The CPP procedure was performed as described in Fig. 1a and Methods. a Clk1+/− mutant mice were divided into three groups: saline, METH 2 mg/kg, and METH 4 mg/kg. Each group received 4 consecutive days of injections in the conditioning phase. Then, following a post-conditioning test, CPP scores were calculated (n = 8–9 animals per group). b After this post-conditioning test, the group receiving METH 2 mg/kg received an additional 4 consecutive days of injections in the conditioning phase (i.e., a total of 8 days). Then, following another post-conditioning test, CPP scores were again calculated (n = 8–9 animals per group). Bar graphs are presented as means ± SEM. Statistical analysis was performed using one-way ANOVA followed by Bonferroni-corrected tests (*P < 0.05 vs saline; NS not significant).