Abstract

Intracellular Mg²⁺ (Mg_i²⁺) inhibits theN-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⁺.