mGlu5 Positive Allosteric Modulators Research Articles

Metabotropic Glutamate Receptor 4 (mGlu4) Positive Allosteric Modulators Lack Efficacy in Rat and Marmoset Models of L-DOPA-Induced Dyskinesia.

Increased activity across corticostriatal glutamatergic synapses may contribute to L-DOPA-induced dyskinesia in Parkinson's disease. Given the weak efficacy and side-effect profile of amantadine, alternative strategies to reduce glutamate transmission are being investigated. Metabotropic glutamate receptor 4 (mGlu4) is a promising target since its activation would reduce glutamate release. We hypothesized that two mGlu4 positive allosteric modulators, Lu AF21934 ((1 S,2 R)-N1-(3,4-dichlorophenyl)cyclohexane-1,2-dicarboxamide) and ADX88178 (5-Methyl-N-(4-methylpyrimidin-2-yl)-4-(1H-pyrazol-4-yl)thiazol-2-amine), would provide relief in rat and primate models of L-DOPA-induced dyskinesia. The ability of Lu AF21934 or ADX88178 to reverse pre-established dyskinesia was examined in L-DOPA-primed 6-hydroxydopamine-lesioned rats expressing abnormal involuntary movements (AIMs) or in 1-methyl-4-phenyl,1,2,3,6-tetrahydropyridine (MPTP)-treated common marmosets expressing L-DOPA-induced dyskinesia. Additionally, the ability of Lu AF21934 to prevent the development of de novo L-DOPA-induced AIMs was explored in the 6-hydroxydopamine-lesioned rats. Neither Lu AF21934 (10 or 30 mg/kg p.o.) nor ADX88178 (10 or 30 mg/kg p.o.) reduced pre-established AIMs in 6-hydroxydopamine-lesioned rats. Similarly, in L-DOPA-primed common marmosets, no reduction in established dyskinesia was observed with Lu AF21934 (3 or 10 mg/kg p.o.). Conversely, amantadine significantly reduced (>40%) the expression of dyskinesia in both models. Lu AF21934 also failed to suppress the development of AIMs in 6-hydroxydopamine-lesioned rats. This study found no benefit of mGlu4 positive allosteric modulators in tackling L-DOPA-induced dyskinesia. These findings are concordant with the recent failure of foliglurax in phase II clinical trials supporting the predictive validity of these pre-clinical dyskinesia models, while raising further doubt on the anti-dyskinetic potential of mGlu4 positive allosteric modulators.

Effect of the mGlu4 positive allosteric modulator ADX-88178 on parkinsonism, psychosis-like behaviours and dyskinesia in the MPTP-lesioned marmoset.

Positive allosteric modulation of metabotropic glutamate type 4 (mGlu4) receptors is a promising strategy to alleviate parkinsonian disability and L-3,4-dihydroxyphenylalanine (L-DOPA) induced dyskinesia. ADX-88178 is a highly selective mGlu4 positive allosteric modulator (PAM) that previously enhanced the anti-parkinsonian action of L-DOPA in the 6-hydroxydopamine-lesioned rat model of Parkinson's disease (PD). We sought to explore the effects of ADX-88178 on psychosis-like behaviours (PLBs) in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmoset. We also aimed to determine the effect of ADX-88178 on parkinsonism and dyskinesia. Six MPTP-lesioned marmosets were administered L-DOPA chronically to induce stable PLBs and dyskinesias. They were then administered ADX-88178 (0.01, 0.1 and 1mg/kg) or vehicle, in combination with L-DOPA/benserazide (15/3.75mg/kg), both sub-cutaneously, in a randomised fashion. PLBs, parkinsonism and dyskinesia were then measured. ADX-88178 mildly worsened global PLBs at the dose of 1mg/kg (by 13%, P = 0.020). L-DOPA alone conferred 158min of on-time, while the duration of on-time was 212min (34% increase, P = 0.011), after adding ADX-88178 1mg/kg to L-DOPA. Accordingly, ADX-88178 1mg/kg reduced global parkinsonian disability, by 38% (P = 0.0096). ADX-88178 1mg/kg diminished peak dose dyskinesia by 34% (P = 0.015). Minimal effects were provided by lower doses. Whereas these results provide additional evidence of the anti-parkinsonian and anti-dyskinetic effects of mGlu4 positive allosteric modulation as an adjunct to L-DOPA, they also suggest that ADX-88178 may exacerbate dopaminergic psychosis. Further studies are needed to evaluate this possible adverse effect of mGlu4 PAMs on PD psychosis.

Structural insights into dimerization and activation of the mGlu2–mGlu3 and mGlu2–mGlu4 heterodimers

Heterodimerization of the metabotropic glutamate receptors (mGlus) has shown importance in the functional modulation of the receptors and offers potential drug targets for treating central nervous system diseases. However, due to a lack of molecular details of the mGlu heterodimers, understanding of the mechanisms underlying mGlu heterodimerization and activation is limited. Here we report twelve cryo-electron microscopy (cryo-EM) structures of the mGlu2–mGlu3 and mGlu2–mGlu4 heterodimers in different conformational states, including inactive, intermediate inactive, intermediate active and fully active conformations. These structures provide a full picture of conformational rearrangement of mGlu2–mGlu3 upon activation. The Venus flytrap domains undergo a sequential conformational change, while the transmembrane domains exhibit a substantial rearrangement from an inactive, symmetric dimer with diverse dimerization patterns to an active, asymmetric dimer in a conserved dimerization mode. Combined with functional data, these structures reveal that stability of the inactive conformations of the subunits and the subunit–G protein interaction pattern are determinants of asymmetric signal transduction of the heterodimers. Furthermore, a novel binding site for two mGlu4 positive allosteric modulators was observed in the asymmetric dimer interfaces of the mGlu2–mGlu4 heterodimer and mGlu4 homodimer, and may serve as a drug recognition site. These findings greatly extend our knowledge about signal transduction of the mGlus.

Distribution of [11C]-JNJ-42491293 in the marmoset brain: a positron emission tomography study.

JNJ-42491293 is a metabotropic glutamate 2 (mGlu2) positive allosteric modulator (PAM) that was radiolabelled with [11C]- to serve as a positron emission tomography (PET) ligand. Indeed, in vitro, the molecule displays high selectivity at mGlu2 receptors. However, PET experiments performed in rats, macaques and humans, have suggested that [11C]-JNJ-42491293 could interact with an unidentified, non-mGlu2 receptor binding site. The brain distribution of [11C]-JNJ-42491293 has not been determined in the brain of the common marmoset, a small non-human primate increasingly used in neuroscience research. Here, we investigated the distribution of [11C]-JNJ-42491293 in the marmoset brain. Three marmosets underwent brain magnetic resonance imaging (MRI) and 90-min dynamic PET scans with [11C]-JNJ-42491293 in combination with vehicle or the mGlu2 PAM AZD8529 (0.1, 1 and 10mg/kg). In the scans in which [11C]-JNJ-42491293 was co-administered with vehicle, the brain areas with the highest standardised uptake values (SUVs) were the midbrain, cerebellum and thalamus, while the lowest SUVs were found in the pons. The addition of AZD8529 (0.1, 1 and 10mg/kg) to [11C]-JNJ-42491293 did not modify the SUVs obtained with [11C]-JNJ-42491293 alone, and ex vivo blocking autoradiography with PAM AZD8529 (10, 100, 300µM) on marmoset brain sections showed increased signals in the blocking conditions compared to vehicle, suggesting that no competition occurred between the 2 ligands. The results we obtained here do not suggest that [11C]-JNJ-42491293 interacts selectively, or even at all, with mGlu2 receptors in the marmoset, in agreement with findings previously reported in macaque and human.

An allosteric potentiator of metabotropic glutamate (mGlu) 2 receptors reduces the cocaine-stimulated ERK1/2 phosphorylation in the mouse striatum

Metabotropic glutamate (mGlu) receptors are involved in the experience-dependent neuroplasticity in the mesolimbic reward circuit. A Gαi/o-coupled mGlu2 subtype is distributed presynaptically in the striatum. These autoreceptors may have a significant influence over striatal neurons in their intracellular signaling pathways in response to a psychostimulant. Here we explored the effect of pharmacological potentiation of mGlu2 receptors on cocaine-stimulated phosphorylation (activation) of extracellular signal-regulated kinases (ERK) in the mouse striatum in vivo. We found that an mGlu2 selective positive allosteric modulator (PAM) LY487379 after a systemic injection did not alter basal phosphorylation of ERK1/2 or c-Jun N-terminal kinases in the striatum. However, pretreatment with LY487379 blocked the ERK1/2 phosphorylation induced by cocaine in the two subdivisions of the striatum, i.e., the caudate putamen and nucleus accumbens. LY487379 also blocked the cocaine-induced phosphorylation of Elk-1, a transcription factor downstream to the ERK pathway. Additionally, LY487379 reduced locomotor behavioral responses to cocaine. These results demonstrate that the mGlu2 PAM LY487379 possesses the ability to attenuate the activation of the ERK1/2 pathway in striatal neurons and reduce locomotor activity in response to cocaine in vivo.

Anti-parkinsonian effect of the mGlu2 positive allosteric modulator LY-487,379 as monotherapy and adjunct to a low L-DOPA dose in the MPTP-lesioned marmoset

In previous experiments, we have discovered that positive allosteric modulation of metabotropic glutamate 2 (mGlu2) receptors enhances the anti-parkinsonian action of an optimal dose of L-3,4-dihydroxyphenylalanine (L-DOPA). Whether selective mGlu2 positive allosteric modulation would also alleviate parkinsonian disability as monotherapy or as adjunct to a sub-optimal dose of L-DOPA has not been determined. Here, we assessed the anti-parkinsonian effect of mGlu2 positive allosteric modulation as monotherapy and adjunct to a sub-optimal dose of L-DOPA in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmosets. The highly selective positive allosteric modulator (PAM) LY-487,379 was utilised to activate mGlu2 receptors. When administered as monotherapy, LY-487,379 10 mg/kg diminished global parkinsonism by 48% (P < 0.001) and increased duration of on-time by 7-fold, when compared to vehicle treatment (P < 0.05). When added to a sub-optimal dose of L-DOPA, LY-487,379 10 mg/kg decreased global parkinsonism by 44% (P < 0.001) and extended duration of on-time by 2.5-fold (P < 0.01). Our results indicate that selective mGlu2 positive allosteric modulation elicits anti-parkinsonian benefits as monotherapy and as adjunct to sub-optimal dose of L-DOPA paradigms, potentially suggesting that mGlu2 PAMs may have a therapeutic niche early in the treatment of PD as DOPA-sparing agents.

MGlu2 mechanism-based interventions to treat alcohol relapse.

Recently we identified a deficiency in metabotropic glutamate receptor 2 (mGlu2) function in the corticoaccumbal pathway, as a common pathological mechanism underlying alcohol-seeking and relapse behavior. Based on this mechanism, we hypothesized that mGlu2/3 agonists and mGlu2 positive allosteric modulators (PAMs) may be effective in reducing relapse-like behavior. Two mGlu2/3 agonists, LY379268 and LY354740 (a structural analog of LY379268 six-fold more potent in activating mGlu2 over mGluR3), were tested in a well-established rat model of relapse, the alcohol deprivation effect (ADE) with repeated deprivation phases. Since these agonists do not readily discriminate between contributions of mGlu2 and mGluR3, we also tested LY487379, a highly specific PAM that potentiates the effect of glutamate on the mGlu2 with less specificity on other mGlu receptor subtypes. Both LY379268 and LY354740 significantly and dose-dependently reduced the expression of the ADE. No significant changes in water intake, body weight and locomotor activity were observed. Importantly, repeated administration of mGlu2/3 agonist did not lead to tolerance development. mGlu2 PAM LY487379 treatment significantly reduced expression of the ADE in both male and female rats. Combination treatment of mGlu2/3 agonist and PAM had similar effect on relapse-like drinking to that seen in mGlu2/3 agonist treatment alone. Together with other preclinical data showing that PAMs can reduce alcohol-seeking behavior we conclude that mGlu2 PAMs should be considered for clinical trials in alcohol-dependent patients.

Evaluation of the effects of the mGlu2/3 antagonist LY341495 on dyskinesia and psychosis-like behaviours in the MPTP-lesioned marmoset.

We have previously demonstrated that the metabotropic glutamate 2 and 3 (mGlu2/3) antagonist LY341495 reverses the anti-dyskinetic and anti-psychotic benefits conferred by mGlu2 activation and serotonin 2A (5-HT2A) antagonism. Here, we hypothesised that a higher dose of LY341495, associated with a higher antagonistic effect at mGlu3 receptors, would result in a reduction of the reversal of mGlu2 activation and 5-HT2A blockade on dyskinesia, in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmoset. After induction of parkinsonism with MPTP, marmosets entered 3 streams of experiments, in which the following treatments were administered, in combination with l-3,4-dihydroxyphenylalanine (L-DOPA), after which dyskinesia, psychosis-like behaviours (PLBs) and parkinsonism were rated: 1. vehicle/vehicle, LY354740 (mGlu2/3 orthosteric agonist)/vehicle, LY354740/LY341495 1mg/kg and LY354740/LY341495 3mg/kg; 2. vehicle/vehicle, LY487379 (mGlu2 positive allosteric modulator)/vehicle, LY487379/LY341495 1mg/kg and LY487379/LY341495 3mg/kg; 3. vehicle/vehicle, EMD-281,014 (5-HT2A antagonist)/vehicle, EMD-281,014/LY341495 1mg/kg and EMD-281,014/LY341495 3mg/kg. Each of LY354740, LY487379 and EMD-281,014 reduced the severity of L-DOPA-induced dyskinesia, by 55%, 39% and 40%, respectively (all p < 0.001), as well as the severity of PLBs, by 48%, 36% and 41%, respectively (all p < 0.001). Adding LY341495 1 and 3mg/kg to each of LY354740, LY487379 and EMD-281,014 resulted in a dose-dependent reversal of their anti-dyskinetic and anti-psychotic actions. No effect on the anti-parkinsonian action of L-DOPA was noted with any treatment combination. These results suggest that an antagonistic effect at mGlu3 receptors may not be sufficient to overcome the deleterious effect of mGlu2 blockade on dyskinesia in PD. It remains to be seen whether similar effects would have been obtained with a selective mGlu3 antagonist.

MGlu1-mediated restoration of prefrontal cortex inhibitory signaling reverses social and cognitive deficits in an NMDA hypofunction model in mice.

Extensive evidence supports the hypothesis that deficits in inhibitory GABA transmission in the prefrontal cortex (PFC) may drive pathophysiological changes underlying symptoms of schizophrenia that are not currently treated by available medications, including cognitive and social impairments. Recently, the mGlu1 subtype of metabotropic glutamate (mGlu) receptor has been implicated as a novel target to restore GABAergic transmission in the PFC. A recent study reported that activation of mGlu1 increases inhibitory transmission in the PFC through excitation of somatostatin-expressing GABAergic interneurons, implicating mGlu1 PAMs as a potential treatment strategy for schizophrenia. Here, we leveraged positive allosteric modulators (PAMs) of mGlu1 to examine whether mGlu1 activation might reverse physiological effects and behavioral deficits induced by MK-801, an NMDA receptor antagonist commonly used to model cortical deficits observed in schizophrenia patients. Using ex vivo whole-cell patch-clamp electrophysiology, we found that MK-801 decreased the frequency of spontaneous inhibitory postsynaptic currents onto layer V pyramidal cells of the PFC and this cortical disinhibition was reversed by mGlu1 activation. Furthermore, acute MK-801 treatment selectively induced inhibitory deficits onto layer V pyramidal cells that project to the basolateral amygdala, but not to the nucleus accumbens, and these deficits were restored by selective mGlu1 activation. Importantly, the mGlu1 PAM VU6004909 effectively reversed deficits in sociability and social novelty preference in a three-chamber assay and improved novel objection recognition following MK-801 treatment. Together, these findings provide compelling evidence that mGlu1 PAMs could serve as a novel approach to reduce social and cognitive deficits associated with schizophrenia by enhancing inhibitory transmission in the PFC, thus providing an exciting improvement over current antipsychotic medication.

Positive Allosteric Modulation of mGlu1 Reverses Cocaine-Induced Behavioral and Synaptic Plasticity Through the Integrated Stress Response and Oligophrenin-1

BackgroundCue-induced cocaine craving progressively intensifies (incubates) during abstinence from cocaine self-administration. Expression of incubated cocaine craving depends on elevated calcium-permeable AMPA receptors (CP-AMPARs) on medium spiny neurons in the nucleus accumbens (NAc) core. After incubation has occurred, stimulation of NAc metabotropic glutamate 1 (mGlu1) receptors or systemic administration of mGlu1 positive allosteric modulators removes CP-AMPARs from NAc synapses via dynamin-dependent internalization (mGlu1 long-term depression [LTD]) and thereby reduces incubated cocaine craving. Because mGlu1 positive allosteric modulators are potential therapeutics for cocaine craving, it is important to further define the mechanism triggering this mGlu1-LTD. MethodsMale and female rats self-administered saline or cocaine (10 days) using a long access regimen (6 h/day). Following ≥40 days of abstinence, we assessed the ability of an mGlu1 positive allosteric modulator to inhibit expression of incubated craving and remove CP-AMPARs from NAc synapses under control conditions, after blocking the integrated stress response (ISR), or after knocking down oligophrenin-1, a mediator of the ISR that can promote AMPAR endocytosis. AMPAR transmission in NAc medium spiny neurons was assessed with ex vivo slice recordings. ResultsmGlu1 stimulation reduced cue-induced craving and removed synaptic CP-AMPARs. When the ISR was blocked prior to mGlu1 stimulation, there was no reduction in cue-induced craving, nor were CP-AMPARs removed from the synapse. Further, selective knockdown of oligophrenin-1 blocked mGlu1-LTD. ConclusionsOur results indicate that mGlu1-LTD in the NAc and consequently the reduction of cue-induced seeking occur through activation of the ISR, which induces translation of oligophrenin-1. We also demonstrate CP-AMPAR accumulation and mGlu1 reversal in female rats, as previously shown in male rats.

Selective mGlu1 Potentiation Reverses Cortical Disinhibition and Schizophrenia‐like Social and Cognitive Deficits

There is a critical need to develop novel therapeutic strategies for addressing the negative and cognitive symptom domains in schizophrenia (SCZ). Multiple clinical and preclinical studies support the hypothesis that loss of GABAergic inhibitory transmission in the prefrontal cortex (PFC) and other forebrain regions may play a critical role in the pathophysiological changes underlying cognitive and social behavioral deficits in SCZ patients. Recently, we demonstrated that activation of the mGlu1 subtype of metabotropic glutamate receptor increases inhibitory transmission in the PFC by selective excitation of somatostatin‐expressing GABA interneurons. Genetic studies in humans reveal an association between deleterious non‐synonymous single nucleotide polymorphisms (nsSNPS) in of the human gene encoding mGlu1 (GRM1) and SCZ, raising the possibility that mGlu1 signaling is critical to the function of brain circuits underlying symptoms associated with this disorder.Here, we examine whether novel positive allosteric modulators (PAMs) selective for mGlu1 reverse electrophysiological effects and behavioral deficits in an MK‐801‐induced animal model of SCZ. We used ex vivo whole‐cell patch‐clamp electrophysiology to show that MK‐801 decreased the frequency of spontaneous inhibitory postsynaptic currents onto layer V pyramidal cells of the PFC, and this cortical disinhibition was reversed by mGlu1 activation. Furthermore, acute MK‐801 treatment induces inhibitory deficits onto PFC‐basolateral amygdala (BLA), but not PFC‐nucleus accumbens, projecting layer V pyramidal cells that can be reversed by selective mGlu1 activation. PFC‐BLA projections are known to contribute functionally to social‐approach behavior, while the BLA plays a critical role in object recognition and cognition broadly. We found that the mGlu1 PAM VU6004909 effectively reversed MK801‐induced deficits in social interaction and social novelty preference in a three‐chamber assay. Additionally, VU6004909 restored novel object recognition following MK‐801 treatment. In combination with our previous reports that mGlu1 PAMs reverse deficits in inhibitory transmission in the PFC, the current study provides strong evidence for mGlu1 PAMs as a promising novel approach for the treatment of social and cognitive deficits in SCZ patients.

Metabotropic Glutamate Receptors As Emerging Targets for the Treatment of Schizophrenia.

Accumulating evidence of glutamatergic abnormalities in the brains of schizophrenia patients has led to efforts to target various components of glutamatergic signaling as potential new approaches for schizophrenia. Exciting research suggests that metabotropic glutamate (mGlu) receptors could provide a fundamentally new approach for better symptomatic relief in patients with schizophrenia. In preclinical studies, the mGlu5 receptor positive allosteric modulators (PAMs) show efficacy in animal models relevant for all symptom domains in schizophrenia. Interestingly, biased pure mGlu5 receptor PAMs that do not potentiate coupling of mGlu5 receptors to N-methyl-D-aspartate (NMDA) receptors lack neurotoxic effects associated with mGlu5 PAMs that enhance coupling to NMDA receptors or have allosteric agonist activity. This provides a better therapeutic profile for treating schizophrenia-like symptoms. Additionally, the mGlu1 receptor PAMs modulate dopamine release in the striatum, which may contribute to their antipsychotic-like effects. Besides group I mGlu (mGlu1 and mGlu5) receptors, agonists of mGlu2/3 receptors also induce robust antipsychotic-like and procognitive effects in rodents and may be effective in treating symptoms of schizophrenia in a selective group of patients. Additionally, mGlu2/4 receptor heterodimers modulate glutamatergic neurotransmission in the prefrontal cortex at selective synapses activated in schizophrenia and therefore hold potential as novel antipsychotics. Excitingly, the mGlu3 receptor activation can enhance cognition in rodents, suggesting that mGlu3 receptor agonist/PAM could provide a novel approach for the treatment of cognitive deficits in schizophrenia. Collectively, the development of mGlu receptor-specific ligands may provide an alternative approach to meet the clinical need for safer and more efficacious therapeutics for schizophrenia. SIGNIFICANCE STATEMENT: The currently available antipsychotic medications do not show significant efficacy for treating negative symptoms and cognitive deficits in schizophrenia. Emerging preclinical and clinical literature suggests that pharmacological targeting of metabotropic glutamate receptors could potentially provide an alternative approach for designing safer and more efficacious therapeutics for treating schizophrenia.

Adolescent Alcohol Exposure Results in Sex-specific Alterations in Conditioned Fear Learning and Memory in Adulthood.

The present study used auditory fear conditioning to assess the impact of repeated binge-like episodes of alcohol exposure during adolescence on conditioned fear in adulthood. Male and female Long-Evans rats were subjected to adolescent intermittent ethanol (AIE) exposure by vapor inhalation between post-natal day 28 and 44. After aging into adulthood, rats then underwent fear conditioning by exposure to a series of tone-shock pairings. This was followed by cued-tone extinction training, and then testing of fear recovery. In male rats, AIE exposure enhanced conditioned freezing but did not alter the time-course of extinction of cued-tone freezing. During subsequent assessment of fear recovery, AIE exposed rats exhibited less freezing during contextual fear renewal, but greater freezing during extinction recall and spontaneous recovery. Compared to males, female rats exhibited significantly lower levels of freezing during fear conditioning, more rapid extinction of freezing behavior, and significantly lower levels of freezing during the tests of fear recovery. Unlike males that were all classified as high conditioners; female rats could be parsed into either a high or low conditioning group. However, irrespective of their level of conditioned freezing, both the high and low conditioning groups of female rats exhibited rapid extinction of conditioned freezing behavior and comparatively low levels of freezing in tests of fear recovery. Regardless of group classification, AIE had no effect on freezing behavior in female rats during acquisition, extinction, or fear recovery. Lastly, exposure of male rats to the mGlu5 positive allosteric modulator CDPPB prevented AIE-induced alterations in freezing. Taken together, these observations demonstrate sex-specific changes in conditioned fear behaviors that are reversible by pharmacological interventions that target mGlu5 receptor activation.

MGlu1 potentiation enhances prelimbic somatostatin interneuron activity to rescue schizophrenia-like physiological and cognitive deficits.

SUMMARYEvidence for prefrontal cortical (PFC) GABAergic dysfunction is one of the most consistent findings in schizophrenia and may contribute to cognitive deficits. Recent studies suggest that the mGlu1 subtype of metabotropic glutamate receptor regulates cortical inhibition; however, understanding the mechanisms through which mGlu1 positive allosteric modulators (PAMs) regulate PFC microcircuit function and cognition is essential for advancing these potential therapeutics toward the clinic. We report a series of electrophysiology, optogenetic, pharmacological magnetic resonance imaging, and animal behavior studies demonstrating that activation of mGlu1 receptors increases inhibitory transmission in the prelimbic PFC by selective excitation of somatostatin-expressing interneurons (SST-INs). An mGlu1 PAM reverses cortical hyperactivity and concomitant cognitive deficits induced by N-methyl-d-aspartate (NMDA) receptor antagonists. Using in vivo optogenetics, we show that prelimbic SST-INs are necessary for mGlu1 PAM efficacy. Collectively, these findings suggest that mGlu1 PAMs could reverse cortical GABAergic deficits and exhibit efficacy in treating cognitive dysfunction in schizophrenia.

Modulation of mGlu5 improves sensorimotor gating deficits in rats neonatally treated with quinpirole through changes in dopamine D2 signaling

This study analyzed whether the positive allosteric modulator of metabotropic glutamate receptor type 5 (mGlu5) 3-Cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl) benzamide (CDPPB) would alleviate deficits in prepulse inhibition (PPI) and affect dopamine (DA) D2 signaling in the dorsal striatum and prefrontal cortex (PFC) in the neonatal quinpirole (NQ) model of schizophrenia (SZ). Male and female Sprague-Dawley rats were neonatally treated with either saline (NS) or quinpirole HCL (1 mg/kg; NQ), a DAD2 receptor agonist, from postnatal days (P) 1–21. Rats were raised to P44 and behaviorally tested on PPI from P44-P48. Before each trial, rats were subcutaneous (sc) administered saline or CDPPB (10 mg/kg or 30 mg/kg). On P50, rats were given a spontaneous locomotor activity test after CDPPB or saline administration. On P51, the dorsal striatum and PFC were evaluated for both arrestin-2 (βA-2) and phospho-AKT protein levels. NQ-treated rats demonstrated a significant deficit in PPI, which was alleviated to control levels by the 30 mg/kg dose of CDPPB. There were no significant effects of CDPPB on locomotor activity. NQ treatment increased βA-2 and decreased phospho-AKT in both the dorsal striatum and PFC, consistent with an increase DAD2 signaling. The 30 mg/kg dose of CDPPB significantly reversed changes in βA-2 in the dorsal striatum and PFC and phospho-AKT in the PFC equivalent to controls. Both doses of CDPPB produced a decrease of phospho-AKT in the PFC compared to controls. This study revealed that a mGlu5 positive allosteric modulator was effective to alleviate PPI deficits and striatal DAD2 signaling in the NQ model of SZ.

Group II metabotropic glutamate receptor (mGlu2 and mGlu3) roles in thalamic processing

As the thalamus underpins almost all aspects of behaviour, it is important to understand how the thalamus operates. Group II metabotropic glutamate (mGlu2 /mGlu3 ) receptor activation reduces inhibition in thalamic nuclei originating from the surrounding thalamic reticular nucleus (TRN). Whilst an mGlu2 component to this effect has been reported, in this study, we demonstrate that it is likely, largely mediated via mGlu3 . The somatosensory ventrobasal thalamus (VB) is an established model for probing fundamental principles of thalamic function. In vitro slices conserving VB-TRN circuitry from wild-type and mGlu3 knockout mouse brains were used to record IPSPs and mIPSCs. In vivo extracellular recordings were made from VB neurons in anaesthetised rats. A range of selective pharmacological agents were used to probe Group II mGlu receptor function (agonist, LY354740; antagonist, LY341495; mGlu2 positive allosteric modulator, LY487379 and mixed mGlu2 agonist/mGlu3 antagonist LY395756). The in vitro and in vivo data are complementary and suggest that mGlu3 receptor activation is largely responsible for potentiating responses to somatosensory stimulation by reducing inhibition from the TRN. mGlu3 receptor activation in the VB likely enables important somatosensory information to be discerned from background activity. These mGlu3 receptors are likely to be endogenously activated via 'glutamate spillover'. In cognitive thalamic nuclei, this mechanism may be of importance in governing attentional processes. Positive allosteric modulation of endogenous mGlu3 receptor activation may therefore enhance cognitive function in pathophysiological disease states, such as schizophrenia, thus representing a highly specific therapeutic target. This article is part of a themed issue on Building Bridges in Neuropharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.8/issuetoc.

Neurochemical changes underlying cognitive impairment in olfactory bulbectomized rats and the impact of the mGlu5-positive allosteric modulator CDPPB

The olfactory bulbectomized (OBX) rat model is a well-established model of depression in which antidepressant drugs reverse deficits in the passive avoidance test 14 days after administration. Recently, the olfactory bulbectomized rat model has been proposed to be a model of Alzheimer’s disease (AD), and the available data indicate similarities between the changes that typically occur in AD and those observed in OBX animals. In the present study, the occurrence of neurochemical impairments related to AD were investigated 8 months after OB ablation. The expression of the nitric oxide synthases eNOS and nNOS, receptor for advanced glycation endproducts (RAGEs) and dimethylarginine dimethylaminohydrolase (DDAH1) in the prefrontal cortices (PFCs), hippocampi and striata of olfactory bulbectomized and sham-operated rats was evaluated. Subsequently, the impact of the administration of a positive allosteric modulator of the mGlu5 receptor, CDPPB (14 days, 2.5 or 5 mg/kg), on OBX-related changes was assessed. OB ablation induced typical deficits in passive avoidance. Significant aberrations in the expression of both isoforms of NOS were observed in the hippocampus and striatum, and the expression of DDAH1 was increased in the PFCs of OBX animals. CDPPB at a dose of 5 mg/kg ameliorated cognitive impairment in the passive avoidance test and partially reversed the changes in eNOS and nNOS expression induced by the lesion. The results of this study confirm that some of the neurochemical changes observed in OBX animals may resemble those associated with AD pathology and that activation of the mGlu5 receptor may partially counteract these pathological alterations.

Selective mGlu 1 potentiation enhances cortical inhibition to rescue schizophrenia‐like cognitive and social deficits

Accumulating evidence suggests that selective activators of the mGlu1 subtype of metabotropic glutamate (mGlu) receptor may provide a promising novel therapeutic approach for the treatment of schizophrenia. Genetic studies in humans reveal an association of the human gene encoding mGlu1 (GRM1) and, specifically, loss of function single nucleotide polymorphisms in GRM1 with schizophrenia, raising the possibility that mGlu1 signaling is critical to the function of brain circuits underlying symptoms associated with this disorder. Multiple clinical and preclinical studies support the hypothesis that loss of GABAergic inhibitory transmission in the prefrontal cortex (PFC) and other forebrain regions may play a critical role in the pathophysiological changes underlying behavioral deficits in schizophrenia patients, including cognitive and social impairment. This is especially interesting in light of our recent studies demonstrating that activation of mGlu1 increases inhibitory transmission in the PFC by selective excitation of somatostatin-expressing GABA interneurons. Here, we leverage recently developed positive allosteric modulators (PAMs) selective for mGlu1 to examine whether mGlu1 activation might reverse electrophysiological effects and behavioral deficits induced by MK-801, an NMDA receptor antagonist commonly used to model cortical pathophysiology relevant to behavioral and cognitive deficits observed in schizophrenia patients. Using ex vivo whole-cell patch-clamp electrophysiology, we found that MK-801 decreased the frequency of spontaneous inhibitory postsynaptic currents onto layer V pyramidal cells of the PFC and this cortical disinhibition was reversed by mGlu1 activation, suggesting that mGlu1 activation is a pertinent mechanism to restore inhibitory signaling in the PFC. Importantly, the mGlu1 PAM VU6004909 effectively reversed MK-801-induced deficits in working memory as measured by spontaneous alternation in a Y-maze task and restored social interaction in a three-chamber assay. In combination with our previous reports that mGlu1 PAMs possess potential antipsychotic-like efficacy and reverse deficits in inhibitory transmission in the PFC, the current study provides strong evidence that mGlu1 PAMs could serve as a novel approach that could reduce positive symptoms and also treat cognitive and social deficits in schizophrenia patients, providing an exciting improvement over current antipsychotics.

Probing The Structural Basis of Metabotropic Glutamate Receptor 5 (mGlu 5 ) Activation and Positive Allosteric Modulation

The metabotropic glutamate receptor 5 (mGlu5) is a class C G protein-coupled receptor (GPCR) that has emerged as a promising drug target for multiple central nervous system disorders. Structurally, mGlu5 forms obligate dimers. Each protomer possesses a 7-transmembrane (7TM) domain which is connected through a cysteine-rich domain (CRD) to a large extracellular domain (ECD), encompassing the orthosteric ligand-binding Venus flytrap domain (VFD). Recent spectroscopic studies on mGlu5 signaling demonstrated large-scale conformational changes underlie signal transmission from the ECD to the G protein coupling 7TM domain. Although these studies reported the necessity of extracellular loops (ECL) for activation of mGlu5 by orthosteric agonists, specific residues related to transmission of receptor activation remain unknown. We used a combination of site-directed mutagenesis, Ca2+ mobilization assays and radioligand binding studies to identify residues within the ECLs that aid in the propagation of orthosteric agonist-induced conformational changes in the VFD to the 7TM. Multiple ECL2 residues markedly reduced activation by glutamate but not allosteric agonists. These same mutations had no effect on receptor expression and thus likely contribute to the mGlu­5 activation network. Select mutations that compromised glutamate activation could be rescued with positive allosteric modulators (PAMs). mGlu5 PAMs are promising potential new therapeutic strategies for schizophrenia and cognition, however, structure-activity relationships for mGlu5 PAMs are notoriously difficult to interpret. A deeper understanding of the structural basis of dynamic receptor activation and modulation processes will aid future rational structure-based drug discovery efforts

Combined 5-HT2A and mGlu2 modulation for the treatment of dyskinesia and psychosis in Parkinson's disease

Antagonising the serotonin 2A (5-HT2A) receptor is an efficacious way to alleviate dyskinesia and psychosis in Parkinson's disease (PD). However, previous research indicates that there might be a limit to the effects conferred by this approach. 5-HT2A receptors were shown to form hetero-dimers with metabotropic glutamate 2 (mGlu2) receptors, in which 5-HT2A blockade and mGlu2 activation elicit equivalent effects at the downstream signalling level. We have previously shown that mGlu2 activation reduces both dyskinesia and psychosis-like behaviours (PLBs) induced by L-3,4-dihydroxyphenylalanine (l-DOPA), in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned primate. Here, we hypothesised that concurrent 5-HT2A antagonism and mGlu2 activation would provide greater anti-dyskinetic and anti-psychotic benefits than either approach alone. We conducted 3 series of experiments in the MPTP-lesioned marmoset. In the first series of experiments, the mGlu2 positive allosteric modulator LY-487,379 and the 5-HT2A antagonist EMD-281,014, either alone or in combination, were added to l-DOPA. In the second series of experiments, the mGlu2/3 orthosteric agonist LY-354,740 and EMD-281,014, either alone or in combination, were added to l-DOPA. In the last series of experiments, we investigated whether mGlu2 blockade would diminish the effects of antagonising 5-HT2A receptors. To this end, the mGlu2/3 orthosteric antagonist LY-341,495 and EMD-281,014, either alone or in combination, were added to l-DOPA. We found that the anti-dyskinetic effect of the combination LY-487,379/EMD-281,014 was greater than the ones conferred by LY-487,379 (by 35%, P < 0.05) and EMD-281,014 (by 38%, P < 0.01). The anti-dyskinetic and anti-psychotic effects of the combination LY-354,740/EMD-281,014 were also greater than the ones conferred by LY-354,740 (by 57% for dyskinesia and 54% for PLBs, both P < 0.001) and EMD-281,014 (by 61% for dyskinesia and 53% for PLBs, both P < 0.001). The anti-parkinsonian action of l-DOPA was maintained with all treatments. Lastly, the addition of LY-341,495 abolished the therapeutic effects of EMD-281,014 on dyskinesia and PLBs. Our results suggest that mGlu2 activation may enhance the anti-dyskinetic and anti-psychotic effects of 5-HT2A blockade and could provide relief to PD patients with dyskinesia and psychotic symptoms beyond what can be achieved with current therapies.