Antidepressant-like and anxiolytic-like actions of the mGlu5 receptor antagonist MTEP, microinjected into lateral septal nuclei of male Wistar rats

Downregulation of surface AMPA receptor expression in the striatum following prolonged social isolation, a role of mGlu5 receptors

Major depressive disorder is a common and serious mood illness. The molecular mechanisms underlying the pathogenesis and symptomatology of depression are poorly understood at present. Multiple neurotransmitter systems are believed to be implicated in depression. Increasing evidence supports glutamatergic transmission as a critical element in depression and antidepressant activity. In this study, we investigated adaptive changes in expression of AMPA receptors in a key limbic reward structure, the striatum, in response to an anhedonic model of depression. Prolonged social isolation in adult rats caused anhedonic/depression- and anxiety-like behavior. In these depressed rats, surface levels of AMPA receptors, mainly GluA1 and GluA3 subunits, were reduced in the nucleus accumbens (NAc). Surface GluA1/A3 expression was also reduced in the caudate putamen (CPu) following chronic social isolation. No change was observed in expression of presynaptic synaptophysin, postsynaptic density-95, and dendritic microtubule-associated protein 2 in the striatum. Noticeably, chronic treatment with the metabotropic glutamate (mGlu) receptor 5 antagonist MTEP reversed the reduction of AMPA receptors in the NAc and CPu. MTEP also prevented depression- and anxiety-like behavior induced by social isolation. These data indicate that adulthood prolonged social isolation induces the adaptive downregulation of GluA1/A3-containing AMPA receptor expression in the limbic striatum. mGlu5 receptor activity is linked to this downregulation, and antagonism of mGlu5 receptors produces an antidepressant effect in this anhedonic model of depression.

Differences in mGluR5 Availability Depending on the Level of Social Avoidance in Drug-Naïve Young Patients with Major Depressive Disorder

BACKGROUND

Previous research has shown that metabotropic glutamate receptor-5 (mGluR5) signaling is significantly involved in social avoidance. We investigated the relationship between levels of social avoidance and mGluR5 availability in drug-naïve young patients with major depressive disorder (MDD).

METHODS

Twenty non-smoking patients and eighteen matched non-smoking healthy controls underwent [¹¹C]ABP688 positron emission tomography (PET) and magnetic resonance imaging scans. The binding potential (BP_ND) of [¹¹C]ABP688 was obtained using the simplified reference tissue model. Patients' level of social avoidance was assessed using the Social Avoidance and Distress Scale (SADS). For [¹¹C]ABP688 BP_ND, the region-of-interest (ROI)-based between-group comparisons and correlations with SADS scores were investigated. The frontal cortices were chosen as a priori ROIs based on previous PET investigations in MDD, and on literature underscoring the importance of the frontal cortex in social avoidance.

RESULTS

Independent samples t-tests revealed no significant differences in [¹¹C]ABP688 BP_ND in the frontal cortices between the MDD patient group as a whole and healthy controls. One-way analysis of variance with post-hoc tests revealed significantly lower BP_ND in the bilateral superior frontal cortex (SFC) and left middle frontal cortex (MFC) in MDD patients with low levels of social avoidance (L-SADS) than in healthy controls. The L-SADS patients also had significantly lower BP_ND in the medial part of the right SFC than both MDD patients with high levels of social avoidance (H-SADS) and healthy controls. The L-SADS patients also showed significantly lower BP_ND in the orbital parts of the SFC, MFC, and inferior frontal cortex than H-SADS patients. No significant group differences were found between H-SADS patients and healthy controls. The ROI-based correlation analysis revealed significant positive correlations between social avoidance levels and frontal [¹¹C]ABP688 BP_ND in the entire patients.

CONCLUSION

Our exploratory study shows significant differences in frontal mGluR5 availability depending on the level of social avoidance in drug-naïve non-smoking MDD patients, suggesting that social avoidance should be considered as one of the clinical factors involved in mGluR5 signaling changes in depression.

Loss of mGluR5 in D1 Receptor-Expressing Neurons Improves Stress Coping

The metabotropic glutamate receptor type 5 (mGluR5) has been proposed to play a crucial role in the selection and regulation of cognitive, affective, and emotional behaviors. However, the mechanisms by which these receptors mediate these effects remain largely unexplored. Here, we studied the role of mGluR5 located in D1 receptor-expressing (D1) neurons in the manifestation of different behavioral expressions. Mice with conditional knockout (cKO) of mGluR5 in D1 neurons (mGluR5^D1 cKO) and littermate controls displayed similar phenotypical profiles in relation to memory expression, anxiety, and social behaviors. However, mGluR5^D1 cKO mice presented different coping mechanisms in response to acute escapable or inescapable stress. mGluR5^D1 cKO mice adopted an enhanced active stress coping strategy upon exposure to escapable stress in the two-way active avoidance (TWA) task and a greater passive strategy upon exposure to inescapable stress in the forced swim test (FST). In summary, this work provides evidence for a functional integration of the dopaminergic and glutamatergic system to mediate control over internal states upon stress exposure and directly implicates D1 neurons and mGluR5 as crucial mediators of behavioral stress responses.

Linkage of Non-receptor Tyrosine Kinase Fyn to mGlu5 Receptors in Striatal Neurons in a Depression Model

The Src family kinase (SFK) is a subfamily of non-receptor tyrosine kinases. The SFK member Fyn is enriched at synaptic sites in the limbic reward circuit and plays a pivotal role in the regulation of glutamate receptors. In this study, we investigated changes in phosphorylation and function of the two key SFK members (Fyn and Src) and SFK interactions with a metabotropic glutamate (mGlu) receptor in the limbic striatum of adult rats in response to chronic passive stress, i.e., prolonged social isolation which is a pre-validated animal paradigm modeling depression in adulthood. In rats that showed typical anhedonic/depression-like behavior after chronic social isolation, phosphorylation of SFKs at a conserved and activation-associated autophosphorylation site (Y416) was not altered in the two subdivisions of the striatum, the nucleus accumbens and caudate putamen. The total level of phosphorylation and kinase activity of individual Fyn and Src immunopurified from the striatum also remained stable after social isolation. Noticeably, Fyn and Src were found to interact with a G_αq-coupled mGlu5 receptor in striatal neurons. The interaction of Fyn with mGlu5 receptors was selectively elevated in socially isolated rats. Moreover, social isolation induced an increase in surface expression of striatal mGlu5 receptors, which was reduced by an SFK inhibitor. These results indicate that Fyn interacts with mGlu5 receptors in striatal neurons. Adulthood social isolation in rats enhances the Fyn-mGlu5 interaction, which appears to be critical for the upregulation of surface mGlu5 receptor expression in striatal neurons.

Blood glutamate scavenging as a novel glutamate-based therapeutic approach for post-stroke depression

Post-stroke depression (PSD) is a major complication of stroke that significantly impacts functional recovery and quality of life. While the exact mechanism of PSD is unknown, recent attention has focused on the association of the glutamatergic system in its etiology and treatment. Minimizing secondary brain damage and neuropsychiatric consequences associated with excess glutamate concentrations is a vital part of stroke management. The blood glutamate scavengers, oxaloacetate and pyruvate, degrade glutamate in the blood to its inactive metabolite, 2-ketoglutarate, by the coenzymes glutamate-oxaloacetate transaminase (GOT) and glutamate-pyruvate transaminase (GPT), respectively. This reduction in blood glutamate concentrations leads to a subsequent shift of glutamate down its concentration gradient from the blood to the brain, thereby decreasing brain glutamate levels. Although there are not yet any human trials that support blood glutamate scavengers for clinical use, there is increasing evidence from animal research of their efficacy as a promising new therapeutic approach for PSD. In this review, we present recent evidence in the literature of the potential therapeutic benefits of blood glutamate scavengers for reducing PSD and other related neuropsychiatric conditions. The evidence reviewed here should be useful in guiding future clinical trials.

Ketamine-induced reduction in mGluR5 availability is associated with an antidepressant response: an [11C]ABP688 and PET imaging study in depression

The mechanisms of action of the rapid antidepressant effects of ketamine, an N-methyl-D-aspartate glutamate receptor antagonist, have not been fully elucidated. This study examined the effects of ketamine on ligand binding to a metabotropic glutamatergic receptor (mGluR5) in individuals with major depressive disorder (MDD) and healthy controls. Thirteen healthy and 13 MDD nonsmokers participated in two [11C]ABP688 positron emission tomography (PET) scans on the same day-before and during intravenous ketamine administration-and a third scan 1 day later. At baseline, significantly lower [11C]ABP688 binding was detected in the MDD as compared with the control group. We observed a significant ketamine-induced reduction in mGluR5 availability (that is, [11C]ABP688 binding) in both MDD and control subjects (average of 14±9% and 19±22%, respectively; P<0.01 for both), which persisted 24 h later. There were no differences in ketamine-induced changes between MDD and control groups at either time point (P=0.8). A significant reduction in depressive symptoms was observed following ketamine administration in the MDD group (P<0.001), which was associated with the change in binding (P<0.04) immediately after ketamine. We hypothesize that glutamate released after ketamine administration moderates mGluR5 availability; this change appears to be related to antidepressant efficacy. The sustained decrease in binding may reflect prolonged mGluR5 internalization in response to the glutamate surge.

Alterations in mGlu5 receptor expression and function in the striatum in a rat depression model

Major depressive disorder is a common form of mental illness. Many brain regions are implicated in the pathophysiology and symptomatology of depression. Among key brain areas is the striatum that controls reward and mood and is involved in the development of core depression-like behavior in animal models of depression. While molecular mechanisms in this region underlying depression-related behavior are poorly understood, the glutamatergic input to the striatum is believed to play a role. In this study, we investigated changes in metabotropic glutamate (mGlu) receptor expression and signaling in the striatum of adult rats in response to prolonged (10-12 weeks) social isolation, a pre-validated animal paradigm modeling depression in adulthood. We found that mGlu5 receptor protein levels in the striatum were increased in rats that showed typical depression- and anxiety-like behavior after chronic social isolation. This increase in mGlu5 receptor expression was seen in both subdivisions of the striatum, the nucleus accumbens and caudate putamen. At subcellular and subsynaptic levels, mGlu5 receptor expression was elevated in surface membranes at synaptic sites. In striatal neurons, the mGlu5-associated phosphoinositide signaling pathway was augmented in its efficacy after prolonged social isolation. These data indicate that the mGlu5 receptor is a sensitive substrate of depression. Adulthood social isolation leads to the up-regulation of mGlu5 receptor expression and function in striatal neurons.

The Emerging Role of Metabotropic Glutamate Receptors in the Pathophysiology of Chronic Stress-Related Disorders

Chronic stress-related psychiatric conditions such as anxiety, depression, and alcohol abuse are an enormous public health concern. The etiology of these pathologies is complex, with psychosocial stressors being among the most frequently discussed risk factors. The brain glutamatergic neurotransmitter system has often been found involved in behaviors and pathophysiologies resulting from acute stress and fear. Despite this, relatively little is known about the role of glutamatergic system components in chronic psychosocial stress, neither in rodents nor in humans. Recently, drug discovery efforts at the metabotropic receptor subtypes of the glutamatergic system (mGlu1-8 receptors) led to the identification of pharmacological tools with emerging potential in psychiatric conditions. But again, the contribution of individual mGlu subtypes to the manifestation of physiological, molecular, and behavioral consequences of chronic psychosocial stress remains still largely unaddressed. The current review will describe animal models typically used to analyze acute and particularly chronic stress conditions, including models of psychosocial stress, and there we will discuss the emerging roles for mGlu receptor subtypes. Indeed, accumulating evidence indicates relevance and potential therapeutic usefulness of mGlu2/3 ligands and mGlu5 receptor antagonists in chronic stress-related disorders. In addition, a role for further mechanisms, e.g. mGlu7-selective compounds, is beginning to emerge. These mechanisms are important to be analyzed in chronic psychosocial stress paradigms, e.g. in the chronic subordinate colony housing (CSC) model. We summarize the early results and discuss necessary future investigations, especially for mGlu5 and mGlu7 receptor blockers, which might serve to suggest improved therapeutic strategies to treat stress-related disorders.

The metabotropic glutamate receptor 5 role on motor behavior involves specific neural substrates

Background

The metabotropic glutamate receptor 5 (mGluR5) is involved in various brain functions, including memory, cognition and motor behavior. Regarding locomotor activity, we and others have demonstrated that pharmacological antagonism of mGluR5 promotes hyperkinesia in mice. Moreover, increased locomotor activity can also be observed in mice following the genetic deletion of mGluR5. However, it is still unclear which specific brain substrates contribute to mGluR5-mediated regulation of motor function.

Results

Thus, to better understand the role of mGluR5 in motor control and to determine which neural substrates are involved in this regulation we performed stereotactic microinfusions of the mGluR5 antagonist, MPEP, into specific brain regions and submitted mice to the open field and rotarod apparatus. Our findings indicate that mGluR5 blockage elicits distinct outcomes in terms of locomotor activity and motor coordination depending on the brain region injected with mGluR5 antagonist. MPEP injection into either the dorsal striatum or dorsal hippocampus resulted in increased locomotor activity, whereas MPEP injection into either the ventral striatum or motor cortex resulted in hypokinesia. Moreover, MPEP injected into the olfactory bulb increased the distance mice traveled in the center of the open field arena. With respect to motor coordination on the rotarod, injection of MPEP into the motor cortex and olfactory bulb elicited decreased latency to fall.

Conclusions

Taken together, our data suggest that not only primarily motor neural substrates, but also limbic and sensory structures are involved in mGluR5-mediated motor behavior.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-015-0113-2) contains supplementary material, which is available to authorized users.

RGS4 overexpression in the rat dorsal striatum modulates mGluR5- and amphetamine-mediated behavior and signaling

RATIONALE

Regulator of G-protein signaling 4 (RGS4) is a brain-enriched negative modulator of G-protein-coupled receptor signaling. Decreased availability of RGS4 in the frontal cortex and striatum has been described in animal models of schizophrenia and drug addiction. However, cellular and behavioral consequences of dysregulated RGS4-dependent receptor signaling in the brain remain poorly understood.

OBJECTIVE

This study aims to investigate whether RGS4, through inhibiting the function of mGluR5 receptors in the dorsal striatum (dSTR), regulates cellular and behavioral responses to acute amphetamine.

METHODS

After herpes simplex virus-RGS4 was infused into the dSTR, RGS4 overexpression as well as binding of recombinant RGS4 to mGluR5 was assessed. The effect of RGS4 overexpression on behavioral activity induced by the intrastriatal mGluR5 agonist, DHPG, or amphetamine was recorded. Activation of extracellular signal-regulated kinase (ERK) and Akt (protein kinase B) was measured in the dSTR tissue at the end of each behavioral experiment.

RESULTS

RGS4 overexpressed in the dSTR coimmunoprecipitated with mGluR5 receptors and suppressed both behavioral activity and phospho-ERK levels induced by DHPG. RGS4 overexpression or the mGluR5 antagonist, 3-((2-methyl-4-thiazolyl)ethynyl)pyridine (MTEP), attenuated amphetamine-induced phospho-ERK (but not phospho-Akt) levels. RGS4 suppressed amphetamine-induced vertical activity and augmented horizontal activity over 90 min. Similarly, MTEP augmented amphetamine-induced horizontal activity, but did not affect vertical activity.

CONCLUSIONS

The present data demonstrate that RGS4 in the dSTR attenuates amphetamine-induced ERK signaling and decreases the behavioral efficacy of acute amphetamine likely by limiting mGluR5 function.

mGlu2/3 and mGlu5 receptors: potential targets for novel antidepressants

Major depressive disorder is among the most prevalent forms of mental illness. All currently available antidepressant medications have stemmed from study of the mechanisms of serendipitously discovered drugs, and only 30-50% of patients exhibit remission and frequently at least 3-4 weeks are required for manifestation of significant therapeutic effects. To overcome these drawbacks, discovering novel neuronal mechanisms of pathophysiology of depression as well as more effective treatments are necessary. This review focuses on the metabotropic glutamate (mGlu) receptors and their potential for drug targets for the treatment of depression. In particular, accumulating evidence has indicated the potential importance and usefulness of agents acting on mGlu2/3 and mGlu5 receptors. Preclinical and clinical evidence of mGlu2/3 receptor ligands and mGlu5 receptor antagonists are described. Moreover, their potential in clinic will be discussed in the context of neuronal mechanisms of ketamine, an agent recently demonstrated a robust effect for patients with treatment-resistant depression. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Intra-lateral septal infusions of folic acid alone or combined with various antidepressant drugs produce antidepressant-like actions in male Wistar rats forced to swim

Intra-cerebral administrations of folic acid produce antidepressant-like effects; either alone or combined with several antidepressant drugs. However, the specific limbic structures implied in the antidepressant-like actions of folic acid are un-known. Thus, intra-lateral septal infusions of folic acid (5.0 nmol, P<0.05; 10.0 nmol, P<0.05) or oral administrations of folic acid (50 mg/kg, P<0.05, p.o.; 75.0; mg/kg, P<0.05, p.o.) or systemic administrations of fluoxetine (20.0 mg/kg, P<0.05; 25.0 mg/kg, P<0.05) reduced immobility by increasing swimming behavior in the forced swimming test (FST) of male Wistar rats. Conversely, desipramine (10.0 mg/kg, P<0.05; 15.0 mg/kg, P<0.05) reduced immobility by increasing climbing behavior. Subthreshold doses of folic acid (2.5 nmol/intra-LSN) combined with subthreshold doses of folic acid (25.0 mg/kg, p.o., P<0.05) or with subthreshold doses of fluoxetine (15.0 mg/kg, P<0.05) and they produced antidepressant-like effects which were canceled by ketanserin. In conclusion, intra-lateral septal infusions of folic acid alone or combined with systemic doses of folic acid or fluoxetine reduced immobility in the FST. These antidepressant-like actions, probably, were due to modifications of the serotonergic system since swimming behavior was increased and these effects were canceled by ketanserin.

Review of pharmacological treatment in mood disorders and future directions for drug development

After a series of serendipitous discoveries of pharmacological treatments for mania and depression several decades ago, relatively little progress has been made for novel hypothesis-driven drug development in mood disorders. Multifactorial etiologies of, and lack of a full understanding of, the core neurobiology of these conditions clearly have contributed to these development challenges. There are, however, relatively novel targets that have raised opportunities for progress in the field, such as glutamate and cholinergic receptor modulators, circadian regulators, and enzyme inhibitors, for alternative treatment. This review will discuss these promising new treatments in mood disorders, the underlying mechanisms of action, and critical issues of their clinical application. For these new treatments to be successful in clinical practice, it is also important to design innovative clinical trials that identify the specific actions of new drugs, and, ideally, to develop biomarkers for monitoring individualized treatment response. It is predicted that future drug development will identify new agents targeting the molecular mechanisms involved in the pathophysiology of mood disorders.

Fluoxetine, 17-β estradiol or folic acid combined with intra-lateral septal infusions of neuropeptide Y produced antidepressant-like actions in ovariectomized rats forced to swim

Folic acid is antidepressant, either alone or combined with several antidepressant drugs. However, the antidepressant-like actions of folic acid combined with intra-lateral septal (LSN) infusions of neuropeptide Y (NPY) in the forced swimming test (FST) have not been tested before. Thus, systemic injections of fluoxetine (20.0mg/kg, P<0.05; s.c.) or 17-β estradiol (10.0 μg/rat, P<0.05; s.c.) or oral administrations of folic acid (50.0 mg/kg, P<0.05; 75.0 mg/kg, P<0.05) or NPY intra-LSN (3.0 μg, P<0.05; 3.5 μg, P<0.05) reduced immobility of ovariectomized Wistar rats. Subthreshold doses of: folic acid (25.0 mg/kg) or 17-β estradiol (5.0 μg/rat, P<0.05) or fluoxetine (15.0 mg/kg, P<0.05; s.c.) combined with subthreshold doses of NPY (2.5 μg/rat, P<0.05; intra-LSN) and these combinations produced antidepressant-like actions; which were canceled by BIBP 3226 (a NPY-Y1 receptor antagonist). It is concluded that folic acid produced antidepressant-like effects probably through the participation of the NPY Y1 receptors found in the lateral septal nuclei.

MRI analysis of mGluR5 and mGluR1 antagonists, MTEP and R214127 in the cerebral forebrain of awake, conscious rats

Metabotropic glutamate receptors mGluR5 and mGluR1 mediate key neuropsychiatric functions in health and disease and their antagonists hold promise to treat anxiety, depression, inflammation, and neuropathic pain. Pharmacological magnetic resonance imaging (phMRI) using a functional MRI approach in awake, conscious rodents can determine the activities of receptor ligands without the potential interference of anesthetics and independent of the specific biochemical mechanism of action of the candidate molecule. In this study we determined the neuronal activation patterns of 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) and 1-(3,4-dihydro-2H-pyrano[2,3-b]quinolin-7-yl0-2phenyl-1-ethanone (R214127), antagonists of mGluR5 and mGluR1 receptors by phMRI. We found that MTEP and R214127 activated specific primary somatosensory, piriform, entorhinal and motor cortices and the caudateputamen each to a different extent and in partly overlapping manners. Additional analysis of the activation data indicated that these brain regions and their connections are involved in mediating neuropathic pain and also, reward and olfaction. Using awake, conscious animals in phMRI can be a useful approach in characterizing candidate mGluR5 and mGlR1 antagonists also allowing a more direct comparison of animal and human phMRI studies.

New perspectives in glutamate and anxiety

Anxiety and stress-related disorders, namely posttraumatic stress disorder (PTSD), generalized anxiety disorder (GAD), obsessive-compulsive disorder (ODC), social and specific phobias, and panic disorder, are a major public health issue. A growing body of evidence suggests that glutamatergic neurotransmission may be involved in the biological mechanisms underlying stress response and anxiety-related disorders. The glutamatergic system mediates the acquisition and extinction of fear-conditioning. Thus, new drugs targeting glutamatergic neurotransmission may be promising candidates for new pharmacological treatments. In particular, N-methyl-d-aspartate receptors (NMDAR) antagonists (AP5, AP7, CGP37849, CGP39551, LY235959, NPC17742, and MK-801), NMDAR partial agonists (DCS, ACPC), α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors (AMPARs) antagonists (topiramate), and several allosteric modulators targeting metabotropic glutamate receptors (mGluRs) mGluR1, mGluR2/3, and mGluR5, have shown anxiolytic-like effects in several animal and human studies. Several studies have suggested that polyamines (agmatine, putrescine, spermidine, and spermine) may be involved in the neurobiological mechanisms underlying stress-response and anxiety-related disorders. This could mainly be attributed to their ability to modulate ionotropic glutamate receptors, especially NR2B subunits. The aim of this review is to establish that glutamate neurotransmission and polyaminergic system play a fundamental role in the onset of anxiety-related disorders. This may open the way for new drugs that may help to treat these conditions.

Roles of glutamate signaling in preclinical and/or mechanistic models of depression

Accumulating evidence suggests that the glutamatergic system plays important roles in the pathophysiology and treatment of major depressive disorder (MDD). Abnormalities in the glutamatergic system are definitely observed in this disorder, and certain glutamatergic agents exhibit antidepressant effects in patients with MDD. In this review, we summarize the preclinical findings suggesting the involvement of glutamate signaling in the pathophysiology and treatment of MDD. Preclinical animal models for depression are often characterized by changes in molecules related to glutamatergic signaling. Some antidepressants exert their effects by affecting glutamatergic system components in animals. Animals with genetically modified glutamatergic function exhibit depression-like behaviors or anti-depressive behavior. In addition, several types of glutamatergic agents have shown antidepressant-like effects in preclinical models for depression. Many types of glutamate receptors (NMDA, AMPA, and metabotropic glutamate receptors) or transporters appear to be involved in the etiology of depression or in the mechanisms of action of antidepressants. These functional proteins related to glutamate signal transduction are potential targets for a new generation of antidepressants with fast-onset effects, such as the NMDA antagonist ketamine.

Potential psychiatric applications of metabotropic glutamate receptor agonists and antagonists

Drugs acting at metabotropic glutamate receptors (mGluRs) are among the most promising agents under development for the treatment of psychiatric disorders. The research in this area is at a relatively early stage, as there are no drugs acting at mGluRs that have been approved for the treatment of any psychiatric disorder. However, in the areas of schizophrenia, anxiety disorders and mood disorders, research conducted in animal models appears to translate well into efficacy in human laboratory-based models of psychopathology and in preliminary clinical trials. Further, the genes coding for mGluRs are implicated in the risk for a growing number of psychiatric disorders. This review highlights the best studied mGluR strategies for psychiatry, based on human molecular genetics, studies in animal models and preliminary clinical trials. It describes the potential value of mGluR2 and mGluR5 agonists and positive allosteric modulators for the treatment of schizophrenia. It also reviews evidence that group II mGluR agonists and positive allosteric modulators as well as group I mGluR antagonists might also treat anxiety disorders and some forms of depression, while mGluR2 and group I mGluR antagonists (particularly mGluR5 antagonists) might have antidepressant properties. This review also links growing insights into the role of glutamate in the pathophysiology of these disorders to hypothesized mGluR-related treatment mechanisms.

Antidepressant-like or anxiolytic-like actions of topiramate alone or co-administered with intra-lateral septal infusions of neuropeptide Y in male Wistar rats

We tested the effects of intra-lateral septal infusions of neuropeptide Y (NPY) combined with systemic injections of topiramate in the DRL-72s paradigm and the elevated plus-maze test in male Wistar rats. Intra-lateral septal infusions of desipramine (5.0 microg/microl; P<0.05) or intra-lateral septal infusions of NPY (3.0 microg/microl, P<0.05; 3.5 microg/microl, P<0.05) or systemic injections of topiramate (20.0mg/kg, P<0.05; 30.0mg/kg, P<0.05) or subthreshold doses of topiramate (10.0mg/kg) combined with intra-lateral septal infusions of subthreshold doses of NPY (2.5 microg/microl; P<0.05) induced a dose-dependent increase in reinforced lever presses and a cohesive rightward shift of the inter-response time distribution in the DRL 72s task. In the elevated plus-maze test, intra-lateral septal infusions of NPY (3.0 microg/microl, P<0.05; 3.5 microg/microl, P<0.05) or midazolam (10.0 microg/microl; P<0.05) or systemic injections of topiramate (20.0mg/kg, P<0.05; 30.0mg/kg, P<0.05) or subthreshold doses of systemic injections of topiramate (10.0mg/kg) combined with intra-lateral septal infusions of subthreshold doses of NPY (2.5 microg/microl; P<0.05) increased the exploration of the open arms without affecting locomotion. In conclusion, intra-septal NPY has anxiolytic effects in the EPM, and antidepressant effects in the DRL 72s test. Similarly, systemic topiramate has anxiolytic effects in the EPM, and antidepressant effects in the DRL 72s test. Finally, a combination of subthreshold doses of NPY and topiramate together also have anxiolytic and antidepressant effects, suggesting a synergistic effect.

Metabotropic glutamate receptors: potential drug targets for psychiatric disorders

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

Metabotropic glutamate receptors in the tripartite synapse as a target for new psychotropic drugs

Mental disorders, such as depression, anxiety and schizophrenia, has become a large medical and social problem recently. Studies performed in animal tests and early clinical investigations brought a new insight in the pharmacotherapy of these disorders. Latest investigations are focused mainly on the glutamatergic system, a main excitatory amino acid neurotransmitter in the brain. Evidence indicates that metabotropic glutamate receptors ligands have excellent antidepressant, anxiolytic and antipsychotic effects. Metabotopic glutamate receptors (mGlu) divaded into three groups (group I, II and III) are localized on nerve terminals, postsynaptic sites and glial cells and thus they can influence and modulate the action of glutamate on different levels in the synapse. Recent advances in the identification of selective and specific compounds (both ortho- and allosteric ligands), and the generation of transgenic animals enabled to have new insight into the pathophysiology and therapy of mood disorders. At present, the most potent seem to be negative allosteric modulators of the first group (mGlu1 and mGlu5), and positive allosteric modulators of the second (mGlu2 and mGlu3) and third (mGlu4/7/8) group of mGlu receptors.

The effects of intra-cerebral drug infusions on animals' unconditioned fear reactions: a systematic review

Intra-cerebral (i.c.) microinfusion of selective receptor agonists and antagonists into behaving animals can provide both neuroanatomical and neurochemical insights into the neural mechanisms of anxiety. However, there have been no systematic reviews of the results of this experimental approach that include both a range of unconditioned anxiety reactions and a sufficiently broad theoretical context. Here we focus on amino acid, monoamine, cholinergic and peptidergic receptor ligands microinfused into neural structures previously implicated in anxiety, and subsequent behavioral effects in animal models of unconditioned anxiety or fear. GABAA receptor agonists and glutamate receptor antagonists produced the most robust anxiolytic-like behavioral effects, in the majority of neural substrates and animal models. In contrast, ligands of the other receptor systems had more selective, site-specific anti-anxiety effects. For example, 5-HT1A receptor agonists produced anxiolytic-like effects in the raphe nuclei, but inconsistent effects in the amygdala, septum, and hippocampus. Conversely, 5-HT3 receptor antagonists produced anxiolytic-like effects in the amygdala but not in the raphe nuclei. Nicotinic receptor agonists produced anxiolytic-like effects in the raphe and anxiogenic effects in the septum and hippocampus. Unexpectedly, physostigmine, a general cholinergic agonist, produced anxiolytic-like effects in the hippocampus. Neuropeptide receptors, although they are popular targets for the development of selective anxiolytic agents, had the least reliable effects across different animal models and brain structures, perhaps due in part to the fact that selective receptor ligands are relatively scarce. While some inconsistencies in the microinfusion data can easily be attributed to pharmacological variables such as dose or ligand selectivity, in other instances pharmacological explanations are more difficult to invoke: e.g., even the same dose of a known anxiolytic compound (midazolam) with a known mechanism of action (the benzodiazepine-GABAA receptor complex), can selectively affect different fear reactions depending upon the different subregions of the nucleus into which it is infused (CeA versus BLA). These particular functional dissociations are important and may depend on the ability of a GABAA receptor agonist to interact with distinct isoforms and combinations of GABAA receptor subunits (e.g., alpha1-6, beta1-3, Upsilon1-2, delta), many of which are unevenly distributed throughout the brain. Although this molecular hypothesis awaits thorough evaluation, the microinfusion data overall give some support for a model of "anxiety" that is functionally segregated along different levels of a neural hierarchy, analogous in some ways to the organization of sensorimotor systems.

Nociceptive and behavioural sensitisation by protein kinase Cepsilon signalling in the CNS

Despite the apparent homology in the protein kinase C (PKC) family, it has become clear that slight structural differences are sufficient to have unique signalling properties for each individual isoform. For PKCepsilon in depth investigation of these aspects revealed unique actions in the CNS and lead to development of specific modulators with clinical perspective. In this review, we describe to which extent PKCepsilon is distinct from other isoforms on the level of tissue expression and protein structure. As this kinase is highly expressed in the brain, we outline three main aspects of PKCepsilon signalling in the CNS. First, its ability to alter the permeability of N-type Ca2+ channels in dorsal root ganglia has been shown to enhance nociception. Secondly, PKCepsilon increases anxiety by diminishing GABA(A)R-induced inhibitory post-synaptic currents in the prefrontal cortex. Another important aspect of the latter inhibition is the reduced sensitivity of GABA(A) receptors to ethanol, a mechanism potentially contributing to abuse. A third signalling cascade improves cognitive functions by facilitating cholinergic signalling in the hippocampus. Collectively, these findings point to a physical and behavioural sensitising role for this kinase.

Anxiolytic-like action of MTEP expressed in the conflict drinking Vogel test in rats is serotonin dependent

The purpose of the present study was to investigate whether the anxiolytic-like action of a selective and brain penetrable group I metabotropic glutamate (mGlu5) receptor antagonist 3-[(2-methyl-1,3-tiazol-4-yl)ethynyl]-pyridine (MTEP) is dependent upon the serotonergic system. Experiments were performed on male Wistar rats. The Vogel conflict drinking test was used to detect anxiolytic-like activity. MTEP administered intraperitoneally at doses of 1, 3 and 6 mg/kg induced anxiolytic-like effect. The potential anxiolytic effect of MTEP (1 mg/kg) was inhibited by a nonselective 5-HT receptor antagonist metergoline (2 mg/kg i.p.) and 5-HT2A/2C receptor antagonist ritanserin (0.5 mg/kg i.p.), but not by a 5-HT1A receptor antagonist N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridynyl)cyclohexane-carboxamide (WAY 100635) (0.1 mg/kg i.p). The anxiolytic effect of MTEP (6 mg/kg) was attenuated by ritanserin (1 mg/kg i.p.). Moreover, MTEP-induced a dose-dependent release of serotonin in the frontal cortex. The obtained results suggest that the potential anxiolytic effect of the mGlu5 receptor antagonist MTEP is due to the increased serotonin release with subsequent activation of 5-HT2A/2C receptors, most probably located postsynaptically, but not by the 5-HT1A receptors.

The role of hippocampus in anxiety: intracerebral infusion studies

Although current models of hippocampal function stress its well-known role in cognitive functions, historically it has also been viewed as a neural mediator of emotion. Here, we review recent evidence from intrahippocampal infusion studies in animals that support a distinctive role of the hippocampus in anxiety, independent of its roles in learning and memory. Specifically, gamma-aminobutyric acid type A receptor agonists, both direct and indirect, reliably inhibit a number of animals' untrained anxiety reactions when microinfused into the hippocampus, whereas gamma-aminobutyric acid type A receptor antagonists do not. Intrahippocampal infusions of glutamatergic, serotonergic and cholinergic compounds also produce statistically reliable antianxiety effects, but the results vary as a function of specific anxiety reactions, and to some extent specific intrahippocampal targets. One hypothesis that may accommodate some of this variability is that anxiety is functionally segregated within the hippocampus, with ventral subregions more involved in anxiety-related processes, and dorsal subregions more involved with cognitive processes. Another possibility is that different hippocampal functions (e.g. memory and anxiety) are mediated by different neurotransmitter systems and/or different receptor subtypes within the hippocampus. Although there is some evidence that supports the latter hypothesis, the evidence for the former is not conclusive. Overall, however, the data clearly suggest that the hippocampus is importantly and directly involved in the mediation of untrained anxiety reactions in animals.

Differential effects of ethanol and midazolam upon the devaluation of an aversive memory in infant rats

In infant rats, low doses of ethanol (EtOH) have been found to attenuate the aversive representation of an unconditioned stimulus (US) as assessed through a revaluation paradigm. This may be explained by early anxiolytic properties of EtOH. The present set of experiments was aimed at analyzing possible mechanisms of these putative antianxiety effects of EtOH. In the first experiment, EtOH's effects upon the expression of citric acid-induced distress calls were compared with varying doses of midazolam (MDZ), a fast-acting gamma-aminobutyric acid type A (GABA(A)) agonist. Similar calming effects of 0.5 g/kg EtOH and 0.09 mg/kg MDZ were observed. Both drugs were then assessed in their capability to alter the expression of a conditioned aversion by devaluing the US. Aversive conditioning was conducted on postnatal day 14 (PD14) by pairing a lemon odor (conditioned stimulus, CS) with intraoral stimulation of citric acid (US). Control animals experienced both stimuli in an explicitly unrelated fashion. On PD15, pups were briefly exposed to the citric acid solution under the effects of 0.5 g/kg EtOH, 0.09 mg/kg MDZ, or the respective vehicle for each drug. Pups were then tested in a two-way odor preference test (lemon vs. cineole). Both vehicle- and MDZ-treated animals spent significantly less time near the lemon CS, thus expressing a citric acid-mediated odor aversion. This conditioned response was completely inhibited in pups that received 0.5 g/kg EtOH. Locomotor patterns at test were not affected by either EtOH or MDZ administration. A higher dose of MDZ (0.18 mg/kg, intraperitoneal) was also ineffective in attenuating the aversive memory. In summary, EtOH's devaluating capabilities are not shared by MDZ, indicating that these effects of EtOH may not be GABA mediated. Appetitive motivational properties of EtOH or non-GABA(A)-mediated antianxiety effects [i.e., N-methyl-D-aspartic acid (NMDA) related] could underlie this devaluation effect of EtOH.