Metabotropic and ionotropic glutamate receptors as neurobiological targets in anxiety and stress-related disorders: Focus on pharmacology and preclinical translational models

Gut microbiota and the tryptophan-kynurenine pathway in anxiety: new insights and treatment strategies

Anxiety disorders are mental health disorders characterized by long-lasting fear, worry, nervousness, and alterations in gut microbiota (GM). The GM is a vital modulator of brain function through the gut-brain axis, which acts as the neural pathway between the central and peripheral nervous systems. Dysbiosis of GM plays an essential role in anxiety development because of alterations in the vagus nerve, increased intestinal permeability, and altered breakdown of tryptophan (TRP). The Kynurenine (KYN) pathway plays a crucial role in the pathogenesis of anxiety disorders, primarily through its neuroprotective (KYNA) and neurotoxic (QUIN) metabolites. Higher ratios of KYNA/QUIN result in neuroprotection, whereas higher KYN/TRP ratios indicate increased QUIN production causing neuroinflammation. Studies on germ-free models exhibit higher plasma TRP levels, which interrupt the metabolic balance of TRP-derived compounds, thus causing brain impairment. A key issue in anxiety disorders is the dysregulation of GM, which disrupts TRP metabolism and neuroinflammatory pathways, however, remains poorly understood. Hence, the proper understanding of these mechanisms is crucial for future therapeutic advancements. Here, we highlight the significance of the TRP-KYN pathway and the potential of modulating KYN pathway enzymes, such as kynurenine aminotransferases (KATs), to adjust KYNA levels and restore neurotransmitter balance. It further discusses new therapeutic methods with a particular focus on probiotics that may restore GM and modulate TRP metabolism. Advancing our understanding of the intricate relationship between GM and anxiety disorders may facilitate novel, microbiota-targeted interventions. This ultimately contributes to precision medicine approaches in mental health care, thereby enhancing treatment efficacy and patient outcomes.

Update on neurobiological mechanisms of fear: illuminating the direction of mechanism exploration and treatment development of trauma and fear-related disorders

Fear refers to an adaptive response in the face of danger, and the formed fear memory acts as a warning when the individual faces a dangerous situation again, which is of great significance to the survival of humans and animals. Excessive fear response caused by abnormal fear memory can lead to neuropsychiatric disorders. Fear memory has been studied for a long time, which is of a certain guiding effect on the treatment of fear-related disorders. With continuous technological innovations, the study of fear has gradually shifted from the level of brain regions to deeper neural (micro) circuits between brain regions and even within single brain regions, as well as molecular mechanisms. This article briefly outlines the basic knowledge of fear memory and reviews the neurobiological mechanisms of fear extinction and relapse, which aims to provide new insights for future basic research on fear emotions and new ideas for treating trauma and fear-related disorders.

Biological embedding of early trauma: the role of higher prefrontal synaptic strength

Background: Early trauma predicts poor psychological and physical health. Glutamatergic synaptic processes offer one avenue for understanding this relationship, given glutamate's abundance and involvement in reward and stress sensitivity, emotion, and learning. Trauma-induced glutamatergic excitotoxicity may alter neuroplasticity and approach/avoidance tendencies, increasing risk for psychiatric disorders. Studies examine upstream or downstream effects instead of glutamatergic synaptic processes in vivo, limiting understanding of how trauma affects the brain.Objective: In a pilot study using a previously published data set, we examine associations between early trauma and a proposed measure of synaptic strength in vivo in one of the largest human samples to undergo Carbon-13 (13C MRS) magnetic resonance spectroscopy. Participants were 18 healthy controls and 16 patients with PTSD (male and female).Method: Energy per cycle (EPC), which represents the ratio of neuronal oxidative energy production to glutamate neurotransmitter cycling, was generated as a putative measure of glutamatergic synaptic strength.Results: Results revealed that early trauma was positively correlated with EPC in individuals with PTSD, but not in healthy controls. Increased synaptic strength was associated with reduced behavioural inhibition, and EPC showed stronger associations between reward responsivity and early trauma for those with higher EPC.Conclusion: In the largest known human sample to undergo 13C MRS, we show that early trauma is positively correlated with EPC, a direct measure of synaptic strength. Our study findings have implications for pharmacological treatments thought to impact synaptic plasticity, such as ketamine and psilocybin.

Targeting NMDA Receptors in Emotional Disorders: Their Role in Neuroprotection

Excitatory glutamatergic neurotransmission mediated through N-methyl-D-Aspartate (NMDA) receptors (NMDARs) is essential for synaptic plasticity and neuronal survival. While under pathological states, abnormal NMDAR activation is involved in the occurrence and development of psychiatric disorders, which suggests a directional modulation of NMDAR activity that contributes to the remission and treatment of psychiatric disorders. This review thus focuses on the involvement of NMDARs in the pathophysiological processes of psychiatric mood disorders and analyzes the neuroprotective mechanisms of NMDARs. Firstly, we introduce NMDAR-mediated neural signaling pathways in brain function and mood regulation as well as the pathophysiological mechanisms of NMDARs in emotion-related mental disorders such as anxiety and depression. Then, we provide an in-depth summary of current NMDAR modulators that have the potential to be developed into clinical drugs and their pharmacological research achievements in the treatment of anxiety and depression. Based on these findings, drug-targeting for NMDARs might open up novel territory for the development of therapeutic agents for refractory anxiety and depression.

Glutamatergic System is Affected in Brain from an Hyperthermia-Induced Seizures Rat Model

One of the most frequent neurological disorders in children is febrile seizures (FS), a risk for epilepsy in adults. Glutamate is the main excitatory neurotransmitter in CNS acting through ionotropic and metabotropic receptors. Excess of glutamate in the extracellular space elicits excitotoxicity and has been associated with neurological disorders, such as epilepsy. The removal of extracellular glutamate by excitatory amino acid transporters (EATT) plays an important neuroprotective role. GLT-1 is the main EAAT present in the cortex brain. On the other hand, an increase in metabotropic glutamate receptors 5 (mGlu5R) levels or their overstimulation have been related to the appearance of seizure events in different animal models and in temporal lobe epilepsy in humans. In this work, the status of several components of the glutamatergic system has been analysed in the cortex brain from an FS rat model at short (48 h) and long (20 days) term after hyperthermia-induced seizures. At the short term, we detected increased GLT-1 levels, reduced glutamate concentration, and unchanged mGlu5R levels, without neuronal loss. However, at the long term, an increase in mGlu5R levels together with a decrease in both GLT-1 and glutamate levels were observed. These changes were associated with the appearance of an anxious phenotype. These results suggest a neuroprotective role of the glutamatergic components mGlu5R and GLT-1 at the short term. However, this neuroprotective effect seems to be lost at the long term, leading to an anxious phenotype and suggesting an increased vulnerability and propensity to epileptic events in adults.

GABA-ergic agents modulated the effects of histamine on male mice behavior in the elevated plus-maze

NEW FINDINGS

What is the main question of this study? Is there an interaction between histamine and GABAergic system on modulation of anxiety in mice? What is the main finding and its importance? There is a synergistic anxiogenic effect between histamine and bicuculline in mice. This effect may be due to a direct or indirect effect of the histaminergic system on the GABAergic system.

ABSTRACT

There are documents that both histaminergic and GABAergic systems are participated in the neurobiology of anxiety behavior. In the current research, we investigated the effects of the histaminergic system and GABAA receptor agents on anxiety-related behaviors and their interaction using the elevated plus-maze (EPM) test in mice. Intraperitoneally (i.p.) administration of muscimol (0.12 and 0.25 mg/kg) increased the open arm time (OAT) (p < 0.001) without affecting the open arm entries (OAE) and locomotor activity, showing an anxiolytic effect. I.p. injection of bicuculline (0.5 and 1 mg/kg) decreased OAT (p < 0.001) but not OAE and locomotor activity, suggesting an anxiogenic behavior. Intracerebroventricularly (i.c.v.) microinjection of histamine (2.5 and 5 μg/mouse) declined OAT (p < 0.001) but not OAE and locomotor activity, indicating an anxiogenic response. Co-administration of histamine with GABAergic agents, muscimol (0.06 mg/kg; i.p.) and bicuculline (0.25 mg/kg; i.p.), decreased (p < 0.001) and increased (p < 0.05) the anxiogenic-like response of the effective dose (5 μg/mouse; i.c.v.) of histamine, respectively. In addition, co-treatment of effective doses of histamine (2.5 and 5 μg/mouse;i.c.v.) along with an effective dose of muscimol (0.12 mg/kg;i.p.) and not-effective dose of bicuculline (0.25 mg/kg; i.p.) significantly decreased OAT (p < 0.001), suggesting a likely interaction between the histaminergic and GABAergic systems on the regulation of anxiety. The results demonstrated a synergistic anxiogenic-like effect between histamine and bicuculline in mice. In conclusion, our results presented an interaction between the histaminergic and GABAergic systems on anxiolytic/anxiogenic-like behaviors in the EPM test. This article is protected by copyright. All rights reserved.

Experimental Drugs for Panic Disorder: An Updated Systematic Review

Several effective pharmacological therapies for panic disorder (PD) are available, but they have some drawbacks, and unsatisfactory outcomes can occur. Expanding the variety of anti-panic medications may allow for improving PD treatment. The authors performed an updated systematic review of preclinical and clinical (Phase I–III) pharmacological studies to look for advances made in the last six years concerning novel-mechanism-based anti-panic compounds or using medications approved for nonpsychiatric medical conditions to treat PD. The study included seven published articles presenting a series of preclinical studies, two Phase I clinical studies with orexin receptor (OXR) antagonists, and two clinical studies investigating the effects of D-cycloserine (DCS) and xenon gas in individuals with PD. The latest preclinical findings confirmed and expanded previous promising indications of OXR1 antagonists as novel-mechanism-based anti-panic compounds. Translating preclinical research into clinical applications remains in the early stages. However, limited clinical findings suggested the selective OXR1 antagonist JNJ-61393115 may exert anti-panic effects in humans. Overall, OXR1 antagonists displayed a favorable profile of short-term safety and tolerability. Very preliminary suggestions of possible anti-panic effects of xenon gas emerged but need confirmation with more rigorous methodology. DCS did not seem promising as an enhancer of cognitive-behavioral therapy in PD. Future studies, including objective panic-related physiological parameters, such as respiratory measures, and expanding the use of panic vulnerability biomarkers, such as hypersensitivity to CO₂ panic provocation, may allow for more reliable conclusions about the anti-panic properties of new compounds.

The neuropsychiatric manifestations of COVID-19: Interactions with psychiatric illness and pharmacological treatment

The recent outbreak of the corona virus disease (COVID-19) has had major global impact. The relationship between severe acute respiratory syndrome coronavirus (SARS-CoV-2) infection and psychiatric diseases is of great concern, with an evident link between corona virus infections and various central and peripheral nervous system manifestations. Unmitigated neuro-inflammation has been noted to underlie not only the severe respiratory complications of the disease but is also present in a range of neuro-psychiatric illnesses. Several neurological and psychiatric disorders are characterized by immune-inflammatory states, while treatments for these disorders have distinct anti-inflammatory properties and effects. With inflammation being a common contributing factor in SARS-CoV-2, as well as psychiatric disorders, treatment of either condition may affect disease progression of the other or alter response to pharmacological treatment. In this review, we elucidate how viral infections could affect pre-existing psychiatric conditions and how pharmacological treatments of these conditions may affect overall progress and outcome in the treatment of SARS-CoV-2. We address whether any treatment-induced benefits and potential adverse effects may ultimately affect the overall treatment approach, considering the underlying dysregulated neuro-inflammatory processes and potential drug interactions. Finally, we suggest adjunctive treatment options for SARS-CoV-2-associated neuro-psychiatric symptoms.

Ketamine effects on anxiety and fear-related behaviors: Current literature evidence and new findings

Ketamine, a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, presents a rapid and sustained antidepressant effect in clinical and preclinical studies. Regarding ketamine effects on anxiety, there is a widespread discordance among pre-clinical studies. To address this issue, the present study reviewed the literature (electronic database MEDLINE) to summarize the profile of ketamine effects in animal tests of anxiety/fear. We found that ketamine anxiety/fear-related effects may depend on the anxiety paradigm, schedule of ketamine administration and tested species. Moreover, there was no report of ketamine effects in animal tests of fear related to panic disorder (PD). Based on that finding, we evaluated if treatment with ketamine and another NMDA antagonist, MK-801, would induce acute and sustained (24 hours later) anxiolytic and/or panicolytic-like effects in animals exposed to the elevated T-maze (ETM). The ETM evaluates, in the same animal, conflict-evoked and fear behaviors, which are related, respectively, to generalized anxiety disorder and PD. Male Wistar rats were systemically treated with racemic ketamine (10, 30 and 80 mg/kg) or MK-801 (0.05 and 0.1 mg/kg) and tested in the ETM in the same day or 24 hours after their administration. Ketamine did not affect the behavioral tasks performed in the ETM acutely or 24 h later. MK-801 impaired inhibitory avoidance in the ETM only at 45 min post-injection, suggesting a rapid but not sustained anxiolytic-like effect. Altogether our results suggest that ketamine might have mixed effects in anxiety tests while it does not affect panic-related behaviors.

Early sexual trauma is related with the tapetum in patients with panic disorder

BACKGROUND

Early trauma exposure is suggested to confer a greater risk for the development of panic disorder (PD) by altering neurodevelopmental processes. A body of studies have shown that white matter microstructures, particularly in the fronto-limbic and callosal regions, could be affected by a history of early trauma in patients with PD. The tapetum, the extended posterior part of the corpus callosum connecting the bilateral temporal lobes, has not yet been studied regarding its relationship with early trauma, especially sexual one.

METHODS

Seventy participants with PD, sixty age- and sex-matched healthy controls were enrolled. The Early Trauma Inventory Self Report-Short Form (ETISR-SF), Neuroticism Scale, and Panic Disorder Severity Scale (PDSS) were administered. Voxel-wise statistical analysis of diffusion tensor imaging data was performed within the bilateral tapetum regions using Tract-Based Spatial Statistics (TBSS).

RESULTS

In participants with PD, higher levels of sexual trauma were significantly associated with the increased fractional anisotropies (FAs) in a cluster of the right tapetum. Exploratory correlation analysis revealed that FAs in this cluster were significantly correlated with higher neuroticism and poorer treatment response after one year of pharmacotherapy. No significant correlation was found between FAs of the right tapetum cluster and the PDSS scores.

LIMITATIONS

The possibility of retrospective recall bias for early trauma cannot be completely ruled out.

CONCLUSIONS

The findings of this study suggest that the right tapetum may serve as a potential neural marker of early sexual trauma in patients with PD and contribute to personal vulnerability and poorer treatment outcome after pharmacotherapy.

The fluctuations of metabotropic glutamate receptor subtype 5 (mGluR5) in the amygdala in fear conditioning model of male Wistar rats following sleep deprivation, reverse circadian and napping

Sleep is involved in metabolic system, mental health and cognitive functions. Evidence shows that sleep deprivation (SD) negatively affects mental health and impairs cognitive functions, including learning and memory. Furthermore, the metabotropic glutamate receptor subtype 5 (mGluR5) is a metabolic biomarker, which is affected by various conditions, including stress, sleep deprivation, and cognitive and psychiatric disorders. In this research, we investigated the effect of SD and reverse circadian (RC), and two models of napping (continuous and non-continuous) combined with SD or RC on fear-conditioning memory, anxiety-like behavior and mGluR5 fluctuations in the amygdala. 64 male Wistar rats were used in this study. The water box apparatus was used to induce SD/RC for 48 h, and fear-conditioning memory apparatus was used to assess fear memory. The results showed, fear-conditioning memory was impaired following SD and RC, especially in contextual stage. However, anxiety-like behavior was increased. Furthermore, mGluR5 was increased in the left amygdala more than the right amygdala. Additionally, continuous napping significantly improved fear-conditioning memory, especially freezing behavior. In conclusion, following SD and RC, fear-conditioning memory in contextual stage is more vulnerable than in auditory stage. Furthermore, increase in anxiety-like behavior is related to increase in the activity of left amygdala and mGluR5 receptors.

Glutamate Systems in DSM-5 Anxiety Disorders: Their Role and a Review of Glutamate and GABA Psychopharmacology

Serotonin reuptake inhibitors and benzodiazepines are evidence-based pharmacological treatments for Anxiety Disorders targeting serotonin and GABAergic systems, respectively. Although clearly effective, these medications fail to improve anxiety symptoms in a significant proportion of patients. New insights into the glutamate system have directed attention toward drugs that modulate glutamate as potential alternative treatments for anxiety disorders. Here we summarize the current understanding of the potential role of glutamate neurotransmission in anxiety disorders and highlight specific glutamate receptors that are potential targets for novel anxiety disorder treatments. We also review clinical trials of medications targeting the glutamate system in DSM-5 anxiety disorders. Understanding the role of the glutamate system in the pathophysiology of anxiety disorder may aid in developing novel pharmacological agents that are effective in treating anxiety disorders.

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.

[Anxiety in the elderly]

Anxiety occurs in about one third of people over 65 years of age. However, its identification in this age has significant difficulties. The clinical manifestations, pathogenetic mechanisms, approaches to the diagnosis and treatment of various types of anxiety are described in the article. Particular attention is paid to the comorbidity of anxiety disorders in elderly patients. A comprehensive approach to the treatment of elderly patients with anxiety includes psychotherapeutic and pharmacotherapeutic approaches. Special attention should be paid to the efficacy and safety of the drugs, which is especially important in this category of patients.

Bisphenol-A exposure induced neurotoxicity in glutamatergic neurons derived from human embryonic stem cells

Bisphenol-A (BPA) is a lipophilic, organic, synthetic compound that has been used as an additive in polycarbonate plastics manufacturing since 1957. Studies have shown that BPA interferes with the development and functions of the brain, but little is known about the effects of BPA on human glutamatergic neurons (hGNs) at the molecular and cellular levels. We investigated the impact of chronic exposure to BPA to hGNs derived from human embryonic stem cells (hESCs). The results showed that chronic exposure of different concentrations of BPA (0, 0.1, 1.0 and 10 μM) to hGNs for 14 days reduced neurite outgrowth in a concentration-dependent manner. Using presynaptic protein synaptophysin and postsynaptic protein PSD-95 antibodies, immunofluorescence staining and western blotting results indicated that BPA exposure altered the morphology of dendritic spines and increased synaptophysin and PSD-95 expression. Furthermore, BPA exposure at concentrations higher than 1.0 μM resulted in the increase of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) expression and deterioration of dendritic spines. In addition, our results suggested that these BPA mediated neurotoxicity effects were due to an increased production of reactive nitrogen species (RNS) and reactive oxygen species (ROS) via increased nitric oxide synthase (iNOS), neuronal nitric oxide synthase (nNOS), 3-nitrotyrosine expression and Ca²⁺ influx. These results imply that hESC-based neuronal differentiation is an excellent cellular model to examine BPA-induced neurotoxicity on human neurons at the cellular and molecular level.

Anxiolytic activity of paraoxon is associated with alterations in rat brain glutamatergic system

Exposure to organophosphate (OP) compounds leads to behavioral alterations. To determine whether paraoxon has effects on anxiety, anxiety-like behaviors were assessed in paraoxon-exposed rats. Protein expression of glutamate transporters has also been measured in hippocampus and prefrontal cortex. Three doses of paraoxon (0.3, 0.7, or 1 mg/kg) or corn oil (vehicle) were intraperitoneally injected to adult male rats. At 14 or 28 days after exposure, behavioral tests were done using elevated plus-maze (EPM) or open field tests. Thereafter, animals were sacrificed and both hippocampi and prefrontal cortices were extracted for cholinesterase assay and western blotting. Animals treated with convulsive doses of paraoxon (0.7 and 1 mg/kg) showed an increase in percentage of time spent in open arms and percentage of open arm entries in the EPM. In the open field test, an increase in the time spent in central area was observed in rats treated with the same doses of paraoxon. These effects of paraoxon were independent of any changes in locomotor activity. There was an increase in both astrocytic glutamate transporter proteins (GLAST and GLT-1) in the hippocampus of animals treated with 0.7 and 1 mg/kg of paraoxon. In the prefrontal cortex, protein levels of the GLAST and GLT-1 increased in 0.7 and decreased in 1 mg/kg groups. Only a significant decrease in EAAC1 protein was observed in the prefrontal cortex at 14 days following exposure to 1 mg/kg of paraoxon. Collectively, this study showed that exposure to convulsive doses of paraoxon induced anxiolytic-like behaviors in both behavioral tests. This effect may be attributed to alterations of glutamate transporter proteins in the rat hippocampus and prefrontal cortex.

Psychological co-morbidities in COPD: Targeting systemic inflammation, a benefit for both?

COPD is a chronic lung disease characterized by persistent respiratory symptoms and airflow limitation due to airway and/or alveolar abnormalities. Furthermore, COPD is often characterized by extrapulmonary manifestations and comorbidities worsening COPD progression and quality of life. A neglected comorbidity in COPD management is mental health impairment defined by anxiety, depression and cognitive problems. This paper summarizes the evidence for impaired mental health in COPD and focuses on current pharmacological intervention strategies. In addition, possible mechanisms in impaired mental health in COPD are discussed with a central role for inflammation. Many comorbidities are associated with multi-organ-associated systemic inflammation in COPD. Considering the accumulative evidence for a major role of systemic inflammation in the development of neurological disorders, it can be hypothesized that COPD-associated systemic inflammation also affects the function of the brain and is an interesting therapeutic target for nutra- and pharmaceuticals.

The Impact and Mechanism of Methylated Metabotropic Glutamate Receptors 1 and 5 in the Hippocampus on Depression-Like Behavior in Prenatal Stress Offspring Rats

An increasing number of epidemiological investigations and animal models research suggest that prenatal stress (PS) could cause depression-like behavior in the offspring, which is sex specific. However, the underlying mechanisms remain to be elucidated. This study is to investigate the promoter methylation of metabotropic glutamate receptor 1 (mGluR1) and metabotropic Glutamate Receptor 5 (mGluR5) gene modification on PS induced depression-like behavior in offspring rats (OR). PS models were established, with or without 5-aza-2′-deoxycytidine (5-azaD, decitabine) treatment. Animal behavior was assessed by the sucrose preference test (SPT), forced swimming test (FST), and open field test (OFT). The mRNA and protein expression levels of mGluR1 and mGluR5 in the hippocampus of offspring were detected with quantitative real-time PCR and Western blot analysis, respectively. The promoter methylation in the hippocampus of mGluR1 and mGluR5 OR were also analyzed. SPT showed significantly reduced sucrose preference in PS induced OR. FST showed significantly prolonged immobility time in PS induced OR. OFT showed significantly reduced central residence time in PS induced OR and no significantly influence in rearing as well as in frequency of micturition. Moreover, the mRNA, protein expression levels, and gene promoter methylation level of mGluR1 and mGluR5 in the hippocampus were significantly increased in the PS induced male OR, while no significantly influence in the PS induced female OR. Furthermore, the PS induced effects in male OR could be reversed by the microinjection of 5-azaD. In conclusion, our results showed that the promoter methylation of mGluR1 and mGluR5 gene modification is only involved in PS induced depression-like behavior in male OR in a sex-specific manner. These findings might contribute to the understanding of the disease pathogenesis and clinical treatment in future.

Stress Response Modulation Underlying the Psychobiology of Resilience

PURPOSE OF REVIEW

This review focuses on the relationship between resilience and the ability to effectively modulate the stress response. Neurobiological and behavioral responses to stress are highly variable. Exposure to a similar stressor can lead to heterogeneous outcomes-manifesting psychopathology in one individual, but having minimal effect, or even enhancing resilience, in another. We highlight aspects of stress response modulation related to early life development and epigenetics, selected neurobiological and neurochemical systems, and a number of emotional, cognitive, psychosocial, and behavioral factors important in resilience. We also briefly discuss interventions with potential to build and promote resilience.

RECENT FINDINGS

Throughout this review, we include evidence from recent preclinical and clinical studies relevant to the psychobiology of resilient stress response modulation. Effective modulation of the stress response is an essential component of resilience and is dependent on a complex interplay of neurobiological and behavioral factors.

A magnetoencephalography investigation of coherence source imaging in panic disorder

Limbic and frontal structures are largely implicated in panic disorder (PD). Decreased coherence imaging values, as determined by magnetoencephalography (MEG), are suggestive of decreased or inefficient communication among these structures. We have previously demonstrated that coherence source imaging (CSI) values could be similar or higher in some PD patients. The purpose of the current investigation was to replicate these finding in a larger sample. Nine strictly diagnosed PD patients and nine age-matched and sex-matched healthy controls were examined. The CSI-MEG values of 26 frontotemporal regions (FTRs) and 28 extra-frontotemporal regions (ex-FTR; Brodmann areas) were determined for each participant. MEG scans were acquired using a 151-channel whole-head biomagnetometer system. Despite the relatively small sample size, CSI values were significantly lower in a number of FTRs in PD patients. In none of the ex-FTRs (i.e. posterior regions) were there differences between panic and control groups. The above data add to the complexity of understanding the nature of the pathophysiology of PD. Our finding of decreased focal coherence imaging values may reflect decreased excitability in these areas. The preliminary finding could be interpreted as an inhibitory process guarding against the spread of activity in closer hyperexcitable areas as seen in epilepsy. The current data provide evidence for dysfunctional communication within the frontotemporal structures. The findings have implications for the understanding of the neural circuitry underlying PD.

Exploring a post-traumatic stress disorder paradigm in Flinders sensitive line rats to model treatment-resistant depression I: bio-behavioural validation and response to imipramine

OBJECTIVE

Co-morbid depression with post-traumatic stress disorder (PTSD) is often treatment resistant. In developing a preclinical model of treatment-resistant depression (TRD), we combined animal models of depression and PTSD to produce an animal with more severe as well as treatment-resistant depressive-like behaviours.

METHODS

Male Flinders sensitive line (FSL) rats, a genetic animal model of depression, were exposed to a stress re-stress model of PTSD [time-dependent sensitisation (TDS)] and compared with stress-naive controls. Seven days after TDS stress, depressive-like and coping behaviours as well as hippocampal and cortical noradrenaline (NA) and 5-hydroxyindoleacetic acid (5HIAA) levels were analysed. Response to sub-chronic imipramine treatment (IMI; 10 mg/kg s.c.×7 days) was subsequently studied.

RESULTS

FSL rats demonstrated bio-behavioural characteristics of depression. Exposure to TDS stress in FSL rats correlated negatively with weight gain, while demonstrating reduced swimming behaviour and increased immobility versus unstressed FSL rats. IMI significantly reversed depressive-like (immobility) behaviour and enhanced active coping behaviour (swimming and climbing) in FSL rats. The latter was significantly attenuated in FSL rats exposed to TDS versus unstressed FSL rats. IMI reversed reduced 5HIAA levels in unstressed FSL rats, whereas exposure to TDS negated this effect. Lowered NA levels in FSL rats were sustained after TDS with IMI significantly reversing this in the hippocampus.

CONCLUSION

Combining a gene-X-environment model of depression with a PTSD paradigm produces exaggerated depressive-like symptoms that display an attenuated response to antidepressant treatment. This work confirms combining FSL rats with TDS exposure as a putative animal model of TRD.

Juvenile treatment with a novel mGluR2 agonist/mGluR3 antagonist compound, LY395756, reverses learning deficits and cognitive flexibility impairments in adults in a neurodevelopmental model of schizophrenia

Schizophrenia (SCZ) is a neurodevelopmental psychiatric disorder, in which cognitive function becomes disrupted at early stages of the disease. Although the mechanisms underlying cognitive impairments remain unclear, N-methyl-D-aspartate receptors (NMDAR) hypofunctioning in the prefrontal cortex (PFC) has been implicated. Moreover, cognitive symptoms in SCZ are usually unresponsive to treatment with current antipsychotics and by onset, disruption of the dopamine system, not NMDAR hypofunctioning, dominates the symptoms. Therefore, treating cognitive deficits at an early stage is a realistic approach. In this study, we tested whether an early treatment targeting mGluR2 would be effective in ameliorating cognitive impairments in the methylazoxymethanol acetate (MAM) model of SCZ. We investigated the effects of an mGluR2 agonist/mGluR3 antagonist, LY395756 (LY39), on the NMDAR expression and function in juveniles, as well as cognitive deficits in adult rats after juvenile treatment. We found that gestational MAM exposure induced a significant decrease in total protein levels of the NMDAR subunit, NR2B, and a significant increase of pNR2BTyr1472 in the juvenile rat PFC. Treatment with LY39 in juvenile MAM-exposed rats effectively recovered the disrupted NMDAR expression. Furthermore, a subchronic LY39 treatment in juvenile MAM-exposed rats also alleviated the learning deficits and cognitive flexibility impairments when tested with a cross-maze based set-shifting task in adults. Therefore, our study demonstrates that targeting dysfunctional NMDARs with an mGluR2 agonist during the early stage of SCZ could be an effective strategy in preventing the development and progression in addition to ameliorating cognitive impairments of SCZ.

Infralimbic Neurotrophin-3 Infusion Rescues Fear Extinction Impairment in a Mouse Model of Pathological Fear

The inability to properly extinguish fear memories constitutes the foundation of several anxiety disorders, including panic disorder. Recent findings show that boosting prefrontal cortex synaptic plasticity potentiates fear extinction, suggesting that therapies that augment synaptic plasticity could prove useful in rescue of fear extinction impairments in this group of disorders. Previously, we reported that mice with selective deregulation of neurotrophic tyrosine kinase receptor, type 3 expression (TgNTRK3) exhibit increased fear memories accompanied by impaired extinction, congruent with an altered activation pattern of the amygdala-hippocampus-medial prefrontal cortex fear circuit. Here we explore the specific role of neurotrophin 3 and its cognate receptor in the medial prefrontal cortex, and its involvement in fear extinction in a pathological context. In this study we combined molecular, behavioral, in vivo pharmacology and ex vivo electrophysiological recordings in TgNTRK3 animals during contextual fear extinction processes. We show that neurotrophin 3 protein levels are increased upon contextual fear extinction in wild-type animals but not in TgNTRK3 mice, which present deficits in infralimbic long-term potentiation. Importantly, infusion of neurotrophin 3 to the medial prefrontal cortex of TgNTRK3 mice rescues contextual fear extinction and ex vivo local application improves medial prefrontal cortex synaptic plasticity. This effect is blocked by inhibition of extracellular signal-regulated kinase phosphorylation through peripheral administration of SL327, suggesting that rescue occurs via this pathway. Our results suggest that stimulating neurotrophin 3-dependent medial prefrontal cortex plasticity could restore contextual fear extinction deficit in pathological fear and could constitute an effective treatment for fear-related disorders.

Glutamate dysregulation and glutamatergic therapeutics for PTSD: Evidence from human studies

Posttraumatic stress disorder (PTSD) is a chronic and debilitating psychiatric disorder afflicting millions of individuals across the world. While the availability of robust pharmacologic interventions is quite lacking, our understanding of the putative neurobiological underpinnings of PTSD has significantly increased over the past two decades. Accumulating evidence demonstrates aberrant glutamatergic function in mood, anxiety, and trauma-related disorders and dysfunction in glutamate neurotransmission is increasingly considered a cardinal feature of stress-related psychiatric disorders including PTSD. As part of a PTSD Special Issue, this mini-review provides a concise discussion of (1) evidence of glutamatergic abnormalities in PTSD, with emphasis on human subjects data; (2) glutamate-modulating agents as potential alternative pharmacologic treatments for PTSD; and (3) selected gaps in the literature and related future directions.

Zebrafish Get Connected: Investigating Neurotransmission Targets and Alterations in Chemical Toxicity

Neurotransmission is the basis of neuronal communication and is critical for normal brain development, behavior, learning, and memory. Exposure to drugs and chemicals can alter neurotransmission, often through unknown pathways and mechanisms. The zebrafish (Danio rerio) model system is increasingly being used to study the brain and chemical neurotoxicity. In this review, the major neurotransmitter systems, including glutamate, GABA, dopamine, norepinephrine, serotonin, acetylcholine, histamine, and glutamate are surveyed and pathways of synthesis, transport, metabolism, and action are examined. Differences between human and zebrafish neurochemical pathways are highlighted. We also review techniques for evaluating neurological function, including the measurement of neurotransmitter levels, assessment of gene expression through transcriptomic analysis, and the recording of neurobehavior. Finally examples of chemical toxicity studies evaluating alterations in neurotransmitter systems in the zebrafish model are reviewed.

Understanding resilience: New approaches for preventing and treating PTSD

All individuals experience stressful life events, and up to 84% of the general population will experience at least one potentially traumatic event. In some cases, acute or chronic stressors lead to the development of posttraumatic stress disorder (PTSD) or other psychopathology; however, the majority of people are resilient to such effects. Resilience is the ability to adapt successfully in the face of stress and adversity. A wealth of research has begun to identify the genetic, epigenetic, neural, and environmental underpinnings of resilience, and has indicated that resilience is mediated by adaptive changes encompassing several environmental factors, neural circuits, numerous neurotransmitters, and molecular pathways. The first part of this review focuses on recent findings regarding the genetic, epigenetic, developmental, psychosocial, and neurochemical factors as well as neural circuits and molecular pathways that underlie the development of resilience. Emerging and exciting areas of research and novel methodological approaches, including genome-wide gene expression studies, immune, endocannabinoid, oxytocin, and glutamatergic systems, are explored to help delineate innovative mechanisms that may contribute to resilience. The second part reviews several interventions and preventative approaches designed to enhance resilience in both developmental and adult populations. Specifically, the review will delineate approaches aimed to bolster resilience in individuals with PTSD. Furthermore, we discuss novel pharmacologic approaches, including the N-methyl-d-aspartate (NMDA) receptor ketamine and neuropeptide Y (NPY), as exciting new prospects for not only the treatment of PTSD but as new targets to enhance resilience. Our growing understanding of resilience and interventions will hopefully lead to the development of new strategies for not just treating PTSD but also screening and early identification of at-risk youth and adults. Taken together, efforts aimed at dissemination and implementation of novel interventions to enhance resilience will have to keep pace with the growth of new preventive and treatment strategies.

Anxiety-related behavior in hyperhomocysteinemia induced by methionine nutritional overload in rats: role of the brain oxidative stress

The aim of this study was to examine the effects of a methionine-enriched diet on anxiety-related behavior in rats and to determine the role of the brain oxidative status in these alterations. Adult male Wistar rats were fed from the 30th to 60th postnatal day with standard or methionine-enriched diet (double content comparing with standard diet: 7.7 g/kg). Rats were tested in open field and light-dark tests and afterwards oxidative status in the different brain regions were determined. Hyperhomocysteinemia induced by methionine-enriched diet in this study decreased the number of rearings, as well as the time that these animals spent in the center of the open field, but increased index of thigmotaxy. Oxidative status was selectively altered in the examined regions. Lipid peroxidation was significantly increased in the cortex and nc. caudatus of rats developing hyperhomocysteinemia, but unaltered in the hippocampus and thalamus. Based on the results of this research, it could be concluded that hyperhomocysteinemia induced by methionine nutritional overload increased anxiety-related behavior in rats. These proanxiogenic effects could be, at least in part, a consequence of oxidative stress in the rat brain.

Social Isolation Stress Induces Anxious-Depressive-Like Behavior and Alterations of Neuroplasticity-Related Genes in Adult Male Mice

Stress is a major risk factor in the onset of several neuropsychiatric disorders including anxiety and depression. Although several studies have shown that social isolation stress during postweaning period induces behavioral and brain molecular changes, the effects of social isolation on behavior during adulthood have been less characterized. Aim of this work was to investigate the relationship between the behavioral alterations and brain molecular changes induced by chronic social isolation stress in adult male mice. Plasma corticosterone levels and adrenal glands weight were also analyzed. Socially isolated (SI) mice showed higher locomotor activity, spent less time in the open field center, and displayed higher immobility time in the tail suspension test compared to group-housed (GH) mice. SI mice exhibited reduced plasma corticosterone levels and reduced difference between right and left adrenal glands. SI showed lower mRNA levels of the BDNF-7 splice variant, c-Fos, Arc, and Egr-1 in both hippocampus and prefrontal cortex compared to GH mice. Finally, SI mice exhibited selectively reduced mGluR1 and mGluR2 levels in the prefrontal cortex. Altogether, these results suggest that anxious- and depressive-like behavior induced by social isolation stress correlates with reduction of several neuroplasticity-related genes in the hippocampus and prefrontal cortex of adult male mice.

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.

Negative childhood experiences alter a prefrontal-insular-motor cortical network in healthy adults: A preliminary multimodal rsfMRI-fMRI-MRS-dMRI study

Research in humans and animals has shown that negative childhood experiences (NCE) can have long-term effects on the structure and function of the brain. Alterations have been noted in grey and white matter, in the brain's resting state, on the glutamatergic system, and on neural and behavioural responses to aversive stimuli. These effects can be linked to psychiatric disorder such as depression and anxiety disorders that are influenced by excessive exposure to early life stressors. The aim of the current study was to investigate the effect of NCEs on these systems. Resting state functional MRI (rsfMRI), aversion task fMRI, glutamate magnetic resonance spectroscopy (MRS), and diffusion magnetic resonance imaging (dMRI) were combined with the Childhood Trauma Questionnaire (CTQ) in healthy subjects to examine the impact of NCEs on the brain. Low CTQ scores, a measure of NCEs, were related to higher resting state glutamate levels and higher resting state entropy in the medial prefrontal cortex (mPFC). CTQ scores, mPFC glutamate and entropy, correlated with neural BOLD responses to the anticipation of aversive stimuli in regions throughout the aversion-related network, with strong correlations between all measures in the motor cortex and left insula. Structural connectivity strength, measured using mean fractional anisotropy, between the mPFC and left insula correlated to aversion-related signal changes in the motor cortex. These findings highlight the impact of NCEs on multiple inter-related brain systems. In particular, they highlight the role of a prefrontal-insular-motor cortical network in the processing and responsivity to aversive stimuli and its potential adaptability by NCEs.

Elevated Plasma C-Terminal Endothelin-1 Precursor Fragment Concentrations Are Associated with Less Anxiety in Patients with Cardiovascular Risk Factors. Results from the Observational DIAST-CHF Study

Background

The role of endothelin-1 (ET-1) in the neurobiology of anxiety is unknown, therefore, we assessed in the observational multicenter DIAST-CHF study whether the C-terminal ET-1 precursor fragment (CT-proET-1) is linked to anxiety.

Methods

Plasma concentrations of CT-proET-1 were measured in a total of 1,410 patients presenting with cardiovascular risk factors (mean age 66.91±8.2 years, 49.3% males, mean left ventricular ejection fraction 60.0±8.2%) who had completed the Hospital Anxiety and Depression Scale (HADS) questionnaire.

Results

Among the total study cohort (n = 1,410), there were 118 subjects (8.4%) with an HADS anxiety score above the cut-off level of 11 suggestive of clinically relevant anxiety. Plasma CT-proET-1 levels were significantly lower in the group of anxious patients as compared to non-anxious patients (p = 0.013). In regression models adjusted for sex, age, systolic blood pressure, and diameters of left atrium and ventricle, plasma CT-proET-1 was again linked to anxiety (Exp(β) = 0.247, 95%-confidence interval [95%-CI] = 0.067–0.914, p = 0.036). Given the high prevalence of depressive disorders in anxious patients, we additionally included the HADS depression score as an independent variable in the models and found that CT-proET-1 remained a significant predictor of anxiety, independent of comorbid depression (Exp(β) = 0.114, 95%-CI = 0.023–0.566, p = 0.008).

Conclusions

Our data from a population-based study in outpatients with cardiovascular risk factors revealed that circulating CT-proET-1 levels are negatively associated with anxiety. Further investigations are required to clarify the putative anxiolytic effect of ET-1 or its precursor molecules in humans and to decipher its mechanistic pathways.

Epilepsy spectrum disorders: A concept in need of validation or refutation

Episodic psychiatric symptoms are not uncommon and range from panic attacks to repeated violent acts. Some evidence has accumulated over the years that at least in a subset of patients exhibiting these symptoms there may be evidence for the presence of focal cortical/subcortical hyperexcitability. In these cases the condition could be conceptualized as an epilepsy spectrum disorder (ESD) with significant treatment implications. There is currently no clear demarcation of this category of symptoms, their prevalence, an understanding of how these symptoms occur, what is appropriate work up and possible treatments. In this article, we propose that milder degrees of increased neural excitability (i.e., a subthreshold excitation insufficient to cause seizures) may nonetheless be capable of causing observable phenotypic changes. The observable phenotypic changes depend on the degree of hyperexcitability and the location of the hyperexcitable neural tissue. The location of the abnormal neural tissue may dictate the initial manifestation of an attack resulting from activation of the hyperexcitable tissue, but the anatomical connectivity of the abnormal region will dictate the breadth of manifestations. We provide some evidence, derived mainly from either electroencephalography studies of these populations or clinical reports of response to anti-epilepsy treatment, for the assumption and propose methods to test the advanced hypothesis.

Hippocampal-Dependent Antidepressant Action of the H3 Receptor Antagonist Clobenpropit in a Rat Model of Depression

BACKGROUND

Histamine is a modulatory neurotransmitter regulating neuronal activity. Antidepressant drugs target modulatory neurotransmitters, thus ultimately regulating glutamatergic transmission and plasticity. Histamine H3 receptor (H3R) antagonists have both pro-cognitive and antidepressant effects; however, the mechanism by which they modulate glutamate transmission is not clear. We measured the effects of the H3R antagonist clobenpropit in the Flinders Sensitive Line (FSL), a rat model of depression with impaired memory and altered glutamatergic transmission.

METHODS

Behavioral tests included the forced swim test, memory tasks (passive avoidance, novel object recognition tests), and anxiety-related paradigms (novelty suppressed feeding, social interaction, light/dark box tests). Hippocampal protein levels were detected by Western blot. Hippocampal plasticity was studied by in slice field recording of CA3-CA1 long-term synaptic potentiation (LTP), and glutamatergic transmission by whole-cell patch clamp recording of excitatory postsynaptic currents (EPSCs) in CA1 pyramidal neurons.

RESULTS

Clobenpropit, administered systemically or directly into the hippocampus, decreased immobility during the forced swim test; systemic injections reversed memory deficits and increased hippocampal GluN2A protein levels. FSL rats displayed anxiety-related behaviors not affected by clobenpropit treatment. Clobenpropit enhanced hippocampal plasticity, but did not affect EPSCs. H1R and H2R antagonists prevented the clobenpropit-induced increase in LTP and, injected locally into the hippocampus, blocked clobenpropit's effect in the forced swim test.

CONCLUSIONS

Clobenpropit's antidepressant effects and the enhanced synaptic plasticity require hippocampal H1R and H2R activation, suggesting that clobenpropit acts through disinhibition of histamine release. Clobenpropit reverses memory deficits and increases hippocampal GluN2A expression without modifying anxiety-related phenotypes or EPSCs in CA1 pyramidal neurons.

From neural to genetic substrates of panic disorder: Insights from human and mouse studies

Fear is an ancestral emotion, an intrinsic defensive response present in every organism. Although fear is an evolutionarily advantageous emotion, under certain pathologies such as panic disorder it might become exaggerated and non-adaptive. Clinical and preclinical work pinpoints that changes in cognitive processes, such as perception and interpretation of environmental stimuli that rely on brain regions responsible for high-level function, are essential for the development of fear-related disorders. This review focuses on the involvement of cognitive function to fear circuitry disorders. Moreover, we address how animal models are contributing to understand the involvement of human candidate genes to pathological fear and helping achieve progress in this field. Multidisciplinary approaches that integrate human genetic findings with state of the art genetic mouse models will allow to elucidate the mechanisms underlying pathology and to develop new strategies for therapeutic targeting.

The role of metabotropic glutamate receptor 5 in the pathogenesis of mood disorders and addiction: combining preclinical evidence with human Positron Emission Tomography (PET) studies

In the present review, we deliver an overview of the involvement of metabotropic glutamate receptor 5 (mGluR5) activity and density in pathological anxiety, mood disorders and addiction. Specifically, we will describe mGluR5 studies in humans that employed Positron Emission Tomography (PET) and combined the findings with preclinical animal research. This combined view of different methodological approaches—from basic neurobiological approaches to human studies—might give a more comprehensive and clinically relevant view of mGluR5 function in mental health than the view on preclinical data alone. We will also review the current research data on mGluR5 along the Research Domain Criteria (RDoC). Firstly, we found evidence of abnormal glutamate activity related to the positive and negative valence systems, which would suggest that antagonistic mGluR5 intervention has prominent anti-addictive, anti-depressive and anxiolytic effects. Secondly, there is evidence that mGluR5 plays an important role in systems for social functioning and the response to social stress. Finally, mGluR5's important role in sleep homeostasis suggests that this glutamate receptor may play an important role in RDoC's arousal and modulatory systems domain. Glutamate was previously mostly investigated in non-human studies, however initial human clinical PET research now also supports the hypothesis that, by mediating brain excitability, neuroplasticity and social cognition, abnormal metabotropic glutamate activity might predispose individuals to a broad range of psychiatric problems.

Prefrontal GABA and glutathione imbalance in posttraumatic stress disorder: preliminary findings

Although posttraumatic stress disorder (PTSD) is associated with a variety of structural and functional brain changes, the molecular pathophysiological mechanisms underlying these macroscopic alterations are unknown. Recent studies support the existence of an altered excitation-inhibition balance in PTSD. Further, there is preliminary evidence from blood-sample studies suggesting heightened oxidative stress in PTSD, potentially leading to neural damage through excessive brain levels of free radicals. In this study we investigated PTSD (n=12) and non-PTSD participants (n=17) using single-voxel proton magnetic resonance spectroscopy (MRS) in dorsolateral prefrontal cortex (DLPFC) and anterior cingulate cortex (ACC). We found significantly higher levels of γ-amino butyric acid (GABA) (a primary inhibitory neurotransmitter) and glutathione (a marker for neuronal oxidative stress) in PTSD participants. Atypically high prefrontal inhibition as well as oxidative stress may be involved in the pathogenesis of PTSD.

Metabotropic glutamate receptor 5 binding in patients with obsessive-compulsive disorder

Obsessive-compulsive disorder (OCD) is a disabling, mostly chronic, psychiatric condition with significant social and economic impairments and is a major public health issue. However, numerous patients are resistant to currently available pharmacological and psychological interventions. Given that recent animal studies and magnetic resonance spectroscopy research points to glutamate dysfunction in OCD, we investigated the metabotropic glutamate receptor 5 (mGluR5) in patients with OCD and healthy controls. We determined mGluR5 distribution volume ratio (DVR) in the brain of ten patients with OCD and ten healthy controls by using [11C]ABP688 positron-emission tomography. As a clinical measure of OCD severity, the Yale-Brown Obsessive Compulsive Scale (Y-BOCS) was employed. We found no significant global difference in mGluR5 DVR between patients with OCD and healthy controls. We did, however, observe significant positive correlations between the Y-BOCS obsession sub-score and mGluR5 DVR in the cortico-striatal-thalamo-cortical brain circuit, including regions of the amygdala, anterior cingulate cortex, and medial orbitofrontal cortex (Spearman's ρ's⩾ = 0.68, p < 0.05). These results suggest that obsessions in particular might have an underlying glutamatergic pathology related to mGluR5. The research indicates that the development of metabotropic glutamate agents would be useful as a new treatment for OCD.

New insights on the antidepressant discontinuation syndrome

OBJECTIVE

Antidepressants are at best 50–55% effective. Non-compliance and the antidepressant discontinuation syndrome (ADS) are causally related yet poorly appreciated. While ADS is associated with most antidepressants, agomelatine seems to be devoid of such risk. We review the neurobiology and clinical consequences of antidepressant non-compliance and the ADS. Agomelatine is presented as a counterpoint to learn more on how ADS risk is determined by pharmacokinetics and pharmacology.

DESIGN

The relevant literature is reviewed through a MEDLINE search via PubMed, focusing on agomelatine and clinical and preclinical research on ADS.

RESULTS

Altered serotonergic dysfunction appears central to ADS so that how an antidepressant targets serotonin will determine its relative risk for inducing ADS and thereby affect later treatment outcome. Low ADS risk with agomelatine versus other antidepressants can be ascribed to its unique pharmacokinetic characteristics as well as its distinctive actions on serotonin, including melatonergic, monoaminergic and glutamatergic-nitrergic systems.

CONCLUSIONS

This review raises awareness of the long-term negative aspects of non-compliance and inappropriate antidepressant discontinuation, and suggests possible approaches to “design-out” a risk for ADS. It reveals intuitive and rational ideas for antidepressant drug design, and provides new thoughts on antidepressant pharmacology, ADS risk and how these affect long-term outcome.

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

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

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

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

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

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

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

Tests of unconditioned anxiety - pitfalls and disappointments

The plus-maze, the light-dark box and the open-field are the main current tests of unconditioned anxiety for mice and rats. Despite their disappointing achievements, they remain as popular as ever and seem to play an important role in an ever-growing demand for behavioral phenotyping and drug screening. Numerous reviews have repeatedly reported their lack of consistency and reliability but they failed to address the core question of whether these tests do provide unequivocal measures of fear-induced anxiety, that these measurements are not confused with measures of fear-induced avoidance or natural preference responses - i.e. discriminant validity. In the present report, I examined numerous issues that undermine the validity of the current tests, and I highlighted various flaws in the aspects of these tests and the methodologies pursued. This report concludes that the evidence in support of the validity of the plus-maze, the light/dark box and the open-field as anxiety tests is poor and methodologically questionable.

Blockade of the metabotropic glutamate (mGluR2) modulates arousal through vigilance states transitions: evidence from sleep-wake EEG in rodents

Accumulating data continue to support the therapeutic potential of glutamate metabotropic (mGluR2) receptors for treatment of psychiatric disorders such as depression, anxiety and schizophrenia. Glutamate neurotransmission is an integral component of sleep-wake mechanisms, which have translational relevance to assess on-target activity of drugs. Here, we investigated the influence of mGluR2 inactivation upon sleep-wake electroencephalogram (EEG) in rodents. Rats were administered with vehicle, the specific mGluR2 antagonist LY341495 (2.5, 5, 10mg/kg) or negative allosteric modulator (NAM) Ro-4491533 (2.5, 10 and 40 mg/kg) at lights onset. mGluR2 (-/-) mice were used to confirm the selectivity of functional response. Both LY341495 and Ro-4491533 induced an immediate and endured desynchronized cortical activity during 3-6h associated with enhanced theta and gamma oscillations and depressed slow oscillations during sleep. The arousal-promoting effect is consistent with the marked lengthening of sleep onset latency, an increased number of state transitions from light sleep to waking and the gradual increase in homeostatic compensatory sleep. The arousal response to mGluR2 blockade was not accompanied by sharp rebound hypersomnolence as found with the classical psycho-stimulant amphetamine. mGluR2 (-/-) mice and their WT littermates exhibited similar sleep-wake phenotype, while Ro-4491533 (40 mg/kg) enhanced waking associated with increased locomotor activity and body temperature in WT but not in mGluR2 (-/-) mice, which confirm the role of mGluR2 inactivation in the arousal response. Our results lend support for a role of mGluR2 blockade in promoting cortical arousal associated with theta/gamma oscillations as well as high thresholds transitions from sleep to waking.

Efficacy of lovastatin on learning and memory deficits caused by chronic intermittent hypoxia-hypercapnia: through regulation of NR2B-containing NMDA receptor-ERK pathway

BACKGROUND

Chronic intermittent hypoxia-hypercapnia (CIHH) exposure leads to learnning and memory deficits in rats. Overactivation of N-methyl-D-aspartate receptors(NMDARs) can lead to the death of neurons through a process termed excitotoxicity, which is involved in CIHH-induced cognitive deficits. Excessively activated NR2B (GluN2B)-containing NMDARs was reported as the main cause of excitotoxicity. The ERK1/2 (extracellular signal-regulated kinase 1/2) signaling cascade acts as a key component in NMDARs-dependent neuronal plasticity and survival. Ca2+/calmodulin-dependent protein kinase II (CaMKII), synapse-associated protein 102 (SAP102) and Ras GTPase-activating protein (SynGAP) have been shown to be involved in the regulation of NMDAR-ERK signalling cascade. Recent studies revealed statins (the HMG-CoA reductase inhibitor) have effect on the expression of NMDARs. The present study intends to explore the potential effect of lovastatin on CIHH-induced cognitive deficits and the NR2B-ERK signaling pathway.

METHODS AND FINDINGS

Eighty male Sprague Dawley rats were randomly divided into five groups. Except for those in the control group, the rats were exposed to chronic intermittent hypoxia-hypercapnia (CIHH) (9 ∼ 11%O2, 5.5 ∼ 6.5%CO2) for 4 weeks. After lovastatin administration, the rats performed better in the Morris water maze test. Electron microscopy showed alleviated hippocampal neuronal synaptic damage. Further observation suggested that either lovastatin or ifenprodil (a selective NR2B antagonist) administration similarly downregulated NR2B subunit expression leading to a suppression of CaMKII/SAP102/SynGAP signaling cascade, which in turn enhanced the phosphorylation of ERK1/2. The phosphorylated ERK1/2 induced signaling cascade involving cAMP-response element-binding protein (CREB) phosphorylation and brain-derived neurotrophic factor (BDNF) activation, which is responsible for neuroprotection.

CONCLUSIONS

These findings suggest that the ameliorative cognitive deficits caused by lovastatin are due to the downregulation of excessive NR2B expression accompanied by increased expression of ERK signaling cascade. The effect of NR2B in upregulating pERK1/2 maybe due, at least in part, to inactivation of CaMKII/SAP102/SynGAP signaling cascade.