Cocaine-induced locomotor activity in GXE mice. (a) Locomotor activity in four groups of mice are shown when mice were injected with saline on Day 0 and with cocaine (10 mg/kg, i.p.) on Days 1–5. Repeated cocaine exposure induced a greater increase in locomotor activity in GXE mice. ** p < 0.01, Bonferroni post hoc tests, GXE vs WT, G, or E at the same day. CTL, n = 7; E, n = 6; G, n = 10; GXE, n = 9. (b) Locomotor activity in four groups of mice are shown when mice were injected with saline on Days 0–5, and an acute administration of cocaine (10 mg/kg) on Day 6. No differences between the groups were seen following repeated saline administration (Days 0–5) or during the cocaine challenge (Day 6). n = 5 each.