Single channel analysis of a novel NMDA channel from Xenopus oocytes expressing recombinant NR1a, NR2A and NR2D subunits.

Two types of subunit (an NR1 subunit and one of the NR2 subunits) are sufficient to form efficient NMDA receptors. In order to investigate whether functional receptors may contain more than one sort of NR2 subunit, NR1, NR2A and NR2D subunits were co-expressed in Xenopus oocytes. Single channel recordings from the oocytes showed, as expected, channel openings that are characteristic of NR1/NR2A receptors (ca 50 and 40 pS conductances), and channels characteristic of NR1/NR2D receptors (ca 40 and 20 pS conductances). However, they also contained a novel channel that contained conductances of ca 30, 40 and 50 pS, with direct transitions between all three levels. It is postulated that the novel channel contains NR1, NR2A and NR2D subunits. The implications of this for receptor stoichiometry are discussed. The novel channel was intermediate between the NR1/NR2A and NR1/NR2D 'duplet' receptors in the length of the channel activations, and in the probability of being open during activation. Its glycine sensitivity was much higher than that of NR1/NR2A, and was comparable with that of the other 'duplet' receptors.