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The N-methyl-D-aspartate (NMDA) receptor is a ligand-gated ion channel involved in excitatory neurotransmission, synaptic plasticity, and neuronal cell death. Two families of NMDA receptor subunits exist: the NR1 subunit (1), which is found in eight alternatively spliced isoforms (2), and the NR2 subunit, for which four subtypes have been described (NR2A-D), each one being a different gene product (3-7). Functional NR1 homomers apparently can be assembled in frog oocytes (1), whereas receptors in mammalian cells likely are formed by the coassembly of the NR1 subunit with at least one type of NR2 subunit (8-13). Although it recently has been suggested that functional NMDA channels most likely contain two NR1 subunits (ref. 14, but see ref. 15), the number of NR2 subunits that coassemble with NR1 has yet to be determined.Individual NR2 subunits coassembling with NR1 produce recombinant NMDA receptors with varying sensitivity to pharmacological agents (16-19) and different channel permeation and kinetic properties (4,6,(20)(21)(22)(23)(24). Hence, the number and type of NR2 subunits in a single receptor likely will influence its overall function. In the present study, we have developed a series of profiles, based on unique biophysical and pharmacological properties, for four different combinations of NR1, NR2A, NR2B, and NR2C subunits transiently expressed in Chinese hamster ovary (CHO) cells (25).

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