Involvement of the secretory pathway for AMPA receptors in NMDA-induced potentiation in hippocampus

A chemical form of synaptic potentiation was produced with a brief bath app lication of NMDA to rat hippocampal slices. Two methods were used to assess changes in membrane-bound AMPA receptors. Traditional subcellular fraction ation was used to isolate synaptic membranes; alternatively, membrane recep tors were cross-linked with the membrane-impermeable reagent bis(sulfosucci nimidyl) suberate, and levels of nonmembrane receptors were determined. In both cases, Western blots were used to determine the content of receptor su bunits in various subcellular fractions. NMDA-induced potentiation was asso ciated with increased levels of glutamate receptor 1 (GluR1) and GluR2/3 su bunits of AMPA receptors in synaptic membrane preparations, whereas no chan ge was observed in whole homogenates. Both KN-62, an inhibitor of calcium/c almodulin kinase, and calpain inhibitor III, a calpain inhibitor, inhibited NMDA-induced potentiation and changes in GluR1 and GluR2/3 subunits of AMP A receptors. Brefeldin A (BFA) inhibits protein trafficking between the Gol gi apparatus and cell membranes. Pretreatment of hippocampal slices with BF A significantly decreased NMDA-induced potentiation and completely prevente d an NMDA-induced increase in GluR1 levels in membrane fractions. Thus, the levels of GluR1 and GluR2/3 subunits of AMPA receptors are rapidly upregul ated in synaptic membranes under conditions associated with potentiation of synaptic responses, and this upregulation requires a functional secretory pathway.