Pharmacological effects of metabotropic glutamate receptor ligands on prepulse inhibition in DBA/2J mice

New Therapeutic Targets and Drugs for Schizophrenia Beyond Dopamine D2 Receptor Antagonists

Schizophrenia is a disease with a complex pathological mechanism that is influenced by multiple genes. The study of its pathogenesis is dominated by the dopamine hypothesis, as well as other hypotheses such as the 5-hydroxytryptamine hypothesis, glutamate hypothesis, immune-inflammatory hypothesis, gene expression abnormality hypothesis, and neurodevelopmental abnormality hypothesis. The first generation of antipsychotics was developed based on dopaminergic receptor antagonism, which blocks dopamine D2 receptors in the brain to exert antipsychotic effects. The second generation of antipsychotics acts by dual blockade of 5-hydroxytryptamine and dopamine receptors. From the third generation of antipsychotics onwards, the therapeutic targets for antipsychotic schizophrenia expanded beyond D2 receptor blockade to explore D2 receptor partial agonism and the antipsychotic effects of new targets such as D3, 5-HT1A, 5-HT7, and mGlu2/3 receptors. The main advantages of the second and third generation antipsychotics over first-generation antipsychotics are the reduction of side effects and the improvement of negative symptoms, and even though third-generation antipsychotics do not directly block D2 receptors, the modulation of the dopamine transmitter system is still an important part of their antipsychotic process. According to recent research, several receptors, including 5-hydroxytryptamine, glutamate, γ-aminobutyric acid, acetylcholine receptors and norepinephrine, play a role in the development of schizophrenia. Therefore, the focus of developing new antipsychotic drugs has shifted towards agonism or inhibition of these receptors. Specifically, the development of NMDARs stimulants, GABA receptor agonists, mGlu receptor modulators, cholinergic receptor modulators, 5-HT2C receptor agonists and alpha-2 receptor modulators has become the main direction. Animal experiments have confirmed the antipsychotic effects of these drugs, but their pharmacokinetics and clinical applicability still require further exploration. Research on alternative targets for antipsychotic drugs, beyond the dopamine D2 receptor, has expanded the potential treatment options for schizophrenia and gives an important way to address the challenge of refractory schizophrenia. This article aims to provide a comprehensive overview of the research on therapeutic targets and medications for schizophrenia, offering valuable insights for both treatment and further research in this field.

Group I mGluRs positive allosteric modulators improved schizophrenia-related behavioral and molecular deficits in the Poly I:C rat model

RATIONALE

Maternal polyinosinic-polycytidylic acid (Poly I:C) exposure leads to an increase in various proinflammatory cytokines and causes schizophrenia-like symptoms in offspring. In recent years, group I metabotropic glutamate receptors (mGluRs) have emerged as a potential target in the pathophysiology of schizophrenia.

OBJECTIVES

The aim of our study was to investigate the behavioral and molecular changes by using the mGlu1 receptor positive allosteric modulator (PAM) agent RO 67-7476, and the negative allosteric modulator (NAM) agent JNJ 16259685 and the mGlu5 receptor PAM agent VU-29, and NAM agent fenobam in the Poly I:C-induced schizophrenia model in rats.

METHODS

Female Wistar albino rats were treated with Poly I:C on day 14 of gestation after mating. On the postnatal day (PND) 35, 56 and 84, behavioral tests were performed in the male offspring. On the PND84, brain tissue was collected and the level of proinflammatory cytokines was determined by ELISA method.

RESULTS

Poly I:C caused impairments in all behavioral tests and increased the levels of proinflammatory cytokines. While PAM agents caused significant improvements in prepulse inhibition (PPI), novel object recognition (NOR), spontaneous alternation and reference memory tests, they brought the levels of proinflammatory cytokines closer to the control group. NAM agents were ineffective on behavioral tests. It was observed that PAM agents significantly improved Poly I:C-induced disruption in behavioral and molecular analyses.

CONCLUSIONS

These results suggest that PAM agents, particularly the mGlu5 receptor VU-29, are also promising and could be a potential target in schizophrenia.

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

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

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

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

Targeting Glia with N-Acetylcysteine Modulates Brain Glutamate and Behaviors Relevant to Neurodevelopmental Disorders in C57BL/6J Mice

An imbalance between excitatory (E) glutamate and inhibitory (I) GABA transmission may underlie neurodevelopmental conditions such as autism spectrum disorder (ASD) and schizophrenia. This may be direct, through alterations in synaptic genes, but there is increasing evidence for the importance of indirect modulation of E/I balance through glial mechanisms. Here, we used C57BL/6J mice to test the hypothesis that striatal glutamate levels can be shifted by N-acetylcysteine (NAC), which acts at the cystine-glutamate antiporter of glial cells. Striatal glutamate was quantified in vivo using proton magnetic resonance spectroscopy. The effect of NAC on behaviors relevant to ASD was examined in a separate cohort. NAC induced a time-dependent decrease in striatal glutamate, which recapitulated findings of lower striatal glutamate reported in ASD. NAC-treated animals were significantly less active and more anxious in the open field test; and NAC-treated females had significantly impaired prepulse inhibition of startle response. This at least partly mimics greater anxiety and impaired sensorimotor gating reported in neurodevelopmental disorders. Thus glial mechanisms regulate glutamate acutely and have functional consequences even in adulthood. Glial cells may be a potential drug target for the development of new therapies for neurodevelopmental disorders across the life-span.

Metabotropic glutamate receptors as targets for new antipsychotic drugs: Historical perspective and critical comparative assessment

In this review, we aim to present, discuss and clarify our current understanding regarding the prediction of possible antipsychotic effects of metabotropic glutamate (mGlu) receptor ligands. The number of preclinical trials clearly indicates, that this group of compounds constitutes an excellent alternative to presently used antipsychotic therapy, being effective not only to positive, but also negative and cognitive symptoms of schizophrenia. Although the results of clinical trials that were performed for the group of mGlu2/3 agonists were not so enthusiastic as in animal studies, they still showed that mGlu ligands do not induced variety of side effects typical for presently used antipsychotics, and were generally well tolerated. The lack of satisfactory effectiveness towards schizophrenia symptoms of mGlu2/3 activators in humans could be a result of variety of uncontrolled factors and unidentified biomarkers different for each schizophrenia patient, that should be taken into consideration in the future set of clinical trials. The subject is still open for further research, and the novel classes of mGlu5 or mGlu2/3 agonists/PAMs were recently introduced, including the large group of compounds from the third group of mGlu receptors, especially of mGlu4 subtype. Finally, more precise treatment based on simultaneous administration of minimal doses of the ligands for two or more receptors, seems to be promising in the context of symptoms-specific schizophrenia treatment.

A novel class of succinimide-derived negative allosteric modulators of metabotropic glutamate receptor subtype 1 provides insight into a disconnect in activity between the rat and human receptors

Recent progress in the discovery of mGlu₁ allosteric modulators has suggested the modulation of mGlu₁ could offer possible treatment for a number of central nervous system disorders; however, the available chemotypes are inadequate to fully investigate the therapeutic potential of mGlu₁ modulation. To address this issue, we used a fluorescence-based high-throughput screening assay to screen an allosteric modulator-biased library of compounds to generate structurally diverse mGlu₁ negative allosteric modulator hits for chemical optimization. Herein, we describe the discovery and characterization of a novel mGlu₁ chemotype. This series of succinimide negative allosteric modulators, exemplified by VU0410425, exhibited potent inhibitory activity at rat mGlu₁ but was, surprisingly, inactive at human mGlu₁. VU0410425 and a set of chemically diverse mGlu₁ negative allosteric modulators previously reported in the literature were utilized to examine this species disconnect between rat and human mGlu₁ activity. Mutation of the key transmembrane domain residue 757 and functional screening of VU0410425 and the literature compounds suggests that amino acid 757 plays a role in the activity of these compounds, but the contribution of the residue is scaffold specific, ranging from critical to minor. The operational model of allosterism was used to estimate the binding affinities of each compound to compare to functional data. This novel series of mGlu₁ negative allosteric modulators provides valuable insight into the pharmacology underlying the disconnect between rat and human mGlu₁ activity, an issue that must be understood to progress the therapeutic potential of allosteric modulators of mGlu₁.

Modeling combined schizophrenia-related behavioral and metabolic phenotypes in rodents

Schizophrenia is a chronic, debilitating disorder with a complex behavioral and cognitive phenotype underlined by a similarly complex etiology involving an interaction between susceptibility genes and environmental factors during early development. Limited progress has been made in developing novel pharmacotherapy, partly due to a lack of valid animal models. The recent recognition of the potentially causal role of central and peripheral energy metabolism in the pathophysiology of schizophrenia raises the need of research on animal models that combine both behavioral and metabolic phenotypic domains, similar to what have been identified in humans. In this review we focus on selected genetic (DBA/2J mice, leptin receptor mutants, and PSD-93 knockout mice), early neurodevelopmental (maternal protein deprivation) and pharmacological (acute phencyclidine) animal models that capture the combined behavioral and metabolic abnormalities shown by schizophrenic patients. In reviewing behavioral phenotypes relevant to schizophrenia we apply the principles established by the Research Domain Criteria (RDoC) for better translation. We demonstrate that etiologically diverse manipulations such as specific breeding, deletion of genes that are primarily involved in metabolic regulation and in synaptic plasticity, as well as early metabolic deprivation and adult pharmacological challenge of the glutamate system can lead to schizophrenia-related behavioral and metabolic phenotypes, which suggest that these pathways might be interlinked. We propose that using animal models that combine different domains of schizophrenia can be used as a translationally valid approach to capture the system-level complex interplay between peripheral and central processes in the development of psychopathology.

CRF1 receptor antagonists do not reverse pharmacological disruption of prepulse inhibition in rodents

RATIONALE

As enhanced corticotropin-releasing factor (CRF) transmission is associated with induction of sensorimotor gating deficits, CRF₁ receptor antagonists may reverse disrupted prepulse inhibition (PPI), an operational measure of sensorimotor gating.

OBJECTIVES

To determine the effects of CRF₁ receptor antagonists in pharmacological models of disrupted PPI and to determine if long-term elevated central CRF levels alter sensitivity towards PPI disrupting drugs.

METHODS

CP154,526 (10-40 mg/kg), SSR125543 (3-30 mg/kg) and DMP695 (40 mg/kg) were tested on PPI disruption provoked by D-amphetamine (2.5, 3 mg/kg), ketamine (5, 30 mg/kg) and MK801 (0.2, 0.5 mg/kg) in Wistar rats, C57Bl/6J and CD1 mice, and on spontaneously low PPI in Iffa Credo rats and DBA/2J mice. PPI-disrupting effects of D-amphetamine (2.5-5 mg/kg) and MK801 (0.3-1 mg/kg) were examined in CRF-overexpressing (CRFtg) mice, which display PPI deficits. Finally, we determined the influence of CP154,526 on D-amphetamine-induced dopamine outflow in nucleus accumbens and prefrontal cortex of CRFtg mice using in vivo microdialysis.

RESULTS

No CRF₁-antagonists improved PPI deficits in any test. CRFtg mice showed blunted PPI disruption in response to MK801, but not D-amphetamine. Further, D-amphetamine-induced dopamine release was less pronounced in CRFtg versus wild-type mice, a response normalized by pretreatment with CP154,526.

CONCLUSION

The inability of CRF₁ receptor antagonists to block pharmacological disruption of sensorimotor gating suggests that the involvement of CRF₁ receptors in the modulation of dopaminergic and glutamatergic neurotransmission relevant for sensory gating is limited. Furthermore, the alterations observed in CRFtg mice support the notion that long-term elevated central CRF levels induce changes in these neurotransmitter systems.

Octahydropyrrolo[3,4-c]pyrrole negative allosteric modulators of mGlu1

Development of SAR in an octahydropyrrolo[3,4-c]pyrrole series of negative allosteric modulators of mGlu1 using a functional cell-based assay is described in this Letter. The octahydropyrrolo[3,4-c]pyrrole scaffold was chosen as an isosteric replacement for the piperazine ring found in the initial hit compound. Characterization of selected compounds in protein binding assays was used to identify the most promising analogs, which were then profiled in P450 inhibition assays in order to further assess the potential for drug-likeness within this series of compounds.

N-Acyl-N'-arylpiperazines as negative allosteric modulators of mGlu1: identification of VU0469650, a potent and selective tool compound with CNS exposure in rats

Development of SAR in an N-acyl-N'-arylpiperazine series of negative allosteric modulators of mGlu1 using a functional cell-based assay is described in this Letter. Characterization of selected compounds in protein binding assays was used to aid in selecting VU0469650 for further profiling in ancillary pharmacology assays and pharmacokinetic studies. VU0469650 demonstrated an excellent selectivity profile and good exposure in both plasma and brain samples following intraperitoneal dosing in rats.

Pharmacological modulation of mGluR7 with AMN082 and MMPIP exerts specific influences on alcohol consumption and preference in rats

Growing evidence supports a role for the central nervous system (CNS) neurotransmitter L-glutamate and its metabotropic receptors (mGluRs) in drug addiction in general and alcohol-use disorders in particular. Alcohol dependence, for instance, has a genetic component, and the recent discovery that variations in the gene coding for mGluR7 modulate alcohol consumption further validates involvement of the L-glutamate system. Consequently, increasing interest emerges in developing L-glutamatergic therapies for the treatment of alcohol abuse and dependence. To this end, we performed a detailed behavioral pharmacology study to investigate the regulation of alcohol consumption and preference following administration of the mGluR7-selective drugs N,N'-dibenzyhydryl-ethane-1,2-diamine dihydrochloride (AMN082) and 6-(4-Methoxyphenyl)-5-methyl-3-(4-pyridinyl)-isoxazolo[4,5-c]pyridin-4(5H)-one hydrochloride (MMPIP). Upon administration of the allosteric agonist AMN082 (10 mg/kg, i.p.) in rats, there was a significant decrease in ethanol consumption and preference, without affecting ethanol blood metabolism. In contrast, mGluR7 blockade with MMPIP (10 mg/kg, i.p.) showed an increase in alcohol intake and reversed AMN082's effect on ethanol consumption and preference. Both mGluR7-directed pharmacological tools had no effect on total fluid intake, taste preference, or on spontaneous locomotor activity. In conclusion, these findings support a specific regulatory role for mGluR7 on alcohol drinking and preference and provide evidence for the use of AMN082-type drugs as potential new treatments for alcohol-use disorders in man.

The selective metabotropic glutamate receptor 7 allosteric agonist AMN082 prevents reinstatement of extinguished ethanol-induced conditioned place preference in mice

Alcohol dependence is considered a major public health problem in modern societies. The role for glutamatergic neurotransmission in the reinforcing effects of ethanol is becoming increasingly evident. Our previous findings have shown that in rats, the mGluR7 positive allosteric agonist AMN082, but not its allosteric antagonist MMPIP, prevented ethanol consumption and preference in the two-bottle choice paradigm. This study was conducted to determine the effects of AMN082 and MMPIP on the extinction and reinstatement of ethanol-elicited place preference (CPP) in C57BL/6 mice. AMN082 and MMPIP were administered during extinction of ethanol CPP to determine whether mGluR7 signaling is required. Furthermore, the effects of AMN082 and MMPIP on reinstatement of CPP were also evaluated. Finally, spontaneous locomotor activity and ethanol pharmacokinetics were assessed following systemic administration of AMN082 and MMPIP. Our results indicate that mGluR7 pharmacological modulation had no effect on ethanol-elicited CPP extinction. In contrast, mGluR7 activation using AMN082 reduced ethanol-induced CPP reinstatement, an effect reversed by co-administration of MMPIP. Collectively, these results indicate, for the first time, that activation of the mGluR7 receptor is effective in reducing the reinstatement of conditioned rewarding effects of ethanol. Taken together, the efficacy of AMN082 on the various phases of alcohol-CPP could represent an interesting pharmacological approach and could open a new line of research for the development of therapies to reduce ethanol intake in patients.

Metabotropic glutamate receptors for new treatments in schizophrenia

Metabotropic glutamate receptors (mGluRs) represent exciting targets for the development of novel therapeutic agents for schizophrenia. Recent studies indicate that selective activation of specific mGluR subtypes may provide potential benefits for not only the positive symptoms, but also the negative symptoms and cognitive impairments observed in individuals with schizophrenia. Although optimization of traditional orthosteric agonists may still offer a feasible approach for the activation of mGluRs, important progress has been made in the discovery of novel subtype-selective allosteric ligands, including positive allosteric modulators (PAMs) of mGluR2 and mGluR5. These allosteric mGluR ligands have improved properties for clinical development and have served as key preclinical tools for a more in-depth understanding of the potential roles of these different mGluR subtypes for the treatment of schizophrenia.

Group I metabotropic glutamate receptor antagonists alter select behaviors in a mouse model for fragile X syndrome

RATIONALE

Studies in the Fmr1 knockout (KO) mouse, a model of fragile X syndrome (FXS), suggest that excessive signaling through group I metabotropic glutamate receptors (mGluRs), comprised of subtypes mGluR1 and mGluR5, may play a role in the pathogenesis of FXS. Currently, no studies have assessed the effect of mGluR1 modulation on Fmr1 KO behavior, and there has not been an extensive behavioral analysis of mGluR5 manipulation in Fmr1 KO mice.

OBJECTIVES

The goals for this study were to determine if pharmacologic blockade of mGluR1 may affect Fmr1 KO behavior as well as to expand on the current literature regarding pharmacologic blockade of mGluR5 on Fmr1 KO behavior.

METHODS

Reduction of mGluR1 or mGluR5 activity was evaluated on a variety of behavioral assays in wild-type (WT) and Fmr1 KO mice through the use of antagonists: JNJ16259685 (JNJ, mGluR1 antagonist) and MPEP (mGluR5 antagonist).

RESULTS

JNJ and MPEP decreased marble burying in both WT and Fmr1 KO mice without reductions in activity. Neither JNJ nor MPEP affected the prepulse inhibition in either WT or Fmr1 KO mice. JNJ did not affect Fmr1 KO motor coordination but did impair WT performance. MPEP improved a measure of motor learning in Fmr1 KO but not WT mice. While both JNJ and MPEP decreased the audiogenic seizures in the Fmr1 KO, MPEP completely abolished the manifestation of seizures.

CONCLUSION

These data illustrate that, while the manipulation of either mGluR1 or mGluR5 can affect select behaviors in the Fmr1 KO, we observe greater effects upon mGluR5 reduction.

Allosteric modulation of family C G-protein-coupled receptors: from molecular insights to therapeutic perspectives

Allosteric receptor modulation is an attractive concept in drug targeting because it offers important potential advantages over conventional orthosteric agonism or antagonism. Allosteric ligands modulate receptor function by binding to a site distinct from the recognition site for the endogenous agonist. They often have no effect on their own and therefore act only in conjunction with physiological receptor activation. This article reviews the current status of allosteric modulation at family C G-protein coupled receptors in the light of their specific structural features on the one hand and current concepts in receptor theory on the other hand. Family C G-protein-coupled receptors are characterized by a large extracellular domain containing the orthosteric agonist binding site known as the "venus flytrap module" because of its bilobal structure and the dynamics of its activation mechanism. Mutational analysis and chimeric constructs have revealed that allosteric modulators of the calcium-sensing, metabotropic glutamate and GABA(B) receptors bind to the seven transmembrane domain, through which they modify signal transduction after receptor activation. This is in contrast to taste-enhancing molecules, which bind to different parts of sweet and umami receptors. The complexity of interactions between orthosteric and allosteric ligands is revealed by a number of adequate biochemical and electrophysiological assay systems. Many allosteric family C GPCR modulators show in vivo efficacy in behavioral models for a variety of clinical indications. The positive allosteric calcium sensing receptor modulator cinacalcet is the first drug of this type to enter the market and therefore provides proof of principle in humans.

Metabotropic glutamate receptors: from the workbench to the bedside

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