Differential cellular and subcellular localization of AMPA receptor-binding protein and glutamate receptor-interacting protein

Excitatory synaptic currents in the mammalian brain are typically mediated by the neurotransmitter glutamate, acting at AMPA receptors. We used immuno cytochemistry to investigate the distribution of AMPA receptor-binding prot ein (ABP) in the cerebral neocortex. ABP was most prominent in pyramidal ne urons, although it was also present (at lower levels) in interneurons. ABP and its putative binding partners, the GluR2/3 subunits of the AMPA recepto r, exhibited prominent cellular colocalization. Under appropriate processin g conditions, colocalization could also be documented in puncta, many of wh ich could be recognized as dendritic spines. However, a sizable minority of GluR2/3-positive puncta were immunonegative for ABP. Because glutamate rec eptor-interacting protein (GRIP) may also anchor GluR2, we studied the rela tive distribution of ABP and GRIP. There was extensive colocalization of th ese two antigens at the cellular level, although GRIP, unlike ABP, was stro ngest in nonpyramidal neurons. Different parts of a single dendrite could s tain selectively for ABP or GRIP. To further characterize this heterogeneit y, we investigated punctate staining of neuropil using synaptophysin and th e membrane tracer DiA to identify probable synapses. Some puncta were compa rably positive for both ABP and GRIP, but the majority were strongly positi ve for one antigen and only weakly positive or immunonegative for the other . This heterogeneity could be seen even within adjacent spines of a single dendrite. These data suggest that ABP may act as a scaffold for AMPA recept ors either in concert with or independently from GRIP.