Memantine as an example of a fast, voltage-dependent, open channel N-methyl-D-aspartate receptor blocker

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.

The role of NMDA receptors in bipolar disorder: A systematic review

OBJECTIVES

Glutamatergic transmission and N-methyl-D-aspartate receptors (NMDARs) have been implicated in the pathophysiology schizophrenic spectrum and major depressive disorders. Less is known about the role of NMDARs in bipolar disorder (BD). The present systematic review aimed to investigate the role of NMDARs in BD, along with its possible neurobiological and clinical implications.

METHODS

We performed a computerized literature research on PubMed in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement, using the following string: (("Bipolar Disorder"[Mesh]) OR (manic-depressive disorder[Mesh]) OR ("BD") OR ("MDD")) AND ((NMDA [Mesh]) OR (N-methyl-D-aspartate) OR (NMDAR[Mesh]) OR (N-methyl-D-aspartate receptor)).

RESULTS

Genetic studies yield conflicting results, and the most studied candidate for an association with BD is the GRIN2B gene. Postmortem expression studies (in situ hybridization and autoradiographic and immunological studies) are also contradictory but suggest a reduced activity of NMDARs in the prefrontal, superior temporal cortex, anterior cingulate cortex, and hippocampus.

CONCLUSIONS

Glutamatergic transmission and NMDARs do not appear to be primarily involved in the pathophysiology of BD, but they might be linked to the severity and chronicity of the disorder. Disease progression could be associated with a long phase of enhanced glutamatergic transmission, with ensuing excitotoxicity and neuronal damage, resulting into a reduced density of functional NMDARs.

Design, synthesis, and in vitro and in vivo characterization of new memantine analogs for Alzheimer's disease

Currently, of the few accessible symptomatic therapies for Alzheimer's disease (AD), memantine is the only N-methyl-d-aspartate receptor (NMDAR) blocker approved by the FDA. This work further explores a series of memantine analogs featuring a benzohomoadamantane scaffold. Most of the newly synthesized compounds block NMDARs in the micromolar range, but with lower potency than previously reported hit IIc, results that were supported by molecular dynamics simulations. Subsequently, electrophysiological studies with the more potent compounds allowed classification of IIc, a low micromolar, uncompetitive, voltage-dependent, NMDAR blocker, as a memantine-like compound. The excellent in vitro DMPK properties of IIc made it a promising candidate for in vivo studies in Caenorhabditis elegans (C. elegans) and in the 5XFAD mouse model of AD. Administration of IIc or memantine improved locomotion and rescues chemotaxis behavior in C. elegans. Furthermore, both compounds enhanced working memory in 5XFAD mice and modified NMDAR and CREB signaling, which may prevent synaptic dysfunction and modulate neurodegenerative progression.

Structural insights into binding of therapeutic channel blockers in NMDA receptors

Excitatory signaling mediated by N-methyl-D-aspartate receptor (NMDAR) is critical for brain development and function, as well as for neurological diseases and disorders. Channel blockers of NMDARs are of medical interest owing to their potential for treating depression, Alzheimer's disease, and epilepsy. However, precise mechanisms underlying binding and channel blockade have remained limited owing to challenges in obtaining high-resolution structures at the binding site within the transmembrane domains. Here, we monitor the binding of three clinically important channel blockers: phencyclidine, ketamine, and memantine in GluN1-2B NMDARs at local resolutions of 2.5-3.5 Å around the binding site using single-particle electron cryo-microscopy, molecular dynamics simulations, and electrophysiology. The channel blockers form different extents of interactions with the pore-lining residues, which control mostly off-speeds but not on-speeds. Our comparative analyses of the three unique NMDAR channel blockers provide a blueprint for developing therapeutic compounds with minimal side effects.

Efficacy of Ion-Channel Inhibitors Amantadine, Memantine and Rimantadine for the Treatment of SARS-CoV-2 In Vitro

We report the in vitro efficacy of ion-channel inhibitors amantadine, memantine and rimantadine against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In VeroE6 cells, rimantadine was most potent followed by memantine and amantadine (50% effective concentrations: 36, 80 and 116 µM, respectively). Rimantadine also showed the highest selectivity index, followed by amantadine and memantine (17.3, 12.2 and 7.6, respectively). Similar results were observed in human hepatoma Huh7.5 and lung carcinoma A549-hACE2 cells. Inhibitors interacted in a similar antagonistic manner with remdesivir and had a similar barrier to viral escape. Rimantadine acted mainly at the viral post-entry level and partially at the viral entry level. Based on these results, rimantadine showed the most promise for treatment of SARS-CoV-2.

Hydroxynorketamine Blocks N-Methyl-d-Aspartate Receptor Currents by Binding to Closed Receptors

Ketamine, a dissociative anesthetic, is experiencing a clinical resurgence as a fast-acting antidepressant. In the central nervous system, ketamine acts primarily by blocking NMDA receptor currents. Although it is generally safe in a clinical setting, it can be addictive, and several of its derivatives are being investigated as preferable alternatives. 2R,6R-Hydroxynorketamine (HNK), a ketamine metabolite, reproduces some of the therapeutic effects of ketamine and appears to lack abuse liability. Here, we report a systematic investigation of the effects of HNK on macroscopic responses elicited from recombinant NMDA receptors expressed in human embryonic kidney 293 cells. We found that, like ketamine, HNK reduced NMDA receptor currents in a dose-, pH-, and voltage-dependent manner. Relative to ketamine, it had 100-fold-lower potency (46 µM at pH 7.2), 10-fold-slower inhibition onset, slower apparent dissociation rate, weaker voltage dependence, and complete competition by magnesium. Notably, HNK inhibition was fully effective when applied to resting receptors. These results revealed unexpected properties of hydroxynorketamine that warrant its further investigation as a possible therapeutic in pathologies associated with NMDA receptor dysfunction. SIGNIFICANCE STATEMENT: NMDA receptors are excitatory ion channels with fundamental roles in synaptic transmission and plasticity, and their dysfunction associates with severe neuropsychiatric disorders. 2R,6R-Hydroxynorketamine, a metabolite of ketamine, mimics some of the neuroactive properties of ketamine and may lack its abuse liability. Results show that 2R,6R-hydroxynorketamine blocks NMDA receptor currents with low affinity and weak voltage dependence and is effective when applied to resting receptors. These properties highlight its effectiveness to a subset of NMDA receptor responses and recommend it for further investigation.

Extremely low frequency magnetic field induces human neuronal differentiation through NMDA receptor activation

Magnetic fields with different frequency and intensity parameters exhibit a wide range of effects on different biological models. Extremely low frequency magnetic field (ELF MF) exposure is known to augment or even initiate neuronal differentiation in several in vitro and in vivo models. This effect holds potential for clinical translation into treatment of neurodegenerative conditions such as autism, Parkinson's disease and dementia by promoting neurogenesis, non-invasively. However, the lack of information on underlying mechanisms hinders further investigation into this phenomenon. Here, we examine involvement of glutamatergic Ca²⁺ channel, N-methyl-D-aspartate (NMDA) receptors in the process of human neuronal differentiation under ELF MF exposure. We show that human neural progenitor cells (hNPCs) differentiate more efficiently under ELF MF exposure in vitro, as demonstrated by the abundance of neuronal markers. Furthermore, they exhibit higher intracellular Ca²⁺ levels as evidenced by c-fos expression and more elongated mature neurites. We were able to neutralize these effects by blocking NMDA receptors with memantine. As a result, we hypothesize that the effects of ELF MF exposure on neuronal differentiation originate from the effects on NMDA receptors, which sequentially triggers Ca²⁺-dependent cascades that lead to differentiation. Our findings identify NMDA receptors as a new key player in this field that will aid further research in the pursuit of effect mechanisms of ELF MFs.

Memantine augmentation in clozapine-refractory schizophrenia: a randomized, double-blind, placebo-controlled crossover study

BACKGROUND

Dysfunction of neuroplasticity due to N-methyl-d-aspartate (NMDA) receptor hypofunction may be a causal factor for memory and executive dysfunctioning in schizophrenia. Deregulation of NMDA transmission in the prefrontal cortex may also explain negative and positive symptoms. Clozapine augmentation with memantine targets altered NMDA receptor-mediated neurotransmission in schizophrenia and showed substantial beneficial effects on several symptom domains in a small proof-of-concept study. We evaluate effects of memantine add-on treatment to clozapine for memory and executive function, and negative and positive symptoms in schizophrenia.

METHOD

Clozapine-treated patients with refractory schizophrenia were randomly assigned to 12 weeks of double-blind adjunctive treatment with memantine (n = 26) or placebo (n = 26). Crossover occurred after a 2-week placebo wash-out period. Primary endpoints were change from baseline to 12 weeks treatment and 14 weeks to 26 weeks treatment on memory and executive function using the Cambridge Neuropsychological Test Automated Battery (CANTAB), Positive and Negative Syndrome Scale (PANSS), and Clinical Global Impression Severity Scale (CGI-S). Side effects were assessed using the Liverpool University Neuroleptic Side-Effect Rating Scale.

RESULTS

When compared with placebo, memantine improved a composite memory score comprising verbal recognition memory and paired associates learning task scores on the CANTAB (effect size = 0.30) and PANSS negative subscale score (effect size = 0.29). Side effects were mild and transient.

CONCLUSIONS

In patients with clozapine-treated refractory schizophrenia, memantine addition significantly improved verbal and visual memory and negative symptoms without serious adverse effects. These results justify further investigations on long-term memantine augmentation to clozapine in treatment-resistant schizophrenia.

Treatment with UDP-glucose, GDNF, and memantine promotes SVZ and white matter self-repair by endogenous glial progenitor cells in neonatal rats with ischemic PVL

Periventricular leukomalacia (PVL) is one of the foremost neurological conditions leading to long-term abnormalities in premature infants. Since it is difficult to prevent initiation of this damage in utero, promoting the innate regenerative potential of the brain after birth may provide a more feasible, prospective therapy for PVL. Treatment with UDP-glucose (UDPG), an endogenous agonist of G protein-coupled receptor 17 (GPR17) that may enhance endogenous self-repair potentiality, glial cell line-derived neurotrophic factor (GDNF), a neurotrophic factor associated with the growth and survival of nerve cells, and memantine, a noncompetitive antagonist of N-methyl-d-aspartate (NMDA) receptors that block ischemia-induced glutamate signal transduction, has been reported to achieve functional, neurological improvement in neonatal rats with PVL. The aim of the present study was to further explore whether UDPG, GDNF and/or memantine could promote corresponding self-repair of the subventricular zone (SVZ) and white matter (WM) in neonatal rats with ischemia-induced PVL. SVZ or WM tissue samples and cultured glial progenitor cells derived from a 5 day-old neonatal rat model of PVL were utilized for studying response to UDPG, GDNF and memantine in vivo and in vitro, respectively. Labeling with 5'-bromo-2'-deoxyuridine and immunofluorescent cell lineage markers after hypoxia-ischemia or oxygen-glucose deprivation (OGD) revealed that UDPG, GDNF and memantine each significantly increased glial progenitor cells and preoligodendrocytes (preOLs), as well as more differentiated immature and mature oligodendrocyte (OL), in both the SVZ and WM in vivo or in vitro. SVZ and WM glial cell apoptosis was also significantly reduced by UDPG, GDNF or memantine, both in vivo and in vitro. These results indicated that UDPG, GDNF or memantine may promote endogenous self-repair by stimulating proliferation of glial progenitor cells derived from both the SVZ and WM, activating their differentiation into more mature OLs, and raising the survival rate of these newly generated glial cells in neonatal rats with ischemic PVL.

Regulation of postsynaptic plasticity genes' expression and topography by sustained dopamine perturbation and modulation by acute memantine: relevance to schizophrenia

A relevant role for dopamine-glutamate interaction has been reported in the pathophysiology and treatment of psychoses. Dopamine and glutamate may interact at multiple levels, including the glutamatergic postsynaptic density (PSD), an electron-dense thickening that has gained recent attention as a switchboard of dopamine-glutamate interactions and for its role in synaptic plasticity. Recently, glutamate-based strategies, such as memantine add-on to antipsychotics, have been proposed for refractory symptoms of schizophrenia, e.g. cognitive impairment. Both antipsychotics and memantine regulate PSD transcripts but sparse information is available on memantine's effects under dopamine perturbation. We tested gene expression changes of the Homer1 and PSD-95 PSD proteins in models of sustained dopamine perturbation, i.e. subchronic treatment by: a) GBR-12909, a dopamine receptor indirect agonist; b) haloperidol, a D2R antagonist; c) SCH-23390, a dopamine D1 receptor (D1R) antagonist; and d) SCH-23390+haloperidol. On the last day of treatment, rats were acutely treated with vehicle or memantine. The Homer1a immediate-early gene was significantly induced by haloperidol and by haloperidol+SCH-23390. The gene was not induced by SCH-23390 per se or by GBR-12909. Expression of the constitutive genes Homer1b/c and PSD-95 was less affected by these dopaminergic paradigms. Acute memantine administration significantly increased Homer1a expression by the dopaminergic compounds used herein. Both haloperidol and haloperidol+SCH-23390 shifted Homer1a/Homer1b/c ratio of expression toward Homer1a. This pattern was sharpened by acute memantine. Dopaminergic compounds and acute memantine also differentially affected topographic distribution of gene expression and coordinated expression of Homer1a among cortical-subcortical regions. These results indicate that dopaminergic perturbations may affect glutamatergic signaling in different directions. Memantine may help partially revert dopamine-mediated glutamatergic dysfunctions.

Memantine prevents memory consolidation failure induced by soluble beta amyloid in rats

It has been well documented that β-amyloid (Aβ) peptide accumulation and aggregation in the brain plays a crucial role in the pathophysiology of Alzheimer's disease (AD). However, a new orientation of the amyloid cascade hypothesis has evidenced that soluble forms of the peptide (sAβ) are involved in Aβ-induced cognitive impairment and cause rapid disruption of the synaptic mechanisms underlying memory. The primary aim of this study was to elucidate the effects of sAβ, acutely injected intracerebrally (i.c.v., 4 μM), on the short term and long term memory of young adult male rats, by using the novel object recognition task. Glutamatergic receptors have been proposed as mediating the effect of Aβ on synaptic plasticity and memory. Thus, we also investigated the effects of sAβ on prefrontal cortex (PFC) glutamate release and the specific contribution of N-methyl-D-aspartate (NMDA) receptor modulation to the effects of sAβ administration on the cognitive parameters evaluated. We found that a single i.c.v. injection of sAβ 2 h before testing did not alter the ability of rats to differentiate between a familiar and a novel object, in a short term memory test, while it was able to negatively affect consolidation/retrieval of long term memory. Moreover, a significant increase of glutamate levels was found in PFC of rats treated with the peptide 2 h earlier. Interestingly, memory deficit induced by sAβ was reversed by a NMDA-receptor antagonist, memantine (5 mg/kg i.p), administered immediately after the familiarization trial (T1). On the contrary, memantine administered 30 min before T1 trial, was not able to rescue long term memory impairment. Taken together, our results suggest that an acute i.c.v. injection of sAβ peptide interferes with the consolidation/retrieval of long term memory. Moreover, such sAβ-induced effect indicates the involvement of glutamatergic system, proposing that NMDA receptor inhibition might prevent or lead to the recovery of early cognitive impairment.

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.

Cerebrovascular and neuroprotective effects of adamantane derivative

Objectives. The influence of 5-hydroxyadamantane-2-on was studied on the rats' brain blood flow and on morphological state of brain tissue under the condition of brain ischemia. The interaction of the substance with NMDA receptors was also studied. Methods. Study has been implemented using the methods of local blood flow registration by laser flowmeter, [³H]-MK-801binding, and morphological examination of the brain tissue. We used the models of global transient ischemia of the brain, occlusion of middle cerebral artery, and hypergravity ischemia of the brain. Results. Unlike memantine, antagonist of glutamatergic receptors, the 5-hydroxyadamantane-2-on does not block NMDA receptors but enhances the cerebral blood flow of rats with brain ischemia. This effect is eliminated by bicuculline. Under conditions of permanent occlusion of middle cerebral artery, 5-hydroxyadamantane-2-on has recovered compensatory regeneration in neural cells, axons, and glial cells, and the number of microcirculatory vessels was increased. 5-Hydroxyadamantane-2-on was increasing the survival rate of animals with hypergravity ischemia. Conclusions. 5-Hydroxyadamantane-2-on, an adamantane derivative, which is not NMDA receptors antagonist, demonstrates significant cerebrovascular and neuroprotective activity in conditions of brain ischemia. Presumably, the GABA-ergic system of brain vessels is involved in mechanisms of cerebrovascular and neuroprotective activity of 5-hydroxyadamantane-2-on.

Effect of memantine on L-DOPA-induced dyskinesia in the 6-OHDA-lesioned rat model of Parkinson's disease

An increasing body of experimental evidence demonstrates that the glutamatergic system is involved in the genesis of l-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia (LID). Indeed, the N-methyl-d-aspartate (NMDA) receptor antagonist amantadine is the only anti-dyskinetic compound used in patients, albeit with limited efficacy and side effects. In this study, we investigated the anti-dyskinetic properties of memantine, a non-competitive NMDA receptor antagonist in clinical use for the treatment of dementia, in the 6-hydroxy-dopamine (6-OHDA)-lesion rat model of Parkinson's disease. For comparison, parallel experiments were also performed with amantadine. First, we investigated the acute effect of different doses of memantine (5, 10, 15 and 20mg/kg), and amantadine (10, 20, 40, 60mg/kg) on established dyskinesia induced by L-DOPA (6mg/kg plus benserazide). Results showed that both memantine and amantadine produced a significant reduction of LID. Afterward, drug-naïve and L-DOPA-primed 6-OHDA-lesioned rats were sub-chronically treated with daily injections of L-DOPA (6mg/kg plus benserazide) alone, or in combination with the effective doses of memantine, while amantadine was tested in already dyskinetic rats. Results showed that memantine significantly dampened dyskinesia in both drug-naïve and L-DOPA-primed rats, but only during the first few days of administration. In fact, the anti-dyskinetic effect of memantine was completely lost already at the fifth administration, indicating a rapid induction of tolerance. Interestingly, a 3-week washout period was not sufficient to restore the anti-dyskinetic effect of the drug. Similarly, amantadine was able to dampen already established dyskinesia only during the first day of administration. Moreover, memantine partially decreased the therapeutic effect of L-DOPA, as showed by the result of the stepping test. Finally, loss of the anti-dyskinetic effect of memantine was associated to increased synaptic GluN2A/GluN2B ratio at striatal synaptic membranes. Our results are in line with clinical observations suggesting that NMDA receptor blockade may only be transiently effective against LID in PD patients.

NMDA Receptors in Glial Cells: Pending Questions

Glutamate receptors of the N-methyl-D-aspartate (NMDA) type are involved in many cognitive processes, including behavior, learning and synaptic plasticity. For a long time NMDA receptors were thought to be the privileged domain of neurons; however, discoveries of the last 25 years have demonstrated their active role in glial cells as well. Despite the large number of studies in the field, there are many unresolved questions connected with NMDA receptors in glia that are still a matter of debate. The main objective of this review is to shed light on these controversies by summarizing results from all relevant works concerning astrocytes, oligodendrocytes and polydendrocytes (also known as NG2 glial cells) in experimental animals, further extended by studies performed on human glia. The results are divided according to the study approach to enable a better comparison of how findings obtained at the mRNA level correspond with protein expression or functionality. Furthermore, special attention is focused on the NMDA receptor subunits present in the particular glial cell types, which give them special characteristics different from those of neurons – for example, the absence of Mg²⁺ block and decreased Ca²⁺ permeability. Since glial cells are implicated in important physiological and pathophysiological roles in the central nervous system (CNS), the last part of this review provides an overview of glial NMDA receptors with respect to ischemic brain injury.

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.

Glutamate system, amyloid ß peptides and tau protein: functional interrelationships and relevance to Alzheimer disease pathology

Alzheimer disease is the most prevalent form of dementia globally and is characterized premortem by a gradual memory loss and deterioration of higher cognitive functions and postmortem by neuritic plaques containing amyloid ß peptide and neurofibrillary tangles containing phospho-tau protein. Glutamate is the most abundant neurotransmitter in the brain and is essential to memory formation through processes such as long-term potentiation and so might be pivotal to Alzheimer disease progression. This review discusses how the glutamatergic system is impaired in Alzheimer disease and how interactions of amyloid ß and glutamate influence synaptic function, tau phosphorylation and neurodegeneration. Interestingly, glutamate not only influences amyloid ß production, but also amyloid ß can alter the levels of glutamate at the synapse, indicating that small changes in the concentrations of both molecules could influence Alzheimer disease progression. Finally, we describe how the glutamate receptor antagonist, memantine, has been used in the treatment of individuals with Alzheimer disease and discuss its effectiveness.

Polycyclic cage structures as lipophilic scaffolds for neuroactive drugs

Polycyclic cage scaffolds have been successfully used in the development of numerous lead compounds demonstrating activity in the central nervous system (CNS). Several neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, schizophrenia, and stroke, as well as drug abuse, can be modulated with polycyclic cage derivatives. These cage moieties, including adamantane and pentacycloundecane derivatives, improve the pharmacokinetic and pharmacodynamic properties of conjugated parent drugs and serve as an important scaffold in the design of therapeutically active agents for the treatment of neurological disorders. In this Minireview, we focus on the recent developments in the field of polycyclic cage compounds, as well as the relationship between the lipophilic character of these cage-derived drugs and the ability of such compounds to target and reach the CNS and improve the pharmacodynamic properties of compounds conjugated to it.

Quantification of the Mg2+-induced potency shift of amantadine and memantine voltage-dependent block in human recombinant GluN1/GluN2A NMDARs

Clinically, amantadine and memantine are drugs whose therapeutic utility is linked to their ability to block N-methyl-D-aspartate receptors (NMDARs) in a voltage-dependent manner. Nevertheless many studies that have characterized the pharmacological actions of amantadine and memantine have done so in the absence of physiological levels of Mg(2+) ions. This study quantifies the extent to which Mg(2+) alters the potency of the block produced by both amantadine and memantine at human recombinant GluN1/GluN2A NMDARs. Human recombinant GluN1/GluN2A NMDARs were expressed in Xenopus laevis oocytes and two-electrode voltage-clamp recordings were made at -80, -60 and -40 mV to quantify amantadine and memantine block in the absence and presence of Mg(2+). Amantadine and memantine blocked human GluN1/GluN2A NMDARs in a voltage-dependent manner with IC(50) values (at -80 mV) of 49 ± 6 μM (n = 7) and 1.0 ± 0.3 μM (n = 7), respectively. In the presence of Mg(2+) (1mM) the equivalent IC(50) values were 165 ± 10 μM (n=6) and 6.6 ± 0.3 μM (n = 5). Similarly in the presence of amantadine or memantine the potency of Mg(2+) in blocking GluN1/GluN2A NMDARs was reduced. The decrease in the potencies of both amantadine and memantine in the presence of physiological concentrations of Mg(2+) indicates that other targets (e.g. α7-nicotinic acetylcholine receptors and 5-HT(3) receptors) in addition to NMDARs may well be sites of the therapeutic action of these channel blockers.

Potential therapeutic interventions for fragile X syndrome

Fragile X syndrome (FXS) is caused by a lack of the fragile X mental retardation protein (FMRP); FMRP deficiency in neurons of patients with FXS causes intellectual disability (IQ<70) and several behavioural problems, including hyperactivity and autistic-like features. In the brain, no gross morphological malformations have been found, although subtle spine abnormalities have been reported. FXS has been linked to altered group I metabotropic glutamate receptor (mGluR)-dependent and independent forms of synaptic plasticity. Here, we discuss potential targeted therapeutic strategies developed to specifically correct disturbances in the excitatory mGluR and the inhibitory gamma-aminobutyric (GABA) receptor pathways that have been tested in animal models and/or in clinical trials with patients with FXS.

Pharmacodynamics of memantine: an update

Memantine received marketing authorization from the European Agency for the Evaluation of Medicinal Products (EMEA) for the treatment of moderately severe to severe Alzheimer s disease (AD) in Europe on 17(th) May 2002 and shortly thereafter was also approved by the FDA for use in the same indication in the USA. Memantine is a moderate affinity, uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist with strong voltage-dependency and fast kinetics. Due to this mechanism of action (MOA), there is a wealth of other possible therapeutic indications for memantine and numerous preclinical data in animal models support this assumption. This review is intended to provide an update on preclinical studies on the pharmacodynamics of memantine, with an additional focus on animal models of diseases aside from the approved indication. For most studies prior to 1999, the reader is referred to a previous review [196].In general, since 1999, considerable additional preclinical evidence has accumulated supporting the use of memantine in AD (both symptomatic and neuroprotective). In addition, there has been further confirmation of the MOA of memantine as an uncompetitive NMDA receptor antagonist and essentially no data contradicting our understanding of the benign side effect profile of memantine.

Open-label memantine in fragile X syndrome

Glutamatergic dysfunction is implicated in the pathophysiology of fragile X syndrome (FXS). The purpose of this pilot study was to examine the effectiveness and tolerability of memantine for a number of target symptoms associated with FXS. Medical records describing open-label treatment with memantine in 6 patients with FXS and a comorbid diagnosis of PDD were reviewed. Six patients received memantine over a mean 34.7 weeks of treatment. Four of 6 (67%) patients showed global clinical benefit on ratings with the CGI-I. Symptom specific rating scales, however, showed no statistically significant improvement. Two patient developed treatment-limiting irritability on memantine. Memantine was modestly effective in several patients with FXS. Further systematic study is warranted.