Blocking kinetics of memantine on NR1a/2A receptors recorded in inside-out and outside-out patches from Xenopus oocytes

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.

Inhibition of NMDA receptors through a membrane-to-channel path

N-methyl-d-aspartate receptors (NMDARs) are transmembrane proteins that are activated by the neurotransmitter glutamate and are found at most excitatory vertebrate synapses. NMDAR channel blockers, an antagonist class of broad pharmacological and clinical significance, inhibit by occluding the NMDAR ion channel. A vast literature demonstrates that NMDAR channel blockers, including MK-801, phencyclidine, ketamine, and the Alzheimer’s disease drug memantine, can bind and unbind only when the NMDAR channel is open. Here we use electrophysiological recordings from transfected tsA201 cells and cultured neurons, NMDAR structural modeling, and custom-synthesized compounds to show that NMDAR channel blockers can enter the channel through two routes: the well-known hydrophilic path from extracellular solution to channel through the open channel gate, and also a hydrophobic path from plasma membrane to channel through a gated fenestration (“membrane-to-channel inhibition” (MCI)). Our demonstration that ligand-gated channels are subject to MCI, as are voltage-gated channels, highlights the broad expression of this inhibitory mechanism.

Wilcox et al. (2022) show that NMDA receptor channel blockers, some of which are clinically important drugs, can access their binding site via 2 routes: a well-known path from the extracellular solution, and another path through the plasma membrane.

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+.

Memantine block depends on agonist presentation at the NMDA receptor in substantia nigra pars compacta dopamine neurones

NMDA glutamate receptors (NMDARs) have critical functional roles in the nervous system but NMDAR over-activity can contribute to neuronal damage. The open channel NMDAR blocker, memantine is used to treat certain neurodegenerative diseases, including Parkinson's disease (PD) and is well tolerated clinically. We have investigated memantine block of NMDARs in substantia nigra pars compacta (SNc) dopamine neurones, which show severe pathology in PD. Memantine (10 μM) caused robust inhibition of whole-cell (synaptic and extrasynaptic) NMDARs activated by NMDA at a high concentration or a long duration, low concentration. Less memantine block of NMDAR-EPSCs was seen in response to low frequency synaptic stimulation, while responses to high frequency synaptic stimulation were robustly inhibited by memantine; thus memantine inhibition of NMDAR-EPSCs showed frequency-dependence. By contrast, MK-801 (10 μM) inhibition of NMDAR-EPSCs was not significantly different at low versus high frequencies of synaptic stimulation. Using immunohistochemistry, confocal imaging and stereological analysis, NMDA was found to reduce the density of cells expressing tyrosine hydroxylase, a marker of viable dopamine neurones; memantine prevented the NMDA-evoked decrease. In conclusion, memantine blocked NMDAR populations in different subcellular locations in SNc dopamine neurones but the degree of block depended on the intensity of agonist presentation at the NMDAR. This profile may contribute to the beneficial effects of memantine in PD, as glutamatergic activity is reported to increase, and memantine could preferentially reduce over-activity while leaving some physiological signalling intact.