Quantitative analysis of AMPA receptor subunit composition in addiction-related brain regions

The subunit composition of α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors (AMPARs) is an important determinant of AMPAR biophysical properties and trafficking. To date, AMPAR subunit composition has been quantitatively evaluated only for the hippocampus, where different experimental approaches have yielded different results. Here, we used quantitative co-immunoprecipitation to characterize GluA1-3 associations in the adult rat nucleus accumbens, dorsal striatum, prefrontal cortex, and hippocampus, and blue native electrophoresis (BNE) to study GluA1-3 assembly state. In all brain regions, co-immunoprecipitation experiments showed that ~90% of GluA1 was associated with GluA2 or GluA3 (most was GluA1A2). All regions contained a small number of GluA1A3 receptors. Homomeric GluA1 receptors may also exist. More than half of the GluA2 (53%-65% depending on the region) was not associated with GluA1. However, this represents an over-estimate of the percent of GluA2 present in GluA2A3 receptors, based on BNE results demonstrating that the majority of GluA2 exists as dimers, rather than functional tetrameric receptors. Relatively more GluA1 was present in tetramers. Together with other findings, our results suggest a dominant role for GluA1A2 receptors in all brain regions examined. They also help explain why different results for hippocampal AMPAR subunit composition were obtained using co-immunoprecipitation, which assesses the total cellular pool of AMPARs including partially assembled AMPARs in intracellular compartments, and electrophysiological approaches, which can selectively assess tetrameric (functional) AMPARs on the cell surface.