a Reprinted with permission of AAAS/Science from Ahmed SH, Koob GF: Transition from moderate to excessive drug intake: change in hedonic set point. Science 1998; 282:298–300, Figures 2A and 2B, p. 299.
b In long-access rats (N=12) but not in short-access rats (N=12), mean total cocaine intake started to increase significantly from session 5 (p<0.05; sessions 5–22 compared to session 1) and continued to increase thereafter (p<0.05; session 5 compared to sessions 8–10, 12, 13, and 17–22).
c During the first hour, long-access rats self-administered more infusions than short-access rats during sessions 5–8, 11, 12, 14, 15, and 17–22 (p<0.05).