a Experimental timeline of self-administration in the nicotine vapor group and vehicle vapor group. b Acquisition and maintenance of nicotine vapor self-administration during 2 weeks of daily short access (1 h) to nicotine vapor (0.5 mg/ml, n = 24 [12 females, 12 males]) or vehicle vapor (1:1, propylene glycol:glycerol, n = 24 [13 females, 11 males]). Error bars represent the SEM of lever presses. c Nicotine vapor and vehicle vapor self-administration following pretreatment with varenicline (1.5 mg/ml, s.c.). Error bars represent the SEM of lever presses.