Memantine preferentially blocks extrasynaptic over synaptic NMDA receptor currents in hippocampal autapses

Abstract

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 microM) 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 microM NMDA/10 microM glycine with a twofold higher potency than its blockade of the NMDAR component of evoked EPSCs (EPSCs(NMDAR)); this effect persists under conditions of pathological depolarization in the presence of 1 mm extracellular Mg(2+). 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.

Figure 1.

Low micromolar concentrations of memantine manifest relatively little effect on synaptic NMDAR-mediated currents. A, At 1 μm, memantine exerted a minimal effect on the NMDAR component of the EPSC. For comparison, nearly complete blockade by 1 μm MK-801 is shown. B, At 100 nm, MK-801 also nearly completely blocked synaptic NMDARs. C, After the inhibitory effect reached a plateau, 1 μm memantine blocked only 27.1 ± 1 3% of the NMDAR-mediated component of the EPSC, whereas equimolar MK-801 rendered virtually complete blockade (93.6 ± 0.5%). To reach a plateau with 100 nm MK-801, we performed prolonged recordings and still observed 91.6 ± 0.7% blockade of synaptic currents; 10 nm MK-801, although not reaching a plateau, also blocked nearly all of the EPSC_NMDAR (data not shown). D, At 1 μm, memantine blockade of the NMDAR-mediated component of the EPSC reached a plateau within 100 s, whereas 1 μm MK-801 required ∼500 s. Electrical pulses to elicit EPSCs were delivered every 10 s. Values are mean ± SEM, measured at peak current (n = 28; *p < 0.0001 by t test).

Figure 2.

Memantine block of extrasynaptic NMDARs activated by bath application of 100 μm NMDA. A, B, Inhibitory effect of 1 μm memantine or MK-801 on NMDA-induced extrasynaptic currents at a holding potential of −70 mV. C, At 100 nm, MK-801 also completely blocked extrasynaptic NMDARs. D, Memantine blocked predominantly extrasynaptic NMDAR-mediated current (50.4 ± 1.3%) compared with synaptic NMDARs (27.1 ± 1.3%), whereas MK-801 blocked both equally well regardless of concentration (∼96% block by 1 μm and ∼92% at 100 nm). Values are mean ± SEM (n = 28; *p < 0.0001).

Figure 3.

Memantine preferentially blocks extrasynaptic NMDARs. A low (1 μm) concentration of memantine blocked extrasynaptic over synaptic NMDARs at a ratio of ∼2:1, whereas 1 μm or 100 nm MK-801 blocked both synaptic and extrasynaptic NMDARs equally. Values are mean ± SEM (n = 28; *p < 0.0001).

Figure 4.

Therapeutic concentrations of memantine block pathological NMDA-induced currents. A, At 10 μm, memantine exerted a minimal effect on the NMDAR-mediated component of the EPSC at a holding potential of −25 mV in the presence of 1 mm extracellular Mg²⁺. B, Under the same conditions, 10 μm memantine displayed a substantial inhibitory effect on 100 μm NMDA-induced extrasynaptic currents. C, Memantine (10 μm) significantly blocked more extrasynaptic than synaptic NMDAR-mediated currents when neurons were pathologically depolarized (V _h = −25 mV; extrasynaptic inhibition, 64.3 ± 2.5%; synaptic inhibition, 39.8 ± 2.5%; n = 7; *p < 0.0005 by paired t test). Values are mean ± SEM.