At excitatory synapses, the postsynaptic scaffolding protein postsynaptic density 95 (PSD-95) couples NMDA receptors (NMDARs) to the Ras GTPase-activating protein SynGAP. The close association of SynGAP and NMDARs suggests that SynGAP may have an important role in NMDAR-dependent activation of Ras signaling pathways, such as the MAP kinase pathway, and in synaptic plasticity. To explore this issue, we examined long-term potentiation (LTP), p42 MAPK (ERK2) signaling, and spatial learning in mice with a heterozygous null mutation of the SynGAP gene (SynGAP(-/+)). In SynGAP(-/+) mutant mice, the induction of LTP in the hippocampal CA1 region was strongly reduced in the absence of any detectable alteration in basal synaptic transmission and NMDAR-mediated synaptic currents. Although basal levels of activated ERK2 were elevated in hippocampal extracts from SynGAP(-/+) mice, NMDAR stimulation still induced a robust increase in ERK activation in slices from SynGAP(-/+) mice. Thus, although SynGAP may regulate the ERK pathway, its role in LTP most likely involves additional downstream targets. Consistent with this, the amount of potentiation induced by stimulation protocols that induce an ERK-independent form of LTP were also significantly reduced in slices from SynGAP(-/+) mice. An elevation of basal phospho-ERK2 levels and LTP deficits were also observed in SynGAP(-/+)/H-Ras(-)/- double mutants, suggesting that SynGAP may normally regulate Ras isoforms other than H-Ras. A comparison of SynGAP and PSD-95 mutants suggests that PSD-95 couples NMDARs to multiple downstream signaling pathways with very different roles in LTP and learning.

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Brain injury, genetic manipulations, and pharmacological treatments can result in alterations of motor skills in mice. Fine motor coordination and balance can be assessed by the beam walking assay. The goal of this test is for the mouse to stay upright and walk across an elevated narrow beam to a safe platform. This test takes place over 3 consecutive days: 2 days of training and 1 day of testing. Performance on the beam is quantified by measuring the time it takes for the mouse to traverse the beam and the number of paw slips that occur in the process. Here we report the protocol used in our laboratory, and representative results from a cohort of C57BL/6 mice. This task is particularly useful for detecting subtle deficits in motor skills and balance that may not be detected by other motor tests, such as the Rotarod.

The membrane-associated guanylate kinases (MAGUKs) PSD-95, PSD-93 and SAP102 are thought to have crucial roles in both AMPA receptor trafficking and formation of NMDA receptor-associated signalling complexes involved in synaptic plasticity. While PSD-95, PSD-93, and SAP102 appear to have similar roles in AMPA receptor trafficking, it is not known whether these MAGUKs also have functionally similar roles in synaptic plasticity. To explore this issue we examined several properties of basal synaptic transmission in the hippocampal CA1 region of PSD-93 and PSD-95 mutant mice and compared the ability of a number of different synaptic stimulation protocols to induce long-term potentiation (LTP) and long-term depression (LTD) in these mutants. We find that while both AMPA and NMDA receptor- content in synapses, probably through TARP binding (Elias & Nicoll, 2007). For example, overexpression of PSD-95, PSD-93 or SAP102 increases AMPAR incorporation into excitatory synapses (El-Husseini et al. 2000;Schnell et al. 2002;Béïque & Andrade, 2003;Stein et al. 2003;Ehrlich & Malinow, 2004;Nakagawa et al. 2004;Elias et al. 2006). Conversely, acute shRNA-mediated knockdown of either PSD-95 or PSD-93 decreases AMPAR-mediated synaptic transmission in cultured hippocampal pyramidal cells (Elias et al. 2006). Thus, these MAGUKs appear to have similar roles in recruiting AMPARs to synapses (Elias & Nicoll, 2007).There is now abundant evidence that MAGUKs also participate in NMDAR-dependent forms of synaptic plasticity such as long-term potentiation (LTP) and long-term depression (Migaud et al. 1998;Béïque & Andrade, 2003;Colledge et al. 2003;Stein et al. 2003;Yao et al. 2004;Béïque et al. 2006;Cuthbert et al. 2007). It is unclear, however, whether different MAGUKs have unique roles in LTP despite their apparently overlapping roles in AMPAR trafficking. To address this issue we examined properties of basal synaptic transmission in PSD-95 and PSD-93 knockout mice as well as the ability of different synaptic stimulation protocols to induce LTP in the hippocampal CA1 region in these mutants. We find that AMPAR-mediated basal synaptic transmission is impaired in PSD-95 mutants but normal in PSD-93 mutant mice. Thus, PSD-95 has a unique role in AMPAR trafficking at excitatory synapses in the adult hippocampus which is not apparent in cultured neurons. Moreover, we find that PSD-93 knockout mice exhibit deficits in LTP and normal LTD, in contrast to the facilitated LTP and impaired LTD observed in the absence of PSD-95. Thus, PSD-95 and PSD-93 appear to have distinct roles in synaptic plasticity, perhaps through differential recruitment of signalling molecules to the NMDAR.

Short-term and long-term changes in the strength of synapses in neural networks underlie working memory and long-term memory storage in the brain. These changes are regulated by many biochemical signalling pathways in the postsynaptic spines of excitatory synapses. Recent findings about the roles and regulation of the small GTPases Ras, Rap and Rac in spines provide new insights into the coordination and cooperation of different pathways to effect synaptic plasticity. Here, we present an initial working representation of the interactions of five signalling cascades that are usually studied individually. We discuss their integrated function in the regulation of postsynaptic plasticity.