Schizophrenia: moving beyond monoamine antagonists

Systematic literature review and network meta-analysis of lurasidone, brexpiprazole and cariprazine for schizophrenia

A systematic review was undertaken to identify randomized controlled trials (RCTs) comparing the efficacy and safety of lurasidone, brexpiprazole and cariprazine (selected because of a shared safety profile) with each other or placebo in adult patients with schizophrenia. Key outcomes included: Positive and Negative Syndrome Scales (PANSS), Clinical Global Impression-Severity (CGI-S) scores and cardiovascular and metabolic parameters. A feasibility assessment evaluated the trials' suitability for inclusion in a Bayesian network meta-analysis (NMA). Random effects models were used. In total, 1138 records were identified and 19 RCTs contributed to the NMA. Lurasidone doses of 160 mg performed best in terms of change in PANSS and CGI-S scores at 6 weeks, with stronger evidence when compared with brexpiprazole than cariprazine. The safety outcomes were variable; for all treatments, the 95% credible intervals usually contained 'no difference'. Active treatments were associated with lower odds of discontinuation due to any cause, and higher odds of experiencing any adverse event. Lurasidone was comparable to brexpiprazole and cariprazine for efficacy and safety outcomes assessed at 6 weeks, with the 160 mg dose being superior for the change in PANSS and CGI-S outcomes. The lurasidone results were relatively consistent across doses compared with brexpiprazole and cariprazine.

An Overview of Glycine Transporter Subtype 1 Inhibitors Under Preclinical and Clinical Evaluation for the Treatment of Alcohol Abuse

Abstract:

Being a historical issue that withstands multiple societal control measures, alcohol abuse remains a major healthcare problem. Despite worldwide efforts to limit consumption and educate people about its effects, consumption rates remain unchanged. Alcohol abuse arises from chronic alcohol exposure-caused permanent synaptic plasticity changes in the brain. These manifest in life-threatening withdrawal symptoms and drive relapse even after detoxification and treatment. Since ethanol has multiple targets in the human brain, it warrants a multiapproach therapy; here we introduce the potential therapeutic effects of glycine transporter subtype 1 inhibitors. We have listed the various glycine transporter 1 inhibitors used in studies of alcoholism and how they influenced glycine release from rat hippocampus was demonstrated in a preliminary study. Glycine transporters modulate both glutamatergic and glycinergic pathways: (i) glutamatergic neurotransmission plays an important role in the development of chronic changes in alcoholism as daily alcohol administration was shown to increase N-methyl-D-aspartic acid receptor activity long-term, and (ii) ethanol has access to the dopaminergic reward system via glycine receptors, being an allosteric modulator of glycine receptors. This manuscript summarises the progress and development of glycine transporter 1 inhibitors, characterizing them by their mode of action, adverse effects, and discusses their clinical applicability. Furthermore, we highlight the progress in the latest clinical trials, outline currently applied treatment methods, and offer suggestions for implementing glycine transporter 1 inhibitors into the long-term treatment of alcohol abuse.

Main characteristics of dermatoglypics associated with schizophrenia and its clinical subtypes

Dermatoglypic patterns are extensively investigated to apply in disease-related risk assessment due to an obvious association between morphological and genetic characteristics. In the current study, we aimed to determine whether the fingerprint and palmar patterns vary between case population with schizophrenia and general population. A cross sectional study was conducted in people diagnosed with schizophrenia (cases) and a control population between 2016 and 2019. In this study, 252 people were participated. Ink and paper method was used to evaluate the difference of fingerprints palm prints between patients with schizophrenia and participants in control group.93 participants were analyzed in schizophrenic group and 142 participants were investigated in the control group. The percentage of arches on the right ring finger was significantly different between the schizophrenic patient group and control group (p = 0.011). The whorl pattern type (U-W-U-W-W-W-W-U-W-U) was dominantly observed in both of the schizophrenic patient group and control group. A-B ridge count in schizophrenic patient group and control group produced a markedly significant difference (p<0.05). Interestingly, a strong significant difference was produced in comparing of A-B ridge count in catatonic schizophrenia group with residual schizophrenia group (p<0.005). In comparison, index of pattern intensity in control group was slightly higher than that in schizophrenic patient group. Taking together, these results showed that the dermatoglypic characteristics might be a valuable tool to describe the nature of schizophrenia and its clinical subtypes and further studies are needed in clinical application.

Functional connectivity of cerebellar dentate nucleus and cognitive impairments in patients with drug-naive and first-episode schizophrenia

Cognitive impairments are the hallmark of schizophrenia and prominent in the early episode stage. However, the underlying pathological mechanisms of cognitive impairments are not fully understood. This study aimed to investigate the abnormal functional connectivity (FC) of the cerebellar dentate nucleus (DN) and its correlation with cognitive impairments in patients with drug-naive and first-episode schizophrenia. Resting-state functional magnetic resonance imaging data were acquired in 47 patients and 43 healthy controls. Cognitive functions were assessed by number sequence span, verbal category fluency, digit-symbol coding tests. The results showed that the patients had deficits in all three cognitive tests compared to the controls. Furthermore, the increased FC of DN with the bilateral postcentral gyrus and decreased FC of DN with the right inferior temporal gyrus and regional cerebellum (e.g., Vermis 4-5 and Crus I) were observed in the patient group compared to the control group. Importantly, these abnormal DN FC significantly correlated with cognitive tests (e.g., number sequence span and digit-symbol coding) and clinical symptoms (e.g., negative symptom) in the patient group. The results suggested that abnormal FC of DN with cortical and subcortical regions was associated with cognitive impairments and symptom severity and might be an underlying neural mechanism in schizophrenia.

Attenuation Effects of Alpha-Pinene Inhalation on Mice with Dizocilpine-Induced Psychiatric-Like Behaviour

α-Pinene, an organic terpene compound found in coniferous trees, is used as a safe food additive and is contained in many essential oils. Moreover, some studies have shown that α-pinene suppresses neuronal activity. In this study, we investigated whether inhalation of α-pinene suppresses dizocilpine (MK-801-) induced schizophrenia-like behavioural abnormalities in mice. Mice inhaled α-pinene 1 h before the first MK-801 injection. Thirty minutes after MK-801 injection, the open field, spontaneous locomotor activity, elevated plus maze, Y-maze, tail suspension, hot plate, and grip strength tests were conducted as behavioural experiments. Inhalation of α-pinene suppressed the activity of mice in the spontaneous locomotor activity test and although it did not suppress the MK-801-induced increased locomotor activity in the open field test, it remarkably decreased the time that the mice remained in the central area. Inhalation of α-pinene suppressed the MK-801-induced increased total distance travelled in the Y-maze test, whereas it did not alter the MK-801-induced reduced threshold of antinociception in the hot plate test. In the tail suspension and grip strength tests, there was no effect on mouse behaviour by administration of MK-801 and inhalation of α-pinene. These results suggest that α-pinene acts to reduce MK-801-induced behavioural abnormalities resembling those seen in neuropsychiatric disorders. Therefore, both medicinal plants and essential oils containing α-pinene may have potential for therapeutic treatment of schizophrenia.

Effects of Amisulpride Adjunctive Therapy on Working Memory and Brain Metabolism in the Frontal Cortex of Patients with Schizophrenia: A Preliminary Positron Emission Tomography/Computerized Tomography Investigation

Objective

Dopamine plays a significant role in working memory by acting as a key neuromodulator between brain networks. Additionally, treatment of patients with schizophrenia using amisulpride, a pure dopamine class 2/3 receptor antagonist, improves their clinical symptoms with fewer side effects. We hypothesized that patients with schizophrenia treated with amisulpride and aripiprazole show increased working memory and glucose metabolism compared with those treated with cognitive behavioral therapy (CBT) and aripiprazole instead.

Methods

Sixteen patients with schizophrenia (eight in the amisulpride group [aripiprazole+amisulpride] and eight in the CBT group [aripiprazole+CBT]) and 15 age- and sex-matched healthy control subjects were recruited for a 12-week-long prospective trial. An [18F]-fluorodeoxyglucose-positron emission tomography/computerized tomography scanner was used to acquire the images.

Results

After 12 weeks of treatment, the amisulpride group showed greater improvement in the Letter-Number Span scores than the CBT group. Additionally, although brain metabolism in the left middle frontal gyrus, left occipital lingual gyrus, and right inferior parietal lobe was increased in all patients with schizophrenia, the amisulpride group exhibited a greater increase in metabolism in both the right superior frontal gyrus and right frontal precentral gyrus than the CBT group.

Conclusion

This study suggests that a small dose of amisulpride improves the general psychopathology, working memory performance, and brain glucose metabolism of patients with schizophrenia treated with aripiprazole.

Global and Specific Profiles of Executive Functioning in Prodromal and Early Psychosis

Objective: Numerous reports on neurocognitive functioning deficits in individuals at clinical high risk (CHR) and first-episode psychosis (FEP) patients suggest particular deficits in executive functioning (EF). However, to date, most of the studies have administered a single or a few EF tests to participants, and few investigations have examined the different components of EF to identify specific subdomains of relative strength and weakness. Method: Forty CHR subjects, 85 FEP patients, and 85 healthy controls (HCs) were assessed with a neuropsychological battery to elucidate the profiles of EF in the subdomains of shift, attention, fluency, and planning. Results: In the subdomains of shift, attention, and fluency, CHR individuals and FEP patients showed deficits compared to HC. The post hoc analysis revealed that CHR individuals had comparable attention shifting and phonemic fluency compared to FEP. CHR showed intermediate deficits between FEP and HCs in spatial working memory and semantic fluency, and the largest effect size was observed in semantic fluency both for CHR and FEP. Conclusion: Overall, the findings of this study, in addition to providing detailed profiles of EF in prodromal and early psychosis patients, highlight the informative value of the specific subdomains of semantic fluency and spatial working memory.

Epigenetic Alterations in Prenatal Stress Mice as an Endophenotype Model for Schizophrenia: Role of Metabotropic Glutamate 2/3 Receptors

Mice subjected to prenatal restraint stress (PRS mice) showed biochemical and behavioral abnormalities consistent with a schizophrenia-like phenotype (Matrisciano et al., 2016). PRS mice are characterized by increased DNA-methyltransferase 1 (DNMT1) and ten-eleven methylcytosine dioxygenase 1 (TET1) expression levels and exhibit an enrichment of 5-methylcytosine (5MC) and 5-hydroxymethylcytosine (5HMC) at neocortical GABAergic and glutamatergic gene promoters. Activation of group II metabotropic glutamate receptors (mGlu2 and−3 receptors) showed a potential epigenetically-induced antipsychotic activity by reversing the molecular and behavioral changes observed in PRS mice. This effect was most likely caused by the increase in the expression of growth arrest and DNA damage 45-β (Gadd45-β) protein, a molecular player of DNA demethylation, induced by the activation of mGlu2/3 receptors. This effect was mimicked by clozapine and valproate but not by haloperidol. Treatment with the selective mGlu2/3 receptors agonist LY379268 also increased the amount of Gadd45-β bound to specific promoter regions of reelin, BDNF, and GAD67. A meta-analysis of several clinical trials showed that treatment with an orthosteric mGlu2/3 receptor agonist improved both positive and negative symptoms of schizophrenia, but only in patients who were early-in-disease and had not been treated with atypical antipsychotic drugs (Kinon et al., 2015). Our findings show that PRS mice are valuable model for the study of epigenetic mechanisms involved in the pathogenesis of schizophrenia and support the hypothesis that pharmacological modulation of mGlu2/3 receptors could impact the early phase of schizophrenia and related neurodevelopmental disorders by regulating epigenetic processes that lie at the core of the disorders.

Whole-Brain Multimodal Neuroimaging Model Using Serotonin Receptor Maps Explains Non-linear Functional Effects of LSD

Understanding the underlying mechanisms of the human brain in health and disease will require models with necessary and sufficient details to explain how function emerges from the underlying anatomy and is shaped by neuromodulation. Here, we provide such a detailed causal explanation using a whole-brain model integrating multimodal imaging in healthy human participants undergoing manipulation of the serotonin system. Specifically, we combined anatomical data from diffusion magnetic resonance imaging (dMRI) and functional magnetic resonance imaging (fMRI) with neurotransmitter data obtained with positron emission tomography (PET) of the detailed serotonin 2A receptor (5-HT_2AR) density map. This allowed us to model the resting state (with and without concurrent music listening) and mechanistically explain the functional effects of 5-HT_2AR stimulation with lysergic acid diethylamide (LSD) on healthy participants. The whole-brain model used a dynamical mean-field quantitative description of populations of excitatory and inhibitory neurons as well as the associated synaptic dynamics, where the neuronal gain function of the model is modulated by the 5-HT_2AR density. The model identified the causative mechanisms for the non-linear interactions between the neuronal and neurotransmitter system, which are uniquely linked to (1) the underlying anatomical connectivity, (2) the modulation by the specific brainwide distribution of neurotransmitter receptor density, and (3) the non-linear interactions between the two. Taking neuromodulatory activity into account when modeling global brain dynamics will lead to novel insights into human brain function in health and disease and opens exciting possibilities for drug discovery and design in neuropsychiatric disorders.

The discovery of VU0486846: steep SAR from a series of M1 PAMs based on a novel benzomorpholine core

This letter describes the chemical optimization of a new series of M1 positive allosteric modulators (PAMs) based on a novel benzomorpholine core, developed via iterative parallel synthesis, and culminating in the highly utilized rodent in vivo tool compound, VU0486846 (7), devoid of adverse effect liability. This is the first report of the optimization campaign (SAR and DMPK profiling) that led to the discovery of VU0486846 and details all of the challenges faced in allosteric modulator programs (both steep and flat SAR, as well as subtle structural changes affecting CNS penetration and overall physiochemical and DMPK properties).

Thalamic miR-338-3p mediates auditory thalamocortical disruption and its late onset in models of 22q11.2 microdeletion

Although 22q11.2 deletion syndrome (22q11DS) is associated with early-life behavioral abnormalities, affected individuals are also at high risk for the development of schizophrenia symptoms, including psychosis, later in life. Auditory thalamocortical (TC) projections recently emerged as a neural circuit that is specifically disrupted in mouse models of 22q11DS (hereafter referred to as 22q11DS mice), in which haploinsufficiency of the microRNA (miRNA)-processing-factor-encoding gene Dgcr8 results in the elevation of the dopamine receptor Drd2 in the auditory thalamus, an abnormal sensitivity of thalamocortical projections to antipsychotics, and an abnormal acoustic-startle response. Here we show that these auditory TC phenotypes have a delayed onset in 22q11DS mice and are associated with an age-dependent reduction of miR-338-3p, a miRNA that targets Drd2 and is enriched in the thalamus of both humans and mice. Replenishing depleted miR-338-3p in mature 22q11DS mice rescued the TC abnormalities, and deletion of Mir338 (which encodes miR-338-3p) or reduction of miR-338-3p expression mimicked the TC and behavioral deficits and eliminated the age dependence of these deficits. Therefore, miR-338-3p depletion is necessary and sufficient to disrupt auditory TC signaling in 22q11DS mice, and it may mediate the pathogenic mechanism of 22q11DS-related psychosis and control its late onset.

Positive Allosteric Modulation of the Muscarinic M1 Receptor Improves Efficacy of Antipsychotics in Mouse Glutamatergic Deficit Models of Behavior

Current antipsychotics are effective in treating the positive symptoms associated with schizophrenia, but they remain suboptimal in targeting cognitive dysfunction. Recent studies have suggested that positive allosteric modulation of the M₁ muscarinic acetylcholine receptor (mAChR) may provide a novel means of improving cognition. However, very little is known about the potential of combination therapies in extending coverage across schizophrenic symptom domains. This study investigated the effect of the M₁ mAChR positive allosteric modulator BQCA [1-(4-methoxybenzyl)-4-oxo-1,4-dihydroquinoline-3-carboxylic acid], alone or in combination with haloperidol (a first-generation antipsychotic), clozapine (a second-generation atypical antipsychotic), or aripiprazole (a third-generation atypical antipsychotic), in reversing deficits in sensorimotor gating and spatial memory induced by the N-methyl-d-aspartate receptor antagonist, MK-801 [(5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine]. Sensorimotor gating and spatial memory induction are two models that represent aspects of schizophrenia modeled in rodents. In prepulse inhibition (an operational measure of sensorimotor gating), BQCA alone had minimal effects but exhibited different levels of efficacy in reversing MK-801-induced prepulse inhibition disruptions when combined with a subeffective dose of each of the three (currently prescribed) antipsychotics. Furthermore, the combined effect of BQCA and clozapine was absent in M₁^-/- mice. Interestingly, although BQCA alone had no effect in reversing MK-801-induced memory impairments in a Y-maze spatial test, we observed a reversal upon the combination of BQCA with atypical antipsychotics, but not with haloperidol. These findings provide proof of concept that a judicious combination of existing antipsychotics with a selective M₁ mAChR positive allosteric modulator can extend antipsychotic efficacy in glutamatergic deficit models of behavior.

New Pharmacotherapy Targeting Cognitive Dysfunction of Schizophrenia via Modulation of GABA Neuronal Function

Schizophrenia is considered a neurodevelopmental and neurodegenerative disorder. Cognitive impairment is a core symptom in patients with the illness, and has been suggested a major predictor of functional outcomes. Reduction of parvalbumin (PV)-positive γ-aminobutyric acid (GABA) interneurons has been associated with the pathophysiology of schizophrenia, in view of the link between the abnormality of GABA neurons and cognitive impairments of the disease. It is assumed that an imbalance of excitatory and inhibitory (E-I) activity induced by low activity of glutamatergic projections and PV-positive GABA interneurons in the prefrontal cortex resulted in sustained neural firing and gamma oscillation, leading to impaired cognitive function. Therefore, it is important to develop novel pharmacotherapy targeting GABA neurons and their activities. Clinical evidence suggests serotonin (5-HT) 1A receptor agonist improves cognitive disturbances of schizophrenia, consistent with results from preclinical studies, through mechanism that corrects E-I imbalance via the suppression of GABA neural function. On the other hand, T-817MA, a novel neurotrophic agent, ameliorated loss of PV-positive GABA neurons in the medial prefrontal cortex and reduction of gamma-band activity, as well as cognitive dysfunction in animal model of schizophrenia. In conclusion, a pharmacotherapy to alleviate abnormalities in GABA neurons through 5-HT1A agonists and T-817MA is expected to prevent the onset and/or progression of schizophrenia.

Chemical modulation of mutant mGlu1 receptors derived from deleterious GRM1 mutations found in schizophrenics

Schizophrenia is a complex and highly heterogeneous psychiatric disorder whose precise etiology remains elusive. While genome-wide association studies (GWAS) have identified risk genes, they have failed to determine if rare coding single nucleotide polymorphisms (nsSNPs) contribute in schizophrenia. Recently, two independent studies identified 12 rare, deleterious nsSNPS in the GRM1 gene, which encodes the metabotropic glutamate receptor subtype 1 (mGlu1), in schizophrenic patients. Here, we generated stable cell lines expressing the mGlu1 mutant receptors and assessed their pharmacology. Using both the endogenous agonist glutamate and the synthetic agonist DHPG, we found that several of the mutant mGlu1 receptors displayed a loss of function that was not due to a loss in plasma membrane expression. Due to a lack of mGlu1 positive allosteric modulators (PAM) tool compounds active at human mGlu1, we optimized a known mGlu4 PAM/mGlu1 NAM chemotype into a series of potent and selective mGlu1 PAMs by virtue of a double "molecular switch". Employing mGlu1 PAMs from multiple chemotypes, we demonstrate that the mutant receptors can be potentiated by small molecules and in some cases efficacy restored to that comparable to wild type mGlu1 receptors, suggesting deficits in patients with schizophrenia due to these mutations may be amenable to intervention with an mGlu1 PAM. However, in wild type animals, mGlu1 negative allosteric modulators (NAMs) are efficacious in classic models predictive of antipsychotic activity, whereas we show that mGlu1 PAMs have no effect to slight potentiation in these models. These data further highlight the heterogeneity of schizophrenia and the critical role of patient selection strategies in psychiatric clinical trials to match genotype with therapeutic mechanism.

Development of allosteric modulators of GPCRs for treatment of CNS disorders

The discovery of allosteric modulators of G protein-coupled receptors (GPCRs) provides a promising new strategy with potential for developing novel treatments for a variety of central nervous system (CNS) disorders. Traditional drug discovery efforts targeting GPCRs have focused on developing ligands for orthosteric sites which bind endogenous ligands. Allosteric modulators target a site separate from the orthosteric site to modulate receptor function. These allosteric agents can either potentiate (positive allosteric modulator, PAM) or inhibit (negative allosteric modulator, NAM) the receptor response and often provide much greater subtype selectivity than orthosteric ligands for the same receptors. Experimental evidence has revealed more nuanced pharmacological modes of action of allosteric modulators, with some PAMs showing allosteric agonism in combination with positive allosteric modulation in response to endogenous ligand (ago-potentiators) as well as "bitopic" ligands that interact with both the allosteric and orthosteric sites. Drugs targeting the allosteric site allow for increased drug selectivity and potentially decreased adverse side effects. Promising evidence has demonstrated potential utility of a number of allosteric modulators of GPCRs in multiple CNS disorders, including neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and Huntington's disease, as well as psychiatric or neurobehavioral diseases such as anxiety, schizophrenia, and addiction.

Chronic nicotine attenuates phencyclidine-induced impulsivity in a mouse serial reaction time task

Schizophrenia is a disorder characterized by positive, negative, and cognitive symptoms. While positive symptoms can be effectively treated with typical antipsychotic medication, which generally affects the dopaminergic system, negative and cognitive symptoms, including attentional deficits and impulsive behavior, are less sensitive to standard treatments. It has further been well documented that schizophrenic patients use tobacco products at a rate much higher than the general population, and this persists despite treatment. It has been argued this behavior may be a form of self-medication, to alleviate some symptoms of schizophrenia. It has further been posited that prefrontal glutamatergic hypofunction may underlie some aspects of schizophrenia, and in accordance with this model, systemic phencyclidine has been used to model the disease. We employed a modified 5-choice serial reaction time test, a paradigm that is often used to investigate many of the treatment-resistant symptoms of schizophrenia including impulsivity, selective attention, and sustained attention/cognitive vigilance, to determine the medicinal effects of nicotine. We demonstrate that chronic oral, but not acute injections of nicotine can selectively attenuate phencyclidine-induced increases in impulsivity without affecting other measures of attention. This suggests that nicotine use by schizophrenics may provide some relief of distinct symptoms that involve impulsive behaviors.

Pomaglumetad methionil: no significant difference as an adjunctive treatment for patients with prominent negative symptoms of schizophrenia compared to placebo

This study tested whether treatment with pomaglumetad methionil (LY2140023 monohydrate), a metabotropic glutamate receptor 2/3 agonist compared with placebo (PBO), when added to a fixed-dose second-generation antipsychotic (SGA) demonstrated significantly greater reduction of negative symptoms, as assessed by the 16-item Negative Symptom Assessment scale (NSA-16), in patients with schizophrenia. This parallel-group, 16-week study enrolled adults with schizophrenia who were receiving standard of care (SOC) therapy, which included ≥3months treatment with one of four SGAs: aripiprazole, olanzapine, risperidone, or quetiapine. Patients received either 20mg of twice daily LY2140023 monohydrate (LY2140023) or concurrent PBO SGA. The primary efficacy measure was change from baseline to final visit in NSA-16 total score. Secondary measures included additional measures of efficacy, cognition, and assessments of safety. Of 352 patients screened, 167 were randomly assigned to treatment, and 110 patients completed the study. Patients treated with LY2140023 and SOC failed to demonstrate a statistically significant improvement over patients treated with PBO and SOC on NSA-16 total score at endpoint or at any point during the study (all p>0.131). Changes in secondary efficacy measures were not significantly different between groups at endpoint. With the exception of vomiting which was greater in the LY2140023 group, there were no statistically significant differences in safety and tolerability measures. This study found no benefit of adjunctive LY2140023 versus PBO for negative symptoms in patients with schizophrenia receiving treatment with SOC. LY2140023 was generally well-tolerated in these patients.

Classics in chemical neuroscience: clozapine

Clozapine was the first true breakthrough in schizophrenia treatment since the discovery of chlorpromazine in 1950, effectively treating positive, negative, and some cognitive symptoms, as well as possessing unprecedented efficacy in treatment-resistant patients. Despite over 30 years of intense study, the precise molecular underpinnings that account for clozapine's unique efficacy remain elusive. In this Viewpoint, we will showcase the history and importance of clozapine to neuroscience in general, as well as for the treatment of schizophrenia, and review the synthesis, pharmacology, drug metabolism, and adverse events of clozapine.

Group II metabotropic glutamate receptor agonist LY379268 regulates AMPA receptor trafficking in prefrontal cortical neurons

Group II metabotropic glutamate receptor (mGluR) agonists have emerged as potential treatment drugs for schizophrenia and other neurological disorders, whereas the mechanisms involved remain elusive. Here we examined the effects of LY379268 (LY37) on the expression and trafficking of the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor subunits GluA1 and GluA2 in prefrontal neurons. We show that LY37 significantly increased the surface and total expression of both GluA1 and GluA2 subunits in cultured prefrontal neurons and in vivo. This effect was mimicked by the selective mGluR2 agonist LY395756 and was blocked by mGluR2/3 antagonist LY341495. Moreover, we found that both GluA1 and GluA2 subunits were colocalized with PSD95 but not synapsin I, suggesting a postsynaptic localization. Consistently, treatment with LY37 significantly increased the amplitude, but not frequency, of miniature excitatory postsynaptic currents. Further, actinomycin-D blocked LY37's effects, suggesting a transcriptional regulation. In addition, application of glycogen synthase kinase-3beta (GSK-3β) inhibitor completely blocked LY37's effect on GluA2 surface expression, whereas GSK-3β inhibitor itself induced decreases in the surface and total protein levels of GluA1, but not GluA2 subunits. This suggests that GSK-3β differentially mediates GluA1 and GluA2 trafficking. Further, LY37 significantly increased the phosphorylation, but not total protein, of extracellular signal-regulated kinase 1/2 (ERK1/2). Neither ERK1/2 inhibitor PD98059 alone nor PD98059 combined with LY37 treatment induced changes in GluA1 or GluA2 surface expression or total protein levels. Our data thus suggest that mGluR2/3 agonist regulates postsynaptic AMPA receptors by affecting the synaptic trafficking of both GluA1 and GluA2 subunits and that the regulation is likely through ERK1/2 signaling in GluA1 and/or both ERK1/2 and GSK-3β signaling pathways in the GluA2 subunit.

Metabotropic glutamate receptor 5-positive allosteric modulators for the treatment of schizophrenia (2004–2012)

The mGlu5, a class C G-protein-coupled receptor and member of the group I mGlu receptor family, has been demonstrated to play a role in a number of therapeutic areas within the CNS, including schizophrenia, dementia, epilepsy, cognition, drug abuse, and fragile X syndrome. Small-molecule modulation of mGlu5 via positive allosteric modulators (PAMs) is being pursued as a promising approach for the treatment of schizophrenia and has been validated preclinically in a number of animal models. This article provides a brief historical overview of mGlu5 PAMs in the primary literature followed by a comprehensive overview of the patent literature since 2004. Schizophrenia is a complex disorder and although no mGlu5 PAMs have progressed into clinical trials in patients, the target continues to show promise as an attractive non-dopaminergic therapy. The successful development of mGlu5 PAMs for clinical testing must address several issues, including challenges associated with ‘molecular switches’, allosteric-agonist activity and stimulus bias.

Pharmacology of metabotropic glutamate receptor allosteric modulators: structural basis and therapeutic potential for CNS disorders

The metabotropic glutamate receptors (mGlus) mediate a neuromodulatory role throughout the brain for the major excitatory neurotransmitter, glutamate. Seven of the eight mGlu subtypes are expressed within the CNS and are attractive targets for a variety of psychiatric and neurological disorders including anxiety, depression, schizophrenia, Parkinson's disease, and Fragile X syndrome. Allosteric modulation of these class C 7-transmembrane spanning receptors represents a novel approach to facilitate development of mGlu subtype-selective probes and therapeutics. Allosteric modulators that interact with sites topographically distinct from the endogenous ligand-binding site offer a number of advantages over their competitive counterparts. In particular for CNS therapeutics, allosteric modulators have the potential to maintain the spatial and temporal aspects of endogenous neurotransmission. The past 15 years have seen the discovery of numerous subtype-selective allosteric modulators for the majority of the mGlu family members, including positive, negative, and neutral allosteric modulators, with a number of mGlu allosteric modulators now in clinical trials.

Further exploration of M₁ allosteric agonists: subtle structural changes abolish M₁ allosteric agonism and result in pan-mAChR orthosteric antagonism

This letter describes the further exploration of two series of M(1) allosteric agonists, TBPB and VU0357017, previously reported from our lab. Within the TPBP scaffold, either electronic or steric perturbations to the central piperidine ring led to a loss of selective M(1) allosteric agonism and afforded pan-mAChR antagonism, which was demonstrated to be mediated via the orthosteric site. Additional SAR around a related M(1) allosteric agonist family (VU0357017) identified similar, subtle 'molecular switches' that modulated modes of pharmacology from allosteric agonism to pan-mAChR orthosteric antagonism. Therefore, all of these ligands are best classified as bi-topic ligands that possess high affinity binding at an allosteric site to engender selective M(1) activation, but all bind, at higher concentrations, to the orthosteric ACh site, leading to non-selective orthosteric site binding and mAChR antagonism.

Pharmacological treatment of schizophrenia: a critical review of the pharmacology and clinical effects of current and future therapeutic agents

Since the introduction of chlorpromazine and throughout the development of the new-generation antipsychotic drugs (APDs) beginning with clozapine, the D(2) receptor has been the target for the development of APDs. Pharmacologic actions to reduce neurotransmission through the D(2) receptor have been the only proven therapeutic mechanism for psychoses. A number of novel non-D(2) mechanisms of action of APDs have been explored over the past 40 years but none has definitively been proven effective. At the same time, the effectiveness of treatments and range of outcomes for patients are far from satisfactory. The relative success of antipsychotics in treating positive symptoms is limited by the fact that a substantial number of patients are refractory to current medications and by their lack of efficacy for negative and cognitive symptoms, which often determine the level of functional impairment. In addition, while the newer antipsychotics produce fewer motor side effects, safety and tolerability concerns about weight gain and endocrinopathies have emerged. Consequently, there is an urgent need for more effective and better-tolerated antipsychotic agents, and to identify new molecular targets and develop mechanistically novel compounds that can address the various symptom dimensions of schizophrenia. In recent years, a variety of new experimental pharmacological approaches have emerged, including compounds acting on targets other than the dopamine D(2) receptor. However, there is still an ongoing debate as to whether drugs selective for singe molecular targets (that is, 'magic bullets') or drugs selectively non-selective for several molecular targets (that is, 'magic shotguns', 'multifunctional drugs' or 'intramolecular polypharmacy') will lead to more effective new medications for schizophrenia. In this context, current and future drug development strategies can be seen to fall into three categories: (1) refinement of precedented mechanisms of action to provide drugs of comparable or superior efficacy and side-effect profiles to existing APDs; (2) development of novel (and presumably non-D(2)) mechanism APDs; (3) development of compounds to be used as adjuncts to APDs to augment efficacy by targeting specific symptom dimensions of schizophrenia and particularly those not responsive to traditional APD treatment. In addition, efforts are being made to determine if the products of susceptibility genes in schizophrenia, identified by genetic linkage and association studies, may be viable targets for drug development. Finally, a focus on early detection and early intervention aimed at halting or reversing progressive pathophysiological processes in schizophrenia has gained great influence. This has encouraged future drug development and therapeutic strategies that are neuroprotective. This article provides an update and critical review of the pharmacology and clinical profiles of current APDs and drugs acting on novel targets with potential to be therapeutic agents in the future.

Targeting selective activation of M(1) for the treatment of Alzheimer's disease: further chemical optimization and pharmacological characterization of the M(1) positive allosteric modulator ML169

The M(1) muscarinic acetylcholine receptor is thought to play an important role in memory and cognition, making it a potential target for the treatment of Alzheimer's disease (AD) and schizophrenia. Moreover, M(1) interacts with BACE1 and regulates its proteosomal degradation, suggesting selective M(1) activation could afford both palliative cognitive benefit as well as disease modification in AD. A key challenge in targeting the muscarinic acetylcholine receptors is achieving mAChR subtype selectivity. Our lab has previously reported the M(1) selective positive allosteric modulator ML169. Herein we describe our efforts to further optimize this lead compound by preparing analogue libraries and probing novel scaffolds. We were able to identify several analogues that possessed submicromolar potency, with our best example displaying an EC(50) of 310 nM. The new compounds maintained complete selectivity for the M(1) receptor over the other subtypes (M(2)-M(5)), displayed improved DMPK profiles, and potentiated the carbachol (CCh)-induced excitation in striatal MSNs. Selected analogues were able to potentiate CCh-mediated nonamyloidogenic APPsα release, further strengthening the concept that M(1) PAMs may afford a disease-modifying role in the treatment of AD.

Novel allosteric agonists of M1 muscarinic acetylcholine receptors induce brain region-specific responses that correspond with behavioral effects in animal models

M(1) muscarinic acetylcholine receptors (mAChRs) represent a viable target for treatment of multiple disorders of the central nervous system (CNS) including Alzheimer's disease and schizophrenia. The recent discovery of highly selective allosteric agonists of M(1) receptors has provided a major breakthrough in developing a viable approach for the discovery of novel therapeutic agents that target these receptors. Here we describe the characterization of two novel M(1) allosteric agonists, VU0357017 and VU0364572, that display profound differences in their efficacy in activating M(1) coupling to different signaling pathways including Ca(2+) and β-arrestin responses. Interestingly, the ability of these agents to differentially activate coupling of M(1) to specific signaling pathways leads to selective actions on some but not all M(1)-mediated responses in brain circuits. These novel M(1) allosteric agonists induced robust electrophysiological effects in rat hippocampal slices, but showed lower efficacy in striatum and no measureable effects on M(1)-mediated responses in medial prefrontal cortical pyramidal cells in mice. Consistent with these actions, both M(1) agonists enhanced acquisition of hippocampal-dependent cognitive function but did not reverse amphetamine-induced hyperlocomotion in rats. Together, these data reveal that M(1) allosteric agonists can differentially regulate coupling of M(1) to different signaling pathways, and this can dramatically alter the actions of these compounds on specific brain circuits important for learning and memory and psychosis.

Pharmacological activation of group-II metabotropic glutamate receptors corrects a schizophrenia-like phenotype induced by prenatal stress in mice

Prenatal exposure to restraint stress causes long-lasting changes in neuroplasticity that likely reflect pathological modifications triggered by early-life stress. We found that the offspring of dams exposed to repeated episodes of restraint stress during pregnancy (here named 'prenatal restraint stress mice' or 'PRS mice') developed a schizophrenia-like phenotype, characterized by a decreased expression of brain-derived neurotrophic factor and glutamic acid decarboxylase 67, an increased expression of type-1 DNA methyl transferase (DNMT1) in the frontal cortex, and a deficit in social interaction, locomotor activity, and prepulse inhibition. PRS mice also showed a marked decrease in metabotropic glutamate 2 (mGlu2) and mGlu3 receptor mRNA and protein levels in the frontal cortex, which was manifested at birth and persisted in adult life. This decrease was associated with an increased binding of DNMT1 to CpG-rich regions of mGlu2 and mGlu3 receptor promoters and an increased binding of MeCP2 to the mGlu2 receptor promoter. Systemic treatment with the selective mGlu2/3 receptor agonist LY379268 (0.5 mg/kg, i.p., twice daily for 5 days), corrected all the biochemical and behavioral abnormalities shown in PRS mice. Our data show for the first time that PRS induces a schizophrenia-like phenotype in mice, and suggest that epigenetic changes in mGlu2 and mGlu3 receptors lie at the core of the pathological programming induced by early-life stress.

Metabotropic glutamate receptors as therapeutic targets for schizophrenia

Treatment options for schizophrenia that address all symptom categories (positive, negative, and cognitive) are lacking in current therapies for this disorder. Compounds targeting the metabotropic glutamate (mGlu) receptors hold promise as a more comprehensive therapeutic alternative to typical and atypical antipsychotics and may avoid the occurrence of extrapyramidal side effects that accompany these treatments. Activation of the group II mGlu receptors (mGlu(2) and mGlu(3)) and the group I mGlu(5) are hypothesized to normalize the disruption of thalamocortical glutamatergic circuitry that results in abnormal glutamaterigic signaling in the prefrontal cortex (PFC). Agonists of mGlu(2) and mGlu(3) have demonstrated efficacy for the positive symptom group in both animal models and clinical trials with mGlu(2) being the subtype most likely responsible for the therapeutic effect. Limitations in the chemical space tolerated by the orthosteric site of the mGlu receptors has led to the pursuit of compounds that potentiate the receptor's response to glutamate by acting at less highly conserved allosteric sites. Several series of selective positive allosteric modulators (PAMs) for mGlu(2) and mGlu(5) have demonstrated efficacy in animal models used for the evaluation of antipsychotic agents. In addition, evidence from animal studies indicates that mGlu(5) PAMs hold promise for the treatment of cognitive deficits that occur in schizophrenia. Hopefully, further optimization of allosteric modulators of mGlu receptors will yield clinical candidates that will allow full evaluation of the potential efficacy of these compounds in the treatment of multiple symptom domains in schizophrenia patients in the near future.

Muscarinic mechanisms in psychotic disorders

Schizophrenia is a devastating disease with several broad symptom clusters and the current monoamine-based treatments do not adequately treat the disease, especially negative and cognitive symptoms. A proposed alternative approach for treating schizophrenia is through the use of compounds that activate certain muscarinic receptor subtypes, the so-called muscarinic cholinergic hypothesis theory. This theory has been revitalized with a number of recent and provocative findings including postmortem reports in schizophrenia patients showing decreased numbers of muscarinic M(1) and M(4) receptors in brain regions associated with schizophrenia as well as decreased muscarinic receptors in an in vivo imaging study. Studies with M(4) knockout mice have shown that there is a reciprocal relationship between M(4) and dopamine receptor function, and a number of muscarinic agonists have shown antidopaminergic activity in a variety of preclinical assays predictive of antipsychotic efficacy in the clinic. Furthermore, the M(1)/M(4) preferring partial agonist xanomeline has been shown to have antipsychotic-like and pro-cognitive activity in preclinical models and in clinical trials to decrease psychotic-like behaviors in Alzheimer's patients and positive, negative, and cognitive symptoms in patients with schizophrenia. Therefore, we propose that an agonist with M(1) and M(4) interactions would effectively treat core symptom clusters associated with schizophrenia. Currently, research is focused on developing subtype-selective muscarinic agonists and positive allosteric modulators that have reduced propensity for parasympathetic side-effects, but retain the therapeutic benefit observed with their less selective predecessors.

Translating glutamate: from pathophysiology to treatment

The neurotransmitter glutamate is the primary excitatory neurotransmitter in mammalian brain and is responsible for most corticocortical and corticofugal neurotransmission. Disturbances in glutamatergic function have been implicated in the pathophysiology of several neuropsychiatric disorders-including schizophrenia, drug abuse and addiction, autism, and depression-that were until recently poorly understood. Nevertheless, improvements in basic information regarding these disorders have yet to translate into Food and Drug Administration-approved treatments. Barriers to translation include the need not only for improved compounds but also for improved biomarkers sensitive to both structural and functional target engagement and for improved translational models. Overcoming these barriers will require unique collaborative arrangements between pharma, government, and academia. Here, we review a recent Institute of Medicine-sponsored meeting, highlighting advances in glutamatergic theories of neuropsychiatric illness as well as remaining barriers to treatment development.

Progress toward positive allosteric modulators of the metabotropic glutamate receptor subtype 5 (mGluR5)

This Review describes recent trends in the development of small molecule mGlu(5) positive allosteric modulators (PAMs). A large body of pharmacological, genetic, electrophysiological, and in vivo behavioral evidence has accumulated over the past decade which continues to support the hypothesis and rationale for the activation of the metabotropic glutamate receptor subtype 5 (mGlu(5)) as a viable and promising target for the development of novel antipsychotics. Until recently, functionally efficacious and potent mGlu(5) PAMs have been somewhat structurally limited in scope and slow to emerge. This Review will discuss efforts since late 2008 which have provided novel mGlu(5) PAM chemotypes, offering ligands with a diverse range of pharmacological, physicochemical, and DMPK properties that were previously unavailable. In addition, significant biological studies of importance in the past few years using the well established PAMs known as DFB, CPPHA, CDPPB, and ADX-47273 will be discussed.

Glutamatergic targets for enhancing extinction learning in drug addiction

The persistence of the motivational salience of drug-related environmental cues and contexts is one of the most problematic obstacles to successful treatment of drug addiction. Behavioral approaches to extinguishing the salience of drug-associated cues, such as cue exposure therapy, have generally produced disappointing results which have been attributed to, among other things, the context specificity of extinction and inadequate consolidation of extinction learning. Extinction of any behavior or conditioned response is a process of new and active learning, and increasing evidence suggests that glutamatergic neurotransmission, a key component of the neural plasticity that underlies normal learning and memory, is also involved in extinction learning. This review will summarize findings from both animal and human studies that suggest that pharmacological enhancement of glutamatergic neurotransmission facilitates extinction learning in the context of drug addiction. Pharmacological agents that have shown potential efficacy include NMDA partial agonists, mGluR5 receptor positive allosteric modulators, inhibitors of the GlyT1 glycine transporter, AMPA receptor potentiators, and activators of the cystine-glutamate exchanger. These classes of cognition-enhancing compounds could potentially serve as novel pharmacological adjuncts to cue exposure therapy to increase success rates in attenuating cue-induced drug craving and relapse.

Group II metabotropic glutamate receptor agonist ameliorates MK801-induced dysfunction of NMDA receptors via the Akt/GSK-3β pathway in adult rat prefrontal cortex

Pharmacological intervention targeting mGluRs has emerged as a potential treatment for schizophrenia, whereas the mechanisms involved remain elusive. We explored the antipsychotic effects of an mGluR2/3 agonist in the MK-801 model of schizophrenia in the rat prefrontal cortex. We found that the mGluR2/3 agonist LY379268 effectively recovered the disrupted expression of NMDA receptors induced by MK-801 administration. This effect was attributable to the direct regulatory action of LY379268 on NMDA receptors via activation of the Akt/GSK-3β signaling pathway. As occurs with the antipsychotic drug clozapine, acute treatment with LY379268 significantly increased the expression and phosphorylation of NMDA receptors, as well as Akt and GSK-3β. Physiologically, LY379268 significantly enhanced NMDA-induced current in prefrontal neurons and a GSK-3β inhibitor occluded this effect. In contrast to the widely proposed mechanism of modulating presynaptic glutamate release, our results strongly argue that mGluR2/3 agonists modulate the function of NMDA receptors through postsynaptic actions and reverse the MK-801-induced NMDA dysfunction via the Akt/GSK-3β pathway. This study provides novel evidence for postsynaptic mechanisms of mGluR2/3 in regulation of NMDA receptors and presents useful insights into the mechanistic actions of mGluR2/3 agonists as potential antipsychotic agents for treating schizophrenia.

Activation of group II metabotropic glutamate receptors promotes DNA demethylation in the mouse brain

Activation of group II metabotropic glutamate receptors (mGlu2 and -3 receptors) has shown a potential antipsychotic activity, yet the underlying mechanism is only partially known. Altered epigenetic mechanisms contribute to the pathogenesis of schizophrenia and currently used medications exert chromatin remodeling effects. Here, we show that systemic injection of the brain-permeant mGlu2/3 receptor agonist (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylic acid (LY379268; 0.3-1 mg/kg i.p.) increased the mRNA and protein levels of growth arrest and DNA damage 45-β (Gadd45-β), a molecular player of DNA demethylation, in the mouse frontal cortex and hippocampus. Induction of Gadd45-β by LY379268 was abrogated by the mGlu2/3 receptor antagonist (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (LY341495; 1 mg/kg i.p.). Treatment with LY379268 also increased the amount of Gadd45-β bound to specific promoter regions of reelin, brain-derived neurotrophic factor (BDNF), and glutamate decarboxylase-67 (GAD67). We directly assessed gene promoter methylation in control mice and in mice pretreated for 7 days with the methylating agent methionine (750 mg/kg i.p.). Both single and repeated injections with LY379268 reduce cytosine methylation in the promoters of the three genes, although the effect on the GAD67 was significant only in response to repeated injections. Single and repeated treatment with LY379268 could also reverse the defect in social interaction seen in mice pretreated with methionine. The action of LY379268 on Gadd45-β was mimicked by valproate and clozapine but not haloperidol. These findings show that pharmacological activation of mGlu2/3 receptors has a strong impact on the epigenetic regulation of genes that have been linked to the pathophysiology of schizophrenia.

Positive allosteric modulators of type 5 metabotropic glutamate receptors (mGluR5) and their therapeutic potential for the treatment of CNS disorders

Studies utilizing selective pharmacological antagonists or targeted gene deletion have demonstrated that type 5 metabotropic glutamate receptors (mGluR5) are critical mediators and potential therapeutic targets for the treatment of numerous disorders of the central nervous system (CNS), including depression, anxiety, drug addiction, chronic pain, Fragile X syndrome, Parkinson’s disease, and gastroesophageal reflux disease. However, in recent years, the development of positive allosteric modulators (PAMs) of the mGluR5 receptor have revealed that allosteric activation of this receptor may also be of potential therapeutic benefit for the treatment of other CNS disorders, including schizophrenia, cognitive deficits associated with chronic drug use, and deficits in extinction learning. Here we summarize the discovery and characterization of various mGluR5 PAMs, with an emphasis on those that are systemically active. We will also review animal studies showing that these molecules have potential efficacy as novel antipsychotic agents. Finally, we will summarize findings that suggest that mGluR5 PAMs have pro-cognitive effects such as the ability to enhance synaptic plasticity, improve performance in various learning and memory tasks, including extinction of drug-seeking behavior, and reverse cognitive deficits produced by chronic drug use.

Discovery of novel positive allosteric modulators of the metabotropic glutamate receptor 5 (mGlu5)

Novel in vitro mGlu(5) positive allosteric modulators with good potency, solubility, and low lipophilicity are described. Compounds were identified which did not rely on the phenylacetylene and carbonyl functionalities previously observed to be required for in vitro activity. Investigation of the allosteric binding requirements of a series of dihydroquinolinone analogs led to phenylacetylene azachromanone 4 (EC(50) 11.5 nM). Because of risks associated with potential metabolic and toxicological liabilities of the phenylacetylene, this moiety was successfully replaced with a phenoxymethyl group (27; EC(50) 156.3 nM). Derivation of a second-generation of mGlu(5) PAMs lacking a ketone carbonyl resulted in azaindoline (33), azabenzimidazole (36), and N-methyl 8-azaoxazine (39) phenylacetylenes. By scoping nitrogen substituents and phenylacetylene replacements in 39, we identified phenoxymethyl 8-azaoxazine 47 (EC(50) 50.1 nM) as a potent and soluble mGlu(5) PAM devoid of both undesirable phenylacetylene and carbonyl functionalities.

Functional interaction of mGlu5 and NMDA receptors in aversive learning in rats

Metabotropic glutamate receptor 5 (mGlu5) has been implicated in a variety of learning processes and is important for inhibitory avoidance and conditioned taste aversion learning. MGlu5 receptors are physically connected with NMDA receptors and they interact with, and modulate, the function of one another in several brain regions. The present studies used systemic co-administration of an mGlu5 receptor positive allosteric modulator, 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB) and an NMDA receptor antagonist dizocilpine maleate (MK-801) to characterize the interactions of these receptors in two aversive learning tasks. Male Sprague-Dawley rats were trained in a single-trial step-down inhibitory avoidance or conditioned taste aversion task. CDPPB (3 or 10mg/kg, s.c.), delivered by itself prior to the conditioning trial, did not have any effect on performance in either task 48 h after training. However, CDPPB (at 3mg/kg) attenuated the MK-801 (0.2mg/kg, i.p.) induced learning deficit in both tasks. CDPPB also reduced MK-801-induced hyperactivity. These results underlie the importance of mGlu5 and NMDA receptor interactions in modulating memory processing, and are consistent with findings showing the efficacy of positive allosteric modulators of mGlu5 receptors in reversing the negative effects of NMDA receptor antagonists on other behaviors such as stereotypy, sensorimotor gating, or working, spatial and recognition memory.

Stuck in a rut: rethinking depression and its treatment

The current definition of major depressive disorder (MDD) emerged from efforts to create reliable diagnostic criteria for clinical and research use. However, despite decades of research, the neurobiology of MDD is largely unknown, and treatments are no more effective today than they were 50-70 years ago. Here, we propose that the current conception of depression is misguiding basic and clinical research. Redefinition is necessary and could include a focus on a more narrowly defined set of core symptoms. However, we conclude that depression is better defined as the tendency to enter into, and inability to disengage from, a negative mood state rather than the mood state per se. We also discuss the implications of this revised definition for future clinical and basic research.

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'.

Past and present progress in the pharmacologic treatment of schizophrenia

Despite treatment advances over the past decades, schizophrenia remains one of the most severe psychiatric disorders that is associated with a chronic relapsing course and marked functional impairment in a substantial proportion of patients. In this article, a historical overview of the pharmacologic advances in the treatment of schizophrenia over the past 50 years is presented. This is followed by a review of the current developments in optimizing the treatment and outcomes in patients with schizophrenia. Methodological challenges, potential solutions, and areas of particular need for further research are highlighted. Although treatment goals of response, remission, and recovery have been defined more uniformly, a good "effectiveness" measure mapping onto functional outcomes is still lacking. Moreover, the field must advance in transferring measurement-based approaches from research to clinical practice. There is an ongoing debate regarding whether and which first- or second-generation antipsychotics should be used. However, especially when considering individual adverse effect profiles, the differentiation into first- and second-generation antipsychotics as unified classes cannot be upheld, and a more differentiated view and treatment selection are required. The desired, individualized treatment approach needs to consider current symptoms, comorbid conditions, past therapeutic response, and adverse effects, as well as patient choice and expectations. Acute and long-term goals and effects of medication treatment should be balanced. To date, clozapine is the only evidence-based treatment for refractory patients, and the role of antipsychotic polypharmacy and other augmentation strategies remains unclear, at best. To discover novel treatments with enhanced/broader efficacy and improved tolerability, and to enable personalized treatment, the mechanisms underlying illness development and progression, symptomatic improvement, and side effect development need to be elucidated.

GSK-3β activity and hyperdopamine-dependent behaviors

Dopamine plays important roles in normal brain function and many neuropsychiatric disorders. Classically, dopamine receptors are positively coupled to G protein-mediated signaling to regulate cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA)-dopamine and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) and Ca(2+) pathways. However, emerging evidence indicates that under hyperdopaminergic conditions, the protein kinase B (Akt)-glycogen synthase kinase 3β (GSK-3β) signaling cascade may mediate dopamine actions via D(2)-like receptors. This cAMP-independent signaling pathway involves the regulation of downstream synaptic targets, e.g., AMPA receptor, NMDA receptors, and thus synaptic plasticity. Here we provide an overview of how this novel signaling pathway relays dopamine receptor-mediated responses, particularly hyperdopamine-dependent behaviors. We discuss the relevance of the Akt/GSK-3β signaling cascade for the expression of dopamine-dependent behaviors and the drug actions associated with dopaminergic systems.

Identification of N-(2-(azepan-1-yl)-2-phenylethyl)-benzenesulfonamides as novel inhibitors of GlyT1

A novel series of glycine transporter 1 (GlyT1) inhibitors is described. Scoping of the heterocycle moiety of hit 4-chlorobenzenesulfonamide 1 led to replacement of the piperidine with an azepane for a modest increase in potency. Phenyl sulfonamides proved superior to alkyl and non-phenyl aromatic sulfonamides, while subsequent ortho substitution of the 2-(azepan-1-yl)-2-phenylethanamine aromatic ring yielded 39 (IC(50) 37 nM, solubility 14 microM), the most potent GlyT1 inhibitor in this series. Favorable brain-plasma ratios were observed for select compounds in pharmacokinetic studies to evaluate CNS penetration.

Glycine transporter (GlyT1) inhibitors with reduced residence time increase prepulse inhibition without inducing hyperlocomotion in DBA/2 mice

Inhibition of the glycine transporter type 1 (GlyT1) leading to potentiation of the glycine site (GlyB) on the N-methyl-d-aspartate (NMDA) receptor has been proposed as a novel therapeutic approach for schizophrenia. However, sarcosine-based GlyT1 inhibitors produce undesirable side effects including compulsive walking and respiratory distress. The influence of specific biochemical properties of GlyT1 inhibitors, such as mode of inhibition and residence time, on adverse effects is unknown. Two GlyT1 inhibitors that contain a sarcosine moiety, sarcosine and ALX-5407, and two compounds that do not contain a sarcosine moiety, Roche-7 and Merck (S)-13h, were evaluated for their potency, mode of inhibition, and target residence times in vitro, and modulation of prepulse inhibition (PPI) and locomotor activity in vivo. (S)-13h and sarcosine were competitive inhibitors while ALX-5407 and Roche-7 demonstrated mixed noncompetitive inhibition. Potency of GlyT1 inhibition (ALX-5407>(S)-13h>Roche-7≫sarcosine) did not correlate with residence time on GlyT1 (sarcosine=Roche-7≪(S)-13h<ALX-5407). ALX-5407 and (S)-13h induced compulsive walking, termed obstinate progression (OP), at doses that increased PPI in DBA/2 mice, demonstrating that OP was not a function of mode of inhibition or inhibitor chemotype. Sarcosine and Roche-7 increased PPI without inducing OP, suggesting that compounds with decreased GlyT1 residence time were efficacious without adverse effects. Direct activation of the GlyB site by d-serine did not produce OP. However, OP induced by (S)-13h was blocked by strychnine, a glycine receptor (GlyA) antagonist, suggesting that OP induced by GlyT1 inhibition was mediated by GlyA. Thus, GlyT1 inhibitors with short residence times demonstrated efficacy without mechanism-based adverse effects.

Allosteric modulation of metabotropic glutamate receptors: structural insights and therapeutic potential

Allosteric modulation of G protein-coupled receptors (GPCRs) represents a novel approach to the development of probes and therapeutics that is expected to enable subtype-specific regulation of central nervous system target receptors. The metabotropic glutamate receptors (mGlus) are class C GPCRs that play important neuromodulatory roles throughout the brain, as such they are attractive targets for therapeutic intervention for a number of psychiatric and neurological disorders including anxiety, depression, Fragile X Syndrome, Parkinson's disease and schizophrenia. Over the last fifteen years, selective allosteric modulators have been identified for many members of the mGlu family. The vast majority of these allosteric modulators are thought to bind within the transmembrane-spanning domains of the receptors to enhance or inhibit functional responses. A combination of mutagenesis-based studies and pharmacological approaches are beginning to provide a better understanding of mGlu allosteric sites. Collectively, when mapped onto a homology model of the different mGlu subtypes based on the β(2)-adrenergic receptor, the previous mutagenesis studies suggest commonalities in the location of allosteric sites across different members of the mGlu family. In addition, there is evidence for multiple allosteric binding pockets within the transmembrane region that can interact to modulate one another. In the absence of a class C GPCR crystal structure, this approach has shown promise with respect to the interpretation of mutagenesis data and understanding structure-activity relationships of allosteric modulator pharmacophores.