Cognitive impairment in major depression and the mGlu2 receptor as a therapeutic target

Cognitive impairment and therapeutic response in resistant depression

OBJECTIVES

Therapeutic response in depression is a major challenge since more than one third of patients are not in remission after two attempts of antidepressant treatment and will present a treatment-resistant depression. In order to better adapt therapeutic strategies for treatment-resistant patients, predictive indicators and markers of therapeutic response still need to be identified. In parallel, patients with depression exhibit disturbances in cognitive functioning. This study aims to describe and compare cognitive performances collected at inclusion of patients presenting treatment-resistant depression who will be responders at 6 months to those of non-responders, and to evaluate the predictive value of cognitive indicators on clinical therapeutic response at 6 months after a therapeutic modification.

METHODS

Observational study. Patients were evaluated at the clinical (HDRS and BDI-II) and cognitive levels using standardized tools assessing memory, executive functions, attention, and social cognition, prior to a change in antidepressant treatment. Six months after inclusion, they were reassessed and classified into two groups based on the presence or absence of therapeutic response, defined by a 50% improvement on HDRS and BDI-II. The cognitive scores collected at inclusion were then compared. Additionally, univariate logistic regression models were used.

RESULTS

Thirty patients were included in this study. Only 13 could be evaluated at 6 months. Among these patients, four had responded to the new treatment while nine were non-responders. Both groups of patients presented deviant cognitive performances compared to norms on tests evaluating executive functions and attention. Statistical analyses did not reveal any difference between the cognitive performances of responders and non-responders at 6 months. Regression analyses showed no association between cognitive scores and therapeutic response at 6 months.

CONCLUSION

Executive functioning plays a significant role in treatment-resistant depression. In order to improve the understanding and identification of subtypes of depression, cognitive indicators should be systematically integrated into future research.

The effects and possible mechanisms of whole-body vibration on cognitive function: A narrative review

Whole-body vibration (WBV) is a physical stimulation method that transmits mechanical oscillations to the entire body through a vibration platform or device. Biokinetic and epidemiologic studies have shown that prolonged exposure to high-intensity WBV increases health risks, primarily to the lumbar spine and the nervous system connected to it. There is currently insufficient evidence to demonstrate a quantitative relationship between vibration exposure and risk of health effects. The positive effects of WBV on increasing muscle strength and improving balance and flexibility are well known, but its effects on cognitive function are more complex, with mixed findings, largely related to vibration conditions, including frequency, amplitude, and duration. Studies have shown that short-term low-frequency WBV may have a positive impact on cognitive function, demonstrates potential rehabilitation benefits in enhancing learning and memory, possibly by promoting neuromuscular coordination and enhancing neural plasticity. However, long term exposure to vibration may lead to chronic stress in nerve tissue, affecting nerve conduction efficiency and potentially interfering with neuroprotective mechanisms, thereby having a negative impact on cognitive ability, even causes symptoms such as cognitive decline, mental fatigue, decreased attention, and drowsiness. This literature review aimed to explore the effects of WBV on cognitive function and further to analyze the possible mechanisms. Based on the analysis of literatures, we came to the conclusion that the impact of WBV on cognitive function depends mainly on the frequency and duration of vibration, short-term low-frequency WBV may have a positive impact on cognitive function, while long term exposure to WBV may lead to cognitive decline, and the mechanisms may be involved in neuroinflammation, oxidative stress, synaptic plasticity, and neurotransmitter changes. This review may provide some theoretical foundations and guidance for the prevention and treatment of WBV induced cognitive impairment.

Major Depression in Comorbidity with Substance use Disorders: Patients' Features and Clinical-Neurobiological Rationale of Antidepressant Treatments

The frequent co-occurrence of major depressive disorder (MDD) and substance use disorders (SUDs) entails significant clinical challenges. Compared to patients with MDD alone, patients with MDD and SUD often show increased anhedonia, emotional blunting, and impaired cognitive function. These symptoms lead to an inability to control cravings, more substance use, increased relapse rates, and poor adherence to the treatment. This fosters a detrimental cycle leading to more severe depressive symptoms, functional impairment, and chronicity, culminating in heightened morbidity, mortality, and healthcare resource utilization. Data on antidepressant treatment of MDD-SUD patients are inconclusive and often conflicting because of a number of confounding factors in clinical trials or difficulty in dissecting the specific contributions of pharmacological versus psychological interventions in real-world studies. The patient's unique clinical features and specific SUD and MDD subtypes must be considered when choosing treatments. Ideally, drug treatment for MDD-SUD should act on both conditions and address core symptoms such as anhedonia, craving, and cognitive dysfunction while ensuring minimal emotional blunting, absence of drug interactions, and no addictive potential. This approach aims to address unmet needs and optimize the outcomes in a clinical population often underrepresented in treatment paradigms.

Discovery of 6,7-Dihydropyrazolo[1,5-a]pyrazin-4(5H)-one Derivatives as mGluR2 Negative Allosteric Modulators with In Vivo Activity in a Rodent's Model of Cognition

Allosteric modulators of the metabotropic group II receptors, mGluR2 and mGluR3, have been widely explored due to their ability to modulate cognitive and neurological functions in mood disorders, although none have been approved yet. In our search for new and selective mGluR2 negative allosteric modulators (NAMs), series of 6,7-dihydropyrazolo[1,5-a]pyrazin-4(5H)-one derivatives were identified from our published series of 1,3,5-trisubstituted pyrazoles. SAR evolution of the initial hit resulted in 100-fold improvement in the mGluR2 NAM potency and subsequent selection of compound 11 based on its overall profile, including selectivity and ADMET properties. Further pharmacokinetic-pharmacodynamic (PK-PD) relationship built showed that compound 11 occupied the mGluR2 receptor in a dose-dependent manner. Additionally, the compound revealed in vivo activity in V-maze as a model of cognition from a dose of 0.32 mg/kg. Compound 11 was selected to be evaluated further.

Adaptive Changes in Group 2 Metabotropic Glutamate Receptors Underlie the Deficit in Recognition Memory Induced by Methamphetamine in Mice

Cognitive dysfunction is associated with methamphetamine use disorder (MUD). Here, we used genetic and pharmacological approaches to examine the involvement of either Group 2 metabotropic glutamate (mGlu2) or mGlu3 receptors in memory deficit induced by methamphetamine in mice. Methamphetamine treatment (1 mg/kg, i.p., once a day for 5 d followed by 7 d of withdrawal) caused an impaired performance in the novel object recognition test in wild-type mice, but not in mGlu2-/- or mGlu3-/- mice. Memory deficit in wild-type mice challenged with methamphetamine was corrected by systemic treatment with selectively negative allosteric modulators of mGlu2 or mGlu3 receptors (compounds VU6001966 and VU0650786, respectively). Methamphetamine treatment in wild-type mice caused large increases in levels of mGlu2/3 receptors, the Type 3 activator of G-protein signaling (AGS3), Rab3A, and the vesicular glutamate transporter, vGlut1, in the prefrontal cortex (PFC). Methamphetamine did not alter mGlu2/3-mediated inhibition of cAMP formation but abolished the ability of postsynaptic mGlu3 receptors to boost mGlu5 receptor-mediated inositol phospholipid hydrolysis in PFC slices. Remarkably, activation of presynaptic mGlu2/3 receptors did not inhibit but rather amplified depolarization-induced [3H]-D-aspartate release in synaptosomes prepared from the PFC of methamphetamine-treated mice. These findings demonstrate that exposure to methamphetamine causes changes in the expression and function of mGlu2 and mGlu3 receptors, which might alter excitatory synaptic transmission in the PFC and raise the attractive possibility that selective inhibitors of mGlu2 or mGlu3 receptors (or both) may be used to improve cognitive dysfunction in individuals affected by MUD.

Advancements in research on the effects of panax notoginseng saponin constituents in ameliorating learning and memory disorders

Learning and memory disorder is a cluster of symptoms caused by neuronal aging and other diseases of the central nervous system (CNS). Panax notoginseng saponins (PNS) are a series of saponins derived from the natural active ingredients of traditional Chinese medicine (TCM) that have neuroprotective effects on the central nervous system. In this paper, we review the ameliorative effects and mechanisms of Panax notoginseng saponin-like components on learning and memory disorders to provide valuable references and insights for the development of new drugs for the treatment of learning and memory disorders. Our summary results suggest that Panax ginseng saponins have significant effects on improving learning and memory disorders, and these effects and potential mechanisms are mediated by their anti-inflammatory, anti-apoptotic, antioxidant, β-amyloid lowering, mitochondrial homeostasis in vivo, neuronal structure and function improving, neurogenesis promoting, neurotransmitter release regulating, and probiotic homeostasis in vivo activities. These findings suggest the potential of Panax notoginseng saponin-like constituents as drug candidates for improving learning and memory disorders.

The impact of depressive symptoms on cognitive impairments in chronic ketamine users

BACKGROUND

Chronic ketamine use has been associated with cognitive impairments, while depressive symptoms are commonly observed in individuals using ketamine. However, the influence of depressive symptoms on cognitive impairments in chronic ketamine users remains unclear. This study aimed to examine the impact of depressive symptoms on cognitive function in this population.

METHODS

A cross-sectional study was conducted with a sample of chronic ketamine users. Participants underwent comprehensive cognitive assessments, including measures of attention, executive function, working memory, verbal and visual memory. Depressive symptoms were assessed using Beck Depression Inventory (BDI) scores. Multivariate analyses were utilized to compare the cognitive performance of individuals who use ketamine, both with and without depressive symptoms, as well as a control group, while controlling for relevant covariates.

RESULTS

The results revealed a significant negative impact of depressive symptoms on cognitive impairments, particularly in the domains of memory and executive function, among chronic ketamine users. The analysis of partial correlations revealed that among individuals who use ketamine and have depressive symptoms, those with higher levels of depressive symptoms demonstrated poorer cognitive performance compared to individuals with lower levels of depressive symptoms, controlling for potential confounding factors.

CONCLUSIONS

The findings suggest that depressive symptoms contribute to cognitive impairments, specifically in memory and executive function, in chronic ketamine users. Therefore, it is crucial to evaluate depressive symptoms when considering cognitive enhancement treatment for this population.

mGlu2/3 receptor antagonists for depression: overview of underlying mechanisms and clinical development

Triggered by the ground-breaking finding that ketamine exerts robust and rapid-acting antidepressant effects in patients with treatment-resistant depression, glutamatergic systems have attracted attention as targets for the development of novel antidepressants. Among glutamatergic systems, group II metabotropic glutamate (mGlu) receptors, consisting of mGlu2 and mGlu3 receptors, are of interest because of their modulatory roles in glutamatergic transmission. Accumulating evidence has indicated that mGlu2/3 receptor antagonists have antidepressant-like effects in rodent models that mirror those of ketamine and that mGlu2/3 receptor antagonists also share underlying mechanisms with ketamine that are responsible for these antidepressant-like actions. Importantly, contrary to their antidepressant-like profile, preclinical studies have revealed that mGlu2/3 receptor antagonists are devoid of ketamine-like adverse effects, such as psychotomimetic-like behavior, abuse potential and neurotoxicity. Despite some discouraging results for an mGlu2/3 receptor antagonist decoglurant (classified as a negative allosteric modulator [NAM]) in patients with major depressive disorder, clinical trials of two mGlu2/3 receptor antagonists, a phase 2 trial of TS-161 (an orthosteric antagonist) and a phase 1 trial of DSP-3456 (a NAM), are presently on-going. mGlu2/3 receptors still hold promise for the development of safer and more efficacious antidepressants.

An integrated multi-level analysis reveals learning-memory deficits and synaptic dysfunction in the rat model exposure to austere environment

Austere environment existing in tank, submarine and vessel has many risk factors including high temperature and humidity, confinement, noise, hypoxia, and high level of carbon dioxide, which may cause depression and cognitive impairment. However, the underlying mechanism is not fully understood yet. We attempt to investigate the effects of austere environment (AE) on emotion and cognitive function in a rodent model. After 21 days of AE stress, the rats exhibit depressive-like behavior and cognitive impairment. Compared with control group, the glucose metabolic level of the hippocampus is significantly decreased using whole-brain positron emission tomography (PET) imaging, and the density of dendritic spines of the hippocampus is remarkably reduced in AE group. Then, we employ a label-free quantitative proteomics strategy to investigate the differentially abundant proteins in rats' hippocampus. It is striking that the differentially abundant proteins annotated by KEGG enrich in oxidative phosphorylation pathway, synaptic vesicle cycle pathway and glutamatergic synapses pathway. The synaptic vesicle transport related proteins (Syntaxin-1A, Synaptogyrin-1 and SV-2) are down-regulated, resulting in the accumulation of intracellular glutamate. Furthermore, the concentration of hydrogen peroxide and malondialdehyde is increased while the activity of superoxide dismutase and complex I and IV of mitochondria is decreased, indicating that oxidative damage to hippocampal synapses is associated with the cognitive decline. The results of this study offer direct evidence, for the first time, that austere environment can substantially cause learning and memory deficits and synaptic dysfunction in a rodent model via behavioral assessments, PET imaging, label-free proteomics, and oxidative stress tests. SIGNIFICANCE: The incidence of depression and cognitive decline in military occupations (for example, tanker and submariner) is significantly higher than that of global population. In the present study, we first established novel model to simulate the coexisting risk factors in the austere environment. The results of this study offer the direct evidences, for the first time, that the austere environment can substantially cause learning and memory deficits by altering plasticity of the synaptic transmission in a rodent model via proteomic strategy, PET imaging, oxidative stress and behavioral assessments. These findings provide valuable information to better understand the mechanisms of cognitive impairment.

Rapid-acting antidepressants targeting modulation of the glutamatergic system: clinical and preclinical evidence and mechanisms

Major depressive disorder (MDD) is a devastating mental illness that affects approximately 20% of the world's population. It is a major disease that leads to disability and suicide, causing a severe burden among communities. Currently available medications for treating MDD target the monoaminergic systems. The most prescribed medications include selective serotonin reuptake inhibitors and selective norepinephrine reuptake inhibitors. However, these medications have serious drawbacks, such as a delayed onset requiring weeks or months to reach efficacy and drug resistance, as one-third of patients are unresponsive to the medications. Therefore, it is imperative to develop novel therapies with rapid action, high efficacy and few adverse effects. The discovery of the rapid antidepressant effect of ketamine has triggered tremendous enthusiasm for studying new antidepressants that target the glutamatergic system in the central nervous system. Many agents that directly or indirectly modulate the glutamatergic system have been shown to provide rapid and lasting antidepressant action. Among these agents, ketamine, an antagonist of metabotropic glutamate 2/3 receptors, and scopolamine, an unspecific muscarinic acetylcholine receptor antagonist, have been extensively studied. In this review, we discuss the clinical and preclinical evidence supporting the antidepressant efficacy of these agents and the current understanding of the underlying mechanisms.

Role of vortioxetine in the treatment of neuropathic pain

Neuropathic pain is an important and disabling clinical problem, its management constitutes a challenge for healthcare professionals. Vortioxetine is a new antidepressant drug with multimodal action, which gives it a unique profile. Tricyclic antidepressants, in particular amitriptyline, and serotonin and norepinephrine reuptake inhibitors venlafaxine and duloxetine are first-line drugs in the treatment of neuropathic pain. The interaction between the pain and depression binomial is very frequent, being the most frequent psychological complication in patients with chronic pain. This comprehensive and descriptive review summarizes the most relevant pharmacological data on vortioxetine, as well as the specific literature on vortioxetine in neuropathic pain and chronic pain.

Are mGluR2/3 Inhibitors Potential Compounds for Novel Antidepressants?

Depression is the most common mental illness characterized by anhedonia, avolition and loss of appetite and motivation. The majority of conventional antidepressants are monoaminergic system selective inhibitors, yet the efficacies are not sufficient. Up to 30% of depressed patients are resistant to treatment with available antidepressants, underscoring the urgent need for development of novel therapeutics to meet clinical needs. Recent years, compounds acting on the glutamate system have attracted wide attention because of their strong, rapid and sustained antidepressant effects. Among them, selective inhibitors of metabotropic glutamate receptors 2 and 3 (mGluR2/3) have shown robust antidepressant benefits with fewer side-effects in both preclinical and clinical studies. Thus, we here attempt to summarize the antidepressant effects and underlying mechanisms of these inhibitors revealed in recent years as well as analyze the potential value of mGluR2/3 selective inhibitors in the treatment of depression.

Marine Natural Products from the Russian Pacific as Sources of Drugs for Neurodegenerative Diseases

Neurodegenerative diseases are growing to become one of humanity's biggest health problems, given the number of individuals affected by them. They cause enough mortalities and severe economic impact to rival cancers and infections. With the current diversity of pathophysiological mechanisms involved in neurodegenerative diseases, on the one hand, and scarcity of efficient prevention and treatment strategies, on the other, all possible sources for novel drug discovery must be employed. Marine pharmacology represents a relatively uncharted territory to seek promising compounds, despite the enormous chemodiversity it offers. The current work discusses one vast marine region-the Northwestern or Russian Pacific-as the treasure chest for marine-based drug discovery targeting neurodegenerative diseases. We overview the natural products of neurological properties already discovered from its waters and survey the existing molecular and cellular targets for pharmacological modulation of the disease. We further provide a general assessment of the drug discovery potential of the Russian Pacific in case of its systematic development to tackle neurodegenerative diseases.

A New Look on Long-COVID Effects: The Functional Brain Fog Syndrome

Epidemiological data and etiopathogenesis of brain fog are very heterogeneous in the literature, preventing adequate diagnosis and treatment. Our study aimed to explore the relationship between brain fog, neuropsychiatric and cognitive symptoms in the general population. A sample of 441 subjects underwent a web-based survey, including the PANAS, the DASS-21, the IES-R, the Beck Cognitive Insight Scale, and a questionnaire investigating demographic information, brain fog, subjective cognitive impairments (Scc) and sleep disorders. ANOVA, ANCOVA, correlation and multiple stepwise regression analyses were performed. In our sample, 33% of participants were defined as Healthy Subjects (HS; no brain fog, no Scc), 27% as Probable Brain Fog (PBF; brain fog or Scc), and 40% as Functional Brain Fog (FBF; brain fog plus Scc). PBF and FBF showed higher levels of neuropsychiatric symptoms than HS, and FBF showed the worst psychological outcome. Moreover, worse cognitive symptoms were related to the female gender, greater neuropsychiatric symptoms, sleep disorders, and rumination/indecision. Being a woman and more severe neuropsychiatric symptoms were predictors of FBF severity. Our data pointed out a high prevalence and various levels of severity and impairments of brain fog, suggesting a classificatory proposal and a multifaceted etiopathogenic model, thus facilitating adequate diagnostic and therapeutic approaches.

Clinical investigations of compounds targeting metabotropic glutamate receptors

Pharmacological modulation of glutamate has long been considered to be of immense therapeutic utility. The metabotropic glutamate receptors (mGluRs) are potential targets for safely altering glutamate-driven excitation. Data support the potential therapeutic use of mGluR modulators in the treatment of anxiety, depression, schizophrenia, and other psychiatric disorders, pain, epilepsy, as well as neurodegenerative and neurodevelopmental disorders. For each of the three mGluR groups, compounds have been constructed that produce either potentiation or functional blockade. PET ligands for mGlu5Rs have been studied in a range of patient populations and several mGlu5R antagonists have been tested for potential efficacy in patients including mavoglurant, diploglurant, basimglurant, GET 73, and ADX10059. Efficacy with mGlu5R antagonists has been reported in trials with patients with gastroesophageal reflux disease; data from patients with Parkinson's disease or Fragile X syndrome have not been as robust as hoped. Fenobam was approved for use as an anxiolytic prior to its recognition as an mGlu5R antagonist. mGlu2/3R agonists (pomaglumated methionil) and mGlu2R agonists (JNJ-40411813, AZD 8529, and LY2979165) have been studied in patients with schizophrenia with promising but mixed results. Antagonists of mGlu2/3Rs (decoglurant and TS-161) have been studied in depression where TS-161 has advanced into a planned Phase 2 study in treatment-resistant depression. The Group III mGluRs are the least developed of the mGluR receptor targets. The mGlu4R potentiator, foliglurax, did not meet its primary endpoint in patients with Parkinson's disease. Ongoing efforts to develop mGluR-targeted compounds continue to promise these glutamate modulators as medicines for psychiatric and neurological disorders.

S100A10 and its binding partners in depression and antidepressant actions

S100A10 (p11) is an emerging player in the neurobiology of depression and antidepressant actions. p11 was initially thought to be a modulator of serotonin receptor (5-HTR) trafficking and serotonergic transmission, though newly identified binding partners of p11 and neurobiological studies of these proteins have shed light on multifunctional roles for p11 in the regulation of glutamatergic transmission, calcium signaling and nuclear events related to chromatin remodeling, histone modification, and gene transcription. This review article focuses on direct binding partners of p11 in the brain including 5-HTRs, mGluR5, annexin A2, Ahnak, Smarca3, and Supt6h, as well as their roles in neuronal function, particularly in the context of depressive-like behavior as well as behavioral effects of antidepressant drug treatments in mice. In addition, we discuss neurobiological insights from recently uncovered p11 pathways in multiple types of neurons and non-neuronal cells and cast major remaining questions for future studies.

Role of vortioxetine in the treatment of neuropathic pain

Neuropathic pain is an important and disabling clinical problem, its management constitutes a challenge for healthcare professionals. Vortioxetine is a new antidepressant drug with multimodal action, which gives it a unique profile. Tricyclic antidepressants, in particular amitriptyline, and serotonin and norepinephrine reuptake inhibitors venlafaxine and duloxetine are first-line drugs in the treatment of neuropathic pain. The interaction between the pain and depression binomial is very frequent, being the most frequent psychological complication in patients with chronic pain. This comprehensive and descriptive review summarizes the most relevant pharmacological data on vortioxetine, as well as the specific literature on vortioxetine in neuropathic pain and chronic pain.

Negative allosteric modulators of group II metabotropic glutamate receptors: A patent review (2015 - present)

INTRODUCTION

Group II metabotropic glutamate (mGlu) receptors have emerged as an attractive potential target for the development of novel CNS therapeutics in areas such as Alzheimer's disease (AD), anxiety, cognitive disorders, depression, and others. Several small molecules that act as negative allosteric modulators (NAMs) on these receptors have demonstrated efficacy and/or target engagement in animal models, and one molecule (decoglurant) has been advanced into clinical trials.

AREAS COVERED

This review summarizes patent applications published between January 2015 and November 2020. It is divided into three sections: (1) small molecule nonselective mGlu_2/3 NAMs, (2) small molecule selective mGlu₂ NAMs, and (3) small molecule selective mGlu₃ NAMs.

EXPERT OPINION

Much progress has been made in the discovery of novel small molecule mGlu₂ NAMs. Still, chemical diversity remains somewhat limited and room for expansion remains. Progress with mGlu₃ NAMs has been more limited; however, some promising molecules have been disclosed. The process of elucidating the precise role of each receptor in the diseases associated with group II receptors has begun. Continued studies in animals with selective NAMs for both receptors will be critical in the coming years to inform researchers on the right compound profile and patient population for clinical development.

International Union of Basic and Clinical Pharmacology. CXI. Pharmacology, Signaling, and Physiology of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors respond to glutamate, the major excitatory neurotransmitter in the mammalian brain, mediating a modulatory role that is critical for higher-order brain functions such as learning and memory. Since the first mGlu receptor was cloned in 1992, eight subtypes have been identified along with many isoforms and splice variants. The mGlu receptors are transmembrane-spanning proteins belonging to the class C G protein-coupled receptor family and represent attractive targets for a multitude of central nervous system disorders. Concerted drug discovery efforts over the past three decades have yielded a wealth of pharmacological tools including subtype-selective agents that competitively block or mimic the actions of glutamate or act allosterically via distinct sites to enhance or inhibit receptor activity. Herein, we review the physiologic and pathophysiological roles for individual mGlu receptor subtypes including the pleiotropic nature of intracellular signal transduction arising from each. We provide a comprehensive analysis of the in vitro and in vivo pharmacological properties of prototypical and commercially available orthosteric agonists and antagonists as well as allosteric modulators, including ligands that have entered clinical trials. Finally, we highlight emerging areas of research that hold promise to facilitate rational design of highly selective mGlu receptor-targeting therapeutics in the future. SIGNIFICANCE STATEMENT: The metabotropic glutamate receptors are attractive therapeutic targets for a range of psychiatric and neurological disorders. Over the past three decades, intense discovery efforts have yielded diverse pharmacological tools acting either competitively or allosterically, which have enabled dissection of fundamental biological process modulated by metabotropic glutamate receptors and established proof of concept for many therapeutic indications. We review metabotropic glutamate receptor molecular pharmacology and highlight emerging areas that are offering new avenues to selectively modulate neurotransmission.

The Dual Role of Glutamatergic Neurotransmission in Alzheimer's Disease: From Pathophysiology to Pharmacotherapy

Alzheimer's disease (AD) is an age-related dementia and neurodegenerative disorder, characterized by Aβ and tau protein deposition impairing learning, memory and suppressing synaptic plasticity of neurons. Increasing evidence suggests that there is a link between the glucose and glutamate alterations with age that down-regulates glucose utilization reducing glutamate levels in AD patients. Deviations in brain energy metabolism reinforce the development of AD by hampering glutamate levels in the brain. Glutamate is a nonessential amino acid and the major excitatory neurotransmitter synthesized from glucose. Alterations in cerebral glucose and glutamate levels precede the deposition of Aβ plaques. In the brain, over 40% of neuronal synapses are glutamatergic and disturbances in glutamatergic function have been implicated in pathophysiology of AD. Nevertheless, targeting the glutamatergic system seems to be a promising strategy to develop novel, improved therapeutics for AD. Here, we review data supporting the involvement of the glutamatergic system in AD pathophysiology as well as the efficacy of glutamatergic agents in this neurodegenerative disorder. We also discuss exciting new prospects for the development of improved therapeutics for this devastating disorder.

mGlu2/3 receptor as a novel target for rapid acting antidepressants

Given that ketamine, a noncompetitive N-methyl-d-aspartate receptor antagonist that exerts rapid antidepressant effects in patients with treatment-resistant depression, also has undesirable adverse effects, agents that can be used as alternatives to ketamine have been actively pursued. Group II metabotropic glutamate (mGlu) receptors, consisting of mGlu2 and mGlu3 receptors, have emerged as one of the most promising targets in the development of ketamine-like antidepressants. Indeed, mGlu2/3 receptor antagonists have been demonstrated to exert rapid antidepressant effects in animal models and to be efficacious in animal models refractory to conventional antidepressants. Moreover, there are striking similarities between mGlu2/3 receptor antagonists and ketamine in terms of not only their antidepressant profiles, but also the underlying mechanisms of their antidepressant effects. Nonetheless, studies in rodents have shown that mGlu2/3 receptor antagonists do not cause ketamine-like adverse events, such as psychotomimetic-like behavior, abuse potential or neurotoxicity, supporting the usefulness of mGlu2/3 receptor antagonists as alternatives to ketamine. In this chapter, the past and recent research on the antidepressant effects of mGlu2/3 receptor antagonists will be reviewed. In particular, the potential of mGlu2/3 receptor antagonists as novel ketamine-like antidepressants will be emphasized.

Long-term functioning outcomes are predicted by cognitive symptoms in working patients with major depressive disorder treated with vortioxetine: results from the AtWoRC study

OBJECTIVE

AtWoRC (Assessment in Work productivity and the Relationship with Cognitive symptoms) was an interventional, open-label, Canadian study (NCT02332954) designed to assess the association between cognitive symptoms and workplace productivity in working patients with major depressive disorder (MDD) receiving vortioxetine.

METHODS

Eligible patients with MDD received vortioxetine (10-20 mg/day) and were assessed over 52 weeks at visits emulating a real-life setting (n = 199). Partial correlation between changes in patient-reported cognitive symptoms (20-item Perceived Deficits Questionnaire-Depression; PDQ-D-20) and workplace productivity (Work Limitations Questionnaire; WLQ) was assessed at 12 and 52 weeks. Additional assessments included depression severity, cognitive performance, and patient-reported functioning. Structural equations model (SEM) analyses assessed causal relationships between changes in measures of cognition and functioning over time, adjusted for improvements in depressive symptoms.

RESULTS

Statistically significant improvements in all outcomes from baseline to week 52 were seen in the overall population and both subgroups (first treatment and switch). Response and remission rates were 77% and 56%, respectively. Improvements in PDQ-D-20 and WLQ productivity loss scores at weeks 12 and 52 were significantly correlated. SEM analyses found patient-rated cognitive symptoms (PDQ-D-20) at weeks 12 and 26 were significantly predictive (p < 0.05) of patient-reported functioning (Sheehan Disability Scale) at the subsequent visit. Depression severity and objectively measured cognitive performance did not significantly predict functional outcomes at any timepoint.

CONCLUSION

These results demonstrate the long-term benefits of vortioxetine treatment in working patients with MDD and emphasize the strong association between cognitive symptoms and functioning in a real-world setting.

Further exploration of an N-aryl phenoxyethoxy pyridinone-based series of mGlu3 NAMs: Challenging SAR, enantiospecific activity and in vivo efficacy

This letter describes the further optimization of a series of mGlu₃ NAMs based on an N-aryl phenoxyethoxy pyridinone core. A multidimensional optimization campaign, with focused matrix libraries, quickly established challenging SAR, enantiospecific activity, differences in assay read-outs (Ca²⁺ flux via a promiscuous G protein (G_α15) versus native coupling to GIRK channels), identified both full and partial mGlu₃ NAMs and a new in vivo tool compound, VU6017587. This mGlu₃ NAM showed efficacy in tail suspension, elevated zero maze and marble burying, suggesting selective inhibition of mGlu₃ affords anxiolytic-like and antidepressant-like phenotypes in mice.

Assessment in Work Productivity and the Relationship with Cognitive Symptoms (AtWoRC): primary analysis from a Canadian open-label study of vortioxetine in patients with major depressive disorder (MDD)

OBJECTIVE

The Assessment in Work Productivity and the Relationship with Cognitive Symptoms (AtWoRC) study aimed to assess the association between cognitive symptoms and work productivity in gainfully employed patients receiving vortioxetine for a major depressive episode (MDE).

METHODS

Patients diagnosed with major depressive disorder (MDD) and treated with vortioxetine independently of study enrollment were assessed over 52 weeks at visits that emulated a real-life setting. Patients were classified as those receiving vortioxetine as the first treatment for their current MDE (first treatment) or having shown inadequate response to a previous antidepressant (switch). The primary endpoint was the correlation between changes in patient-reported cognitive symptoms (20-item Perceived Deficits Questionnaire [PDQ-D-20]) and changes in work productivity loss (Work Limitations Questionnaire [WLQ]) at week 12. Additional assessments included changes in symptom and disease severity, cognitive performance, functioning, work loss, and safety.

RESULTS

In the week 12 primary analysis, 196 eligible patients at 26 Canadian sites were enrolled, received at least one treatment dose, and attended at least one postbaseline study visit. This analysis demonstrated a significant, strong correlation between PDQ-D-20 and WLQ productivity loss scores (r=0.634; p<0.001), and this correlation was significant in both first treatment and switch patients (p<0.001). A weaker correlation between Digit Symbol Substitution Test and WLQ scores was found (r=-0.244; p=0.003).

CONCLUSION

At 12 weeks, improvements in cognitive dysfunction were significantly associated with improvements in workplace productivity in patients with MDD, suggesting a role for vortioxetine in functional recovery in MDD.

mGlu2/3 receptor antagonists

Abnormalities of glutamatergic transmission are implicated in neuropsychiatric disorders. Among the glutamate receptors, metabotropic (mGlu) 2/3 receptors have recently gained much attention as molecular targets for the treatment of several neuropsychiatric disorders including depression and anxiety. Both orthosteric and allosteric antagonists of mGlu2/3 receptors have been synthesized, and their therapeutic potential has been examined. These research activities have demonstrated the promise of mGlu2/3 receptor antagonists as potential treatment agents for the above-mentioned neuropsychiatric disorders. In particular, it has been considered that the antidepressant effects of mGlu2/3 receptor antagonists are worthy of pursuing, since the antidepressant profiles as well as synaptic/neural mechanisms involved in the actions of mGlu2/3 receptor antagonists are similar to those of ketamine, which has been demonstrated to show potent, rapid and sustained efficacy in patients with depression, even those resistant to the conventionally prescribed antidepressants. In this chapter, the general pharmacology of mGlu2/3 receptor antagonists and their therapeutic potential are reviewed. In particular, I focus on the usefulness of mGlu2/3 receptor antagonists as novel antidepressants, in comparison with ketamine.

Targeting of Metabotropic Glutamate Receptors for the Development of Novel Antidepressants

Since discovering that ketamine has robust antidepressant effects, the glutamatergic system has been proposed as an attractive target for the development of novel antidepressants. Among the glutamatergic system, metabotropic glutamate (mGlu) receptors are of interest because mGlu receptors play modulatory roles in glutamatergic transmission, consequently, agents acting on mGlu receptors might not exert the adverse effects associated with ketamine. mGlu receptors have eight subtypes that are classified into three groups, and the roles of each mGlu receptor subtype in depression are being investigated. To date, the potential use of mGlu5 receptor antagonists and mGlu2/3 receptor antagonists as antidepressants has been actively investigated, and the mechanisms underlying these antidepressant effects are being delineated. Although the outcomes of clinical trials using an mGlu5 receptor negative allosteric modulator and an mGlu2/3 receptor negative allosteric modulator have not been encouraging, these trials have been inconclusive, and additional trials using other compounds with more appropriate profiles are needed. In contrast, the roles of group III mGlu receptors have not yet been fully elucidated because of a lack of suitable pharmacological tools. Nonetheless, investigations of the use of mGlu4 and mGlu7 receptors as drug targets for the development of antidepressants have been ongoing, and some interesting evidence has been obtained.

A systematic review on neuropsychological function in bipolar disorders type I and II and subthreshold bipolar disorders-something to think about

Neuropsychological dysfunction is a well-established finding in individuals with bipolar disorder type I (BP-I), even during euthymic periods; however, it is less clear whether this also pertains to bipolar disorder type II (BP-II) or those with subthreshold states (SBP; subthreshold bipolar disorder), such as bipolar not otherwise specified (BP-NOS). Herein, we compare the literature regarding neuropsychological performance in BP-II vs BP-I to determine the extent of relative impairment, and we present and review all related studies on cognition in SBP. After systematically searching PubMed, Medline, PsycINFO, and The Cochrane Library, we found 17 papers that comprise all the published studies relevant for this review. The areas that are consistently found to be impaired in BP are executive function, verbal memory, visual spatial working memory, and attention. More studies than not show no significant difference between BP-I and BP-II, particularly in euthymic samples. Preliminary evidence suggests that patients experiencing major depressive episodes who also meet criteria for SBP show similar profiles to BP-II; however, these results pertain only to a depressed sample. SBP were found to perform significantly better than both MDD and healthy controls in a euthymic sample. A consensus on mood state, patient selection, and neuropsychological testing needs to be agreed on for future research. Furthermore, no studies have used the most recent DSM-5 criteria for SBP; future studies should address this. Finally, the underlying bases of cognitive dysfunction in these diagnostic groups need to be further investigated. We suggest recommendations on all of the above current research challenges.

GSK3β: a plausible mechanism of cognitive and hippocampal changes induced by erythropoietin treatment in mood disorders?

Mood disorders are associated with significant psychosocial and occupational disability. It is estimated that major depressive disorder (MDD) will become the second leading cause of disability worldwide by 2020. Existing pharmacological and psychological treatments are limited for targeting cognitive dysfunctions in mood disorders. However, growing evidence from human and animal studies has shown that treatment with erythropoietin (EPO) can improve cognitive function. A recent study involving EPO-treated patients with mood disorders showed that the neural basis for their cognitive improvements appeared to involve an increase in hippocampal volume. Molecular mechanisms underlying hippocampal changes have been proposed, including the activation of anti-apoptotic, antioxidant, pro-survival and anti-inflammatory signalling pathways. The aim of this review is to describe the potential importance of glycogen synthase kinase 3-beta (GSK3β) as a multi-potent molecular mechanism of EPO-induced hippocampal volume change in mood disorder patients. We first examine published associations between EPO administration, mood disorders, cognition and hippocampal volume. We then highlight evidence suggesting that GSK3β influences hippocampal volume in MDD patients, and how this could assist with targeting more precise treatments particularly for cognitive deficits in patients with mood disorders. We conclude by suggesting how this developing area of research can be further advanced, such as using pharmacogenetic studies of EPO treatment in patients with mood disorders.

Multimodal antidepressant vortioxetine causes analgesia in a mouse model of chronic neuropathic pain

Vortioxetine is a multimodal antidepressant that potently antagonizes 5-HT3 serotonin receptors, inhibits the high-affinity serotonin transporter, activates 5-HT1A and 5-HT1B receptors, and antagonizes 5-HT1D and 5-HT7 receptors. 5-HT3 receptors largely mediate the hyperalgesic activity of serotonin that occurs in response to nerve injury. Activation of 5-HT3 receptors contributes to explain why selective serotonin reuptake inhibitors, such as fluoxetine, are not indicated in the treatment of neuropathic pain. Here, we studied the analgesic action of vortioxetine in the chronic constriction injury model of neuropathic pain in mice. Vortioxetine was injected once a day for 27 days at doses (10 mg/kg, intraperitoneally) that determine >90% 5-HT3 receptor occupancy in the central nervous system. The action of vortioxetine was compared to the action of equal doses of the serotonin-noradrenaline reuptake inhibitor, venlafaxine (one of the gold standard drugs in the treatment of neuropathic pain), and fluoxetine. Vortioxetine caused a robust analgesia in chronic constriction injury mice, and its effect was identical to that produced by venlafaxine. In contrast, fluoxetine was inactive in chronic constriction injury mice. Vortioxetine enhanced mechanical pain thresholds in chronic constriction injury mice without changing motor activity, as assessed by the open-field and horizontal bar tests. None of the three antidepressants caused analgesia in the complete Freund’s adjuvant model of chronic inflammatory pain. These findings raise the attractive possibility that vortioxetine can be effective in the treatment of neuropathic pain, particularly in patients with comorbid depression and cognitive dysfunction.

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

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

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

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

Differential Pharmacology and Binding of mGlu2 Receptor Allosteric Modulators

Allosteric modulation of metabotropic glutamate receptor 2 (mGlu2) has demonstrated efficacy in preclinical rodent models of several brain disorders, leading to industry and academic drug discovery efforts. Although the pharmacology and binding sites of some mGlu2 allosteric modulators have been characterized previously, questions remain about the nature of the allosteric mechanism of cooperativity with glutamate and whether structurally diverse allosteric modulators bind in an identical manner to specific allosteric sites. To further investigate the in vitro pharmacology of mGlu2 allosteric modulators, we developed and characterized a novel mGlu2 positive allosteric modulator (PAM) radioligand in parallel with functional studies of a structurally diverse set of mGlu2 PAMs and negative allosteric modulators (NAMs). Using an operational model of allosterism to analyze the functional data, we found that PAMs affect both the affinity and efficacy of glutamate at mGlu2, whereas NAMs predominantly affect the efficacy of glutamate in our assay system. More importantly, we found that binding of a novel mGlu2 PAM radioligand was inhibited by multiple structurally diverse PAMs and NAMs, indicating that they may bind to the mGlu2 allosteric site labeled with the novel mGlu2 PAM radioligand; however, further studies suggested that these allosteric modulators do not all interact with the radioligand in an identical manner. Together, these findings provide new insights into the binding sites and modes of efficacy of different structurally and functionally distinct mGlu2 allosteric modulators and suggest that different ligands either interact with distinct sites or adapt different binding poses to shared allosteric site(s).

Effects of adolescent social stress and antidepressant treatment on cognitive inflexibility and Bdnf epigenetic modifications in the mPFC of adult mice

Adolescent social stress (ASS) can increase susceptibility to depression in adulthood. However, the underlying psychological and neural mechanisms remain unclear. Cortically mediated cognitive dysfunctions are increasingly recognized as an independent and important risk factor of depression. Using social defeat stress, a classical animal model of depression, our previous studies found that mice subjected to this form of stress during early adolescence displayed cognitive inflexibility (CI) in adulthood. This change was accompanied by a down-regulation of Bdnf gene expression in the medial prefrontal cortex (mPFC); this gene encodes a key molecule involved in depression and antidepressant action. In the present paper, we identified epigenetic modification of Bdnf as a possible mechanism underlying the behavioral and molecular changes. ASS induced a set of depressive phenotypes, including increased social avoidance and CI, as well as reduced levels of total Bdnf and isoform IV but not isoform I or VI transcripts in the mPFC. In parallel with changes in Bdnf gene expression, previously stressed adult mice showed increased levels of dimethylation of histone H3 at lysine K9 (H3K9me2) immediately downstream of the Bdnf IV promoter. On the other hand, no differences were found in trimethylation of histone H3 at lysine K4 (H3K4me3) or in acetylation of histone H3 at lysine K9 (H3K9ac) or at K4 (H3K4ac) in the Bdnf IV promoter. Likewise, no alterations were found in DNA methylation of the Bdnf IV promoter. Additionally, treatment with the chronic antidepressant tranylcypromine reversed Bdnf epigenetic changes and related gene transcription while also reversing CI, but not social avoidance, in previously stressed adult mice. These results suggest that epigenetic changes to the Bdnf gene in the mPFC after adolescent social adversity may be involved in the regulation of cognitive dysfunction in depression and antidepressant action in adulthood.

Fluoxetine, not donepezil, reverses anhedonia, cognitive dysfunctions and hippocampal proteome changes during repeated social defeat exposure

While anhedonia is considered a core symptom of major depressive disorder (MDD), less attention has been paid to cognitive dysfunctions. We evaluated the behavioural and molecular effects of a selective serotonin re-uptake inhibitor (SSRI, fluoxetine) and an acetylcholinesterase inhibitor (AChEI, donepezil) on emotional-cognitive endophenotypes of depression and the hippocampal proteome. A chronic social defeat (SD) procedure was followed up by "reminder" sessions of direct and indirect SD. Anhedonia-related behaviour was assessed longitudinally by intracranial self-stimulation (ICSS). Cognitive dysfunction was analysed by an object recognition test (ORT) and extinction of fear memory. Tandem mass spectrometry (MS^E) and protein-protein-interaction (PPI) network modelling were used to characterise the underlying biological processes of SD and SSRI/AChEI treatment. Independent selected reaction monitoring (SRM) was conducted for molecular validation. Repeated SD resulted in a stable increase of anhedonia-like behaviour as measured by ICSS. Fluoxetine treatment reversed this phenotype, whereas donepezil showed no effect. Fluoxetine improved recognition memory and inhibitory learning in a stressor-related context, whereas donepezil only improved fear extinction. MS^E and PPI network analysis highlighted functional SD stress-related hippocampal proteome changes including reduced glutamatergic neurotransmission and learning processes, which were reversed by fluoxetine, but not by donepezil. SRM validation of molecular key players involved in these pathways confirmed the hypothesis that fluoxetine acts via increased AMPA receptor signalling and Ca²⁺-mediated neuroplasticity in the amelioration of stress-impaired reward processing and memory consolidation. Our study highlights molecular mediators of SD stress reversed by SSRI treatment, identifying potential viable future targets to improve cognitive dysfunctions in MDD patients.

Design and Synthesis of N-Aryl Phenoxyethoxy Pyridinones as Highly Selective and CNS Penetrant mGlu3 NAMs

Herein, we detail the optimization of the mGlu₃ NAM, VU0650786, via a reductionist approach to afford a novel, simplified mGlu₃ NAM scaffold that engenders potent and selective mGlu₃ inhibition (mGlu₃ IC₅₀ = 245 nM, mGlu₂ IC₅₀ > 30 μM) with excellent central nervous system penetration (rat brain/plasma K_p = 1.2, K_p,uu = 0.40). Moreover, this new chemotype, exemplified by VU6010572, requires only four synthetic steps and displays improved physiochemical properties and in vivo efficacy in a mouse tail suspension test (MED = 3 mg/kg i.p.).

Genetic deletion of mGlu2 metabotropic glutamate receptors improves the short-term outcome of cerebral transient focal ischemia

We have recently shown that pharmacological blockade of mGlu2 metabotropic glutamate receptors protects vulnerable neurons in the 4-vessel occlusion model of transient global ischemia, whereas receptor activation amplifies neuronal death. This raised the possibility that endogenous activation of mGlu2 receptors contributes to the pathophysiology of ischemic neuronal damage. Here, we examined this possibility using two models of transient focal ischemia: (i) the monofilament model of middle cerebral artery occlusion (MCAO) in mice, and (ii) the model based on intracerebral infusion of endothelin-1 (Et-1) in rats. Following transient MCAO, mGlu2 receptor knockout mice showed a significant reduction in infarct volume and an improved short-term behavioural outcome, as assessed by a neurological disability scale and the “grip test”. Following Et-1 infusion, Grm2 gene mutated Hannover Wistar rats lacking mGlu2 receptors did not show changes in the overall infarct volume as compared to their wild-type counterparts, although they showed a reduced infarct area in the agranular insular cortex. Interestingly, however, mGlu2 receptor-deficient rats performed better than wild-type rats in the adhesive tape test, in which these rats did not show the laterality preference typically observed after focal ischemia. These findings support the hypothesis that activation of mGlu2 receptors is detrimental in the post-ischemic phase, and support the use of mGlu2 receptor antagonists in the experimental treatment of brain ischemia.

Altered learning, memory, and social behavior in type 1 taste receptor subunit 3 knock-out mice are associated with neuronal dysfunction

The type 1 taste receptor member 3 (T1R3) is a G protein-coupled receptor involved in sweet-taste perception. Besides the tongue, the T1R3 receptor is highly expressed in brain areas implicated in cognition, including the hippocampus and cortex. As cognitive decline is often preceded by significant metabolic or endocrinological dysfunctions regulated by the sweet-taste perception system, we hypothesized that a disruption of the sweet-taste perception in the brain could have a key role in the development of cognitive dysfunction. To assess the importance of the sweet-taste receptors in the brain, we conducted transcriptomic and proteomic analyses of cortical and hippocampal tissues isolated from T1R3 knock-out (T1R3KO) mice. The effect of an impaired sweet-taste perception system on cognition functions were examined by analyzing synaptic integrity and performing animal behavior on T1R3KO mice. Although T1R3KO mice did not present a metabolically disrupted phenotype, bioinformatic interpretation of the high-dimensionality data indicated a strong neurodegenerative signature associated with significant alterations in pathways involved in neuritogenesis, dendritic growth, and synaptogenesis. Furthermore, a significantly reduced dendritic spine density was observed in T1R3KO mice together with alterations in learning and memory functions as well as sociability deficits. Taken together our data suggest that the sweet-taste receptor system plays an important neurotrophic role in the extralingual central nervous tissue that underpins synaptic function, memory acquisition, and social behavior.

Recent advances in drug discovery of GPCR allosteric modulators for neurodegenerative disorders

The activation or the inhibition of G-protein coupled receptors (GPCRs) implicated in the pathophysiology of neurodegenerative disorders is considered as a relevant approach for the treatment of these diseases. The modulation of the relevant GPCRs targets by positive or by negative allosteric modulators appears to be promising, the major challenge remaining the discovery of these molecules. In this review, we highlight the recent development in this field and the therapeutic potential of selected GPCRs allosteric modulators.

Neuroplastic Correlates in the mPFC Underlying the Impairment of Stress-Coping Ability and Cognitive Flexibility in Adult Rats Exposed to Chronic Mild Stress during Adolescence

Using a valid chronic mild stress (CMS) model of depression, we found that adolescent (postnatal days [PND] 28–41) CMS induced transient alterations in anhedonia that did not persist into adulthood after a 3-week recovery period. Previously stressed adult rats exhibited more immobility/despair behaviors in the forced swimming test and a greater number of trials to reach criterion in the set-shifting task, suggesting the impaired ability to cope with stressors and the cognitive flexibility that allows adaptation to dynamic environments during adulthood. In addition, adult rat exposure to adolescent CMS had a relatively inhibited activation in ERK signaling and downstream protein expression of phosphorylated cAMP-response element-binding protein (CREB) and brain-derived neurotrophic factor (BDNF) in the medial prefrontal cortex. Further correlation analysis demonstrated that immobility and set-shifting performance were positively correlated with the inhibition of ERK signaling. These results indicated adolescent CMS can be used as an effective stressor to model an increased predisposition to adult depression.

Beyond Emotional and Spatial Processes: Cognitive Dysfunction in a Depressive Phenotype Produced by Long Photoperiod Exposure

Cognitive dysfunction in depression has recently been given more attention and legitimacy as a core symptom of the disorder. However, animal investigations of depression-related cognitive deficits have generally focused on emotional or spatial memory processing. Additionally, the relationship between the cognitive and affective disturbances that are present in depression remains obscure. Interestingly, sleep disruption is one aspect of depression that can be related both to cognition and affect, and may serve as a link between the two. Previous studies have correlated sleep disruption with negative mood and impaired cognition. The present study investigated whether a long photoperiod-induced depressive phenotype showed cognitive deficits, as measured by novel object recognition, and displayed a cognitive vulnerability to an acute period of total sleep deprivation. Adult male Wistar rats were subjected to a long photoperiod (21L:3D) or a normal photoperiod (12L:12D) condition. Our results indicate that our long photoperiod exposed animals showed behaviors in the forced swim test consistent with a depressive phenotype, and showed significant deficits in novel object recognition. Three hours of total sleep deprivation, however, did not significantly change novel object recognition in either group, but the trends suggest that the long photoperiod and normal photoperiod groups had different cognitive responses to total sleep deprivation. Collectively, these results underline the extent of cognitive dysfunction present in depression, and suggest that altered sleep plays a role in generating both the affective and cognitive symptoms of depression.

Therapeutic Modulation of Glutamate Receptors in Major Depressive Disorder

Current pharmacotherapies for major depressive disorder (MDD) have a distinct lag of onset that can prolong distress and impairment for patients, and realworld effectiveness trials further suggest that antidepressant efficacy is limited in many patients. All currently approved antidepressant medications for MDD act primarily through monoaminergic mechanisms, e.g., receptor/reuptake agonists or antagonists with varying affinities for serotonin, norepinephrine, or dopamine. Glutamate is the major excitatory neurotransmitter in the central nervous system, and glutamate and its cognate receptors are implicated in the pathophysiology of MDD, as well as in the development of novel therapeutics for this disorder. Since the rapid and robust antidepressant effects of the N-methyl-D-aspartate (NMDA) antagonist ketamine were first observed in 2000, other NMDA receptor antagonists have been studied in MDD. These have been associated with relatively modest antidepressant effects compared to ketamine, but some have shown more favorable characteristics with increased potential in clinical practice (for instance, oral administration, decreased dissociative and/or psychotomimetic effects, and reduced abuse/diversion liability). This article reviews the clinical evidence supporting the use of glutamate receptor modulators with direct affinity for cognate receptors: 1) non-competitive NMDA receptor antagonists (ketamine, memantine, dextromethorphan, AZD6765); 2) subunit (NR2B)-specific NMDA receptor antagonists (CP- 101,606/traxoprodil, MK-0657); 3) NMDA receptor glycine-site partial agonists (D-cycloserine, GLYX- 13); and 4) metabotropic glutamate receptor (mGluR) modulators (AZD2066, RO4917523/basimglurant). Several other theoretical glutamate receptor targets with preclinical antidepressant-like efficacy, but that have yet to be studied clinically, are also briefly discussed; these include α-amino-3-hydroxyl-5-methyl-4- isoxazoleproprionic acid (AMPA) agonists, mGluR2/3 negative allosteric modulators, and mGluR7 agonists.

Effects of Duloxetine Treatment on Cognitive Flexibility and BDNF Expression in the mPFC of Adult Male Mice Exposed to Social Stress during Adolescence

Early stress is a significant risk factor for the onset of mood disorders such as depression during adulthood. Impairments in cognitive flexibility mediated by prefrontal cortex (PFC) dysfunction are increasingly recognized as important etiological and pathological factors in the development of depression. Our previous study demonstrated that social defeat stress during early adolescence produced delayed deficits in cognitive flexibility in adult mice. The potential molecular mechanisms underlying these long-term consequences remain unclear. One candidate molecule is brain-derived neurotrophic factor (BDNF), which plays a vital role in neural development and synaptic plasticity. In this study, we initially examined the effects of adolescent social stress on cognitive flexibility and PFC BDNF expression within a week after the last stress exposure and 6 weeks later during adulthood. Adolescent (PND 28) male mice were subjected to stress or control manipulation for 10 days. The attentional set-shifting task (AST) was used to assess cognitive flexibility. Levels of BDNF mRNA and protein in the PFC were examined after behavioral testing. The results demonstrated that previously stressed mice exhibited delayed extra-dimensional set-shifting deficits in AST when tested as adults but not when tested as adolescents. Consistent with the cognitive alterations, adolescent stress induced dynamic alterations in BDNF expression in the medial PFC (mPFC), with a transient increase observed shortly after the stress, followed by a decrease 6 weeks later during adulthood. Next, we further determined the effects of chronic treatment with the antidepressant duloxetine during early adulthood on cognitive and molecular alterations induced by adolescent stress. Compared with the controls, duloxetine treatment reversed the cognitive deficits and increased the BDNF protein expression in the mPFC during adulthood in previously stressed mice. These findings demonstrated that BDNF expression in the mPFC was sensitive to adolescent social stress, which may contribute to the disturbance of the development and later functioning of this brain region.

Practical Strategies and Concepts in GPCR Allosteric Modulator Discovery: Recent Advances with Metabotropic Glutamate Receptors

Allosteric modulation of GPCRs has initiated a new era of basic and translational discovery, filled with therapeutic promise yet fraught with caveats. Allosteric ligands stabilize unique conformations of the GPCR that afford fundamentally new receptors, capable of novel pharmacology, unprecedented subtype selectivity, and unique signal bias. This review provides a comprehensive overview of the basics of GPCR allosteric pharmacology, medicinal chemistry, drug metabolism, and validated approaches to address each of the major challenges and caveats. Then, the review narrows focus to highlight recent advances in the discovery of allosteric ligands for metabotropic glutamate receptor subtypes 1-5 and 7 (mGlu1-5,7) highlighting key concepts ("molecular switches", signal bias, heterodimers) and practical solutions to enable the development of tool compounds and clinical candidates. The review closes with a section on late-breaking new advances with allosteric ligands for other GPCRs and emerging data for endogenous allosteric modulators.