Figure 4.

(A) Effect of drug availability on cocaine intake (mean ± SEM). In 6 h long-access (LgA) rats (n = 12) but not in 1 h short-access (ShA) rats (n = 12), mean total cocaine intake started to increase significantly from session 5 (p < 0.05; sessions 5 to 22 compared with session 1) and continued to increase thereafter (p < 0.05; session 5 compared with sessions 8–10, 12, 13, 17–22). [Taken with permission from Ahmed and Koob, 1998.] (B) Effect of drug availability on total intravenous heroin self-infusions (mean ± SEM). During the escalation phase, rats had access to heroin (40 mg per infusion) for 1 h (ShA rats, n = 5–6) or 11 h per session (LgA rats, n = 5–6). Regular 1 h (ShA rats) or 11 h (LgA rats) sessions of heroin self-administration were performed 6 days per week. The dotted line indicates the mean (± SEM) number of heroin self-infusions of LgA rats during the first 11 h session. *p < 0.05, compared with the first session (paired t-test). [Taken with permission from Ahmed et al., 2000.] (C) Effect of extended access to intravenous methamphetamine self-administration as a function of daily sessions in rats trained to self-administer 0.05 mg/kg/infusion of intravenous methamphetamine during a 6 h session. Short access group (ShA), 1 h session (n = 6). Long access group (LgA), 6 h session (n = 4). All data were analyzed using two-way analysis of variance (dose × escalation session within ShA or LgA group). *p < 0.05, **p < 0.01, ***p < 0.001 vs. Day 1. [Taken with permission from Kitamura et al., 2006.] (D) Total 23 h active and inactive responses after repeated cycles of 72 h of nicotine deprivation (ND) followed by 4 days of self-administration (*p < 0.05 vs. baseline). [Taken with permission from George et al., 2007.] (E) Ethanol deliveries (mean ± SEM) in rats trained to respond for 10% ethanol and then either not exposed to ethanol vapor (control, n = 5) or exposed to intermittent ethanol vapor (14 h on/10 h off) for 2 weeks and then tested either 2 h (n = 6) or 8 h (n = 6) after removal from ethanol vapor. *p < 0.05, significant increase in operant self-administration of ethanol in rats receiving intermittent vapor exposure compared with control. No difference was observed between rats exposed to intermittent vapor and tested either 2 or 8 h after ethanol withdrawal. [Taken with permission from O’Dell et al., 2004.]