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1 Department of Neuroscience, University of Pittsburgh, Pittsburgh, PA 15260, USA
The vital roles played by NMDA receptors in CNS physiology depend critically on powerful voltage-dependent channel block by external Mg2+ (Mg2+o). NMDA receptor channel block by Mg2+o depends on receptor subunit composition: NR1/2A receptors (receptors composed of NR1 and NR2A subunits) and NR1/2B receptors are more strongly inhibited by Mg2+o than are NR1/2C or NR1/2D receptors. We investigated the effects of Mg2+o on single-channel and whole-cell currents recorded from recombinant NR1/2D and NR1/2A receptors expressed in HEK293 and 293T cells. The main conclusions are as follows: (1) Voltage-dependent inhibition by Mg2+o of whole-cell NR1/2D receptor responses was at least 4-fold weaker than inhibition of NR1/2A receptor responses at all voltages tested. (2) Channel block by Mg2+o reduced the duration of NR1/2D receptor single-channel openings; this reduction was used to estimate the apparent blocking rate of Mg2+o (k+,app). The k+,app for NR1/2D receptors was similar to but moderately slower than the k+,app obtained from cortical NMDA receptors composed of NR1, NR2A and NR2B subunits at all voltages tested. (3) Mg2+o blocking events induced an additional component in the closed-duration distribution; this component was used to estimate the apparent unblocking rate of Mg2+o (k–,app). The k–,app for NR1/2D receptors was much faster than the k–,app for cortical receptors at all voltages tested. The voltage-dependence of the k–,app of NR1/2D and cortical receptors differed in a manner that suggested that Mg2+o may permeate NR1/2D receptors more easily than cortical receptors. (4) Mg2+o inhibits NR1/2D receptors less effectively than cortical receptors chiefly because Mg2+o unbinds much more rapidly from NR1/2D receptors.
(Received 4 October 2004;
accepted after revision 27 October 2004;
first published online 28 October 2004)
Corresponding author J. W. Johnson: Department of Neuroscience, 446 Crawford Hall, University of Pittsburgh, Pittsburgh, PA 15260, USA. Email: johnson{at}bns.pitt.edu
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