High level expression of the NMDAR1 glutamate receptor subunit in electroporated COS cells

The rat N-methyl-D-aspartate (NMDA) glutamate receptor subunit NR1-1a was transiently expressed in COS cells using the technique of electroporation, which was fivefold more efficient than the calcium phosphate precipitation method of transfection. The glycine site antagonist 5,7- [³H]dichlorokynurenic acid labeled a single high-affinity site (K(D) = 29.6 ± 6 nM; B(max) = 19.4 ± 1.6 pmol/mg of protein) in membranes derived from COS cells electroporated with NR1-1a. In contrast to previous reports using transiently transfected human embryonic kidney 293 cells, binding of the noncompetitive antagonist (+)-5-[³H]methyl-10,11-dihydro-5H-dibenzo[a,d]- cyclohepten-5,10-imine ([³H]MK-801) was not detected in NR1-1a-transfected COS cells. Although immunofluorescent labeling of electroporated COS cells demonstrated that the NR1-1a protein appears to be associated with the cell membrane, neither NMDA nor glutamate effected an increase in intracellular calcium concentration in fura-2-loaded cells, suggesting that homomeric NR1- 1a receptors do not act as functional ligand-gated ion channels. Therefore, COS cells appear to differ from Xenopus oocytes with respect to the transient expression of functional homomeric NR1 receptors. Although expression of NR1- 1a is sufficient to reconstitute a glycine binding site with wild-type affinity for antagonists in COS cells, recombinant homomeric NR1-1 a receptors do not display properties that are characteristic of native NMDA receptors, such as permeability to Ca²⁺ and channel occupancy by MK-801, when expressed in this mammalian cell line.