Modulation by magnesium of the affinity of NMDA receptors for glycine in murine hippocampal neurones.
1. The effects of the divalent cation Mg2+ on NMDA currents recorded from cultured fetal mouse and acutely isolated neonatal rat hippocampal neurones were studied using the whole-cell patch-clamp technique. 2. Current-voltage relations were measured in the presence or absence of applied Mg2+ and added glycine. NMDA-evoked currents were studied in the absence or in a low concentration (0.2 mM) of applied Ca2+ in order to minimize Ca(2+)-dependent inactivation of the responses. Mg2+ unexpectedly enhanced NMDA-activated currents at positive membrane potentials. At negative membrane potentials Mg2+ caused a previously characterized voltage-dependent block of inward NMDA-activated currents. 3. The potentiation by Mg2+ of outward currents activated by NMDA was concentration dependent (EC50, approximately 3 mM; Hill coefficient, approximately 2). Mg2+ also reduced the desensitization of the NMDA receptor. The maximal enhancement of steady-state NMDA-activated currents was 2.7-fold and at 6 mM the time constant of desensitization was doubled. 4. Comparisons of concentration-response curves for glycine and 7-chloro-kynurenic acid demonstrated that Mg2+ significantly increased the affinity of the NMDA receptor for glycine. The EC50 for glycine was 380 nM in the absence of Mg2+ and 163 nM in 3 mM Mg2+. Mg2+ had little effect on the forward rate of the glycine response but halved the off-rate (2.34 to 1.15 s-1) and thus similarly reduced the apparent dissociation constant. 5. There was a good correlation between the concentration of extracellular Ca2+ and a reduction in the time constant of the glycine-sensitive component of NMDA receptor desensitization. Ca2+ could enhance these NMDA-activated currents briefly following exposure to high concentrations of Ca2+. These results are consistent with a Ca(2+)-dependent enhancement of the affinity of the NMDA receptor for glycine. 6. Mg2+ can enhance NMDA-mediated currents and reduce desensitization of this receptor by allosterically interacting with the glycine binding site. This interaction may be a key physiological mechanism through which modulation of the NMDA receptor is achieved.
Wang
L Y
LY
Department of Physiology, University of Toronto, Ontario, Canada.
MacDonald
J F
JF
D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002478:Cells, Cultured; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology*; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D005998:Glycine / Q000378:metabolism*; D018684:Glycine Agents / Q000494:pharmacology*; D006624:Hippocampus / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects; D007736:Kynurenic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D008274:Magnesium / Q000494:pharmacology*; D051379:Mice; D016202:N-Methylaspartate / Q000494:pharmacology; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D051381:Rats; D017208:Rats, Wistar; D016194:Receptors, N-Methyl-D-Aspartate / Q000819:agonists* / Q000037:antagonists & inhibitors / Q000378:metabolism
D018690:Excitatory Amino Acid Agonists; D018691:Excitatory Amino Acid Antagonists; D018684:Glycine Agents; D016194:Receptors, N-Methyl-D-Aspartate; D016202:N-Methylaspartate; D007736:Kynurenic Acid; D008274:Magnesium; C057013:7-chlorokynurenic acid; D002118:Calcium; D005998:Glycine