To examine cognitive functions in more detail, we extended our cognitive evaluation Bioactive Compound Library in vivo by assessing memory extinction. Memory extinction is a form of inhibitory learning that provides a basis for an adaptive control

of cognition and represents one of the key aspects of mental flexibility (Radulovic and Tronson, 2010, Herry et al., 2010 and Floresco and Jentsch, 2011). While it has been demonstrated that various stages of memory extinction require the recruitment of widespread brain domains, the molecular mechanisms regulating this process remain unclear (Lattal and Abel, 2001, Lattal et al., 2003, Fischer et al., 2004, Fischer et al., 2007, Sananbenesi et al., 2007, Radulovic and Tronson, 2010, Agis-Balboa et al., 2011, Bahari-Javan et al., 2012 and Tronson et al., 2012). In order to test the effects of Tet1 ablation on memory extinction, we performed extinction training using two groups of male littermate Tet1+/+ and Tet1KO mice following contextual fear conditioning as described earlier. Twenty-four hours

after fear conditioning, both Tet1+/+ and Tet1KO groups exhibited similar freezing levels (65%–75%) (p > 0.05; Figure 2D). After contextual fear memory test, Tet1+/+ and Tet1KO animals were placed into the same conditioning cages for a “massed” extinction trial (Cain et al., 2003 and Polack et al., 2012). Twenty-four Adriamycin hours later, memory extinction was assessed by scoring freezing events. Interestingly, we found that while control Tet1+/+ mice exhibited robust memory extinction (∼20% freezing after extinction training), Tet1KO mutants failed to display any memory extinction and retained an average freezing

level of about 60% (p < 0.01 control versus Tet1KO; Figure 2D). To extend cognitive evaluation of the Tet1KO mice, we assessed their hippocampus-dependent spatial reference memory using the Morris water Oxalosuccinic acid maze test (MWM) (Vorhees and Williams, 2006). Two groups of male littermate Tet1+/+ and Tet1KO mice were subjected to two training trials per day for 6 days, and the escape latency was scored for each trial. Probe trial was conducted 24 hr after the last day of training. We observed no significant differences between the groups during either the training or the probe trials (p > 0.05; p > 0.05; Figures 2E and 2F). To test spatial memory extinction, we then exposed control and Tet1KO mice used for spatial learning tests to extinction training in the MWM with the same spatial cues but with the platform removed (Zhang et al., 2011). We discovered that while Tet1+/+ mice demonstrated considerable memory extinction as evidenced by their progressively decreasing target quadrant occupancy (from about 35% on the probe trial to about 15% on the last day of extinction training), the Tet1KO animals persevered searching for the platform in the former target quadrant and did not show any decrease in quadrant preference (p > 0.05 for Tet1KO; p < 0.