Escalation of alcohol self-administration. A, Timeline of the experiment. Dependent and nondependent rats were injected with saline or nicotine (0.8 mg/kg, s.c.) once per day, 15 min before each operant session after 6–8 h of withdrawal from alcohol vapor. The data represent the mean ± SEM number of active lever presses for 10% w/v alcohol in dependent (B) and nondependent (C) rats treated with saline (n = 8–9; black circles) or nicotine (n = 8–9; black squares) and dependent (n = 9; black squares) and nondependent (n = 9; black circles) rats treated with nicotine (0.8 mg/kg, s.c.; D). *p < 0.05, compared with saline group; **p < 0.001, compared with saline group; #p < 0.05, compared with baseline.