeks of treadmill run-training before cerebral ischemia was performed by middle cerebral artery occlusion. The level of glutamate in brain extracellular fluid was detected before, during and after ischemia/reperfusion. The expression of metabotropic glutamate receptor-1 (mGluR1) mRNA in striatum was examined after ischemia for 80 min and reperfusion for 240 min. Neurological defect score and brain infarction volumes were measured. The treadmill pre-training significantly suppressed the release of glutamate, and reduced the expression of mGluR1 mRNA at 59% (P < 0.01) and 62% (P < 0.05), respectively, as compared with the ischemia group. The neurological defect score and infarction volume were significantly improved by 75% (P < 0.01) and 74% (P < 0.01), respectively, in the pre-training group, as compared to the ischemia group. Treadmill pre-training has a significant neuroprotective function against ischemia/reperfusion injury, by suppressing glutamate release resulting from cerebral ischemia, and this effect may be mediated by downregulation of mGluR1.

Neuroprotection against neonatal hypoxia/ischemia-induced cerebral cell death by prevention of calpain-mediated mGluR1alpha truncation

Many cellular events are involved in ischemic neuronal death, and it has been difficult to identify those that play a critical role in the cascade triggered by lack of oxygen and glucose, although it has been widely recognized that overactivation of glutamate receptors represents one of the initiating factors. Different glutamate receptor antagonists, especially those for N-methyl-D-aspartate (NMDA) receptors, have achieved significant success in animal models of hypoxia/ischemia; however, these antagonists have failed in clinical trials. We previously reported that calpain-mediated truncation of metabotropic glutamate receptor 1alpha (mGluR1alpha) played a critical role in excitotoxicity, and that a TAT-mGluR1 peptide consisting of a peptide surrounding the calpain cleavage site of mGluR1alpha and the peptide transduction domain of the transactivating regulatory protein (TAT) of HIV was neuroprotective against excitotoxicity. In the present study we tested the effect of this peptide in in vitro and in vivo models of neonatal hypoxia/ischemia. TAT-mGluR1 peptide prevented oxygen/glucose deprivation- (OGD) and hypoxia/ischemia- (H/I) induced neuronal death in cultured hippocampal slices and neonatal rats, respectively. TAT-mGluR1 blocked H/I-induced mGluR1alpha degradation but had no effect on H/I-induced spectrin degradation, suggesting that neuroprotection was due to prevention of calpain-mediated mGluR1alpha truncation and not to calpain inhibition. Our results therefore suggest that mGluR1alpha truncation plays a critical role in neonatal hypoxia/ischemia and that blockade of this event may prevent the activation of many downstream cytotoxic cascades. Compared to glutamate receptor antagonists and general calpain inhibitors, TAT-mGluR1 may have limited side effects.

Correlation of receptor occupancy of metabotropic glutamate receptor subtype 1 (mGluR1) in mouse brain with in vivo activity of allosteric mGluR1 antagonists

The aim of this study was to clarify the relationship between receptor occupancy and in vivo pharmacological activity of mGluR1 antagonists. The tritiated mGluR1-selective allosteric antagonist [(3)H]FTIDC (4-[1-(2-fluoropyridin-3-yl)-5-methyl-1H-1,2,3-triazol-4-yl]-N-isopropyl-N-methyl-3,6-dihydropyridine-1(2H)-carboxamide) was identified as a radioligand having high affinity for mGluR1-expressing CHO cells (K(D) = 2.1 nM) and mouse cerebellum (K(D) = 3.7 nM). [(3)H]FTIDC bound to mGluR1 was displaced by structurally unrelated allosteric antagonists, suggesting there is a mutual binding pocket shared with different allosteric antagonists. The binding specificity of [(3)H]FTIDC for mGluR1 in brain sections was demonstrated by the lack of significant binding to brain sections prepared from mGluR1-knockout mice. Ex vivo receptor occupancy with [(3)H]FTIDC revealed that the receptor occupancy level by FTIDC correlated well with FTIDC dosage and plasma concentration. Intracerebroventricular administration of (S)-3,5-dihydroxyphenylglycine is known to elicit face washing behavior that is mainly mediated by mGluR1. Inhibition of this behavioral change by FTIDC correlated with the receptor occupancy level of mGluR1 in the brain. A linear relationship between the receptor occupancy and in vivo activity was also demonstrated using structurally diverse mGluR1 antagonists. The receptor occupancy assays could help provide guidelines for selecting appropriate doses of allosteric mGluR1 antagonist for examining the function of mGluR1 in vivo.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

Metabotropic glutamate receptors as targets for multipotential treatment of neurological disorders

Glutamate is a major excitatory neurotransmitter in the CNS that is involved in numerous cellular functions, including cell death and survival. Metabotropic glutamate receptors (mGluR) are G-protein coupled receptors that have been classified into three groups on the basis of signal transduction pathways and pharmacological profiles. Group I, II, and III mGluRs are found on cell types within and peripheral to the CNS, including neurons, microglia, astrocytes, oligodendrocytes, T- and B-cell lymphocytes, osteoblasts, hepatocytes, and endothelial cells, among others. These receptors have a number of effects on cells that can influence outcome after trauma, including reducing neuronal and oligodendroglial cell death, inflammation, and endothelial permeability. Thus, mGluRs are a promising multipotential therapeutic approach. Because the pathology of CNS trauma and neurodegeneration is multifactorial (including, for example, oxidative stress, mitochondrial breakdown, and inflammation), therapies that serve to modulate multiple pathophysiological pathways may prove more effective than those directed at a single target. This review examines the multipotential therapeutic utility of mGluR modulation in acute and chronic injury and neurodegeneration.