Immunocytochemical and in situ hybridization studies of the expression and distribution of three subunits of a complex with N-methyl-D-aspartate receptor-like properties.

A group of four proteins with recognition sites for L-glutamate, N-methyl-D-aspartate, glycine, and competitive and non-competitive inhibitors of N-methyl-D-aspartate receptors was previously purified from rat brain synaptic membranes. The biochemical and immunochemical characteristics of this complex, as well as the sequences of the complementary DNAs of three subunits, are distinct from those of other glutamate receptors, transporters, or enzymes. The function of this complex has not yet been defined, but it appears to be involved in glutamate-induced neuronal excitation and toxicity. It is not known whether all protein components of the complex are expressed in the same populations of brain cells. In the present study, immunohistochemical and in situ hybridization were used to map the distribution of the glutamate-binding, glycine/thienylcyclohexylpiperidine-binding, and carboxypiperazinyl-propylphosphonate-binding protein subunits of the complex. These proteins were abundantly expressed in pyramidal neurons of the hippocampus and cerebral cortex, and in granule cells of the dentate gyrus, cerebellum, and olfactory tubercle. Based on these results, it was concluded that the three subunits of the complex have similar patterns of expression in rat brain. The distribution of one subunit of the complex, glutamate-binding protein, was traced throughout the rat brain, thus providing a potential map of the expression of the complex in rodent brain. In addition, probes were developed in the present study that should be useful in future explorations of the role of these proteins in brain function and of the possible co-localization of the protein subunits in single cells or cell processes.