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

Previous experiments on primary cultures of hippocampal/cortical neurones revealed that the block and unblock of N-Methyl-D-Aspartate (NMDA) receptor channels by memantine showed double exponential kinetics and that the offset kinetics following a voltage-step were much faster than following a concentration jump. There are, however, two major problems when using such cultured primary neurones for these experiments (1) the almost certain expression of heterogeneous NMDA receptor subunits which could underlie double exponential kinetics due to different potencies at receptor subtypes and (2) slow space- and concentration-clamp due to neuronal morphology which could mask even faster kinetics. Therefore, we performed similar experiments with Xenopus oocytes exclusively expressing one NMDA receptor type (NR1a/2A) at high levels which allowed recordings from membrane patches with large currents. The use of inside-out patches for voltage-step and outside-out patches in combination with a piezo driven fast application system largely negated potential space- and concentration-clamp problems. Block and unblock of the NMDA receptor by memantine after both voltage jump and concentration jumps showed triple exponential kinetics. The fast onset kinetics of NMDA receptor channel block following both concentration-clamp and voltage jumps from +70 to -70 mV were similar. In contrast, offset kinetics after a voltage-step from -70 to +70 mV were much faster than following a concentration jump at the holding potential of -70 mV. These results provide further support for the hypothesis that rapid relief of block via strong synaptic membrane depolarisation underlies the good therapeutic profile of memantine.