Abstract
Intracellular Mg2+ (Mgi2+) inhibits theN-methyl-d-aspartate (NMDA) subtype of glutamate receptors in cultured cortical neurons. To examine the effects of Mgi2+ 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 Mgi2+. The voltage dependence of control (0 Mgi2+) 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 Mgi2+ 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 Mgi2+ has uniform effects on the native NMDA receptors expressed in cortical neurons. Furthermore, inhibition by Mgi2+ 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 Mgi2+ inhibition could not result from poor control of voltage or Mgi2+ 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 Mgi2+.
Footnotes
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Send reprint requests to: Jon W. Johnson, Department of Neuroscience, 446 Crawford Hall, University of Pittsburgh, Pittsburgh, PA 15260. E-mail: johnson{at}bns.pitt.edu
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↵1 This work was supported by National Institutes of Health Grants MH45817 and MH00944 (to J.W.J.) and NS29365 (to E.A.), and by National Institute of Mental Health Training Grant T32 MH18273 (to Y.L.-S.).
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↵2 Current address: Molecular Physiology and Biophysics Unit, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892.
- Abbreviations:
- NMDA
- N-methyl-d-aspartate
- CHO
- Chinese hamster ovary
- GFP
- green fluorescent protein
- FC
- fractional current
- I-V
- current-voltage
- Received September 9, 1999.
- Accepted November 28, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
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