Single-channel currents from recombinant NMDA NR1a/NR2D receptors expressed in Xenopus oocytes.
We have investigated the single-channel and whole-cell behaviour of recombinant N-methyl-D-aspartate (NMDA) receptors formed from NR1a and NR2D receptor subunits expressed in Xenopus oocytes. The EC50 for apparent steady-state activation of NR1a/NR2D receptors by glutamate was 450 nM, while extracellular MG2+ produced a voltage-dependent block of glutamate responses with an IC50 of 440 microM at -70 mV. At negative holding potentials glutamate-activated NR1a/NR2D single-channel currents, in 0.85 mM external Ca2+, had slope conductances of 35 pS for the main level, and 17 pS for the sublevel; direct transitions occurred between these two conductance levels. On average 35 pS events had mean open times of 1.01 +/- 0.04 ms, whereas the mean open times of 17 pS events were consistently longer (1.28 +/- 0.06 ms). In 5 mM external Ca2+ the larger conductance level was reduced to 20 pS whereas in Ca(2+)-free solutions it was increased to 50 pS. The frequency of transitions between the main and subconductance levels showed temporal asymmetry: 35-17 pS transitions were more frequent (61%) than 17-35 pS transitions. This asymmetry was not affected by alterations in the external Ca2+ concentration (up to 5 mM). In conclusion, the NR1a/NR2D channel is, like NR1a/NR2C, a 'low conductance' NMDA channel, but it can be distinguished from NR1a/NR2C channels on the basis of transition asymmetry and differences in the open times of its main and sub-conductance levels.
Wyllie
D J
DJ
Department of Pharmacology, University College London, U.K.
Béhé
P
P
Nassar
M
M
Schoepfer
R
R
Colquhoun
D
D
D000818:Animals; D007473:Ion Channels / Q000235:genetics / Q000378:metabolism*; D017136:Ion Transport; D016202:N-Methylaspartate / Q000378:metabolism; D016194:Receptors, N-Methyl-D-Aspartate / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D014981:Xenopus
D007473:Ion Channels; D016194:Receptors, N-Methyl-D-Aspartate; D011994:Recombinant Proteins; D016202:N-Methylaspartate