, 2000, Nithianantharajah and Hannan, 2006 and Baroncelli et al., 2010). Thus, enrichment produces robust and reversible learn more increases in the numbers of excitatory synapses in the CNS, as well as circuit alterations reminiscent of enhanced plasticity in juveniles (Moser et al., 1997, Gogolla et al., 2009 and Baroncelli et al., 2010). In parallel, when mice with targeted mutations that compromise synaptic plasticity and learning are housed in enriched environment, learning deficits due to the mutant background can be overcome (Rampon et al., 2000 and Nithianantharajah and Hannan,

2006). Furthermore, enrichment promotes access to critical period-like plasticity and enhances recovery after lesions in the adult (Kim et al., 2008 and Baroncelli et al., 2010). The powerful behavioral consequences of environmental enrichment may thus involve enhanced synapse turnover and synaptogenesis, but testing this hypothesis has been prevented by the absence of tools to specifically interfere with synaptogenesis processes in the adult. Here, we introduce a mouse model with a specific deficit in the assembly of synapses under conditions

of enhanced plasticity in the adult and exploit the model to investigate a role for enhanced synaptogenesis in supporting learning and memory upon environmental enrichment. While under LY294002 basal conditions, only a minority of synapses turn over in the adult CNS, physiological signals that promote plasticity not only increase

synaptogenesis, but also Urease enhance synapse turnover. For example, (1) the potent enhancer of plasticity BDNF promotes synaptogenesis and spine turnover (Horch et al., 1999 and Yoshii and Constantine-Paton, 2010), and Wnt factors can both destabilize synapses and enhance synaptogenesis (Klassen and Shen, 2007 and Sahores et al., 2010); (2) studies in organotypic slice cultures and in vivo have provided evidence that treatments inducing long-term potentiation of synaptic transmission not only stimulate the establishment and maintenance of new synapses, but also produce a widespread destabilization of spine synapses (De Roo et al., 2008 and Barbosa et al., 2008); (3) oculodominance shift experiments in adult mice have provided evidence for enhanced synapse turnover paired to long-term retention of functionally important synapses in visual cortex (Hofer et al., 2009); (4) enhanced plasticity during circuit maturation is accompanied by both higher synapse densities and higher turnover rates of synapses (Gan et al., 2003). Taken together, these studies in different systems and under different experimental circumstances all suggest that learning-related plasticity may involve enhanced synapse turnover coupled to the establishment and retention of critical synapses.