Potential antidepressive properties of amantadine, memantine and bifemelane

Subanesthetic S-ketamine does not acutely alter striatal dopamine transporter binding in healthy Sprague Dawley female rats

Major depressive disorder is one of the most prevalent mental health disorders, posing a global socioeconomic burden. Conventional antidepressant treatments have a slow onset of action, and 30% of patients show no clinically significant treatment response. The recently approved fast-acting antidepressant S-ketamine, an N-methyl-D-aspartate receptor antagonist, provides a new approach for treatment-resistant patients. However, knowledge of S-ketamine's mechanism of action is still being established. Depressed human subjects have lower striatal dopamine transporter (DAT) availability compared to healthy controls. Rodent studies report increased striatal dopamine concentration in response to acute ketamine administration. In vivo [18F]FE-PE2I ([18F]-(E)-N-(3-iodoprop-2-enyl)-2β-carbofluoroethoxy-3β-(4'-methyl-phenyl) nortropane) positron emission tomography (PET) imaging of the DAT has not previously been applied to assess the effect of acute subanesthetic S-ketamine administration on DAT availability. We applied translational in vivo [18F]FE-PE2I PET imaging of the DAT in healthy female rats to evaluate whether an acute subanesthetic intraperitoneal dose of 15 mg/kg S-ketamine alters DAT availability. We also performed [3H]GBR-12935 autoradiography on postmortem brain sections. We found no effect of acute S-ketamine administration on striatal DAT binding using [18F]FE-PE2I PET or [3H]GBR-12935 autoradiography. This negative result does not support the hypothesis that DAT changes are associated with S-ketamine's rapid antidepressant effects, but additional studies are warranted.

Antidepressant and anxiolytic potential of Citrus reticulata Blanco essential oil: a network pharmacology and animal model study

Background

Citrus reticulata Blanco essential oil (CBEO) has attracted increasing attention as a potential treatment for depression and anxiety in recent years. However, there is limited evidence regarding the active compounds responsible for its therapeutic effects. In addition, substantial amounts of CBEO and prolonged therapy are often required. This study aims to investigate the rapid acting antidepressant and anxiolytic effects of CBEO, identify the underlying composition as well as optimize its dosage and duration.

Methods

CBEO composition was determined using gas chromatography-mass spectrometry (GC-MS), and the corresponding targets were obtained from the SwissTargetPrediction database. Depression-related targets were collected from DisGeNET, GeneCards, Therapeutic Target Database, and Online Mendelian Inheritance in Man. Subsequently, the overlap between CBEO and depression targets was utilized to build a network diagram depicting the relationship between the active ingredients and targets using Cytoscape software. The STRING database facilitated the construction of a protein-protein interaction network, and the Ma'ayan Laboratory Enrichment tool was employed for Gene Ontology (GO) enrichment, Kyoto Encyclopedia of Genes and Genomes (KEGG), and Wiki pathway analyses. Molecular docking was conducted using AutoDock Vina and Discovery Studio Visualizer. Topological analysis predicted the main antidepressant active ingredients in CBEO. A mixture of these compounds was prepared based on their relative GC-MS ratios. Tail suspension test, elevated plus maze, corticosterone-induced PC12 cells, and lipopolysaccharide (LPS)-induced BV2 cells were used to validate the antidepressant and anxiolytic potential of CBEO and CBEO's main bioactive constituents.

Results

CBEO contains 18 components that target 121 proteins. We identified 595 targets associated with depression; among them, 29 targets were located between essential oils and depression. Topological results revealed that linalool, p-cymene, α-terpinene, terpinen-4-ol, and α-terpineol were the major active compounds of CBEO in the management of depression. GO analysis identified G protein-coupled opioid receptor activity, phospholipase C-activating G protein-coupled receptor, and neuron projections that were mostly related to molecular functions, cellular components, and biological processes. Neuroactive ligand-receptor interactions, chemical carcinogenesis, and calcium signaling pathways were the major pathways identified in KEGG analysis. Molecular docking showed that the main bioactive ingredients of CBEO had favorable binding affinities for Protein-Protein Interaction's hub proteins, including OPRM1, PTGS2, ESR1, SLC6A4, DRD2, and NR3C1. These five compounds were then mixed at 0.8:5:0.6:2:1 (w/w) ratio to form a CBEO antidepressant active compound mixture. An acute intranasal treatment of CBEO (25 mg/kg) only demonstrated an antidepressant effect, whereas the main bioactive compounds combination (12.5 mg/kg) illustrated both antidepressant and anxiolytic effects in mice. Linalool, p-cymene, and terpinene-4-ol exhibited neuroprotective and anti-neuroinflammation in the in vitro study, while these effects were not observed for α-terpinene and α-terpineol.

Conclusion

Linalool, p-cymene, α-terpinene, terpinen-4-ol, and α-terpineol cymene might be mainly contributing to CBEO's antidepressant effect by regulating neuroactive ligand-receptor interaction, neuron projection, and receptor signaling pathway. A mixture of these compounds showed rapid antidepressant potential via intranasal administration, which was comparable to that of CBEO. The mixture also exhibited an anxiolytic effect while not seen in CBEO.

The Roles of Glutamate Receptors and Their Antagonists in Status Epilepticus, Refractory Status Epilepticus, and Super-Refractory Status Epilepticus

Status epilepticus (SE) is a neurological emergency with a high mortality rate. When compared to chronic epilepsy, it is distinguished by the durability of seizures and frequent resistance to benzodiazepine (BZD). The Receptor Trafficking Hypothesis, which suggests that the downregulation of γ-Aminobutyric acid type A (GABAA) receptors, and upregulation of N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors play major roles in the establishment of SE is the most widely accepted hypothesis underlying BZD resistance. NMDA and AMPA are ionotropic glutamate receptor families that have important excitatory roles in the central nervous system (CNS). They are both essential in maintaining the normal function of the brain and are involved in a variety of neuropsychiatric diseases, including epilepsy. Based on animal and human studies, antagonists of NMDA and AMPA receptors have a significant impact in ending SE; albeit most of them are not yet approved to be in clinically therapeutic guidelines, due to their psychomimetic adverse effects. Although there is still a dearth of randomized, prospective research, NMDA antagonists such as ketamine, magnesium sulfate, and the AMPA antagonist, perampanel, are regarded to be reasonable optional adjuvant therapies in controlling SE, refractory SE (RSE) or super-refractory SE (SRSE), though there are still a lack of randomized, prospective studies. This review seeks to summarize and update knowledge on the SE development hypothesis, as well as clinical trials using NMDA and AMPA antagonists in animal and human studies of SE investigations.

Targeting NMDA Receptors at the Neurovascular Unit: Past and Future Treatments for Central Nervous System Diseases

The excitatory neurotransmission of the central nervous system (CNS) mainly involves glutamate and its receptors, especially N-methyl-D-Aspartate receptors (NMDARs). These receptors have been extensively described on neurons and, more recently, also on other cell types. Nowadays, the study of their differential expression and function is taking a growing place in preclinical and clinical research. The diversity of NMDAR subtypes and their signaling pathways give rise to pleiotropic functions such as brain development, neuronal plasticity, maturation along with excitotoxicity, blood-brain barrier integrity, and inflammation. NMDARs have thus emerged as key targets for the treatment of neurological disorders. By their large extracellular regions and complex intracellular structures, NMDARs are modulated by a variety of endogenous and pharmacological compounds. Here, we will present an overview of NMDAR functions on neurons and other important cell types involved in the pathophysiology of neurodegenerative, neurovascular, mental, autoimmune, and neurodevelopmental diseases. We will then discuss past and future development of NMDAR targeting drugs, including innovative and promising new approaches.

Fluoroethylnormemantine, A Novel Derivative of Memantine, Facilitates Extinction Learning Without Sensorimotor Deficits

BACKGROUND

Memantine, a noncompetitive N-methyl-D-aspartate receptor antagonist, has been approved for use in Alzheimer's disease, but an increasing number of studies have investigated its utility for neuropsychiatric disorders. Here, we characterized a novel compound, fluoroethylnormemtantine (FENM), which was derived from memantine with an extra Fluor in an optimized position for in vivo biomarker labeling. We sought to determine if FENM produced similar behavioral effects as memantine and/or if FENM has beneficial effects against fear, avoidance, and behavioral despair.

METHODS

We administered saline, FENM, or memantine prior to a number of behavioral assays, including paired-pulse inhibition, open field, light dark test, forced swim test, and cued fear conditioning in male Wistar rats.

RESULTS

Unlike memantine, FENM did not produce nonspecific side effects and did not alter sensorimotor gating or locomotion. FENM decreased immobility in the forced swim test. Moreover, FENM robustly facilitated fear extinction learning when administered prior to either cued fear conditioning training or tone reexposure.

CONCLUSIONS

These results suggest that FENM is a promising, novel compound that robustly reduces fear behavior and may be useful for further preclinical testing.

Novel rapid-acting glutamatergic modulators: Targeting the synaptic plasticity in depression

Major depressive disorder (MDD) is severely prevalent, and conventional monoaminergic antidepressants gradually exhibit low therapeutic efficiency, especially for patients with treatment-resistant depression. A neuroplasticity hypothesis is an emerging advancement in the mechanism of depression, mainly expressed in the glutamate system, e.g., glutamate receptors and signaling. Dysfunctional glutamatergic neurotransmission is currently considered to be closely associated with the pathophysiology of MDD. Biological function, pharmacological action, and signal attributes in the glutamate system both regulate the neural process. Specific functional subunits could be therapeutic targets to explore the novel glutamatergic modulators, which have fast-acting, and relatively sustained antidepressant effects. Here, the present review summarizes the pathophysiology of MDD found in the glutamate system, exploring the role of glutamate receptors and their downstream effects. These convergent mechanisms have prompted the development of other modulators targeting on glutamate system, including N-methyl-d-aspartate receptor antagonists, selective GluN2B-specific antagonists, glycine binding site agents, and regulators of metabotropic glutamate receptors. Relevant researches underly the putative mechanisms of these drugs, which reverse the damage of depression by regulating glutamatergic neurotransmission. It also provides further insight into the mechanism of depression and exploring potential targets for novel agent development.

Involvement of dopamine D2 and glutamate NMDA receptors in the antidepressant-like effect of amantadine in mice

The present study investigated the pharmacological mechanisms of the antidepressant-like effects of amantadine in mice and their influence on hippocampal neurogenesis. To improve the translational validity of preclinical results, reproducibility across laboratories and replication in other animal models and species are crucial. Single amantadine administration at doses of 50 and 75 mg/kg resulted in antidepressant-like effects in mice in the tail suspension test (TST), reflected by an increase in immobility time. The effects of amantadine were seen at doses that did not alter locomotor activity. The tyrosine hydroxylase inhibitor α-methyl-ρ-tyrosine did not influence the anti-immobility effect of amantadine in the TST. Pretreatment with the α₁ adrenergic receptor antagonist prazosin, β adrenergic receptor antagonist propranolol, α₂ adrenergic receptor antagonist yohimbine, and α₂ adrenergic receptor agonist clonidine did not alter the antidepressant-like effect of amantadine. However, amantadine's effect was blocked by the dopamine D₂ receptor antagonist haloperidol and glutamate receptor agonist N-methyl-D-aspartate (NMDA). Repeated amantadine administration (50 mg/kg) also exerted an antidepressant-like effect, paralleled by an increase in hippocampal neurogenesis. The present results demonstrate that the antidepressant-like effects of amantadine may be mediated by its actions on D₂ and NMDA receptors and likely involve hippocampal neurogenesis.

Possible Antidepressant Effects of Memantine-Systematic Review with a Case Study

The treatment of bipolar depression is hampered by the inadequate efficacy of antidepressants, moderate effect of mood stabilizers, and the side effects of some second-generation antipsychotics. There is limited evidence to date regarding the antidepressant effects of memantine in bipolar depression. The aim of the article was to provide a short review of preclinical and clinical studies on the antidepressant effect of memantine, and to present the case of a bipolar depression patient successfully treated with memantine. The described patient with bipolar disorder was unsuccessfully treated with two mood stabilizers. The addition of memantine at a dose of 20 mg/d to the treatment with lamotrigine and valproic acid resulted in a reduction in the severity of depression measured on the HDRS-17 scale by 35%, and by 47.1% after 7 weeks. The discussion presents experimental evidence for the antidepressant effect of memantine, as well as data from clinical trials in recurrent and bipolar depression. The presented case is the second report in the medical literature showing the antidepressant effect of memantine as an add-on treatment for bipolar depression. The described case and literature analysis indicate that memantine may be an effective and safe method of augmentation of mood stabilizing therapy in bipolar depression.

Glycine/NMDA Receptor Pathway Mediates the Rapid-onset Antidepressant Effect of Alkaloids From Trichilia Monadelpha

INTRODUCTION

Major depressive disorder is often associated with suicidal tendencies, and this condition accentuates the need for rapid-acting antidepressants. We previously reported that Alkaloids (ALK) from Trichilia monadelpha possess antidepressant action in acute animal models of depression and that this effect is mediated through the monoamine and L-arginine-NO-cGMP pathways. This study investigated the possible rapid-onset antidepressant effect of ALK from T. monadelpha and its connection with the glycine/NMDA receptor pathway.

METHODS

The onset of ALK action from T. monadelpha was evaluated using the Open Space Swim Test (OSST), a chronic model of depression. The modified forced swimming and tail suspension tests were used to assess the effect of the ALK on the glycine/NMDA receptor pathway. The Instutute of Cancer Research (ICR) mice were treated with either ALK (30-300 mg/kg, orally [PO]), imipramine (3-30 mg/kg, PO), fluoxetine (3-30 mg/kg, PO), or saline. To identify the role of glycine/NMDA receptor pathway in the effect of ALK, we pretreated mice with a partial agonist of the glycine/NMDA receptor, D-cycloserine (2.5 mg/kg, intraperitoneally [IP]), and an agonist of glycine/NMDA receptor, D-serine (600 mg/kg, IP), before ALK administration.

RESULTS

ALK reversed immobility in mice after the second day of drug treatment in the OSST. In contrast, there was a delay in the effects induced by fluoxetine and imipramine. ALK also increased mean swimming and climbing scores in mice. ALK was more efficacious than imipramine and fluoxetine in reducing immobility and increasing distance traveled. It is noteworthy that ALK was less potent than fluoxetine and imipramine. D-cycloserine potentiated mobility observed in the ALK- and fluoxetine-treated mice. In contrast, D-serine decreased mobility in the ALK-treated mice.

CONCLUSION

The study results suggest that ALK from T. monadelpha exhibits rapid antidepressant action in mice, and the glycine/NMDA receptor pathway possibly mediates the observed effect.

Amantadine: reappraisal of the timeless diamond-target updates and novel therapeutic potentials

The aim of the current review was to provide a new, in-depth insight into possible pharmacological targets of amantadine to pave the way to extending its therapeutic use to further indications beyond Parkinson's disease symptoms and viral infections. Considering amantadine's affinities in vitro and the expected concentration at targets at therapeutic doses in humans, the following primary targets seem to be most plausible: aromatic amino acids decarboxylase, glial-cell derived neurotrophic factor, sigma-1 receptors, phosphodiesterases, and nicotinic receptors. Further three targets could play a role to a lesser extent: NMDA receptors, 5-HT3 receptors, and potassium channels. Based on published clinical studies, traumatic brain injury, fatigue [e.g., in multiple sclerosis (MS)], and chorea in Huntington's disease should be regarded potential, encouraging indications. Preclinical investigations suggest amantadine's therapeutic potential in several further indications such as: depression, recovery after spinal cord injury, neuroprotection in MS, and cutaneous pain. Query in the database http://www.clinicaltrials.gov reveals research interest in several further indications: cancer, autism, cocaine abuse, MS, diabetes, attention deficit-hyperactivity disorder, obesity, and schizophrenia.

Possibility of a New Indication for Amantadine in the Treatment of Bipolar Depression-Case Series Study

Bipolar disorder is a chronic and remitting mental illness. Antidepressants are not effective in treating acute bipolar depression, and antipsychotic drugs used in the treatment of bipolar depression cause frequent side effects. This situation justifies the search for new drugs as well as the repurposing of drugs used in other indications. In an open and naturalistic serious case study, 4 patients diagnosed with bipolar I disorder, chronically treated with a mood stabilizer, in whom at least two antidepressants were ineffective in the depressive phase, were treated with amantadine. The woman received 100 mg/day and 3 men received the target dose of 200 mg/day. All patients treated with amantadine improved their depressive symptoms after 1 week of treatment. None of them experienced side effects or manic switch. To reduce the risk of a manic switch, the treatment with amantadine was discontinued 2 weeks after the improvement of depressive symptoms, and no recurrence of depressive symptoms was observed. Amantadine may be a further therapeutic option for the treatment of acute bipolar depression. The drug in this indication may act quickly and be well tolerated. Confirmation of the antidepressant efficacy of amantadine in this indication requires replication of the results and conducting clinical trials.

Ketamine's dose related multiple mechanisms of actions: Dissociative anesthetic to rapid antidepressant

Ketamine induces safe and effective anesthesia and displays unusual cataleptic properties that gave rise to the term dissociative anesthesia. Since 1970, clinicians only utilized the drug as an anesthetic or analgesic for decades, but ketamine was found to have rapid acting antidepressant effects in 1990s. Accumulated evidence exhibits NMDAR antagonism may not be the only mechanism of ketamine. The contributions of AMPA receptor, mTor signal pathway, monoaminergic system, sigma-1 receptor, cholinergic, opioid and cannabinoid systems, as well as voltage-gated calcium channels and hyperpolarization cyclic nucleotide gated channels are discussed for the antidepressant effects. Also the effects of ketamine's enantiomers and metabolites are reviewed. Furthermore ketamine's anesthetic and analgesic mechanisms are briefly revisited. Overall, pharmacology of ketamine, its enantiomers and metabolites is very unique. Insight into multiple mechanisms of action will provide further development and desirable clinical effects of ketamine.

Rodent ketamine depression-related research: Finding patterns in a literature of variability

Discovering that the anesthetic drug ketamine has rapidly acting antidepressant effects in many individuals with major depression is one of the most important findings in clinical psychopharmacology in recent decades. The initial report of these effects in human subjects was based on a foundation of rodent preclinical studies carried out in the 1990s, and subsequent investigation has included both further studies in individuals with depression, as well as reverse translational experiments in animal models, especially rodents. While there is general agreement in the rodent literature that ketamine has rapidly-acting, and generally sustained, antidepressant-like properties, there are also points of contention across studies, including the precise mechanism of action of this drug. In this review, we briefly summarize prominent yet variable findings regarding the mechanism of action. We also discuss a combination of similarities and variances in the rodent literature in the antidepressant-like effects of ketamine as a function of dose, species and strain, test, stressor, and presumably sex of the experimenter. We then present previously unpublished mouse strain comparison data suggesting that subanesthetic ketamine does not have robust antidepressant-like properties in unstressed animals, and may actually promote depression-like behavior, in contrast to widely reported findings. We conclude that the data best support the notion of ketamine action principally via NMDA receptor antagonism, transiently boosting glutamatergic (and possibly other) signaling in diverse brain circuits. We also suggest that future studies should address in greater detail the extent to which antidepressant-like properties of this drug are stress-sensitive, in an effort to better model major depression present in humans.

Differential effects of D-cycloserine and amantadine on motor behavior and D2/3 receptor binding in the nigrostriatal and mesolimbic system of the adult rat

D-cycloserine (DCS) and amantadine (AMA) act as partial NMDA receptor (R) agonist and antagonist, respectively. In the present study, we compared the effects of DCS and AMA on dopamine D_2/3R binding in the brain of adult rats in relation to motor behavior. D_2/3R binding was determined with small animal SPECT in baseline and after challenge with DCS (20 mg/kg) or AMA (40 mg/kg) with [¹²³I]IBZM as radioligand. Immediately post-challenge, motor/exploratory behavior was assessed for 30 min in an open field. The regional binding potentials (ratios of the specifically bound compartments to the cerebellar reference region) were computed in baseline and post-challenge. DCS increased D_2/3R binding in nucleus accumbens, substantia nigra/ventral tegmental area, thalamus, frontal, motor and parietal cortex as well as anterodorsal and posterior hippocampus, whereas AMA decreased D_2/3R binding in nucleus accumbens, caudateputamen and thalamus. After DCS, ambulation and head-shoulder motility were decreased, while sitting was increased compared to vehicle and AMA. Moreover, DCS increased rearing relative to AMA. The regional elevations of D_2/3R binding after DCS reflect a reduction of available dopamine throughout the mesolimbocortical system. In contrast, the reductions of D_2/3R binding after AMA indicate increased dopamine in nucleus accumbens, caudateputamen and thalamus. Findings imply that, after DCS, nigrostriatal and mesolimbic dopamine levels are directly related to motor/exploratory activity, whereas an inverse relationship may be inferred for AMA.

The impact of sex as a biological variable in the search for novel antidepressants

A roadblock to successful treatment for anxiety and depression is the high proportion of individuals that do not respond to existing treatments. Different underlying neurobiological mechanisms may drive similar symptoms, so a more personalized approach to treatment could be more successful. There is increasing evidence that sex is an important biological variable modulating efficacy of antidepressants and anxiolytics. We review evidence for sex-specific effects of traditional monoamine based antidepressants and newer pharmaceuticals targeting kappa opioid receptors (KOR), oxytocin receptors (OTR), and N-methyl-D-aspartate receptors (ketamine). In some cases, similar behavioral effects are observed in both sexes while in other cases strong sex-specific effects are observed. Most intriguing are cases such as ketamine which has similar behavioral effects in males and females, perhaps through sex-specific neurobiological mechanisms. These results show how essential it is to include both males and females in both clinical and preclinical evaluations of novel antidepressants and anxiolytics.

Potential antidepressant effect of amantadine: a review of preclinical studies and clinical trials

Objective:

Amantadine blocks N-methyl-D-aspartate (NMDA) receptors and has dopaminergic and noradrenergic action, a neurochemical profile that suggests its potential as an antidepressant drug. We conducted a systematic review of preclinical and clinical studies addressing the effects of amantadine in animal models of depression and in patients with depression.

Methods:

PubMed, Science Direct, and Web of Science were searched up to September 1, 2017 to identify clinical and preclinical studies. The following search terms were used: “amantadine AND depress*”; “amantadine AND mood”; “amantadine AND animal models AND antidepres*”; and “amantadine AND (forced swim, learned helplessness, reserpine, chronic mild stress, anhedonia, sucrose preference).”

Results:

Amantadine had antidepressant-like effects in animal models and appeared to potentiate the antidepressant effects of other antidepressants. These preclinical findings have received some support from the results of small open-label clinical trials, suggesting that amantadine can reduce depressive symptomatology and potentiate the antidepressant effects of monoaminergic drugs. In addition to its glutamatergic and dopaminergic effects, the potential antidepressant-like effects of amantadine have been linked to molecular and cellular actions, such as increased expression of neurotrophic factors (e.g., brain-derived neurotrophic factor), activation of σ₁ receptors, decreased corticosterone levels, and decreased inflammatory response to stress.

Conclusion:

Amantadine is an interesting candidate as new antidepressant drug for the treatment of depression.

Design, synthesis and biological assessment of N-adamantyl, substituted adamantyl and noradamantyl phthalimidines for nitrite, TNF-α and angiogenesis inhibitory activities

A library of 15 novel and heretofore uncharacterized adamantyl and noradamantyl phthalimidines was synthesized and evaluated for neuroprotective and anti-angiogenic properties. Phthalimidine treatment in LPS-challenged cells effected reductions in levels of secreted TNF-α and nitrite relative to basal amounts. The primary SAR suggests nitration of adamantyl phthalimidines has marginal effect on TNF-α activity but promotes anti-nitrite activity; thioamide congeners retain anti-nitrite activity but are less effective reducing TNF-α. Site-specific nitration and thioamidation provided phthalimidine 24, effecting an 88.5% drop in nitrite concurrent with only a 4% drop in TNF-α. Notable anti-angiogenesis activity was observed for 20, 21 and 22.

The role of memantine in the treatment of major depressive disorder: Clinical efficacy and mechanisms of action

A developing body of evidence indicates that disturbed glutamate neurotransmission especially through N-methyl-d-aspartate (NMDA) is central to the pathophysiology of major depressive disorder (MDD) and NMDA receptor antagonists have shown therapeutic potential in the MDD treatment. Memantine is an uncompetitive NMDA receptor antagonist, approved for treatment of Alzheimer's disease (AD) that in contrast to other NMDA receptor antagonists at therapeutic doses does not induce highly undesirable side effects. Neuroprotective properties and well tolerability of memantine have been attributed to its unique pharmacological features such as moderate affinity, rapid blocking kinetics and strongly voltage-dependency. In this review we summarized clinical trial evidence of antidepressant effectiveness of memantine and its mechanisms of action. Available data indicate contradictory findings relating to clinical efficacy suggesting further research is necessary in determining as to whether memantine will eventually be an advantageous therapy for MDD. Preclinical data proposed various neurobiological mechanisms underlying antidepressant-like properties of memantine that are responsible for synaptic plasticity and cell survival.

Neural basis of major depressive disorder: Beyond monoamine hypothesis

The monoamine hypothesis has been accepted as the most common hypothesis of major depressive disorder (MDD) for a long period because of its simplicity and understandability. Actually, most currently used antidepressants have been considered to act based on the monoamine hypothesis. However, an important problem of the monoamine hypothesis has been pointed out as follows: it fails to explain the latency of response to antidepressants. In addition, many patients with MDD have remained refractory to currently used antidepressants. Therefore, monoamine-alternate hypotheses are required to explain the latency of response to antidepressants. Such hypotheses have been expected to contribute to identifying hopeful new therapeutic targets for MDD. Past studies have revealed that the volume of the hippocampus is decreased in patients with MDD, which is likely caused by the failure of the hypothalamic-pituitary-adrenal axis and following elevation of glucocorticoids. Two hypotheses have been proposed to explain the volume of the hippocampus: (i) the neuroplasticity hypothesis; and (ii) the neurogenesis hypothesis. The neuroplasticity hypothesis explains how the hippocampal volume is decreased by the morphological changes of hippocampal neurons, such as the shortening length of dendrites and the decreased number and density of spines. The neurogenesis hypothesis explains how the hippocampal volume is decreased by the decrease of neurogenesis in the hippocampal dentate gyrus. These hypotheses are able to explain the latency of response to antidepressants. In this review, we first overview how the neuroplasticity and neurogenesis hypotheses have been developed. We then describe the details of these hypotheses.

New targets for rapid antidepressant action

Current therapeutic options for major depressive disorder (MDD) and bipolar disorder (BD) are associated with a lag of onset that can prolong distress and impairment for patients, and their antidepressant efficacy is often limited. All currently approved antidepressant medications for MDD act primarily through monoaminergic mechanisms. Glutamate is the major excitatory neurotransmitter in the central nervous system, and glutamate and its cognate receptors are implicated in the pathophysiology of MDD, and in the development of novel therapeutics for this disorder. The rapid and robust antidepressant effects of the N-methyl-d-aspartate (NMDA) antagonist ketamine were first observed in 2000. Since then, other NMDA receptor antagonists have been studied in MDD. Most have demonstrated relatively modest antidepressant effects compared to ketamine, but some have shown more favorable characteristics. This article reviews the clinical evidence supporting the use of novel glutamate receptor modulators with direct affinity for cognate receptors: (1) non-competitive NMDA receptor antagonists (ketamine, memantine, dextromethorphan, AZD6765); (2) subunit (GluN2B)-specific NMDA receptor antagonists (CP-101,606/traxoprodil, MK-0657); (3) NMDA receptor glycine-site partial agonists (GLYX-13); and (4) metabotropic glutamate receptor (mGluR) modulators (AZD2066, RO4917523/basimglurant). We also briefly discuss several other theoretical glutamate receptor targets with preclinical antidepressant-like efficacy that have yet to be studied clinically; these include α-amino-3-hydroxyl-5-methyl-4-isoxazoleproprionic acid (AMPA) agonists and mGluR2/3 negative allosteric modulators. The review also discusses other promising, non-glutamatergic targets for potential rapid antidepressant effects, including the cholinergic system (scopolamine), the opioid system (ALKS-5461), corticotropin releasing factor (CRF) receptor antagonists (CP-316,311), and others.

Investigational drugs for treating major depressive disorder

INTRODUCTION

Treatment of patients suffering from major depression could be highly challenging for psychiatrists. Intractability as well as relapse is commonly seen among these patients, leading to functional impairment and poor quality of life. The present review discusses some of the novel investigational drugs that are under pre-clinical or clinical phases in the treatment of major depression. Areas covered: Molecules belonging to different classes such as triple reuptake inhibitors, opioid receptors, ionotropic and metabotropic glutamate receptors, and neurotrophin in the treatment of major depression are covered in this article. Expert opinion: Although the historical discovery of earlier antidepressant molecules (iproniazid and imipramine) is through serendipitous discovery, the present research focuses on discovering novel molecules based on our current pathophysiological knowledge of the disease condition. The fast-acting antidepressant property of N-methyl-d-aspartate (NMDA) receptor molecules, including ketamine is an exciting area of research. Other drug molecules such as amitifadine (triple reuptake inhibitor), ALKS-5461 (kappa receptor antagonist and mu opioidergic receptor agonist), rapastinel (NMDA glutamatergic receptor modulator) are under Phase-III clinical trials and could be approved in the near future for the treatment of major depression.

Effect of co-administration of memantine and sertraline on the antidepressant-like activity and brain-derived neurotrophic factor (BDNF) levels in the rat brain

A developing body of data has drawn attention to the N-methyl-d-aspartate (NMDA) receptor antagonists as potential drugs for the treatment of major depressive disorder (MDD). We investigated the possibility of synergistic interactions between the antidepressant sertraline with the uncompetitive NMDA receptor antagonist, memantine. The present study was aimed to evaluate behavioural and molecular effects of the chronic treatment with memantine and sertraline alone or in combination in rats. To this aim, rats were chronically treated with memantine (2.5 and 5mg/kg) and sertraline (5mg/kg) for 14days once a day, and then exposed to the forced swimming test. The brain-derived neurotrophic factor (BDNF) levels were assessed in the hippocampus and prefrontal cortex in all groups by ELISA sandwich assay. Sertraline and memantine (2.5mg/kg) alone did not have effect on the immobility time; however, the effect of sertraline was enhanced by both doses of memantine. Combined treatment with memantine and sertraline produced stronger increases in the BDNF protein levels in the hippocampus and prefrontal cortex. Our results indicate that co-administration of antidepressant memantine with sertraline may induce a more pronounced antidepressant activity than treatment with each antidepressant alone. Antidepressant properties using the combination of memantine and sertraline could be attributed to increased levels of BDNF. This finding may be of particular importance in the case of drug-resistant patients and could suggest a method of obtaining significant antidepressant actions whereas limiting side effects.

Failure of memantine to “reverse” quinpirole-induced hypomotility

AIM: To evaluate antidepressant-like effect of memantine in a rat model.

METHODS: Male Wistar rats were treated intraperitoneally with either vehicle, memantine (10 mg/kg) or imipramine (20 mg/kg), for 3 wk. Twenty-four hour after the last treatment animals were challenged with quinpirole (0.3 mg/kg s.c.) and tested for motor activity. After 1 h habituation to the motility cages, the motor response was recorded for the following 45-min and the data were collected in 5-min time bins.

RESULTS: As expected, chronic treatment with imipramine potentiated the locomotor stimulant effect of quinpirole. On the contrary, chronic memantine administration failed to induce the behavioral supersensitivity to the dopamine agonist.

CONCLUSION: The results show that memantine, at variance with antidepressant treatments, fails to induce dopaminergic behavioral supersensitivity. This observation is consistent with the results of preclinical and clinical studies suggesting that memantine does not have an acute antidepressant action but does have an antimanic and mood-stabilizing effect.

Ultimate Translation: Developing Therapeutics Targeting on N-Methyl-d-Aspartate Receptor

N-Methyl-d-aspartate receptors (NMDARs) are broadly distributed in the central nervous system (CNS), where they mediate excitatory signaling. NMDAR-mediated neurotransmission (NMDARMN) is the molecular engine of learning, memory and cognition, which are the basis for high cortical function. NMDARMN is also critically involved in the development and plasticity of CNS. Due to its essential and critical role, either over- or under-activation of NMDARMN can contribute substantially to the development of CNS disorders. The involvement of NMDARMN has been demonstrated in a variety of CNS disorders, including schizophrenia, depression, posttraumatic stress disorder, aging, mild cognitive impairment and Alzheimer's dementia, amyotrophic lateral sclerosis, and anti-NMDAR encephalitis. Several targets to "correct" or "reset" the NMDARMN in these CNS disorders have been identified and confirmed. With analogy to aminergic treatments, these targets include the glycine/d-serine co-agonist site, channel ionophore, glycine transporter-1, and d-amino acid oxidase. It is still early days in terms of developing novel therapeutics targeting the NMDAR. However, agents modulating NMDARMN hold promise as the next generation of CNS therapeutics.

Antidepressant-like effects and possible mechanisms of amantadine on cognitive and synaptic deficits in a rat model of chronic stress

The aim of this study was to examine whether amantadine (AMA), as a low-affinity noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, is able to improve cognitive deficits caused by chronic stress in rats. Male Wistar rats were divided into four groups: control, control + AMA, stress and stress + AMA groups. The chronic stress model combined chronic unpredictable stress (CUS) with isolated feeding. Animals were exposed to CUS continued for 21 days. AMA (25 mg/kg) was administrated p.o. for 20 days from the 4th day of CUS to the 23rd. Weight and sucrose consumption were measured during model establishing period. Spatial memory was evaluated using the Morris water maze (MWM) test. Following MWM testing, both long-term potentiation (LTP) and depotentiation were recorded in the hippocampal CA1 region. NR2B and postsynaptic density protein 95 (PSD-95) proteins were measured by Western-blot analysis. AMA increased weight and sucrose consumption of stressed rats. Spatial memory and reversal learning in stressed rats were impaired relative to controls, whereas AMA significantly attenuated cognitive impairment. AMA also mitigated the chronic stress-induced impairment of hippocampal synaptic plasticity, in which both the LTP and depotentiation were significantly inhibited in stressed rats. Moreover, AMA enhanced the expression of hippocampal NR2B and PSD-95 in stressed rats. The data suggest that AMA may be an effective therapeutic agent for depression-like symptoms and associated cognitive disturbances.

Ketamine and other glutamate receptor modulators for depression in adults

BACKGROUND

Considering the ample evidence of involvement of the glutamate system in the pathophysiology of depression, pre-clinical and clinical studies have been conducted to assess the antidepressant efficacy of glutamate inhibition, and glutamate receptor modulators in particular. This review focuses on the use of glutamate receptor modulators in unipolar depression.

OBJECTIVES

To assess the effects - and review the acceptability - of ketamine and other glutamate receptor modulators in comparison to placebo (or saline placebo), other pharmacologically active agents, or electroconvulsive therapy (ECT) in alleviating the acute symptoms of depression in people with unipolar major depressive disorder.

SEARCH METHODS

We searched the Cochrane Depression, Anxiety and Neurosis Review Group's Specialised Register (CCDANCTR, to 9 January 2015). This register includes relevant randomised controlled trials (RCTs) from: the Cochrane Library (all years), MEDLINE (1950 to date), EMBASE (1974 to date), and PsycINFO (1967 to date). We did not apply any restrictions to date, language or publication status.

SELECTION CRITERIA

Double- or single-blind RCTs comparing ketamine, memantine, or other glutamate receptor modulators with placebo (or saline placebo), other active psychotropic drugs, or electroconvulsive therapy (ECT) in adults with unipolar major depression.

DATA COLLECTION AND ANALYSIS

Three review authors independently identified studies, assessed trial quality and extracted data. The primary outcomes for this review were response rate and adverse events.

MAIN RESULTS

We included 25 studies (1242 participants) on ketamine (9 trials), memantine (3), AZD6765 (3), D-cycloserine (2), Org26576 (2), atomoxetine (1), CP-101,606 (1), MK-0657 (1), N-acetylcysteine (1), riluzole (1) and sarcosine (1). Twenty-one studies were placebo-controlled and the majority were two-arm studies (23 out of 25). Twenty-two studies defined an inclusion criteria specifying the severity of depression; 11 specified at least moderate depression; eight, severe depression; and the remaining three, mild-moderate depression. Nine studies recruited only treatment-resistant patients.We rated the risk of bias as low or unclear for most domains, though lack of detail regarding masking of treatment in the studies reduced our certainty in the effect for all outcomes. We rated three studies as having high risk for selective outcome reporting. Many trials did not provide information on all the prespecified outcomes and we found no data, or very limited data, on very important issues like suicidality, cognition, quality of life, costs to healthcare services and dropouts due to lack of efficacy.Among all glutamate receptor modulators, only ketamine (administered intravenously) proved to be more efficacious than placebo, though the quality of evidence was limited by risk of bias and small sample sizes. There was low quality evidence that treatment with ketamine increased the likelihood of response after 24 hours (odds ratio (OR) 10.77, 95% confidence interval (CI) 2.00 to 58.00; 3 RCTs, 56 participants), 72 hours (OR 12.59, 95% CI 2.38 to 66.73; 3 RCTs, 56 participants), and one week (OR 2.58, 95% CI 1.08 to 6.16; 4 RCTs, 131 participants). The effect of ketamine was even less certain at two weeks, as data were available from only one trial (OR 0.93, 95% CI 0.31 to 2.83; 51 participants, low quality evidence). This was consistent across all efficacy outcomes. Ketamine caused more confusion and emotional blunting compared to placebo. There was insufficient evidence to determine if this increased the likelihood of leaving the study early (OR 1.90, 95% CI 0.43 to 8.47; 5 RCTs, 139 participants, low quality evidence).One RCT with 72 participants reported higher numbers of responders on ketamine than midazolam at 24 hours (OR 0.36, 95% CI 0.14 to 0.58), 72 hours (OR 0.37, 95% CI 0.16 to 0.59), and one week (OR 0.29, 95% CI 0.08 to 0.49). However, midazolam was better tolerated than ketamine in terms of blurred vision, dizziness, general malaise and nausea/vomiting at 24 hours post-infusion. The evidence contributing to these outcomes was of low quality.We found better efficacy of sarcosine over citalopram at four weeks (OR 6.93, 95% CI 1.53 to 31.38; 1 study, 40 participants), but not at two weeks (OR: 8.14, 95% CI 0.88 to 75.48); fewer participants in the sarcosine group experienced adverse events (OR 0.04, 95% CI 0.00 to 0.68; P = 0.03, 1 study, 40 participants). This was based on low quality evidence. No significant results were found for the remaining glutamate receptor modulators.In one study with 18 participants, ketamine was more effective than ECT at 24 hours (OR 28.00, 95% CI 2.07 to 379.25) and 72 hours (OR 12.25, 95% CI 1.33 to 113.06), but not at one week (OR 3.35, 95% CI 0.12 to 93.83), or two weeks (OR 3.35, 95% CI 0.12 to 93.83). No differences in terms of adverse events were found between ketamine and ECT, however the only adverse events reported were blood pressure and heart rate. This study was rated as very low quality.

AUTHORS' CONCLUSIONS

We found limited evidence for ketamine's efficacy over placebo at time points up to one week in terms of the primary outcome, response rate. The effects were less certain at two weeks post-treatment. No significant results were found for the remaining ten glutamate receptor modulators, except for sarcosine being more effective than citalopram at four weeks. In terms of adverse events, the only significant differences in favour of placebo over ketamine were in regards to confusion and emotional blunting. Despite the promising nature of these preliminary results, our confidence in the evidence was limited by risk of bias and the small number of participants. Many trials did not provide information on all the prespecified outcomes and we found no data, or very limited data, on very important issues like suicidality, cognition, quality of life, costs to healthcare services and dropouts due to lack of efficacy.All included studies administered ketamine intravenously, which can pose practical problems in clinical practice. Very few trials were included in the meta-analyses for each comparison; the majority of comparisons contained only one study. Further RCTs (with adequate blinding) are needed to explore different modes of administration of ketamine with longer follow-up, which test the comparative efficacy of ketamine and the efficacy of repeated administrations.

Effect of NMDAR antagonists in the tetrabenazine test for antidepressants: comparison with the tail suspension test

OBJECTIVE

The N-methyl-d-aspartate receptor (NMDAR) antagonist ketamine, produces rapid and enduring antidepressant effect in patients with treatment-resistant depression. Similar dramatic effects have not been observed in clinical trials with other NMDAR antagonists indicating ketamine may possess unique pharmacological properties. Tetrabenazine induces ptosis (a drooping of the eyelids), and the reversal of this effect, attributed to a sympathomimetic action, has been used to detect first-generation antidepressants, as well as ketamine. Because the actions of other NMDAR antagonists have not been reported in this measure, we examined whether reversal of tetrabenazine-induced ptosis was unique to ketamine, or a class effect of NMDAR antagonists.

METHODS

The effects of ketamine and other NMDAR antagonists to reverse tetrabenazine-induced ptosis were examined and compared with their antidepressant-like effects in the tail suspension test (TST) in mice.

RESULTS

All the NMDAR antagonists tested produced a partial reversal of tetrabenazine-induced ptosis and, as expected, reduced immobility in the TST. Ketamine, memantine, MK-801 and AZD6765 were all about half as potent in reversing tetrabenazine-induced ptosis compared to reducing immobility in the TST, while an NR2B antagonist (Ro 25-6981) and a glycine partial agonist (ACPC) were equipotent in both tests.

CONCLUSION

The ability to reverse tetrabenazine-induced ptosis is a class effect of NMDAR antagonists. These findings are consistent with the hypothesis that the inability of memantine, AZD6765 (lanicemine) and MK-0657 to reproduce the rapid and robust antidepressant effects of ketamine in the clinic result from insufficient dosing rather than a difference in mechanism of action among these NMDAR antagonists.

Ketamine as a novel treatment for major depressive disorder and bipolar depression: a systematic review and quantitative meta-analysis

OBJECTIVE

Given the significant disability, morbidity and mortality associated with depression, the promising recent trials of ketamine highlight a novel intervention. A meta-analysis was conducted to assess the efficacy of ketamine in comparison with placebo for the reduction of depressive symptoms in patients who meet criteria for a major depressive episode.

METHOD

Two electronic databases were searched in September 2013 for English-language studies that were randomized placebo-controlled trials of ketamine treatment for patients with major depressive disorder or bipolar depression and utilized a standardized rating scale. Studies including participants receiving electroconvulsive therapy and adolescent/child participants were excluded. Five studies were included in the quantitative meta-analysis.

RESULTS

The quantitative meta-analysis showed that ketamine significantly reduced depressive symptoms. The overall effect size at day 1 was large and statistically significant with an overall standardized mean difference of 1.01 (95% confidence interval 0.69-1.34) (P<.001), with the effects sustained at 7 days postinfusion. The heterogeneity of the studies was low and not statistically significant, and the funnel plot showed no publication bias.

CONCLUSIONS

The large and statistically significant effect of ketamine on depressive symptoms supports a promising, new and effective pharmacotherapy with rapid onset, high efficacy and good tolerability.

Ketamine and other potential glutamate antidepressants

The need for rapid acting antidepressants is widely recognised. There has been much interest in glutamate mechanisms in major depressive disorder (MDD) as a promising target for the development of new antidepressants. A single intravenous infusion of ketamine, a N-methyl-d-aspartate (NMDA) receptor antagonist anaesthetic agent, can alleviate depressive symptoms in patients within hours of administration. The mechanism of action appears to be in part through glutamate release onto non-NMDA receptors including α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and metabotropic receptors. However these are also reported effects on 5-HT, dopamine and intracellular effects on the mammalian target of rapamycin (mTOR) pathway. The effects of SSRI (Selective Serotonin Reuptake Inhibitor) antidepressants may also involve alterations in NMDA function. The article reviews the effect of current antidepressants on NMDA and examines the efficacy and mechanism of ketamine. Response to ketamine is also discussed and comparison with other glutamate drugs including lamotrigine, amantadine, riluzole, memantine, traxoprodil, GLYX-13, MK-0657, RO4917523, AZD2066 and Coluracetam. Future studies need to link the rapid antidepressant effects seen with ketamine to inflammatory theories in MDD.

Comparison of antidepressant-like and abuse-related effects of phencyclidine in rats

Preclinical Research N-methyl-D-aspartate (NMDA) receptor antagonists, such as ketamine, have emerged as novel candidate treatments for major depressive disorder, but abuse potential of these agents is a concern. The NMDA antagonist phencyclidine has known abuse liability but undefined efficacy as an antidepressant. To further evaluate the relationship between antidepressant-like and abuse-related effects of NMDA antagonists, this study evaluated the effects of phencyclidine (1.0-10.0 mg/kg) in male Sprague-Dawley rats responding under two procedures that have been used to assess antidepressant-like effects (differential-reinforcement-of-low-rate [DRL] 72 s schedule of food reinforcement; n = 9) and abuse-related drug effects (intracranial self-stimulation [ICSS]; n = 6). Under the DRL 72 s schedule, phencyclidine (10.0 mg/kg) increased reinforcers and decreased responses without shifting the peak location of the interresponse time (IRT) distribution. Ketamine (10.0 mg/kg) also increased reinforcers and decreased responses, but unlike phencyclidine, it produced a rightward shift in the peak location of the IRT distribution. The 10.0 mg/kg phencyclidine dose that decreased DRL 72 s responding also decreased rates of ICSS for 50 min after its administration; however, abuse-related ICSS facilitation was observed at later times (100-300 min) or after a lower phencyclidine dose (3.2 mg/kg). These results suggest that phencyclidine produces weaker antidepressant-like effects, but stronger abuse-related effects than ketamine in these procedures.

The role of NMDA receptors in the pathophysiology and treatment of mood disorders

Mood disorders such as major depressive disorder and bipolar disorder are chronic and recurrent illnesses that cause significant disability and affect approximately 350 million people worldwide. Currently available biogenic amine treatments provide relief for many and yet fail to ameliorate symptoms for others, highlighting the need to diversify the search for new therapeutic strategies. Here we present recent evidence implicating the role of N-methyl-D-aspartate receptor (NMDAR) signaling in the pathophysiology of mood disorders. The possible role of NMDARs in mood disorders has been supported by evidence demonstrating that: (i) both BPD and MDD are characterized by altered levels of central excitatory neurotransmitters; (ii) NMDAR expression, distribution, and function are atypical in patients with mood disorders; (iii) NMDAR modulators show positive therapeutic effects in BPD and MDD patients; and (iv) conventional antidepressants/mood stabilizers can modulate NMDAR function. Taken together, this evidence suggests the NMDAR system holds considerable promise as a therapeutic target for developing next generation drugs that may provide more rapid onset relief of symptoms. Identifying the subcircuits involved in mood and elucidating the role of NMDARs subtypes in specific brain circuits would constitute an important step toward the development of more effective therapies with fewer side effects.

Mechanisms underlying the antidepressant response and treatment resistance

Depression is a complex and heterogeneous disorder affecting millions of Americans. There are several different medications and other treatments that are available and effective for many patients with depression. However, a substantial percentage of patients fail to achieve remission with these currently available interventions, and relapse rates are high. Therefore, it is necessary to determine both the mechanisms underlying the antidepressant response and the differences between responders and non-responders to treatment. Delineation of these mechanisms largely relies on experiments that utilize animal models. Therefore, this review provides an overview of the various mouse models that are currently used to assess the antidepressant response, such as chronic mild stress, social defeat, and chronic corticosterone. We discuss how these mouse models can be used to advance our understanding of the differences between responders and non-responders to antidepressant treatment. We also provide an overview of experimental treatment modalities that are used for treatment-resistant depression, such as deep brain stimulation and ketamine administration. We will then review the various genetic polymorphisms and transgenic mice that display resistance to antidepressant treatment. Finally, we synthesize the published data to describe a potential neural circuit underlying the antidepressant response and treatment resistance.

Parallels between major depressive disorder and Alzheimer's disease: role of oxidative stress and genetic vulnerability

The thesis of this review is that oxidative stress is the central factor in major depressive disorder (MDD) and Alzheimer's disease (AD). The major elements involved are inflammatory cytokines, the hypothalamic-pituitary axis, the hypothalamic-pituitary gonadal, and arginine vasopressin systems, which induce glucocorticoid and "oxidopamatergic" cascades when triggered by psychosocial stress, severe life-threatening events, and mental-affective and somatic diseases. In individuals with a genomic vulnerability to depression, these cascades may result in chronic depression-anxiety-stress spectra, resulting in MDD and other known depressive syndromes. In contrast, in subjects with genomic vulnerability to AD, oxidative stress-induced brain damage triggers specific antioxidant defenses, i.e., increased levels of amyloid-β (Aβ) and aggregation of hyper-phosphorylated tau, resulting in paired helical filaments and impaired functions related to the ApoEε4 isoform, leading to complex pathological cascades culminating in AD. Surprisingly, all the AD-associated molecular pathways mentioned in this review have been shown to be similar or analogous to those found in depression, including structural damage, i.e., hippocampal and frontal cortex atrophy. Other interacting molecular signals, i.e., GSK-3β, convergent survival factors (brain-derived neurotrophic factor and heat shock proteins), and transition redox metals are also mentioned to emphasize the vast array of intermediates that could interact via comparable mechanisms in both MDD and AD.

Preclinical evidence of rapid-onset antidepressant-like effect in Radix Polygalae extract

Radix Polygalae (the root of Polygala tenuifolia) is a herb widely used in traditional Asian medicine that is thought to exert a variety of neuropsychiatric effects. Radix Polygalae extract can protect against N-methyl D-aspartate (NMDA) neurotoxicity and induce brain-derived neurotrophic factor (BDNF) expression, suggesting modulatory roles at glutamatergic synapses and possible antidepressant action. In accordance with this hypothesis, Radix Polygalae extract demonstrated antidepressant-like effects in 8-week-old male C57Bl/6 mice by decreasing behavioral despair in the forced swim and tail suspension tasks and increasing hedonic-like behavior in the female urine sniffing test 30 minutes after a single oral administration of 0.1 mg/kg. Reduced latency to acquire a food pellet in the novely suppressed feeding paradigm, without change in anxiety-like behaviors suggested a rapid-onset nature of the antidepressant-like effect. In addition, it decreased the number of failed escapes in the learned helplessness paradigm after two oral administrations 24 hours and 30 minutes before the first test. Finally, it reversed anhedonia as measured by saccharin preference in mice exposed to the chronic stress model after two administrations of 0.1 mg/kg, in contrast to the repeated administration generally needed for similar effect by monoamergic antidepressants. Immobility reduction in tail suspension task was blocked by the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist NBQX, a pattern previously demonstrated by ketamine and other ketamine-like rapid-onset antidepressants. Also similarly to ketamine, Radix Polygalae appeared to acutely decrease phosphorylation of GluR1 serine-845 in the hippocampus while leaving the phosphorylation of hippocampal mTOR serine 2448 unchanged. These findings serve as preclinical evidence that Radix Polygalae extract exerts rapid-onset antidepressant effects by modulating glutamatergic synapses in critical brain circuits of depression and may be worthy of further evaluation as a safe substitute to other rapid-onset antidepressants known to have unacceptable side effects.

Neural correlates of rapid antidepressant response to ketamine in treatment-resistant unipolar depression: a preliminary positron emission tomography study

BACKGROUND

Multiple lines of evidence support a role for the glutamatergic system in the pathophysiology of major depressive disorder (MDD). Ketamine, an N-methyl-D-aspartate antagonist, rapidly improves depressive symptoms in individuals with treatment-resistant depression. The neural mechanisms underlying this effect remain unknown.

METHODS

In this preliminary study, 20 unmedicated participants with treatment-resistant MDD underwent positron emission tomography to measure regional cerebral glucose metabolism at baseline and following ketamine infusion (single dose of .5mg/kg intravenous over 40minutes). Metabolic data were compared between conditions using a combination of region-of-interest and voxelwise analyses, and differences were correlated with the associated antidepressant response.

RESULTS

Whole-brain metabolism did not change significantly following ketamine. Regional metabolism decreased significantly under ketamine in the habenula, insula, and ventrolateral and dorsolateral prefrontal cortices of the right hemisphere. Metabolism increased postketamine in bilateral occipital, right sensorimotor, left parahippocampal, and left inferior parietal cortices. Improvement in depression ratings correlated directly with change in metabolism in right superior and middle temporal gyri. Conversely, clinical improvement correlated inversely with metabolic changes in right parahippocampal gyrus and temporoparietal cortex.

CONCLUSIONS

Although preliminary, these results indicate that treatment-resistant MDD subjects showed decreased metabolism in the right habenula and the extended medial and orbital prefrontal networks in association with rapid antidepressant response to ketamine. Conversely, metabolism increased in sensory association cortices, conceivably related to the illusory phenomena sometimes experienced with ketamine. Further studies are needed to elucidate how these functional anatomical changes relate to the molecular mechanisms underlying ketamine's rapid antidepressant effects.

Memantine for late-life depression and apathy after a disabling medical event: a 12-week, double-blind placebo-controlled pilot study

OBJECTIVE

Preclinical data suggests that memantine, a noncompetitive glutamate N-methyl- D-aspartate-receptor blocker used for the treatment of moderate to severe Alzheimer's disease, could reduce depressive and amotivated behavior occurring in the context of psychosocial stress. Therefore, we examined whether memantine could reduce depressive symptoms and amotivation manifesting in older adults after a disabling medical event, thereby improving their functional recovery.

METHOD

We recruited subjects aged 60 years and older who had recently suffered a disabling medical event and were admitted to a skilled nursing facility for rehabilitation. Participants with significant depressive symptoms, defined as a Hamilton Rating Scale for Depression score of 10 or greater, and/or significant apathy symptoms, defined as an Apathy Evaluation Scale score of 40 or greater, were randomized to memantine (10 mg/d for 1 week, then 10 mg twice daily) or placebo, for 12 weeks. We also recruited participants without depressive or apathy symptoms for naturalistic follow-up as a non-depressed comparison group. Depressive and apathy symptoms were main outcomes; functional recovery, and self-report rating of helplessness, and onset of new depressive disorders were secondary outcomes.

RESULTS

Thirty-five older adults with significant depressive and/or apathy symptoms were randomized, of whom 27 (77.1%) completed the 12 week randomized controlled trial. Both groups showed reduction in depressive symptoms (but no significant reduction in apathy symptoms) and improved function. However, there were no group differences between the memantine-randomized and placebo randomized participants on any outcome.

CONCLUSION

Memantine was not associated with superior affective or functional outcome compared with placebo in medically rehabilitating older adults with depressive and apathy symptoms.

Memantine treatment reverses anhedonia, normalizes corticosterone levels and increases BDNF levels in the prefrontal cortex induced by chronic mild stress in rats

Memantine is a N-methyl-D-aspartate (NMDA) receptor antagonist and several studies have pointed to the NMDA receptor antagonists as a potential therapeutic target for the treatment of depression. The present study was aimed to evaluate the behavioral and physiological effects of administration of memantine in rats exposed to the chronic mild stress (CMS) model. To this aim, after 40 days of exposure to CMS procedure, rats were treated with memantine (20 mg/kg) for 7 days. In this study, sweet food consumption, adrenal gland weight, corticosterone levels, and brain-derived-neurotrophic factor (BDNF) protein levels in the prefrontal cortex, hippocampus and amygdala were assessed. Our results demonstrated that chronic stressful situations induced anhedonia, hypertrophy of adrenal gland weight, and an increase of corticosterone levels in rats, but did not alter BDNF protein levels in the rat brain. Memantine treatment reversed anhedonia and the increase of adrenal gland weight, normalized corticosterone levels and increased BDNF protein levels in the prefrontal cortex in stressed rats. Finally, these findings further support the hypothesis that NMDA receptor antagonists such as memantine could be helpful in the pharmacological treatment of depression.

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

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

Depressive-like behavior in adrenocorticotropic hormone-treated rats blocked by memantine

Hyperactivity of the hypothalamic pituitary-adrenal (HPA) axis plays a role in the pathophysiology of major depressive disorder (MDD). Recent studies suggest the role of the glutamatergic system in the pathophysiology of MDD, and N-methyl-D-aspartate (NMDA) receptor antagonists have shown antidepressant effects in both preclinical and clinical studies. However, little is known about the role of adrenocorticotropic hormone (ACTH) specifically in the glutamatergic response to HPA axis activation. Glutamate is an NMDA receptor agonist, and glycine and D-serine act as co-agonists. Here, we measured brain concentrations of these amino acids in rats given repeated administration of ACTH (100 μg/rat/day, sc, for 14 days). Further, we also evaluated behavioral effects of memantine, a non-competitive NMDA antagonist, on immobility time in the forced swimming test and on locomotor activity in ACTH-treated rats. Compared with control rats, glutamine, glycine, L-serine, and D-serine levels were increased in the hippocampus of ACTH-treated rats; glutamate, glutamine, glycine, L-serine, and D-serine were increased in the cerebellum; and glutamine and glycine were increased in the frontal cortex and striatum, all with statistical significance. Remarkably, these increases in agonists and co-agonists might have led to the augmentation of NMDA receptor activity. ACTH treatment increased immobility time in the forced swimming test and decreased locomotor activity in rats. On the contrary, memantine (10 mg/kg, ip) significantly decreased immobility time in the forced swimming test and increased locomotor activity in ACTH-treated rats. Furthermore, imipramine (15 mg/kg, ip) did not alter immobility time in the forced swimming test whereas this drug significantly decreased locomotor activity in ACTH-treated rats. These results suggest that depressive-like behaviors by chronic ACTH treatment could be blocked by memantine.

Immunohistochemical evidence for impaired nitric oxide signaling of the locus coeruleus in bipolar disorder

Nitric oxide (NO) is an important messenger in brain signaling and influences the balance of monoaminergic and glutamatergic neurotransmission. Alterations of NO signaling are thought to play a crucial role in the pathophysiology of mood disorders. The locus coeruleus (LC) comprises the largest group of norepinephrine containing neurons in the mammalian brain. These norepinephrinergic LC neurons are able to generate NO. Immunohistochemical staining of neuronal nitric oxide synthase (nNOS)-immunoreactive (ir) neurons was performed in the LC of the brains of 10 patients with bipolar I disorder (BD), 8 patients with major depressive disorder (MDD) and 16 control cases (C). Analysis of variance (ANOVA) revealed significant differences between the groups, and post hoc tests indicated a lower nNOS-ir neuron number in bipolar patients than in controls (left -34%, right -17%). The total number of Nissl-stained LC neurons showed no changes between major depressive disorder patients, bipolar patients and controls. In the mood disorder patients, illness duration correlated negatively with nNOS-ir neuronal number (r=-0.74, p=0.002). A reduced relative amount of NO in the LC of bipolar patients is likely a result of a compensation for increased glutamatergic activity. The current data on nNOS suggest a dysregulation of the nitrergic system in bipolar disorder. Future studies may clarify the potential role of glial cells in the context of the described nNOS deficit.