Interaction of N-methyl-D-aspartate and group 5 metabotropic glutamate receptors on behavioral flexibility using a novel operant set-shift paradigm

Neural structures underlying set-shifting: roles of medial prefrontal cortex and anterior cingulate cortex

Impaired attentional set-shifting and inflexible decision-making are problems frequently observed during normal aging and in several psychiatric disorders. To understand the neuropathophysiology of underlying inflexible behavior, animal models of attentional set-shifting have been developed to mimic tasks such as the Wisconsin Card Sorting Task (WCST), which tap into a number of cognitive functions including stimulus-response encoding, working memory, attention, error detection, and conflict resolution. Here, we review many of these tasks in several different species and speculate on how prefrontal cortex and anterior cingulate cortex might contribute to normal performance during set-shifting.

Distinct manifestations of executive dysfunction in aged rats

Different components of executive function such as working memory, attention, and cognitive flexibility can be dissociated behaviorally and mechanistically; however, the within-subject influences of normal aging on different aspects of executive function remain ill-defined. To better define these relationships, young adult and aged male F344 rats were cross-characterized on an attentional set-shifting task that assesses cognitive flexibility and a delayed response task that assesses working memory. Across tasks, aged rats were impaired relative to young; however, there was significant variability in individual performance within the aged cohort. Notably, performance on the set-shifting task and performance at long delays on the delayed response task were inversely related among aged rats. Additional experiments showed no relationship between aged rats' performance on the set-shifting task and performance on a hippocampal-dependent spatial reference memory task. These data indicate that normal aging can produce distinct manifestations of executive dysfunction, and support the need to better understand the unique mechanisms contributing to different forms of prefrontal cortical-supported executive decline across the lifespan.

Unique signaling profiles of positive allosteric modulators of metabotropic glutamate receptor subtype 5 determine differences in in vivo activity

BACKGROUND

Metabotropic glutamate receptor subtype 5 (mGlu5) activators have emerged as a novel approach to the treatment of schizophrenia. Positive allosteric modulators (PAMs) of mGlu5 have generated tremendous excitement and fueled major drug discovery efforts. Although mGlu5 PAMs have robust efficacy in preclinical models of schizophrenia, preliminary reports suggest that these compounds may induce seizure activity. Prototypical mGlu5 PAMs do not activate mGlu5 directly but selectively potentiate activation of mGlu5 by glutamate. This mechanism may be critical to maintaining normal activity-dependence of mGlu5 activation and achieving optimal in vivo effects.

METHODS

Using specially engineered mGlu5 cell lines incorporating point mutations within the allosteric and orthosteric binding sites, as well as brain slice electrophysiology and in vivo electroencephalography and behavioral pharmacology, we found that some mGlu5 PAMs have intrinsic allosteric agonist activity in the absence of glutamate.

RESULTS

Both in vitro mutagenesis and in vivo pharmacology studies demonstrate that VU0422465 is an agonist PAM that induces epileptiform activity and behavioral convulsions in rodents. In contrast, VU0361747, an mGlu5 PAMs optimized to eliminate allosteric agonist activity, has robust in vivo efficacy and does not induce adverse effects at doses that yield high brain concentrations.

CONCLUSIONS

Loss of the absolute dependence of mGlu5 PAMs on glutamate release for their activity can lead to severe adverse effects. The finding that closely related mGlu5 PAMs can differ in their intrinsic agonist activity provides critical new insights that is essential for advancing these molecules through clinical development for treatment of schizophrenia.

Enhancement of social novelty discrimination by positive allosteric modulators at metabotropic glutamate 5 receptors: adolescent administration prevents adult-onset deficits induced by neonatal treatment with phencyclidine

Metabotropic glutamate-5 receptors (mGluR5), which physically and functionally interact with N-methyl-D-Aspartate (NMDA) receptors, likewise control cognitive processes and have been proposed as targets for novel classes of antipsychotic agent. Since social cognition is impaired in schizophrenia and disrupted by NMDA receptor antagonists like dizocilpine, we evaluated its potential modulation by mGluR5. Acute administration (0.63-40 mg/kg) of the mGluR5 positive allosteric modulators (PAMs), 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB) and ADX47273, reversed a delay-induced impairment in social novelty discrimination (SND) in adult rats. The action of CDPPB was blocked by the mGluR5 antagonist, 2-methyl-6-(phenylethynyl)-pyridine (2.5-10 mg/kg), and was also expressed upon microinjection into frontal cortex (0.63-10 μg/side), but not striatum. Supporting an interrelationship between mGluR5 and NMDA receptors, enhancement of SND by CDPPB was blocked by dizocilpine (0.08 mg/kg) while, reciprocally, dizocilpine-induced impairment in SND was attenuated by CDPPB (10 mg/kg). The SND deficit elicited by post-natal administration of phencyclidine (10 mg/kg, days 7-11) was reversed by CDPPB or ADX47273 in adults at week 8. This phencyclidine-induced impairment in cognition emerged in adult rats from week 7 on, and chronic, pre-symptomatic treatment of adolescent rats with CDPPB over weeks 5-6 (10 mg/kg per day) prevented the appearance of SND deficits in adults until at least week 13. In conclusion, as evaluated by a SND procedure, mGluR5 PAMs promote social cognition via actions expressed in interaction with NMDA receptors and exerted in frontal cortex. MGluR5 PAMs not only reverse but also (when given during adolescence) prevent the emergence of cognitive impairment associated with a developmental model of schizophrenia.

Mechanism based neurotoxicity of mGlu5 positive allosteric modulators--development challenges for a promising novel antipsychotic target

Previous work has suggested that activation of mGlu5 receptor augments NMDA receptor function and thereby may constitute a rational approach addressing glutamate hypofunction in schizophrenia and a target for novel antipsychotic drug development. Here, we report the in vitro activity, in vivo efficacy and safety profile of 5PAM523 (4-Fluorophenyl){(2R,5S)-5-[5-(5-fluoropyridin-2-yl)-1,2,4-oxadiazol-3-yl]-2-methylpiperidin-1-yl}methanone), a structurally novel positive allosteric modulator selective of mGlu5. In cells expressing human mGlu5 receptor, 5PAM523 potentiated threshold responses to glutamate in fluorometric calcium assays, but does not have any intrinsic agonist activity. 5PAM523 acts as an allosteric modulator as suggested by the binding studies showing that 5PAM523 did not displace the binding of the orthosteric ligand quisqualic acid, but did partially compete with the negative allosteric modulator, MPyEP. In vivo, 5PAM523 reversed amphetamine-induced locomotor activity in rats. Therefore, both the in vitro and in vivo data demonstrate that 5PAM523 acts as a selective mGlu5 PAM and exhibits anti-psychotic like activity. To study the potential for adverse effects and particularly neurotoxicity, brain histopathological exams were performed in rats treated for 4 days with 5PAM523 or vehicle. The brain exam revealed moderate to severe neuronal necrosis in the rats treated with the doses of 30 and 50 mg/kg, particularly in the auditory cortex and hippocampus. To investigate whether this neurotoxicity is mechanism specific to 5PAM523, similar safety studies were carried out with three other structurally distinct selective mGlu5 PAMs. Results revealed a comparable pattern of neuronal cell death. Finally, 5PAM523 was tested in mGlu5 knock-out (KO) and wild type (WT) mice. mGlu5 WT mice treated with 5PAM523 for 4 days at 100 mg/kg presented significant neuronal death in the auditory cortex and hippocampus. Conversely, mGlu5 KO mice did not show any neuronal loss by histopathology, suggesting that enhancement of mGlu5 function is responsible for the toxicity of 5PAM523. This study reveals for the first time that augmentation of mGlu5 function with selective allosteric modulators results in neurotoxicity.

Metabotropic glutamate receptor 5 in the pathology and treatment of schizophrenia

Metabotropic glutamate receptor 5 (mGluR5) potentiates the NMDA receptor (NMDAR) in brain regions implicated in schizophrenia, making it a viable therapeutic target for the treatment of this disorder. mGluR5 positive allosteric modulators may represent a valuable novel strategy for schizophrenia treatment, given the favourable profile of effects in preclinical paradigms. However it remains unclear whether mGluR5 also plays a causal or epiphenomenal role in NMDAR dysfunction in schizophrenia. Animal and cellular data suggest involvement of mGluR5, whilst post-mortem human studies remain inconclusive. This review will explore the molecular, animal and human data to support and refute the involvement of mGluR5 in the pathology of schizophrenia. Furthermore, this review will discuss the potential of mGluR5 modulators in the therapy of schizophrenia as well as aspects of mGluR5 that require further characterisation.

Inhibition of dopamine transporter activity impairs synaptic depression in rat prefrontal cortex through over-stimulation of D1 receptors

In rat prefrontal cortex (PFC), long-term depression induced by low-frequency single stimuli has never been studied. Combined with the well-documented involvement of dopamine transporters (DATs) in the regulation of PFC-dependent cognitive processes, it is important to test whether this form of plasticity can be modulated by DAT activity in the PFC. Here, we show first that prolonged 3-Hz stimuli successfully induced synaptic depression in rat PFC slices whose induction depended on endogenous stimulation of D1-like and D2-like receptors and the activation of extracellular signal-regulated kinase 1/2 (ERK1/2). This depression was found to be significantly impaired by selective inhibition of the DAT by GBR12909 (1-200 nM) or GBR12935 (100 nM). The excess amount of extracellular dopamine caused by DAT inhibition acted critically on D1-like receptors to impair depression. Furthermore, this impairment by GBR12 909 was cancelled by the allosteric-positive mGluR5 modulator CDPPB, the drug known to reverse hyperdopaminergia-induced abnormal PFC activity, and the associated cognitive disturbances. Finally, these induction, impairment, and restoration of synaptic depression were correlated by an inverted-U shape manner with the phosphorylation level of ERK1/2. We suggest that abnormal increases of the extracellular dopamine level by DAT inhibition impair synaptic depression in the PFC through over-stimulation of D1-like receptors.

Effects of a metabotropic glutamate receptor 5 positive allosteric modulator, CDPPB, on spatial learning task performance in rodents

Metabotropic glutamate receptor 5 (mGlu5) has been implicated in a variety of learning and memory processes and is important for avoidance learning. The present studies used an mGlu5 receptor positive allosteric modulator, 3-cyano-N-(1,3 diphenyl-1H-hyrazol-5-yl)benzamide (CDPPB), to characterize the importance of mGlu5 receptors in aversively- and appetitively-motivated spatial learning tasks (tasks in which the instrumental contingency involves discriminative cues that differ in spatial location). C57Bl/6 male mice were initially trained in the Barnes maze in the absence of drug. Subsequently, CDPPB (30mg/kg, i.p.), administered 20min prior to each of 3 daily reversal learning training sessions in the Barnes maze, significantly enhanced performance compared to vehicle-treated controls and had a significant effect on search strategy. Mice treated with CDPPB also displayed significantly less perseverative behavior than control-treated animals. In a second experiment, male Sprague-Dawley rats were trained in an appetitively-motivated, delayed alternation version of a T-maze. 30mg/kg CDPPB (s.c.), delivered 20min prior to each of 5 daily training sessions, enhanced the delay rats were able to withstand between the sample and choice portions of each T-maze trial. The present results emphasize the role of mGlu5 receptors in spatial learning tasks and support previous studies which report mGlu5 positive allosteric modulators can enhance learning in some tasks and may have potential as nootropic drugs.

Glutamatergic Approaches for the Treatment of Schizophrenia

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system and plays a key role in most aspects of normal brain function including cognition, learning and memory. Dysfunction of glutamatergic neurotransmission has been implicated in a number of neurological and psychiatric disorders with a growing body of evidence suggesting that hypofunction of glutamatergic neurotransmission via the N-methyl-d-aspartate (NMDA) receptor plays an important role in the pathophysiology of schizophrenia. It thus follows that potentiation of NMDA receptor function via pharmacological manipulation may provide therapeutic utility for the treatment of schizophrenia and a number of different approaches are currently being pursued by the pharmaceutical industry with this aim in mind. These include strategies that target the glycine/d-serine site of the NMDA receptor (glycine transporter GlyT1, d-serine transporter ASC-1 and d-amino acid oxidase (DAAO) inhibitors) together with those aimed at enhancing glutamatergic neurotransmission via modulation of AMPA receptor and metabotropic glutamate receptor function. Such efforts are now beginning to bear fruit with compounds such as the GlyT1 inhibitor RG1678 and mGlu2 agonist LY2140023 proving to have clinical meaningful effects in phase II clinical trials. While more studies are required to confirm long-term efficacy, functional outcome and safety in schizophrenic agents, these agents hold real promise for addressing unmet medical needs, in particular refractory negative and cognitive symptoms, not currently addressed by existing antipsychotic agents.

Characterizing cognitive aging of working memory and executive function in animal models

Executive functions supported by prefrontal cortical (PFC) systems provide essential control and planning mechanisms to guide goal-directed behavior. As such, age-related alterations in executive functions can mediate profound and widespread deficits on a diverse array of neurocognitive processes. Many of the critical neuroanatomical and functional characteristics of prefrontal cortex are preserved in rodents, allowing for meaningful cross species comparisons relevant to the study of cognitive aging. In particular, as rodents lend themselves to genetic, cellular and biochemical approaches, rodent models of executive function stand to significantly contribute to our understanding of the critical neurobiological mechanisms that mediate decline of executive processes across the lifespan. Moreover, rodent analogs of executive functions that decline in human aging represent an essential component of a targeted, rational approach for developing and testing effective treatment and prevention therapies for age-related cognitive decline. This paper reviews behavioral approaches used to study executive function in rodents, with a focus on those assays that share a foundation in the psychological and neuroanatomical constructs important for human aging. A particular emphasis is placed on behavioral approaches used to assess working memory and cognitive flexibility, which are sensitive to decline with age across species and for which strong rodent models currently exist. In addition, other approaches in rodent behavior that have potential for providing analogs to functions that reliably decline to human aging (e.g., information processing speed) are discussed.

Therapeutic potential of metabotropic glutamate receptor modulators

Glutamate is the main excitatory neurotransmitter in the central nervous system (CNS) and is a major player in complex brain functions. Glutamatergic transmission is primarily mediated by ionotropic glutamate receptors, which include NMDA, AMPA and kainate receptors. However, glutamate exerts modulatory actions through a family of metabotropic G-protein-coupled glutamate receptors (mGluRs). Dysfunctions of glutamatergic neurotransmission have been implicated in the etiology of several diseases. Therefore, pharmacological modulation of ionotropic glutamate receptors has been widely investigated as a potential therapeutic strategy for the treatment of several disorders associated with glutamatergic dysfunction. However, blockade of ionotropic glutamate receptors might be accompanied by severe side effects due to their vital role in many important physiological functions. A different strategy aimed at pharmacologically interfering with mGluR function has recently gained interest. Many subtype selective agonists and antagonists have been identified and widely used in preclinical studies as an attempt to elucidate the role of specific mGluRs subtypes in glutamatergic transmission. These studies have allowed linkage between specific subtypes and various physiological functions and more importantly to pathological states. This article reviews the currently available knowledge regarding the therapeutic potential of targeting mGluRs in the treatment of several CNS disorders, including schizophrenia, addiction, major depressive disorder and anxiety, Fragile X Syndrome, Parkinson’s disease, Alzheimer’s disease and pain.

Azetidinyl oxadiazoles as potent mGluR5 positive allosteric modulators

A novel series of aryl azetidinyl oxadiazoles are identified as mGluR5 positive allosteric modulators (PAMs) with improved physico-chemical properties. N-substituted cyclohexyl and exo-norbornyl carboxamides, and carbamate analogs of azetidines are moderate to potent mGluR5 PAMs. The aryl, lower alkyl carboxamides analogs and sulfonamide analogs of azetidines are moderate mGluR5 negative allosteric modulators (NAMs). In the aryl oxadiazole moiety, substituents such as fluoro, chloro and methyl are well tolerated at the meta position while para substituents led to either inactive compounds or NAMs. A tight pharmacophore and subtle 'PAM to NAM switching' with close analogs makes the optimization of the series extremely challenging.

The NMDA receptor as a target for cognitive enhancement

NMDA receptors (NMDARs) play an important role in neural plasticity including long-term potentiation and long-term depression, which are likely to explain their importance for learning and memory. Cognitive decline is a major problem facing an ageing human population, so much so that its reversal has become an important goal for scientific research and pharmaceutical development. Enhancement of NMDAR function is a core strategy toward this goal. In this review we indicate some of the major ways of potentiating NMDAR function by both direct and indirect modulation. There is good evidence that both positive and negative modulation can enhance function suggesting that a subtle approach correcting imbalances in particular clinical situations will be required. Excessive activation and the resultant deleterious effects will need to be carefully avoided. Finally we describe some novel positive allosteric modulators of NMDARs, with some subunit selectivity, and show initial evidence of their ability to affect NMDAR mediated events. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

N-Aryl pyrrolidinonyl oxadiazoles as potent mGluR5 positive allosteric modulators

A novel series of N-aryl pyrrolidinonyl oxadiazoles were identified as mGluR5 positive allosteric modulators (PAMs). Optimization of the initial lead compound 6a led to the identification of the 12c (-) enantiomer as a potent compound with acceptable in vitro clearance, CYP, hERG and PK properties. Para substituted N-aryl pyrrolidinonyl oxadiazoles are mGluR5 PAMs while the meta and ortho substituted N-aryl pyrrolidinonyl oxadiazoles are negative allosteric modulators (NAMs). Para fluoro substitution on the N-aryl group and meta chloro or methyl substituents on the aryl oxadiazole moiety are optimal for mGluR5 PAM efficacy. The existence of an exquisitely sensitive 'PAM to NAM switch' within this chemotype making it challenging for simultaneous optimization of potency and drug-like properties.

Interactions between the cannabinoid and dopaminergic systems: evidence from animal studies

There is a prominent role of the cannabinoid system to control basal ganglia function, in respect to reward, psychomotor function and motor control. Cannabinoid dysregulations might have a pathogenetic role in dopamine- and basal ganglia related neuropsychiatric disorders, such as drug addiction, psychosis, Parkinson's disease and Huntington's disease. This review highlights interactions between cannabinoids, and dopamine, to modulate neurotransmitter release and synaptic plasticity in the context of drug addiction, psychosis and cognition. Modulating endocannabinoid function, as a plasticity based therapeutic strategy, in the above pathologies with particular focus on cannabinoid receptor type 1 (CB1 receptor) antagonists/inverse agonists, is discussed. On the basis of the existing literature and of new experimental evidence presented here, CB1 receptor antagonists might be beneficial in disease states associated with hedonic dysregulation, and with cognitive dysfunction in particular in the context of psychosis. It is suggested that this effects might be mediated via a hyperglutamatergic state through metabotropic glutamate activation. Indications for endocannabinoid catabolism inhibitors in psychiatric disorders, that might be CB1 receptor independent and might involve TRPV1 receptors, are also discussed.

Autistic-like social behaviour in Shank2-mutant mice improved by restoring NMDA receptor function

Autism spectrum disorder (ASD) is a group of conditions characterized by impaired social interaction and communication, and restricted and repetitive behaviours. ASD is a highly heritable disorder involving various genetic determinants. Shank2 (also known as ProSAP1) is a multi-domain scaffolding protein and signalling adaptor enriched at excitatory neuronal synapses, and mutations in the human SHANK2 gene have recently been associated with ASD and intellectual disability. Although ASD-associated genes are being increasingly identified and studied using various approaches, including mouse genetics, further efforts are required to delineate important causal mechanisms with the potential for therapeutic application. Here we show that Shank2-mutant (Shank2(-/-)) mice carrying a mutation identical to the ASD-associated microdeletion in the human SHANK2 gene exhibit ASD-like behaviours including reduced social interaction, reduced social communication by ultrasonic vocalizations, and repetitive jumping. These mice show a marked decrease in NMDA (N-methyl-D-aspartate) glutamate receptor (NMDAR) function. Direct stimulation of NMDARs with D-cycloserine, a partial agonist of NMDARs, normalizes NMDAR function and improves social interaction in Shank2(-/-) mice. Furthermore, treatment of Shank2(-/-) mice with a positive allosteric modulator of metabotropic glutamate receptor 5 (mGluR5), which enhances NMDAR function via mGluR5 activation, also normalizes NMDAR function and markedly enhances social interaction. These results suggest that reduced NMDAR function may contribute to the development of ASD-like phenotypes in Shank2(-/-) mice, and mGluR modulation of NMDARs offers a potential strategy to treat ASD.

Therapeutic effects of metabotropic glutamate receptor 5 positive allosteric modulator CDPPB on phencyclidine-induced cognitive deficits in mice

This study was undertaken to examine the effects of CDPPB (3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide), a positive allosteric modulator (PAM) of metabotropic glutamate receptor 5 (mGlu₅), on cognitive deficits in mice after repeated administration of the N-methyl-D-aspartate (NMDA) receptor antagonist phencyclidine (PCP). In the novel object recognition test, PCP (10 mg/kg/day for 10 days)-induced cognitive deficits in mice were not improved by a single administration of CDPPB (10 mg/kg/day). However, PCP (10 mg/kg/day for 10 days)-induced cognitive deficits in mice were significantly improved by subsequent subchronic (14 days) administration of CDPPB (10 mg/kg/day), but not of CDPPB (1.0 mg/kg/day). This study suggests that PCP-induced cognitive deficits in mice are improved by subsequent subchronic administration of CDPPB. Therefore, mGlu₅ PAMs would be potential therapeutic drugs for cognitive deficits in schizophrenia.

Acute elevations of brain kynurenic acid impair cognitive flexibility: normalization by the alpha7 positive modulator galantamine

RATIONALE

Cognitive deficits represent a core symptom cluster in schizophrenia (SZ) that is predictive of outcome but not effectively treated by current antipsychotics. Thus, there is a need for validated animal models for testing potential pro-cognitive drugs.

OBJECTIVE

As kynurenic acid levels are increased in prefrontal cortex (PFC) of individuals with SZ, we acutely increased brain levels of this astrocyte-derived, negative modulator of alpha7 nicotinic acetylcholine receptors (α7nAChRs) by administration of its bioprecursor kynurenine and measured the effects on extracellular kynurenic acid and glutamate levels in PFC and also performance in a set-shifting task.

RESULTS

Injections of kynurenine (100 mg/kg, i.p.) increased extracellular kynurenic acid (1,500%) and decreased glutamate levels (30%) in PFC. Kynurenine also produced selective deficits in set-shifting. Saline- and kynurenine-treated rats similarly acquired the compound discrimination and intra-dimensional shift (saline, 7.0 and 6.3 trials, respectively; kynurenine, 8.0 and 6.7). Both groups required more trials to acquire the initial reversal (saline, 15.3; kynurenine, 22.2). Only kynurenine-treated rats were impaired in acquiring the extra-dimensional shift (saline, 8.2; kynurenine, 21.3). These deficits were normalized by administering the α7nAChR positive allosteric modulator galantamine (3.0 mg/kg, i.p) prior to kynurenine, as trials were comparable between galantamine + kynurenine (7.8) and controls (8.2). Bilateral local perfusion of the PFC with galantamine (5.0 μM) also attenuated kynurenine-induced deficits.

CONCLUSIONS

These results validate the use of animals with elevated brain kynurenic acid levels in SZ research and support studies of drugs that normalize brain kynurenic acid levels and/or positively modulate α7nAChRs as pro-cognitive treatments for SZ.

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.

The mGluR5 Positive Allosteric Modulator CDPPB Does Not Alter Extinction or Contextual Reinstatement of Methamphetamine-Seeking Behavior in Rats

Extinction of drug-seeking behavior is a form of new and active learning. Facilitation of extinction learning is of clinical interest since cue exposure therapies for the treatment of addiction have largely been unsuccessful in preventing relapse, primarily due to the context specificity of extinction learning. Recently, several studies have shown that potentiation of glutamatergic transmission can facilitate extinction learning in rodent models of cocaine addiction. In this study we investigated the effects of the type 5 metabotropic glutamate receptor (mGluR5) positive allosteric modulator (PAM) 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB) on the extinction and contextual reinstatement of methamphetamine-seeking behavior. Rats were trained and allowed to self-administer methamphetamine (0.1 mg/kg/infusion) in 2 hr daily sessions in Context A where self-administration chambers had distinct tactile, visual, auditory, and olfactory cues. Next, CDPPB (60 mg/kg) or vehicle was administered prior to subsequent extinction training sessions that were conducted in modified self-administration chambers (Context B) that were Context A. Following 16 days of extinction training in Context B, animals were placed back in Context A for assessment of contextual reinstatement of methamphetamine-seeking behavior. CDPPB failed to produce significant reductions in extinction responding or in the magnitude of contextual reinstatement of methamphetamine-seeking compared to vehicle treated controls. We postulate that numerous factors, including methamphetamine-induced changes in mGluR5 receptor expression or function, may have contributed to the observed lack of effects. Although these findings initially suggest that mGluR5 PAMs may be ineffective in facilitating extinction learning or preventing context-induced relapse in methamphetamine addiction, additional studies are warranted examining effects of other mGluR5 PAMs, particularly those with improved pharmacological properties and devoid of potential side effects at higher doses.

Targeting glutamate synapses in schizophrenia

Although early clinical observations implicated dopamine dysfunction in the neuropathology of schizophrenia, accumulating evidence suggests that multiple neurotransmitter pathways are dysregulated. The psychotomimetic actions of NMDA receptor antagonists point to an imbalance of glutamatergic signaling. Encouragingly, numerous preclinical and clinical studies have elucidated several potential targets for increasing NMDA receptor function and equilibrating glutamatergic tone, including the metabotropic glutamate receptors 2, 3 and 5, the muscarinic acetylcholine receptors M(1) and M(4), and the glycine transporter GlyT1. Highly specific allosteric and orthosteric ligands have been developed that modify the activity of these novel target proteins, and in this review we summarize both the glutamatergic mechanisms and the novel compounds that are increasing the promise for a multifaceted pharmacological approach to treat schizophrenia.

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.

Complex effects of mGluR5 antagonism on sociability and stereotypic behaviors in mice: possible implications for the pharmacotherapy of autism spectrum disorders

Balb/c mice display deficits of sociability; for example, they show reduced locomotor activity in the presence of an enclosed or freely-moving social stimulus mouse. Transgenic mice with defective or diminished expression of NMDA receptors manifest impaired sociability, while a partial and full agonist of the obligatory glycine co-agonist binding site on the NMDA receptor improved sociability in the Balb/c mouse strain. Because 2-methyl-6-(phenylethynyl)-pyridine (MPEP), an antagonist of the mGluR5 metabotropic glutamate receptor (mGluR), reduced self-grooming behavior in BTBR T+tfJ (BTBR) mice, another inbred genetic mouse model of autism spectrum disorders (ASDs), and mGluR5 antagonism is emerging as an experimental treatment for the 'fragile X syndrome," which has a high prevalence of co-morbid ASDs, we examined the effects of MPEP on sociability and stereotypic behaviors in Balb/c and Swiss Webster mice in a standard paradigm. MPEP had complex effects on sociability, impairing some measures of sociability in both strains, while it reduced the intensity of some spontaneous measures of stereotypic behaviors emerging during free social interaction in Swiss Webster mice. Conceivably, mGluR5 antagonism exacerbates diminished endogenous tone of NMDA receptor-mediated neurotransmission in neural circuits relevant to at least some measures of sociability in Balb/c mice; the mGluR5 receptor contributes to regulation of the phosphorylation status of the NMDA receptor. In any event, although stereotypies are an important therapeutic target in ASDs, medication strategies to attenuate their severity via antagonism of mGluR5 receptors must be pursued cautiously because of their potential to worsen at least some measures of sociability.

Selective mGluR5 antagonism attenuates the stress-induced reduction of MK-801's antiseizure potency in the genetically inbred Balb/c mouse

The ability of MK-801 (dizocilpine), a noncompetitive N-methyl D-aspartate (NMDA) antagonist, to antagonize electrical seizures is reduced in stressed mice. Stress-associated alterations in seizure susceptibility and diminished efficacy of antiseizure medications in humans have been reported [Joëls, 2009; Haut et al., 2007; Moshe et al., 2008]; thus, these experimental observations implicate altered endogenous tone of NMDA receptor-mediated neurotransmission in clinically adverse effects of stress on seizure proneness and treatment. The current exploratory experiment examined the effect of 2-methyl-6-(phenylethynyl)-pyridine (MPEP), an antagonist of mGluR5, administered prior to stress on the stress-induced reduction of MK-801's antiseizure effect in Swiss-Webster and Balb/c mice; the Balb/c mouse is behaviorally hypersensitive to MK-801. Interestingly, the data suggest that MPEP can attenuate the severity of the stress-induced reduction of MK-801's antiseizure effect in the Balb/c strain. Thus, mGluR5 could serve as a target for strategies for adjuvant treatment of seizures exacerbated by stress.

Egocentric working memory impairment and dendritic spine plastic changes in prefrontal neurons after NMDA receptor blockade in rats

Working memory may involve context-dependent allocentric or own movement-dependent egocentric strategies. While allocentric working memory can be disrupted by N-methyl-D-aspartate (NMDA) blockage, the possible effects of NMDA receptor manipulation on the egocentric strategy have not been studied. Because dendritic spine plasticity in part underlies working memory-related behavioral efficiency, egocentric working memory performance was evaluated in adult rats following NMDA receptor blockade with 10mg/kg of the NMDA-receptor antagonist CPP, i.p. Dendritic spine density and the proportion of different spine types (thin, stubby, mushroom, wide, branched and double) were assessed in third-layer pyramidal neurons of the dorsomedial prefrontal cortex, after behavioral testing. Working memory was evaluated by challenging the rats to resolve twelve trials per day in a single-day session over five consecutive days, in a "cross-arm" maze and according to a delayed match-to-sample procedure. In control animals, the dendritic spine density remained unchanged after behavioral testing, although the proportion of mushroom spines decreased while that of the branched spines increased. NMDA receptor blockade impaired the behavioral performance of rats and resulted in a decrease in dendritic spine density when compared to the control animals, and dendritic spine types were unchanged. These results suggest that behavioral efficiency of egocentric working memory is dependent on NMDA receptor activation, and that plastic changes in spine cytoarchitecture may play a key role in behavioral performance.

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.

Loss of object recognition memory produced by extended access to methamphetamine self-administration is reversed by positive allosteric modulation of metabotropic glutamate receptor 5

Chronic methamphetamine (meth) abuse can lead to persisting cognitive deficits. Here, we utilized a long-access meth self-administration (SA) protocol to assess recognition memory and metabotropic glutamate receptor (mGluR) expression, and the possible reversal of cognitive impairments with the mGluR5 allosteric modulator, 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl) benzamide (CDPPB). Male, Long-Evans rats self-administered i.v. meth (0.02 mg/infusion) on an FR1 schedule of reinforcement or received yoked-saline infusions. After seven daily 1-h sessions, rats were switched to 6-h daily sessions for 14 days, and then underwent drug abstinence. Rats were tested for object recognition memory at 1 week after meth SA at 90 min and 24 h retention intervals. In a separate experiment, rats underwent the same protocol, but received either vehicle or CDPPB (30 mg/kg) after familiarization. Rats were killed on day 8 or 14 post-SA and brain tissue was obtained. Meth intake escalated over the extended access period. Additionally, meth-experienced rats showed deficits in both short- and long-term recognition memory, demonstrated by a lack of novel object exploration. The deficit at 90 min was reversed by CDPPB treatment. On day 8, meth intake during SA negatively correlated with mGluR expression in the perirhinal and prefrontal cortex, and mGluR5 receptor expression was decreased 14 days after discontinuation of meth. This effect was specific to mGluR5 levels in the perirhinal cortex, as no differences were identified in the hippocampus or in mGluR2/3 receptors. These results from a clinically-relevant animal model of addiction suggest that mGluR5 receptor modulation may be a potential treatment of cognitive dysfunction in meth addiction.

Cognitive enhancers for facilitating drug cue extinction: insights from animal models

Given the success of cue exposure (extinction) therapy combined with a cognitive enhancer for reducing anxiety, it is anticipated that this approach will prove more efficacious than exposure therapy alone in preventing relapse in individuals with substance use disorders. Several factors may undermine the efficacy of exposure therapy for substance use disorders, but we suspect that neurocognitive impairments associated with chronic drug use are an important contributing factor. Numerous insights on these issues are gained from research using animal models of addiction. In this review, the relationship between brain sites whose learning, memory and executive functions are impaired by chronic drug use and brain sites that are important for effective drug cue extinction learning is explored first. This is followed by an overview of animal research showing improved treatment outcome for drug addiction (e.g. alcohol, amphetamine, cocaine, heroin) when explicit extinction training is conducted in combination with acute dosing of a cognitive-enhancing drug. The mechanism by which cognitive enhancers are thought to exert their benefits is by facilitating consolidation of drug cue extinction memory after activation of glutamatergic receptors. Based on the encouraging work in animals, factors that may be important for the treatment of drug addiction are considered.

Methamphetamine self-administration produces attentional set-shifting deficits and alters prefrontal cortical neurophysiology in rats

BACKGROUND

Chronic methamphetamine abusers exhibit deficits in tasks requiring intact prefrontal cortex function, and prefrontal cortex dysfunction has been implicated in the loss of control over drug use. This study used a combination of behavioral and electrophysiologic assessments in rats with a history of long access methamphetamine self-administration to determine methamphetamine-induced changes in prefrontal cortex-dependent attentional set-shifting performance, drug-seeking, and prefrontal cortex neuronal activity.

METHODS

Male Long-Evans rats self-administered methamphetamine (.02 mg/infusion, intravenous) or received yoked saline infusions for 6 hours a day for 14 days. Cognitive flexibility was assessed using an attentional set-shifting task before 2 weeks of self-administration and 1 day after self-administration. Animals then underwent 11 days of abstinence, followed by three subsequent tests for context-induced drug seeking. Finally, animals were anesthetized, and single-unit in vivo extracellular recordings were performed in the dorsomedial prefrontal cortex.

RESULTS

Methamphetamine-experienced rats showed escalated drug intake and context-induced drug-seeking following abstinence. During the extradimensional set-shift component, meth-experienced rats showed selective impairments that were identical to deficits produced by excitotoxic lesions of the prefrontal cortex. Rats with a history of chronic methamphetamine intake also exhibited higher basal firing frequency and a significantly greater proportion of burst-firing cells in the prefrontal cortex compared with yoked-saline controls.

CONCLUSIONS

Prefrontal cortex-specific alterations in neuronal function may play a key role in methamphetamine-induced attentional deficits and drug-seeking. These data support the possibility that targeting prefrontal cortex pathology may improve treatment outcome in methamphetamine addiction.

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.

Interactive effects of mGlu5 and 5-HT2A receptors on locomotor activity in mice

RATIONALE

Metabotropic glutamate (mGlu) receptors have been suggested to play a role in neuropsychiatric disorders including schizophrenia, drug abuse, and depression. Because serotonergic hallucinogens increase glutamate release and mGlu receptors modulate the response to serotonin (5-HT)(2A) activation, the interactions between serotonin 5-HT(2A) receptors and mGlu receptors may prove to be important for our understanding of these diseases.

OBJECTIVE

We tested the effects of the serotonergic hallucinogen and 5-HT(2A) agonist, 2,5-dimethoxy-4-methylamphetamine (DOM), and the selective 5-HT(2A) antagonist, M100907, on locomotor activity in the mouse behavioral pattern monitor (BPM) in mGlu5 wild-type (WT) and knockout (KO) mice on a C57 background.

RESULTS

Both male and female mGlu5 KO mice showed locomotor hyperactivity and diminished locomotor habituation compared with their WT counterparts. Similarly, the mGlu5-negative allosteric modulator 2-methyl-6-(phenylethynyl)pyridine (MPEP) also increased locomotor hyperactivity, which was absent in mGlu5 KO mice. The locomotor hyperactivity in mGlu5 receptor KO mice was potentiated by DOM (0.5 mg/kg, subcutaneously (SC)) and attenuated by M100907 (1.0 mg/kg, SC). M100907 (0.1 mg/kg, SC) also blocked the hyperactivity induced by MPEP.

CONCLUSIONS

These studies demonstrated that loss of mGlu5 receptor activity either pharmacologically or through gene deletion leads to locomotor hyperactivity in mice. Additionally, the gene deletion of mGlu5 receptors increased the behavioral response to the 5-HT(2A) agonist DOM, suggesting that mGlu5 receptors either mitigate the behavioral effects of 5-HT(2A) hallucinogens or that mGlu5 KO mice show an increased sensitivity to 5-HT(2A) agonists. Taken together, these studies indicate a functional interaction between mGlu5 and 5-HT(2A) receptors.

Clinical physiology and mechanism of dizocilpine (MK-801): electron transfer, radicals, redox metabolites and bioactivity

Dizocilpine (MK-801), an extensively investigated drug possessing secondary amine and benzenoid functions, displays a wide array of biological properties, including anticonvulsant and anesthetic. There is scant discussion of biomechanism. A relevant, important finding is formation of oxidative metabolites in the hydroxylamine and phenolic categories. Analogy to cocaine metabolites suggests participation of redox entities, such as, hydroxylamine, nitroxide and nitrosonium, which can lead to electron transfer and radical formation. There is also similarity to metabolism by 3,3′-iminodipropionitrile and phencyclidine. Alternatively, the phenolic metabolites are well-known precursors of ET quinones. The review documents various physiological effects, mainly involving the central nervous system. Also of interest are the pro- and anti-oxidant properties. Considerable attention has been paid to MK-801 as an antagonist of the N-methyl-D-aspartate receptor in the glutamate category. This aspect is often associated with effects on the central nervous system. The review also provides recent literature dealing with MK-801/NMDA receptor in various areas of bioactivity. Studies were made of MK-801 involvement in working memory processing. Deficits in behavior were noted after administration of the drug. Treatment of mice with dizocilpine induced learning impairment. The influence of MK-801 on fear has been investigated. The substance is known to exert an analgesic effect in pain control. A number of reports deal with anesthetic properties.

Differential effects of the mGluR5 positive allosteric modulator CDPPB in the cortex and striatum following repeated administration

The glutamatergic hypofunction hypothesis of schizophrenia has led to the development of novel therapeutic strategies modulating NMDA receptor function. One of these strategies targets the activation of the metabotropic glutamate receptor 5 (mGlu5 receptor) using positive allosteric modulators (PAMs). Our goal was to evaluate the potential for repeated administration of the mGlu5 receptor PAM, CDPPB (3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide) (30 mg/kg) to induce tolerance to the anti-psychotic like effect using the amphetamine-induced hyperlocomotion rat model, and to produce receptor desensitization in mGlu5 receptor-enriched brain regions. CDPPB dose dependently reduced the locomotor response to amphetamine when administered acutely, and the same effect was observed following 7-day pre-treatment regime. In addition, 7-day dosing of CDPPB did not affect mGlu5 receptor density in the striatum, nor did it change mGlu5 receptor PAM-induced phosphorylation of NMDA, GluN1 and GluN2b, receptor subunits in striatum compared to the levels measured acutely. In contrast, in the frontal cortex, repeated administration of CDPPB decreased mGlu5 receptor density and resulted in a loss of its ability to increase GluN1 and GluN2b levels. Consistent with a reduction of cortical mGlu5 receptor density and phosphorylation, CDPPB (30 mg/kg) significantly affected sleep architecture as determined by cortical EEG at day one however by the seventh day of dosing all sleep changes were absent. Together these results suggest that the development of tolerance induced by the repeated treatment with the mGlu5 receptor PAM, CDPPB, may depend not only on the system being measured (sleep architecture vs psychostimulant induced hyperactivity), but also on the brain region involved with frontal cortex being a more susceptible region to receptor desensitization and internalization than striatum.

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.

Disruption of performance in the five-choice serial reaction time task induced by administration of N-methyl-D-aspartate receptor antagonists: relevance to cognitive dysfunction in schizophrenia

Schizophrenia patients suffer from cognitive impairments that are not satisfactorily treated by currently available medications. Cognitive dysfunction in schizophrenia encompasses deficits in several cognitive modalities that can be differentially responsive to different medications and are likely to be mediated by different neurobiological substrates. Translational animal models of cognitive deficits with relevance to schizophrenia are critical for gaining insights into the mechanisms underlying these impairments and developing more effective treatments. The five-choice serial reaction time task (5-CSRTT) is a cognitive task used in rodents that allows simultaneous assessment of several cognitive modalities, including attention, response inhibition, cognitive flexibility, and processing speed. Administration of N-methyl-D-aspartate (NMDA) glutamate receptor antagonists disrupts multiple 5-CSRTT performance measures in a way that mirrors various cognitive deficits exhibited by schizophrenia patients. Some of these disruptions are partially attenuated by antipsychotic medications that exhibit partial effectiveness on cognitive dysfunction in schizophrenia, suggesting that the model has predictive validity. Examination of the effects of pharmacological manipulations on 5-CSRTT performance disruptions induced by NMDA antagonists have implicated a range of brain regions, neurotransmitter systems, and specific receptor subtypes in schizophrenia-like impairment of different cognitive modalities. Thus, disruption of 5-CSRTT performance by NMDA antagonists represents a valuable tool for exploring the neurobiological bases of cognitive dysfunction in schizophrenia.

Serum glutamine, set-shifting ability and anorexia nervosa

BACKGROUND

Set-shifting is impaired in people with anorexia nervosa (AN), but the underlying physiological and biochemical processes are unclear. Animal studies have established that glutamatergic pathways in the prefrontal cortex play an important role in set-shifting ability. However, it is not yet understood whether levels of serum glutamatergic amino acids are associated with set-shifting performance in humans. The aim of this study was to determine whether serum concentrations of amino acids related to glutamatergic neurotransmission (glutamine, glutamate, glycine, l-serine, d-serine) are associated with set-shifting ability in people with acute AN and those after recovery.

METHODS

Serum concentrations of glutamatergic amino acids were measured in 27 women with current AN (AN group), 18 women recovered from AN (ANRec group) and 28 age-matched healthy controls (HC group). Set-shifting was measured using the Wisconsin Card Sorting Test (WCST) and the Trail Making Task (TMT). Dimensional measures of psychopathology were used, including the Eating Disorder Examination Questionnaire (EDEQ), the Maudsley Obsessive-Compulsive Inventory (MOCI) and the Hospital Anxiety and Depression Scale (HADS).

RESULTS

Serum glutamine concentrations in the AN group (1,310.2 +/- 265.6 muM, mean +/- SD) were significantly higher (by approximately 20%) than those in the HC group (1,102.9 +/- 152.7 muM, mean +/- SD) (F(2, 70) = 6.3, P = 0.003, 95% CI 61.2 to 353.4). Concentrations of serum glutamine were positively associated with markers of the illness severity: a negative correlation was present between serum glutamine concentrations and body mass index (BMI) and lowest BMI and a positive correlation was found between duration of illness and EDEQ. The AN group showed significantly impaired set shifting in the WCST, both total errors, and perseverative errors. In the AN group, there were no correlations between serum glutamine concentrations and set shifting.

CONCLUSIONS

Serum concentrations of glutamine may be a biomarker of illness severity in people with AN. It does not appear to be directly associated with changes in executive function.

Setdb1 histone methyltransferase regulates mood-related behaviors and expression of the NMDA receptor subunit NR2B

Histone methyltransferases specific for the histone H3-lysine 9 residue, including Setdb1 (Set domain, bifurcated 1)/Eset/Kmt1e are associated with repressive chromatin remodeling and expressed in adult brain, but potential effects on neuronal function and behavior remain unexplored. Here, we report that transgenic mice with increased Setdb1 expression in adult forebrain neurons show antidepressant-like phenotypes in behavioral paradigms for anhedonia, despair, and learned helplessness. Chromatin immunoprecipitation in conjunction with DNA tiling arrays (ChIP-chip) revealed that genomic occupancies of neuronal Setdb1 are limited to <1% of annotated genes, which include the NMDA receptor subunit NR2B/Grin2B and other ionotropic glutamate receptor genes. Chromatin conformation capture and Setdb1-ChIP revealed a loop formation tethering the NR2B/Grin2b promoter to the Setdb1 target site positioned 30 kb downstream of the transcription start site. In hippocampus and ventral striatum, two key structures in the neuronal circuitry regulating mood-related behaviors, Setdb1-mediated repressive histone methylation at NR2B/Grin2b was associated with decreased NR2B expression and EPSP insensitivity to pharmacological blockade of NR2B, and accelerated NMDA receptor desensitization consistent with a shift in NR2A/B subunit ratios. In wild-type mice, systemic treatment with the NR2B antagonist, Ro25-6981 [R-(R,S)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperidine propranol], and hippocampal small interfering RNA-mediated NR2B/Grin2b knockdown resulted in behavioral changes similar to those elicited by the Setdb1 transgene. Together, these findings point to a role for neuronal Setdb1 in the regulation of affective and motivational behaviors through repressive chromatin remodeling at a select set of target genes, resulting in altered NMDA receptor subunit composition and other molecular adaptations.

Metabotropic glutamate receptors: potential drug targets for psychiatric disorders

Metabotropic glutamate receptors (mGlu receptors) have emerged as new therapeutic targets for psychiatric disorders, such as schizophrenia, depression and anxiety with their regulatory roles in glutamatergic transmissions. To date, several ligands selective for each mGlu receptor have been synthesized, and pharmacological significances of these ligands have been demonstrated in animal models. Among them, mGlu2/3 receptor agonists have been proven to be effective for treating schizophrenia and anxiety disorders in clinical studies, which may prove utilities of mGlu receptor ligands for the treatment of psychiatric disorders. This article reviews recent advances in development of each mGlu receptor ligands and their therapeutic potential.

Cognitive effects of Group I metabotropic glutamate receptor ligands in the context of drug addiction

Glutamate plays a pivotal role in regulating drug self-administration and drug-seeking behavior, and the past decade has witnessed a substantial surge of interest in the role of Group I metabotropic glutamate receptors (mGlu(1) and mGlu(5) receptors) in mediating these behaviors. As will be reviewed here, Group I mGlu receptors are involved in normal and drug-induced synaptic plasticity, drug reward, reinforcement and relapse-like behaviors, and addiction-related cognitive processes such as maladaptive learning and memory, behavioral inflexibility, and extinction learning. Animal models of addiction have revealed that antagonists of Group I mGlu receptors, particularly the mGlu(5) receptor, reduce self-administration of virtually all drugs of abuse. Since inhibitors of mGlu5 receptor function have now entered clinical trials for other medical conditions and appear to be well-tolerated, a key question that remains unanswered is - what changes in cognition are produced by these compounds that result in reduced drug intake and drug-seeking behavior? Finally, in contrast to mGlu(5) receptor antagonists, recent studies have indicated that positive allosteric modulation of mGlu(5) receptors actually enhances synaptic plasticity and improves various aspects of cognition, including spatial learning, behavioral flexibility, and extinction of drug-seeking behavior. Thus, while inhibition of Group I mGlu receptor function may reduce drug reward, reinforcement, and relapse-related behaviors, positive allosteric modulation of the mGlu5 receptor subtype may actually enhance cognition and potentially reverse some of the cognitive deficits associated with chronic drug use.

Group 5 metabotropic glutamate receptors: role in modulating cortical activity and relevance to cognition

Group 5 metabotropic glutamate (mGlu(5)) receptors are abundant in forebrain and limbic regions and provide a novel pharmacological target for modulation of cognition. Here, we review recent advances in understanding the electrophysiology of these receptors which reveal a role for mGlu(5) receptors in the regulation of tonic and bursting modes of neuronal firing, maintenance of distinct forms of synaptic plasticity, and reversal of detrimental effects of NMDA receptor antagonism on cortical neuronal activity. Furthermore, recordings using recently developed positive allosteric modulators of the mGlu(5) receptor suggest that these agents have an electrophysiological profile comparable to the antipsychotic agent clozapine. These findings, in conjunction with behavioral evidence from preclinical studies of cognition, suggest a potential precognitive profile for the mGlu(5) receptor potentiators.

Metabotropic glutamate receptors: physiology, pharmacology, and disease

The metabotropic glutamate receptors (mGluRs) are family C G-protein-coupled receptors that participate in the modulation of synaptic transmission and neuronal excitability throughout the central nervous system. The mGluRs bind glutamate within a large extracellular domain and transmit signals through the receptor protein to intracellular signaling partners. A great deal of progress has been made in determining the mechanisms by which mGluRs are activated, proteins with which they interact, and orthosteric and allosteric ligands that can modulate receptor activity. The widespread expression of mGluRs makes these receptors particularly attractive drug targets, and recent studies continue to validate the therapeutic utility of mGluR ligands in neurological and psychiatric disorders such as Alzheimer's disease, Parkinson's disease, anxiety, depression, and schizophrenia.

MK-801 produces a deficit in sucrose preference that is reversed by clozapine, D-serine, and the metabotropic glutamate 5 receptor positive allosteric modulator CDPPB: relevance to negative symptoms associated with schizophrenia?

Currently prescribed antipsychotics attenuate the positive symptoms of schizophrenia but fail or only mildly improve negative symptoms. The present study aimed to establish an animal model of negative symptoms by examining the effects of the NMDA receptor antagonist MK-801 on sucrose preference. We sought to validate the model by examining the effects of clozapine and D-serine, for which there are positive clinical data regarding their effects on negative symptoms, and haloperidol which is clinically ineffective. We extended our analysis by examining CDPPB, an mGlu5 receptor positive allosteric modulator. Acute MK-801 produced effects indicative of a shift in the hedonic experience of sucrose not confounded by disruptions in motor abilities or taste as revealed by: 1) a decrease in sucrose intake at low concentrations (0.8% or 1.2%), but no effect on water, 2) an increase in consumption for higher (7%) sucrose concentrations, reflecting a shift to the right in the concentration-consumption curve, and 3) no effect on quinine intake. Sub-chronic clozapine and acute d-serine attenuated the MK-801-induced deficit in 1.2% sucrose consumption, whereas sub-chronic haloperidol (0.02 mg/kg) did not. Finally, acute treatment with CDPPB also attenuated this deficit. These data suggest that this model may be useful for identifying novel agents that improve negative symptoms, and that compounds which enhance NMDA receptor function, such as mGlu5 receptor PAMs, may have clinical utility in this regard.

Glutamatergic (N-methyl-D-aspartate receptor) hypofrontality in schizophrenia: too little juice or a miswired brain?

Dopamine D2 receptor blockade has been an obligate mechanism of action present in all medications that effectively treat positive symptoms of schizophrenia (e.g., delusions and hallucinations) and have been approved by regulatory agencies since the 1950s. Blockade of 5-hydroxytryptamine(2A) receptors plays a contributory role in the actions of the second generation of antipsychotic drugs, the so-called atypical antipsychotics. Nevertheless, substantial unmet medical needs remain for the treatment of negative symptoms and cognitive dysfunction. Recognition that dissociative anesthetics block the N-methyl-D-aspartate (NMDA) receptor channel has inspired a search for glutamatergic therapeutic mechanisms because ketamine and phencyclidine are known to induce psychotic-like symptoms in healthy volunteers and exacerbate the symptoms of patients with schizophrenia. Current pathophysiological theories of schizophrenia emphasize that hypofunction of NMDA receptors at critical sites in local circuits modulate the function of a given brain region or control projections from one region to another (e.g., hippocampal-cortical or thalamocortical projections). The demonstration that a metabotropic glutamate 2/3 (mGlu2/3) receptor agonist prodrug decreased both positive and negative symptoms of schizophrenia raised hopes that glutamatergic mechanisms may provide therapeutic advantages. In addition to discussing the activation of mGlu2 receptors with mGlu2/3 receptor agonists or mGlu2 receptor positive allosteric modulators (PAMs), we discuss other methods that may potentially modulate circuits with hypofunctional NMDA receptors such as glycine transporter inhibitors and mGlu5 receptor PAMs. The hope is that by modulating glutamatergic neurotransmission, the dysfunctional circuitry of the schizophrenic brain (both local circuits and long-loop pathways) will be improved.

The promiscuous mGlu5 receptor--a range of partners for therapeutic possibilities?

The issue of non-specific effects for potential therapeutics is particularly salient in neurological/psychiatric disorders, where adverse drug reactions could impair critical brain functions. The issue of specificity is not limited to candidate molecules, as receptor targets themselves often influence physiological as well as pathological outcomes. Metabotropic glutamate receptor 5 (mGlu5) is an example of a "promiscuous" receptor target that has been implicated in addiction, but also many other processes. However, if receptor modulation could be restricted to specific pathways/brain regions, mGlu5 may still prove to be a viable therapeutic target for various indications. Using this premise, a number of possible methods to refine drug development strategy are discussed, including exploiting specific interactions of mGlu5 with other receptors to narrow the influence of pharmacological agents, and also the use of RNA interference targeted to specific cells/regions of the brain.