Subcellular segregation of distinct heteromeric NMDA glutamate receptors in the striatum

Functional N-methyl-d-aspartate (NMDA) glutamate receptors are composed of heteromeric complexes of NR1, the obligatory subunit for channel activity, and NR2 or NR3 family members, which confer variability in the properties of the receptors. Recent studies have provided evidence for the existence of both binary (containing NR1 and either NR2A or NR2B) and ternary (containing NR1, NR2A, and NR2B) receptor complexes in the adult mammalian brain. However, the mechanisms regulating subunit assembly and receptor localization are not well understood. In the CNS, NMDA subunits are present both at intracellular sites and the post-synaptic membrane of neurons. Using biochemical protein fractionation and co-immunoprecipitation approaches we have found that in rat striatum binary NMDA receptors are widely distributed, and can be identified in the light membrane, synaptosomal membrane, and synaptic vesicle-enriched subcellular compartments. In contrast, ternary receptors are found exclusively in the synaptosomal membranes. When striatal proteins are chemically cross-linked prior to subcellular fractionation, ternary NMDA receptors can be precipitated from the light membrane and synaptic vesicle-enriched fractions where this type of receptor complex is not detectable under normal conditions. These findings suggest differential targeting of distinct types of NMDA receptor assemblies between intracellular and post-synaptic sites based on subunit composition. This targeting may underlie important differences in the regulation of the transport pathways involved in both normal as well as pathological receptor functions.