Regulation of neurotransmitter release by metabotropic glutamate receptors

Effects of microinjections of Group II metabotropic glutamate agents into the amygdala on sleep

Systemic administration of the Group II metabotropic glutamate (mGlu) receptor agonist, LY379268 (LY37), dose-dependently suppresses rapid eye movement sleep (REM) whereas systemic administration of the mGlu II receptor antagonist, LY341495 (LY34), increases arousal. Group II mGlu receptors are highly expressed in the amygdala, a brain region involved in the regulation of sleep and arousal. To determine whether the amygdala is involved in mediating the effects of Group II mGlu agents on sleep, we microinjected LY37 and LY34 into the basal amygdala (BA) and the central nucleus of the amygdala (CNA) and recorded sleep and wakefulness. Wistar rats were implanted with electrodes for recording sleep and with bilateral cannulae aimed into BA for drug administration. Different groups of rats received bilateral microinjections of LY37 into BA at two dosage ranges (3.2 mM, 5.3 mM or 10.7 mM or 0.1 nM, 2.0 nM or 10.0 nM) or one dosage range of LY34 (1.0 nM, 30.0 nM or 60.0 nM). Microinjections into CNA were conducted at one dosage range for LY37 (0.1 nM, 2.0 nM or 10.0 nM) and for LY34 (1.0 nM, 30.0 nM or 60.0 nM). All drugs or vehicle alone were administered in a counterbalanced order at 5-day intervals. Following microinjection, sleep was recorded for 20 h. Microinjection of LY37 into BA at both nM and mM concentrations significantly decreased REM without significantly altering NREM, total sleep or wakefulness. The high dosage of LY34 in BA significantly suppressed NREM and total sleep. Microinjections of LY37 or LY34 into CNA had no significant impact on sleep. We suggest that Group II mGlu receptors may influence specific cells in BA that control descending output (via the CNA or bed nucleus of the stria terminalis) that in turn regulates pontine REM generator regions.

mGlu3 receptor and astrocytes: partners in neuroprotection

Astrocytes are currently studied intensively because of their now highlighted relevance as key players with neurons that modulate a wide range of central functions, from synaptic plasticity and synaptogenesis to regulation of metabolic and neuroinflammatory processes. Since the discovery of mGlu3 receptors on astrocytes, accumulating evidence supports a role of these receptors not only in maintaining synaptic homeostasis and treating psychiatric disorders but also in promoting astrocyte survival in several pathologic conditions. This review focuses on providing up-to-date knowledge regarding effects of activating astroglial mGlu3 receptors on psychiatric disorders, astrocyte and neuronal survival, and neurodegenerative diseases. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Development of GPCR modulation of GABAergic transmission in chicken nucleus laminaris neurons

Neurons in the nucleus laminaris (NL) of birds act as coincidence detectors and encode interaural time difference to localize the sound source in the azimuth plane. GABAergic transmission in a number of CNS nuclei including the NL is subject to a dual modulation by presynaptic GABA_B receptors (GABA_BRs) and metabotropic glutamate receptors (mGluRs). Here, using in vitro whole-cell patch clamp recordings from acute brain slices of the chick, we characterized the following important but unknown properties pertaining to such a dual modulation: (1) emergence of functional GABA synapses in NL neurons; (2) the temporal onset of neuromodulation mediated by GABA_BRs and mGluRs; and (3) the physiological conditions under which GABA_BRs and mGluRs are activated by endogenous transmitters. We found that (1) GABA_AR-mediated synaptic responses were observed in about half of the neurons at embryonic day 11 (E11); (2) GABA_BR-mediated modulation of the GABAergic transmission was detectable at E11, whereas the modulation by mGluRs did not emerge until E15; and (3) endogenous activity of GABA_BRs was induced by both low- (5 or 10 Hz) and high-frequency (200 Hz) stimulation of the GABAergic pathway, whereas endogenous activity of mGluRs was induced by high- (200 Hz) but not low-frequency (5 or 10 Hz) stimulation of the glutamatergic pathway. Furthermore, the endogenous activity of mGluRs was mediated by group II but not group III members. Therefore, autoreceptor-mediated modulation of GABAergic transmission emerges at the same time when the GABA synapses become functional. Heteroreceptor-mediated modulation appears at a later time and is receptor type dependent in vitro.

Orthosteric versus allosteric GPCR activation: the great challenge of group-III mGluRs

Group-III metabotropic glutamate receptors (mGluRs) comprise four structurally related brain and retinal G protein-coupled receptors (GPCRs), mGluR4, mGluR6, mGluR7 and mGluR8, which receive much attention as promising targets for nervous system drugs. In particular, activation of mGluR4 is a major focus for the development of new therapeutics in Parkinson's disease, while mGluR7 activation is considered a potential approach for future treatments of specific psychiatric conditions. The first generation group-III mGluR agonists, e.g.l-AP4 and l-SOP, are characterized by an essential phosphonate functional group, which became a major limitation for the development of systemically active, potent and receptor subtype-selective drugs. Recently however, two approaches emerged in parallel providing resolution to this constraint: in silico high-throughput screening of chemical libraries against a 3D-model of the mGluR4 extracellular domain identified a hit that was optimized into a series of potent and subtype-selective orthosteric agonists with drug-like properties and novel chemotype structures; secondly, high-throughput random screening of chemical libraries against recombinantly expressed group-III receptors identified diverse chemical sets of allosteric agonists and positive modulators, which are drug-like, display selectivity for mGluR4, mGluR7, or mGluR8 and act via novel pharmacological sites. Here, we illustrate new scientific insights obtained via the use of those strategies. Also, we compare advantages and disadvantages of both approaches to identify the desired group-III mGluR activators and we conclude with suggestions how to employ those discovery strategies with success for the identification, optimization, and development of clinical drug candidates; this may have important implications for the entire field of GPCR research.

Metabotropic glutamate 7 (mGlu7) receptor: a target for medication development for the treatment of cocaine dependence

Brain glutamate has been shown to play an important role in reinstatement to drug seeking, a behavior considered to be of relevance to relapse to drug taking in humans. Therefore, glutamate receptors, in particular metabotropic glutamate (mGlu) receptors, have become important targets for medication development for the treatment of drug dependence. In this review article, we focus on the mGlu7 receptor subtype, and discuss recent findings with AMN082, a selective mGlu7 receptor allosteric agonist, in animal models with relevance to drug dependence. Systemic or local administration of AMN082 into the nucleus accumbens (NAc), a critical brain region involved in reward and drug dependence processes, inhibited the reinforcing and motivational effects of cocaine, heroin and ethanol, as assessed by the intravenous drug self-administration procedure. In addition, AMN082 inhibited the reward-enhancing effects induced by cocaine, as assessed in the intracranial self-stimulation procedure, and cocaine- or cue-induced reinstatement of drug-seeking behavior. In vivo microdialysis studies indicated that systemic or intra-NAc administration of AMN082 significantly decreased extracellular γ-aminobutyric acid (GABA) and elevated extracellular glutamate, but had no effect on extracellular dopamine in the NAc, suggesting that a non-dopaminergic mechanism underlies the effects of AMN082 on the actions of cocaine. Further, data indicated that AMN082-induced changes in glutamate were the net effect of two actions: one is the direct inhibition of glutamate release by activation of mGlu7 receptors on glutamatergic neurons; another is the indirect increases of glutamate release mediated by decreases in GABA transmission. These increases in extracellular glutamate functionally antagonized cocaine-induced inhibition of NAc-ventral pallidum GABAergic neurotransmission, and therefore, the rewarding effects of cocaine. In addition, elevated extracellular glutamate activated presynaptic mGlu2/3 autoreceptors which in turn inhibited cocaine priming- or cue-induced enhancement of glutamate release and reinstatement of drug-seeking behavior. Taken together, these findings suggest that the mGlu7 receptor is an important target for medication development for the treatment of drug dependence. AMN082 or other mGlu7 receptor allosteric agonists may have potential as novel pharmacotherapies for cocaine addiction. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Metabotropic glutamate receptor 4 (mGlu4)-positive allosteric modulators for the treatment of Parkinson's disease: historical perspective and review of the patent literature

INTRODUCTION

Metabotropic glutamate receptor 4 (mGlu(4)) is a group III GPCR and has been demonstrated to play a major role in a number of therapeutic areas within the CNS. As the orthosteric site of all glutamate receptors is highly conserved, modulating mGlu(4) via allosteric modulation has emerged as a very attractive mode-of-action and has been validated preclinically in a number of animal models for Parkinson's disease, anxiety, pain, and neuroinflammation.

AREAS COVERED

In this review, the patent literature for mGlu(4)-positive allosteric modulators over the past 4 years will be provided. Patents from all companies are discussed and an overview of the chemical matter and relevant biological properties will be given.

EXPERT OPINION

Although there has yet to be an mGlu(4)-positive allosteric modulator progressed into clinical trials, there is a wealth of preclinical data from the primary literature that shows the promise of this emerging target. A number of academic and industry laboratories have recently published exciting patent data covering a multitude of chemical matter. Positive allosteric modulation of mGlu(4) remains one of the more attractive non-dopaminergic therapies for Parkinson's disease, as well as emerging data for other indications such as pain, neuroinflammation, schizophrenia and diabetes, which could potentially make mGlu(4) a significant therapeutic target going forward.

Intrinsic modulators of auditory thalamocortical transmission

Neurons in layer 4 of the primary auditory cortex receive convergent glutamatergic inputs from thalamic and cortical projections that activate different groups of postsynaptic glutamate receptors. Of particular interest in layer 4 neurons are the Group II metabotropic glutamate receptors (mGluRs), which hyperpolarize neurons postsynaptically via the downstream opening of GIRK channels. This pronounced effect on membrane conductance could influence the neuronal processing of synaptic inputs, such as those from the thalamus, essentially modulating information flow through the thalamocortical pathway. To examine how Group II mGluRs affect thalamocortical transmission, we used an in vitro slice preparation of the auditory thalamocortical pathways in the mouse to examine synaptic transmission under conditions where Group II mGluRs were activated. We found that both pre- and post-synaptic Group II mGluRs are involved in the attenuation of thalamocortical EPSP/Cs. Thus, thalamocortical synaptic transmission is suppressed via the presynaptic reduction of thalamocortical neurotransmitter release and the postsynaptic inhibition of the layer 4 thalamorecipient neurons. This could enable the thalamocortical pathway to autoregulate transmission, via either a gating or gain control mechanism, or both.

Test-retest stability of cerebral mGluR₅ quantification using [¹¹C]ABP688 and positron emission tomography in rats

This study evaluates the reproducibility of the quantification of metabotropic glutamate receptor type 5 (mGluR₅) densities in rats using the PET radiotracer [¹¹C]ABP688 and pharmacokinetic models that are based on an input function, which is derived from a reference tissue. Seven rats underwent dynamic PET scans (60 min) after bolus injection of [¹¹C]ABP688. Kinetic analyses included: binding potential (BP(ND) ) determined by calculating (a) the simplified reference tissue model (SRTM) and (b) its two-steps simplified version (SRTM2); (c) multilinear reference tissue model (MRTM) and (d) its 2-parameter version (MRTM2); (e) noninvasive graphical analysis (NIGA). Parametric images were generated representing BP(ND) by the MRTM2 model. BP(ND) determinations were reproducible with low to acceptable variability ranging from 5 to 10% and reproducibility scores (intraclass correlation coefficient) between 0.51 and 0.88. The pharmacokinetic model that showed lowest overall variability was the SRTM. In contrast, the use of the NIGA was associated with significantly lower reproducibility scores. Comparison of parametric images revealed no significant bias between test and retest measurements and is therefore suitable to compare groups at voxel levels. In conclusion, our results suggest that noninvasive quantification of [¹¹C]ABP688 imaging is reproducible and reliable for PET studies of the cerebral mGluR₅ in rats.

Differentiating the roles of mGlu2 and mGlu3 receptors using LY541850, an mGlu2 agonist/mGlu3 antagonist

Despite the potential therapeutic relevance of group II metabotropic glutamate (mGlu) receptors, there has been a lack of pharmacological tools for separating the roles of mGlu2 and mGlu3 receptor subtypes. LY541850 was claimed from human mGlu receptors expressed in non-neuronal cells to be a selective orthosteric mGlu2 agonist and mGlu3 antagonist. We have verified this pharmacological profile of LY541850 in hippocampal slices. Field excitatory post-synaptic potentials (fEPSPs) evoked by stimulation of the temporo-ammonic path (TAP) input to CA1 stratum lacunosum moleculare (SLM) were inhibited by LY541850 in mGlu3-/- mice (EC(50) 38 nM) and wild-type littermates (EC(50) 42 nM) to a similar extent but were not significantly affected in mGlu2-/- mice. The group II agonist, DCG-IV, inhibited the fEPSP in all three genotypes. Co-application of DCG-IV and LY541850 in mGlu3-/- and wild-type littermates resulted in an additive effect, whereas in mGlu2-/- mice, LY541850 reversed the inhibitory action of DCG-IV. These results confirm the selective mGlu2 agonist and mGlu3 antagonist actions of LY541850. A similar profile of activity was seen in medial perforant path synapse to the dentate gyrus. Systemic administration of LY541850 to wild-type mice, reduced the increase in locomotor activity following both phencyclidine and amphetamine administration. These data support the hypothesis that mGlu2 receptors mediate the antipsychotic effects of mixed group II agonists. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Potentiation of synaptic strength and intrinsic excitability in the nucleus accumbens after 10 days of morphine withdrawal

Neuroadaptations in the nucleus accumbens (NAc) are associated with the development of drug addiction. Plasticity in synaptic strength and intrinsic excitability of NAc medium spiny neurons (MSNs) play critical roles in addiction induced by different classes of abused drugs. However, it is unknown whether morphine exposure influences synaptic strength, intrinsic excitability or both in NAc. Here we show that chronic withdrawal (10 days after the last injection) from repeated morphine exposure elicited potentiation in both glutamatergic synaptic strength and intrinsic excitability of MSNs in NAc shell (NAcSh). The potentiation of synaptic strength was demonstrated by an increase in the frequency of miniature excitatory postsynaptic currents (mEPSCs), a decrease in the paired-pulse ratio (PPR), and an increase in the ratio of α-amino-3-hydroxy-5-methyl-isoxazole propionic acid receptors (AMPAR)- to N-methyl-D-aspartate receptors (NMDAR)-mediated currents. The potentiation of intrinsic excitability was mediated by inhibition of the sustained potassium currents via extrasynaptic NMDAR activation. The function of the presynaptic group II metabotropic glutamate receptors (mGluR2/3) was downregulated, enhancing the probability of glutamate release on synaptic terminals during chronic morphine withdrawal. Pretreatment with the mGluR2/3 agonist LY379268 completely blocked potentiation of both synaptic strength and intrinsic excitability. These results suggest that chronic morphine withdrawal downregulates mGluR2/3 to induce potentiation of MSN glutamatergic synapse via increased glutamate release, leading to potentiation of intrinsic excitability. Such potentiation of both synaptic strength and intrinsic excitability might contribute to neuroadaptations induced by morphine application.

Discovery of 1,4-disubstituted 3-cyano-2-pyridones: a new class of positive allosteric modulators of the metabotropic glutamate 2 receptor

The discovery and characterization of compound 48, a selective and in vivo active mGlu2 receptor positive allosteric modulator (PAM), are described. A key to the discovery was the rational exploration of the initial HTS hit 13 guided by an overlay model built with reported mGlu2 receptor PAM chemotypes. The initial weak in vitro activity of the hit 13 was quickly improved, although compounds still had suboptimal druglike properties. Subsequent modulation of the physicochemical properties resulted in compounds having a more balanced profile, combining good potency and in vivo pharmacokinetic properties. Final refinement by addressing cardiovascular safety liabilities led to the discovery of compound 48. Besides good potency, selectivity, and ADME properties, compound 48 displayed robust in vivo activity in a sleep-wake electroencephalogram (sw-EEG) assay consistent with mGlu2 receptor activation, in accordance with previous work from our laboratories.

Activation of group III metabotropic glutamate receptors by endogenous glutamate protects against glutamate-mediated excitotoxicity in the hippocampus in vivo

Perfusion of 4-aminopyridine (4-AP) by microdialysis in the hippocampus produces intense epileptiform behavioral and electrical activity and neurodegeneration, resulting from a stimulated release of glutamate from nerve endings. In contrast, accumulation of extracellular glutamate by blockade of its transport in vivo in anesthetized rats is innocuous, and studies in vitro in brain slices suggest that under these conditions glutamate may activate presynaptic group III metabotropic glutamate receptors (mGluRs) and inhibit its own release. Therefore, using microdialysis, EEG recording, and histological evaluation, we studied the effect of increased endogenous extracellular glutamate by blockade of its transport with pyrrolidine dicarboxylic acid (PDC) on the excitotoxic action of 4-AP in the hippocampus of awake rats. We found that up to a 20-fold increase in extracellular glutamate during >90 min with PDC does not induce any sign of excitotoxicity. On the contrary, this glutamate increase notably protected against the 4-AP-induced seizures and neurodegeneration, and, remarkably, this protection was dependent on the time of perfusion with PDC and thus on the duration of extracellular glutamate accumulation. To test whether this protective action was mediated by the activation of group III mGluRs, we used specific antagonists of these receptors and found that they clearly prevented the protective effect of PDC, without affecting the accumulation of extracellular glutamate. We conclude that the spillover of the excess extracellular glutamate activates presynaptic group III mGluRs and inhibits the stimulatory effect of 4-AP on its release, thus preventing the activation of postsynaptic N-methyl-D-aspartate receptors and its deleterious consequences.

Measures of anxiety, sensorimotor function, and memory in male and female mGluR4⁻/⁻ mice

Metabotropic glutamate receptors (mGluRs) are coupled to second messenger pathways via G proteins and modulate synaptic transmission. Of the eight different types of mGluRs (mGluR1-mGluR8), mGluR4, mGluR6, mGluR7, and mGluR8 are members of group III. Group III receptors are generally located presynaptically, where they regulate neurotransmitter release. Because of their role in modulating neurotransmission, mGluRs are attractive targets for therapies aimed at treating anxiety disorders. Previously we showed that the mGluR4-selective allosteric agonist VU 0155041 reduces anxiety-like behavior in wild-type male mice. Here, we explore the role of mGluR4 in adult (6-month old) and middle-aged (12-month old) male and female mice lacking this receptor. Compared to age- and sex-matched wild-type mice, middle-aged mGluR4(-/-) male mice showed increased measures of anxiety in the open field and elevated zero maze and impaired sensorimotor function on the rotarod. These changes were not seen in adult 6-month-old male mice. In contrast to the male mice, mGluR4(-/-) female mice showed reduced measures of anxiety in the open field and elevated zero maze and enhanced rotarod performance. During the hidden platform training sessions of the water maze, mGluR4(-/-) mice swam farther away from the platform than wild-type mice at 6, but not at 12, months of age. mGluR4(-/-) mice also showed enhanced amygdala-dependent cued fear conditioning. No genotype differences were seen in hippocampus-dependent contextual fear conditioning. These data indicate that effects of mGluR4 on sensorimotor function and measures of anxiety, but not cued fear conditioning, are critically modulated by sex and age.

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.

Parkinson's disease therapeutics: new developments and challenges since the introduction of levodopa

The demonstration that dopamine loss is the key pathological feature of Parkinson's disease (PD), and the subsequent introduction of levodopa have revolutionalized the field of PD therapeutics. This review will discuss the significant progress that has been made in the development of new pharmacological and surgical tools to treat PD motor symptoms since this major breakthrough in the 1960s. However, we will also highlight some of the challenges the field of PD therapeutics has been struggling with during the past decades. The lack of neuroprotective therapies and the limited treatment strategies for the nonmotor symptoms of the disease (ie, cognitive impairments, autonomic dysfunctions, psychiatric disorders, etc.) are among the most pressing issues to be addressed in the years to come. It appears that the combination of early PD nonmotor symptoms with imaging of the nigrostriatal dopaminergic system offers a promising path toward the identification of PD biomarkers, which, once characterized, will set the stage for efficient use of neuroprotective agents that could slow down and alter the course of the disease.

Opposing efficacy of group III mGlu receptor activators, LSP1-2111 and AMN082, in animal models of positive symptoms of schizophrenia

RATIONALE

Several studies have suggested that modulation of the glutamatergic system via metabotropic glutamate receptors (mGlu) could be a new and efficient way to achieve antipsychotic-like activity.

OBJECTIVES

Here, we decided to investigate the possible role of the group III mGlu receptor ligands, LSP1-2111, the group III mGlu receptor orthosteric agonist, preferentially stimulating mGlu4 receptors especially in low doses, and AMN082, the mGlu7 receptor positive modulator. We used MK-801- and amphetamine-induced hyperactivity tests, as well as DOI-induced head twitches in mice as models for positive symptoms of psychosis. The C57Bl/6J mGlu7 receptor knockout mice were used to confirm that AMN082-induced effect was receptor specific. A non-selective antagonist of the group II/III mGlu receptors, LY341495, was used to block LSP1-2111-induced effects.

RESULTS

LSP1-2111 (1, 2, and 5 mg kg(-1)) dose dependently inhibited both MK-801- and amphetamine-induced hyperactivities. Moreover, the drug antagonized DOI-induced head twitches. The effects of the drug were antagonized by LY341495 administration (1.5 mg kg(-1), i.p.). In contrast, AMN082 (3 and 6 mg kg(-1)) had no effect on amphetamine-induced hyperactivity but induced an enhancement of MK-801-induced hyperactivity and DOI-induced head twitches in mice. In C57Bl/6J mGlu7 receptor knockout animals (KO), those effects of AMN082 were not observed. Moreover, mGlu7 KO animals were less sensitive for DOI-induced effect than their wild type littermates.

CONCLUSIONS

Altogether, we propose that among group III mGlu receptors, mGlu4 receptor may be a promising target for the development of novel antipsychotic drugs.

Pre-treatment with the mGlu2/3 receptor agonist LY379268 attenuates DOI-induced impulsive responding and regional c-Fos protein expression

RATIONALE

Overactivation of serotonin (5-hydroxytryptamine, 5-HT)(2A) receptors causes impulsivity and attentional deficits. Since 5-HT(2A) receptors are known to entertain antagonistic interactions with metabotropic glutamate (mGlu)2/3 receptors, this interaction may provide an alternative target for a novel class of antipsychotics.

OBJECTIVES/METHODS

The study characterizes interactions between 5-HT(2A) and mGlu2/3 receptors implicated in impulse control. Hooded Lister rats were trained in a 5-choice serial reaction time task (5-CSRTT) and treated with the 5-HT(2A/2C) receptor agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropan hydrochloride (DOI, 0.1 mg/kg) and the mGlu2/3 receptor agonist LY379268 (1 mg/kg). In addition, associated drug-induced changes in neuronal activity were assessed via c-Fos immunoreactivity (Fos IR), and co-localization of c-Fos and GABAergic markers was detected using double immunofluorescence labeling.

RESULTS

Systemic DOI caused impulsive overresponding that was attenuated in animals pre-treated with LY379268. LY379268 itself had no significant effect on the rats' performance in the 5-CSRTT. DOI enhanced Fos IR within fronto-cortical and limbic brain structures, and this effect was blocked by LY379268 pre-treatment. Double immunofluorescence labeling showed a specific co-localization of DOI-elicited Fos IR with GABAergic (GAD(67)-positive) cells lacking the calcium-binding protein parvalbumin while LY379268 increased Fos IR in GABAergic and non-GABAergic cells.

CONCLUSION

Our results suggest that impulsivity is possibly due to a primary increase in Glu transmission mediated via 5-HT(2A) receptor activation. Thus, mGlu2/3 receptor agonists might have some potential for treating motor impulsivity-related impairments while their cognitive enhancing effects were not confirmed in this study.

Simulation of postsynaptic glutamate receptors reveals critical features of glutamatergic transmission

Activation of several subtypes of glutamate receptors contributes to changes in postsynaptic calcium concentration at hippocampal synapses, resulting in various types of changes in synaptic strength. Thus, while activation of NMDA receptors has been shown to be critical for long-term potentiation (LTP) and long term depression (LTD) of synaptic transmission, activation of metabotropic glutamate receptors (mGluRs) has been linked to either LTP or LTD. While it is generally admitted that dynamic changes in postsynaptic calcium concentration represent the critical elements to determine the direction and amplitude of the changes in synaptic strength, it has been difficult to quantitatively estimate the relative contribution of the different types of glutamate receptors to these changes under different experimental conditions. Here we present a detailed model of a postsynaptic glutamatergic synapse that incorporates ionotropic and mGluR type I receptors, and we use this model to determine the role of the different receptors to the dynamics of postsynaptic calcium with different patterns of presynaptic activation. Our modeling framework includes glutamate vesicular release and diffusion in the cleft and a glutamate transporter that modulates extracellular glutamate concentration. Our results indicate that the contribution of mGluRs to changes in postsynaptic calcium concentration is minimal under basal stimulation conditions and becomes apparent only at high frequency of stimulation. Furthermore, the location of mGluRs in the postsynaptic membrane is also a critical factor, as activation of distant receptors contributes significantly less to calcium dynamics than more centrally located ones. These results confirm the important role of glutamate transporters and of the localization of mGluRs in postsynaptic sites in their signaling properties, and further strengthen the notion that mGluR activation significantly contributes to postsynaptic calcium dynamics only following high-frequency stimulation. They also provide a new tool to analyze the interactions between metabotropic and ionotropic glutamate receptors.

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

Group II metabotropic glutamate receptors (mGluR2 and mGluR3, encoded by GRM2 and GRM3) are implicated in hippocampal function and cognition, and in the pathophysiology and treatment of schizophrenia and other psychiatric disorders. However, pharmacological and behavioral studies with group II mGluR agonists and antagonists have produced complex results. Here, we studied hippocampus-dependent memory in GRM2/3 double knockout (GRM2/3(-/-)) mice in an iterative sequence of experiments. We found that they were impaired on appetitively motivated spatial reference and working memory tasks, and on a spatial novelty preference task that relies on animals' exploratory drive, but were unimpaired on aversively motivated spatial memory paradigms. GRM2/3(-/-) mice also performed normally on an appetitively motivated, non-spatial, visual discrimination task. These results likely reflect an interaction between GRM2/3 genotype and the arousal-inducing properties of the experimental paradigm. The deficit seen on appetitive and exploratory spatial memory tasks may be absent in aversive tasks because the latter induce higher levels of arousal, which rescue spatial learning. Consistent with an altered arousal-cognition relationship in GRM2/3(-/-) mice, injection stress worsened appetitively motivated, spatial working memory in wild-types, but enhanced performance in GRM2/3(-/-) mice. GRM2/3(-/-) mice were also hypoactive in response to amphetamine. This fractionation of hippocampus-dependent memory depending on the appetitive-aversive context is to our knowledge unique, and suggests a role for group II mGluRs at the interface of arousal and cognition. These arousal-dependent effects may explain apparently conflicting data from previous studies, and have translational relevance for the involvement of these receptors in schizophrenia and other disorders.

The mGluR7 allosteric agonist AMN082 produces antidepressant-like effects by modulating glutamatergic signaling

Currently prescribed antidepressants affect the reuptake and/or metabolism of biogenic amines. Unfortunately for patients, these treatments require several weeks to produce significant symptom remission. However, recently it has been found that ketamine, a dissociative anesthetic agent that noncompetitively antagonizes NMDA (N-Methyl-d-aspartic acid) receptors, has rapid antidepressant effects at sub-anesthetic doses in clinically depressed patients. These findings indicate that modulation of the glutamatergic system could be an efficient way to achieve antidepressant activity. For this reason, other mechanisms influencing glutamatergic functioning have gained interest. For example, the metabotropic glutamate receptor 7 (mGluR7) allosteric agonist AMN082 (N,N'-dibenzyhydryl-ethane-1,2-diamine dihydrochloride) has been shown to be effective in the forced swim and tail-suspension test, behavioral assays sensitive to antidepressants. Here we extend the characterization of AMN082 by demonstrating its effects on differential reinforcement of low rates of responding (DRL)-30, another assay sensitive to antidepressants. Furthermore, we show the engagement of glutamatergic signaling by demonstrating the ability of the selective AMPA (2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid) receptor antagonist NBQX (2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione) to reverse the effects of AMN082 in the tail suspension test. In contrast, NBQX failed to reverse the effects of imipramine in the same behavioral test. Finally, we report that behaviorally efficacious doses of AMN082 modulate phosphorylation of AMPA and NMDA receptor subunits in the hippocampus. These results suggest that the antidepressant-like effects of AMN082 are, at least in part, due to modulation of AMPA and NMDA receptor activity. Therefore, our findings confirm the hypothesis that mGluR7 could represent a novel target for treating depression.

Pain-related deactivation of medial prefrontal cortical neurons involves mGluR1 and GABA(A) receptors

Pain-related hyperactivity in the amygdala leads to deactivation of the medial prefrontal cortex (mPFC) and decision-making deficits. The mechanisms of pain-related inhibition of the mPFC are not yet known. Here, we used extracellular single-unit recordings of prelimbic mPFC neurons to determine the role of GABA(A) receptors and metabotropic glutamate receptor (mGluR) subtypes, mGluR1 and mGluR5, in pain-related activity changes of mPFC neurons. Background and evoked activity of mPFC neurons decreased after arthritis induction. To determine pain-related changes, the same neuron was recorded continuously before and after induction of arthritis in one knee joint by intra-articular injection of kaolin/carrageenan. Stereotaxic administration of a GABA(A) receptor antagonist {[R-(R*,S*)]-5-(6,8-dihydro-8-oxofuro[3,4-e]-1,3-benzodioxol-6-yl)-5,6,7,8-tetrahydro-6,6-dimethyl-1,3-dioxolo[4,5-g]isoquinolinium iodide (bicuculline)} into the mPFC by microdialysis reversed pain-related inhibition, whereas offsite injections into the adjacent anterior cingulate cortex had no or opposite effects on prelimbic mPFC neurons. A selective mGluR1/5 agonist [(S)-3,5-dihydroxyphenylglycine (DHPG)] inhibited background and evoked activity under normal conditions through a GABAergic mechanism, because the inhibitory effect was blocked with bicuculline. In the arthritis pain state, DHPG, alone or in the presence of bicuculline, had no effect. Consistent with the involvement of mGluR1 in pain-related inhibition of the mPFC, a selective mGluR1 antagonist [(S)-(+)-α-amino-4-carboxy-2-methylbenzeneacetic acid] reversed the pain-related decrease of background and evoked activity of mPFC neurons in arthritis, whereas a selective mGluR5 antagonist [2-methyl-6-(phenylethynyl)pyridine hydrochloride] had no effect. The mGluR antagonists had no effect under normal conditions. We interpret our data to suggest that pain-related inhibition of mPFC neurons in the arthritis model depends on mGluR1-mediated endogenous activation of GABA(A) receptors. Exogenous activation of mGluR1/5 produces GABAergic inhibition under normal conditions. Restoring normal activity in the mPFC may be a therapeutic strategy to improve cognitive deficits associated with persistent pain.

Involvement of metabotropic glutamate receptor 5 in pudendal inhibition of nociceptive bladder activity in cats

This study used MTEP, a metabotropic glutamate receptor 5 (mGluR5) antagonist, to examine the role of mGluR5 in the neural control of the urinary bladder and in the inhibition of the micturition reflex by pudendal nerve stimulation (PNS). Experiments were conducted in 11 female cats under α-chloralose anaesthesia when the bladder was infused with either saline or 0.25% acetic acid (AA). AA irritated the bladder, induced bladder overactivity and significantly (P < 0.001) reduced bladder capacity to 14.9 ± 10.3% of the saline control capacity. MTEP (0.1-50 mg kg(-1), i.v.) significantly (P < 0.05) increased bladder capacity during saline distension but not during AA irritation. However, MTEP induced a transient inhibition of isovolumetric bladder contractions under both conditions. PNS (5 Hz), which was tested at the threshold (T) intensity for inducing a complete inhibition of isovolumetric bladder contractions and at an intensity of 3-4T, suppressed AA-induced bladder overactivity and significantly increased bladder capacity to 68.0 ± 31.3% at 1T (P < 0.05) and 98.5 ± 55.3% at 3-4T (P < 0.01) of the saline control capacity. MTEP dose dependently (0.1-50 mg kg(-1), i.v.) suppressed PNS inhibition of bladder overactivity at low intensity (1T) but not at high intensity (3-4T). During saline infusion PNS significantly (P < 0.05) increased bladder capacity to 167.7 ± 27.1% at 1T and 196.0 ± 37.4% at 3-4T. These inhibitory effects were not observed after MTEP (0.1-50 mg kg(-1), i.v.) which also increased bladder capacity. These results indicate that glutamic acid has a transmitter function in bladder and somato-bladder reflex mechanisms and raise the possibility that mGluR5 may be a target for pharmacological treatment of lower urinary tract disorders.

Translating glutamate: from pathophysiology to treatment

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

Sleep-deprivation induces changes in GABA(B) and mGlu receptor expression and has consequences for synaptic long-term depression

Long term depression (LTD) in the CA1 region of the hippocampus, induced with a 20-Hz, 30 s tetanus to Schaffer collaterals, is enhanced in sleep-deprived (SD) rats. In the present study, we investigated the role of metabotropic glutamate receptors (mGluRs), γ-aminobutyric acid (GABA) B receptors (GABA(B)-Rs) and N-methyl-D-aspartic acid receptors (NMDARs) in the LTD of the population excitatory postsynaptic potential (pEPSP). The requirement of Ca(2+) from L- and T-type voltage-gated calcium channels (VGCCs) and intracellular stores was also studied. Results indicate that mGluRs, a release of Ca(2+) from intracellular stores and GABA(B)-Rs are required for LTD. Interestingly, while mGlu1Rs seem to be involved in both short-term depression and LTD, mGlu5Rs appear to participate mostly in LTD. CGP 55845, a GABA(B)-R antagonist, partially suppressed LTD in normally sleeping (NS) rats, while completely blocking LTD in SD rats. Moreover, GS-39783, a positive allosteric modulator for GABA(B)-R, suppressed the pEPSP in SD, but not NS rats. Since both mGluRs and GABA(B)-Rs seem to be involved in the LTD, especially in SD rats, we examined if the receptor expression pattern and/or dimerization changed, using immunohistochemical, co-localization and co-immunoprecipitation techniques. Sleep-deprivation induced an increase in the expression of GABA(B)-R1 and mGlu1αR in the CA1 region of the hippocampus. In addition, co-localization and heterodimerization between mGlu1αR/GABA(B)-R1 and mGlu1αR/GABA(B)-R2 is enhanced in SD rats. Taken together, our findings present a novel form of LTD sensitive to the activation of mGluRs and GABA(B)-Rs, and reveal, for the first time, that sleep-deprivation induces alterations in the expression and dimerization of these receptors.

Group II/III metabotropic glutamate receptors exert endogenous activity-dependent modulation of TRPV1 receptors on peripheral nociceptors

There is pharmacological evidence that group II and III metabotropic glutamate receptors (mGluRs) function as activity-dependent autoreceptors, inhibiting transmission in supraspinal sites. These receptors are expressed by peripheral nociceptors. We investigated whether mGluRs function as activity-dependent autoreceptors inhibiting pain transmission to the rat CNS, particularly transient receptor potential vanilloid 1 (TRPV1)-induced activity. Blocking peripheral mGluR activity by intraplantar injection of antagonists LY341495 [(2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid] (LY) (20, 100 μm, group II/III), APICA [(RS)-1-amino-5-phosphonoindan-1-carboxylic acid] (100 μm, group II), or UBP1112 (α-methyl-3-methyl-4-phosphonophenylglycine) (30 μm, group III) increased capsaicin (CAP)-induced nociceptive behaviors and nociceptor activity. In contrast, group II agonist APDC [(2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate] (0.1 μm) or group III agonist l-(+)-2-amino-4-phosphonobutyric acid (l-AP-4) (10 μm) blocked the LY-induced increase. Ca(2+) imaging in dorsal root ganglion (DRG) cells confirmed LY enhanced CAP-induced Ca(2+) mobilization, which was blocked by APDC and l-AP-4. We hypothesized that excess glutamate (GLU) released by high intensity and/or prolonged stimulation endogenously activated group II/III, dampening nociceptor activation. In support of this, intraplantar GLU + LY produced heat hyperalgesia, and exogenous GLU + LY applied to nociceptors produced enhanced nociceptor activity and thermal sensitization. Intraplantar Formalin, known to elevate extracellular GLU, enhanced pain behaviors in the presence of LY. LY alone produced no pain behaviors, no change in nociceptor discharge rate or heat-evoked responses, and no change in cytosolic Ca(2+) in DRG cells, demonstrating a lack of tonic inhibitory control. Group II/III mGluRs maintain an activity-dependent autoinhibition, capable of significantly reducing TRPV1-induced activity. They are endogenously activated after high-frequency and/or prolonged nociceptor stimulation, acting as built-in negative modulators of TRPV1 and nociceptor function, reducing pain transmission to the CNS.

The metabotropic glutamate 2/3 receptor agonist LY379268 blocked nicotine-induced increases in nucleus accumbens shell dopamine only in the presence of a nicotine-associated context in rats

The metabotropic glutamate 2/3 (mGlu2/3) receptor agonist LY379268 ([-]-2-oxa-4-aminobicyclo [3.1.0] hexane-4,6-dicarboxylate) attenuates both nicotine self-administration and cue-induced nicotine seeking in rats. In this study, the effects of LY379268 (1 mg/kg) or saline pretreatment on nicotine-induced increases in nucleus accumbens (NAcc) shell dopamine were evaluated using in vivo microdialysis under different experimental conditions: (i) nicotine (0.4 mg/kg, base) was experimenter-administered subcutaneously to nicotine-naïve rats; (ii) nicotine was experimenter-administered either subcutaneously (0.4 mg/kg) or by a single experimenter-administered infusion (0.06 mg/kg, base) in rats with a history of nicotine self-administration (nicotine experienced) in the absence of a nicotine-associated context (ie, context and cues associated with nicotine self-administration); (iii) nicotine (0.06 mg/kg) was self-administered or experimenter-administered in nicotine-experienced rats in the presence of a nicotine-associated context. In saline-pretreated nicotine-naïve and nicotine-experienced rats, nicotine increased NAcc shell dopamine regardless of the context used for testing. Interestingly, LY379268 pretreatment blocked nicotine-induced increases in NAcc shell dopamine in nicotine-experienced rats only when tested in the presence of a nicotine-associated context. LY379268 did not block nicotine-induced increases in NAcc shell dopamine in nicotine-naïve or -experienced rats tested in the absence of a nicotine-associated context. These intriguing findings suggest that activation of mGlu2/3 receptors negatively modulates the combined effects of nicotine and nicotine-associated contexts/cues on NAcc dopamine. Thus, these data highlight a critical role for mGlu2/3 receptors in context/cue-induced drug-seeking behavior and suggest a neurochemical mechanism by which mGlu2/3 receptor agonists may promote smoking cessation by preventing relapse induced by the combination of nicotine and nicotine-associated contexts and cues.

Interaction of mGlu2/3 agonism with clozapine and lurasidone to restore novel object recognition in subchronic phencyclidine-treated rats

RATIONALE

Subchronic administration to rodents of the N-methyl-D-aspartate non-competitive antagonist, phencyclidine (PCP), impairs novel object recognition (NOR). Atypical antipsychotic drugs (APDs) reverse the effects of subchronic PCP on NOR. The effect of metabotropic glutamate₂/₃ receptor (mGlu₂/₃) agonists upon NOR is unknown.

OBJECTIVES AND METHODS

We tested the hypotheses that the mGlu₂/₃ agonist, LY379268, by itself, or in combination with APDs or pimavanserin, a 5-HT(2A) inverse agonist, would reverse the deficit in NOR induced by subchronic treatment with PCP (2 mg/kg, b.i.d., for 7 days).

RESULTS

The mGlu₂/₃ agonist LY379268 (1 or 3 mg/kg) did not attenuate the PCP-induced NOR deficit. However, together with sub-effective dose of the atypical APDs, clozapine (0.1 mg/kg) or lurasidone (0.03 mg/kg), but not the typical APD, haloperidol (0.1 mg/kg), or pimavanserin (3 mg/kg), LY379268, 1 mg/kg, significantly reversed the PCP-induced NOR deficit. Moreover, the effect of clozapine was blocked by the mGlu₂/₃ antagonist, LY341495 (1 mg/kg).

CONCLUSIONS

These results indicate that mGlu₂/₃ agonism can potentiate the ability of atypical, but not typical APDs, to ameliorate the effect of subchronic PCP on NOR, that mGlu₂/₃ agonism may contribute to the ability of atypical APDs to acutely reverse the effect of subchronic PCP on NOR, but that by itself, mGlu₂/₃ agonism, is ineffective in this model of cognitive impairment in schizophrenia. These results suggest that mGlu₂/₃ receptor agonism should be investigated as an adjunctive treatment of cognitive impairment in schizophrenia rather than as monotherapy, which may be effective for control of psychosis, but not for cognitive impairment.

Understanding the physical mechanism of transition to epileptic seizures

The mechanisms of generating epileptic seizures are still unknown. To identify the mechanisms that underlie the transition to seizure a combination of features that include firing rate, power spectrum and complexity measures were simultaneously analyzed. Pre-ictal periods are characterized by large fluctuations of firing rate which reflect local dysfunctional regulation of neuronal activity. This local dysfunction in neuronal activity is translated in changes of endogenous electric field within clustered regions with high frequency oscillations (HFO) that act at fundamental level of charge dynamics and lead to chaotic dynamics followed by electrical resonances. Right before the onset of seizures the presence of chaotic behavior becomes persistent and leads all types of cells to fire simultaneously and generate the transition to ictal state. The alteration in neuronal regulation and the nature of physical phenomena involved in this transition supports some models of seizure generation and rules out others.

The role of glutamate on the action of antidepressants

Major depressive disorder (MDD) is a common, chronic, recurrent mental illness that affects millions of individuals worldwide. Currently available antidepressants are known to affect the monoaminergic (e.g., serotonin, norepinephrine, and dopamine) systems in the brain. Accumulating evidence suggests that the glutamatergic neurotransmission via the excitatory amino acid glutamate also plays an important role in the neurobiology and treatment of this disease. Clinical studies have demonstrated that the non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist ketamine has rapid antidepressant effects in treatment-resistant patients with MDD, suggesting the role of glutamate in the pathophysiology of treatment-resistant MDD. Furthermore, a number of preclinical studies demonstrated that the agents which act at glutamate receptors such as NMDA receptors, α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors and metabotropic glutamate receptors (mGluRs) might have antidepressant-like activities in animal models of depression. In this article, the author reviews the role of glutamate in the neuron-glia communication induced by potential antidepressants.

Control of striatal signaling by g protein regulators

Signaling via heterotrimeric G proteins plays a crucial role in modulating the responses of striatal neurons that ultimately shape core behaviors mediated by the basal ganglia circuitry, such as reward valuation, habit formation, and movement coordination. Activation of G protein-coupled receptors (GPCRs) by extracellular signals activates heterotrimeric G proteins by promoting the binding of GTP to their α subunits. G proteins exert their effects by influencing the activity of key effector proteins in this region, including ion channels, second messenger enzymes, and protein kinases. Striatal neurons express a staggering number of GPCRs whose activation results in the engagement of downstream signaling pathways and cellular responses with unique profiles but common molecular mechanisms. Studies over the last decade have revealed that the extent and duration of GPCR signaling are controlled by a conserved protein family named regulator of G protein signaling (RGS). RGS proteins accelerate GTP hydrolysis by the α subunits of G proteins, thus promoting deactivation of GPCR signaling. In this review, we discuss the progress made in understanding the roles of RGS proteins in controlling striatal G protein signaling and providing integration and selectivity of signal transmission. We review evidence on the formation of a macromolecular complex between RGS proteins and other components of striatal signaling pathways, their molecular regulatory mechanisms and impacts on GPCR signaling in the striatum obtained from biochemical studies and experiments involving genetic mouse models. Special emphasis is placed on RGS9-2, a member of the RGS family that is highly enriched in the striatum and plays critical roles in drug addiction and motor control.

Study of novel selective mGlu2 agonist in the temporo-ammonic input to CA1 neurons reveals reduced mGlu2 receptor expression in a Wistar substrain with an anxiety-like phenotype

Group II metabotropic receptors (mGluRs) regulate central synaptic transmission by modulating neurotransmitter release. However, the lack of pharmacological tools differentiating between mGlu2 and mGlu3 receptors has hampered identification of the roles of these two receptor subtypes. We have used LY395756 [(1SR,2SR,4RS,5RS,6SR)-2-amino-4-methylbicyclo[3.1.0]-hexane2,6-dicarboxylic], an agonist at mGlu2 receptors and an antagonist at mGlu3 receptors in cell lines, to investigate the roles of these receptors in the temporo-ammonic path from entorhinal cortex to CA1-stratum lacunosum moleculare in rat hippocampal slices. Surprisingly, the degree of inhibition of the field EPSP induced by LY395756 fell into two distinct groups, with EC(50) values of <1 μm and >100 μm. In "sensitive" slices, LY395756 had additive actions with a mixed mGlu2/mGlu3 agonist, DCG-IV [(2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine], whereas in "insensitive" slices, LY395756 reduced the effect of DCG-IV, with an IC(50) of ∼1 μm. This separation into sensitive and insensitive slices could be explained by LY395756 acting as an mGlu2 agonist and mGlu3 antagonist, respectively, a finding supported by data from mice lacking these receptors. The heterogeneity was correlated with differences in expression levels of mGlu2 receptors within our Wistar colony and other Wistar substrains. The initial search for a behavioral correlate indicated that rats lacking mGlu2 receptors showed anxiety-like behavior in open-field and elevated plus maze assays. These findings have implications for rat models of psychiatric disease and are especially pertinent given that mGlu2 receptors are targets for compounds under development for anxiety.

Comparative analysis of the neurophysiological profile of group II metabotropic glutamate receptor activators and diazepam: effects on hippocampal and cortical EEG patterns in rats

Selective activation of the Group II metabotropic glutamate receptors 2/3 (mGlu2/3) by either full agonists or positive allosteric modulators (PAMs) show anxiolytic activity. In the present study the anxiolytic profile of mGlu2/3 receptor agonists LY-354740 and LY-404039 and the mGlu2 receptor PAM 1-methyl-2-((cis-3-methyl-4-(4-trifluoromethyl-2-methoxy)-phenyl)piperidin-1-yl)-1H-imidazo[4,5-b]pyridine (MTFIP) were evaluated using neurophysiology-based assays. Activation of mGlu2/3 receptors by these compounds, as well as the positive control diazepam, significantly decreased the frequency of hippocampal theta oscillation elicited by stimulation of the brainstem nucleus pontis oralis (nPO), a characteristic action of anxiolytic compounds. Since the nPO is a critical region involved in regulation of rapid eye movement sleep, mGlu2/3 receptor activators were also tested on sleep parameters, as well as on cortical and hippocampal encephalography (EEG) activity. Both mGlu2/3 agonists and the mGlu2 PAM significantly prolonged REM sleep latency and reduced total REM sleep duration while during the active awake state all compounds lowered hippocampal peak theta frequency. However, diazepam and mGlu2/3 agonists/PAM elicited opposite changes in cortical EEG delta and beta bands. Delta power significantly increased after any of the mGlu2/3 compounds but decreased after diazepam. In the beta band, mGlu2/3 receptor agonists dose-dependently decreased beta power in contrast to the well-known beta activation by diazepam. These effects lasted 3-4h and could not be explained by modest, transient changes (<1h) in waking and slow wave sleep. The current observations support the role of mGlu2/3 receptor activators as potential anxiolytic compounds, but indicate a distinct action on cortical EEG activity which is different from the effects of GABA(A) PAMs. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Group II mGluR agonist LY354740 and NAAG peptidase inhibitor effects on prepulse inhibition in PCP and D-amphetamine models of schizophrenia

RATIONALE

Group II metabotropic glutamate receptor (mGluR) agonists represent a novel approach to the treatment of schizophrenia. Inasmuch as the peptide neurotransmitter N-acetylaspartylglutamate (NAAG) activates these receptors, NAAG peptidase inhibitors conceptually represent a parallel path toward development of new antipsychotic drugs. While group II agonists are effective in several animal models of schizophrenia, they are reported to lack efficacy in moderating the effects of phencyclidine (PCP) on prepulse inhibition of acoustic startle in animal models of sensory processing deficits found in this disorder.

OBJECTIVE

The objective of this study was to re-examine the efficacy of a group II metabotropic glutamate agonist and NAAG peptidase inhibitors in prepulse inhibition models of schizophrenia across two strains of mice.

METHODS

The method used was an assay to determine the efficacy of these drugs in moderating the reduction in prepulse inhibition of acoustic startle in mice treated with PCP and D: -amphetamine.

RESULTS

The group II agonist LY354740 (5 and 10 mg/kg) moderated the effects of PCP on prepulse inhibition of acoustic startle in DBA/2 but not C57BL/6 mice. In contrast, two NAAG peptidase inhibitors, ZJ43 (150 mg/kg) and 2-PMPA (50, 100, and 150 mg/kg), did not significantly affect the PCP-induced reduction in prepulse inhibition in either strain.

CONCLUSIONS

These data demonstrate that the efficacy of group II agonists in this model of sensory motor processing is strain-specific in mice. The difference between the effects of the group II agonist and the peptidase inhibitors in the DBA/2 mice may relate to the difference in efficacy of NAAG and the agonist at mGluR2.

Blockade of the NMDA and AMPA/kainate receptors in the dorsal raphe nucleus prevents the 5-HT₃ receptor agonist m-chlorophenylbiguanide-induced suppression of REM sleep in the rat

The effects of the selective 5-HT(3) receptor agonist m-chlorophenylbiguanide (m-CPBG), and of the NMDA (N-methyl-D-aspartate) and AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionate)/kainate antagonists AP-5 [(±)-2-amino-5-phosphono-pentanoic acid] and CNQX (6-cyano-7-nitroquinoxaline-2,3-dione), respectively, were studied in adult male Wistar rats implanted for chronic sleep recordings. The compounds were microinjected directly into the dorsal raphe nucleus (DRN) during the light period of the 12-h light/12-h dark cycle. Infusion of m-CPBG (2 and 4mM) into the DRN induced a significant reduction of rapid-eye-movement sleep (REMS) and of the number of REM periods. Local infusion of AP-5 (0.5-1 mM) and CNQX (2 mM) significantly increased slow wave sleep (SWS). Pretreatment with AP-5 (0.5 mM) or CNQX (0.5 mM) antagonized the m-CPBG-induced suppression of REMS. It is proposed that the reduction of REMS after microinjection of m-CPBG into de DRN is related to the activation of glutamatergic interneurons that express the 5-HT(3) receptor and make synaptic contacts with serotonergic cells. The resultant increase of serotonin release at postsynaptic sites involved in the induction of REMS would provoke the suppression of the behavioral state. Our findings provide, in addition, new details concerning the pharmacology of DRN serotonergic neurons in the rat that may become relevant to the development of drugs for enhancing cortical and subcortical serotonergic neurotransmission.

The excitatory neurotransmitter glutamate stimulates DNA repair to increase neuronal resiliency

Glutamate is the most abundant excitatory neurotransmitter in the vertebrate central nervous system and plays an important role in synaptic plasticity required for learning and memory. Activation of glutamate ionotropic receptors promptly triggers membrane depolarization and Ca(2+) influx, resulting in the activation of several different protein kinases and transcription factors. For example, glutamate-mediated Ca(2+) influx activates Ca(2+)/calmodulin-dependent kinase, protein kinase C, and mitogen activated protein kinases resulting in activation of transcription factors such as cyclic AMP response element binding protein (CREB). Abnormally prolonged exposure to glutamate causes neuronal injury, and such "excitotoxicity" has been implicated in many acute and chronic diseases including ischemic stroke, epilepsy, amyotrophic lateral sclerosis, Alzheimer's, Huntington's and Parkinson's diseases. Interestingly, although glutamate-induced Ca(2+) influx can cause DNA damage by a mitochondrial reactive oxygen species-mediated mechanism, the Ca(2+) simultaneously activates CREB, resulting in up-regulation of the DNA repair and redox protein apurinic/apyrimidinic endonuclease 1. Here, we review connections between physiological or aberrant glutamate receptor activation, Ca(2+)-mediated signaling, oxidative DNA damage and repair efficiency, and neuronal vulnerability. We conclude that glutamate signaling involves an adaptive cellular stress response pathway that enhances DNA repair capability, thereby protecting neurons against injury and disease.

Regulation of Akt and Wnt signaling by the group II metabotropic glutamate receptor antagonist LY341495 and agonist LY379268

Metabotropic glutamate receptors 2/3 (mGlu(2/3)) have been implicated in schizophrenia and as a novel treatment target for schizophrenia. The current study examined whether mGlu(2/3) regulates Akt (protein kinase B) and Wnt (Wingless/Int-1) signaling, two cascades associated with schizophrenia and modified by antipsychotics. Western blotting revealed increases in phosphorylated Akt (pAkt) and phosphorylated glycogen synthase kinase-3 (pGSK-3) following acute and repeated treatment of LY379268 (mGlu(2/3) agonist), whereas increases in dishevelled-2 (Dvl-2), dishevelled-3 (Dvl-3), GSK-3 and β-catenin were only observed following repeated treatment. LY341495 (mGlu(2/3) antagonist) induced the opposite response compared with LY379268. Co-immunoprecipitation experiments showed an association between the mGlu(2/3) complex and Dvl-2 providing a possible mechanism to explain how the mGlu(2/3) can mediate changes in Wnt signaling. However, there was no association between the mGlu(2/3) complex and Akt suggesting that changes in Akt signaling following LY341495 and LY379268 treatments may not be directly mediated by the mGlu(2/3) . Finally, an increase in locomotor activity induced by LY341495 treatment correlated with increased pAkt and pGSK-3 levels and was attenuated by the administration of the GSK-3 inhibitor, SB216763. Overall, the results suggest that mGlu(2/3) regulates Akt and Wnt signaling and LY379268 treatment has overlapping effects with D(2) dopamine receptor antagonists (antipsychotic drugs).

mGluRs modulate strength and timing of excitatory transmission in hippocampal area CA3

Excitatory transmission within hippocampal area CA3 stems from three major glutamatergic pathways: the perforant path formed by axons of layer II stellate cells in the entorhinal cortex, the mossy fiber axons originating from the dentate gyrus granule cells, and the recurrent axon collaterals of CA3 pyramidal cells. The synaptic communication of each of these pathways is modulated by metabotropic glutamate receptors that fine-tune the signal by affecting both the timing and strength of the connection. Within area CA3 of the hippocampus, group I mGluRs (mGluR1 and mGluR5) are expressed postsynaptically, whereas group II (mGluR2 and mGluR3) and III mGluRs (mGluR4, mGluR7, and mGluR8) are expressed presynaptically. Receptors from each group have been demonstrated to be required for different forms of pre- and postsynaptic long-term plasticity and also have been implicated in regulating short-term plasticity. A recent observation has demonstrated that a presynaptically expressed mGluR can affect the timing of action potentials elicited in the postsynaptic target. Interestingly, mGluRs can be distributed in a target-specific manner, such that synaptic input from one presynaptic neuron can be modulated by different receptors at each of its postsynaptic targets. Consequently, mGluRs provide a mechanism for synaptic specialization of glutamatergic transmission in the hippocampus. This review will highlight the variability in mGluR modulation of excitatory transmission within area CA3 with an emphasis on how these receptors contribute to the strength and timing of network activity within pyramidal cells and interneurons.

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

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

Biphasic modulation by galanin of excitatory synaptic transmission in substantia gelatinosa neurons of adult rat spinal cord slices

Although intrathecally administrated galanin modulates nociceptive transmission in a biphasic manner, this has not been fully examined previously. In the present study, the action of galanin on synaptic transmission in the substantia gelatinosa (SG) neurons of adult rat spinal cord slices was examined, using the whole cell patch-clamp technique. Galanin concentration-dependently increased the frequency of spontaneous excitatory postsynaptic current (EPSC; EC50 = 2.0 nM) without changing the amplitude, indicating a presynaptic effect. This effect was reduced in a Ca2+-free, or voltage-gated Ca2+ channel blocker La3+-containing Krebs solution and was produced by a galanin type-2/3 receptor (GalR2/R3) agonist, galanin 2–11, but not by a galanin type-1 receptor (GalR1) agonist, M617. Galanin also concentration-dependently produced an outward current at −70 mV (EC50 = 44 nM), although this appeared to be contaminated by a small inward current. This outward current was mimicked by M617, but not by galanin 2–11. Moreover, galanin reduced monosynaptic Aδ-fiber- and C-fiber-evoked EPSC amplitude; the former reduction was larger than the latter. A similar action was produced by galanin 2–11, but not by M617. Spontaneous and focally evoked inhibitory (GABAergic and glycinergic) transmission was unaffected by galanin. These findings indicate that galanin at lower concentrations enhances the spontaneous release of l-glutamate from nerve terminals by Ca2+ entry from the external solution following GalR2/R3 activation, whereas galanin at higher concentrations also produces a membrane hyperpolarization by activating GalR1. Moreover, galanin reduces l-glutamate release onto SG neurons from primary afferent fibers by activating GalR2/R3. These effects could partially contribute to the behavioral effect of galanin.

Late-life depression and Alzheimer's disease: the glutamatergic system inside of this mirror relationship

Late-life depressive syndromes often arise in the context of predementia, dementia syndromes, and Alzheimer's disease (AD). Conversely, patients with a history of mood disorders are at higher risk of developing cognitive impairment. The high rate of co-occurrence of these two disorders is becoming a major health problem in older subjects for both their epidemiological impact and the negative outcomes in terms of disability and increased mortality. In this perspective, it is possible to speculate on the presence of a mirror relationship between depressive and cognitive disorders in late-life. Indeed, although a causal contribution of genetic, environmental, and social factors is widely recognized in these disorders, the neurobiological links still remain largely unknown. l-glutamic acid and γ-aminobutyric acid are the principal excitatory and inhibitory neurotransmitters in the central nervous system, respectively, and increasing evidence suggests that alterations in this neurotransmitter system may contribute to the neurobiology linking depression and cognitive impairment. In the present review article, we examined the neurobiological bases of the relationship between late-life depressive syndromes and AD, with a particular attention to glutamatergic pathway signalling like a bridge connecting these two conditions. In addition, attempts have been made to explain changes in glutamatergic pathway, depression in older age, and dementia through the analysis of signal transduction mechanisms associated with these disabling disorders.

The metabotropic glutamate receptor 7 allosteric modulator AMN082: a monoaminergic agent in disguise?

Metabotropic glutamate receptor 7 (mGluR7) remains the most elusive of the eight known mGluRs primarily because of the limited availability of tool compounds to interrogate its potential therapeutic utility. The discovery of N,N'-dibenzhydrylethane-1,2-diamine dihydrochloride (AMN082) as the first orally active, brain-penetrable, mGluR7-selective allosteric agonist by Mitsukawa and colleagues (Proc Natl Acad Sci USA 102:18712-18717, 2005) provides a means to investigate this receptor system directly. AMN082 demonstrates mGluR7 agonist activity in vitro and interestingly has a behavioral profile that supports utility across a broad spectrum of psychiatric disorders including anxiety and depression. The present studies were conducted to extend the in vitro and in vivo characterization of AMN082 by evaluating its pharmacokinetic and metabolite profile. Profiling of AMN082 in rat liver microsomes revealed rapid metabolism (t(1/2) < 1 min) to a major metabolite, N-benzhydrylethane-1,2-diamine (Met-1). In vitro selectivity profiling of Met-1 demonstrated physiologically relevant transporter binding affinity at serotonin transporter (SERT), dopamine transporter (DAT), and norepinephrine transporter (NET) (323, 3020, and 3410 nM, respectively); whereas the parent compound AMN082 had appreciable affinity at NET (1385 nM). AMN082 produced antidepressant-like activity and receptor occupancy at SERT up to 4 h postdose, a time point at which AMN082 is significantly reduced in brain and plasma while the concentration of Met-1 continues to increase in brain. Acute Met-1 administration produced antidepressant-like activity as would be expected from its in vitro profile as a mixed SERT, NET, DAT inhibitor. Taken together, these data suggest that the reported in vivo actions of AMN082 should be interpreted with caution, because they may involve other mechanisms in addition to mGluR7.

Social influences on neurobiology and behavior: epigenetic effects during development

The quality of the social environment can have profound influences on the development and activity of neural systems with implications for numerous behavioral and physiological responses, including the expression of emotionality. Though social experiences occurring early in development may be particularly influential on the developing brain, there is continued plasticity within these neural circuits amongst juveniles and into early adulthood. In this review, we explore the evidence derived from studies in rodents which illustrates the social modulation during development of neural systems, with a particular emphasis on those systems in which a long-term effect is observed. One possible explanation for the persistence of dynamic changes in these systems in response to the environment is the involvement of epigenetic mechanisms, and here we discuss recent studies which support the role of these mechanisms in mediating the link between social experiences, gene expression, neurobiological changes, and behavioral variation. This literature raises critical questions about the interaction between neural systems, the concordance between neural and behavioral changes, sexual dimorphism in effects, the importance of considering individual differences in response to the social environment, and the potential of an epigenetic perspective in advancing our understanding of the pathways leading to variations in mental health.

Drug targets for cognitive enhancement in neuropsychiatric disorders

The investigation of novel drug targets for treating cognitive impairments associated with neurological and psychiatric disorders remains a primary focus of study in central nervous system (CNS) research. Many promising new therapies are progressing through preclinical and clinical development, and offer the potential of improved treatment options for neurodegenerative diseases such as Alzheimer's disease (AD) as well as other disorders that have not been particularly well treated to date like the cognitive impairments associated with schizophrenia (CIAS). Among targets under investigation, cholinergic receptors have received much attention with several nicotinic agonists (α7 and α4β2) actively in clinical trials for the treatment of AD, CIAS and attention deficit hyperactivity disorder (ADHD). Both glutamatergic and serotonergic (5-HT) agonists and antagonists have profound effects on neurotransmission and improve cognitive function in preclinical experiments with animals; some of these compounds are now in proof-of-concept studies in humans. Several histamine H3 receptor antagonists are in clinical development not only for cognitive enhancement, but also for the treatment of narcolepsy and cognitive deficits due to sleep deprivation because of their expression in brain sleep centers. Compounds that dampen inhibitory tone (e.g., GABA(A) α5 inverse agonists) or elevate excitatory tone (e.g., glycine transporter inhibitors) offer novel approaches for treating diseases such as schizophrenia, AD and Down syndrome. In addition to cell surface receptors, intracellular drug targets such as the phosphodiesterases (PDEs) are known to impact signaling pathways that affect long-term memory formation and working memory. Overall, there is a genuine need to treat cognitive deficits associated with many neuropsychiatric conditions as well as an increasingly aging population.