Emotion enhances our ability to form vivid memories of even trivial events. Norepinephrine (NE), a neuromodulator released during emotional arousal, plays a central role in the emotional regulation of memory. However, the underlying molecular mechanism remains elusive. Toward this aim, we have examined the role of NE in contextual memory formation and in the synaptic delivery of GluR1-containing alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA)-type glutamate receptors during long-term potentiation (LTP), a candidate synaptic mechanism for learning. We found that NE, as well as emotional stress, induces phosphorylation of GluR1 at sites critical for its synaptic delivery. Phosphorylation at these sites is necessary and sufficient to lower the threshold for GluR1 synaptic incorporation during LTP. In behavioral experiments, NE can lower the threshold for memory formation in wild-type mice but not in mice carrying mutations in the GluR1 phosphorylation sites. Our results indicate that NE-driven phosphorylation of GluR1 facilitates the synaptic delivery of GluR1-containing AMPARs, lowering the threshold for LTP, thereby providing a molecular mechanism for how emotion enhances learning and memory.

Incorporation of GluR1-containing AMPA receptors into synapses is essential to several forms of neural plasticity, including long-term potentiation (LTP). Numerous signaling pathways that trigger this process have been identified, but the direct modifications of GluR1 that control its incorporation into synapses are unclear. Here, we show that phosphorylation of GluR1 by PKC at a highly conserved serine 818 residue is increased during LTP and critical for LTP expression. GluR1 is phosphorylated by PKC at this site in vitro and in vivo. In addition, acute phosphorylation at GluR1 S818 by PKC, as well as a phosphomimetic mutation, promotes GluR1 synaptic incorporation. Conversely, preventing GluR1 S818 phosphorylation reduces LTP and blocks PKC-driven synaptic incorporation of GluR1. We conclude that the phosphorylation of GluR1 S818 by PKC is a critical event in the plasticity-driven synaptic incorporation of AMPA receptors.

PSD-95 is a major protein found in virtually all mature excitatory glutamatergic synapses in the brain. Here, we have addressed the role of PSD-95 in controlling glutamatergic synapse function by generating and characterizing a PSD-95 KO mouse. We found that the ␣-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) subtype of glutamate receptor (AMPAR)-mediated synaptic transmission was reduced in these mice. Two-photon (2P) uncaging of MNI-glutamate onto individual spines suggested that the decrease in AMPAR function in the PSD-95 KO mouse stems from an increase in the proportion of ''silent'' synapses i.e., synapses containing N-methyl-D-aspartate (NMDA) receptors (NMDARs) but no AMPARs. Unexpectedly, the silent synapses in the KO mouse were located onto morphologically mature spines. We also observed that a significant population of synapses appeared unaffected by PSD-95 gene deletion, suggesting that the functional role of PSD-95 displays synapse-specificity. In addition, we report that the decay of NMDAR-mediated current was slower in KO mice: The contribution of NR2B subunit containing receptors to the NMDARmediated synaptic current was greater in KO mice. The greater occurrence of silent synapses might be related to the greater magnitude of potentiation after long-term potentiation induction observed in these mice. Together, these results suggest a synapsespecific role for PSD-95 in controlling synaptic function that is independent of spine morphology.glutamate receptors ͉ hippocampus ͉ spines ͉ synaptic transmission ͉ two-photon uncaging G lutamate, the major excitatory neurotransmitter in the brain, activates ionotropic glutamate receptors of the ␣-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), N-methyl-D-aspartate (NMDA), and kainate subtypes. There is considerable interest in elucidating the molecular mechanisms that controls synaptic targeting and trafficking of these receptors, in part because of their role in the induction and expression of various forms of synaptic plasticity (1). These receptors are imbedded in an electron-dense structure, the postsynaptic density (PSD), which is believed to contain key molecules involved in the regulation of glutamate receptor targeting and trafficking. PSD-95, a member of the membraneassociated guanylate kinase (MAGUK) superfamily of proteins, is a core component of the PSD and is thought to be important in the control of excitatory synapse function (2, 3). Because of its interaction with the cytoplasmic domains of NMDA receptor (NMDAR) subunits, it has long been suspected that PSD-95 might control the synaptic targeting of NMDARs (4, 5). However, more recent studies based on sustained or transient overexpression of PSD-95 in neurons have forced a reassessment of this view by suggesting that the primary role of PSD-95 is restricted to controlling AMPAR synaptic expression (6-10). Intriguingly, previous work on a PSD-95 KO mouse, reported no apparent changes in either AMPAR or NMDAR function (11). The interpretation of these data are, however, ...