Figure 2.

(A) Dose-response function of cocaine by rats responding under a progressive-ratio schedule. Test sessions under a progressive-ratio schedule ended when rats did not achieve reinforcement within 1 h. The data are expressed as the number of injections per session on the left axis and ratio per injection on the right axis. *p < 0.05, compared with ShA rats at each dose of cocaine. [Taken with permission from Wee et al., 2008.] (B) Responding for heroin under a progressive-ratio schedule of reinforcement in ShA and LgA rats. *p < 0.05, LgA significantly different from LgA. [Modified with permission from Barbier et al., 2013.] (C) Dose-response for methamphetamine under a progressive-ratio schedule. Test sessions under a progressive-ratio schedule ended when rats did not achieve reinforcement within 1 h. *p < 0.05, **p < 0.01, LgA significantly different from ShA [Modified from Wee et al., 2007.] (D) Breakpoints on a progressive-ratio schedule in long access (LgA) rats that self-administered nicotine with 48 h abstinence between sessions. LgA rats on an intermittent schedule reached significantly higher breakpoints than LgA rats that self-administered nicotine daily. The data are expressed as mean ± SEM. *p < 0.05. n = 9 rats per group. [Taken with permission from Cohen et al., 2012.] (E) Mean (±SEM) breakpoints for ethanol while in nondependent and ethanol-dependent states. **p < 0.01, main effect of vapor exposure on ethanol self-administration. [Taken with permission from Walker and Koob, 2007.]