NMDA receptor-mediated regulation of AMPA receptor properties in organotypic hippocampal slice cultures.

Activation of the calcium-dependent protease calpain has been proposed to be a necessary step in the formation of long-term potentiation (LTP) in the hippocampus, and stimulation of N-methyl-D-aspartate (NMDA) receptors leads to an increase in intracellular calcium concentration, calpain activation, proteolysis of cytoskeletal elements, and modification of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor properties. In the present study, we evaluated the effects of NMDA treatment of cultured hippocampal slices on the properties of AMPA receptors. Cultured hippocampal slices were treated with NMDA (100 microM) for 15 min and [3H]AMPA binding to membrane fractions was measured. NMDA-treated slices exhibited an increase in both "high-affinity" and "low-affinity" [3H]AMPA binding, with smaller changes in 6-cyano-7-nitro[3H]quinoxaline-2,3-dione binding. The increase in [3H]AMPA binding was significantly reduced by preincubation of cultures with calpain inhibitor I or calpeptin (100 microM). Furthermore, NMDA exposure decreased the number of GluR1 subunits of AMPA receptors detected by an antibody against the C-terminal domain of the subunit in western blots and resulted in the formation of a lower molecular weight species detected by an antibody against the N-terminal domain. Both effects were completely prevented by calpain inhibitors. These results indicate that NMDA receptor activation produces calpain activation and complex modifications of AMPA receptor properties, which could be involved in NMDA receptor-mediated changes in synaptic efficacy.