The NMDA receptor (NMDAR) is a component of excitatory synapses and a key participant in synaptic plasticity. We investigated the role of two domains in the C terminus of the NR2B subunit--the PDZ binding domain and the clathrin adaptor protein (AP-2) binding motif--in the synaptic localization of NMDA receptors. NR2B subunits lacking functional PDZ binding are excluded from the synapse. Mutations in the AP-2 binding motif, YEKL, significantly increase the number of synaptic receptors and allow the synaptic localization of NR2B subunits lacking PDZ binding. Peptides corresponding to YEKL increase the synaptic response within minutes. In contrast, the NR2A subunit localizes to the synapse in the absence of PDZ binding and is not altered by mutations in its motif corresponding to YEKL of NR2B. This study identifies a dynamic regulation of synaptic NR2B-containing NMDARs through PDZ protein-mediated stabilization and AP-2-mediated internalization that is modulated by phosphorylation by Fyn kinase.

The NMDA receptor (NMDAR) plays a central role in the function of excitatory synapses. Recent studies have provided interesting insights into several aspects of the trafficking of this receptor in neurons. The NMDAR is not a static resident of the synapse. Rather, the number and composition of synaptic NMDARs can be modulated by several factors. The interaction of PDZ proteins, generally thought to occur at the synapse, appears to occur early in the secretory pathway; this interaction may play a role in the assembly of the receptor complex and its exit from the endoplasmic reticulum. This review addresses recent advances in our understanding of NMDAR trafficking and its synaptic delivery and maintenance.

Developmental changes in miniature IPSC (mIPSC) kinetics have been demonstrated previously in cerebellar neurons in rodents. We report that these kinetic changes in mice are determined primarily by developmental changes in GABA A receptor subunit expression. mIPSCs were studied by whole-cell recordings in cerebellar slices, prepared from postnatal day 11 (P11) and P35 mice. Similar to reports in granule neurons, wild-type cerebellar stellate neuron mIPSCs at P11 had slow decay kinetics, whereas P35 mIPSCs decayed five times faster. When mIPSCs in cerebellar stellate neurons were compared between wild-type (ϩ/ϩ) and GABA A receptor ␣1 subunitdeficient (Ϫ/Ϫ) littermates at P35, we observed dramatically slower mIPSC decay rates in Ϫ/Ϫ animals. We took advantage of the greater potency of imidazopyridines for GABA current potentiation with ␣1 subunit-containing receptors to characterize the relative contribution of ␣1 subunits in native receptors on inhibitory synapses of cerebellar granule neurons. Zolpideminduced prolongation of mIPSC decay was variable among distinct cells, but it increased during development in wild-type mice. Similarly, Zolpidem prolongation of mIPSC decay rate was significantly greater in adult ϩ/ϩ mice than in knock-outs. We propose that an increased ␣1 subunit assembly in postsynaptic receptors of cerebellar inhibitory synapses is responsible for the fast inhibitory synaptic currents that are normally observed during postnatal development. Key words: GABA receptor; gene knock-out; patch-clamp; inhibitory synapses; development; GABASpontaneous IPSCs (sIPSCs) and miniature IPSCs (mIPSCs) [recorded in the presence of tetrodotoxin (TTX)] in mammalian neurons have been shown to become faster with development (Brickley et al., 1996;Draguhn and Heinemann, 1996;Tia et al., 1996;Brussard et al., 1997;Hollrigel and Soltesz, 1997;Pouzat and Hestrin, 1997;Dunning et al., 1999;Okada et al., 2000). Several mechanisms have been proposed to explain these changes, including changes in expression of specific GABA A receptor (GABA A -R) subunits (Tia et al., 1996; Dunning et al., 2000;Okada et al., 2000) and changes in reuptake mechanisms (Draguhn and Heinemann, 1996). Recent studies with overexpression of the ␣1 subunit of the GABA A receptor in thalamic neurons have indicated that the enhanced expression of the ␣1 subunit could be responsible for faster mIPSCs (Okada et al., 2000). This hypothesis is supported by work in recombinant systems that demonstrate that rapid application of brief pulses of GABA to outside-out excised patches generates faster currents with ␣1␤2␥2 than with other subunit combinations (Verdoorn, 1994;Gingrich et al., 1995;Lavoie et al., 1997;McClellan and Twyman, 1999).sIPSCs have been characterized extensively in granule and stellate neurons in rat cerebellar slices (Llano and Gerschenfeld, 1993;Brickley et al., 1996Brickley et al., , 1999Tia et al., 1996;Nusser et al., 1997;Kondo and Marty, 1998). The decay time of sIPSCs in granule neurons has been shown to undergo a developmental...

NMDA (N-methyl-D-aspartate) receptors (NMDARs) are targeted to dendrites and anchored at the post-synaptic density (PSD) through interactions with PDZ proteins. However, little is known about how these receptors are sorted from the endoplasmic reticulum and Golgi apparatus to the synapse. Here, we find that synapse-associated protein 102 (SAP102) interacts with the PDZ-binding domain of Sec8, a member of the exocyst complex. Our results show that interactions between SAP102 and Sec8 are involved in the delivery of NMDARs to the cell surface in heterologous cells and neurons. Furthermore, they suggest that an exocyst-SAP102-NMDAR complex is an important component of NMDAR trafficking.