Mg2+ and memantine block of rat recombinant NMDA receptors containing chimeric NR2A/2D subunits expressed in Xenopus laevis oocytes.
N-methyl-d-aspartate receptors (NMDARs) display differences in their sensitivity to the channel blockers Mg(2+) and memantine that are dependent on the identity of the NR2 subunit present in the receptor-channel complex. This study used two-electrode voltage-clamp recordings from Xenopus laevis oocytes expressing recombinant NMDARs to investigate the actions of Mg(2+) and memantine at the two NMDARs displaying the largest differences in sensitivity to these blockers, namely NR1/NR2A and NR1/NR2D NMDARs. In addition, NR2A/2D chimeric subunits have been employed to examine the effects of pore-forming elements and ligand-binding domains (LBD) on the potency of the block produced by each of these inhibitors. Our results show that, as previously documented, NR2D-containing NMDARs are less sensitive to voltage-dependent Mg(2+) block than their NR2A-containing counterparts. The reduced sensitivity is determined by the M1M2M3 membrane-associated regions, as replacing these regions in NR2A subunits with those found in NR2D subunits results in a approximately 10-fold reduction in Mg(2+) potency. Intriguingly, replacing the NR2A LBD with that from NR2D subunits results in a approximately 2-fold increase in Mg(2+) potency. Moreover, when responses mediated by NR1/NR2A NMDARs are evoked by the partial agonist homoquinolinate, rather than glutamate, Mg(2+) also displays an increased potency. Memantine block of glutamate-evoked currents is most potent at NR1/NR2D NMDARs, but no differences are observed in its ability to inhibit NR2A-containing or NR2A/2D chimeric NMDARs. We suggest that the potency of block of NMDARs by Mg(2+) is influenced not only by pore-forming regions but also the LBD and the resulting conformational changes that occur following agonist binding.
Wrighton
David C
DC
Centre for Neuroscience Research, Hugh Robson Building, University of Edinburgh, George Square, Edinburgh, UK.
Baker
Edward J
EJ
Chen
Philip E
PE
Wyllie
David J A
DJ
D000818:Animals; D002678:Chimera; D015259:Dopamine Agents / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D005071:Evoked Potentials / Q000187:drug effects; D005260:Female; D018698:Glutamic Acid / Q000494:pharmacology; D008274:Magnesium / Q000494:pharmacology*; D008559:Memantine / Q000494:pharmacology*; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D011805:Quinolinic Acids / Q000494:pharmacology; D051381:Rats; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors* / Q000187:drug effects / Q000378:metabolism; D014982:Xenopus laevis
D015259:Dopamine Agents; C120997:NR2A NMDA receptor; C464436:NR2D NMDA receptor; D011805:Quinolinic Acids; D016194:Receptors, N-Methyl-D-Aspartate; D018698:Glutamic Acid; C040632:homoquinolinic acid; D008274:Magnesium; D008559:Memantine