Partial mGlu5 Negative Allosteric Modulator M-5MPEP Demonstrates Antidepressant-Like Effects on Sleep Without Affecting Cognition or Quantitative EEG

Partial mGlu5 receptor NAM, M-5MPEP, induces rapid and sustained antidepressant-like effects in the BDNF-dependent mechanism and enhances (R)-ketamine action in mice

BACKGROUND

Partial negative allosteric modulators (NAM) of the metabotropic glutamate 5 (mGlu5) receptor are an excellent alternative to full antagonists and NAMs because they retain therapeutic effects and have a much broader therapeutic window. Here, we investigated whether partial mGlu5 NAM, 2-(2-(3-methoxyphenyl)ethynyl)-5-methylpyridine (M-5MPEP), induced a fast and sustained antidepressant-like effect, characteristic of rapid-acting antidepressant drugs (RAADs) like ketamine, in mice.

METHODS

A tail suspension test (TST) was used to investigate acute antidepressant-like effects. Sustained effects were studied 24 h after the four intraperitoneal (ip) administrations using the splash test, designed to measure apathy-like state, the sucrose preference test (SPT), reflecting anhedonia, and the TST. Western blot and ELISA techniques were used to measure brain-derived neurotrophic factor (BDNF) and selected protein levels.

METHODS

A tail suspension test (TST) was used to investigate acute antidepressant-like effects. Sustained effects were studied 24 h after the four intraperitoneal (ip) administrations using the splash test, designed to measure apathy-like state, the sucrose preference test (SPT), reflecting anhedonia, and the TST. Western blot and ELISA techniques were used to measure brain-derived neurotrophic factor (BDNF) and selected protein levels.

CONCLUSION

Partial mGlu5 receptor NAM, M-5MPEP, induced rapid and sustained antidepressant-like effects in the BDNF-dependent mechanism and enhanced (R)-ketamine action in mice, indicating both substances' convergent mechanisms of action and the possibility of their practical use in treating depression as RAAD.

Use of Quantitative Electroencephalography to Inform Age- and Sex-Related Differences in NMDA Receptor Function Following MK-801 Administration

Sex- and age-related differences in symptom prevalence and severity have been widely reported in patients with schizophrenia, yet the underlying mechanisms contributing to these differences are not well understood. N-methyl-D-aspartate (NMDA) receptor hypofunction contributes to schizophrenia pathology, and preclinical models often use NMDA receptor antagonists, including MK-801, to model all symptom clusters. Quantitative electroencephalography (qEEG) represents a translational approach to measure neuronal activity, identify targetable biomarkers in neuropsychiatric disorders and evaluate possible treatments. Abnormalities in gamma power have been reported in patients with schizophrenia and correspond to psychosis and cognitive impairment. Further, as gamma power reflects cortical glutamate and GABA signaling, it is highly sensitive to changes in NMDA receptor function, and NMDA receptor antagonists aberrantly increase gamma power in rodents and humans. To evaluate the role of sex and age on NMDA receptor function, MK-801 (0.03-0.3 mg/kg, SC) was administered to 3- and 9-month-old male and female Sprague-Dawley rats that were implanted with wireless EEG transmitters to measure cortical brain function. MK-801-induced elevations in gamma power were observed in 3-month-old male and female and 9-month-old male rats. In contrast, 9-month-old female rats demonstrated blunted maximal elevations across a wide dose range. Importantly, MK-801-induced hyperlocomotor effects, a common behavioral screen used to examine antipsychotic-like activity, were similar across all groups. Overall, sex-by-age-related differences in gamma power support using qEEG as a translational tool to evaluate pathological progression and predict treatment response across a heterogeneous population.

The Metabotropic Glutamate 5 Receptor in Sleep and Wakefulness: Focus on the Cortico-Thalamo-Cortical Oscillations

Sleep is an essential innate but complex behaviour which is ubiquitous in the animal kingdom. Our knowledge of the distinct neural circuit mechanisms that regulate sleep and wake states in the brain are, however, still limited. It is therefore important to understand how these circuits operate during health and disease. This review will highlight the function of mGlu5 receptors within the thalamocortical circuitry in physiological and pathological sleep states. We will also evaluate the potential of targeting mGlu5 receptors as a therapeutic strategy for sleep disorders that often co-occur with epileptic seizures.

Metabotropic glutamate receptor function and regulation of sleep-wake cycles

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

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

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

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.