Fig. 3.

Differential escalation of intravenous cocaine self-administration in high-impulsive rats (n = 8) as compared to non-impulsive rats (n = 8). On the first 5 days, access to cocaine was restricted to 5 hours and a maximum of 50 infusions. After a withdrawal period of 9 days, access to cocaine was increased on each of the following 5 days to 8 hours and a maximum of 150 infusions. This pattern of intermittent cocaine self-administration was repeated on two further occasions. Impulsive rats showed a differential increase in their rate of cocaine self-administration after extended access to cocaine [session: F(19,133) = 2.04, P = 0.01; group: F(1,14) = 32.82, P < 0.001; session × group: F(19,133) = 1.92, P = 0.017]. Subsequent pairwise comparisons revealed significant differences (P < 0.05) between the first session and sessions 10, 13, 14, and 15 for the non-impulsive group and between the first session and all sessions but nos. 2, 3, 4, 10, and 16 for the high-impulsive group. Black circles, high-impulsive rats; white circles, non-impulsive rats.