EFFECTS OF MEMANTINE ON RECOMBINANT RAT NMDA RECEPTORS EXPRESSED IN HEK-293 CELLS

1 The actions of the uncompetitive N-methyl-D-aspartate (NMDA) recepto r antagonists, memantine (1-amino-3,5-dimethyladamantane) and (+)-MK-8 01 ethyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,1 0-imine maleate, dizocilpine), on recombinant NMDA receptors has been studied by use o f the whole-cell patch clamp technique. 2 Human embryonic kidney (HEK) 293 cells were transiently transfected with different NMDA receptor s ubunit combinations (NR1a/NR2A, NR1a/NR2B and NR1a/NR2D). A mutant for m of the green fluorescent protein (GFP) was cotransfected with the NM DA receptor subunits to enable the visualization of transfected cells. 3 Memantine (0.3-30 mu M) blocked L-glutamate (100 mu M)-mediated cur rents in a concentration-dependent manner in NR1a/NR2A, NR1a/NR2B and NR1a/NR2D transfected cells with IC50 values (at -70 mV) of 0.93 +/- 0 .15 mu M, 0.82 +/- 0.12 mu M and 0.47 +/- 0.06 mu M (mean +/- s.e. mea n), respectively. 4 The memantine-induced block was strongly voltage-d ependent. Alteration of the holding potential from -70 mV to +60 mV re sulted in an e-fold increase in the IC50 values per 30-33 mV change in membrane potential, for all 3 subunit combinations investigated. 5 Th e kinetics of the actions of memantine (30 mu M) were investigated for the NR1a/2A combination, in 6 cells (13 - 15 determinations). At -70 mV, the block and recovery from block were both best described by two exponentials with time-constants of 201 +/- 23 ms (81 +/- 2%) and 3.9 +/- 0.6 s and 597 +/- 94 ms (18 +/- 1%) and 18.6 +/- 2.4 s, respective ly. The predominant effect of depolarization was to increase the weigh t of the faster recovery time-constant. Kinetic analysis suggests that these results are consistent with previously proposed Markov models. 6 (+)-MK-801 was studied briefly for comparative purposes. (+)-MK-801 (200 nM) preferentially blocked NMDA receptor currents (at -70 mV) in NR1a/NR2A and NR1a/NR2B (82 +/- 10% and 93 +/- 2% depressions) compare d to NR1a/NR2D (38 +/- 7%) transfected cells. (+)-MK-801 appeared to b e less voltage-dependent than memantine on all three receptor combinat ions. 7 In conclusion, memantine was a voltage-dependent antagonist of recombinant rat NMDA receptors expressed in HEK 293 cells but showed little selectivity between the subunits investigated. Its actions on t hese recombinant receptor combinations are similar to its actions on n ative NMDA receptors.