Effect of pre-treatment of HDAC inhibitors on conditioned avoidance response in HAL-treated young and aged mice. Data are shown as the percentage of avoidance response of young (a and b) and aged mice (c and d). Data are presented as mean±SEM (n=6–8 per group). In panel a, *p<0.05 vs the saline-, VPA-treated groups. In panel b, *p<0.05 vs the saline-, MS-275-treated groups. In panel c, **p<0.01, ***p<0.001 vs the saline-, HAL-, and VPA-treated groups. In panel d, **p<0.01, ***p<0.001 vs the saline-, HAL-, and MS-275-treated groups.