Single-channel currents from recombinant NMDA NRla/NR2D receptors expressed in Xenopus oocytes

摘要

We have investigated the single-channel and whole-cell behaviour of recombinant N-methyl-D-aspartate (NMDA) receptors formed from NRla and NR2D receptor subunits expressed in Xenopus oocytes. The EC_(50) for apparent steady-state activation of NRla/NR2D receptors by glutamate was 450 nM, while extracellular Mg~(2+) produced a voltage-dependent block of glutamate responses with an IC_(50) of 440μM at -- 70 mV, At negative holding potentials glutamate-activated NRla/NR2D single-channel currents, in 0,85 mM external Ca~(2+), 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+004ms, whereas the mean open times of 17 pS events were consistently longer (1 28 + 0,06 ms). In 5 mM external Ca~(2+) 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 sub-conductance levels showed temporal asymmetry: 35-17 pS transitions were moie frequent (61 %) than 17-35 pS transitions. This asymmetry was not affected by alterations in the external Ga~(2+) concentration (up to 5 mM). In conclusion, the NRla/NR2D channelis, like NRla/NR2C, a 'low conductance' NMDA channel, but it can be distinguished from NRla/NR2C channels on the basis of transition asymmetry and differences in the open times of its main and sub-conductance levels.