Vol. 292, Issue 3, 1104-1110, March 2000

Inhibition by Intracellular Mg²⁺ of Recombinant N-Methyl-D-aspartate Receptors Expressed in Chinese Hamster Ovary Cells¹

Yingying Li-Smerin² , Elias Aizenman and Jon W. Johnson

Department of Neuroscience, University of Pittsburgh (Y.L.-S., J.W.J.) and Department of Neurobiology, University of Pittsburgh School of Medicine (E.A.), Pittsburgh, Pennsylvania

Intracellular Mg²⁺ (Mg_i²⁺) inhibits the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors in cultured cortical neurons. To examine the effects of Mg_i²⁺ on recombinant NMDA receptors composed of subunit combinations found in cortical neurons, we expressed heteromeric receptors composed of NR1/NR2A and of NR1/NR2B subunits in Chinese hamster ovary (CHO) cells. We recorded whole-cell currents from the recombinant receptors in the absence and presence of Mg_i²⁺. The voltage dependence of control (0 Mg_i²⁺) NMDA-activated currents obtained from CHO cells transfected with NR1/NR2A and with NR1/NR2B receptors showed outward rectification, a property that has been observed previously in native cortical NMDA receptors. The magnitude and voltage dependence of inhibition by Mg_i²⁺ of NMDA-activated currents were similar in CHO cells transfected with NR1/NR2A receptors, CHO cells transfected with NR1/NR2B receptors, and in cultured neurons expressing native NMDA receptors. These observations suggest that Mg_i²⁺ has uniform effects on the native NMDA receptors expressed in cortical neurons. Furthermore, inhibition by Mg_i²⁺ must not depend on intracellular factors or post-translational receptor modifications that are specific to neurons. Finally, the results indicate that the previously observed differences between whole-cell and outside-out patch measurements of Mg_i²⁺ inhibition could not result from poor control of voltage or Mg_i²⁺ concentration in the dendrites of neurons. The most likely alternative explanation is that patch excision causes an alteration in NMDA receptors that results in more effective inhibition by Mg_i²⁺.