Memantine Blocks 7* Nicotinic Acetylcholine Receptors More Potently Than N-Methyl-D-aspartate Receptors in Rat Hippocampal Neurons

The N-methyl-D-aspartate (NMDA) receptor antagonist memantine is an approved drug for treatment of Alzheimer’s disease (AD). Other such treatments are cholinesterase inhibitors and nicotinic acetylcholine receptor (nAChR)-sensitizing agents such as galantamine. The present study was designed to test whether memantine exerts any effect on the cholinergic system, in particular the Ca -conducting 7* nAChR, in cultured hippocampal neurons. Memantine caused a concentration-dependent reduction of the amplitudes of whole-cell currents evoked by the 7* nAChR-selective agonist choline (10 mM) or by N-methyl-D-aspartate (NMDA) (50 M) plus glycine (10 M). It also inhibited tonically activated NMDA receptors. Memantine was more potent in inhibiting 7* nAChRs than NMDA receptors; at 60 mV, the IC50 values for memantine were 0.34 and 5.1 M, respectively. Consistent with an open-channel blocking mechanism, memantine-induced NMDA receptor inhibition was voltage and use-dependent; the Hill coefficient (nH) was 1. Memantine-induced 7* nAChR inhibition had an nH 1 and showed a variable voltage dependence; the effect was voltage-independent at 0.1 M, becoming voltage-dependent at 1 M. Thus, memantine interacts with more than one class of sites on the 7* nAChRs. One is voltage-sensitive and therefore likely to be within the receptor channel. The other is voltage-insensitive and therefore likely to be in the extracellular domain of the receptor. It is suggested that blockade of 7* nAChRs by memantine could decrease its effectiveness for treatment of AD, particularly at early stages when the degrees of nAChR dysfunction and of cognitive decline correlate well. Alzheimer’s disease (AD) is a neurodegenerative disorder that is characterized by progressive decline of intellectual abilities, is fatal, and aff1icts millions worldwide. It is estimated that by the year 2050, approximately 14 million Americans will have AD (Hebert et al., 2003). Although the cause of AD remains undetermined, the magnitude of cognitive impairment observed as the disease progresses from mild-to-moderate stages correlates well with the degree of loss of 1) nAChRs, particularly those composed of 4, 2, and 7 subunits, in various brain areas (Francis et al., 1985; Perry et al., 2000; Nordberg, 2001); and 2) basal forebrain cholinergic neurons (Geula, 1998). At more advanced stages of the disease, this correlation does not seem to exist (Sabbagh et al., 2001). To date, one of the most successful therapeutic approaches to delay the advancement of AD consists of increasing brain nicotinic cholinergic functions with the anticholinesterase and nicotinic allosteric-potentiating ligand galantamine, or the anticholinesterases donepezil and rivastigmine (Maelicke and Albuquerque, 1996; Pereira et al., 2002; Doody, 2003). There have been reports that increased tonic (JiménezJiménez et al., 1998) and decreased synaptic (Lin et al., 2003) NMDA receptor activities also contribute to the insufficiency of the AD brain. Excessive tonic and decreased synaptic NMDA receptor activities are known to cause impairment of one of the proposed cellular mechanisms for memory storage, long-term potentiation (Frankiewicz and Parsons, 1999). In addition, excitotoxicity resulting from excessive tonic NMDA receptor activation (Hardingham and Bading, 2002) can be one of the processes underlying neuronal loss in AD. Thus, drugs that selectively reduce tonic without altering synaptic NMDA receptor activity in the brain emerged as a promising therapeutic approach for AD. One such drug is the low-affinity, noncompetitive NMDA receptor antagonist memantine (Frankiewicz and Parsons, 1999; Shimono et al., 2002). This work was supported by U.S. Public Health Service Grant NS25296. Article, publication date, and citation information can be found at http://jpet.aspetjournals.org. doi:10.1124/jpet.104.077172. ABBREVIATIONS: AD, Alzheimer’s disease; NMDA, N-methyl-D-aspartate; nAChR, nicotinic acetylcholine receptor; ACh, acetylcholine. 0022-3565/05/3123-1195–1205$20.00 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 312, No. 3 Copyright © 2005 by The American Society for Pharmacology and Experimental Therapeutics 77172/1191584 JPET 312:1195–1205, 2005 Printed in U.S.A. 1195 at A PE T Jornals on A ril 9, 2017 jpet.asjournals.org D ow nladed from A recent placebo-controlled clinical study reported that memantine improves global clinical impression of change, daily performance, and cognition of patients with moderate-to-severe AD (Reisberg et al., 2003). Improvements of global clinical impression and functional performance had also been reported in an earlier placebo-controlled study of memantine in patients with moderately severe-to-severe AD or vascular dementia (Winblad and Poritis, 1999). In contrast, in placebo-controlled studies of patients with mild-to-moderate vascular dementia, a complex disorder in which the cholinergic system is also compromised (Gratham and Geerts, 2002), memantine caused a small, albeit significant, cognitive improvement that was not clinically perceived, given that it was not accompanied by amelioration of the global clinical impression of change (Wilcock, 2003). Furthermore, there is evidence that, resembling nicotinic antagonists (Levin, 2002), memantine impairs cognition in laboratory animals (Willmore et al., 2001) and healthy human subjects (Schugens et al., 1997). In particular, a single dose of memantine (30 mg) administered orally to young, healthy humans impairs the eyeblink classical conditioning (Schugens et al., 1997), a form of associative learning that is modulated by the nicotinic cholinergic system and differentiates cognitive deficits in normal aging and probable AD (Woodruff-Pak, 2001). Therefore, the question is raised as to whether memantine has actions in the brain nicotinic cholinergic system that would preclude its use when reduction of nicotinic cholinergic activity is accompanied by an increase in the degree of cognitive decline. In the cerebrospinal fluid of patients receiving the recommended maintenance dose of 20 mg/day, the concentrations of memantine range from 0.2 to 0.3 M (Kornhuber and Quack, 1995). However, concentrations as high as 1 M may be reached in the extracellular brain compartment (Danysz and Parsons, 2003). At this concentration, in addition to blocking NMDA receptors, memantine blocks human 7 nAChRs and human 4 2 nAChRs ectopically expressed in Xenopus oocytes (Buisson and Bertrand, 1998; Maskell et al., 2003). At 100 to 60 mV, the IC50 values for memantine to inhibit native NMDA receptors (Chen and Lipton, 1997), recombinant 4 2 nAChRs (Buisson and Bertrand, 1998), and recombinant 7 nAChRs (Maskell et al., 2003) range