Fractionation of spatial memory in GRM2/3 (mGlu2/mGlu3) double knockout mice reveals a role for group II metabotropic glutamate receptors at the interface between arousal and cognition

Impairments in the early consolidation of spatial memories via group II mGluR agonism in the mammillary bodies

mGluR2 receptors are widely expressed in limbic brain regions associated with memory, including the hippocampal formation, retrosplenial and frontal cortices, as well as subcortical regions including the mammillary bodies. mGluR2/3 agonists have been proposed as potential therapeutics for neurological and psychiatric disorders, however, there is still little known about the role of these receptors in cognitive processes, including memory consolidation. To address this, we assessed the effect of the mGluR2/3 agonist, eglumetad, on spatial memory consolidation in both mice and rats. Using the novel place preference paradigm, we found that post-sample injections of eglumetad impaired subsequent spatial discrimination when tested 6 h later. Using the immediate early gene c-fos as a marker of neural activity, we showed that eglumetad injections reduced activity in a network of limbic brain regions including the hippocampus and mammillary bodies. To determine whether the systemic effects could be replicated with more targeted manipulations, we performed post-sample infusions of the mGluR2/3 agonist 2R,4R-APDC into the mammillary bodies. This impaired novelty discrimination on a place preference task and an object-in-place task, again highlighting the role of mGluR2/3 transmission in memory consolidation and demonstrating the crucial involvement of the mammillary bodies in post-encoding processing of spatial information.

Behavioral Analysis of NMDAR Function in Rodents: Tests of Long-Term Spatial Memory

NMDAR-dependent forms of synaptic plasticity in brain regions like the hippocampus are widely believed to provide the neural substrate for long-term associative memory formation. However, the experimental data are equivocal at best and may suggest a more nuanced role for NMDARs and synaptic plasticity in memory. Much of the experimental data available comes from studies in genetically modified mice in which NMDAR subunits have been deleted or mutated in order to disrupt NMDAR function. Behavioral assessment of long-term memory in these mice has involved tests like the Morris watermaze and the radial arm maze. Here we describe these behavioral tests and some of the different testing protocols that can be used to assess memory performance. We discuss the importance of distinguishing selective effects on learning and memory processes from nonspecific effects on sensorimotor or motivational aspects of performance.

Metabotropic glutamate receptor function and regulation of sleep-wake cycles

Metabotropic glutamate (mGlu) receptors are the most abundant family of G-protein coupled receptors and are widely expressed throughout the central nervous system (CNS). Alterations in glutamate homeostasis, including dysregulations in mGlu receptor function, have been indicated as key contributors to multiple CNS disorders. Fluctuations in mGlu receptor expression and function also occur across diurnal sleep-wake cycles. Sleep disturbances including insomnia are frequently comorbid with neuropsychiatric, neurodevelopmental, and neurodegenerative conditions. These often precede behavioral symptoms and/or correlate with symptom severity and relapse. Chronic sleep disturbances may also be a consequence of primary symptom progression and can exacerbate neurodegeneration in disorders including Alzheimer's disease (AD). Thus, there is a bidirectional relationship between sleep disturbances and CNS disorders; disrupted sleep may serve as both a cause and a consequence of the disorder. Importantly, comorbid sleep disturbances are rarely a direct target of primary pharmacological treatments for neuropsychiatric disorders even though improving sleep can positively impact other symptom clusters. This chapter details known roles of mGlu receptor subtypes in both sleep-wake regulation and CNS disorders focusing on schizophrenia, major depressive disorder, post-traumatic stress disorder, AD, and substance use disorder (cocaine and opioid). In this chapter, preclinical electrophysiological, genetic, and pharmacological studies are described, and, when possible, human genetic, imaging, and post-mortem studies are also discussed. In addition to reviewing the important relationships between sleep, mGlu receptors, and CNS disorders, this chapter highlights the development of selective mGlu receptor ligands that hold promise for improving both primary symptoms and sleep disturbances.

Metabotropic glutamate receptors and cognition: From underlying plasticity and neuroprotection to cognitive disorders and therapeutic targets

Metabotropic glutamate (mGlu) receptors are G protein-coupled receptors that play pivotal roles in mediating the activity of neurons and other cell types within the brain, communication between cell types, synaptic plasticity, and gene expression. As such, these receptors play an important role in a number of cognitive processes. In this chapter, we discuss the role of mGlu receptors in various forms of cognition and their underlying physiology, with an emphasis on cognitive dysfunction. Specifically, we highlight evidence that links mGlu physiology to cognitive dysfunction across brain disorders including Parkinson's disease, Alzheimer's disease, Fragile X syndrome, post-traumatic stress disorder, and schizophrenia. We also provide recent evidence demonstrating that mGlu receptors may elicit neuroprotective effects in particular disease states. Lastly, we discuss how mGlu receptors can be targeted utilizing positive and negative allosteric modulators as well as subtype specific agonists and antagonist to restore cognitive function across these disorders.

Modulation of Hippocampal Network Oscillation by PICK1-Dependent Cell Surface Expression of mGlu3 Receptors

Metabotropic glutamate receptor Type 3 (mGlu3) controls the sleep/wake architecture, which plays a role in the glutamatergic pathophysiology of schizophrenia. Interestingly, mGlu3 receptor expression is decreased in the brain of schizophrenic patients. However, little is known about the molecular mechanisms regulating mGlu3 receptors at the cell membrane. Subcellular receptor localization is strongly dependent on protein-protein interactions. Here we show that mGlu3 interacts with PICK1 and that this scaffolding protein is important for mGlu3 surface expression and function in hippocampal primary cultures. Disruption of their interaction via an mGlu3 C-terminal mimicking peptide or an inhibitor of the PDZ domain of PICK1 altered the functional expression of mGlu3 receptors in neurons. We next investigated the impact of disrupting the mGlu3-PICK1 interaction on hippocampal theta oscillations in vitro and in vivo in WT male mice. We found a decreased frequency of theta oscillations in organotypic hippocampal slices, similar to what was previously observed in mGlu3 KO mice. In addition, hippocampal theta power was reduced during rapid eye movement sleep, non-rapid eye movement (NREM) sleep, and wake states after intraventricular administration of the mGlu3 C-terminal mimicking peptide. Targeting the mGlu3-PICK1 complex could thus be relevant to the pathophysiology of schizophrenia.SIGNIFICANCE STATEMENT Dysregulation of the glutamatergic system might play a role in the pathophysiology of schizophrenia. Metabotropic glutamate receptors Type 3 (mGlu3) have been proposed as potential targets for schizophrenia. Understanding the molecular mechanisms regulating mGlu3 receptor at the cell membrane is critical toward comprehending how their dysfunction contributes to the pathogenesis of schizophrenia. Here we describe that the binding of the signaling and scaffolding protein PICK1 to mGlu3 receptors is important for their localization and physiological functions. The identification of new proteins that associate specifically to mGlu3 receptors will advance our understanding of the regulatory mechanisms associated with their targeting and function and ultimately might provide new therapeutic strategies to counter these psychiatric conditions.

Spread of pathological human Tau from neurons to oligodendrocytes and loss of high-firing pyramidal neurons in aging mice

Intracellular aggregation of hyperphosphorylated Tau (pTau) in the brain is associated with cognitive and motor impairments, and ultimately neurodegeneration. We investigate how human pTau affects cells and network activity in the hippocampal formation of the THY-Tau22 tauopathy model mice in vivo. We find that pTau preferentially accumulates in deep-layer pyramidal neurons, leading to neurodegeneration, and we establish that pTau spreads to oligodendrocytes. During goal-directed virtual navigation in aged transgenic mice, we detect fewer high-firing prosubicular pyramidal cells, but the firing population retains its coupling to theta oscillations. Analysis of network oscillations and firing patterns of pyramidal and GABAergic neurons recorded in head-fixed and freely moving mice suggests preserved neuronal coordination. In spatial memory tests, transgenic mice have reduced short-term familiarity, but spatial working and reference memory are surprisingly normal. We hypothesize that unimpaired subcortical network mechanisms maintain cortical neuronal coordination, counteracting the widespread pTau aggregation, loss of high-firing cells, and neurodegeneration.

Exploration of group II metabotropic glutamate receptor modulation in mouse models of Rett syndrome and MECP2 Duplication syndrome

Rett syndrome (RTT) and MECP2 Duplication syndrome (MDS) have opposing molecular origins in relation to expression and function of the transcriptional regulator Methyl-CpG-binding protein 2 (MeCP2). Several clinical and preclinical phenotypes, however, are shared between these disorders. Modulation of MeCP2 levels has recently emerged as a potential treatment option for both of these diseases. However, toxicity concerns remain with these approaches. Here, we focus on pharmacologically modulating the group II metabotropic glutamate receptors (mGlu), mGlu₂ and mGlu₃, which are two downstream targets of MeCP2 that are bidirectionally affected in expression in RTT patients and mice (Mecp2^Null/+) versus an MDS mouse model (MECP2^Tg1/o). Mecp2^Null/+ and MECP2^Tg1/o animals also exhibit contrasting phenotypes in trace fear acquisition, a form of temporal associative learning and memory, with trace fear deficiency observed in Mecp2^Null/+ mice and abnormally enhanced trace fear acquisition in MECP2^Tg1/o animals. In Mecp2^Null/+ mice, treatment with the mGlu_2/3 agonist LY379268 reverses the deficit in trace fear acquisition, and mGlu_2/3 antagonism with LY341495 normalizes the abnormal trace fear learning and memory phenotype in MECP2^Tg1/o mice. Altogether, these data highlight the role of group II mGlu receptors in RTT and MDS and demonstrate that both mGlu₂ and mGlu₃ may be potential therapeutic targets for these disorders.

Apparatus design and behavioural testing protocol for the evaluation of spatial working memory in mice through the spontaneous alternation T-maze

Spatial working memory can be assessed in mice through the spontaneous alternation T-maze test. The T-maze is a T-shaped apparatus featuring a stem (start arm) and two lateral goal arms (left and right arms). The procedure is based on the natural tendency of rodents to prefer exploring a novel arm over a familiar one, which induces them to alternate the choice of the goal arm across repeated trials. During the task, in order to successfully alternate choices across trials, an animal has to remember which arm had been visited in the previous trial, which makes spontaneous alternation T-maze an optimal test for spatial working memory. As this test relies on a spontaneous behaviour and does not require rewards, punishments or pre-training, it represents a particularly useful tool for cognitive evaluation, both time-saving and animal-friendly. We describe here in detail the apparatus and the protocol, providing representative results on wild-type healthy mice.

Prelimbic cortical stimulation disrupts fear memory consolidation through ventral hippocampal dopamine D2 receptors

BACKGROUND AND PURPOSE

Anxiety disorders pose one of the biggest threats to mental health worldwide, yet current therapeutics have been mostly ineffective due to issues with relapse, efficacy and toxicity of the medications. Deep brain stimulation (DBS) is a promising therapy for treatment-resistant psychiatric disorders including anxiety, but very little is known about the effects of deep brain stimulation on fear memories.

EXPERIMENTAL APPROACH

In this study, we employed a standard tone-footshock fear conditioning paradigm and modified plus maze discriminative avoidance task to probe the effects of prelimbic cortex deep brain stimulation on various stages of memory.

KEY RESULTS

We identified memory consolidation stage as a critical time point to disrupt fear memory via prelimbic cortex deep brain stimulation. The observed disruption was partially modulated by the inactivation of the ventral hippocampus and the transient changes in ventral hippocampus dopamine (D₂ ) receptors expression upon prelimbic cortex deep brain stimulation. We also observed wide-scale changes of various neurotransmitters and their metabolites in ventral hippocampus, confirming its important role in response to prelimbic cortex deep brain stimulation.

CONCLUSION AND IMPLICATIONS

These findings highlight the molecular mechanism in the ventral hippocampus in response to prelimbic cortex stimulation and may have translational value, indicating that targeting the prelimbic cortex in the memory consolidation stage via non-invasive neuromodulation techniques may be a feasible therapeutic strategy against anxiety disorders.

Hippocampal Hyperactivity as a Druggable Circuit-Level Origin of Aberrant Salience in Schizophrenia

The development of current neuroleptics was largely aiming to decrease excessive dopaminergic signaling in the striatum. However, the notion that abnormal dopamine creates psychotic symptoms by causing an aberrant assignment of salience that drives maladaptive learning chronically during disease development suggests a therapeutic value of early interventions that correct salience-related neural processing. The mesolimbic dopaminergic output is modulated by several interconnected brain-wide circuits centrally involving the hippocampus and key relays like the ventral and associative striatum, ventral pallidum, amygdala, bed nucleus of the stria terminalis, nucleus reuniens, lateral and medial septum, prefrontal and cingulate cortex, among others. Unraveling the causal relationships between these circuits using modern neuroscience techniques holds promise for identifying novel cellular—and ultimately molecular—treatment targets for reducing transition to psychosis and symptoms of schizophrenia. Imaging studies in humans have implicated a hyperactivity of the hippocampus as a robust and early endophenotype in schizophrenia. Experiments in rodents, in turn, suggested that the activity of its output region—the ventral subiculum—may modulate dopamine release from ventral tegmental area (VTA) neurons in the ventral striatum. Even though these observations suggested a novel circuit-level target for anti-psychotic action, no therapy has yet been developed along this rationale. Recently evaluated treatment strategies—at least in part—target excess glutamatergic activity, e.g. N-acetyl-cysteine (NAC), levetiracetam, and mGluR2/3 modulators. We here review the evidence for the central implication of the hippocampus-VTA axis in schizophrenia-related pathology, discuss its symptom-related implications with a particular focus on aberrant assignment of salience, and evaluate some of its short-comings and prospects for drug discovery.

LY379268 Does Not Have Long-Term Procognitive Effects nor Attenuate Glutamatergic Signaling in AβPP/PS1 Mice

Chronically elevated basal glutamate levels are hypothesized to attenuate detection of physiological signals thereby inhibiting memory formation and retrieval, while inducing excitotoxicity-mediated neurodegeneration observed in Alzheimer's disease (AD). However, current medication targeting the glutamatergic system, such as memantine, shows limited efficacy and is unable to decelerate disease progression, possibly because it modulates postsynaptic N-methyl-D-aspartate receptors rather than glutamate release or clearance. To determine if decreasing presynaptic glutamate release leads to long-term procognitive effects, we treated AβPP/PS1 mice with LY379268 (3.0 mg/kg; i.p.), a metabotropic glutamate receptor (mGluR)2/3 agonist from 2-6 months of age when elevated glutamate levels are first observed but cognition is unaffected. C57BL/6J genetic background control mice and another cohort of AβPP/PS1 mice received normal saline (i.p.) as vehicle controls. After 6 months off treatment, mice receiving LY379268 did not show long-term improvement as assessed by the Morris water maze (MWM) spatial learning and memory paradigm. Following MWM, mice were isoflurane anesthetized and a glutamate selective microelectrode was used to measure in vivo basal and stimulus-evoked glutamate release and clearance independently from the dentate, CA3, and CA1 hippocampal subregions. Immunohistochemistry was used to measure hippocampal astrogliosis and plaque pathology. Similar to previous studies, we observed elevated basal glutamate, stimulus evoked glutamate release, and astrogliosis in AβPP/PS1 vehicle mice versus C57BL/6J mice. Treatment with LY379268 did not attenuate these responses nor diminish plaque pathology. The current study builds upon previous research demonstrating hyperglutamatergic hippocampal signaling in AβPP/PS1 mice; however, long-term therapeutic efficacy of LY379268 in AβPP/PS1 was not observed.

The role of glutamate receptors in attention-deficit/hyperactivity disorder: From physiology to disease

Attention-deficit hyperactivity disorder (ADHD) is the most common psychiatric disorder in children and adolescents, which is characterized by behavioral problems such as attention deficit, hyperactivity, and impulsivity. As the receptors of the major excitatory neurotransmitter in the mammalian central nervous system (CNS), glutamate receptors (GluRs) are strongly linked to normal brain functioning and pathological processes. Extensive investigations have been made about the structure, function, and regulation of GluR family, describing evidences that support the disruption of these mechanisms in mental disorders, including ADHD. In this review, we briefly described the family and function of GluRs in the CNS, and discussed what is recently known about the role of GluRs in ADHD, that including GluR genes, animal models, and the treatment, which would help us further elucidate the etiology of ADHD.

Assessing Spatial Working Memory Using the Spontaneous Alternation Y-maze Test in Aged Male Mice

The global population is aging and the prevalence of age-related diseases, such as Alzheimer's disease and vascular dementia is increasing. Understanding functional impairments and disease processes is of vital importance in order to develop effective therapeutics. Using the natural exploratory behavior of mice, the spontaneous alternation y-maze can assess short-term spatial working memory. The protocol for y-maze testing is straightforward and requires minimal resources, as well as animal training and output. Therefore, it can be broadly applied to study short-term memory in aged rodent models.

A role for N-acetylaspartylglutamate (NAAG) and mGluR3 in cognition

The peptide transmitter N-acetylaspartylglutamate (NAAG) and its receptor, the type 3 metabotropic glutamate receptor (mGluR3, GRM3), are prevalent and widely distributed in the mammalian nervous system. Drugs that inhibit the inactivation of synaptically released NAAG have procognitive activity in object recognition and other behavioral models. These inhibitors also reverse cognitive deficits in animal models of clinical disorders. Antagonists of mGluR3 block these actions and mice that are null mutant for this receptor are insensitive to the actions of these procognitive drugs. A positive allosteric modulator of this receptor also has procognitive activity. While some data suggest that drugs acting on mGluR3 achieve their procognitive action by increasing arousal during acquisition training, exploration of the procognitive efficacy of NAAG is in its early stages and thus substantial opportunities exist to define the breadth and nature of this activity.

Mindfulness-based interventions and cognitive function among breast cancer survivors: a systematic review

Background

Breast cancer survivors have an elevated risk of cognitive impairment compared to age-matched women without cancer. Causes of this impairment are complex, including both treatment and psychological factors. Mindfulness-based interventions, which have been shown to improve cognitive function in the general population, may be one approach to mitigate cognitive impairment in this survivor population. Our objective was to conduct a systematic literature review of studies on the effect of mindfulness-based interventions on cognition among breast cancer survivors.

Methods

We conducted searches of three electronic databases (Scopus, PubMed and Cochrane Database of Systematic Reviews) in September 2017 for studies pertaining mindfulness and cognitive function among breast cancer survivors. Abstracts were manually searched by two reviewers and additional articles were identified through reference lists.

Results

A total of 226 articles were identified through our systematic search and six met inclusion criteria for this review. The reviewed studies lacked consistency in terms of the cognition domains studied (e.g. executive function, recent memory, etc) and in the measures used to assess cognition. Of the included studies, two found no association between mindfulness interventions and cognitive function, two found improvement that was not sustained at the follow-up, and another two found sustained improvement at 2- or 6-months.

Conclusions

Mindfulness-based interventions have shown some evidence for improving cognition among breast cancer survivors, but further research using validated and comprehensive cognitive assessments is needed. More research is also needed related to the timing, duration and content of mindfulness interventions.

Electronic supplementary material

The online version of this article (10.1186/s12885-018-5065-3) contains supplementary material, which is available to authorized users.

Optogenetic induction of the schizophrenia-related endophenotype of ventral hippocampal hyperactivity causes rodent correlates of positive and cognitive symptoms

Pathological over-activity of the CA1 subfield of the human anterior hippocampus has been identified as a potential predictive marker for transition from a prodromal state to overt schizophrenia. Psychosis, in turn, is associated with elevated activity in the anterior subiculum, the hippocampal output stage directly activated by CA1. Over-activity in these subfields may represent a useful endophenotype to guide translationally predictive preclinical models. To recreate this endophenotype and study its causal relation to deficits in the positive and cognitive symptom domains, we optogenetically activated excitatory neurons of the ventral hippocampus (vHPC; analogous to the human anterior hippocampus), targeting the ventral subiculum. Consistent with previous studies, we found that vHPC over-activity evokes hyperlocomotion, a rodent correlate of positive symptoms. vHPC activation also impaired performance on the spatial novelty preference (SNP) test of short-term memory, regardless of whether stimulation was applied during the encoding or retrieval stage of the task. Increasing dopamine transmission with amphetamine produced hyperlocomotion, but was not associated with SNP impairments. This suggests that short-term memory impairments resulting from hippocampal over-activity likely arise independently of a hyperdopaminergic state, a finding that is consistent with the pharmaco-resistance of cognitive symptoms in patients.

Investigating the role of mGluR2 versus mGluR3 in antipsychotic-like effects, sleep-wake architecture and network oscillatory activity using novel Han Wistar rats lacking mGluR2 expression

Group II metabotropic glutamate receptors (mGluR2 and mGluR3) are implicated in a number of psychiatric disorders. They also control sleep-wake architecture and may offer novel therapeutic targets. However, the roles of the mGluR2 versus mGluR3 subtypes are not well understood. Here, we have taken advantage of the recently described mutant strain of Han Wistar rats, which do not express mGluR2 receptors, to investigate behavioural, sleep and EEG responses to mGluR2/3 ligands.

The mGluR2/3 agonist, LY354740 (10 mg/kg), reversed amphetamine- and phencyclidine-induced locomotion and rearing behaviours in control Wistar but not in mGluR2 lacking Han Wistar rats. In control Wistar but not in Han Wistar rats the mGluR2/3 agonist LY379268 (3 & 10 mg/kg) induced REM sleep suppression with dose-dependent effects on wake and NREM sleep. By contrast, the mGluR2/3 antagonist LY3020371 (3 & 10 mg/kg) had wake-promoting effects in both rat strains, albeit smaller in the mGluR2-lacking Han Wistar rats, indicating both mGluR2 and mGluR3-mediated effects on wakefulness. LY3020371 enhanced wake cortical oscillations in the theta (4–9 Hz) and gamma (30–80 Hz) range in both Wistar and Han Wistar rat strains, whereas LY379268 reduced theta and gamma oscillations in control Wistar rats, with minimal effects in Han Wistar rats.

Together these studies illustrate the significant contribution of mGluR2 to the antipsychotic-like, sleep and EEG effects of drugs acting on group II mGluRs. However, we also provide evidence of a role for mGluR3 activity in the control of sleep and wake cortical theta and gamma oscillations.

Handling method alters the hedonic value of reward in laboratory mice

Mice are the most widely used model species for drug discovery and scientific research. Consequently, it is important to refine laboratory procedures and practices to ensure high standards of welfare and scientific data quality. Recent studies have identified that the standard practice of handling laboratory mice by their tails increases behaviours indicative of anxiety, which can be overcome by handling mice using a tunnel. However, despite clear negative effects on mice’s behaviour, tunnel handling has yet to be widely implemented. In this study, we provide the first evidence that tail handling also reduces mice’s responses to reward. Anhedonia is a core symptom of clinical depression, and is measured in rodents by assessing how they consume a sucrose solution: depressed mice consume less sucrose and the size of their licking bouts when drinking (their ‘lick cluster sizes’) also tend to be smaller. We found that tail handled mice showed more anhedonic responses in both measures compared to tunnel handled mice, indicative of a decreased responsiveness to reward and potentially a more depressive-like state. Our findings have significant implications for the welfare of laboratory mice as well as the design and interpretation of scientific studies, particularly those investigating or involving reward.

Activation of Group II Metabotropic Glutamate Receptors Promotes LTP Induction at Schaffer Collateral-CA1 Pyramidal Cell Synapses by Priming NMDA Receptors

It is well established that selective activation of group I metabotropic glutamate (mGlu) receptors induces LTD of synaptic transmission at Schaffer collateral-CA1 synapses. In contrast, application of 1S,3R-ACPD, a mixed agonist at group I and group II mGlu receptors, induces LTP. Using whole-cell recordings from CA1 pyramidal cells and field recordings in the hippocampal CA1 region, we investigated the specific contribution of group II mGlu receptors to synaptic plasticity at Schaffer collateral-CA1 synapses in acute slices of adult mice. Pharmacological activation of group II mGlu receptors (mGlu2 and mGlu3 receptors) with the specific agonist LY354740 in conjunction with electrical stimulation induced postsynaptic LTP. This form of plasticity requires coactivation of NMDA receptors (NMDARs). Group II mGlu receptor activation led to PKC-dependent phosphorylation of the GluN1 subunit. We found that both synaptic and extrasynaptic NMDARs, which are differentially modulated by mGlu2 and mGlu3 receptors, contribute to LTP induction. Furthermore, LTP initiated by activation of group II mGlu receptors was not occluded by LTP induced with high-frequency trains of stimuli. However, the phosphorylation of NMDARs mediated by group II mGlu receptor activation led to a priming effect that enhanced subsequent high-frequency stimulation-induced LTP. These findings reveal a novel metaplastic mechanism through which group II mGlu receptors modulate synaptic function at the Schaffer collateral input to CA1 pyramidal cells, thereby lowering the threshold to induce plasticity.

SIGNIFICANCE STATEMENT

The group II metabotropic glutamate (mGlu II) receptors exert a well characterized action on presynaptic neuron terminals to modulate neurotransmitter release. Here, we show that these receptors also have postsynaptic effects in promoting the induction of synaptic plasticity. Using an electrophysiological approach including field and whole-cell patch recording in hippocampi from wild-type and transgenic mice, we show that activation of group II mGlu receptors enhances NMDA receptor (NMDAR)-mediated currents through PKC-dependent phosphorylation. This priming of NMDARs lowers the threshold for the induction of LTP of synaptic transmission. These findings may also provide new insights into the mechanisms through which drugs targeting mGlu II receptors alleviate hypoglutamatergic conditions such as those occurring in certain brain disorders such as schizophrenia.

Persistent Unresolved Inflammation in the Mecp2-308 Female Mutated Mouse Model of Rett Syndrome

Rett syndrome (RTT) is a rare neurodevelopmental disorder usually caused by mutations in the X-linked gene methyl-CpG-binding protein 2 (MECP2). Several Mecp2 mutant mouse lines have been developed recapitulating part of the clinical features. In particular, Mecp2-308 female heterozygous mice, bearing a truncating mutation, are a validated model of the disease. While recent data suggest a role for inflammation in RTT, little information on the inflammatory status in murine models of the disease is available. Here, we investigated the inflammatory status by proteomic 2-DE/MALDI-ToF/ToF analyses in symptomatic Mecp2-308 female mice. Ten differentially expressed proteins were evidenced in the Mecp2-308 mutated plasma proteome. In particular, 5 positive acute-phase response (APR) proteins increased (i.e., kininogen-1, alpha-fetoprotein, mannose-binding protein C, alpha-1-antitrypsin, and alpha-2-macroglobulin), and 3 negative APR reactants were decreased (i.e., serotransferrin, albumin, and apolipoprotein A1). CD5 antigen-like and vitamin D-binding protein, two proteins strictly related to inflammation, were also changed. These results indicate for the first time a persistent unresolved inflammation of unknown origin in the Mecp2-308 mouse model.

Allosteric Modulation of GPCRs: New Insights and Potential Utility for Treatment of Schizophrenia and Other CNS Disorders

G-protein-coupled receptors (GPCRs) play critical roles in regulating brain function. Recent advances have greatly expanded our understanding of these receptors as complex signaling machines that can adopt numerous conformations and modulate multiple downstream signaling pathways. While agonists and antagonists have traditionally been pursued to target GPCRs, allosteric modulators provide several mechanistic advantages, including the ability to distinguish between closely related receptor subtypes. Recently, the discovery of allosteric ligands that confer bias and modulate some, but not all, of a given receptor's downstream signaling pathways can provide pharmacological modulation of brain circuitry with remarkable precision. In addition, allosteric modulators with unprecedented specificity have been developed that can differentiate between subpopulations of a given receptor subtype based on the receptor's dimerization state. These advances are not only providing insight into the biological roles of specific receptor populations, but hold great promise for treating numerous CNS disorders.

Metabotropic glutamate receptor 3 (mGlu3; mGluR3; GRM3) in schizophrenia: Antibody characterisation and a semi-quantitative western blot study

BACKGROUND

Metabotropic glutamate receptor 3 (mGlu3, mGluR3), encoded by GRM3, is a risk gene for schizophrenia and a therapeutic target. It is unclear whether expression of the receptor is altered in the disorder or related to GRM3 risk genotype. Antibodies used to date to assess mGlu3 in schizophrenia have not been well validated.

OBJECTIVE

To characterise six commercially available anti-mGlu3 antibodies for use in human brain, and then conduct a semi-quantitative study of mGlu3 immunoreactivity in schizophrenia.

METHODS

Antibodies tested using Grm3^-/- and Grm2^-/-/3^-/- mice and transfected HEK293T/17 cells. Western blotting on membrane protein isolated from superior temporal cortex of 70 patients with schizophrenia and 87 healthy comparison subjects, genotyped for GRM3 SNP rs10234440.

RESULTS

One (out of six) anti-mGlu3 antibodies was fully validated, a C-terminal antibody which detected monomeric (~100kDa) and dimeric (~200kDa) mGlu3. A second, N-terminal, antibody detected the 200kDa band but also produced non-specific bands. Using the C-terminal antibody for western blotting in human brain, mGlu3 immunoreactivity was found to decline with age, and was affected by pH and post mortem interval. There were no differences in monomeric or dimeric mGlu3 immunoreactivity in schizophrenia or in relation to GRM3 genotype. The antibody was not suitable for immunohistochemistry.

INTERPRETATION

These data highlight the value of knockout mouse tissue for antibody validation, and the need for careful antibody characterisation. The schizophrenia data show that involvement of GRM3 in the disorder and its genetic risk architecture is not reflected in total membrane mGlu3 immunoreactivity in superior temporal cortex.

The mGluR2 positive allosteric modulator, SAR218645, improves memory and attention deficits in translational models of cognitive symptoms associated with schizophrenia

Normalization of altered glutamate neurotransmission through activation of the mGluR2 has emerged as a new approach to treat schizophrenia. These studies describe a potent brain penetrant mGluR2 positive allosteric modulator (PAM), SAR218645. The compound behaves as a selective PAM of mGluR2 in recombinant and native receptor expression systems, increasing the affinity of glutamate at mGluR2 as inferred by competition and GTPγ³⁵S binding assays. SAR218645 augmented the mGluR2-mediated response to glutamate in a rat recombinant mGluR2 forced-coupled Ca²⁺ mobilization assay. SAR218645 potentiated mGluR2 agonist-induced contralateral turning. When SAR218645 was tested in models of the positive symptoms of schizophrenia, it reduced head twitch behavior induced by DOI, but it failed to inhibit conditioned avoidance and hyperactivity using pharmacological and transgenic models. Results from experiments in models of the cognitive symptoms associated with schizophrenia showed that SAR218645 improved MK-801-induced episodic memory deficits in rats and attenuated working memory impairment in NMDA Nr1^neo-/- mice. The drug reversed disrupted latent inhibition and auditory-evoked potential in mice and rats, respectively, two endophenotypes of schizophrenia. This profile positions SAR218645 as a promising candidate for the treatment of cognitive symptoms of patients with schizophrenia, in particular those with abnormal attention and sensory gating abilities.

Searching for cognitive enhancement in the Morris water maze: better and worse performance in D-amino acid oxidase knockout (Dao(-/-)) mice

A common strategy when searching for cognitive‐enhancing drugs has been to target the N‐methyl‐d‐aspartate receptor (NMDAR), given its putative role in synaptic plasticity and learning. Evidence in favour of this approach has come primarily from studies with rodents using behavioural assays like the Morris water maze. D‐amino acid oxidase (DAO) degrades neutral D‐amino acids such as D‐serine, the primary endogenous co‐agonist acting at the glycine site of the synaptic NMDAR. Inhibiting DAO could therefore provide an effective and viable means of enhancing cognition, particularly in disorders like schizophrenia, in which NMDAR hypofunction is implicated. Indirect support for this notion comes from the enhanced hippocampal long‐term potentiation and facilitated water maze acquisition of ddY/Dao⁻ mice, which lack DAO activity due to a point mutation in the gene. Here, in Dao knockout (Dao^−/−) mice, we report both better and worse water maze performance, depending on the radial distance of the hidden platform from the side wall of the pool. Dao^−/− mice displayed an increased innate preference for swimming in the periphery of the maze (possibly due to heightened anxiety), which facilitated the discovery of a peripherally located platform, but delayed the discovery of a centrally located platform. By contrast, Dao^−/− mice exhibited normal performance in two alternative assays of long‐term spatial memory: the appetitive and aversive Y‐maze reference memory tasks. Taken together, these results question the proposed relationship between DAO inactivation and enhanced long‐term associative spatial memory. They also have generic implications for how Morris water maze studies are performed and interpreted.

Blockage of acquisition and expression of morphine-induced conditioned place preference in rats due to activation of glutamate receptors type II/III in nucleus accumbens

Numerous studies have shown that glutamate in the nucleus accumbens (NAc) is an essential neurotransmitter for the extension of morphine-induced place preference. mGlu2/3 glutamate receptors in the NAc have important roles in the reward pathway. However, less is known about the role of this glutamate receptor subtype in morphine-induced conditioned place preference (CPP). In this study, we examined the effects of bilateral intra-accumbal administration of LY379268, an mGlu2/3 receptor agonist on the acquisition and expression of morphine-induced CPP in rats. Adult male Wistar rats (n=136; 220-250g) were evaluated in a CPP paradigm. Doses of LY379268 (0.3, 1 and 3μg/0.5μL saline per side) were administered into the NAc on both sides during the 3days of the conditioning (acquisition) or post-conditioning (expression) phase. The results show that bilateral intra-accumbal administration of LY379268 (0.3, 1 and 3μg) markedly decreased the acquisition of morphine-induced CPP in a dose-dependent manner. In a second series of experiments, we determined that injection of LY379268 into the NAc considerably attenuated the expression of morphine CPP only at the highest dose (3μg). Our findings suggest that activation of mGlu2/3 receptors in the NAc dose-dependently blocked both the establishment and the maintenance of morphine-induced CPP and confirmed the role of this system as a potential therapeutic target for addiction.

Deletion of Metabotropic Glutamate Receptors 2 and 3 (mGlu2 & mGlu3) in Mice Disrupts Sleep and Wheel-Running Activity, and Increases the Sensitivity of the Circadian System to Light

Sleep and/or circadian rhythm disruption (SCRD) is seen in up to 80% of schizophrenia patients. The co-morbidity of schizophrenia and SCRD may in part stem from dysfunction in common brain mechanisms, which include the glutamate system, and in particular, the group II metabotropic glutamate receptors mGlu2 and mGlu3 (encoded by the genes Grm2 and Grm3). These receptors are relevant to the pathophysiology and potential treatment of schizophrenia, and have also been implicated in sleep and circadian function. In the present study, we characterised the sleep and circadian rhythms of Grm2/3 double knockout (Grm2/3^-/-) mice, to provide further evidence for the involvement of group II metabotropic glutamate receptors in the regulation of sleep and circadian rhythms. We report several novel findings. Firstly, Grm2/3^-/- mice demonstrated a decrease in immobility-determined sleep time and an increase in immobility-determined sleep fragmentation. Secondly, Grm2/3^-/- mice showed heightened sensitivity to the circadian effects of light, manifested as increased period lengthening in constant light, and greater phase delays in response to nocturnal light pulses. Greater light-induced phase delays were also exhibited by wildtype C57Bl/6J mice following administration of the mGlu2/3 negative allosteric modulator RO4432717. These results confirm the involvement of group II metabotropic glutamate receptors in photic entrainment and sleep regulation pathways. Finally, the diurnal wheel-running rhythms of Grm2/3^-/- mice were perturbed under a standard light/dark cycle, but their diurnal rest-activity rhythms were unaltered in cages lacking running wheels, as determined with passive infrared motion detectors. Hence, when assessing the diurnal rest-activity rhythms of mice, the choice of assay can have a major bearing on the results obtained.

Pharmacological stimulation of the brain serotonin receptor 7 as a novel therapeutic approach for Rett syndrome

Rett syndrome (RTT) is a rare neurodevelopmental disorder, characterized by severe behavioral and physiological symptoms. Mutations in the methyl CpG-binding protein 2 gene (MECP2) cause >95% of classic cases, and currently there is no cure for this devastating disorder. The serotonin receptor 7 (5-HT7R) is linked to neuro-physiological regulation of circadian rhythm, mood, cognition, and synaptic plasticity. We presently report that 5-HT7R density is consistently reduced in cortical and hippocampal brain areas of symptomatic MeCP2-308 male mice, a RTT model. Systemic repeated treatment with LP-211 (0.25 mg/kg once/day for 7 days), a brain-penetrant selective 5-HT7R agonist, was able to rescue RTT-related defective performance: anxiety-related profiles in a Light/Dark test, motor abilities in a Dowel test, the exploratory behavior in the Marble Burying test, as well as memory in the Novelty Preference task. In the brain of RTT mice, LP-211 also reversed the abnormal activation of PAK and cofilin (key regulators of actin cytoskeleton dynamics) and of the ribosomal protein (rp) S6, whose reduced activation in MECP2 mutant neurons by mTOR is responsible for the altered protein translational control. Present findings indicate that pharmacological targeting of 5-HT7R improves specific behavioral and molecular manifestations of RTT, thus representing a first step toward the validation of an innovative systemic treatment. Beyond RTT, the latter might be extended to other disorders associated with intellectual disability.

Left-right dissociation of hippocampal memory processes in mice

Left-right asymmetries have likely evolved to make optimal use of bilaterian nervous systems; however, little is known about the synaptic and circuit mechanisms that support divergence of function between equivalent structures in each hemisphere. Here we examined whether lateralized hippocampal memory processing is present in mice, where hemispheric asymmetry at the CA3-CA1 pyramidal neuron synapse has recently been demonstrated, with different spine morphology, glutamate receptor content, and synaptic plasticity, depending on whether afferents originate in the left or right CA3. To address this question, we used optogenetics to acutely silence CA3 pyramidal neurons in either the left or right dorsal hippocampus while mice performed hippocampus-dependent memory tasks. We found that unilateral silencing of either the left or right CA3 was sufficient to impair short-term memory. However, a striking asymmetry emerged in long-term memory, wherein only left CA3 silencing impaired performance on an associative spatial long-term memory task, whereas right CA3 silencing had no effect. To explore whether synaptic properties intrinsic to the hippocampus might contribute to this left-right behavioral asymmetry, we investigated the expression of hippocampal long-term potentiation. Following the induction of long-term potentiation by high-frequency electrical stimulation, synapses between CA3 and CA1 pyramidal neurons were strengthened only when presynaptic input originated in the left CA3, confirming an asymmetry in synaptic properties. The dissociation of hippocampal long-term memory function between hemispheres suggests that memory is routed via distinct left-right pathways within the mouse hippocampus, and provides a promising approach to help elucidate the synaptic basis of long-term memory.

The role of group II metabotropic glutamate receptors in cognition and anxiety: comparative studies in GRM2(-/-), GRM3(-/-) and GRM2/3(-/-) knockout mice

Group II metabotropic glutamate receptors (mGlu2 and mGlu3, encoded by GRM2 and GRM3) have been implicated in both cognitive and emotional processes, although their precise role remains to be established. Studies with knockout (KO) mice provide an important approach for investigating the role of specific receptor genes in behaviour. In the present series of experiments we extended our prior characterisation of GRM2/3^−/− double KO mice and, in complementary experiments, investigated the behavioural phenotype of single GRM2^−/− and GRM3^−/− mice. We found no consistent effect on anxiety in either the double or single KO mice. The lack of an anxiety phenotype in any of the lines contrasts with the clear anxiolytic effects of mGlu2/3 ligands. Motor co-ordination was impaired in GRM2/3^−/− mice, but spared in single GRM2^−/− and GRM3^−/− mice. Spatial working memory (rewarded alternation) testing on the elevated T-maze revealed a deficit in GRM2^−/− mice throughout testing, whereas GRM3^−/− mice exhibited a biphasic effect (initially impaired, but performing better than controls by the end of training). A biphasic effect on activity levels was seen for the GRM2^−/− mice. Overall, the phenotype in both GRM2^−/− and GRM3^−/− mice was less pronounced – if present at all – compared to GRM2/3^−/− mice, across the range of task domains. This is consistent with possible redundancy of function and/or compensation in the single KO lines. Results are discussed with reference to a possible role for group II metabotropic glutamate receptors at the interface between arousal and behavioural performance, according to an inverted U-shaped function.

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GRM2^−/− mice exhibited impaired spatial short-term memory (rewarded alternation).

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GRM3^−/− mice displayed bi-directional effects on this spatial short-term memory task.

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GRM2^−/− mice displayed bi-directional effects on activity levels.

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There was no consistent anxiety effect in either double or single knockout mice.

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Behavioural phenotypes were weaker (or absent) in single than in double knockout mice.

Aberrant Rho GTPases signaling and cognitive dysfunction: in vivo evidence for a compelling molecular relationship

Rho GTPases are key intracellular signaling molecules that coordinate dynamic changes in the actin cytoskeleton, thereby stimulating a variety of processes, including morphogenesis, migration, neuronal development, cell division and adhesion. Deviations from normal Rho GTPases activation state have been proposed to disrupt cognition and synaptic plasticity. This review focuses on the functional consequences of genetic ablation of upstream and downstream Rho GTPases molecules on cognitive function and neuronal morphology and connectivity. Available information on this issue is described and compared to that gained from mice carrying mutations in the most studied Rho GTPases and from pharmacological in vivo studies in which brain Rho GTPases signaling was modulated. Results from reviewed literature provide definitive evidence of a compelling link between Rho GTPases signaling and cognitive function, thus supporting the notion that Rho GTPases and their downstream effectors may represent important therapeutic targets for disorders associated with cognitive dysfunction.

An automated maze task for assessing hippocampus-sensitive memory in mice

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Alternation procedures in rodents are highly sensitive to manipulations of the hippocampus.

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However as they require hand testing, they are low throughput and stressful for the animal.

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An automated maze was developed for assessing alternation performance in mice.

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Alternation performance was shown to be impaired in mice with lesions to the hippocampus.

Memory deficits associated with hippocampal dysfunction are a key feature of a number of neurodegenerative and psychiatric disorders. The discrete-trial rewarded alternation T-maze task is highly sensitive to hippocampal dysfunction. Normal mice have spontaneously high levels of alternation, whereas hippocampal-lesioned mice are dramatically impaired. However, this is a hand-run task and handling has been shown to impact crucially on behavioural responses, as well as being labour-intensive and therefore unsuitable for high-throughput studies. To overcome this, a fully automated maze was designed. The maze was attached to the mouse's home cage and the subject earned all of its food by running through the maze. In this study the hippocampal dependence of rewarded alternation in the automated maze was assessed. Bilateral hippocampal-lesioned mice were assessed in the standard, hand-run, discrete-trial rewarded alternation paradigm and in the automated paradigm, according to a cross-over design. A similarly robust lesion effect on alternation performance was found in both mazes, confirming the sensitivity of the automated maze to hippocampal lesions. Moreover, the performance of the animals in the automated maze was not affected by their handling history whereas performance in the hand-run maze was affected by prior testing history. By having more stable performance and by decreasing human contact the automated maze may offer opportunities to reduce extraneous experimental variation and therefore increase the reproducibility within and/or between laboratories. Furthermore, automation potentially allows for greater experimental throughput and hence suitability for use in assessment of cognitive function in drug discovery.

Decreased striatal dopamine in group II metabotropic glutamate receptor (mGlu2/mGlu3) double knockout mice

BACKGROUND

Group II metabotropic glutamate receptors (mGlu2 and mGlu3, encoded by Grm2 and Grm3) have been the focus of attention as treatment targets for a number of psychiatric conditions. Double knockout mice lacking mGlu2 and mGlu3 (mGlu2/3-/-) show a subtle behavioural phenotype, being hypoactive under basal conditions and in response to amphetamine, and with a spatial memory deficit that depends on the arousal properties of the task. The neurochemical correlates of this profile are unknown. Here, we measured tissue levels of dopamine, 5-HT, noradrenaline and their metabolites in the striatum and frontal cortex of mGlu2/3-/- double knockout mice, using high performance liquid chromatography. We also measured the same parameters in mGlu2-/- and mGlu3-/- single knockout mice.

RESULTS

mGlu2/3-/-mice had reduced dopamine levels in the striatum but not in frontal cortex, compared to wild-types. In a separate cohort we replicated this deficit and, using tissue punches, found it was more prominent in the nucleus accumbens than in dorsolateral striatum. Noradrenaline, 5-HT and their metabolites were not altered in the striatum of mGlu2/3-/- mice, although the noradrenaline metabolite MHPG was increased in the cortex. In mGlu2-/- and mGlu3-/- single knockout mice we found no difference in any monoamine or metabolite, in either brain region, compared to their wild-type littermates.

CONCLUSIONS

Group II metabotropic glutamate receptors impact upon striatal dopamine. The effect may contribute to the behavioural phenotype of mGlu2/3-/- mice. The lack of dopaminergic alterations in mGlu2-/- and mGlu3-/- single knockout mice reveals a degree of redundancy between the two receptors. The findings support the possibility that interactions between mGlu2/3 and dopamine may be relevant to the pathophysiology and therapy of schizophrenia and other disorders.

Improved spatial learning is associated with increased hippocampal but not prefrontal long-term potentiation in mGluR4 knockout mice

Although much information about metabotropic glutamate receptors (mGluRs) and their role in normal and pathologic brain function has been accumulated during the last decades, the role of group III mGluRs is still scarcely documented. Here, we examined mGluR4 knockout mice for types of behavior and synaptic plasticity that depend on either the hippocampus or the prefrontal cortex (PFC). We found improved spatial short- and long-term memory in the radial arm maze, which was accompanied by enhanced long-term potentiation (LTP) in hippocampal CA1 region. In contrast, LTP in the PFC was unchanged when compared with wild-type controls. Changes in paired-pulse facilitation that became overt in the presence of the GABAA antagonist picrotoxin indicated a function of mGluR4 in maintaining the excitation/inhibition balance, which is of crucial importance for information processing in the brain and the deterioration of these processes in neuropsychological disorders such as autism, epilepsy and schizophrenia.

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.

Inhibition of glutamate carboxypeptidase II (GCPII) activity as a treatment for cognitive impairment in multiple sclerosis

Half of all patients with multiple sclerosis (MS) experience cognitive impairment, for which there is no pharmacological treatment. Using magnetic resonance spectroscopy (MRS), we examined metabolic changes in the hippocampi of MS patients, compared the findings to performance on a neurocognitive test battery, and found that N-acetylaspartylglutamate (NAAG) concentration correlated with cognitive functioning. Specifically, MS patients with cognitive impairment had low hippocampal NAAG levels, whereas those with normal cognition demonstrated higher levels. We then evaluated glutamate carboxypeptidase II (GCPII) inhibitors, known to increase brain NAAG levels, on cognition in the experimental autoimmune encephalomyelitis (EAE) model of MS. Whereas GCPII inhibitor administration did not affect physical disabilities, it increased brain NAAG levels and dramatically improved learning and memory test performance compared with vehicle-treated EAE mice. These data suggest that NAAG is a unique biomarker for cognitive function in MS and that inhibition of GCPII might be a unique therapeutic strategy for recovery of cognitive function.

Dissecting spatial knowledge from spatial choice by hippocampal NMDA receptor deletion

Hippocampal NMDA receptors (NMDARs) and NMDAR-dependent synaptic plasticity are widely considered crucial substrates of long-term spatial memory, although their precise role remains uncertain. Here we show that Grin1(ΔDGCA1) mice, lacking GluN1 and hence NMDARs in all dentate gyrus and dorsal CA1 principal cells, acquired the spatial reference memory water maze task as well as controls, despite impairments on the spatial reference memory radial maze task. When we ran a spatial discrimination water maze task using two visually identical beacons, Grin1(ΔDGCA1) mice were impaired at using spatial information to inhibit selecting the decoy beacon, despite knowing the platform's actual spatial location. This failure could suffice to impair radial maze performance despite spatial memory itself being normal. Thus, these hippocampal NMDARs are not essential for encoding or storing long-term, associative spatial memories. Instead, we demonstrate an important function of the hippocampus in using spatial knowledge to select between alternative responses that arise from competing or overlapping memories.

Working memory and the homeostatic control of brain adenosine by adenosine kinase

The neuromodulator adenosine maintains brain homeostasis and regulates complex behaviour via activation of inhibitory and excitatory adenosine receptors (ARs) in a brain region-specific manner. AR antagonists such as caffeine have been shown to ameliorate cognitive impairments in animal disease models but their effects on learning and memory in normal animals are equivocal. An alternative approach to reduce AR activation is to lower the extracellular tone of adenosine, which can be achieved by up-regulating adenosine kinase (ADK), the key enzyme of metabolic adenosine clearance. However, mice that globally over-express an Adk transgene ('Adk-tg' mice) were devoid of a caffeine-like pro-cognitive profile; they instead exhibited severe spatial memory deficits. This may be mechanistically linked to cortical/hippocampal N-methyl-d-aspartate receptor (NMDAR) hypofunction because the motor response to acute MK-801 was also potentiated in Adk-tg mice. Here, we evaluated the extent to which the behavioural phenotypes of Adk-tg mice might be modifiable by up-regulating adenosine levels in the cortex/hippocampus. To this end, we investigated mutant 'fb-Adk-def' mice in which ADK expression was specifically reduced in the telencephalon leading to a selective increase in cortical/hippocampal adenosine, while the rest of the brain remained as adenosine-deficient as in Adk-tg mice. The fb-Adk-def mice showed an even greater impairment in spatial working memory and a more pronounced motor response to NMDAR blockade than Adk-tg mice. These outcomes suggest that maintenance of cortical/hippocampal adenosine homeostasis is essential for effective spatial memory and deviation in either direction is detrimental with increased expression seemingly more disruptive than decreased expression.

Metabotropic glutamate receptors for new treatments in schizophrenia

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

Genetic mouse models relevant to schizophrenia: taking stock and looking forward

Genetic mouse models relevant to schizophrenia complement, and have to a large extent supplanted, pharmacological and lesion-based rat models. The main attraction is that they potentially have greater construct validity; however, they share the fundamental limitations of all animal models of psychiatric disorder, and must also be viewed in the context of the uncertain and complex genetic architecture of psychosis. Some of the key issues, including the choice of gene to target, the manner of its manipulation, gene-gene and gene-environment interactions, and phenotypic characterization, are briefly considered in this commentary, illustrated by the relevant papers reported in this special issue.