MECHANISM OF BLOCK AND BEHAVIORAL EFFECTS OF THE N-METHYL-D-ASPARTATE RECEPTOR ANTAGONISTS MEMANTINE AND KETAMINE

Pharmacological inhibition of NMDA receptor activity by ketamine is accompanied by pyschotomimetic side-effects; however, the Alzheimer's disease therapeutic memantine blocks NMDA receptor activity without debilitating side-effects. This dissertation provides electrophysiological and behavioral characterizations of these two NMDA receptor antagonists in an attempt to understand the unique therapeutic utility of memantine. The following work explores memantine and ketamine inhibition at NMDA receptors, their main site of action, with a focus on the mechanism of inhibition and receptor subtype selectivity in physiologically relevant conditions. This research shows NMDA receptors possess a second binding site at which memantine, but not ketamine, can inhibit activity. The research also shows the dramatic effect physiological concentrations of magnesium has on the ability of these drugs to inhibit NMDA receptor activity. Behavioral and cognitive effects of memantine and ketamine are also assessed and compared directly in rat. The effects of memantine and ketamine in rat were found to be similar at the low doses tested and more divergent as dose increased. Furthermore, memantine's effects appeared to be more pronounced and longer-lasting than those of ketamine. These findings demonstrate the importance of considering the physiological environment in which a drug acts, as well as the principles of drug action, when examining the effects of a drug on central nervous system activity

Disease-associated missense mutations in GluN2B subunit alter NMDA receptor ligand binding and ion channel properties.

Genetic and bioinformatic analyses have identified missense mutations in GRIN2B encoding the NMDA receptor GluN2B subunit in autism, intellectual disability, Lennox Gastaut and West Syndromes. Here, we investigated several such mutations using a near-complete, hybrid 3D model of the human NMDAR and studied their consequences with kinetic modelling and electrophysiology. The mutants revealed reductions in glutamate potency; increased receptor desensitisation; and ablation of voltage-dependent Mg block. In addition, we provide new views on Mg and NMDA channel blocker binding sites. We demonstrate that these mutants have significant impact on excitatory transmission in developing neurons, revealing profound changes that could underlie their associated neurological disorders. Of note, the NMDAR channel mutant GluN2B unusually allowed Mg permeation, whereas nearby N615I reduced Ca permeability. By identifying the binding site for an NMDAR antagonist that is used in the clinic to rescue gain-of-function phenotypes, we show that drug binding may be modified by some GluN2B disease-causing mutations

Influence of GluN2 subunit identity on NMDA receptor function

AbstractN-methyl-d-aspartate receptors (NMDARs) are ligand-gated ion channels (‘ionotropic’ receptors) activated by the major excitatory neurotransmitter, l-glutamate. While the term ‘the NMDAR’ is often used it obscures the fact that this class of receptor contains within it members whose properties are as different as they are similar. This heterogeneity was evident from early electrophysiological, pharmacological and biochemical assessments of the functional properties of NMDARs and while the molecular basis of this heterogeneity has taken many years to elucidate, it indicated from the outset that the diversity of NMDAR phenotypes could allow this receptor family to subserve a variety of functions in the mammalian central nervous system. In this review we highlight some recent studies that have identified structural elements within GluN2 subunits that contribute to the heterogeneous biophysical properties of NMDARs, consider why some recently described novel pharmacological tools may permit better identification of native NMDAR subtypes, examine the evidence that NMDAR subtypes differentially contribute to the induction of long-term potentiation and long-term depression and discuss how through the use of chimeric proteins additional insights have been obtained that account for NMDAR subtype-dependency of physiological and pathophysiological signalling.This article is part of the Special Issue entitled ‘Glutamate Receptor-Dependent Synaptic Plasticity’

Glutamatergic regulation of cognition and functional brain connectivity:insights from pharmacological, genetic and translational schizophrenia research

The pharmacological modulation of glutamatergic neurotransmission to improve cognitive function has been a focus of intensive research, particularly in relation to the cognitive deficits seen in schizophrenia. Despite this effort there has been little success in the clinical use of glutamatergic compounds as procognitive drugs. Here we review a selection of the drugs used to modulate glutamatergic signalling and how they impact on cognitive function in rodents and humans. We highlight how glutamatergic dysfunction, and NMDA receptor hypofunction in particular, is a key mechanism contributing to the cognitive deficits observed in schizophrenia, and outline some of the glutamatergic targets that have been tested as putative procognitive targets for the disorder. Using translational research in this area as a leading exemplar, namely models of NMDA receptor hypofunction, we discuss how the study of functional brain network connectivity can provide new insight into how the glutamatergic system impacts on cognitive function. Future studies characterising functional brain network connectivity will increase our understanding of how glutamatergic compounds regulate cognition and could contribute to the future success of glutamatergic drug validation

Synaptic versus extrasynaptic NMDA receptor signalling: implications for neurodegenerative disorders

There is a long-standing paradox that N-methyl-D-aspartate receptors (NMDARs) can both promote neuronal health and kill neurons. Recent studies show that NMDAR-induced responses depend on the receptor location: stimulation of synaptic NMDARs, acting primarily through nuclear Ca(2+) signaling, leads to the build-up of a neuroprotective ‘shield’, whereas stimulation of extrasynaptic NMDARs promotes cell death. These differences result from the activation of distinct genomic programmes and opposing actions on intracellular signalling pathways. Perturbations in the balance between synaptic and extrasynaptic NMDAR activity contribute to neuronal dysfunction in acute ischaemia and Huntington’s disease and could be a common theme in the aetiology of neurodegenerative diseases. Neuroprotective therapies should aim to both enhance the effect of synaptic activity and disrupt extrasynaptic NMDAR-dependent death signalling

Comparison of behavioral effects of the NMDA receptor channel blockers memantine and ketamine in rats

Memantine and ketamine block N-methyl-d-aspartate (NMDA) receptors with similar affinity and kinetics, yet their behavioral consequences differ: e.g., memantine is used to alleviate symptoms of Alzheimer\u27s disease, whereas ketamine reproduces symptoms of schizophrenia. The two drugs exhibit different pharmacokinetics, which may play a principal role in their differential behavioral effects. To gain insight into the drugs\u27 behavioral consequences, we treated adult male rats acutely with varying doses (0-40 mg/kg i.p.) of memantine or ketamine and assessed exploratory behavior and spatial working memory. To examine the importance of pharmacokinetics, we assessed behavior either 15 or 45 min after drug administration. Both drugs decreased ambulation, fine movements, and rearing at the beginning of the exploratory activity test; however, at the end of the test, high doses of only memantine increased ambulation and fine movements. High doses of both drugs disrupted spontaneous alternation, a measure of working memory, but high doses of only memantine elicited perseverative behavior. Surprisingly, ketamine\u27s effects were influenced by the delay between drug administration and testing no more frequently than were memantine\u27s. Our findings show that, regardless of test delay, memantine and ketamine evoke similar behavioral effects at lower doses, consistent with NMDA receptors being both drugs\u27 principal site of action, but can have divergent effects at higher doses. Our results suggest that the divergence of memantine\u27s and ketamine\u27s behavioral consequences is likely to result from differences in mechanisms of NMDA receptor antagonism or actions at other targets. © 2013 Elsevier Inc. All rights reserved

Distinct pharmacological and functional properties of NMDA receptors in mouse cortical astrocytes

Background and purpose Astrocytes of the mouse neocortex express functional NMDA receptors which are not blocked by Mg2+ions. However, study of the pharmacological profile of glial NMDARs and their subunit composition is far from complete. Experimental approach In the present study we tested the sensitivity of NMDA receptor-mediated currents in the cortical astrocytes and neurons to the novel GluN2C/D subunit-selective antagonist UBP141. We also examined the action of memantine, an antagonist reported to have substantially different affinities for GluN2A/B and GluN2C/D-containing receptors in physiological concentrations of extracellular Mg2+ . Key results UBP141 had a strong inhibitory action on NMDA receptor-mediated transmembrane currents in the cortical layer II/III astrocytes with an IC50 of 2.29 μM and a modest inhibitory action on NMDA-responses in the pyramidal neurons with IC50 of 19.8 μM. Astroglial and neuronal NMDA receptors exhibited different sensitivities to memantine with corresponding IC50 of 2.19 and 10.8 μM, respectively. Consistent with pharmacological differences between astroglial and neuronal NMDA receptors, NMDA receptors in astrocytes showed lower Ca2+-permeability than neuronal ones with PCa/PNa ratio of 3.4. Conclusions and Implications The combination of biophysical and pharmacological properties of astrocytic NMDA receptors strongly suggests they have tri-heteromeric structure composed of GluN1, GluN2C/D and GluN3 subunits. Dramatic difference between astroglial and neuronal NMDA receptors in their sensitivity to UBP141 and memantine may enable selective modulation of astrocytic signalling that could be very helpful for elucidating the mechanisms of neuron-glia communications. Our results may also provide a clue for development novel therapeutic agents specifically targeting glial signalling

Memantine binding to a superficial site on NMDA receptors contributes to partial trapping

Although many nervous system disorders are associated with N-methyl-d-aspartate (NMDA) receptor overactivation, pharmacological inhibition of NMDA receptors has typically demonstrated limited clinical value due to debilitating psychotomimetic side-effects. Memantine, however, induces far fewer behavioural side-effects than other NMDA receptor channel blockers such as ketamine, and slows the progressive cognitive decline associated with Alzheimer's disease. Memantine and ketamine inhibit NMDA receptors with similar affinity and kinetics. A prominent mechanistic difference between memantine and ketamine is the degree to which they are ‘trapped’ within the closed channel of NMDA receptors following removal of agonist: ketamine becomes trapped in nearly all NMDA receptors to which it was bound before agonist removal, whereas some bound memantine molecules dissociate after agonist removal, a phenomenon called partial trapping. Here we investigated the mechanism underlying partial trapping of memantine by recombinant NR1/2A NMDA receptors. We found that memantine dissociation from NR1/2A receptors after agonist removal (the process that results in partial trapping) followed an exponential time course with τ= 0.79 ± 0.32 s. Neither membrane voltage depolarization nor maintained presence of memantine after agonist removal affected partial trapping, suggesting that partial trapping does not result from memantine escape through open channels. We tested the hypothesis that partial trapping results from binding of memantine to two sites, a superficial ‘non-trapping’ site and a deep ‘trapping’ site, which cannot be occupied simultaneously. This hypothesis was supported by the lack of ketamine binding to the superficial site, the voltage dependence of partial trapping, and the effect on partial trapping of a mutation near the deep site. The superficial binding site for memantine may, by causing partial trapping, contribute to memantine's unique therapeutic utility