Patch clamp studies on the kinetics and selectivity of N-methyl-D-aspartate receptor antagonism by memantine (1-amino-3,5-dimethyladamantan)

The impact of chelating compounds on Cu2+, Fe2+/3+, and Zn2+ ions in Alzheimer's disease treatment

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by the accumulation of amyloid - β extracellular plaques and tau interfibrillar tangles, leading to memory loss, cognitive decline, and behavioral changes. With dementia posing a growing global health concern, there is an urgent need for comprehensive strategies to address its challenges. The economic burden of dementia is projected to rise significantly, emphasizing the necessity for collaborative efforts in research and healthcare. In the United States alone, millions are affected by AD, with prevalence increasing with age and even affecting younger individuals. The complexity of AD involves intricate biological processes, including the aggregation of amyloid beta, oxidative stress, and metal ion dysregulation. Metal ions, particularly those from copper, iron, and zinc, play pivotal roles in AD pathology, influencing Aβ deposition and tau protein accumulation. Current treatments offer symptomatic relief but do not address the underlying disease mechanisms. This paper explores the potential of various chelating compounds to target metal ions involved in AD pathology. N-acylhydrazones, morpholine, chrysin, quinoline, oxindole, cyclam, catechol-based, and quinazolinone-based derivatives show promising chelation activity and therapeutic effects. Metal chelation therapy offers a targeted approach to AD treatment by addressing the core pathology. By selectively binding to metal ions implicated in disease progression, chelators may minimize side effects associated with broad-spectrum treatments. Additionally, chelators may offer neuroprotective effects beyond metal binding, further enhancing their therapeutic potential. Overall, metal chelation therapy presents a promising strategy in combating AD, with the potential to significantly impact disease progression and improve patient outcomes.

Memantine Inhibits Calcium-Permeable AMPA Receptors

Memantine is an US Food and Drug Administration (FDA) approved drug that selectively inhibits NMDA-subtype ionotropic glutamate receptors (NMDARs) for treatment of dementia and Alzheimer's. NMDARs enable calcium influx into neurons and are critical for normal brain function. However, increasing evidence shows that calcium influx in neurological diseases is augmented by calcium-permeable AMPA-subtype ionotropic glutamate receptors (AMPARs). Here, we demonstrate that these calcium-permeable AMPARs (CP-AMPARs) are inhibited by memantine. Electrophysiology unveils that memantine inhibition of CP-AMPARs is dependent on their calcium permeability and the presence of their neuronal auxiliary subunit transmembrane AMPAR regulatory proteins (TARPs). Through cryo-electron microscopy we elucidate that memantine blocks CP-AMPAR ion channels in a unique mechanism of action from NMDARs. Furthermore, we demonstrate that memantine reverses a gain of function AMPAR mutation found in a patient with a neurodevelopmental disorder and inhibits CP-AMPARs in nerve injury. Our findings alter the paradigm for the memantine mechanism of action and provide a blueprint for therapeutic approaches targeting CP-AMPARs.

Memantine: Updating a rare success story in pro-cognitive therapeutics

The great potential for NMDA receptor modulators as druggable targets in neurodegenerative disorders has been met with limited success. Considered one of the rare exceptions, memantine has consistently demonstrated restorative and prophylactic properties in many AD models. In clinical trials memantine slows the decline in cognitive performance associated with AD. Here, we provide an overview of the basic properties including pharmacological targets, toxicology and cellular effects of memantine. Evidence demonstrating reductions in molecular, physiological and behavioural indices of AD-like impairments associated with memantine treatment are also discussed. This represents both an extension and homage to Dr. Chris Parson's considerable contributions to our fundamental understanding of a success story in the AD treatment landscape.

Emerging Alzheimer's disease therapeutics: promising insights from lipid metabolism and microglia-focused interventions

More than 55 million people suffer from dementia, with this number projected to double every 20 years. In the United States, 1 in 3 aged individuals dies from Alzheimer's disease (AD) or another type of dementia and AD kills more individuals than breast cancer and prostate cancer combined. AD is a complex and multifactorial disease involving amyloid plaque and neurofibrillary tangle formation, glial cell dysfunction, and lipid droplet accumulation (among other pathologies), ultimately leading to neurodegeneration and neuronal death. Unfortunately, the current FDA-approved therapeutics do not reverse nor halt AD. While recently approved amyloid-targeting antibodies can slow AD progression to improve outcomes for some patients, they are associated with adverse side effects, may have a narrow therapeutic window, and are expensive. In this review, we evaluate current and emerging AD therapeutics in preclinical and clinical development and provide insight into emerging strategies that target brain lipid metabolism and microglial function - an approach that may synergistically target multiple mechanisms that drive AD neuropathogenesis. Overall, we evaluate whether these disease-modifying emerging therapeutics hold promise as interventions that may be able to reverse or halt AD progression.

Computational Modeling of Therapy with the NMDA Antagonist in Neurodegenerative Disease: Information Theory in the Mechanism of Action of Memantine

(1) Background: in patients with neurodegenerative diseases, noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists provide neuroprotective advantages. We performed memantine therapy and proved mathematical and computer modeling of neurodegenerative disease in this study. (2) Methods: a computer simulation environment of the N-methyl-D-aspartate receptor incorporating biological mechanisms of channel activation by high extracellular glutamic acid concentration. In comparison to controls, pathological models were essentially treated with doses of memantine 3−30 µM. (3) Results: the mean values and 95% CI for Shannon entropy in Alzheimer’s disease (AD) and memantine treatment models were 1.760 (95% CI, 1.704−1.818) vs. 2.385 (95% CI, 2.280−2.490). The Shannon entropy was significantly higher in the memantine treatment model relative to AD model (p = 0.0162). The mean values and 95% CI for the positive Lyapunov exponent in AD and memantine treatment models were 0.125 (95% CI, NE−NE) vs. 0.058 (95% CI, 0.044−0.073). The positive Lyapunov exponent was significantly higher in the AD model relative to the memantine treatment model (p = 0.0091). The mean values and 95% CI for transfer entropy in AD and memantine treatment models were 0.081 (95% CI, 0.048−0.114) vs. 0.040 (95% CI, 0.019−0.062). The transfer entropy was significantly higher in the AD model relative to the memantine treatment model (p = 0.0146). A correlation analysis showed positive and statistically significant correlations of the memantine concentrations and the positive Lyapunov exponent (correlation coefficient R = 0.87, p = 0.0023) and transfer entropy (TE) (correlation coefficient R = 0.99, p < 0.000001). (4) Conclusions: information theory results of simulation studies show that the NMDA antagonist, memantine, causes neuroprotective benefits in patients with AD. Our simulation study opens up remarkable new scenarios in which a medical product, drug, or device, can be developed and tested for efficacy based on parameters of information theory.

The Computer Simulation of Therapy with the NMDA Antagonist in Excitotoxic Neurodegeneration in an Alzheimer's Disease-like Pathology

(1) Background: The use of uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists results in neuroprotective benefits in patients with moderate to severe Alzheimer’s disease. In this study, we demonstrated mathematical and computer modelling of the excitotoxicity phenomenon and performed virtual memantine therapy. (2) Methods: A computer simulation environment of the N-methyl-D-aspartate receptor combining biological mechanisms of channel activation by means of excessive extracellular glutamic acid concentration in three models of excitotoxicity severity. The simulation model is based on sliding register tables, where each table is associated with corresponding synaptic inputs. Modelling of the increase in extracellular glutamate concentration, through over-stimulation of NMDA receptors and exacerbation of excitotoxicity, is performed by gradually increasing the parameters of phenomenological events by the power function. Pathological models were virtually treated with 3−30 µM doses of memantine compared to controls. (3) Results: The virtual therapy results of memantine at doses of 3−30 µM in the pathological models of excitotoxicity severity show statistically significant neuroprotective benefits in AD patients with moderate severity, 1.25 (95% CI, 1.18−1.32) vs. 1.76 (95% CI, 1.71−1.80) vs. 1.53 (95% CI, 1.48−1.59), (p < 0.001), to severe, 1.32 (95% CI, 1.12−1.53) vs. 1.77 (95% CI, 1.72−1.82) vs. 1.73 (95% CI, 1.68−1.79), (p < 0.001), in the area of effects on memory. A statistically significant benefit of memantine was demonstrated for all neuronal parameters in pathological models. In the mild severity model, a statistically significant increase in frequency was obtained relative to virtual memantine treatment with a dose of 3 µM, which was 23.5 Hz (95% CI, 15.5−28.4) vs. 38.8 Hz (95% CI, 34.0−43.6), (p < 0.0001). In the intermediate excitotoxicity severity model, a statistically significant increase in frequency was obtained relative to virtual memantine therapy with a 3 µM dose of 26.0 Hz (95% CI, 15.7−36.2) vs. 39.0 Hz (95% CI, 34.2−43.8) and a 10 µM dose of 26.0 Hz (95% CI, 15.7−36.2) vs. 30.9 Hz (95% CI, 26.4−35.4), (p < 0.0001). A statistically significant increase in frequency was obtained in the advanced excitotoxicity severity model as in the medium. (4) Conclusions: The NMDA antagonist memantine causes neuroprotective benefits in patients with moderate to severe AD. One of the most important benefits of memantine is the improvement of cognitive function and beneficial effects on memory. On the other hand, memantine provides only symptomatic and temporary support for AD patients. Memantine is prescribed in the US and Europe if a patient has moderate to severe AD. Memantine has also been approved for mild to moderate AD patients. However, its very modest effect provides motivation for further research into new drugs in AD. We are the first to present a mathematical model of the NMDA receptor that allows the simulation of excitotoxicity and virtual memantine therapy.

Memantine exerts neuroprotective effects by modulating α-synuclein transmission in a parkinsonian model

Ample evidence has demonstrated that α-Synuclein can propagate from one area of the brain to others via cell-to-cell transmission, which might be the underlying mechanism for pathological propagation and the disease progression of Parkinson's disease (PD). Recent reports have demonstrated cell surface receptor-mediated cell-to-cell transmission of α-synuclein. Memantine decreased the levels of internalized cytosolic α-synuclein and led to attenuation in α-synuclein-induced cell death. Specifically, memantine attenuated α-synuclein-induced expression of clathrin and EEA1, and increased expression of NR2A subunits. Moreover, memantine inhibited propagation of extracellular α-synuclein and thus, decreased the expression of the phosphorylated form of α-synuclein in dopaminergic neurons of the substantia nigra, which was accompanied by increased survival of dopaminergic neurons with functional improvement of motor deficits. The present study demonstrated that memantine modulates extracellular α-synuclein propagation by inhibiting interactions between α-synuclein and NR2A subunits, which leads to neuroprotective effects on nigral dopaminergic neurons against α-synuclein-enriched conditions. The repositioning use of memantine in α-synuclein propagation needs to be further evaluated in patients with α-synucleinopathies as an effective therapeutic approach.

The Biology and Pathobiology of Glutamatergic, Cholinergic, and Dopaminergic Signaling in the Aging Brain

The elderly population is growing worldwide, with important health and socioeconomic implications. Clinical and experimental studies on aging have uncovered numerous changes in the brain, such as decreased neurogenesis, increased synaptic defects, greater metabolic stress, and enhanced inflammation. These changes are associated with cognitive decline and neurobehavioral deficits. Although aging is not a disease, it is a significant risk factor for functional worsening, affective impairment, disease exaggeration, dementia, and general disease susceptibility. Conversely, life events related to mental stress and trauma can also lead to accelerated age-associated disorders and dementia. Here, we review human studies and studies on mice and rats, such as those modeling human neurodegenerative diseases, that have helped elucidate (1) the dynamics and mechanisms underlying the biological and pathological aging of the main projecting systems in the brain (glutamatergic, cholinergic, and dopaminergic) and (2) the effect of defective glutamatergic, cholinergic, and dopaminergic projection on disabilities associated with aging and neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. Detailed knowledge of the mechanisms of age-related diseases can be an important element in the development of effective ways of treatment. In this context, we briefly analyze which adverse changes associated with neurodegenerative diseases in the cholinergic, glutaminergic and dopaminergic systems could be targeted by therapeutic strategies developed as a result of our better understanding of these damaging mechanisms.

Synthesis, structure, and antiviral properties of novel 2-adamantyl-5-aryl-2H-tetrazoles

The reaction of 5-aryl-NH-tetrazoles with adamantan-1-ol in concentrated sulfuric acid proceeds regioselectively with the formation of the corresponding 2-adamantyl-5-aryl-2H-tetrazoles. Nitration of these compounds leads to 2-(adamantan-1-yl)-5-(3-nitroaryl)-2Htetrazoles. The structures and composition of the obtained novel 2-adamantyl-5-aryltetrazoles were proven by IR spectroscopy, ¹H and ¹³C NMR spectroscopy, high-resolution mass spectrometry, and also by X-ray structural analysis. According to the simultaneous thermal analysis data, the obtained compounds are thermally stable up to a temperature of about 150°C. In vitro studies have shown that some of the 2-adamantyl-5-aryltetrazoles exhibit moderate inhibitory activity against influenza A (H1N1) virus. The antiviral selectivity index (SI) of 2-[2-(adamantan-1-yl)-2H-tetrazol-5-yl]-6-bromo-4-nitroaniline is significantly higher (SI 11) than that of the reference drug rimantadine (SI 5).

Effect of memantine, an anti-Alzheimer's drug, on rodent microglial cells in vitro

The pathophysiology of Alzheimer's disease (AD) is related to neuroinflammatory responses mediated by microglia. Memantine, an antagonist of N-methyl-D-aspartate (NMDA) receptors used as an anti-Alzheimer's drug, protects from neuronal death accompanied by suppression of proliferation and activation of microglial cells in animal models of AD. However, it remains to be tested whether memantine can directly affect microglial cell function. In this study, we examined whether pretreatment with memantine affects intracellular NO and Ca2+ mobilization using DAF-2 and Fura-2 imaging, respectively, and tested the effects of memantine on phagocytic activity by human β-Amyloid (1-42) phagocytosis assay in rodent microglial cells. Pretreatment with memantine did not affect production of NO or intracellular Ca2+ elevation induced by TNF in rodent microglial cells. Pretreatment with memantine also did not affect the mRNA expression of pro-inflammatory (TNF, IL-1β, IL-6 and CD45) or anti-inflammatory (IL-10, TGF-β and arginase) phenotypes in rodent microglial cells. In addition, pretreatment with memantine did not affect the amount of human β-Amyloid (1-42) phagocytosed by rodent microglial cells. Moreover, we observed that pretreatment with memantine did not affect 11 major proteins, which mainly function in the phagocytosis and degradation of β-Amyloid (1-42), including TREM2, DAP12 and neprilysin in rodent microglial cells. To the best of our knowledge, this is the first report to suggest that memantine does not directly modulate intracellular NO and Ca2+ mobilization or phagocytic activity in rodent microglial cells. Considering the neuroinflammation hypothesis of AD, the results might be important to understand the effect of memantine in the brain.

A neuroactive steroid with a therapeutically interesting constellation of actions at GABAA and NMDA receptors

Neuroactive steroids are an ascendant class of treatment for neuropsychiatric illness. Effects on ligand-gated neurotransmitter receptors appear to be a major mechanism of action. Here we describe a neuroactive steroid with a unique constellation of receptor actions. MQ-221 is a sulfated, 3β-hydroxy neurosteroid analogue that inhibits NMDAR function but also potentiates GABAAR function, thereby exhibiting unusual but potentially clinically desirable effects. Although the compound also exhibited features of other sulfated steroids, namely activation-dependent inhibition of GABAAR function, net potentiation dominated under physiological conditions. Potentiation of GABAAR function was distinct from the mechanism governing potentiation by anesthetic neurosteroids. Inhibition of NMDAR function showed weaker channel activation dependence than pregnanolone sulfate (3α5βPS). MQ-221 was unique among four stereoisomers explored in the pattern of effects at GABAA and NMDARs. Taken together, MQ-221 may represent a new class of compound with unique psychoactive effects and beneficial prospects for treating neuropsychiatric disorders.

Effect of Memantine on Pentylenetetrazol-induced Seizures and EEG Profile in Animal Model of Cortical Malformation

Developmental cortical malformations (DCM) are one of the main causes of refractory epilepsy. Many are the mechanisms underlying the hyperexcitability in DCM, including the important contribution of N-methyl-D-aspartate receptors (NMDAR). NMDAR blockers are shown to abolish seizures and epileptiform activity. Memantine, a NMDAR antagonist used to treat Alzheimeŕs disease, has been recently investigated as a possible treatment for other neurological disorders. However, the effects on preventing or diminishing seizures are controversial. Here we aimed to evaluate the effects of memantine on pentylenetetrazole (PTZ)-induced seizures in the freeze-lesion (FL) model. Bilateral cortical microgyria were induced (FL) or not (Sham) in male Wistar neonate rats. At P30, subdural electrodes were implanted and 7 days later, video-EEG was recorded in animals receiving either memantine (FL-M or Sham-M) or saline (FL-S or Sham-S), followed by PTZ. Seizures were evaluated by video-EEG during one hour and scored according to Racine scale. The video-EEG analyses revealed that the number of seizures and the total duration of stage IV-V seizures developed during the 1 h-period increased after memantine application in all groups. The EEG power spectral density (PSD) analysis showed an increased PSD of pre-ictal delta in Sham-M animals and increased PSD of slow, middle and fast gamma oscillations after memantine injection that persists during the pre-ictal period in all groups. Our findings suggested that memantine was unable to control the PTZ-induced seizures and that the associated enhancement of PSD of gamma oscillations may contribute to the increased probability of seizure development in these animals.

Acute and chronic effects of single dose memantine after controlled cortical impact injury in adult rats

BACKGROUND

Altered glutamatergic neurotransmission after traumatic brain injury (TBI) contributes to excitotoxic cell damage and death. Prevention or suppression of such changes is a desirable goal for treatment of TBI. Memantine (3,5-dimethyl-1-adamantanamine), an uncompetitive NMDA receptor antagonist with voltage-dependent open channel blocking kinetics, was reported to be neuroprotective in preclinical models of excitotoxicity, brain ischemia, and in TBI when administered prophylactically, immediately, or within minutes after injury.

METHODS

The current study examined effects of memantine administered by single intraperitoneal injection to adult male rats at a more clinically relevant delay of one hour after moderate-severe controlled cortical impact (CCI) injury or sham surgery. Histopathology was assessed on days 1, 7, 21, and 90, vestibulomotor function (beam balance and beam walk) was assessed on days 1-5 and 71-75, and spatial memory (Morris water maze test, MWM) was assessed on days 14-21 and 83-90 after CCI injury or sham surgery.

RESULTS

When administered at 10 mg/kg, but not 2.5 or 5 mg/kg, memantine preserved cortical tissue and reduced neuronal degeneration 1 day after injury, and attenuated loss of synaptophysin immunoreactivity in the hippocampus 7 days after injury. No effects of 10 mg/kg memantine were observed on histopathology at 21 and 90 days after CCI injury or sham surgery, or on vestibulomotor function and spatial memory acquisition assessed during any of the testing periods. However, 10 mg/kg memantine resulted in trends for improved search strategy in the MWM memory retention probe trial.

CONCLUSIONS

Administration of memantine at a clinically-relevant delay after moderate-severe CCI injury has beneficial effects on acute outcomes, while more significant improvement on subacute and chronic outcomes may require repeated drug administration or its combination with another therapy.

NMDA-type glutamate receptors mediate the acrosome reaction and motility initiation in newt sperm

The N-methyl d-aspartate type glutamate receptor (NMDAR) is a ligand-gated cation channel that causes Ca²⁺ influx in nerve cells. An NMDAR agonist is effective to the sperm motility in fowls, although the actual role of NMDAR in sperm function is unknown. In the present study, RNA-seq of the spermatogenic testes suggested the presence of NMDAR in the sperm of the newt Cynops pyrrhogaster. Glutamate of at least 0.7 ± 0.5 mM was detected in the egg-jelly substances along with acrosome reaction-inducing substance (ARIS) and sperm motility-initiating substance (SMIS). In the egg-jelly extract (JE) that included the ARIS and SMIS, the acrosome reaction was inhibited by a NMDAR antagonists, memantine and MK801. MK801 also inhibited the spontaneous acrosome reaction in Steinberg's salt solution (ST). Furthermore, memantine and MK801 suppressed the progressive motility of the sperm in JE and spontaneous waving of the undulating membrane, which is the tail structure giving thrust for forward motility, in ST. The spontaneous waving of the undulating membrane was promoted when Mg²⁺ , which blocks Ca²⁺ influx through gated NMDARs, was removed from the ST. In addition, the ARIS-induced acrosome reaction was inhibited by a selective antagonist of the transient receptor potential vanilloid 4, whose activation might result in the membrane depolarization to release Mg²⁺ from the NMDAR. These results suggest that NMDAR acts together with other cation channels in the induction of the acrosome reaction and motility of the sperm during the fertilization process of C. pyrrhogaster.

Memantine ameliorates learning and memory disturbance and the behavioral and psychological symptoms of dementia in thiamine-deficient mice

Several studies have reported on the beneficial effects of memantine on behavioral and psychological symptoms of dementia (BPSD) in patients with Alzheimer's disease. However, the effects of memantine on BPSD-like behaviors in animals have not been well addressed. Here, the effects of memantine on memory disturbance and BPSD-like behaviors were evaluated in thiamine-deficient (TD) mice. Memantine (3 and 10 mg/kg, b.i.d.) was orally administered to ddY mice fed a TD diet for 22 days. During the treatment period, the forced swimming test, elevated plus-maze test, passive avoidance test, and locomotor activity test were performed. Neurotransmitter levels in the brain were analyzed after the treatment period. Daily oral administration of memantine ameliorated the memory disturbances, anxiety-like behavior, and depression-like behavior observed in TD mice. Memantine did not have a significant effect on monoamine levels, but increased glutamate levels in the hippocampus in TD mice. These results suggest that memantine prevents or suppresses the progression of BPSD-like behaviors that develop due to TD. This effect may be mediated in part by the enhancement of glutamatergic neuron activity in the hippocampus.

Basic Study of Drug-Drug Interaction between Memantine and the Traditional Japanese Kampo Medicine Yokukansan

Several basic pharmacokinetic and pharmacological studies were conducted as part of a group of studies to clarify the drug-drug interaction (DDI) between memantine (MEM), a drug used to treat Alzheimer's disease, and yokukansan (YKS), a traditional Japanese Kampo medicine used to treat behavioral and psychological symptoms of dementia. The pharmacokinetic studies showed that there were no statistically significant differences in MEM concentrations in the plasma, brain, and urine between mice treated with MEM alone and with MEM plus YKS. Regarding candidate active ingredients of YKS, there were also no statistically significant differences in concentrations of geissoschizine methyl ether in the plasma and brain, urine, glycyrrhetinic acid in the plasma, and isoliquiritigenin in the urine, in mice treated with YKS alone or with MEM plus YKS. The pharmacological studies showed that isoliquiritigenin, which has an N-methyl-d-aspartic acid (NMDA) receptor antagonistic effect, did not affect the inhibitory effect of MEM on NMDA-induced intracellular Ca²⁺ influx in primary cultured rat cortical neurons. Moreover, YKS did not affect either the ameliorative effects of MEM on NMDA-induced learning and memory impairment, or the MEM-induced decrease in locomotor activities in mice. These results suggest that there is probably no pharmacokinetic or pharmacological interaction between MEM and YKS in mice, but more detailed studies are needed in the future. Our findings provide important information for future studies, to clarify the DDI more regarding the efficacy and safety of combined use of these drugs in a clinical situation.

Structure, function, and allosteric modulation of NMDA receptors

Hansen et al. review recent structural data that have provided insight into the function and allosteric modulation of NMDA receptors.

NMDA-type glutamate receptors are ligand-gated ion channels that mediate a Ca²⁺-permeable component of excitatory neurotransmission in the central nervous system (CNS). They are expressed throughout the CNS and play key physiological roles in synaptic function, such as synaptic plasticity, learning, and memory. NMDA receptors are also implicated in the pathophysiology of several CNS disorders and more recently have been identified as a locus for disease-associated genomic variation. NMDA receptors exist as a diverse array of subtypes formed by variation in assembly of seven subunits (GluN1, GluN2A-D, and GluN3A-B) into tetrameric receptor complexes. These NMDA receptor subtypes show unique structural features that account for their distinct functional and pharmacological properties allowing precise tuning of their physiological roles. Here, we review the relationship between NMDA receptor structure and function with an emphasis on emerging atomic resolution structures, which begin to explain unique features of this receptor.

Effects of Mg2+ on recovery of NMDA receptors from inhibition by memantine and ketamine reveal properties of a second site

Memantine and ketamine are NMDA receptor (NMDAR) open channel blockers that are thought to act via similar mechanisms at NMDARs, but exhibit divergent clinical effects. Both drugs act by entering open NMDARs and binding at a site deep within the ion channel (the deep site) at which the endogenous NMDAR channel blocker Mg2+ also binds. Under physiological conditions, Mg2+ increases the IC50s of memantine and ketamine through competition for binding at the deep site. Memantine also can inhibit NMDARs after associating with a second site accessible in the absence of agonist, a process termed second site inhibition (SSI) that is not observed with ketamine. Here we investigated the effects of 1 mM Mg2+ on recovery from inhibition by memantine and ketamine, and on memantine SSI, of the four main diheteromeric NMDAR subtypes. We found that: recovery from memantine inhibition depended strongly on the concentration of memantine used to inhibit the NMDAR response; Mg2+ accelerated recovery from memantine and ketamine inhibition through distinct mechanisms and in an NMDAR subtype-dependent manner; and Mg2+ occupation of the deep site disrupted memantine SSI in a subtype-dependent manner. Our results support the hypothesis that memantine associates with, but does not inhibit at the second site. After associating with the second site, memantine can either slowly dissociate directly to the extracellular solution, or transit to the deep site, resulting in typical channel block. Memantine's relatively slow dissociation from the second site underlies the dependence of NMDAR recovery from inhibition on both memantine concentration and on Mg2+.

Systematic review of the pharmacological agents that have been tested against spreading depolarizations

Spreading depolarization (SD) occurs alongside brain injuries and it can lead to neuronal damage. Therefore, pharmacological modulation of SD can constitute a therapeutic approach to reduce its detrimental effects and to improve the clinical outcome of patients. The major objective of this article was to produce a systematic review of all the drugs that have been tested against SD. Of the substances that have been examined, most have been shown to modulate certain SD characteristics. Only a few have succeeded in significantly inhibiting SD. We present a variety of strategies that have been proposed to overcome the notorious harmfulness and pharmacoresistance of SD. Information on clinically used anesthetic, sedative, hypnotic agents, anti-migraine drugs, anticonvulsants and various other substances have been compiled and reviewed with respect to the efficacy against SD, in order to answer the question of whether a drug at safe doses could be of therapeutic use against SD in humans.

Inhibition of NMDA Receptors Prevents the Loss of BDNF Function Induced by Amyloid β

Brain-derived neurotrophic factor (BDNF) plays important functions in cell survival and differentiation, neuronal outgrowth and plasticity. In Alzheimer’s disease (AD), BDNF signaling is known to be impaired, partially because amyloid β (Aβ) induces truncation of BDNF main receptor, TrkB-full length (TrkB-FL). We have previously shown that such truncation is mediated by calpains, results in the formation of an intracellular domain (ICD) fragment and causes BDNF loss of function. Since calpains are Ca²⁺-dependent proteases, we hypothesized that excessive intracellular Ca²⁺ build-up could be due to dysfunctional N-methyl-d-aspartate receptors (NMDARs) activation. To experimentally address this hypothesis, we investigated whether TrkB-FL truncation by calpains and consequent BDNF loss of function could be prevented by NMDAR blockade. We herein demonstrate that a NMDAR antagonist, memantine, prevented excessive calpain activation and TrkB-FL truncation induced by Aβ_25–35. When calpains were inhibited by calpastatin, BDNF was able to increase the dendritic spine density of neurons exposed to Aβ₂₅₁₃₅. Moreover, NMDAR inhibition by memantine also prevented Aβ-driven deleterious impact of BDNF loss of function on structural (spine density) and functional outcomes (synaptic potentiation). Collectively, these findings support NMDAR/Ca²⁺/calpains mechanistic involvement in Aβ-triggered BDNF signaling disruption.

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.

Classics in Chemical Neuroscience: Memantine

Memantine was the first breakthrough medication for the treatment of moderate to severe Alzheimer's disease (AD) patients and represents a fundamentally new mechanism of action (moderate-affinity, uncompetitive, voltage-dependent, N-methyl-d-aspartate (NMDA) receptor antagonist that exhibits fast on/off kinetics) to modulate glutamatergic dysfunction. Since its approval by the FDA in 2003, memantine, alone and in combination with donepezil, has improved patient outcomes in terms of cognition, behavioral disturbances, daily functioning, and delaying time to institutionalization. In this review, we will highlight the historical significance of memantine to AD (and other neuropsychiatric disorders) as well as provide an overview of the synthesis, pharmacology, and drug metabolism of this unique NMDA uncompetitive antagonist that clearly secures its place among the Classics in Chemical Neuroscience.

Glioprotection of Retinal Astrocytes After Intravitreal Administration of Memantine in the Mouse Optic Nerve Crush Model

Background

In glaucoma, non-intraocular pressure (IOP)-related risk factors can result in increased levels of extracellular glutamate, which triggers a cascade of neurodegeneration characterized by the excessive activation of N-methyl-D-aspartate (NMDA). The purpose of our study was to evaluate the glioprotective effects of memantine as a prototypic uncompetitive NMDA blocker on retinal astrocytes in the optic nerve crush (ONC) mouse model for glaucoma.

Material/Methods

Optic nerve crush was performed on all of the right eyes (n=8), whereas left eyes served as contralateral healthy controls (n=8) in Balb/c/Sca mice. Four randomly assigned mice received 2-μl intravitreal injections of memantine (1 mg/ml) after ONC in the experimental eye. One week after the experiment, optic nerves were dissected and stained with methylene blue. Retinae were detached from the sclera. The tissue was immunostained. Whole-mount retinae were investigated by fluorescent microscopy. Astrocyte counts for each image were performed manually.

Results

Histological sections of crushed optic nerves showed consistently moderate tissue damage in experimental groups. The mean number of astrocytes per image in the ONC group was significantly lower than in the healthy control group (7.13±1.5 and 10.47±1.9, respectively). Loss of astrocytes in the memantine-treated group was significantly lower (8.83±2.2) than in the ONC group. Assessment of inter-observer reliability showed excellent agreement among observations in control, ONC, and memantine groups.

Conclusions

The ONC is an effective method for investigation of astrocytic changes in mouse retina. Intravitreally administered memantine shows a promising glioprotective effect on mouse retinal astrocytes by preserving astrocyte count after ONC.

Therapeutics of Neurotransmitters in Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease, characterized by the loss of memory, multiple cognitive impairments and changes in the personality and behavior. Several decades of intense research have revealed that multiple cellular changes are involved in disease process, including synaptic damage, mitochondrial abnormalities and inflammatory responses, in addition to formation and accumulation of amyloid-β (Aβ) and phosphorylated tau. Although tremendous progress has been made in understanding the impact of neurotransmitters in the progression and pathogenesis of AD, we still do not have a drug molecule associated with neurotransmitter(s) that can delay disease process in elderly individuals and/or restore cognitive functions in AD patients. The purpose of our article is to assess the latest developments in neurotransmitters research using cell and mouse models of AD. We also updated the current status of clinical trials using neurotransmitters' agonists/antagonists in AD.

NMDA Receptors in the Central Nervous System

NMDA-type glutamate receptors are ligand-gated ion channels that mediate a major component of excitatory neurotransmission in the central nervous system (CNS). They are widely distributed at all stages of development and are critically involved in normal brain functions, including neuronal development and synaptic plasticity. NMDA receptors are also implicated in the pathophysiology of numerous neurological and psychiatric disorders, such as ischemic stroke, traumatic brain injury, Alzheimer's disease, epilepsy, mood disorders, and schizophrenia. For these reasons, NMDA receptors have been intensively studied in the past several decades to elucidate their physiological roles and to advance them as therapeutic targets. Seven NMDA receptor subunits exist that assemble into a diverse array of tetrameric receptor complexes, which are differently regulated, have distinct regional and developmental expression, and possess a wide range of functional and pharmacological properties. The diversity in subunit composition creates NMDA receptor subtypes with distinct physiological roles across neuronal cell types and brain regions, and enables precise tuning of synaptic transmission. Here, we will review the relationship between NMDA receptor structure and function, the diversity and significance of NMDA receptor subtypes in the CNS, as well as principles and rules by which NMDA receptors operate in the CNS under normal and pathological conditions.

Memantine and Kynurenic Acid: Current Neuropharmacological Aspects

Glutamatergic neurotransmission, of special importance in the human brain, is implicated in key brain functions such as synaptic plasticity and memory. The excessive activation of N-methyl- D-aspartate (NMDA) receptors may result in excitotoxic neuronal damage; this process has been implicated in the pathomechanism of different neurodegenerative disorders, such as Alzheimer’s disease (AD). Memantine is an uncompetitive antagonist of NMDA receptors with a favorable pharmacokinetic profile, and is therefore clinically well tolerated. Memantine is approved for the treatment of AD, but may additionally be beneficial for other dementia forms and pain conditions. Kynurenic acid (KYNA) is an endogenous antagonist of NMDA receptors which has been demonstrated under experimental conditions to be neuroprotective. The development of a well-tolerated NMDA antagonist may offer a novel therapeutic option for the treatment of neurodegenerative disease and pain syndromes. KYNA may be a valuable candidate for future drug development.

Enhanced LTP in aged rats: Detrimental or compensatory?

Age-dependent memory deterioration has been well documented and yet an increase in rat hippocampal LTP upon aging has been reported. This poses the question of whether the enhanced LTP is a cause or an attempt to compensate the memory deficits described in aged rats. Hippocampal slices from young, adult and aged Wistar rats were pre-incubated, with an NMDA receptor (NMDAR) antagonist, memantine (1 μM, 4 h), and hippocampal LTP was evaluated. The results show that memantine significantly decreases the larger LTP magnitude recorded in hippocampal slices from aged rats without compromising LTP recorded in slices from young and adult animals. To unveil the impact of in vivo administration of memantine, different doses (1, 5 and 10 mg/kg/day) or saline vehicle solution were intraperitoneally administered, for 15-20 days, to both young and aged animals. Memantine did not significantly affect neither the place learning of young animals, evaluated by Morris Water Maze, nor LTP recorded from hippocampal slices from the same group of animals. However, memantine (5 and 10 mg/kg/day) significantly decreased the large LTP recorded in hippocampal slices from aged animals. Moreover, aged animals treated with memantine (10 mg/kg/day) showed a significantly compromised place learning when compared to aged control animals. Overall, these results suggest that the larger LTP observed in aged animals is a compensatory phenomenon, rather than pathological. The finding that age-dependent blockade of LTP by a NMDAR antagonist leads to learning deficits, implies that the increased LTP observed upon aging may be playing an important role in the learning process.

Amyotrophic Lateral Sclerosis, 2016: existing therapies and the ongoing search for neuroprotection

INTRODUCTION

Amyotrophic lateral sclerosis (ALS), one in a family of age-related neurodegenerative disorders, is marked by predominantly cryptogenic causes, partially elucidated pathophysiology, and elusive treatments. The challenges of ALS are illustrated by two decades of negative drug trials.

AREAS COVERED

In this article, we lay out the current understanding of disease genesis and physiology in relation to drug development in ALS, stressing important accomplishments and gaps in knowledge. We briefly consider clinical ALS, the ongoing search for biomarkers, and the latest in trial design, highlighting major recent and ongoing clinical trials; and we discuss, in a concluding section on future directions, the prion-protein hypothesis of neurodegeneration and what steps can be taken to end the drought that has characterized drug discovery in ALS.

EXPERT OPINION

Age-related neurodegenerative disorders are fast becoming major public health problems for the world's aging populations. Several agents offer promise in the near-term, but drug development is hampered by an interrelated cycle of obstacles surrounding etiological, physiological, and biomarkers discovery. It is time for the type of government-funded, public-supported offensive on neurodegenerative disease that has been effective in other fields.

The effect of memantine on sleep architecture and psychiatric symptoms in patients with Alzheimer's disease

OBJECTIVE

Behavioural and psychological symptoms of dementia (BPSD) are commonly present in patients with Alzheimer's disease (AD). Disturbed sleep quality is also observed in AD patients. However, the effects of memantine on sleep architecture have not been investigated. The purpose of this study was to investigate the effects of memantine on polysomnography (PSG) variables and BPSD.

METHODS

In total, 12 patients with AD (mean age: 79.0±4.1 years old) were enrolled in this study. The following tests were performed: the Neuropsychiatric Inventory for the assessment of BPSD, the Mini-Mental State Examination (MMSE) for cognitive function, and PSG for evaluation of sleep architecture. After baseline examinations, patients were treated with memantine according to a standard prescription protocol. After being treated with 20 mg/day of memantine for 4 weeks, examinations were carried out again.

RESULTS

All subjects completed the trial. The mean MMSE and NPI scores were 22.6±3.4 and 13.8±12.9, respectively. Treatment with memantine significantly decreased the NPI score (5.8±4.3, p<0.01). There were significant decreases in the scores of subscales for anxiety (p=0.04) and irritability/lability (p=0.04). PSG demonstrated a longer total sleep time (TST) (p<0.01), increases in sleep efficiency (p<0.01) and time spent in stage II (% TST, p=0.02), and decreases in nocturnal awakening (p<0.01), the periodic limb movement index (p<0.01), and time spent in stage I (% TST, p=0.02).

CONCLUSION

Memantine was effective for reducing fragmented sleep and improving BPSD, and was well tolerated.

Timosaponin derivative YY-23 acts as a non-competitive NMDA receptor antagonist and exerts a rapid antidepressant-like effect in mice

AIM

N-methyl-D-aspartic acid (NMDA) receptor modulators have shown promising results as potential antidepressant agents, whereas timosaponins extracted from the Chinese herb Rhizoma Anemarrhenae exhibit antidepressant activities. In the present study we examined whether YY-23, a modified metabolite of timosaponin B-III, could affect NMDA receptors in rat hippocampal neurons in vitro, and evaluated its antidepressant-like effects in stressed mice.

METHODS

NMDA-induced currents were recorded in acutely dissociated rat hippocampal CA1 neurons using a whole-cell recording technique. C57BL/6 mice were exposed to a 6-week chronic mild stress (CMS) or a 10-d chronic social defeat stress (CSDS). The stressed mice were treated with YY-23 (20 mg·kg(-1)·d(-1)) or a positive-control drug, fluoxetine (10 mg·kg(-1)·d(-1)) for 3 weeks. Behavioral assessments were carried out every week.

RESULTS

In acutely dissociated rat hippocampal CA1 neurons, YY-23 selectively and reversibly inhibited NMDA-induced currents with an EC50 value of 2.8 μmol/L. This inhibition of NMDA-induced currents by YY-23 was non-competitive, and had no features of voltage-dependency or use-dependency. Treatment of the stressed mice with YY-23 not only reversed CMS-induced deficiency of sucrose preference and immobility time, and CSDS-induced reduction of social interaction, but also had faster onset as compared to fluoxetine.

CONCLUSION

YY-23 is a novel non-competitive antagonist of NMDA receptors with promising rapid antidepressant-like effects in mouse models of CMS and CSDS depression.

Preclinical neuroprotective actions of xenon and possible implications for human therapeutics: a narrative review

PURPOSE

The purpose of this report is to facilitate an understanding of the possible application of xenon for neuroprotection in critical care settings. This narrative review appraises the literature assessing the efficacy and safety of xenon in preclinical models of acute ongoing neurologic injury.

SOURCE

Databases of the published literature (MEDLINE® and EMBASE™) were appraised for peer-reviewed manuscripts addressing the use of xenon in both preclinical models and disease states of acute ongoing neurologic injury. For randomized clinical trials not yet reported, the investigators' declarations in the National Institutes of Health clinical trials website were considered.

PRINCIPAL FINDINGS

While not a primary focus of this review, to date, xenon cannot be distinguished as superior for surgical anesthesia over existing alternatives in adults. Nevertheless, studies in a variety of preclinical disease models from multiple laboratories have consistently shown xenon's neuroprotective properties. These properties are enhanced in settings where xenon is combined with hypothermia. Small randomized clinical trials are underway to explore xenon's efficacy and safety in clinical settings of acute neurologic injury where hypothermia is the current standard of care.

CONCLUSION

According to the evidence to date, the neuroprotective efficacy of xenon in preclinical models and its safety in clinical anesthesia set the stage for the launch of randomized clinical trials to determine whether these encouraging neuroprotective findings can be translated into clinical utility.

Ketamine and phencyclidine: the good, the bad and the unexpected

The history of ketamine and phencyclidine from their development as potential clinical anaesthetics through drugs of abuse and animal models of schizophrenia to potential rapidly acting antidepressants is reviewed. The discovery in 1983 of the NMDA receptor antagonist property of ketamine and phencyclidine was a key step to understanding their pharmacology, including their psychotomimetic effects in man. This review describes the historical context and the course of that discovery and its expansion into other hallucinatory drugs. The relevance of these findings to modern hypotheses of schizophrenia and the implications for drug discovery are reviewed. The findings of the rapidly acting antidepressant effects of ketamine in man are discussed in relation to other glutamatergic mechanisms.

Actions of bupivacaine, a widely used local anesthetic, on NMDA receptor responses

NMDA receptors mediate excitatory neurotransmission in brain and spinal cord and play a pivotal role in the neurological disease state of chronic pain, which is caused by central sensitization. Bupivacaine is the indicated local anesthetic in caudal, epidural, and spinal anesthesia and is widely used clinically to manage acute and chronic pain. In addition to blocking Na(+) channels, bupivacaine affects the activity of many other channels, including NMDA receptors. Importantly, bupivacaine inhibits NMDA receptor-mediated synaptic transmission in the dorsal horn of the spinal cord, an area critically involved in central sensitization. We used recombinant NMDA receptors expressed in HEK293 cells and found that increasing concentrations of bupivacaine decreased channel open probability in GluN2 subunit- and pH-independent manner by increasing the mean duration of closures and decreasing the mean duration of openings. Using kinetic modeling of one-channel currents, we attributed the observed current decrease to two main mechanisms: a voltage-dependent "foot-in-the-door" pore block and an allosteric gating effect. Further, the inhibition was state-independent because it occurred to the same degree whether the drug was applied before or after glutamate stimulation and was mediated by extracellular and intracellular inhibitory sites, via hydrophilic and hydrophobic pathways. These results predict that clinical doses of bupivacaine would decrease the peak and accelerate the decay of synaptic NMDA receptor currents during normal synaptic transmission. These quantitative predictions inform possible applications of bupivacaine as preventative and therapeutic approaches in chronic pain.

Neurotransmitter-based strategies for the treatment of cognitive dysfunction in Down syndrome

Down syndrome (DS) is a multisystem disorder affecting the cardiovascular, respiratory, gastrointestinal, neurological, hematopoietic, and musculoskeletal systems and is characterized by significant cognitive disability and a possible common pathogenic mechanism with Alzheimer's disease. During the last decade, numerous studies have supported the notion that the triplication of specific genes on human chromosome 21 plays a significant role in cognitive dysfunction in DS. Here we reviewed studies in trisomic mouse models and humans, including children and adults with DS. In order to identify groups of genes that contribute to cognitive disability in DS, multiple mouse models of DS with segmental trisomy have been generated. Over-expression of these particular genes in DS can lead to dysfunction of several neurotransmitter systems. Therapeutic strategies for DS have either focused on normalizing the expression of triplicated genes with important roles in DS or restoring the function of these systems. Indeed, our extensive review of studies on the pathogenesis of DS suggests that one plausible strategy for the treatment of cognitive dysfunction is to target the cholinergic, serotonergic, GABA-ergic, glutamatergic, and norepinephrinergic system. However, a fundamental strategy for treatment of cognitive dysfunction in DS would include reducing to normal levels the expression of specific triplicated genes in affected systems before the onset of neurodegeneration.

Differentiation renders susceptibility to excitotoxicity in HT22 neurons

HT22 is an immortalized mouse hippocampal neuronal cell line that does not express cholinergic and glutamate receptors like mature hippocampal neurons in vivo. This in part prevents its use as a model for mature hippocampal neurons in memory-related studies. We now report that HT22 cells were appropriately induced to differentiate and possess properties similar to those of mature hippocampal neurons in vivo, such as becoming more glutamate-receptive and excitatory. Results showed that sensitivity of HT22 cells to glutamate-induced toxicity changed dramatically when comparing undifferentiated with differentiated cells, with the half-effective concentration for differentiated cells reducing approximately two orders of magnitude. Moreover, glutamate-induced toxicity in differentiated cells, but not undifferentiated cells, was inhibited by the N-methyl-D- aspartate receptor antagonists MK-801 and memantine. Evidently, differentiated HT22 cells expressed N-methyl-D-aspartate receptors, while undifferentiated cells did not. Our experimental findings indicated that differentiation is important for immortalized cell lines to render post-mitotic neuronal properties, and that differentiated HT22 neurons represent a better model of hippocampal neurons than undifferentiated cells.

N-methyl-d-aspartate glutamate receptor (NMDA-R) antibodies in mild cognitive impairment and dementias

The N-methyl-d-aspartate glutamate receptor (NMDA-R) plays a central role in learning and memory and has therefore a potential role in the pathophysiology of neuropsychiatric disorders. Recently, we detected NMDA-R autoantibodies in aged healthy volunteers without neuropsychiatric disorders. Since studies showing the involvement of NMDA-R antibodies in mild cognitive impairment and different forms of dementia are rare, we examined NMDA-R antibodies (Abs) in serum of 46 patients with Alzheimer's disease (AD), 26 patients with subcortical ischemic vascular dementia (SIVD), 18 patients with frontotemporal dementia (FTD), 11 patients with Lewy body disease (LBD) and 33 patients with mild cognitive impairment (MCI) and in 21 healthy aged, gender-matched volunteers. While IgM and/or IgA NMDA-R Abs were present in all groups, IgG was only detected in one AD sample. Seropositivity could be correlated with the presence of co-symptoms: MCI and AD patients suffering from depression and AD and SIVD patients with a psychosis were almost all NMDA-R Ab positive. We conclude that the presence of NMDA-R Abs in dementia could influence the incidence of comorbid depressive and/or psychotic states.

Different oxysterols have opposing actions at N-methyl-D-aspartate receptors

Oxysterols have emerged as important biomarkers in disease and as signaling molecules. We recently showed that the oxysterol 24(S)-hydroxycholesterol, the major brain cholesterol metabolite, potently and selectively enhances NMDA receptor function at a site distinct from other modulators. Here we further characterize the pharmacological mechanisms of 24(S)-hydroxycholesterol and its synthetic analog SGE201. We describe an oxysterol antagonist of this positive allosteric modulation, 25-hydroxycholesterol. We found that 24(S)-hydroxycholesterol and SGE201 primarily increased the efficacy of NMDAR agonists but did not directly gate the channel or increase functional receptor number. Rather than binding to a direct aqueous-accessible site, oxysterols may partition into the plasma membrane to access the NMDAR, likely explaining slow onset and offset kinetics of modulation. Interestingly, oxysterols were ineffective when applied to the cytosolic face of inside-out membrane patches or through a whole-cell pipette solution, suggesting a non-intracellular site. We also found that another natural oxysterol, 25-hydroxycholesterol, although exhibiting slight potentiation on its own, non-competitively and enantioselectively antagonized the effects of 24(S)-hydroxycholesterol analogs. In summary, we suggest two novel allosteric sites on NMDARs that separately modulate channel gating, but together oppose each other.

Binding of ArgTX-636 in the NMDA receptor ion channel

The N-methyl-d-aspartate receptors (NMDARs) constitute an important class of ligand-gated cation channels that are involved in the majority of excitatory neurotransmission in the human brain. Compounds that bind in the NMDAR ion channel and act as blockers are use- and voltage-dependent inhibitors of NMDAR activity and have therapeutic potential for treatment of a variety of brain diseases or as pharmacological tools for studies of the neurobiological role of NMDARs. We have performed a kinetic analysis of the blocking mechanism of the prototypical polyamine toxin NMDAR ion channel blocker argiotoxin-636 (ArgTX-636) at recombinant GluN1/2A receptors to provide detailed information on the mechanism of block. The predicted binding site of ArgTX-636 is in the pore region of the NMDAR ion channel formed by residues in the transmembrane M3 and the M2 pore-loop segments of the GluN1 and GluN2A subunits. To assess the predicted binding mode in further detail, we performed an alanine- and glycine-scanning mutational analysis of this pore-loop segment to systematically probe the role of pore-lining M2 residues in GluN1 and GluN2A in the channel block by ArgTX-636. Comparison of M2 positions in GluN1 and GluN2A where mutation influences ArgTX-636 potency suggests differential contribution of the M2-loops of GluN1 and GluN2A to binding of ArgTX-636. The results of the mutational analysis are highly relevant for the future structure-based development of argiotoxin-derived NMDAR channel blockers.

Central antinociception induced by ketamine is mediated by endogenous opioids and μ- and δ-opioid receptors

It is generally believed that NMDA receptor antagonism accounts for most of the anesthetic and analgesic effects of ketamine, however, it interacts at multiple sites in the central nervous system, including NMDA and non-NMDA glutamate receptors, nicotinic and muscarinic cholinergic receptors, and adrenergic and opioid receptors. Interestingly, it was shown that at supraspinal sites, ketamine interacts with the μ-opioid system and causes supraspinal antinociception. In this study, we investigated the involvement of endogenous opioids in ketamine-induced central antinociception. The nociceptive threshold for thermal stimulation was measured in Swiss mice using the tail-flick test. The drugs were administered via the intracerebroventricular route. Our results demonstrated that the opioid receptor antagonist naloxone, the μ-opioid receptor antagonist clocinnamox and the δ-opioid receptor antagonist naltrindole, but not the κ-opioid receptor antagonist nor-binaltorphimine, antagonized ketamine-induced central antinociception in a dose-dependent manner. Additionally, the administration of the aminopeptidase inhibitor bestatin significantly enhanced low-dose ketamine-induced central antinociception. These data provide evidence for the involvement of endogenous opioids and μ- and δ-opioid receptors in ketamine-induced central antinociception. In contrast, κ-opioid receptors not appear to be involved in this effect.

Administration of memantine during withdrawal mitigates overactivity and spatial learning impairments associated with neonatal alcohol exposure in rats

BACKGROUND

Prenatal alcohol exposure can disrupt central nervous system development, manifesting as behavioral deficits that include motor, emotional, and cognitive dysfunction. Both clinical and animal studies have reported binge drinking during development to be highly correlated with an increased risk of fetal alcohol spectrum disorders (FASD). We hypothesized that binge drinking may be especially damaging because it is associated with episodes of alcohol withdrawal. Specifically, we have been investigating the possibility that NMDA receptor-mediated excitotoxicity occurs during alcohol withdrawal and contributes to developmental alcohol-related neuropathology. Consistent with this hypothesis, administration of the NMDA receptor antagonists MK-801 or eliprodil during withdrawal attenuates behavioral alterations associated with early alcohol exposure. In this study, we investigated the effects of memantine, a clinically used NMDA receptor antagonist, on minimizing ethanol-induced overactivity and spatial learning deficits.

METHODS

Sprague-Dawley pups were exposed to 6.0 g/kg ethanol via intubation on postnatal day (PD) 6, a period of brain development that models late gestation in humans. Controls were intubated with a calorically matched maltose solution. During withdrawal, 24 and 36 hours after ethanol exposure, subjects were injected with a total of either 0, 20, or 30 mg/kg memantine. The subjects' locomotor levels were recorded in open field activity monitors on PDs 18 to 21 and on a serial spatial discrimination reversal learning task on PDs 40 to 43.

RESULTS

Alcohol exposure induced overactivity and impaired performance in spatial learning. Memantine administration significantly attenuated the ethanol-associated behavioral alterations in a dose-dependent manner. Thus, memantine may be neuroprotective when administered during ethanol withdrawal.

CONCLUSIONS

These data have important implications for the treatment of EtOH's neurotoxic effects and provide further support that ethanol withdrawal significantly contributes to FASD.

Neuropathology and treatment of Alzheimer disease: did we lose the forest for the trees?

Although amyloid-beta-containing senile plaques and phospho-tau containing neurofibrillary tangles are hallmark lesions of Alzheimer disease (AD), neither is specific for AD, nor even a marker of AD. Rather, they are empirical lesions that require close correlation with age and clinical signs for optimal interpretation. In essence, these lesions represent the effect rather than the cause of disease. In this review, we discuss diagnostic criteria for AD, the relationship between pathology, pathogenesis and multiple treatment approaches that have so far been disappointing, including those that presume to address pathological lesions. An acceptance that lesion-based therapies do not address etiology or rate-limiting pathogenic factors is probably necessary for the best chance of significant advances that have thus far been elusive.

Patch clamp combined with voltage/concentration clamp to determine the kinetics and voltage dependency of N-methyl-D-aspartate (NMDA) receptor open channel blockers

Electrophysiological techniques can be used to great effect to help determine the mechanism of action of a compound. However, many factors can compromise the resulting data and their analysis, such as the speed of solution exchange, expression of additional ion channel populations including other ligand-gated receptors and voltage-gated channels, compounds having multiple binding sites, and current desensitization and rundown. In this chapter, such problems and their solutions are discussed and illustrated using data from experiments involving the uncompetitive NMDA receptor antagonist memantine. Memantine differs from many other NMDA receptor channel blockers in that it is well tolerated and does not cause psychotomimetic effects at therapeutic doses. Various electrophysiological parameters of NMDA-induced current blockade by memantine have been proposed to be important in determining therapeutic tolerability; potency, onset and offset kinetics, and voltage dependency. These were all measured using whole cell patch clamp techniques using hippocampal neurons. Full results are shown here for memantine, and these are summarized and compared to those from similar experiments with other NMDA channel blockers. The interpretation of these results is discussed, as are theories concerning the tolerability of NMDA channel blockers, with the aim of illustrating how electrophysiological data can be used to form and support a physiological hypothesis.

Indistinguishable synaptic pharmacodynamics of the N-methyl-D-aspartate receptor channel blockers memantine and ketamine

Memantine and ketamine, voltage- and activation-dependent channel blockers of N-methyl-d-aspartate (NMDA) receptors (NMDARs), have enjoyed a recent resurgence in clinical interest. Steady-state pharmacodynamic differences between these blockers have been reported, but it is unclear whether the compounds differentially affect dynamic physiologic signaling. In this study, we explored nonequilibrium conditions relevant to synaptic transmission in hippocampal networks in dissociated culture and hippocampal slices. Equimolar memantine and ketamine had indistinguishable effects on the following measures: steady-state NMDA currents, NMDAR excitatory postsynaptic current (EPSC) decay kinetics, progressive EPSC inhibition during repetitive stimulation, and extrasynaptic NMDAR inhibition. Therapeutic drug efficacy and tolerability of memantine have been attributed to fast kinetics and strong voltage dependence. However, pulse depolarization in drug presence revealed a surprisingly slow and similar time course of equilibration for the two compounds, although memantine produced a more prominent fast component (62% versus 48%) of re-equilibration. Simulations predicted that low gating efficacy underlies the slow voltage-dependent relief from block. This prediction was empirically supported by faster voltage-dependent blocker re-equilibration with several experimental manipulations of gating efficacy. Excitatory postsynaptic potential-like voltage commands produced drug differences only with large, prolonged depolarizations unlikely to be attained physiologically. In fact, we found no difference between drugs on measures of spontaneous network activity or acute effects on plasticity in hippocampal slices. Despite indistinguishable synaptic pharmacodynamics, ketamine provided significantly greater neuroprotection from damage induced by oxygen glucose deprivation, consistent with the idea that under extreme depolarizing conditions, the biophysical difference between drugs becomes detectable. We conclude that despite subtle differences in voltage dependence, during physiologic activity, blocker pharmacodynamics are largely indistinguishable and largely voltage independent.

Memantine add-on to risperidone for treatment of negative symptoms in patients with stable schizophrenia: randomized, double-blind, placebo-controlled study

We aimed to evaluate the efficacy of memantine add-on in the treatment of primary negative symptoms of patients with stable schizophrenia. In a double-blind placebo-controlled clinical trial, 40 patients with schizophrenia (Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition) who were stabilized on risperidone for a minimum of 8 weeks were randomized to either memantine (20 mg) or placebo in addition to risperidone, 6 mg/d, for eight weeks. Assessment was done using the Positive and Negative Syndrome Scale at baseline, week 4, and week 8. The Hamilton Depression Rating Scale and the Extrapyramidal Symptom Rating Scale at baseline and week 8 were used to assess depression and extrapyramidal symptoms, respectively. All 40 patients had at least one postbaseline measurement, and 38 patients completed the trial. Patients in the memantine group showed a significantly greater improvement on negative subscale than the placebo group at end point (P < 0.001). The same effect was observed for the total score (P < 0.001) and the general psychopathology subscale score (P = 0.002). There was no significant difference in reduction of positive symptoms score between the 2 groups (P = 0.757). Changes in the Hamilton Depression Rating Scale and the Extrapyramidal Symptom Rating Scale scores and frequency of adverse effects did not differ between the 2 groups. Our study showed that memantine is a tolerable and efficacious add-on treatment for primary negative symptoms of schizophrenia.

Memantine and cholinesterase inhibitors: complementary mechanisms in the treatment of Alzheimer's disease

This review describes the preclinical mechanisms that may underlie the increased therapeutic benefit of combination therapy-with the N-methyl-D-aspartate receptor antagonist, memantine, and an acetylcholinesterase inhibitor (AChEI)-for the treatment of Alzheimer's disease (AD). Memantine, and the AChEIs target two different aspects of AD pathology. Both drug types have shown significant efficacy as monotherapies for the treatment of AD. Furthermore, clinical observations indicate that their complementary mechanisms offer superior benefit as combination therapy. Based on the available literature, the authors have considered the preclinical mechanisms that could underlie such a combined approach. Memantine addresses dysfunction in glutamatergic transmission, while the AChEIs serve to increase pathologically lowered levels of the neurotransmitter acetylcholine. In addition, preclinical studies have shown that memantine has neuroprotective effects, acting to prevent glutamatergic over-stimulation and the resulting neurotoxicity. Interrelations between the glutamatergic and cholinergic pathways in regions of the brain that control learning and memory mean that combination treatment has the potential for a complex influence on disease pathology. Moreover, studies in animal models have shown that the combined use of memantine and the AChEIs can produce greater improvements in measures of memory than either treatment alone. As an effective approach in the clinical setting, combination therapy with memantine and an AChEI has been a welcome advance for the treatment of patients with AD. Preclinical data have shown how these drugs act via two different, but interconnected, pathological pathways, and that their complementary activity may produce greater effects than either drug individually.

Alzheimer's disease, β-amyloid, glutamate, NMDA receptors and memantine--searching for the connections

β-amyloid (Aβ) is widely accepted to be one of the major pathomechanisms underlying Alzheimer's disease (AD), although there is presently lively debate regarding the relative roles of particular species/forms of this peptide. Most recent evidence indicates that soluble oligomers rather than plaques are the major cause of synaptic dysfunction and ultimately neurodegeneration. Soluble oligomeric Aβ has been shown to interact with several proteins, for example glutamatergic receptors of the NMDA type and proteins responsible for maintaining glutamate homeostasis such as uptake and release. As NMDA receptors are critically involved in neuronal plasticity including learning and memory, we felt that it would be valuable to provide an up to date review of the evidence connecting Aβ to these receptors and related neuronal plasticity. Strong support for the clinical relevance of such interactions is provided by the NMDA receptor antagonist memantine. This substance is the only NMDA receptor antagonist used clinically in the treatment of AD and therefore offers an excellent tool to facilitate translational extrapolations from in vitro studies through in vivo animal experiments to its ultimate clinical utility.

Identification of small molecule inhibitors of amyloid β-induced neuronal apoptosis acting through the imidazoline I(2) receptor

Aberrant activation of signaling pathways plays a pivotal role in central nervous system disorders, such as Alzheimer's disease (AD). Using a combination of virtual screening and experimental testing, novel small molecule inhibitors of tPA-mediated extracellular signal-regulated kinase (Erk)1/2 activation were identified that provide higher levels of neuroprotection from Aβ-induced apoptosis than Memantine, the most recently FDA-approved drug for AD treatment. Subsequent target deconvolution efforts revealed that they all share low micromolar affinity for the imidazoline I(2) receptor, while being devoid of any significant affinity to a list of AD-relevant targets, including the N-methyl-d-aspartate receptor (NMDAR), acetylcholinesterase (AChE), and monoamine oxidase B (MAO-B). Targeting the imidazoline I(2) receptor emerges as a new mechanism of action to inhibit tPA-induced signaling in neurons for the treatment of AD and other neurodegenerative diseases.