Abstract
We investigated the role of in vivo synaptic activity upon trafficking of the N-methyl-D-aspartate (NMDA) receptor subunit, NR2B, at mature synapses by electron microscopic immunocytochemistry. In vivo blockade of NMDA receptors was achieved by applying the NMDA receptor antagonist, D-2-amino-5-phosphonovalerate (D-APV), onto the cortical surface of one hemisphere of anesthetized adult rats. Inactive L-2-amino-5-phosphonovalerate (L-APV) was applied to the contralateral hemisphere for within-animal control and to assess basal level of NR2B subunits at synapses. Within 30 min of D-APV treatment, we observed a decrease in the number of layer I axo-spinous asymmetric synapses that are positively immuno-labeled for the NR2B subunits. This decrease was paralleled by reductions in the absolute number of immuno-gold particles found at these synapses. The decrease of NR2B labeling was detectable in all five animals examined. Significant reductions were seen not only at post-synaptic densities, but also within the cytoplasm of spines and axon terminals. The data demonstrate that blockade of NMDA receptors induces trafficking of NR2B subunits out of synaptic membranes, spines, and terminals. This is in sharp contrast to a previous observation that NR2A subunits move into spines and axon terminals following in vivo blockade with D-APV. These findings point to yet unknown, NMDA receptor activity-dependent mechanisms that separately regulate the localization of NR2A and NR2B subunits at synapses.