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Glutamate is the major excitatory neurotransmitter in the brain. The NMDA subtype of glutamate receptors (NMDAR) is known to mediate many physiological neural functions. However, excessive activation of NMDARs contributes to neuronal damage in various acute and chronic neurological disorders. To avoid unwanted adverse side effects, blockade of excessive NMDAR activity must therefore be achieved without affecting its physiological function. Memantine, an adamantane derivative, has been used for the treatment of Alzheimer's disease with an excellent clinical safety profile. We previously showed that memantine preferentially blocked neurotoxicity mediated by excessive NMDAR activity while relatively sparing normal neurotransmission, in part because of its uncompetitive antagonism with a fast off-rate. Here, using rat autaptic hippocampal microcultures, we show that memantine at therapeutic concentrations (1–10 μm) preferentially blocks extrasynaptic rather than synaptic currents mediated by NMDARs in the same neuron. We found that memantine blocks extrasynaptic NMDAR-mediated currents induced by bath application of 100 μmNMDA/10 μmglycine with a twofold higher potency than its blockade of the NMDAR component of evoked EPSCs (EPSCsNMDAR); this effect persists under conditions of pathological depolarization in the presence of 1 mmextracellular Mg2+. Thus, our findings provide the first unequivocal evidence to explain the tolerability of memantine based on differential extrasynaptic/synaptic receptor blockade. At therapeutic concentrations, memantine effectively blocks excessive extrasynaptic NMDAR-mediated currents, while relatively sparing normal synaptic activity.

NMDA receptor activity is modulated by various compounds, including sulfhydryl redox agents and Zn2+. In addition to a slow and persistent component of redox modulation common to all NMDA receptors, NR1/NR2A receptors uniquely have a rapid and reversible component that has been variously attributed to redox or Zn2+effects. Here we show that this rapid modulatory effect can be described by two time constants with relatively fast (∼6 sec) and intermediate (60 sec) half lives, and it is likely to be attributable to both redox agents and Zn2+. Using site-directed mutagenesis, we identified three pairs of cysteine residues that underlie the various kinetic components of redox modulation of NMDA-evoked currents inXenopusoocytes expressing NR1/NR2A receptors: (1) Cys 87 and Cys 320 in NR2A underlie the fast component, (2) Cys 79 and Cys 308 in NR1 underlie the intermediate component, and (3) Cys 744 and Cys 798 in NR1 underlie the persistent component. Mutation of these redox-sensitive cysteine residues also affects high-affinity, voltage-independent Zn2+inhibition that is specific to NR1/NR2A receptors. Exposure to methanethiosulfonate agents that modify cysteine residues did not block the Zn2+inhibition. Thus, these cysteine residues do not appear to coordinate Zn2+directly. Instead, the redox status of these cysteine residues may modulate the sensitivity of the receptor to Zn2+.