SINGLE-CHANNEL CURRENTS FROM RECOMBINANT NMDA NR1A NR2D RECEPTORS EXPRESSED IN XENOPUS OOCYTES/

We have investigated the single-channel and whole-cell behaviour of re combinant N-methyl-D-aspartate (NMDA) receptors formed from NR1a and N R2D receptor subunits expressed in Xenopus oocytes. The EC,, for appar ent steady-state activation of NR1a/NR2D receptors by glutamate was 45 0 nM, while extracellular Mg2+ produced a voltage-dependent block of g lutamate responses with an IC50 of 440 mu M at -70 mV. At negative hol ding potentials glutamate-activated NR1a/NR2D single-channel currents, in 0.85 mM external Ca2+, had slope conductances of 35 pS for the mai n level, and 17 pS for the sublevel; direct transitions occurred betwe en 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 Ca2+-free so lutions it was increased to 50 pS. The frequency of transitions betwee n the main and subconductance levels showed temporal asymmetry: 35-17 pS transitions were more frequent (61%,) than 17-35 pS transitions. Th is asymmetry was not affected by alterations in the external Ca2+ conc entration (up to 5 mM). In conclusion, the NR1a/NR2D channel is, like NR1a/NR2C, a 'low conductance' NMDA channel, but it can be distinguish ed from NR1a/NR2C channels on the basis of transition asymmetry and di fferences in the open times of its main and sub-conductance levels.