The glutamate receptor interacting protein 1 (GRIP1) is a scaffolding protein in postsynaptic density (PSD), tethering AMPA receptors to other signaling proteins. Here we report that glutamate stimulation caused a rapid reduction in protein levels of GRIP1, but not that of glutamate receptor (GluR) 1, GluR2 and protein interacting with C kinase 1 (PICK1) in rat primary cortical neuron cultures. Down-regulation of GRIP1 by glutamate was blocked by carbobenzoxyl-leucinyl-leucinyl-leucinal (MG132), a proteasome inhibitor and by expression of K48R-ubiquitin, a dominant negative form of ubiquitin. The GRIP1 reduction was inhibited by MK-801, an N-methyl-d-aspartate (NMDA) receptor antagonist, but not by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an AMPA receptor antagonist. EGTA and 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetra acetic acid tetrakis (BAPTA), two Ca2+ chelators, but not nifedipine, an L-type Ca2+ channel blocker, prevented GRIP1 degradation. Furthermore, MG132 prevented glutamate-stimulated reduction in surface amount of GluR2, and knockdown of GRIP1 by RNAi against GRIP1 reduced surface GluR2 in neurons. Our results suggest that glutamate induces GRIP1 degradation by proteasome through an NMDA receptor-Ca2+ pathway and that GRIP1 degradation may play an important role in regulating GluR2 surface expression.